2. Introduction
RFC 793 contains a discussion of the TCP design goals and provides examples of its operation, including examples of connection establishment, connection termination, and packet retransmission to repair losses.
This document describes the basic functionality expected in modern TCP implementations and replaces the protocol specification in RFC 793. It does not replicate or attempt to update the introduction and philosophy content in Sections 1 and 2 of RFC 793. Other documents are referenced to provide explanations of the theory of operation, rationale, and detailed discussion of design decisions. This document only focuses on the normative behavior of the protocol.
The "TCP Roadmap" [49] provides a more extensive guide to the RFCs that define TCP and describe various important algorithms. The TCP Roadmap contains sections on strongly encouraged enhancements that improve performance and other aspects of TCP beyond the basic operation specified in this document. As one example, implementing congestion control (e.g., [8]) is a TCP requirement, but it is a complex topic on its own and not described in detail in this document, as there are many options and possibilities that do not impact basic interoperability. Similarly, most TCP implementations today include the high-performance extensions in [47], but these are not strictly required or discussed in this document. Multipath considerations for TCP are also specified separately in [59].
A list of changes from RFC 793 is contained in Section 5.
2.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [3] [12] when, and only when, they appear in all capitals, as shown here.
Each use of RFC 2119 keywords in the document is individually labeled and referenced in Appendix B, which summarizes implementation requirements.
Sentences using "MUST" are labeled as "MUST-X" with X being a numeric identifier enabling the requirement to be located easily when referenced from Appendix B.
Similarly, sentences using "SHOULD" are labeled with "SHLD-X", "MAY" with "MAY-X", and "RECOMMENDED" with "REC-X".
For the purposes of this labeling, "SHOULD NOT" and "MUST NOT" are labeled the same as "SHOULD" and "MUST" instances.
2.2. Key TCP Concepts
TCP provides a reliable, in-order, byte-stream service to applications.
The application byte-stream is conveyed over the network via TCP segments, with each TCP segment sent as an Internet Protocol (IP) datagram.
TCP reliability consists of detecting packet losses (via sequence numbers) and errors (via per-segment checksums), as well as correction via retransmission.
TCP supports unicast delivery of data. There are anycast applications that can successfully use TCP without modifications, though there is some risk of instability due to changes of lower-layer forwarding behavior [46].
TCP is connection oriented, though it does not inherently include a liveness detection capability.
Data flow is supported bidirectionally over TCP connections, though applications are free to send data only unidirectionally, if they so choose.
TCP uses port numbers to identify application services and to multiplex distinct flows between hosts.
A more detailed description of TCP features compared to other transport protocols can be found in Section 3.1 of [52]. Further description of the motivations for developing TCP and its role in the Internet protocol stack can be found in Section 2 of [16] and earlier versions of the TCP specification.