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

   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:

      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

         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

   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

   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])

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.