Transport layer

Question 1

Where do transport protocols run?

Answer 1

End systems only

Question 2

send side

Answer 2

breaks application messages into segments, pass them to network layer

Question 3

receive side

Answer 3

reassembles segments into messages, passes to application layer

Question 4

Network layer vs. Transport layer

Answer 4

Network - logical communication between hosts

Transport - logical communication between processes
relies on network layer services

Question 5

6 functions

Answer 5

• connection management
• packetization and reassembly of app data
• error control
• reliability
• flow control
• congestion control

Question 6

transport layer error/ flow control vs. DLL error/ flow controll

Answer 6

• transport layer flow control and error control is end-to-end
• DLL flow control and error control is per-hop

Question 7

end-to-end flow control

Answer 7

• assumes unreliable delivery of frames by DLL
• works well when few errors b/t links (wastes bandwidth when there’re errors in first hop, b/c corrupted data is transmitted over following links)
• less overhead than per-hop for high quality links

Question 8

TCP socket identified by: (4 things)

Answer 8

• source IP
• source port
• dest IP
• dest port

Web servers have different sockets for each connecting client

Question 9

UDP checksum

Answer 9

• 1’s complement addition of segment contents

- receiver computes checksum of received value and compares with checksum field

Question 10

TCP stream delivery

Answer 10

• data from application is treated as a stream of bytes divided into segments (data boundaries ignored)
• each byte has a sequence #, only 1st sequence # of the segment is sent
• window sizes in bytes (instead of # of frames)

Question 11

PUSH flag

Answer 11

• forced delivery

- if your TCP module decides a segment is still too small to send (waiting for more bytes)

Question 12

Urgent flag

Answer 12

• urgently deliver a part of the data stream regardless of its position in the stream
• encapsulate urgent data into a segment with URG = 1

Urgent pointer in TCP header lets you specify where urgent data ends

Question 13

how are connections identified in TCP?

Answer 13

by a pair of endpoints (IP address, port) pair

Question 14

ISN

Answer 14

• Initial sequence #
• chosen based on current time

-should be unpredictable to defend against TCP sequence prediction attacks

Question 15

if SYN == 1, then

Answer 15

this is the initial sequence #

SYN and FIN are 1 byte in sequence # if set

Question 16

half closed

Answer 16

After A terminates connection, but B hasn’t yet

Data can only flow from B to A now. A HAS to keep receiving data from B and returning Acks (for what sequence # it expects next)

Officially closed when B sends last outgoing segment (FIN = 1) and gets an Ack back

Question 17

how does a sender perceive congestion?

Answer 17

a loss event (either a timeout OR 3 duplicate ACKs)

Question 18

TCP AIMD

Answer 18

Additive increase / multiplicative decrease

Increase cwnd (congestion window size) by 1 MSS (maximum segment size) every RTT in the absence of loss events

Cut cwnd in half after loss event

Question 19

fast retransmission

Answer 19

-when network doesn’t seem congested, but a packet is lost (b/c duplicate Acks)

-retransmits lost segment IMMEDIATELY after 3rd duplicate Ack
(doesn’t wait for timeout)

Question 20

Fast recovery

Answer 20

after fast retransmission, cut value of cwnd (congestion window size) in half
(skip slow start phase)

Question 21

Congestion avoidance

Answer 21

predict when congestion is about to happen and then reduce rate at which hosts send data just before packets start getting discarded

Question 22

source based congestion avoidance

Answer 22

watch for some sign from network that some router’s queue is building up, and that congestion will happen soon

-like increase in RTT for successive packets

Question 23

Random early detection congestion avoidance mechanism

Answer 23

router is programmed to monitor its own queue length, and notifies source to adjust its congestion window when it detects congestion is imminent

Question 24

flow control

Answer 24

end-to-end issue

-preventing senders from over-running the capacity of receivers

(flow control also used for receivers sending window size to sender)

Question 25

congestion control

Answer 25

preventing too much data form being injected into network

Question 26

TCP Fairness

Answer 26

if K TCP sessions share same bottleneck link of bandwidth R, each should have avg rate of R/K

Question 27

when Cwnd is above ssthresh, sender is in ______ phase.

Answer 27

congestion avoidance phase

window grows linearly

Question 28

when a timeout occurs, ssthresh is set to _____ and Cwnd is set to _____

Answer 28

ssthresh = Cwnd / 2

Cwnd = 1 MSS (maximum segment size)