Armed with this latency figure, you or your customer can tune receive window appropriately. While a small receive window size is fine for 10 Mbps LAN traffic, it's the Internet traffic you're trying to optimize in the typical residential broadband access situation. The larger receive window won't noticeably hinder intra-LAN performance between a residential customer's PC and his DSL modem. The receive window is often discussed in terms of packets. MTU is the largest possible amount of data you can send over a network connection without having to break it into pieces or fragment it.
Fragmentation is Evil for several reasons.
First, it uses valuable router and switch resources in the Internet, which must break packets into smaller packets before sending them along towards their destination. Upon receipt, destinations must reassemble these fragments. Historically, an MTU size of 576 bytes has been used in dialup networking. If your customer was a former dialup user who tuned TCP receive window for dialup, it's possible he's set his MTU to this value. Since he's now accessing the Internet via an Ethernet link to a DSL modem, check to see that his MTU is suitably large for the Ethernet and DSL link.