Upper Layer Protocol ID Proposal

[Steve Van Trees <[e-mail]>]

Upper Layer Protocol Identifier Proposal

1. Background

Various industries have expressed immediate needs for
communications that  require minimum overhead in the upper layer
standards.  Coordinated work is  ongoing in ITU-T, ISO/IEC JTC 1/SC
21, OSE Implementors Workshop (OIW), and ANSI to address these
needs.  In particular, the contents of document TD - 5161 and  TD -
 5183, generated at the ITU-T SG7 meeting 7-18th February, 1994,
should be  referenced.  The approach proposed below combines
various efforts going on in  ISO, ANSI and ITU-T.  This combines
Fast Byte proposals in ITU-T with ongoing work for the use of
A2CSE.

2. Technical Approach

This paper proposes an expansion of the initial ITU-T concepts on
fast byte to  allow a single byte to act as an Upper Layer Protocol
Identifier (ULPI).  The  general fast byte approach is described in
Appendix K of the ITU-T report, but  some details are not fully
worked out.  In the context of this paper, the fast  byte would
identify current standard OSI implementations using the first octet 
of the session connection establishment PDU (Session PDU Identifier
= H '0D').   As discussed in ITU-T this allows the possibility of
using a Fast Byte value  (e.g. H '0F'), to indicate a specific
alternate stack option.

Looked at in the wider context, the Fast Byte should be expanded to
recognize  this value as a one octet flat address space indicating
various stack options.   Taking an architectural approach modeled
after the ISO 9577 (Network Layer  Protocol Identifier) standard,
the Fast Byte octet then identifies multiple  stack options above
transport, and acts as default header for layers as needed.

ACSE is being modified to support the eXtended Application Layer
Structure  (XALS).  This work is being progressed both in ANSI and
ISO.  The modification  is in the form of an added functional unit
to ACSE to allow the ability to  support Application Service
Objects (ASOs), recursive use of software, and other benefits of
the XALS structure.  ACSE with extensions is referred to as ASO 
Association Control Service Element (A2CSE).  At various recent
meetings of ANSI and OIW the majority wish was to define a
mechanism allowing minimal to no  Session and Presentation PCI
overhead in an A2CSE stack option.

The immediate need within the aeronautical environment is a method
to support  A2CSE encoded using the unaligned variant of Packed
Encoding Rule (PER),  immediately following the "fast byte" value
indicating this option.  While the  idea of having that value
corresponding to the first octet of the association  establishment,
was explored, for still greater efficiency, this does not appear 
to be possible, because A2CSE encoding options could conflict with
the H'0D'  value for session.

3. Proposal

This paper recommends the creation of a Fast-Byte standards
document and corresponding registration authority.  It recommends
that ANSI reserve the  initial values of:

               > one-bit session   /\   one-bit session
                        0         /  \        1
                                 /    \
                                /      \
                  byte-aligned /\ bit  /\
                      0       /  \1  0/  \ 1
                             /    \  /    \
                            /              \
 
          00'D'  8326 + 8822                00  Transparent
          10'0'  connectionless session     01  BER
          00'1'  Fast Byte                  10  bit-aligned PER
                                            11  byte-aligned PER

'01'   Fast Byte for Session;

'0D'   Fast Byte which indicates and is the first octet of
traditional Session, Presentation, and ACSE;

'20'   Fast Byte which indicates and is the first octet of
connectionless Session, Presentation, and ACSE;

'8' for Fast Byte value which implies byte-aligned transparent data
above the Fast Byte value.

'9'    Fast Byte acting as both byte-aligned Session and
Presentation header, and as the first octet of ACSE encoded using
BER.

'A'    Fast Byte acting as byte-aligned Session and Presentation
header followed by ACSE encoded using the PER unaligned variant;

'B'    Fast Byte acting as both byte-aligned Session and
Presentation header, followed by ACSE encoded the PER octet-aligned
variant;

'C' for Fast Byte value which implies bit-aligned transparent data
above the Fast Byte value.

'D'    Fast Byte acting as both bit-aligned Session and
Presentation header, and as the  first octet of ACSE encoded using
BER.

'E'    Fast Byte acting as bit-aligned Session and Presentation
header followed by ACSE encoded using the PER unaligned variant;

'F'    Fast Byte acting as both bit-aligned Session and
Presentation header, followed by ACSE encoded the PER octet-aligned
variant;