rfc878.txt
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Request for Comments: 878
Obsoletes RFCs: 851, 802
The ARPANET 1822L Host Access Protocol
RFC 878
Andrew G. Malis
ARPANET Mail: malis@bbn-unix
BBN Communications Corp.
50 Moulton St.
Cambridge, MA 02238
December 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.
1822L Host Access Protocol December 1983
RFC 878
Table of Contents
1 INTRODUCTION.......................................... 1
2 THE ARPANET 1822L HOST ACCESS PROTOCOL................ 3
2.1 Addresses and Names................................. 5
2.2 Name Translations................................... 7
2.2.1 Authorization and Effectiveness................... 7
2.2.2 Translation Policies............................. 11
2.2.3 Reporting Destination Host Downs................. 13
2.2.4 1822L and 1822 Interoperability.................. 15
2.3 Uncontrolled Packets............................... 16
2.4 Establishing Host-IMP Communications............... 19
2.5 Counting RFNMs When Using 1822L.................... 20
2.6 1822L Name Server.................................. 23
3 1822L LEADER FORMATS................................. 25
3.1 Host-to-IMP 1822L Leader Format.................... 26
3.2 IMP-to-Host 1822L Leader Format.................... 34
4 REFERENCES........................................... 42
A 1822L-IP ADDRESS MAPPINGS............................ 43
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FIGURES
2.1 1822 Address Format.................................. 5
2.2 1822L Name Format.................................... 6
2.3 1822L Address Format................................. 6
3.1 Host-to-IMP 1822L Leader Format..................... 27
3.2 NDM Message Format.................................. 30
3.3 IMP-to-Host 1822L Leader Format..................... 35
3.4 Name Server Reply Format............................ 38
A.1 1822 Class A Mapping................................ 44
A.2 1822L Class A Mapping............................... 44
A.3 1822L Class B Mapping............................... 45
A.4 1822L Class C Mapping............................... 46
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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).
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 851. The changes from that
RFC are:
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RFC 878
o Section 2.2.4 was rewritten for clarity.
o Section 2.5 was expanded to further discuss the effects of
using 1822L names on host-to-host virtual circuits.
o In section 3.2, the type 1 IMP-to-host message has two new
subtypes, the type 9 message has one new subtype, and the type
15, subtype 4 message is no longer defined.
o An appendix describing the mapping between 1822L names and
internet (IP) addresses has been added.
All of these changes to RFC 851 are marked by revision bars (as |
shown here) in the right margin. |
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RFC 878
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) [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 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
a host and an IMP, and the specification in those leaders of the
source and/or destination host(s). Hosts have the choice of
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RFC 878
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.
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RFC 878
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 |
| | |
+----------------+---------------------------------+
1822 Address Format
Figure 2.1
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:
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1 16
+--------------------------------+
| |
| 1822L name |
| |
+--------------------------------+
1822L Name Format
Figure 2.2
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