rfc907.txt

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     assigned a permanent 16-bit logical address which is  independent
     of  the physical port on the SIMP to which it is attached.  These
     16-bit logical addresses are provided  in  all  Host-to-SIMP  and
     SIMP-to-Host data messages.

          Hosts may also be members of groups.   Group  addressing  is
     provided  primarily  to  support  the  multi-destination delivery
     required for  conferencing  applications.   Like  streams,  group
     addresses are dynamically created and deleted by the use of setup
     messages exchanged between a host and the network.  Membership in
     a  group  may consist of an arbitrary subset of all the permanent
     network hosts.   A  message  addressed  to  a  group  address  is
     delivered to all hosts that are members of that group.

          Although HAP does not guarantee error-free  delivery,  error
     control is an important aspect of the protocol design.  HAP error
     control is concerned with both local transfers between a host and
     its local SIMP and transfers from SIMP-to-SIMP over the satellite
     channel.  The  SIMP  offers  users  a  choice  of  network  error
     protection  options based on the network's ability to selectively
     send messages over the  satellite  channel  at  different  coding
     rates.  These forward error correction (FEC) options are referred
     to as reliability levels.  Three reliability levels (low, medium,
     and high) are available to the host.

          In  addition  to  forward  error  correction,  a  number  of
     checksum  mechanisms are employed in the satellite network to add
     an error detection capability.  A host has  an  opportunity  when
     sending  a  message  to  indicate  whether  the message should be
     delivered to its destination or discarded  if  a  data  error  is
     detected  by  the  network.  Each message received by a host from
     the network will have a flag indicating whether or not  an  error
     was  detected in that particular message.  A host can decide on a



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     RFC 907                                      Host Access Protocol
     July 1984                                           Specification



     per-message basis whether or not it wants to  accept  or  discard
     transmissions containing data errors.

          For connection of a host and SIMP in close proximity,  error
     rates  due  to  external noise or hardware failures on the access
     circuit may reasonably be expected to be much  smaller  than  the
     best satellite channel error rate.  Thus for this case, little is
     gained by using error detection and retransmission on the  access
     circuit.   A  16-bit  header  checksum  is  provided, however, to
     insure that SIMPs do not act on  incorrect  control  information.
     For    relatively    long   distances   or   noisy   connections,
     retransmissions over  the  access  circuit  may  be  required  to
     optimize  performance  for both low and high reliability traffic.
     It is expected that link-level error control procedures (such  as
     HDLC) will be used for this purpose.

          Datagram and stream messages being presented to the  network
     by  a  host may not be accepted for a number of reasons: priority
     too low, destination dead, lack of buffers in  the  source  SIMP,
     etc.  The host faces a similar situation with respect to handling
     messages from the SIMP.  To permit the receiver of a  message  to
     inform  the  sender  of  the local disposition of its message, an
     acceptance/refusal (A/R) mechanism is implemented.  The mechanism
     is  the external manifestation of the SIMP's (or host's) internal
     flow and congestion control algorithm.  If A/Rs are  enabled,  an
     explicit  or  implicit  acceptance or refusal for each message is
     returned to the host by the SIMP (and conversely).   This  allows
     the  host  (or  SIMP) to retry refused messages at its discretion
     and can provide information useful for optimizing the sending  of
     subsequent  messages if the reason for refusals is also provided.
     The A/R mechanism can be disabled to  provide  a  "pure  discard"
     interface.

          Each message submitted to the SIMP by a host  is  marked  as
     being  in one of four priority classes, from priority 3 (highest)
     through priority 0 (lowest).  The priority class is used  by  the
     SIMP  for  arbitrating  contention  for  scarce network resources
     (e.g., channel time).  That is, if the network cannot deliver all
     of the offered messages, high priority messages will be delivered
     in  preference  to  low  priority  messages.   In  the  case   of
     datagrams,  priority  level  is  used  by  the  SIMP for ordering



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     RFC 907                                      Host Access Protocol
     July 1984                                           Specification



     satellite channel reservation requests at  the  source  SIMP  and
     message  delivery  at  the  destination  SIMP.   In  the  case of
     streams, priority is associated with the ability of one stream to
     preempt another stream of lower priority at setup time.

          While the A/R mechanism allows control of individual message
     transfers,  it  does not facilitate regulation of priority flows.
     Such regulation is handled by passing advisory status information
     (GOPRI)   across   the   Host-SIMP   interface  indicating  which
     priorities  are  currently  being  accepted.   As  long  as  this
     information, relative to the change in priority status, is passed
     frequently, the sender can avoid originating messages  which  are
     sure to be refused.

          HAP defines both data messages (datagram messages and stream
     messages)  and  control messages.  Data messages are used to send
     information  between  network  hosts.    Control   messages   are
     exchanged  between  a  host  and  the network to manage the local
     access link.  HAP can also be viewed in  terms  of  two  distinct
     protocol  layers,  the  message  layer  and the setup layer.  The
     message layer is associated with the transmission  of  individual
     datagram  messages and stream messages.  The setup layer protocol
     is associated with the establishment, modification, and  deletion
     of  streams  and  groups.   Setup  layer  exchanges  are actually
     implemented as datagrams transmitted between the user host and an
     internal SIMP "service host."

          Every HAP message consists of an integral number  of  16-bit
     words.   The  first  several  words of the message always contain
     control information and are referred to as  the  message  header.
     The  first  word  of  the  message  header identifies the type of
     message which follows.  The second word of the message header  is
     a  checksum  which  covers  all  header information.  Any message
     whose received  header  checksum  does  not  match  the  checksum
     computed  on  the  received header information must be discarded.
     The format of the rest of the  header  depends  on  the  specific
     message type.

          The formats and use of  the  individual  message  types  are
     detailed  in the following sections.  A common format description
     is used for this  purpose.   Words  in  a  message  are  numbered



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     RFC 907                                      Host Access Protocol
     July 1984                                           Specification



     starting  at  zero  (i.e.,  zero  is  the first word of a message
     header).  Bits within  a  word  are  numbered  from  zero  (least
     significant) to fifteen (most significant).  The notation used to
     identify a particular field location is:

     <WORD#>{-<WORD#>}  [ <BIT#>{-<BIT#>} ]  <description>

     where optional elements in {} are used to specify the (inclusive)
     upper  limit  of a range.  The reader should refer to these field
     identifiers for precise field size specifications.  Fields  which
     are  common  to  several  message  types are defined in the first
     section which uses them.  Only the name of the field will usually
     appear in the descriptions in subsequent sections.

          Link-level protocols used to support HAP can differ  in  the
     order  in which they transmit the bits constituting HAP messages.
     For HDLC  and  ARPANET  VDH,  each  word  of  a  HAP  message  is
     transmitted  starting  with the least significant bit (bit 0) and
     ending with the most significant bit (bit 15).  The words of  the
     message  are transmitted from word 0 to word N.  For ARPANET 1822
     local and distant host interfaces, the order of bit  transmission
     within  each  word is the reverse of that for HDLC and VDH, i.e.,
     the transmission is from bit 15 to bit 0.





















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     RFC 907                                      Host Access Protocol
     July 1984                                           Specification



     3  Datagram Messages

          Datagram messages are one of the two types of message  level
     data  messages  used to support host-to-host communication.  Each
     datagram can contain up to 16,384 bits of  user  data.   Datagram
     messages  transmitted  by  a  host  to  a  host  on a remote SIMP
     experience a nominal two satellite hop end-to-end  network  delay
     (about  0.6  sec),  excluding  delay  on  the access links.  This
     network delay is due to the reservation  per  message  scheduling
     procedure  for datagrams which only allocates channel time to the
     message for the duration of the actual transfer.  Since  datagram
     transfers between permanent hosts on the same SIMP do not require
     satellite channel scheduling prior  to  data  transmission,   the
     network delay in this case will be much smaller and is determined
     strictly  by  SIMP  processing  time.  Datagrams  sent  to  group
     addresses  are treated as if they were addressed to  remote hosts
     and are  always sent over the satellite channel.  It is  expected
     that  datagram  messages  will be used to support the majority of
     computer-to-computer and terminal-to-computer  traffic  which  is
     bursty in nature.

          The format of datagram messages and the purpose of  each  of
     the header control fields is described in Figure 1.





















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     RFC 907                                      Host Access Protocol
     July 1984                                           Specification






              15 14 13 12 11 10  9  8  7  6  5  4  3  2  1  0
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
      0      | 0|LB|GOPRI|  XXXX  | F|     MESSAGE NUMBER    |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
      1      |               HEADER CHECKSUM                 |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
      2      |                      A/R                      |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
      3      | 0|IL| D| E| TTL | PRI | RLY |      RLEN       |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
      4      |            DESTINATION HOST ADDRESS           |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
      5      |              SOURCE HOST ADDRESS              |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
     6-N     |                     DATA                      |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+


                        Figure 1 . DATAGRAM MESSAGE



     0[15]     Message Class.  This bit identifies the  message  as  a
               data message or a control message.

                    0 = Data Message
                    1 = Control Message

     0[14]     Loopback Bit.  This bit allows the sender of a  message
               to determine if its own messages are being looped back.
               The host and the SIMP each use  different  settings  of
               this bit for their transmissions.  If a message arrives
               with the loopback bit set equal to its outgoing  value,
               then the message has been looped.

                    0 = Sent by Host
                    1 = Sent by SIMP




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     RFC 907                                      Host Access Protocol
     July 1984                                           Specification



     0[12-13]  Go-Priority.   In  SIMP-to-Host  messages,  this  field
               provides  advisory  information  concerning  the lowest
               priority currently being accepted  by  the  SIMP.   The
               host  may optionally choose to provide similar priority
               information to the SIMP.

                    0 = Low Priority
                    1 = Medium-Low Priority
                    2 = Medium-High Priority
                    3 = High Priority

     0[9-11]   Reserved.

     0[8]      Force Channel Transmission Flag.  This flag can be  set
               by  the  source  host to force the SIMP to transmit the
               message over the satellite channel even if the  message
               contains   permanent   destination   and   source  host
               addresses corresponding to hosts which  are  physically
               connected to the same SIMP.

                    0 = Normal operation
                    1 = Force channel transmission

     0[0-7]    Message Number.  This field contains the identification
               of  the  message  used  by the acceptance/refusal (A/R)
               mechanism (when enabled).  If  the  message  number  is
               zero,  A/R  is disabled for this specific message.  See
               Section  5  for  a  detailed  description  of  the  A/R
               mechanism.

     1[0-15]   Header Checksum.  This field contains a checksum  which
               covers  words  0-5.   It is computed as the negation of
               the 2's-complement sum  of  words  0-5  (excluding  the
               checksum word itself).

     2[0-15]   Piggybacked   A/R.    This   field   may   contain   an
               acceptance/refusal word providing A/R status on traffic
               flowing in the opposite direction.  Its  inclusion  may
               eliminate  the  need for a separate A/R control message
               (see Section 5).  A value of zero for this word is used
               to  indicate  that  no  piggybacked  A/R information is



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     RFC 907                                      Host Access Protocol
     July 1984                                           Specification



               present.

     3[15]     Data Message Type.  This  bit  identifies  whether  the
               message is a datagram message or a stream message.

                    0 = Datagram Message
                    1 = Stream Message

     3[14]     Internet/Local Flag.  This flag is set by a source host
               to  specify  to  a  destination  host  whether the data
               portion of the message contains a standard DoD Internet
               header.   This  field  is  passed  transparently by the
               source  and  destination  SIMPs  for  traffic   between
               external   satellite   network  hosts.  This  field  is
               examined by internal  SIMP  hosts  (e.g.,  the  network
               service host) in order to support Internet operation.

                    0 = Internet
                    1 = Local


     3[13]     Discard Flag.   This  flag  allows  a  source  host  to
               instruct   the   satellite   network   (including   the
               destination host) what to do with the message when data
               errors  are  detected  (assuming the header checksum is
               correct).