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rfc:rfc938

Network Working Group Trudy Miller Request for Comments: 938 ACC

                                                         February 1985
               Internet Reliable Transaction Protocol
               Functional and Interface Specification

STATUS OF THIS MEMO

 This RFC is being distributed to members of the DARPA research
 community in order to solicit their reactions to the proposals
 contained in it.  While the issues discussed may not be directly
 relevant to the research problems of the DARPA community, they may be
 interesting to a number of researchers and implementors.  This RFC
 suggests a proposed protocol for the ARPA-Internet community, and
 requests discussion and suggestions for improvements.  Distribution
 of this memo is unlimited.

ABSTRACT

 The Internet Reliable Transaction Protocol (IRTP) is a transport
 level host to host protocol designed for an internet environment.  It
 provides reliable, sequenced delivery of packets of data between
 hosts and multiplexes/demultiplexes streams of packets from/to user
 processes representing ports.  It is simple to implement, with a
 minimum of connection management, at the possible expense of
 efficiency.

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RFC 938 February 1985 Internet Reliable Transaction Protocol

TABLE OF CONTENTS

 INTRODUCTION
    1.1   Purpose .........................................  1
    1.2   Underlying Mechanisms ...........................  1
    1.3   Relationship to Other Protocols .................  2
 IRTP HEADERS
    2.1   Header Format ...................................  3
    2.2   Packet Type .....................................  3
    2.3   Port Number .....................................  3
    2.4   Sequence Number .................................  4
    2.5   Length ..........................................  4
    2.6   Checksum ........................................  4
 INTERFACES
    3.1   User Services Provided By IRTP ..................  5
    3.2   IP Services Expected by IRTP ....................  5
 MODEL OF OPERATION
    4.1   State Variables .................................  6
    4.2   IRTP Initialization .............................  7
    4.3   Host-to-Host Synchronization ....................  7
    4.3.1   Response to SYNCH Packets .....................  7
    4.3.2   Response to SYNCH ACK Packet ..................  8
    4.4   Transmitting Data ...............................  8
    4.4.1   Receiving Data From Using Processes ...........  8
    4.4.2   Packet Retransmission ......................... 10
    4.5   Receiving Data .................................. 10
    4.5.1   Receive and Acknowledgment Windows ............ 11
    4.5.2   Invalid Packets ............................... 12
    4.5.3   Sequence Numbers Within Acknowledge Window .... 12
    4.5.4   Sequence Numbers Within the Receive Window .... 12
    4.5.5   Forwarding Data to Using Processes ............ 13
 IMPLEMENTATION ISSUES
    5.1   Retransmission Strategies ....................... 14
    5.2   Pinging ......................................... 14
    5.3   Deleting Connection Tables ...................... 16

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RFC 938 February 1985 Internet Reliable Transaction Protocol

 LIST OF FIGURES
    Figure 1-1    Relationship of IRTP to Other Protocols .  2
    Figure 2-1    IRTP Header Format ......................  3
    Figure 4-1    SYNCH Packet Format .....................  8
    Figure 4-2    SYNCH ACK Packet Format .................  8
    Figure 4-3    DATA Packet Format ......................  9
    Figure 4-4    DATA ACK Packet Format .................. 11
    Figure 4-5    PORT NAK Packet Format .................. 11
 ABBREVIATIONS
    ICMP        Internet Control Message Protocol
    IP          Internet Protocol
    IRTP        Internet Reliable Transaction Protocol
    RDP         Reliable Data Protocol
    TCP         Transmission Control Protocol
    UDP         User Datagram Protocol

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RFC 938 February 1985 Internet Reliable Transaction Protocol

CHAPTER 1 - INTRODUCTION

 The Internet Reliable Transaction Protocol (IRTP) is a full duplex,
 transaction oriented, host to host protocol which provides reliable
 sequenced delivery of packets of data, called transaction packets.
 Note: throughout this document the terms host and internet address
 are used interchangeably.
 1.1 Purpose
    The IRTP was designed for an environment in which one host will
    have to maintain reliable communication with many other hosts.  It
    is assumed that there is a (relatively) sporadic flow of
    information with each destination host, however information flow
    may be initiated at any time at either end of the connection.  The
    nature of the information is in the form of transactions, i.e.
    small, self contained messages.  There may be times at which one
    host will want to communicate essentially the same information to
    all of its known destinations as rapidly as possible.
    In effect, the IRTP defines a constant underlying connection
    between two hosts. This connection is not established and broken
    down, rather it can be resynchronized with minimal loss of data
    whenever one of the hosts has been rebooted.
    Due to the lack of connection management, it is desirable that all
    IRTP processes keep static information about all possible remote
    hosts. However, the IRTP has been designed such that minimal state
    information needs to be associated with each host to host pair,
    thereby allowing one host to communicate with many remote hosts.
    The IRTP is more complex than UDP in that it provides reliable,
    sequenced delivery of packets, but it is less complex than TCP in
    that sequencing is done on a packet by packet (rather than
    character stream) basis, and there is only one connection defined
    between any two internet addresses (that is, it is not a process
    to process protocol.)
 1.2 Underlying Mechanisms
    The IRTP uses retransmission and acknowledgments to guarantee
    delivery. Checksums are used to guarantee data integrity and to
    protect against misrouting.  There is a host to host
    synchronization mechanism and packet sequencing to provide
    duplicate detection and ordered delivery to the user process.  A
    simple mechanism allows IRTP to multiplex and demultiplex streams

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RFC 938 February 1985 Internet Reliable Transaction Protocol

    of transaction packets being exchanged between multiple IRTP users
    on this host and statically paired IRTP users on the same remote
    host.
 1.3 Relationship to Other Protocols
    The IRTP is designed for use in a potentially lossy internet
    environment.  It requires that IP be under it.  The IP protocol
    number of IRTP is 28.
    Conversely, IRTP provides a reliable transport protocol for one or
    more user processes.  User processes must have well-known IRTP
    port numbers, and can communicate only with matching processes
    with the same port number.  (Note that the term port refers to a
    higher level protocol.  IRTP connections exists between two hosts,
    not between a host/port and another host/port.)
    These relationships are depicted below.
       +--------+    +--------+   +-----------+
       | port a |....| port x |   | TCP users |   Application Level
       +--------+    +--------+   +-----------+
             |          |            | ... |
           +--------------+       +-----------+
           |     IRTP     |       |    TCP    |   Host Level
           +--------------+       +-----------+
                  |                     |
       +--------------------------------------+
       |    Internet Protocol and ICMP        |   Internet Level
       +--------------------------------------+
                        |
       +--------------------------------------+
       |      Local Network Protocol          |   Network Level
       +--------------------------------------+
       Figure 1-1.  Relationship of IRTP to Other Protocols

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RFC 938 February 1985 Internet Reliable Transaction Protocol

CHAPTER 2 - IRTP HEADERS

 2.1 Header Format
    Each IRTP packet is preceded by an eight byte header depicted
    below. The individual fields are described in the following
    sections.
       0      7 8     15 16             31
       +--------+--------+--------+--------+
       | packet |  port  |     sequence    |
       |  type  | number |      number     |
       +--------+--------+--------+--------+
       |      length     |    checksum     |
       |                 |                 |
       +-----------------+-----------------+
       |                                   |
       |       optional data octets        |
       + . . . . . . . . . . . . . . . . . |
       Figure 2-1.  IRTP Header Format
 2.2 Packet Type
    Five packet types are defined by the IRTP. These are:
    packet type   numeric code
    SYNCH              0
    SYNCH ACK          1
    DATA               2
    DATA ACK           3
    PORT NAK           4
    The use of individual packet types is discussed in MODEL OF
    OPERATION.
 2.3 Port Number
    This field is used for the multiplexing and demultiplexing of
    packets from multiple user processes across a single IRTP
    connection.  Processes which desire to use IRTP must claim port
    numbers.  A port number represents a higher level protocol, and
    data to/from this port may be exchanged only with a process which
    has claimed the same port number at a remote host.  A process can
    claim multiple port numbers, however, only one process may claim
    an individual port number.  All port numbers are well-known.

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 2.4 Sequence Number
    For each communicating pair of hosts, there are two sequence
    numbers defined, which are the send sequence numbers for the two
    ends.  Sequence numbers are treated as unsigned 16 bit integers.
    Each time a new transaction packet is sent, the sender increases
    the sequence number by one.  Initial sequence numbers are
    established when the connection is resynchronized (see Section
    4.3.)
 2.5 Length
    The length is the number of octets in this transaction packet,
    including the header and the data.  (This means that the minimum
    value of the length is 8.)
 2.6 Checksum
    The checksum is the 16-bit one's complement of the one's
    complement sum of the IRTP header and the transaction packet data
    (padded with an octet of zero if necessary to make an even number
    of octets.)

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CHAPTER 3 - INTERFACES

 3.1 User Services Provided by IRTP
    The exact interface to the TRTP from the using processes is
    implementation dependent, however, IRTP should provide the
    following services to the using processes.
       o  user processes must be able to claim a port number
       o  users must be able to request that data be sent to a
          particular port at an internet address (the port must be one
          which the user has claimed)
       o  users must be able to request transaction data from a
          particular port at any (unspecified) remote internet address
          (the port must be one which the user has claimed)
       o  if a port is determined to be unreachable at a particular
          destination, the using process which has claimed that port
          should be notified
    In addition to these minimal data transfer services, a particular
    implementation may want to have a mechanism by which a
    "supervisory" (that is, port independent) module can define
    dynamically the remote internet addresses which are legal targets
    for host to host communication by this IRTP module.  This
    mechanism might be internal or external to the IRTP module itself.
 3.2 IP Services Expected by IRTP
    IRTP expects a standard interface to IP through which it can send
    and receive transaction packets as IP datagrams.  In addition, if
    possible, it is desirable that IP or ICMP notify IRTP in the event
    that a remote internet address is unreachable.
    If the IP implementation (including ICMP) is able to notify IRTP
    of source quench conditions, individual IRTP implementations may
    be able to perform some dynamic adjustment of transmission
    characteristics.

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RFC 938 February 1985 Internet Reliable Transaction Protocol

CHAPTER 4 - MODEL OF OPERATION

 The basic operation of IRTP is as follows.  The first time two hosts
 communicate (or the first time after both have simultaneously
 failed,) synchronization is established using constant initial
 sequence numbers (there is a sequence number for each direction of
 transmission).  The TCP "quiet time" is used following reboots to
 insure that this will not cause inaccurate acknowledgment processing
 by one side or the other.
 Once synchronization has been achieved data may be passed in both
 directions.  Each transaction packet has a 16 bit sequence number.
 Sequence numbers increase monotonically as new packets are generated.
 The receipt of each sequence number must be acknowledged, either
 implicitly or explicitly.  At most 8 unacknowledged packets may be
 outstanding in one direction.  This number (called MAXPACK) is fixed
 for all IRTP modules. Unacknowledged packets must be periodically
 retransmitted.  Sequence numbers are also used for duplicate
 detection by receiving IRTP modules.
 If synchronization is lost due to the failure of one of the
 communicating hosts, after a reboot that host requests the remote
 host to communicate sequence number information, and data transfer
 continues.
 4.1 State Variables
    Each IRTP is associated with a single internet address.  The
    synchronization mechanism of the IRTP depends on the requirement
    that each IRTP module knows the internet addresses of all modules
    with which it will communicate.  For each remote internet address,
    an IRTP module must maintain the following information (called the
    connection table):
    rem_addr     (32 bit remote internet address)
    conn_state   (8  bit connection state)
    snd_nxt      (16 bit send sequence number)
    rcv_nxt      (16 bit expected next receive sequence number)
    snd_una      (16 bit first unacknowledged sequence number)
    In addition to maintaining the connection tables defined above, it
    is required that every IRTP module have some mechanism which
    generates "retransmission events" such that SYNCH packets are
    periodically retransmitted for any connection in synch_wait state
    (see Section 4.3), and the appropriate DATA packet is periodically
    retransmitted for any connection in data_transfer state (see
    Section 4.4.2).  It is implementation dependent whether this

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    mechanism is connection dependent, or a uniform mechanism for all
    connections, so it has not been made part of the connection state
    table.  See Chapter 5 for more discussion.
 4.2 IRTP Initialization
    Whenever a remote internet address becomes known by an IRTP
    process, a 2 minute "quiet time" as described in the TCP
    specification must be observed before accepting any incoming
    packets or user requests.  This is to insure that no old IRTP
    packets are still in the network.  In addition, a connection table
    is initialized as follows:
    rem_addr     =    known internet address
    conn_state   =    0 = out-of-synch
    snd_nxt      =    0
    rcv_nxt      =    0
    snd_una      =    0
    Strictly speaking, the IRTP specification does not allow
    connection tables to be dynamically deleted and recreated,
    however, if this happens the above procedure must be repeated.
    See Chapter 5 for more discussion.
 4.3 Host-to-Host Synchronization
    An IRTP module must initiate synchronization whenever it receives
    a DATA packet or a user request referencing an internet address
    whose connection state is out-of-synch.  Typically, this will
    happen only the first time that internet address is active
    following the reinitialization of the IRTP module. A SYNCH packet
    as shown below is transmitted.  Having sent this packet, the host
    enters connection state synch_wait (conn_state = 1).  In this
    state, any incoming DATA, DATA ACK or PORT NAK packets are
    ignored.  The SYNCH packet itself must be retransmitted
    periodically until synchronization has been achieved.
    4.3.1 Response to SYNCH Packets -
       Whenever a SYNCH packet is received, the recipient, regardless
       of current connection state, is required to to return a SYNCH
       ACK packet as shown below.  At this point the recipient enters
       data_transfer state (conn_state = 2).

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    4.3.2 Response to SYNCH ACK Packet -
       On receipt of a SYNCH ACK packet, the behavior of the recipient
       depends on its state.  If the recipient is in synch_wait state
       the recipient sets rcv_nxt to the sequence number value, sets
       snd_nxt and snd_una to the value in the two-octet data field,
       and enters data_transfer state (conn_state = 2).  Otherwise,
       the packet is ignored.
          0      7 8     15 16             31
          +--------+--------+--------+--------+
          |00000000|00000000|00000000 00000000|
          +--------+--------+--------+--------+
          |        8        |    checksum     |
          +-----------------+-----------------+
          Figure 4-1.  SYNCH Packet Format
          0      7 8     15 16             31
          +--------+--------+--------+--------+
          |00000001| unused |     snd_una     |
          +--------+--------+--------+--------+
          |        10       |    checksum     |
          +-----------------+-----------------+
          |      rcv_nxt    |
          +-----------------+
          Figure 4-2.  SYNCH ACK Packet Format
 4.4 Transmitting Data
    Once in data_transfer state DATA, DATA ACK and PORT NAK packets
    are used to achieve communication between IRTP processes, subject
    to the constraint that no more than MAXPACK unacknowledged packets
    may be transmitted on a connection at any time.  Note that all
    arithmetic operations and comparisons on sequence numbers
    described in this chapter are to be done modulo 2 to the 16.
    4.4.1 Receiving Data From Using Processes -
       User processes may request IRTP to send packets of at most 512
       user data octets to a remote internet address and IRTP port.
       When such a request is received, the behavior of the IRTP
       depends on the state of the connection with the remote host and
       on implementation dependent considerations.  If the connection

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       between this IRTP module and the remote host is not in
       data_transfer state, that state must be achieved (see Section
       4.3) before acting on the user request.
       Once the connection is in data_transfer state, the behavior of
       the IRTP module in reaction to a write request from a user is
       implementation dependent.  The simplest IRTP implementations
       will not accept write requests when MAXPACK unacknowledged
       packets have been sent to the remote connection and will
       provide interested users a mechanism by which they can be
       notified when the connection is no longer in this state, which
       is called flow controlled.  Such implementations are called
       blocking IRTP implementations.  These implementations check, on
       receipt of a write request, to see if the value of snd_nxt is
       less than snd_una+MAXPACK.  If it is, IRTP prepends a DATA
       packet header as shown below, and transmits the packet.  The
       value of snd_nxt is then incremented by one.  In addition, the
       packet must be retained in a retransmission queue until it is
       acknowledged.
          0       7 8     15 16             31
          +--------+--------+--------+--------+
          |00000010|port num|     snd_nxt     |
          +--------+--------+--------+--------+
          |     length      |    checksum     |
          +-----------------+-----------------+
          |           data octet(s)           |
          + . . . . . . . . . . . . . . . . . +
          Figure 4-3.  DATA Packet Format
       Other implementations may allow (some number of) write requests
       to be accepted even when the connection is flow controlled.
       These implementations, called non-blocking IRTP
       implementations, must maintain, in addition to the
       retransmission queue for each connection, a queue of accepted
       but not yet transmitted packets, in order of request.  This is
       called the pretransmission queue for the connection.
       When a non-blocking implementation receives a write request, if
       the connection is not flow controlled, it behaves exactly as a
       blocking IRTP.  Otherwise, it prepends a DATA packet header
       without a sequence number to the data, and appends the packet
       to the pretransmission queue.  Note that in this case, snd_nxt
       is not incremented.  The value of snd_nxt is incremented only
       when a packet is transmitted for the first time.

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    4.4.2 Packet Retransmission -
       The IRTP protocol requires that the transaction packet with
       sequence number snd_una be periodically retransmitted as long
       as there are any unacknowledged, but previously transmitted,
       packets (that is, as long as the value of snd_una is not equal
       to that of snd_nxt.)
       The value of snd_una increases over time due to the receipt of
       DATA ACK or PORT NAK packets from a remote host (see Sections
       4.5.3 and 4.5.4 below).  When either of these packet types is
       received, if the incoming sequence number in that packet is
       greater than the current value of snd_una, the value of snd_una
       is set to the incoming sequence number in that packet.  Any
       DATA packets with sequence number less than the new snd_una
       which were queued for retransmission are released.
       (If this is a non-blocking IRTP implementation, for each DATA
       packet which is thus released from the retransmission queue,
       the earliest buffered packet may be transmitted from the
       pretransmission queue, as long as the pretransmission queue is
       non-empty.  Prior to transmitting the packet, the current value
       of snd_nxt is put in the sequence number field of the header.
       The value of snd_nxt is then incremented by one.)
       Finally, if the acknowledgment is a PORT NAK, the user process
       with the nacked port number should be notified that the remote
       port is not there.
       It is also to be desired, though it is not required, that IRTP
       modules have some mechanism to decide that a remote host is not
       responding in order to notify user processes that this host is
       apparently unreachable.
 4.5 Receiving Data
    When an IRTP module in data_transfer state receives a DATA packet,
    its behavior depends on the port number, sequence number and
    implementation dependent space considerations.
    DATA ACK and PORT NAK packets are used to acknowledge the receipt
    of DATA packets.  Both of these acknowledgment packets acknowledge
    the receipt of all sequence numbers up to, but not including, the
    sequence number in their headers.  Note that this value is denoted
    "rcv_nxt" in the figures below.  This number is the value of
    rcv_nxt at the source of the acknowledgment packet when the
    acknowledgment was generated.

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       0      7 8     15 16             31
       +--------+--------+--------+--------+
       |00000011|port num|     rcv_nxt     |
       +--------+--------+--------+--------+
       |        8        |    checksum     |
       +-----------------+-----------------+
       Figure 4-4.  DATA ACK Packet Format
       0      7 8     15 16             31
       +--------+--------+--------+--------+
       |00000100|port num|     rcv_nxt     |
       +--------+--------+--------+--------+
       |        8        |    checksum     |
       +-----------------+-----------------+
       Figure 4-5.  PORT NAK Packet Format
    It is not required that a receiving IRTP implementation return an
    acknowledgment packet for every incoming DATA packet, nor is it
    required that the acknowledged sequence number be that in the most
    recently received packet.  The exact circumstances under which
    DATA ACK and PORT NAK packets are sent are detailed below.  The
    net effect is that every sequence number is acknowledged, a sender
    can force reacknowledgment if an ACK is lost, all acknowledgments
    are cumulative, and no out of order acknowledgments are permitted.
    4.5.1 Receive and Acknowledgment Windows -
       Each IRTP module has two windows associated with the receive
       side of a connection.  For convenience in the following
       discussion these are given names.  The sequence number window
       rcv_nxt-MAXPACK =< sequence number < rcv_nxt
       is called the acknowledge window.  All sequence numbers within
       this window represent packets which have previously been acked
       or nacked, however, the ack or nack may have been lost in the
       network.
       The sequence number window
       rcv_nxt =< sequence number < rcv_nxt+MYRCV =< rcv_nxt+MAXPACK
       is called the receive window.  All sequence numbers within this
       window represent legal packets which may be in transit,
       assuming that the remote host has received acks for all packets

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       in the acknowledge window.  The value of MYRCV depends on the
       implementation of the IRTP.  In the simplest case this number
       will be one, effectively meaning that the IRTP will ignore any
       incoming packets not in the acknowledge window or not equal to
       rcv_nxt.  If the IRTP has enough memory to buffer some incoming
       out-of-order packets, MYRCV can be set to some number =<
       MAXPACK and a more complex algorithm can be used to compute
       rcv_nxt, thereby achieving potentially greater efficiency.
       Note that in the latter case, these packets are not
       acknowledged until their sequence number is less than rcv_nxt,
       thereby insuring that acknowledgments are always cumulative.
       (See 4.5.4 below.)
    4.5.2 Invalid Packets -
       When an IRTP receives a DATA packet, it first checks the
       sequence number in the received packet.  If the sequence number
       is not within the acknowledge or receive window, the packet is
       discarded.  Similarly, if the computed checksum does not match
       that in the header, the packet is discarded.  No further action
       is taken.
    4.5.3 Sequence Numbers Within Acknowledge Window -
       When an IRTP receives an incoming DATA packet whose sequence
       number is within the acknowledge window, if the port specified
       in the incoming DATA packet is known to this IRTP, a DATA ACK
       packet is returned.  Otherwise, a PORT NAK is returned.
       In both cases, the value put in the sequence number field of
       the acknowlegement packet is the current value of rcv_nxt at
       the IRTP module which is acknowledging the DATA packet.  The
       DATA packet itself is discarded.
       (Note that the PORT NAK acknowledges reception of all packet
       numbers up to rcv_nxt.  It NAKs the port number, not the
       sequence number.)
    4.5.4 Sequence Numbers Within the Receive Window -
       If the received sequence number is within the receive window,
       rcv_nxt is recomputed.  How this is done is implementation
       dependent.  If MYRCV is one, then rcv_nxt is simply
       incremented.  Otherwise, rcv_nxt is set to the lowest sequence
       number such that all data packets with sequence numbers less

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       than this number have been received and are buffered at the
       receiving IRTP, or have been delivered to their destination
       port.
       Once rcv_nxt has been recomputed, a DATA ACK or PORT NAK is
       returned, depending on whether the port number is known or not
       known.  The value placed in the sequence number field is the
       newly computed value for rcv_nxt.
    4.5.5 Forwarding Data to Using Processes -
       Whenever an incoming DATA packet has been acknowledged (either
       implicitly or explicitly) its header can be stripped off and it
       can be queued for delivery to the user process which has
       claimed its port number.  If the IRTP implementation allows
       MYRCV to be greater than one, care must be taken that data
       which was originally received out of order is forwarded to its
       intended recipient in order of original sequence number.

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RFC 938 February 1985 Internet Reliable Transaction Protocol

CHAPTER 5 - IMPLEMENTATION ISSUES

 The preceding chapter was left intentionally vague in certain ways.
 In particular, no explicit description of the use of a timer or
 timers within an IRTP module was given, nor was there a description
 of how timer events should relate to "retransmission events".  This
 was done to separate the syntactic and operational requirements of
 the protocol from the performance characteristics of its
 implementation.
 It is believed that the protocol is robust.  That is, any
 implementation which strictly conforms to Chapter 4 should provide
 reliable synchronization of two hosts and reliable sequenced transfer
 of transaction data between them.  However, different ways of
 defining the notion of a retransmission event can have potentially
 significant impact on the performance of the protocol in terms of
 throughput and in terms of the load it places on the network.  It is
 up to the implementor to take into account overall requirements of
 the network environment and the intended use of the protocol, if
 possible, to optimize overall characteristics of the implementation.
 Several such issues will be discussed in this chapter.
 5.1 Retransmission Strategies
    The IRTP requires that a timer mechanism exists to somehow trigger
    retransmissions and requires that the packet with sequence number
    snd_una be the one retransmitted.  It is not required that
    retransmission be performed on every timer event, though this is
    one "retransmission strategy".  A possible alternative strategy is
    to perform a retransmission on a timer event only if no ACKs have
    been received since the last event.
    Additionally, the interval of the timer can affect the performance
    of the strategies, as can the value of MYRCV and the lossiness of
    the network environment.
    It is not within the scope of this document to recommend a
    retransmission strategy, only to point out that different
    strategies have different consequences.  It might be desirable to
    allow using processes to "specify" a strategy when a port is
    claimed in order to tailor the service of the protocol to the
    needs of a particular application.
 5.2 Pinging
    It is important to make explicit that IRTP modules ping by
    definition.  That is, as long as a remote internet address is

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    known, and is in use (that is, either synchronization or data
    transfer is being attempted), the protocol requires "periodic
    retransmission" of packets.  Note that this is true even if the
    IRTP module has determined that the remote address is currently
    unreachable.
    It is suggested that this situation can be made more sensible by
    adding two fields to the connection table.  These are:
    num_retries  (number of times current packet has been sent)
    time_out     (current retransmission timeout)
    These fields are to be used as follows.  It is assumed that there
    is some default initial value for time_out called DEFTIME, some
    (relatively long) value for time_out called PINGTIME and some
    value MAX_TRIES.  The exact values of these constants are
    implementation dependent.  The value of DEFTIME may also be
    retransmission strategy dependent.
    At the time that a connection table is initialized, num_retries is
    set to zero, and time_out is set to DEFTIME.  Whenever a
    retransmission event occurs (this will either be a retransmission
    of a SYNCH packet or of the packet with sequence number snd_una),
    num_retries is incremented by one unless it is equal to MAX_TRIES.
    If a destination is determined to be unreachable, either via an
    ICMP message or a Destination Host Dead message, num_retries is
    set to MAX_TRIES.  Whenever num_retries transitions to MAX_TRIES,
    either by being incremented or as above, the destination is is
    presumed unreachable and user processes are notified. At this
    point, time_out is set to PINGTIME, the state of the connection
    does not change and retransmissions occur at PINGTIME intervals
    until the destination becomes reachable.
    Conversely, whenever a SYNCH_ACK is received (in synch_wait
    state), or an (implicit or explicit) acknowledgment of sequence
    number snd_una is received (in data transfer state), time_out is
    set to DEFTIME and num_retries is reset to zero.  If time_out was
    already set to PINGTIME, user processes are notified that the
    destination is now reachable.
    The effect of this system is obvious.  The implementation still
    pings as required, but at presumably very infrequent intervals.
    Alternative solutions, which might place the decision to ping on
    using processes, are considered undesirable because
       o  IRTP itself becomes more complicated in terms of states of
          the connection table

Miller [Page 15]

RFC 938 February 1985 Internet Reliable Transaction Protocol

       o  the user interface becomes both more complicated and more
          rigid
       o  such solutions might be deadlock prone in some instances
       o  it seems appropriate that the host to host protocol should
          be the place to determine destination reachability, if the
          overall application requires that such information be known
          (as it does in the environments intended for IRTP.)
 5.3 Deleting Connection Tables
    The protocol as defined does not allow connection tables to be
    deleted (or for a connection state to transition to out_of_synch
    from any other state).  It might be appropriate to delete a
    connection table if it is known that the destination internet
    address is no longer one which this host wants to communicate
    with.  (The only danger there is that if the destination does not
    know this, it could ping this host forever.)  It is dangerous to
    delete a connection table or to go into out_of_synch state to
    avoid pinging when a destination does not appear to be there.  Two
    hosts with the same such strategy could potentially deadlock and
    fail to resynchronize.

AUTHOR'S ADDRESS

 Trudy Miller
 Advanced Computer Communications
 720 Santa Barbara Street
 Santa Barbara, CA  93101
 (805) 963-9431

Miller [Page 16]

/data/webs/external/dokuwiki/data/pages/rfc/rfc938.txt · Last modified: 1992/09/23 19:44 by 127.0.0.1

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