Network Working Group Seisho Yasukawa
Internet Draft NTT
Category: Informational Adrian Farrel
Expires: August 2006 Old Dog Consulting
February 2006
PCC-PCE Communication Requirements for Point to Multipoint
Traffic Engineering
draft-yasukawa-pce-p2mp-req-00.txt
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Abstract
The Path Computation Element (PCE) provides functions of path
computation in support of traffic engineering in Multi-Protocol Label
Switching (MPLS) and Generalized MPLS (GMPLS) networks.
Extensions to the MPLS and GMPLS signaling and routing protocols have
been made in support of traffic engineered point-to-multipoint (P2MP)
label switched paths (LSPs). Since P2MP LSP routes are sometimes
complex to compute, and given the use of PCE in MPLS networks it is
likely that PCE will be used in P2MP MPLS networks.
Generic requirements for a communication protocol between Path
Computation Clients (PCCs) and PCEs are presented in "PCE
Communication Protocol Generic Requirements". This document
complements the generic requirements and presents a detailed set of
PCC-PCE communication protocol requirements for point-to-multipoint
traffic engineering.
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Conventions used in this document
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].
1. Introduction
The Path Computation Element (PCE) defined in [PCE-ARCH] is an entity
that is capable of computing a network path or route based on a
network graph, and applying computational constraints. The intention
is that the PCE is used to compute the path of traffic engineered
label switched paths (TE LSPs) within Multiprotocol Label Switching
(MPLS) and Generalized MPLS (GMPLS) networks.
Requirements for point-to-multipoint (P2MP) MPLS TE LSPs are
documented in [P2MP-REQ] and signaling protocol extensions are
defined in [P2MP-RSVP]. P2MP MPLS TE networks are currently being
planned by service provides in support of various features including
layer 3 multicast VPNs.
Path computation for P2MP LSPs presents a significant challenge and
network optimization of multiple P2MP TE LSPs will require
considerable computational resources. PCE offers a way to offload
such path computations from LSRs.
The applicability of the PCE-based path computation architecture to
point-to-multipoint MPLS traffic engineering is described in a
companion document [PCE-P2MP-APP]. No further attempt is made to
justify the use of PCE for P2MP MPLS TE within this document.
This document presents a set of PCC-PCE communication protocol
(PCECP) requirements for P2MP MPLS traffic engineering. It
supplements the generic requirements documented in [PCE-COM-REQ].
2. PCC-PCE Communication Requirements for P2MP MPLS Traffic Engineering
This section sets out additional requirements not covered in
[PCE-COM-REQ] specific to the problems of P2MP MPLS TE.
2.1. PCC-PCE Communication
The PCC-PCE communication protocol MUST allow requests and replies for
the computation of paths for P2MP LSPs.
This requires no additional messages, but implies the following
additional constraints to be added to the PCC-PCE communication
protocol.
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2.1.1. Indication of P2MP Path Computation Request
Although the presence of certain parameters (such as a list of more
than one destination) may be used to infer that a path computation
request is for a P2MP LSP, an explicit parameter SHOULD be placed in
a conspicuous place within a Path Computation Request message to
allow a receiving PCE to easily identify that the request is for a
P2MP path.
2.1.2. Non-Support of P2MP Path Computation
Not all PCEs are required to support P2MP path computation. Therefore
it MUST be possible for a PCE to reject a P2MP Path Computation
Request message with a reason code that indicates no support for P2MP
path computation.
2.1.3. Non-Support by Back-Level PCE Implementations
It is possible that initial PCE implementations will be developed
without support for P2MP path computation and without the ability to
recognize the explicit flag described in section 2.1.1. Therefore at
least one parameter required for P2MP path computation (possibly
including the flag described in section 2.1.1) MUST be defined in
such a way as to cause automatic rejection as unprocessable or
unrecognized by a back-level PCE implementation without requiring any
changes to that PCE.
2.1.4. Specification of Destinations
Since P2MP LSPs have more than one destination, it MUST be possible
for a single Path Computation Request to list multiple destinations.
2.1.5. Indication of P2MP Paths
The Path Computation Response MUST be able to carry the path of a
P2MP LSP. This SHOULD be expressed as a compacted series of routes as
described in [P2MP-RSVP] although not necessarily using an identical
encoding. This MAY be expressed as a non-compacted series of
source-to-destination routes.
2.1.6. Multi-Message Requests and Responses
A single P2MP LSP may have very many destinations, and the computed
path (tree) may be very extensive. In these cases it is possible that
the entire Path Computation Request or Response cannot fit within one
PCE message. Therefore it MUST be possible for a single request or
response to be conveyed by a sequence of messages.
Note that there is a requirement in [PCE-COM-REQ] for reliable and
in-order message delivery, so it is assumed that components of the
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sequence will be delivered in order and without missing items.
2.1.7. Non-Specification of Per-Destination Constraints and Parameters
It MUST NOT be possible to set different constraints, traffic
parameters, or quality of service requirements for different
destination of a P2MP LSP within a single computation request.
2.1.8. Path Modification and Path Diversity
No changes are made to the requirement to support path modification
and path diversity as described in [PCE-COM-REQ]. Note, however, that
a consequence of this requirement is that it must be possible to
supply an existing path on a Path Computation Request. This
requirement is unchanged as well, but it is a new requirement that
such paths MUST be able to be P2MP paths.
2.1.9. Capabilities Exchange
PCE capabilities exchange forms part of PCE discovery [PCE-DISCO],
but MAY also be included in the PCECP message exchanges.
In the event that the PCE ability to perform P2MP computation is not
advertised as part of PCE discovery, the PCECP MUST allow a PCC to
discover which PCEs with which it communicates support P2MP path
computation and which objective functions specific to P2MP path
computation are supported by each PCE.
3. Manageability Considerations
Manageability of P2MP MPLS TE with PCE must address the following
considerations.
- Need for a MIB module for control and monitoring.
- Need for built-in diagnostic tools.
- Configuration implications for the protocol.
4. Security Considerations
P2MP computation requests do not raise any additional security issues
for the PCECP.
Note, however, that P2MP computation requests are more CPU-intensive
and also use more link bandwidth. Therefore if the PCECP was
susceptible to denial of service attacks based on the injection of
spurious Path Computation Requests, the support of P2MP path
computation would exacerbate the effect.
It would be possible to consider applying different authorization
policies for P2MP path computation requests compared to other
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requests.
5. IANA Considerations
This document makes no requests for IANA action.
6. Acknowledgments
TBD
7. References
7.1. Normative Reference
[RFC2119] Bradner, S., "Key words for use in RFCs to indicate
requirements levels", RFC 2119, March 1997.
[PCE-COM-REQ] J. Ash, J.L Le Roux et al., "PCE Communication
Protocol Generic Requirements",
draft-ietf-pce-comm-protocol-gen-reqs (work in
progress).
7.2. Informative Reference
[PCE-ARCH] A. Farrel, JP. Vasseur and J. Ash, "Path Computation
Element (PCE) Architecture",
draft-ietf-pce-architecture (work in progress).
[PCE-DISCO] JL Le Roux et al., "Requirements for Path Computation
Element (PCE) Discovery",
draft-ietf-pce-discovery-reqs (work in progress).
[PCE-P2MP-APP] S. Yasukawa et al., "Applicability of the Path
Computation Element to Point-to-Multipoint Traffic
Engineering", draft-yasukawa-pce-p2mp-app, (work in
progress).
[P2MP-REQ] Yasukawa, S. (Editor), "Signaling Requirements for
Point to Multipoint Traffic Engineered MPLS LSPs",
draft-ietf-mpls-p2mp-sig-requirement, (work in
progress).
[P2MP-RSVP] Aggarwal, R., Papadimitriou, D., and Yasukawa, S.,
"Extensions to RSVP-TE for Point to Multipoint TE
LSPs", draft-ietf-mpls-rsvp-te-p2mp, (work in
progress).
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8. Authors' Addresses
Seisho Yasukawa
NTT
3-9-11 Midori-cho,
Musashino-shi, Tokyo 180-8585, Japan
Email: yasukawa.seisho@lab.ntt.co.jp
Adrian Farrel
Old Dog Consulting
Email: adrian@olddog.co.uk
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