• Bowen Zhang, UID 905322977. Contributions include IP prefix lookup code, IP-packet handling code and ICMP Message Parsing Code. Special thanks are given due to his dogged persistence when hunting the bugs nobody else could.
• Varun Pidugu, UID 704961666. Contributions include ARP cache management and ARP-request sending code and IP packet handling code.
• Yash Lala, UID 905159212. Contributions include Ethernet-frame handling code, ARP-request receiving code, ARP-IP-packet-flushing code, bug fix in IP and ICMP handling code, and this README.

Remaining contributions can be found in the commit history.

The design of this code does not deviate in any meaningful way from the skeleton code provided. The code functions as follows:

1. When it receives a packet, the router:
   • Checks if it is an ARP reply, and updates its cache accordingly.
   • Checks if it is an IP packet destined for the router, in which case it either responds to a ping request or returns an ICMP Not Reachable packet.
   • Checks if it is an IP packet destined for another IP, in which case it looks up the packet and forwards it to the appropriate interface. If the destination MAC address is unknown, it sends out an ARP request, then flushes the outbound packet queue when interpreting the response.
2. While undertaking other actions, the router keeps track of IP->MAC address associations for each interface in a cache table. When a cache entry becomes stale, it is removed from the queue.

While some code is duplicated, the code is functional and (as far as the writer knows) bug free given certain assumptions about incoming packets.

This project involved a lot of bugs and associated bug fixing. Mininet was notoriously uncooperative; even the final version of this project intermittently fails given certain factors, including (but not limited to):

1. The timings of the ./router and ./run.py commands.
2. Mininet's 'freshness'; ie. if it has been run before in the container.
3. Mininet's mood.
4. The alignment of the moons of Jupiter.

In order to deal with these challenges, we attempted to coordinate our work in a way that would minimize redundant code and fixes. We did so by using Git to coordinate our feature branches and fixes. While not all of us knew how to use version control at the beginning (and arguably may not know so now), we managed to get a workflow that allowed us to quickly coordinate changes and allow for independent work.

While it may sound hand-wavey to claim 'expedient communication' was our only real method when solving problems, we claim that a project of this size really required nothing more. When we were running close on time, we split our teams as per their strengths. Those new to C/C++ programming spent their time reporting logic errors (the writer would once again like to thank Bowen, without whom this project would still be a buggy mess), while those new to remaining patient opted to spend their time coding the described fixes and features.

GDB was not particularly useful due to an unexpected permissions error, so most of our debugging was done through the tried-and-true printf method.

The provided Makefile provides several targets, including to build router implementation. Additionally, the Makefile contains a clean target, and tarball target to create the submission file as well.

For build dependencies, please refer to Vagrantfile.

When POX controller is restarted, the simpler router needs to be manually stopped and started again. In addition, the code may fail when responding to certain oddly formed IP packets (although this is not expected to occur during the program's operation).

This implementation is based on the original code for Stanford CS144 lab3 (https://bitbucket.org/cs144-1617/lab3).