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frame
PPP frame
Name Number of bytes Description
Protocol 1 or 2 setting of protocol in data field
Information variable (0 or more) datagram
Padding variable (0 or more) optional padding
The Protocol field indicates the kind of payload packet (e.g. LCP, NCP, IP, IPX,
AppleTalk, etc.).
The Information field contains the PPP payload; it has a variable length with a
negotiated maximum. By default the maximum is 1500 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 links is usually encapsulated in a framing similar to HDLC,
described by IETF RFC 1662.
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 setting of protocol in information field
Information variable (0 or more) datagram
Padding variable (0 or more) optional padding
FCS 2 (or 4) error correction sum
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 to determine 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 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 (2bytes)
or 32 bits (4 bytes) in size (default is 16 bits - Polynomial x16 + x12 + x5 +
1).
The FCS is calculated over the Address, Control, Protocol, Information and
Padding fields.
Although these are not standard applications, PPP is also used over broadband
connections. RFC 2516 describes Point-to-Point Protocol over Ethernet (PPPoE), a
method for transmitting PPP over Ethernet that is sometimes used with DSL. RFC
2364 describes Point-to-Point Protocol over ATM (PPPoATM), a method for
transmitting PPP over ATM Adaptation Layer 5 (AAL5), which is also sometimes
used with DSL.
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