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Network Working Group                                    K. RamakrishnanRequest for Comments: 3168                            TeraOptic NetworksUpdates:2474,2401,793                                        S. FloydObsoletes:2481                                                    ACIRICategory: Standards Track                                       D. Black                                                                     EMC                                                          September 2001The Addition of Explicit Congestion Notification (ECN) to IPStatus of this Memo   This document specifies an Internet standards track protocol for the   Internet community, and requests discussion and suggestions for   improvements.  Please refer to the current edition of the "Internet   Official Protocol Standards" (STD 1) for the standardization state   and status of this protocol.  Distribution of this memo is unlimited.Copyright Notice   Copyright (C) The Internet Society (2001).  All Rights Reserved.Abstract   This memo specifies the incorporation of ECN (Explicit Congestion   Notification) to TCP and IP, including ECN's use of two bits in the   IP header.Table of Contents1.  Introduction..................................................32.  Conventions and Acronyms......................................53.  Assumptions and General Principles............................54.  Active Queue Management (AQM).................................65.  Explicit Congestion Notification in IP........................65.1.  ECN as an Indication of Persistent Congestion...............105.2.  Dropped or Corrupted Packets................................115.3.  Fragmentation...............................................116.  Support from the Transport Protocol...........................126.1.  TCP.........................................................136.1.1  TCP Initialization.........................................146.1.1.1.  Middlebox Issues........................................16   6.1.1.2.  Robust TCP Initialization with an Echoed Reserved Field. 176.1.2.  The TCP Sender............................................186.1.3.  The TCP Receiver..........................................196.1.4.  Congestion on the ACK-path................................206.1.5.  Retransmitted TCP packets.................................20Ramakrishnan, et al.        Standards Track                     [Page 1]

RFC 3168               The Addition of ECN to IP          September 20016.1.6.  TCP Window Probes.........................................227.  Non-compliance by the End Nodes...............................228.  Non-compliance in the Network.................................248.1.  Complications Introduced by Split Paths.....................259.  Encapsulated Packets..........................................259.1.  IP packets encapsulated in IP...............................259.1.1.  The Limited-functionality and Full-functionality Options..279.1.2.  Changes to the ECN Field within an IP Tunnel..............289.2.  IPsec Tunnels...............................................299.2.1.  Negotiation between Tunnel Endpoints......................319.2.1.1.  ECN Tunnel Security Association Database Field..........329.2.1.2.  ECN Tunnel Security Association Attribute...............329.2.1.3.  Changes to IPsec Tunnel Header Processing...............339.2.2.  Changes to the ECN Field within an IPsec Tunnel...........359.2.3.  Comments for IPsec Support................................359.3.  IP packets encapsulated in non-IP Packet Headers............3610.  Issues Raised by Monitoring and Policing Devices.............3611.  Evaluations of ECN...........................................3711.1.  Related Work Evaluating ECN................................3711.2.  A Discussion of the ECN nonce..............................3711.2.1.  The Incremental Deployment of ECT(1) in Routers..........3812.  Summary of changes required in IP and TCP....................3813.  Conclusions..................................................4014.  Acknowledgements.............................................4115.  References...................................................4116.  Security Considerations......................................4517.  IPv4 Header Checksum Recalculation...........................4518.  Possible Changes to the ECN Field in the Network.............4518.1.  Possible Changes to the IP Header..........................4618.1.1.  Erasing the Congestion Indication........................4618.1.2.  Falsely Reporting Congestion.............................4718.1.3.  Disabling ECN-Capability.................................4718.1.4.  Falsely Indicating ECN-Capability........................4718.2.  Information carried in the Transport Header................4818.3.  Split Paths................................................4919.  Implications of Subverting End-to-End Congestion Control.....5019.1.  Implications for the Network and for Competing Flows.......5019.2.  Implications for the Subverted Flow........................53   19.3.  Non-ECN-Based Methods of Subverting End-to-end Congestion          Control....................................................5420.  The Motivation for the ECT Codepoints........................5420.1.  The Motivation for an ECT Codepoint........................5420.2.  The Motivation for two ECT Codepoints......................5521.  Why use Two Bits in the IP Header?...........................5722.  Historical Definitions for the IPv4 TOS Octet................5823.  IANA Considerations..........................................6023.1.  IPv4 TOS Byte and IPv6 Traffic Class Octet.................6023.2.  TCP Header Flags...........................................61Ramakrishnan, et al.        Standards Track                     [Page 2]

RFC 3168               The Addition of ECN to IP          September 200123.3. IPSEC Security Association Attributes.......................6224.  Authors' Addresses...........................................6225.  Full Copyright Statement.....................................631.  Introduction   We begin by describing TCP's use of packet drops as an indication of   congestion.  Next we explain that with the addition of active queue   management (e.g., RED) to the Internet infrastructure, where routers   detect congestion before the queue overflows, routers are no longer   limited to packet drops as an indication of congestion.  Routers can   instead set the Congestion Experienced (CE) codepoint in the IP   header of packets from ECN-capable transports.  We describe when the   CE codepoint is to be set in routers, and describe modifications   needed to TCP to make it ECN-capable.  Modifications to other   transport protocols (e.g., unreliable unicast or multicast, reliable   multicast, other reliable unicast transport protocols) could be   considered as those protocols are developed and advance through the   standards process.  We also describe in this document the issues   involving the use of ECN within IP tunnels, and within IPsec tunnels   in particular.   One of the guiding principles for this document is that, to the   extent possible, the mechanisms specified here be incrementally   deployable.  One challenge to the principle of incremental deployment   has been the prior existence of some IP tunnels that were not   compatible with the use of ECN.  As ECN becomes deployed, non-   compatible IP tunnels will have to be upgraded to conform to this   document.   This document obsoletesRFC 2481, "A Proposal to add Explicit   Congestion Notification (ECN) to IP", which defined ECN as an   Experimental Protocol for the Internet Community.  This document also   updatesRFC 2474, "Definition of the Differentiated Services Field   (DS Field) in the IPv4 and IPv6 Headers", in defining the ECN field   in the IP header,RFC 2401, "Security Architecture for the Internet   Protocol" to change the handling of IPv4 TOS Byte and IPv6 Traffic   Class Octet in tunnel mode header construction to be compatible with   the use of ECN, andRFC 793, "Transmission Control Protocol", in   defining two new flags in the TCP header.   TCP's congestion control and avoidance algorithms are based on the   notion that the network is a black-box [Jacobson88,Jacobson90].  The   network's state of congestion or otherwise is determined by end-   systems probing for the network state, by gradually increasing the   load on the network (by increasing the window of packets that are   outstanding in the network) until the network becomes congested and a   packet is lost.  Treating the network as a "black-box" and treatingRamakrishnan, et al.        Standards Track                     [Page 3]

RFC 3168               The Addition of ECN to IP          September 2001   loss as an indication of congestion in the network is appropriate for   pure best-effort data carried by TCP, with little or no sensitivity   to delay or loss of individual packets.  In addition, TCP's   congestion management algorithms have techniques built-in (such as   Fast Retransmit and Fast Recovery) to minimize the impact of losses,   from a throughput perspective.  However, these mechanisms are not   intended to help applications that are in fact sensitive to the delay   or loss of one or more individual packets.  Interactive traffic such   as telnet, web-browsing, and transfer of audio and video data can be   sensitive to packet losses (especially when using an unreliable data   delivery transport such as UDP) or to the increased latency of the   packet caused by the need to retransmit the packet after a loss (with   the reliable data delivery semantics provided by TCP).   Since TCP determines the appropriate congestion window to use by   gradually increasing the window size until it experiences a dropped   packet, this causes the queues at the bottleneck router to build up.   With most packet drop policies at the router that are not sensitive   to the load placed by each individual flow (e.g., tail-drop on queue   overflow), this means that some of the packets of latency-sensitive   flows may be dropped. In addition, such drop policies lead to   synchronization of loss across multiple flows.   Active queue management mechanisms detect congestion before the queue   overflows, and provide an indication of this congestion to the end   nodes.  Thus, active queue management can reduce unnecessary queuing   delay for all traffic sharing that queue.  The advantages of active   queue management are discussed inRFC 2309 [RFC2309].  Active queue   management avoids some of the bad properties of dropping on queue   overflow, including the undesirable synchronization of loss across   multiple flows.  More importantly, active queue management means that   transport protocols with mechanisms for congestion control (e.g.,   TCP) do not have to rely on buffer overflow as the only indication of   congestion.   Active queue management mechanisms may use one of several methods for   indicating congestion to end-nodes. One is to use packet drops, as is   currently done. However, active queue management allows the router to   separate policies of queuing or dropping packets from the policies   for indicating congestion. Thus, active queue management allows   routers to use the Congestion Experienced (CE) codepoint in a packet   header as an indication of congestion, instead of relying solely on   packet drops. This has the potential of reducing the impact of loss   on latency-sensitive flows.Ramakrishnan, et al.        Standards Track                     [Page 4]

RFC 3168               The Addition of ECN to IP          September 2001   There exist some middleboxes (firewalls, load balancers, or intrusion   detection systems) in the Internet that either drop a TCP SYN packet   configured to negotiate ECN, or respond with a RST.  This document   specifies procedures that TCP implementations may use to provide   robust connectivity even in the presence of such equipment.2.  Conventions and Acronyms   The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,   SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this   document, are to be interpreted as described in [RFC2119].3.  Assumptions and General Principles   In this section, we describe some of the important design principles   and assumptions that guided the design choices in this proposal.      * Because ECN is likely to be adopted gradually, accommodating        migration is essential. Some routers may still only drop packets        to indicate congestion, and some end-systems may not be ECN-        capable. The most viable strategy is one that accommodates        incremental deployment without having to resort to "islands" of        ECN-capable and non-ECN-capable environments.      * New mechanisms for congestion control and avoidance need to co-        exist and cooperate with existing mechanisms for congestion        control.  In particular, new mechanisms have to co-exist with        TCP's current methods of adapting to congestion and with        routers' current practice of dropping packets in periods of        congestion.      * Congestion may persist over different time-scales. The time        scales that we are concerned with are congestion events that may        last longer than a round-trip time.      * The number of packets in an individual flow (e.g., TCP        connection or an exchange using UDP) may range from a small        number of packets to quite a large number. We are interested in        managing the congestion caused by flows that send enough packets        so that they are still active when network feedback reaches        them.      * Asymmetric routing is likely to be a normal occurrence in the        Internet. The path (sequence of links and routers) followed by        data packets may be different from the path followed by the        acknowledgment packets in the reverse direction.Ramakrishnan, et al.        Standards Track                     [Page 5]

RFC 3168               The Addition of ECN to IP          September 2001      * Many routers process the "regular" headers in IP packets more        efficiently than they process the header information in IP        options.  This suggests keeping congestion experienced        information in the regular headers of an IP packet.      * It must be recognized that not all end-systems will cooperate in        mechanisms for congestion control. However, new mechanisms        shouldn't make it easier for TCP applications to disable TCP        congestion control.  The benefit of lying about participating in        new mechanisms such as ECN-capability should be small.4.  Active Queue Management (AQM)   Random Early Detection (RED) is one mechanism for Active Queue   Management (AQM) that has been proposed to detect incipient   congestion [FJ93], and is currently being deployed in the Internet   [RFC2309].  AQM is meant to be a general mechanism using one of   several alternatives for congestion indication, but in the absence of   ECN, AQM is restricted to using packet drops as a mechanism for   congestion indication.  AQM drops packets based on the average queue   length exceeding a threshold, rather than only when the queue   overflows.  However, because AQM may drop packets before the queue   actually overflows, AQM is not always forced by memory limitations to   discard the packet.   AQM can set a Congestion Experienced (CE) codepoint in the packet   header instead of dropping the packet, when such a field is provided   in the IP header and understood by the transport protocol.  The use   of the CE codepoint with ECN allows the receiver(s) to receive the   packet, avoiding the potential for excessive delays due to   retransmissions after packet losses.  We use the term 'CE packet' to   denote a packet that has the CE codepoint set.5.  Explicit Congestion Notification in IP   This document specifies that the Internet provide a congestion   indication for incipient congestion (as in RED and earlier work   [RJ90]) where the notification can sometimes be through marking   packets rather than dropping them.  This uses an ECN field in the IP   header with two bits, making four ECN codepoints, '00' to '11'.  The   ECN-Capable Transport (ECT) codepoints '10' and '01' are set by the   data sender to indicate that the end-points of the transport protocol   are ECN-capable; we call them ECT(0) and ECT(1) respectively.  The   phrase "the ECT codepoint" in this documents refers to either of the   two ECT codepoints.  Routers treat the ECT(0) and ECT(1) codepoints   as equivalent.  Senders are free to use either the ECT(0) or the   ECT(1) codepoint to indicate ECT, on a packet-by-packet basis.Ramakrishnan, et al.        Standards Track                     [Page 6]

RFC 3168               The Addition of ECN to IP          September 2001   The use of both the two codepoints for ECT, ECT(0) and ECT(1), is   motivated primarily by the desire to allow mechanisms for the data   sender to verify that network elements are not erasing the CE   codepoint, and that data receivers are properly reporting to the   sender the receipt of packets with the CE codepoint set, as required   by the transport protocol.  Guidelines for the senders and receivers   to differentiate between the ECT(0) and ECT(1) codepoints will be   addressed in separate documents, for each transport protocol.  In   particular, this document does not address mechanisms for TCP end-   nodes to differentiate between the ECT(0) and ECT(1) codepoints.   Protocols and senders that only require a single ECT codepoint SHOULD   use ECT(0).   The not-ECT codepoint '00' indicates a packet that is not using ECN.   The CE codepoint '11' is set by a router to indicate congestion to   the end nodes.  Routers that have a packet arriving at a full queue   drop the packet, just as they do in the absence of ECN.      +-----+-----+      | ECN FIELD |      +-----+-----+        ECT   CE         [Obsolete]RFC 2481 names for the ECN bits.         0     0         Not-ECT         0     1         ECT(1)         1     0         ECT(0)         1     1         CE      Figure 1: The ECN Field in IP.   The use of two ECT codepoints essentially gives a one-bit ECN nonce   in packet headers, and routers necessarily "erase" the nonce when   they set the CE codepoint [SCWA99].  For example, routers that erased   the CE codepoint would face additional difficulty in reconstructing   the original nonce, and thus repeated erasure of the CE codepoint   would be more likely to be detected by the end-nodes.  The ECN nonce   also can address the problem of misbehaving transport receivers lying   to the transport sender about whether or not the CE codepoint was set   in a packet.  The motivations for the use of two ECT codepoints is   discussed in more detail inSection 20, along with some discussion of   alternate possibilities for the fourth ECT codepoint (that is, the   codepoint '01').  Backwards compatibility with earlier ECN   implementations that do not understand the ECT(1) codepoint is   discussed inSection 11.   In RFC 2481 [RFC2481], the ECN field was divided into the ECN-Capable   Transport (ECT) bit and the CE bit.  The ECN field with only the   ECN-Capable Transport (ECT) bit set inRFC 2481 corresponds to the   ECT(0) codepoint in this document, and the ECN field with both theRamakrishnan, et al.        Standards Track                     [Page 7]

RFC 3168               The Addition of ECN to IP          September 2001   ECT and CE bit inRFC 2481 corresponds to the CE codepoint in this   document.  The '01' codepoint was left undefined inRFC 2481, and   this is the reason for recommending the use of ECT(0) when only a   single ECT codepoint is needed.         0     1     2     3     4     5     6     7      +-----+-----+-----+-----+-----+-----+-----+-----+      |          DS FIELD, DSCP           | ECN FIELD |      +-----+-----+-----+-----+-----+-----+-----+-----+        DSCP: differentiated services codepoint        ECN:  Explicit Congestion Notification      Figure 2: The Differentiated Services and ECN Fields in IP.   Bits 6 and 7 in the IPv4 TOS octet are designated as the ECN field.   The IPv4 TOS octet corresponds to the Traffic Class octet in IPv6,   and the ECN field is defined identically in both cases.  The   definitions for the IPv4 TOS octet [RFC791] and the IPv6 Traffic   Class octet have been superseded by the six-bit DS (Differentiated   Services) Field [RFC2474,RFC2780].  Bits 6 and 7 are listed in   [RFC2474] as Currently Unused, and are specified inRFC 2780 as   approved for experimental use for ECN.Section 22 gives a brief   history of the TOS octet.   Because of the unstable history of the TOS octet, the use of the ECN   field as specified in this document cannot be guaranteed to be   backwards compatible with those past uses of these two bits that   pre-date ECN.  The potential dangers of this lack of backwards   compatibility are discussed inSection 22.   Upon the receipt by an ECN-Capable transport of a single CE packet,   the congestion control algorithms followed at the end-systems MUST be   essentially the same as the congestion control response to a *single*   dropped packet.  For example, for ECN-Capable TCP the source TCP is   required to halve its congestion window for any window of data   containing either a packet drop or an ECN indication.   One reason for requiring that the congestion-control response to the   CE packet be essentially the same as the response to a dropped packet   is to accommodate the incremental deployment of ECN in both end-   systems and in routers.  Some routers may drop ECN-Capable packets   (e.g., using the same AQM policies for congestion detection) while   other routers set the CE codepoint, for equivalent levels of   congestion.  Similarly, a router might drop a non-ECN-Capable packet   but set the CE codepoint in an ECN-Capable packet, for equivalentRamakrishnan, et al.        Standards Track                     [Page 8]

RFC 3168               The Addition of ECN to IP          September 2001   levels of congestion.  If there were different congestion control   responses to a CE codepoint than to a packet drop, this could result   in unfair treatment for different flows.   An additional goal is that the end-systems should react to congestion   at most once per window of data (i.e., at most once per round-trip   time), to avoid reacting multiple times to multiple indications of   congestion within a round-trip time.   For a router, the CE codepoint of an ECN-Capable packet SHOULD only   be set if the router would otherwise have dropped the packet as an   indication of congestion to the end nodes. When the router's buffer   is not yet full and the router is prepared to drop a packet to inform   end nodes of incipient congestion, the router should first check to   see if the ECT codepoint is set in that packet's IP header.  If so,   then instead of dropping the packet, the router MAY instead set the   CE codepoint in the IP header.   An environment where all end nodes were ECN-Capable could allow new   criteria to be developed for setting the CE codepoint, and new   congestion control mechanisms for end-node reaction to CE packets.   However, this is a research issue, and as such is not addressed in   this document.   When a CE packet (i.e., a packet that has the CE codepoint set) is   received by a router, the CE codepoint is left unchanged, and the   packet is transmitted as usual. When severe congestion has occurred   and the router's queue is full, then the router has no choice but to   drop some packet when a new packet arrives.  We anticipate that such   packet losses will become relatively infrequent when a majority of   end-systems become ECN-Capable and participate in TCP or other   compatible congestion control mechanisms. In an ECN-Capable   environment that is adequately-provisioned, packet losses should   occur primarily during transients or in the presence of non-   cooperating sources.   The above discussion of when CE may be set instead of dropping a   packet applies by default to all Differentiated Services Per-Hop   Behaviors (PHBs) [RFC 2475].  Specifications for PHBs MAY provide   more specifics on how a compliant implementation is to choose between   setting CE and dropping a packet, but this is NOT REQUIRED.  A router   MUST NOT set CE instead of dropping a packet when the drop that would   occur is caused by reasons other than congestion or the desire to   indicate incipient congestion to end nodes (e.g., a diffserv edge   node may be configured to unconditionally drop certain classes of   traffic to prevent them from entering its diffserv domain).Ramakrishnan, et al.        Standards Track                     [Page 9]

RFC 3168               The Addition of ECN to IP          September 2001   We expect that routers will set the CE codepoint in response to   incipient congestion as indicated by the average queue size, using   the RED algorithms suggested in [FJ93,RFC2309].  To the best of our   knowledge, this is the only proposal currently under discussion in   the IETF for routers to drop packets proactively, before the buffer   overflows.  However, this document does not attempt to specify a   particular mechanism for active queue management, leaving that   endeavor, if needed, to other areas of the IETF.  While ECN is   inextricably tied up with the need to have a reasonable active queue   management mechanism at the router, the reverse does not hold; active   queue management mechanisms have been developed and deployed   independent of ECN, using packet drops as indications of congestion   in the absence of ECN in the IP architecture.5.1.  ECN as an Indication of Persistent Congestion   We emphasize that a *single* packet with the CE codepoint set in an   IP packet causes the transport layer to respond, in terms of   congestion control, as it would to a packet drop.  The instantaneous   queue size is likely to see considerable variations even when the   router does not experience persistent congestion.  As such, it is   important that transient congestion at a router, reflected by the   instantaneous queue size reaching a threshold much smaller than the   capacity of the queue, not trigger a reaction at the transport layer.   Therefore, the CE codepoint should not be set by a router based on   the instantaneous queue size.   For example, since the ATM and Frame Relay mechanisms for congestion   indication have typically been defined without an associated notion   of average queue size as the basis for determining that an   intermediate node is congested, we believe that they provide a very   noisy signal. The TCP-sender reaction specified in this document for   ECN is NOT the appropriate reaction for such a noisy signal of   congestion notification.  However, if the routers that interface to   the ATM network have a way of maintaining the average queue at the   interface, and use it to come to a reliable determination that the   ATM subnet is congested, they may use the ECN notification that is   defined here.   We continue to encourage experiments in techniques at layer 2 (e.g.,   in ATM switches or Frame Relay switches) to take advantage of ECN.   For example, using a scheme such as RED (where packet marking is   based on the average queue length exceeding a threshold), layer 2   devices could provide a reasonably reliable indication of congestion.   When all the layer 2 devices in a path set that layer's own   Congestion Experienced codepoint (e.g., the EFCI bit for ATM, the   FECN bit in Frame Relay) in this reliable manner, then the interface   router to the layer 2 network could copy the state of that layer 2Ramakrishnan, et al.        Standards Track                    [Page 10]

RFC 3168               The Addition of ECN to IP          September 2001   Congestion Experienced codepoint into the CE codepoint in the IP   header.  We recognize that this is not the current practice, nor is   it in current standards. However, encouraging experimentation in this   manner may provide the information needed to enable evolution of   existing layer 2 mechanisms to provide a more reliable means of   congestion indication, when they use a single bit for indicating   congestion.5.2.  Dropped or Corrupted Packets   For the proposed use for ECN in this document (that is, for a   transport protocol such as TCP for which a dropped data packet is an   indication of congestion), end nodes detect dropped data packets, and   the congestion response of the end nodes to a dropped data packet is   at least as strong as the congestion response to a received CE   packet.  To ensure the reliable delivery of the congestion indication   of the CE codepoint, an ECT codepoint MUST NOT be set in a packet   unless the loss of that packet in the network would be detected by   the end nodes and interpreted as an indication of congestion.   Transport protocols such as TCP do not necessarily detect all packet   drops, such as the drop of a "pure" ACK packet; for example, TCP does   not reduce the arrival rate of subsequent ACK packets in response to   an earlier dropped ACK packet.  Any proposal for extending ECN-   Capability to such packets would have to address issues such as the   case of an ACK packet that was marked with the CE codepoint but was   later dropped in the network. We believe that this aspect is still   the subject of research, so this document specifies that at this   time, "pure" ACK packets MUST NOT indicate ECN-Capability.   Similarly, if a CE packet is dropped later in the network due to   corruption (bit errors), the end nodes should still invoke congestion   control, just as TCP would today in response to a dropped data   packet. This issue of corrupted CE packets would have to be   considered in any proposal for the network to distinguish between   packets dropped due to corruption, and packets dropped due to   congestion or buffer overflow.  In particular, the ubiquitous   deployment of ECN would not, in and of itself, be a sufficient   development to allow end-nodes to interpret packet drops as   indications of corruption rather than congestion.5.3.  Fragmentation   ECN-capable packets MAY have the DF (Don't Fragment) bit set.   Reassembly of a fragmented packet MUST NOT lose indications of   congestion.  In other words, if any fragment of an IP packet to be   reassembled has the CE codepoint set, then one of two actions MUST be   taken:Ramakrishnan, et al.        Standards Track                    [Page 11]

RFC 3168               The Addition of ECN to IP          September 2001      * Set the CE codepoint on the reassembled packet.  However, this        MUST NOT occur if any of the other fragments contributing to        this reassembly carries the Not-ECT codepoint.      * The packet is dropped, instead of being reassembled, for any        other reason.   If both actions are applicable, either MAY be chosen.  Reassembly of   a fragmented packet MUST NOT change the ECN codepoint when all of the   fragments carry the same codepoint.   We would note that becauseRFC 2481 did not specify reassembly   behavior, older ECN implementations conformant with that Experimental   RFC do not necessarily perform reassembly correctly, in terms of   preserving the CE codepoint in a fragment.  The sender could avoid   the consequences of this behavior by setting the DF bit in ECN-   Capable packets.   Situations may arise in which the above reassembly specification is   insufficiently precise.  For example, if there is a malicious or   broken entity in the path at or after the fragmentation point, packet   fragments could carry a mixture of ECT(0), ECT(1), and/or Not-ECT   codepoints.  The reassembly specification above does not place   requirements on reassembly of fragments in this case.  In situations   where more precise reassembly behavior would be required, protocol   specifications SHOULD instead specify that DF MUST be set in all   ECN-capable packets sent by the protocol.6.  Support from the Transport Protocol   ECN requires support from the transport protocol, in addition to the   functionality given by the ECN field in the IP packet header. The   transport protocol might require negotiation between the endpoints   during setup to determine that all of the endpoints are ECN-capable,   so that the sender can set the ECT codepoint in transmitted packets.   Second, the transport protocol must be capable of reacting   appropriately to the receipt of CE packets.  This reaction could be   in the form of the data receiver informing the data sender of the   received CE packet (e.g., TCP), of the data receiver unsubscribing to   a layered multicast group (e.g., RLM [MJV96]), or of some other   action that ultimately reduces the arrival rate of that flow on that   congested link.  CE packets indicate persistent rather than transient   congestion (seeSection 5.1), and hence reactions to the receipt of   CE packets should be those appropriate for persistent congestion.   This document only addresses the addition of ECN Capability to TCP,   leaving issues of ECN in other transport protocols to further   research.  For TCP, ECN requires three new pieces of functionality:Ramakrishnan, et al.        Standards Track                    [Page 12]

RFC 3168               The Addition of ECN to IP          September 2001   negotiation between the endpoints during connection setup to   determine if they are both ECN-capable; an ECN-Echo (ECE) flag in the   TCP header so that the data receiver can inform the data sender when   a CE packet has been received; and a Congestion Window Reduced (CWR)   flag in the TCP header so that the data sender can inform the data   receiver that the congestion window has been reduced. The support   required from other transport protocols is likely to be different,   particularly for unreliable or reliable multicast transport   protocols, and will have to be determined as other transport   protocols are brought to the IETF for standardization.   In a mild abuse of terminology, in this document we refer to `TCP   packets' instead of `TCP segments'.6.1.  TCP   The following sections describe in detail the proposed use of ECN in   TCP.  This proposal is described in essentially the same form in   [Floyd94]. We assume that the source TCP uses the standard congestion   control algorithms of Slow-start, Fast Retransmit and Fast Recovery   [RFC2581].   This proposal specifies two new flags in the Reserved field of the   TCP header.  The TCP mechanism for negotiating ECN-Capability uses   the ECN-Echo (ECE) flag in the TCP header.  Bit 9 in the Reserved   field of the TCP header is designated as the ECN-Echo flag.  The   location of the 6-bit Reserved field in the TCP header is shown in   Figure 4 ofRFC 793 [RFC793] (and is reproduced below for   completeness).  This specification of the ECN Field leaves the   Reserved field as a 4-bit field using bits 4-7.   To enable the TCP receiver to determine when to stop setting the   ECN-Echo flag, we introduce a second new flag in the TCP header, the   CWR flag.  The CWR flag is assigned to Bit 8 in the Reserved field of   the TCP header.        0   1   2   3   4   5   6   7   8   9  10  11  12  13  14  15      +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+      |               |                       | U | A | P | R | S | F |      | Header Length |        Reserved       | R | C | S | S | Y | I |      |               |                       | G | K | H | T | N | N |      +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+      Figure 3: The old definition of bytes 13 and 14 of the TCP                header.Ramakrishnan, et al.        Standards Track                    [Page 13]

RFC 3168               The Addition of ECN to IP          September 2001        0   1   2   3   4   5   6   7   8   9  10  11  12  13  14  15      +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+      |               |               | C | E | U | A | P | R | S | F |      | Header Length |    Reserved   | W | C | R | C | S | S | Y | I |      |               |               | R | E | G | K | H | T | N | N |      +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+      Figure 4: The new definition of bytes 13 and 14 of the TCP                Header.   Thus, ECN uses the ECT and CE flags in the IP header (as shown in   Figure 1) for signaling between routers and connection endpoints, and   uses the ECN-Echo and CWR flags in the TCP header (as shown in Figure   4) for TCP-endpoint to TCP-endpoint signaling.  For a TCP connection,   a typical sequence of events in an ECN-based reaction to congestion   is as follows:      * An ECT codepoint is set in packets transmitted by the sender to        indicate that ECN is supported by the transport entities for        these packets.      * An ECN-capable router detects impending congestion and detects        that an ECT codepoint is set in the packet it is about to drop.        Instead of dropping the packet, the router chooses to set the CE        codepoint in the IP header and forwards the packet.      * The receiver receives the packet with the CE codepoint set, and        sets the ECN-Echo flag in its next TCP ACK sent to the sender.      * The sender receives the TCP ACK with ECN-Echo set, and reacts to        the congestion as if a packet had been dropped.      * The sender sets the CWR flag in the TCP header of the next        packet sent to the receiver to acknowledge its receipt of and        reaction to the ECN-Echo flag.   The negotiation for using ECN by the TCP transport entities and the   use of the ECN-Echo and CWR flags is described in more detail in the   sections below.6.1.1  TCP Initialization   In the TCP connection setup phase, the source and destination TCPs   exchange information about their willingness to use ECN.  Subsequent   to the completion of this negotiation, the TCP sender sets an ECT   codepoint in the IP header of data packets to indicate to the network   that the transport is capable and willing to participate in ECN for   this packet. This indicates to the routers that they may mark thisRamakrishnan, et al.        Standards Track                    [Page 14]

RFC 3168               The Addition of ECN to IP          September 2001   packet with the CE codepoint, if they would like to use that as a   method of congestion notification. If the TCP connection does not   wish to use ECN notification for a particular packet, the sending TCP   sets the ECN codepoint to not-ECT, and the TCP receiver ignores the   CE codepoint in the received packet.   For this discussion, we designate the initiating host as Host A and   the responding host as Host B.  We call a SYN packet with the ECE and   CWR flags set an "ECN-setup SYN packet", and we call a SYN packet   with at least one of the ECE and CWR flags not set a "non-ECN-setup   SYN packet".  Similarly, we call a SYN-ACK packet with only the ECE   flag set but the CWR flag not set an "ECN-setup SYN-ACK packet", and   we call a SYN-ACK packet with any other configuration of the ECE and   CWR flags a "non-ECN-setup SYN-ACK packet".   Before a TCP connection can use ECN, Host A sends an ECN-setup SYN   packet, and Host B sends an ECN-setup SYN-ACK packet.  For a SYN   packet, the setting of both ECE and CWR in the ECN-setup SYN packet   is defined as an indication that the sending TCP is ECN-Capable,   rather than as an indication of congestion or of response to   congestion. More precisely, an ECN-setup SYN packet indicates that   the TCP implementation transmitting the SYN packet will participate   in ECN as both a sender and receiver.  Specifically, as a receiver,   it will respond to incoming data packets that have the CE codepoint   set in the IP header by setting ECE in outgoing TCP Acknowledgement   (ACK) packets.  As a sender, it will respond to incoming packets that   have ECE set by reducing the congestion window and setting CWR when   appropriate.  An ECN-setup SYN packet does not commit the TCP sender   to setting the ECT codepoint in any or all of the packets it may   transmit.  However, the commitment to respond appropriately to   incoming packets with the CE codepoint set remains even if the TCP   sender in a later transmission, within this TCP connection, sends a   SYN packet without ECE and CWR set.   When Host B sends an ECN-setup SYN-ACK packet, it sets the ECE flag   but not the CWR flag.  An ECN-setup SYN-ACK packet is defined as an   indication that the TCP transmitting the SYN-ACK packet is ECN-   Capable.  As with the SYN packet, an ECN-setup SYN-ACK packet does   not commit the TCP host to setting the ECT codepoint in transmitted   packets.   The following rules apply to the sending of ECN-setup packets within   a TCP connection, where a TCP connection is defined by the standard   rules for TCP connection establishment and termination.      * If a host has received an ECN-setup SYN packet, then it MAY send        an ECN-setup SYN-ACK packet.  Otherwise, it MUST NOT send an        ECN-setup SYN-ACK packet.Ramakrishnan, et al.        Standards Track                    [Page 15]

RFC 3168               The Addition of ECN to IP          September 2001      * A host MUST NOT set ECT on data packets unless it has sent at        least one ECN-setup SYN or ECN-setup SYN-ACK packet, and has        received at least one ECN-setup SYN or ECN-setup SYN-ACK packet,        and has sent no non-ECN-setup SYN or non-ECN-setup SYN-ACK        packet.  If a host has received at least one non-ECN-setup SYN        or non-ECN-setup SYN-ACK packet, then it SHOULD NOT set ECT on        data packets.      * If a host ever sets the ECT codepoint on a data packet, then        that host MUST correctly set/clear the CWR TCP bit on all        subsequent packets in the connection.      * If a host has sent at least one ECN-setup SYN or ECN-setup SYN-        ACK packet, and has received no non-ECN-setup SYN or non-ECN-        setup SYN-ACK packet, then if that host receives TCP data        packets with ECT and CE codepoints set in the IP header, then        that host MUST process these packets as specified for an ECN-        capable connection.      * A host that is not willing to use ECN on a TCP connection SHOULD        clear both the ECE and CWR flags in all non-ECN-setup SYN and/or        SYN-ACK packets that it sends to indicate this unwillingness.        Receivers MUST correctly handle all forms of the non-ECN-setup        SYN and SYN-ACK packets.      * A host MUST NOT set ECT on SYN or SYN-ACK packets.   A TCP client enters TIME-WAIT state after receiving a FIN-ACK, and   transitions to CLOSED state after a timeout.  Many TCP   implementations create a new TCP connection if they receive an in-   window SYN packet during TIME-WAIT state.  When a TCP host enters   TIME-WAIT or CLOSED state, it should ignore any previous state about   the negotiation of ECN for that connection.6.1.1.1.  Middlebox Issues   ECN introduces the use of the ECN-Echo and CWR flags in the TCP   header (as shown in Figure 3) for initialization.  There exist some   faulty firewalls, load balancers, and intrusion detection systems in   the Internet that either drop an ECN-setup SYN packet or respond with   a RST, in the belief that such a packet (with these bits set) is a   signature for a port-scanning tool that could be used in a denial-   of-service attack.  Some of the offending equipment has been   identified, and a web page [FIXES] contains a list of non-compliant   products and the fixes posted by the vendors, where these are   available.  The TBIT web page [TBIT] lists some of the web servers   affected by this faulty equipment.  We mention this in this document   as a warning to the community of this problem.Ramakrishnan, et al.        Standards Track                    [Page 16]

RFC 3168               The Addition of ECN to IP          September 2001   To provide robust connectivity even in the presence of such faulty   equipment, a host that receives a RST in response to the transmission   of an ECN-setup SYN packet MAY resend a SYN with CWR and ECE cleared.   This could result in a TCP connection being established without using   ECN.   A host that receives no reply to an ECN-setup SYN within the normal   SYN retransmission timeout interval MAY resend the SYN and any   subsequent SYN retransmissions with CWR and ECE cleared.  To overcome   normal packet loss that results in the original SYN being lost, the   originating host may retransmit one or more ECN-setup SYN packets   before giving up and retransmitting the SYN with the CWR and ECE bits   cleared.   We note that in this case, the following example scenario is   possible:   (1) Host A: Sends an ECN-setup SYN.   (2) Host B: Sends an ECN-setup SYN/ACK, packet is dropped or delayed.   (3) Host A: Sends a non-ECN-setup SYN.   (4) Host B: Sends a non-ECN-setup SYN/ACK.   We note that in this case, following the procedures above, neither   Host A nor Host B may set the ECT bit on data packets.  Further, an   important consequence of the rules for ECN setup and usage inSection6.1.1 is that a host is forbidden from using the reception of ECT   data packets as an implicit signal that the other host is ECN-   capable.6.1.1.2.  Robust TCP Initialization with an Echoed Reserved Field   There is the question of why we chose to have the TCP sending the SYN   set two ECN-related flags in the Reserved field of the TCP header for   the SYN packet, while the responding TCP sending the SYN-ACK sets   only one ECN-related flag in the SYN-ACK packet.  This asymmetry is   necessary for the robust negotiation of ECN-capability with some   deployed TCP implementations.  There exists at least one faulty TCP   implementation in which TCP receivers set the Reserved field of the   TCP header in ACK packets (and hence the SYN-ACK) simply to reflect   the Reserved field of the TCP header in the received data packet.   Because the TCP SYN packet sets the ECN-Echo and CWR flags to   indicate ECN-capability, while the SYN-ACK packet sets only the ECN-   Echo flag, the sending TCP correctly interprets a receiver's   reflection of its own flags in the Reserved field as an indication   that the receiver is not ECN-capable.  The sending TCP is not mislead   by a faulty TCP implementation sending a SYN-ACK packet that simply   reflects the Reserved field of the incoming SYN packet.Ramakrishnan, et al.        Standards Track                    [Page 17]

RFC 3168               The Addition of ECN to IP          September 20016.1.2.  The TCP Sender   For a TCP connection using ECN, new data packets are transmitted with   an ECT codepoint set in the IP header.  When only one ECT codepoint   is needed by a sender for all packets sent on a TCP connection,   ECT(0) SHOULD be used.  If the sender receives an ECN-Echo (ECE) ACK   packet (that is, an ACK packet with the 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.  The indication   of congestion should be treated just as a congestion loss in non-   ECN-Capable TCP. That is, the TCP source halves the congestion window   "cwnd" and reduces the slow start threshold "ssthresh".  The sending   TCP SHOULD NOT increase the congestion window in response to the   receipt of an ECN-Echo ACK packet.   TCP should not react to congestion indications more than once every   window of data (or more loosely, more than once every round-trip   time). That is, the TCP sender's congestion window should be reduced   only once in response to a series of dropped and/or CE packets from a   single window of data.  In addition, the TCP source should not   decrease the slow-start threshold, ssthresh, if it has been decreased   within the last round trip time.  However, if any retransmitted   packets are dropped, then this is interpreted by the source TCP as a   new instance of congestion.   After the source TCP reduces its congestion window in response to a   CE packet, incoming acknowledgments that continue to arrive can   "clock out" outgoing packets as allowed by the reduced congestion   window.  If the congestion window consists of only one MSS (maximum   segment size), and the sending TCP receives an ECN-Echo ACK packet,   then the sending TCP should in principle still reduce its congestion   window in half. However, the value of the congestion window is   bounded below by a value of one MSS.  If the sending TCP were to   continue to send, using a congestion window of 1 MSS, this results in   the transmission of one packet per round-trip time.  It is necessary   to still reduce the sending rate of the TCP sender even further, on   receipt of an ECN-Echo packet when the congestion window is one.  We   use the retransmit timer as a means of reducing the rate further in   this circumstance.  Therefore, the sending TCP MUST reset the   retransmit timer on receiving the ECN-Echo packet when the congestion   window is one.  The sending TCP will then be able to send a new   packet only when the retransmit timer expires.   When an ECN-Capable TCP sender reduces its congestion window for any   reason (because of a retransmit timeout, a Fast Retransmit, or in   response to an ECN Notification), the TCP sender sets the CWR flag in   the TCP header of the first new data packet sent after the window   reduction.  If that data packet is dropped in the network, then theRamakrishnan, et al.        Standards Track                    [Page 18]

RFC 3168               The Addition of ECN to IP          September 2001   sending TCP will have to reduce the congestion window again and   retransmit the dropped packet.   We ensure that the "Congestion Window Reduced" information is   reliably delivered to the TCP receiver.  This comes about from the   fact that if the new data packet carrying the CWR flag is dropped,   then the TCP sender will have to again reduce its congestion window,   and send another new data packet with the CWR flag set.  Thus, the   CWR bit in the TCP header SHOULD NOT be set on retransmitted packets.   When the TCP data sender is ready to set the CWR bit after reducing   the congestion window, it SHOULD set the CWR bit only on the first   new data packet that it transmits.   [Floyd94] discusses TCP's response to ECN in more detail.  [Floyd98]   discusses the validation test in the ns simulator, which illustrates   a wide range of ECN scenarios. These scenarios include the following:   an ECN followed by another ECN, a Fast Retransmit, or a Retransmit   Timeout; a Retransmit Timeout or a Fast Retransmit followed by an   ECN; and a congestion window of one packet followed by an ECN.   TCP follows existing algorithms for sending data packets in response   to incoming ACKs, multiple duplicate acknowledgments, or retransmit   timeouts [RFC2581].  TCP also follows the normal procedures for   increasing the congestion window when it receives ACK packets without   the ECN-Echo bit set [RFC2581].6.1.3.  The TCP Receiver   When TCP receives a CE data packet at the destination end-system, the   TCP data receiver sets the ECN-Echo flag in the TCP header of the   subsequent ACK packet.  If there is any ACK withholding implemented,   as in current "delayed-ACK" TCP implementations where the TCP   receiver can send an ACK for two arriving data packets, then the   ECN-Echo flag in the ACK packet will be set to '1' if the CE   codepoint is set in any of the data packets being acknowledged.  That   is, if any of the received data packets are CE packets, then the   returning ACK has the ECN-Echo flag set.   To provide robustness against the possibility of a dropped ACK packet   carrying an ECN-Echo flag, the TCP receiver sets the ECN-Echo flag in   a series of ACK packets sent subsequently.  The TCP receiver uses the   CWR flag received from the TCP sender to determine when to stop   setting the ECN-Echo flag.   After a TCP receiver sends an ACK packet with the ECN-Echo bit set,   that TCP receiver continues to set the ECN-Echo flag in all the ACK   packets it sends (whether they acknowledge CE data packets or non-CERamakrishnan, et al.        Standards Track                    [Page 19]

RFC 3168               The Addition of ECN to IP          September 2001   data packets) until it receives a CWR packet (a packet with the CWR   flag set).  After the receipt of the CWR packet, acknowledgments for   subsequent non-CE data packets do not have the ECN-Echo flag set. If   another CE packet is received by the data receiver, the receiver   would once again send ACK packets with the ECN-Echo flag set.  While   the receipt of a CWR packet does not guarantee that the data sender   received the ECN-Echo message, this does suggest that the data sender   reduced its congestion window at some point *after* it sent the data   packet for which the CE codepoint was set.   We have already specified that a TCP sender is not required to reduce   its congestion window more than once per window of data.  Some care   is required if the TCP sender is to avoid unnecessary reductions of   the congestion window when a window of data includes both dropped   packets and (marked) CE packets.  This is illustrated in [Floyd98].6.1.4.  Congestion on the ACK-path   For the current generation of TCP congestion control algorithms, pure   acknowledgement packets (e.g., packets that do not contain any   accompanying data) MUST be sent with the not-ECT codepoint.  Current   TCP receivers have no mechanisms for reducing traffic on the ACK-path   in response to congestion notification.  Mechanisms for responding to   congestion on the ACK-path are areas for current and future research.   (One simple possibility would be for the sender to reduce its   congestion window when it receives a pure ACK packet with the CE   codepoint set). For current TCP implementations, a single dropped ACK   generally has only a very small effect on the TCP's sending rate.6.1.5.  Retransmitted TCP packets   This document specifies ECN-capable TCP implementations MUST NOT set   either ECT codepoint (ECT(0) or ECT(1)) in the IP header for   retransmitted data packets, and that the TCP data receiver SHOULD   ignore the ECN field on arriving data packets that are outside of the   receiver's current window.  This is for greater security against   denial-of-service attacks, as well as for robustness of the ECN   congestion indication with packets that are dropped later in the   network.   First, we note that if the TCP sender were to set an ECT codepoint on   a retransmitted packet, then if an unnecessarily-retransmitted packet   was later dropped in the network, the end nodes would never receive   the indication of congestion from the router setting the CE   codepoint.  Thus, setting an ECT codepoint on retransmitted data   packets is not consistent with the robust delivery of the congestion   indication even for packets that are later dropped in the network.Ramakrishnan, et al.        Standards Track                    [Page 20]

RFC 3168               The Addition of ECN to IP          September 2001   In addition, an attacker capable of spoofing the IP source address of   the TCP sender could send data packets with arbitrary sequence   numbers, with the CE codepoint set in the IP header.  On receiving   this spoofed data packet, the TCP data receiver would determine that   the data does not lie in the current receive window, and return a   duplicate acknowledgement.  We define an out-of-window packet at the   TCP data receiver as a data packet that lies outside the receiver's   current window.  On receiving an out-of-window packet, the TCP data   receiver has to decide whether or not to treat the CE codepoint in   the packet header as a valid indication of congestion, and therefore   whether to return ECN-Echo indications to the TCP data sender.  If   the TCP data receiver ignored the CE codepoint in an out-of-window   packet, then the TCP data sender would not receive this possibly-   legitimate indication of congestion from the network, resulting in a   violation of end-to-end congestion control.  On the other hand, if   the TCP data receiver honors the CE indication in the out-of-window   packet, and reports the indication of congestion to the TCP data   sender, then the malicious node that created the spoofed, out-of-   window packet has successfully "attacked" the TCP connection by   forcing the data sender to unnecessarily reduce (halve) its   congestion window.  To prevent such a denial-of-service attack, we   specify that a legitimate TCP data sender MUST NOT set an ECT   codepoint on retransmitted data packets, and that the TCP data   receiver SHOULD ignore the CE codepoint on out-of-window packets.   One drawback of not setting ECT(0) or ECT(1) on retransmitted packets   is that it denies ECN protection for retransmitted packets.  However,   for an ECN-capable TCP connection in a fully-ECN-capable environment   with mild congestion, packets should rarely be dropped due to   congestion in the first place, and so instances of retransmitted   packets should rarely arise.  If packets are being retransmitted,   then there are already packet losses (from corruption or from   congestion) that ECN has been unable to prevent.   We note that if the router sets the CE codepoint for an ECN-capable   data packet within a TCP connection, then the TCP connection is   guaranteed to receive that indication of congestion, or to receive   some other indication of congestion within the same window of data,   even if this packet is dropped or reordered in the network.  We   consider two cases, when the packet is later retransmitted, and when   the packet is not later retransmitted.   In the first case, if the packet is either dropped or delayed, and at   some point retransmitted by the data sender, then the retransmission   is a result of a Fast Retransmit or a Retransmit Timeout for either   that packet or for some prior packet in the same window of data.  In   this case, because the data sender already has retransmitted this   packet, we know that the data sender has already responded to anRamakrishnan, et al.        Standards Track                    [Page 21]

RFC 3168               The Addition of ECN to IP          September 2001   indication of congestion for some packet within the same window of   data as the original packet.  Thus, even if the first transmission of   the packet is dropped in the network, or is delayed, if it had the CE   codepoint set, and is later ignored by the data receiver as an out-   of-window packet, this is not a problem, because the sender has   already responded to an indication of congestion for that window of   data.   In the second case, if the packet is never retransmitted by the data   sender, then this data packet is the only copy of this data received   by the data receiver, and therefore arrives at the data receiver as   an in-window packet, regardless of how much the packet might be   delayed or reordered.  In this case, if the CE codepoint is set on   the packet within the network, this will be treated by the data   receiver as a valid indication of congestion.6.1.6.  TCP Window Probes.   When the TCP data receiver advertises a zero window, the TCP data   sender sends window probes to determine if the receiver's window has   increased.  Window probe packets do not contain any user data except   for the sequence number, which is a byte.  If a window probe packet   is dropped in the network, this loss is not detected by the receiver.   Therefore, the TCP data sender MUST NOT set either an ECT codepoint   or the CWR bit on window probe packets.   However, because window probes use exact sequence numbers, they   cannot be easily spoofed in denial-of-service attacks.  Therefore, if   a window probe arrives with the CE codepoint set, then the receiver   SHOULD respond to the ECN indications.7.  Non-compliance by the End Nodes   This section discusses concerns about the vulnerability of ECN to   non-compliant end-nodes (i.e., end nodes that set the ECT codepoint   in transmitted packets but do not respond to received CE packets).   We argue that the addition of ECN to the IP architecture will not   significantly increase the current vulnerability of the architecture   to unresponsive flows.   Even for non-ECN environments, there are serious concerns about the   damage that can be done by non-compliant or unresponsive flows (that   is, flows that do not respond to congestion control indications by   reducing their arrival rate at the congested link).  For example, an   end-node could "turn off congestion control" by not reducing its   congestion window in response to packet drops. This is a concern for   the current Internet.  It has been argued that routers will have to   deploy mechanisms to detect and differentially treat packets fromRamakrishnan, et al.        Standards Track                    [Page 22]

RFC 3168               The Addition of ECN to IP          September 2001   non-compliant flows [RFC2309,FF99].  It has also been suggested that   techniques such as end-to-end per-flow scheduling and isolation of   one flow from another, differentiated services, or end-to-end   reservations could remove some of the more damaging effects of   unresponsive flows.   It might seem that dropping packets in itself is an adequate   deterrent for non-compliance, and that the use of ECN removes this   deterrent.  We would argue in response that (1) ECN-capable routers   preserve packet-dropping behavior in times of high congestion; and   (2) even in times of high congestion, dropping packets in itself is   not an adequate deterrent for non-compliance.   First, ECN-Capable routers will only mark packets (as opposed to   dropping them) when the packet marking rate is reasonably low. During   periods where the average queue size exceeds an upper threshold, and   therefore the potential packet marking rate would be high, our   recommendation is that routers drop packets rather then set the CE   codepoint in packet headers.   During the periods of low or moderate packet marking rates when ECN   would be deployed, there would be little deterrent effect on   unresponsive flows of dropping rather than marking those packets. For   example, delay-insensitive flows using reliable delivery might have   an incentive to increase rather than to decrease their sending rate   in the presence of dropped packets.  Similarly, delay-sensitive flows   using unreliable delivery might increase their use of FEC in response   to an increased packet drop rate, increasing rather than decreasing   their sending rate.  For the same reasons, we do not believe that   packet dropping itself is an effective deterrent for non-compliance   even in an environment of high packet drop rates, when all flows are   sharing the same packet drop rate.   Several methods have been proposed to identify and restrict non-   compliant or unresponsive flows. The addition of ECN to the network   environment would not in any way increase the difficulty of designing   and deploying such mechanisms. If anything, the addition of ECN to   the architecture would make the job of identifying unresponsive flows   slightly easier.  For example, in an ECN-Capable environment routers   are not limited to information about packets that are dropped or have   the CE codepoint set at that router itself; in such an environment,   routers could also take note of arriving CE packets that indicate   congestion encountered by that packet earlier in the path.Ramakrishnan, et al.        Standards Track                    [Page 23]

RFC 3168               The Addition of ECN to IP          September 20018.  Non-compliance in the Network   This section considers the issues when a router is operating,   possibly maliciously, to modify either of the bits in the ECN field.   We note that in IPv4, the IP header is protected from bit errors by a   header checksum;  this is not the case in IPv6.  Thus for IPv6 the   ECN field can be accidentally modified by bit errors on links or in   routers without being detected by an IP header checksum.   By tampering with the bits in the ECN field, an adversary (or a   broken router) could do one or more of the following: falsely report   congestion, disable ECN-Capability for an individual packet, erase   the ECN congestion indication, or falsely indicate ECN-Capability.Section 18 systematically examines the various cases by which the ECN   field could be modified.  The important criterion considered in   determining the consequences of such modifications is whether it is   likely to lead to poorer behavior in any dimension (throughput,   delay, fairness or functionality) than if a router were to drop a   packet.   The first two possible changes, falsely reporting congestion or   disabling ECN-Capability for an individual packet, are no worse than   if the router were to simply drop the packet.  From a congestion   control point of view, setting the CE codepoint in the absence of   congestion by a non-compliant router would be no worse than a router   dropping a packet unnecessarily. By "erasing" an ECT codepoint of a   packet that is later dropped in the network, a router's actions could   result in an unnecessary packet drop for that packet later in the   network.   However, as discussed inSection 18, a router that erases the ECN   congestion indication or falsely indicates ECN-Capability could   potentially do more damage to the flow that if it has simply dropped   the packet.  A rogue or broken router that "erased" the CE codepoint   in arriving CE packets would prevent that indication of congestion   from reaching downstream receivers.  This could result in the failure   of congestion control for that flow and a resulting increase in   congestion in the network, ultimately resulting in subsequent packets   dropped for this flow as the average queue size increased at the   congested gateway.Section 19 considers the potential repercussions of subverting end-   to-end congestion control by either falsely indicating ECN-   Capability, or by erasing the congestion indication in ECN (the CE-   codepoint).  We observe inSection 19 that the consequence of   subverting ECN-based congestion control may lead to potential   unfairness, but this is likely to be no worse than the subversion of   either ECN-based or packet-based congestion control by the end nodes.Ramakrishnan, et al.        Standards Track                    [Page 24]

RFC 3168               The Addition of ECN to IP          September 20018.1.  Complications Introduced by Split Paths   If a router or other network element has access to all of the packets   of a flow, then that router could do no more damage to a flow by   altering the ECN field than it could by simply dropping all of the   packets from that flow.  However, in some cases, a malicious or   broken router might have access to only a subset of the packets from   a flow.  The question is as follows:  can this router, by altering   the ECN field in this subset of the packets, do more damage to that   flow than if it has simply dropped that set of the packets?   This is also discussed in detail inSection 18, which concludes as   follows:  It is true that the adversary that has access only to a   subset of packets in an aggregate might, by subverting ECN-based   congestion control, be able to deny the benefits of ECN to the other   packets in the aggregate.  While this is undesirable, this is not a   sufficient concern to result in disabling ECN.9.  Encapsulated Packets9.1.  IP packets encapsulated in IP   The encapsulation of IP packet headers in tunnels is used in many   places, including IPsec and IP in IP [RFC2003].  This section   considers issues related to interactions between ECN and IP tunnels,   and specifies two alternative solutions.  This discussion is   complemented byRFC 2983's discussion of interactions between   Differentiated Services and IP tunnels of various forms [RFC 2983],   as Differentiated Services uses the remaining six bits of the IP   header octet that is used by ECN (see Figure 2 inSection 5).   Some IP tunnel modes are based on adding a new "outer" IP header that   encapsulates the original, or "inner" IP header and its associated   packet.  In many cases, the new "outer" IP header may be added and   removed at intermediate points along a connection, enabling the   network to establish a tunnel without requiring endpoint   participation.  We denote tunnels that specify that the outer header   be discarded at tunnel egress as "simple tunnels".   ECN uses the ECN field in the IP header for signaling between routers   and connection endpoints.  ECN interacts with IP tunnels based on the   treatment of the ECN field in the IP header.  In simple IP tunnels   the octet containing the ECN field is copied or mapped from the inner   IP header to the outer IP header at IP tunnel ingress, and the outer   header's copy of this field is discarded at IP tunnel egress.  If the   outer header were to be simply discarded without taking care to deal   with the ECN field, and an ECN-capable router were to set the CERamakrishnan, et al.        Standards Track                    [Page 25]

RFC 3168               The Addition of ECN to IP          September 2001   (Congestion Experienced) codepoint within a packet in a simple IP   tunnel, this indication would be discarded at tunnel egress, losing   the indication of congestion.   Thus, the use of ECN over simple IP tunnels would result in routers   attempting to use the outer IP header to signal congestion to   endpoints, but those congestion warnings never arriving because the   outer header is discarded at the tunnel egress point.  This problem   was encountered with ECN and IPsec in tunnel mode, andRFC 2481   recommended that ECN not be used with the older simple IPsec tunnels   in order to avoid this behavior and its consequences.  When ECN   becomes widely deployed, then simple tunnels likely to carry ECN-   capable traffic will have to be changed.  If ECN-capable traffic is   carried by a simple tunnel through a congested, ECN-capable router,   this could result in subsequent packets being dropped for this flow   as the average queue size increases at the congested router, as   discussed inSection 8 above.   From a security point of view, the use of ECN in the outer header of   an IP tunnel might raise security concerns because an adversary could   tamper with the ECN information that propagates beyond the tunnel   endpoint.  Based on an analysis in Sections18 and19 of these   concerns and the resultant risks, our overall approach is to make   support for ECN an option for IP tunnels, so that an IP tunnel can be   specified or configured either to use ECN or not to use ECN in the   outer header of the tunnel.  Thus, in environments or tunneling   protocols where the risks of using ECN are judged to outweigh its   benefits, the tunnel can simply not use ECN in the outer header.   Then the only indication of congestion experienced at routers within   the tunnel would be through packet loss.   The result is that there are two viable options for the behavior of   ECN-capable connections over an IP tunnel, including IPsec tunnels:      * A limited-functionality option in which ECN is preserved in the        inner header, but disabled in the outer header.  The only        mechanism available for signaling congestion occurring within        the tunnel in this case is dropped packets.      * A full-functionality option that supports ECN in both the inner        and outer headers, and propagates congestion warnings from nodes        within the tunnel to endpoints.   Support for these options requires varying amounts of changes to IP   header processing at tunnel ingress and egress.  A small subset of   these changes sufficient to support only the limited-functionality   option would be sufficient to eliminate any incompatibility between   ECN and IP tunnels.Ramakrishnan, et al.        Standards Track                    [Page 26]

RFC 3168               The Addition of ECN to IP          September 2001   One goal of this document is to give guidance about the tradeoffs   between the limited-functionality and full-functionality options.  A   full discussion of the potential effects of an adversary's   modifications of the ECN field is given in Sections18 and19.9.1.1.  The Limited-functionality and Full-functionality Options   The limited-functionality option for ECN encapsulation in IP tunnels   is for the not-ECT codepoint to be set in the outside (encapsulating)   header regardless of the value of the ECN field in the inside   (encapsulated) header.  With this option, the ECN field in the inner   header is not altered upon de-capsulation.  The disadvantage of this   approach is that the flow does not have ECN support for that part of   the path that is using IP tunneling, even if the encapsulated packet   (from the original TCP sender) is ECN-Capable.  That is, if the   encapsulated packet arrives at a congested router that is ECN-   capable, and the router can decide to drop or mark the packet as an   indication of congestion to the end nodes, the router will not be   permitted to set the CE codepoint in the packet header, but instead   will have to drop the packet.   The full-functionality option for ECN encapsulation is to copy the   ECN codepoint of the inside header to the outside header on   encapsulation if the inside header is not-ECT or ECT, and to set the   ECN codepoint of the outside header to ECT(0) if the ECN codepoint of   the inside header is CE.  On decapsulation, if the CE codepoint is   set on the outside header, then the CE codepoint is also set in the   inner header.  Otherwise, the ECN codepoint on the inner header is   left unchanged.  That is, for full ECN support the encapsulation and   decapsulation processing involves the following:  At tunnel ingress,   the full-functionality option sets the ECN codepoint in the outer   header.  If the ECN codepoint in the inner header is not-ECT or ECT,   then it is copied to the ECN codepoint in the outer header.  If the   ECN codepoint in the inner header is CE, then the ECN codepoint in   the outer header is set to ECT(0).  Upon decapsulation at the tunnel   egress, the full-functionality option sets the CE codepoint in the   inner header if the CE codepoint is set in the outer header.   Otherwise, no change is made to this field of the inner header.   With the full-functionality option, a flow can take advantage of ECN   in those parts of the path that might use IP tunneling.  The   disadvantage of the full-functionality option from a security   perspective is that the IP tunnel cannot protect the flow from   certain modifications to the ECN bits in the IP header within the   tunnel.  The potential dangers from modifications to the ECN bits in   the IP header are described in detail in Sections18 and19.Ramakrishnan, et al.        Standards Track                    [Page 27]

RFC 3168               The Addition of ECN to IP          September 2001      (1) An IP tunnel MUST modify the handling of the DS field octet at      IP tunnel endpoints by implementing either the limited-      functionality or the full-functionality option.      (2) Optionally, an IP tunnel MAY enable the endpoints of an IP      tunnel to negotiate the choice between the limited-functionality      and the full-functionality option for ECN in the tunnel.   The minimum required to make ECN usable with IP tunnels is the   limited-functionality option, which prevents ECN from being enabled   in the outer header of the tunnel.  Full support for ECN requires the   use of the full-functionality option.  If there are no optional   mechanisms for the tunnel endpoints to negotiate a choice between the   limited-functionality or full-functionality option, there can be a   pre-existing agreement between the tunnel endpoints about whether to   support the limited-functionality or the full-functionality ECN   option.   All IP tunnels MUST implement the limited-functionality option, and   SHOULD support the full-functionality option.   In addition, it is RECOMMENDED that packets with the CE codepoint in   the outer header be dropped if they arrive at the tunnel egress point   for a tunnel that uses the limited-functionality option, or for a   tunnel that uses the full-functionality option but for which the   not-ECT codepoint is set in the inner header.  This is motivated by   backwards compatibility and to ensure that no unauthorized   modifications of the ECN field take place, and is discussed further   in the next Section (9.1.2).9.1.2.  Changes to the ECN Field within an IP Tunnel.   The presence of a copy of the ECN field in the inner header of an IP   tunnel mode packet provides an opportunity for detection of   unauthorized modifications to the ECN field in the outer header.   Comparison of the ECT fields in the inner and outer headers falls   into two categories for implementations that conform to this   document:      * If the IP tunnel uses the full-functionality option, then the        not-ECT codepoint should be set in the outer header if and only        if it is also set in the inner header.      * If the tunnel uses the limited-functionality option, then the        not-ECT codepoint should be set in the outer header.   Receipt of a packet not satisfying the appropriate condition could be   a cause of concern.Ramakrishnan, et al.        Standards Track                    [Page 28]

RFC 3168               The Addition of ECN to IP          September 2001   Consider the case of an IP tunnel where the tunnel ingress point has   not been updated to this document's requirements, while the tunnel   egress point has been updated to support ECN.  In this case, the IP   tunnel is not explicitly configured to support the full-functionality   ECN option. However, the tunnel ingress point is behaving identically   to a tunnel ingress point that supports the full-functionality   option.  If packets from an ECN-capable connection use this tunnel,   the ECT codepoint will be set in the outer header at the tunnel   ingress point.  Congestion within the tunnel may then result in ECN-   capable routers setting CE in the outer header.  Because the tunnel   has not been explicitly configured to support the full-functionality   option, the tunnel egress point expects the not-ECT codepoint to be   set in the outer header.  When an ECN-capable tunnel egress point   receives a packet with the ECT or CE codepoint in the outer header,   in a tunnel that has not been configured to support the full-   functionality option, that packet should be processed, according to   whether the CE codepoint was set, as follows.  It is RECOMMENDED that   on a tunnel that has not been configured to support the full-   functionality option, packets should be dropped at the egress point   if the CE codepoint is set in the outer header but not in the inner   header, and should be forwarded otherwise.   An IP tunnel cannot provide protection against erasure of congestion   indications based on changing the ECN codepoint from CE to ECT.  The   erasure of congestion indications may impact the network and other   flows in ways that would not be possible in the absence of ECN.  It   is important to note that erasure of congestion indications can only   be performed to congestion indications placed by nodes within the   tunnel; the copy of the ECN field in the inner header preserves   congestion notifications from nodes upstream of the tunnel ingress   (unless the inner header is also erased).  If erasure of congestion   notifications is judged to be a security risk that exceeds the   congestion management benefits of ECN, then tunnels could be   specified or configured to use the limited-functionality option.9.2.  IPsec Tunnels   IPsec supports secure communication over potentially insecure network   components such as intermediate routers.  IPsec protocols support two   operating modes, transport mode and tunnel mode, that span a wide   range of security requirements and operating environments.  Transport   mode security protocol header(s) are inserted between the IP (IPv4 or   IPv6) header and higher layer protocol headers (e.g., TCP), and hence   transport mode can only be used for end-to-end security on a   connection.  IPsec tunnel mode is based on adding a new "outer" IP   header that encapsulates the original, or "inner" IP header and its   associated packet.  Tunnel mode security headers are inserted between   these two IP headers.  In contrast to transport mode, the new "outer"Ramakrishnan, et al.        Standards Track                    [Page 29]

RFC 3168               The Addition of ECN to IP          September 2001   IP header and tunnel mode security headers can be added and removed   at intermediate points along a connection, enabling security gateways   to secure vulnerable portions of a connection without requiring   endpoint participation in the security protocols.  An important   aspect of tunnel mode security is that in the original specification,   the outer header is discarded at tunnel egress, ensuring that   security threats based on modifying the IP header do not propagate   beyond that tunnel endpoint.  Further discussion of IPsec can be   found in [RFC2401].   The IPsec protocol as originally defined in [ESP,AH] required that   the inner header's ECN field not be changed by IPsec decapsulation   processing at a tunnel egress node; this would have ruled out the   possibility of full-functionality mode for ECN.  At the same time,   this would ensure that an adversary's modifications to the ECN field   cannot be used to launch theft- or denial-of-service attacks across   an IPsec tunnel endpoint, as any such modifications will be discarded   at the tunnel endpoint.   In principle, permitting the use of ECN functionality in the outer   header of an IPsec tunnel raises security concerns because an   adversary could tamper with the information that propagates beyond   the tunnel endpoint.  Based on an analysis (included in Sections18   and 19) of these concerns and the associated risks, our overall   approach has been to provide configuration support for IPsec changes   to remove the conflict with ECN.   In particular, in tunnel mode the IPsec tunnel MUST support the   limited-functionality option outlined inSection 9.1.1, and SHOULD   support the full-functionality option outlined inSection 9.1.1.   This makes permission to use ECN functionality in the outer header of   an IPsec tunnel a configurable part of the corresponding IPsec   Security Association (SA), so that it can be disabled in situations   where the risks are judged to outweigh the benefits.  The result is   that an IPsec security administrator is presented with two   alternatives for the behavior of ECN-capable connections within an   IPsec tunnel, the limited-functionality alternative and full-   functionality alternative described earlier.   In addition, this document specifies how the endpoints of an IPsec   tunnel could negotiate enabling ECN functionality in the outer   headers of that tunnel based on security policy.  The ability to   negotiate ECN usage between tunnel endpoints would enable a security   administrator to disable ECN in situations where she believes the   risks (e.g., of lost congestion notifications) outweigh the benefits   of ECN.Ramakrishnan, et al.        Standards Track                    [Page 30]

RFC 3168               The Addition of ECN to IP          September 2001   The IPsec protocol, as defined in [ESP,AH], does not include the IP   header's ECN field in any of its cryptographic calculations (in the   case of tunnel mode, the outer IP header's ECN field is not   included).  Hence modification of the ECN field by a network node has   no effect on IPsec's end-to-end security, because it cannot cause any   IPsec integrity check to fail.  As a consequence, IPsec does not   provide any defense against an adversary's modification of the ECN   field (i.e., a man-in-the-middle attack), as the adversary's   modification will also have no effect on IPsec's end-to-end security.   In some environments, the ability to modify the ECN field without   affecting IPsec integrity checks may constitute a covert channel; if   it is necessary to eliminate such a channel or reduce its bandwidth,   then the IPsec tunnel should be run in limited-functionality mode.9.2.1.  Negotiation between Tunnel Endpoints   This section describes the detailed changes to enable usage of ECN   over IPsec tunnels, including the negotiation of ECN support between   tunnel endpoints.  This is supported by three changes to IPsec:      * An optional Security Association Database (SAD) field indicating        whether tunnel encapsulation and decapsulation processing allows        or forbids ECN usage in the outer IP header.      * An optional Security Association Attribute that enables        negotiation of this SAD field between the two endpoints of an SA        that supports tunnel mode.      * Changes to tunnel mode encapsulation and decapsulation        processing to allow or forbid ECN usage in the outer IP header        based on the value of the SAD field.  When ECN usage is allowed        in the outer IP header, the ECT codepoint is set in the outer        header for ECN-capable connections and congestion notifications        (indicated by the CE codepoint) from such connections are        propagated to the inner header at tunnel egress.   If negotiation of ECN usage is implemented, then the SAD field SHOULD   also be implemented.  On the other hand, negotiation of ECN usage is   OPTIONAL in all cases, even for implementations that support the SAD   field.  The encapsulation and decapsulation processing changes are   REQUIRED, but MAY be implemented without the other two changes by   assuming that ECN usage is always forbidden.  The full-functionality   alternative for ECN usage over IPsec tunnels consists of the SAD   field and the full version of encapsulation and decapsulation   processing changes, with or without the OPTIONAL negotiation support.   The limited-functionality alternative consists of a subset of the   encapsulation and decapsulation changes that always forbids ECN   usage.Ramakrishnan, et al.        Standards Track                    [Page 31]

RFC 3168               The Addition of ECN to IP          September 2001   These changes are covered further in the following three subsections.9.2.1.1.  ECN Tunnel Security Association Database Field   Full ECN functionality adds a new field to the SAD (see [RFC2401]):      ECN Tunnel: allowed or forbidden.      Indicates whether ECN-capable connections using this SA in tunnel      mode are permitted to receive ECN congestion notifications for      congestion occurring within the tunnel.  The allowed value enables      ECN congestion notifications.  The forbidden value disables such      notifications, causing all congestion to be indicated via dropped      packets.      [OPTIONAL.  The value of this field SHOULD be assumed to be      "forbidden" in implementations that do not support it.]   If this attribute is implemented, then the SA specification in a   Security Policy Database (SPD) entry MUST support a corresponding   attribute, and this SPD attribute MUST be covered by the SPD   administrative interface (currently described inSection 4.4.1 of   [RFC2401]).9.2.1.2.  ECN Tunnel Security Association Attribute   A new IPsec Security Association Attribute is defined to enable the   support for ECN congestion notifications based on the outer IP header   to be negotiated for IPsec tunnels (see [RFC2407]).  This attribute   is OPTIONAL, although implementations that support it SHOULD also   support the SAD field defined inSection 9.2.1.1.   Attribute Type           class               value           type     -------------------------------------------------     ECN Tunnel                 10             Basic   The IPsec SA Attribute value 10 has been allocated by IANA to   indicate that the ECN Tunnel SA Attribute is being negotiated; the   type of this attribute is Basic (seeSection 4.5 of [RFC2407]).  The   Class Values are used to conduct the negotiation.  See [RFC2407,RFC2408,RFC2409] for further information including encoding formats   and requirements for negotiating this SA attribute.Ramakrishnan, et al.        Standards Track                    [Page 32]

RFC 3168               The Addition of ECN to IP          September 2001   Class Values   ECN Tunnel   Specifies whether ECN functionality is allowed to be used with Tunnel   Encapsulation Mode.  This affects tunnel encapsulation and   decapsulation processing - seeSection 9.2.1.3.   RESERVED          0   Allowed           1   Forbidden         2   Values 3-61439 are reserved to IANA.  Values 61440-65535 are for   private use.   If unspecified, the default shall be assumed to be Forbidden.   ECN Tunnel is a new SA attribute, and hence initiators that use it   can expect to encounter responders that do not understand it, and   therefore reject proposals containing it.  For backwards   compatibility with such implementations initiators SHOULD always also   include a proposal without the ECN Tunnel attribute to enable such a   responder to select a transform or proposal that does not contain the   ECN Tunnel attribute.RFC 2407 currently requires responders to   reject all proposals if any proposal contains an unknown attribute;   this requirement is expected to be changed to require a responder not   to select proposals or transforms containing unknown attributes.9.2.1.3.  Changes to IPsec Tunnel Header Processing   For full ECN support, the encapsulation and decapsulation processing   for the IPv4 TOS field and the IPv6 Traffic Class field are changed   from that specified in [RFC2401] to the following:                        <-- How Outer Hdr Relates to Inner Hdr -->                        Outer Hdr at                 Inner Hdr at   IPv4                 Encapsulator                 Decapsulator     Header fields:     --------------------         ------------       DS Field         copied from inner hdr (5)    no change       ECN Field        constructed (7)              constructed (8)   IPv6     Header fields:       DS Field         copied from inner hdr (6)    no change       ECN Field        constructed (7)              constructed (8)Ramakrishnan, et al.        Standards Track                    [Page 33]

RFC 3168               The Addition of ECN to IP          September 2001      (5)(6) If the packet will immediately enter a domain for which the      DSCP value in the outer header is not appropriate, that value MUST      be mapped to an appropriate value for the domain [RFC 2474].  Also      see [RFC 2475] for further information.      (7) If the value of the ECN Tunnel field in the SAD entry for this      SA is "allowed" and the ECN field in the inner header is set to      any value other than CE, copy this ECN field to the outer header.      If the ECN field in the inner header is set to CE, then set the      ECN field in the outer header to ECT(0).      (8) If the value of the ECN tunnel field in the SAD entry for this      SA is "allowed" and the ECN field in the inner header is set to      ECT(0) or ECT(1) and the ECN field in the outer header is set to      CE, then copy the ECN field from the outer header to the inner      header.  Otherwise, make no change to the ECN field in the inner      header.      (5) and (6) are identical to match usage in [RFC2401], although      they are different in [RFC2401].   The above description applies to implementations that support the ECN   Tunnel field in the SAD; such implementations MUST implement this   processing instead of the processing of the IPv4 TOS octet and IPv6   Traffic Class octet defined in [RFC2401].  This constitutes the   full-functionality alternative for ECN usage with IPsec tunnels.   An implementation that does not support the ECN Tunnel field in the   SAD MUST implement this processing by assuming that the value of the   ECN Tunnel field of the SAD is "forbidden" for every SA.  In this   case, the processing of the ECN field reduces to:      (7) Set the ECN field to not-ECT in the outer header.      (8) Make no change to the ECN field in the inner header.   This constitutes the limited functionality alternative for ECN usage   with IPsec tunnels.   For backwards compatibility, packets with the CE codepoint set in the   outer header SHOULD be dropped if they arrive on an SA that is using   the limited-functionality option, or that is using the full-   functionality option with the not-ECN codepoint set in the inner   header.Ramakrishnan, et al.        Standards Track                    [Page 34]

RFC 3168               The Addition of ECN to IP          September 20019.2.2.  Changes to the ECN Field within an IPsec Tunnel.   If the ECN Field is changed inappropriately within an IPsec tunnel,   and this change is detected at the tunnel egress, then the receipt of   a packet not satisfying the appropriate condition for its SA is an   auditable event.  An implementation MAY create audit records with   per-SA counts of incorrect packets over some time period rather than   creating an audit record for each erroneous packet.  Any such audit   record SHOULD contain the headers from at least one erroneous packet,   but need not contain the headers from every packet represented by the   entry.9.2.3.  Comments for IPsec Support   Substantial comments were received on two areas of this document   during review by the IPsec working group.  This section describes   these comments and explains why the proposed changes were not   incorporated.   The first comment indicated that per-node configuration is easier to   implement than per-SA configuration.  After serious thought and   despite some initial encouragement of per-node configuration, it no   longer seems to be a good idea. The concern is that as ECN-awareness   is progressively deployed in IPsec, many ECN-aware IPsec   implementations will find themselves communicating with a mixture of   ECN-aware and ECN-unaware IPsec tunnel endpoints.  In such an   environment with per-node configuration, the only reasonable thing to   do is forbid ECN usage for all IPsec tunnels, which is not the   desired outcome.   In the second area, several reviewers noted that SA negotiation is   complex, and adding to it is non-trivial.  One reviewer suggested   using ICMP after tunnel setup as a possible alternative.  The   addition to SA negotiation in this document is OPTIONAL and will   remain so; implementers are free to ignore it.  The authors believe   that the assurance it provides can be useful in a number of   situations.  In practice, if this is not implemented, it can be   deleted at a subsequent stage in the standards process.  Extending   ICMP to negotiate ECN after tunnel setup is more complex than   extending SA attribute negotiation.  Some tunnels do not permit   traffic to be addressed to the tunnel egress endpoint, hence the ICMP   packet would have to be addressed to somewhere else, scanned for by   the egress endpoint, and discarded there or at its actual   destination.  In addition, ICMP delivery is unreliable, and hence   there is a possibility of an ICMP packet being dropped, entailing the   invention of yet another ack/retransmit mechanism.  It seems better   simply to specify an OPTIONAL extension to the existing SA   negotiation mechanism.Ramakrishnan, et al.        Standards Track                    [Page 35]

RFC 3168               The Addition of ECN to IP          September 20019.3.  IP packets encapsulated in non-IP Packet Headers.   A different set of issues are raised, relative to ECN, when IP   packets are encapsulated in tunnels with non-IP packet headers.  This   occurs with MPLS [MPLS], GRE [GRE], L2TP [L2TP], and PPTP [PPTP].   For these protocols, there is no conflict with ECN; it is just that   ECN cannot be used within the tunnel unless an ECN codepoint can be   specified for the header of the encapsulating protocol.  Earlier work   considered a preliminary proposal for incorporating ECN into MPLS,   and proposals for incorporating ECN into GRE, L2TP, or PPTP will be   considered as the need arises.10.  Issues Raised by Monitoring and Policing Devices   One possibility is that monitoring and policing devices (or more   informally, "penalty boxes") will be installed in the network to   monitor whether best-effort flows are appropriately responding to   congestion, and to preferentially drop packets from flows determined   not to be using adequate end-to-end congestion control procedures.   We recommend that any "penalty box" that detects a flow or an   aggregate of flows that is not responding to end-to-end congestion   control first change from marking to dropping packets from that flow,   before taking any additional action to restrict the bandwidth   available to that flow.  Thus, initially, the router may drop packets   in which the router would otherwise would have set the CE codepoint.   This could include dropping those arriving packets for that flow that   are ECN-Capable and that already have the CE codepoint set.  In this   way, any congestion indications seen by that router for that flow   will be guaranteed to also be seen by the end nodes, even in the   presence of malicious or broken routers elsewhere in the path.  If we   assume that the first action taken at any "penalty box" for an ECN-   capable flow will be to drop packets instead of marking them, then   there is no way that an adversary that subverts ECN-based end-to-end   congestion control can cause a flow to be characterized as being   non-cooperative and placed into a more severe action within the   "penalty box".   The monitoring and policing devices that are actually deployed could   fall short of the `ideal' monitoring device described above, in that   the monitoring is applied not to a single flow, but to an aggregate   of flows (e.g., those sharing a single IPsec tunnel).  In this case,   the switch from marking to dropping would apply to all of the flows   in that aggregate, denying the benefits of ECN to the other flows in   the aggregate also.  At the highest level of aggregation, another   form of the disabling of ECN happens even in the absence ofRamakrishnan, et al.        Standards Track                    [Page 36]

RFC 3168               The Addition of ECN to IP          September 2001   monitoring and policing devices, when ECN-Capable RED queues switch   from marking to dropping packets as an indication of congestion when   the average queue size has exceeded some threshold.11.  Evaluations of ECN11.1.  Related Work Evaluating ECN   This section discusses some of the related work evaluating the use of   ECN.  The ECN Web Page [ECN] has pointers to other papers, as well as   to implementations of ECN.   [Floyd94] considers the advantages and drawbacks of adding ECN to the   TCP/IP architecture.  As shown in the simulation-based comparisons,   one advantage of ECN is to avoid unnecessary packet drops for short   or delay-sensitive TCP connections.  A second advantage of ECN is in   avoiding some unnecessary retransmit timeouts in TCP.  This paper   discusses in detail the integration of ECN into TCP's congestion   control mechanisms.  The possible disadvantages of ECN discussed in   the paper are that a non-compliant TCP connection could falsely   advertise itself as ECN-capable, and that a TCP ACK packet carrying   an ECN-Echo message could itself be dropped in the network.  The   first of these two issues is discussed in the appendix of this   document, and the second is addressed by the addition of the CWR flag   in the TCP header.   Experimental evaluations of ECN include [RFC2884,K98].  The   conclusions of [K98] and [RFC2884] are that ECN TCP gets moderately   better throughput than non-ECN TCP; that ECN TCP flows are fair   towards non-ECN TCP flows; and that ECN TCP is robust with two-way   traffic (with congestion in both directions) and with multiple   congested gateways.  Experiments with many short web transfers show   that, while most of the short connections have similar transfer times   with or without ECN, a small percentage of the short connections have   very long transfer times for the non-ECN experiments as compared to   the ECN experiments.11.2.  A Discussion of the ECN nonce.   The use of two ECT codepoints, ECT(0) and ECT(1), can provide a one-   bit ECN nonce in packet headers [SCWA99].  The primary motivation for   this is the desire to allow mechanisms for the data sender to verify   that network elements are not erasing the CE codepoint, and that data   receivers are properly reporting to the sender the receipt of packets   with the CE codepoint set, as required by the transport protocol.   This section discusses issues of backwards compatibility with IP ECN   implementations in routers conformant withRFC 2481, in which only   one ECT codepoint was defined.  We do not believe that theRamakrishnan, et al.        Standards Track                    [Page 37]

RFC 3168               The Addition of ECN to IP          September 2001   incremental deployment of ECN implementations that understand the   ECT(1) codepoint will cause significant operational problems.  This   is particularly likely to be the case when the deployment of the   ECT(1) codepoint begins with routers, before the ECT(1) codepoint   starts to be used by end-nodes.11.2.1.  The Incremental Deployment of ECT(1) in Routers.   ECN has been an Experimental standard since January 1999, and there   are already implementations of ECN in routers that do not understand   the ECT(1) codepoint.  When the use of the ECT(1) codepoint is   standardized for TCP or for other transport protocols, this could   mean that a data sender is using the ECT(1) codepoint, but that this   codepoint is not understood by a congested router on the path.   If allowed by the transport protocol, a data sender would be free not   to make use of ECT(1) at all, and to send all ECN-capable packets   with the codepoint ECT(0).  However, if an ECN-capable sender is   using ECT(1), and the congested router on the path did not understand   the ECT(1) codepoint, then the router would end up marking some of   the ECT(0) packets, and dropping some of the ECT(1) packets, as   indications of congestion.  Since TCP is required to react to both   marked and dropped packets, this behavior of dropping packets that   could have been marked poses no significant threat to the network,   and is consistent with the overall approach to ECN that allows   routers to determine when and whether to mark packets as they see fit   (seeSection 5).12.  Summary of changes required in IP and TCP   This document specified two bits in the IP header to be used for ECN.   The not-ECT codepoint indicates that the transport protocol will   ignore the CE codepoint.  This is the default value for the ECN   codepoint.  The ECT codepoints indicate that the transport protocol   is willing and able to participate in ECN.   The router sets the CE codepoint to indicate congestion to the end   nodes.  The CE codepoint in a packet header MUST NOT be reset by a   router.   TCP requires three changes for ECN, a setup phase and two new flags   in the TCP header. The ECN-Echo flag is used by the data receiver to   inform the data sender of a received CE packet.  The Congestion   Window Reduced (CWR) flag is used by the data sender to inform the   data receiver that the congestion window has been reduced.Ramakrishnan, et al.        Standards Track                    [Page 38]

RFC 3168               The Addition of ECN to IP          September 2001   When ECN (Explicit Congestion Notification) is used, it is required   that congestion indications generated within an IP tunnel not be lost   at the tunnel egress.  We specified a minor modification to the IP   protocol's handling of the ECN field during encapsulation and de-   capsulation to allow flows that will undergo IP tunneling to use ECN.   Two options for ECN in tunnels were specified:   1) A limited-functionality option that does not use ECN inside the IP   tunnel, by setting the ECN field in the outer header to not-ECT, and   not altering the inner header at the time of decapsulation.   2) The full-functionality option, which sets the ECN field in the   outer header to either not-ECT or to one of the ECT codepoints,   depending on the ECN field in the inner header.  At decapsulation, if   the CE codepoint is set in the outer header, and the inner header is   set to one of the ECT codepoints, then the CE codepoint is copied to   the inner header.   For IPsec tunnels, this document also defines an optional IPsec   Security Association (SA) attribute that enables negotiation of ECN   usage within IPsec tunnels and an optional field in the Security   Association Database to indicate whether ECN is permitted in tunnel   mode on a SA.  The required changes to IPsec tunnels for ECN usage   modifyRFC 2401 [RFC2401], which defines the IPsec architecture and   specifies some aspects of its implementation.  The new IPsec SA   attribute is in addition to those already defined inSection 4.5 of   [RFC2407].   This document obsoletesRFC 2481, "A Proposal to add Explicit   Congestion Notification (ECN) to IP", which defined ECN as an   Experimental Protocol for the Internet Community.  The rest of this   section describes the relationship between this document and its   predecessor.RFC 2481 included a brief discussion of the use of ECN with   encapsulated packets, and noted that for the IPsec specifications at   the time (January 1999), flows could not safely use ECN if they were   to traverse IPsec tunnels.RFC 2481 also described the changes that   could be made to IPsec tunnel specifications to made them compatible   with ECN.   This document also incorporates work that was done afterRFC 2481.   First was to describe the changes to IPsec tunnels in detail, and   extensively discuss the security implications of ECN (now included as   Sections18 and19 of this document).  Second was to extend the   discussion of IPsec tunnels to include all IP tunnels.  Because older   IP tunnels are not compatible with a flow's use of ECN, theRamakrishnan, et al.        Standards Track                    [Page 39]

RFC 3168               The Addition of ECN to IP          September 2001   deployment of ECN in the Internet will create strong pressure for   older IP tunnels to be updated to an ECN-compatible version, using   either the limited-functionality or the full-functionality option.   This document does not address the issue of including ECN in non-IP   tunnels such as MPLS, GRE, L2TP, or PPTP.  An earlier preliminary   document about adding ECN support to MPLS was not advanced.   A third new piece of work afterRFC2481 was to describe the ECN   procedure with retransmitted data packets, that an ECT codepoint   should not be set on retransmitted data packets.  The motivation for   this additional specification is to eliminate a possible avenue for   denial-of-service attacks on an existing TCP connection.  Some prior   deployments of ECN-capable TCP might not conform to the (new)   requirement not to set an ECT codepoint on retransmitted packets; we   do not believe this will cause significant problems in practice.   This document also expands slightly on the specification of the use   of SYN packets for the negotiation of ECN.  While some prior   deployments of ECN-capable TCP might not conform to the requirements   specified in this document, we do not believe that this will lead to   any performance or compatibility problems for TCP connections with a   combination of TCP implementations at the endpoints.   This document also includes the specification of the ECT(1)   codepoint, which may be used by TCP as part of the implementation of   an ECN nonce.13.  Conclusions   Given the current effort to implement AQM, we believe this is the   right time to deploy congestion avoidance mechanisms that do not   depend on packet drops alone.  With the increased deployment of   applications and transports sensitive to the delay and loss of a   single packet (e.g., realtime traffic, short web transfers),   depending on packet loss as a normal congestion notification   mechanism appears to be insufficient (or at the very least, non-   optimal).   We examined the consequence of modifications of the ECN field within   the network, analyzing all the opportunities for an adversary to   change the ECN field.  In many cases, the change to the ECN field is   no worse than dropping a packet. However, we noted that some changes   have the more serious consequence of subverting end-to-end congestion   control.  However, we point out that even then the potential damage   is limited, and is similar to the threat posed by end-systems   intentionally failing to cooperate with end-to-end congestion   control.Ramakrishnan, et al.        Standards Track                    [Page 40]

RFC 3168               The Addition of ECN to IP          September 200114.  Acknowledgements   Many people have made contributions to this work and this document,   including many that we have not managed to directly acknowledge in   this document.  In addition, we would like to thank Kenjiro Cho for   the proposal for the TCP mechanism for negotiating ECN-Capability,   Kevin Fall for the proposal of the CWR bit, Steve Blake for material   on IPv4 Header Checksum Recalculation, Jamal Hadi-Salim for   discussions of ECN issues, and Steve Bellovin, Jim Bound, Brian   Carpenter, Paul Ferguson, Stephen Kent, Greg Minshall, and Vern   Paxson for discussions of security issues.  We also thank the   Internet End-to-End Research Group for ongoing discussions of these   issues.   Email discussions with a number of people, including Dax Kelson,   Alexey Kuznetsov, Jamal Hadi-Salim, and Venkat Venkatsubra, have   addressed the issues raised by non-conformant equipment in the   Internet that does not respond to TCP SYN packets with the ECE and   CWR flags set.  We thank Mark Handley, Jitentra Padhye, and others   for discussions on the TCP initialization procedures.   The discussion of ECN and IP tunnel considerations draws heavily on   related discussions and documents from the Differentiated Services   Working Group.  We thank Tabassum Bint Haque from Dhaka, Bangladesh,   for feedback on IP tunnels.  We thank Derrell Piper and Kero Tivinen   for proposing modifications toRFC 2407 that improve the usability of   negotiating the ECN Tunnel SA attribute.   We thank David Wetherall, David Ely, and Neil Spring for the proposal   for the ECN nonce.  We also thank Stefan Savage for discussions on   this issue.  We thank Bob Briscoe and Jon Crowcroft for raising the   issue of fragmentation in IP, on alternate semantics for the fourth   ECN codepoint, and several other topics.  We thank Richard Wendland   for feedback on several issues in the document.   We also thank the IESG, and in particular the Transport Area   Directors over the years, for their feedback and their work towards   the standardization of ECN.15.  References   [AH]         Kent, S. and R. Atkinson, "IP Authentication Header",RFC 2402, November 1998.   [ECN]       "The ECN Web Page", URL                "http://www.aciri.org/floyd/ecn.html".  Reference for                informational purposes only.Ramakrishnan, et al.        Standards Track                    [Page 41]

RFC 3168               The Addition of ECN to IP          September 2001   [ESP]        Kent, S. and R. Atkinson, "IP Encapsulating Security                Payload",RFC 2406, November 1998.   [FIXES]      ECN-under-Linux Unofficial Vendor Support Page, URL                "http://gtf.org/garzik/ecn/".  Reference for                informational purposes only.   [FJ93]       Floyd, S., and Jacobson, V., "Random Early Detection                gateways for Congestion Avoidance", IEEE/ACM                Transactions on Networking, V.1 N.4, August 1993, p.                397-413.   [Floyd94]    Floyd, S., "TCP and Explicit Congestion Notification",                ACM Computer Communication Review, V. 24 N. 5, October                1994, p. 10-23.   [Floyd98]    Floyd, S., "The ECN Validation Test in the NS                Simulator", URL "http://www-mash.cs.berkeley.edu/ns/",                test tcl/test/test-all- ecn.  Reference for                informational purposes only.   [FF99]       Floyd, S., and Fall, K., "Promoting the Use of End-to-                End Congestion Control in the Internet", IEEE/ACM                Transactions on Networking, August 1999.   [FRED]       Lin, D., and Morris, R., "Dynamics of Random Early                Detection", SIGCOMM '97, September 1997.   [GRE]        Hanks, S., Li, T., Farinacci, D. and P. Traina, "Generic                Routing Encapsulation (GRE)",RFC 1701, October 1994.   [Jacobson88] V. Jacobson, "Congestion Avoidance and Control", Proc.                ACM SIGCOMM '88, pp. 314-329.   [Jacobson90] V. Jacobson, "Modified TCP Congestion Avoidance                Algorithm", Message to end2end-interest mailing list,                April 1990. URL                "ftp://ftp.ee.lbl.gov/email/vanj.90apr30.txt".   [K98]        Krishnan, H., "Analyzing Explicit Congestion                Notification (ECN) benefits for TCP", Master's thesis,                UCLA, 1998.  Citation for acknowledgement purposes only.   [L2TP]       Townsley, W., Valencia, A., Rubens, A., Pall, G., Zorn,                G. and B. Palter, "Layer Two Tunneling Protocol "L2TP"",RFC 2661, August 1999.Ramakrishnan, et al.        Standards Track                    [Page 42]

RFC 3168               The Addition of ECN to IP          September 2001   [MJV96]      S. McCanne, V. Jacobson, and M. Vetterli, "Receiver-                driven Layered Multicast", SIGCOMM '96, August 1996, pp.                117-130.   [MPLS]       Awduche, D., Malcolm, J., Agogbua, J., O'Dell, M. and J.                McManus, Requirements for Traffic Engineering Over MPLS,RFC 2702, September 1999.   [PPTP]       Hamzeh, K., Pall, G., Verthein, W., Taarud, J., Little,                W.  and G. Zorn, "Point-to-Point Tunneling Protocol                (PPTP)",RFC 2637, July 1999.   [RFC791]     Postel, J., "Internet Protocol", STD 5,RFC 791,                September 1981.   [RFC793]     Postel, J., "Transmission Control Protocol", STD 7,RFC793, September 1981.   [RFC1141]    Mallory, T. and A. Kullberg, "Incremental Updating of                the Internet Checksum",RFC 1141, January 1990.   [RFC1349]    Almquist, P., "Type of Service in the Internet Protocol                Suite",RFC 1349, July 1992.   [RFC1455]    Eastlake, D., "Physical Link Security Type of Service",RFC 1455, May 1993.   [RFC1701]    Hanks, S., Li, T., Farinacci, D. and P. Traina, "Generic                Routing Encapsulation (GRE)",RFC 1701, October 1994.   [RFC1702]    Hanks, S., Li, T., Farinacci, D. and P. Traina, "Generic                Routing Encapsulation over IPv4 networks",RFC 1702,                October 1994.   [RFC2003]    Perkins, C., "IP Encapsulation within IP",RFC 2003,                October 1996.   [RFC2119]    Bradner, S., "Key words for use in RFCs to Indicate                Requirement Levels",BCP 14,RFC 2119, March 1997.   [RFC2309]    Braden, B., et al., "Recommendations on Queue Management                and Congestion Avoidance in the Internet",RFC 2309,                April 1998.   [RFC2401]    Kent, S. and R. Atkinson, Security Architecture for the                Internet Protocol,RFC 2401, November 1998.Ramakrishnan, et al.        Standards Track                    [Page 43]

RFC 3168               The Addition of ECN to IP          September 2001   [RFC2407]    Piper, D., "The Internet IP Security Domain of                Interpretation for ISAKMP",RFC 2407, November 1998.   [RFC2408]    Maughan, D., Schertler, M., Schneider, M. and J. Turner,                "Internet Security Association and Key Management                Protocol (ISAKMP)",RFC 2409, November 1998.   [RFC2409]    Harkins D. and D. Carrel, "The Internet Key Exchange                (IKE)",RFC 2409, November 1998.   [RFC2474]    Nichols, K., Blake, S., Baker, F. and D. Black,                "Definition of the Differentiated Services Field (DS                Field) in the IPv4 and IPv6 Headers",RFC 2474, December                1998.   [RFC2475]    Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.                and W. Weiss, "An Architecture for Differentiated                Services",RFC 2475, December 1998.   [RFC2481]    Ramakrishnan K. and S. Floyd, "A Proposal to add                Explicit Congestion Notification (ECN) to IP",RFC 2481,                January 1999.   [RFC2581]    Alman, M., Paxson, V. and W. Stevens, "TCP Congestion                Control",RFC 2581, April 1999.   [RFC2884]    Hadi Salim, J. and U. Ahmed, "Performance Evaluation of                Explicit Congestion Notification (ECN) in IP Networks",RFC 2884, July 2000.   [RFC2983]    Black, D., "Differentiated Services and Tunnels",RFC2983, October 2000.   [RFC2780]    Bradner S. and V. Paxson, "IANA Allocation Guidelines                For Values In the Internet Protocol and Related                Headers",BCP 37,RFC 2780, March 2000.   [RJ90]       K. K. Ramakrishnan and Raj Jain, "A Binary Feedback                Scheme for Congestion Avoidance in Computer Networks",                ACM Transactions on Computer Systems, Vol.8, No.2, pp.                158-181, May 1990.   [SCWA99]     Stefan Savage, Neal Cardwell, David Wetherall, and Tom                Anderson, TCP Congestion Control with a Misbehaving                Receiver, ACM Computer Communications Review, October                1999.Ramakrishnan, et al.        Standards Track                    [Page 44]

RFC 3168               The Addition of ECN to IP          September 2001   [TBIT]       Jitendra Padhye and Sally Floyd, "Identifying the TCP                Behavior of Web Servers", ICSI TR-01-002, February 2001.                URL "http://www.aciri.org/tbit/".16.  Security Considerations   Security considerations have been discussed in Sections7,8,18, and   19.17.  IPv4 Header Checksum Recalculation   IPv4 header checksum recalculation is an issue with some high-end   router architectures using an output-buffered switch, since most if   not all of the header manipulation is performed on the input side of   the switch, while the ECN decision would need to be made local to the   output buffer. This is not an issue for IPv6, since there is no IPv6   header checksum. The IPv4 TOS octet is the last byte of a 16-bit   half-word.RFC 1141 [RFC1141] discusses the incremental updating of the IPv4   checksum after the TTL field is decremented.  The incremental   updating of the IPv4 checksum after the CE codepoint was set would   work as follows: Let HC be the original header checksum for an ECT(0)   packet, and let HC' be the new header checksum after the CE bit has   been set.  That is, the ECN field has changed from '10' to '11'.   Then for header checksums calculated with one's complement   subtraction, HC' would be recalculated as follows:        HC' = { HC - 1     HC > 1              { 0x0000     HC = 1   For header checksums calculated on two's complement machines, HC'   would be recalculated as follows after the CE bit was set:        HC' = { HC - 1     HC > 0              { 0xFFFE     HC = 0   A similar incremental updating of the IPv4 checksum can be carried   out when the ECN field is changed from ECT(1) to CE, that is, from '   01' to '11'.18.  Possible Changes to the ECN Field in the Network   This section discusses in detail possible changes to the ECN field in   the network, such as falsely reporting congestion, disabling ECN-   Capability for an individual packet, erasing the ECN congestion   indication, or falsely indicating ECN-Capability.Ramakrishnan, et al.        Standards Track                    [Page 45]

RFC 3168               The Addition of ECN to IP          September 200118.1.  Possible Changes to the IP Header18.1.1.  Erasing the Congestion Indication   First, we consider the changes that a router could make that would   result in effectively erasing the congestion indication after it had   been set by a router upstream.  The convention followed is:  ECN   codepoint of received packet -> ECN codepoint of packet transmitted.   Replacing the CE codepoint with the ECT(0) or ECT(1) codepoint   effectively erases the congestion indication.  However, with the use   of two ECT codepoints, a router erasing the CE codepoint has no way   to know whether the original ECT codepoint was ECT(0) or ECT(1).   Thus, it is possible for the transport protocol to deploy mechanisms   to detect such erasures of the CE codepoint.   The consequence of the erasure of the CE codepoint for the upstream   router is that there is a potential for congestion to build for a   time, because the congestion indication does not reach the source.   However, the packet would be received and acknowledged.   The potential effect of erasing the congestion indication is complex,   and is discussed in depth inSection 19 below.  Note that the effect   of erasing the congestion indication is different from dropping a   packet in the network.  When a data packet is dropped, the drop is   detected by the TCP sender, and interpreted as an indication of   congestion.  Similarly, if a sufficient number of consecutive   acknowledgement packets are dropped, causing the cumulative   acknowledgement field not to be advanced at the sender, the sender is   limited by the congestion window from sending additional packets, and   ultimately the retransmit timer expires.   In contrast, a systematic erasure of the CE bit by a downstream   router can have the effect of causing a queue buildup at an upstream   router, including the possible loss of packets due to buffer   overflow.  There is a potential of unfairness in that another flow   that goes through the congested router could react to the CE bit set   while the flow that has the CE bit erased could see better   performance.  The limitations on this potential unfairness are   discussed in more detail inSection 19 below.   The last of the three changes is to replace the CE codepoint with the   not-ECT codepoint, thus erasing the congestion indication and   disabling ECN-Capability at the same time.   The `erasure' of the congestion indication is only effective if the   packet does not end up being marked or dropped again by a downstream   router.  If the CE codepoint is replaced by an ECT codepoint, theRamakrishnan, et al.        Standards Track                    [Page 46]

RFC 3168               The Addition of ECN to IP          September 2001   packet remains ECN-Capable, and could be either marked or dropped by   a downstream router as an indication of congestion.  If the CE   codepoint is replaced by the not-ECT codepoint, the packet is no   longer ECN-capable, and can therefore be dropped but not marked by a   downstream router as an indication of congestion.18.1.2.  Falsely Reporting Congestion   This change is to set the CE codepoint when an ECT codepoint was   already set, even though there was no congestion.  This change does   not affect the treatment of that packet along the rest of the path.   In particular, a router does not examine the CE codepoint in deciding   whether to drop or mark an arriving packet.   However, this could result in the application unnecessarily invoking   end-to-end congestion control, and reducing its arrival rate.  By   itself, this is no worse (for the application or for the network)   than if the tampering router had actually dropped the packet.18.1.3.  Disabling ECN-Capability   This change is to turn off the ECT codepoint of a packet.  This means   that if the packet later encounters congestion (e.g., by arriving to   a RED queue with a moderate average queue size), it will be dropped   instead of being marked.  By itself, this is no worse (for the   application) than if the tampering router had actually dropped the   packet.  The saving grace in this particular case is that there is no   congested router upstream expecting a reaction from setting the CE   bit.18.1.4.  Falsely Indicating ECN-Capability   This change would incorrectly label a packet as ECN-Capable. The   packet may have been sent either by an ECN-Capable transport or a   transport that is not ECN-Capable.   If the packet later encounters moderate congestion at an ECN-Capable   router, the router could set the CE codepoint instead of dropping the   packet.  If the transport protocol in fact is not ECN-Capable, then   the transport will never receive this indication of congestion, and   will not reduce its sending rate in response.  The potential   consequences of falsely indicating ECN-capability are discussed   further inSection 19 below.   If the packet never later encounters congestion at an ECN-Capable   router, then the first of these two changes would have no effect,   other than possibly interfering with the use of the ECN nonce by the   transport protocol.  The last change, however, would have the effectRamakrishnan, et al.        Standards Track                    [Page 47]

RFC 3168               The Addition of ECN to IP          September 2001   of giving false reports of congestion to a monitoring device along   the path.  If the transport protocol is ECN-Capable, then this change   could also have an effect at the transport level, by combining   falsely indicating ECN-Capability with falsely reporting congestion.   For an ECN-capable transport, this would cause the transport to   unnecessarily react to congestion.  In this particular case, the   router that is incorrectly changing the ECN field could have dropped   the packet. Thus for this case of an ECN-capable transport, the   consequence of this change to the ECN field is no worse than dropping   the packet.18.2.  Information carried in the Transport Header   For TCP, an ECN-capable TCP receiver informs its TCP peer that it is   ECN-capable at the TCP level, conveying this information in the TCP   header at the time the connection is setup.  This document does not   consider potential dangers introduced by changes in the transport   header within the network.  We note that when IPsec is used, the   transport header is protected both in tunnel and transport modes   [ESP,AH].   Another issue concerns TCP packets with a spoofed IP source address   carrying invalid ECN information in the transport header.  For   completeness, we examine here some possible ways that a node spoofing   the IP source address of another node could use the two ECN flags in   the TCP header to launch a denial-of-service attack. However, these   attacks would require an ability for the attacker to use valid TCP   sequence numbers, and any attacker with this ability and with the   ability to spoof IP source addresses could damage the TCP connection   without using the ECN flags.  Therefore, ECN does not add any new   vulnerabilities in this respect.   An acknowledgement packet with a spoofed IP source address of the TCP   data receiver could include the ECE bit set.  If accepted by the TCP   data sender as a valid packet, this spoofed acknowledgement packet   could result in the TCP data sender unnecessarily halving its   congestion window.  However, to be accepted by the data sender, such   a spoofed acknowledgement packet would have to have the correct 32-   bit sequence number as well as a valid acknowledgement number.  An   attacker that could successfully send such a spoofed acknowledgement   packet could also send a spoofed RST packet, or do other equally   damaging operations to the TCP connection.   Packets with a spoofed IP source address of the TCP data sender could   include the CWR bit set.  Again, to be accepted, such a packet would   have to have a valid sequence number.  In addition, such a spoofed   packet would have a limited performance impact.  Spoofing a data   packet with the CWR bit set could result in the TCP data receiverRamakrishnan, et al.        Standards Track                    [Page 48]

RFC 3168               The Addition of ECN to IP          September 2001   sending fewer ECE packets than it would otherwise, if the data   receiver was sending ECE packets when it received the spoofed CWR   packet.18.3.  Split Paths   In some cases, a malicious or broken router might have access to only   a subset of the packets from a flow.  The question is as follows:   can this router, by altering the ECN field in this subset of the   packets, do more damage to that flow than if it had simply dropped   that set of packets?   We will classify the packets in the flow as A packets and B packets,   and assume that the adversary only has access to A packets.  Assume   that the adversary is subverting end-to-end congestion control along   the path traveled by A packets only, by either falsely indicating   ECN-Capability upstream of the point where congestion occurs, or   erasing the congestion indication downstream.  Consider also that   there exists a monitoring device that sees both the A and B packets,   and will "punish" both the A and B packets if the total flow is   determined not to be properly responding to indications of   congestion.  Another key characteristic that we believe is likely to   be true is that the monitoring device, before `punishing' the A&B   flow, will first drop packets instead of setting the CE codepoint,   and will drop arriving packets of that flow that already have the CE   codepoint set.  If the end nodes are in fact using end-to-end   congestion control, they will see all of the indications of   congestion seen by the monitoring device, and will begin to respond   to these indications of congestion. Thus, the monitoring device is   successful in providing the indications to the flow at an early   stage.   It is true that the adversary that has access only to the A packets   might, by subverting ECN-based congestion control, be able to deny   the benefits of ECN to the other packets in the A&B aggregate.  While   this is unfortunate, this is not a reason to disable ECN.   A variant of falsely reporting congestion occurs when there are two   adversaries along a path, where the first adversary falsely reports   congestion, and the second adversary `erases' those reports. (Unlike   packet drops, ECN congestion reports can be `reversed' later in the   network by a malicious or broken router.  However, the use of the ECN   nonce could help the transport to detect this behavior.)  While this   would be transparent to the end node, it is possible that a   monitoring device between the first and second adversaries would see   the false indications of congestion.  Keep in mind our recommendation   in this document, that before `punishing' a flow for not responding   appropriately to congestion, the router will first switch to droppingRamakrishnan, et al.        Standards Track                    [Page 49]

RFC 3168               The Addition of ECN to IP          September 2001   rather than marking as an indication of congestion, for that flow.   When this includes dropping arriving packets from that flow that have   the CE codepoint set, this ensures that these indications of   congestion are being seen by the end nodes.  Thus, there is no   additional harm that we are able to postulate as a result of multiple   conflicting adversaries.19.  Implications of Subverting End-to-End Congestion Control   This section focuses on the potential repercussions of subverting   end-to-end congestion control by either falsely indicating ECN-   Capability, or by erasing the congestion indication in ECN (the CE   codepoint).  Subverting end-to-end congestion control by either of   these two methods can have consequences both for the application and   for the network.  We discuss these separately below.   The first method to subvert end-to-end congestion control, that of   falsely indicating ECN-Capability, effectively subverts end-to-end   congestion control only if the packet later encounters congestion   that results in the setting of the CE codepoint.  In this case, the   transport protocol (which may not be ECN-capable) does not receive   the indication of congestion from these downstream congested routers.   The second method to subvert end-to-end congestion control, `erasing'   the CE codepoint in a packet, effectively subverts end-to-end   congestion control only when the CE codepoint in the packet was set   earlier by a congested router.  In this case, the transport protocol   does not receive the indication of congestion from the upstream   congested routers.   Either of these two methods of subverting end-to-end congestion   control can potentially introduce more damage to the network (and   possibly to the flow itself) than if the adversary had simply dropped   packets from that flow.  However, as we discuss later in this section   and inSection 7, this potential damage is limited.19.1.  Implications for the Network and for Competing Flows   The CE codepoint of the ECN field is only used by routers as an   indication of congestion during periods of *moderate* congestion.   ECN-capable routers should drop rather than mark packets during heavy   congestion even if the router's queue is not yet full.  For example,   for routers using active queue management based on RED, the router   should drop rather than mark packets that arrive while the average   queue sizes exceed the RED queue's maximum threshold.Ramakrishnan, et al.        Standards Track                    [Page 50]

RFC 3168               The Addition of ECN to IP          September 2001   One consequence for the network of subverting end-to-end congestion   control is that flows that do not receive the congestion indications   from the network might increase their sending rate until they drive   the network into heavier congestion.  Then, the congested router   could begin to drop rather than mark arriving packets.  For flows   that are not isolated by some form of per-flow scheduling or other   per-flow mechanisms, but are instead aggregated with other flows in a   single queue in an undifferentiated fashion, this packet-dropping at   the congested router would apply to all flows that share that queue.   Thus, the consequences would be to increase the level of congestion   in the network.   In some cases, the increase in the level of congestion will lead to a   substantial buffer buildup at the congested queue that will be   sufficient to drive the congested queue from the packet-marking to   the packet-dropping regime.  This transition could occur either   because of buffer overflow, or because of the active queue management   policy described above that drops packets when the average queue is   above RED's maximum threshold.  At this point, all flows, including   the subverted flow, will begin to see packet drops instead of packet   marks, and a malicious or broken router will no longer be able to `   erase' these indications of congestion in the network.  If the end   nodes are deploying appropriate end-to-end congestion control, then   the subverted flow will reduce its arrival rate in response to   congestion.  When the level of congestion is sufficiently reduced,   the congested queue can return from the packet-dropping regime to the   packet-marking regime.  The steady-state pattern could be one of the   congested queue oscillating between these two regimes.   In other cases, the consequences of subverting end-to-end congestion   control will not be severe enough to drive the congested link into   sufficiently-heavy congestion that packets are dropped instead of   being marked.  In this case, the implications for competing flows in   the network will be a slightly-increased rate of packet marking or   dropping, and a corresponding decrease in the bandwidth available to   those flows.  This can be a stable state if the arrival rate of the   subverted flow is sufficiently small, relative to the link bandwidth,   that the average queue size at the congested router remains under   control.  In particular, the subverted flow could have a limited   bandwidth demand on the link at this router, while still getting more   than its "fair" share of the link.  This limited demand could be due   to a limited demand from the data source; a limitation from the TCP   advertised window; a lower-bandwidth access pipe; or other factors.   Thus the subversion of ECN-based congestion control can still lead to   unfairness, which we believe is appropriate to note here.Ramakrishnan, et al.        Standards Track                    [Page 51]

RFC 3168               The Addition of ECN to IP          September 2001   The threat to the network posed by the subversion of ECN-based   congestion control in the network is essentially the same as the   threat posed by an end-system that intentionally fails to cooperate   with end-to-end congestion control.  The deployment of mechanisms in   routers to address this threat is an open research question, and is   discussed further inSection 10.   Let us take the example described inSection 18.1.1, where the CE   codepoint that was set in a packet is erased: {'11' -> '10' or '11'   -> '01'}.  The consequence for the congested upstream router that set   the CE codepoint is that this congestion indication does not reach   the end nodes for that flow. The source (even one which is completely   cooperative and not malicious) is thus allowed to continue to   increase its sending rate (if it is a TCP flow, by increasing its   congestion window).  The flow potentially achieves better throughput   than the other flows that also share the congested router, especially   if there are no policing mechanisms or per-flow queuing mechanisms at   that router.  Consider the behavior of the other flows, especially if   they are cooperative: that is, the flows that do not experience   subverted end-to-end congestion control.  They are likely to reduce   their load (e.g., by reducing their window size) on the congested   router, thus benefiting our subverted flow. This results in   unfairness.  As we discussed above, this unfairness could either be   transient (because the congested queue is driven into the packet-   marking regime), oscillatory (because the congested queue oscillates   between the packet marking and the packet dropping regime), or more   moderate but a persistent stable state (because the congested queue   is never driven to the packet dropping regime).   The results would be similar if the subverted flow was intentionally   avoiding end-to-end congestion control.  One difference is that a   flow that is intentionally avoiding end-to-end congestion control at   the end nodes can avoid end-to-end congestion control even when the   congested queue is in packet-dropping mode, by refusing to reduce its   sending rate in response to packet drops in the network.  Thus the   problems for the network from the subversion of ECN-based congestion   control are less severe than the problems caused by the intentional   avoidance of end-to-end congestion control in the end nodes.  It is   also the case that it is considerably more difficult to control the   behavior of the end nodes than it is to control the behavior of the   infrastructure itself.  This is not to say that the problems for the   network posed by the network's subversion of ECN-based congestion   control are small; just that they are dwarfed by the problems for the   network posed by the subversion of either ECN-based or other   currently known packet-based congestion control mechanisms by the end   nodes.Ramakrishnan, et al.        Standards Track                    [Page 52]

RFC 3168               The Addition of ECN to IP          September 200119.2.  Implications for the Subverted Flow   When a source indicates that it is ECN-capable, there is an   expectation that the routers in the network that are capable of   participating in ECN will use the CE codepoint for indication of   congestion. There is the potential benefit of using ECN in reducing   the amount of packet loss (in addition to the reduced queuing delays   because of active queue management policies).  When the packet flows   through an IPsec tunnel where the nodes that the tunneled packets   traverse are untrusted in some way, the expectation is that IPsec   will protect the flow from subversion that results in undesirable   consequences.   In many cases, a subverted flow will benefit from the subversion of   end-to-end congestion control for that flow in the network, by   receiving more bandwidth than it would have otherwise, relative to   competing non-subverted flows.  If the congested queue reaches the   packet-dropping stage, then the subversion of end-to-end congestion   control might or might not be of overall benefit to the subverted   flow, depending on that flow's relative tradeoffs between throughput,   loss, and delay.   One form of subverting end-to-end congestion control is to falsely   indicate ECN-capability by setting the ECT codepoint.  This has the   consequence of downstream congested routers setting the CE codepoint   in vain.  However, as described inSection 9.1.2, if an ECT codepoint   is changed in an IP tunnel, this can be detected at the egress point   of the tunnel, as long as the inner header was not changed within the   tunnel.   The second form of subverting end-to-end congestion control is to   erase the congestion indication by erasing the CE codepoint.  In this   case, it is the upstream congested routers that set the CE codepoint   in vain.   If an ECT codepoint is erased within an IP tunnel, then this can be   detected at the egress point of the tunnel, as long as the inner   header was not changed within the tunnel.  If the CE codepoint is set   upstream of the IP tunnel, then any erasure of the outer header's CE   codepoint within the tunnel will have no effect because the inner   header preserves the set value of the CE codepoint.  However, if the   CE codepoint is set within the tunnel, and erased either within or   downstream of the tunnel, this is not necessarily detected at the   egress point of the tunnel.   With this subversion of end-to-end congestion control, an end-system   transport does not respond to the congestion indication.  Along with   the increased unfairness for the non-subverted flows described in theRamakrishnan, et al.        Standards Track                    [Page 53]

RFC 3168               The Addition of ECN to IP          September 2001   previous section, the congested router's queue could continue to   build, resulting in packet loss at the congested router - which is a   means for indicating congestion to the transport in any case.  In the   interim, the flow might experience higher queuing delays, possibly   along with an increased bandwidth relative to other non-subverted   flows.  But transports do not inherently make assumptions of   consistently experiencing carefully managed queuing in the path.  We   believe that these forms of subverting end-to-end congestion control   are no worse for the subverted flow than if the adversary had simply   dropped the packets of that flow itself.19.3.  Non-ECN-Based Methods of Subverting End-to-end Congestion Control   We have shown that, in many cases, a malicious or broken router that   is able to change the bits in the ECN field can do no more damage   than if it had simply dropped the packet in question.  However, this   is not true in all cases, in particular in the cases where the broken   router subverted end-to-end congestion control by either falsely   indicating ECN-Capability or by erasing the ECN congestion indication   (in the CE codepoint).  While there are many ways that a router can   harm a flow by dropping packets, a router cannot subvert end-to-end   congestion control by dropping packets.  As an example, a router   cannot subvert TCP congestion control by dropping data packets,   acknowledgement packets, or control packets.   Even though packet-dropping cannot be used to subvert end-to-end   congestion control, there *are* non-ECN-based methods for subverting   end-to-end congestion control that a broken or malicious router could   use.  For example, a broken router could duplicate data packets, thus   effectively negating the effects of end-to-end congestion control   along some portion of the path.  (For a router that duplicated   packets within an IPsec tunnel, the security administrator can cause   the duplicate packets to be discarded by configuring anti-replay   protection for the tunnel.)  This duplication of packets within the   network would have similar implications for the network and for the   subverted flow as those described in Sections18.1.1 and18.1.4   above.20.  The Motivation for the ECT Codepoints.20.1.  The Motivation for an ECT Codepoint.   The need for an ECT codepoint is motivated by the fact that ECN will   be deployed incrementally in an Internet where some transport   protocols and routers understand ECN and some do not. With an ECT   codepoint, the router can drop packets from flows that are not ECN-   capable, but can *instead* set the CE codepoint in packets that *are*Ramakrishnan, et al.        Standards Track                    [Page 54]

RFC 3168               The Addition of ECN to IP          September 2001   ECN-capable. Because an ECT codepoint allows an end node to have the   CE codepoint set in a packet *instead* of having the packet dropped,   an end node might have some incentive to deploy ECN.   If there was no ECT codepoint, then the router would have to set the   CE codepoint for packets from both ECN-capable and non-ECN-capable   flows.  In this case, there would be no incentive for end-nodes to   deploy ECN, and no viable path of incremental deployment from a non-   ECN world to an ECN-capable world.  Consider the first stages of such   an incremental deployment, where a subset of the flows are ECN-   capable.  At the onset of congestion, when the packet   dropping/marking rate would be low, routers would only set CE   codepoints, rather than dropping packets.  However, only those flows   that are ECN-capable would understand and respond to CE packets. The   result is that the ECN-capable flows would back off, and the non-   ECN-capable flows would be unaware of the ECN signals and would   continue to open their congestion windows.   In this case, there are two possible outcomes: (1) the ECN-capable   flows back off, the non-ECN-capable flows get all of the bandwidth,   and congestion remains mild, or (2) the ECN-capable flows back off,   the non-ECN-capable flows don't, and congestion increases until the   router transitions from setting the CE codepoint to dropping packets.   While this second outcome evens out the fairness, the ECN-capable   flows would still receive little benefit from being ECN-capable,   because the increased congestion would drive the router to packet-   dropping behavior.   A flow that advertised itself as ECN-Capable but does not respond to   CE codepoints is functionally equivalent to a flow that turns off   congestion control, as discussed earlier in this document.   Thus, in a world when a subset of the flows are ECN-capable, but   where ECN-capable flows have no mechanism for indicating that fact to   the routers, there would be less effective and less fair congestion   control in the Internet, resulting in a strong incentive for end   nodes not to deploy ECN.20.2.  The Motivation for two ECT Codepoints.   The primary motivation for the two ECT codepoints is to provide a   one-bit ECN nonce.  The ECN nonce allows the development of   mechanisms for the sender to probabilistically verify that network   elements are not erasing the CE codepoint, and that data receivers   are properly reporting to the sender the receipt of packets with the   CE codepoint set.Ramakrishnan, et al.        Standards Track                    [Page 55]

RFC 3168               The Addition of ECN to IP          September 2001   Another possibility for senders to detect misbehaving network   elements or receivers would be for the data sender to occasionally   send a data packet with the CE codepoint set, to see if the receiver   reports receiving the CE codepoint.  Of course, if these packets   encountered congestion in the network, the router might make no   change in the packets, because the CE codepoint would already be set.   Thus, for packets sent with the CE codepoint set, the TCP end-nodes   could not determine if some router intended to set the CE codepoint   in these packets.  For this reason, sending packets with the CE   codepoint would have to be done sparingly, and would be a less   effective check against misbehaving network elements and receivers   than would be the ECN nonce.   The assignment of the fourth ECN codepoint to ECT(1) precludes the   use of this codepoint for some other purposes.  For clarity, we   briefly list other possible purposes here.   One possibility might have been for the data sender to use the fourth   ECN codepoint to indicate an alternate semantics for ECN.  However,   this seems to us more appropriate to be signaled using a   differentiated services codepoint in the DS field.   A second possible use for the fourth ECN codepoint would have been to   give the router two separate codepoints for the indication of   congestion, CE(0) and CE(1), for mild and severe congestion   respectively.  While this could be useful in some cases, this   certainly does not seem a compelling requirement at this point.  If   there was judged to be a compelling need for this, the complications   of incremental deployment would most likely necessitate more that   just one codepoint for this function.   A third use that has been informally proposed for the ECN codepoint   is for use in some forms of multicast congestion control, based on   randomized procedures for duplicating marked packets at routers.   Some proposed multicast packet duplication procedures are based on a   new ECN codepoint that (1) conveys the fact that congestion occurred   upstream of the duplication point that marked the packet with this   codepoint and (2) can detect congestion downstream of that   duplication point.  ECT(1) can serve this purpose because it is both   distinct from ECT(0) and is replaced by CE when ECN marking occurs in   response to congestion or incipient congestion.  Explanation of how   this enhanced version of ECN would be used by multicast congestion   control is beyond the scope of this document, as are ECN-aware   multicast packet duplication procedures and the processing of the ECN   field at multicast receivers in all cases (i.e., irrespective of the   multicast packet duplication procedure(s) used).Ramakrishnan, et al.        Standards Track                    [Page 56]

RFC 3168               The Addition of ECN to IP          September 2001   The specification of IP tunnel modifications for ECN in this document   assumes that the only change made to the outer IP header's ECN field   between tunnel endpoints is to set the CE codepoint to indicate   congestion.  This is not consistent with some of the proposed uses of   ECT(1) by the multicast duplication procedures in the previous   paragraph, and such procedures SHOULD NOT be deployed unless this   inconsistency between multicast duplication procedures and IP tunnels   with full ECN functionality is resolved.  Limited ECN functionality   may be used instead, although in practice many tunnel protocols   (including IPsec) will not work correctly if multicast traffic   duplication occurs within the tunnel21.  Why use Two Bits in the IP Header?   Given the need for an ECT indication in the IP header, there still   remains the question of whether the ECT (ECN-Capable Transport) and   CE (Congestion Experienced) codepoints should have been overloaded on   a single bit.  This overloaded-one-bit alternative, explored in   [Floyd94], would have involved a single bit with two values.  One   value, "ECT and not CE", would represent an ECN-Capable Transport,   and the other value, "CE or not ECT", would represent either   Congestion Experienced or a non-ECN-Capable transport.   One difference between the one-bit and two-bit implementations   concerns packets that traverse multiple congested routers.  Consider   a CE packet that arrives at a second congested router, and is   selected by the active queue management at that router for either   marking or dropping.  In the one-bit implementation, the second   congested router has no choice but to drop the CE packet, because it   cannot distinguish between a CE packet and a non-ECT packet.  In the   two-bit implementation, the second congested router has the choice of   either dropping the CE packet, or of leaving it alone with the CE   codepoint set.   Another difference between the one-bit and two-bit implementations   comes from the fact that with the one-bit implementation, receivers   in a single flow cannot distinguish between CE and non-ECT packets.   Thus, in the one-bit implementation an ECN-capable data sender would   have to unambiguously indicate to the receiver or receivers whether   each packet had been sent as ECN-Capable or as non-ECN-Capable.  One   possibility would be for the sender to indicate in the transport   header whether the packet was sent as ECN-Capable.  A second   possibility that would involve a functional limitation for the one-   bit implementation would be for the sender to unambiguously indicate   that it was going to send *all* of its packets as ECN-Capable or as   non-ECN-Capable.  For a multicast transport protocol, this   unambiguous indication would have to be apparent to receivers joining   an on-going multicast session.Ramakrishnan, et al.        Standards Track                    [Page 57]

RFC 3168               The Addition of ECN to IP          September 2001   Another concern that was described earlier (and recommended in this   document) is that transports (particularly TCP) should not mark pure   ACK packets or retransmitted packets as being ECN-Capable.  A pure   ACK packet from a non-ECN-capable transport could be dropped, without   necessarily having an impact on the transport from a congestion   control perspective (because subsequent ACKs are cumulative).  An   ECN-capable transport reacting to the CE codepoint in a pure ACK   packet by reducing the window would be at a disadvantage in   comparison to a non-ECN-capable transport. For this reason (and for   reasons described earlier in relation to retransmitted packets), it   is desirable to have the ECT codepoint set on a per-packet basis.   Another advantage of the two-bit approach is that it is somewhat more   robust.  The most critical issue, discussed inSection 8, is that the   default indication should be that of a non-ECN-Capable transport.  In   a two-bit implementation, this requirement for the default value   simply means that the not-ECT codepoint should be the default.  In   the one-bit implementation, this means that the single overloaded bit   should by default be in the "CE or not ECT" position.  This is less   clear and straightforward, and possibly more open to incorrect   implementations either in the end nodes or in the routers.   In summary, while the one-bit implementation could be a possible   implementation, it has the following significant limitations relative   to the two-bit implementation.  First, the one-bit implementation has   more limited functionality for the treatment of CE packets at a   second congested router.  Second, the one-bit implementation requires   either that extra information be carried in the transport header of   packets from ECN-Capable flows (to convey the functionality of the   second bit elsewhere, namely in the transport header), or that   senders in ECN-Capable flows accept the limitation that receivers   must be able to determine a priori which packets are ECN-Capable and   which are not ECN-Capable. Third, the one-bit implementation is   possibly more open to errors from faulty implementations that choose   the wrong default value for the ECN bit.  We believe that the use of   the extra bit in the IP header for the ECT-bit is extremely valuable   to overcome these limitations.22.  Historical Definitions for the IPv4 TOS OctetRFC 791 [RFC791] defined the ToS (Type of Service) octet in the IP   header.  InRFC 791, bits 6 and 7 of the ToS octet are listed as   "Reserved for Future Use", and are shown set to zero.  The first two   fields of the ToS octet were defined as the Precedence and Type of   Service (TOS) fields.Ramakrishnan, et al.        Standards Track                    [Page 58]

RFC 3168               The Addition of ECN to IP          September 2001             0     1     2     3     4     5     6     7          +-----+-----+-----+-----+-----+-----+-----+-----+          |   PRECEDENCE    |       TOS       |  0  |  0  |RFC 791          +-----+-----+-----+-----+-----+-----+-----+-----+RFC 1122 included bits 6 and 7 in the TOS field, though it did not   discuss any specific use for those two bits:             0     1     2     3     4     5     6     7          +-----+-----+-----+-----+-----+-----+-----+-----+          |   PRECEDENCE    |       TOS                   |RFC 1122          +-----+-----+-----+-----+-----+-----+-----+-----+   The IPv4 TOS octet was redefined inRFC 1349 [RFC1349] as follows:             0     1     2     3     4     5     6     7          +-----+-----+-----+-----+-----+-----+-----+-----+          |   PRECEDENCE    |       TOS             | MBZ |RFC 1349          +-----+-----+-----+-----+-----+-----+-----+-----+   Bit 6 in the TOS field was defined inRFC 1349 for "Minimize Monetary   Cost".  In addition to the Precedence and Type of Service (TOS)   fields, the last field, MBZ (for "must be zero") was defined as   currently unused.RFC 1349 stated that "The originator of a datagram   sets [the MBZ] field to zero (unless participating in an Internet   protocol experiment which makes use of that bit)."RFC 1455 [RFC 1455] defined an experimental standard that used all   four bits in the TOS field to request a guaranteed level of link   security.RFC 1349 andRFC 1455 have been obsoleted by "Definition of the   Differentiated Services Field (DS Field) in the IPv4 and IPv6   Headers" [RFC2474] in which bits 6 and 7 of the DS field are listed   as Currently Unused (CU).RFC 2780 [RFC2780] specified ECN as an   experimental use of the two-bit CU field.RFC 2780 updated the   definition of the DS Field to only encompass the first six bits of   this octet rather than all eight bits; these first six bits are   defined as the Differentiated Services CodePoint (DSCP):            0     1     2     3     4     5     6     7         +-----+-----+-----+-----+-----+-----+-----+-----+         |               DSCP                |    CU     |  RFCs 2474,         +-----+-----+-----+-----+-----+-----+-----+-----+    2780   Because of this unstable history, the definition of the ECN field in   this document cannot be guaranteed to be backwards compatible with   all past uses of these two bits.Ramakrishnan, et al.        Standards Track                    [Page 59]

RFC 3168               The Addition of ECN to IP          September 2001   Prior toRFC 2474, routers were not permitted to modify bits in   either the DSCP or ECN field of packets forwarded through them, and   hence routers that comply only with RFCs prior to 2474 should have no   effect on ECN.  For end nodes, bit 7 (the second ECN bit) must be   transmitted as zero for any implementation compliant only with RFCs   prior to 2474.  Such nodes may transmit bit 6 (the first ECN bit) as   one for the "Minimize Monetary Cost" provision ofRFC 1349 or the   experiment authorized byRFC 1455; neither this aspect ofRFC 1349   nor the experiment inRFC 1455 were widely implemented or used.  The   damage that could be done by a broken, non-conformant router would   include "erasing" the CE codepoint for an ECN-capable packet that   arrived at the router with the CE codepoint set, or setting the CE   codepoint even in the absence of congestion.  This has been discussed   in the section on "Non-compliance in the Network".   The damage that could be done in an ECN-capable environment by a   non-ECN-capable end-node transmitting packets with the ECT codepoint   set has been discussed in the section on "Non-compliance by the End   Nodes".23.  IANA Considerations   This section contains the namespaces that have either been created in   this specification, or the values assigned in existing namespaces   managed by IANA.23.1.  IPv4 TOS Byte and IPv6 Traffic Class Octet   The codepoints for the ECN Field of the IP header are specified by   the Standards Action of this RFC, as is required byRFC 2780.   When this document is published as an RFC, IANA should create a new   registry, "IPv4 TOS Byte and IPv6 Traffic Class Octet", with the   namespace as follows:   IPv4 TOS Byte and IPv6 Traffic Class Octet   Description:  The registrations are identical for IPv4 and IPv6.   Bits 0-5:  see Differentiated Services Field Codepoints Registry           (http://www.iana.org/assignments/dscp-registry)Ramakrishnan, et al.        Standards Track                    [Page 60]

RFC 3168               The Addition of ECN to IP          September 2001   Bits 6-7, ECN Field:   Binary  Keyword                                  References   ------  -------                                  ----------     00     Not-ECT (Not ECN-Capable Transport)     [RFC 3168]     01     ECT(1) (ECN-Capable Transport(1))       [RFC 3168]     10     ECT(0) (ECN-Capable Transport(0))       [RFC 3168]     11     CE (Congestion Experienced)             [RFC 3168]23.2.  TCP Header Flags   The codepoints for the CWR and ECE flags in the TCP header are   specified by the Standards Action of this RFC, as is required byRFC2780.   When this document is published as an RFC, IANA should create a new   registry, "TCP Header Flags", with the namespace as follows:   TCP Header Flags   The Transmission Control Protocol (TCP) included a 6-bit Reserved   field defined inRFC 793, reserved for future use, in bytes 13 and 14   of the TCP header, as illustrated below.  The other six Control bits   are defined separately byRFC 793.     0   1   2   3   4   5   6   7   8   9  10  11  12  13  14  15   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+   |               |                       | U | A | P | R | S | F |   | Header Length |        Reserved       | R | C | S | S | Y | I |   |               |                       | G | K | H | T | N | N |   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+RFC 3168 defines two of the six bits from the Reserved field to be   used for ECN, as follows:     0   1   2   3   4   5   6   7   8   9  10  11  12  13  14  15   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+   |               |               | C | E | U | A | P | R | S | F |   | Header Length |    Reserved   | W | C | R | C | S | S | Y | I |   |               |               | R | E | G | K | H | T | N | N |   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+Ramakrishnan, et al.        Standards Track                    [Page 61]

RFC 3168               The Addition of ECN to IP          September 2001   TCP Header Flags   Bit      Name                                    Reference   ---      ----                                    ---------    8        CWR (Congestion Window Reduced)        [RFC 3168]    9        ECE (ECN-Echo)                         [RFC 3168]23.3. IPSEC Security Association Attributes   IANA allocated the IPSEC Security Association Attribute value 10 for   the ECN Tunnel use described inSection 9.2.1.2 above at the request   of David Black in November 1999.  The IANA has changed the Reference   for this allocation from David Black's request to this RFC.24.  Authors' Addresses   K. K. Ramakrishnan   TeraOptic Networks, Inc.   Phone: +1 (408) 666-8650   EMail: kk@teraoptic.com   Sally Floyd   ACIRI   Phone: +1 (510) 666-2989   EMail: floyd@aciri.org   URL:http://www.aciri.org/floyd/   David L. Black   EMC Corporation   42 South St.   Hopkinton, MA  01748   Phone:  +1 (508) 435-1000 x75140   EMail: black_david@emc.comRamakrishnan, et al.        Standards Track                    [Page 62]

RFC 3168               The Addition of ECN to IP          September 200125.  Full Copyright Statement   Copyright (C) The Internet Society (2001).  All Rights Reserved.   This document and translations of it may be copied and furnished to   others, and derivative works that comment on or otherwise explain it   or assist in its implementation may be prepared, copied, published   and distributed, in whole or in part, without restriction of any   kind, provided that the above copyright notice and this paragraph are   included on all such copies and derivative works.  However, this   document itself may not be modified in any way, such as by removing   the copyright notice or references to the Internet Society or other   Internet organizations, except as needed for the purpose of   developing Internet standards in which case the procedures for   copyrights defined in the Internet Standards process must be   followed, or as required to translate it into languages other than   English.   The limited permissions granted above are perpetual and will not be   revoked by the Internet Society or its successors or assigns.   This document and the information contained herein is provided on an   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING   TASK FORCE DISCLAIMS 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.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Ramakrishnan, et al.        Standards Track                    [Page 63]