Quality of Service for PPPoE Discovery in PTP 650/700

Point-to-Point Protocol (PPP) is a well-established (that is to say, quite ancient) technique used to transport network-layer data packets over a point-to-point link. Although PPP is designed to transport packets from a variety of different network-layer protocols, modern networks are almost exclusively designed for IPv4 over PPP.

PPP was created to connect over a serial link, for example the type of link provided by dial-up modems.  The same sort of approach has more recently proven useful where the lower layer is Ethernet, giving rise to the widely-used PPP over Ethernet or PPPoE encapsulation. PPPoE is useful in Ethernet-based Internet access networks, where it enforces a strict point-to-point connection between a service provider and a subscriber, over a lower layer that otherwise provides LAN-like bridging.

PPPoE connections consist of a Discovery phase (where a control protocol is used to establish the connection) and a Session phase (where PPP encapsulates the user's IP datagrams).

If Discovery phase packets are not reliably delivered between the server (the service provider's access equipment) and the client (the subscriber's equipment) the session may fail to establish, meaning that the subscriber's attempt to connect to the Internet also fails. We can maximise the probability of establishing a new connection over a heavily-loaded PTP 650 or PTP 700 link by transmitting PPPoE Discovery frames with higher priority.

We can't classify Discovery frames based on the DSCP, because Discovery does not carry IP datagrams. Instead, we classify PPPoE Discovery frames based on a distinctive Ethertype value (0x8863). The Ethernet frame looks like this:

The Ethertype (0x8863) tells us that this is a PPPoE Discovery phase frame. The next six bytes (14 to 19) are the PPPoE header. The PPP header starts at Byte 20.

In the PTP 650/700 QoS Configuration page, the classification is selected like this:


The default for PPPoE Discovery is Q7, which is the highest priority queue.

 Also, check out Quality of Service for PPPoE Session in PTP 650/700