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<font face="Helvetica, Arial, sans-serif">Queues and retransmissions
are inseparable. You have to maintain a queue while retransmitting.
Ideally (in coding theory) you never would retransmit the same thing
twice. You would instead transmit a smaller and different piece of
information the second time, so that the combination at the receiver
would regenerate the lost information.<br>
For an erasure channel, you could use low rate Reed-Solomon codes or
other kinds of things over the link. For other kinds of channels, you
could do other kinds of things. But the key thing here regarding
latency and throughput is: don't create a queue behind your
retransmission by focusing on moving the current packet (becoming older
and older) at "heroic cost". This is an "end-to-end argument" about
placing function (reliable delivery) in the wrong place (at the link
layer in wireless).<br>
Larry Roberts fumes (and I support him) about 802.11 systems that won't
stop retransmitting one Ethernet packet until 255 tries have been
made. That means that congestion is not signaled and routing is not
changed for 255 times too long. What the link should do is fragment
packets to tinier deliverable units, reassemble them, and stop creating
a backup in the path.<br>
On 09/18/2009 12:06 PM, Detlef Bosau wrote:
<blockquote cite="mid:4AB3B004.firstname.lastname@example.org" type="cite"><br>
O.k., so that's the case for small queues: small queuing latencies and
endpoints, which back off intelligently.
But what's the reason for doing only a small number of retransmissions
locally on a lossy channel?
One point, you mentioned, is that an application may want to pause and
wait for better channel conditions (you talked about load conditions,
but this is comparable).
Basically, my question is to which extent recovery on lossy links
should be done locally and to which extent recovery should be left to
the end points.