Hi,
You need to modify the template and possibly the .cnf file to insert your changes into the generated code there shouldn’t be a need to modify the ASN1 files.
I have no knowledge of the protocol itself so this is just general statements. If the UDP packets contains multiple PDU’s you probably have to check the PDU
length
And loop trough the UDP data until all of the buffer has been read. Reassembly should be done in a similar way if the PDU size is bigger than the received UDP
segment
You should call the reassembly code until enough data has been received. It looks like it might be unaligned PER which might be a challenge to handle if PDUs
don’t align to byte boundaries, if fragments don’t align shouldn’t be a (big) problem.
Regards
Anders
From: wireshark-dev-bounces@xxxxxxxxxxxxx [mailto:wireshark-dev-bounces@xxxxxxxxxxxxx]
On Behalf Of Victor Xiang
Sent: den 2 oktober 2014 01:17
To: wireshark-dev@xxxxxxxxxxxxx
Subject: [Wireshark-dev] Modify ASN1 developed dissector
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
