One document matched: draft-floyd-incr-init-win-01.txt
Differences from draft-floyd-incr-init-win-00.txt
Internet Engineering Task Force Mark Allman
INTERNET DRAFT NASA Lewis/Sterling Software
File: draft-floyd-incr-init-win-01.txt Sally Floyd
LBL
Craig Partridge
BBN Technologies
March, 1998
Expires: September, 1998
Increasing TCP's Initial Window
Status of this Memo
This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups. Note that other groups may also distribute
working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six
months and may be updated, replaced, or obsoleted by other documents
at any time. It is inappropriate to use Internet-Drafts as
reference material or to cite them other than as ``work in
progress.''
To learn the current status of any Internet-Draft, please check the
``1id-abstracts.txt'' listing contained in the Internet- Drafts
Shadow Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe),
munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or
ftp.isi.edu (US West Coast).
Abstract
This is a note to suggest changing the permitted initial window in
TCP from 1 segment to roughly 4K bytes. This draft considers the
advantages and disadvantages of such a change, as well as outlining
some experimental results that indicate the costs and benefits of
making such a change to TCP, and pointing out remaining research
questions.
1. TCP Modification
This draft suggests allowing the initial window used by a TCP
connection to increase from 1 segment to between 2 and 4 segments.
In most cases, this will result in an initial window of roughly 4K
bytes (although given a large segment size, the initial window could
be significantly larger than 4K bytes). The proposed initial window
size is given in (1):
min (4*MSS, max (2*MSS, 4380 bytes)) (1)
Allman [Page 1]
March 1998
Or, more specifically the initial window size is based on the
maximum segment size (MSS), as follows:
MSS <= 1095 bytes:
win = 4 * MSS
1095 bytes < MSS < 2190 bytes:
win = 4380
MSS => 2190 bytes:
win = 2 * MSS
This increased initial window would be optional: that a TCP MAY
start with a larger initial window, not that it SHOULD.
This change would only apply to the initial window of the
connection, in the first round trip time (RTT) of transmission
following the TCP three-way handshake. That is, the SYN/ACK in the
three way handshake should not increase the initial window size
above that outlined in equation (1). However, if the SYN or SYN/ACK
is lost the initial window used after a correctly transmitted SYN
MUST be 1 segment.
Some TCP implementations use slow start to re-start transmission
after a long idle period. In this case, the initial window used
should be the same as the initial window used at the beginning of
the transfer. The change proposed in this document would not change
the behavior after a retransmit timeout, when the sender would
continue to slow start from an initial window of one segment.
2. Advantages of Larger Initial Windows
1. For connections transmitting only a small amount of data, a
larger initial window would reduce the transmission time
(assuming 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.
2. 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 links.
3. When the initial window is 1 segment, a receiver employing
delayed acknowledgments (ACK) [Bra89] is forced to wait for a
timeout before generating an ACK. With a larger initial window,
the receiver will be able to generate an ACK after the second
data segment arrives. This eliminates the need to wait on the
timeout (0.1 seconds, or more).
Allman [Page 2]
March 1998
3. Implementation Issues
When larger initial windows are implemented along with Path MTU
Discovery [MD90], only one of the segments in the initial window
should have the "Don't Fragment" (DF) bit set. Preliminary analysis
indicates that setting the DF bit in the last segment in the initial
window provides the least chance for needless retransmissions and
large line-rate bursts of segments when the initial segment size is
found to be too large. In addition, if the MSS being used is found
to be too large, the 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. 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 SHOULD NOT be
viewed as an encouragement for web browsers to open multiple
simultaneous TCP connections all with larger initial windows. (Web
browsers should not open four simultaneous TCP connections to the
same destination in any case, because this works against TCP's
congestion control mechanisms [FF98]).
4. Disadvantages of Larger Initial Windows for the Individual
Connection
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 should
not happen in uncongested networks, or in moderately-congested
networks where the congested router used 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
advertised window.
5. Disadvantages of Larger Initial Windows for the Network
We consider two separate potential dangers for the network. The
first danger would be a scenario where a large number of segments on
Allman [Page 3]
March 1998
congested links were duplicate or unnecessarily-retransmitted
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.
Unnecessarily-retransmitted segments:
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 a large number of
unnecessarily-retransmitted 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 path.
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 collapse.
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 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
segment.
For a network with a high segment drop rate, increasing the
initial TCP congestion 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, this should be a second order
effect. Given uncorrelated arrivals for TCP connections, the
larger initial TCP congestion window should generally not
significantly increase the segment drop rate.
6. Network Changes
There are other changes in the network that make a larger initial
window less of a problem. These include the increasing deployment
Allman [Page 4]
March 1998
of higher-speed links where 4K bytes is a rather small quantity of
data and the deployment of queue management mechanisms such as RED
that are more tolerant of transient traffic bursts. The current
dangers of congestion collapse most likely now come not from a 4K
initial burst from TCP connections, but from the increased
deployment of UDP connections without end-to-end congestion control.
7. Concerns
All the experiments (see section 8) with larger initial windows have
tested how the larger window affects the TCP connection that uses
the larger window. No one has thoroughly studied the impact of the
larger window on other TCP connections. In particular, no one has a
thorough set of answers about what happens when a TCP bursts a
larger initial window into or across a path already being shared by
a set of established TCP connections.
Part of the reason for this omission is the assumption that the
effect is small. For example, in much of the Internet bursts of 2
and 3 segments are common and bursts of 4 and 5 segments are not
rare. A delayed ACK (covering two previously unacknowledged
segments) received during congestion avoidance causes the window to
slide and 2 segments to be sent. The same delayed ACK received
during slow start causes the window to slide by 2 segments and then
be incremented by 1 segment, leading to a 3 segment burst. Assuming
delayed ACKs, a single dropped ACK causes the subsequent ACK to
cover 4 previously unacknowledged segments. During congestion
avoidance this leads to a 4 segment burst and during slow start a 5
segment burst is generated.
However, there are some common scenarios where a larger initial
window might have an effect. One example is low speed tail circuits
with routers with small buffers. For instance, imagine a dialup
link connecting routers each of which have a handful of buffers.
Further imagine the link is already being shared by a few TCP
connections. Then a new connection launches a large initial window,
causing losses. How long will it be before the connections resume
sharing the link fairly? Are there any signs of a capture effect,
in which the new TCP gets a large fraction of the bandwidth? (A
capture effect could ensure that, say, an SMTP server got more
bandwidth than a long running FTP).
Another 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?
Allman [Page 5]
March 1998
8. Experimental Results
8.1 Studies of TCP Connections using Larger Initial Windows
A number of studies have been done using larger initial windows.
The first study considers the effects on the global Internet, as
well as on slow dialup modem links [All97a]. These test results
show that for 16 KB transfers to 100 Internet hosts, 4 segment
initial windows resulted in an 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 a 4
segment (512 byte MSS) initial window when compared to an initial
window of 1 segment. Tests over a 28.8 bps dialup channel showed no
increase in the drop rate and a transfer time decrease of roughly
10% over standard TCP when using a 4 segment initial window.
In another study, larger initial windows have been shown to improve
performance over satellite channels [All97b]. In this study, an
initial window of 4 segments (512 byte MSS) resulted in throughput
improvements of up to 30% (depending upon transfer size).
Next, 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]. [HAGT98] also 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 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 [SP97]. This study found that in this environment, larger
initial windows slightly improved performance.
8.2 Studies of Networks using Larger Initial Windows
A simulation study of how the use of a larger initial window impacts
competing network traffic is outlined in [PN98]. In this
investigation, a number of HTTP and FTP flows were sharing a
congested gateway (the exact number of flows was varied in this
study). The study showed improvement in HTTP transfer times on the
order of 30% in many scenarios. In addition, a larger initial
window slightly increased the segment drop rate (only one scenario
increased the drop rate more than 1% above the loss rate experienced
when using an initial window of 1 segment).
Morris [Mor97] investigated larger initial windows in a very congested
network. The loss rate in networks where all TCP connections use an
initial window of 4 segments is shown to be 1-2% greater than in a
network where all connections use an initial window of 1 segment.
In addition, in networks where connections used an initial window of
4 segments, roughly 5-10% more time was spent waiting for the
retransmit timer (RTO) to expire to resend a segment than was spent
when using an initial window of 1 segment. The time spent waiting
for the RTO timer to expire represents idle time when no useful work
Allman [Page 6]
March 1998
was being accomplished. These results show that in a very congested
environment, where each connection's share of the bottleneck
bandwidth is close to 1 segment, using a larger initial window
degrades performance.
9. Conclusion
This draft suggests 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).
10. Acknowledgments
We would like to acknowledge Tim Shepard and the members of the
End-to-End-Interest Mailing List for continuing discussions of these
issues.
References
[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.
[BLFN96] Tim Berners-Lee, R. Fielding, and H. Nielsen. Hypertext
Transfer Protocol -- HTTP/1.0, May 1996. RFC 1945.
[Bra89] Robert Braden. Requirements for Internet Hosts --
Communication Layers, October 1989. RFC 1122.
[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.
[FJGFBL97] R. Fielding, Jeffrey C. Mogul, Jim Gettys, H. Frystyk,
and Tim Berners-Lee. Hypertext Transfer Protocol -- HTTP/1.1,
January 1997. RFC 2068.
[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-nrg.ee.lbl.gov/floyd/.
Allman [Page 7]
March 1998
[HAGT98] 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. To Appear.
[MD90] Jeffrey C. Mogul and Steve Deering. Path MTU Discovery,
November 1990. RFC 1191.
[MMFR96] Matt Mathis, Jamshid Mahdavi, Sally Floyd and Allyn
Romanow. TCP Selective Acknowledgment Options, October 1996.
RFC 2018.
[Mor97] Robert Morris. Private communication.
[Nic97] Kathleen Nichols. Improving Network Simulation with
Feedback. Com21, Inc. Technical Report. Available from
http://www.com21.com/pages/papers/068.pdf.
[PN98] Poduri, K., and Nichols, K., Simulation Studies of Increased
Initial TCP Window Size, February 1998. Internet-Draft
draft-ietf-tcpimpl-poduri-00.txt (work in progress).
[Pos82] Jon Postel. Simple Mail Transfer Protocol, August 1982.
RFC 821.
[RF97] Ramakrishnan, K.K., and Floyd, S., A Proposal to Add Explicit
Congestion Notification (ECN) to IPv6 and to TCP. Internet-Draft
draft-kksjf-ecn-00.txt (work in progress). November 1997.
[SP97] Tim Shepard and Craig Partridge. When TCP Starts Up With
Four Packets Into Only Three Buffers, July 1997. Internet-Draft
draft-shepard-TCP-4-packets-3-buff-00.txt (work in progress).
Appendix A
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. The change to a
larger initial window should not result in a large number of
unnecessarily-retransmitted segments.
If a 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,
there are no unnecessarily-retransmitted segments. 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. In addition, a
single segment dropped from an initial window of three segments may
Allman [Page 8]
March 1998
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.
We now consider the case when multiple segments are dropped from the
initial window. Using the first recovery method, slow-starting from
a window of one segment, the number of unnecessarily-retransmitted
segments is limited [FF96]. In the second case of Fast Recovery
without SACK, multiple segment drops from a window of data generally
result in a retransmit timeout. Again, the number of
unnecessarily-retransmitted segments is small. In the third case,
of Fast Recovery with SACK, there can only be
unnecessarily-retransmitted segments if a precise pattern of ACK
segments are also lost [Flo96], or if segments are
seriously-reordered in the network. In any case, the number of
unnecessarily-retransmitted segments due to a larger initial window
should be small.
Author's Addresses
Mark Allman
NASA Lewis Research Center/Sterling Software
21000 Brookpark Road
MS 54-2
Cleveland, OH 44135
mallman@lerc.nasa.gov
http://gigahertz.lerc.nasa.gov/~mallman/
Sally Floyd
Lawrence Berkeley National Laboratory
One Cyclotron Road
Berkeley, CA 94720
floyd@ee.lbl.gov
Craig Partridge
BBN Technologies
10 Moulton Street
Cambridge, MA 02138
craig@bbn.com
Allman [Page 9]
| PAFTECH AB 2003-2026 | 2026-04-22 05:39:49 |