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This document explains all about MTU (Maximum Transmission Unit) and how to configure your network settings to optimize your ADSL Service.

The Maximum Transmission Unit, measured in bytes, is the maximum size of IP datagram that may be sent without fragmentation through an IP path.

Under normal circumstances you would not need to get involved with setting the MTU. Typically the MTU value is set at 1500 (bytes) and a service called PMTU attempts to discover the largest MTU value that may be used without fragmentation to maximize data transfer throughput automatically.

Problems with the configuration of firewalls and router's used by ISP's on the Internet can cause PMTU Discovery to fail. This results in the default MTU value 1500 being used resulting in unpredictable behavior and reduced performance. BT Wholesale recommend an MTU setting of 1458 (bytes) and state that it is best suited to the performance of the ADSL broadband network. Some firewall configs out there throw away the packets used by the MTU discovery protocol, and suddenly you can't access certain web sites. Sadly this includes sites such as Hotmail and Cisco.

If your system needs to know the lowest MTU in use on the link, it sends out frames of decreasing size, with the DF-flag (Do not Fragment) set, until the ICMP_FRAG_NEEDED (but DF set) replies stop. This mechanism relies on the ability of ICMP type 3 messages to pass unhindered across all parts of the network. It seems that some hosts or routers are blocking ICMP type 3 messages, and this mechanism will fail.

As the BT ADSL-line uses PPPoA with L2TP encapsulation the maximum value is 1458. 'Normal' Ethernet MTU is 1500 bytes, but after encapsulating in PPPoA you have only 1458 bytes left. The encapsulation itself effectively steals 42 bytes from your frames.