Re: DLT_LAPB value assignment

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

Mirko.Karanovic () ttc ca wrote:

>  2Q: Or would it also contain an extra flag to 
> indicate whether the packet is a DTE->DCE or DCE->DTE packet?
>
> 2A:  LAPB contains direction ( DCE to DTE or vice versa) encoded in the
> Address byte.

Actually, what X.25 (10/96) says is:

	Frames containing commands transferred from the DCE to the DTE will 
contain the address A for the single link operation and address C for 
the multilink operation.

	Frames containing responses transferred from the DCE to the DTE will 
contain the address B for the single link operation and address D for 
the multilink operation.

	Frames containing commands transferred from the DTE to the DCE shall 
contain the address B for the single link operation and address D for 
the multilink operation.

	Frames containing responses transferred from the DTE to the DCE shall 
contain the address A for the single link operation and address C for 
the multilink operation.

with the address A being 0x03, B being 0x01, C being 0x0F, and D being 
0x07, so, for example, DCE->DTE frames can contain any one of those four 
addresses, as can DTE->DCE frames.

I.e., for frames going in a given direction, the address encodes whether 
the frame is a request or a response and whether single-link or 
multilink operation is being used.
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