CS352 review

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CS352 review
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Topics

• Fundamentals protocol layering
• Application Layer HTTP, HTML
• Reliable data transfer sliding window,
  go-back-N, selective repeat
• Transport protocols TCP
• Routing subnetting, CIDR routing, IP
  forwarding, ICMP
• Link layer ethernet, bit map protocol
• Security encryption (substitution,
  transposition)
• Queue management fair queuing
• Suggestions go over sample questions and old
  exams posted at my website

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Utilization, Efficiency

• Utilization, Efficiency
• Efficiency (resource used for real work)/(total
  resource)
• Midterm 2, Q8
• You have 64 workstations on a network (only one
  has data to send) and are trying to decide
  between a binary-countdown and a bit-map
  protocol. If frames are 128 bytes in size
  (including header), what is the efficiency of
  each protocol?
• Real work? transmitting data
• total resource ? resource used for transmitting
  both data bytes and meta information (the
  protocol header)
• what if multiple stations have data to send?

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Utilization, Efficiency

• Midterm 1, Q4
• For parts A and B, 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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Utilization, Efficiency

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

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Fundamentals protocol layering
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Application Layer HTTP, HTML
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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?

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TCP

• Sample question set 1 (tcp/udp), Q 4
• Consider a TCP connection in which Congestion
  threshold is 8 KB available buffer size at the
  receiver is 64KB and the current window is also
  8KB. 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 6KB? 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 6KB? Justify your
  work.

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Subnetting

• Midterm 1, Q 7
• Consider a conventional class B network. A
  network administrator decides to give all subnets
  in the class B network a sub-net mask of
  255.255.248.0.
• How many sub-nets can the administrator use if
  all sub-nets use this mask?
• How many hosts are possible on each sub-net?
• The administrator just heard that she only needs
  16 sub-nets for the class B address. What sub-net
  mask maximizes the number of hosts on each
  sub-net?

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IP-forwarding, CIDR

• Understand how to read a routing table
• Longest matching
• Subnet vs. host routing

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(No Transcript)
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ICMP

• Protocol for error detection and reporting
• tightly coupled with IP, unreliable
• ICMP messages delivered in IP packets
• ICMP functions
• Announce network errors
• Announce network congestion
• Assist trouble shooting
• Announce timeouts

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Traceroute

• Traceroute records the route that packets take
• A clever use of the TTL field
• When a router receives a packet, it decrements
  TTL
• If TTL0, it sends an ICMP time exceeded message
  back to the sender
• To determine the route, progressively increase
  TTL
• Every time an ICMP time exceeded message is
  received, record the senders (routers) address
• Repeat until the destination host is reached or
  an error message occurs

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Traceroute (contd)
Te Time exceeded Pu Port unreachable
R1
R2
R3
A
B
TTL1, Dest B, port invalid
Te (R1)
TTL2, Dest B
Te (R2)
Time
TTL3, Dest B
Te (R3)
TTL4, Dest B
Pu (B)
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Ethernet standards

• cable length and repeaters - know the limitations
  and WHY

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Transposition Encryption

• Instead of substituting letters in the plaintext,
  we change their order

A N D R E W 1 4 2 5 3 6 t h i s i s a m e s s a g
e i w o u l d l i k e t o e n c r y p t n o w
Key ANDREW Plaintext thisisamessageiwould
liketoencryptnow Ciphertext
tagltyieiletisokco
hmedopsswinnsauerw
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Fair Queuing

• Virtual Time
• Finish Time
• Scheduling

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Fair Queuing Summary

• On packet arrival
• Compute finish number of packet
• Re-compute virtual time
• Based on number of active queues at time of
  arrival
• On packet completion
• Select packet with lowest finish number to be
  output
• Recompute virtual time

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Virtual Time

• How to keep track of service delivered on each
  queue?
• Virtual Time is the number of rounds of queue
  service completed by a bit-by-bit Round Robin
  (RR) scheduler
• May not be an integer
• increases/decreases with of active queues

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Active Queues

• A queue is active if the largest finish number is
  greater than the current virtual time
• Notice the length of a RR round (set of queue
  services) in real time is proportional to the
  number of active connections
• Allows WFQ to take advantage of idle connections

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Approximate bit-bit RR

• Virtual time is incremented each time a bit is
  transmitted for all flows
• If we have 3 active flows, and transmit 3 bits,
  we increment virtual time by 1.
• If we had 4 flows, and transmit 2 bits, increment
  Vt by 0.5.

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Computing Finish Numbers

• Finish of an arriving packet is computed as the
  size of the packet in bits the greater of
• Finish of previous packet in the same queue
• Current virtual time

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Finish Number

• Define
• - finish of packet k of flow i (in virtual
  time)
• - length of packet k of flow I (bits)
• - Real to virtual time function
• Then The finish of packet k of flow i is

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A Fair Queuing Example

• 3 queues A, B, and C
• At real time 0, 3 packets
• Of size 1 on A, size 2 on B and C
• A packet of size 2 shows up at Real time 4 on
  queue A

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FQ Example

• The finish s for queues A, B and C are set to
  1, 2 and 2
• Virtual time runs at 1/3 real time

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FQ Example- cont.

• After 1 unit of service, each connection has
  received 10.33 0.33 units of service
• Packet from queue A departed at R(t)1
• Packet from queue C is transmitting (break tie
  randomly)

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FQ Example- cont.

• Between T1,3 there are 2 connections virtual
  time V(t) function increases by 0.5 per unit of
  real time

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FQ Example- cont.

• Between T3,4 only 1 connection virtual time
  increases by 1.0 per unit of real time

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Final Schedule
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Fair Queuing

• A fair queuing system solves the scheduling
  problem when several sub-queues compete for the
  same resource (processing)
• There may be multiple processing resources. In
  that case, we have multiple fair queuing system
  and they wont affect each other. For example,
  the multiple output ports in a router can all
  accept input packets and function in parallel.