W5 L2 - Principles of congestion control

Sequence number

Number assigned to each byte of data sent over a connection

• Tracks the order of data, enabling correct reassembly at the receiver’s end

Cumulative ACK

• TCP can only send an ACK if it has positively received all bytes preceeding that ACK

• So if host1 never receives an ACK for 100, but does receive one for 120, it can assume the ACK for seq 100 was lost in traffic

TCP fast retransmit

If sender receives 3 additional ACKs for same data, resend unACKed segment with smallest seq #

TCP flow control

“receive window” in TCP data controls the sender, so that the sender doesn’t overflow the receiver’s buffer by transmitting too quickly

TCP Congestion Control - AIMD

• Senders increase sending rate until packet loss occurs, then decrease sending rate, gradually working back up

Decrease specifics:

• Cut in half when triple duplicate detected

• Cut to 1 MSS (Maximum segment size) when timeout detected

  • MSS is the maximum chunk of data without fragmenting

TCP Congestion Control - TCP Cubic

Improvement upon AIMD

• Increases cubicly rather than linearly, allowing more time in a high sending rate before being reset