RFC 802: The ARPANET 1822L Host Access Protocol
Andrew G. Malis
Bolt Beranek and Newman Inc.
Table of Contents
1 INTRODUCTION.......................................... 1 2 THE ARPANET 1822L HOST ACCESS PROTOCOL................ 4 2.1 Addresses and Names................................. 6 2.2 Name Authorization and Effectiveness................ 8 2.3 Uncontrolled Messages.............................. 14 2.4 The Short-Blocking Feature......................... 15 2.4.1 Host Blocking.................................... 16 2.4.2 Reasons for Host Blockage........................ 19 2.5 Establishing Host-IMP Communications............... 22 3 1822L LEADER FORMATS................................. 25 3.1 Host-to-IMP 1822L Leader Format.................... 26 3.2 IMP-to-Host 1822L Leader Format.................... 34 4 REFERENCES........................................... 42
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1822 Address Format....................................... 6 1822L Name Format......................................... 7 1822L Address Format...................................... 7 Communications between different host types.............. 13 Host-to-IMP 1822L Leader Format.......................... 27 NDM Message Format....................................... 30 IMP-to-Host 1822L Leader Format.......................... 35
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This document proposes two major changes to the current ARPANET
host access protocol. The first change will allow hosts to use
logical addressing (i.e., host addresses that are independent of
their physical location on the ARPANET) to communicate with each
other, and the second 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 15 seconds).
The new host access protocol is known as the ARPANET 1822L (for
Logical) Host Access Protocol, and it represents an addition to
the current ARPANET 1822 Host Access Protocol, which is described
in sections 3.3 and 3.4 of BBN Report 1822 . Although the
1822L protocol uses different Host-IMP leaders than the 1822
protocol, hosts using either protocol can readily communicate
with each other (the IMPs handle the translation automatically).
The new option for shortening the host blocking timeout is called
the short-blocking feature, and it replaces the non-blocking host
interface described in section 3.7 of Report 1822. This feature
will be available to all hosts on C/30 IMPs (see the next
paragraph), regardless of whether they use the 1822 or 1822L
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There is one major restriction to the new capabilities being
described. Both the 1822L protocol and the short-blocking
feature will be implemented on C/30 IMPs only, and will therefore
only be useable by hosts connected to C/30 IMPs, as the Honeywell
and Pluribus IMPs do not have sufficient memory to hold the new
programs and tables. This restriction also means that logical
addressing cannot be used to address a host on a non-C/30 IMP.
However, the ARPANET will shortly be completely converted to C/30
IMPs, and at that time this restriction will no longer be a
I will try to keep my terminology consistent with that used in
Report 1822, and will define new terms when they are first used.
Of course, familiarity with Report 1822 (section 3 in particular)
This document makes many references to Report 1822. As a
convenient abbreviation, I will use "see 1822(x)" instead of
"please refer to Report 1822, section x, for further details".
This document is a proposal, not a description of an implemented
system. Thus, described features are subject to change based
upon responses to this document and restrictions that become
evident during implementation. However, any such changes are
expected to be minor. A new RFC will be made available once the
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implementation is complete containing the actual as-implemented
Finally, I would like to thank Dr. Eric C. Rosen, who wrote most
of section 2.4, and James G. Herman, Dr. Paul J. Santos Jr., John
F. Haverty, and Robert M. Hinden, all of BBN, who contributed
many of the ideas found herein.
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2 THE ARPANET 1822L HOST ACCESS PROTOCOL
The ARPANET 1822L Host Access Protocol, which replaces the
ARPANET 1822 Host Access Protocol described in Report 1822,
sections 3.3 and 3.4, allows a host to use logical addressing to
communicate with other hosts on the ARPANET. Basically, logical
addressing allows hosts to refer to each other using an 1822L
name (see section 2.1) which is independent of a host's physical
location in the network. IEN 183 (also published as BBN Report
4473)  gives the use of logical addressing considerable
justification. Among the advantages it cites are:
o The ability to refer to each host on the network by a name
independent of its location on the network.
o Allowing different hosts to share the same host port on a
o Allowing a host to use multi-homing (where a single host uses
more than one port to communicate with the network).
o And allowing several hosts that provide the same service to
share the same name.
The main differences between the 1822 and 1822L protocols are the
format of the leaders that are used to introduce messages between
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a host and an IMP, and the specification in those leaders of the
source and/or destination host(s). Hosts have the choice of
using the 1822 or the 1822L protocol. When a host comes up on an
IMP, it declares itself to be an 1822 host or an 1822L host host
by the type of NOP message (see section 3.1) it uses. Once up,
hosts can switch from one protocol to the other by issuing an
appropriate NOP. Hosts that do not use the 1822L protocol will
still be addressable by and can communicate with hosts that do,
Another difference between the two protocols is that the 1822
leaders are symmetric, while the 1822L leaders are not. The term
symmetric means that in the 1822 protocol, the exact same leader
format is used for messages in both directions between the hosts
and IMPs. For example, a leader sent from a host over a cable
that was looped back onto itself (via a looping plug or faulty
hardware) would arrive back at the host and appear to be a legal
message from a real host (the destination host of the original
message). In contrast, the 1822L headers are not symmetric, and
a host can detect if the connection to its IMP is looped by
receiving a message with the wrong leader format. This allows
the host to take appropriate action upon detection of the loop.
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2.1 Addresses and Names
The 1822 protocol defines one form of host specification, and the
1822L protocol defines two additional ways to identify network
hosts. These three forms are 1822 addresses, 1822L names, and
1822 addresses are the 24-bit host addresses found in 1822
leaders. They have the following format:
1 8 9 24 +----------------+---------------------------------+ | | | | Host number | IMP number | | | | +----------------+---------------------------------+
Figure 1. 1822 Address Format
These fields are quite large, and the ARPANET will never use more
than a fraction of the available address space. 1822 addresses
are used in 1822 leaders only.
1822L names are 16-bit unsigned numbers that serve as a logical
identifier for one or more hosts. 1822L names have a much
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1 16 +--------------------------------+ | | | 1822L name | | | +--------------------------------+
Figure 2. 1822L Name Format
The 1822L names are just 16-bit unsigned numbers, except that
bits 1 and 2 are not both zeros (see below). This allows over
49,000 hosts to be specified.
1822 addresses cannot be used in 1822L leaders, but there may be
a requirement for an 1822L host to be able to address a specific
physical host port or IMP fake host. 1822L addresses are used
for this function. 1822L addresses form a subset of the 1822L
name space, and have both bits 1 and 2 off.
1 2 3 8 9 16 +---+---+------------+----------------+ | | | | | | 0 | 0 | host # | IMP number | | | | | | +---+---+------------+----------------+
Figure 3. 1822L Address Format
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This format gives 1822L hosts the ability to directly address
hosts 0-59 at IMPs 1-255 (IMP 0 does not exist). Host numbers
60-63 are reserved for addressing the four fake hosts at each
2.2 Name Authorization and Effectiveness
Every host on a C/30 IMP, regardless of whether it is using the
1822 or 1822L protocol to access the network, will be assigned at
least one 1822L name (logical address). Other 1822L hosts will
use this name to address the host, wherever it may be physically
located. Because of the implementation constraints mentioned in
the introduction, hosts on non-C/30 IMPs cannot be assigned 1822L
names. To circumvent this restriction, however, 1822L hosts can
use 1822L addresses to access all other hosts on the network, no
matter where they reside.
At this point, several questions arise: How are these names
assigned, how do they become known to the IMPs (so that
translations to physical addresses can be made), and how do the
IMPs know which host is currently using a shared port? To answer
each question in order:
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Names are assigned by a central network administrator. When each
name is created, it is assigned to a host (or a group of hosts)
at one or more specific host ports. The host(s) are allowed to
reside at those specific host ports, and nowhere else. If a host
moves, it will keep the same name, but the administrator has to
update the central database to reflect the new host port.
Changes to this database are distributed to the IMPs by the
Network Operations Center (NOC) at BBN. For a while, the host
may be allowed to reside at either of (or both) the new and old
ports. Once the correspondence between a name and one or more
hosts ports where it may be used has been made official by the
administrator, that name is said to be authorized. 1822L
addresses, which actually refer to physical host ports, are
always authorized in this sense.
Once a host has been assigned one or more names, it has to let
the IMPs know where it is and what name(s) it is using. There
are two cases to consider, one for 1822L hosts and another for
1822 hosts. The following discussion only pertains to hosts on
When an IMP sees an 1822L host come up on a host port, the IMP
has no way of knowing which host has just come up (several hosts
may share the same port, or one host may prefer to be known by
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different names at different times). This requires the host to
let the IMP know what is happening before it can actually send
and receive messages. This function is performed by a new host-
to-IMP message, the Name Declaration Message (NDM), which lists
the names that the host would like to be known by. The IMP
checks its tables to see if each of the names is authorized, and
sends an NDM Reply to the host saying which names in the list can
be used for sending and receiving messages (i.e., which names are
effective). A host can also use an NDM message to change its list
of effective addresses (it can add to and delete from the list)
at any time. The only constraint on the host is that any names
it wishes to use can become effective only if they are
In the second case, if a host comes up on a C/30 IMP using the
1822 protocol, the IMP automatically makes the first name the IMP
finds in its tables for that host become effective. Thus, even
though the host is using the 1822 protocol, it can still receive
messages from 1822L hosts via its 1822L name. Of course, it can
also receive messages from an 1822L host via its 1822L address as
well. (Remember, the distinction between 1822L names and
addresses is that the addresses correspond to physical locations
on the network, while the names are strictly logical
identifiers). The IMPs translate between the different leaders
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and send the proper leader in each case (more on this below).
The third question above has by now already been answered. When
an 1822L host comes up, it uses the NDM message to tell the IMP
which host it is (which names it is known by). Even if this is a
shared port, the IMP knows which host is currently connected.
Whenever a host goes down, its names automatically become non-
effective. When it comes back up, it has to make them effective
Several hosts can share the same 1822L name. If more than one of
these hosts is up at the same time, any messages sent to that
1822L name will be delivered to just one of the hosts sharing
that name, and a RFNM will be returned as usual. However, the
sending host will not receive any indication of which host
received the message, and subsequent messages to that name are
not guaranteed to be sent to the same host. Typically, hosts
providing exactly the same service could share the same 1822L
name in this manner.
Similarly, when a host is multi-homed, the same 1822L name may
refer to more than one host port (all connected to the same
host). If the host is up on only one of those ports, that port
will be used for all messages addressed to it. However, if the
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host were up on more than one port, the message would be
delivered over just one of those ports, and the subnet would
choose which port to use. This port selection could change from
message to message. If a host wanted to insure that certain
messages were delivered to it on specific ports, these messages
could use either the port's 1822L address or a specific 1822L
name that referred to that port alone.
Some further details are required on communications between 1822
and 1822L hosts. Obviously, when 1822 hosts converse, or when
1822L hosts converse, no conversions between leaders and address
formats are required. However, this becomes more complicated
when 1822 and 1822L hosts converse with each other.
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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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 and 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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2.3 Uncontrolled Messages
Uncontrolled messages (see 1822(3.6)) present a unique problem
for the 1822L protocol. Uncontrolled messages 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 messages need to carry all of their overhead
with them, including source and destination addresses. If 1822L
addresses are used when sending an uncontrolled message,
additional information is now required by the subnetwork when the
message is transferred to the destination IMP. This means that
less host-to-host data can be contained in the message than is
possible between 1822 hosts.
Uncontrolled messages that are sent between 1822 hosts may
contain not more than 991 bits of data. Uncontrolled messages
that are sent to and/or from 1822L hosts are limited to 32 bits
less, or not more than 959 bits. Messages that exceed this
length will result in an error indication to the host, and the
message 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 message
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Other enhancements that are provided for uncontrolled message
service are a notification to the host of any message-related
errors that are detected by the host's IMP when it receives the
message. A host will be notified if an uncontrolled message
contains an error in the 1822L name specification, such as the
name not being authorized or effective, or if the remote host is
unreachable (which is indicated by none of its names being
effective), or if network congestion control throttled the
message before it left the source IMP. The host will not be
notified if the uncontrolled message was lost for some reason
once it was transmitted by the source IMP.
2.4 The Short-Blocking Feature
The short-blocking feature of the 1822 and 1822L protocols is
designed to allow 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 15 seconds). It is a replacement for
the non-blocking host interface described in 1822(3.7), and that
description should be ignored.
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2.4.1 Host Blocking
Most commonly, 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
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
(see below). Therefore, although it might take a long time to
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gather the resources needed to process some 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 message 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
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 not processed, 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
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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 generally fall somewhere in the range of 100 milliseconds to
2 seconds, with its value possibly depending on the reason for
Note, however, that there is a disadvantage to having short
blocking times. Let us say 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
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 of the same priority 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
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that it has in the past.
Up to now, type 0 (regular) messages have only had sub-types
available to request the standard blocking timeout. 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 a short amount of time if the message
cannot be immediately processed. See section 3.1 for a complete
list of the available sub-types.
If sequential delivery by the subnet is a strict requirement, as
would be the case for messages produced by NCP, the short-
blocking feature cannot be used. For messages produced by TCP,
however, the use of the short-blocking feature is allowed and
2.4.2 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-blocking message. The IMP signals this rejection of a
short-blocking message by using the Incomplete Transmission (Type
9) message, using the sub-type field to indicate which of the
above reasons caused the rejection of the message. See section
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3.2 for a summary of the Incomplete Transmission message and a
complete list of its sub-types. The sub-types that apply to the
short-blocking feature are:
6. Connection setup-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 messages (see section 2.3). 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 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
7. End-to-end flow control: For every message that a host
submits to an IMP (except uncontrolled messages) the IMP eventually returns a reply to the host indicating the disposition of the message. Between the time that the
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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.
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.
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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
10. Local resource shortage: Sometimes 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 messages. The
message's size or connection is not relevant.
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 2 messages. A host using these
sub-types should be prepared to correctly handle Incomplete
Transmission messages from the IMP.
2.5 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
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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.
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
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host interface, and the 1822L NOPs will contain the corresponding
Once the IMP and the host have sent each other the above
messages, regular communications can commence. See 1822(3.2) for
further details concerning the ready line, host tardiness, and
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3 1822L LEADER FORMATS
The following sections describe the formats of the leaders that
precede messages between an 1822L host and its IMP. They were
designed to be as compatible with the 1822 leaders as possible.
The second, fifth, and sixth words are identical in the two
leaders, and all of the existing functionality of the 1822
leaders has been retained. The first difference one will note is
in the first word. The 1822 New Format Flag is now also used to
identify the two types of 1822L leaders, and the Handling Type
has been moved to the second byte. The third and fourth words
contain the Source and Destination 1822L Name, respectively.
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3.1 Host-to-IMP 1822L Leader Format
1 4 5 8 9 16 +--------+--------+----------------+ | | 1822L | | | Unused | H2I | Handling Type | | | Flag | | +--------+--------+----------------+ 17 20 21 22 24 25 32 +--------+-+------+----------------+ | |T|Leader| | | Unused |R|Flags | Message Type | | |C| | | +--------+-+------+----------------+ 33 48 +----------------------------------+ | | | Source Host | | | +----------------------------------+ 49 64 +----------------------------------+ | | | Destination Host | | | +----------------------------------+ 65 76 77 80 +-------------------------+--------+ | | | | Message ID |Sub-type| | | | +-------------------------+--------+ 81 96 +----------------------------------+ | | | Unused | | | +----------------------------------+
Figure 5. Host-to-IMP 1822L Leader Format
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Bits 1-4: Unused, must be set to zero.
Bits 5-8: 1822L Host-to-IMP Flag:
This field is set to decimal 13 (1101 in binary).
Bits 9-16: Handling Type:
This field is bit-coded to indicate the transmission characteristics of the connection desired by the host. See
Bit 9: Priority Bit:
Messages with this bit on will be treated as priority
Bits 10-16: Unused, must be zero.
Bits 17-20: Unused, must be zero.
Bit 21: Trace Bit:
If equal to one, this message is designated for tracing as
it proceeds through the network. See 1822(5.5).
Bits 22-24: Leader Flags:
Bit 22: A flag available for use by the destination host.
See 1822(3.3) for a description of its use by the IMP's
TTY fake host.
Bits 23-24: Reserved for future use, must be zero.
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Bits 25-32: Message Type:
Type 0: Regular Message - All host-to-host communication occurs via regular messages, which have several sub-
types, found in bits 77-80. These sub-types are:
0: Standard - The IMP uses its full message and error
control facilities, and host blocking (see section
2.4) may occur.
1: Standard, short-blocking - See section 2.4.
Uncontrolled, short-blocking - See section 2.4.
3: Uncontrolled - The IMP will perform no message- control functions for this type of message, and network flow and congestion control (see section 2.4) may cause loss of the message. Also see
1822(3.6) and section 2.3.
Type 1: Error Without Message ID - See 1822(3.3).
Type 2: Host Going Down - see 1822(3.3).
Type 3: Name Declaration Message (NDM) - This message is
used by the host to declare which of its 1822L names is
or is not effective (see section 2.2), or to make all of its names non-effective. The first 16 bits of the data portion of the NDM message, following the leader and any padding, contains the number of 1822L name
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entries contained in the message. This is followed by
the 1822L name entries, each 32 bits long, of which the
first 16 bits is a 1822L name and the second 16 bits contains either of the integers zero or one. Zero indicates that the name should not be effective, and one indicates that the name should be effective. The IMP will reply with a NDM Reply message (see section 3.2) indicating which of the names are now effective and which are not. Pictorially, a NDM message has the following format (including the leader, which is
printed in hexadecimal):
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1 16 17 32 33 48 +----------------+----------------+----------------+ | | | | | 0D00 | 0003 | 0000 | | | | | +----------------+----------------+----------------+ 49 64 65 80 81 96 +----------------+----------------+----------------+ | | | | | 0000 | 0000 | 0000 | | | | | +----------------+----------------+----------------+ 97 112 113 128 129 144 +----------------+----------------+----------------+ | | | | | # of entries | 1822L name #1 | 0 or 1 | | | | | +----------------+----------------+----------------+ 145 160 161 176 +----------------+----------------+ | | | | 1822L name #2 | 0 or 1 | etc. | | | +----------------+----------------+
Figure 6. NDM Message Format
An NDM with zero entries will cause all current
effective names for the host to become non-effective.
Type 4: NOP - This allows the IMP to know which style of leader the host wishes to use. A 1822L NOP signifies that the host wishes to use 1822L leaders, and an 1822
NOP signifies that the host wishes to use 1822 leaders.
All of the other remarks concerning the NOP message in
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1822(3.3) still hold. The host should always issue NOPs in groups of three to insure proper reception by
the IMP. Also see section 2.5 for a further discussion
on the use of the NOP message.
Type 8: Error with Message ID - see 1822(3.3).
Types 5-7,9-255: Unassigned.
Bits 33-48: Source Host:
This field contains one of the source host's 1822L names (or, alternatively, the 1822L address of the host port the message is being sent over). This field is not
automatically filled in by the IMP, as in the 1822 protocol,
because the host may be known by several names and may wish
to use a particular name as the source of this message. All
messages from the same host need not use the same name in this field. Each source name, when used, is checked for
authorization, effectiveness, and actually belonging to this
host. Messages using names that do not satisfy all of these
requirements will not be delivered, and will instead result in an error message being sent back into the source host. If the host places its 1822L Address in this field, the
address is checked to insure that it actually represents the
host port where the message originated. If the message is
destined for an 1822 host on a non-C/30 IMP, this field MUST
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contain the source host's 1822L address (see Figure 4 in
Bits 49-64: Destination Host:
This field contains the 1822L name or address of the destination host. If it contains a name, the name will be
checked for effectiveness, with an error message returned to
the source host if the name is not effective. If the
message is destined for an 1822 host on a non-C/30 IMP, this
field MUST contain the destination host's 1822L address (see
Figure 4 in section 2.2).
Bits 65-76: Message ID:
This is a host-specified identification used in all type 0 and type 8 messages, and is also used in type 2 messages. When used in type 0 messages, bits 65-72 are also known as the Link Field, and should contain values specified in Assigned Numbers  appropriate for the host-to-host
protocol being used.
Bits 77-80: Sub-type:
This field is used as a modifier by message types 0, 2, 4,
- 32 -
Bits 81-96: Unused, must be zero.
- 33 -
3.2 IMP-to-Host 1822L Leader Format
1 4 5 8 9 16 +--------+--------+----------------+ | | 1822L | | | Unused | I2H | Handling Type | | | Flag | | +--------+--------+----------------+ 17 20 21 22 24 25 32 +--------+-+------+----------------+ | |T|Leader| | | Unused |R|Flags | Message Type | | |C| | | +--------+-+------+----------------+ 33 48 +----------------------------------+ | | | Source Host | | | +----------------------------------+ 49 64 +----------------------------------+ | | | Destination Host | | | +----------------------------------+ 65 76 77 80 +-------------------------+--------+ | | | | Message ID |Sub-type| | | | +-------------------------+--------+ 81 96 +----------------------------------+ | | | Message Length | | | +----------------------------------+
Figure 7. IMP-to-Host 1822L Leader Format
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Bits 1-4: Unused and set to zero.
Bits 5-8: 1822L IMP-to-Host Flag:
This field is set to decimal 14 (1110 in binary).
Bits 9-16: Handling Type:
This has the value assigned by the source host (see section 3.1). This field is only used in message types 0, 5-9, 11
Bits 17-20: Unused and set to zero.
Bit 21: Trace Bit:
If equal to one, the source host designated this message for
tracing as it proceeds through the network. See 1822(5.5).
Bits 22-24: Leader Flags:
Bit 22: Available as a destination host flag.
Bits 23-24: Reserved for future use, set to zero.
Bits 25-32: Message Type:
Type 0: Regular Message - All host-to-host communication occurs via regular messages, which have several sub- types. The sub-type field (bits 77-80) is the same as
sent in the host-to-IMP leader (see section 3.1).
Type 1: Error in Leader - See 1822(3.4).
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Type 2: IMP Going Down - See 1822(3.4).
Type 3: NDM Reply - This is a reply to the NDM host-to-IMP message (see section 3.1). It will have the same number of entries as the NDM message that is being replying to, and each listed 1822L name will be accompanied by a zero or a one. A zero signifies that the name is not effective, and a one means that the
name is now effective.
Type 4: NOP - The host should discard this message. It is used during initialization of the IMP/host
communication. The Destination Host field will contain
the 1822L Address of the host port over which the NOP
is being sent. All other fields are unused.
Type 5: Ready for Next Message (RFNM) - See 1822(3.4).
Type 6: Dead Host Status - See 1822(3.4).
Type 7: Destination Host or IMP Dead (or unknown) - This message is sent in response to a message for a
destination which the IMP cannot reach. The message to
the "dead" destination is discarded. See 1822(3.4) for
a complete list of the applicable sub-types. If this message is in response to a standard (type 0, sub-type 0 or 1) message, it will be followed by a Dead Host Status message, which gives further information about
- 36 -
the status of the dead host. If this message is in response to an uncontrolled (type 0, sub-type 2 or 3) message, only sub-type 1 (The destination host is not
up) will be used, and it will not be followed by a Dead
Host Status message.
Type 8: Error in Data - See 1822(3.4).
Type 9: Incomplete Transmission - The transmission of the named message was incomplete for some reason. An incomplete transmission message is similar to a RFNM, but is a failure indication rather than a success indication. This message is also used by the short-
blocking feature to indicate that the named message was
rejected because it would have caused to IMP to block the host for a long amount of time. See section 2.4
for more details concerning the short-blocking feature.
The message's sub-types are:
0: The destination host did not accept the message
The message was too long.
2: The host took more than 15 seconds to transmit the message to the IMP. This time is measured from
the last bit of the leader through the last bit of
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3: The message was lost in the network due to IMP or
4: The IMP could not accept the entire message within
- 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 the
5: Source IMP I/O failure occurred during receipt of
Sub-types 6-10 are all issued in response to a short- blocking message that timed out (would have caused the
host to become blocked for a long amount of time). The
sub-types are designed to give the host some indication
of why it timed out and what other messages would also time out. See section 2.4.2 for further details
concerning each of these sub-types.
6: The message timed out because of connection set-up delay. Further messages to the same host (if on
the same connection) may also be affected.
7: The message timed out because of end-to-end flow
control. Further messages to the same host on the
same connection will also be affected.
- 38 -
8: Destination IMP buffer shortage caused the message to time out. This affects multi-packet standard messages to the specified host, but shorter messages or messages to hosts on other IMPs may
not be affected.
Network congestion control caused the message to be
rejected. Messages to hosts on other IMPs may not
be affected, however.
10: Local resource shortage kept the IMP from being able to accept the message within the short-
blocking timeout period.
Type 10: Interface Reset - See 1822(3.4).
Type 15: 1822L Name or Address Error - This message is sent in response to a type 0 message from a host that contained an erroneous Source Host or Destination Host
field. Its sub-types are:
0: The Source Host 1822L name is not authorized or not
1: The Source Host 1822L address does not match the
host port used to send the message.
The Destination Host 1822L name is not authorized.
3: The Destination Host 1822L name is authorized but
- 39 -
not effective, even though the named host is up. If the host were actually down, a type 7 message
would be returned, not a type 15.
4: The Source or Destination Host field contains a 1822L name, but the host being addressed is on a
non-C/30 IMP (see Figure 4 in section 2.2).
Types 11-14,16-255: Unassigned.
Bits 33-48: Source Host:
For type 0 messages, this field contains the 1822L name or address of the host that originated the message. All replies to the message should be sent to the host specified herein. For message types 5-9, 11 and 15, this field contains the source host field used in a previous type 0
message sent by this host.
Bits 49-64: Destination Host:
For type 0 messages, this field contains the 1822L name or address that the message was sent to. This allows the destination host to detect how it was specified by the source host. For message types 5-9, 11 and 15, this field contains the destination host field used in a previous type
- message sent by this host.
- 40 -
Bits 65-76: Message ID:
For message types 0, 5, 7-9, 11 and 15, this is the value assigned by the source host to identify the message (see section 3.1). This field is also used by message types 2
Bits 77-80: Sub-type:
This field is used as a modifier by message types 0-2, 4-7,
9, 11 and 15.
Bits 81-96: Message Length:
This field is contained in type 0 and type 3 messages only, and is the actual length in bits of the message (exclusive
of leader, leader padding, and hardware padding) as computed
by the IMP.
- 41 -
 Specifications for the Interconnection of a Host and an IMP,
BBN Report 1822, May 1978 Revision.
 E. C. Rosen et. al., ARPANET Routing Algorithm Improvements, IEN 183 (also published as BBN Report 4473, Vol. 1), August
1980, pp. 55-107.
 J. Postel, Assigned Numbers, RFC 790, September 1981, p. 10.
- 42 -
1822...................................................... 4 1822 address.............................................. 6 1822 host................................................. 5 1822L..................................................... 4 1822L address............................................. 7 1822L host................................................ 5 1822L name................................................ 6 authorized................................................ 9 blocking................................................. 16 congestion control................................... 22, 39 connection........................................... 20, 38 destination host..................................... 32, 40 effective................................................ 10 flow control......................................... 20, 38 handing type......................................... 27, 35 incomplete transmission message...................... 19, 37 leader flags......................................... 27, 35 link field............................................... 32 logical addressing........................................ 4 message ID........................................... 32, 41 message length........................................... 41 message type......................................... 28, 35 multi-homing.............................................. 4 NDM.................................................. 10, 28 NDM reply............................................ 10, 36 NOC....................................................... 9 NOP........................................... 5, 22, 30, 36 outstanding.............................................. 21 priority bit............................................. 27 regular message...................................... 28, 35 RFNM..................................................... 36 short-blocking feature................................... 15 short-blocking message............................... 19, 28 source host.......................................... 31, 40 standard message......................................... 28 sub-type............................................. 32, 41 symmetric................................................. 5 trace bit............................................ 27, 35 uncontrolled message................................. 14, 28