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Understanding the PPPoE-Based Radio-to-Router Protocol


Point-to-Point Protocol over Ethernet (PPPoE)-based radio-to-router protocols include messages that define how an external system will provide the device with timely information about the quality of a link’s connection. They also include a flow control mechanism to indicate how much data the device can forward. The device can then use the information provided in the PPPoE messages to dynamically adjust the interface speed of PPP links.

For example, a high-band networking waveform (HNW) radio provides ground-to-ground or ground-to-air communications with like devices. When the HNW picks up a signal from another device, it initiates a PPPoE session with a directly connected device (router). The PPPoE session encapsulates the packets that are relayed over a PPP link between the local and remote devices. The remote radio then forwards traffic to a remote device using an independent PPPoE session. The two devices exchange Link Control Protocol (LCP) and Internet Protocol Control Protocol (IPCP) messages to configure the link and exchange OSPF messages to establish the network topology.

Each HNW radio monitors the link every 50 milliseconds for changes in the link bandwidth, quality, and utilization. If any changes are detected, the radios announce the new set of metrics to the respective devices through a PPPoE Active Discovery Quality (PADQ) message, which is a nonstandard extension to the PPPoE Discovery Protocol (RFC 2516). The device transforms these metrics into a bandwidth value for the PPP link and compares it to the value currently in use. When the device detects that the difference exceeds a user-specified threshold, it adjusts the speed of the PPP link. OSPF is notified of the change and announces any resulting routing topology changes to its neighbors.

The CLI statement, radio-router, indicates that metrics announcements received on the interface will be processed by the device. When a PPPoE logical interface refers to this as an underlying interface, the device then processes incoming PADQ messages and uses information from the host’s messages to control the flow of traffic and manage the speed of the link, resulting in a corresponding adjustment of the OSPF cost. If this option is not specified, then PADQ messages received over the underlying interface are ignored.

The following options are available within the radio-router configuration statement:

  • bandwidth, resource, latency, and quality —These statements provide control over the weights used when transforming PADQ link metrics into an interface speed for the virtual link:

    • bandwidth—Weight of current (vs. maximum) data rate

    • resource—Resource weight

    • latency—Latency weight

    • quality—Relative link quality weight

    All four weights accept values from 0 through 100. The default value for all four weights is 100.

  • credit—This statement supports the credit-based flow control extensions described in RFC 4938, The statement enables PPPoE peers to grant each other forwarding credits. The grantee is then allowed to forward traffic to the peer only when it has a sufficient number of credits to do so. The subsequent credit interval statement controls how frequently the device generates credit announcement messages. The interval sub-statement, which controls the grant rate interval, accepts values from 1 through 60 seconds.

  • threshold—This statement specifies how much of a difference is required between the calculated and the current interface speeds. The threshold value, expressed as a percentage, defaults to 10.

The following hierarchy provides another view of the radio-router configuration statements.