Internet Engineering Task Force A. Charny
Internet-Draft J. Zhang
Intended status: Informational Cisco Systems
Expires: December 30, 2010 G. Karagiannis
U. Twente
M. Menth
University of Wuerzburg
T. Taylor, Ed.
Huawei Technologies
June 28, 2010
PCN Boundary Node Behaviour for the Single Marking (SM) Mode of
Operation
draft-ietf-pcn-sm-edge-behaviour-03
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 a form of
measurement-based load control using two PCN marking states, not PCN-
marked, and excess-traffic-marked. This behaviour is known
informally as the Single Marking (SM) PCN edge behaviour.
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
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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 December 30, 2010.
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Assumed Core Network Behaviour for SM . . . . . . . . . . . . 4
3. Node Behaviours . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.2. Behaviour of the PCN-Egress-Node . . . . . . . . . . . . . 5
3.2.1. Data Collection . . . . . . . . . . . . . . . . . . . 5
3.2.2. Reporting the PCN Data . . . . . . . . . . . . . . . . 5
3.2.3. Optional Report Suppression . . . . . . . . . . . . . 6
3.2.4. Optional Calculation and Reporting of Congestion
Level Estimate . . . . . . . . . . . . . . . . . . . . 6
3.3. Behaviour at the Decision Point . . . . . . . . . . . . . 6
3.3.1. Flow Admission . . . . . . . . . . . . . . . . . . . . 7
3.3.2. Flow Termination . . . . . . . . . . . . . . . . . . . 7
3.3.3. Decision Point Action For Missing Egress Node
Reports . . . . . . . . . . . . . . . . . . . . . . . 8
3.4. Behaviour of the Ingress Node . . . . . . . . . . . . . . 8
3.5. Summary of Timers . . . . . . . . . . . . . . . . . . . . 9
4. Identifying Ingress-Egress-Aggregates and Their Edge Points . 10
5. Specification of Diffserv Per-Domain Behaviour . . . . . . . . 10
5.1. Applicability . . . . . . . . . . . . . . . . . . . . . . 10
5.2. Technical Specification . . . . . . . . . . . . . . . . . 10
5.3. Attributes . . . . . . . . . . . . . . . . . . . . . . . . 11
5.4. Parameters . . . . . . . . . . . . . . . . . . . . . . . . 11
5.5. Assumptions . . . . . . . . . . . . . . . . . . . . . . . 12
5.6. Example Uses . . . . . . . . . . . . . . . . . . . . . . . 12
5.7. Environmental Concerns . . . . . . . . . . . . . . . . . . 12
5.8. Security Considerations . . . . . . . . . . . . . . . . . 12
6. Security Considerations . . . . . . . . . . . . . . . . . . . 13
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
9.1. Normative References . . . . . . . . . . . . . . . . . . . 13
9.2. Informative References . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 14
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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.
NM-rate
rate of not-marked PCN traffic in octets per second. For further
details see Section 3.2.1.
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ETM-rate
rate of excess-traffic-marked PCN traffic in octets per second.
For further details see Section 3.2.1.
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 3.2.4.
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
flows offered to the aggregate based on the current value of the
PCN-admission-state. For further details see Section 3.3.1.
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 3.3.1.
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.
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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
not-marked and excess-traffic-marked PCN traffic to the Decision
Point. For a detailed description, see Section 3.2.
The PCN ingress node enforces flow admission and termination
decisions. It also reports the rate of PCN traffic admitted to a
given ingress-egress aggregate when requested by the Decision Point.
For details, see Section 3.4.
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 3.3.
3.2. Behaviour of the PCN-Egress-Node
3.2.1. Data Collection
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 packets which are not
PCN- Marked;
o ETM-rate: octets per second of PCN traffic in PCN-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.
3.2.2. Reporting the PCN Data
If the report suppression option described in the next sub-section is
not enabled, the PCN-egress-node MUST report the latest values of NM-
rate and ETM-rate to the Decision Point each time that it calculates
them.
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3.2.3. Optional Report Suppression
Report suppression MUST be provided as a configurable option. If
this option is enabled, the PCN-egress-node MUST NOT send a report to
the Decision Point for a given ingress-egress-aggregate whenever all
of the following conditions are satisfied:
o ETM-rate was zero in the latest interval.
o ETM-rate was zero in the next most recent interval.
o Less than time Tmaxnorep has elapsed since the last time the PCN-
egress-node sent a report to the Decision Point for the given
aggregate, where Tmaxnorep is a configurable value.
The above procedure ensures that at least one report is sent per
period Tmaxnorep. This provides some protection against loss of
egress reports and also demonstrates to the Decision Point that both
the PCN-egress-node and the communication path between the two nodes
are in operation. However, depending on the transport used for
reporting, the operator may choose to set Tmaxnorep to an effectively
infinite value. For example, the transport may include its own keep-
alive signalling at a sufficient frequency that PCN keep-alive is
redundant.
3.2.4. Optional Calculation and Reporting of Congestion Level Estimate
The calculation and reporting of congestion level estimates (CLE)
MUST be provided as a configurable option at the PCN-egress-node. If
this option is enabled, the PCN-egress-node MUST calculate the
current value for CLE for each ingress-egress-aggregate in each
measurement interval and include this in its report (along with the
current values of NM-rate and ETM-rate). The CLE is equal to the
ratio:
ETM-Rate / (NM-rate + ETM-rate)
if any PCN traffic was observed, or zero otherwise.
3.3. Behaviour at the Decision Point
Operators may choose to deploy just flow admission, or just flow
termination, or both. The Decision Point MUST implement both
mechanisms, but configurable options MUST be provided to activate or
deactivate PCN-based flow admission and flow termination
independently of each other at a given Decision Point.
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3.3.1. Flow Admission
The Decision Point determines the PCN-admission-state for a given
ingress-egress-aggregate each time it receives a report from the
egress node. It makes this determination on the basis of the
congestion level estimate (CLE), calculated as described in
Section 3.2.4. If the CLE is provided in the egress node report, the
Decision Point SHOULD use the reported value. If the CLE was not
provided in the report, the Decision Point MUST calculate it. The
Decision Point MUST compare the reported or calculated CLE to an
admission decision threshold CLElimit. 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, but such policy intervention
risks defeating the purpose of using PCN..
3.3.2. Flow Termination
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.
The Decision Point MUST then wait, for both the requested rate from
the ingress node and 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
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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 SHOULD select flows to terminate, until it determines
that the PCN traffic admission rate will no longer be greater than
the estimated sustainable aggregate rate. If the Decision Point
knows the bandwidth required by individual flows (e.g., from resource
signalling used to establish the flows), it MAY choose to complete
its selection of flows to terminate in a single round of decisions.
Alternatively, the Decision Point MAY spread flow termination 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]).
3.3.3. Decision Point Action For Missing Egress Node Reports
If the Decision Point fails to receive reports from a given egress
node for a configurable interval Tfail, 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. If a report is subsequently received from the egress
node concerned, the Decision Point MUST restart failure timing and
resume making admission and termination decisions based on the
reports it receives.
3.4. 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.
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
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observation of at least 30 PCN packets to achieve reasonable
statistical reliability.
3.5. Summary of Timers
Table 1 summarizes the timers implied by the preceding procedures.
Tcol and Trep are reset upon expiry. Tmon is reset by management
action or by receipt of a report from the egress node concerned.
+-------+----------+--------------+-----------+---------------------+
| Timer | Location | Incidence | Limit | Action on Expiry |
+-------+----------+--------------+-----------+---------------------+
| Tcol | Egress | One per node | Tcalc | Calculate and |
| | node | | | possibly report |
| | | | | NM-rate, ETM-rate |
| | | | | and optionally CLE |
| | | | | for each IEA. |
| - | - | - | - | - |
| Trep | Egress | One per IEA | Tmaxnorep | Send a report for |
| | node | if report | | that IEA at the |
| | | suppression | | next expiry of |
| | | is enabled. | | Tcol. |
| - | - | - | - | - |
| Tmon | Decision | One per | Tfail | Assume failure and |
| | point | egress node | | cease to admit |
| | | | | flows passing |
| | | | | through that egress |
| | | | | node. |
+-------+----------+--------------+-----------+---------------------+
IEA = ingress-egress-aggregate
Table 1: Timers Used For the CL Edge Behaviour
The value of Tcalc SHOULD be configurable, and is RECOMMENDED to be
of the order of 100 to 500 ms.
Trep is active only when report suppression is enabled. The value of
Tmaxnorep SHOULD be configurable. The appropriate value depends on
the transport used to carry the egress node reports. For unreliable
transport, Tmaxnorep is RECOMMENDED to be of the order of one second.
The value of Tfail MUST be configurable. When unreliable transport
is used, the value of Tfail is RECOMMENDED to be of the order of 3 *
Tmaxnorep if report suppression is enabled, and of the order of 3 *
Tcalc if report suppression is not enabled. When reliable transport
is used, the operator may choose to provide similar values for Tfail
or may choose to disable report timing by setting an effectively
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infinite value for Tfail.
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].
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
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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 3.3.1. A fraction U must be specified at each PCN-ingress-
node, with a common value over the whole domain; see Section 3.3.2.
In the list that follows, note that most PCN-ingress-nodes are also
egress nodes, and vice versa. Furthermore, the ingress nodes may be
collocated with Decision Points.
Parameters at the PCN-ingress-node:
o Filters for distinguishing PCN from non-PCN inbound traffic.
o The DSCP(s) to be used to mark PCN traffic.
o Reference rates on each inward link for the PCN-excess-rate; see
Section 2.
o The information needed to distinguish PCN traffic belonging to a
given ingress-egress-aggregate.
Parameters at the PCN-egress-node:
o The calculation interval Tcalc.
o Whether report suppression is enabled and, if so, the value of
Tmaxnorep, the maximum interval between reports for a given
ingress-egress-aggregate.
o Whether calculation and reporting of congestion level estimates is
enabled at the PCN-egress-node.
o The information needed to distinguish PCN traffic belonging to a
given ingress-egress-aggregate.
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o The marking rules for re-marking PCN traffic leaving the PCN
domain.
Parameters at each interior node:
o A reference rates on each link for the PCN-excess-rate; see
Section 2.
Parameters at the Decision Point:
o Activation/deactivation of PCN-based flow admission.
o Activation/deactivation of PCN-based flow termination.
o The admission decision threshold CLElimit.
o The fraction U used to derive the supportable aggregate rate from
the NM-rate;
o The maximum interval Tfail between reports from a given egress
node, for detecting failure of communications with that node.
o The information needed to map between each ingress-egress-
aggregate and its edgepoints, particularly the corresponding
ingress node.
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.
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].
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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. Toby
Moncaster provided a detailed review of the CL edge behaviour draft,
the results of which also appear in this document.
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[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
Encoding and Transport of Pre-Congestion Information",
RFC 5696, November 2009.
9.2. Informative References
[I-D.satoh-pcn-performance-termination]
Satoh, D., Ueno, H., and M. Menth, "Performance Evaluation
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of Termination in CL-Algorithm (Work in progress)",
July 2009.
[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>.
[RFC3086] Nichols, K. and B. Carpenter, "Definition of
Differentiated Services Per Domain Behaviors and Rules for
their Specification", RFC 3086, April 2001.
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
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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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