Congestion and Pre-Congestion B. Briscoe Notification T. Moncaster Internet-Draft BT Intended status: Experimental July 1, 2009 Expires: January 2, 2010 PCN 3-State Encoding Extension in a single DSCP draft-ietf-pcn-3-in-1-encoding-00 Status of this Memo This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. 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." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on January 2, 2010. Copyright Notice Copyright (c) 2009 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 (http://trustee.ietf.org/license-info). Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Abstract The objective of Pre-Congestion Notification (PCN) is to protect the quality of service (QoS) of inelastic flows within a Diffserv domain. Briscoe & Moncaster Expires January 2, 2010 [Page 1] Internet-Draft 3-in-1 PCN Encoding July 2009 The overall rate of the PCN-traffic is metered on every link in the PCN-domain, and PCN-packets are appropriately marked when certain configured rates are exceeded. The level of marking allows the boundary nodes to make decisions about whether to admit or block a new flow request, and (in abnormal circumstances) whether to terminate some of the existing flows, thereby protecting the QoS of previously admitted flows. This document specifies how such marks are to be encoded into the IP header by re-using the Explicit Congestion Notification (ECN) codepoints within this controlled domain. This encoding builds on the baseline encoding and provides for three PCN encoding states: Not-marked, Threshold-marked and Excess-traffic-marked. 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 (in abnormal circumstances) flow termination to decide whether to terminate some of the existing flows. To achieve this, the overall rate of PCN-traffic is metered on every link in the domain, and PCN-packets are appropriately marked when certain configured rates are exceeded. These configured rates are below the rate of the link thus providing notification to boundary nodes about overloads before any congestion occurs (hence "pre-congestion notification"). The level of marking allows boundary nodes to make decisions about whether to admit or terminate. This is achieved by marking packets on interior nodes according to some metering function implemented at each node. Excess-traffic-marking marks PCN packets that exceed a certain reference rate on a link while threshold marking marks all PCN packets on a link when the PCN traffic rate exceeds a higher reference rate [I-D.ietf-pcn-marking-behaviour]. These marks are monitored by the egress nodes of the PCN domain. To fully support these two types of marking, three encoding states are needed. The baseline encoding described in [I-D.ietf-pcn-baseline-encoding] provides for deployment scenarios that only require two PCN encoding states using a single Diffserv codepoint. This document describes an experimental extension to the baseline-encoding that adds a third PCN encoding state in the IP header, still using a single Diffserv codepoint. For brevity it will be called the 3-in-1 PCN Encoding. General PCN-related terminology is defined in the PCN architecture Briscoe & Moncaster Expires January 2, 2010 [Page 2] Internet-Draft 3-in-1 PCN Encoding July 2009 [RFC5559], and terminology specific to packet encoding is defined in the PCN baseline encoding [I-D.ietf-pcn-baseline-encoding]. Note that [I-D.ietf-pcn-baseline-encoding] requires the PCN Working Group to maintain a list of all DSCPs used for PCN experiments. 1.1. Changes in This Version (to be removed by RFC Editor) From draft-briscoe-pcn-3-in-1-encoding-00: * Filename changed to draft-ietf-pcn-3-in-1-encoding. * Introduction altered to include new standard description of PCN. * References updated. * Terminology brought into line with [I-D.ietf-pcn-marking-behaviour]. * Minor corrections. 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 RFC 2119 [RFC2119]. 3. The Requirement for Three PCN Encoding States The PCN architecture [RFC5559] describes proposed PCN schemes that expect traffic to be metered and marked using both Threshold and Excess Traffic schemes. In order to achieve this it is necessary to allow for three PCN encoding states: one as a Not Marked (NM) state and the other two to distinguish these two levels of marking severity [I-D.ietf-pcn-marking-behaviour]. The way tunnels process the ECN field severely limits how to encode these states. The two bit ECN field seems to offer four possible encoding states, but one (00) is set aside for traffic controlled by transports that do not understand PCN marking [I-D.ietf-pcn-baseline-encoding], so it would be irregular and risky to use it as a PCN encoding state. Of the three remaining ECN codepoints, only one (11) can be introduced by a congested node within a tunnel and still survive the decapsulation behaviour of a tunnel egress as currently standardised. The two remaining codepoints are (10) and (01). But if a node within the tunnel used either of these two remaining codepoints to try to Briscoe & Moncaster Expires January 2, 2010 [Page 3] Internet-Draft 3-in-1 PCN Encoding July 2009 mark packets with a second severity level, this marking would be removed on decapsulation. The ECN field is constrained to two marking states in this way irrespective of whether regular IP in IP tunnelling [RFC3168] or IPsec tunnelling [RFC4301] is used. One way to provide another encoding state that survives tunnelling is to use a second Diffserv codepoint [I-D.ietf-pcn-3-state-encoding]. Instead, to avoid wasting scarce Diffserv codepoints, we could modify standard tunnels in the PCN region to remove the constraints imposed by standard tunnelling. Therefore this document presupposes tunnels in the PCN region comply with the newly proposed decapsulation rules defined in [I-D.ietf-tsvwg-ecn-tunnel]. Then the constraints of standard tunnels no longer apply so this document can define a 3-state encoding for PCN within one Diffserv codepoint. 4. The 3-in-1 PCN Encoding The 3-in-1 PCN Encoding scheme is based closely on that defined in [I-D.ietf-pcn-baseline-encoding] so that there will be no compatibility issues if a PCN-domain evolves from using the baseline encoding scheme to the experimental scheme described here. The exact manner in which the PCN encoding states are carried in the IP header is shown in Table 1. Codepoint in ECN field of IP header +--------+--------------+-------------+-------------+---------+ | DSCP | 00 | 10 | 01 | 11 | +--------+--------------+-------------+-------------+---------+ | DSCP n | Not-PCN | NM | ThM | ETM | +--------+--------------+-------------+-------------+---------+ Table 1: 3-in-1 PCN Encoding In Table 1 the 3 PCN states are encoded in the ECN field ([RFC3168]) of an IP packet with its Diffserv field ([RFC2474]) set to DSCP n, which is any PCN-Compatible DiffServ codepoint as defined in Section 4.2 of the PCN baseline encoding [I-D.ietf-pcn-baseline-encoding]). The PCN codepoint of a packet defines its marking state as follows: Not-PCN: The packet is controlled by a transport that does not understand PCN marking, therefore the only valid action to notify congestion is to drop the packet; Briscoe & Moncaster Expires January 2, 2010 [Page 4] Internet-Draft 3-in-1 PCN Encoding July 2009 NM: Not marked. A packet in the NM state has not (yet) had its marking state changed to the ThM or ETM states, but it may be changed to one of these states by a node experiencing congestion or pre-congestion; ThM: Threshold-marked. Such a packet has had its marking state changed by the threshold-meter function [I-D.ietf-pcn-marking-behaviour]; ETM: Excess-traffic-marked. Such a packet has had its marking state changed by the excess-traffic-meter function [I-D.ietf-pcn-marking-behaviour]. Packets marked NM, ThM or ETM are termed PCN-packets because their entry into the pcn-domain is controlled by edge nodes that understand how to process PCN markings. 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: o Except where explicitly stated otherwise, it MUST comply with [I-D.ietf-pcn-baseline-encoding] 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 a PCN-Enabled codepoint and MUST NOT change a PCN-Enabled codepoint 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 may increase the severity of packet marking but it MUST NOT decrease it, where the order of severity increases from NM through ThM to ETM. 6. IANA Considerations This memo includes no request to IANA. Briscoe & Moncaster Expires January 2, 2010 [Page 5] Internet-Draft 3-in-1 PCN Encoding July 2009 Note to RFC Editor: this section may be removed on publication as an RFC. 7. Security Considerations The security concerns relating to this extended PCN encoding are essentially the same as those in [I-D.ietf-pcn-baseline-encoding]. 8. Conclusions The 3-in-1 PCN Encoding provides three states to encode PCN markings in the ECN field of an IP packet using just one Diffserv codepoint. One state is for not marked packets while the two others are for PCN nodes to mark packets with increasing levels of severity. Use of this encoding presupposes that any tunnels in the PCN region have been updated to comply with [I-D.ietf-tsvwg-ecn-tunnel]. 9. Acknowledgements Thanks to Phil Eardley for reviewing this. 10. 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. 11. References 11.1. Normative References [I-D.ietf-pcn-baseline-encoding] Moncaster, T., Briscoe, B., and M. Menth, "Baseline Encoding and Transport of Pre-Congestion Information", draft-ietf-pcn-baseline-encoding-04 (work in progress), May 2009. [I-D.ietf-tsvwg-ecn-tunnel] Briscoe, B., "Tunnelling of Explicit Congestion Notification", draft-ietf-tsvwg-ecn-tunnel-02 (work in progress), March 2009. Briscoe & Moncaster Expires January 2, 2010 [Page 6] Internet-Draft 3-in-1 PCN Encoding July 2009 [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. [RFC5559] Eardley, P., "Pre-Congestion Notification (PCN) Architecture", RFC 5559, June 2009. 11.2. Informative References [I-D.ietf-pcn-3-state-encoding] Moncaster, T., Briscoe, B., and M. Menth, "A PCN encoding using 2 DSCPs to provide 3 or more states", draft-ietf-pcn-3-state-encoding-00 (work in progress), April 2009. [I-D.ietf-pcn-marking-behaviour] Eardley, P., "Metering and marking behaviour of PCN- nodes", draft-ietf-pcn-marking-behaviour-04 (work in progress), June 2009. [RFC4301] Kent, S. and K. Seo, "Security Architecture for the Internet Protocol", RFC 4301, December 2005. 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://www.cs.ucl.ac.uk/staff/B.Briscoe/ Briscoe & Moncaster Expires January 2, 2010 [Page 7] Internet-Draft 3-in-1 PCN Encoding July 2009 Toby Moncaster BT c/o B54/70, Adastral Park Martlesham Heath Ipswich IP5 3RE UK Phone: +44 1206 332805 Email: toby.moncaster@bt.com Briscoe & Moncaster Expires January 2, 2010 [Page 8]