Principle: End-to-end reliability
Total cost to send a message is 1 + r ? p
Note that p applies to the entire retransmission
Example:
- TCP retransmits a whole packet if it doesn’t get a positive ack
- If a lower layer is an ATM network, the packet is divided into small ‘cells’ that may be discarded due to overload
- If it takes 100 cells, then ppacket = 100 pcell
Measuring r
- When work(retry) ? work(send), r ? 1
- When retry involves a timeout that may be long compared to the time to send, r needs to reflect time and may thus be much greater than 1