Computer Networks Project 2

Files

• project is the folder to develop codes for future projects.
• grader contains an autograder for you to test your program.
• scenarios contains test cases and inputs to your program.
• docker-compose.yaml and Dockerfile are files configuring the containers.

Environment

• OS: ubuntu 22.04
• IP: 192.168.10.225. NOT accessible from the host machine.
• Files in this repo are in the /project folder. That means, server.cpp is /project/project/server.cpp in the container.
  • When submission, server.cpp should be project/server.cpp in the .zip file.

Project 2 Report

Project Members:
Justin Hu 205-514-102
Randy Gu 305-592-076

High level Design:

We divided the project up into 2 main sections: Processing a single IP packet, and everything else. Justin worked in main.cpp doing connection and map setups, dealing with sockets and select(), and placing a whole IP packet into one continuous char buffer. Randy worked in functions.cpp doing packet processing logic such as checksum calculations, drop condition checks, and address rewrites. In main, the entire IP packet is processed in buffer with one call to the IP processing function and then sent out to the listed destination or dropped.

Major problems:

We ran into two main problems, packets not being sent and checksum problems. Checksums are very annoying to deal with and issues were usually due to small details about the computation that we missed such as TCP padding. We also had a strange issue with packets not being sent out in time because the sockets were being closed, which seems to imply that our program was taking too long to run. Initially, we wrote the select() code as to be able to handle multiple IP packets at once as well as partial IP packets. In an act of desperation after hours of no progress, we deleted everything but the most basic code to only handle single, whole IP packets and our code magically started to work. We still have no explanation as to why this is the case, but we suspect that reading in 0 bytes from the socket was causing it to close prematurely, and so it turns out that the dumber, simpler way of assuming one packet per select was the right way to go.

References:

In learning how to use select(), we heavily referenced TA Boyan Ding's example code at https://github.com/dboyan/CS118-S23-1A/blob/main/Week%207/select.c as well as the Beej guide https://beej.us/guide/bgnet/html/split/slightly-advanced-techniques.html#select

Another student in discussion section 1D suggested the use of unordered maps to store address socket translations as well as the NAT table, which we found to be a good solution.

We referenced:

RFC 791 Internet Protocol
RFC 768 User Datagram Protocol
RFC 9293 Transmission Control Protocol

For details about headers and checksum computations.

How to use the test script

To test your program with the provided checker, go to the root folder of the repo and run python3 grader/executor.py <path-to-server> <path-to-scenario-file>.
For example, to run the first given test case, run the following command:

python3 grader/executor.py project/server scenarios/setting1.json
# Passed check point 1
# Passed check point 2
# OK

If your program passes the test, the last line of output will be OK. Otherwise, the first unexpect/missing packet will be printed in hex. Your program's output to stdout and stderr will be saved to stdout.txt and stderr.txt, respectively. You can use these log files to help you debug your router implementation. You can also read executor.py and modify it (like add extra outputs) to help you. We will not use the grader in your submitted repo for grading.

How to write a test scenario

A test scenario is written in a JSON file. There are 5 example test cases in the scenarios folder. The fields of the JSON file are:

• $schema: Specify the JSON schema file so your text editor can help you validate the format. Should always point to setting_schema.json.
• input: Specify the input file to the program. Should use relative path to the JSON file.
• actions: A list of actions taken in the test scenario. There are 3 types of actions:
  • send: Send a TCP/UDP packet at a specified port (port).
  • expect: Expect to receive a TCP/UDP packet at a specified port (port).
  • check: Delay for some time for your server to process (delay, in seconds). Then, check if all expectations are satisfied. All packets received since the last checkpoint must be exactly the same as specified in expect instructions. There should be no unexpected or missing packets
  • The last action of actions must be check.
• The fields of a packet include:
  • port: The ID of the router port to send/receive the packet, not the port number. The port numbers are specified in src_port and dst_port.
  • src_ip and src_port: The source IP address and port number.
  • dst_ip and dst_port: The destination IP address and port number.
  • proto: The transport layer protocol. Can only be tcp or udp.
  • payload: The application layer payload of the packet. Must be a string.
  • ttl: Hop limit of the packet.
  • seq: TCP sequence number.
  • ack: TCP acknowledge number.
  • flag: The flag field in TCP header. Should be specified in numbers. For example, ACK should be 16.
  • rwnd: TCP flow control window.
  • ip_options_b64: The IP options. Must be encoded in base64 if specified.
  • ip_checksum: The checksum for an IP packet. Automatically computed to be the correct number if not specified.
  • trans_checksum: The checksum in the TCP/UDP header. Automatically computed to be the correct number if not specified.
  • Most of these fields are optional, but omitting mandatory fields may crash the grader.

Please read the example JSON files and the schema JSON for details.

How to examine a test scenario

To print all packets in a test scenario in hex format, run python3 grader/packet_generate.py and input the JSON setting. You may also use < to redirect the input to the JSON file, like

python3 grader/packet_generate.py < scenarios/setting1.json
# ================== SEND @@ 01 ==================
# 45 00 00 1c 00 00 40 00  40 11 b6 54 c0 a8 01 64 
# c0 a8 01 c8 13 88 17 70  00 08 50 69
# ================== ========== ==================
#
# ================== RECV @@ 02 ==================
# 45 00 00 1c 00 00 40 00  3f 11 b7 54 c0 a8 01 64 
# c0 a8 01 c8 13 88 17 70  00 08 50 69
# ================== ========== ==================
#
# Check point 1
#
# ================== SEND @@ 01 ==================
# 46 00 00 20 00 00 40 00  40 11 b4 4f c0 a8 01 64 
# c0 a8 01 c8 01 01 00 00  13 88 17 70 00 08 50 69
# ================== ========== ==================
#
# ================== RECV @@ 02 ==================
# 46 00 00 20 00 00 40 00  3f 11 b5 4f c0 a8 01 64 
# c0 a8 01 c8 01 01 00 00  13 88 17 70 00 08 50 69
# ================== ========== ==================
#
# Check point 2
#