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rfc:rfc2481

Network Working Group K. Ramakrishnan Request for Comments: 2481 AT&T Labs Research Category: Experimental S. Floyd

                                                                  LBNL
                                                          January 1999
   A Proposal to add Explicit Congestion Notification (ECN) to IP

Status 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 1999

2. 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 in
 RFC 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 allows

Ramakrishnan & 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 one

Ramakrishnan & 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 in Section 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 data

Ramakrishnan & 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 during

Ramakrishnan & 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 1999

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
 [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 of RFC
 793 [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-Echo

Ramakrishnan & 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 the

Ramakrishnan & 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 may

Ramakrishnan & 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 decreasing

Ramakrishnan & 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 1999

10.1. 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 1999

11. 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 in Section 8 of this document,
 and the second is addressed by the proposal in Section 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 1999

12. 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, RFC
              793, 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 in Section 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 = 0

17. 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 Sections 8 and 9.

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 in Section 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 the

Ramakrishnan & 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 octet

 RFC 791 [RFC791] defined the ToS (Type of Service) octet in the IP
 header.  In RFC 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 in RFC 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 in RFC 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 in Section 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 in Section 9 on "Non-compliance by the End Nodes".

Ramakrishnan & Floyd Experimental [Page 23] RFC 2481 ECN to IP January 1999

AUTHORS' 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 1999

Full 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]

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