Protocol Independent Multicast - Dense Mode (PIM-DM): Protocol Specification (Revised)
draft-ietf-pim-dm-new-v2-05
The information below is for an old version of the document that is already published as an RFC.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 3973.
|
|
|---|---|---|---|
| Authors | Jonathan Nicholas , Andrew Adams , William Siadak | ||
| Last updated | 2020-01-21 (Latest revision 2004-06-24) | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | Experimental | ||
| Formats | |||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | (None) | |
| Document shepherd | (None) | ||
| IESG | IESG state | Became RFC 3973 (Experimental) | |
| Action Holders |
(None)
|
||
| Consensus boilerplate | Unknown | ||
| Telechat date | (None) | ||
| Responsible AD | Alex D. Zinin | ||
| Send notices to | pusateri@juniper.net, lwei@redback.com, mcbride@cisco.com |
draft-ietf-pim-dm-new-v2-05
Internet Engineering Task Force PIM WG
INTERNET DRAFT Andrew Adams (NextHop Technolgies)
draft-ietf-pim-dm-new-v2-05.txt Jonathan Nicholas (ITT A/CD)
William Siadak (NextHop Technologies)
June 2004
Expires December 2004
Protocol Independent Multicast - Dense Mode (PIM-DM):
Protocol Specification (Revised)
Status of this Document
This document is an Internet Draft and is in full conformance with all
provisions of Section 10 of RFC 2026.
Internet Drafts are working documents of the Internet Engineering Task
Force (IETF), its areas, and its working groups. Note that other groups
may also distribute working documents as Internet Drafts.
Internet Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet Drafts as reference material
or to cite them other than as "work in progress."
The list of current Internet Drafts can be accessed at
http://www.ietf.org/ietf/lid-abstracts.txt.
The list of Internet Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This document is a product of the IETF PIM WG. Comments should be
addressed to the authors, or the WG's mailing list at pim@ietf.org.
Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
This document specifies Protocol Independent Multicast - Dense Mode
(PIM-DM). PIM-DM is a multicast routing protocol that uses the
underlying unicast routing information base to flood multicast datagrams
to all multicast routers. Prune messages are used to prevent future
messages from propagating to routers with no group membership
information.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Definitions . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2. Pseudocode Notation . . . . . . . . . . . . . . . . . . . . 5
3. PIM-DM Protocol Overview . . . . . . . . . . . . . . . . . . 5
4. Protocol Specification . . . . . . . . . . . . . . . . . . . 6
4.1. PIM Protocol State . . . . . . . . . . . . . . . . . . . . . 6
4.1.1. General Purpose State . . . . . . . . . . . . . . . . . . . 7
4.1.2. (S,G) State . . . . . . . . . . . . . . . . . . . . . . . . 7
4.1.3. State Summarization Macros . . . . . . . . . . . . . . . . . 8
4.2. Data Packet Forwarding Rules . . . . . . . . . . . . . . . . 10
4.3. Hello Messages . . . . . . . . . . . . . . . . . . . . . . . 10
4.3.1. Sending Hello Messages . . . . . . . . . . . . . . . . . . . 10
4.3.2. Receiving Hello Messages . . . . . . . . . . . . . . . . . . 11
4.3.3. Hello Message Hold Time . . . . . . . . . . . . . . . . . . 11
4.3.4. Handling Router Failures . . . . . . . . . . . . . . . . . . 11
4.3.5. Reducing Prune Propagation Delay on LANs . . . . . . . . . . 12
4.4. PIM-DM Prune, Join and Graft Messages . . . . . . . . . . . 13
4.4.1. Upstream Prune, Join and Graft Messages . . . . . . . . . . 13
4.4.1.1. Transitions from the Forwarding (F) State . . . . . . . . . 16
4.4.1.2. Transitions from the Pruned (P) State . . . . . . . . . . . 17
4.4.1.3. Transitions from the AckPending (AP) State . . . . . . . . . 18
4.4.2. Downstream Prune, Join and Graft Messages . . . . . . . . . 19
4.4.2.1. Transitions from the NoInfo State . . . . . . . . . . . . . 21
4.4.2.2. Transitions from the PrunePending (PP) State . . . . . . . . 22
4.4.2.3. Transitions from the Prune (P) State . . . . . . . . . . . . 23
4.5. State Refresh . . . . . . . . . . . . . . . . . . . . . . . 24
4.5.1. Forwarding of State Refresh Messages . . . . . . . . . . . . 24
4.5.2. State Refresh Message Origination . . . . . . . . . . . . . 25
4.5.2.1. Transitions from the NotOriginator (NO) State . . . . . . . 27
4.5.2.2. Transitions from the Originator (O) State . . . . . . . . . 27
4.6. PIM Assert Messages . . . . . . . . . . . . . . . . . . . . 28
4.6.1. Assert Metrics . . . . . . . . . . . . . . . . . . . . . . . 28
4.6.2. AssertCancel Messages . . . . . . . . . . . . . . . . . . . 29
4.6.3. Assert State Macros . . . . . . . . . . . . . . . . . . . . 29
4.6.4. (S,G) Assert Message State Machine . . . . . . . . . . . . . 29
4.6.4.1. Transitions from NoInfo State . . . . . . . . . . . . . . . 31
4.6.4.2. Transitions from Winner State . . . . . . . . . . . . . . . 32
4.6.4.3. Transitions from Loser State . . . . . . . . . . . . . . . . 33
4.6.5. Rationale for Assert Rules . . . . . . . . . . . . . . . . . 34
4.7. PIM Packet Formats . . . . . . . . . . . . . . . . . . . . . 35
4.7.1. PIM Header . . . . . . . . . . . . . . . . . . . . . . . . . 35
4.7.2. Encoded Unicast Address . . . . . . . . . . . . . . . . . . 36
4.7.3. Encoded Group Address . . . . . . . . . . . . . . . . . . . 36
4.7.4. Encoded Source Address . . . . . . . . . . . . . . . . . . . 37
4.7.5. Hello Message Format . . . . . . . . . . . . . . . . . . . . 38
4.7.5.1. Hello Hold Time Option . . . . . . . . . . . . . . . . . . . 39
4.7.5.2. LAN Prune Delay Option . . . . . . . . . . . . . . . . . . . 39
4.7.5.3. Generation ID Option . . . . . . . . . . . . . . . . . . . . 40
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4.7.5.4. State Refresh Capable Option . . . . . . . . . . . . . . . . 40
4.7.6. Join/Prune Message Format . . . . . . . . . . . . . . . . . 40
4.7.7. Assert Message Format . . . . . . . . . . . . . . . . . . . 42
4.7.8. Graft Message Format . . . . . . . . . . . . . . . . . . . . 43
4.7.9. Graft Ack Message Format . . . . . . . . . . . . . . . . . . 43
4.7.10. State Refresh Message Format . . . . . . . . . . . . . . . . 44
4.8. PIM-DM Timers . . . . . . . . . . . . . . . . . . . . . . . 45
5. Protocol Interaction Considerations . . . . . . . . . . . . 48
5.1. PIM-SM Interactions . . . . . . . . . . . . . . . . . . . . 48
5.2. IGMP Interactions . . . . . . . . . . . . . . . . . . . . . 49
5.3. Source Specific Multicast (SSM) Interactions . . . . . . . . 49
5.4. Multicast Group Scope Boundary Interactions . . . . . . . . 49
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . 49
6.1. PIM Address Family . . . . . . . . . . . . . . . . . . . . . 49
6.2. PIM Hello Options . . . . . . . . . . . . . . . . . . . . . 50
7. Security Considerations. . . . . . . . . . . . . . . . . . . 50
7.1. Attacks Based on Forged Messages . . . . . . . . . . . . . . 50
7.2. Non-cryptographic Authentication Mechanisms . . . . . . . . 51
7.3. Authentication Using IPsec . . . . . . . . . . . . . . . . . 52
7.4. Denial of Service Attacks . . . . . . . . . . . . . . . . . 53
8. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 53
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 53
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 54
10.1. Normative References . . . . . . . . . . . . . . . . . . . . 54
10.2. Informative References . . . . . . . . . . . . . . . . . . . 54
11. Full Copyright Statement . . . . . . . . . . . . . . . . . . 55
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1. Introduction
This specification defines a multicast routing algorithm for multicast
groups that are densely distributed across a network. This protocol
does not have a topology discovery mechanism often used by a unicast
routing protocol. It employs the same packet formats sparse mode PIM
(PIM-SM) uses. This protocol is called PIM - Dense Mode. The
foundation of this design was largely built on Deering's early work on
IP multicast routing [11].
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" are to be
interpreted as described in RFC 2119 and indicate requirement levels for
compliant PIM-DM implementations.
2.1. Definitions
Multicast Routing Information Base (MRIB)
This is the multicast topology table, which is typically derived from
the unicast routing table, or routing protocols such as MBGP that
carry multicast-specific topology information. PIM-DM uses the MRIB
to make decisions regarding RPF interfaces.
Tree Information Base (TIB)
This is the collection of state maintained by a PIM router and created
by receiving PIM messages and IGMP information from local hosts. It
essentially stores the state of all multicast distribution trees at
that router.
Reverse Path Forwarding (RPF)
RPF is a multicast forwarding mode where a data packet is accepted for
forwarding only if it is received on an interface used to reach the
source in unicast.
Upstream Interface
Interface towards the source of the datagram. Also known as the RPF
Interface.
Downstream Interface
All interfaces that are not the upstream interface, including the
router itself.
(S,G) Pair
Source S and destination group G associated with an IP packet.
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2.2. Pseudocode Notation
We use set notation in several places in this specification.
A (+) B
is the union of two sets A and B.
A (-) B
are the elements of set A that are not in set B.
NULL
is the empty set or list.
Note that operations MUST be conducted in the order specified. This is
due to the fact that (-) is not a true difference operator because B is
not necessarily a subset of A. That is, A (+) B (-) C = A (-) C (+) B
is not a true statement unless C is a subset of both A and B.
In addition we use C-like syntax:
= denotes assignment of a variable.
== denotes a comparison for equality.
!= denotes a comparison for inequality.
Braces { and } are used for grouping.
3. PIM-DM Protocol Overview
This section provides an overview of PIM-DM behavior. It is intended as
an introduction to how PIM-DM works, and is NOT definitive. For the
definitive specification, see Section 4 - Protocol Specification.
PIM-DM assumes that when a source starts sending, all downstream systems
want to receive multicast datagrams. Initially, multicast datagrams are
flooded to all areas of the network. PIM-DM uses RPF to prevent looping
of multicast datagrams while flooding. If some areas of the network do
not have group members, PIM-DM will prune off the forwarding branch by
instantiating prune state.
Prune state has a finite lifetime. When that lifetime expires, data
will again be forwarded down the previously pruned branch.
Prune state is associated with an (S,G) pair. When a new member for a
group G appears in a pruned area, a router can "graft" toward the source
S for the group, thereby turning the pruned branch back into a
forwarding branch.
The broadcast of datagrams followed by pruning of unwanted branches is
often referred to as a flood and prune cycle and is typical of dense
mode protocols.
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In order to minimize repeated flooding of datagrams and subsequent
pruning associated with a particular (S,G) pair, PIM-DM uses a state
refresh message. This message is sent by the router(s) directly
connected to the source and is propagated throughout the network. When
received by a router on its RPF interface, the state refresh message
causes an existing prune state to be refreshed.
Compared with multicast routing protocols with built in topology
discovery mechanisms (e.g. DVMRP [12]) PIM-DM has a simplified design
and is not hard-wired into a specific topology discovery protocol.
However, such a simplification does incur more overhead by causing
flooding and pruning to occur on some links that could be avoided if
sufficient topology information were available, i.e. to decide whether
an interface leads to any downstream members of a particular group.
Additional overhead is chosen in favor of the simplification and
flexibility gained by not depending on a specific topology discovery
protocol.
PIM-DM differs from PIM-SM in two essential ways: 1) There are no
periodic joins transmitted, only explicitly triggered prunes and grafts.
2) There is no Rendezvous Point (RP). This is particularly important in
networks that cannot tolerate a single point of failure. (An RP is the
root of a shared multicast distribution tree. For more details see [4]).
4. Protocol Specification
The specification of PIM-DM is broken into several parts:
* Section 4.1 details the protocol state stored.
* Section 4.2 specifies the data packet forwarding rules.
* Section 4.3 specifies generation and processing of Hello messages.
* Section 4.4 specifies the Join, Prune and Graft generation and
processing rules.
* Section 4.5 specifies the State Refresh generation and forwarding
rules.
* Section 4.6 specifies the Assert generation and processing rules.
* Section 4.7 gives details on PIM-DM Packet Formats.
* Section 4.8 summarizes PIM-DM timers and their defaults.
4.1. PIM Protocol State
This section specifies all the protocol states that a PIM-DM
implementation should maintain in order to function correctly. We term
this state the Tree Information Base or TIB, as it holds the state of
all the multicast distribution trees at this router. In this
specification, we define PIM-DM mechanisms in terms of the TIB.
However, only a very simple implementation would actually implement
packet forwarding operations in terms of this state. Most
implementations will use this state to build a multicast forwarding
table, which would then be updated when the relevant state in the TIB
changes.
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Unlike PIM-SM, PIM-DM does not maintain a keepalive timer associated
with each (S,G) route. Within PIM-DM, route and state information
associated with an (S,G) entry MUST be maintained as long as any timer
associated with that (S,G) entry is active. When no timer associated
with an (S,G) entry is active, all information concerning that (S,G)
route may be discarded.
Although we specify precisely the state to be kept, this does not mean
that an implementation of PIM-DM needs to hold the state in this form.
This is actually an abstract state definition, which is needed in order
to specify the router's behavior. A PIM-DM implementation is free to
hold whatever internal state it requires, and will still be conformant
with this specification so long as it results in the same externally
visible protocol behavior as an abstract router that holds the following
state.
4.1.1. General Purpose State
A router stores the following non-group-specific state:
For each interface:
Hello Timer (HT)
State Refresh Capable
LAN Delay Enabled
Propagation Delay (PD)
Override Interval (OI)
Neighbor State:
For each neighbor:
Information from neighbor's Hello
Neighbor's Gen ID.
Neighbor's LAN Prune Delay
Neighbor's Override Interval
Neighbor's State Refresh Capability
Neighbor Liveness Timer (NLT)
4.1.2. (S,G) State
For every source/group pair (S,G), a router stores the following state:
(S,G) state:
For each interface:
Local Membership:
State: One of {"NoInfo", "Include"}
PIM (S,G) Prune State:
State: One of {"NoInfo" (NI), "Pruned" (P), "PrunePending" (PP)}
Prune Pending Timer (PPT)
Prune Timer (PT)
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(S,G) Assert Winner State:
State: One of {"NoInfo" (NI), "I lost Assert" (L),
"I won Assert" (W)}
Assert Timer (AT)
Assert winner's IP Address
Assert winner's Assert Metric
Upstream interface-specific:
Graft/Prune State:
State: One of {"NoInfo" (NI), "Pruned" (P), "Forwarding" (F),
"AckPending" (AP) }
GraftRetry Timer (GRT)
Override Timer (OT)
Prune Limit Timer (PLT)
Originator State:
Source Active Timer (SAT)
State Refresh Timer (SRT)
4.1.3. State Summarization Macros
Using the state defined above, the following "macros" are defined and
will be used in the descriptions of the state machines and pseudocode in
the following sections.
The most important macros are those defining the outgoing interface list
(or "olist") for the relevant state.
immediate_olist(S,G) = pim_nbrs (-) prunes(S,G) (+)
( pim_include(*,G) (-) pim_exclude(S,G) ) (+)
pim_include(S,G) (-) lost_assert(S,G) (-)
boundary(G)
olist(S,G) = immediate_olist(S,G) (-) RPF_interface(S)
The macros pim_include(*,G) and pim_include(S,G) indicate the interfaces
to which traffic might be forwarded or not forwarded because of hosts
that are local members on those interfaces.
pim_include(*,G) = {all interfaces I such that:
local_receiver_include(*,G,I)}
pim_include(S,G) = {all interfaces I such that:
local_receiver_include(S,G,I)}
pim_exclude(S,G) = {all interfaces I such that:
local_receiver_exclude(S,G,I)}
The macro RPF_interface(S) returns the RPF interface for source S. That
is to say, it returns the interface used to reach S as indicated by the
MRIB.
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The macro local_receiver_include(S,G,I) is true if the IGMP module or
other local membership mechanism has determined that there are local
members on interface I that desire to receive traffic sent specifically
by S to G.
The macro local_receiver_include(*,G,I) is true if the IGMP module or
other local membership mechanism has determined that there are local
members on interface I that desire to receive all traffic sent to G.
Note that this determination is expected to account for membership joins
initiated on or by the router.
The macro local_receiver_exclude(S,G,I) is true if
local_receiver_include(*,G,I) is true but none of the local members
desire to receive traffic from S.
The set pim_nbrs is the set of all interfaces on which the router has at
least one active PIM neighbor.
The set prunes(S,G) is the set of all interfaces on which the router has
received Prune(S,G) messages:
prunes(S,G) = {all interfaces I such that
DownstreamPState(S,G,I) is in Pruned state}
The set lost_assert(S,G) is the set of all interfaces on which the
router has lost an (S,G) Assert.
lost_assert(S,G) = {all interfaces I such that
lost_assert(S,G,I) == TRUE}
boundary(G) = {all interfaces I with an administratively scoped
boundary for group G}
The following pseudocode macro definitions are also used in many places
in the specification. Basically RPF' is the RPF neighbor towards a
source unless a PIM-DM Assert has overridden the normal choice of
neighbor.
neighbor RPF'(S,G) {
if ( I_Am_Assert_loser(S, G, RPF_interface(S) )) {
return AssertWinner(S, G, RPF_interface(S) )
} else {
return MRIB.next_hop( S )
}
}
The macro I_Am_Assert_loser(S, G, I) is true if the Assert state machine
(in section 4.6) for (S,G) on interface I is in the "I am Assert Loser"
state.
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4.2. Data Packet Forwarding Rules
The PIM-DM packet forwarding rules are defined below in pseudocode.
iif is the incoming interface of the packet.
S is the source address of the packet.
G is the destination address of the packet (group address).
RPF_interface(S) is the interface the MRIB indicates would be used to
route packets to S.
First, an RPF check MUST be performed to determine whether the packet
should be accepted based on TIB state and the interface on which that
the packet arrived. Packets that fail the RPF check MUST NOT be
forwarded and the router will conduct an assert process for the (S,G)
pair specified in the packet. Packets for which a route to the source
cannot be found MUST be discarded.
If the RPF check has been passed, an outgoing interface list is
constructed for the packet. If this list is not empty, then the packet
MUST be forwarded to all listed interfaces. If the list is empty, then
the router will conduct a prune process for the (S,G) pair specified in
the packet.
On receipt on a data packet from S addressed to G on interface iif:
if (iif == RPF_interface(S) AND UpstreamPState(S,G) != Pruned) {
oiflist = olist(S,G)
} else {
oiflist = NULL
}
forward packet on all interfaces in oiflist
This pseudocode employs the following "macro" definition:
UpstreamPState(S,G) is the state of the Upstream(S,G) state machine in
section 4.4.1.
4.3. Hello Messages
This section describes the generation and processing of Hello messages.
4.3.1. Sending Hello Messages
PIM-DM uses Hello messages to detect other PIM routers. Hello messages
are sent periodically on each PIM enabled interface. Hello messages are
multicast to the ALL-PIM-ROUTERS group. When PIM is enabled on an
interface or a router first starts, the Hello Timer (HT) MUST be set to
random value between 0 and Triggered_Hello_Delay. This prevents
synchronization of Hello messages if multiple routers are powered on
simultaneously.
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After the initial Hello message, a Hello message MUST be sent every
Hello_Period. A single Hello timer MAY be used to trigger sending
Hello messages on all active interfaces. The Hello Timer SHOULD NOT be
reset except when it expires.
4.3.2. Receiving Hello Messages
When a Hello message is received, the receiving router SHALL record the
receiving interface, the sender and any information contained in
recognized options. This information is retained for a number of
seconds in the Hold Time field of the Hello Message. If a new Hello
message is received from a particular neighbor N, the Neighbor Liveness
Timer (NLT(N,I)) MUST be reset to the newly received Hello Holdtime. If
a Hello message is received from a new neighbor, the receiving router
SHOULD send its own Hello message after a random delay between 0 and
Triggered_Hello_Delay.
4.3.3. Hello Message Hold Time
The Hold Time in the Hello Message should be set to a value that can
reasonably be expected to keep the Hello active until a new Hello
message is received. On most links, this will be 3.5 times the value of
Hello_Period.
If the Hold Time is set to '0xffff', the receiving router MUST NOT time
out that Hello message. This feature might be used for on-demand links
to avoid keeping the link up with periodic Hello messages.
If a Hold Time of '0' is received, the corresponding neighbor state is
expired immediately. When a PIM router takes an interface down or
changes IP address, a Hello message with a zero Hold Time SHOULD be sent
immediately (with the old IP address if the IP address is changed) to
cause any PIM neighbors to remove the old information immediately.
4.3.4. Handling Router Failures
If a Hello message is received from an active neighbor with a different
Generation ID (GenID), the neighbor has restarted and may not contain
the correct (S,G) state. A Hello message SHOULD be sent after a random
delay between 0 and Triggered_Hello_Delay (see 4.8) before any other
messages are sent. If the neighbor is downstream, the router MAY
replay the last State Refresh message for any (S,G) pairs for which it
is the Assert Winner indicating Prune and Assert status to the
downstream router. These State Refresh messages SHOULD be sent out
immediately after the Hello message. If the neighbor is the upstream
neighbor for an (S,G) entry, the router MAY cancel its Prune Limit
Timer to permit sending a prune and reestablishing a Pruned state in the
upstream router.
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Upon startup, a router MAY use any State Refresh messages received
within Hello_Period of its first Hello message on an interface to
establish state information. The State Refresh source will be the
RPF'(S), and Prune status for all interfaces will be set according to
the Prune Indicator bit in the State Refresh message. If the Prune
Indicator is set, the router SHOULD set the PruneLimitTimer to
Prune_Holdtime and set the PruneTimer on all downstream interfaces to
the State Refresh's Interval times two. The router SHOULD then
propagate the State Refresh as described in section 4.5.1.
4.3.5. Reducing Prune Propagation Delay on LANs
If all routers on a LAN support the LAN Prune Delay option, then the PIM
routers on that LAN will use the values received to adjust their
J/P_Override_Interval on that interface and the interface is LAN Delay
Enabled. Briefly, to avoid synchronization of Prune Override (Join)
messages when multiple downstream routers share a multi-access link,
sending of such messages is delayed by a small random amount of time.
The period of randomization is configurable and has a default value of 3
seconds.
Each router on the LAN expresses its view of the amount of randomization
necessary in the Override Interval field of the LAN Prune Delay option.
When all routers on a LAN use the LAN Prune Delay Option, all routers on
the LAN MUST set their Override_Interval to the largest Override value
on the LAN.
The LAN Delay inserted by a router in the LAN Prune Delay option
expresses the expected message propagation delay on the link and SHOULD
be configurable by the system administrator. When all routers on a link
use the LAN Prune Delay Option, all routers on the LAN MUST set
Propagation Delay to the largest LAN Delay on the LAN.
PIM implementers should enforce a lower bound on the permitted values
for this delay to allow for scheduling and processing delays within
their router. Such delays may cause received messages to be processed
later as well as triggered messages to be sent later than intended.
Setting this LAN Prune Delay to too low a value may result in temporary
forwarding outages because a downstream router will not be able to
override a neighbor's prune message before the upstream neighbor stops
forwarding.
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4.4. PIM-DM Prune, Join and Graft Messages
This section describes the generation and processing of PIM-DM Join,
Prune and Graft messages. Prune messages are sent towards the upstream
neighbor for S to indicate that traffic from S addressed to group G is
not desired. In the case of two downstream routers A and B, where A
wishes to continue receiving data and B does not, A will send a Join in
response to B's Prune to override the Prune. This is the only situation
in PIM-DM in which a Join message is used. Finally, a Graft message is
used to re-join a previously pruned branch to the delivery tree.
4.4.1. Upstream Prune, Join and Graft Messages
The Upstream(S,G) state machine for sending Prune, Graft and Join
messages is given below. There are three states.
Forwarding (F)
This is the starting state of the Upsteam(S,G) state machine. The
state machine is in this state if it just started or if
oiflist(S,G) != NULL.
Pruned(P)
The set, olist(S,G), is empty. The router will not forward data
from S addressed to group G.
AckPending(AP)
The router was in the Pruned(P) state but a transition has occurred
in the Downstream(S,G) state machine for one of this (S,G) entry's
outgoing interfaces indicating that traffic from S addressed to G
should again be forwarded. A Graft message has been sent to RPF'(S)
but a Graft Ack message has not yet been received.
In addition there are three state-machine-specific timers:
GraftRetry Timer (GRT(S,G))
This timer is set when a Graft is sent upstream. If a corresponding
GraftAck is not received before the timer expires, then another
Graft is sent and the GraftRetry Timer is reset. The timer is
stopped when a Graft Ack message is received. This timer is normally
set to Graft_Retry_Period (see 4.8).
Override Timer (OT(S,G))
This timer is set when a Prune(S,G) is received on the upstream
interface where olist(S,G) != NULL. When the timer expires, a
Join(S,G) message is sent on the upstream interface. This timer is
normally set to t_override (see 4.8).
Prune Limit Timer (PLT(S,G))
This timer is used to rate-limit Prunes on a LAN. It is only used
when the Upstream(S,G) state machine is in the Pruned state. A Prune
cannot be sent if this timer is running. This timer is normally set
to t_limit (see 4.8).
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+-------------+ +-------------+
| | olist == NULL | |
| Forward |----------------------->| Pruned |
| | | |
+-------------+ +-------------+
^ | ^ |
| | | |
| |RPF`(S) Changes olist == NULL| |
| | | |
| | +-------------+ | |
| +-------->| |----------+ |
| | AckPending | |
+-------------| |<-------------+
Rcv GraftAck OR +-------------+ olist != NULL
Rcv State Refresh
With (P==0) OR
S Directly Connect
Figure 1: Upstream Interface State Machine
In tabular form, the state machine is defined as follows:
+-------------------------------+--------------------------------------+
| | Previous State |
| +------------+------------+------------+
| Event | Forwarding | Pruned | AckPending |
+-------------------------------+------------+------------+------------+
| Data packet arrives on | ->P Send | ->P Send | N/A |
| RPF_Interface(S) AND | Prune(S,G) | Prune(S,G) | |
| olist(S,G) == NULL AND |Set PLT(S,G)|Set PLT(S,G)| |
| PLT(S,G) not running | | | |
+-------------------------------+------------+------------+------------+
| State Refresh(S,G) received | ->F Set | ->P Reset |->AP Set |
| from RPF`(S) AND | OT(S,G) | PLT(S,G) | OT(S,G) |
| Prune Indicator == 1 | | | |
+-------------------------------+------------+------------+------------+
| State Refresh(S,G) received | ->F | ->P Send |->F Cancel |
| from RPF`(S) AND | | Prune(S,G) | GRT(S,G) |
| Prune Indicator == 0 AND | |Set PLT(S,G)| |
| PLT(S,G) not running | | | |
+-------------------------------+------------+------------+------------+
| See Join(S,G) to RPF'(S) | ->F Cancel | ->P |->AP Cancel |
| | OT(S,G) | | OT(S,G) |
+-------------------------------+------------+------------+------------+
| See Prune(S,G) | ->F Set | ->P |->AP Set |
| | OT(S,G) | | OT(S,G) |
+-------------------------------+------------+------------+------------+
| OT(S,G) Expires | ->F Send | N/A |->AP Send |
| | Join(S,G) | | Join(S,G) |
+-------------------------------+------------+------------+------------+
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+-------------------------------+--------------------------------------+
| | Previous State |
| +------------+------------+------------+
| Event | Forwarding | Pruned | AckPending |
+-------------------------------+------------+------------+------------+
| olist(S,G)->NULL | ->P Send | N/A |->P Send |
| | Prune(S,G) | | Prune(S,G) |
| |Set PLT(S,G)| |Set PLT(S,G)|
| | | | Cancel |
| | | | GRT(S,G) |
+-------------------------------+------------+------------+------------+
| olist(S,G)->non-NULL | N/A | ->AP Send | N/A |
| | | Graft(S,G) | |
| | |Set GRT(S,G)| |
+-------------------------------+------------+------------+------------+
| RPF'(S) Changes AND | ->AP Send | ->AP Send |->AP Send |
| olist(S,G) != NULL | Graft(S,G) | Graft(S,G) | Graft(S,G) |
| |Set GRT(S,G)|Set GRT(S,G)|Set GRT(S,G)|
+-------------------------------+------------+------------+------------+
| RPF'(S) Changes AND | ->P | ->P Cancel |->P Cancel |
| olist(S,G) == NULL | | PLT(S,G) | GRT(S,G) |
+-------------------------------+------------+------------+------------+
| S becomes directly connected | ->F | ->P |->F Cancel |
| | | | GRT(S,G) |
+-------------------------------+------------+------------+------------+
| GRT(S,G) Expires | N/A | N/A |->AP Send |
| | | | Graft(S,G) |
| | | |Set GRT(S,G)|
+-------------------------------+------------+------------+------------+
| Receive GraftAck(S,G) from | ->F | ->P |->F Cancel |
| RPF'(S) | | | GRT(S,G) |
+-------------------------------+------------+------------+------------+
The transition event "RcvGraftAck(S,G)" implies receiving a Graft Ack
message targeted to this router's address on the incoming interface for
the (S,G) entry. If the destination address is not correct, the state
transitions in this state machine must not occur.
4.4.1.1. Transitions from the Forwarding (F) State
When the Upstream(S,G) state machine is in the Forwarding (F) state, the
following events may trigger a transition:
Data Packet arrives on RPF_Interface(S) AND olist(S,G) == NULL AND S
NOT directly connected
The Upstream(S,G) state machine MUST transition to the Pruned (P)
state, send a Prune(S,G) to RPF'(S) and set PLT(S,G) to t_limit
seconds.
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State Refresh(S,G) Received from RPF'(S)
The Upstream(S,G) state machine remains in a Forwarding state. If
the received State Refresh has the Prune Indicator bit set to one,
this router must override the upstream router's Prune state after a
short random interval. If OT(S,G) is not running and the Prune
Indicator bit equals one, the router MUST set OT(S,G) to t_override
seconds.
See Join(S,G) to RPF'(S)
This event is only relevant if RPF_interface(S) is a shared medium.
This router sees another router on RPF_interface(S) send a Join(S,G)
to RPF'(S,G). If the OT(S,G) is running, then it means that the
router had scheduled a Join to override a previously received Prune.
Another router has responded more quickly with a Join and so the
local router SHOULD cancel its OT(S,G), if it is running. The
Upstream(S,G) state machine remains in the Forwarding (F) state.
See Prune(S,G) AND S NOT directly connected
This event is only relevant if RPF_interface(S) is a shared medium.
This router sees another router on RPF_interface(S) send a
Prune(S,G). As this router is in Forwarding state, it must
override the Prune after a short random interval. If OT(S,G) is not
running, the router MUST set OT(S,G) to t_override seconds. The
Upstream(S,G) state machine remains in Forwarding (F) state.
OT(S,G) Expires AND S NOT directly connected
The OverrideTimer (OT(S,G)) expires. The router MUST send a
Join(S,G) to RPF'(S) to override a previously detected prune. The
Upstream(S,G) state machine remains in the Forwarding (F) state.
olist(S,G) -> NULL AND S NOT directly connected
The Upstream(S,G) state machine MUST transition to the Pruned (P)
state, send a Prune(S,G) to RPF'(S) and set PLT(S,G) to t_limit
seconds.
RPF'(S) Changes AND olist(S,G) is non-NULL AND S NOT directly
connected
Unicast routing or Assert state causes RPF'(S) to change, including
changes to RPF_Interface(S). The Upstream(S,G) state machine MUST
transition to the AckPending (AP) state, unicast a Graft to the new
RPF'(S) and set the GraftRetry Timer (GRT(S,G)) to
Graft_Retry_Period.
RPF'(S) Changes AND olist(S,G) is NULL
Unicast routing or Assert state causes RPF'(S) to change, including
changes to RPF_Interface(S). The Upstream(S,G) state machine MUST
transition to the Pruned (P) state.
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4.4.1.2. Transitions from the Pruned (P) State
When the Upstream(S,G) state machine is in the Pruned (P) state, the
following events may trigger a transition:
Data arrives on RPF_interface(S) AND PLT(S,G) not running AND S NOT
directly connected
Either another router on the LAN desires traffic from S addressed to
G or a previous Prune was lost. In order to prevent generating a
Prune(S,G) in response to every data packet, the PruneLimit Timer
(PLT(S,G)) is used. Once the PLT(S,G) expires, the router needs to
send another prune in response to a data packet not received
directly from the source. A Prune(S,G) MUST be sent to RPF'(S) and
the PLT(S,G) MUST be set to t_limit.
State Refresh(S,G) Received from RPF'(S)
The Upstream(S,G) state machine remains in a Pruned state. If the
State Refresh has its Prune Indicator bit set to zero and PLT(S,G)
is not running, a Prune(S,G) MUST be sent to RPF'(S) and the
PLT(S,G) MUST be set to t_limit. If the State Refresh has its Prune
Indicator bit set to one, the router MUST reset PLT(S,G) to t_limit.
See Prune(S,G) to RPF'(S)
A Prune(S,G) is seen on RPF_interface(S) to RPF'(S). The
Upstream(S,G) state machine stays in the Pruned (P) state. The
router MAY reset its PLT(S,G) to the value in the Holdtime field of
the received message if greater than the current value of the
PLT(S,G).
olist(S,G)->non-NULL AND S NOT directly connected
The set of interfaces defined by the olist(S,G) macro becomes
non-empty indicating traffic from S addressed to group G must be
forwarded. The Upstream(S,G) state machine MUST cancel PLT(S,G),
transition to the AckPending (AP) state and unicast a Graft message
to RPF'(S). The Graft Retry Timer (GRT(S,G)) MUST be set to
Graft_Retry_Period.
RPF'(S) Changes AND olist(S,G) == non-NULL AND S NOT directly
connected
Unicast routing or Assert state causes RPF'(S) to change, including
changes to RPF_Interface(S). The Upstream(S,G) state machine MUST
cancel PLT(S,G), transition to the AckPending (AP) state, send a
Graft unicast to the new RPF'(S) and set the GraftRetry Timer
(GRT(S,G)) to Graft_Retry_Period.
RPF'(S) Changes AND olist(S,G) == NULL AND S NOT directly connected
Unicast routing or Assert state causes RPF'(S) to change, including
changes to RPF_Interface(S). The Upstream(S,G) state machine stays
in the Pruned (P) state and MUST cancel the PLT(S,G) timer.
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S becomes directly connected
Unicast routing changed so that S is directly connected. The
Upstream(S,G) state machine remains in the Pruned (P) state.
4.4.1.3. Transitions from the AckPending (AP) State
When the Upstream(S,G) state machine is in the AckPending (AP) state,
the following events may trigger a transition:
State Refresh(S,G) Received from RPF'(S) with Prune Indicator == 1
The Upstream(S,G) state machine remains in an AckPending state. The
router must override the upstream router's Prune state after a short
random interval. If OT(S,G) is not running and the Prune Indicator
bit equals one, the router MUST set OT(S,G) to t_override seconds.
State Refresh(S,G) Received from RPF'(S) with Prune Indicator == 0
The router MUST cancel its GraftRetry Timer (GRT(S,G)) and
transition to the Forwarding (F) state.
See Join(S,G) to RPF'(S,G)
This event is only relevant if RPF_interface(S) is a shared medium.
This router sees another router on RPF_interface(S) send a Join(S,G)
to RPF'(S,G). If the OT(S,G) is running, then it means that the
router had scheduled a Join to override a previously received Prune.
Another router has responded more quickly with a Join and so the
local router SHOULD cancel its OT(S,G), if it is running. The
Upstream(S,G) state machine remains in the AckPending (AP) state.
See Prune(S,G)
This event is only relevant if RPF_interface(S) is a shared medium.
This router sees another router on RPF_interface(S) send a
Prune(S,G). As this router is in AckPending (AP) state, it must
override the Prune after a short random interval. If OT(S,G) is not
running, the router MUST set OT(S,G) to t_override seconds. The
Upstream(S,G) state machine remains in AckPending (AP) state.
OT(S,G) Expires
The OverrideTimer (OT(S,G)) expires. The router MUST send a
Join(S,G) to RPF'(S). The Upstream(S,G) state machine remains in
the AckPending (AP) state.
olist(S,G) -> NULL
The set of interfaces defined by the olist(S,G) macro becomes null
indicating traffic from S addressed to group G should no longer be
forwarded. The Upstream(S,G) state machine MUST transition to the
Pruned (P) state. A Prune(S,G) MUST be multicast to the
RPF_interface(S) with RPF'(S) named in the upstream neighbor field.
The GraftRetry Timer (GRT(S,G)) MUST be cancelled and PLT(S,G) MUST
be set to t_limit seconds.
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RPF'(S) Changes AND olist(S,G) does not become NULL AND S NOT directly
connected
Unicast routing or Assert state causes RPF'(S) to change, including
changes to RPF_Interface(S). The Upstream(S,G) state machine stays
in the AckPending (AP) state. A Graft MUST be unicast to the new
RPF'(S) and the GraftRetry Timer (GRT(S,G)) reset to
Graft_Retry_Period.
RPF'(S) Changes AND olist(S,G) == NULL AND S NOT directly connected
Unicast routing or Assert state causes RPF'(S) to change, including
changes to RPF_Interface(S). The Upstream(S,G) state machine MUST
transition to the Pruned (P) state. The GraftRetry Timer (GRT(S,G))
MUST be cancelled.
S becomes directly connected
Unicast routing has changed so that S is directly connected. The
GraftRetry Timer MUST be cancelled and the Upstream(S,G) state
machine MUST transition to the Forwarding(F) state.
GRT(S,G) Expires
The GraftRetry Timer (GRT(S,G)) expires for this (S,G) entry. The
Upstream(S,G) state machine stays in the AckPending (AP) state.
Another Graft message for (S,G) SHOULD be unicasted to RPF'(S) and
the GraftRetry Timer (GRT(S,G)) reset to Graft_Retry_Period. It is
RECOMMENDED that the router retry a configured number of times
before ceasing retries.
See GraftAck(S,G) from RPF'(S)
A GraftAck is received from RPF'(S). The GraftRetry Timer MUST be
cancelled and the Upstream(S,G) state machine MUST transition to the
Forwarding(F) state.
4.4.2 Downstream Prune, Join and Graft Messages
The Prune(S,G) Downstream state machine for receiving Prune, Join and
Graft messages on interface I is given below. This state machine MUST
always be in the NoInfo state on the upstream interface. It contains
three states.
NoInfo(NI)
The interface has no (S,G) Prune state and neither the Prune timer
(PT(S,G,I)) nor the PrunePending timer ((PPT(S,G,I)) is running.
PrunePending(PP)
The router has received a Prune(S,G) on this interface from a
downstream neighbor and is waiting to see whether the prune will be
overridden by another downstream router. For forwarding purposes,
the PrunePending state functions exactly like the NoInfo state.
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Pruned(P)
The router has received a Prune(S,G) on this interface from a
downstream neighbor and the Prune was not overridden. Data from S
addressed to group G is no longer being forwarded on this interface.
In addition there are two timers:
PrunePending Timer (PPT(S,G,I))
This timer is set when a valid Prune(S,G) is received. Expiry of
the PrunePending Timer (PPT(S,G,I)) causes the interface to
transition to the Pruned state.
Prune Timer (PT(S,G,I))
This timer is set when the PrunePending Timer (PT(S,G,I)) expires.
Expiry of the Prune Timer (PT(S,G,I)) causes the interface to
transition to the NoInfo (NI) state, thereby allowing data from S
addressed to group G to be forwarded on the interface.
+-------------+ +-------------+
| | PPT Expires | |
|PrunePending |----------------------->| Pruned |
| | | |
+-------------+ +-------------+
| ^ |
| | |
| |Rcv Prune |
| | |
| | +-------------+ |
| +---------| | |
| | NoInfo |<-------------+
+------------>| | Rcv Join/Graft OR
Rcv Join/Graft OR +-------------+ PT Expires OR
RPF_Interface(S)->I RPF_Interface(S)->I
Figure 2: Downstream Interface State Machine
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In tabular form, the state machine is:
+-------------------------------+--------------------------------------+
| | Previous State |
+ +------------+------------+------------+
| Event | No Info | PrunePend | Pruned |
+-------------------------------+------------+------------+------------+
| Receive Prune(S,G) |->PP Set |->PP |->P Reset |
| | PPT(S,G,I) | | PT(S,G,I) |
+-------------------------------+------------+------------+------------+
| Receive Join(S,G) |->NI |->NI Cancel |->NI Cancel |
| | | PPT(S,G,I) | PT(S,G,I) |
+-------------------------------+------------+------------+------------+
| Receive Graft(S,G) |->NI Send |->NI Send |->NI Send |
| | GraftAck | GraftAck | GraftAck |
| | | Cancel | Cancel |
| | | PPT(S,G,I) | PT(S,G,I) |
+-------------------------------+------------+------------+------------+
| PPT(S,G) Expires | N/A |->P Set | N/A |
| | | PT(S,G,I) | |
+-------------------------------+------------+------------+------------+
| PT(S,G) Expires | N/A | N/A |->NI |
+-------------------------------+------------+------------+------------+
| RPF_Interface(S) becomes I |->NI |->NI Cancel |->NI Cancel |
| | | PPT(S,G,I) | PT(S,G,I) |
+-------------------------------+------------+------------+------------+
| Send State Refresh(S,G) out I |->NI |->PP |->P Reset |
| | | | PT(S,G,I) |
+-------------------------------+------------+------------+------------+
The transition events "Receive Graft(S,G)", "Receive Prune(S,G)" and
"Receive Join(S,G)" denote receiving a Graft, Prune or Join message in
which this router's address on I is contained in the message's upstream
neighbor field. If the upstream neighbor field does not match this
router's address on I, then these state transitions in this state
machine must not occur.
4.4.2.1. Transitions from the NoInfo State
When the Prune(S,G) Downstream state machine is in the NoInfo (NI)
state, the following events may trigger a transition:
Receive Prune(S,G)
A Prune(S,G) is received on interface I with the upstream neighbor
field set to the router's address on I. The Prune(S,G) Downstream
state machine on interface I MUST transition to the PrunePending
(PP) state. The PrunePending Timer (PPT(S,G,I)) MUST be set to
J/P_Override_Interval if the router has more than one neighbor on I.
If the router has only one neighbor on interface I, then it SHOULD
set the PPT(S,G,I) to zero, effectively transitioning immediately to
the Pruned (P) state.
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Receive Graft(S,G)
A Graft(S,G) is received on the interface I with the upstream
neighbor field set to the router's address on I. The Prune(S,G)
Downstream state machine on interface I stays in the NoInfo (NI)
state. A GraftAck(S,G) MUST be unicasted to the originator of the
Graft(S,G) message.
4.4.2.2. Transitions from the PrunePending (PP) State
When the Prune(S,G) downstream state machine is in the PrunePending (PP)
state, the following events may trigger a transition.
Receive Join(S,G)
A Join(S,G) is received on interface I with the upstream neighbor
field set to the router's address on I. The Prune(S,G) Downstream
state machine on interface I MUST transition to the NoInfo (NI)
state. The PrunePending Timer (PPT(S,G,I)) MUST be cancelled.
Receive Graft(S,G)
A Graft(S,G) is received on interface I with the upstream neighbor
field set to the router's address on I. The Prune(S,G) Downstream
state machine on interface I MUST transition to the NoInfo (NI)
state and MUST unicast a Graft Ack message to the Graft originator.
The PrunePending Timer (PPT(S,G,I)) MUST be cancelled.
PPT(S,G,I) Expires
The PrunePending Timer (PPT(S,G,I)) expires indicating that no
neighbors have overridden the previous Prune(S,G) message. The
Prune(S,G) Downstream state machine on interface I MUST transition
to the Pruned (P) state. The Prune Timer (PT(S,G,I)) is started and
MUST be initialized to the received Prune_Hold_Time minus
J/P_Override_Interval. A PruneEcho(S,G) MUST be sent on I if I has
more than one PIM neighbor. A PruneEcho(S,G) is simply a Prune(S,G)
message multicast by the upstream router to a LAN with itself as the
Upstream Neighbor. Its purpose is to add additional reliability so
that if a Join that should have overridden the Prune is lost locally
on the LAN, then the PruneEcho(S,G) may be received and trigger a
new Join message . A PruneEcho(S,G) is OPTIONAL on an interface
with only one PIM neighbor. In addition, the router MUST evaluate
any possible transitions in the Upstream(S,G) state machine.
RPF_Interface(S) becomes interface I
The upstream interface for S has changed. The Prune(S,G) Downstream
state machine on interface I MUST transition to the NoInfo (NI)
state. The PrunePending Timer (PPT(S,G,I)) MUST be cancelled.
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4.4.2.3. Transitions from the Prune (P) State
When the Prune(S,G) Downstream state machine is in the Pruned (P) state,
the following events may trigger a transition.
Receive Prune(S,G)
A Prune(S,G) is received on the interface I with the upstream
neighbor field set to the router's address on I. The Prune(S,G)
Downstream state machine on interface I remains in the Pruned (P)
state. The Prune Timer (PT(S,G,I)) SHOULD be reset to the holdtime
contained in the Prune(S,G) message if it is greater than the
current value.
Receive Join(S,G)
A Join(S,G) is received on the interface I with the upstream
neighbor field set to the router's address on I. The Prune(S,G)
downstream state machine on interface I MUST transition to the
NoInfo (NI) state. The Prune Timer (PT(S,G,I)) MUST be cancelled.
The router MUST evaluate any possible transitions in the
Upstream(S,G) state machine.
Receive Graft(S,G)
A Graft(S,G) is received on interface I with the upstream neighbor
field set to the router's address on I. The Prune(S,G) Downstream
state machine on interface I MUST transition to the NoInfo (NI)
state and send a Graft Ack back to the Graft's source. The Prune
Timer (PT(S,G,I)) MUST be cancelled. The router MUST evaluate any
possible transitions in the Upstream(S,G) state machine.
PT(S,G,I) Expires
The Prune Timer (PT(S,G,I)) expires indicating that it is again time
to flood data from S addressed to group G onto interface I. The
Prune(S,G) Downstream state machine on interface I MUST transition
to the NoInfo (NI) state. The router MUST evaluate any possible
transitions in the Upstream(S,G) state machine.
RPF_Interface(S) becomes interface I
The upstream interface for S has changed. The Prune(S,G) Downstream
state machine on interface I MUST transition to the NoInfo (NI)
state. The PruneTimer (PT(S,G,I)) MUST be cancelled.
Send State Refresh(S,G) out interface I
The router has refreshed the Prune(S,G) state on interface I. The
router MUST reset the Prune Timer (PT(S,G,I)) to the Holdtime from
an active Prune received on interface I. The Holdtime used SHOULD
be the largest active one, but MAY be the most recently received
active Prune Holdtime.
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4.5. State Refresh
This section describes the major portions of the state refresh
mechanism.
4.5.1. Forwarding of State Refresh Messages
When a State Refresh message, SRM, is received, it is forwarded
according to the following pseudo-code.
if (iif != RPF_interface(S))
return;
if (RPF'(S) != srcaddr(SRM))
return;
if (StateRefreshRateLimit(S,G) == TRUE)
return;
for each interface I in pim_nbrs {
if (TTL(SRM) == 0 OR (TTL(SRM) - 1) < Threshold(I))
continue; /* Out of TTL, skip this interface */
if (boundary(I,G))
continue; /* This interface is scope boundary, skip it */
if (I == iif)
continue; /* This is the incoming interface, skip it */
if (lost_assert(S,G,I) == TRUE)
continue; /* Let the Assert Winner do State Refresh */
Copy SRM to SRM'; /* Make a copy of SRM to forward */
if (I contained in prunes(S,G)) {
set Prune Indicator bit of SRM' to 1;
if StateRefreshCapable(I) == TRUE
set PT(S,G) to largest active holdtime read from a Prune message
accepted on I;
} else {
set Prune Indicator bit of SRM' to 0;
}
set srcaddr(SRM') to my_addr(I);
set TTL of SRM' to TTL(SRM) - 1;
set metric of SRM' to metric of unicast route used to reach S;
set pref of SRM' to preference of unicast route used to reach S;
set mask of SRM' to mask of route used to reach S;
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if (AssertState == NoInfo) {
set Assert Override of SRM' to 1;
} else {
set Assert Override of SRM' to 0;
}
transmit SRM' on I;
}
The pseudocode above employs the following macro definitions.
Boundary(I,G) evaluates to TRUE if an administratively scoped boundary
for group G is configured on interface I.
StateRefreshCapable(I) evaluates to TRUE if all neighbors on an
interface use the State Refresh option.
StateRefreshRateLimit(S,G) evaluates to TRUE if the time elapsed since
the last received StateRefresh(S,G) is less than the configured
RefreshLimitInterval.
TTL(SRM) returns the TTL contained in the State Refresh Message, SRM.
This is different from the TTL contained in the IP header.
Threshold(I) returns the minimum TTL that a packet must have before it
can be transmitted on interface I.
srcaddr(SRM) returns the source address contained in the network
protocol (e.g. IPv4) header of the State Refresh Message, SRM.
my_addr(I) returns this node's network (e.g. IPv4) address on interface
I.
4.5.2 State Refresh Message Origination
This section describes the origination of State Refresh messages. These
messages are generated periodically by the PIM-DM router that is
directly connected to a source. One Origination(S,G) state machine
exists per (S,G) entry in a PIM-DM router.
The Origination(S,G) state machine has the following states:
NotOriginator(NO)
This is the starting state of the Origination(S,G) state machine.
While in this state a router will not originate State Refresh
messages for the (S,G) pair.
Originator(O)
When in this state the router will periodically originate State
Refresh messages. Only routers which are directly connected to S
may transition to this state.
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In addition there are two state-machine-specific timers:
State Refresh Timer (SRT(S,G))
This timer is controls when State Refresh messages are generated.
The timer is initially set when that Origination(S,G) state machine
transitions to the O state. It is cancelled when the
Origination(S,G) state machine transitions to the NO state. This
timer is normally set to StateRefreshInterval (see 4.8).
Source Active Timer (SAT(S,G))
This timer is first set when the Origination(S,G) state machine
transitions to the O state and is reset on the receipt of every
data packet from S addressed to group G. When it expires, the
Origination(S,G) state machine transitions to the NO state. This
timer is normally set to SourceLifetime (see 4.8).
+-------------+ Rcv Directly From S +-------------+
| |----------------------->| |
|NotOriginator| | Originator |
| |<-----------------------| |
+-------------+ SAT Expires OR +-------------+
S NOT Direct Connect
Figure 3: State Refresh State Machine
In tabular form, the state machine is defined as follows:
+----------------------------------------------------------------------+
| | Previous State |
| +---------------+-------------------+
| Event | NotOriginator | Originator |
+----------------------------------+---------------+-------------------+
| Receive Data from S AND | ->O | ->O Reset |
| S directly connected | Set SRT(S,G) | SAT(S,G) |
| | Set SAT(S,G) | |
+----------------------------------+---------------+-------------------+
| SRT(S,G) Expires | N/A | ->O Send |
| | | StateRefresh(S,G) |
| | | Reset SRT(S,G) |
+----------------------------------+---------------+-------------------+
| SAT(S,G) Expires | N/A | ->NO Cancel |
| | | SRT(S,G) |
+----------------------------------+---------------+-------------------+
| S no longer directly connected | ->NO | ->NO |
| | | Cancel SRT(S,G) |
| | | Cancel SAT(S,G) |
+----------------------------------+---------------+-------------------+
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4.5.2.1. Transitions from the NotOriginator (NO) State
When the Originating(S,G) state machine is in the NotOriginator (NO)
state, the following event may trigger a transition:
Data Packet received from directly connected Source S addressed to
group G
The router MUST transition to an Originator (O) state, set SAT(S,G)
to SourceLifetime, and set SRT(S,G) to StateRefreshInterval. The
router SHOULD record the TTL of the packet for use in State Refresh
messages.
4.5.2.2. Transitions from the Originator (O) State
When the Originating(S,G) state machine is in the Originator (O) state,
the following events may trigger a transition:
Receive Data Packet from S addressed to G
The router remains in the Originator (O) state and MUST reset
SAT(S,G) to SourceLifetime. The router SHOULD increase its recorded
TTL to match the TTL of the packet, if the packet's TTL is larger
than the previously recorded TTL.
SRT(S,G) Expires
The router remains in the Originator (O) state and MUST reset
SRT(S,G) to StateRefreshInterval. The router MUST also generate
State Refresh messages for transmission as described in the State
Refresh Forwarding rules (section 4.5.1) except for the TTL. If the
TTL of data packets from S to G are being recorded, then the TTL of
each State Refresh message is set to the highest recorded TTL.
Otherwise, the TTL is set to the configured State Refresh TTL. Let
I denote the interface over which a State Refresh message is being
sent. If the Prune(S,G) Downstream state machine for I is in the
NoInfo (NI) state, then the Prune-Indicator bit MUST be set to 0 in
the State Refresh message being sent over I. Otherwise the
Prune-Indicator bit MUST be set to 1.
SAT(S,G) Expires
The router MUST cancel the SRT(S,G) timer and transition to the
NotOriginator (NO) state.
S is no longer directly connected
The router MUST transition to the NotOriginator (NO) state and
cancel both the SAT(S,G) and SRT(S,G).
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4.6. PIM Assert Messages
4.6.1. Assert Metrics
Assert metrics are defined as:
struct assert_metric {
metric_preference;
route_metric;
ip_address;
};
When comparing assert_metrics, the metric_preference and route_metric
field are compared in order, where the first lower value wins. If all
fields are equal, the IP address of the router that sourced the Assert
message is used as a tie-breaker, with the highest IP address winning.
An Assert metric for (S,G) to include in (or compare against) an Assert
message sent on interface I should be computed using the following
pseudocode:
assert_metric
my_assert_metric(S,G,I) {
if (CouldAssert(S,G,I) == TRUE) {
return spt_assert_metric(S,G,I)
} else {
return infinite_assert_metric()
}
}
spt_assert_metric(S,I) gives the Assert metric we use if we're sending
an Assert based on active (S,G) forwarding state:
assert_metric
spt_assert_metric(S,I) {
return {0,MRIB.pref(S),MRIB.metric(S),my_addr(I)}
}
MRIB.pref(X) and MRIB.metric(X) are the routing preference and routing
metrics associated with the route to a particular (unicast) destination
X, as determined by the MRIB. my_addr(I) is simply the router's network
(e.g. IP) address that is associated with the local interface I.
infinite_assert_metric() gives the Assert metric we need to send an
Assert but doesn't match (S,G) forwarding state:
assert_metric
infinite_assert_metric() {
return {1,infinity,infinity,0}
}
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4.6.2. AssertCancel Messages
An AssertCancel(S,G) message is simply an Assert message for (S,G) with
infinite metric. The Assert winner sends such a message when it changes
its upstream interface to this interface. Other routers will see this
metric, causing those with forwarding state to send their own Asserts
and re-establish an Assert winner.
AssertCancel messages are simply an optimization. The original Assert
timeout mechanism will allow a subnet to eventually become consistent;
the AssertCancel mechanism simply causes faster convergence. No special
processing is required for an AssertCancel message, since it is simply
an Assert message from the current winner.
4.6.3. Assert State Macros
The macro lost_assert(S,G,I), is used in the olist computations of
section 4.1.3, and is defined as follows:
bool lost_assert(S,G,I) {
if ( RPF_interface(S) == I ) {
return FALSE
} else {
return (AssertWinner(S,G,I) != me AND
(AssertWinnerMetric(S,G,I) is better than
spt_assert_metric(S,G,I)))
}
}
AssertWinner(S,G,I) defaults to NULL and AssertWinnerMetric(S,G,I)
defaults to Infinity when in the NoInfo state.
4.6.4. (S,G) Assert Message State Machine
The (S,G) Assert state machine for interface I is shown in Figure 4.
There are three states:
NoInfo (NI)
This router has no (S,G) Assert state on interface I.
I am Assert Winner (W)
This router has won an (S,G) Assert on interface I. It is now
responsible for forwarding traffic from S destined for G via
interface I.
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I am Assert Loser (L)
This router has lost an (S,G) Assert on interface I. It must not
forward packets from S destined for G onto interface I.
In addition there is also an Assert Timer (AT(S,G,I)) that is used to
time out Assert state.
+-------------+ +-------------+
| | Rcv Pref Assert or SR | |
| Winner |----------------------->| Loser |
| | | |
+-------------+ +-------------+
^ | ^ |
| | Rcv Pref Assert or| |
| |AT Expires OR State Refresh| |
| |CouldAssert->FALSE | |
| | | |
| | +-------------+ | |
| +-------->| |----------+ |
| | No Info | |
+-------------| |<-------------+
Rcv Data from dnstrm +-------------+ Rcv Inf Assert from Win OR
OR Rcv Inferior Assert Rcv Inf SR from Winner OR
OR Rcv Inferior SR AT Expires OR
CouldAssert Changes OR
Winner's NLT Expires
Figure 4: Assert State Machine
In tabular form the state machine is defined as follows:
+-------------------------------+--------------------------------------+
| | Previous State |
| +------------+------------+------------+
| Event | No Info | Winner | Loser |
+-------------------------------+------------+------------+------------+
| An (S,G) Data packet received | ->W Send | ->W Send | ->L |
| on downstream interface | Assert(S,G)| Assert(S,G)| |
| | Set | Set | |
| | AT(S,G,I) | AT(S,G,I) | |
+-------------------------------+--------------------------------------+
| Receive Inferior (Assert OR | N/A | N/A |->NI Cancel |
| State Refresh) from Assert | | | AT(S,G,I) |
| Winner | | | |
+-------------------------------+--------------------------------------+
| Receive Inferior (Assert OR | ->W Send | ->W Send | ->L |
| State Refresh) from non-Assert| Assert(S,G)| Assert(S,G)| |
| Winner AND CouldAssert==TRUE | Set | Set | |
| | AT(S,G,I) | AT(S,G,I) | |
+-------------------------------+--------------------------------------+
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+-------------------------------+--------------------------------------+
| | Previous State |
| +------------+------------+------------+
| Event | No Info | Winner | Loser |
+-------------------------------+------------+------------+------------+
| Receive Preferred Assert OR | ->L Send | ->L Send | ->L Set |
| State Refresh | Prune(S,G) | Prune(S,G) | AT(S,G,I) |
| | Set | Set | |
| | AT(S,G,I) | AT(S,G,I) | |
+-------------------------------+--------------------------------------+
| Send State Refresh | ->NI | ->W Reset | N/A |
| | | AT(S,G,I) | |
+-------------------------------+--------------------------------------+
| AT(S,G) Expires | N/A | ->NI | ->NI |
+-------------------------------+--------------------------------------+
| CouldAssert -> FALSE | ->NI |->NI Cancel |->NI Cancel |
| | | AT(S,G,I) | AT(S,G,I) |
+-------------------------------+--------------------------------------+
| CouldAssert -> TRUE | ->NI | N/A |->NI Cancel |
| | | | AT(S,G,I) |
+-------------------------------+--------------------------------------+
| Winner's NLT(N,I) Expires | N/A | N/A |->NI Cancel |
| | | | AT(S,G,I) |
+-------------------------------+--------------------------------------+
| Receive Prune(S,G), Join(S,G) | ->NI | ->W | ->L Send |
| or Graft(S,G) | | | Assert(S,G)|
+-------------------------------+--------------------------------------+
Terminology:
A "preferred assert" is one with a better metric than the current
winner. An "inferior assert" is one with a worse metric than
my_assert_metric(S,G,I).
The state machine uses the following macro:
CouldAssert(S,G,I) = (RPF_interface(S) != I)
4.6.4.1. Transitions from NoInfo State
When in NoInfo state, the following events may trigger transitions:
An (S,G) data packet arrives on downstream interface I
An (S,G) data packet arrived on a downstream interface. It is
optimistically assumed that this router will be the Assert winner
for this (S,G). The Assert state machine MUST transition to the "I
am Assert Winner" state, send an Assert(S,G) to interface I, store
its own address and metric as the Assert Winner and set the
Assert_Timer (AT(S,G,I) to Assert_Time, thereby initiating the
Assert negotiation for (S,G).
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Receive Inferior (Assert OR State Refresh) AND
CouldAssert(S,G,I)==TRUE
An Assert or State Refresh is received for (S,G) that is inferior
to our own assert metric on interface I. The Assert state machine
MUST transition to the "I am Assert Winner" state, send an
Assert(S,G) to interface I, store its own address and metric as the
Assert Winner and set the Assert Timer (AT(S,G,I)) to Assert_Time.
Receive Preferred Assert or State Refresh
The received Assert or State Refresh has a better metric than this
router's and therefore the Assert state machine MUST transition to
the "I am Assert Loser" state and store the Assert Winner's address
and metric. If the metric was received in an Assert, the router MUST
set the Assert Timer (AT(S,G,I)) to Assert_Time. If the metric was
received in a State Refresh, the router MUST set the Assert Timer
(AT(S,G,I)) to three times the received State Refresh Interval. If
CouldAssert(S,G,I) == TRUE, the router MUST also multicast a
Prune(S,G) to the Assert winner with a Prune Hold Time equal to the
Assert Timer and evaluate any changes in its Upstream(S,G) state
machine.
4.6.4.2. Transitions from Winner State
When in "I am Assert Winner" state, the following events trigger
transitions:
An (S,G) data packet arrives on downstream interface I
An (S,G) data packet arrived on a downstream interface. The Assert
state machine remains in the "I am Assert Winner" state. The router
MUST send an Assert(S,G) to interface I and set the Assert Timer
(AT(S,G,I) to Assert_Time.
Receive Inferior Assert or State Refresh
An (S,G) Assert is received containing a metric for S that is worse
metric than this router's metric for S. Whoever sent the Assert is
in error. The router MUST send an Assert(S,G) to interface I and
reset the Assert Timer (AT(S,G,I)) to Assert_Time.
Receive Preferred Assert or State Refresh
An (S,G) Assert or State Refresh is received that has a better
metric than this router's metric for S on interface I. The Assert
state machine MUST transition to "I am Assert Loser" state and
store the new Assert Winner's address and metric. If the metric was
received in an Assert, the router MUST set the Assert Timer
(AT(S,G,I)) to Assert_Time. If the metric was received in a State
Refresh, the router MUST set the Assert Timer (AT(S,G,I)) to three
times the State Refresh Interval. The router MUST also multicast a
Prune(S,G) to the Assert winner with a Prune Hold Time equal to the
Assert Timer and evaluate any changes in its Upstream(S,G) state
machine.
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Send State Refresh
The router is sending a State Refresh(S,G) message on interface I.
The router MUST set the Assert Timer (AT(S,G,I)) to three times the
State Refresh Interval contained in the State Refresh(S,G) message.
AT(S,G,I) Expires
The (S,G) Assert Timer (AT(S,G,I)) expires. The Assert state machine
MUST transition to the NoInfo (NI) state.
CouldAssert(S,G,I) -> FALSE
This router's RPF interface changed so as to make CouldAssert(S,G,I)
become false. This router can no longer perform the actions of the
Assert winner, and so the Assert state machine MUST transition to
NoInfo (NI) state, send an AssertCancel(S,G) to interface I, cancel
the Assert Timer (AT(S,G,I)) and remove itself as the Assert Winner.
4.6.4.3. Transitions from Loser State
When in "I am Assert Loser" state, the following transitions can occur:
Receive Inferior Assert or State Refresh from Current Winner
An Assert or State Refresh is received from the current Assert
winner that is worse than this router's metric for S (typically the
winner's metric became worse). The Assert state machine MUST
transition to NoInfo (NI) state and cancel AT(S,G,I). The router
MUST delete the previous Assert Winner's address and metric and
evaluate any possible transitions to its Upstream(S,G) state
machine. Usually this router will eventually re-assert and win when
data packets from S have started flowing again.
Receive Preferred Assert or State Refresh
An Assert or State Refresh is received that has a metric better than
or equal to that of the current Assert winner. The Assert state
machine remains in Loser (L) state. If the metric was received in
an Assert, the router MUST set the Assert Timer (AT(S,G,I)) to
Assert_Time. If the metric was received in a State Refresh, the
router MUST set the Assert Timer (AT(S,G,I)) to three times the
received State Refresh Interval. If the metric is better than the
current Assert Winner, the router MUST store the address and metric
of the new Assert Winner and if CouldAssert(S,G,I) == TRUE, the
router MUST multicast a Prune(S,G) to the new Assert winner.
AT(S,G,I) Expires
The (S,G) Assert Timer (AT(S,G,I)) expires. The Assert state
machine MUST transition to NoInfo (NI) state. The router MUST
delete the Assert Winner's address and metric. If CouldAssert ==
TRUE, the router MUST evaluate any possible transitions to its
Upstream(S,G) state machine.
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CouldAssert -> FALSE
CouldAssert has become FALSE because interface I has become the RPF
interface for S. The Assert state machine MUST transition to NoInfo
(NI) state, cancel AT(S,G,I) and delete information concerning the
Assert Winner on I.
CouldAssert -> TRUE
CouldAssert has become TRUE because interface I used to be the RPF
interface for S, and now it is not. The Assert state machine MUST
transition to NoInfo (NI) state, cancel AT(S,G,I) and delete
information concerning the Assert Winner on I.
Current Assert Winner's NeighborLiveness Timer Expires
The current Assert winner's NeighborLiveness Timer (NLT(N,I)) has
expired. The Assert state machine MUST transition to the NoInfo
(NI) state, delete the Assert Winner's address and metric, and
evaluate any possible transitions to its Upstream(S,G) state
machine.
Receive Prune(S,G), Join(S,G) or Graft(S,G)
A Prune(S,G), Join(S,G) or Graft(S,G) message was received on
interface I with its upstream neighbor address set to the router's
address on I. The router MUST send an Assert(S,G) on the receiving
interface I to initiate an Assert negotiation. The Assert state
machine remains in the Assert Loser(L) state. If a Graft(S,G) was
received, the router MUST respond with a GraftAck(S,G).
4.6.5. Rationale for Assert Rules
The following is a summary of the rules for generating and processing
Assert messages. It is not intended to be definitive (the state
machines and pseudocode provide the definitive behavior). Instead it
provides some rationale for the behavior.
1. The Assert winner for (S,G) must act as the local forwarder for (S,G)
on behalf of all downstream members.
2. PIM messages are directed towards to the RPF' neighbor and not to the
regular RPF neighbor.
3. An Assert loser that receives a Prune(S,G), Join(S,G) or Graft(S,G)
directed to it initiates a new Assert negotiation so the downstream
router can correct its RPF'(S).
4. An Assert winner for (S,G) sends a cancelling assert when it is about
to stop forwarding on an (S,G) entry. Example: if a router is being
taken down, then a canceling assert is sent.
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4.7. PIM Packet Formats
All PIM-DM packets use the same format as PIM-SM packets. In the event
of a discrepancy, PIM-SM [4] should be considered the definitive
specification. All PIM control messages have IP protocol number 103.
All PIM-DM messages MUST be sent with a TTL of 1. All PIM-DM messages
except Graft and Graft Ack messages MUST be sent to the ALL-PIM-ROUTERS
group. Graft messages SHOULD be unicast to the RPF'(S). Graft Ack
messages MUST be unicast to the sender of the Graft.
The IPv4 ALL-PIM-ROUTERS group is 224.0.0.13. The IPv6 ALL-PIM-ROUTERS
group is 'ff02::d'.
4.7.1. PIM Header
All PIM control messages have the following header:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|PIM Ver| Type | Reserved | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
PIM Ver
PIM version number is 2.
Type
Types for specific PIM messages. Available types are:
0 = Hello
1 = Register (PIM-SM only)
2 = Register Stop (PIM-SM only)
3 = Join/Prune
4 = Bootstrap (PIM-SM only)
5 = Assert
6 = Graft
7 = Graft Ack
8 = Candidate RP Advertisement (PIM-SM only)
9 = State Refresh
Reserved
Set to zero on transmission. Ignored upon receipt.
Checksum
The checksum is standard IP checksum, i.e. the 16 bit one's complement
of the one's complement sum of the entire PIM message. For computing
checksum, the checksum field is zeroed.
For IPv6, the checksum also includes the IPv6 "pseudo-header", as
specified in RFC 2460, section 8.1 [13].
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4.7.2. Encoded Unicast Address
An Encoded Unicast Address has the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Addr Family | Encoding Type | Unicast Address
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+...
Addr Family
The PIM Address Family of the 'Unicast Address' field of this address.
Values of 0-127 are as assigned by the IANA for Internet Address
Families in [9]. Values 128-250 are reserved to be assigned by the
IANA for PIM specific Address Families. Values 251-255 are designated
for private use. As there is no assignment authority for this space,
collisions should be expected.
Encoding Type
The type of encoding used with a specific Address Family. The value
'0' is reserved for this field, and represents the native encoding of
the Address Family
Unicast Address
The unicast address as represented by the given Address Family and
Encoding Type.
4.7.3. Encoded Group Address
An Encoded Group address has the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Addr Family | Encoding Type |B| Reserved |Z| Mask Len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Group Multicast Address
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+...
Addr Family
As described above.
Encoding Type
As described above.
B
Indicates the group range should use Bidirectional PIM [16].
Transmitted as zero, ignored upon receipt.
Reserved
Transmitted as zero. Ignored upon receipt.
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Z
Indicates the group range is an admin scope zone. This is used in the
Bootstrap Router Mechanism [18] only. For all other purposes, this bit
is set to zero and ignored on receipt.
Mask Len
The mask length field is 8 bits. The value is the number of
contiguous on bits left justified used as a mask, which combined with
the address, describes a range of addresses. It is less than or equal
to the address length in bits for the given Address Family and
Encoding Type. If the message is sent for a single address then the
mask length MUST equal the address length. PIM-DM routers MUST only
send for a single address.
Group Multicast Address
The address of the multicast group.
4.7.4. Encoded Source Address
An Encoded Source address has the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Addr Family | Encoding Type | Rsrvd |S|W|R| Mask Len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Address
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+...
Addr Family
As described above.
Encoding Type
As described above.
Rsrvd
Reserved. Transmitted as zero. Ignored upon receipt.
S
The Sparse Bit. Set to 0 for PIM-DM. Ignored upon receipt.
W
The Wild Card Bit. Set to 0 for PIM-DM. Ignored upon receipt.
R
The Rendezvous Point Tree bit. Set to 0 for PIM-DM. Ignored upon
receipt.
Mask Len
As described above. PIM-DM routers MUST only send for a single
source address.
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Source Address
The source address.
4.7.5. Hello Message Format
The PIM Hello message, as defined by PIM-SM [4], has the following
format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|PIM Ver| Type | Reserved | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option Type | Option Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option Value |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| . |
| . |
| . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option Type | Option Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option Value |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
PIM Ver, Type, Reserved, Checksum
Described above.
Option Type
The type of option given in the Option Value field. Available types
are:
0 Reserved
1 Hello Hold Time
2 LAN Prune Delay
3-16 Reserved
17 To be assigned by IANA
18 Deprecated and SHOULD NOT be used
19 DR Priority (PIM-SM Only)
20 Generation ID
21 State Refresh Capable
22 Bidir Capable
23-65000 To be assigned by IANA
65001-65535 Reserved for Private Use [9]
Unknown options SHOULD be ignored.
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4.7.5.1. Hello Hold Time Option
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 1 | Length = 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Hold Time |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Hold Time is the number of seconds a receiver MUST keep the neighbor
reachable. If the Hold Time is set to '0xffff', the receiver of this
message never times out the neighbor. This may be used with dial-on-
demand links, to avoid keeping the link up with periodic Hello messages.
Furthermore, if the Holdtime is set to '0', the information is timed out
immediately. The Hello Hold Time option MUST be used by PIM-DM routers.
4.7.5.2. LAN Prune Delay Option
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 2 | Length = 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|T| LAN Prune Delay | Override Interval |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The LAN_Prune_Delay option is used to tune the prune propagation delay
on multi-access LANs. The T bit is used by PIM-SM and SHOULD be set to 0
by PIM-DM routers and ignored upon receipt. The LAN Delay and Override
Interval fields are time intervals in units of milliseconds and are used
to tune the value of the J/P Override Interval and its derived timer
values. Section 4.3.5 describes how these values affect the behavior of
a router. The LAN Prune Delay SHOULD be used by PIM-DM routers.
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4.7.5.3. Generation ID Option
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 20 | Length = 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Generation ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Generation ID is a random value for the interface on which the Hello
message is sent. The Generation ID is regenerated whenever PIM
forwarding is started or restarted on the interface. The Generation ID
option MAY be used by PIM-DM routers.
4.7.5.4. State Refresh Capable Option
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 21 | Length = 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Version = 1 | Interval | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Interval field is the router's configured State Refresh Interval in
seconds. The Reserved field is set to zero and ignored upon reception.
The State Refresh Capable option MUST be used by State Refresh capable
PIM-DM routers.
4.7.6. Join/Prune Message Format
PIM Join/Prune messages, as defined in PIM-SM [4], have the following
format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|PIM Ver| Type | Reserved | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Upstream Neighbor Address (Encoded Unicast Format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Num Groups | Hold Time |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Multicast Group Address 1 (Encoded Group Format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Number of Joined Sources | Number of Pruned Sources |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Joined Source Address 1 (Encoded Source Format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| . |
| . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Joined Source Address n (Encoded Source Format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Pruned Source Address 1 (Encoded Source Format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| . |
| . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Pruned Source Address n (Encoded Source Format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| . |
| . |
| . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Multicast Group Address m (Encoded Group Format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Number of Joined Sources | Number of Pruned Sources |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Joined Source Address 1 (Encoded Source Format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| . |
| . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Joined Source Address n (Encoded Source Format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Pruned Source Address 1 (Encoded Source Format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| . |
| . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Pruned Source Address n (Encoded Source Format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
PIM Ver, Type, Reserved, Checksum
Described above.
Upstream Neighbor Address
The address of the upstream neighbor. The format for this address is
given in the Encoded Unicast address in section 4.7.2. PIM-DM routers
MUST set this field to the RPF next hop.
Reserved
Transmitted as zero. Ignored upon receipt.
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Hold Time
The number of seconds a receiving PIM-DM router MUST keep a Prune
state alive, unless removed by a Join or Graft message. If the Hold
Time is '0xffff', the receiver MUST NOT remove the Prune state unless
a corresponding Join or Graft message is received. The Hold Time is
ignored in Join messages.
Number of Groups
Number of multicast group sets contained in the message.
Multicast Group Address
The multicast group address in the Encoded Multicast address format
given in section 4.7.3.
Number of Joined Sources
Number of Join source addresses listed for a given group.
Number of Pruned Sources
Number of Prune source addresses listed for a given group.
Join Source Address 1..n
This list contains the sources from which the sending router wishes to
continue to receive multicast messages for the given group on this
interface. The addresses use the Encoded Source address format given
in section 4.7.4.
Prune Source Address 1..n
This list contains the sources from which the sending router does not
wish to receive multicast messages for the given group on this
interface. The addresses use the Encoded Source address format given
in section 4.7.4.
4.7.7. Assert Message Format
PIM Assert Messages, as defined in PIM-SM [4], have the following
format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|PIM Ver| Type | Reserved | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Multicast Group Address (Encoded Group Format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Address (Encoded Unicast Format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|R| Metric Preference |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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PIM Ver, Type, Reserved, Checksum
Described above.
Multicast Group Address
The multicast group address in the Encoded Multicast address format
given in section 4.7.3.
Source Address
The source address in the Encoded Unicast address format given in
section 4.7.2.
R
The Rendezvous Point Tree bit. Set to 0 for PIM-DM. Ignored upon
receipt.
Metric Preference
The preference value assigned to the unicast routing protocol that
provided the route to the source.
Metric
The cost metric of the unicast route to the source. The metric is in
units applicable to the unicast routing protocol used.
4.7.8. Graft Message Format
PIM Graft messages use the same format as Join/Prune messages except the
Type field is set to 6. The source address MUST be in the Join section
of the message. The Hold Time field SHOULD be zero and SHOULD be
ignored when a Graft is received.
4.7.9. Graft Ack Message Format
PIM Graft Ack messages are identical in format to the received Graft
message except the Type field is set to 7. The Upstream Neighbor
Address field SHOULD be set to the sender of the Graft message and
SHOULD be ignored upon receipt.
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4.7.10. State Refresh Message Format
PIM State Refresh Messages have the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|PIM Ver| Type | Reserved | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Multicast Group Address (Encoded Group Format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Address (Encoded Unicast Format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Originator Address (Encoded Unicast Format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|R| Metric Preference |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Masklen | TTL |P|N|O|Reserved | Interval |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
PIM Ver, Type, Reserved, Checksum
Described above.
Multicast Group Address
The multicast group address in the Encoded Multicast address format
given in section 4.7.3.
Source Address
The address of the data source in the Encoded Unicast address format
given in section 4.7.2.
Originator Address
The address of the first hop router in the Encoded Unicast address
format given in section 4.7.2.
R
The Rendezvous Point Tree bit. Set to 0 for PIM-DM. Ignored upon
receipt.
Metric Preference
The preference value assigned to the unicast routing protocol that
provided the route to the source.
Metric
The cost metric of the unicast route to the source. The metric is in
units applicable to the unicast routing protocol used.
Masklen
The length of the address mask of the unicast route to the source.
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TTL
Time To Live of the State Refresh message. Decremented each time the
message is forwarded. Note that this is different from the IP Header
TTL, which is always set to 1.
P
Prune indicator flag. This MUST be set to 1 if the State Refresh is
to be sent on a Pruned interface. Otherwise, it MUST be set to 0.
N
Prune Now flag. This SHOULD be set to 1 by the State Refresh
originator on every third State Refresh message and SHOULD be ignored
upon receipt. This is for compatibility with earlier versions of
state refresh.
O
Assert Override flag. This SHOULD be set to 1 by upstream routers on
a LAN if the Assert Timer (AT(S,G)) is not running and SHOULD be
ignored upon receipt. This is for compatibility with earlier versions
of state refresh.
Reserved
Set to zero and ignored upon receipt.
Interval
Set by the originating router to the interval (in seconds) between
consecutive State Refresh messages for this (S,G) pair.
4.8. PIM-DM Timers
PIM-DM maintains the following timers. All timers are countdown timers
- they are set to a value and count down to zero, at which point they
typically trigger an action. Of course they can just as easily be
implemented as count-up timers, where the absolute expiry time is stored
and compared against a real-time clock, but the language in this
specification assumes that they count downwards towards zero.
Global Timers
Hello Timer: HT
Per interface (I):
Per neighbor (N):
Neighbor Liveness Timer: NLT(N,I)
Per (S,G) Pair:
(S,G) Assert Timer: AT(S,G,I)
(S,G) Prune Timer: PT(S,G,I)
(S,G) PrunePending Timer: PPT(S,G,I)
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Per (S,G) Pair:
(S,G) Graft Retry Timer: GRT(S,G)
(S,G) Upstream Override Timer: OT(S,G)
(S,G) Prune Limit Timer: PLT(S,G)
(S,G) Source Active Timer: SAT(S,G)
(S,G) State Refresh Timer: SRT(S,G)
When timer values are started or restarted, they are set to default
values. The following tables summarize those default values.
Timer Name: Hello Timer (HT)
+----------------------+--------+--------------------------------------+
| Value Name | Value | Explanation |
+----------------------+--------+--------------------------------------+
|Hello_Period | 30 sec | Periodic interval for hello messages |
+----------------------+--------+--------------------------------------+
|Triggered_Hello_Delay | 5 sec | Random interval for initial Hello |
| | | message on bootup or triggered Hello |
| | | message to a rebooting neighbor |
+----------------------+--------+--------------------------------------+
Hello message are sent on every active interface once every Hello_Period
seconds. At system power-up, the timer is initialized to
rand(0,Triggered_Hello_Delay) to prevent synchronization. When a new or
rebooting neighbor is detected, a responding Hello is sent within
rand(0,Triggered_Hello_Delay).
Timer Name: Neighbor Liveness Timer (NLT(N,I))
+-------------------+-----------------+--------------------------------+
| Value Name | Value | Explanation |
+-------------------+-----------------+--------------------------------+
| Hello Holdtime | From message | Hold Time from Hello Message |
+-------------------+-----------------+--------------------------------+
Timer Name: PrunePending Timer (PPT(S,G,I))
+-----------------------+---------------+------------------------------+
| Value Name | Value | Explanation |
+-----------------------+---------------+------------------------------+
| J/P_Override_Interval | OI(I) + PD(I) | Short time after a Prune to |
| | | allow other routers on the |
| | | LAN to send a Join |
+-----------------------+---------------+------------------------------+
The J/P_Override_Interval is the sum of the interface's
Override_Interval (OI(I)) and Propagation_Delay (PD(I)). If all routers
on a LAN are using the LAN Prune Delay option, both parameters MUST be
set to the largest value on the LAN. Otherwise, the Override_Interval
(OI(I)) MUST be set to 2.5 seconds and the Propagation_Delay (PD(I))
MUST be set to 0.5 seconds.
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Timer Name: Prune Timer (PT(S,G,I))
+----------------+----------------+------------------------------------+
| Value Name | Value | Explanation |
+----------------+----------------+------------------------------------+
| Prune Holdtime | From message | Hold Time read from Prune Message |
+----------------+----------------+------------------------------------+
Timer Name: Assert Timer (AT(S,G,I))
+--------------------------+---------+---------------------------------+
| Value Name | Value | Explanation |
+--------------------------+---------+---------------------------------+
| Assert Time | 180 sec | Period after last assert before |
| | | assert state is timed out |
+--------------------------+---------+---------------------------------+
Note that for historical reasons, the Assert message lacks a Holdtime
field. Thus changing the Assert Time from the default value is not
recommended. If all members of a LAN are state refresh enabled, the
Assert Time will be three times the received RefreshInterval(S,G).
Timer Name: Graft Retry Timer (GRT(S,G))
+--------------------+-------+-----------------------------------------+
| Value Name | Value | Explanation |
+--------------------+-------+-----------------------------------------+
| Graft_Retry_Period | 3 sec | In the absence of receipt of a GraftAck |
| | | message, the time before retransmission |
| | | of a Graft message |
+--------------------+-------+-----------------------------------------+
Timer Name: Upstream Override Timer (OT(S,G))
+------------+----------------+----------------------------------------+
| Value Name | Value | Explanation |
+------------+----------------+----------------------------------------|
| t_override | rand(0, OI(I)) | Randomized delay to prevent response |
| | | implosion when sending a join message |
| | | to override someone else's prune |
+------------+----------------+----------------------------------------+
t_override is a random value between 0 and the interface's
Override_Interval (OI(I)). If all routers on a LAN are using the LAN
Prune Delay option, the Override_Interval (OI(I)) MUST be set to the
largest value on the LAN. Otherwise, the Override_Interval (OI(I)) MUST
be set to 2.5 seconds.
Timer Name: Prune Limit Timer (PLT(S,G))
+------------+--------------------+------------------------------------+
| Value Name | Value | Explanation |
+------------+--------------------+------------------------------------|
| t_limit | Equal to the Prune | Used to prevent Prune storms on a |
| | Holdtime sent | LAN |
+------------+--------------------+------------------------------------+
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Timer Name: Source Active Timer (SAT(S,G))
+----------------+-------------------+---------------------------------+
| Value Name | Value | Explanation |
+----------------+-------------------+---------------------------------+
| SourceLifetime | Default: 210 secs | Period of time after receiving |
| | | a multicast message a directly |
| | | attached router will continue |
| | | to send State Refresh messages |
+----------------+-------------------+---------------------------------+
Timer Name: State Refresh Timer (SRT(S,G))
+-----------------+------------------+---------------------------------+
| Value Name | Value | Explanation |
+-----------------+------------------+---------------------------------+
| RefreshInterval | Default: 60 secs | Interval between successive |
| | | state refresh messages |
+-----------------+------------------+---------------------------------+
5. Protocol Interaction Considerations
PIM-DM is designed to be independent of underlying unicast routing
protocols and will interact only to the extent needed to perform RPF
checks. It is generally assumed that multicast area and autonomous
system boundaries will correspond to the same boundaries for unicast
routing, though a deployment which does not follow this assumption is
not precluded by this specification.
In general, PIM-DM interactions with other multicast routing protocols
should be in compliance with RFC 2715 [7]. Other specific
interactions are noted below.
5.1. PIM-SM Interactions
PIM-DM is not intended to interact directly with PIM-SM, even though
they share a common packet format. It is particularly important to note
that a router cannot differentiate between a PIM-DM neighbor and a
PIM-SM neighbor based on Hello messages.
In the event that a PIM-DM router becomes a neighbor of a PIM-SM router
they will effectively form a simplex link with the PIM-DM router sending
all multicast messages to the PIM-SM router while the PIM-SM router
sends no multicast messages to the PIM-DM router.
The common packet format permits a hybrid PIM-SM/DM implementation that
would use PIM-SM when a rendezvous point is known and PIM-DM when one is
not. Such an implementation is outside the scope of this document.
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5.2. IGMP Interactions
PIM-DM will forward received multicast data packets to neighboring host
group members in all cases except when the PIM-DM router is in an Assert
Loser state on that interface. Note that a PIM Prune message is not
permitted to prevent the delivery of messages to a network with group
members.
A PIM-DM Router MAY use the DR Priority option described in PIM-SM [13]
to elect an IGMP v1 querier.
5.3. Source Specific Multicast (SSM) Interactions
PIM-DM makes no special considerations for SSM [14]. All Prunes and
Grafts within the protocol are for a specific source, so no additional
checks need be made.
5.4. Multicast Group Scope Boundary Interactions
While multicast group scope boundaries are generally identical to
routing area boundaries, it is conceivable that a routing area might be
partitioned for a particular multicast group. PIM-DM routers MUST NOT
send any messages concerning a particular group across that group's
scope boundary.
6. IANA Considerations
6.1. PIM Address Family
The PIM Address Family field was chosen to be 8 bits as a tradeoff
between packet format and use of the IANA assigned numbers. When the
PIM packet format was designed, only 15 values were assigned for Address
Families and large numbers of new Address Families were not envisioned,
8 bits seemed large enough. However, the IANA assigns Address Families
in a 16 bit value. Therefore, the PIM Address Family is allocated as
follows:
Values 0 through 127 are designated to have the same meaning as IANA
assigned Address Family Numbers [9].
Values 128 through 250 are designated to be assigned by the IANA based
upon IESG approval as defined in [8].
Values 251 through 255 are designated for Private Use, as defined in
[8].
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6.2. PIM Hello Options
Values 17 through 65000 are to be assigned by the IANA. Since the space
is large, they may be assigned as First Come First Served as defined in
[8]. Such assignments are valid for one year, and may be renewed.
Permanent assignments require a specification as defined in [8].
7. Security Considerations
The IPsec authentication header [10] MAY be used to provide data
integrity protection and groupwise data origin authentication of PIM
protocol messages. Authentication of PIM messages can protect against
unwanted behaviors caused by unauthorized or altered PIM messages. In
any case, a PIM router SHOULD NOT accept and process PIM messages from
neighbors unless a valid Hello message has been received from that
neighbor.
We should note that PIM-DM has no rendezvous point, and therefore no
single point of failure that may be vulnerable. It is further worth
noting that because PIM-DM uses unicast routes provided by an unknown
routing protocol, it may suffer collateral effects if the unicast
routing protocol is attacked.
7.1. Attacks Based on Forged Messages
The extent of possible damage depends on the type of counterfeit
messages accepted. We next consider the impact of possible forgeries. A
forged PIM-DM message is link local, and can only reach a LAN if it was
sent by a local host or if it was allowed onto the LAN by a compromised
or non-compliant router.
1. A forged a Hello message can cause multicast traffic to be delivered
to links where there are no legitimate requestors, potentially
wasting bandwidth on that link. On a multi-access LAN, the effects
are limited without the capability to forge a Join message since
other routers will Prune the link if the traffic is not desired.
2. A forged Join/Prune message can cause multicast traffic to be
delivered to links where there are no legitimate requestors,
potentially wasting bandwidth on that link. A forged Prune message
on a multi-access LAN is generally not a significant attack in PIM,
because any legitimately joined router on the LAN would override the
Prune with a Join before the upstream router stops forwarding data
to the LAN.
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3. A forged Graft message can cause multicast traffic to be delivered to
links where there are no legitimate requestors, potentially wasting
bandwidth on that link. In principle, Graft messages could be sent
multiple hops since they are unicast to the upstream router. This
should not be a problem since the remote forger should have no way
to get a Hello message to the target of the attack. Without a valid
Hello message, the receiving router SHOULD NOT accept the Graft.
4. A forged GraftAck message has no impact since it will be ignored
unless the router has recently sent a Graft to its upstream router.
5. By forging an Assert message on a multi-access LAN, an attacker could
cause the legitimate forwarder to stop forwarding traffic to the LAN.
Such a forgery would prevent any hosts downstream of that LAN from
receiving traffic.
6. A forged State Refresh message on a multi-access LAN would have the
same impact as a forged Assert message, having the same general
functions. In addition, forged State Refresh messages would be
propagated downstream and might be used in a denial of service
attack. Therefore, a PIM-DM router SHOULD rate limit State Refresh
messages propagated.
7.2. Non-cryptographic Authentication Mechanisms
A PIM-DM router SHOULD provide an option to limit the set of neighbors
from which it will accept PIM-DM messages. Either static configuration
of IP addresses or an IPSec security association may be used. All
options that restrict the range of addresses from which packets are
accepted MUST default to allowing all packets.
Furthermore, a PIM router SHOULD NOT accept protocol messages from a
router from which it has not yet received a valid Hello message.
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7.3. Authentication Using IPsec
The IPSec [10] transport mode using the Authentication Header (AH) is
the recommended method to prevent the above attacks in PIM. The
specific AH authentication algorithm and parameters, including the
choice of authentication algorithm and the choice of key, are configure
by the network administrator. The Encapsulating Security Payload (ESP)
MAY also be used to provide both encryption and authentication of PIM
protocol messages. When IPsec authentication is used, a PIM router
SHOULD reject (drop without processing) any unauthorized PIM protocol
messages.
To use IPSec, the administrator of a PIM network configures each PIM
router with one or more Security Associations and associated Security
Parameters Indices that are used by senders to sign PIM protocol
messages and are used by receivers to authenticate received PIM protocol
messages. This document does not describe protocols for establishing
Security Associations. It assumes that manual configuration of Security
Associations is performed, but it does not preclude the use of some
future negotiation protocol such as GDOI [17] to establish Security
Associations.
The network administrator defines a Security Association (SA) and
Security Parameters Index (SPI) that is to be used to authenticate all
PIM-DM protocol messages from each router on each link in a PIM-DM
domain.
In order to avoid the problem of allocating individual keys for each
neighbor on a link to each individual router, it is acceptable to
establish only one authentication key for all PIM-DM routers on a link.
This will not specifically authenticate the individual router sending
the message, but will assure that the sender is a PIM-DM router on that
link. If this method is used, the receiver of the message MUST ignore
the received sequence number, thus disabling anti-replay mechanisms.
The effects of disabling anti-replay mechanisms are essentially the same
as the effects of forged messages described in section 7.1 with the
additional protection that the forger can only reuse legitimate
messages.
The Security Policy Database at a PIM-DM router should be configured to
ensure that all incoming and outgoing PIM-DM packets use the SA
associated with the interface to which the packet is sent. Note that,
according to [10], there is nominally a different Security Association
Database (SAD) for each router interface. Thus, the selected Security
Association for an inbound PIM-DM packet can vary depending on the
interface on which the packet arrived. This fact allows the network
administrator to use different authentication methods for each link,
even though the destination address is the same for most PIM-DM packets,
regardless of interface.
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7.4. Denial of Service Attacks
There are a number of possible denial of service attacks against PIM
that can be caused by generating false PIM protocol messages or even by
generating false data traffic. Authenticating PIM protocol traffic
prevents some, but not all of these attacks. The possible attacks
include:
* Sending packets to many different group addresses quickly can be a
denial of service attack in and of itself. These messages will
initially be flooded throughout the network before they are pruned
back. The maintenance of state machines and State Refresh messages
will be a continual drain on network resources.
* Forged State Refresh messages sent quickly could be propagated by
downstream routers, creating a potential denial of service attack.
Therefore, a PIM-DM router SHOULD rate limit State Refresh messages
propagated.
8. Authors' Addresses
Andrew Adams
NextHop Technologies
825 Victors Way, Suite 100
Ann Arbor, MI 48108-2738
ala@nexthop.com
Jonathan Nicholas
ITT Industries
Aerospace/Communications Division
100 Kingsland Rd
Clifton, NJ 07014
jonathan.nicholas@itt.com
William Siadak
NextHop Technologies
825 Victors Way, Suite 100
Ann Arbor, MI 48108-2738
wfs@nexthop.com
9. Acknowledgments
The major features of PIM-DM were originally designed by Stephen
Deering, Deborah Estrin, Dino Farinacci, Van Jacobson, Ahmed Helmy,
David Meyer, and Liming Wei. Additional features for state refresh
were designed by Dino Farinacci, Isidor Kouvelas and Kurt Windisch.
This revision was undertaken to incorporate some of the lessons learned
during the evolution of the PIM-SM specification and early deployments
of PIM-DM.
Thanks the PIM Working Group for their comments.
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10. References
10.1 Normative References
[1] S.E. Deering, "Host Extensions for IP Multicasting", March 1989,
RFC 1112.
[2] W.Fenner, "Internet Group Management Protocol, Version 2",
November 1997, RFC 2236.
[3] B. Cain, S. Deering, B. Fenner, I. Kouvelas, A. Thyagarajan,
"Internet Group Management Protocol, Version 3",
October 2002, RFC 3376.
[4] D. Estrin, D. Farinacci, A. Helmy, D. Thaler, S. Deering,
M. Handley, V. Jacobson, C. Liu, P. Sharma, L. Wei, "Protocol
Independent Multicast-Sparse Mode (PIM-SM): Protocol
Specification", June 1998, RFC 2362.
[5] S. Deering, R. Hinden, "Internet Protocol, Version 6 (IPv6)
Specification", December 1998, RFC 2460.
[6] S. Deering, W. Fenner, B. Haberman, "Multicast Listener Discovery
(MLD) for IPv6", October 1999, RFC 2710.
[7] D. Thaler, "Interoperability Rules for Multicast Routing
Protocols", October 1999, RFC 2715.
[8] T. Narten, H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", October 1998, RFC 2434.
[9] IANA, "Address Family Numbers", linked from
http://www.iana.org/numbers.html.
[10] S. Kent, R. Atkinson, "Security Architecture for the Internet
Protocol", November 1998, RFC 2401.
10.2 Informative References
[11] S.E. Deering, "Multicast Routing in a Datagram Internetwork",
Ph.D. Thesis, Electrical Engineering Dept., Stanford University,
December 1991.
[12] D. Waitzman, B.Partridge, S.Deering, "Distance Vector Multicast
Routing Protocol", November 1988, RFC 1075
[13] W. Fenner, M. Handley, H.Holbrook, I. Kouvelas, "Protocol
Independent Multicast - Sparse Mode (PIM-SM): Protocol
Specification (Revised)", draft-ietf-pim-sm-v2-new-09.txt,
work in progress.
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[14] H.Holbrook, B. Cain, "Source Specific Multicast for IP",
draft-ietf-ssm-arch-04.txt, work in progress.
[15] K.McCloghrie, D.Farinacci, D.Thaler, B.Fenner, "Protocol
Independent Multicast MIB for IPv4", October 2000, RFC 2934
[16] M. Handley, I. Kouvelas, T. Speakman, L. Vicisano, "Bi-directional
Protocol Independent Multicast", draft-ietf-pim-bidir-06.txt,
work in progress.
[17] M. Baugher, B. Weis, T. Hardjono, H. Harney, "The Group Domain of
Interpretation", July 2003, RFC 3547.
[18] W. Fenner, M. Handley, R. Kermode, D. Thaler, "Bootstrap Router
(BSR) Mechanism for PIM Sparse Mode", draft-ietf-pim-sm-bsr-03.txt,
work in progress.
11. Full Copyright Statement
Copyright (C) The Internet Society (2004). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published and
distributed, in whole or in part, without restriction of any kind,
provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Adams, Nicholas, Siadak [Page 55]
Internet Engineering Task Force PIM WG
INTERNET DRAFT Andrew Adams (NextHop Technolgies)
draft-ietf-pim-dm-new-v2-05.txt Jonathan Nicholas (ITT A/CD)
William Siadak (NextHop Technologies)
June 2004
Expires December 2004
Protocol Independent Multicast - Dense Mode (PIM-DM):
Protocol Specification (Revised)
Status of this Document
This document is an Internet Draft and is in full conformance with all
provisions of Section 10 of RFC 2026.
Internet Drafts are working documents of the Internet Engineering Task
Force (IETF), its areas, and its working groups. Note that other groups
may also distribute working documents as Internet Drafts.
Internet Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet Drafts as reference material
or to cite them other than as "work in progress."
The list of current Internet Drafts can be accessed at
http://www.ietf.org/ietf/lid-abstracts.txt.
The list of Internet Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This document is a product of the IETF PIM WG. Comments should be
addressed to the authors, or the WG's mailing list at pim@ietf.org.
Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
This document specifies Protocol Independent Multicast - Dense Mode
(PIM-DM). PIM-DM is a multicast routing protocol that uses the
underlying unicast routing information base to flood multicast datagrams
to all multicast routers. Prune messages are used to prevent future
messages from propagating to routers with no group membership
information.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Definitions . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2. Pseudocode Notation . . . . . . . . . . . . . . . . . . . . 5
3. PIM-DM Protocol Overview . . . . . . . . . . . . . . . . . . 5
4. Protocol Specification . . . . . . . . . . . . . . . . . . . 6
4.1. PIM Protocol State . . . . . . . . . . . . . . . . . . . . . 6
4.1.1. General Purpose State . . . . . . . . . . . . . . . . . . . 7
4.1.2. (S,G) State . . . . . . . . . . . . . . . . . . . . . . . . 7
4.1.3. State Summarization Macros . . . . . . . . . . . . . . . . . 8
4.2. Data Packet Forwarding Rules . . . . . . . . . . . . . . . . 10
4.3. Hello Messages . . . . . . . . . . . . . . . . . . . . . . . 10
4.3.1. Sending Hello Messages . . . . . . . . . . . . . . . . . . . 10
4.3.2. Receiving Hello Messages . . . . . . . . . . . . . . . . . . 11
4.3.3. Hello Message Hold Time . . . . . . . . . . . . . . . . . . 11
4.3.4. Handling Router Failures . . . . . . . . . . . . . . . . . . 11
4.3.5. Reducing Prune Propagation Delay on LANs . . . . . . . . . . 12
4.4. PIM-DM Prune, Join and Graft Messages . . . . . . . . . . . 13
4.4.1. Upstream Prune, Join and Graft Messages . . . . . . . . . . 13
4.4.1.1. Transitions from the Forwarding (F) State . . . . . . . . . 16
4.4.1.2. Transitions from the Pruned (P) State . . . . . . . . . . . 17
4.4.1.3. Transitions from the AckPending (AP) State . . . . . . . . . 18
4.4.2. Downstream Prune, Join and Graft Messages . . . . . . . . . 19
4.4.2.1. Transitions from the NoInfo State . . . . . . . . . . . . . 21
4.4.2.2. Transitions from the PrunePending (PP) State . . . . . . . . 22
4.4.2.3. Transitions from the Prune (P) State . . . . . . . . . . . . 23
4.5. State Refresh . . . . . . . . . . . . . . . . . . . . . . . 24
4.5.1. Forwarding of State Refresh Messages . . . . . . . . . . . . 24
4.5.2. State Refresh Message Origination . . . . . . . . . . . . . 25
4.5.2.1. Transitions from the NotOriginator (NO) State . . . . . . . 27
4.5.2.2. Transitions from the Originator (O) State . . . . . . . . . 27
4.6. PIM Assert Messages . . . . . . . . . . . . . . . . . . . . 28
4.6.1. Assert Metrics . . . . . . . . . . . . . . . . . . . . . . . 28
4.6.2. AssertCancel Messages . . . . . . . . . . . . . . . . . . . 29
4.6.3. Assert State Macros . . . . . . . . . . . . . . . . . . . . 29
4.6.4. (S,G) Assert Message State Machine . . . . . . . . . . . . . 29
4.6.4.1. Transitions from NoInfo State . . . . . . . . . . . . . . . 31
4.6.4.2. Transitions from Winner State . . . . . . . . . . . . . . . 32
4.6.4.3. Transitions from Loser State . . . . . . . . . . . . . . . . 33
4.6.5. Rationale for Assert Rules . . . . . . . . . . . . . . . . . 34
4.7. PIM Packet Formats . . . . . . . . . . . . . . . . . . . . . 35
4.7.1. PIM Header . . . . . . . . . . . . . . . . . . . . . . . . . 35
4.7.2. Encoded Unicast Address . . . . . . . . . . . . . . . . . . 36
4.7.3. Encoded Group Address . . . . . . . . . . . . . . . . . . . 36
4.7.4. Encoded Source Address . . . . . . . . . . . . . . . . . . . 37
4.7.5. Hello Message Format . . . . . . . . . . . . . . . . . . . . 38
4.7.5.1. Hello Hold Time Option . . . . . . . . . . . . . . . . . . . 39
4.7.5.2. LAN Prune Delay Option . . . . . . . . . . . . . . . . . . . 39
4.7.5.3. Generation ID Option . . . . . . . . . . . . . . . . . . . . 40
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4.7.5.4. State Refresh Capable Option . . . . . . . . . . . . . . . . 40
4.7.6. Join/Prune Message Format . . . . . . . . . . . . . . . . . 40
4.7.7. Assert Message Format . . . . . . . . . . . . . . . . . . . 42
4.7.8. Graft Message Format . . . . . . . . . . . . . . . . . . . . 43
4.7.9. Graft Ack Message Format . . . . . . . . . . . . . . . . . . 43
4.7.10. State Refresh Message Format . . . . . . . . . . . . . . . . 44
4.8. PIM-DM Timers . . . . . . . . . . . . . . . . . . . . . . . 45
5. Protocol Interaction Considerations . . . . . . . . . . . . 48
5.1. PIM-SM Interactions . . . . . . . . . . . . . . . . . . . . 48
5.2. IGMP Interactions . . . . . . . . . . . . . . . . . . . . . 49
5.3. Source Specific Multicast (SSM) Interactions . . . . . . . . 49
5.4. Multicast Group Scope Boundary Interactions . . . . . . . . 49
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . 49
6.1. PIM Address Family . . . . . . . . . . . . . . . . . . . . . 49
6.2. PIM Hello Options . . . . . . . . . . . . . . . . . . . . . 50
7. Security Considerations. . . . . . . . . . . . . . . . . . . 50
7.1. Attacks Based on Forged Messages . . . . . . . . . . . . . . 50
7.2. Non-cryptographic Authentication Mechanisms . . . . . . . . 51
7.3. Authentication Using IPsec . . . . . . . . . . . . . . . . . 52
7.4. Denial of Service Attacks . . . . . . . . . . . . . . . . . 53
8. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 53
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 53
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 54
10.1. Normative References . . . . . . . . . . . . . . . . . . . . 54
10.2. Informative References . . . . . . . . . . . . . . . . . . . 54
11. Full Copyright Statement . . . . . . . . . . . . . . . . . . 55
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1. Introduction
This specification defines a multicast routing algorithm for multicast
groups that are densely distributed across a network. This protocol
does not have a topology discovery mechanism often used by a unicast
routing protocol. It employs the same packet formats sparse mode PIM
(PIM-SM) uses. This protocol is called PIM - Dense Mode. The
foundation of this design was largely built on Deering's early work on
IP multicast routing [11].
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" are to be
interpreted as described in RFC 2119 and indicate requirement levels for
compliant PIM-DM implementations.
2.1. Definitions
Multicast Routing Information Base (MRIB)
This is the multicast topology table, which is typically derived from
the unicast routing table, or routing protocols such as MBGP that
carry multicast-specific topology information. PIM-DM uses the MRIB
to make decisions regarding RPF interfaces.
Tree Information Base (TIB)
This is the collection of state maintained by a PIM router and created
by receiving PIM messages and IGMP information from local hosts. It
essentially stores the state of all multicast distribution trees at
that router.
Reverse Path Forwarding (RPF)
RPF is a multicast forwarding mode where a data packet is accepted for
forwarding only if it is received on an interface used to reach the
source in unicast.
Upstream Interface
Interface towards the source of the datagram. Also known as the RPF
Interface.
Downstream Interface
All interfaces that are not the upstream interface, including the
router itself.
(S,G) Pair
Source S and destination group G associated with an IP packet.
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2.2. Pseudocode Notation
We use set notation in several places in this specification.
A (+) B
is the union of two sets A and B.
A (-) B
are the elements of set A that are not in set B.
NULL
is the empty set or list.
Note that operations MUST be conducted in the order specified. This is
due to the fact that (-) is not a true difference operator because B is
not necessarily a subset of A. That is, A (+) B (-) C = A (-) C (+) B
is not a true statement unless C is a subset of both A and B.
In addition we use C-like syntax:
= denotes assignment of a variable.
== denotes a comparison for equality.
!= denotes a comparison for inequality.
Braces { and } are used for grouping.
3. PIM-DM Protocol Overview
This section provides an overview of PIM-DM behavior. It is intended as
an introduction to how PIM-DM works, and is NOT definitive. For the
definitive specification, see Section 4 - Protocol Specification.
PIM-DM assumes that when a source starts sending, all downstream systems
want to receive multicast datagrams. Initially, multicast datagrams are
flooded to all areas of the network. PIM-DM uses RPF to prevent looping
of multicast datagrams while flooding. If some areas of the network do
not have group members, PIM-DM will prune off the forwarding branch by
instantiating prune state.
Prune state has a finite lifetime. When that lifetime expires, data
will again be forwarded down the previously pruned branch.
Prune state is associated with an (S,G) pair. When a new member for a
group G appears in a pruned area, a router can "graft" toward the source
S for the group, thereby turning the pruned branch back into a
forwarding branch.
The broadcast of datagrams followed by pruning of unwanted branches is
often referred to as a flood and prune cycle and is typical of dense
mode protocols.
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In order to minimize repeated flooding of datagrams and subsequent
pruning associated with a particular (S,G) pair, PIM-DM uses a state
refresh message. This message is sent by the router(s) directly
connected to the source and is propagated throughout the network. When
received by a router on its RPF interface, the state refresh message
causes an existing prune state to be refreshed.
Compared with multicast routing protocols with built in topology
discovery mechanisms (e.g. DVMRP [12]) PIM-DM has a simplified design
and is not hard-wired into a specific topology discovery protocol.
However, such a simplification does incur more overhead by causing
flooding and pruning to occur on some links that could be avoided if
sufficient topology information were available, i.e. to decide whether
an interface leads to any downstream members of a particular group.
Additional overhead is chosen in favor of the simplification and
flexibility gained by not depending on a specific topology discovery
protocol.
PIM-DM differs from PIM-SM in two essential ways: 1) There are no
periodic joins transmitted, only explicitly triggered prunes and grafts.
2) There is no Rendezvous Point (RP). This is particularly important in
networks that cannot tolerate a single point of failure. (An RP is the
root of a shared multicast distribution tree. For more details see [4]).
4. Protocol Specification
The specification of PIM-DM is broken into several parts:
* Section 4.1 details the protocol state stored.
* Section 4.2 specifies the data packet forwarding rules.
* Section 4.3 specifies generation and processing of Hello messages.
* Section 4.4 specifies the Join, Prune and Graft generation and
processing rules.
* Section 4.5 specifies the State Refresh generation and forwarding
rules.
* Section 4.6 specifies the Assert generation and processing rules.
* Section 4.7 gives details on PIM-DM Packet Formats.
* Section 4.8 summarizes PIM-DM timers and their defaults.
4.1. PIM Protocol State
This section specifies all the protocol states that a PIM-DM
implementation should maintain in order to function correctly. We term
this state the Tree Information Base or TIB, as it holds the state of
all the multicast distribution trees at this router. In this
specification, we define PIM-DM mechanisms in terms of the TIB.
However, only a very simple implementation would actually implement
packet forwarding operations in terms of this state. Most
implementations will use this state to build a multicast forwarding
table, which would then be updated when the relevant state in the TIB
changes.
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Unlike PIM-SM, PIM-DM does not maintain a keepalive timer associated
with each (S,G) route. Within PIM-DM, route and state information
associated with an (S,G) entry MUST be maintained as long as any timer
associated with that (S,G) entry is active. When no timer associated
with an (S,G) entry is active, all information concerning that (S,G)
route may be discarded.
Although we specify precisely the state to be kept, this does not mean
that an implementation of PIM-DM needs to hold the state in this form.
This is actually an abstract state definition, which is needed in order
to specify the router's behavior. A PIM-DM implementation is free to
hold whatever internal state it requires, and will still be conformant
with this specification so long as it results in the same externally
visible protocol behavior as an abstract router that holds the following
state.
4.1.1. General Purpose State
A router stores the following non-group-specific state:
For each interface:
Hello Timer (HT)
State Refresh Capable
LAN Delay Enabled
Propagation Delay (PD)
Override Interval (OI)
Neighbor State:
For each neighbor:
Information from neighbor's Hello
Neighbor's Gen ID.
Neighbor's LAN Prune Delay
Neighbor's Override Interval
Neighbor's State Refresh Capability
Neighbor Liveness Timer (NLT)
4.1.2. (S,G) State
For every source/group pair (S,G), a router stores the following state:
(S,G) state:
For each interface:
Local Membership:
State: One of {"NoInfo", "Include"}
PIM (S,G) Prune State:
State: One of {"NoInfo" (NI), "Pruned" (P), "PrunePending" (PP)}
Prune Pending Timer (PPT)
Prune Timer (PT)
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(S,G) Assert Winner State:
State: One of {"NoInfo" (NI), "I lost Assert" (L),
"I won Assert" (W)}
Assert Timer (AT)
Assert winner's IP Address
Assert winner's Assert Metric
Upstream interface-specific:
Graft/Prune State:
State: One of {"NoInfo" (NI), "Pruned" (P), "Forwarding" (F),
"AckPending" (AP) }
GraftRetry Timer (GRT)
Override Timer (OT)
Prune Limit Timer (PLT)
Originator State:
Source Active Timer (SAT)
State Refresh Timer (SRT)
4.1.3. State Summarization Macros
Using the state defined above, the following "macros" are defined and
will be used in the descriptions of the state machines and pseudocode in
the following sections.
The most important macros are those defining the outgoing interface list
(or "olist") for the relevant state.
immediate_olist(S,G) = pim_nbrs (-) prunes(S,G) (+)
( pim_include(*,G) (-) pim_exclude(S,G) ) (+)
pim_include(S,G) (-) lost_assert(S,G) (-)
boundary(G)
olist(S,G) = immediate_olist(S,G) (-) RPF_interface(S)
The macros pim_include(*,G) and pim_include(S,G) indicate the interfaces
to which traffic might be forwarded or not forwarded because of hosts
that are local members on those interfaces.
pim_include(*,G) = {all interfaces I such that:
local_receiver_include(*,G,I)}
pim_include(S,G) = {all interfaces I such that:
local_receiver_include(S,G,I)}
pim_exclude(S,G) = {all interfaces I such that:
local_receiver_exclude(S,G,I)}
The macro RPF_interface(S) returns the RPF interface for source S. That
is to say, it returns the interface used to reach S as indicated by the
MRIB.
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The macro local_receiver_include(S,G,I) is true if the IGMP module or
other local membership mechanism has determined that there are local
members on interface I that desire to receive traffic sent specifically
by S to G.
The macro local_receiver_include(*,G,I) is true if the IGMP module or
other local membership mechanism has determined that there are local
members on interface I that desire to receive all traffic sent to G.
Note that this determination is expected to account for membership joins
initiated on or by the router.
The macro local_receiver_exclude(S,G,I) is true if
local_receiver_include(*,G,I) is true but none of the local members
desire to receive traffic from S.
The set pim_nbrs is the set of all interfaces on which the router has at
least one active PIM neighbor.
The set prunes(S,G) is the set of all interfaces on which the router has
received Prune(S,G) messages:
prunes(S,G) = {all interfaces I such that
DownstreamPState(S,G,I) is in Pruned state}
The set lost_assert(S,G) is the set of all interfaces on which the
router has lost an (S,G) Assert.
lost_assert(S,G) = {all interfaces I such that
lost_assert(S,G,I) == TRUE}
boundary(G) = {all interfaces I with an administratively scoped
boundary for group G}
The following pseudocode macro definitions are also used in many places
in the specification. Basically RPF' is the RPF neighbor towards a
source unless a PIM-DM Assert has overridden the normal choice of
neighbor.
neighbor RPF'(S,G) {
if ( I_Am_Assert_loser(S, G, RPF_interface(S) )) {
return AssertWinner(S, G, RPF_interface(S) )
} else {
return MRIB.next_hop( S )
}
}
The macro I_Am_Assert_loser(S, G, I) is true if the Assert state machine
(in section 4.6) for (S,G) on interface I is in the "I am Assert Loser"
state.
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4.2. Data Packet Forwarding Rules
The PIM-DM packet forwarding rules are defined below in pseudocode.
iif is the incoming interface of the packet.
S is the source address of the packet.
G is the destination address of the packet (group address).
RPF_interface(S) is the interface the MRIB indicates would be used to
route packets to S.
First, an RPF check MUST be performed to determine whether the packet
should be accepted based on TIB state and the interface on which that
the packet arrived. Packets that fail the RPF check MUST NOT be
forwarded and the router will conduct an assert process for the (S,G)
pair specified in the packet. Packets for which a route to the source
cannot be found MUST be discarded.
If the RPF check has been passed, an outgoing interface list is
constructed for the packet. If this list is not empty, then the packet
MUST be forwarded to all listed interfaces. If the list is empty, then
the router will conduct a prune process for the (S,G) pair specified in
the packet.
On receipt on a data packet from S addressed to G on interface iif:
if (iif == RPF_interface(S) AND UpstreamPState(S,G) != Pruned) {
oiflist = olist(S,G)
} else {
oiflist = NULL
}
forward packet on all interfaces in oiflist
This pseudocode employs the following "macro" definition:
UpstreamPState(S,G) is the state of the Upstream(S,G) state machine in
section 4.4.1.
4.3. Hello Messages
This section describes the generation and processing of Hello messages.
4.3.1. Sending Hello Messages
PIM-DM uses Hello messages to detect other PIM routers. Hello messages
are sent periodically on each PIM enabled interface. Hello messages are
multicast to the ALL-PIM-ROUTERS group. When PIM is enabled on an
interface or a router first starts, the Hello Timer (HT) MUST be set to
random value between 0 and Triggered_Hello_Delay. This prevents
synchronization of Hello messages if multiple routers are powered on
simultaneously.
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After the initial Hello message, a Hello message MUST be sent every
Hello_Period. A single Hello timer MAY be used to trigger sending
Hello messages on all active interfaces. The Hello Timer SHOULD NOT be
reset except when it expires.
4.3.2. Receiving Hello Messages
When a Hello message is received, the receiving router SHALL record the
receiving interface, the sender and any information contained in
recognized options. This information is retained for a number of
seconds in the Hold Time field of the Hello Message. If a new Hello
message is received from a particular neighbor N, the Neighbor Liveness
Timer (NLT(N,I)) MUST be reset to the newly received Hello Holdtime. If
a Hello message is received from a new neighbor, the receiving router
SHOULD send its own Hello message after a random delay between 0 and
Triggered_Hello_Delay.
4.3.3. Hello Message Hold Time
The Hold Time in the Hello Message should be set to a value that can
reasonably be expected to keep the Hello active until a new Hello
message is received. On most links, this will be 3.5 times the value of
Hello_Period.
If the Hold Time is set to '0xffff', the receiving router MUST NOT time
out that Hello message. This feature might be used for on-demand links
to avoid keeping the link up with periodic Hello messages.
If a Hold Time of '0' is received, the corresponding neighbor state is
expired immediately. When a PIM router takes an interface down or
changes IP address, a Hello message with a zero Hold Time SHOULD be sent
immediately (with the old IP address if the IP address is changed) to
cause any PIM neighbors to remove the old information immediately.
4.3.4. Handling Router Failures
If a Hello message is received from an active neighbor with a different
Generation ID (GenID), the neighbor has restarted and may not contain
the correct (S,G) state. A Hello message SHOULD be sent after a random
delay between 0 and Triggered_Hello_Delay (see 4.8) before any other
messages are sent. If the neighbor is downstream, the router MAY
replay the last State Refresh message for any (S,G) pairs for which it
is the Assert Winner indicating Prune and Assert status to the
downstream router. These State Refresh messages SHOULD be sent out
immediately after the Hello message. If the neighbor is the upstream
neighbor for an (S,G) entry, the router MAY cancel its Prune Limit
Timer to permit sending a prune and reestablishing a Pruned state in the
upstream router.
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Upon startup, a router MAY use any State Refresh messages received
within Hello_Period of its first Hello message on an interface to
establish state information. The State Refresh source will be the
RPF'(S), and Prune status for all interfaces will be set according to
the Prune Indicator bit in the State Refresh message. If the Prune
Indicator is set, the router SHOULD set the PruneLimitTimer to
Prune_Holdtime and set the PruneTimer on all downstream interfaces to
the State Refresh's Interval times two. The router SHOULD then
propagate the State Refresh as described in section 4.5.1.
4.3.5. Reducing Prune Propagation Delay on LANs
If all routers on a LAN support the LAN Prune Delay option, then the PIM
routers on that LAN will use the values received to adjust their
J/P_Override_Interval on that interface and the interface is LAN Delay
Enabled. Briefly, to avoid synchronization of Prune Override (Join)
messages when multiple downstream routers share a multi-access link,
sending of such messages is delayed by a small random amount of time.
The period of randomization is configurable and has a default value of 3
seconds.
Each router on the LAN expresses its view of the amount of randomization
necessary in the Override Interval field of the LAN Prune Delay option.
When all routers on a LAN use the LAN Prune Delay Option, all routers on
the LAN MUST set their Override_Interval to the largest Override value
on the LAN.
The LAN Delay inserted by a router in the LAN Prune Delay option
expresses the expected message propagation delay on the link and SHOULD
be configurable by the system administrator. When all routers on a link
use the LAN Prune Delay Option, all routers on the LAN MUST set
Propagation Delay to the largest LAN Delay on the LAN.
PIM implementers should enforce a lower bound on the permitted values
for this delay to allow for scheduling and processing delays within
their router. Such delays may cause received messages to be processed
later as well as triggered messages to be sent later than intended.
Setting this LAN Prune Delay to too low a value may result in temporary
forwarding outages because a downstream router will not be able to
override a neighbor's prune message before the upstream neighbor stops
forwarding.
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4.4. PIM-DM Prune, Join and Graft Messages
This section describes the generation and processing of PIM-DM Join,
Prune and Graft messages. Prune messages are sent towards the upstream
neighbor for S to indicate that traffic from S addressed to group G is
not desired. In the case of two downstream routers A and B, where A
wishes to continue receiving data and B does not, A will send a Join in
response to B's Prune to override the Prune. This is the only situation
in PIM-DM in which a Join message is used. Finally, a Graft message is
used to re-join a previously pruned branch to the delivery tree.
4.4.1. Upstream Prune, Join and Graft Messages
The Upstream(S,G) state machine for sending Prune, Graft and Join
messages is given below. There are three states.
Forwarding (F)
This is the starting state of the Upsteam(S,G) state machine. The
state machine is in this state if it just started or if
oiflist(S,G) != NULL.
Pruned(P)
The set, olist(S,G), is empty. The router will not forward data
from S addressed to group G.
AckPending(AP)
The router was in the Pruned(P) state but a transition has occurred
in the Downstream(S,G) state machine for one of this (S,G) entry's
outgoing interfaces indicating that traffic from S addressed to G
should again be forwarded. A Graft message has been sent to RPF'(S)
but a Graft Ack message has not yet been received.
In addition there are three state-machine-specific timers:
GraftRetry Timer (GRT(S,G))
This timer is set when a Graft is sent upstream. If a corresponding
GraftAck is not received before the timer expires, then another
Graft is sent and the GraftRetry Timer is reset. The timer is
stopped when a Graft Ack message is received. This timer is normally
set to Graft_Retry_Period (see 4.8).
Override Timer (OT(S,G))
This timer is set when a Prune(S,G) is received on the upstream
interface where olist(S,G) != NULL. When the timer expires, a
Join(S,G) message is sent on the upstream interface. This timer is
normally set to t_override (see 4.8).
Prune Limit Timer (PLT(S,G))
This timer is used to rate-limit Prunes on a LAN. It is only used
when the Upstream(S,G) state machine is in the Pruned state. A Prune
cannot be sent if this timer is running. This timer is normally set
to t_limit (see 4.8).
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+-------------+ +-------------+
| | olist == NULL | |
| Forward |----------------------->| Pruned |
| | | |
+-------------+ +-------------+
^ | ^ |
| | | |
| |RPF`(S) Changes olist == NULL| |
| | | |
| | +-------------+ | |
| +-------->| |----------+ |
| | AckPending | |
+-------------| |<-------------+
Rcv GraftAck OR +-------------+ olist != NULL
Rcv State Refresh
With (P==0) OR
S Directly Connect
Figure 1: Upstream Interface State Machine
In tabular form, the state machine is defined as follows:
+-------------------------------+--------------------------------------+
| | Previous State |
| +------------+------------+------------+
| Event | Forwarding | Pruned | AckPending |
+-------------------------------+------------+------------+------------+
| Data packet arrives on | ->P Send | ->P Send | N/A |
| RPF_Interface(S) AND | Prune(S,G) | Prune(S,G) | |
| olist(S,G) == NULL AND |Set PLT(S,G)|Set PLT(S,G)| |
| PLT(S,G) not running | | | |
+-------------------------------+------------+------------+------------+
| State Refresh(S,G) received | ->F Set | ->P Reset |->AP Set |
| from RPF`(S) AND | OT(S,G) | PLT(S,G) | OT(S,G) |
| Prune Indicator == 1 | | | |
+-------------------------------+------------+------------+------------+
| State Refresh(S,G) received | ->F | ->P Send |->F Cancel |
| from RPF`(S) AND | | Prune(S,G) | GRT(S,G) |
| Prune Indicator == 0 AND | |Set PLT(S,G)| |
| PLT(S,G) not running | | | |
+-------------------------------+------------+------------+------------+
| See Join(S,G) to RPF'(S) | ->F Cancel | ->P |->AP Cancel |
| | OT(S,G) | | OT(S,G) |
+-------------------------------+------------+------------+------------+
| See Prune(S,G) | ->F Set | ->P |->AP Set |
| | OT(S,G) | | OT(S,G) |
+-------------------------------+------------+------------+------------+
| OT(S,G) Expires | ->F Send | N/A |->AP Send |
| | Join(S,G) | | Join(S,G) |
+-------------------------------+------------+------------+------------+
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+-------------------------------+--------------------------------------+
| | Previous State |
| +------------+------------+------------+
| Event | Forwarding | Pruned | AckPending |
+-------------------------------+------------+------------+------------+
| olist(S,G)->NULL | ->P Send | N/A |->P Send |
| | Prune(S,G) | | Prune(S,G) |
| |Set PLT(S,G)| |Set PLT(S,G)|
| | | | Cancel |
| | | | GRT(S,G) |
+-------------------------------+------------+------------+------------+
| olist(S,G)->non-NULL | N/A | ->AP Send | N/A |
| | | Graft(S,G) | |
| | |Set GRT(S,G)| |
+-------------------------------+------------+------------+------------+
| RPF'(S) Changes AND | ->AP Send | ->AP Send |->AP Send |
| olist(S,G) != NULL | Graft(S,G) | Graft(S,G) | Graft(S,G) |
| |Set GRT(S,G)|Set GRT(S,G)|Set GRT(S,G)|
+-------------------------------+------------+------------+------------+
| RPF'(S) Changes AND | ->P | ->P Cancel |->P Cancel |
| olist(S,G) == NULL | | PLT(S,G) | GRT(S,G) |
+-------------------------------+------------+------------+------------+
| S becomes directly connected | ->F | ->P |->F Cancel |
| | | | GRT(S,G) |
+-------------------------------+------------+------------+------------+
| GRT(S,G) Expires | N/A | N/A |->AP Send |
| | | | Graft(S,G) |
| | | |Set GRT(S,G)|
+-------------------------------+------------+------------+------------+
| Receive GraftAck(S,G) from | ->F | ->P |->F Cancel |
| RPF'(S) | | | GRT(S,G) |
+-------------------------------+------------+------------+------------+
The transition event "RcvGraftAck(S,G)" implies receiving a Graft Ack
message targeted to this router's address on the incoming interface for
the (S,G) entry. If the destination address is not correct, the state
transitions in this state machine must not occur.
4.4.1.1. Transitions from the Forwarding (F) State
When the Upstream(S,G) state machine is in the Forwarding (F) state, the
following events may trigger a transition:
Data Packet arrives on RPF_Interface(S) AND olist(S,G) == NULL AND S
NOT directly connected
The Upstream(S,G) state machine MUST transition to the Pruned (P)
state, send a Prune(S,G) to RPF'(S) and set PLT(S,G) to t_limit
seconds.
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State Refresh(S,G) Received from RPF'(S)
The Upstream(S,G) state machine remains in a Forwarding state. If
the received State Refresh has the Prune Indicator bit set to one,
this router must override the upstream router's Prune state after a
short random interval. If OT(S,G) is not running and the Prune
Indicator bit equals one, the router MUST set OT(S,G) to t_override
seconds.
See Join(S,G) to RPF'(S)
This event is only relevant if RPF_interface(S) is a shared medium.
This router sees another router on RPF_interface(S) send a Join(S,G)
to RPF'(S,G). If the OT(S,G) is running, then it means that the
router had scheduled a Join to override a previously received Prune.
Another router has responded more quickly with a Join and so the
local router SHOULD cancel its OT(S,G), if it is running. The
Upstream(S,G) state machine remains in the Forwarding (F) state.
See Prune(S,G) AND S NOT directly connected
This event is only relevant if RPF_interface(S) is a shared medium.
This router sees another router on RPF_interface(S) send a
Prune(S,G). As this router is in Forwarding state, it must
override the Prune after a short random interval. If OT(S,G) is not
running, the router MUST set OT(S,G) to t_override seconds. The
Upstream(S,G) state machine remains in Forwarding (F) state.
OT(S,G) Expires AND S NOT directly connected
The OverrideTimer (OT(S,G)) expires. The router MUST send a
Join(S,G) to RPF'(S) to override a previously detected prune. The
Upstream(S,G) state machine remains in the Forwarding (F) state.
olist(S,G) -> NULL AND S NOT directly connected
The Upstream(S,G) state machine MUST transition to the Pruned (P)
state, send a Prune(S,G) to RPF'(S) and set PLT(S,G) to t_limit
seconds.
RPF'(S) Changes AND olist(S,G) is non-NULL AND S NOT directly
connected
Unicast routing or Assert state causes RPF'(S) to change, including
changes to RPF_Interface(S). The Upstream(S,G) state machine MUST
transition to the AckPending (AP) state, unicast a Graft to the new
RPF'(S) and set the GraftRetry Timer (GRT(S,G)) to
Graft_Retry_Period.
RPF'(S) Changes AND olist(S,G) is NULL
Unicast routing or Assert state causes RPF'(S) to change, including
changes to RPF_Interface(S). The Upstream(S,G) state machine MUST
transition to the Pruned (P) state.
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4.4.1.2. Transitions from the Pruned (P) State
When the Upstream(S,G) state machine is in the Pruned (P) state, the
following events may trigger a transition:
Data arrives on RPF_interface(S) AND PLT(S,G) not running AND S NOT
directly connected
Either another router on the LAN desires traffic from S addressed to
G or a previous Prune was lost. In order to prevent generating a
Prune(S,G) in response to every data packet, the PruneLimit Timer
(PLT(S,G)) is used. Once the PLT(S,G) expires, the router needs to
send another prune in response to a data packet not received
directly from the source. A Prune(S,G) MUST be sent to RPF'(S) and
the PLT(S,G) MUST be set to t_limit.
State Refresh(S,G) Received from RPF'(S)
The Upstream(S,G) state machine remains in a Pruned state. If the
State Refresh has its Prune Indicator bit set to zero and PLT(S,G)
is not running, a Prune(S,G) MUST be sent to RPF'(S) and the
PLT(S,G) MUST be set to t_limit. If the State Refresh has its Prune
Indicator bit set to one, the router MUST reset PLT(S,G) to t_limit.
See Prune(S,G) to RPF'(S)
A Prune(S,G) is seen on RPF_interface(S) to RPF'(S). The
Upstream(S,G) state machine stays in the Pruned (P) state. The
router MAY reset its PLT(S,G) to the value in the Holdtime field of
the received message if greater than the current value of the
PLT(S,G).
olist(S,G)->non-NULL AND S NOT directly connected
The set of interfaces defined by the olist(S,G) macro becomes
non-empty indicating traffic from S addressed to group G must be
forwarded. The Upstream(S,G) state machine MUST cancel PLT(S,G),
transition to the AckPending (AP) state and unicast a Graft message
to RPF'(S). The Graft Retry Timer (GRT(S,G)) MUST be set to
Graft_Retry_Period.
RPF'(S) Changes AND olist(S,G) == non-NULL AND S NOT directly
connected
Unicast routing or Assert state causes RPF'(S) to change, including
changes to RPF_Interface(S). The Upstream(S,G) state machine MUST
cancel PLT(S,G), transition to the AckPending (AP) state, send a
Graft unicast to the new RPF'(S) and set the GraftRetry Timer
(GRT(S,G)) to Graft_Retry_Period.
RPF'(S) Changes AND olist(S,G) == NULL AND S NOT directly connected
Unicast routing or Assert state causes RPF'(S) to change, including
changes to RPF_Interface(S). The Upstream(S,G) state machine stays
in the Pruned (P) state and MUST cancel the PLT(S,G) timer.
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S becomes directly connected
Unicast routing changed so that S is directly connected. The
Upstream(S,G) state machine remains in the Pruned (P) state.
4.4.1.3. Transitions from the AckPending (AP) State
When the Upstream(S,G) state machine is in the AckPending (AP) state,
the following events may trigger a transition:
State Refresh(S,G) Received from RPF'(S) with Prune Indicator == 1
The Upstream(S,G) state machine remains in an AckPending state. The
router must override the upstream router's Prune state after a short
random interval. If OT(S,G) is not running and the Prune Indicator
bit equals one, the router MUST set OT(S,G) to t_override seconds.
State Refresh(S,G) Received from RPF'(S) with Prune Indicator == 0
The router MUST cancel its GraftRetry Timer (GRT(S,G)) and
transition to the Forwarding (F) state.
See Join(S,G) to RPF'(S,G)
This event is only relevant if RPF_interface(S) is a shared medium.
This router sees another router on RPF_interface(S) send a Join(S,G)
to RPF'(S,G). If the OT(S,G) is running, then it means that the
router had scheduled a Join to override a previously received Prune.
Another router has responded more quickly with a Join and so the
local router SHOULD cancel its OT(S,G), if it is running. The
Upstream(S,G) state machine remains in the AckPending (AP) state.
See Prune(S,G)
This event is only relevant if RPF_interface(S) is a shared medium.
This router sees another router on RPF_interface(S) send a
Prune(S,G). As this router is in AckPending (AP) state, it must
override the Prune after a short random interval. If OT(S,G) is not
running, the router MUST set OT(S,G) to t_override seconds. The
Upstream(S,G) state machine remains in AckPending (AP) state.
OT(S,G) Expires
The OverrideTimer (OT(S,G)) expires. The router MUST send a
Join(S,G) to RPF'(S). The Upstream(S,G) state machine remains in
the AckPending (AP) state.
olist(S,G) -> NULL
The set of interfaces defined by the olist(S,G) macro becomes null
indicating traffic from S addressed to group G should no longer be
forwarded. The Upstream(S,G) state machine MUST transition to the
Pruned (P) state. A Prune(S,G) MUST be multicast to the
RPF_interface(S) with RPF'(S) named in the upstream neighbor field.
The GraftRetry Timer (GRT(S,G)) MUST be cancelled and PLT(S,G) MUST
be set to t_limit seconds.
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RPF'(S) Changes AND olist(S,G) does not become NULL AND S NOT directly
connected
Unicast routing or Assert state causes RPF'(S) to change, including
changes to RPF_Interface(S). The Upstream(S,G) state machine stays
in the AckPending (AP) state. A Graft MUST be unicast to the new
RPF'(S) and the GraftRetry Timer (GRT(S,G)) reset to
Graft_Retry_Period.
RPF'(S) Changes AND olist(S,G) == NULL AND S NOT directly connected
Unicast routing or Assert state causes RPF'(S) to change, including
changes to RPF_Interface(S). The Upstream(S,G) state machine MUST
transition to the Pruned (P) state. The GraftRetry Timer (GRT(S,G))
MUST be cancelled.
S becomes directly connected
Unicast routing has changed so that S is directly connected. The
GraftRetry Timer MUST be cancelled and the Upstream(S,G) state
machine MUST transition to the Forwarding(F) state.
GRT(S,G) Expires
The GraftRetry Timer (GRT(S,G)) expires for this (S,G) entry. The
Upstream(S,G) state machine stays in the AckPending (AP) state.
Another Graft message for (S,G) SHOULD be unicasted to RPF'(S) and
the GraftRetry Timer (GRT(S,G)) reset to Graft_Retry_Period. It is
RECOMMENDED that the router retry a configured number of times
before ceasing retries.
See GraftAck(S,G) from RPF'(S)
A GraftAck is received from RPF'(S). The GraftRetry Timer MUST be
cancelled and the Upstream(S,G) state machine MUST transition to the
Forwarding(F) state.
4.4.2 Downstream Prune, Join and Graft Messages
The Prune(S,G) Downstream state machine for receiving Prune, Join and
Graft messages on interface I is given below. This state machine MUST
always be in the NoInfo state on the upstream interface. It contains
three states.
NoInfo(NI)
The interface has no (S,G) Prune state and neither the Prune timer
(PT(S,G,I)) nor the PrunePending timer ((PPT(S,G,I)) is running.
PrunePending(PP)
The router has received a Prune(S,G) on this interface from a
downstream neighbor and is waiting to see whether the prune will be
overridden by another downstream router. For forwarding purposes,
the PrunePending state functions exactly like the NoInfo state.
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Pruned(P)
The router has received a Prune(S,G) on this interface from a
downstream neighbor and the Prune was not overridden. Data from S
addressed to group G is no longer being forwarded on this interface.
In addition there are two timers:
PrunePending Timer (PPT(S,G,I))
This timer is set when a valid Prune(S,G) is received. Expiry of
the PrunePending Timer (PPT(S,G,I)) causes the interface to
transition to the Pruned state.
Prune Timer (PT(S,G,I))
This timer is set when the PrunePending Timer (PT(S,G,I)) expires.
Expiry of the Prune Timer (PT(S,G,I)) causes the interface to
transition to the NoInfo (NI) state, thereby allowing data from S
addressed to group G to be forwarded on the interface.
+-------------+ +-------------+
| | PPT Expires | |
|PrunePending |----------------------->| Pruned |
| | | |
+-------------+ +-------------+
| ^ |
| | |
| |Rcv Prune |
| | |
| | +-------------+ |
| +---------| | |
| | NoInfo |<-------------+
+------------>| | Rcv Join/Graft OR
Rcv Join/Graft OR +-------------+ PT Expires OR
RPF_Interface(S)->I RPF_Interface(S)->I
Figure 2: Downstream Interface State Machine
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In tabular form, the state machine is:
+-------------------------------+--------------------------------------+
| | Previous State |
+ +------------+------------+------------+
| Event | No Info | PrunePend | Pruned |
+-------------------------------+------------+------------+------------+
| Receive Prune(S,G) |->PP Set |->PP |->P Reset |
| | PPT(S,G,I) | | PT(S,G,I) |
+-------------------------------+------------+------------+------------+
| Receive Join(S,G) |->NI |->NI Cancel |->NI Cancel |
| | | PPT(S,G,I) | PT(S,G,I) |
+-------------------------------+------------+------------+------------+
| Receive Graft(S,G) |->NI Send |->NI Send |->NI Send |
| | GraftAck | GraftAck | GraftAck |
| | | Cancel | Cancel |
| | | PPT(S,G,I) | PT(S,G,I) |
+-------------------------------+------------+------------+------------+
| PPT(S,G) Expires | N/A |->P Set | N/A |
| | | PT(S,G,I) | |
+-------------------------------+------------+------------+------------+
| PT(S,G) Expires | N/A | N/A |->NI |
+-------------------------------+------------+------------+------------+
| RPF_Interface(S) becomes I |->NI |->NI Cancel |->NI Cancel |
| | | PPT(S,G,I) | PT(S,G,I) |
+-------------------------------+------------+------------+------------+
| Send State Refresh(S,G) out I |->NI |->PP |->P Reset |
| | | | PT(S,G,I) |
+-------------------------------+------------+------------+------------+
The transition events "Receive Graft(S,G)", "Receive Prune(S,G)" and
"Receive Join(S,G)" denote receiving a Graft, Prune or Join message in
which this router's address on I is contained in the message's upstream
neighbor field. If the upstream neighbor field does not match this
router's address on I, then these state transitions in this state
machine must not occur.
4.4.2.1. Transitions from the NoInfo State
When the Prune(S,G) Downstream state machine is in the NoInfo (NI)
state, the following events may trigger a transition:
Receive Prune(S,G)
A Prune(S,G) is received on interface I with the upstream neighbor
field set to the router's address on I. The Prune(S,G) Downstream
state machine on interface I MUST transition to the PrunePending
(PP) state. The PrunePending Timer (PPT(S,G,I)) MUST be set to
J/P_Override_Interval if the router has more than one neighbor on I.
If the router has only one neighbor on interface I, then it SHOULD
set the PPT(S,G,I) to zero, effectively transitioning immediately to
the Pruned (P) state.
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Receive Graft(S,G)
A Graft(S,G) is received on the interface I with the upstream
neighbor field set to the router's address on I. The Prune(S,G)
Downstream state machine on interface I stays in the NoInfo (NI)
state. A GraftAck(S,G) MUST be unicasted to the originator of the
Graft(S,G) message.
4.4.2.2. Transitions from the PrunePending (PP) State
When the Prune(S,G) downstream state machine is in the PrunePending (PP)
state, the following events may trigger a transition.
Receive Join(S,G)
A Join(S,G) is received on interface I with the upstream neighbor
field set to the router's address on I. The Prune(S,G) Downstream
state machine on interface I MUST transition to the NoInfo (NI)
state. The PrunePending Timer (PPT(S,G,I)) MUST be cancelled.
Receive Graft(S,G)
A Graft(S,G) is received on interface I with the upstream neighbor
field set to the router's address on I. The Prune(S,G) Downstream
state machine on interface I MUST transition to the NoInfo (NI)
state and MUST unicast a Graft Ack message to the Graft originator.
The PrunePending Timer (PPT(S,G,I)) MUST be cancelled.
PPT(S,G,I) Expires
The PrunePending Timer (PPT(S,G,I)) expires indicating that no
neighbors have overridden the previous Prune(S,G) message. The
Prune(S,G) Downstream state machine on interface I MUST transition
to the Pruned (P) state. The Prune Timer (PT(S,G,I)) is started and
MUST be initialized to the received Prune_Hold_Time minus
J/P_Override_Interval. A PruneEcho(S,G) MUST be sent on I if I has
more than one PIM neighbor. A PruneEcho(S,G) is simply a Prune(S,G)
message multicast by the upstream router to a LAN with itself as the
Upstream Neighbor. Its purpose is to add additional reliability so
that if a Join that should have overridden the Prune is lost locally
on the LAN, then the PruneEcho(S,G) may be received and trigger a
new Join message . A PruneEcho(S,G) is OPTIONAL on an interface
with only one PIM neighbor. In addition, the router MUST evaluate
any possible transitions in the Upstream(S,G) state machine.
RPF_Interface(S) becomes interface I
The upstream interface for S has changed. The Prune(S,G) Downstream
state machine on interface I MUST transition to the NoInfo (NI)
state. The PrunePending Timer (PPT(S,G,I)) MUST be cancelled.
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4.4.2.3. Transitions from the Prune (P) State
When the Prune(S,G) Downstream state machine is in the Pruned (P) state,
the following events may trigger a transition.
Receive Prune(S,G)
A Prune(S,G) is received on the interface I with the upstream
neighbor field set to the router's address on I. The Prune(S,G)
Downstream state machine on interface I remains in the Pruned (P)
state. The Prune Timer (PT(S,G,I)) SHOULD be reset to the holdtime
contained in the Prune(S,G) message if it is greater than the
current value.
Receive Join(S,G)
A Join(S,G) is received on the interface I with the upstream
neighbor field set to the router's address on I. The Prune(S,G)
downstream state machine on interface I MUST transition to the
NoInfo (NI) state. The Prune Timer (PT(S,G,I)) MUST be cancelled.
The router MUST evaluate any possible transitions in the
Upstream(S,G) state machine.
Receive Graft(S,G)
A Graft(S,G) is received on interface I with the upstream neighbor
field set to the router's address on I. The Prune(S,G) Downstream
state machine on interface I MUST transition to the NoInfo (NI)
state and send a Graft Ack back to the Graft's source. The Prune
Timer (PT(S,G,I)) MUST be cancelled. The router MUST evaluate any
possible transitions in the Upstream(S,G) state machine.
PT(S,G,I) Expires
The Prune Timer (PT(S,G,I)) expires indicating that it is again time
to flood data from S addressed to group G onto interface I. The
Prune(S,G) Downstream state machine on interface I MUST transition
to the NoInfo (NI) state. The router MUST evaluate any possible
transitions in the Upstream(S,G) state machine.
RPF_Interface(S) becomes interface I
The upstream interface for S has changed. The Prune(S,G) Downstream
state machine on interface I MUST transition to the NoInfo (NI)
state. The PruneTimer (PT(S,G,I)) MUST be cancelled.
Send State Refresh(S,G) out interface I
The router has refreshed the Prune(S,G) state on interface I. The
router MUST reset the Prune Timer (PT(S,G,I)) to the Holdtime from
an active Prune received on interface I. The Holdtime used SHOULD
be the largest active one, but MAY be the most recently received
active Prune Holdtime.
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4.5. State Refresh
This section describes the major portions of the state refresh
mechanism.
4.5.1. Forwarding of State Refresh Messages
When a State Refresh message, SRM, is received, it is forwarded
according to the following pseudo-code.
if (iif != RPF_interface(S))
return;
if (RPF'(S) != srcaddr(SRM))
return;
if (StateRefreshRateLimit(S,G) == TRUE)
return;
for each interface I in pim_nbrs {
if (TTL(SRM) == 0 OR (TTL(SRM) - 1) < Threshold(I))
continue; /* Out of TTL, skip this interface */
if (boundary(I,G))
continue; /* This interface is scope boundary, skip it */
if (I == iif)
continue; /* This is the incoming interface, skip it */
if (lost_assert(S,G,I) == TRUE)
continue; /* Let the Assert Winner do State Refresh */
Copy SRM to SRM'; /* Make a copy of SRM to forward */
if (I contained in prunes(S,G)) {
set Prune Indicator bit of SRM' to 1;
if StateRefreshCapable(I) == TRUE
set PT(S,G) to largest active holdtime read from a Prune message
accepted on I;
} else {
set Prune Indicator bit of SRM' to 0;
}
set srcaddr(SRM') to my_addr(I);
set TTL of SRM' to TTL(SRM) - 1;
set metric of SRM' to metric of unicast route used to reach S;
set pref of SRM' to preference of unicast route used to reach S;
set mask of SRM' to mask of route used to reach S;
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if (AssertState == NoInfo) {
set Assert Override of SRM' to 1;
} else {
set Assert Override of SRM' to 0;
}
transmit SRM' on I;
}
The pseudocode above employs the following macro definitions.
Boundary(I,G) evaluates to TRUE if an administratively scoped boundary
for group G is configured on interface I.
StateRefreshCapable(I) evaluates to TRUE if all neighbors on an
interface use the State Refresh option.
StateRefreshRateLimit(S,G) evaluates to TRUE if the time elapsed since
the last received StateRefresh(S,G) is less than the configured
RefreshLimitInterval.
TTL(SRM) returns the TTL contained in the State Refresh Message, SRM.
This is different from the TTL contained in the IP header.
Threshold(I) returns the minimum TTL that a packet must have before it
can be transmitted on interface I.
srcaddr(SRM) returns the source address contained in the network
protocol (e.g. IPv4) header of the State Refresh Message, SRM.
my_addr(I) returns this node's network (e.g. IPv4) address on interface
I.
4.5.2 State Refresh Message Origination
This section describes the origination of State Refresh messages. These
messages are generated periodically by the PIM-DM router that is
directly connected to a source. One Origination(S,G) state machine
exists per (S,G) entry in a PIM-DM router.
The Origination(S,G) state machine has the following states:
NotOriginator(NO)
This is the starting state of the Origination(S,G) state machine.
While in this state a router will not originate State Refresh
messages for the (S,G) pair.
Originator(O)
When in this state the router will periodically originate State
Refresh messages. Only routers which are directly connected to S
may transition to this state.
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In addition there are two state-machine-specific timers:
State Refresh Timer (SRT(S,G))
This timer is controls when State Refresh messages are generated.
The timer is initially set when that Origination(S,G) state machine
transitions to the O state. It is cancelled when the
Origination(S,G) state machine transitions to the NO state. This
timer is normally set to StateRefreshInterval (see 4.8).
Source Active Timer (SAT(S,G))
This timer is first set when the Origination(S,G) state machine
transitions to the O state and is reset on the receipt of every
data packet from S addressed to group G. When it expires, the
Origination(S,G) state machine transitions to the NO state. This
timer is normally set to SourceLifetime (see 4.8).
+-------------+ Rcv Directly From S +-------------+
| |----------------------->| |
|NotOriginator| | Originator |
| |<-----------------------| |
+-------------+ SAT Expires OR +-------------+
S NOT Direct Connect
Figure 3: State Refresh State Machine
In tabular form, the state machine is defined as follows:
+----------------------------------------------------------------------+
| | Previous State |
| +---------------+-------------------+
| Event | NotOriginator | Originator |
+----------------------------------+---------------+-------------------+
| Receive Data from S AND | ->O | ->O Reset |
| S directly connected | Set SRT(S,G) | SAT(S,G) |
| | Set SAT(S,G) | |
+----------------------------------+---------------+-------------------+
| SRT(S,G) Expires | N/A | ->O Send |
| | | StateRefresh(S,G) |
| | | Reset SRT(S,G) |
+----------------------------------+---------------+-------------------+
| SAT(S,G) Expires | N/A | ->NO Cancel |
| | | SRT(S,G) |
+----------------------------------+---------------+-------------------+
| S no longer directly connected | ->NO | ->NO |
| | | Cancel SRT(S,G) |
| | | Cancel SAT(S,G) |
+----------------------------------+---------------+-------------------+
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4.5.2.1. Transitions from the NotOriginator (NO) State
When the Originating(S,G) state machine is in the NotOriginator (NO)
state, the following event may trigger a transition:
Data Packet received from directly connected Source S addressed to
group G
The router MUST transition to an Originator (O) state, set SAT(S,G)
to SourceLifetime, and set SRT(S,G) to StateRefreshInterval. The
router SHOULD record the TTL of the packet for use in State Refresh
messages.
4.5.2.2. Transitions from the Originator (O) State
When the Originating(S,G) state machine is in the Originator (O) state,
the following events may trigger a transition:
Receive Data Packet from S addressed to G
The router remains in the Originator (O) state and MUST reset
SAT(S,G) to SourceLifetime. The router SHOULD increase its recorded
TTL to match the TTL of the packet, if the packet's TTL is larger
than the previously recorded TTL.
SRT(S,G) Expires
The router remains in the Originator (O) state and MUST reset
SRT(S,G) to StateRefreshInterval. The router MUST also generate
State Refresh messages for transmission as described in the State
Refresh Forwarding rules (section 4.5.1) except for the TTL. If the
TTL of data packets from S to G are being recorded, then the TTL of
each State Refresh message is set to the highest recorded TTL.
Otherwise, the TTL is set to the configured State Refresh TTL. Let
I denote the interface over which a State Refresh message is being
sent. If the Prune(S,G) Downstream state machine for I is in the
NoInfo (NI) state, then the Prune-Indicator bit MUST be set to 0 in
the State Refresh message being sent over I. Otherwise the
Prune-Indicator bit MUST be set to 1.
SAT(S,G) Expires
The router MUST cancel the SRT(S,G) timer and transition to the
NotOriginator (NO) state.
S is no longer directly connected
The router MUST transition to the NotOriginator (NO) state and
cancel both the SAT(S,G) and SRT(S,G).
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4.6. PIM Assert Messages
4.6.1. Assert Metrics
Assert metrics are defined as:
struct assert_metric {
metric_preference;
route_metric;
ip_address;
};
When comparing assert_metrics, the metric_preference and route_metric
field are compared in order, where the first lower value wins. If all
fields are equal, the IP address of the router that sourced the Assert
message is used as a tie-breaker, with the highest IP address winning.
An Assert metric for (S,G) to include in (or compare against) an Assert
message sent on interface I should be computed using the following
pseudocode:
assert_metric
my_assert_metric(S,G,I) {
if (CouldAssert(S,G,I) == TRUE) {
return spt_assert_metric(S,G,I)
} else {
return infinite_assert_metric()
}
}
spt_assert_metric(S,I) gives the Assert metric we use if we're sending
an Assert based on active (S,G) forwarding state:
assert_metric
spt_assert_metric(S,I) {
return {0,MRIB.pref(S),MRIB.metric(S),my_addr(I)}
}
MRIB.pref(X) and MRIB.metric(X) are the routing preference and routing
metrics associated with the route to a particular (unicast) destination
X, as determined by the MRIB. my_addr(I) is simply the router's network
(e.g. IP) address that is associated with the local interface I.
infinite_assert_metric() gives the Assert metric we need to send an
Assert but doesn't match (S,G) forwarding state:
assert_metric
infinite_assert_metric() {
return {1,infinity,infinity,0}
}
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4.6.2. AssertCancel Messages
An AssertCancel(S,G) message is simply an Assert message for (S,G) with
infinite metric. The Assert winner sends such a message when it changes
its upstream interface to this interface. Other routers will see this
metric, causing those with forwarding state to send their own Asserts
and re-establish an Assert winner.
AssertCancel messages are simply an optimization. The original Assert
timeout mechanism will allow a subnet to eventually become consistent;
the AssertCancel mechanism simply causes faster convergence. No special
processing is required for an AssertCancel message, since it is simply
an Assert message from the current winner.
4.6.3. Assert State Macros
The macro lost_assert(S,G,I), is used in the olist computations of
section 4.1.3, and is defined as follows:
bool lost_assert(S,G,I) {
if ( RPF_interface(S) == I ) {
return FALSE
} else {
return (AssertWinner(S,G,I) != me AND
(AssertWinnerMetric(S,G,I) is better than
spt_assert_metric(S,G,I)))
}
}
AssertWinner(S,G,I) defaults to NULL and AssertWinnerMetric(S,G,I)
defaults to Infinity when in the NoInfo state.
4.6.4. (S,G) Assert Message State Machine
The (S,G) Assert state machine for interface I is shown in Figure 4.
There are three states:
NoInfo (NI)
This router has no (S,G) Assert state on interface I.
I am Assert Winner (W)
This router has won an (S,G) Assert on interface I. It is now
responsible for forwarding traffic from S destined for G via
interface I.
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I am Assert Loser (L)
This router has lost an (S,G) Assert on interface I. It must not
forward packets from S destined for G onto interface I.
In addition there is also an Assert Timer (AT(S,G,I)) that is used to
time out Assert state.
+-------------+ +-------------+
| | Rcv Pref Assert or SR | |
| Winner |----------------------->| Loser |
| | | |
+-------------+ +-------------+
^ | ^ |
| | Rcv Pref Assert or| |
| |AT Expires OR State Refresh| |
| |CouldAssert->FALSE | |
| | | |
| | +-------------+ | |
| +-------->| |----------+ |
| | No Info | |
+-------------| |<-------------+
Rcv Data from dnstrm +-------------+ Rcv Inf Assert from Win OR
OR Rcv Inferior Assert Rcv Inf SR from Winner OR
OR Rcv Inferior SR AT Expires OR
CouldAssert Changes OR
Winner's NLT Expires
Figure 4: Assert State Machine
In tabular form the state machine is defined as follows:
+-------------------------------+--------------------------------------+
| | Previous State |
| +------------+------------+------------+
| Event | No Info | Winner | Loser |
+-------------------------------+------------+------------+------------+
| An (S,G) Data packet received | ->W Send | ->W Send | ->L |
| on downstream interface | Assert(S,G)| Assert(S,G)| |
| | Set | Set | |
| | AT(S,G,I) | AT(S,G,I) | |
+-------------------------------+--------------------------------------+
| Receive Inferior (Assert OR | N/A | N/A |->NI Cancel |
| State Refresh) from Assert | | | AT(S,G,I) |
| Winner | | | |
+-------------------------------+--------------------------------------+
| Receive Inferior (Assert OR | ->W Send | ->W Send | ->L |
| State Refresh) from non-Assert| Assert(S,G)| Assert(S,G)| |
| Winner AND CouldAssert==TRUE | Set | Set | |
| | AT(S,G,I) | AT(S,G,I) | |
+-------------------------------+--------------------------------------+
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+-------------------------------+--------------------------------------+
| | Previous State |
| +------------+------------+------------+
| Event | No Info | Winner | Loser |
+-------------------------------+------------+------------+------------+
| Receive Preferred Assert OR | ->L Send | ->L Send | ->L Set |
| State Refresh | Prune(S,G) | Prune(S,G) | AT(S,G,I) |
| | Set | Set | |
| | AT(S,G,I) | AT(S,G,I) | |
+-------------------------------+--------------------------------------+
| Send State Refresh | ->NI | ->W Reset | N/A |
| | | AT(S,G,I) | |
+-------------------------------+--------------------------------------+
| AT(S,G) Expires | N/A | ->NI | ->NI |
+-------------------------------+--------------------------------------+
| CouldAssert -> FALSE | ->NI |->NI Cancel |->NI Cancel |
| | | AT(S,G,I) | AT(S,G,I) |
+-------------------------------+--------------------------------------+
| CouldAssert -> TRUE | ->NI | N/A |->NI Cancel |
| | | | AT(S,G,I) |
+-------------------------------+--------------------------------------+
| Winner's NLT(N,I) Expires | N/A | N/A |->NI Cancel |
| | | | AT(S,G,I) |
+-------------------------------+--------------------------------------+
| Receive Prune(S,G), Join(S,G) | ->NI | ->W | ->L Send |
| or Graft(S,G) | | | Assert(S,G)|
+-------------------------------+--------------------------------------+
Terminology:
A "preferred assert" is one with a better metric than the current
winner. An "inferior assert" is one with a worse metric than
my_assert_metric(S,G,I).
The state machine uses the following macro:
CouldAssert(S,G,I) = (RPF_interface(S) != I)
4.6.4.1. Transitions from NoInfo State
When in NoInfo state, the following events may trigger transitions:
An (S,G) data packet arrives on downstream interface I
An (S,G) data packet arrived on a downstream interface. It is
optimistically assumed that this router will be the Assert winner
for this (S,G). The Assert state machine MUST transition to the "I
am Assert Winner" state, send an Assert(S,G) to interface I, store
its own address and metric as the Assert Winner and set the
Assert_Timer (AT(S,G,I) to Assert_Time, thereby initiating the
Assert negotiation for (S,G).
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Receive Inferior (Assert OR State Refresh) AND
CouldAssert(S,G,I)==TRUE
An Assert or State Refresh is received for (S,G) that is inferior
to our own assert metric on interface I. The Assert state machine
MUST transition to the "I am Assert Winner" state, send an
Assert(S,G) to interface I, store its own address and metric as the
Assert Winner and set the Assert Timer (AT(S,G,I)) to Assert_Time.
Receive Preferred Assert or State Refresh
The received Assert or State Refresh has a better metric than this
router's and therefore the Assert state machine MUST transition to
the "I am Assert Loser" state and store the Assert Winner's address
and metric. If the metric was received in an Assert, the router MUST
set the Assert Timer (AT(S,G,I)) to Assert_Time. If the metric was
received in a State Refresh, the router MUST set the Assert Timer
(AT(S,G,I)) to three times the received State Refresh Interval. If
CouldAssert(S,G,I) == TRUE, the router MUST also multicast a
Prune(S,G) to the Assert winner with a Prune Hold Time equal to the
Assert Timer and evaluate any changes in its Upstream(S,G) state
machine.
4.6.4.2. Transitions from Winner State
When in "I am Assert Winner" state, the following events trigger
transitions:
An (S,G) data packet arrives on downstream interface I
An (S,G) data packet arrived on a downstream interface. The Assert
state machine remains in the "I am Assert Winner" state. The router
MUST send an Assert(S,G) to interface I and set the Assert Timer
(AT(S,G,I) to Assert_Time.
Receive Inferior Assert or State Refresh
An (S,G) Assert is received containing a metric for S that is worse
metric than this router's metric for S. Whoever sent the Assert is
in error. The router MUST send an Assert(S,G) to interface I and
reset the Assert Timer (AT(S,G,I)) to Assert_Time.
Receive Preferred Assert or State Refresh
An (S,G) Assert or State Refresh is received that has a better
metric than this router's metric for S on interface I. The Assert
state machine MUST transition to "I am Assert Loser" state and
store the new Assert Winner's address and metric. If the metric was
received in an Assert, the router MUST set the Assert Timer
(AT(S,G,I)) to Assert_Time. If the metric was received in a State
Refresh, the router MUST set the Assert Timer (AT(S,G,I)) to three
times the State Refresh Interval. The router MUST also multicast a
Prune(S,G) to the Assert winner with a Prune Hold Time equal to the
Assert Timer and evaluate any changes in its Upstream(S,G) state
machine.
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Send State Refresh
The router is sending a State Refresh(S,G) message on interface I.
The router MUST set the Assert Timer (AT(S,G,I)) to three times the
State Refresh Interval contained in the State Refresh(S,G) message.
AT(S,G,I) Expires
The (S,G) Assert Timer (AT(S,G,I)) expires. The Assert state machine
MUST transition to the NoInfo (NI) state.
CouldAssert(S,G,I) -> FALSE
This router's RPF interface changed so as to make CouldAssert(S,G,I)
become false. This router can no longer perform the actions of the
Assert winner, and so the Assert state machine MUST transition to
NoInfo (NI) state, send an AssertCancel(S,G) to interface I, cancel
the Assert Timer (AT(S,G,I)) and remove itself as the Assert Winner.
4.6.4.3. Transitions from Loser State
When in "I am Assert Loser" state, the following transitions can occur:
Receive Inferior Assert or State Refresh from Current Winner
An Assert or State Refresh is received from the current Assert
winner that is worse than this router's metric for S (typically the
winner's metric became worse). The Assert state machine MUST
transition to NoInfo (NI) state and cancel AT(S,G,I). The router
MUST delete the previous Assert Winner's address and metric and
evaluate any possible transitions to its Upstream(S,G) state
machine. Usually this router will eventually re-assert and win when
data packets from S have started flowing again.
Receive Preferred Assert or State Refresh
An Assert or State Refresh is received that has a metric better than
or equal to that of the current Assert winner. The Assert state
machine remains in Loser (L) state. If the metric was received in
an Assert, the router MUST set the Assert Timer (AT(S,G,I)) to
Assert_Time. If the metric was received in a State Refresh, the
router MUST set the Assert Timer (AT(S,G,I)) to three times the
received State Refresh Interval. If the metric is better than the
current Assert Winner, the router MUST store the address and metric
of the new Assert Winner and if CouldAssert(S,G,I) == TRUE, the
router MUST multicast a Prune(S,G) to the new Assert winner.
AT(S,G,I) Expires
The (S,G) Assert Timer (AT(S,G,I)) expires. The Assert state
machine MUST transition to NoInfo (NI) state. The router MUST
delete the Assert Winner's address and metric. If CouldAssert ==
TRUE, the router MUST evaluate any possible transitions to its
Upstream(S,G) state machine.
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CouldAssert -> FALSE
CouldAssert has become FALSE because interface I has become the RPF
interface for S. The Assert state machine MUST transition to NoInfo
(NI) state, cancel AT(S,G,I) and delete information concerning the
Assert Winner on I.
CouldAssert -> TRUE
CouldAssert has become TRUE because interface I used to be the RPF
interface for S, and now it is not. The Assert state machine MUST
transition to NoInfo (NI) state, cancel AT(S,G,I) and delete
information concerning the Assert Winner on I.
Current Assert Winner's NeighborLiveness Timer Expires
The current Assert winner's NeighborLiveness Timer (NLT(N,I)) has
expired. The Assert state machine MUST transition to the NoInfo
(NI) state, delete the Assert Winner's address and metric, and
evaluate any possible transitions to its Upstream(S,G) state
machine.
Receive Prune(S,G), Join(S,G) or Graft(S,G)
A Prune(S,G), Join(S,G) or Graft(S,G) message was received on
interface I with its upstream neighbor address set to the router's
address on I. The router MUST send an Assert(S,G) on the receiving
interface I to initiate an Assert negotiation. The Assert state
machine remains in the Assert Loser(L) state. If a Graft(S,G) was
received, the router MUST respond with a GraftAck(S,G).
4.6.5. Rationale for Assert Rules
The following is a summary of the rules for generating and processing
Assert messages. It is not intended to be definitive (the state
machines and pseudocode provide the definitive behavior). Instead it
provides some rationale for the behavior.
1. The Assert winner for (S,G) must act as the local forwarder for (S,G)
on behalf of all downstream members.
2. PIM messages are directed towards to the RPF' neighbor and not to the
regular RPF neighbor.
3. An Assert loser that receives a Prune(S,G), Join(S,G) or Graft(S,G)
directed to it initiates a new Assert negotiation so the downstream
router can correct its RPF'(S).
4. An Assert winner for (S,G) sends a cancelling assert when it is about
to stop forwarding on an (S,G) entry. Example: if a router is being
taken down, then a canceling assert is sent.
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4.7. PIM Packet Formats
All PIM-DM packets use the same format as PIM-SM packets. In the event
of a discrepancy, PIM-SM [4] should be considered the definitive
specification. All PIM control messages have IP protocol number 103.
All PIM-DM messages MUST be sent with a TTL of 1. All PIM-DM messages
except Graft and Graft Ack messages MUST be sent to the ALL-PIM-ROUTERS
group. Graft messages SHOULD be unicast to the RPF'(S). Graft Ack
messages MUST be unicast to the sender of the Graft.
The IPv4 ALL-PIM-ROUTERS group is 224.0.0.13. The IPv6 ALL-PIM-ROUTERS
group is 'ff02::d'.
4.7.1. PIM Header
All PIM control messages have the following header:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|PIM Ver| Type | Reserved | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
PIM Ver
PIM version number is 2.
Type
Types for specific PIM messages. Available types are:
0 = Hello
1 = Register (PIM-SM only)
2 = Register Stop (PIM-SM only)
3 = Join/Prune
4 = Bootstrap (PIM-SM only)
5 = Assert
6 = Graft
7 = Graft Ack
8 = Candidate RP Advertisement (PIM-SM only)
9 = State Refresh
Reserved
Set to zero on transmission. Ignored upon receipt.
Checksum
The checksum is standard IP checksum, i.e. the 16 bit one's complement
of the one's complement sum of the entire PIM message. For computing
checksum, the checksum field is zeroed.
For IPv6, the checksum also includes the IPv6 "pseudo-header", as
specified in RFC 2460, section 8.1 [13].
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4.7.2. Encoded Unicast Address
An Encoded Unicast Address has the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Addr Family | Encoding Type | Unicast Address
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+...
Addr Family
The PIM Address Family of the 'Unicast Address' field of this address.
Values of 0-127 are as assigned by the IANA for Internet Address
Families in [9]. Values 128-250 are reserved to be assigned by the
IANA for PIM specific Address Families. Values 251-255 are designated
for private use. As there is no assignment authority for this space,
collisions should be expected.
Encoding Type
The type of encoding used with a specific Address Family. The value
'0' is reserved for this field, and represents the native encoding of
the Address Family
Unicast Address
The unicast address as represented by the given Address Family and
Encoding Type.
4.7.3. Encoded Group Address
An Encoded Group address has the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Addr Family | Encoding Type |B| Reserved |Z| Mask Len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Group Multicast Address
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+...
Addr Family
As described above.
Encoding Type
As described above.
B
Indicates the group range should use Bidirectional PIM [16].
Transmitted as zero, ignored upon receipt.
Reserved
Transmitted as zero. Ignored upon receipt.
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Z
Indicates the group range is an admin scope zone. This is used in the
Bootstrap Router Mechanism [18] only. For all other purposes, this bit
is set to zero and ignored on receipt.
Mask Len
The mask length field is 8 bits. The value is the number of
contiguous on bits left justified used as a mask, which combined with
the address, describes a range of addresses. It is less than or equal
to the address length in bits for the given Address Family and
Encoding Type. If the message is sent for a single address then the
mask length MUST equal the address length. PIM-DM routers MUST only
send for a single address.
Group Multicast Address
The address of the multicast group.
4.7.4. Encoded Source Address
An Encoded Source address has the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Addr Family | Encoding Type | Rsrvd |S|W|R| Mask Len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Address
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+...
Addr Family
As described above.
Encoding Type
As described above.
Rsrvd
Reserved. Transmitted as zero. Ignored upon receipt.
S
The Sparse Bit. Set to 0 for PIM-DM. Ignored upon receipt.
W
The Wild Card Bit. Set to 0 for PIM-DM. Ignored upon receipt.
R
The Rendezvous Point Tree bit. Set to 0 for PIM-DM. Ignored upon
receipt.
Mask Len
As described above. PIM-DM routers MUST only send for a single
source address.
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Source Address
The source address.
4.7.5. Hello Message Format
The PIM Hello message, as defined by PIM-SM [4], has the following
format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|PIM Ver| Type | Reserved | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option Type | Option Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option Value |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| . |
| . |
| . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option Type | Option Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option Value |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
PIM Ver, Type, Reserved, Checksum
Described above.
Option Type
The type of option given in the Option Value field. Available types
are:
0 Reserved
1 Hello Hold Time
2 LAN Prune Delay
3-16 Reserved
17 To be assigned by IANA
18 Deprecated and SHOULD NOT be used
19 DR Priority (PIM-SM Only)
20 Generation ID
21 State Refresh Capable
22 Bidir Capable
23-65000 To be assigned by IANA
65001-65535 Reserved for Private Use [9]
Unknown options SHOULD be ignored.
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4.7.5.1. Hello Hold Time Option
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 1 | Length = 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Hold Time |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Hold Time is the number of seconds a receiver MUST keep the neighbor
reachable. If the Hold Time is set to '0xffff', the receiver of this
message never times out the neighbor. This may be used with dial-on-
demand links, to avoid keeping the link up with periodic Hello messages.
Furthermore, if the Holdtime is set to '0', the information is timed out
immediately. The Hello Hold Time option MUST be used by PIM-DM routers.
4.7.5.2. LAN Prune Delay Option
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 2 | Length = 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|T| LAN Prune Delay | Override Interval |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The LAN_Prune_Delay option is used to tune the prune propagation delay
on multi-access LANs. The T bit is used by PIM-SM and SHOULD be set to 0
by PIM-DM routers and ignored upon receipt. The LAN Delay and Override
Interval fields are time intervals in units of milliseconds and are used
to tune the value of the J/P Override Interval and its derived timer
values. Section 4.3.5 describes how these values affect the behavior of
a router. The LAN Prune Delay SHOULD be used by PIM-DM routers.
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4.7.5.3. Generation ID Option
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 20 | Length = 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Generation ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Generation ID is a random value for the interface on which the Hello
message is sent. The Generation ID is regenerated whenever PIM
forwarding is started or restarted on the interface. The Generation ID
option MAY be used by PIM-DM routers.
4.7.5.4. State Refresh Capable Option
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 21 | Length = 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Version = 1 | Interval | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Interval field is the router's configured State Refresh Interval in
seconds. The Reserved field is set to zero and ignored upon reception.
The State Refresh Capable option MUST be used by State Refresh capable
PIM-DM routers.
4.7.6. Join/Prune Message Format
PIM Join/Prune messages, as defined in PIM-SM [4], have the following
format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|PIM Ver| Type | Reserved | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Upstream Neighbor Address (Encoded Unicast Format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Num Groups | Hold Time |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Multicast Group Address 1 (Encoded Group Format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Number of Joined Sources | Number of Pruned Sources |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Joined Source Address 1 (Encoded Source Format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| . |
| . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Joined Source Address n (Encoded Source Format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Pruned Source Address 1 (Encoded Source Format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| . |
| . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Pruned Source Address n (Encoded Source Format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| . |
| . |
| . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Multicast Group Address m (Encoded Group Format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Number of Joined Sources | Number of Pruned Sources |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Joined Source Address 1 (Encoded Source Format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| . |
| . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Joined Source Address n (Encoded Source Format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Pruned Source Address 1 (Encoded Source Format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| . |
| . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Pruned Source Address n (Encoded Source Format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
PIM Ver, Type, Reserved, Checksum
Described above.
Upstream Neighbor Address
The address of the upstream neighbor. The format for this address is
given in the Encoded Unicast address in section 4.7.2. PIM-DM routers
MUST set this field to the RPF next hop.
Reserved
Transmitted as zero. Ignored upon receipt.
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Hold Time
The number of seconds a receiving PIM-DM router MUST keep a Prune
state alive, unless removed by a Join or Graft message. If the Hold
Time is '0xffff', the receiver MUST NOT remove the Prune state unless
a corresponding Join or Graft message is received. The Hold Time is
ignored in Join messages.
Number of Groups
Number of multicast group sets contained in the message.
Multicast Group Address
The multicast group address in the Encoded Multicast address format
given in section 4.7.3.
Number of Joined Sources
Number of Join source addresses listed for a given group.
Number of Pruned Sources
Number of Prune source addresses listed for a given group.
Join Source Address 1..n
This list contains the sources from which the sending router wishes to
continue to receive multicast messages for the given group on this
interface. The addresses use the Encoded Source address format given
in section 4.7.4.
Prune Source Address 1..n
This list contains the sources from which the sending router does not
wish to receive multicast messages for the given group on this
interface. The addresses use the Encoded Source address format given
in section 4.7.4.
4.7.7. Assert Message Format
PIM Assert Messages, as defined in PIM-SM [4], have the following
format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|PIM Ver| Type | Reserved | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Multicast Group Address (Encoded Group Format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Address (Encoded Unicast Format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|R| Metric Preference |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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PIM Ver, Type, Reserved, Checksum
Described above.
Multicast Group Address
The multicast group address in the Encoded Multicast address format
given in section 4.7.3.
Source Address
The source address in the Encoded Unicast address format given in
section 4.7.2.
R
The Rendezvous Point Tree bit. Set to 0 for PIM-DM. Ignored upon
receipt.
Metric Preference
The preference value assigned to the unicast routing protocol that
provided the route to the source.
Metric
The cost metric of the unicast route to the source. The metric is in
units applicable to the unicast routing protocol used.
4.7.8. Graft Message Format
PIM Graft messages use the same format as Join/Prune messages except the
Type field is set to 6. The source address MUST be in the Join section
of the message. The Hold Time field SHOULD be zero and SHOULD be
ignored when a Graft is received.
4.7.9. Graft Ack Message Format
PIM Graft Ack messages are identical in format to the received Graft
message except the Type field is set to 7. The Upstream Neighbor
Address field SHOULD be set to the sender of the Graft message and
SHOULD be ignored upon receipt.
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4.7.10. State Refresh Message Format
PIM State Refresh Messages have the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|PIM Ver| Type | Reserved | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Multicast Group Address (Encoded Group Format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Address (Encoded Unicast Format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Originator Address (Encoded Unicast Format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|R| Metric Preference |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Masklen | TTL |P|N|O|Reserved | Interval |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
PIM Ver, Type, Reserved, Checksum
Described above.
Multicast Group Address
The multicast group address in the Encoded Multicast address format
given in section 4.7.3.
Source Address
The address of the data source in the Encoded Unicast address format
given in section 4.7.2.
Originator Address
The address of the first hop router in the Encoded Unicast address
format given in section 4.7.2.
R
The Rendezvous Point Tree bit. Set to 0 for PIM-DM. Ignored upon
receipt.
Metric Preference
The preference value assigned to the unicast routing protocol that
provided the route to the source.
Metric
The cost metric of the unicast route to the source. The metric is in
units applicable to the unicast routing protocol used.
Masklen
The length of the address mask of the unicast route to the source.
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TTL
Time To Live of the State Refresh message. Decremented each time the
message is forwarded. Note that this is different from the IP Header
TTL, which is always set to 1.
P
Prune indicator flag. This MUST be set to 1 if the State Refresh is
to be sent on a Pruned interface. Otherwise, it MUST be set to 0.
N
Prune Now flag. This SHOULD be set to 1 by the State Refresh
originator on every third State Refresh message and SHOULD be ignored
upon receipt. This is for compatibility with earlier versions of
state refresh.
O
Assert Override flag. This SHOULD be set to 1 by upstream routers on
a LAN if the Assert Timer (AT(S,G)) is not running and SHOULD be
ignored upon receipt. This is for compatibility with earlier versions
of state refresh.
Reserved
Set to zero and ignored upon receipt.
Interval
Set by the originating router to the interval (in seconds) between
consecutive State Refresh messages for this (S,G) pair.
4.8. PIM-DM Timers
PIM-DM maintains the following timers. All timers are countdown timers
- they are set to a value and count down to zero, at which point they
typically trigger an action. Of course they can just as easily be
implemented as count-up timers, where the absolute expiry time is stored
and compared against a real-time clock, but the language in this
specification assumes that they count downwards towards zero.
Global Timers
Hello Timer: HT
Per interface (I):
Per neighbor (N):
Neighbor Liveness Timer: NLT(N,I)
Per (S,G) Pair:
(S,G) Assert Timer: AT(S,G,I)
(S,G) Prune Timer: PT(S,G,I)
(S,G) PrunePending Timer: PPT(S,G,I)
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Per (S,G) Pair:
(S,G) Graft Retry Timer: GRT(S,G)
(S,G) Upstream Override Timer: OT(S,G)
(S,G) Prune Limit Timer: PLT(S,G)
(S,G) Source Active Timer: SAT(S,G)
(S,G) State Refresh Timer: SRT(S,G)
When timer values are started or restarted, they are set to default
values. The following tables summarize those default values.
Timer Name: Hello Timer (HT)
+----------------------+--------+--------------------------------------+
| Value Name | Value | Explanation |
+----------------------+--------+--------------------------------------+
|Hello_Period | 30 sec | Periodic interval for hello messages |
+----------------------+--------+--------------------------------------+
|Triggered_Hello_Delay | 5 sec | Random interval for initial Hello |
| | | message on bootup or triggered Hello |
| | | message to a rebooting neighbor |
+----------------------+--------+--------------------------------------+
Hello message are sent on every active interface once every Hello_Period
seconds. At system power-up, the timer is initialized to
rand(0,Triggered_Hello_Delay) to prevent synchronization. When a new or
rebooting neighbor is detected, a responding Hello is sent within
rand(0,Triggered_Hello_Delay).
Timer Name: Neighbor Liveness Timer (NLT(N,I))
+-------------------+-----------------+--------------------------------+
| Value Name | Value | Explanation |
+-------------------+-----------------+--------------------------------+
| Hello Holdtime | From message | Hold Time from Hello Message |
+-------------------+-----------------+--------------------------------+
Timer Name: PrunePending Timer (PPT(S,G,I))
+-----------------------+---------------+------------------------------+
| Value Name | Value | Explanation |
+-----------------------+---------------+------------------------------+
| J/P_Override_Interval | OI(I) + PD(I) | Short time after a Prune to |
| | | allow other routers on the |
| | | LAN to send a Join |
+-----------------------+---------------+------------------------------+
The J/P_Override_Interval is the sum of the interface's
Override_Interval (OI(I)) and Propagation_Delay (PD(I)). If all routers
on a LAN are using the LAN Prune Delay option, both parameters MUST be
set to the largest value on the LAN. Otherwise, the Override_Interval
(OI(I)) MUST be set to 2.5 seconds and the Propagation_Delay (PD(I))
MUST be set to 0.5 seconds.
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Timer Name: Prune Timer (PT(S,G,I))
+----------------+----------------+------------------------------------+
| Value Name | Value | Explanation |
+----------------+----------------+------------------------------------+
| Prune Holdtime | From message | Hold Time read from Prune Message |
+----------------+----------------+------------------------------------+
Timer Name: Assert Timer (AT(S,G,I))
+--------------------------+---------+---------------------------------+
| Value Name | Value | Explanation |
+--------------------------+---------+---------------------------------+
| Assert Time | 180 sec | Period after last assert before |
| | | assert state is timed out |
+--------------------------+---------+---------------------------------+
Note that for historical reasons, the Assert message lacks a Holdtime
field. Thus changing the Assert Time from the default value is not
recommended. If all members of a LAN are state refresh enabled, the
Assert Time will be three times the received RefreshInterval(S,G).
Timer Name: Graft Retry Timer (GRT(S,G))
+--------------------+-------+-----------------------------------------+
| Value Name | Value | Explanation |
+--------------------+-------+-----------------------------------------+
| Graft_Retry_Period | 3 sec | In the absence of receipt of a GraftAck |
| | | message, the time before retransmission |
| | | of a Graft message |
+--------------------+-------+-----------------------------------------+
Timer Name: Upstream Override Timer (OT(S,G))
+------------+----------------+----------------------------------------+
| Value Name | Value | Explanation |
+------------+----------------+----------------------------------------|
| t_override | rand(0, OI(I)) | Randomized delay to prevent response |
| | | implosion when sending a join message |
| | | to override someone else's prune |
+------------+----------------+----------------------------------------+
t_override is a random value between 0 and the interface's
Override_Interval (OI(I)). If all routers on a LAN are using the LAN
Prune Delay option, the Override_Interval (OI(I)) MUST be set to the
largest value on the LAN. Otherwise, the Override_Interval (OI(I)) MUST
be set to 2.5 seconds.
Timer Name: Prune Limit Timer (PLT(S,G))
+------------+--------------------+------------------------------------+
| Value Name | Value | Explanation |
+------------+--------------------+------------------------------------|
| t_limit | Equal to the Prune | Used to prevent Prune storms on a |
| | Holdtime sent | LAN |
+------------+--------------------+------------------------------------+
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Timer Name: Source Active Timer (SAT(S,G))
+----------------+-------------------+---------------------------------+
| Value Name | Value | Explanation |
+----------------+-------------------+---------------------------------+
| SourceLifetime | Default: 210 secs | Period of time after receiving |
| | | a multicast message a directly |
| | | attached router will continue |
| | | to send State Refresh messages |
+----------------+-------------------+---------------------------------+
Timer Name: State Refresh Timer (SRT(S,G))
+-----------------+------------------+---------------------------------+
| Value Name | Value | Explanation |
+-----------------+------------------+---------------------------------+
| RefreshInterval | Default: 60 secs | Interval between successive |
| | | state refresh messages |
+-----------------+------------------+---------------------------------+
5. Protocol Interaction Considerations
PIM-DM is designed to be independent of underlying unicast routing
protocols and will interact only to the extent needed to perform RPF
checks. It is generally assumed that multicast area and autonomous
system boundaries will correspond to the same boundaries for unicast
routing, though a deployment which does not follow this assumption is
not precluded by this specification.
In general, PIM-DM interactions with other multicast routing protocols
should be in compliance with RFC 2715 [7]. Other specific
interactions are noted below.
5.1. PIM-SM Interactions
PIM-DM is not intended to interact directly with PIM-SM, even though
they share a common packet format. It is particularly important to note
that a router cannot differentiate between a PIM-DM neighbor and a
PIM-SM neighbor based on Hello messages.
In the event that a PIM-DM router becomes a neighbor of a PIM-SM router
they will effectively form a simplex link with the PIM-DM router sending
all multicast messages to the PIM-SM router while the PIM-SM router
sends no multicast messages to the PIM-DM router.
The common packet format permits a hybrid PIM-SM/DM implementation that
would use PIM-SM when a rendezvous point is known and PIM-DM when one is
not. Such an implementation is outside the scope of this document.
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5.2. IGMP Interactions
PIM-DM will forward received multicast data packets to neighboring host
group members in all cases except when the PIM-DM router is in an Assert
Loser state on that interface. Note that a PIM Prune message is not
permitted to prevent the delivery of messages to a network with group
members.
A PIM-DM Router MAY use the DR Priority option described in PIM-SM [13]
to elect an IGMP v1 querier.
5.3. Source Specific Multicast (SSM) Interactions
PIM-DM makes no special considerations for SSM [14]. All Prunes and
Grafts within the protocol are for a specific source, so no additional
checks need be made.
5.4. Multicast Group Scope Boundary Interactions
While multicast group scope boundaries are generally identical to
routing area boundaries, it is conceivable that a routing area might be
partitioned for a particular multicast group. PIM-DM routers MUST NOT
send any messages concerning a particular group across that group's
scope boundary.
6. IANA Considerations
6.1. PIM Address Family
The PIM Address Family field was chosen to be 8 bits as a tradeoff
between packet format and use of the IANA assigned numbers. When the
PIM packet format was designed, only 15 values were assigned for Address
Families and large numbers of new Address Families were not envisioned,
8 bits seemed large enough. However, the IANA assigns Address Families
in a 16 bit value. Therefore, the PIM Address Family is allocated as
follows:
Values 0 through 127 are designated to have the same meaning as IANA
assigned Address Family Numbers [9].
Values 128 through 250 are designated to be assigned by the IANA based
upon IESG approval as defined in [8].
Values 251 through 255 are designated for Private Use, as defined in
[8].
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6.2. PIM Hello Options
Values 17 through 65000 are to be assigned by the IANA. Since the space
is large, they may be assigned as First Come First Served as defined in
[8]. Such assignments are valid for one year, and may be renewed.
Permanent assignments require a specification as defined in [8].
7. Security Considerations
The IPsec authentication header [10] MAY be used to provide data
integrity protection and groupwise data origin authentication of PIM
protocol messages. Authentication of PIM messages can protect against
unwanted behaviors caused by unauthorized or altered PIM messages. In
any case, a PIM router SHOULD NOT accept and process PIM messages from
neighbors unless a valid Hello message has been received from that
neighbor.
We should note that PIM-DM has no rendezvous point, and therefore no
single point of failure that may be vulnerable. It is further worth
noting that because PIM-DM uses unicast routes provided by an unknown
routing protocol, it may suffer collateral effects if the unicast
routing protocol is attacked.
7.1. Attacks Based on Forged Messages
The extent of possible damage depends on the type of counterfeit
messages accepted. We next consider the impact of possible forgeries. A
forged PIM-DM message is link local, and can only reach a LAN if it was
sent by a local host or if it was allowed onto the LAN by a compromised
or non-compliant router.
1. A forged a Hello message can cause multicast traffic to be delivered
to links where there are no legitimate requestors, potentially
wasting bandwidth on that link. On a multi-access LAN, the effects
are limited without the capability to forge a Join message since
other routers will Prune the link if the traffic is not desired.
2. A forged Join/Prune message can cause multicast traffic to be
delivered to links where there are no legitimate requestors,
potentially wasting bandwidth on that link. A forged Prune message
on a multi-access LAN is generally not a significant attack in PIM,
because any legitimately joined router on the LAN would override the
Prune with a Join before the upstream router stops forwarding data
to the LAN.
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3. A forged Graft message can cause multicast traffic to be delivered to
links where there are no legitimate requestors, potentially wasting
bandwidth on that link. In principle, Graft messages could be sent
multiple hops since they are unicast to the upstream router. This
should not be a problem since the remote forger should have no way
to get a Hello message to the target of the attack. Without a valid
Hello message, the receiving router SHOULD NOT accept the Graft.
4. A forged GraftAck message has no impact since it will be ignored
unless the router has recently sent a Graft to its upstream router.
5. By forging an Assert message on a multi-access LAN, an attacker could
cause the legitimate forwarder to stop forwarding traffic to the LAN.
Such a forgery would prevent any hosts downstream of that LAN from
receiving traffic.
6. A forged State Refresh message on a multi-access LAN would have the
same impact as a forged Assert message, having the same general
functions. In addition, forged State Refresh messages would be
propagated downstream and might be used in a denial of service
attack. Therefore, a PIM-DM router SHOULD rate limit State Refresh
messages propagated.
7.2. Non-cryptographic Authentication Mechanisms
A PIM-DM router SHOULD provide an option to limit the set of neighbors
from which it will accept PIM-DM messages. Either static configuration
of IP addresses or an IPSec security association may be used. All
options that restrict the range of addresses from which packets are
accepted MUST default to allowing all packets.
Furthermore, a PIM router SHOULD NOT accept protocol messages from a
router from which it has not yet received a valid Hello message.
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7.3. Authentication Using IPsec
The IPSec [10] transport mode using the Authentication Header (AH) is
the recommended method to prevent the above attacks in PIM. The
specific AH authentication algorithm and parameters, including the
choice of authentication algorithm and the choice of key, are configure
by the network administrator. The Encapsulating Security Payload (ESP)
MAY also be used to provide both encryption and authentication of PIM
protocol messages. When IPsec authentication is used, a PIM router
SHOULD reject (drop without processing) any unauthorized PIM protocol
messages.
To use IPSec, the administrator of a PIM network configures each PIM
router with one or more Security Associations and associated Security
Parameters Indices that are used by senders to sign PIM protocol
messages and are used by receivers to authenticate received PIM protocol
messages. This document does not describe protocols for establishing
Security Associations. It assumes that manual configuration of Security
Associations is performed, but it does not preclude the use of some
future negotiation protocol such as GDOI [17] to establish Security
Associations.
The network administrator defines a Security Association (SA) and
Security Parameters Index (SPI) that is to be used to authenticate all
PIM-DM protocol messages from each router on each link in a PIM-DM
domain.
In order to avoid the problem of allocating individual keys for each
neighbor on a link to each individual router, it is acceptable to
establish only one authentication key for all PIM-DM routers on a link.
This will not specifically authenticate the individual router sending
the message, but will assure that the sender is a PIM-DM router on that
link. If this method is used, the receiver of the message MUST ignore
the received sequence number, thus disabling anti-replay mechanisms.
The effects of disabling anti-replay mechanisms are essentially the same
as the effects of forged messages described in section 7.1 with the
additional protection that the forger can only reuse legitimate
messages.
The Security Policy Database at a PIM-DM router should be configured to
ensure that all incoming and outgoing PIM-DM packets use the SA
associated with the interface to which the packet is sent. Note that,
according to [10], there is nominally a different Security Association
Database (SAD) for each router interface. Thus, the selected Security
Association for an inbound PIM-DM packet can vary depending on the
interface on which the packet arrived. This fact allows the network
administrator to use different authentication methods for each link,
even though the destination address is the same for most PIM-DM packets,
regardless of interface.
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7.4. Denial of Service Attacks
There are a number of possible denial of service attacks against PIM
that can be caused by generating false PIM protocol messages or even by
generating false data traffic. Authenticating PIM protocol traffic
prevents some, but not all of these attacks. The possible attacks
include:
* Sending packets to many different group addresses quickly can be a
denial of service attack in and of itself. These messages will
initially be flooded throughout the network before they are pruned
back. The maintenance of state machines and State Refresh messages
will be a continual drain on network resources.
* Forged State Refresh messages sent quickly could be propagated by
downstream routers, creating a potential denial of service attack.
Therefore, a PIM-DM router SHOULD rate limit State Refresh messages
propagated.
8. Authors' Addresses
Andrew Adams
NextHop Technologies
825 Victors Way, Suite 100
Ann Arbor, MI 48108-2738
ala@nexthop.com
Jonathan Nicholas
ITT Industries
Aerospace/Communications Division
100 Kingsland Rd
Clifton, NJ 07014
jonathan.nicholas@itt.com
William Siadak
NextHop Technologies
825 Victors Way, Suite 100
Ann Arbor, MI 48108-2738
wfs@nexthop.com
9. Acknowledgments
The major features of PIM-DM were originally designed by Stephen
Deering, Deborah Estrin, Dino Farinacci, Van Jacobson, Ahmed Helmy,
David Meyer, and Liming Wei. Additional features for state refresh
were designed by Dino Farinacci, Isidor Kouvelas and Kurt Windisch.
This revision was undertaken to incorporate some of the lessons learned
during the evolution of the PIM-SM specification and early deployments
of PIM-DM.
Thanks the PIM Working Group for their comments.
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10. References
10.1 Normative References
[1] S.E. Deering, "Host Extensions for IP Multicasting", March 1989,
RFC 1112.
[2] W.Fenner, "Internet Group Management Protocol, Version 2",
November 1997, RFC 2236.
[3] B. Cain, S. Deering, B. Fenner, I. Kouvelas, A. Thyagarajan,
"Internet Group Management Protocol, Version 3",
October 2002, RFC 3376.
[4] D. Estrin, D. Farinacci, A. Helmy, D. Thaler, S. Deering,
M. Handley, V. Jacobson, C. Liu, P. Sharma, L. Wei, "Protocol
Independent Multicast-Sparse Mode (PIM-SM): Protocol
Specification", June 1998, RFC 2362.
[5] S. Deering, R. Hinden, "Internet Protocol, Version 6 (IPv6)
Specification", December 1998, RFC 2460.
[6] S. Deering, W. Fenner, B. Haberman, "Multicast Listener Discovery
(MLD) for IPv6", October 1999, RFC 2710.
[7] D. Thaler, "Interoperability Rules for Multicast Routing
Protocols", October 1999, RFC 2715.
[8] T. Narten, H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", October 1998, RFC 2434.
[9] IANA, "Address Family Numbers", linked from
http://www.iana.org/numbers.html.
[10] S. Kent, R. Atkinson, "Security Architecture for the Internet
Protocol", November 1998, RFC 2401.
10.2 Informative References
[11] S.E. Deering, "Multicast Routing in a Datagram Internetwork",
Ph.D. Thesis, Electrical Engineering Dept., Stanford University,
December 1991.
[12] D. Waitzman, B.Partridge, S.Deering, "Distance Vector Multicast
Routing Protocol", November 1988, RFC 1075
[13] W. Fenner, M. Handley, H.Holbrook, I. Kouvelas, "Protocol
Independent Multicast - Sparse Mode (PIM-SM): Protocol
Specification (Revised)", draft-ietf-pim-sm-v2-new-09.txt,
work in progress.
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[14] H.Holbrook, B. Cain, "Source Specific Multicast for IP",
draft-ietf-ssm-arch-04.txt, work in progress.
[15] K.McCloghrie, D.Farinacci, D.Thaler, B.Fenner, "Protocol
Independent Multicast MIB for IPv4", October 2000, RFC 2934
[16] M. Handley, I. Kouvelas, T. Speakman, L. Vicisano, "Bi-directional
Protocol Independent Multicast", draft-ietf-pim-bidir-06.txt,
work in progress.
[17] M. Baugher, B. Weis, T. Hardjono, H. Harney, "The Group Domain of
Interpretation", July 2003, RFC 3547.
[18] W. Fenner, M. Handley, R. Kermode, D. Thaler, "Bootstrap Router
(BSR) Mechanism for PIM Sparse Mode", draft-ietf-pim-sm-bsr-03.txt,
work in progress.
11. Full Copyright Statement
Copyright (C) The Internet Society (2004). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published and
distributed, in whole or in part, without restriction of any kind,
provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
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followed, or as required to translate it into languages other than
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The limited permissions granted above are perpetual and will not be
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This document and the information contained herein is provided on an
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TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
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