Transcript Data Link Part 3

11.7
Point-to-Point
Access:
PPP
PPP
 Used for Point-to-Point Connections only
Used as data link control (encapsulates network layer PDUs ) to
connect :
 Home users (PC) to ISP using a telephone line and a modem
Two routers
Router
Router
Access
Router
Router
Point-to-Point Links
Between Routers
Router
Modems
Dial-Up Access
PPP
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A data link layer protocol
Services provided by PPP
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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
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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.
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Authentication: Authentication is optional (negotiated between the two
devices). PPP supports two authentication protocols: Password Authentication
Protocol (PAP) and Challenge Handshake Authentication Protocol (CHAP).
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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
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NCP protocols allows the network layers at both ends to configure
themselves. For IP network protocol, NCP handles dynamic IP address
assignment
Frame format
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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)
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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).
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Length is 2 bytes or 1byte (negotiated)
Data field (payload) : Contains the encapsulated packet (network layer data, or
PPP control protocols packets)
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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
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Idle state: The link is not being used
Establishing state
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Authentication state:
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Performed by Authentication Protocol
The user identity is verified
Networking state
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Performed by LCP
Options are negotiated between endpoints (frame data length,
keeping address and control fields, use Authentication or not, etc, )
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
IPCP
8021
IPCP