The BGP Tunnel Encapsulation Attribute
RFC 9012

Document Type RFC - Proposed Standard (April 2021; No errata)
Obsoletes RFC 5566, RFC 5512
Updates RFC 5640
Authors Keyur Patel  , Gunter Van de Velde  , Srihari Sangli  , John Scudder 
Last updated 2021-04-27
Replaces draft-vandevelde-idr-remote-next-hop, draft-rosen-idr-tunnel-encaps
Stream Internet Engineering Task Force (IETF)
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Send notices to John Scudder <jgs@juniper.net>, aretana.ietf@gmail.com, Susan Hares <shares@ndzh.com>
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Internet Engineering Task Force (IETF)                          K. Patel
Request for Comments: 9012                                   Arrcus, Inc
Obsoletes: 5512, 5566                                    G. Van de Velde
Updates: 5640                                                      Nokia
Category: Standards Track                                      S. Sangli
ISSN: 2070-1721                                               J. Scudder
                                                        Juniper Networks
                                                              April 2021

                 The BGP Tunnel Encapsulation Attribute

Abstract

   This document defines a BGP path attribute known as the "Tunnel
   Encapsulation attribute", which can be used with BGP UPDATEs of
   various Subsequent Address Family Identifiers (SAFIs) to provide
   information needed to create tunnels and their corresponding
   encapsulation headers.  It provides encodings for a number of tunnel
   types, along with procedures for choosing between alternate tunnels
   and routing packets into tunnels.

   This document obsoletes RFC 5512, which provided an earlier
   definition of the Tunnel Encapsulation attribute.  RFC 5512 was never
   deployed in production.  Since RFC 5566 relies on RFC 5512, it is
   likewise obsoleted.  This document updates RFC 5640 by indicating
   that the Load-Balancing Block sub-TLV may be included in any Tunnel
   Encapsulation attribute where load balancing is desired.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9012.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction
     1.1.  Brief Summary of RFC 5512
     1.2.  Deficiencies in RFC 5512
     1.3.  Use Case for the Tunnel Encapsulation Attribute
     1.4.  Brief Summary of Changes from RFC 5512
     1.5.  Update to RFC 5640
     1.6.  Effects of Obsoleting RFC 5566
   2.  The Tunnel Encapsulation Attribute
   3.  Tunnel Encapsulation Attribute Sub-TLVs
     3.1.  The Tunnel Egress Endpoint Sub-TLV (Type Code 6)
       3.1.1.  Validating the Address Subfield
     3.2.  Encapsulation Sub-TLVs for Particular Tunnel Types (Type
           Code 1)
       3.2.1.  VXLAN (Tunnel Type 8)
       3.2.2.  NVGRE (Tunnel Type 9)
       3.2.3.  L2TPv3 (Tunnel Type 1)
       3.2.4.  GRE (Tunnel Type 2)
       3.2.5.  MPLS-in-GRE (Tunnel Type 11)
     3.3.  Outer Encapsulation Sub-TLVs
       3.3.1.  DS Field (Type Code 7)
       3.3.2.  UDP Destination Port (Type Code 8)
     3.4.  Sub-TLVs for Aiding Tunnel Selection
       3.4.1.  Protocol Type Sub-TLV (Type Code 2)
       3.4.2.  Color Sub-TLV (Type Code 4)
     3.5.  Embedded Label Handling Sub-TLV (Type Code 9)
     3.6.  MPLS Label Stack Sub-TLV (Type Code 10)
     3.7.  Prefix-SID Sub-TLV (Type Code 11)
   4.  Extended Communities Related to the Tunnel Encapsulation
           Attribute
     4.1.  Encapsulation Extended Community
     4.2.  Router's MAC Extended Community
     4.3.  Color Extended Community
   5.  Special Considerations for IP-in-IP Tunnels
   6.  Semantics and Usage of the Tunnel Encapsulation Attribute
   7.  Routing Considerations
     7.1.  Impact on the BGP Decision Process
     7.2.  Looping, Mutual Recursion, Etc.
   8.  Recursive Next-Hop Resolution
   9.  Use of Virtual Network Identifiers and Embedded Labels When
           Imposing a Tunnel Encapsulation
     9.1.  Tunnel Types without a Virtual Network Identifier Field
     9.2.  Tunnel Types with a Virtual Network Identifier Field
       9.2.1.  Unlabeled Address Families
       9.2.2.  Labeled Address Families
   10. Applicability Restrictions
   11. Scoping
   12. Operational Considerations
   13. Validation and Error Handling
   14. IANA Considerations
     14.1.  Obsoleting RFC 5512
     14.2.  Obsoleting Code Points Assigned by RFC 5566
     14.3.  Border Gateway Protocol (BGP) Tunnel Encapsulation
             Grouping
     14.4.  BGP Tunnel Encapsulation Attribute Tunnel Types
     14.5.  Subsequent Address Family Identifiers
     14.6.  BGP Tunnel Encapsulation Attribute Sub-TLVs
     14.7.  Flags Field of VXLAN Encapsulation Sub-TLV
     14.8.  Flags Field of NVGRE Encapsulation Sub-TLV
     14.9.  Embedded Label Handling Sub-TLV
     14.10. Color Extended Community Flags
   15. Security Considerations
   16. References
     16.1.  Normative References
     16.2.  Informative References
   Appendix A.  Impact on RFC 8365
   Acknowledgments
   Contributors
   Authors' Addresses

1.  Introduction

   This document obsoletes [RFC5512].  The deficiencies of [RFC5512],
   and a summary of the changes made, are discussed in Sections 1.1-1.3.
   The material from [RFC5512] that is retained has been incorporated
   into this document.  Since [RFC5566] relies on [RFC5512], it is
   likewise obsoleted.

   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.

1.1.  Brief Summary of RFC 5512

   [RFC5512] defines a BGP path attribute known as the Tunnel
   Encapsulation attribute.  This attribute consists of one or more
   TLVs.  Each TLV identifies a particular type of tunnel.  Each TLV
   also contains one or more sub-TLVs.  Some of the sub-TLVs, for
   example, the Encapsulation sub-TLV, contain information that may be
   used to form the encapsulation header for the specified tunnel type.
   Other sub-TLVs, for example, the "color sub-TLV" and the "protocol
   sub-TLV", contain information that aids in determining whether
   particular packets should be sent through the tunnel that the TLV
   identifies.

   [RFC5512] only allows the Tunnel Encapsulation attribute to be
   attached to BGP UPDATE messages of the Encapsulation Address Family.
   These UPDATE messages have an Address Family Identifier (AFI) of 1 or
   2, and a SAFI of 7.  In an UPDATE of the Encapsulation SAFI, the
   Network Layer Reachability Information (NLRI) is an address of the
   BGP speaker originating the UPDATE.  Consider the following scenario:

   *  BGP speaker R1 has received and selected UPDATE U for local use;

   *  UPDATE U's SAFI is the Encapsulation SAFI;

   *  UPDATE U has the address R2 as its NLRI;

   *  UPDATE U has a Tunnel Encapsulation attribute.

   *  R1 has a packet, P, to transmit to destination D; and

   *  R1's best route to D is a BGP route that has R2 as its next hop.

   In this scenario, when R1 transmits packet P, it should transmit it
   to R2 through one of the tunnels specified in U's Tunnel
   Encapsulation attribute.  The IP address of the tunnel egress
   endpoint of each such tunnel is R2.  Packet P is known as the
   tunnel's "payload".

1.2.  Deficiencies in RFC 5512

   While the ability to specify tunnel information in a BGP UPDATE is
   useful, the procedures of [RFC5512] have certain limitations:

   *  The requirement to use the Encapsulation SAFI presents an
      unfortunate operational cost, as each BGP session that may need to
      carry tunnel encapsulation information needs to be reconfigured to
      support the Encapsulation SAFI.  The Encapsulation SAFI has never
      been used, and this requirement has served only to discourage the
      use of the Tunnel Encapsulation attribute.

   *  There is no way to use the Tunnel Encapsulation attribute to
      specify the tunnel egress endpoint address of a given tunnel;
      [RFC5512] assumes that the tunnel egress endpoint of each tunnel
      is specified as the NLRI of an UPDATE of the Encapsulation SAFI.

   *  If the respective best routes to two different address prefixes
      have the same next hop, [RFC5512] does not provide a
      straightforward method to associate each prefix with a different
      tunnel.

   *  If a particular tunnel type requires an outer IP or UDP
      encapsulation, there is no way to signal the values of any of the
      fields of the outer encapsulation.

   *  In the specification of the sub-TLVs in [RFC5512], each sub-TLV
      has a one-octet Length field.  In some cases, where a sub-TLV may
      require more than 255 octets for its encoding, a two-octet Length
      field may be needed.

1.3.  Use Case for the Tunnel Encapsulation Attribute

   Consider the case of a router R1 forwarding an IP packet P.  Let D be
   P's IP destination address.  R1 must look up D in its forwarding
   table.  Suppose that the "best match" route for D is route Q, where Q
   is a BGP-distributed route whose "BGP next hop" is router R2.  And
   suppose further that the routers along the path from R1 to R2 have
   entries for R2 in their forwarding tables but do NOT have entries for
   D in their forwarding tables.  For example, the path from R1 to R2
   may be part of a "BGP-free core", where there are no BGP-distributed
   routes at all in the core.  Or, as in [RFC5565], D may be an IPv4
   address while the intermediate routers along the path from R1 to R2
   may support only IPv6.

   In cases such as this, in order for R1 to properly forward packet P,
   it must encapsulate P and send P "through a tunnel" to R2.  For
   example, R1 may encapsulate P using GRE, Layer 2 Tunneling Protocol
   version 3 (L2TPv3), IP in IP, etc., where the destination IP address
   of the encapsulation header is the address of R2.

   In order for R1 to encapsulate P for transport to R2, R1 must know
   what encapsulation protocol to use for transporting different sorts
   of packets to R2.  R1 must also know how to fill in the various
   fields of the encapsulation header.  With certain encapsulation
   types, this knowledge may be acquired by default or through manual
   configuration.  Other encapsulation protocols have fields such as
   session id, key, or cookie that must be filled in.  It would not be
   desirable to require every BGP speaker to be manually configured with
   the encapsulation information for every one of its BGP next hops.

   This document specifies a way in which BGP itself can be used by a
   given BGP speaker to tell other BGP speakers, "If you need to
   encapsulate packets to be sent to me, here's the information you need
   to properly form the encapsulation header".  A BGP speaker signals
   this information to other BGP speakers by using a new BGP attribute
   type value -- the BGP Tunnel Encapsulation attribute.  This attribute
   specifies the encapsulation protocols that may be used, as well as
   whatever additional information (if any) is needed in order to
   properly use those protocols.  Other attributes, for example,
   communities or extended communities, may also be included.

1.4.  Brief Summary of Changes from RFC 5512

   This document addresses the deficiencies identified in Section 1.2
   by:

   *  Deprecating the Encapsulation SAFI.

   *  Defining a new "Tunnel Egress Endpoint sub-TLV" (Section 3.1) that
      can be included in any of the TLVs contained in the Tunnel
      Encapsulation attribute.  This sub-TLV can be used to specify the
      remote endpoint address of a particular tunnel.

   *  Allowing the Tunnel Encapsulation attribute to be carried by BGP
      UPDATEs of additional AFI/SAFIs.  Appropriate semantics are
      provided for this way of using the attribute.

   *  Defining a number of new sub-TLVs that provide additional
      information that is useful when forming the encapsulation header
      used to send a packet through a particular tunnel.

   *  Defining the Sub-TLV Type field so that a sub-TLV whose type is in
      the range from 0 to 127 (inclusive) has a one-octet Length field,
      but a sub-TLV whose type is in the range from 128 to 255
      (inclusive) has a two-octet Length field.

   One of the sub-TLVs defined in [RFC5512] is the "Encapsulation sub-
   TLV".  For a given tunnel, the Encapsulation sub-TLV specifies some
   of the information needed to construct the encapsulation header used
   when sending packets through that tunnel.  This document defines
   Encapsulation sub-TLVs for a number of tunnel types not discussed in
   [RFC5512]: Virtual eXtensible Local Area Network (VXLAN) [RFC7348],
   Network Virtualization Using Generic Routing Encapsulation (NVGRE)
   [RFC7637], and MPLS in Generic Routing Encapsulation (MPLS-in-GRE)
   [RFC4023].  MPLS-in-UDP [RFC7510] is also supported, but an
   Encapsulation sub-TLV for it is not needed since there are no
   additional parameters to be signaled.

   Some of the encapsulations mentioned in the previous paragraph need
   to be further encapsulated inside UDP and/or IP.  [RFC5512] provides
   no way to specify that certain information is to appear in these
   outer IP and/or UDP encapsulations.  This document provides a
   framework for including such information in the TLVs of the Tunnel
   Encapsulation attribute.

   When the Tunnel Encapsulation attribute is attached to a BGP UPDATE
   whose AFI/SAFI identifies one of the labeled address families, it is
   not always obvious whether the label embedded in the NLRI is to
   appear somewhere in the tunnel encapsulation header (and if so,
   where), whether it is to appear in the payload, or whether it can be
   omitted altogether.  This is especially true if the tunnel
   encapsulation header itself contains a "virtual network identifier".
   This document provides a mechanism that allows one to signal (by
   using sub-TLVs of the Tunnel Encapsulation attribute) how one wants
   to use the embedded label when the tunnel encapsulation has its own
   Virtual Network Identifier field.

   [RFC5512] defines an Encapsulation Extended Community that can be
   used instead of the Tunnel Encapsulation attribute under certain
   circumstances.  This document describes how the Encapsulation
   Extended Community can be used in a backwards-compatible fashion (see
   Section 4.1).  It is possible to combine Encapsulation Extended
   Communities and Tunnel Encapsulation attributes in the same BGP
   UPDATE in this manner.

1.5.  Update to RFC 5640

   This document updates [RFC5640] by indicating that the Load-Balancing
   Block sub-TLV MAY be included in any Tunnel Encapsulation attribute
   where load balancing is desired.

1.6.  Effects of Obsoleting RFC 5566

   This specification obsoletes RFC 5566.  This has the effect of, in
   turn, deprecating a number of code points defined in that document.
   In the "BGP Tunnel Encapsulation Attribute Tunnel Types" registry
   [IANA-BGP-TUNNEL-ENCAP], the following code points have been marked
   as deprecated: "Transmit tunnel endpoint" (type code 3), "IPsec in
   Tunnel-mode" (type code 4), "IP in IP tunnel with IPsec Transport
   Mode" (type code 5), and "MPLS-in-IP tunnel with IPsec Transport
   Mode" (type code 6).  In the "BGP Tunnel Encapsulation Attribute Sub-
   TLVs" registry [IANA-BGP-TUNNEL-ENCAP], "IPsec Tunnel Authenticator"
   (type code 3) has been marked as deprecated.  See Section 14.2.

2.  The Tunnel Encapsulation Attribute

   The Tunnel Encapsulation attribute is an optional transitive BGP path
   attribute.  IANA has assigned the value 23 as the type code of the
   attribute in the "BGP Path Attributes" registry [IANA-BGP-PARAMS].
   The attribute is composed of a set of Type-Length-Value (TLV)
   encodings.  Each TLV contains information corresponding to a
   particular tunnel type.  A Tunnel Encapsulation TLV, also known as
   Tunnel TLV, is structured as shown in Figure 1.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    Tunnel Type (2 octets)     |        Length (2 octets)      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |                        Value (variable)                       |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 1: Tunnel Encapsulation TLV

   Tunnel Type (2 octets):  Identifies a type of tunnel.  The field
      contains values from the IANA registry "BGP Tunnel Encapsulation
      Attribute Tunnel Types" [IANA-BGP-TUNNEL-ENCAP].  See
      Section 3.4.1 for discussion of special treatment of tunnel types
      with names of the form "X-in-Y".

   Length (2 octets):  The total number of octets of the Value field.

   Value (variable):  Comprised of multiple sub-TLVs.

   Each sub-TLV consists of three fields: A 1-octet type, a 1-octet or
   2-octet length (depending on the type), and zero or more octets of
   value.  A sub-TLV is structured as shown in Figure 2.

                       +--------------------------------+
                       | Sub-TLV Type (1 octet)         |
                       +--------------------------------+
                       | Sub-TLV Length (1 or 2 octets) |
                       +--------------------------------+
                       | Sub-TLV Value (variable)       |
                       +--------------------------------+

                      Figure 2: Encapsulation Sub-TLV

   Sub-TLV Type (1 octet):  Each sub-TLV type defines a certain property
      about the Tunnel TLV that contains this sub-TLV.  The field
      contains values from the IANA registry "BGP Tunnel Encapsulation
      Attribute Sub-TLVs" [IANA-BGP-TUNNEL-ENCAP].

   Sub-TLV Length (1 or 2 octets):  The total number of octets of the
      Sub-TLV Value field.  The Sub-TLV Length field contains 1 octet if
      the Sub-TLV Type field contains a value in the range from 0-127.
      The Sub-TLV Length field contains two octets if the Sub-TLV Type
      field contains a value in the range from 128-255.

   Sub-TLV Value (variable):  Encodings of the Value field depend on the
      sub-TLV type.  The following subsections define the encoding in
      detail.

3.  Tunnel Encapsulation Attribute Sub-TLVs

   This section specifies a number of sub-TLVs.  These sub-TLVs can be
   included in a TLV of the Tunnel Encapsulation attribute.

3.1.  The Tunnel Egress Endpoint Sub-TLV (Type Code 6)

   The Tunnel Egress Endpoint sub-TLV specifies the address of the
   egress endpoint of the tunnel, that is, the address of the router
   that will decapsulate the payload.  Its Value field contains three
   subfields:

   1.  a Reserved subfield

   2.  a two-octet Address Family subfield

   3.  an Address subfield, whose length depends upon the Address
       Family.

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       Reserved (4 octets)                     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Address Family (2 octets)   |           Address             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+          (variable)           +
     ~                                                               ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

            Figure 3: Tunnel Egress Endpoint Sub-TLV Value Field

   The Reserved subfield SHOULD be originated as zero.  It MUST be
   disregarded on receipt, and it MUST be propagated unchanged.

   The Address Family subfield contains a value from IANA's "Address
   Family Numbers" registry [IANA-ADDRESS-FAM].  This document assumes
   that the Address Family is either IPv4 or IPv6; use of other address
   families is outside the scope of this document.

   If the Address Family subfield contains the value for IPv4, the
   Address subfield MUST contain an IPv4 address (a /32 IPv4 prefix).

   If the Address Family subfield contains the value for IPv6, the
   Address subfield MUST contain an IPv6 address (a /128 IPv6 prefix).

   In a given BGP UPDATE, the address family (IPv4 or IPv6) of a Tunnel
   Egress Endpoint sub-TLV is independent of the address family of the
   UPDATE itself.  For example, an UPDATE whose NLRI is an IPv4 address
   may have a Tunnel Encapsulation attribute containing Tunnel Egress
   Endpoint sub-TLVs that contain IPv6 addresses.  Also, different
   tunnels represented in the Tunnel Encapsulation attribute may have
   tunnel egress endpoints of different address families.

   There is one special case: the Tunnel Egress Endpoint sub-TLV MAY
   have a Value field whose Address Family subfield contains 0.  This
   means that the tunnel's egress endpoint is the address of the next
   hop.  If the Address Family subfield contains 0, the Address subfield
   is omitted.  In this case, the Length field of Tunnel Egress Endpoint
   sub-TLV MUST contain the value 6 (0x06).

   When the Tunnel Encapsulation attribute is carried in an UPDATE
   message of one of the AFI/SAFIs specified in this document (see the
   first paragraph of Section 6), each TLV MUST have one, and only one,
   Tunnel Egress Endpoint sub-TLV.  If a TLV does not have a Tunnel
   Egress Endpoint sub-TLV, that TLV should be treated as if it had a
   malformed Tunnel Egress Endpoint sub-TLV (see below).

   In the context of this specification, if the Address Family subfield
   has any value other than IPv4, IPv6, or the special value 0, the
   Tunnel Egress Endpoint sub-TLV is considered "unrecognized" (see
   Section 13).  If any of the following conditions hold, the Tunnel
   Egress Endpoint sub-TLV is considered to be "malformed":

   *  The length of the sub-TLV's Value field is other than 6 added to
      the defined length for the address family given in its Address
      Family subfield.  Therefore, for address family behaviors defined
      in this document, the permitted values are:

      -  10, if the Address Family subfield contains the value for IPv4.

      -  22, if the Address Family subfield contains the value for IPv6.

      -  6, if the Address Family subfield contains the value zero.

   *  The IP address in the sub-TLV's Address subfield lies within a
      block listed in the relevant Special-Purpose IP Address registry
      [RFC6890] with either a "destination" attribute value or a
      "forwardable" attribute value of "false".  (Such routes are
      sometimes colloquially known as "Martians".)  This restriction MAY
      be relaxed by explicit configuration.

   *  It can be determined that the IP address in the sub-TLV's Address
      subfield does not belong to the Autonomous System (AS) that
      originated the route that contains the attribute.  Section 3.1.1
      describes an optional procedure to make this determination.

   Error handling is specified in Section 13.

   If the Tunnel Egress Endpoint sub-TLV contains an IPv4 or IPv6
   address that is valid but not reachable, the sub-TLV is not
   considered to be malformed.

3.1.1.  Validating the Address Subfield

   This section provides a procedure that MAY be applied to validate
   that the IP address in the sub-TLV's Address subfield belongs to the
   AS that originated the route that contains the attribute.  (The
   notion of "belonging to" an AS is expanded on below.)  Doing this is
   thought to increase confidence that when traffic is sent to the IP
   address depicted in the Address subfield, it will go to the same AS
   as it would go to if the Tunnel Encapsulation attribute were not
   present, although of course it cannot guarantee it.  See Section 15
   for discussion of the limitations of this procedure.  The principal
   applicability of this procedure is in deployments that are not
   strictly scoped.  In deployments with strict scope, and especially
   those scoped to a single AS, these procedures may not add substantial
   benefit beyond those discussed in Section 11.

   The Route Origin Autonomous System Number (ASN) of a BGP route that
   includes a Tunnel Encapsulation attribute can be determined by
   inspection of the AS_PATH attribute, according to the procedure
   specified in [RFC6811], Section 2.  Call this value Route_AS.

   In order to determine the Route Origin ASN of the address depicted in
   the Address subfield of the Tunnel Egress Endpoint sub-TLV, it is
   necessary to consider the forwarding route -- that is, the route that
   will be used to forward traffic toward that address.  This route is
   determined by a recursive route-lookup operation for that address, as
   discussed in [RFC4271], Section 5.1.3.  The relevant AS path to
   consider is the last one encountered while performing the recursive
   lookup; the procedures of [RFC6811], Section 2 are applied to that AS
   path to determine the Route Origin ASN.  If no AS path is encountered
   at all, for example, if that route's source is a protocol other than
   BGP, the Route Origin ASN is the BGP speaker's own AS number.  Call
   this value Egress_AS.

   If Route_AS does not equal Egress_AS, then the Tunnel Egress Endpoint
   sub-TLV is considered not to be valid.  In some cases, a network
   operator who controls a set of ASes might wish to allow a tunnel
   egress endpoint to reside in an AS other than Route_AS; configuration
   MAY allow for such a case, in which case the check becomes: if
   Egress_AS is not within the configured set of permitted AS numbers,
   then the Tunnel Egress Endpoint sub-TLV is considered to be
   "malformed".

   Note that if the forwarding route changes, this procedure MUST be
   reapplied.  As a result, a sub-TLV that was formerly considered valid
   might become not valid, or vice versa.

3.2.  Encapsulation Sub-TLVs for Particular Tunnel Types (Type Code 1)

   This section defines Encapsulation sub-TLVs for the following tunnel
   types: VXLAN [RFC7348], NVGRE [RFC7637], MPLS-in-GRE [RFC4023],
   L2TPv3 [RFC3931], and GRE [RFC2784].

   Rules for forming the encapsulation based on the information in a
   given TLV are given in Sections 6 and 9.

   Recall that the tunnel type itself is identified by the Tunnel Type
   field in the attribute header (Section 2); the Encapsulation sub-
   TLV's structure is inferred from this.  Regardless of the tunnel
   type, the sub-TLV type of the Encapsulation sub-TLV is 1.  There are
   also tunnel types for which it is not necessary to define an
   Encapsulation sub-TLV, because there are no fields in the
   encapsulation header whose values need to be signaled from the tunnel
   egress endpoint.

3.2.1.  VXLAN (Tunnel Type 8)

   This document defines an Encapsulation sub-TLV for VXLAN [RFC7348]
   tunnels.  When the tunnel type is VXLAN, the length of the sub-TLV is
   12 octets.  The structure of the Value field in the Encapsulation
   sub-TLV is shown in Figure 4.

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |V|M|R|R|R|R|R|R|          VN-ID (3 octets)                     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                 MAC Address (4 octets)                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  MAC Address (2 octets)       |      Reserved (2 octets)      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

             Figure 4: VXLAN Encapsulation Sub-TLV Value Field

   V:  This bit is set to 1 to indicate that a Virtual Network
      Identifier (VN-ID) is present in the Encapsulation sub-TLV.  If
      set to 0, the VN-ID field is disregarded.  Please see Section 9.

   M:  This bit is set to 1 to indicate that a Media Access Control
      (MAC) Address is present in the Encapsulation sub-TLV.  If set to
      0, the MAC Address field is disregarded.

   R:  The remaining bits in the 8-bit Flags field are reserved for
      further use.  They MUST always be set to 0 by the originator of
      the sub-TLV.  Intermediate routers MUST propagate them without
      modification.  Any receiving routers MUST ignore these bits upon
      receipt.

   VN-ID:  If the V bit is set to 1, the VN-ID field contains a 3-octet
      VN-ID value.  If the V bit is set to 0, the VN-ID field MUST be
      set to zero on transmission and disregarded on receipt.

   MAC Address:  If the M bit is set to 1, this field contains a 6-octet
      Ethernet MAC address.  If the M bit is set to 0, this field MUST
      be set to all zeroes on transmission and disregarded on receipt.

   Reserved:  MUST be set to zero on transmission and disregarded on
      receipt.

   When forming the VXLAN encapsulation header:

   *  The values of the V, M, and R bits are NOT copied into the Flags
      field of the VXLAN header.  The Flags field of the VXLAN header is
      set as per [RFC7348].

   *  If the M bit is set to 1, the MAC Address is copied into the Inner
      Destination MAC Address field of the Inner Ethernet Header (see
      Section 5 of [RFC7348]).

      If the M bit is set to 0, and the payload being sent through the
      VXLAN tunnel is an Ethernet frame, the Destination MAC Address
      field of the Inner Ethernet Header is just the Destination MAC
      Address field of the payload's Ethernet header.

      If the M bit is set to 0, and the payload being sent through the
      VXLAN tunnel is an IP or MPLS packet, the Inner Destination MAC
      Address field is set to a configured value; if there is no
      configured value, the VXLAN tunnel cannot be used.

   *  If the V bit is set to 0, and the BGP UPDATE message has an AFI/
      SAFI other than Ethernet VPNs (SAFI 70, "BGP EVPNs"), then the
      VXLAN tunnel cannot be used.

   *  Section 9 describes how the VNI (VXLAN Network Identifier) field
      of the VXLAN encapsulation header is set.

   Note that in order to send an IP packet or an MPLS packet through a
   VXLAN tunnel, the packet must first be encapsulated in an Ethernet
   header, which becomes the "Inner Ethernet Header" described in
   [RFC7348].  The VXLAN Encapsulation sub-TLV may contain information
   (for example, the MAC address) that is used to form this Ethernet
   header.

3.2.2.  NVGRE (Tunnel Type 9)

   This document defines an Encapsulation sub-TLV for NVGRE [RFC7637]
   tunnels.  When the tunnel type is NVGRE, the length of the sub-TLV is
   12 octets.  The structure of the Value field in the Encapsulation
   sub-TLV is shown in Figure 5.

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |V|M|R|R|R|R|R|R|          VN-ID (3 octets)                     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                 MAC Address (4 octets)                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  MAC Address (2 octets)       |      Reserved (2 octets)      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

             Figure 5: NVGRE Encapsulation Sub-TLV Value Field

   V:  This bit is set to 1 to indicate that a VN-ID is present in the
      Encapsulation sub-TLV.  If set to 0, the VN-ID field is
      disregarded.  Please see Section 9.

   M:  This bit is set to 1 to indicate that a MAC Address is present in
      the Encapsulation sub-TLV.  If set to 0, the MAC Address field is
      disregarded.

   R:  The remaining bits in the 8-bit Flags field are reserved for
      further use.  They MUST always be set to 0 by the originator of
      the sub-TLV.  Intermediate routers MUST propagate them without
      modification.  Any receiving routers MUST ignore these bits upon
      receipt.

   VN-ID:  If the V bit is set to 1, the VN-ID field contains a 3-octet
      VN-ID value, used to set the NVGRE Virtual Subnet Identifier
      (VSID; see Section 9).  If the V bit is set to 0, the VN-ID field
      MUST be set to zero on transmission and disregarded on receipt.

   MAC Address:  If the M bit is set to 1, this field contains a 6-octet
      Ethernet MAC address.  If the M bit is set to 0, this field MUST
      be set to all zeroes on transmission and disregarded on receipt.

   Reserved:  MUST be set to zero on transmission and disregarded on
      receipt.

   When forming the NVGRE encapsulation header:

   *  The values of the V, M, and R bits are NOT copied into the Flags
      field of the NVGRE header.  The Flags field of the NVGRE header is
      set as per [RFC7637].

   *  If the M bit is set to 1, the MAC Address is copied into the Inner
      Destination MAC Address field of the Inner Ethernet Header (see
      Section 3.2 of [RFC7637]).

      If the M bit is set to 0, and the payload being sent through the
      NVGRE tunnel is an Ethernet frame, the Destination MAC Address
      field of the Inner Ethernet Header is just the Destination MAC
      Address field of the payload's Ethernet header.

      If the M bit is set to 0, and the payload being sent through the
      NVGRE tunnel is an IP or MPLS packet, the Inner Destination MAC
      Address field is set to a configured value; if there is no
      configured value, the NVGRE tunnel cannot be used.

   *  If the V bit is set to 0, and the BGP UPDATE message has an AFI/
      SAFI other than Ethernet VPNs (EVPNs), then the NVGRE tunnel
      cannot be used.

   *  Section 9 describes how the VSID field of the NVGRE encapsulation
      header is set.

3.2.3.  L2TPv3 (Tunnel Type 1)

   When the tunnel type of the TLV is L2TPv3 over IP [RFC3931], the
   length of the sub-TLV is between 4 and 12 octets, depending on the
   length of the cookie.  The structure of the Value field of the
   Encapsulation sub-TLV is shown in Figure 6.

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                      Session ID (4 octets)                    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |                        Cookie (variable)                      |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

             Figure 6: L2TPv3 Encapsulation Sub-TLV Value Field

   Session ID:  A non-zero 4-octet value locally assigned by the
      advertising router that serves as a lookup key for the incoming
      packet's context.

   Cookie:  An optional, variable-length (encoded in 0 to 8 octets)
      value used by L2TPv3 to check the association of a received data
      message with the session identified by the Session ID.  Generation
      and usage of the cookie value is as specified in [RFC3931].

      The length of the cookie is not encoded explicitly but can be
      calculated as (sub-TLV length - 4).

3.2.4.  GRE (Tunnel Type 2)

   When the tunnel type of the TLV is GRE [RFC2784], the length of the
   sub-TLV is 4 octets.  The structure of the Value field of the
   Encapsulation sub-TLV is shown in Figure 7.

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                      GRE Key (4 octets)                       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

              Figure 7: GRE Encapsulation Sub-TLV Value Field

   GRE Key:  4-octet field [RFC2890] that is generated by the
      advertising router.  Note that the key is optional.  Unless a key
      value is being advertised, the GRE Encapsulation sub-TLV MUST NOT
      be present.

3.2.5.  MPLS-in-GRE (Tunnel Type 11)

   When the tunnel type is MPLS-in-GRE [RFC4023], the length of the sub-
   TLV is 4 octets.  The structure of the Value field of the
   Encapsulation sub-TLV is shown in Figure 8.

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       GRE Key (4 octets)                      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

          Figure 8: MPLS-in-GRE Encapsulation Sub-TLV Value Field

   GRE Key:  4-octet field [RFC2890] that is generated by the
      advertising router.  Note that the key is optional.  Unless a key
      value is being advertised, the MPLS-in-GRE Encapsulation sub-TLV
      MUST NOT be present.

   Note that the GRE tunnel type defined in Section 3.2.4 can be used
   instead of the MPLS-in-GRE tunnel type when it is necessary to
   encapsulate MPLS in GRE.  Including a TLV of the MPLS-in-GRE tunnel
   type is equivalent to including a TLV of the GRE tunnel type that
   also includes a Protocol Type sub-TLV (Section 3.4.1) specifying MPLS
   as the protocol to be encapsulated.

   Although the MPLS-in-GRE tunnel type is just a special case of the
   GRE tunnel type and thus is not strictly necessary, it is included
   for reasons of backwards compatibility with, for example,
   implementations of [RFC8365].

3.3.  Outer Encapsulation Sub-TLVs

   The Encapsulation sub-TLV for a particular tunnel type allows one to
   specify the values that are to be placed in certain fields of the
   encapsulation header for that tunnel type.  However, some tunnel
   types require an outer IP encapsulation, and some also require an
   outer UDP encapsulation.  The Encapsulation sub-TLV for a given
   tunnel type does not usually provide a way to specify values for
   fields of the outer IP and/or UDP encapsulations.  If it is necessary
   to specify values for fields of the outer encapsulation, additional
   sub-TLVs must be used.  This document defines two such sub-TLVs.

   If an outer Encapsulation sub-TLV occurs in a TLV for a tunnel type
   that does not use the corresponding outer encapsulation, the sub-TLV
   MUST be treated as if it were an unrecognized type of sub-TLV.

3.3.1.  DS Field (Type Code 7)

   Most of the tunnel types that can be specified in the Tunnel
   Encapsulation attribute require an outer IP encapsulation.  The
   Differentiated Services (DS) Field sub-TLV can be carried in the TLV
   of any such tunnel type.  It specifies the setting of the one-octet
   Differentiated Services field in the outer IPv4 or IPv6 encapsulation
   (see [RFC2474]).  Any one-octet value can be transported; the
   semantics of the DSCP (Differentiated Services Code Point) field is
   beyond the scope of this document.  The Value field is always a
   single octet.

      0 1 2 3 4 5 6 7
     +-+-+-+-+-+-+-+-+
     |    DS value   |
     +-+-+-+-+-+-+-+-+

                   Figure 9: DS Field Sub-TLV Value Field

   Because the interpretation of the DSCP field at the recipient may be
   different from its interpretation at the originator, an
   implementation MAY provide a facility to use policy to filter or
   modify the DS field.

3.3.2.  UDP Destination Port (Type Code 8)

   Some of the tunnel types that can be specified in the Tunnel
   Encapsulation attribute require an outer UDP encapsulation.
   Generally, there is a standard UDP destination port value for a
   particular tunnel type.  However, sometimes it is useful to be able
   to use a nonstandard UDP destination port.  If a particular tunnel
   type requires an outer UDP encapsulation, and it is desired to use a
   UDP destination port other than the standard one, the port to be used
   can be specified by including a UDP Destination Port sub-TLV.  The
   Value field of this sub-TLV is always a two-octet field, containing
   the port value.  Any two-octet value other than zero can be
   transported.  If the reserved value zero is received, the sub-TLV
   MUST be treated as malformed, according to the rules of Section 13.

      0                   1
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |       UDP Port (2 octets)     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

            Figure 10: UDP Destination Port Sub-TLV Value Field

3.4.  Sub-TLVs for Aiding Tunnel Selection

3.4.1.  Protocol Type Sub-TLV (Type Code 2)

   The Protocol Type sub-TLV MAY be included in a given TLV to indicate
   the type of the payload packets that are allowed to be encapsulated
   with the tunnel parameters that are being signaled in the TLV.
   Packets with other payload types MUST NOT be encapsulated in the
   relevant tunnel.  The Value field of the sub-TLV contains a 2-octet
   value from IANA's "ETHER TYPES" registry [IANA-ETHERTYPES].  If the
   reserved value 0xFFFF is received, the sub-TLV MUST be treated as
   malformed according to the rules of Section 13.

      0                   1
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |      Ethertype (2 octets)     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 11: Protocol Type Sub-TLV Value Field

   For example, if there are three L2TPv3 sessions, one carrying IPv4
   packets, one carrying IPv6 packets, and one carrying MPLS packets,
   the egress router will include three TLVs of L2TPv3 encapsulation
   type, each specifying a different Session ID and a different payload
   type.  The Protocol Type sub-TLV for these will be IPv4 (protocol
   type = 0x0800), IPv6 (protocol type = 0x86dd), and MPLS (protocol
   type = 0x8847), respectively.  This informs the ingress routers of
   the appropriate encapsulation information to use with each of the
   given protocol types.  Insertion of the specified Session ID at the
   ingress routers allows the egress to process the incoming packets
   correctly, according to their protocol type.

   Note that for tunnel types whose names are of the form "X-in-Y" (for
   example, MPLS-in-GRE), only packets of the specified payload type "X"
   are to be carried through the tunnel of type "Y".  This is the
   equivalent of specifying a tunnel type "Y" and including in its TLV a
   Protocol Type sub-TLV (see Section 3.4.1) specifying protocol "X".
   If the tunnel type is "X-in-Y", it is unnecessary, though harmless,
   to explicitly include a Protocol Type sub-TLV specifying "X".  Also,
   for "X-in-Y" type tunnels, a Protocol Type sub-TLV specifying
   anything other than "X" MUST be ignored; this is discussed further in
   Section 13.

3.4.2.  Color Sub-TLV (Type Code 4)

   The Color sub-TLV MAY be used as a way to "color" the corresponding
   Tunnel TLV.  The Value field of the sub-TLV is eight octets long and
   consists of a Color Extended Community, as defined in Section 4.3.
   For the use of this sub-TLV and extended community, please see
   Section 8.

   The format of the Value field is depicted in Figure 15.

   If the Length field of a Color sub-TLV has a value other than 8, or
   the first two octets of its Value field are not 0x030b, the sub-TLV
   MUST be treated as if it were an unrecognized sub-TLV (see
   Section 13).

3.5.  Embedded Label Handling Sub-TLV (Type Code 9)

   Certain BGP address families (corresponding to particular AFI/SAFI
   pairs, for example, 1/4, 2/4, 1/128, 2/128) have MPLS labels embedded
   in their NLRIs.  The term "embedded label" is used to refer to the
   MPLS label that is embedded in an NLRI, and the term "labeled address
   family" to refer to any AFI/SAFI that has embedded labels.

   Some of the tunnel types (for example, VXLAN and NVGRE) that can be
   specified in the Tunnel Encapsulation attribute have an encapsulation
   header containing a virtual network identifier of some sort.  The
   Encapsulation sub-TLVs for these tunnel types may optionally specify
   a value for the virtual network identifier.

   Suppose a Tunnel Encapsulation attribute is attached to an UPDATE of
   a labeled address family, and it is decided to use a particular
   tunnel (specified in one of the attribute's TLVs) for transmitting a
   packet that is being forwarded according to that UPDATE.  When
   forming the encapsulation header for that packet, different
   deployment scenarios require different handling of the embedded label
   and/or the virtual network identifier.  The Embedded Label Handling
   sub-TLV can be used to control the placement of the embedded label
   and/or the virtual network identifier in the encapsulation.

   The Embedded Label Handling sub-TLV may be included in any TLV of the
   Tunnel Encapsulation attribute.  If the Tunnel Encapsulation
   attribute is attached to an UPDATE of a non-labeled address family,
   then the sub-TLV MUST be disregarded.  If the sub-TLV is contained in
   a TLV whose tunnel type does not have a virtual network identifier in
   its encapsulation header, the sub-TLV MUST be disregarded.  In those
   cases where the sub-TLV is ignored, it MUST NOT be stripped from the
   TLV before the route is propagated.

   The sub-TLV's Length field always contains the value 1, and its Value
   field consists of a single octet.  The following values are defined:

   1:  The payload will be an MPLS packet with the embedded label at the
       top of its label stack.

   2:  The embedded label is not carried in the payload but is either
       carried in the Virtual Network Identifier field of the
       encapsulation header or else ignored entirely.

   If any value other than 1 or 2 is carried, the sub-TLV MUST be
   considered malformed, according to the procedures of Section 13.

   Please see Section 9 for the details of how this sub-TLV is used when
   it is carried by an UPDATE of a labeled address family.

      0 1 2 3 4 5 6 7
     +-+-+-+-+-+-+-+-+
     |     1 or 2    |
     +-+-+-+-+-+-+-+-+

           Figure 12: Embedded Label Handling Sub-TLV Value Field

3.6.  MPLS Label Stack Sub-TLV (Type Code 10)

   This sub-TLV allows an MPLS label stack [RFC3032] to be associated
   with a particular tunnel.

   The length of the sub-TLV is a multiple of 4 octets, and the Value
   field of this sub-TLV is a sequence of MPLS label stack entries.  The
   first entry in the sequence is the "topmost" label, and the final
   entry in the sequence is the "bottommost" label.  When this label
   stack is pushed onto a packet, this ordering MUST be preserved.

   Each label stack entry has the format shown in Figure 13.

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                Label                  |  TC |S|      TTL      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

              Figure 13: MPLS Label Stack Sub-TLV Value Field

   The fields are as defined in [RFC3032] and [RFC5462].

   If a packet is to be sent through the tunnel identified in a
   particular TLV, and if that TLV contains an MPLS Label Stack sub-TLV,
   then the label stack appearing in the sub-TLV MUST be pushed onto the
   packet before any other labels are pushed onto the packet.  (See
   Section 6 for further discussion.)

   In particular, if the Tunnel Encapsulation attribute is attached to a
   BGP UPDATE of a labeled address family, the contents of the MPLS
   Label Stack sub-TLV MUST be pushed onto the packet before the label
   embedded in the NLRI is pushed onto the packet.

   If the MPLS Label Stack sub-TLV is included in a TLV identifying a
   tunnel type that uses virtual network identifiers (see Section 9),
   the contents of the MPLS Label Stack sub-TLV MUST be pushed onto the
   packet before the procedures of Section 9 are applied.

   The number of label stack entries in the sub-TLV MUST be determined
   from the Sub-TLV Length field.  Thus, it is not necessary to set the
   S bit in any of the label stack entries of the sub-TLV, and the
   setting of the S bit is ignored when parsing the sub-TLV.  When the
   label stack entries are pushed onto a packet that already has a label
   stack, the S bits of all the entries being pushed MUST be cleared.
   When the label stack entries are pushed onto a packet that does not
   already have a label stack, the S bit of the bottommost label stack
   entry MUST be set, and the S bit of all the other label stack entries
   MUST be cleared.

   The Traffic Class (TC) field [RFC3270][RFC5129] of each label stack
   entry SHOULD be set to 0, unless changed by policy at the originator
   of the sub-TLV.  When pushing the label stack onto a packet, the TC
   of each label stack SHOULD be preserved, unless local policy results
   in a modification.

   The TTL (Time to Live) field of each label stack entry SHOULD be set
   to 255, unless changed to some other non-zero value by policy at the
   originator of the sub-TLV.  When pushing the label stack onto a
   packet, the TTL of each label stack entry SHOULD be preserved, unless
   local policy results in a modification to some other non-zero value.
   If any label stack entry in the sub-TLV has a TTL value of zero, the
   router that is pushing the stack onto a packet MUST change the value
   to a non-zero value, either 255 or some other value as determined by
   policy as discussed above.

   Note that this sub-TLV can appear within a TLV identifying any type
   of tunnel, not just within a TLV identifying an MPLS tunnel.
   However, if this sub-TLV appears within a TLV identifying an MPLS
   tunnel (or an MPLS-in-X tunnel), this sub-TLV plays the same role
   that would be played by an MPLS Encapsulation sub-TLV.  Therefore, an
   MPLS Encapsulation sub-TLV is not defined.

   Although this specification does not supply detailed instructions for
   validating the received label stack, implementations might impose
   restrictions on the label stack they can support.  If an invalid or
   unsupported label stack is received, the tunnel MAY be treated as not
   feasible, according to the procedures of Section 6.

3.7.  Prefix-SID Sub-TLV (Type Code 11)

   [RFC8669] defines a BGP path attribute known as the "BGP Prefix-SID
   attribute".  This attribute is defined to contain a sequence of one
   or more TLVs, where each TLV is either a Label-Index TLV or an
   Originator SRGB (Source Routing Global Block) TLV.

   This document defines a Prefix-SID (Prefix Segment Identifier) sub-
   TLV.  The Value field of the Prefix-SID sub-TLV can be set to any
   permitted value of the Value field of a BGP Prefix-SID attribute
   [RFC8669].

   [RFC8669] only defines behavior when the BGP Prefix-SID attribute is
   attached to routes of type IPv4/IPv6 Labeled Unicast
   [RFC4760][RFC8277], and it only defines values of the BGP Prefix-SID
   attribute for those cases.  Therefore, similar limitations exist for
   the Prefix-SID sub-TLV: it SHOULD only be included in a BGP UPDATE
   message for one of the address families for which [RFC8669] has a
   defined behavior, namely BGP IPv4/IPv6 Labeled Unicast [RFC4760]
   [RFC8277].  If included in a BGP UPDATE for any other address family,
   it MUST be ignored.

   The Prefix-SID sub-TLV can occur in a TLV identifying any type of
   tunnel.  If an Originator SRGB is specified in the sub-TLV, that SRGB
   MUST be interpreted to be the SRGB used by the tunnel's egress
   endpoint.  The Label-Index, if present, is the Segment Routing SID
   that the tunnel's egress endpoint uses to represent the prefix
   appearing in the NLRI field of the BGP UPDATE to which the Tunnel
   Encapsulation attribute is attached.

   If a Label-Index is present in the Prefix-SID sub-TLV, then when a
   packet is sent through the tunnel identified by the TLV, if that
   tunnel is from a labeled address family, the corresponding MPLS label
   MUST be pushed on the packet's label stack.  The corresponding MPLS
   label is computed from the Label-Index value and the SRGB of the
   route's originator, as specified in Section 4.1 of [RFC8669].

   The corresponding MPLS label is pushed on after the processing of the
   MPLS Label Stack sub-TLV, if present, as specified in Section 3.6.
   It is pushed on before any other labels (for example, a label
   embedded in an UPDATE's NLRI or a label determined by the procedures
   of Section 9) are pushed on the stack.

   The Prefix-SID sub-TLV has slightly different semantics than the BGP
   Prefix-SID attribute.  When the BGP Prefix-SID attribute is attached
   to a given route, the BGP speaker that originally attached the
   attribute is expected to be in the same Segment Routing domain as the
   BGP speakers who receive the route with the attached attribute.  The
   Label-Index tells the receiving BGP speakers what the Prefix-SID is
   for the advertised prefix in that Segment Routing domain.  When the
   Prefix-SID sub-TLV is used, there is no implication that the Prefix-
   SID for the advertised prefix is the same in the Segment Routing
   domains of the BGP speaker that originated the sub-TLV and the BGP
   speaker that received it.

4.  Extended Communities Related to the Tunnel Encapsulation Attribute

4.1.  Encapsulation Extended Community

   The Encapsulation Extended Community is a Transitive Opaque Extended
   Community.

   The Encapsulation Extended Community encoding is as shown in
   Figure 14.

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | 0x03 (1 octet)| 0x0c (1 octet)|       Reserved (2 octets)     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Reserved (2 octets)       |    Tunnel Type (2 octets)     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 14: Encapsulation Extended Community

   The value of the high-order octet of the extended Type field is 0x03,
   which indicates it's transitive.  The value of the low-order octet of
   the extended Type field is 0x0c.

   The last two octets of the Value field encode a tunnel type.

   This extended community may be attached to a route of any AFI/SAFI to
   which the Tunnel Encapsulation attribute may be attached.  Each such
   extended community identifies a particular tunnel type; its semantics
   are the same as semantics of a Tunnel TLV in a Tunnel Encapsulation
   attribute, for which the following three conditions all hold:

   1.  It identifies the same tunnel type.

   2.  It has a Tunnel Egress Endpoint sub-TLV for which one of the
       following two conditions holds:

       a.  Its Address Family subfield contains zero, or

       b.  Its Address subfield contains the address of the Next Hop
           field of the route to which the Tunnel Encapsulation
           attribute is attached.

   3.  It has no other sub-TLVs.

   Such a Tunnel TLV is called a "barebones" Tunnel TLV.

   The Encapsulation Extended Community was first defined in [RFC5512].
   While it provides only a small subset of the functionality of the
   Tunnel Encapsulation attribute, it is used in a number of deployed
   applications and is still needed for backwards compatibility.  In
   situations where a tunnel could be encoded using a barebones TLV, it
   MUST be encoded using the corresponding Encapsulation Extended
   Community.  Notwithstanding, an implementation MUST be prepared to
   process a tunnel received encoded as a barebones TLV.

   Note that for tunnel types of the form "X-in-Y" (for example, MPLS-
   in-GRE), the Encapsulation Extended Community implies that only
   packets of the specified payload type "X" are to be carried through
   the tunnel of type "Y".  Packets with other payload types MUST NOT be
   carried through such tunnels.  See also Section 2.

   In the remainder of this specification, when a route is referred to
   as containing a Tunnel Encapsulation attribute with a TLV identifying
   a particular tunnel type, it implicitly includes the case where the
   route contains an Encapsulation Extended Community identifying that
   tunnel type.

4.2.  Router's MAC Extended Community

   [EVPN-INTER-SUBNET] defines a router's MAC Extended Community.  This
   extended community, as its name implies, carries the MAC address of
   the advertising router.  Since the VXLAN and NVGRE Encapsulation sub-
   TLVs can also optionally carry a router's MAC, a conflict can arise
   if both the Router's MAC Extended Community and such an Encapsulation
   sub-TLV are present at the same time but have different values.  In
   case of such a conflict, the information in the Router's MAC Extended
   Community MUST be used.

4.3.  Color Extended Community

   The Color Extended Community is a Transitive Opaque Extended
   Community with the encoding shown in Figure 15.

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | 0x03 (1 octet)| 0x0b (1 octet)|        Flags (2 octets)       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                      Color Value (4 octets)                   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 15: Color Extended Community

   The value of the high-order octet of the extended Type field is 0x03,
   which indicates it is transitive.  The value of the low-order octet
   of the extended Type field for this community is 0x0b.  The color
   value is user defined and configured locally.  No flags are defined
   in this document; this field MUST be set to zero by the originator
   and ignored by the receiver; the value MUST NOT be changed when
   propagating this extended community.  The Color Value field is
   encoded as a 4-octet value by the administrator and is outside the
   scope of this document.  For the use of this extended community,
   please see Section 8.

5.  Special Considerations for IP-in-IP Tunnels

   In certain situations with an IP fabric underlay, one could have a
   tunnel overlay with the tunnel type IP-in-IP.  The egress BGP speaker
   can advertise the IP-in-IP tunnel endpoint address in the Tunnel
   Egress Endpoint sub-TLV.  When the tunnel type of the TLV is IP-in-
   IP, it will not have a virtual network identifier.  However, the
   tunnel egress endpoint address can be used in identifying the
   forwarding table to use for making the forwarding decisions to
   forward the payload.

6.  Semantics and Usage of the Tunnel Encapsulation Attribute

   The BGP Tunnel Encapsulation attribute MAY be carried in any BGP
   UPDATE message whose AFI/SAFI is 1/1 (IPv4 Unicast), 2/1 (IPv6
   Unicast), 1/4 (IPv4 Labeled Unicast), 2/4 (IPv6 Labeled Unicast),
   1/128 (VPN-IPv4 Labeled Unicast), 2/128 (VPN-IPv6 Labeled Unicast),
   or 25/70 (Ethernet VPN, usually known as EVPN).  Use of the Tunnel
   Encapsulation attribute in BGP UPDATE messages of other AFI/SAFIs is
   outside the scope of this document.

   There is no significance to the order in which the TLVs occur within
   the Tunnel Encapsulation attribute.  Multiple TLVs may occur for a
   given tunnel type; each such TLV is regarded as describing a
   different tunnel.  (This also applies if the Encapsulation Extended
   Community encoding is used.)

   The decision to attach a Tunnel Encapsulation attribute to a given
   BGP UPDATE is determined by policy.  The set of TLVs and sub-TLVs
   contained in the attribute is also determined by policy.

   Suppose that:

   *  a given packet P must be forwarded by router R;

   *  the path along which P is to be forwarded is determined by BGP
      UPDATE U;

   *  UPDATE U has a Tunnel Encapsulation attribute, containing at least
      one TLV that identifies a "feasible tunnel" for packet P.  A
      tunnel is considered feasible if it has the following four
      properties:

      1.  The tunnel type is supported (that is, router R knows how to
          set up tunnels of that type, how to create the encapsulation
          header for tunnels of that type, etc.).

      2.  The tunnel is of a type that can be used to carry packet P
          (for example, an MPLS-in-UDP tunnel would not be a feasible
          tunnel for carrying an IP packet, unless the IP packet can
          first be encapsulated in a MPLS packet).

      3.  The tunnel is specified in a TLV whose Tunnel Egress Endpoint
          sub-TLV identifies an IP address that is reachable.  The
          reachability condition is evaluated as per [RFC4271].  If the
          IP address is reachable via more than one forwarding table,
          local policy is used to determine which table to use.

      4.  There is no local policy that prevents the use of the tunnel.

   Then router R MUST send packet P through one of the feasible tunnels
   identified in the Tunnel Encapsulation attribute of UPDATE U.

   If the Tunnel Encapsulation attribute contains several TLVs (that is,
   if it specifies several feasible tunnels), router R may choose any
   one of those tunnels, based upon local policy.  If any Tunnel TLV
   contains one or more Color sub-TLVs (Section 3.4.2) and/or the
   Protocol Type sub-TLV (Section 3.4.1), the choice of tunnel may be
   influenced by these sub-TLVs.  Many other factors, for example,
   minimization of encapsulation-header overhead, could also be used to
   influence selection.

   The reachability to the address of the egress endpoint of the tunnel
   may change over time, directly impacting the feasibility of the
   tunnel.  A tunnel that is not feasible at some moment may become
   feasible at a later time when its egress endpoint address is
   reachable.  The router may start using the newly feasible tunnel
   instead of an existing one.  How this decision is made is outside the
   scope of this document.

   Once it is determined to send a packet through the tunnel specified
   in a particular Tunnel TLV of a particular Tunnel Encapsulation
   attribute, then the tunnel's egress endpoint address is the IP
   address contained in the Tunnel Egress Endpoint sub-TLV.  If the
   Tunnel TLV contains a Tunnel Egress Endpoint sub-TLV whose Value
   field is all zeroes, then the tunnel's egress endpoint is the address
   of the next hop of the BGP UPDATE containing the Tunnel Encapsulation
   attribute (that is, the Network Address of Next Hop field of the
   MP_REACH_NLRI attribute if the encoding of [RFC4760] is in use or the
   NEXT_HOP attribute otherwise).  The address of the tunnel egress
   endpoint generally appears in a Destination Address field of the
   encapsulation.

   The full set of procedures for sending a packet through a particular
   tunnel type to a particular tunnel egress endpoint depends upon the
   tunnel type and is outside the scope of this document.  Note that
   some tunnel types may require the execution of an explicit tunnel
   setup protocol before they can be used for carrying data.  Other
   tunnel types may not require any tunnel setup protocol.

   Sending a packet through a tunnel always requires that the packet be
   encapsulated, with an encapsulation header that is appropriate for
   the tunnel type.  The contents of the tunnel encapsulation header may
   be influenced by the Encapsulation sub-TLV.  If there is no
   Encapsulation sub-TLV present, the router transmitting the packet
   through the tunnel must have a priori knowledge (for example, by
   provisioning) of how to fill in the various fields in the
   encapsulation header.

   A Tunnel Encapsulation attribute may contain several TLVs that all
   specify the same tunnel type.  Each TLV should be considered as
   specifying a different tunnel.  Two tunnels of the same type may have
   different Tunnel Egress Endpoint sub-TLVs, different Encapsulation
   sub-TLVs, etc.  Choosing between two such tunnels is a matter of
   local policy.

   Once router R has decided to send packet P through a particular
   tunnel, it encapsulates packet P appropriately and then forwards it
   according to the route that leads to the tunnel's egress endpoint.
   This route may itself be a BGP route with a Tunnel Encapsulation
   attribute.  If so, the encapsulated packet is treated as the payload
   and encapsulated according to the Tunnel Encapsulation attribute of
   that route.  That is, tunnels may be "stacked".

   Notwithstanding anything said in this document, a BGP speaker MAY
   have local policy that influences the choice of tunnel and the way
   the encapsulation is formed.  A BGP speaker MAY also have a local
   policy that tells it to ignore the Tunnel Encapsulation attribute
   entirely or in part.  Of course, interoperability issues must be
   considered when such policies are put into place.

   See also Section 13, which provides further specification regarding
   validation and exception cases.

7.  Routing Considerations

7.1.  Impact on the BGP Decision Process

   The presence of the Tunnel Encapsulation attribute affects the BGP
   best route-selection algorithm.  If a route includes the Tunnel
   Encapsulation attribute, and if that attribute includes no tunnel
   that is feasible, then that route MUST NOT be considered resolvable
   for the purposes of the route resolvability condition ([RFC4271],
   Section 9.1.2.1).

7.2.  Looping, Mutual Recursion, Etc.

   Consider a packet destined for address X.  Suppose a BGP UPDATE for
   address prefix X carries a Tunnel Encapsulation attribute that
   specifies a tunnel egress endpoint of Y, and suppose that a BGP
   UPDATE for address prefix Y carries a Tunnel Encapsulation attribute
   that specifies a tunnel egress endpoint of X.  It is easy to see that
   this can have no good outcome.  [RFC4271] describes an analogous case
   as mutually recursive routes.

   This could happen as a result of misconfiguration, either accidental
   or intentional.  It could also happen if the Tunnel Encapsulation
   attribute were altered by a malicious agent.  Implementations should
   be aware that such an attack will result in unresolvable BGP routes
   due to the mutually recursive relationship.  This document does not
   specify a maximum number of recursions; that is an implementation-
   specific matter.

   Improper setting (or malicious altering) of the Tunnel Encapsulation
   attribute could also cause data packets to loop.  Suppose a BGP
   UPDATE for address prefix X carries a Tunnel Encapsulation attribute
   that specifies a tunnel egress endpoint of Y.  Suppose router R
   receives and processes the advertisement.  When router R receives a
   packet destined for X, it will apply the encapsulation and send the
   encapsulated packet to Y.  Y will decapsulate the packet and forward
   it further.  If Y is further away from X than is router R, it is
   possible that the path from Y to X will traverse R.  This would cause
   a long-lasting routing loop.  The control plane itself cannot detect
   this situation, though a TTL field in the payload packets would
   prevent any given packet from looping infinitely.

   During the deployment of techniques described in this document,
   operators are encouraged to avoid mutually recursive route and/or
   tunnel dependencies.  There is greater potential for such scenarios
   to arise when the tunnel egress endpoint for a given prefix differs
   from the address of the next hop for that prefix.

8.  Recursive Next-Hop Resolution

   Suppose that:

   *  a given packet P must be forwarded by router R1;

   *  the path along which P is to be forwarded is determined by BGP
      UPDATE U1;

   *  UPDATE U1 does not have a Tunnel Encapsulation attribute;

   *  the address of the next hop of UPDATE U1 is router R2;

   *  the best route to router R2 is a BGP route that was advertised in
      UPDATE U2; and

   *  UPDATE U2 has a Tunnel Encapsulation attribute.

   Then packet P MUST be sent through one of the tunnels identified in
   the Tunnel Encapsulation attribute of UPDATE U2.  See Section 6 for
   further details.

   However, suppose that one of the TLVs in U2's Tunnel Encapsulation
   attribute contains one or more Color sub-TLVs.  In that case, packet
   P MUST NOT be sent through the tunnel contained in that TLV, unless
   U1 is carrying a Color Extended Community that is identified in one
   of U2's Color sub-TLVs.

   The procedures in this section presuppose that U1's address of the
   next hop resolves to a BGP route, and that U2's next hop resolves
   (perhaps after further recursion) to a non-BGP route.

9.  Use of Virtual Network Identifiers and Embedded Labels When Imposing
    a Tunnel Encapsulation

   If the TLV specifying a tunnel contains an MPLS Label Stack sub-TLV,
   then when sending a packet through that tunnel, the procedures of
   Section 3.6 are applied before the procedures of this section.

   If the TLV specifying a tunnel contains a Prefix-SID sub-TLV, the
   procedures of Section 3.7 are applied before the procedures of this
   section.  If the TLV also contains an MPLS Label Stack sub-TLV, the
   procedures of Section 3.6 are applied before the procedures of
   Section 3.7.

9.1.  Tunnel Types without a Virtual Network Identifier Field

   If a Tunnel Encapsulation attribute is attached to an UPDATE of a
   labeled address family, there will be one or more labels specified in
   the UPDATE's NLRI.  When a packet is sent through a tunnel specified
   in one of the attribute's TLVs, and that tunnel type does not contain
   a Virtual Network Identifier field, the label or labels from the NLRI
   are pushed on the packet's label stack.  The resulting MPLS packet is
   then further encapsulated, as specified by the TLV.

9.2.  Tunnel Types with a Virtual Network Identifier Field

   Two of the tunnel types that can be specified in a Tunnel
   Encapsulation TLV have Virtual Network Identifier fields in their
   encapsulation headers.  In the VXLAN encapsulation, this field is
   called the VNI (VXLAN Network Identifier) field; in the NVGRE
   encapsulation, this field is called the VSID (Virtual Subnet
   Identifier) field.

   When one of these tunnel encapsulations is imposed on a packet, the
   setting of the Virtual Network Identifier field in the encapsulation
   header depends upon the contents of the Encapsulation sub-TLV (if one
   is present).  When the Tunnel Encapsulation attribute is being
   carried in a BGP UPDATE of a labeled address family, the setting of
   the Virtual Network Identifier field also depends upon the contents
   of the Embedded Label Handling sub-TLV (if present).

   This section specifies the procedures for choosing the value to set
   in the Virtual Network Identifier field of the encapsulation header.
   These procedures apply only when the tunnel type is VXLAN or NVGRE.

9.2.1.  Unlabeled Address Families

   This subsection applies when:

   *  the Tunnel Encapsulation attribute is carried in a BGP UPDATE of
      an unlabeled address family,

   *  at least one of the attribute's TLVs identifies a tunnel type that
      uses a virtual network identifier, and

   *  it has been determined to send a packet through one of those
      tunnels.

   If the TLV identifying the tunnel contains an Encapsulation sub-TLV
   whose V bit is set to 1, the Virtual Network Identifier field of the
   encapsulation header is set to the value of the Virtual Network
   Identifier field of the Encapsulation sub-TLV.

   Otherwise, the Virtual Network Identifier field of the encapsulation
   header is set to a configured value; if there is no configured value,
   the tunnel cannot be used.

9.2.2.  Labeled Address Families

   This subsection applies when:

   *  the Tunnel Encapsulation attribute is carried in a BGP UPDATE of a
      labeled address family,

   *  at least one of the attribute's TLVs identifies a tunnel type that
      uses a virtual network identifier, and

   *  it has been determined to send a packet through one of those
      tunnels.

9.2.2.1.  When a Valid VNI Has Been Signaled

   If the TLV identifying the tunnel contains an Encapsulation sub-TLV
   whose V bit is set to 1, the Virtual Network Identifier field of the
   encapsulation header is set to the value of the Virtual Network
   Identifier field of the Encapsulation sub-TLV.  However, the Embedded
   Label Handling sub-TLV will determine label processing as described
   below.

   *  If the TLV contains an Embedded Label Handling sub-TLV whose value
      is 1, the embedded label (from the NLRI of the route that is
      carrying the Tunnel Encapsulation attribute) appears at the top of
      the MPLS label stack in the encapsulation payload.

   *  If the TLV does not contain an Embedded Label Handling sub-TLV, or
      it contains an Embedded Label Handling sub-TLV whose value is 2,
      the embedded label is ignored entirely.

9.2.2.2.  When a Valid VNI Has Not Been Signaled

   If the TLV identifying the tunnel does not contain an Encapsulation
   sub-TLV whose V bit is set to 1, the Virtual Network Identifier field
   of the encapsulation header is set as follows:

   *  If the TLV contains an Embedded Label Handling sub-TLV whose value
      is 1, then the Virtual Network Identifier field of the
      encapsulation header is set to a configured value.

      If there is no configured value, the tunnel cannot be used.

      The embedded label (from the NLRI of the route that is carrying
      the Tunnel Encapsulation attribute) appears at the top of the MPLS
      label stack in the encapsulation payload.

   *  If the TLV does not contain an Embedded Label Handling sub-TLV, or
      if it contains an Embedded Label Handling sub-TLV whose value is
      2, the embedded label is copied into the lower 3 octets of the
      Virtual Network Identifier field of the encapsulation header.

      In this case, the payload may or may not contain an MPLS label
      stack, depending upon other factors.  If the payload does contain
      an MPLS label stack, the embedded label does not appear in that
      stack.

10.  Applicability Restrictions

   In a given UPDATE of a labeled address family, the label embedded in
   the NLRI is generally a label that is meaningful only to the router
   represented by the address of the next hop.  Certain of the
   procedures of Sections 9.2.2.1 or 9.2.2.2 cause the embedded label to
   be carried by a data packet to the router whose address appears in
   the Tunnel Egress Endpoint sub-TLV.  If the Tunnel Egress Endpoint
   sub-TLV does not identify the same router represented by the address
   of the next hop, sending the packet through the tunnel may cause the
   label to be misinterpreted at the tunnel's egress endpoint.  This may
   cause misdelivery of the packet.  Avoidance of this unfortunate
   outcome is a matter of network planning and design and is outside the
   scope of this document.

   Note that if the Tunnel Encapsulation attribute is attached to a VPN-
   IP route [RFC4364], if Inter-AS "option b" (see Section 10 of
   [RFC4364]) is being used, and if the Tunnel Egress Endpoint sub-TLV
   contains an IP address that is not in the same AS as the router
   receiving the route, it is very likely that the embedded label has
   been changed.  Therefore, use of the Tunnel Encapsulation attribute
   in an "Inter-AS option b" scenario is not recommended.

   Other documents may define other ways to signal tunnel information in
   BGP.  For example, [RFC6514] defines the "P-Multicast Service
   Interface Tunnel" (PMSI Tunnel) attribute.  In this specification, we
   do not consider the effects of advertising the Tunnel Encapsulation
   attribute in conjunction with other forms of signaling tunnels.  Any
   document specifying such joint use MUST provide details as to how
   interactions should be handled.

11.  Scoping

   The Tunnel Encapsulation attribute is defined as a transitive
   attribute, so that it may be passed along by BGP speakers that do not
   recognize it.  However, the Tunnel Encapsulation attribute MUST be
   used only within a well-defined scope, for example, within a set of
   ASes that belong to a single administrative entity.  If the attribute
   is distributed beyond its intended scope, packets may be sent through
   tunnels in a manner that is not intended.

   To prevent the Tunnel Encapsulation attribute from being distributed
   beyond its intended scope, any BGP speaker that understands the
   attribute MUST be able to filter the attribute from incoming BGP
   UPDATE messages.  When the attribute is filtered from an incoming
   UPDATE, the attribute is neither processed nor distributed.  This
   filtering SHOULD be possible on a per-BGP-session basis; finer
   granularities (for example, per route and/or per attribute TLV) MAY
   be supported.  For each external BGP (EBGP) session, filtering of the
   attribute on incoming UPDATEs MUST be enabled by default.

   In addition, any BGP speaker that understands the attribute MUST be
   able to filter the attribute from outgoing BGP UPDATE messages.  This
   filtering SHOULD be possible on a per-BGP-session basis.  For each
   EBGP session, filtering of the attribute on outgoing UPDATEs MUST be
   enabled by default.

   Since the Encapsulation Extended Community provides a subset of the
   functionality of the Tunnel Encapsulation attribute, these
   considerations apply equally in its case:

   *  Any BGP speaker that understands it MUST be able to filter it from
      incoming BGP UPDATE messages.

   *  It MUST be possible to filter the Encapsulation Extended Community
      from outgoing messages.

   *  In both cases, this filtering MUST be enabled by default for EBGP
      sessions.

12.  Operational Considerations

   A potential operational difficulty arises when tunnels are used, if
   the size of packets entering the tunnel exceeds the maximum
   transmission unit (MTU) the tunnel is capable of supporting.  This
   difficulty can be exacerbated by stacking multiple tunnels, since
   each stacked tunnel header further reduces the supportable MTU.  This
   issue is long-standing and well-known.  The tunnel signaling provided
   in this specification does nothing to address this issue, nor to
   aggravate it (except insofar as it may further increase the
   popularity of tunneling).

13.  Validation and Error Handling

   The Tunnel Encapsulation attribute is a sequence of TLVs, each of
   which is a sequence of sub-TLVs.  The final octet of a TLV is
   determined by its Length field.  Similarly, the final octet of a sub-
   TLV is determined by its Length field.  The final octet of a TLV MUST
   also be the final octet of its final sub-TLV.  If this is not the
   case, the TLV MUST be considered to be malformed, and the "Treat-as-
   withdraw" procedure of [RFC7606] is applied.

   If a Tunnel Encapsulation attribute does not have any valid TLVs, or
   it does not have the transitive bit set, the "Treat-as-withdraw"
   procedure of [RFC7606] is applied.

   If a Tunnel Encapsulation attribute can be parsed correctly but
   contains a TLV whose tunnel type is not recognized by a particular
   BGP speaker, that BGP speaker MUST NOT consider the attribute to be
   malformed.  Rather, it MUST interpret the attribute as if that TLV
   had not been present.  If the route carrying the Tunnel Encapsulation
   attribute is propagated with the attribute, the unrecognized TLV MUST
   remain in the attribute.

   The following sub-TLVs defined in this document MUST NOT occur more
   than once in a given Tunnel TLV: Tunnel Egress Endpoint (discussed
   below), Encapsulation, DS, UDP Destination Port, Embedded Label
   Handling, MPLS Label Stack, and Prefix-SID.  If a Tunnel TLV has more
   than one of any of these sub-TLVs, all but the first occurrence of
   each such sub-TLV type MUST be disregarded.  However, the Tunnel TLV
   containing them MUST NOT be considered to be malformed, and all the
   sub-TLVs MUST be propagated if the route carrying the Tunnel
   Encapsulation attribute is propagated.

   The following sub-TLVs defined in this document may appear zero or
   more times in a given Tunnel TLV: Protocol Type and Color.  Each
   occurrence of such sub-TLVs is meaningful.  For example, the Color
   sub-TLV may appear multiple times to assign multiple colors to a
   tunnel.

   If a TLV of a Tunnel Encapsulation attribute contains a sub-TLV that
   is not recognized by a particular BGP speaker, the BGP speaker MUST
   process that TLV as if the unrecognized sub-TLV had not been present.
   If the route carrying the Tunnel Encapsulation attribute is
   propagated with the attribute, the unrecognized sub-TLV MUST remain
   in the attribute.

   In general, if a TLV contains a sub-TLV that is malformed, the sub-
   TLV MUST be treated as if it were an unrecognized sub-TLV.  There is
   one exception to this rule: if a TLV contains a malformed Tunnel
   Egress Endpoint sub-TLV (as defined in Section 3.1), the entire TLV
   MUST be ignored and MUST be removed from the Tunnel Encapsulation
   attribute before the route carrying that attribute is distributed.

   Within a Tunnel Encapsulation attribute that is carried by a BGP
   UPDATE whose AFI/SAFI is one of those explicitly listed in the first
   paragraph of Section 6, a TLV that does not contain exactly one
   Tunnel Egress Endpoint sub-TLV MUST be treated as if it contained a
   malformed Tunnel Egress Endpoint sub-TLV.

   A TLV identifying a particular tunnel type may contain a sub-TLV that
   is meaningless for that tunnel type.  For example, perhaps the TLV
   contains a UDP Destination Port sub-TLV, but the identified tunnel
   type does not use UDP encapsulation at all, or a tunnel of the form
   "X-in-Y" contains a Protocol Type sub-TLV that specifies something
   other than "X".  Sub-TLVs of this sort MUST be disregarded.  That is,
   they MUST NOT affect the creation of the encapsulation header.
   However, the sub-TLV MUST NOT be considered to be malformed and
   MUST NOT be removed from the TLV before the route carrying the Tunnel
   Encapsulation attribute is distributed.  An implementation MAY log a
   message when it encounters such a sub-TLV.

14.  IANA Considerations

   IANA has made the updates described in the following subsections.
   All registration procedures listed are per their definitions in
   [RFC8126].

14.1.  Obsoleting RFC 5512

   Because this document obsoletes RFC 5512, IANA has updated references
   to RFC 5512 to point to this document in the following registries:

   *  "Border Gateway Protocol (BGP) Extended Communities" registry
      [IANA-BGP-EXT-COMM]

   *  "Border Gateway Protocol (BGP) Parameters" registry
      [IANA-BGP-PARAMS]

   *  "Border Gateway Protocol (BGP) Tunnel Encapsulation" registry
      [IANA-BGP-TUNNEL-ENCAP]

   *  "Subsequent Address Family Identifiers (SAFI) Parameters" registry
      [IANA-SAFI]

14.2.  Obsoleting Code Points Assigned by RFC 5566

   Since this document obsoletes RFC 5566, the code points assigned by
   that RFC are similarly obsoleted.  Specifically, the following code
   points have been marked as deprecated.

   In the "BGP Tunnel Encapsulation Attribute Tunnel Types" registry
   [IANA-BGP-TUNNEL-ENCAP]:

   +=======+==========================================================+
   | Value | Name                                                     |
   +=======+==========================================================+
   | 3     | Transmit tunnel endpoint (DEPRECATED)                    |
   +-------+----------------------------------------------------------+
   | 4     | IPsec in Tunnel-mode (DEPRECATED)                        |
   +-------+----------------------------------------------------------+
   | 5     | IP in IP tunnel with IPsec Transport Mode (DEPRECATED)   |
   +-------+----------------------------------------------------------+
   | 6     | MPLS-in-IP tunnel with IPsec Transport Mode (DEPRECATED) |
   +-------+----------------------------------------------------------+

                                 Table 1

   And in the "BGP Tunnel Encapsulation Attribute Sub-TLVs" registry
   [IANA-BGP-TUNNEL-ENCAP]:

            +=======+=========================================+
            | Value | Name                                    |
            +=======+=========================================+
            | 3     | IPsec Tunnel Authenticator (DEPRECATED) |
            +-------+-----------------------------------------+

                                  Table 2

14.3.  Border Gateway Protocol (BGP) Tunnel Encapsulation Grouping

   IANA has created a new registry grouping named "Border Gateway
   Protocol (BGP) Tunnel Encapsulation" [IANA-BGP-TUNNEL-ENCAP].

14.4.  BGP Tunnel Encapsulation Attribute Tunnel Types

   IANA has relocated the "BGP Tunnel Encapsulation Attribute Tunnel
   Types" registry to be under the "Border Gateway Protocol (BGP) Tunnel
   Encapsulation" grouping [IANA-BGP-TUNNEL-ENCAP].

14.5.  Subsequent Address Family Identifiers

   IANA has modified the "SAFI Values" registry [IANA-SAFI] to indicate
   that the Encapsulation SAFI (value 7) has been obsoleted.  This
   document is listed as the reference for this change.

14.6.  BGP Tunnel Encapsulation Attribute Sub-TLVs

   IANA has relocated the "BGP Tunnel Encapsulation Attribute Sub-TLVs"
   registry to be under the "Border Gateway Protocol (BGP) Tunnel
   Encapsulation" grouping [IANA-BGP-TUNNEL-ENCAP].

   IANA has included the following note to the registry:

   |  If the Sub-TLV Type is in the range from 0 to 127 (inclusive), the
   |  Sub-TLV Length field contains one octet.  If the Sub-TLV Type is
   |  in the range from 128 to 255 (inclusive), the Sub-TLV Length field
   |  contains two octets.

   IANA has updated the registration procedures of the registry to the
   following:

                   +=========+=========================+
                   | Range   | Registration Procedures |
                   +=========+=========================+
                   | 1-63    | Standards Action        |
                   +---------+-------------------------+
                   | 64-125  | First Come First Served |
                   +---------+-------------------------+
                   | 126-127 | Experimental Use        |
                   +---------+-------------------------+
                   | 128-191 | Standards Action        |
                   +---------+-------------------------+
                   | 192-252 | First Come First Served |
                   +---------+-------------------------+
                   | 253-254 | Experimental Use        |
                   +---------+-------------------------+

                                  Table 3

   IANA has added the following entries to this registry:

              +=======+=========================+===========+
              | Value | Description             | Reference |
              +=======+=========================+===========+
              | 0     | Reserved                | RFC 9012  |
              +-------+-------------------------+-----------+
              | 6     | Tunnel Egress Endpoint  | RFC 9012  |
              +-------+-------------------------+-----------+
              | 7     | DS Field                | RFC 9012  |
              +-------+-------------------------+-----------+
              | 8     | UDP Destination Port    | RFC 9012  |
              +-------+-------------------------+-----------+
              | 9     | Embedded Label Handling | RFC 9012  |
              +-------+-------------------------+-----------+
              | 10    | MPLS Label Stack        | RFC 9012  |
              +-------+-------------------------+-----------+
              | 11    | Prefix-SID              | RFC 9012  |
              +-------+-------------------------+-----------+
              | 255   | Reserved                | RFC 9012  |
              +-------+-------------------------+-----------+

                                  Table 4

14.7.  Flags Field of VXLAN Encapsulation Sub-TLV

   IANA has created a registry named "Flags Field of VXLAN Encapsulation
   Sub-TLVs" under the "Border Gateway Protocol (BGP) Tunnel
   Encapsulation" grouping [IANA-BGP-TUNNEL-ENCAP].  The registration
   policy for this registry is "Standards Action".  The minimum possible
   value is 0, and the maximum is 7.

   The initial values for this new registry are indicated in Table 5.

              +==============+=================+===========+
              | Bit Position | Description     | Reference |
              +==============+=================+===========+
              |      0       | V (VN-ID)       | RFC 9012  |
              +--------------+-----------------+-----------+
              |      1       | M (MAC Address) | RFC 9012  |
              +--------------+-----------------+-----------+

                                 Table 5

14.8.  Flags Field of NVGRE Encapsulation Sub-TLV

   IANA has created a registry named "Flags Field of NVGRE Encapsulation
   Sub-TLVs" under the "Border Gateway Protocol (BGP) Tunnel
   Encapsulation" grouping [IANA-BGP-TUNNEL-ENCAP].  The registration
   policy for this registry is "Standards Action".  The minimum possible
   value is 0, and the maximum is 7.

   The initial values for this new registry are indicated in Table 6.

              +==============+=================+===========+
              | Bit Position | Description     | Reference |
              +==============+=================+===========+
              |      0       | V (VN-ID)       | RFC 9012  |
              +--------------+-----------------+-----------+
              |      1       | M (MAC Address) | RFC 9012  |
              +--------------+-----------------+-----------+

                                 Table 6

14.9.  Embedded Label Handling Sub-TLV

   IANA has created a registry named "Embedded Label Handling Sub-TLVs"
   under the "Border Gateway Protocol (BGP) Tunnel Encapsulation"
   grouping [IANA-BGP-TUNNEL-ENCAP].  The registration policy for this
   registry is "Standards Action".  The minimum possible value is 0, and
   the maximum is 255.

   The initial values for this new registry are indicated in Table 7.

        +=======+=====================================+===========+
        | Value | Description                         | Reference |
        +=======+=====================================+===========+
        |   0   | Reserved                            | RFC 9012  |
        +-------+-------------------------------------+-----------+
        |   1   | Payload of MPLS with embedded label | RFC 9012  |
        +-------+-------------------------------------+-----------+
        |   2   | No embedded label in payload        | RFC 9012  |
        +-------+-------------------------------------+-----------+

                                  Table 7

14.10.  Color Extended Community Flags

   IANA has created a registry named "Color Extended Community Flags"
   under the "Border Gateway Protocol (BGP) Tunnel Encapsulation"
   grouping [IANA-BGP-TUNNEL-ENCAP].  The registration policy for this
   registry is "Standards Action".  The minimum possible value is 0, and
   the maximum is 15.

   This new registry contains columns for "Bit Position", "Description",
   and "Reference".  No values have currently been registered.

15.  Security Considerations

   As Section 11 discusses, it is intended that the Tunnel Encapsulation
   attribute be used only within a well-defined scope, for example,
   within a set of ASes that belong to a single administrative entity.
   As long as the filtering mechanisms discussed in that section are
   applied diligently, an attacker outside the scope would not be able
   to use the Tunnel Encapsulation attribute in an attack.  This leaves
   open the questions of attackers within the scope (for example, a
   compromised router) and failures in filtering that allow an external
   attack to succeed.

   As [RFC4272] discusses, BGP is vulnerable to traffic-diversion
   attacks.  The Tunnel Encapsulation attribute adds a new means by
   which an attacker could cause traffic to be diverted from its normal
   path, especially when the Tunnel Egress Endpoint sub-TLV is used.
   Such an attack would differ from pre-existing vulnerabilities in that
   traffic could be tunneled to a distant target across intervening
   network infrastructure, allowing an attack to potentially succeed
   more easily, since less infrastructure would have to be subverted.
   Potential consequences include "hijacking" of traffic (insertion of
   an undesired node in the path, which allows for inspection or
   modification of traffic, or avoidance of security controls) or denial
   of service (directing traffic to a node that doesn't desire to
   receive it).

   In order to further mitigate the risk of diversion of traffic from
   its intended destination, Section 3.1.1 provides an optional
   procedure to check that the destination given in a Tunnel Egress
   Endpoint sub-TLV is within the AS that was the source of the route.
   One then has some level of assurance that the tunneled traffic is
   going to the same destination AS that it would have gone to had the
   Tunnel Encapsulation attribute not been present.  As RFC 4272
   discusses, it's possible for an attacker to announce an inaccurate
   AS_PATH; therefore, an attacker with the ability to inject a Tunnel
   Egress Endpoint sub-TLV could equally craft an AS_PATH that would
   pass the validation procedures of Section 3.1.1.  BGP origin
   validation [RFC6811] and BGPsec [RFC8205] provide means to increase
   assurance that the origins being validated have not been falsified.

   Many tunnels carry traffic that embeds a destination address that
   comes from a non-global namespace.  One example is MPLS VPNs.  If a
   tunnel crosses from one namespace to another, without the necessary
   translation being performed for the embedded address(es), there
   exists a risk of misdelivery of traffic.  If the traffic contains
   confidential data that's not otherwise protected (for example, by
   end-to-end encryption), then confidential information could be
   revealed.  The restriction of applicability of the Tunnel
   Encapsulation attribute to a well-defined scope limits the likelihood
   of this occurring.  See the discussion of "option b" in Section 10
   for further discussion of one such scenario.

   RFC 8402 specifies that "SR domain boundary routers MUST filter any
   external traffic" ([RFC8402], Section 8.1).  For these purposes,
   traffic introduced into an SR domain using the Prefix-SID sub-TLV
   lies within the SR domain, even though the Prefix-SIDs used by the
   routers at the two ends of the tunnel may be different, as discussed
   in Section 3.7.  This implies that the duty to filter external
   traffic extends to all routers participating in such tunnels.

16.  References

16.1.  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/info/rfc2119>.

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

   [RFC2784]  Farinacci, D., Li, T., Hanks, S., Meyer, D., and P.
              Traina, "Generic Routing Encapsulation (GRE)", RFC 2784,
              DOI 10.17487/RFC2784, March 2000,
              <https://www.rfc-editor.org/info/rfc2784>.

   [RFC2890]  Dommety, G., "Key and Sequence Number Extensions to GRE",
              RFC 2890, DOI 10.17487/RFC2890, September 2000,
              <https://www.rfc-editor.org/info/rfc2890>.

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

   [RFC3270]  Le Faucheur, F., Wu, L., Davie, B., Davari, S., Vaananen,
              P., Krishnan, R., Cheval, P., and J. Heinanen, "Multi-
              Protocol Label Switching (MPLS) Support of Differentiated
              Services", RFC 3270, DOI 10.17487/RFC3270, May 2002,
              <https://www.rfc-editor.org/info/rfc3270>.

   [RFC3931]  Lau, J., Ed., Townsley, M., Ed., and I. Goyret, Ed.,
              "Layer Two Tunneling Protocol - Version 3 (L2TPv3)",
              RFC 3931, DOI 10.17487/RFC3931, March 2005,
              <https://www.rfc-editor.org/info/rfc3931>.

   [RFC4023]  Worster, T., Rekhter, Y., and E. Rosen, Ed.,
              "Encapsulating MPLS in IP or Generic Routing Encapsulation
              (GRE)", RFC 4023, DOI 10.17487/RFC4023, March 2005,
              <https://www.rfc-editor.org/info/rfc4023>.

   [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/info/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/info/rfc4760>.

   [RFC5129]  Davie, B., Briscoe, B., and J. Tay, "Explicit Congestion
              Marking in MPLS", RFC 5129, DOI 10.17487/RFC5129, January
              2008, <https://www.rfc-editor.org/info/rfc5129>.

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

   [RFC6811]  Mohapatra, P., Scudder, J., Ward, D., Bush, R., and R.
              Austein, "BGP Prefix Origin Validation", RFC 6811,
              DOI 10.17487/RFC6811, January 2013,
              <https://www.rfc-editor.org/info/rfc6811>.

   [RFC6890]  Cotton, M., Vegoda, L., Bonica, R., Ed., and B. Haberman,
              "Special-Purpose IP Address Registries", BCP 153,
              RFC 6890, DOI 10.17487/RFC6890, April 2013,
              <https://www.rfc-editor.org/info/rfc6890>.

   [RFC7348]  Mahalingam, M., Dutt, D., Duda, K., Agarwal, P., Kreeger,
              L., Sridhar, T., Bursell, M., and C. Wright, "Virtual
              eXtensible Local Area Network (VXLAN): A Framework for
              Overlaying Virtualized Layer 2 Networks over Layer 3
              Networks", RFC 7348, DOI 10.17487/RFC7348, August 2014,
              <https://www.rfc-editor.org/info/rfc7348>.

   [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/info/rfc7606>.

   [RFC7637]  Garg, P., Ed. and Y. Wang, Ed., "NVGRE: Network
              Virtualization Using Generic Routing Encapsulation",
              RFC 7637, DOI 10.17487/RFC7637, September 2015,
              <https://www.rfc-editor.org/info/rfc7637>.

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

   [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/info/rfc8174>.

   [RFC8669]  Previdi, S., Filsfils, C., Lindem, A., Ed., Sreekantiah,
              A., and H. Gredler, "Segment Routing Prefix Segment
              Identifier Extensions for BGP", RFC 8669,
              DOI 10.17487/RFC8669, December 2019,
              <https://www.rfc-editor.org/info/rfc8669>.

16.2.  Informative References

   [EVPN-INTER-SUBNET]
              Sajassi, A., Salam, S., Thoria, S., Drake, J. E., and J.
              Rabadan, "Integrated Routing and Bridging in EVPN", Work
              in Progress, Internet-Draft, draft-ietf-bess-evpn-inter-
              subnet-forwarding-13, 10 February 2021,
              <https://tools.ietf.org/html/draft-ietf-bess-evpn-inter-
              subnet-forwarding-13>.

   [IANA-ADDRESS-FAM]
              IANA, "Address Family Numbers",
              <https://www.iana.org/assignments/address-family-
              numbers/>.

   [IANA-BGP-EXT-COMM]
              IANA, "Border Gateway Protocol (BGP) Extended
              Communities", <https://www.iana.org/assignments/bgp-
              extended-communities/>.

   [IANA-BGP-PARAMS]
              IANA, "Border Gateway Protocol (BGP) Parameters",
              <https://www.iana.org/assignments/bgp-parameters/>.

   [IANA-BGP-TUNNEL-ENCAP]
              IANA, "Border Gateway Protocol (BGP) Tunnel
              Encapsulation", <https://www.iana.org/assignments/bgp-
              tunnel-encapsulation/>.

   [IANA-ETHERTYPES]
              IANA, "IEEE 802 Numbers: ETHER TYPES",
              <https://www.iana.org/assignments/ieee-802-numbers/>.

   [IANA-SAFI]
              IANA, "Subsequent Address Family Identifiers (SAFI)
              Parameters",
              <https://www.iana.org/assignments/safi-namespace/>.

   [RFC4272]  Murphy, S., "BGP Security Vulnerabilities Analysis",
              RFC 4272, DOI 10.17487/RFC4272, January 2006,
              <https://www.rfc-editor.org/info/rfc4272>.

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

   [RFC5512]  Mohapatra, P. and E. Rosen, "The BGP Encapsulation
              Subsequent Address Family Identifier (SAFI) and the BGP
              Tunnel Encapsulation Attribute", RFC 5512,
              DOI 10.17487/RFC5512, April 2009,
              <https://www.rfc-editor.org/info/rfc5512>.

   [RFC5565]  Wu, J., Cui, Y., Metz, C., and E. Rosen, "Softwire Mesh
              Framework", RFC 5565, DOI 10.17487/RFC5565, June 2009,
              <https://www.rfc-editor.org/info/rfc5565>.

   [RFC5566]  Berger, L., White, R., and E. Rosen, "BGP IPsec Tunnel
              Encapsulation Attribute", RFC 5566, DOI 10.17487/RFC5566,
              June 2009, <https://www.rfc-editor.org/info/rfc5566>.

   [RFC5640]  Filsfils, C., Mohapatra, P., and C. Pignataro, "Load-
              Balancing for Mesh Softwires", RFC 5640,
              DOI 10.17487/RFC5640, August 2009,
              <https://www.rfc-editor.org/info/rfc5640>.

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

   [RFC7510]  Xu, X., Sheth, N., Yong, L., Callon, R., and D. Black,
              "Encapsulating MPLS in UDP", RFC 7510,
              DOI 10.17487/RFC7510, April 2015,
              <https://www.rfc-editor.org/info/rfc7510>.

   [RFC8205]  Lepinski, M., Ed. and K. Sriram, Ed., "BGPsec Protocol
              Specification", RFC 8205, DOI 10.17487/RFC8205, September
              2017, <https://www.rfc-editor.org/info/rfc8205>.

   [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/info/rfc8277>.

   [RFC8365]  Sajassi, A., Ed., Drake, J., Ed., Bitar, N., Shekhar, R.,
              Uttaro, J., and W. Henderickx, "A Network Virtualization
              Overlay Solution Using Ethernet VPN (EVPN)", RFC 8365,
              DOI 10.17487/RFC8365, March 2018,
              <https://www.rfc-editor.org/info/rfc8365>.

   [RFC8402]  Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
              Decraene, B., Litkowski, S., and R. Shakir, "Segment
              Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
              July 2018, <https://www.rfc-editor.org/info/rfc8402>.

Appendix A.  Impact on RFC 8365

   [RFC8365] references RFC 5512 for its definition of the BGP
   Encapsulation Extended Community.  That extended community is now
   defined in this document, in a way consistent with its previous
   definition.

   Section 6 of [RFC8365] talks about the use of the Encapsulation
   Extended Community to allow Network Virtualization Edge (NVE) devices
   to signal their supported encapsulations.  We note that with the
   introduction of this specification, the Tunnel Encapsulation
   attribute can also be used for this purpose.  For purposes where RFC
   8365 talks about "advertising supported encapsulations" (for example,
   in the second paragraph of Section 6), encapsulations advertised
   using the Tunnel Encapsulation attribute should be considered equally
   with those advertised using the Encapsulation Extended Community.

   In particular, a review of Section 8.3.1 of [RFC8365] is called for,
   to consider whether the introduction of the Tunnel Encapsulation
   attribute creates a need for any revisions to the split-horizon
   procedures.

   [RFC8365] also refers to a draft version of this specification in the
   final paragraph of Section 5.1.3.  That paragraph references
   Section 8.2.2.2 of the draft.  In this document, the correct
   reference would be Section 9.2.2.2.  There are no substantive
   differences between the section in the referenced draft version and
   that in this document.

Acknowledgments

   This document contains text from RFC 5512, authored by Pradosh
   Mohapatra and Eric Rosen.  The authors of the current document wish
   to thank them for their contribution.  RFC 5512 itself built upon
   prior work by Gargi Nalawade, Ruchi Kapoor, Dan Tappan, David Ward,
   Scott Wainner, Simon Barber, Lili Wang, and Chris Metz, whom the
   authors also thank for their contributions.  Eric Rosen was the
   principal author of earlier versions of this document.

   The authors wish to thank Lou Berger, Ron Bonica, Martin Djernaes,
   John Drake, Susan Hares, Satoru Matsushima, Thomas Morin, Dhananjaya
   Rao, Ravi Singh, Harish Sitaraman, Brian Trammell, Xiaohu Xu, and
   Zhaohui Zhang for their review, comments, and/or helpful discussions.
   Alvaro Retana provided an especially comprehensive review.

Contributors

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

   Randy Bush
   Internet Initiative Japan
   5147 Crystal Springs
   Bainbridge Island, WA 98110
   United States of America

   Email: randy@psg.com

   Robert Raszuk
   Bloomberg LP
   731 Lexington Ave
   New York City, NY 10022
   United States of America

   Email: robert@raszuk.net

   Eric C. Rosen

Authors' Addresses

   Keyur Patel
   Arrcus, Inc
   2077 Gateway Pl
   San Jose, CA 95110
   United States of America

   Email: keyur@arrcus.com

   Gunter Van de Velde
   Nokia
   Copernicuslaan 50
   2018 Antwerpen
   Belgium

   Email: gunter.van_de_velde@nokia.com

   Srihari R. Sangli
   Juniper Networks

   Email: ssangli@juniper.net

   John Scudder
   Juniper Networks

   Email: jgs@juniper.net