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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.


 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{tar, tar.Z} or http://www-
 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
 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

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
  16      3      1.4     1.6     1.5     1.7
  32      1      0.7     0.9     1.3     0.9
  32      2      0.8     1.0     1.3     1.1
  32      3      0.7     1.0     1.2     1.0
 The above simulations all used http1.0 style web connections, thus, a
 natural question is to ask how results are affected by migration to
 http1.1. A rough model of this behavior was simulated by using one
 connection to send all of the information from both the primary URL
 and the three embedded, or in-line, URLs. Since the transfer size is
 now made up of four web files, the steep improvement in performance
 between an IW of 1 and an IW of two, noted in the previous results,
 has been smoothed. Results are shown in Tables 4 & 5 and Figs. 3 & 4.
 Occasionally an increase in IW from 3 to 4 decreases the network
 power owing to a non-increase or a slight decrease in the throughput.
 TCP connections opening up with a higher window size into a very
 congested network might experience some packet drops and consequently
 a slight decrease in the throughput. This indicates that increase of
 the initial window sizes to further higher values (>4) may not always
 result in a favorable network performance. This can be seen clearly
 in Figure 4 where the network power shows a decrease for the two
 highly congested cases.

Poduri & Nichols Informational [Page 6] RFC 2415 TCP Window Size September 1998

 Table 4. Median web page delay for http1.1
 #Webs   #FTPs   IW=1    IW=2    IW=3    IW=4
                 (s)       (% decrease)
   8      0      0.47   14.9   19.1   21.3
   8      1      0.84   17.9   19.0   25.0
   8      2      0.99   11.5   17.3   23.0
   8      3      1.04   12.1   20.2   28.3
  16      0      0.54   07.4   14.8   20.4
  16      1      0.89   14.6   21.3   27.0
  16      2      1.02   14.7   19.6   25.5
  16      3      1.11   09.0   17.0   18.9
  32      0      0.94   16.0   29.8   36.2
  32      1      1.23   12.2   28.5   21.1
  32      2      1.39   06.5   13.7   12.2
  32      3      1.46   04.0   11.0   15.0
 Table 5. 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      4.2     4.2     4.2     3.7
   8      2      2.7     2.5     2.6     2.3
   8      3      2.1     1.9     2.0     2.0
  16      1      1.8     1.8     1.5     1.4
  16      2      1.5     1.2     1.1     1.5
  16      3      1.0     1.0     1.0     1.0
  32      1      0.3     0.3     0.5     0.3
  32      2      0.4     0.3     0.4     0.4
  32      3      0.4     0.3     0.4     0.5
 For further insight, we returned to the http1.0 model and mixed some
 web-browsing connections with IWs of one with those using IWs of
 three. In this experiment, we first simulated a total of 16 web-
 browsing connections, all using IW of one. Then the clients were
 split into two groups of 8 each, one of which uses IW=1 and the other
 used IW=3.
 We repeated the simulations for a total of 32 and 64 web-browsing
 clients, splitting those into groups of 16 and 32 respectively. Table
 6 shows these results.  We report the goodput (in Mbytes), the web
 page delays (in milli seconds), the percent utilization of the link
 and the percent of packets dropped.

Poduri & Nichols Informational [Page 7] RFC 2415 TCP Window Size September 1998

Table 6. Results for half-and-half scenario

Median Page Delays and Goodput (MB) | Link Utilization (%) & Drops (%) #Webs IW=1 | IW=3 | IW=1 | IW=3

    G.put   dly |  G.put   dly      |  L.util  Drops| L.util   Drops

——————|——————-|—————|————— 16 35.5 0.64| 36.4 0.54 | 67 0.1 | 69 0.7 8/8 16.9 0.67| 18.9 0.52 | 68 0.5 | ——————|——————-|—————|————— 32 48.9 0.91| 44.7 0.68 | 92 3.5 | 85 4.3 16/16 22.8 0.94| 22.9 0.71 | 89 4.6 | ——————|——————-|—————|—————- 64 51.9 1.50| 47.6 0.86 | 98 13.0 | 91 8.6 32/32 29.0 1.40| 22.0 1.20 | 98 12.0 |

 Unsurprisingly, the non-split experiments are consistent with our
 earlier results, clients with IW=3 outperform clients with IW=1. The
 results of running the 8/8 and 16/16 splits show that running a
 mixture of IW=3 and IW=1 has no negative effect on the IW=1
 conversations, while IW=3 conversations maintain their performance.
 However, the 32/32 split shows that web-browsing connections with
 IW=3 are adversely affected. We believe this is due to the
 pathological dynamics of this extremely congested scenario. Since
 embedded URLs open their connections simultaneously, very large
 number of TCP connections are arriving at the bottleneck link
 resulting in multiple packet losses for the IW=3 conversations. The
 myriad problems of this simultaneous opening strategy is, of course,
 part of the motivation for the development of http1.1.

4. Discussion

 The indications from these results are that increasing the initial
 window size to 3 packets (or 4380 bytes) helps to improve perceived
 performance. Many further variations on these simulation scenarios
 are possible and we've made our simulation models and scripts
 available in order to facilitate others' experiments.
 We also used the RED queue management included with ns-2 to perform
 some other simulation studies. We have not reported on those results
 here since we don't consider the studies complete. We found that by
 adding RED to the bottleneck link, we achieved similar performance
 gains (with an IW of 1) to those we found with increased IWs without
 RED. Others may wish to investigate this further.
 Although the simulation sets were run for a T1 link, several
 scenarios with varying levels of congestion and varying number of web
 and ftp clients were analyzed. It is reasonable to expect that the
 results would scale for links with higher bandwidth. However,

Poduri & Nichols Informational [Page 8] RFC 2415 TCP Window Size September 1998

 interested readers could investigate this aspect further.
 We also used the RED queue management included with ns-2 to perform
 some other simulation studies. We have not reported on those results
 here since we don't consider the studies complete. We found that by
 adding RED to the bottleneck link, we achieved similar performance
 gains (with an IW of 1) to those we found with increased IWs without
 RED. Others may wish to investigate this further.

5. References

 [1] B. Mah, "An Empirical Model of HTTP Network Traffic", Proceedings
     of INFOCOM '97, Kobe, Japan, April 7-11, 1997.
 [2] C.R. Cunha, A. Bestavros, M.E. Crovella, "Characteristics of WWW
     Client-based Traces", Boston University Computer Science
     Technical Report BU-CS-95-010, July 18, 1995.
 [3] K.M. Nichols and M. Laubach, "Tiers of Service for Data Access in
     a HFC Architecture", Proceedings of SCTE Convergence Conference,
     January, 1997.
 [4] K.M. Nichols, "Improving Network Simulation with Feedback",
     available from

6. Acknowledgements

 This work benefited from discussions with and comments from Van

7. Security Considerations

 This document discusses a simulation study of the effects of a
 proposed change to TCP. Consequently, there are no security
 considerations directly related to the document. There are also no
 known security considerations associated with the proposed change.

Poduri & Nichols Informational [Page 9] RFC 2415 TCP Window Size September 1998

8. Authors' Addresses

 Kedarnath Poduri
 Bay Networks
 4401 Great America Parkway
 Santa Clara, CA 95052-8185
 Phone: +1-408-495-2463
 Fax:   +1-408-495-1299
 Kathleen Nichols
 Bay Networks
 4401 Great America Parkway
 Santa Clara, CA 95052-8185

Poduri & Nichols Informational [Page 10] RFC 2415 TCP Window Size September 1998

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 Copyright (C) The Internet Society (1998).  All Rights Reserved.
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 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an

Poduri & Nichols Informational [Page 11]

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