Subject area

Transport Layer

22 questions · 9 topics

2013 Q2(a)(i)Transport LayerGo-Back-N· 2 marks

Reliable transport protocols must have at least a unique sequence number for each segment in the window. However, the protocol used can have additional requirements on the minimum unique sequence numbers beyond this requirement. What is the minimum number of unique sequence numbers required for the Go Back N protocol?

GBNwindowsequence numbers

Open question →

2013 Q2(a)(ii)Transport LayerSelective Repeat· 2 marks

Reliable transport protocols must have at least a unique sequence number for each segment in the window. However, the protocol used can have additional requirements on the minimum unique sequence numbers beyond this requirement. What is the minimum number of unique sequence numbers required for the Selective Repeat protocol?

SRwindowsequence numbers

Open question →

2013 Q2(a)(iii)Transport LayerReliable Transport· 4 marks

Reliable transport protocols must have at least a unique sequence number for each segment in the window. However, the protocol used can have additional requirements on the minimum unique sequence numbers beyond this requirement. What is the consequence if these minimum requirements are not met? Give an explanation of precisely why or how they will fail. You may use an example of a failure case or an explanation, but must provide either a general explanation that covers both selective repeat and go back n or a failure case for each of the protocols.

GBNSRsequence numbers

Open question →

2013 Q2(b)Transport LayerBandwidth-Delay Product· 4 marks

The measured average round trip delay on a connection is 50 msec. If the link has a bandwidth of 1Gb/sec, what is the minimum number of Bytes that should be available in the window so that the transfer of the data does not become stop and wait?

TCP windowthroughput

Open question →

2013 Q2(c)Transport LayerCongestion Control· 4 marks

Why does TCP change congestion control behaviour at a threshold value? Specifically what is the significance of the threshold value that makes it the logical point to change from exponential growth to linear growth?

TCPssthreshAIMD

Open question →

2013 Q2(d)Transport LayerFlow Control· 4 marks

What, other than congestion control, limits the sending rate to TCP? Explain how this other limit works.

TCP windowreceive window

Open question →

2013 Q2(e)Transport LayerTCP Connection· 4 marks

TCP’s 4-way handshake used to terminate connections does not guarantee that both sides of the connection will close. Explain why this is and what TCP does to try to maximise the chance that both sides of the connection close once a termination request is sent. You may use an example or diagram.

TCPFINTIME_WAIT

Open question →

2014 Q2(a)Transport LayerMultiplexing / Demultiplexing· 3 marks

Explain the purpose of multiplexing and demultiplexing in the transport layer. How do TCP and UDP provide multiplexing/demultiplexing?

Open question →

2014 Q2(b)(i)Transport LayerCongestion Control· 3 marks

Controlling congestion in a network can be critical to performance. If a network does not have any congestion control, explain what conditions will cause congestion to occur. Be specific. "too much traffic" is not enough detail. You need to explain your answer in terms of router and link resources.

congestionqueuespacket loss

Open question →

2014 Q2(b)(ii)Transport LayerCongestion Control· 6 marks

Controlling congestion in a network can be critical to performance. Explain in detail how TCP provides congestion control.

TCPAIMDslow start

Open question →

2014 Q2(c)Transport LayerRTT / Timeout· 4 marks

TCP uses an estimated RTT plus estimated deviation when determining the timeout value for segments. Why does TCP include the deviation value? What would be more likely to occur if the deviation value were not included?

TCPretransmission

Open question →

2014 Q2(d)(i)Transport LayerReliable Transport· 3 marks

We looked at three protocols for providing reliable transport: Alternating Bit, Go-Back-N and Selective-Repeat. Assume we have a delay-bandwidth product of 1MByte and a maximum segment size (MSS) of 1KByte. Assume the local memory available for buffering is not a limiting factor. Assuming the send window size has been set to the delay-bandwidth product (with the exception of alternating bit which has been set to the MSS), what would be size of the receiver buffer for each of the protocols?

windowbufferGBNSR

Open question →

2014 Q2(d)(ii)Transport LayerReliable Transport· 3 marks

We looked at three protocols for providing reliable transport: Alternating Bit, Go-Back-N and Selective-Repeat. Assume we have a delay-bandwidth product of 1MByte and a maximum segment size (MSS) of 1KByte. Assume the local memory available for buffering is not a limiting factor. Give one advantage for each of the three protocols

GBNSRstop-and-wait

Open question →

2014 Q2(e)Transport LayerFlow Control· 2 marks

TCP flow control relies on the receiver to indicate how much buffer space is available for data at the receiver. How does the receiver provide this information to the sender?

receive windowTCP window

Open question →

2015 Q2(a)Transport LayerMultiplexing / Demultiplexing· 2 marks

How does a web browser know the destination port to connect to reach a web server?

Open question →

2015 Q2(b)Transport LayerCongestion Control· 6 marks

Explain in detail how TCP provides congestion control.

TCPAIMDslow start

Open question →

2015 Q2(c)Transport LayerCongestion Control· 2 marks

Explain why it is reasonable to assume that receiving 3 duplicate ACKs in TCP is an indication that the network is not currently congested.

TCPfast retransmit

Open question →

2015 Q2(d)(i)Transport LayerReliable Transport· 3 marks

We looked at three protocols for providing reliable transport: Alternating Bit, Go-Back-N and Selective-Repeat. Which of these is likely to perform best in the presence of high error rates? Explain.

Open question →

2015 Q2(d)(ii)Transport LayerReliable Transport· 3 marks

We looked at three protocols for providing reliable transport: Alternating Bit, Go-Back-N and Selective-Repeat. What is the optimal size for the sending and receiving window for each of these protocols?

Open question →

2015 Q2(d)(iii)Transport LayerReliable Transport· 3 marks

We looked at three protocols for providing reliable transport: Alternating Bit, Go-Back-N and Selective-Repeat. Given a window size, W , what is the minimum sequence space required for each of these protocols?

sequence numbersGBNSR

Open question →

2015 Q2(e)Transport LayerFlow Control· 3 marks

A TCP sender finds out the available receive buffer space through the ‘receive window’ header field of packets/acknowledgments sent by the receiver to the sender. The flaw in this approach is that if the receiver has no space, it will send a receive window of 0. This indicates that the sender should not send any more data as there is no space in the receiver’s buffer. However, if the sender does not send data, the receiver may never send a packet to tell the sender when there is space. Assuming we do not want the receiver to gratuitously send packets to the sender just to tell it about window space (ie the receiver should only send packets to the sender if it either has data to send or if it is sending an acknowledgement), what can the sender do resolve this problem?

TCPreceive windowpersist timer

Open question →

2015 Q2(f)Transport LayerTCP Connection· 3 marks

Give an example of a case where a two-way handshake to establish a connection could leave one side of the connection live while the other side does not believe there is a connection.

TCPhandshakehalf-open

Open question →