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Communication Networks

Communication Networks. Recitation 3 DNS & ARP. Addressing Schemes. Internet. host names: convenient app-to-app communication IP: efficient large-scale network communication MAC: quick-n-easy LAN forwarding. medellin.cs.columbia.edu. 128.119.40.7. 128.119.40.7. 128.119.40.7.

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Communication Networks

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  1. Communication Networks Recitation 3 DNS & ARP Comnet 2010

  2. Addressing Schemes Internet • host names: convenient app-to-app communication • IP: efficient large-scale network communication • MAC: quick-n-easy LAN forwarding medellin.cs.columbia.edu 128.119.40.7 128.119.40.7 128.119.40.7 128.119.40.7 128.119.40.7 E6-E9-00-17-BB-4B Comnet 2010

  3. Translating between addresses Hostname (medellin.cs.columbia.edu) DNS IP address (128.119.40.7) ARP MAC address (E6-E9-00-17-BB-4B) Comnet 2010

  4. Contacted by local name server when can not resolve name Root name server: contacts authoritative name server if name mapping not known gets mapping returns mapping to local name server 13/Anycast DNS Servers Comnet 2010

  5. DNS Hierarchy edu com org jp rpi albany DNS Distributed Database rpi.edu DNS DB rpi.edu DNS DB rpi.edu DNS DB Comnet 2010

  6. host surf.eurecom.fr wants IP address of gaia.cs.umass.edu 1. Contacts its local DNS server, dns.eurecom.fr 2.dns.eurecom.fr contacts root name server, if necessary 3. root name server contacts authoritative name server, dns.umass.edu, if necessary 2 4 3 5 local name server dns.eurecom.fr 1 6 Simple DNS example root name server authorititive name server dns.umass.edu requesting host surf.eurecom.fr gaia.cs.umass.edu Comnet 2010

  7. Root name server may not know authoritative name server may know intermediate name server: who to contact to find authoritative name server 2 6 3 7 local name server dns.eurecom.fr 5 4 1 8 Intermediate name server root name server intermediate name server dns.umass.edu authoritative name server dns.cs.umass.edu requesting host surf.eurecom.fr gaia.cs.umass.edu Comnet 2010

  8. recursive query: puts burden of name resolution on contacted name server heavy load iterated query: contacted server replies with name of server to contact “I don’t know this name, but ask this server” 2 3 4 5 7 6 local name server dns.eurecom.fr intermediate name server dns.umass.edu 1 8 DNS: iterated queries root name server iterated query authoritative name server dns.cs.umass.edu requesting host surf.eurecom.fr gaia.cs.umass.edu Comnet 2010

  9. DNS library functions struct hostent *gethostbyname( const char *hostname); struct hostent *gethostbyaddr( const char *addr,size_t len, int family); struct hostent *he; he = gethostbyname( “www.yahoo.com” ); printf( “Address: %s\n”, inet_n_ntoa( he->h_addr ) ); struct in_addr addr; inet_aton( “66.94.230.32”, &addr ); he = gethostbyaddr( &addr, sizeof( addr ), AF_INET ); printf( “Name: %s\n”, he->h_name ); Comnet 2010

  10. LAN Addresses and ARP 32-bit IP address: • network-layer address • used to get datagram to destination network LAN (or MAC or physical) address: • used to get datagram from one interface to another physically-connected interface (same network) • 48 bit MAC address (for most LANs) burned into the adapter’s ROM Comnet 2010

  11. LAN Addresses and ARP Each adapter on the LAN has a unique LAN address Comnet 2010

  12. Question: how to determine MAC address of B given B’s IP address? ARP: Address Resolution Protocol • Each IP node (Host, Router) on LAN has ARP module and table • ARP Table: IP/MAC address mappings for some LAN nodes < IP address; MAC address; TTL> < ………………………….. > • TTL (Time To Live): time of day after which address mapping will be forgotten (typically 20 minutes) Comnet 2010

  13. Arp Arp! ARP protocol • A knows B's IP address, wants to learn physical address of B • A broadcasts ARP query packet, containing B's IP address • all machines on LAN receive ARP query • B receives ARP packet, replies to A with its (B's) physical layer address • A caches (saves) IP-to-physical address pairs until information becomes old (times out) • soft state: information that times out (goes away) unless refreshed Comnet 2010

  14. ARP Conversation HEY - Everyone please listen! Will 128.213.1.5 please send me her Ethernet address? not me Hi Green! I’m 128.213.1.5, and my Ethernet address is 87:A2:15:35:02:C3 Comnet 2010

  15. Problem 1 • Calculate the completion time of transferring a file of size 1KB over a link with speed 10Mb/s and length 3000km. Propagation speed is 200,000Km/s Comnet 2010

  16. Solution 1 The completion time is calculated as follows: T = Prop. Delay + Trans. Time We obtain that: • Prop. Delay = 3000 km /(0.2 km/usec) = 15ms • Trans. Time = 1KB/1.25MB/s = 8.2 ms T = 15ms + 8.2 ms = 23.2 ms Comnet 2010

  17. Calculating CRC • For data D and Generator G find bits R such that DR = n*G. • Same as D00…0 = n*G + R • R will always be 1 bit shorter than G • Use long Division with XOR Comnet 2010

  18. Solution 2 • D = 01011000, G=1101, R=??? • 01011000000 | 11011101 11001101 10001101 10101101 1110110111 R=011 n=1100111 DR=01011000011 Comnet 2010

  19. Solution 2 – In reverse • DR = 01011000011, G=1101 • 01011000011 | 11011101 11001101 10001101 10111101 110111010 Comnet 2010

  20. CRC – Concluding Remarks • If R has r bits, there are 2r different CRCs • A random string will not be detected as an error in probability 2-r. • Robustness for common errors depends on G. Comnet 2010

  21. Problem 3 2. In CSMA/CD network there are two computers A and B which collide in round 1. Give the table of possible outcomes of the second round and their probabilities. Assume that the initial delay period after the collision is D=1 (the hosts pick a random number between 0 and D before trying to re-transmit). Comnet 2010

  22. Solution 3 Case A B Probability Comment --------------------------------------------------------------- a 0 0 0.25 Collide in round 2 b 0 1 0.25 A successful in round 2 B successful in round 3 c 1 0 0.25 B successful in round 2 A successful in round 3 d 1 1 0.25 Collide in round 3 Comnet 2010

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