CODEWITHSUNDEEP
cnetwork-programmingtcp
0xfee1dead
0xfee1dead

Reputation: 116

Inconsistent implementations of TCP

I'm implementing TCP in C/Obj-C.
I have noticed that different servers increment the sequence numbers on some occasions while others don't. Namely in the teardown process when the server sends a FIN/ACK some servers increase the ACK number by 1 while others dont.

To clarify the problem:

Server 1: The ACK number has been increased to 2 Working TCP-Teardown

Server 2: The ACK number is still 1. ACK number not increased http://img853.imageshack.us/img853/1248/zf70.png

The output of my program regarding the second server:

    FIN(/ACK)# was 18238 but should have been 18239

How should I deal with these kinds of variations of server-side implementation in my code?

Upvotes: 3

Views: 138

Answers (1)

Gautam
Gautam

Reputation: 1119

Are you referring to some RFC(s) for your implementation? I don't know which RFC contains the correct behaviour (to increment the ACK number or not) when sending a FIN+ACK, but my guess would be that the ACK number should indeed be incremented.

That said, we all know how 'RFC compliant' implementations can be... (and not just TCP implementations)!! That too, for message exchanges that teardown the connection, those who wrote the software must have been extra careless. So now you have two options:

  • handle both cases in your code - when the ACK is incremented and when it is not - and close down your end of the connection properly, or
  • crap out with an RST for an incorrect ACK (refer RFC for correct behaviour), but still close down your end of the connection properly

This is a fairly common problem when dealing with distinct implementations of the same standard (which defeats the whole purpose of a standard in the first place). So be warned that this may just be the first of such problems you might encounter.

I have often had to add additional code to handle specific cases when an implementation of a given RFC would work beautifully in Linux, and fail the most basic test on Windows.

Upvotes: 1

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