Hi, on the top of my head, I was thinking of something like:
(assuming we're at the Service Info. Octet)
if( (tvb_get_guint8(offset + 7) >> 5) == 0
&& (tvb_get_guint8(offset + 9) >> &) == 0 )
format = ANSI;
else if (tvb_get_guint8(offset + 6) >> 4) == 0) format = ITU;
that is, going to some octet, and checking whether the spare bits are effectively spare (ie: zero) or not.
Of course this relies on the fact that spare bits are indeed enconded as zero, which I believe is the general recommendation.
Some special combination of CICs, OPCs and DPCs could make this method fails.
Another idea that comes to mind is going to the octet where the message type (IAM, etc...) is stored and check whether it's a message type octet or not
m_type = tvb_get_guint8(offset + 10)
switch (m_type) {... case IAM: format = ANSI; break;
case ACM: format = ANSI; break ... }
m_type = tvb_get_guint8(offset + 7)
switch (m_type) {... case some_message_type: format = ITU...}
Again, this could fail under some conditions.
Best Regards
Javier
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