Internet Engineering Task Force A. Charny
Internet-Draft Cisco Systems
Intended status: Experimental F. Huang
Expires: December 24, 2011 Huawei Technologies
G. Karagiannis
U. Twente
M. Menth
University of Tuebingen
T. Taylor, Ed.
Huawei Technologies
June 22, 2011
PCN Boundary Node Behaviour for the Controlled Load (CL) Mode of
Operation
draft-ietf-pcn-cl-edge-behaviour-09
Abstract
Pre-congestion notification (PCN) is a means for protecting the
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 three PCN marking states, not-
marked, threshold-marked, and excess-traffic-marked. This behaviour
is known informally as the Controlled Load (CL) PCN-boundary-node
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
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
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 24, 2011.
Copyright Notice
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Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. [CL-Specific] Assumed Core Network Behaviour for CL . . . . . 8
3. Node Behaviours . . . . . . . . . . . . . . . . . . . . . . . 8
3.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.2. Behaviour of the PCN-Egress-Node . . . . . . . . . . . . . 9
3.2.1. Data Collection . . . . . . . . . . . . . . . . . . . 9
3.2.2. Reporting the PCN Data . . . . . . . . . . . . . . . . 10
3.2.3. Optional Report Suppression . . . . . . . . . . . . . 10
3.3. Behaviour at the Decision Point . . . . . . . . . . . . . 11
3.3.1. Flow Admission . . . . . . . . . . . . . . . . . . . . 11
3.3.2. Flow Termination . . . . . . . . . . . . . . . . . . . 12
3.3.3. Decision Point Action For Missing
PCN-Boundary-Node Reports . . . . . . . . . . . . . . 13
3.4. Behaviour of the Ingress Node . . . . . . . . . . . . . . 14
3.5. Summary of Timers and Associated Configurable Durations . 15
3.5.1. Recommended Values For the Configurable Durations . . 16
4. Identifying Ingress and Egress Nodes For PCN Traffic . . . . . 17
5. Specification of Diffserv Per-Domain Behaviour . . . . . . . . 17
5.1. Applicability . . . . . . . . . . . . . . . . . . . . . . 17
5.2. Technical Specification . . . . . . . . . . . . . . . . . 18
5.2.1. Classification and Traffic Conditioning . . . . . . . 18
5.2.2. PHB Configuration . . . . . . . . . . . . . . . . . . 19
5.3. Attributes . . . . . . . . . . . . . . . . . . . . . . . . 19
5.4. Parameters . . . . . . . . . . . . . . . . . . . . . . . . 19
5.5. Assumptions . . . . . . . . . . . . . . . . . . . . . . . 20
5.6. Example Uses . . . . . . . . . . . . . . . . . . . . . . . 21
5.7. Environmental Concerns . . . . . . . . . . . . . . . . . . 21
5.8. Security Considerations . . . . . . . . . . . . . . . . . 21
6. Security Considerations . . . . . . . . . . . . . . . . . . . 21
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 21
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 22
9.1. Normative References . . . . . . . . . . . . . . . . . . . 22
9.2. Informative References . . . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 23
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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 PCN-
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 PCN-boundary-nodes
about incipient overloads before any congestion occurs (hence the
"pre" part of "pre-congestion notification"). The level of marking
allows decisions to be made about whether to admit or terminate PCN-
flows. For more details see [RFC5559].
Section 3 of this document specifies a detailed set of algorithms and
procedures used to implement the PCN mechanisms for the CL mode of
operation. Since the algorithms depend on specific metering and
marking behaviour at the interior nodes, it is also necessary to
specify the assumptions made about PCN-interior-node behaviour
(Section 2). Finally, because PCN uses DSCP values to carry its
markings, a specification of PCN-boundary-node behaviour MUST include
the per domain behaviour (PDB) template specified in [RFC3086],
filled out with the appropriate content (Section 5).
[RFC EDITOR'S NOTE: you may choose to delete the following paragraph
and the "[CL-specific]" tags throughout this document when publishing
it, since they are present primarily to aid reviewers. RFCyyyy is
the published version of draft-ietf-pcn-sm-edge-behaviour.]
A companion document [RFCyyyy] specifies the Single Marking (SM) PCN-
boundary-node behaviour. This document and [RFCyyyy] have a great
deal of text in common. To simplify the task of the reader, the text
in the present document that is specific to the CL PCN-boundary-node
behaviour is preceded by the phrase: "[CL-specific]". A similar
distinction for SM-specific text is made in [RFCyyyy].
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].
This document uses the following terms defined in Section 2 of
[RFC5559]:
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o PCN-domain;
o PCN-ingress-node;
o PCN-egress-node;
o PCN-interior-node;
o PCN-boundary-node;
o PCN-flow;
o ingress-egress-aggregate (IEA);
o [CL-specific] PCN-threshold-rate;
o PCN-excess-rate;
o PCN-admissible-rate;
o PCN-supportable-rate;
o PCN-marked;
o [CL-specific] threshold-marked;
o excess-traffic-marked.
It also uses the terms PCN-traffic and PCN-packet, for which the
definition is repeated from [RFC5559] because of their importance to
the understanding of the text that follows:
PCN-traffic, PCN-packets, PCN-BA
A PCN-domain carries traffic of different Diffserv behaviour
aggregates (BAs) [RFC2474]. The PCN-BA uses the PCN mechanisms to
carry PCN-traffic, and the corresponding packets are PCN-packets.
The same network will carry traffic of other Diffserv BAs. The
PCN-BA is distinguished by a combination of the Diffserv codepoint
and the ECN field.
This document uses the following terms from [RFC5670]:
o [CL-specific] threshold-meter;
o excess-traffic-meter.
To complete the list of borrowed terms, this document reuses the
following terms and abbreviations defined in Section 3 of [RFC5696]:
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o not-PCN codepoint;
o Not-marked (NM) codepoint;
o PCN-marked (PM) codepoint;
o [CL-specific] Experimental (EXP) codepoint.
This document defines the following additional terms:
Decision Point
The node that makes the decision about which flows to admit and to
terminate. In a given network deployment, this can be the PCN-
ingress-node or a centralized control node. In either case, the
PCN-ingress-node is the point where the decisions are enforced.
NM-rate
The rate of not-marked PCN-traffic received at a PCN-egress-node
for a given ingress-egress-aggregate in octets per second. For
further details see Section 3.2.1.
[CL-specific] ThM-rate
The rate of threshold-marked PCN-traffic received at a PCN-egress-
node for a given ingress-egress-aggregate in octets per second.
For further details see Section 3.2.1.
ETM-rate
The rate of excess-traffic-marked PCN-traffic received at a PCN-
egress-node for a given ingress-egress-aggregate in octets per
second. For further details see Section 3.2.1.
PCN-sent-rate
The rate of PCN-traffic received at a PCN-ingress-node and
destined for a given ingress-egress-aggregate in octets per
second. For further details see Section 3.4.
Congestion level estimate (CLE)
The ratio of PCN-marked to total PCN-traffic (measured in octets)
received for a given ingress-egress-aggregate during a given
measurement period. The CLE is used to derive the PCN-admission-
state (Section 3.3.1) and is also used by the report suppression
procedure (Section 3.2.3) if report suppression is activated.
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
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PCN-admission-state. For further details see Section 3.3.1.
Sustainable aggregate rate (SAR)
The estimated maximum rate of PCN-traffic that can be carried in a
given ingress-egress-aggregate at a given moment without risking
degradation of quality of service for the admitted flows. The
intention is that if the PCN-sent-rate of every ingress-egress-
aggregate passing through a given link is limited to its
sustainable aggregate rate, the total rate of PCN-traffic flowing
through the link will be limited to the PCN-supportable-rate for
that link. An estimate of the sustainable aggregate rate for a
given ingress-egress-aggregate is derived as part of the flow
termination procedure, and is used to determine how much PCN-
traffic needs to be terminated. For further details see
Section 3.3.2.
CLE-reporting-threshold
A configurable value against which the CLE is compared as part of
the report suppression procedure. For further details, see
Section 3.2.3.
CLE-limit
A configurable value against which the CLE is compared to
determine the PCN-admission-state for a given ingress-egress-
aggregate. For further details, see Section 3.3.1.
T-meas
A configurable time interval that defines the measurement period
over which the PCN-egress-node collects statistics relating to
PCN-traffic marking. At the end of the interval the PCN-egress-
node calculates the values NM-rate, [CL-specific] ThM-rate, and
ETM-rate as defined and sends a report to the Decision Point,
subject to the operation of the report suppression feature. For
further details see Section 3.2.
T-maxsuppress
A configurable time interval after which the PCN-egress-node MUST
send a report to the Decision Point for a given ingress-egress-
aggregate regardless of the most recent values of the CLE. This
mechanism provides the Decision Point with a periodic confirmation
of liveness when report suppression is activated. For further
details, see Section 3.2.3.
T-fail
A configurable interval after which the Decision Point concludes
that communication from a given PCN-egress-node has failed if it
has received no reports from the PCN-egress-node during that
interval. For further details see Section 3.3.3.
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2. [CL-Specific] Assumed Core Network Behaviour for CL
This section describes the assumed behaviour for PCN-interior-nodes
in the PCN-domain. The CL mode of operation assumes that:
o PCN-interior-nodes perform both threshold-marking and excess-
traffic-marking of PCN-packets, according to the rules specified
in [RFC5670];
o excess-traffic-marking of PCN-packets uses the PCN-Marked (PM)
codepoint defined in [RFC5696];
o threshold-marking of PCN-packets uses the EXP codepoint defined in
[RFC5696];
o the PCN-domain satisfies the conditions specified in [RFC5696];
o on each link the reference rate for the threshold-meter is
configured to be equal to the PCN-admissible-rate for the link;
o on each link the reference rate for the excess-traffic-meter is
configured to be equal to the PCN-supportable-rate for the link;
o the set of valid codepoint transitions is as shown in Section 4.2
of [RFC5696].
3. Node Behaviours
3.1. Overview
This section describes the behaviour of the PCN-ingress-node, PCN-
egress-node, and the Decision Point (which MAY be collocated with the
PCN-ingress-node).
The PCN-egress-node collects the rates of not-marked, [CL-specific]
threshold-marked, and excess-traffic-marked PCN-traffic for each
ingress-egress-aggregate and reports them to the Decision Point.
[CL-specific] It MAY also identify and report PCN-flows that have
experienced excess-traffic-marking. 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 sent 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
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selects flows to terminate based on the information provided by the
PCN-ingress-node and PCN-egress-node 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 the PCN-traffic it receives in order
to calculate the following rates for each ingress-egress-aggregate
passing through it. These rates SHOULD be calculated at the end of
each measurement period based on the PCN-traffic observed during that
measurement period. The duration of a measurement period is equal to
the configurable value T-meas. For further information see
Section 3.5.
o NM-rate: octets per second of PCN-traffic in PCN-packets that are
not-marked (i.e., marked with the NM codepoint);
o [CL-specific] ThM-rate: octets per second of PCN-traffic in PCN-
packets that are threshold-marked (i.e., marked with the EXP
codepoint);
o ETM-rate: octets per second of PCN-traffic in PCN-packets that are
excess-traffic-marked (i.e., marked with the PM codepoint).
Informative note: metering the PCN-traffic continuously and using
equal-length measurement intervals minimizes the statistical variance
introduced by the measurement process itself. On the other hand, the
operation of PCN is not affected if the starting and ending times of
the measurement intervals for different ingress-egress-aggregates are
different.
[CL-specific] As a configurable option, the PCN-egress-node MAY
record flow identifiers of the PCN-flows for which excess-traffic-
marked packets have been observed during this measurement interval.
If this set is large (e.g., more than 20 flows), the PCN-egress-node
MAY record only the most recently excess-traffic-marked PCN-flow
identifiers rather than the complete set.
These can be used by the Decision Point when it selects flows for
termination. In networks using multipath routing it is possible
that congestion is not occurring on all paths carrying a given
ingress-egress-aggregate. Assuming that specific PCN-flows are
routed via specific paths, identifying the PCN-flows that are
experiencing excess-traffic-marking helps to avoid termination of
PCN-flows not contributing to congestion.
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3.2.2. Reporting the PCN Data
Unless the report suppression option described in Section 3.2.3 is
activated, the PCN-egress-node MUST report the latest values of NM-
rate, [CL-specific] ThM-rate, and ETM-rate to the Decision Point each
time that it calculates them.
[CL-specific] If the PCN-egress-node recorded a set of flow
identifiers of PCN-flows for which excess-traffic-marking was
observed in the most recent measurement interval, then it MUST also
include these identifiers in the report.
3.2.3. Optional Report Suppression
Report suppression MUST be provided as a configurable option, along
with two configurable parameters, the CLE-reporting-threshold and the
maximum report suppression interval T-maxsuppress. The default value
of the CLE-reporting-threshold is zero. The CLE-reporting-threshold
MUST NOT exceed the CLE-limit configured at the Decision Point. For
further information on T-maxsuppress see Section 3.5.
If the report suppression option is enabled, the PCN-egress-node MUST
apply the following procedure to decide whether to send a report to
the Decision Point, rather than sending a report automatically at the
end of each measurement interval.
1. As well as the quantities NM-rate, [CLE-specific] ThM-rate, and
ETM-rate, the PCN-egress-node MUST calculate the congestion level
estimate (CLE) for each measurement interval. The CLE is
computed as:
[CL-specific]
CLE = (ThM-rate + ETM-rate) / (NM-rate + ThM-rate + ETM-rate)
if any PCN-traffic was observed, or CLE = 0 if all the rates are
zero.
2. If the CLE calculated for the latest measurement interval is
greater than the CLE-reporting-threshold and/or the CLE
calculated for the immediately previous interval was greater than
the CLE-reporting-threshold, then the PCN-egress-node MUST send a
report to the Decision Point. The contents of the report are
described below.
The reason for taking into account the CLE of the previous
interval is to ensure that the Decision Point gets immediate
feedback if the CLE has dropped below CLE-reporting-threshold.
This is essential if the Decision Point is running the flow
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termination procedure and observing whether (further) flow
termination is needed. See Section 3.3.2.
3. If an interval T-maxsuppress has elapsed since the last report
was sent to the Decision Point, then the PCN-egress-node MUST
send a report to the Decision Point regardless of the CLE value.
4. If neither of the preceding conditions holds, the PCN-egress-node
MUST NOT send a report for the latest measurement interval.
Each report sent to the Decision Point when report suppression has
been activated MUST contain the values of NM-rate, [CL-specific] ThM-
rate, ETM-rate, and CLE that were calculated for the most recent
measurement interval. [CL-specific] If the PCN-egress-node recorded
a set of flow identifiers of PCN-flows for which excess-traffic-
marking was observed in the most recent measurement interval, then it
MUST also include these identifiers in the report.
The above procedure ensures that at least one report is sent per
interval (T-maxsuppress + T-meas). This demonstrates to the Decision
Point that both the PCN-egress-node and the communication path
between that node and the Decision Point are in operation.
3.3. Behaviour at the Decision Point
Operators can choose to use PCN procedures just for flow admission,
or just for flow termination, or for both. A compliant 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.
If PCN-based flow termination is enabled but PCN-based flow admission
is not, flow termination operates as specified in this document.
Logically, some other system of flow admission control is in
operation, but the description of such a system is out of scope of
this document and depends on local arrangements.
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). 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 based on the other values provided in the report, using
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the formula:
[CL-specific]
CLE = (ThM-rate + ETM-rate) / (NM-rate + ThM-rate + ETM-rate)
if any PCN-traffic was observed, or CLE = 0 if all the rates are
zero.
The Decision Point MUST compare the reported or calculated CLE to a
configurable value, the CLE-limit. If the CLE is less than the CLE-
limit, the PCN-admission-state for that aggregate MUST be set to
"admit"; otherwise it MUST be set to "block".
[CL-specific] The outcome of the comparison is not very sensitive
to the value of the CLE-limit in practice, because when threshold-
marking occurs it tends to persist long enough that threshold-
marked traffic becomes a large proportion of the received traffic
in a given interval.
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
[CL-specific] When the report from the PCN-egress-node includes a
non-zero value of the ETM-rate for some ingress-egress-aggregate, the
Decision Point MUST request the PCN-ingress-node to provide an
estimate of the rate (PCN-sent-rate) at which the PCN-ingress-node is
receiving PCN-traffic that is destined for the given ingress-egress-
aggregate.
If the Decision Point is collocated with the PCN-ingress-node, the
request and response are internal operations.
The Decision Point MUST then wait, for both the requested rate from
the PCN-ingress-node and the next report from the PCN-egress-node for
the ingress-egress-aggregate concerned. If this next egress node
report also includes a non-zero value for the ETM-rate, the Decision
Point MUST determine the amount of PCN-traffic to terminate using the
following steps:
1. [CL-specific] The sustainable aggregate rate (SAR) for the given
ingress-egress-aggregate is estimated by the sum:
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SAR = NM-rate + ThM-rate
for the latest reported interval.
2. The amount of traffic to be terminated is the difference:
PCN-sent-rate - SAR,
where PCN-sent-rate is the value provided by the PCN-ingress-
node.
See Section 3.3.3 for a discussion of appropriate actions if the
Decision Point fails to receive a timely response to its request for
the PCN-sent-rate.
If the difference calculated in the second step is positive, the
Decision Point SHOULD select PCN-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 PCN-flows
(e.g., from resource signalling used to establish the flows), it MAY
choose to complete its selection of PCN-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 [IEEE-Satoh] and sections 4.2 and 4.3 of [MeLe10].)
In general, the selection of flows for termination MAY be guided by
policy. [CL-specific] If the egress node has supplied a list of
identifiers of PCN-flows that experienced excess-traffic-marking
(Section 3.2), the Decision Point SHOULD first consider terminating
PCN-flows in that list.
3.3.3. Decision Point Action For Missing PCN-Boundary-Node Reports
The Decision Point SHOULD start a timer t-recvFail when it receives a
report from the PCN-egress-node. t-recvFail is reset each time a new
report is received from the PCN-egress-node. t-recvFail expires if it
reaches the value T-fail. T-fail is calculated according to the
following logic:
a. T-fail = the configurable duration T-crit, if report suppression
is not deployed;
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b. T-fail = T-crit also if report suppression is deployed and the
last report received from the PCN-egress-node contained a CLE
value greater than CLE-reporting-threshold (Section 3.2.3);
c. T-fail = 3 * T-maxsuppress (Section 3.2.3) if report suppression
is deployed and the last report received from the PCN-egress-node
contained a CLE value less than or equal to CLE-reporting-
threshold.
If timer t-recvFail expires for a given PCN-egress-node, the Decision
Point SHOULD raise an alarm to management. A Decision Point
collocated with a PCN-ingress-node SHOULD cease to admit PCN-flows to
the ingress-egress-aggregate associated with the given PCN-egress-
node, until it again receives a report from that node. A centralized
Decision Point MAY cease to admit PCN-flows to all ingress-egress-
aggregates destined to the PCN-egress-node concerned, until it again
receives a report from that node.
A centralized Decision Point SHOULD start a timer t-sndFail when it
sends a request for the estimated value of PCN-sent-rate to a given
PCN-ingress-node. If the Decision Point fails to receive a response
from the PCN-ingress-node before t-sndFail reaches the configurable
value T-crit, the Decision Point SHOULD repeat the request but MAY
also use ETM-rate as an estimate of the amount of traffic to be
terminated in place of the quantity
PCN-sent-rate - SAR
specified in Section 3.3.2. Because this will over-estimate the
amount of traffic to be terminated due to dropping of PCN-packets by
interior nodes, the Decision Point SHOULD use multiple rounds of
termination under these circumstances. If the second request to the
PCN-ingress-node also fails, the Decision Point SHOULD raise an alarm
to management.
The use of T-crit is an approximation. A more precise limit would
be of the order of two round-trip times, plus an allowance for
processing at each end, plus an allowance for variance in these
values.
See Section 3.5 for suggested values of the configurable durations
T-crit and T-maxsuppress.
3.4. Behaviour of the Ingress Node
The PCN-ingress-node MUST provide the estimated current rate of PCN-
traffic received at that node and destined for a given ingress-
egress-aggregate in octets per second (the PCN-sent-rate) when the
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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.
3.5. Summary of Timers and Associated Configurable Durations
Here is a summary of the timers used in the procedures just
described:
t-meas
Where used: PCN-egress-node.
Used in procedure: data collection (Section 3.2.1).
Incidence: one per ingress-egress-aggregate.
Reset: immediately on expiry.
Expiry: when it reaches the configurable duration T-meas.
Action on expiry: calculate NM-rate, [CL-specific] ThM-rate,
and ETM-rate and proceed to the applicable reporting procedure
(Section 3.2.2 or Section 3.2.3).
t-maxsuppress
Where used: PCN-egress-node.
Used in procedure: report suppression (Section 3.2.3).
Incidence: one per ingress-egress-aggregate.
Reset: when the next report is sent after expiry.
Expiry: when it reaches the configurable duration
T-maxsuppress.
Action on expiry: send a report to the Decision Point the next
time the reporting procedure (Section 3.2.3) is invoked,
regardless of the value of CLE.
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t-recvFail
Where used: Decision Point.
Used in procedure: failure detection (Section 3.3.3).
Incidence: one per ingress-egress-aggregate.
Reset: when a report is received for the ingress-egress-
aggregate.
Expiry: when it reaches the calculated duration T-fail. As
described in Section 3.3.3, T-fail is either equal to the
configured duration T-crit or to the calculated value 3 *
T-maxsuppress, where T-maxsuppress is a configured duration.
Action on expiry: raise an alarm to management, and possibly
other actions.
t-sndFail
Where used: centralized Decision Point.
Used in procedure: failure detection (Section 3.3.3).
Incidence: only as required, one per outstanding request to a
PCN-ingress-node.
Started: when a request for the value of PCN-sent-traffic for a
given ingress-egress-aggregate is sent to the PCN-ingress-node.
Terminated without action: when a response is received before
expiry.
Expiry: when it reaches the configured duration T-crit.
Action on expiry: repeat the request, but use an approximation
for the estimate of amount of traffic to terminate. After two
failures, raise an alarm to management and stop repeating the
request.
3.5.1. Recommended Values For the Configurable Durations
The timers just described depend on three configurable durations,
T-meas, T-maxsuppress, and T-crit. The recommendations given below
for the values of these durations are all related to the intended PCN
reaction time of 1 to 3 seconds. However, they are based on
judgement rather than operational experience or mathematical
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derivation.
The value of T-meas is RECOMMENDED to be of the order of 100 to 500
ms to provide a reasonable tradeoff between demands on network
resources (PCN-egress-node and Decision Point processing, network
bandwidth) and the time taken to react to impending congestion.
The value of T-maxsuppress is RECOMMENDED to be on the order of 3 to
6 seconds, for similar reasons to those for the choice of T-meas.
The value of T-crit SHOULD NOT be less than 3 * T-meas. Otherwise it
could cause too many alarms to be raised due to transient conditions
in the PCN-egress-node or along the signalling path. A reasonable
upper bound on T-crit is in the order of 3 seconds.
4. Identifying Ingress and Egress Nodes For PCN Traffic
The operation of PCN depends on the ability of the PCN-ingress-node
to identify the ingress-egress-aggregate to which each new PCN-flow
belongs and the ability of the egress node to identify the ingress-
egress-aggregate to which each received PCN-packet belongs. If the
Decision Point is collocated with the PCN-ingress-node, the PCN-
egress-node also needs to associate each ingress-egress-aggregate
with the address of the PCN-ingress-node to which it sends its
reports.
The means by which this is done depends on the packet routing
technology in use in the network. The procedure to provide the
required information is out of scope for this document.
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 quotes [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 rerouting as a result of
network failures) already-admitted flows MAY be terminated to protect
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the quality of service of the remaining flows. [CL-specific] The
performance results in, e.g., [MeLe10], indicate that the CL boundary
node behaviour provides better service outcomes under such
circumstances than the SM boundary node behaviour described in
[RFCyyyy], because CL is less likely to terminate PCN-flows
unnecessarily.
[RFC EDITOR'S NOTE: please replace RFCyyyy above by the reference to
the published version of draft-ietf-pcn-sm-edge-behaviour.]
5.2. Technical Specification
5.2.1. Classification and Traffic Conditioning
This section paraphrases the applicable portions of Sections 3.6 and
4.2 of [RFC5559].
Packets at the ingress to the domain are classified as either PCN or
non-PCN. Non-PCN packets MAY share the network with PCN packets
within the domain. Because the encoding specified in [RFC5696] and
used in this document requires the use of the ECN fields, PCN-
ingress-nodes MUST prevent ECN-capable traffic that uses the same
DSCP as PCN from entering the PCN-domain directly. The PCN-ingress-
node can accomplish this in three ways. The choice between these
depends on local policy.
o ECN-capable traffic MAY be dropped. This policy is NOT
RECOMMENDED, since it prevents the proper operation of end-to-end
ECN as a means of controlling congestion.
o ECN-capable traffic MAY be assigned a different DSCP from PCN
traffic. This could mean that it is relegated to a lower-priority
behaviour aggregate.
o ECN-capable traffic MAY be tunneled across the PCN-domain. If
this is done, the PCN-ingress-node MUST mark packets as either
not-PCN or PCN-not-marked only after the encapsulation of the
packet, including any initial setting of the ECN field, has been
completed.
PCN packets are further classified as belonging or not belonging to
an admitted flow. PCN packets not belonging to an admitted flow are
dropped. (This assumes that requests for flow admission are
signalled in advance of the arrival of the flows themselves.)
Packets belonging to an admitted flow are policed to ensure that they
adhere to the rate or flowspec that was negotiated during flow
admission.
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5.2.2. PHB Configuration
The PCN CL boundary node behaviour is a metering and marking
behaviour rather than a scheduling behaviour. As a result, while the
encoding uses a single DSCP value, that value MAY vary from one
deployment to another. The PCN working group suggests using
admission control for the following service classes (defined in
[RFC4594]):
o Telephony (EF)
o Real-time interactive (CS4)
o Broadcast Video (CS3)
o Multimedia Conferencing (AF4)
For a fuller discussion, see Section A.1 of Appendix A of [RFC5696].
5.3. Attributes
The purpose of this per-domain behaviour is to achieve low loss and
jitter for the target class of traffic. The design requirement for
PCN was that recovery from overloads through the use of flow
termination SHOULD happen within 1-3 seconds. PCN probably performs
better than that.
5.4. Parameters
In the list that follows, note that most PCN-ingress-nodes are also
PCN-egress-nodes, and vice versa. Furthermore, the PCN-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 markings to be applied to PCN-traffic.
o Reference rates on each link for the [CL-specific] threshold-meter
and the excess-traffic-meter; see Section 2.
o The information needed to distinguish PCN-traffic belonging to a
given ingress-egress-aggregate.
Parameters at the PCN-egress-node:
----------------------------------
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o The measurement interval T-meas.
o Activation/deactivation of report suppression and, if report
suppression is activated, the values of the CLE-reporting-
threshold and T-maxsuppress.
o [CL-specific] Activation/deactivation of recording of individual
flow identifiers when excess-traffic-marked PCN-traffic is
observed.
o The information needed to distinguish PCN-traffic belonging to a
given ingress-egress-aggregate.
o The marking rules for re-marking PCN-traffic leaving the PCN
domain.
Parameters at each interior node:
---------------------------------
o Reference rates on each link for the [CL-specific] threshold-meter
and the excess-traffic-meter; see Section 2.
o The markings to be applied to PCN-traffic, including the
identification of PCN-packets and the encodings to indicate
excess-traffic-marking and [CL-specific] threshold-marking.
Parameters at the Decision Point:
---------------------------------
o Activation/deactivation of PCN-based flow admission.
o Activation/deactivation of PCN-based flow termination.
o The value of CLE-limit.
o The maximum interval T-fail between reports from a given PCN-
egress-node, for detecting failure of communications with that
node.
o The information needed to map each ingress-egress-aggregate to the
corresponding PCN-ingress-node and PCN-egress-node.
5.5. Assumptions
It is assumed that a specific portion of link capacity has been
reserved for PCN-traffic.
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5.6. Example Uses
The PCN CL behaviour MAY be used to carry real-time traffic,
particularly voice and video.
5.7. Environmental Concerns
The PCN CL per-domain behaviour can 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 [RFC5559] 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 content of this memo bears a family resemblance to
[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.
Ruediger Geib, Philip Eardley, and Bob Briscoe have helped to shape
the present document with their comments. Toby Moncaster gave a
careful review to get it into shape for Working Group Last Call.
Amongst the authors, Michael Menth deserves special mention for his
constant and careful attention to both the technical content of this
document and the manner in which it was expressed.
9. References
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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.
[RFC3086] Nichols, K. and B. Carpenter, "Definition of
Differentiated Services Per Domain Behaviors and Rules for
their Specification", RFC 3086, April 2001.
[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
[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.
[IEEE-Satoh]
Satoh, D. and H. Ueno, ""Cause and Countermeasure of
Overtermination for PCN-Based Flow Termination",
Proceedings of IEEE Symposium on Computers and
Communications (ISCC '10), pp. 155-161, Riccione, Italy",
June 2010.
[MeLe10] Menth, M. and F. Lehrieder, "PCN-Based Measured Rate
Termination", Computer Networks Journal (Elsevier) vol.
54, no. 13, pages 2099 - 2116, September 2010.
[RFC4594] Babiarz, J., Chan, K., and F. Baker, "Configuration
Guidelines for DiffServ Service Classes", RFC 4594,
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August 2006.
[RFC6040] Briscoe, B., "Tunnelling of Explicit Congestion
Notification", RFC 6040, November 2010.
[RFCyyyy] Charny, A., Zhang, J., Karagiannis, G., Menth, M., and T.
Taylor, "PCN Boundary Node Behaviour for the Single
Marking (SM) Mode of Operation (Work in progress)",
December 2010.
Authors' Addresses
Anna Charny
Cisco Systems
300 Apollo Drive
Chelmsford, MA 01824
USA
Email: acharny@cisco.com
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
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Michael Menth
University of Tuebingen
Sand 13
Tuebingen D-72076
Germany
Phone: +49-7071-2970505
Email: menth@informatik.uni-tuebingen.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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