Editing Point-to-Point Protocol (section)

===Encapsulation===
PPP frames are encapsulated in a lower-layer protocol that provides framing and may provide other functions such as a [[checksum]] to detect transmission errors. PPP on [[serial link]]s is usually encapsulated in a framing similar to [[HDLC]], described by IETF RFC 1662.
{| class="wikitable"
|-
! Name
! Number of bytes
! Description
|-
| Flag
| 1
| indicates frame's begin or end
|-
| Address
| 1
| broadcast address
|-
| Control
| 1
| control byte
|-
| Protocol
| 1 or 2 or 3
| l in information field
|-
| Information
| variable (0 or more)
| datagram
|-
| Padding
| variable (0 or more)
| optional padding
|-
| FCS
| 2 (or 4)
| error check
|}

The Flag field is present when PPP with HDLC-like framing is used.

The Address and Control fields always have the value hex FF (for "all stations") and hex 03 (for "unnumbered information"), and can be omitted whenever PPP LCP Address-and-Control-Field-Compression (ACFC) is negotiated.

The [[frame check sequence]] (FCS) field is used for determining whether an individual frame has an error. It contains a checksum computed over the frame to provide basic protection against errors in transmission. This is a [[Cyclic redundancy check|CRC]] code similar to the one used for other layer two protocol error protection schemes such as the one used in Ethernet. According to RFC 1662, it can be either 16 bits (2 bytes) or 32 bits (4 bytes) in size (default is 16 bits - Polynomial ''x''<sup>16</sup> + ''x''<sup>12</sup> + ''x''<sup>5</sup> + 1).

The FCS is calculated over the Address, Control, Protocol, Information and Padding fields after the message has been encapsulated.