IEN 192














                      HOST/SATNET  PROTOCOL




                  Bolt Beranek and Newman Inc.


                            July 1981

Host/SATNET Protocol                                      IEN 192



                        TABLE OF CONTENTS



1.  Overview . . . . . . . . . . . . . . . . . . . . . . . . .  1

2.  Satellite IMP Implementation Details . . . . . . . . . . .  4
  2.1  Initialization  . . . . . . . . . . . . . . . . . . . .  5
  2.2  Host-to-Satellite IMP Input . . . . . . . . . . . . . .  6
  2.3  Satellite IMP-to-Host Output  . . . . . . . . . . . . .  8

3.  DATAGRAM ACCESS PROTOCOL . . . . . . . . . . . . . . . . . 10
  3.1  Overview  . . . . . . . . . . . . . . . . . . . . . . . 10
  3.2  Types of Service  . . . . . . . . . . . . . . . . . . . 11
  3.3  Addressing  . . . . . . . . . . . . . . . . . . . . . . 12
  3.4  Message Length  . . . . . . . . . . . . . . . . . . . . 12
  3.5  Host/SATNET Flow Control  . . . . . . . . . . . . . . . 13
  3.6  Status Messages . . . . . . . . . . . . . . . . . . . . 14
  3.7  Hello Messages  . . . . . . . . . . . . . . . . . . . . 14
  3.8  Message Reference Numbers . . . . . . . . . . . . . . . 15
  3.9  Initialization  . . . . . . . . . . . . . . . . . . . . 15
  3.10  Format Errors  . . . . . . . . . . . . . . . . . . . . 16
  3.11  Loop Detection . . . . . . . . . . . . . . . . . . . . 16
  3.12  Piggybacked Control Messages . . . . . . . . . . . . . 17
  3.13  Formats  . . . . . . . . . . . . . . . . . . . . . . . 17
    3.13.1  Control Codes  . . . . . . . . . . . . . . . . . . 17
    3.13.2  Data Messages  . . . . . . . . . . . . . . . . . . 18
      3.13.2.1  Type of Service Word . . . . . . . . . . . . . 19
      3.13.2.2  Acceptance Status Word . . . . . . . . . . . . 20
    3.13.3  ACCEPTED Messages  . . . . . . . . . . . . . . . . 21
    3.13.4  REFUSED Messages . . . . . . . . . . . . . . . . . 21
    3.13.5  STATUS REQUEST Messages  . . . . . . . . . . . . . 22
    3.13.6  STATUS MESSAGES  . . . . . . . . . . . . . . . . . 23
    3.13.7  HELLO Message  . . . . . . . . . . . . . . . . . . 23
    3.13.8  FORMAT ERROR Message . . . . . . . . . . . . . . . 23
    3.13.9  RESTART REQUEST Message  . . . . . . . . . . . . . 24
    3.13.10  RESTART COMPLETE Message  . . . . . . . . . . . . 25

4.  STREAM ACCESS PROTOCOLS  . . . . . . . . . . . . . . . . . 26
  4.1  Overview  . . . . . . . . . . . . . . . . . . . . . . . 26
  4.2  Stream Data Messages  . . . . . . . . . . . . . . . . . 27
  4.3  Stream Request Replies and Notifications  . . . . . . . 28
    4.3.1  CREATE  . . . . . . . . . . . . . . . . . . . . . . 31
    4.3.2  DELETE  . . . . . . . . . . . . . . . . . . . . . . 32
    4.3.3  JOIN  . . . . . . . . . . . . . . . . . . . . . . . 33
    4.3.4  LEAVE . . . . . . . . . . . . . . . . . . . . . . . 34
    4.3.5  CHANGE  . . . . . . . . . . . . . . . . . . . . . . 34
  4.4  SATNET Termination and Suspension of Streams  . . . . . 35





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5.  Land Modem Interface . . . . . . . . . . . . . . . . . . . 37

6.  Local Host Interface . . . . . . . . . . . . . . . . . . . 37

APPENDIX A.    . . . . . . . . . . . . . . . . . . . . . . . . 38
  A.1   Table 1 -- Request Codes . . . . . . . . . . . . . . . 38
  A.2   Table 2 -- Reply Codes . . . . . . . . . . . . . . . . 38
  A.3   Table 3 -- Error Codes in D3 . . . . . . . . . . . . . 39
  A.4   Table 4 -- Notification Codes  . . . . . . . . . . . . 39
  A.5   Table 5 -- SATNET Data Message Types . . . . . . . . . 39
  A.6   Table 6 -- SATNET Logical Address Map  . . . . . . . . 40

APPENDIX B.    . . . . . . . . . . . . . . . . . . . . . . . . 43
  B.1   Figure 1.  Restart State Diagram . . . . . . . . . . . 43
  B.2   Figure 2.  General Message Format  . . . . . . . . . . 44
  B.3   Figure 3.  Block DATA Message  . . . . . . . . . . . . 45
  B.4   Figure 4.  Type of Service Word  . . . . . . . . . . . 46
  B.5   Figure 5.  Acceptance Status Word  . . . . . . . . . . 46
  B.6   Figure 6.  ACCEPTED Message  . . . . . . . . . . . . . 46
  B.7   Figure 7.  REFUSED Message . . . . . . . . . . . . . . 46
  B.8   Figure 8.  STATUS REQUEST Message  . . . . . . . . . . 47
  B.9   Figure 9.  STATUS Message  . . . . . . . . . . . . . . 47
  B.10   Figure 10.  HELLO Message . . . . . . . . . . . . . . 48
  B.11   Figure 11.  FORMAT ERROR Message  . . . . . . . . . . 48
  B.12   Figure 12.  RESTART REQUEST Message . . . . . . . . . 48
  B.13   Figure 13.  RESTART COMPLETE Message  . . . . . . . . 48
  B.14   Figure 14.  Stream Data Format  . . . . . . . . . . . 49
  B.15   Figure 15.  Request Message Format  . . . . . . . . . 50
  B.16   Figure 16.  Reply Message Format  . . . . . . . . . . 51
  B.17   Figure 17.  Notification Message Format . . . . . . . 52
  B.18   Figure 18.  Create Request Words  . . . . . . . . . . 53
  B.19   Figure 19.  Delete, Join, Leave Request Words . . . . 54





















                             - ii -

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                             PREFACE

     This document  describes  the  current  SATNET  Host  Access
protocol.    This  supersedes  PSPWN  #100  (SATNET  Host  Access
protocol) and PSPWN #104 (Host/SATNET  Stream  Access  protocol).
The differences are:

     (1)  The initialization  state  diagram  has  been
          changed  so  that neither side will enter the
          ON state unless both sides of the  connecting
          transmission line are working.

     (2)  A "Host type" field was added to the  Restart
          Request and Restart Complete messages.

     (3)  The numeric values of some error  codes  have
          been  changed,  as have the detailed meanings
          of some of those codes.

     (4)  Some Stream Reply Codes have been changed.

     (5)  Some significant changes have  been  made  to
          the  information  supplied by hosts in Stream
          Create and Change request messages.

     (6)  Hosts must use  the  acceptance/refusal  flow
          control strategy in response to data messages
          from  Satellite IMPs (i.e., this is no longer
          an option).
























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1.  Overview


     In determining an appropriate host access protocol,  several

factors   must  be  considered.   One  set  of  factors  concerns

regulation  of  transfers  in   either   direction   across   the

host-network interface.  A second set of factors concerns actions

associated  with the transmission of messages across the network.

While there are several different protocols  in  existence  which

deal  with  link  and  network access (e.g., SDLC and X.25), none

satisfies the totality of user services and other factors  unique

to  SATNET.  Thus to allow flexible exploration of access issues,

a special protocol was developed  for  the  network  transmission

level.   (For  implementation  convenience,  however, an existing

ARPANET link error control protocol was used to provide  reliable

interface transfers.)


     The  network-level  access  factors  include  the passing of

type-of-service information such as priority and delay class, the

passing of flow and congestion control information,  coordination

of   stream   data  messages  with  their  scheduled  times,  and

mechanisms for dynamic stream and addressing setups.


     The type-of-service information is dealt with by defining an

appropriate field in the message headers.  This  field  currently

consists  of  eight  bits  in  SATNET:  Two bits for message type

(datagram/stream and internet designations), two bits to  specify





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one  of  four  priorities,  two bits to specify one of four delay

classes, one bit to specify a holding time choice, and one bit to

specify reliability.  The delay class choices  presently  consist

of  one  second,  five  seconds, twenty seconds, and two minutes.

The holding time choice consists of either  twice  the  specified

delay  class  or two minutes.  The reliability choice consists of

"high", which causes  channel  retransmissions  to  be  used,  or

"standard",  which inhibits the use of retransmissions and allows

messages with bad data checksums (but  good  checksums  on  their

control   information)   to  be  delivered  to  users.   Standard

reliability is  designed  for  applications  which  can  tolerate

occasional  bit  errors, but cannot tolerate lost or out-of-order

packets (e.g. packet speech).


     Flow and congestion control information is passed by the use

of two distinct mechanisms.  First, status information reflecting

current congestion control  is  sent  in  all  data  and  related

control  messages;  in  the  absence  of other traffic, a special

status message is sent periodically.  This information  indicates

which  priority and delay classes are currently being accepted by

the network.



     The  second  mechanism  consists  of  specific   information

concerning  the  disposition  of  each data message passed to the

network.  Each data message  is  numbered  (modulo  128)  by  the





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sending  host  for  identification  purposes.   Upon receipt, the

network returns an "acceptance" or "refusal"  indication  to  the

host.   An acceptance implies the network believes it can deliver

the message to its  destination  and  is  proceeding  to  do  so;

delivery,  however,  is  not  guaranteed.   A  refusal  means the

network has discarded the message; in this case a refusal subcode

is included to indicate the  reason.   Messages  may  be  refused

because,  for example, the destination is down or does not exist,

because its priority or delay class is  not  being  accepted,  or

because  of temporary flow control reasons associated with source

or destination buffering.  In the  latter  case  the  message  is

assigned  to  one  of  several  categories  used  for  subsequent

notification purposes.   A  bit  representing  each  category  is

passed  along with the priority-delay class status information in

all messages.  When the message is  refused  the  number  of  the

assigned  category  is returned to the sender, with the values of

subsequent category status bits indicating when messages of  that

category will again be accepted.


     In  order  to minimize message delays and to schedule stream

slots efficiently,  the  coordination  of  stream  data  messages

involves  establishment  of  the correct time window in which the

host should pass each message to the network.  The present system

uses explicit accept/refuse messages to accomplish this.   Stream

and addressing setups are accomplished by using datagram messages

between the hosts and the network.



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Host/SATNET Protocol                                      IEN 192



2.  Satellite IMP Implementation Details


     For  each physical host port the software uses the following

data structures:

               Protocol state word,
               Category queues,
               Host output queue,
               Accept/refuse queue (HAQ),
               Interface property table,
               Last-heard word (LSTHRD).


     The protocol state word indicates whether the protocol is in

initialization or running state.  If in initialization state,  no

messages  will  be  accepted  or  delivered  until  the necessary

handshaking procedure, as described below, is completed.


     The four category queues are used  to  store  messages  that

have  been  refused  by  a host.  If a host refuses a message and

provides an appropriate category indication, the message will  be

stored  on  the specified category queue until the host indicates

it is willing to accept messages of that category.  If a  message

reaches  its maximum holding time before the host will accept it,

it will be dropped from the queue.



     The host output queue is a queue of data messages waiting to

be delivered to the appropriate host.  Ordered  by  urgency,  all

messages remain on this queue until they are transferred into the

HAQ  just  prior to being sent to the host or until their holding

time expires.




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     The HAQ is a storage location for a message  that  has  been

sent  to  the  host and is awaiting an accept or refuse response.

If a response is received, the message is removed from the queue.

If no response is received, the message will be removed when  its

maximum holding time has expired.


     The  interface  property table (IPT) is used to indicate any

of the various protocol options selected by  a  host.   One  such

option  is  whether  to allow piggybacking of Host/SATNET control

messages.  Another option is that the  host  may  decline  to  do

accept/reject  decisions on traffic that it receives from SATNET.


     The last-heard word indicates when the host was  last  heard

from.  The host is declared down if it has not been heard from in

a  specified time, currently about thirty seconds.  The host will

remain down until the necessary handshaking is completed.


2.1  Initialization


     Whenever a host or Satellite IMP is restarted,  it  sends  a

RESTART-REQUEST control message to the other party.  This message

will be repeated periodically until a RESTART-COMPLETE message is

received from the other party.  When the RESTART-COMPLETE message

is  received  by  the  first  party,  it  will  then  also send a

RESTART-COMPLETE message.








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2.2  Host-to-Satellite IMP Input


     For  datagram  traffic,   SATNET   accepts   variable-length

messages  of  up  to a fixed maximum length (currently 128 16-bit

words).  The host prefixes  each  message  with  a  header  which

provides   addressing   and  control  information.   The  control

information specifies priority, desired delivery  delay,  maximum

holding  time,  and  selection of message control options such as

reliability.  Each datagram message from a host  is  accepted  or

rejected  by  the  Satellite IMP according to the current network

loading and other factors.  The Satellite IMP  always  returns  a

control message indicating acceptance or refusal.


     The  message  control parameters have the following possible

values:  Priority is an integer from zero to  three,  with  three

being  the  lowest priority and zero the highest.  There are four

possible delay classes:  1 second, 5 seconds, 20 seconds, and 120

seconds.  There are two  possible  choices  for  maximum  holding

time:   twice the delay class or the maximum system holding time.

Reliability may be either normal or  high  reliability  (zero  or

one).


     Initially,  when  a  message is offered by a host, a minimal

amount of buffer is requested at high priority, and the header of

the message is copied into this buffer in SATNET internal format.

The  information  in  the  header  is  then  presented   to   the





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Accept/Reject  module.  If the message is accepted the additional

buffer needed is obtained, the rest of the message is copied into

the buffer, and an accept message is placed on the  accept/reject

notification  list  (ARN).   If  the  message  is  rejected,  the

appropriate rejection code is placed on the ARN  and  the  buffer

space  is  freed.   It  is left for the host to decide whether to

resubmit the message at a later time.  The  rejection  code  will

also  inform  the  host  of  which priority and delay classes are

currently being accepted, to assist the host  in  making  message

offering decisions.


     The  accept/reject  decision  is  performed as follows:  The

first checks are made to  ensure  a  valid  source  ID,  adequate

buffer in the Satellite IMP, and a message size not exceeding the

maximum  allowed  size.   The  Satellite  IMP must also be in the

In-Sync state to be able to accept the message.  For any  message

passing  these  tests,  its  TTG and priority are calculated, and

used to form its urgency value.



     Next a check is made to see if the message can be  delivered

to   the   destination,   which  involves  checking  a  table  of

destination parameters.  The  Satellite  IMP  verifies  that  the

destination  Satellite  IMP  and  host  are  up and are receiving

messages of the same category as the offered message.







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Host/SATNET Protocol                                      IEN 192



     Although  not  implemented,   the   following   checks   are

considered  useful.  If the reservation-making queue contains too

much traffic of higher urgency or if the number of  free  buffers

is below a fixed threshold, the message will be refused.  Also, a

maximum buffer usage may be assigned to each host for messages of

each  category;   messages  will  be  refused if the threshold is

exceeded for messages of the same category.


     A stream packet undergoes a different set of  checks  before

it  is accepted by the Satellite IMP.  The stream must exist, the

length of the message must not exceed the maximum  allowable  for

that stream, and there must be room available on the stream queue

for  another  packet.  Also, the offering host must be designated

as a member of that stream.


2.3  Satellite IMP-to-Host Output


     Whenever there is an opportunity to  send  a  message  to  a

host,  the  Satellite  IMP  will  examine every eligible queue to

determine which message should be sent first.  This includes  the

host  output  queue,  the  four  category queues, and the control

message waiting queue (ARN).  If ARN is longer than  a  specified

threshold,  sending of control messages will take precedence over

sending of data messages, until the ARN length  falls  below  the

threshold.  Otherwise, if piggybacking is allowed, a data message

will  be sent to the host with a control message piggybacked.  If





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no piggybacking  is  allowed,  then  control  messages  and  data

messages  will  compete.   Competition  is always on the basis of

urgency (priority and TTG).  A category queue may  be  ineligible

if  the  host is not accepting messages of that category.  In the

absence of other traffic, a Satellite IMP  will  send  a  'HELLO'

message  every  second.  Once a data message has been selected it

is queued in HAQ and transmitted to the host.


     If the host fails to respond in time, the  message  will  be

deleted  from  HAQ  when  its  holding time expires.  If the host

accepts, the message will be removed from HAQ and discarded.   If

the  host  refuses  and specifies a category, the message will be

removed from HAQ and placed on the specified category queue.   If

the  message  is refused for urgency reasons, the message will be

requeued on the host output queue.



     HAQ, the category queues, the host output  queues,  and  ARN

are  all  kept  ordered  by urgency, so that testing for the most

urgent message consists of testing only the first message on each

queue.















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Host/SATNET Protocol                                      IEN 192



3.  DATAGRAM ACCESS PROTOCOL


3.1  Overview


     For  datagram  traffic,   SATNET   accepts   variable-length

messages  of  up  to 128 words of data.  The source host prefixes

each message with a leader which provides addressing and  control

information.  The control information specifies message priority,

desired  delivery  delay,  delay  at  which the message should be

discarded if not yet  delivered  ("maximum  holding  time"),  and

selection of one or more message control options.


     Each  datagram  message  from  a  source host is accepted or

refused by the source Satellite IMP according to current  network

loading and other factors, with a control message always returned

by the source Satellite IMP indicating the acceptance or refusal.

Upon  acceptance,  SATNET  then  attempts  to deliver the message

within its specified delay.  However, it will continue trying  to

deliver  if  late,  until  its  maximum holding time is exceeded.

Datagram messages always have at least a two-hop delay (about 0.6

seconds) within SATNET.


     A  lower-level  protocol  is  assumed  to  provide  reliable

exchanges  of  data and control messages on the link connecting a

host and  Satellite  IMP,  and  is  assumed  transparent  to  the

protocol  defined  in this document.  The Honeywell 316 Satellite

IMPs use the ARPANET VDH protocol for this purpose.  The new  BBN




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C/30  Satellite  IMPs support in addition ARPANET 1822 local host

and distant host protocol.


3.2  Types of Service


     The type-of-service fields in  the  SATNET  leader  of  each

datagram  message allows the following choices to be specified to

SATNET for each message:


          Priority:  4 choices.  This is used in conjunction with

          the acceptable delivery delay to arbitrate  for  SATNET

          resources.


          Acceptable  Delivery  Delay:   1  second, 5 seconds, 20

          seconds, and 120 seconds.


          Holding Time:  This is the maximum time an  undelivered

          message should be held within SATNET before discarding.

          There   are  two  choices:   (1)  twice  the  specified

          delivery delay, or (2) the maximum system holding  time

          (currently about two minutes).


          Reliability:   There  are  two  choices, "standard" and

          "high".  If standard is specified, a satellite  channel

          acknowledgment  is  not used for the message, and it is

          not retransmitted by the source Satellite IMP  in  case

          of errors; if the packet is received at the destination

          Satellite  IMP with a good message leader but errors in




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          the data portion of the message, it is marked  as  such

          and delivered to its destination.


          If "high" is specified, the message is retransmitted as

          many times as necessary until a positive acknowledgment

          is  received  by  the  source  Satellite IMP, up to the

          specified message holding time (duplicate copies may be

          delivered to the destination host in this case).


3.3  Addressing


     SATNET uses logical addressing, with each  host  assigned  a

16-bit  permanent address.  Each data message sent to SATNET must

contain both a source and destination  logical  address,  and  is

delivered  to  the  specified destination(s) with these addresses

unchanged.  Table 6 in Appendix A contains the  current  list  of

addresses.


3.4  Message Length


     Datagrams may have variable lengths, where these lengths are

integer  multiples  of  16-bit words.  Maximum length of the data

portion of a datagram message is 128 words.  The  "data  portion"

excludes  the  SATNET  leader  but includes an internet header if

present.









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3.5  Host/SATNET Flow Control


     Each message sent by a host is accepted or  refused  by  the

Satellite IMP based upon various network and local congestion and

status  factors.   If  accepted,  an  ACCEPTED control message is

returned to the host containing the message ID  assigned  by  the

host  in  the data message leader.  If refused, a REFUSED control

message is returned containing the message ID and a refusal  code

C.   If the message itself is bad, a FORMAT ERROR control message

is returned containing the message ID.


     The value of C is used to  indicate  to  the  host  when  it

should  subsequently  retry the message.  This is accomplished by

also sending the host an "acceptance  status"  word  at  frequent

intervals   to  inform  it  of  the  categories  currently  being

accepted.  The host may ignore the  category  information  if  it

chooses,   or  map  the  categories  into  a  smaller  subset  if

convenient.  The use of the categories allows the host  to  retry

those  messages first which are most likely to be accepted by the

Satellite IMP.


     The  "acceptance  status"  word  also  contains  information

indicating  which  message  priorities and delivery delay classes

are currently being accepted, allowing the  host  to  also  avoid

unnecessarily  sending  new  messages which will be refused.  The

acceptance status word is sent to the  host  in  every  data  and





                             - 13 -

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control message from the Satellite IMP; in the absence of regular

traffic,  a "Hello" message containing the acceptance status word

is sent once per second.


     Hosts must return an explicit acceptance or refusal for each

message received from the Satellite IMP.  Note that  a  "refusal"

by  the  host  is  a  request to the Satellite IMP to requeue the

message in  question  and  submit  it  again  later.   Currently,

refused  messages are immediately discarded, so as to eliminate a

line congestion problem seen with the catenet.


3.6  Status Messages


     A host can request current status information from SATNET by

sending a "Status Request" control message.   The  Satellite  IMP

will  return  a  status  message containing the acceptance status

word, SATNET global time, and an indicator of the  current  delay

expected  for  each  delivery  delay  class.   (Inclusion of host

status information is still under study.)


3.7  Hello Messages


     When it  is  not  sending  data  or  control  messages,  the

Satellite  IMP  or  host must send a "Hello" control message once

every second.  In addition  to  providing  frequent  updating  of

acceptance  status,  the  hello  message  allows  the receiver to

maintain the up/down status of the  sender.   The  Satellite  IMP





                             - 14 -

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will  do this by resetting a timeout counter whenever anything is

received from the host; if the timeout expires, the host will  be

declared  down  and  the Satellite IMP will begin sending restart

messages as described in the next  section.   The  timeout  value

used by the Satellite IMP is thirty seconds.


3.8  Message Reference Numbers


     To  support  message-by-message  acknowledgements, each data

message is assigned a 7-bit "message  reference  number"  by  its

sender.   Its purpose is to allow referencing of the message in a

subsequent acknowledgement (ACCEPTED, REFUSED, or  FORMAT  ERROR)

message.   Reference numbers may be assigned in any order, except

that a particular number may not be reused until the  message  it

refers  to  has  been  acknowledged.   All  reference numbers are

automatically released  whenever  the  Host/SATNET  interface  is

reinitialized.


3.9  Initialization


     Since   the   Host/SATNET   protocol  requires  an  explicit

acknowledgement for each message,  the  initialization  procedure

ensures  that  full  two-way  communication  is  possible  before

allowing either side to begin transmitting data.  Whenever a host

or a Satellite IMP is  restarted,  it  sends  a  RESTART  REQUEST

control message to the other party once every ten seconds until a

RESTART  COMPLETE  is  received, at which time it sends a RESTART




                             - 15 -

Host/SATNET Protocol                                      IEN 192



COMPLETE.  The procedure in initializing the interface  is  shown

in Figure 1 in Appendix B.


     The  initialization action indicated in Figure 1 consists of

flushing all queued control messages waiting to be sent,  and  of

flushing  all  received  data  messages  for which an ACCEPTED or

REFUSED message has  not  already  been  sent.   Note  that  data

messages  waiting  to  be  sent  need  not  be  flushed; they can

continue to be  queued  during  the  'waiting'  state  and  their

transmission begun once the 'on' state is entered.


3.10  Format Errors


     Whenever  an invalid leader field value or message length is

detected in a received message a FORMAT ERROR control message  is

returned  to the sending host or Satellite IMP.  An error code is

returned in this  message  to  indicate  the  detected  condition

(these codes are defined in the Formats section of this document,

section 3.13).


3.11  Loop Detection


     To  allow  a  Satellite  IMP or host to detect situations in

which the interface may be externally  looped  (crosspatched),  a

Host/Satellite  IMP  address  bit  is  included  in  all data and

control messages, identifying the sender of each message.







                             - 16 -

Host/SATNET Protocol                                      IEN 192



3.12  Piggybacked Control Messages


     Control messages of the ACCEPT, REFUSE,  FORMAT  ERROR,  and

RESTART  COMPLETE  types may be piggybacked onto data messages by

including the control message in the 16-bit "piggyback" field  of

the  data  message  header.   The  Satellite  Imp  interprets all

piggybacked control information before examining the rest of  the

data  message,  so,  for  example, a piggybacked RESTART COMPLETE

takes effect immediately.


3.13  Formats


     The general format used for Host/SATNET interface  exchanges

is  shown  in  Figure  2  (all  figures  are in Appendix B).  The

control code always defines the message format;  for  all  except

data  messages,  it  also  implicitly defines the message length.

Exact data message length is assumed to be derived from the  host

or  Satellite  IMP  interface  transfer, and is always an integer

multiple of 16-bit words.


3.13.1  Control Codes


     Each distinct message type is identified by a  unique  4-bit

control code.  Codes currently defined are:











                             - 17 -

Host/SATNET Protocol                                      IEN 192



               1 = DATA
               2 = ACCEPTED
               3 = REFUSED
               4 = STATUS REQUEST
               5 = STATUS
               6 = HELLO
               7 = DATA WITH ERRORS
              13 = FORMAT ERROR
              14 = RESTART REQUEST
              15 = RESTART COMPLETE


3.13.2  Data Messages


     Figure  3  shows  the format for datagram DATA messages (the

width of each word in this and subsequent figures  is  16  bits).

Words  1,  2,  and 3 are defined by the interface sender (host or

Satellite IMP); words 4, 5, and 6 are defined by the source host.


       Word 1, datagram message control word:

     - H/S, bit 1:  0 = from Satellite IMP, 1 = from host

     - message ID, bits 2-8:  defined in Section 3.6

     - block length,  bits  9-12:   the  number  of  64-word

       blocks of data words following the message leader:  a

       block  length of 1 means the message contains between

       1 and 64 data words; a length of 2 means 65  and  128

       data  words,  etc.   The  maximum  datagram length is

       defined by Section 3.2.  A length of 0 means  a  null

       DATA message.


     - Control Code, bits 13-16:

       1 = DATA (no detected errors)





                             - 18 -

Host/SATNET Protocol                                      IEN 192



       7 = DATA  WITH  ERRORS  --  used  only  if  "standard

           reliability"  service  is  designated  and one or

           more data errors were detected by SATNET  in  the

           data  portion  of  the  message.  Applies only to

           packets delivered by SATNET to hosts.


       Word 2, acceptance status:  (defined below).


       Word 3, piggyback field:  This word may  contain  any

       of  the one-word control messages defined by codes 2,

       3, 13, and 15.  A value of zero means the word is not

       used.


       Word 4, Type of Service Word:  (defined below).


       Word 5, destination host:  a  16-bit  SATNET  logical

       address.


       Word   6,  source  host:   a  16-bit  SATNET  logical

       address.


3.13.2.1  Type of Service Word


     Figure 4 shows the  details  of  word  4  of  datagram  DATA

message leaders.

     M, bits 1-2:  DATA message type;

                   0 = datagram, internet format
                   1 = stream, internet format
                   2 = datagram, local format
                   3 = stream, local format




                             - 19 -

Host/SATNET Protocol                                      IEN 192



     P, bits 3-4:  priority; 0 = highest priority, 3 = lowest.


     D, bits 5-6:  acceptable delivery delay ("delay class");


                   delay class      value

                   -----------      -----

                        0          1 second
                        1          5 seconds
                        2         20 seconds
                        3        120 seconds

     H, bit 7:     holding time; 0 = maximum (120 seconds*),
                                 1 = twice the specified delay class.


     R, bit 8:     reliability;  0 = high, 1 = standard


3.13.2.2  Acceptance Status Word


     Figure  5  shows  the  details for this word.  If the entire

word equals 0, everything is being accepted.


     Category bits, bits  1-4:   each  bit  defines  the  current

     acceptance/refusal  status for the category corresponding to

     the bit number (e.g., bit 3 represents category 3).

          0 = accepting for this category
          1 = refusing


     Delay Class Priority, bits 5-16 (not currently implemented):

     each delay class is identified by its  position  within  the

     acceptance status word; e.g., bits 14-16 contain information

     for delay class 3.  The content of each three-bit field is a

     binary number defining the current priorities being accepted

     for that delay class, interpreted as follows:


                             - 20 -

Host/SATNET Protocol                                      IEN 192



               0 = all priorities accepted
               1 = lowest priority (3) not accepted
               2 = lowest two priorities (2,3) not accepted
               3 = lowest three priorities (1,2,3) not accepted
               7 = none accepted


3.13.3  ACCEPTED Messages


     Figure  6  shows  the  format  of  ACCEPTED  messages.   (An

acceptance status word, as defined in Figure  5,  is  always  the

second word of this and all other messages.)


     H/S, bit 1:  same as for Data messages.


     Data  message ID, bits 2-8:  The message ID of the DATA

     message being accepted.


     Control Code, bits 13-16:  ACCEPTED = 2.


3.13.4  REFUSED Messages


     Figure 7 shows the format of REFUSED messages.


     H/S, bit 1:  same as for DATA messages.


     Data message ID, bits 2-8:  The message ID of the  DATA

     message being refused.


     C,  bits  9-12:   refusal  category.  A value of 0 to 4

     means  a  refusal  due   to   temporarily   unavailable

     resources.   Messages refused with category values 1 to

     4 will not be accepted until the corresponding category




                             - 21 -

Host/SATNET Protocol                                      IEN 192



     bit  in  the  acceptance  status  word  becomes  0.   A

     category value of 0 means the refusal is because of the

     message   priority   and/or   delay  class;  acceptance

     information for this case is given by the  delay  class

     priority bits in the acceptance status word.  Values of

     C  have  the following meanings (acceptance status word

     information does not apply to values 8 to 15):

                 0 = refused for priority/delay class
                 1 = category 1  refusal
                 2 = category 2  refusal
                 3 = category 3  refusal
                 4 = category 4  refusal
                 5 = undefined
                 6 = undefined
                 7 = undefined
                *8 = data length greater than stream length
                 9 = destination host has been declared in a
                     "refusing" state
                10 = destination host is not reachable
                11 = destination host's Satellite IMP is not reachable
                12 = unrecognized destination address
                13 = destination host access not allowed
                14 = illegal source host
               *15 = no active stream with that stream ID
                     *(See Section 4 on stream access)


     Control Code, bits 13-16:  REFUSED = 3.


3.13.5  STATUS REQUEST Messages


     Figure 8 shows the format of STATUS  REQUEST  messages.

     H/S bit 1:  same as DATA messages.


     Control code, bits 13-16:  STATUS REPORT = 4.







                             - 22 -

Host/SATNET Protocol                                      IEN 192



3.13.6  STATUS MESSAGES


     Figure 9 shows the format of STATUS messages.


     Word 1:  H/S bit, bit 1 = same as DATA messages.


     Control Code, bits 13-16:  STATUS = 5.


     Word 2:  ACCEPTANCE STATUS


     Word   3,   SATNET  global  time:   current  internally

     synchronized clock time used by Satellite IMPs; unit of

     time = 10.24 milliseconds, maximum value  equals  2{16}

     -1 units.


     Words  4-7:   current  expected delay for the indicated

     delay class (values to be defined).


3.13.7  HELLO Message


     Figure 10 shows the status of Hello messages.


     H/S bit 1:  same as DATA messages.


     Control Code, bits 13-16:  HELLO = 6.


3.13.8  FORMAT ERROR Message


     Figure 11 shows the format of FORMAT ERROR messages.








                             - 23 -

Host/SATNET Protocol                                      IEN 192



     H/S bit 1:  same as DATA messages.


     Error Message ID, bits 2-8:  ID of  DATA  message  with

     format error (if applicable, otherwise = 0).


     Error Code, bits 9-12:

          0 = undefined error
          1 = data message length exceeds SATNET maximum size
          2 = illegal control code
          3 = block length disagrees with message length.
          4 = illegal control code in piggyback
              word of data message.
          5 = undefined category value (5-7)
              in REFUSED message.


     Control Code, bits 13-16:  FORMAT ERROR = 13.


Note:  Implementation of error codes 2 to 5 is optional; however,

a  FORMAT  ERROR message should be returned with (at least) error

codes 0 and 1 for illegal conditions.


3.13.9  RESTART REQUEST Message


     Figure 12 shows the format of RESTART REQUEST  message.


     H/S bit 1:  same as DATA messages.


     Host Type, bits 9-12:  presently undefined.


     Control Code, bits 13-16:  RESTART REQUEST = 14.










                             - 24 -

Host/SATNET Protocol                                      IEN 192



3.13.10  RESTART COMPLETE Message


     Figure   13   shows  the  format  of  RESTART  COMPLETE

     messages.


     H/S bit 1:  same as DATA messages.


     Host Type, bits 9-12:  presently undefined.


     Control Code, bits 13-16:  RESTART COMPLETE = 15.


Note:  All Restart Request and  Complete  messages  sent  by  the

Satellite IMP have bits 9 to 12 set to zero.

































                             - 25 -

Host/SATNET Protocol                                      IEN 192



4.  STREAM ACCESS PROTOCOLS

4.1  Overview


     In  addition  to datagram message service, SATNET provides a

service called 'stream'.  A stream is a sort of  virtual  circuit

in  which  information  must  be established within Satellite IMP

tables for the duration of  the  stream  use.   This  information

maintains  an  outstanding  reservation  for  the stream, causing

channel time to be scheduled at more or  less  regular  intervals

specified  in  the  stream setup.  This mechanism provides one of

the important performance properties of a stream which  motivates

its  use:   one-hop for each stream data packet (as opposed to at

least two hops for datagrams).


     Any number of hosts can use the same stream; host membership

is accomplished by special Host/SATNET messages.  Each stream  is

identified by a SATNET-assigned Stream ID, which has two distinct

functions.   The first is to allow Satellite IMPs to identify the

transmission properties being used for each stream  data  message

including  verification that the sending host is a stream member.

The  second  Stream  ID  function  is  its  optional  use  as   a

destination address in a stream or datagram data message, causing

delivery to the Stream ID members.  (Its use in datagram messages

allows  "out  of  band"  signaling  messages to be sent while the

stream data messages are also being sent.)






                             - 26 -

Host/SATNET Protocol                                      IEN 192



     The destination address in a  stream  data  message  is  not

limited  to  the  Stream  ID,  however; any SATNET address may be

used.  Thus, a host can set up a simplex stream in  which  it  is

the only member, and therefore the only sender, and send messages

to  different  hosts using the single stream.  Or, a set of hosts

can  use  a  single  stream  in  an  arbitrarily  shared   manner

(determined by the hosts) to send to arbitrary destinations.


     More typically, a stream would be used by a set of hosts for

voice  conferencing,  in which the Stream ID would be used as the

destination address.  Note that, in  all  cases  of  shared  use,

hosts  must  provide  a  protocol  to determine how the stream is

shared.  If every member host presented a stream data  packet  to

its  Satellite  IMP  at  the  same  time,  they would all be sent

simultaneously  in   the   satellite   channel   with   resultant

destructive interference.  (Note:  the addressing use of a Stream

ID   is  not  presently  implemented  --  only  permanent  SATNET

addresses, either point or group, are allowed.)


4.2  Stream Data Messages


     Stream data messages have the format  shown  in  Figure  14.

The  message  type  (bits  1  and  2  of word H4) may be either 1

(stream  data,  internet  format)  or  3  (stream   data,   local

format).(1)   Bits  3  to  16  of  word  H4  contain a Stream ID,

_________________________________________________________________
(1) Table 5 in Appendix A defines all SATNET message types.




                             - 27 -

Host/SATNET Protocol                                      IEN 192



assigned by  SATNET  as  described  in  the  next  section.   The

destination  address  in  word H5 can be any SATNET address.  The

source host address in word H6 must be assigned to the  Satellite

IMP  port  used  to  send  the message into SATNET, and must be a

member of the Stream ID.


     An ACCEPTED or REFUSED  message  will  be  returned  by  the

Satellite  IMP  for  each  stream  data  message, the same as for

datagrams.  Stream data messages will normally always be accepted

unless they are greater than (to  be  determined)  seconds  early

relative  to  the  next  stream slot time, or the stream is being

preempted by higher priority traffic.


     Internal SATNET channel acknowledgments  are  not  used  for

stream data messages.


4.3  Stream Request Replies and Notifications


     Streams  can  be  used  by a Host only by first establishing

certain information within SATNET.  The  following  requests  are

used:

                              CREATE
                              DELETE
                              JOIN
                              LEAVE
                              CHANGE


Each  request  is  sent  by  the  host,  along  with supplemental

information, in the data portion of a  datagram  message  to  the





                             - 28 -

Host/SATNET Protocol                                      IEN 192



SATNET Service Host.  A Request message is accepted or refused by

the  local  Satellite IMP the same as any other datagram message;

if accepted it is delivered to an internal host within the  local

Satellite  IMP  which  acts  on the request, returning a datagram

message reply to the source host indicating its disposition.  The

reply always contains a Request ID supplied by the  host  in  its

Request  message, allowing the host to relate the response to its

earlier request (more than one Request message may be outstanding

from a host; the  maximum  number  outstanding  will  be  limited

implicitly by the datagram message refusal mechanism).


     Figure  15 shows the format used for Request messages, which

are always local datagram (type 2).  The Request message priority

PR in header word H4 may  be  assigned  the  same  as  for  other

datagram  traffic.   The  source host address must be assigned to

the Satellite IMP port used for the message.  The first data word

D1 contains one of the Request Codes defined  in  Table  1.   The

Request  ID  in  word D2 is chosen by the host, and should not be

re-used until a Reply message is received with  the  same  ID  to

avoid ambiguity.


     Figure  16  shows  the format used for Reply messages, which

are always local datagram (type 2).  The values  of  PR  and  the

Request  ID  are  those used by the host in the Request messages;

the source host address of the Request message  is  used  as  the

destination  host address.  Word D1 contains a Reply Code in bits




                             - 29 -

Host/SATNET Protocol                                      IEN 192



2 to 16 indicating the action taken on  the  request;  bit  1  is

always  zero.   The  contents of words D3-D6 depends on the Reply

Code; whether used or not, all Reply messages  will  contain  six

data words.  Table 2 lists the possible Reply Codes.


     In addition to Reply messages, a Notification message may be

sent  to  a  host by SATNET concerning streams for which the host

has previously established membership.  The Notification  message

format  is  shown in Figure 17, and is also a local datagram type

message.  The notification message priority PS in word H4 is that

assigned  to  the  stream;  the  Stream  ID   involved   in   the

notification  is  contained in data word D2.  A Notification Code

is contained in bits 2 to 16 of word D1; bit 1 of  this  word  is

always  set  to 1 to distinguish Notification messages from Reply

messages.  As in  the  case  of  Reply  messages  a  Notification

message  always contains words D3 to D6, whose contents depend on

the Notification Code.  Table 4  lists  the  Notification  Codes.

(Notifications not implemented.)


     Except  when a request is refused as a result of information

local to the source Satellite IMP, Notification messages  have  a

delay of at least one satellite hop (about 0.25 seconds) relative

to  the  request  message, and are initiated at approximately the

same global SATNET time  by  Satellite  IMPs  to  their  involved

hosts.   A  Reply  indicating  successful  completion of a stream

request requires at least five seconds' delay  after  receipt  of

the request.


                             - 30 -

Host/SATNET Protocol                                      IEN 192



4.3.1  CREATE


     The  words  used in the data portion of a CREATE request are

shown in Figure 18.  Word D3 contains a zero if a new  stream  is

being  requested (the use of non-zero values will be defined in a

later version of the Host/SATNET access protocol).   Words  D4-D6

contain  a  Key which is used to authenticate subsequent requests

from any host concerning the stream.  Any  48-bit  value  may  be

used,  including  zero;  however,  all subsequent Request message

Keys for this stream must equal this Key to be honored.


     Words D7-D9 define the parameters to be used for the  stream

by  SATNET.   Ps  is  the priority to be used for the stream data

messages.  Bits 7 to 16 of word D7 indicate the maximum number  L

of 16-bit data words which will be sent in any data message using

the  stream.   The  queue  length, bits 1-4 of D8, is the maximum

number  of  messages  that  may  be  queued  at   once   awaiting

transmission  using  the stream.  The stream interval is a 24-bit

quantity indicating the time (in ten microsecond  units)  between

stream messages.  The high order eight bits are in D8, bits 9-16,

and  the  low 16 bits are in D9.  A suitable time for messages in

this stream will be computed using the stream interval given.


     If the CREATE is honored, Stream Started Reply message  will

be  returned  with  the  Stream  ID assigned by SATNET in word D3

(words D4-D6 are not used).  The first stream channel  allocation





                             - 31 -

Host/SATNET Protocol                                      IEN 192



is begun approximately one stream interval I following generation

of the Reply message.  If refused, a Stream Creation Refused will

be returned with a reason code in Word D3.


     Each  CREATE  message received with word D3 set to zero will

cause the  creation  of  a  new  stream  (resources  permitting),

independently  of  the Key or other field values contained in the

CREATE message.  Note that different streams  may  use  the  same

Key;   the   SATNET-assigned  Stream  ID  supplied  by  hosts  in

subsequent Request messages provides unique  referencing  to  the

stream in question.


4.3.2  DELETE


     Figure  19  shows  the  data words sent in a DELETE Request.

Word D1 contains the DELETE STREAM Request Code; word D2 contains

the host's Request ID (this ID need not relate  to  any  previous

Request  message);  word  D3  contains the Stream ID; words D4-D6

contain the Key.


     If the DELETE STREAM request is honored, the stream  channel

allocations will be terminated and a Stream Deleted Reply message

will  be  returned with the Stream ID in word D3; words D4-D6 are

not used.  In addition, a Notification message  ("Stream  Deleted

by  Host  X")  will  be  sent to all other member hosts, with the

address of the requesting host in D3 (not implemented yet); words

D4-D6 are not used.  All Satellite IMP table  entries  concerning




                             - 32 -

Host/SATNET Protocol                                      IEN 192



the stream will be cleared and the Stream ID released for re-use.


     If  the  DELETE  STREAM  request  is  not  honored, a Stream

Deletion Refused Reply will be returned.  Word D3  will  indicate

why  the  request  was  refused.   Table  3 indicates the refusal

reasons that can appear in D3.


4.3.3  JOIN


     A JOIN Request (not implemented yet) is used by  a  host  to

establish  membership  in  a stream previously created by another

host.   (It  is  the  responsibility  of  the  creating  host  to

distribute  the  assigned  Stream  ID  and  Key to those hosts it

wishes to have participate in the use of the stream.)  The format

shown in Figure 19 is used for  a  JOIN,  with  the  JOIN  STREAM

Request Code in word D1.


     If  the  stream  exists  and  the correct Key is supplied, a

Stream Joined Reply message is returned approximately one  stream

interval   prior to the next scheduled channel allocation for the

stream.  Word D3 of the reply message  contains  the  Stream  ID;

words D4-D6 contain the parameters currently used for the stream,

formatted  according  to  words  D7  to  D9 of Figure 5.  Also, a

Notification Code 3 Message ("Stream Joined by Host X")  is  sent

to  all  other  member hosts--word D3 contains the address of the

newly joined host;  words D4-D6 are not used.






                             - 33 -

Host/SATNET Protocol                                      IEN 192



     If the JOIN is not honored, a  Join  Refused  Reply  message

will be returned with a Table error code as appropriate.


4.3.4  LEAVE


     A  host may leave a stream of which it is a member by use of

a LEAVE Request (not implemented yet).  The format of  Figure  19

is  again  used, with the Request Code = 4.  If the stream exists

and the host is entered as a member in Satellite  IMP  tables,  a

Stream Left Reply message will be returned.  Note that the Key is

not  used  for this request.  Also, a Notification Code 4 message

("Stream Left by Host X") is sent to all member  hosts;  word  D3

contains  the  address  of the departed host; words D4-D6 are not

used.


     A Leave request will always succeed.  However, in some cases

it may be impossible to notify other stream members of the event.

If so,  a  Leave  Without  Notification  Reply  message  will  be

returned.   Word  D3  will indicate why notification could not be

made.


4.3.5  CHANGE


     A host can request changes  to  the  parameters  defining  a

stream  by  use  of a CHANGE request (not implemented yet).  This

message is identical to the CREATE message format of  Figure  18,

except  that  the Request code is set to 5 in word D1 and word D3





                             - 34 -

Host/SATNET Protocol                                      IEN 192



contains the Stream ID.  All parameters of words  D7-D9  must  be

defined,  whether  or  not  their  values are being changed -- if

allowed, the parameters will be  used  to  re-define  the  stream

characteristics  just  as if they were being supplied in a CREATE

request.


     If the changes  can  be  honored,  a  Stream  Changed  Reply

message  will  be  returned  with the Stream ID in word D3; words

D4-D6 are not used.  Also, a Notification code 5 message ("Stream

Changed by Host X") is sent to the other member hosts;   word  D3

contains  the address of the requesting host; words D4-D6 contain

the new parameters.


     If the changes cannot be  made,  a  Stream  Changes  Refused

Reply  message  is returned to the requesting host, with a reason

code in word D3 (see Table 3).


4.4  SATNET Termination and Suspension of Streams


     A stream termination will be initiated by SATNET  if  (1)  a

data message is not sent in the stream by any of the member hosts

for  sixty  seconds,  or  (2)  if the stream's channel allocation

cannot be honored for N (to  be  determined)  consecutive  stream

intervals I due to higher priority traffic (not implemented yet).

If either of these occurs, a Notification code 1 message ("Stream

Deleted  by  SATNET")  will  be  sent to all member hosts with an

appropriate reason code in word D3; words  D4-D6  are  not  used.




                             - 35 -

Host/SATNET Protocol                                      IEN 192



All  Satellite  IMP  table  entries concerning the stream will be

deleted and the Stream ID released for re-use.


     Whenever a stream's channel allocation has not been  honored

by SATNET for M (to be determined) consecutive stream intervals I

following  the  last  allocation,  a  Notification Code 6 message

("Stream Suspended") is sent to all member hosts (M will be  much

smaller  than  N).   If  the  allocations  are  resumed  before N

consecutive  non-allocations,  a  Notification  Code  7   message

("Stream  Resumed")  is sent to all member hosts.  (These are not

implemented yet.)

































                             - 36 -

Host/SATNET Protocol                                      IEN 192



5.  Land Modem Interface


     A Satellite IMP communicates with other IMPs and  with  Very

Distant  Hosts via communication circuits, such as those provided

by the various common carriers  (Bell,  Western  Electric,  etc.)

The  exact  nature  of  the synchronous modems and dedicated full

duplex lines varies from site to site.  The hardware interface to

the modem is  the  standard  BBN  IMP-modem  interface  which  is

logically  identical to the Bell 303 interface with the exception

that the mark and space convention  is  inverted  for  characters

sent  to the modem (i.e., binary "one" equals high current).  The

control lines, however, are not inverted.


6.  Local Host Interface


     Local Host computers interface to the Satellite  IMP  via  a

hardware  interface which is electrically equivalent to that used

in  the  ARPANET  between  hosts  and   IMPs.    The   electrical

specification  for  this interface appears in BBN Report No. 1822

entitled "Specifications for the Interconnection of a host and an

IMP".















                             - 37 -

Host/SATNET Protocol                                      IEN 192



APPENDIX A.


A.1   Table 1 -- Request Codes

          1 -- Create Stream
          2 -- Delete Stream
          3 -- Join Stream
          4 -- Leave Stream
          5 -- Change Stream Parameters




A.2   Table 2 -- Reply Codes

          1 -- Stream Started
          2 -- Stream Deleted
          3 -- Stream Joined
          4 -- Stream Left
          5 -- Stream Changed
          6 -- Stream Creation Refused
          7 -- Stream Deletion Refused
          8 -- Join Refused
          9 -- Leave without Notification
         10 -- Stream Changes Refused
         11 -- Illegal Request Code


























                             - 38 -

Host/SATNET Protocol                                      IEN 192



A.3   Table 3 -- Error Codes in D3

          0 -- System busy; unable to handle request
          1 -- Unimplemented function
          2 -- Invalid protection Key
          3 -- Not member of stream
          4 -- Stream does not exist
          5 -- Net trouble
          6 -- Insufficient resources to handle request
          7 -- Improper format for request
          8 -- Channel protocol does not support streams
          9 -- Illegal argument in stream request
         10 -- Channel access not allowed




A.4   Table 4 -- Notification Codes

          1 -- Stream Deleted by SATNET
          2 -- Stream Deleted by Host X
          3 -- Stream Joined by Host X
          4 -- Stream Left by Host X
          5 -- Stream Changed by Host X
          6 -- Stream Suspended
          7 -- Stream Resumed




A.5   Table 5 -- SATNET Data Message Types

          0 -- Datagram, internet format
          1 -- Stream data, internet format
          2 -- Datagram, local format
          3 -- Stream data, local format

















                             - 39 -

Host/SATNET Protocol                                      IEN 192



A.6   Table 6 -- SATNET Logical Address Map

          0 -- 0 SATNET Service Host
          1 -- Etam EXPAK fake host
          2 -- Goonhilly EXPAK fake host
          3 -- Tanum EXPAK fake host
          4 -- Clarksburg EXPAK fake host
          5 -- Etam Message Generator/Sink #0
          6 -- Etam Message Generator/Sink #1
          7 -- Etam Message Generator/Sink #2
          8 -- Etam Message Generator/Sink #3
          9 -- Goonhilly Message Generator/Sink #0
         10 -- Goonhilly Message Generator/Sink #1
         11 -- Goonhilly Message Generator/Sink #2
         12 -- Goonhilly Message Generator/Sink #3
         13 -- Tanum Message Generator/Sink #0
         14 -- Tanum Message Generator/Sink #1
         15 -- Tanum Message Generator/Sink #2
         16 -- Tanum Message Generator/Sink #3
         17 -- Clarksburg Message Generator/Sink #0
         18 -- Clarksburg Message Generator/Sink #1
         19 -- Clarksburg Message Generator/Sink #2
         20 -- Clarksburg Message Generator/Sink #3
         21 -- Etam Internal Gateway
         22 -- Goonhilly Internal Gateway
         23 -- Tanum Internal Gateway
         24 --   unused
         25 --   unused
         26 --   unused
         27 --   unused
         28 --   unused
         29 --   unused
         30 -- Clarksburg Internal Gateway
         31 --   unused
         32 --   unused
         33 --   unused
         34 --   unused
         35 --   unused
         36 --   unused
         37 -- Universal Message Sink (equivalent name)
         38 -- Tanum NDRE Gateway
         39 -- Clarksburg COMSAT Gateway
         40 -- Universal Satellite Echo (equivalent name)
         41 -- Etam Monitor Fake Host
         42 -- Goonhilly Monitor Fake Host
         43 -- Tanum Monitor Fake Host
         44 -- Clarksburg Monitor Fake Host
         45 -- Etam Packet Core Fake Host
         46 -- Goonhilly Packet Core Fake Host




                             - 40 -

Host/SATNET Protocol                                      IEN 192



         47 -- Tanum Packet Core Fake Host
         48 -- Clarksburg Packet Core Fake Host
         49 -- Etam TTY Fake Host
         50 -- Goonhilly TTY Fake Host
         51 -- Tanum TTY Fake Host
         52 -- Clarksburg TTY Fake Host
         53 -- Etam DDT Fake Host
         54 -- Goonhilly DDT Fake Host
         55 -- Tanum DDT Fake Host
         56 -- Clarksburg DDT Fake Host
         57 --   unused
         58 --   unused
         59 --   unused
         60 -- Goonhilly UCL Gateway
         61 -- Etam BBN Gateway
         62 -- Etam Echo Fake Host
         63 -- Goonhilly Echo Fake Host
         64 -- Tanum Echo Fake Host
         65 -- Clarksburg Echo Fake Host
         66 --   unused
         67 --   unused
         68 --   unused
         69 --   unused
         70 --   unused
         71 --   unused

         72 -- Raisting External Gateway
         73 --   unused
         74 --   unused
         75 --   unused
         76 -- Raisting Internal Gateway
         77 -- Raisting Echo Fake Host
         78 -- Raisting Monitor Fake Host
         79 -- Raisting EXPAK Fake Host
         80 -- Raisting Packet Core Fake Host
         81 -- Raisting DDT Fake Host
         82 -- Raisting TTY Fake Host
         83 -- Raisting Message Generator/Sink #0
         84 -- Raisting Message Generator/Sink #1
         85 -- Raisting Message Generator/Sink #2
         86 -- Raisting Message Generator/Sink #3
         87 --   unused

         88 -- Fucino External Gateway
         89 --   unused
         90 --   unused
         91 --   unused
         92 -- Fucino Internal Gateway
         93 -- Fucino Echo Fake Host




                             - 41 -

Host/SATNET Protocol                                      IEN 192



         94 -- Fucino Monitor Fake Host
         95 -- Fucino EXPAK Fake Host
         96 -- Fucino Packet Core Fake Host
         97 -- Fucino DDT Fake Host
         98 -- Fucino TTY Fake Host
         99 -- Fucino Message Generator/Sink #0
        100 -- Fucino Message Generator/Sink #1
        101 -- Fucino Message Generator/Sink #2
        102 -- Fucino Message Generator/Sink #3
        103 --   unused











































                             - 42 -

Host/SATNET Protocol                                      IEN 192

                                +-------+
                                |       |
                                |  OFF  |
                                |       |
   TO = TIMEOUT                 +-------+
   RR = RESTART REQUEST             |
   RC = RESTART COMPLETE            |INIT & SEND RR
                                    |
                                    V
                                +-------+
                            --->|       |---------------
                          /     |       |                \
                         |      |       |                 \
                         |      |  INIT |                  |
                         |      |       |<---              |
                          \     |       |     \            |
                            ----|       |      |           |
                      10 SEC TO +-------+      |           |
                       -------      |          |           |
                       SEND RR      |RCVD RR   |           |
                                    | -----    |           |
                                    |SEND RC   |           |
                                    V          |           |
                                +-------+      |           |
                 -------------->|       |      |           |
               /                |       |     /            |
              /             --->|       |----              |
             |            /     |WAITING| 100 SEC TO       |
             |           |      |       |  --------        |
             |            \     |       |  SEND RR         |
             |              ----|       |                  |
             |        10 SEC TO +-------+                  |
             |         -------      |                      |
             |         SEND RC      |                      |
             |                      |RCVD RC               |
             |                      |                      |
              \                     V                     /
               \                +-------+                /
                 ---------------|       |<--------------
               RCVD RR          |   ON  |       RCVD RC
             ------------       |       |        -----
            INIT & SEND RC      +-------+       SEND RC



B.1   Figure 1.  Restart State Diagram









                             - 43 -

Host/SATNET Protocol                                      IEN 192



   MSB                                                           LSB

     1   2   3   4   5   6   7   8   9   10  11  12  13  14  15  16
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
H1 |H/S|                                           | CONTROL CODE  |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
H2 |                    ACCEPTANCE STATUS WORD                     |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
   |                                                               |
   |                                                               |
   |                                                               |
   |                                                               |
   |                                                               |
   |                                                               |
   |                                                               |
   |                                                               |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+


B.2   Figure 2.  General Message Format

































                             - 44 -

Host/SATNET Protocol                                      IEN 192



   MSB                                                           LSB

     1   2   3   4   5   6   7   8   9   10  11  12  13  14  15  16
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
H1 |H/S|        MESSAGE ID         | BLOCK LENGTH  | CODE = 1 or 7 |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
H2 |                    ACCEPTANCE STATUS WORD                     |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
H3 |                        PIGGYBACK WORD                         |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
H4 |                        TYPE OF SERVICE                        |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
H5 |                  DESTINATION HOST(S) ADDRESS                  |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
H6 |                      SOURCE HOST ADDRESS                      |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
D1 |                                                               |
   |                                                               |
   |                             DATA                              |
   |                                                               |
DN |                                                               |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+


B.3   Figure 3.  Block DATA Message




























                             - 45 -

Host/SATNET Protocol                                      IEN 192



   MSB                                                           LSB

     1   2   3   4   5   6   7   8   9   10  11  12  13  14  15  16
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
H4 |   M   |   P   |   D   | H | R |           (unused)            |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+

B.4   Figure 4.  Type of Service Word





     1   2   3   4   5   6   7   8   9   10  11  12  13  14  15  16
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
H2 |       C       |     D0    |     D1    |     D2    |     D3    |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+

B.5   Figure 5.  Acceptance Status Word





     1   2   3   4   5   6   7   8   9   10  11  12  13  14  15  16
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
H1 |H/S|      DATA MESSAGE ID      |   (unused)    |   CODE = 2    |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
H2 |                    ACCEPTANCE STATUS WORD                     |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+

B.6   Figure 6.  ACCEPTED Message





     1   2   3   4   5   6   7   8   9   10  11  12  13  14  15  16
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
H1 |H/S|      DATA MESSAGE ID      |       C       |   CODE = 3    |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
H2 |                    ACCEPTANCE STATUS WORD                     |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+

B.7   Figure 7.  REFUSED Message








                             - 46 -

Host/SATNET Protocol                                      IEN 192



   MSB                                                           LSB

     1   2   3   4   5   6   7   8   9   10  11  12  13  14  15  16
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
H1 |H/S|                 (unused)                  |   CODE = 4    |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
H2 |                    ACCEPTANCE STATUS WORD                     |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+


B.8   Figure 8.  STATUS REQUEST Message




   MSB                                                           LSB

     1   2   3   4   5   6   7   8   9   10  11  12  13  14  15  16
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
H1 |H/S|                 (unused)                  |    CODE = 5   |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
H2 |                    ACCEPTANCE STATUS WORD                     |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
H3 |                      SATNET GLOBAL TIME                       |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
H4 |                   CURRENT DELAY FOR CLASS 0                   |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
H5 |                   CURRENT DELAY FOR CLASS 1                   |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
H6 |                   CURRENT DELAY FOR CLASS 2                   |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
H7 |                   CURRENT DELAY FOR CLASS 3                   |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+


B.9   Figure 9.  STATUS Message

















                             - 47 -

Host/SATNET Protocol                                      IEN 192
   MSB                                                           LSB

     1   2   3   4   5   6   7   8   9   10  11  12  13  14  15  16
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
H1 |H/S|                 (unused)                  |   CODE = 6    |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
H2 |                    ACCEPTANCE STATUS WORD                     |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+

B.10   Figure 10.  HELLO Message





     1   2   3   4   5   6   7   8   9   10  11  12  13  14  15  16
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
H1 |H/S|     ERROR MESSAGE ID      |  ERROR CODE   |   CODE = 13   |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
H2 |                    ACCEPTANCE STATUS WORD                     |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+

B.11   Figure 11.  FORMAT ERROR Message





     1   2   3   4   5   6   7   8   9   10  11  12  13  14  15  16
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
H1 |H/S|         (unused)          |   HOST TYPE   |   CODE = 14   |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
H2 |                    ACCEPTANCE STATUS WORD                     |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+

B.12   Figure 12.  RESTART REQUEST Message





     1   2   3   4   5   6   7   8   9   10  11  12  13  14  15  16
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
H1 |H/S|         (unused)          |   HOST TYPE   |   CODE = 15   |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
H2 |                    ACCEPTANCE STATUS WORD                     |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+

B.13   Figure 13.  RESTART COMPLETE Message







                             - 48 -

Host/SATNET Protocol                                      IEN 192



   MSB                                                           LSB

     1   2   3   4   5   6   7   8   9   10  11  12  13  14  15  16
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
H1 |H/S|        MESSAGE ID         | BLOCK LENGTH  | CODE = 1 or 7 |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
H2 |                    ACCEPTANCE STATUS WORD                     |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
H3 |                        PIGGYBACK WORD                         |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
H4 | 1 or 3|                   STREAM ID                           |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
H5 |                  DESTINATION HOST(S) ADDRESS                  |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
H6 |                      SOURCE HOST ADDRESS                      |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
   |                                                               |
   |                                                               |
   |                  (INTERNET HEADER IF TYPE=1)                  |
   |                                                               |
   |                                                               |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
D1 |                                                               |
   |                                                               |
   |                           STREAM DATA                         |
   |                                                               |
DN |                                                               |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+


B.14   Figure 14.  Stream Data Format






















                             - 49 -

Host/SATNET Protocol                                      IEN 192



   MSB                                                           LSB

     1   2   3   4   5   6   7   8   9   10  11  12  13  14  15  16
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
H1 |H/S|        MESSAGE ID         | BLOCK LENGTH  | CODE = 1 or 7 |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
H2 |                    ACCEPTANCE STATUS WORD                     |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
H3 |                        PIGGYBACK WORD                         |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
H4 |0 or 2 |   PR  |   0   | 0 | 0 |           (unused)            |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
H5 |           SATNET SERVICE HOST (CURRENT ADDRESS = 0)           |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
H6 |                      SOURCE HOST ADDRESS                      |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
   |                                                               |
   |                                                               |
   |                  (INTERNET HEADER IF TYPE=0)                  |
   |                                                               |
   |                                                               |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
D1 |                         REQUEST CODE                          |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
D2 |                          REQUEST ID                           |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
D3 |                                                               |
   |                                                               |
   |                      REQUEST INFORMATION                      |
   |                                                               |
DN |                                                               |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+


B.15   Figure 15.  Request Message Format


















                             - 50 -

Host/SATNET Protocol                                      IEN 192



   MSB                                                           LSB

     1   2   3   4   5   6   7   8   9   10  11  12  13  14  15  16
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
H1 |H/S|        MESSAGE ID         | BLOCK LENGTH  | CODE = 1 or 7 |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
H2 |                    ACCEPTANCE STATUS WORD                     |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
H3 |                        PIGGYBACK WORD                         |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
H4 |0 or 2 |   PR  |   0   | 0 | 0 |           (unused)            |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
H5 |                        REQUESTING HOST                        |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
H6 |           SATNET SERVICE HOST (CURRENT ADDRESS = 0)           |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
   |                                                               |
   |                                                               |
   |                  (INTERNET HEADER IF TYPE=0)                  |
   |                                                               |
   |                                                               |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
D1 | 0 |                      REPLY CODE                           |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
D2 |                          REQUEST ID                           |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
D3 |                                                               |
   |                                                               |
   |                       REPLY INFORMATION                       |
   |                                                               |
D6 |                                                               |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+


B.16   Figure 16.  Reply Message Format


















                             - 51 -

Host/SATNET Protocol                                      IEN 192



   MSB                                                           LSB

     1   2   3   4   5   6   7   8   9   10  11  12  13  14  15  16
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
H1 |H/S|        MESSAGE ID         | BLOCK LENGTH  | CODE = 1 or 7 |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
H2 |                    ACCEPTANCE STATUS WORD                     |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
H3 |                        PIGGYBACK WORD                         |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
H4 |0 or 2 |   PS  |   0   | 0 | 0 |           (unused)            |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
H5 |                          MEMBER HOST                          |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
H6 |           SATNET SERVICE HOST (CURRENT ADDRESS = 0)           |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
   |                                                               |
   |                                                               |
   |                  (INTERNET HEADER IF TYPE=0)                  |
   |                                                               |
   |                                                               |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
D1 | 1 |                   NOTIFICATION CODE                       |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
D2 |                           STREAM ID                           |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
D3 |                                                               |
   |                                                               |
   |                    NOTIFICATION INFORMATION                   |
   |                                                               |
D6 |                                                               |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+


B.17   Figure 17.  Notification Message Format


















                             - 52 -

Host/SATNET Protocol                                      IEN 192



   MSB                                                           LSB

     1   2   3   4   5   6   7   8   9   10  11  12  13  14  15  16
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
D1 |                       REQUEST CODE = 1                        |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
D2 |                          REQUEST ID                           |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
D3 |                         STREAM ID = 0                         |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
D4 |                                                               |
   |                                                               |
D5 |                              KEY                              |
   |                                                               |
D6 |                                                               |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
D7 |   (unused)    |   PS  |        MAXIMUM DATA LENGTH (L)        |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
D8 | QUEUE LENGTH  |   (unused)    |  STREAM INTERVAL (HIGH BITS)  |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
D9 |                   STREAM INTERVAL (LOW BITS)                  |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+


B.18   Figure 18.  Create Request Words




























                             - 53 -

Host/SATNET Protocol                                      IEN 192



   MSB                                                           LSB

     1   2   3   4   5   6   7   8   9   10  11  12  13  14  15  16
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
   |                                 = 2:  DELETE                  |
D1 |                    REQUEST CODE = 3:  JOIN                    |
   |                                 = 4:  LEAVE                   |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
D2 |                          REQUEST ID                           |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
D3 |                           STREAM ID                           |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
D4 |                                                               |
   |                                                               |
D5 |                              KEY                              |
   |                                                               |
D6 |                                                               |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+


B.19   Figure 19.  Delete, Join, Leave Request Words
































                             - 54 -