5. ECN: proactive congestion control

As we saw in (§ 5), as TCP is an end-to-end protocol, it has in principle no knowledge of the congestion status of interconnection equipment along the route; IP packet losses are the only signal indicating the existence of a queue (memory) overflow problem in an intermediate router. When it receives this signal, the TCP sender reacts with congestion control. However, this reaction is "late" in the sense that it does not prevent the loss from occurring: network resources will have been used in "upstream" routers to transport a packet that will be lost in a "downstream" router.

With the aim of improving the transport of congestion signals to the TCP sender, a mechanism known as Explicit Congestion Notification (ECN) has been proposed by the IETF [13] . The idea behind ECN is to involve...