RFC: 793
`
` TRANSMISSION CONTROL PROTOCOL
`
` DARPA INTERNET PROGRAM
`
` PROTOCOL SPECIFICATION
`
` September 1981
`
` prepared for
`
` Defense Advanced Research Projects Agency
` Information Processing Techniques Office
` 1400 Wilson Boulevard
` Arlington, Virginia 22209
`
` by
`
` Information Sciences Institute
` University of Southern California
` 4676 Admiralty Way
` Marina del Rey, California 90291
`
`September 1981
` Transmission Control Protocol
`
` TABLE OF CONTENTS
`
` PREFACE ........................................................ iii
`
`

`

`RFC: 793
`
` TRANSMISSION CONTROL PROTOCOL
`
` DARPA INTERNET PROGRAM
`
` PROTOCOL SPECIFICATION
`
` September 1981
`
` prepared for
`
` Defense Advanced Research Projects Agency
` Information Processing Techniques Office
` 1400 Wilson Boulevard
` Arlington, Virginia 22209
`
` by
`
` Information Sciences Institute
` University of Southern California
` 4676 Admiralty Way
` Marina del Rey, California 90291
`
`September 1981
` Transmission Control Protocol
`
` TABLE OF CONTENTS
`
` PREFACE ........................................................ iii
`
`

`

`1. INTRODUCTION ..................................................... 1
`
` 1.1 Motivation .................................................... 1
` 1.2 Scope ......................................................... 2
` 1.3 About This Document ........................................... 2
` 1.4 Interfaces .................................................... 3
` 1.5 Operation ..................................................... 3
`
`2. PHILOSOPHY ....................................................... 7
`
` 2.1 Elements of the Internetwork System ........................... 7
` 2.2 Model of Operation ............................................ 7
` 2.3 The Host Environment .......................................... 8
` 2.4 Interfaces .................................................... 9
` 2.5 Relation to Other Protocols ................................... 9
` 2.6 Reliable Communication ........................................ 9
` 2.7 Connection Establishment and Clearing ........................ 10
` 2.8 Data Communication ........................................... 12
` 2.9 Precedence and Security ...................................... 13
` 2.10 Robustness Principle ......................................... 13
`
`3. FUNCTIONAL SPECIFICATION ........................................ 15
`
` 3.1 Header Format ................................................ 15
` 3.2 Terminology .................................................. 19
` 3.3 Sequence Numbers ............................................. 24
` 3.4 Establishing a connection .................................... 30
` 3.5 Closing a Connection ......................................... 37
` 3.6 Precedence and Security ...................................... 40
` 3.7 Data Communication ........................................... 40
` 3.8 Interfaces ................................................... 44
` 3.9 Event Processing ............................................. 52
`
`GLOSSARY ............................................................ 79
`
`REFERENCES .......................................................... 85
`
` [Page i]
`
`

`

`
` September 1981
`Transmission Control Protocol
`
`[Page ii]
`
`

`

`
`September 1981
` Transmission Control Protocol
`
` PREFACE
`
`This document describes the DoD Standard Transmission Control Protocol
`(TCP). There have been nine earlier editions of the ARPA TCP
`specification on which this standard is based, and the present text
`draws heavily from them. There have been many contributors to this work
`both in terms of concepts and in terms of text. This edition clarifies
`several details and removes the end-of-letter buffer-size adjustments,
`and redescribes the letter mechanism as a push function.
`
` Jon Postel
`
` Editor
`
` [Page iii]
`
`

`

`
`RFC: 793
`Replaces: RFC 761
`IENs: 129, 124, 112, 81,
`55, 44, 40, 27, 21, 5
`
` TRANSMISSION CONTROL PROTOCOL
`
` DARPA INTERNET PROGRAM
` PROTOCOL SPECIFICATION
`
` 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.
`
` 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. Very few
` assumptions are made as to 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.
`
` [Page 1]
`
`

`

`
` September 1981
`Transmission Control Protocol
`Introduction
`
` 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 |
` +---------------------+
` | TCP |
` +---------------------+
` | internet protocol |
` +---------------------+
` |communication network|
` +---------------------+
`
` Figure 1
`
` 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
`
`[Page 2]
`
`

`

`
`September 1981
` Transmission Control Protocol
` Introduction
`
` section offers a very brief view of the protocol interfaces and
` operation. Section 2 summarizes the philosophical basis for the TCP
` design. Section 3 offers both a detailed description of the actions
` required of TCP when various events occur (arrival of new segments,
` user calls, errors, etc.) and the details of the formats of TCP
` segments.
`
`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.
`
` 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. For example, there are
` calls to open and close connections and to send and receive data on
` established connections. It is also expected that the TCP can
` 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.
`
` 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:
`
` Basic Data Transfer
` Reliability
` Flow Control
` Multiplexing
` Connections
` Precedence and Security
`
` The basic operation of the TCP in each of these areas is described in
` the following paragraphs.
`
` [Page 3]
`
`

`

`
` September 1981
`Transmission Control Protocol
`Introduction
`
` 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.
`
` 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.
`
` 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.
`
` 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.
`
` 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.
`
`[Page 4]
`
`

`

`
`September 1981
` Transmission Control Protocol
` Introduction
`
` 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.
`
` The binding of ports to processes is handled independently by each
` Host. However, it proves useful to attach frequently used processes
` (e.g., a "logger" or timesharing service) to fixed sockets which are
` made known to the public. These services can then be accessed
` through the known addresses. Establishing and learning the port
` addresses of other processes may involve more dynamic mechanisms.
`
` 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.
`
` When two processes wish to communicate, their TCP’s must first
` establish a connection (initialize the status information on each
` side). When their communication is complete, the connection is
` terminated or closed to free the resources for other uses.
`
` 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.
`
` 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.
`
` [Page 5]
`
`

`

`
` September 1981
`Transmission Control Protocol
`
`[Page 6]
`
`

`

`
`September 1981
` Transmission Control Protocol
`
` 2. PHILOSOPHY
`
`2.1. Elements of the Internetwork System
`
` The internetwork environment consists of hosts connected to networks
` which are in turn interconnected via gateways. It is assumed here
` that the networks may be either local networks (e.g., the ETHERNET) or
` large networks (e.g., the ARPANET), but in any case are based on
` packet switching technology. The active agents that produce and
` consume messages are processes. Various levels of protocols in the
` networks, the gateways, and the hosts support an interprocess
` communication system that provides two-way data flow on logical
` connections between process ports.
`
` The term packet is used generically here to mean the data of one
` transaction between a host and its network. The format of data blocks
` exchanged within the a network will generally not be of concern to us.
`
` Hosts are computers attached to a network, and from the communication
` network’s point of view, are the sources and destinations of packets.
` Processes are viewed as the active elements in host computers (in
` accordance with the fairly common definition of a process as a program
` in execution). Even terminals and files or other I/O devices are
` viewed as communicating with each other through the use of processes.
` Thus, all communication is viewed as inter-process communication.
`
` Since a process may need to distinguish among several communication
` streams between itself and another process (or processes), we imagine
` that each process may have a number of ports through which it
` communicates with the ports of other processes.
`
`2.2. Model of Operation
`
` Processes transmit data by calling on the TCP and passing buffers of
` data as arguments. The TCP packages the data from these buffers into
` segments and calls on the internet module to transmit each segment to
` the destination TCP. The receiving TCP places the data from a segment
` into the receiving user’s buffer and notifies the receiving user. The
` TCPs include control information in the segments which they use to
` ensure reliable ordered data transmission.
`
` The model of internet communication is that there is an internet
` protocol module associated with each TCP which provides an interface
` to the local network. This internet module packages TCP segments
` inside internet datagrams and routes these datagrams to a destination
` internet module or intermediate gateway. To transmit the datagram
` through the local network, it is embedded in a local network packet.
`
` The packet switches may perform further packaging, fragmentation, or
`
` [Page 7]
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`

`

`
` September 1981
`Transmission Control Protocol
`Philosophy
`
` other operations to achieve the delivery of the local packet to the
` destination internet module.
`
` At a gateway between networks, the internet datagram is "unwrapped"
` from its local packet and examined to determine through which network
` the internet datagram should travel next. The internet datagram is
` then "wrapped" in a local packet suitable to the next network and
` routed to the next gateway, or to the final destination.
`
` A gateway is permitted to break up an internet datagram into smaller
` internet datagram fragments if this is necessary for transmission
` through the next network. To do this, the gateway produces a set of
` internet datagrams; each carrying a fragment. Fragments may be
` further broken into smaller fragments at subsequent gateways. The
` internet datagram fragment format is designed so that the destination
` internet module can reassemble fragments into internet datagrams.
`
` A destination internet module unwraps the segment from the datagram
` (after reassembling the datagram, if necessary) and passes it to the
` destination TCP.
`
` This simple model of the operation glosses over many details. One
` important feature is the type of service. This provides information
` to the gateway (or internet module) to guide it in selecting the
` service parameters to be used in traversing the next network.
` Included in the type of service information is the precedence of the
` datagram. Datagrams may also carry security information to permit
` host and gateways that operate in multilevel secure environments to
` properly segregate datagrams for security considerations.
`
`2.3. The Host Environment
`
` The TCP is assumed to be a module in an operating system. The users
` access the TCP much like they would access the file system. The TCP
` may call on other operating system functions, for example, to manage
` data structures. The actual interface to the network is assumed to be
` controlled by a device driver module. The TCP does not call on the
` network device driver directly, but rather calls on the internet
` datagram protocol module which may in turn call on the device driver.
`
` The mechanisms of TCP do not preclude implementation of the TCP in a
` front-end processor. However, in such an implementation, a
` host-to-front-end protocol must provide the functionality to support
` the type of TCP-user interface described in this document.
`
`[Page 8]
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`

`
`September 1981
` Transmission Control Protocol
` Philosophy
`
`2.4. Interfaces
`
` The TCP/user interface provides for calls made by the user on the TCP
` to OPEN or CLOSE a connection, to SEND or RECEIVE data, or to obtain
` STATUS about a connection. These calls are like other calls from user
` programs on the operating system, for example, the calls to open, read
` from, and close a file.
`
` The TCP/internet interface provides calls to send and receive
` datagrams addressed to TCP modules in hosts anywhere in the internet
` system. These calls have parameters for passing the address, type of
` service, precedence, security, and other control information.
`
`2.5. Relation to Other Protocols
`
` The following diagram illustrates the place of the TCP in the protocol
` hierarchy:
`
` +------+ +-----+ +-----+ +-----+
` |Telnet| | FTP | |Voice| ... | | Application Level
` +------+ +-----+ +-----+ +-----+
` | | | |
` +-----+ +-----+ +-----+
` | TCP | | RTP | ... | | Host Level
` +-----+ +-----+ +-----+
` | | |
` +-------------------------------+
` | Internet Protocol & ICMP | Gateway Level
` +-------------------------------+
` |
` +---------------------------+
` | Local Network Protocol | Network Level
` +---------------------------+
`
` Protocol Relationships
`
` Figure 2.
`
` It is expected that the TCP will be able to support higher level
` protocols efficiently. It should be easy to interface higher level
` protocols like the ARPANET Telnet or AUTODIN II THP to the TCP.
`
`2.6. Reliable Communication
`
` A stream of data sent on a TCP connection is delivered reliably and in
` order at the destination.
`
` [Page 9]
`
`

`

`
` September 1981
`Transmission Control Protocol
`Philosophy
`
` Transmission is made reliable via the use of sequence numbers and
` acknowledgments. Conceptually, each octet of data is assigned a
` sequence number. The sequence number of the first octet of data in a
` segment is transmitted with that segment and is called the segment
` sequence number. Segments also carry an acknowledgment number which
` is the sequence number of the next expected data octet of
` transmissions in the reverse direction. When the TCP transmits a
` segment containing data, it puts a copy on a retransmission queue and
` starts a timer; when the acknowledgment for that data is received, the
` segment is deleted from the queue. If the acknowledgment is not
` received before the timer runs out, the segment is retransmitted.
`
` An acknowledgment by TCP does not guarantee that the data has been
` delivered to the end user, but only that the receiving TCP has taken
` the responsibility to do so.
`
` To govern the flow of data between TCPs, a flow control mechanism is
` employed. The receiving TCP reports a "window" to the sending TCP.
` This window specifies the number of octets, starting with the
` acknowledgment number, that the receiving TCP is currently prepared to
` receive.
`
`2.7. Connection Establishment and Clearing
`
` To identify the separate data streams that a TCP may handle, the TCP
` provides a port identifier. Since port identifiers are selected
` independently by each TCP they might not be unique. To provide for
` unique addresses within each TCP, we concatenate an internet address
` identifying the TCP with a port identifier to create a socket which
` will be unique throughout all networks connected together.
`
` A connection is fully specified by the pair of sockets at the ends. A
` local socket may participate in many connections to different foreign
` sockets. A connection can be used to carry data in both directions,
` that is, it is "full duplex".
`
` TCPs are free to associate ports with processes however they choose.
` However, several basic concepts are necessary in any implementation.
` There must be well-known sockets which the TCP associates only with
` the "appropriate" processes by some means. We envision that processes
` may "own" ports, and that processes can initiate connections only on
` the ports they own. (Means for implementing ownership is a local
` issue, but we envision a Request Port user command, or a method of
` uniquely allocating a group of ports to a given process, e.g., by
` associating the high order bits of a port name with a given process.)
`
` A connection is specified in the OPEN call by the local port and
` foreign socket arguments. In return, the TCP supplies a (short) local
`
`[Page 10]
`
`

`

`
`September 1981
` Transmission Control Protocol
` Philosophy
`
` connection name by which the user refers to the connection in
` subsequent calls. There are several things that must be remembered
` about a connection. To store this information we imagine that there
` is a data structure called a Transmission Control Block (TCB). One
` implementation strategy would have the local connection name be a
` pointer to the TCB for this connection. The OPEN call also specifies
` whether the connection establishment is to be actively pursued, or to
` be passively waited for.
`
` A passive OPEN request means that the process wants to accept incoming
` connection requests rather than attempting to initiate a connection.
` Often the process requesting a passive OPEN will accept a connection
` request from any caller. In this case a foreign socket of all zeros
` is used to denote an unspecified socket. Unspecified foreign sockets
` are allowed only on passive OPENs.
`
` A service process that wished to provide services for unknown other
` processes would issue a passive OPEN request with an unspecified
` foreign socket. Then a connection could be made with any process that
` requested a connection to this local socket. It would help if this
` local socket were known to be associated with this service.
`
` Well-known sockets are a convenient mechanism for a priori associating
` a socket address with a standard service. For instance, the
` "Telnet-Server" process is permanently assigned to a particular
` socket, and other sockets are reserved for File Transfer, Remote Job
` Entry, Text Generator, Echoer, and Sink processes (the last three
` being for test purposes). A socket address might be reserved for
` access to a "Look-Up" service which would return the specific socket
` at which a newly created service would be provided. The concept of a
` well-known socket is part of the TCP specification, but the assignment
` of sockets to services is outside this specification. (See [4].)
`
` Processes can issue passive OPENs and wait for matching active OPENs
` from other processes and be informed by the TCP when connections have
` been established. Two processes which issue active OPENs to each
` other at the same time will be correctly connected. This flexibility
` is critical for the support of distributed computing in which
` components act asynchronously with respect to each other.
`
` There are two principal cases for matching the sockets in the local
` passive OPENs and an foreign active OPENs. In the first case, the
` local passive OPENs has fully specified the foreign socket. In this
` case, the match must be exact. In the second case, the local passive
` OPENs has left the foreign socket unspecified. In this case, any
` foreign socket is acceptable as long as the local sockets match.
` Other possibilities include partially restricted matches.
`
` [Page 11]
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`

`
` September 1981
`Transmission Control Protocol
`Philosophy
`
` If there are several pending passive OPENs (recorded in TCBs) with the
` same local socket, an foreign active OPEN will be matched to a TCB
` with the specific foreign socket in the foreign active OPEN, if such a
` TCB exists, before selecting a TCB with an unspecified foreign socket.
`
` The procedures to establish connections utilize the synchronize (SYN)
` control flag and involves an exchange of three messages. This
` exchange has been termed a three-way hand shake [3].
`
` A connection is initiated by the rendezvous of an arriving segment
` containing a SYN and a waiting TCB entry each created by a user OPEN
` command. The matching of local and foreign sockets determines when a
` connection has been initiated. The connection becomes "established"
` when sequence numbers have been synchronized in both directions.
`
` The clearing of a connection also involves the exchange of segments,
` in this case carrying the FIN control flag.
`
`2.8. Data Communication
`
` The data that flows on a connection may be thought of as a stream of
` octets. The sending user indicates in each SEND call whether the data
` in that call (and any preceeding calls) should be immediately pushed
` through to the receiving user by the setting of the PUSH flag.
`
` A sending TCP is allowed to collect data from the sending user and to
` send that data in segments at its own convenience, until the push
` function is signaled, then it must send all unsent data. When a
` receiving TCP sees the PUSH flag, it must not wait for more data from
` the sending TCP before passing the data to the receiving process.
`
` There is no necessary relationship between push functions and segment
` boundaries. The data in any particular segment may be the result of a
` single SEND call, in whole or part, or of multiple SEND calls.
`
` The purpose of push function and the PUSH flag is to push data through
` from the sending user to the receiving user. It does not provide a
` record service.
`
` There is a coupling between the push function and the use of buffers
` of data that cross the TCP/user interface. Each time a PUSH flag is
` associated with data placed into