Hi.
We are connecting several GPRS-devices (based on Windows CE .NET 4.2) via PPTP-tunnel through the internet to a RouterBOARD. The router itself is connected to the internet via PPPoE-Client on a DSL-connection.
Now there are different MTU values to set:
- Local Ethernet ports of the router: default mtu size is 1500 bytes
- PPPoE-Client on one of the ethernet ports: default mtu size is 1480 bytes
- PPTP-Server on the router: mtu size is 1450 bytes
- PPTP-Client on the gprs-device: mtu size depends on the mtu-size of the pptp-server (see remark)
- PPP-Client on the gprs-device: defined by the gprs-provider, seems to be 1492 bytes
Remark: I tried to send unfragmented ICMP-Echo-Requests from a PPTP-Client to the router. The largest possible request has a size of 1422 bytes (mtu of the pptp-server minus 28 bytes).
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Why 28 bytes difference?
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How do I find the best matching mtu setting? Do I have to analyse all the involved protocol header sizes?
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Do I have to enable the “Change TCP MSS” option in the router’s pptp-profile settings?
Any help would be appreciated
Regards, Thomas