PPP(Point-to-Point Protocol)

PPP is designed for simple links which transport packets between two peers. These links provide full-duplex simultaneous bi-directional operation, and are assumed to deliver packets in order. Although PPP is not tied to a particular type of packets it transports, its most common use is the encapsulation of IP packages over Modem lines. Basically, PPP is similar to SLIP, but it has the advantages of not being limited to one type of Protocol it can transport, a configuration negotiation phase at the start of a connection (for determining connection configuration parameters automatically), and the possibility to use standardized Authentication procedures for automated login.

The two authentication schemes supported by PPP are PAP(Password Authentication Protocol) and CHAP(Challenge Handshake Authentication Protocol).

PPP Frame Format:

The following descriptions summarize the PPP frame fields illustrated in Figure:

• Flag—A single byte that indicates the beginning or end of a frame. The flag field consists of the binary sequence 01111110.

• Address—A single byte that contains the binary sequence 11111111, the standard broadcast address. PPP does not assign individual station addresses.

• Control—A single byte that contains the binary sequence 00000011, which calls for transmission of user data in an unsequenced frame.

• Protocol—Two bytes that identify the protocol encapsulated in the information field of the frame.

• Data—Zero or more bytes that contain the datagram for the protocol specified in the protocol field. The end of the information field is found by locating the closing flag sequence and allowing 2 bytes for the FCS field. The default maximum length
of the information field is 1,500 bytes. By prior agreement, consenting PPP implementations can use other values for the maximum information field length.

• Frame check sequence (FCS)—Normally 16 bits (2 bytes). By prior agreement, consenting PPP implementations can use a 32-bit (4-byte) FCS for improved error detection.