The Hurd presently uses a TCP/IP stack based on code from an old Linux version. This works, but lacks some rather important features (like PPP/PPPoE), and the design is not hurdish at all.
A true hurdish network stack will use a set of translator processes, each implementing a different protocol layer. This way not only the implementation gets more modular, but also the network stack can be used way more flexibly. Rather than just having the standard socket interface, plus some lower-level hooks for special needs, there are explicit (perhaps filesystem-based) interfaces at all the individual levels; special application can just directly access the desired layer. All kinds of packet filtering, routing, tunneling etc. can be easily achieved by stacking components in the desired constellation.
Implementing a complete modular network stack is not feasible as a GSoC project, though. Instead, the task is to take some existing user space TCP/IP implementation, and make it run as a single Hurd server for now, so it can be used in place of the existing pfinet. The idea is to split it up into individual layers later. The initial implementation, and the choice of a TCP/IP stack, should be done with this in mind -- it needs to be modular enough to make such a split later on feasible.
This is GNU Savannah task #5469.
Possible mentors: youpi
Exercise: You could try making some improvement to the existing pfinet implementation; or you could work towards running some existing userspace TCP/IP stack on Hurd. (As a normal program for now, not a proper Hurd server yet.)