What is Point to Point protocol (PPP)? Summarize PPP encapsulation.

Point-to-Point Protocol (PPP) is a network-specific standard protocol and it is described in RFC 1661 and RFC 1662. There are a large number of proposed standard protocols, which specify the operation of PPP over different kinds of point-to-point links. Each has a status of elective. Point-to-point circuits in the form of asynchronous and synchronous lines have long been the mainstay for data communications. In the TCP/IP world, the de facto standard SLIP protocol has served admirably in this area, and is still in widespread use for dial-up TCP/IP connections. However, SLIP has a number of drawbacks that are addressed by the Point-to-Point Protocol.

A summary of the PPP encapsulation is shown in Fig

The encapsulation fields are defined as follows:
Protocol Field: The protocol field is one or two octets, and its value identifies the datagram encapsulated in the Information field of the packet. Up-to-date values of the Protocol field are specified in RFC 3232.

Information Field: The Information field is zero or more octets. The Information field contains the datagram for the protocol specified in the Protocol field. The maximum length for the information field, including padding, but not including the Protocol field, is termed the Maximum Receive Unit (MRU), which defaults to 1500 octets. By negotiation, other values can be used for the MRU.

Padding: On transmission, the information field can be padded with an arbitrary number of octets up to the MRU. It is the responsibility of each protocol to distinguish padding octets from real information. The IP Control Protocol (IPCP) is the NCP for IP and is responsible for configuring, enabling, and disabling the IP protocol on both ends of the point-to-point link. The IPCP options negotiation sequence is the same as for LCP, thus allowing the possibility of reusing the code. One important option used with IPCP is Van Jacobson Header Compression, which is used to reduce the size of the combined IP and TCP headers from 40 bytes to approximately 3-4 by recording the states of a set of TCP connections at each end of the link and replacing the full headers with encoded updates for the normal case, where many of the fields are unchanged or are incremented by small amounts between successive IP datagrams for a session.