Re: DLT for LLCP

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

On Feb 8, 2012, at 3:26 AM, mike wakerly wrote:

> I'd like to request a new encapsulation type for NFC Logical Link
> Control Protocol (LLCP) [1].

        ...

> A new encapsulation type is needed for reasons similar to i2c and
> bluetooth.

No, it's needed because no existing link-layer header type listed on the page at

        http://www.tcpdump.org/linktypes.html

corresponds to the NFC LLCP; even if there were *no* pseudo-header there would *still* need to be a new link-layer 
header type.
        
> Hosts need to record frame directionality in addition to
> raw frame data.  As an example, a psuedo-header prepended to
> individual LLCP frames recorded in this format might look like this:
>
>  struct llcp_phdr {
>    guint16 adapter;  /* Adapter number, typically 0. */
>    guint16 flags;    /* Direction flag (TX/RX) and future use. */
>  };
>
> Can you allocate a new DLT_ for this?

Yes, once there's a firm final specification for what the pseudo-header will look like for all frames.  (If there's 
more than one type of pseudo-header, there will have to be more than one link-layer header type value allocated, one 
for each pseudo-header type.)

Presumably what follows the pseudo-header will be an LLCP PDU of the form described in section 4.1 "LLC PDU Format" of 
the LLCP specification, i.e. a byte containing the DSAP and upper two bits of the PTYPE, followed by a byte containing 
the lower two bits of the PTYPE and the SSAP, followed by the sequence byte if the frame has sequence numbers, followed 
by the bytes of payload.
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