CSCE 515: Computer Network Programming

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CSCE 515Computer Network Programming

• Chin-Tser Huang
• huangct_at_cse.sc.edu
• University of South Carolina

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Midterm Exam Grade

• Before adjustment
• Graduate Avg 10, Highest 12.5
• Undergrad Avg 12.4, Highest 18
• After adjustment
• Graduate 5 to everyone
• Undergrad 2 to everyone

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Final Exam Guide

• Problems similar to those in midterm exam
• Will have one more programming problem that asks
  you to debug a piece of code
• In each section, the one who makes most progress
  in final exam compared to midterm exam grade will
  get 2 bonus points!

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Bulk Data Flow

• With bulk of data to send, it is desired that
  sender is allowed to transmit multiple packets
  before it stops and waits for acknowledgment
• However, cannot keep sending without pause,
  otherwise receiver may run out of buffer
• Use sliding window protocol to control data flow

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Sliding Window Protocol

• Receiver advertises a window to notify sender how
  much data it can send
• Window closes as left edge moves to right
• When data is sent and acknowledged
• Window opens as right edge moves to right
• When receiving process reads acknowledged data
  and freeing up space in TCP receive buffer
• Window shrinks as right edge moves to left
  (discouraged)

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Sliding Window
offered window
(advertised by receiver)
usable window
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cant send until
sent, not ACKed
window moves
sent and
acknowledged
can send ASAP
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TCP Options

• A variable-length list of optional information
  for TCP segments
• Some options defined in TCP include
• End of option list
• No operation
• Maximum segment size
• Window scale factor
• Timestamp
• Selective acknowledgment

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TCP SACK Permitted and SACK Option

• When establishing a connection, either end can
  use SACK permitted option to let the other end
  know that it can accept SACK option
• When some segment is lost, can use SACK option to
  acknowledge received bytes to avoid redundant
  retransmission

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TCP SACK Option Examples

• Assume left window edge is 5000 and sender sends
  a burst of 8 segments, each containing 500 data
  bytes
• Case 1 first 4 segments are received but the
  last 4 are dropped
• receiver will return a normal TCP ACK segment
  acknowledging sequence number 7000, with no SACK
  option

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TCP SACK Option Examples

• Case 2 first segment is dropped but the
  remaining 7 are received
• Upon receiving each of the last seven packets,
  receiver will return a TCP ACK segment that
  acknowledges sequence number 5000 and contains a
  SACK option specifying one block of queued data
• Triggering Segment ACK Left Edge Right Edge
• 5000 (lost)
• 5500 5000 5500 6000
• 6000 5000 5500 6500
• 6500 5000 5500 7000
• 7000 5000 5500 7500
• 7500 5000 5500 8000
• 8000 5000 5500 8500
• 8500 5000 5500 9000

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TCP SACK Option Examples

• Case 3 2nd, 4th, 6th, and 8th (last) segments
  are dropped
• receiver ACKs the first packet normally, and the
  3rd, 5th, and 7th packets trigger SACK options
• Triggering Segment ACK 1st Block 2nd Block 3rd
  Block
• Left Right Left Right Left Right
• 5000 5500
• 5500 (lost)
• 6000 5500 6000 6500
• 6500 (lost)
• 7000 5500 7000 7500 6000 6500
• 7500 (lost)
• 8000 5500 8000 8500 7000 7500 6000 6500
• 8500 (lost)

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TCP PUSH Flag

• Sender uses PUSH flag to notify receiver to pass
  all data it has to the receiving process
• Some cases where PUSH is set
• Send buffer is emptied by sending this segment
• This segment is final data segment

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Urgent Mode

• One end notifies the other end that some urgent
  data is in data stream
• What action to take is up to receiver
• Two fields in TCP header needs to be set
• URG flag set to 1
• Urgent pointer set to a positive offset that is
  added to ISN to get seq of last byte of urgent
  data

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TCP Timeout and Retransmission

• TCP handles lost data segments and
  acknowledgments by setting a timeout when it
  sends data
• If data isnt acknowledged when timeout expires,
  TCP retransmits data
• Two implementation issues to consider
• How to determine timeout interval
• How frequently to retransmit data

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Round-Trip Time Measurement

• RTT changes over time due to route changes and
  network traffic changes
• TCP should track RTT changes and modify its
  timeout accordingly
• R ? ?R(1-?)M, where ?0.9
• RTO R?, where ?2

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Improved RTT Measurement

• Previous approach cant keep up with wide
  fluctuations in RTT and may cause unnecessary
  retransmissions
• Err M-A
• A ? AgErr, where g0.125
• D ? Dh(Err-D), where h0.25
• RTO A4D

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Karns Algorithm

• When receiving an ACK after retransmission, TCP
  cant tell this ACK is for original transmission
  or for retransmission
• Hence cant update RTT estimators using received
  ACK when a timeout and retransmission occur

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TCP Congestion Control

• Assume packet loss is largely due to congestion
• Two indications of packet loss timeout and
  receipt of duplicate ACKs
• When congestion occurs, slow down transmission
  rate, and gradually come back if congestion is
  relieved
• Use two algorithms
• Slow start
• Congestion avoidance

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Slow Start

• The rate at which new packets should be injected
  into network is the rate at which acknowledgments
  are returned
• Use a congestion window (cwnd) in senders TCP
• Initialized to one segment when new connection is
  established
• Increased by one segment each time an ACK is
  received until packet loss occurs exponential
  increase
• Congestion window is flow control by sender while
  advertised window is flow control by receiver

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Congestion Avoidance

• Use a slow start threshold (ssthresh)
• When receiving 3 dup ACKs, cwnd is cut in half,
  and window grows linearly
• 3 dup ACKs indicates network capable of
  delivering some segments
• When timeout occurs, cwnd instead set to 1 MSS,
  and window first grows exponentially until reach
  ssthresh, then grows linearly
• Timeout before 3 dup ACKs implies severe
  congestion

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Congestion Control Algorithm

• When cwnd is below ssthresh, sender in slow-start
  phase, window grows exponentially
• When cwnd is above ssthresh, sender is in
  congestion-avoidance phase, window grows linearly
• When a triple duplicate ACK occurs, ssthresh set
  to cwnd/2 and cwnd set to ssthresh
• When timeout occurs, ssthresh set to cwnd/2 and
  cwnd is set to 1 MSS

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Fast Retransmit and Fast Recovery

• Used when receiving 3 dup ACKs
• Receiving 3 dup ACKs means data can still flow,
  so no need to reduce rate abruptly
• Fast retransmit when receiving 3 dup ACKs,
  retransmit without waiting for retransmission
  timeout
• Fast recovery perform congestion avoidance, not
  slow start

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Repacketization

• When TCP times out and retransmits, it does not
  have to retransmit the identical segment again
• Instead, TCP is allowed to send a bigger segment
  (not exceeding MSS) to increase performance
• Because TCP identifies data by byte number, not
  segment number

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Next Class

• TCP persist and keepalive timers
• Other TCP options
• Read TI Ch. 22, 23, 24