With one of the most popular use for computers being Internet access, changes in Windows Vista, to support improved TCP networking, are significant. I've written about Scalable Networking, which contains 3 identified options - Receive-Side Scaling, TCP AutoTuning, and TCP Offload. Scalable Networking contains changes that are implemented from the client, and only require support from the client equipment.
There are more changes to the Vista TCP stack, though, and some of them require support from equipment outside the client network. Explicit Congestion Notification (ECN) is an option that reduces network problems caused by dropped packets, by letting the routers in the network (which drop packets, when overloaded) warn the client and server that they are approaching overload ("congestion").
Rather than experience packet drop (and require packet retransmission), the client and server can be warned before packet drop is necessary, and voluntarily reduce network use. If the endpoints (client and server) reduce network use, the routers in the network path between the endpoints become less overloaded, and are less likely to drop packets. This reduces network problems, and benefits all members of the network, including other endpoints and routers in other connections. By reducing packet retransmission, ECN can reduce Internet congestion in general.
Used inappropriately, however, ECN can actually increase Internet congestion. All Internet equipment is not ECN friendly, and WikiPedia mentions how enabling ECN might actually cause a problem, rather than preventing one.
Some outdated or buggy network equipment drops packets with the ECN bit set, rather than ignoring the bit.
ECN isn't granular - either you enable it, or you don't - and it potentially affects access to all web sites that you wish to visit. It may be more useful in specialised computers, that are intentionally used for high speed communication with specific web sites. It doesn't appear too useful for web surfing in general, right now.
For this reason, Vista is installed with ECN Disabled. If you try ECN Enabled, and you lose access to one web site, you'll have no choice but to Disable ECN, or face loss of access to the web site in question. As network hardware is upgraded, and becomes ECN friendly, enabling ECN will become a more practical option.
If you wish to use ECN, enter in a Vista command window (Run as Admin)
netsh interface tcp set global ecncapability=enabledIf you detect problems, such as lack of access to various web sites, enter similarly
netsh interface tcp set global ecncapability=disabled