What is TCP (Transmission Control Protocol)?

TCP, or Transmission Control Protocol, plays a critical role in the operation of modern computer networks. Developed in the late 1960s and early 1970s, this protocol facilitated resource sharing between different networks. Over time, networks evolved, and the demand for reliable data transfer increased. Therefore, TCP became a fundamental element in ensuring a smooth connection.

In this article, I will tell you precisely what the TCP protocol is and what its features are. I will also discuss its historical context, its integration into network architecture, and its role in data exchange.

What is TCP Protocol in Networking?

TCP (Transmission Control Protocol) was created in the late 1960s and early 1970s when networks were not made to share resources between different networks.

It has become increasingly crucial for user applications to share resources. However, network administrators must agree on standard technologies for networks to communicate. Therefore, applications such as file transfer and e-mail must also be standardized. With these goals, the Transmission Control Protocol (TCP) was developed.

The primary purpose of TCP is to provide reliable connection services between pairs of processes. This cannot be supplied by lower-level protocols such as IP, so TCP must guarantee reliability on its own.

TCP is located at the transport level of the traditional model. It is situated between the IP and the upper levels and is not loaded on gateways. It protects the integrity of data transfer between endpoints. Usually, this protocol is located on user computers. IP is a connectionless network, so TCP takes on tasks such as reliability, flow control, sequencing, and shutdown.

TCP and IP are closely related, which is why they are called “TCP/IP.” However, it can support other protocols. For example, it can work with non-connection-oriented protocols such as ISO 8473 (CNLP). In addition, application protocols such as SMTP are supported by many of the services offered by TCP.

TCP’s Main Features

TCP protocol provides various services at high levels and is connection-oriented. This means that it keeps information about the status of each user’s data. Also, it manages the data transfer between endpoints in the networks.

TCP ensures that the data is transmitted correctly. It guarantees that the info is received correctly. It transfers each character reliably. It also uses sequence numbers and performs accept/reject operations.

Connection-oriented protocols work as a virtual circuit. Each octet transmitted is assigned a sequence number. The receiving TCP uses checksums to check for data corruption. If the info is accepted, it sends a positive sense (ACK).

If the data is corrupted, the receiver discards the info. The receiver reports the problem using a sequence number. Also, it uses timers before the data is transmitted from the sending node.

TCP supports string-oriented protocols. It sends data byte by byte. The bytes are grouped to form segments. These segments are transmitted to the IP. In short, it determines the length of the segments, and the system developers can decide on this.

Transmission Control Protocol supports the use of variable-length segments. However, applications that work with fixed-length data blocks cannot use it. Therefore, the application level must configure its strings.

On the other hand, the Transmission Control Protocol looks at the copying of data. The receiving TCP discards unnecessary info. In addition, it supports the concept of a push function. This function ensures that all information is transmitted.

Buffer management provides this function. This protocol receives a command to send all traffic to the destination. In addition, it reorders out-of-sequence segments. It eliminates duplicate datagrams.

TCP uses a comprehensive acknowledgment scheme. This number accepts bytes ranging from zero to a missing value. In addition, the acknowledgment timeout may elapse, and the traffic may be resent. This may add additional load to the network.

The receiving controls the info flow. This prevents overflows and saturation. So, it sends a “window” value and does not allow the transmitter to exceed this value. In addition, it supports multi-user sessions.

In short, it provides full-duplex communication. Transmission is performed in both directions without waiting for a direction change signal. In addition, it determines the security levels and ensures the safe closure of virtual circuits.

Active and Passive Clearances

TCP ports can establish two types of connections. Passive open mode allows the higher-level protocol (such as a server) to wait for connection requests to arrive from the remote system. This mode does not require sending an active open to the computer’s operating system.

When the request is received, the operating system assigns a port number. This number is used to communicate with remote users. In addition, application processes that request passive opens can accept incoming requests if the compatibility requirements are met.

If any call can be accepted, the external socket number is reset. However, unspecified external socket numbers are only allowed in passive opens. However, another form of connection establishment is active open mode.

In active open mode, the higher-level protocol uses a specific socket to establish a connection. Typically, an active open is sent to a passively opened port to create a virtual circuit. It supports cases where two active opens are sent simultaneously and establishes the connection.

This feature allows applications to send an open whenever they want. It can establish a connection regardless of whether other applications have sent another open. It also specifies strict rules for how passive and active opens are used.

First, an active exploit identifies a specific socket and its security levels. So, it guarantees the establishment of the connection if a compatible passive exploit or active exploit is available.

The Transmission Control Block (TCB)

TCP stores a lot of information about each virtual connection. It stores this information in the Transmission Control Block (TCB). The TCB contains the local and remote socket numbers.

It also includes pointers for sending and receiving buffers. In addition, pointers to the retransmission queue and the security and priority values ​​of the connection are also stored. It highlights the ongoing segment.

However, the TCB also contains the send and receive sequence numbers. The PDUs exchanged between two modules are called segments. A segment is divided into two parts: the header and the data. The data section follows the header. The first two fields in the segment are the source and destination ports. These 16-bit fields identify the higher-level applications using the connection.

TCP Segment (PDU)

The following field is called the sequence number. This field contains the sequence number of the first octet of the user data field. Its value specifies the location of the bit string of the transmitter module. It also identifies the first octet of user data within the segment.

However, the sequence number is used during connection management. If two entities are utilizing the connection request segment, the sequence number serves specific functions. For example, it specifies the first send sequence number (ISP) to be used for subsequent numbering of user data.

The value of the acknowledge number allows the acceptance of previously received data. This field contains the value of the sequence number of the next octet expected to be received from the transmitter. In addition, this definition allows the acceptance of all octets up to the value of minus 1.

The data offset field specifies the number of 32-bit aligned words that make up the TCP header. This field is used to determine where the info field begins. The reserved field consists of 6 bits that must be zero. These bits are reserved for future use.

The following six bits are called flags. Flags are control bits and indicate specific services or utilities. Some values ​​of these bits indicate how other fields in the header are to be interpreted. These six bits carry various information.

• URG indicates that the urgency marker area is essential.
• ACK indicates whether the acceptance area is essential.
• PSH means that the module will use the push function.
• RST indicates that the connection will start.
• SYN indicates that the sequence numbers will be synchronized; It is used as an indication that some setup operations will be performed in the connection setup segments.
• END indicates that the sender has no other data to send. In other protocols, it can be compared to the end of the transmission signal (EOT).

The following field, called the window, indicates how many octets the receiver will accept. This value is determined by the value of the accept field (accept number). It is also created by adding the values ​​of the window field and the accept number field.

The checksum field contains the 1 to 16-bit complement of the 1-complement of the sum of the 16-bit words in the segment. Its purpose is to determine whether the segment from the transmitter is error-free. In addition, a field in the segment called the urgent pointer is used only if the URG flag is 1.

The purpose of this pointer is to identify the data octet followed by the urgent data. Urgent data is called out-of-band info. However, what is done with the urgent data is up to the application. In other words, only the pointer to the start of the urgent info is specified.

The value in this field is an offset to the sequence number and indicates the octet that the urgent data follows. The options field is designed to allow for future TCP development. Similar to the options field of IP datagrams, it contains a byte indicating the number of each option. It also shows the length of the option and, finally, the option itself.

Currently, the use of the option field is quite limited. The TCP standard specifies only three options: 0 – end of option list, 1 – no operation, and 2 – maximum segment size.

Finally, the user data is below. Internet protocols are one of the most widely used protocol families in the world. These protocols are designed to facilitate communication in computer networks. In addition, network protocols include application-level protocols such as TELNET, FTP, and SMTP.

• 0: end of option list
• 1: no action
• 2: maximum segment size

Finally, the user data is shown below. Internet protocols are one of the most widely used protocol families in the world.

These protocols are designed to facilitate communication across computer networks. Internet protocols consist of many protocols to support communication between networks. In addition, network protocols include many application-level protocols such as TELNET, File Transfer Protocol (FTP), and Simple Mail Transfer Protocol (SMTP).