draft-ietf-pim-drlb-13.txt   draft-ietf-pim-drlb-14.txt 
Network Working Group Y. Cai Network Working Group Y. Cai
Internet-Draft H. Ou Internet-Draft H. Ou
Intended status: Standards Track Alibaba Group Intended status: Standards Track Alibaba Group
Expires: April 25, 2020 S. Vallepalli Expires: June 13, 2020 S. Vallepalli
M. Mishra M. Mishra
S. Venaas S. Venaas
Cisco Systems, Inc. Cisco Systems, Inc.
A. Green A. Green
British Telecom British Telecom
October 23, 2019 December 11, 2019
PIM Designated Router Load Balancing PIM Designated Router Load Balancing
draft-ietf-pim-drlb-13 draft-ietf-pim-drlb-14
Abstract Abstract
On a multi-access network, one of the PIM-SM routers is elected as a On a multi-access network, one of the PIM-SM (PIM Sparse Mode)
Designated Router. One of the responsibilities of the Designated routers is elected as a Designated Router. One of the
Router is to track local multicast listeners and forward data to responsibilities of the Designated Router is to track local multicast
these listeners if the group is operating in PIM-SM. This document listeners and forward data to these listeners if the group is
specifies a modification to the PIM-SM protocol that allows more than operating in PIM-SM. This document specifies a modification to the
one of the PIM-SM routers to take on this responsibility so that the PIM-SM protocol that allows more than one of the PIM-SM routers to
forwarding load can be distributed among multiple routers. take on this responsibility so that the forwarding load can be
distributed among multiple routers.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/. Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 25, 2020. This Internet-Draft will expire on June 13, 2020.
Copyright Notice Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of (https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
skipping to change at page 2, line 27 skipping to change at page 2, line 27
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Applicability . . . . . . . . . . . . . . . . . . . . . . . . 5 3. Applicability . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Functional Overview . . . . . . . . . . . . . . . . . . . . . 5 4. Functional Overview . . . . . . . . . . . . . . . . . . . . . 5
4.1. GDR Candidates . . . . . . . . . . . . . . . . . . . . . 6 4.1. GDR Candidates . . . . . . . . . . . . . . . . . . . . . 6
5. Protocol Specification . . . . . . . . . . . . . . . . . . . 7 5. Protocol Specification . . . . . . . . . . . . . . . . . . . 7
5.1. Hash Mask and Hash Algorithm . . . . . . . . . . . . . . 7 5.1. Hash Mask and Hash Algorithm . . . . . . . . . . . . . . 7
5.2. Modulo Hash Algorithm . . . . . . . . . . . . . . . . . . 8 5.2. Modulo Hash Algorithm . . . . . . . . . . . . . . . . . . 8
5.2.1. Modulo Hash Algorithm Examples . . . . . . . . . . . 9 5.2.1. Modulo Hash Algorithm Examples . . . . . . . . . . . 9
5.2.2. Limitations . . . . . . . . . . . . . . . . . . . . . 10 5.2.2. Limitations . . . . . . . . . . . . . . . . . . . . . 10
5.3. PIM Hello Options . . . . . . . . . . . . . . . . . . . . 10 5.3. PIM Hello Options . . . . . . . . . . . . . . . . . . . . 11
5.3.1. PIM DR Load Balancing Capability (DRLB-Cap) Hello 5.3.1. PIM DR Load Balancing Capability (DRLB-Cap) Hello
Option . . . . . . . . . . . . . . . . . . . . . . . 11 Option . . . . . . . . . . . . . . . . . . . . . . . 11
5.3.2. PIM DR Load Balancing List (DRLB-List) Hello Option . 11 5.3.2. PIM DR Load Balancing List (DRLB-List) Hello Option . 11
5.4. PIM DR Operation . . . . . . . . . . . . . . . . . . . . 13 5.4. PIM DR Operation . . . . . . . . . . . . . . . . . . . . 13
5.5. PIM GDR Candidate Operation . . . . . . . . . . . . . . . 13 5.5. PIM GDR Candidate Operation . . . . . . . . . . . . . . . 14
5.6. DRLB-List Hello Option Processing . . . . . . . . . . . . 14 5.6. DRLB-List Hello Option Processing . . . . . . . . . . . . 14
5.7. PIM Assert Modification . . . . . . . . . . . . . . . . . 15 5.7. PIM Assert Modification . . . . . . . . . . . . . . . . . 15
5.8. Backward Compatibility . . . . . . . . . . . . . . . . . 16 5.8. Backward Compatibility . . . . . . . . . . . . . . . . . 16
6. Operational Considerations . . . . . . . . . . . . . . . . . 16 6. Operational Considerations . . . . . . . . . . . . . . . . . 16
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
7.1. Initial registry . . . . . . . . . . . . . . . . . . . . 17 7.1. Initial registry . . . . . . . . . . . . . . . . . . . . 17
7.2. Assignment of new Hash Algorithms . . . . . . . . . . . . 17 7.2. Assignment of new Hash Algorithms . . . . . . . . . . . . 17
8. Security Considerations . . . . . . . . . . . . . . . . . . . 17 8. Security Considerations . . . . . . . . . . . . . . . . . . . 17
9. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . 18 9. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . 18
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 18 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 18
10.1. Normative References . . . . . . . . . . . . . . . . . . 18 10.1. Normative References . . . . . . . . . . . . . . . . . . 18
10.2. Informative References . . . . . . . . . . . . . . . . . 18 10.2. Informative References . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 19 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 19
1. Introduction 1. Introduction
On a multi-access LAN, such as an Ethernet, with one or more PIM-SM On a multi-access LAN, such as an Ethernet, with one or more PIM-SM
[RFC7761] routers, one of the PIM-SM routers is elected as a (PIM Sparse Mode) [RFC7761] routers, one of the PIM-SM routers is
Designated Router (DR). The PIM DR has two responsibilities in the elected as a Designated Router (DR). The PIM DR has two
PIM-SM protocol. For any active sources on a LAN, the PIM DR is responsibilities in the PIM-SM protocol. For any active sources on a
responsible for registering with the Rendezvous Point (RP) if the LAN, the PIM DR is responsible for registering with the Rendezvous
group is operating in PIM-SM. Also, the PIM DR is responsible for Point (RP) if the group is operating in PIM-SM. Also, the PIM DR is
tracking local multicast listeners and forwarding to these listeners responsible for tracking local multicast listeners and forwarding to
if the group is operating in PIM-SM. these listeners if the group is operating in PIM-SM.
Consider the following LAN in Figure 1: Consider the following LAN in Figure 1:
(core networks) (core networks)
| | | | | |
| | | | | |
R1 R2 R3 R1 R2 R3
| | | | | |
----(LAN)---- ----(LAN)----
| |
| |
(many receivers) (many receivers)
Figure 1: LAN with receivers Figure 1: LAN with receivers
Assume R1 is elected as the DR. According to the PIM-SM protocol, R1 Assume R1 is elected as the DR. According to the PIM-SM protocol, R1
will be responsible for forwarding traffic to that LAN on behalf of will be responsible for forwarding traffic to that LAN on behalf of
any local members. In addition to keeping track of membership all local members. In addition to keeping track of membership
reports, R1 is also responsible for initiating the creation of source reports, R1 is also responsible for initiating the creation of source
and/or shared trees towards the senders or the RPs. The membership and/or shared trees towards the senders or the RPs. The membership
reports would be IGMP or MLD messages. This applies to any versions reports would be IGMP or MLD messages. This applies to any versions
of the IGMP and MLD protocols. The most recent versions are IGMPv3 of the IGMP and MLD protocols. The most recent versions are IGMPv3
[RFC3376] and MLDv2 [RFC3810]. [RFC3376] and MLDv2 [RFC3810].
Having a single router acting as DR and being responsible for data Having a single router acting as DR and being responsible for data
plane forwarding leads to several issues. One of the issues is that plane forwarding leads to several issues. One of the issues is that
the aggregated bandwidth will be limited to what R1 can handle with the aggregated bandwidth will be limited to what R1 can handle with
regards to capacity of incoming links, the interface on the LAN, and regards to capacity of incoming links, the interface on the LAN, and
total forwarding capacity. It is very common that a LAN consists of total forwarding capacity. It is very common that a LAN consists of
switches that run IGMP/MLD or PIM snooping [RFC4541]. This allows switches that run IGMP/MLD or PIM snooping [RFC4541]. This allows
the forwarding of multicast packets to be restricted only to segments the forwarding of multicast packets to be restricted only to segments
leading to receivers who have indicated their interest in multicast leading to receivers that have indicated their interest in multicast
groups using either IGMP or MLD. The emergence of the switched groups using either IGMP or MLD. The emergence of the switched
Ethernet allows the aggregated bandwidth to exceed, sometimes by a Ethernet allows the aggregated bandwidth to exceed, sometimes by a
large number, that of a single link. For example, let us modify large number, that of a single link. For example, let us modify
Figure 1 and introduce an Ethernet switch in Figure 2. Figure 1 and introduce an Ethernet switch in Figure 2.
(core networks) (core networks)
| | | | | |
| | | | | |
R1 R2 R3 R1 R2 R3
| | | | | |
+=gi0===gi1===gi2=+ +=gi1===gi2===gi3=+
+ + + +
+ switch + + switch +
+ + + +
+=gi4===gi5===gi6=+ +=gi4===gi5===gi6=+
| | | | | |
H1 H2 H3 H1 H2 H3
Figure 2: LAN with Ethernet Switch Figure 2: LAN with Ethernet Switch
Let us assume that each individual link is a Gigabit Ethernet. Each Let us assume that each individual link is a Gigabit Ethernet. Each
router, R1, R2 and R3, and the switch have enough forwarding capacity router, R1, R2 and R3, and the switch have enough forwarding capacity
to handle hundreds of Gigabits of data. to handle hundreds of Gigabits of data.
Let us further assume that each of the hosts requests 500 Mbps of 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 unique multicast data. This totals to 1.5 Gbps of data, which is
less than what each switch or the combined uplink bandwidth across less than what each switch or the combined uplink bandwidth across
the routers can handle, even under failure of a single router. 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 On the other hand, the link between R1 and switch, via port gi1, can
only handle a throughput of 1Gbps. And if R1 is the only DR (the PIM only handle a throughput of 1Gbps. And if R1 is the only DR (the PIM
DR elected using the procedure defined by [RFC7761]) at least 500 DR elected using the procedure defined by [RFC7761]) at least 500
Mbps worth of data will be lost because the only link that can be 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. used to draw the traffic from the routers to the switch is via gi1.
In other words, the entire network's throughput is limited by the In other words, the entire network's throughput is limited by the
single connection between the PIM DR and the switch (or LAN as in single connection between the PIM DR and the switch (or LAN as in
Figure 1). Figure 1).
Another important issue is related to failover. If R1 is the only Another important issue is related to failover. If R1 is the only
forwarder on a shared LAN, when R1 goes out of service, multicast forwarder on a shared LAN, when R1 goes out of service, multicast
forwarding for the entire LAN has to be rebuilt by the newly elected forwarding for the entire LAN has to be rebuilt by the newly elected
PIM DR. However, if there was a way that allowed multiple routers to PIM DR. However, if there were a way that allowed multiple routers
forward to the LAN for different groups, failure of one of the 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, routers would only lead to disruption to a subset of the flows,
therefore improving the overall resilience of the network. therefore improving the overall resilience of the network.
This document specifies a modification to the PIM-SM protocol that This document specifies a modification to the PIM-SM protocol that
allows more than one of these routers, called Group Designated allows more than one of these routers, called Group Designated
Routers (GDR) to be selected so that the forwarding load can be Routers (GDR) to be selected so that the forwarding load can be
distributed among a number of routers. distributed among a number of routers.
2. Terminology 2. Terminology
skipping to change at page 5, line 33 skipping to change at page 5, line 33
below) is used to select one of the routers as a GDR. The GDR is below) is used to select one of the routers as a GDR. The GDR is
responsible for initiating the forwarding tree building process responsible for initiating the forwarding tree building process
for the corresponding multicast flow. for the corresponding multicast flow.
o GDR Candidate: a router that has the potential to become a GDR. o GDR Candidate: a router that has the potential to become a GDR.
There might be multiple GDR Candidates on a LAN, but only one can There might be multiple GDR Candidates on a LAN, but only one can
become the GDR for a specific multicast flow. become the GDR for a specific multicast flow.
3. Applicability 3. Applicability
The extension specified in this document applies to PIM-SM when they The extension specified in this document applies to PIM-SM routers
act as last hop routers (there are directly connected receivers). It acting as last hop routers (there are directly connected receivers).
does not alter the behavior of a PIM DR, or any other routers, on the It does not alter the behavior of a PIM DR, or any other routers, on
first hop network (directly connected sources). This is because the the first hop network (directly connected sources). This is because
source tree is built using the IP address of the sender, not the IP the source tree is built using the IP address of the sender, not the
address of the PIM DR that sends the registers towards the RP. The IP address of the PIM DR that sends PIM registers towards the RP.
load balancing between first hop routers can be achieved naturally if The load balancing between first hop routers can be achieved
an IGP provides equal cost multiple paths (which it usually does in naturally if an IGP provides equal cost multiple paths (which it
practice). Also distributing the load to do registering does not usually does in practice). Also distributing the load to do source
justify the additional complexity required to support it. registration does not justify the additional complexity required to
support it.
4. Functional Overview 4. Functional Overview
In the PIM DR election as defined in [RFC7761], when multiple routers In the PIM DR election as defined in [RFC7761], when multiple routers
are connected to a multi-access LAN (for example, an Ethernet), one are connected to a multi-access LAN (for example, an Ethernet), one
of them is elected to act as PIM DR. The PIM DR is responsible for of them is elected to act as PIM DR. The PIM DR is responsible for
sending local Join/Prune messages towards the RP or source. In order 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 to elect the PIM DR, each PIM router on the LAN examines the received
PIM Hello messages and compares its own DR priority and IP address PIM Hello messages and compares its own DR priority and IP address
with those of its neighbors. The router with the highest DR priority with those of its neighbors. The router with the highest DR priority
is the PIM DR. If there are multiple such routers, their IP is the PIM DR. If there are multiple such routers, their IP
addresses are used as the tie-breaker, as described in [RFC7761]. addresses are used as the tie-breaker, as described in [RFC7761].
In order to share forwarding load among last hop routers, besides the In order to share forwarding load among last hop routers, besides the
normal PIM DR election, the GDR is also elected on the multi-access normal PIM DR election, one or more GDRs are elected on the multi-
LAN. There is only one PIM DR on the multi-access LAN, but there access LAN. There is only one PIM DR on the multi-access LAN, but
might be multiple GDR Candidates. there might be multiple GDR Candidates.
For each multicast flow, that is, (*,G) for ASM and (S,G) for SSM, a 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. Hash Algorithm [Section 5.1] is used to select one of the routers to
The new DR Load Balancing Capability (DRLB-Cap) PIM Hello Option is be the GDR. The new DR Load Balancing Capability (DRLB-Cap) PIM
used to announce the Capability as well as the Hash Algorithm type. Hello Option is used to announce the Capability as well as the Hash
Routers with the new DRLB-Cap Option advertised in their PIM Hello, Algorithm type. Routers with the new DRLB-Cap Option advertised in
using the same GDR election Hash Algorithm and the same DR priority their PIM Hello, using the same GDR election Hash Algorithm and the
as the PIM DR, are considered as GDR Candidates. same DR priority as the PIM DR, are considered as GDR Candidates.
Hash Masks are defined for Source, Group and RP separately, in order 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 to handle PIM ASM/SSM. The masks, as well as a sorted list of GDR
Candidate Addresses, are announced by the DR in a new DR Load Candidate Addresses, are announced by the DR in a new DR Load
Balancing List (DRLB-List) PIM Hello Option. Balancing List (DRLB-List) PIM Hello Option.
A Hash Algorithm based on the announced Source, Group, or RP masks A Hash Algorithm based on the announced Source, Group, or RP masks
allows one GDR to be assigned to a corresponding multicast state. allows one GDR to be assigned to a corresponding multicast state.
And that GDR is responsible for initiating the creation of the That GDR is responsible for initiating the creation of the multicast
multicast forwarding tree for multicast traffic. forwarding tree for multicast traffic.
4.1. GDR Candidates 4.1. GDR Candidates
GDR is the new concept introduced by this specification. GDR GDR is the new concept introduced by this specification. GDR
Candidates are routers eligible for GDR election on the LAN. To Candidates are routers eligible for GDR election on the LAN. To
become a GDR Candidate, a router must have the same DR priority and become a GDR Candidate, a router must have the same DR priority and
run the same GDR election Hash Algorithm as the DR on the LAN. run the same GDR election Hash Algorithm as the DR on the LAN.
For example, assume there are 4 routers on the LAN: R1, R2, R3 and For example, assume there are 4 routers on the LAN: R1, R2, R3 and
R4, each announcing a DRLB-Cap option. R1, R2 and R3 have the same R4, each announcing a DRLB-Cap option. R1, R2 and R3 have the same
DR priority while R4's DR priority is less preferred. In this DR priority while R4's DR priority is less preferred. In this
example, R4 will not be eligible for GDR election, because R4 will 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. 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 Furthermore, assume router R1 wins the PIM DR election, R1 and R2
the same Hash Algorithm for GDR election, while R3 runs a different advertise the same Hash Algorithm for GDR election, while R3
one. In this case, only R1 and R2 will be eligible for GDR election, advertises a different one. In this case, only R1 and R2 will be
while R3 will not. eligible for GDR election, while R3 will not.
As a DR, R1 will include its own Load Balancing Hash Masks and the As a DR, R1 will include its own Load Balancing Hash Masks and the
identity of R1 and R2 (the GDR Candidates) in its DRLB-List Hello identity of R1 and R2 (the GDR Candidates) in its DRLB-List Hello
Option. Option.
5. Protocol Specification 5. Protocol Specification
5.1. Hash Mask and Hash Algorithm 5.1. Hash Mask and Hash Algorithm
A Hash Mask is used to extract a number of bits from the A Hash Mask is used to extract a number of bits from the
corresponding IP address field (32 for IPv4, 128 for IPv6) and corresponding IP address field (32 for IPv4, 128 for IPv6) and
calculate a hash value. A hash value is used to select a GDR from calculate a hash value. A hash value is used to select a GDR from
GDR Candidates advertised by the PIM DR. Hash masks allow for GDR Candidates advertised by the PIM DR. Hash masks allow for
certain flows to always be forwarded by the same GDR, by ignoring certain flows to always be forwarded by the same GDR, by ignoring
certain bits in the hash value calculation, so that the hash values certain bits in the hash value calculation, so that the hash values
are the same. For example, 0.0.255.0 defines a Hash Mask for an IPv4 are the same. For example, 0.0.255.0 defines a Hash Mask for an IPv4
address that masks the first, the second, and the fourth octets, address that masks the first, the second, and the fourth octets,
which means that only the third octet will influence the hash value which means that only the third octet will influence the hash value
computed. computed. Note that the masks need not be a contiguous set of bits.
E.g, for IPv4, 15.15.15.15 would be a valid mask.
In the text below, a hash mask is in some places said to be zero. A In the text below, a hash mask is in some places said to be zero. A
hash mask is zero if no bits are set. That is, 0.0.0.0 for IPv4 and hash mask is zero if no bits are set. That is, 0.0.0.0 for IPv4 and
:: for IPv6. Also, a hash mask is said to be an all-bits-set mask if :: for IPv6. Also, a hash mask is said to be an all-bits-set mask if
it is 255.255.255.255 for IPv4 or it is 255.255.255.255 for IPv4 or
FFFF:FFFF:FFFF:FFFF:FFFFF:FFFF:FFFF:FFFF for IPv6. ffff:ffff:ffff:ffff:ffff:ffff:ffff:ffff for IPv6.
There are three Hash Masks defined: There are three Hash Masks defined:
o RP Hash Mask o RP Hash Mask
o Source Hash Mask o Source Hash Mask
o Group Hash Mask o Group Hash Mask
The hash masks need to be configured on the PIM routers that can The hash masks need to be configured on the PIM routers that can
potentially become a PIM DR, unless the implementation provides potentially become a PIM DR, unless the implementation provides
default hash mask values. An implementation SHOULD have default hash default hash mask values. An implementation SHOULD have default hash
mask values as follows. The default RP Hash Mask SHOULD be zero (no mask values as follows. The default RP Hash Mask SHOULD be zero (no
bits set). The default Source and Group Hash Masks SHOULD both be bits set). The default Source and Group Hash Masks SHOULD both be
all-bits-set masks. These default values are likely acceptable for all-bits-set masks. These default values are likely acceptable for
most deployments, and simplify configuration. most deployments, and simplify configuration. There is only a need
to use other masks if one needs to ensure that certain flows are
forwarded by the same GDR.
The DRLB-List Hello Option contains a list of GDR Candidates. The The DRLB-List Hello Option contains a list of GDR Candidates. The
first one listed has ordinal number 0, the second listed ordinal first one listed has ordinal number 0, the second listed ordinal
number 1, and the last one has ordinal number N - 1 if there are N number 1, and the last one has ordinal number N - 1 if there are N
candidates listed. The hash value computed will be the ordinal candidates listed. The hash value computed will be the ordinal
number of the GDR Candidate that is acting as GDR. number of the GDR Candidate that is acting as GDR for the flow in
question.
The input to be hashed is determined as follows:
o If the group is in ASM mode and the RP Hash Mask announced by the o If the group is in ASM mode and the RP Hash Mask announced by the
PIM DR is not zero (at least one bit is set), calculate the value PIM DR is not zero (at least one bit is set), calculate the value
of hashvalue_RP [Section 5.2] to determine the GDR. of hashvalue_RP [Section 5.2] to determine the GDR.
o If the group is in ASM mode and the RP Hash Mask announced by the o If the group is in ASM mode and the RP Hash Mask announced by the
PIM DR is zero (no bits are set), obtain the value of PIM DR is zero (no bits are set), obtain the value of
hashvalue_Group [Section 5.2] to determine the GDR. hashvalue_Group [Section 5.2] to determine the GDR.
o If the group is in SSM mode, use hashvalue_SG [Section 5.2] to o If the group is in SSM mode, use hashvalue_SG [Section 5.2] to
skipping to change at page 8, line 27 skipping to change at page 8, line 35
If different Hash Algorithms are advertised among the routers on a If different Hash Algorithms are advertised among the routers on a
LAN, only the routers advertising the same Hash Algorithm as the DR LAN, only the routers advertising the same Hash Algorithm as the DR
(as well as having the same DR priority as the DR) are eligible for (as well as having the same DR priority as the DR) are eligible for
GDR election. GDR election.
5.2. Modulo Hash Algorithm 5.2. Modulo Hash Algorithm
As part of computing the hash, the notation LSZC(hash_mask) is used As part of computing the hash, the notation LSZC(hash_mask) is used
to denote the number of zeroes counted from the least significant bit to denote the number of zeroes counted from the least significant bit
of a Hash Mask hash_mask. As an example, LSZC(255.255.128) is 7 and of a Hash Mask hash_mask. As an example, LSZC(255.255.128) is 7 and
also LSZC(FFFF:8000::) is 111. If all bits are set, LSZC will be 0. also LSZC(ffff:8000::) is 111. If all bits are set, LSZC will be 0.
If the mask is zero, then LSZC will be 32 for IPv4, and 128 for IPv6. If the mask is zero, then LSZC will be 32 for IPv4, and 128 for IPv6.
The number of GDR Candidates is denoted as GDRC. The number of GDR Candidates is denoted as GDRC.
The idea behind the Modulo Hash Algorithm is in simple terms that the The idea behind the Modulo Hash Algorithm is in simple terms that the
corresponding mask is applied to a value, then the result is shifted corresponding mask is applied to a value, then the result is shifted
right LSZC(mask) bits so that the least significant bits that were right LSZC(mask) bits so that the least significant bits that were
masked out are not considered. Then this result is masked by masked out are not considered. Then this result is masked by
0xFFFFFFFF, keeping only the last 32 bits of the result (this only 0xffffffff, keeping only the last 32 bits of the result (this only
makes a difference for IPv6). Finally, the hash value is this result makes a difference for IPv6). Finally, the hash value is this result
modulo the number of GDR Candidates (GDRC). modulo the number of GDR Candidates (GDRC).
The Modulo Hash Algorithm for computing the values hashvalue_RP, The Modulo Hash Algorithm for computing the values hashvalue_RP,
hashvalue_Group and hashvalue_SG is defined as follows. hashvalue_Group and hashvalue_SG is defined as follows.
hashvalue_RP is calculated as: hashvalue_RP is calculated as:
(((RP_address & RP_mask) >> LSZC(RP_mask)) & 0xFFFFFFFF) % GDRC (((RP_address & RP_mask) >> LSZC(RP_mask)) & 0xffffffff) % GDRC
RP_address is the address of the RP defined for the group and RP_address is the address of the RP defined for the group and
RP_mask is the RP Hash Mask. RP_mask is the RP Hash Mask.
hashvalue_Group is calculated as: hashvalue_Group is calculated as:
(((Group_address & Group_mask) >> LSZC(Group_mask)) & 0xFFFFFFFF) (((Group_address & Group_mask) >> LSZC(Group_mask)) & 0xffffffff)
% GDRC % GDRC
Group_address is the group address and Group_mask is the Group Group_address is the group address and Group_mask is the Group
Hash Mask. Hash Mask.
hashvalue_SG is calculated as: hashvalue_SG is calculated as:
((((Source_address & Source_mask) >> LSZC(Source_mask)) & ((((Source_address & Source_mask) >> LSZC(Source_mask)) &
0xFFFFFFFF) ^ (((Group_address & Group_mask) >> LSZC(Group_mask)) 0xffffffff) ^ (((Group_address & Group_mask) >> LSZC(Group_mask))
& 0xFFFFFFFF)) % GDRC & 0xffffffff)) % GDRC
Group_address is the group address and Group_mask is the Group Group_address is the group address and Group_mask is the Group
Hash Mask. Hash Mask.
5.2.1. Modulo Hash Algorithm Examples 5.2.1. Modulo Hash Algorithm Examples
To help illustrate the algorithm, consider this example. Router X To help illustrate the algorithm, consider this example. Router X
with IPv4 address 203.0.113.1 receives a DRLB-List Hello Option from with IPv4 address 203.0.113.1 receives a DRLB-List Hello Option from
the DR, which announces RP Hash Mask 0.0.255.0 and a list of GDR 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, Candidates, sorted by IP addresses from high to low: 203.0.113.3,
skipping to change at page 9, line 37 skipping to change at page 9, line 44
addresses would be: addresses would be:
0 for 203.0.113.3; 1 for 203.0.113.2; 2 for 203.0.113.1 (Router X). 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 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: for Group2. Following the modulo Hash Algorithm:
LSZC(0.0.255.0) is 8 and GDRC is 3. The hashvalue_RP for Group1 with LSZC(0.0.255.0) is 8 and GDRC is 3. The hashvalue_RP for Group1 with
RP RP1 is: RP RP1 is:
(((192.0.2.1 & 0.0.255.0) >> 8) & 0xFFFFFFFF % 3) = 2 % 3 = 2 (((192.0.2.1 & 0.0.255.0) >> 8) & 0xffffffff % 3) = 2 % 3 = 2
which matches the ordinal number assigned to Router X. Router X will which matches the ordinal number assigned to Router X. Router X will
be the GDR for Group1. be the GDR for Group1.
The hashvalue_RP for Group2 with RP RP2 is: The hashvalue_RP for Group2 with RP RP2 is:
(((198.51.100.2 & 0.0.255.0) >> 8) & 0xFFFFFFFF % 3) = 100 % 3 = 1 (((198.51.100.2 & 0.0.255.0) >> 8) & 0xffffffff % 3) = 100 % 3 = 1
which is different from the ordinal number of Router X (2). Hence,
which is different from the ordinal number of router X (2). Hence,
Router X will not be GDR for Group2. Router X will not be GDR for Group2.
For IPv6 consider this example, similar to the above. Router X with For IPv6 consider this example, similar to the above. Router X with
IPv6 address FE80::1 receives a DRLB-List Hello Option from the DR, IPv6 address fe80::1 receives a DRLB-List Hello Option from the DR,
which announces RP Hash Mask ::FFFF:FFFF:FFFF:0 and a list of GDR which announces RP Hash Mask ::ffff:ffff:ffff:0 and a list of GDR
Candidates, sorted by IP addresses from high to low: FE80::3, FE80::2 Candidates, sorted by IP addresses from high to low: fe80::3, fe80::2
and FE80::1. The ordinal number assigned to those addresses would and fe80::1. The ordinal number assigned to those addresses would
be: be:
0 for FE80::3; 1 for FE80::2; 2 for FE80::1 (Router X). 0 for fe80::3; 1 for fe80::2; 2 for fe80::1 (Router X).
Assume there are 2 RPs: RP1 2001:DB8::1:0:5678:1 for Group1 and RP2 Assume there are 2 RPs: RP1 2001:db8::1:0:5678:1 for Group1 and RP2
2001:DB8::1:0:1234:2 for Group2. Following the modulo Hash 2001:db8::1:0:1234:2 for Group2. Following the modulo Hash
Algorithm: Algorithm:
LSZC(::FFFF:FFFF:FFFF:0) is 16 and GDRC is 3. The hashvalue_RP for LSZC(::ffff:ffff:ffff:0) is 16 and GDRC is 3. The hashvalue_RP for
Group1 with RP RP1 is: Group1 with RP RP1 is:
(((2001:DB8::1:0:5678:1 & ::FFFF:FFFF:FFFF:0) >> 16) & 0xFFFFFFFF % (((2001:db8::1:0:5678:1 & ::ffff:ffff:ffff:0) >> 16) & 0xffffffff %
3) = ((::1:0:5678:0 >> 16) & 0xFFFFFFFF % 3) = (::1:0:5678 & 3) = ((::1:0:5678:0 >> 16) & 0xffffffff % 3) = (::1:0:5678 &
0xFFFFFFFF % 3) = ::5678 % 3 = 2 0xffffffff % 3) = ::5678 % 3 = 2
which matches the ordinal number assigned to Router X. Router X will which matches the ordinal number assigned to Router X. Router X will
be the GDR for Group1. be the GDR for Group1.
The hashvalue_RP for Group2 with RP RP2 is: The hashvalue_RP for Group2 with RP RP2 is:
(((2001:DB8::1:0:1234:1 & ::FFFF:FFFF:FFFF:0) >> 16) & 0xFFFFFFFF % (((2001:db8::1:0:1234:1 & ::ffff:ffff:ffff:0) >> 16) & 0xffffffff %
3) = ((::1:0:1234:0 >> 16) & 0xFFFFFFFF % 3) = (::1:0:1234 & 3) = ((::1:0:1234:0 >> 16) & 0xffffffff % 3) = (::1:0:1234 &
0xFFFFFFFF % 3) = ::1234 % 3 = 1 0xffffffff % 3) = ::1234 % 3 = 1
which is different from the ordinal number of router X (2). Hence, which is different from the ordinal number of Router X (2). Hence,
Router X will not be GDR for Group2. Router X will not be GDR for Group2.
5.2.2. Limitations 5.2.2. Limitations
The Modulo Hash Algorithm has poor failover characteristics when a The Modulo Hash Algorithm has poor failover characteristics when a
shared LAN has more than two GDRs. In the case of more than two GDRs shared LAN has more than two GDRs. In the case of more than two GDRs
on a LAN, when one GDR fails, all of the groups may be reassigned to on a LAN, when one GDR fails, all of the groups may be reassigned to
a different GDR, even if they were not assigned to the failed GDR. a different GDR, even if they were not assigned to the failed GDR.
However, many deployments use only two routers on a shared LAN for However, many deployments use only two routers on a shared LAN for
redundancy purposes. Future work may define new Hash Algorithms redundancy purposes. Future work may define new Hash Algorithms
where only groups assigned to the failed GDR get reassigned. where only groups assigned to the failed GDR get reassigned.
5.3. PIM Hello Options 5.3. PIM Hello Options
All PIM routers include a new option, called "Load Balancing PIM routers include a new option, called "Load Balancing Capability
Capability (DRLB-Cap)" in their PIM Hello messages. (DRLB-Cap)" in their PIM Hello messages.
Besides this DRLB-Cap Hello Option, the elected PIM DR also includes Besides this DRLB-Cap Hello Option, the elected PIM DR also includes
a new "DR Load Balancing List (DRLB-List) Hello Option". The DRLB- a new "DR Load Balancing List (DRLB-List) Hello Option". The DRLB-
List Hello Option consists of three Hash Masks as defined above and List Hello Option consists of three Hash Masks as defined above and
also a sorted list of GDR Candidate addresses on the LAN. also a list of GDR Candidate addresses on the LAN. It is recommended
that the GDR Candidate addresses are sorted in descending order.
This ensures that when using algorithms such as the Modulo algorithm
in this document, that it is predictable which GDR is responsible for
which groups, regardless of the order the DR learned about the
candidates.
5.3.1. PIM DR Load Balancing Capability (DRLB-Cap) Hello Option 5.3.1. PIM DR Load Balancing Capability (DRLB-Cap) Hello Option
0 1 2 3 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 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 = 34 | Length = 4 | | Type = 34 | Length = 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |Hash Algorithm | | Reserved |Hash Algorithm |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: PIM DR Load Balancing Capability Hello Option Figure 3: PIM DR Load Balancing Capability Hello Option
Type: 34 Type: 34
Length: 4 Length: 4
Reserved: Transmitted as zero, ignored on receipt. Reserved: Transmitted as zero, ignored on receipt.
Hash Algorithm: Hash Algorithm type. 0 for the Modulo algorithm Hash Algorithm: Hash Algorithm type. A value listed in the IANA
defined in this document. Designated Router Load Balancing Hash Algorithms registry. 0 is
used for the Modulo algorithm defined in this document.
This DRLB-Cap Hello Option MUST be advertised by routers on all This DRLB-Cap Hello Option MUST be advertised by routers on all
interfaces where DR Load Balancing is enabled. interfaces where DR Load Balancing is enabled. Note that the option
is included at most once.
5.3.2. PIM DR Load Balancing List (DRLB-List) Hello Option 5.3.2. PIM DR Load Balancing List (DRLB-List) Hello Option
0 1 2 3 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 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 = 35 | Length | | Type = 35 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Group Mask | | Group Mask |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Mask | | Source Mask |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RP Mask | | RP Mask |
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RP Mask (32/128 bits): Mask applied to RP addresses as part of RP Mask (32/128 bits): Mask applied to RP addresses as part of
hash computation. hash computation.
All masks MUST have the same number of bits as the IP source All masks MUST have the same number of bits as the IP source
address in the PIM Hello IP header. address in the PIM Hello IP header.
GDR Candidate Address(es) (32/128 bits): List of GDR Candidate(s) GDR Candidate Address(es) (32/128 bits): List of GDR Candidate(s)
All addresses MUST be in the same address family as the PIM All addresses MUST be in the same address family as the PIM
Hello IP header. It is RECOMMENDED that the addresses are Hello IP header. It is recommended that the addresses are
sorted in descending order. sorted in descending order.
If the "Interface ID" option, as specified in [RFC6395], is If the "Interface ID" option, as specified in [RFC6395], is
present in a GDR Candidate's PIM Hello message, and the "Router present in a GDR Candidate's PIM Hello message, and the "Router
Identifier" portion is non-zero: Identifier" portion is non-zero:
+ For IPv4, the "GDR Candidate Address" will be set directly + For IPv4, the "GDR Candidate Address" will be set directly
to the "Router Identifier". to the "Router Identifier".
+ For IPv6, the "GDR Candidate Address" will be 96 bits of + For IPv6, the "GDR Candidate Address" will be 96 bits of
skipping to change at page 13, line 10 skipping to change at page 13, line 28
elected PIM DR. It MUST be ignored if received from a non-DR. elected PIM DR. It MUST be ignored if received from a non-DR.
The option MUST also be ignored if the hash masks are not the The option MUST also be ignored if the hash masks are not the
correct number of bits, or GDR Candidate addresses are in the correct number of bits, or GDR Candidate addresses are in the
wrong address family. wrong address family.
5.4. PIM DR Operation 5.4. PIM DR Operation
The DR election process is still the same as defined in [RFC7761]. The DR election process is still the same as defined in [RFC7761].
The DR advertises the new DRLB-List Hello Option, which contains mask The DR advertises the new DRLB-List Hello Option, which contains mask
values from user configuration (or default values), followed by a values from user configuration (or default values), followed by a
list of GDR Candidate Addresses. It is RECOMMENDED that the list be list of GDR Candidate Addresses. Note that if a router included the
sorted, from the highest value to the lowest value. The reason for "Interface ID" option in the hello message, and the Router ID is non-
sorting the list is to make the behavior deterministic, regardless of zero, the Router ID will be used to form the GDR Candidate address of
the order in which the DR learns of new candidates. Note that, as the router, as discussed in the previous section. It is recommended
non-DR routers, the DR also advertises the DRLB-Cap Hello Option to that the list be sorted, from the highest value to the lowest value.
indicate its ability to support the new functionality and the type of The reason for sorting the list is to make the behavior
GDR election Hash Algorithm. deterministic, regardless of the order in which the DR learns of new
candidates. Note that, as for non-DR routers, the DR also advertises
the DRLB-Cap Hello Option to indicate its ability to support the new
functionality and the type of GDR election Hash Algorithm it uses.
If a PIM DR receives a neighbor DRLB-Cap Hello Option, which contains If a PIM DR receives a neighbor DRLB-Cap Hello Option, which contains
the same Hash Algorithm as the DR, and the neighbor has the same DR the same Hash Algorithm as the DR, and the neighbor has the same DR
priority as the DR, PIM DR SHOULD consider the neighbor as a GDR priority as the DR, PIM DR SHOULD consider the neighbor as a GDR
Candidate and insert the GDR Candidate' Address into the list of the Candidate and insert the GDR Candidate' Address into the list of the
DRLB-List Option. However, the DR may have policies limiting which DRLB-List Option. However, the DR may have policies limiting which
GDR Candidates, or the number of GDR Candidates to include. GDR Candidates, or the number of GDR Candidates to include.
Likewise, the DR SHOULD include itself in the list of GDR Candidates, Likewise, the DR SHOULD include itself in the list of GDR Candidates,
but it is permissable not to do so, if for instance there is some but it is permissible not to do so, if for instance there is some
policy restricting the candidate set. policy restricting the candidate set.
If a PIM neighbor included in the list expires, stops announcing the If a PIM neighbor included in the list expires, stops announcing the
DRLB-Cap Hello Option, changes DR priority, changes Hash Algorithm or DRLB-Cap Hello Option, changes DR priority, changes Hash Algorithm or
otherwise becomes ineligible as a candidate, the DR SHOULD otherwise becomes ineligible as a candidate, the DR SHOULD
immediately send a triggered hello with a new list in the DRLB-List immediately send a triggered hello with a new list in the DRLB-List
option, excluding the neighbor. option, excluding the neighbor.
If a new router becomes eligible as a candidate, there is no urgency If a new router becomes eligible as a candidate, there is no urgency
in sending out an updated list. An updated list SHOULD be included in sending out an updated list. An updated list SHOULD be included
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changes, continue processing as below. Note that if the option does changes, continue processing as below. Note that if the option does
not pass the above checks, the below processing MUST be done as if not pass the above checks, the below processing MUST be done as if
the option was not announced. the option was not announced.
If the contents of the DRLB-List Option, the masks or the candidate If the contents of the DRLB-List Option, the masks or the candidate
list, differs from the previously saved copy, it is received for the list, differs from the previously saved copy, it is received for the
first time, or it is no longer being received or accepted, the option first time, or it is no longer being received or accepted, the option
MUST be processed as below. MUST be processed as below.
1. If the local router is included in the GDR Candidate Address(es) 1. If the local router is included in the GDR Candidate Address(es)
field, for each of the groups, or source and group pairs if the field (it will look for its own address, or its Router ID if it
group is in SSM mode, with local receiver interest, the router announces a non-zero Router ID), for each of the groups, or
MUST run the Hash Algorithm to determine which of them it is the source and group pairs if the group is in SSM mode, with local
GDR for. receiver interest, the router MUST run the Hash Algorithm to
determine which of them it is the GDR for.
If there is no change in the GDR status, then no further If there is no change in the GDR status, then no further
action is required. action is required.
If the router becomes the new GDR, then a multicast forwarding If the router becomes the new GDR, then a multicast forwarding
tree MUST be built [RFC7761]. tree MUST be built [RFC7761].
If the router is no longer the GDR, then it uses an Assert as If the router is no longer the GDR, then it uses an Assert as
explained in [Section 5.7]. explained in [Section 5.7].
skipping to change at page 15, line 20 skipping to change at page 15, line 41
GDR changes may occur due to configuration change, due to GDR GDR changes may occur due to configuration change, due to GDR
candidates going down, and also new routers coming up and becoming candidates going down, and also new routers coming up and becoming
GDR candidates. This may occur while flows are being forwarded. If GDR candidates. This may occur while flows are being forwarded. If
the GDR for an active flow changes, there is likely to be some the GDR for an active flow changes, there is likely to be some
disruption, such as packet loss or duplicates. By using asserts, disruption, such as packet loss or duplicates. By using asserts,
packet loss is minimized, while allowing a small amount of packet loss is minimized, while allowing a small amount of
duplicates. duplicates.
When a router stops acting as the GDR for a group, or source and When a router stops acting as the GDR for a group, or source and
group pair if SSM, it MUST set the Assert metric preference to group pair if SSM, it MUST set the Assert metric preference to
maximum (0x7FFFFFFF) and the Assert metric to one less than maximum maximum (0x7fffffff) and the Assert metric to one less than maximum
(0xFFFFFFFE). This was also mentioned in the previous section. That (0xfffffffe). That is, whenever it sends or receives an Assert for
is, whenever it sends or receives an Assert for the group, it must the group, it must use these values as the metric preference and
use these values as the metric preference and metric rather than the metric rather than the values provided by the unicast routing
values provided by the unicast routing protocol. protocol.
The rest of this section is just for illustration purposes and not The rest of this section is just for illustration purposes and not
part of the protocol definition. part of the protocol definition.
To illustrate the behavior when there is a GDR change, consider the To illustrate the behavior when there is a GDR change, consider the
following scenario where there are two flows G1 and G2. R1 is the following scenario where there are two flows G1 and G2. R1 is the
GDR for G1, and R2 is the GDR for G2. When R3 comes up, it is GDR for G1, and R2 is the GDR for G2. When R3 comes up, it is
possible that R3 becomes GDR for both G1 and G2, hence R3 starts to 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 build the forwarding tree for G1 and G2. If R1 and R2 stop
forwarding before R3 completes the process, packet loss might occur. forwarding before R3 completes the process, packet loss might occur.
skipping to change at page 16, line 15 skipping to change at page 16, line 31
5.8. Backward Compatibility 5.8. Backward Compatibility
In the case of a hybrid Ethernet shared LAN (where some PIM routers In the case of a hybrid Ethernet shared LAN (where some PIM routers
support the functionality defined in this document, and some do not); support the functionality defined in this document, and some do not);
o If the DR does not support the new functionality, then there will o If the DR does not support the new functionality, then there will
be no load-balancing. be no load-balancing.
o If non-DR routers do not support the new functionality, they will o If non-DR routers do not support the new functionality, they will
not be considered as Candidate GDRs and it will not take part in not be considered as Candidate GDRs and it will not take part in
an load-balancing. Load-balancing may still happen on the link. load-balancing. Load-balancing may still happen on the link.
6. Operational Considerations 6. Operational Considerations
An administrator needs to consider what the total bandwidth An administrator needs to consider what the total bandwidth
requirements are and find a set of routers that together has enough requirements are and find a set of routers that together has enough
total capacity, while making sure that each of the routers can handle available capacity, while making sure that each of the routers can
its part, assuming that the traffic is distributed roughly equally handle its part, assuming that the traffic is distributed roughly
among the routers. Ideally, one should also have enough bandwidth to equally among the routers. Ideally, one should also have enough
handle the case where at least one router fails. All routers should bandwidth to handle the case where at least one router fails. All
have reachability to the sources, and RPs if applicable, that is not routers should have reachability to the sources, and RPs if
via the LAN. applicable, that is not via the LAN.
Care must be taken when choosing what hash masks to configure. One Care must be taken when choosing what hash masks to configure. One
would typically configure the same masks on all the routers, so that would typically configure the same masks on all the routers, so that
they are the same, regardless of which router is elected as DR. The they are the same, regardless of which router is elected as DR. The
default masks are likely suitable for most deployment. The RP Hash default masks are likely suitable for most deployment. The RP Hash
Mask must be configured (the default is no bits set) if one wishes to Mask must be configured (the default is no bits set) if one wishes to
hash based on the RP address rather than the group address for ASM. hash based on the RP address rather than the group address for ASM.
The default masks will use the entire group addresses, and source The default masks will use the entire group addresses, and source
addresses if SSM, as part of the hash. An administrator may set addresses if SSM, as part of the hash. An administrator may set
other masks that masks out part of the addresses to ensure that other masks that masks out part of the addresses to ensure that
skipping to change at page 18, line 5 skipping to change at page 18, line 23
If for any reason, the DR includes a GDR in the announced list which If for any reason, the DR includes a GDR in the announced list which
announces a different algorithm from what the DR announces, the GDR announces a different algorithm from what the DR announces, the GDR
is required to ignore the announcement, and there will be no router is required to ignore the announcement, and there will be no router
acting as the DR for the flows that hash to that GDR. acting as the DR for the flows that hash to that GDR.
If a GDR is subverted, it could potentially be made to stop If a GDR is subverted, it could potentially be made to stop
forwarding all the traffic it is expected to forward. This is also forwarding all the traffic it is expected to forward. This is also
similar today to if a DR is subverted. similar today to if a DR is subverted.
An administrator may be able to achieve the desired load-balancing of
known flows, but an attacker may send a single high rate flow which
is served by a single GDR, or send multiple flows that are expected
to be hashed to the same GDR.
9. Acknowledgement 9. Acknowledgement
The authors would like to thank Steve Simlo and Taki Millonis for The authors would like to thank Steve Simlo and Taki Millonis for
helping with the original idea; Alia Atlas, Bill Atwood, Jake helping with the original idea; Alia Atlas, Bill Atwood, Joe Clarke,
Holland, Bharat Joshi, Anish Kachinthaya, Anvitha Kachinthaya and Alissa Cooper, Jake Holland, Bharat Joshi, Anish Kachinthaya, Anvitha
Alvaro Retana for reviews and comments; and Toerless Eckert and Kachinthaya, Benjamin Kaduk, Mirja Kuhlewind, Barry Leiba, Ben Niven-
Jenkins, Alvaro Retana, Adam Roach, Michael Scharf, Eric Vyncke and
Carl Wallace for reviews and comments; and Toerless Eckert and
Rishabh Parekh for helpful conversation on the document. Rishabh Parekh for helpful conversation on the document.
10. References 10. References
10.1. Normative References 10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
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