socket buffer

[Kapil_2]

Hi everyone,

I have a server and client running on the HPUX 11.0

Case 1
------
SO_RCVBUF of server = 32768.
SO_SNDBUF of client = 32768.
The client is able to write 115000 bytes approx. without the server reading a single byte.

Case 2
------
SO_RCVBUF of server = 1024.
SO_SNDBUF of client = 32768.
The client is able to write 35000 bytes approx. without the server reading a single byte.

Case 3
------
SO_RCVBUF of server = 1024.
SO_SNDBUF of client = 1024.
The client is able to write 3200 bytes approx. without the server reading a single byte.

The obvious question is why am i able to write more data than the size of the buffer?
Is SO_RCVBUF,SO_SNDBUF the true size of the buffer?
If no then how do we find the true size?

Kapil

[rick jones]

a partial explanation is that there is a bit of "double buffering" in the HP-UX 11 stack. however, TCP window advertisements are honored

also, even without the double-buffering, the amount that can be sent before the client would block completely is the sum of the client's SO_SNDBUF and the server's SO_RCVBUF. the client sends serverSO_RCVBUF worth of data, which is received and ACKed by the server's tcp. the server's tcp then advertises a zero window.

since the data was ACKed, the client TCP can remove it from his SO_SNDBUF. the client app is now free to put fresh data into the SO_SNDBUF, awaiting a window update from the server, which will be sent once the server app starts reading data from his socket.