CS352 Sample Questions

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CS352 Sample Questions
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Utilization, Efficiency

• Efficiency (resource used for real work)/(total
  resource)
• (time used for real work)/(total time)

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Utilization, Efficiency

• Assume host A is connected to B via a
  point-to-point duplex link which has a data rate
  of 1 Mbps and propagation delay of 10 msec.
• HOST A------------------------------HOST B
• Host A has a large amount of data that must be
  reliably sent to Host B. Assuming Stop-and-Wait
  protocol is used to assure reliable delivery, the
  data frame size is 1000 bytes each, and the ACK
  frame size is 64 bytes.
• Assuming that Host B takes 0 time to process an
  incoming data and send an ACK frame back to A,
  what is the smallest time the retransmission
  timer can be set to at Host A which eliminates
  unnecessary retransmissions?
• Assuming there are no transmission errors, what
  is the link utilization in the direction from A
  to B?
• A new, different channel has a bit rate of 4 Kbps
  and a propagation delay of 20 msec. For what
  range of frame sizes does stop-and-wait protocol
  give an efficiency of at least 50 percent?

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TCP
Two hosts communicating using TCP finish their
data exchange and issue the packets in the
following diagram to complete a symmetric
teardown of the connection
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TCP

• Consider transfering an enormous file of L bytes
  from host A to host B. Assume an MSS of 1,460
  bytes.
• What is the maximum value of L such that TCP
  sequence numbers are not exhausted? Recall that
  the TCP sequence number field has 4 bytes
• For the L you obtain above, find how long it
  takes to transmit the file. Assume that a total
  of 66 bytes of transport, network, and data-link
  header are added to each segment before the
  resulting packet is sent over a 10 Mbps link.
  Ignore flow control and congestion control so A
  can pump out the segments back to back and
  continuously.

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TCP

• Suppose that the TCP congestion window is set to
  64 KB and a timeout occurs. The receive (sliding)
  window is 100 KB and the maximum segment size is
  1 KB. Assume there is no linear mode.
• How big will the congestion window be after the
  next six transmission bursts allowed by the
  congestion window are all successful?
• Now the 7th transmission burst has a timeout. How
  big will the congestion window be after the next
  three transmission bursts are all successful ?

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TCP

• Consider a TCP connection in which Congestion
  threshold is 16 KB available buffer size at the
  receiver is 64KB and the current window is also
  16KB. Assume TCP segment size (MSS) 1KB.
• a) If a packet is dropped and is detected by
  timeout how long will it take the congestion
  window to go up to 16KB? Justify your work.
  Assume round-trip time (RTT) 100ms.
• b) If a packet is dropped and is detected by
  duplicate ACKs, how long will it take the
  congestion window to go up to 16KB? Justify your
  work.

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TCP

• The TCP slow start algorithm by itself is
  inefficient, so a threshold (the default value is
  64KByte) is established to improve throughput.
  Suppose the length of the link between host A and
  host B is 10,000 meters and its propagation delay
  is 20 msec/km. Host A is using TCP over this link
  to transfer a 4M byte file to host B. Assume that
  the TCP has been modified to allow very large
  windows, and so the TCP receiver window size is
  always set to 2M bytes. The segment/packet size
  is 1K bytes. Ignore the packet transmission time
  and handling time.
• A) Suppose no packet is lost, that is, no
  timeout. How many RTTs does it take to send the
  4MB file? Show all work for full credit.
• B) What is the effective throughput for the file
  in part (A)?
• C) Suppose a timeout happens on the 10th
  transmission and no other timeouts occur. How
  many RTTs does it take to send the 4MB file?
• D) What is the effective throughput for the file
  in part (C)?

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TCP

• Suppose you are browsing www.yahoo.com from your
  machine remus.rutgers.edu. If the round-trip time
  between yahoo and remus is 100ms timeout value
  is 200ms Maximum Segment size (MSS) of TCP is
  1KB the congestion window is 1 MSS the
  congestion threshold is 8KB the available buffer
  size is 64KB and transmission bandwidth is 200
  KB/sec.
• What is the transmission time for a TCP segment?
• How long will it take to download a page of 6KB
  from yahoo? Assume there is no prior TCP
  connection between yahoo and your machine, there
  is no packet loss or retransmission. Ignore
  transmission time for this part. Show all your
  work for credit.
• How will the result in (b) change if the 4th. TCP
  segment is dropped and the client gets to detect
  this loss by timeout?

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Sliding window

• A certain satellite channel has the following
  characteristics. The channel has a 200-ms
  propagation delay in either direction. The
  channel is full duplex with 0.5 Mbps raw
  bandwidth in both directions. A sliding window
  protocol for this channel uses 500-bit frames
  with a send window size of 100.
• Suppose, at time t 0, one endpoint of the
  channel starts to send a very large amount of
  data. At what time later does the sending
  station receive an ACK for the 200-th frame?