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Versions: (draft-previdi-idr-segment-routing-te-policy) 00 01 02 03 04

Network Working Group                                    S. Previdi, Ed.
Internet-Draft                                                Individual
Intended status: Standards Track                             C. Filsfils
Expires: January 3, 2019                                    D. Jain, Ed.
                                                     Cisco Systems, Inc.
                                                               P. Mattes
                                                               Microsoft
                                                                E. Rosen
                                                        Juniper Networks
                                                                  S. Lin
                                                                  Google
                                                            July 2, 2018


              Advertising Segment Routing Policies in BGP
              draft-ietf-idr-segment-routing-te-policy-04

Abstract

   This document defines a new BGP SAFI with a new NLRI in order to
   advertise a candidate path of a Segment Routing Policy (SR Policy).
   An SR Policy is a set of candidate paths, each consisting of one or
   more segment lists.  The headend of an SR Policy may learn multiple
   candidate paths for an SR Policy.  Candidate paths may be learned via
   a number of different mechanisms, e.g., CLI, NetConf, PCEP, or BGP.
   This document specifies the way in which BGP may be used to
   distribute candidate paths.  New sub-TLVs for the Tunnel
   Encapsulation Attribute are defined.

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
   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 3, 2019.






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Copyright Notice

   Copyright (c) 2018 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.  Requirements Language . . . . . . . . . . . . . . . . . .   5
   2.  SR Policy Encoding  . . . . . . . . . . . . . . . . . . . . .   5
     2.1.  SR Policy SAFI and NLRI . . . . . . . . . . . . . . . . .   5
     2.2.  SR Policy and Tunnel Encapsulation Attribute  . . . . . .   7
     2.3.  Remote Endpoint and Color . . . . . . . . . . . . . . . .   8
     2.4.  SR Policy Sub-TLVs  . . . . . . . . . . . . . . . . . . .   9
       2.4.1.  Preference Sub-TLV  . . . . . . . . . . . . . . . . .   9
       2.4.2.  Binding SID Sub-TLV . . . . . . . . . . . . . . . . .  10
       2.4.3.  Segment List Sub-TLV  . . . . . . . . . . . . . . . .  11
       2.4.4.  Explicit NULL Label Policy Sub-TLV  . . . . . . . . .  27
       2.4.5.  Policy Priority Sub-TLV . . . . . . . . . . . . . . .  28
       2.4.6.  Policy Name Sub-TLV . . . . . . . . . . . . . . . . .  29
   3.  Extended Color Community  . . . . . . . . . . . . . . . . . .  30
   4.  SR Policy Operations  . . . . . . . . . . . . . . . . . . . .  30
     4.1.  Configuration and Advertisement of SR Policies  . . . . .  30
     4.2.  Reception of an SR Policy NLRI  . . . . . . . . . . . . .  31
       4.2.1.  Acceptance of an SR Policy NLRI . . . . . . . . . . .  31
       4.2.2.  Usable SR Policy NLRI . . . . . . . . . . . . . . . .  32
       4.2.3.  Passing a usable SR Policy NLRI to the SRPM . . . . .  32
       4.2.4.  Propagation of an SR Policy . . . . . . . . . . . . .  32
     4.3.  Flowspec and SR Policies  . . . . . . . . . . . . . . . .  33
   5.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  33
   6.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  34
   7.  Implementation Status . . . . . . . . . . . . . . . . . . . .  34
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  35
     8.1.  Existing Registry: Subsequent Address Family Identifiers
           (SAFI) Parameters . . . . . . . . . . . . . . . . . . . .  35
     8.2.  Existing Registry: BGP Tunnel Encapsulation Attribute
           Tunnel Types  . . . . . . . . . . . . . . . . . . . . . .  35
     8.3.  Existing Registry: BGP Tunnel Encapsulation Attribute



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           sub-TLVs  . . . . . . . . . . . . . . . . . . . . . . . .  35
     8.4.  New Registry: SR Policy List Sub-TLVs . . . . . . . . . .  36
     8.5.  New Registry: SR Policy Binding SID Flags . . . . . . . .  36
     8.6.  New Registry: SR Policy Segment Flags . . . . . . . . . .  37
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  37
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  37
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  37
     10.2.  Informational References . . . . . . . . . . . . . . . .  38
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  39

1.  Introduction

   Segment Routing (SR) allows a headend node to steer a packet flow
   along any path.  Intermediate per-flow states are eliminated thanks
   to source routing [I-D.ietf-spring-segment-routing].

   The headend node is said to steer a flow into a Segment Routing
   Policy (SR Policy).

   The header of a packet steered in an SR Policy is augmented with the
   ordered list of segments associated with that SR Policy.

   [I-D.ietf-spring-segment-routing-policy] details the concepts of SR
   Policy and steering into an SR Policy.  These apply equally to the
   MPLS and SRv6 instantiations of segment routing.

   [I-D.filsfils-spring-sr-policy-considerations] describes some of the
   implementation aspects of the SR Policy Headend Architecture and
   introduces the notion of an SR Policy Module (SRPM) that performs the
   functionality as highlighted in section 2 of
   [I-D.ietf-spring-segment-routing-policy]:

   o  The SRPM may learn multiple candidate paths for an SR Policy via
      various mechanisms (CLI, NetConf, PCEP or BGP).

   o  The SRPM selects the best candidate path for the SR Policy.

   o  The SRPM binds a BSID to the selected candidate path of the SR
      Policy.

   o  The SRPM installs the selected candidate path and its BSID in the
      forwarding plane.

   This document specifies the way to use BGP to distribute one or more
   of the candidate paths of an SR Policy to the headend of that policy.
   The document identifies the functionality that resides in the BGP
   process and for the functionality which is outside the scope of BGP




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   and lies within SRPM on the headend node, it refers to such, as
   appropriate.

   This document specifies a way of representing SR Policies and their
   candidate paths in BGP UPDATE messages.  BGP can then be used to
   propagate the SR Policies and candidate paths.  The usual BGP rules
   for BGP propagation and "bestpath selection" are used.  At the
   headend of a specific policy, this will result in one or more
   candidate paths being installed into the "BGP table".  These paths
   are then passed to the SRPM.  The SRPM may compare them to candidate
   paths learned via other mechanisms, and will choose one or more paths
   to be installed in the data plane.  BGP itself does not install SR
   Policy candidate paths into the data plane.

   This document defines a new BGP address family (SAFI).  In UPDATE
   messages of that address family, the NLRI identifies an SR Policy,
   and the attributes encode the segment lists and other details of that
   SR Policy.

   While for simplicity we may write that BGP advertises an SR Policy,
   it has to be understood that BGP advertises a candidate path of an SR
   policy and that this SR Policy might have several other candidate
   paths provided via BGP (via an NLRI with a different distinguisher as
   defined in this document), PCEP, NETCONF or local policy
   configuration.

   Typically, a controller defines the set of policies and advertise
   them to policy head-end 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 a specific policy head-end.  In this case the
   advertisement may sent on a BGP session to that head-end and not
   propagated any further.

   Alternatively, a router (i.e., a BGP egress router) advertises SR
   Policies representing paths to itself.  In this case, it is possible
   to send the policy to each head-end over a BGP session to that head-
   end, without requiring any further propagation of the policy.

   An SR Policy intended only for the receiver will, in most cases, not
   traverse any Route Reflector (RR, [RFC4456]).

   In some situations, it is undesirable for a controller or BGP egress
   router to have a BGP session to each policy head-end.  In these
   situations, BGP Route Reflectors may be used to propagate the
   advertisements, or it may be necessary for the advertisement to
   propagate through a sequence of one or more ASes.  To make this
   possible, an attribute needs to be attached to the advertisement that



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   enables a BGP speaker to determine whether it is intended to be a
   head-end for the advertised policy.  This is done by attaching one or
   more Route Target Extended Communities to the advertisement
   ([RFC4360]).

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

   o  A new Subsequent Address Family Identifier (SAFI) whose NLRI
      identifies an SR Policy.

   o  A new Tunnel Type identifier for SR Policy, and a set of sub-TLVs
      to be inserted into the Tunnel Encapsulation Attribute (as defined
      in [I-D.ietf-idr-tunnel-encaps]) specifying segment lists of the
      SR Policy, as well as other information about the SR Policy.

   o  One or more IPv4 address format route-target extended community
      ([RFC4360]) attached to the SR Policy advertisement and that
      indicates the intended head-end of such SR Policy advertisement.

   o  The Color Extended Community (as defined in
      [I-D.ietf-idr-tunnel-encaps]) and used in order to steer traffic
      into an SR Policy, as described in section 8.4 in
      [I-D.ietf-spring-segment-routing-policy].  This document
      (Section 3) modifies the format of the Color Extended Community by
      using the two leftmost bits of the RESERVED field.

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 Policy Encoding

2.1.  SR Policy SAFI and NLRI

   A new SAFI is defined: the SR Policy SAFI, (codepoint 73 assigned by
   IANA (see Section 8) from the "Subsequent Address Family Identifiers
   (SAFI) Parameters" registry).

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









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   +------------------+
   |  NLRI Length     | 1 octet
   +------------------+
   |  Distinguisher   | 4 octets
   +------------------+
   |  Policy Color    | 4 octets
   +------------------+
   |  Endpoint        | 4 or 16 octets
   +------------------+

   where:

   o  NLRI Length: 1 octet of length expressed in bits as defined in
      [RFC4760].

   o  Distinguisher: 4-octet value uniquely identifying the policy in
      the context of <color, endpoint> tuple.  The distinguisher has no
      semantic value and is solely used by the SR Policy originator to
      make unique (from an NLRI perspective) multiple occurrences of the
      same SR 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 to steer traffic into the SR Policy
      [I-D.ietf-spring-segment-routing-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).  The Endpoint is an IPv4 (4-octet) address or an IPv6
      (16-octet) address according to the AFI of the NLRI.

   The color and endpoint are used to automate the steering of BGP
   Payload prefixes on SR Policy as described in
   [I-D.ietf-spring-segment-routing-policy].

   The NLRI containing the SR 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 73 (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 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 network address field in SR Policy SAFI (73) updates may
   be either a 4 octet IPv4 address or a 16 octet IPv6 address,



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   independent of the SR Policy AFI.  The length field of the next-hop
   address specifies the next-hop address family.  If the next-hop
   length is 4, then the next-hop is an IPv4 address; if the next-hop
   length is 16, then it is a global IPv6 address; and if the next-hop
   length is 32, then it has a global IPv6 address followed by a link-
   local IPv6 address.  The setting of the next-hop field and its
   attendant processing is governed by standard BGP procedures as
   described in section 3 in [RFC4760].

   It is important to note that any BGP speaker receiving a BGP message
   with an SR Policy NLRI, will process it only if the NLRI is among the
   best paths as per the BGP best path selection algorithm.  In other
   words, this document does not modify the BGP propagation or bestpath
   selection rules.

   It has to be noted that if several candidate paths of the same SR
   Policy (endpoint, color) are signaled via BGP to a head-end, it is
   recommended that each NLRI use a different distinguisher.  If BGP has
   installed into the BGP table two advertisements whose respective
   NLRIs have the same color and endpoint, but different distinguishers,
   both advertisements are passed to the SRPM as different candidate
   paths.  In addition, the originator information corresponding to the
   each candidate path, as described in section 2.4 in
   [I-D.ietf-spring-segment-routing-policy], is passed to the SRPM.

2.2.  SR Policy and Tunnel Encapsulation Attribute

   The content of the SR 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 15, assigned by IANA (see
   Section 8) from the "BGP Tunnel Encapsulation Attribute Tunnel Types"
   registry).

   The SR Policy Encoding structure is as follows:

















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

   o  SR 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 15 (assigned by IANA from the "BGP Tunnel
      Encapsulation Attribute Tunnel Types" registry).

   o  Preference, Binding SID, Priority, Policy Name, ENLP, Segment-
      List, Weight and Segment sub-TLVs are defined in this document.

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

   A Tunnel Encapsulation Attribute MUST NOT contain more than one TLV
   of type "SR Policy".  If more than one TLV of type "SR Policy"
   appears, the update is considered malformed and the "treat-as-
   withdraw" strategy of [RFC7606] is applied.

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

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

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 Policy
   encodings.





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   The Remote Endpoint and Color Sub-TLVs are not used for SR Policy
   encodings and therefore their value is irrelevant in the context of
   the SR Policy SAFI NLRI.  If present, the Remote Endpoint sub-TLV and
   the Color sub-TLV MUST be ignored by the BGP speaker.

2.4.  SR Policy Sub-TLVs

   This section defines the SR Policy sub-TLVs.

   Preference, Binding SID, Segment-List, Priority, Policy Name and
   Explicit NULL Label Policy sub-TLVs are assigned from the "BGP Tunnel
   Encapsulation Attribute Sub-TLVs" registry.

   Weight and Segment sub-TLVs are assigned from a new registry defined
   in this document and called: "SR Policy List Sub-TLVs".  See
   Section 8 for the details of the registry.

2.4.1.  Preference Sub-TLV

   The Preference sub-TLV does not have any effect on the BGP bestpath
   selection or propagation procedures.  The contents of this sub-TLV
   are used by the SRPM as described in section 2.7 in
   [I-D.ietf-spring-segment-routing-policy].

   The Preference sub-TLV is optional and it MUST NOT appear more than
   once in the SR Policy.  If the Preference sub-TLV appears more than
   once, the update is considered malformed and the "treat-as-withdraw"
   strategy of [RFC7606] is applied.

   The Preference sub-TLV 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: 12

   o  Length: 6.

   o  Flags: 1 octet of flags.  None are defined at this stage.  Flags
      SHOULD be set to zero 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.

2.4.2.  Binding SID Sub-TLV

   The Binding SID sub-TLV is not used by BGP.  The contents of this
   sub-TLV are used by the SRPM as described in section 6 in
   [I-D.ietf-spring-segment-routing-policy].

   The Binding SID sub-TLV is optional and it MUST NOT appear more than
   once in the SR Policy.  If the Binding SID sub-TLV appears more than
   once, the update is considered malformed and the "treat-as-withdraw"
   strategy of [RFC7606] is applied.

   The Binding SID sub-TLV 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: 13

   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.  Following flags are defined (to be
      assigned by IANA from the registry "SR Policy Binding SID Flags"
      defined in this document Section 8.5):

    0 1 2 3 4 5 6 7
   +-+-+-+-+-+-+-+-+
   |S|I|           |
   +-+-+-+-+-+-+-+-+

      where:

      *  S-Flag: This flag encodes the "Specified-BSID-only" behavior.
         It is used by SRPM as described in section 6.2.3 in
         [I-D.ietf-spring-segment-routing-policy].




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      *  I-Flag: This flag encodes the "Drop Upon Invalid" behavior.  It
         is used by SRPM as described in section 8.2 in
         [I-D.ietf-spring-segment-routing-policy].

      *  Unused bits in the Flag octet SHOULD be set to zero upon
         transmission and MUST be ignored upon receipt.

   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.

   o  If length is 6 then the Binding SID contains a 4-octet SID.  Below
      format is used to encode the SID.  TC, S, TTL(Total of 12bits) are
      RESERVED and SHOULD be set to Zero and MUST be ignored.

       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     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      If length is 18 then the Binding SID contains a 16-octet IPv6 SID.

2.4.3.  Segment List Sub-TLV

   The Segment List sub-TLV encodes a single explicit path towards the
   endpoint as described in section 5.1 in
   [I-D.ietf-spring-segment-routing-policy].  The Segment List sub-TLV
   includes the elements of the paths (i.e., segments) as well as an
   optional Weight sub-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 codepoint 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 8 for details of codepoints allocation.

   The Segment List sub-TLV is optional and MAY appear multiple times in
   the SR Policy.  The ordering of Segment List sub-TLVs, each sub-TLV
   encoding a Segment List, does not matter.

   The Segment List sub-TLV contains zero or more Segment sub-TLVs and
   MAY contain a Weight sub-TLV.

   The Segment List sub-TLV 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: 128.

   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.

      *  Zero or more Segment sub-TLVs.

   Validation of an explicit path encoded by the Segment List sub-TLV is
   completely within the scope of SRPM as described in section 5 in
   [I-D.ietf-spring-segment-routing-policy].

2.4.3.1.  Weight Sub-TLV

   The Weight sub-TLV specifies the weight associated to a given segment
   list.  The contents of this sub-TLV are used only by the SRPM as
   described in section 2.11 in
   [I-D.ietf-spring-segment-routing-policy].

   The Weight sub-TLV is optional and it MUST NOT appear more than once
   inside the Segment List sub-TLV.  If the Weight sub-TLV appears more
   than once, the update is considered malformed and the "treat-as-
   withdraw" strategy of [RFC7606] is applied.

   The Weight sub-TLV 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 Policy List
   Sub-TLVs" defined in this document).

   Length: 6.

   Flags: 1 octet of flags.  None are defined at this stage.  Flags
   SHOULD be set to zero 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.

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 Policy.

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

   The Segment sub-TLV does not have any effect on the BGP bestpath
   selection or propagation procedures.  The contents of this sub-TLV
   are used only by the SRPM as described in section 4 in
   [I-D.ietf-spring-segment-routing-policy].

   [I-D.ietf-spring-segment-routing-policy] defines several types of
   Segments:














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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 for SR MPLS
Type  5: IPv4 Address + index with optional SID
Type  6: IPv4 Local and Remote addresses with optional SID
Type  7: IPv6 Address + index for local and remote pair with optional SID for SR MPLS
Type  8: IPv6 Local and Remote addresses with optional SID for SR MPLS
Type  9: IPv6 Node Address with optional SID for SRv6
Type 10: IPv6 Address + index for local and remote pair with optional SID for SRv6
Type 11: IPv6 Local and Remote addresses for SRv6

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 Policy List
      Sub-TLVs" defined in this document).

   o  Length is 6.

   o  Flags: 1 octet of flags as defined in Section 2.4.3.2.12.

   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:





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   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.

   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 SRv6 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    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   //                       SRv6 SID (16 octets)                  //
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:

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

   o  Length is 18.

   o  Flags: 1 octet of flags as defined in Section 2.4.3.2.12.

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

   o  SRv6 SID: 16 octets of IPv6 address.

   The IPv6 Segment Identifier (SRv6 SID) is defined in
   [I-D.ietf-6man-segment-routing-header].





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2.4.3.2.3.  Type 3: IPv4 Node Address with optional SID

   The Type-3 Segment Sub-TLV encodes an IPv4 node address, SR Algorithm
   and an optional 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     |  SR Algorithm |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 IPv4 Node Address (4 octets)                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                SID (optional, 4 octets)                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:

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

   o  Length is 6 or 10.

   o  Flags: 1 octet of flags as defined in Section 2.4.3.2.12.

   o  SR Algorithm: 1 octet specifying SR Algorithm as described in
      section 3.1.1 in [I-D.ietf-spring-segment-routing], when A-Flag as
      defined in Section 2.4.3.2.12 is present.  SR Algorithm is used by
      SRPM as described in section 4 in
      [I-D.ietf-spring-segment-routing-policy].  When A-Flag is not
      encoded, this field 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: 4 octet MPLS label.

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

   o  The IPv4 Node Address MUST be present.

   o  The SID is optional and specifies a 4 octet MPLS SID containing
      label, TC, S and TTL as defined in Section 2.4.3.2.1.

   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.



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2.4.3.2.4.  Type 4: IPv6 Node Address with optional SID for SR MPLS

   The Type-4 Segment Sub-TLV encodes an IPv6 node address, SR Algorithm
   and an optional 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     |  SR Algorithm |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   //                IPv6 Node Address (16 octets)                //
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                SID (optional, 4 octets)                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:

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

   o  Length is 18 or 22.

   o  Flags: 1 octet of flags as defined in Section 2.4.3.2.12.

   o  SR Algorithm: 1 octet specifying SR Algorithm as described in
      section 3.1.1 in [I-D.ietf-spring-segment-routing], when A-Flag as
      defined in Section 2.4.3.2.12 is present.  SR Algorithm is used by
      SRPM as described in section 4 in
      [I-D.ietf-spring-segment-routing-policy].  When A-Flag is not
      encoded, this field 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: 4 octet MPLS label.

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

   o  The IPv6 Node Address MUST be present.

   o  The SID is optional and specifies a 4 octet MPLS SID containing
      label, TC, S and TTL as defined in Section 2.4.3.2.1.

   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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2.4.3.2.5.  Type 5: IPv4 Address + Local Interface ID with optional SID

   The Type-5 Segment Sub-TLV encodes an IPv4 node address, a local
   interface Identifier (Local Interface ID) and an optional 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    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Local Interface ID (4 octets)                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 IPv4 Node Address (4 octets)                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                SID (optional, 4 octets)                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:

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

   o  Length is 10 or 14.

   o  Flags: 1 octet of flags as defined in Section 2.4.3.2.12.

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

   o  Local Interface ID: 4 octets of interface index as defined in
      [I-D.ietf-pce-segment-routing].

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

   o  SID: 4 octet MPLS label.

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

   o  The IPv4 Node Address MUST be present.

   o  The Local Interface ID MUST be present.

   o  The SID is optional and specifies a 4 octet MPLS SID containing
      label, TC, S and TTL as defined in Section 2.4.3.2.1.

   o  If length is 10, then the IPv4 Node Address and Local Interface ID
      are present.



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   o  If length is 14, then the IPv4 Node Address, the Local Interface
      ID and the MPLS 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 adjacency local address, an
   adjacency remote address and an optional 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    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                Local IPv4 Address (4 octets)                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                Remote IPv4 Address  (4 octets)                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     SID (optional, 4 octets)                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   where:

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

   o  Length is 10 or 14.

   o  Flags: 1 octet of flags as defined in Section 2.4.3.2.12.

   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.

   o  Remote IPv4 Address: a 4 octet IPv4 address.

   o  SID: 4 octet MPLS label.

   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.




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   o  The SID is optional and specifies a 4 octet MPLS SID containing
      label, TC, S and TTL as defined in Section 2.4.3.2.1.

   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.

2.4.3.2.7.  Type 7: IPv6 Address + Interface ID for local and remote
            pair with optional SID for SR MPLS

   The Type-7 Segment Sub-TLV encodes an IPv6 Link Local adjacency with
   IPv6 local node address, a local interface identifier (Local
   Interface ID), IPv6 remote node address , a remote interface
   identifier (Remote Interface ID) and an optional 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    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Local Interface ID (4 octets)                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   //                IPv6 Local Node Address (16 octets)          //
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Remote Interface ID (4 octets)                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   //                IPv6 Remote Node Address (16 octets)         //
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                SID (optional, 4 octets)                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:

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

   o  Length is 22, 26, 42 or 46.

   o  Flags: 1 octet of flags as defined in Section 2.4.3.2.12.

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

   o  Local Interface ID: 4 octets of interface index as defined in
      [I-D.ietf-pce-segment-routing].



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   o  IPv6 Local Node Address: a 16 octet IPv6 address.

   o  Remote Interface ID: 4 octets of interface index as defined in
      [I-D.ietf-pce-segment-routing].

   o  IPv6 Remote Node Address: a 16 octet IPv6 address.

   o  SID: 4 octet MPLS label.

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

   o  The Local Interface ID and IPv6 Local Node Address MUST be
      present.

   o  The Remote Interface ID and Remote Node Address pair is optional.
      If Remote Interface ID is present, the Remote Node Address MUST be
      present as well.  Similarly, if Remote Node Address is present,
      the Remote Interface ID MUST be present as well.

   o  The SID is optional and specifies a 4 octet MPLS SID containing
      label, TC, S and TTL as defined in Section 2.4.3.2.1.

   o  If length is 22, then the Local Interface ID and the Local IPv6
      Address are present.

   o  If length is 26, then the Local Interface ID, Local IPv6 Address
      and the MPLS SID are present.

   o  If length is 42, then the Local Interface ID, Local IPv6 Node
      Address, Remote Interface ID, and the Remote IPv6 Node Address are
      present.

   o  If length is 46, then the Local Interface ID, Local IPv6 Node
      Address, Remote Interface ID, Remote IPv6 Node Address and the
      MPLS SID are present.

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

   The Type-8 Segment Sub-TLV encodes an adjacency local address, an
   adjacency remote address and an optional SID in the form of an MPLS
   label.  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 (optional, 4 octets)                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   where:

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

   o  Length is 34 or 38.

   o  Flags: 1 octet of flags as defined in Section 2.4.3.2.12.

   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: 4 octet MPLS label.

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

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

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




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2.4.3.2.9.  Type 9: IPv6 Node Address with optional SRv6 SID

   The Type-9 Segment Sub-TLV encodes an IPv6 node address, SR Algorithm
   and an optional 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     | SR Algorithm  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   //                 IPv6 Node Address (16 octets)               //
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   //                     SID (optional, 16 octets)               //
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:

   o  Type: 10 (to be assigned by IANA from the registry "SR Policy List
      Sub-TLVs" defined in this document).

   o  Length is 18 or 34.

   o  Flags: 1 octet of flags as defined in Section 2.4.3.2.12.

   o  SR Algorithm: 1 octet specifying SR Algorithm as described in
      section 3.1.1 in [I-D.ietf-spring-segment-routing], when A-Flag as
      defined in Section 2.4.3.2.12 is present.  SR Algorithm is used by
      SRPM as described in section 4 in
      [I-D.ietf-spring-segment-routing-policy].  When A-Flag is not
      encoded, this field SHOULD be unset on transmission and MUST be
      ignored on receipt.

   o  IPv6 Node Address: a 16 octet IPv6 address.

   o  SID: 16 octet IPv6 address.

   The following applies to the Type-9 Segment sub-TLV:

   o  The IPv6 Node Address MUST be present.

   o  The SID is optional and specifies an SRv6 SID in the form of 16
      octet IPv6 address.

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

   o  If length is 34, then the IPv6 Node Address and the SRv6 SID are
      present.



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2.4.3.2.10.  Type 10: IPv6 Address + Interface ID for local and remote
             pair for SRv6 with optional SID

   The Type-10 Segment Sub-TLV encodes an IPv6 Link Local adjacency with
   local node address, a local interface identifier (Local Interface
   ID), remote IPv6 node address , a remote interface identifier (Remote
   Interface ID) and an optional 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    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Local Interface ID (4 octets)                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   //                IPv6 Local Node Address (16 octets)          //
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Remote Interface ID (4 octets)                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   //                IPv6 Remote Node Address (16 octets)         //
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   //                SID (optional, 16 octets)                    //
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:

   o  Type: 11 (to be assigned by IANA from the registry "SR Policy List
      Sub-TLVs" defined in this document).

   o  Length is 22, 38, 42 or 58.

   o  Flags: 1 octet of flags as defined in Section 2.4.3.2.12.

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

   o  Local Interface ID: 4 octets of interface index as defined in
      [I-D.ietf-pce-segment-routing].

   o  IPv6 Local Node Address: a 16 octet IPv6 address.

   o  Remote Interface ID: 4 octets of interface index as defined in
      [I-D.ietf-pce-segment-routing].

   o  IPv6 Remote Node Address: a 16 octet IPv6 address.

   o  SID: 16 octet IPv6 address.



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   The following applies to the Type-10 Segment sub-TLV:

   o  The Local Interface ID and the Local IPv6 Node Addresses MUST be
      present.

   o  The Remote Interface ID and Remote Node Address pair is optional.
      If Remote Interface ID is present, the Remote Node Address MUST be
      present as well.  Similarly, if Remote Node Address is present,
      the Remote Interface ID MUST be present as well.

   o  The SID is optional and specifies an SRv6 SID in the form of 16
      octet IPv6 address.

   o  If length is 22, then the Local Interface ID, Local IPv6 Node
      Address, are present.

   o  If length is 38, then the Local Interface ID, Local IPv6 Node
      Address and the SRv6 SID are present.

   o  If length is 42, then the Local Interface ID, Local IPv6 Node
      Address, Remote Interface ID, and the Remote IPv6 Node Address are
      present.

   o  If length is 58, then the Local Interface ID, Local IPv6 Node
      Address, Remote Interface ID, Remote IPv6 Node Address and the
      SRv6 SID are present.

2.4.3.2.11.  Type 11: IPv6 Local and Remote addresses for SRv6 with
             optional SID

   The Type-11 Segment Sub-TLV encodes an adjacency local address, an
   adjacency remote address and an optional SID in the form of 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 IPv6 Address (16 octets)                //
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   //               Remote IPv6 Address  (16 octets)              //
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   //                SID (optional, 16 octets)                    //
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   where:





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   o  Type: 12 (to be assigned by IANA from the registry "SR Policy List
      Sub-TLVs" defined in this document).

   o  Length is 34 or 50.

   o  Flags: 1 octet of flags as defined in Section 2.4.3.2.12.

   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: 16 octet IPv6 address.

   The following applies to the Type-11 Segment sub-TLV:

   o  The Local IPv6 Node Address MUST be present.

   o  The Remote IPv6 Node Address MUST be present.

   o  The SID is optional and specifies an SRv6 SID in the form of 16
      octet IPv6 address.

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

   o  If length is 50, then the Local IPv6 Node Address, the Remote IPv6
      Node Address and the SRv6 SID are present.

2.4.3.2.12.  Segment Flags

   The Segment Types described above MAY contain following flags in the
   "Flags" field (codes to be assigned by IANA from the registry "SR
   Policy Segment Flags" defined in this document Section 8.6):

    0 1 2 3 4 5 6 7
   +-+-+-+-+-+-+-+-+
   |V|A|           |
   +-+-+-+-+-+-+-+-+

   where:

      V-Flag: This flag encodes the "Segment Verification" behavior.  It
      is used by SRPM as described in section 5 in
      [I-D.ietf-spring-segment-routing-policy].




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      A-Flag: This flag indicates the presence of SR Algorithm id in the
      "SR Algorithm" field applicable to various Segment Types.  SR
      Algorithm is used by SRPM as described in section 4 in
      [I-D.ietf-spring-segment-routing-policy].

      Unused bits in the Flag octet SHOULD be set to zero upon
      transmission and MUST be ignored upon receipt.

   The following applies to the Segment Flags:

   o  V-Flag is applicable to all Segment Types.

   o  A-Flag is applicable to Segment Types 3, 4 and 9.  If A-Flag
      appears with any other Segment Type, it MUST be ignored.

2.4.4.  Explicit NULL Label Policy Sub-TLV

   In order to steer an unlabeled IP packet into an SR policy, it is
   necessary to create a label stack for that packet, and to push one or
   more labels onto that stack.

   The Explicit NULL Label Policy sub-TLV is used to indicate whether an
   Explicit NULL Label [RFC3032] must be pushed on an unlabeled IP
   packet before any other labels.

   If an Explicit NULL Label Policy Sub-TLV is not present, the decision
   of whether to push an Explicit NULL label on a given packet is a
   matter of local policy.

   The contents of this sub-TLV are used by the SRPM as described in
   section 4.1 in [I-D.ietf-spring-segment-routing-policy].

   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    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     ENLP      |
   +-+-+-+-+-+-+-+-+

   Where:

      Type: TBD1 (to be assigned by IANA from the registry "BGP Tunnel
      Encapsulation Attribute sub-TLVs" defined in this document
      Section 8.3).

      Length: 3.




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      Flags: 1 octet of flags.  None are defined at this stage.  Flags
      SHOULD be set to zero 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.

      ENLP(Explicit NULL Label Policy): Indicates whether Explicit NULL
      labels are to be pushed on unlabeled IP packets that are being
      steered into a given SR policy.  This field has one of the
      following 4 values:

         1: Push an IPv4 Explicit NULL label on an unlabeled IPv4
         packet, but do not push an IPv6 Explicit NULL label on an
         unlabeled IPv6 packet.

         2: Push an IPv6 Explicit NULL label on an unlabeled IPv6
         packet, but do not push an IPv4 Explicit NULL label on an
         unlabeled IPv4 packet.

         3: Push an IPv4 Explicit NULL label on an unlabeled IPv4
         packet, and push an IPv6 Explicit NULL label on an unlabeled
         IPv6 packet.

         4: Do not push an Explicit NULL label.

      The policy signaled in this Sub-TLV MAY be overridden by local
      policy.

2.4.5.  Policy Priority Sub-TLV

   An operator MAY set the Policy Priority sub-TLV to indicate the order
   in which the SR policies are re-computed upon topological change.

   The Priority sub-TLV does not have any effect on the BGP bestpath
   selection or propagation procedures.  The contents of this sub-TLV
   are used by the SRPM as described in section 2.11 in
   [I-D.ietf-spring-segment-routing-policy].

   The Priority sub-TLV is optional and it MUST NOT appear more than
   once in the SR Policy TLV.  If the Priority sub-TLV appears more than
   once, the update is considered malformed and the "treat-as-withdraw"
   strategy of [RFC7606] is applied.

   The Priority sub-TLV has 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      |  Priority     |   RESERVED    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Where:

      Type: TBD2 (to be assigned by IANA from the registry "BGP Tunnel
      Encapsulation Attribute sub-TLVs" defined in this document
      Section 8.3).

      Length: 2.

      Priority: a 1-octet value.

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

2.4.6.  Policy Name Sub-TLV

   An operator MAY set the Policy Name sub-TLV to attach a symbolic name
   to the SR Policy candidate path.

   Usage of Policy Name sub-TLV is described in section 2 in
   [I-D.ietf-spring-segment-routing-policy].

   The Policy Name sub-TLV may exceed 255 bytes length due to long
   policy name.  Therefore a 2-octet length is required.  According to
   [I-D.ietf-idr-tunnel-encaps], the first bit of the sub-TLV codepoint
   defines the size of the length field.  Therefore, for the Policy Name
   sub-TLV a code point of 128 (or higher) is used.  See Section 8 for
   details of codepoints allocation.

   The Policy Name sub-TLV is optional and it MUST NOT appear more than
   once in the SR Policy TLV.  If the Policy Name sub-TLV appears more
   than once, the update is considered malformed and the "treat-as-
   withdraw" strategy of [RFC7606] is applied.

   The Policy Name sub-TLV 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                      |   RESERVED    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   //                        Policy Name                          //
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



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   Where:

      Type: TBD3 (to be assigned by IANA from the registry "BGP Tunnel
      Encapsulation Attribute sub-TLVs" defined in this document
      Section 8.3).

      Length: Variable.

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

      Policy Name: Symbolic name for the policy.  It SHOULD be a string
      of printable ASCII characters, without a NULL terminator.

3.  Extended Color Community

   The Color Extended Community as defined in
   [I-D.ietf-idr-tunnel-encaps] is used to steer traffic into a policy.

   When the Color Extended Community is used for the purpose of steering
   the traffic into an SR Policy, the RESERVED field (as defined in
   [I-D.ietf-idr-tunnel-encaps] is changed as follows:

                        1
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |C O|        RESERVED           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where CO bits are defined as the "Color-Only" bits.
   [I-D.ietf-spring-segment-routing-policy] defines the influence of
   these bits on the automated steering of BGP Payload traffic onto SR
   Policies.

4.  SR Policy Operations

   As described in this document, the consumer of an SR Policy NLRI is
   not the BGP process.  The BGP process is in charge of the origination
   and propagation of the SR Policy NLRI but its installation and use is
   outside the scope of BGP.  The details of SR Policy installation and
   use can be referred from [I-D.ietf-spring-segment-routing-policy].

4.1.  Configuration and Advertisement of SR Policies

   Typically, but not limited to, an SR Policy is configured into a
   controller.





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   Multiple SR Policy NLRIs may be present with the same <color,
   endpoint> tuple but with different content when these SR policies are
   intended to different head-ends.

   The distinguisher of each SR Policy NLRI prevents undesired BGP route
   selection among these SR Policy NLRIs and allow their propagation
   across route reflectors [RFC4456].

   Moreover, one or more route-target SHOULD be attached to the
   advertisement, where each route-target identifies one or more
   intended head-ends for the advertised SR policy.

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

4.2.  Reception of an SR Policy NLRI

   On reception of an SR Policy NLRI, a BGP speaker MUST determine if
   it's first acceptable, then it determines if it is usable.

4.2.1.  Acceptance of an SR Policy NLRI

   When a BGP speaker receives an SR Policy NLRI from a neighbor it has
   to determine if it's acceptable.  The following applies:

   o  The SR Policy NLRI MUST include a distinguisher, color and
      endpoint field which implies that the length of the NLRI MUST be
      either 12 or 24 octets (depending on the address family of the
      endpoint).

   o  The SR Policy update MUST have either the NO_ADVERTISE community
      or at least one route-target extended community in IPv4-address
      format.  If a router supporting this document receives an SR
      policy update with no route-target extended communities and no
      NO_ADVERTISE community, the update MUST NOT be sent to the SRPM.
      Furthermore, it SHOULD be considered to be malformed, and the
      "treat-as-withdraw" strategy of [RFC7606] is applied.

   o  The Tunnel Encapsulation Attribute MUST be attached to the BGP
      Update and MUST have a Tunnel Type TLV set to SR Policy (
      codepoint is 15, assigned by IANA (see Section 8) from the "BGP
      Tunnel Encapsulation Attribute Tunnel Types" registry).

   A router that receives an SR Policy update that is not valid
   according to these criteria MUST treat the update as malformed.  The



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   route MUST NOT be passed to the SRPM, and the "treat-as-withdraw"
   strategy of [RFC7606] is applied.

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

4.2.2.  Usable SR Policy NLRI

   If one or more route-targets are present, then at least one route-
   target MUST match one of the BGP Identifiers 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 any of
   the local BGP Identifiers, then, while the SR Policy NLRI is
   acceptable, it is not usable on the receiver node.  It has to be
   noted that if the receiver has been explicitly configured to do so,
   it MAY propagate the SR Policy NLRI to its neighbors as defined in
   Section 4.2.4.

   The SR Policy candidate paths encoded by the usable SR Policy NLRIs
   are sent to the SRPM.

4.2.3.  Passing a usable SR Policy NLRI to the SRPM

   Once BGP has determined that the SR Policy NLRI is usable, BGP passes
   the SR Policy candidate path to the SRPM.  Note that, along with the
   candidate path details, BGP also passes the originator information
   for breaking ties in the path-selection process as described in
   section 2.4 in [I-D.ietf-spring-segment-routing-policy].

   The SRPM applies the rules defined in section 2 in
   [I-D.ietf-spring-segment-routing-policy] to determine whether the SR
   Policy candidate path is valid and to select the best candidate path
   among the valid SR Policy candidate paths.

4.2.4.  Propagation of an SR Policy

   By default, a BGP node receiving an SR Policy NLRI MUST NOT propagate
   it to any EBGP neighbor.

   However, a node MAY be explicitly configured to advertise a received
   SR Policy NLRI to neighbors according to normal BGP rules (i.e., EBGP
   propagation by an ASBR or iBGP propagation by a Route-Reflector).





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

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

4.3.  Flowspec and SR Policies

   The SR 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 to
   steer them towards the next-hop as specified in the SR Policy SAFI
   NLRI.

5.  Contributors

   Arjun Sreekantiah
   Cisco Systems
   US

   Email: asreekan@cisco.com

   Acee Lindem
   Cisco Systems
   US

   Email: acee@cisco.com

   Siva Sivabalan
   Cisco Systems
   US

   Email: msiva@cisco.com

   Imtiyaz Mohammad
   Arista Networks
   India

   Email: imtiyaz@arista.com

   Gaurav Dawra
   Cisco Systems
   US

   Email: gdawra.ietf@gmail.com



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6.  Acknowledgments

   The authors of this document would like to thank Shyam Sethuram, John
   Scudder, Przemyslaw Krol, Alex Bogdanov, Nandan Saha and Ketan
   Talaulikar for their comments and review of this document.

7.  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
   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: 12

   o  Binding SID sub-TLV: 13

   o  Segment List sub-TLV: 128

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







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8.  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 following new registries:

   o  SR Policy List Sub-TLVs

   o  SR Policy Binding SID Flags

   o  SR Policy Segment Flags

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

   This document defines a new SAFI in the registry "Subsequent Address
   Family Identifiers (SAFI) Parameters" that has been assigned by IANA:

              Codepoint    Description          Reference
              -----------------------------------------------
                 73        SR Policy SAFI       This document

8.2.  Existing Registry: BGP Tunnel Encapsulation Attribute Tunnel Types

   This document defines a new Tunnel-Type in the registry "BGP Tunnel
   Encapsulation Attribute Tunnel Types" that has been assigned by IANA:

            Codepoint     Description            Reference
            --------------------------------------------------
               15        SR Policy Type          This document

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

   This document defines new sub-TLVs in the registry "BGP Tunnel
   Encapsulation Attribute sub-TLVs" to be assigned by IANA:










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          Codepoint       Description              Reference
          ------------------------------------------------------
          12              Preference sub-TLV       This document
          13              Binding SID sub-TLV      This document
          128             Segment List sub-TLV     This document
          TBD1            ENLP sub-TLV             This document
          TBD2            Priority sub-TLV         This document
          TBD3            Policy Name sub-TLV      This document

8.4.  New Registry: SR Policy List Sub-TLVs

   This document defines a new registry called "SR Policy List Sub-
   TLVs".  The allocation policy of this registry is "First Come First
   Served (FCFS)" according to [RFC8126].

   Following Sub-TLV codepoints are defined:

Value    Description                                  Reference
---------------------------------------------------------------------------------
   1     MPLS SID sub-TLV                            This document
   2     SRv6 SID sub-TLV                            This document
   3     IPv4 Node and SID sub-TLV                   This document
   4     IPv6 Node and SID for SR-MPLS 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 for remote and local pair  This document
         and SID for SR-MPLS sub-TLV
   8     IPv6 Local/Remote addresses and SID sub-TLV This document
   9     Weight sub-TLV                              This document
   10    IPv6 Node and SID for SRv6 sub-TLV          This document
   11    IPv6 Node, index for remote and local pair  This document
         and SID for SRv6 sub-TLV
   12    IPv6 Local/Remote addresses and SID for     This document
         SRv6 sub-TLV

8.5.  New Registry: SR Policy Binding SID Flags

   This document defines a new registry called "SR Policy Binding SID
   Flags".  The allocation policy of this registry is "First Come First
   Served (FCFS)" according to [RFC8126].

   Following Flags are defined:

Bit    Description                                  Reference
---------------------------------------------------------------------------------
   0     Drop Upon Invalid Flag (I-Flag)             This document
   1     Specified-BSID-Only Flag (S-Flag)           This document
 2-7     Unassigned



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8.6.  New Registry: SR Policy Segment Flags

   This document defines a new registry called "SR Policy Segment
   Flags".  The allocation policy of this registry is "First Come First
   Served (FCFS)" according to [RFC8126].

   Following Flags are defined:

Bit    Description                                  Reference
---------------------------------------------------------------------------------
   0     Segment Verification Flag (V-Flag)          This document
   1     SR Algorithm Flag (A-Flag)                  This document
 2-7     Unassigned

9.  Security Considerations

   TBD.

10.  References

10.1.  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-09
              (work in progress), February 2018.

   [I-D.ietf-pce-segment-routing]
              Sivabalan, S., Filsfils, C., Tantsura, J., Henderickx, W.,
              and J. Hardwick, "PCEP Extensions for Segment Routing",
              draft-ietf-pce-segment-routing-12 (work in progress), June
              2018.

   [I-D.ietf-spring-segment-routing]
              Filsfils, C., Previdi, S., Ginsberg, L., Decraene, B.,
              Litkowski, S., and R. Shakir, "Segment Routing
              Architecture", draft-ietf-spring-segment-routing-15 (work
              in progress), January 2018.

   [I-D.ietf-spring-segment-routing-policy]
              Filsfils, C., Sivabalan, S., daniel.voyer@bell.ca, d.,
              bogdanov@google.com, b., and P. Mattes, "Segment Routing
              Policy Architecture", draft-ietf-spring-segment-routing-
              policy-01 (work in progress), June 2018.







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   [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>.

   [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>.

   [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>.

   [RFC4360]  Sangli, S., Tappan, D., and Y. Rekhter, "BGP Extended
              Communities Attribute", RFC 4360, DOI 10.17487/RFC4360,
              February 2006, <https://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,
              <https://www.rfc-editor.org/info/rfc4760>.

   [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,
              <https://www.rfc-editor.org/info/rfc5575>.

   [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>.

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

10.2.  Informational References

   [I-D.filsfils-spring-sr-policy-considerations]
              Filsfils, C., Talaulikar, K., Krol, P., Horneffer, M., and
              P. Mattes, "SR Policy Implementation and Deployment
              Considerations", draft-filsfils-spring-sr-policy-
              considerations-01 (work in progress), June 2018.





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   [I-D.ietf-6man-segment-routing-header]
              Filsfils, C., Previdi, S., Leddy, J., Matsushima, S., and
              d. daniel.voyer@bell.ca, "IPv6 Segment Routing Header
              (SRH)", draft-ietf-6man-segment-routing-header-14 (work in
              progress), June 2018.

   [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.

   [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,
              <https://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,
              <https://www.rfc-editor.org/info/rfc7942>.

Authors' Addresses

   Stefano Previdi (editor)
   Individual
   IT

   Email: stefano@previdi.net


   Clarence Filsfils
   Cisco Systems, Inc.
   Brussels
   BE

   Email: cfilsfil@cisco.com


   Dhanendra Jain (editor)
   Cisco Systems, Inc.
   San Jose
   USA

   Email: dhjain@cisco.com






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   Paul Mattes
   Microsoft
   One Microsoft Way
   Redmond, WA  98052
   USA

   Email: pamattes@microsoft.com


   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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