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Versions: (draft-zhao-pce-central-controller-user-cases) 00 01 02

TEAS Working Group                                          Quintin Zhao
Internet-Draft                                                  Robin Li
Intended status: Experimental                        Huawei Technologies
Expires: September 18, 2016                                      King Ke
                                                   Tencent Holdings Ltd.
                                                             Luyuan Fang
                                                               Microsoft
                                                               Chao Zhou
                                                           Cisco Systems
                                                             Boris Zhang
                                                    Telus Communications
                                                          March 17, 2016


  The Use Cases for Using PCE as the Central Controller(PCECC) of LSPs
                   draft-zhao-teas-pcecc-use-cases-00

Abstract

   In certain networks deployment scenarios, service providers would
   like to keep all the existing MPLS functionalities in both MPLS and
   GMPLS network while removing the complexity of existing signaling
   protocols such as LDP and RSVP-TE.  In this document, we propose to
   use the PCE as a central controller so that LSP can be
   calculated/signaled/initiated/downloaded/managed through a
   centralized PCE server to each network devices along the LSP path
   while leveraging the existing PCE technologies as much as possible.

   This draft describes the use cases for using the PCE as the central
   controller where LSPs are calculated/setup/initiated/downloaded/
   maintained through extending the current PCE architectures and
   extending the PCEP.

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 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."




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   This Internet-Draft will expire on September 18, 2016.

Copyright Notice

   Copyright (c) 2016 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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Background  . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Using the PCE as the Central Controller (PCECC) Approach    4
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   7
   3.  PCEP Requirements . . . . . . . . . . . . . . . . . . . . . .   7
   4.  Use Cases of PCECC for Label Resource Reservations  . . . . .   8
   5.  Using PCECC for SR without the IGP Extension  . . . . . . . .   9
     5.1.  Use Cases of PCECC for SR Best Effort(BE) Path  . . . . .  10
     5.2.  Use Cases of PCECC for SR Traffic Engineering (TE) Path .  11
   6.  Use Cases of PCECC for TE LSP . . . . . . . . . . . . . . . .  12
   7.  Use Cases of PCECC for Multicast LSPs . . . . . . . . . . . .  14
     7.1.  Using PCECC for P2MP/MP2MP LSPs' Setup  . . . . . . . . .  14
     7.2.  Use Cases of PCECC for the Resiliency of P2MP/MP2MP LSPs   15
       7.2.1.  PCECC for the End-to-End Protection of the P2MP/MP2MP
               LSPs  . . . . . . . . . . . . . . . . . . . . . . . .  15
       7.2.2.  PCECC for the Local Protection of the P2MP/MP2MP LSPs  16
   8.  Use Cases of PCECC for LSP in the Network Migration . . . . .  17
   9.  Use Cases of PCECC for L3VPN and PWE3 . . . . . . . . . . . .  19
   10. Using PCECC for Traffic Classification Informations . . . . .  19
   11. The Considerations for PCECC Procedure and PCEP extensions  .  20
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  20
   13. Security Considerations . . . . . . . . . . . . . . . . . . .  20
   14. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  20
   15. References  . . . . . . . . . . . . . . . . . . . . . . . . .  20
     15.1.  Normative References . . . . . . . . . . . . . . . . . .  20
     15.2.  Informative References . . . . . . . . . . . . . . . . .  21
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  22





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1.  Introduction

1.1.  Background

   In certain network deployment scenarios, service providers would like
   to have the ability to dynamically adapt to a wide range of
   customer's requests for the sake of flexible network service
   delivery, SDN has provides additional flexibility in how the network
   is operated comparing the traditional network.

   The existing networking ecosystem has become awfully complex and
   highly demanding in terms of robustness, performance, scalability,
   flexibility, agility, etc.  By migrating to the SDN enabled network
   from the existing network, service providers and network operators
   must have a solution which they can evolve easily from the existing
   network into the SDN enabled network while keeping the network
   services remain scalable, guarantee robustness and availability etc.

   Taking the smooth transition between traditional network and the new
   SDN enabled network into account, especially from a cost impact
   assessment perspective, using the existing PCE components from the
   current network to function as the central controller of the SDN
   network is one choice, which not only achieves the goal of having a
   centralized controller to provide the functionalities needed for the
   central controller, but also leverages the existing PCE network
   components.

   The Path Computation Element communication Protocol (PCEP) provides
   mechanisms for Path Computation Elements (PCEs) to perform route
   computations in response to Path Computation Clients (PCCs) requests.
   PCEP Extensions for PCE-initiated LSP Setup in a Stateful PCE Model
   draft [I-D. draft-ietf-pce- stateful-pce] describes a set of
   extensions to PCEP to enable active control of MPLS-TE and GMPLS
   tunnels.

   [I-D.crabbe-pce-pce-initiated-lsp] describes the setup and teardown
   of PCE-initiated LSPs under the active stateful PCE model, without
   the need for local configuration on the PCC, thus allowing for a
   dynamic MPLS network that is centrally controlled and deployed.

   [I-D.ali-pce-remote-initiated-gmpls-lsp] complements [I-D. draft-
   crabbe-pce-pce-initiated-lsp] by addressing the requirements for
   remote-initiated GMPLS LSPs.

   SR technology leverages the source routing and tunneling paradigms.
   A source node can choose a path without relying on hop-by-hop
   signaling protocols such as LDP or RSVP-TE.  Each path is specified
   as a set of "segments" advertised by link-state routing protocols



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   (IS-IS or OSPF).  [I-D.filsfils-spring-segment-routing] provides an
   introduction to SR technology.  The corresponding IS-IS and OSPF
   extensions are specified in [I-D.ietf-isis-segment-routing-
   extensions] and [I-D.psenak-ospf-segment-routing-extensions],
   respectively.

   A Segment Routed path (SR path) can be derived from an IGP Shortest
   Path Tree (SPT).  Segment Routed Traffic Engineering paths (SR-TE
   paths) may not follow IGP SPT.  Such paths may be chosen by a
   suitable network planning tool and provisioned on the source node of
   the SR-TE path.

   It is possible to use a stateful PCE for computing one or more SR-TE
   paths taking into account various constraints and objective
   functions.  Once a path is chosen, the stateful PCE can instantiate
   an SR-TE path on a PCC using PCEP extensions specified in [I-
   D.crabbe-pce-pce-initiated-lsp] using the SR specific PCEP extensions
   described in [I-D.sivabalan-pce-segment-routing].

   By using the solutions provided from above drafts, LSP in both MPLS
   and GMPLS network can be setup/delete/maintained/synchronized through
   a centrally controlled dynamic MPLS network.  Since in these
   solutions, the LSP is need to be signaled through the head end LER to
   the tail end LER, there are either RSVP-TE signaling protocol need to
   be deployed in the MPLS/GMPLS network, or extend TGP protocol with
   node/adjacency segment identifiers signaling capability to be
   deployed.

   The PCECC solution proposed in this document allow for a dynamic MPLS
   network that is eventually controlled and deployed without the
   deployment of RSVP-TE protocol or extended IGP protocol with node/
   adjacency segment identifiers signaling capability while providing
   all the key MPLS functionalities needed by the service providers.
   These key MPLS features include MPLS P2P LSP, P2MP/MP2MP LSP, MPLS
   protection mechanism etc.  In the case that one LSP path consists
   legacy network nodes and the new network nodes which are centrally
   controlled, the PCECC solution provides a smooth transition step for
   users.

1.2.  Using the PCE as the Central Controller (PCECC) Approach

   With PCECC, it not only removes the existing MPLS signaling totally
   from the control plane without losing any existing MPLS
   functionalities, but also PCECC achieves this goal through utilizing
   the existing PCEP without introducing a new protocol into the
   network.

   The following diagram illustrates the PCECC architecture.



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   +----------------------------------------------------------------+
   |                         PCECC                                  |
   |    +-----------------------------------------------------+     |
   |    |  LSP-Database    RSVP-TE Signal Control Module      |     |
   |    |  TE-Database     LDP signaling Control Module       |     |
   |    |  Label-Database  LSP/label/TE MGRs                  |     |
   |    +-----------------------------------------------------+     |
   |    ^              ^           ^             ^        ^         |
   | IGP|LDP/RSVP-TE   |PCEP       |PCEP     PCEP|     IGP|LDP/     |
   |    |PCEP          |           |             |        |RSVP-TE/ |
   |    V              V           V             V        V PCE     |
   | +--------+   +--------+   +--------+   +--------+   +--------+ |
   | |NODE 1  |   | NODE 2 |   | NODE 3 |   | NODE 4 |   | NODE 5 | |
   | |        |...|        |...|        |...|        |...|        | |
   | | Legacy |IGP|        |IGP|        |IGP|  PCC4  |IGP| Legacy | |
   | |  Node  |   |        |   |        |   |        |   |  Node  | |
   | +--------+   +--------+   +--------+   +--------+   +--------+ |
   |                                                                |
   +----------------------------------------------------------------+


   Through the draft, we call the combination of the functionality for
   global label range signaling and the functionality of LSP
   setup/download/cleanup using the combination of global labels and
   local labels as PCECC functionality.

   Current MPLS label has local meaning.  That is, MPLS label allocated
   locally and signaled through the LDP/RSVP-TE/BGP etc dynamic
   signaling protocol.

   As the SDN(Service-Driven Network) technology develops, MPLS global
   label has been proposed again for new solutions.  [I-D.li-mpls-
   global-label-usecases] proposes possible usecases of MPLS global
   label.  MPLS global label can be used for identification of the
   location, the service and the network in different application
   scenarios.  From these usecases we can see that no matter SDN or
   traditional application scenarios, the new solutions based on MPLS
   global label can gain advantage over the existing solutions to
   facilitate service provisions.  The solution choices are described in
   [I-D.li-mpls-global-label-framework].

   To ease the label allocation and signaling mechanism, also with the
   new applications such as concentrated LSP controller is introduced,
   PCE can be conveniently used as a central controller and MPLS global
   label range negotiator.






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   The later section of this draft describes the user cases for PCE
   server and PCE clients to have the global label range negotiation and
   local label range negotiation functionality.

   To empower networking with centralized controllable modules, there
   are many choices for downloading the forwarding entries to the data
   plane, one way is the use of the OpenFlow protocol, which helps
   devices populate their forwarding tables according to a set of
   instructions to the data plane.  There are other candidate protocols
   to convey specific configuration information towards devices also.
   Since the PCEP protocol is already deployed in some of the service
   network, to leverage the PCEP to populated the MPLS forwarding table
   is a possible good choice.

   For the centralized network, the performance achieved through
   distributed system can not be easy matched if all of the forwarding
   path is computed, downloaded and maintained by the centralized
   controller.  The performance can be improved by supporting part of
   the forwarding path in the PCECC network through the segment routing
   mechanism except that the adjacency IDs for all the network nodes and
   links are propagated through the centralized controller instead of
   using the IGP extension.

   The node and link adjacency IDs can be negotiated through the PCECC
   with each PCECC clients and these IDs can be just taken from the
   global label range which has been negotiated already.

   With the capability of supporting SR within the PCECC architecture,
   all the p2p forwarding path protection use cases described in the
   draft [I-D.ietf-spring-resiliency-use-cases] will be supported too
   within the PCECC network.  These protection alternatives include end-
   to-end path protection, local protection without operator management
   and local protection with operator management.

   With the capability of global label and local label existing at the
   same time in the PCECC network, PCECC will use compute, setup and
   maintain the P2MP and MP2MP lsp using the local label range for each
   network nodes.

   With the capability of setting up/maintaining the P2MP/MP2MP LSP
   within the PCECC network, it is easy to provide the end-end managed
   path protection service and the local protection with the operation
   management in the PCECC network for the P2MP/MP2MP LSP, which
   includes both the RSVP-TE P2MP based LSP and also the mLDP based LSP.







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2.  Terminology

   The following terminology is used in this document.

   IGP:  Interior Gateway Protocol.  Either of the two routing
      protocols, Open Shortest Path First (OSPF) or Intermediate System
      to Intermediate System (IS-IS).

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

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

   TE:  Traffic Engineering.

3.  PCEP Requirements

   Following key requirements associated PCECC should be considered when
   designing the PCECC based solution:

   1.  Path Computation Element (PCE) clients supporting this draft MUST
       have the capability to advertise its PCECC capability to the
       PCECC.

   2.  Path Computation Element (PCE) supporting this draft MUST have
       the capability to negotiate a global label range for a group of
       clients.

   3.  Path Computation Client (PCC) MUST be able ask for global label
       range assigned in path request message .

   4.  PCE are not required to support label reserve service.
       Therefore, it MUST be possible for a PCE to reject a Path
       Computation Request message with a reason code that indicates no
       support for label reserve service.

   5.  PCEP SHOULD provide a means to return global label range and LSP
       label assignments of the computed path in the reply message.

   6.  PCEP SHOULD provide a means to download the MPLS forwarding entry
       to the PCECC's clients.







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4.  Use Cases of PCECC for Label Resource Reservations

   Example 1 to 2 are based on network configurations illustrated using
   the following figure:


   +------------------------------+    +------------------------------+
   |         PCE DOMAIN 1         |    |         PCE DOMAIN 2         |
   |          +--------+          |    |          +--------+          |
   |          |        |          |    |          |        |          |
   |          | PCECC1 |  ----------------------- | PCECC2 |          |
   |          |        |          |    |          |        |          |
   |          |        |          |    |          |        |          |
   |          +--------+          |    |          +--------+          |
   |         ^          ^         |    |         ^          ^         |
   |        /            \        |    |        /            \        |
   |       V              V       |    |       V              V       |
   | +--------+        +--------+ |    | +--------+        +--------+ |
   | |NODE 11 |        | NODE 1n| |    | |NODE 21 |        | NODE 2n| |
   | |        | ...... |        | |    | |        | ...... |        | |
   | | PCECC  |        |  PCECC | |    | | PCECC  |        |PCECC   | |
   | |Enabled |        | Enabled|      | |Enabled |        |Enabled | |
   | +--------+        +--------+ |    | +--------+        +--------+ |
   |                              |    |                              |
   +------------------------------+    +------------------------------+


   Example 1: Shared Global Label Range Reservation

   o  PCECC Clients nodes report MPLS label capability to the central
      controller PCECC.

   o  The central controller PCECC collects MPLS label capability of all
      nodes.  Then PCECC can calculate the shared MPLS global label
      range for all the PCECC client nodes.

   o  In the case that the shared global label range need to be
      negotiated across multiple domains, the central controllers of
      these domains need to be communicate to negotiate a common global
      label range.

   o  The central controller PCECC notifies the shared global label
      range to all PCECC client nodes.

   Example 2: Global Label Allocation

   o  PCECC Client node1 send global label allocation request to the
      central controller PCECC1.



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   o  The central controller PCECC1 allocates the global label for FEC1
      from the shared global label range and sends the reply to the
      client node1.

   o  The central controller PCECC1 notifies the allocated label for
      FEC1 to all PCECC client nodes within domain 1.

5.  Using PCECC for SR without the IGP Extension

   For the centralized network, the performance achieved through
   distributed system can not be easy matched if all of the forwarding
   path is computed, downloaded and maintained by the centralized
   controller.  The performance can be improved by supporting part of
   the forwarding path in the PCECC network through the segment routing
   mechanism except that node segment ids and adjacency segment IDs for
   all the network are allocated dynamically and propagated through the
   centralized controller instead of using the IGP extension.

   When the PCECC is used for the distribution of the node segment ID
   and adjacency segment ID, the node segment ID is allocated from the
   global label pool.  For the allocation of adjacency segment ID, there
   are two choices, the first choice is that it is allocated from the
   local label pool, the second choice is that it is allocated from the
   global label pool.  The advantage for the second choice is that the
   depth of the label stack for the forwarding path encoding will be
   reduced since adjacency segment ID can signal the forwarding path
   without adding the node segment ID in front of it.  In this version
   of the draft, we use the fist choice for now.  We may update the
   draft to reflect the use of the second choice.

   Same as the SR solutions, when PCECC is used as the central
   controller, the support of FRR on any topology can be pre-computated
   and setup without any additional signaling (other than the regular
   IGP/BGP protocols) including the support of shared risk constraints,
   support of node and link protection and support of microloop
   avoidance.

   The following example illustrate the use case where the node segment
   ID and adjacency segment ID are allocated from the global label
   allocated for SR path.











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                          192.0.2.1/32
                          +----------+
                          | R1(1001) |
                          +----------+
                               |
                          +----------+
                          | R2(1002) |  192.0.2.2/32
                          +----------+
                         *   |   *    *
                        *    |   *     *
                       *link1|   *      *
        192.0.2.4/32  *      |   *link2  *  192.0.2.5/32
           +-----------+ 9001|   *     +-----------+
           | R4(1004)  |     |   *     | R5(1005)  |
           +-----------+     |   *     +-----------+
                      *      |   *9003  *   +
                       *     |   *     *    +
                        *    |   *    *     +
                        +-----------+   +-----------+
           192.0.2.3/32 | R3(1003)  |   |R6(1006)   |192.0.2.6/32
                        +-----------+   +-----------+
                             |
                        +-----------+
                        | R8(1008)  |  192.0.2.8/32
                        +-----------+


5.1.  Use Cases of PCECC for SR Best Effort(BE) Path

   In this mode of the solution, the PCECC just need to allocate the
   node segment ID and adjacency ID without calculating the explicit
   path for the SR path.  The ingress of the forwarding path just need
   to encapsulate the destination node segment ID on top of the packet.
   All the intermediate nodes will forward the packet based on the final
   destination node segment id.  It is similar to the LDP LSP forwarding
   except that label swapping is using the same global label both for
   the in segment and out segment in each hop.

   The p2p SR BE path examples are explained as bellow:

   Note that the node segment id for each node from the shared global
   labels ranges negotiated already.

   Example 1:

   R1 may send a packet to R8 simply by pushing an SR header with
   segment list {1008}.  The path can be: R1-R2-R3-R8 or R1-R2-R5-R8
   depending on the route calculation on node R2.



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   Example 2: local link/node protection:

   For the packet which has destination of R3 and after that, R2 may
   preinstalled the backup forwarding entry to protect the R4 node, the
   pre-installed the backup path can go through either node5 or link1 or
   link2 between R2 and R3.  The backup path calculation is locally
   decided by R2 and any existing IP FRR algorithms can be used here.

5.2.  Use Cases of PCECC for SR Traffic Engineering (TE) Path

   In the case of traffic engineering path is needed, the PCECC need to
   allocate the node segment ID and adjacency ID, and at the same time
   PCECC calculates the explicit path for the SR path and pass this
   explicit path represented with a sequence of node segment id and
   adjacency id.  The ingress of the forwarding path need to encapsulate
   the stack of node segment id and adjacency id on top of the packet.
   For the case where strict traffic engineering path is needed, all the
   intermediate nodes and links will be specified through the stack of
   labels so that the packet is forwarded exactly as it is wanted.

   Even though it is similar to TE LSP forwarding where forwarding path
   is engineered, but the Qos is only guaranteed through the enforce of
   the bandwidth admission control.  As for the RSVP-TE LSP case, Qos is
   guaranteed through the link bandwidth reservation in each hop of the
   forwarding path.

   The p2p SR traffic engineering path examples are explained as bellow:

   Note that the node segment id for each node is allocated from the
   shared global labels ranges negotiated already and adjacency segment
   ids for each link are allocated from the local label pool for each
   node.

   Example 1:

   R1 may send a packet P1 to R8 simply by pushing an SR header with
   segment list {1008}.  The path should be: R1-R2-R3-R8.

   Example 2:

   R1 may send a packet P2 to R8 by pushing an SR header with segment
   list {1002, 9001, 1008}.  The path should be: R1-R2-(1)link-R3-R8.

   Example 3:

   R1 may send a packet P3 to R8 while avoiding the links between R2 and
   R3 by pushing an SR header with segment list {1004, 1008}.  The path
   should be : R1-R2-R4-R3-R8



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   The p2p local protection examples for SR TE path are explained as
   below:

   Example 4: local link protection:

   o  R1 may send a packet P4 to R8 by pushing an SR header with segment
      list {1002, 9001, 1008}.  The path should be: R1-R2-(1)link-R3-R8.

   o  When node R2 receives the packet from R1 which has the header of
      R2- (1)link-R3-R8, and also find out there is a link failure of
      link1, then it will send out the packet with header of R3-R8
      through link2.

   Example 5: local node protection:

   o  R1 may send a packet P5 to R8 by pushing an SR header with segment
      list {1004, 1008}.  The path should be : R1-R2-R4-R3-R8.

   o  When node R2 receives the packet from R1 which has the header of
      {1004, 1008}, and also find out there is a node failure for node4,
      then it will send out the packet with header of {1005, 1008} to
      node5 instead of node4.

6.  Use Cases of PCECC for TE LSP

   In the previous sections, we have discussed the cases where the SR
   path is setup through the PCECC.  Although those cases give the
   simplicity and scalability, but there are existing functionalities
   for the traffic engineering path such as the bandwidth guarantee
   through the full forwarding path and the multicast forwarding path
   which SR based solution cannot solve.  Also there are cases where the
   depth of the label stack may have been an issue for existing
   deployment and certain vendors.

   So to address these issues, PCECC architecture should also support
   the TE LSP and multicast LSP functionalities.  To achieve this, the
   existing PCEP can be used to communicate between the PCE server and
   PCE's client PCC for exchanging the path request and reply
   information regarding to the TE LSP info.  In this case, the TE LSP
   info is not only the path info itself, but it includes the full
   forwarding info.  Instead of letting the ingress of LSP to initiate
   the LSP setup through the RSVP-TE signaling protocol, with minor
   extensions, we can use the PCEP to download the complete TE LSP
   forwarding entries for each node in the network.







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                          192.0.2.1/32
                         +----------+
                         | R1(1001) |
                         +----------+
                           |       |
                       6001|link1  |
                           |   6002|link2
                          +----------+
                          | R2(1002) |  192.0.2.2/32
                          +----------+
                   link3 *   |   *    * link4
                   7002 *    |   *     *7001
                       *link1|   *      *
        192.0.2.4/32  *      |   *link2  *  192.0.2.5/32
           +-----------+ 5001|   *     +-----------+
           | R4(1004)  |     |   *     | R5(1005)  |
           +-----------+     |   *     +-----------+
                      *      |   *5003  *       +
                   9001*     |   *     *link1   +
                        *    |   *    *9002     +
                        +-----------+   +-----------+
           192.0.2.3/32 | R3(1003)  |   |R6(1006)   |192.0.2.6/32
                        +-----------+   +-----------+
                         |         |
                     3001|link1    |
                         |     3002|link2
                        +-----------+
                        | R8(1008)  |  192.0.2.8/32
                        +-----------+


   TE LSP Setup Example

   o  Node1 sends a path request message for the setup of TE LSP from R1
      to R8.

   o  PCECC program each node along the path from R1 to R8 with the
      primary path: {R1, link1, 6001}, {R2, link3, 7002], {R4, link0,
      9001}, {R3, link1, 3001}, {R8}.

   o  For the end to end protection, PCECC program each node along the
      path from R1 to R8 with the secondary path: {R1, link2, 6002},
      {R2, link4, 7001], {R5, link1, 9002}, {R3, link2, 3002}, {R8}.

   o  It is also possible to have a secondary backup path for the local
      node protection setup by PCECC.  For example, the primary path is
      still same as what we have setup so far, then to protect the node
      R4 locally, PCECC can program the secondary path like this: {R1,



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      link1, 6001}, {R2, link1, 5001}, {R3, link1, 3001}, {R8}. By doing
      this, the node R4 is locally protected.

7.  Use Cases of PCECC for Multicast LSPs

   The current multicast LSPs are setup either using the RSVP-TE P2MP or
   mLDP protocols.  The setup of these LSPs not only need a lot of
   manual configurations, but also it is also complex when the
   protection is considered.  By using the PCECC solution, the multicast
   LSP can be computed and setup through centralized controller which
   has the full picture of the topology and bandwidth usage for each
   link.  It not only reduces the complex configurations comparing the
   distributed RSVP-TE P2MP or mLDP signal lings, but also it can
   compute the disjoint primary path and secondary path efficiently.

7.1.  Using PCECC for P2MP/MP2MP LSPs' Setup

   With the capability of global label and local label existing at the
   same time in the PCECC network, PCECC will use compute, setup and
   maintain the P2MP and MP2MP lsp using the local label range for each
   network nodes.


                          +----------+
                          |    R1    | Root node of the multicast LSP
                          +----------+
                              |6000
                          +----------+
           Transit Node   |    R2    |
                          +----------+
                          *  |   *  *
                     9001*   |   *   *9002
                        *    |   *    *
           +-----------+     |   *     +-----------+
           |    R4     |     |   *     |    R5     | Transit Nodes
           +-----------+     |   *     +-----------+
                      *      |   *      *     +
                   9003*     |   *     *      +9004
                        *    |   *    *       +
                        +-----------+  +-----------+
                        |    R3     |  |    R5     | Leaf Node
                        +-----------+  +-----------+
                         9005|
                        +-----------+
                        |    R8     | Leaf Node
                        +-----------+





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   The P2MP examples are explained here:

   Step1: R1 may send a packet P1 to R2 simply by pushing an label of
   6000 to the packet.

   Step2: After R2 receives the packet with label 6000, it will
   forwarding to R4 by pushing header of 9001 and R5 by pusing header of
   9002.

   Step3: After R4 receives the packet with label 9001, it will
   forwarding to R3 by pushing header of 9003.  After R5 receives the
   packet with label 9002, it will forwarding to R5 by pushing header of
   9004.

   Step3: After R3 receives the packet with label 9003, it will
   forwarding to R8 by pushing header of 9005

7.2.  Use Cases of PCECC for the Resiliency of P2MP/MP2MP LSPs

7.2.1.  PCECC for the End-to-End Protection of the P2MP/MP2MP LSPs

   In this section we describe the end-end managed path protection
   service and the local protection with the operation management in the
   PCECC network for the P2MP/MP2MP LSP, which includes both the RSVP-TE
   P2MP based LSP and also the mLDP based LSP.

   An end-to-end protection (for nodes and links) principle can be
   applied for computing backup P2MP or MP2MP LSPs.  During computation
   of the primarily multicast trees, PCECC server may also be taken into
   consideration to compute a secondary tree.  A PCE may compute the
   primary and backup P2MP or MP2Mp LSP together or sequentially.




















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                               +----+  +----+
              Root node of LSP | R1 |--| R11|
                               +----+  +----+
                                 /         +
                              10/           +20
                               /             +
                       +----------+        +-----------+
        Transit Node   |    R2    |        |     R3    |
                       +----------+        +-----------+
                         |        \       +         +
                         |         \     +          +
                       10|        10\   +20       20+
                         |           \ +            +
                         |            \             +
                         |           + \            +
                       +-----------+      +-----------+ Leaf Nodes
                       |    R4     |      |    R5     | (Downstream LSR)
                       +-----------+      +-----------+


   In the example above, when the PCECC setup the primary multicast tree
   from the root node R1 to the leafs, which is R1->R2->{R4, R5}, at
   same time, it can setup the backup tree, which is R11->R3->{R4, R5}.
   Both the these two primary forwarding tree and secondary forwarding
   tree will be downloaded to each routers along the primary path and
   the secondary path.  The traffic will be forwarded through the
   R1->R2->{R4, R5} path normally, and when there is a node in the
   primary tree, then the root node R1 will switch the flow to the
   backup tree, which is R11->R3->{R4, R5}.  By using the PCECC, the
   path computation and forwarding path downloading can all be done
   without the complex signaling used in the P2MP RSVP-TE or mLDP.

7.2.2.  PCECC for the Local Protection of the P2MP/MP2MP LSPs

   In this section we describe the local protection service in the PCECC
   network for the P2MP/MP2MP LSP.

   While the PCECC sets up the primary multicast tree, it can also build
   the back LSP among PLR, the protected node, and MPs (the downstream
   nodes of the protected node).  In the cases where the amount of
   downstream nodes are huge, this mechanism can avoid unnecessary
   packet duplication on PLR, so that protect the network from traffic
   congestion risk.








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                               +------------+
                               |     R1     | Root Node
                               +------------+
                                      .
                                      .
                                      .
                               +------------+ Point of Local Repair/
                               |     R10     | Switchover Point
                               +------------+ (Upstream LSR)
                                 /         +
                              10/           +20
                               /             +
                       +----------+        +-----------+
        Protected Node |    R20   |        |     R30   |
                       +----------+        +-----------+
                         |        \       +         +
                         |         \     +          +
                       10|        10\   +20       20+
                         |           \ +            +
                         |            \             +
                         |           + \            +
                       +-----------+      +-----------+ Merge Point
                       |    R40    |      |    R50    | (Downstream LSR)
                       +-----------+      +-----------+
                             .                  .
                             .                  .


   In the example above, when the PCECC setup the primary multicast path
   around the PLR node R10 to protect node R20, which is R10->R20->{R40,
   R50}, at same time, it can setup the backup path R10->R30->{R40,
   R50}.  Both the these two primary forwarding path and secondary
   forwarding path will be downloaded to each routers along the primary
   path and the secondary path.  The traffic will be forwarded through
   the R10->R20->{R40, R50} path normally, and when there is a node
   failure for node R20, then the PLR node R10 will switch the flow to
   the backup path, which is R10->R30->{R40, R50}.  By using the PCECC,
   the path computation and forwarding path downloading can all be done
   without the complex signaling used in the P2MP RSVP-TE or mLDP.

8.  Use Cases of PCECC for LSP in the Network Migration

   One of the main advantages for PCECC solution is that it has backward
   compatibility naturally since the PCE server itself can function as a
   proxy node of MPLS network for all the new nodes which don't support
   the existing MPLS signaling protocol anymore.





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   As it is illustrated in the following example, the current network
   will migrate to a total PCECC controlled network gradually by
   replacing the legacy nodes.  During the migration, the legacy nodes
   still need to signal using the existing MPLS protocol such as LDP and
   RSVP-TE, and the new nodes setup their portion of the forwarding path
   through PCECC directly.  With the PCECC function as the proxy of
   these new nodes, MPLS signaling can populate through network as
   normal.

   Example described in this section is based on network configurations
   illustrated using the following figure:


   +------------------------------------------------------------------+
   |                         PCE DOMAIN                               |
   |    +-----------------------------------------------------+       |
   |    |                       PCECC                         |       |
   |    +-----------------------------------------------------+       |
   |     ^              ^                      ^            ^         |
   |     |      RSVP-TE | if22             if44|RSVP-TE     |         |
   |     V              V                      V            V         |
   | +--------+   +--------+   +--------+   +--------+   +--------+   |
   | | NODE 1 |   | NODE 2 |   | NODE x |   | NODE 4 |   | NODE 5 |   |
   | |        |...|        |...|        |...|        |...|        |   |
   | | Legacy |if1| Legacy |if2|PCCEC   |if3| PCECC  |if4| Legacy |   |
   | |  Node  |   |  Node  |   |Enabled |   |Enabled |   |  Node  |   |
   | +--------+   +--------+   +--------+   +--------+   +--------+   |
   |                                                                  |
   +------------------------------------------------------------------+


   Example: PCECC Initiated LSP Setup In the Network Migration

   In this example, there are five nodes for the TE LSP from head end
   (node1) to the tail end (node5).  Where the NodeX is central
   controlled and other nodes are legacy nodes.

   o  Node1 sends a path request message for the setup of LSP
      destinating to Node5.

   o  PCECC sends a reply message for LSP setup with path (node1, if1),
      (node2, if22), (node-PCECC, if44), (node4, if4), Nnode5.

   o  Node1, Node2, Node-PCECC, Node 5 will setup the LSP to Node5
      normally using the local label as normal.

   o  Then the PCECC will program the outsegment of Node2, the insegment
      of Node4, and the insegment/outsegment for NodeX.



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9.  Use Cases of PCECC for L3VPN and PWE3

   The existing services using MPLS LSP tunnels based on MPLS signalling
   mechanism such L3VPN, PWE3 and IPv6 can be simplified by using the
   PCECC to negoitate the label assignments for the L3VPN, PWE3 and
   Ipv6.

   In the case of L3VPN, VPN labels can be negotiated and distributed
   through the PCECC PCEP among the PE router instead of using the BGP
   protocols.

   Example described in this section is based on network configurations
   illustrated using the following figure:



               +-------------------------------------------+
               |                   PCE DOMAIN              |
               |    +-----------------------------------+  |
               |    |                PCECC              |  |
               |    +-----------------------------------+  |
               |           ^          ^              ^     |
               |PWE3/L3VPN | PCEP PCEP|LSP PWE3/L3VPN|PCEP |
               |           V          V              V     |
    +--------+ |     +--------+   +--------+   +--------+  |  +--------+
    |  CE    | |     | PE1    |   | NODE x |   | PE2    |  |  |   CE   |
    |        |...... |        |...|        |...|        |.....|        |
    | Legacy | |if1  | PCECC  |if2|PCCEC   |if3| PCECC  |if4  | Legacy |
    |  Node  | |     | Enabled|   |Enabled |   |Enabled |  |  |  Node  |
    +--------+ |     +--------+   +--------+   +--------+  |  +--------+
               |                                           |
               +-------------------------------------------+


   Example: Using PCECC for L3VPN and PWE3

   In the cast PWE3, instead of using the LDP signalling protocols, the
   lable and port pairs assigned to each pseudowire can be negotiated
   through PCECC among the PE rotuers and the corresponding forwarding
   entries will be distributed into each PE routers through the extended
   PCEP protocols.

10.  Using PCECC for Traffic Classification Informations

   When a TE-LSP is set up, the head end needs to know:

   o  how to use it




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   o  What traffic to send on the LSp

   o  Whether it is a virtual link

   o  Whether to advertise it in the IGP

   o  What bits of this information to signal to the tail end

   PCEP allows an Active PCE to set up or modify LSPs.  But we have no
   way to tell the head end how to use the LSP This is because of
   history.  It used to be the LER that made the request of the PCE, so
   it knew why it wanted the LSP.

   With the PCECC architecture by extending the PCEP protocols, is is
   easy to carry these informations such as how to use the LSP, how to
   advertise the LSP and other extra signaling information.

11.  The Considerations for PCECC Procedure and PCEP extensions

   The PCECC's procedures and PCEP extensions is defined in [I-D.zhao-
   pce-pcep-extension-for-pce-controller].

12.  IANA Considerations

   This document does not require any action from IANA.

13.  Security Considerations

   TBD.

14.  Acknowledgments

   We would like to thank Robert Tao, Changjiang Yan, Tieying Huang and
   Adrian Farrel for their useful comments and suggestions.

15.  References

15.1.  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,
              <http://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,
              <http://www.rfc-editor.org/info/rfc5440>.



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15.2.  Informative References

   [RFC5441]  Vasseur, JP., Ed., Zhang, R., Bitar, N., and JL. Le Roux,
              "A Backward-Recursive PCE-Based Computation (BRPC)
              Procedure to Compute Shortest Constrained Inter-Domain
              Traffic Engineering Label Switched Paths", RFC 5441,
              DOI 10.17487/RFC5441, April 2009,
              <http://www.rfc-editor.org/info/rfc5441>.

   [RFC5541]  Le Roux, JL., Vasseur, JP., and Y. Lee, "Encoding of
              Objective Functions in the Path Computation Element
              Communication Protocol (PCEP)", RFC 5541,
              DOI 10.17487/RFC5541, June 2009,
              <http://www.rfc-editor.org/info/rfc5541>.

   [I-D.filsfils-spring-segment-routing]
              Filsfils, C., Previdi, S., Bashandy, A., Decraene, B.,
              Litkowski, S., Horneffer, M., Milojevic, I., Shakir, R.,
              Ytti, S., Henderickx, W., Tantsura, J., and E. Crabbe,
              "Segment Routing Architecture", draft-filsfils-spring-
              segment-routing-04 (work in progress), July 2014.

   [I-D.ietf-pce-stateful-pce]
              Crabbe, E., Minei, I., Medved, J., and R. Varga, "PCEP
              Extensions for Stateful PCE", draft-ietf-pce-stateful-
              pce-13 (work in progress), December 2015.

   [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.

   [I-D.ali-pce-remote-initiated-gmpls-lsp]
              Ali, Z., Sivabalan, S., Filsfils, C., Varga, R., Lopez,
              V., Dios, O., and X. Zhang, "Path Computation Element
              Communication Protocol (PCEP) Extensions for remote-
              initiated GMPLS LSP Setup", draft-ali-pce-remote-
              initiated-gmpls-lsp-03 (work in progress), February 2014.

   [I-D.ietf-isis-segment-routing-extensions]
              Previdi, S., Filsfils, C., Bashandy, A., Gredler, H.,
              Litkowski, S., Decraene, B., and J. Tantsura, "IS-IS
              Extensions for Segment Routing", draft-ietf-isis-segment-
              routing-extensions-06 (work in progress), December 2015.






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   [I-D.psenak-ospf-segment-routing-extensions]
              Psenak, P., Previdi, S., Filsfils, C., Gredler, H.,
              Shakir, R., Henderickx, W., and J. Tantsura, "OSPF
              Extensions for Segment Routing", draft-psenak-ospf-
              segment-routing-extensions-05 (work in progress), June
              2014.

   [I-D.sivabalan-pce-segment-routing]
              Sivabalan, S., Medved, J., Filsfils, C., Crabbe, E.,
              Raszuk, R., Lopez, V., and J. Tantsura, "PCEP Extensions
              for Segment Routing", draft-sivabalan-pce-segment-
              routing-03 (work in progress), July 2014.

   [I-D.li-mpls-global-label-usecases]
              Li, Z., Zhao, Q., Yang, T., Raszuk, R., and L. Fang,
              "Usecases of MPLS Global Label", draft-li-mpls-global-
              label-usecases-03 (work in progress), October 2015.

   [I-D.li-mpls-global-label-framework]
              Li, Z., Zhao, Q., Chen, X., Yang, T., and R. Raszuk, "A
              Framework of MPLS Global Label", draft-li-mpls-global-
              label-framework-02 (work in progress), July 2014.

   [I-D.zhao-pce-pcep-extension-for-pce-controller]
              Zhao, Q., Li, Z., Dhody, D., and C. Zhou, "PCEP Procedures
              and Protocol Extensions for Using PCE as a Central
              Controller (PCECC) of LSPs", draft-zhao-pce-pcep-
              extension-for-pce-controller-03 (work in progress), March
              2016.

   [I-D.ietf-spring-resiliency-use-cases]
              Francois, P., Filsfils, C., Decraene, B., and R. Shakir,
              "Use-cases for Resiliency in SPRING", draft-ietf-spring-
              resiliency-use-cases-02 (work in progress), December 2015.

Authors' Addresses

   Quintin Zhao
   Huawei Technologies
   125 Nagog Technology Park
   Acton, MA  01719
   US

   EMail: quintin.zhao@huawei.com







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

   EMail: lizhenbin@huawei.com


   King Ke
   Tencent Holdings Ltd.
   Shenzhen
   China

   EMail: kinghe@tencent.com


   Luyuan Fang
   Microsoft

   EMail: lufang@microsoft.com


   Chao Zhou
   Cisco Systems

   EMail: chao.zhou@cisco.com


   Boris Zhang
   Telus Communications

   EMail: Boris.zhang@telus.com


















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