IS 3'4 Transport layer

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IS 3.4Transport layer
Application Layer
Session Layer
Transport Layer
Network Layer
Data Link
Physical Layer
M.H. Rahman
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Outline

• Introduction
• Services provided
• Protocols
• Performance
• Summary

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Introduction

• Transport layer is the heart of the whole
  protocol hierarchy.
• Goal provide efficient, reliable and
  cost-effective service to its users (processes in
  the application layer)
• Makes use of the services provided by the network
  layer
• The transport protocol permits the exchange of
  TPDU between transport entities
• transport entity hardware and/or software within
  the transport layer that does the work.

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Logical relationship
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Transport Layer role

• Transport layer services are very similar to
  network services
• connection-oriented service
• 3 phases establishment, data transfer, release
• connectionless
• Why do we need the transport layer?
• network layer is part of the Communication subnet
  
• makes it possible for transport service to be
  more reliable than the underlying network
  services
• makes it possible to have transport service
  primitives independent of the network service
  primitives
• enhance QoS (Quality of Service)

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Transport Service Primitives
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Elements of Transport Protocols (1)

• Addressing
• TSAP (Transport Service Access Point)
• There is a unique port number for a given service
• Establishing a connection
• problem is delayed duplicates
• solved by imposing a finite lifetime for packets
• 3-way handshake used to agree on the initial
  sequence number
• Releasing a connection
• need to avoid loss of data
• two-army problem (how both sides can be
  convinced?)

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Elements of Transport Protocols (2)

• Flow control buffering
• management of connections while they are in use
• buffer space and carrying capacity of the subnet
  will limit data rate
• Multiplexing
• upward many applications use the same connection
• downward one application uses multiple
  connections
• Crash recovery
• can a client continue working over an open
  connection when a server crashes and then quickly
  reboot?

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Internet Transport Protocol

• There are two transport control protocol in the
  TCP/IP protocol suite
• TCP (Transport Control Protocol)
• This is a connection oriented service and is most
  widely used for most of the internet
  transactions.
• UDP (User Datagram Protocol)
• This is a connectionless service within the
  framework of TCP/IP protocol suite.

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UDP

• UDP is specified in RFC 768.
• It provides a connectionless service to an
  application.
• Basically provides an unreliable service and is
  used when reliability is not critical and time
  lost due to connection and set up and release is
  not justifiable
• Examples
• Inward data collection from sensors etc.
• Outward data dissemination including broadcast
  messages.
• Request response. Application when transaction
  service is provided by c common server.
• Real time application such as telemetry.
• Voice over IP

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UDP

• UDP header
• Length Length of entire UDP segment, including
  header and data.
• Checksum Similar to TCP checksum and applied to
  entire segment plus a pseudo header, used for
  calculation of checksum.

0
16

31
Destination Port
Source Port
Length
Checksum
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TCP

• TCP is designed to provide reliable communication
  between a pair of processes (TCP users) across a
  variety of reliable and unreliable networks.
• It provides a connection oriented full-duplex
  stream transport service that allows two
  application programs to establish a connection
  and then transfer data between them.
• Each connection is terminated gracefully after
  data have been transferred.

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TCP

• TCP header format

Source Port
Destination Port
Sequence number
Acknowledgement number
Data offset
Window
Flags
Reserved
Urgent Pointer
Checksum
Option and padding
U A P R S F R C S S Y I G K H T N N
Flags
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TCP Header format

• Definitions
• Source Port (16 bits) source TCP user.
• Destination Port (16 bit) Destination TCP user
• Sequence Number (32 buts)
• Sequence number of the first data octets in the
  segment except when the SYN flag is set, it is
  the initial sequence number (ISN). Sequence
  number of of data octet is ISN1.
• Acknowledgement Number (32 bits)
• A piggyback acknowledgement. Contains the
  sequence number of the next data octet the TCP is
  expecting.
• Reserved (6 bits) Reserved for future use.

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TCP Header format

• Definitions (contd.)
• Flags
• URG urgent pointer field significant.
• ACK acknowledge field significant.
• PSH push function
• RST rest connection
• SYN synchronize sequence number.
• FIN no more data, this is the last one.
• Window (16 bits)
• flow control credit allocation in octets.
  Contains the number of data octets, beginning
  with the sequence number in the ack filed that
  the sender is willing to accept.

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TCP Header format

• Definitions (contd.)
• Checksum (16 bits)
• the ones complement of the sum modulo 216 - 1 of
  all the 16 bit words in the segment plus a pseudo
  header. The pseudo header includes source and
  destination address and protocol plus a segment
  length field.
• Urgent Pointer (16 bits)
• points to the last octet in the sequence of
  urgent data.
• Options (variable)
• an example is the option that specifies the
  maximum segment size that will be accepted.

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TCP Mechanism

• TCP mechanism has three phases
• Connection establishment
• data transfer and
• connection termination

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TCP Mechanism

• Connection Establishment
• TCP always uses three way handshake.
• To initiate a connection, an entity sends a SYN,
  SNX (ISN).
• The receiver responds with SYN, SNY (ISN) and AN
  X1.
• Finally the initiator responds with ANY1

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Establishing a connection (a)
Host1
Host2
Normal Operation
CR (seq x)
ACK (seq y, ACK x)
CR connection request ACK request accepted
DATA (seq x, ACK y)
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Establishing a connection (b)
Host1
Host2
Old CR duplicate appearing out of nowhere
CR (seq x)
ACK (seq y, ACK x)
REJECT (ACK y)
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Establishing a connection (c)
Host1
Host2
Old CR duplicate and ACK appearing out of nowhere
CR (seq x)
ACK (seq y, ACK x)
DATA (seq x, ACK z)
REJECT (ACK y)
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TCP Mechanism

• Data Transfer
• The TCP layer transfer a block of data to the IP
  layer for transport to a remote location. In most
  cases, a sliding window mechanism, similar to the
  one discussed at DLL, is used.
• Data transfer can be either block oriented or
  stream oriented. This refers to how the window is
  designated number of blocks or number of bytes.
• If the data transfer is block oriented, then the
  window size indicates number of blocks of data
  (packets).
• If the data transfer is stream oriented, then the
  window size indicates number of bytes of data.

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TCP Mechanism

• Data Transfer
• Data transfer is viewed logically as a stream of
  octets. Each segment contains sequence number of
  the first octet of the segment.
• Flow control is exercised using a credit
  allocation scheme in which the credit number is
  in octet rather than number of segments (as in
  X.25).
• Data is buffered by both the sender and receiver
  before delivery to the application. TCP passes
  data to its application at its discretion (when
  to deliver). However the sender can use PSH and
  URG pointer to force the TCP to deliver the data.

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TCP Mechanism

• Data Transfer

Events at Host 1
Events at Host 2
Send packet 1
Receive packet 1
Send ack 1
Receive ack1 Send packet 2
On TCP communication, octet number , instead of
packet number is used.
packet lost
Time out expires Send packet 2
packet 2 received Send ack 2
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TCP Mechanism

• Connection termination
• Each TCP user must issue a CLOSE primitive to
  request termination of its connection.
• The transport entity sets the FIN bit on the last
  segment of data and sends it out.
• Abrupt termination can occur if an ABORT
  primitive is issued by the user.

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Releasing a connection (a)
Host1
Host2
Normal operation
Send DR start timer
DR
Send DR start timer
DR
Release connection
Send ACK
ACK
Release connection
DR disconnect request ACK request accepted
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Releasing a connection (b)
Host1
Host2
Final ACK lost
Send DR start timer
DR
Send DR start timer
DR
Release connection
Send ACK
ACK
(Timeout) Release connection
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Releasing a connection (c)
Host1
Host2
Response lost
Send DR start timer
DR
Send DR start timer
DR
(Timeout) Send DR start timer
DR
Send DR start timer
DR
Release connection
ACK
Send ACK
(Timeout) Release connection
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TCP Implementation

• The TCP standard provides a precise definition of
  the protocol to be used between TCP entities.
  However, certain aspects of the protocol admit
  several possible implementation options.
• Send policy
• In the absence of PSH and close termination
  window, a sending TCP entity is free to transmit
  data at its own convenience. The actual policy
  will depend on performance considerations.
• Deliver policy
• In the absence of push, a receiving TCP entity is
  free to deliver data to the user at its
  convenience. The actual policy will depend on
  performance considerations.

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TCP Implementation

• Accept policy
• When all data segments arrive in order over a
  TCP, TCP places data in a receive buffer for
  delivery tot he user. In case they are out of
  order the receiving TCP has two options
• In-order accept segments arriving in order only.
  Discard if not.
• In-window accept all segment within the receive
  window, even if they are out of order.
• Retransmit policy
• TCP maintains a queue of segments that have been
  sent out but not yet acknowledged. TCP
  specification states that TCP will retransmit if
  it fails to receive an acknowledgement within a
  certain time. There are variation on how to
  handle the timer.

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TCP Implementation

• Acknowledge policy
• When a data segment arrives in sequence, the
  receiving TCP entity has two options concerning
  the timing of acknowledgement
• Immediate When data is accepted, immediately
  transmit an empty data segment with the
  acknowledgement.
• Cumulative When data are accepted, record the
  need for acknowledgement but wait for an outbound
  segment with data I which to piggyback the
  acknowledgement. To avoid long delay, set a
  window timer.

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TCP Congestion Control

• The credit based flow control mechanism was
  designed to enable a destination to restrict flow
  of segments from source to avoid buffer overflow
  at the receiver. The same flow control mechanism
  is now used to provide congestion control. It is
  done with two parameters
• Retransmission timer management. The retransmit
  time is based on the anticipated delay of the
  response. If the network is heavily loaded, RTT
  is increased. There are a variety of ways to
  calculate this time.
• Window management. The size of the window can be
  dynamically adjusted to suit the needs of the
  network. Again, there are a variety of mechanism
  for doing this.

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Summary - TCP

• W have provided a very brief description of how
  the TCP within the TCP/IP protocol suite works.
  For details on some of the items you may refer
  to
• Data and Computer Communications (6th edition),
  by William Stalling, Prentice Hall, 2000.
• Computer Networks and Internet, by Douglas E
  Comer, Prentice Hall, 1997