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Versions: 00 01 02

Internet-Draft                      ARA                       November 1997


Expiration Date: May 1998
File name: draft-ietf-ospf-ara-01.txt






               The OSPF Address Resolution Advertisement Option






                                Rob Coltun
                               FORE Systems
                              (301) 571-2521
                             rcoltun@fore.com


                               Juha Heinanen
                              Telecom Finland
                             +358 400 500 958
                                jh@tele.fi






     Status Of This Memo

     This document is an Internet-Draft.  Internet-Drafts are working docu-
     ments of the Internet Engineering Task Force (IETF), its areas, and
     its working groups.  Note that other groups may also distribute work-
     ing documents as Internet-Drafts.

     Internet-Drafts are draft documents valid for a maximum of six months
     and may be updated, replaced, or obsoleted by other documents at any
     time.  It is inappropriate to use Internet- Drafts as reference
     material or to cite them other than as "work in progress".

     To learn the current status of any Internet-Draft, please check the
     "1id-abstracts.txt" listing contained in the Internet- Drafts Shadow
     Directories on ds.internic.net (US East Coast), nic.nordu.net
     (Europe), ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific Rim).




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     Table Of Contents

          1.0 Abstract ................................................. 4

          2.0 Overview ................................................. 4

          2.1 Address Resolution Advertisements ........................ 5

          2.2 ARA Association Table .................................... 5

          2.3 Logical Network ID List .................................. 5

          2.4 Routing Table Extensions ................................. 5

          2.5 Restricting Shortcut Connectivity ........................ 6

          2.6 Acknowledgments .......................................... 6

          3.0 A Brief Comparison Of Address Resolution Models .......... 7

          4.0 ARA Associations ......................................... 8

          5.0 Examples ................................................. 9

          5.1 Example 1: Intra-Area .................................... 9

          5.2 Example 2: Inter-Area .................................... 10

          5.3 Example 3: Multiple Logical Networks ..................... 11

          6.0 Description Of ARA Packet Formats ........................ 12

          6.1 Vertex Types And Vertex Identifiers ...................... 13

          7.0 Distribution Of ARA Information .......................... 14

          7.1 Originating Inter-Area ARAs .............................. 15

          8.0 ARA Routing Table Extensions ............................. 17

          8.1 Adding ARA Routing Table Extensions ...................... 18

          8.1.1 Modifications To The Intra-Area Route Calculation ...... 18

          8.1.2 Modifications To The Inter-Area Route Calculation ...... 19

          8.1.3 Modifications To The AS External Route Calculation ..... 20




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          9.0 Receiving ARAs ........................................... 21

          10.0 Additional Data Structures And APIs ..................... 21

          Appendix A: ARA Packet Formats ............................... 23

          A.1 The ARA Header ........................................... 23

          A.2 Intra-Area Router ARA .................................... 26

          A.3 Intra-Area Network ARA ................................... 27

          A.4 Inter-Area Router ARA .................................... 28

          A.5 Inter-Area Network ARA ................................... 30

          A.6 Vertex Association ....................................... 31

          A.7 Resolution Information ................................... 32

          A.7.1 ATM Address ............................................ 34

          A.7.2 ATM LIJ Call Identification ............................ 35

          References ................................................... 35


























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1.0  Abstract

     This document defines an OSPF option to enable routers to distribute
     IP to link-layer address resolution information.  An OSPF Address
     Resolution Advertisement (ARA) may include media-specific information
     such as a multipoint-to-point connection identifier along with the
     address resolution information to support media-specific functions.
     The ARA option can be used to support router-to-router inter-subnet
     shortcut architectures such as those described in [HEIN].


2.0  Overview

     Along with the evolution of switched layer 2 technologies comes the
     ability to provide inter-subnet shortcut data switching (bypassing
     router intervention).  Before the ingress devices is able to dynami-
     cally set up the switched path it must have the link-layer address of
     the egress device.  Acquisition of the egress device's link-layer
     address may be through configuration or through a dynamic mechanism
     which resolves an IP address (or an IP end-point identifier) to a
     link-layer address.

     This document introduces a method for IP to link-layer address resolu-
     tion to support router-to-router and router-to-network inter-subnet
     shortcuts.  The ARA option supports both topology-derived and data-
     driven shortcuts architectures with simple extensions to OSPF.  Dis-
     tribution of address resolution information is performed using stan-
     dard OSPF flooding mechanisms.  This document does not define an
     architecture but is meant to be used with architectures such as those
     defined in [HEIN].  The ARA option is designed to support the follow-
     ing operations.


          Shortcuts between core or access routers within ISP Backbones.

          Shortcuts in enterprise networks between routers in the same OSPF
          autonomous system, between OSPF internal routers and autonomous
          system border routers (ASBR) or between routers and servers.

          Distributed router architectures.

          Interoperation with ION NHRP and ATMF MPOA.

          Inter-subnet multicast shortcuts using LIJ or Point-to-MultiPoint
          procedures.






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2.1 Address Resolution Advertisements

     The ARA option defines a set of link-state advertisements called
     address resolution advertisements (ARAs).  ARAs are used to distribute
     link-layer information of routers and their directly connected net-
     works within and between OSPF areas.  ARA information is encapsulated
     in Opaque LSAs (see [OPAQ] for a further description of Opaque LSAs).
     Three LS Types (LS Type 9, 10 and 11) constitute the Opaque class of
     link-state advertisements.  Each of the three Opaque link-state types
     have a scope associated with them so that distribution of the informa-
     tion may be limited appropriately by the originator of the LSA.
     Because the flooding scope for ARAs is always area local, ARAs are
     encapsulated in LS Type 10 LSAs.  Opaque LSAs have a sub-type which
     identifies the specific information that is carried within the LSA.
     ARA uses Opaque-types 1, 2, 3 and 4.  See section 6.0 for a further
     description of the ARA packet formats.


2.2 ARA Association Table

     A router implementing the ARA option maintains a table of link-layer
     associations for each of its OSPF areas.  The ARA Association Table is
     used in calculating the ARA routing table extensions and by area
     border routers in the inter-area ARA origination process.  The indexes
     for an entry in this table entry are the Vertex Type, Vertex ID and
     the Vertex Originator.  The Vertex Type identifies the type of IP
     topology element that the link-layer information is being associated
     with (i.e., a router or a network).  The Vertex ID identifies a piece
     of the OSPF topology (i.e., a router ID or an IP network number).  The
     Vertex Originator is the ARA originator's Router ID.


2.3 Logical Network ID List

     An ARA capable router maintains a configured list of logical networks
     IDs.  This list represents the logical networks that a router is con-
     nected to and may be used to restrict the set of devices that the
     router may setup shortcuts to (see section 2.5 "Restricting Shortcut
     Connectivity").  The absence of entries in the router's list of Logi-
     cal Network IDs means that the router will only activate ARA Associa-
     tion Table entries with the default Logical Network ID (Logical Net-
     work ID 0).


2.4 Routing Table Extensions

     Associations are added to the routing table during the OSPF routing
     table calculation (see section 8.1 entitled "Adding ARA Routing Table



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     Extensions").  That is, in addition to the standard information fields
     contained in the routing table (IP network number, IP mask, next-hop
     interface, etc.), the routing table is extended to contain link-layer
     associations.  However, only 'active' link-layer associations are
     added to the routing table.  Associations containing a logical network
     ID that matches a currently enabled entry in the router's list of log-
     ical network IDs are considered to be active.  Both active and non-
     active link-layer associations may be included in inter-area ARAs that
     are originated by an ABR.

     The routing table (and its ARA routing table extensions) must be
     recalculated if 1) there is a change to the OSPF topology, 2) there is
     a change to the components in the ARA Association Table (see section
     9.0 "Receiving ARAs"), or 3) the router's logical network connectivity
     has changed (e.g., the logical network ID list is modified or the
     status of the router's connections to one of its logical networks has
     changed).

     The use of the routing table extensions are application specific and
     beyond the scope of this document.  See [HEIN] for an example of an
     ARA user application.


2.5 Restricting Shortcut Connectivity

     As a result of setting up shortcuts in an OSPF topology between ARA-
     capable routers, the shortcut connectivity may become fully meshed.
     In many environments this may be desirable whereas in in others this
     may be undesirable.  The ARA option allows for several methods to be
     used which can limit shortcut connectivity.

          o [HEIN] proposes that shortcuts are setup by ingress routers
          only after the sending data rate has passed a configured thres-
          hold.

          o ARA-capable routers may choose not to advertise their resolu-
          tion information until some event has occured.

          o Routers may be associated with "closed user shortcut groups" so
          that only routers that are within the same shortcut group may
          set-up shortcuts to each other. This is done by coordinating the
          configuration of a router's logical network ID list with the log-
          ical network ID advertised in ARA associations.


2.6 Acknowledgments

     The author would like to thank Atul Bansal, Lou Berger, Yiqun Cai,



Coltun, Heinanen                                                   [Page 6]


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     John Moy and the rest of the OSPF Working Group for the ideas and sup-
     port they have given to this project.


3.0 A Brief Comparison Of Address Resolution Models

     Current models of inter-subnet address resolution have taken the form
     of a query/response protocol as in the case of [NHRP].  In this model
     the ingress device originates a resolution request which is forwarded
     hop-by-hop through a series of NHRP servers towards the destination IP
     address contained in the request.  The the last-hop server (the one
     that is closest to the destination) responds to the request with the
     link-layer address that it associates with the requested IP address.
     The address that is returned may be the address of the requested host
     system or the address of a router which is on the path to the destina-
     tion.  Upon receiving a response to its request, the ingress device
     sets up a shortcut path to be used for data transfer.  The resolution
     request mechanism has the following characteristics.


          o Routers and hosts may participate in the request mechanism.
          The participating devices are discovered through polling.

          o The request mechanism requires polling by the ingress device to
          detect topology and reachability changes. Changes in the topology
          could result in packet loss for the polling interval.  Stable
          routing loops may form as a result of topology changes (given a
          limited set of failure conditions and topologies).

          o Requests are unreliable and are subject to packet loss.

          o It is recommended that the request mechanism be limited to
          intra-area shortcuts (although with correctly designed topologies
          this limitation may be over restrictive).

          o The target of a request may be a host or network addresses
          (excluding class D (multicast) networks).

          o The response to the request allows the requesting entity to set
          up a point-to-point shortcut.


     Given the above characteristics, the query-response protocol may not
     be the optimal mechanism for particular applications such as the one
     described in [HEIN]. The ARA option has the following characteristics.


          o Only routers participate in the ARA option.  A router's



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          participation in the ARA option is discovered through its address
          resolution advertisements.

          o The ARA option does not require polling by the ingress device
          to detect topology and reachability changes. Changes in the
          topology and system reachability may result in packet loss (or
          transient loops) for the OSPF convergence time.  Additionally,
          since topology changes are determined as a result of OSPF's SPF
          calculation (which results in loop-free paths), shortcuts derived
          from the ARA option can never result in stable routing loops.

          o Address resolution distribution is reliable and is not subject
          to packet loss.

          o The target of ARA derived shortcuts may be routers and and
          their connected networks within the OSPF autonomous system.
          Shortcuts are also supported when the destination is associated
          with an OSPF AS boundary router advertisement (e.g., networks
          external to the OSPF autonomous system).

          o The ARA option allows the requesting entity to set up point-
          to-point shortcuts as well as shortcuts that join point-to-
          multipoint and multipoint-to-point trees.

          o Routers that run the ARA option can interoperate with systems
          running NHRP.

          o The ARA option may easily be extended to support inter-subnet
          multicast shortcuts.


4.0 ARA Associations

     The ARA option defines four types of advertisements.  These include 1)
     intra-area router associations, 2) intra-area network associations, 3)
     inter-area network associations and 3) inter-area autonomous system
     boundary router (ASBR) associations.  Associations correspond to a
     piece of the OSPF topology.  Intra-area router associations correspond
     to link-layer reachability of a router within the local area, intra-
     area network associations correspond to the link-layer reachability of
     a router's directly connected network (also within the local area),
     inter-area network associations correspond to the link-layer reacha-
     bility of a remote area router's directly connected network, and
     inter-area ASBR associations correspond to ASBRs that are in remote
     OSPF areas.  Note that an inter-area network association may be ori-
     ginated by an area border router (ABR) only if the network is not a
     component of a configured net range.  An ingress router can use these
     associations as follows.



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          Intra-area router associations are used to setup shortcuts to
          routers within the local area.  Data sent over the shortcut will
          be forwarded to destinations local to and beyond the router
          including ones that are in the local area, in a remote area or
          external to the autonomous system.  Destinations that are "beyond
          the router" are determined by the OSPF topology map.

          Intra-area network associations (which may advertise hosts or
          networks) are used to setup intra-area shortcuts to systems whose
          addresses fall within the range of the advertised network.

          Inter-area network associations (which may advertise a host or
          network address) are used to setup inter-area shortcuts to sys-
          tems whose address fall within the range of the advertised net-
          work.

          Inter-area ASBR associations are used to setup shortcuts to ASBRs
          that are in a remote area.  These shortcuts are used to send data
          to destinations that are external to the autonomous system and
          reachable via the ASBR.


5.0 Examples

5.1 Example 1: Intra-Area

     Consider the sample single-area topology in Figure 1 below.  In this
     example RT1, RT2 and RT5 support the ARA option (by definition they
     also support the Opaque LSA option) and RT4 supports the Opaque LSA
     option only (this is necessary so that RT4 redistributes the ARAs ori-
     ginated by RT1, RT2 and RT5).  RT2 and RT5 have each originated a R-
     ARA with an intra-area router association and RT5 has originated a N-
     ARA with an intra-area network association for N5.

     As a result of running the routing table calculation, RT1 has entries
     for N1-N8 in its routing table.  The entry for N2 references the
     link-layer associations distributed in RT2's R-ARA, the entries for
     N3, N4, N6, N7, N8 references the link-layer associations distributed
     in RT5's R-ARA and the entry for N5 references the link-layer associa-
     tions distributed in RT5's intra-area N-ARA.











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                +   ARA
                |  +---+                   N3       N5 (ARA)
              N1|--|RT1|\                    \ N4  /
                |  +---+ \                    \ | /
                +         \                    \|/
                           \+---+            +---+
                            |RT4|------------|RT5|ARA
                            +---+            +---+
                 +   ARA   /                   |     N7
                 |  +---+ /                    |    /
               N2|--|RT2|/                     |   /
                 |  +---+    +---+           +---+/
                 +           |RT3|-----------|RT6|----N8
                             +---+           +---+
                               |
                               |
                          +---------+
                              N6


                      Figure 1: Sample Single-Area Toplogy



5.2 Example 2: Inter-Area

     Consider the sample 2-area topology in Figure 2 below.  In this exam-
     ple RT1, RT2, RT3, RT4, RT6 and RT7 support the ARA option, and RT5
     supports the Opaque option.  N4 is an AS external route (which is
     flooded to all areas) and RT6 is an ASBR.  RT4 is an area-border
     router and originates an LS Type-4 LSA on behalf of RT6 and a LS
     Type-3 LSA for N5 into area 1.1.1.1.

     Within area 1.1.1.1, RT1, RT2, RT3 and RT4 originate intra-area R-
     ARAs.  Within the backbone RT6 and RT7 originate intra-area R-ARAs and
     R7 originates a N-ARA for N5. All backbone ARAs of have their the P-
     bit set (this bit informs ABRs that the ARA may be propagated between
     areas).  RT4 originates an inter-area R-ARA for RT6 (which is an ASBR)
     as well as an inter-area N-ARA for N5 into area 1.1.1.1 RT4 does not
     originate an inter-area R-ARA for RT7 because it is not an ASBR.

     As a result of running the routing table calculation, RT1 has entries
     for N1-N5 in its routing table.  The entry for N2 references the
     link-layer associations distributed in RT3's R-ARA, the entry for N3
     references the link-layer associations distributed in RT4's intra-area
     R-ARA, the entry for N4 references the link-layer associations distri-
     buted in RT4's inter-area R-ARA (indirectly referencing RT6's R-ARA)
     and the entry for N5 references the link-layer associations



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     distributed in RT4's inter-area N5 N-ARA.



             +   ARA      ARA       |
             |  +---+    +---+      |
           N1|--|RT1|----|RT2|\     |        N3          N4<ASE
             |  +---+    +---+ \    |     +-----+        /
             +                  \  ARA       |      ARA /
                                 \+---+    +---+   +---+
                                  |RT4|----|RT5|---|RT6|<ASBR
                                  +---+    +---+   +---+
                       +   ARA   /  |                |
                       |  +---+ /   |               ARA    +
                     N2|--|RT3|/    |              +---+   |
                       |  +---+     |              |RT7|---|N5(ARA)
                       +            |              +---+   |
            ------------------------|--------------------  +
                Area 1.1.1.1        |    OSPF Backbone

                      Figure 2: Sample Area Toplogy


5.3 Example 3: Multiple Logical Networks

     The ARA option supports the existence of disjoint switched networks
     within an OSPF domain. To accomplish this, an ARA may include an iden-
     tifier (the logical network ID) for a specific switched network.  When
     associations are added to the routing table during the OSPF routing
     table calculation (see the section 8.1 "Adding ARA Routing Table
     Extensions") only the associations that include a logical network ID
     that matches one of the router's configured logical network IDs are
     added to the routing table.  This function may also be used to support
     a variation of closed user groups so that shortcuts are limited to
     those routers that are configured to be in the same logical network.

     The single-area topology described in Figure 3 below divides an OSPF
     area into logical networks X and Y.  In this example RT1, RT2 and RT4
     support the ARA option and RT3 supports the Opaque LSA option only.
     RT1 is connected to logical network (LN) X, RT2 is connected LN Y and
     RT4 is connected to both LN X and LN Y.  RT1, RT2 and RT4 all ori-
     ginate R-ARAs.

     As a result of running their routing table calculation, RT1 and RT2
     have entries for N1-N5 in their routing table.  In both routing
     tables, the N3-N5 entries reference the link-layer associations dis-
     tributed in RT4's R-ARA.  However, RT1's routing table does not refer-
     ence RT2's link-layer associations for N2 and RT2's routing table does



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     not reference RT1's link-layer associations for N1 (i.e., they would
     not be able to set up shortcuts to each other and would be forced to
     use a hop-by-hop path to communicate).


                +   ARA (LN=X)
                |  +---+                   N3       N5
              N1|--|RT1|\                    \ N4  /
                |  +---+ \                    \ | /
                +         \                    \|/
                           \+---+            +---+
                            |RT3|------------|RT4| ARA (LN=X,Y)
                            +---+            +---+
                            /
                 +   ARA (LN=Y)
                 |  +---+ /
               N2|--|RT2|/
                 |  +---+
                 +

              Figure 3: Sample Toplogy With Logical Networks




6.0 Description Of ARA Packet Formats

     ARA LSAs (ARAs) include the information necessary to associate an IP
     entity (i.e., a router, network or host) with a link-layer address.
     The ARA option allows further refinement so that an association may
     additionally include information about QoS control services and link-
     layer functionality (e.g., for Point-to-MultiPoint and MultiPoint-to-
     point connections).  ARA advertisements may also include a logical
     network identifier field, which is used when multiple switched net-
     works are present within the OSPF domain.

     The ARA format allows more than one equivalent association to been
     advertised by a router for a specific vertex.  Equivalent associations
     are ones that have identical link service type, integrated service
     type and logical network identifier fields, but have different resolu-
     tion information.  Associations can include a preference which identi-
     fies the advertising router's relative preference for the equivalent
     associations.

     ARA information is encapsulated in Opaque LSAs.  Three LS Types (LS
     Type 9, 10 and 11) constitute the Opaque class of link-state adver-
     tisements.  Each of the three Opaque link-state types have a scope
     associated with them so that distribution may be limited appropriately



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     by the originator of the LSA.  Opaque LSAs have a sub-type which iden-
     tifies the specific information that is carried within the LSA.  The
     ARA Opaque types are Opaque-type 1 - 4.  Because the flooding scope
     for ARAs is always area local, ARAs are encapsulated in LS Type 10
     LSAs.

6.1 Vertex Types And Vertex Identifiers

     The Vertex Type identifies the piece of IP topology that the link-
     layer information is being associated with.  The Vertex Type may be a
     router or a network (a host is considered a network with a mask of
     255.255.255.255).

     Vertex Type 1 ARAs advertise intra-area router resolution associa-
     tions.  These associations distribute the router's link-layer attach-
     ments.  A Vertex Type of 1 is identified by an Opaque type of 1.  The
     Vertex Identifier for a R-ARA is the advertising router field in the
     ARA header.

     Vertex Type 2 ARAs advertise intra-area IP network address resolution
     associations.  These associations distribute the link-layer associa-
     tions for a router's directly connected network.  A Vertex Type of 2
     is identified by an Opaque type of 2.  The Vertex Identifier (the net-
     work and mask) for a N-ARA is contained in the body of the advertise-
     ment.  N-ARAs may only contain a single network (i.e., lists of net-
     works are not permitted).

     Vertex Type 3 ARAs advertise inter-area IP network address resolution
     associations.  These associations are used to distribute link-layer
     associations for networks into remote areas.  A Vertex Type of 3 is
     identified by an Opaque type of 3.  The Vertex Identifier (the network
     and mask) for a inter-area N-ARA is contained in the body of the
     advertisement.  N-ARAs may only identify a single network (i.e., lists
     of networks are not permitted).  Vertex Type 3 N-ARAs are originated
     by an area border router (ABR) into an area when 1) the ABR originates
     a type-3 LSA for the network into the target area, 2) the network is
     not included in any of the area border router's configured area
     ranges, 3) there is a N-ARA for the network in the source area, 4) the
     source N-ARA may be an intra or inter-area N-ARA.  If it is an intra-
     area N-ARA the P-bit must be set in its options field.  The setting of
     the P-bit by the originator denotes that the associations contained in
     the N-ARA are allowed to be propagated into other areas.

     Vertex Type 4 ARAs advertise inter-area router address resolution
     associations. These R-ARAs redistribute associations for ASBRs into
     remote areas.  A Vertex Type of 4 is identified by an Opaque type of
     4.  The Vertex Identifiers for an inter-area R-ARA are the advertising
     router field of the ARA header and the ASBR Router ID found in the



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     body of the ARA.  Vertex Type 4 R-ARAs are originated by an area
     border router (ABR) into a target area when 1) the ABR originates a
     type-4 LSA for the ASBR into the target area, 2) there is a R-ARA for
     the network in the source area, 3) the source R-ARA may be an intra or
     inter-area R-ARA.  If the source R-ARA is an intra-area R-ARA its P-
     bit must be set in the options field. The setting of the P-bit by the
     originator denotes that the associations contained in the R-ARA are
     allowed to be propagated into other areas.

     If a router wishes to advertise several associations for a single ver-
     tex it has two options.  It may originate multiple (N or R) ARAs each
     containing different associations or it may originate a single (N or
     R) ARA containing a list of associations.  An implementation must not
     include identical associations in more than one ARA.


7.0 Distribution Of ARA Information

     In general, OSPF is composed of two components.  It's transport com-
     ponent handles adjacency formation and reliable distribution of topol-
     ogy information.  The second component tracks topology changes and
     organizes the topology information that has been gathered from other
     routers into to a topology map. This map is used to build the router's
     routing table.  The ARA option uses both the OSPF transport component
     and of the topology map component.

     ARA uses the OSPF Opaque LSA as defined in [OPAQ] for distribution of
     resolution information. The Opaque LSA is an optional mechanism to
     allow for distribution of opaque information which may be used
     directly by OSPF or by other protocols and mechanisms.  Opaque LSAs
     use the standard OSPF link-state database flooding mechanisms for dis-
     tribution.  Each of the three Opaque types (LS Types 9, 10 and 11)
     have a scope associated with them (link-local, area-local or domain-
     wide, respectively). Scoping provides an application with a method to
     limit the range of information distribution.  ARA information is dis-
     tributed with area-local scope (i.e., ARA information is encapsulated
     in LS Type 10 LSAs).

     The ARA option uses the topology map component of OSPF to validate the
     information that is received by the distribution mechanism and to
     install the associations into the ARA routing table extensions.  Vali-
     dation is necessary because topology information contained in the OSPF
     link-state database may be stale (e.g., the originator of the informa-
     tion is no longer reachable).

     It is envisioned that an implementor designs an ARA user application
     interface which facilitates 1) flooding of ARA information to other
     routers in the OSPF network, 2) receiving ARA information from other



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     routers in the OSPF network and 3) determines the validity (and change
     of validity) of ARA information.

     For the realization of 1 above, an implementation must provide an API
     to facilitate the ARA user application's "hand off" of resolution
     information to its local OSPF entity which will then be distributed
     throughout the OSPF topology.  In addition, the API must support the
     purging of associations that were previously originated by the router
     if they are no longer valid and send out new versions when the associ-
     ation information has changed.

     For the realization of 2 and 3 above, this document extends the rout-
     ing table to include the associations that have been advertised by the
     ARA capable routers (i.e,. the routing table provides the API for the
     ARA user application).  That is, in addition to the standard informa-
     tion fields contained in the routing table (i.e., IP network number,
     IP mask, next-hop interface, etc.), the routing table is extended to
     contain link-layer associations. The associations are added to the
     routing table during the OSPF routing table calculation.  Section 8.0
     defines the mechanism to calculate the ARA routing table extensions.
     The use of the extensions are ARA user application specific and beyond
     the scope of this document.  See [HEIN] for an example of an ARA user
     application.


7.1 Originating Inter-Area ARAs

     Inter-area ARAs provide a mechanism to distribute link-layer associa-
     tions to other areas.  Inter-area ARAs (consisting of Vertex Type-3
     and Type-4 ARAs) have a one-to-one correspondence to Summary LSAs (LS
     type-3 and type-4 LSAs).  Vertex Type-3 ARAs advertise the link-layer
     associations of IP networks whereas Vertex Type-4 ARAs advertise the
     link-layer associations of autonomous system boundary routers (ASBR).
     As with Summary LSAs, inter-area ARAs are originated by area border
     routers into a target area based on a set of conditions in the source
     area.  For both intra and inter-are ARAs, there may be more than one
     ARA which collectively make up the complete set of link-layer associa-
     tions (recall that an implementation must not include identical asso-
     ciations in more than one ARA).  Inter-area ARAs must include, in one
     or more ARA, all of the link-layer associations contained in their
     'trigger' ARAs (see below for a description of the conditions for ABRs
     to trigger inter-area ARAs).

     The link-layer associations that comprise the 'trigger' ARAs (in the
     source area) may include logical network IDs that are not in the ABR's
     configured list of logical network IDs (i.e., the ABR itself may not
     be able to set up a shortcut because it may be connected to a disjoint
     set of logical networks). Despite the ABR's logical network



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     affiliation, all trigger ARAs' link-layer associations are included in
     the newly originated inter-area ARAs.


     The origination process for type-3 and type-4 Summary LSAs (as dis-
     cussed in section 12.4.3 of [OSPF]) consists of an ABR evaluating each
     entry in the routing table.  If an entry satisfies a set of condi-
     tions, the ABR originates a Summary LSA into the target area.

     This process is extended for inter-area ARA origination so that when a
     Summary LSA is originated into an area by an ABR, the conditions for
     the origination of inter-area ARAs are also evaluated. When these con-
     ditions are satisfied, an inter-area ARA is originated into the target
     area.  Conversely, when a Summary route is withdrawn from an area by
     an ABR and a corresponding ARA was previously originated into the
     area, the ARA must be withdrawn from the target area.  The following
     sections describe the conditions for inter-area ARA origination.

     The conditions for inter-area N-ARA origination are as follows.

          o The ABR is originating a type-3 LSA for a network into the tar-
          get area.  The network and mask that triggered the origination of
          the type-3 LSA must be identical to the network and mask of the
          type-3 LSA.  (i.e., the 'trigger network' is not included in the
          area border router's configured area ranges).

          o There are one or more reachable N-ARAs for the network in the
          source area.  These N-ARAs 1) must be valid (e.g., their ages
          must not be MaxAge), 2) must have the same advertising router as
          the LSA that triggered the origination of the type-3 LSA and 3)
          the Vertex Type must correspond to the 'trigger' network's LSA
          type (recall that only the contents of intra-area ARAs are adver-
          tised into the backbone, whereas the contents of intra-area or
          inter-area ARAs may be advertised into the other areas).  These
          conditions are verified by looking up an entry in source area's
          ARA Association Table.  The Vertex Type is 2 if the 'trigger'
          network is an intra-area LSA and is Vertex Type 3 if the
          'trigger' network is an inter-area LSA. The Vertex Identifier is
          the 'trigger' network's IP network number and mask.  The Vertex
          Originator is the router ID of the trigger network's originator.

          o The set of link-layer associations that are to be included in
          the advertisement are contained in the ARA Association Table
          entry.  However, if the network that triggered the origination of
          the type-3 LSA is an intra-area route, only the link-layer asso-
          ciations whose ARA's P-bit were set may be advertised.  (if no
          associations have their P-bit set the inter-area N-ARA must not
          be originated).  The setting of the P-bit in the N-ARA by its



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          originator gives the ABRs permission to propagate the resolution
          information into other areas.


     Inter-area R-ARAs redistribute link-layer associations for ASBRs to
     other areas. Inter-area R-ARAs have a Vertex Type of 4.  The Vertex
     Identifiers for an inter-area R-ARA are 1) the advertising router
     field of the ARA header and 2) the ASBR's Router ID which is found in
     the body of the ARA.  Vertex Type-4 R-ARAs are originated by an area
     border router (ABR) into an area if the following conditions are met.

          o The ABR originates a type-4 LSA for the ASBR into the target
          area.

          o Only the contents of intra-area ARAs are advertised into the
          backbone, whereas the contents of intra-area or inter-area ARAs
          may be advertised into the other areas.  If the router advertise-
          ment that triggered the origination of a type-4 LSA is an intra-
          area advertisement (i.e., a type-1 LSA) then there must be a
          corresponding intra-area R-ARA in the source area.

          o If the router advertisement that triggered the origination of
          the type-4 LSA is also a type-4 LSA (the source area is the OSPF
          backbone), there must be a corresponding inter-area R-ARA in the
          source area.

          o The set of link-layer associations that are to be included in
          the advertisement are contained in the ARA Association Table
          entry.  However, if the router advertisement that triggered the
          origination of the type-3 LSA is an intra-area route, only the
          link-layer associations whose ARA's P-bit are set may be adver-
          tised in the newly originated inter-area R-ARA (if no associa-
          tions have their P-bit set the inter-area N-ARA must not be ori-
          ginated).  The setting of the P-bit in the R-ARA by its origina-
          tor gives the ABRs permission to propagate the resolution infor-
          mation into other areas.


8.0 ARA Routing Table Extensions

     OSPF determines reachability and topology changes by performing the
     algorithms described in the section 16 of [OSPF] entitled "Calculation
     of the routing table".  ARAs are included in this calculation for the
     purpose of binding link-layer associations to IP routing table
     entries.

     A link-layer association consists of the list of link-layer addresses,
     link-layer service types and other link-layer objects such as Point-



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     to-MultiPoint call identifiers and QoS service specific information
     (see Appendix A for a more complete description of the specific link-
     layer information distributed in ARAs).  The associations that are
     bound to a routing table entry are the associations that are 1)
     closest to the destination and 2) are on the same logical network as
     the calculating router (as identified by the logical network ID).  The
     closest associations are determined during to the construction of the
     OSPF topology map.  The associations that are bound to the routing
     table entries are subsequently used by the ARA user application to
     setup shortcut paths.

     Because a link-layer association may be bound to more than one entry
     in the routing table, an ARA implementation keeps a table of ARA
     derived link-layer associations which is referenced by the routing
     table entry.  Each area has its own ARA Association table.  An entry
     in the ARA Association Table consists of a list of all association for
     a specific vertex and vertex type by a specific originator; the lookup
     keys for an entry in the table include the Vertex Type, Vertex ID and
     the Vertex Originator.


8.1 Adding ARA Routing Table Extensions

     Section 16 of the OSPF specification is modified for the purpose of
     adding the ARA routing table extensions.  Transit vertex data struc-
     tures and the internal representation of Type-3, Type-4 and Type-5
     LSAs are extended to be able to reference a list of link-layer associ-
     ations (i.e., they have a reference to the ARA Association Table).
     The vertex and LSA's list of link-layer associations are added to the
     routing table along with the entry.

     Prior to running the intra-area route calculation the ARA Association
     Table is examined.  Associations containing a logical network ID that
     matches an entry in the router's list of logical network IDs are
     marked 'active'.


8.1.1 Modifications To The Intra-Area Route Calculation

     The intra-area route calculation is enhanced (specifically section
     16.1 step 3) as follows.

          o Call the vertex that is about to be added to the SPF tree ver-
          tex M. If vertex M was originated by the calculating router skip
          this procedure.

          o If vertex M is a transit network vertex lookup the link-layer
          association entry in the ARA Association Table.  This entry's



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          Vertex Type will be 2, the Vertex Identifier will be vertex M's
          network and mask, the Vertex Originator will be vertex M's Router
          ID and its area ID will be the one that is associated with the
          shortest-path calculation.

          o If an active entry is found, reference this entry in vertex M's
          link-layer association field.

          o If vertex M is a router vertex lookup the an entry in the ARA
          Association Table.  This entry's Vertex Type will be 1, the Ver-
          tex Identifier will be vertex M's advertising router, the Vertex
          Originator will be vertex M's advertising router and its area ID
          will be the one that is associated with the shortest-path calcu-
          lation.

          o If an an active entry is found, reference this entry in vertex
          M's link-layer association field.

          o If no active link-layer association entries are found, and ver-
          tex M's parent vertex has link-layer association information,
          vertex M inherits it's parent vertex's information (else the
          information field is left blank).

          o When vertex M is added to the routing table, copy the active
          associations from vertex M's link-layer association list into the
          routing table entry's link-layer association field.


     The following describes the enhancements to section 16.1 step 2 of
     [OSPF] which adds intra-area stub networks to the routing table.


          o Before adding the stub network to the routing table lookup the
          entry in the ARA Association Table.  This entry's Vertex Type
          will be 2, the Vertex Identifier will consist of the network and
          mask of the stub network, the Vertex Originator will be the
          advertising router's Router ID and its area ID will be the one
          that is associated with the shortest-path calculation.

          o If an active entry is found copy the entry's active association
          information into the routing table entry's link-layer association
          field.

          o If an entry is not found and the stub network's advertising
          router vertex has link-layer association information, the routing
          table entry will inherit the advertising router's information
          (else the information field is left blank).




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8.1.2 Modifications To The Inter-Area Route Calculation

     The following describes the enhancements to OSPF sections 16.2, 16.3,
     16.5 which calculate inter-area routes. Before the destination associ-
     ated with the LSA is added to the routing table the following is per-
     formed.


          o If the LSA is a Type-3 Summary LSA, lookup the entry in the ARA
          Association Table. This entry's Vertex Type will be 3, the Vertex
          Identifier will be the Summary LSA's network and mask, the Vertex
          Originator will be the advertising router's Router ID and its
          area ID will be the one that is associated with the shortest-path
          calculation.  If an active entry is found copy the entry's active
          association information into the routing table entry's link-layer
          association field.

          o If the LSA is a Type-4 Summary LSA, lookup a type-4 ARA in the
          ARA Association Table.  This entry's Vertex Type will be 4, the
          Vertex Identifier will be the Summary LSA's ASBR ID, the Vertex
          Originator will be the advertising router's Router ID and its
          area ID will be the one that is associated with the shortest-path
          calculation.  If an active entry is found copy the entry's active
          association information into the routing table entry's link-layer
          association field.

          o If an active entry was not found for the type-3 or type-4 LSA,
          locate the area border router (ABR) that originated the adver-
          tisement. If link-layer association information is available for
          the ABR entry, copy the contents of the ABR's link-layer associa-
          tion information field into the routing table entry's link-layer
          association field.  If no active entry was found for the ABR the
          routing table entry's information field will be left blank.


8.1.3 Modifications To The AS External Route Calculation


     The following describes the enhancements to OSPF sections 16.4 and
     16.6 which calculate AS external routes. Before the destination asso-
     ciated with the LSA is added to the routing table the following is
     performed.

          o If the LSA has a forwarding address, look up the forward
          address in the routing table (this will be an internal OSPF
          route).  Copy the contents of the route's link-layer association
          information field into the external route's routing table entry's
          link-layer association field.  The forwarding address' link-layer



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          association information may have been added as a result of pro-
          cessing intra-area or inter-area N-ARAs.

          o If the LSA does not have a forwarding address, copy the con-
          tents of the advertising ASBR's link-layer association informa-
          tion field into the routing table entry's link-layer association
          field.  The ASBR's link-layer association information may have
          been added as a result of processing intra-area or inter-area R-
          ARAs.




9.0 Receiving ARAs

     After the ARA has been processed according to section 13 of [OSPF] the
     ARA has been determined to be 1) a new ARA, 2) a newer instance of an
     existing ARA with the same contents, 3) a newer instance with dif-
     ferent contents, or 4) an ARA that is being withdrawn by it's origina-
     tor.  Actions need to be taken if the ARA is new, the contents of the
     ARA have changed or the ARA is being withdrawn.

          o Lookup the entry for the ARA in the ARA Association Table.  If
          there is no existing entry, create one which contains the associ-
          ations found in the ARA.  The newly added associations should
          reflect the state of the ARA's P-bit.

          o If the there is an existing entry and the newly received ARA
          contents have changed modify the entry to reflect the associa-
          tions found in the newly received ARA.  The changed associations
          should reflect the state of the ARA's P-bit.

          o If the ARA is being withdrawn and there is an existing entry,
          remove the associations from the link-layer entry that were pre-
          viously included in the ARA. If the contents of the table entry
          is now empty remove the entry from the table.

     If the above process has resulted in a modification to the ARA table,
     the SPF calculation must be rescheduled.  (see section 8.1 entitled
     "Adding ARA Routing Table Extensions").  If the receiving router is an
     ABR the inter-area origination process must be scheduled to be run
     following the SPF calculation (see section 7.1 entitled "Originating
     Inter-area ARAs").




10.0 Additional Data Structures And APIs



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     This section lists the additional data structures and APIs needed to
     support the OSPF ARA option.

          o The implementation must support the Opaque LSA option as
          defined in [OPAQ].

          o A configuration flag to enable the OSPF ARA option.

          o A router implementing the ARA option maintains a table of
          link-layer associations for each of its OSPF areas.  The ARA
          Association Table is used in calculating the ARA routing table
          extensions and in the inter-area ARA origination process.  The
          indexes for an entry in this table entry are the Vertex Type,
          Vertex ID and the Vertex Originator.  The Vertex Type identifies
          the type of IP topology element that the link-layer information
          is being associated with (i.e., a router or a network).  The Ver-
          tex ID identifies a piece of a specific OSPF network's topology
          (i.e., a router ID or an IP network number).  The Vertex Origina-
          tor is the originator of the ARA's router ID.  Entries in this
          table may be either active or non-active.  Active entries are
          ones whose Logical Network IDs match one of the router's config-
          ured (and currently active) Logical Network IDs.

          o Transit vertex data structures and the internal representation
          of Type-3, Type-4 and Type-5 LSAs are extended to contain a
          reference to the an entry in the ARA Association Table.

          o The routing table is extended to contain a reference to the an
          entry in the ARA Association Table.

          o To be able to flood ARA information to other ARA capable
          routers an implementation must provide an API which allows the
          ARA user application to have its local OSPF entity distribute
          resolution information in ARA format (if the scope is area-local,
          a reference to the area must also be supplied).  Additionally,
          the API must allow for associations to be withdrawn when they are
          no longer valid and for new versions of associations to be ori-
          ginated when association information has changed.

          o A router running the ARA option may be configured with a list
          of logical network IDs. This list is used when the router calcu-
          lates the link-layer associations for its routing table and when
          receiving ARAs to determine the change in active status for its
          ARA Association Table entries.  Status information is kept for
          each of the router's attached logical network so that a router
          can determine which logical networks are active at a given point
          in time.  To insure that ARA reachability is up-to-date, a change
          in status of one of the router's connected logical networks must



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          result in the SPF calculation being rerun.

          The absence of entries in the router's list of Logical Network
          IDs means that the router will only activate ARA Association
          Table entries with the default Logical Network ID which is Logi-
          cal Network ID 0.

          A router may originate ARAs with Logical Network IDs that are not
          contained in its list of Logical Network IDs. This may be used,
          for example, to enable shortcuts to be set up from any router to
          any server but to disable shortcuts from being set up between
          routers that are not associated with a server.




Appendix A: ARA Packet Formats

     This document defines four different types of Address Resolution
     Advertisements. Each type of ARA begins with a standard 20-byte Opaque
     LSA header [OPAQ]. This header is described in section A.1. Subsequent
     sections describe the specific advertisements and their content
     including the formats of the resolution information.  An ARA capable
     router may use the ARAs to build shortcut paths to other ARA capable
     routers.

     Each ARA describes a link-layer association for a piece of the OSPF
     routing domain. Any router may originate intra-area router and network
     ARAs.  These ARAs advertise address resolution information for routers
     and networks within the local area and are advertised locally (they
     have an area-local scope).

     Area border routers may originate inter-area network and router ARAs.
     These ARAs advertise address resolution to areas that are beyond the
     source local area. Inter-area network and router ARAs correspond to LS
     Type-3 and LS Type-4 advertisements.

A.1 The ARA Header

     All ARAs begin with a common 20-byte header. This header contains
     enough information to uniquely identify the ARA.  The header, which is
     a subset of the standard LSA header, includes the ARA Vertex Type and
     distribution scope.  The Vertex Type is derived from the Opaque Type
     field; the distribution scope is derived from the LS type field. ARAs
     have an area-local scope (LS Type = 10).

     The Vertex Identifier for an intra-area Router ARA is the advertising
     router field of the ARA header; for inter-area Router ARAs the vertex



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     is identified by the advertising router field and the ASBR Router ID
     field which is in the body of the advertisement.

     The Vertex Identifier for both intra and inter-area Network ARAs is
     contained in the network and mask field (which is in the body of the
     advertisement).  A N-ARA may only identify a single network (i.e.,
     lists of networks are not permitted).

     ARAs make use of the P-bit in the same as the NSSA option [NSSA].
     That is, ARAs may not be advertised beyond area borders unless the P-
     bit is set in the original intra-area ARA.  See the section entitled
     "Originating Inter-Area ARAs" for a further discussion on this topic.

     All of a router's associations for a specific vertex may be described
     in a single ARA or they may distributed over several ARAs.  That is, a
     router may originate multiple (N or R) ARAs each containing different
     associations or may originate a single (N or R) ARA containing a list
     of associations.




        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |            LS age             |     Options   |   LS Type     |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | Opaque Type   |               Opaque ID                       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                      Advertising Router                       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                      LS Sequence Number                       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |         LS checksum           |           Length              |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+




          LS age
               The time in seconds since the ARA was originated.

          Options
               The optional capabilities supported by the described portion
               of the routing domain. The ARA uses two option bits.

                    O-bit
                         This bit describes the router's willingness to



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                         receive and forward Opaque-LSAs as specified in
                         [OPAQ]. All routers supporting the ARA option as
                         described in this document support the Opaque
                         option.

                    P-Bit
                         ARAs make use of the P-Bit in a manner consistent
                         with [NSSA].  An ARA may not be advertised beyond
                         an area border unless the P-bit is set in the ori-
                         ginal intra-area ARA.

               The remainder of OSPF's optional capabilities are documented
               in Section A.2 of [OSPF].

          LS Type
               The type of the LSA. Each LSA type has a separate advertise-
               ment format. The ARA LSA as defined in this document are LS
               Type-10 advertisements (they all have an intra-area scope).

          Opaque Type
               The link-state ID of the Opaque LSA is divided into an
               Opaque Type field (the first 8 bits) and an Opaque ID (the
               remaining 24 bits).  The Address Resolution Advertisements
               are Opaque-types 1 - 4.  The Opaque Type field identifies
               the Vertex Type.

               Opaque Type-1 advertisements are intra-area Router Address
               Resolution Advertisements and contain link-layer associa-
               tions for the advertising router.  These ARAs are advertised
               throughout the local area.

               Opaque Type-2 advertisements are intra-area Network Address
               Resolution Advertisements and contain link-layer associa-
               tions for a router's directly connected IP networks (or
               hosts).  These ARAs are advertised throughout the local
               area.

               Opaque Type-3 advertisements are inter-area Network Address
               Resolution Advertisements and contain link-layer associa-
               tions for IP networks that reside in other areas.  Inter-
               area N-ARAs are coordinated with inter-area network (LS
               Type-3) advertisements.

               Opaque-type 4 advertisement are inter-area Router Address
               Resolution Advertisements and contain link-layer associa-
               tions for ASBR that reside in other areas.  Inter-area R-
               ARAs are coordinated with inter-area ASBR (LS Type-4) adver-
               tisements.



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          Opaque ID
               A 24-bit semantic-less LSA identifier which serves to dif-
               ferentiate between multiple LSAs originated by the same
               router.  The Opaque ID must be unique for an advertising
               router within the advertising scope of the LSA.

          Advertising Router
               The Router ID of the router that originated the ARA.  For
               intra-area R-ARAs the Advertising Router also serves as the
               ARA Vertex Identifier.

          LS Sequence Number
               Detects old or duplicate ARAs.  Successive instances of an
               ARA are given successive LS sequence numbers.  See Section
               12.1.6 of [OSPF] for more details.

          LS Checksum
               The Fletcher checksum of the complete contents of the ARA,
               including the ARA header but excluding the LS age field. See
               Section 12.1.7 of [OSPF] for more details.

          Length
               The length in bytes of the ARA.  This includes the 20 byte
               ARA header.




A.2 Intra-Area Router ARAs

     Opaque Type-1 advertisements are intra-area Router Address Resolution
     Advertisements and contain associations for the advertising router.

     If the originating router is an ASBR and wishes to have the contents
     of the R-ARA distributed beyond the local area (i.e., translated into
     an inter-area R-ARA), the R-ARA must have the P-bit set in its Options
     field.




        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |            LS age             |     Options   |       10      |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |       1       |               Opaque ID                       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



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       |                      Advertising Router                       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                      LS Sequence Number                       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |         LS checksum           |           Length              |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +-+                   Vertex Association                      +-+
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                    ...
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +-+                   Vertex Association                      +-+
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     The body of the R-ARA consists of a list of associations for the
     advertising router.  Each Vertex Association begins with a common 6-
     byte header (described in Section A.6) followed by association-
     specific resolution information (described in Section A.7).




A.3 Intra-Area Network ARAs

     Opaque Type-2 advertisements are intra-area Network Address Resolution
     Advertisements and contain associations for one of the advertising
     router's directly connected IP networks (or hosts).

     If the originating router is wishes the contents of the N-ARA are to
     be distributed beyond the local area (i.e., translated into an inter-
     area N-ARA) the N-ARA must have the P-bit set in its Options field.




        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |            LS age             |     Options   |      10       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |       2       |               Opaque ID                       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                      Advertising Router                       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                      LS Sequence Number                       |



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       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |         LS checksum           |           Length              |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                      IP Network Number                        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                      IP Network Mask                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +-+                   Vertex Association                      +-+
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                    ...
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +-+                   Vertex Association                      +++
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+




          IP Network Number
               One of the router's directly connect network.  This number
               represents an IP network/subnet/supernet.

          IP Network Mask
               A 32-bit number indicating the range of IP addresses resid-
               ing on a single IP network/subnet/supernet.

     The body of the N-ARA consists of a list of associations for this IP
     Network Number.  Each Vertex Association begins with a common 6-byte
     header (described in Section A.6) followed by association-specific
     resolution information (described in Section A.7).




A.4 Inter-Area Network ARAs

     Opaque Type-3 advertisements are inter-area Network Address Resolution
     Advertisements and contain associations for a remote area's IP net-
     works (or hosts).  Inter-area N-ARAs are coordinated with LS type-3
     advertisements.

     Inter-area network ARAs are originated by an area border router into a
     target area if 1) the ABR originates a type-3 LSA for the network into
     the target area, 2) the network is not included in any of the area
     border router's configured area ranges, 3) there is an N-ARA for the



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     network in the source area and 4) the source N-ARA is an intra-area
     N-ARA with a P-bit set in the options field (which denotes that the
     originator of the N-ARA will allow the N-ARA to be propagated into
     other areas) or the source N-ARA is an inter-area N-ARA.




        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |            LS age             |     Options   |      10       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |       3       |               Opaque ID                       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                      Advertising Router                       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                      LS Sequence Number                       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |         LS checksum           |           Length              |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                      IP Network Number                        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                      IP Network Mask                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +-+                   Vertex Association                      +-+
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                    ...
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +-+                   Vertex Association                      +++
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+




          IP Network Number
               One of the router's directly connect network.  This number
               represents an IP network/subnet/supernet.

          IP Network Mask
               A 32-bit number indicating the range of IP addresses resid-
               ing on a single IP network/subnet/supernet.

     The body of the N-ARA consists of a list of associations for this IP



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     Network Number.  Each Vertex Association begins with a common 6-byte
     header (described in Section A.6) followed by association-specific
     resolution information (described in Section A.7).




A.5 Inter-Area Router ARAs

     Opaque Type-4 advertisements are inter-area Router Address Resolution
     Advertisements and contain associations for the an autonomous system
     boundary router.  Inter-area R-ARAs are coordinated with LS type-4
     advertisements.

     Inter-area router ARAs are originated into a target area if 1) the ABR
     originates a type-4 LSA for the ASBR into the target area, 2) there is
     a R-ARA for the ASBR in the source area and 3) the source R-ARA is an
     intra-area R-ARA with a P-bit set in the options field (which denotes
     that the originator of the R-ARA will allow the R-ARA to be propagated
     into other areas) or the source R-ARA is an inter-area R-ARA.




        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |            LS age             |     Options   |       10      |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |       4       |               Opaque ID                       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                      Advertising Router                       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                      LS Sequence Number                       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |         LS checksum           |           Length              |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       ASBR Router ID                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +-+                   Vertex Association                      +-+
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                    ...
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +-+                   Vertex Association                      +-+
       |                                                               |



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       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+




          ASBR Router ID
               The router ID of the ASBR being advertised. This field
               corresponds to the link-state ID of the LS type-4 advertise-
               ment.

     The body of the inter-area R-ARA consists of a list of associations
     for the advertising router.  Each Vertex Association begins with a
     common 6-byte header (described in Section A.6) followed by
     association-specific resolution information (described in Section
     A.7).




A.6 Vertex Association

     The Vertex Association field consists of the link service type,
     IntServ service name, administrative weight, association length, logi-
     cal network ID followed by the association-specific resolution infor-
     mation.




        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | Link Svc Type |  IS Svc Name  | Admin Weight  | Assoc Length  +
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |       Logical Network ID      |     Resolution Information    +
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       + Remaining Octets of Resolution Information padded to 32-bits  +
       |                              ...                              |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+




          Link Svc Type
               Identifies the link-layer functionality for this associa-
               tion.  Link Service Types 1, 2 and 3 are defined by this
               specification.  All other Link Service Types are reserved



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               for definition by the IANA (iana@isi.edu). The current list
               of Link Service Types is described below in Table 1.



                  Link Service Type       Description
                  -------------------------------------------------
                  1                       ATM Point-To-Point
                  2                       ATM MultiPoint-To-Point
                  3                       ATM Point-To-MultiPoint



                                  Table 1

          IS Svc Name
               The IntServ Service Name. Refer to [IS] as a reference for
               the IETF defined service specifications.

          Admin Weight
               When more than one equivalent association has been adver-
               tised for a specific vertex, this field is used to denote
               the advertising router's preference for each association.
               Equivalent associations are ones that have identical Link
               Service Type, IS Svc Name and Logical Network Identifier
               fields.

          Assoc Length
               The length in bytes of this association.

          Logical Network ID
               When more than one overlay network is used to establish
               shortcut paths within the OSPF domain, this number identi-
               fies a specific logical network.  This function may also be
               used to support a variation of closed user groups so that
               shortcuts are limited to those routers that are configured
               to be in the same logical network.  To use the association
               information, a router must have an active attachment to the
               specific logical network identified in the resolution infor-
               mation.  An ARA capable router is configured with a list of
               Logical Network IDs.  The default value (i.e., only one
               overlay network or too lazy to care) for the ID is 0.

          Resolution Information
               The resolution information field includes link-layer and
               service-type specific information.  The contents of this
               field is defined in section A.7 of this document.  The Ver-
               tex Association may include several resolution information



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               items.


A.7 Resolution Information

     The resolution information field is an extensible field that includes
     link-layer and service-type specific information.





       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |    Res Type   |   Res Length  |       Resolution Value        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   remaining octets of  Resolution Value padded to 32-bits     |
       |                            ...                                |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+





          Res Type
               Identifies the resolution being advertised.  Resolution
               Types 1 and 2 are defined by this specification.  Resolution
               Type 1 is defined in A.7.1, Type 2 is defined in A.7.2.  All
               other resolution types are reserved for definition by the
               IANA (iana@isi.edu). The current list of resolution types is
               described below in Table 2.



                  Resolution Type         Description
                  -------------------------------------------------
                  1                       ATM Address
                  2                       ATM LIJ Call Identification



                                  Table 2

          Res Length
               The total length in octets of this resolution information
               field This value includes the Res Type and Res Length
               fields.

          Resolution Value



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               The resolution type-specific data.




A.7.1 ATM Address

     An ATM address is the Resolution Type 1.  This includes the type and
     length of ATM number (8 bits), the type and length of ATM subaddress
     (8 bits), the ATM number (x octets) and possibly the ATM subaddress (y
     octets).

                 8   7   6   5   4   3   2   1
               +---+---+---+---+---+---+---+---+
               |  Type And Len Of ATM Number   |
               +---+---+---+---+---+---+---+---+
               |  Type And Len Of ATM Subaddr  |
               +-----+-----+-----+-----+-----+-----+-----+-----+
               |  ATM Number...
               +-----+-----+-----+-----+-----+-----+-----+-----+
               |  ATM Subaddress...
               +-----+-----+-----+-----+-----+-----+-----+-----+


                             Format Of The ATM Address


     The Type and Length field of ATM number and ATM subaddress are encoded
     as follows.


            MSB   8     7     6     5     4     3     2     1   LSB
               +-----+-----+-----+-----+-----+-----+-----+-----+
               |  0  | 1/0 |   Octet length of address         |
               +-----+-----+-----+-----+-----+-----+-----+-----+



     Where:

             Bit(s)                  Description
             -------------------------------------------------
             8                       Reserved = 0  (for future use)
             7                       Type = 0  ATM Forum NSAPA format
                                          = 1  E.164 format
             6-1                     Length = 6 bit unsigned octet length of
                                     address (MSB = bit.6, LSB = bit.1).
                                     Value range is from 0 to 20 (decimal).



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     A non-existing ATM subaddress is indicated by setting the subaddress
     length to zero.  If the subaddress length is zero, the corresponding
     type field MUST be ignored and the ATM subaddress field MUST NOT con-
     sume any octets in the packet.

     The ATM number and ATM subaddress fields are encoded as defined by the
     ATM Forum UNI 3.1 [AF1] signalling specification.




A.7.2  ATM LIJ Call Identification

     An ATM LIJ Call Identification is the Resolution Type 2.  This
     includes an ATM address as defined in A.7.1 followed by a four octet
     Leaf Initiated Join Call Identifier Value, which together uniquely
     identify an ATM Point-To-Multipoint or Multipoint-To-Point call at a
     root's interface.



             0                   1                   2                   3
             0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
            +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            |              ATM Address as defined in A.7.1                  +
            +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            |                                                               |
            +              Remaining Octets of ATM Address                  +
            |                              ...                              |
            +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            |          Leaf Initiated Join Call Identifier Value            |
            +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                           Format Of LIJ Call Identification


     The Leaf Initiated Join Call Identifier Value is encoded as defined in
     Section 6.1.2.1 of the ATM Forum UNI 4.0 [AF2] signalling specifica-
     tion.




References


    [AF1] ATM Forum, "ATM User-Network Interface (UNI) Specification
        Version 3.1.", ISBN 0-13-393828-X, Prentice-Hall, Inc., Upper



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        Saddle River, NJ, 07458, September, 1994.

    [AF2] ATM Forum, "ATM User-Network Interface (UNI) Signalling
        Specification", July 1996.

    [HEIN] Heinanen, J., "Intra-area IP unicast among routers over legacy ATM",
        Internet Draft, July 1997, <draft-ietf-ion-intra-area-unicast-00.txt>

    [IS] S. Shenker and J. Wroclawski. "Network Element QoS Control
        Service Specification Template". Internet Draft, July 1996, <draft-
        ietf-intserv-svc-template-03.txt>

    [OPAQ] Coltun, R., "The OSPF Opaque LSA Option", Internet Draft
        May 1997, <draft-ietf-ospf-opaque-01.txt>

    [OSPF] Moy, J., "OSPF Version 2", RFC 2178, July 1997

    [NHRP]  Luciani, J., Katz, D., Piscitello, D., Cole, B., "NBMA
        Next-Hop Resolution Protocol", Internet Draft, March 1997,
        <draft-ietf-rolc-nhrp-11.txt>

    [NSSA] Coltun, R. and V. Fuller, "The OSPF NSSA Option", RFC 1587,
        RainbowBridge Communications, Stanford University, March 1994.




























Coltun, Heinanen                                                  [Page 36]


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