Subject: APPN ISR over ATM

From: "Steven H. Berl" <[email-masked]>
Date: Wed, 27 Sep 1995 11:40:49 -0700

Since this is not really an HPR issue, I have carbon copied this
e-mail to the aiw-appn-reference mailing list.

In the ATM-01 document section 1.2.3 entitled "Native ATM DLC", in the
secod to last paragraph, it is stated that:

      "Because HPR is a prerequisite for the native ATM architecture
      described in Chapter 2, "Native ATM DLC", the native ATM architecture
      does not support APPN/ISR FID2 traffic. Likewise, it cannot be used
      for links between DLUR nodes and down-stream nodes with dependant LUs.
      Additional architectural definition is required for such
      configurations. For example, an LLC such as IEEE 802.2 type 2
      providing error recovery must be specified."

In the ATM Forum Technical Committee, a contribution has been
submitted by IBM entitled "Multiprotocol over ATM Adaptation Layer
Type 5 Implementation Agreement". This is ATM Forum/94-0615. This
document describes the encapsulation of 802.2 type 2 traffic for
APPN/ISR FID2 traffic, as well as several other types of SNA traffic.

This sounds like the "additional architectural definition" refered to
in the ATM-01 document. I propose that this be accepted as part of
the APPN architecture as a standard DLC for transporting non-HPR
SNA traffic over and ATM network. 

Our implementation experience has shown that running LLC2 native over
ATM has value. 

- It was extremely easy to implement. It is very similar to
  LLC2 native over Frame Relay.

- In low error rate environments the lack of selective retransmission
  has not been an issue because one very rarely needs to retransmit anything.

- There are a lot of people around who already have a good
  understanding of LLC2 and are able to look at decoded line traces,
  etc. (How many ATM line analyzers are available today that can
  decode RTP?) 

Of course, HPR traffic should be handled using all the extensions and
services presented in the ATM-01 document proposals. For the near
term, it would be good to accept the existing ATM Forum standard for
APPN ISR over ATM.

Steve Berl
Cisco Systems, Inc.