Network Working Group Reiner Ludwig
INTERNET-DRAFT Ericsson Research
Expires: January 2003 Andrei Gurtov
Sonera Corporation
July, 2002
The Eifel Response Algorithm for TCP
<draft-ludwig-tsvwg-tcp-eifel-response-00.txt>
Status of this memo
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Abstract
The Eifel response algorithm uses the Eifel detection algorithm to
detect a posteriori whether the TCP sender has entered loss recovery
unnecessarily. In response to a spurious timeout it avoids the
go-back-N retransmits that would otherwise be sent, and reinitializes
the RTT estimators to avoid further spurious timeouts. Likewise, it
adapts the duplicate acknowledgement threshold in response to a
spurious fast retransmit. In both cases, the Eifel response algorithm
restores the congestion control state in way that avoids packet
bursts.
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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 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 in
[WS95].
We use the TCP sender state variables 'cwnd' (congestion window), and
'ssthresh' (slow start threshold), and the terms 'SMSS', and
'FlightSize' 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. 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 uses the Eifel detection algorithm to
detect a posteriori whether the TCP sender has entered loss recovery
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unnecessarily. In response to a spurious timeout it avoids the
go-back-N retransmits that would otherwise be sent, and reinitializes
the RTT estimators to avoid further spurious timeouts. Likewise, it
adapts the duplicate acknowledgement threshold in response to a
spurious fast retransmit. In both cases, the Eifel response algorithm
restores the congestion control state in way that avoids packet
bursts.
The Eifel response algorithm relies on the Eifel detection algorithm
defined in [LM02]. That document discusses the relevant background
and motivation that also applies to this document. Hence, the reader
is expected to be familiar with [LM02], and should view this document
as a companion document.
2. The Eifel Response Algorithm
The complete algorithm is specified in section 2.1. In sections 2.2
to 2.4, we motivate the different steps of the algorithm.
2.1. The Algorithm
Given that a TCP sender has enabled the Eifel detection algorithm
[LM02] for a connection, a TCP sender MAY use the Eifel response
algorithm as defined in this subsection. Note that this implies that
the TCP Timestamps option [RFC1323] is used for that connection.
Since the Eifel response algorithm is dependent on the Eifel
detection algorithm, we describe it as an extension of the latter.
If the combined Eifel detection and 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.
Note that steps (1)-(5) are an one-to-one copy of the Eifel detection
algorithm specified in [LM02], step (0) has been added, and step
(RESP) from [LM02] has been replaced by steps (RESP)-(ReCC) given
below.
(0) Before the variables cwnd and ssthresh get updated when
loss recovery is initiated, set a "cwnd_prev" variable to
the current value of FlightSize, and set a
"ssthresh_prev" variable to the value of ssthresh.
(1) Set a "RetransmitTS" variable to the value of the
Timestamp Value field of the Timestamps option included
in the retransmit sent when loss recovery is initiated. A
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TCP sender must ensure that RetransmitTS does not get
overwritten as loss recovery progresses, e.g., in case of
a second timeout and subsequent second retransmit of the
same octet.
(2) Set a "SpuriousRecovery" variable to FALSE.
(3) Wait for the arrival of an acceptable ACK. If an
acceptable ACK has arrived, then proceed to step (4).
(4) If the value of the Timestamp Echo Reply field of the
acceptable ACK's Timestamps option is smaller than the
value of the variable RetransmitTS, then proceed to step
(5),
else proceed to step (DONE).
(5) If the loss recovery has been initiated with a timeout-
based retransmit, then set
SpuriousRecovery <- SPUR_TO,
else set
SpuriousRecovery <- dupacks+1
(RESP) If SpuriousRecovery equals SPUR_TO, then proceed to step
(STO.1),
else (spurious fast retransmit) proceed to step (SFR).
(STO.1) Resume transmission off the top:
Set
SND.NXT <- SND.MAX
(STO.2) Reinitialize the RTT estimators:
Set
SRTT <- RTT-SAMPLE
RTTVAR <- RTT-SAMPLE/2,
recalcualte the RTO, and restart the retransmission
timer.
Proceed to step (ReCC).
(SFR) Adapt the duplicate acknowledgement threshold:
Set
DupThresh <- max (DupThresh, SpuriousRecovery)
Proceed to step (ReCC).
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(ReCC) Revert 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, then proceed
to step (DONE),
else set
cwnd <- FlightSize + SMSS
ssthresh <- max (cwnd_prev, ssthresh_prev)
Note: At this point in the algorithm, the value of
FlightSize might be different from the value of
FlightSize in step (0).
Proceed to step (DONE).
(DONE) No further processing.
2.2 Responding to Spurious Timeouts
2.2.1 Suppressing the Spurious go-back-N Retransmits (step STO.1)
Without the Eifel detection algorithm, the TCP sender suffers from
the retransmission ambiguity problem [KP87]. This means that when the
first acceptable ACK arrives after a spurious timeout, the TCP sender
must believe that that 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 also believe that all other segments
outstanding at that point have been lost. Note: the mentioned ACK
cannot carry any SACK option [RFC2018].
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. Any newly
arriving acceptable ACK that was sent in response to an original
transmit will now clock out the segment pointed at by SND.UNA;
whether it was lost or not. In fact, during this phase the TCP sender
breaks 'packet conservation' [Jac88]. This is because the unnecessary
go-back-N retransmits are sent during slow start, i.e., for each
original packet leaving the network, two useless retransmits are sent
into the network (see [LK00] for more detail).
The Eifel detection algorithm reliably eliminates the retransmission
ambiguity problem. Once it 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.
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2.2.2 Re-Initializing the RTT Estimators (step STO.2)
Since the timeout was spurious, the TCP sender's RTT estimators are
likely to be off. On the other hand, since timestamps are 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.
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.
2.3 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
has been 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 spurious fast retransmit. Note that the
Eifel detection algorithm records the latter value in
SpuriousRecovery. This strategy ensures that the TCP sender is able
to cope with the longest reordering length seen on a particular
connection so far.
However, the strategy bears the risk that the retransmission timer
expires before the TCP sender receives the duplicate ACK that would
trigger a fast retransmit of the oldest outstanding segment. To
alleviate that potential problem the TCP sender should implement the
"fast timeout" algorithm proposed in [Lu02].
Also, we believe that this strategy should be implemented together
with an advanced version of the Limited Transmit algorithm [RFC3042].
That is for each duplicate ACK that arrives until DupThresh is
reached, the TCP sender should sent a new data segment if new data is
available, and the TCP receiver's advertised window allows so.
Although, the current Limited Transmit algorithm only allows this for
the first two duplicate ACKs, we believe that this is safe. This is
already implemented in widely deployed TCPs [SK02].
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Other alternatives for responding to spurious fast retransmits are
discussed in [BA02a].
2.4 Reverting 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 has been received. We
therefore believe that it is safe to revert to the previous
congestion control state.
Instead, of simply restoring cwnd, and ssthresh, it is suggested to
set cwnd to one half the previous cwnd, and then enter the slow start
phase. This is more conservative than the original proposal, but it
avoids the packet burst that could otherwise be triggered after a
spurious fast retransmit [LK00]. When the spurious loss recovery has
been triggered during slow start, the previous slow start threshold
is restored. Otherwise, the TCP sender slow starts to the FlightSize
it had before the loss recovery was initiated (cwnd_prev).
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 is true if the TCP sender has
already taken more than three timeouts 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 before beginning transmission
if the TCP sender has not sent data in an interval exceeding the
current RTO. The motivation for doing so is to restart the ACK clock
which is believed to have been lost. The case in step (ReCC) of the
Eifel response algorithm is different. Since, an acceptable ACK has
finally returned, the TCP has reason to believe that the ACK clock
was merely interrupted but has now resumed "ticking" again.
3. Interoperability with Advanced Loss Recovery Schemes
We believe that there are no problems concerning interoperability
with advanced loss recovery schemes such as NewReno [RFC2582], or
SACK-based schemes [2018], [BA02b]. This is because in case loss
recovery has been initiated unnecessarily, the Eifel response
algorithm will have caused the TCP sender to back out of loss
recovery before those schemes would have kicked in.
In fact, we recommend that the Eifel response algorithm is
implemented together with one of those advanced loss recovery scheme;
ideally a SACK-based alternative. In an environment where spurious
timeouts and back-to-back packet losses often coincide, we have found
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that TCP's performance can even suffer if the Eifel response
algorithm is operated without an advanced loss recovery scheme
[GL02].
In that study we among other variants compared TCP-Reno with and
without the Eifel response algorithm (TCP-Reno/Eifel vs. TCP-Reno),
and without an advanced loss recovery scheme for both variants. The
reason that TCP-Reno performed better in the mentioned scenario, is
its 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 within a single round-trip time. On the contrary, the more
conservative TCP-Reno/Eifel was forced into another (backed-off)
timeout in that case. In the study, we found that the best end-to-end
performance was achieved when the TCP sender implemented both the
Eifel response algorithm and SACK-based loss recovery. In case
NewReno is chosen as the advanced loss recovery scheme, we found that
it performs better if the 'bugfix' feature is disabled. That feature
often lead the TCP sender to the wrong decision.
4. Security Considerations
There is a risk that TCP receivers make a genuine retransmit appear
to the TCP sender as a spurious retransmit by forging echoed
timestamps. This could effectively disable congestion control at the
TCP sender. A reliable method to protect against that risk is to
implement the safe variant of the Eifel detection algorithm specified
in [LM02].
Acknowledgments
Many thanks to Keith Sklower, Randy Katz, Michael Meyer, Stephan
Baucke, Sally Floyd, Vern Paxson, Mark Allman, and Ethan Blanton for
very useful discussions that contributed to this work.
References
[RFC2581] M. Allman, V. Paxson, 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.
[BA02a] E. Blanton, M. Allman, On Making TCP More Robust to Packet
Reordering, ACM Computer Communication Review, Vol. 32,
No. 1, January 2002.
[BA02b] E. Blanton, M. Allman, A Conservative SACK-based Loss
Recovery Algorithm for TCP, work in progress, July 2002.
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[RFC2119] S. Bradner, Key words for use in RFCs to Indicate
Requirement Levels, RFC 2119, March 1997.
[RFC2582] S. Floyd, T. Henderson, The NewReno Modification to TCP's
Fast Recovery Algorithm, RFC 2582, April 1999.
[Gu01] A. Gurtov, Effect of Delays on TCP Performance, In
Proceedings of IFIP Personal Wireless Conference,
August 2001.
[GL02] A. Gurtov, R. Ludwig, Evaluating the Eifel Algorithm for
TCP in a GPRS Network, In Proceedings of the European
Wireless Conference, February 2002.
[RFC1323] V. Jacobson, R. Braden, D. Borman, TCP Extensions for High
Performance, RFC 1323, May 1992.
[RFC2018] M. Mathis, J. Mahdavi, S. Floyd, A. Romanow, TCP Selective
Acknowledgement Options, RFC 2018, October 1996.
[KP87] P. Karn, C. Partridge, Improving Round-Trip Time Estimates
in Reliable Transport Protocols, In Proceedings of ACM
SIGCOMM 87.
[LK00] R. Ludwig, R. H. Katz, The Eifel Algorithm: Making TCP
Robust Against Spurious Retransmissions, ACM Computer
Communication Review, Vol. 30, No. 1, January 2000.
[LM02] R. Ludwig, M. Meyer, The Eifel Detection Algorithm for TCP,
work in progress, July 2002.
[Lu02] R. Ludwig, Responding to Fast Timeouts in TCP, work in
progress, July 2002.
[RFC2988] V. Paxson, M. Allman, Computing TCP's Retransmission Timer,
RFC 2988, November 2000.
[RFC793] J. Postel, Transmission Control Protocol, RFC793, September
1981.
[RFC3168] K. Ramakrishnan, S. Floyd, D. Black, The Addition of
Explicit Congestion Notification (ECN) to IP, RFC 3168,
September 2001
[SK02] P. Sarolahti, A. Kuznetsov, Congestion Control in Linux
TCP, In Proceedings of USENIX, June 2002.
[WS95] G. R. Wright, W. R. Stevens, TCP/IP Illustrated, Volume 2
(The Implementation), Addison Wesley, January 1995.
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Author's Address
Reiner Ludwig
Ericsson Research (EED)
Ericsson Allee 1
52134 Herzogenrath, Germany
Email: Reiner.Ludwig@ericsson.com
Andrei Gurtov
Cellular Systems Development
P.O. Box 970, FIN-00051 Sonera
Helsinki, Finland
Phone: +358(0)20401
Fax: +358(0)204064365
Email: andrei.gurtov@sonera.com
Homepage: http://www.cs.helsinki.fi/u/gurtov
This Internet-Draft expires in January 2003.
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