Skip to main content

PCE-based Computation Procedure To Compute Shortest Constrained P2MP Inter-domain Traffic Engineering Label Switched Paths
draft-ietf-pce-pcep-inter-domain-p2mp-procedures-05

The information below is for an old version of the document.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 7334.
Authors Quintin Zhao , Dhruv Dhody , Zafar Ali , Daniel King , Ramon Casellas
Last updated 2013-10-10 (Latest revision 2013-07-14)
Replaces draft-zhao-pce-pcep-inter-domain-p2mp-procedures, draft-zhao-pce-pcep-inter-domain-p2mp-procedures
RFC stream Internet Engineering Task Force (IETF)
Formats
Reviews
Additional resources Mailing list discussion
Stream WG state Submitted to IESG for Publication
Document shepherd Julien Meuric
Shepherd write-up Show Last changed 2013-09-24
IESG IESG state Became RFC 7334 (Experimental)
Consensus boilerplate Unknown
Telechat date (None)
Responsible AD Adrian Farrel
Send notices to pce-chairs@tools.ietf.org, draft-ietf-pce-pcep-inter-domain-p2mp-procedures@tools.ietf.org
draft-ietf-pce-pcep-inter-domain-p2mp-procedures-05
PCE Working Group                                                Q. Zhao
Internet-Draft                                                  D. Dhody
Intended status: Experimental                          Huawei Technology
Expires: January 14, 2014                                         Z. Ali
                                                           Cisco Systems
                                                                 D. King
                                                      Old Dog Consulting
                                                             R. Casellas
                                             CTTC - Centre Tecnologic de
                                          Telecomunicacions de Catalunya
                                                           July 14, 2013

  PCE-based Computation Procedure To Compute Shortest Constrained P2MP
         Inter-domain Traffic Engineering Label Switched Paths
          draft-ietf-pce-pcep-inter-domain-p2mp-procedures-05

Abstract

   The ability to compute paths for constrained point-to-multipoint 
   (P2MP) Traffic Engineering Label Switched Paths (TE LSPs) across
   multiple domains has been identified as a key requirement for the
   deployment of P2MP services in MPLS and GMPLS-controlled networks. 
   The Path Computation Element (PCE) has been recognized as an 
   appropriate technology for the determination of inter-domain paths of
   P2MP TE LSPs.

   This document describes an experiment to provide procedures and 
   extensions to the PCE communication Protocol (PCEP) for the 
   computation of inter-domain paths for P2MP TE LSPs.

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."

   This Internet-Draft will expire on July 14, 2013.

Zhao, et al.                 July 14, 2013                    [Page 1]

Internet-Draft     PCEP P2MP Inter-Domain Procedures      January 2014

Copyright Notice

   Copyright (c) 2013 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.  Scope  . . . . . . . . . . . . . . . . . . . . . . . . . . 3 
     1.2.  Requirements Language  . . . . . . . . . . . . . . . . . . 3 
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . . 3 
   3.  Problem Statement  . . . . . . . . . . . . . . . . . . . . . . 5 
   4.  Assumptions  . . . . . . . . . . . . . . . . . . . . . . . . . 6 
   5.  Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 7 
   6.  Objective Functions and Constraints. . . . . . . . . . . . . . 8 
   7.  P2MP Path Computation Procedures . . . . . . . . . . . . . . . 8 
     7.1.  Core Trees . . . . . . . . . . . . . . . . . . . . . . . . 8 
     7.2.  Core Tree Computation Procedures . . . . . . . . . . . . .10 
     7.3.  Sub-tree Computation Procedures  . . . . . . . . . . . . .11 
     7.4.  PCEP Protocol Extensions . . . . . . . . . . . . . . . . .12 
       7.4.1.  The Extension of RP Object . . . . . . . . . . . . . .12 
       7.4.2.  Domain and PCE Sequence  . . . . . . . . . . . . . . .12 
     7.5.  Relationship with Hierarchical PCE . . . . . . . . . . . .13 
     7.6.  Parallelism  . . . . . . . . . . . . . . . . . . . . . . .13 
   8.  Protection . . . . . . . . . . . . . . . . . . . . . . . . . .13 
     8.1.  End-to-end Protection  . . . . . . . . . . . . . . . . . .14 
     8.2.  Domain Protection  . . . . . . . . . . . . . . . . . . . .14 
   9.  Manageability Considerations . . . . . . . . . . . . . . . . .14 
     9.1.  Control of Function and Policy . . . . . . . . . . . . . .14 
     9.2.  Information and Data Models  . . . . . . . . . . . . . . .15 
     9.3.  Liveness Detection and Monitoring  . . . . . . . . . . . .15 
     9.4.  Verifying Correct Operation  . . . . . . . . . . . . . . .15 
     9.5.  Requirements on Other Protocols and Functional
           Components . . . . . . . . . . . . . . . . . . . . . . . .15 
     9.6.  Impact on Network Operation  . . . . . . . . . . . . . . .16 
     9.7.  Policy Control . . . . . . . . . . . . . . . . . . . . . .16 
   10. Security Considerations  . . . . . . . . . . . . . . . . . . .16 
   11. IANA Considerations  . . . . . . . . . . . . . . . . . . . . .17 

Zhao, et al.                 July 14, 2013                    [Page 2]

Internet-Draft     PCEP P2MP Inter-Domain Procedures      January 2014

   12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .17 
   13. References . . . . . . . . . . . . . . . . . . . . . . . . . .17 
     13.1. Normative References . . . . . . . . . . . . . . . . . . .17 
     13.2. Informative References . . . . . . . . . . . . . . . . . .17 
   14. Contributors' Addresses  . . . . . . . . . . . . . . . . . . .19
   15. Authors'  Addresses  . . . . . . . . . . . . . . . . . . . . .19

1.  Introduction

   Multicast services are increasingly in demand for high-capacity
   applications such as multicast Virtual Private Networks (VPNs), IP-
   television (IPTV) which may be on-demand or streamed, and content-
   rich media distribution (for example, software distribution,
   financial streaming, or database-replication).  The ability to
   compute constrained Traffic Engineering Label Switched Paths (TE
   LSPs) for point-to-multipoint (P2MP) LSPs in Multiprotocol Label
   Switching (MPLS) and Generalized MPLS (GMPLS) networks across
   multiple domains are therefore required.

   The applicability of the Path Computation Element (PCE) [RFC4655] for
   the computation of such paths is discussed in [RFC5671], and the
   requirements placed on the PCE communications Protocol (PCEP) for
   this are given in [RFC5862].

   This document details the requirements for inter-domain P2MP path
   computation, it then describes the experimental procedure 
   "core-tree" path computation, developed to address the requirements
   and objectives for inter-domain P2MP path computation.
   
1.2.  Scope

   The inter-domain P2MP path computation procedures described in this
   document is experimental. The experiment is intended to enable
   research for the Path Computation Element (PCE) to support 
   inter-domain P2MP path computation.  
   
   This document is not intended to replace the intra-domain P2MP path
   computation approach supported by [RFC6006], and will not impact 
   existing PCE procedures and operations.

1.3.  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 RFC 2119 [RFC2119].

2.  Terminology

Zhao, et al.                 July 14, 2013                    [Page 3]

Internet-Draft     PCEP P2MP Inter-Domain Procedures      January 2014

   Terminology used in this document is consistent with the related
   MPLS/GMPLS and PCE documents [RFC4461], [RFC4655], [RFC4875],
   [RFC5376], [RFC5440], [RFC5441], [RFC5671] and [RFC5862].

   ABR: Area Border Router.  Router used to connect two IGP domains
   (areas in OSPF or levels in IS-IS).

   ASBR: Autonomous System Border Router.  Router used to connect
   together ASes of the same or different Service Providers via one or
   more Inter-AS links.

   Boundary Node (BN): is either an ABR in the context
   of inter-area Traffic Engineering or an ASBR in the context of
   inter-AS Traffic Engineering.

   Core Tree: a P2MP tree where the root is the ingress
   LSR, and the leaf nodes are the entry BNs of the leaf domains.

   Domain: a collection of network elements within a common sphere of
   address management or path computational responsibility such as an
   IGP area or an Autonomous System (AS).

   Entry BN of domain(n): a BN connecting domain(n-1) to domain(n) along
   a determined sequence of domains.

   Exit BN of domain(n): a BN connecting domain(n) to domain(n+1) along
   a determined sequence of domains.

   Leaf Domain: a domain with one or more leaf nodes.

   OF: Objective Function.  a set of one or more optimization criterion
   (criteria) used for the computation of paths either for single or for
   synchronized requests (e.g. path cost minimization), or the
   synchronized computation of a set of paths (e.g. aggregate bandwidth
   consumption minimization, etc.).  See [RFC4655] and [RFC5541].

   Path Tree: a set of LSRs and TE links that comprise the path
   of a P2MP TE LSP from the ingress LSR to all egress LSRs.

   Path Domain Sequence: the known sequence of domains for a path
   between root and leaf.

   Path Domain Tree: the tree formed by the domains that the P2MP path
   crosses, where the source (ingress) domain is the root domain.

   PCE(i): a PCE that performs path computations for domain(i).

   Root Domain: the domain that includes the ingress (root) LSR.
   

Zhao, et al.                 July 14, 2013                    [Page 4]

Internet-Draft     PCEP P2MP Inter-Domain Procedures      January 2014

   Sub-tree: used to minimize packet duplication when P2P
   TE sub-LSPs traverse common links.
   
   Transit/branch Domain: a domain that has an upstream and one or more
   downstream neighbor domain.
   
   VSPT: Virtual Shortest Path Tree [RFC5441].

3.  Problem Statement

   The Path Computation Element (PCE) defined in [RFC4655] is an entity
   that is capable of computing a network path or route based on a
   network graph, and applying computational constraints.  A Path
   Computation Client (PCC) may make requests to a PCE for paths to be
   computed.

   [RFC4875] describes how to set up P2MP TE LSPs for use in MPLS and
   GMPLS-controlled networks.  The PCE is identified as a suitable 
   application for the computation of paths for P2MP TE LSPs [RFC5671].

   [RFC5441] specifies a procedure relying on the use of multiple PCEs
   to compute (P2P) inter-domain constrained shortest paths across a
   predetermined sequence of domains, using a Backward Recursive Path
   Computation (BRPC) technique.  The technique can be combined with the
   use of path keys [RFC5520] to preserve confidentiality across
   domains, which is sometimes required when domains are managed by
   different Service Providers.

   The PCE communication Protocol (PCEP) [RFC5440] is extended for
   point-to-multipoint(P2MP) path computation requests in [RFC6006].
   However, [RFC6006] does not provide the necessary mechanisms and
   procedures to request the computation of inter-domain P2MP TE LSPs.

   As discussed in [RFC4461], a P2MP tree is a graphical representation
   of all TE links that are committed for a particular P2MP TE LSP.  In
   other words, a P2MP tree is a representation of the corresponding
   P2MP tunnel on the TE network topology.  A sub-tree is used to 
   minimize packet duplication when P2P TE sub-LSPs traverse common
   links.  As described in [RFC5671] the computation of a P2MP tree
   requires three major pieces of information.  The first is the path
   from the ingress LSR of a P2MP TE LSP to each of the egress LSRs, the
   second is the traffic engineering related parameters, and the third
   is the branch capability information.

   Generally, an inter-domain P2MP tree (i.e., a P2MP tree with source
   and at least one destination residing in different domains) is
   particularly difficult to compute even for a distributed PCE
   architecture.  For instance, while the BRPC may be well-suited for 

Zhao, et al.                 July 14, 2013                    [Page 5]

Internet-Draft     PCEP P2MP Inter-Domain Procedures      January 2014

   P2P paths, P2MP path computation involves multiple branching path 
   segments from the source to the multiple destinations.  As such, 
   inter-domain P2MP path computation may result in a plurality of 
   per-domain path options that may be difficult to coordinate 
   efficiently and effectively between domains.

   That is, when one or more domains have multiple ingress and/or egress
   boundary nodes, there is currently no known technique for one domain 
   to determine which boundary node of another domain will be utilized 
   for the inter-domain P2MP tree, and no way to limit the computation 
   of the P2MP tree to those utilized boundary nodes.

   A trivial solution to the computation of inter-domain P2MP tree would
   be to compute shortest inter-domain P2P paths from source to each
   destination and then combine them to generate an inter-domain,
   shortest-path-to-destination P2MP tree.  This solution, however,
   cannot be used to trade cost to destination for overall tree cost
   (i.e., it cannot produce a Minimum Cost Tree (MCT)) and in the
   context of inter- domain P2MP TE LSPs it cannot be used to reduce the
   number of domain boundary nodes that are transited. Computing P2P TE 
   LSPs individually is not an acceptable solution for computing a P2MP 
   tree.

   Even per domain path computation [RFC5152]
   can be used to compute P2P multi-domain paths, but it does not
   guarantee to find the optimal path which crosses multiple domains.
   Furthermore, constructing a P2MP tree from individual source to leaf
   P2P TE LSPs does not guarantee to produce a Minimum Cost Tree (MCT).
   This approach may also be considered to have scaling issues during 
   LSP setup.  That is, the LSP to each leaf is signaled separately, and
   each boundary node must perform path computation for each leaf.

   P2MP Minimum Cost Tree (MCT), i.e. a computation which guarantees the
   least cost resulting tree, is an NP-complete problem.  Moreover,
   adding and/or removing a single destination to/from the tree may
   result in an entirely different tree.  In this case, frequent MCT
   path computation requests may prove computationally intensive, and
   the resulting frequent tunnel reconfiguration may even cause network
   destabilization.

   This document presents a solution, procedures and extensions to
   PCEP to support P2MP inter-domain path computation.

4.  Assumptions

   Within this document we make the following assumptions:
    
   o Due to deployment and commercial limitations (e.g., inter-AS
     peering agreements), the path domain tree will be known in 
     advance;

Zhao, et al.                 July 14, 2013                    [Page 6]

Internet-Draft     PCEP P2MP Inter-Domain Procedures      January 2014

   o  Each PCE knows about any leaf LSRs in the domain it serves;

   Additional assumptions are documented in [RFC5441] and will 
   not be repeated here.

5.  Requirements

   This section summarizes the requirements specific to computing inter-
   domain P2MP paths.  In these requirements we note that the actual
   computation time taken by any PCE implementation is outside the scope 
   of this document, but we observe that reducing the complexity of the
   required computations has a beneficial effect on the computation time
   regardless of implementation.  Additionally, reducing the number of
   message exchanges and the amount of information exchanged will reduce
   the overall computation time for the entire P2MP tree.  We refer to
   the "complexity of the computation" as the impact on these aspects of
   path computation time as various parameters of the topology and the
   P2MP TE LSP are changed.

   It is also important that the solution preserves confidentiality 
   across domains, which is required when domains are managed by 
   different Service Providers.

   Other than the requirements specified in [RFC5862], a number of
   requirements specific to P2MP are detailed below:

   1.  The computed P2MP TE LSP SHOULD be optimal when only considering 
       the paths among the BNs.

   2.  Grafting and pruning of multicast destinations in a domain SHOULD
       have no impact on other domains and on the paths among BNs.

   3.  The complexity of the computation for each sub-tree within each
       domain SHOULD be dependent only on the topology of the domain and
       it SHOULD be independent of the domain sequence.

   4.  The number of PCReq and PCReq messages SHOULD be independent of
       the number of multicast destinations in each domain.

   5.  It SHOULD be possible to specify the domain entry and exit nodes.

   6.  Specifying which nodes are be used as branch nodes SHOULD be 
       supported.

   7.  Reoptimization of existing sub-trees SHOULD be supported.

   8.  It SHOULD be possible to compute diverse P2MP paths from existing
       P2MP paths.

Zhao, et al.                 July 14, 2013                    [Page 7]

Internet-Draft     PCEP P2MP Inter-Domain Procedures      January 2014

6.  Objective Functions and Constraints

   For the computation of a single or a set of P2MP TE LSPs, a request
   to meet specific optimization criteria, called an Objective Function
   (OF), may be used. Using an OF to select the "best" candidate path, 
   include:

   o  The sub-tree within each domain SHOULD be optimized using minimum
      cost tree [RFC5862], or shortest path tree [RFC5862].
   
   In addition to the OFs, the following constraint optimization may 
   also be beneficial for P2MP path computation:  

   1.  The core tree SHOULD be optimal.

   2.  It SHOULD be possible to limit the number of entry points to a
       domain.

   3.  It SHOULD be possible to force the branches for all leaves within
       a domain to be in that domain.
       
   4.  It SHOULD be possible to combine the aforementioned OFs and 
       constraints for P2MP path computation.
   
   The algorithms used to compute optimal paths using a combination of
   OFs and multiple constraints is out of scope of this document.  

7.  P2MP Path Computation Procedures

   The following sections describe the core tree-based procedures to
   satisfy the requirements specified in the previous section.  A core
   tree-based solution provides an optimal inter-domain P2MP TE LSP.

7.1.  Core Trees

   A core tree is defined as a tree that satisfies the following
   conditions:

   o  The root of the core tree is the ingress LSR in the root domain;

   o  The leaves of the core tree are the entry nodes in the leaf
      domains.
 
   To support confidentiality these nodes and links may be hidden using
   the path-key mechanism [RFC5520], but they MUST be computed and be a
   part of core-tree.
      
   For example, consider the Domain Tree in Figure 1 below,
   representing a domain tree of 6 domains, and part of the resulting
   core tree which satisfies the aforementioned conditions.

Zhao, et al.                 July 14, 2013                    [Page 8]

Internet-Draft     PCEP P2MP Inter-Domain Procedures      January 2014

                             +----------------+
                             |                |Domain D1
                             |        R       |
                             |                |
                             |        A       |
                             |                |
                             +-B------------C-+
                              /              \
                             /                \
                            /                  \
            Domain D2      /                    \ Domain D3
            +-------------D--+             +-----E----------+
            |                |             |                |
            |  F             |             |                |
            |          G     |             |       H        |
            |                |             |                |
            |                |             |                |
            +-I--------------+             +-J------------K-+
             /                              /              \
            /                              /                \
           /                              /                  \
          /                              /                    \
         /                              /                      \
        /                              /                        \
       / Domain D4         Domain D5  /              Domain D6   \
     +-L--------------+       +------P---------+      +-----------T----+
     |                |       |                |      |                |
     |                |       |  Q             |      |   U            |
     |  M        O    |       |         S      |      |                |
     |                |       |                |      |          V     |
     |          N     |       |   R            |      |                |
     +----------------+       +----------------+      +----------------+

                          Figure 1: Domain Tree Example

Zhao, et al.                 July 14, 2013                    [Page 9]

Internet-Draft     PCEP P2MP Inter-Domain Procedures      January 2014

                                    (R)
                                     |
                                    (A)
                                    / \
                                   /   \
                                 (B)   (C)
                                 /       \
                                /         \
                              (D)         (E)
                              /            |
                             /             |
                           (G)            (H)
                           /              / \
                          /              /   \
                        (I)            (J)   (K)
                        /              /       \
                       /              /         \
                     (L)            (P)         (T)

                           Figure 2: Core Tree

   A core tree is computed such that root of the tree is R and the leaf
   node are the entry nodes of the destination domains (L, P and T).
   Path-key mechanism can be used to hide the internal nodes and links
   in the final core tree.
   
7.2.  Core Tree Computation Procedures

   The algorithms to compute the optimal large core tree are outside
   scope of this document.  The following extended BRPC-based procedure
   can be used to compute the core tree.

   A BRPC-based core tree path computation procedure is described below:

   1.  Using the BRPC procedures to compute the VSPT(i) for each leaf
       BN(i), i=1 to n, where n is the total number of entry nodes for
       all the leaf domains.  In each VSPT(i), there are a number of
       P(i) paths.

   2.  When the root PCE has computed all the VSPT(i), i=1 to n, take
       one path from each VSPT and form all possible sets of paths, we
       call them PathSet(j), j=1 to M, where M=P(1)xP(2)...xP(n);

   3.  For each PathSet(j), there are n S2L (Source to Leaf BN) paths
       and form these n paths into a core tree(j);

   4.  There will be M number of core trees computed from step3.  Apply
       the OF to each of these M core trees and find the optimal Core
       Tree.

Zhao, et al.                 July 14, 2013                   [Page 10]

Internet-Draft     PCEP P2MP Inter-Domain Procedures      January 2014

   Note that, since point to point BRPC procedure is used to compute
   VSPT, the path request and response messages as per [RFC5440] are
   used.

   Also note that the application of BRPC in the aforementioned
   procedure differs from the typical one since paths returned from a
   downstream PCE are not necessarily pruned from the solution set by
   intermediate PCEs. The reason for this is that if the PCE in a 
   downstream domain does the pruning and returns the single optimal 
   sub-path to the upstream PCE, BRPC insures that the ingress PCE will 
   get all the best optimal sub-paths for each LN (Leaf Boundary 
   Nodes), but the combination of these single optimal sub-paths into
   a P2MP tree is not necessarily optimal even if each S2L 
   (Source-to-Leaf) sub-path is optimal.

   Without trimming, the ingress PCE will obtain all the possible S2L 
   sub-paths set for LN. The PCE will then, by looking through all
   the combinations and taking one sub-path from each set to build one 
   P2MP tree, can find the optimal tree.

   A PCE MAY add equal cost paths within the domain while constructing 
   an extended VSPT.  This will provide the ingress PCE more candidate
   paths for an optimal P2MP tree.

   The proposed method may present a scalability problem for the dynamic
   computation of the core tree (by iterative checking of all
   combinations of the solution space), specially with dense/meshed
   domains.  Considering a domain sequence D1, D2, D3, D4, where the
   Leaf Boundary Node is at domain D4, PCE(4) will return 1 path.  
   PCE(3) will return N paths, where N is E(3) x X(3), where E(k) x X(k)
   denotes the number of entry nodes times the number of exit nodes for
   that domain.  PCE(2) will return M paths, where M = E(2) x X(2) x N =
   E(2) x X(2) x E(3) x X(3) x 1, etc.  Generally speaking the number of
   potential paths at the ingress PCE Q = \prod E(k) x X(k).

   Consequently, it is expected that the Core Path will be typically
   computed offline, without precluding the use of dynamic, online
   mechanisms such as the one presented here, in which case it SHOULD be
   possible to configure transit PCEs to control the number of paths
   sent upstream during BRPC (trading trimming for optimality at the
   point of trimming and downwards).

7.3.  Sub-tree Computation Procedures

   Once the core tree is built, the grafting of all the leaf nodes from
   each domain to the core tree can be achieved by a number of
   algorithms.  One algorithm for doing this phase is that the root PCE
   will send the request with C bit set (as defined in section 7.4.1 of
   this document) for the path computation to the destination(s) 

Zhao, et al.                 July 14, 2013                   [Page 11]

Internet-Draft     PCEP P2MP Inter-Domain Procedures      January 2014

   directly to the PCE where the destination(s) belong(s) along with the
   core tree computed from section 7.1.

   This approach requires that the root PCE manage a potentially large
   number of adjacencies (either in persistent or non-persistent mode),
   including PCEP adjacencies to PCEs that are not within neighbor
   domains.

   A first alternative would involve establishing PCEP adjacencies that
   correspond to the PCE domain tree.  This would require that branch
   PCEs forward requests and responses from the root PCE towards the
   leaf PCEs and vice-versa.

   Note that the P2MP path request and response format is as per
   [RFC6006], where Record Route Object (RRO) are used to carry the
   core-tree paths in the P2MP grafting request.

   The algorithms to compute the optimal large sub-tree are outside
   scope of this document.  
   
7.4.  PCEP Protocol Extensions

7.4.1.  The Extension of RP Object
 
   This experiment will be carried out by extending the RP (Request 
   Parameters) object (defined in [RFC5440]) used in Path Request and 
   Reply messages.

   The extended format of the RP object body to include the C bit is as
   follows:

   The C bit is added in the flag bits field of the RP object to signal
   the receiver of the message that the request/reply is for inter-
   domain P2MP core tree or not.

     The following flag is added in this draft:

     C bit ( P2MP Core Tree bit - 1 bit):

        0: This indicates that this is normal PCReq/PCRrep for P2MP.

        1: This indicates that this is PCReq or PCRep message for inter-
        domain core tree P2MP.  When the C bit is set, then the request
        message MUST contain the core tree passed along with the
        destinations to be grafted to the tree.

7.4.2.  Domain and PCE Sequence

   The procedure as described in this document requires the domain-tree 
 
 
Zhao, et al.                 July 14, 2013                   [Page 12]

Internet-Draft     PCEP P2MP Inter-Domain Procedures      January 2014

   to be known in advance.  This information may be provided dynamically 
   as documented in the Hierarchical PCE (H-PCE) [RFC6805] framework, or
   obtained through the IGP/BGP routing information. 
   
   [DOMAIN-SEQ] describes the representation of domain in P2MP 
   scenarios.  The use of PCE sequence rather than domain-sequence, is
   based on deployment and implementation preference. 
   
7.5.  Using H-PCE for Scalability 

   The ingress/root PCE is responsible for the grafting of sub-trees 
   into the multi-domain tree. Therefore, the ingress/root PCE will 
   receive all computed sub-trees from all the involved domains. This 
   will require the ingress/root PCE to have a PCEP session with all 
   PCEs providing sub-trees. This may cause an excessive number of 
   sessions or added complexity in implementations.

   The use of the H-PCE framework [RFC6805] may be used to establish a
   dedicated PCE with the capability (memory and CPU) and knowledge to
   maintain the necessary PCEP sessions. The parent PCE would be 
   responsible to request intra-domain sub-trees to the PCEs, combine 
   them and return the overall P2MP tree.

7.6.  Parallelism

   In order to minimize latency in path computation in multi-domain
   networks, intra-domain path segments and intra-domain sub-trees
   SHOULD be computed in parallel when possible.  The proposed
   procedures in this draft present opportunities for parallelism:

   1.  The BRPC procedure for each leaf node can be launched in parallel
       by the ingress/root PCE if the dynamic computation of the Core
       Tree is enabled.

   2.  Intra-domain P2MP paths can also be computed in parallel by the
       PCEs once the entry and exit nodes within a domain are known

   One of the potential issues of parallelism is that the ingress PCE
   would require a potentially high number of PCEP adjacencies to
   "remote" PCEs and that may not be desirable, but a given PCE would
   only receive requests for the destinations that are in its domain (+
   the core nodes), without PCEs forwarding requests.

8.  Protection

   It is envisaged that protection may be required when deploying and
   using inter-domain P2MP TE LSPs.  The procedures and mechanisms 
   defined in this document do not prohibit the use of existing and 

Zhao, et al.                 July 14, 2013                   [Page 13]

Internet-Draft     PCEP P2MP Inter-Domain Procedures      January 2014

   proposed types of protection, including: end-to-end protection 
   [RFC4875] and domain protection schemes.

   Segment or facility (link and node) protection is problematic in
   inter-domain environment due to the limit of Fast-reroute (FRR)
   [RFC4875] requiring knowledge of its next-hop across domain
   boundaries whilst maintaining domain confidentiality.  Although the
   FRR protection might be implemented if next-hop information was known
   in advance.

8.1.  End-to-end Protection

   An end-to-end protection (for nodes and links) principle can be 
   applied for computing backup P2MP TE LSPs.  During computation of the 
   core-tree and sub-trees, may also be taken into consideration. A
   PCE may compute the primary and backup P2MP TE LSP together or 
   sequentially.

8.2.  Domain Protection

   In this protection scheme, backup P2MP Tree can be computed which
   excludes the transit/branch domain completely.  A backup domain path
   tree is needed with the same source domain and destinations domains
   and a new set of transit domains.  The backup path tree can be
   applied to the above procedure to obtain the backup P2MP TE LSP with
   disjoint transit domains.

9.  Manageability Considerations

   [RFC5862] describes various manageability requirements in support of
   P2MP path computation when applying PCEP.  This section describes how
   manageability requirements mentioned in [RFC5862] are supported in
   the context of PCEP extensions specified in this document.

   Note that [RFC5440] describes various manageability considerations in
   PCEP, and most of manageability requirements mentioned in [RFC6006]
   are already covered there.

9.1.  Control of Function and Policy

   In addition to PCE configuration parameters listed in [RFC5440], the
   following additional parameters might be required:

   o  The ability to enable or disable single domain P2MP path
      computations on the PCE.

   o  The ability to enable or disable multi-domain P2MP path
      computations on the PCE.

Zhao, et al.                 July 14, 2013                   [Page 14]

Internet-Draft     PCEP P2MP Inter-Domain Procedures      January 2014

   o  The PCE may be configured to enable or disable the advertisement
      of its single domain and multi-domain P2MP path computation
      capability.

9.2.  Information and Data Models

   A number of MIB objects have been defined for general PCEP control
   and monitoring of P2P computations in [PCEP-MIB].  [RFC5862]
   specifies that MIB objects will be required to support the control
   and monitoring of the protocol extensions defined in this document.
   [PCEP-P2MP-MIB] describes managed objects for modeling of PCEP
   communications between a PCC and PCE, and PCE to PCE, P2MP path
   computation requests and responses.

9.3.  Liveness Detection and Monitoring

   No changes are necessary to the liveness detection and monitoring
   requirements as already embodied in [RFC4657].

   It should be noted that multi-domain P2MP computations are likely to
   take longer than P2P computations, and single domain P2MP
   computations.  The liveness detection and monitoring features of the
   PCEP SHOULD take this into account.

9.4.  Verifying Correct Operation

   There are no additional requirements beyond those expressed in
   [RFC4657] for verifying the correct operation of the PCEP.  Note that
   verification of the correct operation of the PCE and its algorithms
   is out of scope for the protocol requirements, but a PCC MAY send the
   same request to more than one PCE and compare the results.

9.5.  Requirements on Other Protocols and Functional Components

   A PCE operates on a topology graph that may be built using
   information distributed by TE extensions to the routing protocol
   operating within the network.  In order that the PCE can select a
   suitable path for the signaling protocol to use to install the P2MP
   TE LSP, the topology graph MUST include information about the P2MP
   signaling and branching capabilities of each LSR in the network.

   Mechanisms for the knowledge of other domains, the discovery of
   corresponding PCEs and their capabilities should be provided and that
   this information MAY be collected by other mechanisms.

   Whatever means is used to collect the information to build the
   topology graph, the graph MUST include the requisite information.  If
   the TE extensions to the routing protocol are used, these SHOULD be
   as described in [RFC5073].

Zhao, et al.                 July 14, 2013                   [Page 15]

Internet-Draft     PCEP P2MP Inter-Domain Procedures      January 2014

9.6.  Impact on Network Operation

   The use of a PCE to compute P2MP paths is not expected to have
   significant impact on network operations.  However, it should be
   noted that the introduction of P2MP support to a PCE that already
   provides P2P path computation might change the loading of the PCE
   significantly, and that might have an impact on the network behavior,
   especially during recovery periods immediately after a network
   failure.

   The dynamic computation of core trees might also have an impact on
   the load of the involved PCEs as well as path computation times.

9.7.  Policy Control

   [RFC5394] provides additional details on policy within the PCE
   architecture and also provides context for the support of PCE Policy.
   They are also applicable to Inter-domain P2MP Path computation via 
   the core tree mechanism.

10.  Security Considerations

   As described in [RFC5862], P2MP path computation requests are more
   CPU-intensive and also utilize more link bandwidth.  In the event of
   an unauthorized P2MP path computation request, or a denial of service
   attack, the subsequent PCEP requests and processing may be disruptive
   to the network.  Consequently, it is important that implementations
   conform to the relevant security requirements of [RFC5440] that
   specifically help to minimize or negate unauthorized P2MP path
   computation requests and denial of service attacks.  These mechanisms
   include:

   o  Securing the PCEP session requests and responses using TCP
      security techniques (Section 10.2 of [RFC5440]).

   o  Authenticating the PCEP requests and responses to ensure the
      message is intact and sent from an authorized node (Section 10.3
      of [RFC5440]).

   o  Providing policy control by explicitly defining which PCCs, via IP
      access-lists, are allowed to send P2MP path requests to the PCE
      (Section 10.6 of [RFC5440]).

   PCEP operates over TCP, so it is also important to secure the PCE and
   PCC against TCP denial of service attacks.  Section 10.7.1 of
   [RFC5440] outlines a number of mechanisms for minimizing the risk of
   TCP-based denial of service attacks against PCEs and PCCs.

Zhao, et al.                 July 14, 2013                   [Page 16]

Internet-Draft     PCEP P2MP Inter-Domain Procedures      January 2014

   PCEP implementations SHOULD also consider the additional security
   provided by the TCP Authentication Option (TCP-AO) [RFC5925].

11.  IANA Considerations

   Due to the experimental status of this document. No IANA 
   considerations have been requested. 

12.  Acknowledgements

   The authors would like to thank Adrian Farrel, Dan Tappan, Olufemi
   Komolafe, Oscar Gonzalez de Dios and Julien Meuric for their 
   valuable comments on this document.

13.  References

13.1.  Normative References

   [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.
                    
   [RFC5441]        Vasseur, JP., 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, April 2009.
                    
   [RFC5541]        Le Roux, JL., Vasseur, JP., and Y. Lee, "Encoding of
                    Objective Functions in the Path Computation Element
                    Communication Protocol (PCEP)", RFC 5541, June 2009.

   [RFC6006]        Zhao, Q., King, D., Verhaeghe, F., Takeda, T., Ali,
                    Z., and J. Meuric, "Extensions to the Path
                    Computation Element Communication Protocol (PCEP)
                    for Point-to-Multipoint Traffic Engineering Label
                    Switched Paths", RFC 6006, September 2010.

13.2.  Informative References

   [RFC4461]        Yasukawa, S., "Signaling Requirements for Point-to-
                    Multipoint Traffic-Engineered MPLS Label Switched
                    Paths (LSPs)", RFC 4461, April 2006.

Zhao, et al.                 July 14, 2013                   [Page 17]

Internet-Draft     PCEP P2MP Inter-Domain Procedures      January 2014

   [RFC4655]        Farrel, A., Vasseur, J., and J. Ash, "A Path
                    Computation Element (PCE)-Based Architecture",
                    RFC 4655, August 2006.

   [RFC4657]        Ash, J. and J. Le Roux, "Path Computation Element
                    (PCE) Communication Protocol Generic Requirements",
                    RFC 4657, September 2006.

   [RFC4875]        Aggarwal, R., Papadimitriou, D., and S. Yasukawa,
                    "Extensions to Resource Reservation Protocol -
                    Traffic Engineering (RSVP-TE) for Point-to-
                    Multipoint TE Label Switched Paths (LSPs)",
                    RFC 4875, May 2007.

   [RFC5073]        Vasseur, J. and J. Le Roux, "IGP Routing Protocol
                    Extensions for Discovery of Traffic Engineering Node
                    Capabilities", RFC 5073, December 2007.

   [RFC5152]        Vasseur, JP., Ayyangar, A., and R. Zhang, "A Per-
                    Domain Path Computation Method for Establishing
                    Inter-Domain Traffic Engineering (TE) Label Switched
                    Paths (LSPs)", RFC 5152, February 2008.

   [RFC5376]        Bitar, N., Zhang, R., and K. Kumaki, "Inter-AS
                    Requirements for the Path Computation Element
                    Communication Protocol (PCECP)", RFC 5376,
                    November 2008.

   [RFC5394]        Bryskin, I., Papadimitriou, D., Berger, L., and J.
                    Ash, "Policy-Enabled Path Computation Framework",
                    RFC 5394, December 2008.

   [RFC5520]        Bradford, R., Vasseur, JP., and A. Farrel,
                    "Preserving Topology Confidentiality in Inter-Domain
                    Path Computation Using a Path-Key-Based Mechanism",
                    RFC 5520, April 2009.

   [RFC5671]        Yasukawa, S. and A. Farrel, "Applicability of the
                    Path Computation Element (PCE) to Point-to-
                    Multipoint (P2MP) MPLS and GMPLS Traffic Engineering
                    (TE)", RFC 5671, October 2009.

   [RFC5862]        Yasukawa, S. and A. Farrel, "Path Computation
                    Clients (PCC) - Path Computation Element (PCE)
                    Requirements for Point-to-Multipoint MPLS-TE",
                    RFC 5862, June 2010.

   [RFC5925]        Touch, J., Mankin, A., and R. Bonica, "The TCP
                    Authentication Option", RFC 5925, June 2010.

Zhao, et al.                 July 14, 2013                   [Page 18]

Internet-Draft     PCEP P2MP Inter-Domain Procedures      January 2014

   [RFC6805]        King, D. and A. Farrel, "The Application of the Path
                    Computation Element Architecture to the
                    Determination of a Sequence of Domains in MPLS and
                    GMPLS", RFC 6805, November 2012.

   [PCEP-MIB]       Koushik, K., Stephan, E., Zhao, Q., King, D., and J.
                    Hardwick, "PCE communication protocol (PCEP)
                    Management Information Base (Work in Progress)",
                    July 2012.

   [PCEP-P2MP-MIB]  Zhao, Q., Dhody, D., Palle, U., and D. King,
                    "Management Information Base for the PCE
                    Communications Protocol (PCEP) When Requesting
                    Point-to-Multipoint Services (Work in Progress)",
                    Aug 2012.

   [DOMAIN-SEQ]     Dhody, D., Palle, U., and R. Casellas, "Standard
                    Representation Of Domain Sequence (Work in
                    Progress)", Feb 2013.

14. Contributor Addresses

   Siva Sivabalan
   Cisco Systems
   2000 Innovation Drive
   Kanata, Ontario  K2K 3E8
   CANADA

   EMail: msiva@cisco.com

   Tarek Saad
   Cisco Systems, Inc.
   2000 Innovation Drive
   Kanata, Ontario  K2K 3E8
   CANADA

   EMail: tsaad@cisco.com

15. Authors' Addresses

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

   EMail: quintin.zhao@huawei.com

Zhao, et al.                 July 14, 2013                   [Page 19]

Internet-Draft     PCEP P2MP Inter-Domain Procedures      January 2014

   Dhruv Dhody
   Huawei Technology
   Leela Palace
   Bangalore, Karnataka  560008
   INDIA

   EMail: dhruv.dhody@huawei.com

   Zafar Ali
   Cisco Systems
   2000 Innovation Drive
   Kanata, Ontario  K2K 3E8
   CANADA

   EMail: zali@cisco.com

   Daniel King
   Old Dog Consulting
   UK

   EMail: daniel@olddog.co.uk

   Ramon Casellas
   CTTC - Centre Tecnologic de Telecomunicacions de Catalunya
   Av. Carl Friedrich Gauss n7
   Castelldefels, Barcelona  08860
   SPAIN

   EMail: ramon.casellas@cttc.es
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   

Zhao, et al.                 July 14, 2013                   [Page 20]

Internet-Draft     PCEP P2MP Inter-Domain Procedures      January 2014