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Versions: (draft-vandevelde-idr-remote-next-hop) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16

IDR Working Group                                               K. Patel
Internet-Draft                                               Arrcus, Inc
Obsoletes: 5512, 5566, 5640 (if                          G. Van de Velde
           approved)                                               Nokia
Intended status: Standards Track                               S. Sangli
Expires: January 14, 2021                                     J. Scudder
                                                        Juniper Networks
                                                           July 13, 2020


                 The BGP Tunnel Encapsulation Attribute
                    draft-ietf-idr-tunnel-encaps-16

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 use 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.  Since RFCs 5566 and 5640 rely on
   RFC 5512, they are likewise obsoleted.

Status of This Memo

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

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

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any




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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on January 14, 2021.

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   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.  Use Case for The Tunnel Encapsulation Attribute . . . . .   6
   2.  The Tunnel Encapsulation Attribute  . . . . . . . . . . . . .   7
   3.  Tunnel Encapsulation Attribute Sub-TLVs . . . . . . . . . . .   9
     3.1.  The Tunnel Egress Endpoint Sub-TLV  . . . . . . . . . . .   9
       3.1.1.  Validating the Address Field  . . . . . . . . . . . .  11
     3.2.  Encapsulation Sub-TLVs for Particular Tunnel Types  . . .  12
       3.2.1.  VXLAN . . . . . . . . . . . . . . . . . . . . . . . .  12
       3.2.2.  VXLAN GPE . . . . . . . . . . . . . . . . . . . . . .  14
       3.2.3.  NVGRE . . . . . . . . . . . . . . . . . . . . . . . .  15
       3.2.4.  L2TPv3  . . . . . . . . . . . . . . . . . . . . . . .  16
       3.2.5.  GRE . . . . . . . . . . . . . . . . . . . . . . . . .  17
       3.2.6.  MPLS-in-GRE . . . . . . . . . . . . . . . . . . . . .  17
     3.3.  Outer Encapsulation Sub-TLVs  . . . . . . . . . . . . . .  18
       3.3.1.  DS Field  . . . . . . . . . . . . . . . . . . . . . .  18
       3.3.2.  UDP Destination Port  . . . . . . . . . . . . . . . .  18
     3.4.  Sub-TLVs for Aiding Tunnel Selection  . . . . . . . . . .  19
       3.4.1.  Protocol Type Sub-TLV . . . . . . . . . . . . . . . .  19
       3.4.2.  Color Sub-TLV . . . . . . . . . . . . . . . . . . . .  19
     3.5.  Embedded Label Handling Sub-TLV . . . . . . . . . . . . .  20
     3.6.  MPLS Label Stack Sub-TLV  . . . . . . . . . . . . . . . .  21
     3.7.  Prefix-SID Sub-TLV  . . . . . . . . . . . . . . . . . . .  22
   4.  Extended Communities Related to the Tunnel Encapsulation



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       Attribute . . . . . . . . . . . . . . . . . . . . . . . . . .  23
     4.1.  Encapsulation Extended Community  . . . . . . . . . . . .  23
     4.2.  Router's MAC Extended Community . . . . . . . . . . . . .  25
     4.3.  Color Extended Community  . . . . . . . . . . . . . . . .  25
   5.  Special Considerations for  IP-in-IP Tunnels  . . . . . . . .  25
   6.  Semantics and Usage of the Tunnel Encapsulation attribute . .  26
   7.  Routing Considerations  . . . . . . . . . . . . . . . . . . .  28
     7.1.  Impact on the BGP Decision Process  . . . . . . . . . . .  28
     7.2.  Looping, Infinite Stacking, Etc.  . . . . . . . . . . . .  29
   8.  Recursive Next Hop Resolution . . . . . . . . . . . . . . . .  29
   9.  Use of Virtual Network Identifiers and Embedded Labels when
       Imposing a Tunnel Encapsulation . . . . . . . . . . . . . . .  30
     9.1.  Tunnel Types without a Virtual Network Identifier Field .  30
     9.2.  Tunnel Types with a Virtual Network Identifier Field  . .  31
       9.2.1.  Unlabeled Address Families  . . . . . . . . . . . . .  31
       9.2.2.  Labeled Address Families  . . . . . . . . . . . . . .  32
   10. Applicability Restrictions  . . . . . . . . . . . . . . . . .  33
   11. Scoping . . . . . . . . . . . . . . . . . . . . . . . . . . .  33
   12. Validation and Error Handling . . . . . . . . . . . . . . . .  34
   13. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  35
     13.1.  Subsequent Address Family Identifiers  . . . . . . . . .  36
     13.2.  BGP Tunnel Encapsulation Attribute Sub-TLVs  . . . . . .  36
     13.3.  Flags Field of VXLAN Encapsulation sub-TLV . . . . . . .  36
     13.4.  Flags Field of VXLAN GPE Encapsulation sub-TLV . . . . .  37
     13.5.  Flags Field of NVGRE Encapsulation sub-TLV . . . . . . .  37
     13.6.  Embedded Label Handling sub-TLV  . . . . . . . . . . . .  37
     13.7.  Extended Color Community . . . . . . . . . . . . . . . .  37
   14. Security Considerations . . . . . . . . . . . . . . . . . . .  37
   15. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  38
   16. Contributor Addresses . . . . . . . . . . . . . . . . . . . .  38
   17. References  . . . . . . . . . . . . . . . . . . . . . . . . .  39
     17.1.  Normative References . . . . . . . . . . . . . . . . . .  39
     17.2.  Informative References . . . . . . . . . . . . . . . . .  41
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  42

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.  Since [RFC5566] and [RFC5640] rely on RFC 5512,
   they are likewise obsoleted.

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



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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 selected UPDATE U for local use;

   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 route to D is a BGP route that has R2 as its next hop;

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

1.2.  Deficiencies in RFC 5512

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

   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 tunnel egress endpoint address of a given tunnel;
      [RFC5512] assumes that the tunnel egress endpoint of each tunnel
      is specified as the NLRI of an UPDATE of the Encapsulation SAFI.

   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

   This document addresses these deficiencies by:

   o  Deprecating the Encapsulation SAFI.

   o  Defining a new "Tunnel Egress Endpoint 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 0 to 127 inclusive has a one-octet length field,
      but a sub-TLV whose type is in the range from 128 to 255 inclusive
      has a two-octet length field.

   One of the sub-TLVs defined in [RFC5512] is the "Encapsulation sub-
   TLV".  For a given tunnel, the encapsulation sub-TLV specifies some
   of the information needed to construct the encapsulation header used
   when sending packets through that tunnel.  This document defines
   encapsulation sub-TLVs for a number of tunnel types not discussed in
   [RFC5512]: VXLAN (Virtual Extensible Local Area Network, [RFC7348]),



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   VXLAN GPE (Generic Protocol Extension for VXLAN,
   [I-D.ietf-nvo3-vxlan-gpe]), NVGRE (Network Virtualization Using
   Generic Routing Encapsulation [RFC7637]), and MPLS-in-GRE (MPLS in
   Generic Routing Encapsulation [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.  Use Case for The Tunnel Encapsulation Attribute

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

   In cases such as this, in order for R1 to properly forward packet P,
   it must encapsulate P and send P "through a tunnel" to R2.  For
   example, R1 may encapsulate P using GRE, L2TPv3, IP in IP, etc.,



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   where the destination IP address of the encapsulation header is the
   address of R2.

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

   This document specifies a way in which BGP itself can be used by a
   given BGP speaker to tell other BGP speakers, "if you need to
   encapsulate packets to be sent to me, here's the information you need
   to properly form the encapsulation header".  A BGP speaker signals
   this information to other BGP speakers by using a new BGP attribute
   type value, the BGP Tunnel Encapsulation Attribute.  The Tunnel
   Encapsulation attribute MAY be used 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)).

   In a given BGP update, the encapsulation information is specified in
   the BGP Tunnel Encapsulation Attribute.  This attribute specifies the
   encapsulation protocols that may be used as well as whatever
   additional information (if any) is needed in order to properly use
   those protocols.  Other attributes, e.g., communities or extended
   communities, may also be included.

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 Tunnel Encapsulation TLV, also known as
   Tunnel 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 explicitly include a Protocol Type sub-TLV
      specifying "X".  Also, for "X-in-Y" type tunnels, a Protocol Type
      sub-TLV specifying anything other than "X" MUST be ignored; this
      is discussed further in Section 12.

   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: Encapsulation Sub-TLV Value Field

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



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      field contains values from the IANA Registry "BGP Tunnel
      Encapsulation Attribute Sub-TLVs".

   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 0-127.
      The Sub-TLV Length field contains two octets if the Sub-TLV Type
      field contains a value in the range from 128-255.

   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

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

3.1.  The Tunnel Egress Endpoint Sub-TLV

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

   1.  a reserved subfield

   2.  a two-octet Address Family subfield

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

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                            Reserved                           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |      Address Family           |           Address             ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
     ~                                                               ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

           Figure 3: Tunnel Egress Endpoint Sub-TLV Value Field

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




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

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

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

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

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

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

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

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

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

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

      *  0, if the Address Family subfield contains the value zero.






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   o  The IP address in the sub-TLV's address subfield is listed in the
      relevant Special-Purpose IP Address Registry [RFC6890] as either
      not a valid destination, or not forwardable.

   o  It can be determined according to the procedures below
      (Section 3.1.1) that the IP address in the sub-TLV's address
      subfield does not belong to the Autonomous System (AS) that
      originated the route that contains the attribute.

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

   Error Handling is detailed in Section 11.

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

3.1.1.  Validating the Address Field

   This section details a procedure that MAY be applied to validate that
   when traffic is sent to the IP address depicted in the Address Field,
   it will go to the same AS as it would go to if the Tunnel
   Encapsulation Attribute were not present.  See Section 13 for
   discussion of the limitations of this procedure.

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

   In order to determine the Route Origin ASN of the address depicted in
   the Address Field of the Tunnel Egress Endpoint sub-TLV, it is
   necessary to determine the forwarding route, that is, the route
   installed in the Forwarding Information Base that will be used to
   forward traffic toward that address.  The Address Field's Route
   Origin ASN is the Route Origin ASN of that route, or the
   distinguished value "NONE2" if the forwarding route has no AS Path,
   for example if that route's source is a protocol other than BGP.
   (Note that this is a distinct case from a route that has an empty AS
   Path.)  Call this value Egress_AS.

   If Route_AS does not equal Egress_AS, then the Tunnel Egress Endpoint
   sub-TLV is considered not to be valid.  In some cases a network
   operator who controls a set of Autonomous Systems might wish to allow



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   a Tunnel Egress Endpoint to reside in an AS other than Route_AS;
   configuration MAY allow for such a case, in which case the check
   becomes, if Egress_AS is not within the configured set of permitted
   AS numbers, then the Tunnel Egress Endpoint sub-TLV is considered not
   to be valid.

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

3.2.  Encapsulation Sub-TLVs for Particular Tunnel Types

   This section defines Encapsulation sub-TLVs for the following tunnel
   types: VXLAN ([RFC7348]), VXLAN GPE ([I-D.ietf-nvo3-vxlan-gpe]),
   NVGRE ([RFC7637]), MPLS-in-GRE ([RFC4023]), L2TPv3 ([RFC3931]), and
   GRE ([RFC2784]).

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

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

3.2.1.  VXLAN

   This document defines an Encapsulation sub-TLV for VXLAN tunnels.
   When the Tunnel Type is VXLAN (value 8), the length of the sub-TLV is
   12 octets.  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




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      V: This bit is set to 1 to indicate that a VN-ID (Virtual Network
      Identifier) is present in the Encapsulation sub-TLV.  If set to 0,
      the VN-ID field is disregarded.  Please see Section 8.

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

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

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

      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 MUST be
      set to all zeroes on transmission and disregarded on receipt.

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

   When forming the VXLAN encapsulation header:

   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]).

      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  Section 8 describes how the VNI field of the VXLAN encapsulation
      header is set.




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   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 GPE tunnels.
   When the Tunnel Type is VXLAN GPE (value 12), the length of the sub-
   TLV is 8 octets and 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

      Version (Ver): Indicates VXLAN GPE protocol version.  (See the
      "Version Bits" section of [I-D.ietf-nvo3-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.

      V: This bit is set to 1 to indicate that a VN-ID is present in the
      Encapsulation sub-TLV.  If set to 0, the VN-ID field is
      disregarded.  Please see Section 8.

      R: The bits designated "R" above are reserved for future use.
      They MUST always be set to 0 by the originator of the sub-TLV.
      Intermediate routers MUST propagate them without modification.
      Any receiving routers MUST ignore these bits upon a receipt.

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

      Reserved (two fields): MUST be set to zero on transmission and
      disregarded on receipt.

   When forming the VXLAN GPE encapsulation header:




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   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 [I-D.ietf-nvo3-vxlan-gpe].

   o  Section 8 describes 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 (value 9), the length of the sub-TLV is
   12 octets.  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 VN-ID is present in the
      Encapsulation sub-TLV.  If set to 0, the VN-ID field is
      disregarded.  Please see Section 8.

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

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

      VN-ID: If the V bit is set, the VN-ID field contains a 3 octet VN-
      ID value.  If the V bit is not set, the VN-ID field MUST be set to
      zero on transmission and disregarded on receipt.

      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 MUST be
      set to all zeroes on transmission and disregarded on receipt.



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      Reserved (two fields): MUST be set to zero on transmission and
      disregarded on receipt.

   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  Section 8 describes 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 (value 1), the
   length of the sub-TLV is 8 octets.  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 for the incoming
      packet's context.




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

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

3.2.5.  GRE

   When the Tunnel Type of the TLV is GRE (value 2), the length of the
   sub-TLV is 4 octets.  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.  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 (value 11), the length of the
   sub-TLV is 4 octets.  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 9: MPLS-in-GRE Encapsulation Sub-TLV

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





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   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.

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

3.3.1.  DS Field

   Most of the tunnel types that can be specified in the Tunnel
   Encapsulation attribute require an outer IP encapsulation.  The
   Differentiated Services (DS) Field sub-TLV, whose type code is 7, can
   be carried in the TLV of any such Tunnel Type.  It specifies the
   setting of the one-octet Differentiated Services field in the outer
   IPv4 or IPv6 encapsulation (see [RFC2474]).  The value field is
   always a single octet.

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, whose




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   type code is 8.  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, whose type code is 2, MAY be included in a
   given TLV to indicate the type of the payload packets that are
   allowed to be encapsulated with the tunnel parameters that are being
   signaled in the TLV.  Packets with other payload types MUST NOT be
   encapsulated in the relevant tunnel.  The value field of the sub-TLV
   contains a 2-octet value from IANA's "ETHER TYPES" registry
   [Ethertypes].

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

   Note that it is unnecessary to explicitly include this sub-TLV in
   tunnels whose names are of the form "X-in-Y", as discussed in
   Section 2.

3.4.2.  Color Sub-TLV

   The Color sub-TLV, whose type code is 4, MAY be used as a way to
   "color" the corresponding Tunnel TLV.  The value field of the sub-TLV
   is eight octets long, and consists of a Color Extended Community, as
   defined in Section 4.3.  For the use of this sub-TLV and Extended
   Community, please see Section 7.

   If the Length field of a Color sub-TLV has a value other than 8, or
   the first two octets of its value field are not 0x030b, the sub-TLV
   should be treated as if it were an unrecognized sub-TLV (see
   Section 11).








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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.  The term "embedded label" is used to refer to the MPLS
   label that is embedded in an NLRI, and the term "labeled address
   family" to refer to any AFI/SAFI that has embedded labels.

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

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

   The Embedded Label Handling sub-TLV, whose type code is 9, may be
   included in any TLV of the Tunnel Encapsulation attribute.  If the
   Tunnel Encapsulation attribute is attached to an UPDATE of a non-
   labeled address family, then the sub-TLV MUST be disregarded.  If the
   sub-TLV is contained in a TLV whose Tunnel Type does not have a
   virtual network identifier in its encapsulation header, the sub-TLV
   MUST be disregared.  In those cases where the sub-TLV is ignored, it
   SHOULD NOT be stripped from the TLV before the route is propagated.

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

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

   2: The embedded label is not carried in the payload, but is 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.






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3.6.  MPLS Label Stack Sub-TLV

   This sub-TLV, whose type code is 10, allows an MPLS label stack
   ([RFC3032]) to be associated with a particular tunnel.

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

   Each label stack entry has the 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

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

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

   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 procedures 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 being pushed MUST be cleared.
   When the label stack entries are pushed onto a packet that does not
   already have a label stack, the S bit of the bottommost label stack
   entry MUST be set, and the S bit of all the other label stack entries
   MUST be cleared.



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

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

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

3.7.  Prefix-SID Sub-TLV

   [RFC8669] 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, or an
   "Originator SRGB (Source Routing Global Block)" TLV.

   This document defines a Prefix-SID sub-TLV, whose type code is 11.
   The value field of the Prefix-SID sub-TLV can be set to any permitted
   value of the value field of a BGP Prefix-SID attribute [RFC8669].

   [RFC8669] only defines behavior when the Prefix-SID Attribute is
   attached to routes of type IPv4/IPv6 Labeled Unicast ([RFC4760],
   [RFC8277]), and it only defines values of the Prefix-SID Attribute
   when attached to routes of those types.  Therefore, similar
   limitations exist for the Prefix-SID sub-TLV: although it MAY be
   encoded in any BGP UPDATE message where the Tunnel Encapsulation
   attribute is allowed (see Section 5), the encoded information MUST be
   ignored just as the base specification that defines the encoding
   requires.  So, in the case of the values specified in [RFC8669], they
   MUST be ignored if received with routes of type other than IPv4/IPv6
   Labeled Unicast.

   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



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   MUST be interpreted to be the SRGB used by the tunnel's egress
   endpoint.  The Label-Index, if present, is the Segment Routing SID
   that the tunnel's egress endpoint uses to represent the prefix
   appearing in the NLRI field of the BGP UPDATE to which the Tunnel
   Encapsulation attribute is attached.

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

   The corresponding MPLS label is pushed on after the processing of the
   MPLS Label Stack sub-TLV, if present, as specified in Section 3.6.
   It is pushed on before any other labels (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 what the prefix-SID is
   for the advertised prefix in that Segment Routing domain.  When the
   Prefix-SID sub-TLV is used, the receiving BGP speaker need not even
   be in the same Segment Routing Domain as the tunnel's egress
   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.

   The Encapsulation Extended Community encoding is as shown below










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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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |       0x03    |     0x0c      |           Reserved            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |          Reserved             |        Tunnel Type            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 11: Encapsulation Extended Community

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

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

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

   1.  it identifies the same Tunnel Type,

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

       A.  its "Address Family" subfield contains zero, or

       B.  its "Address" subfield contains the address of the next hop
           field of the route to which the Tunnel Encapsulation
           attribute is attached

   3.  it has no other sub-TLVs.

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

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

   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



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   type "Y".  Packets with other payload types MUST NOT be carried
   through such tunnels.  See also Section 2.

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

4.2.  Router's MAC Extended Community

   [I-D.ietf-bess-evpn-inter-subnet-forwarding] 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 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 12: Color Extended Community

   The value of the high-order octet of the extended type field is 0x03,
   which indicates it is transitive.  The value of the low-order octet
   of the extended type field for this community is 0x0b.  The color
   value is user defined and configured locally.  The two octet Reserved
   field MUST be set to zero by the sender and ignored by the receiver.
   The Color Value field is encoded as 4 octet value by the
   administrator and is outside the scope of this document.  For the use
   of this Extended Community please see Section 7.

5.  Special Considerations for IP-in-IP Tunnels

   In certain situations with an IP fabric underlay, one could have a
   tunnel overlay with the tunnel type IP-in-IP.  The egress BGP speaker
   can advertise the IP-in-IP tunnel endpoint address in the Tunnel
   Egress Endpoint sub-TLV.  When the Tunnel type of the TLV is IP-in-



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   IP, it will not have a Virtual Network Identifier.  However, the
   tunnel egress endpoint address can be used in identifying the
   forwarding table to use for making the forwarding decisions to
   forward the payload.  See the second bullet point of Section 9.1 for
   further discussion.

6.  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 messages 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.

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

   Suppose that:

   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



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         carrying an IP packet, UNLESS the IP packet can first be
         encapsulated in a MPLS packet).

      *  The tunnel is specified in a TLV whose Tunnel Egress Endpoint
         sub-TLV identifies an IP address that is reachable.  This IP
         address may be reachable via one or more forwarding tables.
         Local policy may determine these forwarding tables and is
         outside the scope of this document.  The reachability condition
         is evaluated as per [RFC4271].

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

   If the Tunnel Encapsulation attribute contains several TLVs (i.e., if
   it specifies several feasibile tunnels), router R may choose any one
   of those tunnels, based upon local policy.  If any Tunnel TLV
   contains one or more Color sub-TLVs (Section 3.4.2) and/or the
   Protocol Type sub-TLV (Section 3.4.1), the choice of tunnel may be
   influenced by these sub-TLVs.

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

   Once it is determined to send a packet through the tunnel specified
   in a particular Tunnel TLV of a particular Tunnel Encapsulation
   attribute, then the tunnel's egress endpoint address is the IP
   address contained in the sub-TLV.  If the Tunnel TLV contains a
   Tunnel Egress Endpoint sub-TLV whose value field is all zeroes, then
   the tunnel's egress endpoint is the address of the Next Hop of the
   BGP Update containing the Tunnel Encapsulation attribute.  The
   address of the tunnel egress endpoint generally appears in a
   "destination address" field of the encapsulation.

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

   Sending a packet through a tunnel always requires that the packet be
   encapsulated, with an encapsulation header that is appropriate for
   the Tunnel Type.  The contents of the tunnel encapsulation header MAY



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   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 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.  The Tunnel Type codepoint will be assigned in the IANA "BGP
   Tunnel Encapsulation Tunnel Types" registry.

   If a Tunnel Encapsulation attribute specifies several tunnels, the
   way in which a router chooses which one to use is a matter of policy,
   In addition to the reachability to the address of the egress endpoint
   of the tunnel, other policy factors MAY be used to determine the
   feasibility of the tunnel.  The policy factors are beyond the scope
   of this document.

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

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

7.  Routing Considerations

7.1.  Impact on the BGP Decision Process

   The presence of the Tunnel Encapsulation attribute affects the BGP
   best route selection algorithm.  If a route includes the Tunnel
   Encapsulation attribute, and if that attribute includes no tunnel



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   which is feasible, then that route MUST NOT be considered resolvable
   for the purposes of Route Resolvability Condition [RFC4271] section
   9.1.2.1.

7.2.  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 tunnel egress endpoint of Y, and suppose that a BGP
   UPDATE for address prefix Y carries a Tunnel Encapsulation attribute
   that specifies a tunnel egress endpoint of X.  It is easy to see that
   this will cause an infinite number of encapsulation headers to be put
   on the given packet.  [RFC4271] describes an analogous case as
   mutually recursive routes.

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

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

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

8.  Recursive Next Hop Resolution

   Suppose that:

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




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

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

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

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

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

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

9.1.  Tunnel Types without a Virtual Network Identifier Field

   If a Tunnel Encapsulation attribute is attached to an UPDATE of a
   labeled address family, there will be one or more labels specified in
   the UPDATE's NLRI.

   o  If the TLV contains an Embedded Label Handling sub-TLV whose value
      is 1, the label or labels from the NLRI are pushed on the packet's
      label stack.



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   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 ignored completely.  In this case the
      tunnel encapsulation is presumed to provide complete information
      regarding the forwarding context required.

   The resulting MPLS packet is then further encapsulated, as specified
   by the TLV.

9.2.  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.

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

   This section specifies the procedures for choosing the value to set
   in the virtual network identifier field of the encapsulation header.
   These procedures apply only when the Tunnel Type is VXLAN, VXLAN GPE,
   or NVGRE.

9.2.1.  Unlabeled Address Families

   This sub-section applies when:

   o  the Tunnel Encapsulation attribute is carried in 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.



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   Otherwise, the virtual network identifier field of the encapsulation
   header is set to a configured value; if there is no configured value,
   the tunnel cannot be used.

9.2.2.  Labeled Address Families

   This sub-section applies when:

   o  the Tunnel Encapsulation attribute is carried in 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.

9.2.2.1.  When a Valid VNI has been Signaled

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

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

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

9.2.2.2.  When a Valid VNI has not been Signaled

   If the TLV identifying the tunnel does not contain an Encapsulation
   sub-TLV whose V bit is set, 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.





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      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 lower 3 octets of the
      virtual network identifier field of the encapsulation header.

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

10.  Applicability Restrictions

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

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

11.  Scoping

   The Tunnel Encapsulation attribute is defined as a transitive
   attribute, so that it may be passed along by BGP speakers that do not
   recognize it.  However, 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.





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

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

12.  Validation and Error Handling

   The Tunnel Encapsulation attribute is a sequence of TLVs, each of
   which is a sequence of sub-TLVs.  The final octet of a TLV is
   determined by its length field.  Similarly, the final octet of a sub-
   TLV is determined by its length field.  The final octet of a TLV MUST
   also be the final octet of its final sub-TLV.  If this is not the
   case, the TLV MUST be considered to be malformed.  A TLV that is
   found to be malformed for this reason MUST NOT be processed, and MUST
   be stripped from the Tunnel Encapsulation attribute.  In this case,
   the "Treat-as-withdraw" procedure of [RFC7606] is applied.

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

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

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




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   sub-TLVs MUST be propagated if the route carrying the Tunnel
   Encapsulation attribute is propagated.

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

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

   In general, if a TLV contains a sub-TLV that is malformed (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 Tunnel Egress Endpoint sub-TLV (as defined in
   Section 3.1), the entire TLV MUST be ignored, and MUST be removed
   from the Tunnel Encapsulation attribute before the route carrying
   that attribute is distributed.

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

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

13.  IANA Considerations







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13.1.  Subsequent Address Family Identifiers

   IANA is requested to modify the "Subsequent Address Family
   Identifiers" registry to indicate that the Encapsulation SAFI (value
   7) is obsoleted.  This document should be the reference.

   Because this document obsoletes RFC 5512, IANA is asked to change all
   registration information that references [RFC5512] to instead
   reference this document.

13.2.  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 0 to 127 inclusive, the
      Sub-TLV Length field contains one octet.  If the Sub-TLV Type is
      in the range from 128-255 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 has assigned the following codepoints in the "BGP Tunnel
   Encapsulation Attribute Sub-TLVs registry":

      Value 6: Remote Endpoint (note: IANA please rename to "Tunnel
      Egress Endpoint")

      Value 7: IPv4 DS Field (note: IANA please rename to "DS Field")

13.3.  Flags Field of VXLAN Encapsulation sub-TLV

   IANA is requested to add this document as a reference for creating
   the flags field of the VXLAN Encapsulation sub-TLV registry.





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   IANA is requested to add this document as a reference for flag bits V
   and M in the "Flags field of VXLAN Encapsulation sub-TLV" registry.

13.4.  Flags Field of VXLAN GPE Encapsulation sub-TLV

   IANA is requested to add this document as a reference for creating
   the flags field of the VXLAN GPE Encapsulation sub-TLV registry.

   IANA is requested to add this document as a reference for flag bit V
   in the "Flags field of VXLAN GPE Encapsulation sub-TLV" registry.

13.5.  Flags Field of NVGRE Encapsulation sub-TLV

   IANA is requested to add this document as a reference for creating
   the flags field of the NVGRE Encapsulation sub-TLV registry.

   IANA is requested to add this document as a reference for flag bits V
   and M in the "Flags field of NVGRE Encapsulation sub-TLV" registry.

13.6.  Embedded Label Handling sub-TLV

   IANA is requested to add this document as a reference for creating
   the sub-TLV's value field of the Embedded Label Handling sub-TLV
   registry.

   IANA is requested to add this document as a reference for value of 1
   (Payload of MPLS with embedded label) and 2 (no embedded label in
   payload) in the "sub-TLV's value field of the Embedded Label Handling
   sub-TLV" registry.

13.7.  Extended Color Community

   IANA is requested to add this document as a reference for the "Color
   Extended Community" entry in the Transitive Opaque Extended Community
   Sub-Types registry.

14.  Security Considerations

   As Section 11 discusses, 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.  As long as the filtering mechanisms discussed in that
   section are applied diligently, an attacker outside the scope would
   not be able to use the Tunnel Encapsulation attribute in an attack.
   This leaves open the questions of attackers within the scope (for
   example, a compromised router) and failures in filtering that allow
   an external attack to succeed.




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

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

15.  Acknowledgments

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

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

16.  Contributor Addresses

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






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   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

   Eric C. Rosen


17.  References

17.1.  Normative References

   [I-D.ietf-nvo3-vxlan-gpe]
              Maino, F., Kreeger, L., and U. Elzur, "Generic Protocol
              Extension for VXLAN", draft-ietf-nvo3-vxlan-gpe-09 (work
              in progress), December 2019.

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

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

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

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




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

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

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

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

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

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

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

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

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







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

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

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

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

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

17.2.  Informative References

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

   [I-D.ietf-bess-evpn-inter-subnet-forwarding]
              Sajassi, A., Salam, S., Thoria, S., Drake, J., and J.
              Rabadan, "Integrated Routing and Bridging in EVPN", draft-
              ietf-bess-evpn-inter-subnet-forwarding-09 (work in
              progress), June 2020.

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

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




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

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

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

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

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

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

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

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

Authors' Addresses

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

   Email: keyur@arrcus.com





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   Gunter Van de Velde
   Nokia
   Copernicuslaan 50
   Antwerpen 2018
   Belgium

   Email: gunter.van_de_velde@nokia.com


   Srihari R. Sangli
   Juniper Networks

   Email: ssangli@juniper.net


   John Scudder
   Juniper Networks

   Email: jgs@juniper.net
































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