Wednesday, July 17, 2013

kuangalia matokeo ya UCC bofya hapa

follow this link

Ingia hapa

Friday, June 28, 2013

KD:hii ndio ndege atakayokuja nayo USA PRESIDENT OBAMA

Friday, May 31, 2013

kuangalia matokeo ya form 4 na 6 bofya hapa..

Andika hii website

AU bofya hapo chini

matokeo_CSEE_2012

na Matokeo ya form six bofya hapo chini

http://www.necta.go.tz/matokeo_html_2013/ACSEE_2013/alevel.htm

Wednesday, May 29, 2013

BREAKING NEWS:HIP HOP ARTIST ALBERT NGWAIR A.K.A MIMI HAS PASSED AWAY YESTERDAY

Wednesday, April 03, 2013

KD: Shutdown and Restart WindowsXP In Seconds!

Shutdown and Restart WindowsXP In Seconds!

What do you do to shutdown or restart your windows XP? Of course you will do the following,

Click Start Menu ->
Then click Turn Off Computer ->
Then you get a window with three icons saying, Stand by, Turn Off and Restart ->
Then you click on Turn Off button to shutdown windows and Restart button to restart your system

All this process will take 6-7 seconds and if your computer has some programs running in the background, you will get a End Now window, which will further take 4-5 seconds. What if you do the same thing in a fraction of a second.

You can surely do that by creating a Shutdown and Restart Icon on your Desktop. To do that follow the steps below,

Create A Shutdown Icon
To create a shutdown icon on your desktop do the following,
1. Right click anywhere on your desktop and select New > Shortcut
Create-New-Shortcut

2. In the new window that opens Copy paste this command,

C:\Windows\System32\shutdown.exe -s -f -t 00

3. Click Next and Name it Shutdown as shown below

4. You will get an icon as shown below,
Shutdown-icon

Clicking this Icon will Shutdown your Windows Instantly. If you don’t like the Icon Image, you can customize it too. Do the following

Right Click on the shutdown Icon and select Properties a window will open as shown below,

2. Click Change Icon button and choose an Icon from those available,
choose-shutdown-icon

You can also download an icon of your choice and save it to your local drive. To use an icon of your choice simply click the browse button and specify the path where you have saved the icon.
3. Your new Shutdown Icon will look like this,
REd-Shutdown-Icon

Create A Restart Icon
Follow exactly all the steps discussed above to create a Restart Icon. You only need to add a different command for your Restart Icon. Here is the code for your Restart Icon,

C:\Windows\System32\shutdown.exe -r -f -t 00

After selecting an icon image you will get this,
TNT-Restart-Icon

Thats All!

I hope this may help you in saving time in Turning Off and Restarting your Windows.

KD:IT ESSENTIAL CHAPTER 15 ANSWERS

IT ESSENTIAL PC HARDWARE AND SOFTWARE CHAPTER 15 ANSWERS

01.A technician is troubleshooting a networked computer. The technician has gathered as much information as possible from the user and now needs to verify obvious issues. Which two action should the technician perform? ( )Choose two)
• Clear the network log files;
• Change the default gateway;

• Verify the computer IP information;

• Change the router configuration;

• Check activity on the routers.

02.Which statement is true about network maintenance?
• Cable supports are used to identify the devices to which the cables are connected;
• If a network device is making an unusual sound the device should be turned off immediately;

• Educating users on IT policies and procedures is an example of proactive network maintenance;

• Preventive maintenance should not be performed regularly because it interrupts network activity.

03.A technician is concerned that a wireless router is missing current security features. What action could be taken to address this issue?
• Disable the SSID broadcast;

• Check for an updated firmware version;

• Configure mixed mode wireless operation;
• Assign static IP addresses to the wireless devices.

04.A company purchases several wireless NICs at a great price. After installing the NICs, they find that the user cannot connect to the 2,4 GHz wireless network. What is a likely cause of the problem?

• The NICs are designed for the 802.11a standard;

• The NICs are designed for the 802.11b standard;
• The NICs are designed for the 802.11g standard;
• The NICs are designed for the 802.11n standard via USB;

05.In Internet Explorer, after choosing Tools and then Internet Options, which tab would a user select to set the browser to prompt before downloading potentially unsafe content?
• Advanced;
• Content,
• Privacy;
• security

06.A technician is working on a workstation that has an IP address of 169.254.25.34. The technician found that the computer cannot connect to the Internet and the network cable to the computer had been disconnected. Which command can a technician use to get a new IP address once the cable is reconnected?
• nslookup;
• net continue;

• ipconfig /renew;

• nbtstat /new.

07.Which three ports are used by SMTP, POP and IMAP ? (Choose three)
• 21

• 25

• 69

• 110

• 127

• 143

08.A technician has been asked to install a second optical drive in a computer. The technician will need to configure the drive as slave. How can the technician accomplish this desired configuration?
• Use jumpers on the motherboard to configure the drive as slave;
• List the drive as slave in the CMOS;
• Connect the drive after the twist in the PATA cable;

• Use jumpers on the drive to configure the drive as slave.

09.Which protocol provides secure transport of data from the company web server to remote users?
• HTTP

• HTTPS

• NetBEUI
• DNS

10.The network technician is researching the best way connect the branch offices to the main office. What are three main considerations that the technician should remember when choosing an Internet connection? (choose three)

• availability;

• distance;
• operating system;
• protocols;

• reliability;

• speed.

11.Which action should a user take to ensure that personal information that is entered into a website form is not retained on the computer?
• Clear the browser history;
• Change the default browser;
• Reset the file sharing permissions;

• Delete the temporary Internet files.

12.ABC Computer Company is in the process of planning a network for a new client. A network technician has been asked to perform a physical inspection of the facilities to determine the flow of data and protocols required by the client. What is this inspection called?
• data inspection;
• logical survey;
• plant inspection;

• site survey.

13.A user is reporting intermittent loss of connectivity to the network. No other users have reported similar issues. What preventive maintenance task would likely identify the problem?
• Listen for unusual sounds from the core network devices;
• Ensure that all switches have the latest firmware installed;

• Check for a damaged network cable at the user workstation;

• Verify the attachment points between network devices in the core of the network.

14.A user is receiving e-mail on a regular basis from a particular person. A problem occurs when the e-mail from this same person contains an attachment. What is one solution?
• Ask the IT department to permit port 25 through the firewall;

• Request that the person sending the e-mail attach only one picture per e-mail message;

• Check DNS settings on the computer used to receive the e-mail;
• Request that the person sending the photographs archive all e-mail messages before sending e-mail.

15.When planning the network in a new bulding, the technician notes that the company requires cabling that can extend up to 295ft (90m) with enchanged banwith support at an affordable price. Which cable type will the technician pick if he chooses the most common type of cabling used on network?
• Cat3
• Cat5

• Cat5e

• Cat6A

16.Which two protocols are used to receive e-mail? (choose two)
• DHCP
• SNMP

• IMAP

• POP3

• SSH

17.Which two hazard exist when working with fiber optic cabling (choose two)

• Chemicals;

• Copper strands;
• Electricity;

• Glass splinters;

• Connectors.

18.Students on the college network need to be able to view folder names and data in files stored on the Windows based file server, but should never be allowed to add or save information on the server. Occasionally, the students also need to run media programs to watch videos stored on the server. Which permission should the network technician assign to the students accesing the file server?
• Change;
• Execute;

• Read;

• Write.

19.A new employee needs to transfer files to the corporate FTP server. However, the employee is unable to log in to the FTP server. What is the most likely cause of the problem?
• The employee is using the wrong protocol;

• The employee is using an incorrect username or password;

• An incorrect IP address is configured in the FTP client;
• An incorrect port number is configured in the FTP client.

20.What are two ways to assign an IP address to wirekess NIC? (choose two)

• Assign a static address;

• Use ARP;
• Use the net command with the static option;
• Use WEP;
• Use the net command with the automatic option;

• Use DHCP.

Tuesday, April 02, 2013

KD: JUMA NATURE AIBUKA MFALME WA TEMEKE BAADA YA KUMFUNIKA VIBAYA TEMBA NA CHEGE

kD: JUMA NATURE AIBUKA MFALME WA TEMEKE BAADA YA KUMFUNIKA VIBAYA TEMBA NA CHEGE

Sunday, March 31, 2013

KD: Rais Kikwete aenda Muhimbili kuwafariji majeruhi walioumia kwenye ghorofa lililoporomoka Dar es salaam

kD: Rais Kikwete aenda Muhimbili kuwafariji majeruhi walioumia kwenye ghorofa lililoporomoka Dar es salaam

Monday, March 18, 2013

KD:HOW TO MAKE WINDOW 7 GENUINE BY USING COMMAND PROMPT

HOW TO MAKE WINDOW 7 GENUINE BY

USING COMMAND PROMPT
How many of you using Genuine windows 7 ?
we all know that everyone can't buy a genuine windows 7, because cost is too high.
Lots of Windows 7 users have experienced the error message that "Windows 7 is not genuine".
This error message comes if you use pirated or OEM versions of Windows 7.
If windows 7 is not genuine, even you can't change the background wallpaper or any other display related option on your PC.
So now i'm going to show you a easy way to make your windows 7 genuine using command prompt.
How to make Windows 7 genuine using Command Prompt
Press Windows key + R
Now "run" box will appear , in "run" box type CMD
Click on OK
Note : Run CMD as Administrator
Now command prompt will appear
In command prompt type SLMGR -REARM
Hit Enter. Wait 5-10 seconds
Now a message will appear
Done !!!
Now your Windows 7 is genuine .. :D
Restart your PC once. Hereafter you won't see the warning message that "Windows 7 is not genuine"

KD:ACTIVATING WINIDOW 8 WITHOUT USING CRACK

ACTIVATING WINDOW 8 WITHOUT USING CRACK or PATCH

I love windows 8 and its features but to be honest with you guys, I don’t have the money to purchase its license so I decide to search online to see if I’d see a crack or a patch for my windows 8. Hey! For your information guys I had no luck searching for the crack because all I could find was virus, buggy and empty cracks that crashes my system over and over again.We all know that window 8 is a new OS and the crack might be hard to find but I never gave up, so after my long search, I saw just a serial number so I decide to put the serial key manually myself and after many trial and errors, I finally made my windows 8 fully genuine.
Right now, you too can activate your windows 8 with this trick I am about to drop to be fully genuine without the fear of bug or viruses. I am ready if you’re ready…

HOW TO ACTIVATE YOUR WINDOWS 8 TO BE FULLY GENUINE

First of all you will open your command prompt as administrator by pressing the windows key and X key together have you opened the command prompt?

Now type exactly what you see below (Press enter after each line)

slmgr /upk

slmgr /ipk NG4HW-VH26C-733KW-K6F98-J8CK4

slmgr /skms lunar21.no-ip.org:80

slmgr.vbs -ato

Have you completed writing the above command?

If you’re done, restart your pc and voila, you have successfully activated your windows 8 to be fully genuine. Now enjoy your activated windows 8 and be limited no more…

This trick was tested on Windows 8 Professional Retail. Enjoy yourself and activate yours today.

Do you like the post? Then do click the like button, share and recommend us to friends. Oww before you go, subscribe to our feed for latest update to your mail.

Windows 8 is the latest collection to the Microsoft operating system (OS). Windows 8 has a slick design and it is also very nice to use with some extra features that will make you love it even more.

KD:Different steps for finding an ip address

Different steps for finding an ipaddress

The IP address of your PC is unique, just as your phone number is unique. Many organizations, including the authorities, can trace your IP address to find out your host name, ISP location and the geographic area from which you connect to the Internet.^[1] Follow the steps in this article to find the IP address of your PC whether you are using Windows Vista, Windows 7 or Windows XP. You'll also find instructions for finding your IP address on a Mac.

Steps

Method One: Command Prompt

1 Access your Start Menu.
2 Hover over "All Programs".
3 Click on Accessories when the menu pops up.
4 Click on Command Prompt.
5 Type in "ipconfig" once the program comes up and hit the Enter key.
6 Search for your IP address next to IPv4.
- Note: your IP address will be a ten-digit number.

Method Two: Windows 7 Network and Sharing Center

1 Access the Control Panel through the Start menu.
2 In the Control Panel, select Network and Internet.
3 In Network and Internet, select Network and Sharing Center.
4 In Network and Sharing Center, select Wireless Network Connection for wireless internet or Local Network Connection for wired internet.
5 Click on Details.
6 Find your IP address by looking next to IPv4 Address.
- Note: your IP address will be a ten-digit number.

Method Three: Windows Vista

1 Open the Start menu. Right-click on Network and then choose Properties from the drop-down menu.
2 Click the View Status button beside Local Area Connection to find the IP address of a wired connection.
3 Pick the View Status button next to Wireless Network Connection to view the IP address of a wireless adapter.
4 Select Details. Your IP address will show up next to "IPv4 IP address."^[2]

Method Four: Windows XP

1 Choose Start, My Network Places and then Properties.
2 Double-click Local Area Connection to find the IP address of your wired connection.
3 Double-click Wireless Network Connection to find the IP address of your wireless adapter.
4 Click Support and then Details. The window that pops up will display your computer's IP address.

Find Your IP Address on a Mac

1 Look for your IP address under System Preferences.
- Click the Apple menu in the top left corner of your screen. Choose System Preferences.
- Find the Network icon and click on it.
- Find your IP address on the screen under the "Status: Connected" line.
2 Use Terminal to find your IP address.
- Open Finder and click on Applications. Then choose Utilities.
- Choose Terminal from the Utilities menu.
- Type the following command: ifconfig |grep inet.
- Look for your IP address next to the last "inet" entry.^[3]

Friday, March 08, 2013

Wednesday, March 06, 2013

KD:Venezuelan president Hugo Chavez dies

Venezuelan president Hugo Chavez dies

Venezuela's President Hugo Chavez has died aged 58, after 14 years in power.

Mr Chavez had been seriously ill with cancer for more than a year, undergoing several operations in Cuba.
Crowds of supporters gathered outside the Caracas hospital where he died, chanting "We are all Chavez!"
A self-proclaimed revolutionary, Hugo Chavez was a controversial figure in Venezuela and on the world stage. A staunch critic of the US, he inspired a left-wing revival across Latin America.
Following his death, Vice-President Nicolas Maduro will assume the presidency until an election is held within 30 days, Foreign Minister Elias Jaua said.
"It is the mandate that Comandante President Hugo Chavez gave us," Mr Jaua told state television, adding that Mr Maduro would also be the candidate of the governing United Socialist Party (PSUV).
It was not immediately clear when the election would take place.
Mr Chavez's illness prevented him from taking the oath of office after he was re-elected for a fourth term in October.
The exact nature of his cancer was never officially disclosed, leading to continuing speculation about his health, and he had not been seen in public for several months.

Vice-president Nicolas Maduro was emotional as he announced Hugo Chavez's death

Seven days of national mourning have been declared and his body will lie in state until a funeral on Friday.
Opposition leader Henrique Capriles, whom Mr Chavez defeated in October's election, called on the government to "act in strict accordance with its constitutional duties".
Mr Capriles offered his condolences to Mr Chavez's family, saying "we were adversaries, but never enemies"

Thursday, February 28, 2013

KD:IP ADDRESS

IP Addressing

IP address is a unique identifier of a computer on TCP/IP networks and on the internet. Every computer requires a unique IP address to be a part of the internet and the IP address is provided by the internet service providers. Every IP address consists of the 32 bits and a binary system of 0s and 1s. The binary number system consist of only two types of digits 0 and 1. It is easier for us to remember the decimal numbers rather than the binary number system such as 011001101. On a same network segment, all the IP address share the same network address.
There are five classes of the IP addresses such as A, B, C, D and E and only 3 classes are in the use. Class D IP addresses are reserved for the multicast group ant cannot be assigned to hosts and the E class IP addresses are the experimental addresses and cannot be assigned to the people. Every IP address consists of 4 octets and 32 bits. Every participating host and the devices on a network such as servers, routers, switches, DNS, DHCP, gateway, web server, internet fax server and printer have their own unique addresses within the scope of the network.
TCP/IP protocols are installed by default with the Windows based operating systems. After the TCP/IP protocols are successfully installed you need to configure them through the Properties Tab of the Local Area Connection.

IP Addresses Classes

Class A
The binary address for the class A starts with 0. The range of the IP addresses in the class A is between 1 to 126 and the default subnet mask of the class A is 255.0.0.0. Class A supports 16 million hosts on each of 125 networks. An example of the class A is 10.10.1.1. Class A is used for the large networks with many network devices.
Class B
The binary address for the class B starts with 10. The range of the IP address in the class B is between 128 to 191 and the default subnet mast for the class B is 255.255.0.0. Class B supports 65,000 on each of 16,000 networks. An example of the class B address is 150.10.10.10. Class B addresses scheme is used for the medium sized networks.
Class C
The binary address for the class C starts with 110. The range of the IP addresses in the class C is between 192 to 223 and the default subnet mask for the class C is 255.255.255. Class C hosts 254 hosts on each of 2 million networks. An example of the Class C IP address is 210.100.100.50. Class C is used for the small networks with less then 256 devices and nodes in a network.
Class D
The binary addresses for the class D starts with 1110 and the IP addresses range can be between 224 to 239. An example of the class D IP address is 230.50.100.1
Class E
The binary address can starts with 1111 and the decimal can be anywhere from 240 to 255. An example of the class E IP address is 245.101.10.10
It is very important to know that all the computers in the same network segment should have the IP addresses for the same class i.e. form A, B or C.

IP Addressing Tips

A Network ID cannot be All 0s
A host ID cannot be All 1 because this represents a broadcast address for the local network.
Each host must have a unique host portion of the IP address.
All hosts on the same network segment should have the same network id.
A host address cannot be 127 because 127 has been reserved for the loop back functionalities.

Wednesday, February 27, 2013

KD:Networking Notes

Networking Notes

I would like to discuss some basic networking notes especially helpful for beginners who are thinking of establish a good career in this field. These networking notes supply the fundamental details about network types, communication technology, network troubleshooting, network topologies, network products and also the fundamental summary of the WAN and LAN models of communication.
What is computer network?
Internet is simply a type of network. When several devices are attached to one another, generally we are able to refer to it as a network. If these devices are computer systems, we'll refer to it as a computer network.
What's DHCP?
DHCP means Dynamic Host Configuration Protocol. It is used to assign the computer IP addresses and also for other types of network configurations like DNS. DHCP cuts down the administrative tasks of manually assigning IPs to computer systems inside a network.
What's DNS and just how it really works?
DNS means Domain name system that translates host names into the network IPs and IP addresses into host names. Every computer and also domains on the web have designated distinctive IP addresses. Internet communication is dependent on these IP addresses. These addresses have been created in the format of 10.1.1.200, 192.168.2.0 etc. It is difficult to remember these IP addresses; however host names are simpler to remember than their IPs.
What is computer Firewall?
It is a protective boundary for any network. It prevents the access of unauthorized sources over the network. The majority of Microsoft operating-systems, for example Windows XP service pack2, have generated-in firewall utilities.
Types of networks
Essentially, you will find three kinds of computer networks: local area network (LAN), (Metropolitan Area Network (MAN) and Wide Area Network (WAN). The communication inside a WAN is dependent on Hubs. A WAN network covers a town, country or regions. LAN is the interconnection of different hubs, routers and other networking devices in a specific place joined with a cabling system. MAN network covers the range of a limited metropolitan area.
Voice over internet protocol (VOIP) Communication Technology
Voice over internet protocol means VoIP. This technology can be used for voice data transmission over a specific IP based on the long distance of a network to create telephone calls. Voice over internet protocol telephone calls is fairly inexpensive and many organizations, offices and home customers are utilizing Voice over internet protocol technology to make long distance telephone calls.
Wi Max Technology
It is basically a wireless broadband technology that is an advanced form of Wi Fi. Wi Max technology supports data, audio and video communication simultaneously at a good speed of 70 Megabyte per second.
Network Gateway
It can be hardware or software. A gateway is generally a joining reason within a network. It connects normally two systems. Computers connected by using two LAN cards act as a gateway.
Router
It routes visitors to its destination in line with the source and destination IP addresses that are put into the routing software that is referred to as a routing table.
Fiber Optic Cable
Fiber optics offers the quickest medium of communication for data, voice and video transfer. Data travels at the speed of light in these cables. Internet service providers and other corporate offices are often connected with one another using this fiber optic cable to supply high-speed connectivity.
What's File Server?
It is a computer inside a network that provides authentication to network consumers of networks like Home Window 2003 Servers.
OSI Model of seven layers
The fundamental reason for the seven layers OSI model is to comprehend the data transmission and communication system steps. These layers are Application layer, Presentation layer, Transport layer, Network layer, Data Link layer and Physical layer. Each layer has its own functionality for data transmission from source to destination.
GSM Technology
It is wireless short range technology. This technology is usually utilized in cell phones, portable products, audio players, Laptops, computer systems as well as in vehicles.

Thursday, February 21, 2013

KD:Download Bitdefender Antivirus Free For Windows

Download Bitdefender Antivirus Free For Windows

Tuesday, February 19, 2013

KD:Angalia matokeo hapa...

bonyeza hii link.

necta result necta2012

http://www.ajira.go.tz/matokeo2012/csee2012/olevel.htm

exam results

Thursday, February 14, 2013

KD:Differences Between Gprs, Edge, 3g, Hsdpa, Hspa+ WCDMA And 4g Lte

Differences Between Gprs, Edge, 3g, Hsdpa, Hspa+ WCDMA And 4g Lte

KD:25 relationship tips to make a strong relationship with your man

25 relationship tips to make a strong relationship with your man

KD:What Fresh Hell Is This?: How to end an uncomfortable conversation: "You're...

What Fresh Hell Is This?: How to end an uncomfortable conversation:
"You're...: How to end an uncomfortable conversation: "You're starting to cut out... I can barely hear you. If you can still hear me, let's try this an...

Wednesday, February 06, 2013

Operating Systems Lecture Notes

Operating Systems Lecture Notes
Lecture 17
Networking

Martin C. Rinard

Networking deals with interconnected groups of machines talking with each other. Is a very different field than operating systems. Have a lot of standards stuff because everyone must agree on what to do when connect machines together.
What is a network? A collection of machines, links and switches set up so that machines can communicate with each other. Some examples:
- Telephone system. Machines are telephones, links are the telephone lines and switches are the phone switches.
- Ethernet. Machines are computers, there is one link (the ethernet) and no switches.
- Internet. Machines are computers, there are multiple links, both long-haul and local-area links. The switches are gateways.
Message may have to traverse multiple links and multiple switches to go from source to destination.
Circuit-switched versus Packet-switched networks. Basic disadvantage of circuit-switched networks - cannot use resources flexibly. Basic advantage of circuit-switched networks - deliver a guaranteed resource.
Basic Networking Concepts:
- Packetization.
- Addressing.
- Routing.
- Buffering.
- Congestion.
- Flow control.
- Unreliable Delivery.
- Fragmentation.
Local Area Networks. Connect machines in a fairly close geographic area. Standard for many years: Ethernet. Standardized by Xerox, Intel and DEC in 1978. Still in wide use.
Physical hardware technology: coax cable about 1/2 inch thick. Maximum length: 500 meters. Can extend with repeaters. Can only have two repeaters between any two machines, so maximum length is 1500 meters.
Vampire taps to connect machines to Ethernet. Attach an ethernet transceiver to tap; the transceiver does the connection between the Ethernet and the host interface. The host interface then connects to the host machine.
Ethernet is 10 Mbps bus with distributed access control. It is a broadcast medium - all transceivers see all packets and pass all packets to host interface. The host interface chooses packets the host should receive and discards others.
Access scheme: Carrier sense multiple access with collision detection. Each access point senses carrier wave to figure out if machine is idle. To transmit, waits until carrier is idle, then starts transmitting. Each transmission consists of a packet; there is a maximum packet size.
Collision detection and recovery. Transceivers monitor carrier during transimission to detect interference. Interference can happen if two transceivers start sending at same time. If interference happens, transceiver detects a collision.
When collision detected, uses a binary exponential backoff policy to retry the send. Adds on a random delay to avoid synchronized retries.
Is there a fixed bound on how long it will take a packet to get successfully transmitted? Is any packet guaranteed to be transmitted at all?
Addressing. Each host interface has a hardware address built into it. Addresses are 48 bits long. When change host interface hardware, address changes.
Are three kinds of addresses:
- Physical address of one network interface.
- Broadcast address for the network. (All 1's).
- Multicast addresses for a subset of machines on network.
Host interface looks at all packets on the ethernet. It passes a packet on to the host if the address in the packet matches its physical address or the broadcast address. Some host interfaces can also recognize several multicast addresses, and pass packets with those addresses on to the host.
How do vendors avoid ethernet physical address clashes? Buy blocks of addresses from a central authority.
Packet (frame) format.
- Preamble. 64 bits of alternating 1 and 0, to synchronize receivers.
- Destination address. 48 bits.
- Source address. 48 bits.
- Packet type. 16 bits. Helps OS route packets.
- Data. 368-120000 bits.
- CRC. 32 bits.
Ethernet frames are self-identifying. Can just look at frame and know what to do with it. Can multiplex multiple protocols on same machine and network without problems. CRC lets machine identify corrupted packets.
Token-ring networks. Alternative to ethernet style networks. Arrange network in a ring, and pass a token around that lets machine transmit. Message flows around network until reaches destination. Some problems: long latency, token regeneration.
ARPANET. Ancestor of current Internet. Long-haul packet-switched network. Consisted of about 50 C30 and C300 BBN computers in US and Europe connected by long-haul leased data lines. All computers are dedicated packet-switching machines (PSNs).
Interesting fact: ARPANET, like highway system, was initially a DOD project set up officially for defense purposes.
In original ARPANET, each computer connected to ARPANET connected directly to a PSN. Each packet contained address of destination machine and PSN network routed the packet to that machine. Now this is totally impractical and have a much more complex local structure before get onto Internet.
Design of Internet driven by several factors.
- Will have multiple networks. Different vendors compete, plus have different technical tradeoffs for local area, wide area and long haul networks.
- People want universal interconnection.
Will have multiple networks around the world. An internetwork, or internet, connects the different networks. So, job of internet is to route packets between networks.
One goal of internet: Network transparency. Want to have a universal space of machine identifiers and refer to all machines on the internet using this universal space of machine identifiers. Do not want to impose a specific interconnection topology or hardware structure.
Internet architecture. Connect two networks using a gateway machine. The job of the gateway is to route packets from one network to another.
As network topologies become more complicated, gateways must understand how to route data through intermediate networks to reach final destination on a remote network.
In Internet, gateways provide all interconnections between physical networks. All gateways route packets based on the network that the destination is on.
Internet addressing. Each host on the Internet has a unique 32-bit address that is used for all Internet traffic to that host. Each internet address is a (netid, hostid) pair. The network identifies the network that the host is on, the hostid identifies the host within the network.
Three classes of Internet addresses:
- Class A. First Bit: 0. Bits 1-7: Netid. Bits 8-31: Hostid. Can have 128 Class A networks.
- Class B. Bits 0-1: 10. Bits 2-15: Netid. Bits 16-31: Hostid. Can have 16,384 Class B networks.
- Class C. Bits 0-2: 110. Bits 3-23: Netid. Bits 24-31: Hostid. Can have 2 Gig Class C networks.
- Class D. (multicast addresses). Bits 0-3: 1110. Used for Internet multicast.
- Class E. Bits 0-3 1111. Reserved.
See RFC 990 for spec.
Interesting point. Whole structure of internet is available in RFC's (request for comments). Available over the Internet - use the net search functionality for RFC and you'll find pointers. Can read them to figure out what is going on.
Gateways can extract network portion of address quickly. Gateways have two responsibilies:
- Route packets based on network id to a gateway connected to that network.
- If they are connected to destination network, make sure the packet gets delivered to correct machine on that network.
Conceptually, an Internet address identifies a host. Exceptions: gateways have multiple internet addresses, at least one per network that they are connected to.
Because network id is encoded in Internet address, a machine's internet address must change if it switches networks.
Dotted Decimal notation: Reading Internet addresses. Four decimal integers, with each integer representing one byte.
- cs.stanford.edu - 36.8.0.47 (what kind of network is it on).
- cs.ucsb.edu - 128.111.41.20
- ecrc.de - 141.1.1.1
- lcs.mit.edu - 18.26.0.36
- sri.org - 199.88.22.5
Who assigns internet addresses? The Network Information Center! A centralized authority. It just allocates network ids, leaving requesting authority to allocate host ids.
Do example on page 45.
Mapping Internet addresses to Physical Network addresses. Will discuss case when physical network is an Ethernet. Given a 32 bit Internet address, gateway must map to a 48 bit Ethernet address. Uses Address Resolution Protocol (ARP).
Gateway broadcasts a packet containing the Internet address of the machine that it wants to send the packet to. When machine receives packet, it sends back a response containing its physical address. Gateway uses physical address to send packet directly to machine.
Also works for machines on same network even when they are not gateways.
Use a address resolution cache to eliminate ARP traffic.
ARP request and response frames have specific type fields. An ARP request has a type field of 0806, responses have 8035. Standard set up by the Ethernet standard authority.
How does a machine find out its Internet address? Store it on disk, and looks there to find out when it boots up. What if it is diskless? Contacts server and finds it out there using Reverse ARP (RARP). RFC 903 - Ross Finlayson, etc.
RARP request is broadcasted to all machines on network. RARP server looks at physical address of requestor and sends it a RARP response containing the internet address. Usually have a primary RARP server to avoid excessive traffic.
Now switch to talking about IP - the Internet Protocol. The internet conceptually has three kinds of services layered on top of each other: Connectionless, unreliable packet delivery service, reliable transport service, and application services. IP is the lowest level - the packet delivery.
The basic unit of transfer in the Internet is the IP datagram. IP datagram has header and data. Header contains internet addresses and the Internet routes IP datagrams based on Internet addresses in header.
Internet makes a best effort attempt to deliver each datagram, but does not deal with error cases. In particular, can have:
- Lost Packets
- Duplicated Packets
- Out of order Packets
Higher level software layered on top of IP deals with these conditions.
IP packets always travel from gateway to gateway across physical networks. If the IP packet is larger than the physical network frame size, the IP packet will be fragmented: chopped up into multiple physical packets. IP is designed to deal with this situation and provides for fragmentation.
Once a packet has been fragmented, must be reassembled back into a complete packet. Usually reassembled only when fragments reach final destination. But, could build a system that reassembled fragments when got to a physical network with a larger frame size.
Why is there a need for possibility of fragmentation? No good way to impose a uniform packet size on all networks. Some networks may support large packets for performance, while others can only route small packets. Should not prevent some networks from using large packets just because there exists a network somewhere in the world that can not handle large packets. But must be able to route large packets through a network that only handles small packets - network transparency.
Important fields in IP header:
- VERS: protocol version.
- LEN: length of header, in 32-bit words.
- TOTAL LEN: total length of IP packet.
- SOURCE IP ADDRESS: IP address of source machine.
- DEST IP ADDRESS: IP address of destination machine.
- TTL: time to live. How many hops the packet may take without getting removed from Internet. Every time a gateway forwards the packet, it decrements this field. Required to deal with things like cycles in routing, etc.
- IDENT: packet indentifier. Unique for each source. Typically, source maintains a global counter it increments for every IP datagram sent.
- FLAGS: A do not fragment flag (dangerous) and a more fragments flag - 0 marks end of datagram.
- FRAGMENT OFFSET - gives offset of this fragment in original datagram.
How to reassemble a fragmented packet? Allocate a buffer for each packet. Use IDENT and SOURCE IP ADDRESS to identify the original datagram to which the fragment belongs. Use the FRAGMENT OFFSET field to write each fragment into correct spot in the buffer. Use more fragments flag to find end of original datagram. Use some mechanism to make sure all fragments arrived before consider datagram complete.
Routing IP datagrams. There are multiple possible paths between hosts in an internet. How to decide which path for which datagram?
Routing for hosts on same network. Realize that are on same network by looking a netid field of Internet address, and just use underlying physical network.
Routing for hosts on different networks. Gateways pass datagrams from network to network until reach a gateway connected to destination network.
Each gateway must decide next gateway to send datagram to.
- Source routing. The source specifies the route in the datagram. Useful for debugging and other cases in which Internet should be forced to use a certain route.
- Host-specific routes. Can specify a specific route for each host. Used mostly for debugging.
- Table driven routing. Each gateway has a table indexed by destination network id. Each table entry tells where to send datagrams destined for that network. Do example on page 82.
- Default routes. Specify a default next gateway to be used if other routing algorithms don't give a route.
Most routers use a combination of table driven routing and default routing. They know how to route some packets, and pass others along to a default router. Eventually, all defaults point to a router that knows how to route ALL packets.
How are routing tables acquired and maintained? There are a lot of different protocols, but the basic idea is that the gateways send messages back and forth advertising routes. Each advertisement says that a specific network is reachable via N hops. Some protocols also include information about the different hops. The gateways use the route advertisements to build routing tables.
Internet was originally designed to survive military attacks. It has lots of physical redundancy and its routing algorithm is very dynamic and resilient to change. If a link goes away, the network should be able to route around the failure and still deliver packets. So, routing tables change in response to changes in the network.
In practice doesn't always work as well as designed. Chief threat to Internet links these days is backhoes, not bombs. Common error is routing all of the links that are supposed to give physical redundancy in the same fiber run, so are vulnerable to one backhoe.
In original internet, partition gateways into two groups. Core and noncore gateways. Core gateways have complete information about routes. Original core gateways used a protocol called GGP (Gateway to Gateway Protocol) to update routing tables.
GGP messages allow gateways to exchange pairs of messages. Each message advertise that the sender can reach a given network N in D hops. Receiver compares its current route to the new route through the sender, and updates its tables to use the new route if it is better.
Famous case: Harvard gateway bug. Memory fault caused it to advertise a 0 hop route to everybody!
Problem with GGP - distributed shortest path algorithm may take a long time to converge.
Later algorithm (SPF) replicated a complete database of network topology in every gateway. Gateway runs a local shortest path computation to build its tables.
In current Internet, there is no longer any central backbone or authority. Instead, have internet providers. The whole system has switched over to private enterprise.
A top-down view of system. There are 4 Network Access Providers. Each NAP is a very fast router connected via high-capacity lines to other gateways and NAPs. Lines may be T3 (644 Mb/s) lines. Typically big communications companies (MCI, Sprint, ATT) own the lines. Lines are typically fiber.
Organizations go to internet providers to get access to the internet. An internet provider buys a bunch of routers (usually from Cisco) and leases a bunch of lines. The internet provider must also buy access to a NAP or to a gateway that leads to a NAP. The routers talk a route advertisement protocol and implement some routing algorithm.
The internet provider can then turn around and sell internet access to whoever wants to buy it. UCSB buys its internet access from CERFNET, and it pays $23,000 per year for its internet access. All of the UC schools will band together and buy internet access from MCI, getting more bandwidth but at a higher price.
Check out http://www.cerf.net to see an Internet topology.
Organizations tend to chop their communications up into multiple networks, so there are too many networks in the world to give every network an Internet address. For example, the UCSB CS department has more than 10 networks.
The solution is subnetting. Internet views whole organization as having one network. The organization itself chops the host part of IP address up into a pair of local network and local host. For example, UCSB has one class B Internet network. The third byte of every IP address identifies a local network, and the fourth byte is the host on that network.
All IP packets from outside come to one UCSB gateway (by default). As far as the Internet is concerned, all of UCSB has only one network.
Inside UCSB, there is a set of networks connected by routers. These routers interpret the IP address as containing a local network identifier and a host on that network, and route the packet within the UCSB domain. The routers periodically advertise routes using a protocol called RIP.
This is an example of hierarchical routing. Internet routes to UCSB gateway based on Internet network id, then routers within UCSB route based on the subnet id.

traceroute command tells you the gateways packets go through to get to a given location. Here are a few:

cheetah > traceroute minnie (CSIL Lab, UCSB)
traceroute to minnie (128.111.42.17), 30 hops max, 40 byte packets
 1  toons (128.111.49.2)  17 ms *  3 ms
 2  minnie (128.111.42.17)  3 
cheetah > traceroute ecrc.de (Munich, Germany)
traceroute to ecrc.de (141.1.1.1), 30 hops max, 40 byte packets
 1  lo-galaxy (128.111.49.1)  6 ms  3 ms  3 ms
 2  ecigw1-41 (128.111.41.1)  4 ms  12 ms  4 ms
 3  cerfgw (128.111.254.201)  6 ms  12 ms  7 ms
 4  uclagw.cerf.net (134.24.107.104)  24 ms  22 ms  22 ms
 5  sdsc-ucla.cerf.net (134.24.101.100)  44 ms  263 ms  116 ms
 6  nynap-sdsc-atm-ds3.cerf.net (134.24.17.200)  141 ms  111 ms  126 ms
 7  sprintl.sprint.ep.net (192.157.69.9)  124 ms  147 ms *
 8  sl-pen-1-F0/0.sprintlink.net (144.228.60.1)  170 ms  114 ms  122 ms
 9  sl-dc-6-H2/0-T3.sprintlink.net (144.228.10.33)  123 ms  122 ms  132 ms
10  icm-dc-2b-F1/0.icp.net (144.228.20.103)  126 ms  212 ms  119 ms
11  icm-dc-1-F0/0.icp.net (198.67.131.36)  122 ms  214 ms  156 ms
12  icm-ecrc-1-S0-1984k.icp.net (198.67.129.18)  218 ms  209 ms  223 ms
13  ECRC-RBS.ECRC.DE (193.23.5.97)  280 ms  536 ms  315 ms
14  ECRC-GW.ECRC.DE (192.109.251.254)  297 ms  215 ms  236 ms
15  ecrc.de (141.1.1.1)  219 ms *  343 ms
cheetah > traceroute rain.org (Santa Barbara, CA)
traceroute to rain.org (198.68.144.2), 30 hops max, 40 byte packets
 1  lo-galaxy (128.111.49.1)  7 ms  3 ms  3 ms
 2  ecigw1-41 (128.111.41.1)  5 ms  5 ms  5 ms
 3  cerfgw (128.111.254.201)  7 ms  6 ms  6 ms
 4  uclagw.cerf.net (134.24.107.104)  57 ms  40 ms  22 ms
 5  sdsc-ucla.cerf.net (134.24.101.100)  44 ms  54 ms  57 ms
 6  ucop-sdsc.cerf.net (134.24.52.112)  84 ms  62 ms  61 ms
 7  sl-ana-3-S2/6-T1.sprintlink.net (144.228.73.81)  85 ms  81 ms  75 ms
 8  sl-ana-1-F0/0.sprintlink.net (144.228.70.1)  162 ms  139 ms *
 9  sl-fw-6-H2/0-T3.sprintlink.net (144.228.10.29)  125 ms  138 ms  158 ms
10  sl-fw-3-F0/0.sprintlink.net (144.228.30.3)  141 ms  184 ms  121 ms
11  sl-rain-network-1-S0-T1.sprintlink.net (144.228.171.2)  175 ms  165 ms  153 ms
12  coyote.rain.org (198.68.144.2)  149 ms  192 ms  184 ms
cheetah > traceroute cs.orst.edu (Corvallis, Oregon)
traceroute to cs.orst.edu (128.193.32.1), 30 hops max, 40 byte packets
 1  lo-galaxy (128.111.49.1)  7 ms  3 ms  3 ms
 2  ecigw1-41 (128.111.41.1)  5 ms  10 ms  10 ms
 3  cerfgw (128.111.254.201)  8 ms  9 ms  8 ms
 4  * uclagw.cerf.net (134.24.107.104)  37 ms *
 5  uci-la-smds.cerf.net (134.24.95.1)  38 ms  33 ms  30 ms
 6  * * ucop-sf-ds3-smds.cerf.net (134.24.9.112)  64 ms
 7  border3-hssi1-0.SanFrancisco.mci.net (149.20.64.9)  70 ms  71 ms  51 ms
 8  core-fddi-0.SanFrancisco.mci.net (204.70.2.161)  75 ms *  103 ms
 9  core-hssi-2.Seattle.mci.net (204.70.1.49)  74 ms  79 ms  81 ms
10  border1-fddi0-0.Seattle.mci.net (204.70.2.146)  87 ms  72 ms  142 ms
11  nwnet.Seattle.mci.net (204.70.52.6)  68 ms  272 ms  238 ms
12  seabr1-gw.nwnet.net (192.147.179.5)  225 ms  103 ms  75 ms
13  seattle1-gw.nwnet.net (198.104.194.195)  84 ms *  162 ms
14  portland1-gw.nwnet.net (192.80.12.81)  102 ms  137 ms  161 ms
15  osu-gw.nwnet.net (198.104.196.121)  186 ms  151 ms  202 ms
16  orst3-gw.ORST.EDU (192.147.167.1)  127 ms *  91 ms
17  ece-gw-out.ece.ORST.EDU (128.193.8.40)  86 ms *  95 ms
18  CS.ORST.EDU (128.193.32.1)  100 ms  89 ms *

Permission is granted to copy and distribute this material for educational purposes only, provided that the following credit line is included: "Operating Systems Lecture Notes, Copyright 1997 Martin C. Rinard." Permission is granted to alter and distribute this material provided that the following credit line is included: "Adapted from Operating Systems Lecture Notes, Copyright 1997 Martin C. Rinard."

Martin Rinard, rinard@lcs.mit.edu, www.cag.lcs.mit.edu/~rinard
8/22/1998

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