wdiff draft-ietf-tsvwg-tcp-eifel-response-03.txt draft-ietf-tsvwg-tcp-eifel-response-04.txt

Network Working Group Reiner Ludwig
INTERNET-DRAFT Ericsson Research
Expires: September 2003 April 2004 Andrei Gurtov
Sonera Corporation
March,
TeliaSonera
October, 2003

The Eifel Response Algorithm for TCP
<draft-ietf-tsvwg-tcp-eifel-response-03.txt>
<draft-ietf-tsvwg-tcp-eifel-response-04.txt>

Status of this memo

This document is an Internet-Draft and is in full conformance with
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Abstract

The

Based on an appropriate detection algorithm, the Eifel response
algorithm requires provides a detection algorithm to detect way for a posteriori whether the TCP sender has entered loss recovery
unnecessarily. In response to respond to a detected
spurious timeout it timeout. It adapts the retransmission timer to avoid further
spurious timeouts, and can avoid - depending on the detection
algorithm - the often unnecessary go-back-N retransmits that would
otherwise be sent. Likewise, it
adapts the duplicate acknowledgement threshold in response to a
spurious fast retransmit. In both cases, addition, the Eifel response algorithm restores
the congestion control state in such a way that packet bursts are
avoided.

Terminology

The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
document, are to be interpreted as described in [RFC2119].

We refer to the first-time transmission of an octet as the 'original
transmit'. A subsequent transmission of the same octet is referred to
as a 'retransmit'. In most cases this terminology can likewise be
applied to data segments as opposed to octets. However, when
repacketization occurs, a segment can contain both first-time
transmissions and retransmissions of octets. In that case case, this
terminology is only consistent when applied to octets. For the Eifel
detection and response algorithms this makes no difference as they
also operate correctly when repacketization occurs.

We use the term 'acceptable ACK' as defined in [RFC793]. That is an
ACK that acknowledges previously unacknowledged data. We use the term
'duplicate ACK', and the variable 'dupacks' as defined in [WS95]. The
variable 'dupacks' is a counter of duplicate ACKs that have already
been received by the TCP sender before the fast retransmit is sent.
We use the variable 'DupThresh' to refer to the so-called duplicate
acknowledgement threshold, i.e., the number of duplicate ACKs that
need to arrive at the TCP sender to trigger a fast retransmit.
Currently, DupThresh is specified as a fixed value of three
[RFC2581].

Furthermore, we use the TCP
sender state variables 'SND.UNA' and 'SND.NXT' as defined in
[RFC793]. SND.UNA holds the segment sequence number of the oldest
outstanding segment. SND.NXT holds the segment sequence number of the
next segment the TCP sender will (re-)transmit. In addition, we
define as 'SND.MAX' the segment sequence number of the next original
transmit to be sent. The definition of SND.MAX is equivalent to the
definition of snd_max 'snd_max' in [WS95].

We use the TCP sender state variables 'cwnd' (congestion window), and
'ssthresh' (slow start (slow-start threshold), and the terms 'SMSS', 'FlightSize', and
'Initial Window (IW)' as defined in [RFC2581]. FlightSize is the
amount of outstanding data in the network, or
alternatively, the difference between SND.MAX and SND.UNA at a given point in time. The IW is the
size of the sender's congestion window after the three-way handshake
is completed. We use the TCP sender state variables 'SRTT' and
'RTTVAR', and the term 'RTO' as defined in [RFC2988]. In addition, we
assume that the TCP sender maintains in the variable 'RTT-SAMPLE' the
value of the latest round-trip time (RTT) measurement.

1. Introduction

The Eifel response algorithm relies on a detection algorithm such as
the Eifel detection algorithm defined in [RFC***B]. [RFC3522]. That document
discusses the relevant background and motivation that also applies to
this document. Hence, the reader is expected to be familiar with
[RFC***B].
[RFC3522]. Note that alternative response algorithms have been
proposed [BDA03] [BA02] that could also rely on the Eifel detection
algorithm, and vice versa alternative detection algorithms have been
proposed [BA02b], [BA03], [SK03] that could work together with the Eifel
response algorithm.

The

Note: A previous version of the Eifel Response algorithm also
included a response to a detected spurious fast retransmit.
However, since a consensus was not reached about how to adapt the
duplicate acknowledgement threshold in that case, that part of the
algorithm was removed for the time being.

2. Interworking with Detection Algorithms

If the Eifel response algorithm is implemented at the TCP sender, it
MUST be implemented together with a detection algorithm that is
specified in an RFC.

Designers of detection algorithms who want to offer the possibility
that their detection algorithms can to work
together with the Eifel response algorithm MUST should reuse the variable
SpuriousRecovery with the semantics and defined values as specified
in [RFC***B]. [RFC3522]. In addition, we define LATE_SPUR_TO (equal -1) as
another possible value of the variable SpuriousRecovery. Detection
algorithms must should set the value of SpuriousRecovery to LATE_SPUR_TO
if the detection of a spurious retransmit is based upon receiving the
ACK for the retransmit. retransmit (as opposed to the ACK for the original
transmit). For example, this applies to detection algorithms that are
based on the DSACK option. option [BA03].

3. The Eifel Response Algorithm

The complete algorithm is specified in section 2.1. 3.1. In sections 2.2 3.2
to 2.4, 3.5, we motivate the different steps of the algorithm.

3.1. The Algorithm

Given that a TCP sender has enabled a detection algorithm that
complies with the requirements set in Section 2, a TCP sender MAY use
the Eifel response algorithm as defined in this subsection.

If the Eifel response algorithm is used, the following steps MUST be
taken by the TCP sender, but only upon initiation of loss recovery,
i.e., when either the timeout-based retransmit or the fast retransmit is sent. Note: The algorithm
MUST NOT be reinitiated after loss recovery has already started. In
particular, it may not be reinitiated upon subsequent timeouts for
the same segment, and not upon retransmitting segments other than the
oldest outstanding segment.

(0)

(INIT) Before the variables cwnd and ssthresh get updated when
loss recovery is initiated, set a "pipe_prev" variable as
follows:

pipe_prev <- max (FlightSize, ssthresh)

(DTCT)

(DET) This is a placeholder for a detection algorithm that must
be executed at this point. In case [RFC***B] [RFC3522] is used as
the detection algorithm, steps (1) - (6) of that algorithm
go here.

(RESP) If SpuriousRecovery equals FALSE, then proceed to step
(DONE),

else if SpuriousRecovery equals SPUR_TO, then
proceed to step (STO.1),

else if SpuriousRecovery equals LATE_SPUR_TO, then
proceed to step (STO.2),

else (spurious fast retransmit)
proceed to step (SFR). (DONE).

(STO.1) Resume transmission off the top:

Set
SND.NXT <- SND.MAX

(STO.2) Adapt the Conservativeness of the Retransmission Timer:

If the retransmission timer is implemented according to
[RFC2988], then change
if the calculation of SRTT to
SRTT <- SRTT + 1/FlightSize * (RTT-SAMPLE - SRTT)
and TCP Timestamps option [RFC1323] is enabled for
this connection, then set
SRTT <- RTT-SAMPLE
RTTVAR <- RTT-SAMPLE/2,
recalculate the RTO, and restart the retransmission timer,

Note: Even after changing the calculation of SRTT, RTT-SAMPLE/2

else set
RTTVAR <- max (2 * RTTVAR, SRTT)
SRTT <- 2 * SRTT

Set
RTO <- SRTT + max (G, 4*RTTVAR)
Restart the retransmission timer is considered as being
implemented according to [RFC2988].

else
appropriately adapt the conservativeness of the
retransmission
timer.

Proceed to step (ReCC).

(SFR) Adapt the duplicate acknowledgement threshold:

Set
DupThresh <- max (DupThresh, SpuriousRecovery) timer that is implemented.

Proceed to step (ReCC).

(ReCC) Revert Reversing the congestion control state:

If the acceptable ACK has the ECN-Echo flag [RFC3168] set
OR the TCP sender has already taken more than three
timeouts for the oldest outstanding segment, set,
then
proceed to step (DONE),

else set
cwnd <- min (pipe_prev, (FlightSize FlightSize + IW)) min (bytes_acked, IW)
ssthresh <- pipe_prev

Proceed to step (DONE).

(CWV) Interworking with Congestion Window Validation (the
variables 'T_last' and 'tcpnow' are defined in [RFC2861]):

If congestion window validation is implemented according
to [RFC2861], then set
T_last <- tcpnow

(DONE) No further processing.

3.2 Responding to Spurious Timeouts

3.2.1 Suppressing Storing the Unnecessary go-back-N Retransmits Current Congestion Control State (step STO.1)

Without the use of the TCP timestamps option, the INIT)

The TCP sender suffers
from stores in pipe_prev what is considered a "safe" slow-
start threshold (ssthresh) before loss recovery is initiated, i.e.,
before the retransmission ambiguity problem [Zh86], [KP87]. loss indication is taken into account. This means
that when is either the first acceptable ACK arrives after a spurious timeout,
current FlightSize if the TCP sender must believe that that ACK was sent is in response to congestion avoidance or
the
retransmit when current ssthresh if the TCP sender is in fact slow-start. If the TCP
sender later detects that it was sent has entered loss recovery unnecessarily,
then pipe_prev is used in response step (ReCC) to reverse the original
transmit. Furthermore, congestion
control state. Thus, until the loss recovery phase is terminated,
pipe_prev maintains a memory of the congestion control state of the
time right before the loss recovery phase was initiated. A similar
approach is proposed in [RFC2861], where this state is stored in
ssthresh directly after a TCP sender must also believe that all
other has become application-limited.

There had been debates about whether the value of pipe_prev should be
decayed over time, e.g., upon subsequent timeouts for the same
outstanding segment. We do not require the decaying of pipe_prev for
the Eifel response algorithm, and do not believe that such a
conservative approach would be in place. Instead, we follow the idea
of revalidating the congestion window through slow-start as suggested
in [RFC2861]. That is, in step (ReCC), the cwnd is reset to a value
that avoids large packet bursts, while ssthresh is reset to the value
of pipe_prev. Note that [RFC2581] and [RFC2861] also do not require a
decaying of ssthresh after it has been reset in response to a loss
indication, or after a TCP sender has become application-limited.

3.3 Responding to Spurious Timeouts

3.3.1 Suppressing the Unnecessary go-back-N Retransmits (step STO.1)

Without the use of the TCP timestamps option, the TCP sender suffers
from the retransmission ambiguity problem [Zh86], [KP87]. Hence, when
the first acceptable ACK arrives after a spurious timeout, the TCP
sender must assume that this ACK was sent in response to the
retransmit when in fact it was sent in response to the original
transmit. Furthermore, the TCP sender must further assume that all
other segments outstanding at that point were lost.

Note: Except for certain cases where original ACKs were lost, that the
first acceptable ACK cannot carry any DSACK option [RFC2883].

Consequently, once the TCP sender's state has been updated after the
first acceptable ACK has arrived, SND.NXT equals SND.UNA. This is
what causes the often unnecessary go-back-N retransmits. Now From that
point on every arriving acceptable ACK that was sent in response to
an original transmit will advance SND.NXT. But as long as SND.NXT is
smaller than the value that SND.MAX had when the timeout occurred,
those ACKs will clock out retransmits; retransmits, whether those segments were
lost or not.

In fact, during this phase the TCP sender breaks 'packet
conservation' [Jac88]. This is because the go-back-N retransmits are
sent during slow start. slow-start. I.e., for each original transmit leaving the
network, two retransmits are sent into the network as long as SND.NXT
does not equal SND.MAX (see [LK00] for more detail).

The use of the TCP timestamps option reliably eliminates the
retransmission ambiguity problem. Thus, once Once the Eifel detection algorithm
has detected that a timeout was spurious, it is therefore safe to let
the TCP sender resume the transmission with new data. Thus, the Eifel
response algorithm changes the TCP sender's state by setting SND.NXT
to SND.MAX in that case.

3.2.2

3.3.2 Adapting the Retransmission Timer (step STO.2)

There is currently only one retransmission timer standardized for TCP
[RFC2988]. We therefore only address that timer explicitly. Future
standards that might define alternatives to [RFC2988] should propose
similar measures to adapt the conservativeness of the retransmission
timer.

Since the timeout was spurious, the TCP sender's RTT estimators are
likely to be off. However, since timestamps are being used, a new and
valid RTT measurement (RTT-SAMPLE) can be derived from the acceptable
ACK. It is therefore suggested to reinitialize the RTT estimators
from RTT-SAMPLE. Note that this RTT-SAMPLE will be relatively large
since it will include the delay spike that caused the spurious
timeout in the first place. To have the new RTO become effective, the
retransmission timer needs to be restarted. This is consistent with
[RFC2988] which recommends restarting the retransmission timer with
the arrival of an acceptable ACK.

When the path's RTT varies largely, it is recommended to take RTT
samples more frequently than only once per RTT. This allows the TCP
sender to track changes in the RTT more closely. In particular, a TCP
sender can react more quickly to sudden increases of the RTT by
sooner updating the RTO to a more conservative value. The TCP
Timestamps option [RFC1323] provides this capability, allowing the
TCP sender to sample the RTT from every segment that is acknowledged.
Using timestamps across such paths leads to a more conservative TCP
retransmission timer and reduces the risk of triggering spurious
timeouts [IMLGK02].

On the other hand, it is known that executing the RTO calculation
defined in [RFC2988] more often than once per RTT leads to an RTO
that decays too quickly, i.e., that converges to the RTT too quickly.
This is because of the fixed gains (1/8 and 1/4) of RFC2988's RTT
estimators. When timing every segment these gains are increasingly
too large with an increasing FlightSize. This leads to the effect
that the RTT estimators "lose" their memory too soon. This is a known
conflict between [RFC2988] and [RFC1323]. Especially, a large RTO
resulting from an RTT spike will decay within one or two RTTs (e.g.,
see [LS00]). Hence, simply reinitializing RFC2988's RTT estimators
from RTT-SAMPLE is probably not enough to make the retransmission
timer sufficiently conservative for at least the next couple of RTTs.
A solution for the case when every segment is timed according to
[RFC1323] is to make the gains adaptive to the FlightSize [LS00]. We
suggest to adopt this solution for at least the SRTT.

3.3 Responding to Spurious Fast Retransmits (step SFR)

The assumption behind the fast retransmit algorithm [RFC2581] is that
a segment was lost if as many duplicate ACKs have arrived at the TCP
sender as indicated by DupThresh. Currently, DupThresh is specified
as a fixed value of three [RFC2581]. That value is assumed to be
sufficiently conservative so that packet reordering and/or packet
duplication does not falsely trigger the fast retransmit algorithm.
Clearly, this assumption does not hold for a particular TCP
connection once the TCP sender detects that the last fast retransmit
was spurious. It is therefore suggested to dynamically adapt
DupThresh to the reordering characteristics observed over the course
of a particular connection.

At the beginning of a connection DupThresh is initialized with three.
Then for each spurious fast retransmit that is detected, DupThresh is
set to the maximum of the previous DupThresh, and the lowest value
that would have avoided that last spurious fast retransmit. Note that
the Eifel detection algorithm records the latter value in
SpuriousRecovery. This strategy ensures that spurious, the TCP sender is able sender's RTT estimators are
likely to cope with the longest reordering length seen on be off. If timestamps are enabled for this connection, a particular
connection so far. However,
new and valid RTT measurement (RTT-SAMPLE) can be derived from the strategy may lead
acceptable ACK. It is therefore suggested to fast timeouts
[RFC***B], i.e., an event where the retransmission timer expires
before the TCP sender receives reinitialize the duplicate ACK that would trigger a
fast retransmit RTT
estimators from RTT-SAMPLE according to rule (2.2) of the oldest outstanding segment.

Also, we believe RFC2988. Note
that this strategy should RTT-SAMPLE will be implemented together
with an advanced version of relatively large since it will include
the Limited Transmit algorithm [RFC3042].
That is for each duplicate ACK delay spike that arrives until DupThresh is
reached, caused the spurious timeout in the first place.
If timestamps are not enabled for this connection, the TCP sender
should sent a new data segment if allowed by
the TCP receiver's advertised window, instead double SRTT and if also make RTTVAR more conservative.

To have the new data RTO become effective, the retransmission timer needs
to be restarted. This is available.
Although, consistent with [RFC2988] which recommends
restarting the current Limited Transmit algorithm only allows this for retransmission timer with the first two duplicate ACKs, we believe that such arrival of an advanced
limited transmit strategy is safe. It is already implemented in
widely deployed TCPs [SK02].

Other alternatives for responding to spurious fast retransmits are
discussed in [BA02a]. acceptable
ACK.

3.4 Reverting Reversing the Congestion Control State (step ReCC)

When a TCP sender enters loss recovery, it also assumes that is has
received a congestion indication. In response to that it reduces
cwnd, and ssthresh. However, once the TCP sender detects that the
loss recovery has been falsely triggered, this reduction was
unnecessary. In fact, no congestion signal indication has been received. We
therefore believe that it is safe to revert to the previous
congestion control state following the approach of revalidating the
congestion window as outlined below. This is unless the acceptable
ACK signals congestion through the ECN-Echo flag [RFC3168]. In that
case, the TCP sender MUST refrain from reversing congestion control
state.

We suggest to restore

If the ECN-Echo flag is not set, cwnd is reset to the minimum sum of the
current FlightSize and the minimum of IW and the number of bytes that
have been acknowledged by the acceptable ACK. Note that the value of
cwnd must not be changed any further for that ACK, and that the value
of FlightSize at this point in time may be different from the previous FlightSize,
and the current value
of FlightSize plus IW. in step (INIT). The latter avoids large value of IW puts a limit on the
size of the packet
bursts burst that the TCP sender may occur with less careful variants for restoring
congestion control state. For example, send into the original proposal [LK00]
typically causes large bursts
network after packet reordering. the Eifel response algorithm has terminated. The current
proposal limits a potential packet burst to IW, which value
of IW is considered an acceptable burst size. It is the amount of
data that a TCP sender may send into a yet "unprobed" network at the
beginning of a connection.

In addition, we suggest to restore

The TCP sender is then forced into slow-start by resetting ssthresh
to pipe_prev, i.e., the
maximum of the previous value of ssthresh and the value that
FlightSize had when loss recovery was unnecessarily entered. pipe_prev. As a result, the TCP sender either
immediately resumes probing the network for more bandwidth in
congestion avoidance, or it first slow starts
until it has reached its previous share of the available bandwidth.

Clearly, when the acceptable ACK signals congestion through the
ECN-Echo flag [RFC3168], the TCP sender MUST refrain from reverting
congestion control state. The same slow-starts to what is true if the TCP sender has
already taken more than three timeouts considered a
"safe" operating point for the oldest outstanding
segment. Allowing three timeouts while still reverting congestion
control state goes beyond [RFC2581]. That standard recommends setting
cwnd to no more than the restart window (one SMSS) if window. In some cases, this
can mean that the TCP sender
has first few acceptable ACKs that arrive will not sent
clock out any data in an interval exceeding the current RTO. That is
done to restart segments.

3.5 Interworking with the ACK clock which is believed to be lost. The case
in step (ReCC) Congestion Window Validation Algorithm

An implementation of the Eifel response Congestion Window Validation (CWV) algorithm is different. Since,
an acceptable ACK corresponding to an original transmit has finally
returned,
[RFC2861] could potentially misinterpret a delay spike that caused a
spurious timeout as a phase where the TCP has reason to believe that sender had been
application-limited. To prevent the triggering of CWV algorithm in
this case, the ACK clock was merely
interrupted but has now resumed "ticking" again. variable 'T_last' defined in [RFC2861] is reset.

4. Non-Conservative Advanced Loss Recovery after Spurious Timeouts

A TCP sender MAY implement an optimistic form of advanced loss
recovery after a spurious timeout has been detected as motivated in
this section. Such a scheme MUST be terminated after the highest
sequence number outstanding when the spurious timeout was detected
has been acknowledged.

We have studied environments where spurious timeouts and multiple
losses from the same flight of packets often coincide [GL02]. Note
that we refer to the In such
a case were the oldest outstanding segment does arrive at the TCP receiver
receiver, but one or more packets from the remaining outstanding
flight are lost. We found that in such a case In those environments, TCP-Reno's performance can even suffer
suffers if the Eifel response algorithm is operated without an
advanced loss recovery scheme such as NewReno [RFC2582], or SACK-based SACK-
based schemes [2018], [RFC***A]. [RFC2018], [RFC3517]. The reason is TCP-Reno's
aggressiveness after a spurious timeout. Even though it breaks
'packet conservation' (see Section 2.2.1) when blindly retransmitting
all outstanding segments, it usually recovers the
back-to-back packet losses all packets lost from
that flight within a single round-trip time. On the contrary, the
more conservative TCP-Reno/Eifel was is often forced into another
(backed-off) timeout in that case. timeout.

However, in a more recent study [GL03], we found that the mentioned
advanced loss recovery schemes are often too conservative to compete
against TCP-Reno's blind go-back-N in terms of quickly recovering
multiple losses after a spurious timeout. The problem with the
NewReno scheme is that it does not exploit knowledge (e.g., provided
through SACK options) about which segments were lost. The problem
with the conservative SACK-based scheme [RFC***A] [RFC3517] is that it waits
for three SACKs before it retransmits a lost segment. This may often
lead to a second - and in this case genuine - (potentially backed-
off) timeout. In those cases TCP-Reno's loss recovery is often
quicker due the blind go-back-N. This could be viewed as a
disincentive to the deployment of the Eifel response algorithm.

[Making TCP (even) more conservative by fixing a misbehavior in
the name of 'packet conservation' would probably at most result in
credits in the academic world.]

We therefore suggest that a TCP sender MAY implement an optimistic
(non-conservative) form of advanced loss recovery after a spurious
timeout has been detected, if the following guidelines are met:

- Packet Conservation: The TCP sender may not have more segments
(counting both original transmits and retransmits) in flight
than indicated by the congestion window.

- A retransmit may only be sent when a potential loss has been
indicated. For example, a single duplicate ACK is such an
indication; potentially with the corresponding SACK info in case
the SACK option is enabled for the connection.

We have developed and evaluated such a scheme (a variant of NewReno
that exploits SACK info) in [GL03] that shows good results.

5. IPR Considerations

The IETF has been notified of intellectual property rights claimed in
regard to some or all of the specification contained in this
document. For more information consult the online list of claimed
rights at http://www.ietf.org/ipr.

The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it
has made any effort to identify any such rights. Information on the
IETF's procedures with respect to rights in standards-track and
standards-related documentation can be found in BCP-11. Copies of
claims of rights made available for publication and any assurances of
licenses to be made available, or the result of an attempt made to
obtain a general license or permission for the use of such
proprietary rights by implementors or users of this specification can
be obtained from the IETF Secretariat.

6. Security Considerations

There is a risk that a detection algorithm is fooled by spoofed ACKs
that make genuine retransmits appear to the TCP sender as spurious
retransmits. When such a detection algorithm is run together with the
Eifel response algorithm, this could effectively disable congestion
control at the TCP sender. Should this become a concern, the Eifel
response algorithm SHOULD only be run together with detection
algorithms that are known to be safe against such "ACK spoofing
attacks".

For example, the safe variant of the Eifel detection algorithm
[RFC***B],
[RFC3522], is a reliable method to protect against this risk.

Acknowledgments

Many thanks to Keith Sklower, Randy Katz, Michael Meyer, Stephan
Baucke, Sally Floyd, Vern Paxson, Mark Allman, Ethan Blanton, Pasi
Sarolahti, and Alexey Kuznetsov Kuznetsov, and Yogesh Swami for very useful many discussions
that contributed to this work.

Normative References

[RFC2581] M. Allman, V. M., Paxson, V. and W. Stevens, TCP Congestion
Control, RFC 2581, April 1999.

[RFC3042] M. Allman, H. Balakrishnan, S. Floyd, Enhancing TCP's Loss
Recovery Using Limited Transmit, RFC 3042, January 2001.

[RFC2119] S. Bradner, S., Key words for use in RFCs to Indicate
Requirement Levels, RFC 2119, March 1997.

[RFC2582] S. Floyd, S. and T. Henderson, The NewReno Modification to
TCP's Fast Recovery Algorithm, RFC 2582, April 1999.

[RFC2883] S. Floyd, J. S., Mahdavi, M. J., Mathis, M. M., Podolsky, M. and A.
Romanow, An Extension to the Selective Acknowledgement
(SACK) Option for TCP, RFC 2883, July 2000.

[RFC2861] Handley, M., Padhye, J. and S. Floyd, TCP Congestion Window
Validation, RFC 2861, June 2000.

[RFC1323] V. Jacobson, R. V., Braden, R. and D. Borman, TCP Extensions for
High Performance, RFC 1323, May 1992.

[RFC***B] R.

[RFC3522] Ludwig, R. and M. Meyer, The Eifel Detection Algorithm for
TCP,
RFC***B, March RFC3522, April 2003.

[RFC2018] M. Mathis, J. M., Mahdavi, S. J., Floyd, S. and A. Romanow, TCP
Selective Acknowledgement Options, RFC 2018, October 1996.

[RFC2988] V. Paxson, V. and M. Allman, Computing TCP's Retransmission
Timer, RFC 2988, November 2000.

[RFC793] J. Postel, J., Transmission Control Protocol, RFC793,
September 1981.

[RFC3168] K. Ramakrishnan, S. K., Floyd, S. and D. Black, The Addition of
Explicit Congestion Notification (ECN) to IP, RFC 3168,
September 2001

Informative References

[BA02a] E.

[BA02] Blanton, E. and M. Allman, On Making TCP More Robust to
Packet Reordering, ACM Computer Communication Review,
Vol. 32, No. 1, January 2002.

[BA02b] E.

[BA03] Blanton, E. and M. Allman, Using TCP DSACKs and SCTP
Duplicate TSNs to Detect Spurious Retransmissions, draft-blanton-
dsack-use-02.txt draft-
ietf-tsvwg-dsack-use-02.txt (work in progress),
October 2002.

[BDA03] E. Blanton, R. Dimond, M. Allman. Practices for TCP Senders
in the Face of Segment Reordering, draft-blanton-tcp-
reordering-00.txt (work in progress), February 2003..

[RFC***A] E. 2003.

[RFC3517] Blanton, M. E., Allman, K. M., Fall, K. and L. Wang,
A Conservative SACK-based Loss Recovery Algorithm for TCP, RFC***A,
March
RFC3517, April 2003.

[Gu01] A. Gurtov, Effect of Delays on TCP Performance, In
Proceedings of IFIP Personal Wireless Conference,
August 2001.

[GL02] A. Gurtov, A. and R. Ludwig, Evaluating the Eifel Algorithm
for TCP in a GPRS Network, In Proceedings of the European
Wireless Conference, February 2002.

[GL03] A. Gurtov, A. and R. Ludwig, Responding to Spurious Timeouts
in TCP, In Proceedings of IEEE INFOCOM 03, .

[RFC3481] H. Inamura, G. Montenegro, R. Ludwig, A. Gurtov,
F. Khafizov, TCP over Second (2.5G)

[Jac88] Jacobson, V., Congestion Avoidance and Third (3G)
Generation Wireless Networks, RFC3481, February 2003. Control, In
Proceedings of ACM SIGCOMM 88.

[KP87] P. Karn, P. and C. Partridge, Improving Round-Trip Time
Estimates in Reliable Transport Protocols, In Proceedings
of ACM SIGCOMM 87.

[LK00] R. Ludwig, R. and R. H. Katz, The Eifel Algorithm: Making TCP
Robust Against Spurious Retransmissions, ACM Computer
Communication Review, Vol. 30, No. 1, January 2000.

[LS00] R. Ludwig, K. Sklower, The Eifel Retransmission Timer, ACM
Computer Communication Review, Vol. 30, No. 3, July 2000.

[SK02] P. Sarolahti, A. Kuznetsov, Congestion Control in Linux
TCP, In Proceedings of USENIX, June 2002.

[SK03] P. Sarolahti, P. and M. Kojo, F-RTO: A TCP RTO Recovery An Algorithm for Avoiding Unnecessary Retransmissions, draft-sarolahti-
tsvwg-tcp-frto-03.txt
Detecting Spurious Retransmission Timeouts with TCP and
SCTP, draft-ietf-tsvwg-tcp-frto-00.txt (work in progress), January
October 2003.

[WS95] Wright, G. R. Wright, and W. R. Stevens, TCP/IP Illustrated,
Volume 2 (The Implementation), Addison Wesley,
January 1995.

[Zh86] L. Zhang, L., Why TCP Timers Don't Work Well, In Proceedings
of ACM SIGCOMM 88.

Author's Address

Reiner Ludwig
Ericsson Research (EED)
Ericsson Allee 1
52134 Herzogenrath, Germany
Email: Reiner.Ludwig@ericsson.com

Andrei Gurtov
Cellular Systems Development
TeliaSonera Finland
P.O. Box 970, FIN-00051 Sonera
Helsinki, Finland
Phone: +358(0)20401
Fax: +358(0)204064365
Email: andrei.gurtov@sonera.com andrei.gurtov@teliasonera.com
Homepage: http://www.cs.helsinki.fi/u/gurtov

This Internet-Draft expires in September 2003. April 2004.