One document matched: draft-kksjf-ecn-01.txt
Differences from draft-kksjf-ecn-00.txt
Internet Engineering Task Force K. K. Ramakrishnan
INTERNET DRAFT AT&T Labs Research
draft-kksjf-ecn-01.txt Sally Floyd
LBNL
July 1998
Expires: January 1999
A Proposal to add Explicit Congestion Notification (ECN) to IP
Status of this Memo
This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
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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, but could instead set a Congestion
Experienced (CE) bit in the packet header, for 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
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are developed and advance through the standards process.
1. 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.
2. 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) 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.
3. 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.
4. 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.
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 TCP the source TCP halves its
congestion window "cwnd" in response to an ECN indication received by
the data receiver.
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
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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
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.
5. 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
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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.
5.1. 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].
5.1.1. TCP Initialization
In the TCP connection setup phase, the source and destination TCPs
exchange information about their desire and/or capability to use ECN.
As a result of the negotiation, the TCP sender sets the ECT bit in
the IP header 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.
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
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earlier documents.) Bit 9 in the Reserved field of the TCP header is
assigned to the ECN-Echo flag.
When a node sends a TCP SYN packet, it may set the ECN-Echo flag in
the TCP header. For a SYN packet, the ECN-Echo flag is defined as an
indication that the sending TCP is ECN-Capable, rather than as a
return indication of congestion. More precisely, a SYN packet with
the ECN-Echo flag set indicates that that sending TCP implementation
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.
Similarly, for a SYN-ACK packet, the ECN-Echo flag in the TCP header
is defined as an indication that the TCP transmitting the SYN-ACK
packet is ECN-Capable.
5.1.2. 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,
The recommended method for implementing this is as follows. Assume
that at time "t" the source TCP reacts to an ECN-Echo ACK packet by
reducing its congestion window. The source TCP notes the packets
that are outstanding at that time (i.e., packets that have not yet
been acknowledged). Until all these packets are acknowledged, the
source TCP does not react to another ECN indication of congestion.
However, if during this period a packet is retransmitted as a result
of a retransmission timeout or the receipt of the required number
(e.g., 3) of duplicate acknowledgments, then the source TCP will
react to subsequent ECN indications of congestion.
[Floyd94] discusses this further, and [Floyd98] includes a validation
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test illustrating a wide range of ECN scenarios. These scenarios
include the following: an ECN followed by another ECN, a Fast
Retransmit, or a Retransmit Timeout; and a Retransmit Timeout or a
Fast Retransmit followed by an ECN.
When the TCP sender reduces its congestion window in response to an
ECN-Echo ACK packet, there is no need for the sender to slow-start
(as in Tahoe TCP in response to a packet drop) or to stop sending
packets for a period of time to allow the queue to dissipate (as in
Reno TCP for roughly half a round-trip time during Fast Recovery).
The CE packet in the forward direction does not indicate the imminent
possibility of buffer overflow requiring an urgent source action to
reduce the load dramatically. Incoming acknowledgements that
continue to arrive can "clock out" outgoing packets as allowed by the
reduced congestion window.
TCP follows existing algorithms for sending data packets in response
to incoming ACKs, multiple duplicate acknowledgements, or retransmit
timeouts [RFC2001].
5.1.3. 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. 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.
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.
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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 guarantee 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].
5.1.4. 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 ECN-capable 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 can be relegated as an area
for 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.
6. 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
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
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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.
7. 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.
8. 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
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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
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.
9. 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. As one example, a rogue
or broken router could "erase" the CE bit in arriving CE packets,
thus preventing 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.
Concerns regarding the loss of congestion indications from
encapsulated, dropped, or corrupted packets are discussed below.
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9.1. 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.
9.2. 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.
10. 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
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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 a packet from a bulk-
data transfer degrades performance much more severely than marking a
packet.
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 [CKLT98] 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
high 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.
11. Conclusions
Given the current effort to implement RED, we believe this is the
right time for router vendors to examine how to implement congestion
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draft-kksjf-ecn Addition of ECN to IP November 1997
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, depending on packet loss as a
normal congestion notification mechanism appears to be insufficient
(or at the very least, non-optimal).
12. Acknowledgements
A number of people have made contributions to this internet-draft.
In particular, we would like to thank Kenjiro Cho for the proposal
for the TCP mechanism for negotiating ECN-Capability, Steve Blake and
Kevin Fall for the material on IPv4 Header Checksum Recalculation,
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.
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13. References
[CKLTZ97] Chen, C., Krishnan, H., Leung, S., Tang, N., and Zhang, L.,
"Implementing Explicit Congestion Notification (ECN) in TCP over
IPv6", UCLA Technical Report, December 1997, URL
"http://www.cs.ucla.edu/~hari/software/ecn/ecn_rpt.ps.gz".
[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".
[ECN] "The ECN Web Page", URL "http://www-
nrg.ee.lbl.gov/floyd/ecn.html".
[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
"ftp://ftp.ee.lbl.gov/papers/collapse.ps".
[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".
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draft-kksjf-ecn Addition of ECN to IP November 1997
[RFC1141] T. Mallory and A. Kullberg, "Incremental Updating of the
Internet Checksum", RFC 1141, January 1990.
[MJV96], S. McCanne, V. Jacobson, and M. Vetterli, "Receiver-driven
Layered Multicast", SIGCOMM '96, August 1996, pp. 117-130.
[RFC2001] W. Stevens, "TCP Slow Start, Congestion Avoidance, Fast
Retransmit, and Fast Recovery Algorithms", RFC 2001, January 1997.
[RFC2309] B. Braden, D. Clark, J. Crowcroft, B. Davie, S. Deering, D.
Estrin, S. Floyd, V. Jacobson, G. Minshall, C. Partridge, L.
Peterson, K. Ramakrishnan, S. Shenker, J. Wroclawski, 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.
14. Security Considerations
Security considerations have been discussed in Section 9.
15. 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:
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
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16. 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.
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.
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17. 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.
There is only one inherent functional difference between the one-bit
and two-bit implementations. This functional difference 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
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
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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.
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/
This draft was created in July 1998.
It expires January 1999.
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