rfc852.txt

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     seconds.  The Short Blocking Feature  makes  available  new  sub-

     types  that  allow  the  host  to  request  messages  to be short

     blocking, i.e. only cause the host to be blocked for two  seconds

     at most if the message cannot be immediately processed.


     Type 0 messages now have the following subtypes:


     0:  Standard: This subtype instructs the  IMP  to  use  its  full

         message  and  error  control  facilities.   The  host  may be

         blocked up to fifteen seconds during the message submission.


     1:  Standard, Short Blocking: The IMP attempts to  use  the  same

         facilities  as  for  subtype 0, but will block the host for a



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     ARPANET Short Blocking Feature                         April 1983
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         maximum of two seconds.


     3:  Uncontrolled Packet:  The  IMP  performs  no  message-control

         functions,  and the packet is not guaranteed to be delivered.

         The host may be blocked up  to  fifteen  seconds  during  the

         packet submission, although any such blockage is unlikely.


     4:  Uncontrolled, Short  Blocking:  The  IMP  treats  the  packet

         similarly  to  subtype  3, but will only block the host for a

         maximum of two seconds.  Again, actual blockage is unlikely.




     2.2  Reasons for Host Blockage


     There are a number of reasons why a message could  cause  a  long

     blockage  in  the  IMP,  which would result in the rejection of a

     short (or even non-short) blocking message.  The IMP signals this

     rejection of a message by using the Incomplete Transmission (Type

     9) message, using the sub-type field to indicate why the  message

     was  rejected.   The  already-existing  sub-types  for the type 9

     message are:


     0:  The destination host  did  not  accept  the  message  quickly

         enough.


     1:  The message was too long.





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     2:  The host took more  than  fifteen  seconds  to  transmit  the

         message  to the IMP.  This time is measured from the last bit

         of the leader through the last bit of the message.


     3:  The message was lost in the network due  to  IMP  or  circuit

         failures.


     4:  The IMP could not accept the entire  message  within  fifteen

         seconds  because  of unavailable resources.  This sub-type is

         only used in response to non-short blocking messages.   If  a

         short  blocking  message  timed  out, it will be responded to

         with one of sub-types 6-10.


     5:  Source IMP  I/O  failure  occurred  during  receipt  of  this

         message.


     The new sub-types that apply to the Short Blocking Feature are:


     6:  Connection set-up delay: Although the IMP presents  a  simple

         message-at-a-time  interface  to  the  host,  it  provides an

         internal  connection-oriented  (virtual   circuit)   service,

         except in the case of uncontrolled packets.  Two messages are

         considered to be on the same connection if they have the same

         source  host  (i.e.,  they are submitted to the same IMP over

         the same host interface), the same  priority,  and  the  same

         destination  host  name  or  address.   The  subnet maintains




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         internal  connection   set-up   and   tear-down   procedures.

         Connections  are  set  up  as  needed, and are torn down only

         after  a  period  of   inactivity.    Occasionally,   network

         congestion or resource shortage will cause a lengthy delay in

         connection set-up.  During this period, no messages for  that

         connection  can  be  accepted,  but  other  messages  can  be

         accepted.


     7:  End-to-end flow  control:  For  every  message  that  a  host

         submits  to  an  IMP  (except  uncontrolled  packets) the IMP

         eventually  returns  a  reply  to  the  host  indicating  the

         disposition  of  the  message.   Between  the  time  that the

         message is submitted and  the  time  the  host  receives  the

         reply,  the  message  is  said to be outstanding. The ARPANET

         allows  only  eight  outstanding  messages   on   any   given

         connection.   If  there  are  eight outstanding messages on a

         given connection, and a ninth is  submitted,  it  cannot  the

         accepted.  If  a message is refused because its connection is

         blocked due to flow control, messages  on  other  connections

         can still be accepted.


         End-to-end flow control is the  most  common  cause  of  host

         blocking in the ARPANET at present.







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     ARPANET Short Blocking Feature                         April 1983
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     8:  Destination IMP buffer space shortage: If the host submits  a

         message  of  more  than  1008  bits  (exclusive of the 96-bit

         leader), buffer space at the destination IMP must be reserved

         before  the  message  can  be  accepted.  Buffer space at the

         destination IMP is always reserved on a per-connection basis.

         If  the  destination  IMP  is  heavily loaded, there may be a

         lengthy wait for the buffer space;  this  is  another  common

         cause  of  blocking  in  the  present  ARPANET.  Messages are

         rejected  for  this  reason  based  on   their   length   and

         connection;  messages  of  1008 or fewer bits or messages for

         other connections may still be acceptable.


     9:  Congestion control: A message may be refused for  reasons  of

         congestion  control if the path via the intermediate IMPs and

         lines to the destination IMP is too heavily loaded to  handle

         additional  traffic.   Messages  to other destinations may be

         acceptable, however.


     10: Local resource shortage: Occasionally, the source IMP  itself

         is  short  of  buffer  space,  table  entries,  or some other

         resource that it needs to accept a message.  Unlike the other

         reasons  for  message  rejection, this resource shortage will

         affect all messages equally, except for uncontrolled packets.

         The message's size or connection is not relevant.





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     ARPANET Short Blocking Feature                         April 1983
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     The Short Blocking Feature is available  to  all  hosts  on  C/30

     IMPs,  whether they are using the 1822 or 1822L protocol, through

     the use of Type 0, sub-type 1 and 4 messages.  A host using these

     sub-types  should  be  prepared  to  correctly  handle the Type 9

     (Incomplete Transmission) messages from the IMP.









































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     3  REFERENCES


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

          BBN Report 1822, December 1981 Revision.


     [2]  A. Malis, The ARPANET 1822L Host  Access  Protocol,  Request

          for Comments 851, April 1983.







































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