Internet Engineering Task Force A. Charny
Internet-Draft Cisco Systems
Intended status: Informational F. Huang
Expires: January 7, 2010 Huawei Technologies
G. Karagiannis
U. Twente
M. Menth
University of Wuerzburg
T. Taylor, Ed.
Huawei Technologies
July 6, 2009
PCN Boundary Node Behaviour for the Controlled Load (CL) Mode of
Operation
draft-ietf-pcn-cl-edge-behaviour-00
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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 three-state
measurement-based load control, known informally as Controlled Load
(CL).
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Assumed Core Network Behaviour for CL . . . . . . . . . . . . 4
3. Node Behaviours . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.2. Behaviour of the PCN-Egress-Node . . . . . . . . . . . . . 6
3.2.1. PCN-Egress-Node Role In Flow Admission . . . . . . . . 6
3.2.2. PCN-Egress-Node Role in Flow Termination . . . . . . . 7
3.3. Behaviour of the PCN-Ingress-Node . . . . . . . . . . . . 9
3.3.1. PCN-Ingress-Node Role In Flow Admission . . . . . . . 9
3.3.2. PCN-Ingress-Node Role In Flow Termination . . . . . . 9
4. Specification of Diffserv Per-Domain Behaviour . . . . . . . . 10
4.1. Applicability . . . . . . . . . . . . . . . . . . . . . . 10
4.2. Technical Specification . . . . . . . . . . . . . . . . . 10
4.3. Attributes . . . . . . . . . . . . . . . . . . . . . . . . 10
4.4. Parameters . . . . . . . . . . . . . . . . . . . . . . . . 10
4.5. Assumptions . . . . . . . . . . . . . . . . . . . . . . . 10
4.6. Example Uses . . . . . . . . . . . . . . . . . . . . . . . 10
4.7. Environmental Concerns . . . . . . . . . . . . . . . . . . 11
4.8. Security Considerations . . . . . . . . . . . . . . . . . 11
5. Security Considerations . . . . . . . . . . . . . . . . . . . 11
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
8.1. Normative References . . . . . . . . . . . . . . . . . . . 11
8.2. Informative References . . . . . . . . . . . . . . . . . . 12
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 "pre-congestion"
notification). The level of marking allows boundary nodes to make
decisions about whether to admit or terminate. 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 controlled load (CL) mode
of operation.
1.1. Terminology
In addition to the terms defined in [RFC5559], this document uses the
following terms:
Policy Decision Point (PDP)
The node that provides policy input regarding admission and
termination of flows.
PCN-admission-state
The state ("admit" or "block") derived by PCN-egress-node for a
given ingress-egress-aggregate based on PCN packet marking
statistics. The PCN-ingress-node admits or blocks new flows
offered to the aggregate based on the current value of the PCN-
admission-state. Individual decisions may be modified by policy
input from the PDP. For further details see Section 3.2.1 and
Section 3.3.1.
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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 marked to total PCN traffic (measured in octets) over
a short period. In this specification the CLE is an exponentially
weighted moving average of the ratios observed in successive
fixed-length measurement intervals. For further details see
Section 3.2.1.
Admission decision threshold
A fractional value to which the CLE is compared to determine the
PCN-admission-state. 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 3.2.1.
Normal regime
The operating state of the PCN-egress-node with respect to a given
ingress-egress-aggregate during periods when no excess-traffic-
marked packets are received within that aggregate.
Excess traffic regime
The operating state of the PCN-egress-node with respect to a given
ingress-egress-aggregate during periods when excess-traffic-marked
packets are being received within that aggregate. The transition
from normal to excess traffic regime occurs when an excess-
traffic-marked packet is received within the given ingress-egress-
aggregate. The transition from excess traffic regime to normal
regime occurs when a complete measurement interval passes without
receipt of an excess-traffic-marked packet within the given
ingress-egress-aggregate. For further details see Section 3.2.2.
2. Assumed Core Network Behaviour for CL
This section describes the assumed behaviour for nodes of the PCN-
domain when acting in their role as PCN-interior-nodes. The CL mode
of operation assumes that:
o encoding of PCN status within individual packets is based on
[ID.PCN-baseline], extended to provide a third PCN encoding state.
Possible extensions for this purpose are documented in
[ID.PCN3state] or alternatively [ID.PCN3in1];
o the domain satisfies the conditions specified in the applicable
encoding extension document;
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o each link has been configured with a PCN-threshold-rate having a
value equal to the PCN-admissible-rate for the link;
o each link has been configured with a PCN-excess-rate having a
value equal to the PCN-supportable-rate for the link;
o PCN-interior-nodes perform threshold-marking and excess-traffic-
marking of packets according to the rules specified in
[ID.PCN-marking], and any additional rules specified in the
applicable encoding extension document;
According to [ID.PCN-baseline], the encoding extension documents
should specify the allowable transitions between marking states.
However, to be absolutely clear, these allowable transitions are
specified here. At any interior node, the only permitted transitions
are these:
o a PCN packet which is not marked (NM) MAY be threshold-marked
(ThM) or excess-traffic-marked (ETM);
o a PCN packet which is threshold-marked (ThM) MAY be excess-
traffic-marked (ETM).
An interior node MUST NOT re-mark a packet from PCN to non-PCN, or
vice versa.
3. Node Behaviours
3.1. Overview
The Controlled Load (CL) mode of operation supports flow admission
based on the smoothed ratio of threshold-marked to total PCN-traffic
observed by the PCN-egress-node (the congestion level estimate, see
Section 1.1) for each ingress-egress-aggregate. When the PCN-
admission-state (see Section 1.1) for a given ingress-egress-
aggregate changes from "Admit" to "Block" or vice versa, the PCN-
egress-node reports this change. The PCN-ingress-node admits or
blocks new PCN flows offered to a given ingress-egress-aggregate
based on the PCN-admission-state, possibly modified by policy
direction from the Policy Decision Point (PDP).
Flow termination is triggered when the PCN-egress-node observes
excess-traffic-marked packets within a given ingress-egress-
aggregate. The PCN-egress-node performs measurements to produce an
estimate of the edge-to-edge supportable PCN traffic rate for the
ingress-egress-aggregate concerned and reports this estimate. When
it is alerted to the need for flow termination, the PCN-ingress-node
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similarly performs measurements to determine the rate at which it is
admitting PCN traffic to the same ingress-egress-aggregate. The
difference between the measured admission rate and the estimated
edge-to-edge supportable rate is an estimate of the total amount of
flow that must be terminated. The PCN-ingress-node under the
guidance of the PDP terminates selected previously-admitted flows
within the affected ingress-egress-aggregate until no more excess-
marked packets are observed at the PCN-egress-node.
Flow termination may be spread out over a period of time to avoid
over-termination.
When Equal Cost Multipath (ECMP) routing has been configured in the
network, it is possible that some flows within a given ingress-
egress-aggregate pass through the bottleneck that is resulting in
excess-traffic-marking, while others do not. To ensure that the
right set of flows is terminated, the PCN-egress-node supplies a list
of excess-traffic-marked flows along with its estimate of the edge-
to-edge supportable PCN traffic rate. The PDP gives preference to
this list when determining which flows to terminate.
3.2. Behaviour of the PCN-Egress-Node
The egress node generates reports for individual ingress-egress-
aggregates based on measurements of PCN-packets it receives.
Processing of measurements proceeds in two regimes (states). In the
normal regime (Section 1.1) no excess-traffic-marked packets are
being observed within the ingress-egress-aggregate. The reports
generated are reports of changes in the PCN-admission-state
(Section 1.1) for that aggregate. In the excess traffic regime
(Section 1.1), excess-traffic-marked packets are being observed
within the ingress-egress-aggregate. The reports generated contain
estimates of the edge-to-edge supportable rate of PCN-traffic for the
ingress-egress-aggregate. The following sections give details of the
processing within the regimes and the actions taken upon transition
from one regime to the other.
3.2.1. PCN-Egress-Node Role In Flow Admission
For each ingress-egress-aggregate, while no excess-traffic-marked
packets are observed (normal regime), the egress node continuously
measures the following quantities over successive intervals of equal
duration. That duration is suggested to be in the range of 100 to
500ms to provide a reasonable tradeoff between signalling demands on
the network and the time taken to react to impending congestion.
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NM-count:
Number of octets of PCN-traffic contained in received packets
which are neither threshold-marked nor excess-traffic-marked.
ThM-count:
Number of octets of PCN-traffic contained in received packets
which are threshold-marked.
At the end of each measurement interval, the egress node calculates a
ratio R. If both counts are zero for the interval, the ratio R is set
to zero. Otherwise, the egress node calculates the ratio as:
R = ThM-count / (NM-count + ThM-count).
The egress node then updates a congestion level estimate (CLE, see
Section 1.1) with this ratio using exponential smoothing:
new_CLE = k*R + (1-k)*old_CLE,
where k is a constant chosen to put most (say 80%) of the weight in
the accumulated average on the most recent 1 to 3 seconds of data.
The value of k thus depends on the length of the measurement
interval.
The next step is to examine the relationship of old_CLE and new_CLE
to a configured admission decision threshold (Section 1.1). If
old_CLE is below the threshold and new_CLE is above it, the egress
node reports that the PCN-admission-state is now "block" for the
ingress-egress-aggregate. If old_CLE is above the threshold and
new_CLE is below it, the egress node reports that the ingress-egress-
aggregate PCN-admission-state is now "admit" for the ingress-egress-
aggregate. In the absence of one of these two threshold-crossing
events, the egress node issues no report.
Simulation results show that the process is not sensitive to the
value of the decision threshold. A value in the order of 0.5
seems reasonable.
3.2.2. PCN-Egress-Node Role in Flow Termination
When the PCN-egress-node detects an excess-traffic-marked packet, it
transitions to the excess traffic regime with respect to the ingress-
egress-aggregate concerned. As a consequence of this transition, it
immediately resets NM-count and ThM-count and begins a new
measurement interval. In addition, it begins to collect a third
quantity:
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ETM-count:
Number of octets of PCN-traffic contained in received packets
which are excess-traffic-marked.
Finally, if so configured (i.e., because ECMP routing is being used),
the PCN-egress-node begins to record flow identifiers of individual
flows for which excess-traffic-marked packets have been observed.
At the end of the new measurement interval, the PCN-egress-node
calculates the sum
NM-count + ThM-count,
normalizes it to a rate in octets per second, and reports it as an
estimate of the edge-to-edge supportable PCN traffic rate for the
ingress-egress-aggregate concerned. If flow identifiers of excess-
traffic-marked flows were collected, these are also reported.
The PCN-egress-node also calculates the ratio:
R = (ThM-count + ETM-count) / (NM-count + ThM-count + ETM-count)
and then proceeds to update the CLE estimate:
new_CLE = k*R + (1-k)*old_CLE,
with the same value of k as in Section 3.2.1. However, the PCN-
egress-node does not derive or report PCN-admission-state while
excess-marked-traffic is being observed.
The PCN-egress-node repeats the above procedures for successive
measurement intervals until no more excess-marked-traffic is
observed. At the end of the first interval during which ETM-count is
zero, the PCN-egress-node transitions to the normal regime. As part
of the transition, after updating the CLE with the latest results, it
immediately reports the PCN-admission-state for the ingress-egress-
aggregate based on the updated CLE. This report serves to inform the
PCN-ingress-node and PDP that no more flow termination is required.
The PCN-egress-node then reverts to the normal regime procedures
described in Section 3.2.1.
The duration of measurement intervals during the excess traffic
regime is the same as during the normal regime. The only difference
in measurement behaviour is the restart of measurements upon
transition to the excess traffic regime and the collection of ETM-
count while that regime prevails.
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ETM-countcould be collected within the normal regime, but by
definition would always be zero.
3.3. Behaviour of the PCN-Ingress-Node
The PCN-related functions of the PCN-ingress-node are described
briefly in section 4.2 of [RFC5559]. This section focusses on the
specific behaviour associated with admission and flow termination.
Procedures at the PCN-ingress-node for a given ingress-egress-
aggregate can also be classed as occurring within a normal regime or
during an excess traffic regime. The transition from the normal
regime to the excess traffic regime occurs when the PCN-ingress-node
receives a report indicating the estimated edge-to-edge supportable
rate of PCN traffic for the aggregate. The transition back to normal
regime occurs when the PCN-ingress-node receives a report of the
current PCN-admission-state for the aggregate.
3.3.1. PCN-Ingress-Node Role In Flow Admission
When the PCN-ingress-node receives a report indicating that the PCN-
admission-state for a given ingress-egress-aggregate is "admit", it
admits new flows to that aggregate. When the PCN-ingress-node
receives a report indicating that the PCN-admission-state for a given
ingress-egress-aggregate is "block", it ceases to admit new flows to
that aggregate. These actions may be modified by policy input from
the Policy Decision Point (PDP).
3.3.2. PCN-Ingress-Node Role In Flow Termination
When the PCN-ingress-node receives a report providing an estimate of
the edge-to-edge supportable PCN traffic rate for a given ingress-
egress-aggregate, it ceases to admit new flows to that aggregate
(unless directed otherwise for specific flows by the PDP). It
immediately begins to measure the rate of PCN-traffic that it is
currently admitting to that aggregate. When it has accumulated
sufficient data for a reliable estimate [see Table 1 giving traffic
rate vs. meaurement period, to be provided later], it normalizes the
result to a rate in octets per second. It reports the difference
between this result and the estimated edge-to-edge supportable PCN
traffic rate for the aggregate to the PDP.
Subsequently, the PCN-ingress-node terminates already-admitted flows
as directed by the PDP. When it receives a report indicating the
current PCN-admission state for the ingress-egress-aggregate, it
resumes admitting or blocking new flows as described in
Section 3.3.1.
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4. Specification of Diffserv Per-Domain Behaviour
This section provides the specification required by [RFC3086] for a
per-domain behaviour.
4.1. Applicability
This section draws heavily upon points made in the PCN architecture
document, [RFC5559].
The PCN CL 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 CL boundary node behaviour is less likely to terminate too many
flows under such circumstances than some alternative PCN boundary
node behaviours.
4.2. Technical Specification
The technical specification of the PCN CL per domain behaviour is
provided by the contents of [RFC5559], [ID.PCN-baseline],
[ID.PCN-marking], the specification of the encoding extension (e.g.
[ID.PCN3state], [ID.PCN3in1]), and the present document.
4.3. Attributes
TBD -- basically low loss, low jitter. Low delay would be nice but
has to be quantified
4.4. Parameters
TBD. Don't think RFC 3068 is looking for the list of configurable
parameters given in the architecture document.
4.5. Assumptions
Assumed that a specific portion of link capacity has been reserved
for PCN traffic. Assumed that recovery from overloads by flow
termination should happen within 1-3 seconds.
4.6. Example Uses
The PCN CL behaviour may be used to carry real-time traffic,
particularly voice and video.
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4.7. Environmental Concerns
In some markets, traffic preemption is considered to be
impermissible. In such environments, flow termination would not be
enabled.
4.8. Security Considerations
Please see the security considerations in Section 5 as well as those
in [RFC2474] and [RFC2475].
5. Security Considerations
[RFC5559] provides a general description of the security
considerations for PCN. This memo introduces no new considerations.
6. IANA Considerations
This memo includes no request to IANA.
7. Acknowledgements
Excluding the appendices, the content of this memo is drawn from
[ID.briscoe-CL]. The authors of that document were Bob Briscoe,
Philip Eardley, and Dave Songhurst of BT, Anna Charny and Francois Le
Faucheur of Cisco, Jozef Babiarz, Kwok Ho Chan, and Stephen Dudley of
Nortel, Giorgios Karagiannis of U. Twente and Ericsson, and Attila
Bader and Lars Westberg of Ericsson.
8. References
8.1. Normative References
[ID.PCN-baseline]
Moncaster, T., Briscoe, B., and M. Menth, "Baseline
Encoding and Transport of Pre-Congestion Information (Work
in progress)", May 2009.
[ID.PCN-marking]
Eardley, P., "Metering and marking behaviour of PCN-nodes
(Work in progress)", June 2009.
[RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black,
"Definition of the Differentiated Services Field (DS
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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.
8.2. Informative References
[ID.PCN3in1]
Briscoe, B., "PCN 3-State Encoding Extension in a single
DSCP (expired Internet Draft)", October 2008.
[ID.PCN3state]
Moncaster, T., Briscoe, B., and M. Menth, "A PCN encoding
using 2 DSCPs to provide 3 or more states (Work in
progress)", April 2009.
[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.
[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
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Fortune Huang
Huawei Technologies
Section F, Huawei Industrial Base,
Bantian Longgang, Shenzhen 518129
P.R. China
Phone: +86 15013838060
Email: fqhuang@huawei.com
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: tom.taylor@rogers.com
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