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28. Largest MTU shows packet loss from 1465 to 1472 using PPPoE ?

If you are on a PPPoE connection, it is likely for the "Largest MTU" tab in the TCP Optimizer to report packet loss for packets with size over 1464 and under 1472. Note that 1464 + 28 (IP/ICMP headers) = 1492, and that is the maximum possible PPPoE packet size.

You can safely disregard pings with packet size over 1464 under PPPoE, larger packet size is simply not supported.

Details:
The issue occurs since PPPoE adds another 8 header bytes hidden down at the data-link and physical layers, and the OS isn't aware of it. So when pinging under PPPoE, you actually have 3 headers instead of 2: 1464 + 20 IP header + 8 ICMP header + 8 PPPoE header = 1500. On the other hand, for packets over 1472 (plus headers), you reach fragmentation at the TCP/IP layer, before PPPoE encapsulation comes into play.


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