Sliding Transmission Window in TCP

1
Sliding Transmission Window in TCP

• TCP uses a sliding window mechanism to control
  transmission.
• Increases efficiency
• Send all packets within window without waiting
  for an acknowledgement.
• As acknowledgments for segments come in, the
  window is moved.
• Example Consider a transmission window size of
  five (next slide).

2
TCP Retains Three Pointers
0 1 2 3 4 5 6
7
A B C
3
Every packet to the left of A has been sent and
acknowledged. Every packet between A and B has
been transmitted. Packets between B and C are
eligible to send but have not yet been
sent. Packets to the right of C are not eligible
for transmission. All packets between A and C
are sent without waiting for an acknowledgement.
4
Moving the Transmission Window

• Assume packets 0 2 and 5 have been
  acknowledged.
• Slide window to first unacknowledged packet (3 in
  this example).
• Note TCP sets a separate timer for each
  outstanding packet in window.

5
TCP and Sliding Window
0 1 2 3 4 5 6
7
A B C
6
Flow Control

• Do not want to overwhelm capabilities of the
  receiver.
• Transmission window size is variable and set by
  receiver.
• Changed in response to buffer availability.
• Do not want to overwhelm the network.
• Accomplished by limiting number of outstanding
  (unacknowledged) packets to the size of the
  window.
• In terms of performance the larger the window the
  better.
• More efficient to keep the network busy while
  waiting for acknowledgements.

7
Network Congestion

• The routers along a network path have finite
  capacity.
• Limit on their ability to deal with network
  packets.
• If routers get more data than they can handle
  will start queuing packets.
• When buffer space is exhausted will start
  discarding packets.
• Condition termed congestion.
• Results in long delays in transferring data.

8
TCP and Congestion

• Recall TCP uses a timer to determine when to
  retransmit a packet.
• If network is congested then packets and
  acknowledgements will be lost.
• This will trigger more retransmissions.
• Retransmissions will add to network congestion,
  resulting in more packet and acknowledgement
  loss, resulting
• Condition called congestion collapse.

9
Avoiding Congestion Collapse

• TCP defines two mechanisms to avoid congestion
  collapse
• Slow start.
• Multiplicative Decrease.
• Multiplicative Decrease
• TCP maintains a second limit on the size of the
  transmission window called congestion window
  limit.
• The number of packets placed onto the network is
  determined by the minimum of the transmission
  window and congestion window.
• Congestion window is reduced by 50 for each lost
  packet.

10
First Loss
0 1 2 3 4 5 6
7
Transmission and Congestion Window
New Congestion Window
11
Second Loss
0 1 2 3 4 5 6
7
New Congestion Window
Old Congestion Window
Transmission Window
12
Time Between Retransmissions

• In addition to aggressively reducing window size
  also modifies the amount of before the next
  retransmission.
• If one packet lost, TCP doubles the time between
  retransmissions.
• Quite easily reduce window to one packet, and
  wait a long time between retransmissions.

13
Slow Start

• TCP does not immediately double the size of the
  congestion window when packets start flowing
  again.
• Increases the congestion window by one packet per
  acknowledgement.
• Assume congestion window is one.
• Send out packet and wait for acknowledgement.
• When received, increase congestion window to two
  and send next two packets.
• When both acknowledgements are received increase
  congestion window to four.
• When four are received, increase to eight.