Internet Engineering Task Force                        IJ. Wijnands, Ed.
Internet-Draft                                             E. Rosen, Ed.
Intended status: Standards Track                     Cisco Systems, Inc.
Expires: March 26, 2015                                      A. Dolganow
                                                             J. Tantsura
                                                      September 22, 2014

   Encapsulation for Bit Index Explicit Replication in MPLS Networks


   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 the BIER encapsulation to be used in an MPLS network.

Status of This Memo

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   This Internet-Draft will expire on March 26, 2015.

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Copyright Notice

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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  The BIER-MPLS Label . . . . . . . . . . . . . . . . . . . . .   3
   3.  BIER Header . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Imposing and Processing the BIER Encapsulation  . . . . . . .   6
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   8
   8.  Contributor Addresses . . . . . . . . . . . . . . . . . . . .   8
   9.  Normative References  . . . . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   [BIER_ARCH] describes a new architecture for the forwarding of
   multicast data packets.  That architecture provides optimal
   forwarding of multicast data packets through a "multicast domain".
   However, it does not require any explicit tree-building protocol, and
   does not require intermediate nodes to maintain any per-flow state.
   That architecture is known as "Bit Index Explicit Replication"

   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).  Each BFER
   must have a BFR-id as well as a BFR-prefix.  A BFR-id is just a

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   number in the range [1,65535] that, within a BIER domain, identifies
   a BFER uniquely.

   As described in [BIER_ARCH], BIER requires that multicast data
   packets be encapsulated with a header that carries the information
   needed to support the BIER forwarding procedures.  This information
   includes a Set-Id (SI) and a BitString.  Together the SI and the
   BitString identify the set of BFERs to which the packet must be

   This document is applicable when a given BIER domain is both an IGP
   domain and an MPLS network.  In this environment, the BIER
   encapsulation consists of two components:

   o  an MPLS label (which we will call the "BIER-MPLS label"); this
      label appears at the bottom of a packet's MPLS label stack.

   o  a BIER header, as specified in Section 3.

   Following the BIER header is the "payload".  The payload may be an
   IPv4 packet, an IPv6 packet, an ethernet frame, or an MPLS packet.
   If it 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 BIER header contains information
   identifying the type of the payload.

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

2.  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 [BIER_OSPF].

   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 "routing underlay" (see [BIER_ARCH]), a given BFR
   needs to advertise one such label for each SI.  If the domain
   contains multiple routing underlays, a given BFR needs to advertise
   one such label per SI per each underlay in which that BFR has

   The BIER-MPLS labels are locally significant (i.e., unique only to
   the BFR that advertises them) downstream-assigned MPLS labels.  For

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   example, suppose there is a single routing underlay, the network is
   using a BitStringLength of 255, and that all BFERs in the domain 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.

   When a BFR receives an MPLS packet with one of its BIER-MPLS labels
   at the top of the stack, it will assume that a BIER header (see
   Section 3) immediately follows the stack.  It will also infer the
   packet's SI from the label.

3.  BIER Header

   The BIER header is shown in Figure 1.  This header appears after the
   end of the MPLS label stack, immediately after the MPLS-BIER label.

      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
     |0 0 0 0|  Ver  |I|0 0 0 0 0 0 0| Proto |  Len  |   Entropy     |
     |                BitString  (first 32 bits)                     ~
     ~                                                               ~
     ~                BitString  (last 32 bits)                      |
     |                    BFIR-id (optional)                         |

                           Figure 1: BIER Header

   First nibble:

      The first 4 bits of the header are all set to zero; this ensures
      that the BIER header will not be confused with an IP header.


      This 4-bit field identifies the version of the BIER header.  This
      document specifies version 0 of the BIER header.


      This bit is set if and only if header contains the BFIR-id field.
      If this bit is clear, the header does not contain the BFIR-id, and
      the header ends at the end of the BitString.

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      This 4-bit field identifies the type of the payload.  (The
      "payload" is the packet or frame immediately following the BIER
      header.)  The protocol field may take any of the following values:

      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 I
         bit MUST be set, and 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.

      6: IPv6 packet.


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

      All other values of this field are illegal.


      This 8-bit field specifies an "entropy" value that can be used for
      load balancing purposes.  The BIER forwarding process may do equal
      cost load balancing, but the load balancing procedure MUST choose
      the same path for any two packets have the same entropy value.

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      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.


      The BitString that, together with the packet's SI, identifies the
      destination BFERs for this packet.  Note that the SI for the
      packet is inferred from the BIER-MPLS label that precedes the BIER


      This is the BFR-id of the BFIR.  The BFR-id is encoded in the
      32-bit field as an unsigned integer in the range [1,65535].

      This field is optional, and is present only when the I bit is set.

4.  Imposing and Processing the BIER Encapsulation

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

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

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

   3.  The BFIR looks up the BFR-id of each of those BFERs.

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

   5.  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 make a copy of the multicast data packet, and
            encapsulates the copy in a BIER header (see Section 3).  The
            BIER header contains the BitString that represents all the
            destination BFERs whose BFR-ids correspond to the given SI.

       b.   If the payload is an MPLS packet whose label stack begins
            with an upstream-assigned label, the BFIR-id field MUST be
            present.  (Whether a particular MPLS packet payload begins

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            with an upstream-assigned label is learned from the
            multicast flow layer.)  The BFIR-id MAY be included in other
            cases as well.

       c.   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.

       d.   Before transmitting a copy of the packet to a neighboring
            BFR, the BFIR finds the BIER-MPLS label that was advertised
            by the neighbor as corresponding to the given SI.  An MPLS
            label stack is then preprended 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 packet may then be
            transmitted to the neighboring BFR.  (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 receives a packet with one of its own BIER-
   MPLS labels at the top of the label stack, it infers the SI from the
   label, and 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 that
   the neighbor advertised that corresponds to the same SI and routing

   Thus a BIER-encapsulated packet in an MPLS network consists of a
   packet that has:

   o  An MPLS label stack with a BIER-MPLS label at the bottom of the

   o  A BIER header, as described in Section 3.

   o  The payload, a multicast data packet.

   The payload may be an IPv4 packet, an IPv6 packet, an ethernet frame,
   or an MPLS packet.  If it is an MPLS packet, the BIER header is
   followed by a second MPLS label stack; this stack is separate from
   the stack that precedes the BIER header.

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5.  IANA Considerations

   This document has no actions for IANA.

6.  Security Considerations

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

   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.

7.  Acknowledgements

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

8.  Contributor Addresses

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

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   Wim Henderickx
   Copernicuslaan 50
   Antwerp 2018


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


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


   Tony Przygienda


9.  Normative References

              Wijnands, IJ., "Multicast using Bit Index Explicit
              Replication Architecture", internet-draft draft-wijnands-
              bier-architecture-00, September 2014.

              Kumar, N., "OSPF Extension for Bit Index Explicit
              Replication", internet-draft draft-kumar-ospf-bier-
              extension-00, September 2014.

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

   [RFC3032]  Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
              Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
              Encoding", RFC 3032, January 2001.

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   [RFC5331]  Aggarwal, R., Rekhter, Y., and E. Rosen, "MPLS Upstream
              Label Assignment and Context-Specific Label Space", RFC
              5331, August 2008.

Authors' Addresses

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


   Eric C. Rosen (editor)
   Cisco Systems, Inc.
   1414 Massachusetts Avenue
   Boxborough, Massachusetts  01718


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


   Jeff Tantsura
   300 Holger Way
   San Jose, California  95134


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