fall 2012: fcm 708 bridge foundation i

Fall 2012: FCM 708 Bridge Foundation I

Prof. Shamik Sengupta

Instructor’s Website: http://jjcweb.jjay.cuny.edu/ssengupta/ Blackboard Website: https://bbhosted.cuny.edu/

Intro to Computer Networking

What is network: “nuts and bolts” view

1. Numerous connected computing devices: hosts = end systems – running network apps

Home network

Institutional network

Mobile network

Global ISP

Regional ISP

router

PC

server

wireless laptop cellular handheld

wired links

access points

2. communication links fiber, copper, radio,

satellite transmission rate =

bandwidth

3. routers: forward packets (chunks of data)

1-3

Categorization of networks by Application type

• Business Networks • Home Networks • Wireless Networks • Mobile Networks

1-4

Example Network Applications (1)

A network with two clients and one server (typical client-server connection)

1-5


The client-server model involves requests and replies over the public/private network

1-6


Peer-to-peer networking: no fixed clients and servers

1-7

6-8

Example wireless network (4)

network infrastructure

wireless hosts laptop, PDA, IP phone run applications may be stationary (non-

mobile) or mobile wireless does not always

mean mobility

Mobile Users

Combinations of wireless networks and mobile computing

Remember, there’s a difference in terminology between mobile users and wireless users

1-9

Categorization of networks by coverage scale

• Personal area networks (PAN) • Local area networks (LAN) • Metropolitan area networks (MAN) • Wide are networks (WAN) • The Internet (Global network)

1-10

Personal Area Network (PAN)

Bluetooth PAN configuration

1-11

Local Area Networks (LAN)

Wireless and wired LANs. (a) 802.11. (b) Switched Ethernet.

1-12

Metropolitan Area Networks (MAN)

A metropolitan area network 1-13

Wide Area Networks (WAN)

WAN that connects three branch offices in Australia 1-14

Coverage scale (contd.) Classification of interconnected processors by scale

1-15

A different categorization of networks

In terms of communication technology

• Unicasting • Broadcasting • Multicasting

1-16

Lecture 3

The Network Core

• Internet: mesh of interconnected routers

• How is data transferred through networks?

• Two methodologies

– Circuit switching – Packet switching

Lecture 3

Network Core: Circuit Switching

• End-end resources reserved for “call”

– dedicated circuit per call: like

telephone net

– dedicated bandwidth resources: no sharing

– Guaranteed performance

– Overhead: call setup required

Lecture 3

Network Core: Circuit Switching

• Total network resources (e.g., bandwidth) divided into “pieces” – pieces allocated to each call – resource piece idle if not used by owning call (no

sharing)

• dividing link bandwidth into “pieces”…HOW?

– frequency division multiplexing (FDM) • Users use different frequency channels

– time division multiplexing (TDM) • Users use different time slots

Lecture 3

Circuit Switching: FDM and TDM FDM

frequency

time

TDM

frequency

time

4 users

Example:

Lecture 3

Network Core: Packet Switching

each end-end data stream divided into packets

• user A, B packets share network resources

• each packet uses full link bandwidth

• resources used as needed

Circuit switching Bandwidth division into “pieces”

Dedicated allocation Resource reservation

No flexibility

Lecture 3

Packet Switching

A

B

C 100 Mb/s Ethernet

1.5 Mb/s

D E

queue of packets waiting for output

link

Lecture 3

Packet switching versus circuit switching

• Adv.: Packet switching allows users to use the network dynamically! – Lot of flexibility, dynamic sharing – No idle resource wastage – simpler, no call setup

• Disadv.: – No dedicated resources for each user – With excessive users: Excessive congestion – packet delay and loss: performance degrade

How do delay and loss occur in Internet/network?

Course Overview 1-24

How do loss and delay occur? packets queue in router buffers • packet arrival rate to link exceeds output link capacity • packets queue, wait for turn

A

B

packet being transmitted (delay)

packets queueing (delay) free (available) buffers: arriving packets dropped (loss) if no free buffers


Four sources of packet delay

• 1. nodal processing: – check bit errors – determine output link

A

B

propagation

transmission

nodal processing queueing

2. queueing time waiting at output

link for transmission depends on congestion

level of router


Delay in packet-switched networks 3. Transmission delay: • R=link bandwidth (bps) • L=packet length (bits) • time to send bits into link

= L/R

4. Propagation delay: • d = length of physical link • s = propagation speed in

medium (~2x108 m/sec) • propagation delay = d/s

A

B

propagation

transmission

nodal processing queueing

Note: s and R are very different quantities!


Total delay

• dproc = processing delay – typically a few microsecs or less

• dqueue = queuing delay – depends on congestion

• dtrans = transmission delay – = L/R, significant for low-speed links

• dprop = propagation delay – a few microsecs to hundreds of msecs

proptransqueueproctotal ddddd +++=

Lecture 3

Packet loss

• queue (aka buffer) preceding link in buffer has finite capacity

• packet arriving to full queue dropped (aka lost) • lost packet may be retransmitted by previous

node, by source end system, or not at all A

B

packet being transmitted

packet arriving to full buffer is lost

buffer (waiting area)

human protocols:

… specific msgs sent … specific actions taken

when msgs received, or other events

network protocols: • machines rather than

humans • all communication activity

governed by protocols

protocols define format, order of msgs sent and received among network entities, and actions

taken on msg transmission, receipt

What is computer networking: an operational view

1-29

Computer networking: an operational view

Any communication is all about protocol

networking protocol

Hi

Hi Got the time? 2:00

Connection req.

Connection reply.

Get http://jjcweb.jjay.cuny.edu/ssengupta/slide.ppt

<file> time

human protocol

1-30

1-31

Protocol “Layers” Networks are complex! It is not just two machines communicating!

• Millions of components:

– hosts – routers – Access networks – Physical links

• Numerous functionalities

Question: How to manage such vast

amount of components?

Soln: Divide functionalities among multiple layers.

1-32

ticket (purchase)

baggage (check)

gates (load)

runway (takeoff)

airplane routing

departure airport

arrival airport

intermediate air-traffic control centers

airplane routing airplane routing

ticket (complain)

baggage (claim

gates (unload)

runway (land)

airplane routing

ticket

baggage

gate

takeoff/landing

airplane routing

Layering of airline functionality

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

• Another example: Postal Service!

1-33

What are the adv. of layering?

Network is a huge complex system.

• Reduce the design complexity • Ease of updating the system

– change of implementation of layer’s service transparent to rest of system

– e.g., Postal service (overnight flight or overnight ground)

1-34

Internet protocol stack

application

transport

network

link

physical

• application – support host/network applications – Email, FTP, HTTP

• transport – process-process data transfer – TCP, UDP

• network – routing of datagrams from src. to destn. – IP address, routing protocols

• link – data transfer between neighboring network elements – Ethernet

• physical – bits “on the wire”

(Compare with the Postal System!)

Layering: logical communication

application transport network

link physical


link physical application

transport network

link physical


link physical

network link

physical

data

data Each layer: • Only talks with its

peer layer; • performs actions,

exchange messages with peers

• Relying on services provided by layer below

data

transport

transport

ack

35

1-36

source application transport network

link physical

Ht Hn M

segment Ht

datagram

destination application transport network

link physical

Ht Hn Hl M

Ht Hn M

Ht M

M

network link

physical

link physical

Ht Hn Hl M

Ht Hn M

Ht Hn M

Ht Hn Hl M

router

switch

Layering: physical communication

message M

Ht M

Hn frame

Encapsulation

message

1-37

A closer look at network: Access networks and physical media

Q: How to connect end

systems to edge router? • residential access nets • institutional access

networks (school, company) • Wireless access networks

telephone network Internet

home dial-up modem

ISP modem (e.g., AOL)

home PC

central office

Uses existing telephony infrastructure

Home is connected to central office up to 56Kbps direct access to router (often less) Can’t surf and phone at same time: not “always on”

Dial-up Modem

38

telephone network

DSL modem

home PC

home phone

Internet Existing phone line: 0-4KHz phone; 4-50KHz upstream data; 50KHz-1MHz downstream data

splitter

central office

Digital Subscriber Line (DSL)

Also uses existing telephone infrastruture up to 1 Mbps upstream (today typically < 256 kbps) up to 8 Mbps downstream (today typically < 1 Mbps) dedicated physical line to telephone central office

39

1-40

Residential access: cable modems

• Does not use telephone infrastructure – Instead uses cable TV infrastructure

• network of cable attaches homes to ISP router

– homes share access to router – unlike DSL, which has dedicated access

100 Mbps

100 Mbps

100 Mbps 1 Gbps

server

Ethernet switch

Institutional router

To Institution’s ISP

Institutional access: local area networks

Typically used in companies, universities, etc 10 Mbs, 100Mbps, 1Gbps, 10Gbps Ethernet Today, end systems typically connect into Ethernet

switch 41

1-42

Wireless access networks: unguided media

• shared wireless access network connects end system to router – via base station aka “access point”

• wireless LANs: – 802.11b/g (WiFi): 11 or 54 Mbps

• wider-area wireless access

– WiMAX (10’s Mbps) over wide area – wireless access to ISP router via

cellular network

base station

mobile hosts

router

Overview of The Layer Mechanisms

43

Application Layer • Application layer sits on top of all the other layers • Resides in the host machines • Manages various network apps

– Web – e-mail – File transfer – remote login – P2P file sharing – Multi-user network games – streaming video clips – Etc.

application transport network data link physical



1-44 44

45

App-layer protocol defines

• Types of messages exchanged, – e.g., request, response

• Message syntax: – “what” fields in messages &

“how” fields are delineated

• Message semantics – meaning of information in fields

• Rules for how processes send & respond to messages

Public-domain protocols: • defined in RFCs • allows for interoperability • e.g., HTTP, SMTP Proprietary protocols: • e.g., Skype

Application-layer protocols (cont).

Q: how does a process “identify” the other process with which it wants to communicate?

– IP address of host running other process

– “port number” - allows receiving host to determine to which

local process the message should be delivered

46

47

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”

48

Transport service requirements of common apps

Application

file transfer e-mail

Web documents real-time audio/video

stored audio/video interactive games

Data loss no loss no loss no loss loss-tolerant loss-tolerant loss-tolerant

Throughput elastic elastic elastic audio: 5kbps-1Mbps video:10kbps-5Mbps same as above few kbps up

Time Sensitive no no no yes, 100’s msec yes, few secs yes, 100’s msec

What are the major transport layer service “provider” protocols?

49

Internet apps: application, transport protocols

Application

e-mail remote terminal access

Web file transfer

streaming multimedia

Internet telephony

Application layer protocol SMTP [RFC 2821] Telnet [RFC 854] HTTP [RFC 2616] FTP [RFC 959] HTTP (eg Youtube), RTP [RFC 1889] SIP, RTP, proprietary (e.g., Skype)

Underlying transport protocol TCP TCP TCP TCP TCP or UDP typically UDP

The HTTP protocol

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

• http1.0: RFC 1945 • http1.1: RFC 2068

PC running Explorer

Server running

Web server

Mac running Navigator

50

The HTTP protocol continued

HTTP: TCP transport service: • client initiates TCP connection 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

51

HTTP connections

Nonpersistent HTTP • At most one object is sent

over a TCP connection.

Persistent HTTP • Multiple objects can be

sent over single TCP connection between client and server.

1-52 1-52

Nonpersistent HTTP Suppose 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)

1-53 1-53

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

1-54 1-54

Persistent HTTP

Nonpersistent HTTP issues: • requires connection for each

object • Too much time overhead

Persistent HTTP • server leaves connection open

after sending response • subsequent HTTP messages

between same client/server sent over open connection

• client sends requests as soon as it encounters a referenced object

1-55 1-55

56

HTTP Message Format

• two types of HTTP messages: request, response • HTTP request message:

– ASCII (human-readable format)

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

57

HTTP request message: general format

58

Other 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

59

HTTP response message

HTTP/1.1 200 OK Connection close Date: 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 code status phrase)

header lines

data, e.g., requested HTML file

60

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:

FTP: the file transfer protocol

• transfer file to/from remote host • client/server model

– client: side that initiates transfer (either to/from remote) – server: remote host

• ftp server: port 21

file transfer FTP server

FTP user

interface

FTP client

local file system

remote file system

user at host

Lecture 5

FTP: separate control, data connections

1. FTP client contacts FTP server at port 21

2. client authorized over control connection

3. client browses remote directory by sending commands over control connection.

4. when server receives file transfer command, server opens 2nd TCP connection (for file) to client

5. after transferring one file, server closes data connection.

FTP client

FTP server

TCP control connection port 21

TCP data connection port 20

Lecture 5

Application layer

• Email and SMTP (Simple Mail Transfer Protocol)

Lecture 5

Electronic Mail

Three major components: • user agents • mail servers • simple mail transfer protocol:

SMTP

User Agent • a.k.a. “mail reader” • composing, editing, reading mail

messages • e.g., Outlook, Mozilla

Thunderbird • outgoing, incoming messages

stored on server

user mailbox

outgoing message queue

mail server

user agent

user agent

user agent

mail server

user agent

user agent

mail server

user agent

SMTP

SMTP

SMTP

Lecture 5

Electronic Mail: mail servers

Mail Servers • mailbox contains incoming

messages for user • message queue of outgoing (to

be sent) mail messages • SMTP protocol between mail

servers to send email messages – client: sending mail server – “server”: receiving mail

server

mail server

user agent

user agent

user agent

mail server

user agent

user agent

mail server

user agent

SMTP

SMTP

SMTP

Lecture 5

Scenario: Alice sends message to Bob 1) Alice uses UA to compose

message and “to” [email protected]

2) Alice’s UA sends message to her mail server;

3) Message placed in message queue

4) Client side of SMTP opens TCP connection with Bob’s mail server and sends Alice’s message

5) Bob’s mail server places the message in Bob’s mailbox

6) Bob invokes his user agent to read message

user agent

mail server

mail server user

agent

1

2 3 4 5 6

Lecture 5

SMTP: comparison with HTTP

• SMTP uses persistent connections

Comparison with HTTP: • HTTP: pull • SMTP: push

• HTTP: port 80 • SMTP: port 25

• Both require guarantee • Both use TCP connection

Lecture 5

Mail access protocols user

agent

sender’s mail server

user agent

SMTP SMTP access protocol

receiver’s mail server

• Post Office Protocol (POP3)

• Internet Message Access Protocol (IMAP)

Lecture 5

POP3 and IMAP: Comparison POP3 • Uses “download and

delete” mode. • Bob cannot re-read e-mail

if he changes client • POP3 is “stateless” across

sessions

IMAP • Keep all messages in one

place: the server • Allows user to organize

messages in folders • IMAP keeps user state

across sessions: – names of folders and

mappings between message IDs and folder name

Lecture 5

Transport Layer

70

Transport services and protocols

• provide logical communication between app’ processes running on different hosts

• transport protocols run in end systems

• transport vs network layer services:

• network layer: data transfer between end systems

• transport layer: data transfer between processes – relies on network layer

services



network data link physical





71

Transport-layer protocols

Internet transport services: • reliable, in-order unicast delivery

(TCP) – congestion – flow control – connection setup

• unreliable (“best-effort”),

unordered unicast or multicast delivery (UDP)








72

Principles of Reliable data transfer

Sender sends one packet, then waits for receiver response

stop and wait

Sender needs to know whether receiver has received the packets

Simplest implementation:

73

Stop & Wait protocol

74

World is not ideal

Practical scenario: underlying channel can lose packets (data or ACKs) because of many reasons – What if data packet gets lost? – What if Ack packet gets lost?

75

World is not ideal

Practical scenario: underlying channel can lose packets (data or ACKs) because of many reasons

– What if data packet gets lost? – What if Ack packet gets lost?

Approach: sender waits “reasonable” amount of time for ACK

• retransmits if no ACK received in this time

76

Stop & wait protocol with loss

77

New constraint: Stop & wait protocol with delay

Practical scenario: • What if Ack packet lost or

just delayed (not lost)

Approach: sender waits “reasonable” amount of time for ACK

• retransmits if no ACK received in this time

• if pkt (or ACK) just delayed (not lost): – retransmission will be

duplicate

• Sender, receiver need to specify seq # of pkt being handled

78

Stop & wait protocol with delay

79

Performance of stop-and-wait

first packet bit transmitted, t = 0

sender receiver

RTT

last packet bit transmitted, t = L / R

first packet bit arrives last packet bit arrives, send ACK

ACK arrives, send next packet, t = RTT + L / R

U sender =

.008 30+.008

= 0.00027

L / R RTT + L / R

=

Packet size = 8 Kb Transmission rate = 1 Mbps

= 30 sec.

80

Pipelined protocols Pipelining: sender allows multiple, “in-flight”, yet-to-be-

acknowledged pkts – range of sequence numbers must be increased – buffering at sender and/or receiver

81

Pipelining: increased utilization

first packet bit transmitted, t = 0

sender receiver

RTT

last bit transmitted, t = L / R

first packet bit arrives last packet bit arrives, send ACK

ACK arrives, send next packet, t = RTT + L / R

last bit of 2nd packet arrives, send ACK last bit of 3rd packet arrives, send ACK

U sender =

.024 30.008

= 0.0008

3 * L / R RTT + L / R

=

Increase utilization by a factor of 3!

Two generic forms of pipelined protocols: go-Back-N, selective repeat 82

Go-Back-N

Sender: • k-bit seq # in pkt header • “window” of up to N, consecutive unack’ed pkts allowed

ACK(n): ACKs all pkts up to, including seq # n - “cumulative ACK” timer for each in-flight pkt timeout(n): retransmit pkt n and all higher seq # pkts in window

83

GBN in action

84

Selective Repeat

• receiver individually acknowledges all correctly received pkts – buffers pkts, as needed, for eventual in-order delivery to upper

layer

• sender only resends pkts for which ACK not received – sender timer for each unACKed pkt

• sender window – N consecutive seq #’s – again limits seq #s of sent, unACKed pkts

85

Selective repeat in action

86

fall 2012: fcm 708 bridge foundation i

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