The purpose of the OSI model is to open communication between different systems without requiring changes to the logic of the hardware and software.
Is a set of protocols that allows any two difference system to communicate regardless of their underlying architecture.
4 The OSI model is a layered framework for the design of network systems that allows for communication across all types of computer systems. It consists of seven separate but related layers, APPLICATION LAYER PRESENTATION LAYER SESSION LAYER TRANSPORT LAYER NETWORK LAYER DAT LINK LAYER PHYSICAL LAYER
In networking, the Point-to-Point Protocol, or PPP, is a data link protocol.
PPP is commonly used in establishing a direct connection between two networking nodes. It can provide connection authentication, transmission encryption privacy, and compression.
WHY POINT TO POINT: When computer is connected to ISP via the modem, the ISP and the computer made the two common points on the network and hence the protocol which operates between these 2 points is called Point to Point protocol. 5
SLIP was used as framing protocol over serial lines before the advent of PPP.
SLIP has some deficiencies: SLIP does not have error detection mechanism (left to TCP to detect and recover from errors on the transmission line). SLIP only supports IP (no other layer 3 protocols). SLIP does not provide dynamic IP address assignment. 4. SLIP does not provide authentication. 6
PPP was devised in order to overcome the deficiencies of SLIP.
PPP features: PPP affords error detection (checksum). PPP allows to dynamically assign an IP address (NCP). PPP provides authentication (PAP / CHAP) PPP is a symmetric protocol: The 2 parties in a PPP session are the initiator (I, usually client) and the responder (R, usually server). 7
Functions of PPP: PPP is widely used , especially in analog modem access to ISP, where one end is PC and another end is ISP router the functions performed are: The PPP was designed to transport multi-protocol packets between two peers connected by simple links. These links provide Full-Duplex simultaneous bi-directional operation. 8
The PPP allows different types of protocols to function on its platform and on the same link. They are: LCP(Link Control Protocol)
Dynamic assignment of DNS primary and secondary server.
The host must set a default route to the PPP interface since there is no default gateway IP address (the link is point-to-point with no IP address = unnumbered).
The PPP provides three methods of authentication: Password Authentication Protocol(PAP): Uses a password to authenticate. 2. Challenge Handshake Authentication Protocol(CHAP): Which uses the handshake of the server with the dial up as an authentication. 3. Extensible Authentication Protocol (EAP): Is most commonly used for authentication on wireless networks. 11
Flag - Indicates the beginning or end of a frame, consists of the binary sequence 01111110. Address - Contains the binary sequence 11111111, the standard broadcast address. (Note: PPP does not assign individual station addresses.) Control - Contains the binary sequence 00000011, which calls for transmission of user data in an unsequenced frame. Protocol - Identify the protocol encapsulated in the information field of the frame. Information - Zero or more octet(s), contain the datagram for the protocol specified in the protocol field. FCS - Frame Check Sequence (FCS) Field, normally 16 bits. By prior agreement, consenting PPP implementations can use a 32-bit FCS for improved error detection. 14
For “data transparency”, the data field must be allowed to include the pattern <01111110> ; ie, this must not be interpreted as a flag.
To alert the receiver, the transmitter “stuffs” an extra < 01111101> byte after each < 01111110> data byte.
The receiver discards each 01111101 after 01111110, and continues data reception .
PPP Operation: 16 UP OPENED DEAD ESTABLISH AUTHENTICATE FAIL FAIL SUCCESS TERMINATE NETWORK CLOSING
Sequence Of Operations Is Shown Below: USER 17 ROUTER CONFIGURE - REQUEST(LCP) CONFIGURE - ACK(LCP) CONFIGURE – REQUEST IP(NCP) CONFIGURE – ACK IP(NCP) IP DATA PACKETS TERMINATE REQUEST(LCP) TERMINATE – ACK(LCP)
Application of PPP: PPP (Point to Point Protocol) is very popular: used in dial up connection between residential Host and ISP; On SONET/SDH connections. PPP is extremely simple (the simplest in the Data Link protocol family) and very streamlined. 18