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CS 5565 Network Architecture and Protocols

CS 5565 Network Architecture and Protocols. Godmar Back. Lecture 39. Announcements. Project 2B due Wed Apr 26 & May 3 Read posts in forum Reading Assignment Chapter 5. Data Link Layer. Error Detection & Correction. Checksumming: Cyclic Redundancy Check.

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CS 5565 Network Architecture and Protocols

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  1. CS 5565Network Architecture and Protocols Godmar Back Lecture 39

  2. Announcements • Project 2B due Wed Apr 26 & May 3 • Read posts in forum • Reading Assignment Chapter 5 CS 5565 Spring 2006

  3. Data Link Layer Error Detection & Correction

  4. Checksumming: Cyclic Redundancy Check • view data bits, D, as a binary number • choose r+1 bit pattern (generator), G • goal: choose r CRC bits, R, such that • <D,R> exactly divisible by G (modulo 2) • receiver knows G, divides <D,R> by G. If non-zero remainder: error detected! • can detect all burst errors less than r+1 bits • widely used in practice (ATM, HDLC) CS 5565 Spring 2006

  5. CRC Example Want: D.2rXOR R = nG equivalently: D.2r = nG XOR R equivalently: if we divide D.2r by G, want remainder R D.2r G R = remainder[ ] CS 5565 Spring 2006

  6. CRC (cont’d) • Formulation often as polynomials over algebraic field {0, 1}, e.g. • G = x3 + 1 • D = x5 + x3 + x2 + x • CRC-32 IEEE polynomial • x32 + x26 + x23 + x22 + x16 + x12 + x11 + x10 + x8 + x7 + x5 + x4 + x2 + x + 1 • detects all burst errors up to 32bits • detects all burst errors of odd length CS 5565 Spring 2006

  7. Multiple Access Protocols & Link Layer Addressing

  8. Ideal Multiple Access Protocol Broadcast channel of rate R bps 1. When one node wants to transmit, it can send at rate R. 2. When M nodes want to transmit, each can send at average rate R/M 3. Fully decentralized: • no special node to coordinate transmissions • no synchronization of clocks, slots 4. Simple CS 5565 Spring 2006

  9. MAC Protocols: a taxonomy MAC – Media Access Control Three broad classes: • Channel Partitioning • divide channel into smaller “pieces” (time slots, frequency, code) • allocate piece to node for exclusive use • Random Access • channel not divided, allow collisions • “recover” from collisions • “Taking turns” • Nodes take turns, but nodes with more to send can take longer turns CS 5565 Spring 2006

  10. MAC Addresses and ARP • 32-bit IP address: • network-layer address • used to get datagram to destination IP subnet • MAC (or LAN or physical or Ethernet) address: • used to get datagram from one interface to another physically-connected interface (same network) • 48 bit MAC address (for most LANs) burned in the adapter ROM • MAC address allocation administered by IEEE • manufacturer buys portion of MAC address space (to assure uniqueness) CS 5565 Spring 2006

  11. 1A-2F-BB-76-09-AD LAN (wired or wireless) 71-65-F7-2B-08-53 58-23-D7-FA-20-B0 0C-C4-11-6F-E3-98 LAN Addresses and ARP Each adapter on LAN has unique LAN address Broadcast address = FF-FF-FF-FF-FF-FF = adapter CS 5565 Spring 2006

  12. 237.196.7.78 1A-2F-BB-76-09-AD 237.196.7.23 237.196.7.14 LAN 71-65-F7-2B-08-53 58-23-D7-FA-20-B0 Question: how to determine MAC address of B knowing B’s IP address? 0C-C4-11-6F-E3-98 237.196.7.88 ARP: Address Resolution Protocol • Each IP node (Host, Router) on LAN has ARP table • ARP Table: IP/MAC address mappings for some LAN nodes < IP address; MAC address; TTL> • TTL (Time To Live): time after which address mapping will be forgotten (typically 20 min) CS 5565 Spring 2006

  13. A wants to send datagram to B, and B’s MAC address not in A’s ARP table. A broadcasts ARP query packet, containing B's IP address Dest MAC address = FF-FF-FF-FF-FF-FF all machines on LAN receive ARP query B receives ARP packet, replies to A with its (B's) MAC address frame sent to A’s MAC address (unicast) A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state: information that times out (goes away) unless refreshed ARP is “plug-and-play”: nodes create their ARP tables without intervention from net administrator ARP Protocol CS 5565 Spring 2006

  14. Routing to another LAN Send datagram from A to B via R A R B • Two ARP tables in router R, one for each IP network (LAN) • Q. How would you implement ARP at end hosts? • Q. What about security? CS 5565 Spring 2006

  15. Proxy ARP Default Gateway 237.196.7.78 237.196.7.23 • 237.196.7.23 uses 237.196.7/24 • 237.196.7.14 answers ARP requests for 237.196.7.78 • See arp(8), “arp –s … pub” • Forwards IP datagrams along 237.196.7.77 • 237.196.7.78 uses 237.196.7.77 as default gateway 237.196.7.14 LAN 237.196.7.77 Point-To-Point Link, e.g. dialup with subnet 237.196.7.76/30 Upstream Internet 58-23-D7-FA-20-B0 237.196.7.88 CS 5565 Spring 2006

  16. DHCP • Dynamic Host Configuration Protocol • Precursors: RARP – Reverse ARP, bootp • Uses MAC-level broadcast • DHCP Discovery (“what DHCP servers are out there and who can give me an IP address?”) • DHCP Offer (“you may use this address”) • May be more than one • DHCP Request (“I’d like to use this address”) • DHCP Ack (“please do”) • DHCP relay agents for larger networks. CS 5565 Spring 2006

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