📄 rfc851.txt
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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.
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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
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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.
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3.2 IMP-to-Host 1822L Leader Format
1 4 5 8 9 16
+--------+--------+----------------+
| | 1822L | |
| Unused | I2H | Handling Type |
| | Flag | |
+--------+--------+----------------+
17 20 21 22 24 25 32
+--------+-+------+----------------+
| |T|Leader| |
| Unused |R|Flags | Message Type |
| |C| | |
+--------+-+------+----------------+
33 48
+----------------------------------+
| |
| Source Host |
| |
+----------------------------------+
49 64
+----------------------------------+
| |
| Destination Host |
| |
+----------------------------------+
65 76 77 80
+-------------------------+--------+
| | |
| Message ID |Sub-type|
| | |
+-------------------------+--------+
81 96
+----------------------------------+
| |
| Message Length |
| |
+----------------------------------+
Figure 7. IMP-to-Host 1822L Leader Format
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Bits 1-4: Unused and set to zero.
Bits 5-8: 1822L IMP-to-Host Flag:
This field is set to decimal 14 (1110 in binary).
Bits 9-16: Handling Type:
This has the value assigned by the source host (see section
3.1). This field is only used in message types 0, 5-9, 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).
Type 2: IMP Going Down - See 1822(3.4).
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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
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1822L Host Access Protocol April 1983
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.
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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
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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 t⌨️ 快捷键说明
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