when multiple recomputations are requested. The PCECP MUST handle
the congestion in a graceful way so that it does not unduly impact
the rest of the network, and so that it does not gate the ability of
the PCE to perform computation.
5.1.16. Constraints
This section provides a list of generic constraints that MUST be
supported by the PCECP. Other constraints may be added to service
specific applications as identified by separate application-specific
requirements documents. Note that the provisions of Section 5.1.14
mean that new constraints can be added to this list without impacting
the protocol to a level that requires major protocol changes.
The set of supported generic constraints MUST include at least the
following:
o MPLS-TE and GMPLS generic constraints:
- Bandwidth
- Affinities inclusion/exclusion
- Link, Node, Shared Risk Link Group (SRLG) inclusion/exclusion
- Maximum end-to-end IGP metric
- Maximum hop count
- Maximum end-to-end TE metric
- Degree of paths disjointness (Link, Node, SRLG)
o MPLS-TE specific constraints
- Class-type
- Local protection
- Node protection
- Bandwidth protection
o GMPLS specific constraints
- Switching type, encoding type
- Link protection type
5.1.17. Objective Functions Supported
This section provides a list of generic objective functions that MUST
be supported by the PCECP. Other objective functions MAY be added to
service specific applications as identified by separate application-
specific requirements documents. Note that the provisions of Section
5.1.14 mean that new objective functions MAY be added to this list
without impacting the protocol.
The PCECP MUST support at least the following "unsynchronized"
functions:
- Minimum cost path with respect to a specified metric
(shortest path)
- Least loaded path
- Maximum available bandwidth path
Also, the PCECP MUST support at least the following "synchronized"
objective functions:
- Minimize aggregate bandwidth consumption on all links
- Maximize the residual bandwidth on the most loaded link
- Minimize the cumulative cost of a set of diverse paths
5.2. Deployment Support Requirements
5.2.1. Support for Different Service Provider Environments
The PCECP must at least support the following environments:
- MPLS-TE and GMPLS networks
- Packet and non-packet networks
- Centralized and distributed PCE path computation
- Single and multiple PCE path computation
For example, PCECP is possibly applicable to packet networks (e.g.,
IP networks), non-packet networks (e.g., time-division multiplexed
(TDM) transport), and perhaps to multi-layer GMPLS control plane
environments. Definitions of centralized, distributed, single, and
multiple PCE path computation can be found in [RFC4655].
5.2.2. Policy Support
The PCECP MUST allow for the use of policies to accept/reject
requests. It MUST include the ability for a PCE to supply sufficient
detail when it rejects a request for policy reasons to allow the PCC
to determine the reason for rejection or failure. For example,
filtering could be required for a PCE that serves one domain (perhaps
an AS) such that all requests that come from another domain (AS) are
rejected. However, specific policy details are left to application-
specific PCECP requirements. Actual policies, configuration of
policies, and applicability of policies are out of scope.
Note that work on supported policy models and the corresponding
requirements/implications is being undertaken as a separate work item
in the PCE working group.
PCECP messages MUST be able to carry transparent policy information.
5.3. Aliveness Detection & Recovery Requirements
5.3.1. Aliveness Detection
The PCECP MUST allow a PCC/PCE to
- check the liveliness of the PCC-PCE communication,
- rapidly detect PCC-PCE communication failure (indifferently to
partner failure or connectivity failure), and
- distinguish PCC/PCE node failures from PCC-PCE connectivity
failures, after the PCC-PCE communication is recovered.
The aliveness detection mechanism MUST ensure reciprocal knowledge of
PCE and PCC liveness.
5.3.2. Protocol Recovery
In the event of the failure of a sender or of the communication
channel, the PCECP, upon recovery, MUST support resynchronization of
information (e.g., PCE congestion status) and requests between the
sender and the receiver; this SHOULD be arranged so as to minimize
repeat data transfer.
5.3.3. LSP Rerouting & Reoptimization
If an LSP fails owing to the failure of a link or node that it
traverses, a new computation request may be made to a PCE in order to
repair the LSP. Since the PCC cannot know that the PCE’s TED has
been updated to reflect the failure network information, it is useful
to include this information in the new path computation request.
Also, in order to re-use the resources used by the old LSP, it may be
advantageous to indicate the route of the old LSP as part of the new
path computation request.
Hence the path computation request message MUST allow an indication
of whether the computation is for LSP restoration, and it MUST
support the inclusion of the previously computed path as well as the
identity of the failed element. Note that the old path might only be
useful if the old LSP has not yet been torn down. The PCE MAY choose
to take failure indication information carried in a given request
into account when handling subsequent requests. This should be
driven by local policy decision.
Note that a network failure may impact a large number of LSPs. In
this case, a potentially large number of PCCs will simultaneously
send requests to the PCE. The PCECP MUST properly handle such
overload situations, such as, for instance, through throttling of
requests as set forth in Section 5.1.8.
The path computation request message MUST support TE LSP path
reoptimization and the inclusion of a previously computed path. This
will help ensure optimal routing of a reoptimized path, since it will
allow the PCE to avoid double bandwidth accounting and help reduce
blocking issues.
6. Security Considerations
Key management MUST be provided by the PCECP to provide for the
authenticity and integrity of PCECP messages. This will allow
protecting against PCE or PCC impersonation and also against message
content falsification.
The impact of the use of a PCECP MUST be considered in light of the
impact that it has on the security of the existing routing and
signaling protocols and techniques in use within the network.
Intra-domain security is impacted since there is a new interface,
protocol, and element in the network. Any host in the network could
impersonate a PCC and receive detailed information on network paths.
Any host could also impersonate a PCE, both gathering information
about the network before passing the request on to a real PCE and
spoofing responses. Some protection here depends on the security of
the PCE discovery process (see [PCE-DISC-REQ]). An increase in
inter-domain information flows may increase the vulnerability to
security attacks, and the facilitation of inter-domain paths may
increase the impact of these security attacks.
Of particular relevance are the implications for confidentiality
inherent in a PCECP for multi-domain networks. It is not necessarily
the case that a multi-domain PCE solution will compromise security,
but solutions MUST examine their impacts in this area.
Applicability statements for particular combinations of signaling,
routing, and path computation techniques are expected to contain
detailed security sections.
It should be observed that the use of an external PCE introduces
additional security issues. Most notable among these are the
following:
- Interception of PCE requests or responses
- Impersonation of PCE or PCC
- DoS attacks on PCEs or PCCs
The PCECP MUST address these issues in detail using authentication,
encryption, and DoS protection techniques. See also Section 5.1.9.
There are security implications of allowing arbitrary objective
functions, as discussed in Section 5.1.17, and the PCECP MUST allow
mitigating the risk of, for example, a PCC using complex objectives
to intentionally drive a PCE into resource exhaustion.
7. Manageability Considerations
Manageability of the PCECP MUST address the following considerations:
- The need for a MIB module for control and monitoring of PCECP
- The need for built-in diagnostic tools to test the operation of the
protocol (e.g., partner failure detection, Operations
Administration and Maintenance (OAM), etc.)
- Configuration implications for the protocol
PCECP operations MUST be modeled and controlled through appropriate
MIB modules. There are enough specific differences between PCCs and
PCEs to lead to the need of defining separate MIB modules.
Statistics gathering will form an important part of the operation of
the PCECP. The MIB modules MUST provide information that will allow
an operator to determine PCECP historical interactions and the
success rate of requests. Similarly, it is important for an operator
to be able to determine PCECP and PCE load and whether an individual
PCC is responsible for a disproportionate amount of the load. It
MUST be possible, through use of MIB modules, to record and inspect
statistics about the PCECP communications, including issues such as
malformed messages, unauthorized messages, and messages discarded
owing to congestion.
The new MIB modules should also be used to provide notifications
(traps) when thresholds are crossed or when important events occur.
For example, the MIB module may support indication of exceeding the
congestion state threshold or rate limitation state.
PCECP techniques must enable a PCC to determine the liveness of a PCE
both before it sends a request and in the period between sending a
request and receiving a response.
It is also important for a PCE to know about the liveness of PCCs to
gain a predictive view of the likely loading of a PCE in the future
and to allow a PCE to abandon processing of a received request.
The PCECP MUST support indication of congestion state and rate
limitation state, and MAY allow the operator to control such a
function.
8. Contributors
This document is the result of the PCE Working Group PCECP
requirements design team joint effort. In addition to the
authors/editors listed in the "Authors’ Addresses" section, the
following are the design team members who contributed to the
document:
Alia K. Atlas
Google Inc.
1600 Amphitheatre Parkway
Mountain View, CA 94043 USA
EMail: akatlas@alum.mit.edu
Arthi Ayyangar
Nuova Systems,
2600 San Tomas Expressway
Santa Clara, CA 95051
EMail: arthi@nuovasystems.com
Nabil Bitar
Verizon
40 Sylvan Road
Waltham, MA 02145 USA
EMail: nabil.bitar@verizon.com
Igor Bryskin
Independent Consultant
EMail: i_bryskin@yahoo.com
Dean Cheng
Cisco Systems, Inc.
3700 Cisco Way
San Jose CA 95134 USA
Phone: 408 527 0677
EMail: dcheng@cisco.com
Durga Gangisetti
MCI
EMail: durga.gangisetti@mci.com
Kenji Kumaki
KDDI Corporation
Garden Air Tower
Iidabashi, Chiyoda-ku,
Tokyo 102-8460, JAPAN
Phone: 3-6678-3103
EMail: ke-kumaki@kddi.com
Eiji Oki
NTT
Midori-cho 3-9-11
Musashino-shi, Tokyo 180-8585, JAPAN
EMail: oki.eiji@lab.ntt.co.jp
Raymond Zhang
BT INFONET Services Corporation
2160 E. Grand Ave.
El Segundo, CA 90245 USA
EMail: Raymond_zhang@bt.infonet.com
9. Acknowledgements
The authors would like to extend their warmest thanks to (in
alphabetical order) Lou Berger, Ross Callon, Adrian Farrel, Thomas
Morin, Dimitri Papadimitriou, Robert Sparks, and J.P. Vasseur for
their review and suggestions.
10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to
Indicate Requirement Levels", BCP 14, RFC 2119,
March 1997.
[RFC4655] Farrel, A., Vasseur, J.-P., and J. Ash, "A Path
Computation Element (PCE)-Based Architecture",
RFC 4655, August 2006.
10.2. Informative References
[METRIC] Le Faucheur, F., Uppili, R., Vedrenne, A.,
Merckx, P., and T. Telkamp, "Use of Interior
Gateway Protocol (IGP) Metric as a second MPLS
Traffic Engineering (TE) Metric", BCP 87, RFC
3785, May 2004.
[PCE-DISC-REQ] Le Roux, J.L., et al., "Requirements for Path
Computation Element (PCE) Discovery", Work in
Progress.
[PCECP-INTER-AREA] Le Roux, J.L., et al., "PCE Communication
Protocol (PCECP) specific requirements for
Inter-Area (G)MPLS Traffic Engineering", Work in
Progress.
[PCECP-INTER-LAYER] Oki, E., et al., "PCC-PCE Communication
Requirements for Inter-Layer Traffic
Engineering", Work in Progress.
[PCECP-INTER-AS] Bitar, N., Zhang, R., Kumaki, K., "Inter-AS
Requirements for the Path Computation Element
Communication Protocol (PCECP)", Work in
Progress.
[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.
[RFC3127] Mitton, D., St.Johns, M., Barkley, S., Nelson,
D., Patil, B., Stevens, M., and B. Wolff,
"Authentication, Authorization, and Accounting:
Protocol Evaluation", RFC 3127, June 2001.
Authors’ Addresses
Jerry Ash (Editor)
AT&T
Room MT D5-2A01
200 Laurel Avenue
Middletown, NJ 07748, USA
Phone: (732)-420-4578
EMail: gash@att.com
Jean-Louis Le Roux (Editor)
France Telecom
2, avenue Pierre-Marzin
22307 Lannion Cedex, FRANCE
EMail: jeanlouis.leroux@orange-ft.com
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