RFC 1888
This document is obsolete. Please refer to RFC 4048.

Network Working Group                                           J. Bound
Request for Comments: 1888                 Digital Equipment Corporation
Category: Experimental                                      B. Carpenter
                                                           D. Harrington
                                           Digital Equipment Corporation
                                                          J. Houldsworth
                                                     ICL Network Systems
                                                                A. Lloyd
                                                  Datacraft Technologies
                                                             August 1996

                           OSI NSAPs and IPv6

Status of this Memo

   This memo defines an Experimental Protocol for the Internet
   community.  This memo does not specify an Internet standard of any
   kind.  Discussion and suggestions for improvement are requested.
   Distribution of this memo is unlimited.


   This document recommends that network implementors who have planned
   or deployed an OSI NSAP addressing plan, and who wish to deploy or
   transition to IPv6, should redesign a native IPv6 addressing plan to
   meet their needs.  However, it also defines a set of mechanisms for
   the support of OSI NSAP addressing in an IPv6 network.  These
   mechanisms are the ones that MUST be used if such support is
   required.  This document also defines a mapping of IPv6 addresses
   within the OSI address format, should this be required.

Table of Contents

      1. General recommendation on NSAP addressing plans..............2
      2. Summary of defined mechanisms................................4
      3. Restricted NSAPA in a 16-byte IPv6 address for ICD and DCC...4
      3.1 Routing restricted NSAPAs...................................5
      4. Truncated NSAPA used as an IPv6 address......................6
      4.1 Routing truncated NSAPAs....................................8
      5. Carriage of full NSAPAs in IPv6 destination option...........9
      6. IPv6 addresses inside an NSAPA..............................10
      7. Security Considerations.....................................11
      Annex A: Summary of NSAP Allocations...........................13
      Annex B: Additional Rationale..................................14
      Authors' Addresses.............................................16

Bound, et. al.                Experimental                      [Page 1]

RFC 1888                   OSI NSAPs and IPv6                August 1996

1. General recommendation on NSAP addressing plans

   This recommendation is addressed to network implementors who have
   already planned or deployed an OSI NSAP addressing plan for the usage
   of OSI CLNP [IS8473] according to the OSI network layer addressing
   plan [IS8348] using ES-IS and IS-IS routing [IS9542, IS10589].  It
   recommends how they should adapt their addressing plan for use with
   IPv6 [RFC1883].

   The majority of known CLNP addressing plans use either the Digital
   Country Code (DCC) or the International Code Designator (ICD) formats
   defined in [IS8348]. A particular example of this is the US
   Government OSI Profile Version 2 (GOSIP) addressing plan [RFC1629].
   The basic NSAP addressing scheme and current implementations are
   summarised in Annex A.

   [IS8348] specifies a maximum NSAPA (NSAP address) size of 20 bytes
   and some network implementors have designed address allocation
   schemes which make use of this 20 byte address space.

   Other NSAP addressing plans have been specified by the ITU-T for
   public data services, such as X.25 and ISDN, and these can also have
   addresses up to 20 bytes in length.

   The general recommendation is that implementors SHOULD design native
   IPv6 addressing plans according to [RFC1884], but doing so as a
   natural re-mapping of their CLNP addressing plans. While it is
   impossible to give a general recipe for this, CLNP addresses in DCC
   or ICD format can normally be split into two parts: the high order
   part relating to the network service provider and the low order part
   relating to the user network topology and host computers.

   For example, in some applications of US GOSIP the high order part is
   the AFI, ICD, DFI, AA and RD fields, together occupying 9 bytes. The
   low order part is the Area and ID fields, together occupying 8 bytes.
   (The selector byte and the two reserved bytes are not part of the
   addressing plan.) Thus, in such a case, the high-order part could be
   replaced by the provider part of an IPv6 provider-based addressing
   plan.  An 8-byte prefix is recommended for this case and [RFC1884]
   MUST be followed in planning such a replacement. The low order part
   would then be mapped directly in the low-order half of the IPv6
   address space, and user site address plans are unchanged.  A 6-byte
   ID field, exactly as used in US GOSIP and other CLNP addressing
   plans, will be acceptable as the token for IPv6 autoconfiguration

   Analogous rules would be applied for other CLNP addressing plans
   similar to US GOSIP, which is used only as a well known example.

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RFC 1888                   OSI NSAPs and IPv6                August 1996

   Three warnings must be carefully considered in every case:

   1. The ES-IS/IS-IS model employs a routing hierarchy down to the Area
   level, but not all end systems in an Area need to be in the same
   physical subnet (on the same "wire" or "link"). IS routers on
   different links within a given Area exchange information about the
   end systems they can each reach directly.  In contrast, the IPv6
   routing model extends down to the subnet level and all hosts in the
   same subnet are assumed to be on the same link. In mapping a CLNP
   addressing plan into IPv6 format, without changing the physical
   topology, it may be necessary to add an extra level of hierarchy to
   cope with this mismatch. In other words, the Area number cannot
   blindly be mapped as a subnet number, unless the physical network
   topology corresponds to this mapping.

   2. It is highly desirable that subnet addresses can be aggregated for
   wide area routing purposes, to minimise the size of routing tables.
   Thus network implementors should ensure that the address prefix used
   for all their subnets is the same, regardless of whether a particular
   subnet is using a pure IPv6 addressing scheme or one derived from a
   CLNP scheme as above.

   3. Some hosts have more than one physical network interface.  In the
   ES-IS model, an end system may have more than one NSAP address, each
   of which identifies the host as a whole.  Such an end system with
   more than one physical interface may be referenced by any one of the
   NSAPs, and reached via any one of the physical connections.  In the
   IPv6 model, a host may have multiple IPv6 addresses per interface,
   but each of its physical interfaces must have its own unique
   addresses. This restriction must be applied when mapping an NSAP
   addressing plan into an IPv6 addressing plan for such hosts.

   This document does not address the issues associated with migrating
   the routing protocols used with CLNP (ES-IS or IS-IS) and transition
   of their network infrastructure.

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RFC 1888                   OSI NSAPs and IPv6                August 1996

2. Summary of defined mechanisms

   This document defines four distinct mechanisms.  All of these are
   ELECTIVE mechanisms, i.e. they are not mandatory parts of an IPv6
   implementation, but if such mechanisms are needed they MUST be
   implemented as defined in this document.

      1. Restricted NSAPA mapping into 16-byte IPv6 address
      2. Truncated NSAPA for routing, full NSAPA in IPv6 option
      3. Normal IPv6 address, full NSAPA in IPv6 option
      4. IPv6 address carried as OSI address

   To clarify the relationship between the first three mechanisms, note

      If the first byte of an IPv6 address is hexadecimal 0x02 (binary
      00000010), then the remaining 15 bytes SHALL contain a restricted
      NSAPA mapped as in Chapter 3 below. The term "restricted" is used
      to indicate that this format is currently restricted to a subset
      of the ICD and DCC formats.

      If the first byte of an IPv6 address is hexadecimal 0x03 (binary
      00000011), then the remaining 15 bytes SHALL contain a truncated
      NSAPA as described in Chapter 4 below. EITHER a destination option
      containing the complete NSAPA of any format, as described in
      Chapter 5 below, OR an encapsulated CLNP packet, SHALL be present.

      With any other format of IPv6 address, a destination option
      containing a complete NSAPA, as defined in Chapter 5 below, MAY be

3. Restricted NSAPA in a 16-byte IPv6 address for ICD and DCC

   Some organizations may decide for various reasons not to follow the
   above general recommendation to redesign their addressing plan.  They
   may wish to use their existing OSI NSAP addressing plan unchanged for
   IPv6. It should be noted that such a decision has serious
   implications for routing, since it means that routing between such
   organizations and the rest of the Internet is unlikely to be
   optimised. An organization using both native IPv6 addresses and NSAP
   addresses for IPv6 would be likely to have inefficient internal
   routing.  Nevertheless, to cover this eventuality, the present
   document defines a way to map a subset of the NSAP address space into
   the IPv6 address space. The mapping is algorithmic and reversible
   within this subset of the NSAP address space.

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RFC 1888                   OSI NSAPs and IPv6                August 1996

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 0-3  |0 0 0 0 0 0 1 0| AFcode| IDI (last 3 digits)   |Prefix(octet 0)|
 4-7  |             Prefix (octets 1 through 4)                       |
 8-11 | Area (octets 0 and 1)         |  ID (octets 0 and 1)          |
 12-15|             ID (octets 2 through 5)                           |

   The AFcode nibble is overloaded, and encoded as follows

       0000-1001      Implied AFI value is 47 (ICD)
       (0-9 decimal)  AFcode is first BCD digit of the ICD
                      IDI is last three BCD digits of the ICD

       1010           Implied AFI value is 39 (DCC)
       (hex. A)       IDI is the three BCD digits of the DCC

       1011-1111      Reserved, not to be used.
       (hex. B-F)

   The NSEL octet is not included. It is of no use for TCP and UDP
   traffic.  In any case where it is needed, the mechanism described in
   the next chapter should be used.

   The longest CLNP routing prefixes known to be in active use today are
   5 octets (subdivided into AA and RD fields in US GOSIP version 2).
   Thus the semantics of existing 20-octet NSAPAs can be fully mapped.
   DECnet/OSI (Registered Trade Mark) address semantics are also fully

   It is expected that hosts using restricted NSAPAs could be configured
   using IPv6 auto-configuration [RFC1971], and that they could use
   normal IPv6 neighbour discovery mechanisms [RFC1970].

   Restricted NSAPAs, assuming that they can be fully routed using IPv6
   routing protocols, may be used in IPv6 routing headers.

3.1 Routing restricted NSAPAs

   As mentioned in Chapter 1, there is a mismatch between the OSI or
   GOSIP routing model and the IPv6 routing model. Restricted NSAPAs can
   be routed hierarchically down to the Area level but must be flat-
   routed within an Area. Normal IPv6 addresses can be routed

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RFC 1888                   OSI NSAPs and IPv6                August 1996

   hierarchically down to physical subnet (link) level and only have to
   be flat-routed on the physical subnet.

   Thus, packets whose destination address is a restricted NSAPA can be
   routed using any normal IPv6 routing protocol only as far as the
   Area. If the Area contains more than one physical subnet reached by
   more than one router, no IPv6 routing protocol can route the packet
   to the correct final router.  There is no solution to this problem
   within the existing IPv6 mechanisms.  Presumably a flooding
   algorithm, or a suitably adapted implementation of ES-IS, could solve
   this problem.

   In the absence of such a routing protocol, either the Area number
   must be hierarchically structured to correspond to physical subnets,
   or each Area must be limited to one physical subnet.

   It is necessary in an IPv6 network that routes may be aggregated to
   minimise the size of routing tables. If a subscriber is using both
   normal IPv6 addresses [RFC1884] and restricted NSAPAs, these two
   types of address will certainly not aggregate with each other, since
   they differ from the second most significant bit onwards. This means
   that there may be a significant operational penalty for using both
   types of address with currently known routing technology.

4. Truncated NSAPA used as an IPv6 address

   An NSAP address contains routing information (e.g. Routing Domain and
   area/subnet identifiers) in the form of the Area Address (as defined
   in [IS10589]). The format and length of this routing information are
   typically compatible with a 16 byte IPv6 address, and may be
   represented as such using the following format:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 0-3  |0 0 0 0 0 0 1 1|  High order octets of full NSAP address       |
 4-7  |                NSAP address continued                         |
 8-11 |                NSAP address continued                         |
 12-15| NSAP address truncated     ...    zero pads if necessary      |

   If appropriate, when used as a destination IPv6 address, the
   truncated NSAPA may be interpreted as an IPv6 anycast address.  An
   anycast address may be used to identify either an IPv6 node, or
   potentially even an OSI End System or Intermediate System.  For

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RFC 1888                   OSI NSAPs and IPv6                August 1996

   example, it might be configured to identify the endpoints of a CLNP
   tunnel, or it might identify a particular OSI capable system in a
   particular subnet.

   If a truncated NSAPA is used as a source address, it must be
   interpreted as a unicast address and must therefore be uniquely
   assigned within the IPv6 address space.

   If a truncated NSAPA is used as either the source or destination IPv6
   address (or both), EITHER an NSAPA destination option OR an
   encapsulated CLNP packet MUST be present. It is the responsibility of
   the destination system to take the appropriate action for each IPv6
   packet received (e.g. forward, decapsulate, discard) and, if
   necessary, return to the originating host an appropriate ICMP error

   If the truncated NSAPA is used to identify a router, and an NSAPA
   destination option is present, then it is the responsibility of that
   router to forward the complete IPv6 packet to the appropriate host
   based upon the Destination NSAP field in the NSAPA option.  This
   forwarding process may be based upon static routing information (i.e.
   a manual mapping of NSAPs to IPv6 unicast addresses), or it may be
   gathered in an automated fashion analogous to the ES-IS mechanism,
   perhaps using extensions to the Neighbor Discovery protocol
   [RFC1970].  The details of such a mechanism are beyond the scope of
   this document.

   This document does not restrict the formats of NSAP address that may
   be used in truncated NSAPAs, but it is apparent that binary ICD or
   DCC formats will be much easier to accomodate in an IPv6 routing
   infrastructure than the other formats defined in [IS8348].

   It is not expected that IPv6 autoconfiguration [RFC1971] and
   discovery [RFC1970] will work unchanged for truncated NSAPAs.

   Truncated NSAPAs are not meaningful within IPv6 routing headers, and
   there is no way to include full NSAPAs in routing headers.

   If a packet whose source address is a truncated NSAPA causes an ICMP
   message to be returned for whatever reason, this ICMP message may be
   discarded rather than being returned to the true source of the

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RFC 1888                   OSI NSAPs and IPv6                August 1996

4.1 Routing truncated NSAPAs

   This is a grey area. If the truncated NSAPA retains a hierarchical
   structure, it can be routed like a restricted NSAPA, subject to the
   same problem concerning the mismatch between Areas and subnets.  If
   possible, in the case of a GOSIP-like NSAPA, it should be truncated
   immediately after the Area number. In this case the routing
   considerations will be similar to those for restricted NSAPAs, except
   that final delivery of the packet will depend on the last IPv6 router
   being able to interpret the NSAPA destination option (or an
   encapsulated CLNP packet).

   In the general case, nothing can be said since the NSAPA could have
   almost any format and might have very little hierarchical content
   after truncation. There may be many cases in which truncated NSAPAs
   cannot be routed across large regions of the IPv6 network.

   The situation for route aggregation is similar to that described in
   Section 3.1 as long as the truncated NSAPAs have ICD or DCC format.
   However, if arbitrary NSAPAs are used nothing can be predicted about
   route aggregation and we must assume that it will be poor.

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RFC 1888                   OSI NSAPs and IPv6                August 1996

5. Carriage of full NSAPAs in IPv6 destination option

   In the case of a truncated NSAPA used as an IPv6 address other than
   for a CLNP tunnel, the full NSAPA must be carried in a destination
   option. Any format defined in [IS8348] is allowed.

   The NSAPA destination option is illustrated below. It has no
   alignment requirement.

   The option type code is 11-0-00011 = 195 decimal.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       |1 1 0 0 0 0 1 1|  Opt Data Len |Source NSAP Len| Dest. NSAP Len|
       |                                                               |
       +                                                               +
       |                                                               |
       +                         Source NSAP                           +
       |                                                               |
       +                                                               +
       |                                                               |
       +                                                               +
       |                                                               |
       |                                                               |
       +                                                               +
       |                                                               |
       +                       Destination NSAP                        +
       |                                                               |
       +                                                               +
       |                                                               |
       +                                                               +
       |                                                               |

   The length fields are each one octet long and are expressed in
   octets.  The destination node should check the consistency of the
   length fields (Option Data Length = Source NSAP Length + Dest. NSAP
   Length +2).  In case of inconsistency the destination node shall
   discard the packet and send an ICMP Parameter Problem, Code 2,
   message to the packet's source address, pointing to the Option Data
   Length field.

   The boundary between the source NSAP and the destination NSAP is
   simply aligned on an octet boundary. With standard 20 octet NSAPs the
   total option length is 44 bytes and the Option Data Length is 42.

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RFC 1888                   OSI NSAPs and IPv6                August 1996

   The NSAP encodings follow [IS8348] exactly.

   If this option is used, both end systems concerned SHOULD use NSAP
   addresses. In the exceptional case that only one of the end systems
   uses NSAP addresses, the NSAP Length field of the other SHALL be set
   to zero in the NSAP destination option.

   This destination option is used in two cases. Firstly, an IPv6 source
   node using normal IPv6 addresses (unicast address or anycast address)
   MAY supply an NSAP destination option header for interpretation by
   the IPv6 destination node. Secondly, an IPv6 node MAY use a truncated
   NSAP address in place of a normal IPv6 address.

   IPv6 nodes are not required to implement this option, except for
   nodes using truncated NSAPAs other than for CLNP tunnels.

6. IPv6 addresses inside an NSAPA

   If it is required, for whatever reason, to embed an IPv6 address
   inside a 20-octet NSAP address, then the following format MUST be

   A specific possible use of this embedding is to express an IP address
   within the ATM Forum address format.  Another  possible use would be
   to allow CLNP packets that encapsulate IPv6 packets to be routed in a
   CLNP network using the IPv6 address architecture. Several leading
   bytes of the IPv6 address could be used as a CLNP routing prefix.

   The first three octets are an IDP in binary format, using the AFI
   code in the process of being allocated to the IANA. The AFI value
   provisionally allocated is 35, but this requires a formal
   modification to [IS8348].  The encoding format is as for AFI value 47
   [IS8348]. The third octet of the IDP is known as the ICP (Internet
   Code Point) and its value must be zero. All other values are reserved
   for allocation by the IANA.

   Thus an AFI value of 35 with an ICP value of zero means that "this
   NSAPA embeds a 16 byte IPv6 address".

   The last octet is a selector.  To maintain compatibility with both
   NSAP format and IPv6 addressing, this octet must be present, but it
   has no significance for IPv6. Its default value is zero.

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RFC 1888                   OSI NSAPs and IPv6                August 1996

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 0-3  |  AFI = 35     |   ICP = 0000                  | IPv6  (byte 0)|
 4-7  |             IPv6  (bytes 1-4)                                 |
 8-11 |             IPv6  (bytes 5-8)                                 |
 12-15|             IPv6  (bytes 9-12)                                |
 16-19|       IPv6  (bytes 13-15)                     |0 0 0 0 0 0 0 0|

   Theoretically this format would allow recursive address embedding.

   However, this is considered dangerous since it might lead to routing
   table anomalies or to loops (compare [RFC1326]).  Thus embedded IPv6
   address MUST NOT have the prefixes 0x02 or 0x03, and an NSAPA with
   the IANA AFI code MUST NOT be embedded in an IPv6 header.

   An NSAPA with the IANA AFI code and ICP set to zero is converted to
   an IPv6 address by stripping off the first three and the twentieth
   octets. All other formats of NSAPA are handled according to the
   previous Chapters of this document.

7. Security Considerations

   Security issues are not specifically addressed in this document, but
   it is compatible with the IPv6 security mechanisms [RFC1825].


   The authors are pleased to acknowledge the suggestions and comments
   of Ross Callon, Richard Collella, Steve Deering, Dirk Fieldhouse,
   Joel Halpern, Denise Heagerty, Cyndi Jung, Yakov Rekhter, and members
   of the former TUBA and current IPNG working groups of the IETF. The
   support of Scott Bradner and Allison Mankin of the IESG was

   Herb Bertine, Alan Chambers, Dave Marlow, and Jack Wheeler were all
   active in arranging the AFI allocation by ISO/IEC JTC1/SC6.

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RFC 1888                   OSI NSAPs and IPv6                August 1996


   [IS8473] Data communications protocol for providing the
   connectionless-mode network service, ISO/IEC 8473, 1988.

   [IS8348] Annex A, Network Layer Addressing, and Annex B, Rationale
   for the material in Annex A, of ISO/IEC 8348, 1993 (identical to
   CCITT Recommendation X.213, 1992).

   [IS10589] Intermediate system to intermediate system intra-domain-
   routeing routine information exchange protocol for use in
   conjunction with the protocol for providing the connectionless-mode
   Network Service (ISO 8473), ISO 10589, 1992.

   [IS9542] End system to Intermediate system routeing exchange
   protocol for use in conjunction with the Protocol for providing the
   connectionless-mode network service (ISO 8473), ISO 9542, 1988.

   [RFC1629] Colella, R., Callon, R., Gardner, E., and Y. Rekhter,
   "Guidelines for OSI NSAP Allocation in the Internet", RFC 1629, May

   [RFC1326] Tsuchiya, P., "Mutual Encapsulation Considered
   Dangerous", RFC 1326, May 1992.

   [RFC1883] Deering, S., and R. Hinden, "Internet Protocol, Version 6
   (IPv6) Specification", RFC 1883, December 1995.

   [RFC1884] Hinden, R., and S. Deering, "IP Version 6 Addressing
   Architecture", RFC 1884, December 1995.

   [RFC1971] Thompson, S., and T. Narten, "IPv6 Stateless Address
   Autoconfiguration", RFC1971, August 1996.

   [RFC1970] Narten, T., Nordmark, E., and W. Simpson, "Neighbor
   Discovery for IP Version 6 (IPv6)", RFC1970, August 1996.

   [RFC1825] Atkinson, R., "Security Architecture for the Internet
   Protocol", RFC 1825, August 1995.

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RFC 1888                   OSI NSAPs and IPv6                August 1996

Annex A: Summary of NSAP Allocations

            | AFI  | IDI  |     DOMAIN SPECIFIC PART            |
            --------------------20 bytes max---------------------

   The Initial Domain Part (IDP) is split into Authority and Format
   Identifier (AFI) followed by the Initial Domain Identifier (IDI).
   This combination is followed by the Domain Specific Part and
   allocation within that part is domain specific.

   The following is a summary of current allocations:

   ISO DCC Scheme

   AFI = decimal 38 or binary 39 = ISO Data Country Code Scheme.  IDI =
   3 decimal or binary digits specifying the country.  ISO allocate the
   country codes.  The DSP is administered by the standards authority
   for each country.  In the UK, the British Standards Institution have
   delegated administration to the Federation of Electronics Industries
   - FEI

   The UK DSP is split into a single digit UK Format Indicator (UKFI)
   which indicates large, medium or small organisation rather like IP
   addressing and a UK Domain Identifier (UKDI).  Using binary coded
   decimal examples only (there are binary equivalents):

   UKFI = 0 is reserved UKFI = 1, UKDI = nnn, UK Domain Specific Part =
   31 digits.  UKFI = 2, UKDI = nnnnn, UKDSP = 29 digits max.  UKFI = 3,
   UKDI = nnnnnnnn, UKDSP = 26 digits max.

   UKFI = 4 to 9 reserved

   The UK Government have been allocated a UKDI in the UKFI = 1 (large
   organisation) format and have specified the breakdown of the
   Government Domain Specific Part with sub domain addresses followed by
   a station ID (which could be a MAC address) and a selector (which
   could be a TSAP selection).

   ITU-T X.121

   AFI = decimal 36 or 52, binary 37 or 53 indicates that the IDI is a
   14 digit max X.121 International Numbering Plan address (prefix, 3
   digit Data Country Code, dial up data network number).  The full
   X.121 address indicates who controls the formatting of the DSP.

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RFC 1888                   OSI NSAPs and IPv6                August 1996

   ITU-T F.69

   AFI = 40,54 or binary 41,55 indicates that the IDI is a telex number
   up to 8 digits long.

   ITU-T E.163

   AFI = 42,56 or binary 43,57 indicates that the IDI is a normal
   telephone number up to 12 digits long.

   ITU-T E.164

   AFI = 44,58 or binary 45,59 indicates that the IDI is an ISDN number
   up to 15 digits long.

   ISO 6523-ICD

   AFI = 46 or binary 47 indicates that the IDI is an International Code
   Designator allocated according to ISO 6523.  You have to be a global
   organisation to get one of these.  The Organisation to which the ISO
   6523 designator is issued specifies the DSP allocation.

Annex B: Additional Rationale

   This annex is intended to give additional rationale, motivation and
   justification for the support of NSAPAs in an IPv6 network.

   There are several models for OSI-IPv6 convergence, of which address
   mapping is only one. The other models can be identified as

    1. Dual stack coexistence, in which a CLNP network and an IPv6
       network exist side by side indefinitely using multiprotocol

    2. CLNP tunnels over IPv6.

    3. OSI transport over IPv6.

    4. OSI transport over UDP.

    5. OSI transport over TCP (compare RFC 1006)

   The present model is more fundamental, as it attempts to unify and
   reconcile the OSI and IPv6 addressing and routing schemes, and
   replace CLNP by IPv6 at the network level. The rationale for this
   choice is to preserve investment in NSAPA allocation schemes, and to
   open the door for peer-to-peer routing models between IPv6 and bearer
   services (such as ATM) using NSAPA addressing. It should be noted

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RFC 1888                   OSI NSAPs and IPv6                August 1996

   that such peer-to-peer models are contentious at the time of writing,
   but in any case a consistent address mapping is preferable to
   multiple mappings.

   In addition to their use to retain an existing addressing plan,
   certain other uses of restricted NSAPAs could be envisaged.  They
   could be used as an intermediate addressing plan for a network making
   a transition from CLNP to IPv6. They could be used in a header
   translation scheme for dynamic translation between IPv6 and CLNP.
   They could be used to allow CLNP and IPv6 traffic to share the same
   routing architecture within an organization ("Ships in the Day").

   It should be noted that the use of full NSAPA addresses in end
   systems impacts many things. The most obvious are the API and DNS. If
   applications are to work normally, everything that has to be modified
   to cope with IPv6 addresses has to be further modified for full
   NSAPAs.  The mechanisms defined in the present document are only a
   small part of the whole.

   A destination option was chosen to carry full NSAPAs, in preference
   to a dedicated extension header.  In the case of an extension header,
   all IPv6 nodes would have needed to understand its syntax merely in
   order to ignore it. In contrast, intermediate nodes can ignore the
   destination option without any knowledge of its syntax. Thus only
   nodes interested in NSAPAs need to know anything about them.

   Thus we end up with two classes of IPv6 nodes:

   1. Nodes knowing only about 16 byte addresses (including restricted
   NSAPAs, which behave largely like any other IPv6 addresses).

   2. Nodes also knowing about 20 byte NSAPAs, either as an extension of
   the IPv6 address space or as the CLNP address space. In either case,
   regions of the network containing such nodes are connected to each
   other by unicast or anycast tunnels through the 16 byte address
   space. Routing, system configuration, and neighbour discovery in the
   NSAPA regions are outside the scope of the normal IPv6 mechanisms.

Bound, et. al.                Experimental                     [Page 15]

RFC 1888                   OSI NSAPs and IPv6                August 1996

Authors' Addresses

   Jim Bound
   Member Technical Staff                    Phone: (603) 881-0400
   Network Operating Systems                 Fax:   (603) 881-0120
   Digital Equipment Corporation             Email: bound@zk3.dec.com
   110 Spitbrook Road, ZKO3-3/U14
   Nashua, NH 03062

   Brian E. Carpenter
   Group Leader, Communications Systems      Phone:  +41 22 767-4967
   Computing and Networks Division           Fax:    +41 22 767-7155
   CERN                                      Telex:  419000 cer ch
   European Laboratory for Particle Physics  Email: brian@dxcoms.cern.ch
   1211 Geneva 23, Switzerland

   Dan Harrington                            Phone: (508) 486-7643
   Digital Equipment Corp.
   550 King Street (LKG2-2/Q9)               Email: dan@netrix.lkg.dec.com
   Littleton, MA  01460

   Jack Houldsworth            Phone- ICL: +44 438 786112
   ICL Network Systems               Home: +44 438 352997
   Cavendish Road              Fax:        +44 438 786150
   Stevenage                   Email: j.houldsworth@ste0906.wins.icl.co.uk
   UK SG1 4BQ

   Alan Lloyd                  Phone:  +61 3 727 9222
   Datacraft Technologies      Fax:    +61 3 727 1557
   252 Maroondah Highway       Email:  alan.lloyd@datacraft.com.au
   Mooroolbark 3138
   Victoria       Australia

   X.400- G=alan;S=lloyd;O=dcthq;P=datacraft;A=telememo;C=au

Bound, et. al.                Experimental                     [Page 16]