TCP/IP Layer 4 Protocols: TCP and UDP
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The OSI transport layer (Layer 4) defines several functions, the most important of which are
error recovery and flow control. Likewise, the TCP/IP transport layer protocols also implement
these same types of features. Note that both the OSI model and the TCP/IP model
call this layer the transport layer. But as usual, when referring to the TCP/IP model, the
layer name and number are based on OSI, so any TCP/IP transport layer protocols are considered
Layer 4 protocols.
The key difference between TCP and UDP is that TCP provides a wide variety of services
to applications, whereas UDP does not. For example, routers discard packets for many reasons,
including bit errors, congestion, and instances in which no correct routes are known.
As you have read already, most data-link protocols notice errors (a process called error
detection) but then discard frames that have errors. TCP provides retransmission (error
recovery ) and helps to avoid congestion (flow control), whereas UDP does not. As a result,
many application protocols choose to use TCP.
However, do not let UDP’s lack of services make you think that UDP is worse than TCP. By
providing fewer services, UDP needs fewer bytes in its header compared to TCP, resulting
in fewer bytes of overhead in the network. UDP software does not slow down data transfer
in cases where TCP can purposefully slow down. Also, some applications, notably today
Voice over IP (VoIP) and video over IP, do not need error recovery, so they use UDP. So,
UDP also has an important place in TCP/IP networks today.
Table 5-2 lists the main features supported by TCP/UDP. Note that only the first item listed
in the table is supported by UDP, whereas all items in the table are supported by TCP.