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Which is an optional feature IPv4 headers, and is used when both end devices have agreed to use it?
Total length
ECN
Flag
Fragment offset
ECN (Explicit Congestion Notification) works as follows: •The initial TCP SYN handshake includes the addition of ECN-echo capability and Congestion Window Reduced (CWR) capability flags to allow each system to negotiate with its peer as to whether it will properly handle packets with the CE bit set during the data transfer. •The sender sets the ECN Capable Transport (ECT) bit in all packets sent. •If the sender receives a TCP packet with the ECN-echo flag set in the TCP header, the sender will adjust its congestion window, as if it had undergone fast recovery from a single lost packet. •The next sent packet will set the TCP CWR flag, to indicate to the receiver that it has reacted to the congestion. The additional caveat is that the sender will react in this way at most once every RTT interval. Further, TCP packets with the ECN-echo flag set will have no further effect on the sender within the same RTT interval. •The receiver will set the ECN-echo flag in all packets, when it receives a packet with the CE bit set. This will continue until it receives a packet with the CWR bit set, indicating that the sender has reacted to the congestion. The ECT flag is set only in packets that contain a data payload. TCP ACK packets that contain no data payload should be sent with the ECT bit clear.
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