BIER Penultimate Hop Popping
draft-ietf-bier-php-16

BIER                                                            Z. Zhang
Internet-Draft                                          Juniper Networks
Intended status: Standards Track                         4 December 2024
Expires: 7 June 2025

                      BIER Penultimate Hop Popping
                         draft-ietf-bier-php-16

Abstract

   This document specifies a mechanism for Penultimate Hop Popping (PHP)
   in the Bit Index Explicit Replication (BIER) architecture.  PHP
   enables the removal of the BIER header by the penultimate router,
   thereby reducing the processing burden on the final router in the
   delivery path.  This extension to BIER enhances operational
   efficiency by optimizing packet forwarding in scenarios where the
   final hop's capabilities or requirements necessitate such handling.
   The document details the necessary extensions to the BIER
   encapsulation and forwarding processes to support PHP, providing
   guidance for implementation and deployment within BIER-enabled
   networks.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "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 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 7 June 2025.

Zhang                      Expires 7 June 2025                  [Page 1]
Internet-Draft                  bier-php                   December 2024

Copyright Notice

   Copyright (c) 2024 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 (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.  Specifications  . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Signaling . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.2.  BIRT/BIFT Calculation . . . . . . . . . . . . . . . . . .   5
   3.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   4.  Operational Considerations  . . . . . . . . . . . . . . . . .   5
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   6
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   The Bit Index Explicit Replication (BIER) architecture [RFC8279]
   consists of three layers: the "routing underlay", the "BIER layer",
   and the "multicast flow overlay".  The multicast flow overlay is
   responsible for allowing BIER Forwarding Egress Routers (BFERs) to
   signal to BIER Forwarding Ingress Routers (BFIRs) their interest in
   receiving specific multicast flows, enabling BFIRs to encode the
   appropriate bitstring for forwarding by the BIER layer.

   Multicast Virtual Private Network (MVPN) [RFC6513] [RFC6514] and
   Ethernet VPN (EVPN) [RFC7432] are two analogous overlays in which BGP
   Auto-Discovery routes for MVPN/EVPN are exchanged among all Provider
   Edge (PE) routers to signal which PEs should receive multicast
   traffic for all or certain flows.  Typically, a consistent provider
   tunnel type is used for traffic delivery to all receiving PEs.

Zhang                      Expires 7 June 2025                  [Page 2]
Internet-Draft                  bier-php                   December 2024

   In a deployment scenario where MVPN/EVPN is in use and a sufficient
   number of provider routers support BIER, BIER can become the
   preferred provider tunnel type [RFC8556] [RFC9624] . However, some
   PEs may lack the capability to support BIER forwarding (they are
   referred to as BIER-incapable in this document).  While it is
   possible for an ingress PE to send traffic to some PEs using one type
   of tunnel and to others using a different type, such a procedure can
   be complex and may result in suboptimal forwarding.

   A potential solution to this issue is the use of Penultimate Hop
   Popping (PHP), whereby the upstream BIER Forwarding Router (BFR) pops
   the BIER header [RFC8296] and transmits the payload without BIER
   encapsulation.  This transmission can occur either directly or
   indirectly through any type of tunnel to the PE.  This mechanism is
   analogous to Multi-Protocol Label Switching (MPLS) PHP, except that
   the BIER header is removed.

   The transition from an existing MVPN/EVPN deployment with
   conventional provider tunnels to a BIER-based solution, where some
   PEs are not BIER-capable, can be incremental.  Initially, all PEs are
   upgraded to support BIER in the control plane, with those unable to
   perform BIER forwarding requesting PHP.  Subsequently, BIER-capable
   ingress PEs can independently and incrementally switch to BIER
   transport.

   While MVPN/EVPN is used as an example in the above discussion, BIER
   PHP is applicable to any scenario where the multicast flow overlay
   edge router does not support BIER, provided that the edge router does
   not need to identify the transmitting BFIR or participate in BIER
   Operations, Administration, and Maintenance (OAM) procedures.

   This approach is effective when a BIER-incapable PE only needs to
   receive multicast traffic.  However, if the PE also needs to send
   multicast traffic, it must perform Ingress Replication to a BIER-
   capable helper PE, which will then relay the packet to other PEs.
   The helper PE may be a Virtual Hub as defined in [RFC7024] for MVPN
   and [I-D.ietf-bess-evpn-virtual-hub] for EVPN, or an AR-Replicator as
   defined in [RFC9574] for EVPN.

2.  Specifications

   The BIER Penultimate Hop Popping (PHP) mechanism is designed
   specifically for scenarios where a multicast flow overlay router
   within a BIER domain does not support BIER forwarding, either
   completely or for specific BitStringLengths (BSL).  In the latter
   case, PHP applies only to BIER packets with those particular BSLs.
   If the flow overlay router were capable of BIER forwarding, it would
   function as a BFER, and PHP would not be performed by the penultimate

Zhang                      Expires 7 June 2025                  [Page 3]
Internet-Draft                  bier-php                   December 2024

   hop.

   The procedures outlined in this section are applicable only if,
   through means outside the scope of this document, it is established
   that all potential penultimate hop BFRs are capable of supporting PHP
   (i.e., able to remove the BIER header when forwarding to a requesting
   flow overlay router) and that the payload following the BIER header
   is one of the following:

   *  MPLS packets with a downstream-assigned label at the top of the
      stack (i.e., the Proto field in the BIER header is set to 1).  For
      instance, a label from a Domain-wide Common Block (DCB) as
      specified in [RFC9573].

   *  IPv4/IPv6 multicast packets for which the Reverse Path Forwarding
      (RPF) check is disabled.

2.1.  Signaling

   In IS-IS signaling, a sub-TLV nested within another sub-TLV is
   referred to as a sub-sub-TLV (and further levels are possible, such
   as sub-sub-sub-TLV).  In other signaling protocols, a sub-TLV nested
   within another sub-TLV is still referred to as a sub-TLV.  For
   convenience, this document uses the term "sub-TLV" even when
   referring to a sub-sub-TLV in IS-IS, as there is no ambiguity in the
   terminology (e.g., MPLS Encapsulation).

   A BIER-incapable router, when functioning 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], including a PHP sub-TLV within the
   BIER sub-TLV (or TLV in the 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-sub-TLV is TBD, and the length is 0.

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

   In the case of MPLS encapsulation, if a BFER (which supports BIER but
   not a specific BSL) does not support a particular BSL, it MAY
   advertise a corresponding BIER MPLS Encapsulation sub-TLV with the
   Label field set to the Implicit Null Label to request PHP for that
   BSL.  In this scenario, the PHP sub-TLV MUST NOT be included.

Zhang                      Expires 7 June 2025                  [Page 4]
Internet-Draft                  bier-php                   December 2024

   For 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 it MUST set the BIFT-id field in
   the BIER non-MPLS Encapsulation sub-TLV to 0.

   Similarly, for non-MPLS encapsulation, if a BFER (which supports BIER
   but not a specific BSL) does not support a particular 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.  In this
   scenario, the PHP sub-TLV MUST NOT be included.

2.2.  BIRT/BIFT Calculation

   If a BFR adheres to Section 6.9 of [RFC8279] for handling BIER-
   incapable routers, it MUST treat a router as BIER-incapable for a
   specific 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.
   Additionally, the router MUST be treated as BIER-incapable for all
   BSLs if it advertises a PHP sub-TLV.  Consequently, the router will
   not be utilized as a transit BFR for certain or all BSLs.

   When the downstream neighbor (whether determined through IGP
   calculation or indicated in the BIER Nexthop sub-TLV in the case of
   BGP) for a BFR-prefix is the router that advertises the prefix with a
   PHP sub-TLV, an Implicit Null Label in its BIER MPLS Encapsulation
   sub-TLV, or a BIFT-id of 0 in its BIER non-MPLS Encapsulation sub-
   TLV, then, upon the creation or update of the corresponding BIRT or
   BIFT entry, the forwarding behavior MUST be that the BIER header is
   removed and the payload is forwarded 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 [RFC8279] and
   BIER signaling extensions to OSPF [RFC8444], IS-IS [RFC8401] and BGP
   [I-D.ietf-bier-idr-extensions].

4.  Operational Considerations

   BIER PHP can only be used when the conditions specified in Section 2
   are met.  The BIER OAM functionality is not available on the BIER-
   incapable flow overlay routers, but using PHP when the conditions are
   met is simpler than the alternative of using BIER to send to some
   whereas using non-BIER tunnels to send to other flow overlay routers.

Zhang                      Expires 7 June 2025                  [Page 5]
Internet-Draft                  bier-php                   December 2024

   [RFC8279] states that when the BIER header is decapsulated, by
   default its Time to Live (TTL) value is not inherited by the payload.
   That means in the PHP case, the payload's TTL will be decremented
   once more than in the non-PHP case.

5.  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:

        Type    Name                 L2BM
        ----    ----                 ----
        TBD     BIER PHP Request     X

   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

6.  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 the suggestion of requesting PHP if a BFER does
   not support certain BSL.

7.  References

7.1.  Normative References

Zhang                      Expires 7 June 2025                  [Page 6]
Internet-Draft                  bier-php                   December 2024

   [I-D.ietf-bier-idr-extensions]
              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-
              15, 22 November 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-bier-
              idr-extensions-15>.

   [I-D.ietf-bier-lsr-non-mpls-extensions]
              Dhanaraj, S., Yan, G., Wijnands, I., Psenak, P., Zhang, Z.
              J., and J. Xie, "LSR Extensions for BIER non-MPLS
              Encapsulation", Work in Progress, Internet-Draft, draft-
              ietf-bier-lsr-non-mpls-extensions-03, 7 February 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-bier-
              lsr-non-mpls-extensions-03>.

   [I-D.ietf-bier-ospfv3-extensions]
              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,
              <https://datatracker.ietf.org/doc/html/draft-ietf-bier-
              ospfv3-extensions-07>.

   [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>.

   [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,
              <https://www.rfc-editor.org/info/rfc3032>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [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>.

Zhang                      Expires 7 June 2025                  [Page 7]
Internet-Draft                  bier-php                   December 2024

   [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,
              <https://www.rfc-editor.org/info/rfc8401>.

   [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,
              <https://www.rfc-editor.org/info/rfc8444>.

   [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, <https://www.rfc-editor.org/info/rfc8556>.

   [RFC9573]  Zhang, Z., Rosen, E., Lin, W., Li, Z., and IJ. Wijnands,
              "MVPN/EVPN Tunnel Aggregation with Common Labels",
              RFC 9573, DOI 10.17487/RFC9573, May 2024,
              <https://www.rfc-editor.org/info/rfc9573>.

   [RFC9624]  Zhang, Z., Przygienda, T., Sajassi, A., and J. Rabadan,
              "EVPN Broadcast, Unknown Unicast, or Multicast (BUM) Using
              Bit Index Explicit Replication (BIER)", RFC 9624,
              DOI 10.17487/RFC9624, August 2024,
              <https://www.rfc-editor.org/info/rfc9624>.

7.2.  Informative References

   [I-D.ietf-bess-evpn-virtual-hub]
              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,
              <https://datatracker.ietf.org/doc/html/draft-ietf-bess-
              evpn-virtual-hub-00>.

   [RFC6513]  Rosen, E., Ed. and R. Aggarwal, Ed., "Multicast in MPLS/
              BGP IP VPNs", RFC 6513, DOI 10.17487/RFC6513, February
              2012, <https://www.rfc-editor.org/info/rfc6513>.

   [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,
              <https://www.rfc-editor.org/info/rfc6514>.

Zhang                      Expires 7 June 2025                  [Page 8]
Internet-Draft                  bier-php                   December 2024

   [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,
              <https://www.rfc-editor.org/info/rfc7024>.

   [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, <https://www.rfc-editor.org/info/rfc7432>.

   [RFC9574]  Rabadan, J., Ed., Sathappan, S., Lin, W., Katiyar, M., and
              A. Sajassi, "Optimized Ingress Replication Solution for
              Ethernet VPNs (EVPNs)", RFC 9574, DOI 10.17487/RFC9574,
              May 2024, <https://www.rfc-editor.org/info/rfc9574>.

Author's Address

   Zhaohui Zhang
   Juniper Networks
   Email: zzhang@juniper.net

Zhang                      Expires 7 June 2025                  [Page 9]