Internet Engineering Task Force A. Charny
Internet-Draft J. Zhang
Intended status: Informational Cisco Systems
Expires: January 8, 2010 G. Karagiannis
U. Twente
M. Menth
University of Wuerzburg
T. Taylor, Ed.
Huawei Technologies
July 7, 2009
PCN Boundary Node Behaviour for the Single Marking (SM) Mode of
Operation
draft-ietf-pcn-sm-edge-behaviour-00
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and restrictions with respect to this document.
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).
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. PCN-Egress-Node Role In Flow Admission . . . . . . . . 6
3.2.2. PCN-Egress-Node Role In Flow Termination . . . . . . . 6
3.3. Behaviour of the PCN-Ingress-Node . . . . . . . . . . . . 7
3.3.1. PCN-Ingress-Node Role In Flow Admission . . . . . . . 7
3.3.2. PCN-Ingress-Node Role In Flow Termination . . . . . . 7
3.4. Possible Extension to the Basic Algorithm . . . . . . . . 7
4. Specification of Diffserv Per-Domain Behaviour . . . . . . . . 8
4.1. Applicability . . . . . . . . . . . . . . . . . . . . . . 8
4.2. Technical Specification . . . . . . . . . . . . . . . . . 9
4.3. Attributes . . . . . . . . . . . . . . . . . . . . . . . . 9
4.4. Parameters . . . . . . . . . . . . . . . . . . . . . . . . 9
4.5. Assumptions . . . . . . . . . . . . . . . . . . . . . . . 9
4.6. Example Uses . . . . . . . . . . . . . . . . . . . . . . . 9
4.7. Environmental Concerns . . . . . . . . . . . . . . . . . . 9
4.8. Security Considerations . . . . . . . . . . . . . . . . . 9
5. Security Considerations . . . . . . . . . . . . . . . . . . . 9
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
8.1. Normative References . . . . . . . . . . . . . . . . . . . 10
8.2. Informative References . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11
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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 Single Marking (SM) 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 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 each link has been configured with a PCN-excess-rate having a
value 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 [ID.PCN-marking].
o excess-traffic-marking of packets uses the PCN-Marked (PM)
codepoint defined in [ID.PCN-baseline];
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o no link PCN-threshold-rate is configured, and PCN-interior nodes
perform no threshold-metering.
3. Node Behaviours
3.1. Overview
The Single Marking (SM) mode of operation supports flow admission
based on the smoothed ratio of PCN-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).
The decision to terminate flows requires measurement data from both
the PCN-ingress-node and the PCN-egress-node. Hence while the the
PCN-admission-state is "block", the PCN-egress-node reports the
measured rate of flow of unmarked PCN-traffic it receives for each
ingress-egress-aggregate. If the admitted traffic rate measured at
the PCN-ingress-node exceeds the reported unmarked received PCN
traffic rate multiplied by a configured factor, flows are selected
for termination to reduce this difference to zero, with policy
guidance from the PDP. The PCN-ingress-node ceases to admit the
selected flows.
[Not sure what to do about identifying flows for ECMP]
3.2. Behaviour of the PCN-Egress-Node
For each ingress-egress-aggregate, 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.
NM-count:
Number of octets of PCN-traffic contained in received packets
which are not PCN-marked.
PM-count:
Number of octets of PCN-traffic contained in received packets
which are PCN-marked.
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3.2.1. PCN-Egress-Node Role In Flow Admission
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 = PM-count / (NM-count + PM-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 above the threshold and new_CLE is below it, the egress
node reports that the PCN-admission-state is now "admit" for the
ingress-egress-aggregate. If old-CLE and new-CLE are both below the
threshold, no action is required. If new-CLE is above the threshold,
the PCN-admission-state is now "block" for the ingress-egress-
aggregate. The PCN-egress-node procedure in this case is described
in Section 3.2.2.
Note: In the case of SM, the CLE is an indication of where the
actual load is with respect to the PCN-admissible-rate. In fact,
a admission decision threshold of x implies that the expected
behavior of SM is to keep the mean load at the fraction x above
the PCN-admissible-rate. Hence with SM, the admission decision
threshold should be configured with a small value to avoid
unintended over-admission.
3.2.2. PCN-Egress-Node Role In Flow Termination
When the PCN-egress-node determines that the PCN-admission-state
computed on the basis of the updated CLE is "block", it generates a
report indicating the PCN-admission-state and providing the NM-count
normalized to a rate NM-rate in octets per second.
[Not sure what to do about identifying flows for ECMP.]
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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.
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
For each ingress-egress-aggregate, the ingress node continuously
measures the following quantity over successive intervals of equal
duration. That duration is suggested to be in the range of 100 to
500ms, and preferably the same as at the PCN-egress-node.
Sent-count:
Number of octets of PCN-traffic contained in PCN packets which are
admitted to the PCN domain.
When the PCN-ingress-node receives a report containing a value for
the unmarked PCN traffic rate NM-rate for a given ingress-egress-
aggregate, it takes the most recently observed value of Sent-count
and normalizes it to a rate Sent-rate in octets per second. It then
calculates the difference
Sent-rate - U * NM-rate,
where U is a configured network-wide constant. If this difference is
positive, it indicates a required reduction in the rate of admission
of PCN traffic to that ingress-egress-aggregate. Flows are selected
for termination with policy input from the PDP. The PCN-ingress-node
ceases to admit the selected flows.
If the computed difference is negative, the PCN-ingress-node takes no
further action.
3.4. Possible Extension to the Basic Algorithm
The termination mechanisms of SM and CL as described in [I-D.pcn-CL-
edge-behaviour] are both based on excess-rate metering and marking,
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however, there is a subtle difference between the two mechanisms
stemming from the fact that in SM, the bottleneck condition with
respect to the PCN-supportable-rate is not directly conveyed through
the markings. SM meters against the PCN-admissible- rate and infers
the bottleneck condition based on excess-marked traffic. The
inference process is vulnerable to inaccuracies, such as non-
uniformity in the marking distribution, and may result in over-
termination, especially when ingress-egress aggregation is low (< 50
flows).
If SM is used in a low IE-aggregation enviroment, to mitigate this
problem, a possible extension to the basic algorithm is to implement
an additional control, based on smoothing, to counter the inaccuracy
in the interval measurements and to safeguard the triggering of
termination. One such control can be implemented with a CLE-like
value (referred to as CLE-t). Note, the CLE-t is computed in exactly
the same way as described in Section 3.2.1, only with a different
value of k (so that the termination is independent of the admission
decision). The CLE-t is then compared to the value (U-1)/U. If CLE-t
is smaller, no termination should be applied, even if the computed U
* NM-rate is smaller than the Sent-rate. Otherwise, the aggregate
compares the U * NM-rate to Sent-rate to see if (and how much) to
terminate as described in Section 3.2.2.
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 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 [ID.PCN-baseline], thus reducing the work expected to be
performed in the data path of the high-speed routing equipment, and
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saving valuable real estate in the packet header.
4.2. Technical Specification
The technical specification of the PCN SM per domain behaviour is
provided by the contents of [RFC5559], [ID.PCN-baseline],
[ID.PCN-marking], 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 SM behaviour may be used to carry real-time traffic,
particularly voice and video.
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.
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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
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.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
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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
Michael Menth
University of Wuerzburg
Am Hubland
Wuerzburg D-97074
Germany
Phone: +49-931-888-6644
Email: menth@informatik.uni-wuerzburg.de
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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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