Request for Comments: 851
The ARPANET 1822L Host Access Protocol
Andrew G. Malis
ARPANET Mail: malis@bbn-unix
Bolt Beranek and Newman Inc.
50 Moulton St.
Cambridge, MA 02238
April 1983
This RFC specifies the ARPANET 1822L Host Access Protocol, which is a successor to the existing 1822 Host Access Protocol. 1822L allows ARPANET hosts to use logical names as well as 1822's physical port locations to address each other. The RFC is also
being presented as a solicitation of comments on 1822L,
especially from host network software implementers and
maintainers.
RFC 851
Table of Contents
1 INTRODUCTION
2 THE ARPANET 1822L HOST ACCESS PROTOCOL
2.1 Addresses and Names
2.2 Name Translations
2.2.1 Authorization and Effectiveness
2.2.2 Translation Policies
2.2.3 Reporting Destination Host Downs
2.2.4 1822L and 1822 Interoperability
2.3 Uncontrolled Packets
2.4 Establishing Host-IMP Communications
2.5 Counting RFMS When Using 1822L
2.6 1822L Name Server
3 1822L LEADER FORMATS
3.1 Host-to-IMP 1822L Leader Format
3.2 IMP-to-Host 1822L Leader Format
4 REFERENCES
- i -
RFC 851
FIGURES
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
Name Server Reply Format
- ii -
RFC 851
1 INTRODUCTION
This RFC specifies the ARPANET 1822L Host Access Protocol, which
will allow hosts to use logical addressing (i.e., host names that
are independent of their physical location on the ARPANET) to
communicate with each other. This new host access protocol is
known as the ARPANET 1822L (for Logical) Host Access Protocol,
and is a successor 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, the IMPs will continue
to support the 1822 protocol, and hosts using either protocol can
readily communicate with each other (the IMPs will handle the
translation automatically).
There is one major restriction to the new 1822L protocol: it
will be implemented in C/30 IMPs only, and will therefore only be
usable by hosts connected to C/30 IMPs, as 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 identify a host on a non-C/30 IMP. While this
is not a problem on the ARPANET, which only has C/30 IMPs, the
restriction will apply if logical addressing is used on any
network that mixes C/30 and non-C/30 IMPs.
- 1 -
RFC 851
The RFC's terminology is consistent with that used in Report
1822, and any new terms will be defined when they are first used.
Familiarity with Report 1822 (section 3 in particular) is
assumed. As could be expected, the RFC makes many references to
Report 1822. As a result, it uses, as a convenient abbreviation,
"see 1822(x)" instead of "please refer to Report 1822, section x,
for further details".
This RFC updates, and obsoletes, RFC 802. The changes from that
RFC include:
RFC 802, now has its own RFC, RFC 852 [2]. It was moved to its
own RFC, since it is completely independent of logical
addressing.
policies and of host error handling have been added.
further improved. This applies to hosts using 1822 as well as
1822L.
section 2.5.
- 2 -
RFC 851
which makes use of two new Host-to-IMP messages to allow hosts
to do their own name-to-address mapping.
Address Error) IMP-to-Host message have been changed.
- 3 -
RFC 851
2 THE ARPANET 1822L HOST ACCESS PROTOCOL
The ARPANET 1822L Host Access Protocol 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) [3] gives the use of
logical addressing considerable justification. Among the
advantages it cites are:
independent of its location on the network.
time-division basis.
more than one port to communicate with the network).
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
a host and an IMP, and the specification in those leaders of the
source and/or destination host(s). Hosts have the choice of
- 4 -
RFC 851
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 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 -
RFC 851
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 -
RFC 851
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
This format allows 1822L hosts to directly address hosts 0-63 at
IMPs 1-255 (IMP 0 does not exist). Note that the highest host
- 7 -
RFC 851
numbers are reserved for addressing the IMP's internal fake
hosts. At this writing, the IMP has seven fake hosts, so host
numbers 57-63 address the IMP fake hosts, while host numbers 0-56
address real hosts external to the IMP. As the number of IMP
fake hosts changes, this boundary point will also change.
There are a number of factors that determine how an 1822L name is
translated by the IMP into a physical address on the network.
These factors include which translations are legal; in what order
different translations for the same name should be attempted;
which legal translations shouldn't be attempted because a
particular host port is down; and the interoperability between
1822 and 1822L hosts. These issues are discussed in the
following sections.
Every host on a C/30 IMP, regardless of whether it is using the
1822 or 1822L protocol to access the network, can have one or
more 1822L names (logical addresses). Hosts using 1822L can then
use these names to address the hosts in the network independent
of their physical locations. Because of the implementation
- 8 -
RFC 851
constraints mentioned in the introduction, hosts on non-C/30 IMPs
cannot be assigned 1822L names. To circumvent this restriction,
however, 1822L hosts can also use 1822L addresses to access all
of the other hosts.
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:
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). 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.
- 9 -
RFC 851
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
different names at different times). This requires the host to
declare itself to the IMP 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 were actually
authorized and can now 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 names (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 authorized.
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
- 10 -
RFC 851
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
and send the proper leader in each case (see section 2.2.4).
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
again.
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
- 11 -
RFC 851
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 the host. However, if
the 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.
Three different address selection policies are available for the
name mapping process. When translated, each name uses one of the
three policies (the policy is pre-determined on a per-name
basis). The three policies are:
addresses are listed in the IMP's translation tables, to find
- 12 -
RFC 851
the first reachable physical host address. This list is
always searched from the top whenever an uncontrolled packet
is to be sent or an end-to-end connection has to be created.
This is the most commonly used policy.
IMP's routing tables to find the translation to the
destination IMP with the least delay path.
differs in that searching the address list for a valid
translation starts at the address following where the previous
translation search ended. This attempts to spread out the
load from any one IMP's hosts to the various host ports
associated with a particular name. Note that this is NOT
network-wide load leveling, which would require a distributed
algorithm and tables.
As was explained in report 1822, and as will be discussed in
greater detail in section 2.5, whenever regular messages are sent
by a host, the IMP opens a subnetwork connection to each
destination host from the source host. A connection will stay
open at least as long as there are any outstanding (un-RFNMed)
- 13 -
RFC 851
messages using it and both the source and destination hosts stay
up.
However, the destination host may go down for some reason during
the lifetime of a connection. If the host goes down while there
are no outstanding messages to it in the network, then the
connection is closed and no other action is taken until the
source host submits the next message for that destination. At
that time, ONE of the following events will occur:
A1. If 1822 or an 1822L address is being used to specify the
destination host, then the source host will receive a type 7
(Destination Host Dead) message from the IMP.
A2. If an 1822L name is being used to specify the destination
host, and the name maps to only one authorized host port,
then a type 7 message will also be sent to the source host.
A3. If an 1822L name is being used to specify the destination
host, and the name maps to more than one authorized host
port, then the IMP attempts to open a connection to another
authorized and effective host port for that name. If no
such connection can be made, the host will receive a type 15
(1822L Name or Address Error), subtype 5 (no effective
translations) message (see section 3.2). Note that a type 7
message cannot be returned to the source host, since type 7
messages refer to a particular destination host port, and
- 14 -
RFC 851
the name maps to more than one destination port.
Things get a bit more complicated if there are any outstanding
messages on the connection when the destination host goes down.
The connection will be closed, and one of the following will
occur:
B1. If 1822 or an 1822L address is being used to specify the
destination host, then the source host will receive a type 7
message for each outstanding message.
B2. If an 1822L name is being used to specify the destination
host, then the source host will receive a type 9 (Incomplete
Transmission), subtype 3 (message lost due to network
failure) message for each outstanding message. The next
time the source host submits another message for that same
destination name, the previous algorithm will be used
(either step A2 or step A3).
The above two algorithms also apply when a host stays up, but
declares the destination name for an existing connection to no
longer be effective. In this case, however, the type 7 messages
above will be replaced by type 15, subtype 3 (name not effective)
messages.
Section 2.3 discusses how destination host downs are handled for
uncontrolled packets.
- 15 -
RFC 851
As has been previously stated, 1822 and 1822L hosts can
intercommunicate, and the IMPs will automatically handle any
necessary leader and address format conversions. However, not
every combination of 1822 and 1822L hosts allows full
interoperability with regard to the use of 1822L names.
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.
- 16 -
RFC 851
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 an 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
- 17 -
RFC 851
Uncontrolled packets (see 1822(3.6)) present a unique problem for
the 1822L protocol. Uncontrolled packets 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 packets need to carry all of their overhead with
them, including source and destination names. If 1822L names are
used when sending an uncontrolled packet, additional information
is now required by the subnetwork when the packet is transferred
to the destination IMP. This means that less host-to-host data
can be contained in the packet than is possible between 1822
hosts.
Uncontrolled packets that are sent between 1822 hosts may contain
not more than 991 bits of data. Uncontrolled packets that are
sent to and/or from 1822L hosts are limited to 32 bits less, or
not more than 959 bits. Packets that exceed this length will
result in an error indication to the host, and the packet 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 packet without
notification.
- 18 -
RFC 851
Other enhancements that are provided for uncontrolled packet
service are a notification to the host of any errors that are
detected by the host's IMP when it receives the packet. A host
will be notified if an uncontrolled packet contains an error in
the 1822L name specification, such as if the name is not
authorized or effective, if the remote host is unreachable (which
is indicated by none of its names being effective), if network
congestion control throttled the packet before it left the source
IMP, or for any other reason the source IMP was not able to send
the packet on its way.
In most cases, the host will not be notified if the uncontrolled
packet was lost once it was transmitted by the source IMP.
However, the IMP will attempt to notify the source host if a
logically-addressed uncontrolled packet was mistakenly sent to a
host that the source IMP thought was effective, but which turned
out to be dead or non-effective at the destination IMP. This
non-delivery notice is sent back to the source IMP as an
uncontrolled packet from the destination IMP, so the source host
is not guaranteed to receive this indication.
If the source IMP successfully receives the non-delivery notice,
then the source host will receive a type 15 (1822L Name or
Address Error), subtype 6 (down or non-effective port) message.
If the packet is resubmitted or another packet is sent to the
- 19 -
RFC 851
same destination name, and there are no available effective
translations, then the source host will receive a type 15,
subtype 5 (no effective translations) message if the destination
name has more than one mapping; or will receive either a type 7
(Destination Host Dead) or a type 15, subtype 3 (name not
effective) message if the destination name has a single
translation.
Those enhancements to the uncontrolled packet service that are
not specific to logical addressing will be available to hosts
using 1822 as well as 1822L. However, logically-addressed
uncontrolled packets must be used in order to receive any
indication that the packet was lost once it has left the source
IMP.
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
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.
- 20 -
RFC 851
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
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
- 21 -
RFC 851
further details concerning the ready line, host tardiness, and
other issues.
When a host submits a regular message using an 1822 leader, the
IMP checks for an existing simplex virtual circuit connection
from the source host to the destination host. If such a
connection already exists, it is used. Otherwise, a new
connection from the source host port to the destination host port
is opened. In either case, there may be at most eight messages
outstanding on that connection at any one time. If a host
submits a ninth message on that connection before it receives a
reply for the first message, then the host will be blocked until
the reply is sent for the first message.
Such connections can stay open for some time, but are timed out
after three minutes of no activity, or can be closed if there is
contention for the connection blocks in either the source or
destination IMP. However, a connection will never be closed as
long as there are any outstanding messages on it. This allows a
source host to count the number of replies it has received for
messages to each destination host address in order to avoid being
blocked by submitting a ninth outstanding message on any
- 22 -
RFC 851
connection.
When a host submits a regular message using an 1822L leader, a
similar process occurs, except that in this case, connections are
distinguished by the source name/destination name combination.
When the message is received from a host, the IMP first looks for
an open connection for that same source name/destination name
pair. If such a connection is found, then it is used, and no
further name translation is performed. If, however, no open
connection was found, then the destination name is translated,
and a connection opened to the physical host port. As long as
there are any outstanding messages on the connection it will stay
open, and it will have the same restriction that only eight
messages may be outstanding at any one time. Thus, a source host
can still count replies to avoid being blocked, but they must be
counted on a source name/destination name pair basis, instead of
just by destination host address as before.
Since connections are based on the source name as well as the
destination name, this implies that there may be more than one
open connection from physical host port A to physical host port
B, which would allow more than 8 outstanding messages
simultaneously from the first to the second port. However, for
this to occur, either the source or destination names, or both,
must differ from one connection to the next. For example, if the
- 23 -
RFC 851
names "543" and "677" both translate to physical port 3 on IMP
51, then the host on that port could open four connections to
itself by sending messages from "543" to "543", from "543" to
"677", from "677" to "543", and from "677" to "677".
As has already been stated, the destination names in regular
messages are only translated when connections are first opened.
Once a connection is open, that connection, and its destination
physical host port, will continue to be used until it is closed.
If, in the meantime, a "better" destination host port belonging
to the same destination name became available, it would not be
used until the next time a new connection is opened to that
destination name.
There may be times when a host wants to perform its own
translations, or might need the full list of physical addresses
to which a particular name maps. For example, a connection-based
host-to-host protocol may require that the same physical host
port on a multi-homed host be used for all messages using that
host-to-host connection, and the host does not wish to trust the
IMP to always deliver messages using a destination name to the
same host port.
- 24 -
RFC 851
In these cases, the host can submit a type 11 (Name Server
Request) message to the IMP, which requests the IMP to translate
the destination 1822L name and return a list of the addresses to
which it maps. The IMP will respond with a type 11 (Name Server
Reply) message, which contains the selection policy in use for
that name, the number of addresses to which the name maps, the
addresses themselves, and for each address, whether it is
effective and its routing distance from the IMP. See section 3.2
for a complete description of the message's contents.
Using this information, the source host can make an informed
decision on which of the physical host ports corresponding to an
1822L name to use, and can subsequently send the messages to that
port, rather than to the name.
The IMP also supports a different type of name service. A host
needs to issue a Name Declaration Message to the IMP in order to
make its names effective, but it may not wish to keep its names
in some table or file in the host. In this case, it can ask the
IMP to tell it which names it is authorized to use.
In this case, the host submits a type 12 (Port List Request)
message to the IMP, and the IMP replies with a type 12 (Port List
Reply) message. It contains, for the host port over which the
IMP received the request and sent the reply, the number of names
- 25 -
RFC 851
that map to the port, the list of names, and whether or not each
name is effective. The host can then use this information in
order to issue the Name Declaration Message. Section 3.2
contains a complete description of the reply's contents.
- 26 -
RFC 851
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. 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.
- 27 -
RFC 851
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
- 28 -
RFC 851
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
1822(3.3).
Bit 9: Priority Bit:
Messages with this bit on will be treated as priority
messages.
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.
- 29 -
RFC 851
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 may occur.
3: Uncontrolled Packet - The IMP will perform no
message-control functions for this type of
message, and network flow and congestion control
may cause loss of the packet. 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 2.2.1), 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 leader padding, contains the number of 1822L
names 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
- 30 -
RFC 851
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
and which are not. Pictorially, a NDM message has the
following format (including the leader, which is
printed in hexadecimal):
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
- 31 -
RFC 851
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
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 2.4 for a further discussion
on the use of the NOP message.
Type 8: Error with Message ID - see 1822(3.3).
Type 11: Name Server Request - This allows the host to use
the IMP's logical addressing tables as a name server.
The destination name in the 1822L leader is translated,
and the IMP replies with a Name Server Reply message,
which lists the physical host addresses to which the
destination name maps.
Type 12: Port List Request - This allows the physical host
to request the list of names that map to the host port
over which this request was received by the IMP. The
IMP replies with a Port List Reply message, which lists
the names that map to the port.
Types 5-7,9-10,13-255: Unassigned.
- 32 -
RFC 851
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
contain the source host's 1822L address (see figure 4 in
section 2.2.4).
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
- 33 -
RFC 851
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 2.2.4).
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 [4] 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.
Bits 81-96: Unused, must be zero.
- 34 -
RFC 851
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
- 35 -
RFC 851
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
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).
Type 2: IMP Going Down - See 1822(3.4).
- 36 -
RFC 851
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 (see figure 6). 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) - See
1822(3.4).
Type 8: Error in Data - See 1822(3.4).
Type 9: Incomplete Transmission - See 1822(3.4).
Type 10: Interface Reset - See 1822(3.4).
Type 11: Name Server Reply - This reply to the Name Server
Request host-to-IMP message contains a word with the
selection policy and the number of physical addresses
to which the destination name maps, followed by two
words per physical address: the first word contains an
- 37 -
RFC 851
1822L address, and the second word contains a bit
signifying whether or not that particular translation
is effective and the routing distance (in 6.4 ms units)
to the address's IMP. In figure 8, EFF is 1 for
effective and 0 for non-effective, and POL is a two-bit
number indicating the selection policy for the name
(see section 2.2.2):
0: First reachable.
1: Closest physical address.
2: Load leveling.
3: Unused.
- 38 -
RFC 851
1 16 17 32 33 48
+----------------+----------------+----------------+
| | | |
| 0E00 | 000B | 0000 |
| | | |
+----------------+----------------+----------------+
49 64 65 80 81 96
+----------------+----------------+----------------+
| | | |
| dest. name | 0000 | 0000 |
| | | |
+----------------+----------------+----------------+
97 112 113 128 129 144
+-+--------------+----------------+-+--------------+
|P| | |E| |
|O| # of addrs | 1822L addr #1 |F| routing dist |
|L| | |F| |
+-+--------------+----------------+-+--------------+
145 160 161 176
+----------------+-+--------------+
| |E| |
| 1822L addr #2 |F| routine dist | etc.
| |F| |
+----------------+-+--------------+
Figure 8. Name Server Reply Format
Type 12: Port List Reply - This is the reply to the Port
List Request host-to-IMP message. It contains the
number of names that map to this physical host port,
followed by two words per name: the first word contains
an 1822L name that maps to this port, and the second
contains either a zero or a one, signifying whether or
not that particular translation is effective. The
format is identical to the type 3 NDM Reply message
- 39 -
RFC 851
(see figure 6).
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
effective.
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 physical host to which this singly-homed
Destination Host name translated is authorized and
up, but not effective. If the host was 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 2.2.4).
5: The multi-homed Destination Host name is authorized,
but has no available effective translations.
6: A logically-addressed uncontrolled packet was sent
to a dead or non-effective host port. However, if
it is resubmitted, there may be another effective
host port to which the IMP may be able to attempt
- 40 -
RFC 851
to send the packet.
7: Logical addressing is not in use in this network.
8-15: Unassigned.
Types 13-14,16-255: 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 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 and 15, this field
contains the destination host field used in a previous type
0 message sent by this host.
Bits 65-76: Message ID:
For message types 0, 5, 7-9, 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
- 41 -
RFC 851
and 6.
Bits 77-80: Sub-type:
This field is used as a modifier by message types 0-2, 5-7,
9, and 15.
Bits 81-96: Message Length:
This field is contained in type 0, 3, 11, and 12 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.
- 42 -
RFC 851
4 REFERENCES
[1] Specifications for the Interconnection of a Host and an IMP,
BBN Report 1822, December 1981 Revision.
[2] A. Malis, The ARPANET Short Blocking Feature, Request For
Comments 852, April 1983.
[3] E. C. Rosen et. al., ARPANET Routing Algorithm Improvements,
Internet Experimenter's Note 183 (also published as BBN
Report 4473, Vol. 1), August 1980, pp. 55-107.
[4] J. Postel, Assigned Numbers, Request For Comments 820,
January 1983, p. 11.
- 43 -
RFC 851
INDEX
1822
1822 address
1822 host
1822L
1822L address
1822L host
1822L name
address selection policy
authorized
blocking
closest physical address
connection
destination host
effective
first reachable
handing type
host downs
leader flags
link field
load leveling
logical addressing
message ID
message length
message type
multi-homing
name server
NDM
NDM reply
NOC
NOP
priority bit
regular message
RFNM
source host
standard message
sub-type
symmetric
trace bit
uncontrolled packet
- 44 -