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Tree Engineering for Bit Index Explicit Replication (BIER-TE)

The information below is for an old version of the document.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 9262.
Authors Toerless Eckert , Michael Menth , Gregory Cauchie
Last updated 2022-03-10 (Latest revision 2022-01-28)
Replaces draft-eckert-bier-te-arch
RFC stream Internet Engineering Task Force (IETF)
Additional resources Mailing list discussion
Stream WG state Submitted to IESG for Publication
Document shepherd Xuesong Geng
Shepherd write-up Show Last changed 2021-05-26
IESG IESG state Became RFC 9262 (Proposed Standard)
Consensus boilerplate Yes
Telechat date (None)
Responsible AD Alvaro Retana
Send notices to Xuesong Geng <>,
IANA IANA review state Version Changed - Review Needed
Network Working Group                                 T.T.E. Eckert, Ed.
Internet-Draft                                                 Futurewei
Intended status: Standards Track                              M.M. Menth
Expires: 1 August 2022                           University of Tuebingen
                                                            G.C. Cauchie
                                                        Bouygues Telecom
                                                            January 2022

     Tree Engineering for Bit Index Explicit Replication (BIER-TE)


   This memo describes per-packet stateless strict and loose path
   steered replication and forwarding for "Bit Index Explicit
   Replication" (BIER, RFC8279) packets.  It is called BIER Tree
   Engineering (BIER-TE) and is intended to be used as the path steering
   mechanism for Traffic Engineering with BIER.

   BIER-TE introduces a new semantic for "bit positions" (BP).  They
   indicate adjacencies of the network topology, as opposed to (non-TE)
   BIER in which BPs indicate "Bit-Forwarding Egress Routers" (BFER).  A
   BIER-TE packets BitString therefore indicates the edges of the (loop-
   free) tree that the packet is forwarded across by BIER-TE.  BIER-TE
   can leverage BIER forwarding engines with little changes.  Co-
   existence of BIER and BIER-TE forwarding in the same domain is
   possible, for example by using separate BIER "sub-domains" (SDs).
   Except for the optional routed adjacencies, BIER-TE does not require
   a BIER routing underlay, and can therefore operate without depending
   on an "Interior Gateway Routing protocol" (IGP).

   As it operates on the same per-packet stateless forwarding
   principles, BIER-TE can also be a good fit to support multicast path
   steering in "Segment Routing" (SR) networks.

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

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   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 5 July 2022.

Copyright Notice

   Copyright (c) 2022 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 (
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   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   5
   2.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   5
     2.1.  Basic Examples  . . . . . . . . . . . . . . . . . . . . .   5
     2.2.  BIER-TE Topology and adjacencies  . . . . . . . . . . . .   8
     2.3.  Relationship to BIER  . . . . . . . . . . . . . . . . . .   9
     2.4.  Accelerated/Hardware forwarding comparison  . . . . . . .  11
   3.  Components  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     3.1.  The Multicast Flow Overlay  . . . . . . . . . . . . . . .  12
     3.2.  The BIER-TE Control Plane . . . . . . . . . . . . . . . .  12
       3.2.1.  The BIER-TE Controller  . . . . . . . . . . . . . . .  14  BIER-TE Topology discovery and creation . . . . .  14  Engineered Trees via BitStrings . . . . . . . . .  15  Changes in the network topology . . . . . . . . .  16  Link/Node Failures and Recovery . . . . . . . . .  16
     3.3.  The BIER-TE Forwarding Plane  . . . . . . . . . . . . . .  16
     3.4.  The Routing Underlay  . . . . . . . . . . . . . . . . . .  17
     3.5.  Traffic Engineering Considerations  . . . . . . . . . . .  17
   4.  BIER-TE Forwarding  . . . . . . . . . . . . . . . . . . . . .  18
     4.1.  The BIER-TE Bit Index Forwarding Table (BIFT) . . . . . .  18
     4.2.  Adjacency Types . . . . . . . . . . . . . . . . . . . . .  20
       4.2.1.  Forward Connected . . . . . . . . . . . . . . . . . .  21
       4.2.2.  Forward Routed  . . . . . . . . . . . . . . . . . . .  21
       4.2.3.  ECMP  . . . . . . . . . . . . . . . . . . . . . . . .  21
       4.2.4.  Local Decap(sulation) . . . . . . . . . . . . . . . .  22

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     4.3.  Encapsulation / Co-existence with BIER  . . . . . . . . .  22
     4.4.  BIER-TE Forwarding Pseudocode . . . . . . . . . . . . . .  23
     4.5.  Basic BIER-TE Forwarding Example  . . . . . . . . . . . .  26
     4.6.  BFR Requirements for BIER-TE forwarding . . . . . . . . .  28
   5.  BIER-TE Controller Operational Considerations . . . . . . . .  29
     5.1.  Bit Position Assignments  . . . . . . . . . . . . . . . .  29
       5.1.1.  P2P Links . . . . . . . . . . . . . . . . . . . . . .  29
       5.1.2.  BFER  . . . . . . . . . . . . . . . . . . . . . . . .  29
       5.1.3.  Leaf BFERs  . . . . . . . . . . . . . . . . . . . . .  29
       5.1.4.  LANs  . . . . . . . . . . . . . . . . . . . . . . . .  31
       5.1.5.  Hub and Spoke . . . . . . . . . . . . . . . . . . . .  32
       5.1.6.  Rings . . . . . . . . . . . . . . . . . . . . . . . .  32
       5.1.7.  Equal Cost MultiPath (ECMP) . . . . . . . . . . . . .  33
       5.1.8.  Forward Routed adjacencies  . . . . . . . . . . . . .  36  Reducing bit positions  . . . . . . . . . . . . .  36  Supporting nodes without BIER-TE  . . . . . . . .  37
       5.1.9.  Reuse of bit positions (without DNC)  . . . . . . . .  37
       5.1.10. Summary of BP optimizations . . . . . . . . . . . . .  38
     5.2.  Avoiding duplicates and loops . . . . . . . . . . . . . .  39
       5.2.1.  Loops . . . . . . . . . . . . . . . . . . . . . . . .  40
       5.2.2.  Duplicates  . . . . . . . . . . . . . . . . . . . . .  40
     5.3.  Managing SI, sub-domains and BFR-ids  . . . . . . . . . .  41
       5.3.1.  Why SI and sub-domains  . . . . . . . . . . . . . . .  41
       5.3.2.  Assigning bits for the BIER-TE topology . . . . . . .  42
       5.3.3.  Assigning BFR-id with BIER-TE . . . . . . . . . . . .  43
       5.3.4.  Mapping from BFR to BitStrings with BIER-TE . . . . .  44
       5.3.5.  Assigning BFR-ids for BIER-TE . . . . . . . . . . . .  45
       5.3.6.  Example bit allocations . . . . . . . . . . . . . . .  45  With BIER . . . . . . . . . . . . . . . . . . . .  45  With BIER-TE  . . . . . . . . . . . . . . . . . .  46
       5.3.7.  Summary . . . . . . . . . . . . . . . . . . . . . . .  47
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  48
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  49
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  49
   9.  Change log [RFC Editor: Please remove]  . . . . . . . . . . .  50
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  62
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  62
     10.2.  Informative References . . . . . . . . . . . . . . . . .  63
   Appendix A.  BIER-TE and Segment Routing  . . . . . . . . . . . .  66
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  67

1.  Overview

   BIER-TE is based on the (non-TE) BIER architecture, terminology and
   packet formats as described in [RFC8279] and [RFC8296].  This
   document describes BIER-TE in the expectation that the reader is
   familiar with these two documents.

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   BIER-TE introduces a new semantic for "bit positions" (BP).  They
   indicate adjacencies of the network topology, as opposed to (non-TE)
   BIER in which BPs indicate "Bit-Forwarding Egress Routers" (BFER).  A
   BIER-TE packets BitString therefore indicates the edges of the (loop-
   free) tree that the packet is forwarded across by BIER-TE.  With
   BIER-TE, the "Bit Index Forwarding Table" (BIFT) of each "Bit
   Forwarding Router" (BFR) is only populated with BP that are adjacent
   to the BFR in the BIER-TE Topology.  Other BPs are empty in the BIFT.
   The BFR replicate and forwards BIER packets to adjacent BPs that are
   set in the packet.  BPs are normally also cleared upon forwarding to
   avoid duplicates and loops.

   BIER-TE can leverage BIER forwarding engines with little or no
   changes.  It can also co-exist with BIER forwarding in the same
   domain, for example by using separate BIER sub-domains.  Except for
   the optional routed adjacencies, BIER-TE does not require a BIER
   routing underlay, and can therefore operate without depending on an
   "Interior Gateway Routing protocol" (IGP).

   As it operates on the same per-packet stateless forwarding
   principles, BIER-TE can also be a good fit to support multicast path
   steering in "Segment Routing" (SR) networks ([RFC8402]).

   This document is structured as follows:

   *  Section 2 introduces BIER-TE with two forwarding examples,
      followed by an introduction of the new concepts of the BIER-TE
      (overlay) topology and finally a summary of the relationship
      between BIER and BIER-TE and a discussion of accelerated hardware

   *  Section 3 describes the components of the BIER-TE architecture,
      Flow overlay, BIER-TE layer with the BIER-TE control plane
      (including the BIER-TE controller) and BIER-TE forwarding plane,
      and the routing underlay.

   *  Section 4 specifies the behavior of the BIER-TE forwarding plane
      with the different type of adjacencies and possible variations of
      BIER-TE forwarding pseudocode, and finally the mandatory and
      optional requirements.

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   *  Section 5 describes operational considerations for the BIER-TE
      controller, foremost how the BIER-TE controller can optimize the
      use of BP by using specific type of BIER-TE adjacencies for
      different type of topological situations, but also how to assign
      bits to avoid loops and duplicates (which in BIER-TE does not come
      for free), and finally how "Set Identifier" (SI), "sub-domain"
      (SD) and BFR-ids can be managed by a BIER-TE controller, examples
      and summary.

   *  Appendix A concludes the technology specific sections of the
      document by further relating BIER-TE to SR.

   Note that related work, [I-D.ietf-roll-ccast] uses Bloom filters
   [Bloom70] to represent leaves or edges of the intended delivery tree.
   Bloom filters in general can support larger trees/topologies with
   fewer addressing bits than explicit BitStrings, but they introduce
   the heuristic risk of false positives and cannot clear bits in the
   BitString during forwarding to avoid loops.  For these reasons, BIER-
   TE uses explicit BitStrings like BIER.  The explicit BitStrings of
   BIER-TE can also be seen as a special type of Bloom filter, and this
   is how related work [ICC] describes it.

1.1.  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.

2.  Introduction

2.1.  Basic Examples

   BIER-TE forwarding is best introduced with simple examples.  These
   examples use formal terms defined later in the document (Figure 4),
   including forward_connected(), forward_routed() and local_decap().

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   BIER-TE Topology:


                       p5    p6
                     --- BFR3 ---
                  p3/    p13     \p7          p15
      BFR1 ---- BFR2              BFR5 ----- BFR6
         p1   p2  p4\    p14     /p10 p11   p12
                     --- BFR4 ---
                       p8    p9

      (simplified) BIER-TE Bit Index Forwarding Tables (BIFT):

      BFR1:   p1  -> local_decap()
              p2  -> forward_connected() to BFR2

      BFR2:   p1  -> forward_connected() to BFR1
              p5  -> forward_connected() to BFR3
              p8  -> forward_connected() to BFR4

      BFR3:   p3  -> forward_connected() to BFR2
              p7  -> forward_connected() to BFR5
              p13 -> local_decap()

      BFR4:   p4  -> forward_connected() to BFR2
              p10 -> forward_connected() to BFR5
              p14 -> local_decap()

      BFR5:   p6  -> forward_connected() to BFR3
              p9  -> forward_connected() to BFR4
              p12 -> forward_connected() to BFR6

      BFR6:   p11 -> forward_connected() to BFR5
              p15 -> local_decap()

                      Figure 1: BIER-TE basic example

   Consider the simple network in the above BIER-TE overview example
   picture with 6 BFRs. p1...p15 are the bit positions used.  All BFRs
   can act as an ingress BFR (BFIR), BFR1, BFR3, BFR4 and BFR6 can also
   be BFERs.  Forward_connected() is the name for adjacencies that are
   representing subnet adjacencies of the network.  Local_decap() is the
   name of the adjacency to decapsulate BIER-TE packets and pass their
   payload to higher layer processing.

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   Assume a packet from BFR1 should be sent via BFR4 to BFR6.  This
   requires a BitString (p2,p8,p10,p12,p15).  When this packet is
   examined by BIER-TE on BFR1, the only bit position from the BitString
   that is also set in the BIFT is p2.  This will cause BFR1 to send the
   only copy of the packet to BFR2.  Similarly, BFR2 will forward to
   BFR4 because of p8, BFR4 to BFR5 because of p10 and BFR5 to BFR6
   because of p12. p15 finally makes BFR6 receive and decapsulate the

   To send a copy to BFR6 via BFR4 and also a copy to BFR3, the
   BitString needs to be (p2,p5,p8,p10,p12,p13,p15).  When this packet
   is examined by BFR2, p5 causes one copy to be sent to BFR3 and p8 one
   copy to BFR4.  When BFR3 receives the packet, p13 will cause it to
   receive and decapsulate the packet.

   If instead the BitString was (p2,p6,p8,p10,p12,p13,p15), the packet
   would be copied by BFR5 towards BFR3 because of p6 instead of being
   copied by BFR2 to BFR3 because of p5 in the prior case.  This is
   showing the ability of the shown BIER-TE Topology to make the traffic
   pass across any possible path and be replicated where desired.

   BIER-TE has various options to minimize BP assignments, many of which
   are based on out-of-band knowledge about the required multicast
   traffic paths and bandwidth consumption in the network, such as from
   pre-deployment planning.

   Figure 2 shows a modified example, in which Rtr2 and Rtr5 are assumed
   not to support BIER-TE, so traffic has to be unicast encapsulated
   across them.  To emphasize non-L2, but routed/tunneled forwarding of
   BIER-TE packets, these adjacencies are called "forward_routed".
   Otherwise, there is no difference in their processing over the
   aforementioned forward_connected() adjacencies.

   In addition, bits are saved in the following example by assuming that
   BFR1 only needs to be BFIR but not BFER or transit BFR.

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   BIER-TE Topology:


                      p1  p3  p7
                   ....> BFR3 <....       p5
           ........                ........>
      BFR1       (Rtr2)          (Rtr5)      BFR6
           ........                ........> p9
                   ....> BFR4 <....       p6
                      p2  p4  p8

      (simplified) BIER-TE Bit Index Forwarding Tables (BIFT):

      BFR1:   p1  -> forward_routed() to BFR3
              p2  -> forward_routed() to BFR4

      BFR3:   p3  -> local_decap()
              p5  -> forward_routed() to BFR6

      BFR4:   p4  -> local_decap()
              p6  -> forward_routed() to BFR6

      BFR6:   p7  -> forward_routed() to BFR3
              p8  -> forward_routed() to BFR4
              p9  -> local_decap()

                  Figure 2: BIER-TE basic overlay example

   To send a BIER-TE packet from BFR1 via BFR3 to be received by BFR6,
   the BitString is (p1,p5,p9).  From BFR1 via BFR4 to be received by
   BFR6, the BitString is (p2,p6,p9).  A packet from BFR1 to be received
   by BFR3,BFR4 and from BFR3 to be received by BFR6 uses
   (p1,p2,p3,p4,p5,p9).  A packet from BFR1 to be received by BFR3,BFR4
   and from BFR4 to be received by BFR6 uses (p1,p2,p3,p4,p6,p9).  A
   packet from BFR1 to be received by BFR4, and from BFR4 to be received
   by BFR6 and from there to be received by BFR3 uses
   (p2,p3,p4,p6,p7,p9).  A packet from BFR1 to be received by BFR3, and
   from BFR3 to be received by BFR6 there to be received by BFR4 uses

2.2.  BIER-TE Topology and adjacencies

   The key new component in BIER-TE compared to (non-TE) BIER is the
   BIER-TE topology as introduced through the two examples in
   Section 2.1.  It is used to control where replication can or should
   happen and how to minimize the required number of BP for adjacencies.

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   The BIER-TE Topology consists of the BIFTs of all the BFR and can
   also be expressed as a directed graph where the edges are the
   adjacencies between the BFRs labelled with the BP used for the
   adjacency.  Adjacencies are naturally unidirectional.  BP can be
   reused across multiple adjacencies as long as this does not lead to
   undesired duplicates or loops as explained in Section 5.2.

   If the BIER-TE topology represents (a subset of) the underlying
   (layer 2) topology of the network as shown in the first example, this
   may be called a "native" BIER-TE topology.  A topology consisting
   only of "forward_routed" adjacencies as shown in the second example
   may be called an "overlay" BIER-TE topology.  A BIER-TE topology with
   both forward_connected() and forward_routed() adjacencies may be
   called a "hybrid" BIER-TE topology.

2.3.  Relationship to BIER

   BIER-TE is designed so that its forwarding plane is a simple
   extension to the (non-TE) BIER forwarding plane, hence allowing for
   it to be added to BIER deployments where it can be beneficial.

   BIER-TE is also intended as an option to expand the BIER architecture
   into deployments where (non-TE) BIER may not be the best fit, such as
   statically provisioned networks with needs for path steering but
   without desire for distributed routing protocols.

   1.  BIER-TE inherits the following aspects from BIER unchanged:

       1.  The fundamental purpose of per-packet signaled replication
           and delivery via a BitString.

       2.  The overall architecture consisting of three layers, flow
           overlay, BIER(-TE) layer and routing underlay.

       3.  The supported encapsulations [RFC8296].

       4.  The semantic of all [RFC8296] header elements used by the
           BIER-TE forwarding plane other than the semantic of the BP in
           the BitString.

       5.  The BIER forwarding plane, except for how bits have to be
           cleared during replication.

   2.  BIER-TE has the following key changes with respect to BIER:

       1.  In BIER, bits in the BitString of a BIER packet header
           indicate a BFER and bits in the BIFT indicate the BIER
           control plane calculated next-hop toward that BFER.  In BIER-

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           TE, a bit in the BitString of a BIER packet header indicates
           an adjacency in the BIER-TE topology, and only the BFR that
           is the upstream of that adjacency has its BP populated with
           the adjacency in its BIFT.

       2.  In BIER, the implied reference option for the core part of
           the BIER layer control plane are the BIER extensions for
           distributed routing protocols.  This includes ISIS/OSPF
           extensions for BIER, [RFC8401] and [RFC8444].

       3.  The reference option for the core part of the BIER-TE control
           plane is the BIER-TE controller.  Nevertheless, both the BIER
           and BIER-TE BIFTs forwarding plane state could equally be
           populated by any mechanism.

       4.  Assuming the reference options for the control plane, BIER-TE
           replaces in-network autonomous path calculation by explicit
           paths calculated by the BIER-TE controller.

   3.  The following elements/functions described in the BIER
       architecture are not required by the BIER-TE architecture:

       1.  "Bit Index Routing Tables" (BIRTs) are not required on BFRs
           for BIER-TE when using a BIER-TE controller because the
           controller can directly populate the BIFTs.  In BIER, BIRTs
           are populated by the distributed routing protocol support for
           BIER, allowing BFRs to populate their BIFTs locally from
           their BIRTs.  Other BIER-TE control plane or management plane
           options may introduce requirements for BIRTs for BIER-TE

       2.  The BIER-TE layer forwarding plane does not require BFRs to
           have a unique BP and therefore also no unique BFR-id.  See
           Section 5.1.3.

       3.  Identification of BFRs by the BIER-TE control plane is
           outside the scope of this specification.  Whereas the BIER
           control plane uses BFR-ids in its BFR to BFR signaling, a
           BIER-TE controller may choose any form of identification
           deemed appropriate.

       4.  BIER-TE forwarding does not require the BFIR-id field of the
           BIER packet header.

   4.  Co-existence of BIER and BIER-TE in the same network requires the

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       1.  The BIER/BIER-TE packet header needs to allow addressing both
           BIER and BIER-TE BIFTs.  Depending on the encapsulation
           option, the same SD may or may not be reusable across BIER
           and BIER-TE.  See Section 4.3.  In either case, a packet is
           always only forwarded end-to-end via BIER or via BIER-TE
           (ships in the nights forwarding).

       2.  BIER-TE deployments will have to assign BFR-ids to BFRs and
           insert them into the BFIR-id field of BIER packet headers as
           BIER does, whenever the deployment uses (unchanged)
           components developed for BIER that use BFR-id, such as
           multicast flow overlays or BIER layer control plane elements.
           See also Section 5.3.3.

2.4.  Accelerated/Hardware forwarding comparison

   BIER-TE forwarding rules, especially the BitString parsing are
   designed to be as close as possible to those of BIER in the
   expectation that this eases the programming of BIER-TE forwarding
   code and/or BIER-TE forwarding hardware on platforms supporting BIER.
   The pseudocode in Section 4.4 shows how existing (non-TE) BIER/BIFT
   forwarding can be modified to support the required BIER-TE forwarding
   functionality (Section 4.6), by using BIER BIFT's "Forwarding Bit
   Mask" (F-BM): Only the clearing of bits to avoid duplicate packets to
   a BFR's neighbor is skipped in BIER-TE forwarding because it is not
   necessary and could not be done when using BIER F-BM.

   Whether to use BIER or BIER-TE forwarding is simply a choice of the
   mode of the BIFT indicated by the packet (BIER or BIER-TE BIFT).
   This is determined by the BFR configuration for the encapsulation,
   see Section 4.3.

3.  Components

   BIER-TE can be thought of being constituted from the same three
   layers as BIER: The "multicast flow overlay", the "BIER layer" and
   the "routing underlay".  The following picture also shows how the
   "BIER layer" is constituted from the "BIER-TE forwarding plane" and
   the "BIER-TE control plane" represent by the "BIER-TE Controller".

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      |<-IGMP/PIM->  multicast flow   <-PIM/IGMP->|

          BIER-TE  [BIER-TE Controller] <=> [BIER-TE Topology]
          control     ^      ^     ^
          plane      /       |      \   BIER-TE control protocol
                    |        |       |  e.g. YANG/NETCONF/RESTCONF
                    |        |       |       PCEP/...
                    v        v       v
    Src -> Rtr1 -> BFIR-----BFR-----BFER -> Rtr2 -> Rcvr

                 BIER-TE forwarding plane

                   |<- BIER-TE domain->|

                     Routing underlay

                       Figure 3: BIER-TE architecture

3.1.  The Multicast Flow Overlay

   The Multicast Flow Overlay has the same role as described for BIER in
   [RFC8279], Section 4.3.  See also Section

   When a BIER-TE controller is used, then the signaling for the
   Multicast Flow Overlay may also be preferred to operate through a
   central point of control.  For BGP based overlay flow services such
   as "Multicast VPN Using BIER" ([RFC8556]) this can be achieved by
   making the BIER-TE controller operate as a BGP Route Reflector
   ([RFC4456]) and combining it with signaling through BGP or a
   different protocol for the BIER-TE controller calculated BitStrings.
   See Section and Section 5.3.4.

3.2.  The BIER-TE Control Plane

   In the (non-TE) BIER architecture [RFC8279], the BIER control plane
   is not explicitly separated from the BIER forwarding plane, but
   instead their functions are summarized together in Section 4.2.
   Example standardized options for the BIER control plane include ISIS/
   OSPF extensions for BIER, [RFC8401] and [RFC8444].

   For BIER-TE, the control plane includes at minimum the following

   1.  BIER-TE topology control:  During initial provisioning of the

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      network and/or during modifications of its topology and/or
      services, the protocols and/or procedures to establish BIER-TE

      1.  Determine the desired BIER-TE topology for a BIER-TE sub-
          domains: the native and/or overlay adjacencies that are
          assigned to BPs.  Topology discovery is discussed in
          Section and the various aspects of the BIER-TE
          controllers determinations about the topology are discussed
          throughout Section 5

      2.  Determine the per-BFR BIFT from the BIER-TE topology.  This is
          achieved by simply extracting the adjacencies of the BFR from
          the BIER-TE topology and populating the BFRs BIFT with them.

      3.  Optionally assign BFR-ids to BFIRs for later insertion into
          BIER headers on BFIRs as BFIR-id.  Alternatively, BFIR-id in
          BIER packet headers may be managed solely by the flow overlay
          layer and/or be unused.  This is discussed in Section 5.3.3.

      4.  Install/update the BIFTs into the BFRs and optionally BFR-ids
          into BFIRs.  This is discussed in Section

   2.  BIER-TE tree control:  During operations of the network,
      protocols and/or procedures to support creation/change/removal of
      overlay flows on BFIRs:

      1.  Process the BIER-TE requirements for the multicast overlay
          flow: BFIR and BFERs of the flow as well as policies for the
          path selection of the flow.  This is discussed in Section 3.5.

      2.  Determine the BitStrings and optionally Entropy.  This is
          discussed in Section, Section 3.5 and Section 5.3.4.

      3.  Install state on the BFIR to impose the desired BIER packet
          header(s) for packets of the overlay flow.  Different aspects
          of this and the next point are discussed throughout
          Section 3.2.1 and in Section 4.3, but the main responsibility
          of these two points is with the Multicast Flow Overlay
          (Section 3.1), which is architecturally inherited from BIER.

      4.  Install the necessary state on the BFERs to decapsulate the
          BIER packet header and properly dispatch its payload.

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3.2.1.  The BIER-TE Controller

   [RFC-Editor: the following text has three references to anchors
   topology-control, topology-control-1 and tree-control.
   Unfortunately, XMLv2 does not offer any tagging that reasonable
   references are generated (i had this problem already in RFCs last
   year.  Please make sure there are useful-to-read cross-references in
   the RFC in these three places after you convert to XMLv3.]

   This architecture describes the BIER-TE control plane as shown in
   Figure 3 to consist of:

   *  A BIER-TE controller.

   *  BFR data-models and protocols to communicate between controller
      and BFRs in support of BIER-TE topology control (Section 3.2),
      such as YANG/NETCONF/RESTCONF ([RFC7950]/[RFC6241]/[RFC8040]).

   *  BFR data-models and protocols to communicate between controller
      and BFIR in support of BIER-TE tree control (Section 3.2), such as
      BIER-TE extensions for [RFC5440].

   The single, centralized BIER-TE controller is used in this document
   as reference option for the BIER-TE control plane but other options
   are equally feasible.  The BIER-TE control plane could equally be
   implemented without automated configuration/protocols, by an operator
   via CLI on the BFRs.  In that case, operator configured local policy
   on the BFIR would have to determine how to set the appropriate BIER
   header fields.  The BIER-TE control plane could also be decentralized
   and/or distributed, but this document does not consider any
   additional protocols and/or procedures which would then be necessary
   to coordinate its (distributed/decentralized) entities to achieve the
   above described functionality.  BIER-TE Topology discovery and creation

   The first item of BIER-TE topology control (Section 3.2, Paragraph 3,
   Item 2.2.1) includes network topology discovery and BIER-TE topology
   creation.  The latter describes the process by which a Controller
   determines which routers are to be configured as BFRs and the
   adjacencies between them.

   In statically managed networks, such as in industrial environments,
   both discovery and creation can be a manual/offline process.

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   In other networks, topology discovery may rely on protocols including
   extending a "Link-State-Protocol" based IGP into the BIER-TE
   controller itself, [RFC7752] (BGP-LS) or [RFC8345] (YANG topology) as
   well as BIER-TE specific methods, for example via
   [I-D.ietf-bier-te-yang].  These options are non-exhaustive.

   Dynamic creation of the BIER-TE topology can be as easy as mapping
   the network topology 1:1 to the BIER-TE topology by assigning a BP
   for every network subnet adjacency.  In larger networks, it likely
   involves more complex policy and optimization decisions including how
   to minimize the number of BPs required and how to assign BPs across
   different BitStrings to minimize the number of duplicate packets
   across links when delivering an overlay flow to BFER using different
   SIs/BitStrings.  These topics are discussed in Section 5.

   When the BIER-TE topology is determined, the BIER-TE Controller then
   pushes the BitPositions/adjacencies to the BIFT of the BFRs.  On each
   BFR only those SI:BitPositions are populated that are adjacencies to
   other BFRs in the BIER-TE topology.

   Communications between the BIER-TE Controller and BFRs for both BIER-
   TE topology control and BIER-TE tree control is ideally via
   standardized protocols and data-models such as NETCONF/RESTCONF/YANG/
   PCEP.  Vendor-specific CLI on the BFRs is also an option (as in many
   other SDN solutions lacking definition of standardized data models).  Engineered Trees via BitStrings

   In BIER, the same set of BFER in a single sub-domain is always
   encoded as the same BitString.  In BIER-TE, the BitString used to
   reach the same set of BFER in the same sub-domain can be different
   for different overlay flows because the BitString encodes the paths
   towards the BFER, so the BitStrings from different BFIR to the same
   set of BFER will often be different.  Likewise, the BitString from
   the same BFIR to the same set of BFER can be different for different
   overlay flows for policy reasons such as shortest path trees, Steiner
   trees (minimum cost trees), diverse path trees for redundancy and so

   See also [I-D.ietf-bier-multicast-http-response] for an application
   leveraging BIER-TE engineered trees.

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   If the network topology changes (not failure based) so that
   adjacencies that are assigned to bit positions are no longer needed,
   the BIER-TE Controller can re-use those bit positions for new
   adjacencies.  First, these bit positions need to be removed from any
   BFIR flow state and BFR BIFT state, then they can be repopulated,
   first into BIFT and then into the BFIR.  Link/Node Failures and Recovery

   When link or nodes fail or recover in the topology, BIER-TE could
   quickly respond with FRR procedures such as [I-D.eckert-bier-te-frr],
   the details of which are out of scope for this document.  It can also
   more slowly react by recalculating the BitStrings of affected
   multicast flows.  This reaction is slower than the FRR procedure
   because the BIER-TE Controller needs to receive link/node up/down
   indications, recalculate the desired BitStrings and push them down
   into the BFIRs.  With FRR, this is all performed locally on a BFR
   receiving the adjacency up/down notification.

3.3.  The BIER-TE Forwarding Plane

   [RFC-editor Q: "is constituted from" / "consists of" / "composed
   from..." ???]

   The BIER-TE Forwarding Plane is constituted from the following

   1.  On a BFIR, imposition of the BIER header for packets from overlay
       flows.  This is driven by a combination of state established by
       the BIER-TE control plane and/or the multicast flow overlay as
       explained in Section 3.1.

   2.  On BFRs (including BFIR and BFER), forwarding/replication of BIER
       packets according to their SD, SI, "BitStringLength" (BSL),
       BitString and optionally Entropy fields as explained in
       Section 4.  Processing of other BIER header fields such as DSCP
       is outside the scope of this document.

   3.  On BFERs, removal of the BIER header and dispatching of the
       payload according to state created by the BIER-TE control plane
       and/or overlay layer.

   When the BIER-TE Forwarding Plane receives a packet, it simply looks
   up the bit positions that are set in the BitString of the packet in
   the BIFT that was populated by the BIER-TE Controller.  For every BP
   that is set in the BitString, and that has one or more adjacencies in

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   the BIFT, a copy is made according to the type of adjacencies for
   that BP in the BIFT.  Before sending any copy, the BFR clears all BPs
   in the BitString of the packet for which the BFR has one or more
   adjacencies in the BIFT.  Clearing these bits inhibits packets from
   looping when the BitStrings erroneously includes a forwarding loop.
   When a forward_connected() adjacency has the "DoNotClear" (DNC) flag
   set, then this BP is re-set for the packet copied to that adjacency.
   See Section 4.2.1.

3.4.  The Routing Underlay

   For forward_connected() adjacencies, BIER-TE is sending BIER packets
   to directly connected BIER-TE neighbors as L2 (unicasted) BIER
   packets without requiring a routing underlay.  For forward_routed()
   adjacencies, BIER-TE forwarding encapsulates a copy of the BIER
   packet so that it can be delivered by the forwarding plane of the
   routing underlay to the routable destination address indicated in the
   adjacency.  See Section 4.2.2 for the adjacency definition.

   BIER relies on the routing underlay to calculate paths towards BFERs
   and derive next-hop BFR adjacencies for those paths.  This commonly
   relies on BIER specific extensions to the routing protocols of the
   routing underlay but may also be established by a controller.  In
   BIER-TE, the next-hops of a packet are determined by the BitString
   through the BIER-TE Controller established adjacencies on the BFR for
   the BPs of the BitString.  There is thus no need for BFR specific
   routing underlay extensions to forward BIER packets with BIER-TE

   Encapsulation parameters can be provisioned by the BIER-TE controller
   into the forward_connected() or forward_routed() adjacencies directly
   without relying on a routing underlay.

   If the BFR intends to support FRR for BIER-TE, then the BIER-TE
   forwarding plane needs to receive fast adjacency up/down
   notifications: Link up/down or neighbor up/down, e.g. from BFD.
   Providing these notifications is considered to be part of the routing
   underlay in this document.

3.5.  Traffic Engineering Considerations

   Traffic Engineering ([I-D.ietf-teas-rfc3272bis]) provides performance
   optimization of operational IP networks while utilizing network
   resources economically and reliably.  The key elements needed to
   effect TE are policy, path steering and resource management.  These
   elements require support at the control/controller level and within
   the forwarding plane.

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   Policy decisions are made within the BIER-TE control plane, i.e.,
   within BIER-TE Controllers.  Controllers use policy when composing
   BitStrings and BFR BIFT state.  The mapping of user/IP traffic to
   specific BitStrings/BIER-TE flows is made based on policy.  The
   specific details of BIER-TE policies and how a controller uses them
   are out of scope of this document.

   Path steering is supported via the definition of a BitString.
   BitStrings used in BIER-TE are composed based on policy and resource
   management considerations.  For example, when composing BIER-TE
   BitStrings, a Controller must take into account the resources
   available at each BFR and for each BP when it is providing
   congestion-loss-free services such as Rate Controlled Service
   Disciplines [RCSD94].  Resource availability could be provided for
   example via routing protocol information, but may also be obtained
   via a BIER-TE control protocol such as NETCONF or any other protocol
   commonly used by a Controller to understand the resources of the
   network it operates on.  The resource usage of the BIER-TE traffic
   admitted by the BIER-TE controller can be solely tracked on the BIER-
   TE Controller based on local accounting as long as no
   forward_routed() adjacencies are used (see Section 4.2.1 for the
   definition of forward_routed() adjacencies).  When forward_routed()
   adjacencies are used, the paths selected by the underlying routing
   protocol need to be tracked as well.

   Resource management has implications on the forwarding plane beyond
   the BIER-TE defined steering of packets.  This includes allocation of
   buffers to guarantee the worst case requirements of admitted RCSD
   traffic and potentially policing and/or rate-shaping mechanisms,
   typically done via various forms of queuing.  This level of resource
   control, while optional, is important in networks that wish to
   support congestion management policies to control or regulate the
   offered traffic to deliver different levels of service and alleviate
   congestion problems, or those networks that wish to control latencies
   experienced by specific traffic flows.

4.  BIER-TE Forwarding

4.1.  The BIER-TE Bit Index Forwarding Table (BIFT)

   The BIER-TE BIFT is the equivalent to the BIER BIFT for (non-TE)
   BIER.  It exists on every BFR running BIER-TE.  For every BIER sub-
   domain (SD) in use for BIER-TE, it is a table as shown shown in
   Figure 4.  That example BIFT assumes a BSL of 8 bit positions (BPs)
   in the packets BitString.  As in [RFC8279] this BSL is purely used
   for the example and not a BIER/BIER-TE supported BSL (minimum BSL is

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   A BIER-TE BIFT compares to a BIER BIFT as shown in [RFC8279] as

   In both BIER and BIER-TE, BIFT rows/entries are indexed in their
   respective BIER pseudocode ([RFC8279] Section 6.5) and BIER-TE
   pseudocode (Section 4.4) by the BIFT-index derived from the packets
   SI, BSL and the one bit position of the packets BitString (BP)
   addressing the BIFT row: BIFT-index = SI * BSL + BP - 1.  BP within a
   BitString are numbered from 1 to BSL, hence the - 1 offset when
   converting to a BIFT-index.  This document also uses the notion SI:BP
   to indicate BIFT rows, [RFC8279] uses the equivalent notion
   SI:BitString, where the BitString is filled with only the BP for the
   BIFT row.

   In BIER, each BIFT-index addresses one BFER by its BFR-id = BIFT-
   index + 1 and is populated on each BFR with the next-hop "BFR
   Neighbor" (BFR-NBR) towards that BFER.

   In BIER-TE, each BIFT-index and therefore SI:BP indicates one or more
   adjacencies between BFRs in the topology and is only populated with
   those adjacencies forwarding entries on the BFR that is the upstream
   for these adjacencies.  The BIFT entry are empty on all other BFRs.

   In BIER, each BIFT rows also requires a "Forwarding Bit Mask" (F-BM)
   entry for BIER forwarding rules.  In BIER-TE forwarding, F-BM is not
   required, but can be used when implementing BIER-TE on forwarding
   hardware derived from BIER forwarding, that must use F-BM.  This is
   discussed in the first BIER-TE forwarding pseudocode in Section 4.4.

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     | BIFT-index |     | Adjacencies:                                |
     | (SI:BP)    |(FBM)| <empty> or one or more per entry            |
     |       BIFT indices for Packets with SI=0                       |
     | 0 (0:1)    | ... | forward_connected(interface,neighbor{,DNC}) |
     | 1 (0:2)    | ... | forward_connected(interface,neighbor{,DNC}) |
     |            | ... | forward_connected(interface,neighbor{,DNC}) |
     |  ...       | ... | ...                                         |
     | 4 (0:5)    | ... | local_decap({VRF})                          |
     | 5 (0:6)    | ... | forward_routed({VRF,}l3-neighbor)           |
     | 6 (0:7)    | ... | <empty>                                     |
     | 7 (0:8)    | ... | ECMP((adjacency1,...adjacencyN){,seed})     |
     |       BIFT indices for BitString/Packet with SI=1              |
     | 9 (1:1)    |     | ...                                         |
     |  ...       |...  | ...                                         |
                  BIER-TE Bit Index Forwarding Table (BIFT)

             Figure 4: BIER-TE BIFT with different adjacencies

   The BIFT is configured for the BIER-TE data plane of a BFR by the
   BIER-TE Controller through an appropriate protocol and data-model.
   The BIFT is then used to forward packets, according to the rules
   specified in the BIER-TE Forwarding Procedures.

   Note that a BIFT index (SI:BP) may be populated in the BIFT of more
   than one BFR to save BPs.  See Section 5.1.6 for an example of how a
   BIER-TE controller could assign BPs to (logical) adjacencies shared
   across multiple BFRs, Section 5.1.3 for an example of assigning the
   same BP to different adjacencies, and Section 5.1.9 for general
   guidelines regarding re-use of BPs across different adjacencies.

   {VRF} indicates the Virtual Routing and Forwarding context into which
   the BIER payload is to be delivered.  This is optional and depends on
   the multicast flow overlay.

4.2.  Adjacency Types

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4.2.1.  Forward Connected

   A "forward_connected()" adjacency is towards a directly connected BFR
   neighbor using an interface address of that BFR on the connecting
   interface.  A forward_connected() adjacency does not route packets
   but only L2 forwards them to the neighbor.

   Packets sent to an adjacency with "DoNotClear" (DNC) set in the BIFT
   MUST NOT have the bit position for that adjacency cleared when the
   BFR creates a copy for it.  The bit position will still be cleared
   for copies of the packet made towards other adjacencies.  This can be
   used for example in ring topologies as explained in Section 5.1.6.

   For protection against loops from misconfiguration (see
   Section 5.2.1), DNC is only permissible for forward_connected()
   adjacencies.  No need or benefit of DNC for other type of adjacencies
   was identified and their risk was not analyzed.

4.2.2.  Forward Routed

   A "forward_routed()" adjacency is an adjacency towards a BFR that
   uses a (tunneling) encapsulation which will cause the packet to be
   forwarded by the routing underlay toward the adjacent BFR.  This can
   leverage any feasible encapsulation, such as MPLS or tunneling over
   IP/IPv6, as long as the BIER-TE packet can be identified as a
   payload.  This identification can either rely on the BIER/BIER-TE co-
   existence mechanisms described in Section 4.3, or by explicit support
   for a BIER-TE payload type in the tunneling encapsulation.

   forward_routed() adjacencies are necessary to pass BIER-TE traffic
   across non BIER-TE capable routers or to minimize the number of
   required BP by tunneling over (BIER-TE capable) routers on which
   neither replication nor path-steering is desired, or simply to
   leverage path redundancy and FRR of the routing underlay towards the
   next BFR.  They may also be useful to a multi-subnet adjacent BFR to
   leverage the routing underlay ECMP independent of BIER-TE ECMP
   (Section 4.2.3).

4.2.3.  ECMP

   (non-TE) BIER ECMP is tied to the BIER BIFT processing semantic and
   is therefore not directly usable with BIER-TE.

   A BIER-TE "Equal Cost Multipath" (ECMP()) adjacency as shown in
   Figure 4 for BIFT-index 7 has a list of two or more non-ECMP
   adjacencies as parameters and an optional seed parameter.  When a
   BIER-TE packet is copied onto such an ECMP() adjacency, an
   implementation specific so-called hash function will select one out

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   of the list's adjacencies to which the packet is forwarded.  If the
   packet's encapsulation contains an entropy field, the entropy field
   SHOULD be respected; two packets with the same value of the entropy
   field SHOULD be sent on the same adjacency.  The seed parameter
   allows to design hash functions that are easy to implement at high
   speed without running into polarization issues across multiple
   consecutive ECMP hops.  See Section 5.1.7 for more explanations.

4.2.4.  Local Decap(sulation)

   A "local_decap()" adjacency passes a copy of the payload of the BIER-
   TE packet to the protocol ("NextProto") within the BFR (IPv4/IPv6,
   Ethernet,...) responsible for that payload according to the packet
   header fields.  A local_decap() adjacency turns the BFR into a BFER
   for matching packets.  Local_decap() adjacencies require the BFER to
   support routing or switching for NextProto to determine how to
   further process the packet.

4.3.  Encapsulation / Co-existence with BIER

   Specifications for BIER-TE encapsulation are outside the scope of
   this document.  This section gives explanations and guidelines.

   Like [RFC8279], handling of "Maximum Transmission Unit" (MTU)
   limitations is outside the scope of this document and instead part of
   the BIER-TE packet encapsulation and/or flow overlay.  See for
   example [RFC8296], Section 3.  It applies equally to BIER-TE as it
   does to BIER.

   Because a BFR needs to interpret the BitString of a BIER-TE packet
   differently from a (non-TE) BIER packet, it is necessary to
   distinguish BIER from BIER-TE packets.  In the BIER encapsulation
   [RFC8296], the BIFT-id field of the packet indicates the BIFT of the
   packet.  BIER and BIER-TE can therefore be run simultaneously, when
   the BIFT-id address space is shared across BIER BIFT and BIER-TE
   BIFT.  Partitioning the BIFT-id address space is subject to BIER-TE/
   BIER control plane procedures.

   When [RFC8296] is used for BIER with MPLS, BIFT-id address ranges can
   be dynamically allocated from MPLS label space only for the set of
   actually used SD:BSL BIFT.  This allows to also allocate non-
   overlapping label ranges for BIFT-id that are to be used with BIER-TE

   With MPLS, it is also possible to reuse the same SD space for both
   BIER-TE and BIER, so that the same SD has both a BIER BIFT with a
   corresponding range of BIFT-ids and disjoint BIER-TE BIFTs with a
   non-overlapping range of BIFT-ids.

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   When a fixed mapping from BSL, SD and SI to BIFT-id is used which
   does not explicitly partition the BIFT-id space between BIER and
   BIER-TE, such as proposed for non-MPLS forwarding with [RFC8296]
   encapsulation in [I-D.ietf-bier-non-mpls-bift-encoding] revision 04,
   section 5, then it is necessary to allocate disjoint SDs to BIER and
   BIER-TE BIFTs so that both can be addressed by the BIFT-ids.  The
   encoding proposed in section 6. of the same document does not
   statically encode BSL or SD into the BIFT-id, but allows for a
   mapping, and hence could provide for the same freedom as when MPLS is
   being used (same or different SD for BIER/BIER-TE).

   forward_routed() requires an encapsulation that permits to direct
   unicast encapsulated BIER-TE packets to a specific interface address
   on a target BFR.  With MPLS encapsulation, this can simply be done
   via a label stack with that addresses label as the top label -
   followed by the label assigned to the (BSL,SD,SI) BitString.  With
   non-MPLS encapsulation, some form of IP encapsulation would be
   required (for example IP/GRE).

   The encapsulation used for forward_routed() adjacencies can equally
   support existing advanced adjacency information such as "loose source
   routes" via e.g.  MPLS label stacks or appropriate header extensions
   (e.g. for IPv6).

4.4.  BIER-TE Forwarding Pseudocode

   The following pseudocode, Figure 5, for BIER-TE forwarding is based
   on the (non-TE) BIER forwarding pseudocode of [RFC8279], section 6.5
   with one modification.

      void ForwardBitMaskPacket_withTE (Packet)
          for (Index = GetFirstBitPosition(Packet->BitString); Index ;
               Index = GetNextBitPosition(Packet->BitString, Index)) {
              F-BM = BIFT[Index+Offset]->F-BM;
              if (!F-BM) continue;                            [3]
              BFR-NBR = BIFT[Index+Offset]->BFR-NBR;
              PacketCopy = Copy(Packet);
              PacketCopy->BitString &= F-BM;                  [2]
              PacketSend(PacketCopy, BFR-NBR);
              // The following must not be done for BIER-TE:
              // Packet->BitString &= ~F-BM;                  [1]

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      Figure 5: BIER-TE Forwarding Pseudocode for required functions,
                          based on BIER Pseudocode

   In step [2], the F-BM is used to clear bit(s) in PacketCopy.  This
   step exists in both BIER and BIER-TE, but the F-BMs need to be
   populated differently for BIER-TE than for BIER for the desired

   In BIER, multiple bits of a BitString can have the same BFR-NBR.
   When a received packets BitString has more than one of those bits
   set, the BIER replication logic has to avoid that more than one
   PacketCopy is sent to that BFR-NBR ([1]).  Likewise, the PacketCopy
   sent to a BFR-NBR must clear all bits in its BitString that are not
   routed across BFR-NBR.  This protects against BIER replication on any
   possible further BFR to create duplicates ([2]).

   To solve both [1] and [2] for BIER, the F-BM of each bit index needs
   to have all bits set that this BFR wants to route across BFR-NBR. [2]
   clears all other bits in PacketCopy->BitString, and [1] clears those
   bits from Packet->BitString after the first PacketCopy.

   In BIER-TE, a BFR-NBR in this pseudocode is an adjacency,
   forward_connected(), forward_routed() or local_decap().  There is no
   need for [2] to suppress duplicates in the way BIER does because in
   general, different BP would never have the same adjacency.  If a
   BIER-TE controller actually finds some optimization in which this
   would be desirable, then the controller is also responsible to ensure
   that only one of those bits is set in any Packet->BitString, unless
   the controller explicitly wants for duplicates to be created.

   The following points describe how the forwarding bit mask (F-BM) for
   each BP is configured in the BIFT and how this impacts the BitString
   of the packet being processed with that BIFT:

   1.  The F-BMs of all BIFT BPs without an adjacency have all their
       bits clear.  This will cause [3] to skip further processing of
       such a BP.

   2.  All BIFT BPs with an adjacency (with DNC flag clear) have an F-BM
       that has only those BPs set for which this BFR does not have an
       adjacency.  This causes [2] to clear all bits from
       PacketCopy->BitString for which this BFR does have an adjacency.

   3.  [1] is not performed for BIER-TE.  All bit clearing required by
       BIER-TE is performed by [2].

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   This Forwarding Pseudocode can support the required BIER-TE
   forwarding functions (see Section 4.6), forward_connected(),
   forward_routed() and local_decap(), but not the recommended functions
   DNC flag and multiple adjacencies per bit nor the optional function,
   ECMP() adjacencies.  The DNC flag cannot be supported when using only
   [1] to mask bits.

   The modified and expanded Forwarding Pseudocode in Figure 6 specifies
   how to support all BIER-TE forwarding functions (required,
   recommended and optional):

   *  This pseudocode eliminates per-bit F-BM, therefore reducing the
      size of BIFT state by BSL^2*SI and eliminating the need for per-
      packet-copy BitString masking operations except for adjacencies
      with the DNC flag set:

      -  AdjacentBits[SI] are bit positions with a non-empty list of
         adjacencies in this BFR BIFT.  This can be computed whenever
         the BIER-TE Controller updates (add/removes) adjacencies in the

      -  The BFR needs to create packet copies for these adjacent bits
         when they are set in the packets BitString.  This set of bits
         is calculated in PktAdjacentBits.

      -  All bit positions to which the BFR creates copies have to be
         cleared in packet copies to avoid loops.  This is done by
         masking the BitString of the packet with ~AdjacentBits[SI].
         When an adjacency has DNC set, this bit position is set again
         only for the packet copy towards that bit position.

   *  BIFT entries may contain more than one adjacency in support of
      specific configurations such as Section 5.1.5.  The code therefore
      includes a loop over these adjacencies.

   *  The ECMP() adjacency is shown.  Its parameters are a seed and a
      ListOfAdjacencies from which one is picked.

   *  The forward_connected(), forward_routed(), local_decap()
      adjacencies are shown with their parameters.

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    void ForwardBitMaskPacket_withTE (Packet)
        SI = GetPacketSI(Packet);
        Offset = SI * BitStringLength;
        // Determine adjacent bits in the Packets BitString
        PktAdjacentBits = Packet->BitString & AdjacentBits[SI];

        // Clear adjacent bits in Packet header to avoid loops
        Packet->BitString &= ~AdjacentBits[SI];

        // Loop over PktAdjacentBits to create packet copies
        for (Index = GetFirstBitPosition(PktAdjacentBits); Index ;
             Index = GetNextBitPosition(PktAdjacentBits, Index)) {
            for adjacency in BIFT[Index+Offset]->Adjacencies {
                if(adjacency.type == ECMP(ListOfAdjacencies,seed) ) {
                    I = ECMP_hash(sizeof(ListOfAdjacencies),
                    adjacency = ListOfAdjacencies[I];
                PacketCopy = Copy(Packet);
                switch(adjacency.type) {
                    case forward_connected(interface,neighbor,DNC):
                            PacketCopy->BitString |= 1<<(Index-1);

                    case forward_routed({VRF,}l3-neighbor):

                    case local_decap({VRF},neighbor):

       Figure 6: Complete BIER-TE Forwarding Pseudocode for required,
                     recommended and optional functions

4.5.  Basic BIER-TE Forwarding Example

   [RFC Editor: remove this section.]


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   Step-by-step example of basic BIER-TE forwarding.  This example does
   not use ECMP() or forward_routed() adjacencies nor does it try to
   minimize the number of required BitPositions for the topology.

                  [BIER-TE Controller]
                   /       |       \
                  v        v        v
              .                          .
           | p13   p1 |                  .
           +- BFIR2 --+          |       .
           |  .       | p2   p6  |       .   LAN2
           |  .       +-- BFR3 --+       .   |
           |  .       |          |  p7  p11  |
      Src -+  .                  +-- BFER1 --+
           |  .       | p3   p8  |       .   |
           |  .       +-- BFR4 --+       .   +-- Rcv1
           |  .       |          |       .   |
           |  .       |                  .
           | p14  p4  |                  .
           +- BFIR1 --+          |       .
           |  .       +-- BFR5 --+ p10  p12  |
         LAN1 .       | p5   p9  +-- BFER2 --+
              .                  |       .   +-- Rcv2
              .                          .   |
              .                          .   LAN3
              .                          .
          IP  |..... BIER-TE network.....| IP

                    Figure 7: BIER-TE Forwarding Example

   pXX indicate the BitPositions number assigned by the BIER-TE
   Controller to adjacencies in the BIER-TE topology.  For example, p9
   is the adjacency towards BFR5 on the LAN connecting to BFER2.

      BIFT BFIR2:
        p13: local_decap
         p2: forward_connected(BFR3)

      BIFT BFR3:
         p1: forward_connected(BFIR2)
         p7: forward_connected(BFER1)
         p8: forward_connected(BFR4)

      BIFT BFER1:
        p11: local_decap
         p6: forward_connected(BFR3)
         p8: forward_connected(BFR4)

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              Figure 8: BIER-TE Forwarding Example Adjacencies

   ...and so on.

   For example, we assume that some multicast traffic seen on LAN1 needs
   to be sent via BIER-TE by BFIR2 towards Rcv1 and Rcv2.  The BIER-TE
   Controller determines it wants it to pass this traffic across the
   following paths:

                 -> BFER1 ---------------> Rcv1
    BFIR2 -> BFR3
                 -> BFR4 -> BFR5 -> BFER2 -> Rcv2

                 Figure 9: BIER-TE Forwarding Example Paths

   These paths equal to the following BitString: p2, p5, p7, p8, p10,
   p11, p12.

   This BitString is assigned by BFIR2 to the example multicast traffic
   received from LAN1.

   Then BFIR2 forwards this multicast traffic with BIER-TE based on that
   BitString.  The BIFT of BFIR2 has only p2 and p13 populated.  Only p2
   is in the BitString and this is an adjacency towards BFR3.  BFIR2
   therefore clears p2 in the BitString and sends a copy towards BFR2.

   BFR3 sees a BitString of p5,p7,p8,p10,p11,p12.  For those BPs, it has
   only adjacencies for p7,p8.  It creates a copy of the packet to BFER1
   (due to p7) and one to BFR4 (due to p8).  It clears both p7 and p8
   before sending.

   BFER1 sees a BitString of p5,p10,p11,p12.  For those BPs, it only has
   an adjacency for p11. p11 is a local_decap() adjacency installed by
   the BIER-TE Controller to receive a copy of the BIER packet - dispose
   of the BIER header and pass the payload to IP multicast.  IP
   multicast will then forward the packet out to LAN2 because it did
   receive PIM or IGMP joins on LAN2 for the traffic.

   Further processing of the packet in BFR4, BFR5 and BFER2 accordingly.

4.6.  BFR Requirements for BIER-TE forwarding

   BFR that support BIER-TE and BIER MUST support configuration that
   enables BIER-TE instead of (non-TE) BIER forwarding rules for all
   BIFT of one or more BIER sub-domains.  Every BP in a BIER-TE BIFT
   MUST support to have zero or one adjacency.  BIER-TE forwarding MUST
   support the adjacency types forward_connected() with the DNC flag not
   set, forward_routed() and local_decap().  As explained in

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   Section 4.4, these required BIER-TE forwarding functions can be
   implemented via the same Forwarding Pseudocode as BIER forwarding
   except for one modification (skipping one masking with F-BM).

   BIER-TE forwarding SHOULD support forward_connected() adjacencies
   with a set DNC flag, as this is highly useful to save bits in rings
   (see Section 5.1.6).

   BIER-TE forwarding SHOULD support more than one adjacency on a bit.
   This allows to save bits in hub and spoke scenarios (see
   Section 5.1.5).

   BIER-TE forwarding MAY support ECMP() adjacencies to save bits in
   ECMP scenarios, see Section 5.1.7 for an example.  This is an
   optional requirement, because for ECMP deployments using BIER-TE one
   can also leverage ECMP of the routing underlay via forwarded_routed
   adjacencies and/or might prefer to have more explicit control of the
   path chosen via explicit BP/adjacencies for each ECMP path

5.  BIER-TE Controller Operational Considerations

5.1.  Bit Position Assignments

   This section describes how the BIER-TE Controller can use the
   different BIER-TE adjacency types to define the bit positions of a
   BIER-TE domain.

   Because the size of the BitString limits the size of the BIER-TE
   domain, many of the options described exist to support larger
   topologies with fewer bit positions.

5.1.1.  P2P Links

   On a P2P link that connects two BFRs, the same bit position can be
   used on both BFRs for the adjacency to the neighboring BFR.  A P2P
   link requires therefore only one bit position.

5.1.2.  BFER

   Every non-Leaf BFER is given a unique bit position with a
   local_decap() adjacency.

5.1.3.  Leaf BFERs

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           BFR1(P) BFR2(P)             BFR1(P)  BFR2(P)
             |  \ /  |                    |       |
             |   X   |                    |       |
             |  / \  |                    |       |
        BFER1(PE)  BFER2(PE)        BFER1(PE)----BFER2(PE)

                                              ^ U-turn link

            Leaf BFER /               Non-Leaf BFER /
             PE-router                  PE-router

                 Figure 10: Leaf vs. non-Leaf BFER Example

   A leaf BFER is one where incoming BIER-TE packets never need to be
   forwarded to another BFR but are only sent to the BFER to exit the
   BIER-TE domain.  For example, in networks where Provider Edge (PE)
   router are spokes connected to Provider (P) routers, those PEs are
   Leaf BFERs unless there is a U-turn between two PEs.

   Consider how redundant disjoint traffic can reach BFER1/BFER2 in
   Figure 10: When BFER1/BFER2 are Non-Leaf BFER as shown on the right-
   hand side, one traffic copy would be forwarded to BFER1 from BFR1,
   but the other one could only reach BFER1 via BFER2, which makes BFER2
   a non-Leaf BFER.  Likewise, BFER1 is a non-Leaf BFER when forwarding
   traffic to BFER2.  Note that the BFERs in the left-hand picture are
   only guaranteed to be leaf-BFER by fitting routing configuration that
   prohibits transit traffic to pass through the BFERs, which is
   commonly applied in these topologies.

   In most situations, leaf-BFER that are to be addressed via the same
   BitString can share a single bit position for their local_decap()
   adjacency in that BitString and therefore save bit positions.  On a
   non-leaf BFER, a received BIER-TE packet may only need to transit the
   BFER or it may need to also be decapsulated.  Whether or not to
   decapsulate the packet therefore needs to be indicated by a unique
   bit position populated only on the BIFT of this BFER with a
   local_decap() adjacency.  On a leaf-BFER, packets never need to pass
   through; any packet received is therefore usually intended to be
   decapsulated.  This can be expressed by a single, shared bit position
   that is populated with a local_decap() adjacency on all leaf-BFER
   addressed by the BitString.

   The possible exception from this leaf-BFER bit position optimization
   can be cases where the bit position on the prior BIER-TE BFR (which
   created the packet copy for the leaf-BFER in question) is populated
   with multiple adjacencies as an optimization, such as in
   Section 5.1.4 or Section 5.1.5.  With either of these two
   optimizations, the sender of the packet could only control explicitly

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   whether the packet was to be decapsulated on the leaf-BFER in
   question, if the leaf-BFER has a unique bit position for its
   local_decap() adjacency.

   However, if the bit position is shared across leaf-BFER, and packets
   are therefore decapsulated potentially unnecessarily, this may still
   be appropriate if the decapsulated payload of the BIER-TE packet does
   indicate whether or not the packet needs to be further processed/
   received.  This is typically true for example if the payload is IP
   multicast because IP multicast on a BFER would know the membership
   state of the IP multicast payload and be able to discard it if the
   packet was delivered unnecessarily by the BIER-TE layer.  If the
   payload has no such membership indication, and the BFIR wants to have
   explicit control about which BFER are to receive and decapsulate a
   packet, then these two optimizations can not be used together with
   shared bit positions optimization for leaf-BFER.

5.1.4.  LANs

   In a LAN, the adjacency to each neighboring BFR is given a unique bit
   position.  The adjacency of this bit position is a
   forward_connected() adjacency towards the BFR and this bit position
   is populated into the BIFT of all the other BFRs on that LAN.

                                  p3|  p4|   p2|
                                  BFR3 BFR4  BFR7

                           Figure 11: LAN Example

   If Bandwidth on the LAN is not an issue and most BIER-TE traffic
   should be copied to all neighbors on a LAN, then bit positions can be
   saved by assigning just a single bit position to the LAN and
   populating the bit position of the BIFTs of each BFRs on the LAN with
   a list of forward_connected() adjacencies to all other neighbors on
   the LAN.

   This optimization does not work in the case of BFRs redundantly
   connected to more than one LAN with this optimization because these
   BFRs would receive duplicates and forward those duplicates into the
   opposite LANs.  Adjacencies of such BFRs into their LAN still need a
   separate bit position.

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5.1.5.  Hub and Spoke

   In a setup with a hub and multiple spokes connected via separate p2p
   links to the hub, all p2p adjacencies from the hub to the spokes
   links can share the same bit position.  The bit position on the hub's
   BIFT is set up with a list of forward_connected() adjacencies, one
   for each Spoke.

   This option is similar to the bit position optimization in LANs:
   Redundantly connected spokes need their own bit positions, unless
   they are themselves Leaf-BFER.

   This type of optimized BP could be used for example when all traffic
   is "broadcast" traffic (very dense receiver set) such as live-TV or
   many-to-many telemetry including situation-awareness (SA).  This BP
   optimization can then be used to explicitly steer different traffic
   flows across different ECMP paths in Data-Center or broadband-
   aggregation networks with minimal use of BPs.

5.1.6.  Rings

   In L3 rings, instead of assigning a single bit position for every p2p
   link in the ring, it is possible to save bit positions by setting the
   "DoNotClear" (DNC) flag on forward_connected() adjacencies.

   For the rings shown in Figure 12, a single bit position will suffice
   to forward traffic entering the ring at BFRa or BFRb all the way up
   to BFR1:

   On BFRa, BFRb, BFR30,... BFR3, the bit position is populated with a
   forward_connected() adjacency pointing to the clockwise neighbor on
   the ring and with DNC set.  On BFR2, the adjacency also points to the
   clockwise neighbor BFR1, but without DNC set.

   Handling DNC this way ensures that copies forwarded from any BFR in
   the ring to a BFR outside the ring will not have the ring bit
   position set, therefore minimizing the chance to create loops.

                  v        v
                  |        |
           L1     |   L2   |   L3
       /-------- BFRa ---- BFRb --------------------\
       |                                            |
       \- BFR1 - BFR2 - BFR3 - ... - BFR29 - BFR30 -/
           |      |    L4               |      |
        p33|                         p15|
           BFRd                       BFRc

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                          Figure 12: Ring Example

   Note that this example only permits for packets intended to make it
   all the way around the ring to enter it at BFRa and BFRb, and that
   packets will always travel clockwise.  If packets should be allowed
   to enter the ring at any ring BFR, then one would have to use two
   ring bit positions.  One for each direction: clockwise and

   Both would be set up to stop rotating on the same link, e.g.  L1.
   When the ingress ring BFR creates the clockwise copy, it will clear
   the counterclockwise bit position because the DNC bit only applies to
   the bit for which the replication is done.  Likewise for the
   clockwise bit position for the counterclockwise copy.  As a result,
   the ring ingress BFR will send a copy in both directions, serving
   BFRs on either side of the ring up to L1.

5.1.7.  Equal Cost MultiPath (ECMP)

   [RFC-Editor: A reviewer (Lars Eggert) noted that the infinite "to
   use" in the following sentence is not correct.  The same was also
   noted for several other similar instances.  The following URL seems
   to indicate though that this is a per-case decision, which seems
   infinitive-and-gerund-to-do-or-doing.  What exactly should be done
   about this ?].

   An ECMP() adjacency allows to use just one BP to deliver packets to
   one one of N adjacencies instead of one BP for each adjacency.  In
   the common example case Figure 13, a link-bundle of three links
   L1,L2,L3 connects BFR1 and BFR2, and only one BP is used instead of
   three BP to deliver packets from BFR1 to BFR2.

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           BFR1 --L2----- BFR2

     BIFT entry in BFR1:
     | Index |  Adjacencies                                           |
     | 0:6   |  ECMP({forward_connected(L1, BFR2),                    |
     |       |        forward_connected(L2, BFR2),                    |
     |       |        forward_connected(L3, BFR2)}, seed)             |

     BIFT entry in BFR2:
     | Index |  Adjacencies                                           |
     | 0:6   |  ECMP({forward_connected(L1, BFR1),                    |
     |       |        forward_connected(L2, BFR1),                    |
     |       |        forward_connected(L3, BFR1)}, seed)             |

                          Figure 13: ECMP Example

   This document does not standardize any ECMP algorithm because it is
   sufficient for implementations to document their freely chosen ECMP
   algorithm.  Figure 14 shows an example ECMP algorithm, and would
   double as its documentation: A BIER-TE controller could determine
   which adjacency is chosen based on the seed and adjacencies
   parameters and the packet entropy.

      forward(packet, ECMP(adj(0), adj(1),... adj(N-1), seed)):
         i = (packet(bier-header-entropy) XOR seed) % N
         forward packet to adj(i)

                     Figure 14: ECMP algorithm Example

   In the following example, all traffic from BFR1 towards BFR10 is
   intended to be ECMP load split equally across the topology.  This
   example is not meant as a likely setup, but to illustrate that ECMP
   can be used to share BPs not only across link bundles, but also
   across alternative paths across different transit BFR, and it
   explains the use of the seed parameter.

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                    BFR1         (BFIR)
                  /L11  \L12
                 /       \
             BFR2         BFR3
            /L21 \L22    /L31 \L32
           /      \     /      \
          BFR4  BFR5   BFR6  BFR7
           \      /     \      /
            \    /       \    /
             BFR8         BFR9
                 \       /
                  \     /
                   BFR10         (BFER)

     BIFT entry in BFR1:
     | 0:6   |  ECMP({forward_connected(L11, BFR2),                   |
     |       |        forward_connected(L12, BFR3)}, seed1)           |

     BIFT entry in BFR2:
     | 0:7   |  ECMP({forward_connected(L21, BFR4),                   |
     |       |        forward_connected(L22, BFR5)}, seed1)           |

     BIFT entry in BFR3:
     | 0:7   |  ECMP({forward_connected(L31, BFR6),                   |
     |       |        forward_connected(L32, BFR7)}, seed1)           |

     BIFT entry in BFR4, BFR5:
     | 0:8   |  forward_connected(Lxx, BFR8)  |xx differs on BFR4/BFR5|

     BIFT entry in BFR6, BFR7:
     | 0:8   |  forward_connected(Lxx, BFR9)  |xx differs on BFR6/BFR7|

     BIFT entry in BFR8, BFR9:
     | 0:9   |  forward_connected(Lxx, BFR10) |xx differs on BFR8/BFR9|

                      Figure 15: Polarization Example

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   Note that for the following discussion of ECMP, only the BIFT ECMP
   adjacencies on BFR1, BFR2, BFR3 are relevant.  The re-use of BP
   across BFR in this example is further explained in Section 5.1.9

   With the setup of ECMP in the topology above, traffic would not be
   equally load-split.  Instead, links L22 and L31 would see no traffic
   at all: BFR2 will only see traffic from BFR1 for which the ECMP hash
   in BFR1 selected the first adjacency in the list of 2 adjacencies
   given as parameters to the ECMP.  It is link L11-to-BFR2.  BFR2
   performs again ECMP with two adjacencies on that subset of traffic
   using the same seed1, and will therefore again select the first of
   its two adjacencies: L21-to-BFR4.  And therefore L22 and BFR5 sees no
   traffic.  Likewise for L31 and BFR6.

   This issue in BFR2/BFR3 is called polarization.  It results from the
   re-use of the same hash function across multiple consecutive hops in
   topologies like these.  To resolve this issue, the ECMP() adjacency
   on BFR1 can be set up with a different seed2 than the ECMP()
   adjacencies on BFR2/BFR3.  BFR2/BFR3 can use the same hash because
   packets will not sequentially pass across both of them.  Therefore,
   they can also use the same BP 0:7.

   Note that ECMP solutions outside of BIER often hide the seed by auto-
   selecting it from local entropy such as unique local or next-hop
   identifiers.  Allowing the BIER-TE Controller to explicitly set the
   seed gives the ability for it to control same/different path
   selection across multiple consecutive ECMP hops.

5.1.8.  Forward Routed adjacencies  Reducing bit positions

   Forward_routed() adjacencies can reduce the number of bit positions
   required when the path steering requirement is not hop-by-hop
   explicit path selection, but loose-hop selection.  Forward_routed()
   adjacencies can also allow to operate BIER-TE across intermediate hop
   routers that do not support BIER-TE.

            ...BFR1--...           ...--L1-- BFR2...
                     ... .Routers. ...--L2--/
            ...BFR4--...           ...--L3-- BFR3...
                     ...           ...--L4--/ |
                      ...............         |
                       Network Area 1

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               Figure 16: Forward Routed Adjacencies Example

   Assume the requirement in Figure 16 is to explicitly steer traffic
   flows that have arrived at BFR1 or BFR4 via a path in the routing
   underlay "Network Area 1" to one of the following three next
   segments: (1) BFR2 via link L1, (2) BFR2 via link L2, or (3) via BFR3
   and then nor caring whether the packet is forwarded via L3 or L4.

   To enable this, both BFR1 and BFR4 are set up with a forward_routed
   adjacency bit position towards an address of BFR2 on link L1, another
   forward_routed() bit position towards an address of BFR2 on link L2
   and a third forward_routed() bit position towards a node address LO
   of BFR3.  Supporting nodes without BIER-TE

   Forward_routed() adjacencies also enable incremental deployment of
   BIER-TE.  Only the nodes through which BIER-TE traffic needs to be
   steered - with or without replication - need to support BIER-TE.
   Where they are not directly connected to each other, forward_routed
   adjacencies are used to pass over non BIER-TE enabled nodes.

5.1.9.  Reuse of bit positions (without DNC)

   Bit positions can be re-used across multiple BFRs to minimize the
   number of BP needed.  This happens when adjacencies on multiple BFRs
   use the DNC flag as described above, but it can also be done for non-
   DNC adjacencies.  This section only discusses this non-DNC case.

   Because BP are cleared when passing a BFR with an adjacency for that
   BP, reuse of BP across multiple BFRs does not introduce any problems
   with duplicates or loops that do not also exist when every adjacency
   has a unique BP.  Instead, the challenge when reusing BP is whether
   it allows to still achieve the desired Tree Engineering goals.

   BP cannot be reused across two BFRs that would need to be passed
   sequentially for some path: The first BFR will clear the BP, so those
   paths cannot be built.  BP can be set across BFR that would (A) only
   occur across different paths or (B) across different branches of the
   same tree.

   An example of (A) was given in Figure 15, where BP 0:7, BP 0:8 and BP
   0:9 are each reused across multiple BFRs because a single packet/path
   would never be able to reach more than one BFR sharing the same BP.

   Assume the example was changed: BFR1 has no ECMP() adjacency for BP
   0:6, but instead BP 0:5 with forward_connected() to BFR2 and BP 0:6
   with forward_connected() to BFR3.  Packets with both BP 0:5 and BP

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   0:6 would now be able to reach both BFR2 and BFR3 and the still
   existing re-use of BP 0:7 between BFR2 and BFR3 is a case of (B)
   where reuse of BP is perfect because it does not limit the set of
   useful path choices:

   If instead of reusing BP 0:7, BFR3 used a separate BP 0:10 for its
   ECMP() adjacency, no useful additional path steering options would be
   enabled.  If duplicates at BFR10 where undesirable, this would be
   done by not setting BP 0:5 and BP 0:6 for the same packet.  If the
   duplicates where desirable (e.g.: resilient transmission), the
   additional BP 0:10 would also not render additional value.

                      BFR1a BFR1b
                        /    \
           .                Core              .
           |    /       \    /           \  |
         BFR2a BFR2b  BFR3a BFR3b      BFR6a BFR6b
          /-------\   /---------\      /--------\
          | area2 |   |  area3  | ...  | area6  |
          | ring  |   |  ring   |      | ring   |
          \-------/   \---------/      \--------/
           more BFR     more BFR        more BFR

                           Figure 17: Reuse of BP

   Reuse may also save BPs in larger topologies.  Consider the topology
   shown in Figure 17.  A BFIR/sender (e.g.: video headend) is attached
   to area 1, and area 2...6 contain receivers/BFER.  Assume each area
   had a distribution ring, each with two BPs to indicate the direction
   (as explained before).  These two BPs could be reused across the 5
   areas.  Packets would be replicated through other BPs for the Core to
   the desired subset of areas, and once a packet copy reaches the ring
   of the area, the two ring BPs come into play.  This reuse is a case
   of (B), but it limits the topology choices: Packets can only flow
   around the same direction in the rings of all areas.  This may or may
   not be acceptable based on the desired path steering options: If
   resilient transmission is the path engineering goal, then it is
   likely a good optimization, if the bandwidth of each ring was to be
   optimized separately, it would not be a good limitation.

5.1.10.  Summary of BP optimizations

   This section reviewed a range of techniques by which a BIER-TE
   Controller can create a BIER-TE topology in a way that minimizes the
   number of necessary BPs.

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   Without any optimization, a BIER-TE Controller would attempt to map
   the network subnet topology 1:1 into the BIER-TE topology and every
   subnet adjacent neighbor requires a forward_connected() BP and every
   BFER requires a local_decap() BP.

   The optimizations described are then as follows:

   *  P2P links require only one BP (Section 5.1.1).

   *  All leaf-BFER can share a single local_decap() BP (Section 5.1.3).

   *  A LAN with N BFR needs at most N BP (one for each BFR).  It only
      needs one BP for all those BFR that are not redundantly connected
      to multiple LANs (Section 5.1.4).

   *  A hub with p2p connections to multiple non-leaf-BFER spokes can
      share one BP to all spokes if traffic can be flooded to all
      spokes, e.g.: because of no bandwidth concerns or dense receiver
      sets (Section 5.1.5).

   *  Rings of BFR can be built with just two BP (one for each
      direction) except for BFR with multiple ring connections - similar
      to LANs (Section 5.1.6).

   *  ECMP() adjacencies to N neighbors can replace N BP with 1 BP.
      Multihop ECMP can avoid polarization through different seeds of
      the ECMP algorithm (Section 5.1.7).

   *  Forward_routed() adjacencies allow to "tunnel" across non-BIER-TE
      capable routers and across BIER-TE capable routers where no
      traffic-steering or replications are required (Section 5.1.8).

   *  BP can generally be reused across a set of nodes where it can be
      guaranteed that no path will ever need to traverse more than one
      node of the set.  Depending on scenario, this may limit the
      feasible path steering options (Section 5.1.9).

   Note that the described list of optimizations is not exhaustive.
   Especially when the set of required path steering choices is limited
   and the set of possible subsets of BFERs that should be able to
   receive traffic is limited, further optimizations of BP are possible.
   The hub and spoke optimization is a simple example of such traffic
   pattern dependent optimizations.

5.2.  Avoiding duplicates and loops

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5.2.1.  Loops

   Whenever BIER-TE creates a copy of a packet, the BitString of that
   copy will have all bit positions cleared that are associated with
   adjacencies on the BFR.  This inhibits looping of packets.  The only
   exception are adjacencies with DNC set.

                  v        v
                  |        |
           L1     |   L2   |   L3
       /-------- BFRa ---- BFRb ---------------------\
       |        .                                    |
       |         ......  Wrong link wiring           |
       |               .                             |
       \- BFR1 - BFR2   BFR3 - ... - BFR29 - BFR30 -/
           |      |    L4               |      |
        p33|                         p15|
           BFRd                       BFRc

                      Figure 18: Miswired Ring Example

   With DNC set, looping can happen.  Consider in Figure 18 that link L4
   from BFR3 is (inadvertently) plugged into the L1 interface of BFRa
   (instead of BFR2).  This creates a loop where the rings clockwise bit
   position is never cleared for copies of the packets traveling
   clockwise around the ring.

   To inhibit looping in the face of such physical misconfiguration,
   only forward_connected() adjacencies are permitted to have DNC set,
   and the link layer port unique unicast destination address of the
   adjacency (e.g.  MAC address) protects against closing the loop.
   Link layers without port unique link layer addresses should not be
   used with the DNC flag set.

5.2.2.  Duplicates

                   /    \
                  / p2   \ p3
                 BFR2   BFR3
                  \ p4   / p5
                   \    /

                       Figure 19: Duplicates Example

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   Duplicates happen when the graph expressed by a BitString is not a
   tree but redundantly connecting BFRs with each other.  In Figure 19,
   a BitString of p2,p3,p4,p5 would result in duplicate packets to
   arrive on BFER4.  The BIER-TE Controller must therefore ensure to
   only create BitStrings that are trees.

   When links are incorrectly physically re-connected before the BIER-TE
   Controller updates BitStrings in BFIRs, duplicates can happen.  Like
   loops, these can be inhibited by link layer addressing in
   forward_connected() adjacencies.

   If interface or loopback addresses used in forward_routed()
   adjacencies are moved from one BFR to another, duplicates can equally
   happen.  Such re-addressing operations must be coordinated with the
   BIER-TE Controller.

5.3.  Managing SI, sub-domains and BFR-ids

   When the number of bits required to represent the necessary hops in
   the topology and BFER exceeds the supported BitStringLength (BSL),
   multiple SIs and/or sub-domains must be used.  This section discusses

   BIER-TE forwarding does not require the concept of BFR-id, but
   routing underlay, flow overlay and BIER headers may.  This section
   also discusses how BFR-ids can be assigned to BFIR/BFER for BIER-TE.

5.3.1.  Why SI and sub-domains

   For (non-TE) BIER and BIER-TE forwarding, the most important result
   of using multiple SI and/or sub-domains is the same: Packets that
   need to be sent to BFERs in different SIs or sub-domains require
   different BIER packets: each one with a BitString for a different
   (SI,sub-domain) combination.  Each such BitString uses one BSL sized
   SI block in the BIFT of the sub-domain.  We call this a BIFT:SI

   For BIER and BIER-TE forwarding themselves there is also no
   difference whether different SIs and/or sub-domains are chosen, but
   SI and sub-domain have different purposes in the BIER architecture
   shared by BIER-TE.  This impacts how operators are managing them and
   how especially flow overlays will likely use them.

   By default, every possible BFIR/BFER in a BIER network would likely
   be given a BFR-id in sub-domain 0 (unless there are > 64k BFIR/BFER).

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   If there are different flow services (or service instances) requiring
   replication to different subsets of BFERs, then it will likely not be
   possible to achieve the best replication efficiency for all of these
   service instances via sub-domain 0.  Ideal replication efficiency for
   N BFER exists in a sub-domain if they are split over not more than
   ceiling(N/BitStringLength) SI.

   If service instances justify additional BIER:SI state in the network,
   additional sub-domains will be used: BFIR/BFER are assigned BFR-id in
   those sub-domains and each service instance is configured to use the
   most appropriate sub-domain.  This results in improved replication
   efficiency for different services.

   Even if creation of sub-domains and assignment of BFR-id to BFIR/BFER
   in those sub-domains is automated, it is not expected that individual
   service instances can deal with BFER in different sub-domains.  A
   service instance may only support configuration of a single sub-
   domain it should rely on.

   To be able to easily reuse (and modify as little as possible)
   existing BIER procedures including flow-overlay and routing underlay,
   when BIER-TE forwarding is added, we therefore reuse SI and sub-
   domain logically in the same way as they are used in BIER: All
   necessary BFIR/BFER for a service use a single BIER-TE BIFT and are
   split across as many SIs as necessary (see Section 5.3.2).  Different
   services may use different sub-domains that primarily exist to
   provide more efficient replication (and for BIER-TE desirable path
   steering) for different subsets of BFIR/BFER.

5.3.2.  Assigning bits for the BIER-TE topology

   In BIER, BitStrings only need to carry bits for BFERs, which leads to
   the model that BFR-ids map 1:1 to each bit in a BitString.

   In BIER-TE, BitStrings need to carry bits to indicate not only the
   receiving BFER but also the intermediate hops/links across which the
   packet must be sent.  The maximum number of BFER that can be
   supported in a single BitString or BIFT:SI depends on the number of
   bits necessary to represent the desired topology between them.

   "Desired" topology because it depends on the physical topology, and
   on the desire of the operator to allow for explicit path steering
   across every single hop (which requires more bits), or reducing the
   number of required bits by exploiting optimizations such as unicast
   (forward_routed()), ECMP() or flood (DNC) over "uninteresting" sub-
   parts of the topology - e.g. parts where different trees do not need
   to take different paths due to path steering reasons.

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   The total number of bits to describe the topology vs. the number of
   BFERs in a BIFT:SI can range widely based on the size of the topology
   and the amount of alternative paths in it.  In a BIER-TE topology
   crafted by a BIER-TE expert, the higher the percentage of non-BFER
   bits, the higher the likelihood, that those topology bits are not
   just BIER-TE overhead without additional benefit, but instead that
   they will allow to express desirable path steering alternatives.

5.3.3.  Assigning BFR-id with BIER-TE

   BIER-TE forwarding does not use the BFR-id, nor does it require for
   the BFIR-id field of the BIER header to be set to a particular value.
   However, other parts of a BIER-TE deployment may need a BFR-id,
   specifically multicast flow overlay signaling and multicast flow
   overlay packet disposition, and in that case BFRs need to also have
   BFR-ids for BIER-TE SDs.

   For example, for BIER overlay signaling, BFIRs need to have a BFR-id,
   because this BFIR BFR-id is carried in the BFIR-id field of the BIER
   header to indicate to the overlay signaling on the receiving BFER
   which BFIR originated the packet.

   In BIER, BFR-id = SI * BSL + BP, such that the SI and BP of a BFER
   can be calculated from the BFR-id and vice versa.  This also means
   that every BFR with a BFR-id has a reserved BP in an SI, even if that
   is not necessary for BIER forwarding, because the BFR may never be a
   BFER but only a BFIR.

   In BIER-TE, for a non-leaf BFER, there is usually a single BP for
   that BFER with a local_decap() adjacency on the BFER.  The BFR-id for
   such a BFER can therefore be determined using the same procedure as
   in (non-TE) BIER: BFR-id = SI * BSL + BP.

   As explained in Section 5.1.3, leaf BFERs do not need such a unique
   local_decap() adjacency.  Likewise, BFIRs that are not also BFERs may
   not have a unique local_decap() adjacency either.  For all those
   BFIRs and (leaf) BFERs, the controller needs to determine unique BFR-
   ids that do not collide with the BFR-ids derived from the non-leaf
   BFER local_decap() BPs.

   While this document defines no requirements on how to allocate such
   BFR-id, a simple option is to derive it from the (SI,BP) of an
   adjacency that is unique to the BFR in question.  For a BFIR this can
   be the first adjacency only populated on this BFIR, for a leaf-BFER,
   this could be the first BP with an adjacency towards that BFER.

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5.3.4.  Mapping from BFR to BitStrings with BIER-TE

   In BIER, applications of the flow overlay on a BFIR can calculate the
   (SI,BP) of a BFER from the BFR-id of the BFER and can therefore
   easily determine the BitStrings for a BIER packet to a set of BFERs
   with known BFR-ids.

   In BIER-TE this mapping needs to be equally supported for flow
   overlays.  This section outlines two core options, based on what type
   of Tree Engineering the BIER-TE controller needs to performs for a
   particular application.

   "Independent branches": For a given flow overlay instance, the
   branches from a BFIR to every BFER are calculated by the BIER-TE
   controller to be independent of the branches to any other BFER.
   Shortest path trees are the most common examples of trees with
   independent branches.

   "Interdependent branches": When a BFER is added or deleted from a
   particular distribution tree, the BIER-TE controller has to
   recalculate the branches to other BFER, because they may need to
   change.  Steiner trees are examples of interdependent branch trees.

   If "independent branches" are used, the BIER-TE Controller can signal
   to the BFIR flow overlay for every BFER an SI:BitString that
   represents the branch to that BFER.  The flow overlay on the BIFR can
   then independently of the controller calculate the SI:BitString for
   all desired BFERs by OR'ing their BitStrings.  This allows for flow
   overlay applications to operate independently of the controller
   whenever it needs to determine which subset of BFERs need to receive
   a particular packet.

   If "interdependent branches" are required, the application would need
   to inquire the SI:BitString for a given set of BFER whenever the set

   Note that in either case (unlike in BIER), the bits may need to
   change upon link/node failure/recovery, network expansion and network
   resource consumption by other traffic as part of traffic engineering
   goals (e.g.: re-optimization of lower priority traffic flows).
   Interactions between such BFIR applications and the BIER-TE
   Controller do therefore need to support dynamic updates to the

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   Communications between the BFIR flow overlay and the BIER-TE
   controller requires some way to identify the BFER.  If BFR-ids are
   used in the deployment, as outlined in Section 5.3.3, then those are
   the natural BFR identifier.  If BFR-ids are not used, then any other
   unique identifier, such as the BFR-prefix of the BFR ([RFC8279])
   could be used.

5.3.5.  Assigning BFR-ids for BIER-TE

   It is not currently determined if a single sub-domain could or should
   be allowed to forward both (non-TE) BIER and BIER-TE packets.  If
   this should be supported, there are two options:

   A.  BIER and BIER-TE have different BFR-id in the same sub-domain.
   This allows higher replication efficiency for BIER because their BFR-
   id can be assigned sequentially, while the BitStrings for BIER-TE
   will have also the additional bits for the topology.  There is no
   relationship between a BFR BIER BFR-id and its BIER-TE BFR-id.

   B.  BIER and BIER-TE share the same BFR-id.  The BFR-ids are assigned
   as explained above for BIER-TE and simply reused for BIER.  The
   replication efficiency for BIER will be as low as that for BIER-TE in
   this approach.

5.3.6.  Example bit allocations  With BIER

   Consider a network setup with a BSL of 256 for a network topology as
   shown in Figure 20.  The network has 6 areas, each with 170 BFERs,
   connecting via a core with 4 (core) BFRs.  To address all BFERs with
   BIER, 4 SIs are required.  To send a BIER packet to all BFER in the
   network, 4 copies need to be sent by the BFIR.  On the BFIR it does
   not make a difference how the BFR-ids are allocated to BFER in the
   network, but for efficiency further down in the network it does make
   a difference.

                area1           area2        area3
               BFR1a BFR1b  BFR2a BFR2b   BFR3a BFR3b
                 |  \         /    \        /  |
                 .                Core          .
                 |    /       \    /        \  |
               BFR4a BFR4b  BFR5a BFR5b   BFR6a BFR6b
                area4          area5        area6

                  Figure 20: Scaling BIER-TE bits by reuse

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   With random allocation of BFR-id to BFER, each receiving area would
   (most likely) have to receive all 4 copies of the BIER packet because
   there would be BFR-id for each of the 4 SIs in each of the areas.
   Only further towards each BFER would this duplication subside - when
   each of the 4 trees runs out of branches.

   If BFR-ids are allocated intelligently, then all the BFER in an area
   would be given BFR-id with as few as possible different SIs.  Each
   area would only have to forward one or two packets instead of 4.

   Given how networks can grow over time, replication efficiency in an
   area will then also go down over time when BFR-ids are only allocated
   sequentially, network wide.  An area that initially only has BFR-id
   in one SI might end up with many SIs over a longer period of growth.
   Allocating SIs to areas with initially sufficiently many spare bits
   for growths can help to alleviate this issue.  Or renumber BFERs
   after network expansion.  In this example one may consider to use 6
   SIs and assign one to each area.

   This example shows that intelligent BFR-id allocation within at least
   sub-domain 0 can even be helpful or even necessary in BIER.  With BIER-TE

   In BIER-TE one needs to determine a subset of the physical topology
   and attached BFERs so that the "desired" representation of this
   topology and the BFER fit into a single BitString.  This process
   needs to be repeated until the whole topology is covered.

   Once bits/SIs are assigned to topology and BFERs, BFR-id is just a
   derived set of identifiers from the operator/BIER-TE Controller as
   explained above.

   Every time that different sub-topologies have overlap, bits need to
   be repeated across the BitStrings, increasing the overall amount of
   bits required across all BitString/SIs.  In the worst case, one
   assigns random subsets of BFERs to different SIs.  This will result
   in an outcome much worse than in (non-TE) BIER: It maximizes the
   amount of unnecessary topology overlap across SI and therefore
   reduces the number of BFER that can be reached across each individual
   SI.  Intelligent BFER to SI assignment and selecting specific
   "desired" subtopologies can minimize this problem.

   To set up BIER-TE efficiently for the topology of Figure 20, the
   following bit allocation method can be used.  This method can easily
   be expanded to other, similarly structured larger topologies.

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   Each area is allocated one or more SIs depending on the number of
   future expected BFERs and number of bits required for the topology in
   the area.  In this example, 6 SIs, one per area.

   In addition, we use 4 bits in each SI: bia, bib, bea, beb: (b)it
   (i)ngress (a), (b)it (i)ngress (b), (b)it (e)gress (a), (b)it
   (e)gress (b).  These bits will be used to pass BIER packets from any
   BFIR via any combination of ingress area a/b BFR and egress area a/b
   BFR into a specific target area.  These bits are then set up with the
   right forward_routed() adjacencies on the BFIR and area edge BFR:

   On all BFIRs in an area j|j=1...6, bia in each BIFT:SI is populated
   with the same forward_routed(BFRja), and bib with
   forward_routed(BFRjb).  On all area edge BFR, bea in
   BIFT:SI=k|k=1...6 is populated with forward_routed(BFRka) and beb in
   BIFT:SI=k with forward_routed(BFRkb).

   For BIER-TE forwarding of a packet to a subset of BFERs across all
   areas, a BFIR would create at most 6 copies, with SI=1...SI=6, In
   each packet, the bits indicate bits for topology and BFER in that
   topology plus the four bits to indicate whether to pass this packet
   via the ingress area a or b border BFR and the egress area a or b
   border BFR, therefore allowing path steering for those two "unicast"
   legs: 1) BFIR to ingress area edge and 2) core to egress area edge.
   Replication only happens inside the egress areas.  For BFER in the
   same area as in the BFIR, these four bits are not used.

5.3.7.  Summary

   BIER-TE can, like BIER, support multiple SIs within a sub-domain.
   This allows to apply the mapping BFR-id = SI * BSL + BP.  This allows
   to re-use the BIER architecture concept of BFR-id and therefore
   minimize BIER-TE specific functions in possible BIER layer control
   plane mechanisms with BIER-TE, including flow overlay methods and
   BIER header fields.

   The number of BFIR/BFER possible in a sub-domain is smaller than in
   BIER because BIER-TE uses additional bits for topology.

   Sub-domains (SDs) in BIER-TE can be used like in BIER to create more
   efficient replication to known subsets of BFERs.

   Assigning bits for BFERs intelligently into the right SI is more
   important in BIER-TE than in BIER because of replication efficiency
   and overall amount of bits required.

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6.  Security Considerations

   If [RFC8296] is used, BIER-TE shares its security considerations.

   BIER-TE shares the security considerations of BIER, [RFC8279], with
   the following overriding or additional considerations.

   BIER-TE forwarding explicitly supports unicast "tunneling" of BIER
   packets via forward_routed() adjacencies.  The BIER domain security
   model is based on a subset of interfaces on a BFR that connect to
   other BFRs of the same BIER domain.  For BIER-TE, this security model
   equally applies to such unicast "tunneled" BIER packets.  This does
   not only include the need to filter received unicast "tunneled" BIER
   packets to prohibit injection of such "tunneled" BIER packets from
   outside the BIER domain, but also prohibiting forward_routed()
   adjacencies to leak BIER packets from the BIER domain.  It SHOULD be
   possible to configure interfaces to be part of a BIER domain solely
   for sending and receiving of unicast "tunneled" BIER packets even if
   the interface can not send/receive BIER encapsulated packets.

   In BIER, the standardized methods for the routing underlays are IGPs
   with extensions to distribute BFR-ids and BFR-prefixes.  [RFC8401]
   specifies the extensions for IS-IS and [RFC8444] specifies the
   extensions for OSPF.  Attacking the protocols for the BIER routing
   underlay or (non-TE) BIER layer control plane, or impairment of any
   BFR in a domain may lead to successful attacks against the results of
   the routing protocol, enabling DoS attacks against paths or the
   addressing (BFR-id, BFR-prefixes) used by BIER.

   The reference model for the BIER-TE layer control plane is a BIER-TE
   controller.  When such a controller is used, impairment of an
   individual BFR in a domain causes no impairment of the BIER-TE
   control plane on other BFRs.  If a routing protocol is used to
   support forward_routed() adjacencies, then this is still an attack
   vector as in BIER, but only for BIER-TE forward_routed() adjacencies,
   and not other adjacencies.

   Whereas IGP routing protocols are most often not well secured through
   cryptographic authentication and confidentiality, communications
   between controllers and routers such as those to be considered for
   the BIER-TE controller/control-plane can be and are much more
   commonly secured with those security properties, for example by using
   Secure SHell (SSH), [RFC4253] for NETCONF ([RFC6242]), or via
   Transport Layer Security (TLS), such as [RFC8253] for PCEP,
   [RFC5440], or [RFC7589] for NETCONF.  BIER-TE controllers SHOULD use
   security equal to or better than these mechanisms.

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   When any of these security mechanisms/protocols are used for
   communications between a BIER-TE controller and BFRs, their security
   considerations apply to BIER-TE.  In addition, the security
   considerations of PCE, [RFC4655] apply.

   The most important attack vector in BIER-TE is misconfiguration,
   either on the BFR themselves or via the BIER-TE controller.
   Forwarding entries with DNC could be set up to create persistent
   loops, in which packets only expire because of TTL.  To minimize the
   impact of such attacks (or more likely unintentional misconfiguration
   by operators and/or bad BIER-TE controller software), the BIER-TE
   forwarding rules are defined to be as strict in clearing bits as
   possible.  The clearing of all bits with an adjacency on a BFR
   prohibits that a looping packet creates additional packet
   amplification through the misconfigured loop on the packet's second
   or further times around the loop, because all relevant adjacency bits
   would have been cleared on the first round through the loop.  In
   result, BIER-TE has the same degree of looping packets as possible
   with unintentional or malicious loops in the routing underlay with
   BIER or even with unicast traffic.

   Deployments where BIER-TE would likely be beneficial may include
   operational models where actual configuration changes from the
   controller are only required during non-production phases of the
   network's life-cycle, such as in embedded networks or in
   manufacturing networks during e.g. plant reworking/repairs.  In these
   type of deployments, configuration changes could be locked out when
   the network is in production state and could only be (re-)enabled
   through reverting the network/installation into non-production state.
   Such security designs would not only allow to provide additional
   layers of protection against configuration attacks, but would
   foremost protect the active production process from such
   configuration attacks.

7.  IANA Considerations

   This document requests no action by IANA.

8.  Acknowledgements

   The authors would like to thank Greg Shepherd, Ijsbrand Wijnands,
   Neale Ranns, Dirk Trossen, Sandy Zheng, Lou Berger, Jeffrey Zhang,
   Carsten Borman and Wolfgang Braun for their reviews and suggestions.

   Special thanks to Xuesong Geng for shepherding the document and for
   IESG review/suggestions by Alvaro Retana (responsible AD/RTG),
   Benjamin Kaduk (SEC), Tommy Pauly (TSV), Zaheduzzaman Sarker (TSV),
   Eric Vyncke (INT), Martin Vigoureux (RTG), Robert Wilton (OPS), Eric

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   Kline (INT), Lars Eggert (GEN), Roman Danyliv (SEC), Ines Robles
   (RTGDIR), Robert Sparks (Gen-ART), Yingzhen Qu (RTGdir), Martin Duke

9.  Change log [RFC Editor: Please remove]



      AD review Alvaro Retana.

      Various textual/editorial nits including adding () to all
      instances of forwarding adjacency name instances.

      3.1 Added new paragraph outlining possible use of BGP as RR in
      BIER-TE controller as core of multicast flow overlay component of

      3.2 added xref's to relevant sections to the listed control plane

      4.1 rewrote paragraphs of 4.1 leading up to Figure 4. to eliminate
      any confusion in how the BIFT work and how it compares to the
      notions in rfc8279, as well as better linking it to the

      Moved SR section into appendix.

      TSV review Martin Duke.

      Text/editorial nits.

      4.4 improved text describing handling of F-BM.

      RTGdir review Yingzhen Qu.

      Various text/editorial nits.

      Added notion that BitStrings represent loop free tree for packet
      to abstract and intro.

      Various text nit and editorial improvements.

      Fixed some BFR-id field -> BFIR-id field mistakes.

      Capitalized NETCONF/RESTCONF/YANG, added RFC references.

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      Improved Figure 16 with explicitly two links into BFR3 and
      explanatory text.

      Gen-ART review Robert Sparks.

      Various textual nits, editorial improvements.

      3.2 Introduced terms "BIER-TE topology control" and "BIER-TE tree
      control" for the two functional components of the control plane.

      3.2.1 - 3.2 change introduces the open RFC-editor issue of
      appropriate xrfs (to be resolved by RFC-editor).

      3.3 Rewrote last paragraph to better describe loop prevention
      through clearing of bits in BitString.

      4.1 Fixed up text/formula describing mapping between bfr-id, SI:BP
      and SI,BSL and BP.  Fix offset bug. Improved description paragraph explaining overlap of
      topology for different SI.

      5.3.7 Improved first summary paragraph.

      7.  Rephrased applicability statement of control plane protocol
      security considerations to BIER-TE security.

      RTGDIR review Ines Robles.

      Fixed up adjacencies in Example 2 and explanation text to be
      explicit about which BFR not only passes, but also receives the

      7. (security considerations).  Added paragraph about
      forward_routed() and prohibiting BIER packet leaking in/out of

      IESG review Roman Danyliv (SEC).

      Several textual/sentence nits/editorials.

      IESG review Lars Eggert (GEN).

      Various good editorial word fixed.

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      Pointer to non-false-positive bloom filter work that looks like it
      happened after our IETF discussions documented in this doc, so
      will not add it to doc, but here is URL for folks interested:

      Did not change "native" to a different word for inclusivity
      because of my worry there is no estavblished single replacement
      word, making reading/searching/understanding more difficult.

      IESG review Martin Vigeureux (RTG).

      Added back reference to RFC8402.  Textual fixes.

      IESG review Eric Kline (INT).

      2.1 Fixed typo in BFR* explanations.

      4.3 Added explanatio about MTU handling.

      IESG review Eric Vyncke (INT).

      Fixed up initial text to introduce various abbreviations.

      2.4 refined wording to "with the _intent_ to easily build common
      forwarding planes...".

      4.2.3 refined text about entropy in ECMP - now taken text from

      IESG review Zaheduzzaman Sarker (TSV).

      5.1.7 Refined text explaining documentation of ECMP algorithm. fixed range of areas/SI over which to build the example
      large network BPs - removed explanation of the large network shown
      to be only used for sources in area 1 (IPTV), because it was a
      stale explanation.

      IESG review Ben Kaduk (round 2):

      4.4 Advanced pseudocode still had one wrong "~".  Root cause seems
      to have been day 0 problem in pseudocode written for -01, "~" was
      inserted in the wrong one of two code lines.  Also enhanced
      textual description and comments in pseudocode, changed variable
      name AdjacentBits to PktAdjacentBits to avoid confusion with

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      5.1.3 Rewrote last two paragraphs explaining the sharing of bit
      positions for lead-BFER hopefully better.  Also detailled how it
      interacts with other optimizations and the type of payload BIER-TE
      packets may carry.

      4.4 (from Carsten Borman) changed spacing in pseudocode to be
      consistent.  Fixed {VRF}, clarified pseudocode object syntax,

      11: IESG review Ben Kaduk, summary:

      One discuss for bug in pseudocode. turned out to be one cahrcter

      Added (non-TE) prefix in places where BIER by itsels had to be
      better disambiguated.

      enhanced text for hub-and-spoke to indicate we're only talking
      about hub to spoke traffic.

      long list ot language fixes/improvement (nits).  Thanks a lot!.

      add suggestion to SHOULD use known confidentiality protocols
      between controller and BFR.

      10: AD review Alvaro Retana, summary:

      Note: rfcdiff shows more changes than actually exist because text
      moved around.


      1.  restructuring: merged all controller sections under common
          controller ops main section, moved unfitting stuff out to
          other parts of doc.  Split Intro section into Overview and
          Intro.  Shortened Abstract, moved text into Overview, added
          sections overview.

      2.  enhanced/rewrote: 2.3 Comparison with -> Relationship to BIER-

      3.  enhanced/rewrote: 3.2 BIER-TE controller -> BIER-TE control
          plane, 3.2.1 BIER-TE controller, for consistency with rfc8279

      4.  additional subsections for Alvaros asks

      5.  added to: 3.3 BIER-TE forwarding plane (consistency with

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      6.  Enhanced description of 4.3/encap considerations to better
          explain how BIER/BIER-TE can run together.

      Notation: Markers (a),(b),... at end of points are references from
      the review discussion with Alvaro to the changes made.


      Throughout text: changed term spelling to rfc8279 - bit positions,
      sub-domain, ... (i).

      Reset changed to clear, also DNR changed to DNC (Do Not Clear)

      Abstract: Shortened.  Removed name explanation note (Tree
      Engineering), (a).

      1.  Introduction -> Overview: Moved important explanation
      paragraph from abstract to Introduction.  Fixed text, (a).

      Added bullet point list explanation of structure of document (e).

      Renamed to Overview because that is now more factually correct.

      1.1.  Fixed bug in example adding bit p15.(l).

      2.  (New - Introduction): Moved section 1.1 - 1.3 (examples,
      comparison with BIER-TE) from Introduction into new "Overview"
      section.  Primarily so that "requirements language" section (at
      end of Introduction) is not in middle of document after all the

      2.1 Removed discussion of encap, moved to 4.2.2 (m).

      2.2 enhanced paragraph suggesting native/overlay topology types,
      also sugest type hybrid (n).

      2.3 Overhauled comparison text BIER/BIER-TE, structured into
      common, different, not-required-by-te, integration-bier-bier-te.
      Changed title to "Relationship" to allow including last point.

      2.4 moved Hardware forwarding comparison section into section 2 to
      allow coalescing of sections into section 5 about the controller
      operations (hardware forwarding was in the middle of it, wrong
      place).  Shortened/improved third paragraph by pointing to BIFT as
      deciding element for selection between BIER/BIER-TE.  Removed
      notion of experimentation (this now targets standard) (g).

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      3.  (Components): Aligned component name and descriptions better
      with RFC8279.  Now describe exactly same three layers.  BIER layer
      constituted from BIER-TE control plane and BIER-TE forwarding
      plane.  BIER-TE controller is now simply component of BIER-TE
      control plane. (b).

      3.1. shortened/improved paragraph explaining use of SI:BP instead
      of also bfr-id as index into BIFT, rewrote paragraph talking about
      reuse of BPs(o).

      3.2. rewrote explanation of BIER-TE control plane in the style of
      RFC8729 Section 4.2 (BIER layer) with numbered points.  Note that
      RFC8729 mixes control and forwarding plane bullet points (this doc
      does not).  Merged text from old sections 2.2.1 and 2.2.3 into
      list. (b).

      3.2.1.  Expanded/improved explanation of BIER-TE Controller (b).  Added subsection for topology discovery and creation
      (d).  Added subsection for engineered BitStrings as key novel
      aspect not found in BIER.  (X).

      3.3.  Added numbered list for components of BIER-TE forwarding
      plane (completing the comparable text from RFC8729 Section 4.2).

      3.4 Alvaro does not mind additional example, fixed bugs.

      3.5 Removed notion about using IGP BIER extensions for BIER-TE,
      such as BIFT address ranges.  After -10 making use of BIFT
      clearer, it now looks to authors as if use of IGP extensions would
      not be beneficial, as long as we do need to use the BIER-TE
      controller, e.g. unlike in BIER, a BFR could not learn from the
      IGP information what traffic to send towards a particular BIFT-ID,
      but instead that is the core of what the controller needs to

      4.2.2 Improved text to explain requirement to identify BIER-TE in
      the tunnel encap and compress description of use-cases (m).

      4.2.3 enhanced ECMP text (p).

      4.3. rewrote most of Encapsulation Considerations to better
      explain to Alvaros question re sharing or not sharing SD via BIER/
      BIER-TE.  Added reference to I-D.ietf-bier-non-mpls-bift-encoding
      as a very helpful example. (f).

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      4.3 Renamed title to "...Co-Existence with BIER" as this is what
      it is about and to help finding it from abstract/intro ("co-
      exist") (j).

      4.4.  Moved BIER-TE Forwarding Pseudocode here to coalesce text
      logically.  Changed text to better compare with BIER pseudo
      forwarding code.  Numerical list of how F-BM works for BIER-TE.
      Removed efficiency comparison with BIER (too difficult to provide
      sufficient justification, derails from focus of section) (j).

      4.6.  (Requirements) Restructured: Removed notion of "basic" BIER-
      TE forwarding, simply referring to it now as "mandatory" BIER-TE
      forwarding.  Cleaned up text to have requirements for different
      adjacencies in different paragraphs. (c).

      5.  Created new main section "BIER-TE Controller operational
      considerations", coalesced old sections 4., 5., 7. into this new
      main section.  No text changes. (k).

      5.1.9 Added new separate picture instead of referring to a picture
      later in text, adjusted text (r).

      5.3.2 Changed title to not include word "comparison" to avoid this
      being accounted against Alvaros concern about scattering
      comparison (IMHO text already has little comparison, so title was
      misleading) (h).

      co-authors internal review:

      4.4 Added xref to Figure 5.

      5.2.1 Duplicated ring picture, added visuals for described
      miswiring (s).

      5.2.2 replace "topology" with graph (wrong word).

      5.3.3 rewrote explanation of how to map BFR-id to SI:BP and assign
      them, clarified BFR-id is option.  Retitled to better explain
      scope of section.

      5.3.4 Removed considerations in 5.3.4 for sharing BFR-id across
      BIER/BIER-TE (t), changed title to explain how BFIR/BIER-TE
      controller interactions need some form of identifying BFR but this
      does not have to be BFR-id.

      7.  Added new security considerations (u).

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      09: Incorporated fixes for feedback from Shepherd (Xuesong Geng).

      Added references for Bloom Filters and Rate Controlled Service

      1.1 Fixed numbering of example 1 topology explanation.  Improved
      language on second example (less abbreviating to avoid confusion
      about meaning).

      1.2 Improved explanation of BIER-TE topology, fixed terminology of
      graphs (BIER-TE topology is a directed graph where the edges are
      the adjacencies).

      2.4 Fixed and amended routing underlay explanations: detailed why
      no need for BFER routing underlay routing protocol extensions, but
      potential to re-use BIER routing underlay routing protocol
      extensions for non-BFER related extensions.

      3.1 Added explanation for VRF and its use in adjacencies.

      08: Incorporated (with hopefully acceptable fixes) for Lou
      suggested section 2.5, TE considerations.

      Fixes are primarily to the point to a) emphasize that BIER-TE does
      not depend on the routing underlay unless forward_routed()
      adjacencies are used, and b) that the allocation and tracking of
      resources does not explicitly have to be tied to BPs, because they
      are just steering labels.  Instead, it would ideally come from
      per-hop resource management that can be maintained only via local
      accounting in the controller.

      07: Further reworking text for Lou.

      Renamed BIER-PE to BIER-TE standing for "Tree Engineering" after
      votes from BIER WG.

      Removed section 1.1 (introduced by version 06) because not
      considered necessary in this doc by Lou (for framework doc).

      Added [RFC editor pls. remove] Section to explain name change to
      future reviewers.

      06: Concern by Lou Berger re.  BIER-TE as full traffic engineering

      Changed title "Traffic Engineering" to "Path Engineering"

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      Added intro section of relationship BIER-PE to traffic

      Changed "traffic engineering" term in text" to "path engineering",
      where appropriate


      Shortened "BIER-TE Controller Host" to "BIER-TE Controller".
      Fixed up all instances of controller to do this.

      05: Review Jeffrey Zhang.

      Part 2:

      4.3 added note about leaf-BFER being also a propery of routing

      4.7 Added missing details from example to avoid confusion with
      routed adjacencies, also compressed explanatory text and better
      justification why seed is explicitly configured by controller.

      4.9 added section discussing generic reuse of BP methods.

      4.10 added section summarizing BP optimizations of section 4.

      6.  Rewrote/compressed explanation of comparison BIER/BIER-TE
      forwarding difference.  Explained benefit of BIER-TE per-BP
      forwarding being independent of forwarding for other BPs.

      Part 1:

      Explicitly ue forwarded_connected adjcency in ECMP adjcency
      examples to avoid confusion.

      4.3 Add picture as example for leav vs. non-leaf BFR in topology.
      Improved description.

      4.5 Exampe for traffic that can be broadcast -> for single BP in

      4.8.1 Simplified example picture for routed adjacency, explanatory

      Review from Dirk Trossen:

      Fixed up explanation of ICC paper vs. bloom filter.

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      04: spell check run.

      Addded remaining fixes for Sandys (Zhang Zheng) review:

      4.7 Enhance ECMP explanations:

      example ECMP algorithm, highlight that doc does not standardize
      ECMP algorithm.

      Review from Dirk Trossen:

      1.  Added mentioning of prior work for traffic engineered paths
      with bloom filters.

      2.  Changed title from layers to components and added "BIER-TE
      control plane" to "BIER-TE Controller" to make it clearer, what it

      2.2.3.  Added reference to I-D.ietf-bier-multicast-http-response
      as an example solution.

      2.3. clarified sentence about resetting BPs before sending copies
      (also forgot to mention DNR here).

      3.4.  Added text saying this section will be removed unless IESG
      review finds enough redeeming value in this example given how -03
      introduced section 1.1 with basic examples.

      7.2.  Removed explicit numbers 20%/80% for number of topology bits
      in BIER-TE, replaced with more vague (high/low) description,
      because we do not have good reference material Added text saying
      this section will be removed unless IESG review finds enough
      redeeming value in this example given how -03 introduced section
      1.1 with basic examples.

      many typos fixed.  Thanks a lot.

      03: Last call textual changes by authors to improve readability:

      removed Wolfgang Braun as co-authors (as requested).

      Improved abstract to be more explanatory.  Removed mentioning of
      FRR (not concluded on so far).

      Added new text into Introduction section because the text was too
      difficult to jump into (too many forward pointers).  This
      primarily consists of examples and the early introduction of the
      BIER-TE Topology concept enabled by these examples.

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      Amended comparison to SR.

      Changed syntax from [VRF] to {VRF} to indicate its optional and to
      make idnits happy.

      Split references into normative / informative, added references.

      02: Refresh after IETF104 discussion: changed intended status back
      to standard.  Reasoning:

      Tighter review of standards document == ensures arch will be
      better prepared for possible adoption by other WGs (e.g.  DetNet)
      or std. bodies.

      Requirement against the degree of existing implementations is self
      defined by the WG.  BIER WG seems to think it is not necessary to
      apply multiple interoperating implementations against an
      architecture level document at this time to make it qualify to go
      to standards track.  Also, the levels of support introduced in -01
      rev. should allow all BIER forwarding engines to also be able to
      support the base level BIER-TE forwarding.

      01: Added note comparing BIER and SR to also hopefully clarify
      BIER-TE vs. BIER comparison re.  SR.

      - added requirements section mandating only most basic BIER-TE
      forwarding features as MUST.

      - reworked comparison with BIER forwarding section to only
      summarize and point to pseudocode section.

      - reworked pseudocode section to have one pseudocode that mirrors
      the BIER forwarding pseudocode to make comparison easier and a
      second pseudocode that shows the complete set of BIER-TE
      forwarding options and simplification/optimization possible vs.
      BIER forwarding.  Removed MyBitsOfInterest (was pure

      - Added captions to pictures.

      - Part of review feedback from Sandy (Zhang Zheng) integrated.

      00: Changed target state to experimental (WG conclusion), updated
      references, mod auth association.

      - Source now on

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      - Please open issues on the github for change/improvement requests
      to the document - in addition to posting them on the list
      (bier@ietf.).  Thanks!.


      06: Added overview of forwarding differences between BIER, BIER-

      05: Author affiliation change only.

      04: Added comparison to Live-Live and BFIR to FRR section

      04: Removed FRR content into the new FRR draft [I-D.eckert-bier-
      te-frr] (Braun).

      - Linked FRR information to new draft in Overview/Introduction

      - Removed BTAFT/FRR from "Changes in the network topology"

      - Linked new draft in "Link/Node Failures and Recovery"

      - Removed FRR from "The BIER-TE Forwarding Layer"

      - Moved FRR section to new draft

      - Moved FRR parts of Pseudocode into new draft

      - Left only non FRR parts

      - removed FrrUpDown(..) and //FRR operations in

      - New draft contains FrrUpDown(..) and ForwardBierTePacket(Packet)
      from bier-arch-03

      - Moved "BIER-TE and existing FRR to new draft

      - Moved "BIER-TE and Segment Routing" section one level up

      - Thus, removed "Further considerations" that only contained this

      - Added Changes for version 04

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      03: Updated the FRR section.  Added examples for FRR key concepts.
      Added BIER-in-BIER tunneling as option for tunnels in backup
      paths.  BIFT structure is expanded and contains an additional
      match field to support full node protection with BIER-TE FRR.

      03: Updated FRR section.  Explanation how BIER-in-BIER
      encapsulation provides P2MP protection for node failures even
      though the routing underlay does not provide P2MP.

      02: Changed the definition of BIFT to be more inline with BIER.
      In revs. up to -01, the idea was that a BIFT has only entries for
      a single BitString, and every SI and sub-domain would be a
      separate BIFT.  In BIER, each BIFT covers all SI.  This is now
      also how we define it in BIER-TE.

      02: Added Section 5.3 to explain the use of SI, sub-domains and
      BFR-id in BIER-TE and to give an example how to efficiently assign
      bits for a large topology requiring multiple SI.

      02: Added further detailed for rings - how to support input from
      all ring nodes.

      01: Fixed BFIR -> BFER for section 4.3.

      01: Added explanation of SI, difference to BIER ECMP,
      consideration for Segment Routing, unicast FRR, considerations for
      encapsulation, explanations of BIER-TE Controller and CLI.

      00: Initial version.

10.  References

10.1.  Normative References

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

   [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,

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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, <>.

10.2.  Informative References

   [Bloom70]  Bloom, B. H., "Space/time trade-offs in hash coding with
              allowable errors", Comm. ACM 13(7):422-6, July 1970,

              Eckert, T., Cauchie, G., Braun, W., and M. Menth,
              "Protection Methods for BIER-TE", Work in Progress,
              Internet-Draft, draft-eckert-bier-te-frr-03, 5 March 2018,

              Trossen, D., Rahman, A., Wang, C., and T. Eckert,
              "Applicability of BIER Multicast Overlay for Adaptive
              Streaming Services", Work in Progress, Internet-Draft,
              draft-ietf-bier-multicast-http-response-06, 10 July 2021,

              Wijnands, I., Mishra, M., Xu, X., and H. Bidgoli, "An
              Optional Encoding of the BIFT-id Field in the non-MPLS
              BIER Encapsulation", Work in Progress, Internet-Draft,
              draft-ietf-bier-non-mpls-bift-encoding-04, 30 May 2021,

              Zhang, Z., Wang, C., Chen, R., Hu, F., Sivakumar, M., and
              H. Chen, "A YANG data model for Tree Engineering for Bit
              Index Explicit Replication (BIER-TE)", Work in Progress,
              Internet-Draft, draft-ietf-bier-te-yang-04, 7 November
              2021, <

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              Bergmann, O., Bormann, C., Gerdes, S., and H. Chen,
              "Constrained-Cast: Source-Routed Multicast for RPL", Work
              in Progress, Internet-Draft, draft-ietf-roll-ccast-01, 30
              October 2017, <

              Farrel, A., "Overview and Principles of Internet Traffic
              Engineering", Work in Progress, Internet-Draft, draft-
              ietf-teas-rfc3272bis-13, 8 November 2021,

   [ICC]      Reed, M. J., Al-Naday, M., Thomos, N., Trossen, D.,
              Petropoulos, G., and S. Spirou, "Stateless multicast
              switching in software defined networks",  IEEE
              International Conference on Communications (ICC), Kuala
              Lumpur, Malaysia, 2016, May 2016,

   [RCSD94]   Zhang, H. and D. Domenico, "Rate-Controlled Service
              Disciplines",  Journal of High-Speed Networks, 1994, May
              1994, <>.

   [RFC4253]  Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
              Transport Layer Protocol", RFC 4253, DOI 10.17487/RFC4253,
              January 2006, <>.

   [RFC4456]  Bates, T., Chen, E., and R. Chandra, "BGP Route
              Reflection: An Alternative to Full Mesh Internal BGP
              (IBGP)", RFC 4456, DOI 10.17487/RFC4456, April 2006,

   [RFC4655]  Farrel, A., Vasseur, J.-P., and J. Ash, "A Path
              Computation Element (PCE)-Based Architecture", RFC 4655,
              DOI 10.17487/RFC4655, August 2006,

   [RFC5440]  Vasseur, JP., Ed. and JL. Le Roux, Ed., "Path Computation
              Element (PCE) Communication Protocol (PCEP)", RFC 5440,
              DOI 10.17487/RFC5440, March 2009,

   [RFC6241]  Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed.,
              and A. Bierman, Ed., "Network Configuration Protocol
              (NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011,

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   [RFC6242]  Wasserman, M., "Using the NETCONF Protocol over Secure
              Shell (SSH)", RFC 6242, DOI 10.17487/RFC6242, June 2011,

   [RFC7589]  Badra, M., Luchuk, A., and J. Schoenwaelder, "Using the
              NETCONF Protocol over Transport Layer Security (TLS) with
              Mutual X.509 Authentication", RFC 7589,
              DOI 10.17487/RFC7589, June 2015,

   [RFC7752]  Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and
              S. Ray, "North-Bound Distribution of Link-State and
              Traffic Engineering (TE) Information Using BGP", RFC 7752,
              DOI 10.17487/RFC7752, March 2016,

   [RFC7950]  Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language",
              RFC 7950, DOI 10.17487/RFC7950, August 2016,

   [RFC7988]  Rosen, E., Ed., Subramanian, K., and Z. Zhang, "Ingress
              Replication Tunnels in Multicast VPN", RFC 7988,
              DOI 10.17487/RFC7988, October 2016,

   [RFC8040]  Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF
              Protocol", RFC 8040, DOI 10.17487/RFC8040, January 2017,

   [RFC8253]  Lopez, D., Gonzalez de Dios, O., Wu, Q., and D. Dhody,
              "PCEPS: Usage of TLS to Provide a Secure Transport for the
              Path Computation Element Communication Protocol (PCEP)",
              RFC 8253, DOI 10.17487/RFC8253, October 2017,

   [RFC8345]  Clemm, A., Medved, J., Varga, R., Bahadur, N.,
              Ananthakrishnan, H., and X. Liu, "A YANG Data Model for
              Network Topologies", RFC 8345, DOI 10.17487/RFC8345, March
              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,

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   [RFC8402]  Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
              Decraene, B., Litkowski, S., and R. Shakir, "Segment
              Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
              July 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, <>.

Appendix A.  BIER-TE and Segment Routing

   SR (xref target="RFC8402"/>) aims to enable lightweight path steering
   via loose source routing.  Compared to its more heavy-weight
   predecessor RSVP-TE, SR does for example not require per-path
   signaling to each of these hops.

   BIER-TE supports the same design philosophy for multicast.  Like in
   SR, it relies on source-routing - via the definition of a BitString.
   Like SR, it only requires to consider the "hops" on which either
   replication has to happen, or across which the traffic should be
   steered (even without replication).  Any other hops can be skipped
   via the use of routed adjacencies.

   BIER-TE bit position (BP) can be understood as the BIER-TE equivalent
   of "forwarding segments" in SR, but they have a different scope than
   SR forwarding segments.  Whereas forwarding segments in SR are global
   or local, BPs in BIER-TE have a scope that is the group of BFR(s)
   that have adjacencies for this BP in their BIFT.  This can be called
   "adjacency" scoped forwarding segments.

   Adjacency scope could be global, but then every BFR would need an
   adjacency for this BP, for example a forward_routed() adjacency with
   encapsulation to the global SR SID of the destination.  Such a BP
   would always result in ingress replication though (as in [RFC7988]).
   The first BFR encountering this BP would directly replicate to it.
   Only by using non-global adjacency scope for BPs can traffic be
   steered and replicated on non-ingress BFR.

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   SR can naturally be combined with BIER-TE and help to optimize it.
   For example, instead of defining bit positions for non-replicating
   hops, it is equally possible to use segment routing encapsulations
   (e.g.  SR-MPLS label stacks) for the encapsulation of
   "forward_routed" adjacencies.

   Note that (non-TE) BIER itself can also be seen to be similar to SR.
   BIER BPs act as global destination Node-SIDs and the BIER BitString
   is simply a highly optimized mechanism to indicate multiple such SIDs
   and let the network take care of effectively replicating the packet
   hop-by-hop to each destination Node-SID.  What BIER does not allow is
   to indicate intermediate hops, or in terms of SR the ability to
   indicate a sequence of SID to reach the destination.  This is what
   BIER-TE and its adjacency scoped BP enables.

Authors' Addresses

   Toerless Eckert (editor)
   Futurewei Technologies Inc.
   2330 Central Expy
   Santa Clara,  95050
   United States of America


   Michael Menth
   University of Tuebingen


   Gregory Cauchie
   Bouygues Telecom


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