HTTP

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HTTP
EECS 325/425, Fall 2005 September 12
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Chapter 2 Application layer

• 2.1 Principles of network applications
• app architectures
• app requirements
• 2.2 Web and HTTP
• 2.4 Electronic Mail
• SMTP, POP3, IMAP
• 2.5 DNS

• 2.6 P2P file sharing
• 2.7 Socket programming with TCP
• 2.8 Socket programming with UDP
• 2.9 Building a Web server

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Web and HTTP

• Killer Internet applications, started in
  mid-1990s
• News, games, e-commerce, etc
• Some concepts
• A Web page consists of objects
• Objects can be HTML files, JPEG images, Java
  applets, audio files,
• A Web page consists of base HTML-file which may
  include multiple referenced objects
• Each object is addressable by a uniform resource
  locator (URL). They can be from different hosts.
• Example URL

www.someschool.edu/someDept/pic.gif
path name
host name
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HTTP overview

• HTTP hypertext transfer protocol
• Webs application layer protocol
• client/server model
• client browser that requests, receives,
  displays Web objects
• server Web server sends objects in response to
  requests
• HTTP 1.0 RFC 1945
• HTTP 1.1 RFC 2068

HTTP request
PC running Explorer
HTTP response
HTTP request
Server running Apache Web server
HTTP response
Mac running Navigator
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HTTP overview (continued)

• Uses TCP (reliable)
• client initiates TCP connection (creates socket)
  to server, port 80
• server accepts TCP connection from client
• HTTP messages (application-layer protocol
  messages) exchanged between browser (HTTP client)
  and Web server (HTTP server)
• TCP connection closed

• HTTP is stateless
• server maintains no information about past client
  requests

aside

• Protocols that maintain state are complex!
• past history (state) must be maintained
• if server/client crashes, their views of state
  may be inconsistent, must be reconciled

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HTTP connections

• Non-persistent HTTP
• At most one object is sent over a TCP connection.
• HTTP/1.0 uses non-persistent HTTP

• Persistent HTTP
• Multiple objects can be sent over single TCP
  connection between client and server.
• HTTP/1.1 uses persistent connections in default
  mode
• Q is persistent HTTP also stateless?

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Non-persistent HTTP
(contains short text, references to 10 jpeg
images)

• Suppose user enters URL www.someSchool.edu/someDep
  artment/home.index

• 1a. HTTP client initiates TCP connection to HTTP
  server (process) at www.someSchool.edu on port 80

1b. HTTP server at host www.someSchool.edu
waiting for TCP connection at port 80. accepts
connection, notifying client
2. HTTP client sends HTTP request message
(containing URL) into TCP connection socket.
Message indicates that client wants object
someDepartment/home.index
3. HTTP server receives request message, forms
response message containing requested object, and
sends message into its socket
time
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Nonpersistent HTTP (cont.)
4. HTTP server closes TCP connection.

• 5. HTTP client receives response message
  containing html file, displays html. Parsing
  html file, finds 10 referenced jpeg objects

time
6. Steps 1-5 repeated for each of 10 jpeg objects
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Response time modeling

• Definition of RRT time to send a small packet to
  travel from client to server and back.
• Q Do you remember what contribute to the RTT?
• Response time (assumed no packet losses)
• one RTT to initiate TCP connection
• one RTT for HTTP request and first few bytes of
  HTTP response to return
• file transmission time
• total 2RTTtransmit time

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Persistent HTTP

• Persistent without pipelining
• client issues new request only when previous
  response has been received
• one RTT for each referenced object
• Persistent with pipelining
• default in HTTP/1.1
• client sends requests as soon as it encounters a
  referenced object
• as little as one RTT for all the referenced
  objects

• Non-persistent HTTP issues
• requires gt2 RTTs per object
• OS must work and allocate host resources for each
  TCP connection
• but browsers often open parallel TCP connections
  to fetch referenced objects
• Can dramatically affect server performance!
• Persistent HTTP
• server leaves connection open after sending
  response
• subsequent HTTP messages between same
  client/server are sent over this connection

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HTTP request message

• two types of HTTP messages request, response
• HTTP request message
• ASCII (human-readable format)

request line (GET, POST, HEAD commands)
GET /somedir/page.html HTTP/1.1 Host
www.someschool.edu User-agent
Mozilla/4.0 Connection close Accept-languagefr
(extra carriage return, line feed)
header lines
Carriage return, line feed indicates end of
message
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HTTP request message general format
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Uploading form input

• Post method
• Web page often includes form input
• Input is uploaded to server in entity body

• URL method
• Uses GET method
• Input is uploaded in URL field of request line
  (same for google)

www.somesite.com/animalsearch?monkeysbanana
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Method types

• HTTP/1.0
• GET
• POST
• HEAD
• asks server to leave requested object out of
  response
• Why is it useful?

• HTTP/1.1
• GET, POST, HEAD
• PUT
• uploads file in entity body to path specified in
  URL field
• DELETE
• deletes file specified in the URL field

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HTTP response message
status line (protocol status code status phrase)
HTTP/1.1 200 OK Connection close Date Thu, 06
Aug 1998 120015 GMT Server Apache/1.3.0
(Unix) Last-Modified Mon, 22 Jun 1998 ...
Content-Length 6821 Content-Type text/html
data data data data data ...
header lines
data, e.g., requested HTML file
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HTTP response status codes
In first line in server-gtclient response
message. A few sample codes

• 200 OK
• request succeeded, requested object later in this
  message
• 301 Moved Permanently
• requested object moved, new location specified
  later in this message (Location)
• 304 Not modified
• Can use cached copy
• 400 Bad Request
• request message not understood by server
• 403 Forbidden
• Authorization will not help
• 404 Not Found
• requested document not found on this server
• 505 HTTP Version Not Supported

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Trying out HTTP (client side) for yourself

• 1. Telnet to your favorite Web server

Opens TCP connection to port 80 (default HTTP
server port) at cis.poly.edu. Anything typed in
sent to port 80 at cis.poly.edu
telnet cis.poly.edu 80

• 2. Type in a GET HTTP request

By typing this in (hit carriage return twice),
you send this minimal (but complete) GET request
to HTTP server
GET /ross/ HTTP/1.1 Host cis.poly.edu
3. Look at response message sent by HTTP server!
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sxj63_at_easy telnet cis.poly.edu 80 Trying
128.238.32.126... Connected to cis.poly.edu. Escap
e character is ''. HEAD /ross/ HTTP/1.1 Host
cis.poly.edu HTTP/1.1 200 OK Date Mon, 12 Sep
2005 163440 GMT Server Apache/1.2.5 Last-Modifi
ed Fri, 22 Jul 2005 191130 GMT ETag
"15834-2161-42e144e2" Content-Length
8545 Accept-Ranges bytes Content-Type
text/html Connection to cis.poly.edu closed by
foreign host. sxj63_at_easy telnet cis.poly.edu
80 Trying 128.238.32.126... Connected to
cis.poly.edu. Escape character is ''. HEAD
/banana/ HTTP/1.1 Host cis.poly.edu HTTP/1.1
404 File Not Found Date Mon, 12 Sep 2005
163610 GMT Server Apache/1.2.5 Content-Type
text/html Connection to cis.poly.edu closed by
foreign host. sxj63_at_easy
sxj63_at_easy telnet vorlon.cwru.edu 80 Trying
129.22.150.75... Connected to vorlon.EECS.cwru.edu
. Escape character is ''. GET /index.html
HTTP/1.0 HTTP/1.1 200 OK Date Mon, 12 Sep 2005
164427 GMT Server Apache Last-Modified Wed,
24 Nov 2004 040425 GMT ETag "4f4c67-a5-2f6be440
" Accept-Ranges bytes Content-Length
165 Connection close Content-Type text/html
charsetISO-8859-1 lthtmlgt ltheadgt ltmeta
http-equiv"refresh" content"5urlhttp//www.eec
s.cwru.edu/"gt lt/headgt ltbodygt lta
hrefwww.eecs.cwru.edugtwww.eecs.cwru.edult/agt lt/bod
ygt lt/htmlgt Connection to vorlon.EECS.cwru.edu
closed by foreign host. sxj63_at_easy
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User-server state cookies

• Many major Web sites use cookies (users may like
  them or hate them)
• Four components
• 1) cookie header line in the HTTP response
  message
• 2) cookie header line in HTTP request message
• 3) cookie file kept on users host and managed by
  users browser
• 4) back-end database at Web site

• Example
• Susan access Internet always from same PC
• She visits a specific e-commerce site for first
  time
• When initial HTTP requests arrives at site, site
  creates a unique ID and creates an entry in
  backend database for ID

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Cookies keeping state (cont.)
server creates ID 1678 for user
entry in backend database
access
access
one week later
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Cookies (continued)
aside

• Cookies and privacy
• cookies permit sites to learn a lot about you
• you may supply name and e-mail to sites
• search engines use redirection cookies to
  learn yet more
• advertising companies obtain info across sites

• What cookies can bring
• authorization
• shopping carts
• recommendations
• user session state (Web e-mail)

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Conditional GET
server
cache

• Goal dont send object if cache has up-to-date
  cached version
• cache specify date of cached copy in HTTP
  request
• If-modified-since ltdategt
• server response contains no object if cached
  copy is up-to-date
• HTTP/1.0 304 Not Modified
• Q does IMS make HTTP stateful?

HTTP request msg If-modified-since ltdategt
object not modified
HTTP request msg If-modified-since ltdategt
object modified
HTTP response HTTP/1.0 200 OK ltdatagt
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Readings etc

• Section 2.1-2.5
• Is it difficult to implement a Web crawler to
  find out more about the Web, e.g.,
• the links between Web pages
• the versions of Web servers
• the versions of HTTP