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rfc:rfc8279

Internet Engineering Task Force (IETF) IJ. Wijnands, Ed. Request for Comments: 8279 Cisco Systems, Inc. Category: Experimental E. Rosen, Ed. ISSN: 2070-1721 Juniper Networks, Inc.

                                                           A. Dolganow
                                                                 Nokia
                                                         T. Przygienda
                                                Juniper Networks, Inc.
                                                             S. Aldrin
                                                          Google, Inc.
                                                         November 2017
       Multicast Using Bit Index Explicit Replication (BIER)

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 a
 protocol for explicitly building multicast distribution trees, 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 bit string 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.  The procedures for
 forwarding a packet based on its BIER header are specified in this
 document.  Elimination of the per-flow state and the explicit tree-
 building protocols results in a considerable simplification.

Wijnands, et al. Experimental [Page 1] RFC 8279 Multicast with BIER November 2017

Status of This Memo

 This document is not an Internet Standards Track specification; it is
 published for examination, experimental implementation, and
 evaluation.
 This document defines an Experimental Protocol for the Internet
 community.  This document is a product of the Internet Engineering
 Task Force (IETF).  It represents the consensus of the IETF
 community.  It has received public review and has been approved for
 publication by the Internet Engineering Steering Group (IESG).  Not
 all documents approved by the IESG are a candidate for any level of
 Internet Standard; see Section 2 of RFC 7841.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 https://www.rfc-editor.org/info/rfc8279.

Copyright Notice

 Copyright (c) 2017 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (https://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.

Wijnands, et al. Experimental [Page 2] RFC 8279 Multicast with BIER November 2017

Table of Contents

 1. Introduction ....................................................4
 2. The BFR Identifier and BFR-Prefix ...............................7
 3. Encoding BFR Identifiers in BitStrings ..........................8
 4. Layering .......................................................11
    4.1. The Routing Underlay ......................................11
    4.2. The BIER Layer ............................................12
    4.3. The Multicast Flow Overlay ................................13
 5. Advertising BFR-ids and BFR-Prefixes ...........................13
 6. BIER Intra-Domain Forwarding Procedures ........................15
    6.1. Overview ..................................................15
    6.2. BFR Neighbors .............................................17
    6.3. The Bit Index Routing Table ...............................18
    6.4. The Bit Index Forwarding Table ............................19
    6.5. The BIER Forwarding Procedure .............................20
    6.6. Examples of BIER Forwarding ...............................23
         6.6.1. Example 1 ..........................................23
         6.6.2. Example 2 ..........................................24
    6.7. Equal-Cost Multipath Forwarding ...........................26
         6.7.1. Non-deterministic ECMP .............................27
         6.7.2. Deterministic ECMP .................................28
    6.8. Prevention of Loops and Duplicates ........................29
    6.9. When Some Nodes Do Not Support BIER .......................30
    6.10. Use of Different BitStringLengths within a Domain ........33
         6.10.1. BitStringLength Compatibility Check ...............34
         6.10.2. Handling BitStringLength Mismatches ...............36
         6.10.3. Transitioning from One BitStringLength to
                 Another ...........................................36
 7. Operational Considerations .....................................37
    7.1. Configuration .............................................37
 8. IANA Considerations ............................................37
 9. Security Considerations ........................................38
 10. References ....................................................39
    10.1. Normative References .....................................39
    10.2. Informative References ...................................39
 Acknowledgements ..................................................40
 Contributors ......................................................41
 Authors' Addresses ................................................43

Wijnands, et al. Experimental [Page 3] RFC 8279 Multicast with BIER November 2017

1. Introduction

 This document specifies a new architecture for the forwarding of
 multicast data packets.  The architecture provides optimal forwarding
 of multicast data packets through a "multicast domain".  However, it
 does not require the use of a protocol for explicitly building
 multicast distribution trees, and it 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).  The BIER control-plane protocols (see Section 4.2) run within
 a "BIER domain", allowing the BFRs within that domain to exchange the
 information needed for them to forward packets to each other
 using BIER.
 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, for a
 given packet, a BFR may be a BFIR and/or a transit BFR and/or (one
 of) the BFER(s) for that packet.
 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 0").  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 positive integer.  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 will be
 sent.  The BFIR also determines the sub-domain in which the packet
 will be sent.  Determining the sub-domain in which a given packet

Wijnands, et al. Experimental [Page 4] RFC 8279 Multicast with BIER November 2017

 will be sent is known as "assigning the packet to a sub-domain".
 Procedures for choosing the sub-domain to which a particular packet
 is assigned are outside the scope of this document.  However, once a
 particular packet has been assigned to a particular sub-domain, it
 remains assigned to that sub-domain until it leaves the BIER domain.
 That is, the sub-domain to which a packet is assigned MUST NOT be
 changed while the packet is in flight through the BIER domain.
 Once the BFIR determines the sub-domain and the set of BFERs for a
 given packet, the BFIR 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 is 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 the 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 deployments 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

Wijnands, et al. Experimental [Page 5] RFC 8279 Multicast with BIER November 2017

 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.
 It is generally advantageous to assign the BFR-ids of a given
 sub-domain so that as many BFERs as possible can be represented in a
 single bit string.
 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.  BFR-A will then 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.
 Detailed procedures for forwarding a BIER-encapsulated packet through
 a BIER domain can be found in Section 6.
 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

Wijnands, et al. Experimental [Page 6] RFC 8279 Multicast with BIER November 2017

 limited to the number of neighboring BFRs; this can be much smaller
 than the number of BFERs.  See Section 6 (especially Section 6.5) for
 details.
 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
 "OPTIONAL" in this document are to be interpreted as described in
 BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
 capitals, as shown here.

2. The BFR Identifier and BFR-Prefix

 Each BFR MUST be assigned a single "BFR-prefix" for each sub-domain
 to which it belongs.  A BFR's BFR-prefix MUST be an IP address
 (either IPv4 or IPv6) of the BFR.  It is RECOMMENDED that the
 BFR-prefix be a loopback address of the BFR.
 If a BFR belongs to more than one sub-domain, it may (though it need
 not) have a different BFR-prefix in each sub-domain.
 All BFR-prefixes used within a given sub-domain MUST belong to the
 same address family (either IPv4 or IPv6).
 The BFR-prefix of a given BFR in a given sub-domain MUST be routable
 in that sub-domain.  Whether a particular BFR-prefix is routable in a
 given sub-domain depends on the "routing underlay" associated with
 that sub-domain.  The notion of "routing underlay" is described in
 Section 4.1.
 A "BFR Identifier" (BFR-id) is a number in the range [1,65535].
 Within a given sub-domain, every BFR that may need to function as a
 BFIR or BFER MUST have a single BFR-id, which identifies it uniquely
 within that sub-domain.  A BFR that does not need to function as a
 BFIR or BFER in a given sub-domain does not need to have a BFR-id in
 that sub-domain.
 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

Wijnands, et al. Experimental [Page 7] RFC 8279 Multicast with BIER November 2017

 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.
 In some deployments, it may not be possible to support (in a given
 sub-domain) the full range of 65,535 BFR-ids.  For example, if the
 BFRs in a given sub-domain only support 16 SIs and if they only
 support BitStringLengths of 256 or less, then only 16*256=4,096
 BFR-ids can be supported in that sub-domain.

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 of
 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.
 Within a given BIER domain (or even within a given BIER sub-domain),
 different values of BitStringLength may be used.  Each BFR MUST be
 provisioned to know the following:
 o  The BitStringLength ("Imposition BitStringLength") and sub-domain
    ("Imposition sub-domain") to use when it imposes (as a BFIR) a
    BIER encapsulation on a particular set of packets, and
 o  The BitStringLengths ("Disposition BitStringLengths") that it will
    process when (as a BFR or BFER) it receives packets from a
    particular sub-domain.
 It is not required that a BFIR use the same Imposition
 BitStringLength or the same Imposition sub-domain for all packets on
 which it imposes the BIER encapsulation.  However, if a particular
 BFIR is provisioned to use a particular Imposition BitStringLength
 and a particular Imposition sub-domain when imposing the
 encapsulation on a given set of packets, all other BFRs with BFR-ids
 in that sub-domain SHOULD be provisioned to process received BIER

Wijnands, et al. Experimental [Page 8] RFC 8279 Multicast with BIER November 2017

 packets with that BitStringLength (i.e., all other BFRs with BFR-ids
 in that sub-domain SHOULD be provisioned with that BitStringLength as
 a Disposition BitStringLength for that sub-domain).  Exceptions to
 this rule MAY be made under certain conditions; this is discussed in
 Section 6.10.
 When a BIER encapsulation is specified, the specification MUST define
 a default BitStringLength for the encapsulation.  Every BFIR
 supporting that encapsulation MUST be capable of being provisioned
 with that default BitStringLength as its Imposition BitStringLength.
 Every BFR and BFER supporting that encapsulation MUST be capable of
 being provisioned with that default BitStringLength as a Disposition
 BitStringLength.
 The specification of a BIER encapsulation MAY also allow the use of
 other BitStringLengths.
 If a BFR is capable of being provisioned with a given value of
 BitStringLength as an Imposition BitStringLength, it MUST also be
 capable of being provisioned with that same value as one of its
 Disposition BitStringLengths.  It SHOULD be capable of being
 provisioned with each legal smaller value of BitStringLength as (a)
 its Imposition BitStringLength, and (b) one of its Disposition
 BitStringLengths.
 In order to support transition from one BitStringLength to another,
 every BFR MUST be capable of being provisioned to simultaneously use
 two different Disposition BitStringLengths.
 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.)  Note that for a given
 BitStringLength, the total number of BFR-ids that can be represented
 is the product of the BitStringLength and the number of supported
 SIs.  For example, if a deployment uses (in a given sub-domain) a
 BitStringLength of 64 and supports 256 SIs, that deployment can only
 support 16384 BFR-ids in that sub-domain.  Even a deployment that
 supports 256 SIs will not be able to support 65,535 BFR-ids unless it
 uses a BitStringLength of at least 256.
 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

Wijnands, et al. Experimental [Page 9] RFC 8279 Multicast with BIER November 2017

 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.
 In order to minimize the number of copies that must be made of a
 given multicast packet, it is RECOMMENDED that the BFR-ids used in a
 given sub-domain be assigned "densely" (see Section 2) from the
 numbering space.  This will minimize the number of SIs that have to
 be used in that sub-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 intended either for the "east coast" or for
 the "west coast", but not for both coasts.  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]; that document also describes a very similar
 encapsulation that can be used in non-MPLS networks.

Wijnands, et al. Experimental [Page 10] RFC 8279 Multicast with BIER November 2017

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
 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
 BFR-A to BFER-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.  BIER could then be used to
 forward multicast data packets along the multicast-specific tree,
 while unicast packets follow the "ordinary" OSPF best path.  (In a
 case like this, many multicast flows could be traveling along a
 single tree, and the BitString carried by a particular packet would
 identify those nodes of the tree that need to receive that packet.)
 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.

Wijnands, et al. Experimental [Page 11] RFC 8279 Multicast with BIER November 2017

 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.
 In scenarios where External BGP (EBGP) is used as the IGP, the
 underlay adjacencies, by default, are the BGP adjacencies.
 Specification of the protocols and procedures of the routing underlay
 is outside the scope of this document.

4.2. The BIER Layer

 The BIER layer consists of the protocols 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 a given BFR uses to advertise, to
    all other BFRs in the same BIER domain:
  • its BFR-prefix;
  • its BFR-id in each sub-domain for which it has been provisioned

with a BFR-id;

  • the set of Disposition BitStringLengths it has been provisioned

to use for each sub-domain;

  • optionally, information about the routing underlay associated

with each sub-domain.

 o  The procedures used by a BFIR to impose a BIER header on a
    multicast data packet.
 o  The procedures for forwarding BIER-encapsulated packets and for
    modifying the BIER header during transit.
 o  The procedures used by a BFER to decapsulate a BIER packet and
    properly dispatch it.

Wijnands, et al. Experimental [Page 12] RFC 8279 Multicast with BIER November 2017

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 through
 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 are specified in [BIER_MVPN].
 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 (by provisioning) 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 (by provisioning) a BFR-prefix (for a given BIER domain)
 and must advertise this assignment to all the other BFRs in the
 domain.  Finally, each BFR has been provisioned to use a certain set
 of Disposition BitStringLengths for each sub-domain and must
 advertise these to all other BFRs in the domain.

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 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].)
 If, in a particular deployment, the BIER domain is not an OSPF or
 IS-IS domain, procedures suitable to the deployment must be used to
 advertise this information.  Details of the necessary procedures will
 be provided in companion documents.  For example, if BGP is the only
 routing algorithm used in the BIER domain, the procedures of
 [BGP_BIER_EXTENSIONS] may be used.
 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  Suppose that BFR-A has been provisioned with BFR-id N for a
    particular sub-domain but that it has not yet advertised its
    ownership of BFR-id N for that sub-domain.  Suppose also that it
    has received an advertisement from a different BFR (say BFR-B)
    that is advertising ownership of BFR-id N for the same sub-domain.
    In such a case, 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.

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    This procedure may prevent the accidental misconfiguration of a
    new BFR from impacting an existing BFR.
 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.  These rules are not intended to
 specify an implementation strategy; to conform to this specification,
 an implementation need only produce the same results that these rules
 produce.

6.1. Overview

 This section provides a brief overview of the BIER forwarding
 procedures.  Subsequent subsections specify the procedures in more
 detail.
 To forward a BIER-encapsulated packet:
 1.  Determine the packet's sub-domain.
 2.  Determine the packet's BitStringLength and BitString.
 3.  Determine the packet's SI.
 4.  From the sub-domain, the SI, and the BitString, determine the set
     of destination BFERs for the packet.
 5.  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.
 6.  It is possible that the packet's BitString will have one or more
     bits that correspond to BFR-ids that are not in use.  It is also
     possible that the packet's BitString will have one or more bits
     that correspond to BFERs that are unreachable, i.e., that have no
     next-hop adjacency.  In the following, we will consider the
     "next-hop adjacency" for all such bit positions to be the "null"
     next hop.
 7.  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.

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 8.  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 the
         next hop is the null next hop, the packet is discarded.)
 If a BFR receives a BIER-encapsulated packet whose <sub-domain, SI,
 BitString> triple identifies 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 bits set for other
 BFRs, 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 domain, the bit representing the BFR's own BFR-id
 MUST be clear in all the copies.
 When BIER on a BFER is to pass a packet to the multicast flow
 overlay, it of course decapsulates the packet by removing the BIER
 header.  However, it may be necessary to provide the multicast flow
 overlay with contextual information obtained from the BIER
 encapsulation.  The information that needs to pass between the BIER
 layer and the multicast flow overlay is specific to the multicast
 flow overlay.  Specification of the interaction between the BIER
 layer and the multicast flow overlay is outside the scope of this
 specification.
 If the BIER encapsulation contains a "Time to Live" (TTL) value, this
 value is not, by default, inherited by the payload.  If a particular
 multicast flow overlay needs to know the TTL value, this needs to be
 specified in whatever specification defines the interaction between
 BIER and that multicast flow overlay.
 If the BIER encapsulation contains a Traffic Class field, a
 Type of Service field, a Differentiated Services field, or any field
 of that sort, the value of that field is not, by default, passed to
 the multicast flow overlay.  If a particular multicast flow overlay
 needs to know the values of such fields, this fact needs to be
 specified in whatever specification defines the interaction between
 BIER and that multicast flow overlay.
 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.

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 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 overlay.
 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 (a) the sub-domain of the packet
 and (b) 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.  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.
    ( A ) ------------ ( B ) ------------ ( C ) ------------ ( D )
   4 (0:1000)             \                  \           1 (0:0001)
                           \                  \
                           ( E )              ( F )
                         3 (0:0100)         2 (0:0010)
                       Figure 1: BIER Topology 1
 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.
  1. ——————————————-

| 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

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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,
 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.
  1. ————————————

| 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 BIFT is programmed into the data plane and used to forward
 packets, applying the rules specified below in Section 6.5.

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6.5. The BIER Forwarding Procedure

 Below is the procedure that a BFR uses for forwarding a
 BIER-encapsulated packet.
 1.  Determine the packet's SI, BitStringLength, and sub-domain.
 2.  If the BitString consists entirely of zeroes, discard the packet;
     the forwarding process has been completed.  Otherwise, proceed to
     step 3.
 3.  Find the position (call it "k") of the least significant (i.e.,
     of the rightmost) bit that is set in the packet's BitString.
     (Remember, bits are numbered from 1, starting with the least
     significant bit.)
 4.  If bit k identifies the BFR itself, copy the packet, and send the
     copy to the multicast flow overlay.  Then clear bit k in the
     original packet, and go to step 2.  Otherwise, proceed to step 5.
 5.  Use the value k, together with the SI, sub-domain, and
     BitStringLength, as the "index" into the BIFT.
 6.  Extract from the BIFT the F-BM and the BFR-NBR.
 7.  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.  (If the BFR-NBR is null, the copy is just
     discarded.)  Note that when a packet is forwarded to a particular
     BFR-NBR, its BitString identifies only those BFERs that are to be
     reached via that BFR-NBR.
 8.  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.
 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.
 When a packet is sent to a particular BFR-NBR, the BitString is not
 the only part of the BIER header that needs to be modified.  If there
 is a TTL field in the BIER header, it will need to be decremented.
 In addition, when either of the encapsulations of [MPLS_BIER_ENCAPS]
 is used, the BIFT-id field is likely to require modification, based
 on signaling from the BFR-NBR to which the packet is being sent.  The

Wijnands, et al. Experimental [Page 20] RFC 8279 Multicast with BIER November 2017

 BIFT-id field of an incoming BIER packet implicitly identifies an SI,
 a sub-domain, and a BitStringLength.  If the packet is sent to a
 particular BFR-NBR, the BIFT-id field must be changed to whatever
 value that BFR-NBR has advertised for the same SI, sub-domain, and
 BitStringLength.  (If the encapsulation of Section 2.1 of
 [MPLS_BIER_ENCAPS] is used, this is essentially an MPLS label swap
 operation.)
 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 Multipath" (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
 sent 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.

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 The rules given in this section can be represented by the following
 pseudocode:
 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
 This pseudocode assumes 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.  It also assumes that the corresponding F-BM has only
 one bit set, the bit representing the BFER itself.  In this case, the
 "PacketSend" function sends the packet to the multicast flow overlay.
 This pseudocode also assumes that the F-BM for the null next hop
 contains a 1 in a given bit position if and only if that bit position
 corresponds to either an unused BFR-id or an unreachable BFER.  When
 the BFR-NBR is null, the "PacketSend" function discards the packet.

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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 Used in the Forwarding Examples

6.6.1. Example 1

 BFR-D, BFR-E, and BFR-F are BFERs.  BFR-A is the BFIR.  Suppose that
 BFIR-A has learned from the multicast flow overlay 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.
 BFIR-A then follows the procedures of Section 6.5, as follows:
 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 the 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.

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 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 0001 and a BFR-NBR of BFR-D itself.  This will cause a copy
 of the packet to be delivered to the multicast flow overlay 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 overlay 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
 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.  That is, the BitString (assuming again a
 BitStringLength of 4) is 0101.  To forward the packet, BFIR-A follows
 the procedures of Section 6.5, as follows:
 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.

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 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.
 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.

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 At BFER-D, the BIFT entry (not pictured) for BFR-id 1 will specify an
 F-BM of 0001 and a BFR-NBR of BFR-D itself.  This will cause a copy
 of the packet to be delivered to the multicast flow overlay 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 0100 and a BFR-NBR of BFR-E itself.  This will cause a copy
 of the packet to be delivered to the multicast flow overlay 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 Multipath 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 Multipath (ECMP) forwarding".
 BIER supports ECMP forwarding, 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) routing, (b) the packet's entropy, and (c) 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.
 There are trade-offs 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.

Wijnands, et al. Experimental [Page 26] RFC 8279 Multicast with BIER November 2017

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 Non-deterministic 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.
 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.

Wijnands, et al. Experimental [Page 27] RFC 8279 Multicast with BIER November 2017

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 flow no longer needs to be
    received by D, but still needs to be received by F.
 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,

Wijnands, et al. Experimental [Page 28] RFC 8279 Multicast with BIER November 2017

 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 trade-off is made
 is an implementation or deployment decision.

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 BFER will first clear the bit that
 identifies itself.  As a result, duplicate delivery of packets is not
 possible with BIER.

Wijnands, et al. Experimental [Page 29] RFC 8279 Multicast with BIER November 2017

 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 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 describe procedures that can be used
 if 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 its 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.
 3.  BFR-B then continues to look at each of its child nodes,
     including any nodes that have been re-parented to BFR-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, BFR-B's children on the BIER-SPF tree will
 all be BFRs.

Wijnands, et al. Experimental [Page 30] RFC 8279 Multicast with BIER November 2017

 When the BIFT is constructed, BFR-B's child nodes on the BIER-SPF
 tree are considered to be the BFR-NBRs.  The F-BMs must be computed
 appropriately, based on the BFR-NBRs.
 BFR-B may now have BFR-NBRs that are not "directly connected" to
 BFR-B via Layer 2.  To send a packet to one of these BFR-NBRs, BFR-B
 will have to send the packet through a unicast tunnel.  In an MPLS
 network, this may be as simple as finding the IGP unicast next hop to
 the child node and pushing on (above the BIER encapsulation header)
 an MPLS label that the IGP next hop has bound to an address of the
 child node.  (This assumes that the packet is using an MPLS-based
 BIER encapsulation, such as the one specified in Section 2.1 of
 [MPLS_BIER_ENCAPS].)  Of course, the BIFT-id in the BIER
 encapsulation header must be the BIFT-id advertised by the child node
 for the packet's SI, sub-domain, and BitStringLength.
 If for some reason the unicast tunnel cannot be an MPLS tunnel, any
 other kind of tunnel can be used, as long as the encapsulation for
 that tunnel type has a way of indicating that the payload is a
 BIER-encapsulated packet.
 Note that if a BIER-encapsulated packet is not using an MPLS-based
 BIER encapsulation, it will not be possible to send it through an
 MPLS tunnel unless it is known that the tunnel only carries BIER
 packets; this is because MPLS has no "next protocol type" field.
 This is not a problem if an MPLS-based BIER encapsulation is used,
 because in that case the BIER encapsulation begins with an MPLS label
 that identifies the packet as a BIER-encapsulated packet.
 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 underlays.
 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, BFR-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 BFR-B's own
 child nodes on the tree (i.e., so that they appear to be BFR-B's
 BFR-NBRs).  The usual BIER forwarding procedures then apply.
 However, getting the packet from BFR-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.

Wijnands, et al. Experimental [Page 31] RFC 8279 Multicast with BIER November 2017

 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.
 This variant is simpler because the set of BFERs that are reached
 through a particular child node of BFR-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 BFR-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:
 o  The TTL of the MPLS label entry representing the tunnel SHOULD be
    set to a large value, rather than being copied from the TTL value
    from the BIER encapsulation header, and
 o  When the tunnel labels are popped off, the TTL from the tunnel
    labels SHOULD NOT be copied to the BIER encapsulation header.
 In other words, the TTL processing for the tunnel SHOULD be as
 specified in [RFC3443] for "Pipe Model" and "Short Pipe Model" Label
 Switched Paths (LSPs).  The same principle applies if the tunnels are
 not MPLS tunnels; the BIER packet SHOULD NOT inherit the TTL from the
 tunnel encapsulation.  That way, the TTL of the BIER encapsulation
 header constrains only the number of BFRs that the packet may
 traverse, not the total number of hops.
 If two BIER packets have the same value in the entropy field of their
 respective BIER headers and if both are transmitted through a given
 tunnel, it is desirable for the tunnel encapsulation to preserve the
 fact that the two packets have the same entropy.
 The material in this section presupposes that if a given router is a
 BFR, then it supports BIER on all its interfaces.  It is, however,
 possible that a router will have some line cards that support BIER
 and some that do not.  In such a case, one can think of the router as
 a "partial BFR" that supports BIER only on some of its interfaces.
 If it is desired to deploy such partial BFRs, one can use the
 multi-topology features of the IGP to set up a BIER-specific
 topology.  This topology would exclude all the non-BIER-capable
 interfaces that attach to BFRs.  BIER would then have to be run in a
 sub-domain that is bound to this topology.  If unicast tunnels are
 used to bypass non-BFRs, either (a) the tunnels have to be restricted
 to this topology or (b) the tunnel endpoints have to be BFRs that do
 not have any non-BIER-capable interfaces.

Wijnands, et al. Experimental [Page 32] RFC 8279 Multicast with BIER November 2017

6.10. Use of Different BitStringLengths within a Domain

 The procedures of this section apply only when the same encapsulation
 is used throughout the BIER domain.  Consideration of the scenario
 where both multiple encapsulations and multiple BitStringLengths are
 used in a given BIER domain is outside the scope of this document.
 It is possible for different BFRs within a BIER domain to be using
 different Imposition and/or Disposition BitStringLengths.  As stated
 in Section 3:
 "if a particular BFIR is provisioned to use a particular Imposition
 BitStringLength and a particular Imposition sub-domain when imposing
 the encapsulation on a given set of packets, all other BFRs with
 BFR-ids in that sub-domain SHOULD be provisioned to process received
 BIER packets with that BitStringLength (i.e., all other BFRs with
 BFR-ids in that sub-domain SHOULD be provisioned with that
 BitStringLength as a Disposition BitStringLength for that
 sub-domain)."
 Note that mis-provisioning can result in "black holes".  If a BFIR
 creates a BIER packet with a particular BitStringLength and if that
 packet needs to travel through a BFR that cannot process received
 BIER packets with that BitStringLength, then it may be impossible to
 forward the packet to all of the BFERs identified in its BIER header.
 Section 6.10.1 defines a procedure, the "BitStringLength
 Compatibility Check", that can be used to detect the possibility of
 such black holes.
 However, failure of the BitStringLength Compatibility Check does not
 necessarily result in the creation of black holes; Section 6.10.2
 specifies OPTIONAL procedures that allow BIER forwarding to proceed
 without black holes, even if the BitStringLength Compatibility Check
 fails.
 If the procedures of Section 6.10.2 are not deployed but the
 BitStringLength Compatibility Check fails at some BFIR, the BFIR has
 two choices:
 o  Create BIER packets with the provisioned Imposition
    BitStringLength, even though the packets may not be able to reach
    all the BFERs identified in their BitStrings.
 o  Use an Imposition BitStringLength that passes the Compatibility
    Check (assuming that there is one), even if this is not the
    provisioned Imposition BitStringLength.

Wijnands, et al. Experimental [Page 33] RFC 8279 Multicast with BIER November 2017

 Section 6.10.1 discusses the implications of making one or the other
 of these choices.
 There will be times when an operator wishes to change the
 BitStringLengths used in a particular BIER domain.  Section 6.10.3
 specifies a simple procedure that can be used to transition a BIER
 domain from one BitStringLength to another.

6.10.1. BitStringLength Compatibility Check

 When a BFIR needs to encapsulate a packet, the BFIR first assigns the
 packet to a sub-domain.  The BFIR then chooses an Imposition
 BitStringLength L for the packet.  The choice of Imposition
 BitStringLength is determined by provisioning.  However, the BFIR
 should also perform the BitStringLength Compatibility Check defined
 below.
 The combination of sub-domain S and Imposition BitStringLength L
 passes the BitStringLength Compatibility Check if and only if the
 following condition holds:
    Every BFR that has advertised its membership in sub-domain S has
    also advertised that it is using Disposition BitStringLength L
    (and possibly other BitStringLengths as well) in that sub-domain.
    (If MPLS encapsulation (Section 2.1 of [MPLS_BIER_ENCAPS]) is
    being used, this means that every BFR that is advertising a label
    for sub-domain S is advertising a label for the combination of
    sub-domain S and Disposition BitStringLength L.)
 If a BFIR has been provisioned to use a particular Imposition
 BitStringLength and a particular sub-domain for some set of packets,
 and if that combination of Imposition BitStringLength and sub-domain
 does not pass the BitStringLength Compatibility Check, the BFIR
 SHOULD log this fact as an error.  It then has the following two
 choices about what to do with the packets:
 1.  The BFIR MAY use the provisioned Imposition BitStringLength
     anyway.  If the procedure of either option 2 or option 3 of
     Section 6.10.2 is deployed, this will not cause black holes and
     may actually be the optimal result.  It should be understood,
     though, that the BFIR cannot determine by signaling whether those
     procedures have been deployed.
 2.  If the BFIR is capable of using an Imposition BitStringLength
     that does pass the BitStringLength Compatibility Check for the
     particular sub-domain, the BFIR MAY use that Imposition
     BitStringLength instead.

Wijnands, et al. Experimental [Page 34] RFC 8279 Multicast with BIER November 2017

 Which of these two choices to make is itself determined by
 provisioning.
 Note that discarding the packets is not one of the allowable choices.
 Suppose, for example, that all the BFIRs are provisioned to use
 Imposition BitStringLength L for a particular sub-domain S but one
 BFR has not been provisioned to use Disposition BitStringLength L for
 sub-domain S.  This will cause the BitStringLength Compatibility
 Check to fail.  If the BFIR sends packets with BitStringLength L and
 sub-domain S, the mis-provisioned BFR will not be able to forward
 those packets, and thus the packets may only be able to reach a
 subset of the BFERs to which they are destined.  However, this is
 still better than having the BFIRs drop the packets; if the BFIRs
 discard the packets, the packets won't reach any of the BFERs to
 which they are destined at all.
 If the procedures of Section 6.10.2 have not been deployed, choice 2
 above might seem like a better option.  However, there might not be
 any Imposition BitStringLength that a given BFIR can use that also
 passes the BitStringLength Compatibility Check.  If it is desired to
 use choice 2 in a particular deployment, then there should be a
 "Fallback Disposition BitStringLength" (call it "F") such that:
 o  Every BFR advertises that it uses BitStringLength F as a
    Disposition BitStringLength for every sub-domain, and
 o  If a BFIR is provisioned to use Imposition BitStringLength X and
    Imposition sub-domain S for a certain class of packets but the
    BitStringLength Compatibility Check fails for the combination of
    BitStringLength X and sub-domain S, then the BFIR will fall back
    to using BitStringLength F as the Imposition BitStringLength
    whenever the Imposition sub-domain is S.
 It is RECOMMENDED that the value of F be the default BitStringLength
 for the encapsulation being used.

Wijnands, et al. Experimental [Page 35] RFC 8279 Multicast with BIER November 2017

6.10.2. Handling BitStringLength Mismatches

 Suppose that a packet has been BIER-encapsulated with a
 BitStringLength value of X and that the packet has arrived at BFR-A.
 Now suppose that according to the routing underlay the next hop is
 BFR-B, but BFR-B is not using X as one of its Disposition
 BitStringLengths.  What should BFR-A do with the packet?  BFR-A has
 three options.  It MUST do one of the three, but the choice of which
 procedure to follow is a local matter.  The three options are:
 1.  BFR-A MAY discard the packet.
 2.  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 uses Disposition BitStringLength values
     that are smaller than the BitStringLength value of the packet,
     this may require creating additional copies of the packet.
     Whether additional copies actually have to be created depends
     upon the bits that are actually set in the original packet's
     BitString.
 3.  BFR-A MAY treat BFR-B as if BFR-B did not support BIER at all,
     in which case BFR-A applies the rules of Section 6.9.
 Note that there is no signaling that enables a BFR to advertise which
 of the three options it will use.
 Option 2 can be useful if there is a region of the BIER domain where
 the BFRs are capable of using a long BitStringLength as well as a
 region where the BFRs are only capable of using a shorter
 BitStringLength.

6.10.3. Transitioning from One BitStringLength to Another

 Suppose one wants to migrate the BitStringLength used in a particular
 BIER domain from one value (X) to another value (Y).  The following
 migration procedure can be used.  This procedure allows the BFRs to
 be reprovisioned one at a time and does not require a "flag day".
 1.  Upgrade all the BFRs in the domain so that they use both value X
     and value Y as their Disposition BitStringLengths.
 2.  Reprovision the BFIRs so that they use BitStringLength value Y as
     the Imposition BitStringLength.
 3.  One may then optionally reprovision all the BFRs so that they no
     longer use Disposition BitStringLength X.

Wijnands, et al. Experimental [Page 36] RFC 8279 Multicast with BIER November 2017

7. Operational Considerations

 BIER offers a radical simplification over current IP multicast
 operations: no tree-building control plane, no per-flow forwarding
 state, no Reverse Path Forwarding (RPF), no Rendezvous Point (RP),
 etc.  BIER packet forwarding/replication is along the unicast paths
 to each bit position set in the packet, ensuring that the
 encapsulated multicast packets follow the same path as unicast to
 each set bit in the header.  The BIER FIB can be derived from the
 SPF-calculated unicast FIB or from any other forwarding-path
 calculation in or out of band.  Each bit will follow this unicast
 path from the entry point of the BIER domain to the edge device with
 that assigned bit.
 Due to these differences, operational expectations from traditional
 multicast solutions do not apply to a BIER domain.  There is no
 granular per-flow state at each node defining a tree.  Monitoring
 flows at the forwarding-plane level ((S,G) entries) is not provided
 in a BIER node.  BIER FIB packet counters may be maintained for
 BFR-ids or next-hop neighbors.  Any flow-based metrics will require
 deeper packet inspection; this topic is outside the scope of this
 document.  In this way, BIER is again more like unicast.
 It is this reduction in state that allows for one of the key
 operational benefits of BIER: deterministic convergence.  The BIER
 FIB can converge immediately after the unicast FIB regardless of how
 many multicast flows are transiting the links.  Careful monitoring of
 (S,G) utilization is not required within a BIER domain.

7.1. Configuration

 A BIER domain requires that each edge node (BFER) be given a unique
 bit position in the BIER mask (BFR-id).  The BFR-id must be
 configured on each BFER and associated with a unique IP address of
 that BFER.  Any existing manual or automated configuration tools must
 provide access to BIER-specific configuration.  The association of
 the BFR-id with a unique address of the BFER to which it is assigned
 must also be advertised into the IGP of the BIER domain.  This may be
 implied from the BIER configuration or require IGP-specific
 configuration.  This document does not dictate any specific
 configuration methodology.

8. IANA Considerations

 This document does not require any IANA actions.

Wijnands, et al. Experimental [Page 37] RFC 8279 Multicast with BIER November 2017

9. Security Considerations

 When BIER is paired with a particular multicast flow overlay, 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.  Some
 modifications of the BIER encapsulation, e.g., setting every bit in
 the BitString, may result in (intentional or unintentional)
 denial-of-service (DoS) attacks.
 If a BFIR is compromised, it may impose a BIER encapsulation with all
 the bits in the BitString set; this would also result in a DoS
 attack.
 Every BFR MUST be provisioned to know which of its interfaces lead to
 a BIER domain and which do not.  BIER-encapsulated packets MUST NOT
 be accepted from outside the BIER domain.  (Reception of
 BIER-encapsulated packets from outside the BIER domain would create
 an attack vector for DoS attacks, as an attacker might set all the
 bits in the BitString.)
 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.
 DoS attacks may also result from incorrect provisioning (through
 error or malfeasance) of the BFRs.
 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.

Wijnands, et al. Experimental [Page 38] RFC 8279 Multicast with BIER November 2017

10. References

10.1. Normative References

 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119,
            DOI 10.17487/RFC2119, March 1997,
            <https://www.rfc-editor.org/info/rfc2119>.
 [RFC3443]  Agarwal, P. and B. Akyol, "Time To Live (TTL) Processing
            in Multi-Protocol Label Switching (MPLS) Networks",
            RFC 3443, DOI 10.17487/RFC3443, January 2003,
            <https://www.rfc-editor.org/info/rfc3443>.
 [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in
            RFC 2119 Key Words", BCP 14, RFC 8174,
            DOI 10.17487/RFC8174, May 2017,
            <https://www.rfc-editor.org/info/rfc8174>.

10.2. Informative References

 [BGP_BIER_EXTENSIONS]
            Xu, X., Ed., Chen, M., Patel, K., Wijnands, IJ., and A.
            Przygienda, "BGP Extensions for BIER", Work in Progress,
            draft-ietf-bier-idr-extensions-03, August 2017.
 [BIER_MVPN]
            Rosen, E., Ed., Sivakumar, M., Aldrin, S., Dolganow, A.,
            and T. Przygienda, "Multicast VPN Using BIER", Work in
            Progress, draft-ietf-bier-mvpn-09, November 2017.
 [Boivie_Feldman]
            Boivie, R. and N. Feldman, "Small Group Multicast", Work
            in Progress, draft-boivie-sgm-02, February 2001.
 [ISIS_BIER_EXTENSIONS]
            Ginsberg, L., Ed., Przygienda, A., Aldrin, S., and J.
            Zhang, "BIER Support via ISIS", Work in Progress,
            draft-ietf-bier-isis-extensions-06, October 2017.
 [MPLS_BIER_ENCAPS]
            Wijnands, IJ., Ed., Rosen, E., Ed., Dolganow, A.,
            Tantsura, J., Aldrin, S., and I. Meilik, "Encapsulation
            for Bit Index Explicit Replication in MPLS and non-MPLS
            Networks", Work in Progress, draft-ietf-bier-mpls-
            encapsulation-12, October 2017.

Wijnands, et al. Experimental [Page 39] RFC 8279 Multicast with BIER November 2017

 [OSPF_BIER_EXTENSIONS]
            Psenak, P., Ed., Kumar, N., Wijnands, IJ., Dolganow, A.,
            Przygienda, T., Zhang, J., and S. Aldrin, "OSPF Extensions
            for BIER", Work in Progress, draft-ietf-bier-ospf-bier-
            extensions-09, October 2017.
 [RFC6513]  Rosen, E., Ed., and R. Aggarwal, Ed., "Multicast in
            MPLS/BGP IP VPNs", RFC 6513, DOI 10.17487/RFC6513,
            February 2012, <https://www.rfc-editor.org/info/rfc6513>.
 [RFC6514]  Aggarwal, R., Rosen, E., Morin, T., and Y. Rekhter, "BGP
            Encodings and Procedures for Multicast in MPLS/BGP IP
            VPNs", RFC 6514, DOI 10.17487/RFC6514, February 2012,
            <https://www.rfc-editor.org/info/rfc6514>.

Acknowledgements

 The authors wish to thank Rajiv Asati, Alia Atlas, John Bettink, Ross
 Callon (who contributed much of the text on deterministic ECMP),
 Nagendra Kumar, Christian Martin, Neale Ranns, Albert Tian, Ramji
 Vaithianathan, Xiaohu Xu, and Jeffrey Zhang for their ideas and
 contributions to this work.
 The authors also wish to thank Sue Hares, Victor Kuarsingh, and Dan
 Romascanu for their reviews of this document.

Wijnands, et al. Experimental [Page 40] RFC 8279 Multicast with BIER November 2017

Contributors

 The following people contributed significantly to the content of this
 document and should be considered co-authors:
 Gregory Cauchie
 Bouygues Telecom
 Email: gcauchie@bouyguestelecom.fr
 Mach(Guoyi) Chen
 Huawei
 Email: mach.chen@huawei.com
 Arkadiy Gulko
 Thomson Reuters
 195 Broadway
 New York, NY  10007
 United States of America
 Email: arkadiy.gulko@thomsonreuters.com
 Wim Henderickx
 Nokia
 Copernicuslaan 50
 Antwerp  2018
 Belgium
 Email: wim.henderickx@nokia.com
 Martin Horneffer
 Deutsche Telekom
 Hammer Str. 216-226
 Muenster  48153
 Germany
 Email: Martin.Horneffer@telekom.de
 Luay Jalil
 Verizon
 1201 East Arapaho Rd.
 Richardson, TX  75081
 United States of America
 Email: luay.jalil@verizon.com
 Uwe Joorde
 Deutsche Telekom
 Hammer Str. 216-226
 Muenster  D-48153
 Germany
 Email: Uwe.Joorde@telekom.de

Wijnands, et al. Experimental [Page 41] RFC 8279 Multicast with BIER November 2017

 Greg Shepherd
 Cisco Systems
 170 West Tasman Drive
 San Jose, CA  95134
 United States of America
 Email: shep@cisco.com
 Jeff Tantsura
 Email: jefftant.ietf@gmail.com

Wijnands, et al. Experimental [Page 42] RFC 8279 Multicast with BIER November 2017

Authors' Addresses

 IJsbrand Wijnands (editor)
 Cisco Systems, Inc.
 De Kleetlaan 6a
 Diegem  1831
 Belgium
 Email: ice@cisco.com
 Eric C. Rosen (editor)
 Juniper Networks, Inc.
 10 Technology Park Drive
 Westford, Massachusetts  01886
 United States of America
 Email: erosen@juniper.net
 Andrew Dolganow
 Nokia
 438B Alexandra Rd #08-07/10
 Alexandra Technopark
 Singapore  119968
 Singapore
 Email: andrew.dolganow@nokia.com
 Tony Przygienda
 Juniper Networks, Inc.
 1194 N. Mathilda Ave.
 Sunnyvale, California  94089
 United States of America
 Email: prz@juniper.net
 Sam K. Aldrin
 Google, Inc.
 1600 Amphitheatre Parkway
 Mountain View, California  94043
 United States of America
 Email: aldrin.ietf@gmail.com

Wijnands, et al. Experimental [Page 43]

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