TCP over wireless Networks ~ Webtechnology5

TCP (Transmission Control Protocol)

TCP (Transmission Control Protocol) is a standard that defines how to establish and maintain a network conversation through which application programs can exchange data. TCP works with the Internet Protocol (IP), which defines how computers send packets of data to each other.

The Transmission Control Protocol (TCP) is one of the main protocols of the Internet protocol suite. It originated in the initial network implementation in which it complemented the Internet Protocol (IP). Therefore, the entire suite is commonly referred to as TCP/IP. TCP provides reliable, ordered, and error- checked delivery of a stream of octets (bytes) between applications running on hosts communicating via an IP network. Major internet applications such as the world Web, email, remote administration, and file transfer rely on TCP, which is part of the Transport Layer of the TCP/IP suite. SSL/TLS often runs on top of TCP.

TCP is connection-oriented, and a connection between client and server is established (passive open) before data can be sent. Three-way handshake (active open), retransmission, and error-detection adds to reliability but lengthens latency. Applications that do not require reliable data stream service may use the User Datagram Protocol (UDP), which provides a connectionless datagram service that prioritizes time over reliability. TCP employs network congestion avoidance. However, there are vulnerabilities to TCP including denial of service, connection hijacking, TCP veto, and reset attack. For network security, monitoring, and debugging, TCP traffic can be intercepted and logged with a packet sniffer.

figure show: Connection establishment

To establish a connection, TCP uses a three-way handshake. Before a client attempts to connect with a server, the server must first bind to and listen at a port to open it up for connections: this is called a passive open. ... ACK: Finally, the client sends an ACK back to the server.

Protocol operation

TCP protocol operations may be divided into three phases. Connections must be properly established in a multi-step handshake process (connection establishment) before entering the data transfer phase. After data transmission is completed, the connection termination closes established virtual circuits and releases all allocated resources.

A TCP connection is managed by an operating system through a resource that represents the local end-point for communications, the Internet socket. During the lifetime of a TCP connection, the local end-point undergoes a series of state changes:
LISTEN
(server) represents waiting for a connection request from any remote TCP and port.

SYN-SENT
(client) represents waiting for a matching connection request after having sent a connection request.

SYN-RECEIVED
(server) represents waiting for a confirming connection request acknowledgment after having both received and sent a connection request.

ESTABLISHED
(both server and client) represents an open connection, data received can be delivered to the user. The normal state for the data transfer phase of the connection.

FIN-WAIT-1
(both server and client) represents waiting for a connection termination request from the remote TCP, or an acknowledgment of the connection termination request previously sent.

FIN-WAIT-2
(both server and client) represents waiting for a connection termination request from the remote TCP.

CLOSE-WAIT
(both server and client) represents waiting for a connection termination request from the local user.

CLOSING
(both server and client) represents waiting for a connection termination request acknowledgment from the remote TCP.

LAST-ACK
(both server and client) represents waiting for an acknowledgment of the connection termination request previously sent to the remote TCP (which includes an acknowledgment of its connection termination request).

TIME-WAIT
(either server or client) represents waiting for enough time to pass to be sure the remote TCP received the acknowledgment of its connection termination request. [According to RFC 793 a connection can stay in TIME-WAIT for a maximum of four minutes known as two maximum segment lifetime (MSL).]

CLOSED
(both server and client) represents no connection state at all.

What TCP is used for

TCP is used for organizing data in a way that ensures the secure transmission between the server and client. It guarantees the integrity of data sent over the network, regardless of the amount. For this reason, it is used to transmit data from other higher-level protocols that require all transmitted data to arrive. Examples include:

Secure Shell (SSH), File Transfer Protocol (FTP), Telnet: For peer-to-peer file sharing, and, in Telnet's case, logging into another user's computer to access a file.
Simple Mail Transfer Protocol (SMTP), Post Office Protocol (POP), Internet Message Access Protocol (IMAP): For sending and receiving email
HTTP: For web access

These examples all exist at the application layer of the TCP/IP stack and send data downwards toward TCP on the transport layer.

Why TCP is important

TCP is important because it establishes the rules and standard procedures for the way information is communicated over the internet. It is the foundation for the internet as it exists today and ensures that data transmission is carried out uniformly, regardless of the location, hardware or software involved. For this reason, it is flexible and highly scalable, meaning new protocols can be introduced to it and it will accommodate them. It is also nonproprietary, meaning no one person or company owns it.

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