Editing Point-to-Point Protocol (section)

==PPP frame==

===Structure===
PPP frames are variants of [[HDLC]] frames:

{| class="wikitable"
|-
! Name
! Number of bytes
! Description
|-
| Flag
| 1
| 0x7E, the beginning of a PPP frame
|-
| Address
| 1
| 0xFF, standard broadcast address
|-
| Control
| 1
| 0x03, unnumbered data
|-
| Protocol
| 2
| PPP ID of embedded data 
|-
| Information
| variable (0 or more)
| datagram
|-
| Padding
| variable (0 or more)
| optional padding
|-
| Frame Check Sequence
| 2
| frame checksum
|-
| Flag
| 1
| 0x7E, omitted for successive PPP packets
|}

If both peers agree to Address field and Control field compression during LCP, then those fields are omitted. Likewise if both peers agree to Protocol field compression, then the 0x00 byte can be omitted.

The Protocol field indicates the type of payload packet: 0xC021 for [[Link Control Protocol|LCP]], 0x80xy for various [[#NCP|NCP]]s, 0x0021 for IP, 0x0029 AppleTalk, 0x002B for [[IPX]], 0x003D for Multilink, 0x003F for [[NetBIOS]], 0x00FD for [[Microsoft Point-to-Point Compression|MPPC]] and [[Microsoft Point-to-Point Encryption|MPPE]], etc.<ref>{{cite web|title=Point-to-Point (PPP) Protocol Field Assignments|url=https://www.iana.org/assignments/ppp-numbers/ppp-numbers.xhtml|publisher=IANA|access-date=3 September 2015}}</ref> PPP is limited, and cannot contain general [[Network layer|Layer 3]] data, unlike [[EtherType|Ether Type]].

The Information field contains the PPP payload; it has a variable length with a negotiated maximum called the [[Maximum Transmission Unit]]. By default, the maximum is 1500 [[octet (computing)|octets]]. It might be padded on transmission; if the information for a particular protocol can be padded, that protocol must allow information to be distinguished from padding.

===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.