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Re: [RRG] Why delaying initial packets matters



David Conrad wrote:
Lars,

On Feb 15, 2008, at 1:30 AM, Lars Eggert wrote:
We've had to change (mostly) the network layer of hosts stacks for IPv6. But here we're talking about changing transport layers and (UDP-speaking) applications. That's a different magnitude of change.

The lack of backward compatibility has meant we have had to change every network aware application ever written to support IPv6. Granted, many of those changes are 'simple' API changes (some of which might even be hidden by compatibility libraries), but they are still changes.

What I presume you are concerned about is the potential need to change application (or transport layer) logic to support a potentially longer RTT for an initial packet. However, since applications (and transport layer implementations) already need to deal with the potential of a delayed or dropped initial packet, it seems to me this is a question of

My experience is that some applications don't handle delayed or dropped
initial packets all that well. This may be okay since it rarely happens,
but it's a problem if it becomes the norm. Part of the problem here is
how the application/transport can distinguish between a delay/drop due
to a cache-miss (i.e. mapping not in place) and a packet dropped for
other reasons. From a single application point of view it may be good
to retransmit the first packet multiple times in a short period of time,
to make sure that one gets through as soon as possible. I'm not sure if
it's good if applications did that in general.

Anyway, this bring me back to the point I made earlier, that we really
would need to look at some applications. I guess you have some knowledge
how say BIND behaves? E.g., it looks like the host command will wait for
5s if the first request is dropped. Is a 5s delay to DNS lookups
acceptable?

Stig

optimization rather than feasibility.

Regards,
-drc


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