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| 6.3 |  | Troubleshooting Transport Layer Issues on Network Hosts |
| 6.3.1 |  | Common transport layer issues with IP networks |

TCP window size can dramatically impact the performance of the network.

If the TCP window size is too small, the result will be additional network overhead from segment acknowledgments. A lot of time is also wasted when the sending host waits for acknowledgments. Increasing the window size allows the sending host to transmit more than one packet at a time, increase network bandwidth utilization and decrease the time spent waiting for acknowledgements.

Having a TCP window too large can also cause problems, especially on a congested network. If the network is dropping a lot of TCP packets and acknowledgements, the sending host wastes a lot of time waiting for retransmit timers to expire. The sending host also adds to the congestion problem by sending large amounts of retransmission traffic. Making the window smaller forces the sending host to send less data at once, reduces its impact on the congestion, and improves the chances of TCP packets and acknowledgements getting through the network successfully.

When operating correctly, TCP self-tunes its behavior according to network conditions. Recall from previous content that TCP window sizes are self-tuning. This self-tuning mechanism can be seen in action when downloading a large file from the Internet. When the download starts, the rate of transfer is quite slow, as the initial window size is relatively small. As the download proceeds, the rate of download increases rapidly as the window size expands. As the TCP window approaches its optimal size, the increase in the download rate slows until the rate is almost constant. For the remainder of the download, the window size self-tunes to suit prevailing network conditions.