Performance-based Path Selection for Explicitly Routed LSPs using TE Metric Extensions
draft-ietf-teas-te-express-path-01
TEAS Working Group A. Atlas
Internet-Draft J. Drake
Intended status: Informational Juniper Networks
Expires: September 27, 2015 S. Giacalone
Thomson Reuters
D. Ward
S. Previdi
C. Filsfils
Cisco Systems
March 26, 2015
Performance-based Path Selection for Explicitly Routed LSPs using TE
Metric Extensions
draft-ietf-teas-te-express-path-01
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 performance
objectives and non-RSVP TE traffic load. This specification uses
network performance data, such as is advertised via the OSPF and ISIS
TE metric extensions (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.
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 27, 2015.
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Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Basic Requirements . . . . . . . . . . . . . . . . . . . 4
1.2. Oscillation and Stability Considerations . . . . . . . . 4
2. Using Performance Data Constraints . . . . . . . . . . . . . 5
2.1. End-to-End Constraints . . . . . . . . . . . . . . . . . 5
2.2. Link Constraints . . . . . . . . . . . . . . . . . . . . 6
2.3. Links out of compliance with Link Performance Objectives 6
2.3.1. Use of Anomalous Links for New Paths . . . . . . . . 7
2.3.2. Links entering the Anomalous State . . . . . . . . . 7
2.3.3. Links leaving the Anomalous State . . . . . . . . . . 8
3. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
4. Security Considerations . . . . . . . . . . . . . . . . . . . 8
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
6.1. Normative References . . . . . . . . . . . . . . . . . . 8
6.2. Informative References . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
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. Network performance information
can be obtained via either the TE Metric Extensions in OSPF [RFC7471]
or ISIS [I-D.ietf-isis-te-metric-extensions] or via a management
system. As with other TE information flooded via OSPF or ISIS, the
TE metric extensions have a flooding scope limited to the local area
or level. This document describes how a path computation function,
whether in an ingress LSR or a PCE[RFC4655] , can use that
information for path selection for explicitly routed LSPs. The
selected path may be signaled via RSVP-TE [RFC3209] or simply used by
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the ingress with segment routing
[I-D.ietf-spring-segment-routing-mpls] to properly forward the
packet. Methods of optimizing path selection for multiple parameters
are generally computationally complex. However, there are good
heuristics for the delay-constrained lowest-cost (DCLC) computation
problem [k-Paths_DCLC] that can be applied to consider both path cost
and a maximum delay bound. Some of the network performance
information can also be used to prune links from a topology before
computing the path.
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 to latency beyond just the links. Clearly
end-to-end latency is a combination of router latency (e.g. latency
from traversing a router without queueing delay), queuing latency,
physical link latency and other factors. While traversing a router
can cause delay, that router latency can be included in the
advertised link delay. As described in [RFC7471] and
[I-D.ietf-isis-te-metric-extensions], queuing delay must not be
included in the measurements advertised by OSPF or ISIS.
Queuing latency is specifically excluded to insure freedom from
oscillations and stability issues that have plagued prior attempts to
use delay as a routing metric. If application traffic follows a path
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 potentially very substantial per-hop
queuing delay. Only traffic which experiences relatively low
congestion, such as Expedited Forwarding traffic, will experience
delays very close to the sum of the reported link delays.
This document does not specify how a router determines what values to
advertise by the IGP; it does assume that the constraints specified
in [RFC7471] and [I-D.ietf-isis-te-metric-extensions] are followed.
Additionally, 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 path calculation can also determine whether a path under
consideration would be in compliance.
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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
performance objectives 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 with the TE LSDB that a TE
tunnel's current LSP complies with its configured end-to-end
performance requirements.
4. Ability to move tunnels, using make-before-break, based upon
computed end-to-end performance complying with constraints.
5. Ability to move tunnels away from any link that is violating an
underlying link performance objective.
6. Ability to optionally avoid setting up tunnels using any link
that is violating a link performance objective, regardless of
whether end-to-end performance would still meet requirements.
7. Ability to revert back using make-before-break to the best path
after a configurable period.
1.2. Oscillation and Stability Considerations
Past attempts to use unbounded delay or loss as metric sufferred from
severe oscillations. The use of performance based data must be such
that undampened oscillations are not possible and stability cannot be
impacted.
The use of timers is often cited as a cure. Oscillation that is
damped by timers is known as "slosh". If advertisement timers are
very short relative to the jitter applied to RSVP-TE CSPF timers,
then a partial oscillation occurs. If RSVP-TE CSPF timers are short
relative to advertisement timers, full oscillation (all traffic
moving back and forth) can occur. Even a partial oscillation causes
unnecessary reordering which is considered at least minimally
disruptive.
Delay variation or jitter is affected by even small traffic levels.
At even tiny traffic levels, the probability of a queue occupancy of
one can produce a measured jitter proportional to or equal to the
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packet serialization delay. Very low levels of traffic can increase
the probability of queue occupancies of two or three packets enough
to further increase the measured jitter. Because jitter measurement
is extremely sensitive to even very low traffic levels, any use of
jitter is likely to oscillate. There may be legitimate use of a
jitter measurement in path computation that can be considered free of
oscillation.
Delay measurements that are not sensitive to traffic loads may be
safely used in path computation. Delay measurements made at the link
layer or measurements made at a queuing priority higher than any
significant traffic (such as DSCP CS7 or CS6 [RFC4594], but not CS2
if traffic levels at CS3 and higher or EF and AF can affect the
measurement). Making delay measurements at the same priority as the
traffic on affected paths is likely to cause oscillations.
2. Using Performance Data Constraints
2.1. End-to-End Constraints
The per-link performance data available in the IGP [RFC7471]
[I-D.ietf-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.
It is 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. In practical
scenarios, latency constraints are typically a bound constraint
rather than a minimization objective. An end-to-end latency upper
bound merely requires that the path computed be no more than that
bound and does not require that it be the minimum latency path. The
latter is exactly the delay-constrained lowest-cost (DCLC) problem to
which good heuristics have been proposed in the literature (e.g.
[k-Paths_DCLC]).
An end-to-end bound on delay variation can be used similarly as a
constraint in the path computation 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 fraction is 1 - (1 - loss_L1)*(1 -
loss_L2)*...*(1 - loss_Ln), where the links along the path are L1 to
Ln with loss_Li in fractions. This computation is discussed in more
detail in Sections 5.1.4 and 5.1.5 in [RFC6049]. The end-to-end link
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loss bound, computed in this fashion, can also be used as a
constraint in the path computation.
The heuristic algorithms for DCLC only address one constraint bound
but having a CSPF that limits the paths explored (i.e. based on hop-
count) can be combined [hop-count_DCLC].
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 conformance for
link performance objectives 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 includes the
bandwidth used by non-RSVP-TE traffic. In [RFC7471]
[I-D.ietf-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 minimum 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 path computation 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.
2.3. Links out of compliance with Link Performance Objectives
Link conformance to a link performance objective 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 two questions to be asked:
a. Should the link be trusted and used for the setup of new LSPs?
b. Should LSPs using this link automatically be moved to a secondary
path?
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2.3.1. Use of Anomalous Links for New Paths
If the answer to (a) is no for link latency performance objectives,
then any link which has the Anomalous bit set in the Unidirectional
Link Delay sub-TLV[RFC7471] [I-D.ietf-isis-te-metric-extensions]
should be removed from the topology before a path 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 performance
objectives, 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 link jitter
performance objectives, then any link that has the Anomalous bit set
in the Unidirectional Delay Variation sub-
TLV[I-D.ietf-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
compliance of a link with respect to a particular link performance
objective 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
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.
It is not sufficient to just look at the Anomalous bit in order to
determine when TE tunnels must have their compliance verified. When
changing to set, the Anomalous bit merely provides a hint that
interested TE tunnels should have their continued compliance
verified.
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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 due to administrative policy when the link entered the
Anomalous state, may want to reoptimize to a better path. The hint
provided by the Anomalous state change may help optimize when to
recompute for 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. Acknowledgements
The authors would like to thank Curtis Villamizar for his extensive
detailed comments and suggested text in the Section 1 and
Section 1.2. The authors would like to thank Dhruv Dhody for his
useful comments, and his care and persistence in making sure that
these important corrections weren't missed. The authors would also
like to thank Xiaohu Xu and Sriganesh Kini for their review.
6. References
6.1. Normative References
[I-D.ietf-isis-te-metric-extensions]
Previdi, S., Giacalone, S., Ward, D., Drake, J., Atlas,
A., Filsfils, C., and W. Wu, "IS-IS Traffic Engineering
(TE) Metric Extensions", draft-ietf-isis-te-metric-
extensions-04 (work in progress), October 2014.
[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.
[RFC7471] Giacalone, S., Ward, D., Drake, J., Atlas, A., and S.
Previdi, "OSPF Traffic Engineering (TE) Metric
Extensions", RFC 7471, March 2015.
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6.2. Informative References
[I-D.ietf-spring-segment-routing-mpls]
Filsfils, C., Previdi, S., Bashandy, A., Decraene, B.,
Litkowski, S., Horneffer, M., Shakir, R., Tantsura, J.,
and E. Crabbe, "Segment Routing with MPLS data plane",
draft-ietf-spring-segment-routing-mpls-00 (work in
progress), December 2014.
[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.
[RFC4594] Babiarz, J., Chan, K., and F. Baker, "Configuration
Guidelines for DiffServ Service Classes", RFC 4594, August
2006.
[RFC4655] Farrel, A., Vasseur, J., and J. Ash, "A Path Computation
Element (PCE)-Based Architecture", RFC 4655, August 2006.
[RFC6049] Morton, A. and E. Stephan, "Spatial Composition of
Metrics", RFC 6049, January 2011.
[hop-count_DCLC]
Agrawal, H., Grah, M., and M. Gregory, "Optimization of
QoS Routing", 6th IEEE/AACIS International Conference on
Computer and Information Science 2007, 2007,
<http://ieeexplore.ieee.org/xpl/
articleDetails.jsp?arnumber=4276447>.
[k-Paths_DCLC]
Jia, Z. and P. Varaiya, "Heuristic methods for delay
constrained least cost routing using k-shortest-paths",
IEEE Transactions on Automatic Control 51(4), 2006,
<http://paleale.eecs.berkeley.edu/~varaiya/papers_ps.dir/
kdclc-ieeev4.pdf>.
Authors' Addresses
Alia Atlas
Juniper Networks
10 Technology Park Drive
Westford, MA 01886
USA
Email: akatlas@juniper.net
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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
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
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