PCEP extension to support Segment Routing Policy Candidate Paths
draft-barth-pce-segment-routing-policy-cp-00

PCE Working Group                                               C. Barth
Internet-Draft                                    Juniper Networks, Inc.
Intended status: Standards Track                            M. Koldychev
Expires: December 20, 2018                           Cisco Systems, Inc.
                                                                D. Dhody
                                                       Huawei Technology
                                                            S. Sivabalan
                                                     Cisco Systems, Inc.
                                                           June 18, 2018

    PCEP extension to support Segment Routing Policy Candidate Paths
              draft-barth-pce-segment-routing-policy-cp-00

Abstract

   This document introduces a mechanism to specify an SR policy, as a
   collection of SR candidate paths.  An SR policy is identified by
   <headend, color, destination end-point> tuple.  An SR policy can be
   associated with one or more candidate paths where each candidate path
   is represented in PCEP by an LSP.  This document proposes extension
   to PCEP to support association among candidate paths of a given SR
   policy.  The mechanism proposed in this document is applicable to
   both MPLS and IPv6 data plane.

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 https://datatracker.ietf.org/drafts/current/.

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

   This Internet-Draft will expire on December 20, 2018.

Barth, et al.           Expires December 20, 2018               [Page 1]
Internet-Draft                  SR Policy                      June 2018

Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Motivation  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Instantiation of SR policy candidate paths  . . . . . . .   4
     3.2.  Color based path computation  . . . . . . . . . . . . . .   4
     3.3.  Avoid computing lower preference candidate paths  . . . .   5
     3.4.  Minimal signaling overhead  . . . . . . . . . . . . . . .   5
   4.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   5.  SR Candidate Path Association Group . . . . . . . . . . . . .   6
     5.1.  SR Candidate Path Association Group Information TLV . . .   7
   6.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .   8
     6.1.  PCC Initiated SR Policy with single candidate-path  . . .   8
     6.2.  PCC Initiated SR Policy with multiple candidate-paths . .   8
     6.3.  PCE Initiated SR Policy with single candidate-path  . . .   9
     6.4.  PCE Initiated SR Policy with multiple candidate-paths . .   9
   7.  Acknowledgement . . . . . . . . . . . . . . . . . . . . . . .  10
   8.  Normative References  . . . . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   Path Computation Element (PCE) Communication Protocol (PCEP)
   [RFC5440] describes the Path Computation Element communication
   Protocol (PCEP) which enables the communication between a Path
   Computation Client (PCC) and a Path Control Element (PCE), or between
   two PCEs based on the PCE architecture [RFC4655].

   PCEP Extensions for Stateful PCE Model [RFC8231] describes a set of
   extensions to PCEP to enable active control of Multiprotocol Label
   Switching Traffic Engineering (MPLS-TE) and Generalized MPLS (GMPLS)
   tunnels.  [RFC8281] describes the setup and teardown of PCE-initiated

Barth, et al.           Expires December 20, 2018               [Page 2]
Internet-Draft                  SR Policy                      June 2018

   LSPs under the active stateful PCE model, without the need for local
   configuration on the PCC, thus allowing for a dynamic network.

   PCEP Extensions for Segment Routing [I-D.ietf-pce-segment-routing]
   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.

   PCEP Extensions for Establishing Relationships Between Sets of LSPs
   [I-D.ietf-pce-association-group] introduces a generic mechanism to
   create a grouping of LSPs which can then be used to define
   associations between a set of LSPs and a set of attributes (such as
   configuration parameters or behaviors) and is equally applicable to
   stateful PCE (active and passive modes) and stateless PCE.

   Segment Routing Policy for Traffic Engineering [I-D.ietf-spring-
   segment-routing-policy] details the concepts of SR Policy and
   approaches to steering traffic into an SR Policy.

   An SR policy can be associated with one or more candidate paths where
   one or more such paths can be computed via PCE.  This document
   specifies PCEP extensions to specify a candidate path of an SR
   policy.  Each candidate path being represented by a PCEP LSP.  By
   associating multiple candidate paths together, a PCE becomes aware of
   the hierarchical structure of an SR policy.  Thus the PCE can take
   computation and control decisions about the candidate paths, with the
   additional knowledge that these candidate paths belong to the same SR
   policy.  This is accomplished via the use of the existing PCEP
   Association object, by defining a new association type specifically
   for associating SR candidate paths into a single SR policy.

2.  Terminology

   The following terminologies are used in this document:

   Destination Endpoint:  The destination IPv4 or IPv6 endpoint address
      of the SR policy in question, as described in [I-D.ietf-spring-
      segment-routing-policy].

   Association parameters:  As described in [I-D.ietf-pce-association-
      group], the combination of the mandatory fields Association type,
      Association ID and Association Source in the ASSOCIATION object
      uniquely identify the association group.  If the optional TLVs -
      Global Association Source or Extended Association ID are included,
      then they MUST be included in combination with mandatory fields to
      uniquely identifying the association group.

Barth, et al.           Expires December 20, 2018               [Page 3]
Internet-Draft                  SR Policy                      June 2018

   Association information:  As described in [I-D.ietf-pce-association-
      group], the ASSOCIATION object MAY include other optional TLVs
      based on the association types, that provides 'information'
      related to the association type.

   PCC:  Path Computation Client.  Any client application requesting a
      path computation to be performed by a Path Computation Element.

   PCE:  Path Computation Element.  An entity (component, application,
      or network node) that is capable of computing a network path or
      route based on a network graph and applying computational
      constraints.

   PCEP:  Path Computation Element Protocol.

3.  Motivation

   The new Association Type (SR policy candidate path) and the new TLVs
   of the ASSOCIATION object, defined in this document, allow PCEP peers
   to exchange additional parameters of SR policy, defined in [I-D.ietf-
   spring-segment-routing-policy].  These additional parameters include
   the color, endpoint, and preference.  More parameters may be added in
   the future.

   The motivation for signaling these parameters is summarized in the
   following subsections.

3.1.  Instantiation of SR policy candidate paths

   A PCE may want to instantiate one or more candidate paths on the PCC,
   as specified in [RFC8281].  In this scenario, the PCE needs to signal
   to a PCC <headend, color, destination end-point, preference> tuple
   using which the PCC can instantiate a candidate path for the SR
   policy identified.  Current PCEP standards (as of the time of this
   writing) do not provide a way to signal color and preference.
   Although destination end-point can be signaled via the PCEP END-
   POINTS object, this object may not be suitable because the
   destination end-point to which the path is computed is not required
   to be the same IPv4/IPv6 address as the actual endpoint of the SR
   policy.  Thus, a separate way to specify SR policy's destination end-
   point is provided in this draft.

3.2.  Color based path computation

   A PCE can make use of the color of the SR policy, when computing the
   path.  For example, the PCE can map a certain set of colors to a
   specific set of constraints or optimization objectives based on its
   local path computation policy.

Barth, et al.           Expires December 20, 2018               [Page 4]
Internet-Draft                  SR Policy                      June 2018

3.3.  Avoid computing lower preference candidate paths

   When a PCE knows that a given set of LSPs all belong to the same SR
   policy, then path computation can be done on only the highest
   preference candidate-path(s).  Path computation for lower preference
   paths is not necessary if one or two higher preference paths are
   already computed.  Since computing their paths will not affect
   traffic steering, it may be postponed until the higher preference
   paths become invalid, thus saving a lot of computation resources on
   the PCE.

   A PCE MAY take candidate path preference into consideration in path
   computation subject to its local path computation policy.  For
   example, it MAY choose to compute path with higher preference first
   before considering path with lower order preference in the context of
   a given SR policy.  It is also possible for the PCE to compute only
   the path with the highest preference for a given SR policy.

3.4.  Minimal signaling overhead

   When an SR policy contains multiple candidate paths computed by PCE,
   such candidate paths can be created, updated and deleted
   independently of each other.  This is achieved by making each
   candidate path correspond to a unique LSP.  For example, if an SR
   policy has 4 candidate paths, then if the PCE wants to update one of
   those candidate paths, only one set of PCUpd and PCRpt messages needs
   to be exchanged.

4.  Overview

   As per [I-D.ietf-pce-association-group], LSPs are placed into an
   association group.  In our case, each LSP corresponds to a candidate
   path of an SR policy, and the association group corresponds to the SR
   policy itself.

   A new Association Type is defined in this document, based on the
   generic ASSOCIATION object.  Association type = TBD1 ("SR Candidate
   Path Association Type") for SR Candidate Path Association Group
   (SRCPAG).

   The SRCPAG Association is only meant to be used for SR LSPs and with
   PCEP peers which advertise SR capability.

   An Association object of SRCPAG group contains the following
   information:

   o  Color of SR policy, represented by a 32-bit unsigned integer.

Barth, et al.           Expires December 20, 2018               [Page 5]
Internet-Draft                  SR Policy                      June 2018

   o  Destination end-point of SR policy, represented by IPv4 or IPv6
      address.

   o  Preference of SR policy, represented by an unsigned integer.

   The values of Color, Destination Endpoint and Preference are encoded
   as a TLV in the SRCPAG ASSOCIATION object.  This TLV is part of
   Association Information.

   The Association Parameters (see Section 2) are derived from the Color
   and Destination End-point.  The mapping from Color and Destination
   End-point to Association Parameters is left up to the implementation.
   An implementation MAY choose Association Parameters in such a way
   that every possible Color and Destination End-point maps to a unique
   value of Association Parameters, thus requiring the use of Extended
   Association ID TLV.  Alternatively, an implementation MAY implement a
   lookup table to generate Association Parameters incrementally as new
   Color and Destination End-point values are created, thus not
   requiring the use of Extended Association ID TLV.

   As per the processing rules specified in section 5.4 of [I-D.ietf-
   pce-association-group], if a PCEP speaker does not support the SRCPAG
   association type, it MUST return a PCErr message with Error-Type 26
   (Early allocation by IANA) "Association Error" and Error-Value 1
   "Association-type is not supported".  Please note that the
   corresponding PCEP session is not reset.

5.  SR Candidate Path Association Group

   Two object types for IPv4 and IPv6 are defined.  The ASSOCIATION
   object includes "Association type" indicating the type of the
   association group.  This document adds a new Association type.

   Association type = TBD1 ("SR Candidate Path Association Type") for SR
   Candidate Path Association Group (SRCPAG).

   The operator configured Association Range SHOULD NOT be set for this
   association type and MUST be ignored.

   SRCPAG may carry additional TLVs to communicate Association
   Information.  This document specifies a new such TLV called "SRCPAG-
   INFO-TLV" which is used to carry Color, Destination End-point and
   Preference.

   A given LSP MUST belong to at most one SRCPAG, since a candidate path
   cannot belong to multiple SR policies.  If a PCEP speaker receives a
   PCEP message with more than one SRCPAG for an LSP, then the PCEP
   speaker MUST send a PCErr message with Error-Type 26 "Association

Barth, et al.           Expires December 20, 2018               [Page 6]
Internet-Draft                  SR Policy                      June 2018

   Error" and Error-Value TBD "Multiple SRCPAG for one LSP".  If the
   message is a PCRpt message, then the PCEP speaker MUST close the PCEP
   connection.  Closing the PCEP connection is necessary because
   ignoring PCRpt messages may lead to inconsistent LSP DB state between
   the two PCEP peers.

5.1.  SR Candidate Path Association Group Information TLV

   The SRCPAG-INFO-TLV is a mandatory TLV for the SRCPAG Association.
   Only one SRCPAG-INFO-TLV can be carried and only the first occurrence
   is processed and any others MUST be ignored.  SRCPAG-INFO-TLV is part
   of the Association Information (see Section 2).

       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            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                             Color                             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Destination End-point                    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~                           Preference                          ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 1: The SRCPAG-INFO-TLV format

   Type: TBD2.

   Length: variable, depending on length of Destination End-point (IPv4
   or IPv6) and the length of the Preference.

   Color: any unsigned 32-bit number.

   Destination End-point: can be either IPv4 or IPv6, depending on
   whether the policy endpoint IPv4 or IPv6.  This value may be
   different from the one contained in the existing END-POINTS object,
   or in the LSP IDENTIFIERS TLV of the LSP object.  Destination
   Endpoint is meant to strictly correspond to the endpoint of the SR
   policy, as it is defined in [I-D.ietf-spring-segment-routing-policy].

   Preference: value of the candidate path preference.

Barth, et al.           Expires December 20, 2018               [Page 7]
Internet-Draft                  SR Policy                      June 2018

6.  Examples

6.1.  PCC Initiated SR Policy with single candidate-path

   PCReq and PCRep messages are exchanged in the following sequence:

   1.  PCC sends PCReq message to the PCE, encoding the SRCPAG
       ASSOCIATION object and TLVs in the PCReq message.

   2.  PCE returns the path in PCRep message, and echoes back the SRCPAG
       object that was used in the computation.

   PCRpt and PCUpd messages are exchanged in the following sequenece:

   1.  PCC sends PCRpt message to the PCE, including the LSP object and
       the SRCPAG ASSOCIATION object.

   2.  PCE computes path, possibly making use of the Color, Endpoint and
       Preference that were extracted from the SRCPAG ASSOCIATION
       object.

   3.  PCE updates the SR policy candidate path's ERO using PCUpd
       message.

6.2.  PCC Initiated SR Policy with multiple candidate-paths

   PCReq and PCRep messages are exchanged using the sequence specified
   in section 6.1 with individual query for each candidate-path.

   PCRpt and PCUpd messages are exchanged in the following sequence:

   1.  Step 1: For each candidate path of the SR policy, the PCC
       generates a different PLSP-ID and symbolic-name and sends
       multiple PCRpt messages (or one message with multiple LSP
       objects) to the PCE.  Each LSP object is followed by SRCPAG
       ASSOCIATION object with identical Color and Endpoint values.

   2.  Step 2: PCE takes into account that all the LSPs belong to the
       same SR policy.  PCE prioritizes computation for the highest
       preference LSP and sends PCUpd message(s) back to the PCC.

   3.  Step 3: If a new candidate path is added on the PCC by the
       operator, then a new PLSP-ID and symbolic name is generated for
       that candidate path and a new PCRpt is sent to the PCE.

   4.  Step 4: If an existing candidate path is removed from the PCC by
       the operator, then that PLSP-ID is deleted from the PCE by
       sending PCRpt with the R-flag set.

Barth, et al.           Expires December 20, 2018               [Page 8]
Internet-Draft                  SR Policy                      June 2018

6.3.  PCE Initiated SR Policy with single candidate-path

   A candidate-path is created using the following steps:

   1.  PCE sends PCInit message, as usual containing the SRCPAG
       Association object.  PCE needs to generate a symbolic-name for
       this LSP that will not clash with other symbolic names on the
       same PCC.

   2.  PCC uses the color, destination endpoint and preference from the
       SRCPAG object to create a new candidate path.  If no SR policy
       exists to hold the candidate path, then a new SR policy is
       created to hold the new candidate-path.  The Originator of the
       candidate path is set to be the address of the PCE that is
       sending the PCInit message.

   3.  PCC allocates a locally unique PLSP-ID for the newly created
       candidate path.  This PLSP-ID is sent to the PCE in the PCRpt
       message.

   A candidate-path is deleted using the following steps:

   1.  PCE sends PCInit message, setting the R-flag in the LSP object.

   2.  PCC uses the PLSP-ID from the LSP object to find the candidate
       path and delete it.  If this is the last candidate path under the
       SR policy, then the containing SR policy is deleted as well.

6.4.  PCE Initiated SR Policy with multiple candidate-paths

   A candidate-path is created using the following steps:

   1.  PCE sends a separate PCInit message for every candidate path that
       it wants to create, or it sends multiple LSP objects within a
       single PCInit message.  Each candidate-path must get a unique
       symbolic-name generated on the PCE.  SRCPAG object is sent for
       every LSP in the PCInit message.

   2.  PCC creates multiple candidate paths under the same SR policy,
       identified by Color and Endpoint.  PCC generates a unique PLSP-ID
       for every candidate path.

   3.  PCC allocates a locally unique PLSP-ID for each newly created
       candidate path.  This PLSP-ID is sent to the PCE in the PCRpt
       message.

   A candidate path is deleted using the following steps:

Barth, et al.           Expires December 20, 2018               [Page 9]
Internet-Draft                  SR Policy                      June 2018

   1.  PCE sends PCInit message, setting the R-flag in the LSP object.

   2.  PCC uses the PLSP-ID from the LSP object to find the candidate
       path and delete it.

7.  Acknowledgement

8.  Normative References

   [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.

   [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-11 (work in progress),
              November 2017.

   [I-D.ietf-pce-stateful-pce]
              Crabbe, E., Minei, I., Medved, J., and R. Varga, "PCEP
              Extensions for Stateful PCE", draft-ietf-pce-stateful-
              pce-21 (work in progress), June 2017.

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

   [I-D.sivabalan-pce-binding-label-sid]
              Sivabalan, S., Tantsura, J., Filsfils, C., Previdi, S.,
              Hardwick, J., and D. Dhody, "Carrying Binding Label/
              Segment-ID in PCE-based Networks.", draft-sivabalan-pce-
              binding-label-sid-04 (work in progress), March 2018.

   [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>.

Authors' Addresses

Barth, et al.           Expires December 20, 2018              [Page 10]
Internet-Draft                  SR Policy                      June 2018

   Colby Barth
   Juniper Networks, Inc.

   Email: cbarth@juniper.net

   Mike Koldychev
   Cisco Systems, Inc.
   2000 Innovation Drive
   Kanata, Ontario  K2K 3E8
   Canada

   Email: mkoldych@cisco.com

   Dhruv Dhody
   Huawei Technology

   Email: dhruv.dhody@huawei.com

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

   Email: msiva@cisco.com

Barth, et al.           Expires December 20, 2018              [Page 11]