Client To Server: Initiation (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. Only DSL-AC equipment that can serve the "Service-Name" tag should reply.
Read more about this topic: Point-to-point Protocol Over Ethernet, PPPoE Discovery (PPPoED)
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