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
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    1  INTRODUCTION


    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

    seconds).


    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

    assumed.


    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    been  replaced  by  the  combination of this RFC and RFC 851, The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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    which (1822 or 1822L) host access  protocol  they  are  using  to

    communicate with the IMP.















































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    ARPANET Short Blocking Feature                         April 1983
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    2  THE ARPANET SHORT BLOCKING FEATURE


    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

    resources.


    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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    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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    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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    ARPANET Short Blocking Feature                         April 1983
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    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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    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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    ARPANET Short Blocking Feature                         April 1983
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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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    ARPANET Short Blocking Feature                         April 1983
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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
    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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    ARPANET Short Blocking Feature                         April 1983
    RFC 852



    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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