On Thu, 25 Jun 2015, Jonathan Morton wrote:
On 25 Jun, 2015, at 20:49, Dave Taht <[email protected]> wrote:
In my case I just managed to show that congestive (rather than path)
loss can be a factor in the reliability of even a low rate, CS6
prioritized, link local multicast routing protocol (babel), over
present day linux wifi, even using a modern fq+aqm+ecn system.
The conventional wisdom certainly is that ECN should be left off simple 1-RTT
request-response protocols, where there is presumed to be no way to convey and
act on the congestion information in the future.
DNS is such a protocol, at least for simple queries that fit into UDP. Ergo,
DNS generally doesn’t use ECN at present.
But in practice, a DNS resolver makes several queries in rapid succession, and
often the resolver itself has sufficient persistence to be able to relay
congestion state from one query to the next (especially if it’s a proxy in a
CPE router). DNS is also a critical latency factor in many practical Internet
applications, especially Web traffic. ECN capability effectively increases
reliability of delivery when the bottleneck has AQM, and DNS should respond
well to that, since upon loss (of either request or response) it has to wait
for a exponential-backoff timeout.
I think that’s a concept worth pursuing.
From a purely pragmatic point of view, if a router will mark a packet as ECN
instead of dropping it, DNS packets should be marked with ECN because other
requests are serialized on the DNS lookup, so a dropped packet/timeout results
in a very user-visible delay.
David Lang
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