RFC 851
This document is obsolete. Please refer to RFC 878.

     Request for Comments: 851
     Obsoletes RFC: 802

                  The ARPANET 1822L Host Access Protocol

                                  RFC 851

                              Andrew G. Malis
                       ARPANET Mail: malis@bbn-unix

                       Bolt Beranek and Newman Inc.
                              50 Moulton St.
                           Cambridge, MA  02238

                                April 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.  The RFC is also
     being  presented  as  a  solicitation  of  comments   on   1822L,
     especially   from   host   network   software   implementers  and

     1822L Host Access Protocol                             April 1983
     RFC 851

                             Table of Contents

     1   INTRODUCTION.......................................... 1
     2   THE ARPANET 1822L HOST ACCESS PROTOCOL................ 4
     2.1   Addresses and Names................................. 6
     2.2   Name Translations................................... 8
     2.2.1   Authorization and Effectiveness................... 8
     2.2.2   Translation Policies............................. 11
     2.2.3   Reporting Destination Host Downs................. 13
     2.2.4   1822L and 1822 Interoperability.................. 16
     2.3   Uncontrolled Packets............................... 18
     2.4   Establishing Host-IMP Communications............... 20
     2.5   Counting RFMS When Using 1822L..................... 22
     2.6   1822L Name Server.................................. 24
     3   1822L LEADER FORMATS................................. 27
     3.1   Host-to-IMP 1822L Leader Format.................... 28
     3.2   IMP-to-Host 1822L Leader Format.................... 35
     4   REFERENCES........................................... 43

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     1822L Host Access Protocol                             April 1983
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     1822 Address Format....................................... 6
     1822L Name Format......................................... 7
     1822L Address Format...................................... 7
     Communications between different host types.............. 17
     Host-to-IMP 1822L Leader Format.......................... 28
     NDM Message Format....................................... 31
     IMP-to-Host 1822L Leader Format.......................... 35
     Name Server Reply Format................................. 39

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     1822L Host Access Protocol                             April 1983
     RFC 851


     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).

     There is one major restriction to the  new  1822L  protocol:   it

     will be implemented in C/30 IMPs only, and will therefore only be

     usable by hosts connected to C/30 IMPs, as Honeywell and Pluribus

     IMPs  do  not have sufficient memory to hold the new programs and

     tables.  This restriction  also  means  that  logical  addressing

     cannot  be used to identify a host on a non-C/30 IMP.  While this

     is not a problem on the ARPANET, which only has  C/30  IMPs,  the

     restriction  will  apply  if  logical  addressing  is used on any

     network that mixes C/30 and non-C/30 IMPs.

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     1822L Host Access Protocol                             April 1983
     RFC 851

     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 802.  The changes from  that

     RFC include:

     o The Short Blocking Feature, which had also  been  described  in

       RFC 802, now has its own RFC, RFC 852 [2].  It was moved to its

       own  RFC,  since  it  is  completely  independent  of   logical


     o In section 2.2, descriptions of  the  three  address  selection

       policies and of host error handling have been added.

     o In section 2.3, the IMP's uncontrolled packet service has  been

       further  improved.  This applies to hosts using 1822 as well as


     o Pointers on using RFNM counting with 1822L have been  added  as

       section 2.5.

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     1822L Host Access Protocol                             April 1983
     RFC 851

     o Section 2.6 describes the new "1822L name server" in  the  IMP,

       which  makes use of two new Host-to-IMP messages to allow hosts

       to do their own name-to-address mapping.

     o In section 3.2, the subtypes for the type  15  (1822L  Name  or

       Address Error) IMP-to-Host message have been changed.

                                   - 3 -

     1822L Host Access Protocol                             April 1983
     RFC 851


     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) [3] 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

                                   - 4 -

     1822L Host Access Protocol                             April 1983
     RFC 851

     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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     1822L Host Access Protocol                             April 1983
     RFC 851

     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            |
           |                |                                 |

                      Figure 1. 1822 Address Format

     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                             April 1983
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                     1                             16
                    |                                |
                    |           1822L name           |
                    |                                |

                       Figure 2. 1822L Name Format

     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   |
                  |   |   |            |                |

                      Figure 3. 1822L Address Format

     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

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     1822L Host Access Protocol                             April 1983
     RFC 851

     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

     use  these  names to address the hosts in the network independent

     of their  physical  locations.   Because  of  the  implementation

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     1822L Host Access Protocol                             April 1983
     RFC 851

     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

     addresses, which actually  refer  to  physical  host  ports,  are

     always authorized in this sense.

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     1822L Host Access Protocol                             April 1983
     RFC 851

     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.

     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

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     1822L Host Access Protocol                             April 1983
     RFC 851

     finds in its tables for that host become effective.   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


     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

                                  - 11 -

     1822L Host Access Protocol                             April 1983
     RFC 851

     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.

     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

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     1822L Host Access Protocol                             April 1983
     RFC 851

        the first reachable  physical  host  address.   This  list  is

        always  searched  from the top whenever an uncontrolled packet

        is to be sent or an end-to-end connection has to  be  created.

        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.

     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.   This  attempts to spread out the

        load from any one  IMP's  hosts  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  subnetwork  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)

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     1822L Host Access Protocol                             April 1983
     RFC 851

     messages  using it and both the source and destination hosts stay


     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

          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

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     1822L Host Access Protocol                             April 1983
     RFC 851

          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


     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  3  (message  lost  due  to  network

          failure)  message  for  each  outstanding 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)


     Section 2.3 discusses how destination host downs are handled  for

     uncontrolled packets.

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     1822L Host Access Protocol                             April 1983
     RFC 851

     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.

     The   following   figure   illustrates   how   these   addressing

     combinations  are  handled,  showing  how  each  type of host can

     access every other type of host.  There are three types of hosts:

     "1822  on  C/30"  signifies  an  1822 host that is on a C/30 IMP,

     "1822L" signifies an 1822L host (on a C/30  IMP),  and  "1822  on

     non-C/30"  signifies  a  host  on  an  non-C/30 IMP (which cannot

     support the 1822L protocol).  The table entry shows the  protocol

     and  host address format(s) that the source host can use to reach

     the destination host.

                                  - 16 -

     1822L Host Access Protocol                             April 1983
     RFC 851

                                 Destination Host
       Host    | 1822 on C/30   | 1822L          | 1822 on non-C/30
               |                |                |
       1822 on | 1822           | 1822           | 1822
       C/30    |                | (note 1)       |
               |                |                |
               |                |                |
               | 1822L, using   | 1822L, using   | 1822L, using
       1822L   | 1822L name or  | 1822L name or  | 1822L address
               |address (note 2)| address        | only (note 2)
               |                |                |
               |                |                |
       1822 on | 1822           | 1822           | 1822
       non-C/30|                | (note 1)       |
               |                |                |

       Note 1: The message is presented  to  the  destination  host
               with  an 1822L leader containing the 1822L addresses
               of the source  and  destination  hosts.   If  either
               address  cannot be encoded as an 1822L address, then
               the message is not delivered and an error message is
               sent to the source host.

       Note 2: The message is presented  to  the  destination  host
               with  an  1822 leader containing the 1822 address of
               the source host.

          Figure 4. Communications between different host types

                                  - 17 -

     1822L Host Access Protocol                             April 1983
     RFC 851

     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  subnetwork  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


     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


                                  - 18 -

     1822L Host Access Protocol                             April 1983
     RFC 851

     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.

     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

                                  - 19 -

     1822L Host Access Protocol                             April 1983
     RFC 851

     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


     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,  logically-addressed

     uncontrolled  packets  must  be  used  in  order  to  receive any

     indication that the packet was lost once it has left  the  source


     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.

                                  - 20 -

     1822L Host Access Protocol                             April 1983
     RFC 851

     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

     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

                                  - 21 -

     1822L Host Access Protocol                             April 1983
     RFC 851

     further details concerning the ready line,  host  tardiness,  and

     other issues.

     2.5  Counting RFMS When Using 1822L

     When a host submits a regular message using an 1822  leader,  the

     IMP  checks  for  an  existing simplex virtual circuit connection

     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.

     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

                                  - 22 -

     1822L Host Access Protocol                             April 1983
     RFC 851


     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  name/destination  name  combination.

     When the message is received from a host, the IMP first looks for

     an open connection for that  same  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 name/destination name pair basis, instead of

     just by 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

                                  - 23 -

     1822L Host Access Protocol                             April 1983
     RFC 851

     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.

     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.

                                  - 24 -

     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

                                  - 25 -

     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.

                                  - 26 -

     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.

                                  - 27 -

     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

                                  - 28 -

     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


          Bit 9: Priority Bit:

               Messages with this bit on will be treated  as  priority


          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.

                                  - 29 -

     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

                                  - 30 -

     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

                                  - 31 -

     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.

                                  - 32 -

     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

          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

                                  - 33 -

     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.

                                  - 34 -

     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

                                  - 35 -

     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


     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).

                                  - 36 -

     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


          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

                                  - 37 -

     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.

                                  - 38 -

     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


               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

                                  - 41 -

     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  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.

                                  - 42 -

     1822L Host Access Protocol                             April 1983
     RFC 851


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

          BBN Report 1822, December 1981 Revision.

     [2]  A. Malis, The ARPANET Short Blocking  Feature,  Request  For

          Comments 852, April 1983.

     [3]  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.

     [4]  J. Postel,  Assigned  Numbers,  Request  For  Comments  820,

          January 1983, p. 11.

                                  - 43 -

     1822L Host Access Protocol                             April 1983
     RFC 851


     1822...................................................... 4
     1822 address.............................................. 6
     1822 host................................................. 5
     1822L..................................................... 4
     1822L address............................................. 7
     1822L host................................................ 5
     1822L name................................................ 6
     address selection policy................................. 12
     authorized................................................ 9
     blocking................................................. 22
     closest physical address................................. 13
     connection............................................... 22
     destination host..................................... 33, 41
     effective................................................ 10
     first reachable.......................................... 12
     handing type......................................... 29, 36
     host downs............................................... 13
     leader flags......................................... 29, 36
     link field............................................... 34
     load leveling............................................ 13
     logical addressing........................................ 4
     message ID........................................... 34, 41
     message length........................................... 42
     message type......................................... 30, 36
     multi-homing.............................................. 4
     name server...................................... 24, 32, 37
     NDM.................................................. 10, 30
     NDM reply............................................ 10, 37
     NOC....................................................... 9
     NOP........................................... 5, 20, 32, 37
     priority bit............................................. 29
     regular message...................................... 30, 36
     RFNM................................................. 22, 37
     source host.......................................... 33, 41
     standard message......................................... 30
     sub-type............................................. 34, 42
     symmetric................................................. 5
     trace bit............................................ 29, 36
     uncontrolled packet.................................. 18, 30

                                  - 44 -