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Denis Kirjanov authored
there is a problem with the CWR flag set in an incoming ACK segment and it leads to the situation when the ECE flag is latched forever the following packetdrill script shows what happens: // Stack receives incoming segments with CE set +0.1 <[ect0] . 11001:12001(1000) ack 1001 win 65535 +0.0 <[ce] . 12001:13001(1000) ack 1001 win 65535 +0.0 <[ect0] P. 13001:14001(1000) ack 1001 win 65535 // Stack repsonds with ECN ECHO +0.0 >[noecn] . 1001:1001(0) ack 12001 +0.0 >[noecn] E. 1001:1001(0) ack 13001 +0.0 >[noecn] E. 1001:1001(0) ack 14001 // Write a packet +0.1 write(3, ..., 1000) = 1000 +0.0 >[ect0] PE. 1001:2001(1000) ack 14001 // Pure ACK received +0.01 <[noecn] W. 14001:14001(0) ack 2001 win 65535 // Since CWR was sent, this packet should NOT have ECE set +0.1 write(3, ..., 1000) = 1000 +0.0 >[ect0] P. 2001:3001(1000) ack 14001 // but Linux will still keep ECE latched here, with packetdrill // flagging a missing ECE flag, expecting // >[ect0] PE. 2001:3001(1000) ack 14001 // in the script In the situation above we will continue to send ECN ECHO packets and trigger the peer to reduce the congestion window. To avoid that we can check CWR on pure ACKs received. v3: - Add a sequence check to avoid sending an ACK to an ACK v2: - Adjusted the comment - move CWR check before checking for unacknowledged packets Signed-off-by:
Denis Kirjanov <denis.kirjanov@suse.com> Acked-by:
Neal Cardwell <ncardwell@google.com> Signed-off-by:
Eric Dumazet <edumazet@google.com> Signed-off-by:
David S. Miller <davem@davemloft.net>
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