in reply to Network programming with encryption
This is good practice no matter what, partly just so that you can ensure that there's no tampering with your messages.
If you'd like an example of some of this, check out (shameless plug) the packet code in Net::SSH::Perl, my Perl implementation of an ssh client. SSH network packets are encrypted and contain message verification (in the form of checksums), so you might get some ideas there. The packet code is in Net::SSH::Perl::Packet.