MPLS Working Group A. Atlas
Internet-Draft J. Drake
Intended status: Informational Juniper Networks
Expires: August 10, 2013 S. Giacalone
Thomson Reuters
D. Ward
S. Previdi
C. Filsfils
Cisco Systems
February 6, 2013
Performance-based Path Selection for Explicitly Routed LSPs
draft-atlas-mpls-te-express-path-02
Abstract
In certain networks, it is critical to consider network performance
criteria when selecting the path for an explicitly routed RSVP-TE
LSP. Such performance criteria can include latency, jitter, and loss
or other indications such as the conformance to link SLAs and non-
RSVP TE traffic load. This specification uses IGP extension data
(which is defined outside the scope of this document) to perform such
path selections.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
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."
This Internet-Draft will expire on August 10, 2013.
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Atlas, et al. Expires August 10, 2013 [Page 1]
Internet-Draft MPLS-TE-Express-Path February 2013
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
1. Introduction
In certain networks, such as financial information networks, network
performance information is becoming as critical to data path
selection as other existing metrics. The ability to distribute
network performance information in OSPF
[I-D.ietf-ospf-te-metric-extensions] and in ISIS
[I-D.previdi-isis-te-metric-extensions] is being defined (outside the
scope of this document). This document describes how to use that
information for path selection for explicitly routed LSPs signaled
via RSVP-TE [RFC3209]. The method suggested is not optimal for both
minimizing path cost and additional constraints, such as latency;
optimal solutions are computationally complex.
The path selection mechanisms described in this document apply to
paths that are fully computed by the head-end of the LSP and then
signaled in an ERO where every sub-object is strict. This allows the
head-end to consider IGP-distributed performance data without
requiring the ability to signal the performance constraints in an
object of the RSVP Path message.
When considering performance-based data, it is obvious that there are
additional contributors beyond just the links. Clearly end-to-end
latency is a combination of router latency, queuing latency, physical
link latency and other factors. However, if application traffic
requires paths to be selected based upon latency constraints, the
same traffic might be in an Expedited Forwarding Per-Hop-
Behavior[RFC3246] with minimal queuing delay or another PHB with
known maximal per-hop queuing delay. While traversing a router can
cause delay, that can be included in the advertised link delay.
This document does not specify how a router determines what values to
advertise by the IGP. However, the end-to-end performance that is
computed for an LSP path SHOULD be built from the individual link
data. Any end-to-end characterization used to determine an LSP's
performance compliance should be fully reflected in the Traffic
Engineering Database so that a CSPF calculation can also determine
whether a path under consideration would be in compliance.
Atlas, et al. Expires August 10, 2013 [Page 2]
Internet-Draft MPLS-TE-Express-Path February 2013
1.1. Basic Requirements
The following are the requirements that motivate this solution.
1. Select a TE tunnel's path based upon a combination of existing
constraints as well as on link-latency, packet loss, jitter, link
SLA conformance, and bandwidth consumed by non-RSVP-TE traffic.
2. Ability to define different end-to-end performance requirements
for each TE tunnel regardless of common use of resources.
3. Ability to periodically verify that a TE tunnel's current LSP
complies with its configured end-to-end perforance requirements.
4. Ability to move tunnels, using make-before-break, based upon
computed end-to-end performance complying with configuration
5. Ability to move tunnels away from any link that is violating an
underlying SLA
6. Ability to optionally avoid setting up tunnels using any link
that is violating an SLA, regardless of whether end-to-end
performance would still meet requirements.
7. Ability to revert back to the best path after a configurable
period.
2. Using Performance Data Constraints
2.1. End-to-End Constraints
The per-link performance data available in the IGP
[I-D.ietf-ospf-te-metric-extensions]
[I-D.previdi-isis-te-metric-extensions] includes: unidirectional link
delay, unidirectional delay variation, and link loss. Each (or all)
of these parameters can be used to create the path-level link-based
parameter.
While it has been possible to compute a CSPF where the link latency
values are used instead of TE metrics, this results in ignoring the
TE metrics and causing LSPs to prefer the lowest-latency paths.
Instead of this approach to minimize path latency, an end-to-end
latency bound merely requires that the path computed be no more than
that bound without being the minimum. This bound can be used as a
constraint in CSPF to prevent exploring links that would create a
path over the end-to-end latency bound.
Atlas, et al. Expires August 10, 2013 [Page 3]
Internet-Draft MPLS-TE-Express-Path February 2013
This is illustrated as follows. Let the LSP have an end-to-end
latency bound of 20ms. Assume that the path to node X has been
minimized and its latency is 12ms. When X's links are to be
explored, the link X<->Y has a link latency of 5ms and the link X<->Z
has a link latency of 9ms. The path via X to Y along link X<->Y
would have a path latency of 12ms + 5ms = 17ms < 20ms; therefore, the
link X<->Y can be explored. In contrast, reaching Z via link X<->Z
would result in a path latency of 12ms + 9ms = 21ms > 20ms; therefore
the link X<->Z would not be explored in the CSPF.
An end-to-end bound on delay variation can be used similarly as a
constraint in the CSPF on what links to explore where the path's
delay variation is the sum of the used links' delay variations.
For link loss, the path loss is not the sum of the used links'
losses. Instead, the path loss percentage is (100 - loss_L1)*(100 -
loss_L2)*...*(100 - loss_Ln), where the links along the path are L1
to Ln. The end-to-end link loss bound, computed in this fashion, can
also be used as a constraint in the CSPF on what links to explore.
2.2. Link Constraints
In addition to selecting paths that conform to a bound on performance
data, it is also useful to avoid using links that do not meet a
necessary constraint. Naturally, if such a parameter were a known
fixed value, then resource attribute flags could be used to express
this behavior. However, when the parameter associated with a link
may vary dynamically, there is not currently a configuration-time
mechanism to enforce such behavior. An example of this is described
in Section 2.3, where links may move in and out of SLA-conformance
with regards to latency, delay variation, and link loss.
When doing path selection for TE tunnels, it has not been possible to
know how much actual bandwidth is available that inludes the
bandwidth used by non-RSVP-TE traffic. In
[I-D.ietf-ospf-te-metric-extensions]
[I-D.previdi-isis-te-metric-extensions], the Unidirectional Available
Bandwidth is advertised as is the Residual Bandwidth. When computing
the path for a TE tunnel, only links with at least a configurable
amount of Unidirectional Available Bandwidth might be permitted.
Similarly, only links whose loss is under a configurable value might
be acceptable. For these constraints, each link can be tested
against the constraint and only explored in the CSPF if the link
passes. In essence, a link that fails the constraint test is treated
as if it contained a resource attribute in the exclude-any filter.
Atlas, et al. Expires August 10, 2013 [Page 4]
Internet-Draft MPLS-TE-Express-Path February 2013
2.3. Links out of SLA
Link conformance to an SLA can change as a result of rerouting at
lower layers. This could be due to optical regrooming or simply
rerouting of a FA-LSP. When this occurs, there are three questions
to be asked:
a. Should the link be trusted and used for the setup of new LSPs?
b. Should LSPs using this link be immediately verified for continued
compliance to their end-to-end constraints?
c. Should LSPs using this link automatically be moved to a secondary
path?
2.3.1. Use of Anomalous Links for New Paths
If the answer to (a) is no for latency SLAs, then any link which has
the Anomalous bit set in the Unidirectional Link Delay sub-
TLV[I-D.ietf-ospf-te-metric-extensions]
[I-D.previdi-isis-te-metric-extensions] should be removed from the
topology before a CSPF calculation is used to compute a new path. In
essence, the link should be treated exactly as if it fails the
exclude-any resource attributes filter.[RFC3209].
Similarly, if the answer to (a) is no for link loss SLAs, then any
link which has the Anomalous bit set in the Link Los sub-TLV should
be treated as if it fails the exclude-any resource attributes filter.
If the answer to (a) is no for jitter SLAs, then any link that has
the Anomalous bit set in the Unidirectional Delay Variation sub-
TLV[I-D.previdi-isis-te-metric-extensions] should be treated as if it
fails the exclude-any resource attributes filter.
2.3.2. Links entering the Anomalous State
When a link enters the Anomalous state with respect to a parameter,
this is an indication that LSPs using that link might also no longer
be in compliance with their performance bounds. It can also be
considered an indication that something is changing that link and so
it might no longer be trustworthy to carry performance-critical
traffic. Naturally, which performance criteria are important for a
particular LSP is dependent upon the LSP's configuration and thus the
SLA compliance of a link is indicated per performance criterion.
At the ingress of a TE tunnel, a TE tunnel may be configured to be
sensitive to the Anomalous state of links in reference to latency,
delay variation, and/or loss. Additionally, such a TE tunnel may be
configured to either verify continued compliance, to switch
Atlas, et al. Expires August 10, 2013 [Page 5]
Internet-Draft MPLS-TE-Express-Path February 2013
immediately to a standby LSP, or to move to a different path.
When a sub-TLV is received with the Anomalous bit set when previously
it was clear, the list of interested TE tunnels must be scanned.
Each such TE tunnel should either have its continued compliance
verified, be switched to a hot standby, or do a make-before-break to
a secondary path.
2.3.3. Links leaving the Anomalous State
When a link leaves the Anomalous state with respect to a parameter,
this can serve as an indication that those TE tunnels, whose LSPs
were changed when the link entered the Anomalous state, may want to
reoptimize to a better path.
3. IANA Considerations
This document includes no request to IANA.
4. Security Considerations
This document is not currently believed to introduce new security
concerns.
5. References
5.1. Normative References
[I-D.ietf-ospf-te-metric-extensions]
Giacalone, S., Ward, D., Drake, J., Atlas, A., and S.
Previdi, "OSPF Traffic Engineering (TE) Metric
Extensions", draft-ietf-ospf-te-metric-extensions-02 (work
in progress), December 2012.
[I-D.previdi-isis-te-metric-extensions]
Previdi, S., Giacalone, S., Ward, D., Drake, J., Atlas,
A., and C. Filsfils, "IS-IS Traffic Engineering (TE)
Metric Extensions",
draft-previdi-isis-te-metric-extensions-02 (work in
progress), October 2012.
[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.
Atlas, et al. Expires August 10, 2013 [Page 6]
Internet-Draft MPLS-TE-Express-Path February 2013
5.2. Informative References
[RFC3246] Davie, B., Charny, A., Bennet, J., Benson, K., Le Boudec,
J., Courtney, W., Davari, S., Firoiu, V., and D.
Stiliadis, "An Expedited Forwarding PHB (Per-Hop
Behavior)", RFC 3246, March 2002.
[RFC5420] Farrel, A., Papadimitriou, D., Vasseur, JP., and A.
Ayyangarps, "Encoding of Attributes for MPLS LSP
Establishment Using Resource Reservation Protocol Traffic
Engineering (RSVP-TE)", RFC 5420, February 2009.
Authors' Addresses
Alia Atlas
Juniper Networks
10 Technology Park Drive
Westford, MA 01886
USA
Email: akatlas@juniper.net
John Drake
Juniper Networks
1194 N. Mathilda Ave.
Sunnyvale, CA 94089
USA
Email: jdrake@juniper.net
Spencer Giacalone
Thomson Reuters
195 Broadway
New York, NY 10007
USA
Email: Spencer.giacalone@thomsonreuters.com
Atlas, et al. Expires August 10, 2013 [Page 7]
Internet-Draft MPLS-TE-Express-Path February 2013
Dave Ward
Cisco Systems
170 West Tasman Dr.
San Jose, CA 95134
USA
Email: dward@cisco.com
Stefano Previdi
Cisco Systems
Via Del Serafico 200
Rome 00142
Italy
Email: sprevidi@cisco.com
Clarence Filsfils
Cisco Systems
Brussels
Belgium
Email: cfilsfil@cisco.com
Atlas, et al. Expires August 10, 2013 [Page 8]
