Transmission Control Protocol

[1] The Transmission Control Protocol (TCP) is one of the main protocols of the Internet protocol suite, providing reliable, ordered, and error-checked delivery of a stream of octets (bytes) between applications running on hosts communicating via an IP network. It originated in the initial network implementation in which it complemented the Internet Protocol (IP). So the whole suite is often called TCP/IP.

Unlike UDP (user Datagram Protocol), TCP is a connection-oriented protocol, needing a logical connection to be established between endpoints prior data transmissions.

Characteristics and features

Connection-oriented

Prior sending data, TCP makes a connection between the sender and receiver, via the 3-way handshake, after which the connection is established.

Reliable delivery

TCP guarantees data reaches its destination. If a packet is lost, the receiver doesn't acknowledge it. The sender sends it again.

Ordered delivery

Packets may travel different routes. So TCP uses sequence numbers to put them back in the correct order.

E.g., if the following are sent

Packet 1

Packet 2

Packet 3

But it receives:

Packet 3

Packet 1

Packet 2

Then, TCP rearranges them to:

Packet 1

Packet 2

Packet 3

Error checking

TCP uses a checksum to detect corrupted data.

If corruption is detected:

The packet is discarded.

It is retransmitted.

Flow control

TCP prevents a fast sender from overwhelming a slow receiver.

The receiver tells the sender how much data it can accept using the window size.

window (computer network protocol): a buffer management and flow control mechanism defining number of data packets/bytes sender can transmit prior needing an ACK from receiver, preventing fast sender from overwhelming slow receiver, keeps data moving efficiently without waiting each packet.

Congestion control

If the network is busy, TCP slows down its sending rate to reduce congestion. Congestion control protects the intermediate network infrastructure from router queue overflows. The sender calculates a Congestion Window (cwnd) based on network feedback: effective window size = min(rwnd, cwnd)

congestion: blocked, crowded
Slow start: By initiating/recovering a connection, cwnd starts small (e.g., 1 MSS) and doubles each Round-Trip Time (RTT), growing exponentially til reaching a slow-start threshold (ssthresh).
Congestion avoidance: Once cwnd \ge ssthresh$, growth switches to additive linear increments (+1 MSS per RTT) to probe network capacity safely.
Fast recovery is used alongside Fast Retransmit to adjust cwnd downward after loss without reverting all the way back to the initial Slow Start state.

Maximum segment size

The maximum segment size (MSS) defines the max amount of TCP payload data (excluding IP and TCP headers) that a device can transmit in a single segment. It is determined using the Maximum Transmission Unit (MTU):

MSS = MTU - IP header size - TCP header size

For standard Ethernet, with MTU of 15 kBm stabdard 20-byte IP and 20-byte TCP headers yield an MSS of 1460 B.

Sequence and acknowledgment numbers

TCP views datas as unstrctured ordered stream of bytes. A transmitted byte is assigned a sequential sequence number.

The sequence number (seq) specifies the sequence number of the first data byte in the current segment.
The acknowledgement number (ACK) uses cumulative acknowledgements, representing the next byte sequence number the receiver expects to receive.
E.g., if host A sends bytes 1000–1499 to B, then B responds with ACK = 1500, signaling all bytes up to 1499 are received successfully.
- cumulative: growing/increasing by adding more overtime

Retransmission mechanisms

Retransmission mechanisms (RTO): If an ACK isn't received prior adaptive RTO tiemr expires, the sender assumes packet loss and retransmits the segment.
Fast retransmit: If a receiver detects a gap in sequence numbers, it repeatedly sends duplicate ACKs for the last expected byte. When the sender receives 3 duplicate ACKs (4 identical ACKs total), it immediately retransmits the missing segment without waiting for the RTO timer to expire.

Addressing and sockets

TCP uses port numbers combined to IP addresses to multiplex data streams across many applications on the same host. A TCP socket pair (or 4-tuple) uniquely identifies any active internet connection:

socket pair = source IP, source port, destination IP, destination port)

Well-known TCP ports are:

Port 20 / 21: FTP (File Transfer Protocol)
Port 22: SSH (Secure Shell)
Port 23: Telnet
Port 25: SMTP (Simple Mail Transfer Protocol)
Port 80: HTTP (Hypertext Transfer Protocol)
Port 110: POP3 (Post Office Protocol v3)
Port 143: IMAP (Internet Message Access Protocol)
Port 443: HTTPS (HTTP Secure)

TCP and UDP comparisons

Headers

In networking, the TCP Header refers to the control infos attached to the start of a computer's TCP data packet (segment) for reliable data delivery.

control information: data used in computer and management system to direct/manage/coordinate operations, for control, system logs, configuration parameters

But none of these are the true data we're trying to send (like a webpage's HTML, an image, or a text message). It's instead the instructions and bookkeeping data TCP uses behind the scenes so both systems know:

Where the data is from and goes (ports)
How to order the data (sequence numbers)
If data arrived safely (ACKs and checksums)
Data sent speed (window size)
The connection's state it's in (flags like SYN, FIN, RST)

A TCP packet can be thought as a physical shipping box, where:

The payload (data): The actual message or file content inside the box.
The TCP header: The shipping label outside the box.

It has all the meta-info needed for the network and the receiving device to handle, track, verify, and reassemble the data correctly.

Source and destination ports identify which specific application on the sending device is talking to which application on the receiving device (e.g., web traffic on Port 80/443).
Sequence number tracks the order of data chunks so the receiving device can put them back together in the exact right sequence.
Acknowledgment (ACK) Number confirms to the sender which data packets have been successfully received so far.
Flags (SYN, ACK, FIN, etc.) control signals used to manage the lifecycle of the connection (e.g., initiating, acknowledging, or closing a connection).
Window size is used for flow control—tells the sender how much data the receiver can handle before pausing.
Checksum is a mathematical check value used to detect if any data got corrupted during transmission.

TCP flags

TCP flags are control infos in form of 1-bit binary switches in the TCP header to control a connection state and manage how data is handled between the sender and receiver. If a flag is set to 1 (logic ON), it signals a specific instruction.

SYN (Synchronize) is used to initiate and establish a connection during the 3-way handshake.
ACK (Acknowledgment) confirms a packet/data set is successfully received.
FIN (Finish) requests to gracefully close and terminate a connection when data transfer is complete.
RST (Reset) instantly aborts/closes a connection due to an error, unexpected packet, or security issue.
PSH (Push) forces the receiving system to deliver buffered data immediately to the application without waiting for a buffer filling up.
URG (Urgent) indicates data in the segment must be prioritized and processed immediately by the receiver.

Other flags:

ECE (ECN-Echo): Signals network congestion back to the sender.

CWR (Congestion Window Reduced): Confirms to the receiver that the sender reduced its transmission speed after receiving an ECE signal.

Connection connection and termination

The TCP starts a connection via a 3-way handshake prior sending data to create a connection between the sender and receiver.

It then closes a connection with a 4-Way Handshake:

Applications using TCP

TCP is used for reliability:
HTTP/HTTPS (web browsing)
FTP (file transfer)
SMTP (email sending)
IMAP/POP3 (email retrieval)
SSH (secure remote login)

Operation

Connection establishment (3-way handshake)

Prior data transmission, TCP establishes a stateful connection between the client and server through a 3-step sequence, the 3-way handshake, while using the following elements in the TCP header file:

[3] seq is often either represented as 'm' or 'x'.

'seq' represents the client's initial number (ISN). 'seq = m' means that the 32-bit number the client generates, prior data transfer, is represented as the variable m. Each data byte sent over TCP needs a sequence number so both sides can keep track of order and delivery. The SYN packet consumes 1 sequence number 'm'.

It links to the ACK (ack = m+1). If the server receives 'seq = m', it repleies via 'ack = m+1', acknowledging them that the sequence number 'm' is received and that the next sequence number expected is m+1.

The following depicts the 3 steps in establishing a connection via 3-way handshake in TCP:

[3] The client sends the SYN to the server: for the client to connect to the server, it sets a 'SYN' flag as 1 and sends the message to the server, with additional info like the sequence number (a random 32 bits number). The ACK is set here to 0, the window size, and the max segment size. E.g., if the window size is 2000 bits, and the maximum segment size is 200 bits then a maximum of 10 data segments (2000/200 = 10) can be transmitted in the connection.
The server replies with the SYN and the ACK to the client: After receiving the client's synchronization request, the server sends an acknowledge to the client by setting the ACK flag to '1'. The acknowledgement number of the ACK is one more than the received sequence number. For Example, if the client has sent the SYN with sequence number = 1000, then the server will send the ACK with acknowledgement number = 10001. Also, the server sets the SYN flag to '1' and sends it to the client, if the server also wants to establish the connection. The sequence number used here for the SYN will be different from the client's SYN. The server also advertises its window size and maximum segment size to the client. After completion of this step, the connection is established from the client to the server-side.
The client sends the ACK to the server: After receiving the SYN from the server, the client sets the ACK flag to '1' and sends it with an acknowledgement number 1 greater than the server's SYN sequence number to the client. Here, the SYN flag is kept '0'. After completion of this step, the connection is now established from the server to the client-side also. After the connection is being established, the minimum of the sender's and receiver's maximum segment size is taken under consideration for data transmission.

Control infos:

[3] Sequence number is a random 32 bits (in the range of 0 to (2^32 -1)) number assigned to the first data bit. Often, a sequence number is used once in a connection. In other data transmission in the same connection, some other random sequence number can be used.
Acknowledgement number: It is the next sequence number that the acknowledgement sending device expects from the sender. It is generally, 1 greater than the sequence number received from the sender.
Window size/Buffer capacity is the buffer size, the capacity up to which data can be received in the buffer, i.e., how much data (in bytes) a device receives and stores in memory any time without forcing the sende to stop and wait for an acknowledgement. The term comes form the Slidign windopw protocol algorithm TCP uses.

[3] Max segment size is the max acceptable size per data segment by the connected device. Above this size, the device will not be able to receive the data segments.
SYN (synchronization) flag is a request for establishing a connection. If SYN is 1, it means that the device wants to establish a secure connection, else not.
ACK (acknowledgement) flag is the response of SYN. If ACK is 1, it means the device has received the SYN message and acknowledges it, else not.
- ACK = 1: The ACK value is valid and can be used to update the tracking of what's received.
- ACK = 0: Ignore the ACK value field entirely.
FIN (finished) flag: After the data transmission is completed, devices must terminate the connection via the FIN flag. If FIN is 1, the device wants to terminate the connection, else not.

[4]

Sender-side flow control tracking

The receive window is how much data a host receives and processes at once. By advertising this value, a receiver tells the sender how many bytes it can buffer prior needing an ACK.

The send window is a local tracking value the sender uses. It isn't a fixed field in a TCP header, but a calculation the sender maintains based on the receive window to ensure it won't transmit more data than the receiver can handle.

[4] E.g., If the receive window is 5 kB, the server can only send 5 kB prior receiving an ACK.

If the send window size is 10 kB, it means the server is telling the client it can send the server 10 kB prior receiving an ACK.

References

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Transmission Control Protocol

Characteristics and features

Connection-oriented

Reliable delivery

Ordered delivery

Error checking

Flow control

Congestion control

Maximum segment size

Sequence and acknowledgment numbers

Retransmission mechanisms

Addressing and sockets

TCP and UDP comparisons

Headers

TCP flags

Connection connection and termination

Applications using TCP

Operation

Connection establishment (3-way handshake)

Sender-side flow control tracking

References

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