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RFC793 - Transmission Control Protocol

王朝other·作者佚名  2008-05-31
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RFC: 793

TRANSMISSION CONTROL PROTOCOL

DARPA INTERNET PROGRAM

PROTOCOL SPECIFICATION

September 1981

prepared for

Defense Advanced Research Projects Agency

Information Processing Techniques Office

1400 Wilson Boulevard

Arlington, Virginia 22209

by

Information Sciences Institute

University of Southern California

4676 Admiralty Way

Marina del Rey, California 90291

September 1981

Transmission Control Protocol

TABLE OF CONTENTS

PREFACE ........................................................ iii

1. INTRODUCTION ..................................................... 1

1.1 Motivation .................................................... 1

1.2 Scope ......................................................... 2

1.3 About This Document ........................................... 2

1.4 Interfaces .................................................... 3

1.5 Operation ..................................................... 3

2. PHILOSOPHY ....................................................... 7

2.1 Elements of the Internetwork System ........................... 7

2.2 Model of Operation ............................................ 7

2.3 The Host Environment .......................................... 8

2.4 Interfaces .................................................... 9

2.5 Relation to Other Protocols ................................... 9

2.6 Reliable Communication ........................................ 9

2.7 Connection Establishment and Clearing ........................ 10

2.8 Data Communication ........................................... 12

2.9 Precedence and Security ...................................... 13

2.10 Robustness Principle ......................................... 13

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

3.1 Header Format ................................................ 15

3.2 Terminology .................................................. 19

3.3 Sequence Numbers ............................................. 24

3.4 Establishing a connection .................................... 30

3.5 Closing a Connection ......................................... 37

3.6 Precedence and Security ...................................... 40

3.7 Data Communication ........................................... 40

3.8 Interfaces ................................................... 44

3.9 Event Processing ............................................. 52

GLOSSARY ............................................................ 79

REFERENCES .......................................................... 85

[Page i]

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Transmission Control Protocol

[Page ii]

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Transmission Control Protocol

PREFACE

This document describes the DoD Standard Transmission Control Protocol

(TCP). There have been nine earlier editions of the ARPA TCP

specification on which this standard is based, and the present text

draws heavily from them. There have been many contributors to this work

both in terms of concepts and in terms of text. This edition clarifies

several details and removes the end-of-letter buffer-size adjustments,

and redescribes the letter mechanism as a push function.

Jon Postel

Editor

RFC: 793

Replaces: RFC761

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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Transmission Control Protocol

Introduction

TCP is based on concepts first described by Cerf and Kahn in [1]. The

TCP fits into a layered protocol architecture just above a basic

Internet Protocol [2] which provides a way for the TCP to send and

receive variable-length segments of information enclosed in internet

datagram "envelopes". The internet datagram provides a means for

addressing source and destination TCPs in different networks. The

internet protocol also deals with any fragmentation or reassembly of

the TCP segments required to achieve transport and delivery through

multiple networks and interconnecting gateways. The internet protocol

also carries information on the precedence, security classification

and compartmentation of the TCP segments, so this information can be

communicated end-to-end across multiple networks.

Protocol Layering

+---------------------+

higher-level

+---------------------+

TCP

+---------------------+

internet protocol

+---------------------+

communication network

+---------------------+

Figure 1

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

which are co-resident with higher level protocols in the host

computer. Some computer systems will be connected to networks via

front-end computers which house the TCP and internet protocol layers,

as well as network specific software. The TCP specification describes

an interface to the higher level protocols which appears to be

implementable even for the front-end case, as long as a suitable

host-to-front end protocol is implemented.

1.2. Scope

The TCP is intended to provide a reliable process-to-process

communication service in a multinetwork environment. The TCP is

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

networks.

1.3. About this Document

This document represents a specification of the behavior required of

any TCP implementation, both in its interactions with higher level

protocols and in its interactions with other TCPs. The rest of this

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Transmission Control Protocol

Introduction

section offers a very brief view of the protocol interfaces and

operation. Section 2 summarizes the philosophical basis for the TCP

design. Section 3 offers both a detailed description of the actions

required of TCP when various events occur (arrival of new segments,

user calls, errors, etc.) and the details of the formats of TCP

segments.

1.4. Interfaces

The TCP interfaces on one side to user or application processes and on

the other side to a lower level protocol such as Internet Protocol.

The interface between an application process and the TCP is

illustrated in reasonable detail. This interface consists of a set of

calls much like the calls an operating system provides to an

application process for manipulating files. For example, there are

calls to open and close connections and to send and receive data on

established connections. It is also eXPected that the TCP can

asynchronously communicate with application programs. Although

considerable freedom is permitted to TCP implementors to design

interfaces which are appropriate to a particular operating system

environment, a minimum functionality is required at the TCP/user

interface for any valid implementation.

The interface between TCP and lower level protocol is essentially

unspecified except that it is assumed there is a mechanism whereby the

two levels can asynchronously pass information to each other.

Typically, one expects the lower level protocol to specify this

interface. TCP is designed to work in a very general environment of

interconnected networks. The lower level protocol which is assumed

throughout this document is the Internet Protocol [2].

1.5. Operation

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

securable logical circuit or connection service between pairs of

processes. To provide this service on top of a less reliable internet

communication system requires facilities in the following areas:

Basic Data Transfer

Reliability

Flow Control

Multiplexing

Connections

Precedence and Security

The basic operation of the TCP in each of these areas is described in

the following paragraphs.

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Transmission Control Protocol

Introduction

Basic Data Transfer:

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

direction between its users by packaging some number of octets into

segments for transmission through the internet system. In general,

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

convenience.

Sometimes users need to be sure that all the data they have

submitted to the TCP has been transmitted. For this purpose a push

function is defined. To assure that data submitted to a TCP is

actually transmitted the sending user indicates that it should be

pushed through to the receiving user. A push causes the TCPs to

promptly forward and deliver data up to that point to the receiver.

The exact push point might not be visible to the receiving user and

the push function does not supply a record boundary marker.

Reliability:

The TCP must recover from data that is damaged, lost, duplicated, or

delivered out of order by the internet communication system. This

is achieved by assigning a sequence number to each octet

transmitted, and requiring a positive acknowledgment (ACK) from the

receiving TCP. If the ACK is not received within a timeout

interval, the data is retransmitted. At the receiver, the sequence

numbers are used to correctly order segments that may be received

out of order and to eliminate duplicates. Damage is handled by

adding a checksum to each segment transmitted, checking it at the

receiver, and discarding damaged segments.

As long as the TCPs continue to function properly and the internet

system does not become completely partitioned, no transmission

errors will affect the correct delivery of data. TCP recovers from

internet communication system errors.

Flow Control:

TCP provides a means for the receiver to govern the amount of data

sent by the sender. This is achieved by returning a "window" with

every ACK indicating a range of acceptable sequence numbers beyond

the last segment successfully received. The window indicates an

allowed number of octets that the sender may transmit before

receiving further permission.

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Transmission Control Protocol

Introduction

Multiplexing:

To allow for many processes within a single Host to use TCP

communication facilities simultaneously, the TCP provides a set of

addresses or ports within each host. Concatenated with the network

and host addresses from the internet communication layer, this forms

a socket. A pair of sockets uniquely identifies each connection.

That is, a socket may be simultaneously used in multiple

connections.

The binding of ports to processes is handled independently by each

Host. However, it proves useful to attach frequently used processes

(e.g., a "logger" or timesharing service) to fixed sockets which are

made known to the public. These services can then be Accessed

through the known addresses. Establishing and learning the port

addresses of other processes may involve more dynamic mechanisms.

Connections:

The reliability and flow control mechanisms described above require

that TCPs initialize and maintain certain status information for

each data stream. The combination of this information, including

sockets, sequence numbers, and window sizes, is called a connection.

Each connection is uniquely specified by a pair of sockets

identifying its two sides.

When two processes wish to communicate, their TCP's must first

establish a connection (initialize the status information on each

side). When their communication is complete, the connection is

terminated or closed to free the resources for other uses.

Since connections must be established between unreliable hosts and

over the unreliable internet communication system, a handshake

mechanism with clock-based sequence numbers is used to avoid

erroneous initialization of connections.

Precedence and Security:

The users of TCP may indicate the security and precedence of their

communication. Provision is made for default values to be used when

these features are not needed.

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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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Philosophy

other operations to achieve the delivery of the local packet to the

destination internet module.

At a gateway between networks, the internet datagram is "unwrapped"

from its local packet and examined to determine through which network

the internet datagram should travel next. The internet datagram is

then "wrapped" in a local packet suitable to the next network and

routed to the next gateway, or to the final destination.

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

internet datagram fragments if this is necessary for transmission

through the next network. To do this, the gateway produces a set of

internet datagrams; each carrying a fragment. Fragments may be

further broken into smaller fragments at subsequent gateways. The

internet datagram fragment format is designed so that the destination

internet module can reassemble fragments into internet datagrams.

A destination internet module unwraps the segment from the datagram

(after reassembling the datagram, if necessary) and passes it to the

destination TCP.

This simple model of the operation glosses over many details. One

important feature is the type of service. This provides information

to the gateway (or internet module) to guide it in selecting the

service parameters to be used in traversing the next network.

Included in the type of service information is the precedence of the

datagram. Datagrams may also carry security information to permit

host and gateways that operate in multilevel secure environments to

properly segregate datagrams for security considerations.

2.3. The Host Environment

The TCP is assumed to be a module in an operating system. The users

access the TCP much like they would access the file system. The TCP

may call on other operating system functions, for example, to manage

data structures. The actual interface to the network is assumed to be

controlled by a device driver module. The TCP does not call on the

network device driver directly, but rather calls on the internet

datagram protocol module which may in turn call on the device driver.

The mechanisms of TCP do not preclude implementation of the TCP in a

front-end processor. However, in such an implementation, a

host-to-front-end protocol must provide the functionality to support

the type of TCP-user interface described in this document.

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Philosophy

2.4. Interfaces

The TCP/user interface provides for calls made by the user on the TCP

to OPEN or CLOSE a connection, to SEND or RECEIVE data, or to obtain

STATUS about a connection. These calls are like other calls from user

programs on the operating system, for example, the calls to open, read

from, and close a file.

The TCP/internet interface provides calls to send and receive

datagrams addressed to TCP modules in hosts anywhere in the internet

system. These calls have parameters for passing the address, type of

service, precedence, security, and other control information.

2.5. Relation to Other Protocols

The following diagram illustrates the place of the TCP in the protocol

hierarchy:

+------+ +-----+ +-----+ +-----+

Telnet FTP Voice ... Application Level

+------+ +-----+ +-----+ +-----+

+-----+ +-----+ +-----+

TCP RTP ... Host Level

+-----+ +-----+ +-----+

+-------------------------------+

Internet Protocol & ICMP Gateway Level

+-------------------------------+

+---------------------------+

Local Network Protocol Network Level

+---------------------------+

Protocol Relationships

Figure 2.

It is expected that the TCP will be able to support higher level

protocols efficiently. It should be easy to interface higher level

protocols like the ARPANET Telnet or AUTODIN II THP to the TCP.

2.6. Reliable Communication

A stream of data sent on a TCP connection is delivered reliably and in

order at the destination.

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Philosophy

Transmission is made reliable via the use of sequence numbers and

acknowledgments. Conceptually, each octet of data is assigned a

sequence number. The sequence number of the first octet of data in a

segment is transmitted with that segment and is called the segment

sequence number. Segments also carry an acknowledgment number which

is the sequence number of the next expected data octet of

transmissions in the reverse direction. When the TCP transmits a

segment containing data, it puts a copy on a retransmission queue and

starts a timer; when the acknowledgment for that data is received, the

segment is deleted from the queue. If the acknowledgment is not

received before the timer runs out, the segment is retransmitted.

An acknowledgment by TCP does not guarantee that the data has been

delivered to the end user, but only that the receiving TCP has taken

the responsibility to do so.

To govern the flow of data between TCPs, a flow control mechanism is

employed. The receiving TCP reports a "window" to the sending TCP.

This window specifies the number of octets, starting with the

acknowledgment number, that the receiving TCP is currently prepared to

receive.

2.7. Connection Establishment and Clearing

To identify the separate data streams that a TCP may handle, the TCP

provides a port identifier. Since port identifiers are selected

independently by each TCP they might not be unique. To provide for

unique addresses within each TCP, we concatenate an internet address

identifying the TCP with a port identifier to create a socket which

will be unique throughout all networks connected together.

A connection is fully specified by the pair of sockets at the ends. A

local socket may participate in many connections to different foreign

sockets. A connection can be used to carry data in both directions,

that is, it is "full duplex".

TCPs are free to associate ports with processes however they choose.

However, several basic concepts are necessary in any implementation.

There must be well-known sockets which the TCP associates only with

the "appropriate" processes by some means. We envision that processes

may "own" ports, and that processes can initiate connections only on

the ports they own. (Means for implementing ownership is a local

issue, but we envision a Request Port user command, or a method of

uniquely allocating a group of ports to a given process, e.g., by

associating the high order bits of a port name with a given process.)

A connection is specified in the OPEN call by the local port and

foreign socket arguments. In return, the TCP supplies a (short) local

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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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Philosophy

If there are several pending passive OPENs (recorded in TCBs) with the

same local socket, an foreign active OPEN will be matched to a TCB

with the specific foreign socket in the foreign active OPEN, if such a

TCB exists, before selecting a TCB with an unspecified foreign socket.

The procedures to establish connections utilize the synchronize (SYN)

control flag and involves an exchange of three messages. This

exchange has been termed a three-way hand shake [3].

A connection is initiated by the rendezvous of an arriving segment

containing a SYN and a waiting TCB entry each created by a user OPEN

command. The matching of local and foreign sockets determines when a

connection has been initiated. The connection becomes "established"

when sequence numbers have been synchronized in both directions.

The clearing of a connection also involves the exchange of segments,

in this case carrying the FIN control flag.

2.8. Data Communication

The data that flows on a connection may be thought of as a stream of

octets. The sending user indicates in each SEND call whether the data

in that call (and any preceeding calls) should be immediately pushed

through to the receiving user by the setting of the PUSH flag.

A sending TCP is allowed to collect data from the sending user and to

send that data in segments at its own convenience, until the push

function is signaled, then it must send all unsent data. When a

receiving TCP sees the PUSH flag, it must not wait for more data from

the sending TCP before passing the data to the receiving process.

There is no necessary relationship between push functions and segment

boundaries. The data in any particular segment may be the result of a

single SEND call, in whole or part, or of multiple SEND calls.

The purpose of push function and the PUSH flag is to push data through

from the sending user to the receiving user. It does not provide a

record service.

There is a coupling between the push function and the use of buffers

of data that cross the TCP/user interface. Each time a PUSH flag is

associated with data placed into the receiving user's buffer, the

buffer is returned to the user for processing even if the buffer is

not filled. If data arrives that fills the user's buffer before a

PUSH is seen, the data is passed to the user in buffer size units.

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

at some point further along in the data stream than the receiver is

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Philosophy

currently reading there is urgent data. TCP does not attempt to

define what the user specifically does upon being notified of pending

urgent data, but the general notion is that the receiving process will

take action to process the urgent data quickly.

2.9. Precedence and Security

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

security option to provide precedence and security on a per connection

basis to TCP users. Not all TCP modules will necessarily function in

a multilevel secure environment; some may be limited to unclassified

use only, and others may operate at only one security level and

compartment. Consequently, some TCP implementations and services to

users may be limited to a subset of the multilevel secure case.

TCP modules which operate in a multilevel secure environment must

properly mark outgoing segments with the security, compartment, and

precedence. Such TCP modules must also provide to their users or

higher level protocols such as Telnet or THP an interface to allow

them to specify the desired security level, compartment, and

precedence of connections.

2.10. Robustness Principle

TCP implementations will follow a general principle of robustness: be

conservative in what you do, be liberal in what you accept from

others.

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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 UAPRSF

Offset Reserved RCssYI Window

GKHTNN

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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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Functional Specification

Sequence Number: 32 bits

The sequence number of the first data octet in this segment (except

when SYN is present). If SYN is present the sequence number is the

initial sequence number (ISN) and the first data octet is ISN+1.

Acknowledgment Number: 32 bits

If the ACK control bit is set this field contains the value of the

next sequence number the sender of the segment is expecting to

receive. Once a connection is established this is always sent.

Data Offset: 4 bits

The number of 32 bit Words in the TCP Header. This indicates where

the data begins. The TCP header (even one including options) is an

integral number of 32 bits long.

Reserved: 6 bits

Reserved for future use. Must be zero.

Control Bits: 6 bits (from left to right):

URG: Urgent Pointer field significant

ACK: Acknowledgment field significant

PSH: Push Function

RST: Reset the connection

SYN: Synchronize sequence numbers

FIN: No more data from sender

Window: 16 bits

The number of data octets beginning with the one indicated in the

acknowledgment field which the sender of this segment is willing to

accept.

Checksum: 16 bits

The checksum field is the 16 bit one's complement of the one's

complement sum of all 16 bit words in the header and text. If a

segment contains an odd number of header and text octets to be

checksummed, the last octet is padded on the right with zeros to

form a 16 bit word for checksum purposes. The pad is not

transmitted as part of the segment. While computing the checksum,

the checksum field itself is replaced with zeros.

The checksum also covers a 96 bit pseudo header conceptually

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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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Transmission Control Protocol

Functional Specification

Currently defined options include (kind indicated in octal):

Kind Length Meaning

---- ------ -------

0 - End of option list.

1 - No-Operation.

2 4 Maximum Segment Size.

Specific Option Definitions

End of Option List

+--------+

00000000

+--------+

Kind=0

This option code indicates the end of the option list. This

might not coincide with the end of the TCP header according to

the Data Offset field. This is used at the end of all options,

not the end of each option, and need only be used if the end of

the options would not otherwise coincide with the end of the TCP

header.

No-Operation

+--------+

00000001

+--------+

Kind=1

This option code may be used between options, for example, to

align the beginning of a subsequent option on a word boundary.

There is no guarantee that senders will use this option, so

receivers must be prepared to process options even if they do

not begin on a word boundary.

Maximum Segment Size

+--------+--------+---------+--------+

0000001000000100 max seg size

+--------+--------+---------+--------+

Kind=2 Length=4

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Transmission Control Protocol

Functional Specification

Maximum Segment Size Option Data: 16 bits

If this option is present, then it communicates the maximum

receive segment size at the TCP which sends this segment.

This field must only be sent in the initial connection request

(i.e., in segments with the SYN control bit set). If this

option is not used, any segment size is allowed.

Padding: variable

The TCP header padding is used to ensure that the TCP header ends

and data begins on a 32 bit boundary. The padding is composed of

zeros.

3.2. Terminology

Before we can discuss very much about the operation of the TCP we need

to introduce some detailed terminology. The maintenance of a TCP

connection requires the remembering of several variables. We conceive

of these variables being stored in a connection record called a

Transmission Control Block or TCB. Among the variables stored in the

TCB are the local and remote socket numbers, the security and

precedence of the connection, pointers to the user's send and receive

buffers, pointers to the retransmit queue and to the current segment.

In addition several variables relating to the send and receive

sequence numbers are stored in the TCB.

Send Sequence Variables

SND.UNA - send unacknowledged

SND.NXT - send next

SND.WND - send window

SND.UP - send urgent pointer

SND.WL1 - segment sequence number used for last window update

SND.WL2 - segment acknowledgment number used for last window

update

ISS - initial send sequence number

Receive Sequence Variables

RCV.NXT - receive next

RCV.WND - receive window

RCV.UP - receive urgent pointer

IRS - initial receive sequence number

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Transmission Control Protocol

Functional Specification

The following diagrams may help to relate some of these variables to

the sequence space.

Send Sequence Space

1 2 3 4

----------------------------------------

SND.UNA SND.NXT SND.UNA

+SND.WND

1 - old sequence numbers which have been acknowledged

2 - sequence numbers of unacknowledged data

3 - sequence numbers allowed for new data transmission

4 - future sequence numbers which are not yet allowed

Send Sequence Space

Figure 4.

The send window is the portion of the sequence space labeled 3 in

figure 4.

Receive Sequence Space

1 2 3

------------------------------

RCV.NXT RCV.NXT

+RCV.WND

1 - old sequence numbers which have been acknowledged

2 - sequence numbers allowed for new reception

3 - future sequence numbers which are not yet allowed

Receive Sequence Space

Figure 5.

The receive window is the portion of the sequence space labeled 2 in

figure 5.

There are also some variables used frequently in the discussion that

take their values from the fields of the current segment.

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Functional Specification

Current Segment Variables

SEG.SEQ - segment sequence number

SEG.ACK - segment acknowledgment number

SEG.LEN - segment length

SEG.WND - segment window

SEG.UP - segment urgent pointer

SEG.PRC - segment precedence value

A connection progresses through a series of states during its

lifetime. The states are: LISTEN, SYN-SENT, SYN-RECEIVED,

ESTABLISHED, FIN-WAIT-1, FIN-WAIT-2, CLOSE-WAIT, CLOSING, LAST-ACK,

TIME-WAIT, and the fictional state CLOSED. CLOSED is fictional

because it represents the state when there is no TCB, and therefore,

no connection. Briefly the meanings of the states are:

LISTEN - represents waiting for a connection request from any remote

TCP and port.

SYN-SENT - represents waiting for a matching connection request

after having sent a connection request.

SYN-RECEIVED - represents waiting for a confirming connection

request acknowledgment after having both received and sent a

connection request.

ESTABLISHED - represents an open connection, data received can be

delivered to the user. The normal state for the data transfer phase

of the connection.

FIN-WAIT-1 - represents waiting for a connection termination request

from the remote TCP, or an acknowledgment of the connection

termination request previously sent.

FIN-WAIT-2 - represents waiting for a connection termination request

from the remote TCP.

CLOSE-WAIT - represents waiting for a connection termination request

from the local user.

CLOSING - represents waiting for a connection termination request

acknowledgment from the remote TCP.

LAST-ACK - represents waiting for an acknowledgment of the

connection termination request previously sent to the remote TCP

(which includes an acknowledgment of its connection termination

request).

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Functional Specification

TIME-WAIT - represents waiting for enough time to pass to be sure

the remote TCP received the acknowledgment of its connection

termination request.

CLOSED - represents no connection state at all.

A TCP connection progresses from one state to another in response to

events. The events are the user calls, OPEN, SEND, RECEIVE, CLOSE,

ABORT, and STATUS; the incoming segments, particularly those

containing the SYN, ACK, RST and FIN flags; and timeouts.

The state diagram in figure 6 illustrates only state changes, together

with the causing events and resulting actions, but addresses neither

error conditions nor actions which are not connected with state

changes. In a later section, more detail is offered with respect to

the reaction of the TCP to events.

NOTE BENE: this diagram is only a summary and must not be taken as

the total specification.

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Functional Specification

+---------+ ---------\ active OPEN

CLOSED \ -----------

+---------+<---------\ \ create TCB

^ \ \ snd SYN

passive OPEN CLOSE \ \

------------ ---------- \ \

create TCB delete TCB \ \

V \ \

+---------+ CLOSE \

LISTEN ----------

+---------+ delete TCB

rcv SYN SEND

----------- ------- V

+---------+ snd SYN,ACK / \ snd SYN +---------+

<----------------- ------------------>

SYN rcv SYN SYN

RCVD <----------------------------------------------- SENT

snd ACK

------------------ -------------------

+---------+ rcv ACK of SYN \ / rcv SYN,ACK +---------+

-------------- -----------

x snd ACK

V V

CLOSE +---------+

------- ESTAB

snd FIN +---------+

CLOSE rcv FIN

V ------- -------

+---------+ snd FIN / \ snd ACK +---------+

FIN <----------------- ------------------> CLOSE

WAIT-1 ------------------ WAIT

+---------+ rcv FIN \ +---------+

rcv ACK of FIN ------- CLOSE

-------------- snd ACK -------

V x V snd FIN V

+---------+ +---------+ +---------+

FINWAIT-2 CLOSING LAST-ACK

+---------+ +---------+ +---------+

rcv ACK of FIN rcv ACK of FIN

rcv FIN -------------- Timeout=2MSL --------------

------- x V ------------ x V

\ snd ACK +---------+delete TCB +---------+

------------------------>TIME WAIT------------------> CLOSED

+---------+ +---------+

TCP Connection State Diagram

Figure 6.

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Functional Specification

3.3. Sequence Numbers

A fundamental notion in the design is that every octet of data sent

over a TCP connection has a sequence number. Since every octet is

sequenced, each of them can be acknowledged. The acknowledgment

mechanism employed is cumulative so that an acknowledgment of sequence

number X indicates that all octets up to but not including X have been

received. This mechanism allows for straight-forward duplicate

detection in the presence of retransmission. Numbering of octets

within a segment is that the first data octet immediately following

the header is the lowest numbered, and the following octets are

numbered consecutively.

It is essential to remember that the actual sequence number space is

finite, though very large. This space ranges from 0 to 2**32 - 1.

Since the space is finite, all arithmetic dealing with sequence

numbers must be performed modulo 2**32. This unsigned arithmetic

preserves the relationship of sequence numbers as they cycle from

2**32 - 1 to 0 again. There are some subtleties to computer modulo

arithmetic, so great care should be taken in programming the

comparison of such values. The symbol "=<" means "less than or equal"

(modulo 2**32).

The typical kinds of sequence number comparisons which the TCP must

perform include:

(a) Determining that an acknowledgment refers to some sequence

number sent but not yet acknowledged.

(b) Determining that all sequence numbers occupied by a segment

have been acknowledged (e.g., to remove the segment from a

retransmission queue).

(c) Determining that an incoming segment contains sequence numbers

which are expected (i.e., that the segment "overlaps" the

receive window).

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Transmission Control Protocol

Functional Specification

In response to sending data the TCP will receive acknowledgments. The

following comparisons are needed to process the acknowledgments.

SND.UNA = oldest unacknowledged sequence number

SND.NXT = next sequence number to be sent

SEG.ACK = acknowledgment from the receiving TCP (next sequence

number expected by the receiving TCP)

SEG.SEQ = first sequence number of a segment

SEG.LEN = the number of octets occupied by the data in the segment

(counting SYN and FIN)

SEG.SEQ+SEG.LEN-1 = last sequence number of a segment

A new acknowledgment (called an "acceptable ack"), is one for which

the inequality below holds:

SND.UNA < SEG.ACK =< SND.NXT

A segment on the retransmission queue is fully acknowledged if the sum

of its sequence number and length is less or equal than the

acknowledgment value in the incoming segment.

When data is received the following comparisons are needed:

RCV.NXT = next sequence number expected on an incoming segments, and

is the left or lower edge of the receive window

RCV.NXT+RCV.WND-1 = last sequence number expected on an incoming

segment, and is the right or upper edge of the receive window

SEG.SEQ = first sequence number occupied by the incoming segment

SEG.SEQ+SEG.LEN-1 = last sequence number occupied by the incoming

segment

A segment is judged to occupy a portion of valid receive sequence

space if

RCV.NXT =< SEG.SEQ < RCV.NXT+RCV.WND

or

RCV.NXT =< SEG.SEQ+SEG.LEN-1 < RCV.NXT+RCV.WND

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Functional Specification

The first part of this test checks to see if the beginning of the

segment falls in the window, the second part of the test checks to see

if the end of the segment falls in the window; if the segment passes

either part of the test it contains data in the window.

Actually, it is a little more complicated than this. Due to zero

windows and zero length segments, we have four cases for the

acceptability of an incoming segment:

Segment Receive Test

Length Window

------- ------- -------------------------------------------

0 0 SEG.SEQ = RCV.NXT

0 >0 RCV.NXT =< SEG.SEQ < RCV.NXT+RCV.WND

>0 0 not acceptable

>0 >0 RCV.NXT =< SEG.SEQ < RCV.NXT+RCV.WND

or RCV.NXT =< SEG.SEQ+SEG.LEN-1 < RCV.NXT+RCV.WND

Note that when the receive window is zero no segments should be

acceptable except ACK segments. Thus, it is be possible for a TCP to

maintain a zero receive window while transmitting data and receiving

ACKs. However, even when the receive window is zero, a TCP must

process the RST and URG fields of all incoming segments.

We have taken advantage of the numbering scheme to protect certain

control information as well. This is achieved by implicitly including

some control flags in the sequence space so they can be retransmitted

and acknowledged without confusion (i.e., one and only one copy of the

control will be acted upon). Control information is not physically

carried in the segment data space. Consequently, we must adopt rules

for implicitly assigning sequence numbers to control. The SYN and FIN

are the only controls requiring this protection, and these controls

are used only at connection opening and closing. For sequence number

purposes, the SYN is considered to occur before the first actual data

octet of the segment in which it occurs, while the FIN is considered

to occur after the last actual data octet in a segment in which it

occurs. The segment length (SEG.LEN) includes both data and sequence

space occupying controls. When a SYN is present then SEG.SEQ is the

sequence number of the SYN.

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Functional Specification

Initial Sequence Number Selection

The protocol places no restriction on a particular connection being

used over and over again. A connection is defined by a pair of

sockets. New instances of a connection will be referred to as

incarnations of the connection. The problem that arises from this is

-- "how does the TCP identify duplicate segments from previous

incarnations of the connection?" This problem becomes apparent if the

connection is being opened and closed in quick succession, or if the

connection breaks with loss of memory and is then reestablished.

To avoid confusion we must prevent segments from one incarnation of a

connection from being used while the same sequence numbers may still

be present in the network from an earlier incarnation. We want to

assure this, even if a TCP crashes and loses all knowledge of the

sequence numbers it has been using. When new connections are created,

an initial sequence number (ISN) generator is employed which selects a

new 32 bit ISN. The generator is bound to a (possibly fictitious) 32

bit clock whose low order bit is incremented roughly every 4

microseconds. Thus, the ISN cycles approximately every 4.55 hours.

Since we assume that segments will stay in the network no more than

the Maximum Segment Lifetime (MSL) and that the MSL is less than 4.55

hours we can reasonably assume that ISN's will be unique.

For each connection there is a send sequence number and a receive

sequence number. The initial send sequence number (ISS) is chosen by

the data sending TCP, and the initial receive sequence number (IRS) is

learned during the connection establishing procedure.

For a connection to be established or initialized, the two TCPs must

synchronize on each other's initial sequence numbers. This is done in

an exchange of connection establishing segments carrying a control bit

called "SYN" (for synchronize) and the initial sequence numbers. As a

shorthand, segments carrying the SYN bit are also called "SYNs".

Hence, the solution requires a suitable mechanism for picking an

initial sequence number and a slightly involved handshake to exchange

the ISN's.

The synchronization requires each side to send it's own initial

sequence number and to receive a confirmation of it in acknowledgment

from the other side. Each side must also receive the other side's

initial sequence number and send a confirming acknowledgment.

1) A --> B SYN my sequence number is X

2) A <-- B ACK your sequence number is X

3) A <-- B SYN my sequence number is Y

4) A --> B ACK your sequence number is Y

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Functional Specification

Because steps 2 and 3 can be combined in a single message this is

called the three way (or three message) handshake.

A three way handshake is necessary because sequence numbers are not

tied to a global clock in the network, and TCPs may have different

mechanisms for picking the ISN's. The receiver of the first SYN has

no way of knowing whether the segment was an old delayed one or not,

unless it remembers the last sequence number used on the connection

(which is not always possible), and so it must ask the sender to

verify this SYN. The three way handshake and the advantages of a

clock-driven scheme are discussed in [3].

Knowing When to Keep Quiet

To be sure that a TCP does not create a segment that carries a

sequence number which may be duplicated by an old segment remaining in

the network, the TCP must keep quiet for a maximum segment lifetime

(MSL) before assigning any sequence numbers upon starting up or

recovering from a crash in which memory of sequence numbers in use was

lost. For this specification the MSL is taken to be 2 minutes. This

is an engineering choice, and may be changed if experience indicates

it is desirable to do so. Note that if a TCP is reinitialized in some

sense, yet retains its memory of sequence numbers in use, then it need

not wait at all; it must only be sure to use sequence numbers larger

than those recently used.

The TCP Quiet Time Concept

This specification provides that hosts which "crash" without

retaining any knowledge of the last sequence numbers transmitted on

each active (i.e., not closed) connection shall delay emitting any

TCP segments for at least the agreed Maximum Segment Lifetime (MSL)

in the internet system of which the host is a part. In the

paragraphs below, an explanation for this specification is given.

TCP implementors may violate the "quiet time" restriction, but only

at the risk of causing some old data to be accepted as new or new

data rejected as old duplicated by some receivers in the internet

system.

TCPs consume sequence number space each time a segment is formed and

entered into the network output queue at a source host. The

duplicate detection and sequencing algorithm in the TCP protocol

relies on the unique binding of segment data to sequence space to

the extent that sequence numbers will not cycle through all 2**32

values before the segment data bound to those sequence numbers has

been delivered and acknowledged by the receiver and all duplicate

copies of the segments have "drained" from the internet. Without

such an assumption, two distinct TCP segments could conceivably be

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Functional Specification

assigned the same or overlapping sequence numbers, causing confusion

at the receiver as to which data is new and which is old. Remember

that each segment is bound to as many consecutive sequence numbers

as there are octets of data in the segment.

Under normal conditions, TCPs keep track of the next sequence number

to emit and the oldest awaiting acknowledgment so as to avoid

mistakenly using a sequence number over before its first use has

been acknowledged. This alone does not guarantee that old duplicate

data is drained from the net, so the sequence space has been made

very large to reduce the probability that a wandering duplicate will

cause trouble upon arrival. At 2 megabits/sec. it takes 4.5 hours

to use up 2**32 octets of sequence space. Since the maximum segment

lifetime in the net is not likely to exceed a few tens of seconds,

this is deemed ample protection for foreseeable nets, even if data

rates escalate to l0's of megabits/sec. At 100 megabits/sec, the

cycle time is 5.4 minutes which may be a little short, but still

within reason.

The basic duplicate detection and sequencing algorithm in TCP can be

defeated, however, if a source TCP does not have any memory of the

sequence numbers it last used on a given connection. For example, if

the TCP were to start all connections with sequence number 0, then

upon crashing and restarting, a TCP might re-form an earlier

connection (possibly after half-open connection resolution) and emit

packets with sequence numbers identical to or overlapping with

packets still in the network which were emitted on an earlier

incarnation of the same connection. In the absence of knowledge

about the sequence numbers used on a particular connection, the TCP

specification recommends that the source delay for MSL seconds

before emitting segments on the connection, to allow time for

segments from the earlier connection incarnation to drain from the

system.

Even hosts which can remember the time of day and used it to select

initial sequence number values are not immune from this problem

(i.e., even if time of day is used to select an initial sequence

number for each new connection incarnation).

Suppose, for example, that a connection is opened starting with

sequence number S. Suppose that this connection is not used much

and that eventually the initial sequence number function (ISN(t))

takes on a value equal to the sequence number, say S1, of the last

segment sent by this TCP on a particular connection. Now suppose,

at this instant, the host crashes, recovers, and establishes a new

incarnation of the connection. The initial sequence number chosen is

S1 = ISN(t) -- last used sequence number on old incarnation of

connection! If the recovery occurs quickly enough, any old

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Transmission Control Protocol

Functional Specification

duplicates in the net bearing sequence numbers in the neighborhood

of S1 may arrive and be treated as new packets by the receiver of

the new incarnation of the connection.

The problem is that the recovering host may not know for how long it

crashed nor does it know whether there are still old duplicates in

the system from earlier connection incarnations.

One way to deal with this problem is to deliberately delay emitting

segments for one MSL after recovery from a crash- this is the "quite

time" specification. Hosts which prefer to avoid waiting are

willing to risk possible confusion of old and new packets at a given

destination may choose not to wait for the "quite time".

Implementors may provide TCP users with the ability to select on a

connection by connection basis whether to wait after a crash, or may

informally implement the "quite time" for all connections.

Obviously, even where a user selects to "wait," this is not

necessary after the host has been "up" for at least MSL seconds.

To summarize: every segment emitted occupies one or more sequence

numbers in the sequence space, the numbers occupied by a segment are

"busy" or "in use" until MSL seconds have passed, upon crashing a

block of space-time is occupied by the octets of the last emitted

segment, if a new connection is started too soon and uses any of the

sequence numbers in the space-time footprint of the last segment of

the previous connection incarnation, there is a potential sequence

number overlap area which could cause confusion at the receiver.

3.4. Establishing a connection

The "three-way handshake" is the procedure used to establish a

connection. This procedure normally is initiated by one TCP and

responded to by another TCP. The procedure also works if two TCP

simultaneously initiate the procedure. When simultaneous attempt

occurs, each TCP receives a "SYN" segment which carries no

acknowledgment after it has sent a "SYN". Of course, the arrival of

an old duplicate "SYN" segment can potentially make it appear, to the

recipient, that a simultaneous connection initiation is in progress.

Proper use of "reset" segments can disambiguate these cases.

Several examples of connection initiation follow. Although these

examples do not show connection synchronization using data-carrying

segments, this is perfectly legitimate, so long as the receiving TCP

doesn't deliver the data to the user until it is clear the data is

valid (i.e., the data must be buffered at the receiver until the

connection reaches the ESTABLISHED state). The three-way handshake

reduces the possibility of false connections. It is the

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Functional Specification

implementation of a trade-off between memory and messages to provide

information for this checking.

The simplest three-way handshake is shown in figure 7 below. The

figures should be interpreted in the following way. Each line is

numbered for reference purposes. Right arrows (-->) indicate

departure of a TCP segment from TCP A to TCP B, or arrival of a

segment at B from A. Left arrows (<--), indicate the reverse.

Ellipsis (...) indicates a segment which is still in the network

(delayed). An "XXX" indicates a segment which is lost or rejected.

Comments appear in parentheses. TCP states represent the state AFTER

the departure or arrival of the segment (whose contents are shown in

the center of each line). Segment contents are shown in abbreviated

form, with sequence number, control flags, and ACK field. Other

fields such as window, addresses, lengths, and text have been left out

in the interest of clarity.

TCP A TCP B

1. CLOSED LISTEN

2. SYN-SENT --> <SEQ=100><CTL=SYN> --> SYN-RECEIVED

3. ESTABLISHED <-- <SEQ=300><ACK=101><CTL=SYN,ACK> <-- SYN-RECEIVED

4. ESTABLISHED --> <SEQ=101><ACK=301><CTL=ACK> --> ESTABLISHED

5. ESTABLISHED --> <SEQ=101><ACK=301><CTL=ACK><DATA> --> ESTABLISHED

Basic 3-Way Handshake for Connection Synchronization

Figure 7.

In line 2 of figure 7, TCP A begins by sending a SYN segment

indicating that it will use sequence numbers starting with sequence

number 100. In line 3, TCP B sends a SYN and acknowledges the SYN it

received from TCP A. Note that the acknowledgment field indicates TCP

B is now expecting to hear sequence 101, acknowledging the SYN which

occupied sequence 100.

At line 4, TCP A responds with an empty segment containing an ACK for

TCP B's SYN; and in line 5, TCP A sends some data. Note that the

sequence number of the segment in line 5 is the same as in line 4

because the ACK does not occupy sequence number space (if it did, we

would wind up ACKing ACK's!).

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Simultaneous initiation is only slightly more complex, as is shown in

figure 8. Each TCP cycles from CLOSED to SYN-SENT to SYN-RECEIVED to

ESTABLISHED.

TCP A TCP B

1. CLOSED CLOSED

2. SYN-SENT --> <SEQ=100><CTL=SYN> ...

3. SYN-RECEIVED <-- <SEQ=300><CTL=SYN> <-- SYN-SENT

4. ... <SEQ=100><CTL=SYN> --> SYN-RECEIVED

5. SYN-RECEIVED --> <SEQ=100><ACK=301><CTL=SYN,ACK> ...

6. ESTABLISHED <-- <SEQ=300><ACK=101><CTL=SYN,ACK> <-- SYN-RECEIVED

7. ... <SEQ=101><ACK=301><CTL=ACK> --> ESTABLISHED

Simultaneous Connection Synchronization

Figure 8.

The principle reason for the three-way handshake is to prevent old

duplicate connection initiations from causing confusion. To deal with

this, a special control message, reset, has been devised. If the

receiving TCP is in a non-synchronized state (i.e., SYN-SENT,

SYN-RECEIVED), it returns to LISTEN on receiving an acceptable reset.

If the TCP is in one of the synchronized states (ESTABLISHED,

FIN-WAIT-1, FIN-WAIT-2, CLOSE-WAIT, CLOSING, LAST-ACK, TIME-WAIT), it

aborts the connection and informs its user. We discuss this latter

case under "half-open" connections below.

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TCP A TCP B

1. CLOSED LISTEN

2. SYN-SENT --> <SEQ=100><CTL=SYN> ...

3. (duplicate) ... <SEQ=90><CTL=SYN> --> SYN-RECEIVED

4. SYN-SENT <-- <SEQ=300><ACK=91><CTL=SYN,ACK> <-- SYN-RECEIVED

5. SYN-SENT --> <SEQ=91><CTL=RST> --> LISTEN

6. ... <SEQ=100><CTL=SYN> --> SYN-RECEIVED

7. SYN-SENT <-- <SEQ=400><ACK=101><CTL=SYN,ACK> <-- SYN-RECEIVED

8. ESTABLISHED --> <SEQ=101><ACK=401><CTL=ACK> --> ESTABLISHED

Recovery from Old Duplicate SYN

Figure 9.

As a simple example of recovery from old duplicates, consider

figure 9. At line 3, an old duplicate SYN arrives at TCP B. TCP B

cannot tell that this is an old duplicate, so it responds normally

(line 4). TCP A detects that the ACK field is incorrect and returns a

RST (reset) with its SEQ field selected to make the segment

believable. TCP B, on receiving the RST, returns to the LISTEN state.

When the original SYN (pun intended) finally arrives at line 6, the

synchronization proceeds normally. If the SYN at line 6 had arrived

before the RST, a more complex exchange might have occurred with RST's

sent in both directions.

Half-Open Connections and Other Anomalies

An established connection is said to be "half-open" if one of the

TCPs has closed or aborted the connection at its end without the

knowledge of the other, or if the two ends of the connection have

become desynchronized owing to a crash that resulted in loss of

memory. Such connections will automatically become reset if an

attempt is made to send data in either direction. However, half-open

connections are expected to be unusual, and the recovery procedure is

mildly involved.

If at site A the connection no longer exists, then an attempt by the

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Functional Specification

user at site B to send any data on it will result in the site B TCP

receiving a reset control message. Such a message indicates to the

site B TCP that something is wrong, and it is expected to abort the

connection.

Assume that two user processes A and B are communicating with one

another when a crash occurs causing loss of memory to A's TCP.

Depending on the operating system supporting A's TCP, it is likely

that some error recovery mechanism exists. When the TCP is up again,

A is likely to start again from the beginning or from a recovery

point. As a result, A will probably try to OPEN the connection again

or try to SEND on the connection it believes open. In the latter

case, it receives the error message "connection not open" from the

local (A's) TCP. In an attempt to establish the connection, A's TCP

will send a segment containing SYN. This scenario leads to the

example shown in figure 10. After TCP A crashes, the user attempts to

re-open the connection. TCP B, in the meantime, thinks the connection

is open.

TCP A TCP B

1. (CRASH) (send 300,receive 100)

2. CLOSED ESTABLISHED

3. SYN-SENT --> <SEQ=400><CTL=SYN> --> (??)

4. (!!) <-- <SEQ=300><ACK=100><CTL=ACK> <-- ESTABLISHED

5. SYN-SENT --> <SEQ=100><CTL=RST> --> (Abort!!)

6. SYN-SENT CLOSED

7. SYN-SENT --> <SEQ=400><CTL=SYN> -->

Half-Open Connection Discovery

Figure 10.

When the SYN arrives at line 3, TCP B, being in a synchronized state,

and the incoming segment outside the window, responds with an

acknowledgment indicating what sequence it next expects to hear (ACK

100). TCP A sees that this segment does not acknowledge anything it

sent and, being unsynchronized, sends a reset (RST) because it has

detected a half-open connection. TCP B aborts at line 5. TCP A will

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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

[Page 50]

September 1981

Transmission Control Protocol

Functional Specification

TCP/Lower-Level Interface

The TCP calls on a lower level protocol module to actually send and

receive information over a network. One case is that of the ARPA

internetwork system where the lower level module is the Internet

Protocol (IP) [2].

If the lower level protocol is IP it provides arguments for a type

of service and for a time to live. TCP uses the following settings

for these parameters:

Type of Service = Precedence: routine, Delay: normal, Throughput:

normal, Reliability: normal; or 00000000.

Time to Live = one minute, or 00111100.

Note that the assumed maximum segment lifetime is two minutes.

Here we explicitly ask that a segment be destroyed if it cannot

be delivered by the internet system within one minute.

If the lower level is IP (or other protocol that provides this

feature) and source routing is used, the interface must allow the

route information to be communicated. This is especially important

so that the source and destination addresses used in the TCP

checksum be the originating source and ultimate destination. It is

also important to preserve the return route to answer connection

requests.

Any lower level protocol will have to provide the source address,

destination address, and protocol fields, and some way to determine

the "TCP length", both to provide the functional equivlent service

of IP and to be used in the TCP checksum.

[Page 51]

September 1981

Transmission Control Protocol

Functional Specification

3.9. Event Processing

The processing depicted in this section is an example of one possible

implementation. Other implementations may have slightly different

processing sequences, but they should differ from those in this

section only in detail, not in substance.

The activity of the TCP can be characterized as responding to events.

The events that occur can be cast into three categories: user calls,

arriving segments, and timeouts. This section describes the

processing the TCP does in response to each of the events. In many

cases the processing required depends on the state of the connection.

Events that occur:

User Calls

OPEN

SEND

RECEIVE

CLOSE

ABORT

STATUS

Arriving Segments

SEGMENT ARRIVES

Timeouts

USER TIMEOUT

RETRANSMISSION TIMEOUT

TIME-WAIT TIMEOUT

The model of the TCP/user interface is that user commands receive an

immediate return and possibly a delayed response via an event or

pseudo interrupt. In the following descriptions, the term "signal"

means cause a delayed response.

Error responses are given as character strings. For example, user

commands referencing connections that do not exist receive "error:

connection not open".

Please note in the following that all arithmetic on sequence numbers,

acknowledgment numbers, windows, et cetera, is modulo 2**32 the size

of the sequence number space. Also note that "=<" means less than or

equal to (modulo 2**32).

[Page 52]

September 1981

Transmission Control Protocol

Functional Specification

A natural way to think about processing incoming segments is to

imagine that they are first tested for proper sequence number (i.e.,

that their contents lie in the range of the expected "receive window"

in the sequence number space) and then that they are generally queued

and processed in sequence number order.

When a segment overlaps other already received segments we reconstruct

the segment to contain just the new data, and adjust the header fields

to be consistent.

Note that if no state change is mentioned the TCP stays in the same

state.

[Page 53]

September 1981

Transmission Control Protocol

Functional Specification

OPEN Call

OPEN Call

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

Create a new transmission control block (TCB) to hold connection

state information. Fill in local socket identifier, foreign

socket, precedence, security/compartment, and user timeout

information. Note that some parts of the foreign socket may be

unspecified in a passive OPEN and are to be filled in by the

parameters of the incoming SYN segment. Verify the security and

precedence requested are allowed for this user, if not return

"error: precedence not allowed" or "error: security/compartment

not allowed." If passive enter the LISTEN state and return. If

active and the foreign socket is unspecified, return "error:

foreign socket unspecified"; if active and the foreign socket is

specified, issue a SYN segment. An initial send sequence number

(ISS) is selected. A SYN segment of the form <SEQ=ISS><CTL=SYN>

is sent. Set SND.UNA to ISS, SND.NXT to ISS+1, enter SYN-SENT

state, and return.

If the caller does not have access to the local socket specified,

return "error: connection illegal for this process". If there is

no room to create a new connection, return "error: insufficient

resources".

LISTEN STATE

If active and the foreign socket is specified, then change the

connection from passive to active, select an ISS. Send a SYN

segment, set SND.UNA to ISS, SND.NXT to ISS+1. Enter SYN-SENT

state. Data associated with SEND may be sent with SYN segment or

queued for transmission after entering ESTABLISHED state. The

urgent bit if requested in the command must be sent with the data

segments sent as a result of this command. If there is no room to

queue the request, respond with "error: insufficient resources".

If Foreign socket was not specified, then return "error: foreign

socket unspecified".

[Page 54]

September 1981

Transmission Control Protocol

Functional Specification

OPEN Call

SYN-SENT STATE

SYN-RECEIVED STATE

ESTABLISHED STATE

FIN-WAIT-1 STATE

FIN-WAIT-2 STATE

CLOSE-WAIT STATE

CLOSING STATE

LAST-ACK STATE

TIME-WAIT STATE

Return "error: connection already exists".

[Page 55]

September 1981

Transmission Control Protocol

Functional Specification

SEND Call

SEND Call

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

If the user does not have access to such a connection, then return

"error: connection illegal for this process".

Otherwise, return "error: connection does not exist".

LISTEN STATE

If the foreign socket is specified, then change the connection

from passive to active, select an ISS. Send a SYN segment, set

SND.UNA to ISS, SND.NXT to ISS+1. Enter SYN-SENT state. Data

associated with SEND may be sent with SYN segment or queued for

transmission after entering ESTABLISHED state. The urgent bit if

requested in the command must be sent with the data segments sent

as a result of this command. If there is no room to queue the

request, respond with "error: insufficient resources". If

Foreign socket was not specified, then return "error: foreign

socket unspecified".

SYN-SENT STATE

SYN-RECEIVED STATE

Queue the data for transmission after entering ESTABLISHED state.

If no space to queue, respond with "error: insufficient

resources".

ESTABLISHED STATE

CLOSE-WAIT STATE

Segmentize the buffer and send it with a piggybacked

acknowledgment (acknowledgment value = RCV.NXT). If there is

insufficient space to remember this buffer, simply return "error:

insufficient resources".

If the urgent flag is set, then SND.UP <- SND.NXT-1 and set the

urgent pointer in the outgoing segments.

[Page 56]

September 1981

Transmission Control Protocol

Functional Specification

SEND Call

FIN-WAIT-1 STATE

FIN-WAIT-2 STATE

CLOSING STATE

LAST-ACK STATE

TIME-WAIT STATE

Return "error: connection closing" and do not service request.

[Page 57]

September 1981

Transmission Control Protocol

Functional Specification

RECEIVE Call

RECEIVE Call

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

If the user does not have access to such a connection, return

"error: connection illegal for this process".

Otherwise return "error: connection does not exist".

LISTEN STATE

SYN-SENT STATE

SYN-RECEIVED STATE

Queue for processing after entering ESTABLISHED state. If there

is no room to queue this request, respond with "error:

insufficient resources".

ESTABLISHED STATE

FIN-WAIT-1 STATE

FIN-WAIT-2 STATE

If insufficient incoming segments are queued to satisfy the

request, queue the request. If there is no queue space to

remember the RECEIVE, respond with "error: insufficient

resources".

Reassemble queued incoming segments into receive buffer and return

to user. Mark "push seen" (PUSH) if this is the case.

If RCV.UP is in advance of the data currently being passed to the

user notify the user of the presence of urgent data.

When the TCP takes responsibility for delivering data to the user

that fact must be communicated to the sender via an

acknowledgment. The formation of such an acknowledgment is

described below in the discussion of processing an incoming

segment.

[Page 58]

September 1981

Transmission Control Protocol

Functional Specification

RECEIVE Call

CLOSE-WAIT STATE

Since the remote side has already sent FIN, RECEIVEs must be

satisfied by text already on hand, but not yet delivered to the

user. If no text is awaiting delivery, the RECEIVE will get a

"error: connection closing" response. Otherwise, any remaining

text can be used to satisfy the RECEIVE.

CLOSING STATE

LAST-ACK STATE

TIME-WAIT STATE

Return "error: connection closing".

[Page 59]

September 1981

Transmission Control Protocol

Functional Specification

CLOSE Call

CLOSE Call

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

If the user does not have access to such a connection, return

"error: connection illegal for this process".

Otherwise, return "error: connection does not exist".

LISTEN STATE

Any outstanding RECEIVEs are returned with "error: closing"

responses. Delete TCB, enter CLOSED state, and return.

SYN-SENT STATE

Delete the TCB and return "error: closing" responses to any

queued SENDs, or RECEIVEs.

SYN-RECEIVED STATE

If no SENDs have been issued and there is no pending data to send,

then form a FIN segment and send it, and enter FIN-WAIT-1 state;

otherwise queue for processing after entering ESTABLISHED state.

ESTABLISHED STATE

Queue this until all preceding SENDs have been segmentized, then

form a FIN segment and send it. In any case, enter FIN-WAIT-1

state.

FIN-WAIT-1 STATE

FIN-WAIT-2 STATE

Strictly speaking, this is an error and should receive a "error:

connection closing" response. An "ok" response would be

acceptable, too, as long as a second FIN is not emitted (the first

FIN may be retransmitted though).

[Page 60]

September 1981

Transmission Control Protocol

Functional Specification

CLOSE Call

CLOSE-WAIT STATE

Queue this request until all preceding SENDs have been

segmentized; then send a FIN segment, enter CLOSING state.

CLOSING STATE

LAST-ACK STATE

TIME-WAIT STATE

Respond with "error: connection closing".

[Page 61]

September 1981

Transmission Control Protocol

Functional Specification

ABORT Call

ABORT Call

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

If the user should not have access to such a connection, return

"error: connection illegal for this process".

Otherwise return "error: connection does not exist".

LISTEN STATE

Any outstanding RECEIVEs should be returned with "error:

connection reset" responses. Delete TCB, enter CLOSED state, and

return.

SYN-SENT STATE

All queued SENDs and RECEIVEs should be given "connection reset"

notification, delete the TCB, enter CLOSED state, and return.

SYN-RECEIVED STATE

ESTABLISHED STATE

FIN-WAIT-1 STATE

FIN-WAIT-2 STATE

CLOSE-WAIT STATE

Send a reset segment:

<SEQ=SND.NXT><CTL=RST>

All queued SENDs and RECEIVEs should be given "connection reset"

notification; all segments queued for transmission (except for the

RST formed above) or retransmission should be flushed, delete the

TCB, enter CLOSED state, and return.

CLOSING STATE

LAST-ACK STATE

TIME-WAIT STATE

Respond with "ok" and delete the TCB, enter CLOSED state, and

return.

[Page 62]

September 1981

Transmission Control Protocol

Functional Specification

STATUS Call

STATUS Call

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

If the user should not have access to such a connection, return

"error: connection illegal for this process".

Otherwise return "error: connection does not exist".

LISTEN STATE

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

SYN-SENT STATE

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

SYN-RECEIVED STATE

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

ESTABLISHED STATE

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

FIN-WAIT-1 STATE

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

FIN-WAIT-2 STATE

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

CLOSE-WAIT STATE

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

CLOSING STATE

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

LAST-ACK STATE

Return "state = LAST-ACK", and the TCB pointer.

[Page 63]

September 1981

Transmission Control Protocol

Functional Specification

STATUS Call

TIME-WAIT STATE

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

[Page 64]

September 1981

Transmission Control Protocol

Functional Specification

SEGMENT ARRIVES

SEGMENT ARRIVES

If the state is CLOSED (i.e., TCB does not exist) then

all data in the incoming segment is discarded. An incoming

segment containing a RST is discarded. An incoming segment not

containing a RST causes a RST to be sent in response. The

acknowledgment and sequence field values are selected to make the

reset sequence acceptable to the TCP that sent the offending

segment.

If the ACK bit is off, sequence number zero is used,

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

If the ACK bit is on,

<SEQ=SEG.ACK><CTL=RST>

Return.

If the state is LISTEN then

first check for an RST

An incoming RST should be ignored. Return.

second check for an ACK

Any acknowledgment is bad if it arrives on a connection still in

the LISTEN state. An acceptable reset segment should be formed

for any arriving ACK-bearing segment. The RST should be

formatted as follows:

<SEQ=SEG.ACK><CTL=RST>

Return.

third check for a SYN

If the SYN bit is set, check the security. If the

security/compartment on the incoming segment does not exactly

match the security/compartment in the TCB then send a reset and

return.

<SEQ=SEG.ACK><CTL=RST>

[Page 65]

September 1981

Transmission Control Protocol

Functional Specification

SEGMENT ARRIVES

If the SEG.PRC is greater than the TCB.PRC then if allowed by

the user and the system set TCB.PRC<-SEG.PRC, if not allowed

send a reset and return.

<SEQ=SEG.ACK><CTL=RST>

If the SEG.PRC is less than the TCB.PRC then continue.

Set RCV.NXT to SEG.SEQ+1, IRS is set to SEG.SEQ and any other

control or text should be queued for processing later. ISS

should be selected and a SYN segment sent of the form:

<SEQ=ISS><ACK=RCV.NXT><CTL=SYN,ACK>

SND.NXT is set to ISS+1 and SND.UNA to ISS. The connection

state should be changed to SYN-RECEIVED. Note that any other

incoming control or data (combined with SYN) will be processed

in the SYN-RECEIVED state, but processing of SYN and ACK should

not be repeated. If the listen was not fully specified (i.e.,

the foreign socket was not fully specified), then the

unspecified fields should be filled in now.

fourth other text or control

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

must have an ACK and thus would be discarded by the ACK

processing. An incoming RST segment could not be valid, since

it could not have been sent in response to anything sent by this

incarnation of the connection. So you are unlikely to get here,

but if you do, drop the segment, and return.

If the state is SYN-SENT then

first check the ACK bit

If the ACK bit is set

If SEG.ACK =< ISS, or SEG.ACK > SND.NXT, send a reset (unless

the RST bit is set, if so drop the segment and return)

<SEQ=SEG.ACK><CTL=RST>

and discard the segment. Return.

If SND.UNA =< SEG.ACK =< SND.NXT then the ACK is acceptable.

second check the RST bit

[Page 66]

September 1981

Transmission Control Protocol

Functional Specification

SEGMENT ARRIVES

If the RST bit is set

If the ACK was acceptable then signal the user "error:

connection reset", drop the segment, enter CLOSED state,

delete TCB, and return. Otherwise (no ACK) drop the segment

and return.

third check the security and precedence

If the security/compartment in the segment does not exactly

match the security/compartment in the TCB, send a reset

If there is an ACK

<SEQ=SEG.ACK><CTL=RST>

Otherwise

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

If there is an ACK

The precedence in the segment must match the precedence in the

TCB, if not, send a reset

<SEQ=SEG.ACK><CTL=RST>

If there is no ACK

If the precedence in the segment is higher than the precedence

in the TCB then if allowed by the user and the system raise

the precedence in the TCB to that in the segment, if not

allowed to raise the prec then send a reset.

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

If the precedence in the segment is lower than the precedence

in the TCB continue.

If a reset was sent, discard the segment and return.

fourth check the SYN bit

This step should be reached only if the ACK is ok, or there is

no ACK, and it the segment did not contain a RST.

If the SYN bit is on and the security/compartment and precedence

[Page 67]

September 1981

Transmission Control Protocol

Functional Specification

SEGMENT ARRIVES

are acceptable then, RCV.NXT is set to SEG.SEQ+1, IRS is set to

SEG.SEQ. SND.UNA should be advanced to equal SEG.ACK (if there

is an ACK), and any segments on the retransmission queue which

are thereby acknowledged should be removed.

If SND.UNA > ISS (our SYN has been ACKed), change the connection

state to ESTABLISHED, form an ACK segment

<SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK>

and send it. Data or controls which were queued for

transmission may be included. If there are other controls or

text in the segment then continue processing at the sixth step

below where the URG bit is checked, otherwise return.

Otherwise enter SYN-RECEIVED, form a SYN,ACK segment

<SEQ=ISS><ACK=RCV.NXT><CTL=SYN,ACK>

and send it. If there are other controls or text in the

segment, queue them for processing after the ESTABLISHED state

has been reached, return.

fifth, if neither of the SYN or RST bits is set then drop the

segment and return.

[Page 68]

September 1981

Transmission Control Protocol

Functional Specification

SEGMENT ARRIVES

Otherwise,

first check sequence number

SYN-RECEIVED STATE

ESTABLISHED STATE

FIN-WAIT-1 STATE

FIN-WAIT-2 STATE

CLOSE-WAIT STATE

CLOSING STATE

LAST-ACK STATE

TIME-WAIT STATE

Segments are processed in sequence. Initial tests on arrival

are used to discard old duplicates, but further processing is

done in SEG.SEQ order. If a segment's contents straddle the

boundary between old and new, only the new parts should be

processed.

There are four cases for the acceptability test for an incoming

segment:

Segment Receive Test

Length Window

------- ------- -------------------------------------------

0 0 SEG.SEQ = RCV.NXT

0 >0 RCV.NXT =< SEG.SEQ < RCV.NXT+RCV.WND

>0 0 not acceptable

>0 >0 RCV.NXT =< SEG.SEQ < RCV.NXT+RCV.WND

or RCV.NXT =< SEG.SEQ+SEG.LEN-1 < RCV.NXT+RCV.WND

If the RCV.WND is zero, no segments will be acceptable, but

special allowance should be made to accept valid ACKs, URGs and

RSTs.

If an incoming segment is not acceptable, an acknowledgment

should be sent in reply (unless the RST bit is set, if so drop

the segment and return):

<SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK>

After sending the acknowledgment, drop the unacceptable segment

and return.

[Page 69]

September 1981

Transmission Control Protocol

Functional Specification

SEGMENT ARRIVES

In the following it is assumed that the segment is the idealized

segment that begins at RCV.NXT and does not exceed the window.

One could tailor actual segments to fit this assumption by

trimming off any portions that lie outside the window (including

SYN and FIN), and only processing further if the segment then

begins at RCV.NXT. Segments with higher begining sequence

numbers may be held for later processing.

second check the RST bit,

SYN-RECEIVED STATE

If the RST bit is set

If this connection was initiated with a passive OPEN (i.e.,

came from the LISTEN state), then return this connection to

LISTEN state and return. The user need not be informed. If

this connection was initiated with an active OPEN (i.e., came

from SYN-SENT state) then the connection was refused, signal

the user "connection refused". In either case, all segments

on the retransmission queue should be removed. And in the

active OPEN case, enter the CLOSED state and delete the TCB,

and return.

ESTABLISHED

FIN-WAIT-1

FIN-WAIT-2

CLOSE-WAIT

If the RST bit is set then, any outstanding RECEIVEs and SEND

should receive "reset" responses. All segment queues should be

flushed. Users should also receive an unsolicited general

"connection reset" signal. Enter the CLOSED state, delete the

TCB, and return.

CLOSING STATE

LAST-ACK STATE

TIME-WAIT

If the RST bit is set then, enter the CLOSED state, delete the

TCB, and return.

[Page 70]

September 1981

Transmission Control Protocol

Functional Specification

SEGMENT ARRIVES

third check security and precedence

SYN-RECEIVED

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

exactly match the security/compartment and precedence in the TCB

then send a reset, and return.

ESTABLISHED STATE

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

exactly match the security/compartment and precedence in the TCB

then send a reset, any outstanding RECEIVEs and SEND should

receive "reset" responses. All segment queues should be

flushed. Users should also receive an unsolicited general

"connection reset" signal. Enter the CLOSED state, delete the

TCB, and return.

Note this check is placed following the sequence check to prevent

a segment from an old connection between these ports with a

different security or precedence from causing an abort of the

current connection.

fourth, check the SYN bit,

SYN-RECEIVED

ESTABLISHED STATE

FIN-WAIT STATE-1

FIN-WAIT STATE-2

CLOSE-WAIT STATE

CLOSING STATE

LAST-ACK STATE

TIME-WAIT STATE

If the SYN is in the window it is an error, send a reset, any

outstanding RECEIVEs and SEND should receive "reset" responses,

all segment queues should be flushed, the user should also

receive an unsolicited general "connection reset" signal, enter

the CLOSED state, delete the TCB, and return.

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

and an ack would have been sent in the first step (sequence

number check).

[Page 71]

September 1981

Transmission Control Protocol

Functional Specification

SEGMENT ARRIVES

fifth check the ACK field,

if the ACK bit is off drop the segment and return

if the ACK bit is on

SYN-RECEIVED STATE

If SND.UNA =< SEG.ACK =< SND.NXT then enter ESTABLISHED state

and continue processing.

If the segment acknowledgment is not acceptable, form a

reset segment,

<SEQ=SEG.ACK><CTL=RST>

and send it.

ESTABLISHED STATE

If SND.UNA < SEG.ACK =< SND.NXT then, set SND.UNA <- SEG.ACK.

Any segments on the retransmission queue which are thereby

entirely acknowledged are removed. Users should receive

positive acknowledgments for buffers which have been SENT and

fully acknowledged (i.e., SEND buffer should be returned with

"ok" response). If the ACK is a duplicate

(SEG.ACK < SND.UNA), it can be ignored. If the ACK acks

something not yet sent (SEG.ACK > SND.NXT) then send an ACK,

drop the segment, and return.

If SND.UNA < SEG.ACK =< SND.NXT, the send window should be

updated. If (SND.WL1 < SEG.SEQ or (SND.WL1 = SEG.SEQ and

SND.WL2 =< SEG.ACK)), set SND.WND <- SEG.WND, set

SND.WL1 <- SEG.SEQ, and set SND.WL2 <- SEG.ACK.

Note that SND.WND is an offset from SND.UNA, that SND.WL1

records the sequence number of the last segment used to update

SND.WND, and that SND.WL2 records the acknowledgment number of

the last segment used to update SND.WND. The check here

prevents using old segments to update the window.

[Page 72]

September 1981

Transmission Control Protocol

Functional Specification

SEGMENT ARRIVES

FIN-WAIT-1 STATE

In addition to the processing for the ESTABLISHED state, if

our FIN is now acknowledged then enter FIN-WAIT-2 and continue

processing in that state.

FIN-WAIT-2 STATE

In addition to the processing for the ESTABLISHED state, if

the retransmission queue is empty, the user's CLOSE can be

acknowledged ("ok") but do not delete the TCB.

CLOSE-WAIT STATE

Do the same processing as for the ESTABLISHED state.

CLOSING STATE

In addition to the processing for the ESTABLISHED state, if

the ACK acknowledges our FIN then enter the TIME-WAIT state,

otherwise ignore the segment.

LAST-ACK STATE

The only thing that can arrive in this state is an

acknowledgment of our FIN. If our FIN is now acknowledged,

delete the TCB, enter the CLOSED state, and return.

TIME-WAIT STATE

The only thing that can arrive in this state is a

retransmission of the remote FIN. Acknowledge it, and restart

the 2 MSL timeout.

sixth, check the URG bit,

ESTABLISHED STATE

FIN-WAIT-1 STATE

FIN-WAIT-2 STATE

If the URG bit is set, RCV.UP <- max(RCV.UP,SEG.UP), and signal

the user that the remote side has urgent data if the urgent

pointer (RCV.UP) is in advance of the data consumed. If the

user has already been signaled (or is still in the "urgent

mode") for this continuous sequence of urgent data, do not

signal the user again.

[Page 73]

September 1981

Transmission Control Protocol

Functional Specification

SEGMENT ARRIVES

CLOSE-WAIT STATE

CLOSING STATE

LAST-ACK STATE

TIME-WAIT

This should not occur, since a FIN has been received from the

remote side. Ignore the URG.

seventh, process the segment text,

ESTABLISHED STATE

FIN-WAIT-1 STATE

FIN-WAIT-2 STATE

Once in the ESTABLISHED state, it is possible to deliver segment

text to user RECEIVE buffers. Text from segments can be moved

into buffers until either the buffer is full or the segment is

empty. If the segment empties and carries an PUSH flag, then

the user is informed, when the buffer is returned, that a PUSH

has been received.

When the TCP takes responsibility for delivering the data to the

user it must also acknowledge the receipt of the data.

Once the TCP takes responsibility for the data it advances

RCV.NXT over the data accepted, and adjusts RCV.WND as

apporopriate to the current buffer availability. The total of

RCV.NXT and RCV.WND should not be reduced.

Please note the window management suggestions in section 3.7.

Send an acknowledgment of the form:

<SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK>

This acknowledgment should be piggybacked on a segment being

transmitted if possible without incurring undue delay.

[Page 74]

September 1981

Transmission Control Protocol

Functional Specification

SEGMENT ARRIVES

CLOSE-WAIT STATE

CLOSING STATE

LAST-ACK STATE

TIME-WAIT STATE

This should not occur, since a FIN has been received from the

remote side. Ignore the segment text.

eighth, check the FIN bit,

Do not process the FIN if the state is CLOSED, LISTEN or SYN-SENT

since the SEG.SEQ cannot be validated; drop the segment and

return.

If the FIN bit is set, signal the user "connection closing" and

return any pending RECEIVEs with same message, advance RCV.NXT

over the FIN, and send an acknowledgment for the FIN. Note that

FIN implies PUSH for any segment text not yet delivered to the

user.

SYN-RECEIVED STATE

ESTABLISHED STATE

Enter the CLOSE-WAIT state.

FIN-WAIT-1 STATE

If our FIN has been ACKed (perhaps in this segment), then

enter TIME-WAIT, start the time-wait timer, turn off the other

timers; otherwise enter the CLOSING state.

FIN-WAIT-2 STATE

Enter the TIME-WAIT state. Start the time-wait timer, turn

off the other timers.

CLOSE-WAIT STATE

Remain in the CLOSE-WAIT state.

CLOSING STATE

Remain in the CLOSING state.

LAST-ACK STATE

Remain in the LAST-ACK state.

[Page 75]

September 1981

Transmission Control Protocol

Functional Specification

SEGMENT ARRIVES

TIME-WAIT STATE

Remain in the TIME-WAIT state. Restart the 2 MSL time-wait

timeout.

and return.

[Page 76]

September 1981

Transmission Control Protocol

Functional Specification

USER TIMEOUT

USER TIMEOUT

For any state if the user timeout expires, flush all queues, signal

the user "error: connection aborted due to user timeout" in general

and for any outstanding calls, delete the TCB, enter the CLOSED

state and return.

RETRANSMISSION TIMEOUT

For any state if the retransmission timeout expires on a segment in

the retransmission queue, send the segment at the front of the

retransmission queue again, reinitialize the retransmission timer,

and return.

TIME-WAIT TIMEOUT

If the time-wait timeout expires on a connection delete the TCB,

enter the CLOSED state and return.

[Page 77]

September 1981

Transmission Control Protocol

[Page 78]

September 1981

Transmission Control Protocol

GLOSSARY

1822

BBN Report 1822, "The Specification of the Interconnection of

a Host and an IMP". The specification of interface between a

host and the ARPANET.

ACK

A control bit (acknowledge) occupying no sequence space, which

indicates that the acknowledgment field of this segment

specifies the next sequence number the sender of this segment

is expecting to receive, hence acknowledging receipt of all

previous sequence numbers.

ARPANET message

The unit of transmission between a host and an IMP in the

ARPANET. The maximum size is about 1012 octets (8096 bits).

ARPANET packet

A unit of transmission used internally in the ARPANET between

IMPs. The maximum size is about 126 octets (1008 bits).

connection

A logical communication path identified by a pair of sockets.

datagram

A message sent in a packet switched computer communications

network.

Destination Address

The destination address, usually the network and host

identifiers.

FIN

A control bit (finis) occupying one sequence number, which

indicates that the sender will send no more data or control

occupying sequence space.

fragment

A portion of a logical unit of data, in particular an internet

fragment is a portion of an internet datagram.

FTP

A file transfer protocol.

[Page 79]

September 1981

Transmission Control Protocol

Glossary

header

Control information at the beginning of a message, segment,

fragment, packet or block of data.

host

A computer. In particular a source or destination of messages

from the point of view of the communication network.

Identification

An Internet Protocol field. This identifying value assigned

by the sender aids in assembling the fragments of a datagram.

IMP

The Interface Message Processor, the packet switch of the

ARPANET.

internet address

A source or destination address specific to the host level.

internet datagram

The unit of data exchanged between an internet module and the

higher level protocol together with the internet header.

internet fragment

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

header.

IP

Internet Protocol.

IRS

The Initial Receive Sequence number. The first sequence

number used by the sender on a connection.

ISN

The Initial Sequence Number. The first sequence number used

on a connection, (either ISS or IRS). Selected on a clock

based procedure.

ISS

The Initial Send Sequence number. The first sequence number

used by the sender on a connection.

leader

Control information at the beginning of a message or block of

data. In particular, in the ARPANET, the control information

on an ARPANET message at the host-IMP interface.

[Page 80]

September 1981

Transmission Control Protocol

Glossary

left sequence

This is the next sequence number to be acknowledged by the

data receiving TCP (or the lowest currently unacknowledged

sequence number) and is sometimes referred to as the left edge

of the send window.

local packet

The unit of transmission within a local network.

module

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

procedure.

MSL

Maximum Segment Lifetime, the time a TCP segment can exist in

the internetwork system. Arbitrarily defined to be 2 minutes.

octet

An eight bit byte.

Options

An Option field may contain several options, and each option

may be several octets in length. The options are used

primarily in testing situations; for example, to carry

timestamps. Both the Internet Protocol and TCP provide for

options fields.

packet

A package of data with a header which may or may not be

logically complete. More often a physical packaging than a

logical packaging of data.

port

The portion of a socket that specifies which logical input or

output channel of a process is associated with the data.

process

A program in execution. A source or destination of data from

the point of view of the TCP or other host-to-host protocol.

PUSH

A control bit occupying no sequence space, indicating that

this segment contains data that must be pushed through to the

receiving user.

RCV.NXT

receive next sequence number

[Page 81]

September 1981

Transmission Control Protocol

Glossary

RCV.UP

receive urgent pointer

RCV.WND

receive window

receive next sequence number

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

receive.

receive window

This represents the sequence numbers the local (receiving) TCP

is willing to receive. Thus, the local TCP considers that

segments overlapping the range RCV.NXT to

RCV.NXT + RCV.WND - 1 carry acceptable data or control.

Segments containing sequence numbers entirely outside of this

range are considered duplicates and discarded.

RST

A control bit (reset), occupying no sequence space, indicating

that the receiver should delete the connection without further

interaction. The receiver can determine, based on the

sequence number and acknowledgment fields of the incoming

segment, whether it should honor the reset command or ignore

it. In no case does receipt of a segment containing RST give

rise to a RST in response.

RTP

Real Time Protocol: A host-to-host protocol for communication

of time critical information.

SEG.ACK

segment acknowledgment

SEG.LEN

segment length

SEG.PRC

segment precedence value

SEG.SEQ

segment sequence

SEG.UP

segment urgent pointer field

[Page 82]

September 1981

Transmission Control Protocol

Glossary

SEG.WND

segment window field

segment

A logical unit of data, in particular a TCP segment is the

unit of data transfered between a pair of TCP modules.

segment acknowledgment

The sequence number in the acknowledgment field of the

arriving segment.

segment length

The amount of sequence number space occupied by a segment,

including any controls which occupy sequence space.

segment sequence

The number in the sequence field of the arriving segment.

send sequence

This is the next sequence number the local (sending) TCP will

use on the connection. It is initially selected from an

initial sequence number curve (ISN) and is incremented for

each octet of data or sequenced control transmitted.

send window

This represents the sequence numbers which the remote

(receiving) TCP is willing to receive. It is the value of the

window field specified in segments from the remote (data

receiving) TCP. The range of new sequence numbers which may

be emitted by a TCP lies between SND.NXT and

SND.UNA + SND.WND - 1. (Retransmissions of sequence numbers

between SND.UNA and SND.NXT are expected, of course.)

SND.NXT

send sequence

SND.UNA

left sequence

SND.UP

send urgent pointer

SND.WL1

segment sequence number at last window update

SND.WL2

segment acknowledgment number at last window update

[Page 83]

September 1981

Transmission Control Protocol

Glossary

SND.WND

send window

socket

An address which specifically includes a port identifier, that

is, the concatenation of an Internet Address with a TCP port.

Source Address

The source address, usually the network and host identifiers.

SYN

A control bit in the incoming segment, occupying one sequence

number, used at the initiation of a connection, to indicate

where the sequence numbering will start.

TCB

Transmission control block, the data structure that records

the state of a connection.

TCB.PRC

The precedence of the connection.

TCP

Transmission Control Protocol: A host-to-host protocol for

reliable communication in internetwork environments.

TOS

Type of Service, an Internet Protocol field.

Type of Service

An Internet Protocol field which indicates the type of service

for this internet fragment.

URG

A control bit (urgent), occupying no sequence space, used to

indicate that the receiving user should be notified to do

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

sequence numbers less than the value indicated in the urgent

pointer.

urgent pointer

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

field communicates the value of the urgent pointer which

indicates the data octet associated with the sending user's

urgent call.

[Page 84]

September 1981

Transmission Control Protocol

REFERENCES

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

Intercommunication", IEEE Transactions on Communications,

Vol. COM-22, No. 5, pp 637-648, May 1974.

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

Protocol Specification", RFC791, 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", RFC790, USC/Information Sciences

Institute, September 1981.

 
 
 
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