"Its true that jumbo frames send more data per ACK. It is common practice to ACK every other frame, so Jumbo frames can transfer 18 KBytes with a single ACK where regular frames require six. Nonetheless the cost of processing ACKs just isn't that significant, not enough to match the benefit of LSO. As you note, a significant bottleneck is in the window size chosen. The amount of window needed is proportional to the link speed times the round trip time. RTT has remained relatively constant for WAN links, as it is dominated by the speed of light in fiber, but WAN link speed has increased substantially in a relatively few years. Window sizes appropriate for DS3 get pressed into service for the shiny new gigabit link. Negotiated window size is only weakly related to MTU. Most OSes won't allow the window size to be set below one MSS, but Windows now seems to use 17.5K by default. It should be the same for both regular and jumbo frames. MTU does strongly influence the congestion window,..." - DGentry