Discussion

The switch mechanisms described in this section illustrate several key points about the Anypoint architecture. Most importantly, transport equivalence says that Anypoint does not affect the transport connection semantics perceived by the end nodes. This architectural choice yields several benefits:

• End nodes use the same transport code for point-to-point and Anypoint connections, and do not distinguish between them. All Anypoint-specific functions are local to the switch.

• Transport equivalence helps to ensure that Anypoint is compatible with all clients and servers implementing the transport, conforms to congestion conventions, and composes well with other components of the Internet architecture. For example, given appropriate ALRM support, both ends of an Anypoint connection could be ensembles, or Anypoint ensembles could nest arbitrarily in a hierarchy.

• Transport functions for sequencing and reliable at-most-once delivery continue to operate in an end-to-end fashion. The Anypoint switch never buffers data for rate control or transport reassembly; its role is to transform frames to coordinate these functions at the end nodes. The switch buffers packet data only as needed for port queues and frame assembly. This improves scalability of the switch architecture.

Although the switch maintains per-flow control state, it is bounded by the flow window w. Because acknowledgments and buffering remain end-to-end in Anypoint, a failed switch does not lose user data. However connections maintained by the failed switch must be re-initiated for the service to recover a failed session. Support for session recovery is common in new service protocols including iSCSI and DAFS [16].

Note also that no mutable state is shared across connections within an Anypoint switch. Thus an Anypoint switch design could spread frame processing load across processors at each external switch port. An ensemble may also partition communication traffic from different peers across multiple switches to further improve scalability.