📄 rfc851.txt
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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
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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.
2.6 1822L Name Server
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.
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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
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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.
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3 1822L LEADER FORMATS
The following sections describe the formats of the leaders that
precede messages between an 1822L host and its IMP. They were
designed to be as compatible with the 1822 leaders as possible.
The second, fifth, and sixth words are identical in the two
leaders, and all of the existing functionality of the 1822
leaders has been retained. In the first word, the 1822 New
Format Flag is now also used to identify the two types of 1822L
leaders, and the Handling Type has been moved to the second byte.
The third and fourth words contain the Source and Destination
1822L Name, respectively.
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3.1 Host-to-IMP 1822L Leader Format
1 4 5 8 9 16
+--------+--------+----------------+
| | 1822L | |
| Unused | H2I | Handling Type |
| | Flag | |
+--------+--------+----------------+
17 20 21 22 24 25 32
+--------+-+------+----------------+
| |T|Leader| |
| Unused |R|Flags | Message Type |
| |C| | |
+--------+-+------+----------------+
33 48
+----------------------------------+
| |
| Source Host |
| |
+----------------------------------+
49 64
+----------------------------------+
| |
| Destination Host |
| |
+----------------------------------+
65 76 77 80
+-------------------------+--------+
| | |
| Message ID |Sub-type|
| | |
+-------------------------+--------+
81 96
+----------------------------------+
| |
| Unused |
| |
+----------------------------------+
Figure 5. Host-to-IMP 1822L Leader Format
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Bits 1-4: Unused, must be set to zero.
Bits 5-8: 1822L Host-to-IMP Flag:
This field is set to decimal 13 (1101 in binary).
Bits 9-16: Handling Type:
This field is bit-coded to indicate the transmission
characteristics of the connection desired by the host. See
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.
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Bits 25-32: Message Type:
Type 0: Regular Message - All host-to-host communication
occurs via regular messages, which have several sub-
types, found in bits 77-80. These sub-types are:
0: Standard - The IMP uses its full message and error
control facilities, and host blocking 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
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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):
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
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