Network Working Group D. Piscitello
Request for Comments: 1526 Bellcore
Category: Informational September 1993

Assignment of System Identifiers for TUBA/CLNP Hosts

Status of this Memo

This memo provides information for the Internet community. It does
not specify an Internet standard. Distribution of this memo is
unlimited.

Abstract

This document describes conventions whereby the system identifier
portion of an RFC1237 style NSAP address may be guaranteed
uniqueness within a routing domain for the purpose of
autoconfiguration in TUBA/CLNP internets. The mechanism is extensible
and can provide a basis for assigning system identifiers in a
globally unique fashion.

IntrodUCtion

This memo specifies methods for assigning a 6 octet system identifier
portion of the OSI NSAP address formats described in "Guidelines for
OSI NSAP Allocation in the Internet" [1], in a fashion that ensures
that the ID is unique within a routing domain. It also recommends
methods for assigning system identifiers having lengths other than 6
octets. The 6 octet system identifiers recommended in this RFCare
assigned from 2 globally administered spaces (IEEE 802 or "Ethernet",
and IP numbers, administered by the Internet Assigned Numbers
Authority, IANA).

At this time, the primary purpose for assuring uniqueness of system
identifiers is to aid in autoconfiguration of NSAP addresses in
TUBA/CLNP internets [2]. The guidelines in this paper also establish
an initial framework within which globally unique system identifiers,
also called endpoint identifiers, may be assigned.

Acknowledgments

Many thanks to Radia Perlman, Allison Mankin, and Ross Callon of for
their insights and assistance. Thanks also to the Ethernet connector
to my MAC, which conveniently and quite inoBTrusively fell out,
enabling me to get an entire day's worth of work done without email
interruptions.

1. Background

The general format of OSI network service Access point (NSAP)
addresses is illustrated in Figure 1.

_______________________________________________
____IDP____________________DSP______________
__AFI__IDI______HO-DSP_________ID____SEL_

IDP Initial Domain Part
AFI Authority and Format Identifier
IDI Initial Domain Identifier
DSP Domain Specific Part
HO-DSP High-order DSP
ID System Identifier
SEL NSAP Selector

Figure 1: OSI NSAP Address Structure.

The recommended encoding and allocation of NSAP addresses in the
Internet is specified in RFC1237. RFC1237 makes the following
statements regarding the system identifier (ID) field of the NSAPA:

1. the ID field may be from one to eight octets in length

2. the ID must have a single known length in any particular
routing domain

3. the ID field must be unique within an area for ESs and
level 1 ISs, and unique within the routing domain for level
2 ISs.

4. the ID field is assumed to be flat

RFC1237 further indicates that, within a routing domain that
conforms to the OSI intradomain routing protocol [3] the lower-order
octets of the NSAP should be structured as the ID and SEL fields
shown in Figure 1 to take full advantage of intradomain IS-IS
routing. (End systems with addresses which do not conform may require
additional manual configuration and be subject to inferior routing
performance.)

Both GOSIP Version 2 (under DFI-80h, see Figure 2a) and ANSI DCC NSAP
addressing (Figure 2b) define a common DSP structure in which the
system identifier is assumed to be a fixed length of 6 octets.

_______________
<--__IDP_-->____________________________________
AFI___IDI______________<--_DSP_-->____________
_47___0005__DFI_AA_Rsvd__RD_Area_ID_Sel_
octets _1_____2_____1___3___2___2___2____6__1__

Figure 2 (a): GOSIP Version 2 NSAP structure.
______________
<--_IDP_-->______________________________________
AFI___IDI______________<--_DSP_-->_____________
_39___840__DFI__ORG_Rsvd_RD_Area__ID_Sel_
octets _1_____2____1____3___2____2___2___6___1__

IDP Initial Domain Part
AFI Authority and Format Identifier
IDI Initial Domain Identifier
DSP Domain Specific Part
DFI DSP Format Identifier
ORG Organization Name (numeric form)
Rsvd Reserved
RD Routing Domain Identifier
Area Area Identifier
ID System Identifier
SEL NSAP Selector

Figure 2(b): ANSI NSAP address format for DCC=840

2. Autoconfiguration

There are provisions in OSI for the autoconfiguration of area
addresses. OSI end systems may learn their area addresses
automatically by observing area address identified in the IS-Hello
packets transmitted by routers using the ISO 9542 End System to
Intermediate System Routing Protocol, and may then insert their own
system identifier. (In particular, RFC1237 eXPlains that when the ID
portion of the address is assigned using IEEE style 48-bit
identifiers, an end system can reconfigure its entire NSAP address
automatically without the need for manual intervention.) End systems
that have not been pre-configured with an NSAPA may also request an
address from an intermediate system their area using [5].

2.1 Autoconfiguration and 48-bit addresses

There is a general misassumption that the 6-octet system identifier
must be a 48-bit IEEE assigned (ethernet) address. Generally
speaking, autoconfiguration does not rely on the use of a 48-bit
ethernet style address; any system identifier that is globally

administered and is unique will do. The use of 48-bit/6 octet system
identifiers is "convenient...because it is the same length as an 802
address", but more importantly, choice of a single, uniform ID length
allows for "efficient packet forwarding", since routers won't have to
make on the fly decisions about ID length (see [6], pages 156-157).
Still, it is not a requirement that system identifiers be 6 octets to
operate the the IS-IS protocol, and IS-IS allows for the use of IDs
with lengths from 1 to 8 octets.

3. System Identifiers for TUBA/CLNP

Autoconfiguration is a desirable feature for TUBA/CLNP, and is viewed
by some as "essential if a network is to scale to a truly large size"
[6].

For this purpose, and to accommodate communities who do not wish to
use ethernet style addresses, a generalized format that satisfies the
following criteria is desired:

o the format is compatible with installed end systems
complying to RFC1237

o the format accommodates 6 octet, globally unique system
identifiers that do not come from the ethernet address space

o the format accommodates globally unique system identifiers
having lengths other than 6 octets

The format and encoding of a globally unique system identifier that
meets these requirements is illustrated in Figure 3:

Octet 1 Octet 2 Octet 3 ... Octet LLL-1 Octet LLLL
+-----------+-----------+-----------+- ...-+-----------+-----------+
xxxx TTGM xxxx xxxx xxxx xxxx xxxx xxxx xxxx xxxx
+-----------+-----------+-----------+- ...-+-----------+-----------+

Figure 3. General format of the system identifier

3.1 IEEE 802 Form of System Identifier

The format is compatible with globally assigned IEEE 802 addresses,
since it carefully preserves the semantics of the global/local and
group/individual bits. Octet 1 identifies 2 qualifier bits, G and M,
and a subtype (TT) field whose semantics are associated with the
qualifier bits. When a globally assigned IEEE 802 address is used as
a system identifier, the qualifier bit M, representing the
multicast/unicast bit, must always be set to zero to denote a unicast
address. The qualifier bit G may be either 0 or 1, depending on

whether the individual address is globally or locally assigned. In
these circumstances, the subtype bits are "don't care", and the
system identifier shall be interpreted as a 48-bit, globally unique
identifier assigned from the IEEE 802 committee (an ethernet
address). The remaining bits in octet 1, together with octets 2 and
3 are the vendor code or OUI (organizationally unique identifier), as
illustrated in Figure 4. The ID is encoded in IEEE 802 canonical
form (low order bit of low order hex digit of leftmost octet is the
first bit transmitted).

Octet 1 Octet 2 Octet 3 Octet 4 Octet 5 Octet 6
+-----------+-----------+-----------+-----------+-----------+-----------+
VVVV VV00 VVVV VVVV VVVV VVVV SSSS SSSS SSSS SSSS SSSS SSSS
+-----------+-----------+-----------+-----------+-----------+-----------+

------------vendor code -------------------station code---------------

Figure 4. IEEE 802 form of system identifier

4. Embedded IP Address as System Identifier

To distinguish 48-bit IEEE 802 addresses used as system identifiers
from other forms of globally admininistered system identifiers, the
qualifer bit M shall be set to 1. The correct interpretation of the M
bit set to 1 should be, "this can't be an IEEE 802 multicast address,
since use of multicast addresses is by convention illegal, so it must
be some other form of system identifier". The subtype (TT) bits
illustrated in Figure 3 thus become relevant.

When the subtype bits (TT) are set to a value of 0, the system
identifier contains an embedded IP address. The remainder of the 48-
bit system identifier is encoded as follows. The remaining nibble in
octet 1 shall be set to zero. Octet 2 is reserved and shall be set
to a pre-assigned value (see Figure 5). Octets 3 through 6 shall
contain a valid IP address, assigned by IANA. Each octet of the IP
address is encoded in binary, in internet canonical form, i.e., the
leftmost bit of the network number first.

Octet 1 Octet 2 Octet 3 Octet 4 Octet 5 Octet 6
+-----------+-----------+-----------+-----------+-----------+-----------+
0000 0001 1010 1010 aaaa aaaa bbbb bbbb cccc cccc dddd dddd
+-----------+-----------+-----------+-----------+-----------+-----------+

-len&Type----reserved----------IP address----------------------------

Figure 5. Embedded IP address as system identifier

As an example, the host "eve.bellcore.com = 128.96.90.55" could retain
its IP address as a system identifier in a TUBA/CLNP network. The
encoded ID is illustrated in Figure 6.

Octet 1 Octet 2 Octet 3 Octet 4 Octet 5 Octet 6
+-----------+-----------+-----------+-----------+-----------+-----------+
0000 0001 1010 1010 1000 0000 0110 0000 0101 1010 0011 0111
+-----------+-----------+-----------+-----------+-----------+-----------+

-len&Type----reserved----------IP address----------------------------

Figure 6. Example of IP address encoded as ID

H 2 "Other forms of System Identifiers"

To allow for the future definition of additional 6-octet system
identifiers, the remaining subtype values are reserved.

It is also possible to identify system identifiers with lengths other
than 6 octets. Communities who wish to use 8 octet identifiers (for
example, embedded E.164 international numbers for the ISDN ERA) must
use a GOSIP/ANSI DSP format that allows for the specification of 2
additional octets in the ID field, perhaps at the expense of the
"Rsvd" fields; this document recommends that a separate Domain Format
Indicator value be assigned for such purposes; i.e., a DFI value that
is interpreted as saying, among other things, "the system identifier
encoded in this DSP is 64-bits/8 octets. The resulting ANSI/GOSIP DSP
formats under such circumstances are illustrated in Figure 7:

______________
<--_IDP_-->_______________________________
AFI___IDI______________<--_DSP_-->_______
_39___840__DFI__ORG_RD_Area__ID_Sel_
octets _1_____2____1____3___2___2___8___1__

Figure 7a: ANSI NSAP address format for DCC=840, DFI=foo

_______________
<--__IDP_-->____________________________________
AFI___IDI______________<--_DSP_-->____________
_47___0005__DFI_AA__RD_Area_ID_Sel_
octets _1_____2_____1___3__2___2____8__1__

IDP Initial Domain Part
AFI Authority and Format Identifier
IDI Initial Domain Identifier
DSP Domain Specific Part
DFI DSP Format Identifier
AA Administrative Authority
RD Routing Domain Identifier
Area Area Identifier
ID System Identifier
SEL NSAP Selector

Figure 7b: GOSIP Version 2 NSAP structure, DFI=bar

Similar address engineering can be applied for those communities who
wish to have shorter system identifiers; have another DFI assigned,
and expand the reserved field.

5. Conclusions

This proposal should debunk the "if it's 48-bits, it's gotta be an
ethernet address" myth. It demonstrates how IP addresses may be
encoded within the 48-bit system identifier field in a compatible
fashion with IEEE 802 addresses, and offers guidelines for those who
wish to use system identifiers other than those enumerated here.

6. References

[1] Callon, R., Gardner, E., and R. Colella, "Guidelines for OSI NSAP
Allocation in the Internet", RFC1237, NIST, Mitre, DEC, June
1991.

[2] Callon, R., "TCP and UDP with Bigger Addresses (TUBA), A Simple
Proposal for Internet Addressing and Routing", RFC1347, DEC,
June 1992.

[3] ISO, "Intradomain routing protocol for use in conjunction with
ISO 8473, Protocol for providing the OSI connectionless network
service", ISO 10589.

[4] ISO, End-system and intermediate-system routing protocol for use
in conjunction with ISO 8473, Protocol for providing the OSI
connectionless network service, ISO 9542.

[5] ISO, "End-system and intermediate-system routing protocol for use
in conjunction with ISO 8473, Protocol for providing the OSI
connectionless network service. Amendment 1: Dynamic Discovery
of OSI NSAP Addresses End Systems", ISO 9542/DAM1.

[6] Perlman, R., "Interconnections: Bridges and Routers", Addison-
Wesley Publishers, Reading, MA. 1992.

7. Security Considerations

Security issues are not discussed in this memo.

8. Author's Address

David M. Piscitello
Bell Communications Research
NVC 1C322
331 Newman Springs Road
Red Bank, NJ 07701

EMail: dave@mail.bellcore.com