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BIER Penultimate Hop Popping

Document Type Active Internet-Draft (bier WG)
Author Zhaohui (Jeffrey) Zhang
Last updated 2023-03-09
Replaces draft-zzhang-bier-php
RFC stream Internet Engineering Task Force (IETF)
Additional resources Mailing list discussion
Stream WG state WG Consensus: Waiting for Write-Up
Doc Shepherd Follow-up Underway
Document shepherd Xiao Min
IESG IESG state I-D Exists
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BIER                                                            Z. Zhang
Internet-Draft                                          Juniper Networks
Intended status: Standards Track                            9 March 2023
Expires: 10 September 2023

                      BIER Penultimate Hop Popping


   Bit Index Explicit Replication (BIER) can be used as provider tunnel
   for Multicast Virtual Private Network (MVPN), Global Table Multicast
   or Ethernet Virtual Private Network (EVPN).  It is possible that not
   all routers in the provider network support BIER and there are
   various methods to handle BIER-incapable transit routers.  However
   those methods assume the MVPN/EVPN Provider Edges (PEs) are BIER-
   capable.  This document specifies a method to allow BIER-incapable
   routers to act as MVPN/EVPN PEs with BIER as the transport, by having
   the upstream BIER Forwarding Router (BFR) that is connected directly
   or indirectly via a tunnel to a BIER-incapable PE remove the BIER
   header and send the payload to the PE.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "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.

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

   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 10 September 2023.

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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 (
   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.  Specifications  . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Signaling . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.2.  BIRT/BIFT Calculation . . . . . . . . . . . . . . . . . .   5
   3.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   5.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   6
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   The BIER architecture includes three layers: the "routing underlay",
   the "BIER layer", and the "multicast flow overlay".  The multicast
   flow overlay is responsible for the BIER Forwarding Egress Routers
   (BFERs) to signal to BIER Forwarding Ingress Routers (BFIRs) that
   they are interested in receiving certain multicast flows so that
   BFIRs can encode the correct bitstring for BIER forwarding by the
   BIER layer.

   MVPN [RFC6513] [RFC6514] and EVPN [RFC7432] are two similar overlays
   where BGP Auto-Discovery routes for MVPN/EVPN are exchanged among all
   PEs to signal which PEs need to receive multicast traffic for all or
   certain flows.  Typically the same provider tunnel type is used for
   traffic to reach all receiving PEs.

   Consider an MVPN/EVPN deployment where enough provider routers are
   BIER-capable for BIER to become the preferred choice of provider
   tunnel [RFC8556] [I-D.ietf-bier-evpn].  However, some PEs cannot be
   upgraded to support BIER forwarding.  While there are ways to allow

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   an ingress PE to send traffic to some PEs with one type of tunnel and
   send traffic to some other PEs with a different type of tunnel, the
   procedure becomes complicated and forwarding is not optimized.

   One way to solve this problem is to use Penultimate Hop Popping (PHP)
   so that the upstream BFR can pop the BIER header [RFC8296] and send
   the payload "natively" (note that the upstream BFR can be connected
   directly or indirectly via any type of tunnel to the PE).  This is
   similar to Multi-Protocol Label Switching (MPLS) PHP though it is the
   BIER header that is popped.

   The transition of an existing MVPN/EVPN deployment with traditional
   provider tunnels to using BIER with some PEs not capable of receiving
   BIER packets can be incremental.  All PEs are first upgraded to
   support BIER at least in the control plane, with those not capable of
   BIER forwarding requesting PHP.  Then BIER-capable ingress PEs
   independently and incrementally switch to BIER transport.

   While the above text uses MVPN/EVPN as example, BIER PHP is
   applicable to any scenario where the multicast flow overlay edge
   router does not support BIER, as long as the edge router does not
   need to know the transmitting BFIR or participate in BIER OAM

   This works well if a BIER-incapable PE only needs to receive
   multicast traffic.  If it needs to send multicast traffic as well,
   then it must Ingress Replicate to a BIER-capable helper PE, who will
   in turn relay the packet to other PEs.  The helper PE is either a
   Virtual Hub as specified in [RFC7024] for MVPN and
   [I-D.ietf-bess-evpn-virtual-hub] for EVPN, or an AR-Replicator as
   specified in [I-D.ietf-bess-evpn-optimized-ir] for EVPN.

2.  Specifications

   The BIER Penultimate Hop Popping is intended only for the scenario
   where a multicast flow overlay router for a BIER domain does not
   support BIER forwarding, either entirely or just for some particular
   BitStringLengths (BSL).  In the latter case, PHP is only for BIER
   packets with those BSL.  The flow overlay router would be a BFER if
   it did support BIER forwarding, and PHP would not be done by its
   penultimate hop.

   The procedures in this section apply only if, by means outside the
   scope of this document, it is known that all potential penultimate
   hop BFRs support PHP (i.e., able to pop the BIER header when sending
   to a requesting flow overlay router) , and that the payload after
   BIER header is one of the following:

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   *  MPLS packets with downstream-assigned label at top of stack (i.e.,
      the Proto field in the BIER header is 1).  For example, a label
      from a Domain-wide Common Block (DCB) is used as specified in

   *  IPv4/IPv6 multicast packets for which Reverse Path Forwarding
      check is disabled.

2.1.  Signaling

   With IS-IS signaling, a sub-TLV in another sub-TLV is called sub-sub-
   TLV (and more sub-levels are possible like sub-sub-sub-TLV).  With
   other signaling protocols, a sub-TLV in another sub-TLV is still
   called sub-TLV.  For convenience, in this document we use sub-TLV
   even when it is sub-sub-TLV in IS-IS, as there is no ambiguity with
   the name itself (e.g.  MPLS Encapsulation).

   A BIER-incapable router, if acting as a multicast flow overlay router
   for BIER, MUST signal its BIER information as specified in [RFC8401],
   [RFC8444], [I-D.ietf-bier-ospfv3-extensions], or
   [I-D.ietf-bier-idr-extensions] with a PHP sub-TLV included in the
   BIER sub-TLV (or TLV in case of BGP) attached to the BIER-incapable
   router's BFR-prefix to request BIER PHP from other BFRs.  The type of
   the sub-TLV or sub-TLV is TBD, and the length is 0.

   With MPLS encapsulation, the BIER-incapable multicast flow overlay
   router MAY omit the BIER MPLS Encapsulation sub-LV, or MUST set the
   Label field in BIER MPLS Encapsulation sub-TLV to Implicit Null Label

   With MPLS encapsulation, if a BFER (that does support BIER but) does
   not support a certain BSL, it MAY advertise a corresponding BIER MPLS
   Encapsulation sub-TLV with the Label field to Implicit Null Label to
   request PHP for that BSL.  It MUST NOT include the PHP sub-TLV in
   this case.

   With non-MPLS encapsulation [I-D.ietf-bier-lsr-non-mpls-extensions],
   the BIER-incapable multicast flow overlay router MAY omit the BIER
   non-MPLS Encapsulation sub-TLV, or MUST set the BIFT-id field in the
   BIER non-MPLS Encapsulation sub-TLV to 0.

   With non-MPLS encapsulation, if a BFER (that does support BIER but)
   does not support certain BSL, it MAY advertise a corresponding BIER
   non-MPLS Encapsulation sub-TLV but set the BIFT-id field to 0 to
   request PHP for that BSL.  It MUST NOT include the PHP sub-TLV in
   this case.

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2.2.  BIRT/BIFT Calculation

   If a BFR follows section 6.9 of [RFC8279] to handle BIER-incapable
   routers, it MUST treat a router as BIER-incapable for a BSL if the
   label in the corresponding MPLS Encapsulation sub-TLV advertised by
   the router is Implicit Null, or if the BIFT-id in the corresponding
   non-MPLS Encapsulation sub-TLV is 0.  It MUST treat the router as
   BIER-incapable for all BSLs if the router advertises a PHP sub-TLV.
   That way, the router will not used as a transit BFR for certain or
   for all BSLs.

   If the downstream neighbor (either resulting in IGP calculation or
   carried in the BIER Nexthop sub-TLV in case of BGP) for a BFR-prefix
   is the one advertising the prefix with a PHP sub-TLV or with an
   Implicit Null Label in its BIER MPLS Encapsulation sub-TLV, or with
   BIFT-id 0 in its BIER non-MPLS Encapsulation sub-TLV, then when the
   corresponding BIRT or BIFT entry is created/updated, the forwarding
   behavior MUST be that the BIER header is removed and the payload be
   sent to the downstream router without the BIER header, either
   directly or over any type of tunnel.

3.  Security Considerations

   This specification does not introduce additional security concerns
   beyond those already discussed in BIER architecture and OSPF/IS-IS/
   BGP extensions for BIER signaling.

4.  IANA Considerations

   This document requests a new sub-sub-TLV type value from the "Sub-
   sub-TLVs for BIER Info Sub-TLV" registry within the "IS-IS TLV
   Codepoints" registry:

        Type    Name
        ----    ----
        TBD     BIER PHP Request

   This document requests a new sub-TLV type value from the OSPFv2
   Extended Prefix TLV Sub-TLV registry:

        Type    Name
        ----    ----
        TBD     BIER PHP Request

   This document requests a new sub-TLV type value from the OSPFv3
   Extended LSA Sub-TLVs registry:

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        Type    Name
        ----    ----
        TBD     BIER PHP Request

   This document requests a new sub-TLV type value from the BGP BIER TLV
   sub-TLV Types registry requested in [I-D.ietf-bier-idr-extensions]:

        Type    Name
        ----    ----
        TBD     BIER PHP Request

5.  Acknowledgements

   The author wants to thank Eric Rosen and Antonie Przygienda for their
   review, comments and suggestions.  The author also wants to thank
   Senthil Dhanaraj for his suggestion of requesting PHP if a BFER does
   not support certain BSL.

6.  References

6.1.  Normative References

              Zhang, Z. J., Rosen, E. C., Lin, W., Li, Z., and I.
              Wijnands, "MVPN/EVPN Tunnel Aggregation with Common
              Labels", Work in Progress, Internet-Draft, draft-ietf-
              bess-mvpn-evpn-aggregation-label-09, 12 December 2022,

              Zhang, Z. J., Przygienda, T., Sajassi, A., and J. Rabadan,
              "EVPN BUM Using BIER", Work in Progress, Internet-Draft,
              draft-ietf-bier-evpn-08, 4 January 2023,

              Xu, X., Chen, M., Patel, K., Wijnands, I., Przygienda, T.,
              and Z. J. Zhang, "BGP Extensions for BIER", Work in
              Progress, Internet-Draft, draft-ietf-bier-idr-extensions-
              09, 15 February 2023,

              Dhanaraj, S., Yan, G., Wijnands, I., Psenak, P., Zhang, Z.
              J., and J. Xie, "LSR Extensions for BIER non-MPLS

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              Encapsulation", Work in Progress, Internet-Draft, draft-
              ietf-bier-lsr-non-mpls-extensions-01, 19 September 2022,

              Psenak, P., Nainar, N. K., and I. Wijnands, "OSPFv3
              Extensions for BIER", Work in Progress, Internet-Draft,
              draft-ietf-bier-ospfv3-extensions-07, 1 December 2022,

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

   [RFC3032]  Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
              Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
              Encoding", RFC 3032, DOI 10.17487/RFC3032, January 2001,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 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, <>.

   [RFC8401]  Ginsberg, L., Ed., Przygienda, T., Aldrin, S., and Z.
              Zhang, "Bit Index Explicit Replication (BIER) Support via
              IS-IS", RFC 8401, DOI 10.17487/RFC8401, June 2018,

   [RFC8444]  Psenak, P., Ed., Kumar, N., Wijnands, IJ., Dolganow, A.,
              Przygienda, T., Zhang, J., and S. Aldrin, "OSPFv2
              Extensions for Bit Index Explicit Replication (BIER)",
              RFC 8444, DOI 10.17487/RFC8444, November 2018,

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   [RFC8556]  Rosen, E., Ed., Sivakumar, M., Przygienda, T., Aldrin, S.,
              and A. Dolganow, "Multicast VPN Using Bit Index Explicit
              Replication (BIER)", RFC 8556, DOI 10.17487/RFC8556, April
              2019, <>.

6.2.  Informative References

              Rabadan, J., Sathappan, S., Lin, W., Katiyar, M., and A.
              Sajassi, "Optimized Ingress Replication Solution for
              Ethernet VPN (EVPN)", Work in Progress, Internet-Draft,
              draft-ietf-bess-evpn-optimized-ir-12, 25 January 2022,

              Patel, K., Sajassi, A., Drake, J., Zhang, Z. J., and W.
              Henderickx, "Virtual Hub-and-Spoke in BGP EVPNs", Work in
              Progress, Internet-Draft, draft-ietf-bess-evpn-virtual-
              hub-00, 26 January 2020,

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

   [RFC6514]  Aggarwal, R., Rosen, E., Morin, T., and Y. Rekhter, "BGP
              Encodings and Procedures for Multicast in MPLS/BGP IP
              VPNs", RFC 6514, DOI 10.17487/RFC6514, February 2012,

   [RFC7024]  Jeng, H., Uttaro, J., Jalil, L., Decraene, B., Rekhter,
              Y., and R. Aggarwal, "Virtual Hub-and-Spoke in BGP/MPLS
              VPNs", RFC 7024, DOI 10.17487/RFC7024, October 2013,

   [RFC7432]  Sajassi, A., Ed., Aggarwal, R., Bitar, N., Isaac, A.,
              Uttaro, J., Drake, J., and W. Henderickx, "BGP MPLS-Based
              Ethernet VPN", RFC 7432, DOI 10.17487/RFC7432, February
              2015, <>.

Author's Address

   Zhaohui Zhang
   Juniper Networks

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