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73
•
Congestion control
measures ensure that a network’s performance is not eroded due to overload conditions.
•
Internetworking facilitates the communication between two stations situated on
different networks subnets. This requires protocol conversions and is handled by Interworking Units IWUs.
•
A router is a protocol converter which interconnects two networks of the same architecture. A gateway is a protocol converter which interconnects two
networks of different architectures.
•
The network layer is subdivided into three sublayers to facilitate internetworking: the subnet access sublayer deals with the services of a single subnet, the
subnet dependence convergence
sublayer deals with service mapping
between the subnets, the subnet independence convergence sublayer deal with internetworking services.
•
X.25
is a widely-known network layer standard. It provides no routing or switching functions, but rather an interface between DTEs and network DCEs.
X.25 supports three types of packets: data packets, control packets, and interrupt
packets.
•
Conventional character-based terminal can be connected to X.25 using a Packet AssemblerDisassembler
PAD. X.3 defines the PAD parameters. X.28
defines the interface between the PAD and the terminal, and X.29 defines
the interface between the PAD and the remote host.
•
X.75
supports the internetworking of X.25 subnets. The subnets are
interconnected by Signaling Terminals STEs which act as partial IWUs or routers.
•
The Internet Protocol IP is a connectionless datagram protocol developed by the DARPA. ISO 8473 is a standardization and simplification of IP.
www.pragsoft.com Chapter 5: The Transport Layer
65
5. The Transport Layer
This chapter describes the transport layer of the OSI model. The transport layer is concerned with the provision of host-to-host user connections for the reliable and cost-
effective transfer of user data. Although it may use a variety of networks and network protocols to realize its service, it hides such details from its users by offering a
transparent transport service.
The transport layer is similar to the network layer in that both attempt to offer a reliable data transfer service. Their main difference lies in that the transport layer looks
at the problem from an end-user’s point of view denoted by a process running on a host, while the network layer’s perspective encompasses lower level concerns such
as routing, connection modes, and internetworking. Also, the transport layer operates only on the end hosts, while the network layer also operates on intermediate network
nodes.
The transport layer is the lowest user layer in the OSI model. Its scope of responsibility is a function of the quality of service required of it by the user and the
quality of service provided by the network layer. The transport layer simply has to bridge this gap, which may be wide in some cases and negligible or nil in other cases.
We will first look at the transport service primitives and see how these define the transport service. Then we will describe the transport protocol and related issues, such
as segmentation, multiplexing, addressing, error control, and flow control. Finally, we will discuss TCP as an example of a popular transport layer standard.
5.1. Transport Services
As with the network layer, the transport services are defined in terms of transport primitives. Figure 5.57 summarizes the primitives together with their possible types and
parameters.
66 Communication Networks
Copyright © 2005 PragSoft
Figure 5.57 Transport service primitives.
Primitive Types
Parameters Purpose
T-CONNECT request
indicate response
confirm addresses,
option, QOS,
user data Used for initiating a transport connection
connections are always initiated by service users.
T-DISCONNECT request
indicate reason,
user data Used for disconnected a connection may
be initiated by service users or the service provider.
T-DATA request
indicate user data
Used for normal data transfer. T-EXPEDITED-DATA
request indicate
user data Used for high priority data transfer.
Addresses refer to Transport Service Access Points TSAP; these denote entities at the transport layer to which connections are made. Quality Of Service QOS
denotes a set of parameters such as connectionreleasetransfer delay, connectionreleasetransfer probability of failure, throughput, error rate which
collectively describe the quality of the transport service requested by the user. User data refers to actual user data provided by the service user for transfer by the service
provider. Option refers to the expedited data function being available in the service.
Figure 5.58 Sample scenario of transport services.
service user A
Transport
T-CONNECT request T-CONNECT indication
T-CONNECT confirm T-CONNECT response
T-DATA request T-DATA indication
T-EXPEDITED-DATA indication T-EXPEDITED-DATA request
T-DATA request T-DATA indication
T-DISCONNECT indication
service provider
Transport service
user B Transport
T-DISCONNECT indication
Figure 5.36 illustrates the use of the transport services in a sample scenario. Transport service user A first requests a connection, which is indicated to transport
service user B by the service provider. B responds to the request and the service provider confirms with A. Then B expedites some data which is indicated to A by the
service provider. Two normal data transfers from A to B, and from B to A then follow. Finally, the service provider simultaneously sends disconnect requests to both A
and B and terminates the connection.
