TCP and UDP ports

[Dave Rajnauth]

I have been trying to figure out how TCP and UDP ports are used.  From
the assigned numbers RFC I can see what the destination port numbers
should be in TCP or UDP packets.  But what are the source port numbers?
Is there some kind of "unwritten" convention?

Also does anyone how NFS allocates its ports.  As far as I can tell
(from my tcpdump program) ports 111 and 801 are used on the server.  What
is port 801 for (111 is RPC).

Thanks
Dave Rajnauth