� � � � ✁ ✁ ✁ � � ✁ � � � � Application Layer in the Internet ���� ���������������� ���� ���������������� 15 February, 2001 1 DNS: Domain Name System Domain Name System: Domain Name System: People: many identifiers: many identifiers: People: distributed database distributed database implemented implemented SSN, name, Passport # SSN, name, Passport # in hierarchy of many name servers in hierarchy of many name servers Internet hosts, routers: Internet hosts, routers: application-layer protocol application-layer protocol host, host, IP address (32 bit) - used IP address (32 bit) - used routers, name servers to routers, name servers to for addressing for addressing datagrams datagrams communicate to communicate to resolve resolve names names name � , e.g., � name � , e.g., � (address/name translation) (address/name translation) gaia. gaia .cs cs. .umass umass. .edu edu - - note: core Internet function note: core Internet function used by humans used by humans implemented as application- implemented as application- Q: map between IP addresses Q: map between IP addresses layer protocol layer protocol and name ? and name ? complexity at network complexity at network � � s s � � edge edge � � 15 February, 2001 The Computer Communications Course 2 1
� � � � ✁ ✁ � � ✁ ✁ ✁ ✁ ✁ � � ✁ ✁ ✁ ✁ ✁ ✁ � � � DNS name servers no server has all name-to-IP no server has all name-to-IP Why not centralize DNS? Why not centralize DNS? address mappings address mappings single point of failure single point of failure local name servers: local name servers: traffic volume traffic volume each ISP, company has local each ISP, company has local (default) name server (default) name server distant centralized database distant centralized database host DNS query first goes to host DNS query first goes to maintenance maintenance local name server local name server authoritative name server: authoritative name server: doesn � doesn � t t scale! scale! for a host: stores that host � for a host: stores that host � s s IP address, name IP address, name can perform name/address can perform name/address translation for that host � s translation for that host � s name name 15 February, 2001 The Computer Communications Course 3 DNS: Root Name Servers ����������������������� ������������������� ������������ ����������������� �������� ������������������ �������������� ����������������� ������������ ������������������ ����������������� ����������������� ����������������� 15 February, 2001 The Computer Communications Course 4 2
Domain Name Servers root root root root ... nsf.gov purdue.edu huji.ac.il co.il cs.purdue.edu math. cs. huji. huji. ac. ac. il il 15 February, 2001 The Computer Communications Course 5 Simple DNS example ���������������� Host Host harmony. harmony.cs cs. .huji huji.ac. .ac.il il 2 4 wants IP address of wants IP address of 3 gaia gaia. .cs cs. .umass umass. .edu edu 5 1. 1. Contacts its local DNS server, Contacts its local DNS server, shuldig shuldig. .cs cs. .huji huji.ac. .ac.il il 2. 2. shuldig shuldig. .cs cs. .huji huji.ac. .ac.il il contacts root name server, if contacts root name server, if ����������������� �✂✁☎✄☎✆✞✝✠✟☛✡ ✄☎✡ ✄☎✡ ☞✍✌✏✎✑�✠✒✓✌✕✔✍✌✖✟✍☞✍✌✖✟ shuldig.cs.huji.ac.il dns.umass.edu necessary necessary 3. 3. root name server contacts root name server contacts 1 6 authoritative name server, authoritative name server, , if necessary if necessary dns dns. .dmass dmass. .edu edu, ��������������� gaia.cs.umass.edu harmony.cs.huji.ac.il 15 February, 2001 The Computer Communications Course 6 3
✂ ✁ ✒ ✌ ✂ ✌ ✡ ✎ ✡ ✂ ✔ ✔ ✡ ✁ ✡ ✎ ✂ ✂ ✂ ✒ ✂ ✂ ✁ ✁ DNS example ���������������� ����������������� 6 2 ������������ 3 7 ������������������ ������ �������� ����������������� ����������������� ✄✑✌✖✟☛✒✓✌ �✂✄✑✌ ✎✍�✠✒✓✌ ✌✖✟✑☞ ✌✖✟ ������� ������� shuldig.cs.huji.ac.il dns.umass.edu 5 4 ��������������� 1 8 ������������������ ������ �✂✁☎✄☎✆✞✝✠✟☛✡ ✄✑�✂✄☎✡ ☞✍✌ ✎✍� ✔✍✌ ✟✑☞ ✌✖✟ dns.cs.umass.edu ��������������� harmony.cs.huji.ac.il gaia.cs.umass.edu 15 February, 2001 The Computer Communications Course 7 DNS: iterated queries ���������������� recursive query: recursive query: �������������� 2 puts burden of name puts burden of name 3 resolution on contacted resolution on contacted name server name server 4 heavy load? heavy load? 7 iterated query: iterated query: ����������������� ✄✑✌✖✟☛✒✓✌ �✂✄✑✌ ✎✍�✠✒✓✌ ✌✖✟✑☞ ✌✖✟ contacted server replies contacted server replies shuldig.cs.huji.ac.il dns.umass.edu with name of server to with name of server to 5 6 1 8 contact contact � I don I don � � t know this name, t know this name, � but ask this server � but ask this server �✂✁☎✄☎✆✞✝✠✟☛✡ ✄✑�✂✄☎✡ ☞✍✌ ✎✍� ✔✍✌ ✟✑☞ ✌✖✟ � dns.cs.umass.edu ��������������� harmony.cs.huji.ac.il gaia.cs.umass.edu 15 February, 2001 The Computer Communications Course 8 4
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