Request for Comments: 852
                    The ARPANET Short Blocking Feature
                                  RFC 852
                              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 Short Blocking Feature, which will
     allow ARPANET hosts to optionally shorten the IMP's host blocking
     timer.  This Feature is a replacement of the ARPANET non-blocking
     host   interface,  which  was  never  implemented,  and  will  be
     available to hosts using either the 1822  or  1822L  Host  Access
     Protocol.   The  RFC is also being presented as a solicitation of
     comments on the Short  Blocking  Feature,  especially  from  host
     network software implementers and maintainers.

ARPANET Short Blocking Feature                         April 1983
     RFC 852
     This RFC specifies the ARPANET Short Blocking Feature, which will
     allow a host to shorten the amount of time that it may be blocked
     by its IMP after it presents a message to the network (currently,
     the  IMP  can  block  further input from a host for up to fifteen
     The Feature is an addition to the ARPANET  1822  and  1822L  Host
     Access  Protocols,  and  replaces the non-blocking host interface
     described in section 3.7 of BBN Report 1822 [1], which was  never
     implemented.   This  Feature  will  be available to hosts on C/30
     IMPs only.  This will not present a problem on the ARPANET, which
     only  has  C/30 IMPs, but hosts on non-C/30 IMPs in networks that
     mix C/30 and non-C/30 IMPs will not be  able  to  use  the  Short
     Blocking Feature.
     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
     This RFC was once part of RFC 802, which is now obsolete and  has
     been  replaced  by  the  combination of this RFC and RFC 851, The
     ARPANET 1822L Host  Access  Protocol  [2].   The  Short  Blocking
     Feature  will  be  available to all hosts on C/30 IMPs, no matter
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     RFC 852
     which (1822 or 1822L) host access  protocol  they  are  using  to
     communicate with the IMP.
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     The Short Blocking Feature of the 1822 and 1822L protocols allows
     a  host to present messages to the IMP without causing the IMP to
     not accept further messages from the host  for  long  amounts  of
     time  (up  to fifteen seconds).  It is a replacement for the non-
     blocking host interface described in section 3.7 of Report  1822,
     and that description should be ignored.
     2.1  Host Blocking
     Usually, when a source host submits a message to an IMP, the  IMP
     immediately processes that message and sends it on its way to its
     destination host.  Sometimes, however, the IMP  is  not  able  to
     process the message immediately.  Processing a message requires a
     significant number of resources, and when the network is  heavily
     loaded,  there can sometimes be a long delay before the necessary
     resources become available.  In such cases, the IMP must  make  a
     decision  as  to  what to do while it is attempting to gather the
     One possibility is for the IMP to stop  accepting  messages  from
     the  source  host  until  it has gathered the resources needed to
     process the message just submitted.  This strategy  is  known  as
     blocking  the  host,  and is basically the strategy that has been
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     RFC 852
     used in the ARPANET up to the present.  When  a  host  submits  a
     message  to  an  IMP, all further transmissions from that host to
     that IMP are blocked until the message can be processed.
     It is important to note, however, that not all  messages  require
     the  same  set  of resources in order to be processed by the IMP.
     The particular set of resources needed will depend on the message
     type,  the  message  length,  and  the  destination  host  of the
     message.  Therefore, although it might take a long time to gather
     the  resources  needed  to process a particular message, it might
     take only a short time to gather the resources needed to  process
     some other message.  This fact exposes a significant disadvantage
     in the strategy of blocking the host.  A host  which  is  blocked
     may  have many other messages to submit which, if only they could
     be submitted, could be processed immediately.  It is "unfair" for
     the  IMP to refuse to accept these messages until it has gathered
     the resources for some  other,  unrelated  message.   Why  should
     messages for which the IMP has plenty of resources be delayed for
     an arbitrarily long amount of time just because the IMP lacks the
     resources needed for some other message?
     A simple way to alleviate the problem would be to place  a  limit
     on  the  amount of time during which a host can be blocked.  This
     amount  of  time  should  be  long  enough  so  that,   in   most
     circumstances,  the  IMP  will  be  able  to gather the resources
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     RFC 852
     needed to process the message within the given time period.   If,
     however, the resources cannot be gathered in this period of time,
     the IMP will flush the message, sending a  reply  to  the  source
     host  indicating that the message was rejected and specifying the
     reason that it could not be  processed.   However,  the  resource
     gathering process would continue.  The intention is that the host
     resubmit the message  in  a  short  time,  when,  hopefully,  the
     resource  gathering  process  has concluded successfully.  In the
     meantime, the host  can  submit  other  messages,  which  may  be
     processed   sooner.    This   strategy  does  not  eliminate  the
     phenomenon of host blocking, but  only  limits  the  time  during
     which  a host is blocked.  This shorter time limit will always be
     less than or equal to two seconds.
     Note, however, that there  is  a  disadvantage  to  having  short
     blocking  times.  Let us assume that the IMP accepts a message if
     it has all the resources  needed  to  process  it.   The  ARPANET
     provides a sequential delivery service, whereby messages with the
     same priority, source host, and destination host are delivered to
     the  destination host in the same order as they are accepted from
     the source host.  With short blocking times, however,  the  order
     in  which  the IMP accepts messages from the source host need not
     be the same as the order in  which  the  source  host  originally
     submitted  the messages.  Since the two data streams (one in each
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     RFC 852
     direction) between the host and the IMP are not synchronized, the
     host  may  not  receive the reply to a rejected message before it
     submits subsequent messages for the same destination host.  If  a
     subsequent  message  is accepted, the order of acceptance differs
     from the order of original submission, and the ARPANET  will  not
     provide  the  same type of sequential delivery that it has in the
     past.   If  sequential  delivery  by  the  subnet  is  a   strict
     requirement,  the Short Blocking Feature should not be used.  For
     messages without this requirement, however,  the  Short  Blocking
     Feature can be used.
     Up to now, type 0 (Regular)  messages  have  only  had  sub-types
     available  to  request  the  standard  blocking  timeout, fifteen
     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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         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
     1:  The message was too long.
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     RFC 852
     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
     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
     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
     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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     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
     RFC 852
     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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     [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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