RFC7823: Performance-Based Path Selection for Explicitly Routed Label Switched Paths (LSPs) Using TE Metric Extensions
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Related keywords:
(Path Computation)
(traffic engineering)
Internet Engineering Task Force (IETF) A. Atlas
Request for Comments: 7823 J. Drake
Category: Informational Juniper Networks
ISSN: 2070-1721 S. Giacalone
Microsoft
S. Previdi
Cisco Systems
May 2016
Performance-Based Path Selection for
Explicitly Routed Label Switched Paths (LSPs) Using TE Metric Extensions
Abstract
In certain networks, it is critical to consider network performance
criteria when selecting the path for an explicitly routed RSVP-TE
Label Switched Path (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 describes how a path computation function
may use network performance data, such as is advertised via the OSPF
and IS-IS TE metric extensions (defined outside the scope of this
document) to perform such path selections.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7823.
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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. Security Considerations . . . . . . . . . . . . . . . . . . . 8
4. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
4.1. Normative References . . . . . . . . . . . . . . . . . . 8
4.2. Informative References . . . . . . . . . . . . . . . . . 8
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 9
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
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 IS-IS [RFC7810] or via a management system. As with other TE
information flooded via OSPF or IS-IS, 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] [RFC3473] or simply used by the ingress with segment
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routing [SEG-ROUTE-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 Explicit Route Object (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 [RFC7810],
queuing delay must not be included in the measurements advertised by
OSPF or IS-IS.
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 (PHB) [RFC3246] with minimal
queuing delay or another PHB with potentially very substantial per-
hop queuing delay. Only traffic that 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 [RFC7810] 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 considered for a path computation
function that uses network performance criteria.
1. Select a TE tunnel's path based upon a combination of existing
constraints as well as on link-latency, packet loss, jitter,
conformance with link performance objectives, 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 Link State Database
(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 a metric suffered
from severe oscillations. The use of performance based data must be
such that undamped 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 Constrained
Shortest Path First (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 that is
considered at least minimally disruptive.
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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
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 very low traffic levels, any use of jitter
is likely to oscillate. However, there may be uses 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 Differentiated Services Code Point
(DSCP) CS7 or CS6 [RFC4594], but not CS2 if traffic levels at CS3 and
higher or Expedited Forwarding and Assured Forwarding 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]
[RFC7810] 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 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
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more detail in Sections 5.1.4 and 5.1.5 in [RFC6049]. The end-to-end
link 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] and [RFC7810],
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 an 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 that has the Anomalous bit set in the Unidirectional
Link Delay sub-TLV [RFC7471] [RFC7810] 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 that has the Anomalous bit set in the Link
Loss sub-TLV should be treated as if it fails the exclude-any
resource attributes filter.
2.3.2. Links Entering the Anomalous State
When the Anomalous bit transitions from clear to set, this indicates
that the associated link has entered the Anomalous state with respect
to the associated parameter; similarly, a transition from set to
clear indicates that the Anomalous state has been exited for that
link and associated parameter.
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; 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 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
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interested TE tunnels should have their continued compliance
verified.
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. Security Considerations
This document is not currently believed to introduce new security
concerns.
4. References
4.1. Normative References
[RFC3209] Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
Tunnels", RFC 3209, DOI 10.17487/RFC3209, December 2001,
<http://www.rfc-editor.org/info/rfc3209>.
[RFC7471] Giacalone, S., Ward, D., Drake, J., Atlas, A., and S.
Previdi, "OSPF Traffic Engineering (TE) Metric
Extensions", RFC 7471, DOI 10.17487/RFC7471, March 2015,
<http://www.rfc-editor.org/info/rfc7471>.
[RFC7810] Previdi, S., Ed., Giacalone, S., Ward, D., Drake, J., and
Q. Wu, "IS-IS Traffic Engineering (TE) Metric Extensions",
RFC 7810, DOI 10.17487/7810, May 2016,
<http://www.rfc-editor.org/info/rfc7810>.
4.2. Informative References
[hop-count_DCLC]
Agrawal, H., Grah, M., and M. Gregory, "Optimization of
QoS Routing", 6th IEEE/AACIS International Conference on
Computer and Information Science,
DOI 10.1109/ICIS.2007.144, July 2007,
<http://ieeexplore.ieee.org/xpl/
articleDetails.jsp?arnumber=4276447>.
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RFC 7823 Path Selection with TE Metric Extensions May 2016
[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, vol. 51, no. 4,
April 2006, <http://dx.doi.org/10.1109/TAC.2006.872827>.
[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, DOI 10.17487/RFC3246, March 2002,
<http://www.rfc-editor.org/info/rfc3246>.
[RFC3473] Berger, L., Ed., "Generalized Multi-Protocol Label
Switching (GMPLS) Signaling Resource ReserVation Protocol-
Traffic Engineering (RSVP-TE) Extensions", RFC 3473,
DOI 10.17487/RFC3473, January 2003,
<http://www.rfc-editor.org/info/rfc3473>.
[RFC4594] Babiarz, J., Chan, K., and F. Baker, "Configuration
Guidelines for DiffServ Service Classes", RFC 4594,
DOI 10.17487/RFC4594, August 2006,
<http://www.rfc-editor.org/info/rfc4594>.
[RFC4655] Farrel, A., Vasseur, J., and J. Ash, "A Path Computation
Element (PCE)-Based Architecture", RFC 4655,
DOI 10.17487/RFC4655, August 2006,
<http://www.rfc-editor.org/info/rfc4655>.
[RFC6049] Morton, A. and E. Stephan, "Spatial Composition of
Metrics", RFC 6049, DOI 10.17487/RFC6049, January 2011,
<http://www.rfc-editor.org/info/rfc6049>.
[SEG-ROUTE-MPLS]
Filsfils, C., Ed., Previdi, S., Ed., Bashandy, A.,
Decraene, B., Litkowski, S., Horneffer, M., Shakir, R.,
Tantsura, J., and E. Crabbe, "Segment Routing with MPLS
data plane", Work in Progress, draft-ietf-spring-segment-
routing-mpls-04, March 2016.
Acknowledgements
The authors would like to thank Curtis Villamizar for his extensive
detailed comments and suggested text in Sections 1 and 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 reviews.
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Contributors
Dave Ward and Clarence Filsfils contributed to this document.
Authors' Addresses
Alia Atlas
Juniper Networks
10 Technology Park Drive
Westford, MA 01886
United States
Email: akatlas@juniper.net
John Drake
Juniper Networks
1194 N. Mathilda Ave.
Sunnyvale, CA 94089
United States
Email: jdrake@juniper.net
Spencer Giacalone
Microsoft
Email: spencer.giacalone@gmail.com
Stefano Previdi
Cisco Systems
Via Del Serafico 200
Rome 00142
Italy
Email: sprevidi@cisco.com
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