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Re: [RRG] Why delaying initial packets matters
Noel,
> From: Marshall Eubanks <tme@multicasttech.com>
> many simple web pages have in them pieces from other servers and other
> IP addresses (banner ads are frequently done this way) and so this
> could put a real performance hit on web page load times in the real
> world.
Perhaps, but I'll note that interspersing of content in this manner also, in
theory, requires a DNS lookup for each of these included pieces, since they
are generally referred to via a DNS-based URL. (I can make that assertion
because I regularly trawl my web cache to find new sources of advertising
content which I can block, and I don't see many which are IP-address based.)
If the DNS resolution isn't proving problematic, it's probably because a
relatively small number of sites are involved (my list of blocked sites is
not that large), and the DNS records for them are getting cached relatively
close to the user machines (e.g. in a local DNS server
Several researchers have analysed the performance of DNS. MIT did an
interesting analysis in 2000, the main results form this analysis are
the following :
Our most surprising, non-obvious findings and conclusions are:
* About a quarter of all DNS lookups never get an answer. More than
50% of the DNS-related packets in the wide-area correspond to such lookups!
* The DNS retransmission protocol appears to be overly persistent:
while most successful answers are received in at most 2-3
retransmissions, the lack of an answer or response causes a much larger
number of retransmissions and a corresponding number of DNS packets
traverse the wide-area.
* Replacing all (i.e., for non-name server hosts) the A-record
TTL's to a value as small of 10 minutes is not likely to degrade the
scalability of DNS in any noticeable way. This is because of the
heavy-tailed nature of accesses to names.
* The scalability of DNS has little to do with its hierarchical
organization or to good A-record caching. Most of the DNS name space is
a flat, two-level structure. A-record caching does not seem to add much
more to the per-host or per-application caching done by end clients
today. Rather, the scalability derives from the good name space
partitioning achieved by the cacheability of NS records, which avoid
load on the root and top-level name servers.
See http://nms.csail.mit.edu/projects/dns/
I think that for the DNS the RRG should probably also consider looking
at real data...
Olivier
--
http://inl.info.ucl.ac.be , Universite catholique de Louvain, Belgium
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