HTTP and Sockets Lecture 6

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HTTP and SocketsLecture 6

• cs193i Internet Technologies
• Summer 2004
• Stanford University

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Administrative Stuff

• Lab 2 due today
• HW 2 due July 21
• Midterm Monday, July 19, in class

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Midterm

• In class, Monday, July 19
• Previous quarters exam online
• Closed-book
• 1 8x11 cheat sheet
• Short-answer

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Todays Discussion

• HTTP Wrap-up
• HTTP 1.1 additions
• HTTP Proxies
• Sockets in Java

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Persistent Connections

• HTTP 1.0 -- Connections close by Default
• No need for Content-length, end signaled by EOF
  (in-band signal)
• HTTP 1.1 -- Persistent by Default
• Must use Content-length

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Chunked-Transfer Encoding

• Problem Content-length costly for server
• May not know if generated dynamically
• Solution
• Server omits Content-Length
• Transfer-encoding chunked
• Send Data in Chunks, Prefixed by length in Hex
• End is marked with Chunk Length 0

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HTTP 1.1 Virtual Hosts

• Problem
• Want multiple web sites hosted on single server
• Incoming requests must be directed to return
  appropriate web sites page
• Solution
• Request includes host name header
• http//www.foo.com
• hostwww.foo.com

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Try This At Home

• telnet cva.stanford.edu 80
• GET / HTTP/1.1
• Hostcva.stanford.edu
• ConnectionKeep-Alive
• GET /group_mtg.html HTTP/1.1
• Hostcva.stanford.edu
• ConnectionClose

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HTTP Proxy Servers

• "Middlemen" between clients and servers
• May "rewrite" HTTP requests and responses

Proxy Server
Looks like a server sending responses
Looks like a client making requests
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Why Proxy Servers?

• Content filtering
• Packet filter
• Prevent children from accessing sites
• Used to contact server in special way
• Firewall or circuit relay
• Security
• Rewrite content
• Swedish chef
• Redirect requests
• Content router
• To improve performance
• Logging activity

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Why Proxy Servers?
Web cache is also a type of proxy!
RT latency reduced
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Why Proxy Servers?
Content Router
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Why Proxy Servers?
Content Router
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Why Content Routers?
Content Router
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Why Proxy Servers?
Content Router
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Why Proxy Servers?
Anonymizer
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Why Proxy Servers?
Translation
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Why Proxy Servers?
Translation
http//translate.google.com/translate?hlensles
uhttp//espanol.yahoo.com/prev/search3Fq3Dspa
nish26hl3Den26lr3D26ie3DUTF-8
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Requests via Proxy

• Client must realize talking to proxy
• Request header
• GET path HTTP/1.0
• Client must use absolute path!
• GET / HTTP/1.0
• GET http//www.yahoo.com/ HTTP/1.0

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Five Minute Break
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Sockets in Java (Client)

• java.net.socket
• Create Socket
• Socket c new Socket(hostname, port)
• OR
• Socket c new Socket(IP addr, port)

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Reading/Writing to Sockets

• Just like performing file I/O in Java
• Create input/output streams to read/write to/from
  socket
• Output example PrintWriter class
• new PrintWriter(c.getOutputStream(), true)
• Input example BufferedReader class
• new BufferedReader(new InputSreamReader(c.getInput
  Stream())

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Sockets in Java (Server)

• Create ServerSocket
• Socket listen new ServerSocket(port)
• Create Socket
• ServerSocket.accept()
• Blocks until connection established
• Socket s new listenSocket.accept()
• Create input/output streams
• Close when finished

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Exceptions

• Sockets throw exceptions when something goes
  wrong
• UnknownHostExcpetion
• Handle using try/catch pairs
• try
• (statement)
• catch(ExceptionType e)
• (statement)
• finally
• (statement)
• Can have multiple catch clauses for handling
  different exceptions

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URL Class

• Automatically decomposes string into parts
• getHost()
• getPort()
• getProtocol()
• getFile() // path

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What do you do with this stuff?

• StringTokenizer class
• Allows you to grab tokenized strings one at a
  time
• You can specify what delineates strings
• Example could be delineators
• Pattern and Matcher classes
• Create pattern
• Use Matcher class to compare strings to pattern