rfc878.txt

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                           1822L Address Format
                                Figure 2.3







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     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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     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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     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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     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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     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