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Versions: (draft-vandevelde-idr-remote-next-hop) 00 01 02 03 04 05 06 07

IDR Working Group                                          E. Rosen, Ed.
Internet-Draft                                    Juniper Networks, Inc.
Obsoletes: 5512 (if approved)                                   K. Patel
Intended status: Standards Track                                  Arrcus
Expires: January 18, 2018                                G. Van de Velde
                                                                   Nokia
                                                           July 17, 2017


                 The BGP Tunnel Encapsulation Attribute
                    draft-ietf-idr-tunnel-encaps-07

Abstract

   RFC 5512 defines a BGP Path Attribute known as the "Tunnel
   Encapsulation Attribute".  This attribute allows one to specify a set
   of tunnels.  For each such tunnel, the attribute can provide the
   information needed to create the tunnel and the corresponding
   encapsulation header.  The attribute can also provide information
   that aids in choosing whether a particular packet is to be sent
   through a particular tunnel.  RFC 5512 states that the attribute is
   only carried in BGP UPDATEs that have the "Encapsulation Subsequent
   Address Family (Encapsulation SAFI)".  This document deprecates the
   Encapsulation SAFI (which has never been used in production), and
   specifies semantics for the attribute when it is carried in UPDATEs
   of certain other SAFIs.  This document adds support for additional
   tunnel types, and allows a remote tunnel endpoint address to be
   specified for each tunnel.  This document also provides support for
   specifying fields of any inner or outer encapsulations that may be
   used by a particular tunnel.

   This document obsoletes RFC 5512.

Status of This Memo

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

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

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




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Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   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  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Brief Summary of RFC 5512 . . . . . . . . . . . . . . . .   4
     1.2.  Deficiencies in RFC 5512  . . . . . . . . . . . . . . . .   4
     1.3.  Brief Summary of Changes from RFC 5512  . . . . . . . . .   5
     1.4.  Impact on RFC 5566  . . . . . . . . . . . . . . . . . . .   6
   2.  The Tunnel Encapsulation Attribute  . . . . . . . . . . . . .   6
   3.  Tunnel Encapsulation Attribute Sub-TLVs . . . . . . . . . . .   8
     3.1.  The Remote Endpoint Sub-TLV . . . . . . . . . . . . . . .   8
     3.2.  Encapsulation Sub-TLVs for Particular Tunnel Types  . . .  10
       3.2.1.  VXLAN . . . . . . . . . . . . . . . . . . . . . . . .  11
       3.2.2.  VXLAN-GPE . . . . . . . . . . . . . . . . . . . . . .  12
       3.2.3.  NVGRE . . . . . . . . . . . . . . . . . . . . . . . .  13
       3.2.4.  L2TPv3  . . . . . . . . . . . . . . . . . . . . . . .  14
       3.2.5.  GRE . . . . . . . . . . . . . . . . . . . . . . . . .  15
       3.2.6.  MPLS-in-GRE . . . . . . . . . . . . . . . . . . . . .  15
     3.3.  Outer Encapsulation Sub-TLVs  . . . . . . . . . . . . . .  16
       3.3.1.  IPv4 DS Field . . . . . . . . . . . . . . . . . . . .  16
       3.3.2.  UDP Destination Port  . . . . . . . . . . . . . . . .  17
     3.4.  Sub-TLVs for Aiding Tunnel Selection  . . . . . . . . . .  17
       3.4.1.  Protocol Type Sub-TLV . . . . . . . . . . . . . . . .  17
       3.4.2.  Color Sub-TLV . . . . . . . . . . . . . . . . . . . .  17
     3.5.  Embedded Label Handling Sub-TLV . . . . . . . . . . . . .  17
     3.6.  MPLS Label Stack Sub-TLV  . . . . . . . . . . . . . . . .  18
     3.7.  Prefix-SID Sub-TLV  . . . . . . . . . . . . . . . . . . .  20
   4.  Extended Communities Related to the Tunnel Encapsulation
       Attribute . . . . . . . . . . . . . . . . . . . . . . . . . .  21
     4.1.  Encapsulation Extended Community  . . . . . . . . . . . .  21
     4.2.  Router's MAC Extended Community . . . . . . . . . . . . .  22
     4.3.  Color Extended Community  . . . . . . . . . . . . . . . .  23



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   5.  Semantics and Usage of the Tunnel Encapsulation
       attribute . . . . . . . . . . . . . . . . . . . . . . . . . .  23
   6.  Routing Considerations  . . . . . . . . . . . . . . . . . . .  27
     6.1.  No Impact on BGP Decision Process . . . . . . . . . . . .  27
     6.2.  Looping, Infinite Stacking, Etc.  . . . . . . . . . . . .  27
   7.  Recursive Next Hop Resolution . . . . . . . . . . . . . . . .  28
   8.  Use of Virtual Network Identifiers and Embedded Labels
       when Imposing a Tunnel Encapsulation  . . . . . . . . . . . .  29
     8.1.  Tunnel Types without a Virtual Network Identifier
           Field . . . . . . . . . . . . . . . . . . . . . . . . . .  29
     8.2.  Tunnel Types with a Virtual Network Identifier Field  . .  29
       8.2.1.  Unlabeled Address Families  . . . . . . . . . . . . .  30
       8.2.2.  Labeled Address Families  . . . . . . . . . . . . . .  30
         8.2.2.1.  When a Valid VNI has been Signaled  . . . . . . .  31
         8.2.2.2.  When a Valid VNI has not been Signaled  . . . . .  31
   9.  Applicability Restrictions  . . . . . . . . . . . . . . . . .  32
   10. Scoping . . . . . . . . . . . . . . . . . . . . . . . . . . .  32
   11. Error Handling  . . . . . . . . . . . . . . . . . . . . . . .  33
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  34
     12.1.  Subsequent Address Family Identifiers  . . . . . . . . .  34
     12.2.  BGP Path Attributes  . . . . . . . . . . . . . . . . . .  35
     12.3.  Extended Communities . . . . . . . . . . . . . . . . . .  35
     12.4.  BGP Tunnel Encapsulation Attribute Sub-TLVs  . . . . . .  35
     12.5.  Tunnel Types . . . . . . . . . . . . . . . . . . . . . .  36
   13. Security Considerations . . . . . . . . . . . . . . . . . . .  36
   14. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  37
   15. Contributor Addresses . . . . . . . . . . . . . . . . . . . .  38
   16. References  . . . . . . . . . . . . . . . . . . . . . . . . .  38
     16.1.  Normative References . . . . . . . . . . . . . . . . . .  38
     16.2.  Informative References . . . . . . . . . . . . . . . . .  38
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  41

1.  Introduction

   This document obsoletes RFC 5512.  The deficiencies of RFC 5512, and
   a summary of the changes made, are discussed in Sections 1.1-1.3.
   The material from RFC 5512 that is retained has been incorporated
   into this document.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL", when and only when appearing in all capital letters, are
   to be interpreted as described in [RFC2119].








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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, e.g., the
   "Encapsulation sub-TLV", contain information that may be used to form
   the encapsulation header for the specified tunnel type.  Other sub-
   TLVs, e.g., 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 AFI (Address Family Identifier) of 1 or
   2, and a SAFI of 7.  In an UPDATE of the Encapsulation SAFI, the NLRI
   (Network Layer Reachability Information) is an address of the BGP
   speaker originating the UPDATE.  Consider the following scenario:

   o  BGP speaker R1 has received and installed UPDATE U;

   o  UPDATE U's SAFI is the Encapsulation SAFI;

   o  UPDATE U has the address R2 as its NLRI;

   o  UPDATE U has a Tunnel Encapsulation attribute.

   o  R1 has a packet, P, to transmit to destination D;

   o  R1's best path 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 remote 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:

   o  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.




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

   o  If the respective best paths 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.

   o  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.

   o  In [RFC5512]'s specification of the sub-TLVs, each sub-TLV has
      one-octet length field.  In some cases, a two-octet length field
      may be needed.

1.3.  Brief Summary of Changes from RFC 5512

   In this document we address these deficiencies by:

   o  Deprecating the Encapsulation SAFI.

   o  Defining a new "Remote Endpoint Address sub-TLV" 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.

   o  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.

   o  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.

   o  Defining the sub-TLV type field so that a sub-TLV whose type is in
      the range from 1 to 127 inclusive has a one-octet length field,
      but a sub-TLV whose type is in the range from 128 to 254 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]: VXLAN (Virtual Extensible Local Area Network, [RFC7348]),



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   VXLAN-GPE (Generic Protocol Extension for VXLAN, [VXLAN-GPE]), NVGRE
   (Network Virtualization Using Generic Routing Encapsulation
   [RFC7637]), and MPLS-in-GRE (MPLS in Generic Routing Encapsulation
   [RFC2784], [RFC2890], [RFC4023]).  MPLS-in-UDP [RFC7510] is also
   supported, but an Encapsulation sub-TLV for it is not needed.

   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), or 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 a Tunnel Encapsulation Extended Community, that can
   be used instead of the Tunnel Encapsulation attribute under certain
   circumstances.  This document addresses the issue of how to handle a
   BGP UPDATE that carries both a Tunnel Encapsulation attribute and one
   or more Tunnel Encapsulation Extended Communities.

1.4.  Impact on RFC 5566

   [RFC5566] uses the mechanisms defined in [RFC5512].  While this
   document obsoletes [RFC5512], it does not address the issue of how to
   use the mechanisms of [RFC5566] without also using the Encapsulation
   SAFI.  Those issues are considered to be outside the scope of this
   document.

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.  The attribute is composed of a set of Type-Length-Value
   (TLV) encodings.  Each TLV contains information corresponding to a
   particular tunnel type.  A TLV is structured as shown in Figure 1:





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

              Figure 1: Tunnel Encapsulation TLV Value Field

   o  Tunnel Type (2 octets): identifies a type of tunnel.  The field
      contains values from the IANA Registry "BGP Tunnel Encapsulation
      Attribute Tunnel Types".

      Note that for tunnel types whose names are of the form "X-in-Y",
      e.g., "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 include a Protocol Type sub-TLV specifying
      "X".

   o  Length (2 octets): the total number of octets of the value field.

   o  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 field (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: Tunnel Encapsulation Sub-TLV Format

   o  Sub-TLV Type (1 octet): each sub-TLV type defines a certain
      property about the tunnel TLV that contains this sub-TLV.





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   o  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 1-127.
      The Sub-TLV Length field contains two octets if the Sub-TLV Type
      field contains a value in the range from 128-254.

   o  Sub-TLV Value (variable): encodings of the value field depend on
      the sub-TLV type as enumerated above.  The following sub-sections
      define the encoding in detail.

3.  Tunnel Encapsulation Attribute Sub-TLVs

   In this section, we specify a number of sub-TLVs.  These sub-TLVs can
   be included in a TLV of the Tunnel Encapsulation attribute.

3.1.  The Remote Endpoint Sub-TLV

   The Remote Endpoint sub-TLV is a sub-TLV whose value field contains
   three sub-fields:

   1.  a four-octet Autonomous System (AS) number sub-field

   2.  a two-octet Address Family sub-field

   3.  an address sub-field, 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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                  Autonomous System Number                     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |      Address Family           |           Address             ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
     ~                                                               ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

               Figure 3: Remote Endpoint Sub-TLV Value Field

   The Address Family subfield contains a value from IANA's "Address
   Family Numbers" registry.  In this document, we assume 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).



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   In this case, the length field of Remote Endpoint sub-TLV must
   contain the value 10 (0xa).

   If the Address Family subfield contains the value for IPv6, the
   address sub-field must contain an IPv6 address (a /128 IPv6 prefix).
   In this case, the length field of Remote Endpoint sub-TLV must
   contain the value 22 (0x16).  IPv6 link local addresses are not valid
   values of the IP address field.

   In a given BGP UPDATE, the address family (IPv4 or IPv6) of a Remote
   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 Remote Endpoint sub-
   TLVs that contain IPv6 addresses.  Also, different tunnels
   represented in the Tunnel Encapsulation attribute may have Remote
   Endpoints of different address families.

   A two-octet AS number can be carried in the AS number field by
   setting the two high order octets to zero, and carrying the number in
   the two low order octets of the field.

   The AS number in the sub-TLV MUST be the number of the AS to which
   the IP address in the sub-TLV belongs.

   There is one special case: the Remote Endpoint sub-TLV MAY have a
   value field whose Address Family subfield contains 0.  This means
   that the tunnel's remote endpoint is the UPDATE's BGP next hop.  If
   the Address Family subfield contains 0, the Address subfield is
   omitted, and the Autonomous System number field is set to 0.

   If any of the following conditions hold, the Remote Endpoint sub-TLV
   is considered to be "malformed":

   o  The sub-TLV contains the value for IPv4 in its Address Family
      subfield, but the length of the sub-TLV's value field is other
      than 10 (0xa).

   o  The sub-TLV contains the value for IPv6 in its Address Family
      subfield, but the length of the sub-TLV's value field is other
      than 22 (0x16).

   o  The sub-TLV contains the value zero in its Address Family field,
      but the length of the sub-TLV's value field is other than 6, or
      the Autonomous System subfield is not set to zero.

   o  The IP address in the sub-TLV's address subfield is not a valid IP
      address (e.g., it's an IPv4 broadcast address).




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   o  It can be determined that the IP address in the sub-TLV's address
      subfield does not belong to the non-zero AS whose number is in the
      its Autonomous System subfield.  (See section Section 13 for
      discussion of one way to determine this.)

   If the Remote Endpoint sub-TLV is malformed, the TLV containing it is
   also considered to be malformed, and the entire TLV MUST be ignored.
   However, the Tunnel Encapsulation attribute SHOULD NOT be considered
   to be malformed in this case; other TLVs in the attribute SHOULD be
   processed (if they can be parsed correctly).

   When redistributing a route that is carrying a Tunnel Encapsulation
   attribute containing a TLV that itself contains a malformed Remote
   Endpoint sub-TLV, the TLV SHOULD be removed from the attribute before
   redistribution.

   See Section 11 for further discussion of how to handle errors that
   are encountered when parsing the Tunnel Encapsulation attribute.

   If the Remote Endpoint sub-TLV contains an IPv4 or IPv6 address that
   is valid but not reachable, the sub-TLV is NOT considered to be
   malformed, and the containing TLV SHOULD NOT be removed from the
   attribute before redistribution.  However, the tunnel identified by
   the TLV containing that sub-TLV cannot be used until such time as the
   address becomes reachable.  See Section 5.

3.2.  Encapsulation Sub-TLVs for Particular Tunnel Types

   This section defines Tunnel Encapsulation sub-TLVs for the following
   tunnel types: VXLAN ([RFC7348]), VXLAN-GPE ([VXLAN-GPE]), NVGRE
   ([RFC7637]), MPLS-in-GRE ([RFC2784], [RFC2890], [RFC4023]), L2TPv3
   ([RFC3931]), and GRE ([RFC2784], [RFC2890], [RFC4023]).

   Rules for forming the encapsulation based on the information in a
   given TLV are given in Sections 5 and 8.

   For some tunnel types, the rules are obvious and not mentioned in
   this document.

   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 remote
   endpoint.








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3.2.1.  VXLAN

   This document defines an encapsulation sub-TLV for VXLAN tunnels.
   When the tunnel type is VXLAN, the following is the structure of the
   value field in the encapsulation sub-TLV:


      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |V|M|R|R|R|R|R|R|          VN-ID (3 Octets)                     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                 MAC Address (4 Octets)                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  MAC Address (2 Octets)       |          Reserved             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 4: VXLAN Encapsulation Sub-TLV

      V: This bit is set to 1 to indicate that a "valid" VN-ID (Virtual
      Network Identifier) is present in the encapsulation sub-TLV.
      Please see Section 8.

      M: This bit is set to 1 to indicate that a valid MAC Address is
      present in the encapsulation sub-TLV.

      R: The remaining bits in the 8-bit flags field are reserved for
      further use.  They SHOULD always be set to 0.

      VN-ID: If the V bit is set, the VN-id field contains a 3 octet VN-
      ID value.  If the V bit is not set, the VN-id field SHOULD be set
      to zero.

      MAC Address: If the M bit is set, this field contains a 6 octet
      Ethernet MAC address.  If the M bit is not set, this field SHOULD
      be set to all zeroes.

   When forming the VXLAN encapsulation header:

   o  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].

   o  If the M bit is set, the MAC Address is copied into the Inner
      Destination MAC Address field of the Inner Ethernet Header (see
      section 5 of [RFC7348].





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      If the M bit is not set, 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 not set, 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.

   o  See Section 8 to see how the VNI 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
   (e.g.,the MAC address) that is used to form this ethernet header.

3.2.2.  VXLAN-GPE

   This document defines an encapsulation sub-TLV for VXLAN tunnels.
   When the tunnel type is VXLAN-GPE, the following is the structure of
   the value field in the encapsulation sub-TLV:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |Ver|V|R|R|R|R|R|                 Reserved                      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       VN-ID                   |   Reserved    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                 Figure 5: VXLAN GPE Encapsulation Sub-TLV

      V: This bit is set to 1 to indicate that a "valid" VN-ID is
      present in the encapsulation sub-TLV.  Please see Section 8.

      R: The bits designated "R" above are reserved for future use.
      They SHOULD always be set to zero.

      Version (Ver): Indicates VXLAN GPE protocol version.  (See the
      "Version Bits" section of [VXLAN-GPE].)  If the indicated version
      is not supported, the TLV that contains this Encapsulation sub-TLV
      MUST be treated as specifying an unsupported tunnel type.  The
      value of this field will be copied into the corresponding field of
      the VXLAN encapsulation header.



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      VN-ID: If the V bit is set, this field contains a 3 octet VN-ID
      value.  If the V bit is not set, this field SHOULD be set to zero.

   When forming the VXLAN-GPE encapsulation header:

   o  The values of the V and R bits are NOT copied into the flags field
      of the VXLAN-GPE header.  However, the values of the Ver bits are
      copied into the VXLAN-GPE header.  Other bits in the flags field
      of the VXLAN-GPE header are set as per [VXLAN-GPE].

   o  See Section 8 to see how the VNI field of the VXLAN-GPE
      encapsulation header is set.

3.2.3.  NVGRE

   This document defines an encapsulation sub-TLV for NVGRE tunnels.
   When the tunnel type is NVGRE, the following is the structure of the
   value field in the encapsulation sub-TLV:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |V|M|R|R|R|R|R|R|          VN-ID (3 Octets)                     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                 MAC Address (4 Octets)                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  MAC Address (2 Octets)       |           Reserved            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 6: NVGRE Encapsulation Sub-TLV

      V: This bit is set to 1 to indicate that a "valid" VN-ID is
      present in the encapsulation sub-TLV.  Please see Section 8.

      M: This bit is set to 1 to indicate that a valid MAC Address is
      present in the encapsulation sub-TLV.

      R: The remaining bits in the 8-bit flags field are reserved for
      further use.  They SHOULD always be set to 0.

      VN-ID: If the V bit is set, the VN-id field contains a 3 octet VN-
      ID value.  If the V bit is not set, the VN-id field SHOULD be set
      to zero.

      MAC Address: If the M bit is set, this field contains a 6 octet
      Ethernet MAC address.  If the M bit is not set, this field SHOULD
      be set to all zeroes.




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   When forming the NVGRE encapsulation header:

   o  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 VXLAN header is
      set as per [RFC7637].

   o  If the M bit is set, 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 not set, 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 not set, 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.

   o  See Section 8 to see how the VSID (Virtual Subnet Identifier)
      field of the NVGRE encapsulation header is set.

3.2.4.  L2TPv3

   When the tunnel type of the TLV is L2TPv3 over IP, the following is
   the structure of the value field of the encapsulation sub-TLV:


      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                      Session ID (4 octets)                    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |                        Cookie (Variable)                      |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                  Figure 7: L2TPv3 Encapsulation Sub-TLV

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

      Cookie: an optional, variable length (encoded in octets -- 0 to 8
      octets) value used by L2TPv3 to check the association of a



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      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).

3.2.5.  GRE

   When the tunnel type of the TLV is GRE, the following is the
   structure of the value field of the encapsulation sub-TLV:

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


                    Figure 8: GRE Encapsulation Sub-TLV

      GRE Key: 4-octet field [RFC2890] that is generated by the
      advertising router.  The actual method by which the key is
      obtained is beyond the scope of this document.  The key is
      inserted into the GRE encapsulation header of the payload packets
      sent by ingress routers to the advertising router.  It is intended
      to be used for identifying extra context information about the
      received payload.

      Note that the key is optional.  Unless a key value is being
      advertised, the GRE encapsulation sub-TLV MUST NOT be present.

3.2.6.  MPLS-in-GRE

   When the tunnel type is MPLS-in-GRE, the following is the structure
   of the value field in an optional encapsulation sub-TLV:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       GRE-Key (4 Octets)                      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 9: MPLS-in-GRE Encapsulation Sub-TLV

      GRE-Key: 4-octet field [RFC2890] that is generated by the
      advertising router.  The actual method by which the key is
      obtained is beyond the scope of this document.  The key is



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      inserted into the GRE encapsulation header of the payload packets
      sent by ingress routers to the advertising router.  It is intended
      to be used for identifying extra context information about the
      received payload.  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 3.2.5 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.  That is, if a TLV specifies
   MPLS-in-GRE or if it includes a Protocol Type sub-TLV specifying
   MPLS, the GRE tunnel advertised in that TLV MUST NOT be used for
   carrying IP packets.

   While it is not really necessary to have both the GRE and MPLS-in-GRE
   tunnel types, both are included for reasons of backwards
   compatibility.

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
   is treated as if it were an unknown type of sub-TLV.

3.3.1.  IPv4 DS Field

   Most of the tunnel types that can be specified in the Tunnel
   Encapsulation attribute require an outer IP encapsulation.  The IPv4
   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 IP encapsulation (see
   [RFC2474]).  The value field is always a single octet.






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3.3.2.  UDP Destination Port

   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 non-standard 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.

3.4.  Sub-TLVs for Aiding Tunnel Selection

3.4.1.  Protocol Type Sub-TLV

   The protocol type sub-TLV MAY be included in a given TLV to indicate
   the type of the payload packets that may be encapsulated with the
   tunnel parameters that are being signaled in the TLV.  The value
   field of the sub-TLV contains a 2-octet value from IANA's ethertype
   registry [Ethertypes].

   For example, if we want to use 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.

3.4.2.  Color Sub-TLV

   The color sub-TLV MAY be encoded as a way to "color" the
   corresponding tunnel TLV.  The value field of the sub-TLV 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 7.

3.5.  Embedded Label Handling Sub-TLV

   Certain BGP address families (corresponding to particular AFI/SAFI
   pairs, e.g., 1/4, 2/4, 1/128, 2/128) have MPLS labels embedded in
   their NLRIs.  We will use the term "embedded label" to refer to the




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   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 (e.g., VXLAN, VXLAN-GPE, and NVGRE) that can
   be specified in the Tunnel Encapsulation attribute have an
   encapsulation header containing "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
   an embedded 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,
   the sub-TLV is treated as a no-op.  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 is treated as a no-op.  In
   those cases where the sub-TLV is treated as a no-op, it SHOULD NOT be
   stripped from the TLV before the UPDATE is forwarded.

   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 carried
      either in the virtual network identifier field of the
      encapsulation header, or else is ignored entirely.

   Please see Section 8 for the details of how this sub-TLV is used when
   it is carried by an UPDATE of a labeled address family.

3.6.  MPLS Label Stack Sub-TLV

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

   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, the



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   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 following format:

      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 10: MPLS Label Stack Sub-TLV

   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.  This label stack MUST be pushed onto the packet before any
   other labels are pushed onto the packet.

   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 8),
   the contents of the MPLS label stack sub-TLV MUST be pushed onto the
   packet before the procdures of Section 8 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 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..

   By default, the TC (Traffic Class) field ([RFC3032], [RFC5462]) of
   each label stack entry is set to 0.  This may of course be changed by
   policy at the originator of the sub-TLV.  When pushing the label
   stack onto a packet, the TC of the label stack entries is preserved
   by default.  However, local policy at the router that is pushing on
   the stack MAY cause modification of the TC values.





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   By default, the TTL (Time to Live) field of each label stack entry is
   set to 255.  This may be changed by policy at the originator of the
   sub-TLV.  When pushing the label stack onto a packet, the TTL of the
   label stack entries is preserved by default.  However, local policy
   at the router that is pushing on the stack MAY cause modification of
   the TTL values.  If any label stack entry in the sub-TLV has a TTL
   value of zero, the router that is pushing the stack on a packet MUST
   change the value to a non-zero value.

   Note that this sub-TLV can be 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.

3.7.  Prefix-SID Sub-TLV

   [Prefix-SID-Attribute] defines a BGP Path attribute known as the
   "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, an "IPv6 SID (Segment Identifier)" TLV, or an "Originator
   SRGB (Source Routing Global Block)" TLV.

   In this document, we define a Prefix-SID sub-TLV.  The value field of
   the Prefix-SID sub-TLV can be set to any valid value of the value
   field of a BGP Prefix-SID attribute, as defined in
   [Prefix-SID-Attribute].

   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 Remote
   Endpoint.  The Label-Index, if present, is the Segment Routing SID
   that the tunnel's Remote 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, 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.

   If the Originator SRGB is not present,it is assumed that the
   originator's SRGB is known by other means.  Such "other means" are
   outside the scope of this document.





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   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 (e.g., a label embedded in
   UPDATE's NLRI, or a label determined by the procedures of Section 8
   are pushed on the stack.

   The Prefix-SID sub-TLV has slightly different semantics than the
   Prefix-SID attribute.  When the 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 that the prefix-SID is
   for the advertised prefix in that Segment Routing domain.  When the
   Prefix-SID sub-TLV is used, the BGP speaker at the head end of the
   tunnel need even not be in the same Segment Routing Domain as the
   tunnel's Remote Endpoint, and 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.  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.  If
   the Encapsulation Extended Community identifies a particular tunnel
   type, its semantics are exactly equivalent to the semantics of a
   Tunnel Encapsulation attribute Tunnel TLV for which the following
   three conditions all hold:

   1.  it identifies the same tunnel type,

   2.  it has a Remote 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 same IP address that
           appears in the next hop field of the route to which the
           Tunnel Encapsulation attribute is attached

   3.  it has no other sub-TLVs.

   We will refer to such a Tunnel TLV as a "barebones" Tunnel TLV.




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   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.  To
   ensure backwards compatibility, this specification establishes the
   following rules:

   1.  If the Tunnel Encapsulation attribute of a given route contains a
       barebones Tunnel TLV identifying a particular tunnel type, an
       Encapsulation Extended Community identifying the same tunnel type
       SHOULD be attached to the route.

   2.  If the Encapsulation Extended Community identifying a particular
       tunnel type is attached to a given route, the corresponding
       barebones Tunnel TLV MAY be omitted from the Tunnel Encapsulation
       attribute.

   3.  Suppose a particular route has both (a) an Encapsulation Extended
       Community specifying a particular tunnel type, and (b) a Tunnel
       Encapsulation attribute with a barebones Tunnel TLV specifying
       that same tunnel type.  Both (a) and (b) MUST be interpreted as
       denoting the same tunnel.

   In short, in situations where one could use either the Encapsulation
   Extended Community or a barebones Tunnel TLV, one may use either or
   both.  However, to ensure backwards compatibility with applications
   that do not support the Tunnel Encapsulation attribute, it is
   preferable to use the Encapsulation Extended Community.  If the
   Extended Community (identifying a particular tunnel type) is present,
   the corresponding Tunnel TLV is optional.

   Note that for tunnel types of the form "X-in-Y", e.g., 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".

   In the remainder of this specification, when we speak of a route as
   containing a Tunnel Encapsulation attribute with a TLV identifying a
   particular tunnel type, we are implicitly including the case where
   the route contains a Tunnel 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 provides information that may conflict with
   information in one or more of the Encapsulation Sub-TLVs of a Tunnel




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   Encapsulation attribute.  In case of such a conflict, the information
   in the Encapsulation Sub-TLV takes precedence.

4.3.  Color Extended Community

   The Color Extended Community is a Transitive Opaque Extended
   Community with the following encoding:

      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    |     0x0b      |           Reserved            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                          Color Value                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 11: Color Extended Community

   For the use of this Extended Community please see Section 7.

5.  Semantics and Usage of the Tunnel Encapsulation attribute

   [RFC5512] specifies the use of the Tunnel Encapsulation attribute in
   BGP UPDATE messages of AFI/SAFI 1/7 and 2/7.  That document restricts
   the use of this attribute to UPDATE messsages of those SAFIs.  This
   document removes that restriction.

   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.

   It has been suggested that it may sometimes be useful to attach a
   Tunnel Encapsulation attribute to a BGP UPDATE message that is also
   carrying a PMSI (Provider Multicast Service Interface) Tunnel
   attribute [RFC6514].  If the PMSI Tunnel attribute specifies an IP
   tunnel, the Tunnel Encapsulation attribute could be used to provide
   additional information about the IP tunnel.  The usage of the Tunnel
   Encapsulation attribute in combination with the PMSI Tunnel attribute
   is outside the scope of this document.

   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.




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   When the Tunnel Encapsulation attribute is carried in an UPDATE of
   one of the AFI/SAFIs specified in the previous paragraph, each TLV
   MUST have a Remote Endpoint sub-TLV.  If a TLV that does not have a
   Remote Endpoint sub-TLV, that TLV should be treated as if it had a
   malformed Remote Endpoint sub-TLV (see Section 3.1).

   Suppose that:

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

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

   o  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 three
      properties:

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

      *  The tunnel is of a type that can be used to carry packet P
         (e.g., an MPLS-in-UDP tunnel would not be a feasible tunnel for
         carrying an IP packet, UNLESS the IP packet can first be
         converted to an MPLS packet).

      *  The tunnel is specified in a TLV whose Remote Endpoint sub-TLV
         identifies an IP address that is reachable.

   Then router R SHOULD 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 (i.e., if
   it specifies several tunnels), router R may choose any one of those
   tunnels, based upon local policy.  If any of tunnels' TLVs contain
   the Color sub-TLV(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.

   Note that if none of the TLVs specifies the MPLS tunnel type, a Label
   Switched Path SHOULD NOT be used unless none of the TLVs specifies a
   feasible tunnel.

   If a particular tunnel is not feasible at some moment because its
   Remote Endpoint cannot be reached at that moment, the tunnel may
   become feasible at a later time (when its endpoint becomes
   reachable).  Router R SHOULD take note of this.  If router R is



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   already using a different tunnel, it MAY switch to the tunnel that
   just became feasible, or it MAY decide to continue using the tunnel
   that it is already using.  How this decision is made is outside the
   scope of this document.

   A TLV specifying a non-feasible tunnel is not considered to be
   malformed or erroneous in any way, and the TLV SHOULD NOT be stripped
   from the Tunnel Encapsulation attribute before redistribution.

   In addition to the sub-TLVs already defined, additional sub-TLVs may
   be defined that affect the choice of tunnel to be used, or that
   affect the contents of the tunnel encapsulation header.  The
   documents that define any such additional sub-TLVs must specify the
   effect that including the sub-TLV is to have.

   Once it is determined to send a packet through the tunnel specified
   in a particular TLV of a particular Tunnel Encapsulation attribute,
   then the tunnel's remote endpoint address is the IP address contained
   in the sub-TLV.  If the TLV contains a Remote Endpoint sub-TLV whose
   value field is all zeroes, then the tunnel's remote endpoint is the
   IP address specified as the Next Hop of the BGP Update containing the
   Tunnel Encapsulation attribute.  The address of the remote 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 remote 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 (e.g., by
   provisioning) of how to fill in the various fields in the
   encapsulation header.

   Whenever a new Tunnel Type TLV is defined, the specification of that
   TLV should describe (or reference) the procedures for creating the
   encapsulation header used to forward packets through that tunnel
   type.  If a tunnel type codepoint is assigned in the IANA "BGP Tunnel
   Encapsulation Tunnel Types" registry, but there is no corresponding
   specification that defines an Encapsulation sub-TLV for that tunnel




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   type, the transmitting endpoint of such a tunnel is presumed to know
   a priori how to form the encapsulation header for that tunnel type.

   If a Tunnel Encapsulation attribute specifies several tunnels, the
   way in which a router chooses which one to use is a matter of policy,
   subject to the following constraint: if a router can determine that a
   given tunnel is not functional, it MUST NOT use that tunnel.  In
   particular, if the tunnel is identified in a TLV that has a Remote
   Endpoint sub-TLV, and if the IP address specified in the sub-TLV is
   not reachable from router R, then the tunnel SHOULD be considered
   non-functional.  Other means of determining whether a given tunnel is
   functional MAY be used; specification of such means is outside the
   scope of this specification.  Of course, if a non-functional tunnel
   later becomes functional, router R SHOULD reevaluate its choice of
   tunnels.

   If router R determines that it cannot use any of the tunnels
   specified in the Tunnel Encapsulation attribute, it MAY either drop
   packet P, or it MAY transmit packet P as it would had the Tunnel
   Encapsulation attribute not been present.  This is a matter of local
   policy.  By default, the packet SHOULD be transmitted as if the
   Tunnel Encapsulation attribute had not been present.

   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 Remote 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 remote 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 is 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.








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6.  Routing Considerations

6.1.  No Impact on BGP Decision Process

   The presence of the Tunnel Encapsulation attribute does not affect
   the BGP bestpath selection algorithm.

   Under certain circumstances, this may lead to counter-intuitive
   consequences.  For example, suppose:

   o  router R1 receives a BGP UPDATE message from router R2, such that

      *  the NLRI of that UPDATE is prefix X,

      *  the UPDATE contains a Tunnel Encapsulation attribute specifying
         two tunnels, T1 and T2,

      *  R1 cannot use tunnel T1 or tunnel T2, either because the tunnel
         remote endpoint is not reachable or because R1 does not support
         that kind of tunnel

   o  router R1 receives a BGP UPDATE message from router R3, such that

      *  the NLRI of that UPDATE is prefix X,

      *  the UPDATE contains a Tunnel Encapsulation attribute specifying
         two tunnels, T3 and T4,

      *  R1 can use at least one of the two tunnels

   Since the Tunnel Encapsulation attribute does not affect bestpath
   selection, R1 may well install the route from R2 rather than the
   route from R3, even though R2's route contains no usable tunnels.

   This possibility must be kept in mind whenever a Remote Endpoint sub-
   TLV carried by a given UPDATE specifies an IP address that is
   different than the next hop of that UPDATE.

6.2.  Looping, Infinite Stacking, Etc.

   Consider a packet destined for address X.  Suppose a BGP UPDATE for
   address prefix X carries a Tunnel Encapsulation attribute that
   specifies a remote tunnel endpoint of Y.  And suppose that a BGP
   UPDATE for address prefix Y carries a Tunnel Encapsulation attribute
   that specifies a Remote Endpoint of X.  It is easy to see that this
   will cause an infinite number of encapsulation headers to be put on
   the given packet.




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   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 of this.  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 remote tunnel endpoint of Y.  Suppose router R
   receives and processes the update.  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
   presumably prevent any given packet from looping infinitely.

   These possibilities must also be kept in mind whenever the Remote
   Endpoint for a given prefix differs from the BGP next hop for that
   prefix.

7.  Recursive Next Hop Resolution

   Suppose that:

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

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

   o  UPDATE U1 does not have a Tunnel Encapsulation attribute;

   o  the next hop of UPDATE U1 is router R2;

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

   o  UPDATE U2 has a Tunnel Encapsulation attribute.

   Then packet P SHOULD be sent through one of the tunnels identified in
   the Tunnel Encapsulation attribute of UPDATE U2.  See Section 5 for
   further details.

   However, suppose that one of the TLVs in U2's Tunnel Encapsulation
   attribute contains the Color Sub-TLV.  In that case, packet P SHOULD
   NOT be sent through the tunnel identified in that TLV, unless U1 is



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   carrying the Color Extended Community that is identified in U2's
   Color Sub-TLV.

   Note that if UPDATE U1 and UPDATE U2 both have Tunnel Encapsulation
   attributes, packet P will be carried through a pair of nested
   tunnels.  P will first be encapsulated based on the Tunnel
   Encapsulation attribute of U1.  This encapsulated packet then becomes
   the payload, and is encapsulated based on the Tunnel Encapsulation
   attribute of U2.  This is another way of "stacking" tunnels (see also
   Section 5.

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

8.  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.

8.1.  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.

8.2.  Tunnel Types with a Virtual Network Identifier Field

   Three 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 and VXLAN-GPE encapsulations,
   this field is called the VNI (Virtual Network Identifier) field; in
   the NVGRE encapsulation, this field is called the VSID (Virtual
   Subnet Identifier) field.





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   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 on 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, VXLAN-GPE,
   or NVGRE.

8.2.1.  Unlabeled Address Families

   This sub-section applies when:

   o  the Tunnel Encapsulation attribute is carried on a BGP UPDATE of
      an unlabeled address family, and

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

   o  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, 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.

8.2.2.  Labeled Address Families

   This sub-section applies when:

   o  the Tunnel Encapsulation attribute is carried on a BGP UPDATE of a
      labeled address family, and

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

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




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8.2.2.1.  When a Valid VNI has been Signaled

   If the TLV identifying the tunnel contains an Encapsulation sub-TLV
   whose V bit is set, the virtual network identifier field of the
   encapsulation header is set as follows:

   o  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 the value of the virtual network
      identifier field of the Encapsulation sub-TLV.

      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.

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

8.2.2.2.  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, the virtual network identifier field of
   the encapsulation header is set as follows:

   o  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.

   o  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 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 lable stack, the embedded label does not appear in that
      stack.




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9.  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
   whose address appears as the next hop.  Certain of the procedures of
   Section 8.2.2.1 or Section 8.2.2.2 cause the embedded label to be
   carried by a data packet to the router whose address appears in the
   Remote Endpoint sub-TLV.  If the Remote Endpoint sub-TLV does not
   identify the same router that is the next hop, sending the packet
   through the tunnel may cause the label to be misinterpreted at the
   tunnel's remote endpoint.  This may cause misdelivery of the packet.

   Therefore the embedded label MUST NOT be carried by a data packet
   traveling through a tunnel unless it is known that the label will be
   properly interpreted at the tunnel's remote endpoint.  How this is
   known is outside the scope of this document.

   Note that if the Tunnel Encapsulation attribute is attached to a VPN-
   IP route [RFC4364], and if Inter-AS "option b" (see section 10 of
   [RFC4364] is being used, and if the Remote Endpoint sub-TLV contains
   an IP address that is not in 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 supported.

10.  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, it is intended that the Tunnel Encapsulation
   attribute be used only within a well-defined scope, e.g., within a
   set of Autonomous Systems 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 redistributed.  This
   filtering SHOULD be possible on a per-BGP-session basis.  For each
   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




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   session, filtering of the attribute on outgoing UPDATEs MUST be
   enabled by default.

11.  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.  A TLV that is
   found to be malformed for this reason MUST NOT be processed, and MUST
   be stripped from the Tunnel Encapsulation attribute before the
   attribute is propagated.  Subsequent TLVs in the Tunnel Encapsulation
   attribute may still be valid, in which case they MUST be processed
   and redistributed normally.

   If a Tunnel Encapsulation attribute does not have any valid TLVs, or
   it does not have the transitive bit set, the "Attribute Discard"
   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, the TLV with the unrecognized tunnel type MUST be
   ignored, and the BGP speaker 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 SHOULD remain in the attribute.

   If a TLV of a Tunnel Encapsulation attribute contains a sub-TLV that
   is not recognized by a particular BGP speaker, the BGP speaker SHOULD
   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 TLV SHOULD remain in
   the attribute.

   If the type code of a sub-TLV appears as "reserved" in the IANA "BGP
   Tunnel Encapsulation Attribute Sub-TLVs" registry, the sub-TLV MUST
   be treated as an unrecognized sub-TLV.

   In general, if a TLV contains a sub-TLV that is malformed (e.g.,
   contains a length field whose value is not legal for that sub-TLV),
   the sub-TLV should be treated as if it were an unrecognized sub-TLV.
   This document specifies one exception to this rule -- if a TLV
   contains a malformed Remote Endpoint sub-TLV (as defined in
   Section 3.1, the entire TLV MUST be ignored, and SHOULD be removed




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   from the Tunnel Encapsulation attribute before the route carrying
   that attribute is redistributed.

   A TLV that does not contain exactly one Remote Endpoint sub-TLV MUST
   be treated as if it contained a malformed Remote 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.  Sub-TLVs of this sort
   SHOULD be treated as no-ops.  That is, they SHOULD 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
   redistributed.  (This allows for the possibility that such sub-TLVs
   may be given a meaning, in the context of the specified tunnel type,
   in the future.)

   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.

   The following sub-TLVs defined in this document SHOULD NOT occur more
   than once in a given Tunnel TLV: Remote Endpoint (discussed above),
   Encapsulation, IPv4 DS, UDP Destination Port, Embedded Label
   Handling, MPLS Label Stack, 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 treated as a no-op.  However, the
   Tunnel TLV containing them MUST NOT be considered to be malformed,
   and all the sub-TLVs SHOULD 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, 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.

12.  IANA Considerations

12.1.  Subsequent Address Family Identifiers

   IANA is requested to modify the "Subsequent Address Family
   Identifiers" registry to indicate that the Encapsulation SAFI is
   deprecated.  This document should be the reference.





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12.2.  BGP Path Attributes

   IANA has assigned value 23 from the "BGP Path Attributes" Registry,
   to "Tunnel Encapsulation Attribute".  IANA is requested to add this
   document as a reference.

12.3.  Extended Communities

   IANA has assigned values from the "Transitive Opaque Extended
   Community" type Registry to the "Color Extended Community" (sub-type
   0x0b), and to the "Encapsulation Extended Community"(0x030c).  IANA
   is requested to add this document as a reference for both
   assignments.

12.4.  BGP Tunnel Encapsulation Attribute Sub-TLVs

   IANA is requested to add the following note to the "BGP Tunnel
   Encapsulation Attribute Sub-TLVs" registry:

      If the Sub-TLV Type is in the range from 1 to 127 inclusive, the
      Sub-TLV Length field contains one octet.  If the Sub-TLV Type is
      in the range from 128-254 inclusive, the Sub-TLV Length field
      contains two octets.

   IANA is requested to change the registration policy of the "BGP
   Tunnel Encapsulation Attribute Sub-TLVs" registry to the following:

   o  The values 0 and 255 are reserved.

   o  The values in the range 1-63 and 128-191 are to be allocated using
      the "Standards Action" registration procedure.

   o  The values in the range 64-125 and 192-252 are to be allocated
      using the "First Come, First Served" registration procedure.

   o  The values in the range 126-127 and 253-254 are reserved for
      experimental use; IANA shall not allocate values from this range.

   IANA is requested to assign a codepoint, from the range 1-63 of the
   "BGP Tunnel Encapsulation Attribute Sub-TLVs" registry, for "Remote
   Endpoint", with this document being the reference.

   IANA is requested to assign a codepoint, from the range 1-63 of the
   "BGP Tunnel Encapsulation Attribute Sub-TLVs" registry, for "IPv4 DS
   Field", with this document being the reference.






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   IANA is requested to assign a codepoint, from the range 1-63 of the
   "BGP Tunnel Encapsulation Attribute Sub-TLVs" registry for "UDP
   Destination Port", with this document being the reference.

   IANA is requested to assign a codepoint, from the range 1-63 of the
   "BGP Tunnel Encapsulation Attribute Sub-TLVs" registry, for "Embedded
   Label Handling", with this document being the reference.

   IANA is requested to assign a codepoint, from the range 1-63 of the
   "BGP Tunnel Encapsulation Attribute Sub-TLVs" registry, for "MPLS
   Label Stack", with this document being the reference.

   IANA is requested to assign a codepoint, from the range 1-63 of the
   "BGP Tunnel Encapsulation Attribute Sub-TLVs" registry, for "Prefix
   SID", with this document being the reference.

   IANA has assigned codepoints from the "BGP Tunnel Encapsulation
   Attribute Sub-TLVs" registry for "Encapsulation", "Protocol Type",
   and "Color".  IANA is requested to add this document as a reference.

12.5.  Tunnel Types

   IANA is requested to add this document as a reference for tunnel
   types 8 (VXLAN), 9 (NVGRE), 11 (MPLS-in-GRE), and 12 (VXLAN-GPE) in
   the "BGP Tunnel Encapsulation Tunnel Types" registry.

   IANA is requested to assign a codepoint from the "BGP Tunnel
   Encapsulation Tunnel Types" registry for "GTP".

   IANA is requested to add this document as a reference for tunnel
   types 1 (L2TPv3), 2 (GRE), and 7 (IP in IP) in the "BGP Tunnel
   Encapsulation Tunnel Types" registry.

13.  Security Considerations

   The Tunnel Encapsulation attribute can cause traffic to be diverted
   from its normal path, especially when the Remote Endpoint sub-TLV is
   used.  This can have serious consequences if the attribute is added
   or modified illegitimately, as it enables traffic to be "hijacked".

   The Remote Endpoint sub-TLV contains both an IP address and an AS
   number.  BGP Origin Validation [RFC6811] can be used to obtain
   assurance that the given IP address belongs to the given AS.  While
   this provides some protection against misconfiguration, it does not
   prevent a malicious agent from inserting a sub-TLV that will appear
   valid.





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   Before sending a packet through the tunnel identified in a particular
   TLV of a Tunnel Encapsulation attribute, it may be advisable to use
   BGP Origin Validation to obtain the following additional assurances:

   o  the origin AS of the route carrying the Tunnel Encapsulation
      attribute is correct;

   o  the origin AS of the route to the IP address specified in the
      Remote Endpoint sub-TLV is correct, and is the same AS that is
      specified in the Remote Endpoint sub-TLV.

   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.  However, this may
   not suit all use cases, and in any event is not very strong
   protection against hijacking.

   For these reasons, BGP Origin Validation should not be relied upon
   exclusively, and the filtering procedures of Section 10 should always
   be in place.

   Increased protection can be obtained by using BGP Path Validation
   [BGPSEC] to ensure that the route carrying the Tunnel Encapsulation
   attribute, and the routes to the Remote Endpoint of each specified
   tunnel, have not been altered illegitimately.

   If BGP Origin Validation is used as specified above, and the tunnel
   specified in a particular TLV of a Tunnel Encapsulation attribute is
   therefore regarded as "suspicious", that tunnel should not be used.
   Other tunnels specified in (other TLVs of) the Tunnel Encapsulation
   attribute may still be used.

14.  Acknowledgments

   This document contains text from RFC5512, co-authored by Pradosh
   Mohapatra.  The authors of the current document wish to thank Pradosh
   for his contribution.  RFC5512 itself built upon prior work by Gargi
   Nalawade, Ruchi Kapoor, Dan Tappan, David Ward, Scott Wainner, Simon
   Barber, and Chris Metz, whom we also thank for their contributions.

   The authors wish to thank Lou Berger, Ron Bonica, Martin Djaernes,
   John Drake, Satoru Matsushima, Dhananjaya Rao, John Scudder, Ravi
   Singh, Thomas Morin, Xiaohu Xu, and Zhaohui Zhang for their review,
   comments, and/or helpful discussions.







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15.  Contributor Addresses

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

   Randy Bush
   Internet Initiative Japan
   5147 Crystal Springs
   Bainbridge Island, Washington  98110
   United States

   Email: randy@psg.com

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

   Email: robert@raszuk.net


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

   [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,
              <http://www.rfc-editor.org/info/rfc5512>.

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

16.2.  Informative References

   [BGPSEC]   Lepinski, M. and S. Turner, "An Overview of BGPsec",
              internet-draft draft-ietf-sidr-bgpsec-overview-08, June
              2016.





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   [Ethertypes]
              "IANA Ethertype Registry",
              <http://www.iana.org/assignments/ieee-802-numbers/
              ieee-802-numbers.xhtml>.

   [EVPN-Inter-Subnet]
              Sajassi, A., Salem, S., Thoria, S., Drake, J., Rabadan,
              J., and L. Yong, "Integrated Routing and Bridging in
              EVPN", internet-draft draft-ietf-bess-evpn-inter-subnet-
              forwarding-03, February 2017.

   [Prefix-SID-Attribute]
              Previdi, S., Filsfils, C., Lindem, A., Patel, K.,
              Sreekantiah, A., Ray, S., and H. Gredler, "Segment Routing
              Prefix SID extensions for BGP", internet-draft draft-ietf-
              idr-bgp-prefix-sid-06, June 2017.

   [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,
              <http://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,
              <http://www.rfc-editor.org/info/rfc2784>.

   [RFC2890]  Dommety, G., "Key and Sequence Number Extensions to GRE",
              RFC 2890, DOI 10.17487/RFC2890, September 2000,
              <http://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,
              <http://www.rfc-editor.org/info/rfc3032>.

   [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,
              <http://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,
              <http://www.rfc-editor.org/info/rfc4023>.





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

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

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

   [RFC6514]  Aggarwal, R., Rosen, E., Morin, T., and Y. Rekhter, "BGP
              Encodings and Procedures for Multicast in MPLS/BGP IP
              VPNs", RFC 6514, DOI 10.17487/RFC6514, February 2012,
              <http://www.rfc-editor.org/info/rfc6514>.

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

   [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,
              <http://www.rfc-editor.org/info/rfc7348>.

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

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

   [VXLAN-GPE]
              Maino, F., Kreeger, L., and U. Elzur, "Generic Protocol
              Extension for VXLAN", internet-draft draft-ietf-nvo3-
              vxlan-gpe, April 2017.







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Authors' Addresses

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

   Email: erosen@juniper.net


   Keyur Patel
   Arrcus

   Email: keyur@arrcus.com


   Gunter Van de Velde
   Nokia
   Copernicuslaan 50
   Antwerpen  2018
   Belgium

   Email: gunter.van_de_velde@nokia.com



























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