User Datagram protocol For Msc CS

11
User Datagram
Protocol
(UDP)

UDP is a communications protocol that offers a limited amount of service
when messages are exchanged between computers in a network that
uses the Internet Protocol (IP). UDP is an alternative to the
Transmission Control Protocol (TCP) and, together with IP, is sometimes
referred to as UDP/IP. Like the Transmission Control Protocol, UDP uses
the Internet Protocol to actually get a data unit (called a datagram) from
one computer to another.
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Unlike TCP, however, UDP does not provide the service of
dividing a message into packets (datagrams) and reassembling it at the
other end. Specifically, UDP doesn't provide sequencing of the packets
that the data arrives in. This means that the application program that
uses UDP must be able to make sure that the entire message has arrived
and is in the right order. Network applications that want to save
processing time because they have very small data units to exchange
(and therefore very little message reassembling to do) may prefer UDP
to TCP. The Trivial File Transfer Protocol (TFTP) uses UDP instead of
TCP.

The User Datagram Protocol (UDP) is one of the
core members of the Internet protocol suite (the set
of network protocols used for the Internet). With
UDP, computer applications can send messages, in
this case referred to as datagrams , to other hosts
on an Internet Protocol (IP) network without prior
communications to set up special transmission
channels or data paths. The protocol was designed
by David P. Reed in 1980 and formally defined in
RFC 768.
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UDP is located between the application layer
and the IP layer, and serves as the intermediary
between the application programs and the
network operations.
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77
Figure 14.1 Position of UDP in the TCP/IP protocol suite

Applications
Numerous key Internet applications use UDP, including: the
Domain Name System (DNS), where queries must be fast and
only consist of a single request followed by a single reply
packet, the Simple Network Management Protocol (SNMP),
the Routing Information Protocol (RIP)[2] and the Dynamic Host
Configuration Protocol (DHCP).
Voice and video traffic is generally transmitted using UDP
many businesses are finding that a recent increase in UDP
traffic from these real-time applications is hindering the
performance of applications using TCP, such as point of sale,
accounting, and database systems. When TCP detects packet
loss,
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USER DATAGRAM
UDP packets, called user datagrams, have a
fixed-size header of 8 bytes.

Example
The following is a dump of a UDP header in
hexadecimal format.
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a. What is the source port number?
b. What is the destination port number?
c. What is the total length of the user datagram?
d. What is the length of the data?
f. What is the client process?

Solution
a. The source port number is the first four
hexadecimal digits (CB84)16 or 52100.
b. The destination port number is the second four
hexadecimal digits (000D)16 or 13.
c. The third four hexadecimal digits (001C)16 define
the length of the whole UDP packet as 28 bytes.
d. The length of the data is the length of the whole
packet
minus the length of the header, or 28 – 8 = 20
bytes.
f. The client process is the Daytime (see Table 14.1).
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Encapsulation and decapsulation

Multiplexing and demultiplexing
TCP/IIPP PPrroottooccooll SSuuiittee 1155

1166
UDP is an example of the
Note
connectionless simple protocol with the
exception of an optional checksum
added to packets for error detection.

A client-server application such as DNS uses the
services of UDP because a client needs to send a
short request to a server and to receive a quick
response from it. The request and response can each
fit in one user datagram. Since only one message is
exchanged in each direction, the connectionless
feature is not an issue; the client or server does not
worry that messages are delivered out of order.
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Why UDP?

A client-server application such as SMTP which is
used in electronic mail, cannot use the services of
UDP because a user can send a long e-mail
message, which may include multimedia (images,
audio, or video). If the application uses UDP and the
message does not fit in one single user datagram, the
message must be split by the application into
different user datagrams. Here the connectionless
service may create problems. The user datagrams
may arrive and be delivered to the receiver
application out of order. The receiver application may
not be able to reorder the pieces. This means the
connectionless service has a disadvantage for an
application program that sends long messages.
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Assume we are downloading a very large text file
from the Internet. We definitely need to use a
transport layer that provides reliable service. We
don’t want part of the file to be missing or corrupted
when we open the file. The delay created between
the delivery of the parts are not an overriding
concern for us; we wait until the whole file is
composed before looking at it. In this case, UDP is
not a suitable transport layer.
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2200
UDP PACKAGE
To show how UDP handles the sending and
receiving of UDP packets, we present a
simple version of the UDP package.
We can say that the UDP package
involves five components: a control-block
table, input queues, a control-block module,
an input module, and an output module.

UDP design
TCP/IIPP PPrroottooccooll SSuuiittee 2211

The first activity is the arrival of a user datagram with destination port number
52,012. The input module searches for this port number and finds it. Queue
number 38 has been assigned to this port, which means that the port has been
previously used. The input module sends the data to queue 38. The control-block
table does not change.
After a few seconds, a process starts. It asks the operating system for a port
number and is granted port number 52,014. Now the process sends its ID
(4,978) and the port number to the control-block module to create an entry in
the table. The module takes the first FREE entry and inserts the information
received. The module does not allocate a queue at this moment because no
user datagrams have arrived for this destination (see Table 14.6).
A user datagram now arrives for port 52,011. The input module checks
the table and finds that no queue has been allocated for this
destination since this is the first time a user datagram has arrived for
this destination. The module creates a queue and gives it a number
(43). See Table 14.7.
After a few seconds, a user datagram arrives for port 52,222. The
input module checks the table and cannot find an entry for this
destination. The user datagram is dropped and a request is made
to ICMP to send an unreachable port message to the source.
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Reliability and congestion control solutions
Lacking reliability, UDP applications must generally be willing
to accept some loss, errors or duplication. Some applications,
such as TFTP, may add rudimentary reliability mechanisms
into the application layer as needed.
Most often, UDP applications do not employ reliability
mechanisms and may even be hindered by them. Streaming
media, real-time multiplayer games and voice over IP (VoIP)
are examples of applications that often use UDP. In these
particular applications, loss of packets is not usually a fatal
problem. If an application requires a high degree of reliability,
a protocol such as the Transmission Control Protocol may be
used instead.

User Datagram protocol For Msc CS

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User Datagram protocol For Msc CS