Transmission Control Protocol TCP

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Transmission Control Protocol (TCP)
2
TCP

• TCP is a Reliable end-to-end communication.
• There is a TCP transport entity
• It runs on machine that supports TCP.
• It has Interfaces to the IP layer.
• It Manages TCP streams.
• Accepts user data, breaks it down and sends it
  as separate IP datagrams.
• At receiver, reconstructs original byte stream
  from IP datagrams.

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TCP Reliability Service

• On of TCPs main characteristics is that it
  imposes reliable delivery on the stream.
• It does this with ACKs and Timeouts and
  retransmissions.
• It also provides Ordered delivery.
• TCP Service Model - Obtained by creating TCP end
  points.
• Example UNIX sockets.
• TSAP address
• IP address 16-bit port number.
• Multiple connections can share same port pair.
• Port numbers below 1024 well-known ports
  reserved for standard services.
• List of well-known ports in RFC 1700.

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TCP Service Model .

• TCP connections are full-duplex and
  point-to-point.
• Byte stream (not message stream).
• Message boundaries are not preserved e2e.

A B C D
A
B
C
D
4 512-byte segments sent as separate IP datagrams
2048 bytes of data delivered to application in
single READ
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TCP Byte Stream

• When application passes data to TCP, it may send
  it immediately or buffer it.
• Sometimes application wants to send data
  immediately such as interactive applications.
• Here - Use PUSH flag to force transmission.
• URGENT flag.
• Also forces TCP to transmit at once.

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TCP Protocol Overview

• TCPs TPDU segment.
• 20-byte header options.
• Data.
• TCP entity decides the size of segment.
• 2 limits 64KByte IP payload and MTU.
• Segments that are too large are fragmented.
• More overhead by addition of IP header.
• Sequence numbers.
• Reliability, ordering, and flow control.
• Assigned to every byte.
• 32-bit sequence numbers.

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TCP Segment Header
Source port
Destination port
Sequence number
Acknowledgment number
Header length
A
Window size
P
R
S
F
U
Checksum
Urgent pointer
Options (0 or more 32-bit words)
Data
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TCP Header Fields

• Source and destination ports identify connection
  end points.
• Sequence number.
• Acknowledgment number specifies next byte
  expected.
• TCP header length how many 32-bit words are
  contained in header.
• 6-bit unused field.
• 6 1-bit flags
• URG indicate urgent data present urgent
  pointer gives byte offset from current sequence
  number where urgent data is.
• ACK indicates whether segment contains
  acknowledgment if 0, acknowledgement number
  field ignored.
• PUSH indicates PUSHed data so receiver delivers
  it to application immediately.

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.TCP Header Fields

• Flags (contd)
• RST used to reset connection, reject invalid
  segment, or refuse to open connection.
• SYN used to establish connection connection
  request, SYN1, ACK0.
• FIN used to release connection.
• Window size how many bytes can be sent starting
  at acknowledgment number.
• Checksum checksums the headerdatapseudo-header.
  
• Options provide way to add extra information.
• Examples
• Maximum payload host is willing to accept can be
  advertised during connection setup.
• Window scale factor that allows sender and
  receiver to negotiate larger window sizes.

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TCP Connection Setup

• 3-way handshake.

Host 2
Host 1
SYN (SEQx)
SYN(SEQy,ACKx1)
(SEQx1, ACKy1)
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TCP Connection Release

• Abrupt release
• Send RESET.
• May cause data loss.
• Graceful release
• Each side of the connection released
  independently.
• Either side send TCP segment with FIN1.
• When FIN acknowledged, that direction is shut
  down for data.
• Connection released when both sides shut down.

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TCP Transmission

• Sender process initiates connection.
• Once connection established, TCP can start
  sending data.
• Sender writes bytes to TCP stream.
• TCP sender breaks byte stream into segments.
• Each byte assigned sequence number.
• Segment sent and timer started.
• If timer expires, retransmit segment.
• After retransmitting segment for maximum number
  of times, assumes connection is dead and closes
  it.

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TCP Flow Control

• Sliding window.
• Receivers advertised window.
• Size of advertised window related to receivers
  buffer space.
• Sender can send data up to receivers advertised
  window.

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TCP Flow Control Example
App. writes 2K of data
4K
2KSEQ0
2K
ACK2048 WIN2048
App. does 3K write
2K SEQ2048
0
Sender blocked
App. reads 2K of data
ACK4096 WIN0
2K
ACK4096 WIN2048
Sender may send up to 2K
1K SEQ4096
1K
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TCP Flow Control Observations

• TCP sender not required to transmit data as soon
  as it comes in form application.
• Example when first 2KB of data comes in, could
  wait for more data since window is 4KB.
• Receiver not required to send ACKs as soon as
  possible.
• Wait for data so ACK is piggybacked.

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Delayed ACKs

• Tries to optimize ACK transmission.
• Delay ACKs and window update (500msec) hoping to
  piggyback on data segment.
• Example telnet to interactive editor
• Send 1 character at a time 20-byte TCP header
  1-byte data20-byte IP header.
• Receiver ACKs immediately 40-byte ACK.
• When editor reads character, window update
  40-byte datagram.
• Then echoes character back 41-byte datagram.

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Congestion Control

• Why do it at the transport layer?
• Real fix to congestion is to slow down sender.
• Use law of conservation of packets.
• Keep number of packets in the network constant.
• Dont inject new packet until old one leaves.
• Congestion indicator packet loss.

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TCP Congestion Control

• Like, flow control, also window based.
• Sender keeps congestion window (cwin).
• Each sender keeps 2 windows receivers
  advertised window and congestion window.
• Number of bytes that may be sent is
  min(advertised window, cwin).
• Slow start Jacobson 1988
• Connections congestion window starts at 1
  segment.
• If segment ACKed before time out, cwincwin1.
• As ACKs come in, current cwin is increased by 1.
• Exponential increase.
• Congestion Avoidance
• Third parameter threshold.
• Initially set to 64KB.
• If timeout, thresholdcwin/2 and cwin1.
• Re-enters slow-start until cwinthreshold.
• Then, cwin grows linearly until it reaches
  receivers advertised window.

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TCP Congestion Control Example
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TCP Retransmission Timer

• When segment sent, retransmission timer starts.
• If segment ACKed, timer stops.
• If time out, segment retransmitted and timer
  starts again.
• How to set the Timer
• Based on round-trip time time between a segment
  is sent and ACK comes back.
• If timer is too short, unnecessary
  retransmissions.
• If timer is too long, long retransmission delay.

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Keepalive Timer

• Goes off when a connection is idle for a long
  time.
• Causes one side to check whether the other side
  is still alive.
• If no answer, connection terminated.
• TIME_WAIT
• 2MSL.
• Makes sure all segments die after connection is
  closed.

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Finally.Wireless TCP

• According to layered system design principles,
  transport protocol should be independent of
  underlying technology.
• However, wireless networks invalidate this
  principle.
• Ignoring properties of wireless medium can lead
  to poor TCP performance.
• Problem TCPs congestion control.
• Problem packet loss as congestion indicator.
• When retransmission timer times out, sender
  slows down.
• Wireless links are lossy!
• Dealing with losses in this case should be
  re-sending lost segments asap.