rfc851.txt

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          requirements will not be delivered, and will instead  result

          in  an  error  message being sent back into the source host.

          If the host places its 1822L  address  in  this  field,  the

          address is checked to insure that it actually represents the

          host port where the message originated.  If the  message  is

          destined for an 1822 host on a non-C/30 IMP, this field MUST

          contain the source host's 1822L address  (see  figure  4  in

          section 2.2.4).


     Bits 49-64: Destination Host:

          This field  contains  the  1822L  name  or  address  of  the

          destination  host.   If it contains a name, the name will be

          checked for effectiveness, with an error message returned to




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     1822L Host Access Protocol                             April 1983
     RFC 851



          the  source  host  if  the  name  is  not effective.  If the

          message is destined for an 1822 host on a non-C/30 IMP, this

          field MUST contain the destination host's 1822L address (see

          figure 4 in section 2.2.4).


     Bits 65-76: Message ID:

          This is a host-specified identification used in all  type  0

          and  type  8  messages, and is also used in type 2 messages.

          When used in type 0 messages, bits 65-72 are also  known  as

          the  Link  Field,  and  should  contain  values specified in

          Assigned  Numbers  [4]  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.


     Bits 81-96: Unused, must be zero.


















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     1822L Host Access Protocol                             April 1983
     RFC 851



     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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     1822L Host Access Protocol                             April 1983
     RFC 851



     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, 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).

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



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     1822L Host Access Protocol                             April 1983
     RFC 851



          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 (see figure 6).  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)  -  See

               1822(3.4).

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

          Type 9: Incomplete Transmission - See 1822(3.4).

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

          Type 11: Name Server Reply - This reply to the  Name  Server

               Request  host-to-IMP  message  contains a word with the

               selection policy and the number of  physical  addresses

               to  which  the  destination  name maps, followed by two

               words per physical address: the first word contains  an



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     1822L Host Access Protocol                             April 1983
     RFC 851



               1822L  address,  and  the  second  word  contains a bit

               signifying whether or not that  particular  translation

               is effective and the routing distance (in 6.4 ms units)

               to the address's IMP.   In  figure  8,  EFF  is  1  for

               effective and 0 for non-effective, and POL is a two-bit

               number indicating the selection  policy  for  the  name

               (see section 2.2.2):

               0: First reachable.

               1: Closest physical address.

               2: Load leveling.

               3: Unused.





























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     1822L Host Access Protocol                             April 1983
     RFC 851






                 1             16 17            32 33            48
                +----------------+----------------+----------------+
                |                |                |                |
                |      0E00      |      000B      |      0000      |
                |                |                |                |
                +----------------+----------------+----------------+
                 49            64 65            80 81            96
                +----------------+----------------+----------------+
                |                |                |                |
                |   dest. name   |      0000      |      0000      |
                |                |                |                |
                +----------------+----------------+----------------+
                 97           112 113          128 129          144
                +-+--------------+----------------+-+--------------+
                |P|              |                |E|              |
                |O|  # of addrs  |  1822L addr #1 |F| routing dist |
                |L|              |                |F|              |
                +-+--------------+----------------+-+--------------+
                145           160 161          176
                +----------------+-+--------------+
                |                |E|              |
                |  1822L addr #2 |F| routine dist |       etc.
                |                |F|              |
                +----------------+-+--------------+

                    Figure 8. Name Server Reply Format



          Type 12: Port List Reply - This is the  reply  to  the  Port

               List  Request  host-to-IMP  message.   It  contains the

               number of names that map to this  physical  host  port,

               followed by two words per name: the first word contains

               an 1822L name that maps to this port,  and  the  second

               contains  either a zero or a one, signifying whether or

               not that  particular  translation  is  effective.   The

               format  is  identical  to  the type 3 NDM Reply message



                                  - 39 -



     1822L Host Access Protocol                             April 1983
     RFC 851



               (see figure 6).

          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  physical  host  to  which  this   singly-homed

                    Destination Host name translated is authorized and

                    up, but not effective.  If the host  was  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 2.2.4).

               5: The multi-homed Destination Host name is authorized,

                    but has no available effective translations.

               6: A logically-addressed uncontrolled packet  was  sent

                    to a dead or non-effective host port.  However, if

                    it is resubmitted, there may be another  effective

                    host  port to which the IMP may be able to attempt



                                  - 40 -



     1822L Host Access Protocol                             April 1983
     RFC 851



                    to send the packet.

               7: Logical addressing is not in use in this network.

               8-15: Unassigned.

          Types 13-14,16-255: 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 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  and 15, this field

          contains the destination host field used in a previous  type

          0 message sent by this host.


     Bits 65-76: Message ID:

          For message types 0, 5, 7-9,  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




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     1822L Host Access Protocol                             April 1983
     RFC 851



          and 6.


     Bits 77-80: Sub-type:

          This field is used as a modifier by message types 0-2,  5-7,

          9, and 15.


     Bits 81-96: Message Length:

          This field is contained in type 0, 3, 11,  and  12  message