SRv6 Bitmap Multicast
draft-anish-spring-bimap-multicast-01

Network Working Group A. Peter, Ed.
Internet-Draft Individual
Intended status: Standards Track 1 December 2025
Expires: 4 June 2026
 SRv6 Bitmap Multicast
 draft-anish-spring-bimap-multicast-01
Abstract
 Multicast forwarding in a network provides advantages in improving
 the network usage and performance. In some cases it helps improve
 the operations in managing network. The major challenge in multicast
 operations is in managing the per-flow states in the network as
 required by all the legacy multicast frameworks.
 This document specifies a bitmap forwarding extension to SRv6 to
 support state-free forwarding model in a network.
Requirements Language
 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in RFC2119 [RFC2119].
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 4 June 2026.
Copyright Notice
 Copyright (c) 2025 IETF Trust and the persons identified as the
 document authors. All rights reserved.
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 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
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Table of Contents
 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
 2. Network Overview . . . . . . . . . . . . . . . . . . . . . . 3
 3. IPv6 Bit-Index Format . . . . . . . . . . . . . . . . . . . . 3
 4. Segment Routing Header . . . . . . . . . . . . . . . . . . . 4
 5. BI6 with multiple sub-domains . . . . . . . . . . . . . . . . 6
 6. Interworking with non compatible BI6 Routers . . . . . . . . 6
 6.1. Node-SID insertion for intermediate node tunneling . . . 6
 6.2. Receiving a BI6 packet . . . . . . . . . . . . . . . . . 6
 7. Routing extension header for BIER . . . . . . . . . . . . . . 7
 8. Scope for future work . . . . . . . . . . . . . . . . . . . . 9
 8.1. Define egress functions based on FUNC and ARG bits . . . 9
 8.2. IGP extension to support underlay . . . . . . . . . . . . 9
 9. Subscriber management . . . . . . . . . . . . . . . . . . . . 9
 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
 11. Security Considerations . . . . . . . . . . . . . . . . . . . 9
 12. Appendix 1: Bit-Index string length . . . . . . . . . . . . . 10
 12.1. Private IPv6 address for operations . . . . . . . . . . 10
 13. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
 13.1. Normative References . . . . . . . . . . . . . . . . . . 10
 13.2. Informative References . . . . . . . . . . . . . . . . . 10
 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
 Segment routing with IPv6 as specified in RFC8986 [RFC8986] provides
 a source-routing solution for next generation network requirements.
 More applications and use-cases are finding a better solutions using
 SRv6. Along with this comes the need to support multicasting and
 broadcasting in such networks. The various use-cases for this would
 be stated in the subsequent sections.
 Broadcasting typically needs a point-to-multipoint (p2mp)
 distribution with all the nodes in the network being receivers.
 Multicasting would imply p2mp distribution along with multipoint-to-
 multipoint (mp2mp) packet distribution with the participants being
 pre-determined via a discovery or provisioning mechanism.
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 Bit-Index-Explicit-Replication specified in RFC8279 [RFC8279]
 introduced a per-flow-state-free forwarding for multicast using a
 bit-indexed addressing of multicast receivers.
 This document introduces a bit-map based distribution schema in IPv6
 networks to achieve p2mp distribution patterns. SRv6 introduced a
 new semantic to IPv6 address by fragmenting the address bits into
 Locator:Function:Argument construct to achieve the desired SR
 functionality. This document proposes a similar treatment of IPv6
 address to achieve BIER forwarding.
 Though-out this document non-reduced SID encap is presented. But a
 reduced SRH can also be used. This convention is followed to improve
 clarity.
2. Network Overview
 BIER architecture puts forward a multicast forwarding based on "Bit-
 Index-Explicit-Replication". This architecture defines a BIER domain
 in which an ingress router would encapsulate p2mp packet with a BIER
 header RFC8296 [RFC8296] . This BIER packet would be replicated to
 the egress routers identified by the ingress in its BIER header, over
 an optimal per-flow-stateless tree discovered with the underlay.
3. IPv6 Bit-Index Format
 This document provides a new semantic to the IPv6 address as
 SI_LOCATOR:BITSTRING:FUNCTION:ARGUMENT. This structure is partly
 borrowed from SRv6. The BITSTRING part is introduced to address the
 egress routers in the BIER subdomain by its bit index. From here on
 this format is called as Bit-Index-6 (BI6)
 BIER architecture envisages forwarding by identifying each egress
 router with an BFR-id. These BFR-id in forwarding translates to a
 Set-Identifier (SI) and Bitstring. In the IPv6 Bit-Index format, the
 SI is identified by the SI_LOCATOR and bitstring is encoded in the
 BITSTRING part of the BI6. The FUNCTION and ARGUMENT bits are part
 of the format. But depending on the network requirement their
 lengths may be set to 0 for using this bits for extended bitstring.
 SI_LOCATOR is defined as an anycast routable prefix to reach any one
 of the specific set of routers in a SetIdentifier. Once a BI6 packet
 reaches a router that is part of a SI, The bit-index based part is
 referred to for forwarding towards the BFER's with the BIER
 forwarding principles. The semantics of the FUNC and ARG bits is
 global in the Sub-domain. The attributes of FUNCTION and ARGUMENT
 bits must be pre-determined for a BFER's in a BIER Set-Identifier.
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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+...+-+-+-+-+-+-+-+-+-+-...-+-+...+-+
 | SI_LOCATOR | BITSTRING | FUNC | ARG |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+...+-+-+-+-+-+-+-+-+-+-...-+-+...+-+
 Figure 1: Syntax of BIER6 address
 The SI_Locator present in the destination (anycast) address in the
 ipv6 header would provide a map to identify the BIFT-ID.
4. Segment Routing Header
 In some scenarios there is a need to have multiple SID's to achieve
 the desired network forwarding. The scenarios could be
 a. For sending a packet though a predefined path to the first
 router in a BIER subdomain. The application for this is stated in
 the sub-sequent sections.
 b. For sending a packet though a set of legacy systems that may
 not support BI6 forwarding.
 c. When sending packet on to a node reachable over a transit LAN.
 In such scenarios this document provides the structure of the SRH
 with a SID vector in addition to BIER SID.
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 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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Next Header | Hdr Ext Len | Routing Type | Segments Left |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Last Entry | Flags | Tag |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | |
 | 128bit BI6 SID (SI_LOC::FUNC:ARG) |
 | |
 | |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | |
 | Segment List[1] (128-bit IPv6 address) |
 | |
 | |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | |
 | |
 ~ ... ~
 | |
 | |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | |
 | Segment List[n] (128-bit IPv6 address) |
 | |
 | |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 // //
 // Optional Type Length Value objects (variable) //
 // //
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Figure 2: Syntax of BIER SRH
 where:
 Next Header: Defined in [RFC8200], Section 4.4.
 Hdr Ext Len: Defined in [RFC8200], Section 4.4.
 Routing Type: 4, defined in [RFC8754], Section 2.
 Segments Left: Defined in [RFC8200], Section 4.4.
 Last Entry: contains the index (zero based), in the Segment List
 including the SI6 SID, of the last element of the Segment List.
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 Flags: 8 bits of flags. [RFC8754].
 Tag: Tag: is described in [RFC8754], Section 4.3.1.
 Segment List[0..n]: Is described in [RFC8754], Section 4.3.1.
 TLV: Type Length Value (TLV) is described in [RFC8754], Section 2.1.
5. BI6 with multiple sub-domains
 In a larger network having multiple sub-domains, a router may be
 programmed to do ingress replication of the traffic to multiple BIER
 subdomains. The ingress router may introduce a path vector as a SID
 list on each of this packet.
6. Interworking with non compatible BI6 Routers
 A network topology may have legacy devices which may not be capable
 of BI6 processing. When deploying BI6 the traffic may have to pass
 through some of these devices for loop-free forwarding.
6.1. Node-SID insertion for intermediate node tunneling
 A router may come to know about the BI6 capability of all routers in
 an IGP area via the capabilities it has published in its IGP
 advertisement. Based on this IGP may form a map of adjacent BFR's.
 An adj-BFR may be reachable over a few hops of legacy nodes. If the
 BFR is not directly connected, then that node must compute the list
 of legacy nodes that must be passed through to reach that adjcent
 BFR. Attached to adjacency map the BFR must maintain the SID vector
 to reach that adj-BFR.
 When a packet gets forwarded to such an adjacency this SID vector
 would be inserted in that packet after doing the forwarding updates
 in the bitmap.
6.2. Receiving a BI6 packet
 When receiving a BI6 packet with the sid penultimate to BI6 being
 that of self. The router may strip the SRH of all the SID's other
 than the BI6-SID. Post this it must do BIER forwarding. The process
 of doing BIER forwarding involves BIT string updation according to
 BIER principles.
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7. Routing extension header for BIER
 For the topologies needing longer bitstring to address more BFER's,
 the SRH as specified for SRV6 itself can be used for sending the
 bitmap. This section provides the procedures involved in using this
 extension.
 The existing SRH format supports encoding a SID stack to specify the
 multihop routing chain.
 This SRH can be used for BIER bitmap by encoding the BITSTRING in the
 initial segment ID locations. The bitstring length is supposed to be
 infered from BIFT-ID (or from locator) in BI6, for this specification
 the bitsting length can be any multiple of 128.
 The value of segments left must be set to roundup(length of the
 bitstream/128) + the number of element present in the segment list.
 After the bitsting, the BI6 top-sid must follow, which would have the
 SI_Locator, ARGUMENT and FUNCTION bits.
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 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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Next Header | Hdr Ext Len | Routing Type | Segments Left |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Last Entry | Flags |B| Tag |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | |
 | BIER bit-string (m * 128) |
 ~ ... ~
 | |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | |
 | 128bit BI6 SID (SI_LOC::FUNC:ARG) |
 | |
 | |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | |
 | Segment List[n-m-1] (128-bit IPv6 address) |
 | |
 | |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | |
 | |
 ~ ... ~
 | |
 | |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | |
 | Segment List[n] (128-bit IPv6 address) |
 | |
 | |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 // //
 // Optional Type Length Value objects (variable) //
 // //
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Figure 3: Syntax of BIER SRH for accomodating longer bitstrings
 where fields are as defined before unless stated below.
 Flags: 8 bits of flags. [RFC8754].
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 0 1 2 3 4 5 6 7
 +-+-+-+-+-+-+-+-+
 |U U U U U U U B|
 +-+-+-+-+-+-+-+-+
 U: Unused and for future use. MUST be 0 on transmission and
 ignored on receipt.
 B: In this last bit is set to 1 of the SRH includes a BIER
 BITSTRING. It must not set when the SRH has the SI6 SID without
 an explicit BIER bitmap.
 BIER Bitsring: BITSRING for BIER forwarding. The size of this
 BITSRING is already known from tbe BIER configurations. It can
 also be figured out from a packet as the bits in between the BI6
 SID and the common header.
8. Scope for future work
8.1. Define egress functions based on FUNC and ARG bits
8.2. IGP extension to support underlay
9. Subscriber management
 In BIER architecture the multicast egress routers must be learned by
 the ingress router. This discovery may happen via some out-of-band
 mechanism beyond the scope of this document.
10. IANA Considerations
 This specification introduces new semantics for IPv6 address. Though
 this draft does not need any allocations, New IANA allocations would
 be required for the supplimentary specifications.
11. Security Considerations
 This document proposes a semantic for IPv6 address. The security
 challenges that apply to IPv6 and in the BIER architecture applies to
 the intended BI6 forwarding model specified here.
 Firewall/ACL/QoS policy filters usualy applied on multicast/broadcast
 traffic may not be applicable as such on a BI6 packet.
 With BI6 it becomes possible to a remote node to inject p2mp traffic
 into a network. Making important to have packet source validations.
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 The further security scenarios would be added in the due course.
12. Appendix 1: Bit-Index string length
12.1. Private IPv6 address for operations
 The Unique Local IPv6 address allocation RFC4193 [RFC4193] provides
 free to use address blocks with SI_LOCATOR size of 48. This provides
 a maximum BI6 addressing space of 80 bit length.
 The Bit-index string length that can be used would be determined by
 the SI_locator prefix length and the need for FUNC and ARG bits.
 Hence if Unique local address space is used, upto 80 BFER's can be
 addressed.
13. References
13.1. 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>.
 [RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6
 (IPv6) Specification", STD 86, RFC 8200,
 DOI 10.17487/RFC8200, July 2017,
 <https://www.rfc-editor.org/info/rfc8200>.
 [RFC8279] Wijnands, IJ., Ed., Rosen, E., Ed., Dolganow, A.,
 Przygienda, T., and S. Aldrin, "Multicast Using Bit Index
 Explicit Replication (BIER)", RFC 8279,
 DOI 10.17487/RFC8279, November 2017,
 <https://www.rfc-editor.org/info/rfc8279>.
 [RFC8296] Wijnands, IJ., Ed., Rosen, E., Ed., Dolganow, A.,
 Tantsura, J., Aldrin, S., and I. Meilik, "Encapsulation
 for Bit Index Explicit Replication (BIER) in MPLS and Non-
 MPLS Networks", RFC 8296, DOI 10.17487/RFC8296, January
 2018, <https://www.rfc-editor.org/info/rfc8296>.
 [RFC8754] Filsfils, C., Ed., Dukes, D., Ed., Previdi, S., Leddy, J.,
 Matsushima, S., and D. Voyer, "IPv6 Segment Routing Header
 (SRH)", RFC 8754, DOI 10.17487/RFC8754, March 2020,
 <https://www.rfc-editor.org/info/rfc8754>.
13.2. Informative References
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 [RFC4193] Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
 Addresses", RFC 4193, DOI 10.17487/RFC4193, October 2005,
 <https://www.rfc-editor.org/info/rfc4193>.
 [RFC8986] Filsfils, C., Ed., Camarillo, P., Ed., Leddy, J., Voyer,
 D., Matsushima, S., and Z. Li, "Segment Routing over IPv6
 (SRv6) Network Programming", RFC 8986,
 DOI 10.17487/RFC8986, February 2021,
 <https://www.rfc-editor.org/info/rfc8986>.
Author's Address
 Anish Peter (editor)
 Individual
 Bangalore
 KA
 India
 Email: anish.ietf@gmail.com
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