Retransmission Delay

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Retransmission Delay

Retransmission Delay is the additional delay in the Network Round Trip Time caused by packets needing retransmission.

The data shown is an average across all observations, not the actual retransmission time for one transaction. Given a total of five transactions, four with no retransmissions, and one with a 5-second Retransmission Delay, the view shows a 1-second Retransmission Delay.

The management console calculates Retransmission Delay by observing duplicate packets in the network from the monitoring device's vantage point next to the server. A monitoring device can see packets retransmitted by the server because of data losses in the server-to-client direction along the network path. These observations are included in Retransmission Delay. When data loss occurs in the client-to-server direction (for example, in the network path before reaching the server), the monitoring device cannot observe such packet loss and that delay is not included in the Retransmission Delay metric. The management console includes this associated delay with retransmission in the Network Round Trip Time metric, which includes the server response and client acknowledgment. A delay in client acknowledgment caused by unseen Retransmission Delay increases the NRTT value. This metric does not reveal the impact of losses on the Data Transfer Time because of TCP congestion.

Retransmissions can cause a given session to reduce the transaction time, but the speed of the bytes across the line remains constant unless the path changes or congestion increases.

Look for	Might Indicate
Retransmission Delay increases as the number of observations increases	The network might not be able to handle the higher load. Changing the buffer allocation or routing strategies might alleviate short-term spikes.
Retransmission Delay varies in a consistent pattern	Problem in the link to the region. Correlate the high retransmission periods to other information sources to determine what else is happening on the network at those times. If certain subnets indicate a greater ratio of Retransmission Delay to Network Round Trip Time delay than other regions, there might be a problem in the link to that region. If you suspect that a particular network is experiencing excessive delay because of retransmission, view the Retransmission Delay for the network in question and compare it to the average Retransmission Delay for all networks.
Retransmission Delay for the network is high compared to the average for all networks	Normal behavior as in a wireless environment.
A consistently high Retransmission Delay	Network cannot carry the required load or a device might be malfunctioning.
High values	Dropped packets caused by network congestion, misconfigured load balancing, redundant paths or routes, and network error conditions.