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RFC 5512 - The BGP Encapsulation Subsequent Address Family Ident

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Network Working Group                                       P. Mohapatra
Request for Comments: 5512                                      E. Rosen
Category: Standards Track                            Cisco Systems, Inc.
                                                              April 2009

   The BGP Encapsulation Subsequent Address Family Identifier (SAFI)
               and the BGP Tunnel Encapsulation Attribute

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (c) 2009 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 in effect on the date of
   publication of this document (http://trustee.ietf.org/license-info).
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.


   In certain situations, transporting a packet from one Border Gateway
   Protocol (BGP) speaker to another (the BGP next hop) requires that
   the packet be encapsulated by the first BGP speaker and decapsulated
   by the second.  To support these situations, there needs to be some
   agreement between the two BGP speakers with regard to the
   "encapsulation information", i.e., the format of the encapsulation
   header as well as the contents of various fields of the header.

   The encapsulation information need not be signaled for all
   encapsulation types.  In cases where signaling is required (such as
   Layer Two Tunneling Protocol - Version 3 (L2TPv3) or Generic Routing
   Encapsulation (GRE) with key), this document specifies a method by
   which BGP speakers can signal encapsulation information to each
   other.  The signaling is done by sending BGP updates using the
   Encapsulation Subsequent Address Family Identifier (SAFI) and the
   IPv4 or IPv6 Address Family Identifier (AFI).  In cases where no
   encapsulation information needs to be signaled (such as GRE without

   key), this document specifies a BGP extended community that can be
   attached to BGP UPDATE messages that carry payload prefixes in order
   to indicate the encapsulation protocol type to be used.

Table of Contents

   1. Introduction ....................................................2
   2. Specification of Requirements ...................................4
   3. Encapsulation NLRI Format .......................................4
   4. Tunnel Encapsulation Attribute ..................................5
      4.1. Encapsulation sub-TLV ......................................6
      4.2. Protocol Type Sub-TLV ......................................7
      4.3. Color Sub-TLV ..............................................8
           4.3.1. Color Extended Community ............................8
      4.4. Tunnel Type Selection ......................................8
      4.5. BGP Encapsulation Extended Community .......................9
   5. Capability Advertisement .......................................10
   6. Error Handling .................................................10
   7. Security Considerations ........................................10
   8. IANA Considerations ............................................10
   9. Acknowledgements ...............................................11
   10. References ....................................................12
      10.1. Normative References .....................................12
      10.2. Informative References ...................................12

1.  Introduction

   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 [MESH], 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.,
   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.

   In this document, we specify 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 distinguished SAFI
   value, the Encapsulation SAFI.  The Encapsulation SAFI can be used
   with the AFI for IPv4 or with the AFI for IPv6.  The IPv4 AFI is used
   when the encapsulated packets are to be sent using IPv4; the IPv6 AFI
   is used when the encapsulated packets are to be sent using IPv6.

   In a given BGP update, the Network Layer Reachability Information
   (NLRI) of the Encapsulation SAFI consists of the IP address (in the
   family specified by the AFI) of the originator of that update.  The
   encapsulation information is specified in the BGP "tunnel
   encapsulation attribute" (specified herein).  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.

   Since the encapsulation information is coded as an attribute, one
   could ask whether a new SAFI is really required.  After all, a BGP
   speaker could simply attach the tunnel encapsulation attribute to
   each prefix (like Q in our example) that it advertises.  But with
   that technique, any change in the encapsulation information would
   cause a very large number of updates.  Unless one really wants to
   specify different encapsulation information for each prefix, it is
   much better to have a mechanism in which a change in the
   encapsulation information causes a BGP speaker to advertise only a
   single update.  Conversely, when prefixes get modified, the tunnel
   encapsulation information need not be exchanged.

   In this specification, a single SAFI is used to carry information for
   all encapsulation protocols.  One could have taken an alternative
   approach of defining a new SAFI for each encapsulation protocol.
   However, with the specified approach, encapsulation information can
   pass transparently and automatically through intermediate BGP
   speakers (e.g., route reflectors) that do not necessarily understand
   the encapsulation information.  This works because the encapsulation
   attribute is defined as an optional transitive attribute.  New
   encapsulations can thus be added without the need to reconfigure any

   intermediate BGP system.  If adding a new encapsulation required
   using a new SAFI, the information for that encapsulation would not
   pass through intermediate BGP systems unless those systems were
   reconfigured to support the new SAFI.

   For encapsulation protocols where no encapsulation information needs
   to be signaled (such as GRE without key), the egress router MAY still
   want to specify the protocol to use for transporting packets from the
   ingress router.  This document specifies a new BGP extended community
   that can be attached to UPDATE messages that carry payload prefixes
   for this purpose.

2.  Specification of Requirements

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

3.  Encapsulation NLRI Format

   The NLRI, defined below, is carried in BGP UPDATE messages [RFC4271]
   using BGP multiprotocol extensions [RFC4760] with an AFI of 1 or 2
   (IPv4 or IPv6) [IANA-AF] and a SAFI value of 7 (called an
   Encapsulation SAFI).

   The NLRI is encoded in a format defined in Section 5 of [RFC4760] (a
   2-tuple of the form <length, value>).  The value field is structured
   as follows:

            |       Endpoint Address (Variable)             |

   - Endpoint Address: This field identifies the BGP speaker originating
     the update.  It is typically one of the interface addresses
     configured at the router.  The length of the endpoint address is
     dependent on the AFI being advertised.  If the AFI is set to IPv4
     (1), then the endpoint address is a 4-octet IPv4 address, whereas
     if the AFI is set to IPv6 (2), the endpoint address is a 16-octet
     IPv6 address.

   An update message that carries the MP_REACH_NLRI or MP_UNREACH_NLRI
   attribute with the Encapsulation SAFI MUST also carry the BGP
   mandatory attributes:  ORIGIN, AS_PATH, and LOCAL_PREF (for IBGP
   neighbors), as defined in [RFC4271].  In addition, such an update
   message can also contain any of the BGP optional attributes, like the
   Community or Extended Community attribute, to influence an action on
   the receiving speaker.

   When a BGP speaker advertises the Encapsulation NLRI via BGP, it uses
   its own address as the BGP nexthop in the MP_REACH_NLRI or
   MP_UNREACH_NLRI attribute.  The nexthop address is set based on the
   AFI in the attribute.  For example, if the AFI is set to IPv4 (1),
   the nexthop is encoded as a 4-byte IPv4 address.  If the AFI is set
   to IPv6 (2), the nexthop is encoded as a 16-byte IPv6 address of the
   router.  On the receiving router, the BGP nexthop of such an update
   message is validated by performing a recursive route lookup operation
   in the routing table.

   Bestpath selection of Encapsulation NLRIs is governed by the decision
   process outlined in Section 9.1 of [RFC4271].  The encapsulation data
   carried through other attributes in the message are to be used by the
   receiving router only if the NLRI has a bestpath.

4.  Tunnel Encapsulation Attribute

   The Tunnel Encapsulation attribute is an optional transitive
   attribute that is composed of a set of Type-Length-Value (TLV)
   encodings.  The type code of the attribute is 23.  Each TLV contains
   information corresponding to a particular tunnel technology.  The TLV
   is structured as follows:

       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                             |
      |                                                               |

   * Tunnel Type (2 octets): identifies the type of tunneling technology
     being signaled.  This document defines the following types:

     - L2TPv3 over IP [RFC3931]: Tunnel Type = 1
     - GRE [RFC2784]: Tunnel Type = 2
     - IP in IP [RFC2003] [RFC4213]: Tunnel Type = 7

     Unknown types are to be ignored and skipped upon receipt.

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

   * Value (variable): comprised of multiple sub-TLVs.  Each sub-TLV
     consists of three fields: a 1-octet type, 1-octet length, and zero
     or more octets of value.  The sub-TLV is structured as follows:

                     |      Sub-TLV Type (1 Octet)       |
                     |     Sub-TLV Length (1 Octet)      |
                     |     Sub-TLV Value (Variable)      |
                     |                                   |

   * Sub-TLV Type (1 octet): each sub-TLV type defines a certain
     property about the tunnel TLV that contains this sub-TLV.  The
     following are the types defined in this document:

     - Encapsulation: sub-TLV type = 1
     - Protocol type: sub-TLV type = 2
     - Color: sub-TLV type = 4

     When the TLV is being processed by a BGP speaker that will be
     performing encapsulation, any unknown sub-TLVs MUST be ignored and
     skipped.  However, if the TLV is understood, the entire TLV MUST
     NOT be ignored just because it contains an unknown sub-TLV.

   * Sub-TLV Length (1 octet): the total number of octets of the sub-TLV
     value field.

   * 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.

4.1.  Encapsulation Sub-TLV

   The syntax and semantics of the encapsulation sub-TLV is determined
   by the tunnel type of the TLV that contains this sub-TLV.

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

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

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

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

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

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

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

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

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

4.2.  Protocol Type Sub-TLV

   The protocol type sub-TLV MAY be encoded to indicate the type of the
   payload packets that will be encapsulated with the tunnel parameters
   that are being signaled in the TLV.  The value field of the sub-TLV
   contains a 2-octet protocol type that is one of the types defined in

   For example, if we want to use three L2TPv3 sessions, one carrying
   IPv4 packets, one carrying IPv6 packets, and one carrying MPLS
   packets, the egress router will include three TLVs of L2TPv3
   encapsulation type, each specifying a different Session ID and a
   different payload type.  The protocol type sub-TLV for these will be
   IPv4 (protocol type = 0x0800), IPv6 (protocol type = 0x86dd), and
   MPLS (protocol type = 0x8847), respectively.  This informs the
   ingress routers of the appropriate encapsulation information to use

   with each of the given protocol types.  Insertion of the specified
   Session ID at the ingress routers allows the egress to process the
   incoming packets correctly, according to their protocol type.

   Inclusion of this sub-TLV depends on the tunnel type.  It MUST be
   encoded for L2TPv3 tunnel type.  On the other hand, the protocol type
   sub-TLV is not required for IP in IP or GRE tunnels.

4.3.  Color Sub-TLV

   The color sub-TLV MAY be encoded as a way to color the corresponding
   tunnel TLV.  The value field of the sub-TLV contains an extended
   community that is defined as follows:

4.3.1.  Color Extended Community

   The Color Extended Community is an opaque extended community
   [RFC4360] 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                          |

   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 on the routers.  The
   same Color Extended Community can then be attached to the UPDATE
   messages that contain payload prefixes.  This way, the BGP speaker
   can express the fact that it expects the packets corresponding to
   these payload prefixes to be received with a particular tunnel
   encapsulation header.

4.4.  Tunnel Type Selection

   A BGP speaker may include multiple tunnel TLVs in the tunnel
   attribute.  The receiving speaker MAY have local policies defined to
   choose different tunnel types for different sets/types of payload
   prefixes received from the same BGP speaker.  For instance, if a BGP
   speaker includes both L2TPv3 and GRE tunnel types in the tunnel
   attribute and it also advertises IPv4 and IPv6 prefixes, the ingress
   router may have local policy defined to choose L2TPv3 for IPv4
   prefixes (provided the protocol type received in the tunnel attribute
   matches) and GRE for IPv6 prefixes.

   Additionally, the Encapsulation SAFI UPDATE message can contain a
   color sub-TLV for some or all of the tunnel TLVs.  The BGP speaker
   SHOULD then attach a Color Extended Community to payload prefixes to
   select the appropriate tunnel types.

   In a multi-vendor deployment that has routers supporting different
   tunneling technologies, including color sub-TLV to the Encapsulation
   SAFI UPDATE message can serve as a classification mechanism (for
   example, set A of routers for GRE and set B of routers for L2TPv3).
   The ingress router can then choose the encapsulation data
   appropriately while sending packets to an egress router.

   If a BGP speaker originates an update for prefix P with color C and
   with itself as the next hop, then it MUST also originate an
   Encapsulation SAFI update that contains the color C.

   Suppose that a BGP speaker receives an update for prefix P with color
   C, that the BGP decision procedure has selected the route in that
   update as the best route to P, and that the next hop is node N, but
   that an Encapsulation SAFI update originating from node N containing
   color C has not been received.  In this case, no route to P will be
   installed in the forwarding table unless and until the corresponding
   Encapsulation SAFI update is received, or the BGP decision process
   selects a different route.

   Suppose that a BGP speaker receives an "uncolored" update for prefix
   P, with next hop N, and that the BGP speaker has also received an
   Encapsulation SAFI originated by N, specifying one or more
   encapsulations that may or may not be colored.  In this case, the
   choice of encapsulation is a matter of local policy.  The only
   "default policy" necessary is to choose one of the encapsulations
   supported by the speaker.

4.5.  BGP Encapsulation Extended Community

   Here, we define a BGP opaque extended community that can be attached
   to BGP UPDATE messages to indicate the encapsulation protocol to be
   used for sending packets from an ingress router to an egress router.
   Considering our example from the Section 1, R2 MAY include this
   extended community, specifying a particular tunnel type to be used in
   the UPDATE message that carries route Q to R1.  This is useful if
   there is no explicit encapsulation information to be signaled using
   the Encapsulation SAFI for a tunneling protocol (such as GRE without

        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          |

   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 as
   defined in this document.

   For interoperability, a speaker supporting Encapsulation SAFI MUST
   implement the Encapsulation Extended Community.

5.  Capability Advertisement

   A BGP speaker that wishes to exchange tunnel endpoint information
   must use the Multiprotocol Extensions Capability Code as defined in
   [RFC4760], to advertise the corresponding (AFI, SAFI) pair.

6.  Error Handling

   When a BGP speaker encounters an error while parsing the tunnel
   encapsulation attribute, the speaker MUST treat the UPDATE as a
   withdrawal of existing routes to the included Encapsulation SAFI
   NLRIs, or discard the UPDATE if no such routes exist.  A log entry
   should be raised for local analysis.

7.  Security Considerations

   Security considerations applicable to softwires can be found in the
   mesh framework [MESH].  In general, security issues of the tunnel
   protocols signaled through Encapsulation SAFI are inherited.

   If a third party is able to modify any of the information that is
   used to form encapsulation headers, to choose a tunnel type, or to
   choose a particular tunnel for a particular payload type, user data
   packets may end up getting misrouted, misdelivered, and/or dropped.

8.  IANA Considerations

   IANA assigned value 7 from the "Subsequent Address Family" Registry,
   in the "Standards Action" range, to "Encapsulation SAFI", with this
   document as the reference.

   IANA assigned value 23 from the "BGP Path Attributes" Registry, to
   "Tunnel Encapsulation Attribute", with this document as the

   IANA assigned two new values from the "BGP Opaque Extended Community"
   type Registry.  Both are from the transitive range.  The first new
   value is called "Color Extended Community" (0x030b), and the second
   is called "Encapsulation Extended Community"(0x030c).  This document
   is the reference for both assignments.

   IANA set up a registry for "BGP Tunnel Encapsulation Attribute Tunnel
   Types".  This is a registry of two-octet values (0-65535), to be
   assigned on a first-come, first-served basis.  The initial
   assignments are as follows:

      Tunnel Name                             Type
      ---------------                         -----
      L2TPv3 over IP                            1
      GRE                                       2
      IP in IP                                  7

   IANA set up a registry for "BGP Tunnel Encapsulation Attribute Sub-
   TLVs".  This is a registry of 1-octet values (0-255), to be assigned
   on a "standards action/early allocation" basis.  This document is the
   reference.  The initial assignments are:

      Sub-TLV name                            Type
      -------------                           -----
      Encapsulation                             1
      Protocol Type                             2
      Color                                     4

9.  Acknowledgements

   This specification builds on prior work by Gargi Nalawade, Ruchi
   Kapoor, Dan Tappan, David Ward, Scott Wainner, Simon Barber, and
   Chris Metz.  The current authors wish to thank all these authors for
   their contribution.

   The authors would like to thank John Scudder, Robert Raszuk, Keyur
   Patel, Chris Metz, Yakov Rekhter, Carlos Pignataro, and Brian
   Carpenter for their valuable comments and suggestions.

10.  References

10.1.  Normative References

   [RFC4271]  Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
              Border Gateway Protocol 4 (BGP-4)", RFC 4271, January

   [RFC4760]  Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
              "Multiprotocol Extensions for BGP-4", RFC 4760, January

   [RFC4360]  Sangli, S., Tappan, D., and Y. Rekhter, "BGP Extended
              Communities Attribute", RFC 4360, February 2006.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3931]  Lau, J., Ed., Townsley, M., Ed., and I. Goyret, Ed.,
              "Layer Two Tunneling Protocol - Version 3 (L2TPv3)", RFC
              3931, March 2005.

   [RFC2784]  Farinacci, D., Li, T., Hanks, S., Meyer, D., and P.
              Traina, "Generic Routing Encapsulation (GRE)", RFC 2784,
              March 2000.

   [RFC2890]  Dommety, G., "Key and Sequence Number Extensions to GRE",
              RFC 2890, September 2000.

   [RFC2003]  Perkins, C., "IP Encapsulation within IP", RFC 2003,
              October 1996.

   [RFC4213]  Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms
              for IPv6 Hosts and Routers", RFC 4213, October 2005.

10.2.  Informative References

   [IANA-AF]  "Address Family Numbers," http://www.iana.org.

   [MESH]     Wu, J., Cui, Y., Metz, C., and E. Rosen, "Softwire Mesh
              Framework," Work in Progress, February 2009.

Authors' Addresses

   Pradosh Mohapatra
   Cisco Systems, Inc.
   170 Tasman Drive
   San Jose, CA, 95134
   EMail: pmohapat@cisco.com

   Eric Rosen
   Cisco Systems, Inc.
   1414 Massachusetts Avenue
   Boxborough, MA, 01719
   EMail: erosen@cisco.com


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