Re: Linux Frame-Relay and misc

[Krzysztof Halasa <khc () pm waw pl>]

Guy Harris <guy () alum mit edu> writes:

> Do ARPHRD_DLCI devices supply a useful link-layer header (from which the
> protocol running atop Frame Relay can be determined), or not?

No. It's a virtual device and FR headers are added/removed by underlying
physical device code.

> It shouldn't break anything, unless in some kernel the right ARPHRD_
> type for Cisco HDLC has the name ARPHRD_HDLC and a value other than 513,
> which I presume isn't the case.

Sure.

> So are Q.933a and ANSI LMI the same thing, or is one an extension of the
> other, or are the two different such that, to correctly dissect a frame,
> you have to look at, for example, the DLCI?

No, they aren't the same thing. You can tell if it's ANSI by looking
for ANSI locking shift byte, 0x95. It should be just after "call
reference" and "message type" byte fields.

They are quite similar, though - in addition to lock shift byte, ANSI
uses 0x1, 0x3 and 0x7 report types while Q.933a uses 0x51, 0x53 and 0x57
for respectively status, link integrity verification and PVC status.
There might be other differences, but I'm not aware of any.

Both ANSI (ANSI T1.617 annex D) and ITU-T/CCITT (ITU-T Q.933 annex A)
use the same DLCI 0 for LMI. Cisco (the original) LMI uses DLCI 1023.

> I assume it's intended to handle the Q.933 "single octet information
> elements" such as Shift and Repeat indicator, although it doesn't
> actually do any shifting....

Hmm... Never heard of such elements. I will investigate a little
further.

Thanks.
-- 
Krzysztof Halasa, B*FH
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