Transport Layer

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Transport Layer

• Rong Wang
• CGS3285
• Spring2004

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TCP RELIABLE TRANSPORT SERVICES

• Internet Protocol (IP) provides unreliable
  datagram service'' between hosts
• Transport protocols provide end-to-end delivery
  between endpoints of a connection e.g.,
  processes or programs
• User Datagram Protocol (UDP) provides datagram
  service
• Transmission Control Protocol (TCP) provides
  reliable data delivery

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USER DATAGRAM PROTOCOL

• UDP delivers independent messages, called
  datagrams between applications or processes on
  host computers
• Best effort'' delivery - datagrams may be lost,
  delivered out of order, etc.
• Checksum (optionally) guarantees integrity of
  data
• For generality, endpoints of UDP are called
  protocol ports or ports
• Each UDP data transmission identifies the
  internet address and port number of the
  destination and the source of the message
• Destination port and source port may be different

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UDP HEADERS

• UDP datagrams have a header that follows the
  hardware and IP headers
• Hardware IP UDP Application Data
• UDP header is very simple
• Port numbers
• Message length
• Checksum
• UDP source port UDP destination port UDP message
  length UDP checksum Data

Hardware IP UDP Application Data
UDP Source Port UDP Destination Port
UDP Message Length UDP Checksum
Data Data
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TCP

• Transmission Control Protocol (TCP) is most
  widely used transport protocol
• Provides reliable data delivery by using IP
  unreliable datagram delivery
• Compensates for loss, delay, duplication and
  similar problems in Internet components
• Reliable delivery is high-level, familiar model
  for construction of applications

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FEATURES OF TCP

• Connection oriented Application requests
  connection to destination and then uses
  connection to deliver data to transfer data
• Point-to-point A TCP connection has two
  endpoints
• Reliability TCP guarantees data will be
  delivered without loss, duplication or
  transmission errors
• Full duplex The endpoints of a TCP connection
  can exchange data in both directions
  simultaneously
• Stream interface Application delivers data to
  TCP as a continuous stream, with no record
  boundaries TCP makes no guarantees that data
  will be received in same blocks as transmitted
• Reliable connection startup Three-way handshake
  guarantees reliable, synchronized startup between
  endpoints
• Graceful connection shutdown TCP guarantees
  delivery of all data after endpoint shutdown by
  application

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STREAM DELIVERY
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USING IP FOR DATA DELIVERY

• TCP uses IP for data delivery (like UDP)
• Endpoints are identified by ports (like UDP)
• Allows multiple connections on each host
• Ports may be associated with an application or a
  process
• IP treats TCP like data and does not interpret
  any contents of the TCP message

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SENDING AND RECEIVING BUFFERS
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TCP SEGMENTS
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Note
The bytes of data being transferred in each
connection are numbered by TCP. The numbering
starts with a randomly generated number.
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Note
The value of the sequence number field in a
segment defines the number of the first data byte
contained in that segment.
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Note
The value of the acknowledgment field in a
segment defines the number of the next byte a
party expects to receive. The acknowledgment
number is cumulative.
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TCP SEGMENT FORMAT
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CONTROL FIELD
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DESCRIPTION OF FLAGS IN THE CONTROL FIELD
Flag Description
URG The value of the urgent pointer field is valid.
ACK The value of the acknowledgment field is valid.
PSH Push the data.
RST The connection must be reset.
SYN Synchronize sequence numbers during connection.
FIN Terminate the connection.
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THREE-STEP CONNECTION ESTABLISHMENT
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FOUR-STEP CONNECTION TERMINATION
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States for TCP
State Description
CLOSED There is no connection.
LISTEN The server is waiting for calls from the client.
SYN-SENT A connection request is sent waiting for acknowledgment.
SYN-RCVD A connection request is received.
ESTABLISHED Connection is established.
FIN-WAIT-1 The application has requested the closing of the connection.
FIN-WAIT-2 The other side has accepted the closing of the connection.
TIME-WAIT Waiting for retransmitted segments to die.
CLOSE-WAIT The server is waiting for the application to close.
LAST-ACK The server is waiting for the last acknowledgment.
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STATE TRANSITION DIAGRAM
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Note
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.
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SENDER BUFFER
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RECEIVER WINDOW
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SENDER BUFFER AND SENDER WINDOW
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SLIDING THE SENDER WINDOW
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EXPANDING THE SENDER WINDOW
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SHRINKING THE SENDER WINDOW
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Note
In TCP, the sender window size is totally
controlled by the receiver window value (the
number of empty locations in the receiver
buffer). However, the actual window size can be
smaller if there is congestion in the network.
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Note
Some points about TCPs sliding windows
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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LOST SEGMENT
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LOST ACKNOWLEDGEMENT
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TCP TIMERS