| RFC: 793
TRANSMISSION CONTROL PROTOCOL
DARPA INTERNET PROGRAM
PROTOCOL SPECIFICATION
September 1981
prepared for
Defense Advanced Research
Projects Agency
Information Processing
Techniques Office
1400 Wilson Boulevard
Arlington, Virginia 22209
by
Information Sciences
Institute
University of Southern
California
4676 Admiralty Way
Marina del Rey, California
90291
September 1981
Transmission Control Protocol
TABLE OF CONTENTS
PREFACE
........................................................ iii
1. INTRODUCTION
..................................................... 1
1.1 Motivation
.................................................... 1
1.2 Scope
......................................................... 2
1.3 About This Document
........................................... 2
1.4 Interfaces
.................................................... 3
1.5 Operation
..................................................... 3
2. PHILOSOPHY
....................................................... 7
2.1 Elements of the
Internetwork System ........................... 7
2.2 Model of Operation
............................................ 7
2.3 The Host Environment
.......................................... 8
2.4 Interfaces
.................................................... 9
2.5 Relation to Other
Protocols ................................... 9
2.6 Reliable Communication
........................................ 9
2.7 Connection Establishment
and Clearing ........................ 10
2.8 Data Communication
........................................... 12
2.9 Precedence and Security
...................................... 13
2.10 Robustness Principle
......................................... 13
3. FUNCTIONAL SPECIFICATION
........................................ 15
3.1 Header Format
................................................ 15
3.2 Terminology
.................................................. 19
3.3 Sequence Numbers
............................................. 24
3.4 Establishing a connection
.................................... 30
3.5 Closing a Connection
......................................... 37
3.6 Precedence and Security
...................................... 40
3.7 Data Communication
........................................... 40
3.8 Interfaces
................................................... 44
3.9 Event Processing
............................................. 52
GLOSSARY
............................................................
79
REFERENCES
.......................................................... 85
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Transmission Control Protocol
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Transmission Control Protocol
PREFACE
This document describes the
DoD Standard Transmission Control Protocol
(TCP). There have been nine
earlier editions of the ARPA TCP
specification on which this
standard is based, and the present text
draws heavily from them.
There have been many contributors to this work
both in terms of concepts and
in terms of text. This edition clarifies
several details and removes
the end-of-letter buffer-size adjustments,
and redescribes the letter
mechanism as a push function.
Jon Postel
Editor
RFC: 793
Replaces: RFC 761
IENs: 129, 124, 112, 81,
55, 44, 40, 27, 21, 5
TRANSMISSION CONTROL PROTOCOL
DARPA INTERNET PROGRAM
PROTOCOL SPECIFICATION
1. INTRODUCTION
The Transmission Control
Protocol (TCP) is intended for use as a highly
reliable host-to-host
protocol between hosts in packet-switched computer
communication networks, and
in interconnected systems of such networks.
This document describes the
functions to be performed by the
Transmission Control Protocol,
the program that implements it, and its
interface to programs or
users that require its services.
1.1. Motivation
Computer communication
systems are playing an increasingly important
role in military, government,
and civilian environments. This
document focuses its
attention primarily on military computer
communication requirements,
especially robustness in the presence of
communication unreliability
and availability in the presence of
congestion, but many of these
problems are found in the civilian and
government sector as well.
As strategic and tactical
computer communication networks are
developed and deployed, it is
essential to provide means of
interconnecting them and to
provide standard interprocess
communication protocols which
can support a broad range of
applications. In anticipation
of the need for such standards, the
Deputy Undersecretary of
Defense for Research and Engineering has
declared the Transmission
Control Protocol (TCP) described herein to
be a basis for DoD-wide
inter-process communication protocol
standardization.
TCP is a connection-oriented,
end-to-end reliable protocol designed to
fit into a layered hierarchy
of protocols which support multi-network
applications. The TCP
provides for reliable inter-process
communication between pairs
of processes in host computers attached to
distinct but interconnected
computer communication networks. Very few
assumptions are made as to
the reliability of the communication
protocols below the TCP layer.
TCP assumes it can obtain a simple,
potentially unreliable
datagram service from the lower level
protocols. In principle, the
TCP should be able to operate above a
wide spectrum of
communication systems ranging from hard-wired
connections to
packet-switched or circuit-switched networks.
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Transmission Control Protocol
Introduction
TCP is based on concepts
first described by Cerf and Kahn in [1]. The
TCP fits into a layered
protocol architecture just above a basic
Internet Protocol [2] which
provides a way for the TCP to send and
receive variable-length
segments of information enclosed in internet
datagram "envelopes".
The internet datagram provides a means for
addressing source and
destination TCPs in different networks. The
internet protocol also deals
with any fragmentation or reassembly of
the TCP segments required to
achieve transport and delivery through
multiple networks and
interconnecting gateways. The internet protocol
also carries information on
the precedence, security classification
and compartmentation of the
TCP segments, so this information can be
communicated end-to-end
across multiple networks.
Protocol Layering
+---------------------+
| higher-level |
+---------------------+
| TCP |
+---------------------+
| internet protocol |
+---------------------+
|communication network|
+---------------------+
Figure 1
Much of this document is
written in the context of TCP implementations
which are co-resident with
higher level protocols in the host
computer. Some computer
systems will be connected to networks via
front-end computers which
house the TCP and internet protocol layers,
as well as network specific
software. The TCP specification describes
an interface to the higher
level protocols which appears to be
implementable even for the
front-end case, as long as a suitable
host-to-front end protocol is
implemented.
1.2. Scope
The TCP is intended to
provide a reliable process-to-process
communication service in a
multinetwork environment. The TCP is
intended to be a host-to-host
protocol in common use in multiple
networks.
1.3. About this Document
This document represents a
specification of the behavior required of
any TCP implementation, both
in its interactions with higher level
protocols and in its
interactions with other TCPs. The rest of this
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Transmission Control Protocol
Introduction
section offers a very brief
view of the protocol interfaces and
operation. Section 2
summarizes the philosophical basis for the TCP
design. Section 3 offers both
a detailed description of the actions
required of TCP when various
events occur (arrival of new segments,
user calls, errors, etc.) and
the details of the formats of TCP
segments.
1.4. Interfaces
The TCP interfaces on one
side to user or application processes and on
the other side to a lower
level protocol such as Internet Protocol.
The interface between an
application process and the TCP is
illustrated in reasonable
detail. This interface consists of a set of
calls much like the calls an
operating system provides to an
application process for
manipulating files. For example, there are
calls to open and close
connections and to send and receive data on
established connections. It
is also expected that the TCP can
asynchronously communicate
with application programs. Although
considerable freedom is
permitted to TCP implementors to design
interfaces which are
appropriate to a particular operating system
environment, a minimum
functionality is required at the TCP/user
interface for any valid
implementation.
The interface between TCP and
lower level protocol is essentially
unspecified except that it is
assumed there is a mechanism whereby the
two levels can asynchronously
pass information to each other.
Typically, one expects the
lower level protocol to specify this
interface. TCP is designed to
work in a very general environment of
interconnected networks. The
lower level protocol which is assumed
throughout this document is
the Internet Protocol [2].
1.5. Operation
As noted above, the primary
purpose of the TCP is to provide reliable,
securable logical circuit or
connection service between pairs of
processes. To provide this
service on top of a less reliable internet
communication system requires
facilities in the following areas:
Basic Data Transfer
Reliability
Flow Control
Multiplexing
Connections
Precedence and Security
The basic operation of the
TCP in each of these areas is described in
the following paragraphs.
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Transmission Control Protocol
Introduction
Basic Data Transfer:
The TCP is able to transfer a
continuous stream of octets in each
direction between its users
by packaging some number of octets into
segments for transmission
through the internet system. In general,
the TCPs decide when to block
and forward data at their own
convenience.
Sometimes users need to be
sure that all the data they have
submitted to the TCP has been
transmitted. For this purpose a push
function is defined. To
assure that data submitted to a TCP is
actually transmitted the
sending user indicates that it should be
pushed through to the
receiving user. A push causes the TCPs to
promptly forward and deliver
data up to that point to the receiver.
The exact push point might
not be visible to the receiving user and
the push function does not
supply a record boundary marker.
Reliability:
The TCP must recover from
data that is damaged, lost, duplicated, or
delivered out of order by the
internet communication system. This
is achieved by assigning a
sequence number to each octet
transmitted, and requiring a
positive acknowledgment (ACK) from the
receiving TCP. If the ACK is
not received within a timeout
interval, the data is
retransmitted. At the receiver, the sequence
numbers are used to correctly
order segments that may be received
out of order and to eliminate
duplicates. Damage is handled by
adding a checksum to each
segment transmitted, checking it at the
receiver, and discarding
damaged segments.
As long as the TCPs continue
to function properly and the internet
system does not become
completely partitioned, no transmission
errors will affect the
correct delivery of data. TCP recovers from
internet communication system
errors.
Flow Control:
TCP provides a means for the
receiver to govern the amount of data
sent by the sender. This is
achieved by returning a "window" with
every ACK indicating a range
of acceptable sequence numbers beyond
the last segment successfully
received. The window indicates an
allowed number of octets that
the sender may transmit before
receiving further permission.
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Introduction
Multiplexing:
To allow for many processes
within a single Host to use TCP
communication facilities
simultaneously, the TCP provides a set of
addresses or ports within
each host. Concatenated with the network
and host addresses from the
internet communication layer, this forms
a socket. A pair of sockets
uniquely identifies each connection.
That is, a socket may be
simultaneously used in multiple
connections.
The binding of ports to
processes is handled independently by each
Host. However, it proves
useful to attach frequently used processes
(e.g., a "logger"
or timesharing service) to fixed sockets which are
made known to the public.
These services can then be accessed
through the known addresses.
Establishing and learning the port
addresses of other processes
may involve more dynamic mechanisms.
Connections:
The reliability and flow
control mechanisms described above require
that TCPs initialize and
maintain certain status information for
each data stream. The
combination of this information, including
sockets, sequence numbers,
and window sizes, is called a connection.
Each connection is uniquely
specified by a pair of sockets
identifying its two sides.
When two processes wish to
communicate, their TCP's must first
establish a connection (initialize
the status information on each
side). When their
communication is complete, the connection is
terminated or closed to free
the resources for other uses.
Since connections must be
established between unreliable hosts and
over the unreliable internet
communication system, a handshake
mechanism with clock-based
sequence numbers is used to avoid
erroneous initialization of
connections.
Precedence and Security:
The users of TCP may indicate
the security and precedence of their
communication. Provision is
made for default values to be used when
these features are not needed.
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2. PHILOSOPHY
2.1. Elements of the
Internetwork System
The internetwork environment
consists of hosts connected to networks
which are in turn
interconnected via gateways. It is assumed here
that the networks may be
either local networks (e.g., the ETHERNET) or
large networks (e.g., the
ARPANET), but in any case are based on
packet switching technology.
The active agents that produce and
consume messages are
processes. Various levels of protocols in the
networks, the gateways, and
the hosts support an interprocess
communication system that
provides two-way data flow on logical
connections between process
ports.
The term packet is used
generically here to mean the data of one
transaction between a host
and its network. The format of data blocks
exchanged within the a
network will generally not be of concern to us.
Hosts are computers attached
to a network, and from the communication
network's point of view, are
the sources and destinations of packets.
Processes are viewed as the
active elements in host computers (in
accordance with the fairly
common definition of a process as a program
in execution). Even terminals
and files or other I/O devices are
viewed as communicating with
each other through the use of processes.
Thus, all communication is
viewed as inter-process communication.
Since a process may need to
distinguish among several communication
streams between itself and
another process (or processes), we imagine
that each process may have a
number of ports through which it
communicates with the ports
of other processes.
2.2. Model of Operation
Processes transmit data by
calling on the TCP and passing buffers of
data as arguments. The TCP
packages the data from these buffers into
segments and calls on the
internet module to transmit each segment to
the destination TCP. The
receiving TCP places the data from a segment
into the receiving user's
buffer and notifies the receiving user. The
TCPs include control
information in the segments which they use to
ensure reliable ordered data
transmission.
The model of internet
communication is that there is an internet
protocol module associated
with each TCP which provides an interface
to the local network. This
internet module packages TCP segments
inside internet datagrams and
routes these datagrams to a destination
internet module or
intermediate gateway. To transmit the datagram
through the local network, it
is embedded in a local network packet.
The packet switches may
perform further packaging, fragmentation, or
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Philosophy
other operations to achieve
the delivery of the local packet to the
destination internet module.
At a gateway between networks,
the internet datagram is "unwrapped"
from its local packet and
examined to determine through which network
the internet datagram should
travel next. The internet datagram is
then "wrapped" in a
local packet suitable to the next network and
routed to the next gateway,
or to the final destination.
A gateway is permitted to
break up an internet datagram into smaller
internet datagram fragments
if this is necessary for transmission
through the next network. To
do this, the gateway produces a set of
internet datagrams; each
carrying a fragment. Fragments may be
further broken into smaller
fragments at subsequent gateways. The
internet datagram fragment
format is designed so that the destination
internet module can
reassemble fragments into internet datagrams.
A destination internet module
unwraps the segment from the datagram
(after reassembling the
datagram, if necessary) and passes it to the
destination TCP.
This simple model of the
operation glosses over many details. One
important feature is the type
of service. This provides information
to the gateway (or internet
module) to guide it in selecting the
service parameters to be used
in traversing the next network.
Included in the type of
service information is the precedence of the
datagram. Datagrams may also
carry security information to permit
host and gateways that
operate in multilevel secure environments to
properly segregate datagrams
for security considerations.
2.3. The Host Environment
The TCP is assumed to be a
module in an operating system. The users
access the TCP much like they
would access the file system. The TCP
may call on other operating
system functions, for example, to manage
data structures. The actual
interface to the network is assumed to be
controlled by a device driver
module. The TCP does not call on the
network device driver
directly, but rather calls on the internet
datagram protocol module
which may in turn call on the device driver.
The mechanisms of TCP do not
preclude implementation of the TCP in a
front-end processor. However,
in such an implementation, a
host-to-front-end protocol
must provide the functionality to support
the type of TCP-user
interface described in this document.
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2.4. Interfaces
The TCP/user interface
provides for calls made by the user on the TCP
to OPEN or CLOSE a
connection, to SEND or RECEIVE data, or to obtain
STATUS about a connection.
These calls are like other calls from user
programs on the operating
system, for example, the calls to open, read
from, and close a file.
The TCP/internet interface
provides calls to send and receive
datagrams addressed to TCP
modules in hosts anywhere in the internet
system. These calls have
parameters for passing the address, type of
service, precedence, security,
and other control information.
2.5. Relation to Other
Protocols
The following diagram
illustrates the place of the TCP in the protocol
hierarchy:
+------+ +-----+ +-----+ +-----+
|Telnet| | FTP | |Voice| ...
| | Application Level
+------+ +-----+ +-----+ +-----+
| | | |
+-----+ +-----+ +-----+
| TCP | | RTP | ... | | Host
Level
+-----+ +-----+ +-----+
| | |
+-------------------------------+
| Internet Protocol &
ICMP | Gateway Level
+-------------------------------+
|
+---------------------------+
| Local Network Protocol |
Network Level
+---------------------------+
Protocol Relationships
Figure 2.
It is expected that the TCP
will be able to support higher level
protocols efficiently. It
should be easy to interface higher level
protocols like the ARPANET
Telnet or AUTODIN II THP to the TCP.
2.6. Reliable Communication
A stream of data sent on a
TCP connection is delivered reliably and in
order at the destination.
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Transmission is made reliable
via the use of sequence numbers and
acknowledgments. Conceptually,
each octet of data is assigned a
sequence number. The sequence
number of the first octet of data in a
segment is transmitted with
that segment and is called the segment
sequence number. Segments
also carry an acknowledgment number which
is the sequence number of the
next expected data octet of
transmissions in the reverse
direction. When the TCP transmits a
segment containing data, it
puts a copy on a retransmission queue and
starts a timer; when the
acknowledgment for that data is received, the
segment is deleted from the
queue. If the acknowledgment is not
received before the timer
runs out, the segment is retransmitted.
An acknowledgment by TCP does
not guarantee that the data has been
delivered to the end user,
but only that the receiving TCP has taken
the responsibility to do so.
To govern the flow of data
between TCPs, a flow control mechanism is
employed. The receiving TCP
reports a "window" to the sending TCP.
This window specifies the
number of octets, starting with the
acknowledgment number, that
the receiving TCP is currently prepared to
receive.
2.7. Connection Establishment
and Clearing
To identify the separate data
streams that a TCP may handle, the TCP
provides a port identifier.
Since port identifiers are selected
independently by each TCP
they might not be unique. To provide for
unique addresses within each
TCP, we concatenate an internet address
identifying the TCP with a
port identifier to create a socket which
will be unique throughout all
networks connected together.
A connection is fully
specified by the pair of sockets at the ends. A
local socket may participate
in many connections to different foreign
sockets. A connection can be
used to carry data in both directions,
that is, it is "full
duplex".
TCPs are free to associate
ports with processes however they choose.
However, several basic
concepts are necessary in any implementation.
There must be well-known
sockets which the TCP associates only with
the "appropriate"
processes by some means. We envision that processes
may "own" ports,
and that processes can initiate connections only on
the ports they own. (Means
for implementing ownership is a local
issue, but we envision a
Request Port user command, or a method of
uniquely allocating a group
of ports to a given process, e.g., by
associating the high order
bits of a port name with a given process.)
A connection is specified in
the OPEN call by the local port and
foreign socket arguments. In
return, the TCP supplies a (short) local
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Philosophy
connection name by which the
user refers to the connection in
subsequent calls. There are
several things that must be remembered
about a connection. To store
this information we imagine that there
is a data structure called a
Transmission Control Block (TCB). One
implementation strategy would
have the local connection name be a
pointer to the TCB for this
connection. The OPEN call also specifies
whether the connection
establishment is to be actively pursued, or to
be passively waited for.
A passive OPEN request means
that the process wants to accept incoming
connection requests rather
than attempting to initiate a connection.
Often the process requesting
a passive OPEN will accept a connection
request from any caller. In
this case a foreign socket of all zeros
is used to denote an
unspecified socket. Unspecified foreign sockets
are allowed only on passive
OPENs.
A service process that wished
to provide services for unknown other
processes would issue a
passive OPEN request with an unspecified
foreign socket. Then a
connection could be made with any process that
requested a connection to
this local socket. It would help if this
local socket were known to be
associated with this service.
Well-known sockets are a
convenient mechanism for a priori associating
a socket address with a
standard service. For instance, the
"Telnet-Server"
process is permanently assigned to a particular
socket, and other sockets are
reserved for File Transfer, Remote Job
Entry, Text Generator, Echoer,
and Sink processes (the last three
being for test purposes). A
socket address might be reserved for
access to a
"Look-Up" service which would return the specific
socket
at which a newly created
service would be provided. The concept of a
well-known socket is part of
the TCP specification, but the assignment
of sockets to services is
outside this specification. (See [4].)
Processes can issue passive
OPENs and wait for matching active OPENs
from other processes and be
informed by the TCP when connections have
been established. Two
processes which issue active OPENs to each
other at the same time will
be correctly connected. This flexibility
is critical for the support
of distributed computing in which
components act asynchronously
with respect to each other.
There are two principal cases
for matching the sockets in the local
passive OPENs and an foreign
active OPENs. In the first case, the
local passive OPENs has fully
specified the foreign socket. In this
case, the match must be exact.
In the second case, the local passive
OPENs has left the foreign
socket unspecified. In this case, any
foreign socket is acceptable
as long as the local sockets match.
Other possibilities include
partially restricted matches.
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If there are several pending
passive OPENs (recorded in TCBs) with the
same local socket, an foreign
active OPEN will be matched to a TCB
with the specific foreign
socket in the foreign active OPEN, if such a
TCB exists, before selecting
a TCB with an unspecified foreign socket.
The procedures to establish
connections utilize the synchronize (SYN)
control flag and involves an
exchange of three messages. This
exchange has been termed a
three-way hand shake [3].
A connection is initiated by
the rendezvous of an arriving segment
containing a SYN and a
waiting TCB entry each created by a user OPEN
command. The matching of
local and foreign sockets determines when a
connection has been initiated.
The connection becomes "established"
when sequence numbers have
been synchronized in both directions.
The clearing of a connection
also involves the exchange of segments,
in this case carrying the FIN
control flag.
2.8. Data Communication
The data that flows on a
connection may be thought of as a stream of
octets. The sending user
indicates in each SEND call whether the data
in that call (and any
preceeding calls) should be immediately pushed
through to the receiving user
by the setting of the PUSH flag.
A sending TCP is allowed to
collect data from the sending user and to
send that data in segments at
its own convenience, until the push
function is signaled, then it
must send all unsent data. When a
receiving TCP sees the PUSH
flag, it must not wait for more data from
the sending TCP before
passing the data to the receiving process.
There is no necessary
relationship between push functions and segment
boundaries. The data in any
particular segment may be the result of a
single SEND call, in whole or
part, or of multiple SEND calls.
The purpose of push function
and the PUSH flag is to push data through
from the sending user to the
receiving user. It does not provide a
record service.
There is a coupling between
the push function and the use of buffers
of data that cross the TCP/user
interface. Each time a PUSH flag is
associated with data placed
into the receiving user's buffer, the
buffer is returned to the
user for processing even if the buffer is
not filled. If data arrives
that fills the user's buffer before a
PUSH is seen, the data is
passed to the user in buffer size units.
TCP also provides a means to
communicate to the receiver of data that
at some point further along
in the data stream than the receiver is
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Transmission Control Protocol
Philosophy
currently reading there is
urgent data. TCP does not attempt to
define what the user
specifically does upon being notified of pending
urgent data, but the general
notion is that the receiving process will
take action to process the
urgent data quickly.
2.9. Precedence and Security
The TCP makes use of the
internet protocol type of service field and
security option to provide
precedence and security on a per connection
basis to TCP users. Not all
TCP modules will necessarily function in
a multilevel secure
environment; some may be limited to unclassified
use only, and others may
operate at only one security level and
compartment. Consequently,
some TCP implementations and services to
users may be limited to a
subset of the multilevel secure case.
TCP modules which operate in
a multilevel secure environment must
properly mark outgoing
segments with the security, compartment, and
precedence. Such TCP modules
must also provide to their users or
higher level protocols such
as Telnet or THP an interface to allow
them to specify the desired
security level, compartment, and
precedence of connections.
2.10. Robustness Principle
TCP implementations will
follow a general principle of robustness: be
conservative in what you do,
be liberal in what you accept from
others.
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Transmission Control Protocol
3. FUNCTIONAL SPECIFICATION
3.1. Header Format
TCP segments are sent as
internet datagrams. The Internet Protocol
header carries several
information fields, including the source and
destination host addresses
[2]. A TCP header follows the internet
header, supplying information
specific to the TCP protocol. This
division allows for the
existence of host level protocols other than
TCP.
TCP Header Format
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4
5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Port | Destination
Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Acknowledgment Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data | |U|A|P|R|S|F| |
| Offset| Reserved
|R|C|S|S|Y|I| Window |
| | |G|K|H|T|N|N| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum | Urgent Pointer |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
TCP Header Format
Note that one tick mark
represents one bit position.
Figure 3.
Source Port: 16 bits
The source port number.
Destination Port: 16 bits
The destination port number.
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Transmission Control Protocol
Functional Specification
Sequence Number: 32 bits
The sequence number of the
first data octet in this segment (except
when SYN is present). If SYN
is present the sequence number is the
initial sequence number (ISN)
and the first data octet is ISN+1.
Acknowledgment Number: 32
bits
If the ACK control bit is set
this field contains the value of the
next sequence number the
sender of the segment is expecting to
receive. Once a connection is
established this is always sent.
Data Offset: 4 bits
The number of 32 bit words in
the TCP Header. This indicates where
the data begins. The TCP
header (even one including options) is an
integral number of 32 bits
long.
Reserved: 6 bits
Reserved for future use. Must
be zero.
Control Bits: 6 bits (from
left to right):
URG: Urgent Pointer field
significant
ACK: Acknowledgment field
significant
PSH: Push Function
RST: Reset the connection
SYN: Synchronize sequence
numbers
FIN: No more data from sender
Window: 16 bits
The number of data octets
beginning with the one indicated in the
acknowledgment field which
the sender of this segment is willing to
accept.
Checksum: 16 bits
The checksum field is the 16
bit one's complement of the one's
complement sum of all 16 bit
words in the header and text. If a
segment contains an odd
number of header and text octets to be
checksummed, the last octet
is padded on the right with zeros to
form a 16 bit word for
checksum purposes. The pad is not
transmitted as part of the
segment. While computing the checksum,
the checksum field itself is
replaced with zeros.
The checksum also covers a 96
bit pseudo header conceptually
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September 1981
Transmission Control Protocol
Functional Specification
prefixed to the TCP header.
This pseudo header contains the Source
Address, the Destination
Address, the Protocol, and TCP length.
This gives the TCP protection
against misrouted segments. This
information is carried in the
Internet Protocol and is transferred
across the TCP/Network
interface in the arguments or results of
calls by the TCP on the IP.
+--------+--------+--------+--------+
| Source Address |
+--------+--------+--------+--------+
| Destination Address |
+--------+--------+--------+--------+
| zero | PTCL | TCP Length |
+--------+--------+--------+--------+
The TCP Length is the TCP
header length plus the data length in
octets (this is not an
explicitly transmitted quantity, but is
computed), and it does not
count the 12 octets of the pseudo
header.
Urgent Pointer: 16 bits
This field communicates the
current value of the urgent pointer as a
positive offset from the
sequence number in this segment. The
urgent pointer points to the
sequence number of the octet following
the urgent data. This field
is only be interpreted in segments with
the URG control bit set.
Options: variable
Options may occupy space at
the end of the TCP header and are a
multiple of 8 bits in length.
All options are included in the
checksum. An option may begin
on any octet boundary. There are two
cases for the format of an
option:
Case 1: A single octet of
option-kind.
Case 2: An octet of
option-kind, an octet of option-length, and
the actual option-data octets.
The option-length counts the
two octets of option-kind and
option-length as well as the
option-data octets.
Note that the list of options
may be shorter than the data offset
field might imply. The
content of the header beyond the
End-of-Option option must be
header padding (i.e., zero).
A TCP must implement all
options.
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Transmission Control Protocol
Functional Specification
Currently defined options
include (kind indicated in octal):
Kind Length Meaning
---- ------ -------
0 - End of option list.
1 - No-Operation.
2 4 Maximum Segment Size.
Specific Option Definitions
End of Option List
+--------+
|00000000|
+--------+
Kind=0
This option code indicates
the end of the option list. This
might not coincide with the
end of the TCP header according to
the Data Offset field. This
is used at the end of all options,
not the end of each option,
and need only be used if the end of
the options would not
otherwise coincide with the end of the TCP
header.
No-Operation
+--------+
|00000001|
+--------+
Kind=1
This option code may be used
between options, for example, to
align the beginning of a
subsequent option on a word boundary.
There is no guarantee that
senders will use this option, so
receivers must be prepared to
process options even if they do
not begin on a word boundary.
Maximum Segment Size
+--------+--------+---------+--------+
|00000010|00000100| max seg
size |
+--------+--------+---------+--------+
Kind=2 Length=4
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September 1981
Transmission Control Protocol
Functional Specification
Maximum Segment Size Option
Data: 16 bits
If this option is present,
then it communicates the maximum
receive segment size at the
TCP which sends this segment.
This field must only be sent
in the initial connection request
(i.e., in segments with the
SYN control bit set). If this
option is not used, any
segment size is allowed.
Padding: variable
The TCP header padding is
used to ensure that the TCP header ends
and data begins on a 32 bit
boundary. The padding is composed of
zeros.
3.2. Terminology
Before we can discuss very
much about the operation of the TCP we need
to introduce some detailed
terminology. The maintenance of a TCP
connection requires the
remembering of several variables. We conceive
of these variables being
stored in a connection record called a
Transmission Control Block or
TCB. Among the variables stored in the
TCB are the local and remote
socket numbers, the security and
precedence of the connection,
pointers to the user's send and receive
buffers, pointers to the
retransmit queue and to the current segment.
In addition several variables
relating to the send and receive
sequence numbers are stored
in the TCB.
Send Sequence Variables
SND.UNA - send unacknowledged
SND.NXT - send next
SND.WND - send window
SND.UP - send urgent pointer
SND.WL1 - segment sequence
number used for last window update
SND.WL2 - segment
acknowledgment number used for last window
update
ISS - initial send sequence
number
Receive Sequence Variables
RCV.NXT - receive next
RCV.WND - receive window
RCV.UP - receive urgent
pointer
IRS - initial receive
sequence number
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Transmission Control Protocol
Functional Specification
The following diagrams may
help to relate some of these variables to
the sequence space.
Send Sequence Space
1 2 3 4
----------|----------|----------|----------
SND.UNA SND.NXT SND.UNA
+SND.WND
1 - old sequence numbers
which have been acknowledged
2 - sequence numbers of
unacknowledged data
3 - sequence numbers allowed
for new data transmission
4 - future sequence numbers
which are not yet allowed
Send Sequence Space
Figure 4.
The send window is the
portion of the sequence space labeled 3 in
figure 4.
Receive Sequence Space
1 2 3
----------|----------|----------
RCV.NXT RCV.NXT
+RCV.WND
1 - old sequence numbers
which have been acknowledged
2 - sequence numbers allowed
for new reception
3 - future sequence numbers
which are not yet allowed
Receive Sequence Space
Figure 5.
The receive window is the
portion of the sequence space labeled 2 in
figure 5.
There are also some variables
used frequently in the discussion that
take their values from the
fields of the current segment.
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Transmission Control Protocol
Functional Specification
Current Segment Variables
SEG.SEQ - segment sequence
number
SEG.ACK - segment
acknowledgment number
SEG.LEN - segment length
SEG.WND - segment window
SEG.UP - segment urgent
pointer
SEG.PRC - segment precedence
value
A connection progresses
through a series of states during its
lifetime. The states are:
LISTEN, SYN-SENT, SYN-RECEIVED,
ESTABLISHED, FIN-WAIT-1,
FIN-WAIT-2, CLOSE-WAIT, CLOSING, LAST-ACK,
TIME-WAIT, and the fictional
state CLOSED. CLOSED is fictional
because it represents the
state when there is no TCB, and therefore,
no connection. Briefly the
meanings of the states are:
LISTEN - represents waiting
for a connection request from any remote
TCP and port.
SYN-SENT - represents waiting
for a matching connection request
after having sent a
connection request.
SYN-RECEIVED - represents
waiting for a confirming connection
request acknowledgment after
having both received and sent a
connection request.
ESTABLISHED - represents an
open connection, data received can be
delivered to the user. The
normal state for the data transfer phase
of the connection.
FIN-WAIT-1 - represents
waiting for a connection termination request
from the remote TCP, or an
acknowledgment of the connection
termination request
previously sent.
FIN-WAIT-2 - represents
waiting for a connection termination request
from the remote TCP.
CLOSE-WAIT - represents
waiting for a connection termination request
from the local user.
CLOSING - represents waiting
for a connection termination request
acknowledgment from the
remote TCP.
LAST-ACK - represents waiting
for an acknowledgment of the
connection termination
request previously sent to the remote TCP
(which includes an
acknowledgment of its connection termination
request).
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Transmission Control Protocol
Functional Specification
TIME-WAIT - represents
waiting for enough time to pass to be sure
the remote TCP received the
acknowledgment of its connection
termination request.
CLOSED - represents no
connection state at all.
A TCP connection progresses
from one state to another in response to
events. The events are the
user calls, OPEN, SEND, RECEIVE, CLOSE,
ABORT, and STATUS; the
incoming segments, particularly those
containing the SYN, ACK, RST
and FIN flags; and timeouts.
The state diagram in figure 6
illustrates only state changes, together
with the causing events and
resulting actions, but addresses neither
error conditions nor actions
which are not connected with state
changes. In a later section,
more detail is offered with respect to
the reaction of the TCP to
events.
NOTE BENE: this diagram is
only a summary and must not be taken as
the total specification.
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September 1981
Transmission Control Protocol
Functional Specification
+---------+ ---------\ active
OPEN
| CLOSED | \ -----------
+---------+<---------\ \
create TCB
| ^ \ \ snd SYN
passive OPEN | | CLOSE \ \
------------ | | ---------- \
\
create TCB | | delete TCB \ \
V | \ \
+---------+ CLOSE | \
| LISTEN | ---------- | |
+---------+ delete TCB | |
rcv SYN | | SEND | |
----------- | | ------- | V
+---------+ snd SYN,ACK / \
snd SYN +---------+
| |<-----------------
------------------>| |
| SYN | rcv SYN | SYN |
| RCVD |<-----------------------------------------------|
SENT |
| | snd ACK | |
| |------------------
-------------------| |
+---------+ rcv ACK of SYN \
/ rcv SYN,ACK +---------+
| -------------- | |
-----------
| x | | snd ACK
| V V
| CLOSE +---------+
| ------- | ESTAB |
| snd FIN +---------+
| CLOSE | | rcv FIN
V ------- | | -------
+---------+ snd FIN / \ snd
ACK +---------+
| FIN |<-----------------
------------------>| CLOSE |
| WAIT-1 |------------------
| WAIT |
+---------+ rcv FIN \ +---------+
| rcv ACK of FIN ------- |
CLOSE |
| -------------- snd ACK |
------- |
V x V snd FIN V
+---------+ +---------+ +---------+
|FINWAIT-2| | CLOSING | |
LAST-ACK|
+---------+ +---------+ +---------+
| rcv ACK of FIN | rcv ACK of
FIN |
| rcv FIN -------------- |
Timeout=2MSL -------------- |
| ------- x V ------------ x
V
\ snd ACK +---------+delete
TCB +---------+
------------------------>|TIME
WAIT|------------------>| CLOSED |
+---------+ +---------+
TCP Connection State Diagram
Figure 6.
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Transmission Control Protocol
Functional Specification
3.3. Sequence Numbers
A fundamental notion in the
design is that every octet of data sent
over a TCP connection has a
sequence number. Since every octet is
sequenced, each of them can
be acknowledged. The acknowledgment
mechanism employed is
cumulative so that an acknowledgment of sequence
number X indicates that all
octets up to but not including X have been
received. This mechanism
allows for straight-forward duplicate
detection in the presence of
retransmission. Numbering of octets
within a segment is that the
first data octet immediately following
the header is the lowest
numbered, and the following octets are
numbered consecutively.
It is essential to remember
that the actual sequence number space is
finite, though very large.
This space ranges from 0 to 2**32 - 1.
Since the space is finite,
all arithmetic dealing with sequence
numbers must be performed
modulo 2**32. This unsigned arithmetic
preserves the relationship of
sequence numbers as they cycle from
2**32 - 1 to 0 again. There
are some subtleties to computer modulo
arithmetic, so great care
should be taken in programming the
comparison of such values.
The symbol "=<" means "less than or equal"
(modulo 2**32).
The typical kinds of sequence
number comparisons which the TCP must
perform include:
(a) Determining that an
acknowledgment refers to some sequence
number sent but not yet
acknowledged.
(b) Determining that all
sequence numbers occupied by a segment
have been acknowledged (e.g.,
to remove the segment from a
retransmission queue).
(c) Determining that an
incoming segment contains sequence numbers
which are expected (i.e.,
that the segment "overlaps" the
receive window).
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Transmission Control Protocol
Functional Specification
In response to sending data
the TCP will receive acknowledgments. The
following comparisons are
needed to process the acknowledgments.
SND.UNA = oldest
unacknowledged sequence number
SND.NXT = next sequence
number to be sent
SEG.ACK = acknowledgment from
the receiving TCP (next sequence
number expected by the
receiving TCP)
SEG.SEQ = first sequence
number of a segment
SEG.LEN = the number of
octets occupied by the data in the segment
(counting SYN and FIN)
SEG.SEQ+SEG.LEN-1 = last
sequence number of a segment
A new acknowledgment (called
an "acceptable ack"), is one for which
the inequality below holds:
SND.UNA < SEG.ACK =<
SND.NXT
A segment on the
retransmission queue is fully acknowledged if the sum
of its sequence number and
length is less or equal than the
acknowledgment value in the
incoming segment.
When data is received the
following comparisons are needed:
RCV.NXT = next sequence
number expected on an incoming segments, and
is the left or lower edge of
the receive window
RCV.NXT+RCV.WND-1 = last
sequence number expected on an incoming
segment, and is the right or
upper edge of the receive window
SEG.SEQ = first sequence
number occupied by the incoming segment
SEG.SEQ+SEG.LEN-1 = last
sequence number occupied by the incoming
segment
A segment is judged to occupy
a portion of valid receive sequence
space if
RCV.NXT =< SEG.SEQ <
RCV.NXT+RCV.WND
or
RCV.NXT =<
SEG.SEQ+SEG.LEN-1 < RCV.NXT+RCV.WND
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Transmission Control Protocol
Functional Specification
The first part of this test
checks to see if the beginning of the
segment falls in the window,
the second part of the test checks to see
if the end of the segment
falls in the window; if the segment passes
either part of the test it
contains data in the window.
Actually, it is a little more
complicated than this. Due to zero
windows and zero length
segments, we have four cases for the
acceptability of an incoming
segment:
Segment Receive Test
Length Window
------- -------
-------------------------------------------
0 0 SEG.SEQ = RCV.NXT
0 >0 RCV.NXT =< SEG.SEQ
< RCV.NXT+RCV.WND
>0 0 not acceptable
>0 >0 RCV.NXT =<
SEG.SEQ < RCV.NXT+RCV.WND
or RCV.NXT =<
SEG.SEQ+SEG.LEN-1 < RCV.NXT+RCV.WND
Note that when the receive
window is zero no segments should be
acceptable except ACK
segments. Thus, it is be possible for a TCP to
maintain a zero receive
window while transmitting data and receiving
ACKs. However, even when the
receive window is zero, a TCP must
process the RST and URG
fields of all incoming segments.
We have taken advantage of
the numbering scheme to protect certain
control information as well.
This is achieved by implicitly including
some control flags in the
sequence space so they can be retransmitted
and acknowledged without
confusion (i.e., one and only one copy of the
control will be acted upon).
Control information is not physically
carried in the segment data
space. Consequently, we must adopt rules
for implicitly assigning
sequence numbers to control. The SYN and FIN
are the only controls
requiring this protection, and these controls
are used only at connection
opening and closing. For sequence number
purposes, the SYN is
considered to occur before the first actual data
octet of the segment in which
it occurs, while the FIN is considered
to occur after the last
actual data octet in a segment in which it
occurs. The segment length (SEG.LEN)
includes both data and sequence
space occupying controls.
When a SYN is present then SEG.SEQ is the
sequence number of the SYN.
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Transmission Control Protocol
Functional Specification
Initial Sequence Number
Selection
The protocol places no
restriction on a particular connection being
used over and over again. A
connection is defined by a pair of
sockets. New instances of a
connection will be referred to as
incarnations of the
connection. The problem that arises from this is
-- "how does the TCP
identify duplicate segments from previous
incarnations of the
connection?" This problem becomes apparent if the
connection is being opened
and closed in quick succession, or if the
connection breaks with loss
of memory and is then reestablished.
To avoid confusion we must
prevent segments from one incarnation of a
connection from being used
while the same sequence numbers may still
be present in the network
from an earlier incarnation. We want to
assure this, even if a TCP
crashes and loses all knowledge of the
sequence numbers it has been
using. When new connections are created,
an initial sequence number (ISN)
generator is employed which selects a
new 32 bit ISN. The generator
is bound to a (possibly fictitious) 32
bit clock whose low order bit
is incremented roughly every 4
microseconds. Thus, the ISN
cycles approximately every 4.55 hours.
Since we assume that segments
will stay in the network no more than
the Maximum Segment Lifetime
(MSL) and that the MSL is less than 4.55
hours we can reasonably
assume that ISN's will be unique.
For each connection there is
a send sequence number and a receive
sequence number. The initial
send sequence number (ISS) is chosen by
the data sending TCP, and the
initial receive sequence number (IRS) is
learned during the connection
establishing procedure.
For a connection to be
established or initialized, the two TCPs must
synchronize on each other's
initial sequence numbers. This is done in
an exchange of connection
establishing segments carrying a control bit
called "SYN" (for
synchronize) and the initial sequence numbers. As a
shorthand, segments carrying
the SYN bit are also called "SYNs".
Hence, the solution requires
a suitable mechanism for picking an
initial sequence number and a
slightly involved handshake to exchange
the ISN's.
The synchronization requires
each side to send it's own initial
sequence number and to
receive a confirmation of it in acknowledgment
from the other side. Each
side must also receive the other side's
initial sequence number and
send a confirming acknowledgment.
1) A --> B SYN my sequence
number is X
2) A <-- B ACK your
sequence number is X
3) A <-- B SYN my sequence
number is Y
4) A --> B ACK your
sequence number is Y
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September 1981
Transmission Control Protocol
Functional Specification
Because steps 2 and 3 can be
combined in a single message this is
called the three way (or
three message) handshake.
A three way handshake is
necessary because sequence numbers are not
tied to a global clock in the
network, and TCPs may have different
mechanisms for picking the
ISN's. The receiver of the first SYN has
no way of knowing whether the
segment was an old delayed one or not,
unless it remembers the last
sequence number used on the connection
(which is not always possible),
and so it must ask the sender to
verify this SYN. The three
way handshake and the advantages of a
clock-driven scheme are
discussed in [3].
Knowing When to Keep Quiet
To be sure that a TCP does
not create a segment that carries a
sequence number which may be
duplicated by an old segment remaining in
the network, the TCP must
keep quiet for a maximum segment lifetime
(MSL) before assigning any
sequence numbers upon starting up or
recovering from a crash in
which memory of sequence numbers in use was
lost. For this specification
the MSL is taken to be 2 minutes. This
is an engineering choice, and
may be changed if experience indicates
it is desirable to do so.
Note that if a TCP is reinitialized in some
sense, yet retains its memory
of sequence numbers in use, then it need
not wait at all; it must only
be sure to use sequence numbers larger
than those recently used.
The TCP Quiet Time Concept
This specification provides
that hosts which "crash" without
retaining any knowledge of
the last sequence numbers transmitted on
each active (i.e., not closed)
connection shall delay emitting any
TCP segments for at least the
agreed Maximum Segment Lifetime (MSL)
in the internet system of
which the host is a part. In the
paragraphs below, an
explanation for this specification is given.
TCP implementors may violate
the "quiet time" restriction, but only
at the risk of causing some
old data to be accepted as new or new
data rejected as old
duplicated by some receivers in the internet
system.
TCPs consume sequence number
space each time a segment is formed and
entered into the network
output queue at a source host. The
duplicate detection and
sequencing algorithm in the TCP protocol
relies on the unique binding
of segment data to sequence space to
the extent that sequence
numbers will not cycle through all 2**32
values before the segment
data bound to those sequence numbers has
been delivered and
acknowledged by the receiver and all duplicate
copies of the segments have
"drained" from the internet. Without
such an assumption, two
distinct TCP segments could conceivably be
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September 1981
Transmission Control Protocol
Functional Specification
assigned the same or
overlapping sequence numbers, causing confusion
at the receiver as to which
data is new and which is old. Remember
that each segment is bound to
as many consecutive sequence numbers
as there are octets of data
in the segment.
Under normal conditions, TCPs
keep track of the next sequence number
to emit and the oldest
awaiting acknowledgment so as to avoid
mistakenly using a sequence
number over before its first use has
been acknowledged. This alone
does not guarantee that old duplicate
data is drained from the net,
so the sequence space has been made
very large to reduce the
probability that a wandering duplicate will
cause trouble upon arrival.
At 2 megabits/sec. it takes 4.5 hours
to use up 2**32 octets of
sequence space. Since the maximum segment
lifetime in the net is not
likely to exceed a few tens of seconds,
this is deemed ample
protection for foreseeable nets, even if data
rates escalate to l0's of
megabits/sec. At 100 megabits/sec, the
cycle time is 5.4 minutes
which may be a little short, but still
within reason.
The basic duplicate detection
and sequencing algorithm in TCP can be
defeated, however, if a
source TCP does not have any memory of the
sequence numbers it last used
on a given connection. For example, if
the TCP were to start all
connections with sequence number 0, then
upon crashing and restarting,
a TCP might re-form an earlier
connection (possibly after
half-open connection resolution) and emit
packets with sequence numbers
identical to or overlapping with
packets still in the network
which were emitted on an earlier
incarnation of the same
connection. In the absence of knowledge
about the sequence numbers
used on a particular connection, the TCP
specification recommends that
the source delay for MSL seconds
before emitting segments on
the connection, to allow time for
segments from the earlier
connection incarnation to drain from the
system.
Even hosts which can remember
the time of day and used it to select
initial sequence number
values are not immune from this problem
(i.e., even if time of day is
used to select an initial sequence
number for each new
connection incarnation).
Suppose, for example, that a
connection is opened starting with
sequence number S. Suppose
that this connection is not used much
and that eventually the
initial sequence number function (ISN(t))
takes on a value equal to the
sequence number, say S1, of the last
segment sent by this TCP on a
particular connection. Now suppose,
at this instant, the host
crashes, recovers, and establishes a new
incarnation of the
connection. The initial sequence number chosen is
S1 = ISN(t) -- last used
sequence number on old incarnation of
connection! If the recovery
occurs quickly enough, any old
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September 1981
Transmission Control Protocol
Functional Specification
duplicates in the net bearing
sequence numbers in the neighborhood
of S1 may arrive and be
treated as new packets by the receiver of
the new incarnation of the
connection.
The problem is that the
recovering host may not know for how long it
crashed nor does it know
whether there are still old duplicates in
the system from earlier
connection incarnations.
One way to deal with this
problem is to deliberately delay emitting
segments for one MSL after
recovery from a crash- this is the "quite
time" specification.
Hosts which prefer to avoid waiting are
willing to risk possible
confusion of old and new packets at a given
destination may choose not to
wait for the "quite time".
Implementors may provide TCP
users with the ability to select on a
connection by connection
basis whether to wait after a crash, or may
informally implement the
"quite time" for all connections.
Obviously, even where a user
selects to "wait," this is not
necessary after the host has
been "up" for at least MSL seconds.
To summarize: every segment
emitted occupies one or more sequence
numbers in the sequence
space, the numbers occupied by a segment are
"busy" or "in
use" until MSL seconds have passed, upon crashing a
block of space-time is
occupied by the octets of the last emitted
segment, if a new connection
is started too soon and uses any of the
sequence numbers in the
space-time footprint of the last segment of
the previous connection
incarnation, there is a potential sequence
number overlap area which
could cause confusion at the receiver.
3.4. Establishing a
connection
The "three-way handshake"
is the procedure used to establish a
connection. This procedure
normally is initiated by one TCP and
responded to by another TCP.
The procedure also works if two TCP
simultaneously initiate the
procedure. When simultaneous attempt
occurs, each TCP receives a
"SYN" segment which carries no
acknowledgment after it has
sent a "SYN". Of course, the arrival of
an old duplicate "SYN"
segment can potentially make it appear, to the
recipient, that a
simultaneous connection initiation is in progress.
Proper use of
"reset" segments can disambiguate these cases.
Several examples of
connection initiation follow. Although these
examples do not show
connection synchronization using data-carrying
segments, this is perfectly
legitimate, so long as the receiving TCP
doesn't deliver the data to
the user until it is clear the data is
valid (i.e., the data must be
buffered at the receiver until the
connection reaches the
ESTABLISHED state). The three-way handshake
reduces the possibility of
false connections. It is the
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Transmission Control Protocol
Functional Specification
implementation of a trade-off
between memory and messages to provide
information for this checking.
The simplest three-way
handshake is shown in figure 7 below. The
figures should be interpreted
in the following way. Each line is
numbered for reference
purposes. Right arrows (-->) indicate
departure of a TCP segment
from TCP A to TCP B, or arrival of a
segment at B from A. Left
arrows (<--), indicate the reverse.
Ellipsis (...) indicates a
segment which is still in the network
(delayed). An "XXX"
indicates a segment which is lost or rejected.
Comments appear in
parentheses. TCP states represent the state AFTER
the departure or arrival of
the segment (whose contents are shown in
the center of each line).
Segment contents are shown in abbreviated
form, with sequence number,
control flags, and ACK field. Other
fields such as window,
addresses, lengths, and text have been left out
in the interest of clarity.
TCP A TCP B
1. CLOSED LISTEN
2. SYN-SENT --> <SEQ=100><CTL=SYN>
--> SYN-RECEIVED
3. ESTABLISHED <-- <SEQ=300><ACK=101><CTL=SYN,ACK>
<-- SYN-RECEIVED
4. ESTABLISHED --> <SEQ=101><ACK=301><CTL=ACK>
--> ESTABLISHED
5. ESTABLISHED --> <SEQ=101><ACK=301><CTL=ACK><DATA>
--> ESTABLISHED
Basic 3-Way Handshake for
Connection Synchronization
Figure 7.
In line 2 of figure 7, TCP A
begins by sending a SYN segment
indicating that it will use
sequence numbers starting with sequence
number 100. In line 3, TCP B
sends a SYN and acknowledges the SYN it
received from TCP A. Note
that the acknowledgment field indicates TCP
B is now expecting to hear
sequence 101, acknowledging the SYN which
occupied sequence 100.
At line 4, TCP A responds
with an empty segment containing an ACK for
TCP B's SYN; and in line 5,
TCP A sends some data. Note that the
sequence number of the
segment in line 5 is the same as in line 4
because the ACK does not
occupy sequence number space (if it did, we
would wind up ACKing ACK's!).
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Simultaneous initiation is
only slightly more complex, as is shown in
figure 8. Each TCP cycles
from CLOSED to SYN-SENT to SYN-RECEIVED to
ESTABLISHED.
TCP A TCP B
1. CLOSED CLOSED
2. SYN-SENT --> <SEQ=100><CTL=SYN>
...
3. SYN-RECEIVED <--
<SEQ=300><CTL=SYN> <-- SYN-SENT
4. ... <SEQ=100><CTL=SYN>
--> SYN-RECEIVED
5. SYN-RECEIVED -->
<SEQ=100><ACK=301><CTL=SYN,ACK> ...
6. ESTABLISHED <-- <SEQ=300><ACK=101><CTL=SYN,ACK>
<-- SYN-RECEIVED
7. ... <SEQ=101><ACK=301><CTL=ACK>
--> ESTABLISHED
Simultaneous Connection
Synchronization
Figure 8.
The principle reason for the
three-way handshake is to prevent old
duplicate connection
initiations from causing confusion. To deal with
this, a special control
message, reset, has been devised. If the
receiving TCP is in a
non-synchronized state (i.e., SYN-SENT,
SYN-RECEIVED), it returns to
LISTEN on receiving an acceptable reset.
If the TCP is in one of the
synchronized states (ESTABLISHED,
FIN-WAIT-1, FIN-WAIT-2,
CLOSE-WAIT, CLOSING, LAST-ACK, TIME-WAIT), it
aborts the connection and
informs its user. We discuss this latter
case under "half-open"
connections below.
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TCP A TCP B
1. CLOSED LISTEN
2. SYN-SENT --> <SEQ=100><CTL=SYN>
...
3. (duplicate) ... <SEQ=90><CTL=SYN>
--> SYN-RECEIVED
4. SYN-SENT <-- <SEQ=300><ACK=91><CTL=SYN,ACK>
<-- SYN-RECEIVED
5. SYN-SENT --> <SEQ=91><CTL=RST>
--> LISTEN
6. ... <SEQ=100><CTL=SYN>
--> SYN-RECEIVED
7. SYN-SENT <-- <SEQ=400><ACK=101><CTL=SYN,ACK>
<-- SYN-RECEIVED
8. ESTABLISHED --> <SEQ=101><ACK=401><CTL=ACK>
--> ESTABLISHED
Recovery from Old Duplicate
SYN
Figure 9.
As a simple example of
recovery from old duplicates, consider
figure 9. At line 3, an old
duplicate SYN arrives at TCP B. TCP B
cannot tell that this is an
old duplicate, so it responds normally
(line 4). TCP A detects that
the ACK field is incorrect and returns a
RST (reset) with its SEQ
field selected to make the segment
believable. TCP B, on
receiving the RST, returns to the LISTEN state.
When the original SYN (pun
intended) finally arrives at line 6, the
synchronization proceeds
normally. If the SYN at line 6 had arrived
before the RST, a more
complex exchange might have occurred with RST's
sent in both directions.
Half-Open Connections and
Other Anomalies
An established connection is
said to be "half-open" if one of the
TCPs has closed or aborted
the connection at its end without the
knowledge of the other, or if
the two ends of the connection have
become desynchronized owing
to a crash that resulted in loss of
memory. Such connections will
automatically become reset if an
attempt is made to send data
in either direction. However, half-open
connections are expected to
be unusual, and the recovery procedure is
mildly involved.
If at site A the connection
no longer exists, then an attempt by the
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user at site B to send any
data on it will result in the site B TCP
receiving a reset control
message. Such a message indicates to the
site B TCP that something is
wrong, and it is expected to abort the
connection.
Assume that two user
processes A and B are communicating with one
another when a crash occurs
causing loss of memory to A's TCP.
Depending on the operating
system supporting A's TCP, it is likely
that some error recovery
mechanism exists. When the TCP is up again,
A is likely to start again
from the beginning or from a recovery
point. As a result, A will
probably try to OPEN the connection again
or try to SEND on the
connection it believes open. In the latter
case, it receives the error
message "connection not open" from the
local (A's) TCP. In an
attempt to establish the connection, A's TCP
will send a segment
containing SYN. This scenario leads to the
example shown in figure 10.
After TCP A crashes, the user attempts to
re-open the connection. TCP
B, in the meantime, thinks the connection
is open.
TCP A TCP B
1. (CRASH) (send 300,receive
100)
2. CLOSED ESTABLISHED
3. SYN-SENT --> <SEQ=400><CTL=SYN>
--> (??)
4. (!!) <-- <SEQ=300><ACK=100><CTL=ACK>
<-- ESTABLISHED
5. SYN-SENT --> <SEQ=100><CTL=RST>
--> (Abort!!)
6. SYN-SENT CLOSED
7. SYN-SENT --> <SEQ=400><CTL=SYN>
-->
Half-Open Connection
Discovery
Figure 10.
When the SYN arrives at line
3, TCP B, being in a synchronized state,
and the incoming segment
outside the window, responds with an
acknowledgment indicating
what sequence it next expects to hear (ACK
100). TCP A sees that this
segment does not acknowledge anything it
sent and, being
unsynchronized, sends a reset (RST) because it has
detected a half-open
connection. TCP B aborts at line 5. TCP A will
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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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Functional Specification
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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Transmission Control Protocol
Functional Specification
In providing interprocess
communication facilities, the TCP must
not only accept commands, but
must also return information to the
processes it serves. The
latter consists of:
(a) general information about
a connection (e.g., interrupts,
remote close, binding of
unspecified foreign socket).
(b) replies to specific user
commands indicating success or
various types of failure.
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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Transmission Control Protocol
Functional Specification
precedence or
security/compartment. The absence of precedence
or security/compartment
specification in the OPEN call indicates
the default values must be
used.
TCP will accept incoming
requests as matching only if the
security/compartment
information is exactly the same and only if
the precedence is equal to or
higher than the precedence
requested in the OPEN call.
The precedence for the
connection is the higher of the values
requested in the OPEN call
and received from the incoming
request, and fixed at that
value for the life of the
connection.Implementers may
want to give the user control of
this precedence negotiation.
For example, the user might be
allowed to specify that the
precedence must be exactly matched,
or that any attempt to raise
the precedence be confirmed by the
user.
A local connection name will
be returned to the user by the TCP.
The local connection name can
then be used as a short hand term
for the connection defined by
the <local socket, foreign socket>
pair.
Send
Format: SEND (local
connection name, buffer address, byte
count, PUSH flag, URGENT flag
[,timeout])
This call causes the data
contained in the indicated user buffer
to be sent on the indicated
connection. If the connection has
not been opened, the SEND is
considered an error. Some
implementations may allow
users to SEND first; in which case, an
automatic OPEN would be done.
If the calling process is not
authorized to use this
connection, an error is returned.
If the PUSH flag is set, the
data must be transmitted promptly
to the receiver, and the PUSH
bit will be set in the last TCP
segment created from the
buffer. If the PUSH flag is not set,
the data may be combined with
data from subsequent SENDs for
transmission efficiency.
If the URGENT flag is set,
segments sent to the destination TCP
will have the urgent pointer
set. The receiving TCP will signal
the urgent condition to the
receiving process if the urgent
pointer indicates that data
preceding the urgent pointer has not
been consumed by the
receiving process. The purpose of urgent
is to stimulate the receiver
to process the urgent data and to
indicate to the receiver when
all the currently known urgent
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Transmission Control Protocol
Functional Specification
data has been received. The
number of times the sending user's
TCP signals urgent will not
necessarily be equal to the number
of times the receiving user
will be notified of the presence of
urgent data.
If no foreign socket was
specified in the OPEN, but the
connection is established
(e.g., because a LISTENing connection
has become specific due to a
foreign segment arriving for the
local socket), then the
designated buffer is sent to the implied
foreign socket. Users who
make use of OPEN with an unspecified
foreign socket can make use
of SEND without ever explicitly
knowing the foreign socket
address.
However, if a SEND is
attempted before the foreign socket
becomes specified, an error
will be returned. Users can use the
STATUS call to determine the
status of the connection. In some
implementations the TCP may
notify the user when an unspecified
socket is bound.
If a timeout is specified,
the current user timeout for this
connection is changed to the
new one.
In the simplest
implementation, SEND would not return control to
the sending process until
either the transmission was complete
or the timeout had been
exceeded. However, this simple method
is both subject to deadlocks
(for example, both sides of the
connection might try to do
SENDs before doing any RECEIVEs) and
offers poor performance, so
it is not recommended. A more
sophisticated implementation
would return immediately to allow
the process to run
concurrently with network I/O, and,
furthermore, to allow
multiple SENDs to be in progress.
Multiple SENDs are served in
first come, first served order, so
the TCP will queue those it
cannot service immediately.
We have implicitly assumed an
asynchronous user interface in
which a SEND later elicits
some kind of SIGNAL or
pseudo-interrupt from the
serving TCP. An alternative is to
return a response
immediately. For instance, SENDs might return
immediate local
acknowledgment, even if the segment sent had not
been acknowledged by the
distant TCP. We could optimistically
assume eventual success. If
we are wrong, the connection will
close anyway due to the
timeout. In implementations of this
kind (synchronous), there
will still be some asynchronous
signals, but these will deal
with the connection itself, and not
with specific segments or
buffers.
In order for the process to
distinguish among error or success
indications for different
SENDs, it might be appropriate for the
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Transmission Control Protocol
Functional Specification
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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Transmission Control Protocol
Functional Specification
allocate buffer storage, or
the TCP might share a ring buffer
with the user.
Close
Format: CLOSE (local
connection name)
This command causes the
connection specified to be closed. If
the connection is not open or
the calling process is not
authorized to use this
connection, an error is returned.
Closing connections is
intended to be a graceful operation in
the sense that outstanding
SENDs will be transmitted (and
retransmitted), as flow
control permits, until all have been
serviced. Thus, it should be
acceptable to make several SEND
calls, followed by a CLOSE,
and expect all the data to be sent
to the destination. It should
also be clear that users should
continue to RECEIVE on
CLOSING connections, since the other side
may be trying to transmit the
last of its data. Thus, CLOSE
means "I have no more to
send" but does not mean "I will not
receive any more." It
may happen (if the user level protocol is
not well thought out) that
the closing side is unable to get rid
of all its data before timing
out. In this event, CLOSE turns
into ABORT, and the closing
TCP gives up.
The user may CLOSE the
connection at any time on his own
initiative, or in response to
various prompts from the TCP
(e.g., remote close executed,
transmission timeout exceeded,
destination inaccessible).
Because closing a connection
requires communication with the
foreign TCP, connections may
remain in the closing state for a
short time. Attempts to
reopen the connection before the TCP
replies to the CLOSE command
will result in error responses.
Close also implies push
function.
Status
Format: STATUS (local
connection name) -> status data
This is an implementation
dependent user command and could be
excluded without adverse
effect. Information returned would
typically come from the TCB
associated with the connection.
This command returns a data
block containing the following
information:
local socket,
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Transmission Control Protocol
Functional Specification
foreign socket,
local connection name,
receive window,
send window,
connection state,
number of buffers awaiting
acknowledgment,
number of buffers pending
receipt,
urgent state,
precedence,
security/compartment,
and transmission timeout.
Depending on the state of the
connection, or on the
implementation itself, some
of this information may not be
available or meaningful. If
the calling process is not
authorized to use this
connection, an error is returned. This
prevents unauthorized
processes from gaining information about a
connection.
Abort
Format: ABORT (local
connection name)
This command causes all
pending SENDs and RECEIVES to be
aborted, the TCB to be
removed, and a special RESET message to
be sent to the TCP on the
other side of the connection.
Depending on the
implementation, users may receive abort
indications for each
outstanding SEND or RECEIVE, or may simply
receive an
ABORT-acknowledgment.
TCP-to-User Messages
It is assumed that the
operating system environment provides a
means for the TCP to
asynchronously signal the user program. When
the TCP does signal a user
program, certain information is passed
to the user. Often in the
specification the information will be
an error message. In other
cases there will be information
relating to the completion of
processing a SEND or RECEIVE or
other user call.
The following information is
provided:
Local Connection Name Always
Response String Always
Buffer Address Send &
Receive
Byte count (counts bytes
received) Receive
Push flag Receive
Urgent flag Receive
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Transmission Control Protocol
Functional Specification
TCP/Lower-Level Interface
The TCP calls on a lower
level protocol module to actually send and
receive information over a
network. One case is that of the ARPA
internetwork system where the
lower level module is the Internet
Protocol (IP) [2].
If the lower level protocol
is IP it provides arguments for a type
of service and for a time to
live. TCP uses the following settings
for these parameters:
Type of Service = Precedence:
routine, Delay: normal, Throughput:
normal, Reliability: normal;
or 00000000.
Time to Live = one minute, or
00111100.
Note that the assumed maximum
segment lifetime is two minutes.
Here we explicitly ask that a
segment be destroyed if it cannot
be delivered by the internet
system within one minute.
If the lower level is IP (or
other protocol that provides this
feature) and source routing
is used, the interface must allow the
route information to be
communicated. This is especially important
so that the source and
destination addresses used in the TCP
checksum be the originating
source and ultimate destination. It is
also important to preserve
the return route to answer connection
requests.
Any lower level protocol will
have to provide the source address,
destination address, and
protocol fields, and some way to determine
the "TCP length",
both to provide the functional equivlent service
of IP and to be used in the
TCP checksum.
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Transmission Control Protocol
Functional Specification
3.9. Event Processing
The processing depicted in
this section is an example of one possible
implementation. Other
implementations may have slightly different
processing sequences, but
they should differ from those in this
section only in detail, not
in substance.
The activity of the TCP can
be characterized as responding to events.
The events that occur can be
cast into three categories: user calls,
arriving segments, and
timeouts. This section describes the
processing the TCP does in
response to each of the events. In many
cases the processing required
depends on the state of the connection.
Events that occur:
User Calls
OPEN
SEND
RECEIVE
CLOSE
ABORT
STATUS
Arriving Segments
SEGMENT ARRIVES
Timeouts
USER TIMEOUT
RETRANSMISSION TIMEOUT
TIME-WAIT TIMEOUT
The model of the TCP/user
interface is that user commands receive an
immediate return and possibly
a delayed response via an event or
pseudo interrupt. In the
following descriptions, the term "signal"
means cause a delayed
response.
Error responses are given as
character strings. For example, user
commands referencing
connections that do not exist receive "error:
connection not open".
Please note in the following
that all arithmetic on sequence numbers,
acknowledgment numbers,
windows, et cetera, is modulo 2**32 the size
of the sequence number space.
Also note that "=<" means less than or
equal to (modulo 2**32).
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Transmission Control Protocol
Functional Specification
A natural way to think about
processing incoming segments is to
imagine that they are first
tested for proper sequence number (i.e.,
that their contents lie in
the range of the expected "receive window"
in the sequence number space)
and then that they are generally queued
and processed in sequence
number order.
When a segment overlaps other
already received segments we reconstruct
the segment to contain just
the new data, and adjust the header fields
to be consistent.
Note that if no state change
is mentioned the TCP stays in the same
state.
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Transmission Control Protocol
Functional Specification
OPEN Call
OPEN Call
CLOSED STATE (i.e., TCB does
not exist)
Create a new transmission
control block (TCB) to hold connection
state information. Fill in
local socket identifier, foreign
socket, precedence,
security/compartment, and user timeout
information. Note that some
parts of the foreign socket may be
unspecified in a passive OPEN
and are to be filled in by the
parameters of the incoming
SYN segment. Verify the security and
precedence requested are
allowed for this user, if not return
"error: precedence not
allowed" or "error: security/compartment
not allowed." If passive
enter the LISTEN state and return. If
active and the foreign socket
is unspecified, return "error:
foreign socket
unspecified"; if active and the foreign socket is
specified, issue a SYN
segment. An initial send sequence number
(ISS) is selected. A SYN
segment of the form <SEQ=ISS><CTL=SYN>
is sent. Set SND.UNA to ISS,
SND.NXT to ISS+1, enter SYN-SENT
state, and return.
If the caller does not have
access to the local socket specified,
return "error:
connection illegal for this process". If there is
no room to create a new
connection, return "error: insufficient
resources".
LISTEN STATE
If active and the foreign
socket is specified, then change the
connection from passive to
active, select an ISS. Send a SYN
segment, set SND.UNA to ISS,
SND.NXT to ISS+1. Enter SYN-SENT
state. Data associated with
SEND may be sent with SYN segment or
queued for transmission after
entering ESTABLISHED state. The
urgent bit if requested in
the command must be sent with the data
segments sent as a result of
this command. If there is no room to
queue the request, respond
with "error: insufficient resources".
If Foreign socket was not
specified, then return "error: foreign
socket unspecified".
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Transmission Control Protocol
Functional Specification
OPEN Call
SYN-SENT STATE
SYN-RECEIVED STATE
ESTABLISHED STATE
FIN-WAIT-1 STATE
FIN-WAIT-2 STATE
CLOSE-WAIT STATE
CLOSING STATE
LAST-ACK STATE
TIME-WAIT STATE
Return "error:
connection already exists".
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Transmission Control Protocol
Functional Specification
SEND Call
SEND Call
CLOSED STATE (i.e., TCB does
not exist)
If the user does not have
access to such a connection, then return
"error: connection
illegal for this process".
Otherwise, return
"error: connection does not exist".
LISTEN STATE
If the foreign socket is
specified, then change the connection
from passive to active,
select an ISS. Send a SYN segment, set
SND.UNA to ISS, SND.NXT to
ISS+1. Enter SYN-SENT state. Data
associated with SEND may be
sent with SYN segment or queued for
transmission after entering
ESTABLISHED state. The urgent bit if
requested in the command must
be sent with the data segments sent
as a result of this command.
If there is no room to queue the
request, respond with
"error: insufficient resources". If
Foreign socket was not
specified, then return "error: foreign
socket unspecified".
SYN-SENT STATE
SYN-RECEIVED STATE
Queue the data for
transmission after entering ESTABLISHED state.
If no space to queue, respond
with "error: insufficient
resources".
ESTABLISHED STATE
CLOSE-WAIT STATE
Segmentize the buffer and
send it with a piggybacked
acknowledgment
(acknowledgment value = RCV.NXT). If there is
insufficient space to
remember this buffer, simply return "error:
insufficient resources".
If the urgent flag is set,
then SND.UP <- SND.NXT-1 and set the
urgent pointer in the
outgoing segments.
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Transmission Control Protocol
Functional Specification
SEND Call
FIN-WAIT-1 STATE
FIN-WAIT-2 STATE
CLOSING STATE
LAST-ACK STATE
TIME-WAIT STATE
Return "error:
connection closing" and do not service request.
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Transmission Control Protocol
Functional Specification
RECEIVE Call
RECEIVE Call
CLOSED STATE (i.e., TCB does
not exist)
If the user does not have
access to such a connection, return
"error: connection
illegal for this process".
Otherwise return "error:
connection does not exist".
LISTEN STATE
SYN-SENT STATE
SYN-RECEIVED STATE
Queue for processing after
entering ESTABLISHED state. If there
is no room to queue this
request, respond with "error:
insufficient resources".
ESTABLISHED STATE
FIN-WAIT-1 STATE
FIN-WAIT-2 STATE
If insufficient incoming
segments are queued to satisfy the
request, queue the request.
If there is no queue space to
remember the RECEIVE, respond
with "error: insufficient
resources".
Reassemble queued incoming
segments into receive buffer and return
to user. Mark "push
seen" (PUSH) if this is the case.
If RCV.UP is in advance of
the data currently being passed to the
user notify the user of the
presence of urgent data.
When the TCP takes
responsibility for delivering data to the user
that fact must be
communicated to the sender via an
acknowledgment. The formation
of such an acknowledgment is
described below in the
discussion of processing an incoming
segment.
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Transmission Control Protocol
Functional Specification
RECEIVE Call
CLOSE-WAIT STATE
Since the remote side has
already sent FIN, RECEIVEs must be
satisfied by text already on
hand, but not yet delivered to the
user. If no text is awaiting
delivery, the RECEIVE will get a
"error: connection
closing" response. Otherwise, any remaining
text can be used to satisfy
the RECEIVE.
CLOSING STATE
LAST-ACK STATE
TIME-WAIT STATE
Return "error:
connection closing".
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Transmission Control Protocol
Functional Specification
CLOSE Call
CLOSE Call
CLOSED STATE (i.e., TCB does
not exist)
If the user does not have
access to such a connection, return
"error: connection
illegal for this process".
Otherwise, return
"error: connection does not exist".
LISTEN STATE
Any outstanding RECEIVEs are
returned with "error: closing"
responses. Delete TCB, enter
CLOSED state, and return.
SYN-SENT STATE
Delete the TCB and return
"error: closing" responses to any
queued SENDs, or RECEIVEs.
SYN-RECEIVED STATE
If no SENDs have been issued
and there is no pending data to send,
then form a FIN segment and
send it, and enter FIN-WAIT-1 state;
otherwise queue for
processing after entering ESTABLISHED state.
ESTABLISHED STATE
Queue this until all
preceding SENDs have been segmentized, then
form a FIN segment and send
it. In any case, enter FIN-WAIT-1
state.
FIN-WAIT-1 STATE
FIN-WAIT-2 STATE
Strictly speaking, this is an
error and should receive a "error:
connection closing"
response. An "ok" response would be
acceptable, too, as long as a
second FIN is not emitted (the first
FIN may be retransmitted
though).
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Transmission Control Protocol
Functional Specification
CLOSE Call
CLOSE-WAIT STATE
Queue this request until all
preceding SENDs have been
segmentized; then send a FIN
segment, enter CLOSING state.
CLOSING STATE
LAST-ACK STATE
TIME-WAIT STATE
Respond with "error:
connection closing".
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Transmission Control Protocol
Functional Specification
ABORT Call
ABORT Call
CLOSED STATE (i.e., TCB does
not exist)
If the user should not have
access to such a connection, return
"error: connection
illegal for this process".
Otherwise return "error:
connection does not exist".
LISTEN STATE
Any outstanding RECEIVEs
should be returned with "error:
connection reset"
responses. Delete TCB, enter CLOSED state, and
return.
SYN-SENT STATE
All queued SENDs and RECEIVEs
should be given "connection reset"
notification, delete the TCB,
enter CLOSED state, and return.
SYN-RECEIVED STATE
ESTABLISHED STATE
FIN-WAIT-1 STATE
FIN-WAIT-2 STATE
CLOSE-WAIT STATE
Send a reset segment:
<SEQ=SND.NXT><CTL=RST>
All queued SENDs and RECEIVEs
should be given "connection reset"
notification; all segments
queued for transmission (except for the
RST formed above) or
retransmission should be flushed, delete the
TCB, enter CLOSED state, and
return.
CLOSING STATE
LAST-ACK STATE
TIME-WAIT STATE
Respond with "ok"
and delete the TCB, enter CLOSED state, and
return.
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Transmission Control Protocol
Functional Specification
STATUS Call
STATUS Call
CLOSED STATE (i.e., TCB does
not exist)
If the user should not have
access to such a connection, return
"error: connection
illegal for this process".
Otherwise return "error:
connection does not exist".
LISTEN STATE
Return "state =
LISTEN", and the TCB pointer.
SYN-SENT STATE
Return "state =
SYN-SENT", and the TCB pointer.
SYN-RECEIVED STATE
Return "state =
SYN-RECEIVED", and the TCB pointer.
ESTABLISHED STATE
Return "state =
ESTABLISHED", and the TCB pointer.
FIN-WAIT-1 STATE
Return "state =
FIN-WAIT-1", and the TCB pointer.
FIN-WAIT-2 STATE
Return "state =
FIN-WAIT-2", and the TCB pointer.
CLOSE-WAIT STATE
Return "state =
CLOSE-WAIT", and the TCB pointer.
CLOSING STATE
Return "state =
CLOSING", and the TCB pointer.
LAST-ACK STATE
Return "state =
LAST-ACK", and the TCB pointer.
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Transmission Control Protocol
Functional Specification
STATUS Call
TIME-WAIT STATE
Return "state =
TIME-WAIT", and the TCB pointer.
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Transmission Control Protocol
Functional Specification
SEGMENT ARRIVES
SEGMENT ARRIVES
If the state is CLOSED (i.e.,
TCB does not exist) then
all data in the incoming
segment is discarded. An incoming
segment containing a RST is
discarded. An incoming segment not
containing a RST causes a RST
to be sent in response. The
acknowledgment and sequence
field values are selected to make the
reset sequence acceptable to
the TCP that sent the offending
segment.
If the ACK bit is off,
sequence number zero is used,
<SEQ=0><ACK=SEG.SEQ+SEG.LEN><CTL=RST,ACK>
If the ACK bit is on,
<SEQ=SEG.ACK><CTL=RST>
Return.
If the state is LISTEN then
first check for an RST
An incoming RST should be
ignored. Return.
second check for an ACK
Any acknowledgment is bad if
it arrives on a connection still in
the LISTEN state. An
acceptable reset segment should be formed
for any arriving ACK-bearing
segment. The RST should be
formatted as follows:
<SEQ=SEG.ACK><CTL=RST>
Return.
third check for a SYN
If the SYN bit is set, check
the security. If the
security/compartment on the
incoming segment does not exactly
match the
security/compartment in the TCB then send a reset and
return.
<SEQ=SEG.ACK><CTL=RST>
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Transmission Control Protocol
Functional Specification
SEGMENT ARRIVES
If the SEG.PRC is greater
than the TCB.PRC then if allowed by
the user and the system set
TCB.PRC<-SEG.PRC, if not allowed
send a reset and return.
<SEQ=SEG.ACK><CTL=RST>
If the SEG.PRC is less than
the TCB.PRC then continue.
Set RCV.NXT to SEG.SEQ+1, IRS
is set to SEG.SEQ and any other
control or text should be
queued for processing later. ISS
should be selected and a SYN
segment sent of the form:
<SEQ=ISS><ACK=RCV.NXT><CTL=SYN,ACK>
SND.NXT is set to ISS+1 and
SND.UNA to ISS. The connection
state should be changed to
SYN-RECEIVED. Note that any other
incoming control or data
(combined with SYN) will be processed
in the SYN-RECEIVED state,
but processing of SYN and ACK should
not be repeated. If the
listen was not fully specified (i.e.,
the foreign socket was not
fully specified), then the
unspecified fields should be
filled in now.
fourth other text or control
Any other control or
text-bearing segment (not containing SYN)
must have an ACK and thus
would be discarded by the ACK
processing. An incoming RST
segment could not be valid, since
it could not have been sent
in response to anything sent by this
incarnation of the
connection. So you are unlikely to get here,
but if you do, drop the
segment, and return.
If the state is SYN-SENT then
first check the ACK bit
If the ACK bit is set
If SEG.ACK =< ISS, or
SEG.ACK > SND.NXT, send a reset (unless
the RST bit is set, if so
drop the segment and return)
<SEQ=SEG.ACK><CTL=RST>
and discard the segment.
Return.
If SND.UNA =< SEG.ACK
=< SND.NXT then the ACK is acceptable.
second check the RST bit
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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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September 1981
Transmission Control Protocol
Functional Specification
SEGMENT ARRIVES
are acceptable then, RCV.NXT
is set to SEG.SEQ+1, IRS is set to
SEG.SEQ. SND.UNA should be
advanced to equal SEG.ACK (if there
is an ACK), and any segments
on the retransmission queue which
are thereby acknowledged
should be removed.
If SND.UNA > ISS (our SYN
has been ACKed), change the connection
state to ESTABLISHED, form an
ACK segment
<SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK>
and send it. Data or controls
which were queued for
transmission may be included.
If there are other controls or
text in the segment then
continue processing at the sixth step
below where the URG bit is
checked, otherwise return.
Otherwise enter SYN-RECEIVED,
form a SYN,ACK segment
<SEQ=ISS><ACK=RCV.NXT><CTL=SYN,ACK>
and send it. If there are
other controls or text in the
segment, queue them for
processing after the ESTABLISHED state
has been reached, return.
fifth, if neither of the SYN
or RST bits is set then drop the
segment and return.
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Functional Specification
SEGMENT ARRIVES
Otherwise,
first check sequence number
SYN-RECEIVED STATE
ESTABLISHED STATE
FIN-WAIT-1 STATE
FIN-WAIT-2 STATE
CLOSE-WAIT STATE
CLOSING STATE
LAST-ACK STATE
TIME-WAIT STATE
Segments are processed in
sequence. Initial tests on arrival
are used to discard old
duplicates, but further processing is
done in SEG.SEQ order. If a
segment's contents straddle the
boundary between old and new,
only the new parts should be
processed.
There are four cases for the
acceptability test for an incoming
segment:
Segment Receive Test
Length Window
------- -------
-------------------------------------------
0 0 SEG.SEQ = RCV.NXT
0 >0 RCV.NXT =< SEG.SEQ
< RCV.NXT+RCV.WND
>0 0 not acceptable
>0 >0 RCV.NXT =<
SEG.SEQ < RCV.NXT+RCV.WND
or RCV.NXT =<
SEG.SEQ+SEG.LEN-1 < RCV.NXT+RCV.WND
If the RCV.WND is zero, no
segments will be acceptable, but
special allowance should be
made to accept valid ACKs, URGs and
RSTs.
If an incoming segment is not
acceptable, an acknowledgment
should be sent in reply
(unless the RST bit is set, if so drop
the segment and return):
<SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK>
After sending the
acknowledgment, drop the unacceptable segment
and return.
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September 1981
Transmission Control Protocol
Functional Specification
SEGMENT ARRIVES
In the following it is
assumed that the segment is the idealized
segment that begins at
RCV.NXT and does not exceed the window.
One could tailor actual
segments to fit this assumption by
trimming off any portions
that lie outside the window (including
SYN and FIN), and only
processing further if the segment then
begins at RCV.NXT. Segments
with higher begining sequence
numbers may be held for later
processing.
second check the RST bit,
SYN-RECEIVED STATE
If the RST bit is set
If this connection was
initiated with a passive OPEN (i.e.,
came from the LISTEN state),
then return this connection to
LISTEN state and return. The
user need not be informed. If
this connection was initiated
with an active OPEN (i.e., came
from SYN-SENT state) then the
connection was refused, signal
the user "connection
refused". In either case, all segments
on the retransmission queue
should be removed. And in the
active OPEN case, enter the
CLOSED state and delete the TCB,
and return.
ESTABLISHED
FIN-WAIT-1
FIN-WAIT-2
CLOSE-WAIT
If the RST bit is set then,
any outstanding RECEIVEs and SEND
should receive
"reset" responses. All segment queues should be
flushed. Users should also
receive an unsolicited general
"connection reset"
signal. Enter the CLOSED state, delete the
TCB, and return.
CLOSING STATE
LAST-ACK STATE
TIME-WAIT
If the RST bit is set then,
enter the CLOSED state, delete the
TCB, and return.
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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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Transmission Control Protocol
Functional Specification
SEGMENT ARRIVES
fifth check the ACK field,
if the ACK bit is off drop
the segment and return
if the ACK bit is on
SYN-RECEIVED STATE
If SND.UNA =< SEG.ACK
=< SND.NXT then enter ESTABLISHED state
and continue processing.
If the segment acknowledgment
is not acceptable, form a
reset segment,
<SEQ=SEG.ACK><CTL=RST>
and send it.
ESTABLISHED STATE
If SND.UNA < SEG.ACK =<
SND.NXT then, set SND.UNA <- SEG.ACK.
Any segments on the
retransmission queue which are thereby
entirely acknowledged are
removed. Users should receive
positive acknowledgments for
buffers which have been SENT and
fully acknowledged (i.e.,
SEND buffer should be returned with
"ok" response). If
the ACK is a duplicate
(SEG.ACK < SND.UNA), it
can be ignored. If the ACK acks
something not yet sent
(SEG.ACK > SND.NXT) then send an ACK,
drop the segment, and return.
If SND.UNA < SEG.ACK =<
SND.NXT, the send window should be
updated. If (SND.WL1 <
SEG.SEQ or (SND.WL1 = SEG.SEQ and
SND.WL2 =< SEG.ACK)), set
SND.WND <- SEG.WND, set
SND.WL1 <- SEG.SEQ, and
set SND.WL2 <- SEG.ACK.
Note that SND.WND is an
offset from SND.UNA, that SND.WL1
records the sequence number
of the last segment used to update
SND.WND, and that SND.WL2
records the acknowledgment number of
the last segment used to
update SND.WND. The check here
prevents using old segments
to update the window.
[Page 72]
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Transmission Control Protocol
Functional Specification
SEGMENT ARRIVES
FIN-WAIT-1 STATE
In addition to the processing
for the ESTABLISHED state, if
our FIN is now acknowledged
then enter FIN-WAIT-2 and continue
processing in that state.
FIN-WAIT-2 STATE
In addition to the processing
for the ESTABLISHED state, if
the retransmission queue is
empty, the user's CLOSE can be
acknowledged ("ok")
but do not delete the TCB.
CLOSE-WAIT STATE
Do the same processing as for
the ESTABLISHED state.
CLOSING STATE
In addition to the processing
for the ESTABLISHED state, if
the ACK acknowledges our FIN
then enter the TIME-WAIT state,
otherwise ignore the segment.
LAST-ACK STATE
The only thing that can
arrive in this state is an
acknowledgment of our FIN. If
our FIN is now acknowledged,
delete the TCB, enter the
CLOSED state, and return.
TIME-WAIT STATE
The only thing that can
arrive in this state is a
retransmission of the remote
FIN. Acknowledge it, and restart
the 2 MSL timeout.
sixth, check the URG bit,
ESTABLISHED STATE
FIN-WAIT-1 STATE
FIN-WAIT-2 STATE
If the URG bit is set, RCV.UP
<- max(RCV.UP,SEG.UP), and signal
the user that the remote side
has urgent data if the urgent
pointer (RCV.UP) is in
advance of the data consumed. If the
user has already been
signaled (or is still in the "urgent
mode") for this
continuous sequence of urgent data, do not
signal the user again.
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Transmission Control Protocol
Functional Specification
SEGMENT ARRIVES
CLOSE-WAIT STATE
CLOSING STATE
LAST-ACK STATE
TIME-WAIT
This should not occur, since
a FIN has been received from the
remote side. Ignore the URG.
seventh, process the segment
text,
ESTABLISHED STATE
FIN-WAIT-1 STATE
FIN-WAIT-2 STATE
Once in the ESTABLISHED
state, it is possible to deliver segment
text to user RECEIVE buffers.
Text from segments can be moved
into buffers until either the
buffer is full or the segment is
empty. If the segment empties
and carries an PUSH flag, then
the user is informed, when
the buffer is returned, that a PUSH
has been received.
When the TCP takes
responsibility for delivering the data to the
user it must also acknowledge
the receipt of the data.
Once the TCP takes
responsibility for the data it advances
RCV.NXT over the data
accepted, and adjusts RCV.WND as
apporopriate to the current
buffer availability. The total of
RCV.NXT and RCV.WND should
not be reduced.
Please note the window
management suggestions in section 3.7.
Send an acknowledgment of the
form:
<SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK>
This acknowledgment should be
piggybacked on a segment being
transmitted if possible
without incurring undue delay.
[Page 74]
September 1981
Transmission Control Protocol
Functional Specification
SEGMENT ARRIVES
CLOSE-WAIT STATE
CLOSING STATE
LAST-ACK STATE
TIME-WAIT STATE
This should not occur, since
a FIN has been received from the
remote side. Ignore the
segment text.
eighth, check the FIN bit,
Do not process the FIN if the
state is CLOSED, LISTEN or SYN-SENT
since the SEG.SEQ cannot be
validated; drop the segment and
return.
If the FIN bit is set, signal
the user "connection closing" and
return any pending RECEIVEs
with same message, advance RCV.NXT
over the FIN, and send an
acknowledgment for the FIN. Note that
FIN implies PUSH for any
segment text not yet delivered to the
user.
SYN-RECEIVED STATE
ESTABLISHED STATE
Enter the CLOSE-WAIT state.
FIN-WAIT-1 STATE
If our FIN has been ACKed
(perhaps in this segment), then
enter TIME-WAIT, start the
time-wait timer, turn off the other
timers; otherwise enter the
CLOSING state.
FIN-WAIT-2 STATE
Enter the TIME-WAIT state.
Start the time-wait timer, turn
off the other timers.
CLOSE-WAIT STATE
Remain in the CLOSE-WAIT
state.
CLOSING STATE
Remain in the CLOSING state.
LAST-ACK STATE
Remain in the LAST-ACK state.
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Transmission Control Protocol
Functional Specification
SEGMENT ARRIVES
TIME-WAIT STATE
Remain in the TIME-WAIT
state. Restart the 2 MSL time-wait
timeout.
and return.
[Page 76]
September 1981
Transmission Control Protocol
Functional Specification
USER TIMEOUT
USER TIMEOUT
For any state if the user
timeout expires, flush all queues, signal
the user "error:
connection aborted due to user timeout" in general
and for any outstanding
calls, delete the TCB, enter the CLOSED
state and return.
RETRANSMISSION TIMEOUT
For any state if the
retransmission timeout expires on a segment in
the retransmission queue,
send the segment at the front of the
retransmission queue again,
reinitialize the retransmission timer,
and return.
TIME-WAIT TIMEOUT
If the time-wait timeout
expires on a connection delete the TCB,
enter the CLOSED state and
return.
[Page 77]
September 1981
Transmission Control Protocol
[Page 78]
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Transmission Control Protocol
GLOSSARY
1822
BBN Report 1822, "The
Specification of the Interconnection of
a Host and an IMP". The
specification of interface between a
host and the ARPANET.
ACK
A control bit (acknowledge)
occupying no sequence space, which
indicates that the
acknowledgment field of this segment
specifies the next sequence
number the sender of this segment
is expecting to receive,
hence acknowledging receipt of all
previous sequence numbers.
ARPANET message
The unit of transmission
between a host and an IMP in the
ARPANET. The maximum size is
about 1012 octets (8096 bits).
ARPANET packet
A unit of transmission used
internally in the ARPANET between
IMPs. The maximum size is
about 126 octets (1008 bits).
connection
A logical communication path
identified by a pair of sockets.
datagram
A message sent in a packet
switched computer communications
network.
Destination Address
The destination address,
usually the network and host
identifiers.
FIN
A control bit (finis)
occupying one sequence number, which
indicates that the sender
will send no more data or control
occupying sequence space.
fragment
A portion of a logical unit
of data, in particular an internet
fragment is a portion of an
internet datagram.
FTP
A file transfer protocol.
[Page 79]
September 1981
Transmission Control Protocol
Glossary
header
Control information at the
beginning of a message, segment,
fragment, packet or block of
data.
host
A computer. In particular a
source or destination of messages
from the point of view of the
communication network.
Identification
An Internet Protocol field.
This identifying value assigned
by the sender aids in
assembling the fragments of a datagram.
IMP
The Interface Message
Processor, the packet switch of the
ARPANET.
internet address
A source or destination
address specific to the host level.
internet datagram
The unit of data exchanged
between an internet module and the
higher level protocol
together with the internet header.
internet fragment
A portion of the data of an
internet datagram with an internet
header.
IP
Internet Protocol.
IRS
The Initial Receive Sequence
number. The first sequence
number used by the sender on
a connection.
ISN
The Initial Sequence Number.
The first sequence number used
on a connection, (either ISS
or IRS). Selected on a clock
based procedure.
ISS
The Initial Send Sequence
number. The first sequence number
used by the sender on a
connection.
leader
Control information at the
beginning of a message or block of
data. In particular, in the
ARPANET, the control information
on an ARPANET message at the
host-IMP interface.
[Page 80]
September 1981
Transmission Control Protocol
Glossary
left sequence
This is the next sequence
number to be acknowledged by the
data receiving TCP (or the
lowest currently unacknowledged
sequence number) and is
sometimes referred to as the left edge
of the send window.
local packet
The unit of transmission
within a local network.
module
An implementation, usually in
software, of a protocol or other
procedure.
MSL
Maximum Segment Lifetime, the
time a TCP segment can exist in
the internetwork system.
Arbitrarily defined to be 2 minutes.
octet
An eight bit byte.
Options
An Option field may contain
several options, and each option
may be several octets in
length. The options are used
primarily in testing
situations; for example, to carry
timestamps. Both the Internet
Protocol and TCP provide for
options fields.
packet
A package of data with a
header which may or may not be
logically complete. More
often a physical packaging than a
logical packaging of data.
port
The portion of a socket that
specifies which logical input or
output channel of a process
is associated with the data.
process
A program in execution. A
source or destination of data from
the point of view of the TCP
or other host-to-host protocol.
PUSH
A control bit occupying no
sequence space, indicating that
this segment contains data
that must be pushed through to the
receiving user.
RCV.NXT
receive next sequence number
[Page 81]
September 1981
Transmission Control Protocol
Glossary
RCV.UP
receive urgent pointer
RCV.WND
receive window
receive next sequence number
This is the next sequence
number the local TCP is expecting to
receive.
receive window
This represents the sequence
numbers the local (receiving) TCP
is willing to receive. Thus,
the local TCP considers that
segments overlapping the
range RCV.NXT to
RCV.NXT + RCV.WND - 1 carry
acceptable data or control.
Segments containing sequence
numbers entirely outside of this
range are considered
duplicates and discarded.
RST
A control bit (reset),
occupying no sequence space, indicating
that the receiver should
delete the connection without further
interaction. The receiver can
determine, based on the
sequence number and
acknowledgment fields of the incoming
segment, whether it should
honor the reset command or ignore
it. In no case does receipt
of a segment containing RST give
rise to a RST in response.
RTP
Real Time Protocol: A
host-to-host protocol for communication
of time critical information.
SEG.ACK
segment acknowledgment
SEG.LEN
segment length
SEG.PRC
segment precedence value
SEG.SEQ
segment sequence
SEG.UP
segment urgent pointer field
[Page 82]
September 1981
Transmission Control Protocol
Glossary
SEG.WND
segment window field
segment
A logical unit of data, in
particular a TCP segment is the
unit of data transfered
between a pair of TCP modules.
segment acknowledgment
The sequence number in the
acknowledgment field of the
arriving segment.
segment length
The amount of sequence number
space occupied by a segment,
including any controls which
occupy sequence space.
segment sequence
The number in the sequence
field of the arriving segment.
send sequence
This is the next sequence
number the local (sending) TCP will
use on the connection. It is
initially selected from an
initial sequence number curve
(ISN) and is incremented for
each octet of data or
sequenced control transmitted.
send window
This represents the sequence
numbers which the remote
(receiving) TCP is willing to
receive. It is the value of the
window field specified in
segments from the remote (data
receiving) TCP. The range of
new sequence numbers which may
be emitted by a TCP lies
between SND.NXT and
SND.UNA + SND.WND - 1.
(Retransmissions of sequence numbers
between SND.UNA and SND.NXT
are expected, of course.)
SND.NXT
send sequence
SND.UNA
left sequence
SND.UP
send urgent pointer
SND.WL1
segment sequence number at
last window update
SND.WL2
segment acknowledgment number
at last window update
[Page 83]
September 1981
Transmission Control Protocol
Glossary
SND.WND
send window
socket
An address which specifically
includes a port identifier, that
is, the concatenation of an
Internet Address with a TCP port.
Source Address
The source address, usually
the network and host identifiers.
SYN
A control bit in the incoming
segment, occupying one sequence
number, used at the
initiation of a connection, to indicate
where the sequence numbering
will start.
TCB
Transmission control block,
the data structure that records
the state of a connection.
TCB.PRC
The precedence of the
connection.
TCP
Transmission Control
Protocol: A host-to-host protocol for
reliable communication in
internetwork environments.
TOS
Type of Service, an Internet
Protocol field.
Type of Service
An Internet Protocol field
which indicates the type of service
for this internet fragment.
URG
A control bit (urgent),
occupying no sequence space, used to
indicate that the receiving
user should be notified to do
urgent processing as long as
there is data to be consumed with
sequence numbers less than
the value indicated in the urgent
pointer.
urgent pointer
A control field meaningful
only when the URG bit is on. This
field communicates the value
of the urgent pointer which
indicates the data octet
associated with the sending user's
urgent call.
[Page 84]
September 1981
Transmission Control Protocol
REFERENCES
[1] Cerf, V., and R. Kahn,
"A Protocol for Packet Network
Intercommunication",
IEEE Transactions on Communications,
Vol. COM-22, No. 5, pp
637-648, May 1974.
[2] Postel, J. (ed.),
"Internet Protocol - DARPA Internet Program
Protocol Specification",
RFC 791, USC/Information Sciences
Institute, September 1981.
[3] Dalal, Y. and C.
Sunshine, "Connection Management in Transport
Protocols", Computer
Networks, Vol. 2, No. 6, pp. 454-473,
December 1978.
[4] Postel, J.,
"Assigned Numbers", RFC 790, USC/Information
Sciences
Institute, September 1981.
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