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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.

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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