Network Working Group Yiqun. Cai
Internet-Draft Heidi. Ou
Intended status: Standards Track Alibaba Group
Expires: December 21, 2018 Sri. Vallepalli
Mankamana. Mishra
Stig. Venaas
Cisco Systems
Andy. Green
British Telecom
June 19, 2018
PIM Designated Router Load Balancing
draft-ietf-pim-drlb-08
Abstract
On a multi-access network, one of the PIM routers is elected as a
Designated Router (DR). On the last hop LAN, the PIM DR is
responsible for tracking local multicast listeners and forwarding
traffic to these listeners if the group is operating in PIM-SM. In
this document, we propose a modification to the PIM-SM protocol that
allows more than one of these last hop routers to be selected so that
the forwarding load can be distributed among these routers.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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Task Force (IETF). Note that other groups may also distribute
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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 December 21, 2018.
Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Applicability . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Functional Overview . . . . . . . . . . . . . . . . . . . . . 6
4.1. GDR Candidates . . . . . . . . . . . . . . . . . . . . . 6
4.2. Hash Mask and Hash Algorithm . . . . . . . . . . . . . . 7
4.3. Modulo Hash Algorithm . . . . . . . . . . . . . . . . . . 8
4.4. PIM Hello Options . . . . . . . . . . . . . . . . . . . . 9
5. Hello Option Formats . . . . . . . . . . . . . . . . . . . . 9
5.1. PIM DR Load Balancing Capability (DRLBC) Hello Option . . 9
5.2. PIM DR Load Balancing GDR (DRLBGDR) Hello Option . . . . 10
6. Protocol Specification . . . . . . . . . . . . . . . . . . . 11
6.1. PIM DR Operation . . . . . . . . . . . . . . . . . . . . 11
6.2. PIM GDR Candidate Operation . . . . . . . . . . . . . . . 12
6.2.1. Router Receives New DRLBGDR . . . . . . . . . . . . . 13
6.2.2. Router Receives Updated DRLBGDR . . . . . . . . . . . 13
6.3. PIM Assert Modification . . . . . . . . . . . . . . . . . 14
7. Compatibility . . . . . . . . . . . . . . . . . . . . . . . . 15
8. Manageability Considerations . . . . . . . . . . . . . . . . 15
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
10. Security Considerations . . . . . . . . . . . . . . . . . . . 16
11. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . 16
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 16
12.1. Normative References . . . . . . . . . . . . . . . . . . 16
12.2. Informative References . . . . . . . . . . . . . . . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 17
1. Introduction
On a multi-access LAN such as an Ethernet, one of the PIM routers is
elected as a DR. The PIM DR has two roles in the PIM-SM protocol.
On the first hop network, the PIM DR is responsible for registering
an active source with the Rendezvous Point (RP) if the group is
operating in PIM-SM. On the last hop LAN, the PIM DR is responsible
for tracking local multicast listeners and forwarding to these
listeners if the group is operating in PIM-SM.
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Consider the following last hop LAN in Figure 1:
( core networks )
| | |
| | |
R1 R2 R3
| | |
--(last hop LAN)--
|
|
(many receivers)
Figure 1: Last Hop LAN
Assume R1 is elected as the Designated Router. According to
[RFC4601], R1 will be responsible for forwarding traffic to that LAN
on behalf of any local members. In addition to keeping track of IGMP
and MLD membership reports, R1 is also responsible for initiating the
creation of source and/or shared trees towards the senders or the
RPs.
Forcing sole data plane forwarding responsibility on the PIM DR
uncovers a limitation in the protocol. In comparison, even though an
OSPF DR or an IS-IS DIS handles additional duties while running the
OSPF or IS-IS protocols, they are not required to be solely
responsible for forwarding packets for the network. On the other
hand, on a last hop LAN, only the PIM DR is asked to forward packets
while the other routers handle only control traffic (and perhaps drop
packets due to RPF failures). Hence the forwarding load of a last
hop LAN is concentrated on a single router.
This leads to several issues. One of the issues is that the
aggregated bandwidth will be limited to what R1 can handle towards
this particular interface. It is very common that the last hop LAN
usually consists of switches that run IGMP/MLD or PIM snooping. This
allows the forwarding of multicast packets to be restricted only to
segments leading to receivers who have indicated their interest in
multicast groups using either IGMP or MLD. The emergence of the
switched Ethernet allows the aggregated bandwidth to exceed,
sometimes by a large number, that of a single link. For example, let
us modify Figure 1 and introduce an Ethernet switch in Figure 2.
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( core networks )
| | |
| | |
R1 R2 R3
| | |
+=gi0===gi1===gi2=+
+ +
+ switch +
+ +
+=gi4===gi5===gi6=+
| | |
H1 H2 H3
Figure 2: Last Hop Network with Ethernet Switch
Let us assume that each individual link is a Gigabit Ethernet. Each
router, R1, R2 and R3, and the switch have enough forwarding capacity
to handle hundreds of Gigabits of data.
Let us further assume that each of the hosts requests 500 Mbps of
unique multicast data. This totals to 1.5 Gbps of data, which is
less than what each switch or the combined uplink bandwidth across
the routers can handle, even under failure of a single router.
On the other hand, the link between R1 and switch, via port gi0, can
only handle a throughput of 1Gbps. And if R1 is the only DR (the PIM
DR elected using the procedure defined by [RFC4601]) at least 500
Mbps worth of data will be lost because the only link that can be
used to draw the traffic from the routers to the switch is via gi0.
In other words, the entire network's throughput is limited by the
single connection between the PIM DR and the switch (or the last hop
LAN as in Figure 1).
The problem may also manifest itself in a different way. For
example, R1 happens to forward 500 Mbps worth of unicast data to H1,
and at the same time, H2 and H3 each request 300 Mbps of different
multicast data. R1 experiences packet drop once again. while, in the
meantime, there is sufficient forwarding capacity left on R2 and R3
and unused link capacity between the switch and R2/R3.
Another important issue is related to failover. If R1 is the only
forwarder on the last hop router for shared LAN, when R1 goes out of
service, multicast forwarding for the entire LAN has to be rebuilt by
the newly elected PIM DR. However, if there was a way that allowed
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multiple routers to forward to the LAN for different groups, failure
of one of the routers would only lead to disruption to a subset of
the flows, therefore improving the overall resilience of the network.
There is limitation in the hash algorithm used in this document, but
this draft provides the option to have different and more consistent
hash algorithms in the future.
In this document, we propose a modification to the PIM-SM protocol
that allows more than one of these routers, called Group Designated
Routers (GDR) to be selected so that the forwarding load can be
distributed among a number of routers.
2. Terminology
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 [RFC2119].
With respect to PIM, this document follows the terminology that has
been defined in [RFC4601].
This document also introduces the following new acronyms:
o GDR: GDR stands for "Group Designated Router". For each multicast
flow, either a (*,G) for ASM, or an (S,G) for SSM, a hash
algorithm (described below) is used to select one of the routers
as a GDR. The GDR is responsible for initiating the forwarding
tree building process for the corresponding multicast flow.
o GDR Candidate: a last hop router that has the potential to become
a GDR. A GDR Candidate must have the same DR priority and must
run the same GDR election hash algorithm as the DR router. It
must send and process new PIM Hello Options as defined in this
document. There might be more than one GDR Candidate on a LAN,
but only one can become GDR for a specific multicast flow.
3. Applicability
The proposed change described in this specification applies to PIM-SM
last hop routers only.
It does not alter the behavior of a PIM DR on the first hop network.
This is because the source tree is built using the IP address of the
sender, not the IP address of the PIM DR that sends the registers
towards the RP. The load balancing between first hop routers can be
achieved naturally if an IGP provides equal cost multiple paths
(which it usually does in practice). Also distributing the load to
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do registering does not justify the additional complexity required to
support it.
4. Functional Overview
In the existing PIM DR election, when multiple last hop routers are
connected to a multi-access LAN (for example, an Ethernet), one of
them is selected to act as PIM DR. The PIM DR is responsible for
sending local Join/Prune messages towards the RP or source. In order
to elect the PIM DR, each PIM router on the LAN examines the received
PIM Hello messages and compares its DR priority and IP address with
those of its neighbors. The router with the highest DR priority is
the PIM DR. If there are multiple such routers, their IP addresses
are used as the tie-breaker, as described in [RFC4601].
In order to share forwarding load among last hop routers, besides the
normal PIM DR election, the GDR is also elected on the last hop
multi-access LAN. There is only one PIM DR on the multi-access LAN,
but there might be multiple GDR Candidates.
For each multicast flow, that is, (*,G) for ASM and (S,G) for SSM, a
hash algorithm is used to select one of the routers to be the GDR. A
new DR Load Balancing Capability (DRLBC) PIM Hello Option, which
contains hash algorithm type, is announced by routers on interfaces
where this specification is enabled. Last hop routers with the new
DRLBC Option advertised in its Hello, and using the same GDR election
hash algorithm and the same DR priority as the PIM DR, are considered
as GDR Candidates.
Hash Masks are defined for Source, Group and RP separately, in order
to handle PIM ASM/SSM. The masks, as well as a sorted list of GDR
Candidates' Addresses, are announced by DR in a new DR Load Balancing
GDR (DRLBGDR) PIM Hello Option.
A hash algorithm based on the announced Source, Group, or RP masks
allows one GDR to be assigned to a corresponding multicast state.
And that GDR is responsible for initiating the creation of the
multicast forwarding tree for multicast traffic.
4.1. GDR Candidates
GDR is the new concept introduced by this specification. GDR
Candidates are routers eligible for GDR election on the LAN. To
become a GDR Candidate, a router MUST support this specification,
have the same DR priority and run the same GDR election hash
algorithm as the DR on the LAN.
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For example, assume there are 4 routers on the LAN: R1, R2, R3 and
R4, which all support this specification. R1, R2 and R3 have the
same DR priority while R4's DR priority is less preferred. In this
example, R4 will not be eligible for GDR election, because R4 will
not become a PIM DR unless all of R1, R2 and R3 go out of service.
Furthermore, assume router R1 wins the PIM DR election, R1 and R2 run
the same hash algorithm for GDR election, while R3 runs a different
one. In this case, only R1 and R2 will be eligible for GDR election,
while R3 will not.
As a DR, R1 will include its own Load Balancing Hash Masks and the
identity of R1 and R2 (the GDR Candidates) in its DRLBGDR Hello
Option.
4.2. Hash Mask and Hash Algorithm
A Hash Mask is used to extract a number of bits from the
corresponding IP address field (32 for v4, 128 for v6) and calculate
a hash value. A hash value is used to select a GDR from GDR
Candidates advertised by PIM DR. For example, 0.0.255.0 defines a
Hash Mask for an IPv4 address that masks the first, the second, and
the fourth octets.
There are three Hash Masks defined,
o RP Hash Mask
o Source Hash Mask
o Group Hash Mask
The hash masks need to be configured on the PIM routers that can
potentially become a PIM DR, unless the implementation provides
default Hash Mask. An implementation SHOULD provide masks with
default values 255.255.255.255 (IPv4) and
FFFF:FFFF:FFFF:FFFF:FFFFF:FFFF:FFFF:FFFF (IPv6).
o If the group is ASM and the RP Hash Mask announced by the PIM DR
is not 0, calculate the value of hashvalue_RP [Section 4.3] to
determine GDR.
o If the group is ASM and the RP Hash Mask announced by the PIM DR
is 0, obtain the value of hashvalue_Group [Section 4.3 ] to
determine GDR.
o If the group is SSM, use hashvalue_SG [Section 4.3] to determine
GDR.
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A simple Modulo hash algorithm will be discussed in this document.
However, to allow another hash algorithms to be used, a 4-bytes "Hash
Algorithm Type" field is included in DRLBC Hello Option to specify
the hash algorithm used by a last hop router.
If different hash algorithm types are advertised among last hop
routers, only last hop routers running the same hash algorithm as the
DR (and having the same DR priority as the DR) are eligible for GDR
election.
4.3. Modulo Hash Algorithm
Modulo hash algorithm is discussed here with a detailed description
on hashvalue_RP. The same algorithm is described in brief for
hashvalue_Group using the group address instead of the RP address for
an ASM group with RP_hashmask==0, and also with hashvalue_SG for a
the source address of an (S,G), instead of the RP address,
o For ASM groups, with a non-zero RP_Hash Mask, hash value is
calculated as:
hashvalue_RP = (((RP_address & RP_hashmask) >> N) & 0xFFFF) % M
RP_address is the address of the RP defined for the group. N
is the number of zeros, counted from the least significant bit
of the RP_hashmask. M is the number of GDR Candidates.
For example, Router X with IPv4 address 203.0.113.1 receives a
DRLBGDR Hello Option from the DR, which announces RP Hash Mask
0.0.255.0 and a list of GDR Candidates, sorted by IP addresses
from high to low: 203.0.113.3, 203.0.113.2 and 203.0.113.1.
The ordinal number assigned to those addresses would be:
0 for 203.0.113.3; 1 for 203.0.113.2; 2 for 203.0.113.1 (Router
X)
Assume there are 2 RPs: RP1 192.0.2.1 for Group1 and RP2
198.51.100.2 for Group2. Following the modulo hash algorithm:
N is 8 for 0.0.255.0, and M is 3 for the total number of GDR
Candidates. The hashvalue_RP for RP1 192.0.2.1 is:
(((192.0.2.1 & 0.0.255.0) >> 8) & 0xFFFF % 3) = 2 % 3 = 2
matches the ordinal number assigned to Router X. Router X will
be the GDR for Group1, which uses 192.0.2.1 as the RP.
The hashvalue_RP for RP2 198.51.100.2 is:
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(((198.51.100.2 & 0.0.255.0) >> 8) & 0xFFFF % 3) = 100 % 3 = 1
which is different from Router X's ordinal number(2) hence,
Router X will not be GDR for Group2.
o If RP_hashmask is 0, a hash value for ASM group is calculated
using the group Hash Mask:
hashvalue_Group = (((Group_address & Group_hashmask) >> N) &
0xFFFF) % M
Compare hashvalue_Group with Ordinal number assigned to Router
X, to decide if Router X is the GDR.
o For SSM groups, a hash value is calculated using both the source
and group Hash Mask:
hashvalue_SG = ((((Source_address & Source_hashmask) >> N_S) &
0xFFFF) ^ (((Group_address & Group_hashmask) >> N_G) & 0xFFFF))
% M
4.4. PIM Hello Options
When a last hop PIM router sends a PIM Hello from an interface with
this specification enabled, it includes a new option, called "Load
Balancing Capability (DRLBC)".
Besides this DRLBC Hello Option, the elected PIM DR also includes a
new "DR Load Balancing GDR (DRLBGDR) Hello Option". The DRLBGDR
Hello Option consists of three Hash Masks as defined above and also
the sorted list of all GDR Candidates' Address on the last hop LAN.
The elected PIM DR uses DRLBC Hello Option advertised by all routers
on the last hop LAN to compose its DRLBGDR. The GDR Candidates use
DRLBGDR Hello Option advertised by PIM DR to calculate hash value.
5. Hello Option Formats
5.1. PIM DR Load Balancing Capability (DRLBC) Hello Option
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = TBD | Length = 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Hash Algorithm Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: Capability Hello Option
Type: TBD.
Length: 4 octets
Hash Algorithm Type: 0 for Modulo hash algorithm
This DRLBC Hello Option SHOULD be advertised by last hop routers from
interfaces with this specification enabled.
5.2. PIM DR Load Balancing GDR (DRLBGDR) Hello 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 = TBD | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Group Mask |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Mask |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RP Mask |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| GDR Candidate Address(es) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: GDR Hello Option
Type: TBD
Length: 3 x (4 byte or 16 byte) + n x (4 byte or 16 byte) where n
is the number of GDR candidates.
Group Mask (32/128 bits): Mask
Source Mask (32/128 bits): Mask
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RP Mask (32/128 bits): Mask
All masks MUST be in the same address family as the Hello IP
header.
GDR Address (32/128 bits): Address(es) of GDR Candidate(s)
All addresses must be in the same address family as the Hello
IP header. The addresses are sorted in descending order. The
order is converted to the ordinal number associated with each
GDR candidate in hash value calculation. For example,
addresses advertised are R3, R2, R1, the ordinal number
assigned to R3 is 0, to R2 is 1 and to R1 is 2.
If "Interface ID" option, as described in [RFC6395], presents
in a GDR Candidate's PIM Hello message, and the "Router ID"
portion is non-zero,
+ For IPv4, the "GDR Candidate Address" will be set directly
to "Router ID".
+ For IPv6, the "GDR Candidate Address" will be set to the
IPv4-IPv6 translated address of "Router ID", as described in
[RFC4291] , that is the "Router-ID" is appended to the
prefix of 96-bits zeros.
If the "Interface ID" option is not present in a GDR
Candidate's PIM Hello message, or if the "Interface ID" option
is present but the "Router ID" field is zero, the "GDR
Candidate Address" will be the IPv4 or IPv6 source address from
PIM Hello message.
This DRLBGDR Hello Option MUST only be advertised by the
elected PIM DR.
6. Protocol Specification
6.1. PIM DR Operation
The DR election process is still the same as defined in [RFC4601]. A
DR that has this specification enabled on the interface advertises
the new DRLBGDR Hello Option, which contains value of masks from user
configuration, followed by a sorted list of all GDR Candidates'
Addresses, from the highest value to the lowest value. Moreover,
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same as non-DR routers, DR also advertises DRLBC Hello Option to
indicate its capability of supporting this specification and the type
of its GDR election hash algorithm.
If a PIM DR receives a PIM Hello with DRLBGDR Option, the PIM DR
SHOULD ignore the TLV.
If a PIM DR receives a neighbor DRLBC Hello Option, which contains
the same hash algorithm type as the DR, and the neighbor has the same
DR priority as the DR, PIM DR SHOULD consider the neighbor as a GDR
Candidate and insert the GDR Candidate's Address into the sorted list
of DRLBGDR Option.
6.2. PIM GDR Candidate Operation
When an IGMP/MLD join is received, without this specification, only
PIM DR will handle the join and potentially run into the issues
described earlier. Using this specification, a hash algorithm is
used on GDR Candidate to determine which router is going to be
responsible for building forwarding trees on behalf of the host.
If a router supports this specification then each of the interfaces
where multicast protocol is enabled, it MUST advertise DRLBC Hello
Option in its PIM Hello. Though DRLBC option in PIM hello does not
guarantee that this router would be considered as a GDR candidate.
For example, this router may have lower priority configured on shared
LAN compare to other PIM routers. Once DR election is done, DRLBGDR
Hello option would be received from the current PIM DR on the link
which would contain list of GDR.
A GDR Candidate may receive a DRLBGDR Hello Option from PIM DR with
different Hash Masks from those configured on it. The GDR Candidate
must use the Hash Masks advertised by the PIM DR to calculate the
hash value.
A GDR Candidate may receive a DRLBGDR Hello Option from a PIM router
which is not DR. The GDR Candidate MUST ignore such DRLBGDR Hello
Option.
A GDR Candidate may receive a Hello from the elected PIM DR, and the
PIM DR does not support this specification. The GDR election
described by this specification will not take place, that is only the
PIM DR joins the multicast tree.
A router only acts as GDR if it is included in the GDR list of
DRLBGDR Hello Option
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6.2.1. Router Receives New DRLBGDR
When a router receives a new DRLBGDR from the current PIM DR, it need
to process and check if router is in list of of GDR
1. If a router is not listed as a GDR candidate in DRLBGDR, no
action is needed.
2. If a router is listed as a GDR candidate in DRLBGDR, then it need
to process each of the groups in the IGMP/MLD reports. The masks
are announced in the PIM Hello by DR as DRLBGDR Hello option.
For each of groups in the reports it (PIM Router) needs to run
hash algorithm (described in section 4.3) based on the announced
Source, Group or RP masks to determine if it is GDR for specified
group. If the hash result is to be the GDR for the multicast
flow, it does build the multicast forwarding tree. If it is not
the GDR for the multicast flow, no action is needed.
6.2.2. Router Receives Updated DRLBGDR
If a router (GDR or non GDR) receives an unchanged DRLBGDR from the
current PIM DR, no action is needed.
If a router (GDR or non GDR) receives a new or modified DRLBGDR from
the current PIM DR. It requires processing as described below:
1. If it was GDR and still included in current GDR list: it needs to
process each of the groups and run the hash algorithm to check if
it is still the GDR for the given group.
If it was the GDR for group G and the new hash result chose it
as the GDR, then no processing is required.
If it was the GDR for a group earlier and now it is no longer
the GDR, then it sets its assert metric for the multicast flow
to be (PIM_ASSERT_INFINITY - 1), as explained in Sec 6.3
If it was not the GDR for a group earlier, than even the new
hash does not make it GDR. For the multicast group no
processing is required.
If it was not the GDR for an earlier group and now becomes the
GDR, it starts building multicast forwarding tree for this
flow.
2. If it was not the GDR , and updated DRLBGDR from current PIM DR
contains this router as one of the GDR. In this case this router
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being new GDR candidate MUST run hash algorithm for each of the
groups (multicast flows) and for given group,
If it is not the GDR, no processing is required.
If it is hashed as the GDR , it needs to build multicast
forwarding tree.
6.3. PIM Assert Modification
It is possible that the identity of the GDR might change in the
middle of an active flow. Examples this could happen include:
When a new PIM router comes up
When a GDR restarts
When the GDR changes, existing traffic might be disrupted.
Duplicates or packet losses might be observed. To illustrate the
case, consider the following scenario where there are two streams G1
and G2. R1 is the GDR for G1, and R2 is the GDR for G2. When R3
comes up online, it is possible that R3 becomes GDR for both G1 and
G2, hence R3 starts to build the forwarding tree for G1 and G2. If
R1 and R2 stop forwarding before R3 completes the process, packet
loss might occur. On the other hand, if R1 and R2 continue
forwarding while R3 is building the forwarding trees, duplicates
might occur.
This is not a typical deployment scenario but might still happen.
Here we describe a mechanism to minimize the impact. We essentially
want to minimize packet loss. Therefore, we would allow a small
amount of duplicates and depend on PIM Assert to minimize the
duplication.
When the role of GDR changes as above, instead of immediately
stopping forwarding, R1 and R2 continue forwarding to G1 and G2
respectively, while, at the same time, R3 build forwarding trees for
G1 and G2. This will lead to PIM Asserts.
With the introduction of GDR, the following modification to the
Assert packet MUST be done: if a router enables this specification on
its downstream interface, but it is not a GDR (before network event
it was GDR), it would adjust its Assert metric to
(PIM_ASSERT_INFINITY - 1).
Using the above example, for G1, assume R1 and R3 agree on the new
GDR, which is R3. R1 will set its Assert metric as
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(PIM_ASSERT_INFINITY - 1). That will make R3, which has normal
metric in its Assert as the Assert winner.
For G2, assume it takes a slightly longer time for R2 to find out
that R3 is the new GDR and still considers itself being the GDR while
R3 already has assumed the role of GDR. Since both R2 and R3 think
they are GDRs, they further compare the metric and IP address. If R3
has the better routing metric, or the same metric but a better tie-
breaker, the result will be consistent during GDR selection. If
unfortunately, R2 has the better metric or the same metric but a
better tie-breaker, R2 will become the Assert winner and continues to
forward traffic. This will continue until:
The next PIM Hello option from DR selects R3 as the GDR. R3 will
then build the forwarding tree and send an Assert.
The process continues until R2 agrees to the selection of R3 as the
GDR, and set its own Assert metric to (PIM_ASSERT_INFINITY - 1),
which will make R3 the Assert winner. During the process, we will
see intermittent duplication of traffic but packet loss will be
minimized. In the unlikely case that R2 never relinquishes its role
as GDR (while every other router thinks otherwise), the proposed
mechanism also helps to keep the duplication to a minimum until
manual intervention takes place to remedy the situation.
7. Compatibility
In case of the hybrid Ethernet shared LAN ( where some PIM router
enables specification defined in this draft and some do not enable)
o If a router which does not support specification defined in this
draft becomes DR on link, it MUST be only DR on link as [RFC4601]
and there would be no router which would act as GDR.
o If a router which does not support specification defined in this
draft becomes non DR on link, then it should act as non-DR defined
in [RFC4601].
8. Manageability Considerations
o All of the routers in LAN that support this specification MUST use
identical Hash Algorithm Type (described in section 5.1). In the
case of a hybrid Hash Algorithm Type, one MUST go backward to use
DR election method defined in PIM-SM [RFC4601]. Migration between
different algorithm type is out of the scope of this document.
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9. IANA Considerations
IANA has temporarily assigned type 34 for the PIM DR Load Balancing
Capability (DRLBC) Hello Option, and type 35 for the PIM DR Load
Balancing GDR (DRLBGDR) Hello Option. IANA is requested to make
these assignments permanent when this document is published as an
RFC. The string TBD should be replaced by the assigned values
accordingly.
10. Security Considerations
Security of the new DR Load Balancing PIM Hello Options is only
guaranteed by the security of PIM Hello message, so the security
considerations for PIM Hello messages as described in PIM-SM
[RFC4601] apply here.
11. Acknowledgement
The authors would like to thank Steve Simlo, Taki Millonis for
helping with the original idea, Bill Atwood, Bharat Joshi for review
comments, Toerless Eckert and Rishabh Parekh for helpful conversation
on the document.
Special thanks to Anish Kachinthaya, Anvitha Kachinthaya and Jake
Holland for reviewing the document and providing comments.
12. References
12.1. 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,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, DOI 10.17487/RFC4291, February
2006, <https://www.rfc-editor.org/info/rfc4291>.
[RFC4601] Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas,
"Protocol Independent Multicast - Sparse Mode (PIM-SM):
Protocol Specification (Revised)", RFC 4601,
DOI 10.17487/RFC4601, August 2006,
<https://www.rfc-editor.org/info/rfc4601>.
[RFC6395] Gulrajani, S. and S. Venaas, "An Interface Identifier (ID)
Hello Option for PIM", RFC 6395, DOI 10.17487/RFC6395,
October 2011, <https://www.rfc-editor.org/info/rfc6395>.
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12.2. Informative References
[HELLO-OPT]
IANA, "PIM Hello Options", IANA PIM-HELLO-OPTIONS, March
2007.
Authors' Addresses
Yiqun Cai
Alibaba Group
Email: yiqun.cai@alibaba-inc.com
Heidi Ou
Alibaba Group
Sri Vallepalli
Cisco Systems
3625 Cisco Way,
Sanjose, CALIFORNIA 95134
UNITED STATES
Email: svallepa@cisco.com
Mankamana Mishra
Cisco Systems
821 Alder Drive,
MILPITAS, CALIFORNIA 95035
UNITED STATES
Email: mankamis@cisco.com
Stig Venaas
Cisco Systems
821 Alder Drive,
MILPITAS, CALIFORNIA 95035
UNITED STATES
Email: stig@cisco.com
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Andy Green
British Telecom
Adastral Park
Ipswich IP5 2RE
United Kingdom
Email: andy.da.green@bt.com
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