Module 6: Transport Layer - TCP

Question 1

When is a TCP segment sent?

Answer 1

Segment sent when…
1. Segment is full (max segment size)
2. not full , but times out
3. “pushed” by application

Question 2

TCP segment

Answer 2

Question 3

Initial Sequence Number

Answer 3

Why not start with 0?
- IP addresses and port numbers uniquely id a connection
- eventually though these port numbers do get used again

Question 4

How do sender/receiver independently determine ISN?

Answer 4

3 way handshake
1. open connection: 3-way handshake
2. reliable byte stream transfer
3. close (tear down) connection

Question 5

open connection: 3-way handshake

Answer 5

Question 6

What if the SYN packet gets lost?

Answer 6

eventually no SYN-ACK arrives
- sender sets a timer and wait from SYN-ACK
- receiver sets a timer
- after timeout, resend

Question 7

close connection

Answer 7

4-ways tear down

Question 8

Why is stop and wait inefficient?

Answer 8

only one TCP segment is in flight at a time

Question 9

Sliding Window

Answer 9

Question 10

Go Back N (GBN)

Answer 10

transmit up to n unacknowledged packets
if timeout for ACK(k), k+1,…
not efficient if windows are large: high speed networks

Question 11

What is an improvement on GBN?

Answer 11

Selective Repeat
GBN forces sender to retransmit all packets in window even if some have been received correctly

to avoid, we need a “finer granularity”of acknowledgements (individual acks as opposed to cumulative acknowledgements)

sender: transmit up to n unacknowledged packets

Question 12

SR is great for _____

Answer 12

isolated loss
does not recover quickly from burst loss

Question 13

What does TCP use for sliding window?

Answer 13

hybrid of GBN and SR

Question 14

General observations about Sliding Windows

Answer 14

Question 15

TCP Flow Control

Answer 15

Question 16

Which of the following in the TCP header is especially important to flow control?

Answer 16

acknowledgement
advertised window
sequence number

Question 17

retransmission timeout

Answer 17

how to set timeout?
if too long: connection has low throughput
if too short: what if retransmit packet that was just delayed

solution: make timeout proportional to RTT

Question 18

RTT Estimation

Answer 18

Question 19

smaller alpha means slower reaction time. t/f

Answer 19

false

Question 20

karn/partridge algo

Answer 20

Question 21

The Karn/Partridge algorithm doubles the last timeout because if you are getting successive needs to timeout, it means there are losses in the network. T/F

Answer 21

True