Chapter 12 Transmission Control Protocol (TCP)

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Chapter 12 Transmission Control Protocol (TCP)
Mi-Jung Choi Dept. of Computer Science and
Engineering mjchoi_at_postech.ac.kr
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Contents

• 12.1 TCP SERVICES
• 12.2 TCP FEATURES
• 12.3 SEGMENT
• 12.4 A TCP CONNECTION
• 12.5 STATE TRANSITION DIAGRAM
• 12.6 FLOW CONTROL
• 12.7 ERROR CONTROL
• 12.8 CONGESTION CONTROL
• 12.9 TCP TIMERS
• 12.10 OPTIONS
• 12.12 TCP PACKAGE

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Objectives

• Be able to name and understand the services
  offered by TCP
• Understand TCPs flow and error control and
  congestion control
• Be familiar with the fields in a TCP segment
• Understand the phases in a connection-oriented
  connection
• Understand the TCP transition state diagram
• Be able to name and understand the timers used
  in TCP
• Be familiar with the TCP options

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12.6 Flow Control
Flow control regulates the amount of data a
source can send before receiving an
acknowledgment from the destination. TCP defines
a window that is imposed on the buffer of data
delivered from the application program.
The topics discussed in this section include
Sliding Window Protocol Silly Window Syndrome
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12.6 Flow Control

• defines the amount of data a source can send
  before receiving an acknowledge from the
  destination
• Sender does not flood the receiver, but maximizes
  throughput
• Sliding window Protocol
• 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
• TCPs sliding windows are byte oriented
• Window Set of sequence numbers to send/receive
• Sender window
• Sender window increases when ack received
• Packets in sender window must be buffered at
  source

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Sequence Number

• using 32 bit field, sequence numbers in set 0
  232-1,
• sending window seq. numbers of bytes that may be
  sent
• receiving window seq. numbers that may be
  accepted into buffers

• TCP
• buffer space at sender and receiver with a
  credit scheme for flow control
• receiver advertises credit to sender ack is
  next expected packet

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Sliding Window

• rwnd receiver window
• cwnd congestion window

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Example 3
What is the value of the receiver window (rwnd)
for host A if the receiver, host B, has a buffer
size of 5,000 bytes and 1,000 bytes of received
and unprocessed data?
SolutionThe value of rwnd 5,000 - 1,000
4,000. Host B can receive only 4,000 bytes of
data before overflowing its buffer. Host B
advertises this value in its next segment to A.
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Example 4
What is the size of the window for host A if the
value of rwnd is 3,000 bytes and the value of
cwnd is 3,500 bytes?
SolutionThe size of the window is the smaller of
rwnd and cwnd, which is 3,000 bytes.
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Sliding Window Protocol

• Send buffer
• Receiver window

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Sliding Window Protocol

• Expanding the sender window
• Shrinking the sender window

To avoid shrinking the sender window, the
receiver must wait until more space is available
in its buffer.
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Example 5
Figure 12.21 shows an unrealistic example of a
sliding window. The sender has sent bytes up to
202. We assume that cwnd is 20 (in reality this
value is thousands of bytes). The receiver has
sent an acknowledgment number of 200 with an rwnd
of 9 bytes (in reality this value is thousands of
bytes). The size of the sender window is the
minimum of rwnd and cwnd or 9 bytes. Bytes 200 to
202 are sent, but not acknowledged. Bytes 203 to
208 can be sent without worrying about
acknowledgment. Bytes 209 and above cannot be
sent.
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Example 6
In Figure 12.21 the server receives a packet with
an acknowledgment value of 202 and an rwnd of 9.
The host has already sent bytes 203, 204, and
205. The value of cwnd is still 20. Show the new
window.
SolutionFigure 12.22 shows the new window. Note
that this is a case in which the window closes
from the left and opens from the right by an
equal number of bytes the size of the window has
not been changed. The acknowledgment value, 202,
declares that bytes 200 and 201 have been
received and the sender needs not worry about
them the window can slide over them.
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Example 7
In Figure 12.22 the sender receives a packet with
an acknowledgment value of 206 and an rwnd of 12.
The host has not sent any new bytes. The value of
cwnd is still 20. Show the new window.
SolutionThe value of rwnd is less than cwnd, so
the size of the window is 12. Figure 12.23 shows
the new window. Note that the window has been
opened from the right by 7 and closed from the
left by 4 the size of the window has increased.
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Example 8
In Figure 12.23 the host receives a packet with
an acknowledgment value of 210 and an rwnd of 5.
The host has sent bytes 206, 207, 208, and 209.
The value of cwnd is still 20. Show the new
window.
SolutionThe value of rwnd is less than cwnd, so
the size of the window is 5. Figure 12.24 shows
the situation. Note that this is a case not
allowed by most implementations. Although the
sender has not sent bytes 215 to 217, the
receiver does not know this.
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Example 8 (cont.)
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Example 9
How can the receiver avoid shrinking the window
in the previous example?
SolutionThe receiver needs to keep track of the
last acknowledgment number and the last rwnd. If
we add the acknowledgment number to rwnd we get
the byte number following the right wall. If we
want to prevent the right wall from moving to the
left (shrinking), we must always have the
following relationship.
new ack new rwnd last ack last rwndornew
rwnd (last ack last rwnd) - new ack
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Note

• To avoid shrinking the sender window, the
  receiver must wait until more space is available
  in its buffer.
• Window shutdown
• The receiver can temporarily shut down the window
  by sending an rwnd of 0, if the receiver does not
  want to receive any data from the sender for a
  while.
• The sender does not actually shrink the size of
  the window, but stops sending data until a new
  advertisement has arrived.
• Even when the window is shut down by an order
  from the receiver, the sender can always send a
  segment with one byte of data (probing) to
  prevent a deadlock.

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12.6 FLOW CONTROL (cont.)

• In TCP, the sender window size is totally
  controlled by the receiver window value. However,
  the actual window size can be smaller if there is
  congestion in the network.

Some Points about TCPs Sliding Windows ? The
size of the window is the lesser of rwnd and
cwnd.? The source does not have to send a full
windows worth of data.? The window can be
opened or closed by the receiver, but should
not be shrunk.? The destination can send an
acknowledgment at any time as long as it
does not result in a shrinking window.? The
receiver can temporarily shut down the window
the sender, however, can always send a segment
of one byte after the window is shut down.
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12.6 FLOW CONTROL (cont.)

• Credit-based Window management
• Increase and decrease by receiving TCP

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SILLY WINDOW SYNDROME

• Silly window syndrome created by sender
• if sending TCP is serving an application program
  that creates data slowly, for example, 1 byte at
  a time.
• ? 1 byte data 20 bytes TCP header 20 byte IP
  header
• Nagles Solution
• To prevent the sending TCP from sending data byte
  by byte.
• TCP must be forced to wait as it collects data to
  send in a large block
• Nagle algorithm by sender
• sending TCP? ?? ? ????? ?? ?? ???????? ???? ?
  ???? ????? ?? ? ????.
• ??? ????? ??? ??, sending TCP? receiving TCP???
  ????? ?????, ?? ?? ??? ????? ??? ? ?? ??? ???
  ???? ?? ??? ???? ???? ???? ?? ??? ????. ? ?? ?
  ??? ????, sending TCP? ????? ????.
• ??? ?? ???? 2?? ??? ????. ? ???? 2? ?? ?? ???
  ????? ?? ??? ????? ?? ? ?? ??? ??? ???? ?? ???
  ???? ???? 3? ????.

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SILLY WINDOW SYNDROME

• Silly window syndrome created by receiver
• if receiving TCP is serving an application
  program that consumes data slowly, for example, 1
  byte at a time.
• Clarks Solution
• To send an ACK as soon as the arrives, but to
  announce a window size of 0 until either there is
  enough space to accommodate a segment of maximum
  size or until half of buffer is empty.
• 2nd Solution Delayed Acknowledgement
• To delay sending the ACK
• The receiver waits until there is decent amount
  of space in its incoming buffer before
  acknowledging the arrived segments.
• ?? ??? ??(? ????? ??? ????? ??? ??? ???)
• ?? ?????? ????? ?? ?? ????? ? ??? ???? ?? ???
  TCP??? ?? ??? 0.5? ?? ???? ??? ???? ??.

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12.7 ERROR CONTROL
TCP provides reliability using error control,
which detects corrupted, lost, out-of-order, and
duplicated segments. Error control in TCP is
achieved through the use of the checksum,
acknowledgment, and time-out.
The topics discussed in this section include

• Checksum
• Acknowledgment
• Acknowledgment Type
• Retransmission
• Out-of-Order Segments
• Some Scenarios

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12.7 ERROR CONTROL

• Error types
• Corrupted Segments
• Lost Segments
• Out-of-order Segments
• Duplicated Segments
• Error detection mechanism
• Error detection tools checksum,
  acknowledgement, and time-out
• Checksum field is used to check for a corrupted
  segment
• Acknowledgement is used to confirm the receipt of
  those segment uncorrupted
• If a segment is not acknowledged before the
  time-out, it considered to be either corrupted or
  lost
• Error correction mechanism
• time-out timer retransmission

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Acknowledgement

• ACK segments do not consume sequence numbers and
  are not acknowledged.
• ACK rules
• Rule 1 piggybacking
• Rule 2
• When the receiver has no data to send and it
  receives an in-order segment, the receiver
  delays sending an ACK until another segment
  arrived or until a period of time (normally 500
  ms).
• Rule 3
• no more than 2 in-order unacknowledged segments
  at any time
• Rule 4
• When a segment arrives with an out-of-order seg.
  number, the receiver immediately sends an ACK
  announcing the next expected seg. number.
• Rule 5
• When a missing segment arrives, the receiver
  immediately sends an ACK.
• Rule 6
• If a duplicate segment arrives, the receiver
  immediately sends an ACK.
• Acknowledgement types
• Accumulate Acknowledgement (ACK)
• Selective Acknowledgement (SACK)

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Retransmission

• In modern implementations, a retransmission
  occurs if the retransmission timer expires or
  three duplicate ACK segments have arrived.
• No retransmission timer is set for an ACK
  segment.
• Retransmission after RTO (retransmission timeout)
• Retransmission after Three Duplicate ACK segments
• Out-of-Order segment
• Data may arrive out of order and be temporarily
  stored by the receiving TCP, but TCP guarantees
  that no out-of-order segment is delivered to the
  process.

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Normal Operation
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Lost segment
The receiver TCP delivers only ordered data to
the process.
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Fast retransmission
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• Delayed segment
• Delayed TCP segment are treated the same way as
  lost or corrupted segments by the receiver.
• The delayed segment may arrive after it has been
  resent (a duplicate segment)
• Duplicate segment
• When a segment arrives that contains a sequence
  number less than the previously acknowledged
  bytes, it is discarded.

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Automatically Corrected Lost Acknowledgment
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Lost acknowledgment corrected by resending a
segment
Lost acknowledgments may create deadlock if they
are not properly handled