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Re: [ns] a small bug in delayed ACK ?
Maybe I had misconcept in terminology, "out-of-order".
I use "out-of-order" as the packet at the arrival of which the receiver
should send an ACK without delaying it.
So, what I am willing to indicate is that an ACK of some "next expected"
packets should not be delayed.
for example, The sender send W packets from seq. 1. Assuming the first W-1
packets are dropped and the last W-th packet arrived at the receiver,
after any kind of retransmission method, the sender retransmit the dropped
packet from seq. 1. At this time, the receiver has "next expected" seq. 1
but the receiver should not delayed the ACK of it.
But unfortunely in current ns implementation, the receiver delayed it.
So, I think that if we use maxseen_ instead of next_, we can correct it.
Thanks
jchee
I observe this behavior in some ns simulations.
On Mon, 3 Apr 2000, Lloyd Wood wrote:
> On Sun, 2 Apr 2000, [ks_c_5601-1987] ��� wrote:
>
> > I think there is a small bug in the implementation of delayed ACK in "tcp-sink.cc"
> >
> > At line 300 of tcp-sink.cc of ns 2.1b6, (DelAckSink::recv).
> > To delay an ACK, packet seq. "th->seqno()" is compared to "acker_->Seqno()"
> >
> > It can cause a problem, because received packet can be "next expected"
> > and simultaneously "out-of-order".
> > Hence the comparison should be changed to between packet seq. "th->seqno()" and
> > _max seq._ "maxseen_".
>
> Definitely not.
>
> maxseen_ is the highest sequence number seen to date, so can be from
> an out of order packet.
>
> A received packet cannot be next expected and out-of-order, where
> 'out-of-order' means not-what-is-needed-to-move-the-left-window-edge-
> along. The delack timer is only started when a received packet is
> in-order, ie was expected, and maxseen_ doesn't tell you that.
>
> L.
>
> <[email protected]>PGP<http://www.ee.surrey.ac.uk/Personal/L.Wood/>
>
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Changhee Joo [email protected]
School of Electrical Engineering, Seoul National Univ., Korea
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