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Old December 17th, 2010, 12:48 PM   #1
iJAF
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Schneider M340+NIP2212 STB - TTL issue!

Hi, I'm using a schneider M340 modicon PLC with some STBs NIP2212 in a big gigabit optic fiber ring topology network !!!

Everything is working fine !

But when testing the redundancy of the ring network (by disconnecting network segments) and the packets have to travel by the longest side of the ring (lot of network nodes) the communications are lost !!

Through packet monitoring, we believe that the default TTL value (time to live) of 10 is not enough (more than 10 jumps) !!!

I have contacted the local schneider support and they said that I can't change the TTL of TCP packets :O !!!

Which is the best way to solve this problem !?!

Thank you !!
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Old December 18th, 2010, 05:06 AM   #2
robertkjonesjr
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Are you using IOScanner? Try with READ_VAR instructions - I think their TTL is longer. Is this 10 hops through routers? I didn't think Switches decrement the TTL. Can you prove it is a TTL issue, can you ping? My windows machine has a TTL of 128 with ping.
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Old December 20th, 2010, 04:00 AM   #3
iJAF
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Quote:
Originally Posted by robertkjonesjr View Post
Are you using IOScanner? Try with READ_VAR instructions - I think their TTL is longer. Is this 10 hops through routers? I didn't think Switches decrement the TTL. Can you prove it is a TTL issue, can you ping? My windows machine has a TTL of 128 with ping.
Yes, I'm using the IOscanner !!!

And yes the network is very large and has a some routers!

Some packets were analised by the network managers and they concluded that the TTL of 10 is not enough (more than 10 jumps)!
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Old December 20th, 2010, 04:54 PM   #4
robertkjonesjr
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Not sure what else to suggest besides changing to READ_VARs. I have another idea, but it isn't fully developed, so not sure if it will work or not.

When you do NAT translation through a router (i.e. a typical home router, like Linksys, DLINK, what have you) it will create a new IP packet with different IP addresses, ports, etc. I wonder if it changes the ttl as well... if it does, you could put the IOScanner behind a NAT router and run this way. I can't easily test and there is a good chance the TTL is copied from the incoming to outgoing packet, which would render this idea useless. I know it's possible to use Linux and IPtables to change the ttl in the firewall rules, but I am not sure I would suggest this for a production system.
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Old December 22nd, 2010, 03:56 AM   #5
iJAF
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The problem is that I already have the solution working fine and it is completely validated !

The application was done based in ioscan and now it is very difficult to rebuild the whole app with read_var and write_var !!

By the way, do you know which is the best way to emulate the ioscan via read and write vars with a aprox. 250ms rate ?!

read_var(ip_1)
read_var(ip_2)
read_var(ip_3)
read_var(ip_4)

[WAIT 50ms]

write_var(ip_1)
write_var(ip_2)
write_var(ip_3)
write_var(ip_4)

[WAIT 200ms]

read_var(ip_1)
read_var(ip_2)
read_var(ip_3)
read_var(ip_4)
...

I have attached the print screen of my ioscan config !
I never used the manual messaging, sorry for my "rookiness"

Thanks
Attached Images
File Type: jpg scan.jpg (28.0 KB, 30 views)

Last edited by iJAF; December 22nd, 2010 at 04:01 AM.
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Old January 21st, 2011, 05:21 AM   #6
iJAF
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I have decided to advance to change my solution from IOScan to manual READ/WRITE_VAR, emulating the IOScan !! Read/Write_Var requests have TTL = 64 !!!

Have you ever used STB polling with Read/write var ?!

Do you know if we could do write/read requests to the same IP at the same time ?!

Could I "fire" 7 read_var requests @ the same time wait 200ms and then "fire again" ?! and the same for the 7 writes !

Have you got some experience with this !?

I must poll with a rate of aprox 200msecs !
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