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.
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
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) . 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
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
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.
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
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
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
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
which fit the current window, and may repackage segments on the
retransmission queue. Such repackaging is not required, but may be
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
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
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.
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.
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,
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.
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.
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
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
The TCP or some component of the operating system will verify
the users authority to open a connection with the specified
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
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>
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
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
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
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
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
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
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
allocate buffer storage, or the TCP might share a ring buffer
with the user.
Format: CLOSE (local connection name)
This command causes the connection specified to be closed. If
the connection is not open or the calling process is not
authorized to use this connection, an error is returned.
Closing connections is intended to be a graceful operation in
the sense that outstanding SENDs will be transmitted (and
retransmitted), as flow control permits, until all have been
serviced. Thus, it should be acceptable to make several SEND
calls, followed by a CLOSE, and expect all the data to be sent
to the destination. It should also be clear that users should
continue to RECEIVE on CLOSING connections, since the other side
may be trying to transmit the last of its data. Thus, CLOSE
means "I have no more to send" but does not mean "I will not
receive any more." It may happen (if the user level protocol is
not well thought out) that the closing side is unable to get rid
of all its data before timing out. In this event, CLOSE turns
into ABORT, and the closing TCP gives up.
The user may CLOSE the connection at any time on his own
initiative, or in response to various prompts from the TCP
(e.g., remote close executed, transmission timeout exceeded,
Because closing a connection requires communication with the
foreign TCP, connections may remain in the closing state for a
short time. Attempts to reopen the connection before the TCP
replies to the CLOSE command will result in error responses.
Close also implies push function.
Format: STATUS (local connection name) -> status data
This is an implementation dependent user command and could be
excluded without adverse effect. Information returned would
typically come from the TCB associated with the connection.
This command returns a data block containing the following
local connection name,
number of buffers awaiting acknowledgment,
number of buffers pending receipt,
and transmission timeout.
Depending on the state of the connection, or on the
implementation itself, some of this information may not be
available or meaningful. If the calling process is not
authorized to use this connection, an error is returned. This
prevents unauthorized processes from gaining information about a
Format: ABORT (local connection name)
This command causes all pending SENDs and RECEIVES to be
aborted, the TCB to be removed, and a special RESET message to
be sent to the TCP on the other side of the connection.
Depending on the implementation, users may receive abort
indications for each outstanding SEND or RECEIVE, or may simply
receive an ABORT-acknowledgment.
It is assumed that the operating system environment provides a
means for the TCP to asynchronously signal the user program. When
the TCP does signal a user program, certain information is passed
to the user. Often in the specification the information will be
an error message. In other cases there will be information
relating to the completion of processing a SEND or RECEIVE or
other user call.
The following information is provided:
Local Connection Name Always
Response String Always
Buffer Address Send & Receive
Byte count (counts bytes received) Receive
Push flag Receive
Urgent flag Receive
The TCP calls on a lower level protocol module to actually send and
receive information over a network. One case is that of the ARPA
internetwork system where the lower level module is the Internet
Protocol (IP) .
If the lower level protocol is IP it provides arguments for a type
of service and for a time to live. TCP uses the following settings
for these parameters:
Type of Service = Precedence: routine, Delay: normal, Throughput:
normal, Reliability: normal; or 00000000.
Time to Live = one minute, or 00111100.
Note that the assumed maximum segment lifetime is two minutes.
Here we explicitly ask that a segment be destroyed if it cannot
be delivered by the internet system within one minute.
If the lower level is IP (or other protocol that provides this
feature) and source routing is used, the interface must allow the
route information to be communicated. This is especially important
so that the source and destination addresses used in the TCP
checksum be the originating source and ultimate destination. It is
also important to preserve the return route to answer connection
Any lower level protocol will have to provide the source address,
destination address, and protocol fields, and some way to determine
the "TCP length", both to provide the functional equivlent service
of IP and to be used in the TCP checksum.
3.9. Event Processing
The processing depicted in this section is an example of one possible
implementation. Other implementations may have slightly different
processing sequences, but they should differ from those in this
section only in detail, not in substance.
The activity of the TCP can be characterized as responding to events.
The events that occur can be cast into three categories: user calls,
arriving segments, and timeouts. This section describes the
processing the TCP does in response to each of the events. In many
cases the processing required depends on the state of the connection.
Events that occur:
The model of the TCP/user interface is that user commands receive an
immediate return and possibly a delayed response via an event or
pseudo interrupt. In the following descriptions, the term "signal"
means cause a delayed response.
Error responses are given as character strings. For example, user
commands referencing connections that do not exist receive "error:
connection not open".
Please note in the following that all arithmetic on sequence numbers,
acknowledgment numbers, windows, et cetera, is modulo 2**32 the size
of the sequence number space. Also note that "=<" means less than or
equal to (modulo 2**32).
A natural way to think about processing incoming segments is to
imagine that they are first tested for proper sequence number (i.e.,
that their contents lie in the range of the expected "receive window"
in the sequence number space) and then that they are generally queued
and processed in sequence number order.
When a segment overlaps other already received segments we reconstruct
the segment to contain just the new data, and adjust the header fields
to be consistent.
Note that if no state change is mentioned the TCP stays in the same