Just thought about this again. What exactly is it you are trying to do?
I'm assuming you are creating a modem as the only reason you need the DSP is usually to convert Analogue signals to Digital or vice versia. You are taking analogue signals from an un-explained interface, the DSP processes this into a digital signal and then outputs it via the ethernet port.
The question is, what is between the DSP and the Ethernet port? A microcontroller?
There are plenty of algorithms out there. The key points are to know what modulation and frequency of encoding is used and replace any variables accordingly. As far as implementing TCP/IP stack, good luck
On a final note, I found very little information on the subject of DSP & C on the net. I found much better results in my library (books...they still have their uses)