Networks prio1

Question 1

What is stop-and-wait ARQ?

Answer 1

Stop and wait are when a sender sends a packet and then waits for an ACK from reciever. The window size for receiver is 1.

Question 2

What is TCP congestion window?

Answer 2

Congestion window is a window which have a size of x number of bytes. This is the allowed maximimum of data that a sender can send.

Question 3

Explain how ping adminstration utility works.

Answer 3

Ping administration utility is a utility that uses the ICMP protocol. It sends an ICMP packet to a destination specified by an IP adress.

Question 4

What is the key feature of the routing within virtual circuit networks?

Answer 4

The key feature of the routing within virtual circuit networks is that it reservers a path for communication.

Question 5

What is the key feature of routing within datagram networks?

Answer 5

In datagram networks, a packet being sent can take any available path to a given destination. The packets a numbered so when packets arrive they can be re-arrange to the correct order.

Question 6

Is it possible that congestion window exceeds network bandwidth-delay product and why?

Answer 6

Yes, it is possible for the congestion window to exceed the network bandwidth-delay product. This can happen if the network has a high bandwidth and a low delay, or if the congestion window is set too high. (The network bandwidth-delay product is the maximum amount of data that can be in transit at any given time.)

Question 7

What is the relationship between inter-arrival times of acks at the sender and capacity of bottleneck link?

Answer 7

If the bottleneck link has a higher capacity, the sender will receive acks more quickly and the inter-arrival times will be shorter. (intervals between the arrival of successive packets at a given node)

Question 8

What is an asymmetric key cryptography and an symmetric key cryptography?

Answer 8

The symmetric key is a key that is used to encrypt and decrypt. For asymmetric one key encrypts and the other decrypts.

Question 9

What is the ALOHA protocol?

Answer 9

ALOHA is a protocol that is used to avoid channel allocation problem, a channel that is used by more than just one station. It works on the principle that if there is anything to send, send it.
There are two variants of ALOHA. Pure ALOHA and Slotted ALOHA.
slotted ALOHA instead relyies on “slots” where the sender has to wait when a slot is available. This is synchronized by a master clock.

Question 10

What is TCP congestion control?

Answer 10

TCP congestion control avoids routers buffers overflow. It automatically adjusts the transmission of packets so that the network links are fully utilized while limiting congestion losses. Normally it uses packet losses as a congestion signal. That said, it is a combined responsibility of the network and transport layer for congestion control but is ultimately caused by traffic sent into the network layer by the transport layer. Congestion control is when the transport layer sends traffic to the network layer more slowly. Here different algorithm is used to solve this. The main one that TCP uses is the AIMD algorithm.

Question 11

What is TCP congestion window?

Answer 11

TCP congestion window is a set size of x amount of bytes that the sender can send. This is used to avoid congestion.

Question 12

What is http and https?

Answer 12

Hypertext transfer protocol used to transport information between web servers and clients. Normally runs over TCP.
HTTPS is more secure as it uses TLS encryption.

Question 13

what is RSA?

Answer 13

Is a symmetric-key algorithm.
D(E(P)) = P
Difficult to deduce D from E
E cannot be broken by a chosen plaintext attack.

Question 14

What is TCP flow control?

Answer 14

Makes sure that the source does not generate data faster than the destination is able to absorb.

Question 15

What is AIMD?

Answer 15

AIMD Additive Increase, Multiplicative decrease is a congestion algorithm that TCP uses. AIMD combines linear growth of the congestion window when there is no congestion and then with an exponential reduction when congestion is detected.

Question 16

What is an “ACK clock” in TCP?

Answer 16

An ACK clock is part of TCP congestion control. ACKs will here return to the sender at about the same rate that the packets can be send over the slowest link in the path. With this TCP smoothens out traffic and avoid unnecassary queues at routers.

Question 17

What is slow start in TCP AIMD?

Answer 17

Slow start is TCP starts sending a few segments and would then for each segment that the sender receives an ACK for, It would add one more segment of bytes to the congestion window.

Question 18

What is Additive increase in TCP AIMD?

Answer 18

When slow start threshold is crossed, TCP will switch to this. Here the congestion window is increase by one segment every round trip time.

Question 19

What is TCP Tahoe?

Answer 19

Besides the AIMD algorithm with a slow start, Tahoe uses Fast Retransmit.
Here 3 Acks assumes a lost packet. The window is increased every ACK and after a certain threshold, it is increased by one segment every RTT.

Question 20

What is TCP Reno?

Answer 20

Is an improved version of TCP Tahoe. This one uses a faster retransmit algorithm.

Question 21

What is Jitter?

Answer 21

Jitter is the variation in time delay(latency) between when a signal is transmitted and when it’s received over a network connection, measuring the variability in ping. Jitter is often caused by congestion

Question 22

What is a “Tracker”?

Answer 22

A server that leads peers to the content of the torrent.

Question 23

What is throughput?

Answer 23

Is the amount of data moved from one place to another in a given time period. Typically measured in bits per second bps or Mbps, Gbps. For example, if a typical round trip time on the Internet is about 40ms, If the packets have 1,500 bytes then in Kbps this is 1500*8/0.04 = 300kbps.

Question 24

What is coderate?

Answer 24

Is the fraction of a codeword that carries information that is not redundant. m/n n=m+r

Question 25

What is Go Back N ARQ?

Answer 25

The basic version of TCP uses this. It uses a “transmit window” of size N. All N packets within this window are transmitted. If a packet isn’t receiving an ACK all packages within this window are retransmitted.

Question 26

What is Go Back N Selective Acknowledgment ARQ?

Answer 26

Compare to Go Back N, with Selective Acknowledgments the receiver sends a positive ACK for all the packets it has received correctly. nack

Question 27

What is CSMA?

Answer 27

CSMA (Carrier sense multiple access) is an improvement of ALOHA. CSMA ensures no station begins to transmit while the channel is busy.

Question 28

Describe CSMA/CD.

Answer 28

CSMA with collision detection is An algorithm that classic ethernet uses within its MAC sublayer. Station sense the medium when they have a frame to send and send the frame as soon as the medium becomes idle. They monitior the channel for collisions as they send. If there is a collision they abort the transmission and retransmits after a random interval.

Question 29

Describe CSMA/CA

Answer 29

CSMA collision avoidance is used in wifi 802.11 within its MAC sublayer protocol. Is similar to Ethernet’s CSMA/CD but here when a station has a frame to send it starts with a random “backoff”. The number of chosen slots to backoff could be 0-15. The station waits until the channel is idle and then counts down idle slots, sending when the counter reaches 0. If no ACK has been received the sender doubles the backoff period and tries again.

Question 30

What is QoS?

Answer 30

QoS stands for Quality of Service.
In the network layer, a stream of packets from source to destination, is called a flow. This flow have different quality of service which is characterized by four parameters bandwidth, delay, jitter and loss.

Question 31

What is LLC?

Answer 31

Is Logical Link control protocol which together with the MAC sublayer compose the data link layer of the OSI model. While the MAC sublayer handles the transfer of data this layer instead figures out what to do with an incoming frame.

Question 32

Explain the phases a TCP connection goes through.

Answer 32

TCP goes through “setup, send data and close”. C connection is established with a “three-way handshake”, SYN, SYN.ACK, then first package is sent with the ACK set. Data is exchanged (with data and acks); half-close(sender)(with FIN and FIN.ACK); then a second half-close(receiver)(with FIN, timed wait, and FIN.ACK).

Question 33

What is the hidden node/terminal problem?

Answer 33

The hidden terminal/node problem refers to the situation where two sending devices cannot hear each other (i.e they are hidden from each other), and hence one of them can begin transmission even if the other device is already transmitting.