Lect-2: BasicsComputer Networks1 371-1-0291 : An Introduction to Computer Networks Handout #2:...

Lect-2: Basics Computer Networks 1

371-1-0291: An Introduction to Computer Networks

Handout #2: Foundations and Basic Concepts

Additional Reading Text book: Chaps. 1.2,3.1

Homepagehttp://www.cse.bgu.ac.il/Courses/course.asp?ID=63


Outline

A Detailed FTP Example Layering Packet Switching and Circuit Switching Some terms

Data rate, “Bandwidth” and “throughput” Propagation delay Packet, header, address Bandwidth-delay product, RTT


Example: FTP over the Internet Using TCP/IP and Ethernet

App

OS

R2R2 R3R3

R4R4

R1R1 R5R5

Ethernet

“A” (BGU) “B” (TECH)

Ethernet

App

OS

1

234

67

20

191817

5

910

81213

11 1516

14


In the sending host

1. Application-Programming Interface (API) Application requests TCP connection with “B”

2. Transmission Control Protocol (TCP) Creates TCP “Connection setup” packet TCP requests IP packet to be sent to “B”

TCPData

TCPHeader

TCP Packet

Type = Connection Setup

Empty


In the sending host (cont.)

3. Internet Protocol (IP) Creates IP packet with correct addresses. IP requests packet to be sent to router.

IPData

TCP Packet

Encapsulation

IPHeader

IP Packet

Destination Address: IP “B”Source Address: IP “A”Protocol = TCP

TCPData

TCPHeader


In the sending host (cont.)

4. Link (“MAC” or Ethernet) Protocol Creates MAC frame with Frame Check Sequence (FCS). Wait for Access to the line. MAC requests PHY to send each bit of the frame.

EthernetData

IP Packet

EthernetFCS

EthernetHeader

Ethernet Packet

Destination Address: MAC “R1”Source Address: MAC “A”Protocol = IP

IPData

IPHeader

Encapsulation


In Router R1

5. Link (“MAC” or Ethernet) Protocol Accept MAC frame, check address and Frame Check

Sequence (FCS). Pass data to IP Protocol.

EthernetData

IP Packet

EthernetFCS

EthernetHeader

Ethernet Packet

Destination Address: MAC “R1”Source Address: MAC “A”Protocol = IP

IPData

IPHeader

De-capsulation


In Router R1

6. Internet Protocol (IP) Use IP destination address to decide where to send

packet next (“next-hop routing”). Request Link Protocol to transmit packet.

IPData

IPHeader

IP Packet



In Router R1


EthernetData

IP Packet

EthernetFCS

EthernetHeader

Ethernet Packet

Destination Address: MAC “R2”Source Address: MAC “R1”Protocol = IP

IPData

IPHeader

Encapsulation


In Routers R2, R3, R5 Same operations as Router R1


EthernetData

IP Packet

EthernetFCS

EthernetHeader

Ethernet Packet

Destination Address: MAC “B”Source Address: MAC “R5”Protocol = IP

IPData

IPHeader

Encapsulation


In the receiving host

17. Link (“MAC” or Ethernet) Protocol Accept MAC frame, check address and Frame Check

Sequence (FCS). Pass data to IP Protocol.

EthernetData

IP Packet

EthernetFCS

EthernetHeader

Ethernet Packet

Destination Address: MAC “B”Source Address: MAC “R5”Protocol = IP

IPData

IPHeader

De-capsulation


In the receiving host (2)

18. Internet Protocol (IP) Verify IP address. Extract/de-capsulate TCP packet from IP packet. Pass TCP packet to TCP Protocol.

IPData

TCP Packet

De-capsulation

IPHeader

IP Packet


TCPData

TCPHeader


In the receiving host (3)

19. Transmission Control Protocol (TCP) Accepts TCP “Connection setup” packet Establishes connection by sending “Ack”.

20. Application-Programming Interface (API)

Application receives request for TCP connection with “A”.

TCPData

TCPHeader

TCP Packet

Type = Connection Setup

Empty


Outline




Layering: The OSI Model

Session

Network

Link

PhysicalPhysicalPhysical

Application

Presentation

Transport

Network

Link Link

Network

Transport

Session

Presentation

Application

Network

Link

Physical

Peer-layer communication

layer-to-layer communication

Router Router

1

2

3

4

5

6

7

1

2

3

4

5

6

7


Layering: Our FTP Example

Network

Link

Transport

Application

Presentation

Session

Transport

Network

Link

Physical

The 7-layer OSI Model The 4-layer Internet model

ApplicationFTP

ASCII/Binary

IP

TCP

Ethernet


Outline




Circuit SwitchingA B

Source Destination

It’s the method used by the telephone network. A call has three phases:

1. Establish circuit from end-to-end (“dialing”),2. Communicate,3. Close circuit (“tear down”).

Originally, a circuit was an end-to-end physical wire. Nowadays, a circuit is like a virtual private wire: each

call has its own private, guaranteed data rate from end-to-end.


Circuit Switching Telephone Network

Source“Caller”

Central Office“C.O.”

Destination“Callee”

Central Office“C.O.”

TrunkExchange

Each phone call is allocated 64kb/s. So, a 2.5Gb/s trunk line can carry about 39,000

calls.


Packet Switching

A

R1

R2

R4

R3

B

Source Destination

It’s the method used by the Internet. Each packet is individually routed packet-by-packet,

using the router’s local routing table. The routers maintain no per-flow state. Different packets may take different paths. Several packets may arrive for the same output link at

the same time, therefore a packet switch has buffers.


Packet SwitchingSimple router model

R1Link 1

Link 2

Link 3

Link 4

Link 1, ingress Link 1, egress





Link rate, RX(t)

Dropped packets

B

Dropped packetsQueue LengthX(t)

Time

Packet buffer

Packets for one output

Packet SwitchingStatistical Multiplexing

Data Hdr

Data Hdr

Data Hdr

RR

R

Because the buffer absorbs temporary bursts, the egress link need not operate at rate N.R. But the buffer has finite size, B, so losses will occur.

1

2

N


Virtual Circuit SwitchingA B

Source Destination

A widely used method for packet switching. Differs from datagram switching:

1. Connection oriented delivery of packets2. A virtual connection is setup from source to destination3. All packets follow the same circuit4. A circuit link can be shared by several virtual circuits

Each Virtual Circuit (VC) has an Identifier (VCI) A session gets a VCI and its packets are routed based on

VCI There are two types: Permanent VC (PVC), Switched VC

(SVC)


Statistical Multiplexing

B

A

time

time

rate

rate

x

x

A x

B x


Statistical Multiplexing Gain

A

B

C

2x

C < 2x

A+B

time

rate

Statistical multiplexing gain = 2x/C

Note: the gain could be defined for a particular loss probability (in this case, x and C were chosen so that there

were no losses).


Why does the Internet usepacket switching?

1. Efficient use of expensive links: The links are assumed to be expensive and scarce. Packet switching allows many, bursty flows to share the

same link efficiently. “Circuit switching is rarely used for data networks, ...

because of very inefficient use of the links” - Gallager

2. Resilience to failure of links & routers: ”For high reliability, ... [the Internet] was to be a

datagram subnet, so if some lines and [routers] were destroyed, messages could be ... rerouted” - Tanenbaum

Source: Networking 101


Some Definitions Packet length, P, is the length of a packet in bits. Link length, L, is the length of a link in meters. Data rate, R, is the rate at which bits can be sent, in

bits/second, or b/s.1

Propagation delay, PROP, is the time for one bit to travel along a link of length, L.

PROP = L/c. Transmission time, TRANSP, is the time to transmit a

packet of length P. TRANSP = P/R.

Latency is the time from when the first bit begins transmission, until the last bit has been received. On a link:

Latency = PROP + TRANSP.1. Note that a kilobit/second, kb/s, is 1000 bits/second, not 1024 bits/second.


Packet Switching

Host A

Host B

R1

R2

R3

A

R1

R2

R4

R3

B

TRANSP1

TRANSP2

TRANSP3

TRANSP4

PROP1

PROP2

PROP3

PROP4

Source Destination

“Store-and-Forward” at each Router

( )i ii

TRANSP PROP Minimum end to end latency


Packet SwitchingWhy not send the entire message in one packet?

Breaking message into packets allows parallel transmission across all links, reducing end to end latency. It also prevents a

link from being “hogged” for a long time by one message.

Host A

Host B

R1

R2

R3

M/R

min/ ii

M R PROP Latency

Host A

Host B

R1

R2

R3

( / )i ii

PROP M R Latency

M/R


Packet SwitchingQueueing Delay

Host A

Host B

R1

R2

R3

TRANSP1

TRANSP2

TRANSP3

TRANSP4

PROP1

PROP2

PROP3

PROP4

( )i i ii

TRANSP PROP Q Actual end to end latency

Q2

Because the egress link is not necessarily free when a packet arrives, it may be queued in a buffer. If the network is busy, packets might have to wait a long time.


Packet Switching

It’s often useful to model a router as a queue per output link

A A

R

A

BC B

C

B

C


Packet SwitchingLittle’s Result

Where:

is the average number of customers in the queue,

is the arrival rate, in customers per second, and

is the average time that a customer

waits in the queue.

Result holds so long as

L

w

L w

no customers are lost/ dropped.


Packet SwitchingA useful, simple fluid model

A(t)

D(t)Cumulative number of

departed bits up until time t.

time

Service process

Cumulativenumber of bits

Cumulative number of bits that arrived up until time t.

R

A(t)

D(t)

X(t)

Properties of A(t), D(t): A(t), D(t) are non-decreasing A(t) >= D(t)


D(t)

A(t)

time

Q(t)

d(t)

Queue occupancy: Q(t) = A(t) - D(t).

Queueing delay, d(t), is the time spent in the queue by a bit that arrived at time t, and if the queue is served first-come-first-served (FCFS or FIFO)

Fluid Model

Cumulativenumber of bits


D(t)

A(t)

time

Q(t)

d(t)

Fluid Model ExampleCumulative

number of bits

Example: Every second, a train of 100 bits arrive at rate 1000b/s. The maximum departure rate is 500b/s.What is the average queue occupancy?

( ( ) ( ) 0) 0.5 (0.1 5Q t Q t

: During each cycle, the queue fi lls at rate 500b/ s f or 0.1s,

then drains at rate 500b/ s f or 0.1s.The average queue occupancy when

the queue is non-empty is theref ore:

Solution

00) 25 .

( ) (0.2 25) (0.8 0) 5 .Q t

bits

The queue is empty f or 0.8s each cycle, and so: bits

Lect-2: BasicsComputer Networks1 371-1-0291 : An Introduction to Computer Networks Handout #2:...

Documents

Transcript of Lect-2: BasicsComputer Networks1 371-1-0291 : An Introduction to Computer Networks Handout #2:...