Week 8

Question 1

What is TCP? What are its features?

Answer 1

Transmission Control Protocol

• Connection-oriented
• Reliable

Question 2

What is meant by TCP being connection oriented?

Answer 2

• A virtual circuit established between two devices
• To the application, connections appears to be a point-to-point full duplex connection
• Messages split into segments for transmission

Question 3

What is meant by TCP being reliable?

Answer 3

• Errors are detected and corrected

- Segments are re-assembled in the correct order

Question 4

Difference between simplex, duplex and half duplex?

Answer 4

Simplex = One device sends and one recieves
Duplex = Both devices send and recieve at the same time
Half duplex = Both devices can send and recieve but not at same time, have to wait turns recieving and sending

Question 5

Are TCP segments enveloped?

Answer 5

Yes, evnveloped in IP packets

Question 6

What use TCP?

Answer 6

HTTP
SMTP
IMAP
SSH

Question 7

Why can it be detrimental to send too many TCP segments at once?

Answer 7

May overload recieved or intermediate path with lower bandwidth

Question 8

Why should we transmit TCP segments?

Answer 8

• Sending too large segments requires IP to fragment

- Large segments also increase errors

Question 9

What are the two approaches to determine TCP segment size?

Answer 9

1. Use reasonable MTU

2. Use Path MTU Discovery (PMTUD)

Question 10

What is a ‘reasonable’ MTU?

Answer 10

This is accepting that sometimes IP needs to fragment.

Question 11

What is Path MTU Discovery?

Answer 11

• Sends IP packets to destinations, asking routers to NEVER fragment
• If router HAS to fragmentm, return ERROR
• Increase packet length until error occurs, use last known error-free MTU

Question 12

How to manage TCP congestion?

Answer 12

1. Start slow: Wait for ACK after each segment
2. Increase speed with each ACK: sending more segments each time
3. Fall back to slower speed when no ACK arrived (packet lost)

Question 13

What does the data link layer do to frames that have errors?

Answer 13

Discards frames with errors

Question 14

Why is TCP reliable?

Answer 14

Because of TCP ARQ (Automatic Repeat ReQuest)

• Echange ACKs letting sender know that packets where recieved correctly
• Sender re-transmits if no ACK recieved within certain time

Question 15

What numbers does a TCP packet include?

Answer 15

1. Sequence Number - How many bytes we’ve transmitted
2. Acknowledgement number - How many bytes reciever recieved from sender

IF seq != ack then recived incorrectly

Question 16

How does TCP establish a conenction?

Answer 16

Three way handshake:
- Client sends SYN packet with random sequence number A

• Reciever replies with SYN, ACK, random seq number B and acknowledgement number A+1
• Client sends ACK with seq number A+1 and ackowledgement number B+1
• Connection established

Question 17

What are NAKs? Are there any with TCP ARQ?

Answer 17

negative acknowledgment, No

Question 18

What is UDP?

Answer 18

User datagram protocol

Question 19

Features of UDP?

Answer 19

Connectionless:

• each packet send individually
• no virtual circuit (unlike TCP)
• no ack (less relaible)

Small header

• 8 bytes (TCP is 20 bytes(
• Less overhead
• Faster

Question 20

Why use UDP?

Answer 20

For applications that either send very small messages (DNS, DHCP etc.)
OR
Where loss of segments is acceptable e.g. video streaming

Question 21

What is quality of service?

Answer 21

This refers to the sound quality and availability of network and emergency contacts of VoIP (voice over ip)

Good quality has low latency (lag) and low jitter (rubberbanding/dropping voice)

Question 22

How to achieve quality of service?

Answer 22

Two strats:
- Integrated services: Applications request a channel with certain guarantess. Implemented by resource reservation protocol (RSVP) ON TOP OF UDP

• Differentiated services: Each individual packet requests a certain service class, which is given corresponding priority. Implemented using special feld in IP packet header.