VMware ESX 4.1 and vCenter Server 4.1 Edition

ESX Configuration Guide : Networking : Advanced Networking : Networking Policies : Traffic Shaping Policy

Traffic Shaping Policy
A traffic shaping policy is defined by three characteristics: average bandwidth, peak bandwidth, and burst size. You can establish a traffic shaping policy for each port group and each dvPort or dvPort group.
ESX shapes outbound network traffic on vSwitches and both inbound and outbound traffic on a vNetwork Distributed Switch. Traffic shaping restricts the network bandwidth available on a port, but can also be configured to allow bursts of traffic to flow through at higher speeds.
The maximum number of bytes to allow in a burst. If this parameter is set, a port might gain a burst bonus if it does not use all its allocated bandwidth. Whenever the port needs more bandwidth than specified by Average Bandwidth, it might be allowed to temporarily transmit data at a higher speed if a burst bonus is available. This parameter limits the number of bytes that have accumulated in the burst bonus and thus transfers at a higher speed.