📄 rfc793.txt
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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).
[Page 21]
September 1981
Transmission Control Protocol
Functional Specification
TIME-WAIT - represents waiting for enough time to pass to be sure
the remote TCP received the acknowledgment of its connection
termination request.
CLOSED - represents no connection state at all.
A TCP connection progresses from one state to another in response to
events. The events are the user calls, OPEN, SEND, RECEIVE, CLOSE,
ABORT, and STATUS; the incoming segments, particularly those
containing the SYN, ACK, RST and FIN flags; and timeouts.
The state diagram in figure 6 illustrates only state changes, together
with the causing events and resulting actions, but addresses neither
error conditions nor actions which are not connected with state
changes. In a later section, more detail is offered with respect to
the reaction of the TCP to events.
NOTE BENE: this diagram is only a summary and must not be taken as
the total specification.
[Page 22]
September 1981
Transmission Control Protocol
Functional Specification
+---------+ ---------\ active OPEN
| CLOSED | \ -----------
+---------+<---------\ \ create TCB
| ^ \ \ snd SYN
passive OPEN | | CLOSE \ \
------------ | | ---------- \ \
create TCB | | delete TCB \ \
V | \ \
+---------+ CLOSE | \
| LISTEN | ---------- | |
+---------+ delete TCB | |
rcv SYN | | SEND | |
----------- | | ------- | V
+---------+ snd SYN,ACK / \ snd SYN +---------+
| |<----------------- ------------------>| |
| SYN | rcv SYN | SYN |
| RCVD |<-----------------------------------------------| SENT |
| | snd ACK | |
| |------------------ -------------------| |
+---------+ rcv ACK of SYN \ / rcv SYN,ACK +---------+
| -------------- | | -----------
| x | | snd ACK
| V V
| CLOSE +---------+
| ------- | ESTAB |
| snd FIN +---------+
| CLOSE | | rcv FIN
V ------- | | -------
+---------+ snd FIN / \ snd ACK +---------+
| FIN |<----------------- ------------------>| CLOSE |
| WAIT-1 |------------------ | WAIT |
+---------+ rcv FIN \ +---------+
| rcv ACK of FIN ------- | CLOSE |
| -------------- snd ACK | ------- |
V x V snd FIN V
+---------+ +---------+ +---------+
|FINWAIT-2| | CLOSING | | LAST-ACK|
+---------+ +---------+ +---------+
| rcv ACK of FIN | rcv ACK of FIN |
| rcv FIN -------------- | Timeout=2MSL -------------- |
| ------- x V ------------ x V
\ snd ACK +---------+delete TCB +---------+
------------------------>|TIME WAIT|------------------>| CLOSED |
+---------+ +---------+
TCP Connection State Diagram
Figure 6.
[Page 23]
September 1981
Transmission Control Protocol
Functional Specification
3.3. Sequence Numbers
A fundamental notion in the design is that every octet of data sent
over a TCP connection has a sequence number. Since every octet is
sequenced, each of them can be acknowledged. The acknowledgment
mechanism employed is cumulative so that an acknowledgment of sequence
number X indicates that all octets up to but not including X have been
received. This mechanism allows for straight-forward duplicate
detection in the presence of retransmission. Numbering of octets
within a segment is that the first data octet immediately following
the header is the lowest numbered, and the following octets are
numbered consecutively.
It is essential to remember that the actual sequence number space is
finite, though very large. This space ranges from 0 to 2**32 - 1.
Since the space is finite, all arithmetic dealing with sequence
numbers must be performed modulo 2**32. This unsigned arithmetic
preserves the relationship of sequence numbers as they cycle from
2**32 - 1 to 0 again. There are some subtleties to computer modulo
arithmetic, so great care should be taken in programming the
comparison of such values. The symbol "=<" means "less than or equal"
(modulo 2**32).
The typical kinds of sequence number comparisons which the TCP must
perform include:
(a) Determining that an acknowledgment refers to some sequence
number sent but not yet acknowledged.
(b) Determining that all sequence numbers occupied by a segment
have been acknowledged (e.g., to remove the segment from a
retransmission queue).
(c) Determining that an incoming segment contains sequence numbers
which are expected (i.e., that the segment "overlaps" the
receive window).
[Page 24]
September 1981
Transmission Control Protocol
Functional Specification
In response to sending data the TCP will receive acknowledgments. The
following comparisons are needed to process the acknowledgments.
SND.UNA = oldest unacknowledged sequence number
SND.NXT = next sequence number to be sent
SEG.ACK = acknowledgment from the receiving TCP (next sequence
number expected by the receiving TCP)
SEG.SEQ = first sequence number of a segment
SEG.LEN = the number of octets occupied by the data in the segment
(counting SYN and FIN)
SEG.SEQ+SEG.LEN-1 = last sequence number of a segment
A new acknowledgment (called an "acceptable ack"), is one for which
the inequality below holds:
SND.UNA < SEG.ACK =< SND.NXT
A segment on the retransmission queue is fully acknowledged if the sum
of its sequence number and length is less or equal than the
acknowledgment value in the incoming segment.
When data is received the following comparisons are needed:
RCV.NXT = next sequence number expected on an incoming segments, and
is the left or lower edge of the receive window
RCV.NXT+RCV.WND-1 = last sequence number expected on an incoming
segment, and is the right or upper edge of the receive window
SEG.SEQ = first sequence number occupied by the incoming segment
SEG.SEQ+SEG.LEN-1 = last sequence number occupied by the incoming
segment
A segment is judged to occupy a portion of valid receive sequence
space if
RCV.NXT =< SEG.SEQ < RCV.NXT+RCV.WND
or
RCV.NXT =< SEG.SEQ+SEG.LEN-1 < RCV.NXT+RCV.WND
[Page 25]
September 1981
Transmission Control Protocol
Functional Specification
The first part of this test checks to see if the beginning of the
segment falls in the window, the second part of the test checks to see
if the end of the segment falls in the window; if the segment passes
either part of the test it contains data in the window.
Actually, it is a little more complicated than this. Due to zero
windows and zero length segments, we have four cases for the
acceptability of an incoming segment:
Segment Receive Test
Length Window
------- ------- -------------------------------------------
0 0 SEG.SEQ = RCV.NXT
0 >0 RCV.NXT =< SEG.SEQ < RCV.NXT+RCV.WND
>0 0 not acceptable
>0 >0 RCV.NXT =< SEG.SEQ < RCV.NXT+RCV.WND
or RCV.NXT =< SEG.SEQ+SEG.LEN-1 < RCV.NXT+RCV.WND
Note that when the receive window is zero no segments should be
acceptable except ACK segments. Thus, it is be possible for a TCP to
maintain a zero receive window while transmitting data and receiving
ACKs. However, even when the receive window is zero, a TCP must
process the RST and URG fields of all incoming segments.
We have taken advantage of the numbering scheme to protect certain
control information as well. This is achieved by implicitly including
some control flags in the sequence space so they can be retransmitted
and acknowledged without confusion (i.e., one and only one copy of the
control will be acted upon). Control information is not physically
carried in the segment data space. Consequently, we must adopt rules
for implicitly assigning sequence numbers to control. The SYN and FIN
are the only controls requiring this protection, and these controls
are used only at connection opening and closing. For sequence number
purposes, the SYN is considered to occur before the first actual data
octet of the segment in which it occurs, while the FIN is considered
to occur after the last actual data octet in a segment in which it
occurs. The segment length (SEG.LEN) includes both data and sequence
space occupying controls. When a SYN is present then SEG.SEQ is the
sequence number of the SYN.
[Page 26]
September 1981
Transmission Control Protocol
Functional Specification
Initial Sequence Number Selection
The protocol places no restriction on a particular connection being
used over and over again. A connection is defined by a pair of
sockets. New instances of a connection will be referred to as
incarnations of the connection. The problem that arises from this is
-- "how does the TCP identify duplicate segments from previous
incarnations of the connection?" This problem becomes apparent if the
connection is being opened and closed in quick succession, or if the
connection breaks with loss of memory and is then reestablished.
To avoid confusion we must prevent segments from one incarnation of a
connection from being used while the same sequence numbers may still
be present in the network from an earlier incarnation. We want to
assure this, even if a TCP crashes and loses all knowledge of the
sequence numbers it has been using. When new connections are created,
an initial sequence number (ISN) generator is employed which selects a
new 32 bit ISN. The generator is bound to a (possibly fictitious) 32
bit clock whose low order bit is incremented roughly every 4
microseconds. Thus, the ISN cycles approximately every 4.55 hours.
Since we assume that segments will stay in the network no more than
the Maximum Segment Lifetime (MSL) and that the MSL is less than 4.55
hours we can reasonably assume that ISN's will be unique.
For each connection there is a send sequence number and a receive
sequence number. The initial send sequence number (ISS) is chosen by
the data sending TCP, and the initial receive sequence number (IRS) is
learned during the connection establishing procedure.
For a connection to be established or initialized, the two TCPs must
synchronize on each other's initial sequence numbers. This is done in
an exchange of connection establishing segments carrying a control bit
called "SYN" (for synchronize) and the initial sequence numbers. As a
shorthand, segments carrying the SYN bit are also called "SYNs".
Hence, the solution requires a suitable mechanism for picking an
initial sequence number and a slightly involved handshake to exchange
the ISN's.
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