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rfc:rfc1195

Network Working Working Group R. Callon Request for Comments: 1195 Digital Equipment Corporation

                                                         December 1990
    Use of OSI IS-IS for Routing in TCP/IP and Dual Environments

Status of this Memo

 This RFC specifies a protocol on the IAB Standards Track for the
 Internet community, and requests discussion and suggestions for
 improvements. Please refer to the current edition of the "IAB
 Official Protocol Standards" for the standardization state and status
 of this protocol. Distribution of this memo is unlimited.
 This RFC is available in both postscript and text versions. Where
 possible, use of the postscript version is recommended. For example,
 this text version may have figures which are less informative or
 missing.

Abstract

 This RFC specifies an integrated routing protocol, based on the OSI
 Intra-Domain IS-IS Routing Protocol, which may be used as an interior
 gateway protocol (IGP) to support TCP/IP as well as OSI. This allows
 a single routing protocol to be used to support pure IP environments,
 pure OSI environments, and dual environments. This specification was
 developed by the IS-IS working group of the Internet Engineering Task
 Force.
 The OSI IS-IS protocol has reached a mature state, and is ready for
 implementation and operational use. The most recent version of the
 OSI IS-IS protocol is contained in ISO DP 10589 [1]. The proposed
 standard for using IS-IS for support of TCP/IP will therefore make
 use of this version (with a minor bug correction, as discussed in
 Annex B).  We expect that future versions of this proposed standard
 will upgrade to the final International Standard version of IS-IS
 when available.
 Comments should be sent to "isis@merit.edu".

Contents

  1   Introduction: Overview of the Protocol
      1.1     What the Integrated IS-IS offers
      1.2     Overview of the ISO IS-IS Protocol
      1.3     Overview of the Integrated IS-IS
      1.4     Support of Mixed Routing Domains

Callon [Page 1] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

      1.5     Advantages of Using Integrated IS-IS
  2   Symbols and Abbreviations
  3   Subnetwork Independent Functions
      3.1     Exchange of Routing Information
      3.2     Hierarchical Abbreviation of IP Reachability Information
      3.3     Addressing Routers in IS-IS Packets
      3.4     External Links
      3.5     Type of Service Routing
      3.6     Multiple LSPs and SNPs
      3.7     IP-Only Operation
      3.8     Encapsulation
      3.9     Authentication
      3.10    Order of Preference of Routes / Dijkstra Computation
  4   Subnetwork Dependent Functions
      4.1     Link Demultiplexing
      4.2     Multiple IP Addresses per Interface
      4.3     LANs, Designated Routers, and Pseudonodes
      4.4     Maintaining Router Adjacencies
      4.5     Forwarding to Incompatible Routers
  5   Structure and Encoding of PDUs
      5.1     Overview of IS-IS PDUs
      5.2     Overview of IP-Specific Information for IS-IS
      5.3     Encoding of IP-Specific Fields in IS-IS PDUs
  6   Security Considerations
  7   Author's Address
  8   References
  A   Inter-Domain Routing Protocol Information
      A.1     Inter-Domain Information Type
      A.2     Encoding
  B   Encoding of Sequence Number Packets
      B.1     Level 1 Complete Sequence Numbers PDU
      B.2     Level 2 Complete Sequence Numbers PDU
      B.3     Level 1 Partial Sequence Numbers PDU
      B.4     Level 2 Partial Sequence Numbers PDU
  C   Dijkstra Calculation and Forwarding
      C.1     SPF Algorithm for IP and Dual Use
      C.2     Forwarding of IP packets

Callon [Page 2] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

  D   Use of the Authentication Field
      D.1     Authentication Field in IS-IS packets
      D.2     Authentication Type 1 - Simple Password
  E   Interaction of the Integrated IS-IS with Brouters
      E.1     The Problem
      E.2     Possible Solutions

Figures

      1       ISO Hierarchical Address Structure
      2       An Example
      3       Encoding of Variable Length Fields

1 Introduction: Overview of the Protocol

 The TCP/IP protocol suite has been growing in importance as a multi-
 vendor communications architecture. With the anticipated emergence of
 OSI, we expect coexistence of TCP/IP and OSI to continue for an
 extended period of time. There is a critical need for routers to
 support both IP traffic and OSI traffic in parallel.
 There are two main methods that are available for routing protocols
 to support dual OSI and IP routers. One method, known as "Ships in
 the Night", makes use of completely independent routing protocols for
 each of the two protocol suites. This specification presents an
 alternate approach, which makes use of a single integrated protocol
 for interior routing (i.e., for calculating routes within a routing
 domain) for both protocol suites.
 This integrated protocol design is based on the OSI Intra-domain IS-
 IS routing protocol [1], with IP-specific functions added. This RFC
 is considered a companion to the OSI IS-IS Routing spec, and will
 only describe the required additional features.
 By supporting both IP and OSI traffic, this integrated protocol
 design supports traffic to IP hosts, OSI end systems, and dual end
 systems.  This approach is "integrated" in the sense that the IS-IS
 protocol can be used to support pure-IP environments, pure-OSI
 environments, and dual environments. In addition, this approach
 allows interconnection of dual (IP and OSI) routing domains with
 other dual domains, with IP-only domains, and with OSI-only domains.
 The protocol specified here is based on the work of the IETF IS-IS
 working group.

1.1 What the Integrated IS-IS offers

 The integrated IS-IS provides a single routing protocol which will

Callon [Page 3] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

 simultaneously provide an efficient routing protocol for TCP/IP, and
 for OSI. This design makes use of the OSI IS-IS routing protocol,
 augmented with IP-specific information. This design provides explicit
 support for IP subnetting, variable subnet masks, TOS-based routing,
 and external routing. There is provision for authentication
 information, including the use of passwords or other mechanisms. The
 precise form of authentication mechanisms (other than passwords) is
 outside of the scope of this document.
 Both OSI and IP packets are forwarded "as is" -- i.e., they are
 transmitted directly over the underlying link layer services without
 the need for mutual encapsulation. The integrated IS-IS is a dynamic
 routing protocol, based on the SPF (Dijkstra) routing algorithm.
 The protocol described in this specification allows for mixing of
 IP-only, OSI-only, and dual (IP and OSI) routers, as defined below.
 An IP-only IS-IS router (or "IP-only" router) is defined to be a
 router which: (i) Uses IS-IS as the routing protocol for IP, as
 specified in this report; and (ii) Does not otherwise support OSI
 protocols. For example, such routers would not be able to forward OSI
 CLNP packets.
 An OSI-only router is defined to be a router which uses IS-IS as the
 routing protocol for OSI, as specified in [1]. Generally, OSI-only
 routers may be expected to conform to OSI standards, and may be
 implemented independent of this specification.
 A dual IS-IS router (or "dual" router) is defined to be a router
 which uses IS-IS as a single integrated routing protocol for both IP
 and OSI, as specified in this report.
 This approach does not change the way that IP packets are handled.
 IP-only and dual routers are required to conform to the requirements
 of Internet Gateways [4]. The integrated IS-IS protocol described in
 this report outlines an Interior Gateway Protocol (IGP) which will
 provide routing within a TCP/IP routing domain (i.e., autonomous
 system). Other aspects of router functionality (e.g., operation of
 ICMP, ARP, EGP, etc.) are not affected by this proposal.
 Similarly, this approach does not change the way that OSI packets are
 handled. There will be no change at all to the contents nor to the
 handling of ISO 8473 Data packets and Error Reports, nor to ISO 9542
 Redirects and ES Hellos. ISO 9542 IS Hellos transmitted on LANs are
 similarly unchanged. ISO 9542 IS Hellos transmitted on point-to-point
 links are unchanged except for the addition of IP-related
 information.  Similarly, other OSI packets (specifically those
 involved in the IS-IS intra-domain routing protocol) remain unchanged

Callon [Page 4] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

 except for the addition of IP-related information.
 This approach makes use of the existing IS-IS packets, with IP-
 specific fields added. Specifically: (i) authentication information
 may be added to all IS-IS packets; (ii) the protocols supported by
 each router, as well as each router's IP addresses, are specified in
 ISO 9542 IS Hello, IS-IS Hello and Link State Packets; (iii)
 internally reachable IP addresses are specified in all Link State
 Packets; and (iv) externally reachable IP addresses, and external
 routing protocol information, may be specified in level 2 Link State
 Packets. The detailed encoding and interpretation of this in
 formation is specified in sections 3, 4, and 5 of this RFC.
 The protocol described in this report may be used to provide routing
 in an IP-only routing domain, in which all routers are IP-only.
 Similarly, this protocol may be used to provide routing in a pure
 dual domain, in which all routers are dual. Finally, this protocol
 may be used to provide routing in a mixed domain, in which some
 routers are IP-only, some routers are OSI-only, and some routers are
 dual. The specific topological restrictions which apply in this
 latter case are described in detail in section 1.4 ("Support of Mixed
 Routing Domains").  The use of IS-IS for support of pure OSI domains
 is specified in [1].
 This protocol specification does not constrain which network
 management protocol(s) may be used to manage IS-IS-based routers.
 Management information bases (MIBs) for managing IP-only, OSI-only,
 and dual routers, compatible with CMIP, CMOT, and/or SNMP, are the
 subject of a separate, companion document [8].

1.2 Overview of the ISO IS-IS Protocol

 The IS-IS Routing Protocol has been developed in ISO to provide
 routing for pure OSI environments. In particular, IS-IS is designed
 to work in conjunction with ISO 8473 (The ISO Connectionless Network
 Layer Protocol [2]), and ISO 9542 (The ISO End System to Intermediate
 System Protocol [3]). This section briefly describes the manner in
 which IS-IS is used to support pure OSI environments. Enhancements
 for support of IP and dual environments are specified elsewhere in
 this report.
 In IS-IS, the network is partitioned into "routing domains". The
 boundaries of routing domains are defined by network management, by
 setting some links to be "exterior links". If a link is marked as
 "exterior", no IS-IS routing messages are sent on that link.
 Currently, ISO does not have a standard for inter-domain routing
 (i.e., for routing between separate autonomous routing domains).

Callon [Page 5] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

 Instead, manual configuration is used. The link is statically
 configured with the set of address prefixes reachable via that link,
 and with the method by which they can be reached (such as the DTE
 address to be dialed to reach that address, or the fact that the DTE
 address should be extracted from the IDP portion of the ISO address).
 OSI IS-IS routing makes use of two-level hierarchical routing. A
 routing domain is partitioned into areas. Level 1 routers know the
 topology in their area, including all routers and end systems in
 their area. However, level 1 routers do not know the identity of
 routers or destinations outside of their area. Level 1 routers
 forward all traffic for destinations outside of their area to a level
 2 router in their area. Similarly, level 2 routers know the level 2
 topology, and know which addresses are reachable via each level 2
 router. However, level 2 routers do not need to know the topology
 within any level 1 area, except to the extent that a level 2 router
 may also be a level 1 router within a single area. Only level 2
 routers can exchange data packets or routing information directly
 with external routers located outside of the routing domains.
  +----------------------+-------------------------------+
  |        IDP           |              DSP              |
  +----------------------+-------------------------------+
  .                      .                               .
  .                      .                               .
  .                      .                               .
  +-----+----------------+----------+--------------+-----+
  | AFI |      IDI       |  HO-DSP  |      ID      | SEL |
  +-----+----------------+----------+--------------+-----+
       Figure 1 - ISO Hierarchical Address Structure
 As illustrated in figure 1, ISO addresses are subdivided into the
 Initial Domain Part (IDP), and the Domain Specific Part (DSP). The
 IDP is the part which is standardized by ISO, and specifies the
 format and authority responsible for assigning the rest of the
 address. The DSP is assigned by whatever addressing authority is
 specified by the IDP. The DSP is further subdivided into a "High
 Order Part of DSP" (HO-DSP), a system identifier (ID), and an NSAP
 selector (SEL). The HO-DSP may use any format desired by the
 authority which is identified by the IDP. Together, the combination
 of [IDP, HO-DSP] identify both the routing domain and the area within
 the routing domain. The combination of [IDP,HO-DSP] may therefore be
 referred to as the "Area Address".
 Usually, all nodes in an area have the same area address. However,
 sometimes an area might have multiple addresses. Motivations for

Callon [Page 6] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

 allowing this are:
  1. It might be desirable to change the address of an area. The most

graceful way of changing an area from having address A to having

   address B is to first allow it to have both addresses A and B, and
   then after all nodes in the area have been modified to recognize
   both addresses, then one by one the nodes can be modified to
   "forget" address A.
  1. It might be desirable to merge areas A and B into one area. The

method for accomplishing this is to, one by one, add knowledge of

   address B into the A partition, and similarly add knowledge of
   address A into the B  partition.
  1. It might be desirable to partition an area C into two areas, A

and B (where "A" might equal "C", in which case this example

   becomes one of removing a portion of an area). This would be
   accomplished by first introducing knowledge of address A into
   the appropriate nodes (those destined to become area A), and
   knowledge of address B into the appropriate nodes, and then one
   by one removing knowledge of address C.
 Since OSI addressing explicitly identifies the area, it is very easy
 for level 1 routers to identify packets going to destinations outside
 of their area, which need to be forwarded to level 2 routers.
 In IS-IS, there are two types of routers:
  1. Level 1 intermediate systems – these nodes route based on the ID

portion of the ISO address. They route within an area. They

   recognize, based on the destination address in a packet, whether
   the destination is within the area. If so, they route towards
   the destination. If not, they route to the nearest level 2 router.
  1. Level 2 intermediate systems – these nodes route based on the area

address (i.e., on the combination of [IDP, HO-DSP]). They route

   towards areas, without regard to the internal structure of an area.
   A level 2 IS may also be a level 1 IS in one area.
 A level 1 router will have the area portion of its address manually
 configured. It will refuse to become a neighbor with a node whose
 area addresses do not overlap its area addresses. However, if level 1
 router has area addresses A,  B, and C, and a neighbor has area
 addresses B and D, then the level 1 router will accept the other node
 as a neighbor.
 A level 2 router will accept another level 2 router as a neighbor,
 regardless of area address. However, if the area addresses do not

Callon [Page 7] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

 overlap, the link would be considered by both routers to be "level 2
 only", and only level 2 LSPs would flow on the link. External links
 (to other routing domains) must be from level 2 routers.
 IS-IS provides an optional partition repair function. In the unlikely
 case that a level 1 area become partitioned, this function, if
 implemented, allows the partition to be repaired via use of level 2
 routes.
 IS-IS requires that the set of level 2 routers be connected. Should
 the level 2 backbone become partitioned, there is no provision for
 use of level 1 links to repair a level 2 partition.
 In unusual cases, a single level 2 router may lose connectivity to
 the level 2 backbone. In this case the level 2 router will indicate
 in its level 1 LSPs that it is not "attached", thereby allowing level
 1 routers in the area to route traffic for outside of the domain to a
 different level 2 router. Level 1 routers therefore route traffic to
 destinations outside of their area only to level 2 routers which
 indicate in their level 1 LSPs that they are "attached".
 An end system may autoconfigure the area portion of its address by
 extracting the area portion of a neighboring router's address. If
 this is the case, then an endnode will always accept a router as a
 neighbor. Since the standard does not specify that the end system
 MUST autoconfigure its area address, an end system may be configured
 with an area address. In this case the end system would ignore router
 neighbors with non-matching area addresses.
 Special treatment is necessary for broadcast subnetworks, such as
 LANs. This solves two sets of issues: (i) In the absence of special
 treatment, each router on the subnetwork would announce a link to
 every other router on the subnetwork, resulting in n-squared links
 reported; (ii) Again, in the absence of special treatment, each
 router on the LAN would report the same identical list of end systems
 on the LAN, resulting in substantial duplication.
 These problems are avoided by use of a "pseudonode", which represents
 the LAN. Each router on the LAN reports that it has a link to the
 pseudonode (rather than reporting a link to every other router on the
 LAN). One of the routers on the LAN is elected "designated router".
 The designated router then sends out an LSP on behalf of the
 pseudonode, reporting links to all of the routers on the LAN. This
 reduces the potential n-squared links to n links. In addition, only
 the pseudonode LSP includes the list of end systems on the LAN,
 thereby eliminating the potential duplication (for further
 information on designated routers and pseudonodes, see [1]).

Callon [Page 8] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

 The IS-IS provides for optional Quality of Service (QOS) routing,
 based on throughput (the default metric), delay, expense, or residual
 error probability. This is described in greater detail in section
 3.5, and in [1].

1.3 Overview of the Integrated IS-IS

 The integrated IS-IS allows a single routing protocol to be used to
 route both IP and OSI packets. This implies that the same two-level
 hierarchy will be used for both IP and OSI routing. Each area will be
 specified to be either IP-only (only IP traffic can be routed in that
 particular area), OSI-only (only OSI traffic can be routed in that
 area), or dual (both IP and OSI traffic can be routed in the area).
 This proposal does not allow for partial overlap of OSI and IP areas.
 For example, if one area is OSI-only, and an other area is IP-only,
 then it is not permissible to have some routers be in both areas.
 Similarly, a single backbone is used for the routing domain. There is
 no provision for independent OSI and IP backbones.
 Similarly, within an IP-only or dual area, the amount of knowledge
 maintained by routers about specific IP destinations will be as
 similar as possible as for OSI. For example, IP-capable level 1
 routers will maintain the topology within the area, and will be able
 to route directly to IP destinations within the area. However, IP-
 capable level 1 routers will not maintain information about
 destinations outside of the area. Just as in normal OSI routing,
 traffic to destinations outside of the area will be forwarded to the
 nearest level 2 router. Since IP routes to subnets, rather than to
 specific end systems, IP routers will not need to keep nor distribute
 lists of IP host identifiers (note that routes to hosts can be
 announced by using a subnet mask of all ones).
 The IP address structure allows networks to be partitioned into
 subnets, and allows subnets to be recursively subdivided into smaller
 subnets. However, it is undesireable to require any specific
 relationship between IP subnet addresses and IS-IS areas. For
 example, in many cases, the dual routers may be installed into
 existing environments, which already have assigned IP and/or OSI
 addresses. In addition, even if IP addresses are not already pre-
 assigned, the address limitations of IP constrain what addresses may
 be assigned. We therefore will not require any specific relationship
 between IP addresses and the area structure. The IP addresses can be
 assigned completely independently of the OSI addresses and IS-IS area
 structure. As will be described in section 3.2 ("Hierarchical
 Abbreviation of IP Reachability Information"), greater efficiency and
 scaling of the routing algorithm can be achieved if there is some
 correspondence between the IP address assignment structure and the

Callon [Page 9] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

 area structure.
 Within an area, level 1 routers exchange link state packets which
 identify the IP addresses reachable by each router. Specifically,
 zero or more [IP address, subnet mask, metric] combinations may be
 included in each Link State Packet. Each level 1 router is manually
 configured with the [IP address, subnet mask, metric] combinations
 which are reachable on each interface. A level 1 router routes as
 follows:
  1. If a specified destination address matches an [IP address, subnet

mask, metric] reachable within the area, the packet is routed via

   level 1 routing.
  1. If a specified destination address does not match any [IP address,

subnet mask, metric] combination listed as reachable within the

   area, the packet is routed towards the nearest level 2 router.
 Flexible use of the limited IP address space is important in order to
 cope with the anticipated growth of IP environments. Thus an area
 (and by implication a routing domain) may simultaneously make use of
 a variety of different address masks for different subnets in the
 area (or domain). Generally, if a specified destination address
 matches more than one [IP address, subnet mask] pair, the more
 specific address is the one routed towards (the one with more "1"
 bits in the mask -- this is known as "best match" routing).
 Level 2 routers include in their level 2 LSPs a complete list of [IP
 address, subnet mask, metric] specifying all IP addresses reachable
 in their area. As described in section 3, this information may be
 obtained from a combination of the level 1 LSPs (obtained from level
 1 routers in the same area), and/or by manual configuration. In
 addition, Level 2 routers may report external reachability
 information, corresponding to addresses which can be reached via
 routers in other routing domains (autonomous systems)
 Default routes may be announced by use of a subnet mask containing
 all zeroes. Default routes should be used with great care, since they
 can result in "black holes". Default routes are permitted only at
 level 2 as external routes (i.e., included in the "IP External
 Reachability Information" field, as explained in sections 3 and 5).
 Default routes are not permitted at level 1.
 The integrated IS-IS provides optional Type of Service (TOS) routing,
 through use of the QOS feature from IS-IS.

Callon [Page 10] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

1.4 Support of Mixed Routing Domains

 The integrated IS-IS proposal specifically allows for three types of
 routing domains:
  1. Pure IP
  1. Pure OSI
  1. Dual
 In a pure IP routing domain, all routers must be IP-capable. IP-only
 routers may be freely mixed with dual routers. Some fields
 specifically related to OSI operation may be included by dual
 routers, and will be ignored by IP-only routers. Only IP traffic will
 be routed in an pure IP domain. Any OSI traffic may be discarded
 (except for the IS-IS packets necessary for operation of the routing
 protocol).
 In a pure OSI routing domain, all routers must be OSI-capable.  OSI-
 only routers may be freely mixed with dual routers. Some fields
 specifically related to IP operation may be included by dual routers,
 and will be ignored by OSI-only routers. Only OSI traffic will be
 routed in a pure OSI domain. Any IP traffic may be discarded.
 In a dual routing domain, IP-only, OSI-only, and dual routers may be
 mixed on a per-area basis. Specifically, each area may itself be
 defined to be pure IP, pure OSI, or dual.
 In a pure IP area within a dual domain, IP-only and dual routers may
 be freely mixed. Only IP traffic can be routed by level 1 routing
 within a pure-IP area.
 In a pure-OSI area within a dual domain, OSI-only and dual routers
 may be freely mixed. Only OSI traffic can be routed by level 1
 routing within a pure OSI area.
 In a dual area within a dual routing domain only dual routers may be
 used. Both IP and OSI traffic can be routed within a dual area.
 Within a dual domain, if both IP and OSI traffic are to be routed
 between areas then all level 2 routers must be dual.

1.5 Advantages of Using Integrated IS-IS

 Use of the integrated IS-IS protocol, as a single protocol for
 routing both IP and OSI packets in a dual environment, has
 significant advantages over using separate protocols for

Callon [Page 11] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

 independently routing IP and OSI traffic.
 An alternative approach is known as "Ships In the Night" (S.I.N.).
 With the S.I.N. approach, completely separate routing protocols are
 used for IP and for OSI. For example, OSPF [5] may be used for
 routing IP traffic, and IS-IS [1] may be used for routing OSI
 traffic. With S.I.N., the two routing protocols operate more or less
 independently. However, dual routers will need to implement both
 routing protocols, and therefore there will be some degree of
 competition for resources.
 Note that S.I.N. and the integrated IS-IS approach are not really
 completely separate options. In particular, if the integrated IS-IS
 is used within a routing domain for routing of IP and OSI traffic, it
 is still possible to use other independent routing protocols for
 routing other protocol suites.
 In the future, optional extensions to IS-IS may be defined for
 routing other common protocol suites. However, such future options
 are outside of the scope of this document. This section will compare
 integrated IS-IS and S.I.N. for routing of IP and OSI only.
 A primary advantage of the integrated IS-IS relates to the network
 management effort required. Since the integrated IS-IS provides a
 single routing protocol, within a single coordinated routing domain
 using a single backbone, this implies that there is less information
 to configure. This combined with a single coordinated MIB simplifies
 network management.
 Note that the operation of two routing protocols with the S.I.N.
 approach are not really independent, since they must share common
 resources. However, with the integrated IS-IS, the interactions are
 explicit, whereas with S.I.N., the interactions are implicit. Since
 the interactions are explicit, again it may be easier to manage and
 debug dual routers.
 Another advantage of the integrated IS-IS is that, since it requires
 only one routing protocol, it uses fewer resources. In particular,
 less implementation resources are needed (since only one protocol
 needs to be implemented), less CPU and memory resources are used in
 the router (since only one protocol needs to be run), and less
 network resources are used (since only one set of routing packets
 need to be transmitted). Primarily this translates into a financial
 savings, since each of these three types of resources cost money.
 This implies that dual routers based on the integrated IS-IS should
 be less expensive to purchase and operate than dual routers based on
 S.I.N.

Callon [Page 12] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

 Note that the operation of two routing protocols with the S.I.N.
 approach are not really independent, since they must share common
 resources. For example, if one routing protocol becomes unstable and
 starts to use excessive resources, the other protocol is likely to
 suffer. A bug in one protocol could crash the other. However, with
 the integrated IS-IS, the interactions are explicit and are defined
 into the protocol and software interactions. With S.I.N., the
 interactions are implicit.
 The use of a single integrated routing protocol similarly reduces the
 likely frequency of software upgrades. Specifically, if you have two
 different routing protocols in your router, then you have to upgrade
 the software any time EITHER of the protocols change. If you make use
 of a single integrated routing protocol, then software changes are
 still likely to be needed, but less frequently.
 Finally, routing protocols have significant real time requirements.
 In IS-IS, these real time requirements have been explicitly
 specified. In other routing protocols, these requirements are
 implicit. However, in all routing protocols, there are real time
 guarantees which must be met in order to ensure correct operation. In
 general, it is difficult enough to ensure compliance with real time
 requirements in the implementation of a single real time system. With
 S.I.N., implementation of two semi-independent real-time protocols in
 a single device makes this more difficult.
 Note that both integrated IS-IS and S.I.N. allow for independence of
 external routes (for traffic from/to outside of the routing domain),
 and allow for independent assignment of OSI and TCP/IP addresses.

2 Symbols and Abbreviations

AA Administrative Authority

              (a three octet field in the GOSIP version 2.0 NSAP
              address format)

AFI Authority and Format Identifier

              (the first octet of all OSI NSAP addresses -- identifies
              format of the rest of the address)

CLNP Connection-Less Network Protocol

              (ISO 8473, the OSI connectionless network layer protocol
              -- very similar to IP)

DFI DSP Format Identifier

              (a one octet field in the GOSIP version 2.0 NSAP address
              format)

Callon [Page 13] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

ES End System

              (The OSI term for a host)

ES-IS End System to Intermediate System Routeing Exchange

              Protocol (ISO 9542 -- OSI protocol between routers
              and end systems)

ICD International Code Designator

              (ISO standard for identifying organizations)

IP Internetwork Protocol

              (an Internet Standard Network Layer Protocol)

IS Intermediate System

              (The OSI term for a router)

IS-IS Intermediate System to Intermediate System Routeing

              Exchange Protocol
              (the ISO protocol for routing within a single
              routing domain)

IS-IS Hello An Hello packet defined by the IS-IS protocol

              (a type of packet used by the IS-IS protocol)

ISH An Hello packet defined by ISO 9542 (ES-IS protocol).

              (not the same as IS-IS Hello)

ISO International Organization for Standardization

              (an international body which is authorized to write
              standards of many kinds)

LSP Link State Packet

              (a type of packet used by the IS-IS protocol)

NLPID Network Layer Protocol ID

              (A one-octet field identifying a network layer protocol)

NSAP Network Service Access Point

              (a conceptual interface point at which the network
              service is made available)

SEL NSAP Selector

              (the last octet of NSAP addresses, also called NSEL)

OSI Open Systems Interconnection

              (an international standard protocol architecture)

Callon [Page 14] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

RD Routing Domain

              (the set of routers and end systems using a single
              instance of a routing protocol such as IS-IS)

SNPA Subnetwork Point of Attachment

              (a conceptual interface at which a subnetwork service
              is provided)

TCP Transmission Control Protocol

              (an Internet Standard Transport Layer Protocol)

TCP/IP The protocol suite based on TCP, IP, and related

              protocols (the Internet standard protocol
              architecture)

3 Subnetwork Independent Functions

3.1 Exchange of Routing Information

 The exchange of routing information between routers makes use of the
 normal routing packet exchange as defined in the OSI IS-IS routing
 spec, with additional IP-specific information added to the IS-IS
 routing packets.
 The IS-IS protocol provides for the inclusion of variable length
 fields in all IS-IS packets. These fields are encoded using a "Code,
 Length, Value" triplet, where the code and length are encoded in one
 octet each, and the value has the length specified (from 0 to 254
 octets). IS-IS requires that: "Any codes in a received PDU that are
 not recognised are ignored and passed through unchanged". This
 requirement applies to all routers implementing IS-IS, including
 OSI-only, IP-only, and dual routers. This allows IP-specific
 information to be encoded in a manner which OSI-only routers will
 ignore, and also allows OSI-specific information to be encoded in a
 manner which IP-only routers will ignore.
 IP-capable (i.e., all IP-only and dual) routers need to know what
 network layer protocols are supported by other routers in their area.
 This information is made available by inclusion of a "protocols
 supported" field in all IS-IS Hello and Link State Packets. This
 field makes use of the NLPID (Network Layer Protocol Identifier),
 which is a one-octet value assigned by ISO to identify network level
 protocols. NLPID values have been assigned to ISO 8473 and to IP.
 IP-capable routers need to know the IP address of the adjacent
 interface of neighboring routers. This is required for sending ICMP
 redirects (when an IP-capable router sends an ICMP redirect to a
 host, it must include the IP address of the appropriate interface of

Callon [Page 15] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

 the correct next-hop router). This information is made available by
 inclusion of the IP interface address in the IS-IS Hello packets.
 Specifically, each IS-IS Hello packet contains the IP address(es) of
 the interface over which the Hello is transmitted. The IS-IS allows
 multiple IP addresses to be assigned to each physical interface.
 In some cases, it will be useful for IP-capable routers to be able to
 determine an IP address(es) of all other routers at their level
 (i.e., for level 1 routers: all other routers in their area; for
 level 2 routers: all other level 2 routers in the routing domain).
 This is useful whenever an IP packet is to be sent to a router, such
 as for encapsulation or for transmission of network management
 packets. This information is made available by inclusion of IP
 address in LSPs. Specifically, each IS-IS LSP includes one or more IP
 addresses of the router which transmits the LSP. An IP-capable router
 is required to include at least one of its IP addresses in its LSPs,
 and may optionally include several or all of its IP addresses. Where
 a single router operates as both a level 1 and a level 2 router, it
 is required to include the same IP address(es) in its level 1 and
 level 2 LSPs.
 IP-capable routers need to know, for any given IP destination
 address, the correct route to that destination. Specifically, level 1
 routers need to know what IP addresses are reachable from each level
 1 router in their area. In addition, level 1 routers need to find
 level 2 routers (for traffic to IP addresses outside of their area).
 Level 2 routers need to know what IP addresses are reachable
 internally (either directly, or via level 1 routing) from other level
 2 routers, and what addresses are reachable externally from other
 level 2 routers. All of this information is made available by
 inclusion of IP reachable address information in the Link State
 Packets.
 Internal (within the routing domain) and external (outside the
 domain) reachability information is announced separately in level 2
 LSPs. Reachable IP addresses include a default metric, and may
 include multiple TOS-specific metrics. In general, for external
 routes, metrics may be of type "internal" (i.e., directly comparable
 with internal metrics) or of type "external" (i.e., not comparable
 with the internal metric). A route using internal metrics (i.e.,
 either announced as "IP internal reachability information", or
 announced as "IP external reachability information" with an internal
 metric) is always preferred to a route using external metrics (i.e.,
 announced as "IP external reachability information", with an external
 metric).
 The detailed encoding of the IP-specific information included in
 routing packets is provided in section 5 (Structure and Encoding of

Callon [Page 16] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

 PDUs).

3.2 Hierarchical Abbreviation of IP Reachability Information

 Level 2 routers include in their level 2 LSPs a list of all [IP
 address, subnet mask, metric] combinations reachable in their area.
 In general, this information may be determined from the level 1 LSPs
 from all routers in the area. If we ignore resource constraints, then
 it would be permissible for a level 2 router to simply duplicate all
 [IP address, subnet mask, metric] entries from all level 1 routers in
 its area (with appropriate metric adjustment), for inclusion in its
 level 2 LSP. However, in order for hierarchical routing to scale to
 large routing domain sizes, it is highly desired to abbreviate the
 reachable address information.
 This is accomplished by manual configuration of summary addresses.
 Each level 2 router may be configured with one or more [IP address,
 subnet mask, metric] entries for announcement in their level 2 LSPs.
 The set of reachable addresses obtained from level 1 LSPs is compared
 with the configured reachable addresses. Redundant information
 obtained from level 1 LSPs is not included in level 2 LSPs. Generally
 it is expected that the level 2 configured information will specify
 more inclusive addresses (corresponding to a subnet mask with fewer
 bits set to 1). This will therefore allow one configured
 address/submask pair (or a small number of such pairs) to
 hierarchically supercede the information corresponding to multiple
 entries in level 1 LSPs.
 The manually configured addresses are included in level 2 LSPs only
 if they correspond to at least one address which is reachable in the
 area. For manually configured level 2 addresses, the associated
 metric values to announce in level 2 LSPs are also manually
 configured. The configured addresses will supercede reachable address
 entries from level 1 LSPs based only on the IP address and subnet
 mask -- metric values are not considered when determining if a given
 configured address supercedes an address obtained from a level 1 LSP.
 Any address obtained from a level 1 LSP which is not superceded by
 the manually configured information is included in the level 2 LSPs.
 In this case, the metric value announced in the level 2 LSPs is
 calculated from the sum of the metric value announced in the
 corresponding level 1 LSP, plus the distance from the level 2 router
 to the appropriate level 1 router. Note: If this sum results in a
 metric value greater than 63 (the maximum value that can be reported
 in level 2 LSPs), then the value 63 must be used. Delay, expense, and
 error metrics (i.e., those TOS metrics other than the default metric)
 will be included only if (i) the level 2 router supports the specific

Callon [Page 17] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

 TOS; (ii) the path from the level 2 router to the appropropriate
 level 1 router is made up of links which support the specific TOS;
 and (iii) the level 1 router which can reach the address directly
 also supports the specific TOS for this route, as indicated in its
 level 1 LSP.
 In general, the same [IP address, subnet mask] pair may be announced
 in level 1 LSPs sent by multiple level 1 routers in the same area. In
 this case (assuming the entry is not superceded by a manually
 configured entry), then only one such entry shall be included in the
 level 2 LSP. The metric value(s) announced in level 2 LSPs correspond
 to the minimum of the metric value(s) that would be calculated for
 each of the level 1 LSP entries.
 A level 2 router will have IP addresses which are directly reachable
 via its own interfaces. For purposes of inclusion of IP reachable
 address information in level 2 LSPs, these "directly reachable"
 addresses are treated exactly the same as addresses received in level
 1 LSPs.
 Manually configured addresses may hierarchically supercede multiple
 level 1 reachable address entries. However, there may be some IP
 addresses which match the manually configured addresses, but which
 are not reachable via level 1 routing. If a level 2 router receives
 an IP packet whose IP address matches a manually configured address
 which it is including in its level 2 LSP, but which is not reachable
 via level 1 routing in the area, then the packet must be discarded.
 In this case, an error report may be returned (as specified in RFC
 1009), with the reason for discard specifying destination
 unreachable.
         Figure 2 - An Example Routing Domain (not shown)
 An example is illustrated in figure 2. Suppose that the network
 number for the entire routing domain is 17 (a class A network).
 Suppose each area is assigned a subnet number consisting of the next
 8 bits. The area may be further subdivided by assigning the next
 eight bits to each LAN in the area, giving each a 24 bit subnet mask
 (counting the network and subnet fields). Finally 8 bits are left for
 the host field. Suppose that for a particular area (given subnet
 number 17.133) there are a number of IP capable level 1 routers
 announcing (in the special IP entry in their level 1 LSPs) subnets
 17.133.5, 17.133.43, and 17.133.57.

Callon [Page 18] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

 Suppose that in this example, in order to save space in level 2 LSPs,
 the level 2 routers in this area are configured to announce subnet
 17.133. Only this one address needs to be announced in level 2 LSPs.
 Thus if an IP packet comes along for an address in subnet 17.133.5,
 17.133.43 or 17.133.57, then other level 2 routers, in other areas,
 will know to pass the traffic to this area.
 The inclusion of 17.133 in level 2 LSPs means that the three subnet
 addresses starting with 17.133 do not all have to be listed
 separately in level 2 LSPs.
 If any traffic comes along that is for an unreachable address such as
 17.133.124.7, then level 2 routers in other areas in this particular
 domain will think that this area can handle this traffic, will
 forward traffic to level 2 routers in this area, which will have to
 discard this traffic.
 Suppose that subnet number 17.133.125 was actually reachable via some
 other area, such as the lower right hand area. In this case, the
 level 2 router in the left area would be announcing (in its level 2
 LSPs according to manually configured information) reachability to
 subnet 17.133. However, the level 2 router in the lower right area
 would be announcing (in its level 2 LSPs according to information
 taken from its received level 1 LSPs), reachability to subnet
 17.133.125. Due to the use of best match routing, this works
 correctly. All traffic from other areas destined to subnet 17.133.125
 would be sent to the level 2 router in the lower right area, and all
 other traffic to subnet 17.133 (i.e., traffic to any IP address
 starting with 17.133, but not starting with 17.133.125) would be sent
 to the level 2 router in the leftmost area.

3.3 Addressing Routers in IS-IS Packets

 The IS-IS packet formats explicitly require that OSI-style addresses
 of routers appear in the IS-IS packets. For example, these addresses
 are used to determine area membership of routers. It is therefore
 necessary for all routers making use of the IS-IS protocol to have
 OSI style addresses assigned. For IP-only routers, these addresses
 will be used only in the operation of the IS-IS protocol, and are not
 used for any other purpose (such as the operation of EGP, ICMP, or
 other TCP/IP protocols).
 For OSI-only and dual routers, assignment of NSAP addresses is
 straight forward, but is outside of the scope of this specification.
 Address assignment mechanisms are being set up by standards bodies
 which allow globally unique OSI NSAP addresses to be assigned. All
 OSI-only and dual routers may therefore make use of normal OSI
 addresses in the operation of the IS-IS protocol.

Callon [Page 19] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

 For IP-only routers, there are two ways in which NSAP addresses may
 be obtained for use with the IS-IS protocol.
 1) For those environments in which OSI is being used, or in which it
    is anticipated that OSI will be used in the future, it is
    permissible to obtain NSAP address assignments in the normal
    manner, assign normal NSAP addresses to IP-only routers, and use
    these addresses in the operation of IS-IS. This approach is
    recommended even for pure IP routing domains, as it will simplify
    future migration from IP-only to dual operation.
 2) In some cases, routers may have only TCP/IP addresses, and it may
    be undesireable to have to go through the normal mechanisms for
    assignment of NSAP addresses. Instead, an alternate mechanim is
    provided below for algorithmically generating a valid OSI style
    address from existing IP address and autonomous system number
    assignments.
 Where desired, for IP-only routers, for use in IS-IS packet formats
 only, OSI-style addresses (compatible with the USA GOSIP version 2.0
 NSAP address format [9]) may be derived as follows:
      AFI       1 octet       value "47" (specifies ICD format)
      ICD       2 octet       value "00 05" (specifies Internet/Gosip)
      DFI       1 octet       value "xx"
      AA        3 octets      value "xx xx xx" (specifies special
                              IP-only use of NSAPs)
      Reserved  2 octets      must be "00 00"
      RD        2 octets      contains autonomous system number
      Area      2 octets      must be assigned as described below
      ID        6 octets      must be assigned as described below
      SEL       1 octet       used as described below
 The AFI value of "47" and the ICD value of "00 05" specifies the
 Gosip Version 2.0 addressing format. The DFI number of "xx" and the
 AA of "xx xx xx" specify that this special NSAP address format is
 being used, solely for IS-IS packet formats in an IP-only
 environment. The reserved field must contain "00 00", as specified in
 GOSIP version 2.0.

Callon [Page 20] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

 The routing domain field contains the Autonomous System number.
 Strictly speaking, this is not necessary, since the IS-IS packets are
 exchanged within a single AS only. However, inclusion of the AS
 number in this address format will ensure correct operation in the
 event that routers from separate routing domains/ASs are incorrectly
 placed on the same link. The AS number in this context is used only
 for definition of unique NSAP addresses, and does not imply any
 coupling with exterior routing protocols.
 The Area field must be assigned by the authority responsible for the
 routing domain, such that each area in the routing domain must have a
 unique Area value.
 The ID must be assigned by the authority responsible for the routing
 domain. The ID must be assigned such that every router in the routing
 domain has a unique value. It is recommended that one of the
 following methods is used:
 1)use a unique IEEE 802 48 bit station ID
 2)use the value hex "02 00" prepended to an IP address of the router.
 IEEE 802 addresses, if used, must appear in IEEE canonical format.
 Since the IEEE 802 station IDs are assigned to be globally unique,
 use of these values clearly assures uniqueness in the area. Also, all
 assigned IEEE 802 station IDs have the global/local bit set to zero.
 Prepending the indicated pattern to the front of the IP address
 therefore assures that format (2) illustrated above cannot produce
 addresses which collide with format (1). Finally, to the extent that
 IP addresses are also globally unique, format (2) will produce unique
 IDs for routers.
 The indicated hex value is specified in IEEE 802 canonical form [10].
 In IEEE 802 addresses, the multicast bit is the least significant bit
 of the first byte. The global/local bit is the next least significant
 bit of the first byte. The indicated prefix therefore sets the
 global/local bit to 1, and all other bits in the first two octets to
 0.
 Note that within an area, whether ISO addresses are configured into
 the routers through ISO address assignment, or whether the ISO-style
 address is generated directly from the AS number and IP address, all
 routers within an area must have the same high order part of address
 (AFI, ICD, DFI, AA, RD, and Area). This ISO-style address is used in
 IS-IS Hello messages and is the basis by which routers recognize
 whether neighbor nodes are in or out of their area.

Callon [Page 21] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

3.4 External Links

 External connectivity (i.e., communications with routers outside of
 the routing domain) is done only by level 2 routers. The ISO version
 of IS-IS allows external OSI routes to be reported as "reachable
 address prefixes" in level 2 LSPs. The integrated IS-IS also allows
 external IP reachable addresses (i.e., IP addresses reachable via
 inter-domain routing) to be reported in level 2 LSPs in the "IP
 external reachability information" field. External OSI and external
 IP routes are handled independently.
 The routes announced in IP external reachability information entries
 include all routes to outside of the routing domain. This includes
 routes learned from OSPF, EGP, RIP, or any other external protocol.
 External routes may make use of "internal" or "external" metrics.
 Internal metrics are comparable with the metrics used for internal
 routes. Thus in choosing between an internal route, and an external
 route using internal metrics, the metric values may be directly
 compared. In contrast, external metrics cannot be directly compared
 with internal metrics. Any route defined solely using internal
 metrics is always preferred to any route defined using external
 metrics. When an external route using external metrics must be used,
 the lowest value of the external metric is preferred regardless of
 the internal cost to reach the appropriate exit point.
 It is useful, in the operation of external routing protocols, to
 provide a mechanism for border routers (i.e., routers in the same
 routing domain, which have the ability to route externally to other
 domains) to determine each other's existence, and to exchange
 external information (in a form understood only by the border routers
 themselves). This is made possible by inclusion of "inter-domain
 routing protocol information" fields in level 2 LSPs. The inter-
 domain routing protocol information field is not included in
 pseudonode LSPs.
 In general there may be multiple types of external inter-domain
 routing protocol information exchanged between border routers. The
 IS-IS therefore specifies that each occurance of the inter-domain
 routing protocol information field include a "type" field, which
 indicates the type of inter-domain routing protocol information
 enclosed. Values to be used in the type field will be specified in
 future versions of the "Assigned Numbers" RFC. Initial values for
 this field are specified in Annex A of this specification.
 Information contained in the inter-domain routing protocol
 information field will be carried in level 2 LSPs, and will therefore
 need to be stored by all level 2 routers in the domain. However, only

Callon [Page 22] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

 those level 2 routers which are directly involved in external routing
 will use this information. In designing the use of this field, it is
 important to carefully consider the implications that this may have
 on storage requirements in level 2 routers (including those level 2
 routers which are not directly involved in external routing).
 The protocols used to exchange routing information directly between
 border routers, and external routers (in other routing domains /
 autonomous systems) are outside of the scope of this specification.

3.5 Type of Service Routing

 The integrated IS-IS protocol provides IP Type of Service (TOS)
 routing, through use of the Quality of Service (QOS) feature of IS-
 IS. This allows for routing on the basis of throughput (the default
 metric), delay, expense, or residual error probability. Note than any
 particular packet may be routed on the basis of any one of these four
 metrics. Routing on the basis of general combinations of metrics is
 not supported.
 The support for TOS/QOS is optional. If a particular packet calls for
 a specific TOS, and the correct path from the source to destination
 is made up of routers all of which support that particular TOS, then
 the packet will be routed on the optimal path. However, if there is
 no path from the source to destination made up of routers which
 support that particular type of service, then the packet will be
 forwarded using the default metric instead. This allows for TOS
 service in those environments where it is needed, while still
 providing acceptable service in the case where an unsupported TOS is
 requested.
 NOTE - IP does not have a cost TOS. There is therefore no mapping of
 IP TOS metrics which corresponds to the minimum cost metric.
 The IP TOS field is mapped onto the four available metrics as
 follows:
 Bits 0-2 (Precedence):  This field does not affect the route, but
                         rather may affect other aspects of packet
                         forwarding.
 Bits 3 (Delay), 4 (Throughput) and 5 (Reliability):
         000     (all normal)            Use default metric
         100     (low delay)             Use delay metric
         010     (high throughput)       Use default metric

Callon [Page 23] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

         001     (high reliabiity)       Use reliability metric
         other                           Use default metric

3.6 Multiple LSPs and SNPs

 In some cases, IS-IS packets (specifically Link State Packets and
 Complete Sequence Number Packets) may be too large to fit into one
 packet. The OSI IS-IS [1] allows for LSPs and CSNPs to be split into
 multiple packets. This is independent of ISO 8473 segmentation, and
 is also independent of IP fragmentation. Use of independent multiple
 packets has the advantages (with respect to segmentation or
 fragmentation) that: (i) when information in the IS-IS changes, only
 those packets effected need to be re-issued; (ii) when a single
 packet is received, it can be processed without the need to receive
 all other packets of the same type from the same router before
 beginning processing.
 The Integrated IS-IS makes use of the same multiple packet function,
 as defined in [1]. IP-specific fields in IS-IS packets may be split
 across multiple packets. As specified in section 5 ("Structure and
 Encoding of PDUs"), some of the IP-specific fields (those which may
 be fairly long) may be split into several occurences of the same
 field, thereby allowing splitting of the fields across different
 packets.
 Multiple LSPs from the same router are distinguished by LSP number.
 Generally, most variable length fields may occur in an LSP with any
 LSP number. Some specific variable length fields may be required to
 occur in LSP number 0. Except where explicitly stated otherwise, when
 an IS-IS router issues multiple LSPs, the IP-specific fields may
 occur in an LSP with any LSP number.
 Complete Sequence Number Packets may be split into multiple packets,
 with the range to which each packet applies explicitly reported in
 the packet. Partial Sequence Number Packets are inherently partial,
 and so can easily be split into multiple packets if this is
 necessary. Again, where applicable, IP-specific fields may occur in
 any SNP.

3.7 IP-Only Operation

 For IP-only routers, the format for IS-IS packets remains unchanged.
 However, there are some variable length fields from the IS-IS packets
 that can be omitted. Specifically:

Callon [Page 24] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

 IS-IS Hello Packets:
  1. no change
 IS-IS Link State Packets:
  1. the "End Systems Neighbours" entries are omitted
  1. the "Prefix Neighbours" entries are omitted
 IS-IS Sequence Number Packets:
  1. no change

3.8 Encapsulation

 Future versions of the Integated IS-IS may specify optional
 encapsulation mechanisms for partition repair, and for forwarding
 packets through incompatible routers (i.e., for forwarding OSI
 packets through IP-only routers, and forwarding IP packets through
 OSI-only routers). The details of encapsulation and decapsulation are
 for further study. Routers complying with the Integrated IS-IS are
 not required to implement encapsulation nor decapsulation.

3.9 Authentication

 The authentication field allows each IS-IS packet to contain
 information used to authenticate the originator and/or contents of
 the packet.  The authentication information contained in each packet
 is used to authenticate the entire packet, including OSI and IP
 parts. If a packet is received which contains invalid authentication
 information, then the entire packet is discarded. If an LSP or SNP is
 split into multiple packets (as described in section 3.6), then each
 is authenticated independently.
 Use of the authentication field is optional. Routers are not required
 to be able to interpret authentication information. As with other
 fields in the integrated IS-IS, if a router does not implement
 authentication then it will ignore any authentication field that may
 be present in an IS-IS packet.
 Annex D specifies a proposed use of the authentication field.

3.10 Order of Preference of Routes / Dijkstra Computation

 We define the term "IP reachability entry" to mean the combination of
 the [IP address, subnet mask]. The Dijkstra calculation must
 calculate routes to each distinct IP reachability entry. For the

Callon [Page 25] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

 Dijkstra calculation, each IP reachability entry can be treated in
 much the same manner as an OSI end system. Naturally, each IP
 reachability entry is treated as distinct from any OSI end systems
 which may also be reachable in the same area or routing domain.
 For any particular IP reachability entry, this is the same as another
 entry if and only if: (i) the subnet masks are identical; and (ii)
 for each bit in the subnet mask which has the value "1", the IP
 address is identical. This can easily be tested by zeroing those bits
 in the IP address which correspond to a zero bit in the mask, and
 then treating the entry as a 64 bit quantity, and testing for
 equality between different 64 bit quantities. The actual calculation
 of routes to IP reachability entries is therefore no more complex
 than calculation of routes to OSI end systems (except for the
 replacement of a 48-bit test with a 64-bit test).
 The Dijkstra computation does not take into consideration whether a
 router is IP-only, OSI-only, or dual. The topological restrictions
 specified in section 1.4 ensure that IP packets will only be sent via
 IP-capable routers, and OSI packets will only be sent via OSI-capable
 routers.
 The Integrated IS-IS prefers routes within the area (via level 1
 routing) whenever possible. If level 2 routes must be used, then
 routes within the routing domain (specifically, those routes using
 internal metrics) are prefered to routes outside of the routing
 domain (using external metrics).
 The Integrated IS-IS protocol makes use of "best match" routing of IP
 packets. This implies that a particular destination address may match
 more than one entry in the forwarding database. If a particular IP
 packet has a destination address which matches two different IP
 reachability entries, then the entry who's mask contains the most "1"
 bits is preferred.
 IP packets whose destination is a router are routed the same way as
 any other IP packet, by forwarding first to the appropriate subnet,
 and then forwarding on that subnet to the destination host (which
 just happens to be a router in this case). In particular, the IP
 forwarding database does not contain explicit routes to the
 individual "IP interface addresses" listed by each router in its LSP.
 However, host routes (routes with a subnet mask of all ones) may of
 course be included in the IP reachability entries, and will be
 handled in the same manner as other IP reachability entries.
 In order to ensure correct interoperation of different router
 implementations, it is necessary to specify the order of preference

Callon [Page 26] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

 of possible routes. For OSI destinations, this is outside of the
 scope of this report. For IP destinations, this is specified in
 section 3.10.1 and 3.10.2 below. Annex C specifies a detailed
 Dijkstra calculation and forwarding algorithm which is compatible
 with the order of preference of routes specified here.
 With IS-IS, if a route to a given destination is advertised, or a
 link between routers is advertised, then metric values associated
 with some or all of the specified TOS metric types may be associated
 with that destination or link. However, the default metric must
 always be available. Normally this ensures that if a route using any
 TOS metric is available, then a route using the default metric will
 also be available. The only exception to this is where the
 corresponding route using the default metric has a total cost (within
 the area, or within the level 2 backbone) greater than MaxPathMetric.
 In determining the route to a particular destination for a specified
 TOS, only routes using either the requested TOS metric, or the
 default TOS metric, are considered.

3.10.1 Order of Preference of Routes In Level 1 Routing

 If a given destination is reachable within an area via a route using
 either the requested TOS or the default TOS, then the IS-IS will
 always make use of a path within the area (via level 1 routing),
 regardless of whether an alternate path exists outside of the area
 (via level 2 routing). In this case, routes within the area are
 selected as follows:
 1) Amongst routes in the area, if the specified destination
    address matches more than one [IP address, subnet mask] pair,
    then the more specific address match (the one with more "1"
    bits in the mask) is prefered.
 2) Amongst routes in the area to equally specific address
    matches, routes on which the requested TOS (if any) is
    supported are always prefered to routes on which the
    requested TOS is not supported.
 3) Amongst routes in the area of the same TOS to equally
    specific address matches, the shortest routes are prefered.
    For determination of the shortest path, if a route on which
    the specified TOS is supported is available, then the
    specified TOS metric is used, otherwise the default metric
    is used. Amongst routes of equal cost, load splitting may
    be performed as specified in [1].
 For a level 1 only router (i.e., a router which does not take part in

Callon [Page 27] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

 level 2 routing, or a level 2 router which is not "attached"), if a
 given destination is not reachable within an area, level 1 routing
 will always route to a level 2 router as follows:
 1) Amongst routes in the area to attached level 2 routers,
    routes on which the requested TOS (if any) is supported
    are always prefered to routes on which the requested TOS
    is not supported.
 2) Amongst routes in the area of the same TOS to attached
    level 2 routers, the shortest routes are prefered. For
    determination of the shortest path, if a route on which
    the specified TOS is supported is available, then the
    specified TOS metric is used, otherwise the default
    metric is used. Amongst routes of equal cost,
    loadsplitting may be performed as specified in [1].

3.10.2 Order of Preference of Routes in Level 2 Routing

 For those level 2 routers which also take part in level 1 routing,
 routes learned via level 1 routing, using either the requested TOS or
 the default TOS, are always prefered to routes learned through level
 2 routing. For destinations which are not reachable via level 1
 routing, or for level 2 only routers (routers which do not take part
 in level 1 routing), then level 2 routes are selected as follows:
 1) Routes using internal metrics only are always preferred
    to routes using external metrics.
 2) If a route using internal metrics only is available:
    a) If the specified destination address matches more
       than one [IP address, subnet mask] pair, then the more
       specific address match (i.e., the largest number of
       "1"s present in the subnet mask) is prefered.
    b) Amongst routes with equally specific address matches
       (i.e., an equal number of "1"s present in the subnet
       mask), routes on which the requested TOS (if any) is
       supported are always preferred to routes on which the
       requested TOS is not supported.
    c) Amongst routes of the same TOS with an equally specific
       address matches, the shortest path is prefered. For
       determination of the shortest path, if a route on which
       the specified TOS is supported is available, then the
       specified TOS metric is used, otherwise the default
       metric is used. Amongst routes of equal cost,

Callon [Page 28] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

       loadsplitting may be performed as specified in [1].
       NOTE: Internal routes (routes to destinations announced
       in the "IP Internal Reachability Information" field),
       and external routes using internal metrics (routes to
       destinations announced in the "IP External Reachability
       Information" field, with a metric of type "internal")
       are treated identically for the purpose of the order of
       preference of routes, and the Dijkstra calculation.
 3) If a route using internal metrics only is not available,
    but a route using external metrics is available:
    a) If the specified destination address matches more than
       one [IP address, subnet mask] pair, then the more
       specific address match is prefered.
       NOTE: For external routes, the subnet mask will normally
       correspond precisely to the network number. This implies
       that this test will always discover equal length matching
       strings.  However, this test is included to allow future
       migration to more general handling of external addresses.
    b) Amongst routes with equally specific matches, routes on
       which the requested TOS (if any) is supported are always
       preferred to routes on which the requested TOS is not
       supported. NOTE: for external routes, the route is
       considered to support the requested TOS only if the
       internal route to the appropriate border router
       supports the requested TOS, and the external route
       reported by the border router also supports the
       requested TOS.
    c) Amongst routes of the same TOS with an equal length
       matching address string, the shortest path is prefered.
       For determination of the shortest path:
       (i)  Routes with a smaller announced external metric
            are always prefered.
       (ii) Amongst routes with an equal external metric,
            routes with a shorter internal metric are prefered.
            Amongst routes of equal cost, loadsplitting may be
            performed as specified in [1].
 For level 2 routers which are announcing manually configured summary
 addresses in their level 2 LSPs, in some cases there will exist IP
 addresses which match the manually configured addresses, but which do

Callon [Page 29] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

 not match any addresses which are actually reachable via level 1
 routing in the area. Generally, packets to such addresses are handled
 according to the following rules:
 1) If the specified destination is reachable via level 1 routing,
    then according to the order of preference of routes specified
    above, the packet will be delivered via level 1 routing.
 2) If the specified destination is not reachable via level 1 routing,
    but is reachable via 2 routing, and there are other level 2
    routers which offer more desireable routes according to the
    rules specified above (for example a route with a more specific
    match, or a route with an equally specific match which supports
    the correct TOS), then level 2 routing will forward the packet
    according to the more desireable route.
 3) If the specified destination is not reachable via level 1 routing,
    and the manually configured summary address advertised by this
    router (the router which has received the packet and is trying
    to forward it) represents the most desireable route, then the
    destination is unreachable and the packet must be discarded.

4 Subnetwork Dependent Functions

4.1 Link Demultiplexing

 Dual routers may receive a combination of OSI packets, and IP
 packets. It is necessary for the dual routers to be able to clearly
 and unambiguously distinguish the two protocol suites.
 This problem is not unique to the integrated IS-IS routing protocol.
 In fact, this problem will occur in any multi-protocol environment.
 This problem is currently being worked on independently, and is
 outside of the scope of this specification.
 In general, the link type is a configuration parameter. For example,
 whether to use PPP, HDLC, or some other point-to-point protocol over
 a point-to-point link would be configured. For any particular link
 type, a method must be defined for encapsulation of both OSI and IP
 packets. Definition of such methods for common link types is outside
 of the scope of this specification.
 IP packets are encapsulated directly over the underlying link layer
 service, using the normal method for transmssion of IP packets over
 each type of link. Similarly OSI packets are encapsulated directly
 over the underlying link layer service, using the normal method for
 transmission of OSI packets over each type of link. Finally, note
 that IS-IS packets are encapsulated using the normal method for

Callon [Page 30] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

 transmission of OSI packets over any particular link type. This
 implies that all IS-IS routers, including IP-only routers, must be
 able to receive IS-IS packets using the normal encapsulation for OSI
 packets.

4.2 Multiple IP Addresses per Interface

 The integrated IS-IS allows each router to have multiple IP addresses
 for each physical interface, up to the maximum number which may be
 contained in a single "IP Interface Address" field (i.e., up to a
 maximum of 63 addresses per interface). For example, where there are
 two logical subnets on the same LAN, the interface may have two IP
 addresses, one corresponding to each logical subnet. Each IS-IS Hello
 packet contains a list of IP addresses associated with the physical
 interface over which the Hello is transmitted.
 It is permissible to implement routers which conform to the
 Integrated IS-IS specification which restrict the number of IP
 addresses per interface. However, IP-capable routers must be able to
 interact correctly with other routers which assign multiple IP
 addresses per physical interface (up to the maximum of 63 addresses
 per interface).
 Where appropriate (for example, in some cases on point-to-point
 links), some interfaces may have no IP addresses assigned. In this
 case, the IS-IS Hello transmitted on that interface may omit the IP
 Interface Address field, or may include the IP Interface Address
 field with zero entries.

4.3 LANs, Designated Routers, and Pseudonodes

 The maintenance of designated routers and pseudonodes is specified in
 [1], and is not changed by this proposal. In the case that IP-only
 and dual routers (or OSI-only and dual routers) are mixed on the same
 LAN in a pure IP area (or a pure OSI area, respectively), any router
 on the LAN may be elected designated router.
 However, there is a fundamental difference in the way that OSI and
 TCP/IP deal with LANs, and other broadcast subnetworks.
 With OSI, the use of the ES-IS protocol (ISO 9542) allows the end
 systems and routers to automatically determine their connectivity,
 thereby allowing all end systems on the LAN to potentially route via
 any of the routers on the LAN.
 In contract, TCP/IP explictly assigns subnet identifiers to each
 local area network. In some cases, a single physical LAN could have
 multiple subnet identifiers assigned to it. In this case, end systems

Callon [Page 31] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

 (hosts) which have an address on one logical subnet are explicitly
 precluded from sending IP packets directly to a router whose address
 places it on a different logical subnet. Each router is manually
 configured to know which subnets it can reach on each interface. In
 the case that there are multiple logical subnets on the same LAN,
 each router can only exchange IP packets with those end systems which
 are on the same logical subnet. This implies that it is not
 sufficient for the pseudonode LSP to announce all subnets on the LAN
 (i.e., all [IP address, subnet mask] pairs reachable on the LAN).
 It is therefore necessary for each router to announce in its LSPs
 those subnets which it can reach on each interface, including
 interfaces to broadcast subnetworks such as LANs. The pseudonode LSP
 does not specify the IP addresses which are reachable on the LAN
 (i.e., does not contain the the IP reachability field).
 As specified elsewhere (see the forthcoming update to the
 "Requirements of IP Gateways" [4]), routers may send ICMP redirects
 only if: (i) the IP packet is being forwarded over the same physical
 interface over which it arrived; and (ii) the source address of the
 forwarded IP packet, the IP address of this router's interface (as
 indicated by the source address of the ICMP redirect), and the IP
 address of the router to which the packet is being redirected (again,
 as indicated in the ICMP redirect) are all on the same IP subnet.

4.4 Maintaining Router Adjacencies

 The IS-IS determines whether an adjacency is to be established
 between two routers using means which are independent of the IP
 interface addresses of the routers. Where multiple logical subnets
 occur on the same physical LAN, this potentially allows adjacencies
 to be brought up between two routers which share physical
 connectivity to each other, but which don't have a logical subnet in
 common. IP-capable IS-IS routers therefore must be able to forward IP
 packets over existing adjacencies to routers with which they share
 physical connectivity, even when the IP address of the adjacent
 interface of the neighboring router is on a different logical IP
 subnet.
 For point-to-point links, IS-IS requires exchange of ISO 9542 ISHs,
 as the first step in establishing the link between routers. All IS-IS
 routers are therefore required to transmit and receive ISO 9542 ISH
 packets on point-to-point links.
 The "protocols supported" field (defined in section 5 below) must be
 present in all IS-IS Hello packets sent by dual and IP-only routers.
 If this field is missing, then it is assumed that the packet was
 transmitted by an OSI-only router. Similarly, those 9542 ISHs sent

Callon [Page 32] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

 over point-to-point links, where there is (or may be) another IS-IS
 router at the other end of the point-to-point link, must also
 contains the "protocols supported" field. Note that if this field is
 mistakenly sent in a 9542 ISH where there is an ordinary OSI-only End
 System at the other end of the link, then (in accordance to ISO 9542)
 the End System is required to ignore the field and interpret the ISH
 correctly. It is therefore safe to always include this field in ISHs
 sent over point-to-point links.
 Dual routers must operate in a dual fashion on every link in the
 routing domain over which they are running IS-IS. Thus, the value of
 the "protocols supported" field must be identical on every link
 (i.e., for any one router running IS-IS, all of the Hellos and LSPs
 transmitted by it must contain the same "protocols supported"
 values).

4.5 Forwarding to Incompatible Routers

 There may be times when a dual router has to forward an IP packet to
 an OSI-only router, or forward an OSI packet to an IP-only router. In
 this case the packet must be discarded. An error report may be
 transmitted, in accordance with the IP or ISO 8473 specification
 (respectively). The reason for discard specified in the error report
 should specify "destination host unreachable" (for IP), or
 "destination unreachable" (for OSI).
 Similarly, due to errors, in some cases an IP-only router may have to
 forward an IP packet to an OSI-only router. Again, the packet must be
 discarded, as specified above. This may only occur if IP-only and
 OSI-only routers occur in the same area, which is a configuration
 error.

5 Structure and Encoding of PDUs

 This clause describes the additional packet fields for use of the ISO
 IS-IS Intra-Domain Routing protocol in pure IP and dual environments.
 Specifically, the same packet types are used as in IS-IS [1], and all
 fixed fields remain the same. Additional variable length fields are
 defined in this section.

5.1 Overview of IS-IS PDUs

 The packets used in IS-IS routing protocol fall into three main
 classes: (i) Hello Packets; (ii) Link State Packets (LSPs); and (iii)
 Sequence Number Packets (SNPs).
 Hello packets are used to initialize and maintain adjacencies between
 neighboring routers. There are three types of IS-IS Hello packets:

Callon [Page 33] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

 (i) "Level 1 LAN IS to IS Hello PDUs" are used by level 1 routers on
 broadcast LANs. (ii) "Level 2 LAN IS to IS Hello PDUs" are used by
 level 2 routers on broadcast LANs. (iii) "Point-to-Point IS to IS
 Hello PDUs" are used on non-broadcast media, such as point-to-point
 links, or general topology subnetworks.
 On point-to-point links, the exchange of ISO 9542 ISHs (intermediate
 system Hellos) is used to initialize the link, and to allow each
 router to know if there is a router on the other end of the link,
 before IS-IS Hellos are exchanged. All routers implementing IS-IS
 (whether IP-only, OSI-only, or dual), if they have any interfaces on
 point-to-point links, must therefore be able to transmit ISO 9542
 ISHs on their point-to-point links.
 Link State Packets (LSPs) are used to exchange link state
 information. There are two types of LSPs: (i) "Level 1 Link State
 PDUs" are transmitted by level 1 routers. (ii) "Level 2 Link State
 PDUs" are transmitted by level 2 routers. Note that level 2 routers
 will, in most cases, also be level 1 routers, and will therefore
 transmit both sorts of LSPs.
 Sequence number PDUs are used to ensure that neighboring routers have
 the same notion of what is the most recent LSP from each other
 router. The sequence number PDUs therefore serve a similar function
 to acknowledgement packets, but allow more efficient operation. There
 are four types of sequence number packets: (i) "Level 1 Complete
 Sequence Numbers PDU"; (ii) "Level 2 Complete Sequence Numbers PDU";
 (iii) "Level 1 Partial Sequence Numbers PDU"; and (iv) "Level 2
 Partial Sequence Numbers PDU". A partial sequence number packet lists
 the most recent sequence number of one or more LSPs, and operates
 much like an acknowlegement. A partial sequence number packet differs
 from an conventional acknowledgement in the sense that it may
 acknowlege multiple LSPs at once, and in the sense that it may act as
 a request for information. A complete sequence number packet contains
 the most recent sequence number of all LSPs in the database. A
 complete sequence number packet may therefore be used to ensure
 synchronization of the database between adjacent routers either
 periodically, or when a link first comes up.

5.2 Overview of IP-Specific Information for IS-IS

 There are six new fields defined for the Integrated IS-IS: (i)
 "Protocols Supported"; (ii) "IP Interface Address"; (iii)
 "Authentication Information"; (iv) "IP Internal Reachability
 Information"; (v) "IP External Reachability Information"; and (vi)
 "Inter-Domain Routing Protocol Information".
 The "Protocols Supported" field identifies the protocols which are

Callon [Page 34] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

 supported by each router. This field must be included in all IS-IS
 Hello packets and all LSPs with LSP number 0 transmitted by IP-
 capable routers. If this field is not included in an IS-IS Hello
 packet or an LSP with LSP number 0, it may be assumed that the packet
 was transmitted by an OSI-only router. The "Protocols Supported"
 field must also be included in ISO 9542 ISHs send by IP-capable
 routers over point-to-point links to other IS-IS routers.
 The "IP Interface Address" is included in all IS-IS Hello packets and
 LSPs transmitted by IP-only and dual routers. In the Hello packets,
 this field occurs once only, and contains the IP address(es) of the
 interface on which the Hello packet is transmitted (up to a maximum
 of 63 IP addresses on each interface). If an IS-IS Hello is
 transmitted over an interface which does not have an IP address
 assigned, then this field may be omitted, or may be included with
 zero entries. In Link State Packets, this field contains a list of
 one or more IP addresses corresponding to one or more interfaces of
 the router which originates the LSP. Each IP-capable router must
 include this field in its LSPs. This field may occur multiple times
 in an LSP, and may occur in an LSP with any LSP number.
 The "Authentication Information" field is optional in all IS-IS PDUs.
 If used, it contains information used to authenticate the packet. All
 IS-IS packets (including 9542 IS Hellos) may be authenticated by use
 of this field.
 The "IP Internal Reachability Information" field may be present in
 all LSPs transmitted by IP-capable routers. If present, it identifies
 a list of zero or more [IP address, subnet mask, metrics] reachable
 by the router which originates the LSP. Each entry must contain a
 default metric, and may contain delay, expense, and error metrics. If
 an IP-capable router does not directly reach any IP addresses, then
 it may omit this field, or may include the field with zero [IP
 address, subnet mask, metrics] entries. If included in level 1 LSPs,
 this field includes only entries directly reachable by the router
 which originates the LSP, via one of its interfaces. If included in
 level 2 LSPs, this field includes only entries reachable by the
 router which originates the LSP, either via one of its interfaces, or
 indirectly via level 1 routing. This field may occur multiple times
 in an LSP, and may occur in an LSP with any LSP number.
 The "IP External Reachability Information" field may be present in
 level 2 LSPs transmitted by level 2 IP-capable routers. If present,
 it identifies a list of zero or more [IP address, subnet mask,
 metrics] entries reachable by the router which originates the level 2
 LSP. Each entry must contain a default metric, and may contain delay,
 expense, and error metrics. Each entry may contain metrics of type
 "internal", or of type "external". If a level 2 router does not have

Callon [Page 35] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

 any external routes (via neighboring routers in other routing
 domains), when it may omit this field, or may include the field with
 zero entries. This field includes only entries reachable by the
 router which originates the LSP, via a direct link to an external
 router. This field may occur multiple times in a level 2 LSP, and may
 occur in an LSP with any LSP number.
 The "Inter-Domain Routing Protocol Information" field may be present
 in level 2 LSPs transmitted by level 2 IP-capable routers. This field
 is transmitted for the convenience of the external routing protocol,
 and is not used by the IS-IS. For example, this may be used to allow
 border routers to find each other. This field may occur multiple
 times in a level 2 LSP, and may occur in an LSP with any LSP number.
 The DP 10589 version of the OSI IS-IS does not currently allow
 addition of TLV-encoded variable length fields to Sequence Number
 Packets. However, this is being corrected in future versions of
 10589. In addition, this is expected to be the only correction to
 future versions of 10589 that is not backward-compatible with the DP
 version. The Integrated IS-IS therefore makes use of a corrected
 version of DP 10589, such that the encoding of SNPs has been fixed.
 The correct encoding of sequence number packets (as is expected to
 appear in future versions of ISO 10589) is given in Annex B of this
 specification.
 All IP-specific information is encoded in IS-IS packets as variable
 length fields. All variable length fields in IS-IS are encoded as
 follows:
                                       No. of Octets
        +---------------------------+
        |           CODE            |      1
        +---------------------------+
        |          LENGTH           |      1
        +---------------------------+
        |           VALUE           |      LENGTH
        +---------------------------+
      Figure 3 - Encoding of Variable Length Fields
 Any codes in a received PDU that are not recognised shall be ignored
 and, for those packets which are forwarded (specifically Link State
 Packets), passed on unchanged.
 In general, an IS-IS PDU may contain multiple variable length fields,
 some of which contain OSI-specific information (specified in [1]) and
 some of which contain IP-specific information (specified below).
 Except where explicitly stated otherwise, these variable length

Callon [Page 36] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

 fields may occur in any order.

5.3 Encoding of IP-Specific Fields in IS-IS PDUs

 This section specifies the detailed encoding of all IP-specific
 fields in IS-IS PDUs. Where a particular field may be present in more
 than one type of PDU, the field is repeated for each type of PDU to
 which it applies.
 Bit and octet numbering is the same as in [1]. In particular, octets
 in a PDU are numbered starting from 1, in increasing order. Bits in
 an octet are numbered from 1 to 8, where bit 1 is the least
 significant bit and is pictured on the right. When consecutive octets
 are used to represent a number, the lower octet number has the most
 significant value.

5.3.1 Level 1 LAN IS to IS Hello PDU

- Additional codes for IP support are:

7 Protocols Supported -- the set Network Layer Protocol Identifiers
  for Network Layer protocols that this Intermediate System is
  capable of relaying
  x CODE - 129
  x LENGTH - total length of the value field (one octet per
    protocol supported).
  x VALUE - one octet NLPID (as assigned by ISO/TR 9577) for
    each supported data protocol.
                                        No. of Octets
        +---------------------------+
        |           NLPID           |       1
        +---------------------------+
        :                           :
        :                           :
        |---------------------------|
        |           NLPID           |       1
        +---------------------------+
      NLPID - ISO/TR 9577 registered Network Layer Protocol Identifier.
7 IP Interface Address -- the IP address(es) of the interface
  corresponding to the SNPA over which this PDU is to be transmitted.
  x CODE - 132
  x LENGTH - total length of the value field (four octets per address).

Callon [Page 37] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

  x VALUE -
                                        No. of Octets
        +----------------------------+
        |         IP ADDRESS         |      4
        +----------------------------+
        :                            :
        :                            :
        +----------------------------+
        |         IP ADDRESS         |      4
        +----------------------------+
      IP ADDRESS - 4 octet IP Address of the Interface.
7 Authentication Information -- Information used to authenticate the
  PDU
  x CODE - 133
  x LENGTH - total length of the value field.
  x VALUE - TBD.

5.3.2 Level 2 LAN IS to IS Hello PDU

- Additional codes for IP support are:

7 Protocols Supported -- the set Network Layer Protocol Identifiers
  for Network Layer protocols that this Intermediate System is
  capable of relaying
  x CODE - 129
  x LENGTH  - total length of the value field (one octet per protocol
              supported).
  x VALUE - one octet NLPID (as assigned by ISO/TR 9577) for each
            supported data protocol.
                                        No. of Octets
        +----------------------------+
        |           NLPID            |      1
        +----------------------------+
        :                            :
        :                            :
        +----------------------------+
        |           NLPID            |      1
        +----------------------------+
      NLPID - ISO/TR 9577 registered Network Layer Protocol Identifier.
7 IP Interface Address -- The IP address(es) of the interface

Callon [Page 38] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

  corresponding to the SNPA over which this PDU is to be transmitted.
  x CODE - 132
  x LENGTH - total length of the value field (four octets per address).
  x VALUE -
                                   No. of Octets
        +----------------------------+
        |        IP ADDRESS          |      4
        +----------------------------+
        :                            :
        :                            :
        +----------------------------+
        |         IP ADDRESS         |      4
        +----------------------------+
      IP ADDRESS - 4 octet IP Address of the Interface.
7 Authentication Information -- Information used to authenticate
  the PDU
  x CODE - 133
  x LENGTH - total length of the value field
  x VALUE - TBD

5.3.3 Point-to-Point IS to IS Hello PDU

- Additional codes for IP support are:

7 Protocols Supported -- the set Network Layer Protocol Identifiers
  for Network Layer protocols that this Intermediate System is
  capable of relaying
  x CODE - 129
  x LENGTH - total length of the value field (one octet per protocol
             supported).
  x VALUE - one octet NLPID (as assigned by ISO/TR 9577) for each
            supported data protocol.

Callon [Page 39] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

                                   No. of Octets
        +----------------------------+
        |           NLPID            |      1
        +----------------------------+
        :                            :
        :                            :
        +----------------------------+
        |           NLPID            |      1
        +----------------------------+
      NLPID - ISO/TR 9577 registered Network Layer Protocol Identifier.
7 IP Interface Address -- The IP address(es) of the interface
  corresponding to the SNPA over which this PDU is to be transmitted.
  x CODE - 132
  x LENGTH - total length of the value field (four octets per address).
  x VALUE -
                                        No. of Octets
        +----------------------------+
        |         IP ADDRESS         |      4
        +----------------------------+
        :                            :
        :                            :
        +----------------------------+
        |         IP ADDRESS         |      4
        +----------------------------+
      IP ADDRESS - 4 octet IP Address of the Interface.
7 Authentication Information -- Information used to authenticate
  the PDU
  x CODE - 133
  x LENGTH - total length of the value field
  x VALUE - TBD

5.3.4 Level 1 Link State PDU

- Additional codes for IP support are:

7 Protocols Supported -- the set Network Layer Protocol Identifiers
  for Network Layer protocols that this Intermediate System is
  capable of relaying.
  This must appear once in LSP number 0.

Callon [Page 40] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

  x CODE - 129
  x LENGTH - total length of the value field (one octet per protocol
             supported).
  x VALUE - one octet NLPID (as assigned by ISO/TR 9577) for each
            supported data protocol.
                                        No. of Octets
        +----------------------------+
        |           NLPID            |      1
        +----------------------------+
        :                            :
        :                            :
        +----------------------------+
        |           NLPID            |      1
        +----------------------------+
      NLPID - ISO/TR 9577 registered Network Layer Protocol Identifier.
7 IP Interface Addresses -- The IP addresss of one or more interfaces
  corresponding to the SNPAs enabled on this Intermediate system
  (i.e., one or more IP addresses of this router).
  This is permitted to appear multiple times, and in an LSP with
  any LSP number.
  x CODE - 132
  x LENGTH - total length of the value field (four octets per address).
  x VALUE -
                                        No. of Octets
        +----------------------------+
        |         IP ADDRESS         |      4
        +----------------------------+
        :                            :
        :                            :
        +----------------------------+
        |         IP ADDRESS         |      4
        +----------------------------+
      IP ADDRESS - 4 octet IP Address
7 Authentication Information -- Information used to authenticate
  the PDU
  x CODE - 133
  x LENGTH - total length of the value field

Callon [Page 41] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

  x VALUE - TBD
7 IP Internal Reachability Information -- IP addresses within the
  routing domain reachable directly via one or more interfaces on
  this Intermediate system.
  This is permitted to appear multiple times, and in an LSP with any
  LSP number. However, this field must not appear in pseudonode LSPs.
  x CODE - 128.
  x LENGTH - a multiple of 12.
  x VALUE -
                                        No. of Octets
        +----------------------------+
        | 0 |I/E|   DEFAULT METRIC   |      1
        +----------------------------+
        | S | R |    DELAY METRIC    |      1
        +----------------------------+
        | S | R |   EXPENSE METRIC   |      1
        +----------------------------+
        | S | R |    ERROR METRIC    |      1
        +----------------------------+
        |         IP ADDRESS         |      4
        +----------------------------+
        |        SUBNET MASK         |      4
        +----------------------------+
        :                            :
        :                            :
        +----------------------------+
        | 0 |I/E|   DEFAULT METRIC   |      1
        +----------------------------+
        | S | R |    DELAY METRIC    |      1
        +----------------------------+
        | S | R |   EXPENSE METRIC   |      1
        +----------------------------+
        | S | R |    ERROR METRIC    |      1
        +----------------------------+
        |         IP ADDRESS         |      4
        +----------------------------+
        |        SUBNET MASK         |      4
        +----------------------------+
    DEFAULT METRIC is the value of the default metric for the link
    to the listed neighbor. Bit 8 of this field is reserved, and
    must be set to zero on tranmission and ignored on reception.
    Bit 7 of this field (marked I/E) indicates the metric type

Callon [Page 42] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

    (internal or external) for all four TOS metrics, and must be
    set to zero indicating internal metrics.
    DELAY METRIC is the value of the delay metric for the link to the
    listed neighbor. If this IS does not support this metric it shall
    set the bit "S" to 1 to indicate that the metric is unsupported.
    Bit 7 of this field is reserved, and must be set to zero on
    transmission and ignored on reception.
    EXPENSE METRIC is the value of the expense metric for the link to
    the listed neighbor. If this IS does not support this metric it
    shall set the bit "S" to 1 to indicate that the metric is
    unsupported. Bit 7 of this field is reserved, and must be set to
    zero on transmission and ignored on reception.
    ERROR METRIC is the value of the error metric for the link to
    the listed neighbor. If this IS does not support this metric it
    shall set the bit "S" to 1 to indicate that the metric is
    unsupported. Bit 7 of this field is reserved, and must be set
    to zero on transmission and ignored on reception.
    IP ADDRESS is a 4-octet Internet address
    SUBNET MASK is a 4 octet IP subnet mask.

5.3.5 Level 2 Link State PDU

- Additional codes for IP support are:

7 Protocols Supported -- the set Network Layer Protocol Identifiers
  for Network Layer protocols that this Intermediate System is
  capable of relaying.
  This must appear once in LSP number 0.
  x CODE - 129
  x LENGTH - total length of the value field (one octet per
    protocol supported).

Callon [Page 43] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

  x VALUE - one octet NLPID (as assigned by ISO/TR 9577) for
    each supported data protocol.
                                        No. of Octets
        +----------------------------+
        |           NLPID            |      1
        +----------------------------+
        :                            :
        :                            :
        +----------------------------+
        |           NLPID            |      1
        +----------------------------+
      NLPID - ISO/TR 9577 registered Network Layer Protocol Identifier.
7 IP Interface Addresses -- The IP addresss of one or more interfaces
  corresponding to the SNPAs enabled on this Intermediate system
  (i.e., one or more IP addresses of this router).
  This is permitted to appear multiple times, and in an LSP with
  any LSP number. Where a router is both a level 1 and level 2 router,
  it must include the same IP addresses in its level 1 and level 2 LSPs.
  x CODE - 132
  x LENGTH - total length of the value field (four octets per address).
  x VALUE-
                                        No. of Octets
        +----------------------------+
        |         IP ADDRESS         |      4
        +----------------------------+
        :                            :
        :                            :
        +----------------------------+
        |         IP ADDRESS         |      4
        +----------------------------+
      IP ADDRESS - 4 octet IP Address
7 Authentication Information -- Information used to authenticate
  the PDU
  x CODE - 133
  x LENGTH - total length of the value field
  x VALUE - TBD
7 IP Internal Reachability Information -- IP addresses within the
  routing domain reachable directly via one or more interfaces on

Callon [Page 44] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

  this Intermediate system.
  This is permitted to appear multiple times, and in an LSP with
  any LSP number. However, this field must not appear in pseudonode
  LSPs.
  x CODE - 128.
  x LENGTH -  a multiple of 12.
  x VALUE -
                                        No. of Octets
        +----------------------------+
        | 0 |I/E|   DEFAULT METRIC   |      1
        +----------------------------+
        | S | R |    DELAY METRIC    |      1
        +----------------------------+
        | S | R |   EXPENSE METRIC   |      1
        +----------------------------+
        | S | R |    ERROR METRIC    |      1
        +----------------------------+
        |         IP ADDRESS         |      4
        +----------------------------+
        |        SUBNET MASK         |      4
        +----------------------------+
        :                            :
        :                            :
        +----------------------------+
        | 0 |I/E|   DEFAULT METRIC   |      1
        +----------------------------+
        | S | R |    DELAY METRIC    |      1
        +----------------------------+
        | S | R |   EXPENSE METRIC   |      1
        +----------------------------+
        | S | R |    ERROR METRIC    |      1
        +----------------------------+
        |         IP ADDRESS         |      4
        +----------------------------+
        |        SUBNET MASK         |      4
        +----------------------------+
    DEFAULT METRIC is the value of the default metric for the link
    to the listed neighbor. Bit 8 of this field is reserved, and must
    be set to zero on transmission and ignored on reception. Bit 7
    of this field indicates the metric type (internal or external)
    for all four TOS metrics, and must be set to zero indicating
    internal metrics.

Callon [Page 45] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

    DELAY METRIC is the value of the delay metric for the link to
    the listed neighbor. If this IS does not support this metric it
    shall set the bit "S" to 1 to indicate that the metric is
    unsupported. Bit 7 of this field is reserved, and must be set
    to zero on transmission and ignored on reception.
    EXPENSE METRIC is the value of the expense metric for the link to
    the listed neighbor. If this IS does not support this metric it
    shall set the bit "S" to 1 to indicate that the metric is
    unsupported. Bit 7 of this field is reserved, and must be set
    to zero on transmission and ignored on reception.
    ERROR METRIC is the value of the error metric for the link to the
    listed neighbor. If this IS does not support this metric it shall
    set the bit "S" to 1 to indicate that the metric is unsupported.
    Bit 7 of this field is reserved, and must be set to zero on
    transmission and ignored on reception.
    IP ADDRESS is a 4-octet Internet address
    SUBNET MASK is a 4 octet IP subnet mask.
7 IP External Reachability Information -- IP addresses outside the
  routing domain reachable via interfaces on this Intermediate
  system.
  This is permitted to appear multiple times, and in an LSP with
  any LSP number. However, this field must not appear in pseudonode LSPs.
  x CODE - 130.
  x LENGTH - a multiple of 12.
  x VALUE -

Callon [Page 46] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

                                        No. of Octets
        +----------------------------+
        | 0 |I/E|   DEFAULT METRIC   |      1
        +----------------------------+
        | S | R |    DELAY METRIC    |      1
        +----------------------------+
        | S | R |   EXPENSE METRIC   |      1
        +----------------------------+
        | S | R |    ERROR METRIC    |      1
        +----------------------------+
        |         IP ADDRESS         |      4
        +----------------------------+
        |        SUBNET MASK         |      4
        +----------------------------+
        :                            :
        :                            :
        +----------------------------+
        | 0 |I/E|   DEFAULT METRIC   |      1
        +----------------------------+
        | S | R |    DELAY METRIC    |      1
        +----------------------------+
        | S | R |   EXPENSE METRIC   |      1
        +----------------------------+
        | S | R |    ERROR METRIC    |      1
        +----------------------------+
        |         IP ADDRESS         |      4
        +----------------------------+
        |        SUBNET MASK         |      4
        +----------------------------+
    DEFAULT METRIC is the value of the default metric for the
    path to the listed IP addresses. Bit 8 of this field is
    reserved, and must be set to zero on transmission and ignored
    on reception.  Bit 7 of this field indicates the metric type
    (internal or external) for all four TOS metrics, and may be
    set to zero indicating internal metrics, or may be set to 1
    indicating external metrics.
    DELAY METRIC is the value of the delay metric for the path
    to the listed IP addresses. If this IS does not support this
    metric it shall set the bit "S" to 1 to indicate that the metric
    is unsupported. Bit 7 of this field is reserved, and must be
    set to zero on transmission and ignored on reception.
    EXPENSE METRIC is the value of the expense metric for the link
    to the listed IP addresses. If this IS does not support this
    metric it shall set the bit "S" to 1 to indicate that the metric
    is unsupported.  Bit 7 of this field is reserved, and must be

Callon [Page 47] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

    set to zero on transmission and ignored on reception.
    ERROR METRIC is the value of the error metric for the link to
    the listed IP addresses. If this IS does not support this metric
    it shall set the bit "S" to 1 to indicate that the metric is
    unsupported. Bit 7 of this field is reserved, and must be set to
    zero on transmission and ignored on reception.
    IP ADDRESS is a 4-octet Internet address
    SUBNET MASK is a 4 octet IP subnet mask
7 Inter-Domain Routing Protocol Information -- Inter-domain routing
  protocol information carried transparently through level 2 for
  the convenience of any Inter-Domain protocol that may be running
  in the boundary ISs.
  This is permitted to appear multiple times, and in an LSP with
  any LSP number.
  x CODE - 131.
  x LENGTH - total length of the value field
  x VALUE -
                                           No. of Octets
        +-------------------------------+
        | Inter-Domain Information Type |      1
        +-------------------------------+
        |     External Information      |      VARIABLE
        +-------------------------------+
    INTER-DOMAIN INFORMATION TYPE indicates the type of the
    external information which is encoded in the field.
    EXTERNAL INFORMATION contains inter-domain routing protocol
    information, and is passed transparently by the IS-IS protocol.

5.3.6 Level 1 Complete Sequence Numbers PDU

- Additional codes for IP support are:

7 Authentication Information -- Information used to authenticate
  the PDU
  x CODE - 133
  x LENGTH - total length of the value field

Callon [Page 48] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

  x VALUE - TBD

5.3.7 Level 2 Complete Sequence Numbers PDU

- Additional codes for IP support are:

7 Authentication Information -- Information used to authenticate
  the PDU
  x CODE - 133
  x LENGTH - total length of the value field
  x VALUE - TBD

5.3.8 Level 1 Partial Sequence Numbers PDU

- Additional codes for IP support are:

7 Authentication Information -- Information used to authenticate
  the PDU
  x CODE - 133
  x LENGTH - total length of the value field
  x VALUE - TBD

5.3.9 Level 2 Partial Sequence Numbers PDU

- Additional codes for IP support are:

7 Authentication Information -- Information used to authenticate
  the PDU
  x CODE - 133
  x LENGTH - total length of the value field
  x VALUE - TBD

5.3.10 ISO 9542 ISH PDU

- Additional codes for IP support are:

7 Protocols Supported -- the set Network Layer Protocol Identifiers
  for Network Layer protocols that this Intermediate System is
  capable of relaying.

Callon [Page 49] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

  This appears in ISO 9542 ISH PDUs transmitted on point-to-point
  links.
  x CODE - 129
  x LENGTH - total length of the value field (one octet per
    protocol supported).
  x VALUE - one octet NLPID (as assigned by ISO/TR 9577) for
    each supported data protocol.
                                        No. of Octets
        +----------------------------+
        |           NLPID            |      1
        +----------------------------+
        :                            :
        :                            :
        +----------------------------+
        |           NLPID            |      1
        +----------------------------+
      NLPID - ISO/TR 9577 registered Network Layer Protocol Identifier.
7 Authentication Information -- Information used to authenticate
  the PDU
  x CODE - 133
  x LENGTH - total length of the value field
  x VALUE - TBD

6 Security Considerations

 The integrated IS-IS has a provision for carrying authentication
 information in all IS-IS packets. This is extensible to multiple
 authentication mechanisms. However, currently the only defined
 mechanism is a simple password, transmitted in the clear without
 encryption (see Annex D). The use of a simple password does not
 provide useful protection against intentional misbehavior. Rather,
 this should be thought of as a weak protection against accidental
 errors such as accidental mis-configuration. Definition of other
 authentication mechanisms is beyond the scope of this document.
 Other aspects of security are not discussed in this document.

Callon [Page 50] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

7 Author's Address

  Ross Callon
  Digital Equipment Corporation
  550 King Street, LKG 1-2/A19
  Littleton, MA 01460-1289
  508-486-5009

8 References

[1] "Intermediate System to Intermediate System Intra-Domain

      Routeing Exchange Protocol for use in Conjunction with the
      Protocol for Providing the Connectionless-mode Network Service
      (ISO 8473)", ISO DP 10589, February 1990.

[2] "Protocol for Providing the Connectionless-Mode Network

      Service", ISO 8473, March 1987.

[3] "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,
      March 1988.

[4] Braden,R., and Postel,J., "Requirements for Internet Gateways",

      RFC 1009, June 1987.

[5] Moy,J., "The OSPF Specification", RFC 1131, October 1989.

[6] Postel,J., "Internetwork Protocol", RFC 791, September 1981.

[7] Postel,J., "Internet Control Message Protocol", RFC 792,

      September 1981.

[8] "MIB for Use with the Extended OSI IS-IS in TCP/IP and Dual

      Environments", forthcoming.

[9] GOSIP Advanced Requirements Group, "Government Open Systems

      Interconnection Profile (GOSIP) Version 2.0 [Final Text]",
      Federal Information Processing Standard, U.S. Department of
      Commerce, National Institute of Standards and Technology,
      Gaithersburg, MD, October 1990.

[10] "Standard for Local Area Networks and Metropolitan Area

      Networks: Overview and Architecture of Network Standards",
      IEEE Standard 802.1a-1990.

Callon [Page 51] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

                              Annex A
             Inter-Domain Routing Protocol Information
 This annex specifies the contents and encoding of the Inter-Domain
 Routing Protocol Information (IDRPI) field. This annex is an integral
 part of the Integrated IS-IS specification. However, it is expected
 that this annex may be augmented or superceded by future efforts
 outside of the scope of the IS-IS specification.

A.1 Inter-Domain Information Type

 As specified in sections 3.4 and 5.3, the IDRPI field consists of a
 one-octet inter-domain information type field, plus a variable
 external information field. This section specifies initial values for
 the inter-domain information type field.  Other values for inter-
 domain information type will be assigned and maintained in future
 versions of the "Assigned Numbers" RFC.
 The following types have been assigned:
      Type = 0        reserved
      Type = 1        local (uses routing-domain specific format)
      Type = 2        AS Number Tag
 Type = 1 indicates that the inter-domain routing protocol information
 uses a format which is local to the routing domain.
 Type = 2 indicates that the inter-domain routing protocol information
 includes autonomous system information used to tag IP external
 reachability information. In this case the inter-domain routing
 protocol information entry must include a single AS number, which is
 used to tag all subsequent External IP Reachability entries until the
 end of the LSP, or until the next occurence of the Inter-Domain
 Routing Protocol Information field.

A.2 Encoding

 As specified in section 5.3.5, the IDPRI entry is encoded as a
 variable length field, as follows:
  x CODE - 131
  x LENGTH - total length of the value field
  x VALUE -

Callon [Page 52] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

                                           No. of Octets
        +-------------------------------+
        | Inter-Domain Information Type |      1
        +-------------------------------+
        |     External Information      |      VARIABLE
        +-------------------------------+
    INTER-DOMAIN INFORMATION TYPE indicates the type of the
    external information which is encoded in the field.
    EXTERNAL INFORMATION contains inter-domain routing protocol
    information, and is passed transparently by the IS-IS protocol.
 The Inter-domain information type field indicates the type of
 information which is contained in the external information field, as
 follow:
Type = 0 is reserved (must not be sent, and must be ignored on receipt).
Type = 1 indicates that the external information field contains
information which follows a locally specified format.
Type = 2 indicates that the external information field contains an
autonomous system number tag, to be applied to subsequent IP external
reachability information entries. In this case, this "inter-domain
routing protocol information" entry must contain precisely one 2
octet AS number. The AS tag is associated with subsequent IP External
Reachability entries, until the end of the LSP, or until the next
occurence of the Inter-Domain Routing Protocol Information field.
In this case, the VALUE contains the following:
  x VALUE -
                                             No. of Octets
        +---------------------------------+
        | Inter-Domain Information Type=2 |      1
        +---------------------------------+
        |   Autonomous System Number      |      2
        +---------------------------------+

Callon [Page 53] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

                              Annex B
                Encoding of Sequence Number Packets
 The Integrated IS-IS protocol defined in this specification makes use
 of the ISO Draft Proposed standard for Intra-domain routing (ISO DP
 10589 [1]) as the base routing protocol, upon which IP support may be
 added.
 However, DP 10589 contains a bug regarding encoding of the variable
 length fields in Sequence Number Packets. In particular, DP 10589
 encodes the variable length fields in SNPs in a manner which is not
 flexible (additional variable length fields cannot be defined for
 sequence number packets), and which is inconsistent with the encoding
 of the variable length fields in all other IS-IS and ES-IS packets.
 The encoding of the variable length fields in SNPs is expected to be
 fixed in future versions of 10589. Also, this bug represents the only
 expected change to 10589 which cannot be made backward compatible
 with existing DP 10589 implementations. For these reasons, the
 current version of the Integrated IS-IS will use the anticipated
 future encoding of the variable length part of the SNPs. This should
 allow future versions of this specification to be compatible with
 implementations based on this specification.
 This annex specifies the encoding of SNPs, as amended to fix the
 encoding of variable length fields. This annex is an integral part of
 the Integrated IS-IS specification.
 The encoding of SNPs for OSI-only use is shown in this section. For
 IP-only or Integrated use, the additional variable length fields
 specified in sections 5.3.6 through 5.3.9 are also applicable to
 SNPs.

Callon [Page 54] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

B.1 Level 1 Complete Sequence Numbers PDU

                                            No. of Octets
        +--------------------------------+
        |     INTRA-DOMAIN ROUTEING      |      1
        |     PROTOCOL DISCRIMINATOR     |
        +--------------------------------+
        |        LENGTH INDICATOR        |      1
        +--------------------------------+
        |    VERSION/PROTOCOL ID EXT     |      1
        +--------------------------------+
        |            RESERVED            |      1
        +--------------------------------+
        | R | R | R |        TYPE        |      1
        +--------------------------------+
        |            VERSION             |      1
        +--------------------------------+
        |              ECO               |      1
        +--------------------------------+
        |            USER ECO            |      1
        +--------------------------------+
        |           PDU LENGTH           |      2
        +--------------------------------+
        |           SOURCE ID            |      7
        +--------------------------------+
        |          START LSP ID          |      8
        +--------------------------------+
        |           END LSP ID           |      8
        +================================+====================
        |     VARIABLE LENGTH FIELDS     |      VARIABLE
        +--------------------------------+

- INTRADOMAIN ROUTEING PROTOCOL DISCRIMINATOR - architectural constant

- LENGTH INDICATOR - Header Length in octets (33.)

- VERSION/PROTOCOL ID EXTENSION - 1

- RESERVED - transmitted as 0, ignored on receipt

- TYPE (bits 1 through 5) - 24. Note bits 6, 7 and 8 are Reserved,

which means they are transmitted as 0 and ignored on receipt.

- VERSION - 1

- ECO - transmitted as zero, ignored on receipt

Callon [Page 55] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

- USER ECO - transmitted as zero, ignored on receipt

- PDU LENGTH - Entire Length of this PDU, in octets, including header

- SOURCE ID - 7 octet ID of Intermediate System (with zero Circuit ID)

generating this Sequence Numbers PDU.

- START LSP ID - 8 octet ID of first LSP in the range covered by this

Complete Sequence Numbers PDU.

- END LSP ID - 8 octet ID of last LSP in the range covered by this

Complete Sequence Numbers PDU.

- VARIABLE LENGTH FIELDS - fields of the form:

                                            No. of Octets
        +--------------------------------+
        |              CODE              |      1
        +--------------------------------+
        |             LENGTH             |      1
        +--------------------------------+
        |             VALUE              |      LENGTH
        +--------------------------------+

Any codes in a received CSNP that are not recognised are ignored.

Currently defined codes are:

7 LSP Entries -- This may appear multiple times. The option fields,
  if they appear more than once, shall appear sorted into ascending
  LSPID order.
  x CODE - 9
  x LENGTH - total length of the value field.
  x VALUE - a list of LSP entries of the form:

Callon [Page 56] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

                                            No. of Octets
        +--------------------------------+
        |       REMAINING LIFETIME       |      2
        +--------------------------------+
        |             LSP ID             |      8
        +--------------------------------+
        |         LSP SEQ NUMBER         |      4
        +--------------------------------+
        |            CHECKSUM            |      2
        +--------------------------------+
        :                                :
        :                                :
        +--------------------------------+
        |       REMAINING LIFETIME       |      2
        +--------------------------------+
        |             LSP ID             |      8
        +--------------------------------+
        |         LSP SEQ NUMBER         |      4
        +--------------------------------+
        |            CHECKSUM            |      2
        +--------------------------------+

7 REMAINING LIFETIME - Remaining Lifetime of LSP.

7 LSP ID - 8 octet ID of the LSP to which this entry refers.

7 LSP SEQ NUMBER - Sequence number of LSP.

7 CHECKSUM - Checksum reported in LSP.

The entries shall be sorted into ascending LSPID order (the LSP number octet of the LSPID is the least significant octet).

Callon [Page 57] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

B.2 Level 2 Complete Sequence Numbers PDU

                                            No. of Octets
        +--------------------------------+
        |     INTRA-DOMAIN ROUTEING      |      1
        |     PROTOCOL DISCRIMINATOR     |
        +--------------------------------+
        |        LENGTH INDICATOR        |      1
        +--------------------------------+
        |    VERSION/PROTOCOL ID EXT     |      1
        +--------------------------------+
        |            RESERVED            |      1
        +--------------------------------+
        | R | R | R |        TYPE        |      1
        +--------------------------------+
        |            VERSION             |      1
        +--------------------------------+
        |              ECO               |      1
        +--------------------------------+
        |            USER ECO            |      1
        +--------------------------------+
        |           PDU LENGTH           |      2
        +--------------------------------+
        |           SOURCE ID            |      7
        +--------------------------------+
        |          START LSP ID          |      8
        +--------------------------------+
        |           END LSP ID           |      8
        +================================+====================
        |     VARIABLE LENGTH FIELDS     |      VARIABLE
        +--------------------------------+

- INTRADOMAIN ROUTEING PROTOCOL DISCRIMINATOR - architectural constant

- LENGTH INDICATOR - Header Length in octets (33.)

- VERSION/PROTOCOL ID EXTENSION - 1

- RESERVED - transmitted as 0, ignored on receipt

- TYPE (bits 1 through 5) - 25. Note bits 6, 7 and 8 are Reserved,

which means they are transmitted as 0 and ignored on receipt.

- VERSION - 1

- ECO - transmitted as zero, ignored on receipt

Callon [Page 58] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

- USER ECO - transmitted as zero, ignored on receipt

- PDU LENGTH - Entire Length of this PDU, in octets, including header

- SOURCE ID - 7 octet ID of Intermediate System (with zero Circuit ID)

generating this Sequence Numbers PDU.

- START LSP ID - 8 octet ID of first LSP in the range covered by this

Complete Sequence Numbers PDU.

- END LSP ID - 8 octet ID of last LSP in the range covered by this

Complete Sequence Numbers PDU.

- VARIABLE LENGTH FIELDS - fields of the form:

                                            No. of Octets
        +--------------------------------+
        |              CODE              |      1
        +--------------------------------+
        |             LENGTH             |      1
        +--------------------------------+
        |             VALUE              |      LENGTH
        +--------------------------------+

Any codes in a received CSNP that are not recognised are ignored.

Currently defined codes are:

7 LSP Entries – this may appear multiple times. The option fields,

if they appear more than once, shall appear sorted into ascending
LSPID order.
x CODE - 9
x LENGTH - total length of the value field.
x VALUE - a list of LSP entries of the form:

Callon [Page 59] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

                                            No. of Octets
        +--------------------------------+
        |       REMAINING LIFETIME       |      2
        +--------------------------------+
        |             LSP ID             |      8
        +--------------------------------+
        |         LSP SEQ NUMBER         |      4
        +--------------------------------+
        |            CHECKSUM            |      2
        +--------------------------------+
        :                                :
        :                                :
        +--------------------------------+
        |       REMAINING LIFETIME       |      2
        +--------------------------------+
        |             LSP ID             |      8
        +--------------------------------+
        |         LSP SEQ NUMBER         |      4
        +--------------------------------+
        |            CHECKSUM            |      2
        +--------------------------------+

7 REMAINING LIFETIME - Remaining Lifetime of LSP.

7 LSP ID - 8 octet ID of the LSP to which this entry refers.

7 LSP SEQ NUMBER - Sequence number of LSP.

7 CHECKSUM - Checksum reported in LSP.

The entries shall be sorted into ascending LSPID order (the LSP number octet of the LSPID is the least significant octet).

Callon [Page 60] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

B.3 Level 1 Partial Sequence Numbers PDU

                                            No. of Octets
        +--------------------------------+
        |     INTRA-DOMAIN ROUTEING      |      1
        |     PROTOCOL DISCRIMINATOR     |
        +--------------------------------+
        |        LENGTH INDICATOR        |      1
        +--------------------------------+
        |    VERSION/PROTOCOL ID EXT     |      1
        +--------------------------------+
        |            RESERVED            |      1
        +--------------------------------+
        | R | R | R |        TYPE        |      1
        +--------------------------------+
        |            VERSION             |      1
        +--------------------------------+
        |              ECO               |      1
        +--------------------------------+
        |            USER ECO            |      1
        +--------------------------------+
        |           PDU LENGTH           |      2
        +--------------------------------+
        |           SOURCE ID            |      7
        +================================+====================
        |     VARIABLE LENGTH FIELDS     |      VARIABLE
        +--------------------------------+

- INTRADOMAIN ROUTEING PROTOCOL DISCRIMINATOR - architectural constant

- LENGTH INDICATOR - Header Length in octets (17.)

- VERSION/PROTOCOL ID EXTENSION - 1

- RESERVED - transmitted as 0, ignored on receipt

- TYPE (bits 1 through 5) 26. Note bits 6, 7 and 8 are Reserved,

which means they are transmitted as 0 and ignored on receipt.

- VERSION - 1

- ECO - transmitted as zero, ignored on receipt

- USER ECO - transmitted as zero, ignored on receipt

- PDU LENGTH - Entire Length of this PDU, in octets, including header

- SOURCE ID - 7 octet ID of Intermediate system (with zero Circuit ID)

Callon [Page 61] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

generating this Sequence Numbers PDU.

- VARIABLE LENGTH FIELDS - fields of the form:

                                            No. of Octets
        +--------------------------------+
        |              CODE              |      1
        +--------------------------------+
        |             LENGTH             |      1
        +--------------------------------+
        |             VALUE              |      LENGTH
        +--------------------------------+

Any codes in a received PSNP that are not recognised are ignored.

Currently defined codes are:

7 LSP Entries - this may appear multiple times. The option fields,

 if they appear more than once, shall appear sorted into ascending
 LSPID order.
 x CODE - 9
 x LENGTH - total length of the value field.

Callon [Page 62] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

 x VALUE - a list of LSP entries of the form:
                                            No. of Octets
        +--------------------------------+
        |       REMAINING LIFETIME       |      2
        +--------------------------------+
        |             LSP ID             |      8
        +--------------------------------+
        |         LSP SEQ NUMBER         |      4
        +--------------------------------+
        |            CHECKSUM            |      2
        +--------------------------------+
        :                                :
        :                                :
        +--------------------------------+
        |       REMAINING LIFETIME       |      2
        +--------------------------------+
        |             LSP ID             |      8
        +--------------------------------+
        |         LSP SEQ NUMBER         |      4
        +--------------------------------+
        |            CHECKSUM            |      2
        +--------------------------------+

7 REMAINING LIFETIME - Remaining Lifetime of LSP.

7 LSP ID - 8 octet ID of the LSP to which this entry refers.

7 LSP SEQ NUMBER - Sequence number of LSP.

7 CHECKSUM - Checksum reported in LSP.

The entries shall be sorted into ascending LSPID order (the LSP number octet of the LSPID is the least significant octet).

Callon [Page 63] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

B.4 Level 2 Partial Sequence Numbers PDU

                                            No. of Octets
        +--------------------------------+
        |     INTRA-DOMAIN ROUTEING      |      1
        |     PROTOCOL DISCRIMINATOR     |
        +--------------------------------+
        |        LENGTH INDICATOR        |      1
        +--------------------------------+
        |    VERSION/PROTOCOL ID EXT     |      1
        +--------------------------------+
        |            RESERVED            |      1
        +--------------------------------+
        | R | R | R |        TYPE        |      1
        +--------------------------------+
        |            VERSION             |      1
        +--------------------------------+
        |              ECO               |      1
        +--------------------------------+
        |            USER ECO            |      1
        +--------------------------------+
        |           PDU LENGTH           |      2
        +--------------------------------+
        |           SOURCE ID            |      7
        +================================+====================
        |    VARIABLE LENGTH FIELDS      |      VARIABLE
        +--------------------------------+

- INTRADOMAIN ROUTEING PROTOCOL DISCRIMINATOR - architectural constant

- LENGTH INDICATOR - Header Length in octets (17.)

- VERSION/PROTOCOL ID EXTENSION - 1

- RESERVED - transmitted as 0, ignored on receipt

- TYPE (bits 1 through 5) - 27. Note bits 6, 7 and 8 are Reserved,

which means they are transmitted as 0 and ignored on receipt.

- VERSION - 1

- ECO - transmitted as zero, ignored on receipt

- USER ECO - transmitted as zero, ignored on receipt

- PDU LENGTH - Entire Length of this PDU, in octets, including header

- SOURCE ID - 7 octet ID of Intermediate system (with zero Circuit ID)

Callon [Page 64] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

generating this Sequence Numbers PDU.

- VARIABLE LENGTH FIELDS - fields of the form:

                                            No. of Octets
        +--------------------------------+
        |              CODE              |      1
        +--------------------------------+
        |             LENGTH             |      1
        +--------------------------------+
        |             VALUE              |      LENGTH
        +--------------------------------+

Any codes in a received PSNP that are not recognised are ignored.

Currently defined codes are:

7 LSP Entries – this may appear multiple times. The option fields,

if they appear more than once, shall appear sorted into ascending
LSPID order.
x CODE - 9
x LENGTH - total length of the value field.
x VALUE - a list of LSP entries of the form:

Callon [Page 65] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

                                            No. of Octets
        +--------------------------------+
        |       REMAINING LIFETIME       |      2
        +--------------------------------+
        |             LSP ID             |      8
        +--------------------------------+
        |         LSP SEQ NUMBER         |      4
        +--------------------------------+
        |            CHECKSUM            |      2
        +--------------------------------+
        :                                :
        :                                :
        +--------------------------------+
        |       REMAINING LIFETIME       |      2
        +--------------------------------+
        |             LSP ID             |      8
        +--------------------------------+
        |         LSP SEQ NUMBER         |      4
        +--------------------------------+
        |            CHECKSUM            |      2
        +--------------------------------+

7 REMAINING LIFETIME - Remaining Lifetime of LSP.

7 LSP ID - 8 octet ID of the LSP to which this entry refers.

7 LSP SEQ NUMBER -Sequence number of LSP.

7 CHECKSUM - Checksum reported in LSP.

The entries shall be sorted into ascending LSPID order (the LSP number octet of the LSPID is the least significant octet).

Callon [Page 66] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

                              Annex C
                Dijkstra Calculation and Forwarding
 Annex C.2 of ISO DP 10589 [1] specifies the SPF (Dikskstra) algorithm
 for calculating routes with the IS-IS routing protocol. This annex
 specifies modifications to the SPF algorithm for supporting IP and
 dual routing, and specifies a compatible method for forwarding IP
 packets. This will result in an order of preference of routes which
 is compatible with that specified in section 3.10.
 This annex is included for informational purposes.

C.1 SPF Algorithm for IP and Dual Use

 This section specifies an SPF Algorithm for calculating routes with
 the IS-IS routing protocol, for support of both TCP/IP and OSI. This
 is based on an extention to the algorithm specified in annex C.2 of
 ISO DP 10589 [1].
 An algorithm invented by Dijkstra known as shortest path first (SPF)
 is used as the basis for the route calculation. It has a
 computational complexity of the square of the number of nodes, which
 can be decreased to the number of links in the domain times the log
 of the number of nodes for sparse networks (networks which are not
 highly connected).
 A number of additional optimizations are possible:
 1) If the routing metric is defined over a small finite field (as in
    this standard), the factor of log n may be removed by using data
    structures which maintain a separate list of systems for each value
    of the metric rather than sorting the systems by logical distance.
 2) Updates can be performed incrementally without requiring a complete
    recalculation. However, a full update must be done periodically to
    ensure recovery from data corruption, and studies suggest that with
    a very small number of link changes (perhaps 2) the expected
    computation complexity of the incremental update exceeds the
    complete recalculation. Thus, this annex specifies the algorithm
    only for the full update.
 3) If only End System LSP information has changed, it is not necessary
    to re-compute the entire Dijkstra tree. If the proper data
    structures are used, End Systems (including IP reachability
    entries) may be attached and detached as leaves of the tree and
    their forwarding information base entries altered as appropriate.
 The original SPF algorithm does not support load splitting over

Callon [Page 67] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

 multiple paths. The algorithm in this annex does permit load
 splitting by identifying a set of equal cost paths to each
 destination rather than a single least cost path.

C.1.1 Databases

PATHS -- This represents an acyclic directed graph of shortest paths
from the system S performing the calculation. It is stored as a set
of triples of the form <N,d(N),{Adj(N)}>, where:
    N is a system identifier. In the level 1 algorithm, N is a
    6 octet ID for OSI end systems, a 7 octet ID for routers, or
    an 8 octet IP Internal Reachability Information entry. For a
    router which is not a pseudonode, it is the 6 octet system ID,
    with a 0 appended octet. For a pseudonode it is a true 7 octet
    quantity, comprised of the 6 octet Designated Intermediate
    System ID and the extra octet assigned by the Destinated Router.
    The IP Internal Reachability Information entries consist of a
    4 octet IP address plus a 4 octet subnet mask, and will always
    be a leaf, i.e., "End System" in PATHS.
    In the level 2 algorithm, N is either a 7 octet router or
    pseudonode ID (as in the level 1 algorithm); a variable
    length OSI address prefix; an 8 octet IP Internal Reachability
    Information Entry, or an 8 octet IP External Reachability
    Information entry. The variable length OSI address prefixes,
    and 8 octet IP Reachability Information entries will always
    be a leaf, i.e., "End System" in PATHS. As above, the IP
    Reachability Information entries consist of an [IP address,
    subnet mask] combination.
    d(N) is N's distance from S (i.e., the total metric value
    from N to S).
    {Adj(N)} is a set of valid adjacencies that S may use for
    forwarding to N.
 When a system is placed on PATHS, the path(s) designated by its
 position in the graph is guaranteed to be a shortest path.
TENT -- This is a list of triples of the form <N,d(N),{Adj(N)}>,
where N, d(N), and {Adj(N)} are as defined above for PATHS.
TENT can intuitively be thought of as a tentative placement
of a system in PATHS. In other words, the triple <N,x,{A}>
in TENT means that if N were placed in PATHS, d(N) would be x,
but N cannot be placed on PATHS until is is guaranteed that
no path shorter than x exists.

Callon [Page 68] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

Similarly, the triple <N,x,{A,B}> in TENT means that if N
were placed in PATHS, then d(N) would be x via either
adjacency A or B.
 Note: It is suggested that the implementation maintain the database
 TENT as a set of list of triples of the form <*,Dist,*>, sorted by
 distance Dist. In addition, it is necessary to be able to process
 those systems which are pseudonodes before any non-pseudonodes at the
 same distance Dist.
 The 8 octet system identifiers which specify IP reachability entries
 must always be distinguishable from other system identifiers. As
 specified in section 3.10, two IP reachability entries which differ
 only in the subnet mask are still considered to be separate, and will
 therefore have distinct system identifiers N. The SPF algorithm will
 therefore calculate routes to each such entry, and the correct entry
 will be selected in the forwarding process.

C.1.2 Use of Metrics in the SPF Algorithm

 Internal metrics are not comparable to external metrics. For external
 routes (routes to destinations outside of the routing domain), the
 cost d(N) of the path from N to S may include both internal and
 external metrics. d(N) may therefore be maintained as a two-
 dimensioned vector quantity (specifying internal and external metric
 values).
 d(N) is initialized to [internal metric = 0, external metric = 0].
 In incrementing d(N) by 1, if the internal metric value is less than
 the maximum value MaxPathMetric, then the internal metric value is
 incremented by one and the external metric value left unchanged; if
 the internal metric value is equal to the maximum value
 MaxPathMetric, then the internal metric value is set to 0 and the
 external metric value is incremented by 1. Note that this can be
 implemented in a straightforward manner by maintaining the external
 metric as the high order bits of the distance.
 In the code of the algorithm below, the current path length is held
 in the variable "tentlength". This variable is a two-dimensional
 quantity tentlength=[internal metric, external metric], and is used
 for comparing the current path length with d(N) as described above.
 Tentlength is incremented in the same manner as d(N).

C.1.3 Overview of the Algorithm

 The basic algorithm, which builds PATHS from scratch, starts out by
 putting the system doing the computation on PATHS (no shorter path to

Callon [Page 69] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

 SELF can possibly exist). TENT is then pre-loaded from the local
 adjacency database.
 Note that a system is not placed on PATHS unless no shorter path to
 that system exists. When a system N is placed on PATHS, the path to
 each neighbor M of N, through N, is examined, as the path to N plus
 the link from N to M. If <M,*,*> is in PATHS, this new path will be
 longer, and thus ignored.
 If <M,*,*> is in TENT, and the new path is shorter, the old entry is
 removed from TENT and the new path is placed in TENT. If the new path
 is the same length as the one in TENT, then the set of potential
 adjacencies {Adj(M)} is set to the union of the old set (in TENT) and
 the new set {Adj(N)}. If M is not in TENT, then the path is added to
 TENT.
 Next the algorithm finds the triple <N,x,{Adj(N)}> in TENT, with
 minimal x. Note: This is done efficiently because of the optimization
 described above. When the list of triples for distance Dist is
 exhausted, the algorithm then increments Dist until it finds a list
 with a triple of the form <*,Dist,*>.
 N is placed in PATHS. We know that no path to N can be shorter than x
 at this point because all paths through systems already in PATHS have
 already been considered, and paths through systems in TENT still have
 to be greater than x because x is minimal in TENT.
 When TENT is empty, PATHS is complete.
 Note that external metrics can only occur in "IP External
 Reachability Information" entries, which correspond to a leaf (i.e.,
 End System in PATHS). Any route utilizing an entry with an external
 metric will always be considered to be less desireable than any entry
 which uses an internal metric. This implies that in the addition of
 systems to PATHS, all systems reachable via internal routes are
 always added before any system reachable via external routes.

C.1.4 The Algorithm

 The Decision Process Algorithm must be run once for each supported
 routing metric (i.e., for each supported Type of Service). A level 1
 router runs the algorithm using the level 1 LSP database to compute
 level 1 paths (for those level 1 routers which are not level 2
 routers, this includes the path to the nearest attached level 2
 router). Level 2 routers also separately run the algorithm using the
 level 2 LSP database to compute level 2 paths. IP-capable level 2
 routers must keep level 2 internal IP routes separate from level 2
 external IP routes.

Callon [Page 70] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

 Note that this implies that routers which are both level 1 and level
 2 routers, and which support all four routing metrics, must run the
 SPF algorithm 8 times (assuming partition repair is not implemented).
 If this system is a Level 2 Router which supports the partition
 repair optional function the Decision Process algorithm for computing
 Level 1 paths must be run twice for the default metric. This first
 execution is done to determine which of the area's
 manualAreaAddresses are reachable in this partition, and to elect a
 Partition Designated Level 2 Router for the partition. The partition
 Designated Level 2 Router will determine if the area is partitioned
 and will create virtual Level 1 links to the other Partition
 Designated Level 2 Routers in the area in order to repair the Level 1
 partition. This is further described in section 7.2.10 of [1].
 The SPF algorithm specified here will calculate routes for both OSI
 and IP. In particular, routes are calculated to all system
 identifiers N, where N may specify an OSI End System, the OSI address
 of a router, or an IP reachability entry. In computing the forwarding
 database, it is an implementation specific issue whether the IP
 forwarding database is kept separately from the OSI forwarding
 database. Where appropriate, this annex will refer separately to
 entries in these two forwarding data bases. This is not meant to
 preclude any specific implementation method.
 OSI and IP use separate mechanisms to determine whether a packet is
 in the area (in particular, OSI makes use of area addresses, and IP
 determines that a destination is not in an area by looking in the
 level 1 forwarding database and determining that no entry exists for
 that destination within the area). The route to the nearest level 2
 router will result in separate entries in the forwarding database for
 OSI and IP. For IP, the route to the nearest attached level 2 router
 may be entered in the forwarding database as a default route (i.e., a
 route with a subnet mask of all 0).
 One approach would be to put the results of each Dijkstra algorithm
 in a separate forwarding database. For a router which supports both
 level 1 and level 2 routing (including level 2 internal and level 2
 external routes), and which supports all four types of service, this
 would result in twelve separate forwarding databases for IP.
 Implementations may choose to minimize the number of forwarding
 databases by combining the information from the multiple Dijkstra
 calculations into a single database per supported TOS. This is
 discussed in section C.2 below.
 The SPF algorithm specified in section C.2.3 of [1] is amended to
 appear as follows:

Callon [Page 71] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

 Step 0: Initialize TENT and PATHS to empty. Initialize tentlength to
 [internalmetric=0, externalmetric=0].
 (tentlength is the pathlength of elements in TENT that we are
 examining.)
 1) Add <SELF,0,W> to PATHS, where W is a special value indicating
    traffic to SELF is passed up to internal processes (rather than
    forwarded).
 2) Now pre-load TENT with the local adjacency database (Each
    entry made to TENT must be marked as being either an End System
    or a router to enable the check at the end of Step 2 to be made
    correctly - Note that each local IP reachability entry is
    included as an adjacency, and is marked as being an End System).
    For each adjacency Adj(N) (including level 1 OSI Manual
    Adjacencies, or level 2 OSI enabled reachable addresses, and
    IP reachability entries) on enabled circuits, to system N of
    SELF in state "Up" compute:
       d(N) = cost of the parent circuit of the adjacency (N),
       obtained from metric.k , where k = one of {default metric,
       delay metric, monetary metric, error metric}
       Adj(N) = the adjacency number of the adjacency to N
 3) If a triple <N,x,{Adj(M)}> is in TENT, then:
       If x = d(N), then {Adj(M)} <--- {Adj(M)} U {Adj(N)}.
 4) If N is a router or an OSI End System entry, and there are now
    more adjacencies in {Adj(M)} than maximumPathSplits, then remove
    excess adjacencies as described in Clause 7.2.7 of [1]. If N
    is an IP Reachability Entry, then excess adjacencies may be
    removed as desired. This will not effect the correctness of
    routing, but may eliminate the determinism for IP routes (i.e.,
    IP packets still follow optimal routes within an area, but
    where multiple equally good routes exist, will not necessarily
    follow precisely the route that any one particular router
    would have anticipated).
 5) If x < d(N), do nothing.
 6) If x > d(N), remove <N,x,{Adj(M)}> from TENT and add the triple
    <N,d(N),{Adj(N)}>.
 7) If no triple <N,x,{Adj(M)}> is in TENT, then add <N,d(N),{Adj(N)}>
    to TENT.

Callon [Page 72] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

 8) Now add systems to which the local router does not have adjacencies,
    but which are mentioned in neighboring pseudonode LSPs. The
    adjacency for such systems is set to that of the designated router.
    Note that this does not include IP reachability entries from
    neighboring pseudonode LSPs. In particular, the pseudonode LSPs
    do not include IP reachability entries.
 9) For all broadcast circuits in state "On", find the pseudonode
    LSP for that circuit (specifically, the LSP with number zero and
    with the first 7 octets of LSPID equal to LnCircuitID for that
    circuit, where n is 1 (for level 1 routing) or 2 (level 2
    routing)). If it is present, for all the neighbors N reported in
    all the LSPs of this pseudonode which do not exist in TENT add
    an entry <N,d(N),{Adj(N)}> to TENT, where:
       d(N) = metric.k  of the circuit.
       Adj(N) = the adjacency number of the adjacency to the DR.
 10) Go to Step 2.
 Step 1: Examine the zeroeth link state PDU of P, the system just
 placed on PATHS (i.e., the LSP with the same first 7 octets of LSPID
 as P, and LSP number zero).
 1) If this LSP is present, and the "Infinite Hippity Cost" bit is
    clear, then for each LSP of P (i.e., all LSPs with the same
    first 7 octets of LSPID and P, irrespective of the value of
    LSP number) compute:
       dist(P,N) = d(P) + metric.k(P,N)
 for each neighbor N (both End System and router) of the system P. If
 the "Infinite Hippity Cost" bit is set, only consider the End System
 neighbors of the system P. Note that the End Systems neighbors of the
 system P includes IP reachable address entries included in the LSPs
 from system P. Here, d(P) is the second element of the triple
       <P,d(P),{Adj(P)}>
 and metric.k(P,N) is the cost of the link from P to N as reported in
 P's link state PDU.
 2) If dist(P,N) > MaxPathMetric, then do nothing.
 3) If <N,d(N),{Adj(N)}> is in PATHS, then do nothing.
       Note: d(N) must be less than dist(P,N), or else N would not
       have been put into PATHS. An additional sanity check may be

Callon [Page 73] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

       done here to ensure that d(N) is in fact less than dist(P,N)
 4) If a triple <N,x,{Adj(N)}> is in TENT, then:
   a) If x = dist(P,N), then {Adj(N)} <-- {Adj(N)} U {Adj(P)}.
   b) If N is a router or an OSI end system, and there are now more
      adjacencies in {Adj(N)} than maximumPath Splits, then remove
      excess adjacencies, as described in clause 7.2.7 of [1]. For
      IP Reachability Entries, excess adjacencies may be removed as
      desired. This will not effect the correctness of routing, but
      may eliminate the determinism for IP routes (i.e., IP packets
      will still follow optimal routes within an area, but where
      multiple equally good routes exist, will not necessarily follow
      precisely the route that any one particular router would have
      anticipated).
   c) if x < dist(P,N), do nothing.
   d) if x > dist(P,N), remove <N,x,{Adj(N)}> from TENT, and add
      <N,dist(P,N),{Adj(P)}>
 5) if no triple <N,x,{Adj(N)}> is in TENT, then add
    <N,dist(P,N),{Adj(P)}> to TENT.
 Step 2: If TENT is empty, stop. Else:
 1) Find the element <P,x,{Adj(P)}>, with minimal x as follows:
   a) If an element <*,tentlength,*> remains in TENT in the list for
      tentlength, choose that element. If there are more than one
      elements in the list for tentlength, choose one of the elements
      (if any) for a system which is a pseudonode in preference to one
      for a non-pseudonode. If there are no more elements in the list
      for tentlength, increment tentlength and repeat Step 2.
   b) Remove <P,tentlength,{Adj(P)}> from TENT.
   c) Add <P,d(P),{Adj(P)}> to PATHS.
   d) If this is the Level 2 Decision Process running, and the system
      just added to PATHS listed itself as Partition Designated Level 2
      Intermediate system, then additionally add <AREA.P,d(P),{Adj(P)}>
      to PATHS, where AREA.P is the Network Entity Title of the other
      end of the Virtual Link, obtained by taking the first AREA
      listed in P's LSP and appending P's ID.
   e) If the system just added to PATHS was an end system, go to

Callon [Page 74] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

      step 2. Else go to Step 1.
 NOTE - In the level 2 context, the "End Systems" are the set of
 Reachable Address Prefixes (for OSI), the set of Area Addresses with
 zero cost (again, for OSI), plus the set of IP reachability entries
 (including both internal and external).

C.2 Forwarding of IP packets

 The SPF algorithm specified in section C.1 may be used to calculate
 (logically) separate IP forwarding tables for each type of service,
 and for level 1, level 2 internal, and level 2 external routes.
 Section C.2.1 describes how to forward IP packets, based on these
 multiple forwarding databases. Section C.2.2 describes how the
 multiple forwarding databases can be combined into a single
 forwarding database per supported TOS.

C.2.1 Basic Method for Forwarding IP packets

 For level 1-only routers:
  1. Determine if the IP destination address matches any entry in the

level 1 forwarding table for the specified TOS.

  1. Determine if the IP destination address matches any entry in the

level 1 forwarding table for the default TOS.

  1. If default TOS resulted in more specific entry, forward according

to default TOS.

  1. If equally specific entries found, or specified TOS resulted in

more specific entry, forward according to specified TOS

  1. If no entry was found (which includes no default route entry), then

destination is unreachable.

 Note: For level 1 only routers, the route to the nearest attached
 level 2 router will be entered into the forwarding database as a
 default route (i.e., a route with a subnet mask which is all 0). Thus
 this last event (no entry found) can occur only if there is no
 attached level 2 router reachable in the area.
 For routers which are both level 1 and level 2 routers:
  1. Determine if the IP destination address matches any entry in the

level 1 forwarding table for the specified TOS.

  1. Determine if the IP destination address matches any entry in the

Callon [Page 75] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

   level 1 forwarding table for the default TOS.
  1. If default TOS resulted in more specific entry (i.e., more bits in

the subnet mask take the value 1), forward according to default TOS.

  1. If equally specific entries found, or specified TOS resulted in

more specific entry, forward according to specified TOS

  1. If no entry found:
  1. Determine if the IP destination address matches any entry in the

level 2 internal forwarding table for the specified TOS.

  1. Determine if the IP destination address matches any entry in the

level 2 internal forwarding table for the default TOS.

  1. If default TOS resulted in more specific entry, forward according

to default TOS.

  1. If equally specific entries found, or specified TOS resulted in

more specific entry, forward according to specified TOS

  1. If no entry found:
  1. Determine if the IP destination address matches any entry in the

level 2 external forwarding table for the specified TOS.

  1. Determine if the IP destination address matches any entry in the

level 2 external forwarding table for the default TOS.

  1. If default TOS resulted in more specific entry, forward according

to default TOS.

  1. If equally specific entries found, or specified TOS resulted in

more specific entry, forward according to specified TOS

  1. If no entry is found, then destination is unreachable
 For level 2-only routers, the above algorithm can be used, except
 since there is no level 1 forwarding database, the corresponding
 steps can be skipped.
 As discussed in section 3.10.2, for level 2 routers which are
 announcing manually configured summary addresses in their level 2
 LSPs, in some cases there will exist IP addresses which match the
 manually configured addresses, but which do not match any addresses
 which are reachable via level 1 routing in the area. Packets to such
 addresses are handled according to the rules specified in section

Callon [Page 76] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

 3.10.2. This may be accomplished by adding the manually configured
 [IP address, subnet mask] entry to the level 2 forwarding database
 (for the appropriate TOS), with a special "next hop" address which
 specifies that packets for which this entry is selected are to be
 discarded. This will work correctly because more desireable entries
 (such as delivering the packet via level 1 routing to the correct
 destination, or a more specific level 2 route) will automatically
 take precedence according to the forwarding rules specified above.
 Less desireable routes (such as using a level 2 external route to the
 "default route" entry) are not possible because other level 2 routers
 will believe the summary addresses advertised by this router.

C.2.2 Reduction of IP Forwarding Databases

 The multiple forwarding databases used in the basic forwarding method
 in section C.2.1 can be reduced, by combining the multiple databases
 into one database for each supported TOS.
 For reduction of IP forwarding databases, it is assumed that for any
 two overlapping address entries, either the entries are identical, or
 one range contains the other. In other words, for any two [IP
 address, subnet mask] entries A and B, if there is at least one IP
 address which matches both entries, then either: (i) the two entries
 are identical; or (ii) entry A contains entry B (i.e., any IP address
 which matches entry B also matches entry A); or (iii) entry B
 contains entry A (any IP address which matches entry A also matches
 entry B).
 Non-contiguous subnet masks can be configured to violate this
 assumption. For example, consider the two entries:
  1. A=[address="01010101 00000101 00000000 00000000",

mask="11111111 00001111 00000000 00000000"]

  1. B=[address="01010101 01010000 00000000 00000000",

mask="11111111 11110000 00000000 00000000"]

 In this case neither entry contains the other. Specifically;
  1. there are IP addresses which match both A and B (e.g.,

"01010101 01010101 xxxxxxxx xxxxxxxx"),

  1. there are IP addresses which match A but not B (e.g.,

"01010101 11110101 xxxxxxxx xxxxxxxx")

  1. there are IP addresses which match B but not A (e.g.,

"01010101 01011111 xxxxxxxx xxxxxxxx").

Callon [Page 77] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

 The reduction of the multiple forwarding databases for each TOS to a
 single database for each TOS is based on the use of "best match"
 routing, combined with reduction of the entries placed in the
 forwarding database in order to eliminate entries which are not to be
 selected (based on the order of preference of routes specified in
 section 3.10). The specific algorithm for creation of the IP
 forwarding database can be described as follows:
 1) Make use of the the Dijkstra algorithm described in section C.1 to
    create separate forwarding databases for each supported TOS for
    level 1 routes, level 2 internal routes, and level 2 external
    routes. (Note that each entry in the forwarding database will
    specify an [IP address, subnet mask] combination, as well as the
    next hop router for IP packets which match that entry).
 2) For each level 1 route entry which has been placed in the level 1
    IP forwarding database for a specific TOS, copy that entry into
    the overall IP forwarding database for that TOS.
 3) For each route entry X which has been placed in the level 2 internal
    IP forwarding database for a specific TOS, search for overlapping
    entries in the level 1 IP forwarding database for the specific TOS,
    and also for the default TOS:
    a) If there is any overlapping entry Y in the level 1 forwarding
       database (for the specfic TOS, or for the default TOS) such
       that either (i) Y contains X; or (ii) Y is identically specific
       to X; then ignore entry X.
    b) Otherwise, copy entry X into the overall IP forwarding database
       for the specific TOS.
 4) For each route entry X which has been placed in the level 2
    external IP forwarding database for a specific TOS, search for
    overlapping entries in the level 1 IP forwarding database for
    the specific TOS, and for the default TOS, and the level 2
    internal IP forwarding database for the specific TOS, and for
    the default TOS.
    a) If there is an overlapping entry Y such that either (i) Y
       contains X; or (ii) Y is identically specific to X; then
       ignore entry X.
    b) Otherwise, copy entry X into the overall IP forwarding database
       for the specific TOS.
 This method will result in one forwarding database for each supported
 TOS. The forwarding of packets can then be simplified to be as follows:

Callon [Page 78] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

 1) For IP packets which map to the default TOS metric (or to an
    unsupported TOS metric), search the default TOS forwarding
    database and select the entry which has the most specific match.
    Forward the packet accordingly.
 2) For packets which map to a specific (non-default) TOS metric,
    search the specific TOS forwarding database and select the entry
    j which has the most specific match. Also search the default TOS
    forwarding database and select the entry k which has the most
    specific match. Forward the packet as follows:
    a) If k is more specific than j, forward according to entry k
    b) If j and k are equally specific, forward according to entry j
    c) If j is more specific than k, forward according to entry j

Callon [Page 79] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

                                Annex D
                    Use of the Authentication Field
 The use of the Authentication field is outside of the scope of this
 specification. However, there is a urgent need for simple error
 detection/authentication mechanisms (such as a simple password) to
 protect against certain types of errors. This annex therefore
 proposes a possible use of this field.
 This annex is included for informational purposes.

D.1 Authentication Field in IS-IS packets

 All IS-IS packets may optionally include the authentication field, as
 described in sections 3.9 and 5 of this specification. As described
 in section 5, the authentication field is encoded as a (Code, Length,
 Value) triplet. This annex proposes that the contents of the Value
 field consist of a one octet "Authentication Type" field, plus a
 variable length "Authentication Information" field. A specific value
 of the "Authentication Type" is assigned to passwords, transmitted in
 the clear without encryption. The authentication field is encoded as
 follows:
7 Authentication Information -- Information used to authenticate
  the PDU
  x CODE - 133
  x LENGTH - total length of the value field
  x VALUE -
                                            No. of Octets
        +--------------------------------+
        |      Authentication Type       |      1
        +--------------------------------+
        |   Authentication Information   |      VARIABLE
        +--------------------------------+

The Authentication Type is assigned as follows:

    Type  =  0        reserved
    Type  =  1        simple password
    Type  >  1        reserved

Callon [Page 80] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

D.2 Authentication Type 1 - Simple Password

 Using this authentication type, a variable length password is passed
 in the clear (i.e., not encrypted) in the Authentication Information
 field.
 WARNING: The use of a simple password does not provide useful
 protection against intentional misbehavior. In particular, since the
 password is transmitted in the clear without encryption, it is easy
 for a hostile system to intercept the passwords, and to transmit
 authenticated packets. The use of simple passwords should be
 considered only as a weak protection against accidental errors such
 as accidental misconfiguration.
 The password shall be configured on a per-link, per-area, and per-
 domain basis. Specifically, when this form of authentication is used:
  1. IS-IS Hello and 9542 IS Hello packets shall contain the

per-link password

  1. Level 1 Link State Packets shall contain the per-area password
  1. Level 2 Link State Packets shall contain the per-domain password
  1. Level 1 Sequence Number Packets shall contain the per-area password
  1. Level 2 Sequence Number Packets shall contain the per-domain

password

 Also, each of these three passwords shall be configured with: (i)
 "Transmit Password", whose value is a single password, and (ii)
 "Receive Passwords", whose value is a set of passwords. The transmit
 password value is always transmitted. However, any password contained
 in the receive password set will be accepted on receipt. This method
 allows the graceful changing of passwords without temporary loss of
 connectivity.
 For example, consider the case that an area has the configured area
 password "OLDAREAPASSWORD". In this case, the per-area transmit
 password value is set to OLDAREAPASSWORD, and the per-area receive
 password value is set to {OLDAREAPASSWORD}. Suppose that it is
 desired to change the per-area password to "NEWERPASSWORD".  The
 first step would be to manually configure all of the routers in the
 area to set the per-area receive password value to {OLDAREAPASSWORD,
 NEWERPASSWORD}. When this step is complete, then all routers in the
 area will still be using the old password OLDAREAPASSWORD in their
 level 1 LSPs and SNPs. However, they will also accept the alternate
 password NEWERPASSWORD. The second step would be to configure the

Callon [Page 81] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

 routers in the area to set the per-area transmit password to
 NEWERPASSWORD. When the second step is complete, then all routers
 will be using the new value of the per-area password, but will accept
 the old value as well. Finally, the third step is to change all
 routers in the area to have the per-area receive password set to
 {NEWERPASSWORD}.
 By configuring transmit and receive values for the passwords in this
 manner, it is possible to maintain continuous correct operation. For
 example, in the middle of the second step in the above example, some
 of the routers in the area will be transmitting level 1 LSPs and SNPs
 using the old password OLDAREAPASSWORD, and some will be transmitting
 level 1 LSPs and SNPs using the new password NEWERPASSWORD. However,
 during the second step of the transition all routers in the area will
 accept level 1 LSPs and SNPs using either password.

Callon [Page 82] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

                                Annex E
           Interaction of the Integrated IS-IS with Brouters
 A "brouter" is a device which operates an both a bridge and a router.
 One possible type of brouter acts as a router for IP traffic, and
 acts as a bridge for all other types of traffic.
 Depending upon the manner in which a brouter is implemented, and
 depending upon the network topology, there is an obscure bug which
 can result from the interaction of the Integrated IS-IS protocol, and
 brouters. This appendix gives an example of the bug, and proposes a
 simple correction to the operation of brouters to correct the
 problem.
 This annex is included for informational purposes.

E.1 The Problem

 Suppose that we have a brouter which treats IP packets as if it were
 a normal IP router, and which treats all other packets as if it is a
 bridge.
 Suppose that two routers "X" and "Y" (which implement the integrated
 IS-IS protocol), two Ethernets, and a brouter B are all connected as
 follows:
                   |                               |
              +----+---+                      +----+---+
              | Router |                      | Router |
              |   X    |                      |   Y    |
              +----+---+                      +----+---+
                   |                               |
              -----+------------+-   -+------------+----
                                |     |
                              +-+-----+-+
                              | Brouter |
                              |    B    |
                              +---------+
 Here suppose that X and Y are running the Integrated IS-IS protocol,
 and are both level 1 routers in the same area. Thus X and Y send IS-
 IS Hello packets on the LAN. These Hello packets are received and
 forwarded by the brouter (using normal bridge functions). Similarly,
 X and Y receive each other's IS-IS LSP packets. In this way, it
 appears to the Brouter that X and Y are exchanging OSI packets, and
 so they are forwarded using normal bridge functions. It appears to X

Callon [Page 83] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

 and Y as if they are on the same LAN, and so they learn each others
 48-bit Ethernet addresses and exchange routing information.
 Now, suppose that X receives an IP packet, which it needs to forward
 via Y. Since X thinks that it and Y are on the same Ethernet, it just
 forwards the IP packet directly, using normal Ethernet encapsulation
 and using the 48-bit Ethernet address of Y as the destination address
 in the Ethernet header. Brouter B, when thinking as a bridge says:
 "this is an IP packet, I don't forward this as a bridge". Brouter B,
 when thinking like an IP router says: "this is an IP packet, I know
 how to forward IP packets. However, this is sent to an Ethernet
 address which is not me, thus I will ignore it". The result is that
 the IP packet does not get forwarded.
 This problem relates directly to the fact that X and Y are exchanging
 OSI packets to determine the connectivity of the path between them,
 but then are trying to send IP packets over the path. Also, there is
 a device between X and Y on the path which treats OSI and IP packets
 differently.
 Also note that this problem can also occur in more complex
 topologies, whenever a brouter is treating OSI and IP packets in a
 fundamentally different manner.

E.2 Possible Solutions

E.2.1 More Sophisticated Brouter

 One solution is that brouter B needs to be a little more
 sophisticated. In particular, it needs to use the following rules:
  1. For packets which are not IP packets, act as a bridge (this is the

same as before).

  1. For IP packets sent to an Ethernet broadcast or multicast address,

act as an IP router (this is also the same as before).

  1. For IP packets sent to my own Ethernet 48-bit address(es), act as

an IP router (this is also the same as before).

  1. For IP packets sent to a single station 48-bit address which is not

one of my addresses, act at a bridge (THIS IS NEW).

 With this change, the IP packet transmitted from X to Y is forwarded
 by the brouter, acting as a bridge. This allows the Brouter and the
 multiprotocol routers to interoperate properly.

Callon [Page 84] RFC 1195 OSI ISIS for IP and Dual Environments December 1990

E.2.2 Dual Router / Brouter

 An alternate solution would be for the Brouter to route both OSI and
 IP equally. If the Brouter used the integrated IS-IS for this
 purpose, then it could be part of the same routing domain and
 interoperate like any other dual router (except for the ability to
 bridge other protocol suites).  If it used other protocols for
 routing OSI and IP, then it would need to be part of another routing
 domain, and could interoperate with integrated IS-IS routers like any
 other external router.

Callon [Page 85]

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