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o ACK indicates if the information in the Acknowledgment
field is relevant.
o RST resets TCP connections.
o PSH tells the destination that the DATA should be
delivered to the ULP upon delivery.
o FIN ends the TCP connection.
• Window—This field is used to provide flow control information.
The value is the amount of data the sender can accept.
UDP
The connectionless transport service of the TCP/IP protocol suite is user datagram
protocol (UDP). UDP uses the same port process abstraction that TCP uses. Its
function is to provide low-overhead, fast-transport service for ULP data transport.
UDP has none of the flow control and data synchronization mechanisms that TCP
offers. It processes one packet at a time and offers best-effort delivery service.
The UDP header (see Figure 2.17) is 32 bits long and contains information on the
source and destination ports, the packet size, and an optional checksum for
applications that require checksum support (BOOTP and DHCP).
Figure 2.17. The UDP header.
Like TCP, UDP has a set of reserved ports used for different application server data
exchange points. The most commonly used UDP ports are shown in Table 2.13.
Table 2.13. Commonly Used UDP Ports
Port Number Service
49 TACACS authentication server