BGP Next Hop Dependent Capabilities Attribute
draft-ietf-idr-entropy-label-13

Internet Engineering Task Force                         B. Decraene, Ed.
Internet-Draft                                                    Orange
Updates: 6790, 7447 (if approved)                     J. G. Scudder, Ed.
Intended status: Standards Track                        Juniper Networks
Expires: 11 April 2024                                     W. Henderickx
                                                                   Nokia
                                                             K. Kompella
                                                        Juniper Networks
                                                              S. Mohanty
                                                           Cisco Systems
                                                               J. Uttaro
                                                 Independent Contributor
                                                                  B. Wen
                                                                 Comcast
                                                          9 October 2023

             BGP Next Hop Dependent Capabilities Attribute
                    draft-ietf-idr-entropy-label-13

Abstract

   RFC 5492 allows a BGP speaker to advertise its capabilities to its
   peer.  When a route is propagated beyond the immediate peer, it is
   useful to allow certain capabilities to be conveyed further.  In
   particular, it is useful to advertise forwarding plane features.

   This specification defines a BGP transitive attribute to carry such
   capability information, the "Next Hop Dependent Capabilities
   Attribute," or NHC.  Unlike the capabilities defined by RFC 5492,
   those conveyed in the NHC apply solely to the routes advertised by
   the BGP UPDATE that contains the particular NHC.

   This specification also defines an NHC capability that can be used to
   advertise the ability to process the MPLS Entropy Label as an egress
   LSR for all NLRI advertised in the BGP UPDATE.  It updates RFC 6790
   and RFC 7447 concerning this BGP signaling.

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/.

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   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
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   This Internet-Draft will expire on 11 April 2024.

Copyright Notice

   Copyright (c) 2023 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   4
   2.  BGP Next Hop Dependent Capabilities Attribute . . . . . . . .   4
     2.1.  Encoding  . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.2.  Sending the NHC . . . . . . . . . . . . . . . . . . . . .   6
       2.2.1.  Aggregation . . . . . . . . . . . . . . . . . . . . .   7
     2.3.  Receiving the NHC . . . . . . . . . . . . . . . . . . . .   7
     2.4.  Attribute Error Handling  . . . . . . . . . . . . . . . .   8
     2.5.  Network Operation Considerations  . . . . . . . . . . . .   9
   3.  Entropy Label Capability (ELCv3)  . . . . . . . . . . . . . .   9
     3.1.  Encoding  . . . . . . . . . . . . . . . . . . . . . . . .  10
     3.2.  Sending the ELCv3 . . . . . . . . . . . . . . . . . . . .  10
       3.2.1.  Aggregation . . . . . . . . . . . . . . . . . . . . .  11
     3.3.  Receiving the ELCv3 . . . . . . . . . . . . . . . . . . .  11
     3.4.  ELCv3 Error Handling  . . . . . . . . . . . . . . . . . .  11
   4.  Legacy ELC  . . . . . . . . . . . . . . . . . . . . . . . . .  11
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
     6.1.  Considerations for the NHC  . . . . . . . . . . . . . . .  13
     6.2.  Considerations for the ELCv3 Capability . . . . . . . . .  14
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  14
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  15
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  16
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  16

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   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  17

1.  Introduction

   [RFC5492] allows a Border Gateway Protocol (BGP) speaker to advertise
   its capabilities to its peer.  When a route is propagated beyond the
   immediate peer, it is useful to allow certain capabilities to be
   conveyed further.  In particular, it may be useful to advertise
   forwarding plane features.

   This specification defines a BGP optional transitive attribute to
   carry such capability information, the "Next Hop Dependent
   Capabilities Attribute", or NHC.  (Note that this specification
   should not be confused with RFC 5492 BGP Capabilities.)

   Since the NHC is intended chiefly for conveying information about
   forwarding plane features, it needs to be regenerated whenever the
   BGP route's next hop is changed.  Since owing to the properties of
   BGP transitive attributes this can't be guaranteed (an intermediate
   router that doesn't implement this specification would be expected to
   propagate the NHC as opaque data), the NHC encodes the next hop of
   its originator, or the router that most recently updated the
   attribute.  If the NHC passes through a router that changes the next
   hop without regenerating the NHC, they will fail to match when later
   examined, and the recipient can act accordingly.  This scheme allows
   NHC support to be introduced into a network incrementally.
   Informally, the intent is that,

   *  If a router is not changing the next hop, it can obliviously
      propagate the NHC just like any other optional transitive
      attribute.

   *  If a router is changing the next hop, then it has to be able to
      vouch for every capability it includes in the NHC.

   Complete details are provided in Section 2.

   An NHC carried in a given BGP UPDATE message conveys information that
   relates to all Network Layer Reachability Information (NLRI)
   advertised in that particular UPDATE, and only to those NLRI.  A
   different UPDATE message originated by the same source might not
   include an NHC, and if so, NLRI carried in that UPDATE would not be
   affected by the NHC.  By implication, if a router wishes to use NHC
   to describe all NLRI it originates, it needs to include an NHC with
   each UPDATE it sends.  In this respect, despite its similar naming,
   the NHC is unlike RFC 5492 BGP Capabilities.

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   Informally, a capability included in a given NHC should not be
   thought of as a capability of the next hop, but rather a capability
   of the path, that depends on the ability of the next hop to support
   it.  Hence it is said to be "dependent on" the next hop.

   This specification also defines an NHC capability, called "ELCv3", to
   advertise the ability to process the Multiprotocol Label Switching
   (MPLS) Entropy Label as an egress Label Switching Router (LSR) for
   all NLRI advertised in the BGP UPDATE.  It updates [RFC6790] and
   [RFC7447] with regard to this BGP signaling, this is further
   discussed in Section 3.  Although ELCv3 is only relevant to NLRI of
   labeled address families, a future NHC capability might be applicable
   to non-labeled NLRI, or to both, irrespective of labels.  (The term
   "labeled address family" is defined in the first paragraph of
   Section 3.5 of [RFC9012].  In this document, we use the term "labeled
   NLRI" as a short form of "NLRI of a labeled address family.")

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  BGP Next Hop Dependent Capabilities Attribute

2.1.  Encoding

   The BGP Next Hop Dependent Capabilities attribute (NHC attribute, or
   just NHC) is an optional, transitive BGP path attribute with type
   code 39.  The NHC always includes a network layer address identifying
   the next hop of the route the NHC accompanies.  The NHC signals
   potentially useful information related to the forwarding plane
   features, so it is desirable to make it transitive to ensure
   propagation across BGP speakers (e.g., route reflectors) that do not
   change the next hop and are therefore not in the forwarding path.
   The next hop data is to ensure correctness if it traverses BGP
   speakers that do not understand the NHC.  This is further explained
   below.

   The Attribute Data field of the NHC attribute is encoded as a header
   portion that identifies the router that created or most recently
   updated the attribute, followed by one or more capability Type-
   Length-Value (TLV) triples:

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        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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   Address Family Identifier   |     SAFI      | Next Hop Len  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~             Network Address of Next Hop (variable)            ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~                   Capability TLVs (variable)                  ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                            Figure 1: NHC Format

   The meanings of the header fields (Address Family Identifier, SAFI or
   Subsequent Address Family Identifier, Length of Next Hop, and Network
   Address of Next Hop) are as given in Section 3 of [RFC4760].

   In turn, each Capability is a TLV:

        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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |        Capability Code        |        Capability Length      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~                  Capability Value (variable)                  ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 2: Capability TLV Format

   Capability Code: a two-octet unsigned integer that indicates the type
   of capability advertised and unambiguously identifies an individual
   capability.

   Capability Length: a two-octet unsigned integer that indicates the
   length, in octets, of the Capability Value field.  A length of 0
   indicates that the Capability Value field is zero-length, i.e. it has
   a null value.

   Capability Value: a variable-length field.  It is interpreted
   according to the value of the Capability Code.

   A BGP speaker MUST NOT include more than one instance of a capability
   with the same Capability Code, Capability Length, and Capability
   Value.  Note, however, that processing multiple instances of such a
   capability does not require special handling, as additional instances
   do not change the meaning of the announced capability; thus, a BGP
   speaker MUST be prepared to accept such multiple instances.

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   BGP speakers MAY include more than one instance of a capability (as
   identified by the Capability Code) with different Capability Value.
   Processing of these capability instances is specific to the
   Capability Code and MUST be described in the document introducing the
   new capability.

   Capability TLVs MUST be placed in the NHC in increasing order of
   Capability Code.  (In the event of multiple instances of a capability
   with the same Capability Code as discussed above, no further sorting
   order is defined here.)  Although the major sorting order is
   mandated, an implementation MUST elect to be prepared to consume
   capabilities in any order, for robustness reasons.

2.2.  Sending the NHC

   Suppose a BGP speaker S has a route R it wishes to advertise with
   next hop N to its peer.

   If S is originating R into BGP, it MAY include an NHC attribute with
   it, that carries capability TLVs that describe aspects of R.  S MUST
   set the next hop depicted in the header portion of the NHC to be
   equal to N, using the encoding given above.

   If S has received R from some other BGP speaker, two possibilities
   exist.  First, S could be propagating R without changing N.  In that
   case, S need take no special action, it SHOULD simply propagate the
   NHC unchanged unless specifically configured otherwise.  Indeed, we
   observe that this is no different from the default action a BGP
   speaker takes with an unrecognized optional transitive attribute --
   it is treated as opaque data and propagated.

   Second, S could be changing R in some way, and in particular, it
   could be changing N.  If S has changed N it MUST NOT propagate the
   NHC unchanged.  It SHOULD include a newly-constructed NHC attribute
   with R, constructed as described above in the "originating R into
   BGP" case.  Any given capability TLV carried by the newly-constructed
   NHC attribute might use information from the received NHC attribute
   as input to its construction, possibly as straightforwardly as simply
   copying the TLV.  The details of how the capabilities in the new NHC
   are constructed are specific to the definition of each capability.
   Any capability TLVs received by S that are for capabilities not
   supported by S will not be included in the newly-constructed NHC
   attribute S includes with R.

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   An implementation SHOULD propagate the NHC and its contained
   capabilities by default.  An implementation SHOULD provide
   configuration control of whether any given capability is propagated.
   An implementation MAY provide finer-grained control on propagation
   based on attributes of the peering session, as discussed in
   Section 6.1.

   Due to the nature of BGP optional transitive path attributes, any BGP
   speaker that does not implement this specification will propagate the
   NHC, the requirements of this section notwithstanding.  Such a
   speaker will not update the NHC, however.

   Certain NLRI formats do not include a next hop at all, one example
   being the Flow Specification NLRI [RFC8955].  The NHC MUST NOT be
   sent with such NLRI.

2.2.1.  Aggregation

   When aggregating routes, the above rules for constructing a new NHC
   MUST be followed.  The decision of whether to include the NHC with
   the aggregate route and what its form will be, depends in turn on
   whether any capabilities are eligible to be included with the
   aggregate route.  If there are no eligible capabilities, the NHC MUST
   NOT be included.

   The specification for an individual capability must define how that
   capability is to be aggregated.  If no rules are defined for a given
   capability, that capability MUST NOT be aggregated.  Rules for
   aggregating the ELCv3 are found in Section 3.2.1.

   (Route aggregation is described in [RFC4271].  Although prefix
   aggregation -- combining two or more more-specific prefixes to form
   one less-specific prefix -- is one application of aggregation, we
   note that another is when two or more routes for the same prefix are
   selected to be used for multipath forwarding.)

2.3.  Receiving the NHC

   An implementation receiving routes with a NHC SHOULD NOT discard the
   attribute or its contained capabilities by default.  An
   implementation SHOULD provide configuration control of whether any
   given capability is processed.  An implementation MAY provide finer-
   grained control on propagation based on attributes of the peering
   session, as discussed in Section 6.1.

   When a BGP speaker receives a BGP route that includes the NHC, it
   MUST compare the address given in the header portion of the NHC and
   illustrated in Figure 1 to the next hop of the BGP route.  If the two

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   match, the NHC may be further processed.  If the two do not match, it
   means some intermediate BGP speaker that handled the route in transit
   both does not support NHC, and changed the next hop of the route.  In
   this case, the contents of the NHC cannot be used, and the NHC MUST
   be discarded without further processing, except that the contents MAY
   be logged.

   In considering whether the next hop "matches", a semantic match is
   sought.  While bit-for-bit equality is a trivial test of matching,
   there may be certain cases where the two are not bit-for-bit equal,
   but still "match".  An example is when an MP_REACH Next Hop encodes
   both a global and a link-local IPv6 address.  In that case, the link-
   local address might be removed during Internal BGP (IBGP)
   propagation, the two would still be considered to match if they were
   equal on the global part.  See Section 3 of [RFC2545].

   A BGP speaker receiving a Capability Code that it supports behaves as
   defined in the document defining the Capability Code.  A BGP speaker
   receiving a Capability Code that it does not support MUST ignore that
   Capability Code.  In particular, the receipt of an unrecognized
   Capability Code MUST NOT be handled as an error.

   The presence of a capability SHOULD NOT influence route selection or
   route preference, unless tunneling is used to reach the BGP next hop,
   the selected route has been learned from External BGP (that is, the
   next hop is in a different Autonomous System), or by configuration
   (see following).  Indeed, it is in general impossible for a node to
   know that all BGP routers of the Autonomous System (AS) will
   understand a given capability, and if different routers within an AS
   were to use a different preference for a route, forwarding loops
   could result unless tunneling is used to reach the BGP next hop.
   Following this reasoning, if the administrator of the network is
   confident that all routers within the AS will interpret the presence
   of the capability in the same way, they could relax this restriction
   by configuration.

2.4.  Attribute Error Handling

   An NHC is considered malformed if the length of the attribute,
   encoded in the Attribute Length field of the BGP Path Attribute
   header (Section 4.3 of [RFC4271]), is inconsistent with the lengths
   of the contained capability TLVs.  In other words, the sum of the
   sizes (Capability Length plus 4) of the contained capability TLVs,
   plus the length of the NHC header (Figure 1), must be equal to the
   overall Attribute Length.

   A BGP UPDATE message with a malformed NHC SHALL be handled using the
   approach of "attribute discard" defined in [RFC7606].

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   Unknown Capability Codes MUST NOT be considered to be an error.

   An NHC that contains no capability TLVs MAY be considered malformed,
   although it is observed that the prescribed behavior of "attribute
   discard" is semantically no different from that of having no TLVs to
   process.  There is no reason to propagate an NHC that contains no
   capability TLVs.

   A document that specifies a new NHC Capability should provide
   specifics regarding what constitutes an error for that NHC
   Capability.

   If a capability TLV is malformed, that capability TLV SHOULD be
   ignored and removed.  Other capability TLVs SHOULD be processed as
   usual.  If a given capability TLV requires different error-handling
   treatment than described in the previous sentences, its specification
   should provide specifics.

2.5.  Network Operation Considerations

   In the corner case where multiple nodes use the same IP address as
   their BGP next hop, such as with anycast nodes as described in
   [RFC4786], a BGP speaker MUST NOT advertise a given capability unless
   all nodes sharing this same IP address support this capability.  The
   network operator operating those anycast nodes is responsible for
   ensuring that an anycast node does not advertise a capability not
   supported by all nodes sharing this anycast address.  The means for
   accomplishing this are beyond the scope of this document.

3.  Entropy Label Capability (ELCv3)

   The foregoing sections define the NHC as a container for capability
   TLVs.  The Entropy Label Capability is one such capability.

   When BGP [RFC4271] is used for distributing labeled NLRI as described
   in, for example, [RFC8277], the route may include the ELCv3 as part
   of the NHC.  The inclusion of this capability with a route indicates
   that the egress of the associated Label Switched Path (LSP) can
   process entropy labels as an egress LSR for that route -- see
   Section 4.1 of [RFC6790].  Below, we refer to this for brevity as
   being "EL-capable."

   For historical reasons, this capability is referred to as "ELCv3", to
   distinguish it from the prior Entropy Label Capability (ELC) defined
   in [RFC6790] and deprecated in [RFC7447], and the ELCv2 described in
   [I-D.scudder-bgp-entropy-label].

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   This section (and its subsections) replaces Section 5.2 of [RFC6790],
   which was previously deprecated by [RFC7447].

3.1.  Encoding

   The ELCv3 has capability code 1, capability length 0, and carries no
   value:

        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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      Capability Code = 1      |       Capability Length = 0   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                         Figure 3: ELCv3 TLV Format

3.2.  Sending the ELCv3

   When a BGP speaker S has a route R it wishes to advertise with next
   hop N to its peer, it SHOULD include the ELCv3 capability if it knows
   that the egress of the associated LSP L is EL-capable, otherwise it
   MUST NOT include the ELCv3 capability.  Specific conditions where S
   would know that the egress is EL-capable are if S:

   *  Is itself the egress, and knows itself to be EL-capable, or

   *  Is re-advertising a BGP route it received with a valid ELCv3
      capability, and is preserving the value of N as received, or

   *  Is re-advertising a BGP route it received with a valid ELCv3
      capability, and is changing the next hop that it has received to
      N, and knows that this new next hop (normally itself) is EL-
      capable, or

   *  Is re-advertising a BGP route it received with a valid ELCv3
      capability, and is changing the next hop that it has received to
      N, and knows (for example, through configuration) that the new
      next hop (normally itself) even if not EL-capable will simply swap
      labels without popping the BGP-advertised label stack and
      processing the label below, as with a transit LSR, or

   *  Knows by implementation-specific means that the egress is EL-
      capable, or

   *  Is redistributing a route learned from another protocol, and that
      other protocol conveyed the knowledge that the egress of L was EL-
      capable.  (For example, this might be known through the Label
      Distribution Protocol (LDP) ELC TLV, Section 5.1 of [RFC6790].)

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   The ELCv3 MAY be advertised with routes that are labeled, such as
   those using SAFI 4 [RFC8277].  It MUST NOT be advertised with
   unlabeled routes.

3.2.1.  Aggregation

   When forming an aggregate (see Section 2.2.1), the aggregate route
   thus formed MUST NOT include the ELCv3 unless each constituent route
   would be eligible to include the ELCv3 according to the criteria
   given above.

3.3.  Receiving the ELCv3

   (Below, we assume that "includes the ELCv3" implies that the
   containing NHC has passed the checks specified in Section 2.3.  If it
   had not passed, then the NHC would have been discarded and the ELCv3
   would be deemed not to have been included.)

   When a BGP speaker receives an unlabeled route that includes the
   ELCv3, it MUST discard the ELCv3.

   When a BGP speaker receives a labeled route that includes the ELCv3,
   it indicates the associated LSP supports entropy labels.  This
   implies that the receiving BGP speaker if acting as ingress, MAY
   insert an entropy label as per Section 4.2 of [RFC6790].

3.4.  ELCv3 Error Handling

   The ELCv3 is considered malformed and must be disregarded if its
   length is other than zero.

   If more than one instance of the ELCv3 is included in an NHC,
   instances beyond the first MUST be disregarded.

4.  Legacy ELC

   The ELCv3 functionality introduced in this document replaces the "BGP
   Entropy Label Capability Attribute" (ELC attribute) that was
   introduced by [RFC6790], and deprecated by [RFC7447].  The latter RFC
   specifies that the ELC attribute, BGP path attribute 28, "MUST be
   treated as any other unrecognized optional, transitive attribute".
   This specification revises that requirement.

   As the current specification was developed, it became clear that due
   to incompatibilities between how the ELC attribute is processed by
   different fielded implementations, the most prudent handling of
   attribute 28 is not to propagate it as an unrecognized optional,
   transitive attribute, but to discard it.  Therefore, this

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   specification updates [RFC7447], by instead requiring that an
   implementation that receives the ELC attribute MUST discard any
   received ELC attribute.

5.  IANA Considerations

   IANA has made a temporary allocation in the BGP Path Attributes
   registry of the Border Gateway Protocol (BGP) Parameters group.  IANA
   is requested to make this allocation permanent.

    +=======+===========================================+============+
    | Value | Code                                      | Reference  |
    +=======+===========================================+============+
    | 39    | BGP Next Hop Dependent Capabilities (NHC) | (this doc) |
    +-------+-------------------------------------------+------------+

                                 Table 1

   IANA is requested to create a new registry called "BGP Next Hop
   Dependent Capability Codes" within the Border Gateway Protocol (BGP)
   Parameters group.  The registry's allocation policy is First Come,
   First Served, except where designated otherwise in Table 2.  It is
   seeded with the following values:

   +===============+==================+============+===================+
   | Value         | Description      | Reference  | Change Controller |
   +===============+==================+============+===================+
   | 0             | reserved         | (this doc) | IETF              |
   +---------------+------------------+------------+-------------------+
   | 1             | ELCv3            | (this doc) | IETF              |
   +---------------+------------------+------------+-------------------+
   | 65400 -       | private use      | (this doc) | IETF              |
   | 65499         |                  |            |                   |
   +---------------+------------------+------------+-------------------+
   | 65500 -       | reserved for     | (this doc) | IETF              |
   | 65534         | experimental use |            |                   |
   +---------------+------------------+------------+-------------------+
   | 65535         | reserved         | (this doc) | IETF              |
   +---------------+------------------+------------+-------------------+

                                  Table 2

6.  Security Considerations

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6.1.  Considerations for the NHC

   The header portion of the NHC contains the next hop the attribute's
   originator included when sending it, or that an intermediate router
   included when updating the attribute (in the latter case, the
   "contract" with the intermediate router is that it performed the
   checks in Section 2.3 before propagating the attribute).  This will
   typically be an IP address of the router in question.  This may be an
   infrastructure address the network operator does not intend to
   announce beyond the border of its Autonomous System, and it may even
   be considered in some weak sense, confidential information.

   A motivating application for this attribute is to convey information
   between Autonomous Systems that are under the control of the same
   administrator.  In such a case, it would not need to be sent to other
   Autonomous Systems.  At time of writing, work
   [I-D.uttaro-idr-bgp-oad] is underway to standardize a method of
   distinguishing between the two categories of external Autonomous
   Systems, and if such a distinction is available, an implementation
   can take advantage of it by constraining the NHC and its contained
   capabilities to only propagate by default to and from the former
   category of Autonomous Systems.  If such a distinction is not
   available, a network operator may prefer to configure routers peering
   with Autonomous Systems not under their administrative control to not
   send or accept the NHC or its contained capabilities, unless there is
   an identified need to do so.

   The foregoing notwithstanding, control of NHC propagation can't be
   guaranteed in all cases -- if a border router doesn't implement this
   specification, the attribute, like all BGP optional transitive
   attributes, will propagate to neighboring Autonomous Systems.  (This
   can be seen as a specific case of the general "attribute escape"
   phenomenon discussed in [I-D.haas-idr-bgp-attribute-escape].)
   Similarly, if a border router receiving the attribute from an
   external Autonomous System doesn't implement this specification, it
   will store and propagate the attribute, the requirements of
   Section 2.3 notwithstanding.  So, sometimes this information could
   leak beyond its intended scope.  (Note that it will only propagate as
   far as the first router that does support this specification, at
   which point it will typically be discarded due to a non-matching next
   hop, per Section 2.3.)

   If the attribute leaks beyond its intended scope, capabilities within
   it would potentially be exposed.  Specifications for individual
   capabilities should consider the consequences of such unintended
   exposure, and should identify any necessary constraints on
   propagation.

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6.2.  Considerations for the ELCv3 Capability

   Insertion of an ELCv3 by an attacker could cause forwarding to fail.
   Deletion of an ELCv3 by an attacker could cause one path in the
   network to be overutilized and another to be underutilized.  However,
   we note that an attacker able to accomplish either of these (below,
   an "on-path attacker") could equally insert or remove any other BGP
   path attribute or message.  The former attack described above denies
   service for a given route, which can be accomplished by an on-path
   attacker in any number of ways even absent ELCv3.  The latter attack
   defeats an optimization but nothing more; it seems dubious that an
   attacker would go to the trouble of doing so rather than launching
   some more damaging attack.

7.  References

7.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/rfc/rfc2119>.

   [RFC2545]  Marques, P. and F. Dupont, "Use of BGP-4 Multiprotocol
              Extensions for IPv6 Inter-Domain Routing", RFC 2545,
              DOI 10.17487/RFC2545, March 1999,
              <https://www.rfc-editor.org/rfc/rfc2545>.

   [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,
              <https://www.rfc-editor.org/rfc/rfc4271>.

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

   [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/rfc/rfc6790>.

   [RFC7447]  Scudder, J. and K. Kompella, "Deprecation of BGP Entropy
              Label Capability Attribute", RFC 7447,
              DOI 10.17487/RFC7447, February 2015,
              <https://www.rfc-editor.org/rfc/rfc7447>.

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   [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,
              <https://www.rfc-editor.org/rfc/rfc7606>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.

   [RFC9012]  Patel, K., Van de Velde, G., Sangli, S., and J. Scudder,
              "The BGP Tunnel Encapsulation Attribute", RFC 9012,
              DOI 10.17487/RFC9012, April 2021,
              <https://www.rfc-editor.org/rfc/rfc9012>.

7.2.  Informative References

   [I-D.haas-idr-bgp-attribute-escape]
              Haas, J., "BGP Attribute Escape", Work in Progress,
              Internet-Draft, draft-haas-idr-bgp-attribute-escape-00, 9
              July 2023, <https://datatracker.ietf.org/doc/html/draft-
              haas-idr-bgp-attribute-escape-00>.

   [I-D.ietf-idr-next-hop-capability]
              Decraene, B., Kompella, K., and W. Henderickx, "BGP Next-
              Hop dependent capabilities", Work in Progress, Internet-
              Draft, draft-ietf-idr-next-hop-capability-08, 8 June 2022,
              <https://datatracker.ietf.org/doc/html/draft-ietf-idr-
              next-hop-capability-08>.

   [I-D.scudder-bgp-entropy-label]
              Scudder, J. and K. Kompella, "BGP Entropy Label
              Capability, Version 2", Work in Progress, Internet-Draft,
              draft-scudder-bgp-entropy-label-00, 28 April 2022,
              <https://datatracker.ietf.org/doc/html/draft-scudder-bgp-
              entropy-label-00>.

   [I-D.uttaro-idr-bgp-oad]
              Uttaro, J., Retana, A., Mohapatra, P., Patel, K., and B.
              Wen, "One Administrative Domain using BGP", Work in
              Progress, Internet-Draft, draft-uttaro-idr-bgp-oad-02, 10
              July 2023, <https://datatracker.ietf.org/doc/html/draft-
              uttaro-idr-bgp-oad-02>.

   [RFC4786]  Abley, J. and K. Lindqvist, "Operation of Anycast
              Services", BCP 126, RFC 4786, DOI 10.17487/RFC4786,
              December 2006, <https://www.rfc-editor.org/rfc/rfc4786>.

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

   [RFC8277]  Rosen, E., "Using BGP to Bind MPLS Labels to Address
              Prefixes", RFC 8277, DOI 10.17487/RFC8277, October 2017,
              <https://www.rfc-editor.org/rfc/rfc8277>.

   [RFC8955]  Loibl, C., Hares, S., Raszuk, R., McPherson, D., and M.
              Bacher, "Dissemination of Flow Specification Rules",
              RFC 8955, DOI 10.17487/RFC8955, December 2020,
              <https://www.rfc-editor.org/rfc/rfc8955>.

Acknowledgements

   The authors of this specification thank Randy Bush, Wes Hardaker,
   Jeff Haas, Susan Hares, Ketan Talaulikar, and Gyan Mishra for their
   review and comments.

   This specification derives from two earlier documents,
   [I-D.ietf-idr-next-hop-capability] and
   [I-D.scudder-bgp-entropy-label].

   [I-D.ietf-idr-next-hop-capability] included the following
   acknowledgements:

     The Entropy Label Next-Hop Capability defined in this document is
     based on the ELC BGP attribute defined in section 5.2 of [RFC6790].

     The authors wish to thank John Scudder for the discussions on this
     topic and Eric Rosen for his in-depth review of this document.

     The authors wish to thank Jie Dong and Robert Raszuk for their
     review and comments.

   [I-D.scudder-bgp-entropy-label] included the following
   acknowledgements:

       Thanks to Swadesh Agrawal, Alia Atlas, Bruno Decraene, Martin
       Djernaes, John Drake, Adrian Farrell, Keyur Patel, Toby Rees, and
       Ravi Singh, for their discussion of this issue.

Contributors

   Serge Krier
   Cisco Systems
   Email: sekrier@cisco.com

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   Kevin Wang
   Juniper Networks
   Email: kfwang@juniper.net

Authors' Addresses

   Bruno Decraene (editor)
   Orange
   Email: bruno.decraene@orange.com

   John G. Scudder (editor)
   Juniper Networks
   Email: jgs@juniper.net

   Wim Henderickx
   Nokia
   Email: wim.henderickx@nokia.com

   Kireeti Kompella
   Juniper Networks
   Email: kireeti@juniper.net

   Satya Mohanty
   Cisco Systems
   Email: satyamoh@cisco.com

   James Uttaro
   Independent Contributor
   Email: juttaro@ieee.org

   Bin Wen
   Comcast
   Email: Bin_Wen@comcast.com

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