Speculative File Transfers

Transport protocol changes when speculative


"Lazy file transdfer"

This is the file-transfer equivalent of lazy-evalutation. File transfer occurs as usual, except that gaps are left where components are missing. All packets are labelled with an absolute offset from the file start, to allow placement out-of-order.

Only when a file is requested from the cache is it checked to see if it's complete. If not, NACKs are issued for missing components.

If the file is incomplete and not ready to use, the cache either uses other caches' NACK responses, or can perform its own NACKs during otherwise idle periods.

The result is that speculative files don't impact the user (CPU of their system, etc.)until requested.

Amoritized response

Using multicast to amortize responses requires either a queue of pending inputs or some sort of speculation. Queues at the servers are a sign of overloaded servers, so as performance increases the potential for scanning a queue for duplicate requests decreases. Speculation, loading caches at or near clients with requests of possible future interest, is the only alternative.

At ISI, we're developing a protocol that allows the client to decide what to be interested in receiving, the server to decide what to be interested in broadcasting, and the two to join and split associations dynamically.


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Last modified Mar. 21, 1996.