T/TCP (TCP for Transactions) is a recent extension of TCP to improve TCP's performance with applications like the WWW. T/TCP is part of FreeBSD since 2.0.5R. The most recent Volume (nr. 3) of TCP/IP Illustrated by Richard Stevens covers the working and implementation of the protocol. The book is an excellent source to learn the FreeBSD implementation because the code he uses in the book is derived from the FreeBSD code. Rich Stevens also maintains a T/TCP home page at http://www.noao.edu/~rstevens/ttcp.html with further info about T/TCP. He's also collecting a list of sites out on the net which use T/TCP. If you run some public services on your FreeBSD machine and you didn't disable T/TCP (tcp_extensions=YES is in your /etc/sysconfig), then you may consider sending him a note about your site. Andras Olah (maintainer of the FreeBSD T/TCP implementation)