CCNA Cisco Certified Network Associate Study Guide - FTP Server

FIGURE 3.4 UDP segment
TCP/IP and the DoD Model 111
UDP receives upper-layer blocks of information, instead of data streams
as TCP does, and breaks them into segments. Like TCP, each UDP segment
is given a number for reassembly into the intended block at the destination.
However, UDP does not sequence the segments and does not care in which
order the segments arrive at the destination. At least it numbers them,
though. But after that, UDP sends the segments off and forgets about them.
It doesn’t follow through, check up on them, or even allow for an acknowledgment
of safe arrival—complete abandonment. Because of this, it’s
referred to as an unreliable protocol. This does not mean that UDP is ineffective,
only that it doesn’t handle issues of reliability.
Further, UDP doesn’t create a virtual circuit, nor does it contact the destination
before delivering information to it. It is, therefore, also considered
a connectionless protocol. Since UDP assumes that the application will use
its own reliability method, it doesn’t use any. This gives an application developer
a choice when running the Internet Protocol stack: TCP for reliability
or UDP for faster transfers.
UDP Segment Format
The very low overhead of UDP compared to TCP, which doesn’t use windowing
or acknowledgments, is shown in Figure 3.4.
Bit 0 Bit 15
Source port (16) Destination port (16)
Length (16) Checksum (16)
Data (if any)
Bit 16 Bit 31
You need to understand what each field in the UDP segment is. The UDP
segment contains the following fields:
Source port Port number of the host sending the data
Destination port Port number of the application requested on the destination
host
8 bytes