1.5 Protocol Layers and their service models

Question 1

Define protocol stack.

Answer 1

When taken together, the protocols of the various layers are called the protocol stack.

Question 2

What does the Internet protocol stack consist of?

Answer 2

The internet protocol stack consists of five layers:

• physical
• link
• network
• transport
• application

Question 3

Application layer.

Answer 3

Is where network apps and their app-layer protocols reside.

An app-layer protocol is distributed over many systems, with the application in one end system using the protocol to exchange packets of information (messages) with the app. in another end system.

Question 4

Mention app-layer protocols.

Answer 4

• HTTP
• SMTP
• FTP
• DNS

Question 5

Transport Layer

Answer 5

It transports app-layer msgs between application endpoints.

Transport layer packet: segment.

Question 6

Transport protocols.

Answer 6

• TCP

- UDP

Question 7

Link layer

Answer 7

To move the packet from one node (host or router) to the next node in the route, the network layer relies on the services of the link layer. In particular, at each node, the network layer passes the datagram down to the link layer, which delivers the datagram to the next node along the route. At this next one, the link-layer passes the datagram up to the network layer.

Examples: Ethernet, WiFi, DOCSIS

Link-layer packets: frames.

Question 8

Physical layer

Answer 8

Its role is to move the individual bits from one node to the next.

Question 9

The OSI model.

Answer 9

1970s: ISO proposed that computer nets. be organized around 7 layers called the Open Systems Communication (OSI) model.

The functionality of 5 of these 7 layers is roughly the same as their similarly named Internet counterparts.

Question 10

What are the layers of the OSI model?

Answer 10

• application
• presentation
• session
• transport
• network
• data
• physical

Question 11

Describe the two remaining layers.

Answer 11

• Presentation layer: provides services that allow communicating applications to interpret the meaning of data exchanged.
  These services include data compression and encryption, as well as data description.
• session layer: provides for delimiting and synchronization of data exchange, including the means to build a checkpointing and recovery scheme.

Question 12

Encapsulation

Answer 12

At the sending host, an app-layer msg is passed to the transport layer. In the simplest case, the transport layer takes the msg and appends transport - layer header info that will be used by the receiver side transport layer.

App layer msg + transport layer header = transport layer segment.

The transport layer then passes the segment to the net. layer, which adds net.-layer header info. (such as source and destination end system addresses) creating a network layer datagram.

The datagram is then passed to the link layer, which will add its own link-layer header info. and create a link-layer frame.

At each layer, a packet has two types of fields: header fields and a payload field.