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Conference noted::decnis

Title:	DEC Network Integration Server (DECNIS)
Notice:	Please read note 1 to use this conference effectively
Moderator:	MARVIN::WELCH

Created:	Wed Sep 18 1991
Last Modified:	Thu Jun 05 1997
Last Successful Update:	Fri Jun 06 1997
Number of topics:	3660
Total number of notes:	15082

3642.0. "Routing reports checksum error on IP packets" by RULLE::KLASSON (Sven-Olof Klasson @GOO) Fri May 16 1997 11:29

Hi,

A customer see the following event messages reported by DECNIS routers.

%%%%%%%%%%%  OPCOM   5-APR-1997 06:36:05.18  %%%%%%%%%%%
Message from user SYSTEM on GRIPEN
Event: IP Packet Discard from: Node DAGAB:.mal.MALNIS Routing,
        at: 1997-04-05-06:30:31.796+02:00Iinf
        Receiving Entity=Routing Circuit HUV-4-0,
        IP Header='4500002E861A00007F06FFFFC20E9F37C20E925B'H,
        IP Discard Reason=Incorrect Checksum

%%%%%%%%%%%  OPCOM   8-APR-1997 00:23:50.64  %%%%%%%%%%%
Message from user SYSTEM on GRIPEN
Event: IP Packet Discard from: Node DAGAB:.huv.HUVNIS Routing,
        at: 1997-04-08-00:18:29.655+02:00Iinf
        Receiving Entity=Routing Circuit SKE-8-0,
        IP Header='45000034E4EC00001D06FFFFC20E9583C20E9F37'H,
        IP Discard Reason=Incorrect Checksum

%%%%%%%%%%%  OPCOM  19-APR-1997 10:41:32.86  %%%%%%%%%%%
Message from user SYSTEM on GRIPEN
Event: IP Packet Discard from: Node DAGAB:.jor.JORNIS Routing,
        at: 1997-04-19-10:40:10.156+02:00Iinf
        Receiving Entity=Routing Circuit HUV-4-0,
        IP Header='45000029881A00007F06FFFFC20E9F3FC20E9058'H,
        IP Discard Reason=Incorrect Checksum

The events are seen quite seldom (approx. once/week). They are reported
be several DECNIS routers. All of them are running v3.1-6.
One thing in common is that the packets involved has passed one particular
DECNIS.

These messages look strange to me. A checksum error is detected by routing.
I expected that packets with checksum errors should have been taken care of
by lower layers, and that packet with checksum errors should never reach 
routing.

Is this a kown problem?
Could this be caused by a hardware error in a DECNIS where a packet becomes
corrupted while beeing stored in internal memory?

Sven-Olof Klasson, CSC Sweden

T.R	Title	User	Personal Name	Date	Lines
3642.1	Possible linecard software bug	MARVIN::MCCLURE	Tony McClure, DECnis Engineering RE02 FD9 7830-3564	`Mon May 19 1997 05:38`	161
	Hi Sven-Olof, If you pipe the packets through the DTF program then you see what appears to be the problem IP Header='4500002E861A00007F06FFFFC20E9F37C20E925B'H, IP Header='45000034E4EC00001D06FFFFC20E9583C20E9F37'H, IP Header='45000029881A00007F06FFFFC20E9F3FC20E9058'H, ---------------------------------- IP Packet: IP ------------- Type of service 0x00 Precedence Routine Total length 46 Packet identifier 0x861A Fragment offset 0x0000 Time to live 127 * BAD CHECKSUM * Packet=0xFFFF, Correct=0x0000 Source Address 194.14.159.55 Destination Address 194.14.146.91 Protocol TCP IP protocol: TCP ---------------------------------- IP Packet: IP ------------- Type of service 0x00 Precedence Routine Total length 52 Packet identifier 0xE4EC Fragment offset 0x0000 Time to live 29 * BAD CHECKSUM * Packet=0xFFFF, Correct=0x0000 Source Address 194.14.149.131 Destination Address 194.14.159.55 Protocol TCP IP protocol: TCP ---------------------------------- IP Packet: IP ------------- Type of service 0x00 Precedence Routine Total length 41 Packet identifier 0x881A Fragment offset 0x0000 Time to live 127 * BAD CHECKSUM * Packet=0xFFFF, Correct=0x0000 Source Address 194.14.159.63 Destination Address 194.14.144.88 Protocol TCP So we can see that all three of these packet should have a checksum the is 0x0000 but in all cases the checksum is 0xFFFF. You say that the one thing is common is that all these packets had passed through onw of the DECnis's. If this is true which device was it?? (W622, W618 etc.) I think that with must be a bug in the linecard software. I will do some experiments here and see if I can re-produce the problem.. >>I expected that packets with checksum errors should have been taken care of >>by lower layers, and that packet with checksum errors should never reach >>routing. This is a IP header checksum, not a level 2 datalink checksum. Note there is also TCP header checksums too.. >> Could this be caused by a hardware error in a DECNIS where a packet becomes >> corrupted while beeing stored in internal memory? No, I don't think so I will let you know what I discover. Cheers Tony.
3642.2	Bug in MPC software	MARVIN::MCCLURE	Tony McClure, DECnis Engineering RE02 FD9 7830-3564	`Mon May 19 1997 10:50`	21
	Hi Sven-Olof, I have re-produced the problem is lab here. The problem is only present when the linecards give the packets to the MPC for forwarding, (for example when the ARP cache has timed-out) The MPC code doesn't accept 0xFFFF as a valid checksum for this packet. Please can you submit a CLD on this problem. Cheers Tony.
3642.3	IPMT CFS.51358/SOO100969	RULLE::KLASSON	Sven-Olof Klasson @GOO	`Thu May 22 1997 07:55`	18
	Hi Tony, I have submitted a IPMT on this. IPMT case number is CFS.51358 SOO100969 I have one more question. I assume these packets are lost in the DECnis. What happens when the packet is retransmitted? The IP header should be identical in the retransmitted packet. Could the retransmitted packet be lost in the same way? Or is routing information in the linecards updated, so the linecards are able to handle this by themself without involving the MPC. The customer only see one of these OPCOM messages at a time. Next message of this type may come a week later. I think the retransmitted packet are not lost. If that would be the case, there would be several OPCOM messages with a few seconds interval. Thanks, Sven-Olof
3642.4		MARVIN::MCCLURE	Tony McClure, DECnis Engineering RE02 FD9 7830-3564	`Fri May 23 1997 05:35`	31
	Hi Sven-Olof, >I assume these packets are lost in the DECnis. Yes >What happens when the packet is retransmitted? The IP header should be identical >in the retransmitted packet. Could the retransmitted packet be lost in the same way? Yes this is correct >Or is routing information in the linecards updated, so the linecards are able to >handle this by themself without involving the MPC. >The customer only see one of these OPCOM messages at a time. Next message of >this type may come a week later. I think the retransmitted packet are not lost. >If that would be the case, there would be several OPCOM messages with a few >seconds interval. I guess that what must happen is that another packet must arrive to be sent to the same destination and this would cause the MPC to fix the ARP cache. Cheers Tony