Chapter 1 Introduction Computer Networking: A Top Down Approach Featuring the Internet, 3 rd...
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Chapter 1Introduction
Computer Networking: A Top Down Approach Featuring the Internet,
3rd edition. Jim Kurose, Keith RossAddison-Wesley, July
2004.
A note on the use of these ppt slides:We’re making these slides freely available to all (faculty, students, readers). They’re in PowerPoint form so you can add, modify, and delete slides (including this one) and slide content to suit your needs. They obviously represent a lot of work on our part. In return for use, we only ask the following: If you use these slides (e.g., in a class) in substantially unaltered form, that you mention their source (after all, we’d like people to use our book!) If you post any slides in substantially unaltered form on a www site, that you note that they are adapted from (or perhaps identical to) our slides, and note our copyright of this material.
Thanks and enjoy! JFK/KWR
All material copyright 1996-2005J.F Kurose and K.W. Ross, All Rights Reserved
What’s the Internet: “nuts and bolts” view
• millions of connected computing devices: hosts = end systems
• running network apps• communication links
– fiber, copper, radio, satellite
– transmission rate = bandwidth
• routers: forward packets (chunks of data)
local ISP
companynetwork
regional ISP
router workstation
servermobile
What’s the Internet: “nuts and bolts” view
• protocols control sending, receiving of msgs– e.g., TCP, IP, HTTP, FTP, PPP
• Internet: “network of networks”– loosely hierarchical
– public Internet versus private intranet
• Internet standards– RFC: Request for comments
– IETF: Internet Engineering Task Force
local ISP
companynetwork
regional ISP
router workstation
servermobile
What’s a protocol?human protocols:• “what’s the time?”• “I have a question”• introductions
… specific msgs sent… specific actions
taken when msgs received, or other events
network protocols:• machines rather than
humans• all communication
activity in Internet governed by protocols
protocols define format, order of msgs sent and received among network entities,
and actions taken on msg transmission, receipt
What’s a protocol?a human protocol and a computer network protocol:
Q: Other human protocols?
Hi
Hi
Got thetime?
2:00
TCP connection request
TCP connectionresponseGet http://www.awl.com/kurose-ross
<file>time
Some network apps
• E-mail• Web• Instant messaging• Remote login• P2P file sharing• Multi-user network
games• Streaming stored
video clips
• Internet telephone• Real-time video
conference• Massive parallel
computing
“Cool” internet appliances
World’s smallest web serverhttp://www-ccs.cs.umass.edu/~shri/iPic.html
IP picture framehttp://www.ceiva.com/
Web-enabled toaster +weather forecaster
Internet phones
Internet protocol stack• application: (L7 & L6 of OSI) supporting
network applications– FTP, SMTP, HTTP
• transport: (L5 & L4 of OSI) host-host data transfer– TCP, UDP
• network: routing of datagrams from source to destination– IP, routing protocols
• link: data transfer between neighboring network elements– PPP, Ethernet
• physical: bits “on the wire”
application
transport
network
link
physical
messagesegment
datagram
frame
sourceapplicatio
ntransportnetwork
linkphysical
HtHnHl M
HtHn M
Ht M
M
destination
application
transportnetwork
linkphysical
HtHnHl M
HtHn M
Ht M
M
networklink
physical
linkphysical
HtHnHl M
HtHn M
HtHnHl M
HtHn M
HtHnHl M HtHnHl M
router
switch
Encapsulation
Background: Addressing• For a process to receive
messages, it must have an identifier
• A host has a unique32-bit IP address
• Q: does the IP address of the host on which the process runs suffice for identifying the process?
• Answer: No, many processes can be running on same host
• Identifier includes both the IP address and port numbers associated with the process on the host.
• Example port numbers:– HTTP server: 80
– Mail server: 25
Web and HTTPFirst some jargon• Web page consists of objects• Object can be HTML file, JPEG image, Java
applet, audio file,…• Each object is addressable by a URL• Web page consists of base HTML-file which
includes several referenced objects• Example URL:
www.someschool.edu/someDept/pic.gif
host name path name
HTTP overview• HTTP: hypertext transfer protocol• Web’s 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://www.rfc-editor.org/rfc/rfc1945.txt
• HTTP 1.1: RFC 2068– http://www.rfc-editor.org/rfc/rfc2068.txt
• HTTP state management (cookies): RFC 2109
– http://www.rfc-editor.org/rfc/rfc2109.txt
PC runningExplorer
Server running
Apache Webserver
Mac runningNavigator
HTTP request
HTTP request
HTTP response
HTTP response
HTTP overview (continued)Uses TCP:• client initiates bi-directional
TCP connection (via 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)– Messages encoded in text
• TCP connection closed
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
aside
URL(Uniform Resource Locator)
Way of identifying and accessing a web page:
Examplehttp://www.lclark.edu/~jmache/index.html
“how”Type of transaction
(protocol)
“where”Address or name of server
“what”Resource requested
URI(Uniform Resource Identifier)
Identifies a resource and includes URLs, but broader in context.
See http://www.w3.org/Addressing/ for more details
Mark-up Languages
• A way of describing information in a document.• Standard Generalized Mark-Up Language
(SGML) - a specification for a mark-up language ratified in 1986.
• Key aspect - using pairs of tags that surround information - a begin tag <tag_name> and a matching end tag </tag_name> .
Example<title> CS 393 home page </title>
HyperText Markup Language (HTML)
A mark-up language used in web pages.
“Hypertext” refers to the text’s ability to link to other documents.
“Markup” refers to providing information to tell browser how to display page and other things.
HTML page format
<HTML><HEAD>
</HEAD><BODY>
</BODY></HTML>
Signifies an HTML document
Head section includes information about document - “metadata”
Body section contains text and references to images to be displayed
End of document
HTML Tags
• Tags specify details such as type of text.
Example
<B> to start bold text
</B> to end bold text
<I> to start italic text
</I> to end italic text
HTML page<HTML><HEAD>
</HEAD><BODY><BR>Hello world <P><I> My name is <B>Jens</B> </I>
</BODY></HTML>
Line break tag - some tags in HTML are not in pairs
Attributes
Many tags can have attributes which specify something about the body between tag pair.
Example
<FONT COLOR=red SIZE=3 FACE=Times>This text is displayed in red in Times font, about 12 pt.</FONT>
Attributes
HTTP 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 line (methods)
HTTP/1.0• GET
– Return object specified by URI
• POST– Send data to server (forms)
• HEAD– asks server to leave
requested object out of response
– Return headers only of GET 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
• OPTIONS, TRACE, CONNECT
HTTP request line (cont.)
• URI– Object to retrieve
• E.g. http://www.cs.pdx.edu/index.html with a proxy• E.g. /index.html if no proxy
• HTTP version– Version being used– HTTP 1.1
• Host: header required• Connection: header supported
Common HTTP request headers• Accept
– Acceptable document types, encodings, languages, character sets
• If-Modified-Since– For use with caching
• Referer – URL which caused this page to be requested
• User-Agent• Host
– For multiple web sites hosted on same server
• Connection– Keep connection alive for subsequent request or close connection
Other HTTP request headers• Authorization
– Authentication info for HTTP authentication
• From– User email (when privacy is disabled)
Rest of HTTP request
Blank-line Separate request headers from POST information End of request
Body If POST, send POST information
Handling user input (forms)
POST method:• Input is uploaded to
server in entity body
GET method:• Input is uploaded in
URL field of request line
GET search?name=george&animal=monkey HTTP/1.1Host: www.somesite.com
POST search HTTP/1.1Host: www.somesite.comContent-type: application/x-www-form-urlencoded
name=george&animal=monkey
HTTP 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 format
• Status-line– HTTP version– 3 digit response code
• 1XX – informational• 2XX – success• 3XX – redirection• 4XX – client error• 5XX – server error
– Reason phrase
HTTP 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 server
404 Not Found– requested document not found on this server
505 HTTP Version Not Supported
A few sample codes:
Common HTTP response headers
• Server– server software
• Content-Encoding – x-gzip
• Content-Length• Content-Type• Expires• Last-Modified• ETag
Other HTTP response headers
• Location – redirection
• WWW-Authenticate – request for authentication
• Allow– list of methods supported (GET, HEAD, etc)
Rest of HTTP response Blank-line
Separate headers from data Body
Data being returned to client
HTTP headers by function
• Authentication– Client
• Authorization, Proxy-Authorization
– Server• WWW-authenticate,
Proxy-Authenticate
• User, server tracking– Client
• Cookie, Referer, From, User-agent
– Server• Set-cookie, Server
• Caching– General
• Cache-control, Pragma
– Client• If-Modified-Since, If-
Unmodified-Since, If-Match
– Server• Last-Modified, Expires,
ETag, Age
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:
GET /~ross/ HTTP/1.1Host: cis.poly.edu
By typing this in (hit carriagereturn twice), you sendthis minimal (but complete) GET request to HTTP server
3. Look at response message sent by HTTP server!
User-server state: cookies
Many major Web sites use cookies
Four components:1) cookie header line of HTTP
response messageSet-cookie:
2) cookie header line in HTTP request messageCookie:
3) cookie file kept on user’s host, managed by user’s 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
Cookies: keeping “state” (cont.)client server
usual http request msgusual http response
+Set-cookie: 1678
usual http request msg
cookie: 1678usual http response
msg
usual http request msg
cookie: 1678usual http response msg
cookie-specificaction
cookie-spectificaction
servercreates ID
1678 for user
entry in backend
database
access
acce
ss
Cookie file
amazon: 1678ebay: 8734
Cookie file
ebay: 8734
Cookie file
amazon: 1678ebay: 8734
one week later:
Cookies (continued)What cookies can bring:• authorization• shopping carts• Site preferences• recommendations• user session state
(Web e-mail)
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
aside
Web caches (proxy server)
Why Web caching?• Reduce response time for client
request.
• Reduce traffic on an institution’s access link.
• Reduce load on servers.
• Enables “poor” content providers to effectively deliver content (but so does P2P file sharing)
Goal: satisfy client request without involving origin server (i.e. do not send content that has not changed)
Info on web caching http://www.ircache.net/ http://www.squid.org ICP
http://www.rfc-editor.org/rfc/rfc2186.txt
http://www.rfc-editor.org/rfc/rfc2187.txt
More about Web caching• Browser sends all HTTP requests
to cache– object in cache: cache returns object – else cache requests object from origin
server, then returns object to client
• Done directly at client– Via browser web cache
• Along path from client to origin server– Via proxy web cache– Proxy acts as both client and server– Typically cache is installed by ISP
(university, company, residential ISP)
client
Proxyserver
client
HTTP request
HTTP request
HTTP response
HTTP response
HTTP request
HTTP response
origin server
origin server
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
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
institutionalnetwork 10 Mbps LAN
1.5 Mbps access link
institutionalcache
Caching example (cont)Possible 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
institutionalnetwork 10 Mbps LAN
10 Mbps access link
institutionalcache
Caching example (cont)
Install cache• suppose hit rate is .4
Consequence• 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
institutionalnetwork 10 Mbps LAN
1.5 Mbps access link
institutionalcache
Conditional GET
• Goal: don’t send object if cache has up-to-date cached version
• cache: specify date of cached copy in HTTP requestIf-modified-since: <date>
• server: response contains no object if cached copy is up-to-date: HTTP/1.0 304 Not Modified
cache 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
HTTP caching
• Additional caching methods– ETag and If-Match
• HTTP 1.1 has file signature as well
• When/how often should the original be checked for changes?– Check every time?
– Check each session? Day? Etc?
– Use Expires header• If no Expires, often use Last-Modified as estimate
Example Cache Check Request
GET / HTTP/1.1Accept: */*Accept-Language: en-usAccept-Encoding: gzip, deflateIf-Modified-Since: Mon, 29 Jan 2001 17:54:18 GMTIf-None-Match: "7a11f-10ed-3a75ae4a"User-Agent: Mozilla/4.0 (compatible; MSIE 5.5;
Windows NT 5.0)Host: www.cs.pdx.eduConnection: Keep-Alive