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Network Working Group                                          T. Eckert
Internet-Draft                                                    Huawei
Intended status: Standards Track                              G. Cauchie
Expires: December 23, 2017                              Bouygues Telecom
                                                                W. Braun
                                                                M. Menth
                                                 University of Tuebingen
                                                           June 21, 2017


     Traffic Engineering for Bit Index Explicit Replication BIER-TE
                      draft-eckert-bier-te-arch-05

Abstract

   This document proposes an architecture for BIER-TE: Traffic
   Engineering for Bit Index Explicit Replication (BIER).

   BIER-TE shares part of its architecture with BIER as described in
   [I-D.ietf-bier-architecture].  It also proposes to share the packet
   format with BIER.

   BIER-TE forwards and replicates packets like BIER based on a
   BitString in the packet header but it does not require an IGP.  It
   does support traffic engineering by explicit hop-by-hop forwarding
   and loose hop forwarding of packets.  It does support Fast ReRoute
   (FRR) for link and node protection and incremental deployment.
   Because BIER-TE like BIER operates without explicit in-network tree-
   building but also supports traffic engineering, it is more similar to
   SR than RSVP-TE.

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 http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on December 23, 2017.




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Copyright Notice

   Copyright (c) 2017 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
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Requirements Language . . . . . . . . . . . . . . . . . .   4
   2.  Layering  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  The Multicast Flow Overlay  . . . . . . . . . . . . . . .   5
     2.2.  The BIER-TE Controller Host . . . . . . . . . . . . . . .   5
       2.2.1.  Assignment of BitPositions to adjacencies of the
               network topology  . . . . . . . . . . . . . . . . . .   6
       2.2.2.  Changes in the network topology . . . . . . . . . . .   6
       2.2.3.  Set up per-multicast flow BIER-TE state . . . . . . .   6
       2.2.4.  Link/Node Failures and Recovery . . . . . . . . . . .   6
     2.3.  The BIER-TE Forwarding Layer  . . . . . . . . . . . . . .   7
     2.4.  The Routing Underlay  . . . . . . . . . . . . . . . . . .   7
   3.  BIER-TE Forwarding  . . . . . . . . . . . . . . . . . . . . .   7
     3.1.  The Bit Index Forwarding Table (BIFT) . . . . . . . . . .   7
     3.2.  Adjacency Types . . . . . . . . . . . . . . . . . . . . .   8
       3.2.1.  Forward Connected . . . . . . . . . . . . . . . . . .   8
       3.2.2.  Forward Routed  . . . . . . . . . . . . . . . . . . .   9
       3.2.3.  ECMP  . . . . . . . . . . . . . . . . . . . . . . . .   9
       3.2.4.  Local Decap . . . . . . . . . . . . . . . . . . . . .   9
     3.3.  Encapsulation considerations  . . . . . . . . . . . . . .  10
     3.4.  Basic BIER-TE Forwarding Example  . . . . . . . . . . . .  10
   4.  BIER-TE Controller Host BitPosition Assignments . . . . . . .  12
     4.1.  P2P Links . . . . . . . . . . . . . . . . . . . . . . . .  12
     4.2.  BFER  . . . . . . . . . . . . . . . . . . . . . . . . . .  13
     4.3.  Leaf BFERs  . . . . . . . . . . . . . . . . . . . . . . .  13
     4.4.  LANs  . . . . . . . . . . . . . . . . . . . . . . . . . .  13
     4.5.  Hub and Spoke . . . . . . . . . . . . . . . . . . . . . .  14
     4.6.  Rings . . . . . . . . . . . . . . . . . . . . . . . . . .  14
     4.7.  Equal Cost MultiPath (ECMP) . . . . . . . . . . . . . . .  15
     4.8.  Routed adjacencies  . . . . . . . . . . . . . . . . . . .  17



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       4.8.1.  Reducing BitPositions . . . . . . . . . . . . . . . .  17
       4.8.2.  Supporting nodes without BIER-TE  . . . . . . . . . .  17
   5.  Avoiding loops and duplicates . . . . . . . . . . . . . . . .  17
     5.1.  Loops . . . . . . . . . . . . . . . . . . . . . . . . . .  17
     5.2.  Duplicates  . . . . . . . . . . . . . . . . . . . . . . .  18
   6.  BIER-TE Forwarding Pseudocode . . . . . . . . . . . . . . . .  18
   7.  Managing SI, subdomains and BFR-ids . . . . . . . . . . . . .  19
     7.1.  Why SI and sub-domains  . . . . . . . . . . . . . . . . .  20
     7.2.  Bit assignment comparison BIER and BIER-TE  . . . . . . .  21
     7.3.  Using BFR-id with BIER-TE . . . . . . . . . . . . . . . .  21
     7.4.  Assigning BFR-ids for BIER-TE . . . . . . . . . . . . . .  22
     7.5.  Example bit allocations . . . . . . . . . . . . . . . . .  23
       7.5.1.  With BIER . . . . . . . . . . . . . . . . . . . . . .  23
       7.5.2.  With BIER-TE  . . . . . . . . . . . . . . . . . . . .  24
     7.6.  Summary . . . . . . . . . . . . . . . . . . . . . . . . .  25
   8.  BIER-TE and Segment Routing . . . . . . . . . . . . . . . . .  25
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  26
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  26
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  26
   12. Change log [RFC Editor: Please remove]  . . . . . . . . . . .  26
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  27
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  28

1.  Introduction

1.1.  Overview

   This document specifies the architecture for BIER-TE: traffic
   engineering for Bit Index Explicit Replication BIER.

   BIER-TE shares architecture and packet formats with BIER as described
   in [I-D.ietf-bier-architecture].

   BIER-TE forwards and replicates packets like BIER based on a
   BitString in the packet header but it does not require an IGP.  It
   does support traffic engineering by explicit hop-by-hop forwarding
   and loose hop forwarding of packets.  It does support incremental
   deployment and a Fast ReRoute (FRR) extension for link and node
   protection is given in [I-D.eckert-bier-te-frr].  Because BIER-TE
   like BIER operates without explicit in-network tree-building but also
   supports traffic engineering, it is more similar to Segment Routing
   (SR) than RSVP-TE.

   The key differences over BIER are:

   o  BIER-TE replaces in-network autonomous path calculation by
      explicit paths calculated offpath by the BIER-TE controller host.




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   o  In BIER-TE every BitPosition of the BitString of a BIER-TE packet
      indicates one or more adjacencies - instead of a BFER as in BIER.

   o  BIER-TE in each BFR has no routing table but only a BIER-TE
      Forwarding Table (BIFT) indexed by SI:BitPosition and populated
      with only those adjacencies to which the BFR should replicate
      packets to.

   BIER-TE headers use the same format as BIER headers.

   BIER-TE forwarding does not require/use the BFIR-ID.  The BFIR-ID can
   still be useful though for coordinated BFIR/BFER functions, such as
   the context for upstream assigned labels for MPLS payloads in MVPN
   over BIER-TE.

   If the BIER-TE domain is also running BIER, then the BFIR-ID in BIER-
   TE packets can be set to the same BFIR-ID as used with BIER packets.

   If the BIER-TE domain is not running full BIER or does not want to
   reduce the need to allocate bits in BIER bitstrings for BFIR-ID
   values, then the allocation of BFIR-ID values in BIER-TE packets can
   be done through other mechanisms outside the scope of this document,
   as long as this is appropriately agreed upon between all BFIR/BFER.

1.2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

2.  Layering

   End to end BIER-TE operations consists of four components: The
   "Multicast Flow Overlay", the "BIER-TE Controller Host", the "Routing
   Underlay" and the "BIER-TE forwarding layer".
















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      Picture 2: Layers of BIER-TE

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

                   [Bier-TE Controller Host]
                      ^      ^     ^
                     /       |      \   BIER-TE control protocol
                    |        |       |  eg.: Netconf/Restconf/Yang
                    v        v       v
    Src -> Rtr1 -> BFIR-----BFR-----BFER -> Rtr2 -> Rcvr

                   |--------------------->|
                   BIER-TE forwarding layer

                   |<- BIER-TE domain-->|

                  |<--------------------->|
                      Routing underlay

2.1.  The Multicast Flow Overlay

   The Multicast Flow Overlay operates as in BIER.  See
   [I-D.ietf-bier-architecture].  Instead of interacting with the BIER
   layer, it interacts with the BIER-TE Controller Host

2.2.  The BIER-TE Controller Host

   The BIER-TE controller host is representing the control plane of
   BIER-TE.  It communicates two sets of information with BFRs:

   During bring-up or modifications of the network topology, the
   controller discovers the network topology, assigns BitPositions to
   adjacencies and signals the resulting mapping of BitPositions to
   adjacencies to each BFR connecting to the adjacency.

   During day-to-day operations of the network, the controller signals
   to BFIRs what multicast flows are mapped to what BitStrings.

   Communications between the BIER-TE controller host to BFRs is ideally
   via standardized protocols and data-models such as Netconf/Retconf/
   Yang.  This is currently outside the scope of this document.  Vendor-
   specific CLI on the BFRs is also a possible stopgap option (as in
   many other SDN solutions lacking definition of standardized data
   model).





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   For simplicity, the procedures of the BIER-TE controller host are
   described in this document as if it is a single, centralized
   automated entity, such as an SDN controller.  It could equally be an
   operator setting up CLI on the BFRs.  Distribution of the functions
   of the BIER-TE controller host is currently outside the scope of this
   document.

2.2.1.  Assignment of BitPositions to adjacencies of the network
        topology

   The BIER-TE controller host tracks the BFR topology of the BIER-TE
   domain.  It determines what adjacencies require BitPositions so that
   BIER-TE explicit paths can be built through them as desired by
   operator policy.

   The controller then pushes the BitPositions/adjacencies to the BIFT
   of the BFRs, populating only those SI:BitPositions to the BIFT of
   each BFR to which that BFR should be able to send packets to -
   adjacencies connecting to this BFR.

2.2.2.  Changes in the network topology

   If the network topology changes (not failure based) so that
   adjacencies that are assigned to BitPositions are no longer needed,
   the controller can re-use those BitPositions for new adjacencies.
   First, these BitPositions 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.

2.2.3.  Set up per-multicast flow BIER-TE state

   The BIER-TE controller host tracks the multicast flow overlay to
   determine what multicast flow needs to be sent by a BFIR to which set
   of BFER.  It calculates the desired distribution tree across the
   BIER-TE domain based on algorithms outside the scope of this document
   (eg.: CSFP, Steiner Tree,...).  It then pushes the calculated
   BitString into the BFIR.

2.2.4.  Link/Node Failures and Recovery

   When link or nodes fail or recover in the topology, BIER-TE can
   quickly respond with the optional FRR procedures described in [I-
   D.eckert-bier-te-frr].  It can also more slowly react by
   recalculating the BitStrings of affected multicast flows.  This
   reaction is slower than the FRR procedure because the controller
   needs to receive link/node up/down indications, recalculate the
   desired BitStrings and push them down into the BFIRs.  With FRR, this




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   is all performed locally on a BFR receiving the adjacency up/down
   notification.

2.3.  The BIER-TE Forwarding Layer

   When the BIER-TE Forwarding Layer receives a packet, it simply looks
   up the BitPositions that are set in the BitString of the packet in
   the Bit Index Forwarding Table (BIFT) that was populated by the BIER-
   TE controller host.  For every BP that is set in the BitString, and
   that has one or more adjacencies in the BIFT, a copy is made
   according to the type of adjacencies for that BP in the BIFT.  Before
   sending any copy, the BFR resets all BitPositions in the BitString of
   the packet to which it can create a copy.  This is done to inhibit
   that packets can loop.

2.4.  The Routing Underlay

   BIER-TE is sending BIER packets to directly connected BIER-TE
   neighbors as L2 (unicasted) BIER packets without requiring a routing
   underlay.  BIER-TE forwarding uses the Routing underlay for
   forward_routed adjacencies which copy BIER-TE packets to not-
   directly-connected BFRs (see below for adjacency definitions).

   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, eg.: from BFD.
   Providing these notifications is considered to be part of the routing
   underlay in this document.

3.  BIER-TE Forwarding

3.1.  The Bit Index Forwarding Table (BIFT)

   The Bit Index Forwarding Table (BIFT) exists in every BFR.  For every
   subdomain in use, it is a table indexed by SI:BitPosition and is
   populated by the BIER-TE control plane.  Each index can be empty or
   contain a list of one or more adjacencies.

   BIER-TE can support multiple subdomains like BIER.  Each one with a
   separate BIFT

   In the BIER architecture, indices into the BIFT are explained to be
   both BFR-id and SI:BitString (BitPosition).  This is because there is
   a 1:1 relationship between BFR-id and SI:BitString - every bit in
   every SI is/can be assigned to a BFIR/BFER.  In BIER-TE there are
   more bits used in each BitString than there are BFIR/BFER assigned to
   the bitstring.  This is because of the bits required to express the
   (traffic engineered) path through the topology.  The BIER-TE



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   forwarding definitions do therefore not use the term BFR-id at all.
   Instead, BFR-ids are only used as required by routing underlay, flow
   overlay of BIER headers.  Please refer to Section 7 for explanations
   how to deal with SI, subdomains and BFR-id in BIER-TE.

     ------------------------------------------------------------------
     | Index:          |  Adjacencies:                                |
     | SI:BitPosition  |  <empty> or one or more per entry            |
     ==================================================================
     | 0:1             |  forward_connected(interface,neighbor,DNR)   |
     ------------------------------------------------------------------
     | 0:2             |  forward_connected(interface,neighbor,DNR)   |
     |                 |  forward_connected(interface,neighbor,DNR)   |
     ------------------------------------------------------------------
     | 0:3             |  local_decap([VRF])                          |
     ------------------------------------------------------------------
     | 0:4             |  forward_routed([VRF,]l3-neighbor)           |
     ------------------------------------------------------------------
     | 0:5             |  <empty>                                     |
     ------------------------------------------------------------------
     | 0:6             |  ECMP({adjacency1,...adjacencyN}, seed)      |
     ------------------------------------------------------------------
     ...
     | BitStringLength |  ...                                         |
     ------------------------------------------------------------------
                      Bit Index Forwarding Table


   The BIFT is programmed into the data plane of BFRs by the BIER-TE
   controller host and used to forward packets, according to the rules
   specified in the BIER-TE Forwarding Procedures.

   Adjacencies for the same BP when populated in more than one BFR by
   the controller do not have to have the same adjacencies.  This is up
   to the controller.  BPs for p2p links are one case (see below).

3.2.  Adjacency Types

3.2.1.  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 "DoNotReset" (DNR) set in the BIFT
   will not have the BitPosition for that adjacency reset when the BFR
   creates a copy for it.  The BitPosition will still be reset for



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   copies of the packet made towards other adjacencies.  The can be used
   for example in ring topologies as explained below.

3.2.2.  Forward Routed

   A "forward_routed" adjacency is an adjacency towards a BFR that is
   not a forward_connected adjacency: towards a loopback address of a
   BFR or towards an interface address that is non-directly connected.
   Forward_routed packets are forwarded via the Routing Underlay.

   If the Routing Underlay has multiple paths for a forward_routed
   adjacency, it will perform ECMP independent of BIER-TE for packets
   forwarded across a forward_routed adjacency.

   If the Routing Underlay has FRR, it will perform FRR independent of
   BIER-TE for packets forwarded across a forward_routed adjacency.

3.2.3.  ECMP

   The ECMP mechanisms in BIER are tied to the BIER BIFT and are are
   therefore not directly useable with BIER-TE.  The following
   procedures describe ECMP for BIER-TE that we consider to be
   lightweight but also well manageable.  It leverages the existing
   entropy parameter in the BIER header to keep packets of the flows on
   the same path and it introduces a "seed" parameter to allow
   engineering traffic to be polarized or randomized across multiple
   hops.

   An "Equal Cost Multipath" (ECMP) adjacency has a list of two or more
   adjacencies included in it.  It copies the BIER-TE to one of those
   adjacencies based on the ECMP hash calculation.  The BIER-TE ECMP
   hash algorithm must select the same adjacency from that list for all
   packets with the same "entropy" value in the BIER-TE header if the
   same number of adjacencies and same seed are given as parameters.
   Further use of the seed parameter is explained below.

3.2.4.  Local Decap

   A "local_decap" adjacency passes a copy of the payload of the BIER-TE
   packet to the packets NextProto within the BFR (IPv4/IPv6,
   Ethernet,...).  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.







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3.3.  Encapsulation considerations

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

   Because a BFR needs to interpret the BitString of a BIER-TE packet
   differently from a BIER packet, it is necessary to distinguish BIER
   from BIER-TE packets.  This is subject to definitions in BIER
   encapsulation specifications.

   MPLS encapsulation [I-D.ietf-bier-mpls-encapsulation] for example
   assigns one label by which BFRs recognizes BIER packets for every
   (SI,subdomain) combination.  If it is desirable that every subdomain
   can forward only BIER or BIER-TE packets, then the label allocation
   could stay the same, and only the forwarding model (BIER/BIER-TE)
   would have to be defined per subdomain.  If it is desirable to
   support both BIER and BIER-TE forwarding in the same subdomain, then
   additional labels would need to be assigned for BIER-TE forwarding.

   "forward_routed" requires an encapsulation permitting to unicast
   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 (SI,subdomain) - and if necessary (see above) BIER-TE.
   With non-MPLS encapsulation, some form of IP tunneling (IP in IP,
   LISP, GRE) would be required.

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

3.4.  Basic BIER-TE Forwarding Example

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














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     Picture 1: Forwarding Example

               [Bier-Te Controller Host]
                       /   | \
                      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

   pXX indicate the BitPositions number assigned by the BIER-TE
   controller host to adjacencies in the BIER-TE topology.  For example,
   p9 is the adjacency towards BFR9 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)

   ...and so on.

   Traffic needs to flow from BFIR2 towards Rcv1, Rcv2.  The controller
   determines it wants it to pass across the following paths:



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

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

   This BitString is set up in BFIR2.  Multicast packets arriving at
   BFIR2 from Src are assigned this BitString.

   BFIR2 forwards based on that BitString.  It has p2 and p13 populated.
   Only p13 is in BitString which has an adjacency towards BFR3.  BFIR2
   resets p2 in BitString and sends a copy towards BFR2.

   BFR3 sees a BitString of p5,p7,p8,p10,p11,p12.  It is only interested
   in p1,p7,p8.  It creates a copy of the packet to BFER1 (due to p7)
   and one to BFR4 (due to p8).  It resets p7, p8 before sending.

   BFER1 sees a BitString of p5,p10,p11,p12.  It is only interested in
   p6,p7,p8,p11 and therefore considers only p11. p11 is a "local_decap"
   adjacency installed by the BIER-TE controller host because BFER1
   should pass packets to IP multicast.  The local_decap adjacency
   instructs BFER1 to create a copy, decapsulate it from the BIER header
   and pass it on to the NextProtocol, in this example 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.  BIER-TE Controller Host BitPosition Assignments

   This section describes how the BIER-TE controller host can use the
   different BIER-TE adjacency types to define the BitPositions of a
   BIER-TE domain.

   Because the size of the BitString is limiting the size of the BIER-TE
   domain, many of the options described exist to support larger
   topologies with fewer BitPositions (4.1, 4.3, 4.4, 4.5, 4.6, 4.7,
   4.8).

4.1.  P2P Links

   Each P2p link in the BIER-TE domain is assigned one unique
   BitPosition with a forward_connected adjacency pointing to the
   neighbor on the p2p link.






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4.2.  BFER

   Every BFER is given a unique BitPosition with a local_decap
   adjacency.

4.3.  Leaf BFERs

   Leaf BFERs are BFERs 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 PEs are spokes
   connected to P routers, those PEs are Leaf BFIRs unless there is a
   U-turn between two PEs.

   All leaf-BFER in a BIER-TE domain can share a single BitPosition.
   This is possible because the BitPosition for the adjacency to reach
   the BFER can be used to distinguish whether or not packets should
   reach the BFER.

   This optimization will not work if an upstream interface of the BFER
   is using a BitPosition optimized as described in the following two
   sections (LAN, Hub and Spoke).

4.4.  LANs

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

            BFR1
             |p1
      LAN1-+-+---+-----+
          p3|  p4|   p2|
          BFR3 BFR4  BFR7

   If Bandwidth on the LAN is not an issue and most BIER-TE traffic
   should be copied to all neighbors on a LAN, then BitPositions can be
   saved by assigning just a single BitPosition to the LAN and
   populating the BitPosition 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 face of BFRs redundantly
   connected to more than one LANs with this optimization because these
   BFRs would receive duplicates and forward those duplicates into the
   opposite LANs.  Adjacencies of such BFRs into their LANs still need a
   separate BitPosition.




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4.5.  Hub and Spoke

   In a setup with a hub and multiple spokes connected via separate p2p
   links to the hub, all p2p links can share the same BitPosition.  The
   BitPosition on the hubs BIFT is set up with a list of
   forward_connected adjacencies, one for each Spoke.

   This option is similar to the BitPosition optimization in LANs:
   Redundantly connected spokes need their own BitPositions.

4.6.  Rings

   In L3 rings, instead of assigning a single BitPosition for every p2p
   link in the ring, it is possible to save BitPositions by setting the
   "Do Not Reset" (DNR) flag on forward_connected adjacencies.

   For the rings shown in the following picture, a single BitPosition
   will suffice to forward traffic entering the ring at BFRa or BFRb all
   the way up to BFR1:

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

   Handling DNR this way ensures that copies forwarded from any BFR in
   the ring to a BFR outside the ring will not have the ring BitPosition
   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

   Note that this example only permits for packets to enter the ring 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 BitPositions.  One for clockwise, one for
   counterclockwise.

   Both would be set up to stop rotating on the same link, eg: L1.  When
   the ingress ring BFR creates the clockwise copy, it will reset the
   counterclockwise BitPosition because the DNR bit only applies to the



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   bit for which the replication is done.  Likewise for the clockwise
   BitPosition for the counterclockwise copy.  In result, the ring
   ingress BFR will send a copy in both directions, serving BFRs on
   either side of the ring up to L1.

4.7.  Equal Cost MultiPath (ECMP)

   The ECMP adjacency allows to use just one BP per link bundle between
   two BFRs instead of one BP for each p2p member link of that link
   bundle.  In the following picture, one BP is used across L1,L2,L3 and
   BFR1/BFR2 have for the BP

                --L1-----
           BFR1 --L2----- BFR2
                --L3-----

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

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

   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 mean as a likely setup, but to illustrate that ECMP
   can be used to share BPs not only across link bundles, and it
   explains the use of the seed parameter.
















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

     BIFT entry in BFR1:
     ------------------------------------------------------------------
     | 0:6   |  ECMP({L11-to-BFR2,L12-to-BFR3}, seed)                 |
     ------------------------------------------------------------------

     BIFT entry in BFR2:
     ------------------------------------------------------------------
     | 0:6   |  ECMP({L21-to-BFR4,L22-to-BFR5}, seed)                 |
     ------------------------------------------------------------------

     BIFT entry in BFR3:
     ------------------------------------------------------------------
     | 0:6   |  ECMP({L31-to-BFR6,L32-to-BFR7}, seed)                 |
     ------------------------------------------------------------------

   With the setup of ECMP in above topology, 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 a list of 2 adjacencies: link
   L11-to-BFR2.  When forwarding in BFR2 performs again an ECMP with two
   adjacencies on that subset of traffic, then it will again select the
   first of its two adjacencies to it: L21-to-BFR4.  And therefore L22
   and BFR5 sees no traffic.

   To resolve this issue, the ECMP adjacency on BFR1 simply needs to be
   set up with a different seed than the ECMP adjacencies on BFR2/BFR3

   This issue is called polarization.  It depends on the ECMP hash.  It
   is possible to build ECMP that does not have polarization, for
   example by taking entropy from the actual adjacency members into
   account, but that can make it harder to achieve evenly balanced load-
   splitting on all BFR without making the ECMP hash algorithm
   potentially too complex for fast forwarding in the BFRs.




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4.8.  Routed adjacencies

4.8.1.  Reducing BitPositions

   Routed adjacencies can reduce the number of BitPositions required
   when the traffic engineering requirement is not hop-by-hop explicit
   path selection, but loose-hop selection.

              ...............             ...............
       BFR1--... Redundant ...--L1-- BFR2... Redundant ...---
          \--... Network   ...--L2--/    ... Network   ...---
       BFR4--... Segment 1 ...--L3-- BFR3... Segment 2 ...---
              ...............             ...............

   Assume the requirement in above network is to explicitly engineer
   paths such that specific traffic flows are passed from segment 1 to
   segment 2 via link L1 (or via L2 or via L3).

   To achieve this, BFR1 and BFR4 are set up with a forward_routed
   adjacency BitPosition towards an address of BFR2 on link L1 (or link
   L2 BFR3 via L3).

   For paths to be engineered through a specific node BFR2 (or BFR3),
   BFR1 and BFR4 are set up up with a forward_routed adjacency
   BitPosition towards a loopback address of BFR2 (or BFR3).

4.8.2.  Supporting nodes without BIER-TE

   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.  Avoiding loops and duplicates

5.1.  Loops

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

   With DNR set, looping can happen.  Consider in the ring picture that
   link L4 from BFR3 is plugged into the L1 interface of BFRa.  This
   creates a loop where the rings clockwise BitPosition is never reset
   for copies of the packets traveling clockwise around the ring.




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

5.2.  Duplicates

   Duplicates happen when the topology of the BitString is not a tree
   but redundantly connecting BFRs with each other.  The controller must
   therefore ensure to only create BitStrings that are trees in the
   topology.

   When links are incorrectly physically re-connected before the
   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
   controller.

6.  BIER-TE Forwarding Pseudocode

   The following sections of Pseudocode are meant to illustrate the
   BIER-TE forwarding plane.  This code is not meant to be normative but
   to serve both as a potentially easier to read and more precise
   representation of the forwarding functionality and to illustrate how
   simple BIER-TE forwarding is and that it can be efficiently be
   implemented.

   The following procedure is executed on a BFR whenever the BIFT is
   changed by the BIER-TE controller host:

      global MyBitsOfInterest

      void BIFTChanged()
      {
          for (Index = 0; Index++ ; Index <= BitStringLength)
              if(BIFT[Index] != <empty>)
                  MyBitsOfInterest != 2<<(Index-1)
      }

   The following procedure is executed whenever a BIER-TE packet is to
   be forwarded:





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      void ForwardBierTePacket (Packet)
      {
          // We calculate in BitMask the subset of BPs of the BitString
          // for which we have adjacencies. This is purely an
          // optimization to avoid to replicate for every BP
          // set in BitString only to discover that for most of them,
          // the BIFT has no adjacency.

          local BitMask = Packet->BitString
          Packet->BitString &= ~MyBitsOfInterest
          BitMask &= MyBitsOfInterest

          // Replication
          for (Index = GetFirstBitPosition(BitMask); Index ;
               Index = GetNextBitPosition(BitMask, Index))
              foreach adjacency BIFT[Index]

                  if(adjacency == ECMP(ListOfAdjacencies, seed) )
                      I = ECMP_hash(sizeof(ListOfAdjacencies),
                                    Packet->Entropy, seed)
                      adjacency = ListOfAdjacencies[I]

                  PacketCopy = Copy(Packet)

                  switch(adjacency)
                      case forward_connected(interface,neighbor,DNR):
                          if(DNR)
                              PacketCopy->BitString |= 2<<(Index-1)
                          SendToL2Unicast(PacketCopy,interface,neighbor)

                      case forward_routed([VRF],neighbor):
                          SendToL3(PacketCopy,[VRF,]l3-neighbor)

                      case local_decap([VRF],neighbor):
                          DecapBierHeader(PacketCopy)
                          PassTo(PacketCopy,[VRF,]Packet->NextProto)
      }

7.  Managing SI, subdomains and BFR-ids

   When the number of bits required to represent the necessary hops in
   the topology and BFER exceeds the supported bitstring length,
   multiple SI and/or subdomains must be used.  This section discusses
   how.

   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-id can be assigned to BFIR/BFER for BIER-TE.



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7.1.  Why SI and sub-domains

   For BIER and BIER-TE forwarding, the most important result of using
   multiple SI and/or subdomains is the same: Packets that need to be
   sent to BFER in different SI or subdomains require different BIER
   packets: each one with a bitstring for a different (SI,subdomain)
   bitstring.  Each such bitstring uses one bitstring length sized SI
   block in the BIFT of the subdomain.  We call this a BIFT:SI (block).

   For BIER and BIER-TE forwarding itself there is also no difference
   whether different SI and/or sub-domains are chosen, but SI and
   subdomain 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 subdomain 0 (unless there are > 64k BFIR/BFER).

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

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

   Even if creation of subdomains and assignment of BFR-id to BFIR/BFER
   in those subdomains is automated, it is not expected that individual
   service instances can deal with BFER in different subdomains.  A
   service instance may only support configuration of a single subdomain
   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 subdomain
   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 SI as necessary (see below).  Different services may
   use different subdomains that primarily exist to provide more
   efficient replication (and for BIER-TE desirable traffic engineering)
   for different subsets of BFIR/BFER.





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7.2.  Bit assignment comparison BIER and BIER-TE

   In BIER, bitstrings only need to carry bits for BFER, which lead 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 traffic
   engineering across every single hop (which requires more bits), or
   reducing the number of required bits by exploiting optimizations such
   as unicast (forward_route), ECMP or flood (DNR) over "uninteresting"
   sub-parts of the topology - eg: parts where different trees do not
   need to take different paths due to traffic-engineering reasons.

   The total number of bits to describe the topology in a BIFT:SI can
   therefore easily be as low as 20% or as high as 80%. The higher the
   percentage, the higher the likelihood, that those topology bits are
   not just BIER-TE overhead without additional benefit, but instead
   they will allow to express the desired traffic-engineering
   alternatives.

7.3.  Using BFR-id with BIER-TE

   Because there is no 1:1 mapping between bits in the bitstring and
   BFER, BIER-TE can not simply rely on the BIER 1:1 mapping between
   bits in a bitstring and BFR-id.

   In BIER, automatic schemes could assign all possible BFR-ids
   sequentially to BFERs.  This will not work in BIER-TE.  In BIER-TE,
   the operator or BIER-TE controller host has to determine a BFR-id for
   each BFER in each required subdomain.  The BFR-id may or may not have
   a relationship with a bit in the bitstring.  Suggestions are detailed
   below.  Once determined, the BFR-id can then be configured on the
   BFER and used by flow overlay, routing underlay and the BIER header
   almost the same as the BFR-id in BIER.

   The one exception are application/flow-overlays that automatically
   calculate the bitstring(s) of BIER packets by converting BFR-id to
   bits.  In BIER-TE, this operation can be done in two ways:

   "Independent branches": For a given application or (set of) trees,
   the branches from a BFIR to every BFER are independent of the




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   branches to any other BFER.  For example, shortest part trees have
   independent branches.

   "Interdependent branches": When a BFER is added or deleted from a
   particular distribution tree, branches to other BFER still in the
   tree may need to change.  Steiner tree are examples of dependent
   branch trees.

   If "independent branches" are sufficient, the BIER-TE controller host
   can provide to such applications for every BFR-id a SI:bitstring with
   the BIER-TE bits for the branch towards that BFER.  The application
   can then independently calculate the SI:bitstring for all desired
   BFER by OR'ing their bitstrings.

   If "interdependent branches" are required, the application could call
   a BIER-TE controller host API with the list of required BFER-id and
   get the required bitstring back.  Whenever the set of BFER-id
   changes, this is repeated.

   Note that in either case (unlike in BIER), the bits in BIER-TE may
   need to change upon link/node failure/recovery, network expansion and
   network load by other traffic (as part of traffic engineering goals).
   Interactions between such BFIR applications and the BIER-TE
   controller host do therefore need to support dynamic updates to the
   bitstrings.

7.4.  Assigning BFR-ids for BIER-TE

   For non-leaf BFER, there is usually a single bit k for that BFER with
   a local_decap() adjacency on the BFER.  The BFR-id for such a BFER is
   therefore most easily the one it would have in BIER: SI * bitstring-
   length + k.

   As explained earlier in the document, leaf BFER do not need such a
   separate bit because the fact alone that the BIER-TE packet is
   forwarded to the leaf BFER indicates that the BFER should decapsulate
   it.  Such a BFER will have one or more bits for the links leading
   only to it.  The BFR-id could therefore most easily be the BFR-id
   derived from the lowest bit for those links.

   These two rules are only recommendations for the operator or BIER-TE
   controller assigning the BFR-ids.  Any allocation scheme can be used,
   the BFR-ids just need to be unique across BFRs in each subdomain.

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




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   A.  BIER and BIER-TE have different BFR-id in the same subdomain.
   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 BIER-TE BFR-id.

   B.  BIER and BIER-TE share the same BFR-id.  The BFR-id 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.  Depending on topology, only the same 20%..80% of bits
   as possible for BIER-TE can be used for BIER.

7.5.  Example bit allocations

7.5.1.  With BIER

   Consider a network setup with a bitstring length of 256 for a network
   topology as shown in the picture below.  The network has 6 areas,
   each with ca. 180 BFR, connecting via a core with some larger (core)
   BFR.  To address all BFER with BIER, 4 SI 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-id
   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

   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 SI 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-id are allocated intelligently, then all the BFER in an area
   would be given BFR-id with as few as possible different SI.  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 also easily go down over time when BFR-id are network wide
   allocated sequentially over time.  An area that initially only has



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   BFR-id in one SI might end up with many SI 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
   BFR-id after network expansion.  In this example one may consider to
   use 6 SI and assign one to each area.

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

7.5.2.  With BIER-TE

   In BIER-TE one needs to determine a subset of the physical topology
   and attached BFER 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 BFER, 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, random
   subsets of BFER are assigned to different SI.  This is much worse
   than in BIER because it not only reduces replication efficiency with
   the same number of overall bits, but even further - because more bits
   are required due to duplication of bits for topology across multiple
   SI.  Intelligent BFER to SI assignment and selecting specific
   "desired" subtopologies can minimize this problem.

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

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

   In addition, we use 4 bits in each SI: bia, bib, bea, beb: bit
   ingress a, bit ingress b, bit egress a, bit egress 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 BFIR in an area j, 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 is populated with



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   forward_routed(BFRka) and beb in BIFT:SI=k with
   forward_routed(BFRkb).

   For BIER-TE forwarding of a packet to some subset of BFER 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 engineering for those two
   "unicast" legs: 1) BFIR to ingress are 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.

7.6.  Summary

   BIER-TE can like BIER support multiple SI within a sub-domain to
   allow re-using the concept of BFR-id and therefore minimize BIER-TE
   specific functions in underlay routing, flow overlay methods and BIER
   headers.

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

   Subdomains can in BIER-TE be used like in BIER to create more
   efficient replication to known subsets of BFER.

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

8.  BIER-TE and Segment Routing

   Segment Routing aims to achieve lightweight path engineering via
   loose source routing.  Compared for example to RSVP-TE, it does not
   require per-path signaling to each of these hops.

   BIER-TE is 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.

   Instead of defining BitPositions for non-replicating hops, it is
   equally possible to use segment routing encapsulations (eg: MPLS
   label stacks) for "forward_routed" adjacencies.





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9.  Security Considerations

   The security considerations are the same as for BIER with the
   following differences:

   BFR-ids and BFR-prefixes are not used in BIER-TE, nor are procedures
   for their distribution, so these are not attack vectors against BIER-
   TE.

10.  IANA Considerations

   This document requests no action by IANA.

11.  Acknowledgements

   The authors would like to thank Greg Shepherd, Ijsbrand Wijnands and
   Neale Ranns for their extensive review and suggestions.

12.  Change log [RFC Editor: Please remove]

      04: Added comparison to Live-Live and BFIR to FRR section
      (Eckert).

      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
      ForwardBierTePacket(..)

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

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




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

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

      - Added Changes for version 04



      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 subdomain 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 7 to explain the use of SI, subdomains 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 host and CLI.

      00: Initial version.

13.  References

   [I-D.ietf-bier-architecture]
              Wijnands, I., Rosen, E., Dolganow, A., Przygienda, T., and
              S. Aldrin, "Multicast using Bit Index Explicit
              Replication", draft-ietf-bier-architecture-06 (work in
              progress), April 2017.





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   [I-D.ietf-bier-mpls-encapsulation]
              Wijnands, I., Rosen, E., Dolganow, A., Tantsura, J.,
              Aldrin, S., and I. Meilik, "Encapsulation for Bit Index
              Explicit Replication in MPLS and non-MPLS Networks",
              draft-ietf-bier-mpls-encapsulation-07 (work in progress),
              June 2017.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

Authors' Addresses

   Toerless Eckert
   Futurewei Technologies Inc.
   2330 Central Expy
   Santa Clara  95050
   USA

   Email: tte+ietf@cs.fau.de


   Gregory Cauchie
   Bouygues Telecom

   Email: GCAUCHIE@bouyguestelecom.fr


   Wolfgang Braun
   University of Tuebingen

   Email: wolfgang.braun@uni-tuebingen.de


   Michael Menth
   University of Tuebingen

   Email: menth@uni-tuebingen.de












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