Module 6.1: TCP Flow and Congestion Control

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Module 6.1 TCP Flow and Congestion Control

• Connection establishment Termination
• Flow Control
• Congestion Control
• QoS

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Connection Establishment
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Connection Termination
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Flow Control

• Some Points about TCPs Sliding Windows
• A sliding window is used to make transmission
  more efficient as well as to control the flow of
  data so that the destination does not become
  overwhelmed with data.
• It is called Selective Repeat ARQ.
• Built on top of go-back-n ARQ
• Accepts segments out of order, but error free
• Repeat transmission of only selective segments
• TCPs sliding windows are byte oriented. TCP is
  a stream-oriented protocol. Data is presented as
  streams of characters.
• The window size specifies the receivers current
  buffer size.
• The source does not have to send a full windows
  worth of data.
• The size of the window can be increased or
  decreased by the destination.
• The destination can send an acknowledgment at any
  time.

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TCP Sliding Window
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Corrupted Segment
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Lost Acknowledgement
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Silly Window Syndrome (SWS)

• SWS occurs when each acknowledgement advertises a
  small amount of space available and each segment
  carries a small amount of data.
• Avoidance
• Receive-Side SWS Avoidance Before sending an
  update window advertisement after advertising a
  zero window, wait for space to become available
  that is either at least 50 of the total buffer
  size or equal to a maximum sized segment.
• Implementation using Delayed Acknowledgements
• Ack every segment but with no increase in window
  size until sufficient size becomes available.
• Delay Acks and Ack one with window size after
  sufficient size is available. Standards
  recommend this.
• Advantage decrease in traffic and overhead.
• Disadvantage sender may re-transmit due to long
  delays.

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Timers

• Retransmission Timeout
• If an acknowledgment is received for this
  particular segment before the timer goes off,
  the timer is destroyed.
• If the timer goes off before the acknowledgment
  arrives, the segment is retransmitted and the
  timer is reset.
• Round Trip delay may vary as packets and acks
  travel through the Internet.
• Calculation
• To accommodate the varying delays encountered in
  an internet environment, TCP uses an adaptive
  transmission algorithm that monitors delays on
  each connection and adjusts its timeout
  parameters accordingly.
• RTT (? Old_RTT) ((1-?)New_Round_Trip_Sample
  ), 0lt ? lt1
• RTT ? RTT, ? recommended to be 2

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Timers

• Persistence Timer
• TCP uses a persistence timer for each connection.
  When the sending TCP receives an acknowledgment
  with a window size of zero, it starts a
  persistence timer.
• When the persistence timer goes off, the sending
  TCP sends a special segment called a probe.
• A probe queries the receiver periodically to find
  out if the window has increased. The value of
  the timer changes and increases exponentially.
• A probe contains only one byte of data. TCP is
  allowed to send 1 byte of data beyond the end of
  closed window.
• Keepalive Timer
• Used by the server to check if the client has
  crashed when the connection has been idle for a
  long time.
• A probe message is sent for query.

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Congestion Control

• Remember flow control is between Sender and
  Receiver, and Congestion Control takes care of
  congestion in the intermediate nodes (routers).
• To avoid congestion collapse (I.e., network
  throughput to be zero), receiver controls the
  window size.
• Allowed_Window Min(receiver_advertisement,
  congestion_window)
• Host can inject into the network as many segments
  as the receiver_advertisement (I.e. window size
  of the receiver) allows.
• Congestion_window is for controlling congestion
  at the intermediate router
• Initially congestion_window receiver MSS
• Congestion Avoidance is a combination of
• Slow-Start and Additive Increase
• Whenever starting traffic on a new connection or
  increasing traffic after a period of congestion,
  start the congestion window at the size of a
  single segment and increase the congestion window
  by one segment each time an acknowledgement
  arrives.
• In order to avoid increasing the window size too
  quickly (I.e. exponential increase) and causing
  more congestion, apply additive increase, meaning
  only increase window size by 1 after the size
  reaches the threshold. Initially threshold ½
  receiver_advertisement.
• Multiplicative Decrease
• Upon loss of a segment, reduce the congestion
  window down to one segment, and threshold ½
  last allowed window size.

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Increasing Strategy
Initially advertisement_window 32 segments.
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QoS Metrics

• Reliability Lack of reliability means losing a
  packet or acknowledgement, which entails
  retransmission
• Delay Source-to-destination delay.
• Jitter Variation in delay for packets belonging
  to the same flow.
• Bandwidth bits per second