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`SAMSUNG 1026
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`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
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`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
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`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
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`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.
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`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
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`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.
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`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.
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`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.
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`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
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`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.
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`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.
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`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
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`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.
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`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 the receiving user's buffer, the
`buffer is returned to the user for processing even if the buffer is
`not filled. If data arrives that fills the user's buffer before a
`PUSH is seen, the data is passed to the user in buffer size units.
`TCP also provides a means to communicate to the receiver of data that
`at some point further along in the data stream than the receiver is
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`currently reading there is urgent data. TCP does not attempt to
`define what the user specifically does upon being notified of pending
`urgent data, but the general notion is that the receiving process will
`take action to process the urgent data quickly.
`2.9. Precedence and Security
`The TCP makes use of the internet protocol type of service field and
`security option to provide precedence and security on a per connection
`basis to TCP users. Not all TCP modules will necessarily function in
`a multilevel secure environment; some may be limited to unclassified
`use only, and others may operate at only one security l