Lecture: Introduction

Introduction to Data Networking

Introduction to this class

• Me: Stephan Bohacek

• [email protected], 302-831-4274, skype: Stephan.Bohacek

• http://www.eecis.udel.edu/~bohacek

• Syllabus (online)

Syllabus (also online)

• Textbook: Kurose and Rose. Computer Networking, 2007 (the 4th edition). This book is required

• Prerequisites: Introduction to probability, C/C++ programming

• Grading: homework=1/3, projects=1/3, final=1/3. Homework and projects turned in late will be marked off 2.5% per day (including weekends). Grades of online discussion are based on the number and qualtiy of postings. These postings may be in the form of questions and answers. A good question will count toward your discussion grade.

• There will be programming assignments. This can be done on Linux or on Windows with Visual Studio. Evans 132 has linux machines and remote access is possible. Also, with your EECIS user name and password, MS visual studio can be downloaded for free from here

Syllabus (also online)

Today – networking basics

• Movie on the history of the Internet

• Core components of the Internet – the protocol stack

• Multiplexing, circuit switching, and packet switching

• Loss and delays

• The structure of the Internet

• This lecture covers much of chapter 1 in the textbook.





• Loss and delays







• Loss and delays



Core components

• End-hosts

• Applications– ?

• Packets– ?

• Routers and gateways and groups of routers (ISPs)

• Links– ?

• Protocols

Core components

• End-hosts

• Applications– Web

– Email

– File transfer

– File sharing

• Packets– TCP

– UDP


• Links– Fiber

– Coaxial

– Twisted pair

– Wireless

• Protocols

Application Layer – where the applications live

• Email:– Rules/protocols for how an end-host gets mail from the mail server

• Web: – Rules/protocols for how the end-hosts gets a web page from the web servers

• Question:– How is a networking application different from a non-networking application

(e.g., MS Word). That is, why do we say that an application is a bunch of rules? MS-Word is not a bunch of rules?

– Answer: The networking applications must communicate, and rules are required to define the communication.

• Roles that end-hosts play: – Client, server, and peer

– The client asks the server for a service.• E.g., The client asks the server to send a mail for it.

• The client asks the server for a web page

• The client asks the server to translate a web address to an IP address.

– Peer: A host can act as both a client and a server. But usually in one transaction, the host takes only one role

• End-hosts


– Email

– File transfer

– File sharing

• Packets– TCP

– UDP


• Links– Fiber

– Coaxial

– Twisted pair

– Wireless

• Protocols

Layers 1-4

• End-hosts


– Email

– File transfer

– File sharing

• Packets– TCP

– UDP


• Links– Fiber

– Coaxial

– Twisted pair

– Wireless

• Protocols

clientserver

Which are the end-host?

Routers

Layers 1-4

• End-hosts


– Email

– File transfer

– File sharing

• Packets– TCP

– UDP


• Links– Fiber

– Coaxial

– Twisted pair

– Wireless

• Protocols

clientserver

Goal: move messages from server to the clientApproach: break the problem into little pieces.Each piece is a layer in the “protocol stack”

Why is this a good approach?

1.Small problems are easier to understand/solve.

2.Different solutions can be mixed and matched

Layers 1-4

• End-hosts


– Email

– File transfer

– File sharing

• Packets– TCP

– UDP


• Links– Fiber

– Coaxial

– Twisted pair

– Wireless

• Protocols

clientserver

Top down approach of breaking problems into small pieces1. Transport layer

1.Reliability: The server must make sure that the client gets the dataCongestion control (or lack there of)

2.Congestion Control: The server should send data as fast as possible, but not too fast3.TCP provides these features (services), while UDP does not

2. Network layer (could be called the routing layer, but it isn’t)1.The packets must find their way through the network.2.Each packet has the IP address of the destination3.By examining the IP address, routers decide where to send the packet next

3. Link Layer or MAC layer1.Links connect the routers/gateways and end-hosts2.This layer provides logical and control for communicating across links.3.Services that this layer might provide include

1. congestion control, media access, error detection/correction

http://youtube.com/watch?v=RjrEQaG5jPM

Layers 1-4

• End-hosts


– Email

– File transfer

– File sharing

• Packets– TCP

– UDP


• Links– Fiber

– Coaxial

– Twisted pair

– Wireless

• Protocols

Top down approach of breaking problems into small pieces…..3. Link Layer or MAC layer

1. Links connect the routers/gateways and end-hosts2. This layer provides logical and control for communicating across links.3. Services that this layer might provide include

1. congestion control, media access, error detection/correction

1. Media access. The “air” is a shared medium. If two nodes transmit at the same time, there will be a collision. Thus, a scheme must be developed to determine which node transmits when.

2. Error detection/correction. If interference does occur, then errors might occur. If an error is detected, then

1. the error could be corrected with forward error correction, or2. the receiving link could request a retransmission

Layers 1-4

• End-hosts


– Email

– File transfer

– File sharing

• Packets– TCP

– UDP


• Links– Fiber

– Coaxial

– Twisted pair

– Wireless

• Protocols

clientserver

Top down approach of breaking problems into small pieces1. Transport layer

1. Reliability: The server must make sure that the client gets the dataCongestion control (or lack there of)

2. Congestion Control: The server should send data as fast as possible, but not too fast3. TCP provides these features (services), while UDP does not

2. Network layer (could be called the routing layer, but it isn’t)1. The packets must find their way through the network.2. Each packet has the IP address of the destination3. By examining the IP address, routers decide where to send the packet next

3. Link Layer or MAC layer1. Links connect the routers/gateways and end-hosts2. This layer provides logical and control for communicating across links.3. Services that this layer might provide include

1. congestion control, media access, error detection/correction4. Physical layer

1. Logical bits are encoded as physical quantities, e.g., as voltage levels, as shifts in phase, …2. This course does not cover the physical layer

http://youtube.com/watch?v=RjrEQaG5jPM

Protocols

• End-hosts


– Email

– File transfer

– File sharing

• Packets– TCP

– UDP


• Links– Fiber

– Coaxial

– Twisted pair

– Wireless

• Protocols

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

transmission, receipt

Hi

Hi

Got thetime?

2:00

TCP connection request

TCP connectionresponse

Get http://www.awl.com/kurose-ross

<file>

time

Internet protocol stack

• application: supporting network applications– FTP, SMTP, HTTP

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

• network: routing of datagrams from source to destination

– IP, routing protocols

• link: data transfer between neighboring network elements

– PPP, Ethernet

• physical: bits “on the wire”

application

transport

network

link

physical

ISO/OSI reference model

• presentation: allow applications to interpret meaning of data, e.g., encryption, compression, machine-specific conventions

• session: synchronization, checkpointing, recovery of data exchange

• Internet stack “missing” these layers!

– these services, if needed, must be implemented in application

– needed?

application

presentation

session

transport

network

link

physical





• Loss and delays



Circuit switching versus Packet switching

• Packet switching brought the networking revolution

• Circuit switching

• Virtual circuit networking– A half-way point between packet switched and circuit switched

networking

Circuit switching

• Circuit switching– Old style phone system– Each connection gets its own wire or

bandwidth– Note: calls must be set-up.– E.g.,

• Me: operator, get my the president.• Operator: one moment please.• Then she plugs a cable into a socket so

now I have a physical wired between me and the president.

– Instead of each connection getting a whole wire, connections can share a wire via multiplexing

– The first automatic circuit switching was developed by Almon Strowger – an undertaker. There were two undertakers in a small town and the switch board operator was the wife of the other undertaker. So Strowger invented an automatic circuit switch to rid both husband and wife of employment.

Frequency division multiplexing

toll office End office phonephone End office

300 3400 100300 103400 200300 203400 300 3400

On each hop, the connection gets its own bandwidth

TV is frequency division multiplexing

Time division multiplexing

64kbits

1 2 3

bytes

1/8000 sec per byte

7 bits of data and one bit for control (data or not), so it really 56kbps of

data

1 211Multiplex 24 channels= 24*64kbps + overhead= 1.544Mbps DS1 (T1)

Overhead is 1 bit per 8*24 bits= 8000bps

Note all the control overhead: if the bit is 1, then payload is control. Lots of control is needed to setup a circuit. How is it possible to get channels at each hop?Also, if there is not data, then nothing is sent. This wastes data.But the circuit is yours, guaranteed!

Multiplex 28 DS1= 28*24*64kbps + overhead = 44.736Mbps DS-3

Multiplexing 810 channels + overhead = 51.84 = STS-1/OC-1STS is electrical and oc is opticalOC3 = 155.52Mbps (150.336 payload)OC12 = 633.08 Mbps (601.344 payload)OC48 = 2.488Gbps (2.405Gbps)OC192 = 9.953Gbps (9.6Gbps payload)

There are standard bit-rates that support multiplexing different numbers of calls

Packet switching - Statistical multiplexing

• Data is in packets, not streams.

• Must be digital

• Each packet has an address

• A switch/router reads the whole packet, then reads the address and forwards the packet – store and forward

clientServer: address = 1

1data

packet. Note format specification specifies where the address is



• Must be digital



clientServer: address = 1

1data

AB

C

D

F E

If destination is 1, then next

hop is B


hop is C


hop is

1data

1data

1data



• Must be digital



• No reservations are needed. First come first serve.

• Major benefit:– If you need more bandwidth, then you can get it, it you don’t need it, then maybe someone else

can use it.

• Major drawback:– What happens if two packets arrive at a switch and both need to go to the same output

interface. Picture. One packet is either dropped, or is placed in a buffer. Either way, something bad has happened, the packet is gone, or is delayed. This would never happen on a circuit switched network. queuing delay and packet loss

Packet vs. Circuit Switching

If usage is random (e.g., web surfing) statistical multiplexing is better.

Suppose that

1. A 5Mbps link

2. Each user needs 50kbps

3. And each user is active 20% of the time. (note that this condition does not matter for circuit switching. Why?)

Circuit switching caseThe total number of users that can be accommodated with circuit switching is 5e6/50e3 = 100 users

How many users can be accommodated under packet switching and how many can be accommodated under packet switching?

Packet Switching Case

Now if there are 200 users, what is the probability that there are 150 or more active users?

In this case, there would be a problem, since the network cannot support more than 100 active users.

Simpler questions: What is the probability of 150 particular users being active and 50 other being inactive?

Packet Switching Case

What is the probability of more than 100 users being active?

The probability of 101 users being active plus, 102 users being active, plus, …., 200 users being active, which is

Packet Switching vs. Circuit Switching

A couple of things:

This means that• when you walk into the switching center, the probability of finding overload is 10^-8.• Or, if you random access the link, the probability of finding it in overload.• Once you find it in overload, or not, the probability that is will be in overload in the next second is more

complicated and requires queuing theory. This analysis might reveal worst performance.

8300

101

300 102.012.0300

k

kkk

What does this probability really mean?

In this example, we assumed 20% user utilization (they were active 20% of the time)Is this large or small?If it the user utilization is smaller, then the difference between packet switching and circuit switching is even larger. But it is smaller, then there is less of a difference.What is your user utilization? •For web surfing •For cell phone usage•For music streaming

Packet Switching vs. Circuit Switching

• If loss and delay are permissible and usage is random, then packet switching is better than circuit switching.

• If usage is very regular (e.g. TV!), circuit switching is best.

• If losses and delay are not permissible, then circuit switching is best (e.g., remote controlled surgery).

• With packet switching, congestion control is required. Also, there is more overhead for each packet.

• For circuit switching, once the circuit is setup, it can be very efficient. But circuits must be set-up.

• So, for short file transfer, packet switching is good but for long file transfers, circuit switching might be better.

There is a subtle difference between packet switching and statistical multiplexing. Statistical multiplexing means to use the resource as needed. This leads to the performance improvements mentioned but also the complications (delay and loss). The phone network uses circuit switching, but the circuits are statistically multiplexed between users. In packet switching, links are statistically multiplexed.

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

Packet-switching: store-and-forward

• takes L/R seconds to transmit (push out) packet of L bits on to link at R bps

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

• delay = 3L/R (assuming zero propagation delay)

Example:

• L = 7.5 Mbits

• R = 1.5 Mbps

• transmission delay = 15 sec

R R RL

more on delay shortly …





• Loss and delays



Losses and delay in packet switched networks

• Losses– Transmission losses

• In fiber links, bit-error is 10^-12 or better (i.e., less).– What is the probability of packet error when there are 1400 bytes in a packet?

• In wireless links, the bit-error rate can be very high

– Congestion losses. • If too many packets arrive at the same time, then the buffers will fill up and

packets are lost.

• Increasing the link speeds or reducing the number of users can reduce the probability of loss.

• Increasing the size of the buffer reduces losses, but also increases delay.

• Delay– Queuing delay

– Transmission delay

– Propagation delay

– Processing delay

A

B

packet being transmitted (delay)

packets queueing (delay)

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

Queuing delay

• Queuing delay occurs for the same reason as congestion losses.

• The more the network is utilized, the high the queueing delay (and losses)

• Utilization = := actual use / maximum possible use

A

B

packet being transmitted (delay)

packets queueing (delay)free (available) buffers: arriving packets

dropped (loss) if no free buffers

Suppose that

• the link bit-rate is Z,

• there are X users

• Each users uses data rate Y, with probability P, and use no bandwidth with probability 1-p.

= X*P/Z

Queuing delay

Is it possible to have a network run at full utilization?

No! The average delay would be infinite!

From queuing theoryDelay = /(1- )

Delay in packet switched networks





How long does it take to transmit a packet?How long does it take to get all the bits from node on to the wire/air/fiber?

Suppose•Link bit rate is 10 Mbps•Packet size is 1400 bytesHow long to transmit the packet?

10*10^6 bits / sec

1400 *8 bits / packet= .0011 sec = 1.1 ms






How long does it take for a bit to travel along a wire/fiber/through the air?

– Suppose• Speed of light in a vacuum 3e8 m/s while in a fiber it is 2e8m/s• How long does it take to transmit a bit from NY to LA = 3962km

– 20ms propagation delay

• How about from NY to Jakarta, Indonesia = 16,179km– 80ms

• How about to a Geostationary satellite?– 250-300 (up and back)

• Medium orbit satellites (e.g., GPS) – 120ms

• Low-earth orbit satellites (low earth? What about middle-earth?)– Iridium at 10ms

» Note, Iridium paid 5 billion for the network and sold for 25million (1/2%->on sale 99.5% off, everything must go)– Teledesic. 10ms

Fun with Propagation Delay

How long is a bit?Suppose that a links transmits at 10mbps. How long is a bit?

How long does it take to a bit? 1/10*10^6 = 10^-7

How far does the electric signal go in 10^7 sec?10^-7 * 2e8 = 20 meters.

How long many bits fit in a fiber at 10Mbps from NY to Jakarta?16,179km*10^3/20 = 0.1 MB

How long many bits fit in a fiber at 10 Gbps from NY to Jakarta?16,179km*10^3/20 = 100 MB

Satellite transmissions are subject to transmission loss (e.g., rain can cause interference),The satellite sending station could wait for and ACK from the other side and resend the data if no ACK appeared (a link layer solution)But this would cause out-of-order deliverySo the satellite could hold the packets until the lost one is retransmitted.How large would the buffer need to be if the bit rate was 3Gbps? Answer .5*3e9/8=187MB (assuming no processor delay)


Routers take a bit of time to process packets.

• moving packets inside the router

• Finding which is the next hop

• Applying security or QoS





How to measure delay?

• Ping: > ping 216.109.124.73

• Ping gives help

• (linux) Ping –I 10 216.109.124.73 > file.txt

• Then read it in excel and plot delay

• Traceroute (linux), tracert (windows)

• Traceroute 216.109.124.73 gives the routers and an estimate of the delay to each router.





• Loss and delays



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

Tier-1 ISP: e.g., Sprint

…

to/from customers

peering

to/from backbone

….

………

POP: point-of-presence


• “Tier-2” ISPs: smaller (often regional) ISPs– Connect to one or more tier-1 ISPs, possibly other tier-2 ISPs

Tier 1 ISP

Tier 1 ISP

Tier 1 ISP

Tier-2 ISPTier-2 ISP

Tier-2 ISP Tier-2 ISP

Tier-2 ISP

Tier-2 ISP pays tier-1 ISP for connectivity to rest of Internet tier-2 ISP is customer oftier-1 provider

Tier-2 ISPs also peer privately with each other.


• “Tier-3” ISPs and local ISPs – last hop (“access”) network (closest to end systems)

Tier 1 ISP

Tier 1 ISP

Tier 1 ISP



Tier-2 ISP

localISPlocal

ISPlocalISP

localISP

localISP Tier 3

ISP

localISP

localISP

localISP

Local and tier- 3 ISPs are customers ofhigher tier ISPsconnecting them to rest of Internet


• a packet passes through many networks!

Tier 1 ISP

Tier 1 ISP

Tier 1 ISP



Tier-2 ISP

localISPlocal

ISPlocalISP

localISP

localISP Tier 3

ISP

localISP

localISP

localISP

ISPs and the structure of the Internet

• Video of a Network Access Point (NAP) in Los Angeles

http://www.eecis.udel.edu/~bohacek/classes/1Wilshire-web300.mov

Said to be the most interconnected space in the world and the most expensive real estate in North America, the “Meet Me Room” (a telco industry term) is the heart of One Wilshire. Here the primary fiber optic cables are routed, split, and shared. Because of the presence of so many telcos in this room and the ability to freely interconnect between them, rackspace here becomes extremely valuable. For comparison, the average price for office space in downtown Los Angeles is $1.75 per square foot per month. At the Meet Me Room, $250 per square foot would be a bargain.

MEET ME ROOM

Some 1,800 known conduits contain the fiber optic cables that flow through the building’s stairwells and vertical utility corridors, called “risers.” Cable connects the commercial telco tenants on floors 5 through 29 to the 4th floor Meet Me Room, and to a new, “wireless” Meet Me Room constructed on the 30th floor.

CABLE RISERS

Whenever a permit is pulled by a city contractor for any underground repairs outside One Wilshire, the various telco companies with cable in the area come out and paint the cable routes on the asphalt, creating a visible graphic of the complexity of what lies just under the surface.

SURFACE CABLE MAP

Computers generate a lot of heat, and maintaining a stable, cool temperature and a low humidity is essential in telco hotels, so tenants sometimes demand to install their own cooling systems to safeguard their equipment. At One Wilshire, these units are installed primarily on the third floor roof. A new closed loop cooling system has been installed on the 30th floor roof.

HVAC

As tenants’ needs change, cables can go unused. Cable mining is performed to thin out the obsolete cables and future congestion is alleviated through the installation of dedicated new ducts.

CABLE MINING

Power is supplied by DWP, but in the event of a blackout, the building’s five generators will kick in. It takes the generators three seconds to start up and stabilize. During this brief period, the entire building runs on batteries. There are 11,000 gallons of diesel stored on site, enough to run the generators for 24 hours before being refueled.

ELECTRICITY

ELECTRICITY

ELECTRICITY

ELECTRICITY

ELECTRICITY

On the roof, microwave antennas link up One Wilshire to transmission towers located around the city. Though fiber’s higher capacity has given it dominance over microwave at One Wilshire, microwave’s relatively low cost over long distances continues to make it economical for some applications. The roof’s clear line of sight to the south, west, and to other high-rises, along with the ability to interface with the fiber inside, continues to make One Wilshire an attractive location for microwave-based transmission.

MICROWAVE

Much can be learned about a building’s function by examining its roof. The existence of telco hotels in the region around One Wilshire is indicated by the presence of new and extensive cooling units on the roofs of adjacent buildings, many of which were nearly vacant until the telco companies moved in.

READING A ROOF

The main fiber optic cables connecting One Wilshire to the world enter the building from under the street through closets in the walls of the building’s parking garage. Given the importance of the building to the global communications network, access to the parking garage is controlled, and the building is said to be monitored continuously by federal security officials.

POINT OF ENTRY

Said to be the most interconnected space in the world and the most expensive real estate in North America, the “Meet Me Room” (a telco industry term) is the heart of One Wilshire. Here the primary fiber optic cables are routed, split, and shared. Because of the presence of so many telcos in this room and the ability to freely interconnect between them, rackspace here becomes extremely valuable. For comparison, the average price for office space in downtown Los Angeles is $1.75 per square foot per month. At the Meet Me Room, $250 per square foot would be a bargain.

MEET ME ROOM

Some 1,800 known conduits contain the fiber optic cables that flow through the building’s stairwells and vertical utility corridors, called “risers.” Cable connects the commercial telco tenants on floors 5 through 29 to the 4th floor Meet Me Room, and to a new, “wireless” Meet Me Room constructed on the 30th floor.

CABLE RISERS

Whenever a permit is pulled by a city contractor for any underground repairs outside One Wilshire, the various telco companies with cable in the area come out and paint the cable routes on the asphalt, creating a visible graphic of the complexity of what lies just under the surface.

SURFACE CABLE MAP

Computers generate a lot of heat, and maintaining a stable, cool temperature and a low humidity is essential in telco hotels, so tenants sometimes demand to install their own cooling systems to safeguard their equipment. At One Wilshire, these units are installed primarily on the third floor roof. A new closed loop cooling system has been installed on the 30th floor roof.

HVAC

As tenants’ needs change, cables can go unused. Cable mining is performed to thin out the obsolete cables and future congestion is alleviated through the installation of dedicated new ducts.

CABLE MINING

Power is supplied by DWP, but in the event of a blackout, the building’s five generators will kick in. It takes the generators three seconds to start up and stabilize. During this brief period, the entire building runs on batteries. There are 11,000 gallons of diesel stored on site, enough to run the generators for 24 hours before being refueled.

ELECTRICITY

ELECTRICITY

ELECTRICITY

ELECTRICITY

ELECTRICITY

On the roof, microwave antennas link up One Wilshire to transmission towers located around the city. Though fiber’s higher capacity has given it dominance over microwave at One Wilshire, microwave’s relatively low cost over long distances continues to make it economical for some applications. The roof’s clear line of sight to the south, west, and to other high-rises, along with the ability to interface with the fiber inside, continues to make One Wilshire an attractive location for microwave-based transmission.

MICROWAVE

Much can be learned about a building’s function by examining its roof. The existence of telco hotels in the region around One Wilshire is indicated by the presence of new and extensive cooling units on the roofs of adjacent buildings, many of which were nearly vacant until the telco companies moved in.

READING A ROOF

The main fiber optic cables connecting One Wilshire to the world enter the building from under the street through closets in the walls of the building’s parking garage. Given the importance of the building to the global communications network, access to the parking garage is controlled, and the building is said to be monitored continuously by federal security officials.

POINT OF ENTRY

Homework

• Page 61. Questions (3, 7), 8, (9), (10), 11, 13, 14, 19, (20), 21, (22), (23)• Page 63. Problems 2, (3), 6, 7, 8, (10), (11), (12)• Use trace route to determine the average number of hops between 10

destinations of your choice.• Use ping to determine the propagation delay. Specifically, send very small

packets (these will be 24 bytes).Then send ICMP packets with larger payload. Compare the difference in the RTT and determine the transmission time.

• Do links have time-varying delay? To answer this questions run trace route at different times of the day (e.g., the middle of the night, morning, afternoon, etc) and compare the delay times.

Documents

Lecture: Introduction