    NWG/RFC 721                                           1 SEP 76 LLG 36636
    Out-of-Band Control Signals

    Network Working Group                                      Larry Garlick
    Request for Comments 721                                         SRI-ARC
    NIC 36636                                                 1 September 76

                           Out-of-Band Control Signals
                                      in a
                             Host-to-Host Protocol

    This note addresses the problem of implementing a reliable out-of-band
    signal for use in a host-to-host protocol.  It is motivated by the fact
    that such a satisfactory mechanism does not exist in the Transmission
    Control Protocol (TCP) of Cerf et. al. [reference 4, 6]  In addition to
    discussing some requirements for such an out-of-band signal (interrupts)
    and the implications for the implementation of the requirements, a
    discussion of the problem for the TCP case will be presented.

    While the ARPANET host-to-host protocol does not support reliable
    transmission of either data or controls, it does meet the other
    requirements we have for an out-of-band control signal and will be drawn
    upon to provide a solution for the TCP case.

    The TCP currently handles all data and controls on the same logical
    channel.  To achieve reliable transmission, it provides positive
    acknowledgement and retransmission of all data and most controls.  Since
    interrupts are on the same channel as data, the TCP must flush data
    whenever an interrupt is sent so as not to be subject to flow control.

    Functional Requirements

       It is desirable to insure reliable delivery of an interrupt.  The
       sender must be assured that one and only one interrupt is delivered
       at the destination for each interrupt it sends.  The protocol need
       not be concerned about the order of delivery of interrupts to the
       user.

       The interrupt signal must be independent of data flow control
       mechanisms.  An interrupt must be delivered whether or not there are
       buffers provided for data, whether or not other controls are being
       handled.  The interrupt should not interfere with the reliable
       delivery of other data and controls.

       The host-to-host protocol need not provide synchronization between
       the interrupt channel and the data-control channel.  In fact, if
       coupling of the channels relies on the advancement of sequence
       numbers on the data-control channel, then the interrupt channel is no
       longer independent of flow control as required above.  The
       synchronization with the data stream can be performed by the user by

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    NWG/RFC 721                                           1 SEP 76 LLG 36636
    Out-of-Band Control Signals

       marking the data stream when an interrupt is generated.  The
       interrupt need not be coupled with other controls since it in no way
       affects the state of a connection.

       Once the interrupt has been delivered to the user, no other semantics
       are associated with it at the host-to-host level.

    Implications

       To provide for reliable delivery and accountability of interrupt
       delivery, an acknowledgement scheme is required.  To associate
       interrupt acknowledgements with the correct interrupt, some naming
       convention for interrupts is necessary.  Sequence numbers provide
       such a naming convention, along with the potential for providing an
       ordering mechanism.

       A separate interrupt channel is required to make interrupts
       independent of flow control.  A separate sequence number space for
       naming interrupts is also necessary.  If the sequence numbers are
       from the same sequence number space as some other channel, then
       sending an interrupt can be blocked by the need to resynchronize the
       sequence numbers on that channel.

       In the current TCP, which uses one channel for data, controls, and
       interrupts, flushing of data is combined with the interrupt to bypass
       the flow control mechanism.  However, flushing of resynchronization
       controls is not allowed and receipt of these controls is dependent on
       the acknowledgement of all previous data.  The ARPANET protocol,
       while not providing for reliable transmission, does provide for the
       separation of the interrupt-control channel and the data channel.

    Multiple Channels and Sequence Numbers

       If multiple channels are to be used for a connection, then it becomes
       interesting to determine how the sequence numbers of the channels can
       be coupled so that sequence number maintenance can be done
       efficiently.

       Assigning sequence numbers to each octet of data and control, as in
       the TCP, allows positive acknowledgement and ordering.  However,
       since packets are retransmitted on timeout, and since multi-path
       packet switch networks can cause a packet to stay around for a long
       time, the presence of duplicate packets and out-of-order packets must
       be accounted for.  A sequence number acceptability test must be
       performed on each packet received to determine if one of the
       following actions should be taken:

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    NWG/RFC 721                                           1 SEP 76 LLG 36636
    Out-of-Band Control Signals

          Acknowledge the packet and pass it on to the user.

          Acknowledge the packet, but do not send it to the user, since it
          has already been delivered.

          Discard the packet; the sequence number is not believable.

       Acceptability on Channel 0

          To determine the action to be taken for a packet, acceptability
          ranges are defined.  The following three ranges are mutually
          exclusive and collectively exhaustive of the sequence number space
          (see Figure 1):

             Ack-deliver range (ADR)

             Ack-only range (AOR)

             Discard range (DR)

                        ACCEPTABILITY RANGES

              DR       AOR             ADR              DR
          \\=====)[===========)[===================](========\\
                  ^           ^^                   ^^
                  !           !\                   !\
                  !           ! FCLE               ! DRLE
                AOLE       AORE                 ADRE

                              Figure 1

          Let  S = size of sequence number space (number per octet)

             x = sequence number to be tested

             FCLE = flow control left window edge

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    NWG/RFC 721                                           1 SEP 76 LLG 36636
    Out-of-Band Control Signals

             ADRE = (FCLE+ADR) mod S = Ack-deliver right edge (Discard
                      left edge - 1)

             AOLE = (FCLE-AOR) mod S =  Ack-only left edge (Discard
                      right edge + 1)

             TSE = time since connection establishment (in sec)

             MPL = maximum packet lifetime (in sec)

             TB = TCP bandwidth (in octets/sec)

          For any sequence number, x, and packet text length, l, if

             (AOLE <= x <= ADRE) mod S  and

             (AOLE <= x+l-1 <= ADRE) mod S

          then the packet should be acknowledged.

          If x and l satisfy

             (FCLE <= x <= ADRE) mod S  and

             (FCLE <= x+l-1 <= ADRE) mod S

          then x can also be delivered to the user; however, ordered
          delivery requires that x = FCLE.

          A packet is not in a range only if all of it lies outside a range.
          When a packet falls in more than one range, precedence is ADR,
          then AOR, then DR.  When a packet falls in the AOR then an ACK
          should be sent, even if a packet has to be created.  The ACK will
          specify the current left window edge.  This assures acknowledgment
          of all duplicates.

          ADRE is exactly the maximum sequence number ever "advertised"
          through the flow control window, plus one.  This allows for
          controls to be accepted even though permission for them may never
          have been explicitly given.  Of course, each time a control with a
          sequence number equal to the ADRE is sent, the ADRE must be
          incremented by one.

          AOR is set so that old duplicates (from previous incarnations of
          the connection) can be detected and discarded.  Thus

             AOR = Min(TSE, MPL) * TB

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    NWG/RFC 721                                           1 SEP 76 LLG 36636
    Out-of-Band Control Signals

       Synchronization and Resynchronization Problems

          A special problem arises concerning detection of packets (old
          duplicates) in the network that have sequence numbers assigned by
          old instances of a connection.  To handle this reliably, careful
          selection of the initial sequence number is required [ref. 2, 3]
          as well as periodic checks to determine if resynchronization of
          sequence numbers is necessary.  The overhead of such elaborate
          machinery is expensive and repeating it for each additional
          channel is undesirable.

       Acceptability on Channel i

          We have concluded that the only savings realizable in the muiltple
          channel case is to use channel zero's initial sequence number and
          resynchronization maintenance mechanism for the additional
          channels.  This can be accomplished by coupling each additional
          channel to channel zero's sequence numbers (CZSN), so that each
          item on channel i carries a pair of sequence numbers, the current
          CZSN and the current channel i's sequence number (CISN).

          The acceptablility test of items on channel i is a composite test
          of both sequence numbers.  First the CZSN is checked to see if it
          would be acknowledged if it were an octet received on channel
          zero.  Only if it would have been discarded would the item on
          channel i be discarded.  Having passed the CZSN test, the CISN is
          checked to see if the item is deliverable and acknowledgable with
          respect to the CISN sequence number space.  The CISN test is a
          check for everything but the existence of old duplicates from old
          instances of the connection and is performed like the check for
          channel zero items.

          It has been shown that to implement additional channels for a TCP
          connection, two alternatives are available-- (1) provide each
          channel with its own initial sequence number and resynchronization
          maintenance mechanism or (2) provide one initial sequence number
          and resynchronization maintenance mechanism for all channels
          through channel zero's mechanism.  It is hard for us to compare
          the two alternatives, since we have no experience implementing any
          resynchronization maintenance mechanism.

    TCP Case

       To implement a completely reliable separate interrupt channel for TCP
       requires a channel with a full sequence number space.  It is possible
       to compromise here and make the interrupt number space smaller than
       that required to support consumption of numbers at the TCP's

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    NWG/RFC 721                                           1 SEP 76 LLG 36636
    Out-of-Band Control Signals

       bandwidth.  What is lost is the total independence of the flow
       control from the data-control channel.  Normally, the data-control
       sequence numbers will change often enough so that wraparound in the
       interrupt number space causes no problems.

       Things become slightly messy when many interrupts are generated in
       quick succession.  Even if the interrupt numbers are acknowledged,
       they cannot be reused if they refer to the same data-control sequence
       number, until a full packet lifetime has elapsed.  This can be
       remedied in all but one case by sending a NOP on the data-control
       channel so that the next set of interrupts can refer to a new
       data-control sequence number.  However, if the data-control channel
       is blocked due to flow control and a resynchronizing control (DSN in
       the TCP case) was just sent, a NOP cannot be created until the
       resynchronization is complete and a new starting sequence number is
       chosen.  Thus to send another interrupt, the TCP must wait for a
       packet lifetime or an indication that it can send a NOP on the
       data-control channel.  (In reality, a connection would probably be
       closed long before a packet lifetime elapsed if the sender is not
       accepting data from the receiver. [reference 1])

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    NWG/RFC 721                                           1 SEP 76 LLG 36636
    Out-of-Band Control Signals

    REFERENCES

       (1)  J. Postel, L. Garlick, R. Rom, "TCP Specification (AUTODIN II),"
            ARC Catalog #35938, July 1976.

       (2)  R. Tomlinson, "Selecting Sequence Numbers," INWG Protocol Note
            #2, September 1974.

       (3)  Y. Dalal, "More on Selecting Sequence Numbers," INWG Protocol
            Note #4, October 1974.

       (4)  V. Cerf, Y. Dalal, C. Sunshine, "Specification of Internet
            Transmission Control Program," INWG General Note #72, December
            1974 (Revised). [Also as RFC 675, NIC Catalog #31505.]

       (5)  Cerf, V., "TCP Resynchronization," SU-DSL Technical Note #79,
            January 1976.

       (6)  Cerf, V. and R. Kahn, "A Protocol for Packet Network
            Intercommunication," IEEE Transactions on Communication, Vol
            COM-20, No. 5, May 1974.

       (7)  C. Sunshine, "Interprocess Communication Protocols for Computer
            Networks," Digital Systems Laboratory Technical Note #105,
            December 1975.

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