On Dec 21, 2007, at 4:11 PM, Abhik Sarkar wrote:
> I am facing an issue with dissection of SMPP PDUs with invalid length
> (though the matter might effect any other dissector which use TCP
> reassembly too).
...as well as affecting the receiving SMPP implementation. :-)
> In my test, I send two PDUs in two different TCP frames each of length
> 40.
The term that's used is "TCP segment".
TCP is a byte-stream protocol, so it doesn't expose a notion of
"frames" to protocols running atop it - the receiving system should
operate the same regardless of whether you sent:
one TCP segment with length 80;
two TCP segments with length 40;
80 TCP segments with length 1;
etc..
> However, the PDUs have their PDU length field incorrectly set to
> 41. With reassembly on in this case, the TCP dissector "steals" one
> byte from the second frame for the dissection of the first SMPP PDU.
> However, because the second PDU has now lost the first byte, it is no
> longer recognized as a valid SMPP PDU and hence is not dissected.
>
> With TCP reassembly turned off, the behaviour is that each frame is
> dissected as an SMPP PDU, but as malformed PDUs.
>
> Is this something to be expected,
Yes. The packets are being interpreted the way the SMPP spec says
they should be, i.e. there's a 41-byte message split between two TCP
segments, with the rest of the second TCP segment being the first 39
bytes of another 41-byte message.
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