Messages in this thread | | | Subject | Re: IP: optimize as a router not host | From | Matthias Urlichs <> | Date | Tue, 19 Mar 1996 11:38:04 +0100 (MET) |
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Hi,
Alan Cox wrote: > >The selective ack RFC never went anywhere because you can use a scheme >known as fast retransmit instead to avoid pipeline stalls on a loss of >1 frame/window or less. What happens is the sender sends 1 2 3 4 5 6 >the receive gets 1 2 4 5 6. The acks from the receiver thus go >1 2 2 2 2 .. seeing 3 acks for an old frame in a row the sender now sends >frame 3 again immediately, then will get an ack of 7 and continue.
Umm, yes of course. But selective reject still has advantages. For instance, the receiver can tell the sender which range(s) of bytes it's missing, whereas with fast retransmit the sender has to remember the packet boundaries.
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.
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.
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