Network Working Group                                     T. Eckert, Ed.
Internet-Draft                                                    Huawei
Intended status: Standards Track                              G. Cauchie
Expires: January 9, 2020                                Bouygues Telecom
                                                                M. Menth
                                                 University of Tuebingen
                                                            July 8, 2019

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


   This memo introduces per-packet stateless strict and loose path
   engineered replication and forwarding for Bit Index Explicit
   Replication packets ([RFC8279]).  This is called BIER-TE.

   BIER-TE leverages the BIER architecture ([RFC8279]) and extends it
   with a new semantic for bits in the bitstring.  BIER-TE can leverage
   BIER forwarding engines with little or no changes.

   In BIER, the BitPositions (BP) of the packets bitstring indicate BIER
   Forwarding Egress Routers (BFER), and hop-by-hop forwarding uses a
   Routing Underlay such as an IGP.

   In BIER-TE, BitPositions indicate adjacencies.  The BIFT of each BFR
   are only populated with BPs that are adjacent to the BFR in the BIER-
   TE topology.  The BIER-TE topology can consist of layer 2 or remote
   (route) adjacencies.  The BFR then replicates and forwards BIER
   packets to those adjacencies.  This results in the aforementioned
   strict and loose path forwarding.

   BIER-TE can co-exist with BIER forwarding in the same domain, for
   example by using seperate sub-domains.  In the absence of routed
   adjacencies, BIER-TE does not require a BIER routing underlay, and
   can then be operated without requiring an IGP routing protocol.

   BIER-TE operates without explicit in-network tree-building and
   carries the multicast distribution tree in the packet header.  It can
   therefore 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.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Basic Examples  . . . . . . . . . . . . . . . . . . . . .   4
     1.2.  BIER-TE Topology and adjacencies  . . . . . . . . . . . .   7
     1.3.  Comparison with BIER  . . . . . . . . . . . . . . . . . .   8
     1.4.  Requirements Language . . . . . . . . . . . . . . . . . .   8
   2.  Layering  . . . . . . . . . . . . . . . . . . . . . . . . . .   8
     2.1.  The Multicast Flow Overlay  . . . . . . . . . . . . . . .   9
     2.2.  The BIER-TE Controller Host . . . . . . . . . . . . . . .   9
       2.2.1.  Assignment of BitPositions to adjacencies of the
               network topology  . . . . . . . . . . . . . . . . . .  10
       2.2.2.  Changes in the network topology . . . . . . . . . . .  10
       2.2.3.  Set up per-multicast flow BIER-TE state . . . . . . .  10
       2.2.4.  Link/Node Failures and Recovery . . . . . . . . . . .  10
     2.3.  The BIER-TE Forwarding Layer  . . . . . . . . . . . . . .  11
     2.4.  The Routing Underlay  . . . . . . . . . . . . . . . . . .  11
   3.  BIER-TE Forwarding  . . . . . . . . . . . . . . . . . . . . .  11
     3.1.  The Bit Index Forwarding Table (BIFT) . . . . . . . . . .  11
     3.2.  Adjacency Types . . . . . . . . . . . . . . . . . . . . .  12
       3.2.1.  Forward Connected . . . . . . . . . . . . . . . . . .  12

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       3.2.2.  Forward Routed  . . . . . . . . . . . . . . . . . . .  13
       3.2.3.  ECMP  . . . . . . . . . . . . . . . . . . . . . . . .  13
       3.2.4.  Local Decap . . . . . . . . . . . . . . . . . . . . .  13
     3.3.  Encapsulation considerations  . . . . . . . . . . . . . .  14
     3.4.  Basic BIER-TE Forwarding Example  . . . . . . . . . . . .  14
     3.5.  Forwarding comparison with BIER . . . . . . . . . . . . .  16
     3.6.  Requirements  . . . . . . . . . . . . . . . . . . . . . .  17
   4.  BIER-TE Controller Host BitPosition Assignments . . . . . . .  17
     4.1.  P2P Links . . . . . . . . . . . . . . . . . . . . . . . .  18
     4.2.  BFER  . . . . . . . . . . . . . . . . . . . . . . . . . .  18
     4.3.  Leaf BFERs  . . . . . . . . . . . . . . . . . . . . . . .  18
     4.4.  LANs  . . . . . . . . . . . . . . . . . . . . . . . . . .  18
     4.5.  Hub and Spoke . . . . . . . . . . . . . . . . . . . . . .  19
     4.6.  Rings . . . . . . . . . . . . . . . . . . . . . . . . . .  19
     4.7.  Equal Cost MultiPath (ECMP) . . . . . . . . . . . . . . .  20
     4.8.  Routed adjacencies  . . . . . . . . . . . . . . . . . . .  23
       4.8.1.  Reducing BitPositions . . . . . . . . . . . . . . . .  23
       4.8.2.  Supporting nodes without BIER-TE  . . . . . . . . . .  23
   5.  Avoiding loops and duplicates . . . . . . . . . . . . . . . .  23
     5.1.  Loops . . . . . . . . . . . . . . . . . . . . . . . . . .  23
     5.2.  Duplicates  . . . . . . . . . . . . . . . . . . . . . . .  24
   6.  BIER-TE Forwarding Pseudocode . . . . . . . . . . . . . . . .  24
   7.  Managing SI, subdomains and BFR-ids . . . . . . . . . . . . .  27
     7.1.  Why SI and sub-domains  . . . . . . . . . . . . . . . . .  28
     7.2.  Bit assignment comparison BIER and BIER-TE  . . . . . . .  29
     7.3.  Using BFR-id with BIER-TE . . . . . . . . . . . . . . . .  29
     7.4.  Assigning BFR-ids for BIER-TE . . . . . . . . . . . . . .  30
     7.5.  Example bit allocations . . . . . . . . . . . . . . . . .  31
       7.5.1.  With BIER . . . . . . . . . . . . . . . . . . . . . .  31
       7.5.2.  With BIER-TE  . . . . . . . . . . . . . . . . . . . .  32
     7.6.  Summary . . . . . . . . . . . . . . . . . . . . . . . . .  33
   8.  BIER-TE and Segment Routing (SR)  . . . . . . . . . . . . . .  33
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  34
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  34
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  34
   12. Change log [RFC Editor: Please remove]  . . . . . . . . . . .  35
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  38
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  38
     13.2.  Informative References . . . . . . . . . . . . . . . . .  38
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  38

1.  Introduction

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

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   In BIER-TE, BitPositions (BP) indicate adjacecies.  The BIFT of each
   BFR is only populated with BP that are adjacent to the BFR in the
   BIER-TE Topology.  Other BPs are left without adjacency.  The BFR
   replicate and forwards BIER packets to adjacent BPs that are set in
   the packet.  BPs are normally also reset upon forwarding to avoid
   duplicates and loops.  This is detailled further below.

1.1.  Basic Examples

   BIER-TE forwarding is best introduced with simple examples.

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   BIER-TE Topology:


                       p5    p6
                     --- BFR3 ---
                  p3/    p13     \p7
      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
              p12 -> local_decap

                      Figure 1: BIER-TE basic example

   Consider the simple network in the above BIER-TE overview example
   picture with 6 BFR. p1...p14 are the BitPositions (BP) used.  All BFR
   can act as ingres BFR (BFIR), BFR1, BFR3, BFR4 and BFR6 can also be
   egres BFR (BFER).  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).  When this packet is examined
   by BIER-TE on BFR1, the only BitPosition from the bitstring that is
   also set in the BIFT is p2.  This will cause BFR1 to to send the only
   copy of the packet to BFR2.  Similarily, BFR2 will forward to BFR4
   because of p8, BFR4 to BFR5 because of p10 and BFR5 to BFR6 because
   of p12. p12 also makes BFR6 receive and decapsulate the packet.

   To send in addition to BFR6 via BFR4 also a copy to BFR3, the
   bitstring needs to be (p2,p5,p8,p10,p12,p13).  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), the packet would
   be copied by BFR5 towards BFR3 because p6 instead of BFR2 to BFR5
   because of p6 in the prior casse.  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 assumptions about the required multicast traffic paths
   and bandwidth consumption in the network.

   The following picture 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.  Unicast tunneling of BIER-TE packets can
   leverage any feasible mechanism such as MPLS or IP, these
   encapsulations are out of scope of this document.  To emphasize non-
   native forwarding of BIER-TE packets, these adjacencies are called
   "forward_routed", but 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
           ........                ........>
                   ....> 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:   p5  -> local_decap
              p6  -> local_decap
              p7  -> forward_routed to BFR3
              p8  -> forward_routed to BFR4

                  Figure 2: BIER-TE basic overlay example

   To send a BIER-TE packet from BFR1 via BFR3 to BFR6, the bitstring is
   (p1,p5).  From BFR1 via BFR4 to BFR6 it is (p2,p6).  A packet from
   BFR1 to BFR3,BFR4 and BFR6 can use (p1,p2,p3,p4,p5) or
   (p1,p2,p3,p4,p6), or via BFR6 (p2,p3,p4,p6,p7) or (p1.p3,p4,p5,p8).

1.2.  BIER-TE Topology and adjacencies

   The key new component in BIER-TE to control where replication can or
   should happens and how to minimize the required BP for segments is -
   as shown in these two examples - the BIER-TE topology.

   The BIER-TE Topology effectively consists of the BIFT of all the the
   BFR and can also be expressed in a diagram as a graph where the edges
   are the adjacencies between the BFR.  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
   further down in the text.

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   If the BIER-TE topology represents the underlying (layer 2) topology
   of the network, this is called "native" BIER-TE as shown in the first
   example.  This can be freely mixed with "overlay" BIER-TE, in
   "forward_routed" adjacencies are used.

1.3.  Comparison with BIER

   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.

   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.

1.4.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

2.  Layering

   End to end BIER-TE operations consists of four layers: The "Multicast
   Flow Overlay", the "BIER-TE Controller Host", the "Routing Underlay"
   and the "BIER-TE forwarding layer".  The Bier-TE Controller Host is
   the new architectural element in BIER-TE compared toBIER .

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      Picture 2: Layers of BIER-TE

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

                  [BIER-TE Controller Host] <=> [BIER-TE Topology]
                      ^      ^     ^
                     /       |      \   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

                      Figure 3: BIER-TE architecture

2.1.  The Multicast Flow Overlay

   The Multicast Flow Overlay operates as in BIER.  See [RFC8279].
   Instead of interacting with the BIER forwarding layer layer (as in
   BIER), 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 and creates the BIER-TE
   topology from it: determine which adjacencies are required/desired
   and assign BitPositions to them.  Then it signals the resulting of
   BitPositions and their adjacencies to each BFR to set up their BIER-
   TE BIFTs.

   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-

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   specific CLI on the BFRs is also a possible stopgap option (as in
   many other SDN solutions lacking definition of standardized data

   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

2.2.1.  Assignment of BitPositions to adjacencies of the network

   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

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

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

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   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
   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

                        Figure 4: BIFT adjacencies

   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

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   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
   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

   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

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   support routing or switching for NextProto to determine how to
   further process the packet.

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 [RFC8296] 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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               [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

          IP  |..... BIER-TE network......| IP

                   Figure 5: BIER-TE Forwarding Example

   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)

             Figure 6: BIER-TE Forwarding Example Adjacencies

   ...and so on.

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

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

                Figure 7: BIER-TE Forwarding Example Paths

   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.

3.5.  Forwarding comparison with BIER

   Forwarding of BIER-TE is designed to allow common forwarding hardware
   with BIER.  In fact, one of the main goals of this document is to
   encourage the building of forwarding hardware that can not only
   support BIER, but also BIER-TE - to allow experimentation with BIER-
   TE and support building of BIER-TE control plane code.

   The pseudocode in Section 6 shows how existing BIER/BIFT forwarding
   can be amended to support basic BIER-TE forwarding, by using BIER
   BIFT's F-BM.  Only the masking of bits due to avoid duplicates must
   be skipped when forwarding is for BIER-TE.

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   Whether to use BIER or BIER-TE forwarding can simply be a configured
   choice per subdomain and accordingly be set up by a BIER-TE
   controller host.  The BIER packet encapsulation [RFC8296] too can be
   reused without changes except that the currently defined BIER-TE ECMP
   adjacency does not leverage the entropy field so that field would be
   unused when BIER-TE forwarding is used.

3.6.  Requirements

   Basic BIER-TE forwarding MUST support to configure Subdomains to use
   basic BIER-TE forwarding rules (instead of BIER).  With basic BIER-TE
   forwarding, every bit MUST support to have zero or one adjacency.  It
   MUST support the adjacency types forward_connected without DNR flag,
   forward_routed and local_decap.  All other BIER-TE forwarding
   features are optional.  This Basic BIER-TE requirements make BIER-TE
   forwarding exactly the same as BIER forwarding with the exception of
   skipping the aforementioned F-BM masking on egres.

   BIER-TE forwarding SHOULD support the DNR flag, as this is highly
   useful to save bits in rings (see Section 4.6).

   BIER-TE forwarding MAY support more than one djacency on a bit and
   ECMP adjacencies.  The importance of ECMP adjacencies is unclear when
   traffic engineering is used because it may be more desirable to
   explicitly steer traffic across non-ECMP paths to make per-path
   traffic calculation easier for controllers.  Having more than one
   adjacency for a bit allows further savings of bits in hub&spoke
   scenarios, but unlike rings it is less "natural" to flood traffic
   across multuple links unconditional.  Both ECMP and multiple
   adjacencies are forwarding plane features that should be possible to
   support later when needed as they do not impact the basic BIER-TE
   replication loop.  This is true because there is no inter-copy
   depency through resetting of F-BM as in BIER.

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,

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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.

4.2.  BFER

   Every BFER is given a unique BitPosition with a local_decap

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.

          p3|  p4|   p2|
          BFR3 BFR4  BFR7

                           Figure 8: 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 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

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   a list of forward_connected adjacencies to all other neighbors on the

   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.

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.

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

                          Figure 9: Ring Example

   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

   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
   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

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           BFR1 --L2----- BFR2

     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)        |

                          Figure 10: ECMP 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 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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                  /     \
                 /L11    \L12
             BFR2         BFR3
            /    \       /    \
           /L21   \L22  /L31   \L32
          BFR4  BFR5   BFR6  BFR7
           \      /     \      /
            \    /       \    /
             BFR8         BFR9
                 \       /
                  \     /

     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)                 |

                      Figure 11: Polarization Example

   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-

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   splitting on all BFR without making the ECMP hash algorithm
   potentially too complex for fast forwarding in the BFRs.

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 ...---
              ...............             ...............

                   Figure 12: Routed Adjacencies Example

   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.

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   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.

   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

   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

6.  BIER-TE Forwarding Pseudocode

   The following simplified pseudocode for BIER-TE forwarding is using
   BIER forwarding pseudocode of [RFC8279], section 6.5 with the one
   modification necessary to support basic BIER-TE forwarding.  Like the
   BIER pseudo forwarding code, for simplicity it does hide the details
   of the adjacency processing inside PacketSend() which can be
   forward_connected, forward_routed or local_decap.

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      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;
              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]

            Figure 13: Simplified BIER-TE Forwarding Pseudocode

   The difference is that in BIER-TE, step [1] must not be performed.

   In BIER, this step is necessary to avoid duplicates when two or more
   BFER are reachable via the same neighbor.  The F-BM of all those BFER
   bits will indicate each others bits, and step [1] will reset all
   these bits on the first copy made for the first of those BFER bits
   set in the BitString, hence skipping any further copies to that

   Whereas in BIER, the F-BM of bits toward a specific neighbor contain
   only the bits of those BFER destined to be forwarded across this
   neighbor, in BIER-TE the F-BM for a neighbor needs to have all bits
   set except all those bits that are actual (non-empty) adjacencies of
   this BFR.  Step [2] will reset those adjacency bits to avoid loops,
   but all the other bits that are not adjacencies of this BFR need to
   stay untouched by [2] so that they can be processed by further BFR
   along the path.  If [1] was performed as in BIER, then those non-
   adjacency bits would erroneously get reset during replication.

   To support the DNR (Do Not Reset) flag of forward_connected()
   adjacencies, the F-BM must also have its own bit set in the F-BM of
   such an adjacency , so that for the packet copy made for this
   adjacency the bit stays on, whereas it will not be set in the F-BM of
   other bits so that it will be reset for any other packet copy made.

   Eliminating the need to perform [1] also makes processing of bits in
   the BIER-TE bitstring independent of processing other bits, which may
   also simplify forwarding plane implementations.

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   The following pseudocode is comprehensive:

   o  This pseudocode eliminates per-bit F-BM, therefore reducing state
      by BitStringLength^2*SI and eliminating the need for per-packet-
      copy masking operation except for adjacencies with DNR flag set:

      *  AdjacentBits[SI] are bits with a non-empty list of adjcencies.
         This can be computed whenever the BIER-TE controller host
         updates the adjacencies.

      *  Only the AdjacentBits need to be examined in the loop for
         packet copies.

      *  The packets BitString is masked with those AdjacentBits on
         ingres to avoid packet loopings.

   o  The code loops over the adjacencies because there may be more than
      one adjacency for a bit.

   o  When an adjacency has the DNR bit, the bit is set in the packet
      copy (to save bits in rings for example).

   o  The ECMP adjacency is shown.  Its parameters are a
      ListOfAdjacencies from which one is picked.

   o  The forward_local, forward_routed, local_decap adjacencies are
      shown with their parameters.

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     void ForwardBitMaskPacket_withTE (Packet)
         AdjacentBitstring = Packet->BitString &= ~AdjacentBits[SI];
         Packet->BitString &= AdjacentBits[SI];
         for (Index = GetFirstBitPosition(AdjacentBits); Index ;
              Index = GetNextBitPosition(AdjacentBits, Index)) {
             foreach adjacency BIFT[Index+Offset] {
                 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):
                             PacketCopy->BitString |= 2<<(Index-1);

                     case forward_routed({VRF},neighbor):

                     case local_decap({VRF},neighbor):

                 Figure 14: BIER-TE Forwarding Pseudocode

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

   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

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

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

                 Figure 15: Scaling BIER-TE bits by reuse

   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.

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   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
   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:

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

   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

   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 (SR)

   Segment Routing (SR ([RFC8402])) aims to enable lightweight path
   engineering via loose source routing.  Compared to its more heavy-
   weight predecessor RSVP-TE ([RFC3209]), 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.

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   BIER-TE BitPosition (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 ingres replication though.  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-ingres BFR.

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

   Note that 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 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.

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-

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.

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12.  Change log [RFC Editor: Please remove]


      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 conluded on so far).

      Added new text into Introduction setion 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.

      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 implementions 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.

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      - reworked pseudocode section to have one pseodcode 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.

      - Added captions to pictures.

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

      - Source now on

      - 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

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      - 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

      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.

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13.  References

13.1.  Normative References

   [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,

   [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, <>.

13.2.  Informative References

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

   [RFC3209]  Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
              and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
              Tunnels", RFC 3209, DOI 10.17487/RFC3209, December 2001,

   [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, <>.

Authors' Addresses

   Toerless Eckert (editor)
   Huawei USA - Futurewei Technologies Inc.
   2330 Central Expy
   Santa Clara  95050


   Gregory Cauchie
   Bouygues Telecom


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   Michael Menth
   University of Tuebingen


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