System information

Troubleshooting TCP/IP 7-97
The Transport Layer
IRDP offers several advantages over other methods of discovering addresses of neighboring routers.
Primarily, it does not require hosts to recognize routing protocols, nor does it require manual
configuration by an administrator.
Router advertisement messages allow hosts to discover the existence of neighboring routers, but not
which router is best to reach a particular destination. If a host uses a poor first-hop router to reach a
particular destination, it receives a redirect message identifying a better choice.
The Transport Layer
The Internet transport layer is implemented by TCP and the User Datagram Protocol (UDP). TCP
provides connection-oriented data transport, whereas UDP operation is connectionless.
TCP
TCP provides full-duplex, acknowledged, and flow-controlled service to upper-layer protocols. It
moves data in a continuous, unstructured byte stream where bytes are identified by sequence
numbers. TCP can also support numerous simultaneous upper-layer conversations. The TCP packet
format is shown in Figure 7-8.
Figure 7-8 The TCP Packet Format
The fields of the TCP packet are as follows:
• Source port and destination port—Identify the points at which upper-layer source and destination
processes receive TCP services.
• Sequence number—Usually specifies the number assigned to the first byte of data in the current
message. Under certain circumstances, it can also be used to identify an initial sequence number
to be used in the upcoming transmission.
• Acknowledgment number—Contains the sequence number of the next byte of data the sender of
the packet expects to receive.
• Data offset—Indicates the number of 32-bit words in the TCP header.
• Reserved—Reserved for future use.
• Flags—Carries a variety of control information.
Data offset Reserved Flags Window
Sequence number
Acknowledgment number
Checksum Urgent pointer
Options (+ padding)
Data (variable)
Source port Destination port