📄 rfc2415.txt
字号:
Network Working Group K. Poduri
Request for Comments: 2415 K. Nichols
Category: Informational Bay Networks
September 1998
Simulation Studies of Increased Initial TCP Window Size
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (1998). All Rights Reserved.
Abstract
An increase in the permissible initial window size of a TCP
connection, from one segment to three or four segments, has been
under discussion in the tcp-impl working group. This document covers
some simulation studies of the effects of increasing the initial
window size of TCP. Both long-lived TCP connections (file transfers)
and short-lived web-browsing style connections were modeled. The
simulations were performed using the publicly available ns-2
simulator and our custom models and files are also available.
1. Introduction
We present results from a set of simulations with increased TCP
initial window (IW). The main objectives were to explore the
conditions under which the larger IW was a "win" and to determine the
effects, if any, the larger IW might have on other traffic flows
using an IW of one segment.
This study was inspired by discussions at the Munich IETF tcp-impl
and tcp-sat meetings. A proposal to increase the IW size to about 4K
bytes (4380 bytes in the case of 1460 byte segments) was discussed.
Concerns about both the utility of the increase and its effect on
other traffic were raised. Some studies were presented showing the
positive effects of increased IW on individual connections, but no
studies were shown with a wide variety of simultaneous traffic flows.
It appeared that some of the questions being raised could be
addressed in an ns-2 simulation. Early results from our simulations
were previously posted to the tcp-impl mailing list and presented at
the tcp-impl WG meeting at the December 1997 IETF.
Poduri & Nichols Informational [Page 1]
RFC 2415 TCP Window Size September 1998
2. Model and Assumptions
We simulated a network topology with a bottleneck link as shown:
10Mb, 10Mb,
(all 4 links) (all 4 links)
C n2_________ ______ n6 S
l n3_________\ /______ n7 e
i \\ 1.5Mb, 50ms // r
e n0 ------------------------ n1 v
n n4__________// \ \_____ n8 e
t n5__________/ \______ n9 r
s s
URLs --> <--- FTP & Web data
File downloading and web-browsing clients are attached to the nodes
(n2-n5) on the left-hand side. These clients are served by the FTP
and Web servers attached to the nodes (n6-n9) on the right-hand side.
The links to and from those nodes are at 10 Mbps. The bottleneck link
is between n1 and n0. All links are bi-directional, but only ACKs,
SYNs, FINs, and URLs are flowing from left to right. Some simulations
were also performed with data traffic flowing from right to left
simultaneously, but it had no effect on the results.
In the simulations we assumed that all ftps transferred 1-MB files
and that all web pages had exactly three embedded URLs. The web
clients are browsing quite aggressively, requesting a new page after
a random delay uniformly distributed between 1 and 5 seconds. This is
not meant to realistically model a single user's web-browsing
pattern, but to create a reasonably heavy traffic load whose
individual tcp connections accurately reflect real web traffic. Some
discussion of these models as used in earlier studies is available in
references [3] and [4].
The maximum tcp window was set to 11 packets, maximum packet (or
segment) size to 1460 bytes, and buffer sizes were set at 25 packets.
(The ns-2 TCPs require setting window sizes and buffer sizes in
number of packets. In our tcp-full code some of the internal
parameters have been set to be byte-oriented, but external values
must still be set in number of packets.) In our simulations, we
varied the number of data segments sent into a new TCP connection (or
initial window) from one to four, keeping all segments at 1460 bytes.
A dropped packet causes a restart window of one segment to be used,
just as in current practice.
Poduri & Nichols Informational [Page 2]
RFC 2415 TCP Window Size September 1998
For ns-2 users: The tcp-full code was modified to use an
"application" class and three application client-server pairs were
written: a simple file transfer (ftp), a model of http1.0 style web
connection and a very rough model of http1.1 style web connection.
The required files and scripts for these simulations are available
under the contributed code section on the ns-simulator web page at
the sites ftp://ftp.ee.lbl.gov/IW.{tar, tar.Z} or http://www-
nrg.ee.lbl.gov/floyd/tcp_init_win.html.
Simulations were run with 8, 16, 32 web clients and a number of ftp
clients ranging from 0 to 3. The IW was varied from 1 to 4, though
the 4-packet case lies beyond what is currently recommended. The
figures of merit used were goodput, the median page delay seen by the
web clients and the median file transfer delay seen by the ftp
clients. The simulated run time was rather large, 360 seconds, to
ensure an adequate sample. (Median values remained the same for
simulations with larger run times and can be considered stable)
3. Results
In our simulations, we varied the number of file transfer clients in
order to change the congestion of the link. Recall that our ftp
clients continuously request 1 Mbyte transfers, so the link
utilization is over 90% when even a single ftp client is present.
When three file transfer clients are running simultaneously, the
resultant congestion is somewhat pathological, making the values
recorded stable. Though all connections use the same initial window,
the effect of increasing the IW on a 1 Mbyte file transfer is not
detectable, thus we focus on the web browsing connections. (In the
tables, we use "webs" to indicate number of web clients and "ftps" to
indicate the number of file transfer clients attached.) Table 1 shows
the median delays experienced by the web transfers with an increase
in the TCP IW. There is clearly an improvement in transfer delays
for the web connections with increase in the IW, in many cases on the
order of 30%. The steepness of the performance improvement going
from an IW of 1 to an IW of 2 is mainly due to the distribution of
files fetched by each URL (see references [1] and [2]); the median
size of both primary and in-line URLs fits completely into two
packets. If file distributions change, the shape of this curve may
also change.
Poduri & Nichols Informational [Page 3]
RFC 2415 TCP Window Size September 1998
Table 1. Median web page delay
#Webs #FTPs IW=1 IW=2 IW=3 IW=4
(s) (% decrease)
----------------------------------------------
8 0 0.56 14.3 17.9 16.1
8 1 1.06 18.9 25.5 32.1
8 2 1.18 16.1 17.1 28.9
8 3 1.26 11.9 19.0 27.0
16 0 0.64 11.0 15.6 18.8
16 1 1.04 17.3 24.0 35.6
16 2 1.22 17.2 20.5 25.4
16 3 1.31 10.7 21.4 22.1
32 0 0.92 17.6 28.6 21.0
32 1 1.19 19.6 25.0 26.1
32 2 1.43 23.8 35.0 33.6
32 3 1.56 19.2 29.5 33.3
Table 2 shows the bottleneck link utilization and packet drop
percentage of the same experiment. Packet drop rates did increase
with IW, but in all cases except that of the single most pathological
overload, the increase in drop percentage was less than 1%. A
decrease in packet drop percentage is observed in some overloaded
situations, specifically when ftp transfers consumed most of the link
bandwidth and a large number of web transfers shared the remaining
bandwidth of the link. In this case, the web transfers experience
severe packet loss and some of the IW=4 web clients suffer multiple
packet losses from the same window, resulting in longer recovery
times than when there is a single packet loss in a window. During the
recovery time, the connections are inactive which alleviates
congestion and thus results in a decrease in the packet drop
percentage. It should be noted that such observations were made only
in extremely overloaded scenarios.
Poduri & Nichols Informational [Page 4]
RFC 2415 TCP Window Size September 1998
Table 2. Link utilization and packet drop rates
Percentage Link Utilization | Packet drop rate
#Webs #FTPs IW=1 IW=2 IW=3 IW=4 |IW=1 IW=2 IW=3 IW=4
-----------------------------------------------------------------------
8 0 34 37 38 39 | 0.0 0.0 0.0 0.0
8 1 95 92 93 92 | 0.6 1.2 1.4 1.3
8 2 98 97 97 96 | 1.8 2.3 2.3 2.7
8 3 98 98 98 98 | 2.6 3.0 3.5 3.5
-----------------------------------------------------------------------
16 0 67 69 69 67 | 0.1 0.5 0.8 1.0
16 1 96 95 93 92 | 2.1 2.6 2.9 2.9
16 2 98 98 97 96 | 3.5 3.6 4.2 4.5
16 3 99 99 98 98 | 4.5 4.7 5.2 4.9
-----------------------------------------------------------------------
32 0 92 87 85 84 | 0.1 0.5 0.8 1.0
32 1 98 97 96 96 | 2.1 2.6 2.9 2.9
32 2 99 99 98 98 | 3.5 3.6 4.2 4.5
32 3 100 99 99 98 | 9.3 8.4 7.7 7.6
To get a more complete picture of performance, we computed the
network power, goodput divided by median delay (in Mbytes/ms), and
plotted it against IW for all scenarios. (Each scenario is uniquely
identified by its number of webs and number of file transfers.) We
plot these values in Figure 1 (in the pdf version), illustrating a
general advantage to increasing IW. When a large number of web
clients is combined with ftps, particularly multiple ftps,
pathological cases result from the extreme congestion. In these
cases, there appears to be no particular trend to the results of
increasing the IW, in fact simulation results are not particularly
stable.
To get a clearer picture of what is happening across all the tested
scenarios, we normalized the network power values for the non-
pathological scenario by the network power for that scenario at IW of
one. These results are plotted in Figure 2. As IW is increased from
one to four, network power increased by at least 15%, even in a
congested scenario dominated by bulk transfer traffic. In simulations
where web traffic has a dominant share of the available bandwidth,
the increase in network power was up to 60%.
The increase in network power at higher initial window sizes is due
to an increase in throughput and a decrease in the delay. Since the
(slightly) increased drop rates were accompanied by better
performance, drop rate is clearly not an indicator of user level
performance.
Poduri & Nichols Informational [Page 5]
RFC 2415 TCP Window Size September 1998
The gains in performance seen by the web clients need to be balanced
against the performance the file transfers are seeing. We computed
ftp network power and show this in Table 3. It appears that the
improvement in network power seen by the web connections has
negligible effect on the concurrent file transfers. It can be
observed from the table that there is a small variation in the
network power of file transfers with an increase in the size of IW
but no particular trend can be seen. It can be concluded that the
network power of file transfers essentially remained the same.
However, it should be noted that a larger IW does allow web transfers
to gain slightly more bandwidth than with a smaller IW. This could
mean fewer bytes transferred for FTP applications or a slight
decrease in network power as computed by us.
Table 3. Network power of file transfers with an increase in the TCP
IW size
#Webs #FTPs IW=1 IW=2 IW=3 IW=4
--------------------------------------------
8 1 4.7 4.2 4.2 4.2
8 2 3.0 2.8 3.0 2.8
8 3 2.2 2.2 2.2 2.2
16 1 2.3 2.4 2.4 2.5
16 2 1.8 2.0 1.8 1.9
⌨️ 快捷键说明
复制代码
Ctrl + C
搜索代码
Ctrl + F
全屏模式
F11
切换主题
Ctrl + Shift + D
显示快捷键
?
增大字号
Ctrl + =
减小字号
Ctrl + -