One document matched: draft-ietf-tewg-te-metric-igp-02.txt
Differences from draft-ietf-tewg-te-metric-igp-01.txt
Francois Le Faucheur
Ramesh Uppili
Cisco Systems, Inc.
Alain Vedrenne
Pierre Merckx
Equant
Thomas Telkamp
Global Crossing
IETF Internet Draft
Expires: March, 2003
Document: draft-ietf-tewg-te-metric-igp-02.txt September, 2002
Use of Interior Gateway Protocol (IGP) Metric as a second
MPLS Traffic Engineering Metric
Status of this Memo
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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).
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IGP Metric as second TE Metric September 2002
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].
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
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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 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
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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.
3. Migration Considerations
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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].
5. Acknowledgment
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IGP Metric as second TE Metric September 2002
This document has benefited from discussion with Jean-Philippe
Vasseur.
6. Normative References
[TE-REQ] Awduche et al, Requirements for Traffic Engineering over
MPLS, RFC2702, September 1999.
[OSPF-TE] Katz et al, Traffic Engineering Extensions to OSPF Version
2, draft-katz-yeung-ospf-traffic-07.txt, August 2002.
[ISIS-TE] Smit, Li, IS-IS extensions for Traffic Engineering, draft-
ietf-isis-traffic-04.txt, August 2001.
[RSVP-TE] Awduche et al, "RSVP-TE: Extensions to RSVP for LSP
Tunnels", RFC3209, December 2001.
[CR-LDP] Jamoussi et al., "Constraint-Based LSP Setup using LDP",
RFC3212, January 2002
7. Informative References
[METRICS] Fedyk et al, "Multiple Metrics for Traffic Engineering
with IS-IS and OSPF", draft-fedyk-isis-ospf-te-metrics-01.txt,
November 2000.
[DIFF-TE] Le Faucheur et al, "Requirements for support of Diff-Serv-
aware MPLS Traffic Engineering", draft-ietf-tewg-diff-te-reqts-
05.txt, June 2002.
[PATH-COMP] Vasseur et al, "RSVP Path computation request and reply
messages", draft-vasseur-mpls-path-computation-rsvp-03.txt, June
2002.
[LSP-HIER] Kompella et al, "LSP Hierarchy with Generalized MPLS TE",
draft-ietf-mpls-lsp-hierarchy-07.txt, June 2002.
Authors' Address:
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
Le Faucheur et. al 6
IGP Metric as second TE Metric September 2002
Cisco Systems, Inc.
300 Apollo Drive
Chelmsford, Massachussets 01824
USA
Phone: +1 978 244-4949
Email: ruppili@cisco.com
Alain Vedrenne
EQUANT
400 Galleria Parkway
Atlanta, Georgia 30339
USA
Phone: +1 (678)-346-3466
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
Oudkerkhof 51
3512 GJ Utrecht
The Netherlands
Phone: +31 30 238 1250
E-mail: telkamp@gblx.net
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