Modify ASN1 developed dissector

[Victor Xiang <victorxiang28 () gmail com>]

Hi,



I am trying to use the TETRA dissector, which is developed in ASN1.



After a couple of weeks analyzing through the code with my TETRA packets, I
wasn’t able to see some of the MAC procedures described in the ETSI’s
drafts of this mobile communications protocol and the author of the
dissector confirmed me they are not implemented.

Specifically, I couldn’t see any dissociation nor reassembling after
fragmentation.



The process of association means that a logical channel of a packet can
contain multiple PDUs. So to do the dissociation I should add some loops
inside a logical channel to go through all the PDUs by checking the length
field of each one and separating them in a new subtree.

The fragmentation of a PDU is done in bits and not in bytes. I mean, the
fragmented part could begin in the bit number 10 of a PDU (not beginning in
a new byte). I would like to see the reassembled SDU in the last fragmented
packet.



As I am new in Wireshark development, I would like to know if it would be
possible to modify easily an ASN1 developed dissector to include this
features and a couple of tips of how should I begin proceeding. As there
are many files generated for this dissector, I am a bit lost and don’t know
what’s the road I should follow.



Thanks in advance



Victor Xiang

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