|Title||Using TCP Duplicate Selective Acknowledgement (DSACKs) and Stream
Control Transmission Protocol (SCTP) Duplicate Transmission Sequence
Numbers (TSNs) to Detect Spurious Retransmissions
|Author||E. Blanton, M.
Network Working Group E. Blanton
Request for Comments: 3708 Purdue University
Category: Experimental M. Allman
Using TCP Duplicate Selective Acknowledgement (DSACKs) and
Stream Control Transmission Protocol (SCTP) Duplicate
Transmission Sequence Numbers (TSNs) to Detect Spurious
Status of this Memo
This memo defines an Experimental Protocol for the Internet
community. It does not specify an Internet standard of any kind.
Discussion and suggestions for improvement are requested.
Distribution of this memo is unlimited.
Copyright (C) The Internet Society (2004). All Rights Reserved.
TCP and Stream Control Transmission Protocol (SCTP) provide
notification of duplicate segment receipt through Duplicate Selective
Acknowledgement (DSACKs) and Duplicate Transmission Sequence Number
(TSN) notification, respectively. This document presents
conservative methods of using this information to identify
unnecessary retransmissions for various applications.
TCP [RFC793] and SCTP [RFC2960] provide notification of duplicate
segment receipt through duplicate selective acknowledgment (DSACK)
[RFC2883] and Duplicate TSN notifications, respectively. Using this
information, a TCP or SCTP sender can generally determine when a
retransmission was sent in error. This document presents two methods
for using duplicate notifications. The first method is simple and
can be used for accounting applications. The second method is a
conservative algorithm to disambiguate unnecessary retransmissions
from loss events for the purpose of undoing unnecessary congestion
This document is intended to outline reasonable and safe algorithms
for detecting spurious retransmissions and discuss some of the
considerations involved. It is not intended to describe the only
possible method for achieving the goal, although the guidelines in
this document should be taken into consideration when designing
alternate algorithms. Additionally, this document does not outline
what a TCP or SCTP sender may do after a spurious retransmission is
detected. A number of proposals have been developed (e.g.,
[RFC3522], [SK03], [BDA03]), but it is not yet clear which of these
proposals are appropriate. In addition, they all rely on detecting
spurious retransmits and so can share the algorithm specified in this
Finally, we note that to simplify the text much of the following
discussion is in terms of TCP DSACKs, while applying to both TCP and
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
2. Counting Duplicate Notifications
For certain applications a straight count of duplicate notifications
will suffice. For instance, if a stack simply wants to know (for
some reason) the number of spuriously retransmitted segments,
counting all duplicate notifications for retransmitted segments
should work well. Another application of this strategy is to monitor
and adapt transport algorithms so that the transport is not sending
large amounts of spurious data into the network. For instance,
monitoring duplicate notifications could be used by the Early
Retransmit [AAAB03] algorithm to determine whether fast
retransmitting [RFC2581] segments with a lower than normal duplicate
ACK threshold is working, or if segment reordering is causing
More speculatively, duplicate notification has been proposed as an
integral part of estimating TCP's total loss rate [AEO03] for the
purposes of mitigating the impact of corruption-based losses on
transport protocol performance. [EOA03] proposes altering the
transport's congestion response to the fraction of losses that are
actually due to congestion by requiring the network to provide the
corruption-based loss rate and making the transport sender estimate
the total loss rate. Duplicate notifications are a key part of
estimating the total loss rate accurately [AEO03].
3. Congestion/Duplicate Disambiguation Algorithm
When the purpose of detecting spurious retransmissions is to "undo"
unnecessary changes made to the congestion control state, as
suggested in [RFC2883], the data sender ideally needs to determine:
(a) That spurious retransmissions in a particular window of data do
not mask real segment loss (congestion).
For example, assume segments N and N+1 are retransmitted even
though only segment N was dropped by the network (thus, segment
N+1 was needlessly retransmitted). When the sender receives the
notification that segment N+1 arrived more than once it can
conclude that segment N+1 was needlessly resent. However, it
cannot conclude that it is appropriate to revert the congestion
control state because the window of data contained at least one
valid congestion indication (i.e., segment N was lost).
(b) That network duplication is not the cause of the duplicate
Determining whether a duplicate notification is caused by network
duplication of a packet or a spurious retransmit is a nearly
impossible task in theory. Since [Pax97] shows that packet
duplication by the network is rare, the algorithm in this section
simply ceases to function when network duplication is detected
(by receiving a duplication notification for a segment that was
not retransmitted by the sender).
The algorithm specified below gives reasonable, but not complete,
protection against both of these cases.
We assume the TCP sender has a data structure to hold selective
acknowledgment information (e.g., as outlined in [RFC3517]). The
following steps require an extension of such a 'scoreboard' to
incorporate a slightly longer history of retransmissions than called
for in [RFC3517]. The following steps MUST be taken upon the receipt
of each DSACK or duplicate TSN notification:
(A) Check the corresponding sequence range or TSN to determine
whether the segment has been retransmitted.
(A.1) If the SACK scoreboard is empty (i.e., the TCP sender has
received no SACK information from the receiver) and the
left edge of the incoming DSACK is equal to SND.UNA,
processing of this DSACK MUST be terminated and the
congestion control state MUST NOT be reverted during the
current window of data. This clause intends to cover the
case when an entire window of acknowledgments have been
dropped by the network. In such a case, the reverse path
seems to be in a congested state and so reducing TCP's
sending rate is the conservative approach.
(A.2) If the segment was retransmitted exactly one time, mark it
as a duplicate.
(A.3) If the segment was retransmitted more than once processing
of this DSACK MUST be terminated and the congestion control
state MUST NOT be reverted to its previous state during the
current window of data.
(A.4) If the segment was not retransmitted the incoming DSACK
indicates that the network duplicated the segment in
question. Processing of this DSACK MUST be terminated. In
addition, the algorithm specified in this document MUST NOT
be used for the remainder of the connection, as future
DSACK reports may be indicating network duplication rather
than unnecessary retransmission. Note that some techniques
to further disambiguate network duplication from
unnecessary retransmission (e.g., the TCP timestamp option
[RFC1323]) may be used to refine the algorithm in this
document further. Using such a technique in conjunction
with an algorithm similar to the one presented herein may
allow for the continued use of the algorithm in the face of
duplicated segments. We do not delve into such an
algorithm in this document due the current rarity of
network duplication. However, future work should include
tackling this problem.
(B) Assuming processing is allowed to continue (per the (A) rules),
check all retransmitted segments in the previous window of data.
(B.1) If all segments or chunks marked as retransmitted have also
been marked as acknowledged and duplicated, we conclude
that all retransmissions in the previous window of data
were spurious and no loss occurred.
(B.2) If any segment or chunk is still marked as retransmitted
but not marked as duplicate, there are outstanding
retransmissions that could indicate loss within this window
of data. We can make no conclusions based on this
particular DSACK/duplicate TSN notification.
In addition to keeping the state mentioned in [RFC3517] (for TCP) and
[RFC2960] (for SCTP), an implementation of this algorithm must track
all sequence numbers or TSNs that have been acknowledged as
4. Related Work
In addition to the mechanism for detecting spurious retransmits
outlined in this document, several other proposals for finding
needless retransmits have been developed.
[BA02] uses the algorithm outlined in this document as the basis for
investigating several methods to make TCP more robust to reordered
The Eifel detection algorithm [RFC3522] uses the TCP timestamp option
[RFC1323] to determine whether the ACK for a given retransmit is for
the original transmission or a retransmission. More generally,
[LK00] outlines the benefits of detecting spurious retransmits and
reverting from needless congestion control changes using the
timestamp-based scheme or a mechanism that uses a "retransmit bit" to
flag retransmits (and ACKs of retransmits). The Eifel detection
algorithm can detect spurious retransmits more rapidly than a DSACK-
based scheme. However, the tradeoff is that the overhead of the 12-
byte timestamp option must be incurred in every packet transmitted
for Eifel to function.
The F-RTO scheme [SK03] slightly alters TCP's sending pattern
immediately following a retransmission timeout and then observes the
pattern of the returning ACKs. This pattern can indicate whether the
retransmitted segment was needed. The advantage of F-RTO is that the
algorithm only needs to be implemented on the sender side of the TCP
connection and that nothing extra needs to cross the network (e.g.,
DSACKs, timestamps, special flags, etc.). The downside is that the
algorithm is a heuristic that can be confused by network pathologies
(e.g., duplication or reordering of key packets). Finally, note that
F-RTO only works for spurious retransmits triggered by the
transport's retransmission timer.
Finally, [AP99] briefly investigates using the time between
retransmitting a segment via the retransmission timeout and the
arrival of the next ACK as an indicator of whether the retransmit was
needed. The scheme compares this time delta with a fraction (f) of
the minimum RTT observed thus far on the connection. If the time
delta is less than f*minRTT then the retransmit is labeled spurious.
When f=1/2 the algorithm identifies roughly 59% of the needless
retransmission timeouts and identifies needed retransmits only 2.5%
of the time. As with F-RTO, this scheme only detects spurious
retransmits sent by the transport's retransmission timer.
5. Security Considerations
It is possible for the receiver to falsely indicate spurious
retransmissions in the case of actual loss, potentially causing a TCP
or SCTP sender to inaccurately conclude that no loss took place (and
possibly cause inappropriate changes to the senders congestion
Consider the following scenario: A receiver watches every segment or
chunk that arrives and acknowledges any segment that arrives out of
order by more than some threshold amount as a duplicate, assuming
that it is a retransmission. A sender using the above algorithm will
assume that the retransmission was spurious.
The ECN nonce sum proposal [RFC3540] could possibly help mitigate the
ability of the receiver to hide real losses from the sender with
modest extension. In the common case of receiving an original
transmission and a spurious retransmit a receiver will have received
the nonce from the original transmission and therefore can "prove" to
the sender that the duplication notification is valid. In the case
when the receiver did not receive the original and is trying to
improperly induce the sender into transmitting at an inappropriately
high rate, the receiver will not know the ECN nonce from the original
segment and therefore will probabilistically not be able to fool the
sender for long. [RFC3540] calls for disabling nonce sums on
duplicate ACKs, which means that the nonce sum is not directly
suitable for use as a mitigation to the problem of receivers lying
about DSACK information. However, future efforts may be able to use
[RFC3540] as a starting point for building protection should it be
Sourabh Ladha and Reiner Ludwig made several useful comments on an
earlier version of this document. The second author thanks BBN
Technologies and NASA's Glenn Research Center for supporting this
7.1. Normative References
[RFC793] Postel, J., "Transmission Control Protocol", STD 7, RFC
793, September 1981.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2883] Floyd, S., Mahdavi, J., Mathis, M. and M. Podolsky, "An
Extension to the Selective Acknowledgement (SACK) Option
for TCP", RFC 2883, July 2000.
[RFC2960] Stewart, R., Xie, Q., Morneault, K., Sharp, C.,
Schwarzbauer, H., Taylor, T., Rytina, I., Kalla, M., Zhang,
L. and V. Paxson, "Stream Control Transmission Protocol",
RFC 2960, October 2000.
7.2. Informative References
[AAAB03] Allman, M., Avrachenkov, K., Ayesta, U. and J. Blanton,
"Early Retransmit for TCP", Work in Progress, June 2003.
[AEO03] Allman, M., Eddy, E. and S. Ostermann, "Estimating Loss
Rates With TCP", Work in Progress, August 2003.
[AP99] Allman, M. and V. Paxson, "On Estimating End-to-End Network
Path Properties", SIGCOMM 99.
[BA02] Blanton, E. and M. Allman. On Making TCP More Robust to
Packet Reordering. ACM Computer Communication Review,
32(1), January 2002.
[BDA03] Blanton, E., Dimond, R. and M. Allman, "Practices for TCP
Senders in the Face of Segment Reordering", Work in
Progress, February 2003.
[EOA03] Eddy, W., Ostermann, S. and M. Allman, "New Techniques for
Making Transport Protocols Robust to Corruption-Based
Loss", Work in Progress, July 2003.
[LK00] R. Ludwig, R. H. Katz. The Eifel Algorithm: Making TCP
Robust Against Spurious Retransmissions. ACM Computer
Communication Review, 30(1), January 2000.
[Pax97] V. Paxson. End-to-End Internet Packet Dynamics. In ACM
SIGCOMM, September 1997.
[RFC1323] Jacobson, V., Braden, R. and D. Borman, "TCP Extensions
for High Performance", RFC 1323, May 1992.
[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.
[RFC3522] Ludwig, R. and M. Meyer, "The Eifel Detection Algorithm for
TCP," RFC 3522, April 2003.
[RFC3540] Spring, N., Wetherall, D. and D. Ely, "Robust Explicit
Congestion Notification (ECN) Signaling with Nonces", RFC
3540, June 2003.
[SK03] Sarolahti, P. and M. Kojo, "F-RTO: An Algorithm for
Detecting Spurious Retransmission Timeouts with TCP and
SCTP", Work in Progress, June 2003.
8. Authors' Addresses
Purdue University Computer Sciences
1398 Computer Science Building
West Lafayette, IN 47907
ICSI Center for Internet Research
1947 Center Street, Suite 600
Berkeley, CA 94704-1198
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