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Network Working Group                                    K. RamakrishnanRequest for Comments: 2481                            AT&T Labs ResearchCategory: Experimental                                          S. Floyd                                                                    LBNL                                                            January 1999A Proposal to add Explicit Congestion Notification (ECN) to IPStatus of this Memo   This memo defines an Experimental Protocol for the Internet   community.  It does not specify an Internet standard of any kind.   Discussion and suggestions for improvement are requested.   Distribution of this memo is unlimited.Copyright Notice   Copyright (C) The Internet Society (1999).  All Rights Reserved.Abstract   This note describes a proposed addition of ECN (Explicit Congestion   Notification) to IP.  TCP is currently the dominant transport   protocol used in the Internet. We begin by describing TCP's use of   packet drops as an indication of congestion.  Next we argue 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 could instead set a Congestion   Experienced (CE) bit in the packet header of packets from ECN-capable   transport protocols.  We describe when the CE bit would be set in the   routers, and describe what modifications would be 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.1.  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 [B97].Ramakrishnan & Floyd          Experimental                      [Page 1]

RFC 2481                       ECN to IP                    January 19992. Introduction   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 treating   loss as an indication of congestion in the network is appropriate for   pure best-effort data carried by TCP which has 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   (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 (for reliable data delivery such as 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, this means that some of   the packets of latency-sensitive flows are going to be dropped.   Active queue management mechanisms detect congestion before the queue   overflows, and provide an indication of this congestion to the end   nodes.  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   congestion control (e.g., TCP) do not have to rely on buffer overflow   as the only indication of congestion.  This can reduce unnecessary   queueing delay for all traffic sharing that queue.   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 queueing or dropping packets from the policies   for indicating congestion. Thus, active queue management allowsRamakrishnan & Floyd          Experimental                      [Page 2]

RFC 2481                       ECN to IP                    January 1999   routers to use the Congestion Experienced (CE) bit in a packet header   as an indication of congestion, instead of relying solely on packet   drops.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.   (1) 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.   (2) 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.   (3) 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.   (4) 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.   (5) 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.   (6) 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.   (7) 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. Random Early Detection (RED)   Random Early Detection (RED) is a mechanism for active queue   management that has been proposed to detect incipient congestion   [FJ93], and is currently being deployed in the Internet backbone   [RFC2309].  Although RED is meant to be a general mechanism using oneRamakrishnan & Floyd          Experimental                      [Page 3]

RFC 2481                       ECN to IP                    January 1999   of several alternatives for congestion indication, in the current   environment of the Internet RED is restricted to using packet drops   as a mechanism for congestion indication.  RED drops packets based on   the average queue length exceeding a threshold, rather than only when   the queue overflows.  However, when RED drops packets before the   queue actually overflows, RED is not forced by memory limitations to   discard the packet.   RED could set a Congestion Experienced (CE) bit in the packet header   instead of dropping the packet, if such a bit was provided in the IP   header and understood by the transport protocol.  The use of the CE   bit would allow 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 bit set.5. Explicit Congestion Notification in IP   We propose 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 would require an ECN field in the IP header with   two bits.  The ECN-Capable Transport (ECT) bit would be set by the   data sender to indicate that the end-points of the transport protocol   are ECN-capable.  The CE bit would be set by the router to indicate   congestion to the end nodes.  Routers that have a packet arriving at   a full queue would drop the packet, just as they do now.   Bits 6 and 7 in the IPv4 TOS octet are designated as the ECN field.   Bit 6 is designated as the ECT bit, and bit 7 is designated as the CE   bit.  The IPv4 TOS octet corresponds to the Traffic Class octet in   IPv6.  The definitions for the IPv4 TOS octet [RFC791] and the IPv6   Traffic Class octet are intended to be superseded by the DS   (Differentiated Services) Field [DIFFSERV].  Bits 6 and 7 are listed   in [DIFFSERV] as Currently Unused.Section 19 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 all past uses of these two bits.  The   potential dangers of this lack of backwards compatibility are   discussed inSection 19.   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 dataRamakrishnan & Floyd          Experimental                      [Page 4]

RFC 2481                       ECN to IP                    January 1999   containing either a packet drop or an ECN indication.  However, we   would like to point out some notable exceptions in the reaction of   the source TCP, related to following the shorter-time-scale details   of particular implementations of TCP.  For TCP's response to an ECN   indication, we do not recommend such behavior as the slow-start of   Tahoe TCP in response to a packet drop, or Reno TCP's wait of roughly   half a round-trip time during Fast Recovery.   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 RED policies for congestion detection) while   other routers set the CE bit, for equivalent levels of congestion.   Similarly, a router might drop a non-ECN-Capable packet but set the   CE bit in an ECN-Capable packet, for equivalent levels of congestion.   Different congestion control responses to a CE bit indication and to   a packet drop could result in unfair treatment for different flows.   An additional requirement is that the end-systems should react to   congestion at most once per window of data (i.e., at most once per   roundtrip time), to avoid reacting multiple times to multiple   indications of congestion within a roundtrip time.   For a router, the CE bit 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 bit is set in that packet's IP header.  If so, then   instead of dropping the packet, the router MAY instead set the CE bit   in the IP header.   An environment where all end nodes were ECN-Capable could allow new   criteria to be developed for setting the CE bit, 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 is received by a router, the CE bit is left   unchanged, and the packet 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 adequately-provisioned network in such an ECN-   Capable environment, packet losses should occur primarily duringRamakrishnan & Floyd          Experimental                      [Page 5]

RFC 2481                       ECN to IP                    January 1999   transients or in the presence of non-cooperating sources.   We expect that routers will set the CE bit 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 active queue management at the router, the   reverse does not hold; active queue management mechanisms have been   developed and deployed independently from ECN, using packet drops as   indications of congestion in the absence of ECN in the IP   architecture.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 bit 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 to that   receiver.   This document only addresses the addition of ECN Capability to TCP,   leaving issues of ECN and other transport protocols to further   research.  For TCP, ECN requires three new mechanisms:  negotiation   between the endpoints during setup to determine if they are both   ECN-capable; an ECN-Echo 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, particular for   unreliable or reliable multicast transport protocols, and will have   to be determined as other transport protocols are brought to the IETF   for standardization.Ramakrishnan & Floyd          Experimental                      [Page 6]

RFC 2481                       ECN to IP                    January 19996.1. 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   [RFC 2001].   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 flag in the TCP header.  (This was called the ECN Notify   flag in some earlier documents.)  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 3 ofRFC793 [RFC793].   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   Congestion Window Reduced (CWR) flag.  The CWR flag is assigned to   Bit 8 in the Reserved field of the TCP header.   The use of these flags is described in the sections below.6.1.1.  TCP Initialization   In the TCP connection setup phase, the source and destination TCPs   exchange information about their desire and/or capability to use ECN.   Subsequent to the completion of this negotiation, the TCP sender sets   the ECT bit 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 will indicate to the routers that they may   mark this packet with the CE bit, 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 ECT bit equal to 0 (i.e., not set), and the TCP receiver   ignores the CE bit in the received packet.   When a node sends a TCP SYN packet, it may set the ECN-Echo and CWR   flags in the TCP header.  For a SYN packet, the setting of both the   ECN-Echo and CWR flags are 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, a SYN packet with both the   ECN-Echo and CWR flags set indicates that the TCP implementation   transmitting the SYN packet will participate in ECN as both a sender   and receiver.  As a receiver, it will respond to incoming data   packets that have the CE bit set in the IP header by setting the   ECN-Echo flag in outgoing TCP Acknowledgement (ACK) packets.  As a   sender, it will respond to incoming packets that have the ECN-EchoRamakrishnan & Floyd          Experimental                      [Page 7]

RFC 2481                       ECN to IP                    January 1999   flag set by reducing the congestion window when appropriate.   When a node sends a SYN-ACK packet, it may set the ECN-Echo flag, but   it does not set the CWR flag.  For a SYN-ACK packet, the pattern of   the ECN-Echo flag set and the CWR flag not set in the TCP header is   defined as an indication that the TCP transmitting the SYN-ACK packet   is ECN-Capable.   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 deployed   TCP implementations.  There exists at least one 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.6.1.2.  The TCP Sender   For a TCP connection using ECN, data packets are transmitted with the   ECT bit set in the IP header (set to a "1").  If the sender receives   an ECN-Echo 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 does NOT increase the   congestion window in response to the receipt of an ECN-Echo ACK   packet.   A critical condition is that TCP does 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 or have   the CE bit set, then this is interpreted by the source TCP as a new   instance of congestion.Ramakrishnan & Floyd          Experimental                      [Page 8]

RFC 2481                       ECN to IP                    January 1999   After the source TCP reduces its congestion window in response to a   CE packet, incoming acknowledgements 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.  We believe it is   desirable 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 to reduce the rate   further in this circumstance.  Therefore, the sending TCP should also   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 when the retransmit timer expires.   [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 acknowledgements, or retransmit   timeouts [RFC2001].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 the OR of the CE bits   of all 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 must set the ECN-Echo   flag in a series of ACK packets. The TCP receiver uses the CWR flag   to determine when to stop setting the ECN-Echo flag.Ramakrishnan & Floyd          Experimental                      [Page 9]

RFC 2481                       ECN to IP                    January 1999   When an ECN-Capable TCP reduces its congestion window for any reason   (because of a retransmit timeout, a Fast Retransmit, or in response   to an ECN Notification), the TCP sets the CWR flag in the TCP header   of the first data packet sent after the window reduction.  If that   data packet is dropped in the network, then the sending TCP will have   to reduce the congestion window again and retransmit the dropped   packet.  Thus, the Congestion Window Reduced message is reliably   delivered to the data receiver.   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 ACK packets   until it receives a CWR packet (a packet with the CWR flag set).   After the receipt of the CWR packet, acknowledgements 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 indicate that the data sender reduced its   congestion window at some point *after* it sent the data packet for   which the CE bit was set.   We have already specified that a TCP sender reduces its congestion   window at most once per window of data.  This mechanism requires some   care to make sure that the sender reduces its congestion window at   most once per ECN indication, and that multiple ECN messages over   several successive windows of data are properly reported to the ECN   sender.  This is discussed further 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) should be sent with the ECT bit off. 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 bit   set). For current TCP implementations, a single dropped ACK generally   has only a very small effect on the TCP's sending rate.7. Summary of changes required in IP and TCP   Two bits need to be specified in the IP header, the ECN-Capable   Transport (ECT) bit and the Congestion Experienced (CE) bit.  The ECT   bit set to "0" indicates that the transport protocol will ignore theRamakrishnan & Floyd          Experimental                     [Page 10]

RFC 2481                       ECN to IP                    January 1999   CE bit.  This is the default value for the ECT bit.  The ECT bit set   to "1" indicates that the transport protocol is willing and able to   participate in ECN.   The default value for the CE bit is "0".  The router sets the CE bit   to "1" to indicate congestion to the end nodes.  The CE bit in a   packet header should never be reset by a router from "1" to "0".   TCP requires three changes, a negotiation phase during setup to   determine if both end nodes are ECN-capable, and two new flags in the   TCP header, from the "reserved" flags in the TCP flags field.  The   ECN-Echo flag is used by the data receiver to inform the data sender   of a received CE packet.  The Congestion Window Reduced flag is used   by the data sender to inform the data receiver that the congestion   window has been reduced.8. Non-relationship to ATM's EFCI indicator or Frame Relay's FECN   Since the ATM and Frame Relay mechanisms for congestion indication   have typically been defined without any 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 draft for ECN is NOT the appropriate   reaction for such a noisy signal of congestion notification. It is   our expectation that ATM's EFCI and Frame Relay's FECN mechanisms   would be phased out over time within the ATM network.  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 emphasize that a *single* packet with the CE bit set in an IP   packet causes the transport layer to respond, in terms of congestion   control, as it would to a packet drop.  As such, the CE bit is not a   good match to a transient signal such as one based on the   instantaneous queue size.  However, experiments in techniques at   layer 2 (e.g., in ATM switches or Frame Relay switches) should be   encouraged.  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 bit (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 2   Congestion Experienced bit into the CE bit 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 mayRamakrishnan & Floyd          Experimental                     [Page 11]

RFC 2481                       ECN to IP                    January 1999   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.9. 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 bit in   transmitted packets but do not respond to received CE packets).  We   argue that the addition of ECN to the IP architecture would 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 from   non-compliant flows.  It has also been argued 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 has been argued that dropping packets in itself may be 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   bit 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 decreasingRamakrishnan & Floyd          Experimental                     [Page 12]

RFC 2481                       ECN to IP                    January 1999   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.   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 bit 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.10. Non-compliance in the Network   The breakdown of effective congestion control could be caused not   only by a non-compliant end-node, but also by the loss of the   congestion indication in the network itself.  This could happen   through a rogue or broken router that set the ECT bit in a packet   from a non-ECN-capable transport, or "erased" the CE bit in arriving   packets.  As one example, a rogue or broken router that "erased" the   CE bit 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.   The actions of a rogue or broken router could also result in an   unnecessary indication of congestion to the end-nodes.  These actions   can include a router dropping a packet or setting the CE bit in the   absence of congestion. From a congestion control point of view,   setting the CE bit in the absence of congestion by a non-compliant   router would be no different than a router dropping a packet   unecessarily. By "erasing" the ECT bit 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.   Concerns regarding the loss of congestion indications from   encapsulated, dropped, or corrupted packets are discussed below.Ramakrishnan & Floyd          Experimental                     [Page 13]

RFC 2481                       ECN to IP                    January 199910.1. Encapsulated packets   Some care is required to handle the CE and ECT bits appropriately   when packets are encapsulated and de-encapsulated for tunnels.   When a packet is encapsulated, the following rules apply regarding   the ECT bit.  First, if the ECT bit in the encapsulated ('inside')   header is a 0, then the ECT bit in the encapsulating ('outside')   header MUST be a 0.  If the ECT bit in the inside header is a 1, then   the ECT bit in the outside header SHOULD be a 1.   When a packet is de-encapsulated, the following rules apply regarding   the CE bit.  If the ECT bit is a 1 in both the inside and the outside   header, then the CE bit in the outside header MUST be ORed with the   CE bit in the inside header.  (That is, in this case a CE bit of 1 in   the outside header must be copied to the inside header.)  If the ECT   bit in either header is a 0, then the CE bit in the outside header is   ignored.  This requirement for the treatment of de-encapsulated   packets does not currently apply to IPsec tunnels.   A specific example of the use of ECN with encapsulation occurs when a   flow wishes to use ECN-capability to avoid the danger of an   unnecessary packet drop for the encapsulated packet as a result of   congestion at an intermediate node in the tunnel.  This functionality   can be supported by copying the ECN field in the inner IP header to   the outer IP header upon encapsulation, and using the ECN field in   the outer IP header to set the ECN field in the inner IP header upon   decapsulation.  This effectively allows routers along the tunnel to   cause the CE bit to be set in the ECN field of the unencapsulated IP   header of an ECN-capable packet when such routers experience   congestion.10.2.  IPsec Tunnel Considerations   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 outer IP header's ECN field can be zeroed at the tunnel   ingress and egress nodes.Ramakrishnan & Floyd          Experimental                     [Page 14]

RFC 2481                       ECN to IP                    January 1999   The IPsec protocol currently requires that the inner header's ECN   field not be changed by IPsec decapsulation processing at a tunnel   egress node.  This ensures 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.  This document makes no   change to that IPsec requirement. As a consequence of the current   specification of the IPsec protocol, we suggest that experiments with   ECN not be carried out for flows that will undergo IPsec tunneling at   the present time.   If the IPsec specifications are modified in the future to permit a   tunnel egress node to modify the ECN field in an inner IP header   based on the ECN field value in the outer header (e.g., copying part   or all of the outer ECN field to the inner ECN field), or to permit   the ECN field of the outer IP header to be zeroed during   encapsulation, then experiments with ECN may be used in combination   with IPsec tunneling.   This discussion of ECN and IPsec tunnel considerations draws heavily   on related discussions and documents from the Differentiated Services   Working Group.10.3.  Dropped or Corrupted Packets   An additional issue concerns a packet that has the CE bit set at one   router and is dropped by a subsequent router.  For the proposed use   for ECN in this paper (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.   However, 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   concerns raised by CE packets that were later dropped in the network.   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.Ramakrishnan & Floyd          Experimental                     [Page 15]

RFC 2481                       ECN to IP                    January 199911. A summary of related work.   [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 inSection 8 of this document,   and the second is addressed by the proposal inSection 5.1.3 for a   CWR flag in the TCP header.   [CKLTZ97] reports on an experimental implementation of ECN in IPv6.   The experiments include an implementation of ECN in an existing   implementation of RED for FreeBSD.  A number of experiments were run   to demonstrate the control of the average queue size in the router,   the performance of ECN for a single TCP connection as a congested   router, and fairness with multiple competing TCP connections.  One   conclusion of the experiments is that dropping packets from a bulk-   data transfer can degrade performance much more severely than marking   packets.   Because the experimental implementation in [CKLTZ97] predates some of   the developments in this document, the implementation does not   conform to this document in all respects.  For example, in the   experimental implementation the CWR flag is not used, but instead the   TCP receiver sends the ECN-Echo bit on a single ACK packet.   [K98] and [CKLTZ98] build on [CKLTZ97] to further analyze the   benefits of ECN for TCP. The conclusions 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, 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.  This increased transfer time is particularly   dramatic for those short connections that have their first packet   dropped in the non-ECN experiments, and that therefore have to wait   six seconds for the retransmit timer to expire.   The ECN Web Page [ECN] has pointers to other implementations of ECN   in progress.Ramakrishnan & Floyd          Experimental                     [Page 16]

RFC 2481                       ECN to IP                    January 199912. Conclusions   Given the current effort to implement RED, we believe this is the   right time for router vendors to examine how to implement 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).13. Acknowledgements   Many people have made contributions to this RFC.  In particular, 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.14. References   [AH]         Kent, S. and R. Atkinson, "IP Authentication Header",RFC 2402, November 1998.   [B97]        Bradner, S., "Key words for use in RFCs to Indicate                Requirement Levels",BCP 14,RFC 2119, March 1997.   [CKLT98]     Chen, C., Krishnan, H., Leung, S., Tang, N., and Zhang,                L., "Implementing ECN for TCP/IPv6", presentation to the                ECN BOF at the L.A. IETF, March 1998, URL                "http://www.cs.ucla.edu/~hari/ecn-ietf.ps".   [DIFFSERV]   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.   [ECN]        "The ECN Web Page", URL "http://www-nrg.ee.lbl.gov/floyd/ecn.html".   [ESP]        Kent, S. and R. Atkinson, "IP Encapsulating Security                Payload",RFC 2406, November 1998.Ramakrishnan & Floyd          Experimental                     [Page 17]

RFC 2481                       ECN to IP                    January 1999   [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.  URL "ftp://ftp.ee.lbl.gov/papers/early.pdf".   [Floyd94]    Floyd, S., "TCP and Explicit Congestion Notification",                ACM Computer Communication Review, V. 24 N. 5, October                1994, p. 10-23.  URL                "ftp://ftp.ee.lbl.gov/papers/tcp_ecn.4.ps.Z".   [Floyd97]    Floyd, S., and Fall, K., "Router Mechanisms to Support                End-to-End Congestion Control", Technical report,                February 1997.  URL "http://www-nrg.ee.lbl.gov/floyd/end2end-paper.html".   [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.   [K98]        Krishnan, H., "Analyzing Explicit Congestion                Notification (ECN) benefits for TCP", Master's thesis,                UCLA, 1998, URL                "http://www.cs.ucla.edu/~hari/software/ecn/ecn_report.ps.gz".   [FRED]       Lin, D., and Morris, R., "Dynamics of Random Early                Detection", SIGCOMM '97, September 1997.  URL                "http://www.inria.fr/rodeo/sigcomm97/program.html#ab078".   [Jacobson88] V. Jacobson, "Congestion Avoidance and Control", Proc.                ACM SIGCOMM '88, pp. 314-329.  URL                "ftp://ftp.ee.lbl.gov/papers/congavoid.ps.Z".   [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".   [MJV96]      S. McCanne, V. Jacobson, and M. Vetterli, "Receiver-                driven Layered Multicast", SIGCOMM '96, August 1996, pp.                117-130.   [RFC791]     Postel, J., "Internet Protocol", STD 5,RFC 791,                September 1981.   [RFC793]     Postel, J., "Transmission Control Protocol", STD 7,RFC793, September 1981.Ramakrishnan & Floyd          Experimental                     [Page 18]

RFC 2481                       ECN to IP                    January 1999   [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.   [RFC2001]    Stevens, W., "TCP Slow Start, Congestion Avoidance, Fast                Retransmit, and Fast Recovery Algorithms",RFC 2001,                January 1997.   [RFC2309]    Braden, B., Clark, D., Crowcroft, J., Davie, B.,                Deering, S., Estrin, D., Floyd, S., Jacobson, V.,                Minshall, G., Partridge, C., Peterson, L., Ramakrishnan,                K., Shenker, S., Wroclawski, J. and L. Zhang,                "Recommendations on Queue Management and Congestion                Avoidance in the Internet",RFC 2309, April 1998.   [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 1990.15. Security Considerations   Security considerations have been discussed inSection 9.16. 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 bit was set would work as   follows: Let HC be the original header checksum, and let HC' be the   new header checksum after the CE bit has been set.  Then for header   checksums calculated with one's complement subtraction, HC' would be   recalculated as follows:Ramakrishnan & Floyd          Experimental                     [Page 19]

RFC 2481                       ECN to IP                    January 1999      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 = 017. The motivation for the ECT bit.   The need for the ECT bit 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 the ECT bit, the   router can drop packets from flows that are not ECN-capable, but can   *instead* set the CE bit in flows that *are* ECN-capable. Because the   ECT bit allows an end node to have the CE bit 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 indication, then the router would have to set the   CE bit 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 bits, 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 bit 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 bits is functionally equivalent to a flow that turns off   congestion control, as discussed in Sections8 and9.Ramakrishnan & Floyd          Experimental                     [Page 20]

RFC 2481                       ECN to IP                    January 1999   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.18. 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) indications should be overloaded on a   single bit.  This overloaded-one-bit alternative, explored in   [Floyd94], would involve 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 bit   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.   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 ECT bit should be `OFF' by default.  In theRamakrishnan & Floyd          Experimental                     [Page 21]

RFC 2481                       ECN to IP                    January 1999   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.19.  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.            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         +-----+-----+-----+-----+-----+-----+-----+-----+Ramakrishnan & Floyd          Experimental                     [Page 22]

RFC 2481                       ECN to IP                    January 1999   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 is obsoleted by "Definition of the Differentiated Services   Field (DS Field) in the IPv4 and IPv6 Headers" [DIFFSERV], in which   bits 6 and 7 of the DS field are listed as Currently Unused (CU).   The first six bits of the DS field are defined as the Differentiated   Services CodePoint (DSCP):            0     1     2     3     4     5     6     7         +-----+-----+-----+-----+-----+-----+-----+-----+         |               DSCP                |    CU     |         +-----+-----+-----+-----+-----+-----+-----+-----+   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.  The damage that could be done by a   non-ECN-capable router would be to "erase" the CE bit for an ECN-   capable packet that arrived at the router with the CE bit set, or set   the CE bit even in the absence of congestion.  This has been   discussed inSection 10 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 bit set   has been discussed inSection 9 on "Non-compliance by the End Nodes".Ramakrishnan & Floyd          Experimental                     [Page 23]

RFC 2481                       ECN to IP                    January 1999AUTHORS' ADDRESSES   K. K. Ramakrishnan   AT&T Labs. Research   Phone: +1 (973) 360-8766   EMail: kkrama@research.att.com   URL:http://www.research.att.com/info/kkrama   Sally Floyd   Lawrence Berkeley National Laboratory   Phone: +1 (510) 486-7518   EMail: floyd@ee.lbl.gov   URL:http://www-nrg.ee.lbl.gov/floyd/Ramakrishnan & Floyd          Experimental                     [Page 24]

RFC 2481                       ECN to IP                    January 1999Full Copyright Statement   Copyright (C) The Internet Society (1999).  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.Ramakrishnan & Floyd          Experimental                     [Page 25]