The User Datagram Protocol (UDP) is a connectionless protocol on the Transport layer of the
OSI model. The overall structure of UDP is simpler than TCP, because UDP is connectionless
and therefore does not have overhead to maintain connection information. UDP is commonly used for real-time applications such as video and voice. In these time-sensitive applications,
when a packet is lost or corrupted there is not enough time for the applications to recognize that
a packet is missing and request that it be resent, and for this retransmitted packet to arrive.
Therefore, the overhead that comes with TCP is not warranted for this type of data transfer.
The following frame snippet was taken using EtherPeek and is of a DNS request:
UDP - User Datagram Protocol
Source Port: 1213
Destination Port: 53 domain
Length: 38
Checksum: 0xBFBA
As you can see, all of the overhead that is associated with the connection-oriented nature of
the TCP frame, such as sequence and acknowledgment number, has been removed in UDP. As
a result, the UDP packet is condensed down to four fields.
The first two of these fields, Source Port and Destination Port, are both 16 bits long. The
Destination Port field must be filled in with the destination port of the service that is being
requested; however, the Source Port field only needs a value when the sending station needs
a reply from the receiver. When the conversation is unidirectional and the source port is not
used, this field should be set to 0. When a reply is needed, the receiving station will reply to the
sender on the port indicated in the original packet’s source field.
The last two fields in a UDP header are Length and Checksum. Like the source and destination
port information, the length and checksum are both 16 bits long. The Length field shows the total
number of bytes in the UDP packet, including the UDP header and user data. Checksum, though
optional, allows the receiving station to verify the integrity of the UDP header as well as the data
that is contained in the packet. If Checksum is not used, it should be set to a value of 0.
A great deal of information is covered in this chapter, with the focus on Network and Data Link
layer protocols. It is important to understand this information in order to facilitate your troubleshooting
efforts. If you do not sufficiently understand the protocols present in layers 2 and 3 of
the OSI model, you should study them in depth. The majority of networking problems occur in
these two layers.
Many encapsulation types are available at the second layer of the OSI model. The ones
discussed in this chapter were Ethernet, PPP, SDLC, Frame Relay, and ISDN. Each has its
own strengths and weaknesses that make it better suited for a particular installation.
There are two major protocol classifications: connection-oriented and connectionless.
Connection-oriented protocols allow for sequenced data transfer, flow control, and error
control. Examples of connection-oriented protocols include ATM and TCP. Connectionless
protocols require less overhead; however, they do so at the expense of the sequenced data
transfer and the error and flow control offered by connection-oriented protocols. The connectionless
protocol discussed in this chapter is UDP.
Know the differences between connectionless and connection-oriented protocols. Connectionoriented
protocols have flow-control and error-checking methodologies that are not present in
connectionless protocols. Connectionless protocols offer better performance characteristics for
real-time voice and video applications.
Know the Data Link protocols and technologies. The major technologies covered in this
section include Ethernet, PPP, SDLC (HDLC), Frame Relay, and ISDN.
Know how to calculate subnet masks. Understand how VLSM functions, and know how to
determine an appropriate address and subnet mask combination. 1122