Network Working Group Ross Callon (BBN)
Request for Comments: 986 Hans-Werner Braun (UMich)
June 1986
WORKING DRAFT
Guidelines for the use of Internet-IP addresses in the
ISO Connectionless-Mode Network Protocol
Status of This Memo
This RFC suggests a method to allow the existing IP addressing,
including the IP protocol field, to be used for the ISO
Connectionless Network Protocol (CLNP). This is a draft solution to
one of the problems inherent in the use of "ISO-grams" in the DOD
Internet. Related issues will be discussed in subsequent RFCs. This
RFC suggests a proposed protocol for the ARPA-Internet community, and
requests discussion and suggestions for improvements. Distribution
of this memo is unlimited.
1. Introduction
The CLNP is documented in [1], but for matters of completeness the
following illustration of the CLNP header is included here as
Figure 1.
The addressing part of the header is the subject of this RFC, i.e.,
the source and the destination address, respectively. These
addresses are generally discussed in [2] and [3], with this document
presenting a specific method for addressing in the DOD Internetwork
environment, consistent with the international standardized NSAP
addresses.
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DOD IP Addresses in ISO CLNP
Octet
+--------------------------------------+ +--------
| Network Layer Protocol Identifier | 1 :
|--------------------------------------| :
| Length Indicator | 2 :
|--------------------------------------| :
| Version/Protocol Id Extension | 3 : Fixed
|--------------------------------------| :
| Lifetime | 4 : Part
|--------------------------------------| :
|SP|MS|E/R| Type | 5 :
|--------------------------------------| :
| Segment Length | 6,7 :
|--------------------------------------| :
| Checksum | 8,9 :
|--------------------------------------| +--------
| Destination Address Length Indicator | 10 :
|--------------------------------------| :
| Destination Address | 11 through m-1 : Address
|--------------------------------------| :
| Source Address Length Indicator | m : Part
|--------------------------------------| :
| Source Address | m+1 through n-1 :
|--------------------------------------| +--------
| Data Unit Identifier | n,n+1 :
|--------------------------------------| : Segment
| Segment Offset | n+2,n+3 : ation
|--------------------------------------| :
| Total Length | n+4,n+5 : Part
|--------------------------------------| +--------
| Options | n+6 through p : Options
Part
|--------------------------------------| +--------
| Data | p+1 through z : Data
+--------------------------------------+ +--------
Figure 1: PDU Header Format
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2. Addresses for Use in the Internet
This section describes the primary addresses used to address NSAPs in
the Internet. A later section will describe a separate address
format for end systems and individual simple LANs that are attached
to the Internet only through intervening Public Data Networks.
The appropriate Authority and Format Identifier (AFI) is one octet in
length.
"The AFI consists of an integer with a value between 0 and 99 with
an abstract syntax of two decimal digits" [3], that is, the AFI
codes are binary coded decimal (BCD).
It specifies an ISO-6523-ICD assignment, and also that the Domain
Specific Part (DSP) of the address is based on binary. The AFI octet
uses the value "47". The ISO-6523-ICD format is used to emphasize
that this is an administrative assignment. The usage of an ISO DCC
(Data Country Code) would be possible, but could be misleading due to
the fairly far spread geographical extent of the Internet-IP.
As required by the ISO addressing standard, the next two octets of
the address, in this case, specify the Initial Domain Identifier.
This two octet value is the International Code Designator (ICD)
assigned to the DOD Internet, "0006".
The remainder of the NSAP address is the Domain Specific Part (DSP).
This is assigned by the Internet administration, which is considered
to be an addressing domain. The remainder of the address specifies a
one byte version number, the four byte Internet Protocol address and
a one byte IP user protocol field. The version number allows for
future extensions. The IP address used is the same as the current
four octet IP address. The user protocol field is the same as the
user protocol field in the current IP header. This is necessary
because the ISO protocol considers identification of the user
protocol to be an addressing issue, and therefore does not allow for
the user protocol to be specified in the protocol header
independently from the address.
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DOD IP Addresses in ISO CLNP
Therefore a source or destination address within the ISO
Connectionless Protocol, when used in the DOD Internet, looks as
follows:
Octet
+------------------------+
| AFI | 1
+------------------------+
| IDI / ICD | 2
+-- --+
|(specifies DOD Internet)| 3
+------------------------+
| Version Number | 4
+------------------------+
| | 5
+--- four byte ---+
| | 6
+--- Internet ---+
| | 7
+--- Address (IP) ---+
| | 8
+------------------------+
| IP User Protocol ID | 9
+------------------------+
Figure 2: ISO IP address structure
The Authority and Format Identifier (AFI) is "47" (BCD). The Initial
Domain Identifier (IDI) consists of the International Code Designator
(ICD) assigned to the Internet, and must contain the value "0006".
The Version Number must contain the value "01". The Current IP
addresses and IP user protocol numbers can be found in [4].
3. Devices Attached to PDNs
Otherwise isolated end systems, which are attached to the Internet
only indirectly via public data networks, and simple LANs which are
similarly attached only via Public Data Networks, will make use of a
separate address format based on their X.121 address.
Figure 3 specifies the address for use by end systems attached to
PDNs. Here the AFI specifies an ISO-X.121 address format, with the
DSP based on binary. The AFI occupies a single octet, and must take
the value "37" (hexadecimal). The IDI contains the X.121 addresses
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DOD IP Addresses in ISO CLNP
encoded in binary (using BCD), padded at the end if necessary with
all ones (binary "1111") to make up 7 full octets. Finally, the DSP
contains a single octet, which specifies the user protocol.
Figure 3: ISO IP address structure for isolated end systems
Figure 4 specifies the address for use by simple LANs attached to
PDNs. This address is the same as the address used for end systems
directly attached to PDNs, except for the addition of the (variable
length) local address as used on the LAN. Whether the address is of
the form shown in figure 3, or of the form shown in figure 4, is
determined by looking at the length of the address.
Figure 4: ISO IP address structure for isolated LANs
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References
[1] ISO, "Protocol for Providing the Connectionless-Mode Network
Services", RFC-926, ISO, December 1984.
[2] ANSI, "Guidelines for the Specification of the Structure of the
Domain Specific Part (DSP) of the ISO Standard NSAP Address",
RFC-982, ANSI Working Document X3S3.3/85-258, April 1986.
[3] ISO, Draft International Standard 8348/DAD2, "Information
Processing Systems -- Data Communications -- Network Service
Definition, Addendum 2 Covering Network Layer Addressing",
RFC-941, April 1985.
[4] Reynolds, J. and J. Postel, "Assigned Numbers", RFC-960, USC
Information Sciences Institute, December 1985.
