[ns] Re: ns problem: FullTcp packet size (fwd)

[Prithwish Basu <pbasu@bu.edu>]

I am using the latest version i.e. ns-2.1b7a. If I do the following at the
beginning of FullTcpAgent::recv(), then it works...

datalen = commonhdr->size() - tcphdr->size() - dsrhdr->size() - 20;

instead of:
datalen = hdrcmn->size() - tcphdr->size(); /* current */

Note that I subtract dsrhdr->size() since I'm using DSR as a routing
agent. But I want to know where are the residual 20 bytes getting added?

Thanks,
-Prithwish

On Fri, 23 Feb 2001, Lloyd Wood wrote:

> I've never used ad-hoc networking, and my comments related to the
> satellite routing code, which was adding (but not later
> subtracting) 20 bytes overhead for tunnelling. That copied some mobile
> code.
>
> And you don't say what version of ns you're using; I'd hope this was
> fixed throughout by now.
>
> anyone?
>
> munging packet sizes (rather than having specific flags) just isn't
> that robust.
>
> L.
>
> <L.Wood@surrey.ac.uk>PGP<http://www.ee.surrey.ac.uk/Personal/L.Wood/>
>
> ---------- Forwarded message ----------
> Date: Thu, 22 Feb 2001 17:46:58 -0500 (EST)
> From: Prithwish Basu <pbasu@bu.edu>
> To: l.wood@eim.surrey.ac.uk
> Subject: ns problem: FullTcp packet size
>
> Hello Lloyd,
>
> This is regarding the ns-2 simulator - in particular the FullTcp agent..
> Upon searching the ns users mailing lists I could see that you had written
> a response (sometime in June 2000) which could solve my problem; hence
> this mail...
>
> I was having trouble with the initial SYN/ACK exchange in FullTcp over a
> simulated wireless ad hoc network (currently static) using DSR for
> routing. I am getting errors like "Bad ACK (33) for my SYN (1)" etc. at a
> node that is expecting a SYN/ACK and there was a gap of 32 in all the
> subsequent retries. Then I traced that this is due to the "datalen"
> variable being set to 32 after the first SYN reaches. This happens in the
> beginning of FullTcpAgent::recv() as datalen = common hdr size - tcp
> header size = 72 - 40 = 32.
>
> Now, before sending the SYN packet, I traced that the common header size
> is assigned to the TCP base hdr size i.e. 40 but somewhere 32 is added to
> 40 in the lower layers and when the SYN packet shows up at the receiving
> node! I also traced that the routing protocol DSR was adding 12 bytes of
> header in my case, so that leaves the magical number 20 that is to be
> accounted for. In your response on the ns users list, you had mentioned
> something similar but it was not very specific. Can you please give me
> some more details on where this magic number "20" is added so that I can
> subtract it accordingly?
>
> As a quick test I tried subtracting 32 in FullTcpAgent::recv() and the
> initial ACK exchange was successful (!) but subsequent TcpApp data
> transfers failed... that's how I figured that a uniform solution is needed
> for this header size problem.
>
> Any help will be greatly appreciated!
>
> Thanks very much,
>

-- 
-Prithwish