RFC 793 - Transmission Control Protocol

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

1. Introduction

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

2. Philosophy

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

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

References

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Preface

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

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

Transmission Control Protocol
DARPA INTERNET PROGRAM

PROTOCOL SPECIFICATION

1. Introduction

The Transmission Control Protocol (TCP) is intended for use as a highly

reliable host-to-host protocol between hosts in packet-switched computer
communication networks, and in interconnected systems of such networks.

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

1.1. Motivation

Computer communication systems are playing an increasingly important
role in military, government, and civilian environments. This

document focuses its attention primarily on military computer
communication requirements, especially robustness in the presence of

communication unreliability and availability in the presence of
congestion, but many of these problems are found in the civilian and government sector as well.

As strategic and tactical computer communication networks are
developed and deployed, it is essential to provide means of

interconnecting them and to provide standard interprocess

communication protocols which can support a broad range of
applications. In anticipation of the need for such standards, the

Deputy Undersecretary of Defense for Research and Engineering has

declared the Transmission Control Protocol (TCP) described herein to

be a basis for DoD-wide inter-process communication protocol
standardization. TCP is a connection-oriented, end-to-end reliable protocol designed to

fit into a layered hierarchy of protocols which support multi-network
applications. The TCP provides for reliable inter-process

communication between pairs of processes in host computers attached to
distinct but interconnected computer communication networks. Very few

assumptions are made as to the reliability of the communication
protocols below the TCP layer. TCP assumes it can obtain a simple,

potentially unreliable datagram service from the lower level
protocols. In principle, the TCP should be able to operate above a

wide spectrum of communication systems ranging from hard-wired
connections to packet-switched or circuit-switched networks.
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Introduction
TCP is based on concepts first described by Cerf and Kahn in [1]. The

TCP fits into a layered protocol architecture just above a basic
Internet Protocol [2] which provides a way for the TCP to send and

receive variable-length segments of information enclosed in internet
datagram "envelopes". The internet datagram provides a means for addressing source and destination TCPs in different networks. The

internet protocol also deals with any fragmentation or reassembly of

the TCP segments required to achieve transport and delivery through
multiple networks and interconnecting gateways. The internet protocol also carries information on the precedence, security classification and compartmentation of the TCP segments, so this information can be communicated end-to-end across multiple networks.

Protocol Layering
+---------------------+
| higher-level |
+---------------------+
| TCP |
+---------------------+
| internet protocol |
+---------------------+
|communication network|
+---------------------+
Figure 1

Much of this document is written in the context of TCP implementations

which are co-resident with higher level protocols in the host
computer. Some computer systems will be connected to networks via front-end computers which house the TCP and internet protocol layers, as well as network specific software. The TCP specification describes

an interface to the higher level protocols which appears to be
implementable even for the front-end case, as long as a suitable host-to-front end protocol is implemented.

1.2. Scope

The TCP is intended to provide a reliable process-to-process
communication service in a multinetwork environment. The TCP is

intended to be a host-to-host protocol in common use in multiple
networks.

1.3. About This Document

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

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

1.4. Interfaces

The TCP interfaces on one side to user or application processes and on
the other side to a lower level protocol such as Internet Protocol.

The interface between an application process and the TCP is
illustrated in reasonable detail. This interface consists of a set of

calls much like the calls an operating system provides to an
application process for manipulating files. For example, there are

calls to open and close connections and to send and receive data on
established connections. It is also expected that the TCP can asynchronously communicate with application programs. Although

considerable freedom is permitted to TCP implementors to design

interfaces which are appropriate to a particular operating system
environment, a minimum functionality is required at the TCP/user interface for any valid implementation.

The interface between TCP and lower level protocol is essentially

unspecified except that it is assumed there is a mechanism whereby the
two levels can asynchronously pass information to each other. Typically, one expects the lower level protocol to specify this interface. TCP is designed to work in a very general environment of interconnected networks. The lower level protocol which is assumed throughout this document is the Internet Protocol [2].

1.5. Operation

As noted above, the primary purpose of the TCP is to provide reliable,

securable logical circuit or connection service between pairs of
processes. To provide this service on top of a less reliable internet communication system requires facilities in the following areas:

Basic Data Transfer

Reliability

Flow Control

Multiplexing

Connections

Precedence and Security

The basic operation of the TCP in each of these areas is described in
the following paragraphs.
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Introduction
Basic Data Transfer:

The TCP is able to transfer a continuous stream of octets in each

direction between its users by packaging some number of octets into
segments for transmission through the internet system. In general,

the TCPs decide when to block and forward data at their own
convenience.

Sometimes users need to be sure that all the data they have
submitted to the TCP has been transmitted. For this purpose a push function is defined. To assure that data submitted to a TCP is

actually transmitted the sending user indicates that it should be
pushed through to the receiving user. A push causes the TCPs to promptly forward and deliver data up to that point to the receiver.

The exact push point might not be visible to the receiving user and
the push function does not supply a record boundary marker. Reliability: The TCP must recover from data that is damaged, lost, duplicated, or delivered out of order by the internet communication system. This

is achieved by assigning a sequence number to each octet
transmitted, and requiring a positive acknowledgment (ACK) from the receiving TCP. If the ACK is not received within a timeout interval, the data is retransmitted. At the receiver, the sequence

numbers are used to correctly order segments that may be received
out of order and to eliminate duplicates. Damage is handled by adding a checksum to each segment transmitted, checking it at the receiver, and discarding damaged segments.

As long as the TCPs continue to function properly and the internet
system does not become completely partitioned, no transmission errors will affect the correct delivery of data. TCP recovers from internet communication system errors. Flow Control:

TCP provides a means for the receiver to govern the amount of data
sent by the sender. This is achieved by returning a "window" with

every ACK indicating a range of acceptable sequence numbers beyond
the last segment successfully received. The window indicates an

allowed number of octets that the sender may transmit before
receiving further permission.
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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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2. Philosophy

2.1. Elements Of The Internetwork System

The internetwork environment consists of hosts connected to networks
which are in turn interconnected via gateways. It is assumed here that the networks may be either local networks (e.g., the ETHERNET) or large networks (e.g., the ARPANET), but in any case are based on packet switching technology. The active agents that produce and consume messages are processes. Various levels of protocols in the networks, the gateways, and the hosts support an interprocess

communication system that provides two-way data flow on logical
connections between process ports.

The term packet is used generically here to mean the data of one
transaction between a host and its network. The format of data blocks exchanged within the a network will generally not be of concern to us. Hosts are computers attached to a network, and from the communication network’s point of view, are the sources and destinations of packets.

Processes are viewed as the active elements in host computers (in

accordance with the fairly common definition of a process as a program
in execution). Even terminals and files or other I/O devices are viewed as communicating with each other through the use of processes. Thus, all communication is viewed as inter-process communication.

Since a process may need to distinguish among several communication
streams between itself and another process (or processes), we imagine

that each process may have a number of ports through which it
communicates with the ports of other processes.

2.2. Model Of Operation

Processes transmit data by calling on the TCP and passing buffers of
data as arguments. The TCP packages the data from these buffers into

segments and calls on the internet module to transmit each segment to
the destination TCP. The receiving TCP places the data from a segment into the receiving user’s buffer and notifies the receiving user. The

TCPs include control information in the segments which they use to
ensure reliable ordered data transmission.

The model of internet communication is that there is an internet

protocol module associated with each TCP which provides an interface
to the local network. This internet module packages TCP segments

inside internet datagrams and routes these datagrams to a destination
internet module or intermediate gateway. To transmit the datagram through the local network, it is embedded in a local network packet. The packet switches may perform further packaging, fragmentation, or
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other operations to achieve the delivery of the local packet to the
destination internet module. At a gateway between networks, the internet datagram is "unwrapped"

from its local packet and examined to determine through which network
the internet datagram should travel next. The internet datagram is then "wrapped" in a local packet suitable to the next network and routed to the next gateway, or to the final destination.

A gateway is permitted to break up an internet datagram into smaller

internet datagram fragments if this is necessary for transmission
through the next network. To do this, the gateway produces a set of internet datagrams; each carrying a fragment. Fragments may be further broken into smaller fragments at subsequent gateways. The

internet datagram fragment format is designed so that the destination
internet module can reassemble fragments into internet datagrams.

A destination internet module unwraps the segment from the datagram
(after reassembling the datagram, if necessary) and passes it to the destination TCP. This simple model of the operation glosses over many details. One important feature is the type of service. This provides information

to the gateway (or internet module) to guide it in selecting the
service parameters to be used in traversing the next network.

Included in the type of service information is the precedence of the
datagram. Datagrams may also carry security information to permit

host and gateways that operate in multilevel secure environments to
properly segregate datagrams for security considerations.

2.3. The Host Environment

The TCP is assumed to be a module in an operating system. The users access the TCP much like they would access the file system. The TCP may call on other operating system functions, for example, to manage data structures. The actual interface to the network is assumed to be controlled by a device driver module. The TCP does not call on the network device driver directly, but rather calls on the internet datagram protocol module which may in turn call on the device driver.

The mechanisms of TCP do not preclude implementation of the TCP in a
front-end processor. However, in such an implementation, a

host-to-front-end protocol must provide the functionality to support
the type of TCP-user interface described in this document.
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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 is made reliable via the use of sequence numbers and
acknowledgments. Conceptually, each octet of data is assigned a sequence number. The sequence number of the first octet of data in a

segment is transmitted with that segment and is called the segment
sequence number. Segments also carry an acknowledgment number which

is the sequence number of the next expected data octet of
transmissions in the reverse direction. When the TCP transmits a segment containing data, it puts a copy on a retransmission queue and starts a timer; when the acknowledgment for that data is received, the segment is deleted from the queue. If the acknowledgment is not received before the timer runs out, the segment is retransmitted.

An acknowledgment by TCP does not guarantee that the data has been
delivered to the end user, but only that the receiving TCP has taken the responsibility to do so. To govern the flow of data between TCPs, a flow control mechanism is employed. The receiving TCP reports a "window" to the sending TCP. This window specifies the number of octets, starting with the acknowledgment number, that the receiving TCP is currently prepared to receive.

2.7. Connection Establishment And Clearing

To identify the separate data streams that a TCP may handle, the TCP provides a port identifier. Since port identifiers are selected independently by each TCP they might not be unique. To provide for unique addresses within each TCP, we concatenate an internet address

identifying the TCP with a port identifier to create a socket which
will be unique throughout all networks connected together. A connection is fully specified by the pair of sockets at the ends. A

local socket may participate in many connections to different foreign
sockets. A connection can be used to carry data in both directions, that is, it is "full duplex". TCPs are free to associate ports with processes however they choose. However, several basic concepts are necessary in any implementation.

There must be well-known sockets which the TCP associates only with
the "appropriate" processes by some means. We envision that processes may "own" ports, and that processes can initiate connections only on the ports they own. (Means for implementing ownership is a local issue, but we envision a Request Port user command, or a method of uniquely allocating a group of ports to a given process, e.g., by associating the high order bits of a port name with a given process.)

A connection is specified in the OPEN call by the local port and
foreign socket arguments. In return, the TCP supplies a (short) local
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Philosophy

connection name by which the user refers to the connection in
subsequent calls. There are several things that must be remembered about a connection. To store this information we imagine that there is a data structure called a Transmission Control Block (TCB). One

implementation strategy would have the local connection name be a
pointer to the TCB for this connection. The OPEN call also specifies whether the connection establishment is to be actively pursued, or to be passively waited for.

A passive OPEN request means that the process wants to accept incoming
connection requests rather than attempting to initiate a connection.

Often the process requesting a passive OPEN will accept a connection
request from any caller. In this case a foreign socket of all zeros is used to denote an unspecified socket. Unspecified foreign sockets are allowed only on passive OPENs.

A service process that wished to provide services for unknown other

processes would issue a passive OPEN request with an unspecified
foreign socket. Then a connection could be made with any process that requested a connection to this local socket. It would help if this local socket were known to be associated with this service.

Well-known sockets are a convenient mechanism for a priori associating
a socket address with a standard service. For instance, the "Telnet-Server" process is permanently assigned to a particular socket, and other sockets are reserved for File Transfer, Remote Job Entry, Text Generator, Echoer, and Sink processes (the last three being for test purposes). A socket address might be reserved for access to a "Look-Up" service which would return the specific socket at which a newly created service would be provided. The concept of a well-known socket is part of the TCP specification, but the assignment of sockets to services is outside this specification. (See [4].)

Processes can issue passive OPENs and wait for matching active OPENs

from other processes and be informed by the TCP when connections have
been established. Two processes which issue active OPENs to each other at the same time will be correctly connected. This flexibility

is critical for the support of distributed computing in which
components act asynchronously with respect to each other.

There are two principal cases for matching the sockets in the local
passive OPENs and an foreign active OPENs. In the first case, the local passive OPENs has fully specified the foreign socket. In this case, the match must be exact. In the second case, the local passive OPENs has left the foreign socket unspecified. In this case, any foreign socket is acceptable as long as the local sockets match. Other possibilities include partially restricted matches.
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If there are several pending passive OPENs (recorded in TCBs) with the
same local socket, an foreign active OPEN will be matched to a TCB with the specific foreign socket in the foreign active OPEN, if such a TCB exists, before selecting a TCB with an unspecified foreign socket.

The procedures to establish connections utilize the synchronize (SYN)
control flag and involves an exchange of three messages. This exchange has been termed a three-way hand shake [3].

A connection is initiated by the rendezvous of an arriving segment

containing a SYN and a waiting TCB entry each created by a user OPEN
command. The matching of local and foreign sockets determines when a connection has been initiated. The connection becomes "established" when sequence numbers have been synchronized in both directions. The clearing of a connection also involves the exchange of segments, in this case carrying the FIN control flag.

2.8. Data Communication

The data that flows on a connection may be thought of as a stream of
octets. The sending user indicates in each SEND call whether the data

in that call (and any preceeding calls) should be immediately pushed
through to the receiving user by the setting of the PUSH flag.

A sending TCP is allowed to collect data from the sending user and to
send that data in segments at its own convenience, until the push function is signaled, then it must send all unsent data. When a receiving TCP sees the PUSH flag, it must not wait for more data from the sending TCP before passing the data to the receiving process.

There is no necessary relationship between push functions and segment
boundaries. The data in any particular segment may be the result of a single SEND call, in whole or part, or of multiple SEND calls.

The purpose of push function and the PUSH flag is to push data through
from the sending user to the receiving user. It does not provide a record service.

There is a coupling between the push function and the use of buffers
of data that cross the TCP/user interface. Each time a PUSH flag is associated with data placed into the receiving user’s buffer, the

buffer is returned to the user for processing even if the buffer is
not filled. If data arrives that fills the user’s buffer before a PUSH is seen, the data is passed to the user in buffer size units.

TCP also provides a means to communicate to the receiver of data that

at some point further along in the data stream than the receiver is
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currently reading there is urgent data. TCP does not attempt to

define what the user specifically does upon being notified of pending
urgent data, but the general notion is that the receiving process will take action to process the urgent data quickly.

2.9. Precedence And Security

The TCP makes use of the internet protocol type of service field and

security option to provide precedence and security on a per connection
basis to TCP users. Not all TCP modules will necessarily function in a multilevel secure environment; some may be limited to unclassified use only, and others may operate at only one security 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.
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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.
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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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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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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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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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+---------+ ---------\ 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 232 - 1. Since the space is finite, all arithmetic dealing with sequence numbers must be performed modulo 232. This unsigned arithmetic

preserves the relationship of sequence numbers as they cycle from
232 - 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 232).

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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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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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
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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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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
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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> --> 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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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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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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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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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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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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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 232 the size of the sequence number space. Also note that "=<" means less than or equal to (modulo 232).
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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
CLOSED STATE (i.e., TCB does not exist)

OPEN Call

Create a new transmission control block (TCB) to hold connection
state information. Fill in local socket identifier, foreign socket, precedence, security/compartment, and user timeout information. Note that some parts of the foreign socket may be

unspecified in a passive OPEN and are to be filled in by the
parameters of the incoming SYN segment. Verify the security and precedence requested are allowed for this user, if not return "error: precedence not allowed" or "error: security/compartment not allowed." If passive enter the LISTEN state and return. If active and the foreign socket is unspecified, return "error: foreign socket unspecified"; if active and the foreign socket is specified, issue a SYN segment. An initial send sequence number (ISS) is selected. A SYN segment of the form <SEQ=ISS><CTL=SYN> is sent. Set SND.UNA to ISS, SND.NXT to ISS+1, enter SYN-SENT state, and return. If the caller does not have access to the local socket specified, return "error: connection illegal for this process". If there is no room to create a new connection, return "error: insufficient resources".

LISTEN STATE
If active and the foreign socket is specified, then change the connection from passive to active, select an ISS. Send a SYN segment, set SND.UNA to ISS, SND.NXT to ISS+1. Enter SYN-SENT state. Data associated with SEND may be sent with SYN segment or queued for transmission after entering ESTABLISHED state. The

urgent bit if requested in the command must be sent with the data
segments sent as a result of this command. If there is no room to queue the request, respond with "error: insufficient resources". If Foreign socket was not specified, then return "error: foreign socket unspecified".
[Page 54]

September 1981

Transmission Control Protocol
Functional Specification

OPEN Call

SYN-SENT STATE

SYN-RECEIVED STATE

ESTABLISHED STATE

FIN-WAIT-1 STATE

FIN-WAIT-2 STATE

CLOSE-WAIT STATE

CLOSING STATE

LAST-ACK STATE

TIME-WAIT STATE
Return "error: connection already exists".
[Page 55]

September 1981

Transmission Control Protocol
Functional Specification

SEND Call
CLOSED STATE (i.e., TCB does not exist)

SEND Call
If the user does not have access to such a connection, then return "error: connection illegal for this process". Otherwise, return "error: connection does not exist".

LISTEN STATE
If the foreign socket is specified, then change the connection from passive to active, select an ISS. Send a SYN segment, set SND.UNA to ISS, SND.NXT to ISS+1. Enter SYN-SENT state. Data

associated with SEND may be sent with SYN segment or queued for
transmission after entering ESTABLISHED state. The urgent bit if

requested in the command must be sent with the data segments sent
as a result of this command. If there is no room to queue the request, respond with "error: insufficient resources". If Foreign socket was not specified, then return "error: foreign socket unspecified".

SYN-SENT STATE

SYN-RECEIVED STATE
Queue the data for transmission after entering ESTABLISHED state. If no space to queue, respond with "error: insufficient resources".

ESTABLISHED STATE

CLOSE-WAIT STATE

Segmentize the buffer and send it with a piggybacked
acknowledgment (acknowledgment value = RCV.NXT). If there is insufficient space to remember this buffer, simply return "error: insufficient resources". If the urgent flag is set, then SND.UP <- SND.NXT-1 and set the urgent pointer in the outgoing segments.
[Page 56]

September 1981

Transmission Control Protocol
Functional Specification

SEND Call

FIN-WAIT-1 STATE

FIN-WAIT-2 STATE

CLOSING STATE

LAST-ACK STATE

TIME-WAIT STATE
Return "error: connection closing" and do not service request.
[Page 57]

September 1981

Transmission Control Protocol
Functional Specification

RECEIVE Call
CLOSED STATE (i.e., TCB does not exist)

RECEIVE Call
If the user does not have access to such a connection, return "error: connection illegal for this process". Otherwise return "error: connection does not exist".

LISTEN STATE

SYN-SENT STATE

SYN-RECEIVED STATE
Queue for processing after entering ESTABLISHED state. If there is no room to queue this request, respond with "error: insufficient resources".

ESTABLISHED STATE

FIN-WAIT-1 STATE

FIN-WAIT-2 STATE

If insufficient incoming segments are queued to satisfy the
request, queue the request. If there is no queue space to remember the RECEIVE, respond with "error: insufficient resources".

Reassemble queued incoming segments into receive buffer and return
to user. Mark "push seen" (PUSH) if this is the case. If RCV.UP is in advance of the data currently being passed to the user notify the user of the presence of urgent data.

When the TCP takes responsibility for delivering data to the user

that fact must be communicated to the sender via an
acknowledgment. The formation of such an acknowledgment is

described below in the discussion of processing an incoming
segment.
[Page 58]

September 1981

Transmission Control Protocol
Functional Specification

RECEIVE Call

CLOSE-WAIT STATE
Since the remote side has already sent FIN, RECEIVEs must be satisfied by text already on hand, but not yet delivered to the user. If no text is awaiting delivery, the RECEIVE will get a "error: connection closing" response. Otherwise, any remaining text can be used to satisfy the RECEIVE.

CLOSING STATE

LAST-ACK STATE

TIME-WAIT STATE
Return "error: connection closing".
[Page 59]

September 1981

Transmission Control Protocol
Functional Specification

CLOSE Call
CLOSED STATE (i.e., TCB does not exist)

CLOSE Call
If the user does not have access to such a connection, return "error: connection illegal for this process". Otherwise, return "error: connection does not exist".

LISTEN STATE
Any outstanding RECEIVEs are returned with "error: closing" responses. Delete TCB, enter CLOSED state, and return.

SYN-SENT STATE
Delete the TCB and return "error: closing" responses to any queued SENDs, or RECEIVEs.

SYN-RECEIVED STATE
If no SENDs have been issued and there is no pending data to send, then form a FIN segment and send it, and enter FIN-WAIT-1 state; otherwise queue for processing after entering ESTABLISHED state.

ESTABLISHED STATE
Queue this until all preceding SENDs have been segmentized, then form a FIN segment and send it. In any case, enter FIN-WAIT-1 state.

FIN-WAIT-1 STATE

FIN-WAIT-2 STATE
Strictly speaking, this is an error and should receive a "error: connection closing" response. An "ok" response would be acceptable, too, as long as a second FIN is not emitted (the first FIN may be retransmitted though).
[Page 60]

September 1981

Transmission Control Protocol
Functional Specification

CLOSE Call

CLOSE-WAIT STATE

Queue this request until all preceding SENDs have been
segmentized; then send a FIN segment, enter CLOSING state.

CLOSING STATE

LAST-ACK STATE

TIME-WAIT STATE
Respond with "error: connection closing".
[Page 61]

September 1981

Transmission Control Protocol
Functional Specification

ABORT Call
CLOSED STATE (i.e., TCB does not exist)

ABORT Call
If the user should not have access to such a connection, return "error: connection illegal for this process". Otherwise return "error: connection does not exist".

LISTEN STATE
Any outstanding RECEIVEs should be returned with "error: connection reset" responses. Delete TCB, enter CLOSED state, and return.

SYN-SENT STATE
All queued SENDs and RECEIVEs should be given "connection reset" notification, delete the TCB, enter CLOSED state, and return.

SYN-RECEIVED STATE

ESTABLISHED STATE

FIN-WAIT-1 STATE

FIN-WAIT-2 STATE

CLOSE-WAIT STATE
Send a reset segment: <SEQ=SND.NXT><CTL=RST> All queued SENDs and RECEIVEs should be given "connection reset" notification; all segments queued for transmission (except for the RST formed above) or retransmission should be flushed, delete the TCB, enter CLOSED state, and return.

CLOSING STATE

LAST-ACK STATE

TIME-WAIT STATE
Respond with "ok" and delete the TCB, enter CLOSED state, and return.
[Page 62]

September 1981

Transmission Control Protocol
Functional Specification

STATUS Call

STATUS Call
CLOSED STATE (i.e., TCB does not exist) If the user should not have access to such a connection, return "error: connection illegal for this process". Otherwise return "error: connection does not exist".

LISTEN STATE
Return "state = LISTEN", and the TCB pointer.

SYN-SENT STATE
Return "state = SYN-SENT", and the TCB pointer.

SYN-RECEIVED STATE
Return "state = SYN-RECEIVED", and the TCB pointer.

ESTABLISHED STATE
Return "state = ESTABLISHED", and the TCB pointer.

FIN-WAIT-1 STATE
Return "state = FIN-WAIT-1", and the TCB pointer.

FIN-WAIT-2 STATE
Return "state = FIN-WAIT-2", and the TCB pointer.

CLOSE-WAIT STATE
Return "state = CLOSE-WAIT", and the TCB pointer.

CLOSING STATE
Return "state = CLOSING", and the TCB pointer.

LAST-ACK STATE
Return "state = LAST-ACK", and the TCB pointer.
[Page 63]

September 1981

Transmission Control Protocol
Functional Specification

STATUS Call

TIME-WAIT STATE
Return "state = TIME-WAIT", and the TCB pointer.
[Page 64]

September 1981

Transmission Control Protocol
Functional Specification

SEGMENT ARRIVES

SEGMENT ARRIVES
If the state is CLOSED (i.e., TCB does not exist) then all data in the incoming segment is discarded. An incoming segment containing a RST is discarded. An incoming segment not containing a RST causes a RST to be sent in response. The

acknowledgment and sequence field values are selected to make the

reset sequence acceptable to the TCP that sent the offending
segment. If the ACK bit is off, sequence number zero is used, <SEQ=0><ACK=SEG.SEQ+SEG.LEN><CTL=RST,ACK> If the ACK bit is on, <SEQ=SEG.ACK><CTL=RST> Return.

If the state is LISTEN then

first check for an RST
An incoming RST should be ignored. Return.

second check for an ACK

Any acknowledgment is bad if it arrives on a connection still in
the LISTEN state. An acceptable reset segment should be formed for any arriving ACK-bearing segment. The RST should be formatted as follows: <SEQ=SEG.ACK><CTL=RST> Return.

third check for a SYN
If the SYN bit is set, check the security. If the

security/compartment on the incoming segment does not exactly

match the security/compartment in the TCB then send a reset and
return. <SEQ=SEG.ACK><CTL=RST>
[Page 65]

September 1981

Transmission Control Protocol
Functional Specification

SEGMENT ARRIVES
If the SEG.PRC is greater than the TCB.PRC then if allowed by the user and the system set TCB.PRC<-SEG.PRC, if not allowed send a reset and return. <SEQ=SEG.ACK><CTL=RST> If the SEG.PRC is less than the TCB.PRC then continue. Set RCV.NXT to SEG.SEQ+1, IRS is set to SEG.SEQ and any other control or text should be queued for processing later. ISS should be selected and a SYN segment sent of the form: <SEQ=ISS><ACK=RCV.NXT><CTL=SYN,ACK> SND.NXT is set to ISS+1 and SND.UNA to ISS. The connection state should be changed to SYN-RECEIVED. Note that any other

incoming control or data (combined with SYN) will be processed
in the SYN-RECEIVED state, but processing of SYN and ACK should not be repeated. If the listen was not fully specified (i.e., the foreign socket was not fully specified), then the unspecified fields should be filled in now.

fourth other text or control

Any other control or text-bearing segment (not containing SYN)

must have an ACK and thus would be discarded by the ACK
processing. An incoming RST segment could not be valid, since

it could not have been sent in response to anything sent by this
incarnation of the connection. So you are unlikely to get here, but if you do, drop the segment, and return.

If the state is SYN-SENT then

first check the ACK bit

If the ACK bit is set
If SEG.ACK =< ISS, or SEG.ACK > SND.NXT, send a reset (unless the RST bit is set, if so drop the segment and return) <SEQ=SEG.ACK><CTL=RST> and discard the segment. Return. If SND.UNA =< SEG.ACK =< SND.NXT then the ACK is acceptable.

second check the RST bit
[Page 66]

September 1981

Transmission Control Protocol
Functional Specification

SEGMENT ARRIVES

If the RST bit is set
If the ACK was acceptable then signal the user "error: connection reset", drop the segment, enter CLOSED state, delete TCB, and return. Otherwise (no ACK) drop the segment and return.

third check the security and precedence

If the security/compartment in the segment does not exactly
match the security/compartment in the TCB, send a reset

If there is an ACK
<SEQ=SEG.ACK><CTL=RST>

Otherwise
<SEQ=0><ACK=SEG.SEQ+SEG.LEN><CTL=RST,ACK>

If there is an ACK

The precedence in the segment must match the precedence in the
TCB, if not, send a reset <SEQ=SEG.ACK><CTL=RST>

If there is no ACK

If the precedence in the segment is higher than the precedence

in the TCB then if allowed by the user and the system raise
the precedence in the TCB to that in the segment, if not allowed to raise the prec then send a reset. <SEQ=0><ACK=SEG.SEQ+SEG.LEN><CTL=RST,ACK>

If the precedence in the segment is lower than the precedence
in the TCB continue. If a reset was sent, discard the segment and return.

fourth check the SYN bit
This step should be reached only if the ACK is ok, or there is no ACK, and it the segment did not contain a RST.

If the SYN bit is on and the security/compartment and precedence
[Page 67]

September 1981

Transmission Control Protocol
Functional Specification

SEGMENT ARRIVES
are acceptable then, RCV.NXT is set to SEG.SEQ+1, IRS is set to SEG.SEQ. SND.UNA should be advanced to equal SEG.ACK (if there is an ACK), and any segments on the retransmission queue which are thereby acknowledged should be removed. If SND.UNA > ISS (our SYN has been ACKed), change the connection state to ESTABLISHED, form an ACK segment <SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK> and send it. Data or controls which were queued for transmission may be included. If there are other controls or

text in the segment then continue processing at the sixth step
below where the URG bit is checked, otherwise return. Otherwise enter SYN-RECEIVED, form a SYN,ACK segment <SEQ=ISS><ACK=RCV.NXT><CTL=SYN,ACK> and send it. If there are other controls or text in the segment, queue them for processing after the ESTABLISHED state has been reached, return. fifth, if neither of the SYN or RST bits is set then drop the segment and return.
[Page 68]

September 1981

Transmission Control Protocol
Functional Specification

SEGMENT ARRIVES
Otherwise,

first check sequence number

SYN-RECEIVED STATE

ESTABLISHED STATE

FIN-WAIT-1 STATE

FIN-WAIT-2 STATE

CLOSE-WAIT STATE

CLOSING STATE

LAST-ACK STATE

TIME-WAIT STATE
Segments are processed in sequence. Initial tests on arrival are used to discard old duplicates, but further processing is done in SEG.SEQ order. If a segment’s contents straddle the boundary between old and new, only the new parts should be processed.

There are four cases for the acceptability test for an incoming
segment:

Segment Receive Test

Length Window
------- ------- ------------------------------------------ 0 0 SEG.SEQ = RCV.NXT 0 >0 RCV.NXT =< SEG.SEQ < RCV.NXT+RCV.WND >0 0 not acceptable >0 >0 RCV.NXT =< SEG.SEQ < RCV.NXT+RCV.WND or RCV.NXT =< SEG.SEQ+SEG.LEN-1 < RCV.NXT+RCV.WND If the RCV.WND is zero, no segments will be acceptable, but special allowance should be made to accept valid ACKs, URGs and RSTs. If an incoming segment is not acceptable, an acknowledgment should be sent in reply (unless the RST bit is set, if so drop the segment and return): <SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK> After sending the acknowledgment, drop the unacceptable segment and return.
[Page 69]

September 1981

Transmission Control Protocol
Functional Specification

SEGMENT ARRIVES

In the following it is assumed that the segment is the idealized
segment that begins at RCV.NXT and does not exceed the window.

One could tailor actual segments to fit this assumption by

trimming off any portions that lie outside the window (including
SYN and FIN), and only processing further if the segment then begins at RCV.NXT. Segments with higher begining sequence numbers may be held for later processing. second check the RST bit,

SYN-RECEIVED STATE

If the RST bit is set
If this connection was initiated with a passive OPEN (i.e., came from the LISTEN state), then return this connection to LISTEN state and return. The user need not be informed. If this connection was initiated with an active OPEN (i.e., came from SYN-SENT state) then the connection was refused, signal the user "connection refused". In either case, all segments on the retransmission queue should be removed. And in the active OPEN case, enter the CLOSED state and delete the TCB, and return.

ESTABLISHED

FIN-WAIT-1

FIN-WAIT-2

CLOSE-WAIT
If the RST bit is set then, any outstanding RECEIVEs and SEND should receive "reset" responses. All segment queues should be flushed. Users should also receive an unsolicited general "connection reset" signal. Enter the CLOSED state, delete the TCB, and return.

CLOSING STATE

LAST-ACK STATE

TIME-WAIT
If the RST bit is set then, enter the CLOSED state, delete the TCB, and return.
[Page 70]

September 1981

Transmission Control Protocol
Functional Specification

SEGMENT ARRIVES

third check security and precedence

SYN-RECEIVED

If the security/compartment and precedence in the segment do not

exactly match the security/compartment and precedence in the TCB
then send a reset, and return.

ESTABLISHED STATE

If the security/compartment and precedence in the segment do not

exactly match the security/compartment and precedence in the TCB
then send a reset, any outstanding RECEIVEs and SEND should receive "reset" responses. All segment queues should be flushed. Users should also receive an unsolicited general "connection reset" signal. Enter the CLOSED state, delete the TCB, and return.

Note this check is placed following the sequence check to prevent

a segment from an old connection between these ports with a

different security or precedence from causing an abort of the
current connection. fourth, check the SYN bit,

SYN-RECEIVED

ESTABLISHED STATE

FIN-WAIT STATE-1

FIN-WAIT STATE-2

CLOSE-WAIT STATE

CLOSING STATE

LAST-ACK STATE

TIME-WAIT STATE
If the SYN is in the window it is an error, send a reset, any outstanding RECEIVEs and SEND should receive "reset" responses, all segment queues should be flushed, the user should also receive an unsolicited general "connection reset" signal, enter the CLOSED state, delete the TCB, and return.

If the SYN is not in the window this step would not be reached

and an ack would have been sent in the first step (sequence
number check).
[Page 71]

September 1981

Transmission Control Protocol
Functional Specification
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

SEGMENT ARRIVES
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

CLOSE-WAIT STATE

CLOSING STATE

LAST-ACK STATE

TIME-WAIT

SEGMENT ARRIVES
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 ACK BBN Report 1822, "The Specification of the Interconnection of a Host and an IMP". The specification of interface between a host and the ARPANET. 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 fragment FTP A control bit (finis) occupying one sequence number, which

indicates that the sender will send no more data or control
occupying sequence space. A portion of a logical unit of data, in particular an internet fragment is a portion of an internet datagram. A file transfer protocol.
[Page 79]

September 1981

Transmission Control Protocol

Glossary

header host Control information at the beginning of a message, segment, fragment, packet or block of data. 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 IRS ISN ISS leader
[Page 80]
Internet Protocol. The Initial Receive Sequence number. The first sequence number used by the sender on a connection. The Initial Sequence Number. The first sequence number used on a connection, (either ISS or IRS). Selected on a clock based procedure. The Initial Send Sequence number. The first sequence number used by the sender on a connection.

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.

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 MSL octet Options packet port process PUSH RCV.NXT An implementation, usually in software, of a protocol or other procedure. Maximum Segment Lifetime, the time a TCP segment can exist in the internetwork system. Arbitrarily defined to be 2 minutes. An eight bit byte. 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.

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.

The portion of a socket that specifies which logical input or
output channel of a process is associated with the data. A program in execution. A source or destination of data from the point of view of the TCP or other host-to-host protocol. A control bit occupying no sequence space, indicating that

this segment contains data that must be pushed through to the
receiving user.

receive next sequence number
[Page 81]

September 1981

Transmission Control Protocol

Glossary

RCV.UP RCV.WND

receive urgent pointer

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 RTP SEG.ACK SEG.LEN SEG.PRC SEG.SEQ SEG.UP
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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. Real Time Protocol: A host-to-host protocol for communication of time critical information.

segment acknowledgment

segment length

segment precedence value

segment sequence

segment urgent pointer field

September 1981

Transmission Control Protocol

Glossary

SEG.WND segment

segment window field
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 SND.UNA SND.UP SND.WL1 SND.WL2

send sequence

left sequence

send urgent pointer

segment sequence number at last window update

segment acknowledgment number at last window update
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September 1981

Transmission Control Protocol

Glossary

SND.WND socket

send window
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 TCB TCB.PRC TCP TOS 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. Transmission control block, the data structure that records the state of a connection. The precedence of the connection. Transmission Control Protocol: A host-to-host protocol for reliable communication in internetwork environments. Type of Service, an Internet Protocol field.

Type of Service

An Internet Protocol field which indicates the type of service
for this internet fragment. URG A control bit (urgent), occupying no sequence space, used to

indicate that the receiving user should be notified to do

urgent processing as long as there is data to be consumed with

sequence numbers less than the value indicated in the urgent
pointer.

urgent pointer
A control field meaningful only when the URG bit is on. This

field communicates the value of the urgent pointer which
indicates the data octet associated with the sending user’s urgent call.
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September 1981

Transmission Control Protocol

References

[1] Cerf, V., and R. Kahn, "A Protocol for Packet Network
Intercommunication", IEEE Transactions on Communications, Vol. COM-22, No. 5, pp 637-648, May 1974.
[2] Postel, J. (ed.), "Internet Protocol - DARPA Internet Program
Protocol Specification", RFC 791, USC/Information Sciences Institute, September 1981.
[3] Dalal, Y. and C. Sunshine, "Connection Management in Transport
Protocols", Computer Networks, Vol. 2, No. 6, pp. 454-473, December 1978.
[4] Postel, J., "Assigned Numbers", RFC 790, USC/Information Sciences
Institute, September 1981.
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