21.9 Per-Route Metrics

Newer TCP implementations maintain many of the metrics that we've described in this chapter in the routing table entry. When a TCP connection is closed, if enough data was sent to obtain meaningful statistics, and if the routing table entry for the destination is not a default route, the following information is saved in the routing table entry, for the next use of the entry: the smoothed RTT, the smoothed mean deviation, and the slow start threshold. The quantity "enough data" is 16 windows of data. This gives 16 RTT samples, which allows the smoothed RTT filter to converge within 5% of the correct value.

Additionally, the route (8) command can be used by the administrator to set the metrics for a given route: the three values mentioned in the preceding paragraph, along with the MTU, the outbound bandwidth-delay product (Section 20.7), and the inbound bandwidth-delay product.

When a new TCP connection is established, either actively or passively , if the routing table entry being used for the connection has values for these metrics, the corresponding variable is initialized from the metrics.