Assignment: Packet Demultiplexing

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This assignment is part of the Networking With Xinu track for professors that are Teaching With Xinu and it is intended to be completed in groups of two or three.


You will have to familiarize yourself with several common UNIX tools for this assignment. The first of these is tar, a utility originally devised to create tape archives for the purpose of backing files up onto computer tapes.

While tar is still used to create tape backups of file systems, it has become far more common to use tar to group files and/or directories together into a single entity, typically called a "tar-ball." (So common is the use of tar that it has been verbed in computer science terminology: We speak of "tarring" files, or files that have been "tarred up.") Tar syntax is somewhat arcane, as tar came into existence before modern standards for command-line options.

Change to your working directory and execute the following command. This untars the files into your working directory, if the tar-ball was created properly all the files should go into a subdirectory.

tar xvzf <tar-ball location> 

For more information on tar, please see the UNIX man pages.


While the gcc command-line options provide a great deal of flexibility when compiling programs, things quickly become unmanageable when the number of source files exceeds what you can conveniently type in a few seconds.

The make utility can be thought of as a companion to the compiler infrastructure (preprocessor, compiler, assembler, and linker) that allows the build rules for large projects to be explicitly encoded in Makefiles. A Makefile typically consists of common definitions, (such as, which compiler to use), and a set of rules. Each rule has a target, such as the file that is to be built, and can be followed by a list of dependencies and a sequence of steps to perform in order to build that target. In addition, make has quite a few common rules built into it.

You will not have to write your own Makefiles for this course, but you will have to use and possibly modify some for all of our remaining assignments. The Makefile is always human-readable, so feel free to open them up and look around.

To build the Xinu operating system, perform the following steps:

  • Change directory into the top level produced by the tar-ball.
  • Change directory into the subdirectory "compile". This directory contains the XINU project Makefile, and is where all of the compiled ".o" files will go.
  • Execute the the following command:
  make clean 
  • By standard convention, almost all Makefiles include a target called "clean" that removes everything except the source code. The tar-ball you unpacked already should be clean, but it never hurts to make sure that you are starting from a clean slate. You may find yourself using this command often.
  • Execute the the following command:

This should produce about a page of output as each source file is compiled, and the resulting object files are linked together to form the operating system, a simple set of library functions, and the boot loader. If all goes as it should, you should find the directory full of .o files from all of the source code in the other subdirectories, and most importantly, a newly compiled operating system image called "xinu.boot."

For more information on make, please see the UNIX man pages.


Your Xinu image is now ready to be run on a backend machine. To transfer it there, we have a special utility called mips-console. Execute mips-console in the compile directory where your xinu.boot file resides. Mips-console will connect your terminal to the first available backend machine, and you should see a message like:

connection 'xinurouter', class 'mips', host 'xinuserver-hostname'

This will be immediately followed by a stream of automated commands as the embedded target system boots, configures its network settings, and uploads your xinu.boot kernel.

The most important thing to remember about mips-console is that it is modal, like vi. You start out in direct connection mode, in which your terminal connects directly through special hardware to the serial console on your backend machine. To get out of mips-console, hit Control-Space, followed by the 'q' key.

Packet Demultiplexing

The packet demultiplexing assignment will have students implement a way for the Embedded Xinu operating system to interact with the ethernet driver, both receiving and sending of packets. Incoming packets should be properly divided up (demultiplexed) and passed along to the next part of the system or stored. Outgoing packets should have the final ethernet header fields filled in before sending it off to the driver.

Assignment Notes

  • Implementation can vary based on how the professor intends to grade the assignment
    • Have the students insert print outs of what type of packet is received and print something out when one is sent out
    • Give the students some portion of code so they can pass the packet onward to a piece of the operating system they have yet to write

Student Outcomes

With the completion of the assignment students should understand how demultiplexing works and have knowledge about the complexities of optimizing packet demultiplexing. Students should have a feel for how the Embedded Xinu operating system interacts with the ethernet driver and understand that initial/final crucial step for network communication.