Computing – Communications and Networks

Transport Layer
 Why a transport layer ?
 transport layer = makes network service available to programs
 is end-to-end only, not in routers
 In TCP/IP there are two transport protocols
 UDP (user datagram protocol)
 unreliable
 offers a datagram service to the application (unit of information is a message)
 TCP (transmisssion control protocol)
 reliable
 offers a stream service (unit of information is a byte)


Physical, data link and network layers are sufficient to build a packet transport system between
computers. However, this is not enough for the programmer. When you write a low-level
program which uses the network (as we will do in this lecture), you do not handle packets, but
data. The primary goal of the transport layer is to provide the programmer with an interface to
the network.
Second, the transport layer uses the concept of port. A port is a number which is used locally (on
one machine) and identifies the source and destination of the packet inside the machine. We will
come back to the concept of ports later in this chapter.
The transport layer exists in two varieties: unreliable and reliable. The unreliable variety simply
sends packets, and does not attempt to guarantee any delivery. The reliable variety, in contrast,
makes sure that data does reach the destination, even if some packets may be lost from time to
time. In the Internet there are two transport protocols, TCP and UDP, either of which can
transport application-layer messages. TCP provides a connection-oriented service to its
applications. This service includes guaranteed delivery of application-layer messages to the
destination and flow control (that is, sender/receiver speed matching). TCP also segments long
messages into shorter segments and provides a congestion control mechanism, so that a source
throttles its transmission rate when the network is congested. The UDP protocol provides its
applications a connectionless service, which is very much a no-frills service.