Explicit Congestion Notification (ECN) is a mechanism where network
nodes can mark IP packets instead of dropping them to indicate
incipient congestion to the endpoints. Receivers with an ECN-capable
transport protocol feed back this information to the sender. ECN was
originally specified for TCP in such a way that only one feedback
signal can be transmitted per Round-Trip Time (RTT). Recent new TCP
mechanisms like Congestion Exposure (ConEx), Data Center TCP (DCTCP)
or Low Latency, Low Loss, and Scalable Throughput (L4S) need more
Accurate ECN (AccECN) feedback information whenever more than one
marking is received in one RTT. This document updates the original
ECN specification in RFC 3168 to specify a scheme that provides more
than one feedback signal per RTT in the TCP header. Given TCP header
space is scarce, it allocates a reserved header bit previously
assigned to the ECN-Nonce. It also overloads the two existing ECN
flags in the TCP header. The resulting extra space is additionally
exploited to feed back the IP-ECN field received during the TCP
connection establishment. Supplementary feedback information can
optionally be provided in two new TCP option alternatives, which are
never used on the TCP SYN. The document also specifies the treatment
of this updated TCP wire protocol by middleboxes.
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