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Lecture 4 (Jan 18, 2018)

Persistent vs. Non-Persistent HTTP

Non-Persistent HTTP Issues

  • Requires 2 RTTs (round-trip delay times) per object
  • OS overhead for each TCP connection
  • Browsers often open parallel TCP connections to fetch referenced objects

Persistent HTTP

  • Server leaves connection open after sending response
  • Subsequent HTTP messages between same client/server sent over open connection
  • Client sends requests as soon as it encounters a referenced object
  • As little as one RTT for all the referenced objects

HTTP Request Message

  • Two types of HTTP messages:
    1. Request
      • ASCII (human-readable format) 
        GET /index.html HTTP/1.1\r\n
Host: www-net.cs.umass.edu\r\n
User-Agent: Firefox/3.6.10\r\n
Accept: text/html,application/xhtml+xml\r\n
Accept-Language: en-us,en;q=0.5\r\n
Accept-Encoding: gzip,deflate\r\n
Accept-Charset: ISO-8859-1,utf-8;q=0.7\r\n
Keep-Alive: 115\r\n
Connection: keep-alive\r\n
\r\n
    2. Response

Uploading Form Input

  • POST method
    • Web page often includes form input
    • Output is uploaded to server in entity body
  • URL method
    • Uses GET method
    • Input is uploaded in URL field of request line

Method Types

  • HTTP/1.0
    • GET
    • POST
    • HEAD
      • Asks server to leave requested object out of response
  • HTTP/1.1
    • GET, POST, HEAD
    • PUT
      • Uploads file in entity body to path specified in URL field
    • DELETE
      • Deletes file specified in URL field

HTTP Response Status Codes

  • Status code appears in 1st line in server-to-client response method
  • Examples:
    • 200 OK
      • Request succeeded, requested object later in this msg
    • 301 Moved Permanently
      • Requested object moved, new location specified later in this msg (Location:)
    • 400 Bad Request
      • Request msg not understood by server
    • 404 Not Found
      • Requested document not found on this server
    • 505 HTTP
      • Version not supported

Trying HTTP (Client Side) for Yourself

  1. Telnet to a Web server:
    • telnet gaia.cs.umass.edu 80.
      • Opens TCP connection to port 80 (default HTTP server port) at gaia.cs.umass.edu. Anything typed in will be sent to port 80 at gaia.cs.umass.edu
  2. Type GET HTTP request 
    GET /kurose_ross/interactive/index.php HTTP/1.1 
Host: gaia.cs.umass.edu
  3. Look at response message sent by HTTP server

Stateless HTTP

  • In general, HTTP is stateless
    • Server maintains no information about past client requests
  • Uses TCP
    • 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

User-Server State: Cookies

  • Many Web sites use cookies
  • Four components:
    1. Cookie header line of HTTP response message
    2. Cookie header line in next HTTP request message
    3. Cookie file kept on user's host, managed by user's browser
    4. Back-end database at Web site
  • Used for:
    • Authorization
    • Shopping carts
    • Recommendations
    • User session state (Web e-mail)
  • How to keep "state":
    • Protocol endpoints: maintain state at sender/receiver over multiple transactions
    • Cookies: HTTP messages keep state
  • Privacy
    • Cookies permit sites to learn a lot about you
    • You may supply name and e-mail to sites
    • Data can be used for advertising
  • Example
    • Naim always accesses Internet from PC
    • Visits specific e-commerce site for first time
    • When initial HTTP requests arrive at site, site creates
      • Unique ID
      • Entry in backend database for ID

Web Caches (Proxy Server)

Goal: Satisfy client request without involving origin server

  • User sets browser: Web accesses via cahce
  • Browser sends all HTTP requests to cache
    • Objects in cache: cache returns object
    • Else cache requests object from origin server, then returns object to client
  • Cache acts as both client and server
    • Server for original requesting client
    • Client to origin server
  • Typically cache is installed by ISP (ex. University, company, residential ISP)
  • Why Web caching?
    • reduce response time for client request
    • Reduce traffic on an institution's access link
    • Internet dense with caches: enables "poor" content providers to effectively delever content (so too does P2P file sharing)

Conditional GET

  • Goal: Don't send object if cache has up-to-date cached version
    • No object transmition delay
    • Lower link utilization
  • Cache: specify date of cached copy in HTTP request 
    If-modified-since: 
  <date>
  • Server: response contains no object if cached copy is up-to-date 
    HTTP/1.0 304 Not Modified

Ongoing Effort on HTTP 2.0

  • The next planned version after HTTP1.1 (RFC 2616 in 1999)
    • 1.4% of Web sites support it (Sept. 2015); 11.4% by Jan 2017
  • Designed to improve throughput of client-server connections
  • Based on Google's SPDY
  • Backward compatible with transaction semantics of HTTP 1.1
  • New features:
    • Multiplexing multiple streams over one HTTP 2.0 connection; request-response pipelining
    • Header compression
    • Server push (preemptive transfer to clients)

Web 2.0

  • No update on the technical specs, more of a marketing idea
  • Rather to cumulative changes in the way software developers and users use the Web
  • "Network as a platform" computing
    • Build applications on Web, not desktops
  • Use rich, user-friendly interfaced based on Ajax and similar clienr-side interactivity tools

Web 3.0

  • Extends the hyperlinked human-readable web pages by inserting machine-readable metadata about pages and how they are related to each other
  • Using extensible markup language (XML), resource description framework (RDF), web ontology language (OWL)
    • XML: syntax for conent structures within documents
    • RDF: expressing data models
    • OWL: adds more vocabularies on describing properties and classes

Electronic Mail

  • Three major components:
    1. User agents
    2. Mail servers
    • Simple mail transfer protocol: SMTP
  • User Agent
    • AKA "mail reader" (ex. Outlook, Thunderbird)
    • Composing, editing, reading mail messages
    • Outgoing, incoming messages stored on server

Mail Servers

  • Mailbox contains incoming messages from user
  • Message queue of outgoing (to be sent) mail messages
  • SMTP protocl between mail servers to send email messages
    • Client: sending mail server
    • "Server": receiving mail server

SMTP [RFC 2821]

  • Uses TCP to reliably transfer email message from client to server, port 25
  • Direct transfer: sending server to receiving server
  • Three phases of transfer
    1. Handshaking (greeting)
    2. Transfer of messages
    3. Closure
  • Uses persistent connections
  • Requires message (header & body) to be in 7-bit ASCII
  • SMTP server uses CRLF.CRLF to determine end of message

Comparison with HTTP:

  • HTTP: pull
  • SMTP: push
  • Both have ASCII command/response interaction, status codes
  • HTTP: each object encapsulated in its own response message
  • SMTP: multiple objects sent in multipart message

Example Scenario: Alice Sends Message to Bob

  1. Alice uses UA (user agent) to compose message "to" bob@someschool.edu
  2. Alice's UA sends message to her mail server; message placed in message queue
  3. Client side of SMTp opens TCP connection with Bob's mail server
  4. SMTP client sends Alice's message over the TCP connection
  5. Bob's mail erver places the message in Bob's mailbox
  6. Bob invokes his UA to read message

Try SMPT Interaction for Yourself

  • telnet servername 25
  • See 220 reply from server
  • Enter HELO, MAIL FROM, RCPT TO, DATA, QUIT commands
  • Sends email without email client

Mail Access Protocols

  • SMTP: Delivery/storage to receiver's server
  • Mail access protocol: retreival from server
    • POP: Post Office Protocol [RFC 1939]: authorization, download
    • IMAP: Internet Mail Access Protocol [RCF 1730]: more features, including manipulation of stored messages
    • HTTP: Gmail, Yahoo, Hotmail

POP3 Protocol

  • Authorization phase
    • Client commands
      • user: declare username
      • pass: password
    • Server responses
      • +OK
      • -ERR
    • Transaction phase, client
      • list: list message numbers
      • retr: retrieve message by number
      • dele: delete
      • quit

IMAP

  • Keeps all messages in one place: at server
  • Allows user to organize message in folders
  • Keeps user state across sessions
    • Names of folders and mappings between message IDs and folder name

DNS (Domain Name System)

  • People: many identifiers
    • SSN, name, passport number
  • Internet hosts, routers
    • IP address (32 bit), used for addressing diagrams
    • "name" (ex. naimayat.me), used by humans
  • How do we map between IP address and name, and vice-versa?
    • Distrubuted database: implemented in a hierarchy of many name servers
    • Application-layer protocol: hots, name servers communicate to resolve names (address/name translation)
      • Note: core Internet function, implemented as application-layer protocol
      • Complexity at network "edge"

DNS Services

  • Hostname to IP address translation
  • Host aliasing
    • Canonical, alias names
  • Mail server aliasing
  • Load distribution
    • Replaced Web servers: many IP addresses correspond to one name
  • Cons:
    • Single point of failure
    • Traffic volume
    • Distant centralized database
    • Maintenance
    • Doesn't scale