Lecture 9 (Feb 8 2018)

TCP Slow Start

When connection begins, increase rate exponentially when cwnd<ssthresh
- Double cwnd every RTT by setting cwnd += 1 MSS for every ACK received
Summary: initial rate is slow but ramps up exponentially

Fast retransmit: to detect and repair loss, based on incoming duplicate ACKs
- Use 3 duplicate ACKs to infer packet loss
- Set ssthresh = cwnd/2
- cwnd = ssthresh + 3MSS
- Retransmit the lost packet
Fast recovery: governs the transmission of new data until a non-duplicate ACK arrives
- Increase cwnd by 1 MSS upon every duplicate ACK
Upon 3rd duplicate ACK
- ssthresh = max(cwnd/2, 2*MSS)
  - cwnd should be flight size to be more accurate
- cwnd = ssthresh + 3*MSS
- Retransmit the lost TCP packet
Upon each additional duplicate ACK
- cwnd += 1 MSS
- Transmit a new packet if allowed by the updated cwnd and rwnd
Upon a new (ie. non-duplicate) ACK
- cwnd = ssthresh
  - Deflating the congestion window size

Increase cwnd by 1 MSS per RTT until congestion (loss) is detected
- Conditions: when cwnd > ssthresh and no loss occurs
- Actions: cwnd += (MSS/cwnd)*MSS (bytes) upon every incoming non-duplicate ACK

3 duplicate ACKs to infer losses and differentiate from transient out-of-order delivery
Receiving each duplicate ACK indicates one more packet left at the network and arrived at the receiver