Networking – how to calculate the size of MSS for a “mythical set of protocols”

In this post: A detailed, step-by-step guide demonstrating the steps I use to solve a problem similar to one encountered in my CS 372 Intro to Networking class.

Imagine a mythical set of protocols with the following details:

  • Maximum Link-Layer data frame: 1,331 bytes
  • Network-Layer header size: 29 bytes
  • Transport-Layer header size: 34 bytes

What is the size, in bytes, of the MSS?

Answer:

1331 - 29 - 34 = 1268

You just subtract the header sizes from the data frame. Easy.

You probably could’ve guessed that, but if you’re in CS372 and you’re reading this then it might be because you weren’t 100% sure wanted to check your work but the class materials didn’t really cover it. And that’s the thing that irked me about this problem: the class just threw it at us as a surprise on the weekly (graded but open-book) exercises in week 6. Then you go a-Googlin’ and end up somewhere like chegg.com who wants $15 to sell you a solution! That’s just silly.

Now you know: calculating the maximum segment size is just a simple subtraction problem.

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