Path: utzoo!utgpu!news-server.csri.toronto.edu!rutgers!bellcore!uunet!olivea!apple!apple.com!kerlyn
From: kerlyn@apple.com (Kerry Lynn)
Newsgroups: comp.protocols.appletalk
Subject: Re: AppleTalk Phase2 confusion
Message-ID: <12363@goofy.Apple.COM>
Date: 1 Mar 91 23:07:10 GMT
References: <1991Feb25.174948.29698@isis.cs.du.edu>
Sender: usenet@Apple.COM
Distribution: usa
Organization: Apple Computer, Inc.
Lines: 33

In article <1991Feb25.174948.29698@isis.cs.du.edu> bediger@isis.cs.du.edu 
(bruce allen ediger) writes:
> Which is it: the local network, or a wildcard symbol?

Both.  Non-extended nodes use 0 internally to specify their network
address when there are no routers active on their cable, thus they
will accept long-DDP packets addressed to any network and their node
ID (but such packets are not generated by Apple implementations.)
Extended nodes ALWAYS have an internal net number != 0 or 0xFFFF.

The only acceptable use of network number = 0 in a Phase 2 packet is in
a cable-wide broadcast where dest net = 0 and dest node = 0xFF.  The
flowchart that matches the Mac DDPRead implementation can be found in
"AppleTalk Phase 2 Protocol Specification" (APDA #C0144LL/A) and
unambiguously demonstrates the use of net = 0 in extended and
non-extended cases.


> Hidden assumption in routing algorithm

The above mentioned flowchart expands somewhat on the one contained in
"Inside AppleTalk, 2e."  However, it has a hidden assumption that the
routing code (Figure 4-4) discards pkts addressed to net = 0.


> It would appear that packets entering the diagram have already been
> filtered on the basis on hardware address.

Indeed, the algorithm routes packets based on DDP address which implies
that the packet has already passed through the LAP layer and thus has
been "filtered."  A packet travelling through the internet gets a new
link level address for every router it passes through, but its DDP
destination address never changes.