Point-to-PointAccess:

PPP

11.7

PPP

Dial-Up Access

AccessRouter

Modems

Point-to-Point Links

Router

Router

Router Router

Between Routers

Used for Point-to-Point Connections onlyUsed as data link control (encapsulates network layer PDUs ) to connect :

Home users (PC) to ISP using a telephone line and a modemTwo routers

PPP A data link layer protocol Services provided by PPP

Defines the format of the frame to be exchanged between devices

Supports multiple network layer protocols simultaneously (like in multiprotocol routers)

Defines how two devices authenticate each other

PPP Support protocols

Link management: The link control protocol (LCP) is responsible for establishing, configuring, and negotiating a data-link connection. LCP also monitors the link quality and is used to terminate the data-link connection.

Authentication: Authentication is optional (negotiated between the two devices). PPP supports two authentication protocols: Password Authentication Protocol (PAP) and Challenge Handshake Authentication Protocol (CHAP).

Network protocol configuration: PPP has a family of network control protocols (NCPs) that are responsible for establishing and configuring different network layer protocols such as IP, AppleTalk , Decnet

NCP protocols allows the network layers at both ends to configure themselves. For IP network protocol, NCP handles dynamic IP address assignment

Frame format

Flag fields: 01111110, identify the frame boundaries (start and end of frame) Address field: For point-to-point link address is not necessary. It uses 11111111 (can

be removed – negotiated). Control field: 00000011 (can be removed – negotiated) In general, PPP does not provide frame sequence numbers or acknowledgement

numbers which means No reliable transmission for the data . Protocol: It tells what is carried in the data field. The PPP receiver uses this code

to determine if the data field carries LCP packet (C021h ), Authentication protocols (PAP C023 or CHAP C223) or NCP packet (8021h) or data packet for a specific network protocol (IP 0021h, AppleTalk 0029h, Decnet 0027h).

Length is 2 bytes or 1byte (negotiated) Data field (payload) : Contains the encapsulated packet (network layer data, or

PPP control protocols packets) Variable length, default maximum 1500 bytes can be changed at link

configuration phase. (negotiated maximum) Checksum (FCS): the standard HDLC CRC-16 or CRC-32 (negotiated length)

Multiplexing in PPP

Table 11.3 Common options

LCP packet encapsulated in a frame

Length field contains the length of the whole LCP control packet

PPP frame

Table 11.2 LCP packets

PAP

Figure 11.36 PAP packets encapsulated in a PPP frame

Message

Message

CHAP

Figure 11.37 CHAP packets encapsulated in a PPP frame

Server

Client

IPCP packet encapsulated in PPP frame

IPCP: Internetwork Protocol Control Protocol

NCP protocol for IP packets

Used to establish, configure (IP address, compression of IP packets if required) and terminate a network layer connection for IP packets

Table 11.4 Code value for IPCP packets

IP datagram encapsulated in a PPP frame

IP data is carried

PPP Transition states Idle state: The link is not being used Establishing state

Performed by LCP Options are negotiated between endpoints (frame data length,

keeping address and control fields, use Authentication or not, etc, ) Authentication state:

Performed by Authentication Protocol The user identity is verified

Networking state Performed by NCP Control (dynamic IP address, Compression of IP packets) Exchange of Data packets

Terminating state: The link is terminated (NCP first then LCP)

Figure 11.33 Transition phases

An example

02 Message

An example (continued)

8021

8021

IPCP

IPCP

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