1 Higher level protocols Domain Naming System, DNS HTTP

2 Overview What do names do? –identify objects –help locate objects –define membership in a group –specify a role –convey knowledge of a secret Name space –defines set of possible names –consists of a set of name to value bindings

3 Properties Names versus addresses Location transparent versus location-dependent Flat versus hierarchical Global versus local Absolute versus relative By architecture versus by convention Unique versus ambiguous

4 Examples Hosts cheltenham.cs.princeton.edu :23:A8:33:5B:9F Files /usr/llp/tmp/foo (server, fileid) Users Larry Peterson

5 Examples (cont) Mailboxes Name server Mail program User TCP IP 2 cs.princeton.edu cs.princeton.edu

6 Domain Naming System Hierarchy Name chinstrap.cs.princeton.edu

7 Name Servers Partition hierarchy into zones educom princeton … mit csee ux01ux04 physics cisco … yahoonasa … nsfarpa … navyacm … ieee govmilorgnetukfr Root name server Princeton name server Cisco name server CS name server EE name server … … Each zone implemented by two or more name servers

8 Resource Records Each name server maintains a collection of resource records (Name, Value, Type, Class, TTL) Name/Value: not necessarily host names to IP addresses Type –NS: Value gives domain name for host running name server that knows how to resolve names within specified domain. –CNAME: Value gives canonical name for particle host; used to define aliases. –MX: Value gives domain name for host running mail server that accepts messages for specified domain. Class: allow other entities to define types TTL: how long the resource record is valid

9 Root Server (princeton.edu, cit.princeton.edu, NS, IN) (cit.princeton.edu, , A, IN) (cisco.com, thumper.cisco.com, NS, IN) (thumper.ciscoe.com, , A, IN) …

10 Princeton Server (cs.princeton.edu, optima.cs.princeton.edu, NS, IN) (optima.cs.princeton.edu, , A, IN) (ee.princeton.edu, helios.ee.princeton.edu, NS, IN) (helios.ee.princeton.edu, , A, IN) (jupiter.physics.princeton.edu, , A, IN) (saturn.physics.princeton.edu, , A, IN) (mars.physics.princeton.edu, , A, IN) (venus.physics.princeton.edu, , A, IN)

11 CS Server (cs.princeton.edu, optima.cs.princeton.edu, MX, IN) (cheltenham.cs.princeton.edu, , A, IN) (che.cs.princeton.edu, cheltenham.cs.princeton.edu, CNAME, IN) (optima.cs.princeton.edu, , A, IN) (opt.cs.princeton.edu, optima.cs.princeton.edu, CNAME, IN) (baskerville.cs.princeton.edu, , A, IN) (bas.cs.princeton.edu, baskerville.cs.princeton.edu, CNAME, IN)

12 Name Resolution Strategies –forward –iterative –recursive Local server –need to know root at only one place (not each host) –site-wide cache Root name server Princeton name server CS name server Local name server Client 1 cicada.cs.princeton.edu cicada.cs.princeton.edu princeton.edu, cicada.cs.princeton.edu cicada.cs.princeton.edu, cicada.cs.princeton.edu cs.princeton.edu,

13 The user clicks on a link to indicate which document is to be retrieved. The browser must determine the address that contains the document. It does this by sending a query to its local name server. Once the address is known the browser establishes a connection to the specified machine, usually a TCP connection. In order for the connection to be successful, the specified machine must be ready to accept TCP connections. The browser runs a client version of HTTP, which issues a request specifying both the name of the document and the possible document formats it can handle. The machine that contains the requested document runs a server version of HTTP. It reacts to the HTTP request by sending an HTTP response which contains the desired document in the appropriate format. The TCP connection is then closed and the user may view the document Web Transfers Revisited

14 HTTP server HTTP client Request Response

15 HTTP server HTTP client TCP GET 80, # #, 80 STATUS Port 80 Ephemeral Port # HTTP Client-Server Interaction

16 HTTP Request Operations OPTIONS: Request information about available options GET: Retrieve document in URL HEAD: Retrieve metainformation about document in URL POST: give information to server PUT: store document under specified URL DELETE: delete specified URL TRACE: loopback request message

17 HTTP Result codes Code TypeExample Reasons 1xxInformational request received 2xxSuccessaction successfully received, accepted 3xxRedirection Page moved, further action necessary 4xxClient error bad syntax 5xxServer errorserver failed for valid req.

18 Application Layer Presentation Layer Session Layer Transport Layer Network Layer Data Link Layer Physical Layer Application Layer Presentation Layer Session Layer Transport Layer Network Layer Data Link Layer Physical Layer Network Layer Electrical and/or Optical Signals Application A Application B Data Link Layer Physical Layer Network Layer Data Link Layer Physical Layer Communication Network Layered Architecture Revisited

19 User Interface User PI User DTP PI = Protocol interpreter DTP = Data transfer process User FTP Server PI Server DTP Server FTP Control Connection Data Connection FTP Protocol

20 C:\WINDOWS>ping nal.toronto.edu Pinging nal.toronto.edu [ ] with 32 bytes of data: Reply from : bytes=32 time=118ms TTL=243 C:\WINDOWS> PING