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rfc:rfc878
   Request for Comments: 878
   Obsoletes RFCs: 851, 802
                The ARPANET 1822L Host Access Protocol
                                RFC 878
                            Andrew G. Malis
                     ARPANET Mail: malis@bbn-unix
                       BBN Communications Corp.
                            50 Moulton St.
                         Cambridge, MA  02238
                             December 1983
   This RFC specifies the ARPANET 1822L Host Access Protocol,  which
   is  a successor to the existing 1822 Host Access Protocol.  1822L
   allows ARPANET hosts to use  logical  names  as  well  as  1822's
   physical port locations to address each other.
   1822L Host Access Protocol                          December 1983
   RFC 878
                           Table of Contents
   1   INTRODUCTION.......................................... 1
   2   THE ARPANET 1822L HOST ACCESS PROTOCOL................ 3
   2.1   Addresses and Names................................. 5
   2.2   Name Translations................................... 7
   2.2.1   Authorization and Effectiveness................... 7
   2.2.2   Translation Policies............................. 11
   2.2.3   Reporting Destination Host Downs................. 13
   2.2.4   1822L and 1822 Interoperability.................. 15
   2.3   Uncontrolled Packets............................... 16
   2.4   Establishing Host-IMP Communications............... 19
   2.5   Counting RFNMs When Using 1822L.................... 20
   2.6   1822L Name Server.................................. 23
   3   1822L LEADER FORMATS................................. 25
   3.1   Host-to-IMP 1822L Leader Format.................... 26
   3.2   IMP-to-Host 1822L Leader Format.................... 34
   4   REFERENCES........................................... 42
   A   1822L-IP ADDRESS MAPPINGS............................ 43
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   1822L Host Access Protocol                          December 1983
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                                FIGURES
   2.1  1822 Address Format.................................. 5
   2.2  1822L Name Format.................................... 6
   2.3  1822L Address Format................................. 6
   3.1  Host-to-IMP 1822L Leader Format..................... 27
   3.2  NDM Message Format.................................. 30
   3.3  IMP-to-Host 1822L Leader Format..................... 35
   3.4  Name Server Reply Format............................ 38
   A.1  1822 Class A Mapping................................ 44
   A.2  1822L Class A Mapping............................... 44
   A.3  1822L Class B Mapping............................... 45
   A.4  1822L Class C Mapping............................... 46
  1. ii -
   1822L Host Access Protocol                          December 1983
   RFC 878
   1  INTRODUCTION
   This RFC specifies the ARPANET 1822L Host Access Protocol,  which
   will allow hosts to use logical addressing (i.e., host names that
   are independent of their physical location  on  the  ARPANET)  to
   communicate  with  each  other.  This new host access protocol is
   known as the ARPANET 1822L (for Logical)  Host  Access  Protocol,
   and  is  a  successor  to  the  current  ARPANET 1822 Host Access
   Protocol, which is described in  sections  3.3  and  3.4  of  BBN
   Report  1822  [1].   Although  the  1822L protocol uses different
   Host-IMP leaders than the 1822 protocol, the IMPs  will  continue
   to support the 1822 protocol, and hosts using either protocol can
   readily communicate with each other (the  IMPs  will  handle  the
   translation automatically).
   The RFC's terminology is consistent  with  that  used  in  Report
   1822, and any new terms will be defined when they are first used.
   Familiarity  with  Report  1822  (section  3  in  particular)  is
   assumed.   As could be expected, the RFC makes many references to
   Report 1822.  As a result, it uses, as a convenient abbreviation,
   "see 1822(x)" instead of "please refer to Report 1822, section x,
   for further details".
   This RFC updates, and obsoletes, RFC 851.  The changes from  that
   RFC are:
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   1822L Host Access Protocol                          December 1983
   RFC 878
   o Section 2.2.4 was rewritten for clarity.
   o Section 2.5 was expanded to  further  discuss  the  effects  of
     using 1822L names on host-to-host virtual circuits.
   o In section 3.2, the type 1  IMP-to-host  message  has  two  new
     subtypes,  the type 9 message has one new subtype, and the type
     15, subtype 4 message is no longer defined.
   o An appendix describing the  mapping  between  1822L  names  and
     internet (IP) addresses has been added.
   All of these changes to RFC 851 are marked by revision  bars  (as  |
   shown here) in the right margin.                                   |
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   1822L Host Access Protocol                          December 1983
   RFC 878
   2  THE ARPANET 1822L HOST ACCESS PROTOCOL
   The ARPANET 1822L Host Access  Protocol  allows  a  host  to  use
   logical  addressing  to  communicate  with  other  hosts  on  the
   ARPANET.  Basically, logical addressing allows hosts to refer  to
   each  other  using  an  1822L  name  (see  section  2.1) which is
   independent of a host's physical location in  the  network.   IEN
   183  (also  published  as  BBN  Report 4473) [2] gives the use of
   logical  addressing  considerable   justification.    Among   the
   advantages it cites are:
   o The ability to refer to each host on  the  network  by  a  name
     independent of its location on the network.
   o Allowing different hosts to share  the  same  host  port  on  a
     time-division basis.
   o Allowing a host to use multi-homing (where a single  host  uses
     more than one port to communicate with the network).
   o Allowing several hosts that provide the same service  to  share
     the same name.
   The main differences between the 1822 and 1822L protocols are the
   format of the leaders that are used to introduce messages between
   a host and an IMP, and the specification in those leaders of  the
   source  and/or  destination  host(s).   Hosts  have the choice of
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   RFC 878
   using the 1822 or the 1822L protocol.  When a host comes up on an
   IMP,  it  declares  itself to be an 1822 host or an 1822L host by
   the type of NOP message (see section  3.1)  it  uses.   Once  up,
   hosts  can  switch  from  one protocol to the other by issuing an
   appropriate NOP.  Hosts that do not use the 1822L  protocol  will
   still  be  addressable by and can communicate with hosts that do,
   and vice-versa.
   Another difference between the two protocols  is  that  the  1822
   leaders are symmetric, while the 1822L leaders are not.  The term
   symmetric means that in the 1822 protocol, the exact same  leader
   format  is used for messages in both directions between the hosts
   and IMPs.  For example, a leader sent from a host  over  a  cable
   that  was  looped  back onto itself (via a looping plug or faulty
   hardware) would arrive back at the host and appear to be a  legal
   message  from  a  real host (the destination host of the original
   message).  In contrast, the 1822L headers are not symmetric,  and
   a  host  can  detect  if  the  connection to its IMP is looped by
   receiving a message with the wrong leader  format.   This  allows
   the host to take appropriate action upon detection of the loop.
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   2.1  Addresses and Names
   The 1822 protocol defines one form of host specification, and the
   1822L  protocol  defines  two additional ways to identify network
   hosts.  These three forms are 1822 addresses,  1822L  names,  and
   1822L addresses.
   1822 addresses are  the  24-bit  host  addresses  found  in  1822
   leaders.  They have the following format:
          1              8 9                              24
         +----------------+---------------------------------+
         |                |                                 |
         |  Host number   |           IMP number            |
         |                |                                 |
         +----------------+---------------------------------+
                          1822 Address Format
                              Figure 2.1
   These fields are quite large, and the ARPANET will never use more
   than  a  fraction of the available address space.  1822 addresses
   are used in 1822 leaders only.
   1822L names are 16-bit unsigned numbers that serve as  a  logical
   identifier  for  one  or  more  hosts.   1822L  names have a much
   simpler format:
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   1822L Host Access Protocol                          December 1983
   RFC 878
                   1                             16
                  +--------------------------------+
                  |                                |
                  |           1822L name           |
                  |                                |
                  +--------------------------------+
                           1822L Name Format
                              Figure 2.2
   The 1822L names are just 16-bit  unsigned  numbers,  except  that
   bits  1  and  2 are not both zeros (see below).  This allows over
   49,000 hosts to be specified.
   1822 addresses cannot be used in 1822L leaders, but there may  be
   a  requirement for an 1822L host to be able to address a specific
   physical host port or IMP fake host.  1822L  addresses  are  used
   for  this  function.   1822L addresses form a subset of the 1822L
   name space, and have both bits 1 and 2 off.
                  1   2  3          8 9             16
                +---+---+------------+----------------+
                |   |   |            |                |
                | 0 | 0 |   host #   |   IMP number   |
                |   |   |            |                |
                +---+---+------------+----------------+
                         1822L Address Format
                              Figure 2.3
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   1822L Host Access Protocol                          December 1983
   RFC 878
   This format allows 1822L hosts to directly address hosts 0-63  at
   IMPs  1-255  (IMP  0 does not exist).  Note that the highest host
   numbers are reserved  for  addressing  the  IMP's  internal  fake
   hosts.   At  this  writing, the IMP has seven fake hosts, so host
   numbers 57-63 address the IMP fake hosts, while host numbers 0-56
   address  real  hosts  external  to the IMP.  As the number of IMP
   fake hosts changes, this boundary point will also change.
   2.2  Name Translations
   There are a number of factors that determine how an 1822L name is
   translated  by  the  IMP  into a physical address on the network.
   These factors include which translations are legal; in what order
   different  translations  for  the  same name should be attempted;
   which  legal  translations  shouldn't  be  attempted  because   a
   particular  host  port  is down; and the interoperability between
   1822  and  1822L  hosts.   These  issues  are  discussed  in  the
   following sections.
   2.2.1  Authorization and Effectiveness
   Every host on a C/30 IMP, regardless of whether it is  using  the
   1822  or  1822L  protocol  to access the network, can have one or
   more 1822L names (logical addresses).  Hosts using 1822L can then
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   RFC 878
   use  these  names to address the hosts in the network independent
   of their  physical  locations.   Because  of  the  implementation
   constraints mentioned in the introduction, hosts on non-C/30 IMPs
   cannot be assigned 1822L names.  To circumvent this  restriction,
   however,  1822L  hosts can also use 1822L addresses to access all
   of the other hosts.
   At this point, several questions  arise:   How  are  these  names
   assigned,  how  do  they  become  known  to  the  IMPs  (so  that
   translations to physical addresses can be made), and how  do  the
   IMPs know which host is currently using a shared port?  To answer
   each question in order:
   Names are assigned by a central network administrator.  When each
   name  is  created, it is assigned to a host (or a group of hosts)
   at one or more specific host ports.  The host(s) are  allowed  to
   reside at those specific host ports, and nowhere else.  If a host
   moves, it will keep the same name, but the administrator  has  to
   update  the  central  database  to  reflect  the  new  host port.
   Changes to this database are  distributed  to  the  IMPs  by  the
   Network  Operations  Center  (NOC).  For a while, the host may be
   allowed to reside at either of (or both) the new and  old  ports.
   Once  the  correspondence  between  a  name and one or more hosts
   ports where it  may  be  used  has  been  made  official  by  the
   administrator,   that  name  is  said  to  be  authorized.  1822L
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   RFC 878
   addresses, which actually  refer  to  physical  host  ports,  are
   always authorized in this sense.
   Once a host has been assigned one or more names, it  has  to  let
   the  IMPs  know  where it is and what name(s) it is using.  There
   are two cases to consider, one for 1822L hosts  and  another  for
   1822  hosts.   The following discussion only pertains to hosts on
   C/30 IMPs.
   When an IMP sees an 1822L host come up on a host  port,  the  IMP
   has  no way of knowing which host has just come up (several hosts
   may share the same port, or one host may prefer to  be  known  by
   different  names  at different times).  This requires the host to
   declare itself to the IMP before it can actually send and receive
   messages.   This  function  is  performed  by  a  new host-to-IMP
   message, the Name Declaration  Message  (NDM),  which  lists  the
   names  that  the  host would like to be known by.  The IMP checks
   its tables to see if each of the names is authorized,  and  sends
   an  NDM  Reply  to  the  host  saying  which  names were actually
   authorized and can now be used for sending and receiving messages
   (i.e.,  which  names  are  effective). A host can also use an NDM
   message to change its list of effective names (it can add to  and
   delete  from  the  list) at any time.  The only constraint on the
   host is that any names it wishes to use can become effective only
   if they are authorized.
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   In the second case, if a host comes up on a C/30  IMP  using  the
   1822 protocol, the IMP automatically makes the first name the IMP
   finds in its tables  for  that  host  become  effective  when  it
   receives the first 1822 NOP from the host.  Thus, even though the
   host is using the 1822 protocol, it can  still  receive  messages
   from  1822L  hosts  via  its  1822L name.  Of course, it can also
   receive messages from an 1822L host  via  its  1822L  address  as
   well.    (Remember,  the  distinction  between  1822L  names  and
   addresses is that the addresses correspond to physical  locations
   on   the   network,   while   the   names  are  strictly  logical
   identifiers).  The IMPs translate between the  different  leaders
   and send the proper leader in each case (see section 2.2.4).
   The third question above has by now already been answered.   When
   an  1822L  host comes up, it uses the NDM message to tell the IMP
   which host it is (which names it is known by).  Even if this is a
   shared port, the IMP knows which host is currently connected.
   Whenever a host goes down, its names  automatically  become  non-
   effective.   When it comes back up, it has to make them effective
   again.
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   1822L Host Access Protocol                          December 1983
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   2.2.2  Translation Policies
   Several hosts can share the same 1822L name.  If more than one of
   these  hosts  is  up  at the same time, any messages sent to that
   1822L name will be delivered to just one  of  the  hosts  sharing
   that  name,  and  a RFNM will be returned as usual.  However, the
   sending host will  not  receive  any  indication  of  which  host
   received  the  message,  and subsequent messages to that name are
   not guaranteed to be sent to the  same  host.   Typically,  hosts
   providing  exactly  the  same  service could share the same 1822L
   name in this manner.
   Similarly, when a host is multi-homed, the same  1822L  name  may
   refer  to  more  than  one  host  port (all connected to the same
   host).  If the host is up on only one of those ports,  that  port
   will be used for all messages addressed to the host.  However, if
   the host were up on more than one  port,  the  message  would  be
   delivered  over  just  one  of  those ports, and the subnet would
   choose which port to use.  This port selection could change  from
   message  to  message.   If  a  host wanted to insure that certain
   messages were delivered to it on specific ports,  these  messages
   could  use  either  the  port's 1822L address or a specific 1822L
   name that referred to that port alone.
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   1822L Host Access Protocol                          December 1983
   RFC 878
   Three different address selection policies are available for  the
   name mapping process.  When translated, each name uses one of the
   three policies  (the  policy  is  pre-determined  on  a  per-name
   basis).  The three policies are:
   o  Attempt each translation in the order in  which  the  physical
      addresses  are listed in the IMP's translation tables, to find
      the first reachable  physical  host  address.   This  list  is
      always  searched  from the top whenever an uncontrolled packet
      is to be sent or a new virtual circuit connection  has  to  be
      created  (see  section  2.5).   This is the most commonly used
      policy.
   o  Selection of the closest  physical  address,  which  uses  the
      IMP's   routing   tables   to  find  the  translation  to  the
      destination  IMP  with  the  least  delay  path  whenever   an
      uncontrolled  packet  is  to  be sent or a new virtual circuit
      connection has to be created.
   o  Use load leveling. This is similar to the second  policy,  but
      differs  in  that  searching  the  address  list  for  a valid
      translation starts at the address following where the previous
      translation search ended whenever an uncontrolled packet is to
      be sent or a new virtual circuit connection has to be created.
      This  attempts to spread out the load from any one IMP's hosts
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      to the various host ports associated with a  particular  name.
      Note  that this is NOT network-wide load leveling, which would
      require a distributed algorithm and tables.
   2.2.3  Reporting Destination Host Downs
   As was explained in report 1822, and  as  will  be  discussed  in
   greater detail in section 2.5, whenever regular messages are sent
   by a host, the IMP opens a virtual  circuit  connection  to  each
   destination  host  from  the source host.  A connection will stay
   open at least as long as there are  any  outstanding  (un-RFNMed)
   messages  using it and both the source and destination hosts stay
   up.
   However, the destination host may go down for some reason  during
   the  lifetime of a connection.  If the host goes down while there
   are no outstanding messages  to  it  in  the  network,  then  the
   connection  is  closed  and  no  other  action is taken until the
   source host submits the next message for  that  destination.   At
   that time, ONE of the following events will occur:
   A1.  If 1822 or an 1822L address is being  used  to  specify  the
        destination host, then the source host will receive a type 7
        (Destination Host Dead) message from the IMP.
   A2.  If an 1822L name is being used to  specify  the  destination
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        host,  and  the  name maps to only one authorized host port,
        then a type 7 message will also be sent to the source host.
   A3.  If an 1822L name is being used to  specify  the  destination
        host,  and  the  name  maps to more than one authorized host
        port, then the IMP attempts to open a connection to  another
        authorized  and  effective  host  port for that name.  If no
        such connection can be made, the host will receive a type 15
        (1822L  Name  or  Address  Error),  subtype  5 (no effective
        translations) message (see section 3.2).  Note that a type 7
        message  cannot be returned to the source host, since type 7
        messages refer to a particular destination  host  port,  and
        the name maps to more than one destination port.
   Things get a bit more complicated if there  are  any  outstanding
   messages  on  the connection when the destination host goes down.
   The connection will be closed, and  one  of  the  following  will
   occur:
   B1.  If 1822 or an 1822L address is being  used  to  specify  the
        destination host, then the source host will receive a type 7
        message for each outstanding message.
   B2.  If an 1822L name is being used to  specify  the  destination
        host, then the source host will receive a type 9 (Incomplete  |
        Transmission), subtype 6  (message  lost  due  to  logically  |
        addressed  host  going  down)  message  for each outstanding  |
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        message.  The next time  the  source  host  submits  another
        message   for  that  same  destination  name,  the  previous
        algorithm will be used (either step A2 or step A3).
   The above two algorithms also apply when a  host  stays  up,  but
   declares  the  destination  name for an existing connection to no
   longer be effective.  In this case, however, the type 7  messages
   above will be replaced by type 15, subtype 3 (name not effective)
   messages.
   Section 2.3 discusses how destination host downs are handled  for
   uncontrolled packets.
   2.2.4  1822L and 1822 Interoperability
   As  has  been  previously  stated,  1822  and  1822L  hosts   can
   intercommunicate,  and  the  IMPs  will  automatically handle any
   necessary leader and address format  conversions.   However,  not
   every   combination   of   1822   and  1822L  hosts  allows  full
   interoperability with regard to the use  of  1822L  names,  since
   1822 hosts are restricted to using physical addresses.
   There are two possible situations where any incompatibility could  |
   arise:                                                             |
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   1822L Host Access Protocol                          December 1983
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   o  An 1822 host sending a message to an  1822L  host:   The  1822  |
      host  specifies the destination host by its 1822 address.  The  |
      destination host will receive the message with an 1822L leader  |
      containing  the  1822L addresses of the source and destination  |
      hosts.                                                          |
   o  An 1822L host sending a message to an 1822  host:   The  1822L  |
      host  can  use  1822L  names  or addresses to specify both the  |
      source and  destination  hosts.   The  destination  host  will  |
      receive  the  message  with an 1822 leader containing the 1822  |
      address of the source host.                                     |
   2.3  Uncontrolled Packets
   Uncontrolled packets (see 1822(3.6)) present a unique problem for
   the  1822L protocol.  Uncontrolled packets use none of the normal
   ordering and error-control mechanisms in the IMP, and do not  use
   the  normal  virtual circuit connection facilities.  As a result,
   uncontrolled packets need to carry all  of  their  overhead  with
   them, including source and destination names.  If 1822L names are
   used when sending an uncontrolled packet, additional  information
   is  now required by the subnetwork when the packet is transferred
   to the destination IMP.  This means that less  host-to-host  data
   can  be  contained  in  the  packet than is possible between 1822
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   hosts.
   Uncontrolled packets that are sent between 1822 hosts may contain
   not  more  than  991 bits of data.  Uncontrolled packets that are
   sent to and/or from 1822L hosts are limited to 32 bits  less,  or
   not  more  than  959  bits.  Packets that exceed this length will
   result in an error indication to the host, and  the  packet  will
   not  be sent.  This error indication represents an enhancement to
   the previous level of service provided by the  IMP,  which  would
   simply   discard  an  overly  long  uncontrolled  packet  without
   notification.
   Other enhancements that  are  provided  for  uncontrolled  packet
   service  are  a  notification  to the host of any errors that are
   detected by the host's IMP when it receives the packet.   A  host
   will  be  notified if an uncontrolled packet contains an error in
   the 1822L  name  specification,  such  as  if  the  name  is  not
   authorized or effective, if the remote host is unreachable (which
   is indicated by none of its names being  effective),  if  network
   congestion control throttled the packet before it left the source
   IMP, or for any other reason the source IMP was not able to  send
   the packet on its way.
   In most cases, the host will not be notified if the  uncontrolled
   packet  was  lost  once  it  was  transmitted  by the source IMP.
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   However, the IMP will attempt to notify  the  source  host  if  a
   logically-addressed  uncontrolled packet was mistakenly sent to a
   host that the source IMP thought was effective, but which  turned
   out  to  be  dead  or non-effective at the destination IMP.  This
   non-delivery notice  is  sent  back  to  the  source  IMP  as  an
   uncontrolled  packet from the destination IMP, so the source host
   is not guaranteed to receive this indication.
   If the source IMP successfully receives the non-delivery  notice,
   then  the  source  host  will  receive  a  type 15 (1822L Name or
   Address Error), subtype 6 (down or non-effective  port)  message.
   If  the  packet  is  resubmitted or another packet is sent to the
   same destination name,  and  there  are  no  available  effective
   translations,  then  the  source  host  will  receive  a type 15,
   subtype 5 (no effective translations) message if the  destination
   name  has  more than one mapping; or will receive either a type 7
   (Destination Host Dead)  or  a  type  15,  subtype  3  (name  not
   effective)   message   if  the  destination  name  has  a  single
   translation.
   Those enhancements to the uncontrolled packet  service  that  are
   not  specific  to  logical  addressing will be available to hosts
   using 1822 as well as 1822L.  However, uncontrolled packets  must
   be  sent  using  1822L leaders in order to receive any indication
   that the packet was lost once it has left the source IMP.
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   2.4  Establishing Host-IMP Communications
   When a host comes up on an IMP, or after there has been  a  break
   in   the  communications  between  the  host  and  its  IMP  (see
   1822(3.2)), the orderly flow of messages between the host and the
   IMP  needs  to  be properly (re)established.  This allows the IMP
   and host to recover from most any failure  in  the  other  or  in
   their communications path, including a break in mid-message.
   The first messages that a host should send to its IMP  are  three
   NOP  messages.   Three  messages  are  required to insure that at
   least one message will be properly read by the IMP (the first NOP
   could be concatenated to a previous message if communications had
   been broken in mid-stream, and the third provides redundancy  for
   the   second).    These   NOPs   serve  several  functions:  they
   synchronize the IMP with the host, they tell  the  IMP  how  much
   padding  the  host  requires  between  the message leader and its
   body, and they also tell the IMP whether the host will  be  using
   1822 or 1822L leaders.
   Similarly, the IMP will send three  NOPs  to  the  host  when  it
   detects  that  the host has come up.  Actually, the IMP will send
   six NOPs, alternating three 1822  NOPs  with  three  1822L  NOPs.
   Thus, the host will see three NOPs no matter which protocol it is
   using.   The  NOPs  will  be  followed  by  two  Interface  Reset
  1. 19 -
   1822L Host Access Protocol                          December 1983
   RFC 878
   messages,  one of each style.  If the IMP receives a NOP from the
   host while the above sequence is occurring,  the  IMP  will  only
   send  the  remainder  of  the NOPs and the Interface Reset in the
   proper style.  The 1822 NOPs will contain the 1822 address of the
   host interface, and the 1822L NOPs will contain the corresponding
   1822L address.
   Once the IMP  and  the  host  have  sent  each  other  the  above
   messages, regular communications can commence.  See 1822(3.2) for
   further details concerning the ready line,  host  tardiness,  and
   other issues.
   2.5  Counting RFNMs When Using 1822L
   When a host submits a regular message using an 1822  leader,  the
   IMP  checks  for  an  existing simplex virtual circuit connection
   (see 1822(3.1)) from the source host to the destination host.  If
   such  a  connection already exists, it is used.  Otherwise, a new
   connection from the source host port to the destination host port
   is  opened.   In either case, there may be at most eight messages
   outstanding on that connection  at  any  one  time.   If  a  host
   submits  a  ninth message on that connection before it receives a
   reply for the first message, then the host will be blocked  until
   the reply is sent for the first message.
  1. 20 -
   1822L Host Access Protocol                          December 1983
   RFC 878
   Such connections can stay open for some time, but are  timed  out
   after  three minutes of no activity, or can be closed if there is
   contention for the connection blocks  in  either  the  source  or
   destination  IMP.   However, a connection will never be closed as
   long as there are any outstanding messages on it.  This allows  a
   source  host  to  count the number of replies it has received for
   messages to each destination host address in order to avoid being
   blocked   by  submitting  a  ninth  outstanding  message  on  any
   connection.
   When a host submits a regular message using an  1822L  leader,  a
   similar process occurs, except that in this case, connections are
   distinguished by the  source  port/source  name/destination  name
   combination.   When  the message is received from a host, the IMP
   first looks for an open connection for that same port and  source
   name/destination  name pair.  If such a connection is found, then
   it is used, and no further name translation  is  performed.   If,
   however,  no open connection was found, then the destination name
   is translated, and a connection opened to the physical host port.
   As  long  as there are any outstanding messages on the connection
   it will stay open, and it will have  the  same  restriction  that
   only  eight messages may be outstanding at any one time.  Thus, a
   source host can still count replies to avoid being  blocked,  but
   they must be counted on a source port and source name/destination
  1. 21 -
   1822L Host Access Protocol                          December 1983
   RFC 878
   name pair basis, instead of just by source port  and  destination
   host address as before.
   Since connections are based on the source name  as  well  as  the
   destination  name,  this  implies that there may be more than one
   open connection from physical host port A to physical  host  port
   B,   which   would   allow   more  than  8  outstanding  messages
   simultaneously from the first to the second port.   However,  for
   this  to  occur, either the source or destination names, or both,
   must differ from one connection to the next.  For example, if the
   names  "543"  and  "677" both translate to physical port 3 on IMP
   51, then the host on that port could  open  four  connections  to
   itself  by  sending  messages  from "543" to "543", from "543" to
   "677", from "677" to "543", and from "677" to "677".
   As has already been stated,  the  destination  names  in  regular
   messages  are  only translated when connections are first opened.
   Once a connection is open, that connection, and  its  destination
   physical  host port, will continue to be used until it is closed.
   If, in the meantime, a "better" destination host  port  belonging
   to  the  same  destination name became available, it would not be
   used until the next time a  new  connection  is  opened  to  that
   destination name.
  1. 22 -
   1822L Host Access Protocol                          December 1983
   RFC 878
   Also, the act of making an 1822L name be non-effective  will  not  |
   automatically cause any connections using that name to be closed.  |
   However, they will be closed  after  at  most  three  minutes  of  |
   inactivity.  A host can, if it wishes, make all of its names at a  |
   port be noneffective and close all of its connections to and from  |
   the port by flapping the host's ready line to that IMP port.       |
   2.6  1822L Name Server
   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.
   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
  1. 23 -
   1822L Host Access Protocol                          December 1983
   RFC 878
   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 could  make  an  informed
   decision  on which of the physical host ports corresponding to an
   1822L name to use and then 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
   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.
  1. 24 -
   1822L Host Access Protocol                          December 1983
   RFC 878
   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.
  1. 25 -
   1822L Host Access Protocol                          December 1983
   RFC 878
   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              |
                 |                                  |
                 +----------------------------------+
                    Host-to-IMP 1822L Leader Format
                              Figure 3.1
  1. 26 -
   1822L Host Access Protocol                          December 1983
   RFC 878
   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.
  1. 27 -
   1822L Host Access Protocol                          December 1983
   RFC 878
   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.
             1-2,4-15: 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
  1. 28 -
   1822L Host Access Protocol                          December 1983
   RFC 878
             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,  and   without   any   leader
             padding):
  1. 29 -
   1822L Host Access Protocol                          December 1983
   RFC 878
               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.
              |                |                |
              +----------------+----------------+
                          NDM Message Format
                              Figure 3.2
             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
  1. 30 -
   1822L Host Access Protocol                          December 1983
   RFC 878
             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.
   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
  1. 31 -
   1822L Host Access Protocol                          December 1983
   RFC 878
        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
        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.
   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
        the source host if the name is not effective.
   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  [3]  appropriate  for  the   host-to-host
        protocol being used.
  1. 32 -
   1822L Host Access Protocol                          December 1983
   RFC 878
   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.
  1. 33 -
   1822L Host Access Protocol                          December 1983
   RFC 878
   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          |
                 |                                  |
                 +----------------------------------+
                    IMP-to-Host 1822L Leader Format
                              Figure 3.3
  1. 34 -
   1822L Host Access Protocol                          December 1983
   RFC 878
   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).  In addition to its  |
             already  defined  sub-types,  this  message has two new  |
  1. 35 -
   1822L Host Access Protocol                          December 1983
   RFC 878
             sub-types:                                               |
             4: Illegal Leader Style - The host submitted  a  leader  |
                  in  which  bits  5-8 did not contain the value 13,  |
                  14, or 15 decimal.                                  |
             5: Wrong Leader Style - The  host  submitted  an  1822L  |
                  leader  when the IMP was expecting an 1822 leader,  |
                  or vice-versa.                                      |
        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  (see  figure  3.2).   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).
  1. 36 -
   1822L Host Access Protocol                          December 1983
   RFC 878
        Type 8: Error in Data - See 1822(3.4).
        Type  9:  Incomplete  Transmission  -  See  1822(3.4).    In  |
             addition to its already defined sub-types, this message  |
             has one new sub-type:                                    |
             6: Logically Addressed Host Went  Down  -  A  logically  |
                  addressed  message was lost in the network because  |
                  the  destination  host  to  which  it  was   being  |
                  delivered   went  down.   The  message  should  be  |
                  resubmitted by the source host, since there may be  |
                  another  effective  host port to which the message  |
                  could be delivered (see section 2.2.3).             |
        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,  following the
             leader  and  any  leader  padding,  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
             1822L address, and  the  second  word  contains  a  bit
             signifying  whether  or not that particular translation
             is effective and the routing distance (expected network
             transmission  delay,  in 6.4 ms units) to the address's
             IMP.  In figure 3.4, which includes the leader  without
             any  leader  padding,  EFF is 1 for effective and 0 for
  1. 37 -
   1822L Host Access Protocol                          December 1983
   RFC 878
             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.
               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| routing dist |       etc.
              |                |F|              |
              +----------------+-+--------------+
                       Name Server Reply Format
                              Figure 3.4
  1. 38 -
   1822L Host Access Protocol                          December 1983
   RFC 878
        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
             (see figure 3.2).
        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.
             5: The multi-homed Destination Host name is authorized,
  1. 39 -
   1822L Host Access Protocol                          December 1983
   RFC 878
                  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
                  to send the packet.
             7: Logical addressing is not in use in this network.
             8-15: Unassigned.
        Types 4,13-14,16-255: 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.
  1. 40 -
   1822L Host Access Protocol                          December 1983
   RFC 878
   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
        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  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.
  1. 41 -
   1822L Host Access Protocol                          December 1983
   RFC 878
   4  REFERENCES
   [1]  "Specifications for the Interconnection of  a  Host  and  an
        IMP", BBN Report 1822, December 1981 Revision.
   [2]  E.  C.   Rosen   et.   al.,   "ARPANET   Routing   Algorithm
        Improvements",   Internet   Experimenter's  Note  183  (also
        published as BBN Report 4473, Vol. 1), August 1980, pp.  55-
        107.
   [3]  J. Reynolds and J. Postel, "Assigned Numbers",  Request  For
        Comments 870, October 1983, p. 14.
   [4]  J. Postel, ed., "Internet Protocol - DARPA Internet  Program
        Protocol Specification", Request for Comments 791, September
        1981.
   [5]  J. Postel, "Address Mappings",  Request  for  Comments  796,
        September 1981.
  1. 42 -
   1822L Host Access Protocol                          December 1983
   RFC 878
                              APPENDIX A
                       1822L-IP ADDRESS MAPPINGS
   Once logical addressing is in active  (or  universal)  use  in  a  |
   network,  to  the extent that the "official" host tables for that  |
   network specify hosts by their logical names rather than by their  |
   physical  network  addresses,  it would be desirable for hosts on  |
   other networks to also be able to use the same logical  names  to  |
   specify these hosts when sending traffic to them via the internet  |
   [4].                                                               |
   Happily, there exists a natural mapping between logical names and  |
   internet  addresses  that  fits  very  nicely  with  the  already  |
   standard ARPANET-style address mapping as specified in  RFC  796,  |
   Address  Mappings [5].  The current ARPANET-style class A mapping  |
   is as follows (from RFC 796):                                      |
  1. 43 -
   1822L Host Access Protocol                          December 1983
   RFC 878
           +--------+ +--------+--------+
           |  HOST  | |  ZERO  |  IMP   |    1822 Address
           +--------+ +--------+--------+
               8          8        8
           +--------+--------+--------+--------+
           | net #  |  HOST  |   LH   |  IMP   |   IP Address
           +--------+--------+--------+--------+
               8        8        8        8
                         1822 Class A Mapping
                              Figure A.1
   For 1822L names and addresses, the mapping would be:               |
           +--------+--------+
           | upper  | lower  |     1822L Name or Address
           +--------+--------+
               8        8
           +--------+--------+--------+--------+
           | net #  | upper  |   LH   | lower  |   IP Address
           +--------+--------+--------+--------+
               8        8        8        8
                         1822L Class A Mapping
                              Figure A.2
   For 1822L addresses,  this  mapping  is  identical  to  the  1822  |
   mapping.   For  1822L  names,  the  IP address would appear to be  |
   addressing a high-numbered (64-255) 1822 host.  Although  the  LH  |
   (logical  host)  field  is still defined, its use is discouraged;  |
   multiple logical names should now be used to  multiplex  multiple  |
  1. 44 -
   1822L Host Access Protocol                          December 1983
   RFC 878
   functions onto one physical host port.                             |
   This mapping extends to class B networks:                          |
           +--------+--------+
           | upper  | lower  |     1822L Name or Address
           +--------+--------+
               8        8
           +----------------+--------+--------+
           | network number | upper  | lower  |   IP Address
           +----------------+--------+--------+
                   16            8        8
                         1822L Class B Mapping
                              Figure A.3
   Finally, logical addressing will allow IMP-based class C networks  |
   for  the  first  time.   Previously,  it  was very hard to try to  |
   divide the 8 bits of host specification into some number of  host  |
   bits  and  some  number  of  IMP  bits.   However,  if ALL of the  |
   internet-accessible hosts on  the  network  have  logical  names,  |
   there is no reason why networks with up to 256 such logical names  |
   cannot now use class C addresses, as follows:                      |
  1. 45 -
   1822L Host Access Protocol                          December 1983
   RFC 878
           +--------+--------+
           |01000000| lower  |     1822L Name
           +--------+--------+
               8        8
           +------------------------+--------+
           |     network number     | lower  |   IP Address
           +------------------------+--------+
                       24                8
                         1822L Class C Mapping
                              Figure A.4
   Those hosts on the network  desiring  internet  access  would  be  |
   assigned  logical  names in the range 40000 to 40377 (octal), and  |
   the  gateway(s)  connected  to  that  network  would   make   the  |
   translation  from IP addresses to 1822L names as specified above.  |
   Note that the network could have many more than 256 hosts, or 256  |
   defined  names;  the  only  restriction is that hosts that desire  |
   internet support or access be addressable by a name in the  range  |
   40000  -  40377.   Traffic that was strictly local to the network  |
   could use other names or even 1822L addresses.                     |
  1. 46 -
   1822L Host Access Protocol                          December 1983
   RFC 878
                                 INDEX
   1822...................................................... 3
   1822 address.............................................. 5
   1822 host................................................. 4
   1822L..................................................... 3
   1822L address............................................. 6
   1822L and 1822 interoperability.......................... 15
   1822L host................................................ 4
   1822L name................................................ 5
   address selection policy................................. 12
   authorized................................................ 8
   blocking................................................. 20
   closest physical address................................. 12
   connection............................................... 20
   destination host..................................... 32, 40
   effective............................................. 9, 23
   first reachable.......................................... 12
   handing type......................................... 27, 35
   host downs............................................... 13
   interoperability......................................... 15
   leader flags......................................... 27, 35
   link field............................................... 32
   load leveling............................................ 12
   logical addressing........................................ 3
   message ID........................................... 32, 41
   message length........................................... 41
   message type......................................... 28, 35
   multi-homing.............................................. 3
   name server...................................... 23, 31, 37
   NDM................................................... 9, 28
   NDM reply............................................. 9, 36
   NOC....................................................... 8
   NOP........................................... 4, 19, 30, 36
   priority bit............................................. 27
   regular message...................................... 28, 35
   RFNM................................................. 20, 36
   source host.......................................... 31, 40
   standard message......................................... 28
   sub-type............................................. 33, 41
   symmetric................................................. 4
   trace bit............................................ 27, 35
  1. 47 -
   1822L Host Access Protocol                          December 1983
   RFC 878
   uncontrolled packet.................................. 16, 28
   virtual circuit connection............................... 20
  1. 48 -
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