From: glen@Cayman.COM (Glen B. Glater)
Newsgroups: comp.protocols.appletalk
Subject: Checksum bug in LW NTR
Date: 24 Jul 92 14:48:36 GMT
Distribution: comp.protocols.appletalk, comp.sys.mac.comm
Organization: Cayman Systems Inc., Cambridge, MA

I am posting this message so that people with NTR printers will
understand the causes for NBP LkUp problems that they may be experiencing.
It is not a flame.  As you will read below, Apple understands the 
importance of fixing this problem.

Apple has a new Personal LaserWriter called the NTR. 

It has a bug in its ROM code.  If the printer gets a packet with
a DDP checksum that is not zero, it will ignore the packet.

Why am I posting this?  Because two major players at our customer sites, 
CAP 6.0 and Cisco Routers, both add DDP checksums to packets in their 
default configurations.  That means that the LaserWriter NTR will not  
respond to lookups from a CAP 6.0 server or respond to lookups that have 
been generated by a Cisco Router with DDP checksums turned on (in response 
to an NBP BrRq or forward request, or in response to a directed Phase 2 
broadcast).  

We have already had one customer who thought that this was a Cayman
problem since, after all, we are connecting the printers to the
Ethernet.  This has nothing to do with the GatorBox, and the same
problem will occur on networks connected with any other LocalTalk-
Ethernet gateway (FastPath, EtherRoute, MultiGate, Liason, Apple Internet
Router, etc.)

Here is the official response from Apple.  

>Glen,
 
>I've spoken with the LaserWriter NTR test engineer, who forwarded the
>following informaion.
 
>"The problem your customer is experiencing is due to a bug in the Personal
>LaserWriter NTR ROM. It appears that the Personal LaserWriter NTR ignores
>AppleTalk packets that have a non-zero value in the DDP checksum field of DDP
>Long packets.
 
>To workaround this it is necessary to disable the DDP checksums in the CAP
>modules that talk to printers (i.e. atlook*, papif and tlw).
 
>CAP uses DDP checksums by default. The "unofficial recommended interim
>workaround" for this is to add the following lines to the m4.features file,
>rerun gen.makes and remake CAP:
 
>    # + DONT_DOCHKSUM no DDP checksums
>    define(`specialcflags',concat(specialcflags,` -DDONT_DOCHKSUM'))
>    #
 
>No other code changes are necessary. If you feel adventurous, you can patch an
>unstriped papif with adb, set the variable 'dochecksum' to zero.
 
>Apparently, an upcoming CAP patch will provide a command line argument to
>disable DDP checksums.
 
>There is another related situation we have discovered here at Apple. We use
>Cisco routers and some of our cisco routers have DDP checksums turned on. If a
>NBP lookup is done through a cisco router with DDP checksums turned on, the
>Personal LaserWriter NTR will not reply to the NBP lookup and not show up in
>the Chooser. The workaround here is to disable DDP checksums on the cisco
>router."
 
I've been told by Apple that this is "a high profile problem to be 
resolved ASAP".

By the way, here is info from Cisco on how to disable this feature in 
their products:

>In configuration mode on the cisco, enter the command
  >
  >---> no apple checksum
  >
  >
  >*** The above is a router configuration command. If the customer is
  >familiar with our product, the above should make sense. (to enter
  >configuration mode, type 'configure terminal' from the '#' prompt)
  >
  >If the customer would like to determine if checksums are being
  >calculated, the EXEC command 'sho apple global' will give that
  >information. Here is a sample display:
  >
  >#sho apple global
  >
  >AppleTalk global information:
  >  Internet is incompatible with older, AT Phase1, routers.
  >  There are 3 routes in the internet.
  >  There are 4 zones defined.
  >  All significant events will be logged.
  >  ZIP resends queries every 10 seconds.
  >  RTMP updates are sent every 10 seconds.
  >  RTMP entries are considered BAD after 20 seconds.
  >  RTMP entries are discarded after 60 seconds.
  >  AARP probe retransmit count: 10, interval: 200.
  >  AARP request retransmit count: 5, interval: 1000.
  >  DDP datagrams will be checksummed.
  >  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  >  RTMP datagrams will be strictly checked.
  >  RTMP routes may not be propogated without zones.
  >  Alternate node address format will not be displayed.
  >  Access control of any networks of a zone hides the zone.
  > 
  >
  >It would show: "DDP datagrams will not be checksummed" if it was
  >disabled.
  >
  >It is on by default. If the customer enters the command 'no apple
  >checksum' into the configuration of the router, it will also be
  >visible when the customer executes the EXEC command 'write terminal'

I hope that this helps.

Glen
--

	*************************************************************
	Glen B. Glater				Phone: (617) 494-1999         
	Technical Support Engineer		Fax:   (617) 494-5167         
	Cayman Systems Inc.			Internet: glen@cayman.com     
	26 Landsdowne Street			AppleLink:  CAYMAN.TECH	      
	Cambridge, MA   02139            SneakerNet:  3rd cube on the left