wdiff rfc7334.original rfc7334.txt

PCE Working Group
Internet Engineering Task Force (IETF) Q. Zhao
Internet-Draft
Request for Comments: 7334 D. Dhody
Intended status:
Category: Experimental Huawei Technology
Expires: December 17, 2014
ISSN: 2070-1721 D. King
Old Dog Consulting
Z. Ali
Cisco Systems
R. Casellas
CTTC
June 17,
August 2014

PCE-based

PCE-Based Computation Procedure To to Compute Shortest Constrained P2MP
Inter-domain
Point-to-Multipoint (P2MP) Inter-Domain Traffic Engineering
Label Switched Paths
draft-ietf-pce-pcep-inter-domain-p2mp-procedures-08

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 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 Communication Protocol (PCEP) for the
computation of inter-domain paths for P2MP TE LSPs.

Status of This Memo

This Internet-Draft document is submitted in full conformance with the
provisions of BCP 78 not an Internet Standards Track specification; it is
published for examination, experimental implementation, and BCP 79.

Internet-Drafts are working documents
evaluation.

This document defines an Experimental Protocol for the Internet
community. This document is a product of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list It represents the consensus of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid the IETF
community. It has received public review and has been approved for
publication by the Internet Engineering Steering Group (IESG). Not
all documents approved by the IESG are a maximum candidate for any level of
Internet Standard; see Section 2 of six months RFC 5741.

Information about the current status of this document, any errata,
and how to provide feedback on it may be updated, replaced, or obsoleted by other documents obtained 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 December 17, 2014.
http://www.rfc-editor.org/info/rfc7334.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . .2 ....................................................4
1.1. Scope . . . . . . . . . . . . . . . . . . . . . . . . . .2 ......................................................4
1.2. Requirements Language . . . . . . . . . . . . . . . . . .2 ......................................4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . .2 .....................................................5
3. Examination of Existing Mechanisms . . . . . . . . . . . . .3 ..............................6
4. Assumptions . . . . . . . . . . . . . . . . . . . . . . . . .5 .....................................................7
5. Requirements . . . . . . . . . . . . . . . . . . . . . . . . .5 ....................................................8
6. Objective Functions and Constraints. . . . . . . . . . . . . .7 Constraints .............................9
7. P2MP Path Computation Procedures . . . . . . . . . . . . . . .8 ...............................10
7.1. General . . . . . . . . . . . . . . . . . . . . . . . . .8 ...................................................10
7.2. Core-Trees . . . . . . . . . . . . . . . . . . . . . . . .9 ................................................10
7.3. Optimal Core-Tree Computation Procedure. . . . . . . . . .12 Procedure ...................13
7.4. Sub-tree Computation Procedures . . . . . . . . . . . . .13 ...........................15
7.5. PCEP Protocol Extensions . . . . . . . . . . . . . . . . .13 ..................................15
7.5.1. The Extension of RP Object . . . . . . . . . . . . . .13 .............................15
7.5.2. Domain and PCE Sequence . . . . . . . . . . . . . . .14 ............................16
7.6. Relationship with Hierarchical PCE . . . . . . . . . . . .14 Using H-PCE for Scalability ...............................16
7.7. Parallelism . . . . . . . . . . . . . . . . . . . . . . .15 ...............................................17
8. Protection . . . . . . . . . . . . . . . . . . . . . . . . . .15 .....................................................17
8.1. End-to-end End-to-End Protection . . . . . . . . . . . . . . . . . .15 .....................................17
8.2. Domain Protection . . . . . . . . . . . . . . . . . . . .15 .........................................18
9. Manageability Considerations . . . . . . . . . . . . . . . . .16 ...................................18
9.1. Control of Function and Policy . . . . . . . . . . . . . .16 ............................18
9.2. Information and Data Models . . . . . . . . . . . . . . .16 ...............................18
9.3. Liveness Detection and Monitoring . . . . . . . . . . . .16 .........................19
9.4. Verifying Correct Operation . . . . . . . . . . . . . . .16 ...............................19
9.5. Requirements on Other Protocols and Functional Components.17
Components ................................................19
9.6. Impact on Network Operation . . . . . . . . . . . . . . .17 ...............................19
9.7. Policy Control . . . . . . . . . . . . . . . . . . . . . .17 ............................................20
10. Security Considerations . . . . . . . . . . . . . . . . . . .17 .......................................20
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .18 ...........................................21
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .19 ..............................................21
13. References . . . . . . . . . . . . . . . . . . . . . . . . . .19 ....................................................21
13.1. Normative References . . . . . . . . . . . . . . . . . . .19 .....................................21
13.2. Informative References . . . . . . . . . . . . . . . . . .19 ...................................22
14. Contributors' Addresses . . . . . . . . . . . . . . . . . . .21
15. Authors' Addresses . . . . . . . . . . . . . . . . . . . . .21 .......................................24

1. Introduction

Multicast services are increasingly in demand for high-capacity
applications such as multicast Virtual Private Networks (VPNs), IP-
television (IPTV) which VPNs, IPTV (which may be on-demand or streamed,
streamed), and content-
rich content-rich media distribution (for example, software
distribution, financial streaming, or database-replication). 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) and GMPLS
networks across multiple domains are is therefore required.

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

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

When results of implementation and deployment are available, this
document will be updated and refined, and it will then be moved from
Experimental status to Standards Track.

1.2.

1.1. Scope

The inter-domain P2MP path computation procedures described in this
document is are experimental. The experiment is intended to enable
research for the usage of the PCE to support inter-domain P2MP path
computation.

This document is not intended to replace the intra-domain P2MP path
computation approach defined by [RFC6006], [RFC6006] and will not impact
existing PCE procedures and operations.

1.3.

1.2. 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

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

The additional

Additional terms Core-Tree, Leaf Domain, Path Tree, Path Domain
Sequence, Path Domain Tree, Root Domain, Sub-Tree and Transit/branch
Domain are further defined below. below:

Core-Tree: a P2MP tree where the root is the ingress Label Switching
Router (LSR), (LSR) and the leaf nodes are the entry BNs boundary nodes of the
leaf domains.

Entry BN of domain(n): a Boundary Node boundary node (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.

H-PCE: Hierarchical PCE (as per [RFC6805]).

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

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 (the leaf nodes).

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

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.

Sub-tree: a P2MP tree where the root is the selected entry BN of the
leaf domain and the leaf nodes are the destinations (leaves) in that
domain. The sub-trees are grafted to the core-tree.

Transit/branch

Transit/Branch Domain: a domain that has an upstream and one or more
downstream neighbor domain. domains.

3. Examination of Existing Mechanisms

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, 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 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 Point to Point point-to-point (P2P) inter-domain constrained shortest
paths across a predetermined sequence of domains, using a Backward Backward-
Recursive Path PCE-Based 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.

PCEP [RFC5440] was extended for point-to-multipoint (P2MP) path
computation requests in [RFC6006].

As discussed in [RFC4461], a P2MP tree is the ordered set of LSRs and
TE links that comprise the path of a P2MP TE LSP from its ingress LSR
to all of its egress LSRs. A P2MP LSP is set up with TE constraints
and allows efficient packet or data replication at various branching
points in the network. As per [RFC5671] branch point [RFC5671], selection of branch points
is fundamental to the determination of the paths for a P2MP TE LSP.
Not only is this selection constrained by the network topology and
available network resources, but it is also determined by the
objective functions (OF) (OFs) that may be applied to path computation.

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
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
(i.e., when the domains are multiply inter-connected), existing
techniques may be convoluted when used to determine which boundary
node of another domain will be utilized for the inter-domain P2MP
tree, and there is no way to limit the computation of the P2MP tree
to those utilized boundary nodes.

A trivial solution to the computation of the 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)) (MCT)), and in the
context of inter-domain P2MP TE LSPs LSPs, it cannot be used to reduce the
number of domain boundary nodes that are transited. Computing P2P TE
LSPs individually does not guarantee the generation of an optimal
P2MP tree for every definition of "optimal" in every topology.

Per Domain

Per-domain path computation [RFC5152] may be used to compute P2MP
multi-domain paths, paths but may encounter the issues previously described.
Furthermore, 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 needs to perform path computation
for each leaf.

P2MP Minimum Cost Tree (MCT), i.e. MCT, i.e., a computation which that guarantees the least cost
resulting tree, typically 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 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
(Autonomous System) peering agreements), the path domain tree will
be known in advance; advance.

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

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

5. Requirements

This section summarizes the requirements specific to computing inter-
domain
inter-domain P2MP paths. In these requirements 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 can preserve confidentiality
across domains, which is required when domains are managed by
different Service Providers via the Path-Key mechanism [RFC5520].

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

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

2. The number of PCReq (Path Computation Request) and PCRep (Path
Computation Reply) messages SHOULD be independent of the number
of multicast destinations in each domain.

3. It SHOULD be possible to specify the domain entry and exit nodes
in the PCReq.

4. Specifying which nodes are to be used as branch nodes SHOULD be
supported in the PCReq.

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

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

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 objective function
(OF), MAY be used. Using an SPT (Shortest Path Tree) and MCT (Minimum Cost
Tree), defined in [RFC6006], are two such OF to select optimization criteria
for 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]. used to select the "best"
candidate path.

In addition to the OFs, the following constraints MAY also be
beneficial for inter-domain P2MP path computation:

1. The computed P2MP "core-tree" SHOULD be optimal when only
considering the paths to the leaf domain entry BNs.

2. Grafting and pruning of multicast destinations (sub-tree) (sub-trees) within
a leaf domain SHOULD ensure minimal impact on other domains and
on the core-tree.

3. It SHOULD be possible to choose to optimize the core-tree.

4. It SHOULD be possible to choose to optimize the entire tree (P2MP
LSP).

5. It SHOULD be possible to combine the aforementioned OFs and
constraints for P2MP path computation.

When implementing and operating P2MP LSPs, the following needs need to be
taken into consideration:

o The complexity of computation.

o The optimality of the tree (core-tree as well as full P2MP LSP
tree).

o The stability of the core-tree.

The solution SHOULD allow these trade-offs to be made at computation
time.

The algorithms used to compute optimal paths using a combination of
OFs and multiple constraints is are out of the scope of this document.

7. P2MP Path Computation Procedures

7.1. General

A P2MP path computation can be broken down into two steps of steps: core-tree
computation and grafting of sub-trees. Breaking the procedure into
these specific steps has the following impact: impact, allowing the core-
tree-based solution to provide an optimal inter-domain P2MP TE LSP:

o The core-tree and sub-tree are smaller in comparison to the full
P2MP Tree tree and are thus easier to compute.

o An implementation MAY choose to keep the core-tree fairly static
or computed offline (trade-off with optimality).

o Adding/Pruning Adding/pruning of leaves which require requires changes to the sub-tree in leaf-
domain the
leaf-domain only.

o The PCEP message size is smaller in comparison.

Allowing the core-tree based solution to provide an optimal
inter-domain P2MP TE LSP.

The following sub-sections describe the core-tree based core-tree-based mechanism,
including procedures and PCEP extensions, extensions that satisfy the
requirements and objectives specified in Section Sections 5 and Section 6 of this
document.

7.2. 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; domain.

o The leaves of the core-tree are the entry boundary nodes in the
leaf domains.

To support confidentiality confidentiality, these nodes and links MAY be hidden using
the path-key Path-Key mechanism [RFC5520], but they MUST be computed and be a
part of the core-tree.

For example, consider the Domain Tree domain tree in Figure 1 below, 1, representing a
domain tree of 6 domains, domains and part of the resulting
core-tree core-tree, which
satisfies the aforementioned conditions.

+----------------+
| |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-------------W+ +------P---------+ +-----------T----+
| | | | | |
| | | Q | | U |
| M O | | S | | |
| | | | | V |
| N | | R | | |
+----------------+ +----------------+ +----------------+

Figure 1: Domain Tree Example
(R)
|
(A)
/ \
/ \
(B) (C)
/ \
/ \
(D) (E)
/ |
/ |
(G) (H)
/ / \
/ / \
(I) (J) (K)
/ \ / \
/ \ / \
(L) (W) (P) (T)

Figure 2: Core-Tree

A core-tree is computed such that the root of the tree is R and the
leaf
node nodes are the entry nodes of the destination domains (L, W, P P,
and T).
Path-key The Path-Key mechanism can be used to hide the internal
nodes and links
(node G and H are hidden via Path-Key PK1 and PK2 respectively) in the final core-tree as shown below for domain domains D2
and D3. D3 (nodes G and H are hidden via Path-Keys PK1 and PK2,
respectively).

(R)
|
(A)
/ \
/ \
(B) (C)
/ \
/ \
(D) (E)
/ |
/ |
|PK1| |PK2|
/ / \
/ / \
(I) (J) (K)
/ \ / \
/ \ / \
(L) (W) (P) (T)

Figure 3: Core-Tree with Path-Key

7.3. Optimal Core-Tree Computation Procedure

Applying the core-tree procedure to large groups of domains, such as
the Internet, is not considered feasible or desirable, desirable and is out of
the scope for of this document.

The following extended BRPC-based procedure can be used to compute
the core-tree. Note that a root PCE MAY further use its own enhanced
optimization techniques in the future to compute the core-tree.

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

1. Using Use the BRPC procedures to compute the VSPT(i) (Virtual Shortest
Path Tree) 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 paths. We
call them PathSet(j), j=1 to M, where M=P(1)xP(2)...xP(n); M=P(1)xP(2)...xP(n).

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

4. There will be M number core-trees computed from step 3. An
optimal core-tree is selected based on the OF and constraints.

Note that, that since point to point the point-to-point BRPC procedure is used to compute
VSPT, the path request and response message format formats defined in
[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
(extended VSPT) 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, the combination of these
single optimal sub-paths into a core-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 the entry boundary nodes of the leaf domain. The
PCE will then, by By
looking through all the combinations and taking one sub-path from
each set to build one tree, can the PCE will then select the optimal
core-tree.

A PCE MAY add equal cost equal-cost paths within the domain while constructing
an extended VSPT. This will provide the ingress PCE more candidate
paths for an optimal core-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 especially with dense/meshed
domains. Considering a domain sequence D1, D2, D3, D4, where the
Leaf Boundary Node
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 speaking, the number
of potential paths at the ingress PCE Q = prod E(k) x X(k).

Consequently, it is expected that the core-tree will be typically be
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).

7.4. 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 the C-bit set (as defined in section 7.4.1 Section 7.5.1
of this document) for the path computation to the destination(s)
directly to the PCE where the destination(s) belong(s) along with the
core-tree computed from section per Section 7.2.

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.

An 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. vice versa.

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

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

7.5. PCEP Protocol Extensions

7.5.1. The Extension of RP Object

This experiment will be carried out by extending the RP (Request
Parameters) object (defined in [RFC5440]) used in PCEP requests and
responses.

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

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

The following flag is added in this draft: document:

Bit Number Name Flag
TBA
17 Core-tree computation (C-bit)

C bit

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

0: This indicates Indicates that this is not for an inter-domain P2MP core-tree.

1: This indicates Indicates that this is a PCEP request or a response for the
computation of a an inter-domain core-tree or for the grafting
of a sub-tree to a an inter-domain core-tree.

7.5.2. Domain and PCE Sequence

The procedure described in this document requires the domain-tree domain tree to
be known in advance. This information MAY be either administratively
predetermined or dynamically discovered by some
means means, such as the
Hierarchical PCE (H-PCE) [RFC6805] framework, framework [RFC6805], or derived through the
IGP/BGP routing information.

Examples of ways to encode the domain path tree include [RFC5886]
using are found in
[RFC5886], which uses PCE-ID Object Objects, and [DOMAIN-SEQ].

7.6. Using H-PCE for Scalability

The ingress/root PCE is responsible for the core-tree computation as
well as grafting of sub-trees into the multi-domain tree. Therefore,
the ingress/root PCE will receive all computed path segments from all
the involved domains. When the ingress/root PCE chooses to have a
PCEP session with all involved PCEs, 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 for
sending an intra-domain path computation request to the PCEs, combine them
combining them, and return returning the overall P2MP tree.

7.7. Parallelism

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

1. The BRPC procedure for each leaf boundary node can be launched in
parallel by the ingress/root PCE for dynamic computation of the
core-tree.

2. The grafting of sub-trees can be triggered in parallel once the
core-tree is computed.

One of the potential issues of parallelism is that the ingress PCE
would require a potentially high number of PCEP adjacencies to
"remote" PCEs at the same time and that time; this situation may not be desirable.

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
proposed types of protection, including: including end-to-end protection
[RFC4875]
[RFC4872] and domain protection schemes.

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

8.1. End-to-end 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, protection 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, a backup P2MP Tree tree can be computed which that
excludes the transit/branch domain completely. A backup domain path
tree is needed with the same source domain and destinations destination 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
the 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 the manageability requirements mentioned in
[RFC6006] are already covered there.

9.1. Control of Function and Policy

In addition to the PCE configuration parameters listed in [RFC5440]
and [RFC6006], the following additional parameters might be required:

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

o The Configuration of the PCE may be configured to enable or disable the advertisement of
its 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, communication between PCEs, 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, computations and single domain 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 the scope for of 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, domains and the discovery of
corresponding PCEs and their capabilities SHOULD be provided 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].

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.

It should be noted that pre-computing and maintaining domain-trees domain trees
might be a introduce considerable administration effort on for the operator.

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 inter-domain P2MP Path 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, request or a denial of service 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 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,
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 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 denial-of-service attacks against PCEs and PCCs.

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

Finally, any multi-domain operation necessarily involves the exchange
of information across domain boundaries. This may represent a
significant security and confidentiality risk risk, especially when the
domains are controlled by different commercial entities. PCEP allows
individual PCEs to maintain confidentiality of their domain path
information by using path-keys Path-Keys [RFC5520] and would allow for securing
of domain path information when performing core-tree
based core-tree-based path
computations.

11. IANA Considerations

IANA maintains the "Path Computation Element Protocol (PCEP) Numbers"
registry with and the "RP Object Flag Field" sub-registry.

IANA is requested to allocate has allocated a new bit from this registry as follows:

Bit Description Reference
TBA
17 Core-tree computation (C-bit) [This.I-D] [RFC7334]

12. Acknowledgements

The authors would like to thank Adrian Farrel, Dan Tappan, Olufemi
Komolafe, Oscar Gonzalez de Dios 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. JP., Ed., and JL. Le Roux, Ed., "Path
Computation Element (PCE) Communication Protocol
(PCEP)", RFC 5440, March 2009.

[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
Inter-Domain Traffic Engineering Label Switched
Paths", RFC 5441, April 2009.

[RFC6006] Zhao, Q., Ed., King, D., Ed., 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., Ed., "Signaling Requirements for Point-to-
Multipoint
Point-to-Multipoint Traffic-Engineered MPLS Label
Switched Paths (LSPs)", RFC 4461, April 2006.

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

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

[RFC4872] Lang, J., Ed., Rekhter, Y., Ed., and D.
Papadimitriou, Ed., "RSVP-TE Extensions in Support
of End-to-End Generalized Multi-Protocol Label
Switching (GMPLS) Recovery", RFC 4872, May 2007.

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

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

[RFC5152] Vasseur, JP., Ed., Ayyangar, A., Ed., and R. Zhang,
"A Per-
Domain 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., Ed., 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, Ed., "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.

[RFC5886] Vasseur, JP., Ed., Le Roux, JL., and Y. Ikejiri, "A
Set of Monitoring Tools for Path Computation Element
(PCE)-Based Architecture", RFC 5886, June 2010.

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

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

[PCEP-MIB] Koushik, K., A., Stephan, E., Zhao, Q., King, D., and J.
Hardwick, "PCE communication protocol "Path Computation Element Protocol (PCEP)
Management Information Base (Work Base", Work in Progress)",
April Progress,
July 2014.

[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 Services", Work in Progress)",
Aug Progress,
August 2012.

[DOMAIN-SEQ] Dhody, D., Palle, U., and R. Casellas, "Standard
Representation Of Domain Sequence (Work Domain-Sequence", Work in
Progress)",
Progress, July 2014.

14. Contributor Contributors' Addresses

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

EMail: msiva@cisco.com

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

Dhruv Dhody
Huawei Technology
Leela Palace
Bangalore, Karnataka 560008
INDIA
India

EMail: dhruv.dhody@huawei.com

Daniel King
Old Dog Consulting
UK

EMail: daniel@olddog.co.uk

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

EMail: zali@cisco.com

Daniel King
Old Dog Consulting
UK

EMail: daniel@olddog.co.uk

Ramon Casellas
CTTC
Av. Carl Friedrich Gauss n7
Castelldefels, Barcelona 08860
SPAIN
Spain

EMail: ramon.casellas@cttc.es