Path: utzoo!attcan!uunet!tut.cis.ohio-state.edu!snorkelwacker!apple!aux.support.apple.com!winders
From: winders@aux.support.apple.com (Scott Winders)
Newsgroups: comp.protocols.appletalk
Subject: Re: AppleTalk Phase 2 and FastPath Manager II?
Message-ID: <43310@apple.Apple.COM>
Date: 24 Jul 90 20:40:56 GMT
Sender: nntp@Apple.COM
Organization: Apple Computer, Inc.
Lines: 27
References:<9007231855.AA11208@jessica.stanford.edu> <4af6C1i00WCo40unYa@andrew.cmu.edu>

In AppleTalk Phase 2, the short-form DDP header is not "outlawed". The 
short-form DDP header is, and will continue to be, a valid DDP header 
format. 

The extended-form DDP header must be used on all AppleTalk Phase 2 
extended networks. Examples of AppleTalk Phase 2 extended networks include 
EtherTalk 2.x and TokenTalk 2.0. 

The only time the short-form DDP header can be used is on non-extended 
networks such as LocalTalk or EtherTalk 1.x. On a non-extended network, 
the short-form DDP header can be used for packets whose source and 
destination nodes have the same network number. The Link Access Protocol 
(LAP) header for LocalTalk and EtherTalk 1.0 packets contain the source 
and destination node IDs. Because of this, the source and destination 
network numbers and node IDs in the extended-form DDP packet are redundant 
when the packets source and destination nodes have the same network number.

Short-form DDP headers are used solely for efficiency reasons; in fact, an 
implementation of DDP is permitted to send extended-header DDP packets 
even when the source and destination nodes are on the same AppleTalk Phase 
1 or non-extended AppleTalk Phase 2 network. For example, the AppleTalk 
Internet Router uses extended-form DDP headers exclusively, even on 
non-extended networks such as EtherTalk 1.x.

Scott Winders
internet: winders@aux.support.apple.com
AppleLink: winders.s@applelink.apple.com