11.2 A Discussion of the ECN nonce

11.2 A Discussion of the ECN nonce

The use of two ECT codepoints ECT(0) and ECT(1) enables a one-bit ECN nonce in the packet header [SCWA99]. The primary motivation for this is the desire to allow the data sender to verify that network elements have not cleared the CE codepoint, and that the data receiver is correctly reporting to the sender the receipt of packets with the CE codepoint set, as required by the transport protocol. This section discusses issues of backward compatibility with RFC 2481-compliant IP ECN implementations in routers, where only one ECT codepoint is defined. We do not believe that the incremental deployment of ECN implementations that understand the ECT(1) codepoint will lead to significant operational problems. This is especially likely to be the case when the deployment of the ECT(1) codepoint starts with routers, before end nodes begin using the ECT(1) codepoint.