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Versions: 00 01 02 draft-ietf-pce-stateful-sync-optimizations

PCE Working Group                                              E. Crabbe
Internet-Draft                                              Google, Inc.
Intended status: Standards Track                               J. Medved
Expires: September 4, 2014                           Cisco Systems, Inc.
                                                                I. Minei
                                                            Google, Inc.
                                                                R. Varga
                                               Pantheon Technologies SRO
                                                                X. Zhang
                                                                D. Dhody
                                                     Huawei Technologies
                                                           March 3, 2014


 Optimizations of Label Switched Path State Synchronization Procedures
                           for a Stateful PCE
             draft-minei-pce-stateful-sync-optimizations-02

Abstract

   A stateful Path Computation Element (PCE) has access to not only the
   information disseminated by the network's Interior Gateway Protocol
   (IGP), but also the set of active paths and their reserved resources
   for its computation.  The additional Label Switched Path (LSP) state
   information allows the PCE to compute constrained paths while
   considering individual LSPs and their interactions.  This requires a
   reliable state synchronization mechanism between the PCE and the
   network, PCE and path computation clients (PCCs), and between
   cooperating PCEs.  The basic mechanism for state synchronization is
   part of the stateful PCE specification.  This draft presents
   motivations for optimizations to the base state synchronization
   procedure and specifies the required Path Computation Element
   Communication Protocol (PCEP) extensions.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-



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   Drafts is at http://datatracker.ietf.org/drafts/current/.

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

   This Internet-Draft will expire on September 4, 2014.

Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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




























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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  State Synchronization Avoidance  . . . . . . . . . . . . . . .  4
     3.1.  Motivation . . . . . . . . . . . . . . . . . . . . . . . .  4
     3.2.  State Synchronization Avoidance Procedure  . . . . . . . .  5
     3.3.  PCEP Extensions  . . . . . . . . . . . . . . . . . . . . .  9
       3.3.1.  LSP State Database Version Number TLV  . . . . . . . .  9
       3.3.2.  Speaker Entity Identifier TLV  . . . . . . . . . . . . 10
   4.  PCE-triggered State Synchronization  . . . . . . . . . . . . . 11
     4.1.  Motivation . . . . . . . . . . . . . . . . . . . . . . . . 11
     4.2.  PCE-triggered State Synchronization Procedure  . . . . . . 11
   5.  Incremental State Synchronization  . . . . . . . . . . . . . . 11
     5.1.  Motivation . . . . . . . . . . . . . . . . . . . . . . . . 12
     5.2.  Incremental Synchronization Procedure  . . . . . . . . . . 13
   6.  Advertising Support of Synchronization Optimizations . . . . . 15
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 16
     7.1.  PCEP-Error Object  . . . . . . . . . . . . . . . . . . . . 16
     7.2.  PCEP TLV Type Indicators . . . . . . . . . . . . . . . . . 17
     7.3.  STATEFUL-PCE-CAPABILITY TLV  . . . . . . . . . . . . . . . 17
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 17
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 17
   10. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 17
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
     11.1. Normative References . . . . . . . . . . . . . . . . . . . 18
     11.2. Informative References . . . . . . . . . . . . . . . . . . 18
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 18























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1.  Introduction

   The Path Computation Element Communication Protocol (PCEP) provides
   mechanisms for Path Computation Elements (PCEs) to perform path
   computations in response to Path Computation Clients (PCCs) requests.

   [I-D.ietf-pce-stateful-pce] describes a set of extensions to PCEP to
   provide stateful control.  A stateful PCE has access to not only the
   information carried by the network's Interior Gateway Protocol (IGP),
   but also the set of active paths and their reserved resources for its
   computations.  The additional state allows the PCE to compute
   constrained paths while considering individual LSPs and their
   interactions.  This requires a reliable state synchronization
   mechanism between the PCE and the network, PCE and PCC, and between
   cooperating PCEs.  [I-D.ietf-pce-stateful-pce] describes the basic
   mechanism for state synchronization.  This draft specifies
   optimizations for state synchronization and the correspoding PCEP
   extensions.


2.  Terminology

   This document uses the following terms defined in [RFC5440]: PCC,
   PCE, PCEP Peer.

   This document uses the following terms defined in
   [I-D.ietf-pce-stateful-pce] : Delegation, Redelegation Timeout
   Interval, LSP State Report, LSP Update Request, LSP State Database.

   Within this document, when describing PCE-PCE communications, the
   requesting PCE fills the role of a PCC.  This provides a saving in
   documentation without loss of function.

   The message formats in this document are specified using Routing
   Backus-Naur Format (RBNF) encoding as specified in [RFC5511].


3.  State Synchronization Avoidance

3.1.  Motivation

   The purpose of state synchronization is to provide a checkpoint-in-
   time state replica of a PCC's LSP state in a stateful PCE.  State
   synchronization is performed immediately after the initialization
   phase ([RFC5440]).  [I-D.ietf-pce-stateful-pce] describes the basic
   mechanism for state synchronization.

   State synchronization is not always necessary following a PCEP



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   session restart.  If the state of both PCEP peers did not change, the
   synchronization phase may be skipped.  This can result in significant
   savings in both control-plane data exchanges and the time it takes
   for the stateful PCE to become fully operational.

3.2.  State Synchronization Avoidance Procedure

   State synchronization MAY be skipped following a PCEP session restart
   if the state of both PCEP peers did not change during the period
   prior to session re-initialization.  To be able to make this
   determination, state must be exchanged and maintained by both PCE and
   PCC during normal operation.  This is accomplished by keeping track
   of the changes to the LSP state database, using a version tracking
   field called the LSP State Database Version Number.

   The LSP State Database Version Number, carried in LSP-DB-VERSION TLV
   (see Section 3.3.1), is owned by a PCC and it MUST be incremented by
   1 for each successive change in the PCC's LSP state database.  The
   LSP State Database Version Number MUST start at 1 and may wrap
   around.  Values 0 and 0xFFFFFFFFFFFFFFFF are reserved.  If either of
   the two values are used during LSP state (re)-synchronization, the
   PCE speaker receiving this node should send back a PCErr with Error-
   type 20 Error-value 6 'Received an invalid LSP DB Version Number',
   and close the PCEP session.  Operations that trigger a change to the
   local LSP state database include a change in the LSP operational
   state, delegation of an LSP, removal or setup of an LSP or change in
   any of the LSP attributes that would trigger a report to the PCE.

   State synchronization avoidance is advertised on a PCEP session
   during session startup using the INCLUDE-DB-VERSION (IDB) bit in the
   capabilities TLV (see Section 6).  The peer may move in the network,
   either physically or logically, which may cause its connectivity
   details and transport-level identity (such as IP address) to change.
   To ensure that a PCEP peer can recognize a previously connected peer
   even in face of such mobility, each PCEP peer includes the SPEAKER-
   ENTITY-ID TLV described in Section 3.3.2 in the OPEN message.

   If both PCEP speakers set the IDB flag in the OPEN object's STATEFUL-
   PCE-CAPABILITY TLV to 1, the PCC MUST include the LSP-DB-VERSION TLV
   in each LSP object of the PCRpt message.  If the LSP-DB-VERSION TLV
   is missing in a PCRpt message, the PCE will generate an error with
   error-type 6 (mandatory object missing) and Error Value 12 (LSP-DB-
   VERSION TLV missing) and close the session.  If state synchronization
   avoidance has not been enabled on a PCEP session, the PCC SHOULD NOT
   include the LSP-DB-VERSION TLV in the LSP Object and the PCE SHOULD
   ignore it were to receive one.

   If a PCE's LSP state database survived the restart of a PCEP session,



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   the PCE will include the LSP-DB-VERSION TLV in its OPEN object, and
   the TLV will contain the last LSP State Database Version Number
   received on an LSP State Report from the PCC in the previous PCEP
   session.  If a PCC's LSP State Database survived the restart of a
   PCEP session, the PCC will include the LSP-DB-VERSION TLV in its OPEN
   object and the TLV will contain the latest LSP State Database Version
   Number.  If a PCEP speaker's LSP state database did not survive the
   restart of a PCEP session, the PCEP speaker MUST NOT include the LSP-
   DB-VERSION TLV in the OPEN object.

   If both PCEP speakers include the LSP-DB-VERSION TLV in the OPEN
   Object and the TLV values match, the PCC MAY skip state
   synchronization.  Otherwise, the PCC MUST perform state
   synchronization to the stateful PCE.  If the PCC attempts to skip
   state synchronization (i.e., the SYNC Flag = 0 on the first LSP State
   Report from the PCC as per [I-D.ietf-pce-stateful-pce]), the PCE MUST
   send back a PCErr with Error-type 20 Error-value 2 'LSP Database
   version mismatch', and close the PCEP session.

   If state synchronization is required, then prior to completing the
   initialization phase, the PCE MUST mark any LSPs in the LSP database
   that were previously reported by the PCC as stale.  When the PCC
   reports an LSP during state synchronization, if the LSP already
   exists in the LSP database, the PCE MUST update the LSP database and
   clear the stale marker from the LSP.  When it has finished state
   synchronization, the PCC MUST immediately send an end of
   synchronization marker.  The end of synchronization marker is a Path
   Computation State Report (PCRpt) message with an LSP object
   containing a PLSP-ID of 0 and with the SYNC flag set to 0
   ([I-D.ietf-pce-stateful-pce]).  The LSP-DB-VERSION TLV MUST be
   included in this PCRpt message.  On receiving this state report, the
   PCE MUST purge any LSPs from the LSP database that are still marked
   as stale.

   Note that a PCE/PCC MAY force state synchronization by not including
   the LSP-DB-VERSION TLV in its OPEN object.

   Since a PCE does not make changes to the LSP State Database Version
   Number, a PCC should never encounter this TLV in a message from the
   PCE (other than the OPEN message).  A PCC SHOULD ignore the LSP-DB-
   VERSION TLV, were it to receive one from a PCE.

   If state synchronization avoidance is enabled, a PCC MUST increment
   its LSP State Database Version Number when the 'Redelegation Timeout
   Interval' timer expires (see [I-D.ietf-pce-stateful-pce]) for the use
   of the Redelegation Timeout Interval).

   Figure 1 shows an example sequence where the state synchronization is



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   skipped.

                     +-+-+                    +-+-+
                     |PCC|                    |PCE|
                     +-+-+                    +-+-+
                       |                        |
                       |--Open--,               |
                       |  DBv=42 \    ,---Open--|
                       |   IDB=1  \  /   DBv=42 |
                       |           \/     IDB=1 |
                       |           /\           |
                       |          /   `-------->| (OK to skip sync)
           (Skip sync) |<--------`              |
                       |            .           |
                       |            .           |
                       |            .           |
                       |                        |
                       |--PCRpt,DBv=43,SYNC=0-->| (Regular
                       |                        |  LSP State Report)
                       |--PCRpt,DBv=44,SYNC=0-->| (Regular
                       |                        |  LSP State Report)
                       |--PCRpt,DBv=45,SYNC=0-->|
                       |                        |

                  Figure 1: State Synchronization Skipped

   Figure 2 shows an example sequence where the state synchronization is
   performed due to LSP state database version mismatch during the PCEP
   session setup.  Note that the same state synchronization sequence
   would happen if either the PCC or the PCE would not include the LSP-
   DB-VERSION TLV in their respective Open messages.




















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                     +-+-+                    +-+-+
                     |PCC|                    |PCE|
                     +-+-+                    +-+-+
                       |                        |
                       |--Open--,               |
                       |  DBv=46 \    ,---Open--|
                       |   IDB=1  \  /   DBv=42 |
                       |           \/     IDB=1 |
                       |           /\           |
                       |          /   `-------->| (Expect sync)
             (Do sync) |<--------`              |
                       |                        |
                       |--PCRpt,DBv=46,SYNC=1-->| (Sync start)
                       |            .           |
                       |            .           |
                       |            .           |
                       |--PCRpt,DBv=46,SYNC=0-->| (Sync done)
                       |            .           |(Purge LSP State
                       |            .           | if applicable)
                       |            .           |
                       |--PCRpt,DBv=47,SYNC=0-->| (Regular
                       |                        |  LSP State Report)
                       |--PCRpt,DBv=48,SYNC=0-->| (Regular
                       |                        |  LSP State Report)
                       |--PCRpt,DBv=49,SYNC=0-->|
                       |                        |

                 Figure 2: State Synchronization Performed

   Figure 3 shows an example sequence where the state synchronization is
   skipped, but because one or both PCEP speakers set the IDB Flag to 0,
   the PCC does not send LSP-DB-VERSION TLVs in subsequent PCRpt
   messages to the PCE.  If the current PCEP session restarts, the PCEP
   speakers will have to perform state synchronization, since the PCE
   does not know the PCC's latest LSP State Database Version Number
   information.















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                     +-+-+                    +-+-+
                     |PCC|                    |PCE|
                     +-+-+                    +-+-+
                       |                        |
                       |--Open--,               |
                       |  DBv=42 \    ,---Open--|
                       |   IDB=0  \  /   DBv=42 |
                       |           \/     IDB=0 |
                       |           /\           |
                       |          /   `-------->| (OK to skip sync)
           (Skip sync) |<--------`              |
                       |            .           |
                       |            .           |
                       |            .           |
                       |------PCRpt,SYNC=0----->| (Regular
                       |                        |  LSP State Report)
                       |------PCRpt,SYNC=0----->| (Regular
                       |                        |  LSP State Report)
                       |------PCRpt,SYNC=0----->|
                       |                        |

   Figure 3: State Synchronization Skipped, no LSP-DB-VERSION TLVs sent
                                 from PCC

3.3.  PCEP Extensions

3.3.1.  LSP State Database Version Number TLV

   The LSP State Database Version Number (LSP-DB-VERSION) TLV is an
   optional TLV that MAY be included in the OPEN object and the LSP
   object.

   The format of the LSP-DB-VERSION TLV is shown in the following
   figure:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |           Type=[TBD]          |            Length=8           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                 LSP State DB Version Number                   |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 4: LSP-DB-VERSION TLV format

   The type of the TLV is [TBD] and it has a fixed length of 8 octets.
   The value contains a 64-bit unsigned integer, representing the LSP



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   State DB Version Number.

3.3.2.  Speaker Entity Identifier TLV

   The Speaker Entity Identifier TLV (SPEAKER-ENTITY-ID) is an optional
   TLV that MAY be included in the OPEN Object when a PCEP speaker
   wishes to determine if state synchronization can be skipped when a
   PCEP session is restarted.  It contains a unique identifier for the
   node that does not change during the lifetime of the PCEP speaker.
   It identifies the PCEP speaker to its peers even if the speaker's IP
   address is changed.

   The format of the SPEAKER-ENTITY-ID TLV is shown in the following
   figure:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |           Type=[TBD]          |       Length (variable)       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     //                 Speaker Entity Identifier                    //
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 5: SPEAKER-ENTITY-ID TLV format

   The type of the TLV is [TBD] and it has a a variable length, which
   MUST be greater than 0 and padded to 4-octet alignment (, and padding
   is not included in the Length field).  The value contains the entity
   identifier of the speaker transmitting this TLV.  This identifier is
   required to be unique within its scope of visibility, which is
   usually limited to a single domain.  It MAY be configured by the
   operator.  Alternatively, it can be derived automatically from a
   suitably-stable unique identifier, such as a MAC address, serial
   number, Traffic Engineering Router ID, or similar.  In the case of
   inter-domain connections, the speaker SHOULD prefix its usual
   identifier with the domain identifier of its residence, such as
   Autonomous System number, IGP area identifier, or similar.

   The relationship between this identifier and entities in the Traffic
   Engineering database is intentionally left undefined.

   From a manageability point of view, a PCE or PCC implementation
   SHOULD allow the operator to configure this Speaker Entity
   Identifier.





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4.  PCE-triggered State Synchronization

4.1.  Motivation

   The accuracy of the computations performed by the PCE is tied to the
   accuracy of the view the PCE has on the state of the LSPs.
   Therefore, it can be beneficial to be able to resynchronize this
   state even after the session has been established.  The PCE may use
   this approach to continuously sanity check its state against the
   network, or to recover from error conditions without having to tear
   down sessions.

4.2.  PCE-triggered State Synchronization Procedure

   Support of PCE-triggered state synchronization is advertised during
   session startup using the TRIGGERED-SYNC (T) bit in the STATEFUL-PCE-
   CAPABILITY TLV (see Section 6).  The PCE can choose to resynchronize
   its entire LSP database or a single LSP.

   To trigger resynchronization for an LSP, the PCE MUST first mark the
   LSP as stale and then send a Path Computation State Update (PCUpd)
   for it, with the SYNC flag in the LSP object set to 1.  The PCE
   SHOULD NOT include any parameter updates for the LSP, and the PCC
   SHOULD ignore such updates if the SYNC flag is set.  The PCC MUST
   respond with a PCRpt message and SHOULD include the SRP-ID-number of
   the PCUpd that triggered the resynchronization.

   The PCE can also trigger resynchronization of the entire LSP
   database.  The PCE MUST first mark all LSPs in the LSP database that
   were previously reported by the PCC as stale and then send a PCUpd
   with an LSP object containing a PLSP-ID of 0 and with the SYNC flag
   set to 1.  This PCUpd message is the trigger for the PCC to enter the
   synchronization phase as described in [I-D.ietf-pce-stateful-pce] and
   start sending PCRpt messages.  After the receipt of the end-of-
   synchronization marker, the PCE will purge LSPs which were not
   refreshed.  The SRP-ID-number of the PCUpd that triggered the
   resynchronization SHOULD be included in each of the PCRpt messages.

   If the TRIGGERED-SYNC capability is not advertised and the PCC
   receives a PCUpd with the SYNC flag set to 1, it MUST send a PCErr
   with the SRP-ID-number of the PCUpd, error-type 20 and error-value
   4.(see Section 7.1)


5.  Incremental State Synchronization

   [I-D.ietf-pce-stateful-pce] describes the LSP state synchronization
   mechanism between PCCs and stateful PCEs.  During the state



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   synchronization, a PCC sends the information of all its LSPs (full
   LSP-DB) to the stateful PCE.  In order to save the state
   synchronization overhead when there is a small number of LSP state
   change in the network between PCEP session restart as well as
   avoiding overloading a PCE during state (re-)synchronization phase,
   this section proposes a mechanism for incremental (Delta) LSP
   Database (LSP-DB) synchronization as well as allowing PCE to control
   the timing of the LSP-DB synchronization process during incremental
   syncronization.

5.1.  Motivation

   According to [I-D.ietf-pce-stateful-pce] , if a PCE restarts and its
   LSP-DB survived, PCCs with mismatched LSP State Database Version
   Number will send all their LSPs information (full LSP-DB) to the
   stateful PCE, even if only a small number of LSPs underwent state
   change.  It can take a long time and consume large communication
   channel bandwidth.  Moreover, the stateful PCE can get overloaded
   with all the PCC performing full synchronization with it at the same
   time.

   Figure 6 shows an example of LSP state synchronization.

                                       +-----+
                                       | PCE |
                                       +-----+
                                      /
                                     /
                                    /
                                   /
                            +------+            +------+
                            | PCC1 |------------| PCC2 |
                            +------+            +------+
                               |                   |
                               |                   |
                            +------+            +------+
                            | PCC3 |------------| PCC4 |
                            +------+            +------+

                        Figure 6: Topology Example

   Assuming there are 320 LSPs in the network, with each PCC having 80
   LSPs.  During the time when the PCEP session is down, 20 LSPs of each
   PCC (i.e., 80 LSPs in total), are changed.  Hence when PCEP session
   restarts, the stateful PCE needs to synchronize 320 LSPs with all
   PCCs.  But actually, 240 LSPs stay the same.  If performing full LSP
   state synchronization, it can take a long time to carry out the
   synchronization of all LSPs.  It is especially true when only a low



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   bandwidth communication channel is available and there is a
   substantial number of LSPs in the network.  Another disadvantage of
   full LSP synchronization is that it is a waste of communication
   bandwidth to perform full LSP synchronization given the fact that the
   number of LSP changes can be small during the time when PCEP session
   is down.

   An incremental (Delta) LSP Database (LSP-DB) state synchronization is
   described in this section, where only the LSPs underwent state change
   are synchronized between the session restart.  This may include new/
   modified/deleted LSPs.  Furthermore, to avoid overloading the PCE,
   the proposed method enable a stateful PCE to trigger the LSP
   synchronization (similar to Section 4).

   PCEP extensions for stateful PCEs to perform LSP synchronization
   SHOULD allow:

   o  Incremental LSP state synchronization between session restarts.
      Note this does not exclude the need for a stateful PCE to request
      a full LSP DB synchronization.

   o  A stateful PCE to control the timing of PCC synchronizing its LSP
      state with the PCE during incremental synchronisation.

5.2.  Incremental Synchronization Procedure

   [I-D.ietf-pce-stateful-pce] describes state synchronization and
   Section 3 describes state synchronization avoidance by using LSP-DB-
   VERSION TLV in its OPEN object.  This section extends this idea to
   only synchronize the delta (changes) in case of version mismatch as
   well as to allow a stateful PCE to control the timing of this
   process.

   If both PCEP speakers include the LSP-DB-VERSION TLV in the OPEN
   object and the LSP-DB-VERSION TLV values match, the PCC MAY skip
   state synchronization.  Otherwise, the PCC MUST perform state
   synchronization.  Instead of dumping full LSP-DB to PCE again, the
   PCC synchronizes the delta (changes) as described in Figure 7 when
   DELTA-LSP-SYNC-CAPABILITY (D flag) is set to 1 by both PCC and PCE
   (see Section 6).  Other combinations of D flag setting by PCC and PCE
   result in full LSP-DB synchronization procedure as described in
   [I-D.ietf-pce-stateful-pce].  If a PCC has to force full LSP DB
   synchronization due to reasons including but not limited: (1) local
   policy configured at the PCC; (2) no sufficient LSP state caches for
   incremental update, the PCC can set the D flag to 0.  Note a PCC may
   have to bring down the current session and force a full LSPDB
   synchronization with D flag set to 0 in the subsequent open message.




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                       +-+-+                    +-+-+
                       |PCC|                    |PCE|
                       +-+-+                    +-+-+
                         |                        |
                         |--Open--,               |
                         |  DBv=46 \    ,---Open--|
                         |   IDB=1  \  /   DBv=42 |
                         |   D=1     \/     IDB=1 |
                         |   T=1     /\      D=1  |
                         |          /  \     T=1  |
                         |         /    `-------->| (Expect Delta sync)
                (Do sync)|<--------`              | (DONOT Purge LSP
                (Delta)  |                        | State)
     (Wait for PCE to    |                        |
      trigger LSP state  |                        |
      sync)              |                        |
                         |<----PCUpd, SYNC=1------| (ask for LSP Sync,
                         |                        |  PLSP-ID =0)
     (Delta Sync starts) |--PCRpt,DBv=43,SYNC=1-->|
                         |            .           |
                         |            .           |
                         |            .           |
                         |            .           |
                         |--PCRpt,DBv=46,SYNC=0-->| (Sync done,
                         |                        | PLSP-ID=0)
                         |                        |
                         |--PCRpt,DBv=47,SYNC=0-->| (Regular
                         |                        |  LSP State Report)
                         |--PCRpt,DBv=48,SYNC=0-->| (Regular
                         |                        |  LSP State Report)
                         |--PCRpt,DBv=49,SYNC=0-->|
                         |                        |


              Figure 7: Incremental Synchronization Procedure

   A stateful PCE MAY choose to control the LSP-DB synchronization
   process.  To allow PCE to do so, PCEP speakers MUST set T bit to 1 to
   indicate this (as described in Section 4).  If the LSP-DB Version is
   mis-matched, it can send a PCUpd message with PLSP-ID = 0 and SYNC =
   1 in order to trigger the LSP-DB synchronization process.  In this
   way, the PCE can control the sequence of LSP synchronization among
   all the PCCs that are re-establishing PCEP sessions with it.  When
   the capability of PCE control is enabled, only after a PCC receives
   this message, it will start sending information that PCE does not
   possess, which is inferred from the LSP-DB version information
   exchanged in the OPEN message.  Note that the PCE should not mark the
   existing LSPs as stale for incremental state synchronisation



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   procedure.

   As per Section 3, the LSP State Database Version Number is
   incremented each time a change is made to the PCC's local LSP State
   Database.  Each LSP is associated with the DB version at the time of
   its state change.  This is needed to determine which LSP and what
   information needs to be synchronized in incremental state
   synchronization.

   It is not necessary for a PCC to store a complete history of LSP
   Database change, but rather remember the LSP state changes (including
   LSP modification, setup and deletion) that happend between the PCEP
   session(s) restart in order to carry out incremental state
   synchronization.  After the synchronization procedure finishes, the
   PCC can dump this history information.  In the example shown in
   Figure 7, the PCC needs to store the LSP state changes that happend
   between DB Version 43 to 46 and synchronizes these changes only when
   performing incremental LSP state update.  So a PCC needs to remember
   the LSP state changes that happened when an existing PCEP session to
   a stateful PCE goes down in the hope of doing incremental
   synchronisation when the session is re-established.

   If a PCC finds out it does not have sufficient information to
   complete incremental synchronisation after advertising incremental
   LSP state synchronization capability, it MUST send a PCErr with
   error-type 20 and error-value 5(see Section 7.1) and terminate the
   session.


6.  Advertising Support of Synchronization Optimizations

   Support for each of the optimizations described in this document
   requires advertising the corresponding capabilities during session
   establishment time.

   New flags are defined for the STATEFUL-PCE-CAPABILITY TLV defined in
   [I-D.ietf-pce-stateful-pce].  Its format is shown in the following
   figure:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |               Type            |            Length=4           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                             Flags                   |D|T|I|S|U|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

               Figure 8: STATEFUL-PCE-CAPABILITY TLV Format



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   The value comprises a single field - Flags (32 bits):

   U (LSP-UPDATE-CAPABILITY - 1 bit):  defined in
      [I-D.ietf-pce-stateful-pce].

   S (INCLUDE-DB-VERSION - 1 bit):  if set to 1 by both PCEP Speakers,
      the PCC will include the LSP-DB-VERSION TLV in each LSP Object.

   I (LSP-INSTANTIATION-CAPABILITY - 1 bit):  defined in
      [I-D.crabbe-pce-pce-initiated-lsp].

   T (TRIGGERED-SYNC - 1 bit):  if set to 1 by both PCEP Speakers, the
      PCE can trigger synchronization of LSPs at any point in the life
      of the session.

   D (DELTA-LSP-SYNC-CAPABILITY - 1 bit):  if set to 1 by a PCEP
      speaker, it indicates that the PCEP speaker allows incremental
      state synchronization.


7.  IANA Considerations

   This document requests IANA actions to allocate code points for the
   protocol elements defined in this document.  Values shown here are
   suggested for use by IANA.

7.1.  PCEP-Error Object

   This document defines new Error-Value values for the LSP state
   synchronization error defined in [I-D.ietf-pce-stateful-pce].

    Error-Type  Meaning
       6        Mandatory Object missing
                 Error-value=12: LSP-DB-VERSION TLV missing
       20       LSP State synchronization error
                 Error-value=2:  LSP Database version mismatch.
                 Error-value=3:  The LSP-DB-VERSION TLV Missing when
                                 state synchronization avoidance is
                                 enabled.
                 Error-value=4:  Attempt to trigger a synchronization
                                 when the TRIGGERED-SYNC capability has
                                 not been advertised.
                 Error-value=5:  No sufficient LSP change information
                                 for incremental LSP state
                                 synchronization.






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                 Error-value=6:  Received an invalid LSP DB Version
                                 Number

7.2.  PCEP TLV Type Indicators

   This document defines the following new PCEP TLVs:

       Value     Meaning                     Reference
         23       LSP-DB-VERSION             This document
         24       SPEAKER-ENTITY-ID          This document

7.3.  STATEFUL-PCE-CAPABILITY TLV

   The following values are defined in this document for the Flags field
   in the STATEFUL-PCE-CAPABILITY-TLV in the OPEN object:

      Bit    Description                Reference

       28    DELTA-LSP-SYNC-CAPABILITY  This document
       29    TRIGGERED-SYNC             This document
       30    INCLUDE-DB-VERSION         This document


8.  Security Considerations

   The security considerations listed in [I-D.ietf-pce-stateful-pce]
   apply to this document as well.


9.  Acknowledgements

   We would like to thank Young Lee for his contributions.


10.  Contributors

   Gang Xie
   Huawei Technologies
   F3-5-B R&D Center, Huawei Industrial Base, Bantian, Longgang District
   Shenzhen, Guangdong, 518129
   P.R. China
   Email: xiegang09@huawei.com


11.  References






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11.1.  Normative References

   [I-D.ietf-pce-stateful-pce]
              Crabbe, E., Medved, J., Minei, I., and R. Varga, "PCEP
              Extensions for Stateful PCE",
              draft-ietf-pce-stateful-pce-08 (work in progress),
              February 2014.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC5440]  Vasseur, JP. and JL. Le Roux, "Path Computation Element
              (PCE) Communication Protocol (PCEP)", RFC 5440,
              March 2009.

   [RFC5511]  Farrel, A., "Routing Backus-Naur Form (RBNF): A Syntax
              Used to Form Encoding Rules in Various Routing Protocol
              Specifications", RFC 5511, April 2009.

11.2.  Informative References

   [I-D.crabbe-pce-pce-initiated-lsp]
              Crabbe, E., Minei, I., Sivabalan, S., and R. Varga, "PCEP
              Extensions for PCE-initiated LSP Setup in a Stateful PCE
              Model", draft-crabbe-pce-pce-initiated-lsp-03 (work in
              progress), October 2013.


Authors' Addresses

   Edward Crabbe
   Google, Inc.
   1600 Amphitheatre Parkway
   Mountain View, CA  94043
   US

   Email: edc@google.com


   Jan Medved
   Cisco Systems, Inc.
   170 West Tasman Dr.
   San Jose, CA  95134
   US

   Email: jmedved@cisco.com





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   Ina Minei
   Google, Inc.
   1600 Amphitheatre Parkway
   Mountain View, CA  94043
   US

   Email: inaminei@google.com


   Robert Varga
   Pantheon Technologies SRO
   Mlynske Nivy 56
   Bratislava  821 05
   Slovakia

   Email: robert.varga@pantheon.sk


   Xian Zhang
   Huawei Technologies
   F3-5-B R&D Center, Huawei Industrial Base, Bantian, Longgang District
   Shenzhen, Guangdong  518129
   P.R.China

   Email: zhang.xian@huawei.com


   Dhruv Dhody
   Huawei Technologies
   Leela Palace
   Bangalore, Karnataka  560008
   INDIA

   Email: dhruv.ietf@gmail.com

















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