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Versions: 00 01 02 03 04 05 draft-ietf-bier-architecture

Internet Engineering Task Force                        IJ. Wijnands, Ed.
Internet-Draft                                       Cisco Systems, Inc.
Intended status: Standards Track                           E. Rosen, Ed.
Expires: September 7, 2015                        Juniper Networks, Inc.
                                                             A. Dolganow
                                                          Alcatel-Lucent
                                                           T. Przygienda
                                                                Ericsson
                                                               S. Aldrin
                                                     Huawei Technologies
                                                           March 6, 2015


             Multicast using Bit Index Explicit Replication
                  draft-wijnands-bier-architecture-05

Abstract

   This document specifies a new architecture for the forwarding of
   multicast data packets.  It provides optimal forwarding of multicast
   packets through a "multicast domain".  However, it does not require
   any explicit tree-building protocol, nor does it require intermediate
   nodes to maintain any per-flow state.  This architecture is known as
   "Bit Index Explicit Replication" (BIER).  When a multicast data
   packet enters the domain, the ingress router determines the set of
   egress routers to which the packet needs to be sent.  The ingress
   router then encapsulates the packet in a BIER header.  The BIER
   header contains a bitstring in which each bit represents exactly one
   egress router in the domain; to forward the packet to a given set of
   egress routers, the bits corresponding to those routers are set in
   the BIER header.  Elimination of the per-flow state and the explicit
   tree-building protocols results in a considerable simplification.

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."




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   This Internet-Draft will expire on September 7, 2015.

Copyright Notice

   Copyright (c) 2015 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
   2.  The BFR Identifier and BFR-Prefix . . . . . . . . . . . . . .   5
   3.  Encoding BFR Identifiers in BitStrings  . . . . . . . . . . .   6
   4.  Layering  . . . . . . . . . . . . . . . . . . . . . . . . . .   8
     4.1.  The Routing Underlay  . . . . . . . . . . . . . . . . . .   8
     4.2.  The BIER Layer  . . . . . . . . . . . . . . . . . . . . .   9
     4.3.  The Multicast Flow Overlay  . . . . . . . . . . . . . . .  10
   5.  Advertising BFR-ids and BFR-Prefixes  . . . . . . . . . . . .  10
   6.  BIER Intra-Domain Forwarding Procedures . . . . . . . . . . .  12
     6.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .  12
     6.2.  BFR Neighbors . . . . . . . . . . . . . . . . . . . . . .  13
     6.3.  The Bit Index Routing Table . . . . . . . . . . . . . . .  14
     6.4.  The Bit Index Forwarding Table  . . . . . . . . . . . . .  14
     6.5.  The BIER Forwarding Procedure . . . . . . . . . . . . . .  15
     6.6.  Examples of BIER Forwarding . . . . . . . . . . . . . . .  17
       6.6.1.  Example 1 . . . . . . . . . . . . . . . . . . . . . .  18
       6.6.2.  Example 2 . . . . . . . . . . . . . . . . . . . . . .  18
     6.7.  Equal Cost Multi-path Forwarding  . . . . . . . . . . . .  20
       6.7.1.  Non-deterministic ECMP  . . . . . . . . . . . . . . .  21
       6.7.2.  Deterministic ECMP  . . . . . . . . . . . . . . . . .  22
     6.8.  Prevention of Loops and Duplicates  . . . . . . . . . . .  24
     6.9.  When Some Nodes do not Support BIER . . . . . . . . . . .  24
     6.10. Use of Different BitStringLengths within a Domain . . . .  26
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  27
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  27
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  27
   10. Contributor Addresses . . . . . . . . . . . . . . . . . . . .  27
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  29
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  29



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     11.2.  Informative References . . . . . . . . . . . . . . . . .  29
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  29

1.  Introduction

   This document specifies a new architecture for the forwarding of
   multicast data packets.  It provides optimal forwarding of multicast
   data packets through a "multicast domain".  However, it does not
   require any explicit tree-building protocol, and does not require
   intermediate nodes to maintain any per-flow state.  This architecture
   is known as "Bit Index Explicit Replication" (BIER).

   A router that supports BIER is known as a "Bit-Forwarding Router"
   (BFR).  A BIER domain is a connected set of BFRs.  The BIER control
   plane protocols (see Section 4.2) run within a BIER domain, allowing
   the BFRs within that domain to exchange the necessary information.

   A multicast data packet enters a BIER domain at a "Bit-Forwarding
   Ingress Router" (BFIR), and leaves the BIER domain at one or more
   "Bit-Forwarding Egress Routers" (BFERs).  A BFR that receives a
   multicast data packet from another BFR in the same BIER domain, and
   forwards the packet to another BFR in the same BIER domain, will be
   known as a "transit BFR" for that packet.  A single BFR may be a BFIR
   for some multicast traffic while also being a BFER for some multicast
   traffic and a transit BFR for some multicast traffic.  In fact, a BFR
   may be the BFIR for a given packet and may also be (one of) the
   BFER(s), for that packet; it may also forward that packet to one or
   more additional BFRs.

   A BIER domain may contain one or more sub-domains.  Each BIER domain
   MUST contain at least one sub-domain, the "default sub-domain" (also
   denoted "sub-domain zero").  If a BIER domain contains more than one
   sub-domain, each BFR in the domain MUST be provisioned to know the
   set of sub-domains to which it belongs.  Each sub-domain is
   identified by a sub-domain-id in the range [0,255].

   For each sub-domain to which a given BFR belongs, if the BFR is
   capable of acting as a BFIR or a BFER, it MUST be provisioned with a
   "BFR-id" that is unique within the sub-domain.  A BFR-id is a small
   unstructured number.  For instance, if a particular BIER sub-domain
   contains 1,374 BFRs, each one could be given a BFR-id in the range
   1-1374.

   If a given BFR belongs to more than one sub-domain, it may (though it
   need not) have a different BFR-id for each sub-domain.

   When a multicast packet arrives from outside the domain at a BFIR,
   the BFIR determines the set of BFERs to which the packet must be



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   sent.  The BFIR also determines the sub-domain over which the packet
   must be sent.  (Procedures for assigning a particular packet to a
   particular sub-domain are outside the scope of this document.)  The
   BFIR then encapsulates the packet in a "BIER header".  The BIER
   header contains a bit string in which each bit represents a single
   BFR-id.  To indicate that a particular BFER needs to receive a given
   packet, the BFIR sets the bit corresponding to that BFER's BFR-id in
   the sub-domain to which the packet has been assigned.  We will use
   term "BitString" to refer to the bit string field in the BIER header.
   We will use the term "payload" to refer to the packet that has been
   encapsulated.  Thus a "BIER-encapsulated" packet consists of a "BIER
   header" followed by a "payload".

   The number of BFERs to which a given packet can be forwarded is
   limited only by the length of the BitString in the BIER header.
   Different deployments can use different BitString lengths.  We will
   use the term "BitStringLength" to refer to the number of bits in the
   BitString.  It is possible that some deployment will have more BFERs
   in a given sub-domain than there are bits in the BitString.  To
   accommodate this case, the BIER encapsulation includes both the
   BitString and a "Set Identifier" (SI).  It is the BitString and the
   SI together that determine the set of BFERs to which a given packet
   will be delivered:

   o  by convention, the least significant (rightmost) bit in the
      BitString is "bit 1", and the most significant (leftmost) bit is
      "bit BitStringLength".

   o  if a BIER-encapsulated packet has an SI of n, and a BitString with
      bit k set, then the packet must be delivered to the BFER whose
      BFR-id (in the sub-domain to which the packet has been assigned)
      is n*BitStringLength+k.

   For example, suppose the BIER encapsulation uses a BitStringLength of
   256 bits.  By convention, the least significant (rightmost) bit is
   "bit 1", and the most significant (leftmost) bit is "bit 256".
   Suppose that a given packet has been assigned to sub-domain 0, and
   needs to be delivered to three BFERs, where those BFERs have BFR-ids
   in sub-domain 0 of 13, 126, and 235 respectively.  The BFIR would
   create a BIER encapsulation with the SI set to zero, and with bits
   13, 126, and 235 of the BitString set.  (All other bits of the
   BitString would be clear.)  If the packet also needs to be sent to a
   BFER whose BFR-id is 257, the BFIR would have to create a second copy
   of the packet, and the BIER encapsulation would specify an SI of 1,
   and a BitString with bit 1 set and all the other bits clear.

   Note that it is generally advantageous to assign the BFR-ids so that
   as many BFERs as possible can be represented in a single bit string.



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   Suppose a BFR, call it BFR-A, receives a packet whose BIER
   encapsulation specifies an SI of 0, and a BitString with bits 13, 26,
   and 235 set.  Suppose BFR-A has two BFR neighbors, BFR-B and BFR-C,
   such that the best path to BFERs 13 and 26 is via BFR-B, but the best
   path to BFER 235 is via BFR-C.  Then BFR-A will replicate the packet,
   sending one copy to BFR-B and one copy to BFR-C.  However, BFR-A will
   clear bit 235 in the BitString of the packet copy it sends to BFR-B,
   and will clear bits 13 and 26 in the BitString of the packet copy it
   sends to BFR-C.  As a result, BFR-B will forward the packet only
   towards BFERs 13 and 26, and BFR-C will forward the packet only
   towards BFER 235.  This ensures that each BFER receives only one copy
   of the packet.

   With this forwarding procedure, a multicast data packet can follow an
   optimal path from its BFIR to each of its BFERs.  Further, since the
   set of BFERs for a given packet is explicitly encoded into the BIER
   header, the packet is not sent to any BFER that does not need to
   receive it.  This allows for optimal forwarding of multicast traffic.
   This optimal forwarding is achieved without any need for transit BFRs
   to maintain per-flow state, or to run a multicast tree-building
   protocol.

   The idea of encoding the set of egress nodes into the header of a
   multicast packet is not new.  For example, [Boivie_Feldman] proposes
   to encode the set of egress nodes as a set of IP addresses, and
   proposes mechanisms and procedures that are in some ways similar to
   those described in the current document.  However, since BIER encodes
   each BFR-id as a single bit in a bit string, it can represent up to
   128 BFERs in the same number of bits that it would take to carry the
   IPv6 address of a single BFER.  Thus BIER scales to a much larger
   number of egress nodes per packet.

   BIER does not require that each transit BFR look up the best path to
   each BFER that is identified in the BIER header; the number of
   lookups required in the forwarding path for a single packet can be
   limited to the number of neighboring BFRs; this can be much smaller
   than the number of BFERs.  See Section 6 (especially Section 6.4) for
   details.

   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.  The BFR Identifier and BFR-Prefix

   Each BFR MUST be assigned a "BFR-Prefix".  A BFR's BFR-Prefix MUST be
   an IP address (either IPv4 or IPv6) of the BFR, and MUST be unique
   and routable within the BIER domain.  It is RECOMMENDED that the



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   BFR-prefix be a loopback address of the BFR.  Two BFRs in the same
   BIER domain MUST NOT be assigned the same BFR-Prefix.  Note that a
   BFR in a given BIER domain has the same BFR-prefix in all the sub-
   domains of that BIER domain.

   A "BFR Identifier" (BFR-id) is a number in the range [1,65535].  In
   general, each BFR in a given BIER sub-domain must be assigned a
   unique number from this range (i.e., two BFRs in the same BIER sub-
   domain MUST NOT have the same BFR-id in that sub-domain).  However,
   if it is known that a given BFR will never need to function as a BFER
   in a given sub-domain, then it is not necessary to assign a BFR-id
   for that sub-domain to that BFR.

   Note that the value 0 is not a legal BFR-id.

   The procedure for assigning a particular BFR-id to a particular BFR
   is outside the scope of this document.  However, it is RECOMMENDED
   that the BFR-ids for each sub-domain be assigned "densely" from the
   numbering space, as this will result in a more efficient encoding
   (see Section 3).  That is, if there are 256 or fewer BFERs, it is
   RECOMMENDED to assign all the BFR-ids from the range [1,256].  If
   there are more than 256 BFERs, but less than 512, it is RECOMMENDED
   to assign all the BFR-ids from the range [1,512], with as few "holes"
   as possible in the earlier range.  However, in some deployments, it
   may be advantageous to depart from this recommendation; this is
   discussed further in Section 3.

3.  Encoding BFR Identifiers in BitStrings

   To encode a BFR-id in a BIER data packet, one must convert the BFR-id
   to an SI and a BitString.  This conversion depends upon the parameter
   we are calling "BitStringLength".  The conversion is done as follows.
   If the BFR-id is N, then

   o  SI is the integer part of the quotient (N-1)/BitStringLength

   o  The BitString has one bit position set.  If the low-order bit is
      bit 1, and the high-order bit is bit BitStringLength, the bit
      position that represents BFR-id N is
      ((N-1) modulo BitStringLength)+1.

   If several different BFR-ids all resolve to the same SI, then all
   those BFR-ids can be represented in a single BitString.  The
   BitStrings for all of those BFR-ids are combined using a bitwise
   logical OR operation.

   Different BIER domains may use different values of BitStringLength.
   Each BFIR in a given BIER domain MUST be provisioned to know the



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   BitStringLength to use when imposing a BIER encapsulation on a
   particular set of packets.  This value of BitStringLength SHOULD be a
   value that is supported by all the BFRs in the domain.  (That is, the
   BitStringLength value used by a BFIR when imposing a BIER
   encapsulation on a particular packet SHOULD be a value that is
   supported by all the BFRs and BFERs in the domain that might have to
   forward or receive the packet.)  However, under certain
   circumstances, it is possible to make exceptions to this rule.  This
   is discussed in Section 6.10.

   Every BFIR MUST be able to impose a BIER encapsulation whose
   BitStringLength of 256.  Every BFR MUST be able to forward a BIER-
   encapsulated packet whose BitStringLength is 256.  Every BFER MUST be
   able to receive and properly process a BIER-encapsulated packet whose
   BitStringLength is 256.

   Particular BIER encapsulation types MAY allow other BitStringLengths
   to be OPTIONALLY supported.  For example, when using the
   encapsulation specified in [MPLS_BIER_ENCAPS], a BFR may support any
   or all of the following BitStringLengths: 64, 128, 256, 512, 1024,
   2048, and 4096.

   A BFR MUST support SI values in the range [0,15], and MAY support SI
   values in the range [0,255].  ("Supporting the values in a given
   range" means, in this context, that any value in the given range is
   legal, and will be properly interpreted.)

   When a BFIR determines that a multicast data packet, assigned to a
   given sub-domain, needs to be forwarded to a particular set of
   destination BFERs, the BFIR partitions that set of BFERs into
   subsets, where each subset contains the target BFERs whose BFR-ids in
   the given sub-domain all resolve to the same SI.  Call these the
   "SI-subsets" for the packet.  Each SI-subset can be represented by a
   single BitString.  The BFIR creates a copy of the packet for each
   SI-subset.  The BIER encapsulation is then applied to each packet.
   The encapsulation specifies a single SI for each packet, and contains
   the BitString that represents all the BFR-ids in the corresponding
   SI-subset.  Of course, in order to properly interpret the BitString,
   it must be possible to infer the sub-domain-id from the encapsulation
   as well.

   Suppose, for example, that a BFIR determines that a given packet
   needs to be forwarded to three BFERs, whose BFR-ids (in the
   appropriate sub-domain) are 27, 235, and 497.  The BFIR will have to
   forward two copies of the packet.  One copy, associated with SI=0,
   will have a BitString with bits 27 and 235 set.  The other copy,
   associated with SI=1, will have a BitString with bit 241 set.




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   In order to minimize the number of copies that must be made of a
   given multicast packet, it is RECOMMENDED that the BFR-ids be
   assigned "densely" (see Section 2) from the numbering space.  This
   will minimize the number of SIs that have to be used in the domain.
   However, depending upon the details of a particular deployment, other
   assignment methods may be more advantageous.  Suppose, for example,
   that in a certain deployment, every multicast flow is either intended
   for the "east coast" or for the "west coast".  In such a deployment,
   it would be advantageous to assign BFR-ids so that all the "west
   coast" BFR-ids fall into the same SI-subset, and so that all the
   "east coast" BFR-ids fall into the same SI-subset.

   When a BFR receives a BIER data packet, it will infer the SI from the
   encapsulation.  The set of BFERs to which the packet needs to be
   forwarded can then be inferred from the SI and the BitString.

   In some of the examples given later in this document, we will use a
   BitStringLength of 4, and will represent a BFR-id in the form
   "SI:xyzw", where SI is the Set Identifier of the BFR-id (assuming a
   BitStringLength of 4), and xyzw is a string of 4 bits.  A
   BitStringLength of 4 is used only in the examples; we would not
   expect actual deployments to have such a small BitStringLength.

   It is possible that several different forms of BIER encapsulation
   will be developed.  If so, the particular encapsulation that is used
   in a given deployment will depend on the type of network
   infrastructure that is used to realize the BIER domain.  Details of
   the BIER encapsulation(s) will be given in companion documents.  An
   encapsulation for use in MPLS networks is described in
   [MPLS_BIER_ENCAPS]

4.  Layering

   It is helpful to think of the BIER architecture as consisting of
   three layers: the "routing underlay", the "BIER layer", and the
   "multicast flow overlay".

4.1.  The Routing Underlay

   The "routing underlay" establishes "adjacencies" between pairs of
   BFRs, and determines one or more "best paths" from a given BFR to a
   given set of BFRs.  Each such path is a sequence of BFRs <BFR(k),
   BFR(k+1), ..., BFR(k+n)> such that BFR(k+j) is "adjacent" to
   BFR(k+j+1) (for 0<=j<n).

   At a given BFR, say BFR-A, for every IP address that is the address
   of a BFR in the BIER domain, the routing underlay will map that IP
   address into a set of one or more "equal cost" adjacencies.  If a



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   BIER data packet has to be forwarded by BFR-A to a given BFER, say
   BFER-B, the packet will follow the path from BFR-A to BFER-B that is
   determined by the routing underlay.

   It is expected that in a typical deployment, the routing underlay
   will be the default topology that the Interior Gateway Protocol
   (IGP), e.g., OSPF, uses for unicast routing.  In that case, the
   underlay adjacencies are just the OSPF adjacencies.  A BIER data
   packet traveling from BFR-A to BFER-B will follow the path that OSPF
   has selected for unicast traffic from BFR-A to BFER-B.

   If one wants to have multicast traffic from BFR-A to BFER-B travel a
   path that is different from the path used by the unicast traffic from
   A to B, one can use a different underlay.  For example, if multi-
   topology OSPF is being used, one OSPF topology could be used for
   unicast traffic, and the other for multicast traffic.  (Each topology
   would be considered to be a different underlay.)  Alternatively, one
   could deploy a routing underlay that creates a multicast-specific
   tree of some sort, perhaps a Steiner tree.  Then BIER could be used
   to forward multicast data packets along the multicast-specific tree,
   while unicast packets follow the "ordinary" OSPF best path.  It is
   even possible to have multiple routing underlays used by BIER, as
   long as one can infer from a data packet's BIER encapsulation which
   underlay is being used for that packet.

   If multiple routing underlays are used in a single BIER domain, each
   BIER sub-domain MUST be associated with a single routing underlay.
   (Though multiple sub-domains may be associated with the same routing
   underlay.)  A BFR that belongs to multiple sub-domains MUST be
   provisioned to know which routing underlay is used by each sub-
   domain.  By default (i.e., in the absence of any provisioning to the
   contrary), each sub-domain uses the default topology of the unicast
   IGP as the routing underlay.

   Note that specification of the protocol and procedures of the routing
   underlay is outside the scope of this document.

4.2.  The BIER Layer

   The BIER layer consists of the protocol and procedures that are used
   in order to transmit a multicast data packet across a BIER domain,
   from its BFIR to its BFERs.  This includes the following components:

   o  Protocols and procedures that advertise, to all other BFRs in the
      same BIER domain, each BFR's BFR-prefix.

   o  Protocols and procedures that advertise, to all other BFRs in the
      same BIER domain, each BFR's BFR-id for each sub-domain.



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   o  The imposition by a BFIR of a BIER header on a multicast data
      packet.

   o  The procedures for forwarding BIER-encapsulated packets, and for
      modifying the BIER header during transit.

4.3.  The Multicast Flow Overlay

   The "multicast flow overlay" consists of the set of protocols and
   procedures that enable the following set of functions.

   o  When a BFIR receives a multicast data packet from outside the BIER
      domain, the BFIR must determine the set of BFERs for that packet.
      This information is provided by the multicast flow overlay.

   o  When a BFER receives a BIER-encapsulated packet from inside the
      BIER domain, the BFER must determine how to further forward the
      packet.  This information is provided by the multicast flow
      overlay.

   For example, suppose the BFIR and BFERs are Provider Edge (PE)
   routers providing Multicast Virtual Private Network (MVPN) service.
   The multicast flow overlay consists of the protocols and procedures
   described in [RFC6513] and [RFC6514].  The MVPN signaling described
   in those RFCs enables an ingress PE to determine the set of egress
   PEs for a given multicast flow (or set of flows); it also enables an
   egress PE to determine the "Virtual Routing and Forwarding Tables"
   (VRFs) to which multicast packets from the backbone network should be
   sent.  MVPN signaling also has several components that depend on the
   type of "tunneling technology" used to carry multicast data though
   the network.  Since BIER is, in effect, a new type of "tunneling
   technology", some extensions to the MVPN signaling are needed in
   order to properly interface the multicast flow overlay with the BIER
   layer.  These will be specified in a companion document.

   MVPN is just one example of a multicast flow overlay.  Protocols and
   procedures for other overlays will be provided in companion
   documents.  It is also possible to implement the multicast flow
   overlay by means of a "Software Defined Network" (SDN) controller.
   Specification of the protocols and procedures of the multicast flow
   overlay is outside the scope of this document.

5.  Advertising BFR-ids and BFR-Prefixes

   As stated in Section 2, each BFER is assigned a BFR-id (for a given
   BIER sub-domain).  Each BFER must advertise these assignments to all
   the other BFRs in the domain.  Similarly, each BFR is assigned a
   BFR-prefix (for a given BIER domain), and must advertise this



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   assignment to all the other BFRs in the domain.  Finally, it is
   useful for each BFR to advertise its supported values of
   BitStringLength (for a given BIER domain).

   If the BIER domain is also a link state routing IGP domain (i.e., an
   OSPF or IS-IS domain), the advertisement of the BFR-prefix, <sub-
   domain-id,BFR-id> and BitStringLength can be done using the
   advertisement capabilities of the IGP.  For example, if a BIER domain
   is also an OSPF domain, these advertisements can be done using the
   OSPF "Opaque Link State Advertisement" (Opaque LSA) mechanism.
   Details of the necessary extensions to OSPF and IS-IS will be
   provided in companion documents.  (See [OSPF_BIER_EXTENSIONS] and
   [ISIS_BIER_EXTENSIONS].)

   These advertisements enable each BFR to associate a given <sub-
   domain-id, BFR-id> with a given BFR-prefix.  As will be seen in
   subsequent sections of this document, knowledge of this association
   is an important part of the forwarding process.

   Since each BFR needs to have a unique (in each sub-domain) BFR-id,
   two different BFRs will not advertise ownership of the same <sub-
   domain-id, BFR-id> unless there has been a provisioning error.

   o  If BFR-A determines that BFR-B and BFR-C have both advertised the
      same BFR-id for the same sub-domain, BFR-A MUST log an error.
      Suppose that the duplicate BFR-id is "N".  When BFR-A is
      functioning as a BFIR, it MUST NOT encode the BFR-id value N in
      the BIER encapsulation of any packet that has been assigned to the
      given sub-domain, even if it has determined that the packet needs
      to be received by BFR-B and/or BFR-C.

      This will mean that BFR-B and BFR-C cannot receive multicast
      traffic at all in the given sub-domain until the provisioning
      error is fixed.  However, that is preferable to having them
      receive each other's traffic.

   o  If BFR-A has been provisioned with BFR-id N for a particular sub-
      domain, has not yet advertised its ownership of BFR-id N for that
      sub-domain, but has received an advertisement from a different BFR
      (say BFR-B) that is advertising ownership of BFR-id N for the same
      sub-domain, then BFR-A SHOULD log an error, and MUST NOT advertise
      its own ownership of BFR-id N for that sub-domain as long as the
      advertisement from BFR-B is extant.

      This procedure may prevent the accidental misconfiguration of a
      new BFR from impacting an existing BFR.





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   If a BFR advertises that it has a BFR-id of 0 in a particular sub-
   domain, other BFRs receiving the advertisement MUST interpret that
   advertisement as meaning that the advertising BFR does not have a
   BFR-id in that sub-domain.

6.  BIER Intra-Domain Forwarding Procedures

   This section specifies the rules for forwarding a BIER-encapsulated
   data packet within a BIER domain.

6.1.  Overview

   This section provides a brief overview of the BIER forwarding
   procedures.  Subsequent sub-sections specify the procedures in more
   detail.

   To forward a BIER-encapsulated packet:

   1.  Determine the packet's sub-domain.

   2.  Determine the packet's SI.

   3.  From the sub-domain, the SI and the BitString, determine the set
       of destination BFERs for the packet.

   4.  Using information provided by the routing underlay associated
       with the packet's sub-domain, determine the next hop adjacency
       for each of the destination BFERs.

   5.  Partition the set of destination BFERs such that all the BFERs in
       a single partition have the same next hop.  We will say that each
       partition is associated with a next hop.

   6.  For each partition:

       a.  Make a copy of the packet.

       b.  Clear any bit in the packet's BitString that identifies a
           BFER that is not in the partition.

       c.  Transmit the packet to the associated next hop.

   If a BFR receives a BIER-encapsulated packet whose sub-domain, SI and
   BitString identify that BFR itself, then the BFR is also a BFER for
   that packet.  As a BFER, it must pass the payload to the multicast
   flow overlay.  If the BitString has more than one bit set, the packet
   also needs to be forwarded further within the BIER domain.  If the
   BF(E)R also forwards one or more copies of the packet within the BIER



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   domain, the bit representing the BFR's own BFR-id will be cleared in
   all the copies.

   When BIER on a BFER passes a packet to the multicast flow overlay, it
   may need to provide contextual information obtained from the BIER
   encapsulation.  The information that needs to pass between the BIER
   layer and the multicast flow layer is specific to the multicast flow
   layer.  Specification of the interaction between the BIER layer and
   the multicast flow layer is outside the scope of this specification.

   When BIER on a BFER passes a packet to the multicast flow overlay,
   the overlay will determine how to further dispatch the packet.  If
   the packet needs to be forwarded into another BIER domain, then the
   BFR will act as a BFER in one BIER domain and as a BFIR in another.
   A BIER-encapsulated packet cannot pass directly from one BIER domain
   to another; at the boundary between BIER domains, the packet must be
   decapsulated and passed to the multicast flow layer.

   Note that when a BFR transmits multiple copies of a packet within a
   BIER domain, only one copy will be destined to any given BFER.
   Therefore it is not possible for any BIER-encapsulated packet to be
   delivered more than once to any BFER.

6.2.  BFR Neighbors

   The "BFR Neighbors" (BFR-NBRs) of a given BFR, say BFR-A, are those
   BFRs that, according to the routing underlay, are adjacencies of
   BFR-A.  Each BFR-NBR will have a BFR-prefix.

   Suppose a BIER-encapsulated packet arrives at BFR-A.  From the
   packet's encapsulation, BFR-A learns the sub-domain of the packet,
   and the BFR-ids (in that sub-domain) of the BFERs to which the packet
   is destined.  Then using the information advertised per Section 5,
   BFR-A can find the BFR-prefix of each destination BFER.  Given the
   BFR-prefix of a particular destination BFER, say BFER-D, BFR-A learns
   from the routing underlay (associated with the packet's sub-domain)
   an IP address of the BFR that is the next hop on the path from BFR-A
   to BFER-D.  Let's call this next hop BFR-B.  BFR-A must then
   determine the BFR-prefix of BFR-B.  (This determination can be made
   from the information advertised per Section 5.)  This BFR-prefix is
   the BFR-NBR of BFR-A on the path from BFR-A to BFER-D.

   Note that if the routing underlay provides multiple equal cost paths
   from BFR-A to BFER-D, BFR-A may have multiple BFR-NBRs for BFER-D.

   Under certain circumstances, a BFR may have adjacencies (in a
   particular routing underlay) that are not BFRs.  Please see
   Section 6.9 for a discussion of how to handle those circumstances.



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6.3.  The Bit Index Routing Table

   The Bit Index Routing Table (BIRT) is a table that maps from the
   BFR-id (in a particular sub-domain) of a BFER to the BFR-prefix of
   that BFER, and to the BFR-NBR on the path to that BFER.


     ( A ) ------------ (  B  ) ------------ ( C ) ------------ ( D )
    4 (0:1000)              \                  \            1 (0:0001)
                             \                  \
                             ( E )              ( F )
                           3 (0:0100)         2 (0:0010)

                         Figure 1: BIER Topology 1

   As an example, consider the topology shown in Figure 1.  In this
   diagram, we represent the BFR-id of each BFR in the SI:xyzw form
   discussed in Section 3.  This topology will result in the BIRT of
   Figure 2 at BFR-B.  The first column shows the BFR-id as a number and
   also (in parentheses) in the SI:BitString format that corresponds to
   a BitStringLength of 4.  (The actual minimum BitStringLength is 64,
   but we use 4 in the examples.)

   Note that a BIRT is specific to a particular BIER sub-domain.

               --------------------------------------------
               |     BFR-id     |  BFR-Prefix  | BFR-NBR  |
               | (SI:BitString) | of Dest BFER |          |
               ============================================
               |   4 (0:1000)   |     A        |     A    |
               --------------------------------------------
               |   1 (0:0001)   |     D        |     C    |
               --------------------------------------------
               |   3 (0:0100)   |     E        |     E    |
               --------------------------------------------
               |   2 (0:0010)   |     F        |     C    |
               --------------------------------------------


                Figure 2: Bit Index Routing Table at BFR-B

6.4.  The Bit Index Forwarding Table

   The "Bit Index Forwarding Table" (BIFT) is derived from the BIRT as
   follows.  (Note that a BIFT is specific to a particular sub-domain.)

   Suppose that several rows in the BIRT have the same SI and the same
   BFR-NBR.  By taking the logical OR of the BitStrings of those rows,



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   we obtain a bit mask that corresponds to that combination of SI and
   BFR-NBR.  We will refer to this bit mask as the "Forwarding Bit Mask"
   (F-BM) for that <SI,BFR-NBR> combination.

   For example, in Figure 2, we see that two of the rows have the same
   SI (0) and same BFR-NBR (C).  The Bit Mask that corresponds to <SI=0,
   BFR-NBR-C> is 0011 ("0001" OR'd with "0010").

   The BIFT is used to map from the BFR-id of a BFER to the
   corresponding F-BM and BFR-NBR.  For example, Figure 3 shows the BIFT
   that is derived from the BIRT of Figure 2.  Note that BFR-ids 1 and 2
   have the same SI and the same BFR-NBR, hence they have the same F-BM.

                   -------------------------------------
                   |      BFR-id    |  F-BM  | BFR-NBR |
                   | (SI:Bitstring) |        |         |
                   =====================================
                   |   1 (0:0001)   |  0011  |    C    |
                   -------------------------------------
                   |   2 (0:0010)   |  0011  |    C    |
                   -------------------------------------
                   |   3 (0:0100)   |  0100  |    E    |
                   -------------------------------------
                   |   4 (0:1000)   |  1000  |    A    |
                   -------------------------------------

                   Figure 3: Bit Index Forwarding Table

   This Bit Index Forwarding Table (BIFT) is programmed into the data-
   plane and used to forward packets, applying the rules specified below
   in Section 6.5.

6.5.  The BIER Forwarding Procedure

   Below is the procedure for forwarding a BIER-encapsulated packet.

   1.  Determine the packet's SI.

   2.  Find the position of least significant (rightmost) bit in the
       packet's BitString that is set.  (Remember, bits are numbered
       from 1, starting with the least significant bit.)  Use that bit
       position, together with the SI, as the 'index' into the BIFT.

   3.  Extract from the BIFT the F-BM and the BFR-NBR.

   4.  Copy the packet.  Update the copy's BitString by AND'ing it with
       the F-BM (i.e., PacketCopy->BitString &= F-BM).  Then forward the
       copy to the BFR-NBR.  Note that when a packet is forwarded to a



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       particular BFR-NBR, its BitString identifies only those BFERs
       that are to be reached via that BFR-NBR.

   5.  Now update the original packet's BitString by AND'ing it with the
       INVERSE of the F-BM (i.e., Packet->Bitstring &= ~F-BM).  (This
       clears the bits that identify the BFERs to which a copy of the
       packet has just been forwarded.)  Go to step 2.

   Note that this procedure causes the packet to be forwarded to a
   particular BFR-NBR only once.  The number of lookups in the BIFT is
   the same as the number of BFR-NBRs to which the packet must be
   forwarded; it is not necessary to do a separate lookup for each
   destination BFER.

   Suppose it has been decided (by the above rules) to send a packet to
   a particular BFR-NBR.  If that BFR-NBR is connected via multiple
   parallel interfaces, it may be desirable to apply some form of load
   balancing.  Load balancing algorithms are outside the scope of this
   document.  However, if the packet's encapsulation contains an
   "entropy" field, the entropy field SHOULD be respected; two packets
   with the same value of the entropy field SHOULD be sent on the same
   interface (if possible).

   In some cases, the routing underlay may provide multiple equal cost
   paths (through different BFR-NBRs) to a given BFER.  This is known as
   "Equal Cost Multiple Paths" (ECMP).  The procedures described in this
   section must be augmented in order to support load balancing over
   ECMP.  The necessary augmentations can be found in Section 6.7.

   In the event that unicast traffic to the BFR-NBR is being sent via a
   "bypass tunnel" of some sort, the BIER-encapsulated multicast traffic
   send to the BFR-NBR SHOULD also be sent via that tunnel.  This allows
   any existing "Fast Reroute" schemes to be applied to multicast
   traffic as well as to unicast traffic.

   Some examples of these forwarding procedures can be found in
   Section 6.6.

   The rules given in this section can be represented by the following
   pseudocode:











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   void ForwardBitMaskPacket (Packet)
   {
       SI=GetPacketSI(Packet);
       Offset=SI*BitStringLength;
       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;
           PacketSend(PacketCopy, BFR-NBR);
           Packet->BitString &= ~F-BM;
       }
   }


                           Figure 4: Pseudocode

   Note that at a given BFER, the BFR-NBR entry corresponding to the
   BFER's own BFR-id will be the BFER's own BFR-prefix.  In this case,
   the "PacketSend" function sends the packet to the multicast flow
   layer.

6.6.  Examples of BIER Forwarding

   In this section, we give two examples of BIER forwarding, based on
   the topology in Figure 1.  In these examples, all packets have been
   assigned to the default sub-domain, all packets have SI=0, and the
   BitStringLength is 4.  Figure 5 shows the BIFT entries for SI=0 only.
   For compactness, we show the first column of the BIFT, the BFR-id,
   only as an integer.

           BFR-A BIFT            BFR-B BIFT            BFR-C BIFT
      -------------------   -------------------   -------------------
      | Id | F-BM | NBR |   | Id | F-BM | NBR |   | Id | F-BM | NBR |
      ===================   ===================   ===================
      |  1 | 0111 |  B  |   |  1 | 0011 |  C  |   |  1 | 0001 |  D  |
      -------------------   -------------------   -------------------
      |  2 | 0111 |  B  |   |  2 | 0011 |  C  |   |  2 | 0010 |  F  |
      -------------------   -------------------   -------------------
      |  3 | 0111 |  B  |   |  3 | 0100 |  E  |   |  3 | 1100 |  B  |
      -------------------   -------------------   -------------------
      |  4 | 1000 |  A  |   | 4 |  1000 |  A  |   |  4 | 1100 |  B  |
      -------------------   -------------------   -------------------

                  Figure 5: BIFTs for Forwarding Examples




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6.6.1.  Example 1

   BFR-D, BFR-E and BFR-F are BFER's.  BFR-A is the BFIR.  Suppose that
   BFIR-A has learned from the multicast flow layer that BFER-D is
   interested in a given multicast flow.  If BFIR-A receives a packet of
   that flow from outside the BIER domain, BFIR-A applies the BIER
   encapsulation to the packet.  The encapsulation must be such that the
   SI is zero.  The encapsulation also includes a BitString, with just
   bit 1 set and with all other bits clear (i.e., 0001).  This indicates
   that BFER-D is the only BFER that needs to receive the packet.  Then
   BFIR-A follows the procedures of Section 6.5:

   o  Since the packet's BitString is 0001, BFIR-A finds that the first
      bit in the string is bit 1.  Looking at entry 1 in its BIFT, BFR-A
      determines that the bit mask F-BM is 0111 and the BFR-NBR is
      BFR-B.

   o  BFR-A then makes a copy of the packet, and applies F-BM to the
      copy: Copy->BitString &= 0111.  The copy's Bitstring is now 0001
      (0001 & 0111).

   o  The copy is now sent to BFR-B.

   o  BFR-A then updates the packet's BitString by applying the inverse
      of the F-BM: Packet->Bitstring &= ~F-BM.  As a result, the
      packet's BitString is now 0000 (0001 & 1000).

   o  As the packet's BitString is now zero, the forwarding procedure is
      complete.

   When BFR-B receives the multicast packet from BFR-A, it follows the
   same procedure.  The result is that a copy of the packet, with a
   BitString of 0001, is sent to BFR-C.  BFR-C applies the same
   procedures, and as a result sends a copy of the packet, with a
   BitString of 0001, to BFR-D.

   At BFER-D, the BIFT entry (not pictured) for BFR-id 1 will specify an
   F-BM of 0000 and a BFR-NBR of BFR-D itself.  This will cause a copy
   of the packet to be delivered to the multicast flow layer at BFR-D.
   The packet's BitString will be set to 0000, and the packet will not
   be forwarded any further.

6.6.2.  Example 2

   This example is similar to Example 1, except that BFIR-A has learned
   from the multicast flow layer that both BFER-D and BFER-E are
   interested in a given multicast flow.  If BFIR-A receives a packet of
   that flow from outside the BIER domain, BFIR-A applies the BIER



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   encapsulation to the packet.  The encapsulation must be such that the
   SI is zero.  The encapsulation also includes a BitString with two
   bits set: bit 1 is set (as in example 1) to indicate that BFR-D is a
   BFER for this packet, and bit 3 is set to indicate that BFR-E is a
   BFER for this packet.  I.e., the BitString (assuming again a
   BitStringLength of 4) is 0101.  To forward the packet, BFIR-A follows
   the procedures of Section 6.5:

   o  Since the packet's BitString is 0101, BFIR-A finds that the first
      bit in the string is bit 1.  Looking at entry 1 in its BIFT, BFR-A
      determines that the bit mask F-BM is 0111 and the BFR-NBR is
      BFR-B.

   o  BFR-A then makes a copy of the packet, and applies the F-BM to the
      copy: Copy->BitString &= 0111.  The copy's Bitstring is now 0101
      (0101 & 0111).

   o  The copy is now sent to BFR-B.

   o  BFR-A then updates the packet's BitString by applying the inverse
      of the F-BM: Packet->Bitstring &= ~F-BM.  As a result, the
      packet's BitString is now 0000 (0101 & 1000).

   o  As the packet's BitString is now zero, the forwarding procedure is
      complete.

   When BFR-B receives the multicast packet from BFR-A, it follows the
   procedure of Section 6.5, as follows:

   o  Since the packet's BitString is 0101, BFR-B finds that the first
      bit in the string is bit 1.  Looking at entry 1 in its BIFT, BFR-B
      determines that the bit mask F-BM is 0011 and the BFR-NBR is
      BFR-C.

   o  BFR-B then makes a copy of the packet, and applies the F-BM to the
      copy: Copy->BitString &= 0011.  The copy's Bitstring is now 0001
      (0101 & 0011).

   o  The copy is now sent to BFR-C.

   o  BFR-B then updates the packet's BitString by applying the inverse
      of the F-BM: Packet->Bitstring &=  F-BM.  As a result, the
      packet's BitString is now 0100 (0101 & 1100).

   o  Now BFR-B finds the next bit in the packet's (modified) BitString.
      This is bit 3.  Looking at entry 3 in its BIFT, BFR-B determines
      that the F-BM is 0100 and the BFR-NBR is BFR-E.




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   o  BFR-B then makes a copy of the packet, and applies the F-BM to the
      copy: Copy->BitString &= 0100.  The copy's Bitstring is now 0100
      (0100 & 0100).

   o  The copy is now sent to BFR-E.

   o  BFR-B then updates the packet's BitString by applying the inverse
      of the F-BM: Packet->Bitstring &= ~F-BM.  As a result, the
      packet's BitString is now 0000 (0100 & 1011).

   o  As the packet's BitString is now zero, the forwarding procedure is
      complete.

   Thus BFR-B forwards two copies of the packet.  One copy of the
   packet, with BitString 0001, has now been sent from BFR-B to BFR-C.
   Following the same procedures, BFR-C will forward the packet to
   BFER-D.

   At BFER-D, the BIFT entry (not pictured) for BFR-id 1 will specify an
   F-BM of 0000 and a BFR-NBR of BFR-D itself.  This will cause a copy
   of the packet to be delivered to the multicast flow layer at BFR-D.
   The packet's BitString will be set to 0000, and the packet will not
   be forwarded any further.

   The other copy of the packet has been sent from BFR-B to BFER-E, with
   BitString 0100.

   At BFER-E, the BIFT entry (not pictured) for BFR-id 3 will specify an
   F-BM of 0000 and a BFR-NBR of BFR-E itself.  This will cause a copy
   of the packet to be delivered to the multicast flow layer at BFR-E.
   The packet's BitString will be set to 0000, and the packet will not
   be forwarded any further.

6.7.  Equal Cost Multi-path Forwarding

   In many networks, the routing underlay will provide multiple equal
   cost paths from a given BFR to a given BFER.  When forwarding
   multicast packets through the network, it can be beneficial to take
   advantage of this by load balancing among those paths.  This feature
   is known as "equal cost multiple path forwarding", or "ECMP".

   BIER supports ECMP, but the procedures of Section 6.5 must be
   modified slightly.  Two ECMP procedures are defined.  In the first
   (described in Section 6.7.1), the choice among equal-cost paths taken
   by a given packet from a given BFR to a given BFER depends on (a) the
   packet's entropy, and (b) the other BFERs to which that packet is
   destined.  In the second (described in Section 6.7.2), the choice
   depends only upon the packet's entropy.



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   There are tradeoffs between the two forwarding procedures described
   here.  In the procedure of Section 6.7.1, the number of packet
   replications is minimized.  The procedure in Section 6.7.1 also uses
   less memory in the BFR.  In the procedure of Section 6.7.2, the path
   traveled by a given packet from a given BFR to a given BFER is
   independent of the other BFERs to which the packet is destined.
   While the procedures of Section 6.7.2 may cause more replications,
   they provide a more predictable behavior.

   The two procedures described here operate on identical packet formats
   and will interoperate correctly.  However, if deterministic behavior
   is desired, then all BFRs would need to use the procedure from
   Section 6.7.2.

6.7.1.  Non-deterministic ECMP

   Figure 6 shows the operation of non-deterministic ECMP in BIER.

         BFR-A BIFT            BFR-B BIFT            BFR-C BIFT
    -------------------   -------------------   -------------------
    | Id | F-BM | NBR |   | Id | F-BM | NBR |   | Id | F-BM | NBR |
    ===================   ===================   ===================
    | 1  | 0111 |  B  |   | 1  | 0011 |  C  |   | 1  | 0001 |  D  |
    -------------------   -------------------   -------------------
    | 2  | 0111 |  B  |   | 2  | 0011 |  C  |   | 2  | 0010 |  F  |
    -------------------   |    | 0110 |  E  |   -------------------
    | 3  | 0111 |  B  |   -------------------   | 3  | 1100 |  B  |
    -------------------   | 3  | 0110 |  E  |   -------------------
    | 4  | 1000 |  A  |   ------------------|   | 4  | 1100 |  B  |
    -------------------   | 4  | 1000 |  A  |   -------------------
                          -------------------

     ( A ) ------------ (  B  ) ------------ ( C ) ------------ ( D )
    4 (0:1000)              \                  \            1 (0:0001)
                             \                  \
                             ( E ) ------------ ( F )
                           3 (0:0100)         2 (0:0010)

                         Figure 6: Example of ECMP

   In this example, BFR-B has two equal cost paths to reach BFER-F, one
   via BFR-C and one via BFR-E.  Since the BFR-id of BFER-F is 2, this
   is reflected in entry 2 of BFR-B's BIFT.  Entry 2 shows that BFR-B
   has a choice of two BFR-NBRs for BFER-B, and that a different F-BM is
   associated with each choice.  When BFR-B looks up entry 2 in the
   BIFT, it can choose either BFR-NBR.  However, when following the
   procedures of Section 6.5, it MUST use the F-BM corresponding to the
   BFR-NBR that it chooses.



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   How the choice is made is an implementation matter.  However, the
   usual rules for ECMP apply: packets of a given flow SHOULD NOT be
   split among two paths, and any "entropy" field in the packet's
   encapsulation SHOULD be respected.

   Note however that by the rules of Section 6.5, any packet destined
   for both BFER-D and BFER-F will be sent via BFR-C.

6.7.2.  Deterministic ECMP

   With the procedures of Section 6.7.1, where ECMP paths exist, the
   path a packet takes to reach any particular BFER depends not only on
   routing and on the packet's entropy, but also on the set of other
   BFERs to which the packet is destined.

   For example consider the following scenario in the network of
   Figure 6.

   o  There is a sequence of packets being transmitted by BFR-A, some of
      which are destined for both D and F, and some of which are
      destined only for F.

   o  All the packets in this sequence have the same entropy value, call
      it "Q".

   o  At BFR-B, when a packet with entropy value Q is forwarded via
      entry 2 in the BIFT, the packet is sent to E.

   Using the forwarding procedure of Section 6.7.1, packets of this
   sequence that are destined for both D and F are forwarded according
   to entry 1 in the BIFT, and thus will reach F via the path A-B-C-F.
   However, packets of this sequence that are destined only for F are
   forwarded according to entry 2 in the BIFT, and thus will reach F via
   the path A-B-E-F.

   That procedure minimizes the number of packets transmitted by BFR B.
   However, consider the following scenario:

   o  Beginning at time t0, the multicast flow in question needs to be
      received ONLY by BFER-F;

   o  Beginning at a later time, t1, the flow needs to be received by
      both BFER-D and BFER-F.

   o  Beginning at a later time, t2, the no longer needs to be received
      by D, but still needs to be received by F.





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   Then from t0 until t1, the flow will travel to F via the path
   A-B-E-F.  From t1 until t2, the flow will travel to F via the path
   A-B-C-F.  And from t2, the flow will again travel to F via the path
   A-B-E-F.

   The problem is that if D repeatedly joins and leaves the flow, the
   flow's path from B to F will keep switching.  This could cause F to
   receive packets out of order.  It also makes troubleshooting
   difficult.  For example, if there is some problem on the E-F link,
   receivers at F will get good service when the flow is also going to D
   (avoiding the E-F link), but bad service when the flow is not going
   to D.  Since it is hard to know which path is being used at any given
   time, this may be hard to troubleshoot.  Also, it is very difficult
   to perform a traceroute that is known to follow the path taken by the
   flow at any given time.

   The source of this difficulty is that, in the procedures of
   Section 6.7.1, the path taken by a particular flow to a particular
   BFER depends upon whether there are lower numbered BFERs that are
   also receiving the flow.  Thus the choice among the ECMP paths is
   fundamentally non-deterministic.

   Deterministic forwarding can be achieved by using multiple BIFTs,
   such that each row in a BIFT has only one path to each destination,
   but the multiple ECMP paths to any particular destination are spread
   across the multiple tables.  When a BIER-encapsulated packet arrives
   to be forwarded, the BFR uses a hash of the BIER Entropy field to
   determine which BIFT to use, and then the normal BIER forwarding
   algorithm (as described in Sections 6.5 and 6.6) is used with the
   selected BIFT.

   As an example, suppose there are two paths to destination X (call
   them X1 and X2), and four paths to destination Y (call them Y1, Y2,
   Y3, and Y4).  If there are, say, four BIFTs, one BIFT would have
   paths X1 and Y1, one would have X1 and Y2, one would have X2 and Y3,
   and one would have X2 and Y4.  If traffic to X is split evenly among
   these four BIFTs, the traffic will be split evenly between the two
   paths to X; if traffic to Y is split evenly among these four BIFTs,
   the traffic will be split evenly between the four paths to Y.

   Note that if there are three paths to one destination and four paths
   to another, 12 BIFTs would be required in order to get even splitting
   of the load to each of those two destinations.  Of course, each BIFT
   uses some memory, and one might be willing to have less optimal
   splitting in order to have fewer BIFTs.  How that tradeoff is made is
   an implementation or deployment decision.





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6.8.  Prevention of Loops and Duplicates

   The BitString in a BIER-encapsulated packet specifies the set of
   BFERs to which that packet is to be forwarded.  When a BIER-
   encapsulated packet is replicated, no two copies of the packet will
   ever have a BFER in common.  If one of the packet's BFERs forwards
   the packet further, that will first clear the bit that identifies
   itself.  As a result, duplicate delivery of packets is not possible
   with BIER.

   As long as the routing underlay provides a loop free path between
   each pair of BFRs, BIER-encapsulated packets will not loop.  Since
   the BIER layer does not create any paths of its own, there is no need
   for any BIER-specific loop prevention techniques beyond the
   forwarding procedures specified in Section 6.5.

   If, at some time, the routing underlay is not providing a loop free
   path between BFIR-A and BFER-B, then BIER encapsulated packets may
   loop while traveling from BFIR-A to BFER-B.  However, such loops will
   never result in delivery of duplicate packets to BFER-B.

   These properties of BIER eliminate the need for the "reverse path
   forwarding" (RPF) check that is used in conventional IP multicast
   forwarding.

6.9.  When Some Nodes do not Support BIER

   The procedures of section Section 6.2 presuppose that, within a given
   BIER domain, all the nodes adjacent to a given BFR in a given routing
   underlay are also BFRs.  However, it is possible to use BIER even
   when this is not the case, as long as the ingress and egress nodes
   are BFRs.  In this section, we assume that the routing underlay is an
   SPF-based IGP that computes a shortest path tree from each node to
   all other nodes in the domain.

   At a given BFR, say BFR B, start with a copy of the IGP-computed
   shortest path tree from BFR B to each router in the domain.  (This
   tree is computed by the SPF algorithm of the IGP.)  Let's call this
   copy the "BIER-SPF tree rooted at BFR B."  BFR B then modifies this
   BIER-SPF tree as follows.

   1.  BFR B looks in turn at each of B's child nodes on the BIER-SPF
       tree.

   2.  If one of the child nodes does not support BIER, BFR B removes
       that node from the tree.  The child nodes of the node that has
       just been removed are then re-parented on the tree, so that BFR B
       now becomes their parent.



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   3.  BFR B then continues to look at each of its child nodes,
       including any nodes that have been re-parented to B as a result
       of the previous step.

   When all of the child nodes (the original child nodes plus any new
   ones) have been examined, B's children on the BIER-SPF tree will all
   be BFRs.

   When the BIFT is constructed, B's child nodes on the BIER-SPF tree
   are considered to be the BFR-NBRs.  The F-BMs and outgoing BIER-MPLS
   labels must be computed appropriately, based on the BFR-NBRs.

   B may now have BFR-NBRs that are not "directly connected" to B via
   layer 2.  To send a packet to one of these BFR-NBRs, B will have to
   send the packet through a unicast tunnel.  This may be as simple as
   finding the IGP unicast next hop to the child node, and pushing on
   (above the BIER-MPLS label advertised by the child) the MPLS label
   that the IGP next hop has bound to an address of the child node.  (If
   for some reason the unicast tunnel cannot be an MPLS tunnel, any
   other kind of tunnel can be used, as long as it is possible to
   encapsulate MPLS within that kind of tunnel.)

   Of course, the above is not meant as an implementation technique,
   just as a functional description.

   While the above description assumes that the routing underlay
   provides an SPF tree, it may also be applicable to other types of
   routing underlay.

   Note that the technique above can also be used to provide "node
   protection" (i.e., to provide fast reroute around nodes that are
   believed to have failed).  If BFR B has a failed BFR-NBR, B can
   remove the failed BFR-NBR from the BIER-SPF tree, and can then re-
   parent the child BFR-NBRs of the failed BFR-NBR so that they appear
   to be B's own child nodes on the tree (i.e., so that they appear to
   be B's BFR-NBRs).  Then the usual BIER forwarding procedures apply.
   However, getting the packet from B to the child nodes of the failed
   BFR-NBR is a bit more complicated, as it may require using a unicast
   bypass tunnel to get around the failed node.

   A simpler variant of step 2 above would be the following:

      If one of the child nodes does not support BIER, BFR B removes
      that node from the tree.  All BFERs that are reached through that
      child node are then re-parented on the tree, so that BFR B now
      becomes their parent.





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   This variant is simpler because the set of BFERs that are reached
   through a particular child node of B can be determined from the F-BM
   in the BIFT.  However, if this variant is used, the results are less
   optimal, because packets will be unicast directly from B to the BFERs
   that are reachable through the non-BIER child node.

   When using a unicast MPLS tunnel to get a packet to a BFR-NBR, it may
   be advantageous to (a) set the TTL of the MPLS label entry
   representing the "tunnel" to a large value, rather than copying the
   TTL value from the BIER-MPLS label, and (b) when the tunnel labels
   are popped off, to avoid copying the TTL from the tunnel labels to
   the BIER-MPLS label.  That way, the TTL of the BIER-MPLS label would
   only control the number of "BFR hops" that the packet may traverse.

6.10.  Use of Different BitStringLengths within a Domain

   When a BFIR imposes a BIER header on a particular packet, it uses the
   value of BitStringLength that it has been provisioned to use when
   imposing a BIER header.  For the BIER forwarding procedures to work
   properly, this BitStringLength must be supported by the intermediate
   BFRs and by the BFERs that may receive that packet.

   Suppose one wants to migrate the BitStringLength used in a particular
   domain from one value (X) to another value (Y).  The following
   migration procedure can be used.  First, upgrade all the BFRs in the
   domain so that they support both value X and value Y.  Once this is
   done, reprovision the BFIRs so that they use BitStringLength value Y.

   However, it is always possible that the following situation will
   occur.  Suppose a packet has been BIER-encapsulated with a
   BitStringLength value of X, and that the packet has arrived at BFR-A.
   How suppose that according to the routing underlay, the next hop is
   BFR-B, but BFR-B does not support the BitStringLength value of X.
   What should BFR-A do with the packet?  BFR-A has three choices.  It
   MUST be able to do one of the three, but the choice of which
   procedure to follow is a local matter.  The three choices are:

   o  BFR-A MAY discard the packet.

   o  BFR-A MAY re-encapsulate the packet, using a BIER header whose
      BitStringLength value is supported by BFR-B.  (Note that if BFR-B
      only supports BitStringLength values that are smaller than the
      BitStringLength value of the packet, this may require creating an
      additional copy of the packet.)

   o  BFR-A MAY treat BFR-B as if BFR-B did not support BIER at all, and
      apply the rules of Section 6.9.




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7.  IANA Considerations

   This document contains no actions for IANA.

8.  Security Considerations

   When BIER is paired with a particular multicast flow layer, it
   inherits the security considerations of that layer.  Similarly, when
   BIER is paired with a particular routing underlay, it inherits the
   security considerations of that layer.

   If the BIER encapsulation of a particular packet specifies an SI or a
   BitString other than the one intended by the BFIR, the packet is
   likely to be misdelivered.  If the BIER encapsulation of a packet is
   modified (through error or malfeasance) in a way other than that
   specified in this document, the packet may be misdelivered.

   If the procedures used for advertising BFR-ids and BFR-prefixes are
   not secure, an attack on those procedures may result in incorrect
   delivery of BIER-encapsulated packets.

   Every BFR must be provisioned to know which of its interfaces lead to
   a BIER domain and which do not.  If two interfaces lead to different
   BIER domains, the BFR must be provisioned to know that those two
   interfaces lead to different BIER domains.  If the provisioning is
   not correct, BIER-encapsulated packets from one BIER domain may
   "leak" into another; this is likely to result in misdelivery of
   packets.

9.  Acknowledgements

   The authors wish to thank Rajiv Asati, John Bettink, Ross Callon (who
   contributed much of the text on deterministic ECMP), Nagendra Kumar,
   Christian Martin, Neale Ranns, Greg Shepherd, Albert Tian, Ramji
   Vaithianathan, Xiaohu Xu and Jeffrey Zhang for their ideas and
   contributions to this work.

10.  Contributor Addresses

   Below is a list of other contributing authors in alphabetical order:

   Gregory Cauchie
   Bouygues Telecom

   Email: gcauchie@bouyguestelecom.fr

   Mach (Guoyi) Chen
   Huawei



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   Email: mach.chen@huawei.com


   Arkadiy Gulko
   Thomson Reuters
   195 Broadway
   New York  NY 10007
   US

   Email: arkadiy.gulko@thomsonreuters.com


   Wim Henderickx
   Alcatel-Lucent
   Copernicuslaan 50
   Antwerp 2018
   BE

   Email: wim.henderickx@alcatel-lucent.com

   Martin Horneffer
   Deutsche Telekom
   Hammer Str. 216-226
   Muenster 48153
   DE

   Email: Martin.Horneffer@telekom.de

   Uwe Joorde
   Deutsche Telekom
   Hammer Str. 216-226
   Muenster  D-48153
   DE

   Email: Uwe.Joorde@telekom.de

   Luay Jalil
   Verizon
   1201 E Arapaho Rd.
   Richardson, TX  75081
   US

   Email: luay.jalil@verizon.com

   Jeff Tantsura
   Ericsson
   300 Holger Way
   San Jose, CA  95134



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   US

   Email: jeff.tantsura@ericsson.com

11.  References

11.1.  Normative References

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

11.2.  Informative References

   [Boivie_Feldman]
              Boivie, R. and N. Feldman, "Small Group Multicast",
              (expired) draft-boivie-sgm-02.txt, February 2001.

   [ISIS_BIER_EXTENSIONS]
              Przygienda, T., Ginsberg, L., Aldrin, S., and J. Zhang,
              "OSPF Extensions for Bit Index Explicit Replication",
              internet-draft draft-przygienda-bier-isis-ranges-01.txt,
              October 2014.

   [MPLS_BIER_ENCAPS]
              Wijnands, IJ., "BIER Encapsulation for MPLS Networks",
              internet-draft draft-wijnands-mpls-bier-encaps-02.txt,
              December 2014.

   [OSPF_BIER_EXTENSIONS]
              Psenak, P., Kumar, N., Wijnands, IJ., Dolganow, A.,
              Przygienda, T., and J. Zhang, "OSPF Extensions for Bit
              Index Explicit Replication", internet-draft draft-psenak-
              ospf-bier-extensions-01.txt, October 2014.

   [RFC6513]  Rosen, E. and R. Aggarwal, "Multicast in MPLS/BGP IP
              VPNs", RFC 6513, February 2012.

   [RFC6514]  Aggarwal, R., Rosen, E., Morin, T., and Y. Rekhter, "BGP
              Encodings and Procedures for Multicast in MPLS/BGP IP
              VPNs", RFC 6514, February 2012.

Authors' Addresses









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   IJsbrand Wijnands (editor)
   Cisco Systems, Inc.
   De Kleetlaan 6a
   Diegem  1831
   BE

   Email: ice@cisco.com


   Eric C. Rosen (editor)
   Juniper Networks, Inc.
   10 Technology Park Drive
   Westford, Massachusetts  01886
   US

   Email: erosen@juniper.net


   Andrew Dolganow
   Alcatel-Lucent
   600 March Rd.
   Ottawa, Ontario  K2K 2E6
   CA

   Email: andrew.dolganow@alcatel-lucent.com


   Tony Przygienda
   Ericsson
   300 Holger Way
   San Jose, California  95134
   US

   Email: antoni.przygienda@ericsson.com


   Sam K Aldrin
   Huawei Technologies
   2330 Central Express Way
   Santa Clara, California
   US

   Email: aldrin.ietf@gmail.com








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