For the last few years, large Web content providers interested in
improving their scalability and availability have increasingly turned
to three techniques: mirroring, content distribution, and ISP
multihoming.  The Domain Name System (DNS) has gained a prominent role
in the way each of these techniques directs client requests to achieve
the goals of scalability and availability.  The DNS is thought to
offer the transparent and agile control necessary to react quickly to
ISP link failures or phenomenon such as flash crowds.

In this paper, we investigate this assumption with the objective of
quantifying the degree of responsiveness that can be expected from
DNS.  We use a combination of Web and DNS access measurements from
several busy Web sites, as well as a large content distribution
network, to characterize the behavior of end-systems and local DNS
servers in terms of their adherence to DNS-based controls. Our results
suggest that DNS is at best a coarse-grained mechanism, and poorly
suited for applications, such as route control, which require quick
response to link failures or performance degradations. We then propose
several proactive techniques that, when deployed in cooperation
between large content providers and important clients, have the
potential to improve the responsiveness of DNS-based control.