Copyright Goodheart-Willcox Co., Inc. Chapter 8 Overview of Network Security and Network Threats 263 destination machine to ensure the frame has not been corrupted or damaged dur- ing transit. Think of the CRC as a packing slip for your online order. It identifi es everything that should be in the package, such as which DVDs you purchased or the number of shirts included in the shipment. If the CRC is not what is expected, the frame is refused. To use a protocol analyzer and similar tools, you must be able to decipher the information being transmitted. To understand this, fi rst compare the TCP and UDP segment information. TCP Figure 8-14 shows the detailed information that will be found in a TCP seg- ment. This segment is created at the Transport layer of the OSI model (layer 4). The ports are important. They show where the data are entering and exiting the system. For example, a data request asking for a specifi c web page will show the destination port of 80 or 443. The source port will be dynamically assigned. The sequence number is a 32-bit number used to track how much data are sent. During trans- missions, the Acknowledgement fi eld will refl ect a value indicating the data were successfully received. Figure 8-15 explains the meaning of fl ags that are found in the TCP segment. Flags are 1-bit fi elds that may signal special conditions in the header. The checksum value is used to detect if there was any corruption of the data sent. It is important that the data integrity be maintained. This is a 16-bit fi eld in which data bytes are added together. It is computed on the receiving host to verify they are the same. To help understand the layers in a packet, think of the Russian stacking or nesting dolls. Each doll fi ts inside the next larger doll. FYI TCP or UDP segment IP Packet Frame Goodheart-Willcox Publisher photo: Ibreakstock/Shutterstock.com Figure 8-13. A packet is constructed by encapsulating data in layers. To under- stand this, it can help to picture Russian stacking dolls. Goodheart-Willcox Publisher Figure 8-14. This is how header information of a TCP segment is assembled. Source Port Destination Port Sequence Number Acknowledgement Header Length Reserved URG ACK PSH RST SYN FIN Sliding window size Checksum Urgent Pointer Options Padding Data