GENWiki

Premier IT Outsourcing and Support Services within the UK

User Tools

Site Tools


rfc:rfc916

Network Working Group G. Finn Request for Comments: 916 ISI

                                                          October 1984
           RELIABLE ASYNCHRONOUS TRANSFER PROTOCOL (RATP)

Status of This Memo

 This RFC suggests 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.

Finn [Page 1]

RFC 916 October 1984 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.
  1. 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.
  1. 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.
  1. 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

Finn [Page 2]

RFC 916 October 1984 Reliable Asynchronous Transfer Protocol

 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.

Finn [Page 3]

RFC 916 October 1984 Reliable Asynchronous Transfer Protocol

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.

Finn [Page 4]

RFC 916 October 1984 Reliable Asynchronous Transfer Protocol

 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).

Finn [Page 5]

RFC 916 October 1984 Reliable Asynchronous Transfer Protocol

    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

Finn [Page 6]

RFC 916 October 1984 Reliable Asynchronous Transfer Protocol

          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

Finn [Page 7]

RFC 916 October 1984 Reliable Asynchronous Transfer Protocol

       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.

Finn [Page 8]

RFC 916 October 1984 Reliable Asynchronous Transfer Protocol

    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].

Finn [Page 9]

RFC 916 October 1984 Reliable Asynchronous Transfer Protocol

       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.

Finn [Page 10]

RFC 916 October 1984 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:

Finn [Page 11]

RFC 916 October 1984 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.

Finn [Page 12]

RFC 916 October 1984 Reliable Asynchronous Transfer Protocol

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

Finn [Page 13]

RFC 916 October 1984 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.

Finn [Page 14]

RFC 916 October 1984 Reliable Asynchronous Transfer Protocol

    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>
  1. → <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.

Finn [Page 15]

RFC 916 October 1984 Reliable Asynchronous Transfer Protocol

    Side A                                             Side
    1. ESTABLISHED                                     ESTABLISHED
  1. → <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

Finn [Page 16]

RFC 916 October 1984 Reliable Asynchronous Transfer Protocol

    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).

Finn [Page 17]

RFC 916 October 1984 Reliable Asynchronous Transfer Protocol

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.

Finn [Page 18]

RFC 916 October 1984 Reliable Asynchronous Transfer Protocol

 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.

Finn [Page 19]

RFC 916 October 1984 Reliable Asynchronous Transfer Protocol

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.

Finn [Page 20]

RFC 916 October 1984 Reliable Asynchronous Transfer Protocol

       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.

Finn [Page 21]

RFC 916 October 1984 Reliable Asynchronous Transfer Protocol

 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
  1. A CLOSE request is made by the user. Delete the half-open

TCB and go to the CLOSED state.

  1. 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
  1. A CLOSE request is made by the user. Delete the TCB and

go to the CLOSED state.

  1. A packet arrives with the SYN flag set. Retrieve the

sender's MDL he placed into the LENGTH field. Set AN to

Finn [Page 22]

RFC 916 October 1984 Reliable Asynchronous Transfer Protocol

            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.
  1. 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
  1. A CLOSE request is made by the user. Create a packet with

FIN set. Send it and go to the FIN-WAIT state.

  1. 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.
  1. 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
  1. 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.
  1. A packet containing a FIN is received. Set AN to be

Finn [Page 23]

RFC 916 October 1984 Reliable Asynchronous Transfer Protocol

            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
  1. A FIN, ACK packet is received which acknowledges the FIN

just sent. Go to the TIME-WAIT state.

  1. 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
  1. 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
  1. 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.

Finn [Page 24]

RFC 916 October 1984 Reliable Asynchronous Transfer Protocol

       Departures
  1. 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
  1. 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.
  1. 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.

Finn [Page 25]

RFC 916 October 1984 Reliable Asynchronous Transfer Protocol

 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.

Finn [Page 26]

RFC 916 October 1984 Reliable Asynchronous Transfer Protocol

    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.

Finn [Page 27]

RFC 916 October 1984 Reliable Asynchronous Transfer Protocol

    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>

Finn [Page 28]

RFC 916 October 1984 Reliable Asynchronous Transfer Protocol

       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:
          <SN=received AN><AN=received SN+1 modulo 2><CTL=ACK>
       Discard the duplicate packet and return to the current state
       without any further processing.
    C2 --------------------------------------------------------
       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 SYN was set we assume that the other end crashed and has
       attempted to open a new connection.  We respond by sending a
       legal reset:
          <SN=received AN><AN=received SN+1 modulo 2><CTL=RST, ACK>
       This will cause the other end, currently in the SYN-SENT state,
       to close.  Flush the retransmission queue, inform the user
       "Error: Connection reset", discard the packet, delete the TCB,
       and go to the CLOSED state without any further processing.

Finn [Page 29]

RFC 916 October 1984 Reliable Asynchronous Transfer Protocol

       If neither RST, FIN, nor SYN flags were set it is assumed that
       this packet is a duplicate of one already received.  Send an
       ACK back:
          <SN=received AN><AN=received SN+1 modulo 2><CTL=ACK>
       Discard the duplicate packet and return to the current state
       without any further processing.
    D1 --------------------------------------------------------
       The packet is examined for a RST flag.  If RST is not set then
       return and continue the processing associated with this state.
       RST is now assumed to have been set.  If the connection was
       originally initiated from the LISTEN state (it was passively
       opened) then flush the retransmission queue, discard the
       packet, and go to the LISTEN state without any further
       processing.
       If instead the connection was initiated actively (came from the
       SYN-SENT state) then flush the retransmission queue, inform the
       user "Error: Connection refused", discard the packet, delete
       the TCB, and go to the CLOSED state without any further
       processing.
    D2 --------------------------------------------------------
       The packet is examined for a RST flag.  If RST is not set then
       return and continue the processing associated with this state.
       RST is now assumed to have been set.  Any data remaining to be
       sent is flushed.  The retransmission queue is flushed, the user
       is informed "Error: Connection reset.", discard the packet,
       delete the TCB, and go to the CLOSED state without any further
       processing.
    D3 --------------------------------------------------------
       The packet is examined for a RST flag.  If RST is not set then
       return and continue the processing associated with this state.
       RST is now assumed to have been set.  Discard the packet,
       delete the TCB, and go to the CLOSED state without any further
       processing.

Finn [Page 30]

RFC 916 October 1984 Reliable Asynchronous Transfer Protocol

    E  --------------------------------------------------------
       Check the presence of the SYN flag.  If the SYN flag is not set
       then return and continue the processing associated with this
       state.
       We now assume that the SYN flag was set.  The presence of a SYN
       here is an error.  Flush the retransmission queue, send a legal
       RST packet.
          If the ACK flag was set then send:
             <SN=received AN><CTL=RST>
          If the ACK flag was not set then send:
             <SN=0><CTL=RST>
       The user should receive the message "Error: Connection reset.",
       then delete the TCB and go to the CLOSED state without any
       further processing.
    F1 --------------------------------------------------------
       Check the presence of the ACK flag.  If ACK is not set then
       discard the packet and return without any further processing.
       We now assume that the ACK flag was set.  If the AN field value
       was as expected then return and continue the processing
       associated with this state.
       We now assume that the ACK flag was set and that the AN field
       value was unexpected.  If the connection was originally
       initiated from the LISTEN state (it was passively opened) then
       flush the retransmission queue, discard the packet, and send a
       legal RST packet:
          <SN=received AN><CTL=RST>
       Then delete the TCB and go to the LISTEN state without any
       further processing.
       Otherwise the connection was initiated actively (came from the
       SYN-SENT state) then inform the user "Error: Connection
       refused", flush the retransmission queue, discard the packet,
       and send a legal RST packet:

Finn [Page 31]

RFC 916 October 1984 Reliable Asynchronous Transfer Protocol

          <SN=received AN><CTL=RST>
       Then delete the TCB and go to the CLOSED state without any
       further processing.
    F2 --------------------------------------------------------
       Check the presence of the ACK flag.  If ACK is not set then
       discard the packet and return without any further processing.
       We now assume that the ACK flag was set.  If the AN field value
       was as expected then flush the retransmission queue and inform
       the user with an "Ok" if a buffer has been entirely
       acknowledged.  Another packet containing data may now be sent.
       Return and continue the processing associated with this state.
       We now assume that the ACK flag was set and that the AN field
       value was unexpected.  This is assumed to indicate a duplicate
       acknowledgment.  It is ignored, return and continue the
       processing associated with this state.
    F3 --------------------------------------------------------
       Check the presence of the ACK flag.  If ACK is not set then
       discard the packet and return without any further processing.
       We now assume that the ACK flag was set.  If the AN field value
       was as expected then continue the processing associated with
       this state.
       We now assume that the ACK flag was set and that the AN field
       value was unexpected.  This is ignored, return and continue
       with the processing associated with this state.
    G  --------------------------------------------------------
       This procedure represents the behavior of the CLOSED state of a
       connection.  All incoming packets are discarded.  If the packet
       had the RST flag set take no action.  Otherwise it is necessary
       to build a RST packet.  Since this end is closed the other end
       of the connection has incorrect data about the state of the
       connection and should be so informed.
          If the ACK flag was set then send:
             <SN=received AN><CTL=RST>

Finn [Page 32]

RFC 916 October 1984 Reliable Asynchronous Transfer Protocol

          If the ACK flag was not set then send:
             <SN=0><AN=received SN+1 modulo 2><CTL=RST, ACK>
       After sending the reset packet return to the current state
       without any further processing.
    H1 --------------------------------------------------------
       Our SYN has been acknowledged.  At this point we are
       technically in the ESTABLISHED state.  Send 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 and execute procedure I1 to process
       any data which might be in this packet.
       Any packet not satisfying the above tests is discarded and
       ignored.  Return to the current state without any further
       processing.
    H2 --------------------------------------------------------
       Check the presence of the FIN flag.  If FIN is not set then
       continue the processing associated with this state.
       We now assume that the FIN flag was set.  This means the other
       end has decided to close the connection.  Flush the
       retransmission queue.  If any data remains to be sent then
       inform the user "Warning: Data left unsent."  The user must
       also be informed "Connection closing."  An acknowledgment for
       the FIN must be sent which also indicates this end is closing:
          <SN=received AN><AN=received SN + 1 modulo 2><CTL=FIN, ACK>
       Go to the LAST-ACK state without any further processing.

Finn [Page 33]

RFC 916 October 1984 Reliable Asynchronous Transfer Protocol

    H3 --------------------------------------------------------
       This state represents the final behavior of the FIN-WAIT state.
       If the packet did not contain a FIN we assume this packet is a
       duplicate and that the other end of the connection has not seen
       the FIN packet we sent earlier.  Rely upon retransmission of
       our earlier FIN packet to inform the other end of our desire to
       close.  Discard the packet and return without any further
       processing.
       At this point we have a packet which should contain a FIN.  By
       the rules of this protocol an ACK of a FIN requires a FIN, ACK
       in response and no data.  If the packet contains data we have
       detected an illegal condition.  Send a reset:
       <SN=received AN><AN=received SN+1 modulo 2><CTL=RST, ACK>
       Discard the packet, flush the retransmission queue, inform the
       user "Error: Connection reset.", delete the TCB, and go to the
       CLOSED state without any further processing.
       We now assume that the FIN flag was set and no data was
       contained in the packet.  If the AN field value was expected
       then this packet acknowledges a previously sent FIN packet.
       The other end of the connection is then also assumed to be
       closing and expects an acknowledgment.  Send an acknowledgment
       of the FIN:
          <SN=received AN><AN=received SN+1 modulo 2><CTL=ACK>
       Start the 2*SRTT timer associated with the TIME-WAIT state,
       discard the packet, and go to the TIME-WAIT state without any
       further processing.
       Otherwise the AN field value was unexpected.  This indicates a
       simultaneous closing by both sides of the connection.  Send an
       acknowledgment of the FIN:
          <SN=received AN><AN=received SN+1 modulo 2><CTL=ACK>
       Discard the packet, and go to the CLOSING state without any
       further processing.

Finn [Page 34]

RFC 916 October 1984 Reliable Asynchronous Transfer Protocol

    H4 --------------------------------------------------------
       This state represents the final behavior of the LAST-ACK state.
       If the AN field value is expected then this ACK is in response
       to the FIN, ACK packet recently sent.  This is the final
       acknowledging message indicating both side's agreement to close
       the connection.  Discard the packet, flush all queues, delete
       the TCB, and go to the CLOSED state without any further
       processing.
       Otherwise the AN field value was unexpected.  Discard the
       packet and remain in the current state without any further
       processing.
    H5 --------------------------------------------------------
       This state represents the final behavior of the CLOSING state.
       If the AN field value was expected then this packet
       acknowledges the FIN packet recently sent.  This is the final
       acknowledging message indicating both side's agreement to close
       the connection.  Start the 2*SRTT timer associated with the
       TIME-WAIT state, discard the packet, and go to the TIME-WAIT
       state without any further processing.
       Otherwise the AN field value was unexpected.  Discard the
       packet and remain in the current state without any further
       processing.
    H6 --------------------------------------------------------
       This state represents the behavior of the TIME-WAIT state.
       Check the presence of the ACK flag.  If ACK is not set then
       discard the packet and return without any further processing.
       Check the presence of the FIN flag.  If FIN is not set then
       discard the packet and return without any further processing.
       We now assume that the FIN flag was set.  This situation
       indicates that the last acknowledgment of the FIN packet sent
       by the other end of the connection did not arrive.  Resend the
       acknowledgment:
          <SN=received AN><AN=received SN+1 modulo 2><CTL=ACK>

Finn [Page 35]

RFC 916 October 1984 Reliable Asynchronous Transfer Protocol

       Restart the 2*SRTT timer, discard the packet, and remain in the
       current state without any further processing.
    I1 --------------------------------------------------------
       This represents that stage of processing in the ESTABLISHED
       state in which all the flag bits have been processed and only
       data may remain.  The packet is examined to see if it contains
       data.  If not the packet is now discarded, return to the
       current state without any further processing.
       We assume the packet contained data, that either the SO flag
       was set or LENGTH is positive.  That data is placed into the
       user's receive buffers.  As these become full the user should
       be informed "Receive buffer full."  An acknowledgment is sent:
          <SN=received AN><AN=received SN+1 modulo 2><CTL=ACK>
       If data is queued to send then it is most efficient to
       'piggyback' this acknowledgment on that data packet.
       The packet is now discarded, return to the ESTABLISHED state
       without any further processing.
 5.4. Timers
    There are three timers associated with this protocol.  Their
    purpose will now be briefly discussed as will the actions taken
    when a timer expires.  The particular nature these timeouts take
    and the methods by which they are set is the responsibility of the
    protocol implementation.
    5.4.1. User Timeout
       For practical implementation reasons it is desirable to have a
       user controllable timeout associated with the successful
       opening of a connection, successful acknowledgment of data, and
       successful closing of a connection.  Consider the situations in
       which a connection is so noisy that no data gets through, or a
       connection is physically cut.  Without an overriding timeout
       these situations would result in unbounded retransmissions.
       When this timeout expires the user is informed "Error:
       Connection aborted due to user timeout.", all queues are
       flushed, the TCB is deleted, and the CLOSED state is entered.

Finn [Page 36]

RFC 916 October 1984 Reliable Asynchronous Transfer Protocol

    5.4.2. Retransmission Timeout
       This timer ensures that any packet sent for which the SN is
       significant is acknowledged.  When such a packet is sent it is
       placed in a retransmission queue and the retransmission timer
       is begun.  If an acknowledgment has not arrived within the
       timer's period then the packet is retransmitted and the timer
       is restarted.  If the acknowledgment does arrive in time then
       the timer is stopped and the packet is removed from the
       retransmission queue.  The next packet with a significant SN
       may now be sent.
       This timeout is expected to operate in conjunction with a
       counter which keeps track of the number of times a packet has
       been retransmitted.  Normally an upper limit is set on
       retransmissions.  If that limit is exceeded then the connection
       is aborted.  This event is similar to the user timeout.  The
       user is informed "Error: Connection aborted due to
       retransmission failure", all queues are flushed, the TCB is
       deleted, and the CLOSED state is entered.
    5.4.3. TIME-WAIT Timeout
       This timeout is used to catch any FIN packets which might be
       retransmitted from the other end of a connection in response to
       a dropped acknowledgment packet.  The timeout period should be
       at least as long as 2*SRTT.  After this timeout expires the
       other end of the connection is assumed to be closed, the TCB is
       deleted, and this end enters the CLOSED state also.

Finn [Page 37]

RFC 916 October 1984 Reliable Asynchronous Transfer Protocol

6. Data Error Handling

 This chapter discusses in detail the types of data errors an
 established connection may encounter.  These are distinct from
 protocol errors discussed above.  In order of discussion these are:
  1. Framing Errors
  1. Missing SYNCH pattern
  1. Unacknowledged packets
  1. Bad packets
  1. Duplicate packets
  1. Outside flow control
  1. Packets that are too large
  1. Packets that are too small
 6.1. Framing Errors
    The RS-232 specification provides framing only for an individual
    octet.  Link level protocols for computer networking normally
    provide framing for each packet.  The SYNCH pattern provides a
    boundary for the beginning of a packet.  No similar pattern was
    chosen to mark the end and completely frame the packet.
    Any bit pattern can appear in the data portion of a packet.  For
    any particular pattern to reliably mark the end of a packet that
    terminating pattern cannot be allowed to appear in the data.  This
    is usually accomplished by the sender altering any occurrence of
    the terminating pattern in the data so that it is both no longer
    recognizable as that pattern and also restorable upon receipt.
    Both the sender and the receiver are required by this technique to
    examine all the data.  In the absence of a protocol chip to
    perform this function, it is a source of some overhead.
    6.1.1. Synthetic Framing
       In the absence of framing, the end of the packet must be
       synthetically determined.  The start of a packet is indicated
       by the SYNCH pattern.  The expected end of a packet can now
       only be determined by examining the LENGTH octet of the header.
       It is important to know whether or not the LENGTH data can be

Finn [Page 38]

RFC 916 October 1984 Reliable Asynchronous Transfer Protocol

       trusted.  This is accomplished by employing a one octet header
       checksum to cover the first two octets following the SYNCH
       pattern.  If the header passes the checksum test and neither
       the SYN, FIN, RST, nor SO flag bits were set then LENGTH is
       trusted and the number of octets expected beyond the header is
       LENGTH+2. (For those packets in which any of the above flag
       bits are set the packet length is fixed and includes only a
       header portion.)
       If the header fails the checksum test we are in some
       difficulty.  The length is incorrect so it may be too small or
       too large.  To recover from this error do the following.
       Beginning immediately after the SYNCH pattern rescan looking
       for the next SYNCH pattern.  Throw away all octets until a
       SYNCH is seen and then attempt to reinterpret it as a packet.
       The sender's retransmission timeout guarantees that a new copy
       of the packet will be transmitted.  This ensures that in
       discarding the initial SYNCH pattern, the SYNCH pattern from
       the beginning of the retransmitted packet will eventually be
       seen.
    6.1.2. Costs of Synthetic Framing
       This framing strategy causes no overhead unless data errors
       occur in the packet.  This is presumed to be a low probability
       occurrence.  In addition it removes the overhead of both sender
       and receiver passing over the data to process any termination
       pattern which might appear in the data.
       The worst case behavior would require a packet header to fail
       its checksum, a new SYNCH pattern to appear in the next few
       octets, that header failing its checksum, etc., until the SYNCH
       pattern of the retransmitted packet were finally seen.
       Consistently bad behavior of this type indicates an extremely
       noisy communications link.
 6.2. Missing SYNCH Pattern
    Any valid packet must begin with the SYNCH pattern.  Any receiver
    must discard all input octets until the SYNCH pattern is seen.
    The data which immediately follows a SYNCH pattern is interpreted
    as a packet.  The header checksum test is applied, then LENGTH+2
    octets are read, the data checksum test is applied, etc.

Finn [Page 39]

RFC 916 October 1984 Reliable Asynchronous Transfer Protocol

 6.3. Unacknowledged Packets
    If an ACK for a packet is not obtained within the retransmission
    timeout interval that packet is retransmitted.  Because
    significant variability in response can be expected from either
    end of a connection it is best to dynamically calculate the
    retransmission timeout interval.  An example of such a calculation
    is provided below.  The protocol will operate successfully,
    although not with as high an effective transmission rate, if a
    realistic upper bound time is used instead.
    A realistic upper bound time depends upon the packet size and line
    speed.  If the baud rate of the connection is 300 or above let B
    be the baud rate (for clarity assume it is the same in both
    directions), let L be the MDL of the receiver, let P be the packet
    processing time of the receiver.  Then an Upper Bound for the
    Reception Time (UBRT) is:
       UBRT = L/(B/10) seconds + P seconds
    and a realistic upper bound time is 2*UBRT seconds.
    6.3.1. Calculation of Retransmission Timeout Interval
       For the purpose of detecting retransmission time out the
       protocol must have access to a clock which provides at least
       single second resolution.  One technique for calculating the
       round trip time is:
          Measure the elapsed time between sending a packet with a
          particular SN and receiving an ACK with an AN which covers
          that SN.  The measured elapsed time is the Round Trip Time
          (RTT).  Next a Smoothed Round Trip Time (SRTT) is calculated
          as:
             SRTT = (ALPHA * SRTT) + ((1- ALPHA) * RTT)
          and based upon this you compute the Retransmission Time Out
          (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).

Finn [Page 40]

RFC 916 October 1984 Reliable Asynchronous Transfer Protocol

 6.4. Bad Packets
    A bad packet is received when it fails either the header or data
    checksum tests.  When this happens the sender will retransmit the
    packet after the retransmission timeout interval.
 6.5. Duplicate Packets
    A duplicate packet is a packet which passes the checksum tests but
    for which the SN received is significant but not the expected
    value.  This is normally caused when the sender did not get the
    ACK last sent by the receiver.  This situation is diagrammed
    below.
    Side A                                             Side B
    ESTABLISHED                                        ESTABLISHED
    1.      --> <SN=1><AN=0><CTL=ACK><DATA>       ...
                                                  -->
    2.      XXX <SN=0><AN=0><CTL=ACK><OTHER-DATA> <--
    3. (after SRTT)
            --> <SN=1><AN=0><CTL=ACK><DATA>       ...
    4.                                            -->
            ... <SN=0><AN=0><CTL=ACK><OTHER-DATA> <--
    5.      <--
    In line 2, B's packet was lost in transit, it may have failed its
    checksum tests when it reached A or its initial SYNCH pattern was
    smashed, etc..  In line 3 side A comes to the decision that its
    packet from line 1 was not received after SRTT time passes and
    retransmits that packet.
    In line 4 side B receives the packet.  It detects a duplicate
    because it already sent a packet acknowledging A's SN=1 (although
    that packet was lost).  B now discards the duplicate and
    immediately retransmits its last packet to A. Side A finally
    receives the retransmitted packet in line 5.

Finn [Page 41]

RFC 916 October 1984 Reliable Asynchronous Transfer Protocol

 6.6. Outside Flow Control
    There are many large computer systems which make use of flow
    control to regulate their input side of an RS-232 link.  Flow
    control based upon two special characters such as <Ctrl-S> (ASCII
    DC3) and <Ctrl-Q> (ASCII DC1) is almost universally in use today.
    So it becomes important for the protocol to be able to either:
       (1) Recognize and obey the flow control of the host
           computer(s), or
       (2) Ignore the flow control but still guarantee reliable data
           reception.
    It is the latter approach which this protocol takes.  This
    decision was made because the number of differing flow control
    characters in use would make it difficult to obey them all.
       There is a particular type of flow control with which this
       protocol will not operate.  The ENQUIRE, ACKNOWLEDGE method of
       flow control requires that the receiver of an inquiry respond
       with an acknowledge before any more data will be sent to it.
       This type of flow control also usually prohibits unrestricted
       8-bit data transmission because the inquiry character is
       forbidden as a data byte.
    For the other class of flow control methods a proof is required
    that data may still be reliably transmitted and received if flow
    control is ignored.  For the purposes of this discussion assume
    <Ctrl-S> is sent when the receiving end of the connection wishes
    the sender to stop transmitting.  A <Ctrl-Q> is sent when the
    receiver wishes the sender to resume.  The choice of these
    particular two characters is arbitrary.  If the sender does not
    immediately cease transmission upon receipt of the <Ctrl-S>,
    characters may be discarded.  Since this protocol chooses to
    ignore the flow control characters any part of a packet may be
    discarded.
    More precisely stated consider X to be the receiver and Y to be
    the sender.  The packet sent is represented by the string abc
    where a, b, and c are data segments of unspecified size.  X may
    receive one of:
       1. abc
       2. ab
       3. ac
       4. bc

Finn [Page 42]

RFC 916 October 1984 Reliable Asynchronous Transfer Protocol

    For case [1] the correct data is received and no special action
    need be taken.
    For cases [2], [3], and [4] we have a situation identical to data
    dropped during transmission.  This is handled by the same
    checksum, time-out and retransmission strategy already described.
    Assume Y is not now in the act of receiving a packet, then Y sees
    the two characters <Ctrl-S> and <Ctrl-Q> appear as input in that
    order.  Y is waiting for a message to appear and so expects to see
    a SYNCH pattern.  If the two characters "<Ctrl-S><Ctrl-Q>" are not
    part of a SYNCH pattern then they will be immediately discarded.
    If Y is receiving a packet then the <Ctrl-S> and <Ctrl-Q> are seen
    to be added noise characters and would be detected by the checksum
    tests.  The packet being received would require retransmission.
    The question of which character to pick for the SYNCH pattern is
    slightly muddied by the above observation.  To the author's
    knowledge <SOH> is rarely if ever picked for flow control.  This
    is part of the motivation in using it as the SYNCH pattern.
    How does one guarantee that any data will actually arrive
    successfully?  The initial choice of maximum data counts during
    connection establishment is very important.  Some knowledge of
    one's own operating system must be assumed.  If it is known for
    example, that streams of data in excess of a certain length will
    often trigger flow control at the connection baud rate, then the
    maximum data count should be chosen sufficiently lower that flow
    control rarely will be employed.  An intelligent choice of the
    maximum data count will guarantee that some packets will arrive
    without encountering flow control.
 6.7. Packets that are too Large
    Assume a packet arrives which passes its header checksum test but
    whose LENGTH is larger than the MDL of the receiver.  In such a
    case the sender has violated the protocol or a packet has a data
    error in the LENGTH octet and has passed the header checksum test.
    The latter is unlikely so that we assume the former.  The receiver
    will abort his connection.  The sender must inform the user
    "Error: Connection aborted due to MDL error", and go to the CLOSED
    state.
    When the MDL is exceeded the receiver will transmit a legal reset:
       <SN=received AN><CTL=RST>

Finn [Page 43]

RFC 916 October 1984 Reliable Asynchronous Transfer Protocol

 6.8. Packets that are too Small
    Assume that a packet has passed its header checksum test but some
    of the data octets have been dropped by the link.  In such a case
    the receiver's routine which reads data and builds packets is
    expecting octets which do not arrive.  After SRTT the sender will
    retransmit this packet to the receiver.  The receiver will now
    have enough data to complete the packet.  Almost certainly however
    it will fail the data checksum test.  As with any bad packet the
    receiver will rescan from the octet immediately following the
    SYNCH pattern for the next SYNCH pattern.  In this manner the
    receiver will eventually see the SYNCH pattern of the
    retransmitted packet.

Finn [Page 44]

RFC 916 October 1984 Reliable Asynchronous Transfer Protocol

I. Inability to Transmit/Receive 8-bit Data

 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.  Where this is an unavoidable problem
 both ends of the connection must have a protocol layer beneath this
 protocol.  This lower layer will unpack packets it sends over the
 RS-232 link.  It will also repack packets it receives over the RS-232
 link.  RATP will automatically determine whether or not full 8-bit or
 restricted 7-bit communication is being used (see below).
 The strategy chosen for restricted 7-bit communication is called 4/8
 packing.  That is, each octet to be sent will be broken up into two
 4-bit nibbles.  The order of transmission is the high order four bits
 followed by the low order bits.  Each octet to be received will be
 repacked by the inverse function.  The high order nibble will be
 received first then the low order nibble.  These two nibbles will be
 reassembled into an octet.
 I.1.  Encoding for Transmission
    For those systems which are incapable of 8-bit data transmission
    over RS-232 links, there are operating systems which in addition
    place special restrictions on the non-printable ASCII characters.
    The encoding for 4/8 packing should restrict itself to
    transmitting data only in the printable 7-bit ASCII range.
 I.2.  Framing an Octet
    The seventh and highest order bit of a transmitted 7-bit ASCII
    byte is a flag used to indicate whether the high or low order
    nibble of an octet is contained in this character.  This flag bit
    if set implies that a new octet is being received and that this
    printable ASCII character contains the high order nibble of an
    octet in its four low order bits.  In addition it implies the next
    ASCII character received should not have its highest order bit
    set.
    This high order flag bit is set by adding the ASCII character "@"
    (octal 100) to a data byte.  Thus the first nibble of an octet is
    always transmitted with "@" added to its value.  The high order
    nibble will be transformed into the characters "@" through letter
    "O".
    The lower order nibble of an octet is transmitted with zero "0"
    added to its value.  The low order nibble will be transformed into

Finn [Page 45]

RFC 916 October 1984 Reliable Asynchronous Transfer Protocol

    characters "0" through "?".  When receiving 4/8 packed data, any
    characters not within the range "0" through letter "O" are
    discarded.
    The octet whose octal value is 45 will be transmitted as two 7-bit
    printable ASCII characters:
               +-------------+
    High order |1|0|0|0|1|0|0| First transmitted ("@" + data) = D
               +-------------+
    Low order  |0|1|1|0|1|0|1| Second transmitted ("0" + data) = 5
               +-------------+
    Since data bytes may be dropped or added at any time it is
    important to know always which portion of an octet is expected and
    to deliver only complete octets to the higher protocol level.  If
    a single 7-bit character were completely dropped without being
    noticed the data stream delivered to the higher level could be
    shifted by an odd multiple of four bits.  In the worst case this
    condition could remain indefinitely and the higher level would
    never receive an octet correctly.  In such a case no packets would
    be correctly received, leading to an unusable connection.
    To avoid this problem octets are assembled using a state machine
    driven by the presence of the high order flag bit.  The presence
    of that bit in the 7-bit printable character indicates the
    beginning of a new octet.  The two state machine which assembles
    octets is described below.  A byte received with the high order
    flag bit set is called "HIGH", the byte without "LOW".
       State 0
          [Start state] Read a byte from the legal restricted set.
          This is determined by seeing if the byte is in the legal
          range "@" to the letter "O".  If it was not discard the byte
          and return to this state.
          A HIGH byte was read.  Place the four low order bits of the
          byte into the four high order bits of the assembled octet
          and go to state 1.  Otherwise discard the byte and return to
          this state.

Finn [Page 46]

RFC 916 October 1984 Reliable Asynchronous Transfer Protocol

       State 1
          Read a byte from the legal restricted set.  This is
          determined by seeing if the byte is in the legal range zero
          "0" to the letter "O".  If it was not discard the byte and
          return to this state.
          If a LOW byte was read subtract zero "0" from the byte
          placing the four low order bits of the result into the four
          low order bits of the assembled octet.  A full octet has now
          been assembled.  Pass the octet to the higher level and go
          to state 0.
          Otherwise a HIGH byte was read.  Place the four low order
          bits of the byte into the four high order bits of the
          assembled octet and return to this state.
    Utilizing this state machine to receive 4/8 packed data ensures
    that the data stream delivered to the higher level will not
    permanently remain shifted an odd multiple of four bits.  The
    restriction placed upon bytes read removes obviously bad data and
    in some cases would handle uncontrolled padding or blocking
    insertion.
 I.3.  Automatic Detection of 8-bit or 4/8 Packed Data
    It is an unavoidable problem that some machines cannot handle
    unrestricted 8-bit data.  Since this is given, it is desirable to
    be able to automatically detect whether unrestricted 8-bit or
    restricted 4/8 packing is being used to transmit data on a
    connection.  For the purposes of this discussion those machines
    capable of transmitting and receiving both unrestricted 8-bit and
    4/8 packed data are called smart.  Machines are called dumb if
    they can only transmit and receive 4/8 packed data.
    When initiating a connection there are four possible machine
    configurations and they are:
       1. A (smart) opens a connection to B (smart).
       2. A (dumb) opens a connection to B (smart).
       3. A (dumb) opens a connection to B (dumb).
       4. A (smart) opens a connection to B (dumb).

Finn [Page 47]

RFC 916 October 1984 Reliable Asynchronous Transfer Protocol

    Each case is examined and extensions to the behavior for the
    LISTEN and SYN-SENT states are provided which allow both types of
    machines to initiate or receive a connection.
    Cases 1 and 2: LISTEN Behavior for a Smart Machine
       In these cases machine A initiates a connection to B who is
       assumed to be in the LISTEN state.  B must be able to passively
       detect whether 8-bit or 4/8 packing is being used and respond
       accordingly.  The method B uses relies upon the detection of a
       valid first packet.  In the LISTEN state B attempts to
       simultaneously treat the incoming data as if it were both
       unrestricted 8-bit and 4/8 packed.
       The incoming data is in effect fed to two different receiving
       algorithms.  The detection of a valid header will occur to one
       of these algorithms before the other.  If the first valid
       header was read assuming unrestricted 8-bit data then any
       resulting connection is assumed to use unrestricted 8-bit data
       for the life of the connection.  If the first valid header
       assumed 4/8 packing then the resulting connection is assumed to
       use 4/8 packing for the life of the connection.  In the case of
       the detection of illegal condition in the LISTEN state the
       protocol will reply with a RST packet in kind.
    Case 3: LISTEN Behavior for a Dumb Machine
       In this case machine B is the recipient of a connection request
       and is capable of handling only 4/8 packed data.  The LISTEN
       behavior for machine B assumes that all connections are 4/8
       packed.  It never deals with unrestricted 8-bit data.  As a
       result it will refuse to open a connection request from a smart
       machine (see case 4 below).
    Case 4: SYN-SENT Behavior for a Smart Machine
       In this case machine A attempts to open a connection to machine
       B. However, A has no knowledge of B's capabilities.  A will
       send its connection request assuming B is smart using
       unrestricted 8-bit transmission.  It will await a reply
       assuming the response will be unrestricted 8-bit also.  If B is
       in fact dumb it will not return a SYN-ACK because of the
       restriction imposed by case 3 above.  If no connection is made
       with B using 8-bit data the entire connection initiation is
       restarted assuming B is dumb, 4/8 packing is used and the
       response is assumed to be 4/8 packed as well.

Finn [Page 48]

RFC 916 October 1984 Reliable Asynchronous Transfer Protocol

       The cost of this approach is a longer time to determine whether
       or not it is possible to open a connection to B. It is twice as
       long.  The advantages of being able to automatically adjust to
       either unrestricted 8-bit or 4/8 packed data out weigh this
       disadvantage.  RATP will not exhibit the schizophrenic behavior
       of many other asynchronous protocols when dealing with both
       classes of machines.

Finn [Page 49]

RFC 916 October 1984 Reliable Asynchronous Transfer Protocol

II. A Brief Survey of Some Asynchronous Link Protocols

 II.1.  DDCMP
    DDCMP, Copyright (c) 1978 Digital Equipment Corporation [DDCMP
    78], is a reliable point-to-point and multi-point transmission
    protocol is used by many of that manufacturer's computers.  DDCMP
    does provide reliable asynchronous two way data transmission.
    Some of the decisions taken in the design of DDCMP reflect its
    orientation toward multi-point data links.  This leads to headers
    which are substantially longer than needed for two way
    point-to-point communications.
    DDCMP allows as many as 255 outstanding unacknowledged messages.
    DDCMP does specifically mention that a particular end of a
    connection may choose to limit the send queue to one outstanding
    unacknowledged message.  It also allows sending a stream of
    outstanding unacknowledged packets.  Unless all RS-232
    implementations of DDCMP were limited to a single outstanding
    packet, the collision with existing flow control restrictions
    could lead to very low thruput. (DDCMP is assumed to have control
    over the link driver.  Dealing with various differing flow control
    mechanisms is not a consideration.)
    DDCMP uses a CRC polynomial for data protection which is difficult
    to calculate for many machines without special hardware [TCP
    Checksum 78].  Many Digital Equipment computers have such
    hardware.
    DDCMP does not provide the receiver with the ability to restrict
    incoming packet size.  It is true that all the higher level
    protocols built on top of DDCMP could separately negotiate packet
    size.  But this burden would then be moved away from the link
    level where it properly resides.
    Generally, a full implementation of DDCMP is too complex for
    consideration.  If one were to implement 'part' of the protocol
    then issues of compatibility with already existing implementations
    on other computers are raised.

Finn [Page 50]

RFC 916 October 1984 Reliable Asynchronous Transfer Protocol

 II.2.  MODEM Protocol
    This is a protocol in common use amongst microcomputers.  The
    description here comes from
       MODEM/XMODEM Protocol Explained by Kelly Smith, CP/M-Net
       "SYSOP" January 8,1980
       .... Data is sent in 128-byte sequentially numbered blocks,
       with a single checksum byte appended to the end of each block.
       As the receiving computer acquires the incoming data, it
       performs its own checksum and upon each completion of a block,
       it compares its checksum result with that of the sending
       computers.  If the receiving computer matches the checksum of
       the sending computer, it transmits an ACK (ASCII code protocol
       character for ACKNOWLEDGE (06 Hex, Control-F)) back to the
       sending computer.  The ACK therefore means "all's well on this
       end, send some more...".
       The sending computer will transmit an "initial NAK" (ASCII
       protocol character for NEGATIVE ACKNOWLEDGE (15 Hex,
       Control-U))...or, "that wasn't quite right, please send again".
       Due to the asynchronous nature of the initial "hook-up" between
       the two computers, the receiving computer will "time-out"
       looking for data, and send the NAK as the "cue" for the sending
       computer to begin transmission.  The sending computer knows
       that the receiving computer will "time-out", and uses this fact
       to "get in sync"...  The sending computer responds to the
       "initial NAK" with a SOH (ASCII code protocol character for
       START OF HEADING (01 Hex, Control-A)), sends the first block
       number, sends the 1's complement of the block number, sends 128
       bytes of 8 bit data, and finally a checksum, where the checksum
       is calculated by summing the SOH, the block number, the block
       number 1's complement, and the 128 bytes of data.
       Receiving Computer:
  1. –/NAK/————————/ACK/——————

15H 06H

       Sending Computer:
  1. –/SOH/BLK#/BLK#/DATA/CSUM/—/SOH/BLK#/BLK#/DATA/etc.

01H 01H FEH 8bit 8bit 01H 02H FDH 8bit ….

Finn [Page 51]

RFC 916 October 1984 Reliable Asynchronous Transfer Protocol

       This process continues, with the next 128 bytes.  If the block
       was ACK'ed by the receiving computer, and then the next
       sequential block number and its 1's complement, etc. ....
    As can be seen from this partial description the MODEM protocol is
    unidirectional, data can only pass from the sender to the receiver
    in a stream.  In order for data to flow simultaneously in the
    other direction another connection over another RS-232 line would
    be required.
    In addition this protocol is restricted to a fixed 128 octet
    packet size.  Many front-end concentrators are unable to service
    such large incoming packets.  It has been observed many times that
    the concentrator of a busy DECsystem-20 can invoke flow control on
    input at 1200 baud for packets as small as 64 characters.
 II.3.  KERMIT System
    The KERMIT system, Copyright (c) 1981 Columbia University, is a
    file transfer environment developed recently.  It has
    implementations which run on DECsystem-20, IBM 370 VM/CMS, 8080
    CP/M based systems, and the IBM PC among others.
    KERMIT combines both the reliable transfer and file transfer into
    a single package.  Extension to other applications and higher
    level protocols would be possible but the boundary between the
    reliable transfer and application layers is very indistinct.  It
    violates the layering design strategy the Internet employs.
    There is a limitation of transmission to the restricted printable
    ASCII set for certain computers but not for others.  This leads to
    confusion.  KERMIT allows both restricted ASCII and 8-bit
    transmission.
    The KERMIT protocol does have a method of setting MDL at
    connection initiation.  It is limited to a smaller maximum packet
    size, 96 as opposed to 261 octets.  Kermit originally used a
    checksumming algorithm limited to six bits.  This is considered to
    provide too low a level of error detection capability for data
    packets.  Kermit now allows two other checksumming algorithms in
    addition to the original.  There must be a negotiation between
    sender and receiver regarding which algorithm to use.
    The KERMIT protocol does not appear to make provision for both
    sides of a connection attempting an active open simultaneously.
    One side must be an initial "sending Kermit" and the other a
    "receiving Kermit".  The code published as a KERMIT implementation

Finn [Page 52]

RFC 916 October 1984 Reliable Asynchronous Transfer Protocol

    guide cannot recover from simultaneous active opens, it
    immediately ABORTs.  This reflects a bias towards unidirectional
    data flow.
    The KERMIT packet type (similar to RATP control flags) specifies
    whether an ACK/NAK is contained in the packet, or data, etc.
    These are mutually exclusive and piggybacking an ACK on a data
    packet is not possible.  This can be a source of overhead.  In
    addition KERMIT restricts the sender to a single outstanding
    unacknowledged packet as does RATP.  It allocates an entire byte
    to the sequence number which is unnecessary.
    On the subject of error recovery, the size of a packet is
    contained in the second byte of the packet and is not protected by
    a header checksum.  If the length field was in error due to noise
    on the link, it could be longer than the correct packet size.  The
    code published as the KERMIT implementation guide relies upon the
    detection of the <SOH> character anywhere in a packet to indicate
    the beginning of a packet header.  It re-SYNCHs using this
    technique.  This is only possible if binary data in a packet is
    quoted.  If full eight bit data is transmitted it does not appear
    that the KERMIT protocol rescans for a new MARK (SYNCH) character
    within the bad packet data just consumed.  It will under these
    circumstances throw away the retransmitted packet or portions
    thereof.  Re-SYNCHing under such conditions is problematical.

Finn [Page 53]

RFC 916 October 1984 Reliable Asynchronous Transfer Protocol

REFERENCES

 [Cohen 81]
    Cohen, D. On Holy Wars and a Plea for Peace. IEEE Computer,
    October, 1981.
 [DDCMP 78]
    DDCMP AA-D599A-TC edition, Digital Equipment Corporation, 1978.
    Version 4.0.
 [IP 81]
    Postel, J. DOD Standard Internet Protocol [RFC-791] Defense
    Advanced Research Projects Agency, 1981.
 [TCP 81]
    Postel, J. Transmission Control Protocol [RFC-793] Defense
    Advanced Research Projects Agency, 1981.
 [TCP Checksum 78]
    Plummer, W. W. TCP Checksum Function Design. Technical Report,
    Bolt Beranek and Newman, Inc., 1978.

EDITORS NOTES

 This memo was prepared in essentially this form in June 1983, and set
 aside.  Distribution at this time is prompted by the the "Thinwire"
 proposal described in RFC-914.
  1. -jon postel

Finn [Page 54]

/data/webs/external/dokuwiki/data/pages/rfc/rfc916.txt · Last modified: 1992/09/22 20:58 by 127.0.0.1

Donate Powered by PHP Valid HTML5 Valid CSS Driven by DokuWiki