The Point-to-Point Protocol (PPP) originally emerged as an encapsulation protocol for transporting IP traffic over point-to-point links. PPP also established a standard for the assignment and management of IP addresses, asynchronous (start/stop) and bit-oriented synchronous encapsulation, network protocol multiplexing, link configuration, link quality testing, error detection, and option negotiation for such capabilities as network layer address negotiation and data-compression negotiation.
PPP Components
PPP provides a method for transmitting datagrams over serial point-to-point links. PPP contains three main components:
•A method for encapsulating datagrams over serial links. PPP uses the High-Level Data Link Control (HDLC) protocol as a basis for encapsulating datagrams over point-to-point links.
•An extensible LCP to establish, configure, and test the data link connection.
•A family of NCPs for establishing and configuring different network layer protocols. PPP is designed to allow the simultaneous use of multiple network layer protocols.
PPP is capable of operating across any DTE/DCE interface. PPP does not impose any restriction regarding transmission rate other than those imposed by the particular DTE/DCE interface in use.
Six fields make up the PPP frame. The PPP LCP provides a method of establishing, configuring, maintaining, and terminating the
point-to-point connection.
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. A connectionless link service similar to that of Logical Link Control (LLC) Type 1 is provided. (For more information about LLC types and frame types, refer to Chapter 16.)
•Protocol—Two bytes that identify the protocol encapsulated in the information field of the frame. The most up-to-date values of the protocol field are specified in the most recent Assigned Numbers Request For Comments (RFC).
•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.
PPP Components
PPP provides a method for transmitting datagrams over serial point-to-point links. PPP contains three main components:
•A method for encapsulating datagrams over serial links. PPP uses the High-Level Data Link Control (HDLC) protocol as a basis for encapsulating datagrams over point-to-point links.
•An extensible LCP to establish, configure, and test the data link connection.
•A family of NCPs for establishing and configuring different network layer protocols. PPP is designed to allow the simultaneous use of multiple network layer protocols.
PPP is capable of operating across any DTE/DCE interface. PPP does not impose any restriction regarding transmission rate other than those imposed by the particular DTE/DCE interface in use.
Six fields make up the PPP frame. The PPP LCP provides a method of establishing, configuring, maintaining, and terminating the
point-to-point connection.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. A connectionless link service similar to that of Logical Link Control (LLC) Type 1 is provided. (For more information about LLC types and frame types, refer to Chapter 16.)
•Protocol—Two bytes that identify the protocol encapsulated in the information field of the frame. The most up-to-date values of the protocol field are specified in the most recent Assigned Numbers Request For Comments (RFC).
•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.