Network Working Group M. Allman
Request for Comments: 3390 BBN/NASA GRC
Obsoletes: 2414 S. Floyd
Updates: 2581 ICIR
Category: Standards Track C. Partridge
October 2002 Increasing TCP's Initial Window
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 (C) The Internet Society (2002). All Rights Reserved.
This document specifies an optional standard for TCP to increase the
permitted initial window from one or two segment(s) to roughly 4K
bytes, replacing RFC 2414. It discusses the advantages and
disadvantages of the higher initial window, and includes discussion
of experiments and simulations showing that the higher initial window
does not lead to congestion collapse. Finally, this document
provides guidance on implementation issues.
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 obsoletes [RFC2414] and updates [RFC2581] and specifies
an increase in the permitted upper bound for TCP's initial window
from one or two segment(s) 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 may be significantly larger than 4K
The upper bound for the initial window is given more precisely in
min (4*MSS, max (2*MSS, 4380 bytes)) (1)
Note: Sending a 1500 byte packet indicates a maximum segment size
(MSS) of 1460 bytes (assuming no IP or TCP options). Therefore,
limiting the initial window's MSS to 4380 bytes allows the sender to
transmit three segments initially in the common case when using 1500
Equivalently, the upper bound for the initial window size is based on
the 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: a TCP MAY start with a
larger initial window. However, we expect that most general-purpose
TCP implementations would choose to use the larger initial congestion
window given in equation (1) above.
This upper bound for the initial window size represents a change from
RFC 2581 [RFC2581], which specified that the congestion window be
initialized to one or two segments.
This change applies to the initial window of the connection in the
first round trip time (RTT) of data 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 consisting of MSS bytes.
TCP implementations use slow start in as many as three different
ways: (1) to start a new connection (the initial window); (2) to
restart transmission after a long idle period (the restart window);
and (3) to restart transmission after a retransmit timeout (the loss
window). The change specified 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 of MSS bytes (to
permit the lowest possible window size in the case of severe
2. Implementation Issues
When larger initial windows are implemented along with Path MTU
Discovery [RFC1191], 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 [RFC1191], 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 as
to which of these two alternatives is best; we would hope that
implementation experiences will shed light on this question. 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 specified in this document is not intended
as 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, they are
working against TCP's congestion control mechanisms [FF99],
regardless of the size of the initial window. Combining this
behavior with larger initial windows further increases the unfairness
to other traffic in the network. We suggest the use of HTTP/1.1
[RFC2068] (persistent TCP connections and pipelining) as a way to
achieve better performance of web transfers.
3. Advantages of Larger Initial Windows
1. When the initial window is one segment, a receiver employing
delayed ACKs [RFC1122] 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, and possibly up to 500 msec [RFC1122]).
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 [RFC1945, RFC2068]) 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
will 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 larger
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 judgment is that
while there are dangers of congestion collapse in the current
Internet (see [FF99] 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. Interactions with the Retransmission Timer
Using a larger initial burst of data can exacerbate existing problems
with spurious retransmit timeouts on low-bandwidth paths, assuming
the standard algorithm for determining the TCP retransmission timeout
(RTO) [RFC2988]. The problem is that across low-bandwidth network
paths on which the transmission time of a packet is a large portion
of the round-trip time, the small packets used to establish a TCP
connection do not seed the RTO estimator appropriately. When the
first window of data packets is transmitted, the sender's retransmit
timer could expire before the acknowledgments for those packets are
received. As each acknowledgment arrives, the retransmit timer is
generally reset. Thus, the retransmit timer will not expire as long
as an acknowledgment arrives at least once a second, given the one-
second minimum on the RTO recommended in RFC 2988.
For instance, consider a 9.6 Kbps link. The initial RTT measurement
will be on the order of 67 msec, if we simply consider the
transmission time of 2 packets (the SYN and SYN-ACK), each consisting
of 40 bytes. Using the RTO estimator given in [RFC2988], this yields
an initial RTO of 201 msec (67 + 4*(67/2)). However, we round the
RTO to 1 second as specified in RFC 2988. Then assume we send an
initial window of one or more 1500-byte packets (1460 data bytes plus
overhead). Each packet will take on the order of 1.25 seconds to
transmit. Therefore, the RTO will fire before the ACK for the first
packet returns, causing a spurious timeout. In this case, a larger
initial window of three or four packets exacerbates the problems
caused by this spurious timeout.
One way to deal with this problem is to make the RTO algorithm more
conservative. During the initial window of data, for instance, the
RTO could be updated for each acknowledgment received. In addition,
if the retransmit timer expires for some packet lost in the first
window of data, we could leave the exponential-backoff of the
retransmit timer engaged until at least one valid RTT measurement,
that involves a data packet, is received.
Another method would be to refrain from taking an RTT sample during
connection establishment, leaving the default RTO in place until TCP
takes a sample from a data segment and the corresponding ACK. While
this method likely helps prevent spurious retransmits, it also may
slow the data transfer down if loss occurs before the RTO is seeded.
The use of limited transmit [RFC3042] to aid a TCP connection in
recovering from loss using fast retransmit rather than the RTO timer
mitigates the performance degradation caused by using the high
default RTO during the initial window of data transmission.
This specification leaves the decision about what to do (if anything)
with regards to the RTO, when using a larger initial window, to the
implementer. However, the RECOMMENDED approach is to refrain from
sampling the RTT during the three-way handshake, keeping the default
RTO in place until an RTT sample involving a data packet is taken.
In addition, it is RECOMMENDED that TCPs use limited transmit
7. 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.
8. Simulations and Experimental Results
8.1 Studies of TCP Connections using that Larger Initial Window
This section surveys simulations and experiments that explore the
effect of larger initial windows on TCP connections. The first set
of experiments explores performance over satellite links. Larger
initial windows have been shown to improve the 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 [Nic98].
A second set of experiments 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 simulation study [RFC2416] 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.
Finally, [All00] illustrates that the percentage of connections at a
particular web server that experience loss in the initial window of
data transmission increases with the size of the initial congestion
window. However, the increase is in line with what would be expected
from sending a larger burst into the network.
8.2 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.
A simulation study in [RFC2415] 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 highly
congested network with transfers of 20K in size. 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 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
9. 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 specifies a small change to TCP that will likely 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 and for feedback on this document.
[AHO98] Mark Allman, Chris Hayes, and Shawn Ostermann, An
Evaluation of TCP with Larger Initial Windows, March 1998.
ACM Computer Communication Review, 28(3), July 1998. URL
[All97a] Mark Allman. An Evaluation of TCP with Larger Initial
Windows. 40th IETF Meeting -- TCP Implementations WG.
December, 1997. Washington, DC.
[All97b] Mark Allman. Improving TCP Performance Over Satellite
Channels. Master's thesis, Ohio University, June 1997.
[All00] Mark Allman. A Web Server's View of the Transport Layer.
ACM Computer Communication Review, 30(5), October 2000.
[FF96] Fall, K., and Floyd, S., Simulation-based Comparisons of
Tahoe, Reno, and SACK TCP. Computer Communication Review,
26(3), July 1996.
[FF99] Sally Floyd, Kevin Fall. Promoting the Use of End-to-End
Congestion Control in the Internet. IEEE/ACM Transactions
on Networking, August 1999. URL
[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.
[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
[Mor97] Robert Morris. Private communication, 1997. Cited for
acknowledgement purposes only.
[Nic98] Kathleen Nichols. Improving Network Simulation With
Feedback, Proceedings of LCN 98, October 1998. URL
[Pos82] Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC
821, August 1982.
[RFC1122] Braden, R., "Requirements for Internet Hosts --
Communication Layers", STD 3, RFC 1122, October 1989.
[RFC1191] Mogul, J. and S. Deering, "Path MTU Discovery", RFC 1191,
[RFC1945] Berners-Lee, T., Fielding, R. and H. Nielsen, "Hypertext
Transfer Protocol -- HTTP/1.0", RFC 1945, May 1996.
[RFC2068] Fielding, R., Mogul, J., Gettys, J., Frystyk, H. and T.
Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1", RFC
2616, January 1997.
[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.
[RFC2414] Allman, M., Floyd, S. and C. Partridge, "Increasing TCP's
Initial Window", RFC 2414, September 1998.
[RFC2415] Poduri, K. and K. Nichols, "Simulation Studies of Increased
Initial TCP Window Size", RFC 2415, September 1998.
[RFC2416] Shepard, T. and C. Partridge, "When TCP Starts Up With Four
Packets Into Only Three Buffers", RFC 2416, September 1998.
[RFC2581] Allman, M., Paxson, V. and W. Stevens, "TCP Congestion
Control", RFC 2581, April 1999.
[RFC2821] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
[RFC2988] Paxson, V. and M. Allman, "Computing TCP's Retransmission
Timer", RFC 2988, November 2000.
[RFC3042] Allman, M., Balakrishnan, H. and S. Floyd, "Enhancing TCP's
Loss Recovery Using Limited Transmit", RFC 3042, January
[RFC3168] Ramakrishnan, K.K., Floyd, S. and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP", RFC 3168,
Appendix A - Duplicate Segments
In the current environment (without Explicit Congestion Notification
[Flo94] [RFC2481]), 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 arrived 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 by 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, in the absence of Limited Transmit [RFC3042]. However,
dropping the third segment will always allow recovery via the fast
retransmit algorithm, as long as no ACKs are lost.
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 in
which more than one duplicate segment will be transmitted. Our
conclusion is than the number of duplicate segments transmitted as a
result of a larger initial window should be small.
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