rfc802.txt

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     the  Link  Field,  and  should  contain  values specified in

     Assigned  Numbers  [3]  appropriate  for  the   host-to-host

     protocol being used.


Bits 77-80: Sub-type:

     This field is used as a modifier by message types 0,  2,  4,

     and 8.







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RFC 802                                           Andrew G. Malis



Bits 81-96: Unused, must be zero.
















































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RFC 802                                           Andrew G. Malis



3.2  IMP-to-Host 1822L Leader Format





               1      4 5      8 9             16
              +--------+--------+----------------+
              |        |  1822L |                |
              | Unused |  I2H   | Handling Type  |
              |        |  Flag  |                |
              +--------+--------+----------------+
               17    20 21 22 24 25            32
              +--------+-+------+----------------+
              |        |T|Leader|                |
              | Unused |R|Flags |  Message Type  |
              |        |C|      |                |
              +--------+-+------+----------------+
               33                              48
              +----------------------------------+
              |                                  |
              |           Source Host            |
              |                                  |
              +----------------------------------+
               49                              64
              +----------------------------------+
              |                                  |
              |         Destination Host         |
              |                                  |
              +----------------------------------+
               65                     76 77    80
              +-------------------------+--------+
              |                         |        |
              |       Message ID        |Sub-type|
              |                         |        |
              +-------------------------+--------+
               81                              96
              +----------------------------------+
              |                                  |
              |          Message Length          |
              |                                  |
              +----------------------------------+

           Figure 7. IMP-to-Host 1822L Leader Format





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RFC 802                                           Andrew G. Malis



Bits 1-4: Unused and set to zero.


Bits 5-8: 1822L IMP-to-Host Flag:

     This field is set to decimal 14 (1110 in binary).


Bits 9-16: Handling Type:

     This has the value assigned by the source host (see  section

     3.1).   This  field is only used in message types 0, 5-9, 11

     and 15.


Bits 17-20: Unused and set to zero.


Bit 21: Trace Bit:

     If equal to one, the source host designated this message for

     tracing as it proceeds through the network.  See 1822(5.5).


Bits 22-24: Leader Flags:

     Bit 22: Available as a destination host flag.

     Bits 23-24: Reserved for future use, set to zero.


Bits 25-32: Message Type:

     Type 0: Regular Message  -  All  host-to-host  communication

          occurs  via  regular  messages, which have several sub-

          types.  The sub-type field (bits 77-80) is the same  as

          sent in the host-to-IMP leader (see section 3.1).

     Type 1: Error in Leader - See 1822(3.4).




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RFC 802                                           Andrew G. Malis



     Type 2: IMP Going Down - See 1822(3.4).

     Type 3: NDM Reply - This is a reply to the  NDM  host-to-IMP

          message  (see  section  3.1).   It  will  have the same

          number of entries as the  NDM  message  that  is  being

          replying  to,  and  each  listed  1822L  name  will  be

          accompanied by a zero or a one.  A zero signifies  that

          the  name  is  not  effective, and a one means that the

          name is now effective.

     Type 4: NOP - The host should discard this message.   It  is

          used    during    initialization    of   the   IMP/host

          communication.  The Destination Host field will contain

          the  1822L  Address of the host port over which the NOP

          is being sent.  All other fields are unused.

     Type 5: Ready for Next Message (RFNM) - See 1822(3.4).

     Type 6: Dead Host Status - See 1822(3.4).

     Type 7: Destination Host or IMP Dead  (or  unknown)  -  This

          message  is  sent  in  response  to  a  message  for  a

          destination which the IMP cannot reach.  The message to

          the "dead" destination is discarded.  See 1822(3.4) for

          a complete list of the applicable sub-types.   If  this

          message  is in response to a standard (type 0, sub-type

          0 or 1) message, it will be followed  by  a  Dead  Host

          Status  message,  which gives further information about




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RFC 802                                           Andrew G. Malis



          the status of the dead host.  If  this  message  is  in

          response  to  an uncontrolled (type 0, sub-type 2 or 3)

          message, only sub-type 1 (The destination host  is  not

          up) will be used, and it will not be followed by a Dead

          Host Status message.

     Type 8: Error in Data - See 1822(3.4).

     Type 9: Incomplete Transmission - The  transmission  of  the

          named  message  was  incomplete  for  some  reason.  An

          incomplete transmission message is similar to  a  RFNM,

          but  is  a  failure  indication  rather  than a success

          indication.  This message is also used  by  the  short-

          blocking feature to indicate that the named message was

          rejected because it would have caused to IMP  to  block

          the  host  for  a long amount of time.  See section 2.4

          for more details concerning the short-blocking feature.

          The message's sub-types are:

          0: The destination host  did  not  accept  the  message

               quickly enough.

          1: The message was too long.

          2: The host took more than 15 seconds to  transmit  the

               message  to  the  IMP.  This time is measured from

               the last bit of the leader through the last bit of

               the message.




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RFC 802                                           Andrew G. Malis



          3: The message was lost in the network due  to  IMP  or

               circuit failures.

          4: The IMP could not accept the entire  message  within

               15 seconds because of unavailable resources.  This

               sub-type is only used in  response  to  non-short-

               blocking  messages.   If  a short-blocking message

               timed out, it will be responded to with one of the

               sub-types 6-10.

          5: Source IMP I/O failure occurred  during  receipt  of

               this message.

          Sub-types 6-10 are all issued in response to  a  short-

          blocking  message that timed out (would have caused the

          host to become blocked for a long amount of time).  The

          sub-types are designed to give the host some indication

          of why it timed out and what other messages would  also

          time  out.   See  section  2.4.2  for  further  details

          concerning each of these sub-types.

          6: The message timed out because of  connection  set-up

               delay.   Further  messages to the same host (if on

               the same connection) may also be affected.

          7: The message timed out  because  of  end-to-end  flow

               control.  Further messages to the same host on the

               same connection will also be affected.




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RFC 802                                           Andrew G. Malis



          8: Destination IMP buffer shortage caused  the  message

               to  time  out.  This affects multi-packet standard

               messages  to  the  specified  host,  but   shorter

               messages  or  messages  to hosts on other IMPs may

               not be affected.

          9: Network congestion control caused the message to  be

               rejected.  Messages to hosts on other IMPs may not

               be affected, however.

          10: Local resource shortage kept  the  IMP  from  being

               able  to  accept  the  message  within  the short-

               blocking timeout period.

          11-15: Unassigned.

     Type 10: Interface Reset - See 1822(3.4).

     Type 15: 1822L Name or Address Error - This message is  sent

          in  response  to  a  type  0  message  from a host that

          contained an erroneous Source Host or Destination  Host

          field.  Its sub-types are:

          0: The Source Host 1822L name is not authorized or  not

               effective.

          1: The Source Host 1822L address  does  not  match  the

               host port used to send the message.

          2: The Destination Host 1822L name is not authorized.

          3: The Destination Host 1822L name  is  authorized  but




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RFC 802                                           Andrew G. Malis



               not  effective,  even though the named host is up.

               If the host were actually down, a type  7  message

               would be returned, not a type 15.

          4: The Source or  Destination  Host  field  contains  a

               1822L  name,  but the host being addressed is on a

               non-C/30 IMP (see Figure 4 in section 2.2).

          5-15: Unassigned.

     Types 11-14,16-255: Unassigned.


Bits 33-48: Source Host:

     For type 0 messages, this field contains the 1822L  name  or

     address  of  the  host  that  originated  the  message.  All

     replies to the message should be sent to the host  specified

     herein.   For  message  types  5-9,  11  and  15, this field

     contains the source host field used in  a  previous  type  0

     message sent by this host.


Bits 49-64: Destination Host:

     For type 0 messages, this field contains the 1822L  name  or

     address  that  the  message  was  sent  to.  This allows the

     destination host to detect  how  it  was  specified  by  the

     source  host.   For message types 5-9, 11 and 15, this field

     contains the destination host field used in a previous  type

     0 message sent by this host.




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RFC 802                                           Andrew G. Malis



Bits 65-76: Message ID:

     For message types 0, 5, 7-9, 11 and 15, this  is  the  value

     assigned  by  the  source  host to identify the message (see

     section 3.1).  This field is also used by  message  types  2

     and 6.


Bits 77-80: Sub-type:

     This field is used as a modifier by message types 0-2,  4-7,

     9, 11 and 15.


Bits 81-96: Message Length:

     This field is contained in type 0 and type 3 messages  only,

     and  is  the actual length in bits of the message (exclusive

     of leader, leader padding, and hardware padding) as computed

     by the IMP.






















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RFC 802                                           Andrew G. Malis



4  REFERENCES


[1]  Specifications for the Interconnection of a Host and an IMP,

     BBN Report 1822, May 1978 Revision.


[2]  E. C. Rosen et. al., ARPANET Routing Algorithm Improvements,

     IEN  183 (also published as BBN Report 4473, Vol. 1), August

     1980, pp. 55-107.


[3]  J. Postel, Assigned Numbers, RFC 790, September 1981, p. 10.

































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RFC 802                                           Andrew G. Malis



                              INDEX




1822...................................................... 4
1822 address.............................................. 6
1822 host................................................. 5
1822L..................................................... 4
1822L address............................................. 7
1822L host................................................ 5
1822L name................................................ 6
authorized................................................ 9
blocking................................................. 16
congestion control................................... 22, 39
connection........................................... 20, 38
destination host..................................... 32, 40
effective................................................ 10
flow control......................................... 20, 38
handing type......................................... 27, 35
incomplete transmission message...................... 19, 37
leader flags......................................... 27, 35
link field............................................... 32
logical addressing........................................ 4
message ID........................................... 32, 41
message length........................................... 41
message type......................................... 28, 35
multi-homing.............................................. 4
NDM.................................................. 10, 28
NDM reply............................................ 10, 36
NOC....................................................... 9
NOP........................................... 5, 22, 30, 36
outstanding.............................................. 21
priority bit............................................. 27
regular message...................................... 28, 35
RFNM..................................................... 36
short-blocking feature................................... 15
short-blocking message............................... 19, 28
source host.......................................... 31, 40
standard message......................................... 28
sub-type............................................. 32, 41
symmetric................................................. 5
trace bit............................................ 27, 35
uncontrolled message................................. 14, 28





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