Mind your MTUs and MSSs

In the last couple of weeks, I have encountered an issue that is very important in the realm of network level Web performance: MTU and MSS size. Now, the problems tend not to be on the client side, but on the server side.

Maximum Transmission Unit (MTU) is the maximum size of packet that can be put onto a local area network. On Ethernet, this size is usually 1500 bytes, although 576 bytes is also frequently seen. Maximum Segment Size (MSS) is the maximum amount of data that can be packaged in that packet, excluding all of the necessary headers needed for transmission.

The issue that has surfaced recently in my analysis of the Web performance of a couple of sites running AIX is that this operating system is running with an MSS of 512 bytes. With this setting, AIX servers are more likely to flood the network with unecessary number of small packets that not only increase network overhead, but also increase client processing time, as these packets are more likely to arrive out of order.

Information on tuning the default MSS for AIX can be found here.

The best idea is to tune your MTU based on the MTU that your clients most often send. If you see a lot of client traffice with small MTUs, it may be in your best interest to actually conform to this configuration. However, remember that the more packets you place on the wire, the higher the probability that a packet will be lost.