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