Network Working Group Seisho Yasukawa
Internet Draft NTT
Category: Informational Adrian Farrel
Expires: March 2007 Old Dog Consulting
September 2006
PCC-PCE Communication Requirements for Point to Multipoint
Multiprotocol Label Switching Traffic Engineering (MPLS-TE)
draft-yasukawa-pce-p2mp-req-01.txt
Status of this Memo
By submitting this Internet-Draft, each author represents that any
applicable patent or other IPR claims of which he or she is aware
have been or will be disclosed, and any of which he or she becomes
aware will be disclosed, in accordance with Section 6 of BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as
Internet-Drafts.
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."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
Abstract
The Path Computation Element (PCE) provides path computation
functions 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 point-to-multipoint (P2MP) Traffic Engineered
(TE) Label Switched Paths (LSPs). Since P2MP TE 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-TE networks.
Generic requirements for a communication protocol between Path
Computation Clients (PCCs) and PCEs are presented in "Path
Computation Element (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 MPLS traffic engineering.
Yasukawa Expires March 2007 [Page 1]
draft-yasukawa-pce-p2mp-req-01.txt September 2006
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 [RFC4655] 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 [RFC4461] and signaling protocol extensions for
setting up P2MP MPLS TE LSPs are defined in [P2MP-RSVP]. P2MP MPLS TE
networks are considered in support of various features including
layer 3 multicast VPNs.
Path computation for P2MP TE LSPs presents a significant challenge
and network optimization of multiple P2MP TE LSPs requires
considerable computational resources. PCE offers a way to offload
such path computations from LSRs.
The applicability of the PCE-based path computation architecture to
P2MP MPLS TE 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.
Yasukawa Expires March 2007 [Page 2]
draft-yasukawa-pce-p2mp-req-01.txt September 2006
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 parameter described in section 2.1.1.
Therefore, at least one parameter required for a P2MP path
computation request 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. It is
RECOMMENDED that the parameter that causes this result is the
parameter described in section 2.1.1.
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
Yasukawa Expires March 2007 [Page 3]
draft-yasukawa-pce-p2mp-req-01.txt September 2006
in-order message delivery, so it is assumed that components of the
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. Reoptimization of P2MP TE LSPs
Reoptimization MUST be supported for P2MP TE LSPs as described for
P2P LSPs in [PCE-COM-REQ]. To support this, the existing path MUST
be supplied as described in Section 2.1.8.
Because P2MP LSPs are more complex it is often the case that small
optimization improvements can be made after changes in network
resource availablity, but re-signaling any LSP introduces risks to
the stability of the service provided to the customer and the
stability of the network even when techniques like make-before-break
[RFC3209] are used. Therefore, a path computation request SHOULD
contain a parameter that allows the PCC to express a cost-benefit
reoptimization threshold for the whole LSP as well as per
destination. The setting of this parameter is subject to local policy
at the PCC and SHOULD be subject to policy at the PCE [PCE-POLICY].
For path reoptimization responses SHOULD indicate which of the routes
(as supplied according to Section 2.1.5) have been modified from the
paths supplied on the request.
2.1.10. Addition and Removal of Destinations from Existing Paths
A varitation of path modification described in Section 2.1.8 is that
destinations may be added to or removed from existing P2MP TE LSPs.
In the case of the addition of one or more destinations it is
necessary to compute a path for a new branch of the P2MP LSP. It may
be desirable to recompute the whole P2MP tree, to add the new branch
as a simple spur from the existing tree, or to recompute part of the
P2MP tree.
Yasukawa Expires March 2007 [Page 4]
draft-yasukawa-pce-p2mp-req-01.txt September 2006
To support this function for leaf additions it MUST be possible to
make the following indications on a path computation request:
- The path of an existing P2MP LSP (as described in Section 2.1.8)
- Which destinations are new additions to the tree
- Which destinations of the existing tree must not have their paths
modified.
It MAY also be possible to indicate on a path computation request a
cost-benefit reoptimization threshold such that the tree and/or a new
path to any individual destination is not supplied unless a certain
improvement is made. Compare with Section 2.1.9.
In the case of the deletion of one or more destination it is not
necessary to compute a new path for the P2MP TE LSP, but such a
computation may yield optimizations over a simple pruning of the
tree. The recomputation function in this case is essentially the same
as that described in Section 2.1.9, but note that is MAY be possible
to supply the full previous path of the entire P2MP TE LSP (that is,
before the deletion of the destinations) on the path computation
request.
For both addition and deletion of destinations, the path computation
response SHOULD indicate which of the routes (as supplied according
to Section 2.1.5) have been modified from the paths supplied on the
request as described in Section 2.1.9.
Note that the selection of all of these options is subject to local
policy at the PCC, and SHOULD be subject to policy at the PCE
[PCE-POLICY].
2.1.11. Capabilities Exchange
PCE capabilities exchange forms part of PCE discovery [PCE-DISCO],
but MAY also be included in the PCECP message exchanges.
The ability to perform P2MP path computation SHOULD be advertised as
part of PCE discovery. 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.
Yasukawa Expires March 2007 [Page 5]
draft-yasukawa-pce-p2mp-req-01.txt September 2006
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
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.
[PCE-POLICY] Bryskin, I., Papadimitriou, D., and Berger, L.,
"Policy-Enabled Path Computation Framework",
draft-ietf-pce-policy-enabled-path-comp, work in
progress.
Yasukawa Expires March 2007 [Page 6]
draft-yasukawa-pce-p2mp-req-01.txt September 2006
7.2. Informative Reference
[RFC3209] Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan,
V., and G. Swallow, "RSVP-TE: Extensions to RSVP for
LSP Tunnels", RFC 3209, December 2001.
[RFC4461] S. Yasukawa, Editor "Signaling Requirements for
Point-to-Multipoint Traffic Engineered MPLS LSPs",
RFC4461, April 2006.
[RFC4655] Farrel, A., Vasseur, J.P., and Ash, G., "A Path
Computation Element (PCE)-Based Architecture",
RFC 4655, August 2006.
[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-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.
8. Authors' Addresses
Seisho Yasukawa
NTT Corporation
(R&D Strategy Department)
3-1, Otemachi 2-Chome Chiyodaku, Tokyo 100-8116 Japan
Phone: +81 3 5205 5341
Email: s.yasukawa@hco.ntt.co.jp
Adrian Farrel
Old Dog Consulting
Email: adrian@olddog.co.uk
9. Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Yasukawa Expires March 2007 [Page 7]
draft-yasukawa-pce-p2mp-req-01.txt September 2006
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at
ietf-ipr@ietf.org.
Disclaimer of Validity
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Copyright Statement
Copyright (C) The Internet Society (2006). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.
Yasukawa Expires March 2007 [Page 8]