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+RFC: 793
+                                    
+                                    
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+                                    
+                     TRANSMISSION CONTROL PROTOCOL
+                                    
+                                    
+                         DARPA INTERNET PROGRAM
+                                    
+                         PROTOCOL SPECIFICATION
+                                    
+                                    
+                                    
+                             September 1981
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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]                                                               
+
+
+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]                                                                
+
+
+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.
+
+
+
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+
+
+
+[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]
+
+
+                                                          September 1981
+Transmission Control Protocol
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+[Page 6]                                                                
+
+
+September 1981                                                          
+                                           Transmission Control Protocol
+
+
+
+                             2.  PHILOSOPHY
+
+2.1.  Elements of the Internetwork System
+
+  The internetwork environment consists of hosts connected to networks
+  which are in turn interconnected via gateways.  It is assumed here
+  that the networks may be either local networks (e.g., the ETHERNET) or
+  large networks (e.g., the ARPANET), but in any case are based on
+  packet switching technology.  The active agents that produce and
+  consume messages are processes.  Various levels of protocols in the
+  networks, the gateways, and the hosts support an interprocess
+  communication system that provides two-way data flow on logical
+  connections between process ports.
+
+  The term packet is used generically here to mean the data of one
+  transaction between a host and its network.  The format of data blocks
+  exchanged within the a network will generally not be of concern to us.
+
+  Hosts are computers attached to a network, and from the communication
+  network's point of view, are the sources and destinations of packets.
+  Processes are viewed as the active elements in host computers (in
+  accordance with the fairly common definition of a process as a program
+  in execution).  Even terminals and files or other I/O devices are
+  viewed as communicating with each other through the use of processes.
+  Thus, all communication is viewed as inter-process communication.
+
+  Since a process may need to distinguish among several communication
+  streams between itself and another process (or processes), we imagine
+  that each process may have a number of ports through which it
+  communicates with the ports of other processes.
+
+2.2.  Model of Operation
+
+  Processes transmit data by calling on the TCP and passing buffers of
+  data as arguments.  The TCP packages the data from these buffers into
+  segments and calls on the internet module to transmit each segment to
+  the destination TCP.  The receiving TCP places the data from a segment
+  into the receiving user's buffer and notifies the receiving user.  The
+  TCPs include control information in the segments which they use to
+  ensure reliable ordered data transmission.
+
+  The model of internet communication is that there is an internet
+  protocol module associated with each TCP which provides an interface
+  to the local network.  This internet module packages TCP segments
+  inside internet datagrams and routes these datagrams to a destination
+  internet module or intermediate gateway.  To transmit the datagram
+  through the local network, it is embedded in a local network packet.
+
+  The packet switches may perform further packaging, fragmentation, or
+
+
+                                                                [Page 7]
+
+
+                                                          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]                                                               
+
+
+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.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
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+
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+
+
+                                                               [Page 85]
+