IEN: 42                                                   Winston Edmond
Section:                                                   BBN
                                                            12 June 1978

                           SATNET Information



      The internet  gateway is that part of the PDP-11 code which passes
      messages between networks.  It is a Host on two different nets.

      The speech host is a different piece of PDP-11 code.

      The internal  gateway  is part of the  SIMP  code  which  performs
      limited  gateway  functions.  It is a Host both on SATNET  and one
      other network.

   List of Hosts on SATNET.

      At Etam:

         The BBN gateway  is ARPANET Host 3 on IMP 40 and is SATNET Host
         61 (75 octal).
         The PDP-11 speech Host is SATNET Host 31.
         There  is an internal  gateway, ARPANET Host 4 on IMP 39 (SDAC)
         and SATNET Host 32.

      At Goonhilly:

         The UCL gateway  is ARPANET  Host 3 on IMP 42 (LONDON),  and is
         SATNET Host 60 (74).
         The PDP-11 speech Host is SATNET Host 32.

      At Tanum:

         The NDRE gateway  is ARPANET  Host 3 on IMP 41 (NORSAR), and is
         SATNET Host 38 (46).
         The PDP-11 speech host is SATNET Host 33.

      At Clarksburg:

         The PDP-11 internet gateway is SATNET Host 39.
         The PDP-11 speech Host is SATNET Host 34.
         There  is an internal  gateway, RCCNET (Net 3) Host 1 on IMP 51
         (63), and SATNET Host 30 (36).

Winston Edmond                                                  [page 1]

IEN: 42                                                       12 June 78
SATNET Information

      In summary then:

SATNET           !    ETAM    ! Goonhilly  !    Tanum   ! Clarksburg !
Internet Gateway !   61 (75)  !   60 (74)  !   38 (46)  !   39 (47)  !
Speech Host      !   31 (37)  !   32 (40)  !   33 (41)  !   34 (42)  !
Internal Gateway !   21 (25)  !     -      !     -      !   30 (36)  !

            The notation "xx (yy)" indicates the Host number where xx is
            the decimal value and yy is the octal value.

ARPANET          !    ETAM    ! Goonhilly  !    Tanum   ! Clarksburg !
Internet Gateway !     40-3   !     42-3   !     41-3   !      -     !
Speech Host      !      -     !      -     !      -     !      -     !
Internal Gateway !     39-4   !      -     !      -     ! RCC-51-1   !

            The notation  "xx-y"  means  IMP xx and Host y on  that  IMP
            (both expressed in decimal).

   There are also some permanently  assigned  group names which  may  be
   used to  broadcast  a  message.   These  group  names  are  valid  as
   Destination addresses, but are not valid as Source addresses.

      Group ID         Rate     Members
      ----------       ----     -------
      128. (200)        16K     E G T C
      129. (201)        64K     E G T
      130. (202)        16K     E G   C
      131. (203)        16K     E   T C
      132. (204)        16K       G T C
      133. (205)        64K     E G
      134. (206)        64K     E   T
      135. (207)        16K     E     C
      136. (210)        64K       G T
      137. (211)        16K       G   C
      138. (212)        16K         T C
      140. (214)        16K     Etam and Clarksburg, internal gateways

      For groups 128 to 138, it is the PDP-11 Host that is the member of
      the group at the specified  location.   For example,  group ID 134
      sends a message to the PDP-11 at Etam and the PDP-11 at Tanum. The

Winston Edmond                                                  [page 2]

IEN: 42                                                       12 June 78
SATNET Information

      PDP-11  will determine  which of its internal  parts is to process
      the message.

   Note that when the Host-SIMP  protocol  arrives,  the message will be
   delivered  to the destination  with the group ID in  the  destination

Buffer Size

   The  current   size  of  the  buffer   used  for  satellite   channel
   communication  is 425 (octal)  words.   Of this, 400 (256.) words are
   for data.   "Data"  is considered  to begin immediately following the
   three word Host-SIMP  leader,  and continues  to the last word of the
   message.  The Internet-Header, if any, is counted as part of the data
   area.  For those who think in terms of bits, 256. words = 4096 bits.


   Below is a description  of each timestamp  implemented in SIMP 3. The
   order is the order in which a message would ordinarily be stamped.

   1. VDH interface (Input from Host to SIMP)

      (A) Implementation

         When the hardware  detects the end of a packet, it initiates an
         interrupt  at level  2 (0 is highest  priority,  7 is  lowest).
         Very shortly  after the interrupt  happens,  local  time  (10us
         clock) is read, and the time is saved with the incoming packet.
         The packet is then passed down to a lower processing level.

         A little while later, the lower level will take the packet.  If
         it should  turn out that this packet  was the last packet  of a
         message  (and if the message was not previously rejected by the
         Host-SIMP  protocol)  then the local time will be converted  to
         global time and written as the value of the timestamp.

         This stamp is called level 3.

      (B) Comments

         If the message  is more than one VDH packet  long,  the earlier
         packets may have already been copied over into chunk buffers by
         the time the last packet arrives.  This means the stamp ignores
         the time the SIMP has already  spent  on the message. This time
         will show up as a general processing delay.  If the VDH line is
         continuously  receiving  messages,  the intermessage delay time

Winston Edmond                                                  [page 3]

IEN: 42                                                       12 June 78
SATNET Information

         may be used to estimate  the time spent.   If the message  fits
         entirely in one packet (SIMP 3.1) the problem vanishes.

         It would not be too difficult to timestamp the message with the
         arrival  time  of  the  first  packet,  if  this  should  prove

   2. Satellite channel output

      (A) Implementation

         The time that the message is to be sent out is known in advance
         by the chosen  protocol.   That time is used for the timestamp.
         It is the time when the satellite channel will be turned on for
         the sending of the message.

         This stamp is called level 0.

      (B) Notes.

         The message  is timestamped  only the first  time it  is  sent.
         Retransmissions will not cause additional stamping.

   3. Satellite channel input.

      (A) Implementation

         When the first sixteen  data bits of an incoming  message  have
         been received  from the satellite channel, the hardware records
         the time.   This time is later noted and kept with the incoming
         If the message is from the leader, global time will be updated.
         This means the relationship  between  local and global time may
         change!  This change will cause the local SIMP's global time to
         be brought  into  line  with  that of the leader, if it was not
         The hardware  receive  time of the message is then converted to
         global time, and this value is used for the timestamp.

         This stamp is called level 0.

      (B) Notes

         To figure  out when the packet  has fully  arrived in the SIMP,
         one must add the  packet  transmission  time  to  the  declared
         receive  time.   In the case of a mixed  rate incoming message,
         this calculation may be moderately complex.

Winston Edmond                                                  [page 4]

IEN: 42                                                       12 June 78
SATNET Information

   4. VDH transmit side -- SIMP to Host.

      (A) Implementation

         When the message  is removed from the Host output queue, global
         time is computed and written as the value of the timestamp.

         This stamp is called level 3.

      (B) Notes

         The following events occur after the timestamp time:

            1) Host-SIMP to 1822 leader conversion;

            2) Copying message from chunk buffer to VDH packets;

            3) Sending packets across VDH line.

         Items (2)  and (3)  occur in parallel.  Processing for the next
         message  may be begun  when the last  packet  of  the  previous
         message has been queued (software) for eventual output.

   5. Message Received

      (A) Implementation

         When a message is received from the satellite channel, it is in
         a contiguous buffer.  If the message is not destined for a Host
         on this  SIMP,  it is thrown  away.  If kept, it is passed to a
         lower interrupt  level.  There the message is copied into chunk
         buffers  and some processing  of poda header  info occurs.  The
         chunk  buffer  message  is then taken  off  the  queue  by  the
         background  code,  the current time in global time is computed,
         and this value is used for the timestamp. It is timestamp level

         After the message  is stamped,  the message's  checksum will be
         computed.  The message is discarded if the checksum test fails.
         An ACK will then be created,  if one is wanted,  to acknowledge
         the correct  receipt of the message.  Finally, the message will
         be handed to Delivery, in the Host protocol module.

      (B) Notes

         This timestamp may be used to estimate:

Winston Edmond                                                  [page 5]

IEN: 42                                                       12 June 78
SATNET Information

            1) when the SIMP has finally decided to accept the message;

            2) when the ACK for the message is entered;

            3) when the message is given to the HPM.

Host Status Measurements

   There is a cryptic printout in the Host Status line printed by MON25.
   Perhaps this will clarify what's happening.

   "PS=1/45"  means "packets  sent".  Specifically, the SIMP VDH sent 45
   total data packets,  1 of which was a retransmission, during the last
   reporting interval (approx. one minute intervals).

   "PR=2/67"  means  "packets  received".   During  the  last  reporting
   interval,   the  SIMP  VDH  received   67  good  packets  other  than
   Hello/I-heard-You   packets.    Two  packets  arrived  with  hardware
   checksum  errors,  and it is unknown  what type of packet these might
   have been originally.

   "HI=67/66"  means "Hello/I-Heard-You".  These packets are required by
   the VDH protocol.  They are used to determine that the VDH connection
   works  well enough  to send real data.   They are also used to detect
   the other  host going  down in the absence  of other  traffic. In the
   example,  during  the last reporting  interval,  the  reporting  SIMP
   received 67 Hellos and received 66 I-Heard-You's.

Winston Edmond                                                  [page 6]