In some email I received from Christian E. Hopps, sie wrote:
[...]
>         sdl_index/sdl_nlen+sdl_data
>                         -- specifies which interface to receive frame
>                         from.  If no interface is specified then every
>                         interface is eligible.
>         
>         sdl_type        -- if no interface is chosen the user
>                         can select a specific interface type,
>                         e.g., IFT_ETHER.
> 
>         sdl_slen+sdl_data
>                         -- specifies a selector in the logical frame.
>                         e.g. for FRAMETYPE_E2 it would be a 2 octet
>                         value specifying the ethertype, and for
>                         FRAMETYPE_LLC it would be the destination
>                         service access point (i.e., llc_dsap).
[...]

sdl_data is used twice here...are these three therefore mutually exclusive
options ?  I think it would be useful to be able to specify a combination
of these things.

Darren