RFC 9436 PROPOSED STANDARD MULTICAST · PIM · IANA
PIM Type Extension: PIM Message Type Space Extension and Reserved Bits
Gives the spare bits in the PIM message header a job and makes room for 48 new PIM message types.
RFC EDITOR
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>.
BIBTEX
@misc{rfc9436,
series = {Request for Comments},
number = 9436,
howpublished = {RFC 9436},
publisher = {RFC Editor},
doi = {10.17487/RFC9436},
url = {https://www.rfc-editor.org/info/rfc9436},
author = {S. Venaas and A. Retana},
title = {{PIM Message Type Space Extension and Reserved Bits}},
pagetotal = 7,
year = 2023,
month = aug,
} RFC text
Contents
Internet Engineering Task Force (IETF) S. Venaas
Request for Comments: 9436 Cisco Systems, Inc.
Obsoletes: 8736 A. Retana
Updates: 3973, 5015, 5059, 6754, 7761, Futurewei Technologies, Inc.
8364 August 2023
Category: Standards Track
ISSN: 2070-1721
PIM Message Type Space Extension and Reserved Bits
Abstract
The PIM version 2 messages share a common message header format. The
common header definition contains eight reserved bits. This document
specifies how these bits may be used by individual message types and
extends the PIM type space.
This document updates RFCs 7761 and 3973 by defining the use of the
Reserved field in the PIM common header. This document further
updates RFCs 7761 and 3973, along with RFCs 5015, 5059, 6754, and
8364, by specifying the use of the bits for each PIM message.
This document obsoletes RFC 8736.
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/rfc9436.
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
2. Conventions Used in This Document
3. PIM Header Common Format
4. Flag Bit Definitions
4.1. Flag Bits for Type 4 (Bootstrap)
4.2. Flag Bits for Type 10 (DF Election)
4.3. Flag Bits for Type 12 (PIM Flooding Mechanism)
4.4. Flag Bits for Types 13, 14, and 15 (Type Space Extension)
5. PIM Type Space Extension
6. Security Considerations
7. IANA Considerations
8. References
8.1. Normative References
8.2. Informative References
Authors' Addresses
1. Introduction
The PIM version 2 messages share a common message header format
defined in the PIM Sparse Mode specification [RFC7761]. The common
header definition contains eight reserved bits. While all message
types use this common header, there is no document formally
specifying that these bits are to be used per message type.
This document updates the definition of the Reserved field and refers
to it as the "Flag Bits field". It specifies that the flag bits are
to be separately used on a per-message-type basis. It updates the
"PIM Message Types" registry to indicate the per-message-type usage.
This document updates [RFC7761] and [RFC3973] by defining the use of
the Reserved field in the PIM common header. This document further
updates [RFC7761] and [RFC3973], along with [RFC5015], [RFC5059],
[RFC6754], and [RFC8364], by specifying the use of the bits for each
PIM message.
The originally defined PIM message types were in the range from 0 to
15. Message type 15 had been reserved by [RFC6166] for type space
extension. In Section 5, this document specifies the use of the Flag
Bits field for message types 13, 14, and 15 in order to extend the
PIM type space. The type space extension in [RFC6166] was made
obsolete by [RFC8736]. This document obsoletes [RFC8736].
2. 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.
3. PIM Header Common Format
The common PIM header is defined in Section 4.9 of [RFC7761]. This
document updates the definition of the Reserved field and refers to
it as the "Flag Bits field". The updated common header format is as
below.
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 | Flag Bits | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Updated Common Header
The Flag Bits field is defined in Section 4. All other fields remain
unchanged.
4. Flag Bit Definitions
Unless otherwise specified, all the flag bits for each PIM type are
Unassigned [RFC8126]. They MUST be set to zero on transmission, and
they MUST be ignored upon receipt. The specification of a new PIM
type MUST indicate whether the bits should be treated differently.
When defining flag bits, it is helpful to have a well-defined way of
referring to a particular bit. The most significant of the flag
bits, the bit immediately following the Type field, is referred to as
bit 7. The least significant, the bit right in front of the Checksum
field, is referred to as bit 0. This is shown in the diagram below.
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 0| Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Flag Bits
4.1. Flag Bits for Type 4 (Bootstrap)
PIM message type 4 (Bootstrap) [RFC5059] defines flag bit 7 as No-
Forward. The usage of the bit is defined in that document. The
remaining flag bits are unassigned.
4.2. Flag Bits for Type 10 (DF Election)
PIM message type 10 (DF Election) [RFC5015] specifies that the four
most significant flag bits (bits 4-7) are to be used as a subtype.
The usage of those bits is defined in that document. The remaining
flag bits are unassigned.
4.3. Flag Bits for Type 12 (PIM Flooding Mechanism)
PIM message type 12 (PIM Flooding Mechanism) [RFC8364] defines flag
bit 7 as No-Forward. The usage of the bit is defined in that
document. The remaining flag bits are unassigned.
4.4. Flag Bits for Types 13, 14, and 15 (Type Space Extension)
These types and the corresponding flag bits are defined in Section 5.
5. PIM Type Space Extension
This document extends types 13, 14, and 15 such that each becomes 16
new types, resulting in 48 types available for future PIM extensions.
This extension is achieved by defining a Subtype field (see Figure 3)
using the four most significant flag bits (bits 4-7). The notation
type.subtype is used to reference the new extended types. The
remaining four flag bits (bits 0-3, abbreviated as FB below) are to
be defined by each extended type.
Each of the extended types is represented by the eight bits resulting
from the concatenation of the Type and Subtype fields. No
relationship is expected or implied between extended type messages
with a common Type field.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: Subtypes
6. Security Considerations
This document clarifies the use of the flag bits in the common PIM
header, and it extends the PIM type space. As such, there is no
impact on security or changes to the considerations in [RFC7761] and
[RFC3973].
7. IANA Considerations
This document updates the "PIM Message Types" registry to indicate
which flag bits are defined for use by each of the PIM message types
and changes their registration status to Unassigned except where the
bits have already been specified, as shown in Table 1. The
registration policy remains IETF Review [RFC8126]. Assignments to
this registry MUST define any non-default usage (see Section 4) of
the flag bits in addition to the type.
Extended type 15.15 is Reserved [RFC8126] for future extensions.
Because this document obsoletes [RFC8736], IANA has changed the
references to [RFC8736] in the registry to point to this document
instead.
The updated "PIM Message Types" registry is shown below.
+============+===============+=================+===========+
| Type | Name | Flag Bits | Reference |
+============+===============+=================+===========+
| 0 | Hello | 0-7: Unassigned | [RFC3973] |
| | | | [RFC7761] |
+------------+---------------+-----------------+-----------+
| 1 | Register | 0-7: Unassigned | [RFC7761] |
+------------+---------------+-----------------+-----------+
| 2 | Register Stop | 0-7: Unassigned | [RFC7761] |
+------------+---------------+-----------------+-----------+
| 3 | Join/Prune | 0-7: Unassigned | [RFC3973] |
| | | | [RFC7761] |
+------------+---------------+-----------------+-----------+
| 4 | Bootstrap | 0-6: Unassigned | [RFC5059] |
| | | | [RFC7761] |
| | +-----------------+-----------+
| | | 7: No-Forward | [RFC5059] |
+------------+---------------+-----------------+-----------+
| 5 | Assert | 0-7: Unassigned | [RFC3973] |
| | | | [RFC7761] |
+------------+---------------+-----------------+-----------+
| 6 | Graft | 0-7: Unassigned | [RFC3973] |
+------------+---------------+-----------------+-----------+
| 7 | Graft-Ack | 0-7: Unassigned | [RFC3973] |
+------------+---------------+-----------------+-----------+
| 8 | Candidate RP | 0-7: Unassigned | [RFC7761] |
| | Advertisement | | |
+------------+---------------+-----------------+-----------+
| 9 | State Refresh | 0-7: Unassigned | [RFC3973] |
+------------+---------------+-----------------+-----------+
| 10 | DF Election | 0-3: Unassigned | [RFC5015] |
| | +-----------------+-----------+
| | | 4-7: Subtype | [RFC5015] |
+------------+---------------+-----------------+-----------+
| 11 | ECMP Redirect | 0-7: Unassigned | [RFC6754] |
+------------+---------------+-----------------+-----------+
| 12 | PIM Flooding | 0-6: Unassigned | [RFC8364] |
| | Mechanism +-----------------+-----------+
| | | 7: No-Forward | [RFC8364] |
+------------+---------------+-----------------+-----------+
| 13.0-15.14 | Unassigned | 0-3: Unassigned | |
+------------+---------------+-----------------+-----------+
| 15.15 | Reserved | 0-3: Reserved | RFC 9436 |
+------------+---------------+-----------------+-----------+
Table 1: Updated PIM Message Types Registry
The unassigned types above, as explained in Section 5, use the
extended type notation of type.subtype. Each extended type only has
4 flag bits available. New extended message types should be assigned
consecutively, starting with 13.0, then 13.1, etc.
8. References
8.1. 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>.
[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>.
[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>.
[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>.
8.2. Informative References
[RFC3973] Adams, A., Nicholas, J., and W. Siadak, "Protocol
Independent Multicast - Dense Mode (PIM-DM): Protocol
Specification (Revised)", RFC 3973, DOI 10.17487/RFC3973,
January 2005, <https://www.rfc-editor.org/info/rfc3973>.
[RFC5015] Handley, M., Kouvelas, I., Speakman, T., and L. Vicisano,
"Bidirectional Protocol Independent Multicast (BIDIR-
PIM)", RFC 5015, DOI 10.17487/RFC5015, October 2007,
<https://www.rfc-editor.org/info/rfc5015>.
[RFC5059] Bhaskar, N., Gall, A., Lingard, J., and S. Venaas,
"Bootstrap Router (BSR) Mechanism for Protocol Independent
Multicast (PIM)", RFC 5059, DOI 10.17487/RFC5059, January
2008, <https://www.rfc-editor.org/info/rfc5059>.
[RFC6166] Venaas, S., "A Registry for PIM Message Types", RFC 6166,
DOI 10.17487/RFC6166, April 2011,
<https://www.rfc-editor.org/info/rfc6166>.
[RFC6754] Cai, Y., Wei, L., Ou, H., Arya, V., and S. Jethwani,
"Protocol Independent Multicast Equal-Cost Multipath
(ECMP) Redirect", RFC 6754, DOI 10.17487/RFC6754, October
2012, <https://www.rfc-editor.org/info/rfc6754>.
[RFC8364] Wijnands, IJ., Venaas, S., Brig, M., and A. Jonasson, "PIM
Flooding Mechanism (PFM) and Source Discovery (SD)",
RFC 8364, DOI 10.17487/RFC8364, March 2018,
<https://www.rfc-editor.org/info/rfc8364>.
[RFC8736] Venaas, S. and A. Retana, "PIM Message Type Space
Extension and Reserved Bits", RFC 8736,
DOI 10.17487/RFC8736, February 2020,
<https://www.rfc-editor.org/info/rfc8736>.
Authors' Addresses
Stig Venaas
Cisco Systems, Inc.
Tasman Drive
San Jose, CA 95134
United States of America
Email: stig@cisco.com
Alvaro Retana
Futurewei Technologies, Inc.
2330 Central Expressway
Santa Clara, CA 95050
United States of America
Email: alvaro.retana@futurewei.com
AI ELI5
On this page
PROBLEMAll PIM version 2 messages share a common header with eight reserved bits, but no document formally said how individual message types may use them, and the type space was running low.
MECHANISMThe reserved field is renamed the Flag Bits field and is defined per message type. Types 13, 14 and 15 each gain a 4-bit Subtype taken from the flag bits, giving 48 extended types.
WHY IT MATTERSIt obsoletes RFC 8736, updates six other PIM specifications and updates the IANA PIM Message Types registry.
§1Introduction
PIM version 2 messages share a common header defined in the PIM Sparse Mode specification, RFC 7761. That header contains eight reserved bits, but no document formally specified that these bits are to be used per message type.
RFC 9436 renames the Reserved field the Flag Bits field and says the bits are used separately for each message type. The originally defined message types ran from 0 to 15, and type 15 had been reserved by RFC 6166 for type space extension. This document obsoletes RFC 8736, which had made the RFC 6166 extension obsolete.
A form had some blank boxes nobody had rules for. This document says who may write in which box.
§3The Common Header and Flag Bits
The updated common header has a PIM version field, a Type field, the Flag Bits field and a Checksum. All fields other than Flag Bits remain unchanged.
Unless a message type says otherwise, its flag bits are Unassigned: they MUST be set to zero on transmission and MUST be ignored on receipt. A new PIM type MUST say whether its bits are treated differently. Bits are numbered from bit 7, the most significant and immediately after the Type field, down to bit 0, right before the Checksum.
Each blank box is numbered. If no rule says what a box is for, you leave it empty and ignore anything someone puts in it.
§4Flag Bits in Existing Message Types
The document records how three existing message types already use the bits.
- Type 4 (Bootstrap) defines bit 7 as No-Forward; the other bits are unassigned.
- Type 10 (DF Election) uses the four most significant bits (bits 4 to 7) as a subtype; the rest are unassigned.
- Type 12 (PIM Flooding Mechanism) defines bit 7 as No-Forward; the rest are unassigned.
Each usage is defined in the document that specifies that message type.
Three kinds of messages already had a use for some boxes. This document just writes that down in one place.
§5Extending the Type Space
Types 13, 14 and 15 each become 16 new types, for 48 types available to future PIM extensions. This is done with a Subtype field made from the four most significant flag bits (bits 4 to 7). The notation type.subtype, such as 13.0, names an extended type. The remaining four flag bits (bits 0 to 3) are defined by each extended type.
An extended type is the eight bits made by joining Type and Subtype. No relationship is expected or implied between extended types that share a Type value.
Three kinds of labels each get sixteen sub-labels, like three shelves each split into sixteen boxes, so there is room for many new message kinds.
| Field | Name | Size | Value | Description |
|---|---|---|---|---|
VER | PIM Ver | 4 bits | PIM version. | |
TYPE | Type | 4 bits | Message type; 13, 14 or 15 for extended types. | |
SUBTYPE | Subtype | 4 bits | Flag bits 4 to 7, selecting one of 16 subtypes. | |
FB | FB | 4 bits | Flag bits 0 to 3, defined by each extended type. | |
CHECKSUM | Checksum | 2 bytes | Checksum of the message. |
§7Registry Changes and Security
The IANA "PIM Message Types" registry is updated to show which flag bits each type uses, with bits unassigned except where already specified. The registration policy stays IETF Review, and new assignments MUST define any non-default flag bit usage. Extended type 15.15 is Reserved for future extensions, and new extended types should be assigned consecutively starting with 13.0, then 13.1. IANA also changed references to RFC 8736 in the registry to point to RFC 9436.
On security, the document says it only clarifies flag bit use and extends the type space, so it has no impact on security and does not change the considerations in RFC 7761 and RFC 3973.
The official list of message types was updated. The last slot is kept empty on purpose, and new ones are handed out in order. Nothing about safety changes.
§6References
[RFC7761]RFC 7761Fenner, B., et al., "Protocol Independent Multicast - Sparse Mode (PIM-SM): Protocol Specification (Revised)", STD 83, RFC 7761, March 2016.March 2016[RFC3973]RFC 3973Adams, A., Nicholas, J., and W. Siadak, "Protocol Independent Multicast - Dense Mode (PIM-DM): Protocol Specification (Revised)", RFC 3973, January 2005.January 2005[RFC5015]RFC 5015Handley, M., et al., "Bidirectional Protocol Independent Multicast (BIDIR-PIM)", RFC 5015, October 2007.October 2007[RFC5059]RFC 5059Bhaskar, N., et al., "Bootstrap Router (BSR) Mechanism for Protocol Independent Multicast (PIM)", RFC 5059, January 2008.January 2008[RFC6166]RFC 6166Venaas, S., "A Registry for PIM Message Types", RFC 6166, April 2011.April 2011[RFC8364]RFC 8364Wijnands, IJ., et al., "PIM Flooding Mechanism (PFM) and Source Discovery (SD)", RFC 8364, March 2018.March 2018[RFC8736]RFC 8736Venaas, S. and A. Retana, "PIM Message Type Space Extension and Reserved Bits", RFC 8736, February 2020.February 2020