The tcp_recvspace parameter may require tuning for slow throughput links, and for compatibility with TCP implementations based on 4.2BSD. For slow throughput links (such as a Wide Area Net with a lower speed PPP or SLIP component), lower the value of this and the tcp_sendspace parameters on systems where the connection goes from fast (ethernet, or FDDI, etc) to slow (PPP or SLIP) until the best performance level is found. Very slow links such as PPP or SLIP over modems usually find a 3 Kbyte or 4 Kbyte buffer size to be the most efficient.
Conversely, high speed connections may profit from increasing the buffer size incrementally from 60 KBytes until maximum performance is achieved.
See the setsockopt(2) reference page for more information on setting specific individual socket options.