Re: Request for a new DLT for MTP2 with FCS

[Guy Harris <guy () alum mit edu>]

On Feb 19, 2007, at 12:07 PM, Stephen Donnelly wrote:

> It seems that if it is worth making the change, it is also worth  
> using a
> couple of bits to indicate whether a 16 or 32-bit CRC/FCS is present  
> as
> Guy suggested. This could then be used on linktypes such as PPP_SERIAL
> which can have either length.

I checked in a change, based on Florent Drouin's patch, which splits  
DLT_ and LINKTYPE_ values into:

	a 10-bit class field, where a class of 0 is for regular DLT_/ 
LINKTYPE_ values and a class of 0x224 is for NetBSD "RAWAF" values (I  
don't know whether they're still using them or not);

	a 16-bit value within that class, which, for a class of 0, is a  
regular DLT_/LINKTYPE_ value (note that the linktype field in a pcap- 
NG file is 16 bits);

with the class field at the top (so the old DLT_/LINKTYPE_ values  
still work), and splits the linktype field in the libpcap file header  
into:

        a 4-bit FCS length field at the top;

        a reserved bit below that;

	a 1-bit field below that, which, if 1, indicates that the FCS length  
is present;

        a 26-bit link type, split as per the other split described above.

(note that pcap-NG already has a mechanism to supply FCS length  
information), adds a pcap_datalink_ext() API to get the extra 6 bits  
of information from the linktype field, and adds macros to manipulate  
that field.
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