[VoiceOps] What does an ALG actually do?

[John Levine]

I realize that an ALG is a hack in a router that is supposed to allow
SIP packets to go through a NAT router.  I also realize that for
modern SIP equipment, ALG usually causes more problems than it solves,
and that it's described in RFCs 2663, 3424, and others.

What I can't find anywhere is what a SIP ALG actually does to the
packets.  Is that written down anywhere, or is it just network
folklore?