Effects of MTU

The packet size used by TCP, often called Maximum Transmission Unit (MTU), effects efficiency in at least two conflicting ways. The host interfaces interrupt on each packet, and a good deal of TCP processing time is per packet (not per byte), so the hosts can send and receive faster with larger packets. On the other hand, TCP works best if it can keep a window of at least four packets in flight, since it detects a dropped packet by noting acknowledgments prompted by the rest of the packets in the window. Since the propagation delays are all small in these experiments, the packets in the window can only be stored in host and switch buffers. In many of these experiments the switch buffering will be limited to a few hundred 53-byte cells per VC, so packet sizes much larger than 2K bytes cannot be expected to work well.

This graph shows the effects of MTU on a single TCP connection through the switch. Since there is no congestion, switch buffering is not an issue. The only factor involved is the host's efficiency at processing packets.

Large MTUs approaching 10K bytes allow the hosts to use TCP at close to the network limit of 17 megabytes/second. Note that with an MTU of 2304 bytes, a single TCP connection can make use of half the link bandwidth. This will be important for the rest of the experiments, in which two TCP connections compete for the link bandwidth.