Wireshark-dev: Re: [Wireshark-dev] Conversation tracking
From: Evan Huus <[email protected]>
Date: Fri, 11 May 2012 12:58:48 -0400
On Fri, May 11, 2012 at 12:25 PM, Tobias Weiss <[email protected]> wrote:

Thanks for your quick replies (Jeff & Lars).

I guess I have to explain my real problem in more detail. I want to implement a dissector for a quite old protocol that has 2 stages. The packets of the first stage have a fixed length (4 byte) and the packets of the second stage can have an arbitrary length but with a fixed header (8 bytes).

Unfortunately the content of a 4 byte frame can be the beginning of the 8 byte header. So I have to figure out where stage 1 ends and stage 2 starts. But as the payload of one TCP frame can contain the last stage 1 frame(s) and the first stage 2 frame, this is not straightforward. So my idea was to do this just once with the first packet and save the state in the conversation data and subsequently reuse that information. Because detecting the end of stage 1 is pretty easy if you know where you are in the stream.

And now I'm not sure if something like this could happen within the same conversation:

TCP connect -> stage 1 frames -> stage 2 frames -> TCP disconnect -> TCP connect -> stage 1 frames -> stage 2 frames

In this case I cannot just save the packet number where stage 1 ends if my dissector gets the same conversation for multiple connects/disconnects.

It should return two different conversations because the setup_frame should be different (search "guint32 setup_frame" in README.developer).  Perhaps this is a bug in the conversation backing? I can't check now, but I will take a peek tonight at some point.

Evan