Wireshark · Wireshark-dev: Re: [Wireshark-dev] behavior of tcp_dissect_pdus when protocol pdu is across tcp segments

[wsgd <wsgd@xxxxxxx>]

I think that
the start of tvb (given to get_pdu_len) corresponds to the start of the pdu.
So offset will always be zero.

Olivier


Sudarshan Raghavan a écrit :
> I am currently working on adding a RTMP (Real Time Messaging Protocol)
> dissector to wireshark. RTMP sends out messages by splitting them into
> chunks. It starts with a default chunk size and sets it to a different
> value later if required. Each RTMP chunk will contain a chunk header
> and optionally a message header also.
>
> It is possible that a RTMP chunk starts at an offset inside the
> current TCP segment and spills over to the next TCP segment or later.
> My length function (get_pdu_len arg of tcp_dissect_pdus) returns the
> correct value to be able to get hold of the entire chunk. What i am
> seeing in this case (chunk across TCP segments) is that my length
> function is getting called as soon as the next TCP segment is seen and
> the offset argument passed is 0. I was expecting that tcp_dissect_pdus
> will call the length function at the appropriate offset in the next
> segment based on the length returned previously. Looking at the
> implementation of tcp_dissect_pdus in packet-tcp.c seems to confirm my
> analysis. Am I missing something here? How do I make tcp_dissect_pdus
> work correctly with chunks across TCP segments.
>
> Please note that it works fine if the chunk is contained entirely
> within a TCP segment.
>
> Regards,
> Sudarshan
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