Introduction 1-1 03 - Protocol Layers. Introduction 1-2 Chapter 1 Introduction Computer Networking:...

Introduction 1-1

03 - Protocol Layers

Introduction 1-2

Chapter 1Introduction

Computer Networking: A Top Down Approach Featuring the Internet, 3rd edition. Jim Kurose, Keith RossAddison-Wesley, July 2004.

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Thanks and enjoy! JFK/KWR

All material copyright 1996-2004J.F Kurose and K.W. Ross, All Rights Reserved

Introduction 1-3

Protocol “Layers”Networks are

complex! many “pieces”:

hosts routers links of various

media applications protocols hardware,

software

Questions: Is there any hope of

organizing structure of network?

Or at least our discussion of networks?

Introduction 1-4

Organization of air travel: a layered view

Layers: each layer implements a service via its own internal-layer actions relying on services provided by layer below

ticket (purchase)

baggage (check)

gates (load)

runway takeoff

airplane routing

ticket (complain)

baggage (claim)

gates (unload)

runway landing

airplane routing

airplane routing

Introduction 1-5

Distributed implementation of layer functionality

ticket (purchase)

baggage (check)

gates (load)

runway takeoff

airplane routing

ticket (complain)

baggage (claim)

gates (unload)

runway landing

airplane routing

airplane routing

Dep

art

ing

air

port

arr

ivin

g

air

port

intermediate air traffic sites

airplane routing airplane routing

Introduction 1-6

Why layering?

Dealing with complex systems: explicit structure allows identification,

relationship of complex system’s pieces layered reference model for discussion

modularization eases maintenance, updating of system change of implementation of layer’s service

transparent to rest of system e.g., change in gate procedure doesn’t

affect rest of system

Introduction 1-7

Internet protocol stack application: supporting network

applications FTP, SMTP

transport: process-process data transfer

network: routing of datagrams from source host to destination host IP, routing protocols

link: data transfer between neighboring network elements PPP, Ethernet

physical: bits “on the wire”

application

transport

network

link

physical

5

4

3

2

1

Introduction 1-8

Layering: logical communication

applicationtransportnetwork

linkphysical


linkphysical


linkphysical


linkphysical

networklink

physical

Each layer: distributed “entities”

implement layer functions at each node

entities perform actions, exchange messages with peers

Introduction 1-9

Layering: logical communication


linkphysical


linkphysical


linkphysical


linkphysical

networklink

physical

data

data

E.g.: transport take data from

app add

addressing, reliability check info to form “datagram”

send datagram to peer

wait for peer to ack receipt

data

transport

transport

ack

Introduction 1-10

Layering: physical communication


linkphysical


linkphysical


linkphysical


linkphysical

networklink

physical

data

data

Introduction 1-11

Protocol layering and data

Each layer takes data from above adds header information to create new Protocol Data

Unit (PDU) – may also break into smaller segments passes new data unit to layer below


linkphysical


linkphysical

source destination

M

M

M

M

Ht

HtHn

HtHnHl

M

M

M

M

Ht

HtHn

HtHnHl

message

segment

datagram

frame

Introduction 1-12

Chapter 1: Summary

Covered a “ton” of material! Internet overview what’s a protocol? network edge, core,

access network packet-switching versus

circuit-switching Internet/ISP structure performance: loss, delay layering and service

models

You now have: context, overview,

“feel” of networking more depth, detail

to follow!

2: Application Layer 13

World Wide Web (WWW)


Processes communicating across network

process sends/receives messages to/from its socket

socket analogous to door sending process shoves

message out door sending process assumes

transport infrastructure on other side of door will deliver message to socket at receiving process

process

TCP withbuffers,variables

socket

host orserver

process

TCP withbuffers,variables

socket

host orserver

Internet

controlledby OS

controlled byapp developer


Addressing processes: For a process to receive

messages, it must have an identifier

Every host has a unique 32-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

More on this later


What transport service does an app need?Data loss some apps (e.g., audio)

can tolerate some loss other apps (e.g., file

transfer, telnet) require 100% reliable data transfer

Timing some apps (e.g.,

Internet telephony, interactive games) require low delay to be “effective”

Bandwidth some apps (e.g.,

multimedia) require minimum amount of bandwidth to be “effective”

other apps (“elastic apps”) make use of whatever bandwidth they get


Internet transport protocols services

TCP service: connection-oriented: setup

required between client and server processes

reliable transport between sending and receiving process

flow control: sender won’t overwhelm receiver

congestion control: throttle sender when network overloaded

does not provide: timing, minimum bandwidth guarantees

UDP service: unreliable data transfer

between sending and receiving process

does not provide: connection setup, reliability, flow control, congestion control, timing, or bandwidth guarantee

No setup required.


Web and HTTP

First some jargon Web page consists of objects Object can be HTML file, JPEG image, Java

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

includes several referenced objects Each object is addressable by a URL 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 1.1: RFC 2068

*NetCraft's Web Server Survey

PC runningExplorer

Server running

Apache Webserver*

Mac runningNavigator

HTTP request

HTTP request

HTTP response

HTTP response

http://news.netcraft.com/archives/web_server_survey.html


HTTP overview (continued)

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

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


HTTP connections

Nonpersistent 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 HTTPSuppose user enters URL www.someSchool.edu/someDepartment/home.index

1a. HTTP client initiates TCP connection to HTTP server (process) 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 someDepartment/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 socket

time

(contains text, references to 10

jpeg images)


Nonpersistent HTTP (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


Response time modeling

Definition of Round Trip Time (RTT): time to send 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


Persistent HTTP

Nonpersistent HTTP issues: requires 2 RTTs per object browsers often open

parallel TCP connections to fetch referenced objects

server must maintain a separate connection for each object

Persistent HTTP server leaves connection

open after sending response

subsequent HTTP messages between same client/server are sent over connection

Persistent 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 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: 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: general format


Uploading form input

Post method: Web page often

includes form input Input is uploaded to

server in entity body

URL method: Uses GET method Input is uploaded in

URL field of request line:

www.somesite.com/animalsearch?monkeys&banana


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 the URL field


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., requested

HTML file


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

In first line in server->client response message.A few sample codes:


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 www.rose-hulman.edu.Anything typed in sent to port 80 at www.rose-hulman.edu

telnet www.google.com 80

2. Type in a GET HTTP request:

GET /index.html HTTP/1.1 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!

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