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Inter Stateful Path Computation Element (PCE) Communication Procedures.
draft-ietf-pce-state-sync-03

Document Type Active Internet-Draft (pce WG)
Authors Stephane Litkowski , Siva Sivabalan , Cheng Li , Haomian Zheng
Last updated 2022-07-10
Replaces draft-litkowski-pce-state-sync
Stream Internet Engineering Task Force (IETF)
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draft-ietf-pce-state-sync-03
PCE Working Group                                           S. Litkowski
Internet-Draft                                                     Cisco
Intended status: Standards Track                            S. Sivabalan
Expires: 11 January 2023                               Ciena Corporation
                                                                   C. Li
                                                                H. Zheng
                                                     Huawei Technologies
                                                            10 July 2022

Inter Stateful Path Computation Element (PCE) Communication Procedures.
                      draft-ietf-pce-state-sync-03

Abstract

   The Path Computation Element (PCE) Communication Protocol (PCEP)
   provides mechanisms for PCEs to perform path computation in response
   to a Path Computation Client (PCC) request.  The Stateful PCE
   extensions allow stateful control of Multi-Protocol Label Switching
   (MPLS) Traffic Engineering (TE) Label Switched Paths (LSPs) using
   PCEP.

   A Path Computation Client (PCC) can synchronize an LSP state
   information to a Stateful Path Computation Element (PCE).  A PCC can
   have multiple PCEP sessions towards multiple PCEs.  There are some
   use cases, where an inter-PCE stateful communication can bring
   additional resiliency in the design, for instance when some PCC-PCE
   session fails.

   This document describes the procedures to allow a stateful
   communication between PCEs for various use-cases and also the
   procedures to prevent computations loops.

Status of This Memo

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

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

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

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   This Internet-Draft will expire on 11 January 2023.

Copyright Notice

   Copyright (c) 2022 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
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   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction and Problem Statement  . . . . . . . . . . . . .   3
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   4
     1.2.  Reporting LSP Changes . . . . . . . . . . . . . . . . . .   4
     1.3.  Split-Brain . . . . . . . . . . . . . . . . . . . . . . .   6
     1.4.  Applicability to H-PCE  . . . . . . . . . . . . . . . . .  13
   2.  Solution  . . . . . . . . . . . . . . . . . . . . . . . . . .  13
     2.1.  State-sync Session  . . . . . . . . . . . . . . . . . . .  13
     2.2.  Primary/Secondary Relationship between PCE  . . . . . . .  15
   3.  Procedures and Protocol Extensions  . . . . . . . . . . . . .  15
     3.1.  Opening a state-sync session  . . . . . . . . . . . . . .  15
       3.1.1.  Capability Advertisement  . . . . . . . . . . . . . .  16
     3.2.  State Synchronization . . . . . . . . . . . . . . . . . .  16
     3.3.  Incremental Updates and Report Forwarding Rules . . . . .  17
     3.4.  Maintaining LSP States from Different Sources . . . . . .  18
     3.5.  Computation Priority between PCEs and Sub-delegation  . .  19
     3.6.  Passive Stateful Procedures . . . . . . . . . . . . . . .  20
     3.7.  PCE Initiation Procedures . . . . . . . . . . . . . . . .  21
   4.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .  21
     4.1.  Example 1 . . . . . . . . . . . . . . . . . . . . . . . .  21
     4.2.  Example 2 . . . . . . . . . . . . . . . . . . . . . . . .  23
     4.3.  Example 3 . . . . . . . . . . . . . . . . . . . . . . . .  24
   5.  Using Primary/Secondary Computation and State-sync Sessions to
           increase Scaling  . . . . . . . . . . . . . . . . . . . .  26
   6.  PCEP-PATH-VECTOR TLV  . . . . . . . . . . . . . . . . . . . .  28
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  29
   8.  Implementation Status . . . . . . . . . . . . . . . . . . . .  29
   9.  Manageability Considerations  . . . . . . . . . . . . . . . .  30
     9.1.  Control of Function and Policy  . . . . . . . . . . . . .  30
     9.2.  Information and Data Models . . . . . . . . . . . . . . .  30
     9.3.  Liveness Detection and Monitoring . . . . . . . . . . . .  30

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     9.4.  Verify Correct Operations . . . . . . . . . . . . . . . .  31
     9.5.  Requirements On Other Protocols . . . . . . . . . . . . .  31
     9.6.  Impact On Network Operations  . . . . . . . . . . . . . .  31
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  31
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  31
     11.1.  PCEP-Error Object  . . . . . . . . . . . . . . . . . . .  31
     11.2.  PCEP TLV Type Indicators . . . . . . . . . . . . . . . .  32
     11.3.  STATEFUL-PCE-CAPABILITY TLV  . . . . . . . . . . . . . .  32
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  32
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  32
     12.2.  Informative References . . . . . . . . . . . . . . . . .  33
   Appendix A.  Contributors . . . . . . . . . . . . . . . . . . . .  34
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  35

1.  Introduction and Problem Statement

   The Path Computation Element communication Protocol (PCEP) [RFC5440]
   provides mechanisms for Path Computation Elements (PCEs) to perform
   path computations in response to Path Computation Clients' (PCCs)
   requests.

   A stateful PCE [RFC8231] is capable of considering, for the purposes
   of path computation, not only the network state in terms of links and
   nodes (referred to as the Traffic Engineering Database or TED) but
   also the status of active services (previously computed paths, and
   currently reserved resources, stored in the Label Switched Paths
   Database (LSP-DB).

   [RFC8051] describes general considerations for a stateful PCE
   deployment and examines its applicability and benefits, as well as
   its challenges and limitations through a number of use cases.

   A PCC can synchronize an LSP state information to a Stateful PCE.
   The stateful PCE extension allows a redundancy scenario where a PCC
   can have redundant PCEP sessions towards multiple PCEs.  In such a
   case, a PCC gives control of a LSP to only a single PCE, and only one
   PCE is responsible for path computation for this delegated LSP.

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   There are some use cases, where an inter-PCE stateful communication
   can bring additional resiliency in the design, for instance when some
   PCC-PCE session fails.  The inter-PCE stateful communication may also
   provide a faster update of the LSP states when such an event occurs.
   Finally, when, in a redundant PCE scenario, there is a need to
   compute a set of paths that are part of a group (so there is a
   dependency between the paths), there may be some cases where the
   computation of all paths in the group is not handled by the same PCE:
   this situation is called a split-brain.  This split-brain scenario
   may lead to computation loops between PCEs or suboptimal path
   computation.

   This document describes the procedures to allow a stateful
   communication between PCEs for various use-cases and also the
   procedures to prevent computations loops.

   Further, the examples in this section are for illustrative purpose to
   showcase the need for inter-PCE stateful PCEP sessions.

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.

1.2.  Reporting LSP Changes

   When using a stateful PCE ([RFC8231]), a PCC can synchronize an LSP
   state information to the stateful PCE.  If the PCC grants the control
   of the LSP to the PCE (called delegation [RFC8231]), the PCE can
   update the LSP parameters at any time.

   In a multi PCE deployment (redundancy, loadbalancing...), with the
   current specification defined in [RFC8231], when a PCE makes an
   update, it is the PCC that is in charge of reporting the LSP status
   to all PCEs with LSP parameter change which brings additional hops
   and delays in notifying the overall network of the LSP parameter
   change.

   This delay may affect the reaction time of the other PCEs if they
   need to take action after being notified of the LSP parameter change.

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   Apart from the synchronization from the PCC, it is also useful if
   there is a synchronization mechanism between the stateful PCEs.  As
   stateful PCE make changes to its delegated LSPs, these changes
   (pending LSPs and the sticky resources [RFC7399]) can be synchronized
   immediately to the other PCEs.

             +----------+
             |   PCC1   |  LSP1
             +----------+
                /    \
               /      \
      +---------+    +---------+
      |  PCE1   |    |  PCE2   |
      +---------+    +---------+
              \       /
               \     /
             +----------+
             |   PCC2   |  LSP2
             +----------+

   In the figure above, we consider a load-balanced PCE architecture, so
   PCE1 is responsible to compute paths for PCC1 and PCE2 is responsible
   to compute paths for PCC2.  When PCE1 triggers an LSP update for
   LSP1, it sends a PCUpd message to PCC1 containing the new parameters
   for LSP1.  PCC1 will take the parameters into account and will send a
   PCRpt message to PCE1 and PCE2 reflecting the changes.  PCE2 will so
   be notified of the change only after receiving the PCRpt message from
   PCC1.

   Let's consider that the LSP1 parameters changed in such a way that
   LSP1 will take over resources from LSP2 with a higher priority.
   After receiving the report from PCC1, PCE2 will therefore try to find
   a new path for LSP2.  If we consider that there is a round trip delay
   of about 150 milliseconds (ms) between the PCEs and PCC1 and a round
   trip delay of 10 ms between the two PCEs if will take more than 150
   ms for PCE2 to be notified of the change.

   Adding a PCEP session between PCE1 and PCE2 may allow to reduce the
   synchronization time, so PCE2 can react more quickly by taking the
   pending LSPs and attached resources into account during path
   computation and re-optimization.

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1.3.  Split-Brain

   In a resiliency case, a PCC has redundant PCEP sessions towards
   multiple PCEs.  In such a case, a PCC gives control on an LSP to a
   single PCE only, and only this PCE is responsible for the path
   computation for the delegated LSP: the PCC achieves this by setting
   the D flag only towards the active PCE [RFC8231] selected for
   delegation.  The election of the active PCE to delegate an LSP is
   controlled by each PCC.  The PCC usually elects the active PCE by a
   local configured policy (by setting a priority).  Upon PCEP session
   failure, or active PCE failure, PCC may decide to elect a new active
   PCE by sending new PCRpt message with D flag set to this new active
   PCE.  When the failed PCE or PCEP session comes back online, it will
   be up to the implementation to do preemption.  Doing preemption may
   lead to some disruption on the existing path if path results from
   both PCEs are not exactly the same.  By considering a network with
   multiple PCCs and implementing multiple stateful PCEs for redundancy
   purpose, there is no guarantee that at any time all the PCCs delegate
   their LSPs to the same PCE.

             +----------+
             |   PCC1   |  LSP1
             +----------+
                /    \
               /      \
      +---------+    +---------+
      |  PCE1   |    |  PCE2   |
      +---------+    +---------+
              \       /
      *fail*   \     /
             +----------+
             |   PCC2   |  LSP2
             +----------+

   In the example above, we consider that by configuration, both PCCs
   will firstly delegate their LSPs to PCE1.  So, PCE1 is responsible
   for computing a path for both LSP1 and LSP2.  If the PCEP session
   between PCC2 and PCE1 fails, PCC2 will delegate LSP2 to PCE2.  So
   PCE1 becomes responsible only for LSP1 path computation while PCE2 is
   responsible for the path computation of LSP2.  When the PCC2-PCE1
   session is back online, PCC2 will keep using PCE2 as active PCE
   (consider no preemption in this example).  So the result is a
   permanent situation where each PCE is responsible for a subset of
   path computation.

   This situation is called a split-brain scenario, as there are
   multiple computation brains running at the same time while a central
   computation unit was required in some deployments/use cases.

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   Further, there are use cases where a particular LSP path computation
   is linked to another LSP path computation: the most common use case
   is path disjointness (see [RFC8800]).  The set of LSPs that are
   dependent to each other may start from a different head-end.

         _________________________________________
        /                                         \
       /        +------+            +------+       \
      |         | PCE1 |            | PCE2 |        |
      |         +------+            +------+        |
      |                                             |
      | +------+                          +------+  |
      | | PCC1 | ---------------------->  | PCC2 |  |
      | +------+                          +------+  |
      |                                             |
      |                                             |
      | +------+                          +------+  |
      | | PCC3 | ---------------------->  | PCC4 |  |
      | +------+                          +------+  |
      |                                             |
       \                                           /
        \_________________________________________/

         _________________________________________
        /                                         \
       /        +------+            +------+       \
      |         | PCE1 |            | PCE2 |        |
      |         +------+            +------+        |
      |                                             |
      | +------+           10             +------+  |
      | | PCC1 | ----- R1 ---- R2 ------- | PCC2 |  |
      | +------+       |        |         +------+  |
      |                |        |                   |
      |                |        |                   |
      | +------+       |        |         +------+  |
      | | PCC3 | ----- R3 ---- R4 ------- | PCC4 |  |
      | +------+                          +------+  |
      |                                             |
       \                                           /
        \_________________________________________/

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   In the figure above, the requirement is to create two link-disjoint
   LSPs: PCC1->PCC2 and PCC3->PCC4.  In the topology, all links cost
   metric is set to 1 except for the link 'R1-R2' which has a metric of
   10.  The PCEs are responsible for the path computation and PCE1 is
   the active primary PCE for all PCCs in the nominal case.

   Scenario 1:

   In the normal case (PCE1 as active primary PCE), consider that
   PCC1->PCC2 LSP is configured first with the link disjointness
   constraint, PCE1 sends a PCUpd message to PCC1 with the ERO:
   R1->R3->R4->R2->PCC2 (shortest path).  PCC1 signals and installs the
   path.  When PCC3->PCC4 is configured, the PCEs already knows the path
   of PCC1->PCC2 and can compute a link-disjoint path: the solution
   requires to move PCC1->PCC2 onto a new path to let room for the new
   LSP.  PCE1 sends a PCUpd message to PCC1 with the new ERO:
   R1->R2->PCC2 and a PCUpd to PCC3 with the following ERO:
   R3->R4->PCC4.  In the normal case, there is no issue for PCE1 to
   compute a link-disjoint path.

   Scenario 2:

   Consider that PCC1 lost its PCEP session with PCE1 (all other PCEP
   sessions are UP).  PCC1 delegates its LSP to PCE2.

             +----------+
             |   PCC1   |  LSP: PCC1->PCC2
             +----------+
                     \
                      \ D=1
      +---------+    +---------+
      |  PCE1   |    |  PCE2   |
      +---------+    +---------+
          D=1 \       / D=0
               \     /
             +----------+
             |   PCC3   |  LSP: PCC3->PCC4
             +----------+

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   Consider that the PCC1->PCC2 LSP is configured first with the link
   disjointness constraint, PCE2 (which is the new active primary PCE
   for PCC1) sends a PCUpd message to PCC1 with the ERO:
   R1->R3->R4->R2->PCC2 (shortest path).  When PCC3->PCC4 is configured,
   PCE1 is not aware of LSPs from PCC1 any more, so it cannot compute a
   disjoint path for PCC3->PCC4 and will send a PCUpd message to PCC3
   with the shortest path ERO: R3->R4->PCC4.  When PCC3->PCC4 LSP will
   be reported to PCE2 by PCC3, PCE2 will ensure disjointness
   computation and will correctly move PCC1->PCC2 (as it owns delegation
   for this LSP) on the following path: R1->R2->PCC2.  With this
   sequence of event and these PCEP sessions, disjointness is ensured.

   Scenario 3:

             +----------+
             |   PCC1   |  LSP: PCC1->PCC2
             +----------+
               /     \
          D=1 /       \ D=0
      +---------+    +---------+
      |  PCE1   |    |  PCE2   |
      +---------+    +---------+
                      / D=1
                     /
             +----------+
             |   PCC3   |  LSP: PCC3->PCC4
             +----------+

   Consider the above PCEP sessions and the PCC1->PCC2 LSP is configured
   first with the link disjointness constraint, PCE1 computes the
   shortest path as it is the only LSP in the disjoint association group
   that it is aware of: R1->R3->R4->R2->PCC2 (shortest path).  When
   PCC3->PCC4 is configured, PCE2 must compute a disjoint path for this
   LSP.  The only solution found is to move PCC1->PCC2 LSP on another
   path, but PCE2 cannot do it as it does not have delegation for this
   LSP.  In this set-up, PCEs are not able to find a disjoint path.

   Scenario 4:

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             +----------+
             |   PCC1   |  LSP: PCC1->PCC2
             +----------+
               /     \
          D=1 /       \ D=0
      +---------+    +---------+
      |  PCE1   |    |  PCE2   |
      +---------+    +---------+
           D=0 \      / D=1
                \    /
             +----------+
             |   PCC3   |  LSP: PCC3->PCC4
             +----------+

   Consider the above PCEP sessions and that PCEs are configured to
   fall-back to the shortest path if disjointness cannot be found as
   described in [RFC8800].  The PCC1->PCC2 LSP is configured first, PCE1
   computes the shortest path as it is the only LSP in the disjoint
   association group that it is aware of: R1->R3->R4->R2->PCC2 (shortest
   path).  When PCC3->PCC4 is configured, PCE2 must compute a disjoint
   path for this LSP.  The only solution found is to move PCC1->PCC2 LSP
   on another path, but PCE2 cannot do it as it does not have delegation
   for this LSP.  PCE2 then provides the shortest path for PCC3->PCC4:
   R3->R4->PCC4.  When PCC3 receives the ERO, it reports it back to both
   PCEs.  When PCE1 becomes aware of the PCC3->PCC4 path, it recomputes
   the constrained shortest path first (CSPF) algorithm and provides a
   new path for PCC1->PCC2: R1->R2->PCC2.  The new path is reported back
   to all PCEs by PCC1.  PCE2 recomputes also CSPF to take into account
   the new reported path.  The new computation does not lead to any path
   update.

   Scenario 5:

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         _____________________________________
        /                                     \
       /        +------+        +------+       \
      |         | PCE1 |        | PCE2 |        |
      |         +------+        +------+        |
      |                                         |
      | +------+         100          +------+  |
      | |      | -------------------- |      |  |
      | | PCC1 | ----- R1 ----------- | PCC2 |  |
      | +------+       |              +------+  |
      |    |           |                  |     |
      |  6 |           | 2                | 2   |
      |    |           |                  |     |
      | +------+       |              +------+  |
      | | PCC3 | ----- R3 ----------- | PCC4 |  |
      | +------+               10     +------+  |
      |                                         |
       \                                       /
        \_____________________________________/

   Now, consider a new network topology with the same PCEP sessions as
   the previous example.  Suppose that both LSPs are configured almost
   at the same time.  PCE1 will compute a path for PCC1->PCC2 while PCE2
   will compute a path for PCC3->PCC4.  As each PCE is not aware of the
   path of the second LSP in the association group (not reported yet),
   each PCE is computing the shortest path for the LSP.  PCE1 computes
   ERO: R1->PCC2 for PCC1->PCC2 and PCE2 computes ERO:
   R3->R1->PCC2->PCC4 for PCC3->PCC4.  When these shortest paths will be
   reported to each PCE.  Each PCE will recompute disjointness.  PCE1
   will provide a new path for PCC1->PCC2 with ERO: PCC1->PCC2.  PCE2
   will provide also a new path for PCC3->PCC4 with ERO: R3->PCC4.  When
   those new paths will be reported to both PCEs, this will trigger CSPF
   again.  PCE1 will provide a new more optimal path for PCC1->PCC2 with
   ERO: R1->PCC2 and PCE2 will also provide a more optimal path for
   PCC3->PCC4 with ERO: R3->R1->PCC2->PCC4.  So we come back to the
   initial state.  When those paths will be reported to both PCEs, this
   will trigger CSPF again.  An infinite loop of CSPF computation is
   then happening with a permanent flap of paths because of the split-
   brain situation.

   This permanent computation loop comes from the inconsistency between
   the state of the LSPs as seen by each PCE due to the split-brain:
   each PCE is trying to modify at the same time its delegated path
   based on the last received path information which de facto
   invalidates this received path information.

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   Scenario 6: multi-domain

            Domain/Area 1        Domain/Area 2
         ________________      ________________
        /                \    /                \
       /        +------+ |   |  +------+        \
      |         | PCE1 | |   |  | PCE3 |        |
      |         +------+ |   |  +------+        |
      |                  |   |                  |
      |         +------+ |   |  +------+        |
      |         | PCE2 | |   |  | PCE4 |        |
      |         +------+ |   |  +------+        |
      |                  |   |                  |
      | +------+         |   |        +------+  |
      | | PCC1 |         |   |        | PCC2 |  |
      | +------+         |   |        +------+  |
      |                  |   |                  |
      |                  |   |                  |
      | +------+         |   |        +------+  |
      | | PCC3 |         |   |        | PCC4 |  |
      | +------+         |   |        +------+  |
       \                 |   |                  |
        \_______________/     \________________/

   In the example above, suppose that the disjoint LSPs from PCC1 to
   PCC2 and from PCC4 to PCC3 are created.  All the PCEs have the
   knowledge of both domain topologies (e.g. using BGP-LS [RFC7752]).
   For operation/management reasons, each domain uses its own group of
   redundant PCEs.  PCE1/PCE2 in domain 1 have PCEP sessions with PCC1
   and PCC3 while PCE3/PCE4 in domain 2 have PCEP sessions with PCC2 and
   PCC4.  As PCE1/2 does not know about LSPs from PCC2/4 and PCE3/4 do
   not know about LSPs from PCC1/3, there is no possibility to compute
   the disjointness constraint.  This scenario can also be seen as a
   split-brain scenario.  This multi-domain architecture (with multiple
   groups of PCEs) can also be used in a single domain, where an
   operator wants to limit the failure domain by creating multiple
   groups of PCEs maintaining a subset of PCCs.  As for the multi-domain
   example, there will be no possibility to compute the disjoint path
   starting from head-ends managed by different PCE groups.

   In this document, we propose a solution that addresses the
   possibility to compute LSP association based constraints (like
   disjointness) in split-brain scenarios while preventing computation
   loops.

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1.4.  Applicability to H-PCE

   [RFC8751] describes general considerations and use cases for the
   deployment of Stateful PCE(s) using the Hierarchical PCE [RFC6805]
   architecture.  In this architecture, there is a clear need to
   communicate between a child stateful PCE and a parent stateful PCE.
   The procedures and extensions as described in Section 3 are equally
   applicable to the H-PCE scenario.

2.  Solution

   The solution specified in this document is based on:

   *  The creation of the inter-PCE stateful PCEP session with specific
      procedures.

   *  A Primary/Secondary relationship between stateful PCEs.

2.1.  State-sync Session

   This document proposes to set-up a PCEP session between the stateful
   PCEs.  Creating such a session is already authorized by multiple
   scenarios like the one described in [RFC4655] (multiple PCEs that are
   handling part of the path computation) and [RFC6805] (hierarchical
   PCE) but was only focused on the stateless PCEP sessions.  As
   stateful PCE brings additional features (LSP state synchronization,
   path update, delegation, ...), thus some new behaviors need to be
   defined.

   This inter-PCE PCEP session will allow the exchange of LSP states
   between PCEs that would help some scenarios where PCEP sessions are
   lost between PCC and PCE.  This inter-PCE PCEP session is henceforth
   called a state-sync session.

   For example, in the scenario below, there is no possibility to
   compute disjointness as there is no PCE that is aware of both LSPs.

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             +----------+
             |   PCC1   |  LSP: PCC1->PCC2
             +----------+
               /
          D=1 /
      +---------+       +---------+
      |  PCE1   |       |  PCE2   |
      +---------+       +---------+
                        / D=1
                       /
             +----------+
             |   PCC3   |  LSP: PCC3->PCC4
             +----------+

   If we add a state-sync session, PCE1 will be able to do state
   synchronization via PCRpt messages for its LSP to PCE2 and PCE2 will
   do the same.  All the PCEs will be aware of all LSPs even if a
   PCC->PCE session is down.  PCEs will then be able to compute disjoint
   paths.

             +----------+
             |   PCC1   |  LSP : PCC1->PCC2
             +----------+
               /
          D=1 /
      +---------+ PCEP  +---------+
      |  PCE1   | ----- |  PCE2   |
      +---------+       +---------+
                        / D=1
                       /
             +----------+
             |   PCC3   |  LSP : PCC3->PCC4
             +----------+

   The procedures associated with this state-sync session are defined in
   Section 3.

   By just adding this state-sync session, it does not ensure that a
   path with LSP association based constraints can always be computed
   and does not prevent the computation loop, but it increases
   resiliency and ensures that PCEs will have the state information for
   all LSPs.  Also, this session will allow for a PCE to update the
   other PCEs providing a faster synchronization mechanism than relying
   on PCCs only.

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2.2.  Primary/Secondary Relationship between PCE

   As seen in Section 1, performing a path computation in a split-brain
   scenario (multiple PCEs responsible for computation) may provide a
   non-optimal LSP placement, no path, or computation loops.  To provide
   the best efficiency, an LSP association constraint-based computation
   requires that a single PCE performs the path computation for all LSPs
   in the association group.  Note that, it could be all LSPs belonging
   to a particular association group, or all LSPs from a particular PCC,
   or all LSPs in the network that need to be delegated to a single PCE
   based on the deployment scenarios.

   This document proposes to add a priority mechanism between PCEs to
   elect a single computing 'primary' PCE.  Using this priority
   mechanism, PCEs can agree on the PCE that will be responsible for the
   computation for a particular association group, or set of LSPs.  The
   priority could be set per association, per PCC, or for all LSPs.  How
   this priority is set or advertised is out of the scope of this
   document.  The rest of the text considers the association group as an
   example.

   [Editor's note- Confirm with WG if it is okay for the priority
   setting to be out of scope or should it be added in this I-D.]

   When a single PCE is performing the computation for a particular
   association group, no computation loop can happen and an optimal
   placement will be provided.  The other PCEs will only act as state
   collectors and forwarders.

   In the scenario described in Section 2.1, PCE1 and PCE2 will decide
   that PCE1 will be responsible for the path computation of both LSPs.
   If we first configure PCC1->PCC2, PCE1 computes the shortest path at
   it is the only LSP in the disjoint-group that it is aware of:
   R1->R3->R4->R2->PCC2 (shortest path).  When PCC3->PCC4 is configured,
   PCE2 will not perform computation even if it has delegation but
   forwards the delegation via PCRpt message to PCE1 through the state-
   sync session.  PCE1 will then perform disjointness computation and
   will move PCC1->PCC2 onto R1->R2->PCC2 and provides an ERO to PCE2
   for PCC3->PCC4: R3->R4->PCC4.  The PCE2 will further update the PCC3
   with the new path.

3.  Procedures and Protocol Extensions

3.1.  Opening a state-sync session

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3.1.1.  Capability Advertisement

   A PCE indicates its support of state-sync procedures during the PCEP
   Initialization phase [RFC5440].  The OPEN object in the Open message
   MUST contains the "Stateful PCE Capability" TLV defined in [RFC8231].
   A new P (INTER-PCE-CAPABILITY) flag is introduced to indicate the
   support of state-sync.

   This document adds a new bit in the Flags field with :

   *  P (INTER-PCE-CAPABILITY - 1 bit - TBD4): If set to 1 by a PCEP
      Speaker, the PCEP speaker indicates that the session MUST follow
      the state-sync procedures as described in this document.  The P
      bit MUST be set by both speakers: if a PCEP Speaker receives a
      STATEFUL-PCE-CAPABILITY TLV with P=0 while it advertised P=1 or if
      both set P flag to 0, the session SHOULD be set-up but the state-
      sync procedures MUST NOT be applied on this session.

   The U flag [RFC8231] MUST be set when sending the STATEFUL-PCE-
   CAPABILITY TLV with the P flag set.  In case the U flag is not set
   along with the P flag, the state sync capability is not enabled and
   it is considered as if the P flag is not set.  The S flag MAY be set
   if optimized synchronization is required as per [RFC8232].

3.2.  State Synchronization

   When the state sync capability has been negotiated between stateful
   PCEs, each PCEP speaker will behave as a PCE and as a PCC at the same
   time regarding the state synchronization as defined in [RFC8231].
   This means that each PCEP Speaker:

   *  MUST send a PCRpt message towards its neighbor with S flag set for
      each LSP in its LSP database learned from a PCC.  (PCC role)

   *  MUST send the End Of Synchronization Marker towards its neighbor
      when all LSPs have been reported.  (PCC role)

   *  MUST wait for the LSP synchronization from its neighbor to end
      (receiving an End Of Synchronization Marker).  (PCE role)

   The process of synchronization runs in parallel on each PCE (with no
   defined order).

   The optimized state synchronization procedures MAY be used, as
   defined in [RFC8232].

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   When a PCEP Speaker sends a PCRpt on a state-sync session, it MUST
   add the SPEAKER-IDENTITY-TLV (defined in [RFC8232]) in the LSP
   Object, the value used will refer to the 'owner' PCC of the LSP.  If
   a PCEP Speaker receives a PCRpt on a state-sync session without this
   TLV, it MUST discard the PCRpt message and it MUST reply with a PCErr
   message using error-type=6 (Mandatory Object missing) and error-
   value=TBD1 (SPEAKER-IDENTITY-TLV missing).

3.3.  Incremental Updates and Report Forwarding Rules

   During the life of an LSP, its state may change (path, constraints,
   operational state...) and a PCC will advertise a new PCRpt to the PCE
   for each such change.

   When propagating LSP state changes from a PCE to other PCEs, it is
   mandatory to ensure that a PCE always uses the freshest state coming
   from the PCC.

   When a PCE receives a new PCRpt from a PCC with the LSP-DB-VERSION,
   the PCE MUST forward the PCRpt to all its state-sync sessions and
   MUST add the appropriate SPEAKER-IDENTITY-TLV in the PCRpt.  In
   addition, it MUST add a new ORIGINAL-LSP-DB-VERSION TLV (described
   below).  The ORIGINAL-LSP-DB-VERSION contains the LSP-DB-VERSION
   coming from the PCC.

   When a PCE receives a new PCRpt from a PCC without the LSP-DB-
   VERSION, it SHOULD NOT forward the PCRpt on any state-sync sessions
   and log such an event on the first occurrence.

   When a PCE receives a new PCRpt from a PCC with the R flag (Remove)
   set and an LSP-DB-VERSION TLV, the PCE MUST forward the PCRpt to all
   its state-sync sessions keeping the R flag set (Remove) and MUST add
   the appropriate SPEAKER-IDENTITY-TLV and ORIGINAL-LSP-DB-VERSION TLV
   in the PCRpt message.

   When a PCE receives a PCRpt from a state-sync session, it MUST NOT
   forward the PCRpt to other state-sync sessions.  This helps to
   prevent message loops between PCEs.  As a consequence, a full mesh of
   PCEP sessions between PCEs are REQUIRED.

   When a PCRpt is forwarded, all the original objects and values are
   kept.  As an example, the PLSP-ID used in the forwarded PCRpt will be
   the same as the original one used by the PCC.  Thus an implementation
   supporting this document MUST consider SPEAKER-IDENTITY-TLV and PLSP-
   ID together to uniquely identify an LSP on the state-sync session.

   The ORIGINAL-LSP-DB-VERSION TLV is encoded as follows and MUST always
   contain the LSP-DB-VERSION received from the owner PCC of the LSP:

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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=TBD2           |            Length=8           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 LSP State DB Version Number                   |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Using the ORIGINAL-LSP-DB-VERSION TLV allows a PCE to keep using
   optimized synchronization ([RFC8232]) with another PCE.  In such a
   case, the PCE will send a PCRpt to another PCE with both ORIGINAL-
   LSP-DB-VERSION TLV and LSP-DB-VERSION TLV.  The ORIGINAL-LSP-DB-
   VERSION TLV will contain the version number as allocated by the PCC
   while the LSP-DB-VERSION will contain the version number allocated by
   the local PCE.

3.4.  Maintaining LSP States from Different Sources

   When a PCE receives a PCRpt on a state-sync session, it stores the
   LSP information into the original PCC address context (as the LSP
   belongs to the PCC).  A PCE SHOULD maintain a single state for a
   particular LSP and SHOULD maintain the list of sources it learned a
   particular state from.

   A PCEP speaker may receive state information for a particular LSP
   from different sources: the PCC that owns the LSP (through a regular
   PCEP session) and some PCEs (through PCEP state-sync sessions).  A
   PCEP speaker MUST always keep the freshest state in its LSP database,
   overriding the previously received information.

   A PCE, receiving a PCRpt from a PCC, updates the state of the LSP in
   its LSP-DB with the newly received information.  When receiving a
   PCRpt from another PCE, a PCE SHOULD update the LSP state only if the
   ORIGINAL-LSP-DB-VERSION present in the PCRpt is greater than the
   current ORIGINAL-LSP-DB-VERSION of the stored LSP state.  This
   ensures that a PCE never tries to update its stored LSP state with an
   old information.  Each time a PCE updates an LSP state in its LSP-DB,
   it SHOULD reset the source list associated with the LSP state and
   SHOULD add the source speaker address in the source list.  When a PCE
   receives a PCRpt which has an ORIGINAL-LSP-DB-VERSION (if coming from
   a PCE) or an LSP-DB-VERSION (if coming from the PCC) equals to the
   current ORIGINAL-LSP-DB-VERSION of the stored LSP state, it SHOULD
   add the source speaker address in the source list.

   When a PCE receives a PCRpt requesting an LSP deletion from a
   particular source, it SHOULD remove this particular source from the
   list of sources associated with this LSP.

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   When the list of sources becomes empty for a particular LSP, the LSP
   state MUST be removed.  This means that all the sources must send a
   PCRpt with R=1 for an LSP to make the PCE remove the LSP state.

3.5.  Computation Priority between PCEs and Sub-delegation

   A computation priority is necessary to ensure that a single PCE will
   perform the computation for all the LSPs in an association group:
   this will allow for a more optimized LSP placement and will prevent
   computation loops.

   All PCEs in the network that are handling LSPs in a common LSP
   association group SHOULD be aware of each other including the
   computation priority of each PCE.  Note that there is no need for PCC
   to be aware of this.  The computation priority is a number and the
   PCE having the highest priority SHOULD be responsible for the
   computation.  If several PCEs have the same priority value, their IP
   address SHOULD be used as a tie-breaker to provide a rank: the
   highest IP address has more priority.  How PCEs are aware of the
   priority of each other is out of the scope of this document, but as
   example learning priorities could be done through PCE discovery or
   local configuration.

   [Editor's note- Confirm with WG if it is okay for the priority
   setting to be out of scope or should it be added in this I-D.]

   The definition of the priority could be global so the highest
   priority PCE will handle all path computations or more granular, so a
   PCE may have the highest priority for only a subset of LSPs or
   association-groups.

   A PCEP Speaker receiving a PCRpt from a PCC with the D flag set that
   does not have the highest computation priority, SHOULD forward the
   PCRpt on all state-sync sessions (as per Section 3.3) and SHOULD set
   D flag on the state-sync session towards the highest priority PCE, D
   flag will be unset to all other state-sync sessions.  This behavior
   is similar to the delegation behavior handled at the PCC side and is
   called a sub-delegation (the PCE sub-delegates the control of the LSP
   to another PCE).  When a PCEP Speaker sub-delegates an LSP to another
   PCE, it loose control of the LSP and cannot update it anymore by its
   own decision.  When a PCE receives a PCRpt with D flag set on a
   state-sync session, as a regular PCE, it is granted control over the
   LSP.

   If the highest priority PCE is failing or if the state-sync session
   between the local PCE and the highest priority PCE failed, the local
   PCE MAY decide to delegate the LSP to the next highest priority PCE
   or to take back control of the LSP.  It is a local policy decision.

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   When a PCE has the delegation for an LSP and needs to update this
   LSP, it MUST send a PCUpd message to all state-sync sessions and to
   the PCC session on which it received the delegation.  The D-Flag
   would be unset in the PCUpd for state-sync sessions whereas the
   D-Flag would be set for the PCC.  In the case of sub-delegation, the
   computing PCE will send the PCUpd only to all state-sync sessions (as
   it has no direct delegation from a PCC).  The D-Flag would be set for
   the state-sync session to the PCE that sub-delegated this LSP and the
   D-Flag would be unset for other state-sync sessions.

   The PCUpd sent over a state-sync session MUST contain the SPEAKER-
   IDENTITY-TLV in the LSP Object (the value used must identify the
   target PCC).  The PLSP-ID used is the original PLSP-ID generated by
   the PCC and learned from the forwarded PCRpt.  If a PCE receives a
   PCUpd on a state-sync session without the SPEAKER-IDENTITY-TLV, it
   MUST discard the PCUpd and MUST reply with a PCErr message using
   error-type=6 (Mandatory Object missing) and error-value=TBD1
   (SPEAKER-IDENTITY-TLV missing).

   When a PCE receives a valid PCUpd on a state-sync session, it SHOULD
   forward the PCUpd to the appropriate PCC (identified based on the
   SPEAKER-IDENTITY-TLV value) that delegated the LSP originally and
   SHOULD remove the SPEAKER-IDENTITY-TLV from the LSP Object.  The
   acknowledgment of the PCUpd is done through a cascaded mechanism, and
   the PCC is the only responsible for triggering the acknowledgment:
   when the PCC receives the PCUpd from the local PCE, it acknowledges
   it with a PCRpt as per [RFC8231].  When receiving the new PCRpt from
   the PCC, the local PCE uses the defined forwarding rules on the
   state-sync session so the acknowledgment is relayed to the computing
   PCE.

   A PCE SHOULD NOT compute a path using an association-group constraint
   if it has delegation for only a subset of LSPs in the group.  In this
   case, an implementation MAY use a local policy on PCE to decide if
   PCE does not compute path at all for this set of LSP or if it can
   compute a path by relaxing the association-group constraint.

3.6.  Passive Stateful Procedures

   In the passive stateful PCE architecture, the PCC is responsible for
   triggering a path computation request using a PCReq message to its
   PCE.  Similarly to PCRpt Message, which remains unchanged for passive
   mode, if a PCE receives a PCReq for an LSP and if this PCE finds that
   it does not have the highest computation priority of this LSP, or
   groups..., it MUST forward the PCReq message to the highest priority
   PCE over the state-sync session.  When the highest priority PCE
   receives the PCReq, it computes the path and generates a PCRep
   message towards the PCE that made the request.  This PCE will then

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   forward the PCRep to the requesting PCC.  The handling of LSP object
   and the SPEAKER-IDENTITY-TLV in PCReq and PCRep is similar to PCRpt/
   PCUpd messages.

3.7.  PCE Initiation Procedures

   It is possible that a PCE does not have a PCEP session with the
   headend to initiate a LSP as per [RFC8281].  A PCE could send the
   PCInitiate message on the state-sync sessions to other PCE to request
   it to create a PCE-Initiated LSP on its behalf.  If the PCE is able
   to initiate the LSP it would report it on the state-sync session via
   PCRpt message.  If the PCE does not have a session to the headend, it
   MUST send a PCErr message with Error-type=24 (PCE instantiation
   error) and Error-value=TBD5 (No PCEP session with the headend).  PCE
   could try to initiate via another state-sync PCE if available.

4.  Examples

   The examples in this section are for illustrative purpose to show how
   the behavior of the state sync inter-PCE sessions.

4.1.  Example 1

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         _________________________________________
        /                                         \
       /        +------+            +------+       \
      |         | PCE1 |            | PCE2 |        |
      |         +------+            +------+        |
      |                                             |
      | +------+           10             +------+  |
      | | PCC1 | ----- R1 ---- R2 ------- | PCC2 |  |
      | +------+       |        |         +------+  |
      |                |        |                   |
      |                |        |                   |
      | +------+       |        |         +------+  |
      | | PCC3 | ----- R3 ---- R4 ------- | PCC4 |  |
      | +------+                          +------+  |
      |                                             |
       \                                           /
        \_________________________________________/

             +----------+
             |   PCC1   |  LSP : PCC1->PCC2
             +----------+
               /
          D=1 /
      +---------+    +---------+
      |  PCE1   |----|  PCE2   |
      +---------+    +---------+
                      / D=1
                     /
             +----------+
             |   PCC3   |  LSP : PCC3->PCC4
             +----------+

   PCE1 computation priority 100
   PCE2 computation priority 200

   Consider the PCEP sessions as shown above, where computation priority
   is global for all the LSPs and link disjoint between LSPs PCC1->PCC2
   and PCC3->PCC4 is required.

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   Consider the PCC1->PCC2 is configured first and PCC1 delegates the
   LSP to PCE1, but as PCE1 does not have the highest computation
   priority, it sub-delegates the LSP to PCE2 by sending a PCRpt with
   D=1 and including the SPEAKER-IDENTITY-TLV over the state-sync
   session.  PCE2 receives the PCRpt and as it has delegation for this
   LSP, it computes the shortest path: R1->R3->R4->R2->PCC2.  It then
   sends a PCUpd to PCE1 (including the SPEAKER-IDENTITY-TLV) with the
   computed ERO.  PCE1 forwards the PCUpd to PCC1 (removing the SPEAKER-
   IDENTITY-TLV).  PCC1 acknowledges the PCUpd by a PCRpt to PCE1.  PCE1
   forwards the PCRpt to PCE2.

   When PCC3->PCC4 is configured, PCC3 delegates the LSP to PCE2, PCE2
   can compute a disjoint path as it has knowledge of both LSPs and has
   delegation also for both.  The only solution found is to move
   PCC1->PCC2 LSP on another path, PCE2 can move PCC1->PCC2 as it has
   sub-delegation for it.  It creates a new PCUpd with a new ERO:
   R1->R2-PCC2 towards PCE1 which forwards to PCC1.  PCE2 sends a PCUpd
   to PCC3 with the path: R3->R4->PCC4.

   In this set-up, PCEs are able to find a disjoint path while without
   state-sync and computation priority they could not.

4.2.  Example 2

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         _____________________________________
        /                                     \
       /        +------+        +------+       \
      |         | PCE1 |        | PCE2 |        |
      |         +------+        +------+        |
      |                                         |
      | +------+         100          +------+  |
      | |      | -------------------- |      |  |
      | | PCC1 | ----- R1 ----------- | PCC2 |  |
      | +------+       |              +------+  |
      |    |           |                  |     |
      |  6 |           | 2                | 2   |
      |    |           |                  |     |
      | +------+       |              +------+  |
      | | PCC3 | ----- R3 ----------- | PCC4 |  |
      | +------+               10     +------+  |
      |                                         |
       \                                       /
        \_____________________________________/

             +----------+
             |   PCC1   |  LSP : PCC1->PCC2
             +----------+
               /     \
          D=1 /       \ D=0
      +---------+    +---------+
      |  PCE1   |----|  PCE2   |
      +---------+    +---------+
           D=0 \      / D=1
                \    /
             +----------+
             |   PCC3   |  LSP : PCC3->PCC4
             +----------+

   PCE1 computation priority 200
   PCE2 computation priority 100

   In this example, suppose both LSPs are configured almost at the same
   time.  PCE1 sub-delegates PCC1->PCC2 to PCE2 while PCE2 keeps
   delegation for PCC3->PCC4, PCE2 computes a path for PCC1->PCC2 and
   PCC3->PCC4 and can achieve disjointness computation easily.  No
   computation loop happens in this case.

4.3.  Example 3

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         _________________________________________
        /                                         \
       /        +------+            +------+       \
      |         | PCE1 |            | PCE2 |        |
      |         +------+            +------+        |
      |                                             |
      | +------+           10             +------+  |
      | | PCC1 | ----- R1 ---- R2 ------- | PCC2 |  |
      | +------+       |        |         +------+  |
      |                |        |                   |
      |                |        |                   |
      | +------+       |        |         +------+  |
      | | PCC3 | ----- R3 ---- R4 ------- | PCC4 |  |
      | +------+                          +------+  |
      |                                             |
       \                                           /
        \_________________________________________/

             +----------+
             |   PCC1   |  LSP : PCC1->PCC2
             +----------+
               /
          D=1 /
      +---------+    +---------+    +---------+
      |  PCE1   |----|  PCE2   |----|  PCE3   |
      +---------+    +---------+    +---------+
                      / D=1
                     /
             +----------+
             |   PCC3   |  LSP : PCC3->PCC4
             +----------+

   PCE1 computation priority 100
   PCE2 computation priority 200
   PCE3 computation priority 300

   With the PCEP sessions as shown above, consider the need to have link
   disjoint LSPs PCC1->PCC2 and PCC3->PCC4.

   Suppose PCC1->PCC2 is configured first, PCC1 delegates the LSP to
   PCE1, but as PCE1 does not have the highest computation priority, it
   will sub-delegate the LSP to PCE2 (as it not aware of PCE3 and has no
   way to reach it).  PCE2 cannot compute a path for PCC1->PCC2 as it
   does not have the highest priority and is not allowed to sub-delegate
   the LSP again towards PCE3 as per Section 3.

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   When PCC3->PCC4 is configured, PCC3 delegates the LSP to PCE2 that
   performs sub-delegation to PCE3.  As PCE3 will have knowledge of only
   one LSP in the group, it cannot compute disjointness and can decide
   to fall-back to a less constrained computation to provide a path for
   PCC3->PCC4.  In this case, it will send a PCUpd to PCE2 that will be
   forwarded to PCC3.

   Disjointness cannot be achieved in this scenario because of lack of
   state-sync session between PCE1 and PCE3, but no computation loop
   happens.  Thus it is advised for all PCEs that support state-sync to
   have a full mesh sessions between each other.

5.  Using Primary/Secondary Computation and State-sync Sessions to
    increase Scaling

   The Primary/Secondary computation and state-sync sessions
   architecture can be used to increase the scaling of the PCE
   architecture.  If the number of PCCs is really high, it may be too
   resource consuming for a single PCE to maintain all the PCEP sessions
   while at the same time performing all path computations.  Using
   primary/secondary computation and state-sync sessions may allow to
   create groups of PCEs that manage a subset of the PCCs and perform
   some or no path computations.  Decoupling PCEP session maintenance
   and computation will allow increasing scaling of the PCE
   architecture.

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               +----------+
               |  PCC500  |
             +----------+-+
             |   PCC1   |
             +----------+
               /     \
              /       \
      +---------+   +---------+
      |  PCE1   |---|  PCE2   |
      +---------+   +---------+
           |    \  /    |
           |     \/     |
           |     /\     |
           |    /  \    |
      +---------+   +---------+
      |  PCE3   |---|  PCE4   |
      +---------+   +---------+
              \       /
               \     /
             +----------+
             |  PCC501  |
             +----------+-+
               |  PCC1000 |
               +----------+

   In the figure above, two groups of PCEs are created: PCE1/2 maintain
   PCEP sessions with PCC1 up to PCC500, while PCE3/4 maintain PCEP
   sessions with PCC501 up to PCC1000.  A granular primary/secondary
   policy is set-up as follows to load-share computation between PCEs:

   *  PCE1 has priority 200 for association ID 1 up to 300, association
      source 0.0.0.0.  All other PCEs have a decreasing priority for
      those associations.

   *  PCE3 has priority 200 for association ID 301 up to 500,
      association source 0.0.0.0.  All other PCEs have a decreasing
      priority for those associations.

   If some PCCs delegate LSPs with association ID 1 up to 300 and
   association source 0.0.0.0, the receiving PCE (if not PCE1) will sub-
   delegate the LSPs to PCE1.  PCE1 becomes responsible for the
   computation of these LSP associations while PCE3 is responsible for
   the computation of another set of associations.

   The procedures described in this document could help greatly in load-
   sharing between a group of stateful PCEs.

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6.  PCEP-PATH-VECTOR TLV

   This document allows PCEP messages to be propagated among PCEP
   speaker.  It may be useful to track information about the propagation
   of the messages.  One of the use cases is a message loop detection
   mechanism, but other use cases like hop by hop information recording
   may also be implemented.

   This document introduces the PCEP-PATH-VECTOR TLV (type TBD3) with
   the following format:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               Type=TBD3       |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              PCEP-SPEAKER-INFORMATION#1                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              ...                                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              ...                                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              PCEP-SPEAKER-INFORMATION#n                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The TLV format and padding rules are as per [RFC5440].

   The PCEP-SPEAKER-INFORMATION field has the following format:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Length                    |      ID Length                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   //              Speaker Entity identity (variable)             //
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   //              SubTLVs (optional)                             //
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   *  Length: defines the total length of the PCEP-SPEAKER-INFORMATION
      field.

   *  ID Length: defines the length of the Speaker identity actual field
      (non-padded).

   *  Speaker Entity identity: same possible values as the SPEAKER-
      IDENTIFIER-TLV.  Padded with trailing zeros to a 4-byte boundary.

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   *  The PCEP-SPEAKER-INFORMATION may also carry some optional subTLVs
      so each PCEP speaker can add local information that could be
      recorded.  This document does not define any sub-TLV.

   The PCEP-PATH-VECTOR TLV MAY be carried in the LSP Object.  Its usage
   is purely optional.

   The list of speakers within the PCEP-PATH-VECTOR TLV MUST be ordered.
   When sending a PCEP message (PCRpt, PCUpd, or PCInitiate), a PCEP
   Speaker MAY add the PCEP-PATH-VECTOR TLV with a PCEP-SPEAKER-
   INFORMATION containing its own information.  If the PCEP message sent
   is the result of a previously received PCEP message, and if the PCEP-
   PATH-VECTOR TLV was already present in the initial message, the PCEP
   speaker MAY append a new PCEP-SPEAKER-INFORMATION containing its own
   information.

7.  Security Considerations

   The security considerations described in [RFC8231] and [RFC5440]
   apply to the extensions described in this document as well.
   Additional considerations related to state synchronization and sub-
   delegation between stateful PCEs are introduced, as it could be
   spoofed and could be used as an attack vector.  An attacker could
   attempt to create too much state in an attempt to load the PCEP peer.
   The PCEP peer responds with a PCErr message as described in
   [RFC8231].  An attacker could impact LSP operations by creating bogus
   state.  Further, state synchronization between stateful PCEs could
   provide an adversary with the opportunity to eavesdrop on the
   network.  Thus, securing the PCEP session using Transport Layer
   Security (TLS) [RFC8253], as per the recommendations and best current
   practices in [RFC7525], is RECOMMENDED.

8.  Implementation Status

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

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   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".

   At the time of posting the -03 version of this document, there are no
   known implementations of this mechanism.  It is believed that some
   vendors are considering implementations, but these plans are too
   vague to make any further assertions.

9.  Manageability Considerations

9.1.  Control of Function and Policy

   An operator MUST be allowed to configure the capability to support
   state-sync procedures for a inter-PCE session.  They SHOULD also
   allow configuration of a computation priority at a PCE and at an
   association level.

9.2.  Information and Data Models

   An implementation SHOULD allow the operator to view the capability
   defined in this document.  To serve this purpose, the PCEP YANG
   module [I-D.ietf-pce-pcep-yang] could be extended in the future.

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].

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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].

9.5.  Requirements On Other Protocols

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

9.6.  Impact On Network Operations

   Mechanisms defined in this document improves the network operations
   by alleviating the problems described in Section 1.

10.  Acknowledgements

   Thanks to [I-D.knodel-terminology] urging for better use of terms.

11.  IANA Considerations

   This document requests IANA actions to allocate code points for the
   protocol elements defined in this document.

11.1.  PCEP-Error Object

   IANA is requested to allocate a new Error Value for the Error Type 9.

   +============+============================+===========+
   | Error-Type | Meaning                    | Reference |
   +============+============================+===========+
   |     6      | Mandatory Object Missing   | [RFC5440] |
   +------------+----------------------------+-----------+
   |            | Error-value=TBD1: SPEAKER- | This      |
   |            | IDENTITY-TLV missing       | document  |
   +------------+----------------------------+-----------+
   |     24     | LSP instantiation error    | [RFC8281] |
   +------------+----------------------------+-----------+
   |            | Error-value=TBD5: No PCEP  | This      |
   |            | session with the headend   | document  |
   +------------+----------------------------+-----------+

                           Table 1

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11.2.  PCEP TLV Type Indicators

   IANA is requested to allocate new TLV Type Indicator values within
   the "PCEP TLV Type Indicators" sub-registry of the PCEP Numbers
   registry, as follows:

   +=======+=============================+===============+
   | Value |           Meaning           |   Reference   |
   +=======+=============================+===============+
   |  TBD2 | ORIGINAL-LSP-DB-VERSION TLV | This document |
   +-------+-----------------------------+---------------+
   |  TBD3 |     PCEP-PATH-VECTOR TLV    | This document |
   +-------+-----------------------------+---------------+

                           Table 2

11.3.  STATEFUL-PCE-CAPABILITY TLV

   IANA is requested to allocate a new bit value in the STATEFUL-PCE-
   CAPABILITY TLV Flag Field sub-registry.

   +======+======================+===============+
   | Bit  |     Description      |   Reference   |
   +======+======================+===============+
   | TBD4 | INTER-PCE-CAPABILITY | This document |
   +------+----------------------+---------------+

                       Table 3

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>.

   [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>.

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   [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>.

   [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>.

   [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>.

12.2.  Informative References

   [I-D.ietf-pce-pcep-yang]
              Dhody, D., Hardwick, J., Beeram, V. P., and J. Tantsura,
              "A YANG Data Model for Path Computation Element
              Communications Protocol (PCEP)", Work in Progress,
              Internet-Draft, draft-ietf-pce-pcep-yang-18, 25 January
              2022, <https://datatracker.ietf.org/doc/html/draft-ietf-
              pce-pcep-yang-18>.

   [I-D.knodel-terminology]
              Knodel, M. and N. T. Oever, "Terminology, Power, and
              Exclusionary Language in Internet-Drafts and RFCs", Work
              in Progress, Internet-Draft, draft-knodel-terminology-09,
              9 February 2022, <https://datatracker.ietf.org/doc/html/
              draft-knodel-terminology-09>.

   [RFC4655]  Farrel, A., Vasseur, J.-P., 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>.

   [RFC6805]  King, D., Ed. and A. Farrel, Ed., "The Application of the
              Path Computation Element Architecture to the Determination
              of a Sequence of Domains in MPLS and GMPLS", RFC 6805,
              DOI 10.17487/RFC6805, November 2012,
              <https://www.rfc-editor.org/info/rfc6805>.

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   [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>.

   [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>.

   [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>.

   [RFC8051]  Zhang, X., Ed. and I. Minei, Ed., "Applicability of a
              Stateful Path Computation Element (PCE)", RFC 8051,
              DOI 10.17487/RFC8051, January 2017,
              <https://www.rfc-editor.org/info/rfc8051>.

   [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>.

   [RFC8751]  Dhody, D., Lee, Y., Ceccarelli, D., Shin, J., and D. King,
              "Hierarchical Stateful Path Computation Element (PCE)",
              RFC 8751, DOI 10.17487/RFC8751, March 2020,
              <https://www.rfc-editor.org/info/rfc8751>.

   [RFC8800]  Litkowski, S., Sivabalan, S., Barth, C., and M. Negi,
              "Path Computation Element Communication Protocol (PCEP)
              Extension for Label Switched Path (LSP) Diversity
              Constraint Signaling", RFC 8800, DOI 10.17487/RFC8800,
              July 2020, <https://www.rfc-editor.org/info/rfc8800>.

Appendix A.  Contributors

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

   Email: dhruv.ietf@gmail.com

Authors' Addresses

   Stephane Litkowski
   Cisco
   Email: slitkows.ietf@gmail.com

   Siva Sivabalan
   Ciena Corporation
   Email: msiva282@gmail.com

   Cheng Li
   Huawei Technologies
   Huawei Campus, No. 156 Beiqing Rd.
   Beijing
   100095
   China
   Email: c.l@huawei.com

   Haomian Zheng
   Huawei Technologies
   H1, Huawei Xiliu Beipo Village, Songshan Lake
   Dongguan
   Guangdong, 523808
   China
   Email: zhenghaomian@huawei.com

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