Mark Russinovich, one of our Technical Fellows and a prominent member of the Windows community, posted today on a topic that's received much discussion in the past few days -- that being the manner in which Windows Vista throttles network traffic on gigabit Ethernet networks when the user is engaged in multimedia playback.

As Mark puts it, "many people have correctly surmised that the degradation in network performance during multimedia playback is directly connected with mechanisms employed by the Multimedia Class Scheduler Service (MMCSS), a feature new to Windows Vista."  This throttling of network traffic is mainly apparent on networks with infrastructure allowing gigabit throughput and is experienced by a user receiving, as opposed to sending, data.

The explanation is quite technical and rather esoteric, but the gist is that when receiving data on faster networks, the number of system interrupts is increased and because network-driven system interrupts are handled at higher priority than media playback, multimedia playback can be affected if the number of network-driven interrupts outpaces content refilling the multimedia playback buffer.  Mark's full explanation is quite a bit more detailed -- I've only described it here in outline.

While this behavior is by design, the throttling parameters as shipped caused greater-than-anticipated degradation on gigabit Ethernet systems.  In addition, there's also a related bug we've identified in scenarios involving multiple NICs but for which we're scheduling a fix.  If you'd like all the details in their technical glory, visit Mark's blog.  He'll have more news on this topic as it develops.