The OSI Reference Model
EECC694 - Shaaban
#1 lec #11 Spring2000 4-18-2000
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.
EECC694 - Shaaban
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Nesting of TPDUs,
Packets And Frames
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Data Link Layer Vs. Transport Layer
Data Link Layer Environment:
Adjacent routers.
Transport Layer Environment:
End-to-End from source to destination.
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Network, Transport, And Application
Layers Interfaces
(Transport Protocol Data Unit)
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Simple Transport Layer
Primitives
Primitives used to provide transport services to applications
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Transport Layer:
Quality of Service (QoS) Parameters
Application QoS requirements are specified using these parameters
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Transport Layer Connection Management
State diagram for a simple connection scheme.
Solid lines: Client state sequence.
Dashed lines: Server state sequence.
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Transport
Connection
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)
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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].
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Transport Flow Control
• Once an initial TDPU sequence number is assigned, it’s
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.
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TPDU Sequencing
TPDUs may not be issued
in the forbidden region
T = Maximum Packet Lifetime
The re-synchronization
problem.
Connection data rate less than initial sequence
number generation clock rate
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Normal operation
Old duplicate CONNECTION
REQUEST
Transport Connection
Protocol:
Three-Way
Handshake
Duplicate CONNECTION REQUEST and duplicate ACK
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Abrupt Asymmetric Transport Disconnection
Leads to loss of data and incorrect operation.
CR = Connection Request
DR = Disconnect Request
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Normal case of three-way handshake
Final ACK lost
Transport
Connection
Release
Scenarios
Response lost and
subsequent DRS lost
Response lost
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Transport Layer:
Flow Control and Buffering
Chained fixed-size
buffers
Chained variable-size
buffers
One large circular
buffer per connection
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