Internet Engineering Task Force A. Charny
Internet-Draft J. Zhang
Intended status: Informational Cisco Systems
Expires: September 7, 2010 G. Karagiannis
U. Twente
M. Menth
University of Wuerzburg
T. Taylor, Ed.
Huawei Technologies
March 6, 2010
PCN Boundary Node Behaviour for the Single Marking (SM) Mode of
Operation
draft-ietf-pcn-sm-edge-behaviour-02
Abstract
Precongestion notification (PCN) is a means for protecting quality of
service for inelastic traffic admitted to a Diffserv domain. The
overall PCN architecture is described in RFC 5559. This memo is one
of a series describing possible boundary node behaviours for a PCN
domain. The behaviour described here is that for two-state
measurement-based load control, known informally as Single Marking
(SM).
Status of this Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on September 7, 2010.
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Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. Assumed Core Network Behaviour for SM . . . . . . . . . . . . 5
3. Node Behaviours . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.2. Behaviour of the PCN-Egress-Node . . . . . . . . . . . . . 6
3.2.1. Reporting the PCN Data . . . . . . . . . . . . . . . . 6
3.3. Behaviour of the Ingress Node . . . . . . . . . . . . . . 6
3.4. Behaviour at the Decision Point . . . . . . . . . . . . . 7
3.4.1. Flow Admission . . . . . . . . . . . . . . . . . . . . 7
3.4.2. Flow Termination . . . . . . . . . . . . . . . . . . . 7
3.4.3. Decision Point Action For Missing Egress Node
Reports . . . . . . . . . . . . . . . . . . . . . . . 8
3.5. Summary of Timers . . . . . . . . . . . . . . . . . . . . 9
4. Identifying Ingress-Egress-Aggregates and Their Edge Points . 9
5. Specification of Diffserv Per-Domain Behaviour . . . . . . . . 9
5.1. Applicability . . . . . . . . . . . . . . . . . . . . . . 9
5.2. Technical Specification . . . . . . . . . . . . . . . . . 10
5.3. Attributes . . . . . . . . . . . . . . . . . . . . . . . . 10
5.4. Parameters . . . . . . . . . . . . . . . . . . . . . . . . 10
5.5. Assumptions . . . . . . . . . . . . . . . . . . . . . . . 10
5.6. Example Uses . . . . . . . . . . . . . . . . . . . . . . . 10
5.7. Environmental Concerns . . . . . . . . . . . . . . . . . . 11
5.8. Security Considerations . . . . . . . . . . . . . . . . . 11
6. Security Considerations . . . . . . . . . . . . . . . . . . . 11
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
9.1. Normative References . . . . . . . . . . . . . . . . . . . 11
9.2. Informative References . . . . . . . . . . . . . . . . . . 12
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Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12
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1. Introduction
The objective of Pre-Congestion Notification (PCN) is to protect the
quality of service (QoS) of inelastic flows within a Diffserv domain,
in a simple, scalable, and robust fashion. Two mechanisms are used:
admission control, to decide whether to admit or block a new flow
request, and (in abnormal circumstances) flow termination to decide
whether to terminate some of the existing flows. To achieve this,
the overall rate of PCN-traffic is metered on every link in the
domain, and PCN-packets are appropriately marked when certain
configured rates are exceeded. These configured rates are below the
rate of the link thus providing notification to boundary nodes about
overloads before any congestion occurs (hence the "pre" of "pre-
congestion notification"). The level of marking allows decisions to
be made about whether to admit or terminate individual flows. For
more details see [RFC5559].
Boundary node behaviours specify a detailed set of algorithms and
edge node behaviours used to implement the PCN mechanisms. Since the
algorithms depend on specific metering and marking behaviour at the
interior nodes, it is also necessary to specify the assumptions made
about interior node behaviour. Finally, because PCN uses DSCP values
to carry its markings, a specification of boundary node behaviour
must include the per domain behaviour (PDB) template specified in
[RFC3086], filled out with the appropriate content. The present
document accomplishes these tasks for the Single Marking (SM) mode of
operation.
1.1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC2119 [RFC2119].
In addition to the terms defined in [RFC5559], this document uses the
following terms:
decision point
The node that makes the decision about which flows to admit and to
terminate. In a given network deployment, this may be the ingress
node or a centralized control node. Regardless of the location of
the decision point, the ingress node is the point where the
decisions are enforced.
PCN-admission-state
The state ("admit" or "block") derived by the decision point for a
given ingress-egress-aggregate based on PCN packet marking
statistics. The decision point decides to admit or block new
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flows offered to the aggregate based on the current value of the
PCN-admission-state. For further details see Section 3.4.1.
Congestion level estimate (CLE)
A value derived from the measurement of PCN packets received at a
PCN-egress-node for a given ingress-egress-aggregate, representing
the ratio of excess-traffic-marked to total PCN traffic (measured
in octets) over a short period. For further details see
Section 3.4.1.
Admission decision threshold
A fractional value to which decision point compares the CLE to
determine the PCN-admission-state for a given ingress-egress-
aggregate. If the CLE is below the admission decision threshold
the PCN-admission-state is set to "admit". If the CLE is above
the admission decision threshold the PCN-admission-state is set to
"block". For further details see Section 3.4.1.
2. Assumed Core Network Behaviour for SM
This section describes the assumed behaviour for nodes of the PCN-
domain when acting in their role as PCN-interior-nodes. The SM mode
of operation assumes that:
o on each link the reference rate for the excess traffic meter is
configured with a PCN-excess-rate to be equal to the PCN-
admissible-rate for the link;
o PCN-interior-nodes perform excess-traffic-metering of packets
according to the rules specified in [RFC5670].
o excess-traffic-marking of packets uses the PCN-Marked (PM)
codepoint defined in [RFC5696];
o no link PCN-threshold-rate is configured, and PCN-interior nodes
perform no threshold-metering.
3. Node Behaviours
3.1. Overview
This section describes the behaviour of the PCN ingress and egress
nodes and the decision point (which may be collocated with the
ingress node). The PCN egress node collects and reports the rates of
total, threshold-marked, and excess-traffic-marked PCN traffic to the
decision point. For a detailed description, see Section 3.2.
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The PCN ingress node reports the rate of PCN traffic admitted to a
given ingress-egress aggregate when requested by the decision point.
It also enforces flow admission and termination decisions. For
details, see Section 3.3.
Finally, the decision point makes flow admission decisions and
selects flows to terminate based on the information provided by the
ingress and egress nodes for a given ingress-egress-aggregate. For
details, see Section 3.4.
3.2. Behaviour of the PCN-Egress-Node
The PCN-egress-node MUST meter received PCN traffic in order to
derive periodically the following rates for each ingress-egress-
aggregate passing through it:
o NM-rate: octets per second of PCN traffic in PCN-unmarked packets;
o ETM-rate: octets per second of PCN traffic in PCN-excess-marked
packets.
It is RECOMMENDED that the interval Tcalc between calculation of
these quantities be in the range of 100 to 500 ms to provide a
reasonable tradeoff between signalling demands on the network and the
time taken to react to impending congestion.
The PCN-traffic SHOULD be metered continuously and the intervals
themselves SHOULD be of equal length, to minimize the statistical
variance introduced by the measurement process itself.
3.2.1. Reporting the PCN Data
At the end of each interval, the PCN-egress-node SHOULD report the
latest calculated rates to the decision point. To reduce the volume
of signalling, the egress node MAY choose not to send a report if no
PCN traffic was received either during the present interval or during
the previous one. The egress node MUST send a report at least once
per configurable interval Tmax (of the order of a second) to
demonstrate liveness, even if all of the rates have value zero.
3.3. Behaviour of the Ingress Node
The PCN-ingress-node MUST provide the estimated current rate of
admitted PCN traffic (octets per second) for a specific ingress-
egress-aggregate when the decision point requests it. The way this
rate estimate is derived is a matter of implementation.
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For example, the rate that the PCN-ingress-node supplies MAY be
based on a quick sample taken at the time the information is
required. It is RECOMMENDED that such a sample be based on
observation of at least 30 PCN packets to achieve reasonable
statistical reliability.
3.4. Behaviour at the Decision Point
3.4.1. Flow Admission
When the decision point (e.g., the PCN-ingress-node) receives a
report from the egress node for a given ingress-egress-aggregate that
contains non-zero rates, it MUST calculate a congestion level
estimate (CLE) for the interval, where
CLE = ETM-Rate/(NM-Rate + ETM-Rate).
The decision point MUST compare the CLE to an admission decision
threshold. If the CLE is less than the threshold, the PCN-admission-
state for that aggregate MUST be set to "admit"; otherwise it MUST be
set to "block".
It is RECOMMENDED that the admission decision threshold for SM be
set fairly low, in the order of 0.05. The admission decision
threshold MAY vary for different flows based on policy.
If the PCN-admission-state for a given ingress-egress-aggregate is
"admit", the decision point SHOULD allow new flows to be admitted to
that aggregate. If the PCN-admission-state for a given ingress-
egress-aggregate is "block", the decision point SHOULD NOT allow new
flows to be admitted to that aggregate. These actions MAY be
modified by policy in specific cases.
3.4.2. Flow Termination
Not all operators will wish to deploy flow termination. Hence
deactivation of flow termination at the decision node MUST be a
configurable option.
When the report from the egress node that the PCN-admission-state
computed on the basis of the CLE is "block" for the given ingress-
egress-aggregate, the decision point MUST request the PCN-ingress-
node to provide an estimate of the rate (Admit-Rate) at which PCN-
traffic is being admitted to the aggregate.
If the decision point is collocated with the ingress node, the
request and response are internal operations.
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The decision point MUST then wait, both for the requested rate from
the ingress node and for the next report from the egress node. If
this next egress node report also includes a non-zero value for the
ETM-Rate, the decision point MUST determine an amount of flow to
terminate in the following steps:
1. The sustainable aggregate rate (SAR) for the given ingress-
egress-aggregate is estimated by the product:
SAR = U * NM-Rate
for the latest reported interval, where U is a configurable
factor less than one which is the same for all ingress-egress-
aggregates.
2. The amount of traffic that must be terminated is the difference:
Admit-Rate - SAR,
where Admit-Rate is the value provided by the ingress node.
If the difference calculated in the second step is positive, the
decision point MUST select flows to terminate using its knowledge of
the bandwidth required by individual flows gained, e.g., from
resource signalling, until it determines that the PCN traffic
admission rate will no longer be greater than the estimated
sustainable aggregate rate.
Flow termination MAY be spread out over multiple rounds to avoid
over-termination. If this is done, it is RECOMMENDED that enough
time elapse between successive rounds of termination to allow the
effects of previous rounds to be reflected in the measurements
upon which the termination decisions are based (see
[I-D.satoh-pcn-performance-termination] and sections 4.2 and 4.3
of [Menth08-sub-9]).
3.4.3. Decision Point Action For Missing Egress Node Reports
As mentioned in Section 3.2.1, the egress node MAY choose not to send
reports for a configurable interval Tmax while it does not receive
any PCN traffic for a given ingress-egress-aggregate. However, if
the decision point fails to receive reports for a given ingress-
egress-aggregate for a configurable interval Tfail (of the order of 2
* Tmax or less), it SHOULD cease to admit flows to that aggregate and
raise an alarm to management. This provides some protection against
the case where congestion is preventing the transfer of reports from
the egress node to the decision point.
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3.5. Summary of Timers
This section has referred to three timers:
o Tcalc: a timer which SHOULD be configurable, specifying the
frequency with which the PCN-egress-node calculates NM-Rate, ThM-
Rate, and ETM-Rate and reports them to the decision point. This
timer is RECOMMENDED to be of the order of 100 to 500 ms.
o Tmax: a configurable timer, specifying the maximum amount of time
between successive reports from the PCN-egress-node for a given
ingress-egress-aggregate. This is RECOMMENDED to be of the order
of one second.
o Tfail: a configurable timer, specifying the maximum amount of time
between successive reports for a given ingress-egress-aggregate
received at the decision point, after which the latter SHOULD
cease to admit flows to the aggregate concerned and raise an alarm
to management. This is RECOMMENDED to be of the order of 2 * Tmax
or less.
4. Identifying Ingress-Egress-Aggregates and Their Edge Points
The operation of PCN depends on the ability of the ingress and egress
nodes to identify the aggregate to which each flow belongs. The
egress node also needs to associate an aggregate with the address of
the ingress node for receiving reports, if the ingress node is the
decision point.
The means by which this is done depends on the packet routing
technology in use in the network. In general, classification of
individual packets at the ingress node (for enforcement and metering
of admission rates) and at the egress node must use the content of
the outer packet header. The process may well require configuration
of routing information in the ingress and egress nodes.
5. Specification of Diffserv Per-Domain Behaviour
This section provides the specification required by [RFC3086] for a
per-domain behaviour.
5.1. Applicability
This section draws heavily upon points made in the PCN architecture
document, [RFC5559].
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The PCN SM boundary node behaviour specified in this document is
applicable to inelastic traffic (particularly video and voice) where
quality of service for admitted flows is protected primarily by
admission control at the ingress to the domain. In exceptional
circumstances (e.g. due to network failures) already-admitted flows
may be terminated to protect the quality of service of the remainder.
The SM boundary node behaviour is more likely to terminate too many
flows under such circumstances than some alternative PCN boundary
node behaviours.
Single-Marking requires no extension to the baseline PCN encoding
described in [RFC5696], thus reducing the work expected to be
performed in the data path of the high-speed routing equipment, and
saving valuable real estate in the packet header.
5.2. Technical Specification
The technical specification of the PCN SM per domain behaviour is
provided by the contents of [RFC5559], [RFC5696], [RFC5670], and the
present document.
5.3. Attributes
The purpose of this per-domain behaviour is to achieve low loss and
jitter for the target class of traffic. Recovery from overloads by
flow termination should happen within 1-3 seconds.
5.4. Parameters
The SM per-domain behaviour specifies three timers, two at the PCN-
egress-node and one at the PCN-ingress-node; see Section 3.5.
Reference rates must be specified at each interior router for the
PCN-excess-rate on each link; see Section 2. An admission decision
threshold must be specified at each PCN-ingress-node; see
Section 3.4.1. A fraction U must be specified at each PCN-ingress-
node, with a common value over the whole domain; see Section 3.4.2.
5.5. Assumptions
Assumed that a specific portion of link capacity has been reserved
for PCN traffic.
5.6. Example Uses
The PCN SM behaviour may be used to carry real-time traffic,
particularly voice and video.
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5.7. Environmental Concerns
The PCN SM per-domain behaviour may interfere with the use of end-to-
end ECN due to reuse of ECN bits for PCN marking. See Appendix B of
[RFC5696] for details.
5.8. Security Considerations
Please see the security considerations in Section 6 as well as those
in [RFC2474] and [RFC2475].
6. Security Considerations
[RFC5559] provides a general description of the security
considerations for PCN. This memo introduces no new considerations.
7. IANA Considerations
This memo includes no request to IANA.
8. Acknowledgements
The authors thank Ruediger Geib for his useful comments.
9. References
9.1. Normative References
[RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black,
"Definition of the Differentiated Services Field (DS
Field) in the IPv4 and IPv6 Headers", RFC 2474,
December 1998.
[RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
and W. Weiss, "An Architecture for Differentiated
Services", RFC 2475, December 1998.
[RFC5559] Eardley, P., "Pre-Congestion Notification (PCN)
Architecture", RFC 5559, June 2009.
[RFC5670] Eardley, P., "Metering and Marking Behaviour of PCN-
Nodes", RFC 5670, November 2009.
[RFC5696] Moncaster, T., Briscoe, B., and M. Menth, "Baseline
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Encoding and Transport of Pre-Congestion Information",
RFC 5696, November 2009.
9.2. Informative References
[I-D.babiarz-pcn-explicit-marking]
Liu, X. and J. Babiarz, "Simulations Results for 3sM
(expired Internet Draft)", July 2007.
[I-D.satoh-pcn-performance-termination]
Satoh, D., Ueno, H., and M. Menth, "Performance Evaluation
of Termination in CL-Algorithm (Work in progress)",
July 2009.
[I-D.zhang-pcn-performance-evaluation]
Zhang, X., "Performance Evaluation of CL-PHB Admission and
Termination Algorithms (expired Internet Draft)",
July 2007.
[ID.briscoe-CL]
Briscoe, B., "An edge-to-edge Deployment Model for Pre-
Congestion Notification: Admission Control over a
DiffServ Region (expired Internet Draft)", 2006.
[Menth08-sub-9]
Menth, M. and F. Lehrieder, "PCN-Based Measured Rate
Termination", July 2009, <http://
www3.informatik.uni-wuerzburg.de/~menth/Publications/
papers/Menth08-Sub-9.pdf>.
[Menth08f]
Menth, M. and F. Lehrieder, "Performance Evaluation of
PCN-Based Admission Control", in Proceedings of the 16th
International Workshop on Quality of Service (IWQoS)",
June 2008, <http://www3.informatik.uni-wuerzburg.de/
~menth/Publications/papers/Menth08f.pdf>.
[RFC3086] Nichols, K. and B. Carpenter, "Definition of
Differentiated Services Per Domain Behaviors and Rules for
their Specification", RFC 3086, April 2001.
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Authors' Addresses
Anna Charny
Cisco Systems
300 Apollo Drive
Chelmsford, MA 01824
USA
Email: acharny@cisco.com
Xinyan (Joy) Zhang
Cisco Systems
300 Apollo Drive
Chelmsford, MA 01824
USA
Georgios Karagiannis
U. Twente
Phone:
Email: karagian@cs.utwente.nl
Michael Menth
University of Wuerzburg
Am Hubland
Wuerzburg D-97074
Germany
Phone: +49-931-888-6644
Email: menth@informatik.uni-wuerzburg.de
Tom Taylor (editor)
Huawei Technologies
1852 Lorraine Ave
Ottawa, Ontario K1H 6Z8
Canada
Phone: +1 613 680 2675
Email: tom111.taylor@bell.net
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