Subject: Frame-Relay Formats for HPR

From: "jim (j.) cobban" <[email-masked]>
Date: Mon, 20 Feb 1995 10:16:00 -0500

It is my belief that the proposed encapsulation for HPR over Frame-Relay is
unworkable in practice.  The encapsulation was proposed under the apparent
misunderstanding that there would never be more than one HPR connection over
a given Frame-Relay virtual circuit.  Considering that Frame-Relay is popular
as a connection between gateway or router boxes which act on behalf of
multiple down-stream stations, this is not a supportable conclusion.
Therefore the Frame-Relay encapsulation for HPR must have a data link layer
which provides the multiplexing functionality of 802.2 LLC.

An additional argument, and actually the one which led me to this
conclusion, is that it simplifies the implementation of the transmission
group setup.  That is because only a single implementation of 802.2 LLC
setup is required which can be shared between LANs and Frame-Relay.  The
null data link layer proposed in section 6.2.1 and documented in appendix
A.2.2 would force changes in the contents of the XID3s exchanged during route
setup.  For 802.2 LLC on a LAN the XID3 contains a CV61 with a subfield
X'80'.  With the proposed Frame-Relay encapsulation it would either not
contain the subfield, or the subfield would be ignored.  That requires CS to
be aware of whether this particular LLC connection is going over a LAN or a
Frame-Relay connection.  I don't know about the rest of the implementors, but
certainly our implementation does not have an option to make that
distinction.

The requirement to support multiplexing of multiple link connections between
a single pair of stations, whether on Frame-Relay or natively on a LAN,
causes problems for the proposal in section 6.2.2, to use a separate SAP for
HPR traffic in order to bypass the error checking.  I would like to see
discussion of how that would be implemented in the case of gateways which may
already be using all of the valid SAPs just to carry normal traffic.  Even if
there are TWO saps in use, how do they resolve the conflict over the use of
X'C8'?  Is it really that much more work to check for a UI than to check for
a special SAP?  Could IBM present actual performance measurements of the two
implementation choices to demonstrate that this complication is justified?

---

From: "Steven H. Berl" <[email-masked]>
Date: Wed, 08 Mar 1995 17:19:23 -0800

Has there every been any further discussion on this issue? It seems
like Jim has some excellent points. I'd much prefer that we used the
full LLC UI format encapsulated instead of the null dlc layer
specified in the HPR document.

Steve Berl
Cisco Systems, Inc.

> Date:    Mon, 20 Feb 1995 10:45:01 EST
> To:      [email-masked]
> 
> From:    "jim (j.) cobban" <[email-masked]>
> Subject: Frame-Relay Formats for HPR 
>