Network Working Group IJ. Wijnands
Internet-Draft S. Venaas
Intended status: Experimental Cisco Systems, Inc.
Expires: September 18, 2016 M. Brig
Aegis BMD Program Office
A. Jonasson
Swedish Defence Material Administration (FMV)
March 17, 2016
PIM flooding mechanism and source discovery
draft-ietf-pim-source-discovery-bsr-04
Abstract
PIM Sparse-Mode uses a Rendezvous Point and shared trees to forward
multicast packets from new sources. Once last hop routers receive
packets from a new source, they may join the Shortest Path Tree for
the source for optimal forwarding. This draft defines a new protcol
that provides a way to support PIM Sparse Mode (SM) without the need
for PIM registers, RPs or shared trees. Multicast source information
is flooded throughout the multicast domain using a new generic PIM
flooding mechanism. This allows last hop routers to learn about new
sources without receiving initial data packets.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
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."
This Internet-Draft will expire on September 18, 2016.
Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Conventions used in this document . . . . . . . . . . . . 3
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Testing and deployment experiences . . . . . . . . . . . . . 3
3. A generic PIM flooding mechanism . . . . . . . . . . . . . . 4
3.1. PFP message format . . . . . . . . . . . . . . . . . . . 4
3.2. Processing PFP messages . . . . . . . . . . . . . . . . . 6
3.2.1. Initial checks . . . . . . . . . . . . . . . . . . . 6
3.2.2. Processing messages of supported PFP type . . . . . . 7
3.2.3. Processing messages of unsupported PFP type . . . . . 7
4. Distributing Source to Group Mappings . . . . . . . . . . . . 7
4.1. Group Source Holdtime TLV . . . . . . . . . . . . . . . . 8
4.2. Originating SG messages . . . . . . . . . . . . . . . . . 8
4.3. Processing SG messages . . . . . . . . . . . . . . . . . 9
4.4. The first packets and bursty sources . . . . . . . . . . 9
4.5. Resiliency to network partitioning . . . . . . . . . . . 10
5. Security Considerations . . . . . . . . . . . . . . . . . . . 11
6. IANA considerations . . . . . . . . . . . . . . . . . . . . . 11
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 11
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
8.1. Normative References . . . . . . . . . . . . . . . . . . 11
8.2. Informative References . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
PIM Sparse-Mode uses a Rendezvous Point (RP) and shared trees to
forward multicast packets to Last Hop Routers (LHR). After the first
packet is received by a LHR, the source of the multicast stream is
learned and the Shortest Path Tree (SPT) can be joined. This draft
defines a new protocol that provides a way to support PIM Sparse Mode
(SM) without the need for PIM registers, RPs or shared trees.
Multicast source information is flooded throughout the multicast
domain using a new generic PIM flooding mechanism. This mechanism is
defined in this document, and is modeled after the Bootstrap Router
protocol [RFC5059]. By removing the need for RPs and shared trees,
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the PIM-SM procedures are simplified, improving router operations,
management and making the protocol more robust. Also the data
packets are only sent on the SPTs, providing optimal forwarding.
1.1. Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
1.2. Terminology
RP: Rendezvous Point.
BSR: Bootstrap Router.
RPF: Reverse Path Forwarding.
SPT: Shortest Path Tree.
FHR: First Hop Router, directly connected to the source.
LHR: Last Hop Router, directly connected to the receiver.
SG Mapping: Multicast source to group mapping.
SG Message: A PIM message containing SG Mappings.
2. Testing and deployment experiences
A prototype of this specification has been implemented and there has
been some limited testing in the field. The prototype was tested in
a network with low bandwidth radio links. In this network with
frequent topology changes and link or router failures PIM-SM with RP
election is found to be too slow. With PIM-DM issues were observed
with new multicast sources starving low bandwidth links even when
there are no receivers, in some cases such that there were no
bandwidth left for prune message. For the tests, all routers were
configured to send PFP-SA for directly connected source and to cache
received announcements. Applications such as SIP with multicast
subscriber discovery, multicast voice conferencing, position tracking
and NTP were successfully tested. The tests went quite well.
Packets were rerouted as needed and there were no unnecessary
forwarding of packets. Ease of configuration was seen as a plus.
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3. A generic PIM flooding mechanism
The Bootstrap Router protocol (BSR) [RFC5059] is a commonly used
protocol for distributing dynamic Group to RP mappings in PIM. It is
responsible for flooding information about such mappings throughout a
PIM domain, so that all routers in the domain can have the same
information. BSR as defined, is only able to distribute Group to RP
mappings. We are defining a more generic mechanism that can flood
any kind of information throughout a PIM domain. It is not
necessarily a domain though, it depends on the administrative
boundaries being configured. The forwarding rules are identical to
BSR, except that there is no BSR election and that one can control
whether routers should forward messages of unsupported types. For
some types of information it is quite useful that it can be
distributed without all routers having to support the particular
type, while there may also be types where it is necessary for every
single router to support it. The protocol includes an originator
address which is used for RPF checking to restrict the flooding, just
like BSR. Just like BSR it is also sent hop by hop. Note that there
is no built in election mechanism as in BSR, so there can be multiple
originators. It is still possible to add such an election mechanism
on a type by type bases if this protocol is used in scenarios where
this is desirable. We include a type field, which can allow
boundaries to be defined, and election to take place, independently
per type. We call this protocol the PIM Flooding Protocol (PFP).
3.1. PFP message format
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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 |N| Reserved | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Originator Address (Encoded-Unicast format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PFP Type | Reserved |U|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type 1 | Length 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value 1 |
| . |
| . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| . |
| . |
| Type n | Length n |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value n |
| . |
| . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
PIM Version: Reserved, Checksum Described in [RFC7761].
Type: PIM Message Type. Value (pending IANA) for a PFP message.
[N]o-Forward bit: When set, this bit means that the PFP message is
not to be forwarded. This is irrespective of the value of the
Unsupported-No-Forwarding bit defined below.
Originator Address: The address of the router that originated the
message. This can be any address assigned to the originating
router, but MUST be routable in the domain to allow successful
forwarding (just like BSR address). The format for this address
is given in the Encoded-Unicast address in [RFC7761].
PFP Type: There may be different sub protocols or different uses
for this generic protocol. The PFP Type specifies which sub
protocol it is used for.
[U]nsupported-No-Forwarding bit: When the No-Forward bit defined
above is not set, whether to forward the message depends on
whether the PFP type is supported and the setting of the
Unsupported-No-Forwarding bit. Some sub protocols may require
that each router do some processing of the contents and not simply
forward the message. When Unsupported-No-Forwarding bit is set, a
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router MUST NOT forward the message when the PFP Type is
unsupported. When it is not set, a router MUST forward the
message when possible. If the PFP Type is supported, then the
specification of that type will specify how to handle the message,
including whether the message should be forwarded.
Type 1..n: A message contains one or more TLVs, in this case n
TLVs. The Type specifies what kind of information is in the
Value. Note that the Type space is shared between all PFP types.
Not all types make sense for all PFP types though.
Length 1..n: The length of the the value field.
Value 1..n: The value associated with the type and of the specified
length.
3.2. Processing PFP messages
A router that receives an PFP message MUST perform the initial checks
specified here. If the checks fail, the message MUST be dropped. An
error MAY be logged, but otherwise the message MUST be dropped
silently. If the checks pass, the contents is processed according to
the PFP type if supported. If the type is unsupported it may still
be forwarded if neither the No-Forward bit nor the Unsupported-No-
Forwarding bit are set.
3.2.1. Initial checks
In order to do further processing, a message MUST meet the following
requirements. The message MUST be from a directly connected neighbor
for which we have active Hello state, and it MUST have been sent to
the ALL-PIM-ROUTERS group. Also, the interface MUST NOT be an
administrative boundary for the message's PFP type. If No-Forward is
not set, it MUST have been sent by the RPF neighbor towards the
router that originated the message. If No-Forward is set, we MUST
have restarted within 60 seconds. In pseudo-code the algorithm is as
follows:
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if ((DirectlyConnected(PFP.src_ip_address) == FALSE) OR
(we have no Hello state for PFP.src_ip_address) OR
(PFP.dst_ip_address != ALL-PIM-ROUTERS) OR
(Incoming interface is admin boundary for PFP.type)) {
drop the message silently, optionally log error.
}
if (PFP.no_forward_bit == 0) {
if (PFP.src_ip_address !=
RPF_neighbor(PFP.originator_ip_address)) {
drop the message silently, optionally log error.
}
} else if (more than 60 seconds elapsed since startup)) {
drop the message silently, optionally log error.
}
3.2.2. Processing messages of supported PFP type
When the message is received, the initial checks above must be
performed. If it passes the checks, then we continue as follows. If
the PFP type is supported by the implementation, the processing and
potential forwarding is done according to the specification for that
PFP type. If the PFP type specification does not specify any
particular forwarding rules, the message is forwarded out of all
interfaces with PIM neighbors (including the interface it was
received on).
3.2.3. Processing messages of unsupported PFP type
When the message is received, the initial checks above must be
performed. If it passes the checks, then we continue as follows. If
the PFP type is unsupported, the message MUST be dropped if the
Unsupported-No-Forwarding bit is set. If the bit is not set, the
message is forwarded out of all interfaces with PIM neighbors
(including the interface it was received on).
4. Distributing Source to Group Mappings
We want to provide information about active multicast sources
throughout a PIM domain by making use of the generic flooding
mechanism defined in the previous section. We request PFP Type 0 to
be assigned for this purpose. We call a message with PFP Type 0 an
SG Message. We also define a PFP TLV which we request to be type 0.
How this TLV is used with PFP Type 0 is defined in the next section.
Other PFP Types may specify the use of this TLV for other purposes.
For PFP Type 0 the U-bit MUST NOT be set. This means that routers
not supporting PFP Type 0 would still forward the message.
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4.1. Group Source Holdtime TLV
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 = 0 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Group Address (Encoded-Group format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Src Count | Src Holdtime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Src Address 1 (Encoded-Unicast format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Src Address 2 (Encoded-Unicast format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| . |
| . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Src Address m (Encoded-Unicast format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: This TLV has type 0.
Length: The length of the value.
Group Address: The group we are announcing sources for. The format
for this address is given in the Encoded-Group format in
[RFC7761].
Src Count: How many unicast encoded sources address encodings
follow.
Src Holdtime: The Holdtime (in seconds) for the corresponding
source(s).
Src Address: The source address for the corresponding group. The
format for these addresses is given in the Encoded-Unicast address
in [RFC7761].
4.2. Originating SG messages
An SG Message, that is a PFP message of Type 0, MAY contain one or
more Group Source Holdtime TLVs. This is used to flood information
about active multicast sources. Each FHR that is directly connected
to an active multicast source originates SG BSR messages. How a
multicast router discovers the source of the multicast packet and
when it considers itself the FHR follows the same procedures as the
registering process described in [RFC7761]. When a FHR has decided
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that a register needs to be sent per [RFC7761], the SG is not
registered via the PIM SM register procedures, but the SG mapping is
included in an SG message. Note, only the SG mapping is distributed
in the message, not the entire packet as would have been done with a
PIM register. The router originating the SG messages includes one of
its own addresses in the originator field. Note that this address
SHOULD be routeable due to RPF checking. The SG messages are
periodically sent for as long as the multicast source is active,
similar to how PIM registers are periodically sent. The default
announcement period is 60 seconds, which means that as long as the
source is active, it is included in an SG message originated every 60
seconds. The holdtime for the source is by default 210 seconds.
Other values MAY be configured, but the holdtime MUST be either zero,
or larger than the announcement period. It is RECOMMENDED to be 3.5
times the announcement period. A source MAY be announced with a
holdtime of zero to indicate that the source is no longer active.
4.3. Processing SG messages
A router that receives an SG message SHOULD parse the message and
store the SG mappings with a holdtimer started with the advertised
holdtime for that group. For each group that has directly connected
receivers, this router SHOULD send PIM (S,G) joins for all the SG
mappings advertised in the message for the group. The SG mappings
are kept alive for as long as the holdtimer for the source is
running. Once the holdtimer expires a PIM router SHOULD send a PIM
(S,G) prune to remove itself from the tree. Note that a holdtime of
zero has a special meaning. It is to be treated as if the source
just expired, causing a prune to be sent and state to be removed.
Source information MUST NOT be removed due to it being omitted in a
message. For instance, if there are a large number of sources for a
group, there may be multiple SG messages for the same group, each
message containing a different list of sources.
4.4. The first packets and bursty sources
The PIM register procedure is designed to deliver Multicast packets
to the RP in the absence of a Shortest Path Tree (SPT) from the RP to
the source. The register packets received on the RP are decapsulated
and forwarded down the shared tree to the LHRs. As soon as an SPT is
built, multicast packets would flow natively over the SPT to the RP
or LHR and the register process would stop. The PIM register process
ensures packet delivery until an SPT is in place reaching the FHR.
If the packets were not unicast encapsulated to the RP they would be
dropped by the FHR until the SPT is setup. This functionality is
important for applications where the initial packet(s) must be
received for the application to work correctly. Another reason would
be for bursty sources. If the application sends out a multicast
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packet every 4 minutes (or longer), the SPT is torn down (typically
after 3:30 minutes of inactivity) before the next packet is forwarded
down the tree. This will cause no multicast packet to ever be
forwarded. A well behaved application should be able to deal with
packet loss since IP is a best effort based packet delivery system.
But in reality this is not always the case.
With the procedures defined in this document the packet(s) received
by the FHR will be dropped until the LHR has learned about the source
and the SPT is built. That means for bursty sources or applications
sensitive for the delivery of the first packet this solution would
not be very applicable. This solution is mostly useful for
applications that don't have strong dependency on the initial
packet(s) and have a fairly constant data rate, like video
distribution for example. For applications with strong dependency on
the initial packet(s) we recommend using PIM Bidir [RFC5015] or SSM
[RFC4607]. The protocol operations are much simpler compared to PIM
SM, it will cause less churn in the network and both guarantee best
effort delivery for the initial packet(s).
Another solution to address the problems described above is
documented in [I-D.ietf-magma-msnip]. This proposal allows for a
host to tell the FHR its willingness to act as Source for a certain
Group before sending the data packets. LHRs have time to join the
SPT before the host starts sending which would avoid packet loss.
The SG mappings announced by [I-D.ietf-magma-msnip] can be advertised
directly in SG messages, allowing a nice integration of both
proposals. The life time of the SPT is not driven by the liveliness
of Multicast data packets (which is the case with PIM SM), but by the
announcements driven via [I-D.ietf-magma-msnip]. This will also
prevent packet loss due to bursty sources.
4.5. Resiliency to network partitioning
In a PIM SM deployment where the network becomes partitioned, due to
link or node failure, it is possible that the RP becomes unreachable
to a certain part of the network. New sources that become active in
that partition will not be able to register to the RP and receivers
within that partition are not able to receive the traffic. Ideally
you would want to have a candidate RP in each partition, but you
never know in advance which routers will form a partitioned network.
In order to be fully resilient, each router in the network may end up
being a candidate RP. This would increase the operational complexity
of the network.
The solution described in this document does not suffer from that
problem. If a network becomes partitioned and new sources become
active, the receivers in that partitioned will receive the SG
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Mappings and join the source tree. Each partition works
independently of the other partition(s) and will continue to have
access to sources within that partition. As soon as the network
heals, the SG Mappings are re-flooded into the other partition(s) and
other receivers can join to the newly learned sources.
5. Security Considerations
The security considerations are mainly similar to what is documented
in [RFC5059]. It is a concern that rogue devices can inject packets
that are flooded throughout a domain. PFP packets must only be
accepted from a PIM neighbor. Deployments may use mechanisms for
authenticating PIM neighbors. For PFP-SA it is an issue that
injected packets from a rogue device could send SG mappings for a
large number of source addresses, causing routers to use memory
storing these mappings, and also if they have interest in the groups,
build Shortest Path Trees for sources that are not actually active.
6. IANA considerations
This document requires the assignment of a new PIM Protocol type for
the PIM Flooding Protocol (PFP). IANA is also requested to create a
registry for PFP Types with type 0 assigned to "Source-Group
Message". IANA is also requested to create a registry for PFP TLVs,
with type 0 assigned to the "Source Group Holdtime" TLV. Assignments
for both registries are to be made according to the policy "IETF
Review" as defined in [RFC5226].
7. Acknowledgments
The authors would like to thank Arjen Boers for contributing to the
initial idea and Yiqun Cai for his comments on the draft.
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC5059] Bhaskar, N., Gall, A., Lingard, J., and S. Venaas,
"Bootstrap Router (BSR) Mechanism for Protocol Independent
Multicast (PIM)", RFC 5059, DOI 10.17487/RFC5059, January
2008, <http://www.rfc-editor.org/info/rfc5059>.
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[RFC7761] Fenner, B., Handley, M., Holbrook, H., Kouvelas, I.,
Parekh, R., Zhang, Z., and L. Zheng, "Protocol Independent
Multicast - Sparse Mode (PIM-SM): Protocol Specification
(Revised)", STD 83, RFC 7761, DOI 10.17487/RFC7761, March
2016, <http://www.rfc-editor.org/info/rfc7761>.
8.2. Informative References
[RFC4607] Holbrook, H. and B. Cain, "Source-Specific Multicast for
IP", RFC 4607, DOI 10.17487/RFC4607, August 2006,
<http://www.rfc-editor.org/info/rfc4607>.
[RFC5015] Handley, M., Kouvelas, I., Speakman, T., and L. Vicisano,
"Bidirectional Protocol Independent Multicast (BIDIR-
PIM)", RFC 5015, DOI 10.17487/RFC5015, October 2007,
<http://www.rfc-editor.org/info/rfc5015>.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
DOI 10.17487/RFC5226, May 2008,
<http://www.rfc-editor.org/info/rfc5226>.
[I-D.ietf-magma-msnip]
Fenner, B., Haberman, B., Holbrook, H., Kouvelas, I., and
S. Venaas, "Multicast Source Notification of Interest
Protocol (MSNIP)", draft-ietf-magma-msnip-06 (work in
progress), March 2011.
Authors' Addresses
IJsbrand Wijnands
Cisco Systems, Inc.
De kleetlaan 6a
Diegem 1831
Belgium
Email: ice@cisco.com
Stig Venaas
Cisco Systems, Inc.
Tasman Drive
San Jose CA 95134
USA
Email: stig@cisco.com
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Michael Brig
Aegis BMD Program Office
17211 Avenue D, Suite 160
Dahlgren VA 22448-5148
USA
Email: michael.brig@mda.mil
Anders Jonasson
Swedish Defence Material Administration (FMV)
Loennvaegen 4
Vaexjoe 35243
Sweden
Email: anders@jomac.se
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