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Versions: (draft-sreekantiah-idr-segment-routing-te) 00 01 02 03 04 05 06 07 draft-ietf-idr-segment-routing-te-policy

Network Working Group                                    S. Previdi, Ed.
Internet-Draft                                               C. Filsfils
Intended status: Standards Track                          A. Sreekantiah
Expires: June 20, 2017                                      S. Sivabalan
                                                     Cisco Systems, Inc.
                                                               P. Mattes
                                                               Microsoft
                                                                E. Rosen
                                                        Juniper Networks
                                                                  S. Lin
                                                                  Google
                                                       December 17, 2016


    Advertising Segment Routing Traffic Engineering Policies in BGP
             draft-previdi-idr-segment-routing-te-policy-03

Abstract

   This document defines a new BGP SAFI with a new NLRI in order to
   advertise a Segment Routing Traffic Engineering Policy (SR TE
   Policy).  An SR TE Policy is a set of explicit paths represented by
   one or more segment lists.  The SR TE Policy is advertised along with
   the Tunnel Encapsulation Attribute for which this document also
   defines new sub-TLVs.  An SR TE policy is advertised with the
   information that will be used by the node receiving the advertisement
   in order to instantiate the policy in its forwarding table and to
   steer traffic according to the policy.

Status of This Memo

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

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

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

   This Internet-Draft will expire on June 20, 2017.






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Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   4
   2.  SR TE Policy Encoding . . . . . . . . . . . . . . . . . . . .   4
     2.1.  SR TE Policy SAFI and NLRI  . . . . . . . . . . . . . . .   4
     2.2.  SR TE Policy and Tunnel Encapsulation Attribute . . . . .   6
     2.3.  Remote Endpoint and Color . . . . . . . . . . . . . . . .   7
     2.4.  SR TE Policy Sub-TLVs . . . . . . . . . . . . . . . . . .   7
       2.4.1.  Preference sub-TLV  . . . . . . . . . . . . . . . . .   7
       2.4.2.  SR TE Binding SID Sub-TLV . . . . . . . . . . . . . .   8
       2.4.3.  Segment List Sub-TLV  . . . . . . . . . . . . . . . .   9
   3.  SR TE Policy Operations . . . . . . . . . . . . . . . . . . .  21
     3.1.  Configuration and Advertisement of SR TE Policies . . . .  21
     3.2.  Multipath Operation . . . . . . . . . . . . . . . . . . .  21
     3.3.  Binding SID TLV . . . . . . . . . . . . . . . . . . . . .  22
     3.4.  Reception of an SR TE Policy  . . . . . . . . . . . . . .  22
       3.4.1.  Acceptance of a SR TE Policy Update . . . . . . . . .  22
       3.4.2.  Usable SR TE Policy . . . . . . . . . . . . . . . . .  23
       3.4.3.  Instantiation of an SR TE Policy  . . . . . . . . . .  24
       3.4.4.  Propagation of an SR TE Policy  . . . . . . . . . . .  25
     3.5.  Steering Traffic into a SR TE Policy  . . . . . . . . . .  25
     3.6.  Flowspec and SR TE Policies . . . . . . . . . . . . . . .  26
   4.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  26
   5.  Implementation Status . . . . . . . . . . . . . . . . . . . .  26
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  27
     6.1.  Existing Registry: Subsequent Address Family Identifiers
           (SAFI) Parameters . . . . . . . . . . . . . . . . . . . .  28
     6.2.  Existing Registry: BGP Tunnel Encapsulation Attribute
           Tunnel Types  . . . . . . . . . . . . . . . . . . . . . .  28
     6.3.  Existing Registry: BGP Tunnel Encapsulation Attribute
           sub-TLVs  . . . . . . . . . . . . . . . . . . . . . . . .  28
     6.4.  New Registry: SR TE Policy List Sub-TLVs  . . . . . . . .  28



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   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  29
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  29
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  29
     8.2.  Informational References  . . . . . . . . . . . . . . . .  30
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  31

1.  Introduction

   Segment Routing (SR) technology leverages the source routing and
   tunneling paradigms.  [I-D.ietf-spring-segment-routing] describes the
   SR architecture.  [I-D.ietf-spring-segment-routing-mpls] describes
   its instantiation on the MPLS data plane and
   [I-D.ietf-6man-segment-routing-header] describes the Segment Routing
   instantiation over the IPv6 data plane.

   This document defines the Segment Routing Traffic Engineering Policy
   (SR TE Policy) as a set of unequal equal cost multi-path (UCMP)
   segment lists (representing explicit paths) as well as the mechanism
   allowing a router to steer traffic into an SR TE Policy.

   The SR TE Policy is advertised in the Border Gateway Protocol (BGP)
   by the BGP speaker being a router or a controller and using
   extensions defined in this document.  Among the information encoded
   in the BGP message and representing the SR TE Policy, the steering
   mechanism makes also use of the Extended Color Community currently
   defined in [I-D.ietf-idr-tunnel-encaps]

   Typically, a controller defines the set of policies and advertise
   them to BGP routers (typically ingress routers).  The policy
   advertisement uses BGP extensions defined in this document.  The
   policy advertisement is, in most but not all of the cases, tailored
   for the receiver.  In other words, a policy advertised to a given BGP
   speaker has significance only for that particular router and is not
   intended to be propagated anywhere else.  Then, the receiver of the
   policy instantiate the policy in its routing and forwarding tables
   and steer traffic into it based on both the policy and destination
   prefix color and next-hop.

   Alternatively, a router (i.e.: an BGP egress router) advertises SR TE
   Policies representing paths to itself.  These advertisements are sent
   to BGP ingress nodes who instantiate these policies and steer traffic
   into them according to the color and endpoint/BGP next-hop of both
   the policy and the destination prefix.

   An SR TE Policy being intended only for the receiver of the
   advertisement, the SR TE Policies are sent directly to each receiver
   and, in most of the cases will not traverse any Route Reflector (RR,
   [RFC4456]).



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   However, there are cases where a SR TE Policy is intended to a group
   of nodes.  Also, in a deployment scenario, a controller may also rely
   on the standard BGP update propagation scheme which makes use of
   route reflectors.  This cases require mechanisms that:

   o  Uniquely identify each instance of a given policy.

   o  Uniquely identify the intended receiver of a given SR TE Policy
      advertisement.

   The BGP extensions for the advertisement of SR TE Policies include
   following components:

   o  A new Subsequent Address Family Identifier (SAFI) identifying the
      content of the BGP message (i.e.: the SR TE Policy).

   o  A new NLRI identifying the SR TE Policy.

   o  A set of new TLVs to be inserted into the Tunnel Encapsulation
      Attribute (as defined in [I-D.ietf-idr-tunnel-encaps]) and
      describing the SR TE Policy.

   o  An IPv4 address format route-target extended community ([RFC4360])
      attached to the SR TE Policy advertisement and that indicates the
      intended receiver of such SR TE Policy advertisement.

   o  The Extended Color Community (as defined in
      [I-D.ietf-idr-tunnel-encaps]) and used in order to steer traffic
      into an SR TE Policy.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

2.  SR TE Policy Encoding

2.1.  SR TE Policy SAFI and NLRI

   A new SAFI is defined: the SR TE Policy SAFI (codepoint TBD1 to be
   assigned by IANA from the "Subsequent Address Family Identifiers
   (SAFI) Parameters" registry).

   The SR TE Policy SAFI uses a new NLRI defined as follows:






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   +-----------------------------------------------+
   |           Distinguisher (4 octets)            |
   +-----------------------------------------------+
   |           Policy Color (4 octets)             |
   +-----------------------------------------------+
   |           Endpoint (4 or 16 octets)           |
   +-----------------------------------------------+

   where:

   o  Distinguisher: 4-octet value uniquely identifying the policy in
      the context of <color, endpoint> tuple.  The distinguisher has no
      semantic and it's solely used by the SR TE Policy originator in
      order to make unique (from a NLRI perspective) multiple
      occurrences of the same SR TE Policy.

   o  Policy Color: 4-octet value identifying (with the endpoint) the
      policy.  The color is used to match the color of the destination
      prefixes in order to steer traffic into the SR TE Policy.

   o  Endpoint: identifies the endpoint of a policy.  The Endpoint may
      represent a single node or a set of nodes (e.g.: an anycast
      address or a summary address).  The Endpoint is an IPv4 (4-octet)
      address or an IPv6 (16-octet) address according to the AFI of the
      NLRI.

   The NLRI containing the SR TE Policy is carried in a BGP UPDATE
   message [RFC4271] using BGP multiprotocol extensions [RFC4760] with
   an AFI of 1 or 2 (IPv4 or IPv6) and with a SAFI of TBD1 (to be
   assigned by IANA from the "Subsequent Address Family Identifiers
   (SAFI) Parameters" registry).

   An update message that carries the MP_REACH_NLRI or MP_UNREACH_NLRI
   attribute with the SR TE Policy SAFI MUST also carry the BGP
   mandatory attributes.  In addition, the BGP update message MAY also
   contain any of the BGP optional attributes.

   The next-hop of the SR TE Policy SAFI NLRI is set based on the AFI.
   For example, if the AFI is set to IPv4 (1), then the next-hop is
   encoded as a 4-byte IPv4 address.  If the AFI is set to IPv6 (2),
   then the next-hop is encoded as a 16-byte IPv6 address of the router.
   It is important to note that any BGP speaker receiving a BGP message
   with an SR TE Policy NLRI, will process it only if the NLRI is a best
   path as per the BGP best path selection algorithm.







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2.2.  SR TE Policy and Tunnel Encapsulation Attribute

   The content of the SR TE Policy is encoded in the Tunnel
   Encapsulation Attribute originally defined in
   [I-D.ietf-idr-tunnel-encaps] using a new Tunnel-Type TLV (codepoint
   is TBD2, to be assigned by IANA from the "BGP Tunnel Encapsulation
   Attribute Tunnel Types" registry).

   The SR TE Policy Encoding structure is as follows:

   SR TE Policy SAFI NLRI: <Distinguisher, Policy-Color, Endpoint>
   Attributes:
      Tunnel Encaps Attribute (23)
         Tunnel Type: SR TE Policy
             Binding SID
             Preference
             Segment List
                 Weight
                 Segment
                 Segment
                 ...
             ...
         ...
   where:

   o  SR TE Policy SAFI NLRI is defined in Section 2.1.

   o  Tunnel Encapsulation Attribute is defined in
      [I-D.ietf-idr-tunnel-encaps].

   o  Tunnel-Type is set to TBD2 (to be assigned by IANA from the "BGP
      Tunnel Encapsulation Attribute Tunnel Types" registry).

   o  Preference, Binding SID, Segment-List, Weight and Segment are
      defined in this document.

   o  Additional sub-TLVs may be defined in the future.

   A single occurrence of "Tunnel Type: SR TE Policy" MUST be encoded
   within the same Tunnel Encapsulation Attribute.

   Multiple occurrences of "Segment List" MAY be encoded within the same
   SR TE Policy.

   Multiple occurrences of "Segment" MAY be encoded within the same
   Segment List.





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2.3.  Remote Endpoint and Color

   The Remote Endpoint and Color sub-TLVs, as defined in
   [I-D.ietf-idr-tunnel-encaps], MAY also be present in the SR TE Policy
   encodings.

   If present, the Remote Endpoint sub-TLV MUST match the Endpoint of
   the SR TE Policy SAFI NLRI.

   If present, the Color sub-TLV MUST match the Policy Color of the SR
   TE Policy SAFI NLRI.

2.4.  SR TE Policy Sub-TLVs

   This section defines the SR TE Policy sub-TLVs.

   Preference, Binding SID, Segment-List are allocated from the "BGP
   Tunnel Encapsulation Attribute sub-TLVs" registry.

   Weight and Segment Sub-TLVs are allocated from a new registry defined
   in this document and called: "SR TE Policy List Sub-TLVs".  See
   Section 6 for the details of the registry.

2.4.1.  Preference sub-TLV

   The Preference sub-TLV is used in order to determine the preference
   among multiple SR TE Policy originators.

   The Preference sub-TLV is optional, MAY appear only once in the SR TE
   Policy and has 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |   Length      |     Flags     |   RESERVED    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Preference (4 octets)                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:

   o  Type: TBD3 (to be assigned by IANA from the "BGP Tunnel
      Encapsulation Attribute sub-TLVs" registry).

   o  Length: 6.

   o  Flags: 1 octet of flags.  None is defined at this stage.  Flags
      SHOULD be unset on transmission and MUST be ignored on receipt.



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   o  RESERVED: 1 octet of reserved bits.  SHOULD be unset on
      transmission and MUST be ignored on receipt.

   o  Preference: a 4-octet value.  The highest value is preferred.

   The Preference is used when the same <color,endpoint> policy is
   advertised by multiple originators of the same SR TE Policy.  The
   Preference is used by the receiver in order to determine which of the
   received policies are to be installed.  The following rules apply to
   the Preference:

   o  Preference is to be applied to the <color,endpoint> tuple.  The
      Distinguisher MUST NOT be considered.

   o  Preference is used in order to determine which instance of a given
      SR TE Policy is to be installed.  However, Preference MUST NOT
      influence the BGP selection algorithm and propagation rules.  In
      other words, the preference selection happens after the BGP path
      selection.

2.4.2.  SR TE Binding SID Sub-TLV

   The Binding SID sub-TLV requests the allocation of a Binding Segment
   identifier associated with the SR TE Policy.

   The Binding SID sub-TLV is optional, MAY appear only once in the SR
   TE Policy and 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |   Length      |     Flags     |   RESERVED    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              Binding SID (variable, optional)                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:

   o  Type: TBD4 (to be assigned by IANA from the "BGP Tunnel
      Encapsulation Attribute sub-TLVs" registry).

   o  Length: specifies the length of the value field not including Type
      and Length fields.  Can be 2 or 6 or 18.

   o  Flags: 1 octet of flags.  None is defined at this stage.  Flags
      SHOULD be unset on transmission and MUST be ignored on receipt.





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   o  RESERVED: 1 octet of reserved bits.  SHOULD be unset on
      transmission and MUST be ignored on receipt.

   o  Binding SID: if length is 2, then no Binding SID is present.  If
      length is 6 then the Binding SID contains a 4-octet SID.  If
      length is 18 then the Binding SID contains a 16-octet IPv6 SID.

   The Binding SID sub-TLV is used to instruct the receiver of the BGP
   message to allocate a Binding SID to the SR TE Policy.  The
   allocation of the Binding SID in the receiver is done according to
   following rules:

   o  If length is 2 (no value field is present), then the receiver MUST
      allocate a local Binding SID whose value is chosen by the
      receiver.

   o  If length is 6, then the value field contains the 4-octet Binding
      SID value the receiver SHOULD allocate.

   o  If length is 18, then the value field contains the 16-octet
      Binding SID value the receiver SHOULD allocate.

   When a controller is used in order to define and advertise SR TE
   Policies and when the Binding SID is allocated by the receiver, such
   Binding SID SHOULD be reported to the controller.  The mechanisms
   and/or APIs used for the reporting of the Binding SID are outside the
   scope of this document.

   Further use of the Binding SID is described in a subsequent section.

2.4.3.  Segment List Sub-TLV

   The Segment List sub-TLV is used in order to encode a single explicit
   path towards the endpoint.  The Segment List sub-TLV includes the
   elements of the paths (i.e.: segments) as well as an optional Weight
   TLV.

   The Segment List sub-TLV may exceed 255 bytes length due to large
   number of segments.  Therefore a 2-octet length is required.
   According to [I-D.ietf-idr-tunnel-encaps], the first bit of the sub-
   TLV code point defines the size of the length field.  Therefore, for
   the Segment List sub-TLV a code point of 128 (or higher) is used.
   See Section 6 section for details of codepoints allocation.

   The Segment List sub-TLV is mandatory, MAY appear multiple times in
   the SR TE Policy and has the following format:





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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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |             Length            |   RESERVED    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   //                           sub-TLVs                          //
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:

   o  Type: TBD5 (to be assigned by IANA from the "BGP Tunnel
      Encapsulation Attribute sub-TLVs" registry).

   o  Length: the total length (not including the Type and Length
      fields) of the sub-TLVs encoded within the Segment List sub-TLV.

   o  RESERVED: 1 octet of reserved bits.  SHOULD be unset on
      transmission and MUST be ignored on receipt.

   o  sub-TLVs:

      *  An optional single Weight sub-TLV.

      *  One or more Segment sub-TLVs.

   The Segment List sub-TLV is mandatory.

   Multiple occurrences of the Segment List sub-TLV MAY appear in the SR
   TE Policy.

   When multiple occurrences of the Segment List sub-TLV appear in the
   SR TE Policy, the traffic is load-balanced across them either through
   an ECMP scheme (if no Weight sub-TLV is present) or through a
   weighted UCMP scheme according to Section 2.4.3.1.

   The Segment-List Sub-TLV MUST contain at least one Segment Sub-TLV
   and MAY contain a Weight Sub-TLV.

2.4.3.1.  Weight Sub-TLV

   The Weight sub-TLV specifies the weight associated to a given path
   (i.e.: a given segment list).  The weight is used in order to apply
   weighted UCMP mechanism when steering traffic into a policy that
   includes multiple Segment Lists sub-TLVs (i.e.: multiple explicit
   paths).

   The Weight sub-TLV is optional, MAY only appear once inside the
   Segment List sub-TLV, and has the following format:



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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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |   Length      |     Flags     |   RESERVED    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                              Weight                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:

   Type: 9 (to be assigned by IANA from the registry "SR TE Policy List
   Sub-TLVs" defined in this document).

   Length: 6.

   Flags: 1 octet of flags.  None is defined at this stage.  Flags
   SHOULD be unset on transmission and MUST be ignored on receipt.

   RESERVED: 1 octet of reserved bits.  SHOULD be unset on transmission
   and MUST be ignored on receipt.

   When present, the Weight sub-TLV specifies a weight to be associated
   with the corresponding Segment List, for use in unequal cost multi-
   path.  Weights are applied by summing the total value of all of the
   weights for all Segment Lists, and then assigning a fraction of the
   forwarded traffic to each Segment List in proportion its weight's
   fraction of the total.

2.4.3.2.  Segment Sub-TLV

   The Segment sub-TLV describes a single segment in a segment list
   (i.e.: a single element of the explicit path).  Multiple Segment sub-
   TLVs constitute an explicit path of the SR TE Policy.

   The Segment sub-TLV is mandatory and MAY appear multiple times in the
   Segment List sub-TLV.

   This document defines 8 different types of Segment Sub-TLVs:

   Type 1: SID only, in the form of MPLS Label
   Type 2: SID only, in the form of IPv6 address
   Type 3: IPv4 Node Address with optional SID
   Type 4: IPv6 Node Address with optional SID
   Type 5: IPv4 Address + index with optional SID
   Type 6: IPv4 Local and Remote addresses with optional SID
   Type 7: IPv6 Address + index with optional SID
   Type 8: IPv6 Local and Remote addresses with optional SID




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2.4.3.2.1.  Type 1: SID only, in the form of MPLS Label

   The Type-1 Segment Sub-TLV encodes a single SID in the form of an
   MPLS label.  The format is 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |   Length      |     Flags     |   RESERVED    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Label                        | TC  |S|       TTL     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:

   o  Type: 1 (to be assigned by IANA from the registry "SR TE Policy
      List Sub-TLVs" defined in this document).

   o  Length is 6.

   o  Flags: 1 octet of flags.  None is defined at this stage.  Flags
      SHOULD be unset on transmission and MUST be ignored on receipt.

   o  RESERVED: 1 octet of reserved bits.  SHOULD be unset on
      transmission and MUST be ignored on receipt.

   o  Label: 20 bits of label value.

   o  TC: 3 bits of traffic class.

   o  S: 1 bit of bottom-of-stack.

   o  TTL: 1 octet of TTL.

   The following applies to the Type-1 Segment sub-TLV:

   o  The S bit SHOULD be zero upon transmission, and MUST be ignored
      upon reception.

   o  If the originator wants the receiver to choose the TC value, it
      sets the TC field to zero.

   o  If the originator wants the receiver to choose the TTL value, it
      sets the TTL field to 255.

   o  If the originator wants to recommend a value for these fields, it
      puts those values in the TC and/or TTL fields.




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   o  The receiver MAY override the originator's values for these
      fields.  This would be determined by local policy at the receiver.
      One possible policy would be to override the fields only if the
      fields have the default values specified above.

2.4.3.2.2.  Type 2: SID only, in the form of IPv6 address

   The Type-2 Segment Sub-TLV encodes a single SID in the form of an
   IPv6 address.  The format is 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |   Length      |     Flags     |   RESERVED    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   //                       IPv6 SID (16 octets)                  //
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:

   o  Type: 2 (to be assigned by IANA from the registry "SR TE Policy
      List Sub-TLVs" defined in this document).

   o  Length is 18.

   o  Flags: 1 octet of flags.  None is defined at this stage.  Flags
      SHOULD be unset on transmission and MUST be ignored on receipt.

   o  RESERVED: 1 octet of reserved bits.  SHOULD be unset on
      transmission and MUST be ignored on receipt.

   o  IPv6 SID: 16 octets of IPv6 address.

   The IPv6 Segment Identifier (IPv6 SID) is defined in
   [I-D.ietf-6man-segment-routing-header].

2.4.3.2.3.  Type 3: IPv4 Node Address with optional SID

   The Type-3 Segment Sub-TLV encodes an IPv4 node address and an
   optional SID in the form of either an MPLS label or an IPv6 address.
   The format is as follows:










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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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |   Length      |     Flags     |   RESERVED    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 IPv4 Node Address (4 octets)                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   //                SID (optional, 4 or 16 octets)               //
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:

   o  Type: 3 (to be assigned by IANA from the registry "SR TE Policy
      List Sub-TLVs" defined in this document).

   o  Length is 6 or 10 or 22.

   o  Flags: 1 octet of flags.  None is defined at this stage.  Flags
      SHOULD be unset on transmission and MUST be ignored on receipt.

   o  RESERVED: 1 octet of reserved bits.  SHOULD be unset on
      transmission and MUST be ignored on receipt.

   o  IPv4 Node Address: a 4 octet IPv4 address representing a node.

   o  SID: either 4 octet MPLS SID or a 16 octet IPv6 address.

   The following applies to the Type-3 Segment sub-TLV:

   o  The IPv4 Node Address MUST be present.

   o  The SID is optional and MAY be of one of the following formats:

      *  MPLS SID: a 4 octet label containing label, TC, S and TTL as
         defined in Section 2.4.3.2.1.

      *  IPV6 SID: a 16 octet IPv6 address.

   o  If length is 6, then only the IPv4 Node Address is present.

   o  If length is 10, then the IPv4 Node Address and the MPLS SID are
      present.

   o  If length is 22, then the IPv4 Node Address and the IPv6 SID are
      present.






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2.4.3.2.4.  Type 4: IPv6 Node Address with optional SID

   The Type-4 Segment Sub-TLV encodes an IPv6 node address and an
   optional SID in the form of either an MPLS label or an IPv6 address.
   The format is 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |   Length      |     Flags     |   RESERVED    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   //                IPv6 Node Address (16 octets)                //
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   //                SID (optional, 4 or 16 octets)               //
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:

   o  Type: 4 (to be assigned by IANA from the registry "SR TE Policy
      List Sub-TLVs" defined in this document).

   o  Length is 18 or 22 or 34.

   o  Flags: 1 octet of flags.  None is defined at this stage.  Flags
      SHOULD be unset on transmission and MUST be ignored on receipt.

   o  RESERVED: 1 octet of reserved bits.  SHOULD be unset on
      transmission and MUST be ignored on receipt.

   o  IPv6 Node Address: a 16 octet IPv6 address representing a node.

   o  SID: either 4 octet MPLS SID or a 16 octet IPv6 address.

   The following applies to the Type-4 Segment sub-TLV:

   o  The IPv6 Node Address MUST be present.

   o  The SID is optional and MAY be of one of the following formats:

      *  MPLS SID: a 4 octet label containing label, TC, S and TTL as
         defined in Section 2.4.3.2.1.

      *  IPV6 SID: a 16 octet IPv6 address.

   o  If length is 18, then only the IPv6 Node Address is present.

   o  If length is 22, then the IPv6 Node Address and the MPLS SID are
      present.



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   o  If length is 34, then the IPv6 Node Address and the IPv6 SID are
      present.

2.4.3.2.5.  Type 5: IPv4 Address + index with optional SID

   The Type-5 Segment Sub-TLV encodes an IPv4 node address, an interface
   index (IfIndex) and an optional SID in the form of either an MPLS
   label or an IPv6 address.  The format is 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |   Length      |     Flags     |   RESERVED    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 IfIndex (4 octets)                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 IPv4 Node Address (4 octets)                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   //                SID (optional, 4 or 16 octets)               //
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:

   o  Type: 5 (to be assigned by IANA from the registry "SR TE Policy
      List Sub-TLVs" defined in this document).

   o  Length is 10 or 14 or 26.

   o  Flags: 1 octet of flags.  None is defined at this stage.  Flags
      SHOULD be unset on transmission and MUST be ignored on receipt.

   o  RESERVED: 1 octet of reserved bits.  SHOULD be unset on
      transmission and MUST be ignored on receipt.

   o  IfIndex: 4 octets of interface index.

   o  IPv4 Node Address: a 4 octet IPv4 address representing a node.

   o  SID: either 4 octet MPLS SID or a 16 octet IPv6 address.

   The following applies to the Type-5 Segment sub-TLV:

   o  The IPv4 Node Address MUST be present.

   o  The Interface Index (IfIndex) MUST be present.

   o  The SID is optional and MAY be of one of the following formats:




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      *  MPLS SID: a 4 octet label containing label, TC, S and TTL as
         defined in Section 2.4.3.2.1.

      *  IPV6 SID: a 16 octet IPv6 address.

   o  If length is 10, then the IPv4 Node Address and IfIndex are
      present.

   o  If length is 14, then the IPv4 Node Address, the IfIndex and the
      MPLS SID are present.

   o  If length is 26, then the IPv4 Node Address, the IfIndex and the
      IPv6 SID are present.

2.4.3.2.6.  Type 6: IPv4 Local and Remote addresses with optional SID

   The Type-6 Segment Sub-TLV encodes an IPv4 node address, an adjacency
   local address, an adjacency remote address and an optional SID in the
   form of either an MPLS label or an IPv6 address.  The format is 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |   Length      |     Flags     |   RESERVED    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                Local IPv4 Address (4 octets)                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                Remote IPv4 Address  (4 octets)                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   //                     SID (4 or 16 octets)                    //
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   where:

   o  Type: 6 (to be assigned by IANA from the registry "SR TE Policy
      List Sub-TLVs" defined in this document).

   o  Length is 10 or 14 or 26.

   o  Flags: 1 octet of flags.  None is defined at this stage.  Flags
      SHOULD be unset on transmission and MUST be ignored on receipt.

   o  RESERVED: 1 octet of reserved bits.  SHOULD be unset on
      transmission and MUST be ignored on receipt.

   o  Local IPv4 Address: a 4 octet IPv4 address.



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   o  Remote IPv4 Address: a 4 octet IPv4 address.

   o  SID: either 4 octet MPLS SID or a 16 octet IPv6 address.

   The following applies to the Type-6 Segment sub-TLV:

   o  The Local IPv4 Address MUST be present and represents an adjacency
      local address.

   o  The Remote IPv4 Address MUST be present and represents the remote
      end of the adjacency.

   o  The SID is optional and MAY be of one of the following formats:

      *  MPLS SID: a 4 octet label containing label, TC, S and TTL as
         defined in Section 2.4.3.2.1.

      *  IPV6 SID: a 16 octet IPv6 address.

   o  If length is 10, then only the IPv4 Local and Remote addresses are
      present.

   o  If length is 14, then the IPv4 Local address, IPv4 Remote address
      and the MPLS SID are present.

   o  If length is 26, then the IPv4 Local address, IPv4 Remote address
      and the IPv6 SID are present.

2.4.3.2.7.  Type 7: IPv6 Address + index with optional SID

   The Type-7 Segment Sub-TLV encodes an IPv6 node address, an interface
   index and an optional SID in the form of either an MPLS label or an
   IPv6 address.  The format is 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |   Length      |     Flags     |   RESERVED    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 IfIndex (4 octets)                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   //                IPv6 Node Address (16 octets)                //
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   //                SID (optional, 4 or 16 octets)               //
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:




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   o  Type: 7 (to be assigned by IANA from the registry "SR TE Policy
      List Sub-TLVs" defined in this document).

   o  Length is 22 or 26 or 38.

   o  Flags: 1 octet of flags.  None is defined at this stage.  Flags
      SHOULD be unset on transmission and MUST be ignored on receipt.

   o  RESERVED: 1 octet of reserved bits.  SHOULD be unset on
      transmission and MUST be ignored on receipt.

   o  IfIndex: 4 octets of interface index.

   o  IPv6 Node Address: a 16 octet IPv6 address representing a node.

   o  SID: either 4 octet MPLS SID or a 16 octet IPv6 address.

   The following applies to the Type-7 Segment sub-TLV:

   o  The IPv6 Node Address MUST be present.

   o  The Interface Index MUST be present.

   o  The SID is optional and MAY be of one of the following formats:

      *  MPLS SID: a 4 octet label containing label, TC, S and TTL as
         defined in Section 2.4.3.2.1.

      *  IPV6 SID: a 16 octet IPv6 address.

   o  If length is 22, then the IPv6 Node Address and IfIndex are
      present.

   o  If length is 26, then the IPv6 Node Address, the IfIndex and the
      MPLS SID are present.

   o  If length is 38, then the IPv6 Node Address, the IfIndex and the
      IPv6 SID are present.

2.4.3.2.8.  Type 8: IPv6 Local and Remote addresses with optional SID

   The Type-8 Segment Sub-TLV encodes an IPv6 node address, an adjacency
   local address, an adjacency remote address and an optional SID in the
   form of either an MPLS label or an IPv6 address.  The format is as
   follows:






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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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |   Length      |     Flags     |   RESERVED    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   //               Local IPv6 Address (16 octets)                //
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   //               Remote IPv6 Address  (16 octets)              //
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   //                     SID (4 or 16 octets)                    //
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   where:

   o  Type: 8 (to be assigned by IANA from the registry "SR TE Policy
      List Sub-TLVs" defined in this document).

   o  Length is 34 or 38 or 50.

   o  Flags: 1 octet of flags.  None is defined at this stage.  Flags
      SHOULD be unset on transmission and MUST be ignored on receipt.

   o  RESERVED: 1 octet of reserved bits.  SHOULD be unset on
      transmission and MUST be ignored on receipt.

   o  Local IPv6 Address: a 16 octet IPv6 address.

   o  Remote IPv6 Address: a 16 octet IPv6 address.

   o  SID: either 4 octet MPLS SID or a 16 octet IPv6 address.

   The following applies to the Type-8 Segment sub-TLV:

   o  The Local IPv6 Address MUST be present and represents an adjacency
      local address.

   o  The Remote IPv6 Address MUST be present and represents the remote
      end of the adjacency.

   o  The SID is optional and MAY be of one of the following formats:

      *  MPLS SID: a 4 octet label containing label, TC, S and TTL as
         defined in Section 2.4.3.2.1.

      *  IPV6 SID: a 16 octet IPv6 address.





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   o  If length is 34, then only the IPv6 Local and Remote addresses are
      present.

   o  If length is 38, then the IPv6 Local address, IPv4 Remote address
      and the MPLS SID are present.

   o  If length is 50, then the IPv6 Local address, IPv4 Remote address
      and the IPv6 SID are present.

3.  SR TE Policy Operations

3.1.  Configuration and Advertisement of SR TE Policies

   Typically, but not limited to, a SR TE Policy is configured into a
   controller and on the base of each receiver.  In other words, each SR
   TE Policy configured is related to the intended receiver.  It is
   therefore normal for a given <color,endpoint> SR TE Policy to have
   multiple instances with different content (i.e.: different segment
   lists) where each of these instances (of the same policy) is intended
   to be sent to different receivers.

   Each instance of the same SR TE Policy will have a different
   Distinguisher in order to prevent BGP selection among these instances
   along the distribution of BGP updates.

   Moreover, a Route-Target extended community SHOULD be attached to the
   SR TE Policy and that identifies the intended receiver of the
   advertisement.

   If no route-target is attached to the SR TE Policy NLRI, then it is
   assumed that the originator sends the SR TE Policy update directly
   (e.g.: through iBGP multihop) to the intended receiver.  In such
   case, the NO_ADVERTISE community MUST be attached to the SR TE Policy
   update.

3.2.  Multipath Operation

   The SR TE Policy MAY contain multiple Segment Lists which, in the
   absence of the Weight TLV, signifies equal cost load balancing
   amongst them.

   When a weight sub-TLV is encoded in each Segment List TLV, then the
   weight value SHOULD be used in order to perform an unequal cost load
   balance amongst the Segment Lists as specified in Section 2.4.3.1.







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3.3.  Binding SID TLV

   When the optional Binding SID sub-TLV is present, it indicates an
   instruction, to the receiving BGP speaker to allocate a Binding SID
   for the list of SIDs the Binding sub-TLV is related to.

   Any incoming packet with the Binding SID as active segment (according
   to the terminology described in [I-D.ietf-spring-segment-routing])
   will then have the Binding SID swapped with the list of SIDs
   specified in the Segment List sub-TLVs on the allocating BGP speaker.
   The allocated Binding SID MAY be then advertised by the BGP speaker
   that created it, through, e.g., BGP-LS in order to, typically, feed a
   controller with the updated topology and SR TE Policy information.

3.4.  Reception of an SR TE Policy

   On reception of a SR TE Policy, a BGP speaker MUST determine if the
   SR TE Policy is first acceptable, then usable.

   While only usable SR TE Policies are instantiated, acceptable SR TE
   Policies (i.e.: also the non-usable ones) MAY be propagated.

   Any SR TE Policy update that has been determined acceptable is kept
   in the BGP database.  This includes non-usable SR TE Policies.

3.4.1.  Acceptance of a SR TE Policy Update

   When a BGP speaker receives an SR TE Policy from a neighbor it has to
   determine if the SR TE Policy advertisement is acceptable.  The
   following applies:

   o  The SR TE Policy NLRI MUST have a color value and MUST have an
      endpoint value.

   o  The SR TE Policy NLRI MUST have distinguisher field.

   o  The SR TE Policy update MUST have either the NO_ADV community or
      at least one route-target extended community in IPv4-address
      format.

   o  The Tunnel Encapsulation Attribute MUST be attached to the BGP
      Update and MUST have the Tunnel Type set to SR TE Policy (value to
      be assigned by IANA).

   o  Within the SR TE Policy, at least one Segment List sub-TLV MUST be
      present.





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   o  Within the Segment List sub-TLV at least one Segment sub-TLV MUST
      be present.

   The Remote Endpoint and Color sub-TLVs, as defined in
   [I-D.ietf-idr-tunnel-encaps], MAY also be present in the SR TE Policy
   encodings.  If present, the Remote Endpoint sub-TLV MUST match the
   Endpoint of the SR TE Policy SAFI NLRI.  If they don't match, the SR
   TE Policy advertisement MUST be considered as not acceptable.  If
   present, the Color sub-TLV MUST match the Policy Color of the SR TE
   Policy SAFI NLRI.  If they don't match, the SR TE Policy
   advertisement MUST be considered as not acceptable.

   A non-acceptable SR TE Policy update that has a valid NLRI portion
   with invalid attribute portion MUST be considered as a withdraw of
   the SR TE Policy.

   A non-acceptable SR TE Policy update that has an invalid NLRI portion
   MUST trigger a reset of the BGP session.

3.4.2.  Usable SR TE Policy

   When the receiver has determined that the received SR TE Policy is
   acceptable according to previous section, it has to determine if the
   received SR TE Policy is usable.

   The receiver MUST check whether route-target or NO_ADVERTISE
   communities are attached to it.  If no route-target is present and
   the NO_ADVERTISE community is present, then the SR TE Policy is
   usable.

   If one or more route-targets are present, then at least one route-
   target MUST match the BGP Identifier (BGP Router-ID) of the receiver
   in order for the update to be considered usable.  The BGP Identifier
   is defined in [RFC4271] as a 4 octet IPv4 address.  Therefore the
   route-target extended community MUST be of the same format.

   If one or more route-targets are present and no one matches the local
   BGP router-ID, then, while the SR TE Policy is acceptable, the SR TE
   Policy is not usable.  It has to be noted that if the receiver has
   been explicitly configured to do so, it MAY propagate the SR TE
   Policy to its neighbors as defined in Section 3.4.4.

   The following applies to usable SR TE Policies:

   o  Any segment sub-TLV of type 3 to 8 that is present in the segment
      list MUST be either validated or resolved:





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         if the SID portion of the sub-TLV is present, then the segment
         MUST be validated by the receiver.  Validation consists of
         verifying that the SID value is related to the network address.

         if the SID portion of the sub-TLV is not present, then the
         segment MUST be resolved by the receiver.  Resolution consists
         of taking from the receiver database (e.g.; from the link-state
         or routing information base) that the SID value related to the
         network address in the sub-TLV.

   o  The receiver MUST check the validity of the first SID of each
      Segment List sub-TLV of the SR TE Policy.  The first SID MUST be
      known in the receiver local table either as a label (in the case
      the SID encodes a label value) or as an IPv6 address.

   o  Any invalid segment of the segment list MUST cause an invalidation
      of the whole segment list.  However, the SR TE Policy is still
      usable if at least one segment list is valid.

   o  The receiver much keep track of the validated segment (i.e.: the
      first segment and any segment encoded in Sub-TLV type 3 to 8).  A
      segment who failed validation may become valid after a network
      event and vice versa.  The receiver SHOULD keep the state of the
      received SR TE Policies based on latest state of the segments
      requires validation.

   It has to be noted that an SR-TE policy may be received by a server
   that is not a router, and that does not have the necessary state that
   allows him to infer the next-hop from the first segment.  In that
   case, if the server needs to send a packet according to a particular
   SR-TE policy, it SHOULD push on the label stack that the policy
   specifies, and then send the packet to a default router (or default
   gateway).

3.4.3.  Instantiation of an SR TE Policy

   On reception of an acceptable, valid and usable SR TE Policy, a BGP
   speaker SHOULD instantiate the SR TE Policy in its routing and
   forwarding table with the set of segment lists (i.e.: explicit paths)
   included in the policy and taking into account the Binding SID and
   Weight sub-TLVs.

   The receiver of the SR TE Policy SHOULD program its MPLS or IPv6 data
   planes so that BGP destination prefixes matching their Extended Color
   Community and BGP next-hop with the SR TE Policy SAFI NLRI Color and
   Endpoint are steered into the SR TE Policy and forwarded accordingly.





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   When building the MPLS label stack or the IPv6 Segment list from the
   Segment List sub-TLV, the receiving BGP speaker MUST interpret the
   set of Segment sub-TLVs as follows:

   o  The first Segment sub-TLV represents the topmost label or the
      first IPv6 segment.  In the receiving BGP speaker, it identifies
      the first segment the traffic will be directed towards to (along
      the SR TE explicit path).

   o  The last Segment sub-TLV represents the bottommost label or the
      last IPv6 segment.

   As described in Section 2.4.3.1, when present, the Weight sub-TLV
   specifies a weight to be associated with the corresponding Segment
   List, for use in unequal-cost multi path.  Weights are applied by
   summing the total value of all of the weights for all Segment Lists,
   and then assigning a fraction of the forwarded traffic to each
   Segment List in proportion its weight's fraction of the total.

   If in a SR TE Policy only some of the segment lists have a Weight
   Sub-TLV present, then for those who haven't any weight, a value of 1
   is assumed.

3.4.4.  Propagation of an SR TE Policy

   By default, a BGP node receiving an SR TE Policy MUST NOT not
   propagate it to any eBGP neighbor.

   However, a node MAY be explicitly configured in order to advertise a
   received SR TE Policy update to neighbors according to normal BGP
   rules (iBGP and eBGP propagation), e.g., in the case the node is a
   Route-Reflector.

   SR TE Policies that have been determined acceptable and valid can be
   propagated, even the ones that are not usable.

   Only SR TE Policies that do not have the NO_ADVERTISE community
   attached to them can be propagated.

3.5.  Steering Traffic into a SR TE Policy

   The Color field of the NLRI allows association of destination
   prefixes with a given SR TE Policy.  The BGP speaker SHOULD then
   attach a Color Extended Community (as defined in [RFC5512]) to
   destination prefixes (e.g.: IPv4/IPv6 unicast prefixes) in order to
   allow the receiver of the SR TE Policy and of the destination prefix
   to steer traffic into the SR TE Policy if the destination prefix:




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   o  Has a BGP next-hop matching the SR TE Policy SAFI NLRI Endpoint
      and

   o  Has an attached Extended Color Community with the same value as
      the color of the SR TE Policy NLRI Color.

   On the receiving BGP speaker, all destination prefixes that share the
   same Extended Color Community value and the same BGP next-hop are
   steered to the corresponding SR TE Policy that has been instantiated
   and which matches the Color and Endpoint NLRI values.

   It is assumed that only one Extended Color Community is attached to a
   destination prefix.  In case a destination prefix is received with
   multiple Extended Color Communities, the receiver MUST consider the
   color corresponding to the SR TE Policy having the highest Preference
   TLV value.  In case of multiple policies having the same preference,
   as a breaking tie, the router SHOULD select the policy having the
   lowest color value.

   Different destination prefixes can be steered into distinct SR TE
   Policies by coloring them differently.

3.6.  Flowspec and SR TE Policies

   The SR TE Policy can be carried in context of a Flowspec NLRI
   ([RFC5575]).  In this case, when the redirect to IP next-hop is
   specified as in [I-D.ietf-idr-flowspec-redirect-ip], the tunnel to
   the next-hop is specified by the segment list in the Segment List
   sub-TLVs.  The Segment List (e.g..: label stack or IPv6 segment list)
   is imposed to flows matching the criteria in the Flowspec route in
   order to steer them towards the next-hop as specified in the SR TE
   Policy SAFI NLRI.

4.  Acknowledgments

   The authors of this document would like to thank Dhanendra Jain,
   Shyam Sethuram, Acee Lindem, Imtiyaz Mohammad and John Scudder for
   their comments and review of this document.

5.  Implementation Status

   Note to RFC Editor: Please remove this section prior to publication,
   as well as the reference to RFC 7942.

   This section records the status of known implementations of the
   protocol defined by this specification at the time of posting of this
   Internet-Draft, and is based on a proposal described in [RFC7942].
   The description of implementations in this section is intended to



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

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

   Several early implementations exist and will be reported in detail in
   a forthcoming version of this document.  For purposes of early
   interoperability testing, when no FCFS code point was available,
   implementations have made use of the following values:

   o  Preference sub-TLV: 6

   o  Binding SID sub-TLV: 7

   o  Segment List sub-TLV: 128

   When IANA-assigned values are available, implementations will be
   updated to use them.

6.  IANA Considerations

   This document defines new Sub-TLVs in following existing registries:

   o  Subsequent Address Family Identifiers (SAFI) Parameters

   o  BGP Tunnel Encapsulation Attribute Tunnel Types

   o  BGP Tunnel Encapsulation Attribute sub-TLVs

   This document also defines a new registry: "SR TE Policy List Sub-
   TLVs".









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6.1.  Existing Registry: Subsequent Address Family Identifiers (SAFI)
      Parameters

   This document defines a new SAFI in the registry "Subsequent Address
   Family Identifiers (SAFI) Parameters":

              Codepoint    Description          Reference
              -----------------------------------------------
              TBD1         SR TE Policy SAFI    This document

6.2.  Existing Registry: BGP Tunnel Encapsulation Attribute Tunnel Types

   This document defines a new Tunnel-Type in the registry "BGP Tunnel
   Encapsulation Attribute Tunnel Types":

            Codepoint     Description            Reference
            --------------------------------------------------
            TBD2          SR TE Policy Type      This document

6.3.  Existing Registry: BGP Tunnel Encapsulation Attribute sub-TLVs

   This document defines new sub-TLVs in the registry "BGP Tunnel
   Encapsulation Attribute sub-TLVs":

          Codepoint       Description              Reference
          ------------------------------------------------------
          TBD3            Preference sub-TLV       This document
          TBD4            Binding SID sub-TLV      This document
          TBD5            Segment List sub-TLV     This document

6.4.  New Registry: SR TE Policy List Sub-TLVs

   This document defines a new registry called "SR TE Policy List Sub-
   TLVs".  The allocation policy of this registry is "First Come First
   Served (FCFS)" according to [RFC5226].

   Following Sub-TLV codepoints are defined:














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    Value    Description                                  Reference
    ------------------------------------------------------------------
       1     MPLS SID sub-TLV                            This document
       2     IPv6 SID sub-TLV                            This document
       3     IPv4 Node and SID sub-TLV                   This document
       4     IPv6 Node and SID sub-TLV                   This document
       5     IPv4 Node, index and SID sub-TLV            This document
       6     IPv4 Local/Remote addresses and SID sub-TLV This document
       7     IPv6 Node, index and SID sub-TLV            This document
       8     IPv6 Local/Remote addresses and SID sub-TLV This document
       9     Weight sub-TLV                              This document

7.  Security Considerations

   TBD.

8.  References

8.1.  Normative References

   [I-D.ietf-idr-tunnel-encaps]
              Rosen, E., Patel, K., and G. Velde, "The BGP Tunnel
              Encapsulation Attribute", draft-ietf-idr-tunnel-encaps-03
              (work in progress), November 2016.

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

   [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,
              <http://www.rfc-editor.org/info/rfc4271>.

   [RFC4360]  Sangli, S., Tappan, D., and Y. Rekhter, "BGP Extended
              Communities Attribute", RFC 4360, DOI 10.17487/RFC4360,
              February 2006, <http://www.rfc-editor.org/info/rfc4360>.

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

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              DOI 10.17487/RFC5226, May 2008,
              <http://www.rfc-editor.org/info/rfc5226>.



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

   [RFC5575]  Marques, P., Sheth, N., Raszuk, R., Greene, B., Mauch, J.,
              and D. McPherson, "Dissemination of Flow Specification
              Rules", RFC 5575, DOI 10.17487/RFC5575, August 2009,
              <http://www.rfc-editor.org/info/rfc5575>.

8.2.  Informational References

   [I-D.ietf-6man-segment-routing-header]
              Previdi, S., Filsfils, C., Field, B., Leung, I., Linkova,
              J., Aries, E., Kosugi, T., Vyncke, E., and D. Lebrun,
              "IPv6 Segment Routing Header (SRH)", draft-ietf-6man-
              segment-routing-header-02 (work in progress), September
              2016.

   [I-D.ietf-idr-flowspec-redirect-ip]
              Uttaro, J., Haas, J., Texier, M., Andy, A., Ray, S.,
              Simpson, A., and W. Henderickx, "BGP Flow-Spec Redirect to
              IP Action", draft-ietf-idr-flowspec-redirect-ip-02 (work
              in progress), February 2015.

   [I-D.ietf-spring-segment-routing]
              Filsfils, C., Previdi, S., Decraene, B., Litkowski, S.,
              and R. Shakir, "Segment Routing Architecture", draft-ietf-
              spring-segment-routing-10 (work in progress), November
              2016.

   [I-D.ietf-spring-segment-routing-mpls]
              Filsfils, C., Previdi, S., Bashandy, A., Decraene, B.,
              Litkowski, S., Horneffer, M., Shakir, R.,
              jefftant@gmail.com, j., and E. Crabbe, "Segment Routing
              with MPLS data plane", draft-ietf-spring-segment-routing-
              mpls-05 (work in progress), July 2016.

   [RFC4456]  Bates, T., Chen, E., and R. Chandra, "BGP Route
              Reflection: An Alternative to Full Mesh Internal BGP
              (IBGP)", RFC 4456, DOI 10.17487/RFC4456, April 2006,
              <http://www.rfc-editor.org/info/rfc4456>.

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



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Authors' Addresses

   Stefano Previdi (editor)
   Cisco Systems, Inc.
   Via Del Serafico, 200
   Rome  00142
   Italy

   Email: sprevidi@cisco.com


   Clarence Filsfils
   Cisco Systems, Inc.
   Brussels
   BE

   Email: cfilsfil@cisco.com


   Arjun Sreekantiah
   Cisco Systems, Inc.
   170 W. Tasman Drive
   San Jose, CA  95134
   USA

   Email: asreekan@cisco.com


   Siva Sivabalan
   Cisco Systems, Inc.
   170 W. Tasman Drive
   San Jose, CA  95134
   USA

   Email: msiva@cisco.com


   Paul Mattes
   Microsoft
   One Microsoft Way
   Redmond, WA  98052
   USA

   Email: pamattes@microsoft.com







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   Eric Rosen
   Juniper Networks
   10 Technology Park Drive
   Westford, MA  01886
   US

   Email: erosen@juniper.net


   Steven Lin
   Google

   Email: stevenlin@google.com






































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