TCP

TCP (Transmission Control Protocol)#

TCP Header Reference#

The TCP header is arranged in 32-bit rows. The first row contains the source and destination ports. The next two rows carry the sequence and acknowledgment numbers. The following rows contain header length, control flags, flow-control information, checksum, urgent pointer, options, padding, and payload data.

 0                   15 16                  31
+---------------------+---------------------+
| Source Port         | Destination Port    |
+---------------------+---------------------+
| Sequence Number                           |
+-------------------------------------------+
| Acknowledgment Number                     |
+----+------+----------+--------------------+
|Off |Resvd | Flags    | Window             |
+----+------+----------+--------------------+
| Checksum            | Urgent Pointer      |
+---------------------+---------------------+
| Options and Padding                       |
+-------------------------------------------+
| Data                                      |
+-------------------------------------------+

Header Field Summary#

Field Size Description
Source Port 16 bits Identifies the sending application port.
Destination Port 16 bits Identifies the receiving application port.
Sequence Number 32 bits Identifies the sequence number of the first data byte in this segment.
Acknowledgment Number 32 bits Indicates the next sequence number expected by the sender of the acknowledgment.
Data Offset 4 bits Indicates the TCP header length in 32-bit words.
Reserved Variable Reserved for future use.
Flags Variable TCP control bits such as SYN, ACK, FIN, RST, PSH, and URG.
Window 16 bits Indicates the receive window size.
Checksum 16 bits Provides error detection for the TCP header and payload.
Urgent Pointer 16 bits Points to urgent data when the URG flag is set.
Options and Padding Variable Carries optional TCP parameters such as MSS, SACK, timestamps, and window scaling.
Data Variable Application payload carried by TCP.

Article Body#

Transmission Control Protocol (TCP) is one of the main protocols of the Internet protocol suite. It provides reliable, ordered, and error-checked delivery of a byte stream between applications running on hosts communicating through an IP network.

TCP is connection-oriented. Before application data is exchanged, the endpoints establish a connection using a three-way handshake. This connection model supports reliable delivery, retransmission, flow control, and congestion control.

Major Internet applications such as web browsing, email, remote administration, file transfer, and secure communication commonly rely on TCP.

Historical Origin#

TCP originated from early work on packet-switched internetworking and the original Transmission Control Program. The architecture later separated the functions of TCP and IP, producing the TCP/IP model used by the Internet.

Network Function#

TCP provides a reliable byte-stream service between applications. IP may deliver packets out of order, lose packets, duplicate packets, or delay packets. TCP hides many of these network-layer behaviors from applications by using sequence numbers, acknowledgments, retransmission, checksums, and ordering.

TCP Segment Structure#

A TCP segment contains a TCP header and optional data. The TCP header includes port numbers, sequence and acknowledgment numbers, header length, flags, window size, checksum, urgent pointer, and optional parameters.

Protocol Operation#

Connection Establishment#

TCP commonly establishes connections through a three-way handshake:

Client                                      Server
  |                                           |
  |  SYN                                      |
  |------------------------------------------>|
  |                                           |
  |  SYN, ACK                                 |
  |<------------------------------------------|
  |                                           |
  |  ACK                                      |
  |------------------------------------------>|
  |                                           |

Connection Termination#

TCP connections are usually closed using FIN and ACK segments.

Host A                                      Host B
  |                                           |
  |  FIN                                      |
  |------------------------------------------>|
  |                                           |
  |  ACK                                      |
  |<------------------------------------------|
  |                                           |
  |  FIN                                      |
  |<------------------------------------------|
  |                                           |
  |  ACK                                      |
  |------------------------------------------>|

Data Transfer#

TCP uses sequence numbers and acknowledgments to provide reliable ordered delivery. Missing data can be retransmitted, and out-of-order data can be reordered before being delivered to the application.

Error Detection#

TCP uses a checksum to detect corruption in the TCP header and payload.

Flow Control#

TCP uses a receive window to prevent a sender from overwhelming the receiver.

Congestion Control#

TCP congestion control reduces transmission rate when the network appears congested.

TCP Ports#

TCP uses port numbers to identify application endpoints. A TCP connection is commonly identified by source IP address, source port, destination IP address, destination port, and protocol.

Port Common Use
22 SSH
25 SMTP
80 HTTP
443 HTTPS
993 IMAPS

Vulnerabilities#

TCP can be affected by denial-of-service attacks, connection hijacking, spoofed reset packets, and resource exhaustion. Common mitigations include SYN cookies, randomized initial sequence numbers, firewall filtering, and rate limiting.

Wireshark Filters# 

tcp
tcp.port == 443
tcp.flags.syn == 1
tcp.flags.ack == 1
tcp.flags.reset == 1
tcp.analysis.retransmission
tcp.analysis.duplicate_ack
tcp.window_size == 0
RFC Description
RFC 9293 Current TCP specification
RFC 793 Original TCP specification
RFC 1122 Requirements for Internet Hosts
RFC 2018 TCP Selective Acknowledgment Options
RFC 5681 TCP Congestion Control
RFC 6298 TCP Retransmission Timer
RFC 7323 TCP Extensions for High Performance
  • IPv4
  • IPv6
  • UDP
  • TLS
  • HTTP
  • QUIC
  • SCTP

Source and Attribution#

This page is based on the English Wikipedia article:

ProtocolInfo.net adds the TCP header diagram, field summary, Wireshark filters, RFC mapping, and operational reference sections.