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PCE Working Group                                                  Z. Li
Internet-Draft                                                   S. Peng
Intended status: Standards Track                     Huawei Technologies
Expires: May 29, 2021                                            M. Negi
                                                             RtBrick Inc
                                                                 Q. Zhao
                                                        Etheric Networks
                                                                 C. Zhou
                                                                     HPE
                                                       November 25, 2020


   PCEP Procedures and Protocol Extensions for Using PCE as a Central
  Controller (PCECC) for Segment Routing (SR) MPLS Segment Identifier
                   (SID) Allocation and Distribution.
           draft-zhao-pce-pcep-extension-pce-controller-sr-09

Abstract

   The Path Computation Element (PCE) is a core component of Software-
   Defined Networking (SDN) systems.  It can compute optimal paths for
   traffic across a network and can also update the paths to reflect
   changes in the network or traffic demands.

   PCE was developed to derive paths for MPLS Label Switched Paths
   (LSPs), which are supplied to the head end of the LSP using the Path
   Computation Element Communication Protocol (PCEP).  But SDN has a
   broader applicability than signaled (G)MPLS traffic-engineered (TE)
   networks, and the PCE may be used to determine paths in a range of
   use cases.  PCEP has been proposed as a control protocol for use in
   these environments to allow the PCE to be fully enabled as a central
   controller.

   A PCE-based Central Controller (PCECC) can simplify the processing of
   a distributed control plane by blending it with elements of SDN and
   without necessarily completely replacing it.  Thus, the LSP can be
   calculated/set up/initiated and the label forwarding entries can also
   be downloaded through a centralized PCE server to each network device
   along the path while leveraging the existing PCE technologies as much
   as possible.

   This document specifies the procedures and PCEP extensions when a
   PCE-based controller is also responsible for configuring the
   forwarding actions on the routers, in addition to computing the paths
   for packet flows in a segment routing (SR) network and telling the
   edge routers what instructions to attach to packets as they enter the
   network.  PCECC is further enhanced for SR SID (Segment Identifier)
   allocation and distribution.



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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
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   This Internet-Draft will expire on May 29, 2021.

Copyright Notice

   Copyright (c) 2020 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
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   5
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  PCECC SR  . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  PCEP Requirements . . . . . . . . . . . . . . . . . . . . . .   6
   5.  Procedures for Using the PCE as a Central Controller (PCECC)
       in Segment Routing  . . . . . . . . . . . . . . . . . . . . .   6
     5.1.  Stateful PCE Model  . . . . . . . . . . . . . . . . . . .   6
     5.2.  New LSP Functions . . . . . . . . . . . . . . . . . . . .   6
     5.3.  PCECC Capability Advertisement  . . . . . . . . . . . . .   7
     5.4.  PCEP session IP address and TED Router ID . . . . . . . .   7
     5.5.  LSP Operations  . . . . . . . . . . . . . . . . . . . . .   8
       5.5.1.  PCECC Segment Routing (SR)  . . . . . . . . . . . . .   8
         5.5.1.1.  PCECC SR Node/Prefix SID allocation . . . . . . .   8



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         5.5.1.2.  PCECC SR Adjacency Label allocation . . . . . . .  10
         5.5.1.3.  Redundant PCEs  . . . . . . . . . . . . . . . . .  12
         5.5.1.4.  Re Delegation and Clean up  . . . . . . . . . . .  12
         5.5.1.5.  Synchronization of Label Allocations  . . . . . .  13
         5.5.1.6.  PCC-Based Allocations . . . . . . . . . . . . . .  13
         5.5.1.7.  Binding SID . . . . . . . . . . . . . . . . . . .  13
   6.  PCEP Messages . . . . . . . . . . . . . . . . . . . . . . . .  14
     6.1.  Central Control Instructions  . . . . . . . . . . . . . .  14
       6.1.1.  The PCInitiate Message  . . . . . . . . . . . . . . .  14
       6.1.2.  The PCRpt message . . . . . . . . . . . . . . . . . .  15
   7.  PCEP Objects  . . . . . . . . . . . . . . . . . . . . . . . .  16
     7.1.  OPEN Object . . . . . . . . . . . . . . . . . . . . . . .  16
       7.1.1.  PCECC Capability sub-TLV  . . . . . . . . . . . . . .  16
     7.2.  SR-TE Path Setup  . . . . . . . . . . . . . . . . . . . .  17
     7.3.  CCI Object  . . . . . . . . . . . . . . . . . . . . . . .  17
     7.4.  FEC Object  . . . . . . . . . . . . . . . . . . . . . . .  19
   8.  Implementation Status . . . . . . . . . . . . . . . . . . . .  21
     8.1.  Huawei's Proof of Concept based on ONOS . . . . . . . . .  22
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  22
   10. Manageability Considerations  . . . . . . . . . . . . . . . .  22
     10.1.  Control of Function and Policy . . . . . . . . . . . . .  22
     10.2.  Information and Data Models  . . . . . . . . . . . . . .  23
     10.3.  Liveness Detection and Monitoring  . . . . . . . . . . .  23
     10.4.  Verify Correct Operations  . . . . . . . . . . . . . . .  23
     10.5.  Requirements On Other Protocols  . . . . . . . . . . . .  23
     10.6.  Impact On Network Operations . . . . . . . . . . . . . .  23
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  23
     11.1.  PCECC-CAPABILITY sub-TLV . . . . . . . . . . . . . . . .  23
     11.2.  PCEP Object  . . . . . . . . . . . . . . . . . . . . . .  24
     11.3.  PCEP-Error Object  . . . . . . . . . . . . . . . . . . .  24
     11.4.  CCI Object Flag Field for SR . . . . . . . . . . . . . .  24
   12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  25
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  25
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  25
     13.2.  Informative References . . . . . . . . . . . . . . . . .  27
   Appendix A.  Contributor Addresses  . . . . . . . . . . . . . . .  30
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  31

1.  Introduction

   The Path Computation Element (PCE) [RFC4655] was developed to offload
   the path computation function from routers in an MPLS traffic-
   engineered network.  Since then, the role and function of the PCE has
   grown to cover a number of other uses (such as GMPLS [RFC7025]) and
   to allow delegated control [RFC8231] and PCE-initiated use of network
   resources [RFC8281].





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   According to [RFC7399], Software-Defined Networking (SDN) refers to a
   separation between the control elements and the forwarding components
   so that software running in a centralized system, called a
   controller, can act to program the devices in the network to behave
   in specific ways.  A required element in an SDN architecture is a
   component that plans how the network resources will be used and how
   the devices will be programmed.  It is possible to view this
   component as performing specific computations to place traffic flows
   within the network given knowledge of the availability of network
   resources, how other forwarding devices are programmed, and the way
   that other flows are routed.  This is the function and purpose of a
   PCE, and the way that a PCE integrates into a wider network control
   system (including an SDN system) is presented in [RFC7491].

   In early PCE implementations, where the PCE was used to derive paths
   for MPLS Label Switched Paths (LSPs), paths were requested by network
   elements (known as Path Computation Clients (PCCs)), and the results
   of the path computations were supplied to network elements using the
   Path Computation Element Communication Protocol (PCEP) [RFC5440].
   This protocol was later extended to allow a PCE to send unsolicited
   requests to the network for LSP establishment [RFC8281].

   [RFC8283] introduces the architecture for PCE as a central controller
   as an extension of the architecture described in [RFC4655] and
   assumes the continued use of PCEP as the protocol used between PCE
   and PCC.  [RFC8283] further examines the motivations and
   applicability for PCEP as a Southbound Interface (SBI), and
   introduces the implications for the protocol.
   [I-D.ietf-teas-pcecc-use-cases] describes the use cases for the PCE-
   based Central Controller (PCECC) architecture.

   [I-D.ietf-pce-pcep-extension-for-pce-controller] specify the
   procedures and PCEP extensions for using the PCE as the central
   controller for static LSPs, where LSPs can be provisioned as explicit
   label instructions at each hop on the end-to-end path.

   Segment Routing (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).  [RFC8402] provides an introduction to SR
   architecture.  The corresponding IS-IS and OSPF extensions are
   specified in [RFC8667] and [RFC8665] , respectively.  It relies on a
   series of forwarding instructions being placed in the header of a
   packet.  The segment routing architecture supports operations that
   can be used to steer packet flows in a network, thus providing a form
   of traffic engineering.  [RFC8664] specify the SR specific PCEP
   extensions.



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   PCECC may further use PCEP for SR SID (Segment Identifier) allocation
   and distribution on the SR nodes with some benefits.

   This document specifies the procedures and PCEP extensions when a
   PCE-based controller is also responsible for configuring the
   forwarding actions on the routers (SR SID allocation and distribution
   in this case), in addition to computing the paths for packet flows in
   a segment routing network and telling the edge routers what
   instructions to attach to packets as they enter the network.

   Only SR using MPLS dataplane (SR-MPLS) is in the scope of this
   document.  Refer [I-D.dhody-pce-pcep-extension-pce-controller-srv6]
   for use of PCECC technique for SR in IPv6 (SRv6) dataplane.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  Terminology

   Terminologies used in this document is the same as described in the
   draft [RFC8283] and [I-D.ietf-teas-pcecc-use-cases].

3.  PCECC SR

   [RFC8664] specifies extensions to PCEP that allow a stateful PCE to
   compute, update, or initiate SR-TE paths.  An ingress node of an SR-
   TE path appends all outgoing packets with a list of MPLS labels
   (SIDs).  This is encoded in SR-ERO subobject, capable of carrying a
   label (SID) as well as the identity of the node/adjacency label
   (SID).

   The notion of segment and SID is defined in [RFC8402], which fits the
   MPLS architecture [RFC3031] as the label which is managed by a local
   allocation process of LSR (similarly to other MPLS signaling
   protocols) [RFC8660].  The SR information such as node/adjacency
   label (SID) is flooded via IGP as specified in [RFC8667] and
   [RFC8665].

   As per [RFC8283], PCE as a central controller can allocate and
   provision the node/prefix/adjacency label (SID) via PCEP.

   The rest of the processing is similar to existing stateful PCE with
   SR mechanism.



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   For the purpose of this document, it is assumed that the label range
   to be used by a PCE is set on both PCEP peers.  Further, a global
   label range is assumed to be set on all PCEP peers in the SR domain.
   This document also allows a case where the label space is maintained
   by PCC itself, and the labels are allocated by the PCC, in this case,
   the PCE should request the allocation from PCC as described in
   Section 5.5.1.6.

4.  PCEP Requirements

   Following key requirements for PCECC-SR should be considered when`
   designing the PCECC-based solution:

   o  A PCEP speaker supporting this draft needs to have the capability
      to advertise its PCECC-SR capability to its peers.

   o  PCEP procedures need to allow for PCC-based label/SID allocations.

   o  PCEP procedures need means to update (or clean up) the label-map
      entry to the PCC.

   o  PCEP procedures need to provide a mean to synchronize the SR
      labels allocations between the PCE to the PCC via PCEP messages.

5.  Procedures for Using the PCE as a Central Controller (PCECC) in
    Segment Routing

5.1.  Stateful PCE Model

   Active stateful PCE is described in [RFC8231].  PCE as a Central
   Controller (PCECC) reuses the existing active stateful PCE mechanism
   as much as possible to control the LSPs.

5.2.  New LSP Functions

   Several new functions are required in PCEP to support PCECC as
   described in [I-D.ietf-pce-pcep-extension-for-pce-controller].  This
   document reuses the existing messages to support PCECC-SR.

   The PCEP messages PCRpt, PCInitiate, PCUpd are used to send LSP
   Reports, LSP setup, and LSP update respectively.  The extended
   PCInitiate message described in
   [I-D.ietf-pce-pcep-extension-for-pce-controller] is used to download
   or clean up central controller's instructions (CCIs) (SR SID in the
   scope of this document).  The extended PCRpt message described in
   [I-D.ietf-pce-pcep-extension-for-pce-controller] is also used to
   report the CCIs (SR SIDs) from PCC to PCE.




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   [I-D.ietf-pce-pcep-extension-for-pce-controller] specify an object
   called CCI for the encoding of the central controller's instructions.
   This document extends the CCI by defining a new object-type for
   segment routing.  The PCEP messages are extended in this document to
   handle the PCECC operations for SR.

5.3.  PCECC Capability Advertisement

   During PCEP Initialization Phase, PCEP Speakers (PCE or PCC)
   advertise their support of PCECC extensions.  A PCEP Speaker includes
   the "PCECC Capability" sub-TLV, described in
   [I-D.ietf-pce-pcep-extension-for-pce-controller].

   A new S-bit is added in the PCECC-CAPABILITY sub-TLV to indicate
   support for PCECC-SR.  A PCC MUST set the S-bit in the PCECC-
   CAPABILITY sub-TLV and include the SR-PCE-CAPABILITY sub-TLV
   ([RFC8664]) in the OPEN Object (inside the PATH-SETUP-TYPE-CAPABILITY
   TLV) to support the PCECC SR extensions defined in this document.  If
   the S-bit is set in the PCECC-CAPABILITY sub-TLV and the SR-PCE-
   CAPABILITY sub-TLV is not advertised in the OPEN Object, PCE SHOULD
   send a PCErr message with Error-Type=19 (Invalid Operation) and
   Error-value=TBD4 (SR capability was not advertised) and terminate the
   session.

   The rest of the processing is as per
   [I-D.ietf-pce-pcep-extension-for-pce-controller].

5.4.  PCEP session IP address and TED Router ID

   A PCE may construct its Traffic Engineering Database (TED) by
   participating in the IGP ([RFC3630] and [RFC5305] for MPLS-TE;
   [RFC4203] and [RFC5307] for GMPLS).  An alternative is offered by
   BGP-LS [RFC7752] and [I-D.dhodylee-pce-pcep-ls].

   A PCEP [RFC5440] speaker could use any local IP address while
   creating a TCP session.  It is important to link the session IP
   address with the Router ID in TED for successful PCECC operations.

   During PCEP Initialization Phase, the PCC SHOULD advertise the TE
   mapping information by including the "Node Attributes TLV"
   [I-D.dhodylee-pce-pcep-ls] with "IPv4/IPv6 Router-ID of Local Node",
   in the OPEN Object for this purpose.  [RFC7752] describes the usage
   as auxiliary Router-IDs that the IGP might be using, e.g., for TE
   purposes.  If there are more than one auxiliary Router-ID of a given
   type, then multiple TLVs are used to encode them.

   If "IPv4/IPv6 Router-ID" TLV is not present, the TCP session IP
   address is directly used for mapping purpose.



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5.5.  LSP Operations

   [RFC8664] specify the PCEP extension to allow a stateful PCE to
   compute and initiate SR-TE paths, as well as a PCC to request a path
   subject to certain constraint(s) and optimization criteria in SR
   networks.

   The Path Setup Type for segment routing (PST=1) is used on the PCEP
   session with the Ingress as per [RFC8664].

5.5.1.  PCECC Segment Routing (SR)

   Segment Routing (SR) as described in [RFC8402] depends on "segments"
   that are advertised by Interior Gateway Protocols (IGPs).  The SR-
   node allocates and advertises the SID (node, adj, etc) and flood them
   via the IGP.  This document proposes a new mechanism where PCE
   allocates the SID (label/index/SID) centrally and uses PCEP to
   advertise them.  In some deployments, PCE (and PCEP) are better
   suited than IGP because of the centralized nature of PCE and direct
   TCP based PCEP sessions to the node.

5.5.1.1.  PCECC SR Node/Prefix SID allocation

   Each node (PCC) is allocated a node-SID by the PCECC.  The PCECC
   sends PCInitiate message to update the label map of each node to all
   the nodes in the domain.  The TE router ID is determined from the TED
   or from "IPv4/IPv6 Router-ID" Sub-TLV [I-D.dhodylee-pce-pcep-ls], in
   the OPEN Object Section 5.4.

   It is RECOMMENDED that PCEP session with PCECC-SR capability to use a
   different session IP address during TCP session establishment than
   the node Router ID in TEDB, to make sure that the PCEP session does
   not get impacted by the SR Node/Prefix Label maps (Section 5.4).

   If a node (PCC) receives a PCInitiate message with a CCI encoding a
   SID, out of the range set aside for the SR Global Block (SRGB), it
   MUST send a PCErr message with Error-type=TBD (PCECC failure) and
   Error-value=TBD (Label out of range) (defined in
   [I-D.ietf-pce-pcep-extension-for-pce-controller]) and MUST include
   the SRP object to specify the error is for the corresponding central
   control instruction via the PCInitiate message.

   On receiving the label map, each node (PCC) uses the local routing
   information to determine the next-hop and download the label
   forwarding instructions accordingly.  The PCInitiate message in this
   case does not use the LSP object but uses a new FEC object defined in
   this document.




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                  +---------+                           +-------+
                  |PCC      |                           |  PCE  |
                  |192.0.2.3|                           +-------+
           +------|         |                               |
           | PCC  +---------+                               |
           | 192.0.2.2| |                                   |
    +------|          | |                                   |
    |PCC   +----------+ |                                   |
    |192.0.2.1| |       |                                   |
    +---------+ |       |                                   |
        |       |       |                                   |
        |<--------PCInitiate,FEC=192.0.2.1------------------| Label Map
        |       |       |    CC-ID=X                        | update
        |--------PCRpt,CC-ID=X----------------------------->| CCI
        |Find   |       |                                   |
        |Nexthop|<--------PCInitiate,FEC=192.0.2.1----------| Label Map
        |locally|       |            CC-ID=Y                | update
        |       |-------PCRpt,CC-ID=Y---------------------->| CCI
        |       |       |                                   |
        |       |       |<----PCInitiate,FEC=192.0.2.1------| Label Map
        |       |       |                CC-ID=Z            | update
        |       |       |-----PCRpt,CC-ID=Z---------------->| CCI
        |       |       |                                   |


   The forwarding behavior and the end result is similar to IGP based
   "Node-SID" in SR.  Thus, from anywhere in the domain, it enforces the
   ECMP-aware shortest-path forwarding of the packet towards the related
   node as per [RFC8402].

   PCE relies on the Node/Prefix Label clean up using the same
   PCInitiate message as per [RFC8281].

   The above example Figure 1 depicts the FEC and PCEP speakers that
   uses IPv4 address.  Similarly an IPv6 address (such as 2001:DB8::1)
   can be used during PCEP session establishment in the FEC object as
   described in this specification.

   In the case where the label/SID allocation is made by the PCC itself
   (see Section 5.5.1.6), the PCE could request an allocation to be made
   by the PCC, and where the PCC would send a PCRpt with the allocated
   label/SID encoded in the CC-ID object as shown in Figure 2.









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                  +---------+                           +-------+
                  |PCC      |                           |  PCE  |
                  |192.0.2.3|                           +-------+
           +------|         |                               |
           | PCC  +---------+                               |
           | 192.0.2.2| |                                   |
    +------|          | |                                   |
    |PCC   +----------+ |                                   |
    |192.0.2.1| |       |                                   |
    +---------+ |       |                                   |
        |       |       |                                   |
        |<--------PCInitiate,FEC=192.0.2.1------------------| Label Map
        |       |       |    CC-ID=X,C=1                    | request
        |--------PCRpt,CC-ID=X,Label----------------------->| CCI
        |Find   |       |                                   |
        |Nexthop|<--------PCInitiate,FEC=192.0.2.1----------| Label Map
        |locally|       |            CC-ID=Y,C=0,Label      | update
        |       |-------PCRpt,CC-ID=Y---------------------->| CCI
        |       |       |                                   |
        |       |       |<----PCInitiate,FEC=192.0.2.1------| Label Map
        |       |       |                CC-ID=Z,C=0,Label  | update
        |       |       |-----PCRpt,CC-ID=Z---------------->| CCI
        |       |       |                                   |


   It should be noted that in this example, the request is made to the
   node 192.0.2.1 with C bit set in the CCI object to indicate that the
   allocation needs to be done by this PCC and it responds with the
   allocated label/SID to the PCE.  The PCE would further inform the
   other PCCs in the network about the label-map allocation without
   setting the C bit.

5.5.1.2.  PCECC SR Adjacency Label allocation

   For PCECC-SR, apart from node-SID, Adj-SID is used where each
   adjacency is allocated an Adj-SID by the PCECC.  The PCECC sends the
   PCInitiate message to update the label map of each adjacency to the
   corresponding nodes in the domain.  Each node (PCC) download the
   label forwarding instructions accordingly.  Similar to SR Node/Prefix
   Label allocation, the PCInitiate message in this case does not use
   the LSP object but uses the new FEC object defined in this document.










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                    +---------+                         +-------+
                    |PCC      |                         |  PCE  |
                    |192.0.2.3|                         +-------+
             +------|         |                             |
             | PCC  +---------+                             |
             | 192.0.2.2| |                                 |
      +------|          | |                                 |
      |PCC   +----------+ |                                 |
      |192.0.2.1|  |      |                                 |
      +---------+  |      |                                 |
          |        |      |                                 |
          |<-------PCInitiate,FEC=198.51.100.1--------------| Label Map
          |        |      |       198.51.100.2              | update
          |        |      |   CC-ID=A                       | CCI
          |--------PCRpt,CC-ID=A--------------------------->|
          |        |      |                                 |
          |        |<------PCInitiate,FEC=198.51.100.2------| Label Map
          |        |      |               198.51.100.1      | update
          |        |      |           CC-ID=B               | CCI
          |        |-------PCRpt,CC-ID=B------------------->|
          |        |      |                                 |


   The forwarding behavior and the end result is similar to IGP based
   "Adj-SID" in SR.

   PCE relies on the Adj label clean up using the same PCInitiate
   message as per [RFC8281].

   The above example Figure 3 depicts FEC object and PCEP speakers that
   uses an IPv4 address.  Similarly an IPv6 address (such as
   2001:DB8::1, 2001:DB8::2) can be used during the PCEP session
   establishment in the FEC object as described in this specification.

   The handling of adjacencies on the LAN subnetworks is specified in
   [RFC8402].  PCECC MUST assign Adj-SID for every pair of routers in
   the LAN.  The rest of the protocol mechanism remains the same.

   In the case where the label/SID map allocation is made by the PCC
   itself (see Section 5.5.1.6), the PCE could request an allocation to
   be made by the PCC, and where the PCC would send a PCRpt with the
   allocated label/SID encoded in the CC-ID object as shown in Figure 4.









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                    +---------+                         +-------+
                    |PCC      |                         |  PCE  |
                    |192.0.2.3|                         +-------+
             +------|         |                             |
             | PCC  +---------+                             |
             | 192.0.2.2| |                                 |
      +------|          | |                                 |
      |PCC   +----------+ |                                 |
      |192.0.2.1|  |      |                                 |
      +---------+  |      |                                 |
          |        |      |                                 |
          |<-------PCInitiate,FEC=198.51.100.1--------------| Label Map
          |        |      |        198.51.100.2             | request
          |        |      |    CC-ID=A,C=1                  | CCI
          |--------PCRpt,CC-ID=A,Label1-------------------->|
          |        |      |                                 |
          |        |<------PCInitiate,FEC=198.51.100.2------| Label Map
          |        |      |               198.51.100.1      | request
          |        |      |           CC-ID=B,C=1           | CCI
          |        |-------PCRpt,CC-ID=B,Label2------------>|
          |        |      |                                 |


   In this example, the request is made to the node 192.0.2.1 with the C
   bit set in the CCI object to indicate that the allocation needs to be
   done by this PCC for the adjacency (198.51.100.1 - 198.51.100.2) and
   it responds with the allocated label/SID to the PCE.  Similarly,
   another request is made to the node 192.0.2.2 with the C bit set in
   the CCI object to indicate that the allocation needs to be done by
   this PCC for the adjacency (198.51.100.2 - 198.51.100.1).

5.5.1.3.  Redundant PCEs

   [I-D.litkowski-pce-state-sync] describes the synchronization
   mechanism between the stateful PCEs.  The SR SIDs allocated by a PCE
   MUST also be synchronized among PCEs for PCECC SR state
   synchronization.  Note that the SR SIDs are independent of the SR-TE
   LSPs, and remains intact till any topology change.  The redundant
   PCEs MUST have a common view of all SR SIDs allocated in the domain.

5.5.1.4.  Re Delegation and Clean up

   [I-D.ietf-pce-pcep-extension-for-pce-controller] describes the action
   needed for CCIs for the static LSPs on a terminated session.  Same
   holds true for the CCI for SR SID as well.






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5.5.1.5.  Synchronization of Label Allocations

   [I-D.ietf-pce-pcep-extension-for-pce-controller] describes the
   synchronization of Central Controller's Instructions (CCI) via LSP
   state synchronization as described in [RFC8231] and [RFC8232].  Same
   procedures are applied for the CCI for SR SID as well.

5.5.1.6.  PCC-Based Allocations

   The PCE can request the PCC to allocate the label/SID using the
   PCInitiate message.  The C flag in the CCI object is set to 1 to
   indicate that the allocation needs to be done by the PCC.  The PCC
   would allocate the SID/Label/Index and would report to the PCE using
   the PCRpt message.

   If the value of the SID/Label/Index is 0 and the C flag is set to 1,
   it indicates that the PCE is requesting the allocation to be done by
   the PCC.  If the SID/Label/Index is 'n' and the C flag is set to 1 in
   the CCI object, it indicates that the PCE requests a specific value
   'n' for the SID/Label/Index.  If the allocation is successful, the
   PCC should report via PCRpt message with the CCI object.  Else, it
   MUST send a PCErr message with Error-Type = TBD ("PCECC failure") and
   Error Value = TBD ("Invalid CCI") (defined in
   [I-D.ietf-pce-pcep-extension-for-pce-controller]).  If the value of
   the SID/Label/Index in the CCI object is valid, but the PCC is unable
   to allocate it, it MUST send a PCErr message with Error-Type = TBD
   ("PCECC failure") and Error Value = TBD ("Unable to allocate the
   specified CCI") (defined in
   [I-D.ietf-pce-pcep-extension-for-pce-controller]).

   If the PCC wishes to withdraw or modify the previously assigned
   label/SID, it MUST send a PCRpt message without any SID/Label/Index
   or with the SID/Label/Index containing the new value respectively in
   the CCI object.  The PCE would further trigger the removal of the
   central controller instruction as per this document.

5.5.1.7.  Binding SID

   A PCECC can allocate and provision the node/prefix/adjacency label
   (SID) via PCEP.  Another SID called binding SID is described in
   [I-D.ietf-pce-binding-label-sid], the PCECC mechanism can also be
   used to allocate the binding SID.

   A procedure for binding label/SID allocation is described in
   [I-D.ietf-pce-pcep-extension-for-pce-controller] and is applicable
   for all path setup types (including SR paths).





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6.  PCEP Messages

   As defined in [RFC5440], a PCEP message consists of a common header
   followed by a variable-length body made of a set of objects that can
   be either mandatory or optional.  An object is said to be mandatory
   in a PCEP message when the object must be included for the message to
   be considered valid.  For each PCEP message type, a set of rules is
   defined that specify the set of objects that the message can carry.
   An implementation MUST form the PCEP messages using the object
   ordering specified in this document.

6.1.  Central Control Instructions

6.1.1.  The PCInitiate Message

   The PCInitiate message defined in [RFC8281] and extended in
   [I-D.ietf-pce-pcep-extension-for-pce-controller] is further extended
   to support SR based central control instructions.

   The format of the extended PCInitiate message is as follows:































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        <PCInitiate Message> ::= <Common Header>
                                 <PCE-initiated-lsp-list>
     Where:
        <Common Header> is defined in [RFC5440]

        <PCE-initiated-lsp-list> ::= <PCE-initiated-lsp-request>
                                     [<PCE-initiated-lsp-list>]

        <PCE-initiated-lsp-request> ::=
                             (<PCE-initiated-lsp-instantiation>|
                              <PCE-initiated-lsp-deletion>|
                              <PCE-initiated-lsp-central-control>)

        <PCE-initiated-lsp-central-control> ::= <SRP>
                                                (<LSP>
                                                <cci-list>)|
                                                (<FEC>
                                                <CCI>)

        <cci-list> ::=  <CCI>
                        [<cci-list>]

     Where:
         <PCE-initiated-lsp-instantiation> and
         <PCE-initiated-lsp-deletion> are as per
         [RFC8281].

        The LSP and SRP object is defined in [RFC8231].


   When the PCInitiate message is used to distribute SR SIDs, the SRP,
   the FEC and the CCI objects MUST be present.  The error handling for
   missing SRP or CCI object is as per
   [I-D.ietf-pce-pcep-extension-for-pce-controller].  If the FEC object
   is missing, the receiving PCC MUST send a PCErr message with Error-
   type=6 (Mandatory Object missing) and Error-value=TBD5 (FEC object
   missing).

   To clean up, the R (remove) bit in the SRP object and the
   corresponding FEC and the CCI object are included.

6.1.2.  The PCRpt message

   The PCRpt message can be used to report the SR central controller
   instructions received from the PCECC during the state synchronization
   phase or as an acknowledgment to the PCInitiate message.

   The format of the PCRpt message is as follows:



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         <PCRpt Message> ::= <Common Header>
                             <state-report-list>
      Where:

         <state-report-list> ::= <state-report>[<state-report-list>]

         <state-report> ::= (<lsp-state-report>|
                             <central-control-report>)

         <lsp-state-report> ::= [<SRP>]
                                <LSP>
                                <path>

         <central-control-report> ::= [<SRP>]
                                      (<LSP>
                                      <cci-list>)|
                                      (<FEC>
                                      <CCI>)

         <cci-list> ::=  <CCI>
                         [<cci-list>]


       Where:
         <path> is as per [RFC8231] and the LSP and SRP object are
         also defined in [RFC8231].

   When PCRpt message is used to report the label map allocations, the
   FEC and CCI objects MUST be present.  The error handling for the
   missing CCI object is as per
   [I-D.ietf-pce-pcep-extension-for-pce-controller].  If the FEC object
   is missing, the receiving PCE MUST send a PCErr message with Error-
   type=6 (Mandatory Object missing) and Error-value=TBD5 (FEC object
   missing).

7.  PCEP Objects

7.1.  OPEN Object

7.1.1.  PCECC Capability sub-TLV

   [I-D.ietf-pce-pcep-extension-for-pce-controller] defined the PCECC-
   CAPABILITY sub-TLV.

   A new S-bit is defined in PCECC-CAPABILITY sub-TLV for PCECC-SR:






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       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |               Type=TBD        |            Length=4           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                             Flags                         |S|L|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   [Editor's Note - The above figure is included for ease of the reader
   but should be removed before publication.]

   S (PCECC-SR-CAPABILITY - 1 bit - TBD1): If set to 1 by a PCEP
   speaker, it indicates that the PCEP speaker is capable of PCECC-SR
   capability and the PCE allocates the Node and Adj label/SID on this
   session.

7.2.  SR-TE Path Setup

   The PATH-SETUP-TYPE TLV is defined in [RFC8408].  A PST value of 1 is
   used when Path is setup via SR mode as per [RFC8664].  The procedure
   for SR-TE path setup as specified in [RFC8664] remians unchanged.

7.3.  CCI Object

   The Central Control Instructions (CCI) Object used by the PCE to
   specify the controller instructions is defined in
   [I-D.ietf-pce-pcep-extension-for-pce-controller].  This document
   defines another object-type for SR-MPLS purpose.

   CCI Object-Type is TBD6 for SR-MPLS as 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                            CC-ID                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      MT-ID    |    Algorithm  |    Flags      |B|P|G|C|N|E|V|L|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   //                  SID/Label/Index (variable)                 //
   +---------------------------------------------------------------+
   |                                                               |
   //                        Optional TLV                         //
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+





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   The field CC-ID is as described in
   [I-D.ietf-pce-pcep-extension-for-pce-controller].  Following new
   fields are defined for CCI Object-Type TBD6 -

   MT-ID:  Multi-Topology ID (as defined in [RFC4915]).

   Algorithm:  Single octet identifying the algorithm the SID is
      associated with.  See [RFC8665].

   Flags:  is used to carry any additional information pertaining to the
      CCI.  The following bits are defined -

      *  L-Bit (Local/Global): If set, then the value/index carried by
         the CCI object has local significance.  If not set, then the
         value/index carried by this object has global significance.

      *  V-Bit (Value/Index): If set, then the CCI carries an absolute
         value.  If not set, then the CCI carries an index.

      *  E-Bit (Explicit-Null): If set, any upstream neighbor of the
         node that advertised the SID MUST replace the SID with the
         Explicit-NULL label (0 for IPv4) before forwarding the packet.

      *  N-Bit (No-PHP): If set, then the penultimate hop MUST NOT pop
         the SID before delivering packets to the node that advertised
         the SID.

      *  C-Bit (PCC Allocation): If the bit is set to 1, it indicates
         that the allocation needs to be done by the PCC for this
         central controller instruction.  A PCE set this bit to request
         the PCC to make an allocation from its SR label/ID space.  A
         PCC would set this bit to indicate that it has allocated the
         CC-ID and report it to the PCE.

      *  Following bits are applicable when the SID represents an Adj-
         SID only, it MUST be ignored for others -

         +  G-Bit (Group): When set, the G-Flag indicates that the Adj-
            SID refers to a group of adjacencies (and therefore MAY be
            assigned to other adjacencies as well).

         +  P-Bit (Persistent): When set, the P-Flag indicates that the
            Adj-SID is persistently allocated, i.e., the Adj-SID value
            remains consistent across router restart and/or interface
            flap.

         +  B-Bit (Backup): If set, the Adj-SID refers to an adjacency
            that is eligible for protection (e.g., using IP Fast Reroute



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            or MPLS-FRR (MPLS-Fast Reroute) as described in Section 2.1
            of [RFC8402].

         +  All unassigned bits MUST be set to zero at transmission and
            ignored at receipt.

   SID/Label/Index:  According to the V and L flags, it contains either:

         A 32-bit index defining the offset in the SID/Label space
         advertised by this router.

         A 24-bit label where the 20 rightmost bits are used for
         encoding the label value.

7.4.  FEC Object

   The FEC Object is used to specify the FEC information and MAY be
   carried within PCInitiate or PCRpt message.

   FEC Object-Class is TBD3.


      FEC Object-Type is 1 'IPv4 Node ID'.

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      IPv4 Node ID                             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


      FEC Object-Type is 2 'IPv6 Node ID'.

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      //                      IPv6 Node ID (16 bytes)                //
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      FEC Object-Type is 3 'IPv4 Adjacency'.

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



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

      FEC Object-Type is 4 'IPv6 Adjacency'.

       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)                //
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      FEC Object-Type is 5 'Unnumbered Adjacency with IPv4 NodeIDs'.

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

   FEC Object-Type is 6 'Linklocal IPv6 Adjacency'.


       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 octets)                //
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                    Local Interface ID                         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      //               Remote IPv6 address (16 octets)               //
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                   Remote Interface ID                         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+






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   The FEC objects are as follows:

   IPv4 Node ID: where IPv4 Node ID is specified as an IPv4 address of
   the Node.  FEC Object-type is 1, and the Object-Length is 4 in this
   case.

   IPv6 Node ID: where IPv6 Node ID is specified as an IPv6 address of
   the Node.  FEC Object-type is 2, and the Object-Length is 16 in this
   case.

   IPv4 Adjacency: where Local and Remote IPv4 address is specified as
   pair of IPv4 addresses of the adjacency.  FEC Object-type is 3, and
   the Object-Length is 8 in this case.

   IPv6 Adjacency: where Local and Remote IPv6 address is specified as
   pair of IPv6 addresses of the adjacency.  FEC Object-type is 4, and
   the Object-Length is 32 in this case.

   Unnumbered Adjacency with IPv4 NodeID: where a pair of Node ID /
   Interface ID tuple is used.  FEC Object-type is 5, and the Object-
   Length is 16 in this case.

   Linklocal IPv6 Adjacency: where a pair of (global IPv6 address,
   interface ID) tuple is used.  FEC object-type is 6, and the Object-
   Length is 40 in this case.

8.  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.



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   It is up to the individual working groups to use this information as
   they see fit".

8.1.  Huawei's Proof of Concept based on ONOS

   The PCE function was developed in the ONOS open source platform.
   This extension was implemented on a private version as a proof of
   concept for PCECC.

   o  Organization: Huawei

   o  Implementation: Huawei's PoC based on ONOS

   o  Description: PCEP as a southbound plugin was added to ONOS.  To
      support PCECC-SR, an earlier version of this I-D was implemented.
      Refer https://wiki.onosproject.org/display/ONOS/PCEP+Protocol

   o  Maturity Level: Prototype

   o  Coverage: Partial

   o  Contact: satishk@huawei.com

9.  Security Considerations

   The security considerations described in
   [I-D.ietf-pce-pcep-extension-for-pce-controller] apply to the
   extensions described in this document.

   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 [RFC7525] (unless explicitly set aside in
   [RFC8253]).

10.  Manageability Considerations

10.1.  Control of Function and Policy

   A PCE or PCC implementation SHOULD allow to configure to enable/
   disable PCECC SR capability as a global configuration.  The
   implementation SHOULD also allow setting the local IP address used by
   the PCEP session.







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10.2.  Information and Data Models

   [RFC7420] describes the PCEP MIB, this MIB can be extended to get the
   PCECC SR capability status.

   The PCEP YANG module [I-D.ietf-pce-pcep-yang] could be extended to
   enable/disable PCECC SR capability.

10.3.  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].

10.4.  Verify Correct Operations

   Mechanisms defined in this document do not imply any new operation
   verification requirements in addition to those already listed in
   [RFC5440] and [RFC8231].

10.5.  Requirements On Other Protocols

   PCEP extensions defined in this document do not put new requirements
   on other protocols.

10.6.  Impact On Network Operations

   PCEP implementation SHOULD allow a limit to be placed on the rate of
   PCLabelUpd messages sent by PCE and processed by PCC.  It SHOULD also
   allow sending a notification when a rate threshold is reached.

11.  IANA Considerations

11.1.  PCECC-CAPABILITY sub-TLV

   [I-D.ietf-pce-pcep-extension-for-pce-controller] defines the PCECC-
   CAPABILITY sub-TLV and requests that IANA to create a new sub-
   registry to manage the value of the PCECC-CAPABILITY sub-TLV's Flag
   field.

   IANA is requested to allocate a new bit in the PCECC-CAPABILITY sub-
   TLV Flag Field sub-registry, as follows:

            Bit            Description           Reference
            TBD1           SR                    This document






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11.2.  PCEP Object

   IANA is requested to allocate new code-points for the new FEC object
   and a new Object-Type for CCI object in "PCEP Objects" sub-registry
   as follows:

   Object-Class   Name    Object-Type                     Reference
   Value
   TBD3           FEC     1: IPv4 Node ID                 This document
                          2: IPv6 Node ID                 This document
                          3: IPv4 Adjacency               This document
                          4: IPv6 Adjacency               This document
                          5: Unnumbered Adjacency with    This document
                          IPv4 NodeID
                          6: Linklocal IPv6 Adjacency     This document
   TBD            CCI
                          TBD6: SR-MPLS                   This document

11.3.  PCEP-Error Object

   IANA is requested to allocate a new error-value within the "PCEP-
   ERROR Object Error Types and Values" sub-registry of the PCEP Numbers
   registry for the following errors:


   Error-Type   Meaning
   ----------   -------
   6            Mandatory Object missing.

                 Error-value = TBD5 :                FEC object missing
   19           Invalid operation.

                 Error-value = TBD4 :                SR capability was
                                                     not advertised

11.4.  CCI Object Flag Field for SR

   IANA is requested to create a new sub-registry to manage the Flag
   field of the CCI Object-Type=TBD6 for SR called "CCI Object Flag
   Field for SR".  New values are to be assigned by Standards Action
   [RFC8126].  Each bit should be tracked with the following qualities:

   o  Bit number (counting from bit 0 as the most significant bit)
   o  Capability description
   o  Defining RFC

   Following bits are defined for the CCI Object flag field for SR in
   this document as follows:



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            Bit            Description            Reference
            0-7            Unassigned             This document
            8              B-Bit - Backup         This document
            9              P-Bit - Persistent     This document
            10             G-Bit - Group          This document
            11             C-Bit - PCC Allocation This document
            12             N-Bit - No-PHP         This document
            13             E-Bit - Explicit-Null  This document
            14             V-Bit - Value/Index    This document
            15             L-Bit - Local/Global   This document

12.  Acknowledgments

   We would like to thank Robert Tao, Changjing Yan, Tieying Huang,
   Avantika, and Aijun Wang for their useful comments and suggestions.

13.  References

13.1.  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>.

   [RFC3630]  Katz, D., Kompella, K., and D. Yeung, "Traffic Engineering
              (TE) Extensions to OSPF Version 2", RFC 3630,
              DOI 10.17487/RFC3630, September 2003,
              <https://www.rfc-editor.org/info/rfc3630>.

   [RFC4203]  Kompella, K., Ed. and Y. Rekhter, Ed., "OSPF Extensions in
              Support of Generalized Multi-Protocol Label Switching
              (GMPLS)", RFC 4203, DOI 10.17487/RFC4203, October 2005,
              <https://www.rfc-editor.org/info/rfc4203>.

   [RFC4915]  Psenak, P., Mirtorabi, S., Roy, A., Nguyen, L., and P.
              Pillay-Esnault, "Multi-Topology (MT) Routing in OSPF",
              RFC 4915, DOI 10.17487/RFC4915, June 2007,
              <https://www.rfc-editor.org/info/rfc4915>.

   [RFC5305]  Li, T. and H. Smit, "IS-IS Extensions for Traffic
              Engineering", RFC 5305, DOI 10.17487/RFC5305, October
              2008, <https://www.rfc-editor.org/info/rfc5305>.

   [RFC5307]  Kompella, K., Ed. and Y. Rekhter, Ed., "IS-IS Extensions
              in Support of Generalized Multi-Protocol Label Switching
              (GMPLS)", RFC 5307, DOI 10.17487/RFC5307, October 2008,
              <https://www.rfc-editor.org/info/rfc5307>.



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   [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>.

   [RFC7752]  Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and
              S. Ray, "North-Bound Distribution of Link-State and
              Traffic Engineering (TE) Information Using BGP", RFC 7752,
              DOI 10.17487/RFC7752, March 2016,
              <https://www.rfc-editor.org/info/rfc7752>.

   [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>.

   [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>.

   [RFC8408]  Sivabalan, S., Tantsura, J., Minei, I., Varga, R., and J.
              Hardwick, "Conveying Path Setup Type in PCE Communication
              Protocol (PCEP) Messages", RFC 8408, DOI 10.17487/RFC8408,
              July 2018, <https://www.rfc-editor.org/info/rfc8408>.

   [RFC8664]  Sivabalan, S., Filsfils, C., Tantsura, J., Henderickx, W.,
              and J. Hardwick, "Path Computation Element Communication
              Protocol (PCEP) Extensions for Segment Routing", RFC 8664,
              DOI 10.17487/RFC8664, December 2019,
              <https://www.rfc-editor.org/info/rfc8664>.



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   [I-D.ietf-pce-pcep-extension-for-pce-controller]
              Li, Z., Peng, S., Negi, M., Zhao, Q., 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-08 (work in progress),
              November 2020.

13.2.  Informative References

   [RFC3031]  Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
              Label Switching Architecture", RFC 3031,
              DOI 10.17487/RFC3031, January 2001,
              <https://www.rfc-editor.org/info/rfc3031>.

   [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>.

   [RFC7025]  Otani, T., Ogaki, K., Caviglia, D., Zhang, F., and C.
              Margaria, "Requirements for GMPLS Applications of PCE",
              RFC 7025, DOI 10.17487/RFC7025, September 2013,
              <https://www.rfc-editor.org/info/rfc7025>.

   [RFC7399]  Farrel, A. and D. King, "Unanswered Questions in the Path
              Computation Element Architecture", RFC 7399,
              DOI 10.17487/RFC7399, October 2014,
              <https://www.rfc-editor.org/info/rfc7399>.

   [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>.

   [RFC7491]  King, D. and A. Farrel, "A PCE-Based Architecture for
              Application-Based Network Operations", RFC 7491,
              DOI 10.17487/RFC7491, March 2015,
              <https://www.rfc-editor.org/info/rfc7491>.

   [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>.






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

   [RFC8232]  Crabbe, E., Minei, I., Medved, J., Varga, R., Zhang, X.,
              and D. Dhody, "Optimizations of Label Switched Path State
              Synchronization Procedures for a Stateful PCE", RFC 8232,
              DOI 10.17487/RFC8232, September 2017,
              <https://www.rfc-editor.org/info/rfc8232>.

   [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>.

   [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>.

   [RFC8660]  Bashandy, A., Ed., Filsfils, C., Ed., Previdi, S.,
              Decraene, B., Litkowski, S., and R. Shakir, "Segment
              Routing with the MPLS Data Plane", RFC 8660,
              DOI 10.17487/RFC8660, December 2019,
              <https://www.rfc-editor.org/info/rfc8660>.

   [RFC8665]  Psenak, P., Ed., Previdi, S., Ed., Filsfils, C., Gredler,
              H., Shakir, R., Henderickx, W., and J. Tantsura, "OSPF
              Extensions for Segment Routing", RFC 8665,
              DOI 10.17487/RFC8665, December 2019,
              <https://www.rfc-editor.org/info/rfc8665>.

   [RFC8667]  Previdi, S., Ed., Ginsberg, L., Ed., Filsfils, C.,
              Bashandy, A., Gredler, H., and B. Decraene, "IS-IS
              Extensions for Segment Routing", RFC 8667,
              DOI 10.17487/RFC8667, December 2019,
              <https://www.rfc-editor.org/info/rfc8667>.

   [I-D.ietf-teas-pcecc-use-cases]
              Li, Z., Khasanov, B., Dhody, D., Zhao, Q., Ke, Z., Fang,
              L., Zhou, C., Communications, T., Rachitskiy, A., and A.
              Gulida, "The Use Cases for Path Computation Element (PCE)
              as a Central Controller (PCECC).", draft-ietf-teas-pcecc-
              use-cases-06 (work in progress), September 2020.





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   [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-15 (work in progress), October 2020.

   [I-D.ietf-pce-binding-label-sid]
              Sivabalan, S., Filsfils, C., Tantsura, J., Hardwick, J.,
              Previdi, S., and C. Li, "Carrying Binding Label/Segment-ID
              in PCE-based Networks.", draft-ietf-pce-binding-label-
              sid-05 (work in progress), October 2020.

   [I-D.litkowski-pce-state-sync]
              Litkowski, S., Sivabalan, S., Li, C., and H. Zheng, "Inter
              Stateful Path Computation Element (PCE) Communication
              Procedures.", draft-litkowski-pce-state-sync-09 (work in
              progress), November 2020.

   [I-D.dhodylee-pce-pcep-ls]
              Dhody, D., Peng, S., Lee, Y., Ceccarelli, D., Wang, A.,
              and G. Mishra, "PCEP extensions for Distribution of Link-
              State and TE Information", draft-dhodylee-pce-pcep-ls-19
              (work in progress), November 2020.

   [I-D.dhody-pce-pcep-extension-pce-controller-srv6]
              Li, Z., Peng, S., Geng, X., and M. Negi, "PCEP Procedures
              and Protocol Extensions for Using PCE as a Central
              Controller (PCECC) for SRv6", draft-dhody-pce-pcep-
              extension-pce-controller-srv6-05 (work in progress),
              November 2020.





















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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

   Satish Karunanithi
   Huawei Technologies
   Divyashree Techno Park, Whitefield
   Bangalore, Karnataka  560066
   India

   EMail: satishk@huawei.com

   Adrian Farrel
   Juniper Networks, Inc
   UK

   EMail: adrian@olddog.co.uk

   Xuesong Geng
   Huawei Technologies
   China

   Email: gengxuesong@huawei.com

   Udayasree Palle

   EMail: udayasreereddy@gmail.com

   Katherine Zhao
   Huawei Technologies
   2330 Central Expressway
   Santa Clara, CA  95050
   USA

   EMail: katherine.zhao@huawei.com

   Boris Zhang
   Telus Ltd.
   Toronto
   Canada

   EMail: boris.zhang@telus.com



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   Alex Tokar
   Cisco Systems
   Slovak Republic

   EMail: atokar@cisco.com



Authors' Addresses

   Zhenbin Li
   Huawei Technologies
   Huawei Bld., No.156 Beiqing Rd.
   Beijing    100095
   China

   EMail: lizhenbin@huawei.com


   Shuping Peng
   Huawei Technologies
   Huawei Bld., No.156 Beiqing Rd.
   Beijing  100095
   China

   EMail: pengshuping@huawei.com


   Mahendra Singh Negi
   RtBrick Inc
   N-17L, 18th Cross Rd, HSR Layout
   Bangalore, Karnataka  560102
   India

   EMail: mahend.ietf@gmail.com


   Quintin Zhao
   Etheric Networks
   1009 S CLAREMONT ST
   SAN MATEO, CA  94402
   USA

   EMail: qzhao@ethericnetworks.com







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   Chao Zhou
   HPE

   EMail: chaozhou_us@yahoo.com















































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