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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.
Next: Anypoint Prototype
Up: Inside the Anypoint Switch
Previous: Rate Control
Kenneth G. Yocum
2003-01-20