Networking: Introduction, Lecture notes of Network Technologies and TCP/IP

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CS 4410

Operating Systems

Networking: Introduction

Summer 2016

Cornell University

Today

• The role of OS in computer networking.
• To appreciate this role we give a summary of

the networking principles.

Communication

● People save data, information in computers.

● It was a logical consequence the realization of the communication

between the computers.

● Example: Scientists who exchange data.

● Today: Internet, Email, VoIP, Web, Social network, ...

● But how do the computers exchange data?

Computer Network

● First, computers should be interconnected with

each other, forming a network.

● Depending on the interconnection means, we

have:

● Wired networks

● Wireless networks

● Depending on the size of the network, we have:

● Local Area networks (LANs)

● Wide Area networks (WANs)

Protocol Stack: Case 1

● Suppose:

● We have a reliable, small, wired computer network (LAN). ● The interconnected computers run only one network application. ● The network application is a simple email program, with which a user writes a text and sends it to another interconnected computer.

● Questions:

● How is the data going to be transmitted?

• Probably into packets. ● How can we name the computer that should receive the email?
• Each packet reaches every interconnected computer.
• Only the receiver should read the packet.
• We can assign static ids to computers. ● How do we incorporate this information into the actual data (text)?
• We add a header in-front of each packet (destination, packet id, size).

Protocol Stack: Case 1

● So:

● We have the level of the application, in which the text is created. ● We need a 2nd^ level, which takes the text and the destination, splits the text into packets and adds a header to each packet. ● Finally, we need a 3rd^ layer that will take each packet (in bytes) and will convert it into an electromagnetic signal. ● The electromagnetic signal is transmitted to the common interconnection means. ● The receiver-computer (3rd^ layer) gets the signal and converts it into packet (sequence of bytes). ● The 3rd^ layer passes the packet to the 2nd^ layer. ● The 2nd^ layer reads the header and verifies that this computer is the receiver. ● Then, it removes the header and saves the rest of the packet in a buffer. ● When the data is received, the 2nd^ layer passes the buffer to the application layer. ● The application layer reads the data and prints it as a text to the user.

Protocol Stack: Case 2

● Suppose:

● We have a reliable, small, wired computer network (LAN).

● The interconnected computers run only one network application.

● The network application is a simple email program, with which a user writes a text and sends it to another interconnected computer.

● Questions:

● How will we distinguish the different network applications?

• We should assign an identity number (port) to each application.

● How are we going to deal with packet loss and reordering?

• We can take an action and correct the mistake or we can afford it.

Protocol Stack: Case 2

● So:

● Between the Application and the Link Layer, we add the Transport Layer.

● The Transport Layer splits the text into packets, if need be, and at every packet adds a header.

● The header has at least the port of the corresponding application (email) at the receiver.

● If we want to retransmit lost packets or keep the correct order of received packets at the receiver, the Transport Layer of the sender writes the needed information in the header.

● Then, the Transport Layer forward the packets to the Link Layer.

● The remaining steps are the same as these of the Case 1.

● The Transport Layer of the receiver keeps internal states to correct possible packet lost or reordering. It, also, reassembles the text and forwards it to the application.

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Protocol Stack: Case 3

● Suppose:

● We have a reliable, small, wide wired computer network (WAN). ● The interconnected computers run only one network application. ● The network application is a simple email program, with which a user writes a text and sends it to another interconnected computer.

● Questions:

● How will the computers be connected with each other?

• One common interconnection means is not enough.
• We should define subnetworks of computers.
• The subnetworks will be connected with each other using specific devices (routers). ● How can we identify the computer that should receive the email?
• We should give to the computer a second name (address).
• The name should help the process to find the destination.
• It should be an indication about the subnetwork that contains the destination. ● How can the message find the destination?
• Every packet should ask the routers which direction it should take to reach the destination. (Routing)

Protocol Stack: Case 3

● So:

● We add between the Transport and the Link Layer, the Network Layer.

● The Network Layer takes the packet from the Transport Layer and adds a new header.

● The header contains at least the address of the destination.

● The Network Layer forwards the new packet to the Link Layer.

● When the packet reaches the interconnection means of the subnetwork, a specific device will see that the packet's destination is in a different subnetwork and it will send it to the next router.

● After packet being routed through the network, it reaches the subnetwork of the destination.

● Using the id that exists in the header of the Link Layer, the packet reaches the destination.

Today

• The role of OS in computer networking.
• To appreciate this role we give a summary of

the networking principles.

Coming up…

• Next lecture: Physical and link layer
• HW4 is due today