Re: Defragmenting non-tcp streams

[paul () wsh sbrk co uk]

On Thu, Jan 07, 2010 at 03:26:32AM -0800, Guy Harris wrote:
> On Jan 7, 2010, at 1:48 AM, paul () wsh sbrk co uk wrote:
>
> > TCP has a tcp_dissect_pdus() function that higher protocols can use
> > to  eliminate their dependency on the TCP framing. Is there anything
> > generic for other layers? I'm writing a USB dissector which is really
> > a serial protocol being transported over USB.
>
> That's
>
>       http://www.nordicsemi.com/files/Product/data_sheet/User_guide_ANT.pdf
>
> right?

Correct. I've dissected most of the ANT commands and ANT+ device profiles
and am now doing the device to PC data transfers which are mainly over
burst transfers. That's for dissecting Garmin Forerunner 50, 310XT etc.

> If so, and if you're reassembling the packets in a burst transfer, this
> isn't a byte stream, so you can ignore the TCP dissector, and look instead
> at the IP dissector for an example.  It's not *quite* the same as IP, as

Well, I am reassembling the packets in a burst transfer, but first of all
I have to reassemble the bytes in the USB transfer which is just a byte
stream.
i.e. the stream is a sequence of [A4,len,cmd,[len bytes],chksum] with occasional
trailing null(s) and there maybe be more than one ANT packet in a USB
packet. Potentially, the ANT packets could also span multiple USB packets. So
that's the problem I'm trying to solve at the moment.
 
>       1) you have sequence numbers rather than byte offsets, so you'd
> use fragment_add_seq_check() rather than fragment_add_check();

I am doing that for the burst transfers.
 
>       2) the sequence number wraps around, so you'd have to keep
> some state and provide a "synthesized" sequence number that doesn't
> wrap around.

I'm doing that too.

My code so far is here: git://e2c3.com/nant.git in packet-ant.c

Cheers,

Paul
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