RFC 802: The ARPANET 1822L Host Access Protocol

Andrew G. Malis
Netmail: malis@bbn-unix

Bolt Beranek and Newman Inc.

November 1981

Table of Contents

2.1 Addresses and Names
2.2 Name Authorization and Effectiveness
2.3 Uncontrolled Messages
2.4 The Short-Blocking Feature
2.4.1 Host Blocking
2.4.2 Reasons for Host Blockage
2.5 Establishing Host-IMP Communications
3.1 Host-to-IMP 1822L Leader Format
3.2 IMP-to-Host 1822L Leader Format

                              - i -


1822 Address Format
1822L Name Format
1822L Address Format
Communications between different host types
Host-to-IMP 1822L Leader Format
NDM Message Format
IMP-to-Host 1822L Leader Format

                             - ii -


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 [1]. 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


                              - 1 -

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)

is assumed.

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

                              - 2 -

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.

                              - 3 -


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) [2] 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

time-division basis.

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

                              - 4 -

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,

and vice-versa.

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.

                              - 5 -

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

1822L addresses.

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

simpler format:

                              - 6 -

                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

                              - 7 -

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:

                              - 8 -

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

C/30 IMPs.

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

                              - 9 -

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

                             - 10 -

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

                             - 11 -

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.

                             - 12 -

Destination Host
  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

                             - 13 -

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

without notification.

                             - 14 -

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.

                             - 15 -

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

                             - 16 -

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

                             - 17 -

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

the blocking.

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

                             - 18 -

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

                             - 19 -

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

be accepted.

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

                             - 20 -

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.

                             - 21 -

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: 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

                             - 22 -

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

                             - 23 -

host interface, and the 1822L NOPs will contain the corresponding

1822L address.

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

other issues.

                             - 24 -


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.

                             - 25 -

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

                             - 26 -

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.

                             - 27 -

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.

2: 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.

4-15: Unassigned.

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

                             - 28 -

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):

                             - 29 -

            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

                             - 30 -

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

                             - 31 -

contain the source host's 1822L address (see Figure 4 in

section 2.2).

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  [3]  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,

and 8.

                             - 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

                             - 34 -

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

and 15.

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).

                             - 35 -

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

quickly enough.

1: 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

the message.

                             - 37 -

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

circuit failures.

4: The IMP could not accept the entire message within

15 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

sub-types 6-10.

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

this message.

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.

9: 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.

11-15: Unassigned.

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.

2: 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).

5-15: Unassigned.

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

0 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

and 6.

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 -


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

BBN Report 1822, May 1978 Revision.

[2] E. C. Rosen et. al., ARPANET Routing Algorithm Improvements,

IEN 183 (also published as BBN Report 4473, Vol. 1), August

1980, pp. 55-107.

[3] J. Postel, Assigned Numbers, RFC 790, September 1981, p. 10.

                             - 42 -


1822 address
1822 host
1822L address
1822L host
1822L name
congestion control
destination host
flow control
handing type
incomplete transmission message
leader flags
link field
logical addressing
message ID
message length
message type
NDM reply
priority bit
regular message
short-blocking feature
short-blocking message
source host
standard message
trace bit
uncontrolled message

                             - 43 -