ucsd cse 124 networked services fall 2009

UCSD CSE 124Networked Services

Fall 2009

B. S. Manoj, Ph.Dhttp://cseweb.ucsd.edu/classes/fa09/cse124

10/1/2009 1UCSD CSE 124 Networked Services Fall 2009

Some of these slides are adapted from various sources/individuals including but not limited to the slides from the text books by Kurose and Ross, Tanenbaum, and Peterson and Davie. Use of these slides other than for pedagogical purpose for CSE 124, may require explicit permissions from the respective sources.

Design considerations for Network Protocol stack

• Design for minimum context switches– e.g., Application layer calls send(.) function with

message to be sent• The message can be sent immediately with minimal

delay

– Application layer calls receive(.): context switching can result

– Such cases, call back functions, e.g., deliver(.), can improve performance by minimizing context switches


Design consideration for network protocol stack

• Design for copy-free operation– Message copying is expensive– Define and use a common message abstraction

across layers and protocols of the stack• Permits passing messages up and down the stack with

minimal copying• Manipulation of the messages/packets is done in a

copy-free manner– Two approaches

• Message processing with a priori memory allocation• Message processing with linked lists


Application layer messagePre-assigned memory

Application

Transport

Network

Datalink

PHY

Message copy

Copy-free

Copy-free

Copy-free

Message copy

Message processing with a priori memory allocation


Message processing with linked-list implementation

Application layer messagePre-assigned memory

Application

Transport

Network

Datalink

PHY

Message copy

Copy-free

Copy-free

Copy-free

Message copy10/1/2009 5UCSD CSE 124 Networked Services Fall

2009

Application Layer Protocols • Hiper Text Transfer Protocol

(HTTP) is one of the most popular application layer protocols

• HTTP drives the success of WWW– Handles heterogeniety of clients

and servers

• WWW is a collection of cooperating servers and clients speaking HTTP

• A web browser software helps users to open web locations, referred to as Uniform Resource Locators (URLs)– http://www.ucsd.edu/index.html– http://cseweb.ucsd.edu/classes/

fa09/cse124

PC runningExplorer

Server running

Apache Webserver

Mac runningNavigator

HTTP request

HTTP request

HTTP response

HTTP response


HTTP• HTTP is a text-based protocol

– Readable text-based commands, unlike binary-commands or code-based commands, and information are used for communication

• Defines how the communication between a web browser and web server can take place

• Is stateless – It does not require any state information– However, new methods are added on to gather state or user information

• For example, cookies • Two kinds of messages used

• REQUEST messages• RESPONSE messages

• General message form• START_LINE <CRLF>• MESSAGE_HEADER <CRLF>• <CRLF>• MESSAGE_ BODY <CRLF>


HTTP• A webpage contains one or more objects

– Can be html files, JPEG or GIF images, JAVA applet, or a video clip

• A web page with one html file and n other number of additional objects making it n+1 total objects

• All these objects are located by URLs– e.g.,

http://classweb.ucsd.edu/classes/fa09/cse124/index.html – A URL has two parts

• A host part: classweb.ucsd.edu• A file name with path information: /classes/fa09/cse124/index/html

• A user requests a web server object by – Either entering the URL on the browser’s address bar– Or clicking a hyper link on a web page


UCSD CSE 124 Networked Services Fall 2009 9

An HTTP sessionSuppose user enters URL www.classweb.ucsd.edu/classes/fa09/cse124/index.html

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

2. HTTP client sends HTTP request message (containing URL) into TCP connection socket. Message indicates that client wants object classes/fa09/cse124/index.html

1b. HTTP server at host www.classweb.ucsd.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 socket

time

(contains text, references to 10

jpeg images)

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An HTTP Session (cont.)

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

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HTTP message format

• General format =>

11

GET /somedir/page.html HTTP/1.0Host: www.someschool.edu User-agent: Mozilla/4.0Connection: close Accept-language:en

(extra carriage return, line feed)

request line(GET, POST,

HEAD commands)

header lines

Carriage return, line feed

indicates end of message

10/1/2009 UCSD CSE 124 Networked Services Fall 2009

HTTP REQUEST operations• GET: Fetches a specific web object

• HEAD: Fetch the status of the specific web object (such as time stamp)

• POST: Send information (filled forms, or other info) to the server

• PUT: Store documents at the specific web URL (used for uploading documents)

• DELETE: Delete a specific URL (with sufficient permissions)

• TRACE: Loop back the message (for debugging, or for security verifications)

• CONNECT: For use by proxy servers or for future purpose

• OPTIONS: Query certain options available with servers10/1/2009 12UCSD CSE 124 Networked Services Fall

2009

Response messagesCODE Type Example(s)

1xx Informational 100=Server agrees to handle the client’s request

2xx Success 200=Request successful, 204= no content present

3xx Redirection 301=page moved (see Location: ), 304= cached page still valid, further action may be required to complete the request

4xx Client error 400=Bad request, 403= forbidden pages, 404=page not found

5xx Server error 500=internal server error, 503=try again later, 505= HTTP version not supported

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 content here (NON textual data in MIME encoded)

status line(protocol

status codestatus phrase)

header lines

data, e.g., requestedHTML file



Non-Persistent HTTP: Response time

Definition of RTT: time for a small packet to travel from client to server and back.

Response 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+transmit time

time to transmit file

initiate TCPconnection

RTT

requestfile

RTT

filereceived

time time

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Non-persistent and Persistent HTTP

Non persistent HTTP (HTTP 1.0) Persistent HTTP (HTTP/1.1)

Uses separate TCP connections for each object Allows a single TCP connection to persist over multiple object transfers

High communication overhead: Too many control packets sent

Multiple request/response sessions can happen over the same socket, so less communication overhead

High computing resource overhead: Too many sockets created and closed

Low computing overhead as only fewer sockets are to be maintained

Increased response time (waits for a new connection setup for every object)

Quick response as the socket is ready for communication

Low scalability (high server load) High scalability due to low server load

Communication between a client and server can lost longer (for eg. Web-based email) thus leading to large number of sockets through out the entire session.

• Can one socket get all the data necessary ? (Multiple sockets may still be helpful in parallelizing the communication)

• How long a socket can remain open? (session TIMEOUT is necessary)


Data rendering using HTML• Very simple text-based

format

• Uses a set of tags to render the information in a web browser

• Supports a large number of types of data to be typed– Image, scripts, forms, etc

• Constantly being improved – For example, XML or other

dynamic forms

• Originally defined for static content


Dynamic content• Dynamic content are produced as a result of execution some back-end

program or script at the web-server

• Most recent web-based systems require dynamic content generation and delivery

• Two types– Server side dynamic content generation

• Many popular types– CGI Scripts– Server-based scripts

» PHP» Java Server Pages» Active Server Pages

– Client side dynamic content generation• Executed at the client web-browser• JavaScripts• ActiveX Controls


Common Gateway Interface (CGI) scripts

• Allows web servers to talk to back-end programs or data bases

• CGI scripts accepts data and generate HTML pages in return

• Usually implemented in Perl or scripting languages

• Usually CGI scripts are placed in a folder called CGI-bin• e.g., www.somesite.com/cgi-bin/signup.perl


Server-side scripts for dynamic content generation

• Simpler than CGI-scripts• Can be embedded in the HTML files

– can be executed by the web server directly for generating the dynamic content

• A popular open source solution is PHP– PHP: Hypertext Preprocessor

• Pages created by php has extension .php• An example script• Other solutions

– Java Server Pages– Active Server Pages


Client-side dynamic content generation

• Unlike server-side dynamic content generation, client-side dynamic content generation does not involve server interaction

• JavaScript or ActiveX controls are popular software scripts for generating client-side dynamic content generation


State maintenance in HTTP sessions

• Most recent web servers attempt to maintain state information– To better serve connections and users – To provide differentiated services to users

• Two main methods:– protocol endpoints: maintain state at sender/receiver over multiple transactions– cookies: carried in http messages can contain session state

• Cookies– State information is maintained by storing small text files called cookies– Server can request to store a short text file or information in the client machine’s

hard disk – For subsequent visits, the client web browser transmits the information back to the

server• Four steps for Cookies

– cookie header line of HTTP response message– cookie header line in HTTP request message– cookie file kept on user’s host, managed by user’s browser– back-end database at Web site



Cookies: An exampleclient

server

usual http response msg

usual http response msg

cookie file

one week later:

usual http request msgcookie: 1678 cookie-

specificaction

access

ebay 8734usual http request msg Amazon server

creates ID1678 for user create

entry

usual http response Set-cookie: 1678

ebay 8734amazon 1678

usual http request msgcookie: 1678 cookie-

spectificaction

accessebay 8734amazon 1678

backenddatabase

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Web caches (proxy server)

• user sets browser: Web accesses via cache

• 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

Goal: satisfy client request without involving origin server

client

Proxyserver

client

HTTP request

HTTP response

HTTP request HTTP request

origin server

origin server

HTTP response HTTP response

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More about Web caching

• A Web cache acts as both client and server

• Typically Web cache 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 (but so does P2P file sharing)

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Caching example Assumptions• average object size = 1,000,000

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/sec)x(1Mbits)/100Mbps= 15%• utilization on access link =

(15/sec)x(1Mbits)/15Mbps =100%• total delay = Internet delay + access

delay + LAN delay = 2 sec + minutes + milliseconds

originservers

public Internet

institutionalnetwork 100 Mbps LAN

15 Mbps access link

institutionalcache

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Caching example (cont)

possible solution• increase bandwidth of access link

to, say, 100 Mbps

consequence• 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


100 Mbps access link

institutionalcache

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Caching example (cont)

possible solution: install cache• 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


15 Mbps access link

institutionalcache

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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

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Summary

• Copy free operations for protocol stack efficiency• Application layer protocol: HTTP • Dynamic content generation for HTTP• Web Caching/Proxy servers for HTTP


Homework-1• Capture one or more HTTP session messages using

Wireshark for the following two URLs and briefly explain the trace including the image of the trace– http://cseweb.ucsd.edu/classes/fa09/cse124/index.html– http://blink.ucsd.edu– Deadline: Next week Thursday, the 8th October, 2009– Submission by email with SUBJECT: CSE-124-Fall-2009-

HomeWork-1:<YourFirstName>

• Chapter 2, Problem P9 from Kurose and Ross (Page 183) in 5th edition• Submission either in paper or by email with SUBJECT: CSE-

124-Fall-2009-HomeWork-1:<YourFirstName>10/1/2009 30UCSD CSE 124 Networked Services Fall

2009

ucsd cse 124 networked services fall 2009

Documents

ucsd cse

networked servicesfall

line message

header message

sentthe message

common message abstraction

web server

text books