PCN Working Group (Editor) Internet-Draft BT Intended status: Standards Track April 29, 2008 Expires: October 31, 2008 Marking behaviour of PCN-nodes draft-eardley-pcn-marking-behaviour-00 Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of 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 October 31, 2008. Copyright Notice Copyright (C) The IETF Trust (2008). Abstract This document standardises the two marking behaviours of PCN-nodes: threshold marking and excess traffic marking. Threshold marking marks all PCN-packets if the PCN traffic rate is greater than a first configured rate. Excess traffic marking marks a proportion of PCN- packets, such that the amount marked equals the traffic rate in excess of a second configured rate. (Editor) Expires October 31, 2008 [Page 1] Internet-Draft Marking behaviour of PCN-nodes April 2008 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]. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Specified PCN-marking behaviour . . . . . . . . . . . . . . . 4 2.1. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.2. Classify and condition function . . . . . . . . . . . . . 5 2.3. Threshold meter function . . . . . . . . . . . . . . . . . 6 2.4. Excess traffic meter function . . . . . . . . . . . . . . 6 2.5. Marking function . . . . . . . . . . . . . . . . . . . . . 7 3. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 4. Security Considerations . . . . . . . . . . . . . . . . . . . 8 5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 8 6. Authors . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8 7.1. Normative References . . . . . . . . . . . . . . . . . . . 8 7.2. Informative References . . . . . . . . . . . . . . . . . . 8 Appendix A. Example algorithms . . . . . . . . . . . . . . . . . 9 A.1. Threshold metering and marking . . . . . . . . . . . . . . 9 A.2. Excess traffic metering and marking . . . . . . . . . . . 10 Appendix B. Implementation notes . . . . . . . . . . . . . . . . 11 B.1. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 11 B.2. Classify and condition . . . . . . . . . . . . . . . . . . 11 B.3. Threshold metering . . . . . . . . . . . . . . . . . . . . 12 B.4. Excess traffic metering . . . . . . . . . . . . . . . . . 13 B.5. Marking . . . . . . . . . . . . . . . . . . . . . . . . . 14 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 14 Intellectual Property and Copyright Statements . . . . . . . . . . 15 (Editor) Expires October 31, 2008 [Page 2] Internet-Draft Marking behaviour of PCN-nodes April 2008 1. Introduction [draft-pcn-architecture] describes a general architecture for flow admission and termination based on pre-congestion information in order to protect the quality of service of established inelastic flows within a single DiffServ domain. The pre-congestion information consists of specific markings of PCN-packets. The edge nodes of the DiffServ domain read these markings and convert them into flow admission and termination decisions. Overall the aim is to enable PCN-nodes to give an "early warning" of potential congestion before there is any significant build-up of PCN-packets in their queues. This document standardises the two marking behaviours of PCN-nodes. In summary, their objectives are: o threshold marking: its objective is to mark all PCN-packets (with a "threshold-mark") whenever the rate of PCN-packets is greater than some configured rate ("PCN-threshold-rate"); o excess traffic marking: whenever the rate of PCN-packets is greater than some configured rate ("PCN-excess-rate"), its objective is to mark PCN-packets (with an "excess-traffic-mark") at a rate equal to the difference between the bit rate of PCN- packets and the PCN-excess-rate. [draft-pcn-architecture] describes how the admission control mechanism limits the PCN-traffic on each link to *roughly* its PCN- threshold-rate and how the flow termination mechanism limits the PCN- traffic on each link to *roughly* its PCN-excess-rate. Section 2 specifies the functions involved, which in outline are: o Packet classify and condition - decide whether an incoming packet belongs to a PCN-flow or not; drop (or downgrade) packets if the link is overloaded; o Threshold meter - determine whether the rate of PCN-packets is greater than the configured PCN-threshold-rate; o Excess traffic meter - measure by how much the rate of PCN-packets is greater than the configured PCN-excess-rate; o Mark - actually mark the PCN-packets, if the meter functions indicate to do so; [pcn-encoding] specifies the actual encoding, which uses the DSCP and ECN fields. In a particular deployment the operator may have three (Editor) Expires October 31, 2008 [Page 3] Internet-Draft Marking behaviour of PCN-nodes April 2008 encoding states available (so allowing both threshold marking and excess traffic marking) or may have only two encoding state, which it may use for either threshold marking or excess traffic marking. This leads to the following four use cases: 1. an operator requires both admission control and flow termination, and has three encoding states available. Then admission control is triggered from PCN-packets that are threshold-marked, and flow termination from PCN-packets that are excess-traffic-marked [ref]. 2. an operator requires both admission control and flow termination, and has only two encoding states available. Then both admission control and flow termination are triggered from PCN-packets that are excess-traffic-marked [ref]. 3. an operator requires only admission control. Then admission control is triggered from PCN-packets that are threshold-marked and only two encoding states are needed. 4. an operator requires only flow termination. Then flow termination is triggered from PCN-packets that are excess- traffic-marked and only one encoding states are needed. +---------+ Result +->|Threshold|-------+ | | Meter | | | +---------+ V +---------+ | +--------+ |Classify | | | | Marked Packet ===>| & |==?================>| Marker |===> Packet Stream |Condition| | | | Stream +---------+ | +--------+ | +---------+ ^ | | Excess | | +->| Traffic |-------+ | Meter | Result +---------+ Figure 1: Schematic of functions for PCN-marking 2. Specified PCN-marking behaviour (Editor) Expires October 31, 2008 [Page 4] Internet-Draft Marking behaviour of PCN-nodes April 2008 2.1. Scope The functions defined in the following sub-sections SHOULD be implemented on all links in the PCN-domain. There are three possibilities regarding encoding states: o three encoding states are available, * one for threshold marks, * one for excess rate marks * one for "not PCN-marked"; o two encoding states are available, * one for threshold marks * one for "not PCN-marked"; o two encoding states are available, * one for excess rate marks * one for "not PCN-marked". The same choice MUST be used throughout a PCN-domain. The descriptions in the following sub-sections are functional and are not intended to restrict the implementation. 2.2. Classify and condition function A packet MUST be classified as a PCN-packet if the value of its DSCP and ECN fields are as described in [draft-pcn-encoding]. There may be traffic that is more important than PCN that shares the same capacity as PCN and is priority scheduled over PCN (perhaps an operator's control messages). Such traffic MUST be metered as though it were PCN-traffic, but MUST NOT be PCN-marked. Such packets, together with PCN-packets, are called "metered packets". Otherwise the packet MUST NOT be classified as a PCN-packet. If the level of traffic is sufficiently high to overload the PCN behaviour aggregate(s), then traffic MUST be conditioned. The three possibilities are: (Editor) Expires October 31, 2008 [Page 5] Internet-Draft Marking behaviour of PCN-nodes April 2008 o drop PCN-packets; o downgrade PCN-packets to a lower priority behaviour aggregate, such as best effort or assured forwarding, and perhaps drop lower priority packets; o drop or downgrade other "metered packets". If PCN-packets are dropped (or downgraded) then: o excess-traffic-marked PCN-packets SHOULD be preferentially dropped (downgraded); o PCN-packets that are dropped (downgraded) SHOULD NOT be metered by the Excess traffic Meter. 2.3. Threshold meter function The Threshold Meter MUST have behaviour that is functionally equivalent to the following. The meter is a token bucket, which is sized in bits and has a configured bit rate, termed PCN-threshold-rate. The amount of tokens in the token bucket is termed TB1.fill. Tokens are added at the PCN- threshold-rate, to a maximum value TB1.max. Tokens are removed equal to the size in bits of the metered-packet, to a minimum TB1.fill=0. The token bucket has a configured token bucket depth (between 0 and TB1.max), termed TB1.threshold. If TB1.fill < TB1.threshold, then the meter indicates to the Marking function that the packet is to be threshold-marked; otherwise it does not. 2.4. Excess traffic meter function A packet SHOULD NOT be metered (by this excess traffic meter function) in the following two cases: o If the packet is already excess-traffic-marked; o If this PCN-node drops (downgrades) the packet because the link is overloaded. Otherwise it is metered by the Excess traffic Meter. The Excess traffic Meter MUST have behaviour that is functionally equivalent to the following. The meter is a token bucket, which is sized in bits and has a (Editor) Expires October 31, 2008 [Page 6] Internet-Draft Marking behaviour of PCN-nodes April 2008 configured bit rate, termed PCN-excess-rate. The amount of tokens in the token bucket is termed TB2.fill. Tokens are added at the PCN- excess-rate, to a maximum value TB2.max. Tokens are removed equal to the size in bits of the metered-packet, to a minimum value of 0. The PCN-excess-rate is greater than (or equal to) the PCN-threshold-rate. If the token bucket is empty (TB2.fill = 0), then the meter indicates to the Marking function that the packet is to be excess-traffic- marked. If the token bucket is within an MTU of being empty, then the meter SHOULD indicate to the Marking function that the packet is to be excess-traffic-marked; MTU means the maximum size of PCN- packets on the link. Otherwise the meter does not indicate marking. 2.5. Marking function If the packet is not a PCN-packet, then it MUST NOT be marked. A PCN-packet MUST be marked to reflect the metering results by setting its encoding state appropriately, as specified below. The encoding states are defined values of the DSCP and ECN fields, as specified in [pcn-encoding]. There are three possibilities, depending on how many encoding states are available: o if three encoding states are available (one for threshold-marked, one for excess-traffic-marked and one for "not PCN-marked") then: * the encoding state of a packet that has already been excess- traffic-marked is not altered, whatever the meters indicate; * Otherwise: + if both meters indicate marking, then the packet is excess- traffic-marked; + if one meter indicates marking and the other doesn't, then that marking is applied; + if neither meter indicates marking, then the packet's encoding state is not altered. o if two encoding states are available (one for threshold-marked and one for "not PCN-marked") then: * if the Threshold Meter indicates marking, then the packet is threshold-marked; (Editor) Expires October 31, 2008 [Page 7] Internet-Draft Marking behaviour of PCN-nodes April 2008 * otherwise the packet's encoding state is not altered. o if two encoding states are available (one for excess-traffic- marked and one for "not PCN-marked") then: * if the Excess traffic Meter indicates marking, then the packet is excess-traffic-marked; * otherwise the packet's encoding state is not altered. 3. IANA Considerations This document makes no request of IANA. Note to RFC Editor: this section may be removed on publication as an RFC. 4. Security Considerations See [draft-pcn-architecture] 5. Acknowledgements Michael Menth, Joe Babiarz, Anna Charny reviewed this draft. All the work by many people in the PCN WG. 6. Authors Many people need to be added. 7. References 7.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. 7.2. Informative References [t] "", 2004. (Editor) Expires October 31, 2008 [Page 8] Internet-Draft Marking behaviour of PCN-nodes April 2008 Appendix A. Example algorithms Note: This Appendix is informative, not normative. It is an example of algorithms that implement Section 2 and is based on [draft-charny-pcn-comparison] and [http://www3.informatik.uni-wuerzburg.de/staff/menth/Publications/ Menth08-PCN-Comparison.pdf]. There is no attempt to optimise the algorithms. It implements the metering and marking functions together. It is assumed that three encoding states are available (one for threshold-marked, one for excess-traffic-marked and one for "not PCN-marked"). It is assumed that all metered-packets are PCN-packets and that the link is never overloaded. A.1. Threshold metering and marking A token bucket with the following parameters: o TB1.PCN-threshold-rate: token rate of token bucket (bits/second) o TB1.max: depth of token bucket (bits) o TB1.threshold: marking threshold of token bucket (bits) o TB1.lastUpdate: time the token bucket was last updated (seconds) o TB1.fill: amount of tokens in token bucket (bits) A PCN-packet has the following parameters: o packet.size: the size of the PCN-packet (bits) o packet.mark: the PCN encoding state of the packet In addition there are the parameters: o now: the current time (seconds) The following steps are performed when a PCN-packet arrives on a link: o TB1.fill = min(TB1.max, TB1.fill + (now - TB1.lastUpdate) * TB1.PCN-threshold-rate); // add tokens to token bucket o TB1.fill = max(0, TB1.fill - packet.size); // remove tokens from token bucket (Editor) Expires October 31, 2008 [Page 9] Internet-Draft Marking behaviour of PCN-nodes April 2008 o if ((TB1.fill < TB1.threshold) AND (packet.mark != excess-traffic- marked)) then packet.mark = threshold-marked; // do threshold marking, but don't re-mark packets that are already excess- traffic-marked o TB1.lastUpdate = now A.2. Excess traffic metering and marking A token bucket with the following parameters: o TB2.PCN-excess-rate: token rate of token bucket (bits/second) o TB2.max: depth of TB in token bucket (bits) o TB2.lastUpdate: time the token bucket was last updated (seconds) o TB2.fill: amount of tokens in token bucket (bits) A PCN-packet has the following parameters: o packet.size: the size of the PCN-packet (bits) o packet.mark: the PCN encoding state of the packet In addition there are the parameters: o now: the current time (seconds) o MTU: the maximum transfer unit of the link (or the known maximum size of PCN-packets on the link) (bits) The following steps are performed when a PCN-packet arrives on a link: o TB2.fill = min(TB2.max, TB2.fill + (now - TB2.lastUpdate) * TB2.PCN-excess-rate); // add tokens to token bucket o if (packet.mark != excess-traffic-marked) then TB2.fill = max(0, TB2.fill - packet.size); // remove tokens from token bucket, but do not meter packets that are already excess-traffic-marked o if (TB2.fill < MTU) then packet.mark = excess-traffic-marked; // do (packet size independent) excess traffic marking o TB1.lastUpdate = now (Editor) Expires October 31, 2008 [Page 10] Internet-Draft Marking behaviour of PCN-nodes April 2008 Appendix B. Implementation notes Note: This Appendix is informative, not normative. It comments on Section 2. B.1. Scope It may be known, eg by the design of the network topology, that some links can never be pre-congested (even in unusual circumstances, eg after the failure of some links). There is then no need to implement PCN behaviour on those links. The meter and marker can be implemented on the ingoing or outgoing interface of a PCN-node. It may be that existing hardware can support only one meter and marker per ingoing interface and one per outgoing interface. Then for instance threshold metering and marking could be run on all the ingoing interfaces and excess traffic metering and marking on all the outgoing interfaces; note that the same choice must be made for all the links in a PCN-domain to ensure that the two metering behaviours are applied exactly once for all the links. Note that even if there is only one encoding state both the meters are still implemented, in order to ease compatibility between equipment and remove a configuration option and associated complexity. Although this means that the Marking function ignores indications from one of the meters, they might be logged or acted upon in some other way, for example by the management system or an explicit signalling protocol; such considerations are out of scope of PCN. B.2. Classify and condition Traffic that has a higher DiffServ priority than PCN, but shares the same capacity, is metered as though it were PCN-traffic but cannot be PCN-marked. This means that a meter may indicate a packet is to be PCN-marked, but the Marking function knows it cannot be marked. It is left open to the implementation exactly what to do in this case; one simple possibility is to mark the next PCN-packet. Note that unless the PCN-packets are a large fraction of all the metered- packets then the PCN mechanisms may not work well. Preferential dropping of excess-traffic-marked packets: Section 2.2 specifies: "If the level of traffic is sufficiently high to overload the PCN behaviour aggregate(s), then traffic MUST be conditioned ... excess-traffic-marked PCN-packets SHOULD be preferentially dropped (downgraded)". This avoids over-termination, with the CL/SM edge behaviour, in the event of multiple bottlenecks in the PCN-domain (Editor) Expires October 31, 2008 [Page 11] Internet-Draft Marking behaviour of PCN-nodes April 2008 [ref]. Exactly what "preferentially dropped" means is left to the implementation. It is also left to the implementation what to do if there are no excess-traffic-marked PCN-packets available at a particular instant. Section 2.2 also specifies: "PCN-packets that are dropped (downgraded) SHOULD NOT be metered by the Excess traffic Meter." This avoids over-termination, with the CL/SM edge behaviour, in the event of multiple bottlenecks [ref]. B.3. Threshold metering The description is in terms of a 'token bucket with threshold', however the implementation is not standardised. For example, it could equally well be implemented as a virtual queue [ref]. The behaviour must be functionally equivalent to the description above. "Functionally equivalent" is intended to allow implementation freedom over matters such as: o whether tokens are added to the token bucket at regular time intervals or only when a packet is processed o whether the new token bucket depth is calculated before or after it is decided whether to mark the packet. The effect of this is simply to shift the sequence of marks by one packet. o when the token bucket is very nearly empty and a packet arrives larger than TB1.fill, then the precise change in TB1.fill is up to the implementation. Essentially any behaviour is functionally equivalent if either precisely the same set of packets is marked, or if the set is shifted by one packet. For instance, the following should all be considered as "functionally equivalent": * set TB1.fill = 0 and indicate threshold-mark to the Marking function. (Editor) Expires October 31, 2008 [Page 12] Internet-Draft Marking behaviour of PCN-nodes April 2008 * check whether TB1.fill < TB1.threshold and if it is then indicate threshold-mark to the Marking function; then set TB1.fill = 0. * leave TB1.fill unaltered and indicate threshold-mark to the Marking function. o similarly, when the token bucket is very nearly full and a packet arrives large than (TB1.max - TB1.fill), then the precise change in TB1.fill is up to the implementation. B.4. Excess traffic metering The description is in terms of a token bucket, however the implementation is not standardised. As in Section B.3, "functionally equivalent" allows some implementation flexibility when the token bucket is very nearly empty or very nearly full. Packet size independent marking is specified as a SHOULD in Section 2.4 ( "If the token bucket is within an MTU of being empty, then the meter SHOULD indicate to the Marking function that the packet is to be excess-traffic-marked; MTU means the maximum size of PCN-packets on the link.") Without it, large packets are more likely to be excess-traffic-marked than small packets and this means that, with some edge behaviours, flows with large packets are more likely to be terminated than flows with small packets [refs: http:// www3.informatik.uni-wuerzburg.de/staff/menth/Publications/ Menth08-PCN-MFT.pdf & http://www.ietf.org/internet-drafts/ draft-briscoe-tsvwg-byte-pkt-mark-02.txt]. Section 2.4 specifies: "A metered-packet SHOULD NOT be metered (by this excess traffic meter function) ... If the packet is already excess-traffic-marked". This avoids over-termination (with some edge behaviours) in the event that the PCN-traffic passes through multiple bottlenecks in the PCN-domain [ref]. Note that an implementation could determine whether the packet is already excess-traffic-marked as an integral part of its Classification function. Note that TB2.max is independent of TB1.max; TB2.fill is independent of TB1.fill (except in that a packet changes both); and the two configured rates, PCN-excess-rate and PCN-threshold-rate are independent (except that PCN-excess-rate >= PCN-threshold-rate). (Editor) Expires October 31, 2008 [Page 13] Internet-Draft Marking behaviour of PCN-nodes April 2008 B.5. Marking Although the metering functions are described separately from the Marking function, they can be implemented in an integrated fashion. [pcn-encoding] specifies the encoding states. In some environments encoding states may be scarce, for example MPLS, and then only one encoding state may be preferable. Section 2.5 states: "if three encoding states are available ... if one meter indicates marking and the other doesn't, then that marking is applies". Normally this means that the Threshold Meter indicates marking and the Excess traffic Meter doesn't. However, the reverse is possible for a short time - because the meters react at different speeds when the traffic rate changes. Author's Address Philip Eardley +++ BT Adastral Park, Martlesham Heath Ipswich IP5 3RE UK Email: philip.eardley@bt.com (Editor) Expires October 31, 2008 [Page 14] Internet-Draft Marking behaviour of PCN-nodes April 2008 Full Copyright Statement Copyright (C) The IETF Trust (2008). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. 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Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. Acknowledgment Funding for the RFC Editor function is provided by the IETF Administrative Support Activity (IASA). (Editor) Expires October 31, 2008 [Page 15]