>
> >>In the discussion of error correction, I think an error on my part has been
> >>missed. I was thinking about it and I wonder if this is entirely accurate:
> >>
> >>(concerning what happens after a frame is discarded on WAN link)
> >>
> >>"The end station will respond by acking the next packet it recieves with
> >the
> >>appriopriate (lower numbered) sequence number (of the missed packet). The
> >>originating station will
> >>get this ack (with the lower sequence number) see that the end station is
> >>requesting a packet out of sequence and the originating station will begin
> >>it's next transmission with the data from that particular sequence number."
> >>
> >>Is this correct?
That sounds right, except keep in mind that TCP sequences and acknowledges
bytes, not packets.
From watching TCP sessions with a Sniffer, I have noticed that when bytes
arrive out of order and there's a hole, the recipient's ACK number is the
number of the first byte of the hole.
The sender uses a Positive Acknowledgement with Retransmission (PAR)
mechanism. The sender also uses a sliding window. How much the window can
slide forward depends on which bytes have been acknowledged and the size of
the recipient's receive window. If bytes gets lost (due to a dropped packet
at a router, for example), the sender resends everything from the start of
the loss.
I'm not saying this very well, but there are many good books on TCP. Be
sure to read Comer, for one thing. It sounds like you have it down quite
well already, actually. Also, check out the real-world behavior with a
Sniffer. Implementations are sometimes different than what the spec says.
Priscilla
> >
> >
> >Emphasis: end station. You are describing what TCP does. Routers
> >typically are unconcerned with TCP.
> >
> >And again, not all applications need reliable links, so not all
> >applications will have retransmission ANYWHERE in the path.
> >
________________________
Priscilla Oppenheimer
http://www.priscilla.com
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