Wireshark-dev: Re: [Wireshark-dev] ASN.1 bit string alignment
From: "Neil Piercy" <[email protected]>
Date: Mon, 29 Mar 2010 13:14:14 +0100
Looking at the changes in packet-per.c, it looks like the changes were
deliberate to remove leading pads and return a tvb which had trailing
bits padded - which sounds reasonable to me: I suspect the tvb
bit-offset code assumes no leading bit pads: correct?

If so, it becomes just a display problem.

Given that the bit string is of arbitrary length, and not necessarily
interpreted as an integer, how would we _want_ the example below to be
displayed?

I think the basic problem is the attempt to display a bit string in hex
- would it be better to display it in binary (if say <=32 bits) and have
its (unpadded) integer value (hex or dec?) included too if small enough
- e.g.

dl-HFN: 0000000000000000000000100 (0x4) [bit length 25]

Neil

> -----Original Message-----
> From: [email protected] [mailto:wireshark-dev-
> [email protected]] On Behalf Of Anders Broman
> Sent: 26 March 2010 20:53
> To: Developer support list for Wireshark
> Subject: Re: [Wireshark-dev] ASN.1 bit string alignment
> 
> Looks like a bug, please make a bug report and attach a sample trace.
> Regards
> Anders
> 
> -----Original Message-----
> From: [email protected] [mailto:wireshark-dev-
> [email protected]] On Behalf Of Neil Piercy
> Sent: den 22 mars 2010 19:31
> To: Developer support list for Wireshark
> Subject: [Wireshark-dev] ASN.1 bit string alignment
> 
> In 1.0.x PER ASN.1 bit strings used to be padded in the MSBs, (e.g.
> extract from an RRC message):
> 
> dl-HFN: 00000004 [bit length 25]
> 
> Since 1.2.x they appear to be padded in the LSBs (e.g. the same
> extract):
> 
> dl-HFN: 00000200 [bit length 25]
> 
> The value carried in both cases above is meant to be 4.
> 
> Is this a deliberate change?
> 
> I know bit strings are of arbitrary length, therefore a "natural"
> representation is challenging, but I find the above counter-intuitive.
> 
> Neil
> 
> 
> 
> 
> 
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