Wireshark-dev: Re: [Wireshark-dev] Handling TCP packets reordering
From: Sake Blok <sake@xxxxxxxxxx>
Date: Wed, 4 May 2011 23:29:51 +0200
On 4 mei 2011, at 22:48, Jeff Morriss wrote:
> Sake Blok wrote:
>> On 4 mei 2011, at 22:11, Jeff Morriss wrote:
>>> Max Dmitrichenko wrote:
>>>> Hi!
>>>> I'm continue to write dissector for an encrypted protocol. Everything
>>>> works fine until I receive an out-of-order TCP segment, i.e. previous
>>>> was lost.
>>>> Since I'm trying to decrypt it, I fail with it and break the whole
>>>> decryption context. Is there any way to:
>>>> 1) Detect that this packet is out of order in given conversation?
>>>> 2) Ask the TCP dissector to feed this packet later again when all
>>>> previous segments will be retransmitted?
>>> I would think desegment_tcp() should be able to handle this by not calling your dissector for an out-of-order segment: it should be able to only call your dissector once it has a completely reassembled (desegmented) PDU.  Looking through the code, it's not immediately obvious to me what the problem is.
>> One case that can cause a problem is when the first segment of a PDU is received out-of-order. Or did your recent work also handle this exception, Jeff?
> Yep, that's the case rev 36304 fixed.

Unfortunately not, I just constructed a file in which the first segment of the HTTP-response PDU comes after the second segment of that PDU. And dissection still fails as there is not yet a segment in tcpd->fwd->multisegment_pdus for that seq. 

This could be fixed by adding a segment that comes after a gap to the multisegment list by default, but that would break dissection of single segment PDU's that come out-of-order. I don't know of a good way to solve this dilemma.