CS 381 Introduction to computer networks Chapter 1 - Lecture 3 2/5/2015.

CS 381Introduction to

computer networksChapter 1 - Lecture 3

2/5/2015

Packet Switched Networks

• Packet switches generally use technique called store-and-forward:• All of the bits of a packet must have arrived at input link before switch will put first bit of packet

onto output link. • In other words, no part of a data packet can be on different links at the same time.

• Introduces store-and-forward delay.• Time needed for packet to move from the link into the packet switch• Time needed to process outgoing link from packet switch• Time needed to push packing onto outgoing link

• Switches have multiple input links and output links• Input/Output queues associated with each link in the packet switch

• Output links can hold packets that arrived when switch was transmitting another packet.

• Input links hold bits of a packet until all bits have arrived.

Packet Switched Networks

• Queuing Delay:• The length of time packets spends in the input/output queue of a router/switch• Time in queue determined by multiple factors• Highly variable

• Two delays in packet switched networks introduced:• Store-and-forward delays• Queuing delays• More delays to come!

• Easier now to visualize the idea that the internet is a “best effort” service.

• If packet switch is receiving more packets than it can handle buffers will start to overflow • Dropped packets.• Buffers on the routers/switches are just cache memory, which have finite storage capacities• The application has no knowledge of this and is not informed. • It learns of the dropped packet(s) only after they have not arrived.

• Even then, it doesn’t know if it was dropped or is being delayed in the network.

• Is this a problem?• Of course!

• Solutions?• Flow Control• Congestion Control

Introduction 1-5

Packet Switching: Statistical Multiplexing

• Sequence of A & B packets does not have fixed pattern, bandwidth shared on demand

• statistical multiplexing.

• TDM: each host gets same slot in revolving TDM frame.

A

B

C100 Mb/sEthernet

1.5 Mb/s

D E

statistical multiplexing

queue of packetswaiting for output

link

Introduction 1-6

Packet-switching: store-and-forward

• store and forward: • entire packet must arrive at router before it can be transmitted

on next link

• Let R be the transmission rate in bits per second

• Let L be the number of bits in a packet

• Transmission delay = L/R seconds

R R RL

Introduction 1-7


• Transmission delay of link is L/R seconds. • Takes L/R seconds to transmit (push out) packet of L bits on to link at R

bps• Assuming all links have the same transmission rate, and ignoring

propagation delay

• total transmission delay = NumberofLinks*L/R

• Example:• L = 7.5 Mb• R = 1.5 Mbps• Total transmission delay = 3(7.5) / 1.5 = 15 seconds

• 5 seconds per link, 3 links

R R RL

Introduction 1-8


• Another Example:• L = 8 Mb• R = 2 Mbps• Total transmission delay = 4(8) / 2 = 16 seconds

• 4 seconds per link, 4 links

R R RL

R

Compare Packet Switching and Circuit Switching

• Problems with circuit switching:• Likely to be times when circuit is not being used but is still dedicated to the communication

• No one else can use resource even though it is idle.

• More expensive and more difficult to implement than packet switching • Think about how dedicated bandwidth services would be implemented.

• Each communication stream would require link setup

Packet Switching (can be) More Efficient

• Assume 10 circuits on one megabit/second TDM link

• 10 slots per second (.1 seconds per slot)

• Each communication can transmit at 1 megabit/second for 1/10th of a second for effective transmission rate of 100 kbps.

• One user generates 1000 packets of 1000 bits = 1 megabit

• How long to transmit data?

Packet Switching (can be) More Efficient

• How long to transmit data?• 10 seconds: transmit 100 kbps during its time slot, then waits for next opportunity to send ….• Takes 10 seconds even if no other communication is currently active

Introduction 1-12

Packet Switching

• Can allow more users to use network.

• Consider 1 Mb/s link

• Assume each user: • 100 kb/s when “active”• active 10% of time

• circuit-switching: • 10 users

• packet switching: • with 35 users, probability > 10

active at same time is quite low• 99.99% of time > 10 active users

• Since this is the case, a larger number of users can connect to the network

• Increasing efficiency without increasing bandwidth.

N users

1 Mbps link

Introduction 1-13

Packet switching versus circuit switching

• Packet switching provides• resource sharing• and is simpler, no call setup• Great for so called “bursty” data (I.e., not a continuous stream

but transmitting data from time to time)• Assumes low probability of users being active at exactly the

same time.

• However there are problems with packet switching networks• Varying delays• Difficult to provide constant path resources• congestion: packet delay and loss

• protocols needed for reliable data transfer, congestion control

Is packet switching a “slam dunk winner?”

Introduction 1-14

Packet switching versus circuit switching

• Q: How to provide circuit-like behavior?• bandwidth guarantees needed for audio/video apps• still an unsolved problem (chapter 7)

Introduction 1-15

Internet structure: network of networks

• roughly hierarchical

• at center: “tier-1” ISPs (e.g., Verizon, Sprint, AT&T, Cable and Wireless), national/international coverage

• treat each other as equals

Tier 1 ISP

Tier 1 ISP

Tier 1 ISP

Tier-1 providers interconnect (peer) privately

Introduction 1-16


• Tier 1 ISP• Network that can reach every other network on the Internet without purchasing IP transit

• Tier 2 ISP• Network that peers with one or more tier 1 ISPs, possibly other tier 2 ISPs

• Regional ISP

• Tier 3 ISP• A network that solely purchases transit from other networks to participate in the Internet

• Local ISP

Introduction 1-17


• a packet passes through many networks!

Tier 1 ISP

Tier 1 ISP

Tier 1 ISP

Tier-2 ISPTier-2 ISP

Tier-2 ISP Tier-2 ISP

Tier-2 ISP

localISPlocal

ISPlocalISP

localISP

localISP Tier 3

ISP

localISP

localISP

localISP

Introduction 1-18

Chapter 1: roadmap

1.1 What is the Internet?1.2 Network edge

end systems, access networks, links

1.3 Network core circuit switching, packet switching, network structure

1.4 Delay, loss and throughput in packet-switched networks1.5 Protocol layers, service models1.6 Networks under attack: security1.7 History

Introduction 1-19

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

packets queueing (delay)

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

Introduction 1-20

Four sources of packet delay

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

A

B

propagation

transmission

nodalprocessing queueing

2. queueing time waiting to start

transmission depends on congestion level of

router

Introduction 1-21

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

nodalprocessing queueing

Introduction 1-22

Nodal delay

• Nodal Delay:• Delays experienced at each network switch/router along a path

• dproc = processing delay• Packet header examination time

• typically a few microsecs or less

• dqueue = queuing delay• Input/output queues

• Input Queue delay: time waiting at node before processing

• Output Queue delay: time waiting at node before transmission onto link

• depends on congestion, node CPU

• dtrans = transmission delay• Time needed to “push” packet onto link

• = L/R, significant for low-speed links

• dprop = propagation delay• Time needed to traverse a link (wired or wireless)

• A few microsecs to hundreds of msecs

proptransqueueprocnodal ddddd

Introduction 1-23

Queuing delay (revisited)

• Queuing delay is most highly variable aspect of delay• Important to get a feel for how queuing delay happens



• a=average packet arrival rate (number of packets per second)

• Assume all packets are of length L and that packets arrive at a steady rate.

• Define:• Traffic Intensity (I) = La/R• La --> (mean) number of bits per second arriving at the router• R is the link transmission rate

• What does it mean when I > 1

Introduction 1-24


• What does it mean when Traffic Intensity > 1?

• Router is receiving bits at a faster rate than it can process them

• Queuing delay increases as router cache begins to store packets for processing.

• Can eventually lead to packet loss.• Queue full, nowhere for incoming packets, dropped

Introduction 1-25


• What does it mean when Traffic Intensity is close to 0?

• The number of bits arriving at the router is much lower than the rate at which they can be transmitted on the link.

• Would not expect much queuing at all• Router can handle all of the incoming traffic

Introduction 1-26


• What does it mean when Traffic Intensity is close to1?

• The number of bits arriving at the router is equal to the number of bits that the router can handle.

• If traffic is at all bursty (multiple packets arriving at same time) then there are times when router is overwhelmed.

• Queuing may develop and increase over time.

Introduction 1-27

Queueing delay (revisited)



• a=average packet arrival rate

traffic intensity = La/R

La/R ~ 0: average queuing delay small La/R -> 1: delays become large La/R > 1: more “work” arriving than can be serviced,

average delay infinite!

Introduction 1-28

“Real” Internet delays and routes

• What do “real” Internet delay & loss look like?

• Traceroute program:• Sends UDP packet to each router on the path from source to

destination.• Each router on the path sends back a “special” message to source• Source tracks the time from when it sent the UDP packet and when

it receives the message from the router. • Actually sends three UDP packets to each router to provide three

different timings.

How Traceroute Works

• Recall that TCP and UDP are the two transport mechanisms in the Internet

• When a packet is put onto the network by either protocol, the packet has a header

• Provides information such as the source/destination addresses of the packet.

How Traceroute Works

• One field of the header is Time To Live (TTL) which specifies the maximum number of routers that the packet can traverse

• TTL is decremented by each router it goes through• When TTL reaches 0, the router sends an Internet Control Message Protocol (ICMP) message

to the source informing it that the TTL has expired.

Introduction 1-31

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 tofull buffer is lost

buffer (waiting area)

Introduction 1-32

Throughput

• throughput: • rate (bits/time unit) at which bits transferred between

sender/receiver• instantaneous:

• rate at given point in time

• average: • rate over longer period of time

• Consider transferring a file from host A to host B• Assume the file is F bits and it takes T seconds to complete

transfer. What is the average throughput?• F/T

Introduction 1-33

Throughput

• throughput: rate (bits/time unit) at which bits transferred between sender/receiver

• instantaneous: rate at given point in time• average: rate over longer period of time• What is average throughput?• Minimum of Rs and Rc

server, withfile of F bits

to send to client

link capacity

Rs bits/sec

link capacity

Rc bits/sec pipe that can carry

fluid at rate

Rs bits/sec)

pipe that can carryfluid at rate

Rc bits/sec)

server sends bits

(fluid) into pipe

Introduction 1-34

Throughput (more)

• Rs < Rc What is average end-end throughput?

Rs bits/sec Rc bits/sec

Rs > Rc What is average end-end throughput?

Rs bits/sec Rc bits/sec

link on end-end path that constrains end-end throughput

bottleneck link

Introduction 1-35

Throughput: Internet scenario

10 connections (fairly) share backbone bottleneck link R bits/sec

Rs

Rs

Rs

Rc

Rc

Rc

R

• per-connection end-end throughput: min(Rc,Rs,R/10)

• in practice: Rc or Rs is often bottleneck

CS 381 Introduction to computer networks Chapter 1 - Lecture 3 2/5/2015.

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