Re: Request for a new DLT for MTP2 with FCS

[Florent.Drouin () alcatel-lucent fr]

I agree with you, the problem I am speaking about, seems to be a common
problem.
It is very difficult to differentiate a frame with FCS (Frame Check
Sequence) and a frame without, if we are just looking at the linktype.
But, with the current libpcap, I do not see other solutions, than using
different linktypes.

Is it really a problem to create new linktypes, just for such purpose ?
(I understood that the linktypes are coded on 4 bytes )

Regards
Florent




                                                                                                                        
               
                      Stephen Donnelly                                                                                  
               
                      <stephen () endace com>                 To:      tcpdump-workers () lists tcpdump org             
                     
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                      tcpdump-workers-owner@lists.         Subject: Re: [tcpdump-workers] Request for a new DLT for 
MTP2 with FCS      
                      tcpdump.org                                                                                       
               
                                                                                                                        
               
                                                                                                                        
               
                      07/02/2007 05:37                                                                                  
               
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                      tcpdump-workers                                                                                   
               
                                                                                                                        
               




On Tue, 2007-02-06 at 18:18 +0100, Florent.Drouin () alcatel-lucent fr
wrote:
>       Hello,

> I need a new datalink value (LINKTYPE_MTP2_FCS) to manage MTP2 frame with
> Checksum.
> (See http://www.wireshark.org/lists/wireshark-dev/200702/msg00206.html )
>
> Once I will have the value, I will provide the patch for libpcap.

Endace DAG cards (for instance) can capture MTP2 with or without FCS,
does your proposal mean that MTP2 without FCS should only use
LINKTYPE_MTP2 and MTP2 with FCS should only use LINKTYPE_MTP2_FCS?

What about other linktypes, such as Ethernet?

The issue of whether captured packets should include FCS and how this is
reported in libpcap seems to be a more generic question. I'm not sure
that duplicating all existing DLTs by adding _FCS (or tripling with
_NOFCS as well) at the end would be accepted by the libpcap community.

If people are generally happy with adding LINKTYPE_MTP2_FCS as a special
case I have no problem, and Endace can support both linktypes.

Stephen.
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    Stephen Donnelly BCMS PhD           email: sfd () endace com
    Endace Technology Ltd               phone: +64 7 839 0540
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