Final Exam Review

1
Final Exam Review

• CS460

2
Review ?

• What were the major hurdles to overcome in the
  development of Ethernet?
• users kept passing out from the ether fumes until
  a reliable method of sealing the cables was
  found.
• Which network is prefered by construction
  workers?
• Arcnet - welders seem to prefer this type of
  network
• Who developed Token Ring?
• J.R.R. Token, the celebrated "Lord of the Ring."
• Contributed by Jeremy Stanley and
  http//www.etext.org/Zines/ASCII/RAH/rah9301.txt

3
Questions

• What is the difference between Leakey Bucket and
  Token Bucket and how are they used?

4
Questions

• Suppose hosts A and B are on an Ethernet LAN with
  class C IP network address 200.0.0.0. It is
  desired to attach host C to the network, describe
  how to do this with subnets.

Host B routing table
C
net subnet mask
interface 200.0.0 0/000 0000 1000 0000
Ethernet 200.0.0 1000 00/00 1111 1100 direct link
A
B
A200.0.0.5, B200.0.0.6, C200.0.0.130
5
Questions

• Assume you wish to transfer an n-byte file along
  a path composed of the source, destination, seven
  point-to-point links, and five switches. Suppose
  each link has a propagation delay of 2ms,
  bandwidth of 4Mbps, and that the switches support
  both circuit and packet switching. Thus, you can
  either break the file up into 1KB packets, or set
  up a circuit through the switches and send the
  file as one contiguous bit stream. Suppose that
  packets have 24 bytes of packet header
  information and 1000 bytes of payload, that
  store-and-forward packet processing at each
  switch incurs a 1-ms delay at each switch after
  the message has been completely received, that
  packets may be sent continuously without waiting
  for acknowledgements, and that circuit setup
  requires a 1-KB message to make one round trip on
  the path incurring a 1-ms delay at each switch
  after the message has been completely received.
  Assume switches introduce no delay to data
  traversing a circuit. You may also assume that
  file size is a multiple of 1000 bytes.

6
Questions

• Could you use ARP to translate between a human
  readable name and an IP address? Why or Why not?
• Could you use DNS to translate between an IP
  address and an Ethernet address? Why or Why not?

7
Question

• The silly window syndrome denotes the situation
  when the sender has lots of data to send but
  sends it in tiny increments as the window slides
  forward in time increments.
• Why would this happen?
• How would you fix it?

8
Questions

• Describe the use of RTP, RTCP, SDP, SIP, H.323 in
  IP telephony
• What is the purpose of a SIP proxy?
• Give 4 examples of overlay networks and the
  services they provide.
• Show how an overlay network can decrease overall
  bandwidth consumption for multicast when compared
  to point-to-point connections
• How does the triangle inequality apply to overlay
  networks?

9

• Describe the major differences between Ipv4 and
  IPv6. How will the transition between V4 and V6
  occur?
• Tunnel
• Discuss ASN/BER encoding efficiency. What changes
  might be made to improve this efficiency?
  Contrast XDR(no tags) and ASN.
• Calculate how much time it would take to make
  1000 calls to a method using each technique.
• Describe the steps involved in calling procedure
  0x55FB4 in program 0x55346 using SUN RPC.
• The Port Mapper (at port 111) procedure 3
  converts program number to Port number, Connect
  to the returned port and pass the procedure
  number to make the call.

10

• What must TCP do to guarantee reliable ordered
  delivery on top of an unreliable protocol(IP)?
• What impact on performance(buffering)?

11

• Given a graph similar to 6.11, identify the
  slow-start, linear increase and timeout periods
  as well as the Congestion Window and Congestion
  Threshold.  Indicate the changes that fast
  retransmit, TCP Vegas or RED would have on the
  graph.

12

• Assuming that you are using the version of TCP
  with window sizes much larger than 64kb. Suppose
  that you are using this extended TCP over a 1Gbps
  link with a RTT of 100ms to transfer a 10MB file,
  and the TCP receive window is 1MB. If TCP sends
  1KB segments, and assuming no congestion and no
  lost packets How many RTTs does it take until
  the sender's congestion window reaches 1MB?
  (Recall that the congestion window is initialized
  to the size of a single packet) How many RTTs
  does it take to send the file? If the time to
  send the file is given by the number of required
  RTTs times the link latency, what is the
  effective throughput for the transfer? What
  percentage of the link bandwidth is utilized?

13

• Starting from a window of 1 packet, 1KB, 2KB,
  4KB, 8KB, 16KB, 32KB, 64KB, 128KB, 256KB, 512KB,
  1MB 10RTT
• In this time, we have sent 124816326412825
  6512
• 1023KB of data, we still have 9MB left at 1MB per
  RTT
• Total time 19RTT1900ms1.9sec
• Effective Throughput10MB/1.9sec 5.3MBps 44Mbps

14

• What are the two main problems related to
  scalable routing on the internet and what
  techniques are used to combat these problems?
• Table Size subnetting, AS
• Router Communication Hierarchy
• Assume that you are using TCP (20 bytes header)
  over IPv6 to send a 1MB file. The file contains
  formatted data, so you are sending it 8 bytes at
  a time using xPush to flush unsent bytes after
  each send. If you are running over a 100Mbps
  line, how long will it take to send the file?
  What is your effective throughput?
• TCP 20ByteIP40Byte HeaderEth18Byte Header8
  Bytes data86Bytes Total per message
• ((1MB/8)86)8bits/byte/100Mbps902ms, 9.3Mbps

15

• What is DNS and what are the steps used to
  resolve the name alma.cs.byu.edu from the machine
  foo.ibm.com?
• Detail the SMTP protocol.  How reliable is the
  "MAIL FROM" field in determining the real source
  of an email message?
• What is the difference between flow control and
  congestion control and what mechanisms are used
  to implement each scheme?
• Why would you want to use UDP instead of TCP? Why
  would TCP be better?
• What are the differences in setting up a
  connection
• What are the differences between
  receiver-makes-right and canonical-intermediate-fo
  rm and why would you want to use each of these
  methods?
• Calculate throughput

16

• Use Run Length Encoding and Differential Pulse
  Code Modulation to encode the string
  AAAAAAAAAAAABBBAAAAAAABCBABC. Which scheme
  results in the highest compression? Assume that
  you can represent numbers with a given Huffman
  code and that characters take 8 bits.
• Describe three methods by which JPEG reduces the
  amount of data to be transmitted.
• Why does MPEG result in higher compression ratios
  than JPEG?  Given the DCT, quantization table,
  and Huffman code, show how a macroblock would be
  compressed using MPEG.
• What is motion compensation and what effect does
  it have on compression?

17
Example

• Assume a 3x3 Macroblock after the DCT
• Given the JPEG Quantization table
• The Quantized frequency coefficients
• DCT(1,0)250
• ?250/15.5? ?17.16?17
• RLE(42,5,17,2,0,0,0,0,0)1-42,1-5,1-17,1-2,5-0
• Now do Huffman encoding with symbols 1,42,5,17,2,
  0

18
Huffman Encoding

• Huffman encoding allows you to encode data with
  the minimal number of bits
• We need to encode the RLE 1-42,1-5,1-17,1-2,5-0
• There are 10 occurrences of symbols
• 1 occurs 4/1040 of the time
• 42 occurs 1/1010
• 5 occurs 2/1020
• 17 occurs 1/1010
• 2 occurs 1/1010
• 0 occurs 1/1010

19
Algorithm

• Given a alphabet A, construct a huffman encoding
  tree T Create one node for each character of
  the alphabet A. Let each node have a field
  containing called its weight which is the
  probability of that letter appearing in a
  message. Now repeadly perform the following
  two steps    1) pick two nodes n1 and n2 that
  have the smallest weights    2) and replace them
  with a new node whose children are n1 and n2 and
  whose weight is the sum of the weights of n1 and
  n2. Each time we perform this step will replace
  two nodes in the alphabet with one, until
  eventually only one single node remains as this
  is the root of the tree T.

20
Building Huffman Tree
1-42,1-5,1-17,1-2,5-01 0010 1 01 1 0011 1 0000
01 0001 1001 0101 1001 1100 0001 00010x959c11
W4
I5
0
1
1
I4
W6
1
0
W4
5
I3
0
1
I1
I2
W2
W2
0
0
1
1
2
0
42
17
21

• What is the difference between DES and RSA
  encryption? Which scheme has provable bounds on
  the time necessary to break the encryption? Which
  scheme is public key vs secret key. How is RSA
  used in conjunction with MD5 in Keyed MD5? In
  which circumstances would you choose RSA, DES or
  MD5? Which services would you implement with each
  scheme?
• Given a small p and q, compute the public and
  private keys and USE RSA to encrypt and decrypt a
  message (something similar to page 579).
• Describe PEM message encryption and show the
  detailed steps of the algorithm.
• Random k, encrypt with DES, Encrypt k with
  receivers public, encode message in ascii,
• Convert from ascii, decrypt k with receivers
  private, decrypt with DES
• Describe a email system that would provide both
  privacy and authentication

22

• What are the advantages and disadvantages of
  DECbit vs RED in congestion control?
• Is DECbit adoption probable?
• How do RED gateways signal congestion? Why is
  this scheme preferred to the normal TCP
  congestion control scheme?
• How do the loss characteristics vary
• Compare a filtering firewall to a proxy firewall
  in terms of protection against spoofing attacks
  (exercise 5.17).
• Proxy negotiates sequence number so attacker can
  not just forge the source IP address. A
  filtering firewall does not actually negotiate
  with the source.
• Contrast RIP, OSPF and BGP. Why are routers
  susceptible to attacks by hackers? What kinds of
  damage can occur in such an attack? What part the
  source routing requirement play in making attacks
  easier? How does the CIDR technique allow for
  more efficient use of IP address space?
• Source Routing allows the attacker to specify the
  route back to himself

23

• Assume that there is a maximum latency of 50ms
  for each bridge in the internetwork. What is the
  maximum time between when a IPv4 packet is sent
  and when it is received given that the maximum
  latency between bridges is 180ms (assume we have
  to go through a satellite on every link).
• What limits the number of hops?
• What are the steps to setting up a UDP
  connection?
• What about Accept, Connect calls? Why are they
  not needed?
• Describe the details of the DHCP protocol.  What
  are some of the reasons for DHCP?
• Select a random link-local address and use ARP to
  make sure it is not in use
• DHCPDISCOVER to broadcast address, response with
  IP address allows client to create UDP connection
  to server to get IP address

24

• What is ICMP and how is it used with traceroute?
• TTL increasing
• Given a graph, show how a routing loop could
  occur with Distance Vector Routing. 
• Describe the Split-Horizon and Split-Horizon with
  Poison Reverse heuristics used by RIP. Explain
  how they address the problem of routing loops.

25

• Show how an LSP is floooded through a network. 
  At step 3, which nodes will be communicating,
  given a graph.

26

• Use Dijkstra's algorithm to compute the shortest
  path given a graph.

B
3
5
C
A
2
D
27

• How do the metrics differ for the original
  version of ARPANET and the second version of
  ARPANET?
• Number of packets queued, delay, oscillations
• What are the SYN and ACK flags used for in TCP? 
  What is a SYN/ACK denial of service attack and
  how would it disturb the operation of the TCP
  protocol?
• What is the Karn/Partridge Algorithm (RTT
  estimation) and the Jacobson/Karels Algorithm
  (Variance)?
• Would UDP or TCP be likely to achieve the highest
  bandwidth?  Why?
• What are the different approaches to fair queuing
  and which approaches are most effective and
  practical?

28

• Be able to compute Lempel Ziv compression for a
  set of data( http//www.data-compression.com/lossl
  ess.html)