IP mtu or IP tcp mss adjust to correct fragmentation on my network

[onyx_oyin]

I suspect that fragmentation is causing some intranet sites not to open and I am wondering when or which one to use the IP mtu or IP tcp mss adjust. Or do I use both.

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[Joseph W. Doherty]

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Both.

[Inderjeet Singh]

Hi

You should be setting "tcp adjust mss".. MTU is local to router.. if there is a device in between the path which is using more smaller MTU than yours than you can still face the issue..

Basically MTU = MSS + 40bytes(standard TCP and IP header size)

so assume if you set your MTU to 1480  and another device in the path has set MTU to 1440 and you are sending packets at 1480 size, you may still face the issue depending upon the fragmentation allowed or not on that device.

If you keep MSS small enough from the origin, lets assume you keep mss to 1400 than your packet size will be 1440 and your MTU is 1480 so the packet will pass from your router without any issue.. but since another device is using 1440 on its router, your packet will pass through it as well.

For best results test highest ping packet size that you can ping from your router to other site wihout any drop.. if you find it just subtract 40 from it and put that value as your mss value.. you won't face issue ever unless the header size is different.

Thanks

Inder

[onyx_oyin]

Thanks

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[Inderjeet Singh]

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