UDP Packet Header

241

Table C 3 TCP Packet Header Fields (continued)

Field

Explanation

Checksum

This is a checksum for TCP header and data. It is 16 bits long.

Urgent Pointer

This field is used only when the URG flag is set. It is 16 bits long.

Options

This part is of variable length.

UDP Packet Header

The UDP packet header is simple and is described in RFC 768. It has four fields as

shown in Figure C 5. Each field is 16 bits long. Names of all fields are self explanatory.

Source Port

Destination Port

Length

Checksum

Figure C 5 UDP packet header

ARP Packet Header

ARP packets are used to discover the hardware or MAC addresses when the IP address

is known. In any LAN, you will see a lot of ARP packets being transmitted. This is

because each host has to find out the MAC address of the destination host before send 

ing data. The ARP is a broadcast protocol and its packet header is shown in Figure C 6.

HW Address Type

Protocol Address Type

HW Addr Len

Proto Addr Len

Operation

Source Hardware Address

Source Hardware Address (Continued)

Source Protocol Address

Source Protocol Address (Continued)

Target Hardware Address

Target Hardware Address (Continued)

Target Protocol Address

Figure C 6 ARP header