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Versions: (draft-wijnands-mpls-bier-encapsulation) 00 01 02 03 04 05 06 07 08

Internet Engineering Task Force                        IJ. Wijnands, Ed.
Internet-Draft                                       Cisco Systems, Inc.
Intended status: Experimental                              E. Rosen, Ed.
Expires: March 17, 2018                           Juniper Networks, Inc.
                                                             A. Dolganow
                                                                   Nokia
                                                             J. Tantsura
                                                              Individual
                                                               S. Aldrin
                                                            Google, Inc.
                                                               I. Meilik
                                                                Broadcom
                                                      September 13, 2017


 Encapsulation for Bit Index Explicit Replication in MPLS and non-MPLS
                                Networks
                 draft-ietf-bier-mpls-encapsulation-08

Abstract

   Bit Index Explicit Replication (BIER) is an architecture that
   provides optimal multicast forwarding through a "multicast domain",
   without requiring intermediate routers to maintain any per-flow state
   or to engage in an explicit tree-building protocol.  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.  The details of the encapsulation depend on the type
   of network used to realize the multicast domain.  This document
   specifies a BIER encapsulation that can be used in an MPLS network,
   or with slight differences, in a non-MPLS network.

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 https://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



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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on March 17, 2018.

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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  BIER Header . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  In MPLS Networks  . . . . . . . . . . . . . . . . . . . .   5
       2.1.1.  Encapsulation Initial Four Octets . . . . . . . . . .   5
         2.1.1.1.  The BIER-MPLS Label . . . . . . . . . . . . . . .   5
         2.1.1.2.  Other Fields of the Initial Four Octets . . . . .   7
       2.1.2.  Remainder of Encapsulation  . . . . . . . . . . . . .   8
       2.1.3.  Further Encapsulating a BIER Packet . . . . . . . . .  10
     2.2.  In Non-MPLS Networks  . . . . . . . . . . . . . . . . . .  11
       2.2.1.  Encapsulation Initial Four Octets . . . . . . . . . .  11
         2.2.1.1.  The BIFT-id . . . . . . . . . . . . . . . . . . .  11
         2.2.1.2.  Other Fields of the Initial Four Octets . . . . .  11
       2.2.2.  Remainder of Encapsulation  . . . . . . . . . . . . .  12
       2.2.3.  Further Encapsulating a BIER Packet . . . . . . . . .  13
   3.  Imposing and Processing the BIER Encapsulation  . . . . . . .  14
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  16
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  17
   7.  Contributor Addresses . . . . . . . . . . . . . . . . . . . .  17
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  19
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  19
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  20
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  21






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1.  Introduction

   [BIER_ARCH] describes a new architecture for the forwarding of
   multicast data packets.  Known as "Bit Index Explicit Replication"
   (BIER), that architecture provides optimal forwarding of multicast
   data packets through a "multicast domain".  It does so without
   requiring any explicit tree-building protocol and without requiring
   intermediate nodes to maintain any per-flow state.

   This document will use terminology defined in [BIER_ARCH].

   A router that supports BIER is known as a "Bit-Forwarding Router"
   (BFR).  A "BIER domain" is a connected set of Bit-Forwarding Routers
   (BFRs), each of which has been assigned a BFR-prefix.  A BFR-prefix
   is a routable IP address of a BFR, and is used by BIER to identify a
   BFR.  A packet enters a BIER domain at an ingress BFR (BFIR), and
   leaves the BIER domain at one or more egress BFRs (BFERs).  As
   specified in [BIER_ARCH], each BFR of a given BIER domain is
   provisioned to be in one or more "sub-domains" (SDs).  In the context
   of a given SD, each BFIR and BFER must have a BFR-id that is unique
   within that SD.  A BFR-id is just a number in the range [1,65535]
   that, relative to a BIER SD, identifies a BFR uniquely.

   As described in [BIER_ARCH], BIER requires that multicast data
   packets be encapsulated with a header that provides the information
   needed to support the BIER forwarding procedures.  This information
   includes the SD to which the packet has been assigned, a Set-Id (SI),
   a BitString, and a BitStringLength (BSL) Together these values are
   used to identify the set of BFERs to which the packet must be
   delivered.

   This document defines an encapsulation that can be used in either
   MPLS networks or non-MPLS networks.  However, the construction and
   processing of the BIER header is slightly different in MPLS networks
   than in non-MPLS networks.  In particular:

   o  The handling of certain fields in the encapsulation header (the
      "BIER header") is different depending upon whether the underlying
      network is an MPLS network or not.

   o  In an MPLS network, the first four octets of a BIER header is also
      the bottom entry (the last four octets) of an MPLS label stack.

   The MPLS-based encapsulation is explained in detail in Section 2.1.
   The differences between the MPLS-based encapsulation and the non-MPLS
   encapsulation is explained in Section 2.2.





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   Following the BIER header is the "payload".  The payload may be an
   IPv4 packet, an IPv6 packet, an ethernet frame, an MPLS packet, or an
   OAM packet.  (The use of BIER with other payload types is also
   possible, but is not further discussed in this document.)  The BIER
   header contains information (the Next Protocol field) identifying the
   type of the payload.

   If the payload is an MPLS packet, then an MPLS label stack
   immediately follows the BIER header.  The top label of this MPLS
   label stack may be either a downstream-assigned label [RFC3032] or an
   upstream-assigned label [RFC5331].

   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.  BIER Header

   The BIER header is shown in Figure 1.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |              BIFT-id                  | TC  |S|     TTL       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |Nibble |  Ver  |  BSL  |              Entropy                  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |OAM|Rsv|    DSCP   |   Proto   |            BFIR-id            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                BitString  (first 32 bits)                     ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     ~                                                               ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     ~                BitString  (last 32 bits)                      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                           Figure 1: BIER Header

   The BIFT-id represents a particular Bit Index Forwarding
   Table (BIFT); see Section 6.4 of [BIER_ARCH].  As explained in
   [BIER_ARCH], each BIFT corresponds to a particular combination of SD,
   BSL, and SI.

   Section 2.1 explains how the fields of the encapsulation header are
   used in MPLS networks.  For those fields that are used differently in
   non-MPLS networks, Section 2.2 explains the differences.





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   The default BitStringLength value for the encapsulations defined in
   this document is 256.  See Section 3 of [BIER_ARCH] for a discussion
   of the default BitStringLength value.

2.1.  In MPLS Networks

2.1.1.  Encapsulation Initial Four Octets

2.1.1.1.  The BIER-MPLS Label

   As stated in [BIER_ARCH], when a BIER domain is also an IGP domain,
   IGP extensions can be used by each BFR to advertise the BFR-id and
   BFR-prefix.  The extensions for OSPF are given in
   [OSPF_BIER_EXTENSIONS].  The extensions for ISIS are given in
   [ISIS_BIER_EXTENSIONS].

   When a particular BIER domain is both an IGP domain and an MPLS
   network, we assume that each BFR will also use IGP extensions to
   advertise a set of one or more "BIER-MPLS" labels.  When the domain
   contains a single SD, a given BFR needs to advertise one such label
   for each combination of SI and BSL.  If the domain contains multiple
   SDs, a BFR needs to advertise one such label per SI per BSL for each
   SD.

   In some environments, the only routing protocol in a BIER domain
   might be BGP; in this case, the BGP extensions described in
   [BGP_BIER_EXTENSIONS] can be used to advertise the necessary set of
   BIER-MPLS labels.

   The BIER-MPLS labels are locally significant (i.e., unique only to
   the BFR that advertises them) downstream-assigned MPLS labels.
   Penultimate hop popping ([RFC3031]) MUST NOT be applied to a BIER-
   MPLS label.

   Suppose for example that there is a single SD (the default SD), that
   the network is using a BSL of 256, and that all BFERs in the SD have
   BFR-ids in the range [1,512].  Since each BIER BitString is 256 bits
   long, this requires the use of two SIs: SI=0 and SI=1.  So each BFR
   will advertise, via IGP extensions, two MPLS labels for BIER: one
   corresponding to SI=0 and one corresponding to SI=1.  The
   advertisements of these labels will also bind each label to the
   default SD and to the BSL 256.

   As another example, suppose a particular BIER domain contains 2 SDs
   (SD 0 and SD 1), supports 2 BSLs (256 and 512), and contains 1024
   BFRs.  A BFR that is provisioned for both SDs, and that supports both
   BSLs, would have to advertise the following set of BIER-MPLS labels:




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      L1:   corresponding to SD 0, BSL 256, SI 0.

      L2:   corresponding to SD 0, BSL 256, SI 1.

      L3:   corresponding to SD 0, BSL 256, SI 2.

      L4:   corresponding to SD 0, BSL 256, SI 3.

      L5:   corresponding to SD 0, BSL 512, SI 0.

      L6:   corresponding to SD 0, BSL 512, SI 1.

      L7:   corresponding to SD 1, BSL 256, SI 0.

      L8:   corresponding to SD 1, BSL 256, SI 1.

      L9:   corresponding to SD 1, BSL 256, SI 2.

      L10:  corresponding to SD 1, BSL 256, SI 3.

      L11:  corresponding to SD 1, BSL 512, SI 0.

      L12:  corresponding to SD 1, BSL 512, SI 1.

   The above example should not be taken as implying that the BFRs need
   to advertise 12 individual labels.  For instance, instead of
   advertising a label for <SD 1, BSL 512, SI 0> and a label for <SD 1,
   BSL 512, SI 1>, a BFR could advertise a contiguous range of labels
   (in this case, a range containing exactly two labels) corresponding
   to <SD 1, BSL 512>.  The first label in the range could correspond to
   SI 0, and the second to SI 1.  The precise mechanism for generating
   and forming the advertisements is outside the scope of this document.
   See [OSPF_BIER_EXTENSIONS] and [ISIS_BIER_EXTENSIONS].

   The BIER-MPLS label corresponding to a particular combination of SD,
   SI, and BSL is interpreted as representing the BIFT that corresponds
   to that same combination of SD, SI, and BSL.  That is, the BIER-MPLS
   label performs the function of a BIFT-id.  This label value is
   carried in the BIFT-id field of the BIER encapsulation.

   It is crucial to understand that in an MPLS network, the first four
   octets of the BIER encapsulation header are also the last four octets
   of the MPLS header.  Therefore, any prior MPLS label stack entries
   MUST have the S bit (see [RFC3032]) clear (i.e., the S bit must be
   0).

   When a BFR receives an MPLS packet, and the next label to be
   processed is one of its BIER-MPLS labels, it will assume that the



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   remainder of the BIER header (see Section 2.1.2) immediately follows
   the stack.

   Note that in practice, labels only have to be assigned if they are
   going to be used.  If a particular BIER domain supports BSLs 256 and
   512, but some SD, say SD 1, only uses BSL 256, then it is not
   necessary to assign labels that correspond to the combination of SD 1
   and BSL 512.

2.1.1.2.  Other Fields of the Initial Four Octets

   S bit:

      When a BIER packet is traveling through an MPLS network, the high-
      order 20 bits of the initial four octets of the BIER encapsulation
      contain an MPLS label in the BIFT-id field.  These four octets are
      treated as the final entry in the packet's MPLS label stack.
      Hence the S bit (see [RFC3032]) MUST be set to 1.  If there are
      any MPLS label stack entries immediately preceding the BIER
      encapsulation, the S bit of those label stack entries MUST be set
      to 0.

   TC:

      The "Traffic Class" field ([RFC5462]) has its usual meaning in an
      MPLS label stack entry.

   TTL:

      This is the usual MPLS "Time to Live" field ([RFC3032]).  When a
      BIER packet is received, its "incoming TTL" (see below) is taken
      from this TTL field.

      The BFR MUST perform the MPLS TTL processing correctly.  If the
      packet is forwarded to one or more BFR adjacencies, the BIER-MPLS
      label carried by the forwarded packet MUST have a TTL field whose
      value is one less than that of the incoming TTL.

      Of course, if the incoming TTL is 1, the packet MUST be treated as
      a packet whose TTL has been exceeded.  The packet MUST NOT be
      forwarded, but it MAY be passed to other layers for processing
      (e.g., to cause an ICMP message to be generated, and/or to invoke
      BIER-specific traceroute procedures, and/or to invoke other OAM
      procedures.)







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2.1.2.  Remainder of Encapsulation

   Nibble:

      This field is set to the binary value 0101; this ensures that the
      MPLS ECMP logic will not confuse the remainder of the BIER header
      with an IP header or with the header of a pseudowire packet.  In
      an MPLS network, if a BFR receives a BIER packet with any other
      value in the first nibble after the label stack, it SHOULD discard
      the packet and log an error.

   Ver:

      This 4-bit field identifies the version of the BIER header.  This
      document specifies version 0 of the BIER header.  If a packet is
      received by a particular BFR, and that BFR does not support the
      specified version of the BIER header, the BFR MUST discard the
      packet and log an error.

      The value 0xF is reserved for experimental use; that value MUST
      NOT be assigned by any future IETF document or by IANA.

   BSL:

      This 4-bit field encodes the length in bits of the BitString.

      Note: When parsing the BIER header, a BFR MUST infer the length of
      the BitString from the BIFT-id, and MUST NOT infer it from the
      value of this field.  This field is present only to enable off-
      line tools (such as LAN analyzers) to parse the BIER header.

      If k is the length of the BitString, the value of this field is
      log2(k)-5.  However, only certain values are supported:

         1: 64 bits

         2: 128 bits

         3: 256 bits

         4: 512 bits

         5: 1024 bits

         6: 2048 bits

         7: 4096 bits




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      The value of this field MUST NOT be set to any value other than
      those listed above.  A received packet containing another value in
      this field SHOULD be discarded, and an error logged.  If the value
      in this field is other than what is expected based on the BIER-
      MPLS label, the packet SHOULD be discarded and an error logged.

   Entropy:

      This 20-bit field specifies an "entropy" value that can be used
      for load balancing purposes.  The BIER forwarding process may do
      equal cost load balancing, in which case the load balancing
      procedure MUST choose the same path for any two packets have the
      same entropy value.

      If a BFIR is encapsulating (as the payload) MPLS packets that have
      entropy labels, the BFIR MUST ensure that if two such packets have
      the same MPLS entropy label, they also have the same value of the
      BIER entropy field.

   OAM:

      These two bits are used for the passive performance measurement
      marking method described in [PPM].

   Rsv:

      These 2 bits are currently unused.  They SHOULD be set to zero
      upon transmission, and MUST be ignored upon reception.

   DSCP:

      By default, this 6-bit field is not used in MPLS networks.  The
      default behavior is that all 6 bits SHOULD be set to zero upon
      transmission, and MUST be ignored upon reception.

      Non-default use of this field in MPLS networks is outside the
      scope of this document.

   Proto:

      This 6-bit "Next Protocol" field identifies the type of the
      payload.  (The "payload" is the packet or frame immediately
      following the BIER header.)  IANA has been requested to create a
      registry of "BIER Next Protocol Identifiers".  This field is to be
      populated with the appropriate entry from that registry.






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      If a BFER receives a BIER packet, but does not recognize (or does
      not support) the value of the Next Protocol field, the BFER SHOULD
      discard the packet and log an error.

   BFIR-id:

      By default, this is the BFR-id of the BFIR, in the SD to which the
      packet has been assigned.  The BFR-id is encoded in the 16-bit
      field as an unsigned integer in the range [1,65535].

      Certain applications may require that the BFIR-id field contain
      the BFR-id of a BFR other than the BFIR.  However, that usage of
      the BFIR-id field is outside the scope of the current document.

   BitString:

      The BitString that, together with the packet's SI and SD,
      identifies the destination BFERs for this packet.  Note that the
      SI and SD for the packet are not carried explicitly in the BIER
      header, as a particular BIFT-id always corresponds to a particular
      SI and SD.

2.1.3.  Further Encapsulating a BIER Packet

   Sending a BIER packet from one BFR to another may require the packet
   to be further encapsulated.  For example: in some scenarios it may be
   necessary to encapsulate a BIER packet in an ethernet frame; in other
   scenarios it may be necessary to encapsulate a BIER packet in in a
   UDP packet.  In such cases, the BIER packet itself is the payload of
   an "outer" encapsulation.

   In this document, we assume that the frame or packet carrying a BIER
   packet as its payload is a unicast frame or packet.  That is,
   although a BIER packet is a multicast packet, we assume that the
   frame or packet carrying the BIER packet as its payload is unicast
   from one BFR to the next.

   Generally the outer encapsulation has a codepoint identifying the
   "next protocol".  The outer encapsulation's "next protocol" codepoint
   for MPLS MUST be used.  If a particular outer encapsulation has a
   codepoint for "MPLS with Downstream-Assigned Label" and a different
   codepoint for "MPLS with Upstream-Assigned Label", the codepoint for
   "MPLS with Downstream-Assigned Label" MUST be used.

   For example, if a BIER packet is encapsulated in an ethernet frame,
   the ethertype MUST be 0x8847 ([RFC5332]), which is the ethertype for
   a unicast ethernet frame that carries an MPLS packet whose label
   stack beings with a downstream-assigned label.



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   In the special case where the outer encapsulation is MPLS, the outer
   encapsulation has no "next protocol" codepoint.  All that is needed
   to encapsulate the BIER packet is to push more MPLS label stack
   entries (with S bit clear) on the BIER packet's label stack.

   If two BIER packets have the same value in the entropy field of their
   respective BIER headers, and if both are placed in an outer
   encapsulation, it is desirable for the outer encapsulation to
   preserve the fact that the two packets have the same entropy.  If the
   outer encapsulation is MPLS, and if the MPLS entropy label
   ([RFC6790]) is in use in a given deployment, one way to do this is to
   copy the value of the BIER header entropy field into an MPLS entropy
   label.

2.2.  In Non-MPLS Networks

2.2.1.  Encapsulation Initial Four Octets

2.2.1.1.  The BIFT-id

   In non-MPLS networks, a BIFT-id MUST be assigned for every
   combination of <SD, SI, BSL> that is to be used in that network.  The
   correspondence between a BIFT-id and a particular <SD, SI, BSL>
   triple is unique throughout the BIER domain, and is known to all the
   BFRs in the BIER domain.

   The means by which the BIFT-ids are assigned, and the means by which
   these assignments are made known to the BFRs, are outside the scope
   of this document.

   In an MPLS network, since the BIFT-id is an MPLS label, its value may
   be changed as a BIER packet goes from BFR to BFR.  In a non-MPLS
   network, since the BIFT-id is domain-wide unique, it is not expected
   to change as a BIER packet travels.

2.2.1.2.  Other Fields of the Initial Four Octets

   S bit:

      The S bit has no significance in a non-MPLS network.  It SHOULD be
      set to 1 upon transmission, but it MUST be ignored upon reception.

   TC:

      By default, the TC field has no significance in a non-MPLS
      network.  The default behavior is that this field SHOULD be set to
      the binary value 000 upon transmission, and MUST be ignored upon
      reception.



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      Non-default use of this field in non-MPLS networks is outside the
      scope of this document.

   TTL:

      This is the BIER "Time to Live" field.  Its purpose is to prevent
      BIER packets from looping indefinitely in the event of improper
      operation of the control plane.  When a BIER packet is received,
      its "incoming TTL" (see below) is taken from this TTL field.

      If the incoming TTL is 0 or 1, the packet MUST be treated as a
      packet whose TTL has been exceeded.  The packet MUST NOT be
      forwarded, but it MAY be passed to other layers for processing
      (e.g., to cause an ICMP message to be generated, and/or to invoke
      BIER-specific traceroute procedures, and/or to invoke other OAM
      procedures.)

      If the packet is forwarded to one or more BFR adjacencies, the TTL
      field of the packet MUST be set to a value that is one less than
      the value of the incoming TTL.

2.2.2.  Remainder of Encapsulation

   Nibble:

      This field SHOULD be set to 0000 upon transmission, but MUST be
      ignored upon reception.

   Ver:

      See Section 2.1.2.

   BSL:

      See Section 2.1.2.

   Entropy:

      See Section 2.1.2.

   OAM:

      See Section 2.1.2.

   Rsv:

      See Section 2.1.2.




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   DSCP:

      This 6-bit field MAY be used to hold a Differentiated Services
      Codepoint ([RFC2474]).  The significance of this field is outside
      the scope of this document.

   Proto:

      See Section 2.1.2.

   BFIR-id:

      See Section 2.1.2.

   BitString:

      See Section 2.1.2.

2.2.3.  Further Encapsulating a BIER Packet

   Sending a BIER packet from one BFR to another may require the packet
   to be further encapsulated.  For example: in some scenarios it may be
   necessary to encapsulate a BIER packet in an ethernet frame; in other
   scenarios it may be necessary to encapsulate a BIER packet in in a
   UDP packet.  In such cases, the BIER packet itself is the payload of
   an "outer" encapsulation.

   In this document, we assume that the frame or packet carrying a BIER
   packet as its payload is a unicast frame or packet.  That is,
   although a BIER packet is a multicast packet, we assume that the
   frame or packet carrying the BIER packet as its payload is unicast
   from one BFR to the next.

   Generally the outer encapsulation has a codepoint identifying the
   "next protocol".  This codepoint MUST be set to a value that means
   "Non-MPLS BIER".  In particular, a codepoint that means "MPLS" (with
   either upstream-assigned or downstream-assigned labels) MUST NOT be
   used.

   By requiring the use of a distinct codepoint for "non-MPLS BIER", we
   allow for deployment scenarios where non-MPLS BIER can coexist with
   non-BIER MPLS.  The BIFT-id values used by the former will not
   conflict with MPLS label values used by the latter.

   As an example, if a non-MPLS BIER packet is encapsulated in an
   ethernet header, the ethertype MUST NOT be 0x8847 or 0x8848
   ([RFC5332]).  Rather, a new ethertype would have to be assigned and




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   used.  Specification of the layer 2 codepoints to be used for the
   non-MPLS BIER encapsulation is outside the scope of this document.

   In the special case where the outer encapsulation is MPLS, the outer
   encapsulation has no "next protocol" codepoint.  If it is necessary
   to use MPLS as an outer encapsulation for BIER packets, it is
   RECOMMENDED to use the MPLS encapsulation for BIER.  Procedures for
   encapsulating a non-MPLS BIER packet in MPLS are outside the scope of
   this document.

   If two BIER packets have the same value in the entropy field of their
   respective BIER headers, and if both are placed in an outer
   encapsulation, it is desirable for the outer encapsulation to
   preserve the fact that the two packets have the same entropy.

3.  Imposing and Processing the BIER Encapsulation

   When a BFIR receives a multicast packet from outside the BIER domain,
   the BFIR carries out the following procedure:

   1.  By consulting the "multicast flow overlay" [BIER_ARCH], it
       determines the value of the "Proto" field.

   2.  By consulting the multicast flow overlay, it determines the set
       of BFERs that must receive the packet.

   3.  If more than one SD is supported, the BFIR assigns the packet to
       a particular SD.  Procedures for determining the SD to which a
       particular packet should be assigned are outside the scope of
       this document.

   4.  The BFIR looks up the BFR-id, in the given SD, of each of the
       BFERs.

   5.  The BFIR converts each such BFR-id into (SI, BitString) format,
       as described in [BIER_ARCH].

   6.  All such BFR-ids that have the same SI can be encoded into the
       same BitString.  Details of this encoding can be found in
       [BIER_ARCH].  For each distinct SI that occurs in the list of the
       packet's destination BFERs:

       a.   The BFIR makes a copy of the multicast data packet, and
            encapsulates the copy in a BIER header (see Section 2).  The
            BIER header contains the BitString that represents all the
            destination BFERs whose BFR-ids (in the given SD) correspond
            to the given SI.  It also contains the BFIR's BFIR-id in the
            SD to which the packet has been assigned.



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            N.B.: For certain applications, it may be necessary for the
            BFIR-id field to contain the BFR-id of a BFR other than the
            BFIR that is creating the header.  Such uses are outside the
            scope of this document.

       b.   The BFIR then applies to that copy the forwarding procedure
            of [BIER_ARCH].  This may result in one or more copies of
            the packet (possibly with a modified BitString) being
            transmitted to a neighboring BFR.

       c.   If the non-MPLS BIER encapsulation is being used, the BIFT-
            id field is set to the BIFT-id that corresponds to the
            packet's <SD, SI, BSL>.  The TTL is set according to policy.

            If the MPLS BIER encapsulation is being used, the BFIR finds
            the BIER-MPLS label that was advertised by the neighbor as
            corresponding to the given <SD, SI, BSL>.  An MPLS label
            stack is then prepended to the packet.  This label stack
            [RFC3032] will contain one label, the aforementioned BIER-
            MPLS label.  The "S" bit MUST be set, indicating the end of
            the MPLS label stack.  The TTL field of this label stack
            entry is set according to policy.

       d.   The packet may then be transmitted to the neighboring BFR.
            (In an MPLS network, this may result in additional MPLS
            labels being pushed on the stack.  For example, if an RSVP-
            TE tunnel is used to transmit packets to the neighbor, a
            label representing that tunnel would be pushed onto the
            stack.)

   When an intermediate BFR is processing a received MPLS packet, and
   one of the BFR's own BIER-MPLS labels rises to the top of the label
   stack, the BFR infers the BSL from the label.  The SI and SD are also
   implicitly identified by the label.  The BFR then follows the
   forwarding procedures of [BIER_ARCH].  If it forwards a copy of the
   packet to a neighboring BFR, it first swaps the label at the top of
   the label stack with the BIER-MPLS label, advertised by that
   neighbor, that corresponds to the same <SD, SI, BSL>.  Note that when
   this swap operation is done, the TTL field of the BIER-MPLS label of
   the outgoing packet MUST be one less than the "incoming TTL" of the
   packet, as defined in Section 2.1.1.1.

   When an intermediate BFR is processing a received non-MPLS BIER
   packet, the BFR infers the BSL from the BIFT-id.  The SI and SD are
   also implicitly identified by the BIFT-id.  The BFR then follows the
   forwarding procedures of [BIER_ARCH].





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   Note that if the BIER payload is an MPLS packet, the BIER header is
   followed by an MPLS label stack.  This stack is separate from any
   MPLS stack that may precede the BIER header.  For an example of an
   application where it is useful to carry an MPLS packet as the BIER
   payload, see [BIER_MVPN].

4.  IANA Considerations

   IANA is requested to set up a registry called "BIER Next Protocol
   Identifiers".  The registration policy for this registry is
   "Standards Action" ([RFC8126] and [RFC7120]).

   The initial values in the BIER Next Protocol Identifiers registry
   are:

   0: Reserved.

   1: MPLS packet with downstream-assigned label at top of stack.

   2: MPLS packet with upstream-assigned label at top of stack (see
      [RFC5331]).  If this value of the Proto field is used, the BFR-id
      of the BFIR must be placed in the BFIR-id field.  The BFIR-id
      provides the "context" in which the upstream-assigned label is
      interpreted.

   3: Ethernet frame.

   4: IPv4 packet.

   5: OAM packet [BIER-OAM].

   6: IPv6 packet.

   64:  Reserved.

5.  Security Considerations

   Insofar as this document makes use of MPLS, it inherits any security
   considerations that apply to the use of the MPLS data plane.

   Insofar as this document makes use of IGP extensions, it inherits any
   security considerations that apply to the IGP.

   The security considerations of [BIER_ARCH] also apply.







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6.  Acknowledgements

   The authors wish to thank Rajiv Asati, John Bettink, Nagendra Kumar,
   Christian Martin, Neale Ranns, Greg Shepherd, Ramji Vaithianathan,
   Xiaohu Xu, and Jeffrey Zhang for their ideas and contributions to
   this work.

7.  Contributor Addresses

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









































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   Mach (Guoyi) Chen
   Huawei

   Email: mach.chen@huawei.com

   Arkadiy Gulko
   Thomson Reuters
   195 Broadway
   New York  NY 10007
   United States

   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

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

   Email: Uwe.Joorde@telekom.de

   Tony Przygienda
   Juniper Networks, Inc.
   1194 N. Mathilda Ave.
   Sunnyvale, California  94089
   United States

   Email: prz@juniper.net







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8.  References

8.1.  Normative References

   [BIER_ARCH]
              Wijnands, IJ., Rosen, E., Dolganow, A., Przygienda, T.,
              and S. Aldrin, "Multicast using Bit Index Explicit
              Replication", internet-draft draft-ietf-bier-architecture-
              05, October 2016.

   [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>.

   [RFC2474]  Nichols, K., Blake, S., Baker, F., and D. Black,
              "Definition of the Differentiated Services Field (DS
              Field) in the IPv4 and IPv6 Headers", RFC 2474,
              DOI 10.17487/RFC2474, December 1998,
              <https://www.rfc-editor.org/info/rfc2474>.

   [RFC3031]  Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
              Label Switching Architecture", RFC 3031,
              DOI 10.17487/RFC3031, January 2001,
              <https://www.rfc-editor.org/info/rfc3031>.

   [RFC3032]  Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
              Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
              Encoding", RFC 3032, DOI 10.17487/RFC3032, January 2001,
              <https://www.rfc-editor.org/info/rfc3032>.

   [RFC5331]  Aggarwal, R., Rekhter, Y., and E. Rosen, "MPLS Upstream
              Label Assignment and Context-Specific Label Space",
              RFC 5331, DOI 10.17487/RFC5331, August 2008,
              <https://www.rfc-editor.org/info/rfc5331>.

   [RFC5332]  Eckert, T., Rosen, E., Ed., Aggarwal, R., and Y. Rekhter,
              "MPLS Multicast Encapsulations", RFC 5332,
              DOI 10.17487/RFC5332, August 2008,
              <https://www.rfc-editor.org/info/rfc5332>.

   [RFC5462]  Andersson, L. and R. Asati, "Multiprotocol Label Switching
              (MPLS) Label Stack Entry: "EXP" Field Renamed to "Traffic
              Class" Field", RFC 5462, DOI 10.17487/RFC5462, February
              2009, <https://www.rfc-editor.org/info/rfc5462>.






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   [RFC7120]  Cotton, M., "Early IANA Allocation of Standards Track Code
              Points", BCP 100, RFC 7120, DOI 10.17487/RFC7120, January
              2014, <https://www.rfc-editor.org/info/rfc7120>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

8.2.  Informative References

   [BGP_BIER_EXTENSIONS]
              Xu, X., Chen, M., Patel, K., Wijnands, I., and A.
              Przygienda, "BGP Extensions for BIER", internet-draft
              draft-ietf-bier-idr-extensions-02.txt, June 2017.

   [BIER-OAM]
              Kumar, N., Pignataro, C., Akiya, N., Zheng, L., Chen, M.,
              and G. Mirsky, "BIER Ping and Trace", internet-draft
              draft-ietf-bier-ping-02.txt, July 2017.

   [BIER_MVPN]
              Rosen, E., Ed., Sivakumar, M., Wijnands, IJ., Aldrin, S.,
              Dolganow, A., and T. Przygienda, "Multicast VPN Using
              Bier", internet-draft draft-ietf-bier-mvpn-05, January
              2017.

   [ISIS_BIER_EXTENSIONS]
              Ginsberg, L., Przygienda, T., Aldrin, S., and Z. Zhang,
              "BIER Support via ISIS", internet-draft draft-ietf-bier-
              isis-extensions-04.txt, March 2017.

   [OSPF_BIER_EXTENSIONS]
              Psenak, P., Kumar, N., Wijnands, IJ., Dolganow, A.,
              Przygienda, T., Zhang, Z., and S. Aldrin, "OSPF Extensions
              for Bit Index Explicit Replication", internet-draft draft-
              ietf-ospf-bier-extensions-05.txt, March 2017.

   [PPM]      Chen, M., Zheng, L., Mirsky, G., Fioccola, G., and T.
              Mizrahi, "IP Flow Performance Measurement Framework",
              draft-chen-ippm-coloring-based-ipfpm-framework-06 (work in
              progress), March 2016.

   [RFC6790]  Kompella, K., Drake, J., Amante, S., Henderickx, W., and
              L. Yong, "The Use of Entropy Labels in MPLS Forwarding",
              RFC 6790, DOI 10.17487/RFC6790, November 2012,
              <https://www.rfc-editor.org/info/rfc6790>.




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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

   Email: erosen@juniper.net


   Andrew Dolganow
   Nokia
   600 March Rd.
   Ottawa, Ontario  K2K 2E6
   Canada

   Email: andrew.dolganow@nokia.com


   Jeff Tantsura
   Individual

   Email: jefftant.ietf@gmail.com


   Sam K Aldrin
   Google, Inc.
   1600 Amphitheatre Parkway
   Mountain View, California
   United States

   Email: aldrin.ietf@gmail.com


   Israel Meilik
   Broadcom

   Email: israel@broadcom.com



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