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+
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+RFC: 793
+
+
+
+
+
+
+
+ TRANSMISSION CONTROL PROTOCOL
+
+
+ DARPA INTERNET PROGRAM
+
+ PROTOCOL SPECIFICATION
+
+
+
+ September 1981
+
+
+
+
+
+
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+
+
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+
+
+
+ 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
+
+
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+ [Page i]
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+ September 1981
+Transmission Control Protocol
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+[Page ii]
+
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+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
+
+
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+ [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]
+
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+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.
+
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+ [Page 5]
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+ September 1981
+Transmission Control Protocol
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+[Page 6]
+
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+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]
+
+
+ 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]
+
+
+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]
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+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]
+
+
+ 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 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
+
+
+[Page 12]
+
+
+September 1981
+ Transmission Control Protocol
+ Philosophy
+
+
+
+ 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 level and
+ compartment. Consequently, some TCP implementations and services to
+ users may be limited to a subset of the multilevel secure case.
+
+ TCP modules which operate in a multilevel secure environment must
+ properly mark outgoing segments with the security, compartment, and
+ precedence. Such TCP modules must also provide to their users or
+ higher level protocols such as Telnet or THP an interface to allow
+ them to specify the desired security level, compartment, and
+ precedence of connections.
+
+2.10. Robustness Principle
+
+ TCP implementations will follow a general principle of robustness: be
+ conservative in what you do, be liberal in what you accept from
+ others.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+ [Page 13]
+
+
+ September 1981
+Transmission Control Protocol
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+[Page 14]
+
+
+September 1981
+ Transmission Control Protocol
+
+
+
+ 3. FUNCTIONAL SPECIFICATION
+
+3.1. Header Format
+
+ TCP segments are sent as internet datagrams. The Internet Protocol
+ header carries several information fields, including the source and
+ destination host addresses [2]. A TCP header follows the internet
+ header, supplying information specific to the TCP protocol. This
+ division allows for the existence of host level protocols other than
+ TCP.
+
+ TCP Header Format
+
+
+ 0 1 2 3
+ 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | Source Port | Destination Port |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | Sequence Number |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | Acknowledgment Number |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | Data | |U|A|P|R|S|F| |
+ | Offset| Reserved |R|C|S|S|Y|I| Window |
+ | | |G|K|H|T|N|N| |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | Checksum | Urgent Pointer |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | Options | Padding |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | data |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+
+ TCP Header Format
+
+ Note that one tick mark represents one bit position.
+
+ Figure 3.
+
+ Source Port: 16 bits
+
+ The source port number.
+
+ Destination Port: 16 bits
+
+ The destination port number.
+
+
+
+
+ [Page 15]
+
+
+ September 1981
+Transmission Control Protocol
+Functional Specification
+
+
+
+ Sequence Number: 32 bits
+
+ The sequence number of the first data octet in this segment (except
+ when SYN is present). If SYN is present the sequence number is the
+ initial sequence number (ISN) and the first data octet is ISN+1.
+
+ Acknowledgment Number: 32 bits
+
+ If the ACK control bit is set this field contains the value of the
+ next sequence number the sender of the segment is expecting to
+ receive. Once a connection is established this is always sent.
+
+ Data Offset: 4 bits
+
+ The number of 32 bit words in the TCP Header. This indicates where
+ the data begins. The TCP header (even one including options) is an
+ integral number of 32 bits long.
+
+ Reserved: 6 bits
+
+ Reserved for future use. Must be zero.
+
+ Control Bits: 6 bits (from left to right):
+
+ URG: Urgent Pointer field significant
+ ACK: Acknowledgment field significant
+ PSH: Push Function
+ RST: Reset the connection
+ SYN: Synchronize sequence numbers
+ FIN: No more data from sender
+
+ Window: 16 bits
+
+ The number of data octets beginning with the one indicated in the
+ acknowledgment field which the sender of this segment is willing to
+ accept.
+
+ Checksum: 16 bits
+
+ The checksum field is the 16 bit one's complement of the one's
+ complement sum of all 16 bit words in the header and text. If a
+ segment contains an odd number of header and text octets to be
+ checksummed, the last octet is padded on the right with zeros to
+ form a 16 bit word for checksum purposes. The pad is not
+ transmitted as part of the segment. While computing the checksum,
+ the checksum field itself is replaced with zeros.
+
+ The checksum also covers a 96 bit pseudo header conceptually
+
+
+[Page 16]
+
+
+September 1981
+ Transmission Control Protocol
+ Functional Specification
+
+
+
+ prefixed to the TCP header. This pseudo header contains the Source
+ Address, the Destination Address, the Protocol, and TCP length.
+ This gives the TCP protection against misrouted segments. This
+ information is carried in the Internet Protocol and is transferred
+ across the TCP/Network interface in the arguments or results of
+ calls by the TCP on the IP.
+
+ +--------+--------+--------+--------+
+ | Source Address |
+ +--------+--------+--------+--------+
+ | Destination Address |
+ +--------+--------+--------+--------+
+ | zero | PTCL | TCP Length |
+ +--------+--------+--------+--------+
+
+ The TCP Length is the TCP header length plus the data length in
+ octets (this is not an explicitly transmitted quantity, but is
+ computed), and it does not count the 12 octets of the pseudo
+ header.
+
+ Urgent Pointer: 16 bits
+
+ This field communicates the current value of the urgent pointer as a
+ positive offset from the sequence number in this segment. The
+ urgent pointer points to the sequence number of the octet following
+ the urgent data. This field is only be interpreted in segments with
+ the URG control bit set.
+
+ Options: variable
+
+ Options may occupy space at the end of the TCP header and are a
+ multiple of 8 bits in length. All options are included in the
+ checksum. An option may begin on any octet boundary. There are two
+ cases for the format of an option:
+
+ Case 1: A single octet of option-kind.
+
+ Case 2: An octet of option-kind, an octet of option-length, and
+ the actual option-data octets.
+
+ The option-length counts the two octets of option-kind and
+ option-length as well as the option-data octets.
+
+ Note that the list of options may be shorter than the data offset
+ field might imply. The content of the header beyond the
+ End-of-Option option must be header padding (i.e., zero).
+
+ A TCP must implement all options.
+
+
+ [Page 17]
+
+
+ September 1981
+Transmission Control Protocol
+Functional Specification
+
+
+
+ Currently defined options include (kind indicated in octal):
+
+ Kind Length Meaning
+ ---- ------ -------
+ 0 - End of option list.
+ 1 - No-Operation.
+ 2 4 Maximum Segment Size.
+
+
+ Specific Option Definitions
+
+ End of Option List
+
+ +--------+
+ |00000000|
+ +--------+
+ Kind=0
+
+ This option code indicates the end of the option list. This
+ might not coincide with the end of the TCP header according to
+ the Data Offset field. This is used at the end of all options,
+ not the end of each option, and need only be used if the end of
+ the options would not otherwise coincide with the end of the TCP
+ header.
+
+ No-Operation
+
+ +--------+
+ |00000001|
+ +--------+
+ Kind=1
+
+ This option code may be used between options, for example, to
+ align the beginning of a subsequent option on a word boundary.
+ There is no guarantee that senders will use this option, so
+ receivers must be prepared to process options even if they do
+ not begin on a word boundary.
+
+ Maximum Segment Size
+
+ +--------+--------+---------+--------+
+ |00000010|00000100| max seg size |
+ +--------+--------+---------+--------+
+ Kind=2 Length=4
+
+
+
+
+
+
+[Page 18]
+
+
+September 1981
+ Transmission Control Protocol
+ Functional Specification
+
+
+
+ Maximum Segment Size Option Data: 16 bits
+
+ If this option is present, then it communicates the maximum
+ receive segment size at the TCP which sends this segment.
+ This field must only be sent in the initial connection request
+ (i.e., in segments with the SYN control bit set). If this
+ option is not used, any segment size is allowed.
+
+ Padding: variable
+
+ The TCP header padding is used to ensure that the TCP header ends
+ and data begins on a 32 bit boundary. The padding is composed of
+ zeros.
+
+3.2. Terminology
+
+ Before we can discuss very much about the operation of the TCP we need
+ to introduce some detailed terminology. The maintenance of a TCP
+ connection requires the remembering of several variables. We conceive
+ of these variables being stored in a connection record called a
+ Transmission Control Block or TCB. Among the variables stored in the
+ TCB are the local and remote socket numbers, the security and
+ precedence of the connection, pointers to the user's send and receive
+ buffers, pointers to the retransmit queue and to the current segment.
+ In addition several variables relating to the send and receive
+ sequence numbers are stored in the TCB.
+
+ Send Sequence Variables
+
+ SND.UNA - send unacknowledged
+ SND.NXT - send next
+ SND.WND - send window
+ SND.UP - send urgent pointer
+ SND.WL1 - segment sequence number used for last window update
+ SND.WL2 - segment acknowledgment number used for last window
+ update
+ ISS - initial send sequence number
+
+ Receive Sequence Variables
+
+ RCV.NXT - receive next
+ RCV.WND - receive window
+ RCV.UP - receive urgent pointer
+ IRS - initial receive sequence number
+
+
+
+
+
+
+ [Page 19]
+
+
+ September 1981
+Transmission Control Protocol
+Functional Specification
+
+
+
+ The following diagrams may help to relate some of these variables to
+ the sequence space.
+
+ Send Sequence Space
+
+ 1 2 3 4
+ ----------|----------|----------|----------
+ SND.UNA SND.NXT SND.UNA
+ +SND.WND
+
+ 1 - old sequence numbers which have been acknowledged
+ 2 - sequence numbers of unacknowledged data
+ 3 - sequence numbers allowed for new data transmission
+ 4 - future sequence numbers which are not yet allowed
+
+ Send Sequence Space
+
+ Figure 4.
+
+
+
+ The send window is the portion of the sequence space labeled 3 in
+ figure 4.
+
+ Receive Sequence Space
+
+ 1 2 3
+ ----------|----------|----------
+ RCV.NXT RCV.NXT
+ +RCV.WND
+
+ 1 - old sequence numbers which have been acknowledged
+ 2 - sequence numbers allowed for new reception
+ 3 - future sequence numbers which are not yet allowed
+
+ Receive Sequence Space
+
+ Figure 5.
+
+
+
+ The receive window is the portion of the sequence space labeled 2 in
+ figure 5.
+
+ There are also some variables used frequently in the discussion that
+ take their values from the fields of the current segment.
+
+
+
+
+[Page 20]
+
+
+September 1981
+ Transmission Control Protocol
+ Functional Specification
+
+
+
+ Current Segment Variables
+
+ SEG.SEQ - segment sequence number
+ SEG.ACK - segment acknowledgment number
+ SEG.LEN - segment length
+ SEG.WND - segment window
+ SEG.UP - segment urgent pointer
+ SEG.PRC - segment precedence value
+
+ A connection progresses through a series of states during its
+ lifetime. The states are: LISTEN, SYN-SENT, SYN-RECEIVED,
+ ESTABLISHED, FIN-WAIT-1, FIN-WAIT-2, CLOSE-WAIT, CLOSING, LAST-ACK,
+ TIME-WAIT, and the fictional state CLOSED. CLOSED is fictional
+ because it represents the state when there is no TCB, and therefore,
+ no connection. Briefly the meanings of the states are:
+
+ LISTEN - represents waiting for a connection request from any remote
+ TCP and port.
+
+ SYN-SENT - represents waiting for a matching connection request
+ after having sent a connection request.
+
+ SYN-RECEIVED - represents waiting for a confirming connection
+ request acknowledgment after having both received and sent a
+ connection request.
+
+ ESTABLISHED - represents an open connection, data received can be
+ delivered to the user. The normal state for the data transfer phase
+ of the connection.
+
+ FIN-WAIT-1 - represents waiting for a connection termination request
+ from the remote TCP, or an acknowledgment of the connection
+ termination request previously sent.
+
+ FIN-WAIT-2 - represents waiting for a connection termination request
+ from the remote TCP.
+
+ CLOSE-WAIT - represents waiting for a connection termination request
+ from the local user.
+
+ CLOSING - represents waiting for a connection termination request
+ acknowledgment from the remote TCP.
+
+ LAST-ACK - represents waiting for an acknowledgment of the
+ connection termination request previously sent to the remote TCP
+ (which includes an acknowledgment of its connection termination
+ request).
+
+
+
+ [Page 21]
+
+
+ September 1981
+Transmission Control Protocol
+Functional Specification
+
+
+
+ TIME-WAIT - represents waiting for enough time to pass to be sure
+ the remote TCP received the acknowledgment of its connection
+ termination request.
+
+ CLOSED - represents no connection state at all.
+
+ A TCP connection progresses from one state to another in response to
+ events. The events are the user calls, OPEN, SEND, RECEIVE, CLOSE,
+ ABORT, and STATUS; the incoming segments, particularly those
+ containing the SYN, ACK, RST and FIN flags; and timeouts.
+
+ The state diagram in figure 6 illustrates only state changes, together
+ with the causing events and resulting actions, but addresses neither
+ error conditions nor actions which are not connected with state
+ changes. In a later section, more detail is offered with respect to
+ the reaction of the TCP to events.
+
+ NOTE BENE: this diagram is only a summary and must not be taken as
+ the total specification.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+[Page 22]
+
+
+September 1981
+ Transmission Control Protocol
+ Functional Specification
+
+
+
+
+ +---------+ ---------\ active OPEN
+ | CLOSED | \ -----------
+ +---------+<---------\ \ create TCB
+ | ^ \ \ snd SYN
+ passive OPEN | | CLOSE \ \
+ ------------ | | ---------- \ \
+ create TCB | | delete TCB \ \
+ V | \ \
+ +---------+ CLOSE | \
+ | LISTEN | ---------- | |
+ +---------+ delete TCB | |
+ rcv SYN | | SEND | |
+ ----------- | | ------- | V
+ +---------+ snd SYN,ACK / \ snd SYN +---------+
+ | |<----------------- ------------------>| |
+ | SYN | rcv SYN | SYN |
+ | RCVD |<-----------------------------------------------| SENT |
+ | | snd ACK | |
+ | |------------------ -------------------| |
+ +---------+ rcv ACK of SYN \ / rcv SYN,ACK +---------+
+ | -------------- | | -----------
+ | x | | snd ACK
+ | V V
+ | CLOSE +---------+
+ | ------- | ESTAB |
+ | snd FIN +---------+
+ | CLOSE | | rcv FIN
+ V ------- | | -------
+ +---------+ snd FIN / \ snd ACK +---------+
+ | FIN |<----------------- ------------------>| CLOSE |
+ | WAIT-1 |------------------ | WAIT |
+ +---------+ rcv FIN \ +---------+
+ | rcv ACK of FIN ------- | CLOSE |
+ | -------------- snd ACK | ------- |
+ V x V snd FIN V
+ +---------+ +---------+ +---------+
+ |FINWAIT-2| | CLOSING | | LAST-ACK|
+ +---------+ +---------+ +---------+
+ | rcv ACK of FIN | rcv ACK of FIN |
+ | rcv FIN -------------- | Timeout=2MSL -------------- |
+ | ------- x V ------------ x V
+ \ snd ACK +---------+delete TCB +---------+
+ ------------------------>|TIME WAIT|------------------>| CLOSED |
+ +---------+ +---------+
+
+ TCP Connection State Diagram
+ Figure 6.
+
+
+ [Page 23]
+
+
+ September 1981
+Transmission Control Protocol
+Functional Specification
+
+
+
+3.3. Sequence Numbers
+
+ A fundamental notion in the design is that every octet of data sent
+ over a TCP connection has a sequence number. Since every octet is
+ sequenced, each of them can be acknowledged. The acknowledgment
+ mechanism employed is cumulative so that an acknowledgment of sequence
+ number X indicates that all octets up to but not including X have been
+ received. This mechanism allows for straight-forward duplicate
+ detection in the presence of retransmission. Numbering of octets
+ within a segment is that the first data octet immediately following
+ the header is the lowest numbered, and the following octets are
+ numbered consecutively.
+
+ It is essential to remember that the actual sequence number space is
+ finite, though very large. This space ranges from 0 to 2**32 - 1.
+ Since the space is finite, all arithmetic dealing with sequence
+ numbers must be performed modulo 2**32. This unsigned arithmetic
+ preserves the relationship of sequence numbers as they cycle from
+ 2**32 - 1 to 0 again. There are some subtleties to computer modulo
+ arithmetic, so great care should be taken in programming the
+ comparison of such values. The symbol "=<" means "less than or equal"
+ (modulo 2**32).
+
+ The typical kinds of sequence number comparisons which the TCP must
+ perform include:
+
+ (a) Determining that an acknowledgment refers to some sequence
+ number sent but not yet acknowledged.
+
+ (b) Determining that all sequence numbers occupied by a segment
+ have been acknowledged (e.g., to remove the segment from a
+ retransmission queue).
+
+ (c) Determining that an incoming segment contains sequence numbers
+ which are expected (i.e., that the segment "overlaps" the
+ receive window).
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+[Page 24]
+
+
+September 1981
+ Transmission Control Protocol
+ Functional Specification
+
+
+
+ In response to sending data the TCP will receive acknowledgments. The
+ following comparisons are needed to process the acknowledgments.
+
+ SND.UNA = oldest unacknowledged sequence number
+
+ SND.NXT = next sequence number to be sent
+
+ SEG.ACK = acknowledgment from the receiving TCP (next sequence
+ number expected by the receiving TCP)
+
+ SEG.SEQ = first sequence number of a segment
+
+ SEG.LEN = the number of octets occupied by the data in the segment
+ (counting SYN and FIN)
+
+ SEG.SEQ+SEG.LEN-1 = last sequence number of a segment
+
+ A new acknowledgment (called an "acceptable ack"), is one for which
+ the inequality below holds:
+
+ SND.UNA < SEG.ACK =< SND.NXT
+
+ A segment on the retransmission queue is fully acknowledged if the sum
+ of its sequence number and length is less or equal than the
+ acknowledgment value in the incoming segment.
+
+ When data is received the following comparisons are needed:
+
+ RCV.NXT = next sequence number expected on an incoming segments, and
+ is the left or lower edge of the receive window
+
+ RCV.NXT+RCV.WND-1 = last sequence number expected on an incoming
+ segment, and is the right or upper edge of the receive window
+
+ SEG.SEQ = first sequence number occupied by the incoming segment
+
+ SEG.SEQ+SEG.LEN-1 = last sequence number occupied by the incoming
+ segment
+
+ A segment is judged to occupy a portion of valid receive sequence
+ space if
+
+ RCV.NXT =< SEG.SEQ < RCV.NXT+RCV.WND
+
+ or
+
+ RCV.NXT =< SEG.SEQ+SEG.LEN-1 < RCV.NXT+RCV.WND
+
+
+
+ [Page 25]
+
+
+ September 1981
+Transmission Control Protocol
+Functional Specification
+
+
+
+ The first part of this test checks to see if the beginning of the
+ segment falls in the window, the second part of the test checks to see
+ if the end of the segment falls in the window; if the segment passes
+ either part of the test it contains data in the window.
+
+ Actually, it is a little more complicated than this. Due to zero
+ windows and zero length segments, we have four cases for the
+ acceptability of an incoming segment:
+
+ Segment Receive Test
+ Length Window
+ ------- ------- -------------------------------------------
+
+ 0 0 SEG.SEQ = RCV.NXT
+
+ 0 >0 RCV.NXT =< SEG.SEQ < RCV.NXT+RCV.WND
+
+ >0 0 not acceptable
+
+ >0 >0 RCV.NXT =< SEG.SEQ < RCV.NXT+RCV.WND
+ or RCV.NXT =< SEG.SEQ+SEG.LEN-1 < RCV.NXT+RCV.WND
+
+ Note that when the receive window is zero no segments should be
+ acceptable except ACK segments. Thus, it is be possible for a TCP to
+ maintain a zero receive window while transmitting data and receiving
+ ACKs. However, even when the receive window is zero, a TCP must
+ process the RST and URG fields of all incoming segments.
+
+ We have taken advantage of the numbering scheme to protect certain
+ control information as well. This is achieved by implicitly including
+ some control flags in the sequence space so they can be retransmitted
+ and acknowledged without confusion (i.e., one and only one copy of the
+ control will be acted upon). Control information is not physically
+ carried in the segment data space. Consequently, we must adopt rules
+ for implicitly assigning sequence numbers to control. The SYN and FIN
+ are the only controls requiring this protection, and these controls
+ are used only at connection opening and closing. For sequence number
+ purposes, the SYN is considered to occur before the first actual data
+ octet of the segment in which it occurs, while the FIN is considered
+ to occur after the last actual data octet in a segment in which it
+ occurs. The segment length (SEG.LEN) includes both data and sequence
+ space occupying controls. When a SYN is present then SEG.SEQ is the
+ sequence number of the SYN.
+
+
+
+
+
+
+
+[Page 26]
+
+
+September 1981
+ Transmission Control Protocol
+ Functional Specification
+
+
+
+ Initial Sequence Number Selection
+
+ The protocol places no restriction on a particular connection being
+ used over and over again. A connection is defined by a pair of
+ sockets. New instances of a connection will be referred to as
+ incarnations of the connection. The problem that arises from this is
+ -- "how does the TCP identify duplicate segments from previous
+ incarnations of the connection?" This problem becomes apparent if the
+ connection is being opened and closed in quick succession, or if the
+ connection breaks with loss of memory and is then reestablished.
+
+ To avoid confusion we must prevent segments from one incarnation of a
+ connection from being used while the same sequence numbers may still
+ be present in the network from an earlier incarnation. We want to
+ assure this, even if a TCP crashes and loses all knowledge of the
+ sequence numbers it has been using. When new connections are created,
+ an initial sequence number (ISN) generator is employed which selects a
+ new 32 bit ISN. The generator is bound to a (possibly fictitious) 32
+ bit clock whose low order bit is incremented roughly every 4
+ microseconds. Thus, the ISN cycles approximately every 4.55 hours.
+ Since we assume that segments will stay in the network no more than
+ the Maximum Segment Lifetime (MSL) and that the MSL is less than 4.55
+ hours we can reasonably assume that ISN's will be unique.
+
+ For each connection there is a send sequence number and a receive
+ sequence number. The initial send sequence number (ISS) is chosen by
+ the data sending TCP, and the initial receive sequence number (IRS) is
+ learned during the connection establishing procedure.
+
+ For a connection to be established or initialized, the two TCPs must
+ synchronize on each other's initial sequence numbers. This is done in
+ an exchange of connection establishing segments carrying a control bit
+ called "SYN" (for synchronize) and the initial sequence numbers. As a
+ shorthand, segments carrying the SYN bit are also called "SYNs".
+ Hence, the solution requires a suitable mechanism for picking an
+ initial sequence number and a slightly involved handshake to exchange
+ the ISN's.
+
+ The synchronization requires each side to send it's own initial
+ sequence number and to receive a confirmation of it in acknowledgment
+ from the other side. Each side must also receive the other side's
+ initial sequence number and send a confirming acknowledgment.
+
+ 1) A --> B SYN my sequence number is X
+ 2) A <-- B ACK your sequence number is X
+ 3) A <-- B SYN my sequence number is Y
+ 4) A --> B ACK your sequence number is Y
+
+
+
+ [Page 27]
+
+
+ September 1981
+Transmission Control Protocol
+Functional Specification
+
+
+
+ Because steps 2 and 3 can be combined in a single message this is
+ called the three way (or three message) handshake.
+
+ A three way handshake is necessary because sequence numbers are not
+ tied to a global clock in the network, and TCPs may have different
+ mechanisms for picking the ISN's. The receiver of the first SYN has
+ no way of knowing whether the segment was an old delayed one or not,
+ unless it remembers the last sequence number used on the connection
+ (which is not always possible), and so it must ask the sender to
+ verify this SYN. The three way handshake and the advantages of a
+ clock-driven scheme are discussed in [3].
+
+ Knowing When to Keep Quiet
+
+ To be sure that a TCP does not create a segment that carries a
+ sequence number which may be duplicated by an old segment remaining in
+ the network, the TCP must keep quiet for a maximum segment lifetime
+ (MSL) before assigning any sequence numbers upon starting up or
+ recovering from a crash in which memory of sequence numbers in use was
+ lost. For this specification the MSL is taken to be 2 minutes. This
+ is an engineering choice, and may be changed if experience indicates
+ it is desirable to do so. Note that if a TCP is reinitialized in some
+ sense, yet retains its memory of sequence numbers in use, then it need
+ not wait at all; it must only be sure to use sequence numbers larger
+ than those recently used.
+
+ The TCP Quiet Time Concept
+
+ This specification provides that hosts which "crash" without
+ retaining any knowledge of the last sequence numbers transmitted on
+ each active (i.e., not closed) connection shall delay emitting any
+ TCP segments for at least the agreed Maximum Segment Lifetime (MSL)
+ in the internet system of which the host is a part. In the
+ paragraphs below, an explanation for this specification is given.
+ TCP implementors may violate the "quiet time" restriction, but only
+ at the risk of causing some old data to be accepted as new or new
+ data rejected as old duplicated by some receivers in the internet
+ system.
+
+ TCPs consume sequence number space each time a segment is formed and
+ entered into the network output queue at a source host. The
+ duplicate detection and sequencing algorithm in the TCP protocol
+ relies on the unique binding of segment data to sequence space to
+ the extent that sequence numbers will not cycle through all 2**32
+ values before the segment data bound to those sequence numbers has
+ been delivered and acknowledged by the receiver and all duplicate
+ copies of the segments have "drained" from the internet. Without
+ such an assumption, two distinct TCP segments could conceivably be
+
+
+[Page 28]
+
+
+September 1981
+ Transmission Control Protocol
+ Functional Specification
+
+
+
+ assigned the same or overlapping sequence numbers, causing confusion
+ at the receiver as to which data is new and which is old. Remember
+ that each segment is bound to as many consecutive sequence numbers
+ as there are octets of data in the segment.
+
+ Under normal conditions, TCPs keep track of the next sequence number
+ to emit and the oldest awaiting acknowledgment so as to avoid
+ mistakenly using a sequence number over before its first use has
+ been acknowledged. This alone does not guarantee that old duplicate
+ data is drained from the net, so the sequence space has been made
+ very large to reduce the probability that a wandering duplicate will
+ cause trouble upon arrival. At 2 megabits/sec. it takes 4.5 hours
+ to use up 2**32 octets of sequence space. Since the maximum segment
+ lifetime in the net is not likely to exceed a few tens of seconds,
+ this is deemed ample protection for foreseeable nets, even if data
+ rates escalate to l0's of megabits/sec. At 100 megabits/sec, the
+ cycle time is 5.4 minutes which may be a little short, but still
+ within reason.
+
+ The basic duplicate detection and sequencing algorithm in TCP can be
+ defeated, however, if a source TCP does not have any memory of the
+ sequence numbers it last used on a given connection. For example, if
+ the TCP were to start all connections with sequence number 0, then
+ upon crashing and restarting, a TCP might re-form an earlier
+ connection (possibly after half-open connection resolution) and emit
+ packets with sequence numbers identical to or overlapping with
+ packets still in the network which were emitted on an earlier
+ incarnation of the same connection. In the absence of knowledge
+ about the sequence numbers used on a particular connection, the TCP
+ specification recommends that the source delay for MSL seconds
+ before emitting segments on the connection, to allow time for
+ segments from the earlier connection incarnation to drain from the
+ system.
+
+ Even hosts which can remember the time of day and used it to select
+ initial sequence number values are not immune from this problem
+ (i.e., even if time of day is used to select an initial sequence
+ number for each new connection incarnation).
+
+ Suppose, for example, that a connection is opened starting with
+ sequence number S. Suppose that this connection is not used much
+ and that eventually the initial sequence number function (ISN(t))
+ takes on a value equal to the sequence number, say S1, of the last
+ segment sent by this TCP on a particular connection. Now suppose,
+ at this instant, the host crashes, recovers, and establishes a new
+ incarnation of the connection. The initial sequence number chosen is
+ S1 = ISN(t) -- last used sequence number on old incarnation of
+ connection! If the recovery occurs quickly enough, any old
+
+
+ [Page 29]
+
+
+ September 1981
+Transmission Control Protocol
+Functional Specification
+
+
+
+ duplicates in the net bearing sequence numbers in the neighborhood
+ of S1 may arrive and be treated as new packets by the receiver of
+ the new incarnation of the connection.
+
+ The problem is that the recovering host may not know for how long it
+ crashed nor does it know whether there are still old duplicates in
+ the system from earlier connection incarnations.
+
+ One way to deal with this problem is to deliberately delay emitting
+ segments for one MSL after recovery from a crash- this is the "quite
+ time" specification. Hosts which prefer to avoid waiting are
+ willing to risk possible confusion of old and new packets at a given
+ destination may choose not to wait for the "quite time".
+ Implementors may provide TCP users with the ability to select on a
+ connection by connection basis whether to wait after a crash, or may
+ informally implement the "quite time" for all connections.
+ Obviously, even where a user selects to "wait," this is not
+ necessary after the host has been "up" for at least MSL seconds.
+
+ To summarize: every segment emitted occupies one or more sequence
+ numbers in the sequence space, the numbers occupied by a segment are
+ "busy" or "in use" until MSL seconds have passed, upon crashing a
+ block of space-time is occupied by the octets of the last emitted
+ segment, if a new connection is started too soon and uses any of the
+ sequence numbers in the space-time footprint of the last segment of
+ the previous connection incarnation, there is a potential sequence
+ number overlap area which could cause confusion at the receiver.
+
+3.4. Establishing a connection
+
+ The "three-way handshake" is the procedure used to establish a
+ connection. This procedure normally is initiated by one TCP and
+ responded to by another TCP. The procedure also works if two TCP
+ simultaneously initiate the procedure. When simultaneous attempt
+ occurs, each TCP receives a "SYN" segment which carries no
+ acknowledgment after it has sent a "SYN". Of course, the arrival of
+ an old duplicate "SYN" segment can potentially make it appear, to the
+ recipient, that a simultaneous connection initiation is in progress.
+ Proper use of "reset" segments can disambiguate these cases.
+
+ Several examples of connection initiation follow. Although these
+ examples do not show connection synchronization using data-carrying
+ segments, this is perfectly legitimate, so long as the receiving TCP
+ doesn't deliver the data to the user until it is clear the data is
+ valid (i.e., the data must be buffered at the receiver until the
+ connection reaches the ESTABLISHED state). The three-way handshake
+ reduces the possibility of false connections. It is the
+
+
+
+[Page 30]
+
+
+September 1981
+ Transmission Control Protocol
+ Functional Specification
+
+
+
+ implementation of a trade-off between memory and messages to provide
+ information for this checking.
+
+ The simplest three-way handshake is shown in figure 7 below. The
+ figures should be interpreted in the following way. Each line is
+ numbered for reference purposes. Right arrows (-->) indicate
+ departure of a TCP segment from TCP A to TCP B, or arrival of a
+ segment at B from A. Left arrows (<--), indicate the reverse.
+ Ellipsis (...) indicates a segment which is still in the network
+ (delayed). An "XXX" indicates a segment which is lost or rejected.
+ Comments appear in parentheses. TCP states represent the state AFTER
+ the departure or arrival of the segment (whose contents are shown in
+ the center of each line). Segment contents are shown in abbreviated
+ form, with sequence number, control flags, and ACK field. Other
+ fields such as window, addresses, lengths, and text have been left out
+ in the interest of clarity.
+
+
+
+ TCP A TCP B
+
+ 1. CLOSED LISTEN
+
+ 2. SYN-SENT --> <SEQ=100><CTL=SYN> --> SYN-RECEIVED
+
+ 3. ESTABLISHED <-- <SEQ=300><ACK=101><CTL=SYN,ACK> <-- SYN-RECEIVED
+
+ 4. ESTABLISHED --> <SEQ=101><ACK=301><CTL=ACK> --> ESTABLISHED
+
+ 5. ESTABLISHED --> <SEQ=101><ACK=301><CTL=ACK><DATA> --> ESTABLISHED
+
+ Basic 3-Way Handshake for Connection Synchronization
+
+ Figure 7.
+
+ In line 2 of figure 7, TCP A begins by sending a SYN segment
+ indicating that it will use sequence numbers starting with sequence
+ number 100. In line 3, TCP B sends a SYN and acknowledges the SYN it
+ received from TCP A. Note that the acknowledgment field indicates TCP
+ B is now expecting to hear sequence 101, acknowledging the SYN which
+ occupied sequence 100.
+
+ At line 4, TCP A responds with an empty segment containing an ACK for
+ TCP B's SYN; and in line 5, TCP A sends some data. Note that the
+ sequence number of the segment in line 5 is the same as in line 4
+ because the ACK does not occupy sequence number space (if it did, we
+ would wind up ACKing ACK's!).
+
+
+
+ [Page 31]
+
+
+ September 1981
+Transmission Control Protocol
+Functional Specification
+
+
+
+ Simultaneous initiation is only slightly more complex, as is shown in
+ figure 8. Each TCP cycles from CLOSED to SYN-SENT to SYN-RECEIVED to
+ ESTABLISHED.
+
+
+
+ TCP A TCP B
+
+ 1. CLOSED CLOSED
+
+ 2. SYN-SENT --> <SEQ=100><CTL=SYN> ...
+
+ 3. SYN-RECEIVED <-- <SEQ=300><CTL=SYN> <-- SYN-SENT
+
+ 4. ... <SEQ=100><CTL=SYN> --> SYN-RECEIVED
+
+ 5. SYN-RECEIVED --> <SEQ=100><ACK=301><CTL=SYN,ACK> ...
+
+ 6. ESTABLISHED <-- <SEQ=300><ACK=101><CTL=SYN,ACK> <-- SYN-RECEIVED
+
+ 7. ... <SEQ=101><ACK=301><CTL=ACK> --> ESTABLISHED
+
+ Simultaneous Connection Synchronization
+
+ Figure 8.
+
+ The principle reason for the three-way handshake is to prevent old
+ duplicate connection initiations from causing confusion. To deal with
+ this, a special control message, reset, has been devised. If the
+ receiving TCP is in a non-synchronized state (i.e., SYN-SENT,
+ SYN-RECEIVED), it returns to LISTEN on receiving an acceptable reset.
+ If the TCP is in one of the synchronized states (ESTABLISHED,
+ FIN-WAIT-1, FIN-WAIT-2, CLOSE-WAIT, CLOSING, LAST-ACK, TIME-WAIT), it
+ aborts the connection and informs its user. We discuss this latter
+ case under "half-open" connections below.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+[Page 32]
+
+
+September 1981
+ Transmission Control Protocol
+ Functional Specification
+
+
+
+
+
+ TCP A TCP B
+
+ 1. CLOSED LISTEN
+
+ 2. SYN-SENT --> <SEQ=100><CTL=SYN> ...
+
+ 3. (duplicate) ... <SEQ=90><CTL=SYN> --> SYN-RECEIVED
+
+ 4. SYN-SENT <-- <SEQ=300><ACK=91><CTL=SYN,ACK> <-- SYN-RECEIVED
+
+ 5. SYN-SENT --> <SEQ=91><CTL=RST> --> LISTEN
+
+
+ 6. ... <SEQ=100><CTL=SYN> --> SYN-RECEIVED
+
+ 7. SYN-SENT <-- <SEQ=400><ACK=101><CTL=SYN,ACK> <-- SYN-RECEIVED
+
+ 8. ESTABLISHED --> <SEQ=101><ACK=401><CTL=ACK> --> ESTABLISHED
+
+ Recovery from Old Duplicate SYN
+
+ Figure 9.
+
+ As a simple example of recovery from old duplicates, consider
+ figure 9. At line 3, an old duplicate SYN arrives at TCP B. TCP B
+ cannot tell that this is an old duplicate, so it responds normally
+ (line 4). TCP A detects that the ACK field is incorrect and returns a
+ RST (reset) with its SEQ field selected to make the segment
+ believable. TCP B, on receiving the RST, returns to the LISTEN state.
+ When the original SYN (pun intended) finally arrives at line 6, the
+ synchronization proceeds normally. If the SYN at line 6 had arrived
+ before the RST, a more complex exchange might have occurred with RST's
+ sent in both directions.
+
+ Half-Open Connections and Other Anomalies
+
+ An established connection is said to be "half-open" if one of the
+ TCPs has closed or aborted the connection at its end without the
+ knowledge of the other, or if the two ends of the connection have
+ become desynchronized owing to a crash that resulted in loss of
+ memory. Such connections will automatically become reset if an
+ attempt is made to send data in either direction. However, half-open
+ connections are expected to be unusual, and the recovery procedure is
+ mildly involved.
+
+ If at site A the connection no longer exists, then an attempt by the
+
+
+ [Page 33]
+
+
+ September 1981
+Transmission Control Protocol
+Functional Specification
+
+
+
+ user at site B to send any data on it will result in the site B TCP
+ receiving a reset control message. Such a message indicates to the
+ site B TCP that something is wrong, and it is expected to abort the
+ connection.
+
+ Assume that two user processes A and B are communicating with one
+ another when a crash occurs causing loss of memory to A's TCP.
+ Depending on the operating system supporting A's TCP, it is likely
+ that some error recovery mechanism exists. When the TCP is up again,
+ A is likely to start again from the beginning or from a recovery
+ point. As a result, A will probably try to OPEN the connection again
+ or try to SEND on the connection it believes open. In the latter
+ case, it receives the error message "connection not open" from the
+ local (A's) TCP. In an attempt to establish the connection, A's TCP
+ will send a segment containing SYN. This scenario leads to the
+ example shown in figure 10. After TCP A crashes, the user attempts to
+ re-open the connection. TCP B, in the meantime, thinks the connection
+ is open.
+
+
+
+ TCP A TCP B
+
+ 1. (CRASH) (send 300,receive 100)
+
+ 2. CLOSED ESTABLISHED
+
+ 3. SYN-SENT --> <SEQ=400><CTL=SYN> --> (??)
+
+ 4. (!!) <-- <SEQ=300><ACK=100><CTL=ACK> <-- ESTABLISHED
+
+ 5. SYN-SENT --> <SEQ=100><CTL=RST> --> (Abort!!)
+
+ 6. SYN-SENT CLOSED
+
+ 7. SYN-SENT --> <SEQ=400><CTL=SYN> -->
+
+ Half-Open Connection Discovery
+
+ Figure 10.
+
+ When the SYN arrives at line 3, TCP B, being in a synchronized state,
+ and the incoming segment outside the window, responds with an
+ acknowledgment indicating what sequence it next expects to hear (ACK
+ 100). TCP A sees that this segment does not acknowledge anything it
+ sent and, being unsynchronized, sends a reset (RST) because it has
+ detected a half-open connection. TCP B aborts at line 5. TCP A will
+
+
+
+[Page 34]
+
+
+September 1981
+ Transmission Control Protocol
+ Functional Specification
+
+
+
+ continue to try to establish the connection; the problem is now
+ reduced to the basic 3-way handshake of figure 7.
+
+ An interesting alternative case occurs when TCP A crashes and TCP B
+ tries to send data on what it thinks is a synchronized connection.
+ This is illustrated in figure 11. In this case, the data arriving at
+ TCP A from TCP B (line 2) is unacceptable because no such connection
+ exists, so TCP A sends a RST. The RST is acceptable so TCP B
+ processes it and aborts the connection.
+
+
+
+ TCP A TCP B
+
+ 1. (CRASH) (send 300,receive 100)
+
+ 2. (??) <-- <SEQ=300><ACK=100><DATA=10><CTL=ACK> <-- ESTABLISHED
+
+ 3. --> <SEQ=100><CTL=RST> --> (ABORT!!)
+
+ Active Side Causes Half-Open Connection Discovery
+
+ Figure 11.
+
+ In figure 12, we find the two TCPs A and B with passive connections
+ waiting for SYN. An old duplicate arriving at TCP B (line 2) stirs B
+ into action. A SYN-ACK is returned (line 3) and causes TCP A to
+ generate a RST (the ACK in line 3 is not acceptable). TCP B accepts
+ the reset and returns to its passive LISTEN state.
+
+
+
+ TCP A TCP B
+
+ 1. LISTEN LISTEN
+
+ 2. ... <SEQ=Z><CTL=SYN> --> SYN-RECEIVED
+
+ 3. (??) <-- <SEQ=X><ACK=Z+1><CTL=SYN,ACK> <-- SYN-RECEIVED
+
+ 4. --> <SEQ=Z+1><CTL=RST> --> (return to LISTEN!)
+
+ 5. LISTEN LISTEN
+
+ Old Duplicate SYN Initiates a Reset on two Passive Sockets
+
+ Figure 12.
+
+
+
+ [Page 35]
+
+
+ September 1981
+Transmission Control Protocol
+Functional Specification
+
+
+
+ A variety of other cases are possible, all of which are accounted for
+ by the following rules for RST generation and processing.
+
+ Reset Generation
+
+ As a general rule, reset (RST) must be sent whenever a segment arrives
+ which apparently is not intended for the current connection. A reset
+ must not be sent if it is not clear that this is the case.
+
+ There are three groups of states:
+
+ 1. If the connection does not exist (CLOSED) then a reset is sent
+ in response to any incoming segment except another reset. In
+ particular, SYNs addressed to a non-existent connection are rejected
+ by this means.
+
+ If the incoming segment has an ACK field, the reset takes its
+ sequence number from the ACK field of the segment, otherwise the
+ reset has sequence number zero and the ACK field is set to the sum
+ of the sequence number and segment length of the incoming segment.
+ The connection remains in the CLOSED state.
+
+ 2. If the connection is in any non-synchronized state (LISTEN,
+ SYN-SENT, SYN-RECEIVED), and the incoming segment acknowledges
+ something not yet sent (the segment carries an unacceptable ACK), or
+ if an incoming segment has a security level or compartment which
+ does not exactly match the level and compartment requested for the
+ connection, a reset is sent.
+
+ If our SYN has not been acknowledged and the precedence level of the
+ incoming segment is higher than the precedence level requested then
+ either raise the local precedence level (if allowed by the user and
+ the system) or send a reset; or if the precedence level of the
+ incoming segment is lower than the precedence level requested then
+ continue as if the precedence matched exactly (if the remote TCP
+ cannot raise the precedence level to match ours this will be
+ detected in the next segment it sends, and the connection will be
+ terminated then). If our SYN has been acknowledged (perhaps in this
+ incoming segment) the precedence level of the incoming segment must
+ match the local precedence level exactly, if it does not a reset
+ must be sent.
+
+ If the incoming segment has an ACK field, the reset takes its
+ sequence number from the ACK field of the segment, otherwise the
+ reset has sequence number zero and the ACK field is set to the sum
+ of the sequence number and segment length of the incoming segment.
+ The connection remains in the same state.
+
+
+
+[Page 36]
+
+
+September 1981
+ Transmission Control Protocol
+ Functional Specification
+
+
+
+ 3. If the connection is in a synchronized state (ESTABLISHED,
+ FIN-WAIT-1, FIN-WAIT-2, CLOSE-WAIT, CLOSING, LAST-ACK, TIME-WAIT),
+ any unacceptable segment (out of window sequence number or
+ unacceptible acknowledgment number) must elicit only an empty
+ acknowledgment segment containing the current send-sequence number
+ and an acknowledgment indicating the next sequence number expected
+ to be received, and the connection remains in the same state.
+
+ If an incoming segment has a security level, or compartment, or
+ precedence which does not exactly match the level, and compartment,
+ and precedence requested for the connection,a reset is sent and
+ connection goes to the CLOSED state. The reset takes its sequence
+ number from the ACK field of the incoming segment.
+
+ Reset Processing
+
+ In all states except SYN-SENT, all reset (RST) segments are validated
+ by checking their SEQ-fields. A reset is valid if its sequence number
+ is in the window. In the SYN-SENT state (a RST received in response
+ to an initial SYN), the RST is acceptable if the ACK field
+ acknowledges the SYN.
+
+ The receiver of a RST first validates it, then changes state. If the
+ receiver was in the LISTEN state, it ignores it. If the receiver was
+ in SYN-RECEIVED state and had previously been in the LISTEN state,
+ then the receiver returns to the LISTEN state, otherwise the receiver
+ aborts the connection and goes to the CLOSED state. If the receiver
+ was in any other state, it aborts the connection and advises the user
+ and goes to the CLOSED state.
+
+3.5. Closing a Connection
+
+ CLOSE is an operation meaning "I have no more data to send." The
+ notion of closing a full-duplex connection is subject to ambiguous
+ interpretation, of course, since it may not be obvious how to treat
+ the receiving side of the connection. We have chosen to treat CLOSE
+ in a simplex fashion. The user who CLOSEs may continue to RECEIVE
+ until he is told that the other side has CLOSED also. Thus, a program
+ could initiate several SENDs followed by a CLOSE, and then continue to
+ RECEIVE until signaled that a RECEIVE failed because the other side
+ has CLOSED. We assume that the TCP will signal a user, even if no
+ RECEIVEs are outstanding, that the other side has closed, so the user
+ can terminate his side gracefully. A TCP will reliably deliver all
+ buffers SENT before the connection was CLOSED so a user who expects no
+ data in return need only wait to hear the connection was CLOSED
+ successfully to know that all his data was received at the destination
+ TCP. Users must keep reading connections they close for sending until
+ the TCP says no more data.
+
+
+ [Page 37]
+
+
+ September 1981
+Transmission Control Protocol
+Functional Specification
+
+
+
+ There are essentially three cases:
+
+ 1) The user initiates by telling the TCP to CLOSE the connection
+
+ 2) The remote TCP initiates by sending a FIN control signal
+
+ 3) Both users CLOSE simultaneously
+
+ Case 1: Local user initiates the close
+
+ In this case, a FIN segment can be constructed and placed on the
+ outgoing segment queue. No further SENDs from the user will be
+ accepted by the TCP, and it enters the FIN-WAIT-1 state. RECEIVEs
+ are allowed in this state. All segments preceding and including FIN
+ will be retransmitted until acknowledged. When the other TCP has
+ both acknowledged the FIN and sent a FIN of its own, the first TCP
+ can ACK this FIN. Note that a TCP receiving a FIN will ACK but not
+ send its own FIN until its user has CLOSED the connection also.
+
+ Case 2: TCP receives a FIN from the network
+
+ If an unsolicited FIN arrives from the network, the receiving TCP
+ can ACK it and tell the user that the connection is closing. The
+ user will respond with a CLOSE, upon which the TCP can send a FIN to
+ the other TCP after sending any remaining data. The TCP then waits
+ until its own FIN is acknowledged whereupon it deletes the
+ connection. If an ACK is not forthcoming, after the user timeout
+ the connection is aborted and the user is told.
+
+ Case 3: both users close simultaneously
+
+ A simultaneous CLOSE by users at both ends of a connection causes
+ FIN segments to be exchanged. When all segments preceding the FINs
+ have been processed and acknowledged, each TCP can ACK the FIN it
+ has received. Both will, upon receiving these ACKs, delete the
+ connection.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+[Page 38]
+
+
+September 1981
+ Transmission Control Protocol
+ Functional Specification
+
+
+
+
+
+ TCP A TCP B
+
+ 1. ESTABLISHED ESTABLISHED
+
+ 2. (Close)
+ FIN-WAIT-1 --> <SEQ=100><ACK=300><CTL=FIN,ACK> --> CLOSE-WAIT
+
+ 3. FIN-WAIT-2 <-- <SEQ=300><ACK=101><CTL=ACK> <-- CLOSE-WAIT
+
+ 4. (Close)
+ TIME-WAIT <-- <SEQ=300><ACK=101><CTL=FIN,ACK> <-- LAST-ACK
+
+ 5. TIME-WAIT --> <SEQ=101><ACK=301><CTL=ACK> --> CLOSED
+
+ 6. (2 MSL)
+ CLOSED
+
+ Normal Close Sequence
+
+ Figure 13.
+
+
+
+ TCP A TCP B
+
+ 1. ESTABLISHED ESTABLISHED
+
+ 2. (Close) (Close)
+ FIN-WAIT-1 --> <SEQ=100><ACK=300><CTL=FIN,ACK> ... FIN-WAIT-1
+ <-- <SEQ=300><ACK=100><CTL=FIN,ACK> <--
+ ... <SEQ=100><ACK=300><CTL=FIN,ACK> -->
+
+ 3. CLOSING --> <SEQ=101><ACK=301><CTL=ACK> ... CLOSING
+ <-- <SEQ=301><ACK=101><CTL=ACK> <--
+ ... <SEQ=101><ACK=301><CTL=ACK> -->
+
+ 4. TIME-WAIT TIME-WAIT
+ (2 MSL) (2 MSL)
+ CLOSED CLOSED
+
+ Simultaneous Close Sequence
+
+ Figure 14.
+
+
+
+
+
+ [Page 39]
+
+
+ September 1981
+Transmission Control Protocol
+Functional Specification
+
+
+
+3.6. Precedence and Security
+
+ The intent is that connection be allowed only between ports operating
+ with exactly the same security and compartment values and at the
+ higher of the precedence level requested by the two ports.
+
+ The precedence and security parameters used in TCP are exactly those
+ defined in the Internet Protocol (IP) [2]. Throughout this TCP
+ specification the term "security/compartment" is intended to indicate
+ the security parameters used in IP including security, compartment,
+ user group, and handling restriction.
+
+ A connection attempt with mismatched security/compartment values or a
+ lower precedence value must be rejected by sending a reset. Rejecting
+ a connection due to too low a precedence only occurs after an
+ acknowledgment of the SYN has been received.
+
+ Note that TCP modules which operate only at the default value of
+ precedence will still have to check the precedence of incoming
+ segments and possibly raise the precedence level they use on the
+ connection.
+
+ The security paramaters may be used even in a non-secure environment
+ (the values would indicate unclassified data), thus hosts in
+ non-secure environments must be prepared to receive the security
+ parameters, though they need not send them.
+
+3.7. Data Communication
+
+ Once the connection is established data is communicated by the
+ exchange of segments. Because segments may be lost due to errors
+ (checksum test failure), or network congestion, TCP uses
+ retransmission (after a timeout) to ensure delivery of every segment.
+ Duplicate segments may arrive due to network or TCP retransmission.
+ As discussed in the section on sequence numbers the TCP performs
+ certain tests on the sequence and acknowledgment numbers in the
+ segments to verify their acceptability.
+
+ The sender of data keeps track of the next sequence number to use in
+ the variable SND.NXT. The receiver of data keeps track of the next
+ sequence number to expect in the variable RCV.NXT. The sender of data
+ keeps track of the oldest unacknowledged sequence number in the
+ variable SND.UNA. If the data flow is momentarily idle and all data
+ sent has been acknowledged then the three variables will be equal.
+
+ When the sender creates a segment and transmits it the sender advances
+ SND.NXT. When the receiver accepts a segment it advances RCV.NXT and
+ sends an acknowledgment. When the data sender receives an
+
+
+[Page 40]
+
+
+September 1981
+ Transmission Control Protocol
+ Functional Specification
+
+
+
+ acknowledgment it advances SND.UNA. The extent to which the values of
+ these variables differ is a measure of the delay in the communication.
+ The amount by which the variables are advanced is the length of the
+ data in the segment. Note that once in the ESTABLISHED state all
+ segments must carry current acknowledgment information.
+
+ The CLOSE user call implies a push function, as does the FIN control
+ flag in an incoming segment.
+
+ Retransmission Timeout
+
+ Because of the variability of the networks that compose an
+ internetwork system and the wide range of uses of TCP connections the
+ retransmission timeout must be dynamically determined. One procedure
+ for determining a retransmission time out is given here as an
+ illustration.
+
+ An Example Retransmission Timeout Procedure
+
+ Measure the elapsed time between sending a data octet with a
+ particular sequence number and receiving an acknowledgment that
+ covers that sequence number (segments sent do not have to match
+ segments received). This measured elapsed time is the Round Trip
+ Time (RTT). Next compute a Smoothed Round Trip Time (SRTT) as:
+
+ SRTT = ( ALPHA * SRTT ) + ((1-ALPHA) * RTT)
+
+ and based on this, compute the retransmission timeout (RTO) as:
+
+ RTO = min[UBOUND,max[LBOUND,(BETA*SRTT)]]
+
+ where UBOUND is an upper bound on the timeout (e.g., 1 minute),
+ LBOUND is a lower bound on the timeout (e.g., 1 second), ALPHA is
+ a smoothing factor (e.g., .8 to .9), and BETA is a delay variance
+ factor (e.g., 1.3 to 2.0).
+
+ The Communication of Urgent Information
+
+ The objective of the TCP urgent mechanism is to allow the sending user
+ to stimulate the receiving user to accept some urgent data and to
+ permit the receiving TCP to indicate to the receiving user when all
+ the currently known urgent data has been received by the user.
+
+ This mechanism permits a point in the data stream to be designated as
+ the end of urgent information. Whenever this point is in advance of
+ the receive sequence number (RCV.NXT) at the receiving TCP, that TCP
+ must tell the user to go into "urgent mode"; when the receive sequence
+ number catches up to the urgent pointer, the TCP must tell user to go
+
+
+ [Page 41]
+
+
+ September 1981
+Transmission Control Protocol
+Functional Specification
+
+
+
+ into "normal mode". If the urgent pointer is updated while the user
+ is in "urgent mode", the update will be invisible to the user.
+
+ The method employs a urgent field which is carried in all segments
+ transmitted. The URG control flag indicates that the urgent field is
+ meaningful and must be added to the segment sequence number to yield
+ the urgent pointer. The absence of this flag indicates that there is
+ no urgent data outstanding.
+
+ To send an urgent indication the user must also send at least one data
+ octet. If the sending user also indicates a push, timely delivery of
+ the urgent information to the destination process is enhanced.
+
+ Managing the Window
+
+ The window sent in each segment indicates the range of sequence
+ numbers the sender of the window (the data receiver) is currently
+ prepared to accept. There is an assumption that this is related to
+ the currently available data buffer space available for this
+ connection.
+
+ Indicating a large window encourages transmissions. If more data
+ arrives than can be accepted, it will be discarded. This will result
+ in excessive retransmissions, adding unnecessarily to the load on the
+ network and the TCPs. Indicating a small window may restrict the
+ transmission of data to the point of introducing a round trip delay
+ between each new segment transmitted.
+
+ The mechanisms provided allow a TCP to advertise a large window and to
+ subsequently advertise a much smaller window without having accepted
+ that much data. This, so called "shrinking the window," is strongly
+ discouraged. The robustness principle dictates that TCPs will not
+ shrink the window themselves, but will be prepared for such behavior
+ on the part of other TCPs.
+
+ The sending TCP must be prepared to accept from the user and send at
+ least one octet of new data even if the send window is zero. The
+ sending TCP must regularly retransmit to the receiving TCP even when
+ the window is zero. Two minutes is recommended for the retransmission
+ interval when the window is zero. This retransmission is essential to
+ guarantee that when either TCP has a zero window the re-opening of the
+ window will be reliably reported to the other.
+
+ When the receiving TCP has a zero window and a segment arrives it must
+ still send an acknowledgment showing its next expected sequence number
+ and current window (zero).
+
+ The sending TCP packages the data to be transmitted into segments
+
+
+[Page 42]
+
+
+September 1981
+ Transmission Control Protocol
+ Functional Specification
+
+
+
+ which fit the current window, and may repackage segments on the
+ retransmission queue. Such repackaging is not required, but may be
+ helpful.
+
+ In a connection with a one-way data flow, the window information will
+ be carried in acknowledgment segments that all have the same sequence
+ number so there will be no way to reorder them if they arrive out of
+ order. This is not a serious problem, but it will allow the window
+ information to be on occasion temporarily based on old reports from
+ the data receiver. A refinement to avoid this problem is to act on
+ the window information from segments that carry the highest
+ acknowledgment number (that is segments with acknowledgment number
+ equal or greater than the highest previously received).
+
+ The window management procedure has significant influence on the
+ communication performance. The following comments are suggestions to
+ implementers.
+
+ Window Management Suggestions
+
+ Allocating a very small window causes data to be transmitted in
+ many small segments when better performance is achieved using
+ fewer large segments.
+
+ One suggestion for avoiding small windows is for the receiver to
+ defer updating a window until the additional allocation is at
+ least X percent of the maximum allocation possible for the
+ connection (where X might be 20 to 40).
+
+ Another suggestion is for the sender to avoid sending small
+ segments by waiting until the window is large enough before
+ sending data. If the the user signals a push function then the
+ data must be sent even if it is a small segment.
+
+ Note that the acknowledgments should not be delayed or unnecessary
+ retransmissions will result. One strategy would be to send an
+ acknowledgment when a small segment arrives (with out updating the
+ window information), and then to send another acknowledgment with
+ new window information when the window is larger.
+
+ The segment sent to probe a zero window may also begin a break up
+ of transmitted data into smaller and smaller segments. If a
+ segment containing a single data octet sent to probe a zero window
+ is accepted, it consumes one octet of the window now available.
+ If the sending TCP simply sends as much as it can whenever the
+ window is non zero, the transmitted data will be broken into
+ alternating big and small segments. As time goes on, occasional
+ pauses in the receiver making window allocation available will
+
+
+ [Page 43]
+
+
+ September 1981
+Transmission Control Protocol
+Functional Specification
+
+
+
+ result in breaking the big segments into a small and not quite so
+ big pair. And after a while the data transmission will be in
+ mostly small segments.
+
+ The suggestion here is that the TCP implementations need to
+ actively attempt to combine small window allocations into larger
+ windows, since the mechanisms for managing the window tend to lead
+ to many small windows in the simplest minded implementations.
+
+3.8. Interfaces
+
+ There are of course two interfaces of concern: the user/TCP interface
+ and the TCP/lower-level interface. We have a fairly elaborate model
+ of the user/TCP interface, but the interface to the lower level
+ protocol module is left unspecified here, since it will be specified
+ in detail by the specification of the lowel level protocol. For the
+ case that the lower level is IP we note some of the parameter values
+ that TCPs might use.
+
+ User/TCP Interface
+
+ The following functional description of user commands to the TCP is,
+ at best, fictional, since every operating system will have different
+ facilities. Consequently, we must warn readers that different TCP
+ implementations may have different user interfaces. However, all
+ TCPs must provide a certain minimum set of services to guarantee
+ that all TCP implementations can support the same protocol
+ hierarchy. This section specifies the functional interfaces
+ required of all TCP implementations.
+
+ TCP User Commands
+
+ The following sections functionally characterize a USER/TCP
+ interface. The notation used is similar to most procedure or
+ function calls in high level languages, but this usage is not
+ meant to rule out trap type service calls (e.g., SVCs, UUOs,
+ EMTs).
+
+ The user commands described below specify the basic functions the
+ TCP must perform to support interprocess communication.
+ Individual implementations must define their own exact format, and
+ may provide combinations or subsets of the basic functions in
+ single calls. In particular, some implementations may wish to
+ automatically OPEN a connection on the first SEND or RECEIVE
+ issued by the user for a given connection.
+
+
+
+
+
+[Page 44]
+
+
+September 1981
+ Transmission Control Protocol
+ Functional Specification
+
+
+
+ In providing interprocess communication facilities, the TCP must
+ not only accept commands, but must also return information to the
+ processes it serves. The latter consists of:
+
+ (a) general information about a connection (e.g., interrupts,
+ remote close, binding of unspecified foreign socket).
+
+ (b) replies to specific user commands indicating success or
+ various types of failure.
+
+ Open
+
+ Format: OPEN (local port, foreign socket, active/passive
+ [, timeout] [, precedence] [, security/compartment] [, options])
+ -> local connection name
+
+ We assume that the local TCP is aware of the identity of the
+ processes it serves and will check the authority of the process
+ to use the connection specified. Depending upon the
+ implementation of the TCP, the local network and TCP identifiers
+ for the source address will either be supplied by the TCP or the
+ lower level protocol (e.g., IP). These considerations are the
+ result of concern about security, to the extent that no TCP be
+ able to masquerade as another one, and so on. Similarly, no
+ process can masquerade as another without the collusion of the
+ TCP.
+
+ If the active/passive flag is set to passive, then this is a
+ call to LISTEN for an incoming connection. A passive open may
+ have either a fully specified foreign socket to wait for a
+ particular connection or an unspecified foreign socket to wait
+ for any call. A fully specified passive call can be made active
+ by the subsequent execution of a SEND.
+
+ A transmission control block (TCB) is created and partially
+ filled in with data from the OPEN command parameters.
+
+ On an active OPEN command, the TCP will begin the procedure to
+ synchronize (i.e., establish) the connection at once.
+
+ The timeout, if present, permits the caller to set up a timeout
+ for all data submitted to TCP. If data is not successfully
+ delivered to the destination within the timeout period, the TCP
+ will abort the connection. The present global default is five
+ minutes.
+
+ The TCP or some component of the operating system will verify
+ the users authority to open a connection with the specified
+
+
+ [Page 45]
+
+
+ September 1981
+Transmission Control Protocol
+Functional Specification
+
+
+
+ precedence or security/compartment. The absence of precedence
+ or security/compartment specification in the OPEN call indicates
+ the default values must be used.
+
+ TCP will accept incoming requests as matching only if the
+ security/compartment information is exactly the same and only if
+ the precedence is equal to or higher than the precedence
+ requested in the OPEN call.
+
+ The precedence for the connection is the higher of the values
+ requested in the OPEN call and received from the incoming
+ request, and fixed at that value for the life of the
+ connection.Implementers may want to give the user control of
+ this precedence negotiation. For example, the user might be
+ allowed to specify that the precedence must be exactly matched,
+ or that any attempt to raise the precedence be confirmed by the
+ user.
+
+ A local connection name will be returned to the user by the TCP.
+ The local connection name can then be used as a short hand term
+ for the connection defined by the <local socket, foreign socket>
+ pair.
+
+ Send
+
+ Format: SEND (local connection name, buffer address, byte
+ count, PUSH flag, URGENT flag [,timeout])
+
+ This call causes the data contained in the indicated user buffer
+ to be sent on the indicated connection. If the connection has
+ not been opened, the SEND is considered an error. Some
+ implementations may allow users to SEND first; in which case, an
+ automatic OPEN would be done. If the calling process is not
+ authorized to use this connection, an error is returned.
+
+ If the PUSH flag is set, the data must be transmitted promptly
+ to the receiver, and the PUSH bit will be set in the last TCP
+ segment created from the buffer. If the PUSH flag is not set,
+ the data may be combined with data from subsequent SENDs for
+ transmission efficiency.
+
+ If the URGENT flag is set, segments sent to the destination TCP
+ will have the urgent pointer set. The receiving TCP will signal
+ the urgent condition to the receiving process if the urgent
+ pointer indicates that data preceding the urgent pointer has not
+ been consumed by the receiving process. The purpose of urgent
+ is to stimulate the receiver to process the urgent data and to
+ indicate to the receiver when all the currently known urgent
+
+
+[Page 46]
+
+
+September 1981
+ Transmission Control Protocol
+ Functional Specification
+
+
+
+ data has been received. The number of times the sending user's
+ TCP signals urgent will not necessarily be equal to the number
+ of times the receiving user will be notified of the presence of
+ urgent data.
+
+ If no foreign socket was specified in the OPEN, but the
+ connection is established (e.g., because a LISTENing connection
+ has become specific due to a foreign segment arriving for the
+ local socket), then the designated buffer is sent to the implied
+ foreign socket. Users who make use of OPEN with an unspecified
+ foreign socket can make use of SEND without ever explicitly
+ knowing the foreign socket address.
+
+ However, if a SEND is attempted before the foreign socket
+ becomes specified, an error will be returned. Users can use the
+ STATUS call to determine the status of the connection. In some
+ implementations the TCP may notify the user when an unspecified
+ socket is bound.
+
+ If a timeout is specified, the current user timeout for this
+ connection is changed to the new one.
+
+ In the simplest implementation, SEND would not return control to
+ the sending process until either the transmission was complete
+ or the timeout had been exceeded. However, this simple method
+ is both subject to deadlocks (for example, both sides of the
+ connection might try to do SENDs before doing any RECEIVEs) and
+ offers poor performance, so it is not recommended. A more
+ sophisticated implementation would return immediately to allow
+ the process to run concurrently with network I/O, and,
+ furthermore, to allow multiple SENDs to be in progress.
+ Multiple SENDs are served in first come, first served order, so
+ the TCP will queue those it cannot service immediately.
+
+ We have implicitly assumed an asynchronous user interface in
+ which a SEND later elicits some kind of SIGNAL or
+ pseudo-interrupt from the serving TCP. An alternative is to
+ return a response immediately. For instance, SENDs might return
+ immediate local acknowledgment, even if the segment sent had not
+ been acknowledged by the distant TCP. We could optimistically
+ assume eventual success. If we are wrong, the connection will
+ close anyway due to the timeout. In implementations of this
+ kind (synchronous), there will still be some asynchronous
+ signals, but these will deal with the connection itself, and not
+ with specific segments or buffers.
+
+ In order for the process to distinguish among error or success
+ indications for different SENDs, it might be appropriate for the
+
+
+ [Page 47]
+
+
+ September 1981
+Transmission Control Protocol
+Functional Specification
+
+
+
+ buffer address to be returned along with the coded response to
+ the SEND request. TCP-to-user signals are discussed below,
+ indicating the information which should be returned to the
+ calling process.
+
+ Receive
+
+ Format: RECEIVE (local connection name, buffer address, byte
+ count) -> byte count, urgent flag, push flag
+
+ This command allocates a receiving buffer associated with the
+ specified connection. If no OPEN precedes this command or the
+ calling process is not authorized to use this connection, an
+ error is returned.
+
+ In the simplest implementation, control would not return to the
+ calling program until either the buffer was filled, or some
+ error occurred, but this scheme is highly subject to deadlocks.
+ A more sophisticated implementation would permit several
+ RECEIVEs to be outstanding at once. These would be filled as
+ segments arrive. This strategy permits increased throughput at
+ the cost of a more elaborate scheme (possibly asynchronous) to
+ notify the calling program that a PUSH has been seen or a buffer
+ filled.
+
+ If enough data arrive to fill the buffer before a PUSH is seen,
+ the PUSH flag will not be set in the response to the RECEIVE.
+ The buffer will be filled with as much data as it can hold. If
+ a PUSH is seen before the buffer is filled the buffer will be
+ returned partially filled and PUSH indicated.
+
+ If there is urgent data the user will have been informed as soon
+ as it arrived via a TCP-to-user signal. The receiving user
+ should thus be in "urgent mode". If the URGENT flag is on,
+ additional urgent data remains. If the URGENT flag is off, this
+ call to RECEIVE has returned all the urgent data, and the user
+ may now leave "urgent mode". Note that data following the
+ urgent pointer (non-urgent data) cannot be delivered to the user
+ in the same buffer with preceeding urgent data unless the
+ boundary is clearly marked for the user.
+
+ To distinguish among several outstanding RECEIVEs and to take
+ care of the case that a buffer is not completely filled, the
+ return code is accompanied by both a buffer pointer and a byte
+ count indicating the actual length of the data received.
+
+ Alternative implementations of RECEIVE might have the TCP
+
+
+
+[Page 48]
+
+
+September 1981
+ Transmission Control Protocol
+ Functional Specification
+
+
+
+ allocate buffer storage, or the TCP might share a ring buffer
+ with the user.
+
+ Close
+
+ Format: CLOSE (local connection name)
+
+ This command causes the connection specified to be closed. If
+ the connection is not open or the calling process is not
+ authorized to use this connection, an error is returned.
+ Closing connections is intended to be a graceful operation in
+ the sense that outstanding SENDs will be transmitted (and
+ retransmitted), as flow control permits, until all have been
+ serviced. Thus, it should be acceptable to make several SEND
+ calls, followed by a CLOSE, and expect all the data to be sent
+ to the destination. It should also be clear that users should
+ continue to RECEIVE on CLOSING connections, since the other side
+ may be trying to transmit the last of its data. Thus, CLOSE
+ means "I have no more to send" but does not mean "I will not
+ receive any more." It may happen (if the user level protocol is
+ not well thought out) that the closing side is unable to get rid
+ of all its data before timing out. In this event, CLOSE turns
+ into ABORT, and the closing TCP gives up.
+
+ The user may CLOSE the connection at any time on his own
+ initiative, or in response to various prompts from the TCP
+ (e.g., remote close executed, transmission timeout exceeded,
+ destination inaccessible).
+
+ Because closing a connection requires communication with the
+ foreign TCP, connections may remain in the closing state for a
+ short time. Attempts to reopen the connection before the TCP
+ replies to the CLOSE command will result in error responses.
+
+ Close also implies push function.
+
+ Status
+
+ Format: STATUS (local connection name) -> status data
+
+ This is an implementation dependent user command and could be
+ excluded without adverse effect. Information returned would
+ typically come from the TCB associated with the connection.
+
+ This command returns a data block containing the following
+ information:
+
+ local socket,
+
+
+ [Page 49]
+
+
+ September 1981
+Transmission Control Protocol
+Functional Specification
+
+
+
+ foreign socket,
+ local connection name,
+ receive window,
+ send window,
+ connection state,
+ number of buffers awaiting acknowledgment,
+ number of buffers pending receipt,
+ urgent state,
+ precedence,
+ security/compartment,
+ and transmission timeout.
+
+ Depending on the state of the connection, or on the
+ implementation itself, some of this information may not be
+ available or meaningful. If the calling process is not
+ authorized to use this connection, an error is returned. This
+ prevents unauthorized processes from gaining information about a
+ connection.
+
+ Abort
+
+ Format: ABORT (local connection name)
+
+ This command causes all pending SENDs and RECEIVES to be
+ aborted, the TCB to be removed, and a special RESET message to
+ be sent to the TCP on the other side of the connection.
+ Depending on the implementation, users may receive abort
+ indications for each outstanding SEND or RECEIVE, or may simply
+ receive an ABORT-acknowledgment.
+
+ TCP-to-User Messages
+
+ It is assumed that the operating system environment provides a
+ means for the TCP to asynchronously signal the user program. When
+ the TCP does signal a user program, certain information is passed
+ to the user. Often in the specification the information will be
+ an error message. In other cases there will be information
+ relating to the completion of processing a SEND or RECEIVE or
+ other user call.
+
+ The following information is provided:
+
+ Local Connection Name Always
+ Response String Always
+ Buffer Address Send & Receive
+ Byte count (counts bytes received) Receive
+ Push flag Receive
+ Urgent flag Receive
+
+
+[Page 50]
+
+
+September 1981
+ Transmission Control Protocol
+ Functional Specification
+
+
+
+ TCP/Lower-Level Interface
+
+ The TCP calls on a lower level protocol module to actually send and
+ receive information over a network. One case is that of the ARPA
+ internetwork system where the lower level module is the Internet
+ Protocol (IP) [2].
+
+ If the lower level protocol is IP it provides arguments for a type
+ of service and for a time to live. TCP uses the following settings
+ for these parameters:
+
+ Type of Service = Precedence: routine, Delay: normal, Throughput:
+ normal, Reliability: normal; or 00000000.
+
+ Time to Live = one minute, or 00111100.
+
+ Note that the assumed maximum segment lifetime is two minutes.
+ Here we explicitly ask that a segment be destroyed if it cannot
+ be delivered by the internet system within one minute.
+
+ If the lower level is IP (or other protocol that provides this
+ feature) and source routing is used, the interface must allow the
+ route information to be communicated. This is especially important
+ so that the source and destination addresses used in the TCP
+ checksum be the originating source and ultimate destination. It is
+ also important to preserve the return route to answer connection
+ requests.
+
+ Any lower level protocol will have to provide the source address,
+ destination address, and protocol fields, and some way to determine
+ the "TCP length", both to provide the functional equivlent service
+ of IP and to be used in the TCP checksum.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+ [Page 51]
+
+
+ September 1981
+Transmission Control Protocol
+Functional Specification
+
+
+
+3.9. Event Processing
+
+ The processing depicted in this section is an example of one possible
+ implementation. Other implementations may have slightly different
+ processing sequences, but they should differ from those in this
+ section only in detail, not in substance.
+
+ The activity of the TCP can be characterized as responding to events.
+ The events that occur can be cast into three categories: user calls,
+ arriving segments, and timeouts. This section describes the
+ processing the TCP does in response to each of the events. In many
+ cases the processing required depends on the state of the connection.
+
+ Events that occur:
+
+ User Calls
+
+ OPEN
+ SEND
+ RECEIVE
+ CLOSE
+ ABORT
+ STATUS
+
+ Arriving Segments
+
+ SEGMENT ARRIVES
+
+ Timeouts
+
+ USER TIMEOUT
+ RETRANSMISSION TIMEOUT
+ TIME-WAIT TIMEOUT
+
+ The model of the TCP/user interface is that user commands receive an
+ immediate return and possibly a delayed response via an event or
+ pseudo interrupt. In the following descriptions, the term "signal"
+ means cause a delayed response.
+
+ Error responses are given as character strings. For example, user
+ commands referencing connections that do not exist receive "error:
+ connection not open".
+
+ Please note in the following that all arithmetic on sequence numbers,
+ acknowledgment numbers, windows, et cetera, is modulo 2**32 the size
+ of the sequence number space. Also note that "=<" means less than or
+ equal to (modulo 2**32).
+
+
+
+[Page 52]
+
+
+September 1981
+ Transmission Control Protocol
+ Functional Specification
+
+
+
+ A natural way to think about processing incoming segments is to
+ imagine that they are first tested for proper sequence number (i.e.,
+ that their contents lie in the range of the expected "receive window"
+ in the sequence number space) and then that they are generally queued
+ and processed in sequence number order.
+
+ When a segment overlaps other already received segments we reconstruct
+ the segment to contain just the new data, and adjust the header fields
+ to be consistent.
+
+ Note that if no state change is mentioned the TCP stays in the same
+ state.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+ [Page 53]
+
+
+ September 1981
+Transmission Control Protocol
+Functional Specification
+ OPEN Call
+
+
+
+ OPEN Call
+
+ CLOSED STATE (i.e., TCB does not exist)
+
+ Create a new transmission control block (TCB) to hold connection
+ state information. Fill in local socket identifier, foreign
+ socket, precedence, security/compartment, and user timeout
+ information. Note that some parts of the foreign socket may be
+ unspecified in a passive OPEN and are to be filled in by the
+ parameters of the incoming SYN segment. Verify the security and
+ precedence requested are allowed for this user, if not return
+ "error: precedence not allowed" or "error: security/compartment
+ not allowed." If passive enter the LISTEN state and return. If
+ active and the foreign socket is unspecified, return "error:
+ foreign socket unspecified"; if active and the foreign socket is
+ specified, issue a SYN segment. An initial send sequence number
+ (ISS) is selected. A SYN segment of the form <SEQ=ISS><CTL=SYN>
+ is sent. Set SND.UNA to ISS, SND.NXT to ISS+1, enter SYN-SENT
+ state, and return.
+
+ If the caller does not have access to the local socket specified,
+ return "error: connection illegal for this process". If there is
+ no room to create a new connection, return "error: insufficient
+ resources".
+
+ LISTEN STATE
+
+ If active and the foreign socket is specified, then change the
+ connection from passive to active, select an ISS. Send a SYN
+ segment, set SND.UNA to ISS, SND.NXT to ISS+1. Enter SYN-SENT
+ state. Data associated with SEND may be sent with SYN segment or
+ queued for transmission after entering ESTABLISHED state. The
+ urgent bit if requested in the command must be sent with the data
+ segments sent as a result of this command. If there is no room to
+ queue the request, respond with "error: insufficient resources".
+ If Foreign socket was not specified, then return "error: foreign
+ socket unspecified".
+
+
+
+
+
+
+
+
+
+
+
+
+[Page 54]
+
+
+September 1981
+ Transmission Control Protocol
+ Functional Specification
+OPEN Call
+
+
+
+ SYN-SENT STATE
+ SYN-RECEIVED STATE
+ ESTABLISHED STATE
+ FIN-WAIT-1 STATE
+ FIN-WAIT-2 STATE
+ CLOSE-WAIT STATE
+ CLOSING STATE
+ LAST-ACK STATE
+ TIME-WAIT STATE
+
+ Return "error: connection already exists".
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+ [Page 55]
+
+
+ September 1981
+Transmission Control Protocol
+Functional Specification
+ SEND Call
+
+
+
+ SEND Call
+
+ CLOSED STATE (i.e., TCB does not exist)
+
+ If the user does not have access to such a connection, then return
+ "error: connection illegal for this process".
+
+ Otherwise, return "error: connection does not exist".
+
+ LISTEN STATE
+
+ If the foreign socket is specified, then change the connection
+ from passive to active, select an ISS. Send a SYN segment, set
+ SND.UNA to ISS, SND.NXT to ISS+1. Enter SYN-SENT state. Data
+ associated with SEND may be sent with SYN segment or queued for
+ transmission after entering ESTABLISHED state. The urgent bit if
+ requested in the command must be sent with the data segments sent
+ as a result of this command. If there is no room to queue the
+ request, respond with "error: insufficient resources". If
+ Foreign socket was not specified, then return "error: foreign
+ socket unspecified".
+
+ SYN-SENT STATE
+ SYN-RECEIVED STATE
+
+ Queue the data for transmission after entering ESTABLISHED state.
+ If no space to queue, respond with "error: insufficient
+ resources".
+
+ ESTABLISHED STATE
+ CLOSE-WAIT STATE
+
+ Segmentize the buffer and send it with a piggybacked
+ acknowledgment (acknowledgment value = RCV.NXT). If there is
+ insufficient space to remember this buffer, simply return "error:
+ insufficient resources".
+
+ If the urgent flag is set, then SND.UP <- SND.NXT-1 and set the
+ urgent pointer in the outgoing segments.
+
+
+
+
+
+
+
+
+
+
+[Page 56]
+
+
+September 1981
+ Transmission Control Protocol
+ Functional Specification
+SEND Call
+
+
+
+ FIN-WAIT-1 STATE
+ FIN-WAIT-2 STATE
+ CLOSING STATE
+ LAST-ACK STATE
+ TIME-WAIT STATE
+
+ Return "error: connection closing" and do not service request.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+ [Page 57]
+
+
+ September 1981
+Transmission Control Protocol
+Functional Specification
+ RECEIVE Call
+
+
+
+ RECEIVE Call
+
+ CLOSED STATE (i.e., TCB does not exist)
+
+ If the user does not have access to such a connection, return
+ "error: connection illegal for this process".
+
+ Otherwise return "error: connection does not exist".
+
+ LISTEN STATE
+ SYN-SENT STATE
+ SYN-RECEIVED STATE
+
+ Queue for processing after entering ESTABLISHED state. If there
+ is no room to queue this request, respond with "error:
+ insufficient resources".
+
+ ESTABLISHED STATE
+ FIN-WAIT-1 STATE
+ FIN-WAIT-2 STATE
+
+ If insufficient incoming segments are queued to satisfy the
+ request, queue the request. If there is no queue space to
+ remember the RECEIVE, respond with "error: insufficient
+ resources".
+
+ Reassemble queued incoming segments into receive buffer and return
+ to user. Mark "push seen" (PUSH) if this is the case.
+
+ If RCV.UP is in advance of the data currently being passed to the
+ user notify the user of the presence of urgent data.
+
+ When the TCP takes responsibility for delivering data to the user
+ that fact must be communicated to the sender via an
+ acknowledgment. The formation of such an acknowledgment is
+ described below in the discussion of processing an incoming
+ segment.
+
+
+
+
+
+
+
+
+
+
+
+
+[Page 58]
+
+
+September 1981
+ Transmission Control Protocol
+ Functional Specification
+RECEIVE Call
+
+
+
+ CLOSE-WAIT STATE
+
+ Since the remote side has already sent FIN, RECEIVEs must be
+ satisfied by text already on hand, but not yet delivered to the
+ user. If no text is awaiting delivery, the RECEIVE will get a
+ "error: connection closing" response. Otherwise, any remaining
+ text can be used to satisfy the RECEIVE.
+
+ CLOSING STATE
+ LAST-ACK STATE
+ TIME-WAIT STATE
+
+ Return "error: connection closing".
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+ [Page 59]
+
+
+ September 1981
+Transmission Control Protocol
+Functional Specification
+ CLOSE Call
+
+
+
+ CLOSE Call
+
+ CLOSED STATE (i.e., TCB does not exist)
+
+ If the user does not have access to such a connection, return
+ "error: connection illegal for this process".
+
+ Otherwise, return "error: connection does not exist".
+
+ LISTEN STATE
+
+ Any outstanding RECEIVEs are returned with "error: closing"
+ responses. Delete TCB, enter CLOSED state, and return.
+
+ SYN-SENT STATE
+
+ Delete the TCB and return "error: closing" responses to any
+ queued SENDs, or RECEIVEs.
+
+ SYN-RECEIVED STATE
+
+ If no SENDs have been issued and there is no pending data to send,
+ then form a FIN segment and send it, and enter FIN-WAIT-1 state;
+ otherwise queue for processing after entering ESTABLISHED state.
+
+ ESTABLISHED STATE
+
+ Queue this until all preceding SENDs have been segmentized, then
+ form a FIN segment and send it. In any case, enter FIN-WAIT-1
+ state.
+
+ FIN-WAIT-1 STATE
+ FIN-WAIT-2 STATE
+
+ Strictly speaking, this is an error and should receive a "error:
+ connection closing" response. An "ok" response would be
+ acceptable, too, as long as a second FIN is not emitted (the first
+ FIN may be retransmitted though).
+
+
+
+
+
+
+
+
+
+
+
+[Page 60]
+
+
+September 1981
+ Transmission Control Protocol
+ Functional Specification
+CLOSE Call
+
+
+
+ CLOSE-WAIT STATE
+
+ Queue this request until all preceding SENDs have been
+ segmentized; then send a FIN segment, enter CLOSING state.
+
+ CLOSING STATE
+ LAST-ACK STATE
+ TIME-WAIT STATE
+
+ Respond with "error: connection closing".
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+ [Page 61]
+
+
+ September 1981
+Transmission Control Protocol
+Functional Specification
+ ABORT Call
+
+
+
+ ABORT Call
+
+ CLOSED STATE (i.e., TCB does not exist)
+
+ If the user should not have access to such a connection, return
+ "error: connection illegal for this process".
+
+ Otherwise return "error: connection does not exist".
+
+ LISTEN STATE
+
+ Any outstanding RECEIVEs should be returned with "error:
+ connection reset" responses. Delete TCB, enter CLOSED state, and
+ return.
+
+ SYN-SENT STATE
+
+ All queued SENDs and RECEIVEs should be given "connection reset"
+ notification, delete the TCB, enter CLOSED state, and return.
+
+ SYN-RECEIVED STATE
+ ESTABLISHED STATE
+ FIN-WAIT-1 STATE
+ FIN-WAIT-2 STATE
+ CLOSE-WAIT STATE
+
+ Send a reset segment:
+
+ <SEQ=SND.NXT><CTL=RST>
+
+ All queued SENDs and RECEIVEs should be given "connection reset"
+ notification; all segments queued for transmission (except for the
+ RST formed above) or retransmission should be flushed, delete the
+ TCB, enter CLOSED state, and return.
+
+ CLOSING STATE
+ LAST-ACK STATE
+ TIME-WAIT STATE
+
+ Respond with "ok" and delete the TCB, enter CLOSED state, and
+ return.
+
+
+
+
+
+
+
+
+[Page 62]
+
+
+September 1981
+ Transmission Control Protocol
+ Functional Specification
+STATUS Call
+
+
+
+ STATUS Call
+
+ CLOSED STATE (i.e., TCB does not exist)
+
+ If the user should not have access to such a connection, return
+ "error: connection illegal for this process".
+
+ Otherwise return "error: connection does not exist".
+
+ LISTEN STATE
+
+ Return "state = LISTEN", and the TCB pointer.
+
+ SYN-SENT STATE
+
+ Return "state = SYN-SENT", and the TCB pointer.
+
+ SYN-RECEIVED STATE
+
+ Return "state = SYN-RECEIVED", and the TCB pointer.
+
+ ESTABLISHED STATE
+
+ Return "state = ESTABLISHED", and the TCB pointer.
+
+ FIN-WAIT-1 STATE
+
+ Return "state = FIN-WAIT-1", and the TCB pointer.
+
+ FIN-WAIT-2 STATE
+
+ Return "state = FIN-WAIT-2", and the TCB pointer.
+
+ CLOSE-WAIT STATE
+
+ Return "state = CLOSE-WAIT", and the TCB pointer.
+
+ CLOSING STATE
+
+ Return "state = CLOSING", and the TCB pointer.
+
+ LAST-ACK STATE
+
+ Return "state = LAST-ACK", and the TCB pointer.
+
+
+
+
+
+ [Page 63]
+
+
+ September 1981
+Transmission Control Protocol
+Functional Specification
+ STATUS Call
+
+
+
+ TIME-WAIT STATE
+
+ Return "state = TIME-WAIT", and the TCB pointer.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+[Page 64]
+
+
+September 1981
+ Transmission Control Protocol
+ Functional Specification
+SEGMENT ARRIVES
+
+
+
+ SEGMENT ARRIVES
+
+ If the state is CLOSED (i.e., TCB does not exist) then
+
+ all data in the incoming segment is discarded. An incoming
+ segment containing a RST is discarded. An incoming segment not
+ containing a RST causes a RST to be sent in response. The
+ acknowledgment and sequence field values are selected to make the
+ reset sequence acceptable to the TCP that sent the offending
+ segment.
+
+ If the ACK bit is off, sequence number zero is used,
+
+ <SEQ=0><ACK=SEG.SEQ+SEG.LEN><CTL=RST,ACK>
+
+ If the ACK bit is on,
+
+ <SEQ=SEG.ACK><CTL=RST>
+
+ Return.
+
+ If the state is LISTEN then
+
+ first check for an RST
+
+ An incoming RST should be ignored. Return.
+
+ second check for an ACK
+
+ Any acknowledgment is bad if it arrives on a connection still in
+ the LISTEN state. An acceptable reset segment should be formed
+ for any arriving ACK-bearing segment. The RST should be
+ formatted as follows:
+
+ <SEQ=SEG.ACK><CTL=RST>
+
+ Return.
+
+ third check for a SYN
+
+ If the SYN bit is set, check the security. If the
+ security/compartment on the incoming segment does not exactly
+ match the security/compartment in the TCB then send a reset and
+ return.
+
+ <SEQ=SEG.ACK><CTL=RST>
+
+
+
+ [Page 65]
+
+
+ September 1981
+Transmission Control Protocol
+Functional Specification
+ SEGMENT ARRIVES
+
+
+
+ If the SEG.PRC is greater than the TCB.PRC then if allowed by
+ the user and the system set TCB.PRC<-SEG.PRC, if not allowed
+ send a reset and return.
+
+ <SEQ=SEG.ACK><CTL=RST>
+
+ If the SEG.PRC is less than the TCB.PRC then continue.
+
+ Set RCV.NXT to SEG.SEQ+1, IRS is set to SEG.SEQ and any other
+ control or text should be queued for processing later. ISS
+ should be selected and a SYN segment sent of the form:
+
+ <SEQ=ISS><ACK=RCV.NXT><CTL=SYN,ACK>
+
+ SND.NXT is set to ISS+1 and SND.UNA to ISS. The connection
+ state should be changed to SYN-RECEIVED. Note that any other
+ incoming control or data (combined with SYN) will be processed
+ in the SYN-RECEIVED state, but processing of SYN and ACK should
+ not be repeated. If the listen was not fully specified (i.e.,
+ the foreign socket was not fully specified), then the
+ unspecified fields should be filled in now.
+
+ fourth other text or control
+
+ Any other control or text-bearing segment (not containing SYN)
+ must have an ACK and thus would be discarded by the ACK
+ processing. An incoming RST segment could not be valid, since
+ it could not have been sent in response to anything sent by this
+ incarnation of the connection. So you are unlikely to get here,
+ but if you do, drop the segment, and return.
+
+ If the state is SYN-SENT then
+
+ first check the ACK bit
+
+ If the ACK bit is set
+
+ If SEG.ACK =< ISS, or SEG.ACK > SND.NXT, send a reset (unless
+ the RST bit is set, if so drop the segment and return)
+
+ <SEQ=SEG.ACK><CTL=RST>
+
+ and discard the segment. Return.
+
+ If SND.UNA =< SEG.ACK =< SND.NXT then the ACK is acceptable.
+
+ second check the RST bit
+
+
+[Page 66]
+
+
+September 1981
+ Transmission Control Protocol
+ Functional Specification
+SEGMENT ARRIVES
+
+
+
+ If the RST bit is set
+
+ If the ACK was acceptable then signal the user "error:
+ connection reset", drop the segment, enter CLOSED state,
+ delete TCB, and return. Otherwise (no ACK) drop the segment
+ and return.
+
+ third check the security and precedence
+
+ If the security/compartment in the segment does not exactly
+ match the security/compartment in the TCB, send a reset
+
+ If there is an ACK
+
+ <SEQ=SEG.ACK><CTL=RST>
+
+ Otherwise
+
+ <SEQ=0><ACK=SEG.SEQ+SEG.LEN><CTL=RST,ACK>
+
+ If there is an ACK
+
+ The precedence in the segment must match the precedence in the
+ TCB, if not, send a reset
+
+ <SEQ=SEG.ACK><CTL=RST>
+
+ If there is no ACK
+
+ If the precedence in the segment is higher than the precedence
+ in the TCB then if allowed by the user and the system raise
+ the precedence in the TCB to that in the segment, if not
+ allowed to raise the prec then send a reset.
+
+ <SEQ=0><ACK=SEG.SEQ+SEG.LEN><CTL=RST,ACK>
+
+ If the precedence in the segment is lower than the precedence
+ in the TCB continue.
+
+ If a reset was sent, discard the segment and return.
+
+ fourth check the SYN bit
+
+ This step should be reached only if the ACK is ok, or there is
+ no ACK, and it the segment did not contain a RST.
+
+ If the SYN bit is on and the security/compartment and precedence
+
+
+ [Page 67]
+
+
+ September 1981
+Transmission Control Protocol
+Functional Specification
+ SEGMENT ARRIVES
+
+
+
+ are acceptable then, RCV.NXT is set to SEG.SEQ+1, IRS is set to
+ SEG.SEQ. SND.UNA should be advanced to equal SEG.ACK (if there
+ is an ACK), and any segments on the retransmission queue which
+ are thereby acknowledged should be removed.
+
+ If SND.UNA > ISS (our SYN has been ACKed), change the connection
+ state to ESTABLISHED, form an ACK segment
+
+ <SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK>
+
+ and send it. Data or controls which were queued for
+ transmission may be included. If there are other controls or
+ text in the segment then continue processing at the sixth step
+ below where the URG bit is checked, otherwise return.
+
+ Otherwise enter SYN-RECEIVED, form a SYN,ACK segment
+
+ <SEQ=ISS><ACK=RCV.NXT><CTL=SYN,ACK>
+
+ and send it. If there are other controls or text in the
+ segment, queue them for processing after the ESTABLISHED state
+ has been reached, return.
+
+ fifth, if neither of the SYN or RST bits is set then drop the
+ segment and return.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+[Page 68]
+
+
+September 1981
+ Transmission Control Protocol
+ Functional Specification
+SEGMENT ARRIVES
+
+
+
+ Otherwise,
+
+ first check sequence number
+
+ SYN-RECEIVED STATE
+ ESTABLISHED STATE
+ FIN-WAIT-1 STATE
+ FIN-WAIT-2 STATE
+ CLOSE-WAIT STATE
+ CLOSING STATE
+ LAST-ACK STATE
+ TIME-WAIT STATE
+
+ Segments are processed in sequence. Initial tests on arrival
+ are used to discard old duplicates, but further processing is
+ done in SEG.SEQ order. If a segment's contents straddle the
+ boundary between old and new, only the new parts should be
+ processed.
+
+ There are four cases for the acceptability test for an incoming
+ segment:
+
+ Segment Receive Test
+ Length Window
+ ------- ------- -------------------------------------------
+
+ 0 0 SEG.SEQ = RCV.NXT
+
+ 0 >0 RCV.NXT =< SEG.SEQ < RCV.NXT+RCV.WND
+
+ >0 0 not acceptable
+
+ >0 >0 RCV.NXT =< SEG.SEQ < RCV.NXT+RCV.WND
+ or RCV.NXT =< SEG.SEQ+SEG.LEN-1 < RCV.NXT+RCV.WND
+
+ If the RCV.WND is zero, no segments will be acceptable, but
+ special allowance should be made to accept valid ACKs, URGs and
+ RSTs.
+
+ If an incoming segment is not acceptable, an acknowledgment
+ should be sent in reply (unless the RST bit is set, if so drop
+ the segment and return):
+
+ <SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK>
+
+ After sending the acknowledgment, drop the unacceptable segment
+ and return.
+
+
+ [Page 69]
+
+
+ September 1981
+Transmission Control Protocol
+Functional Specification
+ SEGMENT ARRIVES
+
+
+
+ In the following it is assumed that the segment is the idealized
+ segment that begins at RCV.NXT and does not exceed the window.
+ One could tailor actual segments to fit this assumption by
+ trimming off any portions that lie outside the window (including
+ SYN and FIN), and only processing further if the segment then
+ begins at RCV.NXT. Segments with higher begining sequence
+ numbers may be held for later processing.
+
+ second check the RST bit,
+
+ SYN-RECEIVED STATE
+
+ If the RST bit is set
+
+ If this connection was initiated with a passive OPEN (i.e.,
+ came from the LISTEN state), then return this connection to
+ LISTEN state and return. The user need not be informed. If
+ this connection was initiated with an active OPEN (i.e., came
+ from SYN-SENT state) then the connection was refused, signal
+ the user "connection refused". In either case, all segments
+ on the retransmission queue should be removed. And in the
+ active OPEN case, enter the CLOSED state and delete the TCB,
+ and return.
+
+ ESTABLISHED
+ FIN-WAIT-1
+ FIN-WAIT-2
+ CLOSE-WAIT
+
+ If the RST bit is set then, any outstanding RECEIVEs and SEND
+ should receive "reset" responses. All segment queues should be
+ flushed. Users should also receive an unsolicited general
+ "connection reset" signal. Enter the CLOSED state, delete the
+ TCB, and return.
+
+ CLOSING STATE
+ LAST-ACK STATE
+ TIME-WAIT
+
+ If the RST bit is set then, enter the CLOSED state, delete the
+ TCB, and return.
+
+
+
+
+
+
+
+
+[Page 70]
+
+
+September 1981
+ Transmission Control Protocol
+ Functional Specification
+SEGMENT ARRIVES
+
+
+
+ third check security and precedence
+
+ SYN-RECEIVED
+
+ If the security/compartment and precedence in the segment do not
+ exactly match the security/compartment and precedence in the TCB
+ then send a reset, and return.
+
+ ESTABLISHED STATE
+
+ If the security/compartment and precedence in the segment do not
+ exactly match the security/compartment and precedence in the TCB
+ then send a reset, any outstanding RECEIVEs and SEND should
+ receive "reset" responses. All segment queues should be
+ flushed. Users should also receive an unsolicited general
+ "connection reset" signal. Enter the CLOSED state, delete the
+ TCB, and return.
+
+ Note this check is placed following the sequence check to prevent
+ a segment from an old connection between these ports with a
+ different security or precedence from causing an abort of the
+ current connection.
+
+ fourth, check the SYN bit,
+
+ SYN-RECEIVED
+ ESTABLISHED STATE
+ FIN-WAIT STATE-1
+ FIN-WAIT STATE-2
+ CLOSE-WAIT STATE
+ CLOSING STATE
+ LAST-ACK STATE
+ TIME-WAIT STATE
+
+ If the SYN is in the window it is an error, send a reset, any
+ outstanding RECEIVEs and SEND should receive "reset" responses,
+ all segment queues should be flushed, the user should also
+ receive an unsolicited general "connection reset" signal, enter
+ the CLOSED state, delete the TCB, and return.
+
+ If the SYN is not in the window this step would not be reached
+ and an ack would have been sent in the first step (sequence
+ number check).
+
+
+
+
+
+
+ [Page 71]
+
+
+ September 1981
+Transmission Control Protocol
+Functional Specification
+ SEGMENT ARRIVES
+
+
+
+ fifth check the ACK field,
+
+ if the ACK bit is off drop the segment and return
+
+ if the ACK bit is on
+
+ SYN-RECEIVED STATE
+
+ If SND.UNA =< SEG.ACK =< SND.NXT then enter ESTABLISHED state
+ and continue processing.
+
+ If the segment acknowledgment is not acceptable, form a
+ reset segment,
+
+ <SEQ=SEG.ACK><CTL=RST>
+
+ and send it.
+
+ ESTABLISHED STATE
+
+ If SND.UNA < SEG.ACK =< SND.NXT then, set SND.UNA <- SEG.ACK.
+ Any segments on the retransmission queue which are thereby
+ entirely acknowledged are removed. Users should receive
+ positive acknowledgments for buffers which have been SENT and
+ fully acknowledged (i.e., SEND buffer should be returned with
+ "ok" response). If the ACK is a duplicate
+ (SEG.ACK < SND.UNA), it can be ignored. If the ACK acks
+ something not yet sent (SEG.ACK > SND.NXT) then send an ACK,
+ drop the segment, and return.
+
+ If SND.UNA < SEG.ACK =< SND.NXT, the send window should be
+ updated. If (SND.WL1 < SEG.SEQ or (SND.WL1 = SEG.SEQ and
+ SND.WL2 =< SEG.ACK)), set SND.WND <- SEG.WND, set
+ SND.WL1 <- SEG.SEQ, and set SND.WL2 <- SEG.ACK.
+
+ Note that SND.WND is an offset from SND.UNA, that SND.WL1
+ records the sequence number of the last segment used to update
+ SND.WND, and that SND.WL2 records the acknowledgment number of
+ the last segment used to update SND.WND. The check here
+ prevents using old segments to update the window.
+
+
+
+
+
+
+
+
+
+[Page 72]
+
+
+September 1981
+ Transmission Control Protocol
+ Functional Specification
+SEGMENT ARRIVES
+
+
+
+ FIN-WAIT-1 STATE
+
+ In addition to the processing for the ESTABLISHED state, if
+ our FIN is now acknowledged then enter FIN-WAIT-2 and continue
+ processing in that state.
+
+ FIN-WAIT-2 STATE
+
+ In addition to the processing for the ESTABLISHED state, if
+ the retransmission queue is empty, the user's CLOSE can be
+ acknowledged ("ok") but do not delete the TCB.
+
+ CLOSE-WAIT STATE
+
+ Do the same processing as for the ESTABLISHED state.
+
+ CLOSING STATE
+
+ In addition to the processing for the ESTABLISHED state, if
+ the ACK acknowledges our FIN then enter the TIME-WAIT state,
+ otherwise ignore the segment.
+
+ LAST-ACK STATE
+
+ The only thing that can arrive in this state is an
+ acknowledgment of our FIN. If our FIN is now acknowledged,
+ delete the TCB, enter the CLOSED state, and return.
+
+ TIME-WAIT STATE
+
+ The only thing that can arrive in this state is a
+ retransmission of the remote FIN. Acknowledge it, and restart
+ the 2 MSL timeout.
+
+ sixth, check the URG bit,
+
+ ESTABLISHED STATE
+ FIN-WAIT-1 STATE
+ FIN-WAIT-2 STATE
+
+ If the URG bit is set, RCV.UP <- max(RCV.UP,SEG.UP), and signal
+ the user that the remote side has urgent data if the urgent
+ pointer (RCV.UP) is in advance of the data consumed. If the
+ user has already been signaled (or is still in the "urgent
+ mode") for this continuous sequence of urgent data, do not
+ signal the user again.
+
+
+
+ [Page 73]
+
+
+ September 1981
+Transmission Control Protocol
+Functional Specification
+ SEGMENT ARRIVES
+
+
+
+ CLOSE-WAIT STATE
+ CLOSING STATE
+ LAST-ACK STATE
+ TIME-WAIT
+
+ This should not occur, since a FIN has been received from the
+ remote side. Ignore the URG.
+
+ seventh, process the segment text,
+
+ ESTABLISHED STATE
+ FIN-WAIT-1 STATE
+ FIN-WAIT-2 STATE
+
+ Once in the ESTABLISHED state, it is possible to deliver segment
+ text to user RECEIVE buffers. Text from segments can be moved
+ into buffers until either the buffer is full or the segment is
+ empty. If the segment empties and carries an PUSH flag, then
+ the user is informed, when the buffer is returned, that a PUSH
+ has been received.
+
+ When the TCP takes responsibility for delivering the data to the
+ user it must also acknowledge the receipt of the data.
+
+ Once the TCP takes responsibility for the data it advances
+ RCV.NXT over the data accepted, and adjusts RCV.WND as
+ apporopriate to the current buffer availability. The total of
+ RCV.NXT and RCV.WND should not be reduced.
+
+ Please note the window management suggestions in section 3.7.
+
+ Send an acknowledgment of the form:
+
+ <SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK>
+
+ This acknowledgment should be piggybacked on a segment being
+ transmitted if possible without incurring undue delay.
+
+
+
+
+
+
+
+
+
+
+
+
+[Page 74]
+
+
+September 1981
+ Transmission Control Protocol
+ Functional Specification
+SEGMENT ARRIVES
+
+
+
+ CLOSE-WAIT STATE
+ CLOSING STATE
+ LAST-ACK STATE
+ TIME-WAIT STATE
+
+ This should not occur, since a FIN has been received from the
+ remote side. Ignore the segment text.
+
+ eighth, check the FIN bit,
+
+ Do not process the FIN if the state is CLOSED, LISTEN or SYN-SENT
+ since the SEG.SEQ cannot be validated; drop the segment and
+ return.
+
+ If the FIN bit is set, signal the user "connection closing" and
+ return any pending RECEIVEs with same message, advance RCV.NXT
+ over the FIN, and send an acknowledgment for the FIN. Note that
+ FIN implies PUSH for any segment text not yet delivered to the
+ user.
+
+ SYN-RECEIVED STATE
+ ESTABLISHED STATE
+
+ Enter the CLOSE-WAIT state.
+
+ FIN-WAIT-1 STATE
+
+ If our FIN has been ACKed (perhaps in this segment), then
+ enter TIME-WAIT, start the time-wait timer, turn off the other
+ timers; otherwise enter the CLOSING state.
+
+ FIN-WAIT-2 STATE
+
+ Enter the TIME-WAIT state. Start the time-wait timer, turn
+ off the other timers.
+
+ CLOSE-WAIT STATE
+
+ Remain in the CLOSE-WAIT state.
+
+ CLOSING STATE
+
+ Remain in the CLOSING state.
+
+ LAST-ACK STATE
+
+ Remain in the LAST-ACK state.
+
+
+ [Page 75]
+
+
+ September 1981
+Transmission Control Protocol
+Functional Specification
+ SEGMENT ARRIVES
+
+
+
+ TIME-WAIT STATE
+
+ Remain in the TIME-WAIT state. Restart the 2 MSL time-wait
+ timeout.
+
+ and return.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+[Page 76]
+
+
+September 1981
+ Transmission Control Protocol
+ Functional Specification
+USER TIMEOUT
+
+
+
+ USER TIMEOUT
+
+ For any state if the user timeout expires, flush all queues, signal
+ the user "error: connection aborted due to user timeout" in general
+ and for any outstanding calls, delete the TCB, enter the CLOSED
+ state and return.
+
+ RETRANSMISSION TIMEOUT
+
+ For any state if the retransmission timeout expires on a segment in
+ the retransmission queue, send the segment at the front of the
+ retransmission queue again, reinitialize the retransmission timer,
+ and return.
+
+ TIME-WAIT TIMEOUT
+
+ If the time-wait timeout expires on a connection delete the TCB,
+ enter the CLOSED state and return.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+ [Page 77]
+
+
+ September 1981
+Transmission Control Protocol
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+[Page 78]
+
+
+September 1981
+ Transmission Control Protocol
+
+
+
+ GLOSSARY
+
+
+
+1822
+ BBN Report 1822, "The Specification of the Interconnection of
+ a Host and an IMP". The specification of interface between a
+ host and the ARPANET.
+
+ACK
+ A control bit (acknowledge) occupying no sequence space, which
+ indicates that the acknowledgment field of this segment
+ specifies the next sequence number the sender of this segment
+ is expecting to receive, hence acknowledging receipt of all
+ previous sequence numbers.
+
+ARPANET message
+ The unit of transmission between a host and an IMP in the
+ ARPANET. The maximum size is about 1012 octets (8096 bits).
+
+ARPANET packet
+ A unit of transmission used internally in the ARPANET between
+ IMPs. The maximum size is about 126 octets (1008 bits).
+
+connection
+ A logical communication path identified by a pair of sockets.
+
+datagram
+ A message sent in a packet switched computer communications
+ network.
+
+Destination Address
+ The destination address, usually the network and host
+ identifiers.
+
+FIN
+ A control bit (finis) occupying one sequence number, which
+ indicates that the sender will send no more data or control
+ occupying sequence space.
+
+fragment
+ A portion of a logical unit of data, in particular an internet
+ fragment is a portion of an internet datagram.
+
+FTP
+ A file transfer protocol.
+
+
+
+
+
+ [Page 79]
+
+
+ September 1981
+Transmission Control Protocol
+Glossary
+
+
+
+header
+ Control information at the beginning of a message, segment,
+ fragment, packet or block of data.
+
+host
+ A computer. In particular a source or destination of messages
+ from the point of view of the communication network.
+
+Identification
+ An Internet Protocol field. This identifying value assigned
+ by the sender aids in assembling the fragments of a datagram.
+
+IMP
+ The Interface Message Processor, the packet switch of the
+ ARPANET.
+
+internet address
+ A source or destination address specific to the host level.
+
+internet datagram
+ The unit of data exchanged between an internet module and the
+ higher level protocol together with the internet header.
+
+internet fragment
+ A portion of the data of an internet datagram with an internet
+ header.
+
+IP
+ Internet Protocol.
+
+IRS
+ The Initial Receive Sequence number. The first sequence
+ number used by the sender on a connection.
+
+ISN
+ The Initial Sequence Number. The first sequence number used
+ on a connection, (either ISS or IRS). Selected on a clock
+ based procedure.
+
+ISS
+ The Initial Send Sequence number. The first sequence number
+ used by the sender on a connection.
+
+leader
+ Control information at the beginning of a message or block of
+ data. In particular, in the ARPANET, the control information
+ on an ARPANET message at the host-IMP interface.
+
+
+
+[Page 80]
+
+
+September 1981
+ Transmission Control Protocol
+ Glossary
+
+
+
+left sequence
+ This is the next sequence number to be acknowledged by the
+ data receiving TCP (or the lowest currently unacknowledged
+ sequence number) and is sometimes referred to as the left edge
+ of the send window.
+
+local packet
+ The unit of transmission within a local network.
+
+module
+ An implementation, usually in software, of a protocol or other
+ procedure.
+
+MSL
+ Maximum Segment Lifetime, the time a TCP segment can exist in
+ the internetwork system. Arbitrarily defined to be 2 minutes.
+
+octet
+ An eight bit byte.
+
+Options
+ An Option field may contain several options, and each option
+ may be several octets in length. The options are used
+ primarily in testing situations; for example, to carry
+ timestamps. Both the Internet Protocol and TCP provide for
+ options fields.
+
+packet
+ A package of data with a header which may or may not be
+ logically complete. More often a physical packaging than a
+ logical packaging of data.
+
+port
+ The portion of a socket that specifies which logical input or
+ output channel of a process is associated with the data.
+
+process
+ A program in execution. A source or destination of data from
+ the point of view of the TCP or other host-to-host protocol.
+
+PUSH
+ A control bit occupying no sequence space, indicating that
+ this segment contains data that must be pushed through to the
+ receiving user.
+
+RCV.NXT
+ receive next sequence number
+
+
+
+ [Page 81]
+
+
+ September 1981
+Transmission Control Protocol
+Glossary
+
+
+
+RCV.UP
+ receive urgent pointer
+
+RCV.WND
+ receive window
+
+receive next sequence number
+ This is the next sequence number the local TCP is expecting to
+ receive.
+
+receive window
+ This represents the sequence numbers the local (receiving) TCP
+ is willing to receive. Thus, the local TCP considers that
+ segments overlapping the range RCV.NXT to
+ RCV.NXT + RCV.WND - 1 carry acceptable data or control.
+ Segments containing sequence numbers entirely outside of this
+ range are considered duplicates and discarded.
+
+RST
+ A control bit (reset), occupying no sequence space, indicating
+ that the receiver should delete the connection without further
+ interaction. The receiver can determine, based on the
+ sequence number and acknowledgment fields of the incoming
+ segment, whether it should honor the reset command or ignore
+ it. In no case does receipt of a segment containing RST give
+ rise to a RST in response.
+
+RTP
+ Real Time Protocol: A host-to-host protocol for communication
+ of time critical information.
+
+SEG.ACK
+ segment acknowledgment
+
+SEG.LEN
+ segment length
+
+SEG.PRC
+ segment precedence value
+
+SEG.SEQ
+ segment sequence
+
+SEG.UP
+ segment urgent pointer field
+
+
+
+
+
+[Page 82]
+
+
+September 1981
+ Transmission Control Protocol
+ Glossary
+
+
+
+SEG.WND
+ segment window field
+
+segment
+ A logical unit of data, in particular a TCP segment is the
+ unit of data transfered between a pair of TCP modules.
+
+segment acknowledgment
+ The sequence number in the acknowledgment field of the
+ arriving segment.
+
+segment length
+ The amount of sequence number space occupied by a segment,
+ including any controls which occupy sequence space.
+
+segment sequence
+ The number in the sequence field of the arriving segment.
+
+send sequence
+ This is the next sequence number the local (sending) TCP will
+ use on the connection. It is initially selected from an
+ initial sequence number curve (ISN) and is incremented for
+ each octet of data or sequenced control transmitted.
+
+send window
+ This represents the sequence numbers which the remote
+ (receiving) TCP is willing to receive. It is the value of the
+ window field specified in segments from the remote (data
+ receiving) TCP. The range of new sequence numbers which may
+ be emitted by a TCP lies between SND.NXT and
+ SND.UNA + SND.WND - 1. (Retransmissions of sequence numbers
+ between SND.UNA and SND.NXT are expected, of course.)
+
+SND.NXT
+ send sequence
+
+SND.UNA
+ left sequence
+
+SND.UP
+ send urgent pointer
+
+SND.WL1
+ segment sequence number at last window update
+
+SND.WL2
+ segment acknowledgment number at last window update
+
+
+
+ [Page 83]
+
+
+ September 1981
+Transmission Control Protocol
+Glossary
+
+
+
+SND.WND
+ send window
+
+socket
+ An address which specifically includes a port identifier, that
+ is, the concatenation of an Internet Address with a TCP port.
+
+Source Address
+ The source address, usually the network and host identifiers.
+
+SYN
+ A control bit in the incoming segment, occupying one sequence
+ number, used at the initiation of a connection, to indicate
+ where the sequence numbering will start.
+
+TCB
+ Transmission control block, the data structure that records
+ the state of a connection.
+
+TCB.PRC
+ The precedence of the connection.
+
+TCP
+ Transmission Control Protocol: A host-to-host protocol for
+ reliable communication in internetwork environments.
+
+TOS
+ Type of Service, an Internet Protocol field.
+
+Type of Service
+ An Internet Protocol field which indicates the type of service
+ for this internet fragment.
+
+URG
+ A control bit (urgent), occupying no sequence space, used to
+ indicate that the receiving user should be notified to do
+ urgent processing as long as there is data to be consumed with
+ sequence numbers less than the value indicated in the urgent
+ pointer.
+
+urgent pointer
+ A control field meaningful only when the URG bit is on. This
+ field communicates the value of the urgent pointer which
+ indicates the data octet associated with the sending user's
+ urgent call.
+
+
+
+
+
+[Page 84]
+
+
+September 1981
+ Transmission Control Protocol
+
+
+
+ REFERENCES
+
+
+
+[1] Cerf, V., and R. Kahn, "A Protocol for Packet Network
+ Intercommunication", IEEE Transactions on Communications,
+ Vol. COM-22, No. 5, pp 637-648, May 1974.
+
+[2] Postel, J. (ed.), "Internet Protocol - DARPA Internet Program
+ Protocol Specification", RFC 791, USC/Information Sciences
+ Institute, September 1981.
+
+[3] Dalal, Y. and C. Sunshine, "Connection Management in Transport
+ Protocols", Computer Networks, Vol. 2, No. 6, pp. 454-473,
+ December 1978.
+
+[4] Postel, J., "Assigned Numbers", RFC 790, USC/Information Sciences
+ Institute, September 1981.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+ [Page 85]
+
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