The Addition of Explicit Congestion Notification (ECN) to IP

This document specifies the incorporation of ECN (Explicit Congestion Notification) to TCP and IP, including ECN's use of two bits in the IP header. We begin by describing TCP's use of packet drops as an indication of congestion. Next we explain that with the addition of active queue management (e.g., RED) to the Internet infrastructure, where routers detect congestion before the queue overflows, routers are no longer limited to packet drops as an indication of congestion. Routers can instead set the Congestion Experienced (CE) codepoint in the IP header of packets from ECN-capable transports. We describe when the CE codepoint is to be set in routers, and describe Ramakrishnan, Floyd, Black Proposed Standard [Page 1] draft-ietf-tsvwg-ecn-04 Addition of ECN to IP June 2001 modifications needed to TCP to make it ECN-capable. Modifications to other transport protocols (e.g., unreliable unicast or multicast, reliable multicast, other reliable unicast transport protocols) could be considered as those protocols are developed and advance through the standards process. We also describe in this document the issues involving the use of ECN within IP tunnels, and within IPsec tunnels in particular. One of the guiding principles for this document is that, to the extent possible, the mechanisms specified here be incrementally deployable. One challenge to the principle of incremental deployment has been the prior existence of some IP tunnels that were not compatible with the use of ECN. As ECN becomes deployed, noncompatible IP tunnels will have to be upgraded to conform to this document. This document is intended to obsolete RFC 2481, "A Proposal to add Explicit Congestion Notification (ECN) to IP", which defined ECN as an Experimental Protocol for the Internet Communit...