BIER                                                            Z. Zhang
Internet-Draft                                          Juniper Networks
Intended status: Standards Track                        December 7, 2021
Expires: June 10, 2022

                      BIER Penultimate Hop Popping


   Bit Index Explicit Replication (BIER) can be used as provider tunnel
   for Multicast Virtual Private Network (MVPN).  Global Table Multicast
   [RFC7716] 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",
   document are to be interpreted as described in 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
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   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
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   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 June 10, 2022.

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Copyright Notice

   Copyright (c) 2021 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
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   ( in effect on the date of
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Specifications  . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   5.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   5
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   6
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   7

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 and EVPN 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.  However, some PEs cannot be upgraded to support BIER
   forwarding.  While there are ways to allow an ingress PE to send
   traffic to some PEs with one type of tunnel and send traffic to some

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   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 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.  In the latter case, PHP is only for BIER packets
   with those BitStringLengths.  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 the payload after BIER
   header is one of the following:

   o  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

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      from a Domain-wide Common Block (DCB) is used as specified in [I-

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

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

   With MPLS encapsulation, the BIER-incapable multicast flow overlay
   router MAY omit the BIER MPLS Encapsulation sub-sub-TLV, or MUST set
   the Label field in BIER MPLS Encapsulation sub-sub-TLV to Implicit
   Null Label [RFC3032].

   With MPLS encapsulation, if a BFER does not support certain BSL, it
   MAY still advertise a corresponding BIER MPLS Encapsulation sub-TLV
   but set the Label field to Implicit Null Label.

   If a BFR follows section 6.9 of [RFC8279] to handle BIER-incapable
   routers, it must treat a router as BIER-incapable if the label
   advertised by the router is Implicit Null, or if the router
   advertises a PHP sub-sub-TLV, so that the router is not used as a
   transit BFR.

   If the downstream neighbor for a BIER prefix is the one advertising
   the prefix with a PHP sub-sub-TLV or with an Implicit Null Label in
   the Label field in its BIER MPLS Encapsulation sub-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/ISIS/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 in the "IS-IS TLV
   Codepoints" registry:

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

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

        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., Rosen, E., Lin, W., Li, Z., and I. Wijnands,
              "MVPN/EVPN Tunnel Aggregation with Common Labels", draft-
              ietf-bess-mvpn-evpn-aggregation-label-06 (work in
              progress), April 2021.

              Zhang, Z., Przygienda, A., Sajassi, A., and J. Rabadan,
              "EVPN BUM Using BIER", draft-ietf-bier-evpn-05 (work in
              progress), December 2021.

              Xu, X., Chen, M., Patel, K., Wijnands, I., and A.
              Przygienda, "BGP Extensions for BIER", draft-ietf-bier-
              idr-extensions-07 (work in progress), September 2019.

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

   [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,

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   [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,

6.2.  Informative References

              Rabadan, J., Sathappan, S., Lin, W., Katiyar, M., and A.
              Sajassi, "Optimized Ingress Replication Solution for
              Ethernet VPN (EVPN)", draft-ietf-bess-evpn-optimized-ir-11
              (work in progress), November 2021.

              Patel, K., Sajassi, A., Drake, J. E., Zhang, Z., and W.
              Henderickx, "Virtual Hub-and-Spoke in BGP EVPNs", draft-
              ietf-bess-evpn-virtual-hub-00 (work in progress), January

   [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,

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   [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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