1. Accurate Round Trip Time
Consider using a ping tool to measure the performance of a network
link. Perhaps you wish to measure the RTT (round trip time) to calculate
the bandwidth-delay product and set the appropriate receive window
IPNetMonitor uses the OTGetTimeStamp function to capture times
accurate to less than a millisecond. Time stamps are taken just
before calling Open Transport to send a packet, and again from the
notifier function called at interrupt time by Open Transport when
data is received. Since the receive time stamp is captured during
interrupt processing, it is largely unaffected by other applications.
Some of the other tools measure elapsed time during their next
pass through the applications main event loop, or poll periodically
to see if a response has arrived. Neither technique is very accurate
since WaitNextEvent introduces an arbitrary delay depending on other
Consider the following RTT measurements taken with these ping tools
for a simple Ethernet LAN.
IPNetMonitor. Notice the first ping takes longer because IP needs
to use ARP before it can send the first packet.
AGNetTools. Even though there are no other applications doing anything,
response time varies from .009 to .027 seconds. If another ICMP
tool is running (like AGNetTools Trace Route), Ping misses some
responses and the times can vary more widely.
Mac TCP Watcher doesn't claim to report times accurate to less
than 10ms. Even so, the times are wildly inconsistent.
OTTool does not provide much information in this case. The first
ping reports 16ms (appears to be correct). The rest report 0ms.
WhatRoute. Results are accurate but a little harder to see.
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