Desegment_offset not working properly whem reassembling protocols running atop tcp

[Oleksandr Tyberkevych <oleks101 () gmail com>]

Good Evening,

I've been writing a dissector for a custom protocol running on TCP, and
when trying to use desegment_offset method for reassembly, I encountered
some weird behaviour.
Suppose your current tvb contains a buffer of [a, b], where a and b are
some bytes signifying the start and end of the buffer. Setting
desegment_offset=some_offset and desegment_len=DESEGMENT_ONE_MORE_SEGMENT
should make Wireshark provide a tvb that contains a buffer [a+some_offset,
b+next_packet_data]. However, it sometimes provides [a, b+next_packet_data]
instead -- almost as if it completely ignores the desegment_offset value.
This happens when I have a tvb buffer built up of about 5 pieces of data
from TCP packets, and I try to drop off one of the front packets (I
determine this offset by my own algorithm, but it is always a valid value
within the buffer).

Is this likely a bug, or, alternatively, does anyone know what could be
causing this kind of behaviour?

Thanks in advance,
O.T.

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