Congestion and Pre-Congestion Philip. Eardley (Editor) Notification Working Group BT Internet-Draft August 8, 2007 Intended status: Informational Expires: February 9, 2008 Pre-Congestion Notification Architecture draft-ietf-pcn-architecture-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 February 9, 2008. Copyright Notice Copyright (C) The IETF Trust (2007). Abstract The purpose of this document is to describe a general architecture for flow admission and termination based on aggregated pre-congestion information in order to protect the quality of service of established inelastic flows within a single DiffServ domain. Eardley (Editor) Expires February 9, 2008 [Page 1] Internet-Draft Document August 2007 Status Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6 3. Assumptions and constraints on scope . . . . . . . . . . . . . 7 3.1. Assumption 1: Trust - controlled environment . . . . . . . 8 3.2. Assumption 2: Real-time applications . . . . . . . . . . . 9 3.3. Assumption 3: Many flows and additional load . . . . . . . 9 3.4. Assumption 4: Emergency use out of scope . . . . . . . . . 9 3.5. Other assumptions . . . . . . . . . . . . . . . . . . . . 10 4. High-level functional architecture . . . . . . . . . . . . . . 10 5. Detailed Functional architecture . . . . . . . . . . . . . . . 14 5.1. PCN-interior-node functions . . . . . . . . . . . . . . . 14 5.2. PCN-ingress-node functions . . . . . . . . . . . . . . . . 15 5.3. PCN-egress-node functions . . . . . . . . . . . . . . . . 16 5.4. Admission control functions . . . . . . . . . . . . . . . 16 5.5. Probing functions . . . . . . . . . . . . . . . . . . . . 17 5.6. Flow termination functions . . . . . . . . . . . . . . . . 18 5.7. Addressing . . . . . . . . . . . . . . . . . . . . . . . . 19 5.8. Tunnelling . . . . . . . . . . . . . . . . . . . . . . . . 19 5.9. Fault handling . . . . . . . . . . . . . . . . . . . . . . 20 6. Design goals and challenges . . . . . . . . . . . . . . . . . 21 7. Operations and Management . . . . . . . . . . . . . . . . . . 23 7.1. Fault OAM . . . . . . . . . . . . . . . . . . . . . . . . 23 7.2. Configuration OAM . . . . . . . . . . . . . . . . . . . . 23 7.3. Accounting OAM . . . . . . . . . . . . . . . . . . . . . . 25 7.4. Performance OAM . . . . . . . . . . . . . . . . . . . . . 25 7.5. Security OAM . . . . . . . . . . . . . . . . . . . . . . . 26 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 26 9. Security considerations . . . . . . . . . . . . . . . . . . . 26 10. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 27 11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 27 12. Comments Solicited . . . . . . . . . . . . . . . . . . . . . . 28 13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 28 13.1. Normative References . . . . . . . . . . . . . . . . . . . 28 13.2. Informative References . . . . . . . . . . . . . . . . . . 28 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 31 Intellectual Property and Copyright Statements . . . . . . . . . . 32 Eardley (Editor) Expires February 9, 2008 [Page 2] Internet-Draft Document August 2007 1. Introduction The purpose of this document is to describe a general architecture for flow admission and termination based on aggregated (pre-) congestion information in order to protect the quality of service of flows within a DiffServ domain,[RFC2475]. This document defines an architecture for implementing two mechanisms to protect the quality of service of established inelastic flows within a single DiffServ domain, where all boundary and interior nodes are PCN-enabled and trust each other for correct PCN operation. Flow admission control determines whether a new flow should be admitted and protects the QoS of existing PCN-flows in normal circumstances, by avoiding congestion occurring. However, in abnormal circumstances, for instance a disaster affecting multiple nodes and causing traffic re-routes, then the QoS on existing PCN-flows may degrade even though care was exercised when admitting those flows before those circumstances. Therefore we also propose a mechanism for flow termination, which removes enough traffic in order to protect the QoS of the remaining PCN-flows. As a fundamental building block to enable these two mechanisms, PCN-interior-nodes generate, encode and transport pre- congestion information towards the PCN-egress-nodes. Two rates, a PCN-lower-rate and a PCN-upper-rate, can be associated with each link of the PCN-domain. Each rate is used by an algorithm (specified in another document) that determines how and when a number of PCN-packets are marked, and how the markings are encoded in packet headers. PCN-egress-nodes make measurements of the packet markings and send information as necessary to the nodes that make the decision about which PCN-flows to accept/reject or terminate, based on this information. Another document will describe the decision-making algorithms. 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 the queue; the admission control mechanism limits the PCN-traffic on each link to *roughly* its PCN-lower-rate and the flow termination mechanism limits the PCN-traffic on each link to *roughly* its PCN-upper-rate. We believe that the key benefits of the PCN mechanisms described in this document are that they are simple, scalable, and robust because: o Per flow state is only required at the PCN-ingress-nodes ("stateless core"). This is required for policing purposes (to prevent non-admitted PCN traffic from entering the PCN-domain) and so on. It is not generally required that other network entities are aware of individual flows (although they may be in particular deployment scenarios). Eardley (Editor) Expires February 9, 2008 [Page 3] Internet-Draft Document August 2007 o Admission control is resilient: QoS reservations are decoupled from the routing system and so in general admitted flows can survive capacity, routing or topology changes without additional signalling. The PCN-lower-rates can be chosen small enough that admitted traffic can still be carried after a rerouting in most failure cases. This is an important feature as QoS violations in core networks due to link failures are more likely than QoS violations due to increased traffic volume, [Iyer]. o The PCN-marking algorithms only operate on the overall PCN-traffic on the link, not per flow. o The information of these measurements is signalled to the PCN- egress-nodes by the PCN-marks in the packet headers. No additional signalling protocol is required for transporting the PCN-marks. Therefore no secure binding is required between data packets and separate congestion messages. o The PCN-egress-nodes make separate measurements, operating on the overall PCN-traffic, for each PCN-ingress-node, ie not per flow. Similarly, signalling by the PCN-egress-node of PCN-feedback- information (which is used for flow admission and termination decisions) is at the granularity of the ingress-egress-aggregate. o The admitted PCN-load is controlled dynamically. Therefore it adapts as the traffic matrix changes, and also if the network topology changes (eg after a link failure). Hence an operator can be less conservative when deploying network capacity, and less accurate in their prediction of the PCN-traffic matrix. o The termination mechanism complements admission control. It allows the network to recover from sudden unexpected surges of PCN-traffic on some links, thus restoring QoS to the remaining flows. Such scenarios are expected to be rare but not impossible. They can be caused by large network failures that redirect lots of admitted PCN-traffic to other links, or by malfunction of the measurement-based admission control in the presence of admitted flows that send for a while with an atypically low rate and then increase their rates in a correlated way. o The PCN-upper-rate may be set below the maximum rate that PCN- traffic can be transmitted on a link, in order to trigger termination of some PCN-flows before loss of PCN-packets occurs or to keep the maximum PCN-load on a link below a level configured by the operator. Operators of networks will want to use the PCN mechanisms in various arrangements, for instance depending on how they are performing Eardley (Editor) Expires February 9, 2008 [Page 4] Internet-Draft Document August 2007 admission control outside the PCN-domain (users after all are concerned about QoS end-to-end), what their particular goals and assumptions are, and so on. Several deployment models are possible: o An operator may choose to deploy either admission control or flow termination or both (see Section 7.2). o IntServ over DiffServ [RFC2998]. The DiffServ region is PCN- enabled, RSVP signalling is used end-to-end and the PCN-domain is a single RSVP hop, ie only the PCN-boundary-nodes process RSVP messages. Outside the PCN-domain RSVP messages are processed on each hop. This is described in [I-D.briscoe-tsvwg-cl-architecture] o RSVP signalling is originated and/or terminated by proxies, with application-layer signalling between the end user and the proxy. For instance SIP signalling with a home hub. o Similar to previous bullets but NSIS signalling is used instead of RSVP. o NOTE: Consideration of signalling extensions for specific protocols is outside the scope of the PCN WG, however it will produce a "Requirements for signalling" document as potential input for the appropriate WGs. o Depending on the deployment scenario, the decision-making functionality (about flow admission and termination) could reside at the PCN-ingress-nodes or PCN-egress-nodes or at some central control node in the PCN-domain. NOTE: The Charter restricts us to considering when functionality is at the PCN-boundary-nodes. o There are several PCN-domains on the end-to-end path, each operating PCN mechanisms independently. NOTE: The Charter restricts us to considering a single PCN-domain. A possibility after re-chartering is to consider operating PCN over concatenated DiffServ domains that don't trust each other (ie weakens Assumption 1 about trust, see Section 3.1) o The PCN-domain extends to the end users. NOTE: This is outside the Charter because it breaks Assumption 3 (aggregation, see later; incidentally it doesn't necessarily break Assumption 1 (trust), because in some environments, eg corporate, the end user may have a controlled configuration and so be trusted). The scenario is described in [I-D.babiarz-pcn-sip-cap]. o Pseudowire: PCN may be used as a congestion avoidance mechanism for edge to edge pseudowire emulations Eardley (Editor) Expires February 9, 2008 [Page 5] Internet-Draft Document August 2007 [I-D.ietf-pwe3-congestion-frmwk]. NOTE: Specific consideration of pseudowires is not in the PCN WG Charter. o MPLS: [RFC3270] defines how to support the DiffServ architecture in MPLS networks. [I-D.ietf-tsvwg-ecn-mpls] describes how to add PCN for admission control of microflows into a set of MPLS-TE aggregates (Multi-protocol label switching traffic engineering). PCN-marking is done in MPLS's EXP field. NOTE: This draft is a TSV WG draft, and is also being reviewed in the MPLS WG. o Similarly, it may be possible to extend PCN into Ethernet networks, where PCN-marking is done in the Ethernet header. NOTE: Specific consideration of this extension is outside the PCN WG Charter. 2. Terminology o PCN-domain: a PCN-capable DiffServ domain; a contiguous set of PCN-enabled DiffServ nodes. o PCN-boundary-node: a node that connects one PCN-domain to a node either in another PCN-domain or in a non PCN-domain. o PCN-interior-node: a node in a PCN-domain that is not a PCN- boundary-node. o PCN-node: a PCN-boundary-node or a PCN-interior-node o PCN-egress-node: a PCN-boundary-node in its role in handling traffic as it leaves a PCN-domain. o PCN-ingress-node: a PCN-boundary-node in its role in handling traffic as it enters a PCN-domain. o PCN-traffic: A PCN-domain carries traffic of different DiffServ classes [RFC4594]. Those using the PCN mechanisms are called PCN- classes (collectively called PCN-traffic) and the corresponding packets are PCN-packets. The same network may carry traffic using other DiffServ classes. o Ingress-egress-aggregate: The collection of PCN-packets from all PCN-flows that travel in one direction between a specific pair of PCN-boundary-nodes. o PCN-lower-rate: a reference rate configured for each link in the PCN-domain, which is lower than the PCN-upper-rate. It is used by an algorithm that determines whether a packet should be PCN-marked Eardley (Editor) Expires February 9, 2008 [Page 6] Internet-Draft Document August 2007 with a first encoding. o PCN-upper-rate: a reference rate configured for each link in the PCN-domain, which is higher than the PCN-lower-rate. It is used by an algorithm that determines whether a packet should be PCN- marked with a second encoding. o Threshold-marking: a PCN-marking algorithm such that all PCN- traffic is marked if the PCN-traffic exceeds a particular rate (either the PCN-lower-rate or PCN-upper-rate). NOTE: The definition reflects the overall intent of the algorithm rather than its instantaneous behaviour, since the rate measured at a particular moment depends on the algorithm, its implementation and the traffic's variance as well as its rate. o Excess-rate-marking: a PCN-marking algorithm such that the amount of PCN-traffic that is PCN-marked is equal to the amount that exceeds a particular rate (either the PCN-lower-rate or PCN-upper- rate). NOTE: The definition reflects the overall intent of the algorithm rather than its instantaneous behaviour, since the rate measured at a particular moment depends on the algorithm, its implementation and the traffic's variance as well as its rate. o Pre-congestion: a condition of a link within a PCN-domain in which the PCN-node performs PCN-marking, in order to provide an "early warning" of potential congestion before there is any significant build-up of PCN-packets in the queue. o PCN-marking: the process of setting the header in a PCN-packet based on defined rules, in reaction to pre-congestion. o {{if necessary: PCN-lower-rate-marking and PCN-upper-rate- marking}} o PCN-feedback-information: information signalled by a PCN-egress- node to a PCN-ingress-node or central control node, which is needed for the flow admission and flow termination mechanisms. 3. Assumptions and constraints on scope The PCN WG's charter restricts the initial scope by a set of assumptions. Here we list those assumptions and explain them. 1. these components are deployed in a single DiffServ domain, within which all PCN-nodes are PCN-enabled and trust each other for truthful PCN-marking and transport Eardley (Editor) Expires February 9, 2008 [Page 7] Internet-Draft Document August 2007 2. all flows handled by these mechanisms are inelastic and constrained to a known peak rate through policing or shaping 3. the number of PCN-flows across any potential bottleneck link is sufficiently large that stateless, statistical mechanisms can be effective. To put it another way, the aggregate bit rate of PCN- traffic across any potential bottleneck link needs to be sufficiently large relative to the maximum additional bit rate added by one flow 4. PCN-flows may have different precedence, but the applicability of the PCN mechanisms for emergency use (911, GETS, WPS, MLPP, etc.) is out of scope After completion of the initial phase, the PCN WG may re-charter to develop solutions for specific scenarios where some of these restrictions are not in place. It may also re-charter to consider applying the PCN mechanisms to additional deployment scenarios (operation over concatenated DiffServ domains, PCN-aware application mechanisms etc.). The WG may also re-charter to investigate additional response mechanisms that act on (pre-)congestion information. One example could be flow-rate adaptation by elastic applications (rather than flow admission or termination). Another example of a possible future work item is the operation of PCN over concatenated PCN-domains that don't trust each other (perhaps re- ECN,[I-D.briscoe-re-pcn-border-cheat]). The details of these work items are outside the scope of the initial phase, but the WG may consider their requirements in order to design components that are sufficiently general to support such extensions in the future. The working assumption is that the standards developed in the initial phase should not need to be modified to satisfy the solutions for when these restrictions are removed. 3.1. Assumption 1: Trust - controlled environment We assume that the PCN-domain is a controlled environment, i.e. all the nodes in a PCN-domain run PCN and trust each other. There are several reasons for proposing this assumption: o The PCN-domain has to be encircled by a ring of PCN-boundary- nodes, otherwise PCN-packets could enter the PCN-domain without being subject to admission control, which would potentially destroy the QoS of existing flows. o Similarly, a PCN-boundary-node has to trust that all the PCN-nodes are doing PCN-marking. A non PCN-node wouldn't be able to alert that it is suffering pre-congestion, which potentially would lead to too many PCN-flows being admitted (or too few being Eardley (Editor) Expires February 9, 2008 [Page 8] Internet-Draft Document August 2007 terminated). Worse, a rogue node could perform various attacks, as discussed in the Security Considerations section. One way of assuring the above two points is that the entire PCN- domain is run by a single operator. Another possibility is that there are several operators but they trust each other to a sufficient level, in their handling of PCN-traffic. 3.2. Assumption 2: Real-time applications We assume that PCN-packets come from real time applications generating inelastic traffic [Shenker] like voice and video requiring low delay, jitter and packet loss, for example the Controlled Load Service, [RFC2211], and the Telephony service class, [RFC4594]. This assumption is to help focus the effort where it looks like PCN would be most useful, ie the sorts of applications where per flow QoS is a known requirement. For instance, the impact of this assumption would be to guide simulations work. 3.3. Assumption 3: Many flows and additional load We assume that there are many flows on any bottleneck link in the PCN-domain (or, to put it another way, the aggregate bit rate of PCN- traffic across any potential bottleneck link is sufficiently large relative to the maximum additional bit rate added by one flow). Measurement-based admission control assumes that the present is a reasonable prediction of the future: the network conditions are measured at the time of a new flow request, however the actual network performance must be OK during the call some time later. One issue is that if there are only a few variable rate flows, then the aggregate traffic level may vary a lot, perhaps enough to cause some packets to get dropped. If there are many flows then the aggregate traffic level should be statistically smoothed. How many flows is enough depends on a number of things such as the variation in each flow's rate, the total rate of PCN-traffic, and the size of the "safety margin" between the traffic level at which we start admission-marking and at which packets are dropped. We do not make explicit assumptions on how many PCN-flows are in each ingress-egress-aggregate. Performance evaluation work may clarify whether it is necessary to make any additional assumption on aggregation at the ingress-egress-aggregate level. 3.4. Assumption 4: Emergency use out of scope PCN-flows may have different precedence, but the applicability of the PCN mechanisms for emergency use (911, GETS, WPS, MLPP, etc) is out of scope. Eardley (Editor) Expires February 9, 2008 [Page 9] Internet-Draft Document August 2007 3.5. Other assumptions It is assumed that PCN-marking is being applied to traffic scheduled with the expedited forwarding per-hop behaviour, [RFC3246]. It is assumed that PCN-nodes do not perform ECN, [RFC3246], on PCN- packets. If a packet that is part of a PCN-flow arrives at a PCN-ingress-node with its CE (Congestion experienced) codepoint set, then we assume that the PCN-ingress-node drops the packet. After its initial Charter is complete, the WG may decide to work on a mechanism (such as through a signalling extension) that enables ECN-marking to be carried transparently across the PCN-domain. 4. High-level functional architecture The high-level approach is to split functionality between: o PCN-interior-nodes 'inside' the PCN-domain, which monitor their own state of pre-congestion and mark PCN-packets if appropriate. They are not flow-aware, nor aware of ingress-egress-aggregates. o PCN-boundary-nodes at the edge of the PCN-domain, which control admission of new PCN-flows and termination of existing PCN-flows, based on information from PCN-interior-node. This information is in the form of the PCN-marked data packets (which are intercepted by the PCN-egress-nodes) and not signalling messages. PCN- ingress-nodes are flow-aware (required for policing purposes). In several deployment scenarios PCN-egress-nodes will also be flow aware. (Normally this adds no complexity since a PCN-boundary- node acts as both a PCN-ingress-node and as a PCN-egress-node.) The aim of this split is to keep the bulk of the network simple, scalable and robust, whilst confining policy, application-level and security interactions to the edge of the PCN-domain. For example the lack of flow awareness means that the PCN-interior-nodes don't care about the flow information associated with the PCN-packets that they carry, nor do the PCN-boundary-nodes care about which PCN-interior- nodes its flows traverse. Flow admission: At a high level, flow admission control works as follows. In order to generate information about the current state of the PCN-domain, each PCN-node PCN-marks packets if it is "pre-congested". Exactly how a PCN-node decides if it is "pre-congested" (the algorithm) and Eardley (Editor) Expires February 9, 2008 [Page 10] Internet-Draft Document August 2007 exactly how packets are "PCN-marked" (the encoding) will be defined in a separate standards-track document, but at a high level it is expected to be as follows: o the algorithm: a PCN-node meters the amount of PCN-traffic on each one of its outgoing links. The measurement is made as an aggregate of all PCN-packets, and not per flow. The algorithm has a configured parameter, PCN-lower-rate. As the amount of PCN- traffic exceeds the PCN-lower-rate, then PCN-packets are PCN- marked. See NOTE below for more explanation. o the encoding: a PCN-node PCN-marks a PCN-packet (with a first encoding) by setting fields in the header to specific values. It is expected that the ECN and/or DSCP fields will be used. NOTE: Two main categories of algorithm have been proposed: if the algorithm uses threshold-marking then all PCN-packets are marked if the current rate exceeds the PCN-lower-rate, whereas if the algorithm uses excess-rate-marking the amount marked is equal to the amount in excess of the PCN-lower-rate. However, note that this description reflects the overall intent of the algorithm rather than its instantaneous behaviour, since the rate measured at a particular moment depends on the detailed algorithm, its implementation (eg virtual queue, token bucket...) and the traffic's variance as well as its rate (eg marking may well continue after a recent overload even after the instantaneous rate has dropped). The PCN-boundary-nodes monitor the PCN-marked packets in order to extract information about the current state of the PCN-domain. Based on this monitoring, a decision is made about whether to admit a prospective new flow. Exactly how the admission control decision is made will be defined in separately (at the moment the intention is that there will be one or more informational-track RFCs), but at a high level it is expected to be as follows: o the PCN-egress-node measures (possibly as a moving average) the fraction of the PCN-traffic that is PCN-marked. The fraction is measured for a specific ingress-egress-aggregate. If the fraction is below a threshold value then the new flow is admitted. Note that the PCN-lower-rate is a parameter that can be configured by the operator. It will be set lower than the traffic rate at which the link becomes congested and the node drops packets. (Hence, by analogy with ECN we call our mechanism Pre-Congestion Notification.) Note also that the admission control decision is made for a particular ingress-egress-aggregate. So it is quite possible for a new flow to be admitted between one pair of PCN-boundary-nodes, Eardley (Editor) Expires February 9, 2008 [Page 11] Internet-Draft Document August 2007 whilst at the same time another admission request is blocked between a different pair of PCN-boundary-nodes. Flow termination: At a high level, flow termination control works as follows. Each PCN-node PCN-marks packets in a similar fashion to above. An obvious approach is for the algorithm to use a second configured parameter, PCN-upper-rate, and a second header encoding ("PCN-upper-rate- marking"). However there is also a proposal to use the same rate and the same encoding. Several approaches have been proposed to date about how to convert this information into a flow termination decision; at a high level these are as follows: o One approach measures the rate of unmarked PCN-traffic (ie not PCN-upper-rate-marked) at the PCN-egress-node, which is the amount of PCN-traffic that can actually be supported; the PCN-ingress- node measures the rate of PCN-traffic that is destined for this specific PCN-egress-node, and hence can calculate the excess amount that should be terminated. o Another approach instead measures the rate of PCN-upper-rate- marked traffic and calculates and selects the flows that should be terminated. o Another approach terminates any PCN-flow with a PCN-upper-rate- marked packet. It needs a different marking algorithm, otherwise far too much traffic would be terminated. o Another approach uses only one sort of marking, which is based on the PCN-lower-rate, to decide not only whether to admit more PCN- flows but also whether any PCN-flows need to be terminated. It assumes that the ratio of the (implicit) PCN-upper-rate and the PCN-lower-rate is the same on all links. This approach measures the rate of unmarked PCN-traffic at a PCN-egress-node. The PCN- ingress-node uses this measurement to compute the implicit PCN- upper-rate of the bottleneck link. It then measures the rate of PCN-traffic that is destined for this specific PCN-egress-node and hence can calculate the amount that should be terminated. Since flow termination is designed for "abnormal" circumstances, it is quite likely that some PCN-nodes are congested and hence packets are being dropped and/or significantly queued. The flow termination mechanism must bear this in mind. Note also that the termination control decision is made for a particular ingress-egress-aggregate. So it is quite possible for PCN-flows to be terminated between one pair of PCN-boundary-nodes, Eardley (Editor) Expires February 9, 2008 [Page 12] Internet-Draft Document August 2007 whilst at the same time none are terminated between a different pair of PCN-boundary-nodes. Although designed to work together, flow admission and flow termination are independent mechanisms, and the use of one does not require or prevent the use of the other (discussed further in Section 7.2). Information transport: The transport of pre-congestion information from a PCN-node to a PCN- egress-node is through PCN-markings in data packet headers, no signalling protocol messaging is needed. However, signalling is needed to transport PCN-feedback-information between the PCN- boundary-nodes, for example to convey the fraction of PCN-marked traffic from a PCN-egress-node to the relevant PCN-ingress-node. Exactly what information needs to be transported will be described in the future PCN WG document(s) about the boundary mechanisms. The signalling could be done by an extension of RSVP or NSIS, for instance; protocol work will be done by the relevant WG, but for example [I-D.lefaucheur-rsvp-ecn]describes the extensions needed for RSVP. The following are some high-level points about how PCN works: o There needs to be a way for a PCN-node to distinguish PCN-traffic from non PCN-traffic. They may be distinguished using the DSCP field and/or ECN field. [I-D.chan-pcn-encoding-comparison] discusses further. o The PCN mechanisms may be applied to more than one traffic class (which are distinguished by DSCP). o There may be traffic that is more important than PCN, perhaps a particular application or an operator's control messages. A PCN- node may dedicate capacity to such traffic or priority schedule it over PCN. In the latter case its traffic needs to contribute to the PCN meters. o There will be traffic less important than PCN. For instance best effort or assured forwarding traffic. It will be scheduled at lower priority than PCN, and use a separate queue or queues. However, a PCN-node may dedicate some capacity to lower priority traffic so that it isn't starved. o There may be other traffic with the same priority as PCN-traffic. For instance, Expedited Forwarding sessions that are originated either without capacity admission or with traffic engineering. In Eardley (Editor) Expires February 9, 2008 [Page 13] Internet-Draft Document August 2007 [I-D.ietf-tsvwg-admitted-realtime-dscp] the two traffic classes are called EF and EF-ADMIT. A PCN-node could either use separate queues, or separate policers and a common queue; the draft provides some guidance when each is better, but for instance the latter is preferred when the two traffic classes are carrying the same type of application with the same jitter requirements. 5. Detailed Functional architecture This section is intended to provide a systematic summary of the new functional architecture in the PCN-domain, which maps to the additional functionality required by the PCN-nodes, in addition to their normal router functions. The section discusses the functionality needed for both flow admission control and flow termination. It is split into: 1. functions needed at PCN-interior-nodes 2. functions needed at PCN-ingress-nodes 3. functions needed at PCN-egress-nodes 4. other functions needed for flow admission control 5. other functions needed for probing (which may be needed sometimes) 6. other functions needed for flow termination control The section then discusses some other detailed topics: 1. addressing 2. tunnelling 3. fault handling 5.1. PCN-interior-node functions Each link of the PCN-domain is upgraded with the following functionality: o Packet classify - decide whether an incoming packet is a PCN- packet or not. Another PCN WG document will specify encoding, using the DSCPand/or ECN fields. Eardley (Editor) Expires February 9, 2008 [Page 14] Internet-Draft Document August 2007 o PCN-meter - measure the 'amount of PCN-traffic'. The measurement is made as an aggregate of all PCN-packets, and not per flow. o PCN-mark - algorithms determine whether to PCN-mark PCN-packets and what packet encoding is used (as specified in another PCN WG document). The same general approach of metering and PCN-marking is performed for both flow admission control and flow termination, however the algorithms and encoding may be different. These functions are needed for each link of the PCN-region. They are therefore needed on all links of PCN-interior-nodes, and on the links of PCN-boundary-nodes that are internal to the PCN-domain. There may be more than one PCN-meter and marker installed at a given link, eg one for admission and one for termination. 5.2. PCN-ingress-node functions Each ingress link of the PCN-domain is upgraded with the following functionality: o Packet classify - decide whether an incoming packet is part of a previously admitted microflow, by using a filter spec (eg DSCP, source and destination addresses and port numbers) o Police - police, by dropping or re-marking with a non-PCN DSCP, any packets received with a DSCP demanding PCN transport that do not belong to an admitted flow. Similarly, police packets that are part of a previously admitted microflow, to check that the microflow keeps to the agreed rate or flowspec (eg RFC1633 [RFC1633] and NSIS equivalent). o PCN-colour - set the DSCP field or DSCP and ECN fields to the appropriate value(s) for a PCN-packet. The draft about PCN- encoding will discuss further. o PCN-meter - make "measurements of PCN-traffic". Some approaches to flow termination require the PCN-ingress-node to measure the (aggregate) rate of PCN-traffic towards a particular PCN-egress- node. The first two are policing functions, needed to make sure that PCN- packets let into the PCN-domain belong to a flow that's been admitted (and probably also to ensure that the flow doesn't go at a faster rate than allowed by its service level agreement). The filter spec will for example come from the flow request message (outside scope of PCN WG, see [I-D.briscoe-tsvwg-cl-architecture] for an example using Eardley (Editor) Expires February 9, 2008 [Page 15] Internet-Draft Document August 2007 RSVP). PCN-colouring allows the rest of the PCN-domain to recognise PCN-packets. 5.3. PCN-egress-node functions Each egress link of the PCN-domain is upgraded with the following functionality: o Packet classify - determine which PCN-ingress-node a PCN-packet has come from. o PCN-meter - make "measurements of PCN-traffic". The measurement(s) is made as an aggregate (ie not per flow) of all PCN-packets from a particular PCN-ingress-node. o PCN-colour - for PCN-packets, set the DSCP field or DSCP and ECN fields to the appropriate value(s) for use outside the PCN-domain. Another PCN WG document, about boundary mechanisms, will describe what the "measurements of PCN-traffic" are. This depends on whether the measurement is targeted at admission control or flow termination. It also depends on what encoding and PCN-marking algorithms are specified by the PCN WG. 5.4. Admission control functions Specific admission control functions can be performed at a PCN- boundary-node (PCN-ingress-node or PCN-egress-node) or at a centralised node, but not at normal PCN-interior-nodes. The functions are: o Make decision about admission - compare the required "measurements of PCN-traffic" (output of the PCN-egress-node's PCN-meter function) with some reference level, and hence decide whether to admit the potential new PCN-flow. As well as the PCN measurements, the decision takes account of policy and application layer requirements. o Communicate decision about admission - signal the decision to the node making the admission control request (which may be outside the PCN-region), and to the policer (PCN-ingress-node function) There are various possibilities for how the functionality can be distributed (we assume the operator would configure which is used): o The decision is made at the PCN-egress-node and signalled to the PCN-ingress-node Eardley (Editor) Expires February 9, 2008 [Page 16] Internet-Draft Document August 2007 o The decision is made at the PCN-ingress-node, which requires that the PCN-egress-node signals to the PCN-ingress-node the fraction of PCN-traffic that is PCN-marked (or whatever the PCN WG agrees as the required "measurements of PCN-traffic"). o The decision is made at a centralised node, which requires that the PCN-egress-node signals its measurements to the centralised node, and that the centralised node signals to the PCN-ingress- node about the decision about admission control. It would be possible for the centralised node to be one of the PCN-boundary- nodes, when clearly the signalling would sometimes be replaced by a message internal to the node. 5.5. Probing functions Probing functions are optional, and can be used for admission control. A PCN-ingress-node generates and sends probe packets in order to test the pre-congestion level. Probing is useful or even essential under the following conditions: o when an ingress-egress-aggregate carries no traffic (or too little traffic for the PCN-egress-node to accurately make the "measurements of PCN-traffic" that are required for an admission decision). It may be that the traffic levels on other ingress- egress-aggregates are so high that a new flow shouldn't be admitted on the 'empty' ingress-egress-aggregate. Probing is useful to check this. o in the presence of multipath routing (ECMP) between the PCN- boundary-nodes, when some paths are pre-congested there may be other paths which aren't pre-congested. Probing is useful to determine whether the new flow would follow a path that isn't pre- congested and hence can be admitted. Probe packets may be simple data addressed to the PCN-egress-node and require no protocol standardisation, although there will be best practice for their number, size and rate. There are two possibilities for how probing is triggered: o the PCN-egress-node requests (signals) the PCN-ingress-node to generate probe traffic o if the PCN-ingress-node knows which PCN-egress-node is associated with the destination address in the admission request, then the PCN-ingress-node could know it has no reservation with that PCN- egress-node and unilaterally start probing. The probing functions are: Eardley (Editor) Expires February 9, 2008 [Page 17] Internet-Draft Document August 2007 o Make decision that probing is needed o (if required) Communicate the request that probing is needed - the PCN-egress-node signals to the PCN-ingress-node that probe traffic is needed o Generate probe traffic - the PCN-ingress-node generates the probe traffic. The appropriate number (or rate) of probe packets will depend on the PCN-marking algorithm; for example an excess-rate- marking algorithm generates fewer PCN-marks than a threshold- marking algorithm. o Forward probe packets - as far as PCN-interior-nodes are concerned, probe packets must be handled the same as (ordinary data) PCN-packets. o Consume probe packets - the PCN-egress-node consumes probe packets to ensure that they don't travel beyond the PCN-domain. 5.6. Flow termination functions Specific termination control functions can be performed at a PCN- boundary-node (PCN-ingress-node or PCN-egress-node) or at a centralised node, but not at normal PCN-interior-nodes. There are various possibilities for how the functionality can be distributed, similar to those discussed above in the Admission control section; the flow termination decision could be made at the PCN-ingress-node, the PCN-egress-node or at some centralised node. The functions are: o PCN-meter at PCN-egress-node - (as described in Section 5.3) make "measurements of PCN-traffic" from a particular PCN-ingress-node. o (if required) PCN-meter at PCN-ingress-node - make "measurements of PCN-traffic" being sent towards a particular PCN-egress-node; again, this is done for the ingress-egress-aggregate and not per flow. o (if required) Communicate "measurements of PCN-traffic" to the node that makes the flow termination decision. For example, if the PCN-ingress-node makes the decision then communicate the PCN- egress-node's measurements to it (as in [I-D.briscoe-tsvwg-cl-architecture]). o Make decision about flow termination - use the "measurements of PCN-traffic" to decide which PCN-flow or PCN-flows to terminate. The decision takes account of policy and application layer requirements. Eardley (Editor) Expires February 9, 2008 [Page 18] Internet-Draft Document August 2007 o Communicate decision about flow termination - signal the decision to the node that is able to terminate the flow (which may be outside the PCN-region), and to the policer (PCN-ingress-node function) One particular proposal, [I-D.charny-pcn-single-marking], for PCN- marking and performing flow admission and termination would require a global parameter to be defined on all PCN-boundary-nodes in the PCN- domain. [I-D.charny-pcn-single-marking] discusses in full the impact of this particular proposal on the operation of PCN. 5.7. Addressing PCN-nodes may need to know the address of other PCN-nodes: o in all cases PCN-interior-nodes don't need to know the address of any other PCN-nodes, except their next hop neighbours o in the cases of admission or termination decision by a PCN- boundary-node, the PCN-egress-node needs to know the address of the PCN-ingress-node associated with a flow, at a minimum so that the PCN-ingress-node can be informed to enforce the admission decision through policing. The addressing information can be gathered from signalling, for example as described for RSVP in [I-D.lefaucheur-rsvp-ecn]. Alternatively, if PCN-traffic is always tunnelled across the PCN-domain, then the PCN-ingress- node's address is simply the source address of the outer packet header. o in the cases of admission or termination decision by a central control node, the PCN-egress-node needs to be configured with the address of the centralised node. In addition, depending on the exact deployment scenario and its signalling, the centralised node may need to know the addresses of the PCN-ingress-node and PCN- egress-node, and the PCN-egress-node know the address of the PCN- ingress-node. NOTE: Consideration of the centralised case is out of scope of the initial PCN WG Charter. 5.8. Tunnelling It is possible that tunnels terminate at a PCN-node. It is important that any PCN-marking is preserved after decapsulation, so that it is still seen by the PCN-egress-node. To ensure this, on decapsulation the following rules are applied: o the PCN-marking state of the inner and outer headers are compared Eardley (Editor) Expires February 9, 2008 [Page 19] Internet-Draft Document August 2007 o if the inner header's marking state is more severe then it is preserved o if the outer header's marking state is more severe then it is copied onto the inner header o NB the order of increasing severity is: unmarked; PCN-marking with first encoding (ie associated with the PCN-lower-rate); PCN- marking with second encoding (ie associated with the PCN-upper- rate) Similarly, if encapsulation is done within the PCN-domain, then the following rule is applied: o any PCN-marking is copied into the outer header Tunnelling considerations also depend on which header bits the PCN WG decides to use. If the ECN bits are used then [I-D.briscoe-tsvwg-ecn-tunnel] applies; the rules above conform to its spirit. If the DSCP field is used then [RFC2983] needs to be considered carefully. An operator may wish to tunnel PCN-traffic from PCN-ingress-nodes to PCN-egress-nodes, in which case the rules above aren't needed. The potential reasons for doing such tunnelling are: the PCN-egress-node then automatically knows the address of the relevant PCN-ingress-node for a flow; even if ECMP is running, all PCN-packets on a particular ingress-egress-aggregate follow the same path. But it also has drawbacks: additional overhead in terms of bandwidth and processing; and the effective elimination of ECMP as a load balancing mechanism. 5.9. Fault handling If a PCN-interior-node fails (or one of its links), then lower layer protection mechanisms or the regular IP routing protocol will eventually re-route round it. If the new route can carry all the admitted traffic, flows will gracefully continue. If instead this causes early warning of pre-congestion on the new route, then admission control based on pre-congestion notification will ensure new flows will not be admitted until enough existing flows have departed. Finally re-routing may result in heavy (pre-)congestion, when the flow termination mechanism will kick in. If a PCN-boundary-node fails then we would like the regular QoS signalling protocol to take care of things. As an example [I-D.briscoe-tsvwg-cl-architecture] considers what happens if RSVP is the QoS signalling protocol. The details for a specific signalling protocol are out of scope of the PCN WG, however there is a WG Eardley (Editor) Expires February 9, 2008 [Page 20] Internet-Draft Document August 2007 Milestone on generic "Requirements for signalling". 6. Design goals and challenges Prior work on PCN and similar mechanisms has thrown up a number of considerations about PCN's design goals (things PCN should be good at) and some issues that have been hard to solve in a fully satisfactory manner. Taken as a whole it represents a list of trade- offs (it's unlikely that they can all be 100% achieved) and perhaps as evaluation criteria to help an operator (or the IETF) decide between options. The following are key design goals for PCN (based on [I-D.chan-pcn-problem-statement]): o The PCN-enabled packet forwarding network should be simple, scalable and robust o Compatibility with other traffic (i.e. a proposed solution should work well when non-PCN traffic is also present in the network) o Support of different types of real-time traffic (eg should work well with CBR and VBR voice and video sources treated together) o Reaction time of the mechanisms should be commensurate with the desired application-level requirements (e.g. a termination mechanism needs to terminate flows before significant QoS issues are experienced by real-time traffic, and before most users hang up) o Compatibility with different precedence levels of real-time applications (e.g. preferential treatment of higher precedence calls over lower precedence calls, [ITU-MLPP]. The following are open issues. They are taken from [I-D.briscoe-tsvwg-cl-architecture] which also describes some possible solutions (potential solutions are out of scope for this document). Note that some may be considered unimportant in general or in specific deployment scenarios. o ECMP (Equal Cost Multi-Path) Routing: The level of pre-congestion is measured on a specific ingress-egress-aggregate. However, if the PCN-domain runs ECMP, then traffic on this ingress-egress- aggregate may follow several different paths - some of the paths could be pre-congested whilst others are not. There are two potential problems: Eardley (Editor) Expires February 9, 2008 [Page 21] Internet-Draft Document August 2007 1. over-admission: a new flow is admitted (because the pre- congestion level measured by the PCN-egress-node is sufficiently diluted by unmarked packets from non-congested paths that a new flow is admitted), but its packets travel through a pre-congested PCN-node 2. ineffective termination: flows are terminated, however their path doesn't travel through the (pre-)congested router(s). The overall PCN solution for flow termination must solve the second problem, since flow termination is a 'last resort'. For flow admission, the risk of slight over-admission may be acceptable (particularly with flow termination as a fall-back), at least for some operators. o Bi-Directional Sessions: Many applications have bi-directional sessions - hence there are two flows that should be admitted (or terminated) as a pair - for instance a bi-directional voice call only makes sense if flows in both directions are admitted. However, PCN's mechanisms concern admission and termination of a single flow, and coordination of the decision for both flows is a matter for the signalling protocol and out of scope of PCN. One possible example would use SIP pre-conditions; there are others. o Global Coordination: PCN makes its admission decision based on PCN-markings on a particular ingress-egress-aggregate. Decisions about flows through a different ingress-egress-aggregate are made independently. However, one can imagine network topologies and traffic matrices where from a global perspective it would be better to make a coordinated decision across all the ingress- egress-aggregates for the whole PCN-domain. For example, to block (or even terminate) flows on one ingress-egress-aggregate so that more important flows through a different ingress-egress-aggregate could be admitted. Mechanisms to solve these problems may well be out of scope. o Aggregate Traffic Characteristics: Even when the number of flows is stable, the traffic level through the PCN-domain will vary because the sources vary their traffic rates. PCN works best when there's not too much variability in the total traffic level at a PCN-node's interface (ie in the aggregate traffic from all sources). Too much variation means that a node may (at one moment) not be doing any PCN-marking and then (at another moment) drop packets because it's overloaded. This makes it hard to tune the admission control scheme to stop admitting new flows at the right time. Eardley (Editor) Expires February 9, 2008 [Page 22] Internet-Draft Document August 2007 o Flash crowds and Speed of Reaction: PCN is a measurement-based mechanism and so has a limited speed of reaction. For example, potentially if a big burst of admission requests occurs in a very short space of time (eg prompted by a televote), they could all get admitted before enough PCN-marks are seen to block new flows. In other words, any additional load offered within the reaction time of the mechanism mustn't move the PCN-domain directly from no congestion to overload. This 'vulnerability period' may impact at the signalling level, for instance QoS requests shouldn't be handled any faster than the vulnerability period. o Compatibility of PCN-encoding with ECN-encoding. This issue will be considered further in [I-D.chan-pcn-encoding-comparison]. 7. Operations and Management EDITOR'S NOTE: A re-write of this section is planned; some of the sub-sections are very short! The PCN WG Charter says that the architecture document should include security, manageability and operational considerations. This Section considers operations and management issues, under the FCAPS headings: OAM of Faults, Configuration, Accounting, Performance and Security. 7.1. Fault OAM Fault OAM is about how to tell the management system (or manual operator) that the system has recovered (or not) from a failure. Faults include node or link failures, a wrongly configured address in a node, a wrong address given in a signalling protocol, a wrongly configured parameter in a queueing algorithm, and so on. 7.2. Configuration OAM Perhaps the most important consideration here is that the level of detail of the standardisation affects what can be configured. We would like different implementations and configurations (eg choice of parameters) that are compliant with the PCN standard to work together successfully. Obvious configuration parameters are the PCN-lower-rate and PCN- upper-rate. A larger PCN-lower-rate enables more PCN-traffic to be admitted on a link, hence improving capacity utilisation. A PCN- upper-rate set further above the PCN-lower-rate allows greater increases in traffic (whether due to natural fluctuations or some Eardley (Editor) Expires February 9, 2008 [Page 23] Internet-Draft Document August 2007 unexpected event) before any flows are terminated, ie minimises the chances of unnecessarily triggering the termination mechanism. A greater gap, between the maximum rate at which PCN-traffic can be forwarded on a link and the PCN-lower-rate and PCN-upper-rate, increases the 'safety margin' - which can cover unexpected surges in traffic due to a re-routing event for instance. For instance an operator may want to design their network so that it can cope with a failure of any single PCN-node without terminating any flows. Setting the rates will therefore depend on things like: the operator's requirements, the link's capacity, the typical number of flows and perhaps their traffic characteristics, and so on. Other configurable parameters concern the PCN-boundary-nodes. For example, the amount of PCN-marked traffic above which new flows are blocked. Another configuration choice is the distribution of the functions concerning flows admission and termination, given in Section 5.4 and 5.6, and which could potentially be under the control of a configuration parameter. Another configuration decision is whether to operate both the admission control and termination mechanisms. Although we suggest that an operator uses both, this isn't required and some operators may want to implement only one. For example, an operator could use just admission control, solving heavy congestion (caused by re- routing) by 'just waiting' - as sessions end, existing microflows naturally depart from the system over time, and the admission control mechanism will prevent admission of new microflows that use the affected links. So the PCN-domain will naturally return to normal operation, but with reduced capacity. The drawback of this approach would be that until PCN-flows naturally depart to relieve the congestion, all PCN-flows as well as lower priority services will be adversely affected. On the other hand, an operator could just rely for admission control on statically provisioned capacity per PCN- ingress-node (regardless of the PCN-egress-node of a flow), as is typical in the hose model of the DiffServ architecture [RFC2475]. Such traffic conditioning agreements can lead to focused overload: many flows happen to focus on a particular link and then all flows through the congested link fail catastrophically. The flow termination mechanism could then be used to counteract such a problem. A different possibility is to configure only the PCN-lower-rate and hence only do one type of PCN-marking, but generate admission and flow termination responses from different levels of marking. This is suggested in [I-D.charny-pcn-single-marking] which gives some of the pros and cons of this approach. Eardley (Editor) Expires February 9, 2008 [Page 24] Internet-Draft Document August 2007 Another PCN WG document will specify PCN-marking, in particular how many PCN-packets get PCN-marked according to what measure of PCN- traffic. For instance an algorithm relating the current rate of PCN- traffic to the probability of admission-marking a packet. Depending on how tightly it is decided to specify this, there are potentially quite a few configuration choices, for instance: o does the probability go from 0% at one rate of PCN-traffic (the PCN-lower-rate) to 100% at a slightly higher rate (ie threshold- marking), or does it 'ramp up' gradually (as in RED)? Does the standard allow both? o how is the current rate of PCN-traffic measured? Rate cannot be measured instantaneously, so how is this smoothed? A sliding window or exponentially weighted moving average? o is the PCN-lower-rate a fixed parameter? An idea raised in [Songhurst] is that the PCN-lower-rate on each router should depend on the current amount of non-PCN-traffic; the aim is that resource allocation reflects the traffic mix - for instance more PCN-traffic could be admitted if the fraction of PCN-traffic was higher. Is this allowed? Another question is whether there are any configuration parameters that have to be set once to 'globally' control the whole PCN-domain (as required by some proposals). This may affect operational complexity and the chances of interoperability problems between kit from different vendors. 7.3. Accounting OAM Accounting at the flow level will have to record instances of flow admission, rejection and termination, but accounting itself is outside the scope of PCN. The ability to enable or disable flow accounting for specific classes of flow and to specify retrieval of accounting records in real time for specified classes of flow is a general requirement not specific to PCN that may, however, find specific use when diagnosing faults affecting PCN operation. 7.4. Performance OAM Performance OAM is about monitoring performance at run-time. There are a wide variety of performance metrics that it may be worth collecting at PCN-ingress-nodes, PCN-egress-nodes and PCN-interior- nodes. A detailed list of metrics is not part of this architecture document, but the sorts of things would be: Eardley (Editor) Expires February 9, 2008 [Page 25] Internet-Draft Document August 2007 o can the operator identify 'hot spots' in the network (links which most often do PCN-marking)? This would help them plan to install extra capacity where it is most needed. o what is the rate at which flows are admitted and terminated (for each pair of PCN-boundary-nodes)? Such information would be useful for fault management, networking planning and service level monitoring. 7.5. Security OAM Security OAM is finding out about security breaches or near-misses at run-time. 8. IANA Considerations This memo includes no request to IANA. 9. Security considerations Security considerations essentially come from the Trust Assumption (Section 3.1), ie that all PCN-nodes are PCN-enabled and trust each other for truthful PCN-marking and transport. PCN splits functionality between PCN-interior-nodes and PCN-boundary-nodes, and the security considerations are somewhat different for each, mainly because PCN-boundary-nodes are flow-aware and PCN-interior-nodes are not. o because the PCN-boundary-nodes are flow-aware, they are trusted to use that awareness correctly. The degree of trust required depends on the kinds of decisions they have to make and the kinds of information they need to make them. For example when the PCN- boundary-node needs to know the contents of the sessions for making the admission and termination decisions (perhaps based on the MLPP precedence), or when the contents are highly classified, then the security requirements for the PCN-boundary-nodes involved will also need to be high. o the PCN-ingress-nodes police packets to ensure a flow sticks within its agreed limit, and to ensure that only flows which have been admitted contribute PCN-traffic into the PCN-domain. The policer must drop (or perhaps re-mark to a different DSCP) any PCN-packets received that are outside this remit. This is similar to the existing IntServ behaviour. Between them the PCN-boundary- nodes must encircle the PCN-domain, otherwise PCN-packets could enter the PCN-domain without being subject to admission control, Eardley (Editor) Expires February 9, 2008 [Page 26] Internet-Draft Document August 2007 which would potentially destroy the QoS of existing flows. o PCN-interior-nodes aren't flow-aware. This prevents some security attacks where an attacker targets specific flows in the data plane - for instance for DoS or eavesdropping. o PCN-marking by the PCN-interior-nodes along the packet forwarding path needs to be trusted, because the PCN-boundary-nodes rely on this information. For instance a rogue PCN-interior-node could PCN-mark all packets so that no flows were admitted. Another possibility is that it doesn't PCN-mark any packets, even when it's pre-congested. More subtly, the rogue PCN-interior-node could perform these attacks selectively on particular flows, or it could PCN-mark the correct fraction overall, but carefully choose which flows it marked. o the PCN-boundary-nodes should be able to deal with DoS attacks and state exhaustion attacks based on fast changes in per flow signalling. o the signalling between the PCN-boundary-nodes (and possibly a central control node) must be protected from attacks. For example the recipient needs to validate that the message is indeed from the node that claims to have sent it. Possible measures include digest authentication and protection against replay and man-in- the-middle attacks. For the specific protocol RSVP, hop-by-hop authentication is in [RFC2747], and [I-D.behringer-tsvwg-rsvp-security-groupkeying] may also be useful; for a generic signalling protocol the PCN WG document on "Requirements for signalling" will describe the requirements in more detail. 10. Conclusions {ToDo:} 11. Acknowledgements This document is a revised version of [I-D.eardley-pcn-architecture]. Its authors were: P. Eardley, J. Babiarz, K. Chan, A. Charny, R. Geib, G. Karagiannis, M. Menth, T. Tsou. They are therefore contributors to this document. Thanks to those who've made comments on [I-D.eardley-pcn-architecture]: Bob Briscoe, Michael Menth, Lars Eggert, Steven Blake, Tina Tsou, Tom Taylor, Ruediger Geib, Joe Eardley (Editor) Expires February 9, 2008 [Page 27] Internet-Draft Document August 2007 Babiarz, Anna Charny, Joachim Charzinski, Georgios Karagiannis. This document is the result of discussions in the PCN WG and forerunner activity in the TSVWG. A number of previous drafts were presented to TSVWG: [I-D.chan-pcn-problem-statement], [I-D.briscoe-tsvwg-cl-architecture], [I-D.briscoe-tsvwg-cl-phb], [I-D.charny-pcn-single-marking], [I-D.babiarz-pcn-sip-cap], [I-D.lefaucheur-rsvp-ecn]. The authors of them were: B, Briscoe, P. Eardley, D. Songhurst, F. Le Faucheur, A. Charny, J. Babiarz, K. Chan, S. Dudley, G. Karagiannis, A. Bader, L. Westberg, J. Zhang, V. Liatsos, X-G. Liu. 12. Comments Solicited Comments and questions are encouraged and very welcome. They can be addressed to the IETF PCN working group mailing list . 13. References 13.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. 13.2. Informative References [I-D.briscoe-tsvwg-cl-architecture] Briscoe, B., "An edge-to-edge Deployment Model for Pre- Congestion Notification: Admission Control over a DiffServ Region", draft-briscoe-tsvwg-cl-architecture-04 (work in progress), October 2006. [I-D.briscoe-tsvwg-cl-phb] Briscoe, B., "Pre-Congestion Notification marking", draft-briscoe-tsvwg-cl-phb-03 (work in progress), October 2006. [I-D.charny-pcn-single-marking] Charny, A., "Pre-Congestion Notification Using Single Marking for Admission and Termination", draft-charny-pcn-single-marking-02 (work in progress), July 2007. [I-D.ietf-tsvwg-admitted-realtime-dscp] Baker, F., "DSCPs for Capacity-Admitted Traffic", draft-ietf-tsvwg-admitted-realtime-dscp-01 (work in Eardley (Editor) Expires February 9, 2008 [Page 28] Internet-Draft Document August 2007 progress), March 2007. [I-D.babiarz-pcn-sip-cap] Babiarz, J., "SIP Controlled Admission and Preemption", draft-babiarz-pcn-sip-cap-00 (work in progress), October 2006. [I-D.ietf-tsvwg-ecn-mpls] Davie, B., "Explicit Congestion Marking in MPLS", draft-ietf-tsvwg-ecn-mpls-01 (work in progress), June 2007. [I-D.lefaucheur-rsvp-ecn] Faucheur, F., "RSVP Extensions for Admission Control over Diffserv using Pre-congestion Notification (PCN)", draft-lefaucheur-rsvp-ecn-01 (work in progress), June 2006. [I-D.chan-pcn-problem-statement] Chan, K., "Pre-Congestion Notification Problem Statement", draft-chan-pcn-problem-statement-01 (work in progress), October 2006. [I-D.ietf-pwe3-congestion-frmwk] Bryant, S., "Pseudowire Congestion Control Framework", draft-ietf-pwe3-congestion-frmwk-00 (work in progress), February 2007. [I-D.briscoe-tsvwg-ecn-tunnel] "", . [I-D.briscoe-re-pcn-border-cheat] "", . [I-D.behringer-tsvwg-rsvp-security-groupkeying] "", . [I-D.eardley-pcn-architecture] "", . [I-D.chan-pcn-encoding-comparison] "", . Eardley (Editor) Expires February 9, 2008 [Page 29] Internet-Draft Document August 2007 [RFC4774] Floyd, S., "Specifying Alternate Semantics for the Explicit Congestion Notification (ECN) Field", BCP 124, RFC 4774, November 2006. [RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z., and W. Weiss, "An Architecture for Differentiated Services", RFC 2475, December 1998. [RFC3246] Davie, B., Charny, A., Bennet, J., Benson, K., Le Boudec, J., Courtney, W., Davari, S., Firoiu, V., and D. Stiliadis, "An Expedited Forwarding PHB (Per-Hop Behavior)", RFC 3246, March 2002. [RFC4594] Babiarz, J., Chan, K., and F. Baker, "Configuration Guidelines for DiffServ Service Classes", RFC 4594, August 2006. [RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition of Explicit Congestion Notification (ECN) to IP", RFC 3168, September 2001. [RFC2211] Wroclawski, J., "Specification of the Controlled-Load Network Element Service", RFC 2211, September 1997. [RFC2998] Bernet, Y., Ford, P., Yavatkar, R., Baker, F., Zhang, L., Speer, M., Braden, R., Davie, B., Wroclawski, J., and E. Felstaine, "A Framework for Integrated Services Operation over Diffserv Networks", RFC 2998, November 2000. [RFC3270] Le Faucheur, F., Wu, L., Davie, B., Davari, S., Vaananen, P., Krishnan, R., Cheval, P., and J. Heinanen, "Multi- Protocol Label Switching (MPLS) Support of Differentiated Services", RFC 3270, May 2002. [RFC1633] Braden, B., Clark, D., and S. Shenker, "Integrated Services in the Internet Architecture: an Overview", RFC 1633, June 1994. [RFC2983] Black, D., "Differentiated Services and Tunnels", RFC 2983, October 2000. [RFC2747] Baker, F., Lindell, B., and M. Talwar, "RSVP Cryptographic Authentication", RFC 2747, January 2000. [ITU-MLPP] "Multilevel Precedence and Pre-emption Service (MLPP)", ITU-T Recommendation I.255.3, 1990. Eardley (Editor) Expires February 9, 2008 [Page 30] Internet-Draft Document August 2007 [Iyer] "An approach to alleviate link overload as observed on an IP backbone", IEEE INFOCOM , 2003, . [Shenker] "Fundamental design issues for the future Internet", IEEE Journal on selected areas in communications pp 1176 - 1188, Vol 13 (7), 1995. [Songhurst] "Guaranteed QoS Synthesis for Admission Control with Shared Capacity", BT Technical Report TR-CXR9-2006-001, Feburary 2006, . Author's Address Philip Eardley BT B54/77, Sirius House Adastral Park Martlesham Heath Ipswich, Suffolk IP5 3RE United Kingdom Email: philip.eardley@bt.com Eardley (Editor) Expires February 9, 2008 [Page 31] Internet-Draft Document August 2007 Full Copyright Statement Copyright (C) The IETF Trust (2007). 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. This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 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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). Eardley (Editor) Expires February 9, 2008 [Page 32]