Network Working Group R. Ludwig

Network Working Group R. Ludwig
Request for Comments: 4015 Ericsson Research
Category: Standards Track A. Gurtov
HIIT
February 2005

The Eifel Response Algorithm for TCP

Status of This Memo

This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (2005).

Abstract

Based on an appropriate detection algorithm, the Eifel response
algorithm provides a way for a TCP sender to respond to a detected
spurious timeout. It adapts the retransmission timer to avoid
further spurious timeouts and (depending on the detection algorithm)
can avoid the often unnecessary go-back-N retransmits that would
otherwise be sent. In addition, the Eifel response algorithm
restores the congestion control state in such a way that packet
bursts are avoided.

1. Introduction

The Eifel response algorithm relies on a detection algorithm such as
the Eifel detection algorithm, defined in [RFC3522]. That document
contains informative background and motivation context that may be
useful for implementers of the Eifel response algorithm, but it is
not necessary to read [RFC3522] in order to implement the Eifel
response algorithm. Note that alternative response algorithms have
been proposed [BA02] that could also rely on the Eifel detection
algorithm, and alternative detection algorithms have been proposed
[RFC3708], [SK04] that could work together with the Eifel response
algorithm.

Based on an appropriate detection algorithm, the Eifel response
algorithm provides a way for a TCP sender to respond to a detected
spurious timeout. It adapts the retransmission timer to avoid

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further spurious timeouts and (depending on the detection algorithm)
can avoid the often unnecessary go-back-N retransmits that would
otherwise be sent. In addition, the Eifel response algorithm
restores the congestion control state in such a way that packet
bursts are avoided.

Note: A previous version of the Eifel response algorithm also
included a response to a detected spurious fast retransmit.
However, as 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.

1.1. 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 also be
applied to data segments. 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 'bytes_acked' to refer to the amount (in terms of octets) of
previously unacknowledged data that is acknowledged by the most
recently received acceptable ACK. 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 term 'FlightSize' as
defined in [RFC2581]. FlightSize is the amount (in terms of octets)
of outstanding data at a given point in time. We use the term
'Initial Window' (IW) as defined in [RFC3390]. 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

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'RTTVAR', and the terms 'RTO' and 'G' as defined in [RFC2988]. G is
the clock granularity of the retransmission timer. In addition, we
assume that the TCP sender maintains the value of the latest round-
trip time (RTT) measurement in the (local) variable 'RTT-SAMPLE'.

We use the TCP sender state variable 'T_last', and the term 'tcpnow'
as used in [RFC2861]. T_last holds the system time when the TCP
sender sent the last data segment, whereas tcpnow is the TCP sender's
current system time.

2. Appropriate 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 a standards track or experimental RFC.

Designers of detection algorithms who want their algorithms to work
together with the Eifel response algorithm should reuse the variable
"SpuriousRecovery" with the semantics and defined values specified in
[RFC3522]. In addition, we define the constant LATE_SPUR_TO (set
equal to -1) as another possible value of the variable
SpuriousRecovery. Detection algorithms should set the value of
SpuriousRecovery to LATE_SPUR_TO if the detection of a spurious
retransmit is based on the ACK for the retransmit (as opposed to an
ACK for an original transmit). For example, this applies to
detection algorithms that are based on the DSACK option [RFC3708].

3. The Eifel Response Algorithm

The complete algorithm is specified in section 3.1. In sections 3.2
- 3.6, we discuss 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 a timeout-based
loss recovery. That is when the first timeout-based retransmit is
sent. The algorithm MUST NOT be reinitiated after a timeout-based
loss recovery has already been started but not completed. In
particular, it may not be reinitiated upon subsequent timeouts for
the same segment, or upon retransmitting segments other than the
oldest outstanding segment.

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(0) 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)

Set a "SRTT_prev" variable and a "RTTVAR_prev" variable as
follows:
SRTT_prev <- SRTT + (2 * G)
RTTVAR_prev <- RTTVAR

(DET) This is a placeholder for a detection algorithm that must
be executed at this point, and that sets the variable
SpuriousRecovery as outlined in Section 2. If
[RFC3522] is used as the detection algorithm, steps (1) -
(6) of that algorithm go here.

(7) If SpuriousRecovery equals SPUR_TO, then
proceed to step (8);

else if SpuriousRecovery equals LATE_SPUR_TO, then
proceed to step (9);

else
proceed to step (DONE).

(8) Resume the transmission with previously unsent data:

Set
SND.NXT <- SND.MAX

(9) Reverse the congestion control state:

If the acceptable ACK has the ECN-Echo flag [RFC3168] set,
then
proceed to step (DONE);

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

Proceed to step (DONE).

(10) Interworking with Congestion Window Validation:

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

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(11) Adapt the conservativeness of the retransmission timer:

Upon the first RTT-SAMPLE taken from new data; i.e., the
first RTT-SAMPLE that can be derived from an acceptable
ACK for data that was previously unsent when the spurious
timeout occurred,

if the retransmission timer is implemented according
to [RFC2988], then set
SRTT <- max (SRTT_prev, RTT-SAMPLE)
RTTVAR <- max (RTTVAR_prev, RTT-SAMPLE/2)
RTO <- SRTT + max (G, 4*RTTVAR)

Run the bounds check on the RTO (rules (2.4) and
(2.5) in [RFC2988]), and restart the
retransmission timer;

else
appropriately adapt the conservativeness of the
retransmission timer that is implemented.

(DONE) No further processing.

3.2. Storing the Current Congestion Control State (Step 0)

The TCP sender stores in pipe_prev what is considered a safe slow-
start threshold (ssthresh) before loss recovery is initiated; i.e.,
before the loss indication is taken into account. This is either the
current FlightSize, if the TCP sender is in congestion avoidance, or
the current ssthresh, if the TCP sender is in slow-start. If the TCP
sender later detects that it has entered loss recovery unnecessarily,
then pipe_prev is used in step (9) to reverse the 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 has become idle or 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 decaying pipe_prev for the
Eifel response algorithm and do not believe that such a conservative
approach should be in place. Instead, we follow the idea of
revalidating the congestion window through slow-start, as suggested
in [RFC2861]. That is, in step (9), the cwnd is reset to a value
that avoids large packet bursts, and ssthresh is reset to the value
of pipe_prev. Note that [RFC2581] and [RFC2861] also do not require

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a decaying of ssthresh after it has been reset in response to a loss
indication, or after a TCP sender has become idle or application
limited.

3.3. Suppressing the Unnecessary go-back-N Retransmits (Step 8)

Without the use of the TCP timestamps option [RFC1323], the TCP
sender suffers from the retransmission ambiguity problem [Zh86],
[KP87]. Therefore, 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
an original transmit. Furthermore, the TCP sender must further
assume that all other segments that were outstanding at that point
were lost.

Note: Except for certain cases where original ACKs were lost, the
first acceptable ACK cannot carry a 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. 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, whether or not the
corresponding original transmits were lost.

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. 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).

Once a spurious timeout has been detected (upon receipt of an ACK for
an original transmit), it is safe to let the TCP sender resume the
transmission with previously unsent data. Thus, the Eifel response
algorithm changes the TCP sender's state by setting SND.NXT to
SND.MAX. Note that this step is only executed if the variable
SpuriousRecovery equals SPUR_TO, which in turn requires a detection
algorithm such as the Eifel detection algorithm [RFC3522] or the F-
RTO algorithm [SK04] that detects a spurious retransmit based upon
receiving an ACK for an original transmit (as opposed to the ACK for
the retransmit [RFC3708]).

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3.4. Reversing the Congestion Control State (Step 9)

When a TCP sender enters loss recovery, it reduces cwnd and ssthresh.
However, once the TCP sender detects that the loss recovery has been
falsely triggered, this reduction proves unnecessary. 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.

If the ECN-Echo flag is not set, cwnd is reset to the sum of the
current FlightSize and the minimum of bytes_acked and IW. In some
cases, this can mean that the first few acceptable ACKs that arrive
will not clock out any data segments. Recall that bytes_acked is 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 value of FlightSize in step (0). The value of IW
puts a limit on the size of the packet burst that the TCP sender may
send into the network after the Eifel response algorithm has
terminated. The 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.

Then ssthresh is reset to the value of pipe_prev. As a result, the
TCP sender either immediately resumes probing the network for more
bandwidth in congestion avoidance, or it slow-starts to what is
considered a safe operating point for the congestion window.

3.5. Interworking with the CWV Algorithm (Step 10)

An implementation of the Congestion Window Validation (CWV) algorithm
[RFC2861] could potentially misinterpret a delay spike that caused a
spurious timeout as a phase where the TCP sender had been idle.
Therefore, T_last is reset to prevent the triggering of the CWV
algorithm in this case.

Note: The term 'idle' implies that the TCP sender has no data
outstanding; i.e., all data sent has been acknowledged [Jac88].
According to this definition, a TCP sender is not idle while it is
waiting for an acceptable ACK after a timeout. Unfortunately, the
pseudo-code in [RFC2861] does not include a check for the
condition "idle" (SND.UNA == SND.MAX). We therefore had to add
step (10) to the Eifel response algorithm.

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3.6. Adapting the Retransmission Timer (Step 11)

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.

A spurious timeout often results from a delay spike, which is a
sudden increase of the RTT that usually cannot be predicted. After a
delay spike, the RTT may have changed permanently; e.g., due to a
path change, or because the available bandwidth on a bandwidth-
dominated path has decreased. This may often occur with wide-area
wireless access links. In this case, the RTT estimators (SRTT and
RTTVAR) should be reinitialized from the first RTT-SAMPLE taken from
new data according to rule (2.2) of [RFC2988]. That is, from the
first RTT-SAMPLE that can be derived from an acceptable ACK for data
that was previously unsent when the spurious timeout occurred.

However, a delay spike may only indicate a transient phase, after
which the RTT returns to its previous range of values, or even to
smaller values. Also, a spurious timeout may occur because the TCP
sender's RTT estimators were only inaccurate enough that the
retransmission timer expires "a tad too early". We believe that two
times the clock granularity of the retransmission timer (2 * G) is a
reasonable upper bound on "a tad too early". Thus, when the new RTO
is calculated in step (11), we ensure that it is at least (2 * G)
greater (see also step (0)) than the RTO was before the spurious
timeout occurred.

Note that other TCP sender processing will usually take place between
steps (10) and (11). During this phase (i.e., before step (11) has
been reached), the RTO is managed according to the rules of
[RFC2988]. We believe that this is sufficiently conservative for the
following reasons. First, the retransmission timer is restarted upon
the acceptable ACK that was used to detect the spurious timeout. As
a result, the delay spike is already implicitly factored in for
segments outstanding at that time. This is discussed in more detail
in [EL04], where this effect is called the "RTO offset".
Furthermore, if timestamps are enabled, a new and valid RTT-SAMPLE
can be derived from that acceptable ACK. This RTT-SAMPLE must be
relatively large, as it includes the delay spike that caused the
spurious timeout. Consequently, the RTT estimators will be updated
rather conservatively. Without timestamps the RTO will stay
conservatively backed-off due to Karn's algorithm [RFC2988] until the
first RTT-SAMPLE can be derived from an acceptable ACK for data that
was previously unsent when the spurious timeout occurred.

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For the new RTO to become effective, the retransmission timer has to
be restarted. This is consistent with [RFC2988], which recommends
restarting the retransmission timer with the arrival of an acceptable
ACK.

4. Advanced Loss Recovery is Crucial for the Eifel Response Algorithm

We have studied environments where spurious timeouts and multiple
losses from the same flight of packets often coincide [GL02], [GL03].
In such a case, the oldest outstanding segment arrives at the TCP
receiver, but one or more packets from the remaining outstanding
flight are lost. In those environments, end-to-end performance
suffers if the Eifel response algorithm is operated without an
advanced loss recovery scheme such as a SACK-based scheme [RFC3517]
or NewReno [RFC3782]. The reason is TCP-Reno's aggressiveness after
a spurious timeout. Even though TCP-Reno breaks 'packet
conservation' (see Section 3.3) when blindly retransmitting all
outstanding segments, it usually recovers all packets lost from that
flight within a single round-trip time. On the contrary, the more
conservative TCP-Reno-with-Eifel is often forced into another
timeout. Thus, we recommend that the Eifel response algorithm always
be operated in combination with [RFC3517] or [RFC3782]. Additional
robustness is achieved with the Limited Transmit and Early Retransmit
algorithms [RFC3042], [AAAB04].

Note: The SACK-based scheme we used for our simulations in [GL02]
and [GL03] is different from the SACK-based scheme that later got
standardized [RFC3517]. The key difference is that [RFC3517] is
more robust to multiple losses from the same flight. It is less
conservative in declaring that a packet has left the network, and
is therefore less dependent on timeouts to recover genuine packet
losses.

If the NewReno algorithm [RFC3782] is used in combination with the
Eifel response algorithm, step (1) of the NewReno algorithm SHOULD be
modified as follows, but only if SpuriousRecovery equals SPUR_TO:

(1) Three duplicate ACKs:
When the third duplicate ACK is received and the sender is
not already in the Fast Recovery procedure, go to step 1A.

That is, the entire step 1B of the NewReno algorithm is obsolete
because step (8) of the Eifel response algorithm avoids the case
where three duplicate ACKs result from unnecessary go-back-N
retransmits after a timeout. Step (8) of the Eifel response
algorithm avoids such unnecessary go-back-N retransmits in the first
place. However, recall that step (8) is only executed if the
variable SpuriousRecovery equals SPUR_TO, which in turn requires a

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detection algorithm, such as the Eifel detection algorithm [RFC3522]
or the F-RTO algorithm [SK04], that detects a spurious retransmit
based upon receiving an ACK for an original transmit (as opposed to
the ACK for the retransmit [RFC3708]).

5. 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
[RFC3522], is a reliable method to protect against this risk.

6. Acknowledgements

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

7. References

7.1. Normative References

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

[RFC3390] Allman, M., Floyd, S., and C. Partridge, "Increasing TCP's
Initial Window", RFC 3390, October 2002.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC3782] Floyd, S., Henderson, T., and A. Gurtov, "The NewReno
Modification to TCP's Fast Recovery Algorithm", RFC 3782,
April 2004.

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

[RFC3522] Ludwig, R. and M. Meyer, "The Eifel Detection Algorithm for
TCP", RFC 3522, April 2003.

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

[RFC793] Postel, J., "Transmission Control Protocol", STD 7, RFC
793, September 1981.

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

7.2. Informative References

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

[AAAB04] Allman, M., Avrachenkov, K., Ayesta, U., and J. Blanton,
Early Retransmit for TCP and SCTP, Work in Progress, July
2004.

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

[RFC3708] Blanton, E. and M. Allman, "Using TCP Duplicate Selective
Acknowledgement (DSACKs) and Stream Control Transmission
Protocol (SCTP) Duplicate Transmission Sequence Numbers
(TSNs) to Detect Spurious Retransmissions", RFC 3708,
February 2004.

[RFC3517] Blanton, E., Allman, M., Fall, K., and L. Wang, "A
Conservative Selective Acknowledgment (SACK)-based Loss
Recovery Algorithm for TCP", RFC 3517, April 2003.

[EL04] Ekstrom, H. and R. Ludwig, The Peak-Hopper: A New End-to-
End Retransmission Timer for Reliable Unicast Transport, In
Proceedings of IEEE INFOCOM 04, March 2004.

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

[GL02] 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] Gurtov, A. and R. Ludwig, Responding to Spurious Timeouts
in TCP, In Proceedings of IEEE INFOCOM 03, April 2003.

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RFC 4015 The Eifel Response Algorithm for TCP February 2005

[Jac88] Jacobson, V., Congestion Avoidance and Control, In
Proceedings of ACM SIGCOMM 88.

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

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

[LK00] 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.

[SK04] Sarolahti, P. and M. Kojo, F-RTO: An Algorithm for
Detecting Spurious Retransmission Timeouts with TCP and
SCTP, Work in Progress, November 2004.

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

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

Authors' Addresses

Reiner Ludwig
Ericsson Research (EDD)
Ericsson Allee 1
52134 Herzogenrath, Germany

EMail: [email protected]

Andrei Gurtov
Helsinki Institute for Information Technology (HIIT)
P.O. Box 9800, FIN-02015
HUT, Finland

EMail: [email protected]
Homepage: http://www.cs.helsinki.fi/u/gurtov

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