Meseq is a free and open-source generator of message sequence charts. Meseq is a general purpose tool for illustrating real-life scenarios.
Although (and because) it is not aimed at UML-like modeling nor bound to any programming paradigm, Meseq is suited to engineering design, and all domains where exchange of information matters.
msq
fileExample:
[init]
actor a Alice
actor b Bob
[scenario]
a -> b SYN
a <- b SYN,ACK
a -> b ACK
Meseq v1.1 (direct link): https://github.com/goeb/meseq/raw/v1.1/meseq
On Github.com: https://github.com/goeb/meseq
Git clone: git clone https://github.com/goeb/meseq.git