INF 123 SW ARCH, DIST SYS & INTEROP

LECTURE 5

Prof. Crista Lopes

Objectives

Web history competency Thorough understanding of HTTP

Recap

Distributed System

“Collection of interacting components hosted on different computers that are connected through a computer network”

Component1

Component n

Hardware

Network OS

Host 3

Component1

Component n

Hardware

Network OS

Host 2

Component1

Component n

Hardware

Network OS

Host 1

…

Network

The Origins of the Internet

Heterogeneous computers Decentralized control Many interested players

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OSI Model in Action

Internet

Your laptop

DBH wirelessrouter

UCI routers Google routers

Google server

The Internet

Large-scale infrastructure consisting of 100’s of 1,000’s of routers, cables, wireless links, and millions of hosts.

Traffic through the network consists of small data packets.

Software in each node follows, roughly, the OSI model. Main “contract” between nodes: Internet Protocol (IP)

IP addresses (v4 and now v6) Packets don’t contain routing information Route packets according to their final destination but

depending on local context of router Each packet is routed independently of others

Lecture 5

Context, 1985-1990

Full decade of Internet usage Foundation: TCP/IP [and UDP]

Enabled Client-Server architectures Application: Telnet

Virtual terminal (login to remote machine) Can be used to ‘talk’ to *any* TCP/IP server

Application: Email SMTP: See example next page POP IMAP

Application: News NNTP (before it, Usenet and UUCP)

Application: Instant Messaging Unix’s Talk program Popularized by AOL

Application: File sharing FTP

Client-Server over TCP/IP

Server opens TCP [server] socket, binds to port, listens for connection requests

Client opens TCP [client] socket, connect to server host/port

Server accepts connection, initiates dedicated full-duplex “virtual circuit” Eventually spawns thread for it Main thread goes back to listen for other connections

Client and server send each other messages (byte streams) TCP implementation takes care of protocol details

Example: SMTP over TCP/IP

tagus: crista$ telnet smtp.ics.uci.edu 25Trying 128.195.1.219...Connected to smtp.ics.uci.edu.Escape character is '^]'.220 david-tennant-v0.ics.uci.edu ESMTP mailer ready at Mon, 5 Apr 2010 17:15:01 -0700'HELO smtp.ics.uci.edu250 david-tennant-v0.ics.uci.edu Hello barbara-wright.ics.uci.edu [128.195.1.137], pleased to meet youMAIL FROM:<lopes@ics.uci.edu>250 2.1.0 <lopes@ics.uci.edu>... Sender okRCPT TO:<lopes@ics.uci.edu>250 2.1.5 <lopes@ics.uci.edu>... Recipient okDATA354 Enter mail, end with "." on a line by itselftest.250 2.0.0 o360F1Mo029280 Message accepted for deliveryQUIT221 2.0.0 david-tennant-v0.ics.uci.edu closing connectionConnection closed by foreign host.

Origins of the Web

CERN Conseil Européen pour la Recherche Nucléaire (European Laboratory for Particle Physics; Geneva, Switzerland) Tim Berners-Lee & Robert Cailliou

Originally a system for sharing documents among scientists

First implementation made publicly available quickly became very popular in universities & research institutions

NCSA Mosaic browser made it popular across the board

Main Design Principles, originally Client requests a text document from the server

Server sends back the text document Text document may contain retrieval references (hyperlinks)

to other text documents on that or other servers HyperText Markup Language (HTML)

Client may also send text documents for the server to store Requests/Responses sent over TCP, but

Client makes connection, sends, receives, connection is closed Connection is not maintained among interactions

Requests are self-contained, do not rely on past interactions “Stateless”

(Notice the story based on “text document”; it quickly became apparent that it needed generalization)

Generalization

Document Resource “Page” with markups Actual document, many types Program generating resource

Universal Resource Identifier (URI) Abstract concept Concrete realization: Universal Resource

Locator (URL) Provides a method for finding the resource

http://, file://, ftp://, mailto://, etc.

HTTP URLs

Syntax: http://<host>:<port>[/<path>][?<query>]

Examples Hosts: www.ics.uci.edu, 127.0.0.1 Ports: Number Paths: /wifi/admin/users Queries: first=John&last=Smith

Spec

HyperText Transfer Protocol (HTTP) GET PUT DELETE HEAD OPTIONS TRACE POST CONNECT

Spec

Idempotent methods

HTTP Request Syntax

<OPERATION> <ARGS> <VERSION>[<HEADER_1_NAME>: <HEADER_1_VALUE> …<HEADER_N_NAME >: <HEADER_N_VALUE>]<blank line>[<DATA>]

HTTP Response Syntax

<VERSION> <CODE> <EXPLANATION>[<HEADER_1_NAME>: <HEADER_1_VALUE> …<HEADER_N_NAME >: <HEADER_1_VALUE>]<blank line>[<DATA>]

HTTP Example

GET /index.html HTTP/1.1Host: ics.uci.edu

Blank line here

HTTP/1.1 200 OKDate: Fri, 09 Apr 2010 19:48:36 GMTServer: Apache/2.2.3 (CentOS)Last-Modified: Fri, 19 Feb 2010 22:01:21 GMTETag: "238003-64-47ffb39422e40"Accept-Ranges: bytesContent-Length: 100Connection: closeContent-Type: text/html; charset=UTF-8

<html><head><meta HTTP-EQUIV="REFRESH" content="0; URL=http://www.ics.uci.edu/"></head></html>(show live)

HTTP Headers

Request headers Response headers

Spec

HTTP Status Codes

Informational 1xxx E.g. 100 Continue

Successful 2xx E.g. 200 OK, 201 Created

Redirection 3xx E.g. 300 Multiple Choices, 301 Moved Permanently

Client error 4xx E.g. 400 Bad Request, 404 Not Found

Server error 5xx E.g. 500 Internal Server Error, 503 Service

Unavailable Complete list

Another Example

GET /index.html HTTP/1.1Host: cnn.com

Blank line here

HTTP/1.1 301 Moved PermanentlyDate: Fri, 09 Apr 2010 20:32:14 GMTServer: ApacheLocation: http://www.cnn.com/index.htmlVary: Accept-EncodingContent-Length: 294Content-Type: text/html; charset=iso-8859-1

<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN"><html><head><title>301 Moved Permanently</title></head><body><h1>Moved Permanently</h1><p>The document has moved <a href="http://www.cnn.com/index.html">here</a>.</p><hr><address>Apache Server at cnn.com Port 80</address></body></html>

(show live)

Web Caches

Internet

Proxy

ClientClient …

Internet

ReverseProxy

Server

Server

…

Caches contentfrom Internet

Caches contentfrom servers

Web Caches

Reduce bandwidth Reduce server load Reduce lag

Cache content from Idempotent methods (GET mostly)

Web Caches: Why you need to know about them

github.com demo

Web Cache Control

“Cache-Control” header in responses E.g. Cache-Control: no-cache

“Expires” header in responses E.g. Expires: Fri, 09 Apr 2010 16:00:00 GMT

“Last-Modified” header in responses Proxy can use If-Modified-Since header in

request, server may respond 304 Not Modified

If subsequent POST, PUT, DELETE to same URL, cache should be invalidated

Cookies

Text data sent from the server to the client meant to be sent back in subsequent requests from the client to the same server

Added to Mosaic browser and Web servers in 1994

Uses Session management Personalization Tracking

Setting and Using Cookies

GET /index.html HTTP/1.1Host: www.google.com

HTTP/1.1 200 OKDate: Sat, 10 Apr 2010 14:35:22 GMTExpires: -1Cache-Control: private, max-age=0Content-Type: text/html; charset=ISO-8859-1Set-Cookie: PREF=ID=1bb89b81c47c05fb:TM=1270910122:LM=1270910122:S=YQ3wzhShOas9UStn; expires=Mon, 09-Apr-2012 14:35:22 GMT; path=/; domain=.google.comSet-Cookie: NID=33=CeVJK2EKVB5kcCiguCD1OjG3g5UKlPq78SXCibOjYQOU46P6SMaAKqAhw2hEVPqqnKfFlTzmC-w4Ol5ZwKQqnjyla1DZcS6ZYmb1lLHe2zNuEVnXJRtd4lMrr6gA4o8m; expires=Sun, 10-Oct-2010 14:35:22 GMT; path=/; domain=.google.com; HttpOnlyServer: gwsTransfer-Encoding: chunked

…

Client Server

Server Client

Setting and Using Cookies

GET /index.html HTTP/1.1Host: www.google.comCookie: PREF=ID=1bb89b81c47c05fb:TM=1270910122:LM=1270910122:S=YQ3wzhShOas9UStn

Client Server

Etc.

Uses

Session Management User logs in, server sends cookie Subsequent requests include that cookie

Personalization User visits, server sends cookie User changes preferences, all with cookie Future visits include cookie, server “remembers”

preferences Tracking within same site

Cookie + path + date/time Tracking inter-site

Referer + Cookie (Privacy concerns)