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Point-to-Point Access: PPP

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. Between Routers. A data link layer protocol

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Point-to-Point Access: PPP

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  1. 11.7 Point-to-PointAccess: PPP

  2. 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 Between Routers

  3. 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

  4. 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

  5. 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 maximum1500 bytes can be changed at link configuration phase. (negotiated maximum) Checksum (FCS): the standard HDLC CRC-16 or CRC-32 (negotiated length) Frame format

  6. Multiplexing in PPP

  7. Table 11.3 Common options

  8. LCP packet encapsulated in a frame Length field contains the length of the whole LCP control packet PPP frame

  9. Table 11.2 LCP packets

  10. PAP

  11. Figure 11.36 PAP packets encapsulated in a PPP frame Message Message

  12. CHAP

  13. Figure 11.37 CHAP packets encapsulated in a PPP frame Server Client

  14. 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

  15. Table 11.4 Code value for IPCP packets

  16. IP datagram encapsulated in a PPP frame IP data is carried

  17. 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) PPP Transition states

  18. Figure 11.33 Transition phases

  19. An example Message 02

  20. An example (continued) 8021 IPCP Network 8021 IPCP DataLink

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