Point-to-Point Protocol over Ethernet...!!

Point-to-Point Protocol over Ethernet

PPPoE, Point-to-Point Protocol over Ethernet, is a network protocol for encapsulating Point-to-Point Protocol (PPP) frames inside Ethernet frames. It is used mainly with ADSL services where individual users connect to the ADSL transceiver (modem) over Ethernet and in plain Metro Ethernet networks. It was developed by UUNET, Redback Networks, and Router Ware and is available as an informational RFC 2516.

Ethernet networks are packet-based and have no concept of a connection or circuit and also lack basic security features to protect against IP and MAC conflicts and rogue DHCP servers. By using PPPoE, users can virtually "dial" from one machine to another over an Ethernet network, establish a point to point connection between them and then securely transport data packets over the connection.

The transport protocol used inside the telephone network is still Asynchronous Transfer Mode (ATM). Hence the PPPoE packets must be encapsulated inside ATM frames while entering the telephone network at the Wide area network (WAN) end of the ADSL modem. One way of doing this is to bridge the Ethernet packets containing the PPPoE packets over ATM, using the mechanism specified in RFC 2684; this is sometimes called PPPoEoE (PPP-over-Ethernet-over-Ethernet). Another is to directly encapsulate the PPPoE packets inside ATM AAL5 frames using RFC 2684 and SNAP encapsulation of PPPoE; this is termed PPPoEoA (PPP-over-Ethernet-over-ATM).

PPPoE stages

The PPPoE has two distinct stages:

PPPoE discovery

Since traditional PPP connections are established between two end points over a serial link or over an ATM virtual circuit that has already been established during dial-up, 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. Ethernet packets contain the hardware address of the destination node (MAC address) encoded in the packet. This helps the packet 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 session

Once the MAC address of the peer is known and a session has been established, the two end points have all the information needed to start building a point-to-point connection over Ethernet and exchange packets over the connection. This next stage after Discovery is called the Session stage.

PPPoE Discovery (PPPoED)

Although traditional PPP is a peer-to-peer protocol, PPPoE is inherently a client-server relationship since multiple hosts can connect to a service provider over a single physical connection.

The Discovery process consists of four steps between the host computer which acts as the client and the access concentrator at the internet service provider's end. They are outlined below. The fifth and last step is the way to close an existing session.

PADI

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 (DSL-AC) at the user's Internet service provider's point of presence (POP). Communication over Ethernet is only possible via MAC addresses. As the computer does not know the MAC address of the DSL-AC, it sends out a PADI packet via an Ethernet broadcast(MAC: ff:ff:ff:ff:ff:ff). This PADI packet contains the MAC address of the computer sending it.

Example of a PADI-packet:

Frame 1 (44 bytes on wire, 44 bytes captured)

Ethernet II, Src: 00:50:da:42:d7:df, Dst: ff:ff:ff:ff:ff:ff

PPP-over-Ethernet Discovery

  Version: 1

  Type 1

  Code Active Discovery Initiation (PADI)

  Session ID: 0000

  Payload Length: 24

PPPoE Tags

  Tag: Service-Name

  Tag: Host-Uniq

    Binary Data: (16 bytes)

Src. (=source) holds the MAC address of the computer sending the PADI.
Dst. (=destination) is the Ethernet broadcast address.
The PADI packet can be received by more than one DSL-AC.

PADO

PADO stands for PPPoE Active Discovery Offer.
Once the user's computer has sent the PADI packet, the DSL-AC replies with a PADO packet, using the MAC address supplied in the PADI. The PADO packet contains the MAC address of the DSL-AC, its name (e.g. LEIX11-erx for the T-Com DSL-AC in Leipzig) and the name of the service. 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.

Here is an example of a PADO packet:

Frame 2 (60 bytes on wire, 60 bytes captured)

Ethernet II, Src: 00:0e:40:7b:f3:8a, Dst: 00:50:da:42:d7:df

PPP-over-Ethernet Discovery

  Version: 1

  Type 1

  Code Active Discovery Offer (PADO)

  Session ID: 0000

  Payload Length: 36

PPPoE Tags

  Tag: AC-Name

    String Data: IpzbrOOl

  Tag: Host-Uniq

    Binary Data: (16 bytes)

AC-Name -> String data holds the AC name, in this case “Ipzbr001” (the Arcor DSL-AC in Leipzig)
Src. holds the MAC address of the DSL-AC.
The MAC address of the DSL-AC also reveals the manufacturer of the DSL-AC (in this case Nortel Networks).

PADR

PADR

PADS

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.

PADT

PADT stands for PPPoE Active Discovery Termination.
This packet terminates the connection to the POP. It may be sent from either the user's computer or from the DSL-AC.