rfc2415.txt

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    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.





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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.





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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,



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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 [e-mail]

6. Acknowledgements

   This work benefited from discussions with and comments from Van
   Jacobson.

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.















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8. Authors' Addresses

   Kedarnath Poduri
   Bay Networks
   4401 Great America Parkway
   SC01-04
   Santa Clara, CA 95052-8185

   Phone: [phone]
   Fax:   [phone]
   EMail: [e-mail]


   Kathleen Nichols
   Bay Networks
   4401 Great America Parkway
   SC01-04
   Santa Clara, CA 95052-8185

   EMail: [e-mail]































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RFC 2415                    TCP Window Size               September 1998


Full Copyright Statement

   Copyright (C) The Internet Society (1998).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
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