rfc9768v4.txt		rfc9768.txt
	Internet Engineering Task Force (IETF) B. Briscoe		Internet Engineering Task Force (IETF) B. Briscoe
	Request for Comments: 9768 Independent		Request for Comments: 9768 Independent
	Updates: 3168 M. Kühlewind		Updates: 3168 M. Kühlewind
	Category: Standards Track Ericsson		Category: Standards Track Ericsson
	ISSN: 2070-1721 R. Scheffenegger		ISSN: 2070-1721 R. Scheffenegger
	NetApp		NetApp
	September 2025		October 2025
	More Accurate Explicit Congestion Notification (AccECN) Feedback in TCP		More Accurate Explicit Congestion Notification (AccECN) Feedback in TCP
	Abstract		Abstract
	Explicit Congestion Notification (ECN) is a mechanism by which		Explicit Congestion Notification (ECN) is a mechanism by which
	network nodes can mark IP packets instead of dropping them to		network nodes can mark IP packets instead of dropping them to
	indicate incipient congestion to the endpoints. Receivers with an		indicate incipient congestion to the endpoints. Receivers with an
	ECN-capable transport protocol feed back this information to the		ECN-capable transport protocol feed back this information to the
	sender. ECN was originally specified for TCP in such a way that only		sender. ECN was originally specified for TCP in such a way that only
	skipping to change at line 661 ¶		skipping to change at line 661 ¶
	The procedures for retransmission of SYNs or SYN/ACKs are given in		The procedures for retransmission of SYNs or SYN/ACKs are given in
	Section 3.1.4.		Section 3.1.4.
	It is RECOMMENDED that the AccECN protocol be implemented alongside		It is RECOMMENDED that the AccECN protocol be implemented alongside
	Selective Acknowledgement (SACK) [RFC2018]. If SACK is implemented		Selective Acknowledgement (SACK) [RFC2018]. If SACK is implemented
	with AccECN, Duplicate Selective Acknowledgement (D-SACK) [RFC2883]		with AccECN, Duplicate Selective Acknowledgement (D-SACK) [RFC2883]
	MUST also be implemented.		MUST also be implemented.
	3.1.2. Backward Compatibility		3.1.2. Backward Compatibility
	The three flags set to 1 indicate AccECN support on the SYN has been		The three flags set to 1 to indicate AccECN support on the SYN has
	carefully chosen to enable natural fall-back to prior stages in the		been carefully chosen to enable natural fall-back to prior stages in
	evolution of ECN. Table 2 tabulates all the negotiation		the evolution of ECN. Table 2 tabulates all the negotiation
	possibilities for ECN-related capabilities that involve at least one		possibilities for ECN-related capabilities that involve at least one
	AccECN-capable host. The entries in the first two columns have been		AccECN-capable host. The entries in the first two columns have been
	abbreviated, as follows:		abbreviated, as follows:
	AccECN: Supports more Accurate ECN feedback (the present		AccECN: Supports more Accurate ECN feedback (the present
	specification).		specification).
	Nonce: Supports ECN-nonce feedback [RFC3540].		Nonce: Supports ECN-nonce feedback [RFC3540].
	ECN: Supports 'Classic' ECN feedback [RFC3168].		ECN: Supports 'Classic' ECN feedback [RFC3168].
End of changes. 2 change blocks.
	4 lines changed or deleted		4 lines changed or added
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