Copyrights©2009, All rights Reserved


Home | Free Books Center | Members | MM Guru | Quotes Diary| Contact us | Blog | Sign In | Sign Up


 Education Made Easy

Why PPPoE we uses?

Since traditional 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 because it contains destination mac that helps the frame to reach the intended destination. So without connection they reach to their 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. 

PPPoE and TCP/IP protocol stack

|    Application FTP SMTP HTTP … DNS … |

|     Transport TCP UDP                               |

|     Internet IP Ipv6                                       |

|     Network access PPP                            |

|      PPPoE                                                    |

|      Ethernet                                                  |

How PPPoE Discovery happens?

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 acts as the server. They are outlined below. The fifth and last step is the way to close an existing session

1. Client to server: Initiation (PADI)[edit]

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. Only DSL-AC equipment that can serve the "Service-Name" tag should reply.

2. Server to client: Offer (PADO)[edit]

PADO stands for PPPoE Active Discovery Offer.[11]

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).

3. Client to server: request (PADR)[edit]

PADR stands for PPPoE active discovery request.[12]

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.

4. Server to client: session-confirmation (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.

5. Either end to other end: termination (PADT)

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

hat is the Difference between SDSLC and ADSL?

Symmetric digital subscriber line (SDSL), specific proprietary technology, up to 1,544 kbit/s symmetric over one pair

Asymmetric digital subscriber line (ADSL), umbrella term for xDSL where the bit rate is greater in one direction than the other.

PPPoE Class Notes

How PPPoE fits in the DSL Internet access architecture

Digital subscriber line (DSL; originally digital subscriber loop) is a family of technologies that provide internet access by transmitting digital data using a local telephone network which uses the public switched telephone network. 

The transport protocol used on the telephone network is ATM. The DSL modem encapsulates PPP packets inside ATM cells and sends them over the WAN. There are several encapsulation methods.