Slashdot: Providers Ignoring DNS TTL? (fwd)

Wed Apr 20 18:41:38 UTC 2005

BTW, while it looks like you've shown it to be traditional load balancing,
I ought to explain that this is also not a very good idea. The
loadbalancer is a single point of failure, usually. Loadbalancers are a
good idea for stateful, high-work-request servers such as web servers
running web-apps. This allows you to apply many servers to what then
appears to be a single service. This is well worth the expense of a single
point-of-failure, since what you really wanted was a single, bigger and
exponentially more expensive server.  The multiple small cheap servers
with the load balancer give you that view.  When the load balancer detects
failure and drops that failed server, the loadbalancer isn't really
offering "higher availability" than a single server, but is rather
compensating for the fact that multiple small cheap servers will
collectively crash more frequently.

However, DNS service is comparatively low-work-request, and low latency.  
Generally, people are seeking high availability and load distribution for
DNS caches.  But the work of the traditional load balancer is probably
comparable to the work of the DNS server. So the benefits of multiple DNS
servers behind a single load balancer are probably negligible.

		--Dean

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---------- Forwarded message ----------
Date: Wed, 20 Apr 2005 14:13:33 -0400 (EDT)
From: Dean Anderson <dean at av8.com>
To: Crist Clark <crist.clark at globalstar.com>
Cc: nanog at merit.edu
Subject: Re: Slashdot: Providers Ignoring DNS TTL?

On Wed, 20 Apr 2005, Crist Clark wrote:

> Dean Anderson wrote:
> > I'd rather expect this sort of behavior with anycasted servers... 
> 
> I would not expect this kind of behavior from an anycasted address.
> You'd need a LOT of routing churn to see different caches every few
> seconds. It's much more likely some kind of load balancer in front
> of a DNS server farm.

No, you are thinking of the (wrong) claims originally made by ISC about
how anycast would affect TCP to an anycast authoritative server.  ISC 
wrongly asserted that since BGP routes don't churn very fast compared with 
DNS TCP connection lifetimes, that there should be no problem with anycast 
and TCP.  This view has been shown to be wrong in the face of Per Packet 
Load Balancing (PPLB) which has been demonstrated to work on BGP links by 
haesu at towardex.com. Further, I showed that if you have PPLB on interior
(eg OSPF) links leading to different BGP peers, the problem also happens. 
Packets are sent on a per packet basis to different places.

But caching servers are usually setup to load balance. Usually, the
servers with the same IP address share an ethernet along with multiple
routers.  So the packets are switched on essentially a per-packet basis.
Or possibly a per-arp basis that alters the MAC-based-forwarding behavior
of a switch.  This is fairly fine grained load balancing.

> > With a cache, the behavior is confusing, but also harms DNS TCP support, 
> > just like that described for authoritative servers.
> 
> I verified it wasn't anycast by trying to exploit this very issue. I
> did a query that fell back to TCP while doing multiple small queries.
> I ran a network capture to pick out the short quries that occurred while
> the TCP query was going on. Short quries during the TCP connection
> came back with verying TTLs indicating that I was talking to different
> caches, i.e. different servers. Yet the TCP query continued without
> any hiccups. This indicates there is some type of per-session load
> balancing going on, not anycast routing.

This additional information would seem to indicate they are behind a more
traditional stateful load balancer, rather than anycast.  Without your TCP
connection, I don't think you could distinguish a traditional load
balancer from an anycast cache setup.

		--Dean

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