Your design document should provide a coherent description of what you propose to build and how you plan to build it, using figures as is needed to make your points. I would expect a design document to be on the order of 8-10 pages, although the length will depend of course a lot on font size, number of figures, ...
The design document should have the following 5 components:
Goals: What is the function of the system that you are designing. Who are its users, what will it do for its users? How will it be managed? How does it compare with other systems that have a similar function; this can be either products or research projects. Please include citations.
Architecture: What is the structure of the system. What components does it have and what do they do? How do the components interact? How are the components mapped on the available network infrastructure?
Detailed design: How do you propose to implement each component and interactions between the components. Important design decisions includes things like key data structures and algorithms. What will you build yourself? What existing software will you build on?
Evaluation plan: How will you evaluate the system. What experiments to you plan to run? What metrics will be used? What infrastructure do you need? Are there aspects of the system that cannot be evaluate? In general, an thorough evaluation will test both functionality of your router extensions plus their performance. Also, performance measurements typically come into two forms: microbenchmarks that measure the impact on the performance of the router and end-to-end benchmarks that measure the performance as observed by the application.
Implementation plan: How will your team build the system. What are the main tasks to be performed? Who will do what? What are your project milestones - please have the milestones coincide with the status resport (10/21 and 11/21) and final presentation (12/7)? What are the major risks that might prevent you from finishing the project in time?