PPP (Point to point Protocol)

PPP (Point to Point Protocol)

• majority of home user use
• simple error control : discarding and resending
• no flow control

Framing

PPP frame format

Address field

• fixed 11111111 (broadcast address)

Protocol field

• defines what is being carried
• dafault 2 bytes long

Payload field

• maximun 1500 bytes

FCS field

• CRC

Transition Phase

Transition Phase (FSM)

Dead

• state of start
• no active carrier and the line is quiet

Establish

• when one of the two nodes starts the comuincation
• options are negotiate
• (an extra step) do authentication

Open

• the exchange of data packets can be started
• the connection remains in this state until one of the endpoints wants to terminate the connection

Terminate

• the system remain until the carrier is dropped and goes Dead state

Multiplexing

• Three additional protocol -> makes PPP powerful

Multiplexing in PPP

Link Control Protocol (LCP)

• etablishing, maintaining, configuring, and termlnatlng links
• also provides negotiation mechanisms to set optlons between the two endpoints
• all LCP packets are carried in the payload field

LCP packet encapsulated in a frame

• code field : define the type of LCP
• id field : match a request with a reply; one endpoint inserts a value in this field, which will be copied into the reply packet.
• informatlon field : divided into three fields. optlon type, optlon length, and optlon data

Authentication Protocol (AP)

• PPP hsa created two protocols for authentication; PAP, CHAP

PAP (Password Authentication Protocol)

1. The user who wants to access a system sends an authentication identi fication (usually the user name) and a password.
2. The system checks the validity of the identification and password and either accepts or denies connectlon.

PAP

CHAP (Challenge Handshake Authentication Protocol)

1. The system sends the user a challenge packet containing a challenge value. usually a few bytes
2. The user applies a predefined function that takes the challenge value and the user's own password and creates a result. The user sends the result in the response packet to the system.
3. The system does the same.

CHAP

Network Control Protocol (NCP)

• carry a network-layer data packet from protocols defined by the Internet, OSI Xerox and so on