Path: utzoo!attcan!uunet!cs.utexas.edu!tut.cis.ohio-state.edu!ucbvax!UM.CC.UMICH.EDU!Dave_Katz
From: Dave_Katz@UM.CC.UMICH.EDU
Newsgroups: comp.protocols.tcp-ip
Subject: TCP/IP and OSI over FDDI
Message-ID: <4912147@um.cc.umich.edu>
Date: 20 Sep 89 13:42:44 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The Internet
Lines: 36

>Does anyone know if there is a document that specifies how to
>route an FDDI frame to OSI or TCP/IP protocol stacks?  Even within
>TCP/IP there is a question on how to route ARP and RARP traffic.
>The main burning question is as follows: How can/does the FDDI
>software distinguish between traffic intended for OSI, IP, RARP,
>or ARP?
 
For TCP/IP, RFC 1103 proposes a method of framing ARP and IP
over FDDI using 802.2 LLC and SNAP headers (in which the
classic "ethertype" field is carried).  This document is only
a draft, but the fundamentals points aren't likely to change.
 
For OSI, there's no official document as to how Network Layer
protocols will be carried, although it's essentially a foregone
conclusion that 802.2 LLC will be used, with an LSAP value
of "OSI Network Layer Protocol."
 
So, to answer the question directly, the algorithm is something
like:
  If FDDI LLC frame
    [assume 802.2 LLC in use]
    if LSAP = SNAP
       determine protocol type based on SNAP type
    else if LSAP = OSI NL
       determine protocol type based on first octet of NL packet
    else
       ???
 
It appears likely that the SNAP format will be used for all
protocols that run with Ethernet framing, not just IP and ARP.
 
Protocol identification for OSI is done at the network layer; the
first octet of the packet determines which of several protocols
(CLNP, ES-IS, X.25, etc.) is in use.  An ISO Technical Report,
"Protocol Identification at the Network Layer," (the TR # escapes
me) spells this out.