TCP Packet Loss Response

Two types of TCP losses:

1. Triple duplicate ACK (response: Fast retransmit)
   • Packet $n$ is lost, but packets $n+1$, $n+2$, etc. arrive
   • Then, sender quickly resends packet $n$
   • Do a multiplicative decrease and keep going
2. Timeout (repeat slow start - slow start restart)
   • Packet $n$ is lost and detected via timeout
   • Blasting entire congestion window would cause another burst
   • Better to start over with a new congestion window (CWND=1)
   • Slow-start restart: Slow start until reaching half of previous CWND
     • Why not restart at old CWND?
     • Suppose TCP connection goes idle for a while
     • Network conditions might change in between, maybe link is now backed up
     • Dangerous to start transmitting at old rate - previously idle sender might blast network