Network Working Group
Request for Comments: 938
Trudy Miller
February 1985

Internet Reliable Transaction Protocol

Functional and Interface Specification


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.


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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      1.1   Purpose .........................................  1
      1.2   Underlying Mechanisms ...........................  1
      1.3   Relationship to Other Protocols .................  2
      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
      3.1   User Services Provided By IRTP ..................  5
      3.2   IP Services Expected by IRTP ....................  5
      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
      5.1   Retransmission Strategies ....................... 14
      5.2   Pinging ......................................... 14
      5.3   Deleting Connection Tables ...................... 16

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      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
      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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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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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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   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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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

  • 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)

  • 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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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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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

  • IRTP itself becomes more complicated in terms of states of the connection table

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  • the user interface becomes both more complicated and more rigid
  • such solutions might be deadlock prone in some instances
  • 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.


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