5.1 ECN as an Indication of Persistent Congestion
5.1 ECN as an Indication of Persistent Congestion
We emphasize that a single CE packet set in an IP packet would cause a response at the transport layer in terms of congestion control, just as for a dropped packet. Even in the absence of persistent congestion at a router, transient queue sizes can fluctuate considerably. Therefore, it is important that transient congestion at a router (reflected by the transient queue size reaching a threshold far less than the queue capacity) should not trigger a transport-level reaction. Therefore, routers should not set the CE codepoint based on transient queue size.
For example, because the congestion indication mechanisms for ATM and Frame Relay were typically defined without the relevant notion of average queue size as the basis for determining incipient congestion at intermediate nodes, we view them as providing a very noisy signal. The TCP sender response to ECN specified in this document is not appropriate for such a noisy congestion notification signal. However, if routers interfaced to an ATM network had a way to maintain an average queue at the interface and reliably use this to determine whether the ATM subnet is congested, they could use the ECN notification defined here.
We continue to encourage technical experiments with utilizing ECN at layer 2 (e.g., in ATM switches or Frame Relay switches). For example, using a scheme such as RED (where packet marking is based on average queue length exceeding a threshold), layer 2 devices can provide a fairly reliable indication of congestion. When all layer 2 devices in a path set that layer's own Congestion Experienced codepoint (e.g., the EFCI bit for ATM, the FECN bit in Frame Relay) in such a reliable manner, then the interface router to the layer 2 network could copy the state of that layer 2 Congestion Experienced codepoint to the CE codepoint in the IP header. We recognize that this is not current practice and is not in current standards. However, encouraging experimentation in this manner might provide the information needed to enable existing layer 2 mechanisms to provide a more reliable congestion indication means when using a single bit to indicate congestion.