AIMD

• Add congestion window (CWND)
• Slowly increase to se up new bandwidth
• Quickly decrease to eliminate congestion
• Additive, increase maximum segment size every RTT
• Multiplicative decrease: decrease condo by 50% on every RTT

Improving AIMD

• Exponential increase for initial startup phase
• Reset cwnd to smallest value upon heavy congestion
• 4 algorithms
  1. Slow start
  2. AI
  3. MD
  4. Heavy congestion handling
• On no loss
  • Slow start, quick jump-start by exponential growth at startup
  • Congestion avoidance: switch to linear growth once exceeding threshold

Slow Start

• On TCP start, increase exponentially until ssthresh
• On every RTT (or equivalently new received ACK)
• ssthresh is hard of max cwnd

When loss is detected

• Multiple ACKs, fast retransmit (MD of cwnd)
• Through timeout (reset, start from scratch)

Nothing: