ercm20@castle.ed.ac.uk (Sam Wilson) (03/21/91)
I'm posting the answer to my own question. We were having trouble with Public Folder hanging with 'ATPSendRequest retry count exceeded' when connecting via a cisco. The answer, turning AppleTalk checksumming off, is contained in this extract from a message from Greg Satz at cisco. Michael Peirce (Public Folder author and yes, he does spell it that way) is confused because he just uses the standard Apple routines and doesn't go anywhere near DDP checksums. The fix also seems to have sorted out a problem we were having with access to a System 7 AppleShare server - clients were hanging in mid flight. Sam Wilson Network Services, Edinburgh University Computing Service, Scotland, UK ---------------------------------------------------------------------- > Subject: Re: Problem with ciscos and Public Folder > From: Greg Satz <satz@com.cisco> > > In the 8.1 software, the cisco router would perform checksum operations > against all packets to, from and through the router. You can disable this > with the no apple checksum configuration command. The problem is that the > public folder application sends garbage in the ddp checksum field which the > router discards when performing checksumming. In the 8.2 software release > we never check the checksum field in the ddp header for packets going > through the router. We will continue to check them for correctness on > packets we receive and issue a correct checksum on packets we send. The no > apple checksum in 8.2 turns off checksumming in the input and output cases. > > Greg Mail:
bschmidt@bnr.ca (Ben Schmidt (BNR)) (03/26/91)
Sam, I didn't see your original posting, but I wonder if there might be another source for incorrect DDP checksums in packets from Public Folder. Certain LocalTalk-to-Ethernet routers have an intermittent problem whereby they sometimes incorrectly calculate the DDP checksum, and place otherwise perfectly formed packets onto Ethernet with invalid DDP checksums. Packets which are thus mangled by them on their way from a LocalTalk Mac to a cisco on Ethernet, are thus correctly discarded by the cisco. What cisco has done to provide a "work-around" to this problem, is to provide a way to optionally just ignore DDP checksums. (If the packet is *really* incorrectly formed, a higher-level protocol will then have to detect this.) Is one of your Public Folder users in the above example a LocalTalk-to-Ethernet gateway? Which kind are you using? Ben Schmidt Information Technology, Bell-Northern Research bschmidt@bnr.ca FAX:(613) 763-3283 /* My opinions, not BNR's */