Congestion and Pre-Congestion B. Briscoe Notification BT Internet-Draft T. Moncaster Intended status: Experimental Independent Expires: January 13, 2011 M. Menth University of Wuerzburg July 12, 2010 Encoding 3 PCN-States in the IP header using a single DSCP draft-ietf-pcn-3-in-1-encoding-03 Abstract The objective of Pre-Congestion Notification (PCN) is to protect the quality of service (QoS) of inelastic flows within a Diffserv domain. On every link in the PCN domain, the overall rate of the PCN-traffic is metered, and PCN-packets are appropriately marked when certain configured rates are exceeded. Egress nodes provide decision points with information about the PCN-marks of PCN-packets which allows them to take decisions about whether to admit or block a new flow request, and to terminate some already admitted flows during serious pre- congestion. This document specifies how PCN-marks are to be encoded into the IP header by re-using the Explicit Congestion Notification (ECN) codepoints within a PCN-domain. This encoding builds on the baseline encoding of RFC5696 and provides for three different PCN marking states using a single DSCP: not-marked (NM), threshold-marked (ThM) and excess-traffic-marked (ETM). Hence, it is called the 3-in-1 PCN encoding. 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 January 13, 2011. Briscoe, et al. Expires January 13, 2011 [Page 1] Internet-Draft 3-in-1 PCN Encoding July 2010 Copyright Notice Copyright (c) 2010 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. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Changes in This Version (to be removed by RFC Editor) . . 4 2. Requirements Language . . . . . . . . . . . . . . . . . . . . 5 2.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5 3. Requirements for and Applicability of 3-in-1 PCN Encoding . . 5 3.1. PCN Requirements . . . . . . . . . . . . . . . . . . . . . 5 3.2. Requirements Imposed by Baseline Encoding . . . . . . . . 6 3.3. Applicability of 3-in-1 PCN Encoding . . . . . . . . . . . 6 4. Definition of 3-in-1 PCN Encoding . . . . . . . . . . . . . . 7 5. Behaviour of a PCN Node Compliant with the 3-in-1 PCN Encoding . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 6. Backward Compatibility . . . . . . . . . . . . . . . . . . . . 8 6.1. Backward Compatibility with Pre-existing PCN Implementations . . . . . . . . . . . . . . . . . . . . . 8 6.2. Recommendations for the Use of PCN Encoding Schemes . . . 9 6.2.1. Use of Both Excess-Traffic-Marking and Threshold-Marking . . . . . . . . . . . . . . . . . . 9 6.2.2. Unique Use of Excess-Traffic-Marking . . . . . . . . . 9 6.2.3. Unique Use of Threshold-Marking . . . . . . . . . . . 9 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 8. Security Considerations . . . . . . . . . . . . . . . . . . . 10 9. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 10 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10 11. Comments Solicited . . . . . . . . . . . . . . . . . . . . . . 10 12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11 12.1. Normative References . . . . . . . . . . . . . . . . . . . 11 12.2. Informative References . . . . . . . . . . . . . . . . . . 11 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12 Briscoe, et al. Expires January 13, 2011 [Page 2] Internet-Draft 3-in-1 PCN Encoding July 2010 1. Introduction The objective of Pre-Congestion Notification (PCN) [RFC5559] 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 flow termination to decide whether to terminate some already admitted flows during serious pre-congestion. To achieve this, the overall rate of PCN-traffic is metered on every link in the domain, and PCN-packets are appropriately marked when certain configured rates are exceeded. These configured rates are below the rate of the link thus providing notification to boundary nodes about overloads before any congestion occurs (hence "pre- congestion notification"). Two metering and marking functions are proposed in [RFC5670] that are configured with reference rates. Threshold- marking marks all PCN packets once their traffic rate on a link exceeds the configured reference rate (PCN-threshold-rate). Excess-traffic-marking marks only those PCN packets that exceed the configured reference rate (PCN-excess-rate). The PCN-excess-rate is typically larger than the PCN-threshold-rate [RFC5559]. Egress nodes monitor the PCN-marks of received PCN-packets and provide information about the PCN-marks to decision points which take decisions about flow admission and termination on this basis [I-D.ietf-pcn-cl-edge-behaviour], [I-D.ietf-pcn-sm-edge-behaviour]. The baseline encoding defined in [RFC5696] describes how two PCN marking states can be encoded using a single Diffserv codepoint. However, to support the application of two different marking algorithms in a PCN-domain, for example as required in [I-D.ietf-pcn-cl-edge-behaviour], three PCN marking states are needed. This document describes an extension to the baseline encoding that adds a third PCN marking state in the IP header, still using a single Diffserv codepoint. This encoding scheme is called ao˛3-in-1 PCN encodingaoˇ. All PCN encoding schemes require an additional marking state to indicate non-PCN traffic. Therefore, four codepoints are required to encode three PCN marking states. This document only concerns the PCN wire protocol encoding for all IP headers, whether IPv4 or IPv6. It makes no changes or recommendations concerning algorithms for congestion marking or congestion response. Other documents define the PCN wire protocol for other header types. For example, the MPLS encoding is defined in [RFC5129]. Appendix A provides an informative example for a mapping between the encodings in IP and in MPLS. Briscoe, et al. Expires January 13, 2011 [Page 3] Internet-Draft 3-in-1 PCN Encoding July 2010 1.1. Changes in This Version (to be removed by RFC Editor) From draft-ietf-pcn-3-in-1-encoding-02 to -03: * Corrected mistakes in introduction and improved overall readability. * Added new terminology. * Rewrote a good part of Section 4 and 5 to achieve more clarity. * Added appendix explaining when to use which encoding scheme and how to encode them in MPLS shim headers. * Added new co-author. From draft-ietf-pcn-3-in-1-encoding-01 to -02: * Corrected mistake in introduction, which wrongly stated that the threshold-traffic rate is higher than the excess-traffic rate. Other minor corrections. * Updated acks & refs. From draft-ietf-pcn-3-in-1-encoding-00 to -01: * Altered the wording to make sense if [I-D.ietf-tsvwg-ecn-tunnel] moves to proposed standard. * References updated From draft-briscoe-pcn-3-in-1-encoding-00 to draft-ietf-pcn-3-in-1-encoding-00: * Filename changed to draft-ietf-pcn-3-in-1-encoding. * Introduction altered to include new template description of PCN. * References updated. * Terminology brought into line with [RFC5670]. * Minor corrections. Briscoe, et al. Expires January 13, 2011 [Page 4] Internet-Draft 3-in-1 PCN Encoding July 2010 2. Requirements Language 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]. 2.1. Terminology General PCN-related terminology is defined in the PCN architecture [RFC5559], and terminology specific to packet encoding is defined in the PCN baseline encoding [RFC5696]. Additional terminology is defined below. PCN encoding: mapping of PCN marking states to specific codepoints in the packet header. 3. Requirements for and Applicability of 3-in-1 PCN Encoding 3.1. PCN Requirements The PCN architecture [RFC5559] defines that PCN-ingress-nodes of a PCN-domain control incoming packets. Packets belonging to PCN- controlled flows are subject to PCN metering and marking, they are termed PCN-packets, and PCN-ingress-nodes mark them as not-marked (PCN-colouring). Any node in the PCN-domain may perform PCN metering and marking and mark PCN-packets if needed. There are two different metering and marking schemes: threshold-marking and excess-traffic- marking [RFC5670]. Some edge behaviors require only a single marking scheme [I-D.ietf-pcn-sm-edge-behaviour], others require both [I-D.ietf-pcn-cl-edge-behaviour]. In the latter case, three PCN marking states are needed: not-marked (NM) to indicate not-marked packets, threshold-marked (ThM) to indicate packets marked by the threshold-marker, and excess-traffic-marked (ETM) to indicate packets marked by the excess-traffic-marker [RFC5670]. As threshold-marking and excess-traffic-marking start marking packets at different load conditions, one marking scheme indicates more severe pre-congestion than the other in terms of higher load. If a packet has been marked by both a threshold-marker and an excess-traffic-marker, it is marked with the more severe state. Therefore, a fourth PCN marking state indicating that a packet is marked by both markers is not needed. Nonetheless, in addition to codepoints for the three PCN marking states a fourth codepoint is required to indicate packets that are not PCN-capable (termed the not-PCN codepoint). In all current PCN edge behaviors that use two marking schemes [RFC5559], [I-D.ietf-pcn-cl-edge-behaviour], excess-traffic-marking Briscoe, et al. Expires January 13, 2011 [Page 5] Internet-Draft 3-in-1 PCN Encoding July 2010 is configured with a larger reference rate than threshold-marking. We take this as a rule and define excess-traffic-marked as a more severe PCN-mark than threshold-marked. 3.2. Requirements Imposed by Baseline Encoding The baseline encoding scheme [RFC5696] was defined so that it could be extended to accommodate an additional marking state. It provides rules to embed the encoding of two PCN states in the IP header. Figure 1 shows the structure of the former type-of-service field. It contains the 6-bit Differentiated Services (DS) field that holds the DS codepoint (DSCP) [RFC2474] and the 2-bit ECN field [RFC3168]. 0 1 2 3 4 5 6 7 +-----+-----+-----+-----+-----+-----+-----+-----+ | DS FIELD | ECN FIELD | +-----+-----+-----+-----+-----+-----+-----+-----+ Figure 1: Structure of the former type-of-service field in IP Baseline encoding defines that the DSCP must be set to a PCN- compatible DSCP n and the ECN-field [RFC3168] indicates the specific PCN-mark. Baseline encoding offers four possible encoding states within a single DSCP with the following restrictions. o Codepoint `00' (not-ECT) is used to indicate non-PCN traffic as "not-PCN". This allows the use of a DSCP for both PCN and non-PCN traffic. o Codepoint `10' (ECT(0)) is used to indicate Not-marked PCN traffic. o Codepoint `11' (CE) is used to indicate the most severe PCN-mark. o Codepoint `01' (ECT(1)) is available for experimental use and may be re-used by other PCN encodings such as the presently defined 3-in-1 PCN encoding. 3.3. Applicability of 3-in-1 PCN Encoding When PCN traffic is tunneled IP-in-IP within a PCN-domain, PCN-marks must be preserved in all outer IP headers after encapsulation and decapsulation. This property is violated by legacy encapsulation and decapsulation rules [RFC3168], [RFC4301] due to the way they treat the ECN field. This led to strong limitations regarding how PCN- marks can be encoded using the ECN field of the IP header [I-D.ietf-pcn-encoding-comparison]. Therefore, baseline encoding [RFC5696] was defined which works well with legacy tunnels but supports only two PCN marking states. Briscoe, et al. Expires January 13, 2011 [Page 6] Internet-Draft 3-in-1 PCN Encoding July 2010 Since then, new rules have been defined for IP-in-IP tunneling [I-D.ietf-tsvwg-ecn-tunnel] so that the present 3-in-1 PCN encoding has more freedom to accommodate PCN-marks using the ECN field. From this follows that 3-in-1 PCN encoding may be applied only in PCN- domains that comply with [I-D.ietf-tsvwg-ecn-tunnel] or do not use tunneling. 4. Definition of 3-in-1 PCN Encoding The 3-in-1 PCN encoding scheme is an extension of the baseline encoding scheme defined in [RFC5696]. The PCN requirements and the extension rules for baseline encoding presented in the previous section determine how PCN encoding states are carried in the IP headers. This is shown in Figure 2. +--------+----------------------------------------------------+ | | Codepoint in ECN field of IP header | | DSCP | | | +--------------+-------------+-------------+---------+ | | 00 | 10 | 01 | 11 | +--------+--------------+-------------+-------------+---------+ | DSCP n | Not-PCN | NM | ThM | ETM | +--------+--------------+-------------+-------------+---------+ Figure 2: 3-in-1 PCN Encoding Like baseline encoding, 3-in-1 PCN encoding also uses a PCN compatible DSCP n and the ECN field for the encoding of PCN-marks. The PCN-marks have the following meaning. Not-PCN: indicates a non-PCN-packet, i.e., a packet that is not subject to PCN metering and marking. NM: Not-marked. Indicates a PCN-packet that has not yet been marked by any PCN marker. ThM: Threshold-marked. Indicates a PCN-packet that has been marked by a threshold-marker [RFC5670]. ETM: Excess-traffic-marked. Indicates a PCN-packet that has been marked by an excess-traffic-marker [RFC5670]. 5. Behaviour of a PCN Node Compliant with the 3-in-1 PCN Encoding To be compliant with the 3-in-1 PCN Encoding, an PCN interior node behaves as follows: Briscoe, et al. Expires January 13, 2011 [Page 7] Internet-Draft 3-in-1 PCN Encoding July 2010 o It MUST change NM to ThM if the threshold-meter function indicates to mark the packet. o It MUST change NM or ThM to ETM if the excess-traffic-meter function indicates to mark the packet. o It MUST NOT change not-PCN to NM, ThM, or ETM, and MUST NOT change a NM, ThM, or ETM to not-PCN; o It MUST NOT change ThM to NM; o It MUST NOT change ETM to ThM or to NM; In other words, a PCN interior node MUST NOT mark PCN-packets into non-PCN packets and vice-versa, and it may increase the severity of the PCN-mark of a PCN-packet, but it MUST NOT decrease it. 6. Backward Compatibility Discussion of backward compatibility between PCN encoding schemes and previous uses of the ECN field is given in Section 6 of [RFC5696]. 6.1. Backward Compatibility with Pre-existing PCN Implementations This encoding complies with the rules for extending the baseline PCN encoding schemes in Section 5 of [RFC5696]. The term "compatibility" is meant in the following sense. It is possible to operate nodes with baseline encoding [RFC5696] and 3-in-1 encoding in the same PCN domain. The nodes with baseline encoding MUST perform excess-traffic-marking because the 11 codepoint of 3-in-1 encoding also means excess-traffic-marked. PCN-boundary-nodes of such domains are required to interpret the full 3-in-1 encoding and not just baseline encoding, otherwise they cannot interpret the 01 codepoint. Using nodes that perform only excess-traffic-marking may make sense in networks using the CL edge behavior [I-D.ietf-pcn-cl-edge-behaviour]. Such nodes are able to notify the egress only about severe pre-congestion when traffic needs to be terminated. This seems reasonable for locations that are not expected to see any pre-congestion, but excess-traffic-marking gives them a means to terminate traffic if unexpected overload still occurs. Briscoe, et al. Expires January 13, 2011 [Page 8] Internet-Draft 3-in-1 PCN Encoding July 2010 6.2. Recommendations for the Use of PCN Encoding Schemes This sub-section is informative not normative. +------------------------+------------------------------------+ | Used marking schemes | Recommended PCN encoding scheme | +------------------------+------------------------------------+ | Only threshold-marking | Baseline encoding [RFC5696] | +------------------------+------------------------------------+ | Only excess-traffic- | Baseline encoding [RFC5696] | | marking | or 3-in-1 PCN encoding | +------------------------+------------------------------------+ | Threshold-marking and | 3-in-1 PCN encoding | | excess-traffic-marking | | +------------------------+------------------------------------+ Figure 3: Use of PCN encoding schemes Figure 3 gives guidelines under which conditions baseline encoding and 3-in-1 PCN encoding would typically be used. 6.2.1. Use of Both Excess-Traffic-Marking and Threshold-Marking If both excess-traffic-marking and threshold-marking are enabled in a PCN-domain, 3-in-1 encoding should be used as described in this document. 6.2.2. Unique Use of Excess-Traffic-Marking If only excess-traffic-marking is enabled in a PCN-domain, baseline encoding or 3-in-1 encoding may be used. They lead to the same encoding because PCN-boundary nodes will interpret baseline "PCN- marked (PM)" as "excess-traffic-marked (ETM)". 6.2.3. Unique Use of Threshold-Marking No scheme is currently proposed to solely use threshold-marking. However, if only threshold-marking is enabled in a PCN-domain, baseline encoding SHOULD be used. This is because threshold marking will work in combination with legacy tunnel decapsulators within the PCN-domain, while using threshold marking with the 3-in-1 encoding requires that tunnel decapsulators within a PCN-domain comply with [I-D.ietf-tsvwg-ecn-tunnel]. 7. IANA Considerations This memo includes no request to IANA. Briscoe, et al. Expires January 13, 2011 [Page 9] Internet-Draft 3-in-1 PCN Encoding July 2010 Note to RFC Editor: this section may be removed on publication as an RFC. 8. Security Considerations The security concerns relating to this extended PCN encoding are the same as those in [RFC5696]. In summary, PCN-boundary nodes are responsible for ensuring inappropriate PCN markings do not leak into or out of a PCN domain, and the current phase of the PCN architecture assumes that all the nodes of a PCN-domain are entirely under the control of a single operator, or a set of operators who trust each other. Given the only difference between the baseline encoding and the present 3-in-1 encoding is the use of the 01 codepoint, no new security issues are raised, as this codepoint was already available for experimental use in the baseline encoding. 9. Conclusions The 3-in-1 PCN encoding uses a PCN-compatible DSCP and the ECN field to encode PCN-marks. One codepoint allows non-PCN traffic to be carried with the same PCN-compatible DSCP and three other codepoints support three PCN marking states with different levels of severity. The use of this PCN encoding scheme presupposes that any tunnels in the PCN region have been updated to comply with [I-D.ietf-tsvwg-ecn-tunnel]. 10. Acknowledgements Thanks to Phil Eardley, Teco Boot, and Kwok Ho Chan for reviewing this document. 11. Comments Solicited To be removed by RFC Editor: Comments and questions are encouraged and very welcome. They can be addressed to the IETF Congestion and Pre-Congestion working group mailing list , and/or to the authors. 12. References Briscoe, et al. Expires January 13, 2011 [Page 10] Internet-Draft 3-in-1 PCN Encoding July 2010 12.1. 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. [RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition of Explicit Congestion Notification (ECN) to IP", RFC 3168, September 2001. [RFC4301] Kent, S. and K. Seo, "Security Architecture for the Internet Protocol", RFC 4301, December 2005. [RFC5129] Davie, B., Briscoe, B., and J. Tay, "Explicit Congestion Marking in MPLS", RFC 5129, January 2008. [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. 12.2. Informative References [I-D.ietf-pcn-cl-edge-behaviour] Charny, A., Huang, F., Karagiannis, G., Menth, M., and T. Taylor, "PCN Boundary Node Behaviour for the Controlled Load (CL) Mode of Operation", draft-ietf-pcn-cl-edge-behaviour-06 (work in progress), June 2010. [I-D.ietf-pcn-encoding-comparison] Chan, K., Karagiannis, G., Moncaster, T., Menth, M., Eardley, P., and B. Briscoe, "Pre-Congestion Notification Encoding Comparison", draft-ietf-pcn-encoding-comparison-02 (work in progress), March 2010. [I-D.ietf-pcn-sm-edge-behaviour] Charny, A., Karagiannis, G., Menth, M., and T. Taylor, Briscoe, et al. Expires January 13, 2011 [Page 11] Internet-Draft 3-in-1 PCN Encoding July 2010 "PCN Boundary Node Behaviour for the Single Marking (SM) Mode of Operation", draft-ietf-pcn-sm-edge-behaviour-03 (work in progress), June 2010. [I-D.ietf-tsvwg-ecn-tunnel] Briscoe, B., "Tunnelling of Explicit Congestion Notification", draft-ietf-tsvwg-ecn-tunnel-08 (work in progress), March 2010. Authors' Addresses Bob Briscoe BT B54/77, Adastral Park Martlesham Heath Ipswich IP5 3RE UK Phone: +44 1473 645196 Email: bob.briscoe@bt.com URI: http://bobbriscoe.net/ Toby Moncaster Independent Email: toby@moncaster.com Michael Menth University of Wuerzburg room B206, Institute of Computer Science Am Hubland Wuerzburg 97074 Germany Phone: +49 931 31 86644 Email: menth@informatik.uni-wuerzburg.de Briscoe, et al. Expires January 13, 2011 [Page 12]