rfc851.txt

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     There may  be  times  when  a  host  wants  to  perform  its  own

     translations,  or  might need the full list of physical addresses

     to which a particular name maps.  For example, a connection-based

     host-to-host  protocol  may  require  that the same physical host

     port on a multi-homed host be used for all  messages  using  that

     host-to-host  connection, and the host does not wish to trust the

     IMP to always deliver messages using a destination  name  to  the

     same host port.




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     1822L Host Access Protocol                             April 1983
     RFC 851



     In these cases, the host  can  submit  a  type  11  (Name  Server

     Request)  message to the IMP, which requests the IMP to translate

     the destination 1822L name and return a list of the addresses  to

     which  it maps.  The IMP will respond with a type 11 (Name Server

     Reply) message, which contains the selection policy  in  use  for

     that  name,  the  number of addresses to which the name maps, the

     addresses  themselves,  and  for  each  address,  whether  it  is

     effective and its routing distance from the IMP.  See section 3.2

     for a complete description of the message's contents.


     Using this information, the source  host  can  make  an  informed

     decision  on which of the physical host ports corresponding to an

     1822L name to use, and can subsequently send the messages to that

     port, rather than to the name.


     The IMP also supports a different type of name service.   A  host

     needs  to issue a Name Declaration Message to the IMP in order to

     make its names effective, but it may not wish to keep  its  names

     in  some table or file in the host.  In this case, it can ask the

     IMP to tell it which names it is authorized to use.


     In this case, the host submits a  type  12  (Port  List  Request)

     message to the IMP, and the IMP replies with a type 12 (Port List

     Reply) message.  It contains, for the host port  over  which  the

     IMP  received the request and sent the reply, the number of names




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     1822L Host Access Protocol                             April 1983
     RFC 851



     that map to the port, the list of names, and whether or not  each

     name  is  effective.   The  host can then use this information in

     order  to  issue  the  Name  Declaration  Message.   Section  3.2

     contains a complete description of the reply's contents.











































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     1822L Host Access Protocol                             April 1983
     RFC 851



     3  1822L LEADER FORMATS


     The following sections describe the formats of the  leaders  that

     precede  messages  between  an 1822L host and its IMP.  They were

     designed to be as compatible with the 1822 leaders  as  possible.

     The  second,  fifth,  and  sixth  words  are identical in the two

     leaders, and all  of  the  existing  functionality  of  the  1822

     leaders  has  been  retained.   In  the  first word, the 1822 New

     Format Flag is now also used to identify the two types  of  1822L

     leaders, and the Handling Type has been moved to the second byte.

     The third and fourth words contain  the  Source  and  Destination

     1822L Name, respectively.




























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     1822L Host Access Protocol                             April 1983
     RFC 851



     3.1  Host-to-IMP 1822L Leader Format





                    1      4 5      8 9             16
                   +--------+--------+----------------+
                   |        |  1822L |                |
                   | Unused |  H2I   | 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
                   +----------------------------------+
                   |                                  |
                   |              Unused              |
                   |                                  |
                   +----------------------------------+

                Figure 5. Host-to-IMP 1822L Leader Format






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     1822L Host Access Protocol                             April 1983
     RFC 851



     Bits 1-4: Unused, must be set to zero.


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

          This field is set to decimal 13 (1101 in binary).


     Bits 9-16: Handling Type:

          This  field  is  bit-coded  to  indicate  the   transmission

          characteristics  of  the connection desired by the host. See

          1822(3.3).

          Bit 9: Priority Bit:

               Messages with this bit on will be treated  as  priority

               messages.

          Bits 10-16: Unused, must be zero.


     Bits 17-20: Unused, must be zero.


     Bit 21: Trace Bit:

          If equal to one, this message is designated for  tracing  as

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


     Bits 22-24: Leader Flags:

          Bit 22: A flag available for use by  the  destination  host.

               See 1822(3.3) for a description of its use by the IMP's

               TTY Fake Host.

          Bits 23-24: Reserved for future use, must be zero.






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     1822L Host Access Protocol                             April 1983
     RFC 851



     Bits 25-32: Message Type:

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

               occurs  via  regular  messages, which have several sub-

               types, found in bits 77-80.  These sub-types are:

               0: Standard - The IMP uses its full message  and  error

                    control facilities, and host blocking may occur.

               3: Uncontrolled  Packet  -  The  IMP  will  perform  no

                    message-control   functions   for   this  type  of

                    message, and network flow and  congestion  control

                    may  cause loss of the packet.  Also see 1822(3.6)

                    and section 2.3.

               4-15: Unassigned.

          Type 1: Error Without Message ID - See 1822(3.3).

          Type 2: Host Going Down - see 1822(3.3).

          Type 3: Name Declaration Message (NDM)  -  This  message  is

               used by the host to declare which of its 1822L names is

               or is not effective (see section 2.2.1), or to make all

               of  its  names non-effective.  The first 16 bits of the

               data portion of the NDM message, following  the  leader

               and  any  leader  padding, contains the number of 1822L

               names contained in the message.  This  is  followed  by

               the 1822L name entries, each 32 bits long, of which the

               first 16 bits is a 1822L name and the  second  16  bits

               contains  either  of  the  integers  zero or one.  Zero



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     1822L Host Access Protocol                             April 1983
     RFC 851



               indicates that the name should not  be  effective,  and

               one  indicates  that the name should be effective.  The

               IMP will reply with a NDM Reply  message  (see  section

               3.2)  indicating  which  of the names are now effective

               and which are not.  Pictorially, a NDM message has  the

               following   format  (including  the  leader,  which  is

               printed in hexadecimal):




                 1             16 17            32 33            48
                +----------------+----------------+----------------+
                |                |                |                |
                |      0D00      |      0003      |      0000      |
                |                |                |                |
                +----------------+----------------+----------------+
                 49            64 65            80 81            96
                +----------------+----------------+----------------+
                |                |                |                |
                |      0000      |      0000      |      0000      |
                |                |                |                |
                +----------------+----------------+----------------+
                 97           112 113          128 129          144
                +----------------+----------------+----------------+
                |                |                |                |
                |  # of entries  |  1822L name #1 |     0 or 1     |
                |                |                |                |
                +----------------+----------------+----------------+
                145           160 161          176
                +----------------+----------------+
                |                |                |
                |  1822L name #2 |     0 or 1     |       etc.
                |                |                |
                +----------------+----------------+

                       Figure 6. NDM Message Format







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     1822L Host Access Protocol                             April 1983
     RFC 851



               An  NDM  with  zero  entries  will  cause  all  current

               effective names for the host to become non-effective.

          Type 4: NOP - This allows the IMP to  know  which  style  of

               leader  the  host wishes to use.  A 1822L NOP signifies

               that the host wishes to use 1822L leaders, and an  1822

               NOP signifies that the host wishes to use 1822 leaders.

               All of the other remarks concerning the NOP message  in

               1822(3.3)  still  hold.   The  host should always issue

               NOPs in groups of three to insure proper  reception  by

               the IMP.  Also see section 2.4 for a further discussion

               on the use of the NOP message.

          Type 8: Error with Message ID - see 1822(3.3).

          Type 11: Name Server Request - This allows the host  to  use

               the  IMP's  logical addressing tables as a name server.

               The destination name in the 1822L leader is translated,

               and  the  IMP replies with a Name Server Reply message,

               which lists the physical host addresses  to  which  the

               destination name maps.

          Type 12: Port List Request - This allows the  physical  host

               to  request the list of names that map to the host port

               over which this request was received by the  IMP.   The

               IMP replies with a Port List Reply message, which lists

               the names that map to the port.

          Types 5-7,9-10,13-255: Unassigned.



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     1822L Host Access Protocol                             April 1983
     RFC 851



     Bits 33-48: Source Host:

          This field contains one of the  source  host's  1822L  names

          (or,  alternatively,  the 1822L address of the host port the

          message  is  being  sent   over).    This   field   is   not

          automatically filled in by the IMP, as in the 1822 protocol,

          because the host may be known by several names and may  wish

          to use a particular name as the source of this message.  All

          messages from the same host need not use the  same  name  in

          this  field.   Each  source  name, when used, is checked for

          authorization, effectiveness, and actually belonging to this

          host.  Messages using names that do not satisfy all of these