Author of this application is Tom Poindexter and Keith Vetter.
sockspy [L1 ] "watch[es] the conversation of a Tcp client and server", according to its quite readable README [L2 ]. Although "not a replacement for heavy duty tools such as 'tcpdump' and other passive packet sniffers" (Ethereal [L3 ], ...), it's easy to install, configure, and launch, and pleasingly convenient for "debugging Tcp client/server programs, examining protocols and diagnosing network problems".
sockspy is nearly indispensable for any network programmer or network administrator, quite apart from any interest they might have in Tcl. [Compare to tcpdump, ...]
Version 2.0 of sockspy is available on sourceforge at [L4 ].
Among the new features are: