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.

     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been replaced by the combination of this RFC and RFC 851, The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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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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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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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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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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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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