- September 1981
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TABLE OF CONTENTS
PREFACE
1.
INTRODUCTION
1.1
Motivation
1.2
Scope
1.3
About This Document
1.4
Interfaces
1.5
Operation
2.
PHILOSOPHY
2.1
Elements of the Internetwork System
2.2
Model of Operation
2.3 The
Host Environment
2.4
Interfaces
2.5
Relation to Other Protocols
2.6
Reliable Communication
2.7
Connection Establishment and Clearing
2.8 Data
Communication
2.9
Precedence and Security
2.10
Robustness Principle
3.
FUNCTIONAL SPECIFICATION
3.1
Header Format
3.2
Terminology
3.3
Sequence Numbers
3.4
Establishing a connection
3.5
Closing a Connection
3.6
Precedence and Security
3.7 Data
Communication
3.8
Interfaces
3.9
Event Processing
GLOSSARY
REFERENCES
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PREFACE
- This document describes the DoD Standard Transmission Control Protocol
- (TCP). There have been nine earlier editions of the ARPA TCP
- specification on which this standard is based, and the present text
-
- draws heavily from them. There have been many contributors to this work
-
- both in terms of concepts and in terms of text. This edition clarifies
-
- several details and removes the end-of-letter buffer-size adjustments,
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- and redescribes the letter mechanism as a push function.
Jon Postel
Editor
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- RFC: 793
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- Replaces: RFC 761
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- IENs: 129, 124, 112, 81,
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- 55, 44, 40, 27, 21, 5
TRANSMISSION CONTROL PROTOCOL
DARPA INTERNET PROGRAM
PROTOCOL SPECIFICATION
- 1 INTRODUCTION
- The Transmission Control Protocol (TCP) is intended for use as a highly
-
- reliable host-to-host protocol between hosts in packet-switched computer
-
- communication networks, and in interconnected systems of such networks.
- This document describes the functions to be performed by the
-
- Transmission Control Protocol, the program that implements it, and its
-
- interface to programs or users that require its services.
- 1.1 Motivation
Computer communication systems are playing an increasingly important
role in military, government, and civilian environments. This document focuses
its attention primarily on military computer communication requirements,
especially robustness in the presence of communication unreliability and
availability in the presence of congestion, but many of these problems are
found in the civilian and government sector as well.
As strategic and
tactical computer communication networks are developed and deployed, it is
essential to provide means of interconnecting them and to provide standard
interprocess
communication protocols which can support a broad range of
applications. In anticipation of the need for such standards, the Deputy
Undersecretary of Defense for Research and Engineering has declared the
Transmission Control Protocol (TCP) described herein to be a basis for
DoD-wide inter-process communication protocol standardization.
TCP is
a connection-oriented, end-to-end reliable protocol designed to fit into a
layered hierarchy of protocols which support multi-network applications. The
TCP provides for reliable inter-process communication between pairs of
processes in host computers attached to distinct but interconnected computer
communication networks. Very few assumptions are made as to the reliability of
the communication protocols below the TCP layer. TCP assumes it can obtain a
simple, potentially unreliable datagram service from the lower level
protocols. In principle, the TCP should be able to operate above a wide
spectrum of communication systems ranging from hard-wired connections to
packet-switched or circuit-switched networks.
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- Introduction
TCP is based on concepts first described by Cerf and Kahn in
[1]. The TCP fits into a layered protocol architecture just above a basic
Internet Protocol [2] which provides a way for the TCP to send and receive
variable-length segments of information enclosed in internet datagram
"envelopes". The internet datagram provides a means for addressing source and
destination TCPs in different networks. The internet protocol also deals with
any fragmentation or reassembly of the TCP segments required to achieve
transport and delivery through multiple networks and interconnecting gateways.
The internet protocol also carries information on the precedence, security
classification and compartmentation of the TCP segments, so this information
can be communicated end-to-end across multiple networks.
Protocol
Layering
+---------------------+
| higher-level |
+---------------------+
| TCP |
+---------------------+
| internet protocol |
+---------------------+
|communication network|
+---------------------+
Figure 1
Much of this document is written in the context of TCP implementations
which are co-resident with higher level protocols in the host
computer. Some computer systems will be connected to networks via
front-end computers which house the TCP and internet protocol layers,
as well as network specific software. The TCP specification describes
an interface to the higher level protocols which appears to be
implementable even for the front-end case, as long as a suitable
host-to-front end protocol is implemented.
- 1.2 Scope
The TCP is intended to provide a reliable process-to-process
communication service in a multinetwork environment. The TCP is intended to be
a host-to-host protocol in common use in multiple networks.
- 1.3 About this Document
This document represents a specification of the behavior required of
any TCP implementation, both in its interactions with higher level protocols
and in its interactions with other TCPs. The rest of this
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section offers a
very brief view of the protocol interfaces and operation. Section
2 summarizes the philosophical basis for the TCP design. Section
3 offers both a detailed description of the actions required of TCP when
various events occur (arrival of new segments, user calls, errors, etc.) and
the details of the formats of TCP segments.
- 1.4 Interfaces
The TCP interfaces on one side to user or application processes and on
the other side to a lower level protocol such as Internet Protocol.
The
interface between an application process and the TCP is illustrated in
reasonable detail. This interface consists of a set of calls much like the
calls an operating system provides to an application process for manipulating
files. For example, there are calls to open and close connections and to send
and receive data on established connections. It is also expected that the TCP
can asynchronously communicate with application programs. Although
considerable freedom is permitted to TCP implementors to design interfaces
which are appropriate to a particular operating system environment, a minimum
functionality is required at the TCP/user interface for any valid
implementation.
The interface between TCP and lower level protocol is
essentially unspecified except that it is assumed there is a mechanism whereby
the two levels can asynchronously pass information to each other. Typically,
one expects the lower level protocol to specify this interface. TCP is
designed to work in a very general environment of interconnected networks. The
lower level protocol which is assumed throughout this document is the Internet
Protocol [2].
- 1.5 Operation
As noted above, the primary purpose of the TCP is to provide reliable,
securable logical circuit or connection service between pairs of processes. To
provide this service on top of a less reliable internet communication system
requires facilities in the following areas:
Basic Data Transfer
Reliability
Flow Control
Multiplexing
Connections
Precedence and Security
The basic operation of the TCP in each of
these areas is described in the following paragraphs.
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- Introduction
Basic Data Transfer:
The TCP is able to transfer a
continuous stream of octets in each direction between its users by packaging
some number of octets into segments for transmission through the internet
system. In general, the TCPs decide when to block and forward data at their
own convenience.
Sometimes users need to be sure that all the data
they have submitted to the TCP has been transmitted. For this purpose a push
function is defined. To assure that data submitted to a TCP is actually
transmitted the sending user indicates that it should be pushed through to the
receiving user. A push causes the TCPs to promptly forward and deliver data up
to that point to the receiver. The exact push point might not be visible to
the receiving user and the push function does not supply a record boundary
marker.
Reliability:
The TCP must recover from data that is
damaged, lost, duplicated, or delivered out of order by the internet
communication system. This is achieved by assigning a sequence number to each
octet transmitted, and requiring a positive acknowledgment (ACK) from the
receiving TCP. If the ACK is not received within a timeout interval, the data
is retransmitted. At the receiver, the sequence numbers are used to correctly
order segments that may be received out of order and to eliminate duplicates.
Damage is handled by adding a checksum to each segment transmitted, checking
it at the receiver, and discarding damaged segments.
As long as the
TCPs continue to function properly and the internet system does not become
completely partitioned, no transmission errors will affect the correct
delivery of data. TCP recovers from internet communication system errors.
Flow Control:
TCP provides a means for the receiver to govern
the amount of data sent by the sender. This is achieved by returning a
"window" with every ACK indicating a range of acceptable sequence numbers
beyond the last segment successfully received. The window indicates an allowed
number of octets that the sender may transmit before receiving further
permission.
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Multiplexing:
To allow for many processes within a single Host to use TCP
communication facilities simultaneously, the TCP provides a set of addresses
or ports within each host. Concatenated with the network and host addresses
from the internet communication layer, this forms a socket. A pair of sockets
uniquely identifies each connection. That is, a socket may be simultaneously
used in multiple connections.
The binding of ports to processes is
handled independently by each Host. However, it proves useful to attach
frequently used processes (e.g., a "logger" or timesharing service) to fixed
sockets which are made known to the public. These services can then be
accessed through the known addresses. Establishing and learning the port
addresses of other processes may involve more dynamic mechanisms.
Connections:
The reliability and flow control mechanisms described
above require that TCPs initialize and maintain certain status information for
each data stream. The combination of this information, including sockets,
sequence numbers, and window sizes, is called a connection. Each connection is
uniquely specified by a pair of sockets identifying its two sides.
When two processes wish to communicate, their TCP's must first
establish a connection (initialize the status information on each side). When
their communication is complete, the connection is terminated or closed to
free the resources for other uses.
Since connections must be established
between unreliable hosts and over the unreliable internet communication
system, a handshake mechanism with clock-based sequence numbers is used to
avoid erroneous initialization of connections.
Precedence and
Security:
The users of TCP may indicate the security and precedence of
their communication. Provision is made for default values to be used when
these features are not needed.
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- 2 PHILOSOPHY
- 2.1 Elements of the Internetwork
System
The internetwork environment consists of hosts connected to networks
which are in turn interconnected via gateways. It is assumed here that the
networks may be either local networks (e.g., the ETHERNET) or large networks
(e.g., the ARPANET), but in any case are based on packet switching technology.
The active agents that produce and consume messages are processes. Various
levels of protocols in the networks, the gateways, and the hosts support an
interprocess communication system that provides two-way data flow on logical
connections between process ports.
The term packet is used generically
here to mean the data of one transaction between a host and its network. The
format of data blocks exchanged within the a network will generally not be of
concern to us.
Hosts are computers attached to a network, and from the
communication network's point of view, are the sources and destinations of
packets. Processes are viewed as the active elements in host computers (in
accordance with the fairly common definition of a process as a program in
execution). Even terminals and files or other I/O devices are viewed as
communicating with each other through the use of processes. Thus, all
communication is viewed as inter-process communication.
Since a process
may need to distinguish among several communication streams between itself and
another process (or processes), we imagine that each process may have a number
of ports through which it communicates with the ports of other processes.
- 2.2 Model of Operation
Processes transmit data by calling on the TCP and passing buffers of
data as arguments. The TCP packages the data from these buffers into segments
and calls on the internet module to transmit each segment to the destination
TCP. The receiving TCP places the data from a segment into the receiving
user's buffer and notifies the receiving user. The TCPs include control
information in the segments which they use to ensure reliable ordered data
transmission.
The model of internet communication is that there is an
internet protocol module associated with each TCP which provides an interface
to the local network. This internet module packages TCP segments inside
internet datagrams and routes these datagrams to a destination internet module
or intermediate gateway. To transmit the datagram through the local network,
it is embedded in a local network packet.
The packet switches may perform
further packaging, fragmentation, or
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other operations to achieve the delivery of the local packet
to the destination internet module.
At a gateway between networks, the
internet datagram is "unwrapped" from its local packet and examined to
determine through which network the internet datagram should travel next. The
internet datagram is then "wrapped" in a local packet suitable to the next
network and routed to the next gateway, or to the final destination.
A
gateway is permitted to break up an internet datagram into smaller internet
datagram fragments if this is necessary for transmission through the next
network. To do this, the gateway produces a set of internet datagrams; each
carrying a fragment. Fragments may be further broken into smaller fragments at
subsequent gateways. The internet datagram fragment format is designed so that
the destination internet module can reassemble fragments into internet
datagrams.
A destination internet module unwraps the segment from the
datagram (after reassembling the datagram, if necessary) and passes it to the
destination TCP.
This simple model of the operation glosses over many
details. One important feature is the type of service. This provides
information to the gateway (or internet module) to guide it in selecting the
service parameters to be used in traversing the next network. Included in the
type of service information is the precedence of the datagram. Datagrams may
also carry security information to permit host and gateways that operate in
multilevel secure environments to properly segregate datagrams for security
considerations.
- 2.3 The Host Environment
The TCP is assumed to be a module in an operating system. The users
access the TCP much like they would access the file system. The TCP may call
on other operating system functions, for example, to manage data structures.
The actual interface to the network is assumed to be controlled by a device
driver module. The TCP does not call on the network device driver directly,
but rather calls on the internet datagram protocol module which may in turn
call on the device driver.
The mechanisms of TCP do not preclude
implementation of the TCP in a front-end processor. However, in such an
implementation, a host-to-front-end protocol must provide the functionality to
support the type of TCP-user interface described in this document.
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- 2.4 Interfaces
The TCP/user interface provides for calls made by the user on the TCP
to OPEN or CLOSE a connection, to SEND or RECEIVE data, or to obtain STATUS
about a connection. These calls are like other calls from user programs on the
operating system, for example, the calls to open, read from, and close a file.
The TCP/internet interface provides calls to send and receive
datagrams addressed to TCP modules in hosts anywhere in the internet system.
These calls have parameters for passing the address, type of service,
precedence, security, and other control information.
- 2.5 Relation to Other Protocols
The following diagram illustrates the place of the TCP in the protocol
hierarchy:
+------+ +-----+ +-----+ +-----+
|Telnet| | FTP | |Voice| ... | | Application Level
+------+ +-----+ +-----+ +-----+
| | | |
+-----+ +-----+ +-----+
| TCP | | RTP | ... | | Host Level
+-----+ +-----+ +-----+
| | |
+-------------------------------+
| Internet Protocol & ICMP | Gateway Level
+-------------------------------+
|
+---------------------------+
| Local Network Protocol | Network Level
+---------------------------+
Protocol Relationships
Figure 2.
It is expected that the TCP will be able to support higher level
protocols efficiently. It should be easy to interface higher level
protocols like the ARPANET Telnet or AUTODIN II THP to the TCP.
- 2.6 Reliable Communication
A stream of data sent on a TCP connection is delivered reliably and in
order at the destination.
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Transmission is made reliable via the use of sequence numbers
and acknowledgments. Conceptually, each octet of data is assigned a sequence
number. The sequence number of the first octet of data in a segment is
transmitted with that segment and is called the segment sequence number.
Segments also carry an acknowledgment number which is the sequence number of
the next expected data octet of
transmissions in the reverse direction.
When the TCP transmits a segment containing data, it puts a copy on a
retransmission queue and starts a timer; when the acknowledgment for that data
is received, the segment is deleted from the queue. If the acknowledgment is
not received before the timer runs out, the segment is retransmitted.
An
acknowledgment by TCP does not guarantee that the data has been delivered to
the end user, but only that the receiving TCP has taken the responsibility to
do so.
To govern the flow of data between TCPs, a flow control
mechanism is employed. The receiving TCP reports a "window" to the sending
TCP. This window specifies the number of octets, starting with the
acknowledgment number, that the receiving TCP is currently prepared to
receive.
- 2.7 Connection Establishment and
Clearing
To identify the separate data streams that a TCP may handle, the TCP
provides a port identifier. Since port identifiers are selected independently
by each TCP they might not be unique. To provide for unique addresses within
each TCP, we concatenate an internet address identifying the TCP with a port
identifier to create a socket which will be unique throughout all networks
connected together.
A connection is fully specified by the pair of sockets
at the ends. A local socket may participate in many connections to different
foreign sockets. A connection can be used to carry data in both directions,
that is, it is "full duplex".
TCPs are free to associate ports with
processes however they choose. However, several basic concepts are necessary
in any implementation. There must be well-known sockets which the TCP
associates only with the "appropriate" processes by some means. We envision
that processes may "own" ports, and that processes can initiate connections
only on the ports they own. (Means for implementing ownership is a local
issue, but we envision a Request Port user command, or a method of uniquely
allocating a group of ports to a given process, e.g., by associating the high
order bits of a port name with a given process.)
A connection is specified
in the OPEN call by the local port and foreign socket arguments. In return,
the TCP supplies a (short) local
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connection name by
which the user refers to the connection in subsequent calls. There are several
things that must be remembered about a connection. To store this information
we imagine that there is a data structure called a Transmission Control Block
(TCB). One implementation strategy would have the local connection name be a
pointer to the TCB for this connection. The OPEN call also specifies whether
the connection establishment is to be actively pursued, or to be passively
waited for.
A passive OPEN request means that the process wants to
accept incoming connection requests rather than attempting to initiate a
connection. Often the process requesting a passive OPEN will accept a
connection request from any caller. In this case a foreign socket of all zeros
is used to denote an unspecified socket. Unspecified foreign sockets are
allowed only on passive OPENs.
A service process that wished to
provide services for unknown other processes would issue a passive OPEN
request with an unspecified foreign socket. Then a connection could be made
with any process that requested a connection to this local socket. It would
help if this local socket were known to be associated with this service.
Well-known sockets are a convenient mechanism for a priori associating a
socket address with a standard service. For instance, the "Telnet-Server"
process is permanently assigned to a particular socket, and other sockets are
reserved for File Transfer, Remote Job Entry, Text Generator, Echoer, and Sink
processes (the last three being for test purposes). A socket address might be
reserved for access to a "Look-Up" service which would return the specific
socket at which a newly created service would be provided. The concept of a
well-known socket is part of the TCP specification, but the assignment of
sockets to services is outside this specification. (See [4].)
Processes
can issue passive OPENs and wait for matching active OPENs from other
processes and be informed by the TCP when connections have been established.
Two processes which issue active OPENs to each other at the same time will be
correctly connected. This flexibility is critical for the support of
distributed computing in which components act asynchronously with respect to
each other.
There are two principal cases for matching the sockets in
the local passive OPENs and an foreign active OPENs. In the first case, the
local passive OPENs has fully specified the foreign socket. In this case, the
match must be exact. In the second case, the local passive OPENs has left the
foreign socket unspecified. In this case, any foreign socket is acceptable as
long as the local sockets match. Other possibilities include partially
restricted matches.
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If there are several pending passive OPENs (recorded in TCBs)
with the same local socket, an foreign active OPEN will be matched to a TCB
with the specific foreign socket in the foreign active OPEN, if such a TCB
exists, before selecting a TCB with an unspecified foreign socket.
The
procedures to establish connections utilize the synchronize (SYN) control flag
and involves an exchange of three messages. This exchange has been termed a
three-way hand shake [3].
A connection is initiated by the rendezvous
of an arriving segment containing a SYN and a waiting TCB entry each created
by a user OPEN command. The matching of local and foreign sockets determines
when a connection has been initiated. The connection becomes "established"
when sequence numbers have been synchronized in both directions.
The
clearing of a connection also involves the exchange of segments, in this case
carrying the FIN control flag.
- 2.8 Data Communication
The data that flows on a connection may be thought of as a stream of
octets. The sending user indicates in each SEND call whether the data in that
call (and any preceeding calls) should be immediately pushed through to the
receiving user by the setting of the PUSH flag.
A sending TCP is allowed
to collect data from the sending user and to send that data in segments at its
own convenience, until the push function is signaled, then it must send all
unsent data. When a receiving TCP sees the PUSH flag, it must not wait for
more data from the sending TCP before passing the data to the receiving
process.
There is no necessary relationship between push functions and
segment boundaries. The data in any particular segment may be the result of a
single SEND call, in whole or part, or of multiple SEND calls.
The purpose
of push function and the PUSH flag is to push data through from the sending
user to the receiving user. It does not provide a record service.
There is a coupling between the push function and the use of buffers
of data that cross the TCP/user interface. Each time a PUSH flag is associated
with data placed into the receiving user's buffer, the buffer is returned to
the user for processing even if the buffer is not filled. If data arrives that
fills the user's buffer before a PUSH is seen, the data is passed to the user
in buffer size units.
TCP also provides a means to communicate to the
receiver of data that at some point further along in the data stream than the
receiver is
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currently reading
there is urgent data. TCP does not attempt to define what the user
specifically does upon being notified of pending urgent data, but the general
notion is that the receiving process will take action to process the urgent
data quickly.
- 2.9 Precedence and Security
The TCP makes use of the internet protocol type of service field and
security option to provide precedence and security on a per connection basis
to TCP users. Not all TCP modules will necessarily function in a multilevel
secure environment; some may be limited to unclassified use only, and others
may operate at only one security level and compartment. Consequently, some TCP
implementations and services to users may be limited to a subset of the
multilevel secure case.
TCP modules which operate in a multilevel secure
environment must properly mark outgoing segments with the security,
compartment, and precedence. Such TCP modules must also provide to their users
or higher level protocols such as Telnet or THP an interface to allow them to
specify the desired security level, compartment, and precedence of
connections.
- 2.10 Robustness Principle
TCP implementations will follow a general principle of robustness: be
conservative in what you do, be liberal in what you accept from others.
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- 3 FUNCTIONAL SPECIFICATION
- 3.1 Header Format
TCP segments are sent as internet datagrams. The Internet Protocol
header carries several information fields, including the source and
destination host addresses [2]. A TCP header follows the internet header,
supplying information specific to the TCP protocol. This division allows for
the existence of host level protocols other than TCP.
TCP Header
Format
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Port | Destination Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Acknowledgment Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data | |U|A|P|R|S|F| |
| Offset| Reserved |R|C|S|S|Y|I| Window |
| | |G|K|H|T|N|N| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum | Urgent Pointer |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
TCP Header Format
Note that one tick mark represents one bit position.
Figure 3.
Source Port: 16 bits
The source port number.
Destination Port: 16 bits
The destination port number.
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Sequence Number: 32 bits
The sequence number of the
first data octet in this segment (except when SYN is present). If SYN is
present the sequence number is the initial sequence number (ISN) and the first
data octet is ISN+1.
Acknowledgment Number: 32 bits
If the ACK
control bit is set this field contains the value of the next sequence number
the sender of the segment is expecting to receive. Once a connection is
established this is always sent.
Data Offset: 4 bits
The number of
32 bit words in the TCP Header. This indicates where the data begins. The TCP
header (even one including options) is an integral number of 32 bits long.
Reserved: 6 bits
Reserved for future use. Must be zero.
Control Bits: 6 bits (from left to right):
URG: Urgent Pointer
field significant
ACK: Acknowledgment field significant
PSH: Push
Function
RST: Reset the connection
SYN: Synchronize sequence numbers
FIN: No more data from sender
Window: 16 bits
The number
of data octets beginning with the one indicated in the acknowledgment field
which the sender of this segment is willing to accept.
Checksum: 16
bits
The checksum field is the 16 bit one's complement of the one's
complement sum of all 16 bit words in the header and text. If a segment
contains an odd number of header and text octets to be checksummed, the last
octet is padded on the right with zeros to form a 16 bit word for checksum
purposes. The pad is not transmitted as part of the segment. While computing
the checksum, the checksum field itself is replaced with zeros.
The
checksum also covers a 96 bit pseudo header conceptually
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prefixed to the TCP header. This pseudo header contains the Source
Address, the Destination Address, the Protocol, and TCP length. This gives the
TCP protection against misrouted segments. This information is carried in the
Internet Protocol and is transferred across the TCP/Network interface in the
arguments or results of calls by the TCP on the IP.
+--------+--------+--------+--------+
| Source Address |
+--------+--------+--------+--------+
| Destination Address |
+--------+--------+--------+--------+
| zero | PTCL | TCP Length |
+--------+--------+--------+--------+
The TCP Length is the TCP header length plus the data length in
octets (this is not an explicitly transmitted quantity, but is
computed), and it does not count the 12 octets of the pseudo
header.
Urgent Pointer: 16 bits
This field communicates the current value of the urgent pointer as a
positive offset from the sequence number in this segment. The
urgent pointer points to the sequence number of the octet following
the urgent data. This field is only be interpreted in segments with
the URG control bit set.
Options: variable
Options may occupy space at the end of the TCP header and are a
multiple of 8 bits in length. All options are included in the
checksum. An option may begin on any octet boundary. There are two
cases for the format of an option:
Case 1: A single octet of option-kind.
Case 2: An octet of option-kind, an octet of option-length, and
the actual option-data octets.
The option-length counts the two octets of option-kind and
option-length as well as the option-data octets.
Note that the list of options may be shorter than the data offset
field might imply. The content of the header beyond the
End-of-Option option must be header padding (i.e., zero).
A TCP must implement all options.
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Currently defined options include (kind indicated in octal): Kind Length Meaning
---- ------ -------
0 - End of option list.
1 - No-Operation.
2 4 Maximum Segment Size.
Specific Option Definitions
End of Option List
+--------+
|00000000|
+--------+
Kind=0
This option code indicates the end of the option list. This
might not coincide with the end of the TCP header according to
the Data Offset field. This is used at the end of all options,
not the end of each option, and need only be used if the end of
the options would not otherwise coincide with the end of the TCP
header.
No-Operation
+--------+
|00000001|
+--------+
Kind=1
This option code may be used between options, for example, to
align the beginning of a subsequent option on a word boundary.
There is no guarantee that senders will use this option, so
receivers must be prepared to process options even if they do
not begin on a word boundary.
Maximum Segment Size
+--------+--------+---------+--------+
|00000010|00000100| max seg size |
+--------+--------+---------+--------+
Kind=2 Length=4
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Maximum
Segment Size Option Data: 16 bits
If this option is present, then it
communicates the maximum receive segment size at the TCP which sends this
segment. This field must only be sent in the initial connection request (i.e.,
in segments with the SYN control bit set). If this option is not used, any
segment size is allowed.
Padding: variable
The TCP header
padding is used to ensure that the TCP header ends and data begins on a 32 bit
boundary. The padding is composed of zeros.
- 3.2 Terminology
Before we can discuss very much about the operation of the TCP we need
to introduce some detailed terminology. The maintenance of a TCP connection
requires the remembering of several variables. We conceive of these variables
being stored in a connection record called a Transmission Control Block or
TCB. Among the variables stored in the TCB are the local and remote socket
numbers, the security and precedence of the connection, pointers to the user's
send and receive buffers, pointers to the retransmit queue and to the current
segment. In addition several variables relating to the send and receive
sequence numbers are stored in the TCB.
Send Sequence Variables
SND.UNA - send unacknowledged
SND.NXT - send next
SND.WND -
send window
SND.UP - send urgent pointer
SND.WL1 - segment sequence
number used for last window update SND.WL2 - segment acknowledgment number
used for last window update ISS - initial send sequence number
Receive Sequence Variables
RCV.NXT - receive next
RCV.WND - receive window
RCV.UP - receive urgent pointer
IRS - initial receive sequence number
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The following diagrams may help to relate some of these
variables to the sequence space.
Send Sequence Space
1 2 3 4
----------|----------|----------|----------
SND.UNA SND.NXT SND.UNA
+SND.WND
1 - old sequence numbers which have been acknowledged
2 - sequence numbers of unacknowledged data
3 - sequence numbers allowed for new data transmission
4 - future sequence numbers which are not yet allowed
Send Sequence Space
Figure 4.
The send window is the portion of the sequence space labeled 3 in
figure 4.
Receive Sequence Space
1 2 3
----------|----------|----------
RCV.NXT RCV.NXT
+RCV.WND
1 - old sequence numbers which have been acknowledged
2 - sequence numbers allowed for new reception
3 - future sequence numbers which are not yet allowed
Receive Sequence Space
Figure 5.
The receive window is the portion of the sequence space labeled 2 in
figure 5.
There are also some variables used frequently in the discussion that
take their values from the fields of the current segment.
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Current
Segment Variables
SEG.SEQ - segment sequence number
SEG.ACK -
segment acknowledgment number
SEG.LEN - segment length
SEG.WND -
segment window
SEG.UP - segment urgent pointer
SEG.PRC - segment
precedence value
A connection progresses through a series of states
during its lifetime. The states are: LISTEN, SYN-SENT, SYN-RECEIVED,
ESTABLISHED, FIN-WAIT-1, FIN-WAIT-2, CLOSE-WAIT, CLOSING, LAST-ACK, TIME-WAIT,
and the fictional state CLOSED. CLOSED is fictional because it represents the
state when there is no TCB, and therefore, no connection. Briefly the meanings
of the states are:
LISTEN - represents waiting for a connection
request from any remote TCP and port.
SYN-SENT - represents waiting
for a matching connection request after having sent a connection request.
SYN-RECEIVED - represents waiting for a confirming connection request
acknowledgment after having both received and sent a connection request.
ESTABLISHED - represents an open connection, data received can be
delivered to the user. The normal state for the data transfer phase of the
connection.
FIN-WAIT-1 - represents waiting for a connection
termination request from the remote TCP, or an acknowledgment of the
connection termination request previously sent.
FIN-WAIT-2 -
represents waiting for a connection termination request from the remote TCP.
CLOSE-WAIT - represents waiting for a connection termination request
from the local user.
CLOSING - represents waiting for a connection
termination request acknowledgment from the remote TCP.
LAST-ACK -
represents waiting for an acknowledgment of the connection termination request
previously sent to the remote TCP (which includes an acknowledgment of its
connection termination request).
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TIME-WAIT - represents waiting for enough time to pass to be
sure the remote TCP received the acknowledgment of its connection termination
request.
CLOSED - represents no connection state at all.
A TCP
connection progresses from one state to another in response to events. The
events are the user calls, OPEN, SEND, RECEIVE, CLOSE, ABORT, and STATUS; the
incoming segments, particularly those containing the SYN, ACK, RST and FIN
flags; and timeouts.
The state diagram in figure 6 illustrates only
state changes, together with the causing events and resulting actions, but
addresses neither error conditions nor actions which are not connected with
state changes. In a later section, more detail is offered with respect to the
reaction of the TCP to events.
NOTE BENE: this diagram is only a
summary and must not be taken as the total specification.
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+---------+ ---------\ active OPEN
| CLOSED | \ -----------
+---------+<---------\ \ create TCB
| ^ \ \ snd SYN
passive OPEN | | CLOSE \ \
------------ | | ---------- \ \
create TCB | | delete TCB \ \
V | \ \
+---------+ CLOSE | \
| LISTEN | ---------- | |
+---------+ delete TCB | |
rcv SYN | | SEND | |
----------- | | ------- | V
+---------+ snd SYN,ACK / \ snd SYN +---------+
| |<----------------- ------------------>| |
| SYN | rcv SYN | SYN |
| RCVD |<-----------------------------------------------| SENT |
| | snd ACK | |
| |------------------ -------------------| |
+---------+ rcv ACK of SYN \ / rcv SYN,ACK +---------+
| -------------- | | -----------
| x | | snd ACK
| V V
| CLOSE +---------+
| ------- | ESTAB |
| snd FIN +---------+
| CLOSE | | rcv FIN
V ------- | | -------
+---------+ snd FIN / \ snd ACK +---------+
| FIN |<----------------- ------------------>| CLOSE |
| WAIT-1 |------------------ | WAIT |
+---------+ rcv FIN \ +---------+
| rcv ACK of FIN ------- | CLOSE |
| -------------- snd ACK | ------- |
V x V snd FIN V
+---------+ +---------+ +---------+
|FINWAIT-2| | CLOSING | | LAST-ACK|
+---------+ +---------+ +---------+
| rcv ACK of FIN | rcv ACK of FIN |
| rcv FIN -------------- | Timeout=2MSL -------------- |
| ------- x V ------------ x V
\ snd ACK +---------+delete TCB +---------+
------------------------>|TIME WAIT|------------------>| CLOSED |
+---------+ +---------+
TCP Connection State Diagram
Figure 6.
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- 3.3 Sequence Numbers
A fundamental notion in the design is that every octet of data sent
over a TCP connection has a sequence number. Since every octet is sequenced,
each of them can be acknowledged. The acknowledgment mechanism employed is
cumulative so that an acknowledgment of sequence number X indicates that all
octets up to but not including X have been received. This mechanism allows for
straight-forward duplicate detection in the presence of retransmission.
Numbering of octets within a segment is that the first data octet immediately
following the header is the lowest numbered, and the following octets are
numbered consecutively.
It is essential to remember that the actual
sequence number space is finite, though very large. This space ranges from 0
to 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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In
response to sending data the TCP will receive acknowledgments. The following
comparisons are needed to process the acknowledgments. SND.UNA = oldest unacknowledged sequence number
SND.NXT = next sequence number to be sent
SEG.ACK = acknowledgment from the receiving TCP (next sequence
number expected by the receiving TCP)
SEG.SEQ = first sequence number of a segment
SEG.LEN = the number of octets occupied by the data in the segment
(counting SYN and FIN)
SEG.SEQ+SEG.LEN-1 = last sequence number of a segment
A new acknowledgment (called an "acceptable ack"), is one for which
the inequality below holds:
SND.UNA < SEG.ACK =< SND.NXT
A segment on the retransmission queue is fully acknowledged if the sum
of its sequence number and length is less or equal than the
acknowledgment value in the incoming segment.
When data is received the following comparisons are needed:
RCV.NXT = next sequence number expected on an incoming segments, and
is the left or lower edge of the receive window
RCV.NXT+RCV.WND-1 = last sequence number expected on an incoming
segment, and is the right or upper edge of the receive window
SEG.SEQ = first sequence number occupied by the incoming segment
SEG.SEQ+SEG.LEN-1 = last sequence number occupied by the incoming
segment
A segment is judged to occupy a portion of valid receive sequence
space if
RCV.NXT =< SEG.SEQ < RCV.NXT+RCV.WND
or
RCV.NXT =< SEG.SEQ+SEG.LEN-1 < RCV.NXT+RCV.WND
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The first part of this test checks to see if the beginning of
the segment falls in the window, the second part of the test checks to see if
the end of the segment falls in the window; if the segment passes either part
of the test it contains data in the window.
Actually, it is a little
more complicated than this. Due to zero windows and zero length segments, we
have four cases for the acceptability of an incoming segment:
Segment
Receive Test
Length Window ------- ------- -------------------------------------------
0 0 SEG.SEQ = RCV.NXT
0 >0 RCV.NXT =< SEG.SEQ < RCV.NXT+RCV.WND
>0 0 not acceptable
>0 >0 RCV.NXT =< SEG.SEQ < RCV.NXT+RCV.WND
or RCV.NXT =< SEG.SEQ+SEG.LEN-1 < RCV.NXT+RCV.WND
Note that when the receive window is zero no segments should be
acceptable except ACK segments. Thus, it is be possible for a TCP to
maintain a zero receive window while transmitting data and receiving
ACKs. However, even when the receive window is zero, a TCP must
process the RST and URG fields of all incoming segments.
We have taken advantage of the numbering scheme to protect certain
control information as well. This is achieved by implicitly including
some control flags in the sequence space so they can be retransmitted
and acknowledged without confusion (i.e., one and only one copy of the
control will be acted upon). Control information is not physically
carried in the segment data space. Consequently, we must adopt rules
for implicitly assigning sequence numbers to control. The SYN and FIN
are the only controls requiring this protection, and these controls
are used only at connection opening and closing. For sequence number
purposes, the SYN is considered to occur before the first actual data
octet of the segment in which it occurs, while the FIN is considered
to occur after the last actual data octet in a segment in which it
occurs. The segment length (SEG.LEN) includes both data and sequence
space occupying controls. When a SYN is present then SEG.SEQ is the
sequence number of the SYN.
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Initial
Sequence Number Selection
The protocol places no restriction on a
particular connection being used over and over again. A connection is defined
by a pair of sockets. New instances of a connection will be referred to as
incarnations of the connection. The problem that arises from this is -- "how does the TCP identify duplicate segments from previous
incarnations of the connection?" This problem becomes apparent if the
connection is being opened and closed in quick succession, or if the
connection breaks with loss of memory and is then reestablished.
To avoid confusion we must prevent segments from one incarnation of a
connection from being used while the same sequence numbers may still
be present in the network from an earlier incarnation. We want to
assure this, even if a TCP crashes and loses all knowledge of the
sequence numbers it has been using. When new connections are created,
an initial sequence number (ISN) generator is employed which selects a
new 32 bit ISN. The generator is bound to a (possibly fictitious) 32
bit clock whose low order bit is incremented roughly every 4
microseconds. Thus, the ISN cycles approximately every 4.55 hours.
Since we assume that segments will stay in the network no more than
the Maximum Segment Lifetime (MSL) and that the MSL is less than 4.55
hours we can reasonably assume that ISN's will be unique.
For each connection there is a send sequence number and a receive
sequence number. The initial send sequence number (ISS) is chosen by
the data sending TCP, and the initial receive sequence number (IRS) is
learned during the connection establishing procedure.
For a connection to be established or initialized, the two TCPs must
synchronize on each other's initial sequence numbers. This is done in
an exchange of connection establishing segments carrying a control bit
called "SYN" (for synchronize) and the initial sequence numbers. As a
shorthand, segments carrying the SYN bit are also called "SYNs".
Hence, the solution requires a suitable mechanism for picking an
initial sequence number and a slightly involved handshake to exchange
the ISN's.
The synchronization requires each side to send it's own initial
sequence number and to receive a confirmation of it in acknowledgment
from the other side. Each side must also receive the other side's
initial sequence number and send a confirming acknowledgment.
1) A --> B SYN my sequence number is X
2) A <-- B ACK your sequence number is X
3) A <-- B SYN my sequence number is Y
4) A --> B ACK your sequence number is Y
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Because steps 2 and 3 can be combined in a single message this
is called the three way (or three message) handshake.
A three way
handshake is necessary because sequence numbers are not tied to a global clock
in the network, and TCPs may have different mechanisms for picking the ISN's.
The receiver of the first SYN has no way of knowing whether the segment was an
old delayed one or not, unless it remembers the last sequence number used on
the connection (which is not always possible), and so it must ask the sender
to verify this SYN. The three way handshake and the advantages of a
clock-driven scheme are discussed in [3].
Knowing When to Keep Quiet
To be sure that a TCP does not create a segment that carries a
sequence number which may be duplicated by an old segment remaining in the
network, the TCP must keep quiet for a maximum segment lifetime (MSL) before
assigning any sequence numbers upon starting up or recovering from a crash in
which memory of sequence numbers in use was lost. For this specification the
MSL is taken to be 2 minutes. This is an engineering choice, and may be
changed if experience indicates it is desirable to do so. Note that if a TCP
is reinitialized in some sense, yet retains its memory of sequence numbers in
use, then it need not wait at all; it must only be sure to use sequence
numbers larger than those recently used.
The TCP Quiet Time Concept
This specification provides that hosts which "crash" without retaining
any knowledge of the last sequence numbers transmitted on each active (i.e.,
not closed) connection shall delay emitting any TCP segments for at least the
agreed Maximum Segment Lifetime (MSL) in the internet system of which the host
is a part. In the paragraphs below, an explanation for this specification is
given. TCP implementors may violate the "quiet time" restriction, but only at
the risk of causing some old data to be accepted as new or new data rejected
as old duplicated by some receivers in the internet system.
TCPs
consume sequence number space each time a segment is formed and entered into
the network output queue at a source host. The duplicate detection and
sequencing algorithm in the TCP protocol relies on the unique binding of
segment data to sequence space to the extent that sequence numbers will not
cycle through all 2**32 values before the segment data bound to those sequence
numbers has been delivered and acknowledged by the receiver and all duplicate
copies of the segments have "drained" from the internet. Without such an
assumption, two distinct TCP segments could conceivably be
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assigned the same or overlapping sequence numbers, causing
confusion at the receiver as to which data is new and which is old. Remember
that each segment is bound to as many consecutive sequence numbers as there
are octets of data in the segment.
Under normal conditions, TCPs keep
track of the next sequence number to emit and the oldest awaiting
acknowledgment so as to avoid mistakenly using a sequence number over before
its first use has been acknowledged. This alone does not guarantee that old
duplicate data is drained from the net, so the sequence space has been made
very large to reduce the probability that a wandering duplicate will cause
trouble upon arrival. At 2 megabits/sec. it takes 4.5 hours to use up 2**32
octets of sequence space. Since the maximum segment lifetime in the net is not
likely to exceed a few tens of seconds, this is deemed ample protection for
foreseeable nets, even if data rates escalate to l0's of megabits/sec. At 100
megabits/sec, the cycle time is 5.4 minutes which may be a little short, but
still within reason.
The basic duplicate detection and sequencing
algorithm in TCP can be defeated, however, if a source TCP does not have any
memory of the sequence numbers it last used on a given connection. For
example, if the TCP were to start all connections with sequence number 0, then
upon crashing and restarting, a TCP might re-form an earlier connection
(possibly after half-open connection resolution) and emit packets with
sequence numbers identical to or overlapping with packets still in the network
which were emitted on an earlier incarnation of the same connection. In the
absence of knowledge about the sequence numbers used on a particular
connection, the TCP specification recommends that the source delay for MSL
seconds before emitting segments on the connection, to allow time for segments
from the earlier connection incarnation to drain from the system.
Even
hosts which can remember the time of day and used it to select initial
sequence number values are not immune from this problem (i.e., even if time of
day is used to select an initial sequence number for each new connection
incarnation).
Suppose, for example, that a connection is opened
starting with sequence number S. Suppose that this connection is not used much
and that eventually the initial sequence number function (ISN(t)) takes on a
value equal to the sequence number, say S1, of the last segment sent by this
TCP on a particular connection. Now suppose, at this instant, the host
crashes, recovers, and establishes a new incarnation of the connection. The
initial sequence number chosen is S1 = ISN(t) -- last used sequence number on old incarnation of
connection! If the recovery occurs quickly enough, any old
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duplicates in the net bearing sequence numbers in the
neighborhood of S1 may arrive and be treated as new packets by the receiver of
the new incarnation of the connection.
The problem is that the
recovering host may not know for how long it crashed nor does it know whether
there are still old duplicates in the system from earlier connection
incarnations.
One way to deal with this problem is to deliberately
delay emitting segments for one MSL after recovery from a crash- this is the
"quite time" specification. Hosts which prefer to avoid waiting are willing to
risk possible confusion of old and new packets at a given destination may
choose not to wait for the "quite time". Implementors may provide TCP users
with the ability to select on a connection by connection basis whether to wait
after a crash, or may informally implement the "quite time" for all
connections. Obviously, even where a user selects to "wait," this is not
necessary after the host has been "up" for at least MSL seconds.
To
summarize: every segment emitted occupies one or more sequence numbers in the
sequence space, the numbers occupied by a segment are "busy" or "in use" until
MSL seconds have passed, upon crashing a block of space-time is occupied by
the octets of the last emitted segment, if a new connection is started too
soon and uses any of the sequence numbers in the space-time footprint of the
last segment of the previous connection incarnation, there is a potential
sequence number overlap area which could cause confusion at the receiver.
- 3.4 Establishing a connection
The "three-way handshake" is the procedure used to establish a
connection. This procedure normally is initiated by one TCP and responded to
by another TCP. The procedure also works if two TCP simultaneously initiate
the procedure. When simultaneous attempt occurs, each TCP receives a "SYN"
segment which carries no acknowledgment after it has sent a "SYN". Of course,
the arrival of an old duplicate "SYN" segment can potentially make it appear,
to the recipient, that a simultaneous connection initiation is in progress.
Proper use of "reset" segments can disambiguate these cases.
Several
examples of connection initiation follow. Although these examples do not show
connection synchronization using data-carrying segments, this is perfectly
legitimate, so long as the receiving TCP doesn't deliver the data to the user
until it is clear the data is valid (i.e., the data must be buffered at the
receiver until the connection reaches the ESTABLISHED state). The three-way
handshake reduces the possibility of false connections. It is the
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implementation of a trade-off between memory and messages to
provide information for this checking.
The simplest three-way
handshake is shown in figure 7 below. The figures should be interpreted in the
following way. Each line is numbered for reference purposes. Right arrows
(-->) indicate departure of a TCP segment from TCP A to TCP B, or arrival
of a segment at B from A. Left arrows (<--), indicate the reverse. Ellipsis
(...) indicates a segment which is still in the network (delayed). An "XXX"
indicates a segment which is lost or rejected. Comments appear in parentheses.
TCP states represent the state AFTER the departure or arrival of the segment
(whose contents are shown in the center of each line). Segment contents are
shown in abbreviated form, with sequence number, control flags, and ACK field.
Other fields such as window, addresses, lengths, and text have been left out
in the interest of clarity.
TCP A TCP B
1. CLOSED LISTEN
2. SYN-SENT --> <SEQ=100><CTL=SYN> --> SYN-RECEIVED
3. ESTABLISHED <-- <SEQ=300><ACK=101><CTL=SYN,ACK> <-- SYN-RECEIVED
4. ESTABLISHED --> <SEQ=101><ACK=301><CTL=ACK> --> ESTABLISHED
5. ESTABLISHED --> <SEQ=101><ACK=301><CTL=ACK><DATA> --> ESTABLISHED
Basic 3-Way Handshake for Connection Synchronization
Figure 7.
In line 2 of figure 7, TCP A begins by sending a SYN segment
indicating that it will use sequence numbers starting with sequence
number 100. In line 3, TCP B sends a SYN and acknowledges the SYN it
received from TCP A. Note that the acknowledgment field indicates TCP
B is now expecting to hear sequence 101, acknowledging the SYN which
occupied sequence 100.
At line 4, TCP A responds with an empty segment containing an ACK for
TCP B's SYN; and in line 5, TCP A sends some data. Note that the
sequence number of the segment in line 5 is the same as in line 4
because the ACK does not occupy sequence number space (if it did, we
would wind up ACKing ACK's!).
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Simultaneous initiation is only slightly more complex, as is
shown in figure 8. Each TCP cycles from CLOSED to SYN-SENT to SYN-RECEIVED to
ESTABLISHED.
TCP A TCP B
1. CLOSED CLOSED
2. SYN-SENT --> <SEQ=100><CTL=SYN> ...
3. SYN-RECEIVED <-- <SEQ=300><CTL=SYN> <-- SYN-SENT
4. ... <SEQ=100><CTL=SYN> --> SYN-RECEIVED
5. SYN-RECEIVED --> <SEQ=100><ACK=301><CTL=SYN,ACK> ...
6. ESTABLISHED <-- <SEQ=300><ACK=101><CTL=SYN,ACK> <-- SYN-RECEIVED
7. ... <SEQ=101><ACK=301><CTL=ACK> --> ESTABLISHED
Simultaneous Connection Synchronization
Figure 8.
The principle reason for the three-way handshake is to prevent old
duplicate connection initiations from causing confusion. To deal with
this, a special control message, reset, has been devised. If the
receiving TCP is in a non-synchronized state (i.e., SYN-SENT,
SYN-RECEIVED), it returns to LISTEN on receiving an acceptable reset.
If the TCP is in one of the synchronized states (ESTABLISHED,
FIN-WAIT-1, FIN-WAIT-2, CLOSE-WAIT, CLOSING, LAST-ACK, TIME-WAIT), it
aborts the connection and informs its user. We discuss this latter
case under "half-open" connections below.
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TCP A TCP B
1. CLOSED LISTEN
2. SYN-SENT --> <SEQ=100><CTL=SYN> ...
3. (duplicate) ... <SEQ=90><CTL=SYN> --> SYN-RECEIVED
4. SYN-SENT <-- <SEQ=300><ACK=91><CTL=SYN,ACK> <-- SYN-RECEIVED
5. SYN-SENT --> <SEQ=91><CTL=RST> --> LISTEN
6. ... <SEQ=100><CTL=SYN> --> SYN-RECEIVED
7. SYN-SENT <-- <SEQ=400><ACK=101><CTL=SYN,ACK> <-- SYN-RECEIVED
8. ESTABLISHED --> <SEQ=101><ACK=401><CTL=ACK> --> ESTABLISHED
Recovery from Old Duplicate SYN
Figure 9.
As a simple example of recovery from old duplicates, consider
figure 9. At line 3, an old duplicate SYN arrives at TCP B. TCP B
cannot tell that this is an old duplicate, so it responds normally
(line 4). TCP A detects that the ACK field is incorrect and returns a
RST (reset) with its SEQ field selected to make the segment
believable. TCP B, on receiving the RST, returns to the LISTEN state.
When the original SYN (pun intended) finally arrives at line 6, the
synchronization proceeds normally. If the SYN at line 6 had arrived
before the RST, a more complex exchange might have occurred with RST's
sent in both directions.
Half-Open Connections and Other Anomalies
An established connection is said to be "half-open" if one of the
TCPs has closed or aborted the connection at its end without the
knowledge of the other, or if the two ends of the connection have
become desynchronized owing to a crash that resulted in loss of
memory. Such connections will automatically become reset if an
attempt is made to send data in either direction. However, half-open
connections are expected to be unusual, and the recovery procedure is
mildly involved.
If at site A the connection no longer exists, then an attempt by the
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user at site B to send any data on it will result in the site
B TCP receiving a reset control message. Such a message indicates to the site
B TCP that something is wrong, and it is expected to abort the connection.
Assume that two user processes A and B are communicating with one
another when a crash occurs causing loss of memory to A's TCP. Depending on
the operating system supporting A's TCP, it is likely that some error recovery
mechanism exists. When the TCP is up again, A is likely to start again from
the beginning or from a recovery point. As a result, A will probably try to
OPEN the connection again or try to SEND on the connection it believes open.
In the latter case, it receives the error message "connection not open" from
the local (A's) TCP. In an attempt to establish the connection, A's TCP will
send a segment containing SYN. This scenario leads to the example shown in
figure 10. After TCP A crashes, the user attempts to re-open the connection.
TCP B, in the meantime, thinks the connection is open.
TCP A TCP B
1. (CRASH) (send 300,receive 100)
2. CLOSED ESTABLISHED
3. SYN-SENT --> <SEQ=400><CTL=SYN> --> (??)
4. (!!) <-- <SEQ=300><ACK=100><CTL=ACK> <-- ESTABLISHED
5. SYN-SENT --> <SEQ=100><CTL=RST> --> (Abort!!)
6. SYN-SENT CLOSED
7. SYN-SENT --> <SEQ=400><CTL=SYN> -->
Half-Open Connection Discovery
Figure 10.
When the SYN arrives at line 3, TCP B, being in a synchronized state,
and the incoming segment outside the window, responds with an
acknowledgment indicating what sequence it next expects to hear (ACK
100). TCP A sees that this segment does not acknowledge anything it
sent and, being unsynchronized, sends a reset (RST) because it has
detected a half-open connection. TCP B aborts at line 5. TCP A will
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continue to try to establish the connection; the problem is now
reduced to the basic 3-way handshake of figure 7.
An interesting
alternative case occurs when TCP A crashes and TCP B tries to send data on
what it thinks is a synchronized connection. This is illustrated in figure 11.
In this case, the data arriving at TCP A from TCP B (line 2) is unacceptable
because no such connection exists, so TCP A sends a RST. The RST is acceptable
so TCP B processes it and aborts the connection.
TCP A TCP B
1. (CRASH) (send 300,receive 100)
2. (??) <-- <SEQ=300><ACK=100><DATA=10><CTL=ACK> <-- ESTABLISHED
3. --> <SEQ=100><CTL=RST> --> (ABORT!!)
Active Side Causes Half-Open Connection Discovery
Figure 11.
In figure 12, we find the two TCPs A and B with passive connections
waiting for SYN. An old duplicate arriving at TCP B (line 2) stirs B
into action. A SYN-ACK is returned (line 3) and causes TCP A to
generate a RST (the ACK in line 3 is not acceptable). TCP B accepts
the reset and returns to its passive LISTEN state.
TCP A TCP B
1. LISTEN LISTEN
2. ... <SEQ=Z><CTL=SYN> --> SYN-RECEIVED
3. (??) <-- <SEQ=X><ACK=Z+1><CTL=SYN,ACK> <-- SYN-RECEIVED
4. --> <SEQ=Z+1><CTL=RST> --> (return to LISTEN!)
5. LISTEN LISTEN
Old Duplicate SYN Initiates a Reset on two Passive Sockets
Figure 12.
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A variety of other cases are possible, all of which are
accounted for by the following rules for RST generation and processing.
Reset Generation
As a general rule, reset (RST) must be sent
whenever a segment arrives which apparently is not intended for the current
connection. A reset must not be sent if it is not clear that this is the case.
There are three groups of states:
- 1 If the connection does not exist (CLOSED) then
a reset is sent
- in response to any incoming segment except another reset. In particular,
SYNs addressed to a non-existent connection are rejected by this means.
If the incoming segment has an ACK field, the reset takes its sequence
number from the ACK field of the segment, otherwise the reset has sequence
number zero and the ACK field is set to the sum of the sequence number and
segment length of the incoming segment. The connection remains in the CLOSED
state.
- 2 If the connection is in any non-synchronized
state (LISTEN,
- SYN-SENT, SYN-RECEIVED), and the incoming segment acknowledges something
not yet sent (the segment carries an unacceptable ACK), or if an incoming
segment has a security level or compartment which does not exactly match the
level and compartment requested for the connection, a reset is sent.
If our SYN has not been acknowledged and the precedence level of the
incoming segment is higher than the precedence level requested then either
raise the local precedence level (if allowed by the user and the system) or
send a reset; or if the precedence level of the incoming segment is lower than
the precedence level requested then continue as if the precedence matched
exactly (if the remote TCP cannot raise the precedence level to match ours
this will be detected in the next segment it sends, and the connection will be
terminated then). If our SYN has been acknowledged (perhaps in this incoming
segment) the precedence level of the incoming segment must match the local
precedence level exactly, if it does not a reset must be sent.
If the
incoming segment has an ACK field, the reset takes its sequence number from
the ACK field of the segment, otherwise the reset has sequence number zero and
the ACK field is set to the sum of the sequence number and segment length of
the incoming segment. The connection remains in the same state.
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- 3 If the connection is in a synchronized state
(ESTABLISHED,
- FIN-WAIT-1, FIN-WAIT-2, CLOSE-WAIT, CLOSING, LAST-ACK, TIME-WAIT), any
unacceptable segment (out of window sequence number or unacceptible
acknowledgment number) must elicit only an empty acknowledgment segment
containing the current send-sequence number and an acknowledgment indicating
the next sequence number expected to be received, and the connection remains
in the same state.
If an incoming segment has a security level, or
compartment, or precedence which does not exactly match the level, and
compartment, and precedence requested for the connection,a reset is sent and
connection goes to the CLOSED state. The reset takes its sequence number from
the ACK field of the incoming segment.
Reset Processing
In all
states except SYN-SENT, all reset (RST) segments are validated by checking
their SEQ-fields. A reset is valid if its sequence number is in the window. In
the SYN-SENT state (a RST received in response to an initial SYN), the RST is
acceptable if the ACK field acknowledges the SYN.
The receiver of a
RST first validates it, then changes state. If the receiver was in the LISTEN
state, it ignores it. If the receiver was in SYN-RECEIVED state and had
previously been in the LISTEN state, then the receiver returns to the LISTEN
state, otherwise the receiver aborts the connection and goes to the CLOSED
state. If the receiver was in any other state, it aborts the connection and
advises the user and goes to the CLOSED state.
- 3.5 Closing a Connection
CLOSE is an operation meaning "I have no more data to send." The
notion of closing a full-duplex connection is subject to ambiguous
interpretation, of course, since it may not be obvious how to treat the
receiving side of the connection. We have chosen to treat CLOSE in a simplex
fashion. The user who CLOSEs may continue to RECEIVE until he is told that the
other side has CLOSED also. Thus, a program could initiate several SENDs
followed by a CLOSE, and then continue to RECEIVE until signaled that a
RECEIVE failed because the other side has CLOSED. We assume that the TCP will
signal a user, even if no RECEIVEs are outstanding, that the other side has
closed, so the user can terminate his side gracefully. A TCP will reliably
deliver all buffers SENT before the connection was CLOSED so a user who
expects no data in return need only wait to hear the connection was CLOSED
successfully to know that all his data was received at the destination TCP.
Users must keep reading connections they close for sending until the TCP says
no more data.
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There are essentially three cases:
1) The user
initiates by telling the TCP to CLOSE the connection
2) The remote TCP
initiates by sending a FIN control signal
3) Both users CLOSE
simultaneously
Case 1: Local user initiates the close
In this
case, a FIN segment can be constructed and placed on the outgoing segment
queue. No further SENDs from the user will be accepted by the TCP, and it
enters the FIN-WAIT-1 state. RECEIVEs are allowed in this state. All segments
preceding and including FIN will be retransmitted until acknowledged. When the
other TCP has both acknowledged the FIN and sent a FIN of its own, the first
TCP can ACK this FIN. Note that a TCP receiving a FIN will ACK but not send
its own FIN until its user has CLOSED the connection also.
Case 2: TCP
receives a FIN from the network
If an unsolicited FIN arrives from the
network, the receiving TCP can ACK it and tell the user that the connection is
closing. The user will respond with a CLOSE, upon which the TCP can send a FIN
to the other TCP after sending any remaining data. The TCP then waits until
its own FIN is acknowledged whereupon it deletes the connection. If an ACK is
not forthcoming, after the user timeout the connection is aborted and the user
is told.
Case 3: both users close simultaneously
A
simultaneous CLOSE by users at both ends of a connection causes FIN segments
to be exchanged. When all segments preceding the FINs have been processed and
acknowledged, each TCP can ACK the FIN it has received. Both will, upon
receiving these ACKs, delete the connection.
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TCP A TCP B
1. ESTABLISHED ESTABLISHED
2. (Close)
FIN-WAIT-1 --> <SEQ=100><ACK=300><CTL=FIN,ACK> --> CLOSE-WAIT
3. FIN-WAIT-2 <-- <SEQ=300><ACK=101><CTL=ACK> <-- CLOSE-WAIT
4. (Close)
TIME-WAIT <-- <SEQ=300><ACK=101><CTL=FIN,ACK> <-- LAST-ACK
5. TIME-WAIT --> <SEQ=101><ACK=301><CTL=ACK> --> CLOSED
6. (2 MSL)
CLOSED
Normal Close Sequence
Figure 13.
TCP A TCP B
1. ESTABLISHED ESTABLISHED
2. (Close) (Close)
FIN-WAIT-1 --> <SEQ=100><ACK=300><CTL=FIN,ACK> ... FIN-WAIT-1
<-- <SEQ=300><ACK=100><CTL=FIN,ACK> <--
... <SEQ=100><ACK=300><CTL=FIN,ACK> -->
3. CLOSING --> <SEQ=101><ACK=301><CTL=ACK> ... CLOSING
<-- <SEQ=301><ACK=101><CTL=ACK> <--
... <SEQ=101><ACK=301><CTL=ACK> -->
4. TIME-WAIT TIME-WAIT
(2 MSL) (2 MSL)
CLOSED CLOSED
Simultaneous Close Sequence
Figure 14.
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- 3.6 Precedence and Security
The intent is that connection be allowed only between ports operating
with exactly the same security and compartment values and at the higher of the
precedence level requested by the two ports.
The precedence and security
parameters used in TCP are exactly those defined in the Internet Protocol (IP)
[2]. Throughout this TCP specification the term "security/compartment" is
intended to indicate the security parameters used in IP including security,
compartment, user group, and handling restriction.
A connection
attempt with mismatched security/compartment values or a lower precedence
value must be rejected by sending a reset. Rejecting a connection due to too
low a precedence only occurs after an acknowledgment of the SYN has been
received.
Note that TCP modules which operate only at the default
value of precedence will still have to check the precedence of incoming
segments and possibly raise the precedence level they use on the connection.
The security paramaters may be used even in a non-secure environment
(the values would indicate unclassified data), thus hosts in non-secure
environments must be prepared to receive the security parameters, though they
need not send them.
- 3.7 Data Communication
Once the connection is established data is communicated by the
exchange of segments. Because segments may be lost due to errors (checksum
test failure), or network congestion, TCP uses
retransmission (after a
timeout) to ensure delivery of every segment. Duplicate segments may arrive
due to network or TCP retransmission. As discussed in the section on sequence
numbers the TCP performs certain tests on the sequence and acknowledgment
numbers in the segments to verify their acceptability.
The sender of
data keeps track of the next sequence number to use in the variable SND.NXT.
The receiver of data keeps track of the next sequence number to expect in the
variable RCV.NXT. The sender of data keeps track of the oldest unacknowledged
sequence number in the variable SND.UNA. If the data flow is momentarily idle
and all data sent has been acknowledged then the three variables will be
equal.
When the sender creates a segment and transmits it the sender
advances SND.NXT. When the receiver accepts a segment it advances RCV.NXT and
sends an acknowledgment. When the data sender receives an
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acknowledgment it advances SND.UNA. The extent to which the values
of these variables differ is a measure of the delay in the communication. The
amount by which the variables are advanced is the length of the data in the
segment. Note that once in the ESTABLISHED state all segments must carry
current acknowledgment information.
The CLOSE user call implies a push
function, as does the FIN control flag in an incoming segment.
Retransmission Timeout
Because of the variability of the
networks that compose an internetwork system and the wide range of uses of TCP
connections the retransmission timeout must be dynamically determined. One
procedure for determining a retransmission time out is given here as an
illustration.
An Example Retransmission Timeout Procedure
Measure the elapsed time between sending a data octet with a
particular sequence number and receiving an acknowledgment that covers that
sequence number (segments sent do not have to match segments received). This
measured elapsed time is the Round Trip Time (RTT). Next compute a Smoothed
Round Trip Time (SRTT) as: SRTT = ( ALPHA * SRTT ) + ((1-ALPHA) * RTT)
and based on this, compute the retransmission timeout (RTO) as:
RTO = min[UBOUND,max[LBOUND,(BETA*SRTT)]]
where UBOUND is an upper bound on the timeout (e.g., 1 minute),
LBOUND is a lower bound on the timeout (e.g., 1 second), ALPHA is
a smoothing factor (e.g., .8 to .9), and BETA is a delay variance
factor (e.g., 1.3 to 2.0).
The Communication of Urgent Information
The objective of the TCP urgent mechanism is to allow the sending user
to stimulate the receiving user to accept some urgent data and to
permit the receiving TCP to indicate to the receiving user when all
the currently known urgent data has been received by the user.
This mechanism permits a point in the data stream to be designated as
the end of urgent information. Whenever this point is in advance of
the receive sequence number (RCV.NXT) at the receiving TCP, that TCP
must tell the user to go into "urgent mode"; when the receive sequence
number catches up to the urgent pointer, the TCP must tell user to go
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into "normal mode". If the urgent pointer is updated while the
user is in "urgent mode", the update will be invisible to the user.
The
method employs a urgent field which is carried in all segments transmitted.
The URG control flag indicates that the urgent field is meaningful and must be
added to the segment sequence number to yield the urgent pointer. The absence
of this flag indicates that there is no urgent data outstanding.
To
send an urgent indication the user must also send at least one data octet. If
the sending user also indicates a push, timely delivery of the urgent
information to the destination process is enhanced.
Managing the Window
The window sent in each segment indicates the range of sequence
numbers the sender of the window (the data receiver) is currently prepared to
accept. There is an assumption that this is related to the currently available
data buffer space available for this connection.
Indicating a large
window encourages transmissions. If more data arrives than can be accepted, it
will be discarded. This will result in excessive retransmissions, adding
unnecessarily to the load on the network and the TCPs. Indicating a small
window may restrict the transmission of data to the point of introducing a
round trip delay between each new segment transmitted.
The mechanisms
provided allow a TCP to advertise a large window and to subsequently advertise
a much smaller window without having accepted that much data. This, so called
"shrinking the window," is strongly discouraged. The robustness principle
dictates that TCPs will not shrink the window themselves, but will be prepared
for such behavior on the part of other TCPs.
The sending TCP must be
prepared to accept from the user and send at least one octet of new data even
if the send window is zero. The sending TCP must regularly retransmit to the
receiving TCP even when the window is zero. Two minutes is recommended for the
retransmission interval when the window is zero. This retransmission is
essential to guarantee that when either TCP has a zero window the re-opening
of the window will be reliably reported to the other.
When the
receiving TCP has a zero window and a segment arrives it must still send an
acknowledgment showing its next expected sequence number and current window
(zero).
The sending TCP packages the data to be transmitted into
segments
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which
fit the current window, and may repackage segments on the retransmission
queue. Such repackaging is not required, but may be helpful.
In a
connection with a one-way data flow, the window information will be carried in
acknowledgment segments that all have the same sequence number so there will
be no way to reorder them if they arrive out of order. This is not a serious
problem, but it will allow the window information to be on occasion
temporarily based on old reports from the data receiver. A refinement to avoid
this problem is to act on the window information from segments that carry the
highest acknowledgment number (that is segments with acknowledgment number
equal or greater than the highest previously received).
The window
management procedure has significant influence on the communication
performance. The following comments are suggestions to implementers.
Window Management Suggestions
Allocating a very small window
causes data to be transmitted in many small segments when better performance
is achieved using fewer large segments.
One suggestion for avoiding
small windows is for the receiver to defer updating a window until the
additional allocation is at least X percent of the maximum allocation possible
for the connection (where X might be 20 to 40).
Another suggestion is
for the sender to avoid sending small segments by waiting until the window is
large enough before sending data. If the the user signals a push function then
the data must be sent even if it is a small segment.
Note that the
acknowledgments should not be delayed or unnecessary retransmissions will
result. One strategy would be to send an acknowledgment when a small segment
arrives (with out updating the window information), and then to send another
acknowledgment with new window information when the window is larger.
The segment sent to probe a zero window may also begin a break up of
transmitted data into smaller and smaller segments. If a segment containing a
single data octet sent to probe a zero window is accepted, it consumes one
octet of the window now available. If the sending TCP simply sends as much as
it can whenever the window is non zero, the transmitted data will be broken
into alternating big and small segments. As time goes on, occasional pauses in
the receiver making window allocation available will
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result in breaking the big segments into a small and not quite
so big pair. And after a while the data transmission will be in mostly small
segments.
The suggestion here is that the TCP implementations need to
actively attempt to combine small window allocations into larger windows,
since the mechanisms for managing the window tend to lead to many small
windows in the simplest minded implementations.
- 3.8 Interfaces
There are of course two interfaces of concern: the user/TCP interface
and the TCP/lower-level interface. We have a fairly elaborate model of the
user/TCP interface, but the interface to the lower level protocol module is
left unspecified here, since it will be specified in detail by the
specification of the lowel level protocol. For the case that the lower level
is IP we note some of the parameter values that TCPs might use.
User/TCP Interface
The following functional description of
user commands to the TCP is, at best, fictional, since every operating system
will have different facilities. Consequently, we must warn readers that
different TCP implementations may have different user interfaces. However, all
TCPs must provide a certain minimum set of services to guarantee that all TCP
implementations can support the same protocol hierarchy. This section
specifies the functional interfaces required of all TCP implementations.
TCP User Commands
The following sections functionally
characterize a USER/TCP interface. The notation used is similar to most
procedure or function calls in high level languages, but this usage is not
meant to rule out trap type service calls (e.g., SVCs, UUOs, EMTs).
The user commands described below specify the basic functions the TCP
must perform to support interprocess communication. Individual implementations
must define their own exact format, and may provide combinations or subsets of
the basic functions in single calls. In particular, some implementations may
wish to automatically OPEN a connection on the first SEND or RECEIVE issued by
the user for a given connection.
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In
providing interprocess communication facilities, the TCP must not only accept
commands, but must also return information to the processes it serves. The
latter consists of:
(a) general information about a connection (e.g.,
interrupts, remote close, binding of unspecified foreign socket).
(b)
replies to specific user commands indicating success or various types of
failure.
Open
Format: OPEN (local port, foreign socket,
active/passive [, timeout] [, precedence] [, security/compartment] [,
options]) -> local connection name
We assume that the local TCP is aware of the identity of the
processes it serves and will check the authority of the process
to use the connection specified. Depending upon the
implementation of the TCP, the local network and TCP identifiers
for the source address will either be supplied by the TCP or the
lower level protocol (e.g., IP). These considerations are the
result of concern about security, to the extent that no TCP be
able to masquerade as another one, and so on. Similarly, no
process can masquerade as another without the collusion of the
TCP.
If the active/passive flag is set to passive, then this is a
call to LISTEN for an incoming connection. A passive open may
have either a fully specified foreign socket to wait for a
particular connection or an unspecified foreign socket to wait
for any call. A fully specified passive call can be made active
by the subsequent execution of a SEND.
A transmission control block (TCB) is created and partially
filled in with data from the OPEN command parameters.
On an active OPEN command, the TCP will begin the procedure to
synchronize (i.e., establish) the connection at once.
The timeout, if present, permits the caller to set up a timeout
for all data submitted to TCP. If data is not successfully
delivered to the destination within the timeout period, the TCP
will abort the connection. The present global default is five
minutes.
The TCP or some component of the operating system will verify
the users authority to open a connection with the specified
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precedence or security/compartment. The absence of precedence
or security/compartment specification in the OPEN call indicates the default
values must be used.
TCP will accept incoming requests as matching
only if the security/compartment information is exactly the same and only if
the precedence is equal to or higher than the precedence requested in the OPEN
call.
The precedence for the connection is the higher of the values
requested in the OPEN call and received from the incoming request, and fixed
at that value for the life of the connection.Implementers may want to give the
user control of this precedence negotiation. For example, the user might be
allowed to specify that the precedence must be exactly matched, or that any
attempt to raise the precedence be confirmed by the user.
A local
connection name will be returned to the user by the TCP. The local connection
name can then be used as a short hand term for the connection defined by the
<local socket, foreign socket> pair.
Send
Format: SEND
(local connection name, buffer address, byte count, PUSH flag, URGENT flag
[,timeout])
This call causes the data contained in the indicated user
buffer to be sent on the indicated connection. If the connection has not been
opened, the SEND is considered an error. Some implementations may allow users
to SEND first; in which case, an automatic OPEN would be done. If the calling
process is not authorized to use this connection, an error is returned.
If
the PUSH flag is set, the data must be transmitted promptly to the receiver,
and the PUSH bit will be set in the last TCP segment created from the buffer.
If the PUSH flag is not set, the data may be combined with data from
subsequent SENDs for transmission efficiency.
If the URGENT flag is
set, segments sent to the destination TCP will have the urgent pointer set.
The receiving TCP will signal the urgent condition to the receiving process if
the urgent pointer indicates that data preceding the urgent pointer has not
been consumed by the receiving process. The purpose of urgent is to stimulate
the receiver to process the urgent data and to indicate to the receiver when
all the currently known urgent
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data
has been received. The number of times the sending user's TCP signals urgent
will not necessarily be equal to the number of times the receiving user will
be notified of the presence of urgent data.
If no foreign socket was
specified in the OPEN, but the connection is established (e.g., because a
LISTENing connection has become specific due to a foreign segment arriving for
the local socket), then the designated buffer is sent to the implied foreign
socket. Users who make use of OPEN with an unspecified foreign socket can make
use of SEND without ever explicitly knowing the foreign socket address.
However, if a SEND is attempted before the foreign socket becomes
specified, an error will be returned. Users can use the STATUS call to
determine the status of the connection. In some implementations the TCP may
notify the user when an unspecified socket is bound.
If a timeout is
specified, the current user timeout for this connection is changed to the new
one.
In the simplest implementation, SEND would not return control to
the sending process until either the transmission was complete or the timeout
had been exceeded. However, this simple method is both subject to deadlocks
(for example, both sides of the connection might try to do SENDs before doing
any RECEIVEs) and offers poor performance, so it is not recommended. A more
sophisticated implementation would return immediately to allow the process to
run concurrently with network I/O, and, furthermore, to allow multiple SENDs
to be in progress. Multiple SENDs are served in first come, first served
order, so the TCP will queue those it cannot service immediately.
We have
implicitly assumed an asynchronous user interface in which a SEND later
elicits some kind of SIGNAL or
pseudo-interrupt from the serving TCP. An
alternative is to return a response immediately. For instance, SENDs might
return immediate local acknowledgment, even if the segment sent had not been
acknowledged by the distant TCP. We could optimistically assume eventual
success. If we are wrong, the connection will close anyway due to the timeout.
In implementations of this kind (synchronous), there will still be some
asynchronous signals, but these will deal with the connection itself, and not
with specific segments or buffers.
In order for the process to
distinguish among error or success indications for different SENDs, it might
be appropriate for the
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buffer address to be returned along with the coded response to
the SEND request. TCP-to-user signals are discussed below, indicating the
information which should be returned to the calling process.
Receive
Format: RECEIVE (local connection name, buffer address, byte count)
-> byte count, urgent flag, push flag
This command allocates a
receiving buffer associated with the specified connection. If no OPEN precedes
this command or the calling process is not authorized to use this connection,
an error is returned.
In the simplest implementation, control would
not return to the calling program until either the buffer was filled, or some
error occurred, but this scheme is highly subject to deadlocks. A more
sophisticated implementation would permit several RECEIVEs to be outstanding
at once. These would be filled as segments arrive. This strategy permits
increased throughput at the cost of a more elaborate scheme (possibly
asynchronous) to notify the calling program that a PUSH has been seen or a
buffer filled.
If enough data arrive to fill the buffer before a PUSH
is seen, the PUSH flag will not be set in the response to the RECEIVE. The
buffer will be filled with as much data as it can hold. If a PUSH is seen
before the buffer is filled the buffer will be returned partially filled and
PUSH indicated.
If there is urgent data the user will have been
informed as soon as it arrived via a TCP-to-user signal. The receiving user
should thus be in "urgent mode". If the URGENT flag is on, additional urgent
data remains. If the URGENT flag is off, this call to RECEIVE has returned all
the urgent data, and the user may now leave "urgent mode". Note that data
following the urgent pointer (non-urgent data) cannot be delivered to the user
in the same buffer with preceeding urgent data unless the boundary is clearly
marked for the user.
To distinguish among several outstanding RECEIVEs
and to take care of the case that a buffer is not completely filled, the
return code is accompanied by both a buffer pointer and a byte count
indicating the actual length of the data received.
Alternative
implementations of RECEIVE might have the TCP
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allocate buffer storage, or the TCP might share a ring buffer with
the user.
Close
Format: CLOSE (local connection name)
This command causes the connection specified to be closed. If the
connection is not open or the calling process is not authorized to use this
connection, an error is returned. Closing connections is intended to be a
graceful operation in the sense that outstanding SENDs will be transmitted
(and retransmitted), as flow control permits, until all have been serviced.
Thus, it should be acceptable to make several SEND calls, followed by a CLOSE,
and expect all the data to be sent to the destination. It should also be clear
that users should continue to RECEIVE on CLOSING connections, since the other
side may be trying to transmit the last of its data. Thus, CLOSE means "I have
no more to send" but does not mean "I will not receive any more." It may
happen (if the user level protocol is not well thought out) that the closing
side is unable to get rid of all its data before timing out. In this event,
CLOSE turns into ABORT, and the closing TCP gives up.
The user may
CLOSE the connection at any time on his own initiative, or in response to
various prompts from the TCP (e.g., remote close executed, transmission
timeout exceeded, destination inaccessible).
Because closing a
connection requires communication with the foreign TCP, connections may remain
in the closing state for a short time. Attempts to reopen the connection
before the TCP replies to the CLOSE command will result in error responses.
Close also implies push function.
Status
Format: STATUS
(local connection name) -> status data
This is an implementation
dependent user command and could be excluded without adverse effect.
Information returned would typically come from the TCB associated with the
connection.
This command returns a data block containing the following
information:
local socket,
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foreign socket,
local connection name,
receive window,
send window,
connection state,
number of buffers awaiting
acknowledgment,
number of buffers pending receipt,
urgent state,
precedence,
security/compartment,
and transmission timeout.
Depending on the state of the connection, or on the
implementation
itself, some of this information may not be available or meaningful. If the
calling process is not authorized to use this connection, an error is
returned. This prevents unauthorized processes from gaining information about
a connection.
Abort
Format: ABORT (local connection name)
This command causes all pending SENDs and RECEIVES to be aborted, the
TCB to be removed, and a special RESET message to be sent to the TCP on the
other side of the connection. Depending on the implementation, users may
receive abort indications for each outstanding SEND or RECEIVE, or may simply
receive an ABORT-acknowledgment.
TCP-to-User Messages
It is
assumed that the operating system environment provides a means for the TCP to
asynchronously signal the user program. When the TCP does signal a user
program, certain information is passed to the user. Often in the specification
the information will be an error message. In other cases there will be
information relating to the completion of processing a SEND or RECEIVE or
other user call.
The following information is provided:
Local Connection Name Always
Response String Always
Buffer Address Send & Receive
Byte count (counts bytes received) Receive
Push flag Receive
Urgent flag Receive
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TCP/Lower-Level Interface
The TCP calls on a lower level
protocol module to actually send and receive information over a network. One
case is that of the ARPA internetwork system where the lower level module is
the Internet Protocol (IP) [2].
If the lower level protocol is IP it
provides arguments for a type of service and for a time to live. TCP uses the
following settings for these parameters:
Type of Service = Precedence: routine, Delay: normal, Throughput:
normal, Reliability: normal; or 00000000.
Time to Live = one minute, or 00111100.
Note that the assumed maximum segment lifetime is two minutes.
Here we explicitly ask that a segment be destroyed if it cannot
be delivered by the internet system within one minute.
If the lower level is IP (or other protocol that provides this
feature) and source routing is used, the interface must allow the
route information to be communicated. This is especially important
so that the source and destination addresses used in the TCP
checksum be the originating source and ultimate destination. It is
also important to preserve the return route to answer connection
requests.
Any lower level protocol will have to provide the source address,
destination address, and protocol fields, and some way to determine
the "TCP length", both to provide the functional equivlent service
of IP and to be used in the TCP checksum.
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- 3.9 Event Processing
The processing depicted in this section is an example of one possible
implementation. Other implementations may have slightly different processing
sequences, but they should differ from those in this section only in detail,
not in substance.
The activity of the TCP can be characterized as
responding to events. The events that occur can be cast into three categories:
user calls, arriving segments, and timeouts. This section describes the
processing the TCP does in response to each of the events. In many cases the
processing required depends on the state of the connection.
Events that
occur:
User Calls
OPEN
SEND
RECEIVE
CLOSE
ABORT
STATUS
Arriving Segments
SEGMENT ARRIVES
Timeouts
USER TIMEOUT
RETRANSMISSION TIMEOUT
TIME-WAIT
TIMEOUT
The model of the TCP/user interface is that user commands
receive an immediate return and possibly a delayed response via an event or
pseudo interrupt. In the following descriptions, the term "signal" means cause
a delayed response.
Error responses are given as character strings.
For example, user commands referencing connections that do not exist receive
"error: connection not open".
Please note in the following that all
arithmetic on sequence numbers, acknowledgment numbers, windows, et cetera, is
modulo 2**32 the size of the sequence number space. Also note that "=<"
means less than or equal to (modulo 2**32).
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A
natural way to think about processing incoming segments is to imagine that
they are first tested for proper sequence number (i.e., that their contents
lie in the range of the expected "receive window" in the sequence number
space) and then that they are generally queued and processed in sequence
number order.
When a segment overlaps other already received segments
we reconstruct the segment to contain just the new data, and adjust the header
fields to be consistent.
Note that if no state change is mentioned the
TCP stays in the same state.
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- OPEN Call
OPEN Call
CLOSED STATE (i.e., TCB does not
exist)
Create a new transmission control block (TCB) to hold
connection state information. Fill in local socket identifier, foreign socket,
precedence, security/compartment, and user timeout information. Note that some
parts of the foreign socket may be unspecified in a passive OPEN and are to be
filled in by the parameters of the incoming SYN segment. Verify the security
and precedence requested are allowed for this user, if not return "error:
precedence not allowed" or "error: security/compartment not allowed." If
passive enter the LISTEN state and return. If active and the foreign socket is
unspecified, return "error: foreign socket unspecified"; if active and the
foreign socket is specified, issue a SYN segment. An initial send sequence
number (ISS) is selected. A SYN segment of the form
<SEQ=ISS><CTL=SYN> is sent. Set SND.UNA to ISS, SND.NXT to ISS+1,
enter SYN-SENT state, and return.
If the caller does not have access
to the local socket specified, return "error: connection illegal for this
process". If there is no room to create a new connection, return "error:
insufficient resources".
LISTEN STATE
If active and the
foreign socket is specified, then change the connection from passive to
active, select an ISS. Send a SYN segment, set SND.UNA to ISS, SND.NXT to
ISS+1. Enter SYN-SENT state. Data associated with SEND may be sent with SYN
segment or queued for transmission after entering ESTABLISHED state. The
urgent bit if requested in the command must be sent with the data segments
sent as a result of this command. If there is no room to queue the request,
respond with "error: insufficient resources". If Foreign socket was not
specified, then return "error: foreign socket unspecified".
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- OPEN Call
SYN-SENT STATE
SYN-RECEIVED STATE
ESTABLISHED STATE
FIN-WAIT-1 STATE
FIN-WAIT-2 STATE
CLOSE-WAIT STATE
CLOSING
STATE
LAST-ACK STATE
TIME-WAIT STATE
Return "error: connection
already exists".
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- SEND Call
SEND Call
CLOSED STATE (i.e., TCB does not
exist)
If the user does not have access to such a connection, then
return "error: connection illegal for this process".
Otherwise, return
"error: connection does not exist".
LISTEN STATE
If the foreign
socket is specified, then change the connection from passive to active, select
an ISS. Send a SYN segment, set SND.UNA to ISS, SND.NXT to ISS+1. Enter
SYN-SENT state. Data associated with SEND may be sent with SYN segment or
queued for transmission after entering ESTABLISHED state. The urgent bit if
requested in the command must be sent with the data segments sent as a result
of this command. If there is no room to queue the request, respond with
"error: insufficient resources". If Foreign socket was not specified, then
return "error: foreign socket unspecified".
SYN-SENT STATE
SYN-RECEIVED STATE
Queue the data for transmission after entering
ESTABLISHED state. If no space to queue, respond with "error: insufficient
resources".
ESTABLISHED STATE
CLOSE-WAIT STATE
Segmentize
the buffer and send it with a piggybacked acknowledgment (acknowledgment value = RCV.NXT). If there is
insufficient space to remember this buffer, simply return "error:
insufficient resources".
If the urgent flag is set, then SND.UP <- SND.NXT-1 and set the
urgent pointer in the outgoing segments.
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- SEND Call
FIN-WAIT-1 STATE
FIN-WAIT-2 STATE
CLOSING STATE
LAST-ACK STATE
TIME-WAIT STATE
Return "error: connection
closing" and do not service request.
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- RECEIVE Call
RECEIVE Call
CLOSED STATE (i.e., TCB does
not exist)
If the user does not have access to such a connection,
return "error: connection illegal for this process".
Otherwise return
"error: connection does not exist".
LISTEN STATE
SYN-SENT STATE
SYN-RECEIVED STATE
Queue for processing after entering ESTABLISHED
state. If there is no room to queue this request, respond with "error:
insufficient resources".
ESTABLISHED STATE
FIN-WAIT-1 STATE
FIN-WAIT-2 STATE
If insufficient incoming segments are queued to
satisfy the request, queue the request. If there is no queue space to remember
the RECEIVE, respond with "error: insufficient resources".
Reassemble
queued incoming segments into receive buffer and return to user. Mark "push
seen" (PUSH) if this is the case.
If RCV.UP is in advance of the data
currently being passed to the user notify the user of the presence of urgent
data.
When the TCP takes responsibility for delivering data to the
user that fact must be communicated to the sender via an
acknowledgment.
The formation of such an acknowledgment is described below in the discussion
of processing an incoming segment.
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- RECEIVE Call
CLOSE-WAIT STATE
Since the remote side has already
sent FIN, RECEIVEs must be satisfied by text already on hand, but not yet
delivered to the user. If no text is awaiting delivery, the RECEIVE will get a
"error: connection closing" response. Otherwise, any remaining text can be
used to satisfy the RECEIVE.
CLOSING STATE
LAST-ACK STATE
TIME-WAIT STATE
Return "error: connection closing".
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- CLOSE Call
CLOSE Call
CLOSED STATE (i.e., TCB does not
exist)
If the user does not have access to such a connection, return
"error: connection illegal for this process".
Otherwise, return
"error: connection does not exist".
LISTEN STATE
Any outstanding
RECEIVEs are returned with "error: closing" responses. Delete TCB, enter
CLOSED state, and return.
SYN-SENT STATE
Delete the TCB and return
"error: closing" responses to any queued SENDs, or RECEIVEs.
SYN-RECEIVED STATE
If no SENDs have been issued and there is
no pending data to send, then form a FIN segment and send it, and enter
FIN-WAIT-1 state; otherwise queue for processing after entering ESTABLISHED
state.
ESTABLISHED STATE
Queue this until all preceding SENDs have
been segmentized, then form a FIN segment and send it. In any case, enter
FIN-WAIT-1 state.
FIN-WAIT-1 STATE
FIN-WAIT-2 STATE
Strictly speaking, this is an error and should receive a "error:
connection closing" response. An "ok" response would be acceptable, too, as
long as a second FIN is not emitted (the first FIN may be retransmitted
though).
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- CLOSE Call
CLOSE-WAIT STATE
Queue this request until all
preceding SENDs have been segmentized; then send a FIN segment, enter CLOSING
state.
CLOSING STATE
LAST-ACK STATE
TIME-WAIT STATE
Respond with "error: connection closing".
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- ABORT Call
ABORT Call
CLOSED STATE (i.e., TCB does not
exist)
If the user should not have access to such a connection, return
"error: connection illegal for this process".
Otherwise return "error:
connection does not exist".
LISTEN STATE
Any outstanding
RECEIVEs should be returned with "error: connection reset" responses. Delete
TCB, enter CLOSED state, and return.
SYN-SENT STATE
All queued
SENDs and RECEIVEs should be given "connection reset" notification, delete the
TCB, enter CLOSED state, and return.
SYN-RECEIVED STATE
ESTABLISHED
STATE
FIN-WAIT-1 STATE
FIN-WAIT-2 STATE
CLOSE-WAIT STATE
Send a reset segment:
<SEQ=SND.NXT><CTL=RST>
All queued SENDs and RECEIVEs should be given "connection reset"
notification; all segments queued for transmission (except for the
RST formed above) or retransmission should be flushed, delete the
TCB, enter CLOSED state, and return.
CLOSING STATE
LAST-ACK STATE
TIME-WAIT STATE
Respond with "ok" and delete the TCB, enter CLOSED state, and
return.
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- STATUS Call
STATUS Call
CLOSED STATE (i.e., TCB does not exist)
If the user should not have access to such a connection, return
"error: connection illegal for this process".
Otherwise return "error:
connection does not exist".
LISTEN STATE
Return "state = LISTEN", and the TCB pointer.
SYN-SENT STATE
Return "state = SYN-SENT", and the TCB pointer.
SYN-RECEIVED STATE
Return "state = SYN-RECEIVED", and the TCB pointer.
ESTABLISHED STATE
Return "state = ESTABLISHED", and the TCB pointer.
FIN-WAIT-1 STATE
Return "state = FIN-WAIT-1", and the TCB pointer.
FIN-WAIT-2 STATE
Return "state = FIN-WAIT-2", and the TCB pointer.
CLOSE-WAIT STATE
Return "state = CLOSE-WAIT", and the TCB pointer.
CLOSING STATE
Return "state = CLOSING", and the TCB pointer.
LAST-ACK STATE
Return "state = LAST-ACK", and the TCB pointer.
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- STATUS Call
TIME-WAIT STATE
Return "state = TIME-WAIT", and the TCB pointer.
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- SEGMENT ARRIVES
SEGMENT ARRIVES
If the state is CLOSED (i.e., TCB does
not exist) then
all data in the incoming segment is discarded. An
incoming segment containing a RST is discarded. An incoming segment not
containing a RST causes a RST to be sent in response. The acknowledgment and
sequence field values are selected to make the reset sequence acceptable to
the TCP that sent the offending segment.
If the ACK bit is off,
sequence number zero is used,
<SEQ=0><ACK=SEG.SEQ+SEG.LEN><CTL=RST,ACK>
If the ACK bit is on,
<SEQ=SEG.ACK><CTL=RST>
Return.
If the state is LISTEN then
first check for an RST
An incoming RST should be ignored. Return.
second check for an ACK
Any acknowledgment is bad if it arrives on a connection still in
the LISTEN state. An acceptable reset segment should be formed
for any arriving ACK-bearing segment. The RST should be
formatted as follows:
<SEQ=SEG.ACK><CTL=RST>
Return.
third check for a SYN
If the SYN bit is set, check the security. If the
security/compartment on the incoming segment does not exactly
match the security/compartment in the TCB then send a reset and
return.
<SEQ=SEG.ACK><CTL=RST>
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- SEGMENT ARRIVES
If the SEG.PRC is greater than the TCB.PRC
then if allowed by the user and the system set TCB.PRC<-SEG.PRC, if not
allowed send a reset and return.
<SEQ=SEG.ACK><CTL=RST>
If the SEG.PRC is less than the TCB.PRC then continue.
Set RCV.NXT to SEG.SEQ+1, IRS is set to SEG.SEQ and any other
control or text should be queued for processing later. ISS
should be selected and a SYN segment sent of the form:
<SEQ=ISS><ACK=RCV.NXT><CTL=SYN,ACK>
SND.NXT is set to ISS+1 and SND.UNA to ISS. The connection
state should be changed to SYN-RECEIVED. Note that any other
incoming control or data (combined with SYN) will be processed
in the SYN-RECEIVED state, but processing of SYN and ACK should
not be repeated. If the listen was not fully specified (i.e.,
the foreign socket was not fully specified), then the
unspecified fields should be filled in now.
fourth other text or control
Any other control or text-bearing segment (not containing SYN)
must have an ACK and thus would be discarded by the ACK
processing. An incoming RST segment could not be valid, since
it could not have been sent in response to anything sent by this
incarnation of the connection. So you are unlikely to get here,
but if you do, drop the segment, and return.
If the state is SYN-SENT then
first check the ACK bit
If the ACK bit is set
If SEG.ACK =< ISS, or SEG.ACK > SND.NXT, send a reset (unless
the RST bit is set, if so drop the segment and return)
<SEQ=SEG.ACK><CTL=RST>
and discard the segment. Return.
If SND.UNA =< SEG.ACK =< SND.NXT then the ACK is acceptable.
second check the RST bit
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- 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
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- SEGMENT ARRIVES
are acceptable then, RCV.NXT is set to
SEG.SEQ+1, IRS is set to SEG.SEQ. SND.UNA should be advanced to equal SEG.ACK
(if there is an ACK), and any segments on the retransmission queue which are
thereby acknowledged should be removed.
If SND.UNA > ISS (our SYN
has been ACKed), change the connection state to ESTABLISHED, form an ACK
segment
<SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK>
and send it. Data or controls which were queued for
transmission may be included. If there are other controls or
text in the segment then continue processing at the sixth step
below where the URG bit is checked, otherwise return.
Otherwise enter SYN-RECEIVED, form a SYN,ACK segment
<SEQ=ISS><ACK=RCV.NXT><CTL=SYN,ACK>
and send it. If there are other controls or text in the
segment, queue them for processing after the ESTABLISHED state
has been reached, return.
fifth, if neither of the SYN or RST bits is set then drop the
segment and return.
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- 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.
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- 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.
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- 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).
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- 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.
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- 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.
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- Transmission Control Protocol
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- 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.
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- 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.
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- Transmission Control Protocol
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- Functional Specification
- SEGMENT ARRIVES
TIME-WAIT STATE
Remain in the
TIME-WAIT state. Restart the 2 MSL time-wait timeout.
and return.
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- 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.
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- Transmission Control Protocol
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- 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]
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- Transmission Control Protocol
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- 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.
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- 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
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- Transmission Control Protocol
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- 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
-
- 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
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- Transmission Control Protocol
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- 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.
-
- September 1981
- Transmission Control Protocol
REFERENCES
[1]
Cerf, V., and R. Kahn, "A Protocol for Packet Network
Intercommunication",
IEEE Transactions on Communications, Vol. COM-22, No. 5, pp 637-648, May 1974.
[2] Postel, J. (ed.), "Internet Protocol - DARPA Internet Program
Protocol Specification", RFC 791,
USC/Information Sciences Institute, September 1981.
[3] Dalal, Y. and
C. Sunshine, "Connection Management in Transport Protocols", Computer
Networks, Vol. 2, No. 6, pp. 454-473, December 1978.
[4] Postel, J.,
"Assigned Numbers", RFC 790,
USC/Information Sciences Institute, September 1981.
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