Where can I find file format layouts for various trace files?

[Bill_Royds () pch gc ca]

 Does anyone know where there is description of the file layout for various
trace file formats, in particular the Sniffer .enc files produced by the NAI
(formerly Network General) Sniffer and by Network Ice?
I want to make a perl module for looking at traces but the present Perl  modules
are for gathering data not analyzing it.