Pieces of the Puzzle EE450 Introduction to Computer Networks Professor A Zahid EE450 USC Zahid 1 Burning Questions z z z z How does a host router get the MAC address of another host router on the same LAN z Answer Address Resolution Protocol ARP How does a host get the IP address of another host across the Internet z Answer Domain Name Services DNS How does a host get it s own IP address z Answer Dynamic Host Configuration Protocol DHCP How do we distinguish between two or more applications running on the same host z Answer Port Numbers Sockets EE450 USC Zahid 2 IP MAC Addresses z z z A has a Packet to send to B A A has the IP address of B and it knows that B is located on the same network 223 1 1 1 223 1 2 1 223 1 1 2 223 1 1 4 223 1 2 9 A encapsulate the packet inside a link layer frame B 223 1 1 3 223 1 3 27 A s MAC addr A s IP addr B s IP addr IP payload C 223 1 3 2 223 1 3 1 frame source Packet source destination addresses destination addresses B s MAC addr 223 1 2 2 FCS Packet frame EE450 USC Zahid 3 Question 1 How does a Host Router get the MAC address of another Host Router on the same LAN EE450 USC Zahid 4 Address Resolution Protocol Question how to determine MAC address of B given B s IP address z z z Each IP node Host Router on LAN has ARP module table ARP Cache Table IP MAC address mappings for some LAN nodes Cache lifetime 20 min EE450 USC Zahid 5 ARP Continued z z Address Resolution Protocol binds an IP address to a media link address ARP is a simple request response protocol z z z z z 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 inquiry Host B recognizes its IP address Host B sends a response not a broadcast packet to first host containing its MAC address Host A caches address mapping for later use ARP is a local Plug and Play Protocol EE450 USC Zahid 6 ARP Operation EE450 USC Zahid 7 ARP Packet EE450 USC Zahid 8 Four Cases of ARP use EE450 USC Zahid 9 Four Cases of ARP use Cont EE450 USC Zahid 10 Proxy ARP EE450 USC Zahid 11 Question 2 How does a Host get the IP address of another Host across the Internet EE450 USC Zahid 12 Domain Name Services DNS z z z 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 translation DNS uses a distributed hierarchical naming structure by defining several Domains A domain is a collection of sites that are related in some manner DNS use the services of UDP port 53 Application protocols such as HTTP FTP SMTP etc use the services of DNS EE450 USC Zahid 13 Domain Name Space Root Name Server com net edu usc ucla org mil gov Top Level Domains mit aludra milly EE450 USC Zahid 14 Domains EE450 USC Zahid 15 Domains and Zones EE450 USC Zahid 16 Domain Names Labels EE450 USC Zahid 17 Name Servers z z z Local Name Servers This is the default name server in department university company residential ISP etc that will receive the DNS query from the host z The IP address of the default local name server is configured manually in the host 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 with Authoritative Name Server This is where the host register its name IP address EE450 USC Zahid 18 Root Name Servers Contacted by local name server that can not resolve name Root name server o Contacts authoritative name server if name mapping not known o Gets mapping o Returns mapping to local name server a Verisign Dulles VA c Cogent Herndon VA also LA d U Maryland College Park MD g US DoD Vienna VA h ARL Aberdeen MD j Verisign 21 locations e NASA Mt View CA f Internet Software C Palo Alto CA and 36 other locations k RIPE London also 16 other locations i Autonomica Stockholm plus 28 other locations m WIDE Tokyo also Seoul Paris SF 13 root name servers worldwide b USC ISI Marina del Rey CA l ICANN Los Angeles CA EE450 USC Zahid 19 TLD and Authoritative Name Servers z Top level domain TLD servers Responsible for com org net edu etc and all top level country domains uk fr ca jp in cn z Network Solutions maintains servers for com TLD z Educause for edu TLD z z Authoritative DNS servers 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 provider z EE450 USC Zahid 20 Local Name Server z z Does not strictly belong to hierarchy Each ISP residential ISP company university has one z also z called default name server when host makes DNS query query is sent to its local DNS server z acts as proxy forwards query into hierarchy EE450 USC Zahid 21 Recursive DNS z z z z Root Name Server Host A whose name is John yahoo com wants the IP address of another host B whose name is Mary usc edu 2 5 3 4 Host A Contacts its local DNS server dns yahoo com dns yahoo com contacts root Local Name Server Authoritative Name Server name server if necessary dns yahoo com dns usc edu Root name server contacts authoritative name server dns usc com if necessary 1 6 Requesting Host John yahoo com EE450 USC Zahid Mary usc edu 22 Iterative DNS z z Root Name Server Contacted server replies with name of server to contact 2 I don t know this name but ask this Local Name Server dns yahoo com server 1 8 Requesting Host iterated query 3 4 5 Intermediate Name Server dns usc edu 7 6 Authoritative Name Server dns ee usc edu john yahoo com EE450 USC Zahid Mary ee usc edu 23 Pure Iterative Resolution EE450 USC Zahid 24 DNS Caching z Once any name server learns mapping it caches mapping z Cache entries timeout disappear after some time z TLD servers typically cached in local name servers Thus root name servers not often visited EE450 USC Zahid 25 Why not Centralized DNS z z z z A centralized DNS represent a single point of failure If the name server crashes so would the entire internet All traffic volume would have to be handled by this name …