On Win2K you can persuade a TCP/IP icon to sit in the right end of
your task bar. IF you pop it up it tells you packets sent and received
and speed.

There is a speed tool in http://mindprod.com/jgloss/tweakdun.html

There are various tools for measuring your connection speed -- not app
speed, linked to from
http://mindprod.com/jgloss/transmissionspeed.html

If your app is bursty, you are not really interested in what happens
in idle bits.

Are you using Sockets directly or are you using RMI (or something else)
for client-server communication ?

Can you not use some socks proxy  implementation that will keep track of
the number of bytes written/read ?

Ideally I expected sun to have made it easier to write SocketImpl
classes but they have made class "PlainSocketImpl" package protected
leaving you with only two choices - [1] Make your class part of
"java.net" package and inherit from PlainSocketImpl (sun does not like
this) or [2] Implement all abstract methods in SocketImpl class (which
would not  make sense for the kind of things you need).

If you can write your own SocketImpl class then you can create
ByteCountingInputStream and ByteCountingOutputStream and return these
implementations when asked for the streams. These implementations could
use the underlying socket for all communication and additionally keep
count of bytes. You will still have to design and implement the
mechanism for storing, tracking and accessing the data.

Abhijat