eec-484/584 computer networks lecture 4 wenbing zhao (part of the slides are based on drs. kurose...

EEC-484/584EEC-484/584Computer Computer NetworksNetworks

Lecture 4Lecture 4

Wenbing ZhaoWenbing [email protected] (Part of the slides are based on Drs. Kurose & Ross(Part of the slides are based on Drs. Kurose & Ross’’s s slides for their slides for their Computer Networking Computer Networking bookbook))

mailto:[email protected]

05/04/2305/04/23 EEC-484/584: Computer NetworksEEC-484/584: Computer Networks Wenbing ZhaoWenbing Zhao

Administrative: Lab1 (instruction posted online): Wednesday Lab report requirement:

Typed hardcopy, must include questions/tasks, your answers, and snapshots to backup your answers

Today’s topics Web and HTTP


The World Wide WebThe World Wide Web Creation of Tim Berners-Lee, in 1989 CERN nuclear

physics research Mosaic – first graphical interface, creation of Marc Andersson

(and others), precursor to Netscape Uses a client-server architecture

Web server Web browser

Runs on HTTP over TCP


Web and HTTPWeb and HTTP Web page consists of objects Object can be HTML file, JPEG image, Java applet,

audio file,… A Web page consists of a base HTML-file which

includes several referenced objects Each object is addressable by a URL The idea of having one page point to another is called

hypertext Invented by Vannevar Bush, a MIT EE professor, in 1945


URL – Uniform Resource URL – Uniform Resource LocaterLocater Example URL:

URL encodes three types of information What is the page called – local path name uniquely

indicating the specific page Where is the page located – Host name of the server on

which the page is located How can the page be accessed – protocol, e.g., http, ftp

http://www.someschool.edu/someDept/pic.gif

host name path nameprotocol name


HTTP OverviewHTTP OverviewHTTP: HyperText

Transfer Protocol Web’s application layer

protocol client/server model HTTP 1.0: RFC 1945 HTTP 1.1: RFC 2068

PC runningExplorer

Server running

Apache Webserver

Mac runningNavigator

HTTP request

HTTP request

HTTP response

HTTP response


HTTP OverviewHTTP Overview 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 OverviewHTTP Overview HTTP is “stateless”

Server maintains no information about past client requests

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


HTTP ConnectionsHTTP ConnectionsNonpersistent HTTP At most one object is

sent over a TCP connection

HTTP/1.0 uses nonpersistent 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


Nonpersistent HTTPNonpersistent HTTPSuppose user enters URL http://www.someSchool.edu/someDept/home.index

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

2. HTTP client sends HTTP request message (containing URL) into TCP connection socket. Message indicates that client wants object someDept/home.index

1b. HTTP server at host www.someSchool.edu waiting for TCP connection at port 80. “accepts” connection, notifying client

3. HTTP server receives request message, forms response message containing requested object, and sends message into its sockettime

(contains text, references to 10 jpeg images)


Nonpersistent HTTPNonpersistent HTTP

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

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

4. HTTP server closes TCP connection.

time


Non-Persistent HTTP: Non-Persistent HTTP: Response TimeResponse Time

Definition of RTT: time to send a small packet to travel from client to server and back (Round Trip Time)

time to transmit file

initiate TCPconnection

RTT

requestfile

RTT

filereceived

time time


Non-Persistent HTTP: Response Non-Persistent HTTP: Response TimeTimeResponse time: one RTT to initiate TCP connection one RTT for HTTP request and first few bytes

of HTTP response to return file transmission timeTotal = 2RTT+transmission time


Non-Persistent HTTP Issues Requires 2 RTTs per object OS overhead for each TCP connection

To reduce response time, browsers often open parallel TCP connections to

fetch referenced objects


Persistent HTTPPersistent HTTP Server leaves connection open after sending

response Subsequent HTTP messages between same

client/server sent over open connection


Persistent HTTPPersistent HTTPPersistent 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 Multiple requests are sent over the same connection

concurrently. That is, after the first request, the second request is sent before the reply for the first request is received

As little as one RTT for all the referenced objects


HTTP Request MessageHTTP Request Message Two types of HTTP messages: request, response HTTP request message:

ASCII (human-readable format)

GET /somedir/page.html HTTP/1.1Host: www.someschool.edu User-agent: Mozilla/4.0Connection: close Accept-language:fr

(extra carriage return, line feed)

request line(GET, POST,

HEAD commands)

header lines

Carriage return, line feed

indicates end of message


HTTP Request Message: HTTP Request Message: General FormatGeneral Format

HTTP header is pure ASCII based. It is very different from lower layer protocols such as TCP, which is binary based


Method TypesMethod TypesHTTP/1.0 GET POST HEAD

Asks server to include only the header part in response

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


HTTP Response MessageHTTP Response Message

HTTP/1.1 200 OK Connection closeDate: Thu, 06 Aug 1998 12:00:15 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 ...

status line(protocol

status codestatus phrase)

header lines

data, e.g., requestedHTML file


HTTP Response Status CodesHTTP Response Status 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:)400 Bad Request

request message not understood by server404 Not Found

requested document not found on this server505 HTTP Version Not Supported

Status code is in first line of the response message:


Web CachingWeb Caching

user sets browser: Web accesses via proxy server

browser sends all HTTP requests to proxy server object in cache: returns

cached object else cache requests

object from origin server, then returns object to client

Goal: satisfy client request without involving origin server

client

Proxyserver

client

HTTP request

HTTP request

HTTP response

HTTP response

HTTP request

HTTP response

origin server

origin server


More about Web CachingMore about Web Caching Proxy server acts as both

client and server Typically proxy server is

installed by ISP (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 deliver content


Conditional GET: HTTP Build-Conditional GET: HTTP Build-in Support for Cachingin Support for Caching

Goal: don’t send object if cache is up-to-date

Proxy server: specify date of cached copy in HTTP requestIf-modified-since: <date>

Origin server: response contains no object if cached copy is up-to-date: HTTP/1.0 304 Not Modified

Proxy server

Origin Server

HTTP request msgIf-modified-since:

<date>

HTTP responseHTTP/1.0

304 Not Modified

object not

modified

HTTP request msgIf-modified-since:

<date>

HTTP responseHTTP/1.0 200 OK

<data>

object modified


Non-Caching Example Non-Caching Example Assumptions Average object size =

100,000 bits Avg. request rate from

institution’s browsers to origin servers = 15/sec

Delay from institutional router to any origin server and back to router = 2 sec

originservers

public Internet

institutionalnetwork 10 Mbps LAN

1.5 Mbps access link


Non-Caching Example Non-Caching Example Consequences Utilization on LAN = 15% Utilization on access link =

100% Total delay = Internet delay

+ access delay + LAN delay = 2 sec + minutes +

milliseconds

originservers

public Internet




Non-Caching ExampleNon-Caching ExamplePossible solution Increase bandwidth of

access link to, say, 10 Mbps

Consequences Utilization on LAN = 15% Utilization on access link = 15% Total delay = Internet delay +

access delay + LAN delay = 2 sec + msecs + msecs Often a costly upgrade

originservers

public Internet


10 Mbps access link


Caching ExampleCaching ExampleInstall proxy server Suppose hit rate is 0.4Consequence 40% requests will be satisfied almost

immediately 60% requests satisfied by origin

server Utilization of access link reduced

to 60%, resulting in negligible delays (say 10 msec)

Total avg delay = Internet delay + access delay + LAN delay = .6*(2.01) secs + .4*milliseconds < 1.4 secs

originservers

public Internet



InstitutionalProxy server

eec-484/584 computer networks lecture 4 wenbing zhao (part of the slides are based on drs. kurose...

Documents