The main use is probably to test networks applications : you can construct packets by hand and test the behavior of your program when facing some strange packets. But you can image a lot of other application : e.g. manipulating graphical file or executable headers. Just describe the specification of the structured data.

GASP is divided in two parts : a compiler which take the specification of the protocols and generate the code to handle it, this code is a new Tcl command as GASP in build upon Tcl/Tk and extends the scripting facilities provided by Tcl.

Enough words, let's see how all this works. Let image you're working on a TCP/IP packet. To dump the IP header and change some values, it is quite simple :

% pkt ip configure -version 4 -headerlength 5 -tos {routine {-delay -throughput -reliability -res1 -res2}} -length 69 -identification 12345 -flags {-res -df -mf} -fragoffset 0 -ttl 10 -protocol tcp -checksum 27768 -src 10.0.0.1 -dest 10.0.0.2 -options {} % pkt ip configure -dest 10.0.0.3 -flags +df %