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Versions: (draft-rosen-mpls-rfc3107bis) 00 01

Internet Engineering Task Force                            E. Rosen, Ed.
Internet-Draft                                    Juniper Networks, Inc.
Obsoletes: 3107 (if approved)                             March 13, 2017
Intended status: Standards Track
Expires: September 14, 2017


           Using BGP to Bind MPLS Labels to Address Prefixes
                     draft-ietf-mpls-rfc3107bis-01

Abstract

   This document specifies a set of procedures for using BGP to
   advertise that a specified router has bound a specified MPLS label
   (or a specified sequence of MPLS labels, organized as a contiguous
   part of a label stack) to a specified address prefix.  This can be
   done by sending a BGP UPDATE message whose Network Layer Reachability
   Information field contains both the prefix and the MPLS label(s), and
   whose Next Hop field identifies the node at which said prefix is
   bound to said label(s).  This document obsoletes RFC 3107.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 14, 2017.

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
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect



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   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  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Using BGP to Bind an Address Prefix to One or More MPLS
       Labels  . . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  Multiple Labels Capability  . . . . . . . . . . . . . . .   5
     2.2.  NLRI Encoding when the Multiple Labels Capability is
           Not Used  . . . . . . . . . . . . . . . . . . . . . . . .   8
     2.3.  NLRI Encoding when the Multiple Labels Capability is Used   9
     2.4.  How to Explicitly Withdraw the Binding of a Label to a
           Prefix  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     2.5.  Changing the Label that is Bound to a Prefix  . . . . . .  12
   3.  Installing and/or Propagating SAFI-4 or SAFI-128 Routes . . .  13
     3.1.  Comparability of Routes . . . . . . . . . . . . . . . . .  13
     3.2.  Modification of Label(s) Field When Propagating . . . . .  14
       3.2.1.  When the Next Hop Field is Unchanged  . . . . . . . .  14
       3.2.2.  When the Next Hop Field is Changed  . . . . . . . . .  14
   4.  Data Plane  . . . . . . . . . . . . . . . . . . . . . . . . .  15
   5.  Relationship Between SAFI-4 and SAFI-1 Routes . . . . . . . .  17
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  18
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  18
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  19
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  19
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  20
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  21
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  22

1.  Introduction

   [RFC3107] specifies encodings and procedures for using BGP to
   indicate that a particular router has bound either a single MPLS
   label or a sequence of MPLS labels (organized as a contiguous part of
   an MPLS label stack) ([RFC3031], [RFC3032]) to a particular address
   prefix.  This is done by sending a BGP UPDATE message whose Network
   Layer Reachability Information field contains both the prefix and the
   MPLS label(s), and whose Next Hop field identifies the node at which
   said prefix is bound to said label(s).  Each such UPDATE also
   advertises a path to the specified prefix, via the specified next
   hop.

   Although there are many implementations and deployments of [RFC3107],
   there are a number of issues with [RFC3107] that have impeded




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   interoperability in the past, and may potentially impede
   interoperability in the future:

   o  Although [RFC3107] specifies an encoding that allows a sequence of
      MPLS labels (rather than just a single label) to be bound to a
      prefix, it does not specify the semantics of binding a sequence of
      labels to a prefix.

   o  Many implementations of [RFC3107] assume that only one label will
      be bound to a prefix, and cannot properly process a BGP UPDATE
      message that binds a sequence of labels to a prefix.  Thus an
      implementation attempting to provide this feature is likely to
      experience problems interoperating with other implementations.

   o  [RFC3107]'s procedures for withdrawing the binding of a label or
      sequence of labels to a prefix are not specified clearly and
      correctly.

   o  [RFC3107] specifies an optional feature, known as "Advertising
      Multiple Routes to a Destination", that, to the best of the
      author's knowledge, has never been implemented as specified.  The
      functionality that this feature was intended to provide can and
      has been implemented in a different way using the procedures of
      [RFC7911], which were not available at the time that [RFC3107] was
      written.  In [RFC3107], this feature was controlled by a BGP
      Capability Code that has never been implemented, and is now
      essentially obsolete.

   o  It is possible for a BGP speaker to receive two BGP UPDATEs that
      advertise paths to the same address prefix, where one UPDATE binds
      a label (or sequence of labels) to the prefix and the other does
      not.  [RFC3107] is silent on the issue of how the presence of two
      such UPDATEs impacts the BGP decision process, and does not say
      explicitly whether one or the other or both of these UPDATEs
      should be propagated.  This has led different implementations to
      handle this situation in different ways.

   o  Much of [RFC3107] applies to the VPN-IPv4 ([RFC4364]) and VPN-IPv6
      ([RFC4659]) address families, but those address families are not
      mentioned in [RFC3107].

   This document replaces and obsoletes [RFC3107].  It defines a new BGP
   Capability to be used when binding a sequence of labels to a prefix;
   by using this Capability, the interoperability problems alluded to
   above can be avoided.  This document also removes the unimplemented
   "Advertising Multiple Routes to a Destination" feature, while
   specifying how to use [RFC7911] to provide the same functionality.
   This document also addresses the issue of the how UPDATEs that bind



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   labels to a given prefix interact with UPDATEs that advertise paths
   to that prefix but do not bind labels to it.  However, for backwards
   compatibility, it declares most of these interactions to be matters
   of local policy.

   The places where this specification differs from [RFC3107] are
   indicated in the text.  It is believed that implementations that
   conform to the current document will interoperate correctly with
   existing deployed implementations of [RFC3107].

   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
   [RFC2119].

2.  Using BGP to Bind an Address Prefix to One or More MPLS Labels

   BGP may be used to advertise that a particular node, call it N, has
   bound a particular MPLS label, or a particular sequence of MPLS
   labels (organized as a contiguous part of an MPLS label stack), to a
   particular address prefix.  This is done by sending a Multiprotocol
   BGP UPDATE message, i.e., an UPDATE message with an MP_REACH_NLRI
   attribute ([RFC4760].  The "Network Address of Next Hop" field of
   that attribute contains an IP address of node N.  The label(s) and
   the prefix are encoded in the NLRI field of the MP_REACH_NLRI.  The
   encoding of the NLRI field is specified in Sections 2.2 and 2.3.

   If the prefix is an IPv4 address prefix or a VPN-IPv4 ([RFC4364])
   address prefix, the Address Family Identifier (AFI) of the
   MP_REACH_NLRI attribute is set to 1.  If the prefix is an IPv6
   address prefix or a VPN-IPv6 prefix ([RFC4659], the AFI is set to 2.

   If the prefix is an IPv4 address prefix or an IPv6 address prefix,
   the Subsequent Address Family Identifier (SAFI) field is set to 4.
   If the prefix is a VPN-IPv4 address prefix or a VPN-IPv6 address
   prefix, the SAFI is set to 128.

   The use of SAFI 4 or SAFI 128 when the AFI is other than 1 or 2 is
   outside the scope of this document.

   This document does not specify the format of the "Network Address of
   Next Hop" field of the MP_REACH_NLRI attribute.  The format of the
   next hop field depends upon a number of factors, and is discussed in
   a number of other RFCs: see [RFC4364], [RFC4659], [RFC4798], and
   [RFC5549].

   There are a variety of applications that make use of alternative
   methods of using BGP to advertise MPLS label bindings.  See, e.g.,



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   [RFC7432], [RFC6514], or [TUNNEL-ENCAPS].  The method described in
   the current document is not claimed to be the only way of using BGP
   to advertise MPLS label bindings.  Discussion of which method to use
   for which application is outside the scope of the current document.

   In the remainder of this document, we will use the term "SAFI-x
   UPDATE" to refer to a BGP UPDATE message containing an MP_REACH_NLRI
   attribute or an MP_UNREACH_NLRI attribute ([RFC4760] whose SAFI field
   contains the value x.

   This document defines a BGP Optional Capabilities parameter
   ([RFC5492]) known as the "Multiple Labels Capability".

   o  Unless this Capability is sent on a given BGP session by both of
      that session's BGP speakers, a SAFI-4 or SAFI-128 UPDATE message
      sent on that session from either speaker MUST bind a prefix to
      only a single label, and MUST use the encoding of Section 2.2.

   o  If this Capability is sent by both BGP speakers on a given
      session, an UPDATE message on that session, from either speaker,
      MUST use the encoding of Section 2.3, and MAY bind a prefix to a
      sequence of more than one label.

   The encoding of the Multiple Labels Capability is specified in
   Section 2.1.

   Procedures for explicitly withdrawing a label binding are given in
   Section 2.4.  Procedures for changing the label(s) bound to a given
   prefix by a given node are given in Section 2.5.

   Procedures for propagating SAFI-4 and SAFI-128 UPDATEs are discussed
   in Section 3.

   When a BGP speaker installs and propagates a SAFI-4 or SAFI-128
   update, and if it changes the value of the Network Address of Next
   Hop field, it must program its data plane appropriately.  This is
   discussed in Section 4.

2.1.  Multiple Labels Capability

   [RFC5492] defines the "Capabilities Optional Parameter".  A BGP
   speaker can include a Capabilities Optional Parameter in a BGP OPEN
   message.  The Capabilities Optional Parameter is a triple including a
   one-octet Capability Code, a one-octet Capability length, and a
   variable-length Capability Value.

   This document defines a Capability Code, known as the "Multiple
   Labels Capability" code.  IANA will assign a codepoint for this



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   Capability Code.  (This Capability Code is new to this document and
   does not appear in [RFC3107].)

   If a BGP speaker has not sent the Multiple Labels Capability in its
   BGP Open message on a particular BGP session, or if it has not
   received the Multiple Labels Capability in the BGP Open message from
   its peer on that BGP session, that BGP speaker MUST NOT send on that
   session any UPDATE message that binds more than one MPLS label to any
   given prefix.  Further, when advertising the binding of a single
   label to a prefix, the BGP speaker MUST use the encoding specified in
   Section 2.2.

   The value field of the Multiple Labels Capability (shown in Figure 1)
   consists of one or more triples, where each triple consists of four
   octets.  The first two octets of a triple specify an AFI value, the
   third octet specifies a SAFI value, and the fourth specifies a Count.
   If one of the triples is <AFI,SAFI,Count>, the Count is the maximum
   number of labels that the BGP speaker can process in a received
   UPDATE of the specified AFI/SAFI.

   If the Capability contains more than one triple with a given AFI/
   SAFI, all but the first MUST be ignored.

   Any triple of the form <AFI=x,SAFI=y,Count=0> MUST be ignored.

   If the Capability contains the triple <AFI=x,SAFI=y,Count=255>, then
   no limit has been placed on the number of labels that can be
   advertised in UPDATEs of the corresponding AFI/SAFI.

   A Multiple Labels Capability whose length is not a multiple of four
   MUST be considered to be malformed.

   [RFC4724] ("Graceful Restart Mechanism for BGP") describes a
   procedure that allows routes learned over a given BGP session to be
   maintained when the session fails and then restarts.  This procedure
   requires the entire RIB to be transmitted when the session restarts.
   If the Multiple Labels Capability for a given AFI/SAFI had been
   exchanged on the failed session, but is not exchanged on the
   restarted session, then any prefixes advertised in that AFI/SAFI with
   multiple labels MUST be explicitly withdrawn.  Similarly, if the
   maximum label count, specified in the Capability for a given AFI/SAFI
   is reduced, an prefixes advertised with more labels than are valid
   for the current session MUST be explicitly withdrawn.

   [Enhanced-GR] ("Accelerated Routing Convergence for BGP Graceful
   Restart") describes another procedure that allows the routes learned
   over a given BGP session to be maintained when the session fails and




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   then restarts.  These procedures MUST NOT be applied if either of the
   following conditions hold:

   o  The Multiple Labels Capability for a given AFI/SAFI had been
      exchanged prior to the restart, but has not been exchanged on the
      restarted session.

   o  The Multiple Labels Capability for a given AFI/SAFI had been
      exchanged with a given Count prior to the restart, but has been
      exchanged with a smaller count on the restarted session.

   If either of these conditions holds, the complete set of routes for
   of the given AFI/SAFI MUST be exchanged.

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |              AFI              |    SAFI       |    Count      ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     ~              AFI              |    SAFI       |    Count      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

            Figure 1: Value Field of Multiple Labels Capability

   If a BGP OPEN message contains multiple copies of the Multiple Labels
   Capability, only the first copy is significant; subsequent copies
   MUST BE ignored.

   If a BGP speaker has sent the Multiple Labels Capability in its BGP
   OPEN message for a particular BGP session, and has also received the
   Multiple Labels Capability in its peer's BGP OPEN message for that
   session, and if both Capabilities specify AFI/SAFI x/y, then:

      when using an UPDATE of AFI x and SAFI y to advertise the binding
      of a label or sequence of labels to a given prefix, the BGP
      speaker MUST use the encoding of Section 2.3.  This encoding MUST
      be used even if only one label is being bound to a given prefix.

   If both BGP speakers of a given BGP session have sent the Multiple
   Labels Capability, but AFI/SAFI x/y has not been specified in both
   Capabilities, then UPDATES of AFI/SAFI x/y on that session MUST use
   the encoding of Section 2.2, and such UPDATEs can only bind one label
   to a prefix.

   A BGP speaker SHOULD NOT send an UPDATE that binds more labels to a
   given prefix than its peer is capable of receiving, as specified in
   the Multiple Labels Capability sent by that peer.  If a BGP speaker
   receives an UPDATE that binds more labels to a given prefix than the



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   number of prefixes the BGP speaker is prepared to receive (as
   announced in its Multiple Labels Capability), the BGP speaker MUST
   apply the "treat-as-withdraw" strategy of [RFC7606] to that UPDATE.

   Notwithstanding the number of labels that a BGP speaker has claimed
   to be able to receive, its peer MUST NOT attempt to send more labels
   than can be properly encoded in the NLRI field of the MP_REACH_NLRI
   attribute.  Please note that there is only a limited amount of space
   in the NLRI field for labels:

   o  per [RFC4760] the size of this field is limited to 255 bits (not
      255 octets), including the number of bits in the prefix;

   o  in a SAFI-128 update, the prefix is at least 64 bits long, and may
      be as long as 192 bits (e.g., in a VPN-IPv6 host route).

2.2.  NLRI Encoding when the Multiple Labels Capability is Not Used

   If the Multiple Labels Capability has not been both sent and received
   on a given BGP session, then in a BGP UPDATE on that session whose
   MP_REACH_NLRI attribute contains one of the AFI/SAFI combinations
   specified in Section 2, the NLRI field is encoded as shown in
   Figure 2:

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    Length     |                 Label                 |Rsrv |S|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                          Prefix                               ~
     ~                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 2: NLRI With One Label

   - Length:

      The Length field consists of a single octet.  It specifies the
      length in bits of the remainder of the NLRI field.

      Note that the length will always be the sum of 20 (number of bits
      in label field) plus 3 (number of bits in Rsrv field) plus 1
      (number of bits in S field) plus the length in bits of the prefix.

      In an MP_REACH_NLRI attribute whose AFI/SAFI is 1/4, the prefix
      length will be 32 bits or less.  In an MP_REACH_NLRI attribute
      whose AFI/SAFI is 2/4, the prefix length will be 128 bits or less.
      In an MP_REACH_NLRI attribute whose SAFI is 128, the prefix will



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      be 96 bits or less if the AFI is 1, and will be 192 bits or less
      if the AFI is 2.

      As specified in [RFC4760], actual length of the NLRI field will be
      the number of bits specified in the length field, rounded up to
      the nearest integral number of octets.

   - Label:

      The Label field is a 20-bit field, containing an MPLS label value
      (see [RFC3032]).

   - Rsrv:

      This 3-bit field SHOULD be set to zero on transmission, and MUST
      be ignored on reception.

   - S:

      This 1-bit field MUST be set to one on transmission, and MUST be
      ignored on reception.

   Note that the UPDATE message not only advertises the binding between
   the prefix and the label, it also advertises a path to the prefix via
   the node identified in the "Network Address of Next Hop" field of the
   MP_REACH_NLRI attribute.

   [RFC3107] requires that if only a single label is bound to a prefix,
   the S bit must be set.  If the S bit is not set, [RFC3107] specifies
   that additional labels will appear in the NLRI.  However, some
   implementations assume that the NLRI will contain only a single
   label, and do not check the setting of the S bit.  The procedures
   specified in the current document will interwork with such
   implementations.  As long as the Multiple Labels Capability is not
   sent and received by both BGP speakers on a given BGP session, this
   document REQUIRES that only one label be specified in the NLRI, that
   the S bit be set on transmission, and that it be ignored on
   reception.

   If the procedures of [RFC7911] are being used, a four-octet "path
   identifier" (as defined in Section 3 of [RFC7911]) is part of the
   NLRI, and precedes the Length field.

2.3.  NLRI Encoding when the Multiple Labels Capability is Used

   If the Multiple Labels Capability has been both sent and received on
   a given BGP session, then in a BGP UPDATE on that session whose
   MP_REACH_NLRI attribute contains one of the AFI/SAFI combinations



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   specified in Section 2, the NLRI field is encoded as shown in
   Figure 3:

      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
     +-+-+-+-+-+-+-+-+
     |    Length     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                 Label                 |Rsrv |S~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     ~                 Label                 |Rsrv |S|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                          Prefix                               ~
     ~                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 3: NLRI With Multiple Labels

   - Length:

      The Length field consists of a single octet.  It specifies the
      length in bits of the remainder of the NLRI field.

      Note that for each label, the length is increased by 24 bits (20
      bits in label field plus 3 bits in Rsrv field plus 1 S bit).

      In an MP_REACH_NLRI attribute whose AFI/SAFI is 1/4, the prefix
      length will be 32 bits or less.  In an MP_REACH_NLRI attribute
      whose AFI/SAFI is 2/4, the prefix length will be 128 bits or less.
      In an MP_REACH_NLRI attribute whose SAFI is 128, the prefix will
      be 96 bits or less if the AFI is 1, and will be 192 bits or less
      if the AFI is 2.

      As specified in [RFC4760], actual length of the NLRI field will be
      the number of bits specified in the length field, rounded up to
      the nearest integral number of octets.

   - Label:

      The Label field is a 20-bit field, containing an MPLS label value
      ([RFC3032]).

   - Rsrv:

      This 3-bit field SHOULD be set to zero on transmission, and MUST
      be ignored on reception.

   - S:



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      In all labels except the last (i.e., in all labels except the one
      immediately preceding the prefix), the S bit MUST be 0.  In the
      last label, the S bit MUST be 1.

      Note that failure to set the S bit in the last label will make it
      impossible to parse the NLRI correctly.  See Section 3 paragraph j
      of [RFC7606] for a discussion of error handling when the NLRI
      cannot be parsed.

   Note that the UPDATE message not only advertises the binding between
   the prefix and the labels, it also advertises a path to the prefix
   via the node identified in the "next hop" field of the MP_REACH_NLRI
   attribute.

   If the procedures of [RFC7911] are being used, a four-octet "path
   identifier" (as defined in Section 3 of [RFC7911]) is part of the
   NLRI, and precedes the Length field.

2.4.  How to Explicitly Withdraw the Binding of a Label to a Prefix

   Suppose a BGP speaker has announced, on a given BGP session, the
   binding of a given label or sequence of labels to a given prefix.
   Suppose it now wishes to withdraw that binding.  To do so, it may
   send a BGP UPDATE message with an MP_UNREACH_NLRI attribute.  The
   NLRI field of this attribute is encoded as follows:

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    Length     |        Compatibility                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                          Prefix                               ~
     ~                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 4: NLRI For Withdrawal

   Upon transmission, the Compatibility field SHOULD be set to 0x800000.
   Upon reception, the value of the Compatibility field MUST be ignored.

   This encoding is used for explicitly withdrawing the binding, on a
   given BGP session, between the specified prefix and whatever label or
   sequence of labels had previously been bound by the procedures of
   this document to that prefix on the given session.  This encoding is
   used whether or not the Multiple Labels Capability has been sent or
   received on the session.  Note that label/prefix bindings that were
   not advertised on the given session cannot be withdrawn by this
   method.



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   When using an MP_UNREACH_NLRI attribute to withdraw a route whose
   NLRI was previously specified in an MP_REACH_NLRI attribute, the
   lengths and values of the respective prefixes must match, and the
   respective AFI/SAFIs must match.  If the procedures of [RFC7911] are
   being used, the respective values of the "path identifier" fields
   must match as well.  Note that the prefix length is not the same as
   the NLRI length; to determine the prefix length of a prefix in an
   MP_UNREACH_NLRI, the length of the Compatibility field must be
   subtracted from the length of the NLRI.

   An explicit withdrawal in a SAFI-x UPDATE on a given BGP session not
   only withdraws the binding between the prefix and the label(s), it
   also withdraws the path to that prefix that was previously advertised
   in a SAFI-x UPDATE on that session.

   [RFC3107] allowed one to specify a particular label value in the
   Compatibility field.  However, the functionality that required the
   presence of a particular label value (or sequence of label values)
   was never implemented, and that functionality is not present in the
   current document.  Hence the value of this field is of no
   significance; there is never any reason for this field to contain a
   label value or a sequence of label values.

   [RFC3107] also allowed one to withdraw a binding without specifying
   the label explicitly, by setting the Compatibility field to 0x800000.
   However, some implementations set it to 0x000000.  In order to ensure
   backwards compatibility, this document RECOMMENDS that the
   Compatibility field be set to 0x800000, but REQUIRES that it be
   ignored upon reception.

2.5.  Changing the Label that is Bound to a Prefix

   Suppose a BGP speaker, S1, has received on a given BGP session, a
   SAFI-4 or SAFI-128 UPDATE, U1, that specifies label (or sequence of
   labels) L1, prefix P, and next hop N1.  As specified above, this
   indicates that label (or sequence of labels) L1 is bound to prefix P
   at node N1.  Suppose that S1 now receives, on the same session, an
   UPDATE, U2, of the same AFI/SAFI, that specifies label (or sequence
   of labels) L2, prefix P, and the same next hop, N1.

   o  If [RFC7911] is not being used, UPDATE U2 MUST be interpreted as
      meaning that L2 is now bound to P at N1, and that L1 is no longer
      bound to P at N1.  That is, the UPDATE U1 is implicitly withdrawn,
      and is replaced by UPDATE U2.

   o  Suppose that [RFC7911] is being used, that UPDATE U1 has Path
      Identifier I1, and that UPDATE U2 has Path Identifier I2.




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      *  If I1 is the same as I2, UPDATE U2 MUST be interpreted as
         meaning that L2 is now bound to P at N1, and that L1 is no
         longer bound to P at N1.  UPDATE U1 is implicitly withdrawn.

      *  If I1 is not the same as I2, U2 MUST be interpreted as meaning
         that L2 is now bound to P at N1, but MUST NOT be interpreted as
         meaning that L1 is no longer bound to P at N1.  Under certain
         conditions (specification of which is outside the scope of this
         document), S1 may choose to load balance traffic between the
         path represented by U1 and the path represented by U2.  To send
         traffic on the path represented by U1, S1 uses the label(s)
         advertised in U1; to send traffic on the path represented by
         U2, S1 uses the label(s) advertised in U2.  (Although these two
         paths have the same next hop, one must suppose that they
         diverge further downstream.)

   Suppose a BGP speaker, S1, has received, on a given BGP session, a
   SAFI-4 or SAFI-128 UPDATE that specifies label L1, prefix P, and next
   hop N1.  Suppose that S1 now receives, on a different BGP session, an
   UPDATE, of the same AFI/SAFI, that specifies label L2, prefix P, and
   the same next hop, N1.  BGP speaker S1 SHOULD treat this as
   indicating that N1 has at least two paths to P, and S MAY use this
   fact to do load-balancing of any traffic that it has to send to P.

   Note that this section discusses only the case where two UPDATEs have
   the same next hop.  Procedures for the case where two UPDATEs have
   different next hops are adequately described in [RFC4271].

3.  Installing and/or Propagating SAFI-4 or SAFI-128 Routes

3.1.  Comparability of Routes

   Suppose a BGP speaker has received two SAFI-4 UPDATEs specifying the
   same Prefix, and that either:

   o  the two UPDATEs are received on different BGP sessions, or

   o  the two UPDATES are received on the same session, add-paths is
      used on that session, and the NLRIs of the two updates have
      different path identifiers.

   These two routes MUST be considered to be comparable, even if they
   specify different labels.  Thus the BGP bestpath selection procedures
   (Section 9.1 of [RFC4271]) are applied to select one of them as the
   better path.  If the procedures of [RFC7911] are not being used on a
   particular BGP session, only the best path is propagated on that
   session.  If the procedures of [RFC7911] are being used on a




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   particular BGP session, then both paths may be propagated on that
   session, though with different path identifiers..

   The same applies to SAFI-128 routes.

3.2.  Modification of Label(s) Field When Propagating

3.2.1.  When the Next Hop Field is Unchanged

   When a SAFI-4 or SAFI-128 route is propagated, if the "Network
   Address of Next Hop" field is left unchanged, the label field(s) MUST
   also be left unchanged.

   Note that a given route MUST NOT be propagated to a given peer if the
   route's NLRI has multiple labels, but the peer can only handle a
   single label in the NLRI.  Similarly, a given route SHOULD NOT be
   propagated to a given peer if the route's NLRI has more labels than
   the peer has announced (through its "Multiple Labels" Capability)
   that it can handle.  In either case, if a previous route with the
   same AFI, SAFI, and prefix (but with fewer labels) has already been
   propagated to the peer, that route MUST be withdrawn from that peer,
   using the procedure of Figure 4.

3.2.2.  When the Next Hop Field is Changed

   If the "Network Address of Next Hop" field is changed before a SAFI-4
   or SAFI-128 route is propagated, the label field(s) of the propagated
   route MUST contain the label(s) that that is (are) bound to the
   prefix at the new next hop.

   Suppose BGP speaker S1 has received an UPDATE that binds a particular
   sequence of one or more labels to a particular prefix.  If S1 chooses
   to propagate this route after changing its next hop, S1 may change
   the label in any of the following ways, depending upon local policy:

   o  A single label may be replaced by a single label, of the same or
      different value.

   o  A sequence of multiple labels may be replaced by a single label.

   o  A single label may be replaced by a sequence of multiple labels.

   o  A sequence of multiple labels may be replaced by a sequence of
      multiple labels; the number of labels may be left the same, or may
      be changed.

   Of course, when deciding whether to propagate, to a given BGP peer,
   an UPDATE binding a sequence of more than one label, a BGP speaker



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   must attend to the information provided by the Multiple Labels
   Capability (see Section 2.1).  A BGP speaker MUST NOT send multiple
   labels to a peer with which it has not exchanged the "Multiple
   Labels" Capability, and SHOULD NOT send more labels to a given peer
   than the peer has announced (via the "Multiple Labels" Capability)
   than it can handle.

   It is possible that a BGP speaker's local policy will tell it to
   encode N labels in a given route's NLRI before propagating the route,
   but that one of the BGP speaker's peers cannot handle N labels in the
   NLRI.  In this case, the BGP speaker has two choices:

   o  It can propagate the route to the given peer with fewer than N
      labels.  However, whether this makes sense, and if so, how to
      choose the labels, is also a matter of local policy

   o  It can decide not to propagate the route to the given peer.  In
      that case, if a previous route with the same AFI, SAFI, and prefix
      (but with fewer labels) has already been propagated to that peer,
      that route MUST be withdrawn from that peer, using the procedure
      of Figure 4.

4.  Data Plane

   In the following, we will use the phrase "node S tunnels packet P to
   node N", where packet P is an MPLS packet.  By this phrase, we mean
   that node S encapsulates packet P and causes packet P to be delivered
   to node N, in such a way that P's label stack before encapsulation
   will be seen unchanged by N, but will not be seen by the nodes (if
   any) between S and N.

   If the tunnel is a Label Switched Path (LSP), encapsulating the
   packet may be as simple as pushing on another MPLS label.  If node N
   is a layer 2 adjacency of node S, S a layer 2 encapsulation may be
   all that is needed.  Other sorts of tunnels (e.g., IP tunnels, GRE
   tunnels, UDP tunnels) may also be used, depending upon the particular
   deployment scenario.

   Suppose BGP speaker S1 receives a SAFI-4 or SAFI-128 BGP UPDATE with
   an MP_REACH_NLRI specifying label L1, prefix P, and next hop N1, and
   suppose S1 installs this route as its (or one of its) bestpath(s)
   towards P.  And suppose S1 propagates this route after changing the
   next hop to itself and changing the label to L2.  Suppose further
   that S1 receives an MPLS data packet, and in the process of
   forwarding that MPLS data packet, S1 sees label L2 rise to the top of
   the packet's label stack.  Then to forward the packet further, S1
   must replace L2 with L1 as the top entry in the packet's label stack,
   and S1 must then tunnel the packet to N1.



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   Suppose that the route received by S1 specified not a single label,
   but a sequence of k labels <L11, L12, ..., L1k>, where L11 is the
   first label appearing in the NLRI, and L1k is the last.  And suppose
   again that S1 propagates this route after changing the next hop to
   itself and changing the label field to the single label L2.  Suppose
   further that S1 receives an MPLS data packet, and in the process of
   forwarding that MPLS data packet, S1 sees label L2 rise to the top of
   the packet's label stack.  In this case, instead of simply replacing
   L2 with L1, S1 removes L2 from the top of the label stack, and then
   pushes labels L1k through L11 onto the label stack, such that L11 is
   now at the top of the label stack.  Then S1 must tunnel the packet to
   N1.  (Note that L1k will not be at the bottom of the packet's label
   stack, and hence will not have the "bottom of stack" bit set, unless
   L2 had previously been at the bottom of the packet's label stack.)

   The above paragraph assumes that when S1 propagates a SAFI-4 or
   SAFI-128 route after setting the next hop to itself, it replaces the
   label or labels specified in the NLRI of that route with a single
   label.  However, it is also possible, as determined by local policy,
   for a BGP speaker to specify multiple labels when it propagates a
   SAFI-4 or SAFI-128 route after setting the next hop to itself.

   Suppose, for example, that S1 supports context labels ([RFC5331]).
   Let L21 be a context label supported by S1, and let L22 be a label
   that is in the label space identified (at S1) by L21.  Suppose S1
   receives a SAFI-4 or SAFI-128 UPDATE whose prefix is P, whose label
   field is <L11,L12,...L1k>, and whose next hop is N1.  Before
   propagating the UPDATE, S1 may set the next hop to itself (by
   replacing N1 with S1), and may replace the label stack
   <L11,L12,...L1k> with the pair of labels <L21,L22>.

   In this case, if S1 receives an MPLS data packet whose top label is
   L21 and whose second label is L22, S1 will remove both L21 and L22
   from the label stack, and replace them with <L11,L12,...L1k>.  Note
   that the fact that L21 is a context label is known only to S1; other
   BGP speakers do not know how S1 will interpret L21 (or L22).

   The ability to replace one or more labels by one or more labels can
   provide great flexibility, but must be done carefully.  Let's suppose
   again that S1 receives an UPDATE that specifies prefix P, label stack
   <L11,L12,...,L1k>, and next hop N1.  And suppose that S1 propagates
   this UPDATE to BGP speaker S2 after setting next hop self and after
   replacing the label field with <L21,L22,...L2k>.  Finally, suppose
   that S1 programs its data plane so that when it processes a received
   MPLS packet whose top label is L21, it replaces L21 with
   <L11,L12,...,L1k>, and then tunnels the packet to N1.





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   In this case, BGP speaker S2 will have received a route with prefix
   P, label field <L21,L22,...L2k>, and next hop S1.  If S2 decides to
   forward an IP packet according to this route, it will push
   <L21,L22,...L2k> onto the packet's label stack, and tunnel the packet
   to S1.  S1 will replace L21 with <L11,L12,...,L1k>, and will tunnel
   the packet to N1.  N1 will receive the packet with the following
   label stack: <L11,L12,...L1k,L22,...L2k>.  While this may be useful
   in certain scenarios, it may provide unintended results in other
   scenarios.

   Procedures for choosing, setting up, maintaining, or determining the
   liveness of a particular tunnel or type of tunnel, are outside the
   scope of this document.

   When pushing labels onto a packet's label stack, the Time-to-Live
   (TTL) field ([RFC3032], [RFC3443]) and the Traffic Class (TC) field
   ([RFC3032], [RFC5462]) of each label stack entry must of course be
   set.  This document does not specify any set of rules for setting
   these fields; that is a matter of local policy.

   This document does not specify any new rules for processing the label
   stack of an incoming data packet.

   It is a matter of local policy whether SAFI-4 routes can be used as
   the basis for forwarding IP packets, or whether SAFI-4 routes can
   only be used for forwarding MPLS packets.  If BGP speaker S1 is
   forwarding IP packets according to SAFI-4 routes, then consider an IP
   packet with destination address D, such that P is the "longest prefix
   match" for D from among the routes that are being used to forward IP
   packets.  And suppose the packet is being forwarded according to a
   SAFI-4 route whose prefix is P, whose next hop is N1, and whose
   sequence of labels is L1.  To forward the packet according to this
   route, S1 must create a label stack for the packet, push on the
   sequence of labels L1, and then tunnel the packet to N1.

5.  Relationship Between SAFI-4 and SAFI-1 Routes

   It is possible that a BGP speaker will receive both a SAFI-1 route
   for prefix P and a SAFI-4 route for prefix P.  The significance of
   this is a matter of local policy.

   For example, some implementations may regard SAFI-1 routes and SAFI-4
   routes as completely independent, and may treat them in a "ships in
   the night" fashion.  In this case, bestpath selection for the two
   SAFIs is independent, and there will be a best SAFI-1 route to P as
   well as a best SAFI-4 route to P.  Which packets get forwarded
   according to the routes of which SAFI is then a matter of local
   policy.



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   Other implementations may treat the SAFI-1 and SAFI-4 routes for a
   given prefix as comparable, such that the best route to prefix P is
   either a SAFI-1 route or a SAFI-4 route, but not both.  In
   implementations of this sort, if load balancing is done among a set
   of equal cost routes, some of the equal cost routes may be SAFI-1
   routes and some may be SAFI-4 routes.  Whether this is allowed is
   again a matter of local policy.

   Some implementations may allow a single BGP session to carry UPDATES
   of both SAFI-1 and SAFI-4; other implementations may disallow this.

   A BGP speaker may receive a SAFI-4 route over a given BGP session,
   but may have other BGP sessions for which SAFI-4 is not enabled.  In
   this case, the BGP speaker MAY convert the SAFI-4 route to a SAFI-1
   route and then propagate the result over the session on which SAFI-4
   is not enabled.  Whether this is done is a matter of local policy.

6.  IANA Considerations

   IANA is requested to assign a codepoint for "Multiple Labels
   Capability" in the "BGP Capability Codes" registry, with this
   document as the reference.

   IANA is requested to modify the "BGP Capability Codes" registry to
   mark Capability Code 4 ("Multiple routes to a destination") as
   deprecated, with this document as the reference.

   IANA is requested to change the reference for SAFI 4 in the
   "Subsequent Address Family Identifiers (SAFI) Parameters" to this
   document.  IANA is also requested to add this document as a reference
   for SAFI 128 in that same registry.

7.  Security Considerations

   The security considerations of the BGP specification ([RFC4271])
   apply.

   If a BGP implementation, not conformant with the current document,
   encodes multiple labels in the NLRI but has not sent and received the
   "Multiple Labels" Capability, a BGP implementation that does conform
   with the current document will likely reset the BGP session.

   This document specifies that certain data packets be "tunneled" from
   one BGP speaker to another.  This requires that the packets be
   encapsulated while in flight.  This document does not specify the
   encapsulation to be used.  However, if a particular encapsulation is
   used, the security considerations of that encapsulation are
   applicable.



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   If a particular tunnel encapsulation does not provide integrity and
   authentication, it is possible that a data packet's label stack can
   be modified, through error or malfeasance, while the packet is in
   flight.  This can result in misdelivery of the packet.

   There are various techniques one can use to constrain the
   distribution of BGP UPDATE messages.  If a BGP UPDATE advertises the
   binding of a particular (set of) label(s) to a particular address
   prefix, such techniques can be used to control the set of BGP
   speakers that are intended to learn of that binding.  However, if BGP
   sessions do not provide privacy, other routers may learn of that
   binding.

   When a BGP speaker processes a received MPLS data packet whose top
   label it advertised, there is no guarantee that the label in question
   was put on the packet by a router that was intended to know about the
   label binding.  If a BGP speaker is using the procedures of this
   document, it may be useful for that speaker to distinguish its
   "internal" interfaces from its "external" interfaces, and to avoid
   advertising the same labels to BGP speakers reached on internal
   interfaces as to BGP speakers reached on external interfaces.  Then a
   data packet can be discarded if its top label was not advertised over
   the type of interface from which the packet was received.  This
   reduces the likelihood of forwarding packets whose labels have been
   "spoofed" by untrusted sources.

8.  Acknowledgements

   This draft obsoletes RFC 3107.  We wish to think Yakov Rekhter, co-
   author of RFC 3107, for his work on that document.  We also wish to
   thank Ravi Chandra, Enke Chen, Srihari R.  Sangli, Eric Gray, and
   Liam Casey for their review of and comments on that document.

   We thank Alexander Okonnikov and David Lamparter for pointing out a
   number of the errors in RFC 3107.

   We wish to thank Lili Wang and Kaliraj Vairavakkalai for their help
   and advice during the preparation of this document.

   We also thank Mach Chen, Bruno Decraene, Jie Dong, Adrian Farrel,
   Jeff Haas, Jakob Heitz, Keyur Patel, Kevin Wang, and Lucy Yong for
   their review of and comments on this document.

9.  References







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9.1.  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,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC3031]  Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
              Label Switching Architecture", RFC 3031,
              DOI 10.17487/RFC3031, January 2001,
              <http://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,
              <http://www.rfc-editor.org/info/rfc3032>.

   [RFC3107]  Rekhter, Y. and E. Rosen, "Carrying Label Information in
              BGP-4", RFC 3107, DOI 10.17487/RFC3107, May 2001,
              <http://www.rfc-editor.org/info/rfc3107>.

   [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,
              <http://www.rfc-editor.org/info/rfc3443>.

   [RFC4271]  Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
              Border Gateway Protocol 4 (BGP-4)", RFC 4271,
              DOI 10.17487/RFC4271, January 2006,
              <http://www.rfc-editor.org/info/rfc4271>.

   [RFC4364]  Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
              Networks (VPNs)", RFC 4364, DOI 10.17487/RFC4364, February
              2006, <http://www.rfc-editor.org/info/rfc4364>.

   [RFC4659]  De Clercq, J., Ooms, D., Carugi, M., and F. Le Faucheur,
              "BGP-MPLS IP Virtual Private Network (VPN) Extension for
              IPv6 VPN", RFC 4659, DOI 10.17487/RFC4659, September 2006,
              <http://www.rfc-editor.org/info/rfc4659>.

   [RFC4760]  Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
              "Multiprotocol Extensions for BGP-4", RFC 4760,
              DOI 10.17487/RFC4760, January 2007,
              <http://www.rfc-editor.org/info/rfc4760>.







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   [RFC4798]  De Clercq, J., Ooms, D., Prevost, S., and F. Le Faucheur,
              "Connecting IPv6 Islands over IPv4 MPLS Using IPv6
              Provider Edge Routers (6PE)", RFC 4798,
              DOI 10.17487/RFC4798, February 2007,
              <http://www.rfc-editor.org/info/rfc4798>.

   [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, <http://www.rfc-editor.org/info/rfc5462>.

   [RFC5492]  Scudder, J. and R. Chandra, "Capabilities Advertisement
              with BGP-4", RFC 5492, DOI 10.17487/RFC5492, February
              2009, <http://www.rfc-editor.org/info/rfc5492>.

   [RFC5549]  Le Faucheur, F. and E. Rosen, "Advertising IPv4 Network
              Layer Reachability Information with an IPv6 Next Hop",
              RFC 5549, DOI 10.17487/RFC5549, May 2009,
              <http://www.rfc-editor.org/info/rfc5549>.

   [RFC7606]  Chen, E., Ed., Scudder, J., Ed., Mohapatra, P., and K.
              Patel, "Revised Error Handling for BGP UPDATE Messages",
              RFC 7606, DOI 10.17487/RFC7606, August 2015,
              <http://www.rfc-editor.org/info/rfc7606>.

9.2.  Informative References

   [Enhanced-GR]
              Patel, K., Chen, E., Fernando, R., and J. Scudder,
              "Accelerated Routing Convergence for BGP Graceful
              Restart", internet-draft draft-ietf-idr-enhanced-gr-06,
              June 2016.

   [RFC4724]  Sangli, S., Chen, E., Fernando, R., Scudder, J., and Y.
              Rekhter, "Graceful Restart Mechanism for BGP", RFC 4724,
              DOI 10.17487/RFC4724, January 2007,
              <http://www.rfc-editor.org/info/rfc4724>.

   [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,
              <http://www.rfc-editor.org/info/rfc5331>.

   [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,
              <http://www.rfc-editor.org/info/rfc6514>.




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   [RFC7432]  Sajassi, A., Ed., Aggarwal, R., Bitar, N., Isaac, A.,
              Uttaro, J., Drake, J., and W. Henderickx, "BGP MPLS-Based
              Ethernet VPN", RFC 7432, DOI 10.17487/RFC7432, February
              2015, <http://www.rfc-editor.org/info/rfc7432>.

   [RFC7911]  Walton, D., Retana, A., Chen, E., and J. Scudder,
              "Advertisement of Multiple Paths in BGP", RFC 7911,
              DOI 10.17487/RFC7911, July 2016,
              <http://www.rfc-editor.org/info/rfc7911>.

   [TUNNEL-ENCAPS]
              Rosen, E., Patel, K., and G. Vandevelde, "The BGP Tunnel
              Encapulation Attribute", internet-draft draft-ietf-idr-
              tunnel-encaps-03, November 2016.

Author's Address

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

   Email: erosen@juniper.net



























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