Network Working Group Reiner Ludwig
INTERNET-DRAFT Michael Meyer
Expires: January 2003 Ericsson Research
July, 2002
The Eifel Detection Algorithm for TCP
<draft-ietf-tsvwg-tcp-eifel-alg-04.txt>
Status of this memo
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Abstract
The Eifel detection algorithm allows the TCP sender to detect a
posteriori whether it has entered loss recovery unnecessarily. It
already determines this in the beginning of loss recovery when the
first acceptable ACK arrives after the timeout-based retransmit or
fast retransmit has been sent. The algorithm requires that the TCP
Timestamps option defined in RFC1323 is enabled for a connection. The
idea is to use the timestamps echoed in returning ACKs to eliminate
the retransmission ambiguity in TCP. The Eifel detection algorithm
provides a basis for future TCP enhancements. This includes response
algorithms to back out of loss recovery by restoring a TCP sender's
congestion control state.
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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, with
repacketization 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
algorithm this makes no difference as it also operates 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]. Future TCPs might implement an adaptive DupThresh.
1. Introduction
The retransmission ambiguity problem [KP87] is the TCP sender's
inability to distinguish whether the first acceptable ACK that
arrives after a retransmit, was sent in response to the original
transmit or the retransmit. This problem occurs after a timeout-based
retransmit and after a fast retransmit. The Eifel detection algorithm
uses the TCP Timestamps option defined in [RFC1323] to eliminate the
retransmission ambiguity. It thereby allows the TCP sender to detect
a posteriori whether it has entered loss recovery unnecessarily.
This added capability of the TCP sender is useful in environments
where TCP's loss recovery and congestion control algorithms may often
get falsely triggered. This can be caused by packet reordering,
packet duplication, the loss of a flight of ACKs, or a sudden delay
increase in the data or the ACK path that results in a spurious
timeout. For example, such sudden delay increases can often occur in
wide-area wireless access networks due to handovers, resource
preemption due to higher priority traffic (e.g., voice), or because
the mobile transmitter traverses through a radio coverage hole (e.g.,
see [Gu01]). In such wireless networks, the often unnecessary
go-back-N retransmits that typically occur after a spurious timeout
create a serious problem. They decrease end-to-end throughput, are
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useless load upon a potentially congested network, and waste
transmission (battery) power.
Based on the Eifel detection algorithm, the TCP sender may then
choose to implement dedicated response algorithms. One goal of such a
response algorithm would be to alleviate the consequences of a
falsely triggered loss recovery. This may include restoring the TCP
sender's congestion control state, and avoiding the mentioned
unnecessary go-back-N retransmits. Another goal would be to adapt
protocol parameters such as the duplicate acknowledgement threshold
[RFC2581], and the RTT estimators [RFC2988]. This is to reduce the
risk of falsely triggering TCP's loss recovery again as the
connection progresses. However, such response algorithms are outside
the scope of this document. Note: The original proposal, the "Eifel
algorithm" [LK00], comprises both a detection and a response
algorithm. This document only defines the detection part. The
response part is defined in [LG02].
A key feature of the Eifel detection algorithm is that it already
detects upon the first acceptable ACK that arrives during loss
recovery whether a fast retransmit or a timeout was spurious. This is
crucial to be able to avoid the mentioned go-back-N retransmits.
Another feature is that the Eifel detection algorithm is fairly
robust against the loss of ACKs.
2. Events that Falsely Trigger TCP Loss Recovery
The following events falsely trigger a TCP sender's loss recovery and
congestion control algorithms. This causes a so-called spurious
retransmit, and an unnecessary reduction of the TCP sender's
congestion window and slow start threshold [RFC2581].
- Spurious timeout
- Packet reordering
- Packet duplication
The common understanding of a spurious timeout is a timeout that
would not have occurred had the sender "waited longer". This may be
caused by increased delay that suddenly occurs in the data and/or the
ACK path. That in turn might cause an acceptable ACK to arrive too
late, i.e., only after the TCP sender's retransmission timer has
expired. For the purpose of specifying the algorithm in Section 3, we
define this case as SPUR_TO (equal 1). However, in this document, we
stretch the definition of a spurious timeout. We also speak of a
spurious timeout when the timeout occurred because an entire flight
of ACKs was lost while in fact the original transmit of the oldest
outstanding segment had arrived at the TCP receiver. This case is
also considered to fall under the definition of SPUR_TO since a TCP
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sender is not able to distinguish it from the other case mentioned
above. Such a timeout is certainly unavoidable, i.e., it would not
have made a difference if the TCP sender had "waited longer". Still,
the triggering of loss recovery was spurious since that original
transmit was not lost, i.e., there had been no congestion in the data
path. Hence, we use the term 'spurious timeout' in the sense that
initiating the associated loss recovery was unnecessary. Yet, we
exclude from the definition of a spurious timeout the case where the
retransmit arrives at the TCP receiver before the corresponding
original transmit.
Note: There is another case where a timeout would not have
occurred had the sender "waited longer": the retransmission
timer expires, and afterwards the TCP sender receives the
duplicate ACK that would have triggered a fast retransmit of the
oldest outstanding segment. We call this a "fast timeout" since
in competition with the fast retransmit algorithm the timeout
was faster. However, a fast timeout is not spurious since
apparently a segment was in fact lost, i.e., loss recovery was
initiating rightfully. In this document, we do not consider fast
timeouts. This case is addressed in an independent document
[Lu02].
Packet reordering in the network may occur because IP [RFC791] does
not guarantee in-order delivery of packets. Additionally, a TCP
receiver generates a duplicate ACK for each segment that arrives out-
of-order. This results in a spurious fast retransmit if three or more
data segments arrive out-of-order at the TCP receiver, and at least
three of the resulting duplicate ACKs arrive at the TCP sender. This
assumes that the duplicate acknowledgement threshold is set to three
as defined in [RFC2581].
Packet duplication may occur because a receiving IP does not (cannot)
remove packets that have been duplicated in the network. A TCP
receiver in turn also generates a duplicate ACK for each duplicate
segment. As with packet reordering, this results in a spurious fast
retransmit if duplication of data segments or ACKs results in three
or more duplicate ACKs to arrive at the TCP sender. Again, this
assumes that the duplicate acknowledgement threshold is set to three.
The negative impact on TCP performance caused by packet reordering
and packet duplication is commonly the same: a single spurious
retransmit (the fast retransmit), and the unnecessary halving of the
TCP sender's congestion window as a result of the subsequent fast
recovery phase [RFC2581].
The negative impact on TCP performance caused by a spurious timeout
is more severe. First, the timeout event itself causes a single
spurious retransmit, and unnecessarily forces the TCP sender into
slow start [RFC2581]. Then, as the connection progresses, a chain
reaction gets triggered that further decreases TCP's performance.
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Since the timeout was spurious, at least some ACKs for original
transmits will typically arrive at the TCP sender before the ACK for
the retransmit arrives. (This is unless severe packet reordering
coincided with the spurious timeout in such a way that the ACK for
the retransmit is the first acceptable ACK to arrive at the TCP
sender.) Those ACKs for original transmits will then trigger an
implicit go-back-N loss recovery at the TCP sender. Assuming that
none of the outstanding segments and none of the corresponding ACKs
were lost, all outstanding segments will get retransmitted
unnecessarily. 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. Moreover, some TCPs will in addition suffer from a
spurious fast retransmit. This is because the spurious go-back-N
retransmits will arrive as duplicates at the TCP receiver, which in
turn triggers a series of duplicate ACKs. Note that this last
spurious fast retransmit could be avoided with the careful variant of
'bug fix' [RFC2582].
More detailed explanations including TCP trace plots that visualize
the effects of spurious timeouts and packet reordering can be found
in [LK00].
3. The Eifel Detection Algorithm
3.1 The Idea
The goal of the Eifel detection algorithm is to allow the TCP sender
to detect a posteriori whether it has entered loss recovery
unnecessarily. Furthermore, the TCP sender should be able to make
this decision upon the first acceptable ACK that arrives after the
timeout-based retransmit or the fast retransmit has been sent. This
in turn requires extra information in every ACK by which the TCP
sender can unambiguously distinguish whether that first acceptable
ACK was sent in response to the original transmit or the retransmit.
Such extra information is provided by the TCP Timestamps option
[RFC1323]. Generally speaking, timestamps are monotonously increasing
"serial numbers" added into every segment that are then echoed within
the corresponding ACKs. This is exploited by the Eifel detection
algorithm in the following way.
Given that timestamps are enabled for a connection, the TCP sender
always stores the timestamp of the retransmit sent in the beginning
of loss recovery, i.e., the timestamp of the timeout-based retransmit
or the fast retransmit. If the timestamp of the first acceptable ACK,
arriving after the retransmit was sent, is smaller then the stored
timestamp of that retransmit, then that ACK must have been sent in
response to an original transmit. Hence, the TCP sender must have
entered loss recovery unnecessarily.
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The fact that the Eifel detection algorithm decides upon the first
acceptable ACK is crucial to allow future response algorithms to
avoid the spurious go-back-N retransmits that typically occur after a
spurious timeout. Also, if loss recovery was entered unnecessarily, a
window worth of ACKs are outstanding that all carry a timestamp that
is smaller than the stored timestamp of the retransmit. The arrival
of any one of those ACKs suffices the Eifel detection algorithm to
work. Hence, the solution is fairly robust against ACK losses. Even
the ACK sent in response to the retransmit, i.e., the one that
carries the stored timestamp, may get lost.
Note: Also the DSACK option [RFC2883] can be used to detect a
posteriori whether the TCP sender has entered loss recovery
unnecessarily. However, the first ACK carrying a DSACK option
usually arrives at the TCP sender only after loss recovery has
already terminated. Thus, the DSACK option cannot be used to
eliminate the retransmission ambiguity. Consequently, it cannot
be used to avoid the mentioned spurious go-back-N retransmits.
Moreover, a DSACK-based detection algorithm is less robust
against ACK losses.
3.2 The Algorithm
Given that the TCP Timestamps option [RFC1323] is enabled for a
connection, a TCP sender MAY use the Eifel detection algorithm as
defined in this subsection.
If the Eifel detection 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. The Eifel detection 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, e.g., during selective loss recovery.
(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
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).
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(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) Do nothing (Placeholder for a response algorithm).
(DONE) No further processing.
The comparison "smaller than" in step (4) is conservative. In theory,
when the "timestamp clock" is slow or the network is fast,
RetransmitTS could at most be equal to the timestamp echoed by an ACK
sent in response to an original transmit. In that case, it is assumed
that the loss recovery was not falsely triggered.
3.3 Non-RFC1323-compliant TCP Receivers Mask Certain Spurious Timeouts
Not all TCP implementations strictly follow RFC1323. There are
differences concerning which timestamp a TCP receiver echoes when a
duplicate segment arrives. The relevant case to consider in this
context is a spurious timeout that occurred because an entire flight
of ACKs got lost (recall the definition of a spurious timeout from
Section 2). The question is which timestamp the TCP receiver echoes
in response to the spurious retransmit that typically arrives as a
duplicate segment. RFC1323-compliant TCP receivers (e.g., LINUX
kernel 2.4.x) will echo the timestamp of the last segment that
arrived in-sequence, while some non-RFC1323-compliant TCP receivers
will echo the timestamp of the retransmit.
The Eifel detection algorithm will only detect such spurious timeouts
in case the TCP receiver is RFC1323-compliant in this respect.
Otherwise, the Eifel detection algorithm will simply not kick in.
This is not any different than from running the TCP sender without
the Eifel algorithm.
3.4 Protecting Against Misbehaving TCP Receivers (the Safe Variant)
A TCP receiver can easily make a genuine retransmit appear to the TCP
sender as a spurious retransmit by forging echoed timestamps. This
may pose a security concern.
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Fortunately, there is a way to modify the Eifel detection algorithm
in a way that makes it robust against lying TCP receivers. The idea
is to use timestamps as a "segment's secret" that the TCP receiver
only gets to know if it receives the segment. Conversely, a TCP
receiver will not know the timestamp of a segment that was lost.
Hence, to "prove" that it received the original transmit of a segment
that the TCP sender retransmitted, the TCP receiver would need to
return the timestamp of that original transmit. The Eifel detection
algorithm could then be modified to only decide that loss recovery
has been unnecessarily entered if the first acceptable ACK echoes the
timestamp of the original transmit.
Hence, implementers may choose to implement the algorithm with the
following modifications.
Step (1) is replaced with step (1'):
(1') Set a "RetransmitTS" variable to the value of the
Timestamp Value field of the Timestamps option that was
included in the original transmit corresponding to the
retransmit. Note: This step requires that the TCP sender
stores the timestamps of all outstanding original
transmits.
Step (4) is replaced with step (4'):
(4') If the value of the Timestamp Echo Reply field of the
acceptable ACK's Timestamps option is equal to the value
of the variable RetransmitTS, then proceed to step (5),
else proceed to step (DONE).
These modifications come at a cost: the modified algorithm is fairly
sensitive against ACK losses since it relies on the arrival of the
acceptable ACK that corresponds to the original transmit.
Note: The first acceptable ACK that arrives after loss recovery
has been unnecessarily entered, should echo the timestamp of the
original transmit. This assumes that the ACK corresponding to
the original transmit was not lost, that that ACK was not
reordered in the network, and that the TCP receiver does not
forge timestamps but complies with RFC1323. In case of a
spurious fast retransmit, this is implied by the rules for
generating ACKs for data segments that fill in all or part of a
gap in the sequence space (see section 4.2 of [RFC2581]) and by
the rules for echoing timestamps in that case (see rule (C) in
section 3.4 of [RFC1323]). In case of a spurious timeout, it is
likely that the delay that has caused the spurious timeout has
also caused the TCP receiver's delayed ACK timer [RFC1122] to
expire before the original transmit arrives. Also, in this case
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the rules for generating ACKs and the rules for echoing
timestamps (see rule (A) in section 3.4 of [RFC1323]) ensure
that the original transmit's timestamp is echoed.
A remaining problem is that a TCP receiver might guess a lost
segment's timestamp from observing the timestamps of segments that
arrived earlier. This could be avoided by having the TCP sender add a
"random counter" to the timestamp of every segment it sends, and then
increment that counter by a random value, e.g., between 1 and 100.
This ensures that timestamps remain monotonously increasing while
making it difficult for a TCP receiver to guess the timestamp of a
lost segment.
4. Security Considerations
There do not seem to be any security considerations associated with
the Eifel detection algorithm. This is because the Eifel detection
algorithm does not alter existing protocol state at the TCP sender
nor at the receiver. That is, no value of a TCP sender's state
variable is changed.
Moreover, a variant of the Eifel detection algorithm has been
proposed in Section 3.4 that makes it robust against lying TCP
receivers.
Acknowledgments
Many thanks to Keith Sklower, Randy Katz, Stephan Baucke, Sally
Floyd, Vern Paxson, Mark Allman, Ethan Blanton, Andrei Gurtov, Pasi
Sarolahti, and Alexey Kuznetsov for useful discussions that
contributed to this work.
References
[RFC2581] M. Allman, V. Paxson, W. Stevens, TCP Congestion Control,
RFC 2581, April 1999.
[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.
[RFC2883] S. Floyd, J. Mahdavi, M. Mathis, M. Podolsky, A. Romanow,
An Extension to the Selective Acknowledgement (SACK) Option
for TCP, RFC 2883, July 2000.
[Gu01] A. Gurtov, Effect of Delays on TCP Performance, In
Proceedings of IFIP Personal Wireless Communications,
August 2001.
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[Jac88] V. Jacobson, Congestion Avoidance and Control, In
Proceedings of ACM SIGCOMM 88.
[RFC1323] V. Jacobson, R. Braden, D. Borman, TCP Extensions for High
Performance, RFC 1323, May 1992.
[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,
available at http://www.acm.org/sigcomm/ccr/archive/2000/
jan00/ccr-200001-ludwig.html (easier studied when
viewed/printed in color).
[LG02] R. Ludwig, A. Gurtov, The Eifel Response 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.
[RFC791] J. Postel, Internet Protocol, RFC 791, September 1981.
[RFC793] J. Postel, Transmission Control Protocol, RFC793, September
1981.
[WS95] G. R. Wright, W. R. Stevens, TCP/IP Illustrated, Volume 2
(The Implementation), Addison Wesley, January 1995.
Author's Address
Reiner Ludwig
Ericsson Research
Ericsson Allee 1
52134 Herzogenrath, Germany
Email: Reiner.Ludwig@eed.ericsson.se
Michael Meyer
Ericsson Research
Ericsson Allee 1
52134 Herzogenrath, Germany
Email: Michael.Meyer@eed.ericsson.se
This Internet-Draft expires in January 2003.
Ludwig & Meyer [Page 10]
