Network Working Group K. Ramakrishnan

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].

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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

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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

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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

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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

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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.

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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

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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.

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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.

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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

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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

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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

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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.

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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.

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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.

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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.

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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.

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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.

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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:

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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.

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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

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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
+-----+-----+-----+-----+-----+-----+-----+-----+

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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".

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AUTHORS' ADDRESSES

K. K. Ramakrishnan
AT&T Labs. Research

Phone: +1 (973) 360-8766
EMail: [email protected]
URL: http://www.research.att.com/info/kkrama

Sally Floyd
Lawrence Berkeley National Laboratory

Phone: +1 (510) 486-7518
EMail: [email protected]
URL: http://www-nrg.ee.lbl.gov/floyd/

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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]