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.

To maximize throughput, TCP enables each station to send multiple packets before an acknowledgment arrives. After the sender receives an acknowledgment for an outstanding packet, the sender slides the packet window along the byte stream and sends another packet. This flow control mechanism is known as a sliding window.

TCP can support numerous simultaneous upper-layer conversations. The port numbers in a TCP header identify an upper-layer conversation. Many well-known TCP ports are reserved for File Transfer Protocol (FTP), World Wide Web (WWW), Telnet, and so on.

Within the signaling portion of VoIP, TCP is used to ensure the reliability of the setup of a call. Due to the methods by which TCP operates, it is not feasible to use TCP as the mechanism to carry the actual voice in a VoIP call. With VoIP, packet loss is less important than latency.

The TCP packet fields are as follows:

• Source port and destination port—Identifies 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 also can 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—Carry a variety of control information.

• Window—Specifies the size of the sender's receive window (that is, buffer space available for incoming data).

• Checksum—Indicates whether the header and data were damaged in transit.

• Urgent pointer—Points to the first urgent data byte in the packet.

• Options—Specifies various TCP options.

• Data—Contains upper-layer information.

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