11.2.1 The Incremental Deployment of ECT(1) in Routers

11.2.1 The Incremental Deployment of ECT(1) in Routers

ECN has been an Experimental standard since January 1999, and there are already ECN implementations in routers that do not understand the ECT(1) codepoint. When the use of the ECT(1) codepoint is standardized for TCP or other transport protocols, this could mean that a data sender is using the ECT(1) codepoint, but a congested router on the path does not understand this codepoint.

If the transport protocol permits, data senders will be free not to use ECT(1) at all, and to send all ECN-capable packets with the codepoint ECT(0). However, if an ECN-capable sender is using ECT(1), and a congested router on the path does not understand the ECT(1) codepoint, the router will eventually mark some ECT(0) packets and drop some ECT(1) packets as an indication of congestion. Since TCP is required to respond to both marked and dropped packets, this behavior of dropping packets that could have been marked poses no significant threat to the network, and is consistent with the overall approach of ECN allowing routers to determine at their own discretion when and whether to mark packets (see Section 5).