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PCE                                                         S. Sivabalan
Internet-Draft                                               C. Filsfils
Intended status: Standards Track                     Cisco Systems, Inc.
Expires: May 24, 2018                                        J. Tantsura
                                                              Individual
                                                           W. Henderickx
                                                                   Nokia
                                                             J. Hardwick
                                                     Metaswitch Networks
                                                       November 20, 2017


                  PCEP Extensions for Segment Routing
                   draft-ietf-pce-segment-routing-11

Abstract

   Segment Routing (SR) enables any head-end node to select any path
   without relying on a hop-by-hop signaling technique (e.g., LDP or
   RSVP-TE).  It depends only on "segments" that are advertised by Link-
   State Interior Gateway Protocols (IGPs).  A Segment Routed Path can
   be derived from a variety of mechanisms, including an IGP Shortest
   Path Tree (SPT), explicit configuration, or a Path Computation
   Element (PCE).  This document specifies extensions to the Path
   Computation Element Protocol (PCEP) that allow a stateful PCE to
   compute and initiate Traffic Engineering (TE) paths, as well as a PCC
   to request a path subject to certain constraint(s) and optimization
   criteria in SR networks.

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 [RFC2119].

Status of This Memo

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

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

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




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

   This Internet-Draft will expire on May 24, 2018.

Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Overview of PCEP Operation in SR Networks . . . . . . . . . .   5
   4.  SR-Specific PCEP Message Extensions . . . . . . . . . . . . .   6
   5.  Object Formats  . . . . . . . . . . . . . . . . . . . . . . .   7
     5.1.  The OPEN Object . . . . . . . . . . . . . . . . . . . . .   7
       5.1.1.  The SR PCE Capability sub-TLV . . . . . . . . . . . .   7
       5.1.2.  Exchanging the SR PCE Capability  . . . . . . . . . .   8
     5.2.  The RP/SRP Object . . . . . . . . . . . . . . . . . . . .   9
     5.3.  ERO Object  . . . . . . . . . . . . . . . . . . . . . . .   9
       5.3.1.  SR-ERO Subobject  . . . . . . . . . . . . . . . . . .  10
       5.3.2.  NAI Associated with SID . . . . . . . . . . . . . . .  12
       5.3.3.  ERO Processing  . . . . . . . . . . . . . . . . . . .  13
     5.4.  RRO Object  . . . . . . . . . . . . . . . . . . . . . . .  14
       5.4.1.  RRO Processing  . . . . . . . . . . . . . . . . . . .  14
     5.5.  METRIC Object . . . . . . . . . . . . . . . . . . . . . .  15
   6.  Backward Compatibility  . . . . . . . . . . . . . . . . . . .  15
   7.  Management Considerations . . . . . . . . . . . . . . . . . .  16
     7.1.  Policy  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     7.2.  The PCEP Data Model . . . . . . . . . . . . . . . . . . .  16
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  16
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
     9.1.  PCEP Objects  . . . . . . . . . . . . . . . . . . . . . .  16
     9.2.  PCEP-Error Object . . . . . . . . . . . . . . . . . . . .  17
     9.3.  PCEP TLV Type Indicators  . . . . . . . . . . . . . . . .  18
     9.4.  New Path Setup Type . . . . . . . . . . . . . . . . . . .  18



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     9.5.  New Metric Type . . . . . . . . . . . . . . . . . . . . .  18
   10. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  18
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  18
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  19
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  19
     12.2.  Informative References . . . . . . . . . . . . . . . . .  20
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  21

1.  Introduction

   SR technology leverages the source routing and tunneling paradigms.
   A source node can choose a path without relying on hop-by-hop
   signaling protocols such as LDP or RSVP-TE.  Each path is specified
   as a set of "segments" advertised by link-state routing protocols
   (IS-IS or OSPF).  [I-D.ietf-spring-segment-routing] provides an
   introduction to SR architecture.  The corresponding IS-IS and OSPF
   extensions are specified in
   [I-D.ietf-isis-segment-routing-extensions] and
   [I-D.ietf-ospf-segment-routing-extensions], respectively.  SR
   architecture defines a "segment" as a piece of information advertised
   by a link-state routing protocols, e.g. an IGP prefix or an IGP
   adjacency.  Several types of segments are defined.  A Node segment
   represents an ECMP-aware shortest-path computed by IGP to a specific
   node, and is always global within SR/IGP domain.  An Adjacency
   Segment represents a unidirectional adjacency.  An Adjacency Segment
   is local to the node which advertises it.  Both Node segments and
   Adjacency segments can be used for SR Traffic Engineering (SR-TE).

   The SR architecture can be applied to the MPLS forwarding plane
   without any change, in which case an SR path corresponds to an MPLS
   Label Switching Path (LSP).  This document is relevant to the MPLS
   forwarding plane only.  In this document, "Node-SID" and "Adjacency-
   SID" denote Node Segment Identifier and Adjacency Segment Identifier
   respectively.

   A Segment Routed path (SR path) can be derived from an IGP Shortest
   Path Tree (SPT).  SR-TE paths may not follow an IGP SPT.  Such paths
   may be chosen by a suitable network planning tool and provisioned on
   the ingress node of the SR-TE path.

   [RFC5440] describes the Path Computation Element Protocol (PCEP) for
   communication between a Path Computation Client (PCC) and a Path
   Computation Element (PCE) or between a pair of PCEs.  A PCE, or a PCC
   operating as a PCE (in hierarchical PCE environment), computes paths
   for MPLS Traffic Engineering LSPs (MPLS-TE LSPs) based on various
   constraints and optimization criteria.  [RFC8231] specifies
   extensions to PCEP that allow a stateful PCE to compute and recommend
   network paths in compliance with [RFC4657] and defines objects and



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   TLVs for MPLS-TE LSPs.  Stateful PCEP extensions provide
   synchronization of LSP state between a PCC and a PCE or between a
   pair of PCEs, delegation of LSP control, reporting of LSP state from
   a PCC to a PCE, controlling the setup and path routing of an LSP from
   a PCE to a PCC.  Stateful PCEP extensions are intended for an
   operational model in which LSPs are configured on the PCC, and
   control over them is delegated to the PCE.

   A mechanism to dynamically initiate LSPs on a PCC based on the
   requests from a stateful PCE or a controller using stateful PCE is
   specified in [I-D.ietf-pce-pce-initiated-lsp].  This mechanism is
   useful in Software Defined Networking (SDN) applications, such as on-
   demand engineering, or bandwidth calendaring.

   It is possible to use a stateful PCE for computing one or more SR-TE
   paths taking into account various constraints and objective
   functions.  Once a path is chosen, the stateful PCE can initiate an
   SR-TE path on a PCC using PCEP extensions specified in
   [I-D.ietf-pce-pce-initiated-lsp] using the SR specific PCEP
   extensions specified in this document.  Additionally, using
   procedures described in this document, a PCC can request an SR path
   from either stateful or a stateless PCE.  This specification relies
   on the procedures specified in [I-D.ietf-pce-lsp-setup-type].

2.  Terminology

   The following terminologies are used in this document:

   ERO:  Explicit Route Object

   IGP:  Interior Gateway Protocol

   IS-IS:  Intermediate System to Intermediate System

   LSR:  Label Switching Router

   MSD:  Maximum SID Depth

   NAI:  Node or Adjacency Identifier

   OSPF:  Open Shortest Path First

   PCC:  Path Computation Client

   PCE:  Path Computation Element

   PCEP:  Path Computation Element Protocol




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   RRO:  Record Route Object

   SID:  Segment Identifier

   SR:  Segment Routing

   SR-TE:  Segment Routed Traffic Engineering

   TED:  Traffic Engineering Database

3.  Overview of PCEP Operation in SR Networks

   In SR networks, an ingress node of an SR path appends all outgoing
   packets with an SR header consisting of a list of SIDs (or MPLS
   labels in the context of this document).  The header has all
   necessary information to guide the packets from the ingress node to
   the egress node of the path, and hence there is no need for any
   signaling protocol.

   In a PCEP session, LSP information is carried in the Explicit Route
   Object (ERO), which consists of a sequence of subobjects.  Various
   types of ERO subobjects have been specified in [RFC3209], [RFC3473],
   and [RFC3477].  In SR networks, an ingress node of an SR path appends
   all outgoing packets with an SR header consisting of a list of SIDs
   (or MPLS labels in the context of this document).  SR-TE LSPs
   computed by a PCE can be represented in one of the following forms:

   o  An ordered set of IP address(es) representing network nodes/links:
      In this case, the PCC needs to convert the IP address(es) into the
      corresponding MPLS labels by consulting its Traffic Engineering
      Database (TED).

   o  An ordered set of SID(s).

   o  An ordered set of both MPLS label(s) and IP address(es): In this
      case, the PCC needs to convert the IP address(es) into the
      corresponding SID(s) by consulting its TED.

   This document defines a new ERO subobject denoted by "SR-ERO
   subobject" capable of carrying a SID as well as the identity of the
   node/adjacency represented by the SID.  SR-capable PCEP speakers
   should be able to generate and/or process such ERO subobject.  An ERO
   containing SR-ERO subobjects can be included in the PCEP Path
   Computation Reply (PCRep) message defined in [RFC5440], the PCEP LSP
   Initiate Request message (PCInitiate) defined in
   [I-D.ietf-pce-pce-initiated-lsp], as well as in the PCEP LSP Update
   Request (PCUpd) and PCEP LSP State Report (PCRpt) messages defined in
   [RFC8231].



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   When a PCEP session between a PCC and a PCE is established, both PCEP
   speakers exchange their capabilites to indicate their ability to
   support SR-specific functionality.

   An PCE can update an LSP that is initially established via RSVP-TE
   signaling to use an SR-TE path, by sending a PCUpd to the PCC that
   delegated the LSP to it ([RFC8231]).  Similarly, an LSP initially
   created with an SR-TE path can be updated to use RSVP-TE signaling,
   if necessary.  This capability is useful when a network is migrated
   from RSVP-TE to SR-TE technology.

   A PCC MAY include an RRO object containing the recorded LSP in PCReq
   and PCRpt messages as specified in [RFC5440] and [RFC8231],
   respectively.  This document defines a new RRO subobject for SR
   networks.  The methods used by a PCC to record the SR-TE LSP are
   outside the scope of this document.

   In summary, this document:

   o  Defines a new ERO subobject, a new RRO subobject and new PCEP
      error codes.

   o  Specifies how two PCEP speakers can establish a PCEP session that
      can carry information about SR-TE paths.

   o  Specifies processing rules for the ERO subobject.

   o  Defines a new path setup type to be used in the PATH_SETUP_TYPE
      and PATH_SETUP_TYPE_CAPABILITY TLVs
      ([I-D.ietf-pce-lsp-setup-type]).

   o  Defines a new sub-TLV for the PATH_SETUP_TYPE_CAPABILITY TLV.

   The extensions specified in this document complement the existing
   PCEP specifications to support SR-TE paths.  As such, the PCEP
   messages (e.g., Path Computation Request, Path Computation Reply,
   Path Computation Report, Path Computation Update, Path Computation
   Initiate, etc.,) MUST be formatted according to [RFC5440], [RFC8231],
   [I-D.ietf-pce-pce-initiated-lsp], and any other applicable PCEP
   specifications.

4.  SR-Specific PCEP Message Extensions

   As defined in [RFC5440], a PCEP message consists of a common header
   followed by a variable length body made up of mandatory and/or
   optional objects.  This document does not require any changes in the
   format of the PCReq and PCRep messages specified in [RFC5440],




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   PCInitiate message specified in [I-D.ietf-pce-pce-initiated-lsp], and
   PCRpt and PCUpd messages specified in [RFC8231].

5.  Object Formats

5.1.  The OPEN Object

5.1.1.  The SR PCE Capability sub-TLV

   This document defines a new Path Setup Type (PST) for SR, as follows:

   o  PST = 1: Path is setup using Segment Routing Traffic Engineering.

   A PCEP speaker SHOULD indicate its support of the function described
   in this document by sending a PATH-SETUP-TYPE-CAPABILITY TLV in the
   OPEN object with this new PST included in the PST list.

   This document also defines the SR-PCE-CAPABILITY sub-TLV.  PCEP
   speakers use this sub-TLV to exchange information about their SR
   capability.  If a PCEP speaker includes PST=1 in the PST List of the
   PATH-SETUP-TYPE-CAPABILITY TLV then it MUST also include the SR-PCE-
   CAPABILITY sub-TLV inside the PATH-SETUP-TYPE-CAPABILITY TLV.

   The format of the SR-PCE-CAPABILITY sub-TLV is shown in the following
   figure:

       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=26            |            Length=4           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         Reserved              |     Flags   |L|      MSD      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                Figure 1: SR-PCE-CAPABILITY sub-TLV format

   The code point for the TLV type is 26.  The TLV length is 4 octets.

   The 32-bit value is formatted as follows.  The "Maximum SID Depth" (1
   octet) field (MSD) specifies the maximum number of SIDs (MPLS label
   stack depth in the context of this document) that a PCC is capable of
   imposing on a packet.  The "Reserved" (2 octets) field is unused, and
   MUST be set to zero on transmission and ignored on reception.  The
   "Flags" field is 1 octect long, and this document defines the
   following flag:





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   o  L-flag: A PCC sets this flag to 1 to indicate that it does not
      impose any limit on the MSD.

5.1.2.  Exchanging the SR PCE Capability

   A PCC indicates that it is capable of supporting the head-end
   functions for SR-TE LSP by including the SR-PCE-CAPABILITY sub-TLV in
   the Open message that it sends to a PCE.  A PCE indicates that it is
   capable of computing SR-TE paths by including the SR-PCE-CAPABILITY
   sub-TLV in the Open message that it sends to a PCC.

   If a PCEP speaker receives a PATH-SETUP-TYPE-CAPABILITY TLV with a
   PST list containing PST=1, but the SR-PCE-CAPABILITY sub-TLV is
   absent, then the PCEP speaker MUST send a PCErr message with Error-
   Type 10 (Reception of an invalid object) and Error-Value TBD1 (to be
   assigned by IANA) (Missing PCE-SR-CAPABILITY sub-TLV) and MUST then
   close the PCEP session.  If a PCEP speaker receives a PATH-SETUP-
   TYPE-CAPABILITY TLV with a SR-PCE-CAPABILITY sub-TLV, but the PST
   list does not contain PST=1, then the PCEP speaker MUST ignore the
   SR-PCE-CAPABILITY sub-TLV.

   The number of SIDs that can be imposed on a packet depends on the
   PCC's data plane's capability.  If a PCC sets the L flag to 1 then
   the MSD is not used and MUST be set to zero.  If a PCE receives an
   SR-PCE-CAPABILITY sub-TLV with the L flag set to 1 then it MUST
   ignore the MSD field and MUST assume that the sender can impose a SID
   stack of any depth.  If a PCC sets the L flag to zero, then it sets
   the MSD field to the maximum number of SIDs that it can impose on a
   packet.  If a PCE receives an SR-PCE-CAPABILITY sub-TLV with the L
   flag and MSD both set to zero then it MUST assume that the PCC is not
   capable of imposing a SID stack of any depth and hence is not SR-TE
   capable, unless it learns a non-zero MSD for the PCC through some
   other means.

   Note that the MSD value exchanged via the SR-PCE-CAPABILITY sub-TLV
   indicates the SID/label imposition limit for the PCC node.  However,
   if a PCE learns the MSD value of a PCC node via different means, e.g
   routing protocols, as specified in:
   [I-D.ietf-isis-segment-routing-msd];
   [I-D.ietf-ospf-segment-routing-msd];
   [I-D.ietf-idr-bgp-ls-segment-routing-msd], then it ignores the MSD
   value in the SR-PCE-CAPABILITY sub-TLV.  Furthermore, whenever a PCE
   learns the MSD for a link via different means, it MUST use that value
   for that link regardless of the MSD value exchanged in the SR-PCE-
   CAPABILITY sub-TLV.

   Once an SR-capable PCEP session is established with a non-zero MSD
   value, the corresponding PCE MUST NOT send SR-TE paths with a number



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   of SIDs exceeding that MSD value.  If a PCC needs to modify the MSD
   value, it MUST close the PCEP session and re-establish it with the
   new MSD value.  If a PCEP session is established with a non-zero MSD
   value, and the PCC receives an SR-TE path containing more SIDs than
   specified in the MSD value, the PCC MUST send a PCErr message with
   Error-Type 10 (Reception of an invalid object) and Error-Value 3
   (Unsupported number of Segment ERO subobjects).  If a PCEP session is
   established with an MSD value of zero, then the PCC MAY specify an
   MSD for each path computation request that it sends to the PCE, by
   including a "maximum SID depth" metric object on the request, as
   defined in Section 5.5.

   The L flag and MSD value inside the SR-PCE-CAPABILITY sub-TLV are
   meaningful only in the Open message sent from a PCC to a PCE.  As
   such, a PCE MUST set the L flag and MSD value to zero in an outbound
   message to a PCC.  Similarly, a PCC MUST ignore any MSD value
   received from a PCE.  If a PCE receives multiple SR-PCE-CAPABILITY
   sub-TLVs in an Open message, it processes only the first sub-TLV
   received.

5.2.  The RP/SRP Object

   In order to setup an SR-TE LSP using SR, RP or SRP object MUST
   include PATH-SETUP-TYPE TLV, specified in
   [I-D.ietf-pce-lsp-setup-type], with the PST set to 1 (path setup
   using SR-TE).

   The LSP-IDENTIFIERS TLV MAY be present for the above PST type.

5.3.  ERO Object

   An SR-TE path consists of one or more SID(s) where each SID MAY be
   associated with the identifier that represents the node or adjacency
   corresponding to the SID.  This identifier is referred to as the
   'Node or Adjacency Identifier' (NAI).  As described later, a NAI can
   be represented in various formats (e.g., IPv4 address, IPv6 address,
   etc).  Furthermore, a NAI is used for troubleshooting purposes and,
   if necessary, to derive SID value as described below.

   The ERO object specified in [RFC5440] is used to carry SR-TE path
   information.  In order to carry SID and/or NAI, this document defines
   a new ERO subobject referred to as "SR-ERO subobject" whose format is
   specified in the following section.  An ERO object carrying an SR-TE
   path consists of one or more ERO subobject(s), and MUST carry only
   SR-ERO subobject(s).  Note that an SR-ERO subobject does not need to
   have both SID and NAI.  However, at least one of them MUST be
   present.




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   When building the MPLS label stack from ERO, a PCC MUST assume that
   SR-ERO subobjects are organized as a last-in-first-out stack.  The
   first subobject relative to the beginning of ERO contains the
   information about the topmost label.  The last subobject contains
   information about the bottommost label.

5.3.1.  SR-ERO Subobject

   An SR-ERO subobject consists of a 32-bit header followed by the SID
   and the NAI associated with the SID.  The SID is a 32-bit number.
   The size of the NAI depends on its respective type, as described in
   the following sections.

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |L|    Type     |     Length    |  ST   |     Flags     |F|S|C|M|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                              SID                              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     //                        NAI (variable)                       //
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 2: SR-ERO Subobject format

   The fields in the SR-ERO Subobject are as follows:

   The 'L' Flag  indicates whether the subobject represents a loose-hop
      in the LSP [RFC3209].  If this flag is unset, a PCC MUST not
      overwrite the SID value present in the SR-ERO subobject.
      Otherwise, a PCC MAY expand or replace one or more SID value(s) in
      the received SR-ERO based on its local policy.


   Type  is the type of the SR-ERO subobject.  This document defines the
      SR-ERO subobject type, and requests a new codepoint from IANA.



   Length  contains the total length of the subobject in octets,
      including the L, Type and Length fields.  Length MUST be at least
      8, and MUST be a multiple of 4.  As mentioned earlier, an SR-ERO
      subobject MUST have at least SID or NAI.  The length should take
      into consideration SID or NAI only if they are not null.  The
      flags described below used to indicate whether SID or NAI field is
      null.





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   SID Type (ST)  indicates the type of information associated with the
      SID contained in the object body.  When ST value is 0, SID MUST
      NOT be null and NAI MUST be null.  Other ST values are described
      later in this document.


   Flags  is used to carry any additional information pertaining to SID.
      Currently, the following flag bits are defined:



      *  M: When this bit is set, the SID value represents an MPLS label
         stack entry as specified in [RFC5462] where only the label
         value is specified by the PCE.  Other fields (TC, S, and TTL)
         fields MUST be considered invalid, and PCC MUST set these
         fields according to its local policy and MPLS forwarding rules.



      *  C: When this bit as well as the M bit are set, then the SID
         value represents an MPLS label stack entry as specified in
         [RFC5462], where all the entry's fields (Label, TC, S, and TTL)
         are specified by the PCE.  However, a PCC MAY choose to
         override TC, S, and TTL values according its local policy and
         MPLS forwarding rules.


      *  S: When this bit is set, the SID value in the subobject body is
         null.  In this case, the PCC is responsible for choosing the
         SID value, e.g., by looking up its TED using the NAI which, in
         this case, MUST be present in the subobject.


      *  F: When this bit is set, the NAI value in the subobject body is
         null.


   SID  is the Segment Identifier.


   NAI  contains the NAI associated with the SID.  Depending on the
      value of ST, the NAI can have different format as described in the
      following section.








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5.3.2.  NAI Associated with SID

   This document defines the following NAIs:

   'IPv4 Node ID'  is specified as an IPv4 address.  In this case, ST
      value is 1, and the Length is 8 or 12 depending on either SID or
      NAI or both are included in the subobject.


   'IPv6 Node ID'  is specified as an IPv6 address.  In this case, ST
      and Length are 2, and Length is 8, 20, or 24 depending on either
      SID or NAI or both are included in the subobject.


   'IPv4 Adjacency'  is specified as a pair of IPv4 addresses.  In this
      case, ST value is 3.  The Length is 8, 12, or 16 depending on
      either SID or NAI or both are included in the subobject, and the
      format of the NAI is shown in the following figure:

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                      Local IPv4 address                       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                     Remote IPv4 address                       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 3: NAI for IPv4 adjacency



   'IPv6 Adjacency'  is specified as a pair of IPv6 addresses.  In this
      case, ST valie is 4.  The Length is 8, 36 or 40 depending on
      whether SID or NAI or both included in the subobject,and the
      format of the NAI is shown in the following figure:

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     //               Local IPv6 address (16 bytes)                 //
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     //               Remote IPv6 address (16 bytes)                //
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 4: NAI for IPv6 adjacency






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   'Unnumbered Adjacency with IPv4 NodeIDs'  is specified as a pair of
      Node ID / Interface ID tuples.  In this case, ST value is 5.  The
      Length is 8, 20, or 24 depending on whether SID or NAI or both
      included in the subobject, and the format of the NAI is shown in
      the following figure:

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                      Local Node-ID                            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                    Local Interface ID                         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                      Remote Node-ID                           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                   Remote Interface ID                         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

         Figure 5: NAI for Unnumbered adjacency with IPv4 Node IDs

5.3.3.  ERO Processing

   A PCEP speaker that does not recognize the SR-ERO subobject in PCRep,
   PCInitiate, PCUpd or PCRpt messages MUST reject the entire PCEP
   message and MUST send a PCErr message with Error-Type=3 ("Unknown
   Object") and Error-Value=2 ("Unrecognized object Type") or Error-
   Type=4 ("Not supported object") and Error-Value=2 ("Not supported
   object Type"), defined in [RFC5440].

   When the SID represents an MPLS label (i.e. the M bit is set), its
   value (20 most significant bits) MUST be larger than 15, unless it is
   special purpose label, such as an Entropy Label Indicator (ELI).  If
   a PCEP speaker receives an invalid value, it MUST send a PCErr
   message with Error-Type = 10 ("Reception of an invalid object") and
   Error Value = 2 ("Bad label value").  If both M and C bits of an SR-
   ERO subobject are set, and if a PCEP speaker finds erroneous setting
   in one or more of TC, S, and TTL fields, it MUST send a PCErr message
   with Error-Type = 10 ("Reception of an invalid object") and Error-
   Value = 4 ("Bad label format").

   If a PCC receives a stack of SR-ERO subobjects, and the number of
   stack exceeds the maximum number of SIDs that the PCC can impose on
   the packet, it MAY send a PCErr message with Error-Type = 10
   ("Reception of an invalid object") and Error-Value = 3 ("Unsupported
   number of Segment ERO subobjects").

   When a PCEP speaker detects that all subobjects of ERO are not
   identical, and if it does not handle such ERO, it MUST send a PCErr



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   message with Error-Type = 10 ("Reception of an invalid object") and
   Error-Value = 5 ("Non-identical ERO subobjects").

   If a PCEP speaker receives an SR-ERO subobject in which both SID and
   NAI are absent, it MUST consider the entire ERO object invalid and
   send a PCErr message with Error-Type = 10 ("Reception of an invalid
   object") and Error-Value = 6 ("Both SID and NAI are absent in ERO
   subobject").

   When a PCEP speaker receives an SR-ERO subobject in which ST is 0,
   SID MUST be present and NAI MUST NOT be present(i.e., S-flag MUST be
   0, F-flag MUST be 1, and the Length MUST be 8).  Otherwise, it MUST
   consider the entire ERO object invalid and send a PCErr message with
   Error-Type = 10 ("Reception of an invalid object") and Error-Value =
   11 ("Malformed object").  The PCEP speaker MAY include the malformed
   SR-ERO object in the PCErr message as well.

5.4.  RRO Object

   A PCC can record SR-TE LSP and report the LSP to a PCE via RRO.  An
   RRO object contains one or more subobjects called "SR-RRO subobjects"
   whose format is shown below:

      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    |  ST   |     Flags     |F|S|C|M|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                              SID                              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     //                        NAI (variable)                       //
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 6: SR-RRO Subobject format

   The format of SR-RRO subobject is the same as that of SR-ERO
   subobject without L flag.

   A PCC MUST assume that SR-RRO subobjects are organized such that the
   first subobject relative to the beginning of RRO contains the
   information about the topmost label, and the last subobject contains
   information about the bottommost label of the SR-TE LSP.

5.4.1.  RRO Processing

   Processing rules of SR-RRO subobject are identical to those of SR-ERO
   subobject.




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   If a PCEP speaker receives an SR-RRO subobject in which both SID and
   NAI are absent, it MUST consider the entire RRO object invalid and
   send a PCErr message with Error-Type = 10 ("Reception of an invalid
   object") and Error-Value = 7 ("Both SID and NAI are absent in RRO
   subobject").

   If a PCE detects that all subobjects of RRO are not identical, and if
   it does not handle such RRO, it MUST send a PCErr message with Error-
   Type = 10 ("Reception of an invalid object") and Error-Value = 10
   ("Non-identical RRO subobjects").

5.5.  METRIC Object

   If a PCEP session is established with an MSD value of zero, then the
   PCC MAY specify the MSD for an individual path computation request
   using the METRIC object defined in [RFC5440].  This document defines
   a new type for the METRIC object to be used for this purpose as
   follows:

   o  T = 11: Maximum SID Depth of the requested path.

   The PCC sets the metric-value to the MSD for this path.  The PCC MUST
   set the B (bound) bit to 1 in the METRIC object, which specifies that
   the SID depth for the computed path MUST NOT exceed the metric-value.

   If a PCEP session is established with a non-zero MSD value, then the
   PCC MUST NOT send an MSD METRIC object.  If the PCE receives a path
   computation request with an MSD METRIC object on a session with a
   non-zero MSD value then it MUST consider the request invalid and send
   a PCErr with Error-Type = 10 ("Reception of an invalid object") and
   Error-Value 9 ("Default MSD is specified for the PCEP session").

6.  Backward Compatibility

   A PCEP speaker that does not support the SR PCEP capability cannot
   recognize the SR-ERO or SR-RRO subobjects.  As such, it MUST send a
   PCEP error with Error-Type = 4 (Not supported object) and Error-Value
   = 2 (Not supported object Type) as per [RFC5440].

   Some implementations, which are compliant with an earlier version of
   this specification, do not send the PATH-SETUP-TYPE-CAPABILITY TLV in
   their OPEN objects.  Instead, to indicate that they support SR, these
   implementations include the SR-CAPABILITY-TLV as a top-level TLV in
   the OPEN object.  Unfortunately, some of these implementations made
   it into the field before this document was published in its final
   form.  Therefore, if a PCEP speaker receives an OPEN object in which
   the SR-CAPABILITY-TLV appears as a top-level TLV, then it MUST
   interpret this as though the sender had sent a PATH-SETUP-TYPE-



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   CAPABILITY TLV with a PST list of (0, 1) (that is, both RSVP-TE and
   SR-TE PSTs are supported) and with the SR-CAPABILITY-TLV as a sub-
   TLV.  If a PCEP speaker receives an OPEN object in which both the SR-
   CAPABILITY-TLV and PATH-SETUP-TYPE-CAPABILITY TLV appear as top-level
   TLVs, then it MUST ignore the top-level SR-CAPABILITY-TLV and process
   only the PATH-SETUP-TYPE-CAPABILITY TLV.

7.  Management Considerations

7.1.  Policy

   PCEP implementation:

   o  Can enable SR PCEP capability either by default or via explicit
      configuration.

   o  May generate PCEP error due to unsupported number of SR-ERO or SR-
      RRO subobjects either by default or via explicit configuration.

7.2.  The PCEP Data Model

   A PCEP MIB module is defined in [RFC7420]n eeds be extended to cover
   additional functionality provided by [RFC5440] and
   [I-D.ietf-pce-pce-initiated-lsp].  Such extension will cover the new
   functionality specified in this document.

8.  Security Considerations

   The security considerations described in [RFC5440] and
   [I-D.ietf-pce-pce-initiated-lsp] are applicable to this
   specification.  No additional security measure is required.

9.  IANA Considerations

9.1.  PCEP Objects

   This document defines a new sub-object type for the PCEP explicit
   route object (ERO), and a new sub-object type for the PCEP record
   route object (RRO).  The code points for sub-object types of these
   objects is maintained in the RSVP parameters registry, under the
   EXPLICIT_ROUTE and ROUTE_RECORD objects.  IANA is requested to
   confirm the early allocation of the following code points in the RSVP
   Parameters registry for each of the new sub-object types defined in
   this document.







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    Object                Sub-Object                 Sub-Object Type
    --------------------- -------------------------- ------------------
    EXPLICIT_ROUTE        SR-ERO (PCEP-specific)     36
    ROUTE_RECORD          SR-RRO (PCEP-specific)     36

9.2.  PCEP-Error Object

   IANA is requested to confirm the early allocation of the code-points
   in the PCEP-ERROR Object Error Types and Values registry for the
   following new error-values:


   Error-Type   Meaning
   ----------   -------
   10           Reception of an invalid object.

                 Error-value = 2:                    Bad label value
                 Error-value = 3:                    Unsupported number
                                                     of Segment ERO
                                                     subobjects
                 Error-value = 4:                    Bad label format
                 Error-value = 5:                    Non-identical ERO
                                                     subobjects
                 Error-value = 6:                    Both SID and NAI
                                                     are absent in ERO
                                                     subobject
                 Error-value = 7:                    Both SID and NAI
                                                     are absent in RRO
                                                     subobject
                 Error-value = 9:                    Default MSD is
                                                     specified for the
                                                     PCEP session
                 Error-value = 10:                   Non-identical RRO
                                                     subobjects
                 Error-value = TBD1:                 Missing PCE-SR-
                                                     CAPABILITY sub-TLV

   Note to IANA: this draft originally had an early allocation for
   Error-value=11 (Malformed object) in the above list.  However, we
   have since moved the definition of that code point to draft-ietf-pce-
   lsp-setup-type and we included an instruction in that draft for you
   to update the reference in the indicated registry.  Please ensure
   that this has happened when you process the present draft.

   Note to IANA: the final Error-value (Missing PCE-SR-CAPABILITY sub-
   TLV) in the above list was defined after the early allocation took
   place, and so does not currently have a code point assigned.  Please




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   assign a code point from the indicated registry and replace each
   instance of "TBD1" in this document with the allocated code point.

9.3.  PCEP TLV Type Indicators

   IANA is requested to confirm the early allocation of the following
   code point in the PCEP TLV Type Indicators registry.

   Value                     Meaning                      Reference
   ------------------------- ---------------------------- --------------
   26                        SR-PCE-CAPABILITY            This document

9.4.  New Path Setup Type

   [I-D.ietf-pce-lsp-setup-type] requests that IANA creates a sub-
   registry within the "Path Computation Element Protocol (PCEP)
   Numbers" registry called "PCEP Path Setup Types".  IANA is requested
   to allocate a new code point within this registry, as follows:

   Value                     Description                  Reference
   ------------------------- ---------------------------- --------------
   1                         Traffic engineering path is  This document
                             setup using Segment Routing.

9.5.  New Metric Type

   IANA is requested to confirm the early allocation of the following
   code point in the PCEP METRIC object T field registry:

   Value                     Description                  Reference
   ------------------------- ---------------------------- --------------
   11                        Segment-ID (SID) Depth.      This document

10.  Contributors

   The following people contributed to this document:

      - Lakshmi Sharma
      - Jan Medved
      - Edward Crabbe
      - Robert Raszuk
      - Victor Lopez

11.  Acknowledgements

   We thank Ina Minei, George Swallow, Marek Zavodsky, Dhruv Dhody, Ing-
   Wher Chen and Tomas Janciga for the valuable comments.




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12.  References

12.1.  Normative References

   [I-D.ietf-idr-bgp-ls-segment-routing-msd]
              Tantsura, J., Chunduri, U., Mirsky, G., and S. Sivabalan,
              "Signaling Maximum SID Depth using Border Gateway Protocol
              Link-State", draft-ietf-idr-bgp-ls-segment-routing-msd-01
              (work in progress), October 2017.

   [I-D.ietf-isis-segment-routing-extensions]
              Previdi, S., Filsfils, C., Bashandy, A., Gredler, H.,
              Litkowski, S., Decraene, B., and j. jefftant@gmail.com,
              "IS-IS Extensions for Segment Routing", draft-ietf-isis-
              segment-routing-extensions-13 (work in progress), June
              2017.

   [I-D.ietf-isis-segment-routing-msd]
              Tantsura, J., Chunduri, U., Aldrin, S., and L. Ginsberg,
              "Signaling MSD (Maximum SID Depth) using IS-IS", draft-
              ietf-isis-segment-routing-msd-04 (work in progress), June
              2017.

   [I-D.ietf-ospf-segment-routing-extensions]
              Psenak, P., Previdi, S., Filsfils, C., Gredler, H.,
              Shakir, R., Henderickx, W., and J. Tantsura, "OSPF
              Extensions for Segment Routing", draft-ietf-ospf-segment-
              routing-extensions-21 (work in progress), October 2017.

   [I-D.ietf-ospf-segment-routing-msd]
              Tantsura, J., Chunduri, U., Aldrin, S., and P. Psenak,
              "Signaling MSD (Maximum SID Depth) using OSPF", draft-
              ietf-ospf-segment-routing-msd-05 (work in progress), June
              2017.

   [I-D.ietf-pce-lsp-setup-type]
              Sivabalan, S., Tantsura, J., Minei, I., Varga, R., and J.
              Hardwick, "Conveying path setup type in PCEP messages",
              draft-ietf-pce-lsp-setup-type-05 (work in progress),
              October 2017.

   [I-D.ietf-pce-pce-initiated-lsp]
              Crabbe, E., Minei, I., Sivabalan, S., and R. Varga, "PCEP
              Extensions for PCE-initiated LSP Setup in a Stateful PCE
              Model", draft-ietf-pce-pce-initiated-lsp-11 (work in
              progress), October 2017.





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   [I-D.ietf-spring-segment-routing]
              Filsfils, C., Previdi, S., Decraene, B., Litkowski, S.,
              and R. Shakir, "Segment Routing Architecture", draft-ietf-
              spring-segment-routing-12 (work in progress), June 2017.

   [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>.

   [RFC5440]  Vasseur, JP., Ed. and JL. Le Roux, Ed., "Path Computation
              Element (PCE) Communication Protocol (PCEP)", RFC 5440,
              DOI 10.17487/RFC5440, March 2009, <https://www.rfc-
              editor.org/info/rfc5440>.

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

   [RFC7420]  Koushik, A., Stephan, E., Zhao, Q., King, D., and J.
              Hardwick, "Path Computation Element Communication Protocol
              (PCEP) Management Information Base (MIB) Module",
              RFC 7420, DOI 10.17487/RFC7420, December 2014,
              <https://www.rfc-editor.org/info/rfc7420>.

   [RFC8231]  Crabbe, E., Minei, I., Medved, J., and R. Varga, "Path
              Computation Element Communication Protocol (PCEP)
              Extensions for Stateful PCE", RFC 8231,
              DOI 10.17487/RFC8231, September 2017, <https://www.rfc-
              editor.org/info/rfc8231>.

12.2.  Informative References

   [RFC3209]  Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
              and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
              Tunnels", RFC 3209, DOI 10.17487/RFC3209, December 2001,
              <https://www.rfc-editor.org/info/rfc3209>.

   [RFC3473]  Berger, L., Ed., "Generalized Multi-Protocol Label
              Switching (GMPLS) Signaling Resource ReserVation Protocol-
              Traffic Engineering (RSVP-TE) Extensions", RFC 3473,
              DOI 10.17487/RFC3473, January 2003, <https://www.rfc-
              editor.org/info/rfc3473>.







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   [RFC3477]  Kompella, K. and Y. Rekhter, "Signalling Unnumbered Links
              in Resource ReSerVation Protocol - Traffic Engineering
              (RSVP-TE)", RFC 3477, DOI 10.17487/RFC3477, January 2003,
              <https://www.rfc-editor.org/info/rfc3477>.

   [RFC4657]  Ash, J., Ed. and J. Le Roux, Ed., "Path Computation
              Element (PCE) Communication Protocol Generic
              Requirements", RFC 4657, DOI 10.17487/RFC4657, September
              2006, <https://www.rfc-editor.org/info/rfc4657>.

Authors' Addresses

   Siva Sivabalan
   Cisco Systems, Inc.
   2000 Innovation Drive
   Kanata, Ontario  K2K 3E8
   Canada

   Email: msiva@cisco.com


   Clarence Filsfils
   Cisco Systems, Inc.
   Pegasus Parc
   De kleetlaan 6a, DIEGEM  BRABANT 1831
   BELGIUM

   Email: cfilsfil@cisco.com


   Jeff Tantsura
   Individual
   444 San Antonio Rd, 10A
   Palo Alto, CA  94306
   USA

   Email: jefftant.ietf@gmail.com


   Wim Henderickx
   Nokia
   Copernicuslaan 50
   Antwerp 2018, CA  95134
   BELGIUM

   Email: wim.henderickx@alcatel-lucent.com





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   Jon Hardwick
   Metaswitch Networks
   100 Church Street
   Enfield, Middlesex
   UK

   Email: jonathan.hardwick@metaswitch.com












































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