User Datagram Protocol (UDP) is a connectionless protocol. It is simpler than
TCP in implementation and does not have overheads such as the three-way
handshake. UDP corresponds to the transport layer (Layer 4) of the OSI reference
model. As a complement to the IP protocol, it serves as a transport mechanism to
applications that need efficiency in transmission rather than reliability, and
is used when error correction or even resend does not make sense. Examples of
such applications are video streaming and voice applications.
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| User Datagram Protocol (UDP) Packet Format |
The nature of how the UDP protocol behaves and its format have
the following implications with respect to resiliency:
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As you can see, the UDP datagram format is much simpler than that of TCP. It does not have most of the characteristics of TCP recovery. Because UDP does not provide a recovery function, it depends on higher-layer applications to do so.
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You might wonder why you should implement UDP if it does not provide for a reliable transmission. UDP does one thing better than TCP: performance. It is useful in situations where performance is preferred over reliability of the protocol. Some applications work on the basis of short message passing with a small chance of error. The application might provide its own error-recovery mechanism. The following applications use UDP:
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- Network File System (NFS)
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- Simple Network Management Protocol (SNMP)
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- Domain Name System (DNS)
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- Trivial File Transfer Protocol (TFTP)
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UDP does not have a flow control mechanism like that of TCP. In other words, features such as WRED will not work on the UDP protocol. Therefore, in the event of network congestion, an application that uses UDP cannot be slowed down by the network. The throttling mechanism must come from the application itself. From a network-resiliency perspective, these applications become more difficult to control. Although you may choose to police the transmission rate of these applications, dropping too many packets might affect its quality.
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If you are not familiar with UDP, familiarize yourself now. Most video and VoIP applications run on UDP. Learning to deal with UDP will prove important as you try to keep your voice network running continuously.
So far, this chapter has highlighted the characteristics of the
major protocols such as IP, TCP, and UDP and how their behaviors impact the
resiliency of the network. Another area that contributes to the resiliency of
the network is the high-availability feature of the hardware that supports the
running of these protocols.
This chapter now looks at how fast the hardware can recover
from a failure so as not to affect the running of these protocols, which
ultimately affect the applications that are running on them. The following
section discusses the development of these hardware features and how the
improvements seek to complement the behaviors of the major protocols in
supporting a resilient IP network.
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