|Title||Increasing TCP's Initial Window
|Author||M. Allman, S. Floyd, C. Partridge
Network Working Group M. Allman
Request for Comments: 2414 NASA Lewis/Sterling Software
Category: Experimental S. Floyd
Increasing TCP's Initial Window
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 (1998). All Rights Reserved.
This document specifies an increase in the permitted initial window
for TCP from one segment to roughly 4K bytes. This document
discusses the advantages and disadvantages of such a change,
outlining experimental results that indicate the costs and benefits
of such a change to TCP.
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].
1. TCP Modification
This document specifies an increase in the permitted upper bound for
TCP's initial window from one segment to between two and four
segments. In most cases, this change results in an upper bound on
the initial window of roughly 4K bytes (although given a large
segment size, the permitted initial window of two segments could be
significantly larger than 4K bytes). The upper bound for the initial
window is given more precisely in (1):
min (4*MSS, max (2*MSS, 4380 bytes)) (1)
Equivalently, the upper bound for the initial window size is based on
the maximum segment size (MSS), as follows:
If (MSS <= 1095 bytes)
then win <= 4 * MSS;
If (1095 bytes < MSS < 2190 bytes)
then win <= 4380;
If (2190 bytes <= MSS)
then win <= 2 * MSS;
This increased initial window is optional: that a TCP MAY start with
a larger initial window, not that it SHOULD.
This upper bound for the initial window size represents a change from
RFC 2001 [S97], which specifies that the congestion window be
initialized to one segment. If implementation experience proves
successful, then the intent is for this change to be incorporated
into a revision to RFC 2001.
This change applies to the initial window of the connection in the
first round trip time (RTT) of transmission following the TCP three-
way handshake. Neither the SYN/ACK nor its acknowledgment (ACK) in
the three-way handshake should increase the initial window size above
that outlined in equation (1). If the SYN or SYN/ACK is lost, the
initial window used by a sender after a correctly transmitted SYN
MUST be one segment.
TCP implementations use slow start in as many as three different
ways: (1) to start a new connection (the initial window); (2) to
restart a transmission after a long idle period (the restart window);
and (3) to restart after a retransmit timeout (the loss window). The
change proposed in this document affects the value of the initial
window. Optionally, a TCP MAY set the restart window to the minimum
of the value used for the initial window and the current value of
cwnd (in other words, using a larger value for the restart window
should never increase the size of cwnd). These changes do NOT change
the loss window, which must remain 1 segment (to permit the lowest
possible window size in the case of severe congestion).
2. Implementation Issues
When larger initial windows are implemented along with Path MTU
Discovery [MD90], and the MSS being used is found to be too large,
the congestion window `cwnd' SHOULD be reduced to prevent large
bursts of smaller segments. Specifically, `cwnd' SHOULD be reduced
by the ratio of the old segment size to the new segment size.
When larger initial windows are implemented along with Path MTU
Discovery [MD90], alternatives are to set the "Don't Fragment" (DF)
bit in all segments in the initial window, or to set the "Don't
Fragment" (DF) bit in one of the segments. It is an open question
which of these two alternatives is best; we would hope that
implementation experiences will shed light on this. In the first
case of setting the DF bit in all segments, if the initial packets
are too large, then all of the initial packets will be dropped in the
network. In the second case of setting the DF bit in only one
segment, if the initial packets are too large, then all but one of
the initial packets will be fragmented in the network. When the
second case is followed, setting the DF bit in the last segment in
the initial window provides the least chance for needless
retransmissions when the initial segment size is found to be too
large, because it minimizes the chances of duplicate ACKs triggering
a Fast Retransmit. However, more attention needs to be paid to the
interaction between larger initial windows and Path MTU Discovery.
The larger initial window proposed in this document is not intended
as an encouragement for web browsers to open multiple simultaneous
TCP connections all with large initial windows. When web browsers
open simultaneous TCP connections to the same destination, this works
against TCP's congestion control mechanisms [FF98], regardless of the
size of the initial window. Combining this behavior with larger
initial windows further increases the unfairness to other traffic in
3. Advantages of Larger Initial Windows
1. When the initial window is one segment, a receiver employing
delayed ACKs [Bra89] is forced to wait for a timeout before
generating an ACK. With an initial window of at least two
segments, the receiver will generate an ACK after the second data
segment arrives. This eliminates the wait on the timeout (often
up to 200 msec).
2. For connections transmitting only a small amount of data, a
larger initial window reduces the transmission time (assuming at
most moderate segment drop rates). For many email (SMTP [Pos82])
and web page (HTTP [BLFN96, FJGFBL97]) transfers that are less
than 4K bytes, the larger initial window would reduce the data
transfer time to a single RTT.
3. For connections that will be able to use large congestion
windows, this modification eliminates up to three RTTs and a
delayed ACK timeout during the initial slow-start phase. This
would be of particular benefit for high-bandwidth large-
propagation-delay TCP connections, such as those over satellite
4. Disadvantages of Larger Initial Windows for the Individual
In high-congestion environments, particularly for routers that have a
bias against bursty traffic (as in the typical Drop Tail router
queues), a TCP connection can sometimes be better off starting with
an initial window of one segment. There are scenarios where a TCP
connection slow-starting from an initial window of one segment might
not have segments dropped, while a TCP connection starting with an
initial window of four segments might experience unnecessary
retransmits due to the inability of the router to handle small
bursts. This could result in an unnecessary retransmit timeout. For
a large-window connection that is able to recover without a
retransmit timeout, this could result in an unnecessarily-early
transition from the slow-start to the congestion-avoidance phase of
the window increase algorithm. These premature segment drops are
unlikely to occur in uncongested networks with sufficient buffering
or in moderately-congested networks where the congested router uses
active queue management (such as Random Early Detection [FJ93,
Some TCP connections will receive better performance with the higher
initial window even if the burstiness of the initial window results
in premature segment drops. This will be true if (1) the TCP
connection recovers from the segment drop without a retransmit
timeout, and (2) the TCP connection is ultimately limited to a small
congestion window by either network congestion or by the receiver's
5. Disadvantages of Larger Initial Windows for the Network
In terms of the potential for congestion collapse, we consider two
separate potential dangers for the network. The first danger would
be a scenario where a large number of segments on congested links
were duplicate segments that had already been received at the
receiver. The second danger would be a scenario where a large number
of segments on congested links were segments that would be dropped
later in the network before reaching their final destination.
In terms of the negative effect on other traffic in the network, a
potential disadvantage of larger initial windows would be that they
increase the general packet drop rate in the network. We discuss
these three issues below.
As described in the previous section, the larger initial window
could occasionally result in a segment dropped from the initial
window, when that segment might not have been dropped if the
sender had slow-started from an initial window of one segment.
However, Appendix A shows that even in this case, the larger
initial window would not result in the transmission of a large
number of duplicate segments.
Segments dropped later in the network:
How much would the larger initial window for TCP increase the
number of segments on congested links that would be dropped
before reaching their final destination? This is a problem that
can only occur for connections with multiple congested links,
where some segments might use scarce bandwidth on the first
congested link along the path, only to be dropped later along the
First, many of the TCP connections will have only one congested
link along the path. Segments dropped from these connections do
not "waste" scarce bandwidth, and do not contribute to congestion
However, some network paths will have multiple congested links,
and segments dropped from the initial window could use scarce
bandwidth along the earlier congested links before ultimately
being dropped on subsequent congested links. To the extent that
the drop rate is independent of the initial window used by TCP
segments, the problem of congested links carrying segments that
will be dropped before reaching their destination will be similar
for TCP connections that start by sending four segments or one
An increased packet drop rate:
For a network with a high segment drop rate, increasing the TCP
initial window could increase the segment drop rate even further.
This is in part because routers with Drop Tail queue management
have difficulties with bursty traffic in times of congestion.
However, given uncorrelated arrivals for TCP connections, the
larger TCP initial window should not significantly increase the
segment drop rate. Simulation-based explorations of these issues
are discussed in Section 7.2.
These potential dangers for the network are explored in simulations
and experiments described in the section below. Our judgement would
be, while there are dangers of congestion collapse in the current
Internet (see [FF98] for a discussion of the dangers of congestion
collapse from an increased deployment of UDP connections without
end-to-end congestion control), there is no such danger to the
network from increasing the TCP initial window to 4K bytes.
6. Typical Levels of Burstiness for TCP Traffic.
Larger TCP initial windows would not dramatically increase the
burstiness of TCP traffic in the Internet today, because such traffic
is already fairly bursty. Bursts of two and three segments are
already typical of TCP [Flo97]; A delayed ACK (covering two
previously unacknowledged segments) received during congestion
avoidance causes the congestion window to slide and two segments to
be sent. The same delayed ACK received during slow start causes the
window to slide by two segments and then be incremented by one
segment, resulting in a three-segment burst. While not necessarily
typical, bursts of four and five segments for TCP are not rare.
Assuming delayed ACKs, a single dropped ACK causes the subsequent ACK
to cover four previously unacknowledged segments. During congestion
avoidance this leads to a four-segment burst and during slow start a
five-segment burst is generated.
There are also changes in progress that reduce the performance
problems posed by moderate traffic bursts. One such change is the
deployment of higher-speed links in some parts of the network, where
a burst of 4K bytes can represent a small quantity of data. A second
change, for routers with sufficient buffering, is the deployment of
queue management mechanisms such as RED, which is designed to be
tolerant of transient traffic bursts.
7. Simulations and Experimental Results
7.1 Studies of TCP Connections using that Larger Initial Window
This section surveys simulations and experiments that have been used
to explore the effect of larger initial windows on the TCP connection
using that larger window. The first set of experiments explores
performance over satellite links. Larger initial windows have been
shown to improve performance of TCP connections over satellite
channels [All97b]. In this study, an initial window of four segments
(512 byte MSS) resulted in throughput improvements of up to 30%
(depending upon transfer size). [KAGT98] shows that the use of
larger initial windows results in a decrease in transfer time in HTTP
tests over the ACTS satellite system. A study involving simulations
of a large number of HTTP transactions over hybrid fiber coax (HFC)
indicates that the use of larger initial windows decreases the time
required to load WWW pages [Nic97].
A second set of experiments has explored TCP performance over dialup
modem links. In experiments over a 28.8 bps dialup channel [All97a,
AHO98], a four-segment initial window decreased the transfer time of
a 16KB file by roughly 10%, with no accompanying increase in the drop
rate. A particular area of concern has been TCP performance over low
speed tail circuits (e.g., dialup modem links) with routers with
small buffers. A simulation study [SP97] investigated the effects of
using a larger initial window on a host connected by a slow modem
link and a router with a 3 packet buffer. The study concluded that
for the scenario investigated, the use of larger initial windows was
not harmful to TCP performance. Questions have been raised
concerning the effects of larger initial windows on the transfer time
for short transfers in this environment, but these effects have not
been quantified. A question has also been raised concerning the
possible effect on existing TCP connections sharing the link.
7.2 Studies of Networks using Larger Initial Windows
This section surveys simulations and experiments investigating the
impact of the larger window on other TCP connections sharing the
path. Experiments in [All97a, AHO98] show that for 16 KB transfers
to 100 Internet hosts, four-segment initial windows resulted in a
small increase in the drop rate of 0.04 segments/transfer. While the
drop rate increased slightly, the transfer time was reduced by
roughly 25% for transfers using the four-segment (512 byte MSS)
initial window when compared to an initial window of one segment.
One scenario of concern is heavily loaded links. For instance, a
couple of years ago, one of the trans-Atlantic links was so heavily
loaded that the correct congestion window size for a connection was
about one segment. In this environment, new connections using larger
initial windows would be starting with windows that were four times
too big. What would the effects be? Do connections thrash?
A simulation study in [PN98] explores the impact of a larger initial
window on competing network traffic. In this investigation, HTTP and
FTP flows share a single congested gateway (where the number of HTTP
and FTP flows varies from one simulation set to another). For each
simulation set, the paper examines aggregate link utilization and
packet drop rates, median web page delay, and network power for the
FTP transfers. The larger initial window generally resulted in
increased throughput, slightly-increased packet drop rates, and an
increase in overall network power. With the exception of one
scenario, the larger initial window resulted in an increase in the
drop rate of less than 1% above the loss rate experienced when using
a one-segment initial window; in this scenario, the drop rate
increased from 3.5% with one-segment initial windows, to 4.5% with
four-segment initial windows. The overall conclusions were that
increasing the TCP initial window to three packets (or 4380 bytes)
helps to improve perceived performance.
Morris [Mor97] investigated larger initial windows in a very
congested network with transfers of size 20K. The loss rate in
networks where all TCP connections use an initial window of four
segments is shown to be 1-2% greater than in a network where all
connections use an initial window of one segment. This relationship
held in scenarios where the loss rates with one-segment initial
windows ranged from 1% to 11%. In addition, in networks where
connections used an initial window of four segments, TCP connections
spent more time waiting for the retransmit timer (RTO) to expire to
resend a segment than was spent when using an initial window of one
segment. The time spent waiting for the RTO timer to expire
represents idle time when no useful work was being accomplished for
that connection. These results show that in a very congested
environment, where each connection's share of the bottleneck
bandwidth is close to one segment, using a larger initial window can
cause a perceptible increase in both loss rates and retransmit
8. Security Considerations
This document discusses the initial congestion window permitted for
TCP connections. Changing this value does not raise any known new
security issues with TCP.
This document proposes a small change to TCP that may be beneficial
to short-lived TCP connections and those over links with long RTTs
(saving several RTTs during the initial slow-start phase).
We would like to acknowledge Vern Paxson, Tim Shepard, members of the
End-to-End-Interest Mailing List, and members of the IETF TCP
Implementation Working Group for continuing discussions of these
issues for discussions and feedback on this document.
[All97a] Mark Allman. An Evaluation of TCP with Larger Initial
Windows. 40th IETF Meeting -- TCP Implementations WG.
December, 1997. Washington, DC.
[AHO98] Mark Allman, Chris Hayes, and Shawn Ostermann, An
Evaluation of TCP with Larger Initial Windows, March
1998. Submitted to ACM Computer Communication Review.
[All97b] Mark Allman. Improving TCP Performance Over Satellite
Channels. Master's thesis, Ohio University, June 1997.
[BLFN96] Berners-Lee, T., Fielding, R., and H. Nielsen, "Hypertext
Transfer Protocol -- HTTP/1.0", RFC 1945, May 1996.
[Bra89] Braden, R., "Requirements for Internet Hosts --
Communication Layers", STD 3, RFC 1122, October 1989.
[FF96] Fall, K., and Floyd, S., Simulation-based Comparisons of
Tahoe, Reno, and SACK TCP. Computer Communication
Review, 26(3), July 1996.
[FF98] Sally Floyd, Kevin Fall. Promoting the Use of End-to-End
Congestion Control in the Internet. Submitted to IEEE
Transactions on Networking. URL "http://www-
[FJGFBL97] Fielding, R., Mogul, J., Gettys, J., Frystyk, H., and T.
Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1",
RFC 2068, January 1997.
[FJ93] Floyd, S., and Jacobson, V., Random Early Detection
gateways for Congestion Avoidance. IEEE/ACM Transactions
on Networking, V.1 N.4, August 1993, p. 397-413.
[Flo94] Floyd, S., TCP and Explicit Congestion Notification.
Computer Communication Review, 24(5):10-23, October 1994.
[Flo96] Floyd, S., Issues of TCP with SACK. Technical report,
January 1996. Available from http://www-
[Flo97] Floyd, S., Increasing TCP's Initial Window. Viewgraphs,
40th IETF Meeting - TCP Implementations WG. December,
1997. URL "ftp://ftp.ee.lbl.gov/talks/sf-tcp-ietf97.ps".
[KAGT98] Hans Kruse, Mark Allman, Jim Griner, Diepchi Tran. HTTP
Page Transfer Rates Over Geo-Stationary Satellite Links.
March 1998. Proceedings of the Sixth International
Conference on Telecommunication Systems. URL
[MD90] Mogul, J., and S. Deering, "Path MTU Discovery", RFC
1191, November 1990.
[MMFR96] Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP
Selective Acknowledgment Options", RFC 2018, October
[Mor97] Robert Morris. Private communication, 1997. Cited for
acknowledgement purposes only.
[Nic97] Kathleen Nichols. Improving Network Simulation with
Feedback. Com21, Inc. Technical Report. Available from
[PN98] Poduri, K., and K. Nichols, "Simulation Studies of
Increased Initial TCP Window Size", RFC 2415, September
[Pos82] Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC
821, August 1982.
[RF97] Ramakrishnan, K., and S. Floyd, "A Proposal to Add
Explicit Congestion Notification (ECN) to IPv6 and to
TCP", Work in Progress.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2309] Braden, B., Clark, D., Crowcroft, J., Davie, B., Deering,
S., Estrin, D., Floyd, S., Jacobson, V., Minshall, G.,
Partridge, C., Peterson, L., Ramakrishnan, K., Shenker,
S., Wroclawski, J., and L. Zhang, "Recommendations on
Queue Management and Congestion Avoidance in the
Internet", RFC 2309, April 1998.
[S97] Stevens, W., "TCP Slow Start, Congestion Avoidance, Fast
Retransmit, and Fast Recovery Algorithms", RFC 2001,
[SP97] Shepard, T., and C. Partridge, "When TCP Starts Up With
Four Packets Into Only Three Buffers", RFC 2416,
12. Author's Addresses
NASA Lewis Research Center/Sterling Software
21000 Brookpark Road
Cleveland, OH 44135
Lawrence Berkeley National Laboratory
One Cyclotron Road
Berkeley, CA 94720
10 Moulton Street
Cambridge, MA 02138
13. Appendix - Duplicate Segments
In the current environment (without Explicit Congestion Notification
[Flo94] [RF97]), all TCPs use segment drops as indications from the
network about the limits of available bandwidth. We argue here that
the change to a larger initial window should not result in the sender
retransmitting a large number of duplicate segments that have already
been received at the receiver.
If one segment is dropped from the initial window, there are three
different ways for TCP to recover: (1) Slow-starting from a window of
one segment, as is done after a retransmit timeout, or after Fast
Retransmit in Tahoe TCP; (2) Fast Recovery without selective
acknowledgments (SACK), as is done after three duplicate ACKs in Reno
TCP; and (3) Fast Recovery with SACK, for TCP where both the sender
and the receiver support the SACK option [MMFR96]. In all three
cases, if a single segment is dropped from the initial window, no
duplicate segments (i.e., segments that have already been received at
the receiver) are transmitted. Note that for a TCP sending four
512-byte segments in the initial window, a single segment drop will
not require a retransmit timeout, but can be recovered from using the
Fast Retransmit algorithm (unless the retransmit timer expires
prematurely). In addition, a single segment dropped from an initial
window of three segments might be repaired using the fast retransmit
algorithm, depending on which segment is dropped and whether or not
delayed ACKs are used. For example, dropping the first segment of a
three segment initial window will always require waiting for a
timeout. However, dropping the third segment will always allow
recovery via the fast retransmit algorithm, as long as no ACKs are
Next we consider scenarios where the initial window contains two to
four segments, and at least two of those segments are dropped. If
all segments in the initial window are dropped, then clearly no
duplicate segments are retransmitted, as the receiver has not yet
received any segments. (It is still a possibility that these dropped
segments used scarce bandwidth on the way to their drop point; this
issue was discussed in Section 5.)
When two segments are dropped from an initial window of three
segments, the sender will only send a duplicate segment if the first
two of the three segments were dropped, and the sender does not
receive a packet with the SACK option acknowledging the third
When two segments are dropped from an initial window of four
segments, an examination of the six possible scenarios (which we
don't go through here) shows that, depending on the position of the
dropped packets, in the absence of SACK the sender might send one
duplicate segment. There are no scenarios in which the sender sends
two duplicate segments.
When three segments are dropped from an initial window of four
segments, then, in the absence of SACK, it is possible that one
duplicate segment will be sent, depending on the position of the
The summary is that in the absence of SACK, there are some scenarios
with multiple segment drops from the initial window where one
duplicate segment will be transmitted. There are no scenarios where
more that one duplicate segment will be transmitted. Our conclusion
is that the number of duplicate segments transmitted as a result of a
larger initial window should be small.
14. Full Copyright Statement
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