Point-to-Point Protocol

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 supports these functions by providing an extensible Link Control Protocol (LCP) and a family of Network Control Protocols (NCPs) to negotiate optional configuration parameters and facilities.

In addition to IP, PPP supports other protocols, including Novell's Internetwork Packet Exchange (IPX)

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.

PPP is designed to allow the simultaneous use of multiple network layer protocols.

Physical Layer Requirements:

PPP is capable of operating across any DTE/DCE interface. Examples include EIA/TIA-232-C (formerly RS-232-C), EIA/TIA-422 (formerly RS-422), EIA/TIA-423 (formerly RS-423), and International Telecommunication Union Telecommunication Standardization Sector (ITU-T) (formerly CCITT) V.35.

The only absolute requirement imposed by PPP is the provision of a duplex circuit, either dedicated or switched, that can operate in either an asynchronous or synchronous bit-serial mode, transparent to PPP link layer frames.

PPP does not impose any restrictions regarding transmission rate other than those imposed by the particular DTE/DCE interface in use

PPP Link Layer:

PPP uses the principles, terminology, and frame structure of the International Organization for Standardization (ISO) HDLC procedures (ISO 3309-1979), as modified by ISO 3309:1984/PDAD1 "Addendum 1: Start/Stop Transmission." ISO 3309-1979 specifies the HDLC frame structure for use in synchronous environments.

ISO 3309:1984/PDAD1 specifies proposed modifications to ISO 3309-1979 to allow its use in asynchronous environments.

The PPP control procedures use the definitions and control field encodings standardized in ISO 4335-1979 and ISO 4335-1979.

The PPP frame format appears in Figure:

Six Fields Make Up the PPP Frame.

Figure: Six Fields Make Up the PPP Frame

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.

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 Link-Control Protocol:

The PPP LCP provides a method of establishing, configuring, maintaining, and terminating the point-to-point connection.

LCP goes through four distinct phases.

First, link establishment and configuration negotiation occur. Before any network layer datagrams (for example, IP) can be exchanged, LCP first must open the connection and negotiate configuration parameters.

This phase is complete when a configuration-acknowledgment frame has been both sent and received.

This is followed by link quality determination.

LCP allows an optional link quality determination phase following the link-establishment and configuration-negotiation phase.

In this phase, the link is tested to determine whether the link quality is sufficient to bring up network layer protocols.

This phase is optional. LCP can delay transmission of network layer protocol information until this phase is complete.

At this point, network layer protocol configuration negotiation occurs.

After LCP has finished the link quality determination phase, network layer protocols can be configured separately by the appropriate NCP and can be brought up and taken down at any time.

If LCP closes the link, it informs the network layer protocols so that they can take appropriate action.

Finally, link termination occurs. LCP can terminate the link at any time. This usually is done at the request of a user but can happen because of a physical event, such as the loss of carrier or the expiration of an idle-period timer.

Three classes of LCP frames exist. Link-establishment frames are used to establish and configure a link. Link-termination frames are used to terminate a link, and link-maintenance frames are used to manage and debug a link.