Automatic Route Quality

Name

AUTOQUAL -- Automatic Route Quality.

Description

Automatic Route Quality Measurement" (Autoqual) is an optional tool to help sysops set consistent and meaningful NetRom route qualities.

Background

NetRom makes routing decisions based on a fairly arbitrary metric, i.e. the "route quality", which is assigned by sysops, and disseminated in nodes broadcasts.

In the better-managed parts of the NetRom network, route qualities tend to be assigned according to the baud rate of the link, with adjustments for retry rates, duplex / simplex and shared channels.

However, there is no standard methodology for assigning quality, so not only will each sysop's notion of quality differ from that of other sysops, but also he will probably incorrectly assign the relative qualities of his own links. This leads to inconsistency and distorted routing.

In other parts of the network, route qualities are simply assigned to 192 regardless of how good or bad the link is. This also leads to inconsistency and less-than-optimal routing decisions.

The actual "goodness" of a link may continually change with atmospheric conditions, data throughput, other channel activity, QRM etc. At certain times of day for example, it may be better to use an alternative link.

A more accurate notion of "goodness" is the "Round Trip Time" (RTT) for the link, i.e. the time taken to send a packet and get a reply. After all, this is what *really* matters to users. A link which responds quickly (i.e. with a low RTT) is perceived by users to be better than a link which responds slowly. The RTT will track changes in retry rate, channel loading etc.

The RTT can be easily and consistently measured by software on a continuous basis, thus the "goodness" of the link is accurately known at all times, and all routers of the same type will give comparable values independently of the sysop's notions of quality.

Implementation

XRPi continually measures the RTT of neighbour links and uses the smoothed value to calculate a notional "route quality" every 5 minutes. The maximum, minimum and standard deviation of this quality are calculated and recorded for later display, and the value is further smoothed. The smoothed calculated quality is displayed by the "R Q" command, and can either be used as a guide to allow the sysop to fix the RQ at a sensible value, or if the route has been configured for it, XRPi can use it dynamically, by setting the route's quality to the calculated value.

This RTT to quality conversion is tailored to the British notion of quality, which gives somewhat lower but more meaningful qualities than used elsewhere. For example, a typical 1200 baud half-duplex link with low retry rate would produce a calculated quality around 120. A good 9600 baud half duplex link would equate to around 190, with 210 for a really good full duplex 9k6 link.

RTT measurement primarily uses L3RTT frames, but in their absence it also uses measurements of traffic throughput and retry rate.

Enabling Automatic Route Quality

Route quality calculation is automatic and continuous. However the calculated value is not actually used without the sysop's consent.

In order to allow the route quality to be automatically adjusted, the sysop must specify a RQ between 256 and 511 when adding a route using the "ROUTE ADD" command.

Alternatively, setting the PORT quality between 256 and 511 will cause all *new* routes (not locked in ones) learned on that port to use automatic quality.

A quality between 256 and 511 will instruct XRPi to use "automatic" quality, with an initial value of (qual-256).

See also

ROUTES(2) -- Add, Drop and List Neighbour Routes.
QUALITY(2) -- Display / Set default quality value for a port.