Network Working Group                                         M. McBride
Internet-Draft                                                 Futurewei
Intended status: Standards Track                                  J. Xie
Expires: January 11, 29, 2021                                    S. Dhanaraj
                                                                R. Asati
                                                                  Y. Zhu
                                                           China Telecom
                                                               G. Mishra
                                                            Verizon Inc.
                                                           July 10, 28, 2020

                         BIER IPv6 Requirements


   The BIER WG charter includes work on developing "a mechanism to use
   BIER natively in IPv6".  There have been several proposed solutions
   in this area.  But there hasn't been a document which describes the
   problem and lists the requirements.  The goal of this document is to
   describe the BIER IPv6 requirements, summarize the encapsulation
   modes of the proposed solutions, guide the working group in
   understanding the benefits and drawbacks of the various solutions,
   and help in the development of acceptable solutions.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on January 11, 29, 2021.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Problem Statement . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Conceptual Models For BIER IPv6 Encapsulation and Forwarding    4
     3.1.  Transport-Independent Model . . . . . . . . . . . . . . .   5
     3.2.  Native IPv6 Model . . . . . . . . . . . . . . . . . . . .   6
     3.3.  Encapsulation Approaches Considered . . . . . . . . . . .   7
   4.  Requirements  . . . . . . . . . . . . . . . . . . . . . . . .   7
     4.1.  Mandatory Requirements  . . . . . . . . . . . . . . . . .   8
       4.1.1.  L2 Agnostic . . . . . . . . . . . . . . . . . . . . .   8
       4.1.2.  Support BIER architecture . . . . . . . . . . . . . .   8
       4.1.3.  Conform to existing IPv6 Spec . . . . . . . . . . . .   8
       4.1.4.  Support deployment with Non-BFR routers . . . . . . .   8
       4.1.5.  Support inter-AS multicast deployment . . . . . . . .   8
       4.1.6.  Support Simple Encapsulation  . . . . . . . . . . . .   9
       4.1.7.  Support Deployment Security . . . . . . . . . . . . .   9
     4.2.  Optional Requirements . . . . . . . . . . . . . . . . . .   9
       4.2.1.  Support MVPN  . . . . . . . . . . . . . . . . . . . .   9
       4.2.2.  Support OAM . . . . . . . . . . . . . . . . . . . . .   9
       4.2.3.  Support IPSEC . . . . . . . . . . . . . . . . . . . .   9
       4.2.4.  Support Fragmentation . . . . . . . . . . . . . . . .   9
       4.2.5.  Support hardware fast path  . . . . . . . . . . . . .  10
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   7.  Acknowledgement . . . . . . . . . . . . . . . . . . . . . . .  10
   8.  Normative References  . . . . . . . . . . . . . . . . . . . .  10
   Appendix A.  Solutions Evaluation . . . . . . . . . . . . . . . .  12
     A.1.  BIER-ETH encapsulation in IPv6 networks . . . . . . . . .  12
     A.2.  Encode Bitstring in IPv6 destination address  . . . . . .  13
     A.3.  Add BIER header into IPv6 Extension Header  . . . . . . .  13
     A.4.  Transport BIER as IPv6 payload  . . . . . . . . . . . . .  14
     A.5.  Tunnelling BIER in a IPv6 tunnel  . . . . . . . . . . . .  15
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  15

1.  Introduction

   Bit Index Explicit Replication (BIER) [RFC8279] is an architecture
   that provides optimal multicast forwarding, without requiring
   intermediate routers to maintain per-flow state, through the use of a
   multicast-specific BIER header.  [RFC8296] defines two types of BIER
   encapsulation to run on physical links: one is BIER MPLS
   encapsulation to run on various physical links that support MPLS, the
   other is non-MPLS BIER Ethernet encapsulation to run on ethernet
   links, with an ethertype 0xAB37.  This document describes using BIER
   in non-MPLS IPv6 environments.  We explain the requirements of
   transporting IPv4/IPv6 multicast payloads through an IPv6 network
   using "BIER natively in IPv6".  As clarified in the working-group,
   "BIER natively in IPv6" means BIER not encapsulated in MPLS or
   Ethernet.  This may include native IPv6 encapsulation and generic
   IPv6 tunnelling.  The goal of this document is to help the BIER WG
   evaluate the BIER v6 requirements and solutions in order to begin
   adopting solution drafts.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

1.2.  Terminology

   o  BIER: Bit Index Explicit Replication.  Provides optimal multicast
      forwarding through adding a BIER header and removing state in
      intermediate routers.

   o  BUM: Broadcast, Unknown Unicast, Multicast.  Term used to describe
      the three types of Ethernet modes that will be forwarded to
      multiple destinations

2.  Problem Statement

   The problem is the ability of the network to transport BUM packets,
   with BIER headers, in an IPv6 environment.  In many IPv6 network
   deployments, non-MPLS encapsulation is used for unicast as the data-
   plane and it
   plane.  It is likewise expected to have BIER IPv6 deployments which
   depend on these same unicast technologies. technologies to traverse through non-BFR

   One such case involves supporting a non-BFR router in a network as
   described in section 6.9 of RFC8279.  In the context of this
   document, an IPv6 based unicast tunnel is needed to support such
   deployment where a non-BFR exists.  Another case is to support inter-
   AS multicast deployment as illustrated in
   [I-D.geng-bier-ipv6-inter-domain].  In such deployment, there are
   non-BFR routers, or even an entire non-BIER network, that needs the
   ability to traverse from one BFR to another.
   [I-D.ietf-bier-use-cases] shows it is possible there are other cases
   where inter-AS multicast deployment is required.

   As with IPv6, another problem of BIER IPv6 technology may be
   "Transition Mechanisms and Partial Deployments" which is listed as
   the No.1 charter item of BIER WG.  Therefore, a basic requirement of
   BIER IPv6 is to leverage IPv6 reachability for incremental and inter-
   AS BIER deployment.

   Below is a simple scenario that needs BIER IPv6 encapsulation and

         |                                            |
         |                                         +------+
         |                                         | BFER |
     +------+       +-------+       +-----+        +------+
     | BFIR |       |Non-BFR|       | BFR |           |
     +------+       +-------+       +-----+        +------+
         |                                         | BFER |
         |                IPv6 Network             +------+
         |           (intra-AS or inter-AS)           |

   This scenario depicts the need to replicate bier packets from a BFIR
   to BFERs across an IPv6 core.  The IPv6 environment may include a
   variety of link types, may be entirely IPv6, may be dual stack or any
   type of combination which includes IPv6.  Regardless of the
   environment, there are times when a BIER header, including the BIER
   BitString used to determine the set of BIER forwarding egress
   routers, will need to traverse a IPv6 domain.  The ways in which BIER
   will function in an IPv6 environment is the problem that needs to be

3.  Conceptual Models For BIER IPv6 Encapsulation and Forwarding

   This analysis introduces two conceptual models for BIER IPv6
   encapsulation and forwarding based on the experience and examples
   that have been seen in the IETF community.

3.1.  Transport-Independent Model

   The first conceptual model is a Transport-Independent Model that
   views IP tunnels as links of BIER, and views BIER as an independent

         |<----------(L2.5 BIER(P2MP) Tunnel)-------->|
         |                                            |
         |    +~~~~~~~~~~~~~~~~~~+          +~~~~+    |
         |   /                    \        /      \   |
     +------+       +-------+       +-----+        +------+
     | BFIR |-------|Non-BFR|-------| BFR |--------| BFER |
     +------+       +-------+       +-----+        +------+

     ------- physical link

     ~~~~~~~ IPv6(P2P) tunnel

     <-----> BIER(P2MP) tunnel

   In this model, an IPv6 tunnel works as a link-layer of BIER, and BIER
   works as a transport-independent layer (or layer-2.5) over a virtual-
   link (IPv6 tunnel).  On each BFR, the IPv6 tunnel of the receiving
   packet is decapsulated, and a new IPv6 tunnel is encapsulated before
   sending the packet to the next-hop BFR neighbour.

   From the view of the IPv6 layer, the BIER header is a kind of Upper-
   layer header (Layer-4).  From the view of the BIER layer, the IPv6
   encapsulation is a tunnel working as a "link" of BIER.  With an End-
   to-End view, the tunnel from BFIR to BFERs is a Layer-2.5 BIER (P2MP)
   tunnel, and the BFIR-id is the BIER packet source-origin identifier,
   and is unchanged through the BIER domain from BFIR to BFERs.

   This model is similar to the "MPLS over IP" [RFC4023] or "MPLS over
   UDP" [RFC7510] approach.  A more general output of such approach in
   IETF is "MPLS Segment Routing over IP" [RFC8663].  It makes use of
   IPv4/IPv6 tunnel, IPv4/IPv6 UDP tunnel and IPv4/IPv6 GRE tunnel to
   encapsulate the MPLS-based instructions.  In fact, BIER-MPLS could
   use this approach directly since BIER-MPLS is based on MPLS.

   There may be, however, in certain cases some difficulty with
   allocation of an MPLS label and advertisement through the control-
   plane.  For example, a simple inter-AS BIER deployment may want to
   use the auto-configuration of BIFT-id using Non-MPLS BIER
   encapsulation [RFC8296] as illustrated in
   [I-D.geng-bier-ipv6-inter-domain].  This brings the need of a new
   "Next Header" value to indicate the "Non-MPLS" BIER header.

   For IPv4/IPv6 GRE, the "Next Header" is the 16-bit "Protocol Type"
   field, and has adequate space for such requirement.

   For IPv4/IPv6 UDP, the "Next Header" is the 16-bit "Destination Port"
   field, and has adequate space for such requirement.

   For IPv4/IPv6, the "Next Header" is a 8-bit value and needs to be
   allocated from the "Assigned Internet Protocol Numbers" registry.

   Reassembly/Re-fragmentation of a packet has to be executed on each
   BFR in such case.  This may be common and even friendly for a
   protocol stack in a BFR software implementation, but it may impose
   cost for a BFR hardware implementation.

   IPv6 functions that are expected to be executed from BFIR to BFER are
   assumed to be broken on the BFRs, for example, IPv6 Fragmentation/
   Assembly or IPSEC ESP.  This is because the "IPv6 tunnel" and all its
   functions is "terminated" on the BFRs.  These functions, if desired,
   may need to be re-designed in the "Layer-2.5" BIER mode.

   For deployment security, it is necessary to ensure the "BIER" packet
   is only using the allowed IPv6 tunnel.

3.2.  Native IPv6 Model

   The second conceptual model is a Native IPv6 Model that integrates
   BIER as part of the IPv6 data plane, making it a "Layer-3 BIER"

          |<----------(L3 BIER(P2MP) tunnel)--------->|
          |                                           |
     +------+       +-------+       +-----+        +------+
     | BFIR |-------|Non-BFR|-------| BFR |--------| BFER |
     +------+       +-------+       +-----+        +------+

     ------- physical link

     <-----> BIER(P2MP) tunnel

   In this model, BIER works as part of the IPv6 data plane.  BFIR and
   BFERs work as IPv6 (P2MP) tunnel endpoints, and BFRs work as IPv6
   segment endpoints.  On each BFR, the segment endpoint behaviour of
   IPv6 data plane is executed, and there is no decapsulation of
   receiving IPv6 tunnel and encapsulation of new IPv6 tunnel for

   In this mode, the BIER header is integrated into the IPv6 data plane. extension
   header and processing of the BIER header (e.g., the BitString) is
   implemented as part of the IPv6 extension header processing.  The
   IPv6 source address is the BIER packet source-origin identifier, and
   is unchanged through the BIER domain from BFIR to BFERs.

   This model is similar to many examples emerging in the IETF community
   which soley use the IPv6 data plane.  SRv6 introduced in [RFC8754]
   and [I-D.ietf-spring-srv6-network-programming] is an example.  The
   benefits of such approach includes reducing the number of
   encapsulation layers, capability of deployment with non-capable
   routers in a network, extending the technology in a wider inter-AS
   scope using IP reachability, and capability of integrating the
   functions of the IPv6 data plane.

   This model typically needs an extension to IPv6 data plane, with an
   IPv6 extension header or Option introduced.

   IPv6 functions that are expected to be executed from BFIR to BFER is
   supported if correctly designed, for example, IPv6 Fragmentation/
   Assembly or IPSEC ESP.

   For deployment security, it is necessary to ensure the "BIER" packet
   is in a trusted IPv6-based domain.

3.3.  Encapsulation Approaches Considered

   A number of approaches to the design of BIER-IPv6 encapsulation were
   investigated by the BIER Working Group and were discussed in IETF
   meetings and on the BIER list.  This section divides these approaches
   into the two conceptual models.

   Transport-Independent Model approaches include:

      Transport-Independent BIER [I-D.xu-bier-encapsulation].

      BIERin6 [I-D.zhang-bier-bierin6].

   Native IPv6 Model approaches include:

      BIER-over-IPv6 [I-D.pfister-bier-over-ipv6].

      BIERv6 [I-D.xie-bier-ipv6-encapsulation].

4.  Requirements

   There have been several suggested requirements, on the BIER email
   list and in meetings, which have been used to form BIER IPv6
   requirements used to help the wg evaluate against the proposed
   solutions.  There is also many further discussions on the list about
   BIER IPv6 requirements from different scenarios.

   Considering that the importance of requirement for BIER IPv6 solution
   is different, in this document, the requirements are divided into two
   groups: mandatory and optional.  The requirements in the mandatory
   group are considered necessary for any BIER IPv6 solution, while the
   requirements in the optional group should be considered but are not

4.1.  Mandatory Requirements

4.1.1.  L2 Agnostic

   The solution must be agnostic to the underlying L2 data link type.
   The solution needs to support P2P ethernet links as well as shared
   media ethernet links without requiring the LAN switch to perform BIER

4.1.2.  Support BIER architecture

   The solution must support the BIER architecture.

   Multiple sub-domains bound to one or many topologies or algorithms,
   multiple sets for more BFERs, multiple Bit String Lengths for
   different forwarding capabilities, and multiple BIFTs for ECMP are
   considered essential functions of BIER and need to be supported.

4.1.3.  Conform to existing IPv6 Spec

   The proposed encapsulation must conform to the IPv6 specification and
   guidelines as described in RFC8200.  If new extensions to existing
   IPv6 specification are required, it needs to be discussed and
   reviewed by the 6man working-group.

4.1.4.  Support deployment with Non-BFR routers

   The solution must support deployments with Non-BFR routers.  This is
   beneficial to the deployment of BIER, especially in early deployments
   when some routers do not support BIER forwarding but support IPv6
   forwarding.  This is also the No.1 charter item, "Transition
   Mechanisms and Partial Deployments" of the BIER WG.

4.1.5.  Support inter-AS multicast deployment

   Inter-AS multicast support is needed for ease of provisioning the
   P2MP transport service to enterprises.  This could greatly increase
   the scalability of BIER, as it is usually considered to be suitable
   only for small intra-AS scenarios.

4.1.6.  Support Simple Encapsulation

   The solution must avoid requiring different encapsulation types.  A
   solution needs to do careful trade-off analysis and select one
   encapsulation as its proposal for best coverage of various scenarios.

4.1.7.  Support Deployment Security

   The proposed solution must include careful security considerations,
   including all that is already considered in BIER architecture RFC8279
   and RFC8296, and other security concerns that may raise due to the
   addition of IPv6.

4.2.  Optional Requirements

4.2.1.  Support MVPN

   The solution MAY support MVPN services that is defined in [RFC6513].
   When MVPN is supported, it should work in a "tunnel" mode,
   encapsulating IP or IPv6 multicast packet in an outer IPv6 header.
   When MVPN is supported, it is suggested to think about both intra-AS
   and inter-AS deployment.

4.2.2.  Support OAM

   BIER OAM MAY be supported, either directly using existing method, or
   specify some variant method for the same function.  It may be
   considered essential as part of the BIER architecture in some cases.

4.2.3.  Support IPSEC

   IPSEC is optional to IPv6 and multicast.  It is preferred to support
   IPSEC, including AH/ESP.  If IPSEC is to be supported, it shouldn't
   require hop-by-hop encryption/decryption.

4.2.4.  Support Fragmentation

   As part of IPv6 specification [RFC8200], BIER IPv6 may support
   fragmentation on BFIR and assembly on BFER.  Support of Fragmentation
   can enhance the capability of BIER leveraging the BIER-MTU as
   introduced in section 3 of [RFC8296].  If Fragmentation is to be
   supported, it shouldn't require fragmentation and re-assembly at each

4.2.5.  Support hardware fast path

   If a proposed solution is intended for some scenarios like service-
   provider networks, it should enable the processing and forwarding of
   BIER packets in hardware fast path.

5.  IANA Considerations

   Some BIERv6 encapsulation proposals do not require any action from
   IANA while other proposals require new BIER Destination Option
   codepoints from IPv6 sub-registries, new "Next header" values, or
   require new IP Protocol codes.  This document, however, does not
   require anything from IANA.

6.  Security Considerations

   There are no security issues introduced by this draft.

7.  Acknowledgement

   Thank you to Eric Rosen for his listed set of requirements on the
   bier wg list.

8.  Normative References

              Geng, L., Xie, J., McBride, M., and G. Yan, "Inter-Domain
              Multicast Deployment using BIERv6", draft-geng-bier-ipv6-
              inter-domain-01 (work in progress), January 2020.

              Nainar, N., Asati, R., Chen, M., Xu, X., Dolganow, A.,
              Przygienda, T., Gulko, A., Robinson, D., Arya, V., and C.
              Bestler, "BIER Use Cases", draft-ietf-bier-use-cases-11
              (work in progress), March 2020.

              Filsfils, C., Camarillo, P., Leddy, J., Voyer, D.,
              Matsushima, S., and Z. Li, "SRv6 Network Programming",
              draft-ietf-spring-srv6-network-programming-16 (work in
              progress), June 2020.

              Pfister, P. and I. Wijnands, "An IPv6 based BIER
              Encapsulation and Encoding", draft-pfister-bier-over-
              ipv6-01 (work in progress), October 2016.

              Xie, J., Geng, L., McBride, M., Asati, R., Dhanaraj, S.,
              Zhu, Y., Qin, Z., Shin, M., Mishra, G., and X. Geng,
              "Encapsulation for BIER in Non-MPLS IPv6 Networks", draft-xie-bier-
              ipv6-encapsulation-07 draft-
              xie-bier-ipv6-encapsulation-08 (work in progress), June July

              Xu, X.,, s., Jacquenet,
              C., Raszuk, R., and Z. Zhang, "A Transport-Independent Bit
              Index Explicit Replication (BIER) Encapsulation Header",
              draft-xu-bier-encapsulation-06 (work in progress),
              September 2016.

              Zhang, Z., Przygienda, T., Wijnands, I., Bidgoli, H., and
              M. McBride, "BIER in IPv6 (BIERin6)", draft-zhang-bier-
              bierin6-06 (work in progress), January July 2020.

   [RFC1112]  Deering, S., "Host extensions for IP multicasting", STD 5,
              RFC 1112, DOI 10.17487/RFC1112, August 1989,

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC2473]  Conta, A. and S. Deering, "Generic Packet Tunneling in
              IPv6 Specification", RFC 2473, DOI 10.17487/RFC2473,
              December 1998, <>.

   [RFC4023]  Worster, T., Rekhter, Y., and E. Rosen, Ed.,
              "Encapsulating MPLS in IP or Generic Routing Encapsulation
              (GRE)", RFC 4023, DOI 10.17487/RFC4023, March 2005,

   [RFC6513]  Rosen, E., Ed. and R. Aggarwal, Ed., "Multicast in MPLS/
              BGP IP VPNs", RFC 6513, DOI 10.17487/RFC6513, February
              2012, <>.

   [RFC7510]  Xu, X., Sheth, N., Yong, L., Callon, R., and D. Black,
              "Encapsulating MPLS in UDP", RFC 7510,
              DOI 10.17487/RFC7510, April 2015,

   [RFC8200]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", STD 86, RFC 8200,
              DOI 10.17487/RFC8200, July 2017,

   [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,

   [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, <>.

   [RFC8663]  Xu, X., Bryant, S., Farrel, A., Hassan, S., Henderickx,
              W., and Z. Li, "MPLS Segment Routing over IP", RFC 8663,
              DOI 10.17487/RFC8663, December 2019,

   [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,

Appendix A.  Solutions Evaluation

   The following are solutions that have been proposed to solve BIER in
   IPv6 environments.  Some solutions propose encoding while others
   propose encapsulation.  It is recommended for the wg to evaluate
   these solutions against the requirements listed previously in order
   to make informed decisions on solution readiness.

   As illustrated in these examples, the BIER header, or the BitString,
   may appear in the IPv6 Header, IPv6 Extension Header, IPv6 Payload,
   or IPv6 Tunnel Packet:

A.1.  BIER-ETH encapsulation in IPv6 networks

         |   Ethernet    |   BIER header   | payload
         |  (ethType =   | (BIFT-id, ...)  |
         |    0xAB37)    |                 |
         |               |  Next Header    |

   BIER-ETH encapsulation (BIER header for Non-MPLS networks as defined
   in [RFC8296]) can be used to transport the multicast data in the IPv6
   network by encapsulating the multicast user data payload within the
   BIER-ETH header.  However, BIER-ETH in IPv6 networks is not
   considered to be a "BIER natively in IPv6" solution which utilizes
   the IPv6 header to forward the packet.

   Mixed use of BIER-ETH in a native IPv6 solution is up to the solution
   and is outside the scope of this document.

A.2.  Encode Bitstring in IPv6 destination address

      |  IPv6 header  | payload
      | (BitString in |
      | DA lower bits)|
      |  Next Header  |

   As described in [I-D.pfister-bier-over-ipv6], The information
   required by BIER is stored in the destination IPv6 address.  The BIER
   BitString is encoded in the low-order bits of the IPv6 destination
   address of each packet.  The high-order bits of the IPv6 destination
   address are used by intermediate routers for unicast forwarding,
   deciding whether a packet is a BIER packet, and if so, to identify
   the BIER Sub-Domain, Set Identifier and BitString length.  No
   additional extension or encapsulation header is required.  Instead of
   encapsulating the packet in IPv6, the payload is attached to the BIER
   IPv6 header and the IPv6 protocol number is set to the type of the
   payload.  If the payload is UDP, the UDP checksum needs to change
   when the BitString in the IPv6 destination address changes.

A.3.  Add BIER header into IPv6 Extension Header

      |  IPv6 header  | IPv6 Ext header | payload
      |               | (BIER header in |
      |               |  TLV Type = X)  |
      | Next Header   |   Next Header   |

   According to [RFC8200] In IPv6, optional internet-layer information
   is encoded in separate headers that may be placed between the IPv6
   header and the upper- layer header in a packet.  There is a small
   number of such extension headers, each one identified by a distinct
   Next Header value.  An IPv6 packet may carry zero, one, or more
   extension headers, each identified by the Next Header field of the
   preceding header.  Extension headers (except for the Hop-by-Hop
   Options header) are not processed, inserted, or deleted by any node
   along a packet's delivery path, until the packet reaches the node (or
   each of the set of nodes, in the case of multicast) identified in the
   Destination Address field of the IPv6 header.  The Hop-by-Hop Options
   header is not inserted or deleted, but may be examined or processed
   by any node along a packet's delivery path, until the packet reaches
   the node (or each of the set of nodes, in the case of multicast)
   identified in the Destination Address field of the IPv6 header.  The
   Hop-by-Hop Options header, when present, must immediately follow the
   IPv6 header.  Its presence is indicated by the value zero in the Next
   Header field of the IPv6 header.

   Two of the currently-defined extension headers are the Hop-by-Hop
   Options header and the Destination Options header which carry a
   variable number of type-length-value (TLV) encoded "options".

   In [I-D.xie-bier-ipv6-encapsulation] an IPv6 BIER Destination Option
   is carried by the IPv6 Destination Option Header (indicated by a Next
   Header value 60).  It is initialized in a packet sent by an IPv6 BFIR
   router to inform the following BFR routers in an IPv6 BIER domain to
   replicate to destination BFER routers hop-by-hop.  BIER is generally
   a hop-by-hop and one-to-many architecture and it is required for a
   BIER IPv6 encapsulation to include the BIER Header ([RFC8296]) as an
   IPv6 Extension Header, to pilot the hop-by-hop BIER replication.

   Hop by hop Options Headers may be considered.  The Hop-by-Hop Options
   header is used to carry optional information that may be examined and
   processed by every node along a packet's delivery path.  The Hop-by-
   Hop Options header is identified by a Next Header value of 0 in the
   IPv6 header.

   Defining New Extension Headers and Options may also be considered, if
   the IPv6 Destination Option Header is not good enough and new
   extension headers can solve the problem better.

   Such proposals may include requests to IANA to allocate a "BIER
   Option" code from "Destination Options and Hop-by-Hop Options", and/
   or a "BIER Option Header" code from "IPv6 Extension Header Types".

A.4.  Transport BIER as IPv6 payload

      |  IPv6 header  | IPv6 Ext header | BIER Hdr + payload
      |               |    (optional)   | as IPv6 payload
      |               |                 |
      | Next Header   | Next Header = X |

   There is a proposal for a transport-independent BIER encapsulation
   header which is applicable regardless of the underlying transport
   technology.  As described in [I-D.xu-bier-encapsulation] and
   [I-D.zhang-bier-bierin6], the BIER header, and the payload following
   it, can be combined as an IPv6 payload, and be indicated by a new
   Upper-layer IPv6 Next-Header value.  A unicast IPv6 destination
   address is used for the replication and changes when replicating a
   packet out to a neighbor.

   Such proposals may include a request to IANA to allocate an IPv6
   Next-Header code from "Assigned Internet Protocol Numbers".

A.5.  Tunnelling BIER in a IPv6 tunnel

      |  IPv6 header  | IPv6 Ext header | GRE header |
      |               |    (optional)   |            | BIER Hdr +
      |               |                 |            | payload as GRE
      | Next Header   |   Next Header   | Proto = X  | Payload

   A generic IPv6 Tunnel could be used to encapsulate the bier packet
   within an IPv6 domain.

   GRE is a mechanism by which any ethernet payload can be carried by an
   IP GRE tunnel due to the 16-bits 'Protocol Type' field.  Both IPv4
   and IPv6 can be used to carry GRE.  The Ethernet type codepoint
   0xAB37, defined for BIER, can be used in a GRE header to indicate the
   subsequent BIER header and payload in an IPv6 network.

      |  IPv6 header  | IPv6 Ext header | UDP header |
      |               |    (optional)   |            | BIER Hdr +
      |               |                 |            | payload as UDP
      | Next Header   |   Next Header   | DPort = X  | Payload

   UDP-based tunnelling is another mechanism which uses a specific UDP
   port to indicate a UDP payload format.  Both IPv4 and IPv6 can
   support UDP.  Such UDP-based tunnels can be used for BIER in a IPv6
   network by defining a new UDP port to indicate the BIER header and

Authors' Addresses
   Mike McBride


   Jingrong Xie


   Senthil Dhanaraj


   Rajiv Asati


   Yongqing Zhu
   China Telecom

   Gyan S. Mishra
   Verizon Inc.

               13101 Columbia Pike

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               United States of America

             301 502-1347