19.5 Window Size Advertisements

In Figure 19.4 we see that slip advertises a window of 4096 bytes and vangogh advertises a window of 8192 bytes. Most segments in this figure contain one of these two values.

Segment 5, however, advertises a window of 4095 bytes. This means there is still 1 byte in the TCP buffer for the application (the Rlogin client) to read. Similarly, the next segment from the client advertises a window of 4094 bytes, meaning there are 2 bytes still to be read.

The server normally advertises a window of 8192 bytes, because the server's TCP has nothing to send until the Rlogin server reads the received data and echoes it. The data from the server is sent after the Rlogin server has read its input from the client.

The client TCP, on the other hand, often has data to send when the ACK arrives, since it's buffering the received characters just waiting for the ACK. When the client TCP sends the buffered data, the Rlogin client has not had a chance to read the data received from the server, so the client's advertised window is less than 4096.