Request for Comments: 916 ISI
October 1984
RELIABLE ASYNCHRONOUS TRANSFER PROTOCOL (RATP)
Status of This Memo
This RFCsuggests a proposed protocol for the ARPA-Internet
community, and requests discussion and suggestions for improvements.
Distribution of this memo is unlimited.
This paper proposes and specifies a protocol which allows two
programs to reliably communicate over a communication link. It
ensures that the data entering one end of the link if received
arrives at the other end intact and unaltered. The protocol, named
RATP, is designed to operate over a full duplex point-to-point
connection. It contains some features which tailor it to the RS-232
links now in common use.
Introduction
We are witnessing today an explosive growth in the small or personal
computer market. Such inexpensive computers are not normally
connected to a computer network. They are most likely stand-alone
devices. But virtually all of them have an RS-232 interface. They
also usually have a modem. This allows them to communicate over the
telephone with any other similarly equipped computer.
The telephone system is a pervasive network, but one of the
characteristics of the telephone system is the unpredictable quality
of the circuit. The standard telephone circuit is designed for voice
communication and not data communication. Voice communication
tolerates a much higher degree of 'noise' than does a data circuit,
so a voice circuit is tolerant of a much higher level of noise than
is a data circuit. Thus it is not uncommon for a byte of data
transferred over a telephone circuit to have noise inserted. For the
same reason it is also not uncommon to have spurious data bytes added
to the data stream.
The need for a method of reliably transferring data over an RS-232
point-to-point link has become severe. As the number of powerful
personal computers grows, the need for them to communicate with one
another grows as well. The new markets and new services that these
computers will eventually allow their users to access will rely
heavily upon the telephone system. Services like electronic mail,
electronic banking, ordering merchandise from home with a personal
computer, etc. As the information revolution proceeds data itself
will become a commodity. All require accuracy of the data sent or
received.
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Reliable Asynchronous Transfer Protocol
1. Philosopy of Design
Many tradeoffs were made in designing this protocol. Decisions were
made by above all ensuring reliability and then by favoring
simplicity of implementation. It is hoped that this protocol is
simple enough to be implemented not only by small computers but also
by stand alone devices incorporating microcomputers which accept
commands over RS-232 lines. Sophisticated but unnecessary features
such as dynamic window management [TCP 81] were left out for
simplicity's sake. Having several packets outstanding at a time was
eliminated for the same reason, and data queued to send when a
connection is closed remotely is discarded. This eliminates two
states from the protocol implementation.
The reader may ask why define this protocol at all, there are after
all already RS-232 transport protocols in use. This is true but some
lack one or more features vitally important or are too complex. See
Appendix II for a brief survey.
- A protocol which can only transfer data in one direction is
unable to use a single RS-232 link for a full-duplex connection.
As such it cannot act as a bridge between most computer
networks. Also it is not capable of supporting any applications
requiring the two-way exchange of data. In particular it is not
a platform suitable for the creation of most higher level
applications. Unidirectional flow of data is sufficient for a
weak implementation of file transfer but insufficient for remote
terminal service, transaction oriented processing, etc.
- Some of the existing RS-232 transport protocols allow the use of
only fixed size packets or do not allow the receiver to place a
limit on the sender's packets. Where that block size is too
large for the receiving end concentrator, that concentrator is
likely to immediately invoke flow control. This results in many
dropped and damaged packets. The receiver must be able to
inform the sender at connection initiation what is the maximum
packet size it is prepared to receive.
- Some protocols have a number of features which may or may not be
implemented at each site. Examples are, several checksumming
algorithms, differing data transmission restrictions, sometimes
8-bit data, sometimes restricted ASCII subsets, etc. The
resulting requirement that all sites implement all the various
features is rarely met.
Finally, the size of this document may be imposing. The document
attempts to fully specify the behavior of the protocol. A careful
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exposition of the protocol's behavior under all circumstances is
necessary to answer any questions an implementor might have, to make
it possible to verify the protocol, etc. This size of this
specification should not be taken as an indication of the difficulty
of implementing it.
1.1. The Host Environment
This protocol is designed to operate on any point-to-point
communication link capable of transmitting and receiving data. It
is not necessary that the link be asynchronous. Because neither
end of a connection has control over when the other decides to
transmit, the link should be full duplex. It is expected that in
the vast majority of circumstances an asynchronous full-duplex
RS-232 link will be used.
In practice this protocol could reside anywhere from the RS-232
driver software on a microcomputer in a concentrator all the way
to the user software level. Ideally it properly resides inside
the host operating system or concentrator. It should be an option
associated with communication link which is selectable by the user
program. If reliable data transmission were of great importance
then the software would choose the option. Once the option were
chosen the initial connection handshaking would begin.
There are many cases where this protocol will not reside in a host
operating system (initially this will always be so). In addition
there are many pieces of stand-alone equipment which accept
commands over an RS-232 link. A plotter is such an example. To
have a several hour plot ruined by noise on an unreliable data
line is an all too often occurrence. The sending and receiving
sides of the protocol should be as simple as possible allowing
applications software and stand alone devices to utilize the
protocol with little penalty of time or space.
1.2. Relation to Other Protocols
The "layering" concept has become the accepted way of designing
communications protocols. Because this protocol will operate in a
point-to-point environment it comprises both the datagram and
reliable connection layers. No multi-network capability is
implied. Where a link using this protocol bridges differing
networks it is expected that other protocols like TCP will have
their packets fragmented and encapsulated inside the packets of
this protocol.
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2. Packet Specification
RATP transmits data over a full-duplex communication link. Data may
be transmitted in both directions over the link. A stream of data is
communicated by being broken up into 8-bit pieces called octets.
These octets are serially accumulated to form a packet. The packet
is the unit of data communicated over the link. The protocol
virtually guarantees that the data transmitted at one end, if
received, arrives unaltered and intact at the other end.
Within an octet all eight bits contain data. All eight bits must be
preserved by the link interface and associated device driver. In
many operating systems this is ensured by placing the connection into
RAW or BINARY data mode. During normal operation packets are
transmitted and acknowledged one at a time over the link in each
direction. Each packet is composed of a HEADER followed by a DATA
portion. The DATA portion may be empty.
NOTE: There are some older operating systems and devices which do
not permit 8-bit communication over an RS-232 link. Most of these
allow restricted 7-bit communication. RATP can automatically
detect this situation during connection initiation and utilizes a
special packing strategy when full 8-bit communication is not
possible. This is entirely transparent to any client software.
See Appendix I for a discussion of this case.
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2.1. Header Format
Byte No.
+-------------------------------+
| |
1 | Synch Leader | Hex 01
| |
+-------------------------------+
| S | A | F | R | S | A | E | S |
2 | Y | C | I | S | N | N | O | O | Control
| N | K | N | T | | | R | |
+-------------------------------+
| |
3 | Data length (0-255) |
| |
+-------------------------------+
| |
4 | Header Checksum |
| |
+-------------------------------+
Header Portion of a Packet
2.1.1. Synch Leader
RS-232 provides a self-clocking communications medium. The
wires over which data flows are often placed in 'noisy'
environments where the noise can appear as added unwanted data.
For this reason the beginning of a packet is denoted by a one
octet SYNCH pattern. This allows the receiver to discard noise
which appears on the connection prior to the reception of a
packet. The SYNCH pattern is defined to be the one octet hex
01, the ASCII Start Of Header character <SOH>.
The SYNCH pattern should ideally be unlikely to occur as the
result of noise. Differing modems, etc. have differing
responses to noise so this is hard to achieve. The pattern
chosen is thought to be a good compromise since many modems
manifest noise by setting the high order bits. Situations will
occur in which receiver is scanning for the beginning of a
packet and a spurious SYNCH pattern is seen. To detect
situations of this type a header checksum is provided (see
below).
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2.1.2. Control Bits
The first octet following the SYNCH pattern contains a 5-bit
field of control flags and two 1-bit sequence number fields.
The last bit is reserved and must be zero.
2.1.2.1. SYN - Synchronize Flag
Synchronize the connection. No data may be sent in a packet
which has the SYN flag set.
2.1.2.2. ACK - Acknowledge Flag
Acknowledge number is significant. Data may accompany a
packet which has this flag set as long as neither of SYN,
RST, nor FIN are also set. Once a connection has been
established this is always set.
2.1.2.3. RST - Reset Flag
Reset the connection. This is a method by which one end of
a connection can reset the other when an anomalous condition
is detected. No data may be sent in a packet which has the
RST flag set.
2.1.2.4. FIN - Finishing Flag
This indicates that no more data will be sent to the other
end of the connection. It also indicates that no more data
will be accepted. No data may be sent in a packet which has
the FIN flag set.
2.1.2.5. SN - Sequence Number
The Sequence Number associated with this packet.
2.1.2.6. AN - Acknowledge Number
If the ACK control flag is set this is the next Sequence
Number the sender of the packet is expecting to receive.
2.1.2.7. EOR - End of Record
This bit is provided as an aid for higher level protocols
which may need to fragment their packets. The Internet
protocol for example often uses packets as large as 576
octets. A packet of such size would require fragmentation
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when transported using this protocol. The EOR bit if set
provides information to the higher level that a record is
terminated in this packet. It is for information only and
is the responsibility of the higher level to set/clear it
when building packets to send. The interface to the
protocol must provide a method of reading/setting/clearing
this bit.
2.1.2.8. SO - Single Octet
One application thought to be of special importance is
single character transmission --- a user communicates from
the keyboard of a personal computer to another computer over
an unreliable link. Since rapid interactive response is
desirable it is expected that many of the characters typed
will be transmitted individually. To minimize the overhead
of this special case the SO control flag is provided.
The SO flag has no meaning if either the SYN, RST, or FIN
flags are set. Assume none of those flags are set, then if
the SO flag is set it indicates that a single octet of data
is contained in this packet. Since the amount of data is
known to be one octet the LENGTH field is superfluous and
itself contains the data octet. The data portion of the
packet is not transmitted.
The SO flag removes the need to transmit the data portion of
the packet in this special case. Without the SO flag seven
octets would be required of the packet, with it only four
are needed and so transmission efficiency is improved by 40
percent. The header checksum protects the single octet of
data.
2.1.3. Length
The second octet following the SYNCH pattern holds length
information. If the SYN bit is present this contains the
maximum number of data octets the receiver is allowed to
transmit in any single packet to the sender. This quantity is
called the MDL. A sender may indicate his unwillingness to
accept any data octets by specifying an MDL of zero. In this
case presumably all the data would be moving from the sender to
the receiver. Obviously if data is to be transmitted both
sides of a connection cannot have an MDL of zero.
If neither the SYN, RST, nor FIN flags are set this is an 8-bit
field called LENGTH. In this case if the SO flag bit is set
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then LENGTH contains a single octet of data. Otherwise it
contains the count of data octets in this packet. From zero
(0) to MDL octets of data may appear in a single packet. MDL
is limited to a maximum of 255.
2.1.4. Header Checksum
The header checksum algorithm is the 8-bit equivalent of the
16-bit data checksum detailed below. It is built and processed
in an similar manner but is eight bits wide instead of sixteen.
When sending the header checksum octet is initially cleared.
An 8-bit sum of the control, length, and header checksum octets
is formed employing end-around carry. That sum is then
complemented and stored in the header checksum octet. Upon
receipt the 8-bit end-around carry sum is formed of the same
three octets. If the sum is octal 377 the header is presumed
to be valid. In all other cases the header is assumed to be
invalid.
The reasons for providing this separate protection to the
header are discussed in the chapter dealing with error
handling. The header checksum covers the control and data
length octets. It does not include the SYNCH pattern.
2.2. Data Format
The data portion of a packet immediately follows the header if the
SO flag is not set and LENGTH > 0. It consists of LENGTH data
octets immediately followed by two data checksum octets. If
present the data portion contains LENGTH+2 octets.
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Data Byte No.
+-------------------------------+
1 | | High order \
+-- --+ > Word
2 | | Low order /
+-- --+
. | Data | High order \
+-- --+ > Word
. | | Low order /
+-- --+
LENGTH | | High order \
+-------------------------------+ > Word
| Imaginary padding octet 0 | Low order /
+-------------------------------+
LENGTH+1 | | High order \
+-- Data Checksum --+ > Word
LENGTH+2 | | Low order /
+-------------------------------+
Data Portion of a Packet
2.2.1. Data Checksum
The last two octets of the data portion of a packet are a data
checksum. A 16-bit checksum is used by this protocol to detect
incorrectly transmitted data. This has shown itself to be a
reliable method for detecting most categories of bit drop out
and bit insertion. While it does not guarantee the detection
of all such errors the probability of such an error going
undetected is on the order of 2**(-16).
The checksum octets follow the data to enable the sender of a
packet to compute the checksum while transmitting a packet and
the receiver to compute the checksum while receiving the
packet. Thus neither must store the packet and then process
the data for checksumming in a separate pass.
Order of Transmission
The order in which the 8-bit octets are assembled into
16-bit words, which is the low order octet and which is the
high, must be rigidly specified for the purpose of computing
16-bit checksums. We specify the big endian ordering in the
diagram above [Cohen 81].
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Checksum Algorithm
The checksum algorithm chosen is similar to that used by
IP/TCP protocols [IP 81] [TCP 81]. This algorithm has shown
itself to be both reliable and relatively easy to compute.
The interested reader may refer to [TCP Checksum 78] for a
more thorough discussion of its properties.
The checksum algorithm is:
SENDER
The unsigned sum of the 16-bit words of the data portion
of the packet is formed. Any overflow is added into the
lowest order bit. This sum does not include the header
portion of the packet. For the purpose of building a
packet for transmission the two octet checksum field is
zero. The sum formed is then bit complemented and
inserted into the checksum field before transmission.
If the total number of data octets is odd then the last
octet is padded to the right (low order) with zeros to
form a 16-bit word for checksum purposes. This pad octet
is not transmitted as part of the packet.
RECEIVER
The sum is computed as above but including the values
received in the checksum field. If the 16-bit sum is
octal 177777 then the data is presumed to be valid. In
all other cases the data is presumed to be invalid.
This unsigned 16-bit sum adds 16-bit quantities with any
overflow bit added into the lowest order bit of the sum. This
is called 'end around carry'. End around carry addition
provides several properties: 1) It provides full commutivity of
addition (summing in any order is equivalent), and 2) If you
apply a given rotation to each quantity before addition and
when the final total is formed apply the inverse rotation, then
the result will be equivalent to any other rotation chosen.
The latter property gives little endian machines like a PDP-11
the go ahead to pick up 16-bit quantities and add them in byte
swapped order.
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Reliable Asynchronous Transfer Protocol
The PDP-11 code to calculate the checksum is:
CLR R0 ; R0 will get the checksum
; R2 contains LENGTH count
LOOP: ADD (R1)+,R0 ; Add the next 16-bit byte
ADC R0 ; Make any carry be end around
SOB R2,LOOP ; Loop over entire packet
COM R0 ; Bit complement result
2.3. Sequence Numbers
Sequence numbers work with acknowledge numbers to inform the
sender that his last data packet was received, and to inform the
receiver of the sequence number of the next data packet it expects
to see. When the ACK flag is set in a packet the AN field
contains the sequence number of the next data packet it expects
from the sender. The sender looks at the AN field and by
implication knows that the packet he just sent should have had a
sequence number of:
<AN received-1 modulo 2>
If it did have that number that packet is considered to have been
acknowledged.
Similarly, the receiver expects the next data packet it sees to
have an SN field value equal to the AN field of the last
acknowledge message it sent. If this is not the case then the
receiver assumes that it is receiving a duplicate of a data packet
it earlier acknowledged. This implies that the packet containing
the acknowledgment did not arrive and therefor the packet that
contained the acknowledgment should be retransmitted. The
duplicate data packet is discarded.
The only packets which require acknowledgment are packets
containing status flags (SYN, RST, FIN, or SO) or data. A packet
which contains only an acknowledgment, i.e. <AN=n><CTL=ACK>, does
not require a response (it contains no status flags or data).
Both the AN and SN fields are a single bit wide. Since at most
one packet is in the process of being sent/acknowledged in a
particular direction at any one time a single bit is sufficient to
provide a method of duplicate packet detection and removal of a
packet from the retransmission queue. The arithmetic to advance
these numbers is modulo 2. Thus when a data packet has been
acknowledged the sender's next sequence number will be the current
one, plus one modulo 2:
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Reliable Asynchronous Transfer Protocol
<SN = SN + 1 modulo 2>
The individual acknowledgment of each packet containing data can
mislead one into thinking that side A of a connection cannot send
data to side B until it receives a packet from B. That only then
can it acknowledge B's packet and place in the acknowledging
packet some data of its own. This is not the case.
As long as its last packet sent requiring a response has been
acknowledged each side of a connection is free to send a data
packet whenever it wishes. Naturally, if one side is sending a
data packet and it also must acknowledge receipt of a data packet
from the other side, it is most efficient to combine both
functions in a single packet.
2.4. Maximum Packet Size
The maximum packet size is:
SYNCH + HEADER + Data Checksum + 255 = 261 octets
There is therefor no need to allocate more than that amount of
storage for any received packets.
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3. The Opening and Closing of a Connection
3.1. Opening a Connection
A "three-way handshake" is the procedure used to establish a
connection. It is normally initiated by one end of the connection
and responded to by the other. It will still work if both sides
simultaneously initiate the procedure. Experience has shown that
this strategy of opening a connection reduces the probability of
false connections to an acceptably low level.
The simplest form of the three-way handshake is illustrated in the
diagram below. The time order is line by line from top to bottom
with certain lines numbered for reference. User events are placed
in brackets as in [OPEN]. An arrow (-->) represents the direction
of flow of a packet and an ellipsis (...) indicates a packet in
transit. Side A and side B are the two ends of the connection.
An "XXX" indicates a packet which is lost or rejected. The
contents of the packet are shown on the center of each line. The
state of both connections is that caused by the departure or
arrival of the packet represented on the line. The contents of
the data portion of a packet are left out for clarity.
Side A Side B
1. CLOSED LISTEN
2. [OPEN request]
SYN-SENT -> <SN=0><CTL=SYN><MDL=n> ...
3. --> SYN-RECEIVED
... <SN=0><AN=1><CTL=SYN,ACK><MDL=m> <--
4. ESTABLISHED <--
--> <SN=1><AN=1><CTL=ACK><DATA> ...
5. --> ESTABLISHED
In line 2 above the user at side A has requested that a connection
be opened. Side A then attempts to open a connection by sending a
SYN packet to side B which is in the LISTEN state. It specifies
its initial sequence number, here zero. It places in the LENGTH
field of the header the largest number of data octets it can
consume in any one packet (MDL). The MDL is normally positive.
The action of sending this packet places A in the SYN-SENT state.
In line 3 side B has just received the SYN packet from A. This
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Reliable Asynchronous Transfer Protocol
places B in the SYN-RECEIVED state. B now sends a SYN packet to A
which acknowledges the SYN it just received from A. Note that the
AN field indicates B is now expecting to hear SN=1, thus
acknowledging the SYN packet from A which used SN=0. B also
specifies in the LENGTH field the largest number of data octets it
is prepared to consume.
Side A receives the SYN packet from B which acknowledges A's
original SYN packet in line 4. This places A in the ESTABLISHED
state. Side A can now be confident that B expects to receive more
packets from A.
A is now free to send B the first DATA packet. In line 5 upon
receipt of this packet side B is placed into the ESTABLISHED
state. DATA cannot be sent until the sender is in the ESTABLISHED
state. This is because the LENGTH field is used to specify the
MDL when opening the connection.
3.2. Recovering from a Simultaneous Active OPEN
It is of course possible that both ends of a connection may choose
to perform an active OPEN simultaneously. In this case neither
end of the connection is in the LISTEN state, both send SYN
packets. A reliable bidirectional protocol must recover from this
situation. It should recover in such a manner that the connection
is successfully initiated.
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Side A Side B
1. CLOSED CLOSED
2. [OPEN request]
SYN-SENT --> <SN=0><CTL=SYN><MDL=n> ...
3. ... [OPEN request]
<SN=0><CTL=SYN><MDL=m> <-- SYN-SENT
4. --> SYN-RECEIVED
... <SN=0><AN=1><CTL=SYN,ACK><MDL=m> <--
5. (packet finally arrives)
SYN-RECEIVED <-- <SN=0><CTL=SYN><MDL=m>
--> <SN=0><AN=1><CTL=SYN,ACK><MDL=n> --> ESTABLISHED
... <SN=1><AN=1><CTL=ACK> <--
6. (packet finally arrives)
ESTABLISHED <-- <SN=0><AN=1><CTL=SYN,ACK><MDL=m>
--> <SN=1><AN=1><CTL=ACK> ...
During simultaneous connection both sides of the connection
cycle from the CLOSED state through SYN-SENT to SYN-RECEIVED,
and finally to ESTABLISHED.
3.3. Detecting a Half-Open Connection
Any computer may crash after a connection has been established.
After recovering from the crash it may attempt to open a new
connection. The other end must be able to detect this condition
and treat it as an error.
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Side A Side
1. ESTABLISHED ESTABLISHED
--> <SN=0><AN=1><CTL=ACK><DATA> ...
-->
(crashes)
2. XXX <SN=1><AN=1><CTL=ACK><DATA> <--
3. (attempts to open new connection )
--> <SN=0><CTL=SYN><MDL=m> -->
... <SN=0><AN=1><CTL=RST,ACK> <-- (abort)
CLOSED
4. <--
(connection refused)
CLOSED
3.4. Closing a Connection
Either side may choose to close an established connection. This
is accomplished by sending a packet with the FIN control bit set.
No data may appear in a FIN packet. The other end of the
connection responds by shutting down its end of the connection and
sending a FIN, ACK in response.
Side A Side B
1. ESTABLISHED ESTABLISHED
2. [CLOSE request from user]
FIN-WAIT --> <SN=0><AN=1><CTL=FIN> ...
3. --> LAST-ACK
... <SN=1><AN=1><CTL=FIN,ACK> <--
4. TIME-WAIT <--
--> <SN=1><AN=0><CTL=ACK> ...
5. --> CLOSED
6. (after 2*SRTT time passes)
CLOSED
In line 2 the user on side A of the fully opened connection has
decided to close it down by issuing a CLOSE call. No more data
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will be accepted for sending. If data remains unsent a message
"Warning: Unsent data remains." is communicated to the user. No
more data will be received. A packet containing a FIN but no data
is constructed and sent. Side A goes into the FIN-WAIT state.
Side B sees the FIN sent and immediately builds a FIN, ACK packet
in response. It then goes into the LAST-ACK state. The FIN, ACK
packet is received by side A and an answering ACK is immediately
sent. Side A then goes to the TIME-WAIT state. In line 5 side B
receives the final acknowledgment of its FIN, ACK packet and goes
to the CLOSED state. In line 6 after waiting to be sure its last
acknowledgment was received side A goes to the CLOSED state (SRTT
is the Smoothed Round Trip Time and is defined in section 6.3.1).
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4. Packet Reception
The act of receiving a packet is relatively straightforward. There
are a few points which deserve some discussion. This chapter will
discuss packet reception stage by stage in time order.
Synch Detection
The first stage in the reception of a packet is the discovery of a
SYNCH pattern. Octets are read continuously and discarded until
the SYNCH pattern is seen. Once SYNCH has been observed proceed
to the Header Reception stage.
Header Reception
The remainder of the header is three octets in length. No further
processing can continue until the complete header has been read.
Once read the header checksum test is performed. If this test
fails it is assumed that the current SYNCH pattern was the result
of a data error. Since the correct SYNCH may appear immediately
after the current one, go back to the Synch Detection stage but
treat the three octets of the header following the bad SYNCH as
new input.
If the header checksum test succeeds then proceed to the Data
Reception stage.
Data Reception
A determination of the remaining length of the packet is made. If
either of the SYN, RST, SO, or FIN flags are set then legally the
entire packet has already been read and it is considered to have
'arrived'. No data portion of a packet is present when one of
those flags is set. Otherwise the LENGTH field specifies the
remaining amount of data to read. In this case if the LENGTH
field is zero then the packet contains no data portion and it is
considered to have arrived.
We now assume that a data portion is present and LENGTH was
non-zero. Counting the data checksum LENGTH+2 octets must now be
read. Once read the data checksum test is performed. If this
test fails the entire packet is discarded, return to the Synch
Detection stage. If the test succeeds then the packet is
considered to have arrived.
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Once arrived the packet is released to the upper level protocol
software. In a multiprocess implementation packet reception would
now begin again at the Synch Detection stage.
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5. Functional Specification
A convenient model for the discussion and implementation of protocols
is that of a state machine. A connection can be thought of as
passing through a variety of states, with possible error conditions,
from its inception until it is closed. In such a model each state
represents a known point in the history of a connection. The
connection passes from state to state in response to events. These
events are caused by user calls to the protocol interface (a request
to open or close a connection, data to send, etc.), incoming packets,
and timeouts.
Information about a connection must be maintained at both ends of
that connection. Following the terminology of [TCP 81] the
information necessary to the successful operation of a connection is
called the Transmission Control Block or TCB. The user requests to
the protocol interface are OPEN, SEND, RECEIVE, ABORT, STATUS, and
CLOSE.
This chapter is broken up into three parts. First a brief
description of each protocol state will be presented. Following this
is a slightly more detailed look at the allowed transitions which
occur between states. Finally a detailed discussion of the behavior
of each state is given.
5.1. Protocol States
The states used to describe this protocol are:
LISTEN
This state represents waiting for a connection from the
other end of the link.
SYN-SENT
This represents waiting for a matching connection request
after having sent a connection request.
SYN-RECEIVED
This represents waiting for a confirming connection request
acknowledgment after having both received and sent a
connection request.
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ESTABLISHED
This state represents a connection fully opened at both
ends. This is the normal state for data transfer.
FIN-WAIT
In this state one is waiting for a connection termination
request from the other end of the connection and an
acknowledgment of a termination request previously sent.
LAST-ACK
This end of the connection has seen and acknowledged a
termination request from the other end. This end has
responded with a termination request of its own and is now
expecting an acknowledgment of that request.
CLOSING
This represents waiting for an acknowledgment of a
connection termination request.
TIME-WAIT
This represents waiting for enough time to pass to be sure
that the other end of the connection received the
acknowledgment of its termination request.
CLOSED
A fictional state which represents a completely terminated
connection. If either end of a connection is in this state
it will neither send nor receive data or control packets.
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5.2. State Transitions
This section describes events which cause the protocol to depart
from its current state. A brief mention of each state is
accompanied by a list of departure events and to which state the
protocol goes as a result of those events. Departures due to the
presence of a RST flag are not shown.
5.2.1. LISTEN
This is a request to listen for any connection from the other
end of the link. In this state, no packets are sent. The
connection may be thought of as half-open. A STATUS request
will return to the caller this information.
Arrived at from the CLOSED state in response to a passive OPEN.
In a passive OPEN no packets are sent, the interface is waiting
for the initiation of a connection from the other end of the
link. Also this state can be reached in certain cases in
response to an RST connection reset request.
Departures
- A CLOSE request is made by the user. Delete the half-open
TCB and go to the CLOSED state.
- A packet arrives with the SYN flag set. Retrieve the
sender's MDL he placed into the LENGTH field. Set AN to
be received SN+1 modulo 2. Build a response packet with
SYN, ACK set. Choose your MDL and place it into the
LENGTH octet. Choose your initial SN, place in AN. Send
this packet and go to the SYN-RECEIVED state.
5.2.2. SYN-SENT
Arrived at from the CLOSED state in response to a user's active
OPEN request.
Departures
- A CLOSE request is made by the user. Delete the TCB and
go to the CLOSED state.
- A packet arrives with the SYN flag set. Retrieve the
sender's MDL he placed into the LENGTH field. Set AN to
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be received SN+1 modulo 2. Build a response packet with
ACK set, place in AN. Send this packet and go to the
SYN-RECEIVED state.
- A packet arrives with the SYN, ACK flags set. Retrieve
the sender's MDL he placed into the LENGTH field. Set AN
to be received SN+1 modulo 2. Build a response packet
with ACK set. Set SN to be SN+1 modulo 2, place SN and AN
into the header. Remembering the other end's MDL, build
data portion of packet. Send this packet and go to the
ESTABLISHED state.
5.2.3. SYN-RECEIVED
Arrived at from the LISTEN and SYN-SENT states in response to
an arriving SYN packet.
Departures
- A CLOSE request is made by the user. Create a packet with
FIN set. Send it and go to the FIN-WAIT state.
- A packet arrives with the ACK flag set. This packet
acknowledges a previous SYN packet. Go to the ESTABLISHED
state. The TCB should now note the connection is fully
opened.
- A packet arrives with the FIN flag set. The other end has
decided to close the connection. Create a packet with
FIN, ACK set. Send it and go to the LAST-ACK state.
5.2.4. ESTABLISHED
This state is the normal state for a connection. Data packets
may be exchanged in both directions (MDL allowing). It is
arrived at from the SYN-RECEIVED and SYN-SENT states in
response to the completion of connection initiation.
Departures
- In response to a CLOSE request from the user. Set AN to
be most recently received SN+1 modulo 2. Build a packet
with FIN set. Set SN to be SN+1 modulo 2, place SN and AN
into the header and send the packet. Go to the FIN-WAIT
state.
- A packet containing a FIN is received. Set AN to be
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received SN+1 modulo 2. Build a response packet with both
FIN and ACK set. Set SN to be SN+1 modulo 2, place SN and
AN into the header. No data portion is built. Send this
packet and go to the LAST-ACK state.
5.2.5. FIN-WAIT
Arrived at from either the SYN-RECEIVED state or from the
ESTABLISHED state. In both cases the user had requested a
CLOSE of the connection and a packet with a FIN was sent.
Departures
- A FIN, ACK packet is received which acknowledges the FIN
just sent. Go to the TIME-WAIT state.
- A FIN packet is received which indicates the other end of
the connection has simultaneously decided to close. Set
AN=received SN+1 modulo 2, and SN=SN+1 modulo 2. Send a
response packet with the ACK set. Go to the CLOSING
state.
5.2.6. LAST-ACK
Arrived at from the ESTABLISHED and SYN-RECEIVED states.
Departures
- An ACK is received for the last packet sent which was a
FIN. Delete the TCB and go to the CLOSED state.
5.2.7. CLOSING
Arrived at from the FIN-WAIT state.
Departures
- An ACK is received for the last packet sent which was a
FIN. Go to the TIME-WAIT state.
5.2.8. TIME-WAIT
Arrived at from the FIN-WAIT and CLOSING states.
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Departures
- This states waits until 2*SRTT time has passed. It then
deletes the TCB associated with the connection and goes to
the CLOSED state.
5.2.9. CLOSED
This state can be arrived at for a number of reasons: 1) while
in the LISTEN state the user requests a CLOSE, 2) while in the
SYN-SENT state the user requests a CLOSE, 3) while in the
TIME-WAIT state the 2*SRTT time period has elapsed, and 4)
while in the LAST-ACK state an arriving packet has an ACK of
the previously sent FIN packet.
In this state no data is read or sent over the link. To leave
this state requires an outside request to open a new
connection.
Departures
- User requests an active OPEN. Create a packet with SYN
set. Choose your MDL and place it into the LENGTH octet.
Choose your initial SN. AN is immaterial. Send this
packet and go to the SYN-SENT state. The TCB for this
connection is created. The connection may be thought of
as half-open. A STATUS request will return to the caller
this information.
- User requests a passive OPEN. The TCB for this connection
is created. The connection may be thought of as
half-open. A STATUS request will return to the caller
this information. Go to the LISTEN state.
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5.3. State Behavior
This section discusses in detail the behavior of each state in
response to the arrival of a packet. In what follows a packet is
not considered to have arrived until it has passed a number of
tests (see the chapter entitled: Packet Reception).
The method chosen to describe state behavior is tabular. Each
state is listed opposite a sequence of named procedures to execute
whenever a packet has arrived.
STATE BEHAVIOR
=============+========================
LISTEN | A
-------------+------------------------
SYN-SENT | B
-------------+------------------------
SYN-RECEIVED | C1 D1 E F1 H1
-------------+------------------------
ESTABLISHED | C2 D2 E F2 H2 I1
-------------+------------------------
FIN-WAIT | C2 D2 E F3 H3
-------------+------------------------
LAST-ACK | C2 D3 E F3 H4
-------------+------------------------
CLOSING | C2 D3 E F3 H5
-------------+------------------------
TIME-WAIT | D3 E F3 H6
-------------+------------------------
CLOSED | G
-------------+------------------------
For example, in the ESTABLISHED state the arrival of a packet
causes procedure C2 to be executed, then D2, then E, F2, H2, and
finally I1. Any procedure may terminate the processing which
occurs or cause a state change. Note that these procedures are
executed in sequence, first C2, then D2, etc. The time ordering
cannot be mixed.
The particular actions associated with each procedure are now
described.
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A --------------------------------------------------------
This procedure details the behavior of the LISTEN state. First
check the packet for the RST flag. If it is set then packet is
discarded and ignored, return and continue the processing
associated with this state.
We assume now that the RST flag was not set. Check the packet
for the ACK flag. If it is set we have an illegal condition
since no connection has yet been opened. Send a RST packet
with the correct response SN value:
<SN=received AN><CTL=RST>
Return to the current state without any further processing.
We assume now that neither the RST nor the ACK flags were set.
Check the packet for a SYN flag. If it is set then an attempt
is being made to open a connection. Create a TCB for this
connection. The sender has placed its MDL in the LENGTH field,
also specified is the sender's initial SN value. Retrieve and
place them into the TCB. Note that the presence of the SO flag
is ignored since it has no meaning when either of the SYN, RST,
or FIN flags are set.
Send a SYN packet which acknowledges the SYN received. Choose
the initial SN value and the MDL for this end of the
connection:
<SN=0><AN=received SN+1 modulo 2><CTL=SYN, ACK><LENGTH=MDL>
and go to the SYN-RECEIVED state without any further
processing.
Any packet not satisfying the above tests is discarded and
ignored. Return to the current state without any further
processing.
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B --------------------------------------------------------
This procedure represents the behavior of the SYN-SENT state
and is entered when this end of the connection decides to
execute an active OPEN.
First, check the packet for the ACK flag. If the ACK flag is
set then check to see if the AN value was as expected. If it
was continue below. Otherwise the AN value was unexpected. If
the RST flag was set then discard the packet and return to the
current state without any further processing, else send a
reset:
<SN=received AN><CTL=RST>
Discard the packet and return to the current state without any
further processing.
At this point either the ACK flag was set and the AN value was
as expected or ACK was not set. Second, check the RST flag.
If the RST flag is set there are two cases:
1. If the ACK flag is set then discard the packet, flush the
retransmission queue, inform the user "Error: Connection
refused", delete the TCB, and go to the CLOSED state without
any further processing.
2. If the ACK flag was not set then discard the packet and
return to this state without any further processing.
At this point we assume the packet contained an ACK which was
Ok, or there was no ACK, and there was no RST. Now check the
packet for the SYN flag. If the ACK flag was set then our SYN
has been acknowledged. Store MDL received in the TCB. At this
point we are technically in the ESTABLISHED state. Send an
acknowledgment packet and any initial data which is queued to
send:
<SN=received AN><AN=received SN+1 modulo 2><CTL=ACK><DATA>
Go to the ESTABLISHED state without any further processing.
If the SYN flag was set but the ACK was not set then the other
end of the connection has executed an active open also.
Acknowledge the SYN, choose your MDL, and send:
<SN=0><AN=received SN+1 modulo 2><CTL=SYN, ACK><LENGTH=MDL>
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Go to the SYN-RECEIVED state without any further processing.
Any packet not satisfying the above tests is discarded and
ignored. Return to the current state without any further
processing.
C1 --------------------------------------------------------
Examine the received SN field value. If the SN value was
expected then return and continue the processing associated
with this state.
We now assume the SN value was not what was expected.
If either RST or FIN were set discard the packet and return to
the current state without any further processing.
If neither RST nor FIN flags were set it is assumed that this
packet is a duplicate of one already received. Send an ACK
back: