BIER                                                            Z. Zhang
Internet-Draft                                          Juniper Networks
Intended status: Standards Track                                E. Rosen
Expires: January 11, 2023                                     D. Awduche
                                                             G. Shepherd
                                                           July 10, 2022

                      Multicast/BIER As A Service


   This document describes a framework for providing multicast as a
   service via Bit Index Explicit Replication (BIER) [RFC7279], and
   specifies a few enhancements to [draft-ietf-bier-idr-extensions]
   [RFC8279] [RFC8401] [RFC8444] to enable multicast/BIER as a service.

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
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   This Internet-Draft will expire on January 11, 2023.

Copyright Notice

   Copyright (c) 2022 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminologies . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  A CDN of A Single Provider  . . . . . . . . . . . . . . .   4
       1.2.1.  IGP/BGP Interworking  . . . . . . . . . . . . . . . .   5
     1.3.  A CDN That Involves Another Provider  . . . . . . . . . .   6
       1.3.1.  Providing Independent BAAS To Multiple Customers  . .   6
       1.3.2.  Control and Accounting  . . . . . . . . . . . . . . .   7
     1.4.  Sets and Segmentation . . . . . . . . . . . . . . . . . .   8
       1.4.1.  Multiple Sets . . . . . . . . . . . . . . . . . . . .   8
       1.4.2.  Segmentation  . . . . . . . . . . . . . . . . . . . .   8
   2.  Specifications for Enhancements to BIER Signaling with
       BGP/IGP . . . . . . . . . . . . . . . . . . . . . . . . . . .   9
     2.1.  BGP Procedures  . . . . . . . . . . . . . . . . . . . . .   9
     2.2.  ISIS/OSPF Procedures  . . . . . . . . . . . . . . . . . .  10
   3.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   5.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  11
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  12
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  12
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

1.  Introduction

   Currently multicast is primarily used in the following scenarios:

   o  Enterprise Applications.  For example, large scale financial data

   o  Provider/underlay tunnels for MVPN and for EVPN BUM.

   o  Real-time IPTV offered by a service provider to its customers.

   Besides the above, large scale multicast services, especially transit
   multicast transport provided by large Internet Service Providers is

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   virtually non-existent.  This is mainly because of the following
   chicken and egg dilemma:

   o  Traditional multicast technologies are complicated and lack
      scalability.  The revenue that multicast services bring in cannot
      offset the Capex and Opex that an operator has to invest, so
      provider networks typically do not enable multicast even though
      the deployed equipment does support multicast.

   o  As a result, Content Providers cannot take advantage of multicast
      and instead use less efficient methods like Ingress Replication,
      Peer2Peer, or multicast at application layer.

   A recent multicast technology breakthrough, BIER, provides a simple
   and scalable solution for large scale multicast deployment,
   independent of number of multicast flows.  In the meantime, large
   scale distribution of ultra high definition video content has become
   more and more popular and important.  Service providers simply cannot
   keep on increasing their network capacity even if they could shift
   cost to Content Providers.  With these developments, service
   providers now have both the need and means to provide scalable
   multicast service, potentially across multiple providers.

   This document describes a framework for Multicast As A Service (MAAS)
   enabled by BIER.  We use Content Delivery Network (CDN) as example,
   though it applies to any large scale multicast delivery service.

1.1.  Terminologies

   Readers are assumed to be familiar with multicast, BIER, BGP and
   ISIS/OSPF concepts and procedures.  Some terminologies are listed
   here for convenience.

   o  BFR: BIER Forwarding Router.

   o  BFIR: BIER Forwarding Ingress Router.

   o  BFER: BIER Forwarding Egress Router.

   o  EBFR: Edge BFR.  Including BFIR and BFER.

   o  BSL: BitStrengLength.  Number of bits in the BitString of a BIER

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1.2.  A CDN of A Single Provider

   To make it easier to understand, we first consider a simple example:
   a CDN owned by a single operator, which could be a Content Provider
   itself.  The network spans multiple ASes as shown in the following

                ++++                                       ++++
        EBFR11+      +EBFR12                       EBFR21+     + EBFR22
            +          +                               +          +
           +            +                             +            +
          +    AS100     +                           +    AS200    +
          +              +                            +            +
          +              +                             +          +
           +            ASBR132   +++++++++       ASBR231+      +EBFR23
        EBFR13          +   \   +           +    /        ++++
              +        +    ASBR312          ASBR321
                +++ ASBR131   +              +
                         \    +     AS300    +
                          ASBR311 (BFR)      +
                          ASBR341           ASBR351 (BFR)
                          /    +            +    \
               ++++      /      ++++++++++++      \       +++++*
        EBFR43      ASBR431                        ASBR531        EBFR53
            +        +                                 +          +
           +   AS400  +                               +   AS500    +
           +          +                               +            +
        EBFR41      EBFR42                        EBFR51         EBFR52
            +        +                                 +          +
              ++++++                                     +++++++

   The CDN uses BIER for multicast transport and Edge BIER Forwarding
   Routers (EBFRs) are located throughout the network.  Some of them are
   connected towards multicast content sources and are referred to as
   BIER Forwarding Ingress Routers (BFIRs) in BIER architecture.  Most
   of them are connected towards multicast content receivers and are
   referred to as BIER Forwarding Egress Routers (BFERs).  Notice that
   between content sources and BFIRs there may be Protocol Independent
   Multicast (PIM) in use, while between content receivers and BFERs
   there may be PIM and/or IGMP in use.

   At the initial deployment stage, there might be only a few transit
   BIER Forwarding Routers (BFRs) at strategic points in the network
   (e.g.  ASBR311 and ASBR351).  BGP sessions are established among the
   EBFRs and BFRs, and BGP extensions as defined in

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   [I-D.ietf-bier-idr-extensions] are used to signal BIER information.
   All these are in a single BIER sub-domain.

   In the example of initial stage with only ASBR311 and ASBR351 as
   BFRs, multicast traffic arriving at EBFR11 will be imposed with a
   BIER header and replicated to EBFR12/EBFR13/ASBR311 over tunnels.
   ASBR311 will further replicate traffic to
   ASBR351/EBFR41/EBFR42/EBFR43/EBFR21/EBFR22/EBFR23 over tunnels, and
   ASBR351 will further replicate traffic to EBFR51/EBFR52/EBFR53 over

   The BGP signaling and a necessary enhancement can be explained using
   the following example.  EBFR43 advertises its BIER prefix (a loopback
   address) as /32 IPv4 or /128 IPv6 prefix in BGP with a BIER Path
   Attribute (BPA) [RFC8279] [I-D.ietf-bier-idr-extensions].  ASBR431
   receives it and re-advertises it (with BGP Next Hop changed to
   itself) but does not do anything wrt BIER because it does not support
   BIER.  Same happens on ASBR341.  When ASBR311 and ASBR351 receive it
   from ASBR341, they create a BIFT entry corresponding to EBFR43's BFR-
   ID.  The entry causes a BIER packet with corresponding bit set in its
   BitString to be tunneled to EBFR43.  This cannot be based on BGP Next
   Hop in the advertisement because the BGP Next Hop is ASBR341.  When
   eventually EBFR11 receives the re-advertised route, it creates a BIFT
   entry that causes corresponding packets to be tunneled to ASBR311
   (but not to EBFR43 directly).  Now it is clear that this cannot be
   based on either the BIER prefix itself or the BGP Next Hop.  The
   solution is that the originating EBFR attaches a Tunnel Encap
   Attribute (TEA) [RFC9012] with the tunnel destination set to itself,
   and whenever a BFR re-advertises the route it changes the tunnel
   destination to itself.  When a BFR creates the BIFT entry, it uses
   the Tunnel Egress Endpoint in the TEA to find out where to tunnel

   Over time, more routers in network may be upgraded to support BIER
   and become a BFR.  For example, once ASBR431 is upgraded to a BFR,
   ASBR311 no longer needs to tunnel traffic to EBFR41/EBFR42/EBFR43 but
   only need to tunnel one copy to ASBR431, who will then replicate to

1.2.1.  IGP/BGP Interworking

   Additionally, if enough routers in an AS (or just one of its IGP
   areas) can be upgraded to run BIER, then hop-by-hop BIER forwarding
   can be utilized there, using IGP extensions for BIER signaling
   [RFC8401] [RFC8444].

   Notice that even with this there is still only one BIER sub-domain,
   with mixed IGP and BGP signaling for BIER.  To redistribute BIER

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   information between IGP and BGP, procedures specified in
   [I-D.ietf-bier-prefix-redistribute] and detailed in Section 2.2 are

1.3.  A CDN That Involves Another Provider

   In the above example, the CDN is providing multicast transport
   service, with simplicity and scalability provided by BIER (the per-
   flow state is confined to the edges).  Now let us go one step further
   and consider that AS300 belongs to a different Internet Service
   Provider.  Now the ISP is providing BIER As A Service (BAAS) to the
   CDN, by being part of the CDN's BIER sub-domain.  Notice that, not
   only does the ISP not have per-tree state (it does not have EBFRs),
   but also its BFRs do not need BFR-ID assigned.  The ISP does need to
   learn about all the EBFRs and their corresponding BFR-IDs (through

1.3.1.  Providing Independent BAAS To Multiple Customers

   Now consider that the ISP also provides BAAS for another CDN.  Each
   of the two CDNs has its own BIER domain, with its own BFR-ID or even
   sub-domain ID assignment that could conflict between the two CDNs.
   For example, both have BFR-ID 100 and sub-domain ID 0 assigned but
   they are totally independent of each other.  For an BFR in the ISP to
   support this, with BGP signaling it needs to advertise its own BFR
   prefix multiple times, each time with a different RD that is mapped
   to the corresponding CDN.  A new SAFI BIER (to be allocated by IANA)
   is used.

   In the above example, there are two paths between AS100 and AS300.
   It is possible that while ASBR311 is the BFR, ASBR312 is the unicast
   best path into AS300 and beyond from AS100.  Advertising BIER
   prefixes using a different SAFI with a RD also has the side benefit
   of allowing incongruent topologies for unicast and BIER.

   In the existing BIER architecture and IGP extensions for BIER a sub-
   domain is tied to a single topology (either the one and only topology
   if Multi-topology ISIS/OSPF is not used, or a topology as defined in
   Multi-topology ISIS/OSPF).  In the BIER sub-TLV that ISIS/OSPF
   attaches to a BIER prefix, a Sub-domain-ID value can only appear once
   for a particular topology.  In this document, a BFR in the BAAS
   provider may belong to different and independent BIER domains, and
   the same sub-domain ID needs to be signaled multiple times, once for
   each BIER domain (notice that the same sub-domain-ID actually
   identifies different sub-domains in different BIER domains, so this
   does not really change the architectural requirement that a sub-
   domain is tied to a single topology).  To do so, a new "BIER Domain"
   sub-TLV is introduced, and its value field includes a RD (as in the

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   BGP signaling) and a BIER sub-sub-TLV that is the same as currently
   specified in ISIS/OSPF extensions for BIER.

   This works very well because of the flexible BIER architecture - a
   BIER packet is forwarded based on a Bit Index Forwarding Table (BIFT)
   that is determined by a 20-bit BIFT ID in front of the BIER header,
   and each (subdomain, BSL, set) tuple has its own BIFT.
   Traditionally, a subdomain is identified by a sub-domain ID but in
   this document a subdomain is now identified by a (RD, sub-domain ID)
   tuple in the control plane.

   With this, the scaling aspect on a BFR comes to how many BAAS
   customer the provider needs to support.  For example, if it needs to
   support 16 BAAS customers, one BSL, and four sets (Section 1.4.1) for
   each customer, then the provider needs to support 64 BIFTs (16 x 1 x
   4).  If the BSL is 256, then each BIFT has 256 entries in it and the
   total number of BIFT entries (routes) is 4k (256 x 64).  Notice that
   this 4k number is not related to the number of customers' multicast
   flows, but only related to the number of customers and number of
   customer EBFRs.  The number of customers with their own independent
   BIER domains are likely not very large initially, but if multicast as
   a service gets more widely used, the protocol and procedures defined
   in this document can scale up to the extent of how many BIFTs (and
   BIFT entries) a BFR can support.  Since there is no real difference
   between a BIFT entry and a unicast RIB/FIB entry, as long as the
   scaling requirements are adequately considered in the BIER forwarding
   plane implementation (e.g., enough memory is allocated for the
   BIFTs), scaling will not become a bottleneck.

   Building/updating the BIFTs is the same as in the base BIER
   architecture, except that in the control plane a subdomain is
   identified by a (RD, sub-domain ID) tuple instead of just a sub-
   domain ID.  This is transparent to the forwarding plane - a BIFT is
   always identified by an opaque 20-bit opaque number.  This opaque
   number is either a label for MPLS encapsulation or an opaque number
   for non-MPLS encapsulation, and the optional static encoding as
   specified in [I-D.ietf-bier-non-mpls-bift-encoding] cannot be used.

1.3.2.  Control and Accounting

   With BGP based signaling, internal routers of a BAAS provider does
   not need explicit configuration for the BIER transport services that
   it support.  In the above example, the ASBRs (ASBR311, ASBR312,
   ASBR321, ASBR341, ASBR351) in AS300 only need to have BGP policy
   configured to allow certain received BIER prefix advertisements to
   trigger necessary BIER state and additional signaling of their own.
   For example, when ASBR351 receives the BIER prefix advertisement, if
   its local configuration allows it may create corresponding BIFTs and

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   BIFT entries, and additionally originates or updates its own BIER
   prefix advertisement.  An internal BFR inside AS300, upon receiving
   the BGP advertisements, may or may not need to go through the same
   policy check again (based on the providers operation model).

   When the ASBRs (re-)advertise BIER prefixes toward their external
   peers, they could enable statistics counters for the corresponding
   BIER labels so that they can count incoming BIER packets from
   external peers specifically for this BAAS.  Similarly, the ASBRs can
   enable statistics counters for BIER labels they receive from external
   peers, so that they can count outgoing BIER packets delivered to the
   external peers.  These incoming and outgoing counters can be used for
   accounting and billing purposes.

1.4.  Sets and Segmentation

   The number of EBFRs could very well be larger than the BSL.  There
   are two ways to handle that - multiple sets or segmentation.

1.4.1.  Multiple Sets

   With this method the set of EBFRs are grouped into multiple sets, and
   the number of EBFRs in a set is smaller than the BSL.  A BFIR may
   need to send multiple copies of a multicast packet to reach all
   BFERs, one copy for each set that covers one or more expecting BFERs.
   A separate BIFT is needed for each set (because the same bit in the
   BitString of packets for different sets maps to different BFERs).
   This not only leads to multiple copies to be sent over the same link,
   but also requires additional BIFTs.  In the earlier example, 64 BIFTs
   are needed for 16 BAAS customers because each customer needs 4 BIFTs
   for the multiple sets.

1.4.2.  Segmentation

   With this method, a BIER network is segmented into multiple regions,
   each with its own BIER sub-domain.  In the earlier example, each AS
   could be an independent sub-domain.  A BIER packet from EBFR11 will
   be decapsulated by the segmentation border router ASBR311, and then
   sent into next sub-domain in AS300 with a new BIER header.  The
   segmentation [RFC7524] involves Multicast Flow Overlay [RFC8279]
   [RFC8556] so that the segmentation border routers know what BitString
   to use when sending onto the next segment.  The advantage of
   segmentation is that only a single copy needs to be sent, and the
   number of BIFTs is also reduced on all BFRs.  The disadvantage is
   that the segmentation points need to run multicast flow overlay
   protocol and maintain related state in control plane and data plane.

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   A deployment may start without the need for either multiple sets or
   segmentation when the number of EBFRs is small.  When the number of
   EBFRs grows, segmentation can be introduced incrementally.  A new BFR
   can be added as, or an existing BFR could be converted to, a
   segmentation point, splitting the original sub-domain into two
   independent sub-domains.  The segmentation point does not re-
   advertise BIER information from one sub-domain to another.  Other
   BFRs/EBFRs do not need any configuration changes except to make sure
   that all BIER information exchange is restricted to a single sub-
   domain (for example, two BFRs were BGP peers before and were
   exchanging BIER information but now they belong to two sub-domains
   and only exchange BIER information with the segmentation point and
   other BFRs in the same sub-domain).

   In the earlier example of a CDN of a single provider, using
   segmentation may be acceptable, even though the overlay state needs
   to be kept by the segmentation points.  A BAAS provider may need to
   carefully consider if it wants to keep a customer's overlay state on
   those segmentation points.  On the other hand, the provider may
   consider hosting per-customer segmentation points.  For example,
   tethering small or virtual BFRs to an ASBR and have those BFRs be the
   segmentation points [I-D.ietf-bier-tether].

2.  Specifications for Enhancements to BIER Signaling with BGP/IGP

2.1.  BGP Procedures

   When an EBFR advertises a BIER prefix with a BIER Path Attribute
   (BPA), it SHOULD attach a Tunnel Encap Attribute (TEA) with the
   tunnel destination set to itself.

   A BFR receiving the advertisement MUST use the tunnel destination in
   the TEA to determine where to forward a BIER packet whose BitString
   has a set bit corresponding to the BIER prefix, unless the TEA does
   not exist, in which case the BIER prefix itself is used for the
   determination.  When the BFR re-advertises the BIER prefixes, it MUST
   change the tunnel destination in the TEA to itself, or add a TEA with
   the tunnel destination set to itself if there was no TEA in the
   received advertisement.

   The TEA SHOULD have a Protocol Sub-TLV with protocol type BIER

   A transit BFR that is allowed (by provisioning or based on policy) to
   participate in a BIER sub-domain MUST advertise its own BIER prefix
   with a BPA.  The BFR-id in the BPA SHOULD be 0.  Depending on the
   operational model of the operator, the advertisement MAY be based on

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   received BIER prefixes (subject to certain BGP policy verification),
   or MAY do so only with explicit configuration.

   If a provider provides independent BAAS services to multiple
   customers, when its BFR receives BIER prefixes from a customer it
   MUST re-advetise with a new BIER SAFI.  For simplicity, all BFRs of
   the provider use the same RD that is specifically assigned for the
   customer.  When a BFR re-advertises BIER prefixes to a customer, it
   MUST re-advertise with SAFI 1 or 2.

   If multiple providers together provide BAAS to a customer, then the
   two providers may assign the same RD for the customer or do RD
   rewriting when re-advertising BIER prefixes from one provider to

2.2.  ISIS/OSPF Procedures

   This document defines a new BIER Domain Sub-TLV of ISIS TLVs 135,
   235, 236, and 237.  The sub-TLV type is to be allocated.

   This document also defines a new BIER Domain Sub-TLV of OSPF Extended
   Prefix TLV.  The sub-TLV type is to be allocated.

   The value part of the BIER Domain Sub-TLV includes a 64-bit Route
   Distinguisher followed by one or more BIER Info Sub-TLV (as defined
   in [RFC8401] and [RFC8444] respectively) as its sub-sub-TLVs .

   When a BFR redistribute a BIER prefix from BGP into ISIS/OSPF, if the
   BGP advertisement is of BIER SAFI, a BIER Domain sub-TLV is attached,
   with the RD part of the sub-TLV copied from the BGP advertisement.
   For each BIER TLV in the BPA, a BIER Info sub-sub-TLV is added in the
   BIER Domain sub-TLV, with the subdomain-id and BFR-id copied from the
   corresponding BIER TLV in the BPA, and the Encapsulation sub-sub-sub-
   TLV omitted because it is not needed.

   If the BGP advertisement is of SAFI 1 or 2, BIER Info Sub-TLVs are
   constructed as above directly, without using a BIER Domain sub-TLV.

   When a BFR redistribute a BIER prefix from ISIS/OSPF into BGP, if
   there is a BIER Domain sub-TLV in the corresponding ISIS LSP or OSPF
   LSA, the BGP advertisement is of BIER SAFI and the RD part of the
   NLRI is set to the RD from the BIER Domain sub-TLV.  For each BIER
   Info sub-sub-TLV in the BIER Domain sub-TLV, a BIER TLV is included
   in the BPA, with the subdomain-id and BFR-id copied from the
   corresponding BIER Info sub-sub-TLV.  The MPLS Encapsulation sub-TLV
   is omitted.  The tunnel destination in the TEA is set to the BFR's
   BIER prefix.

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   If there is no BIER Domain sub-TLV in the corresponding ISIS LSP or
   OSPF LSA for the BIER Prefix, the BGP advertisement is of SAFI 1 or
   2, and the BPA is constructed similar to the above (the only
   difference is that in this case BIER Info sub-TLVs are not part of a
   BIER Domain sub-TLV).

3.  IANA Considerations

   This document requests the following IANA assignments:

   o  A sub-TLV type for BIER Domain Sub-TLV from ISIS "Sub-TLVs for
      TLVs 135, 235, 236, and 237" registry.

   o  A sub-TLV type for BIER Domain Sub-TLV from OSPFv2 Extended Prefix
      Sub-TLV registry.

   o  A BIER SAFI from Subsequent Address Family Identifiers (SAFI)

4.  Security Considerations

   There are no security concerns wrt exchange of BIER information
   besides what have been discussed in [I-D.ietf-bier-idr-extensions]
   and [RFC8401] [RFC8444].

   The tunnels between BFRs that are not directly connected are ideally
   auto-configured to reduce provisioning burdens.  Given that they may
   span multiple ASes and MPLS may not always be available, BIER over
   UDP/GRE/IPv4/IPv6 becomes very convenient, though that has the same
   security concerns well discussed in "Security Considerations" of
   [RFC4023] and [RFC7510].

   As one mitigation when the tunnel is not secured, a BFR MAY use
   source address filtering based on pre-provisioned or dynamically
   learned allowable addresses.  With dynamic learning, if a BFR
   receives a BIER prefix with a BPA and a TEA (see Section 2.1), it
   sets up a forwarding filter to allow IP/GRE/UDP tunneling from the
   address encoded in the "Tunnel Egress Endpoint" sub-TLV of Tunnel
   TLVs in the TEA.  While that is the address for this BFR to tunnel
   traffic to, this BFR will also likely receive tunneled traffic from
   that address.

5.  Contributors

   The following people also contributed to this document.

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   Zheng Zhang

   Gyan Mishra

6.  Acknowledgements

   The authors thank Lenny Giuliano and Antoni Przygenda for their
   review and suggestions.

7.  References

7.1.  Normative References

              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.

              Zhang, Z., Wu, B., Zhang, Z., Wijnands, I., Liu, Y., and
              H. Bidgoli, "BIER Prefix Redistribute", draft-ietf-bier-
              prefix-redistribute-02 (work in progress), July 2022.

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

   [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,

   [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, <>.

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   [RFC9012]  Patel, K., Van de Velde, G., Sangli, S., and J. Scudder,
              "The BGP Tunnel Encapsulation Attribute", RFC 9012,
              DOI 10.17487/RFC9012, April 2021,

7.2.  Informative References

              Wijnands, I., Mishra, M., Xu, X., and H. Bidgoli, "An
              Optional Encoding of the BIFT-id Field in the non-MPLS
              BIER Encapsulation", draft-ietf-bier-non-mpls-bift-
              encoding-04 (work in progress), May 2021.

              Zhang, Z., Warnke, N., Wijnands, I., and D. Awduche,
              "Tethering A BIER Router To A BIER incapable Router",
              draft-ietf-bier-tether-01 (work in progress), January

   [RFC7524]  Rekhter, Y., Rosen, E., Aggarwal, R., Morin, T.,
              Grosclaude, I., Leymann, N., and S. Saad, "Inter-Area
              Point-to-Multipoint (P2MP) Segmented Label Switched Paths
              (LSPs)", RFC 7524, DOI 10.17487/RFC7524, May 2015,

   [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,

Authors' Addresses

   Zhaohui Zhang
   Juniper Networks


   Eric Rosen


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   Daniel Awduche


   Greg Shepherd


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