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Link Layer

Link Layer. Ethernet and IEEE 802 Encapsulation. MUST be able to send and receive Ethernet SHOULD be able to receive IEEE 802 MAY be able to send Ethernet and IEEE 802 configurable default Ethernet. SLIP - Serial Line IP. A way to encapsulate IP-datagrams on a serial link

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Link Layer

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  1. Link Layer

  2. Ethernet and IEEE 802 Encapsulation • MUST be able to send and receive Ethernet • SHOULD be able to receive IEEE 802 • MAY be able to send Ethernet and IEEE 802 • configurable • default Ethernet

  3. SLIP - Serial Line IP • A way to encapsulate IP-datagrams on a serial link • Simple - END + ESC character • no address negotiation • no type field - 1 protocol • Error prone - no CRC • CSLIP • Old fashioned

  4. SLIP - Framing IP datagram c0 db ESC ESC END END c0 db dc db dd c0 SLIP encapsulation

  5. PPP - Point to Point Protocol • A way to encapsulate IP-datagrams on a serial link • Asynchronous AND synchronous communication • Link Control Protocol (LCP) • negotiation for datalink options • Conf-req * Conf-ack * Conf-nack * Conf-rej • Network Control Protocol (NCP) • negotiation for network protocol options

  6. flag addr contol protocol information CRC flag protocol Network control data protocol Link control data protocol IP datagram PPP Framing 1 Link setup 2 3

  7. PPP - LCP Negotiation options • Maximum Receive Unit (MRU) • Asynchronous Control Character Map • Authentication Protocol • Address and Control field compression • Multi-link procedure • Call-back

  8. PAP Authentication • Password authentication protocol • Snooping gets you the password • Not compatible with bidirectional authorization

  9. CHAP Authentication • Challenge authentication protocol • Follows establishment of LCP • Identifies user • Random number challenge, crypto response • Snooping does not discover password

  10. CHAP Authentication Challenge CHAP CHAP Response LCP LCP Link (Async, ISDN) Link (Async, ISDN)

  11. IPCP IPXCP MLP Bundle LCP LCP LCP Link Link Link Multilink PPP

  12. PPP - IPCP options • IP-Address • IP-Compression-Protocol • Unnumbered mode • Primary-DNS-Address • Secondary-DNS-Address

  13. PPP summary • Multiple protocols on one link • CRC for every frame • Dynamic IP address negotiation (LCP) • TCP and IP header compression • Negotiation of datalink options (NCP)

  14. Serial line troughput calculation • 33.6 kbps - 8 bits / byte - 1 start - 1 stop = 3360 bytes /sec • 1024-byte + 20(TCP) + 20(IP) + 8(PPP) --> 319 msec • RTT = 638 ms • > 100 - 200 ms is bad for interactive response • for multiple sessions use small frames or better use queuing

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