RFC 8736 PROPOSED STANDARD MULTICAST · PIM · IANA
PIM type space extension: PIM Message Type Space Extension and Reserved Bits
Gives the spare bits in every PIM message header a job, and uses some of them to stretch the message type space.
RFC EDITOR
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>.
BIBTEX
@misc{rfc8736,
series = {Request for Comments},
number = 8736,
howpublished = {RFC 8736},
publisher = {RFC Editor},
doi = {10.17487/RFC8736},
url = {https://www.rfc-editor.org/info/rfc8736},
author = {S. Venaas and A. Retana},
title = {{PIM Message Type Space Extension and Reserved Bits}},
pagetotal = 8,
year = 2020,
month = feb,
} RFC text
Contents
Internet Engineering Task Force (IETF) S. Venaas
Request for Comments: 8736 Cisco Systems, Inc.
Obsoletes: 6166 A. Retana
Updates: 3973, 5015, 5059, 6754, 7761, Futurewei Technologies, Inc.
8364 February 2020
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
creates a registry containing the per-message-type usage. This
document also extends the PIM type space by defining three new
message types. For each of the new types, four of the previously
reserved bits are used to form an extended type range.
This document updates RFCs 7761 and 3973 by defining the use of the
currently 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 currently reserved bits for
each PIM message.
This document obsoletes RFC 6166.
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/rfc8736.
Copyright Notice
Copyright (c) 2020 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 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. 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 (PFM)
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 refers to the bits specified as "reserved" in the
common PIM header [RFC7761] as "PIM message type Flag Bits" or,
simply, "Flag Bits", and it specifies that they are to be separately
used on a per-message-type basis. It creates a registry containing
the per-message-type usage.
This document updates [RFC7761] and [RFC3973] by defining the use of
the currently 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
currently reserved bits for each PIM message.
The currently defined PIM message types are in the range from 0 to
15. That type space is almost exhausted. Message type 15 was
reserved by [RFC6166] for type space extension. In Section 5, this
document specifies the use of the Flag Bits for message types 13, 14,
and 15 in order to extend the PIM type space. This document
obsoletes [RFC6166].
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
that field as "PIM message type Flag Bits" or, simply, "Flag Bits".
The new 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: New 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
Reserved [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 reserved.
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 reserved.
4.3. Flag Bits for Type 12 (PFM)
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 reserved.
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 defines types 13, 14, and 15, such that each of these
types has 16 subtypes, providing a total of 48 subtypes available for
future PIM extensions. This is achieved by defining a new Subtype
field (see Figure 3) using the four most significant flag bits (bits
4-7). The notation type.subtype is used to reference these new
extended types. The remaining four flag bits (bits 0-3) are reserved
to be used by each extended type (abbreviated as FB 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 |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.
The registry now references this document. The registration policy
remains IETF Review [RFC8126]. Assignments into this registry MUST
define any non-default usage (see Section 4) of the flag bits in
addition to the type.
The updated "PIM Message Types" registry is shown below.
+------------+---------------+---------------+--------------------+
| Type | Name | Flag Bits | Reference |
+============+===============+===============+====================+
| 0 | Hello | 0-7: Reserved | [RFC3973][RFC7761] |
+------------+---------------+---------------+--------------------+
| 1 | Register | 0-7: Reserved | [RFC7761] |
+------------+---------------+---------------+--------------------+
| 2 | Register Stop | 0-7: Reserved | [RFC7761] |
+------------+---------------+---------------+--------------------+
| 3 | Join/Prune | 0-7: Reserved | [RFC3973][RFC7761] |
+------------+---------------+---------------+--------------------+
| 4 | Bootstrap | 0-6: Reserved | [RFC5059][RFC7761] |
| | +---------------+--------------------+
| | | 7: No-Forward | [RFC5059] |
+------------+---------------+---------------+--------------------+
| 5 | Assert | 0-7: Reserved | [RFC3973][RFC7761] |
+------------+---------------+---------------+--------------------+
| 6 | Graft | 0-7: Reserved | [RFC3973] |
+------------+---------------+---------------+--------------------+
| 7 | Graft-Ack | 0-7: Reserved | [RFC3973] |
+------------+---------------+---------------+--------------------+
| 8 | Candidate RP | 0-7: Reserved | [RFC7761] |
| | Advertisement | | |
+------------+---------------+---------------+--------------------+
| 9 | State Refresh | 0-7: Reserved | [RFC3973] |
+------------+---------------+---------------+--------------------+
| 10 | DF Election | 0-3: Reserved | [RFC5015] |
| | +---------------+--------------------+
| | | 4-7: Subtype | [RFC5015] |
+------------+---------------+---------------+--------------------+
| 11 | ECMP Redirect | 0-7: Reserved | [RFC6754] |
+------------+---------------+---------------+--------------------+
| 12 | PIM Flooding | 0-6: Reserved | [RFC8364] |
| | Mechanism +---------------+--------------------+
| | | 7: No-Forward | [RFC8364] |
+------------+---------------+---------------+--------------------+
| 13.0-15.15 | Unassigned | 0-3: | RFC 8736 |
| | | Unassigned | |
+------------+---------------+---------------+--------------------+
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>.
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 said how individual message types may use them. The message type space, 0 to 15, was also almost exhausted.
MECHANISMThe reserved bits are renamed Flag Bits and are defined per message type, with a registry recording each type's usage. Types 13, 14 and 15 each use the top four Flag Bits as a Subtype, giving 48 extended types.
STATUS TODAYPIM gained room for future message types and a registry of flag bit usage. The document obsoletes RFC 6166 and has itself been obsoleted by RFC 9436.
§1Why the Reserved Bits Needed a Rule
PIM version 2 messages all start with a common header defined in RFC 7761. That header contains eight reserved bits, yet no document formally said these bits could be used separately by each message type.
This RFC calls those bits Flag Bits and says they are used on a per-message-type basis. It also notes that the defined message types run from 0 to 15, that this space is almost exhausted, and that type 15 had already been reserved for type space extension by RFC 6166, which this document obsoletes.
A form had some empty boxes nobody was sure how to use. This paper says who may fill them in, and how.
§3The Common PIM Header with Flag Bits
The common header keeps its layout: the PIM version, the message type, the eight-bit Flag Bits field (formerly Reserved) and the checksum. Only the Reserved field is renamed and redefined. All other fields remain unchanged.
To talk about single bits, the RFC numbers them from bit 7, the most significant and right after the Type field, down to bit 0, the one just before the Checksum.
The top of every PIM message has a row of little boxes. This paper gives the boxes numbers so everyone can point to the same one.
§4Flag Bit Definitions per Message Type
Unless a document says otherwise, all flag bits are Reserved: they MUST be set to zero on transmission and MUST be ignored upon receipt. The specification of any new PIM type MUST say whether its bits are treated differently.
- Type 4 (Bootstrap): bit 7 is No-Forward, defined in RFC 5059. The rest are reserved.
- Type 10 (DF Election): bits 4 to 7 form a subtype, defined in RFC 5015. The rest are reserved.
- Type 12 (PIM Flooding Mechanism): bit 7 is No-Forward, defined in RFC 8364. The rest are reserved.
- Types 13, 14 and 15: defined by the type space extension.
Unless a message says otherwise, its boxes stay empty and everyone ignores them. A few message kinds already have special uses for certain boxes.
§5Extending the PIM Type Space
Types 13, 14 and 15 each get 16 subtypes, which gives 48 subtypes for future PIM extensions. The Subtype field is made from the four most significant flag bits (bits 4 to 7). The notation type.subtype, such as 13.0, names these extended types.
The four remaining flag bits (bits 0 to 3) are reserved for each extended type to use as it needs.
Running out of message names is like running out of locker numbers. Now three lockers each get sixteen shelves, so there are many more places to put things.
| Field | Name | Size | Value | Description |
|---|---|---|---|---|
VER | PIM Ver | 4 bits | PIM version. | |
TYPE | Type | 4 bits | 13, 14 or 15 for the extended types. | |
SUBTYPE | Subtype | 4 bits | Flag bits 4 to 7 used as a subtype. | |
FB | FB | 4 bits | Flag bits 0 to 3, reserved for each extended type. | |
CHECKSUM | Checksum | 2 bytes | Message checksum. |
§7The Updated Message Types Registry
The IANA "PIM Message Types" registry is updated to show which flag bits each type uses, and it now references this document. The registration policy remains IETF Review, and new assignments MUST define any non-default flag bit usage along with the type.
Types 13.0 through 15.15 are listed as unassigned. Each extended type has only 4 flag bits available, and new extended types should be assigned consecutively, starting with 13.0, then 13.1, and so on. The RFC says it has no impact on security and no change to the considerations in RFC 7761 and RFC 3973.
There is a big list of message kinds and what each box means. New kinds must say how they use the boxes, and the new shelves are handed out in order.
§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