📄 rfc851.txt
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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.
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2.2.4 1822L and 1822 Interoperability
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.
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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 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
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2.3 Uncontrolled Packets
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.
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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
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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.
2.4 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
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.
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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
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further details concerning the ready line, host tardiness, and
other issues.
2.5 Counting RFMS When Using 1822L
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
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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
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