The sender already knows these as it has the packets handy for
retransmission. The BSD one simply spots 3 repeated ack values in a row
and uses that.
> If more than one packet in that sequence is missing, fast retransmit fails
> -- you don't know which packet got lost, you have to wait for the ack, i.e.
> at least one round trip delay for each hole. Selective reject would give
> you one round trip time total.
Quite probably.
> The rule of thumb seems to be that with more than about 10..25% packet
> loss, TCP just doesn't work very well (or not at all, if you want to send
> big files). IMHO, selective reject would increase that number somewhat.
Ask a mathematician. Certainly at 40% loss tcp becomes a bit of a joke.
Alan