I used Google's Protocol Buffers as my serializer/IDL thingie.  I
couldn't really bring myself to use rpcgen/XDR after 10 years, and
protobufs has some cool features -- check out my declaration to see
(I have only one reply message, that flexes to handle all the request
messages).

Sadly, Google does not provide native C generation (only C++/C#/ObjC),
so rather than use some nasty C wrapper around the C++ (which would
suck in all possible ways, I used protobufs-c, which has been around
a long time and is reasonably current and does seem to get hacked on
a little (mostly bugfixes).  The generated code isn't super nice, but
it'll do once it's wrapped in some protocol sugar foo.  I hope.