Internet Engineering Task Force A. Charny
Internet-Draft J. Zhang
Intended status: Informational Cisco Systems
Expires: April 30, 2010 G. Karagiannis
U. Twente
M. Menth
University of Wuerzburg
T. Taylor, Ed.
Huawei Technologies
October 27, 2009
PCN Boundary Node Behaviour for the Single Marking (SM) Mode of
Operation
draft-ietf-pcn-sm-edge-behaviour-01
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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
3.2.3. Reporting the PCN Data . . . . . . . . . . . . . . . . 7
3.3. Behaviour at the Decision Point . . . . . . . . . . . . . 7
3.3.1. Flow Admission . . . . . . . . . . . . . . . . . . . . 7
3.3.2. Flow Termination . . . . . . . . . . . . . . . . . . . 7
3.4. Behaviour of the Ingress Node . . . . . . . . . . . . . . 8
4. Identifying Ingress-Egress-Aggregates and Their Edge Points . 8
5. Specification of Diffserv Per-Domain Behaviour . . . . . . . . 8
5.1. Applicability . . . . . . . . . . . . . . . . . . . . . . 8
5.2. Technical Specification . . . . . . . . . . . . . . . . . 9
5.3. Attributes . . . . . . . . . . . . . . . . . . . . . . . . 9
5.4. Parameters . . . . . . . . . . . . . . . . . . . . . . . . 9
5.5. Assumptions . . . . . . . . . . . . . . . . . . . . . . . 9
5.6. Example Uses . . . . . . . . . . . . . . . . . . . . . . . 9
5.7. Environmental Concerns . . . . . . . . . . . . . . . . . . 9
5.8. Security Considerations . . . . . . . . . . . . . . . . . 10
6. Security Considerations . . . . . . . . . . . . . . . . . . . 10
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
9.1. Normative References . . . . . . . . . . . . . . . . . . . 10
9.2. Informative References . . . . . . . . . . . . . . . . . . 11
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 the "pre" of "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.
Some aspects of this specification are necessary for
interoperability, while others are simply suggestions. This document
attempts to make the distinction as it proceeds.
1.1. Terminology
RFC 2119 requirements language does not seem appropriate for an
Informational document. This document uses three levels of
requirement:
o "must" applies to requirements that affect the integrity of
operation of the complete system;
o "recommended" applies to procedures that appear to give superior
results at time of writing, but may be replaced by other
procedures directed to the same objective without affecting the
integrity of operation of the complete system;
o "suggested" applies to procedures that are not seen as superior at
time of writing, but appear to be valid approaches for meeting a
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particular objective.
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. Of course, 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 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. For further details see Section 3.2.1 and
Section 3.3.1.
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 marked to total PCN traffic (measured in octets) over
a short period. This specification suggests that the CLE be
calculated as an exponentially weighted moving average of the
ratios observed in successive fixed-length measurement intervals,
but the exact algorithms used are not critical to
interoperability. 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.
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;
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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];
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 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. The PCN-egress-node reports the
latest value of the PCN-admission-state to the decision point at
regular intervals. The decision point decides to admit or block new
PCN flows offered to a given ingress-egress-aggregate based on the
PCN-admission-state.
The decision to terminate flows requires measurement data from both
the PCN-ingress-node and the PCN-egress-node. Hence while the PCN-
admission-state is "block" for a given ingress-egress-aggregate, the
PCN-egress-node reports an estimate of the edge-to-edge supportable
PCN traffic rate along with the PCN-admission-state. If the admitted
traffic rate measured at the PCN-ingress-node exceeds the reported
estimate of the edge-to-edge supportable PCN traffic rate, the
decision point selects flows for termination to reduce this
difference to zero. The PCN-ingress-node ceases to admit the
selected flows.
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.
This specification recommends that the interval between calculation
of these quantities be in the range of 100 to 500ms to provide a
reasonable tradeoff between signalling demands on the network and the
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time taken to react to impending congestion.
This specification suggests that PCN-traffic be metered continuously,
that the counts of the number of octets of PCN traffic needed to
calculate the above rates be accumulated continuously throughout the
interval, and that the intervals themselves be of equal length, to
minimize the statistical variance introduced by the measurement
process itself.
3.2.1. PCN-Egress-Node Role In Flow Admission
Each time the egress node has calculated the rates listed above, the
egress node must calculate a ratio R of marked to total traffic. If
both of the rates are zero for the interval, the ratio R must be set
to zero. Otherwise, the egress node must calculate the ratio as:
R = ETM-rate / (NM-rate + ETM-rate).
The egress node must then use this ratio to update a congestion level
estimate (CLE, see Section 1.1).
Exponential smoothing is suggested for this purpose, so that
updated CLE = w*R + (1-w)*previous CLE.
The value of w depends on the length of the measurement interval: for
the equivalent system memory, a shorter interval calls for a smaller
smoothing constant. Simulation results
([I-D.babiarz-pcn-explicit-marking],
[I-D.zhang-pcn-performance-evaluation]) show that the effectiveness
of PCN is not sensitive to the specific value of w used.
The egress node now compares the updated CLE against a decision
threshold. If the CLE is less than the threshold, the PCN-admission-
state for the ingress-egress-aggregate is determined to be "admit",
otherwise it is determined to be "block".
Simulation results ([I-D.zhang-pcn-performance-evaluation] and
[Menth08f]) show that the process is also not sensitive to the
value of the decision threshold.
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 must compute
an estimate of the edge-to-edge supportable rate of PCN traffic for
the ingress-egress-aggregate concerned. This specification
recommends that the estimate be calculated by multiplying the latest
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calculated NM-rate by a configured factor U, which is the same for
all flows and all egress nodes in the domain.
3.2.3. Reporting the PCN Data
The PCN-egress-node must report the latest value of the PCN-
admission-state to the decision point each time it calculates it. If
the PCN-admission-state is "block", the egress node must also report
the estimate of the supportable edge-to-edge rate of PCN traffic
calculated in the previous section.
3.3. Behaviour at the Decision Point
3.3.1. Flow Admission
When the decision point (e.g., 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 decision point 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 in specific cases.
3.3.2. Flow Termination
When the report from the egress node includes an estimate of the
edge-to-edge supportable PCN traffic rate for the given ingress-
egress-aggregate, the decision point must fetch the rate at which
PCN-traffic has been admitted to the aggregate from the PCN-ingress-
node. If the rate of admitted traffic is greater than the estimate
of the edge-to-edge supportable PCN traffic rate for the given
ingress-egress-aggregate, 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 edge-to-edge supportable PCN traffic rate provided by
the egress node.
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].)
If the egress node has supplied a list of flow identifiers
(Section 3.2.2), the decision point first looks to terminate flows
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from that list. Flow selection may be guided by policy in specific
cases.
3.4. Behaviour of the Ingress Node
In a specific deployment, the PCN-ingress-node may be the decision
point. If so, it carries out the procedures described in the
previous section.
Aside from those procedures, the PCN-ingress-node has the
responsibility to provide the rate of admitted PCN traffic (octets
per second) on a specific ingress-egress-aggregate when the decision
point must determine how much flow to terminate in that aggregate.
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.
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. Some cases
will be particularly challenging, as when a packet is carried by an
MPLS tunnel through the ingress node to some node short of the egress
node, and then turns into an ordinary IP packet.
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 [ID.PCN-baseline], 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], [ID.PCN-baseline],
[ID.PCN-marking], and the present document.
5.3. Attributes
TBD -- basically low loss, low jitter. Low delay would be nice but
has to be quantified
5.4. Parameters
TBD. Don't think RFC 3068 is looking for the list of configurable
parameters given in the architecture document.
5.5. 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.
5.6. Example Uses
The PCN SM behaviour may be used to carry real-time traffic,
particularly voice and video.
5.7. Environmental Concerns
In some markets, traffic preemption is considered to be
impermissible. In such environments, flow termination would not be
enabled.
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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
TBD -- no time to rewrite. Ruediger for sure.
9. References
9.1. 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.
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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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