week 3 - More transport Layer

Question 1

Acknowledgement message does not mean you should free buffer

BUFFER ONLY FREED/ PROCESSED WHEN DATA PASSED TO APPLICATION LAYER

Answer 1

Really important

ACK (message) != freeing buffer

Question 2

What is Flow Control

Answer 2

“Flow control is a specific end-to-end mechanism between sender and receiver where the receiver advertises its available buffer space (using rwnd) to prevent the sender from transmitting more data than the receiver can process. This mechanism is implemented through the sliding window protocol.”

Question 3

what is congestion control

Answer 3

“Congestion control is a sender-side mechanism in TCP that prevents network congestion by dynamically adjusting the transmission rate based on network conditions. It uses the congestion window (cwnd) and various algorithms to detect and respond to network congestion, ensuring optimal network utilization without overwhelming network resources.”
Congestion Window (cwnd) Mechanism:
“The congestion window is a dynamic, sender-maintained limit on the amount of unacknowledged data that can be in transit. It starts small (typically 1 MSS) and adjusts using algorithms:

. slow start
.

Question 4

Question: What is the purpose of the sliding window in TCP?

Answer 4

The sliding window in TCP is an end-to-end flow control mechanism that allows the sender to transmit multiple segments up to the window size without waiting for an acknowledgment. It controls how much data can be in transit before requiring confirmation from the receiver, enabling continuous and efficient data transmission.

Question 5

Question: How does the sliding window mechanism contribute to flow control?

Answer 5

The sliding window mechanism helps manage the flow of data by ensuring that the sender only sends data that the receiver can handle based on its advertised window size. The receiver adjusts the window size to reflect its available buffer space, preventing data overflow and ensuring smooth data reception.

Question 6

How does the sliding window mechanism affect throughput?

Answer 6

The sliding window increases throughput by allowing the sender to send multiple segments in sequence without waiting for acknowledgments after each segment. This avoids the delays of a stop-and-wait approach, keeps the network “pipeline” full, and makes the data transmission more efficient.

Question 7

How does persist time help(take example where window size is 0 )

Answer 7

When Window Size = 0:

.Sender stops sending (receiver buffer full)
.Persist timer activates to prevent deadlock
.Timer expires → Small packet called “window probe” is sent
.Receiver must respond with current window size
.If still 0: Timer doubles, repeat probe (5s → 10s → 20s → max 60s)
.If non-zero: Normal transmission resumes

Question 8

throughput

Answer 8

rate at which data is processed

Fast throughput -> data processed quickly and buffer empties faster so data can be sent quicker

Question 9

bandwidth

Answer 9

capacity at which a network can transmit data

Question 10

How does TCP Congestion control work?
basic definiton

Answer 10

Increase the sending rate until packet loss is observed , at which point sending rate is decreased

Question 11

What is TCP Congestion control method called AIMD

Answer 11

AIMD

Question 12

Explain TCP Congestion control now in detail

Answer 12

Additive Increase:
Increase sending rate by 1 MSS (Maximum Segment Size) per RTT (Round trip time) until a loss is detected

AT WHICH POINT

|
Multiplacitve decrease occurs:
Sending rate is cut in half at each loss event

Question 13

Multiplacitve decrease in detail

Answer 13

.When 3 duplicate acknowledements are received sending rate is cut in half

. When timeout occurs sending rate cut to one MSS ( severe)

Question 14

what is ss thresh

Answer 14

half the value of the cwnd just before timeout

at this point you switch from slow start to additive increase

Question 15

explain the transiton from slow start to linear ( additive increase and is slow start actually slower than linear additive increase?

Answer 15

lets say you have just experienced a timeout and so cwnd cut to 1 MSS

We then begin slow start where we double the control widow until ss thresh ( half the value of cwnd before timeout)

at this point we switch from slow start to additive increase where we increased cwnd by 1 mss per rtt until packet loss detected ( either through a timeout - cut to 1 mss - or 3 duplicate acks we cut sending rate to half the control window and resume linear additive increase)

Question 16

RTT

Answer 16

Time for:
segment to go from segment to receiver AND acknowledgement from receiver to sender

Question 17

congestion window

Answer 17

sender side flow control that changes based on network conditions

Question 18

What is the web

Answer 18

A network of resources

Question 19

Are all resources network retrievable? Explain your answer

Answer 19

No not all resoures are network tetrievalbe

humans - can represent as a resource but cant download a human

same with company

abstract things - friendsship relationships , mathematical operations etc

Question 20

URI , URL , URN

Answer 20

URI - Parent ( can be URL or URN or other type

URI (parent category)
├── URL (tells how to access/where it is)
└── URN (just names/identifies something)

URL - (Uniform Resource Locator):
- shows how to access and where to find it
- START WITH HTTP
- https://www.google.com
- ftp://ftp.example.com/downloads/file.txt
- http://localhost:8080/index.html

URN- REMAINS PERMANENET
urn:isbn:0-486-27557-4

This ISBN will always identify the same book

Question 21

how does http message structure look like

Answer 21

start line
headers
empty line
body

Question 22

Concept: The amount of data a sender can transmit is determined by:

EffectiveWindow=min⁡(cwnd,rwnd)

where:

cwnd (Congestion Window): Controls data flow to avoid network congestion.
rwnd (Receiver Window): Indicates how much data the receiver can accept.

Answer 22

asdm;asm