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     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
     RFC 878



     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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     1822L Host Access Protocol                          December 1983
     RFC 878



     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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     1822L Host Access Protocol                          December 1983
     RFC 878



     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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     1822L Host Access Protocol                          December 1983
     RFC 878



     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




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     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.




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     1822L Host Access Protocol                          December 1983
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     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




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     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.







                                  - 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




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     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