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rfc:rfc793

RFC: 793

                                  
                                  
                                  
                                  
                                  
                                  
                                  
                   TRANSMISSION CONTROL PROTOCOL
                                  
                                  
                       DARPA INTERNET PROGRAM
                                  
                       PROTOCOL SPECIFICATION
                                  
                                  
                                  
                           September 1981
                            prepared for
                                  
             Defense Advanced Research Projects Agency
              Information Processing Techniques Office
                       1400 Wilson Boulevard
                     Arlington, Virginia  22209
                                 by
                   Information Sciences Institute
                 University of Southern California
                         4676 Admiralty Way
                 Marina del Rey, California  90291

September 1981

                                         Transmission Control Protocol
                         TABLE OF CONTENTS
  PREFACE ........................................................ iii

1. INTRODUCTION …………………………………………….. 1

1.1  Motivation .................................................... 1
1.2  Scope ......................................................... 2
1.3  About This Document ........................................... 2
1.4  Interfaces .................................................... 3
1.5  Operation ..................................................... 3

2. PHILOSOPHY ………………………………………………. 7

2.1  Elements of the Internetwork System ........................... 7
2.2  Model of Operation ............................................ 7
2.3  The Host Environment .......................................... 8
2.4  Interfaces .................................................... 9
2.5  Relation to Other Protocols ................................... 9
2.6  Reliable Communication ........................................ 9
2.7  Connection Establishment and Clearing ........................ 10
2.8  Data Communication ........................................... 12
2.9  Precedence and Security ...................................... 13
2.10 Robustness Principle ......................................... 13

3. FUNCTIONAL SPECIFICATION …………………………………. 15

3.1  Header Format ................................................ 15
3.2  Terminology .................................................. 19
3.3  Sequence Numbers ............................................. 24
3.4  Establishing a connection .................................... 30
3.5  Closing a Connection ......................................... 37
3.6  Precedence and Security ...................................... 40
3.7  Data Communication ........................................... 40
3.8  Interfaces ................................................... 44
3.9  Event Processing ............................................. 52

GLOSSARY …………………………………………………… 79

REFERENCES …………………………………………………. 85

                                                              [Page i]
                                                        September 1981

Transmission Control Protocol

[Page ii]

September 1981

                                         Transmission Control Protocol
                              PREFACE

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

                                                         Jon Postel
                                                         Editor
                                                            [Page iii]

RFC: 793 Replaces: RFC 761 IENs: 129, 124, 112, 81, 55, 44, 40, 27, 21, 5

                   TRANSMISSION CONTROL PROTOCOL
                       DARPA INTERNET PROGRAM
                       PROTOCOL SPECIFICATION
                          1.  INTRODUCTION

The Transmission Control Protocol (TCP) is intended for use as a highly reliable host-to-host protocol between hosts in packet-switched computer communication networks, and in interconnected systems of such networks.

This document describes the functions to be performed by the Transmission Control Protocol, the program that implements it, and its interface to programs or users that require its services.

1.1. Motivation

Computer communication systems are playing an increasingly important
role in military, government, and civilian environments.  This
document focuses its attention primarily on military computer
communication requirements, especially robustness in the presence of
communication unreliability and availability in the presence of
congestion, but many of these problems are found in the civilian and
government sector as well.
As strategic and tactical computer communication networks are
developed and deployed, it is essential to provide means of
interconnecting them and to provide standard interprocess
communication protocols which can support a broad range of
applications.  In anticipation of the need for such standards, the
Deputy Undersecretary of Defense for Research and Engineering has
declared the Transmission Control Protocol (TCP) described herein to
be a basis for DoD-wide inter-process communication protocol
standardization.
TCP is a connection-oriented, end-to-end reliable protocol designed to
fit into a layered hierarchy of protocols which support multi-network
applications.  The TCP provides for reliable inter-process
communication between pairs of processes in host computers attached to
distinct but interconnected computer communication networks.  Very few
assumptions are made as to the reliability of the communication
protocols below the TCP layer.  TCP assumes it can obtain a simple,
potentially unreliable datagram service from the lower level
protocols.  In principle, the TCP should be able to operate above a
wide spectrum of communication systems ranging from hard-wired
connections to packet-switched or circuit-switched networks.
                                                              [Page 1]
                                                        September 1981

Transmission Control Protocol Introduction

TCP is based on concepts first described by Cerf and Kahn in [1].  The
TCP fits into a layered protocol architecture just above a basic
Internet Protocol [2] which provides a way for the TCP to send and
receive variable-length segments of information enclosed in internet
datagram "envelopes".  The internet datagram provides a means for
addressing source and destination TCPs in different networks.  The
internet protocol also deals with any fragmentation or reassembly of
the TCP segments required to achieve transport and delivery through
multiple networks and interconnecting gateways.  The internet protocol
also carries information on the precedence, security classification
and compartmentation of the TCP segments, so this information can be
communicated end-to-end across multiple networks.
                         Protocol Layering
                      +---------------------+
                      |     higher-level    |
                      +---------------------+
                      |        TCP          |
                      +---------------------+
                      |  internet protocol  |
                      +---------------------+
                      |communication network|
                      +---------------------+
                              Figure 1
Much of this document is written in the context of TCP implementations
which are co-resident with higher level protocols in the host
computer.  Some computer systems will be connected to networks via
front-end computers which house the TCP and internet protocol layers,
as well as network specific software.  The TCP specification describes
an interface to the higher level protocols which appears to be
implementable even for the front-end case, as long as a suitable
host-to-front end protocol is implemented.

1.2. Scope

The TCP is intended to provide a reliable process-to-process
communication service in a multinetwork environment.  The TCP is
intended to be a host-to-host protocol in common use in multiple
networks.

1.3. About this Document

This document represents a specification of the behavior required of
any TCP implementation, both in its interactions with higher level
protocols and in its interactions with other TCPs.  The rest of this

[Page 2]

September 1981

                                         Transmission Control Protocol
                                                          Introduction
section offers a very brief view of the protocol interfaces and
operation.  Section 2 summarizes the philosophical basis for the TCP
design.  Section 3 offers both a detailed description of the actions
required of TCP when various events occur (arrival of new segments,
user calls, errors, etc.) and the details of the formats of TCP
segments.

1.4. Interfaces

The TCP interfaces on one side to user or application processes and on
the other side to a lower level protocol such as Internet Protocol.
The interface between an application process and the TCP is
illustrated in reasonable detail.  This interface consists of a set of
calls much like the calls an operating system provides to an
application process for manipulating files.  For example, there are
calls to open and close connections and to send and receive data on
established connections.  It is also expected that the TCP can
asynchronously communicate with application programs.  Although
considerable freedom is permitted to TCP implementors to design
interfaces which are appropriate to a particular operating system
environment, a minimum functionality is required at the TCP/user
interface for any valid implementation.
The interface between TCP and lower level protocol is essentially
unspecified except that it is assumed there is a mechanism whereby the
two levels can asynchronously pass information to each other.
Typically, one expects the lower level protocol to specify this
interface.  TCP is designed to work in a very general environment of
interconnected networks.  The lower level protocol which is assumed
throughout this document is the Internet Protocol [2].

1.5. Operation

As noted above, the primary purpose of the TCP is to provide reliable,
securable logical circuit or connection service between pairs of
processes.  To provide this service on top of a less reliable internet
communication system requires facilities in the following areas:
  Basic Data Transfer
  Reliability
  Flow Control
  Multiplexing
  Connections
  Precedence and Security
The basic operation of the TCP in each of these areas is described in
the following paragraphs.
                                                              [Page 3]
                                                        September 1981

Transmission Control Protocol Introduction

Basic Data Transfer:
  The TCP is able to transfer a continuous stream of octets in each
  direction between its users by packaging some number of octets into
  segments for transmission through the internet system.  In general,
  the TCPs decide when to block and forward data at their own
  convenience.
  Sometimes users need to be sure that all the data they have
  submitted to the TCP has been transmitted.  For this purpose a push
  function is defined.  To assure that data submitted to a TCP is
  actually transmitted the sending user indicates that it should be
  pushed through to the receiving user.  A push causes the TCPs to
  promptly forward and deliver data up to that point to the receiver.
  The exact push point might not be visible to the receiving user and
  the push function does not supply a record boundary marker.
Reliability:
  The TCP must recover from data that is damaged, lost, duplicated, or
  delivered out of order by the internet communication system.  This
  is achieved by assigning a sequence number to each octet
  transmitted, and requiring a positive acknowledgment (ACK) from the
  receiving TCP.  If the ACK is not received within a timeout
  interval, the data is retransmitted.  At the receiver, the sequence
  numbers are used to correctly order segments that may be received
  out of order and to eliminate duplicates.  Damage is handled by
  adding a checksum to each segment transmitted, checking it at the
  receiver, and discarding damaged segments.
  As long as the TCPs continue to function properly and the internet
  system does not become completely partitioned, no transmission
  errors will affect the correct delivery of data.  TCP recovers from
  internet communication system errors.
Flow Control:
  TCP provides a means for the receiver to govern the amount of data
  sent by the sender.  This is achieved by returning a "window" with
  every ACK indicating a range of acceptable sequence numbers beyond
  the last segment successfully received.  The window indicates an
  allowed number of octets that the sender may transmit before
  receiving further permission.

[Page 4]

September 1981

                                         Transmission Control Protocol
                                                          Introduction
Multiplexing:
  To allow for many processes within a single Host to use TCP
  communication facilities simultaneously, the TCP provides a set of
  addresses or ports within each host.  Concatenated with the network
  and host addresses from the internet communication layer, this forms
  a socket.  A pair of sockets uniquely identifies each connection.
  That is, a socket may be simultaneously used in multiple
  connections.
  The binding of ports to processes is handled independently by each
  Host.  However, it proves useful to attach frequently used processes
  (e.g., a "logger" or timesharing service) to fixed sockets which are
  made known to the public.  These services can then be accessed
  through the known addresses.  Establishing and learning the port
  addresses of other processes may involve more dynamic mechanisms.
Connections:
  The reliability and flow control mechanisms described above require
  that TCPs initialize and maintain certain status information for
  each data stream.  The combination of this information, including
  sockets, sequence numbers, and window sizes, is called a connection.
  Each connection is uniquely specified by a pair of sockets
  identifying its two sides.
  When two processes wish to communicate, their TCP's must first
  establish a connection (initialize the status information on each
  side).  When their communication is complete, the connection is
  terminated or closed to free the resources for other uses.
  Since connections must be established between unreliable hosts and
  over the unreliable internet communication system, a handshake
  mechanism with clock-based sequence numbers is used to avoid
  erroneous initialization of connections.
Precedence and Security:
  The users of TCP may indicate the security and precedence of their
  communication.  Provision is made for default values to be used when
  these features are not needed.
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Transmission Control Protocol

[Page 6]

September 1981

                                         Transmission Control Protocol
                           2.  PHILOSOPHY

2.1. Elements of the Internetwork System

The internetwork environment consists of hosts connected to networks
which are in turn interconnected via gateways.  It is assumed here
that the networks may be either local networks (e.g., the ETHERNET) or
large networks (e.g., the ARPANET), but in any case are based on
packet switching technology.  The active agents that produce and
consume messages are processes.  Various levels of protocols in the
networks, the gateways, and the hosts support an interprocess
communication system that provides two-way data flow on logical
connections between process ports.
The term packet is used generically here to mean the data of one
transaction between a host and its network.  The format of data blocks
exchanged within the a network will generally not be of concern to us.
Hosts are computers attached to a network, and from the communication
network's point of view, are the sources and destinations of packets.
Processes are viewed as the active elements in host computers (in
accordance with the fairly common definition of a process as a program
in execution).  Even terminals and files or other I/O devices are
viewed as communicating with each other through the use of processes.
Thus, all communication is viewed as inter-process communication.
Since a process may need to distinguish among several communication
streams between itself and another process (or processes), we imagine
that each process may have a number of ports through which it
communicates with the ports of other processes.

2.2. Model of Operation

Processes transmit data by calling on the TCP and passing buffers of
data as arguments.  The TCP packages the data from these buffers into
segments and calls on the internet module to transmit each segment to
the destination TCP.  The receiving TCP places the data from a segment
into the receiving user's buffer and notifies the receiving user.  The
TCPs include control information in the segments which they use to
ensure reliable ordered data transmission.
The model of internet communication is that there is an internet
protocol module associated with each TCP which provides an interface
to the local network.  This internet module packages TCP segments
inside internet datagrams and routes these datagrams to a destination
internet module or intermediate gateway.  To transmit the datagram
through the local network, it is embedded in a local network packet.
The packet switches may perform further packaging, fragmentation, or
                                                              [Page 7]
                                                        September 1981

Transmission Control Protocol Philosophy

other operations to achieve the delivery of the local packet to the
destination internet module.
At a gateway between networks, the internet datagram is "unwrapped"
from its local packet and examined to determine through which network
the internet datagram should travel next.  The internet datagram is
then "wrapped" in a local packet suitable to the next network and
routed to the next gateway, or to the final destination.
A gateway is permitted to break up an internet datagram into smaller
internet datagram fragments if this is necessary for transmission
through the next network.  To do this, the gateway produces a set of
internet datagrams; each carrying a fragment.  Fragments may be
further broken into smaller fragments at subsequent gateways.  The
internet datagram fragment format is designed so that the destination
internet module can reassemble fragments into internet datagrams.
A destination internet module unwraps the segment from the datagram
(after reassembling the datagram, if necessary) and passes it to the
destination TCP.
This simple model of the operation glosses over many details.  One
important feature is the type of service.  This provides information
to the gateway (or internet module) to guide it in selecting the
service parameters to be used in traversing the next network.
Included in the type of service information is the precedence of the
datagram.  Datagrams may also carry security information to permit
host and gateways that operate in multilevel secure environments to
properly segregate datagrams for security considerations.

2.3. The Host Environment

The TCP is assumed to be a module in an operating system.  The users
access the TCP much like they would access the file system.  The TCP
may call on other operating system functions, for example, to manage
data structures.  The actual interface to the network is assumed to be
controlled by a device driver module.  The TCP does not call on the
network device driver directly, but rather calls on the internet
datagram protocol module which may in turn call on the device driver.
The mechanisms of TCP do not preclude implementation of the TCP in a
front-end processor.  However, in such an implementation, a
host-to-front-end protocol must provide the functionality to support
the type of TCP-user interface described in this document.

[Page 8]

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                                         Transmission Control Protocol
                                                            Philosophy

2.4. Interfaces

The TCP/user interface provides for calls made by the user on the TCP
to OPEN or CLOSE a connection, to SEND or RECEIVE data, or to obtain
STATUS about a connection.  These calls are like other calls from user
programs on the operating system, for example, the calls to open, read
from, and close a file.
The TCP/internet interface provides calls to send and receive
datagrams addressed to TCP modules in hosts anywhere in the internet
system.  These calls have parameters for passing the address, type of
service, precedence, security, and other control information.

2.5. Relation to Other Protocols

The following diagram illustrates the place of the TCP in the protocol
hierarchy:
                                  
     +------+ +-----+ +-----+       +-----+                    
     |Telnet| | FTP | |Voice|  ...  |     |  Application Level 
     +------+ +-----+ +-----+       +-----+                    
           |   |         |             |                       
          +-----+     +-----+       +-----+                    
          | TCP |     | RTP |  ...  |     |  Host Level        
          +-----+     +-----+       +-----+                    
             |           |             |                       
          +-------------------------------+                    
          |    Internet Protocol & ICMP   |  Gateway Level     
          +-------------------------------+                    
                         |                                     
            +---------------------------+                      
            |   Local Network Protocol  |    Network Level     
            +---------------------------+                      
                       Protocol Relationships
                             Figure 2.
It is expected that the TCP will be able to support higher level
protocols efficiently.  It should be easy to interface higher level
protocols like the ARPANET Telnet or AUTODIN II THP to the TCP.

2.6. Reliable Communication

A stream of data sent on a TCP connection is delivered reliably and in
order at the destination.
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Transmission Control Protocol Philosophy

Transmission is made reliable via the use of sequence numbers and
acknowledgments.  Conceptually, each octet of data is assigned a
sequence number.  The sequence number of the first octet of data in a
segment is transmitted with that segment and is called the segment
sequence number.  Segments also carry an acknowledgment number which
is the sequence number of the next expected data octet of
transmissions in the reverse direction.  When the TCP transmits a
segment containing data, it puts a copy on a retransmission queue and
starts a timer; when the acknowledgment for that data is received, the
segment is deleted from the queue.  If the acknowledgment is not
received before the timer runs out, the segment is retransmitted.
An acknowledgment by TCP does not guarantee that the data has been
delivered to the end user, but only that the receiving TCP has taken
the responsibility to do so.
To govern the flow of data between TCPs, a flow control mechanism is
employed.  The receiving TCP reports a "window" to the sending TCP.
This window specifies the number of octets, starting with the
acknowledgment number, that the receiving TCP is currently prepared to
receive.

2.7. Connection Establishment and Clearing

To identify the separate data streams that a TCP may handle, the TCP
provides a port identifier.  Since port identifiers are selected
independently by each TCP they might not be unique.  To provide for
unique addresses within each TCP, we concatenate an internet address
identifying the TCP with a port identifier to create a socket which
will be unique throughout all networks connected together.
A connection is fully specified by the pair of sockets at the ends.  A
local socket may participate in many connections to different foreign
sockets.  A connection can be used to carry data in both directions,
that is, it is "full duplex".
TCPs are free to associate ports with processes however they choose.
However, several basic concepts are necessary in any implementation.
There must be well-known sockets which the TCP associates only with
the "appropriate" processes by some means.  We envision that processes
may "own" ports, and that processes can initiate connections only on
the ports they own.  (Means for implementing ownership is a local
issue, but we envision a Request Port user command, or a method of
uniquely allocating a group of ports to a given process, e.g., by
associating the high order bits of a port name with a given process.)
A connection is specified in the OPEN call by the local port and
foreign socket arguments.  In return, the TCP supplies a (short) local

[Page 10]

September 1981

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

If there are several pending passive OPENs (recorded in TCBs) with the
same local socket, an foreign active OPEN will be matched to a TCB
with the specific foreign socket in the foreign active OPEN, if such a
TCB exists, before selecting a TCB with an unspecified foreign socket.
The procedures to establish connections utilize the synchronize (SYN)
control flag and involves an exchange of three messages.  This
exchange has been termed a three-way hand shake [3].
A connection is initiated by the rendezvous of an arriving segment
containing a SYN and a waiting TCB entry each created by a user OPEN
command.  The matching of local and foreign sockets determines when a
connection has been initiated.  The connection becomes "established"
when sequence numbers have been synchronized in both directions.
The clearing of a connection also involves the exchange of segments,
in this case carrying the FIN control flag.

2.8. Data Communication

The data that flows on a connection may be thought of as a stream of
octets.  The sending user indicates in each SEND call whether the data
in that call (and any preceeding calls) should be immediately pushed
through to the receiving user by the setting of the PUSH flag.
A sending TCP is allowed to collect data from the sending user and to
send that data in segments at its own convenience, until the push
function is signaled, then it must send all unsent data.  When a
receiving TCP sees the PUSH flag, it must not wait for more data from
the sending TCP before passing the data to the receiving process.
There is no necessary relationship between push functions and segment
boundaries.  The data in any particular segment may be the result of a
single SEND call, in whole or part, or of multiple SEND calls.
The purpose of push function and the PUSH flag is to push data through
from the sending user to the receiving user.  It does not provide a
record service.
There is a coupling between the push function and the use of buffers
of data that cross the TCP/user interface.  Each time a PUSH flag is
associated with data placed into the receiving user's buffer, the
buffer is returned to the user for processing even if the buffer is
not filled.  If data arrives that fills the user's buffer before a
PUSH is seen, the data is passed to the user in buffer size units.
TCP also provides a means to communicate to the receiver of data that
at some point further along in the data stream than the receiver is

[Page 12]

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                                         Transmission Control Protocol
                                                            Philosophy
currently reading there is urgent data.  TCP does not attempt to
define what the user specifically does upon being notified of pending
urgent data, but the general notion is that the receiving process will
take action to process the urgent data quickly.

2.9. Precedence and Security

The TCP makes use of the internet protocol type of service field and
security option to provide precedence and security on a per connection
basis to TCP users.  Not all TCP modules will necessarily function in
a multilevel secure environment; some may be limited to unclassified
use only, and others may operate at only one security level and
compartment.  Consequently, some TCP implementations and services to
users may be limited to a subset of the multilevel secure case.
TCP modules which operate in a multilevel secure environment must
properly mark outgoing segments with the security, compartment, and
precedence.  Such TCP modules must also provide to their users or
higher level protocols such as Telnet or THP an interface to allow
them to specify the desired security level, compartment, and
precedence of connections.

2.10. Robustness Principle

TCP implementations will follow a general principle of robustness:  be
conservative in what you do, be liberal in what you accept from
others.
                                                             [Page 13]
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Transmission Control Protocol

[Page 14]

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                                         Transmission Control Protocol
                    3.  FUNCTIONAL SPECIFICATION

3.1. Header Format

TCP segments are sent as internet datagrams.  The Internet Protocol
header carries several information fields, including the source and
destination host addresses [2].  A TCP header follows the internet
header, supplying information specific to the TCP protocol.  This
division allows for the existence of host level protocols other than
TCP.
TCP Header Format
                                  
  0                   1                   2                   3   
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |          Source Port          |       Destination Port        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                        Sequence Number                        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                    Acknowledgment Number                      |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |  Data |           |U|A|P|R|S|F|                               |
 | Offset| Reserved  |R|C|S|S|Y|I|            Window             |
 |       |           |G|K|H|T|N|N|                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |           Checksum            |         Urgent Pointer        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                    Options                    |    Padding    |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                             data                              |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                          TCP Header Format
        Note that one tick mark represents one bit position.
                             Figure 3.
Source Port:  16 bits
  The source port number.
Destination Port:  16 bits
  The destination port number.
                                                             [Page 15]
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Transmission Control Protocol Functional Specification

Sequence Number:  32 bits
  The sequence number of the first data octet in this segment (except
  when SYN is present). If SYN is present the sequence number is the
  initial sequence number (ISN) and the first data octet is ISN+1.
Acknowledgment Number:  32 bits
  If the ACK control bit is set this field contains the value of the
  next sequence number the sender of the segment is expecting to
  receive.  Once a connection is established this is always sent.
Data Offset:  4 bits
  The number of 32 bit words in the TCP Header.  This indicates where
  the data begins.  The TCP header (even one including options) is an
  integral number of 32 bits long.
Reserved:  6 bits
  Reserved for future use.  Must be zero.
Control Bits:  6 bits (from left to right):
  URG:  Urgent Pointer field significant
  ACK:  Acknowledgment field significant
  PSH:  Push Function
  RST:  Reset the connection
  SYN:  Synchronize sequence numbers
  FIN:  No more data from sender
Window:  16 bits
  The number of data octets beginning with the one indicated in the
  acknowledgment field which the sender of this segment is willing to
  accept.
Checksum:  16 bits
  The checksum field is the 16 bit one's complement of the one's
  complement sum of all 16 bit words in the header and text.  If a
  segment contains an odd number of header and text octets to be
  checksummed, the last octet is padded on the right with zeros to
  form a 16 bit word for checksum purposes.  The pad is not
  transmitted as part of the segment.  While computing the checksum,
  the checksum field itself is replaced with zeros.
  The checksum also covers a 96 bit pseudo header conceptually

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                                         Transmission Control Protocol
                                              Functional Specification
  prefixed to the TCP header.  This pseudo header contains the Source
  Address, the Destination Address, the Protocol, and TCP length.
  This gives the TCP protection against misrouted segments.  This
  information is carried in the Internet Protocol and is transferred
  across the TCP/Network interface in the arguments or results of
  calls by the TCP on the IP.
                   +--------+--------+--------+--------+
                   |           Source Address          |
                   +--------+--------+--------+--------+
                   |         Destination Address       |
                   +--------+--------+--------+--------+
                   |  zero  |  PTCL  |    TCP Length   |
                   +--------+--------+--------+--------+
    The TCP Length is the TCP header length plus the data length in
    octets (this is not an explicitly transmitted quantity, but is
    computed), and it does not count the 12 octets of the pseudo
    header.
Urgent Pointer:  16 bits
  This field communicates the current value of the urgent pointer as a
  positive offset from the sequence number in this segment.  The
  urgent pointer points to the sequence number of the octet following
  the urgent data.  This field is only be interpreted in segments with
  the URG control bit set.
Options:  variable
  Options may occupy space at the end of the TCP header and are a
  multiple of 8 bits in length.  All options are included in the
  checksum.  An option may begin on any octet boundary.  There are two
  cases for the format of an option:
    Case 1:  A single octet of option-kind.
    Case 2:  An octet of option-kind, an octet of option-length, and
             the actual option-data octets.
  The option-length counts the two octets of option-kind and
  option-length as well as the option-data octets.
  Note that the list of options may be shorter than the data offset
  field might imply.  The content of the header beyond the
  End-of-Option option must be header padding (i.e., zero).
  A TCP must implement all options.
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  Currently defined options include (kind indicated in octal):
    Kind     Length    Meaning
    ----     ------    -------
     0         -       End of option list.
     1         -       No-Operation.
     2         4       Maximum Segment Size.
    
  Specific Option Definitions
    End of Option List
      +--------+
      |00000000|
      +--------+
       Kind=0
      This option code indicates the end of the option list.  This
      might not coincide with the end of the TCP header according to
      the Data Offset field.  This is used at the end of all options,
      not the end of each option, and need only be used if the end of
      the options would not otherwise coincide with the end of the TCP
      header.
    No-Operation
      +--------+
      |00000001|
      +--------+
       Kind=1
      This option code may be used between options, for example, to
      align the beginning of a subsequent option on a word boundary.
      There is no guarantee that senders will use this option, so
      receivers must be prepared to process options even if they do
      not begin on a word boundary.
    Maximum Segment Size
      +--------+--------+---------+--------+
      |00000010|00000100|   max seg size   |
      +--------+--------+---------+--------+
       Kind=2   Length=4

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                                         Transmission Control Protocol
                                              Functional Specification
      Maximum Segment Size Option Data:  16 bits
        If this option is present, then it communicates the maximum
        receive segment size at the TCP which sends this segment.
        This field must only be sent in the initial connection request
        (i.e., in segments with the SYN control bit set).  If this
        option is not used, any segment size is allowed.
Padding:  variable
  The TCP header padding is used to ensure that the TCP header ends
  and data begins on a 32 bit boundary.  The padding is composed of
  zeros.

3.2. Terminology

Before we can discuss very much about the operation of the TCP we need
to introduce some detailed terminology.  The maintenance of a TCP
connection requires the remembering of several variables.  We conceive
of these variables being stored in a connection record called a
Transmission Control Block or TCB.  Among the variables stored in the
TCB are the local and remote socket numbers, the security and
precedence of the connection, pointers to the user's send and receive
buffers, pointers to the retransmit queue and to the current segment.
In addition several variables relating to the send and receive
sequence numbers are stored in the TCB.
  Send Sequence Variables
    SND.UNA - send unacknowledged
    SND.NXT - send next
    SND.WND - send window
    SND.UP  - send urgent pointer
    SND.WL1 - segment sequence number used for last window update
    SND.WL2 - segment acknowledgment number used for last window
              update
    ISS     - initial send sequence number
  Receive Sequence Variables
    RCV.NXT - receive next
    RCV.WND - receive window
    RCV.UP  - receive urgent pointer
    IRS     - initial receive sequence number
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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.
  
  
The send window is the portion of the sequence space labeled 3 in
figure 4.
Receive Sequence Space
                     1          2          3      
                 ----------|----------|---------- 
                        RCV.NXT    RCV.NXT        
                                  +RCV.WND        
      1 - old sequence numbers which have been acknowledged  
      2 - sequence numbers allowed for new reception         
      3 - future sequence numbers which are not yet allowed  
                       Receive Sequence Space
                             Figure 5.
  
  
The receive window is the portion of the sequence space labeled 2 in
figure 5.
There are also some variables used frequently in the discussion that
take their values from the fields of the current segment.

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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, CLOSE-WAIT, CLOSING, LAST-ACK,
TIME-WAIT, and the fictional state CLOSED.  CLOSED is fictional
because it represents the state when there is no TCB, and therefore,
no connection.  Briefly the meanings of the states are:
  LISTEN - represents waiting for a connection request from any remote
  TCP and port.
  SYN-SENT - represents waiting for a matching connection request
  after having sent a connection request.
  SYN-RECEIVED - represents waiting for a confirming connection
  request acknowledgment after having both received and sent a
  connection request.
  ESTABLISHED - represents an open connection, data received can be
  delivered to the user.  The normal state for the data transfer phase
  of the connection.
  FIN-WAIT-1 - represents waiting for a connection termination request
  from the remote TCP, or an acknowledgment of the connection
  termination request previously sent.
  FIN-WAIT-2 - represents waiting for a connection termination request
  from the remote TCP.
  CLOSE-WAIT - represents waiting for a connection termination request
  from the local user.
  CLOSING - represents waiting for a connection termination request
  acknowledgment from the remote TCP.
  LAST-ACK - represents waiting for an acknowledgment of the
  connection termination request previously sent to the remote TCP
  (which includes an acknowledgment of its connection termination
  request).
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  TIME-WAIT - represents waiting for enough time to pass to be sure
  the remote TCP received the acknowledgment of its connection
  termination request.
  CLOSED - represents no connection state at all.
A TCP connection progresses from one state to another in response to
events.  The events are the user calls, OPEN, SEND, RECEIVE, CLOSE,
ABORT, and STATUS; the incoming segments, particularly those
containing the SYN, ACK, RST and FIN flags; and timeouts.
The state diagram in figure 6 illustrates only state changes, together
with the causing events and resulting actions, but addresses neither
error conditions nor actions which are not connected with state
changes.  In a later section, more detail is offered with respect to
the reaction of the TCP to events.
NOTE BENE:  this diagram is only a summary and must not be taken as
the total specification.

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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 \ +———+

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

+———+ +———+ +———+ |FINWAIT-2| | CLOSING | | LAST-ACK| +———+ +———+ +———+

 |                rcv ACK of FIN |                 rcv ACK of FIN |  
 |  rcv FIN       -------------- |    Timeout=2MSL -------------- |  
 |  -------              x       V    ------------        x       V  
  \ snd ACK                 +---------+delete TCB         +---------+
   ------------------------>|TIME WAIT|------------------>| CLOSED  |
                            +---------+                   +---------+
                    TCP Connection State Diagram
                             Figure 6.
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3.3. Sequence Numbers

A fundamental notion in the design is that every octet of data sent
over a TCP connection has a sequence number.  Since every octet is
sequenced, each of them can be acknowledged.  The acknowledgment
mechanism employed is cumulative so that an acknowledgment of sequence
number X indicates that all octets up to but not including X have been
received.  This mechanism allows for straight-forward duplicate
detection in the presence of retransmission.  Numbering of octets
within a segment is that the first data octet immediately following
the header is the lowest numbered, and the following octets are
numbered consecutively.
It is essential to remember that the actual sequence number space is
finite, though very large.  This space ranges from 0 to 2**32 - 1.
Since the space is finite, all arithmetic dealing with sequence
numbers must be performed modulo 2**32.  This unsigned arithmetic
preserves the relationship of sequence numbers as they cycle from
2**32 - 1 to 0 again.  There are some subtleties to computer modulo
arithmetic, so great care should be taken in programming the
comparison of such values.  The symbol "=<" means "less than or equal"
(modulo 2**32).
The typical kinds of sequence number comparisons which the TCP must
perform include:
  (a)  Determining that an acknowledgment refers to some sequence
       number sent but not yet acknowledged.
  (b)  Determining that all sequence numbers occupied by a segment
       have been acknowledged (e.g., to remove the segment from a
       retransmission queue).
  (c)  Determining that an incoming segment contains sequence numbers
       which are expected (i.e., that the segment "overlaps" the
       receive window).

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                                         Transmission Control Protocol
                                              Functional Specification
In response to sending data the TCP will receive acknowledgments.  The
following comparisons are needed to process the acknowledgments.
  SND.UNA = oldest unacknowledged sequence number
  SND.NXT = next sequence number to be sent
  SEG.ACK = acknowledgment from the receiving TCP (next sequence
            number expected by the receiving TCP)
  SEG.SEQ = first sequence number of a segment
  SEG.LEN = the number of octets occupied by the data in the segment
            (counting SYN and FIN)
  SEG.SEQ+SEG.LEN-1 = last sequence number of a segment
A new acknowledgment (called an "acceptable ack"), is one for which
the inequality below holds:
  SND.UNA < SEG.ACK =< SND.NXT
A segment on the retransmission queue is fully acknowledged if the sum
of its sequence number and length is less or equal than the
acknowledgment value in the incoming segment.
When data is received the following comparisons are needed:
  RCV.NXT = next sequence number expected on an incoming segments, and
      is the left or lower edge of the receive window
  RCV.NXT+RCV.WND-1 = last sequence number expected on an incoming
      segment, and is the right or upper edge of the receive window
  SEG.SEQ = first sequence number occupied by the incoming segment
  SEG.SEQ+SEG.LEN-1 = last sequence number occupied by the incoming
      segment
A segment is judged to occupy a portion of valid receive sequence
space if
  RCV.NXT =< SEG.SEQ < RCV.NXT+RCV.WND
or
  RCV.NXT =< SEG.SEQ+SEG.LEN-1 < RCV.NXT+RCV.WND
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The first part of this test checks to see if the beginning of the
segment falls in the window, the second part of the test checks to see
if the end of the segment falls in the window; if the segment passes
either part of the test it contains data in the window.
Actually, it is a little more complicated than this.  Due to zero
windows and zero length segments, we have four cases for the
acceptability of an incoming segment:
  Segment Receive  Test
  Length  Window
  ------- -------  -------------------------------------------
     0       0     SEG.SEQ = RCV.NXT
     0      >0     RCV.NXT =< SEG.SEQ < RCV.NXT+RCV.WND
    >0       0     not acceptable
    >0      >0     RCV.NXT =< SEG.SEQ < RCV.NXT+RCV.WND
                or RCV.NXT =< SEG.SEQ+SEG.LEN-1 < RCV.NXT+RCV.WND
Note that when the receive window is zero no segments should be
acceptable except ACK segments.  Thus, it is be possible for a TCP to
maintain a zero receive window while transmitting data and receiving
ACKs.  However, even when the receive window is zero, a TCP must
process the RST and URG fields of all incoming segments.
We have taken advantage of the numbering scheme to protect certain
control information as well.  This is achieved by implicitly including
some control flags in the sequence space so they can be retransmitted
and acknowledged without confusion (i.e., one and only one copy of the
control will be acted upon).  Control information is not physically
carried in the segment data space.  Consequently, we must adopt rules
for implicitly assigning sequence numbers to control.  The SYN and FIN
are the only controls requiring this protection, and these controls
are used only at connection opening and closing.  For sequence number
purposes, the SYN is considered to occur before the first actual data
octet of the segment in which it occurs, while the FIN is considered
to occur after the last actual data octet in a segment in which it
occurs.  The segment length (SEG.LEN) includes both data and sequence
space occupying controls.  When a SYN is present then SEG.SEQ is the
sequence number of the SYN.

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                                         Transmission Control Protocol
                                              Functional Specification
Initial Sequence Number Selection
The protocol places no restriction on a particular connection being
used over and over again.  A connection is defined by a pair of
sockets.  New instances of a connection will be referred to as
incarnations of the connection.  The problem that arises from this is
-- "how does the TCP identify duplicate segments from previous
incarnations of the connection?"  This problem becomes apparent if the
connection is being opened and closed in quick succession, or if the
connection breaks with loss of memory and is then reestablished.
To avoid confusion we must prevent segments from one incarnation of a
connection from being used while the same sequence numbers may still
be present in the network from an earlier incarnation.  We want to
assure this, even if a TCP crashes and loses all knowledge of the
sequence numbers it has been using.  When new connections are created,
an initial sequence number (ISN) generator is employed which selects a
new 32 bit ISN.  The generator is bound to a (possibly fictitious) 32
bit clock whose low order bit is incremented roughly every 4
microseconds.  Thus, the ISN cycles approximately every 4.55 hours.
Since we assume that segments will stay in the network no more than
the Maximum Segment Lifetime (MSL) and that the MSL is less than 4.55
hours we can reasonably assume that ISN's will be unique.
For each connection there is a send sequence number and a receive
sequence number.  The initial send sequence number (ISS) is chosen by
the data sending TCP, and the initial receive sequence number (IRS) is
learned during the connection establishing procedure.
For a connection to be established or initialized, the two TCPs must
synchronize on each other's initial sequence numbers.  This is done in
an exchange of connection establishing segments carrying a control bit
called "SYN" (for synchronize) and the initial sequence numbers.  As a
shorthand, segments carrying the SYN bit are also called "SYNs".
Hence, the solution requires a suitable mechanism for picking an
initial sequence number and a slightly involved handshake to exchange
the ISN's.
The synchronization requires each side to send it's own initial
sequence number and to receive a confirmation of it in acknowledgment
from the other side.  Each side must also receive the other side's
initial sequence number and send a confirming acknowledgment.
  1) A --> B  SYN my sequence number is X
  2) A <-- B  ACK your sequence number is X
  3) A <-- B  SYN my sequence number is Y
  4) A --> B  ACK your sequence number is Y
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Because steps 2 and 3 can be combined in a single message this is
called the three way (or three message) handshake.
A three way handshake is necessary because sequence numbers are not
tied to a global clock in the network, and TCPs may have different
mechanisms for picking the ISN's.  The receiver of the first SYN has
no way of knowing whether the segment was an old delayed one or not,
unless it remembers the last sequence number used on the connection
(which is not always possible), and so it must ask the sender to
verify this SYN.  The three way handshake and the advantages of a
clock-driven scheme are discussed in [3].
Knowing When to Keep Quiet
To be sure that a TCP does not create a segment that carries a
sequence number which may be duplicated by an old segment remaining in
the network, the TCP must keep quiet for a maximum segment lifetime
(MSL) before assigning any sequence numbers upon starting up or
recovering from a crash in which memory of sequence numbers in use was
lost.  For this specification the MSL is taken to be 2 minutes.  This
is an engineering choice, and may be changed if experience indicates
it is desirable to do so.  Note that if a TCP is reinitialized in some
sense, yet retains its memory of sequence numbers in use, then it need
not wait at all; it must only be sure to use sequence numbers larger
than those recently used.
The TCP Quiet Time Concept
  This specification provides that hosts which "crash" without
  retaining any knowledge of the last sequence numbers transmitted on
  each active (i.e., not closed) connection shall delay emitting any
  TCP segments for at least the agreed Maximum Segment Lifetime (MSL)
  in the internet system of which the host is a part.  In the
  paragraphs below, an explanation for this specification is given.
  TCP implementors may violate the "quiet time" restriction, but only
  at the risk of causing some old data to be accepted as new or new
  data rejected as old duplicated by some receivers in the internet
  system.
  TCPs consume sequence number space each time a segment is formed and
  entered into the network output queue at a source host. The
  duplicate detection and sequencing algorithm in the TCP protocol
  relies on the unique binding of segment data to sequence space to
  the extent that sequence numbers will not cycle through all 2**32
  values before the segment data bound to those sequence numbers has
  been delivered and acknowledged by the receiver and all duplicate
  copies of the segments have "drained" from the internet.  Without
  such an assumption, two distinct TCP segments could conceivably be

[Page 28]

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                                         Transmission Control Protocol
                                              Functional Specification
  assigned the same or overlapping sequence numbers, causing confusion
  at the receiver as to which data is new and which is old.  Remember
  that each segment is bound to as many consecutive sequence numbers
  as there are octets of data in the segment.
  Under normal conditions, TCPs keep track of the next sequence number
  to emit and the oldest awaiting acknowledgment so as to avoid
  mistakenly using a sequence number over before its first use has
  been acknowledged.  This alone does not guarantee that old duplicate
  data is drained from the net, so the sequence space has been made
  very large to reduce the probability that a wandering duplicate will
  cause trouble upon arrival.  At 2 megabits/sec. it takes 4.5 hours
  to use up 2**32 octets of sequence space.  Since the maximum segment
  lifetime in the net is not likely to exceed a few tens of seconds,
  this is deemed ample protection for foreseeable nets, even if data
  rates escalate to l0's of megabits/sec.  At 100 megabits/sec, the
  cycle time is 5.4 minutes which may be a little short, but still
  within reason.
  The basic duplicate detection and sequencing algorithm in TCP can be
  defeated, however, if a source TCP does not have any memory of the
  sequence numbers it last used on a given connection. For example, if
  the TCP were to start all connections with sequence number 0, then
  upon crashing and restarting, a TCP might re-form an earlier
  connection (possibly after half-open connection resolution) and emit
  packets with sequence numbers identical to or overlapping with
  packets still in the network which were emitted on an earlier
  incarnation of the same connection.  In the absence of knowledge
  about the sequence numbers used on a particular connection, the TCP
  specification recommends that the source delay for MSL seconds
  before emitting segments on the connection, to allow time for
  segments from the earlier connection incarnation to drain from the
  system.
  Even hosts which can remember the time of day and used it to select
  initial sequence number values are not immune from this problem
  (i.e., even if time of day is used to select an initial sequence
  number for each new connection incarnation).
  Suppose, for example, that a connection is opened starting with
  sequence number S.  Suppose that this connection is not used much
  and that eventually the initial sequence number function (ISN(t))
  takes on a value equal to the sequence number, say S1, of the last
  segment sent by this TCP on a particular connection.  Now suppose,
  at this instant, the host crashes, recovers, and establishes a new
  incarnation of the connection. The initial sequence number chosen is
  S1 = ISN(t) -- last used sequence number on old incarnation of
  connection!  If the recovery occurs quickly enough, any old
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Transmission Control Protocol Functional Specification

  duplicates in the net bearing sequence numbers in the neighborhood
  of S1 may arrive and be treated as new packets by the receiver of
  the new incarnation of the connection.
  The problem is that the recovering host may not know for how long it
  crashed nor does it know whether there are still old duplicates in
  the system from earlier connection incarnations.
  One way to deal with this problem is to deliberately delay emitting
  segments for one MSL after recovery from a crash- this is the "quite
  time" specification.  Hosts which prefer to avoid waiting are
  willing to risk possible confusion of old and new packets at a given
  destination may choose not to wait for the "quite time".
  Implementors may provide TCP users with the ability to select on a
  connection by connection basis whether to wait after a crash, or may
  informally implement the "quite time" for all connections.
  Obviously, even where a user selects to "wait," this is not
  necessary after the host has been "up" for at least MSL seconds.
  To summarize: every segment emitted occupies one or more sequence
  numbers in the sequence space, the numbers occupied by a segment are
  "busy" or "in use" until MSL seconds have passed, upon crashing a
  block of space-time is occupied by the octets of the last emitted
  segment, if a new connection is started too soon and uses any of the
  sequence numbers in the space-time footprint of the last segment of
  the previous connection incarnation, there is a potential sequence
  number overlap area which could cause confusion at the receiver.

3.4. Establishing a connection

The "three-way handshake" is the procedure used to establish a
connection.  This procedure normally is initiated by one TCP and
responded to by another TCP.  The procedure also works if two TCP
simultaneously initiate the procedure.  When simultaneous attempt
occurs, each TCP receives a "SYN" segment which carries no
acknowledgment after it has sent a "SYN".  Of course, the arrival of
an old duplicate "SYN" segment can potentially make it appear, to the
recipient, that a simultaneous connection initiation is in progress.
Proper use of "reset" segments can disambiguate these cases.
Several examples of connection initiation follow.  Although these
examples do not show connection synchronization using data-carrying
segments, this is perfectly legitimate, so long as the receiving TCP
doesn't deliver the data to the user until it is clear the data is
valid (i.e., the data must be buffered at the receiver until the
connection reaches the ESTABLISHED state).  The three-way handshake
reduces the possibility of false connections.  It is the

[Page 30]

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                                         Transmission Control Protocol
                                              Functional Specification
implementation of a trade-off between memory and messages to provide
information for this checking.
The simplest three-way handshake is shown in figure 7 below.  The
figures should be interpreted in the following way.  Each line is
numbered for reference purposes.  Right arrows (-->) indicate
departure of a TCP segment from TCP A to TCP B, or arrival of a
segment at B from A.  Left arrows (<--), indicate the reverse.
Ellipsis (...) indicates a segment which is still in the network
(delayed).  An "XXX" indicates a segment which is lost or rejected.
Comments appear in parentheses.  TCP states represent the state AFTER
the departure or arrival of the segment (whose contents are shown in
the center of each line).  Segment contents are shown in abbreviated
form, with sequence number, control flags, and ACK field.  Other
fields such as window, addresses, lengths, and text have been left out
in the interest of clarity.
    TCP A                                                TCP B
1.  CLOSED                                               LISTEN
2.  SYN-SENT    --> <SEQ=100><CTL=SYN>               --> SYN-RECEIVED
3.  ESTABLISHED <-- <SEQ=300><ACK=101><CTL=SYN,ACK>  <-- SYN-RECEIVED
4.  ESTABLISHED --> <SEQ=101><ACK=301><CTL=ACK>       --> ESTABLISHED
5.  ESTABLISHED --> <SEQ=101><ACK=301><CTL=ACK><DATA> --> ESTABLISHED
        Basic 3-Way Handshake for Connection Synchronization
                              Figure 7.
In line 2 of figure 7, TCP A begins by sending a SYN segment
indicating that it will use sequence numbers starting with sequence
number 100.  In line 3, TCP B sends a SYN and acknowledges the SYN it
received from TCP A.  Note that the acknowledgment field indicates TCP
B is now expecting to hear sequence 101, acknowledging the SYN which
occupied sequence 100.
At line 4, TCP A responds with an empty segment containing an ACK for
TCP B's SYN; and in line 5, TCP A sends some data.  Note that the
sequence number of the segment in line 5 is the same as in line 4
because the ACK does not occupy sequence number space (if it did, we
would wind up ACKing ACK's!).
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Simultaneous initiation is only slightly more complex, as is shown in
figure 8.  Each TCP cycles from CLOSED to SYN-SENT to SYN-RECEIVED to
ESTABLISHED.
    TCP A                                            TCP B
1.  CLOSED                                           CLOSED
2.  SYN-SENT     --> <SEQ=100><CTL=SYN>              ...
3.  SYN-RECEIVED <-- <SEQ=300><CTL=SYN>              <-- SYN-SENT
4.               ... <SEQ=100><CTL=SYN>              --> SYN-RECEIVED
5.  SYN-RECEIVED --> <SEQ=100><ACK=301><CTL=SYN,ACK> ...
6.  ESTABLISHED  <-- <SEQ=300><ACK=101><CTL=SYN,ACK> <-- SYN-RECEIVED
7.               ... <SEQ=101><ACK=301><CTL=ACK>     --> ESTABLISHED
              Simultaneous Connection Synchronization
                             Figure 8.
The principle reason for the three-way handshake is to prevent old
duplicate connection initiations from causing confusion.  To deal with
this, a special control message, reset, has been devised.  If the
receiving TCP is in a  non-synchronized state (i.e., SYN-SENT,
SYN-RECEIVED), it returns to LISTEN on receiving an acceptable reset.
If the TCP is in one of the synchronized states (ESTABLISHED,
FIN-WAIT-1, FIN-WAIT-2, CLOSE-WAIT, CLOSING, LAST-ACK, TIME-WAIT), it
aborts the connection and informs its user.  We discuss this latter
case under "half-open" connections below.

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    TCP A                                                TCP B
1.  CLOSED                                               LISTEN
2.  SYN-SENT    --> <SEQ=100><CTL=SYN>               ...
3.  (duplicate) ... <SEQ=90><CTL=SYN>               --> SYN-RECEIVED
4.  SYN-SENT    <-- <SEQ=300><ACK=91><CTL=SYN,ACK>  <-- SYN-RECEIVED
5.  SYN-SENT    --> <SEQ=91><CTL=RST>               --> LISTEN
6.              ... <SEQ=100><CTL=SYN>               --> SYN-RECEIVED
7.  SYN-SENT    <-- <SEQ=400><ACK=101><CTL=SYN,ACK>  <-- SYN-RECEIVED
8.  ESTABLISHED --> <SEQ=101><ACK=401><CTL=ACK>      --> ESTABLISHED
                  Recovery from Old Duplicate SYN
                             Figure 9.
As a simple example of recovery from old duplicates, consider
figure 9.  At line 3, an old duplicate SYN arrives at TCP B.  TCP B
cannot tell that this is an old duplicate, so it responds normally
(line 4).  TCP A detects that the ACK field is incorrect and returns a
RST (reset) with its SEQ field selected to make the segment
believable.  TCP B, on receiving the RST, returns to the LISTEN state.
When the original SYN (pun intended) finally arrives at line 6, the
synchronization proceeds normally.  If the SYN at line 6 had arrived
before the RST, a more complex exchange might have occurred with RST's
sent in both directions.
Half-Open Connections and Other Anomalies
An established connection is said to be  "half-open" if one of the
TCPs has closed or aborted the connection at its end without the
knowledge of the other, or if the two ends of the connection have
become desynchronized owing to a crash that resulted in loss of
memory.  Such connections will automatically become reset if an
attempt is made to send data in either direction.  However, half-open
connections are expected to be unusual, and the recovery procedure is
mildly involved.
If at site A the connection no longer exists, then an attempt by the
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user at site B to send any data on it will result in the site B TCP
receiving a reset control message.  Such a message indicates to the
site B TCP that something is wrong, and it is expected to abort the
connection.
Assume that two user processes A and B are communicating with one
another when a crash occurs causing loss of memory to A's TCP.
Depending on the operating system supporting A's TCP, it is likely
that some error recovery mechanism exists.  When the TCP is up again,
A is likely to start again from the beginning or from a recovery
point.  As a result, A will probably try to OPEN the connection again
or try to SEND on the connection it believes open.  In the latter
case, it receives the error message "connection not open" from the
local (A's) TCP.  In an attempt to establish the connection, A's TCP
will send a segment containing SYN.  This scenario leads to the
example shown in figure 10.  After TCP A crashes, the user attempts to
re-open the connection.  TCP B, in the meantime, thinks the connection
is open.
    TCP A                                           TCP B
1.  (CRASH)                               (send 300,receive 100)
2.  CLOSED                                           ESTABLISHED
3.  SYN-SENT --> <SEQ=400><CTL=SYN>              --> (??)
4.  (!!)     <-- <SEQ=300><ACK=100><CTL=ACK>     <-- ESTABLISHED
5.  SYN-SENT --> <SEQ=100><CTL=RST>              --> (Abort!!)
6.  SYN-SENT                                         CLOSED
7.  SYN-SENT --> <SEQ=400><CTL=SYN>              -->
                   Half-Open Connection Discovery
                             Figure 10.
When the SYN arrives at line 3, TCP B, being in a synchronized state,
and the incoming segment outside the window, responds with an
acknowledgment indicating what sequence it next expects to hear (ACK
100).  TCP A sees that this segment does not acknowledge anything it
sent and, being unsynchronized, sends a reset (RST) because it has
detected a half-open connection.  TCP B aborts at line 5.  TCP A will

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                                              Functional Specification
continue to try to establish the connection; the problem is now
reduced to the basic 3-way handshake of figure 7.
An interesting alternative case occurs when TCP A crashes and TCP B
tries to send data on what it thinks is a synchronized connection.
This is illustrated in figure 11.  In this case, the data arriving at
TCP A from TCP B (line 2) is unacceptable because no such connection
exists, so TCP A sends a RST.  The RST is acceptable so TCP B
processes it and aborts the connection.
      TCP A                                              TCP B
1.  (CRASH)                                   (send 300,receive 100)
2.  (??)    <-- <SEQ=300><ACK=100><DATA=10><CTL=ACK> <-- ESTABLISHED
3.          --> <SEQ=100><CTL=RST>                   --> (ABORT!!)
         Active Side Causes Half-Open Connection Discovery
                             Figure 11.
In figure 12, we find the two TCPs A and B with passive connections
waiting for SYN.  An old duplicate arriving at TCP B (line 2) stirs B
into action.  A SYN-ACK is returned (line 3) and causes TCP A to
generate a RST (the ACK in line 3 is not acceptable).  TCP B accepts
the reset and returns to its passive LISTEN state.
    TCP A                                         TCP B
1.  LISTEN                                        LISTEN
2.       ... <SEQ=Z><CTL=SYN>                -->  SYN-RECEIVED
3.  (??) <-- <SEQ=X><ACK=Z+1><CTL=SYN,ACK>   <--  SYN-RECEIVED
4.       --> <SEQ=Z+1><CTL=RST>              -->  (return to LISTEN!)
5.  LISTEN                                        LISTEN
     Old Duplicate SYN Initiates a Reset on two Passive Sockets
                             Figure 12.
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A variety of other cases are possible, all of which are accounted for
by the following rules for RST generation and processing.
Reset Generation
As a general rule, reset (RST) must be sent whenever a segment arrives
which apparently is not intended for the current connection.  A reset
must not be sent if it is not clear that this is the case.
There are three groups of states:
  1.  If the connection does not exist (CLOSED) then a reset is sent
  in response to any incoming segment except another reset.  In
  particular, SYNs addressed to a non-existent connection are rejected
  by this means.
  If the incoming segment has an ACK field, the reset takes its
  sequence number from the ACK field of the segment, otherwise the
  reset has sequence number zero and the ACK field is set to the sum
  of the sequence number and segment length of the incoming segment.
  The connection remains in the CLOSED state.
  2.  If the connection is in any non-synchronized state (LISTEN,
  SYN-SENT, SYN-RECEIVED), and the incoming segment acknowledges
  something not yet sent (the segment carries an unacceptable ACK), or
  if an incoming segment has a security level or compartment which
  does not exactly match the level and compartment requested for the
  connection, a reset is sent.
  If our SYN has not been acknowledged and the precedence level of the
  incoming segment is higher than the precedence level requested then
  either raise the local precedence level (if allowed by the user and
  the system) or send a reset; or if the precedence level of the
  incoming segment is lower than the precedence level requested then
  continue as if the precedence matched exactly (if the remote TCP
  cannot raise the precedence level to match ours this will be
  detected in the next segment it sends, and the connection will be
  terminated then).  If our SYN has been acknowledged (perhaps in this
  incoming segment) the precedence level of the incoming segment must
  match the local precedence level exactly, if it does not a reset
  must be sent.
  If the incoming segment has an ACK field, the reset takes its
  sequence number from the ACK field of the segment, otherwise the
  reset has sequence number zero and the ACK field is set to the sum
  of the sequence number and segment length of the incoming segment.
  The connection remains in the same state.

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  3.  If the connection is in a synchronized state (ESTABLISHED,
  FIN-WAIT-1, FIN-WAIT-2, CLOSE-WAIT, CLOSING, LAST-ACK, TIME-WAIT),
  any unacceptable segment (out of window sequence number or
  unacceptible acknowledgment number) must elicit only an empty
  acknowledgment segment containing the current send-sequence number
  and an acknowledgment indicating the next sequence number expected
  to be received, and the connection remains in the same state.
  If an incoming segment has a security level, or compartment, or
  precedence which does not exactly match the level, and compartment,
  and precedence requested for the connection,a reset is sent and
  connection goes to the CLOSED state.  The reset takes its sequence
  number from the ACK field of the incoming segment.
Reset Processing
In all states except SYN-SENT, all reset (RST) segments are validated
by checking their SEQ-fields.  A reset is valid if its sequence number
is in the window.  In the SYN-SENT state (a RST received in response
to an initial SYN), the RST is acceptable if the ACK field
acknowledges the SYN.
The receiver of a RST first validates it, then changes state.  If the
receiver was in the LISTEN state, it ignores it.  If the receiver was
in SYN-RECEIVED state and had previously been in the LISTEN state,
then the receiver returns to the LISTEN state, otherwise the receiver
aborts the connection and goes to the CLOSED state.  If the receiver
was in any other state, it aborts the connection and advises the user
and goes to the CLOSED state.

3.5. Closing a Connection

CLOSE is an operation meaning "I have no more data to send."  The
notion of closing a full-duplex connection is subject to ambiguous
interpretation, of course, since it may not be obvious how to treat
the receiving side of the connection.  We have chosen to treat CLOSE
in a simplex fashion.  The user who CLOSEs may continue to RECEIVE
until he is told that the other side has CLOSED also.  Thus, a program
could initiate several SENDs followed by a CLOSE, and then continue to
RECEIVE until signaled that a RECEIVE failed because the other side
has CLOSED.  We assume that the TCP will signal a user, even if no
RECEIVEs are outstanding, that the other side has closed, so the user
can terminate his side gracefully.  A TCP will reliably deliver all
buffers SENT before the connection was CLOSED so a user who expects no
data in return need only wait to hear the connection was CLOSED
successfully to know that all his data was received at the destination
TCP.  Users must keep reading connections they close for sending until
the TCP says no more data.
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There are essentially three cases:
  1) The user initiates by telling the TCP to CLOSE the connection
  2) The remote TCP initiates by sending a FIN control signal
  3) Both users CLOSE simultaneously
Case 1:  Local user initiates the close
  In this case, a FIN segment can be constructed and placed on the
  outgoing segment queue.  No further SENDs from the user will be
  accepted by the TCP, and it enters the FIN-WAIT-1 state.  RECEIVEs
  are allowed in this state.  All segments preceding and including FIN
  will be retransmitted until acknowledged.  When the other TCP has
  both acknowledged the FIN and sent a FIN of its own, the first TCP
  can ACK this FIN.  Note that a TCP receiving a FIN will ACK but not
  send its own FIN until its user has CLOSED the connection also.
Case 2:  TCP receives a FIN from the network
  If an unsolicited FIN arrives from the network, the receiving TCP
  can ACK it and tell the user that the connection is closing.  The
  user will respond with a CLOSE, upon which the TCP can send a FIN to
  the other TCP after sending any remaining data.  The TCP then waits
  until its own FIN is acknowledged whereupon it deletes the
  connection.  If an ACK is not forthcoming, after the user timeout
  the connection is aborted and the user is told.
Case 3:  both users close simultaneously
  A simultaneous CLOSE by users at both ends of a connection causes
  FIN segments to be exchanged.  When all segments preceding the FINs
  have been processed and acknowledged, each TCP can ACK the FIN it
  has received.  Both will, upon receiving these ACKs, delete the
  connection.

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    TCP A                                                TCP B
1.  ESTABLISHED                                          ESTABLISHED
2.  (Close)
    FIN-WAIT-1  --> <SEQ=100><ACK=300><CTL=FIN,ACK>  --> CLOSE-WAIT
3.  FIN-WAIT-2  <-- <SEQ=300><ACK=101><CTL=ACK>      <-- CLOSE-WAIT
4.                                                       (Close)
    TIME-WAIT   <-- <SEQ=300><ACK=101><CTL=FIN,ACK>  <-- LAST-ACK
5.  TIME-WAIT   --> <SEQ=101><ACK=301><CTL=ACK>      --> CLOSED
6.  (2 MSL)
    CLOSED                                                      
                       Normal Close Sequence
                             Figure 13.
    TCP A                                                TCP B
1.  ESTABLISHED                                          ESTABLISHED
2.  (Close)                                              (Close)
    FIN-WAIT-1  --> <SEQ=100><ACK=300><CTL=FIN,ACK>  ... FIN-WAIT-1
                <-- <SEQ=300><ACK=100><CTL=FIN,ACK>  <--
                ... <SEQ=100><ACK=300><CTL=FIN,ACK>  -->
3.  CLOSING     --> <SEQ=101><ACK=301><CTL=ACK>      ... CLOSING
                <-- <SEQ=301><ACK=101><CTL=ACK>      <--
                ... <SEQ=101><ACK=301><CTL=ACK>      -->
4.  TIME-WAIT                                            TIME-WAIT
    (2 MSL)                                              (2 MSL)
    CLOSED                                               CLOSED
                    Simultaneous Close Sequence
                             Figure 14.
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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 ports.
The precedence and security parameters used in TCP are exactly those
defined in the Internet Protocol (IP) [2].  Throughout this TCP
specification the term "security/compartment" is intended to indicate
the security parameters used in IP including security, compartment,
user group, and handling restriction.
A connection attempt with mismatched security/compartment values or a
lower precedence value must be rejected by sending a reset.  Rejecting
a connection due to too low a precedence only occurs after an
acknowledgment of the SYN has been received.
Note that TCP modules which operate only at the default value of
precedence will still have to check the precedence of incoming
segments and possibly raise the precedence level they use on the
connection.
The security paramaters may be used even in a non-secure environment
(the values would indicate unclassified data), thus hosts in
non-secure environments must be prepared to receive the security
parameters, though they need not send them.

3.7. Data Communication

Once the connection is established data is communicated by the
exchange of segments.  Because segments may be lost due to errors
(checksum test failure), or network congestion, TCP uses
retransmission (after a timeout) to ensure delivery of every segment.
Duplicate segments may arrive due to network or TCP retransmission.
As discussed in the section on sequence numbers the TCP performs
certain tests on the sequence and acknowledgment numbers in the
segments to verify their acceptability.
The sender of data keeps track of the next sequence number to use in
the variable SND.NXT.  The receiver of data keeps track of the next
sequence number to expect in the variable RCV.NXT.  The sender of data
keeps track of the oldest unacknowledged sequence number in the
variable SND.UNA.  If the data flow is momentarily idle and all data
sent has been acknowledged then the three variables will be equal.
When the sender creates a segment and transmits it the sender advances
SND.NXT.  When the receiver accepts a segment it advances RCV.NXT and
sends an acknowledgment.  When the data sender receives an

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acknowledgment it advances SND.UNA.  The extent to which the values of
these variables differ is a measure of the delay in the communication.
The amount by which the variables are advanced is the length of the
data in the segment.  Note that once in the ESTABLISHED state all
segments must carry current acknowledgment information.
The CLOSE user call implies a push function, as does the FIN control
flag in an incoming segment.
Retransmission Timeout
Because of the variability of the networks that compose an
internetwork system and the wide range of uses of TCP connections the
retransmission timeout must be dynamically determined.  One procedure
for determining a retransmission time out is given here as an
illustration.
  An Example Retransmission Timeout Procedure
    Measure the elapsed time between sending a data octet with a
    particular sequence number and receiving an acknowledgment that
    covers that sequence number (segments sent do not have to match
    segments received).  This measured elapsed time is the Round Trip
    Time (RTT).  Next compute a Smoothed Round Trip Time (SRTT) as:
      SRTT = ( ALPHA * SRTT ) + ((1-ALPHA) * RTT)
    and based on this, compute the retransmission timeout (RTO) as:
      RTO = min[UBOUND,max[LBOUND,(BETA*SRTT)]]
    where UBOUND is an upper bound on the timeout (e.g., 1 minute),
    LBOUND is a lower bound on the timeout (e.g., 1 second), ALPHA is
    a smoothing factor (e.g., .8 to .9), and BETA is a delay variance
    factor (e.g., 1.3 to 2.0).
The Communication of Urgent Information
The objective of the TCP urgent mechanism is to allow the sending user
to stimulate the receiving user to accept some urgent data and to
permit the receiving TCP to indicate to the receiving user when all
the currently known urgent data has been received by the user.
This mechanism permits a point in the data stream to be designated as
the end of urgent information.  Whenever this point is in advance of
the receive sequence number (RCV.NXT) at the receiving TCP, that TCP
must tell the user to go into "urgent mode"; when the receive sequence
number catches up to the urgent pointer, the TCP must tell user to go
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into "normal mode".  If the urgent pointer is updated while the user
is in "urgent mode", the update will be invisible to the user.
The method employs a urgent field which is carried in all segments
transmitted.  The URG control flag indicates that the urgent field is
meaningful and must be added to the segment sequence number to yield
the urgent pointer.  The absence of this flag indicates that there is
no urgent data outstanding.
To send an urgent indication the user must also send at least one data
octet.  If the sending user also indicates a push, timely delivery of
the urgent information to the destination process is enhanced.
Managing the Window
The window sent in each segment indicates the range of sequence
numbers the sender of the window (the data receiver) is currently
prepared to accept.  There is an assumption that this is related to
the currently available data buffer space available for this
connection.
Indicating a large window encourages transmissions.  If more data
arrives than can be accepted, it will be discarded.  This will result
in excessive retransmissions, adding unnecessarily to the load on the
network and the TCPs.  Indicating a small window may restrict the
transmission of data to the point of introducing a round trip delay
between each new segment transmitted.
The mechanisms provided allow a TCP to advertise a large window and to
subsequently advertise a much smaller window without having accepted
that much data.  This, so called "shrinking the window," is strongly
discouraged.  The robustness principle dictates that TCPs will not
shrink the window themselves, but will be prepared for such behavior
on the part of other TCPs.
The sending TCP must be prepared to accept from the user and send at
least one octet of new data even if the send window is zero.  The
sending TCP must regularly retransmit to the receiving TCP even when
the window is zero.  Two minutes is recommended for the retransmission
interval when the window is zero.  This retransmission is essential to
guarantee that when either TCP has a zero window the re-opening of the
window will be reliably reported to the other.
When the receiving TCP has a zero window and a segment arrives it must
still send an acknowledgment showing its next expected sequence number
and current window (zero).
The sending TCP packages the data to be transmitted into segments

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                                              Functional Specification
which fit the current window, and may repackage segments on the
retransmission queue.  Such repackaging is not required, but may be
helpful.
In a connection with a one-way data flow, the window information will
be carried in acknowledgment segments that all have the same sequence
number so there will be no way to reorder them if they arrive out of
order.  This is not a serious problem, but it will allow the window
information to be on occasion temporarily based on old reports from
the data receiver.  A refinement to avoid this problem is to act on
the window information from segments that carry the highest
acknowledgment number (that is segments with acknowledgment number
equal or greater than the highest previously received).
The window management procedure has significant influence on the
communication performance.  The following comments are suggestions to
implementers.
  Window Management Suggestions
    Allocating a very small window causes data to be transmitted in
    many small segments when better performance is achieved using
    fewer large segments.
    One suggestion for avoiding small windows is for the receiver to
    defer updating a window until the additional allocation is at
    least X percent of the maximum allocation possible for the
    connection (where X might be 20 to 40).
    Another suggestion is for the sender to avoid sending small
    segments by waiting until the window is large enough before
    sending data.  If the the user signals a push function then the
    data must be sent even if it is a small segment.
    Note that the acknowledgments should not be delayed or unnecessary
    retransmissions will result.  One strategy would be to send an
    acknowledgment when a small segment arrives (with out updating the
    window information), and then to send another acknowledgment with
    new window information when the window is larger.
    The segment sent to probe a zero window may also begin a break up
    of transmitted data into smaller and smaller segments.  If a
    segment containing a single data octet sent to probe a zero window
    is accepted, it consumes one octet of the window now available.
    If the sending TCP simply sends as much as it can whenever the
    window is non zero, the transmitted data will be broken into
    alternating big and small segments.  As time goes on, occasional
    pauses in the receiver making window allocation available will
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    result in breaking the big segments into a small and not quite so
    big pair. And after a while the data transmission will be in
    mostly small segments.
    The suggestion here is that the TCP implementations need to
    actively attempt to combine small window allocations into larger
    windows, since the mechanisms for managing the window tend to lead
    to many small windows in the simplest minded implementations.

3.8. Interfaces

There are of course two interfaces of concern:  the user/TCP interface
and the TCP/lower-level interface.  We have a fairly elaborate model
of the user/TCP interface, but the interface to the lower level
protocol module is left unspecified here, since it will be specified
in detail by the specification of the lowel level protocol.  For the
case that the lower level is IP we note some of the parameter values
that TCPs might use.
User/TCP Interface
  The following functional description of user commands to the TCP is,
  at best, fictional, since every operating system will have different
  facilities.  Consequently, we must warn readers that different TCP
  implementations may have different user interfaces.  However, all
  TCPs must provide a certain minimum set of services to guarantee
  that all TCP implementations can support the same protocol
  hierarchy.  This section specifies the functional interfaces
  required of all TCP implementations.
  TCP User Commands
    The following sections functionally characterize a USER/TCP
    interface.  The notation used is similar to most procedure or
    function calls in high level languages, but this usage is not
    meant to rule out trap type service calls (e.g., SVCs, UUOs,
    EMTs).
    The user commands described below specify the basic functions the
    TCP must perform to support interprocess communication.
    Individual implementations must define their own exact format, and
    may provide combinations or subsets of the basic functions in
    single calls.  In particular, some implementations may wish to
    automatically OPEN a connection on the first SEND or RECEIVE
    issued by the user for a given connection.

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    In providing interprocess communication facilities, the TCP must
    not only accept commands, but must also return information to the
    processes it serves.  The latter consists of:
      (a) general information about a connection (e.g., interrupts,
      remote close, binding of unspecified foreign socket).
      (b) replies to specific user commands indicating success or
      various types of failure.
    Open
      Format:  OPEN (local port, foreign socket, active/passive
      [, timeout] [, precedence] [, security/compartment] [, options])
      -> local connection name
      We assume that the local TCP is aware of the identity of the
      processes it serves and will check the authority of the process
      to use the connection specified.  Depending upon the
      implementation of the TCP, the local network and TCP identifiers
      for the source address will either be supplied by the TCP or the
      lower level protocol (e.g., IP).  These considerations are the
      result of concern about security, to the extent that no TCP be
      able to masquerade as another one, and so on.  Similarly, no
      process can masquerade as another without the collusion of the
      TCP.
      If the active/passive flag is set to passive, then this is a
      call to LISTEN for an incoming connection.  A passive open may
      have either a fully specified foreign socket to wait for a
      particular connection or an unspecified foreign socket to wait
      for any call.  A fully specified passive call can be made active
      by the subsequent execution of a SEND.
      A transmission control block (TCB) is created and partially
      filled in with data from the OPEN command parameters.
      On an active OPEN command, the TCP will begin the procedure to
      synchronize (i.e., establish) the connection at once.
      The timeout, if present, permits the caller to set up a timeout
      for all data submitted to TCP.  If data is not successfully
      delivered to the destination within the timeout period, the TCP
      will abort the connection.  The present global default is five
      minutes.
      The TCP or some component of the operating system will verify
      the users authority to open a connection with the specified
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      precedence or security/compartment.  The absence of precedence
      or security/compartment specification in the OPEN call indicates
      the default values must be used.
      TCP will accept incoming requests as matching only if the
      security/compartment information is exactly the same and only if
      the precedence is equal to or higher than the precedence
      requested in the OPEN call.
      The precedence for the connection is the higher of the values
      requested in the OPEN call and received from the incoming
      request, and fixed at that value for the life of the
      connection.Implementers may want to give the user control of
      this precedence negotiation.  For example, the user might be
      allowed to specify that the precedence must be exactly matched,
      or that any attempt to raise the precedence be confirmed by the
      user.
      A local connection name will be returned to the user by the TCP.
      The local connection name can then be used as a short hand term
      for the connection defined by the <local socket, foreign socket>
      pair.
    Send
      Format:  SEND (local connection name, buffer address, byte
      count, PUSH flag, URGENT flag [,timeout])
      This call causes the data contained in the indicated user buffer
      to be sent on the indicated connection.  If the connection has
      not been opened, the SEND is considered an error.  Some
      implementations may allow users to SEND first; in which case, an
      automatic OPEN would be done.  If the calling process is not
      authorized to use this connection, an error is returned.
      If the PUSH flag is set, the data must be transmitted promptly
      to the receiver, and the PUSH bit will be set in the last TCP
      segment created from the buffer.  If the PUSH flag is not set,
      the data may be combined with data from subsequent SENDs for
      transmission efficiency.
      If the URGENT flag is set, segments sent to the destination TCP
      will have the urgent pointer set.  The receiving TCP will signal
      the urgent condition to the receiving process if the urgent
      pointer indicates that data preceding the urgent pointer has not
      been consumed by the receiving process.  The purpose of urgent
      is to stimulate the receiver to process the urgent data and to
      indicate to the receiver when all the currently known urgent

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                                         Transmission Control Protocol
                                              Functional Specification
      data has been received.  The number of times the sending user's
      TCP signals urgent will not necessarily be equal to the number
      of times the receiving user will be notified of the presence of
      urgent data.
      If no foreign socket was specified in the OPEN, but the
      connection is established (e.g., because a LISTENing connection
      has become specific due to a foreign segment arriving for the
      local socket), then the designated buffer is sent to the implied
      foreign socket.  Users who make use of OPEN with an unspecified
      foreign socket can make use of SEND without ever explicitly
      knowing the foreign socket address.
      However, if a SEND is attempted before the foreign socket
      becomes specified, an error will be returned.  Users can use the
      STATUS call to determine the status of the connection.  In some
      implementations the TCP may notify the user when an unspecified
      socket is bound.
      If a timeout is specified, the current user timeout for this
      connection is changed to the new one.
      In the simplest implementation, SEND would not return control to
      the sending process until either the transmission was complete
      or the timeout had been exceeded.  However, this simple method
      is both subject to deadlocks (for example, both sides of the
      connection might try to do SENDs before doing any RECEIVEs) and
      offers poor performance, so it is not recommended.  A more
      sophisticated implementation would return immediately to allow
      the process to run concurrently with network I/O, and,
      furthermore, to allow multiple SENDs to be in progress.
      Multiple SENDs are served in first come, first served order, so
      the TCP will queue those it cannot service immediately.
      We have implicitly assumed an asynchronous user interface in
      which a SEND later elicits some kind of SIGNAL or
      pseudo-interrupt from the serving TCP.  An alternative is to
      return a response immediately.  For instance, SENDs might return
      immediate local acknowledgment, even if the segment sent had not
      been acknowledged by the distant TCP.  We could optimistically
      assume eventual success.  If we are wrong, the connection will
      close anyway due to the timeout.  In implementations of this
      kind (synchronous), there will still be some asynchronous
      signals, but these will deal with the connection itself, and not
      with specific segments or buffers.
      In order for the process to distinguish among error or success
      indications for different SENDs, it might be appropriate for the
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      buffer address to be returned along with the coded response to
      the SEND request.  TCP-to-user signals are discussed below,
      indicating the information which should be returned to the
      calling process.
    Receive
      Format:  RECEIVE (local connection name, buffer address, byte
      count) -> byte count, urgent flag, push flag
      This command allocates a receiving buffer associated with the
      specified connection.  If no OPEN precedes this command or the
      calling process is not authorized to use this connection, an
      error is returned.
      In the simplest implementation, control would not return to the
      calling program until either the buffer was filled, or some
      error occurred, but this scheme is highly subject to deadlocks.
      A more sophisticated implementation would permit several
      RECEIVEs to be outstanding at once.  These would be filled as
      segments arrive.  This strategy permits increased throughput at
      the cost of a more elaborate scheme (possibly asynchronous) to
      notify the calling program that a PUSH has been seen or a buffer
      filled.
      If enough data arrive to fill the buffer before a PUSH is seen,
      the PUSH flag will not be set in the response to the RECEIVE.
      The buffer will be filled with as much data as it can hold.  If
      a PUSH is seen before the buffer is filled the buffer will be
      returned partially filled and PUSH indicated.
      If there is urgent data the user will have been informed as soon
      as it arrived via a TCP-to-user signal.  The receiving user
      should thus be in "urgent mode".  If the URGENT flag is on,
      additional urgent data remains.  If the URGENT flag is off, this
      call to RECEIVE has returned all the urgent data, and the user
      may now leave "urgent mode".  Note that data following the
      urgent pointer (non-urgent data) cannot be delivered to the user
      in the same buffer with preceeding urgent data unless the
      boundary is clearly marked for the user.
      To distinguish among several outstanding RECEIVEs and to take
      care of the case that a buffer is not completely filled, the
      return code is accompanied by both a buffer pointer and a byte
      count indicating the actual length of the data received.
      Alternative implementations of RECEIVE might have the TCP

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                                              Functional Specification
      allocate buffer storage, or the TCP might share a ring buffer
      with the user.
    Close
      Format:  CLOSE (local connection name)
      This command causes the connection specified to be closed.  If
      the connection is not open or the calling process is not
      authorized to use this connection, an error is returned.
      Closing connections is intended to be a graceful operation in
      the sense that outstanding SENDs will be transmitted (and
      retransmitted), as flow control permits, until all have been
      serviced.  Thus, it should be acceptable to make several SEND
      calls, followed by a CLOSE, and expect all the data to be sent
      to the destination.  It should also be clear that users should
      continue to RECEIVE on CLOSING connections, since the other side
      may be trying to transmit the last of its data.  Thus, CLOSE
      means "I have no more to send" but does not mean "I will not
      receive any more."  It may happen (if the user level protocol is
      not well thought out) that the closing side is unable to get rid
      of all its data before timing out.  In this event, CLOSE turns
      into ABORT, and the closing TCP gives up.
      The user may CLOSE the connection at any time on his own
      initiative, or in response to various prompts from the TCP
      (e.g., remote close executed, transmission timeout exceeded,
      destination inaccessible).
      Because closing a connection requires communication with the
      foreign TCP, connections may remain in the closing state for a
      short time.  Attempts to reopen the connection before the TCP
      replies to the CLOSE command will result in error responses.
      Close also implies push function.
    Status
      Format:  STATUS (local connection name) -> status data
      This is an implementation dependent user command and could be
      excluded without adverse effect.  Information returned would
      typically come from the TCB associated with the connection.
      This command returns a data block containing the following
      information:
        local socket,
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Transmission Control Protocol Functional Specification

        foreign socket,
        local connection name,
        receive window,
        send window,
        connection state,
        number of buffers awaiting acknowledgment,
        number of buffers pending receipt,
        urgent state,
        precedence,
        security/compartment,
        and transmission timeout.
      Depending on the state of the connection, or on the
      implementation itself, some of this information may not be
      available or meaningful.  If the calling process is not
      authorized to use this connection, an error is returned.  This
      prevents unauthorized processes from gaining information about a
      connection.
    Abort
      Format:  ABORT (local connection name)
      This command causes all pending SENDs and RECEIVES to be
      aborted, the TCB to be removed, and a special RESET message to
      be sent to the TCP on the other side of the connection.
      Depending on the implementation, users may receive abort
      indications for each outstanding SEND or RECEIVE, or may simply
      receive an ABORT-acknowledgment.
  TCP-to-User Messages
    It is assumed that the operating system environment provides a
    means for the TCP to asynchronously signal the user program.  When
    the TCP does signal a user program, certain information is passed
    to the user.  Often in the specification the information will be
    an error message.  In other cases there will be information
    relating to the completion of processing a SEND or RECEIVE or
    other user call.
    The following information is provided:
      Local Connection Name                    Always
      Response String                          Always
      Buffer Address                           Send & Receive
      Byte count (counts bytes received)       Receive
      Push flag                                Receive
      Urgent flag                              Receive

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                                              Functional Specification
TCP/Lower-Level Interface
  The TCP calls on a lower level protocol module to actually send and
  receive information over a network.  One case is that of the ARPA
  internetwork system where the lower level module is the Internet
  Protocol (IP) [2].
  If the lower level protocol is IP it provides arguments for a type
  of service and for a time to live.  TCP uses the following settings
  for these parameters:
    Type of Service = Precedence: routine, Delay: normal, Throughput:
    normal, Reliability: normal; or 00000000.
    Time to Live    = one minute, or 00111100.
      Note that the assumed maximum segment lifetime is two minutes.
      Here we explicitly ask that a segment be destroyed if it cannot
      be delivered by the internet system within one minute.
  If the lower level is IP (or other protocol that provides this
  feature) and source routing is used, the interface must allow the
  route information to be communicated.  This is especially important
  so that the source and destination addresses used in the TCP
  checksum be the originating source and ultimate destination. It is
  also important to preserve the return route to answer connection
  requests.
  Any lower level protocol will have to provide the source address,
  destination address, and protocol fields, and some way to determine
  the "TCP length", both to provide the functional equivlent service
  of IP and to be used in the TCP checksum.
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3.9. Event Processing

The processing depicted in this section is an example of one possible
implementation.  Other implementations may have slightly different
processing sequences, but they should differ from those in this
section only in detail, not in substance.
The activity of the TCP can be characterized as responding to events.
The events that occur can be cast into three categories:  user calls,
arriving segments, and timeouts.  This section describes the
processing the TCP does in response to each of the events.  In many
cases the processing required depends on the state of the connection.
  Events that occur:
    User Calls
      OPEN
      SEND
      RECEIVE
      CLOSE
      ABORT
      STATUS
    Arriving Segments
      SEGMENT ARRIVES
    Timeouts
      USER TIMEOUT
      RETRANSMISSION TIMEOUT
      TIME-WAIT TIMEOUT
The model of the TCP/user interface is that user commands receive an
immediate return and possibly a delayed response via an event or
pseudo interrupt.  In the following descriptions, the term "signal"
means cause a delayed response.
Error responses are given as character strings.  For example, user
commands referencing connections that do not exist receive "error:
connection not open".
Please note in the following that all arithmetic on sequence numbers,
acknowledgment numbers, windows, et cetera, is modulo 2**32 the size
of the sequence number space.  Also note that "=<" means less than or
equal to (modulo 2**32).

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A natural way to think about processing incoming segments is to
imagine that they are first tested for proper sequence number (i.e.,
that their contents lie in the range of the expected "receive window"
in the sequence number space) and then that they are generally queued
and processed in sequence number order.
When a segment overlaps other already received segments we reconstruct
the segment to contain just the new data, and adjust the header fields
to be consistent.
Note that if no state change is mentioned the TCP stays in the same
state.
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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.  Note that some parts of the foreign socket may be
    unspecified in a passive OPEN and are to be filled in by the
    parameters of the incoming SYN segment.  Verify the security and
    precedence requested are allowed for this user, if not return
    "error:  precedence not allowed" or "error:  security/compartment
    not allowed."  If passive enter the LISTEN state and return.  If
    active and the foreign socket is unspecified, return "error:
    foreign socket unspecified"; if active and the foreign socket is
    specified, issue a SYN segment.  An initial send sequence number
    (ISS) is selected.  A SYN segment of the form <SEQ=ISS><CTL=SYN>
    is sent.  Set SND.UNA to ISS, SND.NXT to ISS+1, enter SYN-SENT
    state, and return.
    If the caller does not have access to the local socket specified,
    return "error:  connection illegal for this process".  If there is
    no room to create a new connection, return "error:  insufficient
    resources".
  LISTEN STATE
    If active and the foreign socket is specified, then change the
    connection from passive to active, select an ISS.  Send a SYN
    segment, set SND.UNA to ISS, SND.NXT to ISS+1.  Enter SYN-SENT
    state.  Data associated with SEND may be sent with SYN segment or
    queued for transmission after entering ESTABLISHED state.  The
    urgent bit if requested in the command must be sent with the data
    segments sent as a result of this command.  If there is no room to
    queue the request, respond with "error:  insufficient resources".
    If Foreign socket was not specified, then return "error:  foreign
    socket unspecified".

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OPEN Call

  SYN-SENT STATE
  SYN-RECEIVED STATE
  ESTABLISHED STATE
  FIN-WAIT-1 STATE
  FIN-WAIT-2 STATE
  CLOSE-WAIT STATE
  CLOSING STATE
  LAST-ACK STATE
  TIME-WAIT STATE
    Return "error:  connection already exists".
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                                                             SEND Call
SEND Call
  CLOSED STATE (i.e., TCB does not exist)
    If the user does not have access to such a connection, then return
    "error:  connection illegal for this process".
    Otherwise, return "error:  connection does not exist".
  LISTEN STATE
    If the foreign socket is specified, then change the connection
    from passive to active, select an ISS.  Send a SYN segment, set
    SND.UNA to ISS, SND.NXT to ISS+1.  Enter SYN-SENT state.  Data
    associated with SEND may be sent with SYN segment or queued for
    transmission after entering ESTABLISHED state.  The urgent bit if
    requested in the command must be sent with the data segments sent
    as a result of this command.  If there is no room to queue the
    request, respond with "error:  insufficient resources".  If
    Foreign socket was not specified, then return "error:  foreign
    socket unspecified".
  SYN-SENT STATE
  SYN-RECEIVED STATE
    Queue the data for transmission after entering ESTABLISHED state.
    If no space to queue, respond with "error:  insufficient
    resources".
  ESTABLISHED STATE
  CLOSE-WAIT STATE
    Segmentize the buffer and send it with a piggybacked
    acknowledgment (acknowledgment value = RCV.NXT).  If there is
    insufficient space to remember this buffer, simply return "error:
    insufficient resources".
    If the urgent flag is set, then SND.UP <- SND.NXT-1 and set the
    urgent pointer in the outgoing segments.

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SEND Call

  FIN-WAIT-1 STATE
  FIN-WAIT-2 STATE
  CLOSING STATE
  LAST-ACK STATE
  TIME-WAIT STATE
    Return "error:  connection closing" and do not service request.
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                                                          RECEIVE Call
RECEIVE Call
  CLOSED STATE (i.e., TCB does not exist)
    If the user does not have access to such a connection, return
    "error:  connection illegal for this process".
    Otherwise return "error:  connection does not exist".
  LISTEN STATE
  SYN-SENT STATE
  SYN-RECEIVED STATE
    Queue for processing after entering ESTABLISHED state.  If there
    is no room to queue this request, respond with "error:
    insufficient resources".
  ESTABLISHED STATE
  FIN-WAIT-1 STATE
  FIN-WAIT-2 STATE
    If insufficient incoming segments are queued to satisfy the
    request, queue the request.  If there is no queue space to
    remember the RECEIVE, respond with "error:  insufficient
    resources".
    Reassemble queued incoming segments into receive buffer and return
    to user.  Mark "push seen" (PUSH) if this is the case.
    If RCV.UP is in advance of the data currently being passed to the
    user notify the user of the presence of urgent data.
    When the TCP takes responsibility for delivering data to the user
    that fact must be communicated to the sender via an
    acknowledgment.  The formation of such an acknowledgment is
    described below in the discussion of processing an incoming
    segment.

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RECEIVE Call

  CLOSE-WAIT STATE
    Since the remote side has already sent FIN, RECEIVEs must be
    satisfied by text already on hand, but not yet delivered to the
    user.  If no text is awaiting delivery, the RECEIVE will get a
    "error:  connection closing" response.  Otherwise, any remaining
    text can be used to satisfy the RECEIVE.
  CLOSING STATE
  LAST-ACK STATE
  TIME-WAIT STATE
    Return "error:  connection closing".
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                                                            CLOSE Call
CLOSE Call
  CLOSED STATE (i.e., TCB does not exist)
    If the user does not have access to such a connection, return
    "error:  connection illegal for this process".
    Otherwise, return "error:  connection does not exist".
  LISTEN STATE
    Any outstanding RECEIVEs are returned with "error:  closing"
    responses.  Delete TCB, enter CLOSED state, and return.
  SYN-SENT STATE
    Delete the TCB and return "error:  closing" responses to any
    queued SENDs, or RECEIVEs.
  SYN-RECEIVED STATE
    If no SENDs have been issued and there is no pending data to send,
    then form a FIN segment and send it, and enter FIN-WAIT-1 state;
    otherwise queue for processing after entering ESTABLISHED state.
  ESTABLISHED STATE
    Queue this until all preceding SENDs have been segmentized, then
    form a FIN segment and send it.  In any case, enter FIN-WAIT-1
    state.
  FIN-WAIT-1 STATE
  FIN-WAIT-2 STATE
    Strictly speaking, this is an error and should receive a "error:
    connection closing" response.  An "ok" response would be
    acceptable, too, as long as a second FIN is not emitted (the first
    FIN may be retransmitted though).

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CLOSE Call

  CLOSE-WAIT STATE
    Queue this request until all preceding SENDs have been
    segmentized; then send a FIN segment, enter CLOSING state.
  CLOSING STATE
  LAST-ACK STATE
  TIME-WAIT STATE
    Respond with "error:  connection closing".
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                                                            ABORT Call
ABORT Call
  CLOSED STATE (i.e., TCB does not exist)
    If the user should not have access to such a connection, return
    "error:  connection illegal for this process".
    Otherwise return "error:  connection does not exist".
  LISTEN STATE
    Any outstanding RECEIVEs should be returned with "error:
    connection reset" responses.  Delete TCB, enter CLOSED state, and
    return.
  SYN-SENT STATE
    All queued SENDs and RECEIVEs should be given "connection reset"
    notification, delete the TCB, enter CLOSED state, and return.
  SYN-RECEIVED STATE
  ESTABLISHED STATE
  FIN-WAIT-1 STATE
  FIN-WAIT-2 STATE
  CLOSE-WAIT STATE
    Send a reset segment:
      <SEQ=SND.NXT><CTL=RST>
    All queued SENDs and RECEIVEs should be given "connection reset"
    notification; all segments queued for transmission (except for the
    RST formed above) or retransmission should be flushed, delete the
    TCB, enter CLOSED state, and return.
  CLOSING STATE
  LAST-ACK STATE
  TIME-WAIT STATE
    Respond with "ok" and delete the TCB, enter CLOSED state, and
    return.

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                                              Functional Specification

STATUS Call

STATUS Call
  CLOSED STATE (i.e., TCB does not exist)
    If the user should not have access to such a connection, return
    "error:  connection illegal for this process".
    Otherwise return "error:  connection does not exist".
  LISTEN STATE
    Return "state = LISTEN", and the TCB pointer.
  SYN-SENT STATE
    Return "state = SYN-SENT", and the TCB pointer.
  SYN-RECEIVED STATE
    Return "state = SYN-RECEIVED", and the TCB pointer.
  ESTABLISHED STATE
    Return "state = ESTABLISHED", and the TCB pointer.
  FIN-WAIT-1 STATE
    Return "state = FIN-WAIT-1", and the TCB pointer.
  FIN-WAIT-2 STATE
    Return "state = FIN-WAIT-2", and the TCB pointer.
  CLOSE-WAIT STATE
    Return "state = CLOSE-WAIT", and the TCB pointer.
  CLOSING STATE
    Return "state = CLOSING", and the TCB pointer.
  LAST-ACK STATE
    Return "state = LAST-ACK", and the TCB pointer.
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                                                           STATUS Call
  TIME-WAIT STATE
    Return "state = TIME-WAIT", 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 RST
      An incoming RST should be ignored.  Return.
    second check for an ACK
      Any acknowledgment is bad if it arrives on a connection still in
      the LISTEN state.  An acceptable reset segment should be formed
      for any arriving ACK-bearing segment.  The RST should be
      formatted as follows:
        <SEQ=SEG.ACK><CTL=RST>
      Return.
    third check for a SYN
      If the SYN bit is set, check the security.  If the
      security/compartment on the incoming segment does not exactly
      match the security/compartment in the TCB then send a reset and
      return.
        <SEQ=SEG.ACK><CTL=RST>
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                                                       SEGMENT ARRIVES
      If the SEG.PRC is greater than the TCB.PRC then if allowed by
      the user and the system set TCB.PRC<-SEG.PRC, if not allowed
      send a reset and return.
        <SEQ=SEG.ACK><CTL=RST>
      If the SEG.PRC is less than the TCB.PRC then continue.
      Set RCV.NXT to SEG.SEQ+1, IRS is set to SEG.SEQ and any other
      control or text should be queued for processing later.  ISS
      should be selected and a SYN segment sent of the form:
        <SEQ=ISS><ACK=RCV.NXT><CTL=SYN,ACK>
      SND.NXT is set to ISS+1 and SND.UNA to ISS.  The connection
      state should be changed to SYN-RECEIVED.  Note that any other
      incoming control or data (combined with SYN) will be processed
      in the SYN-RECEIVED state, but processing of SYN and ACK should
      not be repeated.  If the listen was not fully specified (i.e.,
      the foreign socket was not fully specified), then the
      unspecified fields should be filled in now.
    fourth other text or control
      Any other control or text-bearing segment (not containing SYN)
      must have an ACK and thus would be discarded by the ACK
      processing.  An incoming RST segment could not be valid, since
      it could not have been sent in response to anything sent by this
      incarnation of the connection.  So you are unlikely to get here,
      but if you do, drop the segment, and return.
  If the state is SYN-SENT then
    first check the ACK bit
      If the ACK bit is set
        If SEG.ACK =< ISS, or SEG.ACK > SND.NXT, send a reset (unless
        the RST bit is set, if so drop the segment and return)
          <SEQ=SEG.ACK><CTL=RST>
        and discard the segment.  Return.
        If SND.UNA =< SEG.ACK =< SND.NXT then the ACK is acceptable.
    second check the RST bit

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                                              Functional Specification

SEGMENT ARRIVES

      If the RST bit is set
        If the ACK was acceptable then signal the user "error:
        connection reset", drop the segment, enter CLOSED state,
        delete TCB, and return.  Otherwise (no ACK) drop the segment
        and return.
    third check the security and precedence
      If the security/compartment in the segment does not exactly
      match the security/compartment in the TCB, send a reset
        If there is an ACK
          <SEQ=SEG.ACK><CTL=RST>
        Otherwise
          <SEQ=0><ACK=SEG.SEQ+SEG.LEN><CTL=RST,ACK>
      If there is an ACK
        The precedence in the segment must match the precedence in the
        TCB, if not, send a reset
          <SEQ=SEG.ACK><CTL=RST>
      If there is no ACK
        If the precedence in the segment is higher than the precedence
        in the TCB then if allowed by the user and the system raise
        the precedence in the TCB to that in the segment, if not
        allowed to raise the prec then send a reset.
          <SEQ=0><ACK=SEG.SEQ+SEG.LEN><CTL=RST,ACK>
        If the precedence in the segment is lower than the precedence
        in the TCB continue.
      If a reset was sent, discard the segment and return.
    fourth check the SYN bit
      This step should be reached only if the ACK is ok, or there is
      no ACK, and it the segment did not contain a RST.
      If the SYN bit is on and the security/compartment and precedence
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                                                       SEGMENT ARRIVES
      are acceptable then, RCV.NXT is set to SEG.SEQ+1, IRS is set to
      SEG.SEQ.  SND.UNA should be advanced to equal SEG.ACK (if there
      is an ACK), and any segments on the retransmission queue which
      are thereby acknowledged should be removed.
      If SND.UNA > ISS (our SYN has been ACKed), change the connection
      state to ESTABLISHED, form an ACK segment
        <SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK>
      and send it.  Data or controls which were queued for
      transmission may be included.  If there are other controls or
      text in the segment then continue processing at the sixth step
      below where the URG bit is checked, otherwise return.
      Otherwise enter SYN-RECEIVED, form a SYN,ACK segment
        <SEQ=ISS><ACK=RCV.NXT><CTL=SYN,ACK>
      and send it.  If there are other controls or text in the
      segment, queue them for processing after the ESTABLISHED state
      has been reached, return.
    fifth, if neither of the SYN or RST bits is set then drop the
    segment and return.

[Page 68]

September 1981

                                         Transmission Control Protocol
                                              Functional Specification

SEGMENT ARRIVES

  Otherwise,
  first check sequence number
    SYN-RECEIVED STATE
    ESTABLISHED STATE
    FIN-WAIT-1 STATE
    FIN-WAIT-2 STATE
    CLOSE-WAIT STATE
    CLOSING STATE
    LAST-ACK STATE
    TIME-WAIT STATE
      Segments are processed in sequence.  Initial tests on arrival
      are used to discard old duplicates, but further processing is
      done in SEG.SEQ order.  If a segment's contents straddle the
      boundary between old and new, only the new parts should be
      processed.
      There are four cases for the acceptability test for an incoming
      segment:
      Segment Receive  Test
      Length  Window
      ------- -------  -------------------------------------------
         0       0     SEG.SEQ = RCV.NXT
         0      >0     RCV.NXT =< SEG.SEQ < RCV.NXT+RCV.WND
        >0       0     not acceptable
        >0      >0     RCV.NXT =< SEG.SEQ < RCV.NXT+RCV.WND
                    or RCV.NXT =< SEG.SEQ+SEG.LEN-1 < RCV.NXT+RCV.WND
      If the RCV.WND is zero, no segments will be acceptable, but
      special allowance should be made to accept valid ACKs, URGs and
      RSTs.
      If an incoming segment is not acceptable, an acknowledgment
      should be sent in reply (unless the RST bit is set, if so drop
      the segment and return):
        <SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK>
      After sending the acknowledgment, drop the unacceptable segment
      and return.
                                                             [Page 69]
                                                        September 1981

Transmission Control Protocol Functional Specification

                                                       SEGMENT ARRIVES
      In the following it is assumed that the segment is the idealized
      segment that begins at RCV.NXT and does not exceed the window.
      One could tailor actual segments to fit this assumption by
      trimming off any portions that lie outside the window (including
      SYN and FIN), and only processing further if the segment then
      begins at RCV.NXT.  Segments with higher begining sequence
      numbers may be held for later processing.
  second check the RST bit,
    SYN-RECEIVED STATE
      If the RST bit is set
        If this connection was initiated with a passive OPEN (i.e.,
        came from the LISTEN state), then return this connection to
        LISTEN state and return.  The user need not be informed.  If
        this connection was initiated with an active OPEN (i.e., came
        from SYN-SENT state) then the connection was refused, signal
        the user "connection refused".  In either case, all segments
        on the retransmission queue should be removed.  And in the
        active OPEN case, enter the CLOSED state and delete the TCB,
        and return.
    ESTABLISHED
    FIN-WAIT-1
    FIN-WAIT-2
    CLOSE-WAIT
      If the RST bit is set then, any outstanding RECEIVEs and SEND
      should receive "reset" responses.  All segment queues should be
      flushed.  Users should also receive an unsolicited general
      "connection reset" signal.  Enter the CLOSED state, delete the
      TCB, and return.
    CLOSING STATE
    LAST-ACK STATE
    TIME-WAIT
      If the RST bit is set then, enter the CLOSED state, delete the
      TCB, and return.

[Page 70]

September 1981

                                         Transmission Control Protocol
                                              Functional Specification

SEGMENT ARRIVES

  third check security and precedence
    SYN-RECEIVED
      If the security/compartment and precedence in the segment do not
      exactly match the security/compartment and precedence in the TCB
      then send a reset, and return.
    ESTABLISHED STATE
      If the security/compartment and precedence in the segment do not
      exactly match the security/compartment and precedence in the TCB
      then send a reset, any outstanding RECEIVEs and SEND should
      receive "reset" responses.  All segment queues should be
      flushed.  Users should also receive an unsolicited general
      "connection reset" signal.  Enter the CLOSED state, delete the
      TCB, and return.
    Note this check is placed following the sequence check to prevent
    a segment from an old connection between these ports with a
    different security or precedence from causing an abort of the
    current connection.
  fourth, check the SYN bit,
    SYN-RECEIVED
    ESTABLISHED STATE
    FIN-WAIT STATE-1
    FIN-WAIT STATE-2
    CLOSE-WAIT STATE
    CLOSING STATE
    LAST-ACK STATE
    TIME-WAIT STATE
      If the SYN is in the window it is an error, send a reset, any
      outstanding RECEIVEs and SEND should receive "reset" responses,
      all segment queues should be flushed, the user should also
      receive an unsolicited general "connection reset" signal, enter
      the CLOSED state, delete the TCB, and return.
      If the SYN is not in the window this step would not be reached
      and an ack would have been sent in the first step (sequence
      number check).
                                                             [Page 71]
                                                        September 1981

Transmission Control Protocol Functional Specification

                                                       SEGMENT ARRIVES
  fifth check the ACK field,
    if the ACK bit is off drop the segment and return
    if the ACK bit is on
      SYN-RECEIVED STATE
        If SND.UNA =< SEG.ACK =< SND.NXT then enter ESTABLISHED state
        and continue processing.
          If the segment acknowledgment is not acceptable, form a
          reset segment,
            <SEQ=SEG.ACK><CTL=RST>
          and send it.
      ESTABLISHED STATE
        If SND.UNA < SEG.ACK =< SND.NXT then, set SND.UNA <- SEG.ACK.
        Any segments on the retransmission queue which are thereby
        entirely acknowledged are removed.  Users should receive
        positive acknowledgments for buffers which have been SENT and
        fully acknowledged (i.e., SEND buffer should be returned with
        "ok" response).  If the ACK is a duplicate
        (SEG.ACK < SND.UNA), it can be ignored.  If the ACK acks
        something not yet sent (SEG.ACK > SND.NXT) then send an ACK,
        drop the segment, and return.
        If SND.UNA < SEG.ACK =< SND.NXT, the send window should be
        updated.  If (SND.WL1 < SEG.SEQ or (SND.WL1 = SEG.SEQ and
        SND.WL2 =< SEG.ACK)), set SND.WND <- SEG.WND, set
        SND.WL1 <- SEG.SEQ, and set SND.WL2 <- SEG.ACK.
        Note that SND.WND is an offset from SND.UNA, that SND.WL1
        records the sequence number of the last segment used to update
        SND.WND, and that SND.WL2 records the acknowledgment number of
        the last segment used to update SND.WND.  The check here
        prevents using old segments to update the window.

[Page 72]

September 1981

                                         Transmission Control Protocol
                                              Functional Specification

SEGMENT ARRIVES

      FIN-WAIT-1 STATE
        In addition to the processing for the ESTABLISHED state, if
        our FIN is now acknowledged then enter FIN-WAIT-2 and continue
        processing in that state.
      FIN-WAIT-2 STATE
        In addition to the processing for the ESTABLISHED state, if
        the retransmission queue is empty, the user's CLOSE can be
        acknowledged ("ok") but do not delete the TCB.
      CLOSE-WAIT STATE
        Do the same processing as for the ESTABLISHED state.
      CLOSING STATE
        In addition to the processing for the ESTABLISHED state, if
        the ACK acknowledges our FIN then enter the TIME-WAIT state,
        otherwise ignore the segment.
      LAST-ACK STATE
        The only thing that can arrive in this state is an
        acknowledgment of our FIN.  If our FIN is now acknowledged,
        delete the TCB, enter the CLOSED state, and return.
      TIME-WAIT STATE
        The only thing that can arrive in this state is a
        retransmission of the remote FIN.  Acknowledge it, and restart
        the 2 MSL timeout.
  sixth, check the URG bit,
    ESTABLISHED STATE
    FIN-WAIT-1 STATE
    FIN-WAIT-2 STATE
      If the URG bit is set, RCV.UP <- max(RCV.UP,SEG.UP), and signal
      the user that the remote side has urgent data if the urgent
      pointer (RCV.UP) is in advance of the data consumed.  If the
      user has already been signaled (or is still in the "urgent
      mode") for this continuous sequence of urgent data, do not
      signal the user again.
                                                             [Page 73]
                                                        September 1981

Transmission Control Protocol Functional Specification

                                                       SEGMENT ARRIVES
    CLOSE-WAIT STATE
    CLOSING STATE
    LAST-ACK STATE
    TIME-WAIT
      This should not occur, since a FIN has been received from the
      remote side.  Ignore the URG.
  seventh, process the segment text,
    ESTABLISHED STATE
    FIN-WAIT-1 STATE
    FIN-WAIT-2 STATE
      Once in the ESTABLISHED state, it is possible to deliver segment
      text to user RECEIVE buffers.  Text from segments can be moved
      into buffers until either the buffer is full or the segment is
      empty.  If the segment empties and carries an PUSH flag, then
      the user is informed, when the buffer is returned, that a PUSH
      has been received.
      When the TCP takes responsibility for delivering the data to the
      user it must also acknowledge the receipt of the data.
      Once the TCP takes responsibility for the data it advances
      RCV.NXT over the data accepted, and adjusts RCV.WND as
      apporopriate to the current buffer availability.  The total of
      RCV.NXT and RCV.WND should not be reduced.
      Please note the window management suggestions in section 3.7.
      Send an acknowledgment of the form:
        <SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK>
      This acknowledgment should be piggybacked on a segment being
      transmitted if possible without incurring undue delay.

[Page 74]

September 1981

                                         Transmission Control Protocol
                                              Functional Specification

SEGMENT ARRIVES

    CLOSE-WAIT STATE
    CLOSING STATE
    LAST-ACK STATE
    TIME-WAIT STATE
      This should not occur, since a FIN has been received from the
      remote side.  Ignore the segment text.
  eighth, check the FIN bit,
    Do not process the FIN if the state is CLOSED, LISTEN or SYN-SENT
    since the SEG.SEQ cannot be validated; drop the segment and
    return.
    If the FIN bit is set, signal the user "connection closing" and
    return any pending RECEIVEs with same message, advance RCV.NXT
    over the FIN, and send an acknowledgment for the FIN.  Note that
    FIN implies PUSH for any segment text not yet delivered to the
    user.
      SYN-RECEIVED STATE
      ESTABLISHED STATE
        Enter the CLOSE-WAIT state.
      FIN-WAIT-1 STATE
        If our FIN has been ACKed (perhaps in this segment), then
        enter TIME-WAIT, start the time-wait timer, turn off the other
        timers; otherwise enter the CLOSING state.
      FIN-WAIT-2 STATE
        Enter the TIME-WAIT state.  Start the time-wait timer, turn
        off the other timers.
      CLOSE-WAIT STATE
        Remain in the CLOSE-WAIT state.
      CLOSING STATE
        Remain in the CLOSING state.
      LAST-ACK STATE
        Remain in the LAST-ACK state.
                                                             [Page 75]
                                                        September 1981

Transmission Control Protocol Functional Specification

                                                       SEGMENT ARRIVES
      TIME-WAIT STATE
        Remain in the TIME-WAIT state.  Restart the 2 MSL time-wait
        timeout.
  and return.

[Page 76]

September 1981

                                         Transmission Control Protocol
                                              Functional Specification

USER TIMEOUT

USER TIMEOUT
  For any state if the user timeout expires, flush all queues, signal
  the user "error:  connection aborted due to user timeout" in general
  and for any outstanding calls, delete the TCB, enter the CLOSED
  state and return.
RETRANSMISSION TIMEOUT
  For any state if the retransmission timeout expires on a segment in
  the retransmission queue, send the segment at the front of the
  retransmission queue again, reinitialize the retransmission timer,
  and return.
TIME-WAIT TIMEOUT
  If the time-wait timeout expires on a connection delete the TCB,
  enter the CLOSED state and return.
                                                             [Page 77]
                                                        September 1981

Transmission Control Protocol

[Page 78]

September 1981

                                         Transmission Control Protocol
                              GLOSSARY

1822

        BBN Report 1822, "The Specification of the Interconnection of
        a Host and an IMP".  The specification of interface between a
        host and the ARPANET.

ACK

        A control bit (acknowledge) occupying no sequence space, which
        indicates that the acknowledgment field of this segment
        specifies the next sequence number the sender of this segment
        is expecting to receive, hence acknowledging receipt of all
        previous sequence numbers.

ARPANET message

        The unit of transmission between a host and an IMP in the
        ARPANET.  The maximum size is about 1012 octets (8096 bits).

ARPANET packet

        A unit of transmission used internally in the ARPANET between
        IMPs.  The maximum size is about 126 octets (1008 bits).

connection

        A logical communication path identified by a pair of sockets.

datagram

        A message sent in a packet switched computer communications
        network.

Destination Address

        The destination address, usually the network and host
        identifiers.

FIN

        A control bit (finis) occupying one sequence number, which
        indicates that the sender will send no more data or control
        occupying sequence space.

fragment

        A portion of a logical unit of data, in particular an internet
        fragment is a portion of an internet datagram.

FTP

        A file transfer protocol.
                                                             [Page 79]
                                                        September 1981

Transmission Control Protocol Glossary

header

        Control information at the beginning of a message, segment,
        fragment, packet or block of data.

host

        A computer.  In particular a source or destination of messages
        from the point of view of the communication network.

Identification

        An Internet Protocol field.  This identifying value assigned
        by the sender aids in assembling the fragments of a datagram.

IMP

        The Interface Message Processor, the packet switch of the
        ARPANET.

internet address

        A source or destination address specific to the host level.

internet datagram

        The unit of data exchanged between an internet module and the
        higher level protocol together with the internet header.

internet fragment

        A portion of the data of an internet datagram with an internet
        header.

IP

        Internet Protocol.

IRS

        The Initial Receive Sequence number.  The first sequence
        number used by the sender on a connection.

ISN

        The Initial Sequence Number.  The first sequence number used
        on a connection, (either ISS or IRS).  Selected on a clock
        based procedure.

ISS

        The Initial Send Sequence number.  The first sequence number
        used by the sender on a connection.

leader

        Control information at the beginning of a message or block of
        data.  In particular, in the ARPANET, the control information
        on an ARPANET message at the host-IMP interface.

[Page 80]

September 1981

                                         Transmission Control Protocol
                                                              Glossary

left sequence

        This is the next sequence number to be acknowledged by the
        data receiving TCP (or the lowest currently unacknowledged
        sequence number) and is sometimes referred to as the left edge
        of the send window.

local packet

        The unit of transmission within a local network.

module

        An implementation, usually in software, of a protocol or other
        procedure.

MSL

        Maximum Segment Lifetime, the time a TCP segment can exist in
        the internetwork system.  Arbitrarily defined to be 2 minutes.

octet

        An eight bit byte.

Options

        An Option field may contain several options, and each option
        may be several octets in length.  The options are used
        primarily in testing situations; for example, to carry
        timestamps.  Both the Internet Protocol and TCP provide for
        options fields.

packet

        A package of data with a header which may or may not be
        logically complete.  More often a physical packaging than a
        logical packaging of data.

port

        The portion of a socket that specifies which logical input or
        output channel of a process is associated with the data.

process

        A program in execution.  A source or destination of data from
        the point of view of the TCP or other host-to-host protocol.

PUSH

        A control bit occupying no sequence space, indicating that
        this segment contains data that must be pushed through to the
        receiving user.

RCV.NXT

        receive next sequence number
                                                             [Page 81]
                                                        September 1981

Transmission Control Protocol Glossary

RCV.UP

        receive urgent pointer

RCV.WND

        receive window

receive next sequence number

        This is the next sequence number the local TCP is expecting to
        receive.

receive window

        This represents the sequence numbers the local (receiving) TCP
        is willing to receive.  Thus, the local TCP considers that
        segments overlapping the range RCV.NXT to
        RCV.NXT + RCV.WND - 1 carry acceptable data or control.
        Segments containing sequence numbers entirely outside of this
        range are considered duplicates and discarded.

RST

        A control bit (reset), occupying no sequence space, indicating
        that the receiver should delete the connection without further
        interaction.  The receiver can determine, based on the
        sequence number and acknowledgment fields of the incoming
        segment, whether it should honor the reset command or ignore
        it.  In no case does receipt of a segment containing RST give
        rise to a RST in response.

RTP

        Real Time Protocol:  A host-to-host protocol for communication
        of time critical information.

SEG.ACK

        segment acknowledgment

SEG.LEN

        segment length

SEG.PRC

        segment precedence value

SEG.SEQ

        segment sequence

SEG.UP

        segment urgent pointer field

[Page 82]

September 1981

                                         Transmission Control Protocol
                                                              Glossary

SEG.WND

        segment window field

segment

        A logical unit of data, in particular a TCP segment is the
        unit of data transfered between a pair of TCP modules.

segment acknowledgment

        The sequence number in the acknowledgment field of the
        arriving segment.

segment length

        The amount of sequence number space occupied by a segment,
        including any controls which occupy sequence space.

segment sequence

        The number in the sequence field of the arriving segment.

send sequence

        This is the next sequence number the local (sending) TCP will
        use on the connection.  It is initially selected from an
        initial sequence number curve (ISN) and is incremented for
        each octet of data or sequenced control transmitted.

send window

        This represents the sequence numbers which the remote
        (receiving) TCP is willing to receive.  It is the value of the
        window field specified in segments from the remote (data
        receiving) TCP.  The range of new sequence numbers which may
        be emitted by a TCP lies between SND.NXT and
        SND.UNA + SND.WND - 1. (Retransmissions of sequence numbers
        between SND.UNA and SND.NXT are expected, of course.)

SND.NXT

        send sequence

SND.UNA

        left sequence

SND.UP

        send urgent pointer

SND.WL1

        segment sequence number at last window update

SND.WL2

        segment acknowledgment number at last window update
                                                             [Page 83]
                                                        September 1981

Transmission Control Protocol Glossary

SND.WND

        send window

socket

        An address which specifically includes a port identifier, that
        is, the concatenation of an Internet Address with a TCP port.

Source Address

        The source address, usually the network and host identifiers.

SYN

        A control bit in the incoming segment, occupying one sequence
        number, used at the initiation of a connection, to indicate
        where the sequence numbering will start.

TCB

        Transmission control block, the data structure that records
        the state of a connection.

TCB.PRC

        The precedence of the connection.

TCP

        Transmission Control Protocol:  A host-to-host protocol for
        reliable communication in internetwork environments.

TOS

        Type of Service, an Internet Protocol field.

Type of Service

        An Internet Protocol field which indicates the type of service
        for this internet fragment.

URG

        A control bit (urgent), occupying no sequence space, used to
        indicate that the receiving user should be notified to do
        urgent processing as long as there is data to be consumed with
        sequence numbers less than the value indicated in the urgent
        pointer.

urgent pointer

        A control field meaningful only when the URG bit is on.  This
        field communicates the value of the urgent pointer which
        indicates the data octet associated with the sending user's
        urgent call.

[Page 84]

September 1981

                                         Transmission Control Protocol
                             REFERENCES

[1] Cerf, V., and R. Kahn, "A Protocol for Packet Network

   Intercommunication", IEEE Transactions on Communications,
   Vol. COM-22, No. 5, pp 637-648, May 1974.

[2] Postel, J. (ed.), "Internet Protocol - DARPA Internet Program

   Protocol Specification", RFC 791, USC/Information Sciences
   Institute, September 1981.

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

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

[4] Postel, J., "Assigned Numbers", RFC 790, USC/Information Sciences

   Institute, September 1981.
                                                             [Page 85]
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