Subject: Proposal for HPR Encapsulation on Frame-Relay

From: "jim (j.) cobban" <[email-masked]>
Date: Tue, 19 Mar 1996 11:51:00 -0500

The Frame-Relay encapsulation currently specified has a number of
limitations:

  1) It is not suitable for implementation in link level gateways, such as
     Frame-Relay Access Devices, because it requires that the FRAD understand
     details of the HPR protocol which are innappropriate in such a low level
     device.
  2) It does not permit multiplexing HPR link connections within a single VC
     something which increases network costs in many configurations.
  3) It places the RTP header on a non word boundary, thus voiding the work
     which was done to ensure that all subfields of the RTP header are 
     aligned on word boundaries for efficient processing.

I therefore propose an enhancement to the current encapsulation.

The existing specification, on page A-35 says "[The 802.2 LLC] subfield is
not used for direct communication over a Frame Relay link even though Frame
Relay uses LLC SAPs"

My proposal is to remove this restriction and have the Frame-Relay
implementation of LLC negotiate in the same way as the LAN implementation.
If the two ends negotiate to the use of the default SAP (X'C8') at both ends
of the link then the encapsulation described in section A.2.2 would be used.
However if either end requests a different SAP then the encapsulation
specified in section A.2.3, however with a UI command code, would be used.

Note that the existing specification is silent as to what happens if the
subfield is exchanged on a Frame Relay link.  The specification states that
the subfield is "not used".  In other words a station which sent this
subfield on an XID would not be compliant to the current specification.

A station which supports this extension, but has selected (or defaulted) to
X'C8' as its own SAP interoperates with stations compliant to the current
specification.  This was verified at the recent Interoperability Workshop.
However, in addition, a station which supports this extension will also
interoperate with stations which support this extension but have selected a
different HPR SAP value.

However note that a station which supports this extension, and selects a SAP
other than X'C8' will not interoperate with a station which is compliant only
to the existing specification, because the existing station, following the
Note on page A-35, ignores the 802.2 LLC subfield.  There is no method,
within the current definition of the XID, for a station to determine whether
or not the remote station honors the 802.2 LLC subfield because the existing
architecture insists that the subfield is always supported by a LAN based
station and is not used on Frame-Relay.  Therefore a frame-relay attached
station should never send an XID with a CV X'61' subfield X'80' unless it
knows that the far end supports it.

---

From: "Steven H. Berl" <[email-masked]>
Date: Tue, 19 Mar 1996 10:54:51 -0800

Jim,

Cisco fully supports this proposal. 

We would also like to see similar rules applied to the ATM environment
(using the RFC1483 encodings instead of RFC1490 of course).

Thanks,

Steve Berl
Cisco Systems, Inc.

> Date:    Tue, 19 Mar 1996 12:06:41 EST
> To:      [email-masked]
> 
> From:    "jim (j.) cobban" <[email-masked]>
> Subject: Proposal for HPR Encapsulation on Frame-Relay 
>