The Transmission Control Protocol (TCP)

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The Transmission Control Protocol (TCP)

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The Transmission Control Protocol (TCP)

• TCP is a protocol that specifies
• How to distinguish among multiple destinations on
  a given machine
• How to initiate and terminate a stream transfer
• Format of the data and acknowledgments that two
  computers exchange to achieve a reliable transfer
• Procedures the computers use to ensure that the
  data arrives correctly

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TCP Segment Format

• TCP divides the data stream into segments for
  transmission

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TCP Segment Header Fields

• Code bits - identify the contents of the segment
• Window - how much data the sender is willing to
  accept (flow control)
• Urgent pointer - specifies the position in the
  segment where urgent data ends

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Out of Band Data

• TCP provides a mechanism to handle urgent data
• Urgent data is received before octets already in
  the stream
• Sender
• Sets urgent bit in segment header
• Puts urgent data at the beginning of the data
  field
• Sets urgent pointer to the end of the urgent data
• Receiver
• Notified of the urgent data as soon as it arrives
• Enters urgent mode until all urgent data has
  been consumed
• Returns to normal mode

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TCP Segment Header Fields (cont)

• Checksum - used to verify the integrity of the
  segment
• Computed over
• Segment header
• Segment data
• Pseudo header
• 1s complement addition algorithm
• Options - signal various options
• Maximum segment size

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TCP - Acknowledgments and Retransmission

• Uses cumulative acknowledgment scheme
• ACK 7 all octets up to but not including
  number 7 have been received correctly
• Advantages
• ACKs are easy to generate and unambiguous
• Lost acknowledgments do not force retransmission
• Disadvantages
• Sender does not receive information about all
  successful transmissions

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TCP - Timeout and Retransmission

• For each segment sent
• Start timer and wait for acknowledgment
• Retransmit if timer expires
• TCP uses an adaptive retransmission algorithm
  because internet delays are so variable
• Round trip time of each connection is recomputed
  every time an acknowledgment arrives
• Timeout value is adjusted accordingly

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TCP - Timeout and Retransmission (cont)

• How should the most recent round trip sample
  (RTS) effect the round trip time (RTT)?
• New RTT (a RTT) ((1-a) RTS)
• a 0
• a 1
• How should RTT be used to compute timeout?
• Timeout b RTT
• b 1
• b gt 1

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Accurate Measurement ofRound Trip Samples

• Simple answer
• Difference in time segment sent and time ACK
  received
• But what about retransmissions?
• Does ACK correspond to first or second copy of
  segment sent?
• Assuming ACK is for the earliest transmission
  causes problems
• Assuming ACK is for the latest transmission
  causes problems
• TCP acknowledgments are ambiguous

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Accurate Measurement ofRound Trip Samples

• To avoid problems with ambiguous ACKs
• TCP should not update the RTT for retransmitted
  segments
• Problem if timeout value is too small all
  segments will cause retransmissions and RTT will
  never be updated
• Timer back-off if the timer expires and causes a
  retransmission TCP increases the timeout
• New timeout c timeout

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Karns Algorithm

• When computing the round trip estimate, ignore
  samples that correspond to retransmitted
  segments, but use a backoff strategy, and retain
  the timeout value from a retransmitted packet for
  subsequent packets until a valid sample is
  obtained.

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Responding to HighVariance in Delay

• Often a good estimate of the round trip time is
  not very useful because internet delays tend to
  have a high variance
• Most TCP implementations estimate
• Average round trip time
• Variance
• Using the variance
• Timeout b RTT

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Responding to Congestion

• Endpoints cannot know the details of where in the
  internet congestion has occurred or why
• Congestion will usually lengthen delays
• TCPs response to lengthened delays
  (retransmission) can cause congestion collapse
• Instead, TCP must reduce transmission rates when
  congestion occurs

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Responding to Congestion (cont)

• Augment the TCP transmission window with a
  congestion window
• Windowmin(receiver advertisement,congestion
  window)
• Multiplicative Decrease Congestion Avoidance
• Upon loss of a segment reduce the congestion
  window by half (down to a minimum of one segment)
• Backoff the retransmission timer exponentially

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Responding to Congestion (cont)

• Slow-start (additive) recovery
• When 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 ACK arrives
• Congestion avoidance phase (during recovery)
• Once the congestion window reaches half of its
  original size before congestion occurred
• Increase the size of the congestion window by 1
  only if all segments in the window have been
  acknowledged

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TCP Timeout and Retransmission - Summary

• What do we get for all this stuff
• Slow-start increase
• Multiplicitive decrease
• Congestion avoidance
• Measurement of RTT and variance
• Exponential timer backoff
• Dramatic improvement of TCP performance without
  adding significant computational overhead

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Establishing a TCP Connection

• The 3-way handshake
• Guarantee that both sides are ready for
  connection
• Allows both sides to agree on initial sequence
  numbers

Receive SYN
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Closing a TCP Connection

• Applications should close a connection when they
  have no more data to transmit
• Connection can be closed in either one or both
  directions
• Site 1 finishes transmitting data and waits for
  ACK from site 2
• Site 1 transmits a segment with the FIN bit set
• Site 2 acknowledges the FIN segment
• Site 2 notifies the application that no more data
  is coming
• Data can still be transmitted from site 2 to site
  1
• Site 1 will still receive and acknowledge data
  from site 2
• Eventually, site 2 will finish transmitting and
  close its connection
• Both endpoints delete record of the connection

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Closing a TCP Connection (cont)
Site 1
Network
Site 2

(app closes connection)
Send FIN seqx
Receive FIN Send ACK x1
(inform application)
Receive ACK
(app closes connection)
Send FIN seqy, ACK x1
Receive FINACK
Send ACK y1
Receive ACK
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TCP Connection Reset

• Applications normally close connections
• Sometimes abnormal conditions arise that break a
  connection
• Broken connections can be reset
• Site 1 sends a segment with the RST bit set
• Site 2 receives segment and aborts the connection
• Transfers in both directions cease immediately
• Resources for the connection are released
• Applications programs are informed

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Forcing Data Delivery

• TCP divides the stream of octets into segments
  for transmission
• This improves efficiency since octets can be
  buffered until a good-sized segment can be sent
• TCP provides a push operation for applications
  that want to force delivery of octets
• Set PSH bit
• Send segment

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Reserved TCP Port Numbers

• Like UDP
• Static port bindings for commonly used services
• Ports 0-1024 are reserved
• Dynamic port bindings
• Port numbers over 1024
• Port numbers for services accessible by both UDP
  and TCP usually match
• ECHO (7)
• TIME (37)

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Reserved TCP Port Numbers

• Port number Service
• 0 Reserved
• 7 Echo
• 17 Quote of the day
• 21 FTP
• 23 TELNET
• 25 SMTP
• 37 Time
• 79 Finger
• 80 HTTP
• 119 NNTP

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TCP Performance

• Silly Window Syndrome
• Sender generates data quickly
• Receiver reads incoming data one octet at a time

Sender
Receiver
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TCP Performance (cont)

• Silly Window Syndrome
• Each ACK advertises a small amount of space
• Each segment carries a small amount of data
• Problems
• Poor use of network bandwidth
• Unnecessary computational overhead

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TCP Performance (cont)

• Avoiding Silly Window Syndrome
• Use heuristics at sender to avoid transmitting a
  small amount of data in each segment
• Use heuristics at receiver to avoid sending small
  window advisements
• Receive-side silly window avoidance
• Monitor receive window size
• Delay advertising an increase until a
  significant increase is possible
• Significant min(half the window, maximum
  segment size)

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Receive-Side Silly Window Avoidance Example

Receive 6 octets, send ACK 7 with window
advisement of 0
Application reads one octet
Application reads one octet
Application reads one octet
Send window advisement of 3, receive 3 octets
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Receive-Side SillyWindow Avoidance

• Two approaches
• Receiver can ACK received octets but does not
  advertise an increase in its window until the
  increase is significant
• Receiver can not send ACKs when the window is not
  large enough to advertise
• Advantages/disadvantages?

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Send-Side SillyWindow Avoidance

• Goal avoid sending small segments
• Application can generate data in small blocks
• TCP must collect data sent by application into a
  single large segment (clump) for transmission
• TCP must delay sending a segment until it
  contains a reasonable amount of data
• How long should TCP wait before transmitting data?

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Send-Side Silly Window Avoidance (cont)

• The Nagle Algorithm
• Application generates data to be sent over a
  connection that has already transmitted some data
• If all previous transmissions have been
  acknowledged send the data immediately
• If any ACKs are still pending do not transmit
  until
• Maximum segment size is reached, or
• An ACK arrives
• Self-clocking - does not compute delays
• Applies even if the application requests a push

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TCP Summary

• Provides reliable stream delivery service
• Full duplex
• Out-of-band for urgent data
• Makes efficient use of the network
• Piggybacking
• Sliding windows
• Efficiency
• End-to-end flow control
• Acknowledgment and retransmission
• Congestion recovery/avoidance