The OSI Reference Model

1
The OSI Reference Model
2
The Transport Layer

• Provides reliable end-to-end service to processes
  in the application layer
• Connection-oriented or connection-less services.
• TPDUs (Transport Protocol Data Units) Refer to
  messages sent between two transport entities.
• Transport service primitives Allow application
  programs to access the transport layer services.
• Data received from application layer is broken
  into TPDUs that should fit into the data or
  payload field of a packet.
• Packets received possibly out-of-order from the
  network layer are reordered and assembled for
  delivery to application layer.
• Transport Entity Hardware/software in the
  transport layer
• In operating system kernel or,
• In a separate user process or,
• In the network interface card.
• Option Negotiation The process of negotiating
  quality of service (QoS) parameters between the
  user and remote transport entities as specified
  by applications.

3
Nesting of TPDUs, Packets And Frames
4
Data Link Layer Vs. Transport Layer
Data Link Layer Environment Adjacent
routers.
Transport Layer Environment End-to-End from
source to destination.
5
Network, Transport, And Application Layers
Interfaces
(Transport Protocol Data Unit)
6
Simple Transport Layer Primitives
Primitives used to provide transport services to
applications
7
Transport Layer Quality of Service (QoS)
Parameters
Application QoS requirements are specified using
these parameters
8
Transport Layer Connection Management
State diagram for a simple connection scheme.
Solid lines Client state sequence.
Dashed lines Server state sequence.
9
TransportConnection Addressing TSAPs, NSAPs

• Network Layer Addresses NSAP (Network Service
  Access Point) i.e IP address.
• Transport Layer Addresses TSAP (Transport
  Service Access Point)
• i.e (IP address, local port)

10
Transport Flow Control

• To accomplish transport flow control a Sliding
  Window protocol is used end-to-end using TDPUs as
  protocol transfer units
• Available receiver capacity and buffering used as
  a receive window RWIN.
• Receiver buffer over-runs are usually not
  allowed.
• Each TPDU must carry an identifier or sequence
  number to distinguish between original TPDUs and
  delayed duplicates.
• To curtail the effect of delayed duplicates
• Packets are not allowed to live forever.
• Each packet has a restricted maximum lifetime
  T.
• The low-order k-bits of a time-of-day clock, of
  the form of a binary counter, are usually used
  to generate initial TPDU sequence numbers for new
  connections.
• This clock is assumed to keep running even if the
  host crashes.
• The clock frequency and k are selected such
  that a generated initial sequence number should
  not repeat (i.e. be assigned to another TPDU) for
  a period longer than the maximum packet lifetime
  T forbidden region.

11
Transport Flow Control

• Once an initial TDPU sequence number is assigned,
  its incremented as required by the connection.
  
• TDPU sequence numbers of a connection may run
  into forbidden region if
• A host sends too much data too fast on a newly
  opened connection
• Here, actual used sequence number vs. time is
  more steep than initial sequence number
  generation vs. time.
• This restricts the maximum data rate of a
  connection to one TDPU per cycle.
• At any connection data rate less than the
  initial sequence number generation clock rate
• The actual sequence numbers used will eventually
  run into the forbidden region from the left.
• This condition must be checked by transport
  entity requiring a TDPU delay of T, or sequence
  number re- synchronization.

12
TPDU Sequencing
TPDUs may not be issued in the forbidden region
The re-synchronization
problem. Connection data rate less than initial
sequence number generation clock rate
T Maximum Packet Lifetime
13
Transport Connection Protocol Three-Way
Handshake
Old duplicate CONNECTION REQUEST

Normal operation
Duplicate CONNECTION REQUEST and duplicate ACK

14
Abrupt Asymmetric Transport Disconnection
Leads to loss of data and incorrect operation.
CR Connection Request DR Disconnect Request
15
TransportConnection Release Scenarios
Normal case of three-way handshake
Final ACK lost
Response lost and subsequent DRS lost
Response lost
16
Transport Layer Flow Control and Buffering
Chained fixed-size buffers
Chained variable-size buffers
One large circular buffer per connection