Point-to-Point Protocol (PPP) Encapsulation

Point-to-Point Protocol (PPP) is basically a Wide Area Network (WAN) protocol that performs or works at layer 2 by simply encapsulating frames for transmission or transferring over different physical links or connections like serial cables, cell phones, fiber optic cable among others, etc. Encapsulation is basically a process in which lower-layer protocol basically receives data from higher layer protocol and then further places this data portion of its frame. In simple words, we can say the encapsulation is process of enclosing one type of packing with help of other types of packet. PPP generally provides encapsulation so that various protocols at the network get supported simultaneously. PPP connections also deliver or transmit packets in sequence and provide full-duplex simultaneous bi-directional operation. PPP usually encapsulates any of the network layer packets in its frame that makes it possible for PPP layer three protocol to become independent and even capable of carrying multiple-layer three packets through a single link or connection. PPP Encapsulation is also required to disambiguate multiprotocol datagrams i.e. removal of ambiguity to make the multiprotocol datagrams clear and easy to be understood. PPP puts data in a frame and transmit it through PPP connection or link. A frame is basically defined as a unit of transmission in Data Link Layer (DLL) of the OSI protocol stack. To form encapsulation, a total of 8-bytes are required. Data is usually transmitted from left to right in frames. General Structure of PPP encapsulation is shown below :
PPP encapsulation frame basically contains three types of fields as given below :

1. Protocol Field – This field is of 1 or 2 bytes i.e., 8 or 16 bits that are used to identify datagram that is being encapsulated in the information field of packet. It simply indicates protocol that is used in the frame. Least significant bit of lower bytes is usually set to 1 and on the other hand, most significant bit is usually set to 0. Types of protocols that can be present are Link Control Protocol (LCP), Password Authentication Protocol (PAP), Challenge Handshake Authentication Protocol (CHAP), etc. 
   

Protocol Number	Protocol Name
0001	Padding Protocol
0003 to 001f	Reserved (transparency inefficient)
00cf	Reserved (PPP NLPID)
8001 to 801	Unused
807d	Unused
0021	IP (Internet Protocol)
8021	IPCP (Internet Protocol Control Protocol) (NCP for IP)
002d	Van Jacobson TCP/IP header compression (RFC 1144)
002f	Van Jacobson IP header compression
c021	Link Control Protocol
c023	Password Authentication Protocol
c025	Link Quality Report
c223	Challenge Handshake Authentication Protocol

1. Information Field – This field is of 0 or more bytes. It also has maximum length of 1500 bytes including padding and excluding protocol field. It usually contains datagram for the protocol that is specified and identified in protocol field. A datagram is basically a transfer unit that is associated with networking.
2. Padding Field – This field is optional. On transmission, Information field may be padded up to the Maximum Receive Unit (MRU). Both of peers must be capable of recognizing and distinguish padding bytes from true data or real information.