



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.




Peter                      Expires 4 June 2026                  [Page 1]

Internet-Draft            SRv6 Bitmap Multicast            December 2025


   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
   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.




Peter                      Expires 4 June 2026                  [Page 2]

Internet-Draft            SRv6 Bitmap Multicast            December 2025


   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.



Peter                      Expires 4 June 2026                  [Page 3]

Internet-Draft            SRv6 Bitmap Multicast            December 2025


     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+...+-+-+-+-+-+-+-+-+-+-...-+-+...+-+
     | 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.

























Peter                      Expires 4 June 2026                  [Page 4]

Internet-Draft            SRv6 Bitmap Multicast            December 2025


    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.




Peter                      Expires 4 June 2026                  [Page 5]

Internet-Draft            SRv6 Bitmap Multicast            December 2025


   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.








Peter                      Expires 4 June 2026                  [Page 6]

Internet-Draft            SRv6 Bitmap Multicast            December 2025


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.































Peter                      Expires 4 June 2026                  [Page 7]

Internet-Draft            SRv6 Bitmap Multicast            December 2025


    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].








Peter                      Expires 4 June 2026                  [Page 8]

Internet-Draft            SRv6 Bitmap Multicast            December 2025


            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.




Peter                      Expires 4 June 2026                  [Page 9]

Internet-Draft            SRv6 Bitmap Multicast            December 2025


   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




Peter                      Expires 4 June 2026                 [Page 10]

Internet-Draft            SRv6 Bitmap Multicast            December 2025


   [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

































Peter                      Expires 4 June 2026                 [Page 11]
