Online services are typically replicated on multiple servers in different datacenters, and have (at best) a loose association with specific end-hosts or locations. To meet the needs of these online services, we introduce SCAFFOLD-an architecture that provides flowbased anycast with (possibly moving) service instances. SCAFFOLD allows addresses to change as end-points move, in order to retain the scalability advantages of hierarchical addressing. Successive refinement in resolving service names limits the scope of churn to ensure scalability, while in-band signaling of new addresses supports seamless communication as end-points move. We design, build, and evaluate a SCAFFOLD prototype that includes an end-host network stack (built as extensions to Linux and the BSD socket API) and a network infrastructure (built on top of OpenFlow and NOX). We demonstrate several applications, including a cluster of web servers, partitioned memcached servers, and migrating virtual machines, running on SCAFFOLD. Dynamism. Modern services operate in a dynamic environment, where a replica may fail, undergo maintenance, migrate to a new location, seek to offload work, or be powered down to save energy; new replicas may be added to handle extra load or tolerate faults. This dynamism stretches across many levels of granularityfrom connections, to virtual machines and physical hosts, to entire datacenters. Rather than hosts retaining their addresses as they move, SCAFFOLD allows end-point addresses to change dynamically. This allows networks to apply whatever hierarchical addressing scheme they wish for more scalable routing, and enables hosts to migrate across layer-two boundaries. Principle: The network addresses associated with a service should be able to change over time as service instances fail, recover, or move. When an end-point moves, SCAFFOLD performs inband signaling to update the remote end-points of established flows. When a service instance fails, recovers, or moves, the network automatically directs new requests to the new location. In contrast, today's network cannot easily allow end-point addresses to change because these addresses are exposed to (and cached by) applications.
