Sometimes, we are feeling slow Internet connection, but don't know how we can measure the speed and what is right speed, download speed, TCP throughput, I can expect. Measuring and calculating TCP throughput is not that hard. See below famous TCP throughput formula.

# RCV buffer size / RTT = Max TCP throughput = ? bps ** Buffer size is normally 65Kbps

ex) (64Kbyte x 8bit) / 0.17 = 3011764 bps = 3Mbps, (RTT=170ms)

To learn more about Internet Speed Issue, please check our article series within cisconet.com

RCV Buffer size / TCP receive window size

- RCV buffer size is denoted as TCP receive window size. Window systems have 64Kbyte of window size as default (Window NT and Millennium have 8Kbytes of window size).  The TCP/IP standard allows for a receive window up to 65,535 bytes in size, which is the maximum value that can be specified in the 16-bit TCP window size field. Why 65Kbytes? Well, more accurate expression will be  65,535 = (2^16)-1. To improve TCP throughput, speed, performance whatever you called, in high speed connection or high delay network, you can increase TCP window size(reference RFC 1323). However, if transport link is not stable, it might give you worse performance. Packet loss or bottleneck in the network is the most likely factors that are leading to the TCP throughput reductions.

Optimal TCP window size

- Optimal RCV buffer size is considered to be 2 x BDP, where BDP is the Bandwidth*Delay Product

ex) RTT is 20 ms, and connection speed is 10 Mbps.
2 x (10Mbps/8 * .020s) = 50Kbytes

Well, default window size 65kbytes is not adequate for today's network. In these days, most of network is 100Mbps or higher.

Round Trip Time(RTT)

- If you are not using TCP window scaling option (RFC 1323), TCP window size will be used as 64Kbytes. If then, Round Trip Time(RTT) is the main factor to decide TCP throughput between locations.

RTT 10 ms => TCP throughput = 52428000 bps = 52Mbps
RTT 20 ms => TCP throughput = 26214000 bps = 26Mbps
RTT 50 ms => TCP throughput = 10485600 bps = 10Mbps
RTT 100 ms => TCP throughput = 5242800 bps = 5.2Mbps
RTT 150 ms => TCP throughput = 3495200 bps = 4.3Mbps
RTT 200 ms => TCP throughput = 2621400 bps = 2.5Mbps
RTT 300 ms => TCP throughput = 1747600 bps = 1.7Mbps
RTT 500 ms => TCP throughput = 1048560 bps = 1Mbps

** Used maximum TCP window size  = 65Kbytes = 65535

You can see the deficiency with the greater RTT.

Last Updated (Monday, 04 January 2010 18:14)

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