EE450, USC, Zahid 1Pieces of the PuzzleEE450: Introduction to Computer NetworksProfessor A. ZahidEE450, USC, Zahid 2Burning Questionsz How does a host/router get the MAC address of another host/router on the same LAN?z Answer: Address Resolution Protocol: ARPz How does a host get the IP address of another host across the Internet?z Answer : Domain Name Services: DNSz How does a host get it’s own IP address?z Answer: Dynamic Host Configuration Protocol (DHCP)z How do we distinguish between two or more applications running on the same host?z Answer: Port Numbers/SocketsEE450, USC, Zahid 3IP & MAC Addresses223.1.1.1223.1.1.2223.1.1.3223.1.1.4223.1.2.9223.1.2.2223.1.2.1223.1.3.2223.1.3.1223.1.3.27ABCz A has a Packet to send to B. z A has the IP address of B and it knows that B is located on the same network. z A encapsulate the packet inside a link-layer frame frame source &destination addressesPacket source &destination addressesB’s MACaddrA’s MACaddrA’s IPaddrB’s IPaddrIP payloadPacketframeFCSEE450, USC, Zahid 4Question #1: How does a Host/Router get the MAC address of another Host/Router on the same LAN?EE450, USC, Zahid 5Address Resolution Protocolz Each IP node (Host, Router) on LAN has ARPmodule/tablez ARP Cache Table: IP/MAC address mappings for some LAN nodesz Cache lifetime ~ 20 minQuestion: how to determineMAC address of Bgiven B’s IP address?EE450, USC, Zahid 6ARP (Continued)z Address Resolution Protocol binds an IP address to a media (link) addressz ARP is a simple request-response protocolz Host “A” broadcasts a request packet containing IP address of “B”. Broadcast MAC address is FF:FF:FF:FF:FF:FF. All hosts receive the ARP inquiryz Host “B” recognizes its IP addressz Host “B” sends a response (not a broadcast) packet to first host containing its MAC addressz Host “A” caches address mapping for later usez ARP is a local, “Plug and Play” ProtocolEE450, USC, Zahid 7ARP OperationEE450, USC, Zahid 8ARP PacketEE450, USC, Zahid 9Four Cases of ARP useEE450, USC, Zahid 10Four Cases of ARP use (Cont.)EE450, USC, Zahid 11Proxy ARPEE450, USC, Zahid 12Question #2: How does a Host get the IP address of another Host across the Internet?EE450, USC, Zahid 13Domain Name Services (DNS)z DNS is a TCP/IP client server application protocol that allow host and name servers to communicate in order to provide host name to IP address translationz DNS uses a distributed, hierarchical naming structure by defining several Domains. A domain is a collection of sites that are related in some manner z DNS use the services of UDP, port # 53z Application protocols such as HTTP, FTP, SMTP, etc... use the services of DNSEE450, USC, Zahid 14Domain Name Spacecom net edu org mil gov Top Level DomainsRoot Name Serveruscuclamitaludra millyEE450, USC, Zahid 15DomainsEE450, USC, Zahid 16Domains and ZonesEE450, USC, Zahid 17Domain Names & LabelsEE450, USC, Zahid 18Name Serversz Local Name Servers: This is the default name server (in department, university, company, residential ISP, etc…) that will receive the DNS query from the hostz The IP address of the default local name server is configured manually in the host z Root Name Servers: There are 13 root name servers most of which are located in US (two of them in Marina Del Rey). When a local name server can’t satisfy the query from a host, it will behave as a DNS client and queries one of the root servers. If the root name server can’t satisfy the query, it consult withz Authoritative Name Server: This is where the host register its name/IP addressEE450, USC, Zahid 19Root Name Servers13 root name servers worldwideb USC-ISI Marina del Rey, CAl ICANN Los Angeles, CAe NASA Mt View, CAf Internet Software C. Palo Alto, CA (and 36 other locations)i Autonomica, Stockholm (plus 28 other locations)k RIPE London (also 16 other locations)m WIDE Tokyo (also Seoul, Paris, SF)a Verisign, Dulles, VAc Cogent, Herndon, VA (also LA)d U Maryland College Park, MDg US DoD Vienna, VAh ARL Aberdeen, MDj Verisign, ( 21 locations) Contacted by local name server that can not resolve name❒ Root name server:o Contacts authoritative name server if name mapping not knowno Gets mappingo Returns mapping to local name serverEE450, USC, Zahid 20TLD and Authoritative Name Serversz Top-level domain (TLD) servers:z Responsible for com, org, net, edu, etc, and all top-level country domains uk, fr, ca, jp, in, cnz Network Solutions maintains servers for com TLDz Educause for edu TLDz Authoritative DNS servers:z Organization’s DNS servers, providing authoritative hostname to IP mappings for organization’s servers (e.g., Web, mail).z Can be maintained by organization or service providerEE450, USC, Zahid 21Local Name Serverz Does not strictly belong to hierarchyz Each ISP (residential ISP, company, university) has one.z also called “default name server”z when host makes DNS query, query is sent to its local DNS serverz acts as proxy, forwards query into hierarchyEE450, USC, Zahid 22Recursive DNSz Host “A” whose name is John.yahoo.com wants the IP address of another host “B” whose name is Mary.usc.eduz Host “A” Contacts its local DNS server, dns.yahoo.comz dns.yahoo.com contacts root name server, if necessaryz Root name server contacts authoritative name server, dns.usc.com, if necessaryRequesting HostJohn.yahoo.comMary.usc.eduRoot Name ServerAuthoritative Name Serverdns.usc.eduLocal Name Serverdns.yahoo.com123456EE450, USC, Zahid 23Iterative DNSz Contacted server replies with name of server to contactz “I don’t know this name, but ask this server”Requesting Hostjohn.yahoo.comMary.ee.usc.eduRoot Name ServerLocal Name Serverdns.yahoo.com1234Authoritative Name Serverdns.ee.usc.eduIntermediate Name Serverdns.usc.edu58iterated query67EE450, USC, Zahid 24Pure Iterative ResolutionEE450, USC, Zahid 25DNS Cachingz Once (any) name server learns mapping, it caches mappingz Cache entries timeout (disappear) after some timez TLD servers typically cached in local name servers●Thus root name servers not often visitedEE450, USC, Zahid 26Why not Centralized DNS?z A centralized DNS represent a single point of failure. If the name server crashes so would the entire internetz All traffic volume would have to be handled by this name serverz A single name server can’t be close to all query clients ⇒ increased delays ⇒ World Wide Wait !!!!!z Maintaining and updating a single name server is a huge task. Just dealing wit