Point-to-Point Access: PPP
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Transcript of Point-to-Point Access: PPP
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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