How to increase ftp socket window size?

[Jeffrey Wang]

Is there any way to increase socket's window size for ftp, so that it will fill the pipe?

I tried:
1. ftp -B block_size
2. increase
tcp_recv_hiwater_def

None of them seems can fill the lan pipe.

[Mariani Alberto]

I don't know what you mean by ftp socket window size.

What i know is that ftp works using tcp, and this means you have to deal with the "slow start threshold" algorithm.

If you are familiar with it, you should know that the amount of tcp packets the sender can trasnmit is a direct function of the receive window of the receiver, but is also influenced by the sender's 'send window' (it sends only the minimum between its own send window and the receiver's receive window - this value is known as the congestion window) and by the state of the lan.

It's also important to note that the slow start algorithm starts over (hence sending the minimum possible amount of data: 1 or 2 packets, depending on how the tcp parameters are set up) every time a timeout for a sent packet happens without a piggybacked ack from the receiver (and note that the timeouts are adaptative, so they tend to shrink the more traffic the connections delivers with no errors, but double, for a few packets at least, the moment an error occurs).

Moreover, when the congestion window tops the value of the 'slow start threshold', it grows in a slower way than when this condition is not true (about linearly instead of quadratically), because in this situation the algorithm opeartes in "congestion avoidance mode".

Lastly, if you are on an ethernet (even a switched one), remember that you cannot saturate its capacity: firstly because of how the protocol is designed, and secondly because the delay with which a packet is delivered (if at all - after 16 collisions a packet is discarded) grows too much, hence impacting on the performance.

On a switched fast ethernet, i wouldn't expect more than 10 MB/sec out of the max of 12.5 MB/sec available in theory.

Hope this helps.

Bye,
Alberto