Advertising Segment Routing Policies in BGP
draft-ietf-idr-segment-routing-te-policy-08

Document Type Active Internet-Draft (idr WG)
Last updated 2019-11-18
Replaces draft-previdi-idr-segment-routing-te-policy
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Network Working Group                                         S. Previdi
Internet-Draft                                                Individual
Intended status: Standards Track                             C. Filsfils
Expires: May 21, 2020                                 K. Talaulikar, Ed.
                                                           Cisco Systems
                                                               P. Mattes
                                                               Microsoft
                                                                E. Rosen
                                                        Juniper Networks
                                                                 D. Jain
                                                                  S. Lin
                                                                  Google
                                                       November 18, 2019

              Advertising Segment Routing Policies in BGP
              draft-ietf-idr-segment-routing-te-policy-08

Abstract

   This document defines a new BGP SAFI with a new NLRI in order to
   advertise a candidate path of a Segment Routing (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 SR Policy candidate paths.  New sub-TLVs for the Tunnel
   Encapsulation Attribute are defined for signaling information about
   these candidate paths.

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
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   Drafts is at https://datatracker.ietf.org/drafts/current/.

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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on May 21, 2020.

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Copyright Notice

   Copyright (c) 2019 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  . . . . . . . . . . . . . . . . . . .   8
       2.4.1.  Preference Sub-TLV  . . . . . . . . . . . . . . . . .   9
       2.4.2.  Binding SID Sub-TLV . . . . . . . . . . . . . . . . .   9
       2.4.3.  Segment List Sub-TLV  . . . . . . . . . . . . . . . .  11
       2.4.4.  Explicit NULL Label Policy Sub-TLV  . . . . . . . . .  24
       2.4.5.  Policy Priority Sub-TLV . . . . . . . . . . . . . . .  25
       2.4.6.  Policy Name Sub-TLV . . . . . . . . . . . . . . . . .  26
   3.  Color Extended Community  . . . . . . . . . . . . . . . . . .  27
   4.  SR Policy Operations  . . . . . . . . . . . . . . . . . . . .  27
     4.1.  Advertisement of SR Policies  . . . . . . . . . . . . . .  27
     4.2.  Reception of an SR Policy NLRI  . . . . . . . . . . . . .  28
       4.2.1.  Acceptance of an SR Policy NLRI . . . . . . . . . . .  28
       4.2.2.  Usable SR Policy NLRI . . . . . . . . . . . . . . . .  29
       4.2.3.  Passing a usable SR Policy NLRI to the SRPM . . . . .  29
       4.2.4.  Propagation of an SR Policy . . . . . . . . . . . . .  29
   5.  Error Handling  . . . . . . . . . . . . . . . . . . . . . . .  30
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  31
     6.1.  Existing Registry: Subsequent Address Family Identifiers
           (SAFI) Parameters . . . . . . . . . . . . . . . . . . . .  31
     6.2.  Existing Registry: BGP Tunnel Encapsulation Attribute
           Tunnel Types  . . . . . . . . . . . . . . . . . . . . . .  31
     6.3.  Existing Registry: BGP Tunnel Encapsulation Attribute
           sub-TLVs  . . . . . . . . . . . . . . . . . . . . . . . .  32
     6.4.  New Registry: SR Policy List Sub-TLVs . . . . . . . . . .  32
     6.5.  New Registry: SR Policy Binding SID Flags . . . . . . . .  32

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     6.6.  New Registry: SR Policy Segment Flags . . . . . . . . . .  33
     6.7.  New Registry: Color Extended Community Field  . . . . . .  33
     6.8.  Guidance for Designated Experts . . . . . . . . . . . . .  33
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  34
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  34
   9.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  34
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  35
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  35
     10.2.  Informational References . . . . . . . . . . . . . . . .  37
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  37

1.  Introduction

   Segment Routing (SR) [RFC8402] allows a headend node to steer a
   packet flow along any path.  Intermediate per-flow states are
   eliminated thanks to source routing.

   The headend node is said to steer a flow into a 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 IPv6 (known as SRv6) data plane instantiations of Segment
   Routing with their respective representations of segments as SR-MPLS
   SID and SRv6 SID as described in [RFC8402].

   [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 describes the functionality that resides in the BGP

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   process and, as appropriate, provides references for the
   functionality which is outside the scope of BGP (i.e. resides within
   SRPM on the headend node).

   This document specifies a way of representing SR Policy candidate
   paths in BGP UPDATE messages.  BGP can then be used to propagate the
   SR Policy candidate paths to the headend nodes in the network.  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
   Candidate Path, and the attributes encode the segment lists and other
   details of that SR Policy Candidate Path.

   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 be 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 AS.  To make this

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   possible, an attribute needs to be attached to the advertisement that
   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", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  SR Policy Encoding

2.1.  SR Policy SAFI and NLRI

   A new SAFI is defined: the SR Policy SAFI with codepoint 73.  The AFI
   used MUST be IPv4(1) or IPv6(2).

   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].  When AFI = 1 value MUST be 12 and when AFI = 2 value
      MUST be 24.

   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 candidate paths 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 as specified in
      [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.

   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,
   independent of the SR Policy AFI.  The length field of the next-hop

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   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 leverages the existing BGP propagation and
   bestpath selection rules.  Details of the procedures are described in
   Section 4.

   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 along with their respective originator information as described
   in section 2.4 of [I-D.ietf-spring-segment-routing-policy].

2.2.  SR Policy and Tunnel Encapsulation Attribute

   The content of the SR Policy is encoded in the Tunnel Encapsulation
   Attribute defined in [I-D.ietf-idr-tunnel-encaps] using a new Tunnel-
   Type called SR Policy Type with codepoint 15.

   The SR Policy Encoding structure is as follows:

   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:

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   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.

   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.

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.

   The Remote Endpoint and Color Sub-TLVs of the Tunnel Encapsulation
   Attribute 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 specifies the sub-TLVs defined for encoding the
   information about the SR Policy.

   Preference, Binding SID, Segment-List, Priority, Policy Name and
   Explicit NULL Label Policy are the new sub-TLVs of the BGP Tunnel
   Encapsulation Attribute [I-D.ietf-idr-tunnel-encaps] being defined in
   this section.

   Weight and Segment are sub-TLVs of the new Segment-List sub-TLV
   mentioned above.

   None of the sub-TLVs defined in the following sub-sections have any
   effect on the BGP bestpath selection or propagation procedures.
   These sub-TLVs are not used by BGP and are instead passed on to SRPM
   as SR Policy Candidate Path information for further processing
   described in [I-D.ietf-spring-segment-routing-policy] .

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2.4.1.  Preference Sub-TLV

   The Preference sub-TLV is used to carry the preference of the SR
   Policy candidate path.  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.

   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.

   o  RESERVED: 1 octet of reserved bits.  SHOULD be set to zero 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 used to signal the binding SID related
   information of the SR Policy candidate path.  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.

   The Binding SID 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    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              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 in the new
      registry "SR Policy Binding SID Flags" as described in
      Section 6.5:

    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].

      *  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 set to zero 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 is encoded in 4 octets using the
      format below.  TC, S, TTL (Total of 12 bits) are RESERVED and
      SHOULD be set to zero and MUST be ignored.

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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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          Label                        | TC  |S|       TTL     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      If length is 18 then the Binding SID contains a 16-octet SRv6 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.

   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:

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

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   o  sub-TLVs currently defined:

      *  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
   beyond the scope of BGP and performed by the 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.

   The Weight 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    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                              Weight                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:

   o  Type: 9.

   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.

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

2.4.3.2.  Segment Sub-TLVs

   A Segment sub-TLV describes a single segment in a segment list (i.e.,
   a single element of the explicit path).  One or more Segment sub-TLVs
   constitute an explicit path of the SR Policy.  The contents of these

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   sub-TLVs are used only by the SRPM as described in section 4 in
   [I-D.ietf-spring-segment-routing-policy].

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

   [I-D.ietf-spring-segment-routing-policy] defines several Segment
   Types:

 Type  A: SID only, in the form of MPLS Label
 Type  B: SID only, in the form of IPv6 address
 Type  C: IPv4 Node Address with optional SID
 Type  D: IPv6 Node Address with optional SID for SR MPLS
 Type  E: IPv4 Address and index with optional SID
 Type  F: IPv4 Local and Remote addresses with optional SID
 Type  G: IPv6 Address and index for local and remote pair with optional
          SID for SR MPLS
 Type  H: IPv6 Local and Remote addresses with optional SID for SR MPLS
 Type  I: IPv6 Node Address with optional SID for SRv6
 Type  J: IPv6 Address and index for local and remote pair with optional
          SID for SRv6
 Type  K: IPv6 Local and Remote addresses for SRv6

   The follow sub-sections specify the sub-TLV used for encoding each of
   these Segment Types.

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

   The Type A Segment Sub-TLV encodes a single SR-MPLS SID.  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.

   o  Length is 6.

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

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   o  RESERVED: 1 octet of reserved bits.  SHOULD be set to zero 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.

   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 B: SID only, in the form of IPv6 address

   The Type B Segment Sub-TLV encodes a single SRv6 SID.  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.

   o  Length is 18.

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

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

   o  SRv6 SID: 16 octets of IPv6 address.

2.4.3.2.3.  Type C: IPv4 Node Address with optional SID

   The Type C Segment Sub-TLV encodes an IPv4 node address, SR Algorithm
   and an optional SR-MPLS SID.  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)                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                SR-MPLS SID (optional, 4 octets)               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:

   o  Type: 3.

   o  Length is 10 when the SR-MPLS SID is present else is 6.

   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 [RFC8402], 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 set to zero on transmission and MUST
      be ignored on receipt.

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

   o  SR-MPLS SID: optional, 4 octet field containing label, TC, S and
      TTL as defined in Section 2.4.3.2.1.

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

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

   where:

   o  Type: 4

   o  Length is 22 when the SR-MPLS SID is present else is 18.

   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 [RFC8402], 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 set to zero on transmission and MUST
      be ignored on receipt.

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

   o  SR-MPLS SID: optional, 4 octet field containing label, TC, S and
      TTL as defined in Section 2.4.3.2.1.

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

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

   where:

   o  Type: 5.

   o  Length is 14 when the SR-MPLS SID is present else is 10.

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

   o  RESERVED: 1 octet of reserved bits.  SHOULD be set to zero 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  SR-MPLS SID: optional, 4 octet field containing label, TC, S and
      TTL as defined in Section 2.4.3.2.1.

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

   The Type F Segment Sub-TLV encodes an adjacency local address, an
   adjacency remote address and an optional SR-MPLS SID.  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 IPv4 Address (4 octets)                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                Remote IPv4 Address  (4 octets)                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                SR-MPLS SID (optional, 4 octets)               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:

   o  Type: 6.

   o  Length is 14 when the SR-MPLS SID is present else is 10.

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

   o  RESERVED: 1 octet of reserved bits.  SHOULD be set to zero 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  SR-MPLS SID: optional, 4 octet field containing label, TC, S and
      TTL as defined in Section 2.4.3.2.1.

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

   The Type G 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 SR-MPLS SID.  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 Interface ID (4 octets)                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   //                IPv6 Local Node Address (16 octets)          //
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Remote Interface ID (4 octets)                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   //                IPv6 Remote Node Address (16 octets)         //
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                SR-MPLS SID (optional, 4 octets)               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:

   o  Type: 7

   o  Length is 46 when the SR-MPLS SID is present else is 42.

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

   o  RESERVED: 1 octet of reserved bits.  SHOULD be set to zero 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].  The value MAY be set to zero when
      the local node address and interface identifiers are sufficient to
      describe the link.

   o  IPv6 Remote Node Address: a 16 octet IPv6 address.  The value MAY
      be set to zero when the local node address and interface
      identifiers are sufficient to describe the link.

   o  SR-MPLS SID: optional, 4 octet field containing label, TC, S and
      TTL as defined in Section 2.4.3.2.1.

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

   The Type H Segment Sub-TLV encodes an adjacency local address, an
   adjacency remote address and an optional SR-MPLS SID.  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)              //
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                SR-MPLS SID (optional, 4 octets)               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:

   o  Type: 8

   o  Length is 38 when the SR-MPLS SID is present else is 34.

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

   o  RESERVED: 1 octet of reserved bits.  SHOULD be set to zero 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  SR-MPLS SID: optional, 4 octet field containing label, TC, S and
      TTL as defined in Section 2.4.3.2.1.

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

   The Type I Segment Sub-TLV encodes an IPv6 node address, SR Algorithm
   and an optional SRv6 SID.  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     | SR Algorithm  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   //                 IPv6 Node Address (16 octets)               //
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   //                    SRv6 SID (optional, 16 octets)           //
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:

   o  Type: 10

   o  Length is 34 when the SRv6 SID is present else is 18.

   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 [RFC8402], 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 set to zero on transmission and MUST
      be ignored on receipt.

   o  IPv6 Node Address: a 16 octet IPv6 address.

   o  SRv6 SID: optional, 16 octet IPv6 address.

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

   The Type J 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 SRv6 SID.  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 Interface ID (4 octets)                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   //                IPv6 Local Node Address (16 octets)          //
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Remote Interface ID (4 octets)                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   //                IPv6 Remote Node Address (16 octets)         //
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   //                SRv6 SID (optional, 16 octets)               //
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:

   o  Type: 11.

   o  Length is 58 when the SRv6 SID is present else is 42.

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

   o  RESERVED: 1 octet of reserved bits.  SHOULD be set to zero 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].  The value MAY be set to zero when
      the local node address and interface identifiers are sufficient to
      describe the link.

   o  IPv6 Remote Node Address: a 16 octet IPv6 address.  The value MAY
      be set to zero when the local node address and interface
      identifiers are sufficient to describe the link.

   o  SRv6 SID: optional, 16 octet IPv6 address.

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

   The Type K Segment Sub-TLV encodes an adjacency local address, an
   adjacency remote address and an optional SRv6 SID.  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)              //
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   //                SRv6 SID (optional, 16 octets)               //
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   where:

   o  Type: 12 .

   o  Length is 50 when the SRv6 SID is present else is 34.

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

   o  RESERVED: 1 octet of reserved bits.  SHOULD be set to zero 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  SRv6 SID: optional, 16 octet IPv6 address.

2.4.3.2.12.  Segment Flags

   The Segment Types sub-TLVs described above MAY contain following
   flags in the "Flags" field defined in Section 6.6:

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

   where:

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      V-Flag: This flag is used by SRPM for the purpose of "SID
      verification" as described in Section 5.1 in
      [I-D.ietf-spring-segment-routing-policy].

      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 (ENLP) 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 ENLP Sub-TLV is not present, the decision of whether to push an
   Explicit NULL label on a given packet is a matter of local
   configuration.

   The ENLP sub-TLV is optional and it MUST NOT appear more than once in
   the SR 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:

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      Type: 14.

      Length: 3.

      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 set to zero 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 values:

         0: Reserved.

         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.

         5 - 255: Reserved.

      The ENLP reserved values may be used for future extensions and
      implementations SHOULD ignore the ENLP Sub-TLV with these values.
      The behavior signaled in this Sub-TLV MAY be overridden by local
      configuration.  The section 4.1 of
      [I-D.ietf-spring-segment-routing-policy] draft describes the
      behavior on the headend for handling of explicit null label.

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 contents of this sub-TLV are used by the SRPM as described in
   section 2.11 in [I-D.ietf-spring-segment-routing-policy].

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   The Priority sub-TLV is optional and it MUST NOT appear more than
   once in the SR Policy TLV.

   The Priority 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      |  Priority     |   RESERVED    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Where:

      Type: 15

      Length: 2.

      Priority: a 1-octet value.

      RESERVED: 1 octet of reserved bits.  SHOULD be set to zero 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.

   The Policy Name sub-TLV is optional and it MUST NOT appear more than
   once in the SR Policy TLV.

   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: 129.

      Length: Variable.

      RESERVED: 1 octet of reserved bits.  SHOULD be set to zero 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.  Color Extended 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, two bits from the RESERVED field (as
   defined in [I-D.ietf-idr-tunnel-encaps]) is used 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 are specified in [I-D.ietf-spring-segment-routing-policy].

4.1.  Advertisement of SR Policies

   Typically, but not limited to, an SR Policy is computed by a
   controller or a path computation engine (PCE) and originated by a BGP
   speaker on its behalf.

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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 for different head-ends.

   The distinguisher of each SR Policy NLRI prevents undesired BGP route
   selection among these SR Policy NLRIs and allows 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 first determines if
   it is acceptable and then 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 MUST
   first determine if it's acceptable.  The following rules apply:

   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 specification receives an SR
      policy update with no route-target extended communities and no
      NO_ADVERTISE community, the update MUST be considered as
      malformed.

   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).

   A router that receives an SR Policy update that is not valid
   according to these criteria MUST treat the update as malformed and
   the SR Policy candidate path MUST NOT be passed to the SRPM.

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4.2.2.  Usable SR Policy NLRI

   A SR Policy update that has been determined to be acceptable is
   further evaluated for its usability by the receiving node.

   An SR Policy NLRI update without any route-target extended community
   but having the NO_ADVERTISE community is considered usable.

   If one or more route-targets are present, then at least one route-
   target MUST match the BGP Identifier of the receiver 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 none matches the local
   BGP Identifier, then, while the SR Policy NLRI is acceptable, it is
   not usable on the receiver node.

4.2.3.  Passing a usable SR Policy NLRI to the SRPM

   Once BGP on the receiving node has determined that the SR Policy NLRI
   is usable, it 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 candidate 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 ones for a given SR Policy.

4.2.4.  Propagation of an SR Policy

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

   SR Policy NLRIs that have the NO_ADVERTISE community attached to them
   MUST NOT be propagated.

   By default, a BGP node receiving an SR Policy NLRI MUST NOT propagate
   it to any EBGP neighbor.  An implementation MAY provide an explicit
   configuration to override this and enable propagation of acceptable
   SR Policy NLRIs to specific EBGP neighbors.

   A BGP node advertises a received SR Policy NLRI to its IBGP neighbors
   according to normal IBGP propagation rules.

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   By default, a BGP node receiving an SR Policy NLRI SHOULD NOT remove
   route-target extended community before propagation.  An
   implementation MAY provide support for configuration to filter and/or
   remove route-target extended community before propagation.

5.  Error Handling

   This section describes the error handling actions, as described in
   [RFC7606], that are to be performed for handling of BGP update
   messages for BGP SR Policy SAFI.

   A BGP Speaker MUST perform the following syntactic validation of the
   SR Policy NLRI to determine if it is malformed.  This includes the
   validation of length of each NLRI and the total length of the
   MP_REACH_NLRI and MP_UNREACH_NLRI attributes.

   When the error determined allows for the router to skip the malformed
   NLRI(s) and continue processing of the rest of the update message,
   then it MUST handle such malformed NLRIs as 'Treat-as-withdraw'.  In
   other cases, where the error in the NLRI encoding results in the
   inability to process the BGP update message (e.g. length related
   encoding errors), then the router SHOULD handle such malformed NLRIs
   as 'AFI/SAFI disable' when other AFI/SAFI besides SR Policy are being
   advertised over the same session.  Alternately, the router MUST
   perform 'session reset' when the session is only being used for SR
   Policy or when it 'AFI/SAFI disable' action is not possible.

   The validation of the TLVs/sub-TLVs introduced in this document and
   defined in their respective sub-sections of Section 2.4 MUST be
   performed to determine if they are malformed or invalid.  The
   validation of the Tunnel Encapsulation Attribute itself and the other
   TLVs/sub-TLVs specified in [I-D.ietf-idr-tunnel-encaps] MUST be done
   as described in that document.  In case of any error detected, either
   at the attribute or its TLV/sub-TLV level, the "treat-as-withdraw"
   strategy of [RFC7606] MUST be applied.  This is because an SR Policy
   update without a valid Tunnel Encapsulation Attribute (comprising of
   all valid TLVs/sub-TLVs) is not usable.

   The validation of the individual fields of the TLVs/sub-TLVs defined
   in Section 2.4 are beyond the scope of BGP as they are handled by the
   SRPM as described in the individual TLV/sub-TLV sub-sections.  A BGP
   implementation MUST NOT perform semantic verification of such fields
   nor consider the SR Policy update to be invalid or not acceptable/
   usable on the basis of such a validation.

   An implementation SHOULD log an error for any errors found during the
   above validation for further analysis.

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6.  IANA Considerations

   This document requests codepoint allocations for new TLVs/sub-TLVs in
   following existing registries:

   o  Subsequent Address Family Identifiers (SAFI) Parameters registry

   o  BGP Tunnel Encapsulation Attribute Tunnel Types registry under the
      BGP Parameters registry

   o  BGP Tunnel Encapsulation Attribute sub-TLVs registry under the BGP
      Parameters registry

   This document also requests creation of the following new registries:

   o  SR Policy List Sub-TLVs under the BGP Parameters registry

   o  SR Policy Binding SID Flags under the BGP Parameters registry

   o  SR Policy Segment Flags under the BGP Parameters registry

   o  Color Extended Community Field under the BGP Extended Communities
      registry

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" that has been assigned a
   codepoint by IANA as follows:

              Codepoint    Description          Reference
              -----------------------------------------------
                 73        SR 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" that has been assigned a
   codepoint by IANA as follows:

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

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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" that has been assigned codepoints
   by IANA as follows:

          Codepoint       Description              Reference
          ------------------------------------------------------
          12              Preference sub-TLV       This document
          13              Binding SID sub-TLV      This document
          128             Segment List sub-TLV     This document
          14              ENLP sub-TLV             This document
          15              Priority sub-TLV         This document
          129             Policy Name sub-TLV      This document

6.4.  New Registry: SR Policy List Sub-TLVs

   This document requests creation of a new registry called "SR Policy
   List Sub-TLVs".  The allocation policy of this registry is
   "Specification Required" according to [RFC8126].

   Following initial Sub-TLV codepoints are assigned by this document:

Value   Description                                          Reference
------------------------------------------------------------------------
  1    Type A MPLS SID sub-TLV                             This document
  2    Type B SRv6 SID sub-TLV                             This document
  3    Type C IPv4 Node and SID sub-TLV                    This document
  4    Type D IPv6 Node and SID for SR-MPLS sub-TLV        This document
  5    Type E IPv4 Node, index and SID sub-TLV             This document
  6    Type F IPv4 Local/Remote addresses and SID sub-TLV  This document
  7    Type G IPv6 Node, index for remote and local pair   This document
       and SID for SR-MPLS sub-TLV
  8    Type H IPv6 Local/Remote addresses and SID sub-TLV  This document
  9    Weight sub-TLV                                      This document
 10    Type I IPv6 Node and SID for SRv6 sub-TLV           This document
 11    Type J IPv6 Node, index for remote and local pair   This document
       and SID for SRv6 sub-TLV
 12    Type K IPv6 Local/Remote addresses and SID for      This document
       SRv6 sub-TLV

6.5.  New Registry: SR Policy Binding SID Flags

   This document requests creation of a new registry called "SR Policy
   Binding SID Flags".  The allocation policy of this registry is
   "Specification Required" according to [RFC8126].

   Following flags are defined:

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

6.6.  New Registry: SR Policy Segment Flags

   This document requests creation of a new registry called "SR Policy
   Segment Flags".  The allocation policy of this registry is
   "Specification Required" 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

6.7.  New Registry: Color Extended Community Field

   This document requests creation of a new registry called "Color
   Extended Community Field".  The allocation policy of this registry is
   "Specification Required" according to [RFC8126].

   Following bits are defined in this 2 octet field:

      Bit     Description                                Reference
    ------------------------------------------------------------------
      0-1     Color-only bits                            This document
     2-15     Unassigned

6.8.  Guidance for Designated Experts

   In all cases of review by the Designated Expert (DE) described here,
   the DE is expected to ascertain the existence of suitable
   documentation (a specification) as described in [RFC8126].  The DE is
   also expected to check the clarity of purpose and use of the
   requested code points.  Additionally, the DE must verify that any
   request for one of these code points has been made available for
   review and comment within the IETF: the DE will post the request to
   the IDR Working Group mailing list (or a successor mailing list
   designated by the IESG).  If the request comes from within the IETF,
   it should be documented in an Internet-Draft.  Lastly, the DE must
   ensure that any other request for a code point does not conflict with
   work that is active or already published within the IETF.

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7.  Security Considerations

   The security mechanisms of the base BGP security model apply to the
   extensions described in this document as well.  See the Security
   Considerations section of [RFC4271] for a discussion of BGP security.
   Also refer to [RFC4272] and [RFC6952] for analysis of security issues
   for BGP.

   The BGP SR Policy extensions specified in this document enable
   traffic engineering and service programming use-cases within the SR
   domain as described in [I-D.ietf-spring-segment-routing-policy] . SR
   operates within a trusted SR domain [RFC8402] and its security
   considerations also apply to BGP sessions when carrying SR Policy
   information.  The SR Policies distributed by BGP are expected to be
   used entirely within this trusted SR domain i.e. within a single AS
   or between multiple AS/domains within a single provider network.
   Therefore, precaution is necessary to ensure that the SR Policy
   information advertised via BGP sessions is limited to nodes in a
   secure manner within this trusted SR domain.  BGP peering sessions
   for address-families other than SR Policy SAFI may be setup to
   routers outside the SR domain.  The isolation of BGP SR Policy SAFI
   peering sessions may be used to ensure that the SR Policy information
   is not advertised by accident or error to an EBGP peering session
   outside the SR domain.

   Additionally, it may be considered that the export of SR Policy
   information as described in this document constitutes a risk to
   confidentiality of mission-critical or commercially sensitive
   information about the network (more specifically endpoint/node
   addresses, SR SIDs and the SR Policies deployed).  BGP peerings are
   not automatic and require configuration; thus, it is the
   responsibility of the network operator to ensure that only trusted
   nodes (that include both routers and controller applications) within
   the SR domain are configured to receive such information.

8.  Acknowledgments

   The authors of this document would like to thank Shyam Sethuram, John
   Scudder, Przemyslaw Krol, Alex Bogdanov, Nandan Saha, Bruno Decraene,
   Gurusiddesh Nidasesi, Kausik Majumdar, Zafar Ali, Swadesh Agarwal,
   Jakob Heitz and Viral Patel for their comments and review of this
   document.

9.  Contributors

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   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

10.  References

10.1.  Normative References

   [I-D.ietf-idr-tunnel-encaps]
              Patel, K., Velde, G., and S. Ramachandra, "The BGP Tunnel
              Encapsulation Attribute", draft-ietf-idr-tunnel-encaps-14
              (work in progress), September 2019.

   [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-16 (work in progress),
              March 2019.

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   [I-D.ietf-spring-segment-routing-policy]
              Filsfils, C., Sivabalan, S., Voyer, D., Bogdanov, A., and
              P. Mattes, "Segment Routing Policy Architecture", draft-
              ietf-spring-segment-routing-policy-03 (work in progress),
              May 2019.

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

   [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>.

   [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>.

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

   [RFC8402]  Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
              Decraene, B., Litkowski, S., and R. Shakir, "Segment
              Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
              July 2018, <https://www.rfc-editor.org/info/rfc8402>.

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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-04 (work in progress), October 2019.

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

   [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>.

   [RFC6952]  Jethanandani, M., Patel, K., and L. Zheng, "Analysis of
              BGP, LDP, PCEP, and MSDP Issues According to the Keying
              and Authentication for Routing Protocols (KARP) Design
              Guide", RFC 6952, DOI 10.17487/RFC6952, May 2013,
              <https://www.rfc-editor.org/info/rfc6952>.

Authors' Addresses

   Stefano Previdi
   Individual
   IT

   Email: stefano@previdi.net

   Clarence Filsfils
   Cisco Systems
   Brussels
   BE

   Email: cfilsfil@cisco.com

   Ketan Talaulikar (editor)
   Cisco Systems
   India

   Email: ketant@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

   Dhanendra Jain
   Google

   Email: dhanendra.ietf@gmail.com

   Steven Lin
   Google

   Email: stevenlin@google.com

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