EECC694 - Shaaban #1 lec #11 Spring2000 4-18-2000 The OSI Reference Model

EECC694 - ShaabanEECC694 - Shaaban#1 lec #11 Spring2000 4-18-2000

The OSI Reference ModelThe OSI Reference Model


The Transport LayerThe 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.


Nesting of TPDUs, Nesting of TPDUs, Packets And FramesPackets And Frames


Data Link Layer Vs. Transport LayerData Link Layer Vs. Transport Layer

Data Link Layer Environment:Data Link Layer Environment: Adjacent routers. Adjacent routers.

Transport Layer Environment:Transport Layer Environment:End-to-End from source to destination.End-to-End from source to destination.


Network, Transport, And Application Network, Transport, And Application Layers InterfacesLayers Interfaces

(Transport Protocol Data Unit)


Simple Transport Layer Simple Transport Layer PrimitivesPrimitives

Primitives used to provide transport services to applications


Transport Layer:Transport Layer: Quality of Service (QoS) Parameters Quality of Service (QoS) Parameters

Application QoS requirements are specified using these parameters


State diagram for a simple connection scheme. Solid lines: Client state sequence. Dashed lines: Server state sequence.

Transport Layer Connection ManagementTransport Layer Connection Management


TransportTransportConnection Connection Addressing:Addressing:

TSAPs, TSAPs, NSAPs 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)


Transport Flow ControlTransport 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].


Transport Flow ControlTransport 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.


TPDU SequencingTPDU Sequencing

TPDUs may not be issued TPDUs may not be issued in the forbidden regionin the forbidden region

The re-synchronizationThe re-synchronization problem.problem.Connection data rate less than initial sequence number generation clock rate

T = Maximum Packet Lifetime


Transport Connection Transport Connection Protocol:Protocol:

Three-Way Three-Way Handshake Handshake

Normal operation Old duplicate CONNECTION REQUEST

Duplicate CONNECTION REQUEST and duplicate ACK


Abrupt Asymmetric Transport DisconnectionAbrupt Asymmetric Transport Disconnection Leads to loss of data and incorrect operation.

CR = Connection RequestDR = Disconnect Request


TransportTransportConnection Connection

Release Release ScenariosScenarios

Normal case of three-way handshake

Response lost and subsequent DRS lost

Final ACK lost

Response lost


Transport Layer: Transport Layer: Flow Control and BufferingFlow Control and Buffering

Chained fixed-size buffers

Chained variable-size buffers

One large circularbuffer per connection

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EECC694 - Shaaban #1 lec #11 Spring2000 4-18-2000 The OSI Reference Model