Protocols

Question 1

What do protocols do?

Answer 1

determine the way of operation and define the procedure and rules that are governing

Question 2

What is a network protocol?

Answer 2

a predefined set of messaging rules and message structures that the communicating entities in a computer network must follow

Question 3

What is a protocol layer?

Answer 3

A group of related functions that are performed in a given level of a hierarchy of groups of related functions

Question 4

What is protocol layering?

Answer 4

The division of complex network protocols into simple network protocols

Question 5

How do protocols in different levels make use of each other?

Answer 5

Protocols in higher levels make use of protocols in lower levels

Question 6

What is a layered protocol stack?

Answer 6

A set of horizontal protocols together with the interactions between the different layers

Question 7

What is a vertical/monolithic protocol design? What does it avoid? When is it preferable?

Answer 7

An alternative protocol design that does not use protocol layering, and a single protocol instead governs everything

Overhead created by layering

Where resource management and optimisation are the main concerns

Question 8

What are the two most widely-known network protocol stacks?

Answer 8

Internet TCP/IP Model

Open Systems Interconnection (OSI) Reference Model

Question 9

How many layers does the Internet Model consist of? What are these layers ordered from highest to lowest?

Answer 9

Five Layers:

Application, Transport, Network, Link, Physical

Question 10

What does the Application Layer of the Internet Model do? What does it ensure if all applications have the same underlying protocol?

Answer 10

Provides message formats and a well-defined interface to applications/software running on end devices

That they all communicate to web servers using the same rules and message formats

Question 11

What are the packets of the Application Layer called? What are they? For example?

Answer 11

Messages

Structured data units exchanged between application programs across a network

HTTP Request Message:”Browser asks the server: ‘Please send me the homepage (index.html).

‘“HTTP Response Message:”Server replies to the browser: ‘Here is the homepage content.’”

SMTP Email Sending Message:”Email client says to the mail server: ‘I want to send an email from alice@example.com to bob@example.com.’

“SMTP Email Received Message:”Mail server says to the email client: ‘You have a new email from alice@example.com.’

FTP File Download Request:”User says to the FTP server: ‘I want to download the file report.pdf.’

“FTP File Download Response:”FTP server replies: ‘Here is the file report.pdf.’

DNS Query Message:”User’s computer asks the DNS server: ‘What is the IP address of www.example.com?’

“DNS Response Message:”DNS server replies: ‘The IP address of www.example.com is 192.0.2.1.’”

Question 12

What is the Transport Layer responsible for? What does it keep track of and what does it do with it?

Answer 12

Responsible for the transport of application layer messages between remote processes

Keeps track application data from several processes and packages it in transport layer packets

Question 13

What are the processes between which the transport layer ensures the transport of messages?

Answer 13

Programs or applications running on computers that communicate over a network

Question 14

What are Transport Layer packets called? Crucially, what do these include?

Answer 14

Segments

Labelling

Question 15

What are further responsibilities of the Transport Layer?

Answer 15

Reliable Data Transfer

Flow Control

Congestion control

Question 16

What is flow control?

Answer 16

Matching the speed between the sender and the receiver

Question 17

What is congestion control?

Answer 17

Keeping the network load below capacity

Question 18

What is the network layer responsible for? What is a network layer packet called?

Answer 18

End-to-end packet delivery between two devices that are attached to different (sub)networks

Datagram

Question 19

What is a network layer packet called? What is its payload?

Answer 19

Datagram

Transport layer segments

Question 20

What are the other functions of the network layer?

Answer 20

Logical Addressing

Routing

Error Handling

Quality of Service Control - capability of networks to provide different priority levels to different types of network traffic, ensuring that certain traffic receives preferential treatment over others

Question 21

What does a datagram pass through on its route?

Answer 21

Many routers and link between routers

Question 22

What is the primary responsibility of the data link layer? What are its other responsibilities? What are the two types of links?

Answer 22

Carrying network layer datagrams to the next node (either a router or the destination host) on the end-to-end route

Physical addressing, medium access control (MAC), link flow control, and error handling

Wired, Wireless

Question 23

What is the set of all devices interconnected via the link layer called?

Answer 23

Network/Subnet

Question 24

What is the primary responsibility of the physical layer?

Answer 24

The transmission of individual bits from one node to the next

Question 25

What are link layer packets called?

Answer 25

Frames

Question 26

What is the OSI Reference Model? What are its two additional layers?

Answer 26

An earlier model than the Internet Model that included two additional layers Presentation and Session Layers

Question 27

Where does the Presentation Layer come? What is its primary responsibility? What are further, possible responsibilities?

Answer 27

Directly under Application Layer Deals with differences in data representations of communcating entities by translating different character sets of hosts using different operating systems to each other Compression/Decompression of Data and Encryption/Decryption

Question 28

Where is the Session Layer situated? What is its primary responsibility?

Answer 28

Underneath the Presentation Layer Establishes, maintains, and closes communication sessions between remote processes

Question 29

What are the four primary principles of protocol layering?

Answer 29

1. Create a layer when a different abstraction is needed 2. Implement in each layer a well-defined service 3. Minimise information flow across interfaces 4. Choose services of the layer by accounting for internationally-standardised protocols

Question 30

What is a service in protocol layering? How does it differ from a protocol?

Answer 30

A set of primitive operations that a layer provides to the layer above it Defines what operations are performed, but not how they are performed (which is what protocols do)

Question 31

What is the purpose of a service interface?

Answer 31

Defining how to access the services of the lower layer

Question 32

What is data encapsulation in protocol layering? What layers is it implemented by?

Answer 32

The practice of packaging data from higher layers of the Internet model within specific headers and trailers as it progresses downward through the protocol stack All layers besides Physical Layer (as this only deals with bits and not data units)

Question 33

What are the four sources of delay in packet switched networks?

Answer 33

1. Nodal Processing Delay 2. Queueing Delay 3. Transmission Delay 4. Propagation Delay

Question 34

What causes nodal processing delay? For example?

Answer 34

Caused by routers processing every packet they receive in one of their network interfaces Checking for bit errors inside the packet and determining the output link at which the packet needs to be transmitted

Question 35

What causes queueing delay? Why does it exist? What does it depend on?

Answer 35

Caused by packets having to wait in line in a packet queue at the selected outgoing link of the router to be transmitted Since packets are transmitted one by one Network packet traffic

Question 36

What is throughput? What must it be greater than?

Answer 36

The rate (bits/second) at which the bits that are pumpted into the network by the sender reach the destination The minimum processing requirements of a given server

Question 37

What is transmission delay? What does it depend on? What is the transmission delay of L bits at throughput R bps?

Answer 37

Caused by the store and forward behaviour employed by routers Throughput of the link L/R seconds

Question 38

What is propagation delay? What is it equal to? When is it especially significant?

Answer 38

The time it takes for a single bit to take the distance across a wire or wireless medium Distance divided by propagation speed For very long links

Question 39

What is total nodal delay equal to?

Answer 39

The sum of the nodal processing delay, queueing delay of the node, transmission delay onto the next link, and the propagation delay experienced on that link

Question 40

What is end-to-end network packet delay? What is it equal to?

Answer 40

The time difference from the transmission instance of the first bit of a packet from the source until the reception of the last bit by the destination The sum of all four delay components for all nodes passed through before reaching the destination

Question 41

When does data packet loss occur? For example?

Answer 41

If a packet arrives to a full queue at a router, as the packet must be discarded If total delay between source and destination is too high, this may also cause situations which effectively translate to data loss - a video frame which takes longer to pass then its playback time

Question 42

What is a bottleneck link?

Answer 42

A unique link on an end-to-end path that constrains max throughput That is, a segment of a network path where the capacity (bandwidth) is significantly lower than other segments along the same path