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Versions: (draft-sivabalan-pce-binding-label-sid) 00 01

PCE Working Group                                           S. Sivabalan
Internet-Draft                                               C. Filsfils
Intended status: Standards Track                     Cisco Systems, Inc.
Expires: May 6, 2020                                         J. Tantsura
                                                            Apstra, Inc.
                                                             J. Hardwick
                                                     Metaswitch Networks
                                                              S. Previdi
                                                                   C. Li
                                                     Huawei Technologies
                                                        November 3, 2019


        Carrying Binding Label/Segment-ID in PCE-based Networks.
                  draft-ietf-pce-binding-label-sid-01

Abstract

   In order to provide greater scalability, network opacity, and service
   independence, Segment Routing (SR) utilizes a Binding Segment
   Identifier (BSID).  It is possible to associate a BSID to RSVP-TE
   signaled Traffic Engineering Label Switching Path or binding Segment-
   ID (SID) to SR Traffic Engineering path.  Such a binding label/SID
   can be used by an upstream node for steering traffic into the
   appropriate TE path to enforce SR policies.  This document proposes
   an approach for reporting binding label/SID to Path Computation
   Element (PCE) for supporting PCE-based Traffic Engineering policies.

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.

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



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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 6, 2020.

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
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Path Binding TLV  . . . . . . . . . . . . . . . . . . . . . .   6
   4.  Operation . . . . . . . . . . . . . . . . . . . . . . . . . .   7
   5.  Binding SID in SR-ERO . . . . . . . . . . . . . . . . . . . .   8
   6.  Binding SID in SRv6-ERO/  . . . . . . . . . . . . . . . . . .   8
   7.  Implementation Status . . . . . . . . . . . . . . . . . . . .   9
     7.1.  Huawei  . . . . . . . . . . . . . . . . . . . . . . . . .   9
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   9.  Manageability Considerations  . . . . . . . . . . . . . . . .  10
     9.1.  Control of Function and Policy  . . . . . . . . . . . . .  10
     9.2.  Information and Data Models . . . . . . . . . . . . . . .  10
     9.3.  Liveness Detection and Monitoring . . . . . . . . . . . .  10
     9.4.  Verify Correct Operations . . . . . . . . . . . . . . . .  10
     9.5.  Requirements On Other Protocols . . . . . . . . . . . . .  10
     9.6.  Impact On Network Operations  . . . . . . . . . . . . . .  11
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
     10.1.  PCEP TLV Type Indicators . . . . . . . . . . . . . . . .  11
       10.1.1.  TE-PATH-BINDING TLV  . . . . . . . . . . . . . . . .  11
     10.2.  PCEP Error Type and Value  . . . . . . . . . . . . . . .  11
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  12
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  12
     12.2.  Informative References . . . . . . . . . . . . . . . . .  13
   Appendix A.  Contributor Addresses  . . . . . . . . . . . . . . .  15
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  15



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1.  Introduction

   A PCE can compute Traffic Engineering paths (TE paths) through a
   network that are subject to various constraints.  Currently, TE paths
   are either set up using the RSVP-TE signaling protocol or Segment
   Routing (SR).  We refer to such paths as RSVP-TE paths and SR-TE
   paths respectively in this document.

   As per [RFC8402] SR allows a headend node to steer a packet flow
   along any path.  The headend node is said to steer a flow into an
   Segment Routing Policy (SR Policy).  Further, as per
   [I-D.ietf-spring-segment-routing-policy], an SR Policy is a framework
   that enables instantiation of an ordered list of segments on a node
   for implementing a source routing policy with a specific intent for
   traffic steering from that node.

   As described in [RFC8402], Binding Segment Identifier (BSID) is bound
   to an Segment Routed (SR) Policy, instantiation of which may involve
   a list of SIDs.  Any packets received with an active segment equal to
   BSID are steered onto the bound SR Policy.  A BSID may be either a
   local (SR Local Block (SRLB)) or a global (SR Global Block (SRGB))
   SID.  As per Section 6.4 of [I-D.ietf-spring-segment-routing-policy]
   a BSID can also be associated with any type of interfaces or tunnel
   to enable the use of a non-SR interface or tunnels as segments in a
   SID-list.

   [RFC5440] describes the Path Computation Element Protocol (PCEP) for
   communication between a Path Computation Client (PCC) and a PCE or
   between a pair of PCEs as per [RFC4655].  [RFC8231] specifies
   extension to PCEP that allows a PCC to delegate its LSPs to a
   stateful PCE.  A stateful PCE can then update the state of LSPs
   delegated to it.  [RFC8281] specifies a mechanism allowing a PCE to
   dynamically instantiate an LSP on a PCC by sending the path and
   characteristics.  The PCEP extension to setup and maintain SR-TE
   paths is specified in [I-D.ietf-pce-segment-routing].

   [I-D.ietf-pce-segment-routing] provides a mechanism for a network
   controller (acting as a PCE) to instantiate candidate paths for an SR
   Policy onto a head-end node (acting as a PCC) using PCEP.  For more
   information on the SR Policy Architecture, see
   [I-D.ietf-spring-segment-routing-policy].

   Binding label/SID has local significance to the ingress node of the
   corresponding TE path.  When a stateful PCE is deployed for setting
   up TE paths, it may be desirable to report the binding label or SID
   to the stateful PCE for the purpose of enforcing end-to-end TE/SR
   policy.  A sample Data Center (DC) use-case is illustrated in the
   following diagram.  In the MPLS DC network, an SR LSP (without



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   traffic engineering) is established using a prefix SID advertised by
   BGP (see [I-D.ietf-idr-bgp-prefix-sid]).  In IP/MPLS WAN, an SR-TE
   LSP is setup using the PCE.  The list of SIDs of the SR-TE LSP is {A,
   B, C, D}. The gateway node 1 (which is the PCC) allocates a binding
   SID X and reports it to the PCE.  In order for the access node to
   steer the traffic over the SR-TE LSP, the PCE passes the SID stack
   {Y, X} where Y is the prefix SID of the gateway node-1 to the access
   node.  In the absence of the binding SID X, the PCE should pass the
   SID stack {Y, A, B, C, D} to the access node.  This example also
   illustrates the additional benefit of using the binding SID to reduce
   the number of SIDs imposed on the access nodes with a limited
   forwarding capacity.


           SID stack
           {Y, X}              +-----+
    _ _ _ _ _ _ _ _ _ _ _ _ _ _| PCE |
   |                           +-----+
   |                              ^
   |                              | Binding
   |           .-----.            | SID (X)     .-----.
   |          (       )           |            (       )
   V       .--(         )--.      |        .--(         )--.
+------+  (                 )  +-------+  (                 )  +-------+
|Access|_(  MPLS DC Network  )_|Gateway|_(    IP/MPLS WAN    )_|Gateway|
| Node | (  ==============>  ) |Node-1 | ( ================> ) |Node-2 |
+------+  (     SR path     )  +-------+  (    SR-TE path   )  +-------+
           '--(         )--'    Prefix     '--(         )--'
               (       )        SID of         (       )
                '-----'         Node-1          '-----'
                                is Y            SIDs for SR-TE LSP:
                                                {A, B, C, D}



                Figure 1: A sample Use-case of Binding SID

   A PCC could report the binding label/SID allocated by it to the
   stateful PCE via Path Computation State Report (PCRpt) message.  It
   is also possible for a stateful PCE to request a PCC to allocate a
   specific binding label/SID by sending an Path Computation Update
   Request (PCUpd) message.  If the PCC can successfully allocate the
   specified binding value, it reports the binding value to the PCE.
   Otherwise, the PCC sends an error message to the PCE indicating the
   cause of the failure.  A local policy or configuration at the PCC
   SHOULD dictate if the binding label/SID needs to be assigned.





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   In this document, we introduce a new OPTIONAL TLV that a PCC can use
   in order to report the binding label/SID associated with a TE LSP, or
   a PCE to request a PCC to allocate a specific binding label/SID
   value.  This TLV is intended for TE LSPs established using RSVP-TE,
   SR, or any other future method.  Also, in the case of SR-TE LSPs, the
   TLV can carry a binding MPLS label (for SR-TE path with MPLS data-
   plane) or a binding IPv6 SID (e.g., IPv6 address for SR-TE paths with
   IPv6 data-plane).  Binding value means either MPLS label or SID
   throughout this document.

   Additionally, to support the PCE based central controller [RFC8283]
   operation where the PCE would take responsibility for managing some
   part of the MPLS label space for each of the routers that it
   controls, the PCE could directly make the binding label/SID
   allocation and inform the PCC.  See
   [I-D.ietf-pce-pcep-extension-for-pce-controller] for details.

2.  Terminology

   The following terminologies are used in this document:

   BSID:  Binding Segment Identifier.

   LER:  Label Edge Router.

   LSP:  Label Switched Path.

   LSR:  Label Switching Router.

   PCC:  Path Computation Client.

   PCE:  Path Computation Element

   PCEP:  Path Computation Element Protocol.

   RSVP-TE:  Resource ReserVation Protocol-Traffic Engineering.

   SID:  Segment Identifier.

   SR:  Segment Routing.

   SRGB:  Segment Routing Global Block.

   SRLB:  Segment Routing Local Block.

   TLV:  Type, Length, and Value.





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3.  Path Binding TLV

   The new optional TLV is called "TE-PATH-BINDING TLV" (whose format is
   shown in the figure below) is defined to carry binding label or SID
   for a TE path.  This TLV is associated with the LSP object specified
   in ([RFC8231]).  The type of this TLV is to be allocated by IANA.

       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            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      BT       |                 Reserved                      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~            Binding Value (variable length)                    ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                       Figure 2: TE-PATH-BINDING TLV

   TE-PATH-BINDING TLV is a generic TLV such that it is able to carry
   MPLS label binding as well as SRv6 Binding SID.  It is formatted
   according to the rules specified in [RFC5440].

   Binding Type (BT): A one byte field identifies the type of binding
   included in the TLV.  This document specifies the following BT
   values:

   o  BT = 0: The binding value is an MPLS label carried in the format
      specified in [RFC5462] where only the label value is valid, and
      other fields (TC, S, and TTL) fields MUST be considered invalid.
      The Length MUST be set to 6.

   o  BT = 1: Similar to the case where BT is 0 except that all the
      fields on the MPLS label entry are set on transmission.  However,
      the receiver MAY choose to override TC, S, and TTL values
      according its local policy.

   o  BT = 2: The binding value is a SRv6 SID with a format of an 16
      byte IPv6 address, representing the binding SID for SRv6.

   Reserved: MUST be set to 0 while sending and ignored on receipt.

   Binding Value: A variable length field, padded with trailing zeros to
   a 4-byte boundary.  For the BT as 0, the 20 bits represents the MPLS
   label.  For the BT as 1, the 32-bits represents the label stack entry
   as per [RFC5462].  For the BT as 2, the 128-bits represent the SRv6
   SID.



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4.  Operation

   The binding value is allocated by the PCC and reported to a PCE via
   PCRpt message.  If a PCE does not recognize the TE-PATH-BINDING TLV,
   it would ignore the TLV in accordance with ([RFC5440]).  If a PCE
   recognizes the TLV but does not support the TLV, it MUST send PCErr
   with Error-Type = 2 (Capability not supported).

   If a TE-PATH-BINDING TLV is absent in PCRpt message, PCE MUST assume
   that the corresponding LSP does not have any binding.  If there are
   more than one TE-PATH-BINDING TLVs, only the first TLV MUST be
   processed and the rest MUST be silently ignored.  If a PCE recognizes
   an invalid binding value (e.g., label value from the reserved label
   space when MPLS label binding is used), it MUST send the PCErr
   message with Error-Type = 10 ("Reception of an invalid object") and
   Error Value = 2 ("Bad label value") as specified in
   [I-D.ietf-pce-segment-routing].

   If a PCE requires a PCC to allocate a specific binding value, it may
   do so by sending a PCUpd or PCInitiate message containing a TE-PATH-
   BINDING TLV.  If the value can be successfully allocated, the PCC
   reports the binding value to the PCE.  If the PCC considers the
   binding value specified by the PCE invalid, it MUST send a PCErr
   message with Error-Type = TBD2 ("Binding label/SID failure") and
   Error Value = TBD3 ("Invalid SID").  If the binding value is valid,
   but the PCC is unable to allocate the binding value, it MUST send a
   PCErr message with Error-Type = TBD2 ("Binding label/SID failure")
   and Error Value = TBD4 ("Unable to allocate the specified label/
   SID").

   If a PCC receives TE-PATH-BINDING TLV in any message other than PCUpd
   or PCInitiate, it MUST close the corresponding PCEP session with the
   reason "Reception of a malformed PCEP message" (according to
   [RFC5440]).  Similarly, if a PCE receives a TE-PATH-BINDING TLV in
   any message other than a PCRpt or if the TE-PATH-BINDING TLV is
   associated with any object other than LSP object, the PCE MUST close
   the corresponding PCEP session with the reason "Reception of a
   malformed PCEP message" (according to [RFC5440]).

   If a PCC wishes to withdraw or modify a previously reported binding
   value, it MUST send a PCRpt message without any TE-PATH-BINDING TLV
   or with the TE-PATH-BINDING TLV containing the new binding value
   respectively.

   If a PCE wishes to modify a previously requested binding value, it
   MUST send a PCUpd message with TE-PATH-BINDING TLV containing the new
   binding value.  Absence of TE-PATH-BINDING TLV in PCUpd message means




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   that the PCE does not specify a binding value in which case the
   binding value allocation is governed by the PCC's local policy.

   If a PCC receives a valid binding value from a PCE which is different
   than the current binding value, it MUST try to allocate the new
   value.  If the new binding value is successfully allocated, the PCC
   MUST report the new value to the PCE.  Otherwise, it MUST send a
   PCErr message with Error-Type = TBD2 ("Binding label/SID failure")
   and Error Value = TBD4 ("Unable to allocate the specified label/
   SID").

   In some cases, a stateful PCE can request the PCC to allocate a
   binding value.  It may do so by sending a PCUpd message containing an
   empty TE-PATH-BINDING TLV, i.e., no binding value is specified
   (making the length field of the TLV as 2).  A PCE can also make the
   request PCC to allocate a binding at the time of initiation by
   sending a PCInitiate message with an empty TE-PATH-BINDING TLV.

5.  Binding SID in SR-ERO

   In PCEP messages, LSP route information is carried in the Explicit
   Route Object (ERO), which consists of a sequence of subobjects.
   [I-D.ietf-pce-segment-routing] defines a new ERO subobject "SR-ERO
   subobject" capable of carrying a SID as well as the identity of the
   node/adjacency (NAI) represented by the SID.  The NAI Type (NT) field
   indicates the type and format of the NAI contained in the SR-ERO.  In
   case of binding SID, the NAI MUST NOT be included and NT MUST be set
   to zero.  So as per Section 5.2.1 of [I-D.ietf-pce-segment-routing],
   for NT=0, the F bit is set to 1, the S bit needs to be zero and the
   Length is 8.  Further the M bit is set.  If these conditions are not
   met, the entire ERO MUST be considered invalid and a PCErr message is
   sent with Error-Type = 10 ("Reception of an invalid object") and
   Error-Value = 11 ("Malformed object").

6.  Binding SID in SRv6-ERO/

   [I-D.ietf-pce-segment-routing] defines a new ERO subobject "SRv6-ERO
   subobject" for SRv6 SID.  The NAI MUST NOT be included and NT MUST be
   set to zero.  So as per Section 5.2.1 of
   [I-D.ietf-pce-segment-routing], for NT=0, the F bit is set to 1, the
   S bit needs to be zero and the Length is 24.  If these conditions are
   not met, the entire ERO is considered invalid and a PCErr message is
   sent with Error-Type = 10 ("Reception of an invalid object") and
   Error-Value = 11 ("Malformed object") (as per
   [I-D.ietf-pce-segment-routing]).






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7.  Implementation Status

   [Note to the RFC Editor - remove this section before publication, as
   well as remove the reference to RFC 7942.]

   This section records the status of known implementations of the
   protocol defined by this specification at the time of posting of this
   Internet-Draft, and is based on a proposal described in [RFC7942].
   The description of implementations in this section is intended to
   assist the IETF in its decision processes in progressing drafts to
   RFCs.  Please note that the listing of any individual implementation
   here does not imply endorsement by the IETF.  Furthermore, no effort
   has been spent to verify the information presented here that was
   supplied by IETF contributors.  This is not intended as, and must not
   be construed to be, a catalog of available implementations or their
   features.  Readers are advised to note that other implementations may
   exist.

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

7.1.  Huawei

   o  Organization: Huawei

   o  Implementation: Huawei's Router and Controller

   o  Description: An experimental code-point is used and plan to
      request early code-point allocation from IANA after WG adoption.

   o  Maturity Level: Production

   o  Coverage: Full

   o  Contact: chengli13@huawei.com

8.  Security Considerations

   The security considerations described in [RFC5440], [RFC8231],
   [RFC8281] and [I-D.ietf-pce-segment-routing] are applicable to this
   specification.  No additional security measure is required.

   As described [I-D.ietf-pce-segment-routing], SR allows a network
   controller to instantiate and control paths in the network.  A rouge



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   PCE can manipulate binding SID allocations to move traffic around for
   some other LSPs that uses BSID in its SR-ERO.

   Thus, as per [RFC8231], it is RECOMMENDED that these PCEP extensions
   only be activated on authenticated and encrypted sessions across PCEs
   and PCCs belonging to the same administrative authority, using
   Transport Layer Security (TLS) [RFC8253], as per the recommendations
   and best current practices in BCP195 [RFC7525] (unless explicitly set
   aside in [RFC8253]).

9.  Manageability Considerations

   All manageability requirements and considerations listed in
   [RFC5440], [RFC8231], and [I-D.ietf-pce-segment-routing] apply to
   PCEP protocol extensions defined in this document.  In addition,
   requirements and considerations listed in this section apply.

9.1.  Control of Function and Policy

   A PCC implementation SHOULD allow the operator to configure the
   policy based on which PCC needs to allocates the binding label/SID.

9.2.  Information and Data Models

   The PCEP YANG module [I-D.ietf-pce-pcep-yang] could be extended to
   include policy configuration for binding label/SID allocation.

9.3.  Liveness Detection and Monitoring

   Mechanisms defined in this document do not imply any new liveness
   detection and monitoring requirements in addition to those already
   listed in [RFC5440].

9.4.  Verify Correct Operations

   Mechanisms defined in this document do not imply any new operation
   verification requirements in addition to those already listed in
   [RFC5440], [RFC8231], and [I-D.ietf-pce-segment-routing].

9.5.  Requirements On Other Protocols

   Mechanisms defined in this document do not imply any new requirements
   on other protocols.








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9.6.  Impact On Network Operations

   Mechanisms defined in [RFC5440], [RFC8231], and
   [I-D.ietf-pce-segment-routing] also apply to PCEP extensions defined
   in this document.  Further, the mechanism described in this document
   can help the operator to request control of the LSPs at a particular
   PCE.

10.  IANA Considerations

10.1.  PCEP TLV Type Indicators

   This document defines a new PCEP TLV; IANA is requested to make the
   following allocations from the "PCEP TLV Type Indicators" sub-
   registry of the PCEP Numbers registry, as follows:

                Value    Name                  Reference

                 TBD1    TE-PATH-BINDING       This document

10.1.1.  TE-PATH-BINDING TLV

   IANA is requested to create a sub-registry to manage the value of the
   Binding Type field in the TE-PATH-BINDING TLV.

                Value    Description           Reference

                  0      MPLS Label            This document
                  1      MPLS Label Stack      This document
                         Entry
                  2      SRv6 SID              This document

10.2.  PCEP Error Type and Value

   This document defines a new Error-type and Error-Values for the PCErr
   message.  IANA is requested to allocate new error-type and error-
   values within the "PCEP-ERROR Object Error Types and Values"
   subregistry of the PCEP Numbers registry, as follows:


   Error-Type   Meaning
   ----------   -------
   TBD2         Binding label/SID failure:

                 Error-value = TBD3:                 Invalid SID
                 Error-value = TBD4:                 Unable to allocate
                                                     the specified
                                                     label/SID



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11.  Acknowledgements

   We like to thank Milos Fabian for his valuable comments.

12.  References

12.1.  Normative References

   [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>.

   [RFC7525]  Sheffer, Y., Holz, R., and P. Saint-Andre,
              "Recommendations for Secure Use of Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
              2015, <https://www.rfc-editor.org/info/rfc7525>.

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

   [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>.

   [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>.








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   [RFC8253]  Lopez, D., Gonzalez de Dios, O., Wu, Q., and D. Dhody,
              "PCEPS: Usage of TLS to Provide a Secure Transport for the
              Path Computation Element Communication Protocol (PCEP)",
              RFC 8253, DOI 10.17487/RFC8253, October 2017,
              <https://www.rfc-editor.org/info/rfc8253>.

   [RFC8281]  Crabbe, E., Minei, I., Sivabalan, S., and R. Varga, "Path
              Computation Element Communication Protocol (PCEP)
              Extensions for PCE-Initiated LSP Setup in a Stateful PCE
              Model", RFC 8281, DOI 10.17487/RFC8281, December 2017,
              <https://www.rfc-editor.org/info/rfc8281>.

   [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>.

   [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.

12.2.  Informative References

   [RFC4655]  Farrel, A., Vasseur, J., and J. Ash, "A Path Computation
              Element (PCE)-Based Architecture", RFC 4655,
              DOI 10.17487/RFC4655, August 2006,
              <https://www.rfc-editor.org/info/rfc4655>.

   [RFC8283]  Farrel, A., Ed., Zhao, Q., Ed., Li, Z., and C. Zhou, "An
              Architecture for Use of PCE and the PCE Communication
              Protocol (PCEP) in a Network with Central Control",
              RFC 8283, DOI 10.17487/RFC8283, December 2017,
              <https://www.rfc-editor.org/info/rfc8283>.

   [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.

   [I-D.ietf-idr-bgp-prefix-sid]
              Previdi, S., Filsfils, C., Lindem, A., Sreekantiah, A.,
              and H. Gredler, "Segment Routing Prefix SID extensions for
              BGP", draft-ietf-idr-bgp-prefix-sid-27 (work in progress),
              June 2018.




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   [I-D.ietf-pce-pcep-extension-for-pce-controller]
              Zhao, Q., Li, Z., Negi, M., and C. Zhou, "PCEP Procedures
              and Protocol Extensions for Using PCE as a Central
              Controller (PCECC) of LSPs", draft-ietf-pce-pcep-
              extension-for-pce-controller-02 (work in progress), July
              2019.

   [I-D.ietf-pce-pcep-yang]
              Dhody, D., Hardwick, J., Beeram, V., and J. Tantsura, "A
              YANG Data Model for Path Computation Element
              Communications Protocol (PCEP)", draft-ietf-pce-pcep-
              yang-13 (work in progress), October 2019.







































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Appendix A.  Contributor Addresses

   Dhruv Dhody
   Huawei Technologies
   Divyashree Techno Park, Whitefield
   Bangalore, Karnataka  560066
   India

   EMail: dhruv.ietf@gmail.com

   Mahendra Singh Negi

   EMail: mahend.ietf@gmail.com

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
   Apstra, Inc.

   EMail: jefftant.ietf@gmail.com


   Jonathan Hardwick
   Metaswitch Networks
   100 Church Street
   Enfield, Middlesex
   UK

   EMail: Jonathan.Hardwick@metaswitch.com




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   Stefano Previdi
   Huawei Technologies

   EMail: stefano@previdi.net


   Cheng Li
   Huawei Technologies
   Huawei Campus, No. 156 Beiqing Rd.
   Beijing  100095
   China

   EMail: chengli13@huawei.com






































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