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<concept id="GUID-8A678AE5-A133-58EA-A4DD-B5BFC94D3AEA" xml:lang="en"><title> Variable

TCP Receive Window</title><shortdesc>Explains how to improve performance of TCP/IP stack.</shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>

<p>The performance of the existing Symbian platform TCP/IP stack is improved

by doing the following:</p>

<ul>

<li id="GUID-00E77774-7DC6-513E-9CC3-46B499B4C196"><p>Setting an initial TCP

receive window size that depends on the <xref href="GUID-8A678AE5-A133-58EA-A4DD-B5BFC94D3AEA.dita#GUID-8A678AE5-A133-58EA-A4DD-B5BFC94D3AEA/GUID-CDAA0409-6ECB-5ABB-A0E5-F7C5C58E6347">bearer

type</xref>. </p> <p> </p> </li>

<li id="GUID-22A70B8B-AD26-5898-8505-79F5434B5971"><p>Allowing the TCP receive

window size to dynamically resize when there is a change in modulation. For

example, GPRS to EDGE or 2G to 3G. </p> </li>

</ul>

<p>Bearers provide different bandwidth for data transmission using TCP/IP.

The connection setup between TCP hosts are through three-way handshake. The

TCP receive window size parameter is set during the connection startup. </p>

<p>To indicate the TCP receive window size, the TCP header contains a 16-bit

window field. The TCP receive window size denotes the maximum number of bytes

that a sender can send and a receiver can receive. The TCP header can have

any value between 0 and 64KB. Use the <xref href="http://www.ietf.org/rfc/rfc1323.txt" scope="external">TCP window scaling</xref> to increase the window size beyond

64kB. The sender and receiver must agree to the scaling factor. The scaling

factor can be multiplied at both the ends to increase the window size. For

example, if the scaling factor is 2 and the window size negotiated is 96KB,

then the stored window size is 48KB. Also a scaling factor of 2 yields the

effective size of 96KB. The TCP receive maximum window size is mentioned in <codeph>tcpip.ini</codeph> file. </p>

<p>At ROM build time, the (NetMCPR) plug-in configures the TCP receive window.

It is possible to enable or disable the adaptive window size based on the

bearer type. This helps some bearers to use adaptive window size and other

bearers to use fixed pre-defined windows. The <xref href="GUID-8A678AE5-A133-58EA-A4DD-B5BFC94D3AEA.dita#GUID-8A678AE5-A133-58EA-A4DD-B5BFC94D3AEA/GUID-E1123CF2-EA87-5F18-8F3C-3FD98DD6ABE8">lookup

table</xref> provides the information on TCP receive window size at licensees

specific MCPR (NetMCPR). Licensees are provided with the TCP receive window

interface. </p>

<p>TCP receive window must be changed due to the following reasons: </p>

<ul>

<li id="GUID-4E46CC4D-3CFA-5078-B801-9E0F20857CC8"><p>Change in modulation

due to the network change - modulation value for bearer varies in different

network. Modulation change in the network triggers the notification to change

the TCP receive window for the connected TCP sockets. For example, change

in network from 2.5G to 3G. </p> </li>

<li id="GUID-41F0FC86-3E72-5372-8C43-B59F3835DD48"><p>Change in bearer - Forces

the TCP/IP stack to supply a new TCP receive window that is specific to the

new bearer. </p> </li>

</ul>

<section id="GUID-CDAA0409-6ECB-5ABB-A0E5-F7C5C58E6347"><title>Bearer types</title> <p>The

following are the bearer types that support the TCP receive window: </p> <ul>

<li id="GUID-302F21B4-FDBA-5052-939E-7FE309C4A7DF"><p>General Packet Radio

Service (GPRS 2.5G) </p> </li>

<li id="GUID-F60D6AF2-0EAA-5693-982B-B0B531E22862"><p>Enhanced Data Rate for

GSM Evolution (EDGE 2.75G or 3G – depends on the modulation scheme) </p> </li>

<li id="GUID-07D37BB9-2BBE-5C22-9D5C-A3477F73E766"><p>Universal Mobile Telecommunications

System (UMTS 3G) </p> </li>

<li id="GUID-1677511B-C52E-5E4D-8657-075468587E97"><p>High-Speed Downlink

Packet Access (HSDPA 3G) </p> </li>

<li id="GUID-DF4013BE-A1DE-5ECA-9124-2376924345C8"><p>WLAN </p> </li>

</ul> </section>

<section id="GUID-E1123CF2-EA87-5F18-8F3C-3FD98DD6ABE8"><title>Lookup table</title> <p>The

lookup table contains the entries of TCP receive window size for each bearer

type. To set the TCP receive window size for bearer, licensees must override

the <xref href="GUID-5AC853D0-31FB-3944-8769-A59977F616B0.dita"><apiname>SetTcpWin()</apiname></xref> function. The licensees can define their

own TCP window lookup table and set the TCP receive window size for a bearer. </p> <p>Symbian

uses the following lookup table for testing purpose. The licensees can replace

the TCP receive window size with their own values: </p> <table id="GUID-0697063E-F323-5824-9775-FE4413399697">

<tgroup cols="2"><colspec colname="col0"/><colspec colname="col1"/>

<tbody>

<row>

<entry><p> <b>Bearer or Modulation</b>  </p> </entry>

<entry><p> <b>TCP receive window size</b>  </p> </entry>

</row>

<row>

<entry><p>GPRS </p> </entry>

<entry><p>12240 – 17520 </p> </entry>

</row>

<row>

<entry><p>EDGE </p> </entry>

<entry><p>21760 – 49640 </p> </entry>

</row>

<row>

<entry><p>UMTS </p> </entry>

<entry><p>29920 – 33580 </p> </entry>

</row>

<row>

<entry><p>HSDPA </p> </entry>

<entry><p>65535 or greater </p> </entry>

</row>

<row>

<entry><p>WLAN </p> </entry>

<entry><p>65535 or greater </p> </entry>

</row>

<row>

<entry><p>Ethernet </p> </entry>

<entry><p>65335 </p> </entry>

</row>

</tbody>

</tgroup>

</table> <p> </p> </section>

<section id="GUID-6A97BEE5-8236-40A3-899B-6A4173D51C97"><title>Handling TCP receive window in TCP/IP stack</title> <p>The

following are the scenarios to handle the TCP receive window: </p> <ul>

<li id="GUID-2E1E9EC2-F481-54AE-A648-D9EC50EF77C4"><p>Increase window size

- Change in the bearer increases the window size. </p> </li>

<li id="GUID-B6C3EC7E-E0A0-521C-8830-4FDB27B409B8"><p>Decrease window size

- Change in the bearer decreases the window size till it reaches the appropriate

size for the new bearer. </p> </li>

</ul> </section>

</conbody><related-links>

<link href="GUID-6822E7FA-D055-5693-8872-8D0E8554A734.dita"><linktext>Configuring

TCP Receive                 Window</linktext></link>

</related-links></concept>