Wire times vs timestamps
Measuring network performance entails the record of
occurrence times of events occurring on the net: but the
user process can only observe (and timestamp) indirect
"echoes" of these events.
- time when first octet is sent. Timestamp taken
before a send-to. Error: measurement process preemption.
- time when last octet is sent. Timestamp taken
immediately after the send-to. Error: measurement process
rescheduling.
- time when first octet is received. Unavailable if
receive-from is blocking.
- time when last octet is received. Timestamp taken
immediately after receive-from. Error: measurement process
rescheduling.
Sources of errors are in the kernel, and therefore are platform
specific.
One-way bandwidth measurement is not immediate: we can't timestamp
the receive of the first octet.
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