Network Working Group B. Davie Internet-Draft Cisco Systems, Inc. Expires: December 20, 2006 B. Briscoe J. Tay BT Research June 18, 2006 Explicit Congestion Marking in MPLS draft-davie-ecn-mpls-00.txt 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 December 20, 2006. Copyright Notice Copyright (C) The Internet Society (2006). Abstract RFC 3270 defines how to support the Diffserv arhitecture in MPLS networks, including how to encode Diffserv Code Points (DSCPs) in an MPLS header. DSCPs may be encoded in the EXP field, while other uses of that field are not precluded. RFC3270 makes no statement about how Explicit Congestion Notification (ECN) marking might be encoded in the MPLS header. This draft defines how an operator might define Davie, et al. Expires December 20, 2006 [Page 1] Internet-Draft ECN for MPLS June 2006 some of the EXP codepoints for explicit congestion notification, without precluding other uses. 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]. Davie, et al. Expires December 20, 2006 [Page 2] Internet-Draft ECN for MPLS June 2006 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.1. Background . . . . . . . . . . . . . . . . . . . . . . . . 4 1.2. Intent . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.3. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5 2. Use of MPLS EXP Field for ECN . . . . . . . . . . . . . . . . 5 3. Per-domain ECT checking . . . . . . . . . . . . . . . . . . . 7 4. ECN-enabled MPLS domain . . . . . . . . . . . . . . . . . . . 8 4.1. Pushing (adding) one or more labels to an IP packet . . . 8 4.2. Pushing one or more labels onto an MPLS labelled packet . 8 4.3. Congestion experienced in an interior MPLS node . . . . . 9 4.4. Crossing a Diffserv Domain Boundary . . . . . . . . . . . 9 4.5. Popping an MPLS label (not the end of the stack) . . . . . 9 4.6. Popping the last MPLS label in the stack . . . . . . . . . 9 4.7. Diffserv Tunneling Models . . . . . . . . . . . . . . . . 10 4.8. Extension to Pre-Congestion Notification . . . . . . . . . 10 4.8.1. Label Push onto IP packet . . . . . . . . . . . . . . 10 4.8.2. Pushing Additional MPLS Labels . . . . . . . . . . . . 10 4.8.3. Admission Control or Pre-emption Marking inside MPLS domain . . . . . . . . . . . . . . . . . . . . . 11 4.8.4. Popping an MPLS Label (not end of stack) . . . . . . . 11 4.8.5. Popping the last MPLS Label to expose IP header . . . 11 5. ECN-disabled MPLS domain . . . . . . . . . . . . . . . . . . . 11 6. The use of more codepoints with E-LSPs and L-LSPs . . . . . . 11 7. Relationship to tunnel behavior in RFC 3168 . . . . . . . . . 12 7.1. Alternative approach to support ECN in an MPLS domain . . 12 8. Example Uses . . . . . . . . . . . . . . . . . . . . . . . . . 13 8.1. RFC3168-style ECN . . . . . . . . . . . . . . . . . . . . 13 8.2. ECN Co-existence with Diffserv E-LSPs . . . . . . . . . . 14 8.3. Congestion-feedback-based Traffic Engineering . . . . . . 14 8.4. PCN flow admission control and flow pre-emption . . . . . 14 9. Deployment Considerations . . . . . . . . . . . . . . . . . . 15 9.1. Marking non-ECN Capable Packets . . . . . . . . . . . . . 15 9.2. Non-ECN capable routers in an MPLS Domain . . . . . . . . 16 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16 11. Security Considerations . . . . . . . . . . . . . . . . . . . 16 12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 17 13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17 13.1. Normative References . . . . . . . . . . . . . . . . . . . 17 13.2. Informative References . . . . . . . . . . . . . . . . . . 18 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 20 Intellectual Property and Copyright Statements . . . . . . . . . . 21 Davie, et al. Expires December 20, 2006 [Page 3] Internet-Draft ECN for MPLS June 2006 1. Introduction 1.1. Background [RFC3270] defines how to support the Diffserv arhitecture in MPLS networks, including how to encode Diffserv Code Points (DSCPs) in an MPLS header. DSCPs may be encoded in the EXP field, while other uses of that field are not precluded. RFC3270 makes no statement about how Explicit Congestion Notification (ECN) marking might be encoded in the MPLS header. This draft defines how an operator might define some of the EXP codepoints for explicit congestion notification, without precluding other uses. In parallel to the activity defining the addition of ECN to IP [RFC3168], two proposals were made to add ECN to MPLS [Floyd][Shayman]. These proposals, however, fell by the way-side. With ECN for IP now being a proposed standard, and developing interest in using pre-congestion notification (PCN) for admission control and flow pre-emption[I-D.briscoe-tsvwg-cl- architecture], there is consequent interest in being able to support ECN across IP networks consisting of MPLS-enabled domains. Therefore it is necessary to specify the protocol for including ECN or PCN in the MPLS shim header, and the protocol behaviour of edge MPLS nodes. We note that in [RFC3168] there are four codepoints used for ECN marking, which are encoded using two bits of the IP header. The MPLS EXP field is the logical place to encode ECN codepoints, but with only 3 bits (8 codepoints) available, and with the same field being used to convey DSCP information as well, there is a clear incentive to conserve the number of codepoints consumed for ECN purposes. Efficient use of the EXP field has been a focus of prior drafts [Floyd] [Shayman] and we draw on those efforts in this draft as well. 1.2. Intent Our intent is to specify how the MPLS shim header[RFC3032] should denote ECN marking and how MPLS nodes should understand whether the transport for a packet will be ECN capable. We offer this as a building block, from which to build different congestion notification systems. We do not intend to specify how the resulting congestion notification is fed back to an upstream node that can mitigate congestion. For instance, unlike [Shayman], we do not specify edge- to-edge MPLS domain feedback, but we also do not preclude it. Nonetheless, we do specify how the egress node of an MPLS domain should copy congestion notification from the MPLS shim into the underlying IP header if the ECN is to be carried onward towards the IP receiver. But we do NOT mandate that MPLS congestion notification must be copied into the IP header for onward transmission. This draft aims to be generic for any use of congestion notification in MPLS. PCN or traffic engineering are merely two of many motivating Davie, et al. Expires December 20, 2006 [Page 4] Internet-Draft ECN for MPLS June 2006 applications (see Section 8.) 1.3. Terminology This document draws freely on the terminology of ECN [RFC3168] and MPLS [RFC3031]. For ease of reference, we have included some definitions here, but refer the reader to the references above for complete specifications of the relevant technologies: o CE: Congestion Experienced. One of the states with which a packet may be marked in a network supporting ECN. A packet is marked in this state by an ECN-capable router, to indicate that this router was experiencing congestion at the time the packet arrived. o ECT: ECN-capable Transport. One of the ECN states which a packet may be in when it is sent by an end system. An end system marks a packet with an ECT codepoint to indicate that the end-points of the transport protocol are ECN-capable. A router may not mark a packet as CE unless the packet was marked ECT when it arrived. o Not-ECT: Not ECN capable transport. An end system marks a packet with this codepoint to indicate that the end-points of the transport protocol are not ECN-capable. A congested router cannot mark such packets as CE, and thus can only drop them to indicate congestion. o EXP field. A 3 bit field in the MPLS label header [RFC3032] which may be used to convey Diffserv information (and used in this draft to carry ECN information). o PHP. Penultimate Hop Popping. An MPLS operation in which the penultimate Label Switching Router (LSR) on a Label Switched Path (LSP) removes the top label from the packet before forwarding the packet to the final LSR on the LSP. 2. Use of MPLS EXP Field for ECN We propose that LSRs configured for explicit congestion notification should use the EXP field in the MPLS shim header. However, RFC 3270 already defines use of codepoints in the EXP field for differentiated services. Although it does not preclude other compatible uses of the EXP field, this clearly seems to limit the space available for ECN, given the field is only 3 bits (8 codepoints). RFC 3270 defines two possible approaches for requesting differentiated service treatment from an LSR. Davie, et al. Expires December 20, 2006 [Page 5] Internet-Draft ECN for MPLS June 2006 o In the E-LSP approach, different codepoints of the EXP field in the MPLS shim header are used to indicate the packet's per hop behaviour (PHB). o In the L-LSP approach, an MPLS label is assigned for each PHB scheduling class (PSC, as defined in [RFC3260], so that an LSR determines both its forwarding and its scheduling behaviour from the label. If an MPLS domain uses the L-LSP approach, there is likely to be space in the EXP field for ECN codepoint(s). Where the E-LSP approach is used, then codepoint space in the EXP field is likely to be scarce. This draft focuses on interworking ECN marking with the E-LSP approach as it is the tougher problem. Consequently the same approach can also be applied with L-LSPs. We recommend that explicit congestion notification in MPLS should use codepoints instead of bits in the EXP field. Since not every DSCP will need an associated ECN codepoint and some DSCPs might need two ECN codepoints [I-D.briscoe-tsvwg-cl-architecture], it would be wasteful and incorrect to assign a bit for ECN. For each PHB that uses ECN marking, we assume one EXP codepoint will be defined meaning not congestion marked (Not-CM), and at least one other codepoint will be defined meaning congestion marked (CM). Therefore, each PHB that uses ECN marking will consume at least two EXP codepoints. But PHBs that do not use ECN marking will only consume one. Further, we wish to use minimal space in the MPLS shim header to tell interior LSRs whether each packet will be received by an ECN-capable transport (ECT). Nonetheless, we must ensure that an end-point that would not understand an ECN mark will not receive one, otherwise it will not be able to respond to congestion as it should. In the past, three solutions to this problem have been proposed: o One possible approach is for congested LSRs to mark the ECN field in the underlying IP header at the bottom of the label stack. Although many commercial LSRs routinely access the IP header for other reasons (ECMP), there are numerous drawbacks to attempting to find an IP header beneath an MPLS label stack. Notably, there is the challenge of detecting the absence of an IP header when non-IP packets are carried on an LSP. Therefore we will not consider this approach further. o In the schemes suggested by [Floyd] and [Shayman], ECT and CE are overloaded into one bit, so that a 0 means ECT while a 1 might either mean Not-ECT or it might mean CE. A packet that has been Davie, et al. Expires December 20, 2006 [Page 6] Internet-Draft ECN for MPLS June 2006 marked as having experienced congestion upstream, and then is picked out for marking at a second congested LSR, will be dropped by the second LSR since it cannot determine whether the packet has previously experienced congestion or if ECN is not supported by the transport. While such an approach seemed potentially palatable for traditional ECN, we do not recommend it here for the following reasons. In some cases we wish to be able to use ECN marking long before actual congestion (e.g. pre-congestion notification). In these circumstances, marking rates at each LSR might be non- negligible most of the time, so the chances of a previously marked packet encountering an LSR that wants to mark it again will also be non-negligible. This will lead to unacceptable drop rates. For instance, if the typical marking rate at every router or LSRs is p, and the typical diameter of the network of LSRs is d, then the probability that a marked packet will be marked again is 1- [1+p(d-1)][1-p]^(d-1). For instance, with 6 LSRs in a row, each marking ECN with 1% probability, this bit overloading scheme would introduce a drop rate of 0.15% unnecessarily. Given most modern core networks are sized to introduce near-zero packet drop, it may be unacceptable to drop over one in a thousand packets unnecessarily. o A third possible approach is for interior LSRs to assume that the endpoints are ECN-capable, but this assumption is checked when the final label is popped. If an interior LSR has marked ECN in the EXP field of the shim, but the IP header says the endpoints are not ECN capable, the edge router (or penultimate if using penultimate hop popping) drops the packet. We recommend this scheme, which we call `per-domain ECT checking'; and define it more precisely in the following section. Its chief drawback is that it can involve packets continuing to be forwarded after encountering congestion only to be dropped at the egress of the MPLS domain. The rationale for this decision is given in Section 9.1. 3. Per-domain ECT checking For the purposes of this discussion, we define the egress nodes of an MPLS domain as the nodes that pop the last MPLS label from the label stack, exposing the IP (or, potentially non-IP) header. Note that such a node may be the ultimate or penultimate hop of an LSP, depending on whether penultimate hop popping (PHP) is employed. In the per-domain ECT checking approach, the egress nodes take responsibility for checking whether the transport is ECN capable. Davie, et al. Expires December 20, 2006 [Page 7] Internet-Draft ECN for MPLS June 2006 This draft does not specify how these nodes should pass on congestion notification, because different approaches are likely in different scenarios. However, if congestion notification in the MPLS header is copied into the IP header, the procedure MUST conform to the specification given here. If congestion notification is passed to the transport without first passing it onward in the IP header, the approach used must take similar care to check that the transport is ECN capable before passing it ECN markings. Specifically, if the transport for a particular congestion marked MPLS packet is found not to be ECN- capable, the packet MUST be dropped at this egress node. In the per-domain ECT checking approach, only the egress nodes check whether an IP packet is destined for an ECN-capable transport. Therefore, any single LSR within an MPLS domain MUST NOT be configured to enable ECN marking unless all the egress LSRs surrounding it are already configured to handle ECN marking. We call a domain surrounded by ECN-capable egress LSRs an ECN-enabled MPLS domain. This term only implies that all the egress LSRs are ECN-enabled; some interior LSRs may not be ECN-enabled. For instance, it would be possible to use legacy LSRs incapable of supporting ECN in the interior of an MPLS domain as long as all the egress LSRs were ECN-capable. Note that if PHP is used, the "penultimate hop" routers which perform the pop operation do need to be ECN-enabled, since they are acting in this context as egress LSRs. 4. ECN-enabled MPLS domain In the following subsections we describe various operations affecting the ECN marking of a packet that may be performed at MPLS edge and core LSRs. 4.1. Pushing (adding) one or more labels to an IP packet On encapsulating an IP packet with an MPLS label stack, the ECN field must be translated from the IP packet into the MPLS EXP field. The Not-CM (not congestion marked) state is set in the MPLS EXP field if the ECN status of the IP packet is "Not ECT" or ECT(1) or ECT(0). The CM state is set if the ECN status of the IP packet is "CE". If more than one label is pushed at one time, the same value should be placed in the EXP value of all label stack entries. 4.2. Pushing one or more labels onto an MPLS labelled packet The EXP field is copied directly from the topmost label before the Davie, et al. Expires December 20, 2006 [Page 8] Internet-Draft ECN for MPLS June 2006 push to the newly added outer label. If more than one label is being pushed, the same EXP value is copied to all label stack entries. 4.3. Congestion experienced in an interior MPLS node If the EXP codepoint of the packet maps to a PHB that uses ECN marking and the marking algorithm requires the packet to be marked, the CM state is set (irrespective of whether it is already in the CM state). If the buffer is full, the packet would be dropped. 4.4. Crossing a Diffserv Domain Boundary If an MPLS-encapsulated packet crosses a Diffserv domain boundary, it may be the case that the two domains use different encodings of the same PHB in the EXP field. In such cases, the EXP field must be rewritten at the domain boundary. If the PHB is one that supports ECN, then the appropriate ECN marking should also be preserved when the EXP field is mapped at the boundary. The related issue of Diffserv tunnel models is discussed in Section 4.7. 4.5. Popping an MPLS label (not the end of the stack) When a packet has more than one MPLS label in the stack and the top label is popped, another MPLS label is exposed. In this case the ECN information should be transferred from the outer EXP field to the inner MPLS label in the following manner. If the inner EXP field is Not-CM, the inner EXP field is set to the same CM or Not-CM state as the outer EXP field. If the inner EXP field is CM, it remains unchanged whatever the outer EXP field. Note that an inner value of CM and an outer value of not-CM should be considered anomalous, and SHOULD be logged in some way by the LSR. 4.6. Popping the last MPLS label in the stack When the last MPLS label is popped from the packet, its payload is exposed. If that packet is not IP, and does not have any capability equivalent to ECT, it is assumed Not-ECT and treated as such. That means that if the EXP value of the MPLS header was CM, the packet MUST be dropped. Assuming an IP packet was exposed, we have to examine whether that packet is ECT or not. If the inner IP packet is Not-ECT, its ECN field remains unchanged if the EXP field is Not-CM. However, a Not- ECT packet MUST be dropped if the EXP field is CM. Davie, et al. Expires December 20, 2006 [Page 9] Internet-Draft ECN for MPLS June 2006 If the ECN field of the inner packet is set to ECT(0), ECT(1) or CE, the ECN field remains unchanged if the EXP field is set to Not-CM. The ECN field is set to CE if the EXP field is CM. Note that an inner value of CE and an outer value of not-CM should be considered anomalous, and SHOULD be logged in some way by the LSR. 4.7. Diffserv Tunneling Models [RFC3270] describes three tunneling models for Diffserv support across MPLS Domains, referred to as the uniform, short pipe, and pipe models. The differences between these models lie in whether the Diffserv treatment that applies to a packet while it travels along a particular LSP is carried to the last hop of the LSP and beyond the last hop. Depending on which mode is preferred by an operator, the EXP value or DSCP value of an exposed header following a label pop may or may not be dependent on the EXP value of the label that is removed by the pop operation. We believe that in the case of ECN marking, the use of these models should only apply to the encoding of the Diffserv PHB in the EXP value, and that the choice of codepoint for ECN should always be made based on the procedures described above, independent of the tunneling model. 4.8. Extension to Pre-Congestion Notification To fully support PCN [I-D.briscoe-tsvwg-cl-architecture] in an MPLS domain for a particular PHB, a total of 3 codepoints need to be allocated for that PHB. (See Section 8.4 for further discussion of PCN and the possibility of using fewer codepoints.) These 3 codepoints represent the admission marked (AM), pre-emption marked (PM) and not marked (NM) states. The procedures described above need to be slightly modified to support this scenario. The following procedures are invoked when the topmost DSCP or EXP value indicates a PHB that supports PCN. 4.8.1. Label Push onto IP packet If the IP packet header indicates AM, set the EXP value of all entries in the label stack to AM. If the IP packet header indicates PM, set the EXP value of all entries in the label stack to PM. For any other marking of the IP header, set the EXP value of all entries in the label stack to NM. 4.8.2. Pushing Additional MPLS Labels The procedures of Section 4.2 apply. Davie, et al. Expires December 20, 2006 [Page 10] Internet-Draft ECN for MPLS June 2006 4.8.3. Admission Control or Pre-emption Marking inside MPLS domain The EXP value can be set to AM or PM according to the same procedures as described in [I-D.briscoe-tsvwg-cl-phb]. 4.8.4. Popping an MPLS Label (not end of stack) When popping an MPLS Label exposes another MPLS label, the AM or PM marking should be transferred to the exposed EXP field in the following manner: if the inner EXP value is NM, then it should be set to the same marking state as the EXP value of the popped label stack entry. If the inner EXP value is AM, it should be unchanged if the popped EXP value was AM, and it should be set to PM if the popped EXP value was PM. If the popped EXP value was NM, this should be logged in some way and the inner EXP value should be unchanged. If the inner EXP value is PM, it should be unchanged whatever the popped EXP value was, but any EXP value other than PM should be logged. 4.8.5. Popping the last MPLS Label to expose IP header When popping the last MPLS Label exposes the IP header, the AM or PM marking should be transferred to the exposed IP header field in the following manner: if the inner IP header value is neither AM nor PM, and the EXP value was NM, then the IP header should be unchanged. For any other EXP value, the IP header should be set to the same marking state as the EXP value of the popped label stack entry. If the inner IP header value is AM, it should be unchanged if the popped EXP value was AM, and it should be set to PM if the popped EXP value was PM. If the popped EXP value was NM, this should be logged in some way and the inner IP header value should be unchanged. If the IP header value is PM, it should be unchanged whatever the popped EXP value was, but any EXP value other than PM should be logged. 5. ECN-disabled MPLS domain If ECN is not enabled on all the egress LSRs of a domain, ECN MUST NOT be enabled on any LSRs throughout the domain. If congestion is experienced on any LSR in an ECN-disabled MPLS domain, packets MUST be dropped NOT marked. The exact algorithm for deciding when to drop packets during congestion (e.g. tail-drop, RED, etc.) is a local matter for the operator of the domain. 6. The use of more codepoints with E-LSPs and L-LSPs RFC 3270 gives different options with E-LSPs and L-LSPs and some of those could potentially provide ample EXP codepoints for ECN/PCN. Davie, et al. Expires December 20, 2006 [Page 11] Internet-Draft ECN for MPLS June 2006 However, deploying L-LSPs vs E-LSPs has many implications such as platform support and operational complexity. The above ECN/PCN MPLS solution should provide some flexibility. If the operator has deployed one L-LSP per PHB scheduling class, then EXP space will be a non-issue and it could be used to achieve more sophisticated ECN/PCN behavior if required. If the operator wants to stick to E-LSPs and uses a handful of EXP codepoints for Diffserv, it may be desirable to operate with a minimum number of extra ECN/PCN codepoints, even if this comes with some compromise on ECN/PCN optimality. See Section 8 for discussion of some possible deployment scenarios. 7. Relationship to tunnel behavior in RFC 3168 [RFC3168] defines two modes of encapsulating ECN-marked IP packets inside additonal IP headers when tunnels are used. The two modes are the "full functionality" and "limited functionality" modes. In the full functionality mode, the ECT information from the inner header is copied to the outer header at the tunnel ingress, but the CE information is not. In the limited functionality mode, neither ECT nor CE information is copied to the outer header, and thus ECN cannot be applied to the encapsulated packet. The behavior that is specified in Section 4 of this document resembles the "full functionality" mode in the sense that it conveys some information from inner to outer header, and in the sense that it enables full ECN support along the MPLS LSP (which is analogous to an IP tunnel in this context). However it differs in one respect, which is that the CE information is conveyed from the inner header to the outer header. Our reason for this different design choice is to give interior routers and LSRs more information about upstream marking in multi-bottleneck cases. For instance, the flow pre-emption marking mechanism proposed for PCN works by only considering packets for marking that have not already been marked upstream. Unless existing pre-emption marking is copied from the inner to the outer header at tunnel ingress, the mechanism doesn't pre-empt enough traffic in cases where anomalous events hit multiple MPLS domains at once. [RFC3168] does not give any reasons against conveying CE information from the inner header to the outer in the "full functionality" mode. So, rather than define different encapsulation methods for ECN and PCN, Section 4 defines a common approach for both. 7.1. Alternative approach to support ECN in an MPLS domain It is possible to define an approach for MPLS support of ECN that more closely resembles that of the full functionality mode of [RFC3168]. This approach would differ from that described in Section 4 in the following ways: Davie, et al. Expires December 20, 2006 [Page 12] Internet-Draft ECN for MPLS June 2006 o when pushing one or more MPLS labels onto an IP packet, the not-CM state is set in the EXP field of all label stack entries o when pushing one or more MPLS labels onto an MPLS packet, the not-CM state is set in the EXP field of all newly added label stack entries o when popping an MPLS label and the exposed header is MPLS (i.e. this is not the end of stack), the EXP field of the MPLS packet should be set to CM if the popped label's EXP value was CM and left unchanged otherwise o when popping an MPLS label and the exposed header is IP, the IP ECN field should be set to CE if the EXP value was CM and if the IP header indicated that the packet was ECN capable. If the IP header indicated not-ECT and the EXP value was CM, the packet MUST be dropped. If the EXP value was not-CM, the ECN field in the IP header is unchanged. The advantages of this scheme over that described in Section 4 are greater similarity to [RFC3168], and the ability to determine, at the end of an LSP, that congestion either did or did not occur along that LSP (since the initial state is always not-CM at the start of an LSP). A disadvantage of this approach is that exceptions to this rule are necessary in cases where the marking process on LSRs needs to depend on whether a packet has already suffered upstream marking. The currently proposed pre-emption marking in PCN is an example where such an exception would be necessary (see the discussion at the start of Section 7). 8. Example Uses 8.1. RFC3168-style ECN [RFC3168] proposes the use of ECN in TCP and introduces the use of ECN-Echo and CWR flags in the TCP header for initialisation. The TCP sender responds accordingly (such as not increasing the congestion window) when it receives an ECN-Echo (ECE) ACK packet (that is, an ACK packet with ECN-Echo flag set in the TCP header), then the sender knows that congestion was encountered in the network on the path from the sender to the receiver. It would be possible to enable ECN in an MPLS domain for Diffserv PHBs like AF and best efforts that are expected to be used by TCP and similar transports (e.g. DCCP [RFC4340]). Then end-to-end Davie, et al. Expires December 20, 2006 [Page 13] Internet-Draft ECN for MPLS June 2006 congestion control in transports capable of understanding ECN would be able to respond to approaching congestion on LSRs without having to rely on packet discard to signal congestion. 8.2. ECN Co-existence with Diffserv E-LSPs Many operators today have deployed Diffserv using the E-LSP approach of [RFC3270]. In many cases the number of PHBs used is less than 8, and hence there remain available codepoints in the EXP space. If an operator wished to support ECN for single PHB, this can be accomplished by simply allocated a second codepoint to the PHB for the "CM" state of that PHB and retaining the old codepoint for the "not-CM" state. An operator with only four deployed PHBs could of course enable ECN marking on all those PHBs. It is easy to imagine cases where some PHBs might benefit more from ECN than others - for example, an operator might use ECN on a premium data service but not on a PHB used for best effort internet traffic. As an illustrative example of how the EXP field might be used in this case, consider the example of an operator who is using the aggregated service classes described in [I-D.chan-tsvwg-diffserv-class-aggr]. He may choose to support ECN only for the Assured Elastic Treatment Aggregate, using the EXP codepoint 010 for the not-CM state and 011 for the CM state. All other codepoints could be the same as in [I-D.chan-tsvwg-diffserv-class-aggr]. Of course any other combination of EXP values can be used according to the specific set of PHBs and marking conventions used within that operator's network. 8.3. Congestion-feedback-based Traffic Engineering Shayman's traffic engineering [Shayman] proposed the use of ECN by an egress LSR feeding back congestion to an ingress LSR to mitigate congestion by employing dynamic traffic engineering techniques such as shifting flows to an alternate path. It proposed a new RSVP TUNNEL CONGESTION message which was sent to the ingress LSR and ignored by transit LSRs. 8.4. PCN flow admission control and flow pre-emption [I-D.briscoe-tsvwg-cl-architecture] proposes using pre-congestion notification (PCN) on routers within an edge-to-edge Diffserv region to control admission of new flows to the region and, if necessary, to pre-empt existing flows in response to disasters and other anomalous routing events. In this approach, the current level of PCN marking is picked up by the signalling used to initiate each flow in order to inform the admission control decision for the whole region at once. As an example, a minor extension to RSVP signalling has been proposed [I-D.lefaucheur-rsvp-ecn] to carry this message, but a similar Davie, et al. Expires December 20, 2006 [Page 14] Internet-Draft ECN for MPLS June 2006 approach has also been proposed that uses NSIS signalling [I-D.ietf- nsis-rmd]. If it is possible for LSRs to signify congestion in MPLS, PCN marking could be used for admission control and flow pre-emption across a Diffserv region, irrespective of whether it contained pure IP routers, MPLS LSRs, or both. Indeed, the solution could be somewhat more efficient to implement if aggregates could identify themselves by their MPLS label. Section 4.8 describes the mechanisms by which the necessary markings for PCN could be carried in the MPLS header. As an illustrative example of how the EXP field might be used in this case, consider the example of an operator who is using the aggregated service classes described in [I-D.chan-tsvwg-diffserv-class-aggr]. He may choose to support PCN only for the Real Time Treatment Aggregate, using the EXP codepoint 100 for the not-marked (NM) state, 101 for the Admission Marked (AM) state, and 111 for the Pre-emption Marked (PM) state. All other codepoints could be the same as in [I-D.chan-tsvwg-diffserv-class-aggr]. Of course any other combination of EXP values can be used according to the specific set of PHBs and marking conventions used within that operator's network. It might also be possible to deploy a similar solution using PCN marking over MPLS for just admission control alone, or just flow pre- emption alone, particularly if codepoint space was at a premium in the MPLS EXP field. However, the feasibility of deploying one without the other would require further study. 9. Deployment Considerations 9.1. Marking non-ECN Capable Packets What is the consequences of marking a packet that is not ECN-capable? Even if it will be dropped before leaving the domain, doesn't this consume resources unnecessarily? The problem only arises if there is congestion downstream of an earlier congested node. It might be that marked packets are carried through this second congested router when, within the underlying IP header they are not ECN capable, so they will be dropped later. Such packets might cause other packets to be marked (or dropped) that would not otherwise have been. We decided to use the per-domain ECT checking approach because it would become optimal as ECN deployment became prevalent. The situation where traffic is carried beyond a congested LSR only to be dropped later should become less prevalent as more transports use Davie, et al. Expires December 20, 2006 [Page 15] Internet-Draft ECN for MPLS June 2006 ECN. This is why we chose not to use the [Floyd] alternative which introduced a low but persistent level of unnecessary packet drop for all time. Although that scheme did not carry droppable traffic to the edge of the MPLS domain, we felt this was a small price to pay, and it was anyway only of concern until ECN had become more widely deployed. A partial solution would be to preferentially drop packets arriving at a congested router that were already marked. There is no solution to the problem of marking a packet congested by another packet that should have been dropped. However, the chance of such an occurrence is very low and the consequences are not significant. It merely causes an application to very occasionally slow down its rate when it did not have to. 9.2. Non-ECN capable routers in an MPLS Domain What if an MPLS domain wants to use ECN, but not all legacy routers are able to support it? If the legacy router(s) are used in the interior, this is not a problem. They will simply have to drop the packets if they are congested, rather than mark them, which is the standard behaviour for IP routers that are not ECN-enabled. If the legacy router were used as an egress router, it would not be able to check the ECN capability of the transport correctly. An operator in this position would not be able to use this solution and therefore MUST NOT enable ECN unless all egress routers are ECN- capable. 10. IANA Considerations This document makes no request of IANA. Note to RFC Editor: this section may be removed on publication as an RFC. 11. Security Considerations We believe no new vulnerabilities are introduced by this draft. We have considered whether malicious sources might be able to exploit the fact that interior LSRs will mark packets that are Not-ECT, relying on their egress LSR to drop them. Although this might allow sources to engineer a situation where more traffic is carried across Davie, et al. Expires December 20, 2006 [Page 16] Internet-Draft ECN for MPLS June 2006 an MPLS domain than should be, we figured that even if we hadn't introduced this feature, these sources would have been able to prevent these LSRs dropping this traffic anyway, simply by setting ECT in the first place. An ECN sender can use the ECN nonce [RFC3540] to detect a misbehaving receiver. The ECN nonce works correctly across an MPLS domain without requiring any specific support from the proposal in this draft. The nonce does not need to be present in the MPLS shim header. As long as the nonce is present in the IP header when the ECN information is copied from the last MPLS shim header, it will be overwritten if congestion has been experienced by an LSR. This is all that is necessary for the sender to detect a misbehaving receiver. An alternative proposal currently in progress in the IETF [I-D.briscoe-tsvwg-re-ecn-tcp] allows the network to prevent misbehaviour by senders or receivers or other routers. Like the ECN nonce, it works correctly without requiring any specific support from the proposal in this draft. It uses a bit in the IP header (the RE bit) which is set by the sender and never changed along the path-it is only read by certain policing elements in the network. There is no need for a copy of this bit in the MPLS shim, as policing nodes can examine the IP header if they need to, particularly given they are intended to only be necessary at domain borders where MPLS headers are often removed. 12. Acknowledgements Thanks to K.K. Ramakrishnan and Sally Floyd for getting us thinking about this in the first place and for providing advice on tunneling of ECN packets, and to Joe Babiarz and Ben Niven-Jenkins for their comments on the draft. 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. [RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z., and W. Weiss, "An Architecture for Differentiated Services", RFC 2475, December 1998. [RFC3031] Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol Davie, et al. Expires December 20, 2006 [Page 17] Internet-Draft ECN for MPLS June 2006 Label Switching Architecture", RFC 3031, January 2001. [RFC3032] Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y., Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack Encoding", RFC 3032, January 2001. [RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition of Explicit Congestion Notification (ECN) to IP", RFC 3168, September 2001. [RFC3260] Grossman, D., "New Terminology and Clarifications for Diffserv", RFC 3260, April 2002. [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. 13.2. Informative References [Floyd] "A Proposal to Incorporate ECN in MPLS", 1999. Work in progress. http://www.icir.org/floyd/papers/ draft-ietf-mpls-ecn-00.txt [I-D.briscoe-tsvwg-cl-architecture] Briscoe, B., "A Framework for Admission Control over DiffServ using Pre-Congestion Notification", draft-briscoe-tsvwg-cl-architecture-02 (work in progress), March 2006. [I-D.briscoe-tsvwg-cl-phb] Briscoe, B., "Pre-Congestion Notification marking", draft-briscoe-tsvwg-cl-phb-01 (work in progress), March 2006. [I-D.briscoe-tsvwg-re-ecn-border-cheat] Briscoe, B., "Emulating Border Flow Policing using Re-ECN on Bulk Data", draft-briscoe-tsvwg-re-ecn-border-cheat-00 (work in progress), February 2006. [I-D.briscoe-tsvwg-re-ecn-tcp] Briscoe, B., "Re-ECN: Adding Accountability for Causing Congestion to TCP/IP", draft-briscoe-tsvwg-re-ecn-tcp-01 (work in progress), March 2006. [I-D.chan-tsvwg-diffserv-class-aggr] Chan, K., "Aggregation of DiffServ Service Classes", Davie, et al. Expires December 20, 2006 [Page 18] Internet-Draft ECN for MPLS June 2006 draft-chan-tsvwg-diffserv-class-aggr-03 (work in progress), January 2006. [I-D.ietf-nsis-rmd] Bader, A., "RMD-QOSM - The Resource Management in Diffserv QOS Model", draft-ietf-nsis-rmd-06 (work in progress), February 2006. [I-D.lefaucheur-rsvp-ecn] Faucheur, F., "RSVP Extensions for Admission Control over Diffserv using Pre-congestion Notification", draft-lefaucheur-rsvp-ecn-00 (work in progress), October 2005. [RFC3540] Spring, N., Wetherall, D., and D. Ely, "Robust Explicit Congestion Notification (ECN) Signaling with Nonces", RFC 3540, June 2003. [RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram Congestion Control Protocol (DCCP)", RFC 4340, March 2006. [Shayman] "Using ECN to Signal Congestion Within an MPLS Domain", 2000. Work in progress. http://www.ee.umd.edu/~shayman/papers.d/ draft-shayman-mpls-ecn-00.txt Davie, et al. Expires December 20, 2006 [Page 19] Internet-Draft ECN for MPLS June 2006 Authors' Addresses Bruce Davie Cisco Systems, Inc. 1414 Mass. Ave. Boxborough, MA 01719 USA Email: bsd@cisco.com Bob Briscoe BT Research B54/77, Sirius House Adastral Park Martlesham Heath Ipswich Suffolk IP5 3RE United Kingdom Email: bob.briscoe@bt.com June Tay BT Research B54/77, Sirius House Adastral Park Martlesham Heath Ipswich Suffolk IP5 3RE United Kingdom Email: june.tay@bt.com Davie, et al. Expires December 20, 2006 [Page 20] Internet-Draft ECN for MPLS June 2006 Intellectual Property Statement The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. 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. Disclaimer of Validity 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 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. Copyright Statement Copyright (C) The Internet Society (2006). 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. Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society. Davie, et al. Expires December 20, 2006 [Page 21]