PIM-SM BFD Failover: Fast Failover in Protocol Independent Multicast - Sparse Mode (PIM-SM) Using Bidirectional Forwarding Detection (BFD) for Multipoint Networks
Using multipoint BFD so PIM-SM routers notice a failed neighbor in under a second instead of 105 seconds.
Mirsky, G. and X. Ji, "Fast Failover in Protocol Independent Multicast - Sparse Mode (PIM-SM) Using Bidirectional Forwarding Detection (BFD) for Multipoint Networks", RFC 9186, DOI 10.17487/RFC9186, January 2022, <https://www.rfc-editor.org/info/rfc9186>.
@misc{rfc9186,
series = {Request for Comments},
number = 9186,
howpublished = {RFC 9186},
publisher = {RFC Editor},
doi = {10.17487/RFC9186},
url = {https://www.rfc-editor.org/info/rfc9186},
author = {G. Mirsky and X. Ji},
title = {{Fast Failover in Protocol Independent Multicast - Sparse Mode (PIM-SM) Using Bidirectional Forwarding Detection (BFD) for Multipoint Networks}},
pagetotal = 7,
year = 2022,
month = jan,
} RFC text
Contents
Internet Engineering Task Force (IETF) G. Mirsky
Request for Comments: 9186 Ericsson
Category: Standards Track X. Ji
ISSN: 2070-1721 ZTE Corporation
January 2022
Fast Failover in Protocol Independent Multicast - Sparse Mode (PIM-SM)
Using Bidirectional Forwarding Detection (BFD) for Multipoint Networks
Abstract
This document specifies how Bidirectional Forwarding Detection (BFD)
for multipoint networks can provide sub-second failover for routers
that participate in Protocol Independent Multicast - Sparse Mode
(PIM-SM). An extension to the PIM Hello message used to bootstrap a
point-to-multipoint BFD session is also defined in this document.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9186.
Copyright Notice
Copyright (c) 2022 IETF Trust and the persons identified as the
document authors. All rights reserved.
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 Revised BSD License text as described in Section 4.e of the
Trust Legal Provisions and are provided without warranty as described
in the Revised BSD License.
Table of Contents
1. Introduction
1.1. Conventions Used in This Document
1.1.1. Terminology
1.1.2. Requirements Language
2. BFD Discriminator PIM Hello Option
2.1. Using P2MP BFD in PIM Router Monitoring
2.2. P2MP BFD in PIM DR Load Balancing
2.3. Multipoint BFD Encapsulation
3. IANA Considerations
4. Security Considerations
5. References
5.1. Normative References
5.2. Informative References
Acknowledgments
Authors' Addresses
1. Introduction
Faster convergence in the control plane minimizes the periods of
traffic loss due to the use of stale routing information, transient
routing loops, and other situations that may negatively affect
service data flow. Faster convergence in the control plane is
beneficial to unicast and multicast routing protocols.
[RFC7761] is the current specification of the Protocol Independent
Multicast - Sparse Mode (PIM-SM) for IPv4 and IPv6 networks. A
conforming implementation of PIM-SM elects a Designated Router (DR)
on each PIM-SM interface. When a group of PIM-SM nodes is connected
to a shared media segment, e.g., Ethernet, the node elected as the DR
acts on behalf of directly connected hosts in the context of the PIM-
SM protocol. Failure of the DR impacts the quality of the multicast
services it provides to directly connected hosts because the default
failure detection interval for PIM-SM routers is 105 seconds.
Bidirectional Forwarding Detection (BFD) [RFC5880] was originally
defined to detect a failure of a point-to-point (P2P) path, single
hop [RFC5881], or multihop [RFC5883]. In some PIM-SM deployments, a
P2P BFD can be used to detect a failure and enable faster failover.
[RFC8562] extends the BFD base specification [RFC5880] for multipoint
and multicast networks, which matches the deployment scenarios for
PIM-SM over a LAN segment. A BFD system in a point-to-multipoint
(P2MP) environment that transmits BFD Control messages using the BFD
Demand mode [RFC5880] creates less BFD state than the Asynchronous
mode. P2MP BFD can enable faster detection of PIM-SM router failure
compared to PIM-SM without BFD and thus minimizes multicast service
disruption. The monitored PIM-SM router acts as the head and other
routers act as tails of a P2MP BFD session. This document defines
the monitoring of a PIM-SM router using P2MP BFD. This document also
defines the extension to PIM-SM [RFC7761] to bootstrap a PIM-SM
router to join in the P2MP BFD session over a shared media segment.
1.1. Conventions Used in This Document
1.1.1. Terminology
This document uses terminology defined in [RFC5880], [RFC8562], and
[RFC7761]. Familiarity with these specifications and the terminology
used is expected.
1.1.2. Requirements Language
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.
2. BFD Discriminator PIM Hello Option
Figure 1 displays the new optional BFD Discriminator PIM Hello Option
to bootstrap a tail of the P2MP BFD session:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OptionType | OptionLength |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| HeadDiscriminator |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: BFD Discriminator PIM Hello Option
where new fields are interpreted as:
OptionType: 39
OptionLength: MUST be set to 4.
HeadDiscriminator: the 4-octet field MUST be included in the BFD
Discriminator PIM-SM Hello Option. The value MUST NOT be zero.
It equals the value of My Discriminator [RFC5880] allocated by the
head.
If the value of the OptionLength field is not equal to 4, the BFD
Discriminator PIM Hello Option is considered malformed, and the
receiver MUST stop processing PIM Hello Options. If the value of the
HeadDiscriminator field equals zero, then the BFD Discriminator PIM
Hello Option MUST be considered invalid, and the receiver MUST ignore
it. The receiver SHOULD log a notification regarding the malformed
or invalid BFD Discriminator Hello Option under the control of a
throttling logging mechanism.
2.1. Using P2MP BFD in PIM Router Monitoring
If the head is no longer serving the function that prompted it to be
monitored, then it MUST cease including the BFD Discriminator PIM
Hello Option in its PIM Hello message, and it SHOULD shut down the
BFD session following the procedures described in [RFC8562],
Section 5.9.
The head MUST create a BFD session of type MultipointHead [RFC8562].
Note that any PIM-SM router, regardless of its role, MAY become a
head of a P2MP BFD session. To control the volume of BFD Control
traffic on a shared media segment, an operator should carefully
select PIM-SM routers configured as a head of a P2MP BFD session.
The head MUST include the BFD Discriminator PIM Hello Option in its
PIM Hello messages.
A PIM-SM router that is configured to monitor the head by using P2MP
BFD is referred to throughout this document as a "tail". When such a
tail receives a PIM Hello packet with the BFD Discriminator PIM Hello
Option, the tail MAY create a P2MP BFD session of type
MultipointTail, as defined in [RFC8562].
The node that includes the BFD Discriminator PIM Hello Option
transmits BFD Control packets periodically. For the tail to
correctly demultiplex BFD [RFC8562], the source address and My
Discriminator of the BFD packets MUST be the same as the source
address and the HeadDiscriminator, respectively, of the PIM Hello
message. If that is not the case, the tail BFD node would not be
able to monitor the state of the PIM-SM node -- that is, the head of
the P2MP BFD session -- though the regular PIM-SM mechanisms remain
fully operational.
If the tail detects a MultipointHead failure [RFC8562], it MUST
delete the corresponding neighbor state and follow procedures defined
in [RFC7761] for the DR and additional neighbor state deletion after
the neighbor timeout expires.
If the head ceases to include the BFD Discriminator PIM Hello Option
in its PIM Hello message, the tail SHOULD close the corresponding
MultipointTail BFD session without affecting the PIM state in any
way. Thus, the tail stops using BFD to monitor the head and reverts
to the procedures defined in [RFC7761].
2.2. P2MP BFD in PIM DR Load Balancing
[RFC8775] specifies the PIM Designated Router Load-Balancing (DRLB)
functionality. Any PIM router that advertises the DR Load-Balancing
Capability (DRLB-Cap) Hello Option can become the head of a P2MP BFD
session, as specified in Section 2.1. The head router
administratively sets the bfd.SessionState to Up in the
MultipointHead session [RFC8562] only if it is a Group Designated
Router (GDR) Candidate, as specified in Sections 5.5 and 5.6 of
[RFC8775]. If the router is no longer the GDR, then it MUST shut
down following the procedures described in [RFC8562], Section 5.9.
For each GDR Candidate that includes the BFD Discriminator Option in
its PIM Hello, the PIM DR MUST create a MultipointTail session
[RFC8562]. PIM DR demultiplexes BFD sessions based on the value of
the My Discriminator field and the source IP address. If PIM DR
detects a failure of one of the sessions, it MUST remove that router
from the GDR Candidate list and immediately transmit a new DRLB-List
option.
2.3. Multipoint BFD Encapsulation
The MultipointHead of a P2MP BFD session when transmitting BFD
Control packets:
* MUST set the TTL or Hop Limit value to 255 ([RFC5881], Section 5).
Similarly, all received BFD Control packets that are demultiplexed
to the session MUST be discarded if the received TTL or Hop Limit
is not equal to 255, and
* MUST use the group address ALL-PIM-ROUTERS ("224.0.0.13" for IPv4
and "ff02::d" for IPv6) as the destination IP address.
3. IANA Considerations
IANA has allocated a new OptionType value in the "PIM-Hello Options"
registry according to Table 1:
+=======+========+==========================+===========+
| Value | Length | Name | Reference |
+=======+========+==========================+===========+
| 39 | 4 | BFD Discriminator Option | RFC 9186 |
+-------+--------+--------------------------+-----------+
Table 1: BFD Discriminator Option Type
4. Security Considerations
This document defines a way to accelerate detection of a failure that
affects PIM functionality by using BFD. The operation of either
protocol is not changed.
The security considerations discussed in [RFC5880], [RFC5881],
[RFC7761], [RFC8562], and [RFC8775] apply to this document.
5. References
5.1. 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>.
[RFC5880] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
(BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010,
<https://www.rfc-editor.org/info/rfc5880>.
[RFC5881] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
(BFD) for IPv4 and IPv6 (Single Hop)", RFC 5881,
DOI 10.17487/RFC5881, June 2010,
<https://www.rfc-editor.org/info/rfc5881>.
[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>.
[RFC8562] Katz, D., Ward, D., Pallagatti, S., Ed., and G. Mirsky,
Ed., "Bidirectional Forwarding Detection (BFD) for
Multipoint Networks", RFC 8562, DOI 10.17487/RFC8562,
April 2019, <https://www.rfc-editor.org/info/rfc8562>.
[RFC8775] Cai, Y., Ou, H., Vallepalli, S., Mishra, M., Venaas, S.,
and A. Green, "PIM Designated Router Load Balancing",
RFC 8775, DOI 10.17487/RFC8775, April 2020,
<https://www.rfc-editor.org/info/rfc8775>.
5.2. Informative References
[RFC5883] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
(BFD) for Multihop Paths", RFC 5883, DOI 10.17487/RFC5883,
June 2010, <https://www.rfc-editor.org/info/rfc5883>.
Acknowledgments
The authors cannot say enough to express their appreciation of the
comments and suggestions that were received from Stig Venaas. The
authors also greatly appreciate the comments and suggestions by
Alvaro Retana that improved the clarity of this document.
Authors' Addresses
Greg Mirsky
Ericsson
Email: gregimirsky@gmail.com
Xiaoli Ji
ZTE Corporation
Yuhuatai District
No. 50 Software Avenue
Nanjing
China
Email: ji.xiaoli@zte.com.cn
AI ELI5
On this page
§1Why PIM-SM Needs Faster Failure Detection
Faster control plane convergence reduces traffic loss caused by stale routing information and transient loops. In PIM-SM, one router on each shared segment (such as Ethernet) is elected as the Designated Router (DR) and acts for the directly connected hosts. If the DR fails, multicast service for those hosts suffers, because the default failure detection interval for PIM-SM routers is 105 seconds.
Bidirectional Forwarding Detection (BFD) was first defined for point-to-point paths. RFC 8562 extends BFD to multipoint and multicast networks, which matches PIM-SM on a LAN segment. In such a point-to-multipoint (P2MP) session the monitored router is the head and the other routers are tails. Using BFD Demand mode creates less BFD state than Asynchronous mode. This RFC defines monitoring of a PIM-SM router with P2MP BFD and a PIM extension to bootstrap the session.
Normally routers wait a very long time, 105 seconds, before they admit a neighbor is gone. That is like waiting almost two minutes before noticing your friend left the room.
§2The BFD Discriminator PIM Hello Option
The new optional PIM Hello option tells neighbors which BFD session to join. It has three fields: OptionType (39), OptionLength (MUST be 4) and HeadDiscriminator, a 4-octet field that MUST be included, MUST NOT be zero and equals the My Discriminator value the head allocated for its BFD session.
If OptionLength is not 4, the option is malformed and the receiver MUST stop processing PIM Hello options. If the discriminator is zero, the option is invalid and the receiver MUST ignore it. In both cases the receiver SHOULD log a notification, under the control of a throttling logging mechanism.
It is a small note in the router's regular greeting that says 'listen for my heartbeat, my code number is this'. A code number of zero is not allowed.
| Field | Name | Size | Value | Description |
|---|---|---|---|---|
OPTION-TYPE | OptionType | 2 bytes | 39 | Set to 39. |
OPTION-LENGTH | OptionLength | 2 bytes | 4 | MUST be 4. |
HEAD-DISCRIMINATOR | HeadDiscriminator | 4 bytes | Equals the My Discriminator value allocated by the head; MUST NOT be zero. |
§3Monitoring a PIM Router with Multipoint BFD
The head MUST create a BFD session of type MultipointHead and MUST include the BFD Discriminator option in its PIM Hellos. Any PIM-SM router, whatever its role, MAY become a head, but operators should choose heads carefully to control the volume of BFD Control traffic on the segment. A router that monitors the head is a tail. When a tail sees the option in a Hello, it MAY create a MultipointTail session.
For the tail to demultiplex correctly, the source address and My Discriminator of the BFD packets MUST match the source address and HeadDiscriminator of the PIM Hello. If they do not, the tail cannot monitor the head, though regular PIM-SM mechanisms stay fully operational.
If the tail detects a MultipointHead failure, it MUST delete the corresponding neighbor state and follow the RFC 7761 procedures for the DR and for neighbor state deletion after the neighbor timeout expires. If the head stops serving the function that prompted monitoring, it MUST stop including the option and SHOULD shut down the BFD session following RFC 8562. When the option disappears, the tail SHOULD close its MultipointTail session without affecting PIM state, reverting to RFC 7761 procedures.
The watched router keeps sending quick heartbeats. Its neighbors listen, and if the heartbeat stops they immediately treat it as gone instead of waiting a long time.
- PIM Hello with BFD Discriminator optionTail MAY create a MultipointTail session.
- BFD Control packets (periodic)Source address and My Discriminator must match the Hello.
§4DR Load Balancing and BFD Encapsulation
With PIM Designated Router Load Balancing (RFC 8775), any router advertising the DRLB-Cap option can become a head. It sets the session state to Up only if it is a Group Designated Router (GDR) Candidate. For each GDR Candidate that includes the BFD Discriminator option, the PIM DR MUST create a MultipointTail session, demultiplexing by My Discriminator and source IP address. If the DR detects a failure, it MUST remove that router from the GDR Candidate list and immediately send a new DRLB-List option.
When the head sends BFD Control packets, it MUST set the TTL or Hop Limit to 255, and received packets for the session MUST be discarded if the TTL or Hop Limit is not 255. It MUST use the group address ALL-PIM-ROUTERS as the destination, which is 224.0.0.13 for IPv4 and ff02::d for IPv6.
If several helpers share the work, the boss drops any helper whose heartbeat stops and tells the others at once. Heartbeats only count if they come from right next door.
§5IANA and Security Considerations
IANA has allocated OptionType value 39, length 4, named BFD Discriminator Option, in the PIM-Hello Options registry.
The document only accelerates detection of a failure that affects PIM functionality, and the operation of either protocol is not changed. The security considerations of RFC 5880, RFC 5881, RFC 7761, RFC 8562 and RFC 8775 apply.
This only makes routers notice problems faster. It does not change how either protocol works, so the same safety rules still apply.