It has been a while, but I don't remember there being a handshake for
connecting to services in general.
I vaguely remember one specific service where the client first has to
send a specific message before doing most other things else but that was
a rather service-specific thing.
My guess for the reason of the absence of a handshake, is that usually
there would be no information to include in the handshake (except
perhaps proof of what user/group is connecting, but that's not part of
the bytes).
Well, I suppose you could say there exists a 'Berkeley handshake' in a
similar way to a 'TCP handhake' existing if (non-localhost) TCP were to
be used instead, but that doesn't seem to be what you mean and would be
more a matter for an operating system or kernel forum than a GNUnet
mailing list.
Best regards,
Amélie Devos