BGP Next Hop Dependent Characteristics Attribute
draft-ietf-idr-nhc-01

Internet Engineering Task Force                         B. Decraene, Ed.
Internet-Draft                                                    Orange
Intended status: Standards Track                             K. Kompella
Expires: 2 September 2026                                            HPE
                                                                S. Krier
                                                           Cisco Systems
                                                              S. Mohanty
                                                                 Zscaler
                                                      J. G. Scudder, Ed.
                                                                 K. Wang
                                                                     HPE
                                                                  B. Wen
                                                                 Comcast
                                                            1 March 2026

            BGP Next Hop Dependent Characteristics Attribute
                         draft-ietf-idr-nhc-01

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 characteristics to be conveyed further.  In
   particular, it is useful to advertise forwarding plane features.

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

About This Document

   This note is to be removed before publishing as an RFC.

   Status information for this document may be found at
   https://datatracker.ietf.org/doc/draft-ietf-idr-nhc/.

   Discussion of this document takes place on the IDR Working Group
   mailing list (mailto:idr@ietf.org), which is archived at
   https://mailarchive.ietf.org/arch/browse/idr/.  Subscribe at
   https://www.ietf.org/mailman/listinfo/idr/.

Status of This Memo

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

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   4
   2.  BGP Next Hop Dependent Characteristics Attribute  . . . . . .   4
     2.1.  Encoding  . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.2.  Sending the NHC . . . . . . . . . . . . . . . . . . . . .   6
       2.2.1.  Link-Local-Only Next Hops . . . . . . . . . . . . . .   7
       2.2.2.  Aggregation . . . . . . . . . . . . . . . . . . . . .   7
       2.2.3.  When Next Hop Resolution is Irrelevant to
               Forwarding  . . . . . . . . . . . . . . . . . . . . .   8
     2.3.  Receiving the NHC . . . . . . . . . . . . . . . . . . . .   8
     2.4.  Attribute Error Handling  . . . . . . . . . . . . . . . .   9
     2.5.  Anycast Next Hops . . . . . . . . . . . . . . . . . . . .  10
   3.  BGP Identifier Characteristic . . . . . . . . . . . . . . . .  10
     3.1.  Encoding  . . . . . . . . . . . . . . . . . . . . . . . .  11
     3.2.  Sending the BGPID . . . . . . . . . . . . . . . . . . . .  11
       3.2.1.  Aggregation . . . . . . . . . . . . . . . . . . . . .  11
     3.3.  Receiving the BGPID . . . . . . . . . . . . . . . . . . .  11
       3.3.1.  Not Receiving the BGPID . . . . . . . . . . . . . . .  12
     3.4.  BGPID Error Handling  . . . . . . . . . . . . . . . . . .  12
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12

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   5.  Implementation Status . . . . . . . . . . . . . . . . . . . .  13
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  15
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  15
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  16
   Appendix A.  A Case Where a Link-Local Next Hop Could Lead to a
           False Positive  . . . . . . . . . . . . . . . . . . . . .  18
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  19
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  20
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  20

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 characteristics 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 information, the "Next Hop Dependent Characteristics
   Attribute", or NHC.

   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 characteristic 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

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   different UPDATE message originated by the same source might not
   include an NHC, and if so, the 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.

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

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 Characteristics Attribute

2.1.  Encoding

   The BGP Next Hop Dependent Characteristics 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 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)            ~
      |                                                               |
      +---------------------------------------------------------------+
      |                                                               |
      ~                 Characteristic 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 Characteristic 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      Characteristic Code      |      Characteristic Length    |
      +-------------------------------+-------------------------------+
      |                                                               |
      ~                Characteristic Value (variable)                ~
      |                                                               |
      +---------------------------------------------------------------+

                    Figure 2: Characteristic TLV Format

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

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

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

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

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

   Characteristic TLVs MUST be placed in the NHC in increasing order of
   Characteristic Code.  (In the event of multiple instances of a
   characteristic with the same Characteristic Code as discussed above,
   no further sorting order is defined here.)  Although the major
   sorting order is mandated, an implementation MUST be prepared to
   consume characteristics 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 characteristic 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 does not need to take any 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 characteristic 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 characteristics in
   the new NHC are constructed are specific to the definition of each

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   characteristic.  Any characteristic TLVs received by S that are for
   characteristics not supported by S will not be included in the newly-
   constructed NHC attribute S includes with R.

   An implementation SHOULD propagate the NHC and its contained
   characteristics by default.  An implementation SHOULD provide
   configuration control of whether any given characteristic is
   propagated.  An implementation MAY provide finer-grained control on
   propagation based on attributes of the peering session, as discussed
   in Section 6.

   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.  Link-Local-Only Next Hops

   In some cases, the BGP speaker sending a route might encode only a
   link-local address and no global address.  In such a case, a problem
   arises because there is no expectation of global uniqueness of such
   an address, and the "semantic match" discussed in Section 2.3 could
   yield a false positive.  An illustration is provided in Appendix A.

   To mitigate this problem, if a BGP speaker originates a route whose
   next hop has no global part, it MUST include a BGPID TLV (Section 3).

2.2.2.  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 characteristics are eligible to be included with the
   aggregate route.  If there are no eligible characteristics, the NHC
   MUST NOT be included.

   The specification for an individual characteristic must define how
   that characteristic is to be aggregated.  If no rules are defined for
   a given characteristic, that characteristic MUST NOT be aggregated.

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   (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.)

2.2.3.  When Next Hop Resolution is Irrelevant to Forwarding

   In some cases, forwarding routes can be derived from a BGP route
   without regard to its next hop.  One example is when the Tunnel
   Encapsulation Attribute [RFC9012] Tunnel Egress Endpoint Sub-TLV is
   used to point to a remote router.  (The final paragraph of
   Section 7.2 of RFC 9012 includes a warning about this case.)

   The use of NHC is not completely precluded in such scenarios.  The
   principle that must be followed is that the router that attaches the
   attribute must have reliable knowledge that the information it
   includes with the NHC accurately depicts the forwarding plane that
   packets will encounter when forwarded according to the route.  If the
   router cannot accurately make that determination, it must not attach
   the NHC.

   A remaining concern pertains to intermediate routers.  It's possible
   that such a router might not support this specification and might
   change some aspect of the route that affects forwarding, without
   changing the next hop.  An example is if a route carried a Tunnel
   Encapsulation Attribute that was stripped by an intermediate router.
   Such scenarios are fraught with danger even in the absence of the
   NHC, but are not precluded by the protocol.

   Owing to these considerations, use of NHC in situations where
   forwarding is, or might be, noncongruent with the next hop, should be
   done with care.

2.3.  Receiving the NHC

   An implementation receiving routes with an NHC SHOULD NOT discard the
   attribute or its contained characteristics by default.  An
   implementation SHOULD provide configuration control of whether any
   given characteristic is processed.  An implementation MAY provide
   finer-grained control on propagation based on attributes of the
   peering session, as discussed in Section 6.

   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
   match, the NHC may be further processed.  If the two do not match, it
   means that some intermediate BGP speaker that handled the route in

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   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, but the two would still be considered to match if they
   were equal on the global part.  See Section 3 of [RFC2545].  In other
   cases, only a link-local address might be present.  This is discussed
   in Section 2.2.1; in such a case, further information is required to
   permit matching.  This is discussed in Section 3.

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

   The presence of a characteristic 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 characteristic, 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 characteristic 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 characteristic TLVs.  In other words, the sum of the
   sizes (Characteristic Length plus 4) of the contained characteristic
   TLVs, plus the length of the NHC header (Figure 1), must be equal to
   the overall Attribute Length.

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

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

   An NHC that contains no characteristic 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 characteristic TLVs.

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

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

2.5.  Anycast Next Hops

   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 characteristic
   unless all nodes sharing this same IP address support this
   characteristic.  The network operator operating those anycast nodes
   is responsible for ensuring that an anycast node does not advertise a
   characteristic not supported by all nodes sharing this anycast
   address.  The means for accomplishing this are beyond the scope of
   this document.

   In cases where a BGP speaker receives a route for some prefix P with
   next hop N that carries an NHC, and receives a different route for P,
   N that carries no NHC or a NHC with conflicting content, that could
   be indicative of a configuration error as described above.  In such a
   case, an implementation MAY log an error to help diagnose the
   potential problem.

3.  BGP Identifier Characteristic

   As discussed in Section 2.2.1, it might be possible that a route
   could be originated that has no global part in its next hop.  To
   provide uniqueness in this case, it is sufficient to associate the
   BGP Identifier and AS Number of the route's sender.  The BGP
   Identifier Characteristic (BGPID) provides a way to convey this
   information if required.

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3.1.  Encoding

   The BGPID has characteristic code 3, characteristic length 8, and
   carries as its value the BGP Identifier and Autonomous System Number
   of its sender:

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    Characteristic Code = 3    |   Characteristic Length = 8   |
      +-------------------------------+-------------------------------+
      |                        BGP Identifier                         |
      +---------------------------------------------------------------+
      |                          AS Number                            |
      +---------------------------------------------------------------+

                         Figure 3: BGPID TLV Format

   BGP Identifier: The BGP Identifier (Section 4.2 of [RFC4271], and
   [RFC6286]) of the route's sender.

   AS Number: The Autonomous System Number [RFC6793] of the route's
   sender.  In cases where the sender might represent different
   Autonomous System Numbers to different peers (for example, [RFC5065],
   [RFC7705]), the value used is the one that was in the sender's BGP
   OPEN to the peer concerned.

3.2.  Sending the BGPID

   Under the circumstances described in Section 2.2.1, the BGPID MUST be
   included.  Under other circumstances, the BGPID MAY be included.

3.2.1.  Aggregation

   Since the BGPID, by definition, is regenerated whenever the next hop
   is changed and provides context to disambiguate the next hop carried
   in the NHC header, there is no case in which it might need to be
   aggregated.

3.3.  Receiving the BGPID

   Under the circumstances described in Section 2.2.1, a next hop
   received from a given peer MUST NOT be considered a "semantic match"
   for the NHC unless the BGP Identifier and Autonomous System of that
   peer match the BGP Identifier and Autonomous System carried in the
   BGPID.

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   Since the only case in which the BGPID might be needed to
   disambiguate the next hop carried in the NHC involves the immediate
   peer (see Appendix A for more detail), the BGP Identifier and
   Autonomous System of the peer are readily derived; they are the
   values that were received in that peer's OPEN message.

   Other uses of the BGPID are beyond the scope of this document.  In
   particular, if a route is received that has a global part to its next
   hop and thus does not match the circumstances described in
   Section 2.2.1, but which nonetheless has a BGPID, this specification
   requires no specific action.  In such a case, the BGPID can be
   disregarded.

3.3.1.  Not Receiving the BGPID

   Under the circumstances described in Section 2.2.1, if a BGPID is not
   present in the NHC, the next hop match described in Section 2.3 MUST
   be considered to have failed.

3.4.  BGPID Error Handling

   The BGPID is considered malformed and must be disregarded if its
   length is other than eight.

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

   The situation where a route is received that fails the test described
   in Section 3.3 is not an error.  However, it might indicate a
   misconfiguration in the network, and a message MAY be logged.

4.  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 and to update its name
   and reference as shown below.

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

                                 Table 1

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   IANA is requested to create a new registry called "BGP Next Hop
   Dependent Characteristic 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        | draft-ietf-idr-      | IETF               |
   |       |              | elc-00               |                    |
   +-------+--------------+----------------------+--------------------+
   | 2     | NNHN         | draft-wang-idr-next- | kfwang@juniper.net |
   |       |              | next-hop-nodes-01    |                    |
   +-------+--------------+----------------------+--------------------+
   | 3     | BGPID        | (this doc)           | IETF               |
   +-------+--------------+----------------------+--------------------+
   | 4     | IFIT         | draft-ietf-idr-bgp-  | IETF               |
   |       |              | ifit-capabilities-05 |                    |
   +-------+--------------+----------------------+--------------------+
   | 5     | AMetric      | draft-ietf-idr-bgp-  | IETF               |
   |       |              | generic-metric-01    |                    |
   +-------+--------------+----------------------+--------------------+
   | 65400 | private use  | (this doc)           | IETF               |
   | -     |              |                      |                    |
   | 65499 |              |                      |                    |
   +-------+--------------+----------------------+--------------------+
   | 65500 | reserved for | (this doc)           | IETF               |
   | -     | experimental |                      |                    |
   | 65534 | use          |                      |                    |
   +-------+--------------+----------------------+--------------------+
   | 65535 | reserved     | (this doc)           | IETF               |
   +-------+--------------+----------------------+--------------------+

                                 Table 2

5.  Implementation Status

      |  RFC Editor: Please remove this entire section before
      |  publication, as well as the reference to RFC 7942.

   This section refers the reader to the status of known implementations
   of the protocol defined by this specification at the time of posting
   of this Internet-Draft, and is based on a proposal described in
   [RFC7942].  The description of implementations referenced by this
   section is intended to assist the IETF in its decision processes in

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   progressing drafts to RFCs.  Please note that the listing of any
   individual implementation does not imply endorsement by the IETF.
   Furthermore, no effort has been spent to verify the information
   presented that was supplied by IETF contributors.  This is not
   intended as, and must not be construed to be, a catalog of available
   implementations or their features.  Readers are advised to note that
   other implementations may exist.

   According to [RFC7942], "this will allow reviewers and working groups
   to assign due consideration to documents that have the benefit of
   running code, which may serve as evidence of valuable experimentation
   and feedback that have made the implemented protocols more mature.
   It is up to the individual working groups to use this information as
   they see fit".

   Implementations are reported at the IDR implementation status Wiki
   (https://wiki.ietf.org/group/idr/implementations/draft-ietf-idr-
   entropy-label).

6.  Security Considerations

   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 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 the 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
   characteristics 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 characteristics, unless there
   is an identified need to do so.

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   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, characteristics
   within it would potentially be exposed.  Specifications for
   individual characteristics should consider the consequences of such
   unintended exposure, and should identify any necessary constraints on
   propagation.

   [RFC8799] discusses Limited Domains and Internet Protocols.  The
   functionality defined in this document might be useful in realizing
   the control plane of some kinds of limited domains.

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>.

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   [RFC6286]  Chen, E. and J. Yuan, "Autonomous-System-Wide Unique BGP
              Identifier for BGP-4", RFC 6286, DOI 10.17487/RFC6286,
              June 2011, <https://www.rfc-editor.org/rfc/rfc6286>.

   [RFC6793]  Vohra, Q. and E. Chen, "BGP Support for Four-Octet
              Autonomous System (AS) Number Space", RFC 6793,
              DOI 10.17487/RFC6793, December 2012,
              <https://www.rfc-editor.org/rfc/rfc6793>.

   [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>.

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-03, 9
              April 2025, <https://datatracker.ietf.org/doc/html/draft-
              haas-idr-bgp-attribute-escape-03>.

   [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-07, 14
              October 2025, <https://datatracker.ietf.org/doc/html/
              draft-uttaro-idr-bgp-oad-07>.

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   [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>.

   [RFC5065]  Traina, P., McPherson, D., and J. Scudder, "Autonomous
              System Confederations for BGP", RFC 5065,
              DOI 10.17487/RFC5065, August 2007,
              <https://www.rfc-editor.org/rfc/rfc5065>.

   [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>.

   [RFC7705]  George, W. and S. Amante, "Autonomous System Migration
              Mechanisms and Their Effects on the BGP AS_PATH
              Attribute", RFC 7705, DOI 10.17487/RFC7705, November 2015,
              <https://www.rfc-editor.org/rfc/rfc7705>.

   [RFC7942]  Sheffer, Y. and A. Farrel, "Improving Awareness of Running
              Code: The Implementation Status Section", BCP 205,
              RFC 7942, DOI 10.17487/RFC7942, July 2016,
              <https://www.rfc-editor.org/rfc/rfc7942>.

   [RFC8799]  Carpenter, B. and B. Liu, "Limited Domains and Internet
              Protocols", RFC 8799, DOI 10.17487/RFC8799, July 2020,
              <https://www.rfc-editor.org/rfc/rfc8799>.

   [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>.

   [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>.

   [I-D.ietf-idr-elc]
              Wen, B., Wang, K., Scudder, J., Satya, M. R., Krier, S.,
              Kompella, K., and B. Decraene, "BGP Entropy Label
              Characteristic", Work in Progress, Internet-Draft, draft-
              ietf-idr-elc-00, 2 November 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-idr-elc-
              00>.

   [I-D.wang-idr-next-next-hop-nodes]
              Wang, K., Haas, J., Lin, C., and J. Tantsura, "BGP Next-
              next Hop Nodes", Work in Progress, Internet-Draft, draft-

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              wang-idr-next-next-hop-nodes-04, 26 August 2025,
              <https://datatracker.ietf.org/doc/html/draft-wang-idr-
              next-next-hop-nodes-04>.

   [I-D.ietf-idr-bgp-ifit-capabilities]
              Fioccola, G., Pang, R., Wang, S., Decraene, B., Zhuang,
              S., and H. Wang, "Advertising In-situ Flow Information
              Telemetry (IFIT) Capabilities in BGP", Work in Progress,
              Internet-Draft, draft-ietf-idr-bgp-ifit-capabilities-08,
              15 October 2025, <https://datatracker.ietf.org/doc/html/
              draft-ietf-idr-bgp-ifit-capabilities-08>.

   [I-D.ietf-idr-bgp-generic-metric]
              Sangli, S. R., Hegde, S., Das, R., Decraene, B., Wen, B.,
              Kozak, M., Dong, J., Jalil, L., and K. Talaulikar,
              "Accumulated Metric in NHC attribute", Work in Progress,
              Internet-Draft, draft-ietf-idr-bgp-generic-metric-02, 6
              January 2026, <https://datatracker.ietf.org/doc/html/
              draft-ietf-idr-bgp-generic-metric-02>.

Appendix A.  A Case Where a Link-Local Next Hop Could Lead to a False
             Positive

   Consider a simple BGP peering topology, with four routers, in three
   Autonomous Systems:

    +----+ +------------+ +----+
    |    | |            | |    |
    | A <---> B <--> C <---> D |
    |    | |            | |    |
    +----+ +------------+ +----+
     AS X       AS Y       AS Z

                    Figure 4: A Trivial Peering Topology

   Suppose A and D support NHC.  B and C do not support NHC.  In this
   case, when A originates a route with an attached NHC, if B propagates
   it to C, and C updates the next hop when propagating it to D, D will
   follow the procedures of Section 2.3 and will discard the NHC without
   further processing.

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   However, now suppose that on the peerings between A and B, and
   between C and D, only link-local addresses are used.  Further,
   suppose that A uses link-local address L as its local address on its
   peering with B, and C also uses the same address, L, as its local
   address on its peering with D.  In the situation described in the
   previous paragraph, D would have no way of detecting that C had
   violated the correctness assumptions of this specification, due to
   the collision between its address and A's.

   It can be seen that since the scope of a link-local address is, of
   course, only the local link, the problem to be solved is restricted
   to knowing whether an immediate peer whose link-local address appears
   in the NHC is truly the originator of that NHC, or if it might be an
   NHC-incapable speaker that has propagated an NHC that originated
   elsewhere, with a colliding address.

   It can further be seen that if the procedures of Section 3 are
   followed, this issue is resolved since A will attach a BGPID TLV
   containing its own BGP Identifier and its AS Number, X.  Even if C's
   BGP Identifier is the same as A's, its AS Number is different, and
   thus D will discard the NHC without further processing.

Acknowledgements

   The authors of this specification thank Randy Bush, Mach Chen,
   Giuseppe Fioccola, 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:

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       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

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

   James Uttaro
   Independent Contributor
   Email: juttaro@ieee.org

Authors' Addresses

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

   Kireeti Kompella
   HPE
   Email: kireeti@juniper.net

   Serge Krier
   Cisco Systems
   Email: sekrier@cisco.com

   Satya Mohanty
   Zscaler
   Email: smohanty@zscaler.com

   John G. Scudder (editor)
   HPE
   Email: jgs@bgp.nu

   Kevin Wang
   HPE
   Email: kfwang@juniper.net

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   Bin Wen
   Comcast
   Email: Bin_Wen@comcast.com

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