Internet Engineering Task Force A. Charny
Internet-Draft J. Zhang
Intended status: Experimental Protocol Cisco Systems
Expires: October 06, 2012 G. Karagiannis
U. Twente
M. Menth
University of Tuebingen
T. Taylor, Ed.
Huawei Technologies
April 06, 2012

PCN Boundary Node Behaviour for the Single Marking (SM) Mode of Operation


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 two PCN marking states, not-marked, and excess-traffic-marked. This behaviour is known informally as the Single Marking (SM) 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

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 October 06, 2012.

Copyright Notice

Copyright (c) 2012 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 ( 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.

Table of Contents

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].

This document describes an experimental edge node behaviour to implement PCN in a network. The experiment may be run in a network in which a substantial proportion of the traffic carried is in the form of inelastic flows and where admission control of micro-flows is applied at the edge. For the effects of PCN to be observable, the committed bandwidth (i.e., level of non-best-effort traffic) on at least some links of the network should be near or at link capacity. The amount of effort required to prepare the network for the experiment (see Section 5.1) may constrain the size of network to which it is applied. The purposes of the experiment are:

For the first two objectives, the experiment should run long enough for the network to experience sharp peaks of traffic in at least some directions. It would also be desirable to observe PCN performance in the face of failures in the network. A period in the order of a month or two in busy season may be enough. The third objective is more difficult, and could require observation over a period long enough for traffic demand to grow to the point where additional capacity must be provisioned at some points in the network.

Section 3 of this document specifies a detailed set of algorithms and procedures used to implement the PCN mechanisms for the SM 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 4).

Note that the terms "block" or "terminate" actually translate to one or more of several possible courses of action, as discussed in Section 3.6 of [RFC5559]. The choice of which action to take for blocked or terminated flows is a matter of local policy.

[RFC EDITOR'S NOTE: RFCyyyy is the published version of draft-ietf-pcn-cl-edge-behaviour.]

A companion document [RFCyyyy] specifies the Controlled Load (CL) 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 SM PCN-boundary-node behaviour is preceded by the phrase: "[SM-specific]". A similar distinction for CL-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]:

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 term from [RFC5670]:

To complete the list of borrowed terms, this document reuses the following terms and abbreviations defined in Section 3 of [ID.pcn-3-in-1]:

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.

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.

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.

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.

The state ("admit" or "block") derived by the Decision Point for a given ingress-egress-aggregate based on PCN packet marking statistics. The Decision Point decides to admit or block new flows offered to the aggregate based on the current value of the PCN-admission-state. For further details see Section 3.3.1.
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.

A configurable value against which the CLE is compared as part of the report suppression procedure. For further details, see Section 3.2.3.

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.

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 and ETM-rate as defined above and sends a report to the Decision Point, subject to the operation of the report suppression feature. For further details see Section 3.2.

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.

An 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.

A configurable interval used in the calculation of T_fail. For further details see Section 3.3.3.

2. [SM-Specific] Assumed Core Network Behaviour for SM

This section describes the assumed behaviour for PCN-interior-nodes in the PCN-domain. The SM mode of operation assumes that:

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 and excess-traffic-marked PCN-traffic for each ingress-egress-aggregate and reports them to the Decision Point. For a detailed description, see Section 3.2.

The PCN-ingress-node enforces flow admission and termination decisions. It also reports the rate of PCN-traffic 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 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.

Specification of a signaling protocol to report rates to the Decision Point is out of scope of this document. If the PCN-ingress-node is chosen as the Decision Point, [I-D.tsvwg-rsvp-pcn] specifies an appropriate signaling protocol.

Section 5.1.2 describes how to derive the filters by means of which PCN-ingress-nodes and PCN-egress-nodes are able to classify incoming packets into ingress-egress-aggregates.

3.2. Behaviour of the PCN-Egress-Node

3.2.1. Data Collection

The PCN-egress-node needs to 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.

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 and ETM-rate to the Decision Point each time that it calculates them.

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 and ETM-rate, the PCN-egress-node MUST calculate the congestion level estimate (CLE) for each measurement interval. The CLE is computed as:

    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.

  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, ETM-rate, and CLE that were calculated for the most recent measurement interval.

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. Decision Points 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.

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 the formula:

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".

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.

A performance study of this admission control method is presented in [MeLe12].

3.3.2. Flow Termination

[SM-specific] When the PCN-admission-state computed on the basis of the CLE is "block" for the given ingress-egress-aggregate, the Decision Point MUST request the PCN-ingress-node to provide an estimate of the rate (PCN-sent-rate) at which the PCN-ingress-node is receiving PCN-traffic that is destined for the given ingress-egress-aggregate. 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.

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. [SM-specific] The sustainable aggregate rate (SAR) for the given ingress-egress-aggregate is estimated using the formula:

    for the latest reported interval, where U is a configurable factor greater than one which is the same for all ingress-egress-aggregates. In effect, the value of the PCN-supportable-rate for each link is approximated by the expression

    rather than being calculated explicitly.

  2. The amount of traffic to be terminated is the difference:

    where PCN-sent-rate is the value provided by the PCN-ingress-node.


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 [Satoh10] and sections 4.2 and 4.3 of [MeLe10].)

In general, the selection of flows for termination MAY be guided by policy.

The Decision Point SHOULD log each round of termination as described in Section

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:

  1. T_fail = the configurable duration T_crit, if report suppression is not deployed;
  2. 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);
  3. 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 notify management. A log format is defined for that purpose in Section Other actions depend on local policy, but MAY include blocking of new flows destined for the PCN-egress-node concerned until another report is received from it. Termination of already-admitted flows is also possible, but could be triggered by "Destination unreachable" messages received at the PCN-ingress-node.

If a centralized Decision Point sends a request for the estimated value of PCN-sent-rate to a given PCN-ingress-node and fails to receive a response in a reasonable amount of time, the Decision Point SHOULD repeat the request once. [SM-specific] If the second request to the PCN-ingress-node also fails, the Decision Point SHOULD notify management. The log format defined in Section is also suitable for this case.

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 Decision Point requests it. The way this rate estimate is derived is a matter of implementation.

3.5. Summary of Timers and Associated Configurable Durations

Here is a summary of the timers used in the procedures just described:





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 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 management notifications 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. Specification of Diffserv Per-Domain Behaviour

This section provides the specification required by [RFC3086] for a per-domain behaviour.

4.1. Applicability

This section quotes [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 rerouting as a result of network failures) already-admitted flows may be terminated to protect the quality of service of the remaining flows. [SM-specific] The performance results in, e.g., [MeLe10], indicate that the SM boundary node behaviour is more likely to terminate too many flows under such circumstances than the CL boundary node behaviour described in [RFCyyyy].

[RFC EDITOR'S NOTE: please replace RFCyyyy above by the reference to the published version of draft-ietf-pcn-cl-edge-behaviour.]

4.2. Technical Specification

4.2.1. Classification and Traffic Conditioning

Packet classification and treatment at the PCN-ingress-node is described in Section 5.1 of [ID.pcn-3-in-1].

PCN packets are further classified as belonging or not belonging to an admitted flow. PCN packets not belonging to an admitted flow are "blocked". (See Section 1 for an understanding of how this term is interpreted.) Packets belonging to an admitted flow are policed to ensure that they adhere to the rate or flowspec that was negotiated during flow admission.

4.2.2. PHB Configuration

The PCN SM 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 can vary from one deployment to another. The PCN working group suggests using admission control for the following service classes (defined in [RFC4594]): [ID.pcn-3-in-1].

For a fuller discussion, see Appendix A of

4.3. 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.

4.4. Parameters

The set of parameters that needs to be configured at each PCN-node and at the Decision Point is described in Section 5.1.

4.5. Assumptions

It is assumed that a specific portion of link capacity has been reserved for PCN-traffic.

4.6. Example Uses

The PCN SM behaviour may be used to carry real-time traffic, particularly voice and video.

4.7. Environmental Concerns

The PCN SM per-domain behaviour could theoretically interfere with the use of end-to-end ECN due to reuse of ECN bits for PCN marking. Section 5.1 of [ID.pcn-3-in-1] describes the actions that can be taken to protect ECN signalling. Appendix B of that document provides further discussion of how ECN and PCN can co-exist.

4.8. Security Considerations

Please see the security considerations in [RFC5559] as well as those in [RFC2474] and [RFC2475].

5. Operational and Management Considerations

5.1. Deployment of the SM Edge Behaviour

Deployment of the PCN Single Marking edge behaviour requires the following steps:

5.1.1. Selection of Deployment Options and Global Parameters

The first set of decisions affects the operation of the network as a whole. To begin with, the operator needs to make basic design decisions such as whether the Decision Point is centralized or collocated with the PCN-ingress-nodes, and whether per-flow and aggregate resource signalling as described in [I-D.tsvwg-rsvp-pcn] is deployed in the network. After that, the operator needs to decide:

The following parameters affect the operation of PCN itself. The operator needs to choose:

5.1.2. Specification of Node- and Link-Specific Parameters

Filters are required at both the PCN-ingress-node and the PCN-egress-node to classify incoming PCN packets by ingress-egress-aggregate. Because of the potential use of multi-path routing in domains upstream of the PCN-domain, it is impossible to do such classification reliably at the PCN-egress-node based on the packet header contents as originally received at the PCN-ingress-node. (Packets with the same header contents could enter the PCN-domain at multiple PCN-ingress-nodes.) As a result, the only way to construct such filters reliably is to tunnel the packets from the PCN-ingress-node to the PCN-egress-node.

The PCN-ingress-node needs filters in order to place PCN packets into the right tunnel in the first instance, and also to satisfy requests from the Decision Point for admission rates into specific ingress-egress-aggregates. These filters select the PCN-egress-node, but not necessarily a specific path through the network to that node. As a result, they are likely to be stable even in the face of failures in the network, except when the PCN-egress-node itself becomes unreachable. If all PCN packets will be tunneled, the PCN-ingress-node also needs to know the address of the peer PCN-egress-node associated with each filter.

Operators may wish to give some thought to the provisioning of alternate egress points for some or all ingress-egress aggregates in case of failure of the PCN-egress-node. This could require the setting up of standby tunnels to these alternate egress points.

Each PCN-egress-node needs filters to classify incoming PCN packets by ingress-egress-aggregate, in order to gather measurements on a per-aggregate basis. If tunneling is used, these filters are constructed on the basis of the identifier of the tunnel from which the incoming packet has emerged (e.g. the source address in the outer header if IP encapsulation is used). The PCN-egress-node also needs to know the address of the Decision Point to which it sends reports for each ingress-egress-aggregate.

A centralized Decision Point needs to have the address of the PCN-ingress-node corresponding to each ingress-egress-aggregate. Security considerations require that information also be prepared for a centralized Decision Point and each PCN-edge-node to allow them to authenticate each other.

Turning to link-specific parameters, the operator needs to derive a value for the PCN-admissible-rate on each link in the network. The first two paragraphs of Section 5.2.2 of [RFC5559] discuss how these values may be derived. ([SM-specific] Confusingly, "PCN-admissible-rate" in the present context corresponds to "PCN-threshold-rate" in the cited paragraphs.)

5.1.3. Installation of Parameters and Policies

As discussed in the previous two sections, every PCN node needs to be provisioned with a number of parameters and policies relating to its behaviour in processing incoming packets. The Diffserv MIB [RFC3289] can be useful for this purpose, although it needs to be extended in some cases. This MIB covers packet classification, metering, counting, policing and dropping, and marking. The required extensions specifically include an encapsulation action following re-classification by ingress-egress-aggregate. In addition, the MIB has to be extended to include objects for marking the ECN field in the outer header at the PCN-ingress-node and an extension to the classifiers to include the ECN field at PCN-interior and PCN-egress-nodes. Finally, a new object may need to be defined at the PCN-interior-nodes to represent the packet-size-independent excess-traffic-marking metering algorithm.

The value for the PCN-admissible-rate on each link on a node appears as a metering parameter. Operators should take note of the need to deploy excess-traffic meters either on the ingress side or the egress of each interior link, but not both (Appendix B.2 of [RFC5670].

The following additional information has to be configured by other means (e.g., additional MIBs, NETCONF models).

At the PCN-egress-node:

At the Decision Point:

Depending on the testing strategy, it may be necessary to install the new configuration data in stages. This is discussed further below.

5.1.4. Activation and Verification of All Behaviours

It is certainly not within the scope of this document to advise on testing strategy, which operators undoubtedly have well in hand. Quite possibly an operator will prefer an incremental approach to activation and testing. Implementing the PCN marking scheme at PCN-ingress-nodes, corresponding scheduling behaviour in downstream nodes, and re-marking at the PCN-egress-nodes is a large enough step in itself to require thorough testing before going further.

Testing will probably involve the injection of packets at individual nodes and tracking of how the node processes them. This work can make use of the counter capabilities included in the Diffserv MIB. The application of these capabilities to the management of PCN is discussed in the next section.

5.2. Management Considerations

This section focuses on the use of event logging and the use of counters supported by the Diffserv MIB [RFC3289] for the various monitoring tasks involved in management of a PCN network.

5.2.1. Event Logging In the PCN Domain

It is anticipated that event logging using SYSLOG [RFC5424] will be needed for fault management and potentially for capacity management. Implementations MUST be capable of generating logs for the following events:

All of these logs are generated by the Decision Point. There is a strong likelihood in the first and third cases that the events are correlated with network failures at a lower level. This has implications for how often specific event types should be reported, so as not to contribute unnecessarily to log buffer overflow. Recommendations on this topic follow for each event report type.

The field names (e.g., HOSTNAME, STRUCTURED-DATA) used in the following subsections are defined in [RFC5424]. Logging Loss and Restoration of Contact

Section 3.3.3 describes the circumstances under which the Decision Point may determine that it has lost contact, either with a PCN-ingress-node or a PCN-egress-node, due to failure to receive an expected report. Loss of contact with a PCN-ingress-node is a case primarily applicable when the Decision Point is in a separate node. However, implementations MAY implement logging in the collocated case if the implementation is such that non-response to a request from the Decision Point function can occasionally occur due to processor load or other reasons.

The log reporting the loss of contact with a PCN-ingress-node or PCN-egress-node MUST include the following content:

The following values are also RECOMMENDED for the indicated fields in this log, subject to local practice:

If contact is not regained with a PCN-egress-node in a reasonable period of time (say, one minute), the log SHOULD be repeated, this time with a PRI value of 113, implying a Facility value of (14) "log alert" and a Severity value of (1) "Alert: action must be taken immediately". The reasoning is that by this time, any more general conditions should have been cleared, and the problem lies specifically with the PCN-egress-node concerned and the PCN application in particular.

Whenever a loss-of-contact log is generated for a PCN-egress-node, a log indicating recovery SHOULD be generated when the Decision Point next receives a report from the node concerned. The log SHOULD have the same content as just described for the loss-of-contact log, with the following differences: Logging Flow Termination Events

Section 3.3.2 describes the process whereby the Decision Point decides that flow termination is required for a given ingress-egress-aggregate, calculates how much flow to terminate, and selects flows for termination. This section describes a log that SHOULD be generated each time such an event occurs. (In the case where termination occurs in multiple rounds, one log SHOULD be generated per round.) The log may be useful in fault management, to indicate the service impact of a fault occuring in a lower-level subsystem. In the absence of network failures, it may also be used as an indication of an urgent need to review capacity utilization along the path of the ingress-egress-aggregate concerned.

The log reporting a flow termination event MUST include the following content:

The following values are also RECOMMENDED for the indicated fields in this log, subject to local practice:

5.2.2. Provision and Use of Counters

The Diffserv MIB [RFC3289] allows for the provision of counters along the various possible processing paths associated with an interface and flow direction. It is RECOMMENDED that the PCN-nodes be instrumented as described below. It is assumed that the cumulative counts so obtained will be collected periodically for use in debugging, fault management, and capacity management.

PCN-ingress-nodes SHOULD provide the following counts for each ingress-egress-aggregate. Since the Diffserv MIB installs counters by interface and direction, aggregation of counts over multiple interfaces may be necessary to obtain total counts by ingress-egress-aggregate. It is expected that such aggregation will be performed by a central system rather than at the PCN-ingress-node.

PCN-interior-nodes SHOULD provide the following counts for each interface, noting that a given packet MUST NOT be counted more than once as it passes through the node:

PCN-egress-nodes SHOULD provide the following counts for each ingress-egress-aggregate. As with the PCN-ingress-node, so with the PCN-egress-node it is expected that any necessary aggregation over multiple interfaces will be done by a central system.

The following continuously cumulative counters SHOULD be provided as indicated, but require new MIBs to be defined. If the Decision Point is not collocated with the PCN-ingress-node, the latter SHOULD provide a count of the number of requests for PCN-sent-rate received from the Decision Point and the number of responses returned to the Decision Point. The PCN-egress-node SHOULD provide a count of the number of reports sent to each Decision Point. Each Decision Point SHOULD provide the following:

6. Security Considerations

[RFC5559] provides a general description of the security considerations for PCN. This memo introduces one new consideration, related to the use of a centralized Decision Point. The Decision Point itself is a trusted entity. However, its use implies the existence of an interface on the PCN-ingress-node through which communication of policy decisions takes place. That interface is a point of vulnerability which must be protected from denial of service attacks.

7. IANA Considerations

This memo includes no request to IANA.

8. Acknowledgements

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.

David Harrington's careful AD review resulted not only in necessary changes throughout the document, but also the addition of the operations and management considerations (Section 5).

Finally, reviews by Joel Halpern and Brian Carpenter helped to clarify how ingress-egress-aggregates are distinguished (Joel) and handling of packets that cannot be carried successfully as PCN-packets (Brian). They also made other suggestions to improve the document, as did Stephen Farrell, Sean Turner, and Pete Resnick.

9. References

9.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2474] Nichols, K., Blake, S., Baker, F. and D.L. Black, "Definition of the Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers", RFC 2474, December 1998.
[RFC2475] Blake, S., Black, D.L., Carlson, M.A., 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.
[RFC3289] Baker, F., Chan, K. and A. Smith, "Management Information Base for the Differentiated Services Architecture", RFC 3289, May 2002.
[RFC5424] Gerhards, R., "The Syslog Protocol", RFC 5424, March 2009.
[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.
[ID.pcn-3-in-1] Briscoe, B., Moncaster, T. and M. Menth, "Encoding 3 PCN-States in the IP header using a single DSCP", March 2012.

9.2. Informative References

[RFC4594] Babiarz, J., Chan, K. and F. Baker, "Configuration Guidelines for DiffServ Service Classes", RFC 4594, August 2006.
[RFCyyyy] Charny, A., Karagiannis, G., Menth, M., Huang, F.Q. and T. Taylor, "PCN Boundary Node Behaviour for the Controlled Load (CL) Mode of Operation (Work in progress)", February 2012.
[MeLe10] Menth, M. and F. Lehrieder, "PCN-Based Measured Rate Termination", Computer Networks Journal (Elsevier) vol. 54, no. 13, pages 2099 - 2116, September 2010.
[Satoh10] 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.
[MeLe12] Menth, M. and F. Lehrieder, "Performance of PCN-Based Admission Control under Challenging Conditions, IEEE/ACM Transactions on Networking, vol. 20, no. 2", April 2012.
[I-D.tsvwg-rsvp-pcn] Karagiannis, G. and A. Bhargava, "Generic Aggregation of Resource ReSerVation Protocol (RSVP) for IPv4 And IPv6 Reservations over PCN domains (Work in progress)", July 2011.

Authors' Addresses

Anna Charny Cisco Systems USA EMail:
Xinyan (Joy) Zhang Cisco Systems 300 Apollo Drive Chelmsford, MA 01824 USA EMail:
Georgios Karagiannis U. Twente EMail:
Michael Menth University of Tuebingen Sand 13 Tuebingen, D-72076 Germany Phone: +49-7071-2970505 EMail:
Tom Taylor editor Huawei Technologies Ottawa, Ontario Canada EMail: