Transport Layer

Question 1

What is “logical communication”?

Answer 1

From an application’s perspective, it is as if the hosts running the processes were directly connected; in reality, the hosts may be on opposite sides of the planet, connected via numerous routers and a side range of link types.

Question 2

Between which layers does the transport layer reside?

Answer 2

The transport layer is below the application layer and above the network layer.

Question 3

How does a household mail example serve as an analogy for explaining how the transport layer relates to the network layer?

Answer 3

application messages: letters in envelopes
processes: cousins
hosts: houses
transport-layer protocol: Ann and Bill
network-layer protocol: postal service

Question 4

What are the two transport layer protocols of the Internet?

Answer 4

UDP and TCP.

Question 5

What is demultiplexing?

Answer 5

At the receiving end, the transport layer examines the fields in the segment to identify the receiving socket and then directs the segment to that socket.

Question 6

What is multiplexing?

Answer 6

The job of gathering data chunks at the source host from different sockets, encapsulating each data chunk with header information to create segments, and passing segments to the network layer.

Question 7

What are the segment fields that help with multiplexing/demultiplexing for UDP?

Answer 7

source port number and destination port number.

Question 8

What are the segment fields that help with multiplexing and demultiplexing for TCP

Answer 8

Source IP address, source port number, destination IP address and destination port number

Question 9

What is a connection establishment request?

Answer 9

A connection establishment request is nothing more than a TCP segment with destination port number and a special connection-establishment bit set in the TCP header.

Question 10

Why choose UDP over TCP if it isn’t reliable?

Answer 10

TCP is considerably slower than UDP because of its handshaking process and also its congestion control. If UDP is selected the application can be designed to add functionality to UDP’s no-frills delivery service. UDP also has a smaller packet header size (8 bytes vs TCP’s 20)

Question 11

Why not choose UDP?

Answer 11

UDP’s lack of congestion control is a large issue. Lack of congestion control in UDP can result in high loss rates between a USP sender and receiver, and the crowding out of TCP sessions

Question 12

Describe the UDP segment structure and each of its parts

Answer 12

Source port, destination port
Length - length of the segment including the header in bytes.
Checksum - a 16 bit number to which all 16 bit words are added. The sum should be 1111111111111111. If one of the bits is a zero we will know there has been an error.
Application data - data being sent

Question 13

What is the end-end principle?

Answer 13

Since certain functionality must be implemented on an end-end basis, functions placeced at the lower levels may be redundant or of little value when compaired to the cost of providing them at a higher level

Question 14

What are ARQ protocols?

Answer 14

The control messages that allow the receiver to let the sender know that the message has been received correctly and what has been received in error and thus required repeating.

Question 15

What are ACK and NAK

Answer 15

The positive ACK and negative NAK acknowledgment replies in the message dictation scenario are examples of feedback from the receiver to the sender`

Question 16

What is a stop-and-wait protocol?

Answer 16

A protocol in which the sender stops and waits for a response from the receiver before sending the next packet

Question 17

What is a sequence number?

Answer 17

A sequence number is a header field of TCP segments that allows sender and receivers to determine whether packets were received in retransmission or not.

Question 18

What is the utilization of the sender/

Answer 18

The reaction of time the sender is actually busy sending bits into the channel.

Usender = L/R / RTT + L/R

Question 19

What is pipelining?

Answer 19

Pipelining is when the sender is allowed to send multiple packets before waiting for a response

Question 20

What are the consequences of pipelining for rdt’s?

Answer 20

• The range of sequence numbers must be increased
• the sender and receiver sides of the protocols may have to buffer more than one packet
• the rand of sequence numbers needed and the buffering requirements will depend on the manner in which a data transfer protocol responds to lost corrupted and overly delayed packets.

Question 21

What is a Go-Back-N protocol?

Answer 21

GBN is when the sender is allowed to transmit multiple packets without waiting for an ACK but is constrained to have no more than some max allowable number N of unacknowledged packets in the pipeline.

Question 22

What is a window size?

Answer 22

The range of permissible sequence numbers of transmitted but not yet acknowledged packets

Question 23

What is a cumulative acknowledgment?

Answer 23

an ack that indicates that all packets with a sequence number up to and including n have been correctly received at the receiver.

Question 24

What is a selective repeat protocol?

Answer 24

A selective repeat protocol is when the sender acknowledges all correctly received packets regardless of packet order.

Question 25

What are the pieces of a TCP segment?

Answer 25

Source port, destination port, sequence number, acknowledgment number, header length, flags, receive window, internet checksum, urgent data pointer, options and data

Question 26

What is the sequence number of a TCP segment?

Answer 26

The sequence number for a segment is the byte-stream number of the first byte in the segment

Question 27

What is a cumulative acknowledgment?

Answer 27

TCP uses cumulative acknowledgements and this TCP only acknowledges bytes up to the first missing byte in the stream

Question 28

How do we calculate EstimatedRTT?

Answer 28

Estimated RTT is a weighted average where the previous RTTs count more than the most recent ones. the formula is 0.875*Previously estimated RTT average + 0.125*most recently estimated RTT

Question 29

What does setting a timeout interval have to do with congestion control?

Answer 29

If there is an expiration of the timer, TCP will retransmit the segment and double the timer time. If the timer is restarted after either of data of ACK being received, it will derive the timeout interval from the Estimated RTT and DevRTT

Question 30

What is a duplicate ACK?

Answer 30

In TCP a duplicate ACK is sent instead of a NAK. The receiver will send a duplicate ACK when they receive a piece of data out of order and detect a gap in the data they have received. They will resend the ACK from the last piece of in-order data.

Question 31

How does TCP provide flow control?

Answer 31

TCP provides flow control by having the sender maintain a variable called the receive window. THe receive window is used to give the sender an idea of how much free buffer space is available at the receiver.

Question 32

What is the "offered load"

Answer 32

The rate at which the transport layer sends segments into the transport layer

Question 33

What are some of the costs of a congested network?

Answer 33

- Large queuing delays are experienced as the packet arrival rate nears the link capacity - The sender must perform retransmissions in order to compensate for dropped packets due to buffer overflow - unneeded retransmissions by the sender in the face of large delays may cause a router to use its bandwidth to forward unneeded copies of a packet - When a packet is dropped the transmission capacity that was used at each of the upstream links has been wasted.

Question 34

What is end-to-end congestion control?

Answer 34

In the end to end approach to congestion control, the network layer provides to support to the transport control layer. TCP segment loss is taken as an indication of congestion and the window size is decreased accordingly

Question 35

what is Network assisted congestion control?

Answer 35

In this approach, the network provides explicit feedback to the sender regarding congestion of the network.

Question 36

What is a choke packet?

Answer 36

A notification from the network router to the sender.

Question 37

What is a congestion window?

Answer 37

The congestion window is a variable that the sender operates. It imposes a constraint on the rate at which a TCP sender can send traffic into the network. Specifically the amount of unacknowledged data that can be sent by the sender is restricted by the congestion window

Question 38

Why is TCP said to be self clocking?

Answer 38

TCP is self clocking because it uses feedback in the form of timeouts or duplicate ACKs to adjust its congestion window.

Question 39

What are the three components of the congestion control algorithm?

Answer 39

- Slow start - Congestion avoidance - Fast recovery

Question 40

Describe slow-start

Answer 40

In the slow start stat the value of the congestion window begins at 1 MSS (maximum segment size) and increases by 1 MSS every time a transmitted segment is first acknowledged.

Question 41

Describe congestion avoidance.

Answer 41

On the entry to the congestion avoidance state, the value of the congestion window will be approx. half the value it was when congestion was last encountered. TCP increases the value of cwnd by a single MSS every RTT.

Question 42

Describe fast recovery/

Answer 42

In fast recovery, the value of cwnd is increased by 1 MSS for every duplicate ACK received. If the ACK arrives for the missing segment, TCP enters congestion avoidance, if there is a timeout, TCP enters slow-start.