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Week 2 Synopsis
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When the router detects congestion before buffer overflow, the router can provide a congestion indication either by dropping a packet or by setting the Congestion Experienced (CE) codepoint in the Explicit Congestion Notification (ECN) field in the IP header [RFC3168].
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RFC 3168 [RFC3168] does not specify the use of ECN for the first SYN/ACK packet itself. However, because of the high cost to the TCP transfer of having a SYN/ACK packet dropped, with the resulting retransmission timeout, rfc5562 specifies the use of ECN for the SYN/ACK packet itself.
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Initiator: The node which initiates the TCP connection establishment. Responder: The node which sends the SYN/ACK packet.
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A TCP node may respond to an initial ECN-setup SYN packet by setting ECT in the responding ECN-setup SYN/ACK packet, indicating to routers that the SYN/ACK packet is ECN-Capable.
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This allows a congested router along the path to mark the packet instead of dropping the packet as an indication of congestion.
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If a SYN/ACK packet with the ECT codepoint is dropped, the responder should resend the SYN/ACK packet without the ECN-Capable codepoint. (responder uses a 3 second retransmission timer)
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if the TCP responder is informed that the SYN/ACK was ECN-marked, the TCP responder immediately sends a SYN/ACK packet that is not ECN-Capable. The TCP responder is only allowed to send data packets after the TCP initiator reports the receipt of a SYN/ACK packet that is not ECN- marked.
- What are the reasons TCP packets can be dropped? Is it only congestion??
- Does ECN help with reducing the unfairness caused by packet drop with the previous congestion control mechanism?? How?