<- BCP Index (1..100)
(also RFC 3785)
Network Working Group F. Le Faucheur
Request for Comments: 3785 R. Uppili
BCP: 87 Cisco Systems, Inc.
Category: Best Current Practice A. Vedrenne
P. Merckx
Equant
T. Telkamp
Global Crossing
May 2004
Use of Interior Gateway Protocol (IGP) Metric
as a second MPLS Traffic Engineering (TE) Metric
Status of this Memo
This document specifies an Internet Best Current Practices for the
Internet Community, and requests discussion and suggestions for
improvements. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
This document describes a common practice on how the existing metric
of Interior Gateway Protocols (IGP) can be used as an alternative
metric to the Traffic Engineering (TE) metric for Constraint Based
Routing of MultiProtocol Label Switching (MPLS) Traffic Engineering
tunnels. This effectively results in the ability to perform
Constraint Based Routing with optimization of one metric (e.g., link
bandwidth) for some Traffic Engineering tunnels (e.g., Data Trunks)
while optimizing another metric (e.g., propagation delay) for some
other tunnels with different requirements (e.g., Voice Trunks). No
protocol extensions or modifications are required. This text
documents current router implementations and deployment practices.
1. Introduction
Interior Gateway Protocol (IGP) routing protocols (OSPF and IS-IS) as
well as MultiProtocol Label Switching (MPLS) signaling protocols
(RSVP-TE and CR-LDP) have been extended (as specified in [ISIS-TE],
[OSPF-TE], [RSVP-TE] and [CR-LDP]) in order to support the Traffic
Engineering (TE) functionality as defined in [TE-REQ].
Le Faucheur, et al. Best Current Practice [Page 1]
RFC 3785 IGP Metric as a second MPLS TE Metric May 2004 These IGP routing protocol extensions currently include advertisement of a single additional MPLS TE metric to be used for Constraint Based Routing of TE tunnels. However, the objective of traffic engineering is to optimize the use and the performance of the network. So it seems relevant that TE tunnel placement may be optimized according to different optimization criteria. For example, some Service Providers want to perform traffic engineering of different classes of service separately so that each class of Service is transported on a different TE tunnel. One example motivation for doing so is to apply different fast restoration policies to the different classes of service. Another example motivation is to take advantage of separate Constraint Based Routing in order to meet the different Quality of Service (QoS) objectives of each Class of Service. Depending on QoS objectives one may require either (a) enforcement by Constraint Based Routing of different bandwidth constraints for the different classes of service as defined in [DS-TE], or (b) optimizing on a different metric during Constraint Based Routing or (c) both. This document discusses how optimizing on a different metric can be achieved during Constraint Based Routing. The most common scenario for a different metric calls for optimization of a metric reflecting delay (mainly propagation delay) when Constraint Based Routing TE Label Switched Paths (LSPs) that will be transporting voice, while optimizing a more usual metric (e.g., reflecting link bandwidth) when Constraint Based Routing TE LSPs that will be transporting data. Additional IGP protocol extensions could be defined so that multiple TE metrics could be advertised in the IGP (as proposed for example in [METRICS]) and would thus be available to Constraint Based Routing in order to optimize on a different metric. However this document describes how optimizing on a different metric can be achieved today by existing implementations and deployments, without any additional IGP extensions beyond [ISIS-TE] and [OSPF-TE], by effectively using the IGP metric as a "second" TE metric. 2. Common Practice In current MPLS TE deployments, network administrators often want Constraint Based Routing of TE LSPs carrying data traffic to be based on the same metric as the metric used for Shortest Path Routing. Where this is the case, this practice allows the Constraint Based Routing algorithm running on the Head-End LSR to use the IGP metric advertised in the IGP to compute paths for data TE LSPs instead of the advertised TE metric. The TE metric can then be used to convey Le Faucheur, et al. Best Current Practice [Page 2]
RFC 3785 IGP Metric as a second MPLS TE Metric May 2004
another metric (e.g., a delay-based metric) which can be used by the
Constraint Based Routing algorithm on the Head-End LSR to compute
path for the TE LSPs with different requirements (e.g., Voice TE
LSP).
In some networks, network administrators configure the IGP metric to
a value factoring the link propagation delay. In that case, this
practice allows the Constraint Based Routing algorithm running on the
Head-End LSR to use the IGP metric advertised in the IGP to compute
paths for delay-sensitive TE LSPs (e.g., Voice TE LSPs) instead of
the advertised TE metric. The TE metric can then be used to convey
another metric (e.g., bandwidth based metric) which can be used by
the Constraint Based Routing algorithm to compute paths for the data
TE LSPs.
More generally, the TE metric can be used to carry any arbitrary
metric that may be useful for Constraint Based Routing of the set of
LSPs which need optimization on another metric than the IGP metric.
2.1. Head-End LSR Implementation Practice
A Head-End LSR implements the current practice by:
(i) Allowing configuration, for each TE LSP to be routed, of
whether the IGP metric or the TE metric is to be used by the
Constraint Based Routing algorithm.
(ii) Enabling the Constraint Based Routing algorithm to make use of
either the TE metric or the IGP metric, depending on the above
configuration for the considered TE-LSP
2.2. Network Deployment Practice
A Service Provider deploys this practice by:
(i) Configuring, on every relevant link, the TE metric to reflect
whatever metric is appropriate (e.g., delay-based metric) for
Constraint Based Routing of some LSPs as an alternative metric
to the IGP metric
(ii) Configuring, for every TE LSP, whether this LSP is to be
constraint based routed according to the TE metric or IGP
metric
Le Faucheur, et al. Best Current Practice [Page 3]
RFC 3785 IGP Metric as a second MPLS TE Metric May 2004
2.3. Constraints
The practice described in this document has the following
constraints:
(i) it only allows TE tunnels to be routed on either of two metrics
(i.e., it cannot allow TE tunnels to be routed on one of three,
or more, metrics). Extensions (for example such as those
proposed in [METRICS]) could be defined in the future if
necessary to relax this constraints, but this is outside the
scope of this document.
(ii) it can only be used where the IGP metric is appropriate as one
of the two metrics to be used for constraint based routing
(i.e., it cannot allow TE tunnels to be routed on either of two
metrics while allowing IGP SPF to be based on a third metric).
Extensions (for example such as those proposed in [METRICS])
could be defined in the future if necessary to relax this
constraints, but this is outside the scope of this document.
(iii) it can only be used on links which support an IGP adjacency so
that an IGP metric is indeed advertised for the link. For
example, this practice can not be used on Forwarding
Adjacencies (see [LSP-HIER]).
Note that, as with [METRICS], this practice does not recommend that
the TE metric and the IGP metric be used simultaneously during path
computation for a given LSP. This is known to be an NP-complete
problem.
2.4. Interoperability
Where path computation is entirely performed by the Head-End (e.g.,
intra-area operations with path computation on Head-end), this
practice does not raise any interoperability issue among LSRs since
the use of one metric or the other is a matter purely local to the
Head-End LSR.
Where path computation involves another component than the Head-End
(e.g., with inter-area operations where path computation is shared
between the Head-End and Area Boundary Routers or a Path Computation
Server), this practice requires that which metric to optimize on, be
signaled along with the other constraints (bandwidth, affinity) for
the LSP. See [PATH-COMP] for an example proposal on how to signal
which metric to optimize, to another component involved in path
computation when RSVP-TE is used as the protocol to signal path
computation information.
Le Faucheur, et al. Best Current Practice [Page 4]
RFC 3785 IGP Metric as a second MPLS TE Metric May 2004
3. Migration Considerations
Service Providers need to consider how to migrate from the current
implementation to the new one supporting this practice.
Although the head-end routers act independently from each other, some
migration scenarios may require that all head-end routers be upgraded
to the new implementation to avoid any disruption on existing TE-LSPs
before two metrics can effectively be used by TE. The reason is that
routers with current implementation are expected to always use the TE
metric for Constraint Based Routing of all tunnels; so when the TE
metric is reconfigured to reflect the "second metric" (say to a
delay-based metric) on links in the network, then all TE-LSPs would
get routed based on the "second metric" metric, while the intent may
be that only the TE-LSPs explicitly configured so should be routed
based on the "second metric".
A possible migration scenario would look like this:
1) upgrade software on all head-end routers in the network to support
this practice.
2) change the TE-LSPs configuration on the head-end routers to use
the IGP metric (e.g., bandwidth-based) for Constraint Based
Routing rather than the TE metric.
3) configure TE metric on the links to reflect the "second metric"
(e.g., delay-based).
4) modify the LSP configuration of the subset of TE-LSPs which need
to be Constraint Based routed using the "second metric" (e.g.,
delay-based), and/or create new TE-LSPs with such a configuration.
It is desirable that step 2 is non-disruptive (i.e., the routing of a
LSP will not be affected in any way, and the data transmission will
not be interrupted) by the change of LSP configuration to use "IGP
metric" as long as the actual value of the "IGP metric" and "TE
metric" are equal on every link at the time of LSP reconfiguration
(as would be the case at step 2 in migration scenario above which
assumed that TE metric was initially equal to IGP metric).
4. Security Considerations
The practice described in this document does not raise specific
security issues beyond those of existing TE. Those are discussed in
the respective security sections of [TE-REQ], [RSVP-TE] and [CR-LDP].
Le Faucheur, et al. Best Current Practice [Page 5]
RFC 3785 IGP Metric as a second MPLS TE Metric May 2004
5. Acknowledgment
This document has benefited from discussion with Jean-Philippe
Vasseur.
6. References
6.1. Normative References
[TE-REQ] Awduche, D., Malcolm, J., Agogbua, J., O'Dell, M. and J.
McManus, Requirements for Traffic Engineering over MPLS,
RFC 2702, September 1999.
[OSPF-TE] Katz, D., Kompella, K. and D. Yeung, "Traffic Engineering
(TE) Extensions to OSPF Version 2", RFC 3630, September
2003.
[ISIS-TE] Smit, H. and T. Li, "Intermediate System to Intermediate
System (IS-IS) Extensions for Traffic Engineering (TE),
RFC 3784, May 2004.
[RSVP-TE] 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.
[CR-LDP] Jamoussi, B., Andersson, L., Callon, R., Dantu, R., Wu,
L., Doolan, P., Worster, T., Feldman, N., Fredette, A.,
Girish, M., Gray, E., Heinanen, J., Kilty, T. and A.
Malis, "Constraint-Based LSP Setup using LDP", RFC 3212,
January 2002.
6.1. Informative References
[METRICS] Fedyk, et al., "Multiple Metrics for Traffic Engineering
with IS-IS and OSPF", Work in Progress, November 2000.
[DIFF-TE] Le Faucheur, F. and W. Lai, "Requirements for Support of
Differentiated Services-aware MPLS Traffic Engineering",
RFC 3564, July 2003.
[PATH-COMP] Vasseur, et al., "RSVP Path computation request and reply
messages", Work in Progress, June 2002.
[LSP-HIER] Kompella, et al., "LSP Hierarchy with Generalized MPLS
TE", Work in Progress, September 2002.
Le Faucheur, et al. Best Current Practice [Page 6]
RFC 3785 IGP Metric as a second MPLS TE Metric May 2004 7. Authors' Addresses Francois Le Faucheur Cisco Systems, Inc. Village d'Entreprise Green Side - Batiment T3 400, Avenue de Roumanille 06410 Biot-Sophia Antipolis France Phone: +33 4 97 23 26 19 EMail: flefauch@cisco.com Ramesh Uppili Cisco Systems, 2000 Innovation Drive Kanata, ONTARIO, Canada - K2K 3E8 Phone: 01-613-254 4578 Email: ruppili@cisco.com Alain Vedrenne Equant Heraklion, 1041 route des Dolines, BP347 06906 Sophia Antipolis Cedex FRANCE Phone: +33 4 92 96 57 22 EMail: alain.vedrenne@equant.com Pierre Merckx Equant 1041 route des Dolines - BP 347 06906 SOPHIA ANTIPOLIS Cedex FRANCE Phone: +33 (0)492 96 6454 EMail: pierre.merckx@equant.com Thomas Telkamp Global Crossing, Ltd. Croeselaan 148 NL-3521CG Utrecht The Netherlands Phone: +31 30 238 1250 EMail: telkamp@gblx.net Le Faucheur, et al. Best Current Practice [Page 7]
RFC 3785 IGP Metric as a second MPLS TE Metric May 2004 8. Full Copyright Statement Copyright (C) The Internet Society (2004). 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. 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. Intellectual Property 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. 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. Acknowledgement Funding for the RFC Editor function is currently provided by the Internet Society. Le Faucheur, et al. Best Current Practice [Page 8]