rfc851.txt

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     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.




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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.



















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     1822L Host Access Protocol                             April 1983
     RFC 851






                                 Destination Host
       Source
       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













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     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

     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.







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     1822L Host Access Protocol                             April 1983
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     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



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     1822L Host Access Protocol                             April 1983
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     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,  logically-addressed

     uncontrolled  packets  must  be  used  in  order  to  receive any

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

     IMP.




     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.






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     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



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     1822L Host Access Protocol                             April 1983
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     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




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     1822L Host Access Protocol                             April 1983
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     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  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



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     1822L Host Access Protocol                             April 1983
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     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