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1. Introduction

The Transmission Control Protocol (TCP) is intended for use as a highly reliable host-to-host protocol between hosts in packet-switched computer communication networks, and in interconnected systems of such networks.

This document describes the functions to be performed by the Transmission Control Protocol, the program that implements it, and its interface to programs or users that require its services.

1.1. Motivation​

Computer communication systems are playing an increasingly important role in military, government, and civilian environments. This document focuses its attention primarily on military computer communication requirements, especially robustness in the presence of communication unreliability and availability in the presence of congestion, but many of these problems are found in the civilian and government sector as well.

As strategic and tactical computer communication networks are developed and deployed, it is essential to provide means of interconnecting them and to provide standard interprocess communication protocols which can support a broad range of applications. In anticipation of the need for such standards, the Deputy Undersecretary of Defense for Research and Engineering has declared the Transmission Control Protocol (TCP) described herein to be a basis for DoD-wide inter-process communication protocol standardization.

Key Features of TCP​

TCP is a connection-oriented, end-to-end reliable protocol designed to fit into a layered hierarchy of protocols which support multi-network applications. The TCP provides for reliable inter-process communication between pairs of processes in host computers attached to distinct but interconnected computer communication networks.

TCP makes very few assumptions about the reliability of the communication protocols below the TCP layer. TCP assumes it can obtain a simple, potentially unreliable datagram service from the lower level protocols. In principle, the TCP should be able to operate above a wide spectrum of communication systems ranging from hard-wired connections to packet-switched or circuit-switched networks.

Theoretical Foundation​

TCP is based on concepts first described by Cerf and Kahn in [1]. The TCP fits into a layered protocol architecture just above a basic Internet Protocol [2] which provides a way for the TCP to send and receive variable-length segments of information enclosed in internet datagram "envelopes". The internet datagram provides a means for addressing source and destination TCPs in different networks. The internet protocol also deals with any fragmentation or reassembly of the TCP segments required to achieve transport and delivery through multiple networks and interconnecting gateways. The internet protocol also carries information on the precedence, security classification and compartmentation of the TCP segments, so this information can be communicated end-to-end across multiple networks.

Protocol Layering​

``` +---------------------+ | higher-level | ← Application Layer Protocols (HTTP, FTP, SMTP, etc.) +---------------------+ | TCP | ← Transport Layer (this protocol) +---------------------+ | Internet Protocol | ← Network Layer (RFC 791) +---------------------+ |communication network| ← Link/Physical Layer +---------------------+ ```

Much of this document is written in the context of TCP implementations which are co-resident with higher level protocols in the host computer. Some computer systems will be connected to networks via front-end computers which house the TCP and internet protocol layers, as well as network specific software. The TCP specification describes an interface to the higher level protocols which appears to be implementable even for the front-end case, as long as a suitable host-to-front end protocol is implemented.

1.2. Scope​

The TCP is intended to provide a reliable process-to-process communication service in a multinetwork environment. The TCP is intended to be a host-to-host protocol in common use in multiple networks.

1.3. About This Document​

This document represents a specification of the behavior required of any TCP implementation, both in its interactions with higher level protocols and in its interactions with other TCPs. The rest of this section offers a very brief view of the protocol interfaces and operation.

Document Structure​

  • Section 2: Summarizes the philosophical basis for the TCP design
  • Section 3: Offers detailed description of:
    • The actions required of TCP when various events occur (arrival of new segments, user calls, errors, etc.)
    • The details of TCP segment formats

1.4. Interfaces​

The TCP interfaces on one side to user or application processes and on the other side to a lower level protocol such as Internet Protocol.

TCP/User Interface​

The interface between an application process and the TCP is illustrated in reasonable detail. This interface consists of a set of calls much like the calls an operating system provides to an application process for manipulating files.

Main Interface Calls:

  • OPEN: Open a connection
  • CLOSE: Close a connection
  • SEND: Send data on an established connection
  • RECEIVE: Receive data on an established connection

The TCP can also asynchronously communicate with application programs. Although considerable freedom is permitted to TCP implementors to design interfaces which are appropriate to a particular operating system environment, a minimum functionality is required at the TCP/user interface for any valid implementation.

TCP/Lower Level Protocol Interface​

The interface between TCP and lower level protocol is essentially unspecified except that it is assumed there is a mechanism whereby the two levels can asynchronously pass information to each other. Typically, one expects the lower level protocol to specify this interface.

TCP is designed to work in a very general environment of interconnected networks. The lower level protocol which is assumed throughout this document is the Internet Protocol [2].

1.5. Operation​

As noted above, the primary purpose of the TCP is to provide reliable, securable logical circuit or connection service between pairs of processes. To provide this service on top of a less reliable internet communication system requires facilities in the following areas:

Core Functions Overview​

1. Basic Data Transfer​

The TCP is able to transfer a continuous stream of octets in each direction between its users by packaging some number of octets into segments for transmission through the internet system. In general, the TCPs decide when to block and forward data at their own convenience.

Push Function: Sometimes users need to be sure that all the data they have submitted to the TCP has been transmitted. For this purpose a push function is defined. To assure that data submitted to a TCP is actually transmitted the sending user indicates that it should be pushed through to the receiving user. A push causes the TCPs to promptly forward and deliver data up to that point to the receiver. The exact push point might not be visible to the receiving user and the push function does not supply a record boundary marker.

2. Reliability​

The TCP must recover from data that is damaged, lost, duplicated, or delivered out of order by the internet communication system. This is achieved by assigning a sequence number to each octet transmitted, and requiring a positive acknowledgment (ACK) from the receiving TCP. If the ACK is not received within a timeout interval, the data is retransmitted. At the receiver, the sequence numbers are used to correctly order segments that may be received out of order and to eliminate duplicates. Damage is handled by adding a checksum to each segment transmitted, checking it at the receiver, and discarding damaged segments.

Reliability Mechanisms:

  • Timeout and Retransmission: If ACK is not received within timeout interval, data is retransmitted
  • Sequencing: At receiver, sequence numbers are used to correctly order segments received out of order
  • Duplicate Elimination: Eliminate duplicate segments
  • Checksum: Handle damage by adding checksum to each transmitted segment, checking at receiver, and discarding damaged segments

As long as the TCPs continue to function properly and the internet system does not become completely partitioned, no transmission errors will affect the correct delivery of data. TCP recovers from internet communication system errors.

3. Flow Control​

TCP provides a means for the receiver to govern the amount of data sent by the sender. This is achieved by returning a "window" with every ACK indicating a range of acceptable sequence numbers beyond the last segment successfully received. The window indicates an allowed number of octets that the sender may transmit before receiving further permission.

Sliding Window: The window indicates the number of allowed octets that the sender may transmit before receiving further permission.

4. Multiplexing​

To allow for many processes within a single Host to use TCP communication facilities simultaneously, the TCP provides a set of addresses or ports within each host. Concatenated with the network and host addresses from the internet communication layer, this forms a socket. A pair of sockets uniquely identifies each connection. That is, a socket may be simultaneously used in multiple connections.

Port Binding:

  • The binding of ports to processes is handled independently by each Host
  • Frequently used processes (e.g., a "logger" or timesharing service) attach to fixed sockets which are made known to the public
  • These services can be accessed through known addresses
  • Establishing and learning the port addresses of other processes may involve more dynamic mechanisms

5. Connections​

The reliability and flow control mechanisms described above require that TCPs initialize and maintain certain status information for each data stream. The combination of this information, including sockets, sequence numbers, and window sizes, is called a connection. Each connection is uniquely specified by a pair of sockets identifying its two sides.

Connection Lifecycle:

  1. Establishment: When two processes wish to communicate, their TCPs must first establish a connection (initialize the status information on each side)
  2. Usage: Data transfer phase
  3. Termination: When communication is complete, the connection is terminated or closed to free the resources for other uses

Three-Way Handshake: Since connections must be established between unreliable hosts and over the unreliable internet communication system, a handshake mechanism with clock-based sequence numbers is used to avoid erroneous initialization of connections.

6. Precedence and Security​

The users of TCP may indicate the security and precedence of their communication. Provision is made for default values to be used when these features are not needed.

Implementation Considerations​

Minimum Functionality Requirements​

Any valid TCP implementation must provide:

  1. Connection management (establishment, maintenance, termination)
  2. Reliable data transmission (acknowledgment, retransmission, sequencing)
  3. Flow control (sliding window)
  4. Multiplexing (port management)
  5. Error detection (checksum)

Flexibility​

While this specification defines TCP behavior, implementers have considerable freedom in:

  • Internal data structure design
  • Integration approach with operating systems
  • Performance optimization strategies
  • Specific forms of user interfaces

Relation to Other Protocols​

``` Application Layer: HTTP, FTP, SMTP, Telnet, etc. ↓ Transport Layer: TCP (this protocol) | UDP (RFC 768) ↓ Network Layer: IP (RFC 791) ↓ Link Layer: Ethernet, Wi-Fi, etc. ```

TCP resides in the transport layer of the protocol stack, providing reliable end-to-end communication services to the application layer, and relies on the IP protocol at the network layer for routing and addressing.

Next: 2. Philosophy - In-depth exploration of TCP's design philosophy and theoretical foundation

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