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6. Metrics

DLEP includes the ability for the router and modem to communicate metrics that reflect the characteristics (e.g., data rate, latency) of the variable-quality link in use. DLEP does not specify how a given metric value is to be calculated; rather, the protocol assumes that metrics have been calculated by a "best effort", incorporating all pertinent data that is available to the modem device. Metrics based on large-enough sample sizes will preclude short traffic bursts from adversely skewing reported values.

DLEP allows for metrics to be sent within two contexts -- metrics for a specific destination within the network (e.g., a specific router), and "per session" (those that apply to all destinations accessed via the modem). Most metrics can be further subdivided into transmit and receive metrics. In cases where metrics are provided at the session level, the router propagates the metrics to all entries in its information base for destinations that are accessed via the modem.

DLEP modems announce all metric Data Items that will be reported during the session, and provide default values for those metrics, in the Session Initialization Response Message (Section 12.6). In order to use a metric type that was not included in the Session Initialization Response Message, modem implementations terminate the session with the router (via the Session Termination Message (Section 12.9)) and establish a new session.

A DLEP modem can send metrics in both (1) a session context, via the Session Update Message (Section 12.7) and (2) a specific destination context, via the Destination Update Message (Section 12.17), at any time. The most recently received metric value takes precedence over any earlier value, regardless of context -- that is:

  1. If the router receives metrics in a specific destination context (via the Destination Update Message), then the specific destination is updated with the new metric.

  2. If the router receives metrics in a session-wide context (via the Session Update Message), then the metrics for all destinations accessed via the modem are updated with the new metric.

It is left to implementations to choose sensible default values based on their specific characteristics. Modems having static (non-changing) link metric characteristics can report metrics only once for a given destination (or once on a session-wide basis, if all connections via the modem are of this static nature).

In addition to communicating existing metrics about the link, DLEP provides a Message allowing a router to request a different data rate or latency from the modem. This Message is the Link Characteristics Request Message (Section 12.18); it gives the router the ability to deal with requisite increases (or decreases) of allocated data rate/latency in demand-based schemes in a more deterministic manner.