Network Working Group Reiner Ludwig
INTERNET-DRAFT Michael Meyer
Expires: April 2003 Ericsson Research
October, 2002
The Eifel Detection Algorithm for TCP
<draft-ietf-tsvwg-tcp-eifel-alg-06.txt>
Status of this memo
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Abstract
The Eifel detection algorithm allows a TCP sender to detect a
posteriori whether it has entered loss recovery unnecessarily. It
requires that the TCP Timestamps option defined in RFC1323 is enabled
for a connection. The Eifel detection algorithm makes use of the fact
that the TCP Timestamps option eliminates the retransmission
ambiguity in TCP. Based on the timestamp of the first acceptable ACK
that arrives during loss recovery, it decides whether loss recovery
was entered unnecessarily. 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 a 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 a 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 [Zh86][KP87] is a 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 a TCP sender to detect a
posteriori whether it has entered loss recovery unnecessarily.
This added capability of a 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, 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 useless load upon the network, and waste
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transmission (battery) power. Note, that across such networks the use
of timestamps is recommended anyway [IMLGK02].
Based on the Eifel detection algorithm, a 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.
Also the DSACK option [RFC2883] can be used to detect a posteriori
whether a TCP sender has entered loss recovery unnecessarily [BA02].
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
unnecessary go-back-N retransmits. Moreover, a DSACK-based detection
algorithm is less robust against ACK losses. A recent proposal
neither based on the TCP timestamps nor the DSACK option does not
have the limitation of DSACK-based schemes, but only addresses the
case of spurious timeouts [SK02].
2. Events that Falsely Trigger TCP Loss Recovery
The following events may 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
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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 a 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).
Note: There is another case where a timeout would not have
occurred had the sender "waited longer": the retransmission timer
expires, and afterwards a 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 initiated
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 a 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 a 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 a
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 a TCP sender into slow
start [RFC2581]. Then, as the connection progresses, a chain reaction
gets triggered that further decreases TCP's performance. Since the
timeout was spurious, at least some ACKs for original transmits
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
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first acceptable ACK to arrive at the TCP sender.) Those ACKs for
original transmits 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
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. Thus,
for each packet that leaves the network and that belongs to the first
half of the original flight, two useless retransmits are sent into
the network. Moreover, some TCPs in addition suffer from a spurious
fast retransmit. This is because the unnecessary go-back-N
retransmits 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 the original proposal [LK00].
3. The Eifel Detection Algorithm
3.1 The Idea
The goal of the Eifel detection algorithm is to allow a 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 ACKs 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,
that arrives 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.
The fact that the Eifel detection algorithm decides upon the first
acceptable ACK is crucial to allow future response algorithms to
avoid the unnecessary go-back-N retransmits that typically occur
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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.
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 a 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 "SpuriousRecovery" variable to FALSE (equal 0).
(2) 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.
(3) Wait for the arrival of an acceptable ACK. When an
acceptable ACK has arrived 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 acceptable ACK does not carry a DSACK option
[RFC2883], then proceed to step (6),
else proceed to step (DONE).
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(6) If the loss recovery has been initiated with a timeout-
based retransmit, then set
SpuriousRecovery <- SPUR_TO (equal 1),
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,
if 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 A Corner Case: "Timeout due to loss of all ACKs"
Even though the oldest outstanding segment arrived at a TCP receiver,
the TCP sender is forced into a timeout when all ACKs are lost.
Although, the resulting retransmit is unnecessary, such a timeout is
unavoidable. It should therefore not be considered to be spurious.
This particular case of a timeout is considered in this section.
In that case, the retransmit arrives as a duplicate at the TCP
receiver. In response to duplicates, RFC1323 mandates that the
timestamp of the last segment that arrived in-sequence should be
echoed. That timestamp is carried by the first acceptable ACK that
arrives at the TCP sender after loss recovery was entered, and is
commonly smaller than the timestamp carried by the retransmit.
Consequently, the Eifel detection algorithm mistakes such a timeout
as spurious, unless that first acceptable ACK also carries a DSACK
option (see step (5) of the algorithm).
Note: Not all TCP implementations strictly follow RFC1323. In
response to a duplicate data segment, some TCP receivers echo the
timestamp of the duplicate. With such TCP receivers, the corner
case discussed in this section does not apply. The timestamp
carried by the retransmit would be echoed in the first acceptable
ACK, and the Eifel detection algorithm would be terminated through
step (4). Thus, even though all ACKs were lost, and independent of
whether the DSACK option was enabled for a connection, the Eifel
detection algorithm would have no effect.
A concern with this corner case arises if the Eifel detection
algorithm is combined with a response algorithm like the Eifel
response algorithm [LG02]. That algorithm backs out of loss recovery
by reversing congestion control state after a spurious timeout has
been detected. The argument against doing so, is that the TCP sender
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should keep its congestion window halved in case all ACKs are lost
since that is a sign of severe congestion on the ACK path.
This is not really a problem as long as data segments were lost in
addition to the entire flight of ACKs. The Eifel detection algorithm
would misinterpret the timeout as spurious, and the Eifel response
algorithm would reverse congestion control state. Still, the TCP
sender would respond to congestion (in the data path) as soon as it
finds out about the first loss in the outstanding flight. I.e., the
TCP sender would still half its congestion window for that flight of
packets.
The concern remains, though, in case the DSACK option is not enabled,
and an entire flight of ACKs is lost while all of the data segments
arrive at the TCP receiver. Without the Eifel detection and response
algorithm the TCP sender would go into slow start. With those
algorithms it would not respond to the congestion in the ACK path.
We do not believe that this is a serious concern. First, we assume
that DSACK will get increasingly deployed. And even if this was not
the case, this special corner case must occur sufficiently often to
become a problem for the progress of a TCP connection. It is unlikely
that any TCP could manage to grow its congestion window much beyond
one maximum segment size with such an ACK path. And in that case, the
reversing of congestion control state becomes meaningless. Moreover,
in an implementation one might choose to disable the Eifel detection
algorithm if such an ACK path is encountered.
3.4 Protecting Against Misbehaving TCP Receivers (the Safe Variant)
A TCP receiver can easily make a genuine retransmit appear to a TCP
sender as a spurious retransmit by forging echoed timestamps. This
may pose a security concern.
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 a 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 a
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.
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Step (2) is replaced with step (2'):
(2') 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 (in the data
path) 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 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 recently
received segments. For example, if segment N was lost while segment
N-1 and N+1 have arrived, a TCP receiver could guess the timestamp
that lies in the middle of the timestamps of segments N-1 and N+1,
and echo it in the ACK for segment N+1. Especially if a TCP sender
implements timestamps with a coarse granularity, a misbehaving TCP
receiver is likely to be successful with such an approach. In fact,
with the 500 ms granularity suggested in [WS95], it even becomes
quite likely that the timestamps of segments N-1, N, N+1 are
identical.
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One way to reduce this risk is to implement fine grained timestamps.
Note that the granularity of the timestamps is independent of the
granularity of the retransmission timer. For example, some TCP
implementations run a timestamp clock that even ticks every
microsecond. This should make it more difficult for a TCP receiver to
guess the timestamp of a lost segment. Alternatively, it might be
possible to combine the timestamps with a nonce, as done for the
Explicit Congestion Notification (ECN) [RFC3168]. One would need to
take care, though, that the timestamps of succeeding segments remain
monotonously increasing and do not interfere with the RTT timing
defined in [RFC1323].
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 a TCP sender.
Note that the Eifel detection algorithm only requires changes to the
implementation of the TCP sender.
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.
Normative 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.
[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.
[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.
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[RFC793] J. Postel, Transmission Control Protocol, RFC793, September
1981.
Informative References
[BA02] E. Blanton, M. Allman, Using TCP DSACKs and SCTP Duplicate
TSNs to Detect Spurious Retransmissions, work in progress,
October 2002..
[RFC1122] R. Braden, Requirements for Internet Hosts - Communication
Layers, RFC 1122, October 1989.
[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 Communications,
August 2001.
[IMLGK02] H. Inamura et. al., TCP over Second (2.5G) and Third (3G)
Generation Wireless Networks, work in progress, July 2002.
[Jac88] V. Jacobson, Congestion Avoidance and Control, In
Proceedings of ACM SIGCOMM 88.
[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.
[LG02] R. Ludwig, A. Gurtov, The Eifel Response Algorithm for TCP,
work in progress, October 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.
[RFC3168] K. Ramakrishnan, S. Floyd, D. Black, The Addition of
Explicit Congestion Notification (ECN) to IP, RFC 3168,
September 2001.
[SK02] P. Sarolahti, M. Kojo, F-RTO: A TCP RTO Recovery Algorithm
for Avoiding Unnecessary Retransmissions, work in progress,
June 2002.
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[WS95] G. R. Wright, W. R. Stevens, TCP/IP Illustrated, Volume 2
(The Implementation), Addison Wesley, January 1995.
[Zh86] L. Zhang, Why TCP Timers Don't Work Well, In Proceedings of
ACM SIGCOMM 88.
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 April 2003.
Ludwig & Meyer [Page 12]
