transport layer

Question 1

goal of the transport layer

Answer 1

provide logical communication between application processes running on different hosts

Question 2

what happens on sender side

Answer 2

breaks app messages into segments and passes to network layer

Question 3

what happens on receiver side

Answer 3

reassembles segmnets into messages and passes to app layer

Question 4

explain multiplexing at sender

Answer 4

handle data from multiple sockets and add transport header

Question 5

explain what is meant by demultiplexing at receiver

Answer 5

use header to deliver received segments to correct socket

Question 6

how does demultiplexing work?

Answer 6

• host receives IP datagram - each has src ip and dest ip address, each contains trans layer segments, each segment contains dest port no
• host uses IP address and port no to direct segment to appropriate socket

Question 7

Explain how connectionless demultiplexing happens

Answer 7

• socket created at local port
• when host receives UDP segment: checks dest port, directs udp segment to socket with same port n #
• IP datagrams with same dest port # directed to same sockets

Question 8

Explain how connection-orientated demux happens

Answer 8

TCP socket identified by-tuple: src ip & port, dest ip and & port
- receiver uses all 4 values to direct to correct socket
server hose may support many simultaneous TCP sockets

Question 9

TCP allows full duplex data - what does this mean

Answer 9

Bi-directional data flow in same connection and MSS (max segment size)

Question 10

what are sequence numbers

Answer 10

byte stream number of first byte in segments data

Question 11

what are acknowledgements

Answer 11

seq# of next byte expected from other side - cumulative ack

Question 12

will 2 way handshake always work?

Answer 12

variable delays
retransmitted messages
message loss
message reordering
cant see other side

Question 13

how can a two-way handshake fail?

Answer 13

1.) retransmitted connection request is delayed, client disconnects because it ack for inital message, server receives retransmitted connection request - establishes another connection with the client (who is disconnected)

Question 14

Explain a three-way hand shake

Answer 14

c - choose initializing seq number x - send tcp syn messahe to establish connection (synbit = 1, seq = x)
s - choose initializing seq # = y, send synAck to client
- synbit = 1, seq = y, ackBit = 1, Ack# = x+1
c - send ack for synack to server (may also contain data)
- Ackbit = 1, Ack# = y+1

Question 15

Explain how closing a tcp connection happens

Answer 15

c - close client socket (can no longer send but can still receive) - FinBit = 1, seq = x
s - can still send data to client - Ack = 1, Ack# = x+1 - client waits for server to close
s - Finbit - 1 seq = y – can now no longer send data but can still receive
c - acknowledges server close - Ackbit = 1, ack# = y+1

Question 16

how do we disambiguate the rtt for tcp

Answer 16

use time stamps and echo

Question 17

What to consider when setting the TCP timeout value?

Answer 17

Longer than RTT
if too short - premature timeout - unecessary retransmissions
too long - slow reaction to segment loss

Question 18

Estimated RTT formula

Answer 18

(1-a)estimatedRTT + asampleRTT

a is usually 0.125

Question 19

what is the timeout interval

Answer 19

estimatedRTT + safety margin … large variation in RTT = larger safety margin

Question 20

Timeout interval calculation

Answer 20

Deviation = (1-b)average of previous devs + b|samplertt-estimatedrtt|, typically b = 0.25
Timeout interval = EstimatedRTT + 4*DevRTT

Question 21

Define flow control

Answer 21

receiver controls sender so that sender doesn’t overflow receiver’s buffer by transmitting too much too fast

Question 22

Explain how flow control works

Answer 22

receiver advertises free buffer space by including a receiver window (rwnd) value in tcp header of receiver-sender segments
sender limits amount of unacked data to receivers rwnd value
guarantees that buffer will not overflow

Question 23

what is congestion

Answer 23

too many sources sending too much data too fast for network to handle

Question 24

manifestations of congestion

Answer 24

lost packets - buffer overflow at routers

long delays - queuing in router buffers

Question 25

What are two broad approaches to congestion control?

Answer 25

end-to-end congestion control

network assisted congestion control

Question 26

Explain network assisted cong control

Answer 26

routers provide feedback to end systems
single bit indicating congestion
explicit rate for sender to send at

Question 27

Explain end-to-end cong control

Answer 27

no explicit feedback from network
congestion inferred from end-system observed loss/delay
approach taken by tcp

Question 28

What does TCP use for cong control?

Answer 28

additive increase multiplicative decrease

Question 29

explain additive increase and multiplicative decrease

Answer 29

sender increases transmission rate (window size) probing for usable bandwith until loss occurs

• add increase - increase sending window by 1 mss every RTT until loss
• mult decrease - decrease sending window (cwnd) by half everyloss

Question 30

TCP sending rate formula

Answer 30

rate = cwnd/rtt bytes per second (sent and not acked cannot exceed cwnd)