PPPoEHeader

PPPoE header refers to the 6-byte header used in PPP over Ethernet (PPPoE) frames, as defined for PPPoE by standards such as RFC 2516. It sits between the Ethernet header and the PPP payload inside the Ethernet frame and precedes the PPP frame carried within the PPPoE envelope.

The header consists of four fields: Version/Type (1 byte), Code (1 byte), Session ID (2 bytes), and Length (2 bytes). The Version/Type field carries a 4-bit version and a 4-bit type; for PPPoE these indicate Version 1 and Type 1, so the combined octet is typically 0x11. The Code field specifies the message type: in the PPPoE discovery stage, common codes are 0x09 for PADI, 0x07 for PADO, 0x19 for PADR, and 0x65 for PADS; in an established session, the Code is typically 0x00 for data frames. The Session ID identifies the PPP session; it is 0x0000 during discovery and is assigned by the server for the session thereafter. The Length field indicates the length of the PPP payload that follows the PPPoE header and can range from 0 to 1492 bytes, reflecting the maximum PPPoE payload allowed by the Ethernet MTU.

In typical operation, the PPPoE header is followed by the PPP header (2 bytes) and the PPP payload (such as an IP packet). The header therefore anchors the transition from the Ethernet domain to the PPP session, enabling multiple discovery and data frames within a single Ethernet link.

See also: PPP over Ethernet, RFC 2516, PPP, Ethernet.