Measurements

IP SLA uses the timestamps in the ICMP timing packet provided by the Tracked Object Manager to measure the round-trip delay.

• EchoTxTime —The timestamp added to the ICMP echo request just prior to calculating the IP checksum and transmitting the packet to its destination.
• EchoRxTime —The timestamp of the arrival time of the ICMP echo request at the destination. The Tracked Object Manager adds this timestamp to the ICMP echo reply prior to the ReplyTxTime timestamp.
• ReplyTxTime —The timestamp added to the ICMP echo reply just prior to calculating the IP checksum and transmitting the packet back to the source of the ICMP echo request.
• ReplyRxTime —The timestamp of the arrival time of the ICMP echo reply.

With the timing information, you can create a statistical model the network. For all measurements, the minimum, average, and maximum values are included in the output of any collection. Also included is the sum of the values for each category and the sum of squares. The sums of the squares partition the variance into manageable portions.

To calculate the standard deviation with the data provided, use the computing formula for standard deviation.

Standard deviation =

The following are the round-trip delay calculations.

• Round-trip delay (RTD) : ReplyRxTime – EchoTxTime
• Wire-time RTD (WRTD): ( ( ReplyRxTime – EchoTxTime) – (ReplyTxTime – EchoRxTime) )
• Host-time RTD (HRTD) : RTD – WRTD

The calculation of the round-trip delay is always available. If the EchoRxTime and ReplyTxTime are available, the wire-time and host-time round-trip delay are calculated.