PIM Designated Router Load Balancing
draft-ietf-pim-drlb-11
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 8775.
|
|
|---|---|---|---|
| Authors | Yiqun Cai , Heidi Ou , Sri Vallepalli , Mankamana Prasad Mishra , Stig Venaas , Andy Green | ||
| Last updated | 2019-10-18 (Latest revision 2019-10-11) | ||
| Replaces | draft-hou-pim-drlb | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Formats | |||
| Reviews |
GENART Last Call review
(of
-13)
by Pete Resnick
Ready w/issues
|
||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | Submitted to IESG for Publication | |
| Document shepherd | Mike McBride | ||
| Shepherd write-up | Show Last changed 2019-01-14 | ||
| IESG | IESG state | Became RFC 8775 (Proposed Standard) | |
| Consensus boilerplate | Unknown | ||
| Telechat date | (None) | ||
| Responsible AD | Alvaro Retana | ||
| Send notices to | aretana.ietf@gmail.com |
draft-ietf-pim-drlb-11
Network Working Group Y. Cai
Internet-Draft H. Ou
Intended status: Standards Track Alibaba Group
Expires: April 13, 2020 S. Vallepalli
M. Mishra
S. Venaas
Cisco Systems, Inc.
A. Green
British Telecom
October 11, 2019
PIM Designated Router Load Balancing
draft-ietf-pim-drlb-11
Abstract
On a multi-access network, one of the PIM-SM routers is elected as a
Designated Router. One of the responsibilities of the Designated
Router is to track local multicast listeners and forward data to
these listeners if the group is operating in PIM-SM. This document
specifies a modification to the PIM-SM protocol that allows more than
one of the PIM-SM routers to take on this responsibility so that the
forwarding load can be distributed among multiple routers.
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 https://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 April 13, 2020.
Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.
Cai, et al. Expires April 13, 2020 [Page 1]
Internet-Draft PIM Designated Router Load Balancing October 2019
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
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
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 . . . . . . . . . . . . . . . . . . . . . 5
4.1. GDR Candidates . . . . . . . . . . . . . . . . . . . . . 6
5. Protocol Specification . . . . . . . . . . . . . . . . . . . 7
5.1. Hash Mask and Hash Algorithm . . . . . . . . . . . . . . 7
5.2. Modulo Hash Algorithm . . . . . . . . . . . . . . . . . . 8
5.2.1. Modulo Hash Algorithm Example . . . . . . . . . . . . 9
5.2.2. Limitations . . . . . . . . . . . . . . . . . . . . . 10
5.3. PIM Hello Options . . . . . . . . . . . . . . . . . . . . 10
5.3.1. PIM DR Load Balancing Capability (DRLB-Cap) Hello
Option . . . . . . . . . . . . . . . . . . . . . . . 10
5.3.2. PIM DR Load Balancing List (DRLB-List) Hello Option . 11
5.4. PIM DR Operation . . . . . . . . . . . . . . . . . . . . 12
5.5. PIM GDR Candidate Operation . . . . . . . . . . . . . . . 13
5.6. DRLB-List Hello Option Processing . . . . . . . . . . . . 13
5.7. PIM Assert Modification . . . . . . . . . . . . . . . . . 14
5.8. Backward Compatibility . . . . . . . . . . . . . . . . . 16
6. Manageability Considerations . . . . . . . . . . . . . . . . 16
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
7.1. Initial registry . . . . . . . . . . . . . . . . . . . . 17
7.2. Assignment of new hash algorithms . . . . . . . . . . . . 17
8. Security Considerations . . . . . . . . . . . . . . . . . . . 17
9. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . 18
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 18
10.1. Normative References . . . . . . . . . . . . . . . . . . 18
10.2. Informative References . . . . . . . . . . . . . . . . . 18
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 19
1. Introduction
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
Designated Router (DR). The PIM DR has two responsibilities in the
PIM-SM protocol. For any active sources on a LAN, the PIM DR is
Cai, et al. Expires April 13, 2020 [Page 2]
Internet-Draft PIM Designated Router Load Balancing October 2019
responsible for registering with the Rendezvous Point (RP) if the
group is operating in PIM-SM. Also, the PIM DR is responsible for
tracking local multicast listeners and forwarding to these listeners
if the group is operating in PIM-SM.
Consider the following LAN in Figure 1:
(core networks)
| | |
| | |
R1 R2 R3
| | |
----(LAN)----
|
|
(many receivers)
Figure 1: LAN with receivers
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
any local members. In addition to keeping track of membership
reports, R1 is also responsible for initiating the creation of source
and/or shared trees towards the senders or the RPs. The membership
reports would be IGMP or MLD messages. This applies to any versions
of the IGMP and MLD protocols. The most recent versions are IGMPv3
[RFC3376] and MLDv2 [RFC3810].
Having a single router acting as DR and being responsible for data
plane forwarding leads to several issues. One of the issues is that
the aggregated bandwidth will be limited to what R1 can handle with
regards to capacity of incoming links, the interface on the LAN, and
total forwarding capacity. It is very common that a LAN consists of
switches that run IGMP/MLD or PIM snooping [RFC4541]. 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.
Cai, et al. Expires April 13, 2020 [Page 3]
Internet-Draft PIM Designated Router Load Balancing October 2019
(core networks)
| | |
| | |
R1 R2 R3
| | |
+=gi0===gi1===gi2=+
+ +
+ switch +
+ +
+=gi4===gi5===gi6=+
| | |
H1 H2 H3
Figure 2: LAN 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 [RFC7761]) 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 LAN as in
Figure 1).
Another important issue is related to failover. If R1 is the only
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
PIM DR. However, if there was a way that allowed 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.
This document specifies a modification to the PIM-SM protocol that
allows more than one of these routers, called Group Designated
Cai, et al. Expires April 13, 2020 [Page 4]
Internet-Draft PIM Designated Router Load Balancing October 2019
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", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
With respect to PIM-SM, this document follows the terminology that
has been defined in [RFC7761].
This document also introduces the following new acronyms:
o GDR: Group Designated Router. For each multicast flow, either a
(*,G) for Any-Source Multicast (ASM), or an (S,G) for Source-
Specific Multicast (SSM) [RFC4607], 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 router that has the potential to become a GDR.
There might be multiple GDR Candidates on a LAN, but only one can
become the GDR for a specific multicast flow.
3. Applicability
The extension specified in this document applies to PIM-SM when they
act as last hop routers (there are directly connected receivers). It
does not alter the behavior of a PIM DR, or any other routers, on the
first hop network (directly connected sources). 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 do registering does not
justify the additional complexity required to support it.
4. Functional Overview
In the PIM DR election as defined in [RFC7761], when multiple routers
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
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 own DR priority and IP address
Cai, et al. Expires April 13, 2020 [Page 5]
Internet-Draft PIM Designated Router Load Balancing October 2019
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 [RFC7761].
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
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 (DRLB-Cap) PIM Hello Option, which
contains hash algorithm type, is announced by routers on interfaces
where this specification is enabled. Routers with the new DRLB-Cap
Option advertised in their PIM Hello, 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
Candidate Addresses, are announced by the DR in a new DR Load
Balancing List (DRLB-List) 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 have the same DR priority and
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
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
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.
Cai, et al. Expires April 13, 2020 [Page 6]
Internet-Draft PIM Designated Router Load Balancing October 2019
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
Option.
5. Protocol Specification
5.1. Hash Mask and Hash Algorithm
A Hash Mask is used to extract a number of bits from the
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
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. Hash masks allow for certain flows to always be
forwarded by the same GDR, since the hash values are the same. For
instance the mask 0.0.255.0 means that only the third octet will be
considered when hashing.
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
:: 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
FFFF:FFFF:FFFF:FFFF:FFFFF:FFFF:FFFF:FFFF for IPv6.
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 values. An implementation SHOULD have default hash
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
all-bits-set masks. These default values are likely acceptable for
most deployments, and simplify configuration.
The DRLB-List Hello Option contains a list of GDR Candidates. The
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
candidates listed. The hash value computed will be the ordinal
number of the GDR Candidate that is acting as GDR.
Cai, et al. Expires April 13, 2020 [Page 7]
Internet-Draft PIM Designated Router Load Balancing October 2019
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
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
PIM DR is zero (no bits are set), obtain the value of
hashvalue_Group [Section 5.2] to determine the GDR.
o If the group is in SSM mode, use hashvalue_SG [Section 5.2] to
determine the GDR.
A simple Modulo hash algorithm is defined in this document. However,
to allow another hash algorithms to be used, a 1-octet "Hash
Algorithm" field is included in the DRLB-Cap Hello Option to specify
the hash algorithm used by the router.
If different hash algorithms are advertised among the routers on a
LAN, only the outers advertising the same hash algorithm as the DR
(as well as having the same DR priority as the DR) are eligible for
GDR election.
5.2. Modulo Hash Algorithm
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
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.
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 idea behind the Modulo hash algorithm is in simple terms that the
corresponding mask is applied to a value, then the result is shifted
right LSZC(mask) bits so that the least significant bits that were
masked out are not considered. Then this result is masked by 0xFFFF,
keeping only the last 32 bits of the result (this only makes a
difference for IPv6). Finally, the hash value is this result modulo
the number of GDR Candidates (GDRC).
The Modulo hash algorithm for computing the values hashvalue_RP,
hashvalue_Group and hashvalue_SG is defined as follows.
hashvalue_RP is calculated as:
(((RP_address & RP_mask) >> LSZC(RP_mask)) & 0xFFFF) % GDRC
RP_address is the address of the RP defined for the group and
RP_mask is the RP Hash Mask.
Cai, et al. Expires April 13, 2020 [Page 8]
Internet-Draft PIM Designated Router Load Balancing October 2019
hashvalue_Group is calculated as:
(((Group_address & Group_mask) >> LSZC(Group_mask)) & 0xFFFF) %
GDRC
Group_address is the group address and Group_mask is the Group
Hash Mask.
hashvalue_SG is calculated as:
((((Source_address & Source_mask) >> LSZC(Source_mask)) & 0xFFFF)
^ (((Group_address & Group_mask) >> LSZC(Group_mask)) & 0xFFFF)) %
GDRC
Group_address is the group address and Group_mask is the Group
Hash Mask.
5.2.1. Modulo Hash Algorithm Example
To help illustrate the algorithm, consider this example. Router X
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
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:
LSZC(0.0.255.0) is 8 and GDRC is 3. The hashvalue_RP for Group1 with
RP RP1 is:
(((192.0.2.1 & 0.0.255.0) >> 8) & 0xFFFF % 3) = 2 % 3 = 2
which matches the ordinal number assigned to Router X. Router X will
be the GDR for Group1.
The hashvalue_RP for Group2 with RP RP2 is:
(((198.51.100.2 & 0.0.255.0) >> 8) & 0xFFFF % 3) = 100 % 3 = 1
which is different from the ordinal number of router X (2). Hence,
Router X will not be GDR for Group2.
Cai, et al. Expires April 13, 2020 [Page 9]
Internet-Draft PIM Designated Router Load Balancing October 2019
5.2.2. Limitations
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
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.
However, many deployments use only two routers on a shared LAN for
redundancy purposes. Future work may define new hash algorithms
where only groups assigned to the failed GDR get reassigned.
5.3. PIM Hello Options
When a PIM router sends a PIM Hello on an interface with this
specification enabled, it includes a new option, called "Load
Balancing Capability (DRLB-Cap)".
Besides this DRLB-Cap Hello Option, the elected PIM DR also includes
a new "DR Load Balancing List (DRLB-List) Hello Option". The DRLB-
List Hello Option consists of three Hash Masks as defined above and
also a sorted list of GDR Candidate addresses on the LAN.
5.3.1. PIM DR Load Balancing Capability (DRLB-Cap) 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 = 34 | Length = 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |Hash Algorithm |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: PIM DR Load Balancing Capability Hello Option
Type: 34
Length: 4
Reserved: Transmitted as zero, ignored on receipt.
Hash Algorithm: Hash algorithm type. 0 for the Modulo algorithm
defined in this document.
This DRLB-Cap Hello Option MUST be advertised by routers on all
interfaces where DR Load Balancing is enabled.
Cai, et al. Expires April 13, 2020 [Page 10]
Internet-Draft PIM Designated Router Load Balancing October 2019
5.3.2. PIM DR Load Balancing List (DRLB-List) 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 = 35 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Group Mask |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Mask |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RP Mask |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| GDR Candidate Address(es) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: PIM DR Load Balancing List Hello Option
Type: 35
Length: (3 + n) x (4 or 16), where n is the number of GDR
candidates.
Group Mask (32/128 bits): Mask applied to group addresses as part
of hash computation.
Source Mask (32/128 bits): Mask applied to source addresses as
part of hash computation.
RP Mask (32/128 bits): Mask applied to RP addresses as part of
hash computation.
All masks MUST have the same number of bits as the IP source
address in the PIM 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 PIM
Hello IP header. It is RECOMMENDED that the addresses are
sorted in descending order.
If the "Interface ID" option, as specified in [RFC6395], is
present in a GDR Candidate's PIM Hello message, and the "Router
ID" portion is non-zero:
Cai, et al. Expires April 13, 2020 [Page 11]
Internet-Draft PIM Designated Router Load Balancing October 2019
+ For IPv4, the "GDR Candidate Address" will be set directly
to the "Router ID".
+ For IPv6, the "GDR Candidate Address" will be 96 bits of
zeroes followed by the 32 bit Router ID.
If the "Interface ID" option is not present in a GDR Candidate'
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 of the PIM Hello
message.
This DRLB-List Hello Option MUST only be advertised by the
elected PIM DR. It MUST be ignored if received from a non-DR.
5.4. PIM DR Operation
The DR election process is still the same as defined in [RFC7761]. A
DR that has this specification enabled on an interface advertises the
new DRLB-List Hello Option, which contains mask values from user
configuration (or default values), followed by a list of GDR
Candidate Addresses. It is RECOMMENDED that the list is sorted, from
the highest value to the lowest value. The reason for sorting the
list is to make the behavior deterministic, regardless of the order
the DR learns of new candidates. Note that same as non-DR routers,
the DR also advertises DRLB-Cap 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 neighbor DRLB-Cap Hello Option, which contains
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
Candidate and insert the GDR Candidate' Address into the list of the
DRLB-List Option. However, the DR may have policies limiting which
GDR Candidates, or the number of GDR Candidates to include. The DR
would normally include itself in the list of GDR Candidates.
If a PIM neighbor included in the list expires, stops announcing the
DRLB-Cap Hello Option, changes DR priority, changes hash algorithm or
otherwise becomes ineligibile as a candidate, the DR should
immediately send a triggered hello with a new list in the DRLB-List
option, excluding the neighbor.
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 the next hello.
Cai, et al. Expires April 13, 2020 [Page 12]
Internet-Draft PIM Designated Router Load Balancing October 2019
5.5. PIM GDR Candidate Operation
When an IGMP/MLD report is received, without this specification, only
the PIM DR will handle the join and potentially run into the issues
described earlier. Using this specification, a hash algorithm is
used by the GDR Candidates to determine which router is going to be
responsible for building forwarding trees on behalf of the host.
If this specification is enabled on an interface, the router MUST
include the DRLB-Cap Hello Option in all PIM Hello messages sent on
that interface. Note that the presence of the DRLB-Cap Option in PIM
Hello does not guarantee that this router would be considered as a
GDR candidate. Once DR election is done, the DRLB-List Hello Option
would be received from the current PIM DR on the link which would
contain a list of GDRs Candidates selected by the PIM DR.
A router only acts as a GDR Candidate if it is included in the GDR
Candidate list of the DRLB-List Hello Option. See next section for
details.
5.6. DRLB-List Hello Option Processing
This section discusses processing of the DRLB-List Hello Option. All
routers MUST ignore the DRLB-List Hello Option if it is received from
a PIM router which is not the DR. The option MUST only be processed
by routers that are announcing the DRLB-Cap Option. Also, the
algorithm announced in the DRLB-Cap Option, MUST be the same as what
was announced by the DR. All GDR Candidates MUST use the Hash Masks
advertised in the Option, even if they differ from those the
candidate was configured with.
A router stores the latest option contents that was announced, if
any, and deletes the previous contents. The router MUST also compare
the new contents with any previous contents, and if there are any
changes, continue processing as below. Note that if the option does
not pass the above checks, the below processing MUST be done as if
the option was not announced.
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
first time, or it is no longer being received or accepted, the option
MUST be processed as below.
1. If the router was not included in the previous GDR list, or there
was no previous GDR list, but it is included in the new GDR list,
the router MUST for each of the groups, or source and group pairs
if the group is in SSM mode, with local receiver interest, run
the hash algorithm to determine which of them it is the GDR for.
Cai, et al. Expires April 13, 2020 [Page 13]
Internet-Draft PIM Designated Router Load Balancing October 2019
If it is not the GDR for a group, or source and group pair if
SSM, no processing is required.
If it is hashed as the GDR, it needs to build a multicast
forwarding tree.
2. If the router was included in the previous GDR list, and still is
included in the new GDR list: The router MUST for each of the
groups, or source and group pairs if the group is in SSM mode,
with local receiver interest, run the hash algorithm to determine
which of them it is the GDR for.
If it was the GDR for a group, or source and group pair if
SSM, and the new hash result chose it as the GDR, then no
processing is required.
If it was the GDR for a group, or source and group pair if
SSM, earlier and now it is no longer the GDR, then it sets the
assert metric preference to maximum (0x7FFFFFFF) and the
assert metric to one less than maximum (0xFFFFFFFE), as
explained in [Section 5.7].
If it was not the GDR for a group, or source and group pair if
SSM, earlier, and the new hash does not make it GDR, then no
processing is required.
If it was not the GDR for an earlier group, or source and
group pair if SSM, and now becomes the GDR, it starts building
multicast forwarding tree for this flow.
3. If the router was included in the previous GDR list, but is not
included in the new GDR list, or there is no new GDR list: The
router MUST for each of the groups, or source and group pairs if
the group is in SSM mode, with local receiver interest do as
follows.
If it was the GDR for a group, or source and group pair if
SSM, it sets the assert metric preference to maximum
(0x7FFFFFFF) and the assert metric to one less than maximum
(0xFFFFFFFE), as explained in [Section 5.7].
If it was not the GDR, then no processing is required.
5.7. PIM Assert Modification
GDR changes may occur due to configuration change, due to GDR
candidates going down, and also new routers coming up and becoming
GDR candidates. This may occur while flows are being forwarded. If
Cai, et al. Expires April 13, 2020 [Page 14]
Internet-Draft PIM Designated Router Load Balancing October 2019
the GDR for an active flow changes, there is likely to be some
disruption, such as packet loss or duplicates. By using asserts,
packet loss is minimized, while allowing a small amount of
duplicates.
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
maximum (0x7FFFFFFF) and the assert metric to one less than maximum
(0xFFFFFFFE). This was also mentioned in the previous section. That
is, whenever it sends or receives an assert for the group, it must
use these values as the metric preference and metric rather than the
values provided by routing. This is similar to what is done for
AssertCancel Messages in [RFC7761], except that the metric value here
is one less.
The rest of this section is just for illustration purposes and not
part of the protocol definition.
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
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
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.
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.
Using the above example, for G1, assume R1 and R3 agree on the new
GDR, which is R3. With the new assert behavior, R1 sets its assert
metric to the near maximum value discussed above. 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 their metric and IP addresses.
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. Shortly after when R2 finds out that it is no
longer the GDR, R2 will change to using the near maximum assert
metric. Next time R2 sends an assert message, it will lose the
Cai, et al. Expires April 13, 2020 [Page 15]
Internet-Draft PIM Designated Router Load Balancing October 2019
assert and stop forwarding. As assert winner, R2 would send periodic
assert messages per [RFC7761].
5.8. Backward Compatibility
In the case of a hybrid Ethernet shared LAN (where some PIM routers
enable the specification defined in this document, and some do not).
o If a router which does not support this specification becomes the
DR on the LAN, then it is the only router acting as a DR, and
there will be no load-balancing.
o If a router which does not support this specification becomes a
non-DR on link, then it acts as non-DR defined in [RFC7761], and
it will not take part in any load-balancing. Load-balancing may
still happen.
6. Manageability Considerations
An administrator needs to consider what the total bandwidth
requirements are and find a set of routers that together has enough
total capacity, while making sure that each of the router can handle
its part, assuming that the traffic is distributed roughly equally
among the routers. Ideally, one should also have enough bandwidth to
handle the case where at least one router fails. Ideally all the
routers should have reachability to the sources, and RPs if
applicable, that is not via the LAN.
Care must be taken when choosing what hash masks to configure. One
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
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
hash based on the RP address rather than the group address for ASM.
The default masks will use the entire group addresses, and source
addresses if SSM, as part of the hash. An administrator may set
other masks that masks out part of the addresses to ensure that
certain flows always get hashed to the same router. How this is
achieved depends on how the group addresses are allocated.
Only the routers announcing the same Hash Algorithm as the DR would
be considered as GDR candidates. Network administrators need to make
sure that the desired set of routers announce the same algorithm.
Migration between different algorithms is not considered in this
document.
Cai, et al. Expires April 13, 2020 [Page 16]
Internet-Draft PIM Designated Router Load Balancing October 2019
7. IANA Considerations
IANA has temporarily assigned type 34 for the PIM DR Load Balancing
Capability (DRLB-Cap) Hello Option, and type 35 for the PIM DR Load
Balancing List (DRLB-List) Hello Option in the PIM-Hello Options
registry. IANA is requested to make these assignments permanent when
this document is published as an RFC. Note that the option names
have changed slightly since the temporary assignments were made.
Also, the length of option 34 is always 4, the registry currently
says it is variable.
This document requests IANA to create a registry called "Designated
Router Load Balancing Hash Algorithms" in the "Protocol Independent
Multicast (PIM)" branch of the registry tree. The registry lists
hash algorithms for use by PIM Designated Router Load Balancing.
7.1. Initial registry
The initial content of the registry should be as follows.
Type Name Reference
------ ---------------------------------------- --------------------
0 Modulo This document
1-255 Unassigned
7.2. Assignment of new hash algorithms
Assignment of new hash algorithms is done according to the "IETF
Review" model, see [RFC8126].
8. Security Considerations
Security of the new DR Load Balancing PIM Hello Options is only
guaranteed by the security of PIM Hello messages, so the security
considerations for PIM Hello messages as described in PIM-SM
[RFC7761] apply here.
If the DR is subverted it could omit or add certain GDRs or announce
an unsupported algorithm. If another router is subverted, it could
be made DR and cause similar issues. While these issues are specific
to this specification, they are not that different from existing
attacks such as subverting a DR and lowering the DR priority, causing
a different router to become the DR.
Cai, et al. Expires April 13, 2020 [Page 17]
Internet-Draft PIM Designated Router Load Balancing October 2019
If a GDR is subverted, it could potentially be made to stop
forwarding all the traffic it is expected to forward. This is also
similar today to if a DR is subverted.
9. Acknowledgement
The authors would like to thank Steve Simlo and Taki Millonis for
helping with the original idea; Alia Atlas, Bill Atwood, Jake
Holland, Bharat Joshi, Anish Kachinthaya, Anvitha Kachinthaya and
Alvaro Retana for reviews and comments; and Toerless Eckert and
Rishabh Parekh for helpful conversation on the document.
10. References
10.1. 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>.
[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>.
[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, <https://www.rfc-editor.org/info/rfc7761>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
10.2. Informative References
[RFC3376] Cain, B., Deering, S., Kouvelas, I., Fenner, B., and A.
Thyagarajan, "Internet Group Management Protocol, Version
3", RFC 3376, DOI 10.17487/RFC3376, October 2002,
<https://www.rfc-editor.org/info/rfc3376>.
[RFC3810] Vida, R., Ed. and L. Costa, Ed., "Multicast Listener
Discovery Version 2 (MLDv2) for IPv6", RFC 3810,
DOI 10.17487/RFC3810, June 2004,
<https://www.rfc-editor.org/info/rfc3810>.
Cai, et al. Expires April 13, 2020 [Page 18]
Internet-Draft PIM Designated Router Load Balancing October 2019
[RFC4541] Christensen, M., Kimball, K., and F. Solensky,
"Considerations for Internet Group Management Protocol
(IGMP) and Multicast Listener Discovery (MLD) Snooping
Switches", RFC 4541, DOI 10.17487/RFC4541, May 2006,
<https://www.rfc-editor.org/info/rfc4541>.
[RFC4607] Holbrook, H. and B. Cain, "Source-Specific Multicast for
IP", RFC 4607, DOI 10.17487/RFC4607, August 2006,
<https://www.rfc-editor.org/info/rfc4607>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
Authors' Addresses
Yiqun Cai
Alibaba Group
Email: yiqun.cai@alibaba-inc.com
Heidi Ou
Alibaba Group
Email: heidi.ou@alibaba-inc.com
Sri Vallepalli
Cisco Systems, Inc.
3625 Cisco Way
San Jose CA 95134
USA
Email: svallepa@cisco.com
Mankamana Mishra
Cisco Systems, Inc.
821 Alder Drive,
Milpitas CA 95035
USA
Email: mankamis@cisco.com
Cai, et al. Expires April 13, 2020 [Page 19]
Internet-Draft PIM Designated Router Load Balancing October 2019
Stig Venaas
Cisco Systems, Inc.
Tasman Drive
San Jose CA 95134
USA
Email: stig@cisco.com
Andy Green
British Telecom
Adastral Park
Ipswich IP5 2RE
United Kingdom
Email: andy.da.green@bt.com
Cai, et al. Expires April 13, 2020 [Page 20]