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SubjectRe: Client receives TCP packets but does not ACK


On Mon, 18 Jun 2001, Jonathan Morton wrote:

> > > Btw: can the aplication somehow ask the tcp/ip stack what was
> >actualy acked?
> >> (ie. how many bytes were acked).
> >
> >no, but it's not necessarily a useful number anyhow -- because it's
> >possible that the remote end ACKd bytes but the ACK never arrives. so you
> >can get into a situation where the remote application has the entire
> >message but the local application doesn't know. the only way to solve
> >this is above the TCP layer. (message duplicate elimination using an
> >unique id.)
>
> No, because if the ACK doesn't reach the sending machine, the sender
> will retry the data until it does get an ACK.

if the network goes down in between, the sender may never get the ACK.
the sender will see a timeout eventually. the receiver may already be
done with the connection and closed it and never see the error. if it
were a protocol such as SMTP then the sender would retry later, and the
result would be a duplicate message. (which you can eliminate above the
TCP layer using unique ids.)

-dean

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