COMPUTER NETWORKS Chapter 06 The Transport Layer

Chapter 6 The Transport Layer

Essence: The transport layer is responsible for completing the services of the underlying network to the extent that application development can take place

6.1 The Transport Service

• 6.1.1 Services Provided to the Upper Layer Ø provide reliable connection-oriented services Ø provide unreliable connectionless services Ø provide parameters for specifying quality of services

• Important: we’re talking about efficient and costeffective services, in particular reliable connections.

• Note: depending on the services offered by the network layer, the added functionality in the transport layer can vary considerably.

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Consequence: If we want to develop applications that are independent of the particular services offered by a carrier, we’ll have to design a standard communication interface and implement that interface at the client’s sites. The transport layer contains such implementations.

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Quality of Service: QoS

• Connection establishment delay: how long does it take to establish a connection. The shorter, the better.

• Connection establishment failure probability: Chance of not being able to establish a connection within the specified maximum delay. Should be as low as possible.

• Throughput: how many bytes per second can a client pass through a connection.

• Transit delay: how long does it take for a message to make it from sender to receiver. The shorter, the better.

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Quality of Service: QoS

• Residual error ratio: fraction of lost or garbled messages. Should be as near to zero as possible.

• Protection: to what extent is a client’s message protected against eavesdropping or tampering.

• Priority: is it possible for a client to indicate that one connection should be serviced at a higher priority than another one.

• Resilience: probability that the transport layer looses a connection (i.e., the connection breaks on account of network failures).

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• 6.1.2 Transport Service Primitives

• The transport service primitives allow transport users (e.g., application programs) to access the transport service. Each transport service has its own access primitives.

Allows application programs to establish, use, and release connections.

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Transport Interface and TPDU TPDU, Transport Protocol Data Unit, for message sent from transport entity to transport entity.

Differences between transport service and network service

network service is intended to model the service, generally unreliable, connection oriented transport service is reliable.

network service is only used by transport entities, transport service must be convenient and easy to use by application programs.

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Connection establishment and release

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