PIM Null-Register pack: PIM Null-Register Packing
Bundle many PIM Null-Register and Register-Stop records into a single message to cut router chatter.
Kamath, V., Chokkanathapuram Sundaram, R., Banthia, R., and A. Gopal, "PIM Null-Register Packing", RFC 9465, DOI 10.17487/RFC9465, September 2023, <https://www.rfc-editor.org/info/rfc9465>.
@misc{rfc9465,
series = {Request for Comments},
number = 9465,
howpublished = {RFC 9465},
publisher = {RFC Editor},
doi = {10.17487/RFC9465},
url = {https://www.rfc-editor.org/info/rfc9465},
author = {V. Kamath and R. Chokkanathapuram Sundaram and R. Banthia and A. Gopal},
title = {{PIM Null-Register Packing}},
pagetotal = 9,
year = 2023,
month = sep,
} RFC text
Contents
Internet Engineering Task Force (IETF) V. Kamath
Request for Comments: 9465 VMware
Category: Standards Track R. Chokkanathapuram Sundaram
ISSN: 2070-1721 Cisco Systems, Inc.
R. Banthia
Apstra
A. Gopal
Cisco Systems, Inc.
September 2023
PIM Null-Register Packing
Abstract
In PIM Sparse Mode (PIM-SM) networks, PIM Null-Register messages are
sent by the Designated Router (DR) to the Rendezvous Point (RP) to
signal the presence of multicast sources in the network. There are
periodic PIM Null-Registers sent from the DR to the RP to keep the
state alive at the RP as long as the source is active. The PIM Null-
Register message carries information about a single multicast source
and group.
This document defines a standard to send information about multiple
multicast sources and groups in a single PIM message. This document
refers to the new messages as the "PIM Packed Null-Register message"
and "PIM Packed Register-Stop message".
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/rfc9465.
Copyright Notice
Copyright (c) 2023 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
1.2. Terminology
2. Packing Capability
3. PIM Packed Null-Register Message Format
4. PIM Packed Register-Stop Message Format
5. Protocol Operation
6. Operational Considerations
6.1. PIM Anycast RP Considerations
6.2. Interoperability between Different Versions
6.3. Disabling PIM Packed Message Support at RP and/or DR
7. Fragmentation Considerations
8. Security Considerations
9. IANA Considerations
10. Normative References
Acknowledgments
Authors' Addresses
1. Introduction
The DR periodically sends PIM Null-Registers to keep the state of
existing multicast sources active on the RP. As the number of
multicast sources increases, the number of PIM Null-Register messages
that are sent also increases. This results in more PIM packet
processing at the RP and the DR.
This document specifies a method to efficiently pack the content of
multiple PIM Null-Register and Register-Stop messages [RFC7761] into
a single message.
The document also discusses interoperability between PIM routers that
support PIM Packed Null-Registers and PIM Packed Register-Stops and
PIM routers that do not.
1.1. Conventions Used in This Document
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.
1.2. Terminology
RP: Rendezvous Point
DR: Designated Router
MSDP: Multicast Source Discovery Protocol
PIM-SM: PIM Sparse Mode
2. Packing Capability
The RP indicates its ability to receive PIM Packed Null-Register
messages (Section 3) and send PIM Packed Register-Stop messages
(Section 4) with a Packing Capability bit (P-bit) in the PIM
Register-Stop message. The P-bit is allocated in Section 9.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|PIM Ver| Type |7 6 5 4 3 2 1|P| Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Group Address (Encoded-Group format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Address (Encoded-Unicast format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: PIM Register-Stop Message with Packing Capability Option
The Group Address and Source Address fields in the PIM Register-Stop
message are defined in Section 4.9.4 of [RFC7761]. The common header
is defined in [RFC9436].
Packing Capability bit (P-bit; flag bit 0): When set, it indicates
the ability of the RP to receive PIM Packed Null-Register messages
and send PIM Packed Register-Stop messages.
3. PIM Packed Null-Register Message Format
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|PIM Ver| Type |Subtype| FB | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Group Address[1] (Encoded-Group format) |
| Source Address[1] (Encoded-Unicast format) |
. .
. .
. .
. .
. Group Address[N] .
| Source Address[N] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: PIM Packed Null-Register Message Format
The Group Address and Source Address fields in the PIM Packed Null-
Register message are defined in Section 4.9.4 of [RFC7761]. The
common header is defined in [RFC9436].
Type, Subtype: PIM Packed Null-Register (13.0).
N: The total number of records; a record consists of a Group Address
and Source Address pair.
After parsing the PIM common header, individual records are then
parsed one by one until the end of the PIM Packed Null-Register
message. This length is inferred from the IP layer.
Sending or receiving a PIM Packed Null-Register message has the
equivalent effect of sending or receiving an individual Null-Register
message for each record represented in the PIM Packed Null-Register
message.
4. PIM Packed Register-Stop Message Format
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|PIM Ver| Type |Subtype| FB | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Group Address[1] (Encoded-Group format) |
| Source Address[1] (Encoded-Unicast format) |
. .
. .
. .
. .
. Group Address[N] .
| Source Address[N] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: PIM Packed Register-Stop Message Format
The Group Address and Source Address fields in the PIM Packed
Register-Stop message are defined in Section 4.9.4 of [RFC7761]. The
common header is defined in [RFC9436].
Type, Subtype: PIM Packed Register-Stop (13.1).
N: The total number of records; a record consists of a Group Address
and Source Address pair.
After parsing the PIM common header, individual records are then
parsed one by one until the end of the PIM Packed Register-Stop
message. This length is inferred from the IP layer.
Sending or receiving a PIM Packed Register-Stop message has the
equivalent effect of sending or receiving an individual Null-Register
message for each record represented in the PIM Packed Register-Stop.
5. Protocol Operation
As specified in [RFC7761], the DR sends PIM Register messages towards
the RP when a new source is detected.
When this feature is enabled/configured, an RP supporting this
specification MUST set the P-bit (flag bit 0) in all Register-Stop
messages.
When a Register-Stop message with the P-bit set is received, the DR
SHOULD send PIM Packed Null-Register messages (Section 3) to the RP
instead of multiple Register messages with the N-bit set [RFC7761].
The DR MAY use a mixture of PIM Packed Null-Register messages and
Register messages. The decision is up to the implementation and out
of the scope of this document. However, it is RECOMMENDED to stick
to the PIM Packed Null-Register and PIM Packed Register-Stop formats
as long as the RP and DR have the feature enabled.
After receiving a PIM Packed Null-Register message, the RP SHOULD
start sending PIM Packed Register-Stop messages (Section 4) to the
corresponding DR instead of individual Register-Stop messages. The
RP MAY use a mixture of PIM Packed Register-Stop messages and
individual Register-Stop messages. The decision is up to the
implementation and out of the scope of this document. However, it is
RECOMMENDED to stick to the PIM Packed Null-Register and PIM Packed
Register-Stop formats as long as the RP and DR have the feature
enabled.
6. Operational Considerations
6.1. PIM Anycast RP Considerations
The PIM Packed Null-Register packet format should be enabled only if
it is supported by all the routers in the Anycast-RP set [RFC4610].
This consideration applies to PIM Anycast RP with Multicast Source
Discovery Protocol (MSDP) [RFC3446] as well.
6.2. Interoperability between Different Versions
A router (DR) can decide to use the PIM Packed Null-Register message
format based on the Packing Capability received from the RP as part
of the PIM Register-Stop. This ensures compatibility with routers
that do not support processing of the new packet format. The Packing
Capability information MUST be indicated by the RP via the PIM
Register-Stop message sent to the DR. Thus, a DR will switch to the
new packet format only when it learns that the RP is capable of
handling the PIM Packed Null-Register messages.
Conversely, a DR that does not support the packed format can continue
generating the PIM Null-Register as defined in Section 4.4 of
[RFC7761].
6.3. Disabling PIM Packed Message Support at RP and/or DR
Consider a PIM RP router that supports PIM Packed Null-Registers and
PIM Packed Register-Stops. In scenarios where this router no longer
supports this feature, for example, in case of a software downgrade,
it will not send a PIM Register-Stop message to the DR in response to
a PIM Packed Null-Register message.
When the DR switches to Data Registers from Null-Registers, it MUST
start a Packed_Register_Probe_Time timer. If no PIM Packed Register-
Stop or Register-Stop with the P-bit set is received within
Packed_Register_Probe_Time seconds, the DR can decide that the RP no
longer supports PIM Packed Null-Registers. The
Packed_Register_Probe_Time timer is configurable; its default value
is 60 seconds.
When Packed_Register_Probe_Time expires, the DR MAY also send an
unpacked PIM Null-Register and check the PIM Register-Stop to see if
the P-bit is set or not. If it is not set, then the DR will continue
sending unpacked PIM Null-Register messages.
In case the network manager disables the Packing Capability at the RP
(or in other words, disables the feature from the RP), the router
MUST NOT advertise the Packing Capability. However, an
implementation MAY choose to still parse any packed registers if they
are received. This may be particularly useful in the transitional
period after the network manager disables it.
7. Fragmentation Considerations
As explained in Section 4.4.1 of [RFC7761], the DR may perform Path
MTU Discovery to the RP before sending PIM Packed Null-Register
messages. Similarly, the RP may perform Path MTU Discovery to the DR
before sending PIM Packed Register-Stop messages. In both cases, the
number of records in a message should be limited such that it can fit
within the Path MTU.
8. Security Considerations
The Security Considerations in [RFC7761] apply to this document. In
particular, the effect of forging a PIM Packed Null-Register or
Register-Stop message would be amplified to all the records included
instead of just one.
By forging a PIM Register-Stop message and setting the P-bit, an
attacker can trigger the use of PIM Packed Null-Register messages by
a DR, thus creating unnecessary churn in the network.
9. IANA Considerations
IANA has assigned a Packing Capability bit (0) in the PIM Register-
Stop common header in the "PIM Message Types" registry.
IANA has assigned a PIM message type (13.0) for PIM Packed Null-
Register in the "PIM Message Types" registry. Flag bits 0-3 for this
message type are "Unassigned".
IANA has assigned a PIM message type (13.1) for PIM Packed Register-
Stop in the "PIM Message Types" registry. The flag bits 0-3 for this
message type are "Unassigned".
10. 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>.
[RFC3446] Kim, D., Meyer, D., Kilmer, H., and D. Farinacci, "Anycast
Rendevous Point (RP) mechanism using Protocol Independent
Multicast (PIM) and Multicast Source Discovery Protocol
(MSDP)", RFC 3446, DOI 10.17487/RFC3446, January 2003,
<https://www.rfc-editor.org/info/rfc3446>.
[RFC4610] Farinacci, D. and Y. Cai, "Anycast-RP Using Protocol
Independent Multicast (PIM)", RFC 4610,
DOI 10.17487/RFC4610, August 2006,
<https://www.rfc-editor.org/info/rfc4610>.
[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>.
[RFC9436] Venaas, S. and A. Retana, "PIM Message Type Space
Extension and Reserved Bits", RFC 9436,
DOI 10.17487/RFC9436, August 2023,
<https://www.rfc-editor.org/info/rfc9436>.
Acknowledgments
The authors would like to thank Stig Venaas, Alvaro Retana, Anish
Peter, Zheng Zhang, and Umesh Dudani for their helpful comments on
the document.
Authors' Addresses
Vikas Ramesh Kamath
VMware
3401 Hillview Ave
Palo Alto, CA 94304
United States of America
Email: vkamath@vmware.com
Ramakrishnan Chokkanathapuram Sundaram
Cisco Systems, Inc.
Tasman Drive
San Jose, CA 95134
United States of America
Email: ramaksun@cisco.com
Raunak Banthia
Apstra
Suite 200
333 Middlefield Rd
Menlo Park, CA 94025
United States of America
Email: rbanthia@apstra.com
Ananya Gopal
Cisco Systems, Inc.
Tasman Drive
San Jose, CA 95134
United States of America
Email: ananygop@cisco.com
AI ELI5
On this page
§1The Problem with One Message per Source
In PIM Sparse Mode (PIM-SM), the Designated Router (DR) periodically sends Null-Register messages to the Rendezvous Point (RP) to keep the state of existing multicast sources alive. Each message describes a single source and group. As the number of sources grows, so does the number of messages, which means more PIM packet processing at both the RP and the DR.
RFC 9465 defines a way to pack the content of multiple Null-Register and Register-Stop messages (from RFC 7761) into a single message. It also covers interoperability with routers that do not support the packed formats.
Every kid on the bus has to say "I'm still here" to the teacher, one at a time. That takes forever when the bus is full.
§2Advertising the Packing Capability
The RP signals that it can receive PIM Packed Null-Register messages and send PIM Packed Register-Stop messages by setting a Packing Capability bit (P-bit, flag bit 0) in the PIM Register-Stop message. The Register-Stop uses the common header defined in RFC 9436, followed by the Group Address (Encoded-Group format) and Source Address (Encoded-Unicast format) from RFC 7761.
When the feature is enabled, an RP supporting this specification MUST set the P-bit in all Register-Stop messages.
The teacher puts up a sign saying "I can read group lists now." Only then do the kids switch to the list.
§3The Packed Message Formats
Two message types are defined. The PIM Packed Null-Register is type and subtype 13.0, and the PIM Packed Register-Stop is 13.1. Both start with the PIM common header (version, type, subtype, flag bits and checksum) followed by N records, where each record is a Group Address and Source Address pair.
After the common header, records are parsed one by one until the end of the message, and the length is inferred from the IP layer. Sending or receiving a packed message has the same effect as sending or receiving an individual message for each record in it.
The packed message is a list. Each line says which group and which sender, and the receiver reads down the list until it runs out of lines.
| Field | Name | Size | Value | Description |
|---|---|---|---|---|
VER | PIM Ver | 4 bits | PIM version. | |
TYPE | Type | 4 bits | Message type; 13 for packed messages. | |
SUBTYPE | Subtype | 4 bits | 0 for Packed Null-Register. | |
FB | FB | 4 bits | Flag bits. | |
CHECKSUM | Checksum | 2 bytes | Checksum. | |
GROUP | Group Address[1] | 4 bytes | Encoded-Group format; records repeat up to N. | |
SOURCE | Source Address[1] | 4 bytes | Encoded-Unicast format. |
§4How the DR and RP Switch Over
A DR still sends Register messages when it detects a new source. When it receives a Register-Stop with the P-bit set, it SHOULD send PIM Packed Null-Register messages instead of multiple Register messages with the N-bit set. It MAY mix packed Null-Registers and ordinary Registers, but it is RECOMMENDED to stick to the packed formats while both ends have the feature enabled.
After receiving a Packed Null-Register, the RP SHOULD start sending Packed Register-Stop messages to that DR instead of individual Register-Stops, with the same freedom to mix and the same recommendation.
A DR that learns of packing capability only from the RP's Register-Stop will never use the new format toward an RP that cannot handle it. A DR that does not support packing keeps sending ordinary Null-Registers as defined in Section 4.4 of RFC 7761.
The kids only switch to the list once the teacher's sign is up. Kids who never learned the list keep calling out one at a time, and that still works.
- Register-Stop with P-bit setThe RP advertises Packing Capability.
- Packed Null-Register (13.0)Many group and source records in one message.
- Packed Register-Stop (13.1)Replaces individual Register-Stops.
§5Operational Considerations
In a PIM Anycast RP setup (RFC 4610, and the MSDP variant in RFC 3446), the packed format should be enabled only if every router in the Anycast-RP set supports it.
If an RP stops supporting the feature, for example after a software downgrade, it will not answer a Packed Null-Register with a Register-Stop. When the DR switches to Data Registers from Null-Registers, it MUST start a Packed_Register_Probe_Time timer, default 60 seconds and configurable. If no Packed Register-Stop or Register-Stop with the P-bit arrives in that time, the DR can conclude the RP no longer supports packing, and MAY send an unpacked Null-Register to check the P-bit again. If the network manager disables the capability at the RP, the router MUST NOT advertise it, though it MAY still parse packed registers it receives.
For fragmentation, the DR or RP may perform Path MTU Discovery as described in Section 4.4.1 of RFC 7761, and the number of records per message should be limited so it fits within the Path MTU.
If the teacher stops reading lists, the kids notice nobody answers after a minute and go back to calling out one by one. Also, do not write a list too long to fit on one page.
§6Security and IANA
The security considerations of RFC 7761 apply. The effect of forging a packed message is amplified to all the records in it instead of just one. An attacker who forges a Register-Stop with the P-bit set can trigger a DR to use Packed Null-Registers, creating unnecessary churn in the network.
IANA assigned the Packing Capability bit (0) in the PIM Register-Stop common header, message type 13.0 for Packed Null-Register and 13.1 for Packed Register-Stop in the "PIM Message Types" registry. Flag bits 0-3 of the two new types are "Unassigned".
A forged list could fool the teacher about many kids at once, so the list makes a trick bigger, not smaller.