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rfc:ien:ien124

IEN: 124

                                  
                                  
                                  
                                  
                                  
                                  
                                  
                            DOD STANDARD
                                  
                   TRANSMISSION CONTROL PROTOCOL
                                  
                                  
                                  
                           December 1979
                            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

December 1979

                                         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 ....................................... 10
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 .................................... 29
3.5  Closing a Connection ......................................... 35
3.6  Precedence and Security ...................................... 38
3.7  Data Communication ........................................... 38
3.8  Interfaces ................................................... 42
3.9  Event Processing ............................................. 52

GLOSSARY …………………………………………………… 75

REFERENCES …………………………………………………. 83

                                                              [Page i]
                                                         December 1979

Transmission Control Protocol

[Page ii]

December 1979

                                         Transmission Control Protocol
                              PREFACE

This document describes the DoD Standard Transmission Control Protocol (TCP). There have been seven 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 incorporates the addition of security, compartmentation, and precedence concepts into the TCP specification.

                                                         Jon Postel
                                                         Editor
                                                            [Page iii]

December 1979 IEN:124 Replaces: IENs 112, 81, 55, 44, 40, 27, 21, 5

                            DOD STANDARD
                   TRANSMISSION CONTROL PROTOCOL
                          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 especially 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 primarily focuses its attention 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.  At most, the 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]
                                                         December 1979

Transmission Control Protocol Introduction

The TCP fits into a layered protocol architecture just above a basic
Internet Protocol [1] 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, and the
internet protocol also deals with any fragmentation or reassembly of
the TCP segments which might be 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.  As a practical matter, many 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
section offers a very brief view of the protocol interfaces and

[Page 2]

December 1979

                                         Transmission Control Protocol
                                                          Introduction
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 letters 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, this TCP specification requires a certain minimum
functionality to be achieved at the TCP/user interface for any valid
implementation.
The interface between TCP and lower level protocol is essentially
unspecified except that it is assume there is a mechanism whereby the
two 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.  Therefore, the lower level protocol which is assumed
throughout this document is the Internet Protocol.

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]
                                                         December 1979

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 this
  stream mode, the TCPs decide when to block and forward data at their
  own convenience.
  For users who desire a record-oriented service, the TCP also permits
  the user to submit records, called letters, for transmission.  When
  the sending user indicates a record boundary (end-of-letter), this
  causes the TCPs to promptly forward and deliver data up to that
  point to the receiver.
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 users.  All errors in the internet
  communication system are recovered by the TCP.
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.  For stream mode, the window
  indicates an allowed number of octets that the sender may transmit
  before receiving further permission.  It is also possible for the
  TCP to operate in a mode where buffer sizes and letter boundaries
  are incorporated in flow control.
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

[Page 4]

December 1979

                                         Transmission Control Protocol
                                                          Introduction
  and host addresses from the internet communication layer, this forms
  a socket.  A pair of sockets uniquely identifies each connection.
  That is, different connections may have a common socket on one side,
  but the sockets on the other sides must be different.
  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 over the unreliable internet
  communication system, a handshake mechanism with clock-based
  sequence numbers is used to avoid erroneous initialization of
  connections.
Precedence and Security:
  The users of TCP may indicate the security and precedence of their
  communication.  Provision is made for default values to be used when
  these features are not needed.
                                                              [Page 5]
                                                         December 1979

Transmission Control Protocol

[Page 6]

December 1979

                                         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.
We specifically assume that data is transmitted from host to host
through means of a set of  networks.  When we say network, we have in
mind a packet switched network (PSN).  This assumption is probably
unnecessary, since a circuit switched network or a hybrid combination
of the two could also be used; but for concreteness, we explicitly
assume that the hosts are connected to one or more packet switches of
a PSN.
The term packet is used generically here to mean the data of one
transaction between a host and a packet switch.  The format of data
blocks exchanged between the packet switches in 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 users buffer and notifies the receiving user.  The
TCPs include control information in the segments which they use to
ensure reliable ordered data transmission.
                                                              [Page 7]
                                                         December 1979

Transmission Control Protocol Philosophy

The model of internet communication is that there is a basic gateway
(or internet protocol module) associated with each TCP which provides
an interface to the local network.  This basic gateway packages TCP
segments inside internet datagrams and routes these datagrams to a
destination 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
other operations to achieve the delivery of the local packet to the
destination gateway.
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.
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 broken
into smaller ones at intermediate gateways.  The internet datagram
fragment format is designed so that the destination gateway can
reassemble fragments into internet datagrams.
A destination gateway 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 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 a time sharing 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.

[Page 8]

December 1979

                                         Transmission Control Protocol
                                                            Philosophy
Though it is assumed here that processes are supported by the host
operating system, 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
above.

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        |  Gateway Level     
          +-------------------------------+                    
                         |                                     
            +---------------------------+                      
            |   Local Network Protocol  |    Network Level     
            +---------------------------+                      
                         |                                     
                       Protocol Relationships
                             Figure 2.
                                                              [Page 9]
                                                         December 1979

Transmission Control Protocol Philosophy

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.
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 the sequence number 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, 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 TCP has taken the
responsibility to do so.
To govern the flow of data into a TCP, a flow control mechanism is
employed.  The the data receiving TCP reports a window to the sending
TCP.  This window is the number of octets starting with the
acknowledgment number that the data 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 operating system, TCP, or user, they might not
be unique.  To provide for unique addresses at 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,
since the same 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 seem necessary in any implementation.

[Page 10]

December 1979

                                         Transmission Control Protocol
                                                            Philosophy
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 only initiate connections 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
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 foreign socket of all zeros is called unspecified.  The purpose
behind unspecified sockets is to provide a sort of "general delivery"
facility (useful for processes offering services).  This is allowed
only for passive OPENs.
A service process that wished to provide services for unknown other
processes could 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 might be permanently assigned to a particular
socket, and other sockets might be 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.
Processes can issue passive OPENs and wait for matching calls from
other processes and be informed by the TCP when connections have been
established.  Two processes which issue calls to each other at the
same time are correctly connected.  This flexibility is critical for
the support of distributed computing in which components act
asynchronously with respect to each other.
                                                             [Page 11]
                                                         December 1979

Transmission Control Protocol Philosophy

There are two cases for matching the sockets in the local request and
an incoming segment.  In the first case, the local request has fully
specified the foreign socket.  In this case, the match must be exact.
In the second case, the local request has left the foreign socket
unspecified.  In this case, any foreign socket is acceptable as long
as the local sockets match.
If there are several pending passive OPENs (recorded in TCBs) with the
same local socket, an incoming segment should be matched to a request
with the specific foreign socket in the segment, if such a request
exists, before selecting a request with an unspecified foreign socket.
The procedures to establish and clear connections utilize synchronize
(SYN) and finis (FIN) control flags and involve 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 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, or as a sequence of records.  In TCP the records are called
letters and are of variable length.  The sending user indicates in
each SEND call whether the data in that call completes a letter by the
setting of the end-of-letter parameter.
The length of a letter may be such that it must be broken into
segments before it can be transmitted to its destination.  We assume
that the segments will normally be reassembled into a letter before
being passed to the receiving process.  A segment may contain all or a
part of a letter, but a segment never contains parts of more than one
letter.  The end of a letter is marked by the appearance of an EOL
control flag in a segment.  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 end of letter is signaled then it must send all
unsent data.  When a receiving TCP has a complete letter, it must not
wait for more data from the sending TCP before passing the letter to
the receiving process.
There is a coupling between letters as sent and the use of buffers of
data that cross the TCP/user interface.  Each time an end-of-letter

[Page 12]

December 1979

                                         Transmission Control Protocol
                                                            Philosophy
(EOL) 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 a letter is longer than the user's buffer,
the letter is passed to the user in buffer size units, the last of
which may be only partly full.
The TCP is responsible for regulating the flow of segments on the
connections, as a way of preventing itself from becoming saturated or
overloaded with traffic.  This is done using a window flow control
mechanism.  The data receiving TCP reports to the data sending TCP a
window which is the range of sequence numbers of data octets that data
receiving TCP is currently prepared to accept.
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
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
should take action to read through the end 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 should
properly mark outgoing segments with the security, compartment, and
precedence.  Such TCP modules should 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 should follow a general principle of robustness:
be conservative in what you do, be liberal in what you accept from
others.
                                                             [Page 13]
                                                         December 1979

Transmission Control Protocol

[Page 14]

December 1979

                                         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 [1].  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|E|R|S|F|                               |
 | Offset| Reserved  |R|C|O|S|Y|I|            Window             |
 |       |           |G|K|L|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]
                                                         December 1979

Transmission Control Protocol Functional Specification

Sequence Number:  32 bits
  The sequence number of the first data octet in this segment.
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.
Data Offset:  4 bits
  The number of 32 bit words in the TCP Header.  This indicates where
  the data begins.  The TCP header including options is an integral
  number of 32 bits long.
Reserved:  6 bits
  Reserved for future use.  Must be zero.
Control Bits:  8 bits (from left to right):
  URG:  Urgent Pointer field significant
  ACK:  Acknowledgment field significant
  EOL:  End of Letter
  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
  prefixed to the TCP header.  This pseudo header contains the Source
  Address, the Destination Address, the Protocol, and TCP length.

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                                         Transmission Control Protocol
                                              Functional Specification
  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 plus the data length in octets
    (this is not a explicitly transmitted quantity, but is computed
    from the total length, and the header length).
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 should 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 should be header padding (i.e., zero).
  A TCP must implement all options.
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  Currently defined options include (kind indicated in octal):
    Kind     Length    Meaning
    ----     ------    -------
     0         -       End of option list.
     1         -       No-Operation.
    100        -       Reserved.
    105        4       Buffer 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.
    Buffer Size
      +--------+--------+---------+--------+
      |01000101|00000100|    buffer size   |
      +--------+--------+---------+--------+
       Kind=105 Length=4

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                                         Transmission Control Protocol
                                              Functional Specification
      Buffer Size Option Data:  16 bits
        If this option is present, then it communicates the receive
        buffer size at the TCP which sends this segment.  This field
        should only be sent in segments with the SYN control bit set.
        If this option is not used, the default buffer size of one
        octet is assumed.
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 sequence
    SND.WND - send window
    SND.BS  - send buffer size
    SND.UP  - send urgent pointer
    SND.WL  - send sequence number used for last window update
    SND.LBB - send last buffer beginning
    ISS     - initial send sequence number
  Receive Sequence Variables
    RCV.NXT - receive sequence
    RCV.WND - receive window
    RCV.BS  - receive buffer size
    RCV.UP  - receive urgent pointer
    RCV.LBB - receive last buffer beginning
    IRS     - initial receive sequence number
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                                                         December 1979

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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.
  
  
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.
  
  
There are also some variables used frequently in the discussion that
take their values from the fields of the current segment.

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                                         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, TIME-WAIT, CLOSE-WAIT, CLOSING,
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 sent and received a
  connection request.
  ESTABLISHED - represents an open connection, ready to transmit and
  receive data segments.
  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.
  TIME-WAIT - represents waiting for enough time to pass to be sure
  the remote TCP received the acknowledgment of its connection
  termination request.
  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.
  CLOSED - represents no connection state at all.
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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 and FIN flags; and timeouts.
The Glossary contains a more complete list of terms and their
definitions.
The state diagram in figure 6 only illustrates 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.

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                                         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 \ / CLOSE +———+

 | rcv ACK of FIN   -------   |     |   -------                      
 | --------------   snd ACK   |     |   snd FIN                      
 V        x                   V     V                                

+———+ +———+ |FINWAIT-2| | CLOSING | +———+ +———+

 | rcv FIN                          | rcv ACK of FIN                 
 | -------    Timeout=2MSL          | --------------                 
 V snd ACK    ------------          V   delete TCB                   

+———+ delete TCB +———+ |TIME WAIT|—————–>| CLOSED | +———+ +———+

                    TCP Connection State Diagram
                             Figure 6.
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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.
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 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).
On send connections the following comparisons are needed:
  older sequence numbers                        newer sequence numbers
                                  
      SND.UNA                SEG.ACK                 SND.NXT  
         |                      |                       |     
     ----|----XXXXXXX------XXXXXXXXXX---------XXXXXX----|---- 
         |    |            |    |             |         |     
              |            |                  |               
           Segment 1    Segment 2          Segment 3          
                    <----- sequence space ----->
                 Sending Sequence Space Information
                             Figure 7.

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                                         Transmission Control Protocol
                                              Functional Specification
  SND.UNA = oldest unacknowledged sequence number
  SND.NXT = next sequence number to be sent
  SEG.ACK = acknowledgment (next sequence number expected by the
            acknowledging TCP)
  SEG.SEQ = first sequence number of a segment
  SEG.SEQ+SEG.LEN-1 = last sequence number of a segment
An acceptable acknowledgment, SEG.ACK, is one for which the inequality
below holds:
  0 < (SEG.ACK - SND.UNA) =< (SND.NXT - SND.UNA)
or:
  SND.UNA < SEG.ACK =< SND.NXT
Note that all arithmetic is modulo 2**32 and that comparisons are
unsigned.  "=<" means "less than or equal".
Similarly, the determination that a particular segment has been fully
acknowledged can be made if the inequality below holds:
  0 < (SEG.SEQ+SEG.LEN-1 - SND.UNA) < (SEG.ACK - SND.UNA)
SEG.LEN is the number of octets occupied by the data in the segment.
It is important to note that SEG.LEN must be non-zero; segments which
do not occupy any sequence space (e.g., empty acknowledgment segments)
are never placed on the retransmission queue, so would not go through
this particular test.
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Transmission Control Protocol Functional Specification

On receive connections the following comparisons are needed:
  older sequence numbers                        newer sequence numbers
                                  
              RCV.NXT                         RCV.NXT+RCV.WND 
                 |                               |            
     ---------XXX|XXX------XXXXXXXXXX---------XXX|XX--------- 
              |  |         |                  |  |            
              |            |                  |               
           Segment 1    Segment 2          Segment 3          
                    <----- sequence space ----->
                Receiving Sequence Space Information
                              Figure 8.
  RCV.NXT = next sequence number expected on incoming segments
  RCV.NXT+RCV.WND = last sequence number expected on incoming
      segments, plus one
  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
   0 =< (SEG.SEQ+SEG.LEN-1 - RCV.NXT) < (RCV.NXT+RCV.WND - RCV.NXT)
SEG.SEQ+SEG.LEN-1 is the last sequence number occupied by the segment;
RCV.NXT is the next sequence number expected on an incoming segment;
and RCV.NXT+RCV.WND is the right edge of the receive 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:

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                                         Transmission Control Protocol
                                              Functional Specification
  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+SEG.LEN =< RCV.NXT+RCV.WND
Note that the acceptance test for a segment, since it requires the end
of a segment to lie in the window, is somewhat more restrictive than
is absolutely necessary.  If at least the first sequence number of the
segment lies in the receive window, or if some part of the segment
lies in the receive window, then the segment might be judged
acceptable.  Thus, in figure 8, at least segments 1 and 2 are
acceptable by the strict rule, and segment 3 may or may not be,
depending on the strictness of interpretation of the rule.
Note that when the receive window is zero no segments should be
acceptable except ACK segments.  Thus, it should be possible for a TCP
to maintain a zero receive window while transmitting data and
receiving ACKs.
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 includes both data and sequence space
occupying controls.
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 owing to this
is -- "how does the TCP identify duplicate segments from previous
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Transmission Control Protocol Functional Specification

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 being emitted with
sequence numbers which duplicate those which are still in the network.
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 tens of seconds or minutes, at worst, 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 messages carrying a control bit
called "SYN" (for synchronize) and the initial sequence numbers.  As a
shorthand, messages 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.  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

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                                         Transmission Control Protocol
                                              Functional Specification
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.
It should be noted that this strategy does not protect against
spoofing or other replay type duplicate message problems.

3.4. Establishing a connection

The "three-way handshake" is essentially a unidirectional attempt to
establish a connection, i.e., there is an initiator and a responder.
The TCP can also establish a connection when a simultaneous initiation
occurs.  A simultaneous attempt occurs when one 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
are offered below.  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 implementation of a trade-off between memory
and messages to provide information for this checking.
The simplest three-way handshake is shown in figure 9 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.
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Transmission Control Protocol Functional Specification

    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 9.
In line 2 of figure 9, 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!).
Simultaneous initiation is only slightly more complex, as is shown in
figure 10.  Each TCP cycles from CLOSED to SYN-SENT to SYN-RECEIVED to
ESTABLISHED.
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.  A TCP which
receives a reset message first verifies that the ACK field of the
reset acknowledges something the TCP sent (otherwise, the message is
ignored).  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, TIME-WAIT, CLOSE-WAIT, CLOSING),
it aborts the connection and informs its user.  We discuss this latter
case under "half-open" connections below.

[Page 30]

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                                         Transmission Control Protocol
                                              Functional Specification
    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=101><ACK=301><CTL=ACK> ...
6.  ESTABLISHED  <-- <SEQ=301><ACK=101><CTL=ACK> <-- SYN-RECEIVED
7.               ... <SEQ=101><ACK=301><CTL=ACK> --> ESTABLISHED
              Simultaneous Connection Synchronization
                             Figure 10.
    TCP A                                                TCP B
1.  CLOSED                                               LISTEN
2.  SYN-SENT    --> <SEQ=100><CTL=SYN>               ...
3.  (duplicate) ... <SEQ=1000><CTL=SYN>              --> SYN-RECEIVED
4.  SYN-SENT    <-- <SEQ=300><ACK=1001><CTL=SYN,ACK> <-- SYN-RECEIVED
5.  SYN-SENT    --> <SEQ=1001><CTL=RST>              --> LISTEN
                                                         (ACK is ok)
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 11.
As a simple example of recovery from old duplicates, consider
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                                                         December 1979

Transmission Control Protocol Functional Specification

figure 11.  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
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 should indicate 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 12.  After TCP A crashes, the user attempts to
re-open the connection.  TCP B, in the meantime, thinks the connection
is open.

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                                         Transmission Control Protocol
                                              Functional Specification
    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.                                                   CLOSED
7.  SYN-SENT --> <SEQ=400><CTL=SYN>              --> CLOSED
8.  (Abort!!)<-- <SEQ=xxx><ACK=401><CTL=RST,ACK> <-- CLOSED
9.  CLOSED                                           CLOSED
                   Half-Open Connection Discovery
                             Figure 12.
When the SYN arrives at line 3, TCP B, being in a synchronized state,
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 continue to retransmit its SYN; and if the user at
TCP B re-opens the connection, eventually everything will work out.
In the figure TCP B does not reopen the connection, and in line 8
sends a reset to reject the offered connection.
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 13.  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.
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      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 13.
In figure 14, 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 14.
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) should be sent whenever a segment
arrives which apparently is not intended for the current or a future
incarnation of the connection.  A reset should not be sent if it is
not clear that this is the case.  Thus, if any segment arrives for a
nonexistent connection, a reset should be sent.  If a segment ACKs

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                                         Transmission Control Protocol
                                              Functional Specification
something which has never been sent on the current connection, then
one of the following two cases applies.
1.  If the connection is in any non-synchronized state (LISTEN,
SYN-SENT, SYN-RECEIVED) or if the connection does not exist, a reset
(RST) should be formed and sent for any segment that acknowledges
something not yet sent.  The RST should take its SEQ field from the
ACK field of the offending segment (if the ACK control bit was set),
and its ACK bit should be reset (zero), except to refuse a initial
SYN.  A reset is also sent if an incoming segment has a security level
or compartment which does not exactly match the level and compartment
requested for the connection.  If the precedence of the incoming
segment is less than the precedence level requested a reset is sent.
2.  If the connection is in a synchronized state (ESTABLISHED,
FIN-WAIT-1, FIN-WAIT-2, TIME-WAIT, CLOSE-WAIT, CLOSING), any
unacceptable segment should elicit only an empty acknowledgment
segment containing the current send-sequence number and an
acknowledgment indicating the next sequence number expected to be
received.
Reset Processing
All reset (RST) segments are validated by checking their SEQ-fields.
A reset is valid if its sequence number is the next expected sequence
number.  In the case of a RST received in response to an initial SYN
any sequence number 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 unilaterally inform a user,
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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.
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.  It should be noted 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 should respond with a CLOSE, upon which the TCP can send a FIN
  to the other TCP.  The TCP then waits until its own FIN is
  acknowledged whereupon it deletes the connection.  If an ACK is not
  forthcoming, after a 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.

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                                         Transmission Control Protocol
                                              Functional Specification
    TCP A                                                TCP B
1.  ESTABLISHED                                          ESTABLISHED
2.  (Close)
    FIN-WAIT-1  --> <SEQ=100><CTL=FIN>               --> CLOSE-WAIT
3.  FIN-WAIT-2  <-- <SEQ=300><ACK=101><CTL=ACK>      <-- CLOSE-WAIT
4.                                                       (Close)
    TIME-WAIT   <-- <SEQ=301><CTL=FIN>               <-- CLOSING
5.  TIME-WAIT   --> <SEQ=100><ACK=301><CTL=ACK>      --> CLOSED
6.  (2 MSL)
    CLOSED
                       Normal Close Sequence
                             Figure 15.
    TCP A                                                TCP B
1.  ESTABLISHED                                          ESTABLISHED
2.  (Close)                                              (Close)
    FIN-WAIT-1  --> <SEQ=100><CTL=FIN>               ... FIN-WAIT-1
                <-- <SEQ=300><CTL=FIN>               <--
                ... <SEQ=100><CTL=FIN>               -->
3.  CLOSING     --> <SEQ=100><ACK=301><CTL=ACK>      ... CLOSING
                <-- <SEQ=300><ACK=101><CTL=ACK>      <--
                ... <SEQ=100><ACK=301><CTL=ACK>      -->
4.  CLOSED                                               CLOSED
                    Simultaneous Close Sequence
                             Figure 16.
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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 parts.
The precedence levels are:
  flash override - 111
  flash          - 110
  immediate      - 10X
  priority       - 01X
  routine        - 00X
The security levels are:
  top secret    - 11
  secret        - 10
  confidential  - 01
  unclassified  - 00
The compartments are assigned by the Defense Communications Agency.
The defaults are precedence:  routine, security:  unclassified,
compartment:  zero.  A host which does not implement precedence or
security feature should clear these fields to zero for segments it
sends.
A connection attempt with mismatched security/compartment values or a
lower precedence value should be rejected by sending a reset.
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.

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

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                                         Transmission Control Protocol
                                              Functional Specification
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
acknowledgment it advances SND.UNA.  The extent to which the values of
these variables differ is a measure of the delay in the communication.
Normally the amount by which the variables are advanced is the length
of the data in the segment.  However, when letters are used there are
special provisions for coordination the sequence numbers, the letter
boundaries, and the receive buffer boundaries.
End of Letter Sequence Number Adjustments
There is provision in TCP for the receiver of data to optionally
communicate to the sender of data on a connection at the time of the
connection synchronization the receiver's buffer size.  If this is
done the receiver must use this fixed size of buffers for the lifetime
of the connection.  If a buffer size is communicated then there is a
coordination between receive buffers, letters, and sequence numbers.
Each time a buffer is completed either due to being filled or due to
an end of letter, the sequence number is incremented through the end
of that buffer.
That is, whenever an EOL is transmitted, the sender advances its send
sequence number, SND.NXT, by an amount sufficient to consume all the
unused space in the receiver's buffer.  The amount of space consumed
in this fashion is subtracted from the send window just as is the
space consumed by actual data.
And, whenever an EOL is received, the receiver advances its receive
sequence number, RCV.NXT, by an amount sufficient to consume all the
unused space in the receiver's buffer.  The amount of space consumed
in this fashion is subtracted from the receive window just as is the
space consumed by actual data.
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  older sequence numbers                        newer sequence numbers
          |           Buffer 1            |   Buffer 2       
          |                               |                  
      ----+-------------------------------+----------------- 
          XXXXXXXXXXXXXXXXXXXXX+++++++++++                   
          |                    |          |                  
          |<-----SEG.LEN------>|          |                  
          |                    |          |                  
          |                    |          |                  
       SEG.SEQ                 A          B                  
                  XXX - data octets from segment 
                  +++ - phantom data             
                    <----- sequence space ----->
                      End of Letter Adjustment
                             Figure 17.
In the case illustrated above, if the segment does not carry an EOL
flag, the next value of SND.NXT or RCV.NXT will be A.  If it does
carry an EOL flag, the next value will be B.
The exchange of buffer size and sequencing information is done in
units of octets.  If no buffer size is stated, then the buffer size is
assumed to be 1 octet.  The receiver tells the sender the size of the
buffer in a SYN segment that contains the 16 bit buffer size data in
an option field in the TCP header.
Each EOL advances the sequence number (SN) to the next buffer boundary
  While LBB < SEG.SEQ+SEG.LEN
  Do LBB <- LBB + BS End
  SN <- LBB
  where LBB is the Last Buffer Beginning, and BS is the buffer size.
The CLOSE user call implies an end of letter, as does the FIN control
flag in an incoming segment.
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.

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                                         Transmission Control Protocol
                                              Functional Specification
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
should tell the user to go into "urgent mode"; when the receive
sequence number catches up to the urgent pointer, the TCP should tell
user to go into "normal mode".  If the urgent pointer is updated while
the user is in "read fast" 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 should be added to the segment sequence number to yield
the urgent pointer.  The absence of this flag indicates that the
urgent pointer has not changed.
To send an urgent indication the user must also send at least one data
octet.  If the sending user also indicates end of letter, 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 number
the sender of the window (the data receiver) is currently prepared to
accept.  There is an assumption that this is somehow related to 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, 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 and send at least one octet
of new data even if the send window is zero.  The sending TCP should
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.
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Users must keep reading connections they close for sending until the
TCP says no more data.
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.

3.8. Interfaces

There are of course two interfaces of concern:  the user/TCP interface
and the TCP/IP interface.  We have a fairly elaborate model of the
user/TCP interface, but only a sketch of the interface to the lower
level protocol module.
User/TCP Interface
  The 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 should 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.

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                                         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.
    Although the means for signaling user processes and the exact
    format of replies will vary from one implementation to another, it
    would promote common understanding and testing if a common set of
    codes were adopted.  Such a set of event codes is described below.
    Open
      Format:  OPEN (local port, foreign socket, active/passive
      [, buffer size] [, timeout] [, precedence]
      [, security/compartment]) -> 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 by
      the processes that serve it (e.g., the program which interfaces
      the TCP network).  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 full-duplex 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 buffer size, if present, indicates that the caller will
      always receive data from the connection in that size of buffers.
      This buffer size is a measure of the buffer between the user and
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      the local TCP.  The buffer size between the two TCPs may be
      different.
      The timeout, if present, permits the caller to set up a timeout
      for all buffers transmitted on the connection.  If a buffer is
      not successfully delivered to the destination within the timeout
      period, the TCP will abort the connection.  The present global
      default is 30 seconds.  The buffer retransmission rate may vary;
      most likely, it will be related to the measured time for
      responses from the remote TCP.
      The TCP or some component of the operating system will verify
      the users authority to open a connection with the specified
      precedence or security/compartment.  The absence of precedence
      or security/compartment specification in the OPEN call indicates
      the default values should 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.
      Depending on the TCP implementation, either a local connection
      name will be returned to the user by the TCP, or the user will
      specify this local connection name (in which case another
      parameter is needed in the call).  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,
      EOL 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 EOL flag is set, the data is the End Of a Letter, and the
      EOL bit will be set in the last TCP segment created from the

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                                              Functional Specification
      buffer.  If the EOL flag is not set, subsequent SENDs will
      appear to be part of the same letter.
      If the URGENT flag is set, segments resulting from this call
      will have the urgent pointer set to indicate that some of the
      data associated with this call is urgent.  This facility, for
      example, can be used to simulate "break" signals from terminals
      or error or completion codes from I/O devices.  The semantics of
      this signal to the receiving process are unspecified.  The
      receiving TCP will signal the urgent condition to the receiving
      process as long as 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 accept some urgent data and to indicate to the
      receiver when all the currently known urgent 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.  In general, 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, then the current 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 highly 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.
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      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 letters.
      NOTA BENE: In order for the process to distinguish among error
      or success indications for different SENDs, it might be
      appropriate for the buffer address to be returned along with the
      coded response to the SEND request.  We will offer an example
      event code format below, showing the information which should be
      returned to the calling process.
    Receive
      Format:  RECEIVE (local connection name, buffer address, byte
      count)
      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
      letters, segments or fragments arrive.  This strategy permits
      increased throughput at the cost of a more elaborate scheme
      (possibly asynchronous) to notify the calling program that a
      letter has been received or a buffer filled.
      If insufficient buffer space is given to reassemble a complete
      letter, the EOL flag will not be set in the response to the
      RECEIVE.  The buffer will be filled with as much data as it can
      hold.  The last buffer required to hold the letter is returned
      with EOL signaled.

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                                              Functional Specification
      The remaining parts of a partly delivered letter will be placed
      in buffers as they are made available via successive RECEIVEs.
      If a number of RECEIVEs are outstanding, they may be filled with
      parts of a single long letter or with at most one letter each.
      The event codes associated with each RECEIVE will indicate what
      is contained in the buffer.
      If a buffer size was given in the OPEN call, then all buffers
      presented in RECEIVE calls must be of exactly that size, or an
      error indication will be returned.
      The URGENT flag will be set only if the receiving user has
      previously been informed via a general event, that urgent data
      is waiting.  The receiving user should thus be in "read-fast"
      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 "read-fast" mode.
      To distinguish among several outstanding RECEIVEs and to take
      care of the case that a letter is smaller than the buffer
      supplied, the event code is accompanied by both a buffer pointer
      and a byte count indicating the actual length of the letter
      received.
      Alternative implementations of RECEIVE might have the TCP
      allocate buffer storage, or the TCP might share a ring buffer
      with the user.  Variations of this kind will produce obvious
      variation in user interface to the TCP.
    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
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      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 end of letter.
    Status
      Format:  STATUS(local connection name)
      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,
        foreign socket,
        local connection name,
        receive window,
        send window,
        connection state,
        number of buffers awaiting acknowledgment,
        number of buffers pending receipt (including partial ones),
        receive buffer size,
        urgent state,
        precedence,
        security/compartment,
        and default 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.

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    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
      End-of-Letter flag                       Receive
      End-of-Urgent flag                       Receive
TCP/Network 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 [1].  In most cases the following simple interface would be
  adequate.
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  The following two calls satisfy the requirements for the TCP to
  internet protocol module communication:
    SEND (dest, TOS, TTL, BufPTR, len, Id, DF, options => result)
      where:
        dest = destination address
        TOS = type of service
        TTL = time to live
        BufPTR = buffer pointer
        len = length of buffer
        Id  = Identifier
        DF = Don't Fragment
        options = option data
        result = response
          OK = datagram sent ok
          Error = error in arguments or local network error
      Note that the precedence is included in the TOS and the
      security/compartment is passed as an option.
    RECV (BufPTR => result, source, dest, prot, TOS, len)
      where:
        BufPTR = buffer pointer
        result = response
          OK = datagram received ok
          Error = error in arguments
        source = source address
        dest = destination address
        prot = protocol
        TOS = type of service
        options = option data
        len = length of buffer
      Note that the precedence is in the TOS, and the
      security/compartment is an option.
    When the TCP sends a segment, it executes the SEND call supplying
    all the arguments.  The internet protocol module, on receiving
    this call, checks the arguments and prepares and sends the
    message.  If the arguments are good and the segment is accepted by
    the local network, the call returns successfully.  If either the
    arguments are bad, or the segment is not accepted by the local
    network, the call returns unsuccessfully.  On unsuccessful
    returns, a reasonable report should be made as to the cause of the

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    problem, but the details of such reports are up to individual
    implementations.
    When a segment arrives at the internet protocol module from the
    local network, either there is a pending RECV call from TCP or
    there is not.  In the first case, the pending call is satisfied by
    passing the information from the segment to the TCP.  In the
    second case, the TCP is notified of a pending segment.
    The notification of a TCP may be via a pseudo interrupt or similar
    mechanism, as appropriate in the particular operating system
    environment of the implementation.
    A TCP's RECV call may then either be immediately satisfied by a
    pending segment, or the call may be pending until a segment
    arrives.
    We note that the Internet Protocol 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:  none, Package:  stream,
      Reliability:  higher, Preference:  speed, Speed:  higher; or
      00011111.
      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.
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3.9. Event Processing

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
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.
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.

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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.
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                                                             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.  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 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 and a SYN segment of the form <SEQ=ISS><CTL=SYN>
    is sent.  Set SND.UNA to ISS, SND.NXT to ISS+1, SND.LBB 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
  SYN-SENT STATE
  SYN-RECEIVED STATE
  ESTABLISHED STATE
  FIN-WAIT-1 STATE
  FIN-WAIT-2 STATE
  TIME-WAIT STATE
  CLOSE-WAIT STATE
  CLOSING STATE
    Return "error:  connection already exists".

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SEND Call

SEND Call
  CLOSED STATE (i.e., TCB does not exist)
    If the user should no 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 and SND.LBB 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 should be sent
    with the first data segment 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
    Queue for processing after the connection is ESTABLISHED.
    Typically, nothing can be sent yet, anyway, because the send
    window has not yet been set by the other side.  If no space,
    return "error:  insufficient resources".
  SYN-RECEIVED STATE
    Queue for later processing after entering ESTABLISHED state.  If
    no space to queue, respond with "error:  insufficient resources".
  ESTABLISHED STATE
    Segmentize the buffer, send or queue it for output, with a
    piggybacked acknowledgment (acknowledgment value = SND.UNA) with
    the data (this is not required, but there is no advantage in not
    doing so).  If there is insufficient space to remember this
    buffer, simply return "error:  insufficient resources".
    If remote buffer size is not one octet, then, if this is the end
    of a letter, do the following end-of-letter/buffer-size adjustment
    processing:
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                                                             SEND Call
      if EOL = 0 then
        SND.NXT <- SEG.SEQ + SEG.LEN
      if EOL = 1 then
        While SND.LBB < SEG.SEQ + SEG.LEN
        Do SND.LBB <- SND.LBB + SND.BS End
        SND.NXT <- SND.LBB
    If the urgent flag is set, then SND.UP <- SND.NXT-1 and set the
    urgent pointer in the outgoing segment.
  FIN-WAIT-1 STATE
  FIN-WAIT-2 STATE
  TIME-WAIT STATE
    Return "error:  connection closing" and do not service request.
  CLOSE-WAIT STATE
    Segmentize any text to be sent and queue for output.  If there is
    insufficient space to remember the SEND, return "error:
    insufficient resources"
  CLOSING STATE
    Respond with "error:  connection closing"

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RECEIVE Call

RECEIVE Call
  CLOSED STATE (i.e., TCB does not exist)
    If the user should no 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
    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 "end of letter" (EOL) 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.
  FIN-WAIT-1 STATE
  FIN-WAIT-2 STATE
    Reassemble and return a letter, or as much as will fit, in the
    user buffer.  Queue the request if it cannot be serviced
    immediately.
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                                                          RECEIVE Call
  TIME-WAIT STATE
  CLOSE-WAIT STATE
    Since the remote side has already sent FIN, RECEIVEs must be
    satisfied by text already reassembled, but not yet delivered to
    the user.  If no reassembled segment text is awaiting delivery,
    the RECEIVE should get a "error:  connection closing" response.
    Otherwise, any remaining text can be used to satisfy the RECEIVE.
  CLOSING STATE
    Return "error:  connection closing"

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CLOSE Call

CLOSE Call
  CLOSED STATE (i.e., TCB does not exist)
    If the user should no 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:  closing"
    responses.  Delete TCB, return "ok".
  SYN-SENT STATE
    Delete the TCB and return "error:  closing" responses to any
    queued SENDs, or RECEIVEs.
  SYN-RECEIVED STATE
    Queue for processing after entering ESTABLISHED state or
    segmentize and send FIN segment.  If the latter, enter FIN-WAIT-1
    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).
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                                                            CLOSE Call
  TIME-WAIT STATE
    Strictly speaking, this is an error and should receive a "error:
    connection closing" response.  An "ok" response would be
    acceptable, too.  However, since the FIN has been sent and
    acknowledged, nothing should be sent (or retransmitted).
  CLOSE-WAIT STATE
    Queue this request until all preceding SENDs have been
    segmentized; then send a FIN segment, enter CLOSING state.
  CLOSING STATE
    Respond with "error:  connection closing"

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ABORT Call

ABORT Call
  CLOSED STATE (i.e., TCB does not exist)
    If the user should no 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, return "ok".
  SYN-SENT STATE
    Delete the TCB and return "reset" responses to any queued SENDs,
    or RECEIVEs.
  SYN-RECEIVED STATE
    Send a RST of the form:
      <SEQ=SND.NXT><ACK=RCV.NXT><CTL=RST,ACK>
    and return any unprocessed SENDs, or RECEIVEs with "reset" code,
    delete the TCB.
  ESTABLISHED STATE
    Send a reset segment:
      <SEQ=SND.NXT><ACK=RCV.NXT><CTL=RST,ACK>
    All queued SENDs and RECEIVEs should be given "reset" responses;
    all segments queued for transmission (except for the RST formed
    above) or retransmission should be flushed, delete the TCB.
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                                                            ABORT Call
  FIN-WAIT-1 STATE
  FIN-WAIT-2 STATE
    A reset segment (RST) should be formed and sent:
      <SEQ=SND.NXT><ACK=RCV.NXT><CTL=RST,ACK>
    Outstanding SENDs, RECEIVEs, CLOSEs, and/or segments queued for
    retransmission, or segmentizing, should be flushed, with
    "connection reset" notification to the user, delete the TCB.
  TIME-WAIT STATE
    Respond with "ok" and delete the TCB.
  CLOSE-WAIT STATE
    Flush any pending SENDs and RECEIVEs, returning "connection reset"
    responses for them.  Form and send a RST segment:
      <SEQ=SND.NXT><ACK=RCV.NXT><CTL=RST,ACK>
    Flush all segment queues and delete the TCB.
  CLOSING STATE
    Respond with "ok" and delete the TCB; flush any remaining segment
    queues.  If a CLOSE command is still pending, respond "error:
    connection reset".

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STATUS Call

STATUS Call
  CLOSED STATE (i.e., TCB does not exist)
    If the user should no 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.
  TIME-WAIT STATE
    Return "state = TIME-WAIT and the TCB pointer.
  CLOSE-WAIT STATE
    Return "state = CLOSE-WAIT", and the TCB pointer.
  CLOSING STATE
    Return "state = CLOSING", and the TCB pointer.
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                                                       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 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, except another RST.  The
      RST should be formatted as follows:
        <SEQ=SEG.ACK><CTL=RST>
      Return.
      An incoming RST should be ignored.  Return.
    if there was no ACK then 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 and check the precedence.  If the SEG.PRC is less than
      the TCB.PRC then send a reset and return.  If the SEG.PRC is
      greater than the TCB.PRC then set TCB.PRC<-SEG.PRC.  Now RCV.NXT
      and RCV.LBB are set 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:

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SEGMENT ARRIVES

        <SEQ=ISS><ACK=RCV.NXT><CTL=SYN,ACK>
      SND.NXT and SND.LBB are 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.
      This segment may also include data and control bits (e.g., URG,
      EOL) which were queued for transmission.
    if there was no SYN but there was other text or control
      Any other control or text-bearing segment (not containing SYN)
      should 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 for an ACK
      If SEG.ACK =< ISS, or SEG.ACK > SND.NXT, or the
      security/compartment in the segment does not exactly match the
      security/compartment in the TCB, or the precedence in the
      segment is less than the precedence in the TCB, send a reset
        <SEQ=SEG.ACK><CTL=RST>
      and discard the segment.  Return.
      If SND.UNA =< SEG.ACK =< SND.NXT and the security/compartment
      and precedence are acceptable then the ACK is acceptable.
      SND.UNA should be advanced to equal SEG.ACK, and any segments on
      the retransmission queue which are thereby acknowledged should
      be removed.
    if the ACK is ok (or there is no ACK), check the RST bit
      If the RST bit is set then signal the user "error:  connection
      reset", enter CLOSED state, drop the segment, delete TCB, and
      return.
    if the ACK is ok (or there is no ACK) and it was not a RST, check
    the SYN bit
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                                                       SEGMENT ARRIVES
      If the SYN bit is on and the security/compartment and precedence
      are acceptable then, RCV.NXT and RCV.LBB are set to SEG.SEQ+1,
      IRS is set to SEG.SEQ.  If SND.UNA > ISS (our SYN has been
      ACKed), change the connection state to ESTABLISHED, otherwise
      enter SYN-RECEIVED.  In any case, 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 SEG.PRC is greater than TCB.PRC set TCB.PRC<-SEG.PRC.
      If there are other controls or text in the segment then continue
      processing at the fifth step below where the URG bit is checked,
      otherwise return.

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SEGMENT ARRIVES

  Otherwise,
  first check sequence number
    SYN-RECEIVED STATE
    ESTABLISHED STATE
    FIN-WAIT-1 STATE
    FIN-WAIT-2 STATE
    TIME-WAIT STATE
    CLOSE-WAIT STATE
    CLOSING 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+SEG.LEN =< RCV.NXT+RCV.WND
      Note that the test above guarantees that the last sequence
      number used by the segment lies in the receive-window.  If the
      RCV.WND is zero, no segments will be acceptable, but special
      allowance should be made to accept valid ACKs.
      If an incoming segment is not acceptable, an acknowledgment
      should be sent in reply:
        <SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK>
      If the incoming segment is unacceptable, drop it and return.
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                                                       SEGMENT ARRIVES
  second check security and precedence
    If the security/compartment and precedence in the segment do not
    exactly match the security/compartment and precedence in the TCB
    then form a reset and return.
    Note this check is placed following the sequence check to prevent
    a segment from an old connection between these parts with a
    different security or precedence from causing an abort of the
    current connection.
  third check the ACK field,
    SYN-RECEIVED STATE
      If the RST bit is off and SND.UNA < SEG.ACK =< SND.NXT then set
      SND.UNA <- SEG.ACK, remove any acknowledged segments from the
      retransmission queue, and enter ESTABLISHED state.
      If the segment acknowledgment is not acceptable, form a reset
      segment,
        <SEQ=SEG.ACK><CTL=RST>
      and send it, unless the incoming segment is an RST (or there is
      no ACK), in which case, it should be discarded, then return.
    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, it can be ignored.
      If the segment passes the sequence number and acknowledgment
      number tests, the send window should be updated.  If
      SND.WL =< SEG.SEQ, set SND.WND <- SEG.WND and set
      SND.WL <- SEG.SEQ.
      If the remote buffer size is not one, then the
      end-of-letter/buffer-size adjustment to sequence numbers may
      have an effect on the next expected sequence number to be
      acknowledged.  It is possible that the remote TCP will
      acknowledge with a SEG.ACK equal to a sequence number of an

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SEGMENT ARRIVES

      octet that was skipped over at the end of a letter.  This a mild
      error on the remote TCPs part, but not cause for alarm.
    FIN-WAIT-1 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.
    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.
    CLOSE-WAIT STATE
    CLOSING STATE
      Do the same processing as for the ESTABLISHED state.
  fourth check the RST bit,
    SYN-RECEIVED STATE
      If the RST bit is set then, if the segment has passed sequence
      and acknowledgment tests, it is valid.  If this connection was
      initiated with a passive OPEN (i.e., came from the LISTEN
      state), then return this connection to LISTEN state.  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.
    ESTABLISHED
    FIN-WAIT-1
    FIN-WAIT-2
    CLOSE-WAIT
    CLOSING STATE
      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.
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                                                       SEGMENT ARRIVES
    TIME-WAIT
      Enter the CLOSED state, delete the TCB, and return.
  fifth, check the SYN bit,
    SYN-RECEIVED
    ESTABLISHED STATE
      If the SYN bit is set, check the segment sequence number against
      the receive window.  The segment sequence number must be in the
      receive window; if not, ignore the segment.  If the SYN is on
      and SEG.SEQ = RCV.NXT-1, then everything is ok and no action is
      needed; but if they are not equal, there is an error and a reset
      must be sent.
        If a reset must be sent it is formed as follows:
          <SEQ=SEG.ACK><CTL=RST>
        The connection must be aborted as if a RST had been received.
    FIN-WAIT STATE-1
    FIN-WAIT STATE-2
    TIME-WAIT STATE
    CLOSE-WAIT STATE
    CLOSING STATE
      This case should not occur, since a duplicate of the SYN which
      started the current connection incarnation will have been
      filtered in the SEG.SEQ processing.  Other SYN's will have been
      rejected by this test as well (see SYN processing for
      ESTABLISHED state).
  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.

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                                         Transmission Control Protocol
                                              Functional Specification

SEGMENT ARRIVES

    TIME-WAIT STATE
    CLOSE-WAIT STATE
    CLOSING
      This should not occur, since a FIN has been received from the
      remote side.  Ignore the URG.
  seventh, process the segment text,
    ESTABLISHED 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 EOL flag, then the
      user is informed, when the buffer is returned, that an EOL has
      been received.
      If buffer size is not one octet, then do  the following
      end-of-letter/buffer-size adjustment processing:
        if EOL = 0 then
          RCV.NXT <- SEG.SEQ + SEG.LEN
        if EOL = 1 then
          While RCV.LBB < SEG.SEQ+SEG.LEN
          Do RCV.LBB <- RCV.LBB + RCV.BS End
          RCV.NXT <- RCV.LBB
      When the TCP takes responsibility for delivering the data to the
      user it must also acknowledge the receipt of the data.  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.
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                                                       SEGMENT ARRIVES
    FIN-WAIT-1 STATE
    FIN-WAIT-2 STATE
      If there are outstanding RECEIVEs, they should be satisfied, if
      possible, with the text of this segment; remaining text should
      be queued for further processing.  If a RECEIVE is satisfied,
      the user should be notified, with "end-of-letter" (EOL) signal,
      if appropriate.
    TIME-WAIT STATE
    CLOSE-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,
    Send an acknowledgment for the FIN.  Signal the user "connection
    closing", and return any pending RECEIVEs with same message.  Note
    that FIN implies EOL for any segment text not yet delivered to the
    user.  If the current state is ESTABLISHED, enter the CLOSE-WAIT
    state.  If the current state is FIN-WAIT-1, enter the CLOSING
    state.  If the current state is FIN-WAIT-2, enter the TIME-WAIT
    state.
  and return.

[Page 72]

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                                         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, 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.
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[Page 74]

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                                         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).

buffer size

        An option (buffer size) used to state the receive data buffer
        size of the sender of this option.  May only be sent in a
        segment that also carries a SYN.

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.

EOL

        A control bit (End of Letter) occupying no sequence space,
        indicating that this segment ends a logical letter with the
        last data octet in the segment.  If this end of letter causes
        a less than full buffer to be released to the user and the
        connection buffer size is not one octet then the
        end-of-letter/buffer-size adjustment to the receive sequence
        number must be made.
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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.

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.

[Page 76]

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                                         Transmission Control Protocol
                                                              Glossary

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.

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.

letter

        A logical unit of data, in particular the logical unit of data
        transmitted between processes via TCP.

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.

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
                                                             [Page 77]
                                                         December 1979

Transmission Control Protocol Glossary

        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.

PSN

        A Packet Switched Network.  For example, the ARPANET.

RCV.BS

        receive buffer size, the remote buffer size

RCV.LBB

        receive last buffer beginning

RCV.NXT

        receive next sequence number

RCV.UP

        receive urgent pointer

RCV.WND

        receive window

receive last buffer beginning

        This is the sequence number of the first octet of the most
        recent buffer.  This value is use in calculating the next
        sequence number when a segment contains an end of letter
        indication.

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.

[Page 78]

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                                         Transmission Control Protocol
                                                              Glossary

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.

Rubber EOL

        An end of letter (EOL) requiring a sequence number adjustment
        to align the beginning of the next letter on a buffer
        boundary.

SEG.ACK

        segment acknowledgment

SEG.LEN

        segment length

SEG.PRC

        segment precedence value

SEG.SEQ

        segment sequence

SEG.UP

        segment urgent pointer field

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.
                                                             [Page 79]
                                                         December 1979

Transmission Control Protocol Glossary

segment sequence

        The number in the sequence field of the arriving segment.

send last buffer beginning

        This is the sequence number of the first octet of the most
        recent buffer.  This value is use in calculating the next
        sequence number when a segment contains an end of letter
        indication.

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 sequence numbers which may be
        emitted by a TCP lies between SND.NXT and
        SND.UNA + SND.WND - 1.

SND.BS

         send buffer size, the local buffer size

SND.LBB

        send last buffer beginning

SND.NXT

        send sequence

SND.UNA

        left sequence

SND.UP

        send urgent pointer

SND.WL

        send sequence number at last window update

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.

[Page 80]

December 1979

                                         Transmission Control Protocol
                                                              Glossary

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.
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[Page 82]

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                                         Transmission Control Protocol
                             REFERENCES

[1] Postel, J. (ed.), "DOD Standard Internet Protocol," Defense

   Advanced Research Projects Agency, Information Processing
   Techniques Office, IEN 123, December 1979.

[2] Feinler, E. and J. Postel, ARPANET Protocol Handbook, Network

   Information Center, Stanford Research Institute, Menlo Park, CA,
   January 1978.

[3] Dalal, Y. and C. Sunshine, "Connection Management in Transport

   Protocols," Computer Networks, Vol. 2, No. 6, pp. 454-473,
   December 1978.
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