[p2p-hackers] UDP file transfer link speed identification

[David Barrett]

Daniel Stutzbach wrote:
> If a loss is detected via 3 duplicate ACKs, cwnd and ssthresh are both
> cut in half, then it goes into Fast Retransmit mode to recover the
> loss.  Once the loss is recovered, it returns to congestion avoidance
> mode.

Can you clarify for me what precisely is a "duplicate ACK"?  Specifically:

- Does it literally mean two ACK packets that are byte-identical?  Or 
just two ACKs that overlap?

- Does the client ever send a duplicate ACK intentionally?  Or is this 
just something that happens occasionally on bad connections?

Basically, I just don't "get" what duplicate ACKs are, why I should 
switch fast transmit in response to three of them in a row (does this 
mean six identical ACK packets?), and so forth.  Can you shed any light 
on this for me?

Thanks!

-david