NETWORKING: PPPoE Theory

In today section, We will discuss about PPPoE theory part …. PPPoE protocol majorly used in broadband sector for connecting end users to ISP for accessing internet.

There are three technologies which are used in Broadband industry to connect users to ISP network to access internet, These are - PPPoE , IPoE and PPPoA. But here I will discuss PPPoE and IPoE.

PPPoE

Point-to-Point Protocol over Ethernet (PPPoE) relies on two widely accepted standards: PPP and Ethernet. PPPoE is a specification for connecting the users on an Ethernet to the Internet through a common broadband medium, such as a single DSL line, wireless device or cable modem. All the users over the Ethernet share a common connection, so the Ethernet principles supporting multiple users in a LAN combine with the principles of PPP, which apply to serial connections

PPPOE stages-

Traditionally PPP connections are established between two end points over a serial link.All PPP frames sent on the wire are sure to reach the other end. But Ethernet networks are multi-access where each node in the network can access every other node. An Ethernet frame contains the hardware address of the destination node .This helps the frame reach the intended destination.

Hence before exchanging PPP control packets to establish the connection over Ethernet, the MAC address of the two end points should be known to each other so that they can be encoded in these control packets. The PPPoE Discovery stage does exactly this. In addition it also helps establish a Session ID that can be used for further exchange of packets.

PPP is a peer-to-peer protocol, PPPoE is inherently a client-server relationship ,multiple hosts can connect to a ISP over single physical connection.

The Discovery process consists of four steps –

Client to server: Initiation (PADI) – 1^st step

PADI stands for PPPoE Active Discovery Initiation

If a user wants to dial up to the Internet using DSL, then his computer first must find the DSL Access Concentrator at the user nearest POP. Communication over Ethernet is only possible via MAC addresses.

As the computer does not know the MAC address of the DSL Access concentrator, it sends out a PADI packet via an Ethernet broadcast . This PADI packet contains the MAC address of the computer sending it.

Server to client: Offer (PADO) – 2^nd step

PADO stands for PPPoE Active Discovery Offer

Once the user's computer/device has sent the PADI packet, the DSL-Access concentrator replies with a PADO packet, using the MAC address supplied in the PADI. The PADO packet contains the MAC address of the DSL-AC. If more than one POP's DSL-AC replies with a PADO packet, the user's computer selects the DSL-AC for a particular POP using the supplied name or service.

Client to server: Request (PADR) – 3^rd step

PADR stands for PPPoE Active Discovery Request

A PADR packet is sent by the user's computer to the DSL-AC following receipt of an acceptable PADO packet from the DSL-AC. It confirms acceptance of the offer of a PPPoE connection made by the DSL-AC issuing the PADO packet.

Server to client: Session-confirmation (PADS) – 4^th step

PADS stands for PPPoE Active Discovery Session-confirmation

The PADR packet above is confirmed by the DSL-AC with a PADS packet, and a Session ID is given out with it. The connection with the DSL-AC for that POP has now been fully established.

After this process to PPP session starts -

PPP was designed to support switched connections such as dial-in users, it is well-suited to support individual subscribers.

First, the link connection is established using Link Control Protocol (LCP).
Second, the subscriber is authenticated as part of the connection establishment.
Third, a Network Control Protocol (NCP) establishes the protocol parameters used for communications .
Internet Protocol Control Protocol (IPCP) is the NCP used to establish an IP connection over the established link, including assigning an IP address to the client.
At this point the subscriber can access the network, and PPP includes a means of monitoring link quality and availability.

Below diagram provides an overview of PPPoE session establishment. Although the entire lifecycle is depicted, the always-on broadband connections are rarely terminated normally.