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Internet-Draft ARA August 1997
Expiration Date: February 1998
File name: draft-ietf-ospf-ara-00.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".
Coltun, Heinanen [Page 1]
Internet-Draft ARA August 1997
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).
Coltun, Heinanen [Page 2]
Internet-Draft ARA August 1997
Table Of Contents
1.0 Abstract ................................................. 5
2.0 Overview ................................................. 5
2.1 Acknowledgments .......................................... 5
3.0 A Brief Comparison Of Address Resolution Models .......... 6
4.0 ARA Associations ......................................... 7
5.0 Examples ................................................. 8
5.1 Example 1: Intra-Area .................................... 8
5.2 Example 2: Inter-Area .................................... 9
6.0 Description Of ARA Packet Formats ........................ 10
6.1 Vertex Types And Vertex Identifiers ...................... 11
7.0 Distribution Of ARA Information .......................... 11
8.0 ARA Routing Table Extensions ............................. 13
8.1 Adding ARA Routing Table Extensions ...................... 13
8.1.1 Modifications To The Intra-Area Route Calculation ...... 14
8.1.2 Modifications To The Inter-Area Route Calculations ..... 15
8.1.3 Modifications To The AS External Route Calculations .... 15
9.0 Receiving ARAs ........................................... 16
10.0 Additional Data Structures And APIs ..................... 17
Appendix A: ARA Data Formats ................................. 17
A.1 ARA Formats .............................................. 17
A.1.1 The ARA Header ......................................... 18
A.1.2 Router ARAs ............................................ 20
A.1.3 Network ARAs ........................................... 20
Coltun, Heinanen [Page 3]
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A.1.4 Vertex Association ..................................... 22
A.1.5 Resolution Information ................................. 23
A.1.6 ATM Address ............................................ 24
References ................................................... 26
Coltun, Heinanen [Page 4]
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1.0 Abstract
This document defines an OSPF option which enables routers to distri-
bute IP to link-layer address resolution information. An OSPF Address
Resolution Advertisement (ARA) may include link-layer specific func-
tionality such as a multipoint-to-point connection identifier along
with the address resolution information. 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 and cut-through data switch-
ing (bypassing router intervention). Before the ingress devices is
able to dynamically 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
discovery 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
area, between OSPF internal routers and autonomous system border
routers (ASBR) and between routers and servers.
Distributed router architectures.
Interoperation with ION NHRP and MPOA.
Inter-subnet multicast shortcuts using LIJ or Point-to-MultiPoint
procedures.
2.1 Acknowledgments
Coltun, Heinanen [Page 5]
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The author would like to thank Lou Berger, John Moy and Yiqun Cai, and
the rest of the OSPF Working Group for the ideas and support 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 returned address may be the address of the requested host system
or the address of a router which is on the path to the destination.
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 limi-
tation may be over restrictive).
o The target of a request may be a host or network addresses (exclud-
ing 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. Participation in the
Coltun, Heinanen [Page 6]
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ARA option is discovered through 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 will 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 an OSPF area. Inter-area shortcuts are sup-
ported 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 run-
ning NHRP.
o The ARA option may easily be extended to support inter-subnet multi-
cast shortcuts.
4.0 ARA Associations
The ARA option defines three types of advertisements. These include
1) intra-area router associations, 2) intra-area network associations,
and 3) inter-area 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 link-layer reachability of a
router's directly connected networks (also within the local area), and
inter-area ASBR associations correspond to ASBRs that may be in the
local or in a remote area. An ingress router can use these associa-
tions as follows.
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. A destination that is "beyond
Coltun, Heinanen [Page 7]
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the router" is determined by the OSPF topology map.
Intra-area network associations (which may advertise a host, net-
work or IP multicast address) are used to setup a shortcuts to
systems whose address fall within the range of the advertised
network.
Inter-area ASBR associations are used to setup shortcuts to ASBRs
that are in the local or remote area. These shortcuts are used
to send data to destinations that are external to the autonomous
system and reachable via the ASBR.
The ARA option maintains a list of all association for a specific ver-
tex and vertex type in an ARA Association Table. The lookup keys to
an entry in the table include the vertex type, vertex ID and, if the
scope is area-local, the area ID.
Associations are added to the routing table during the OSPF routing
table calculation (see the section entitled Adding ARA Routing Table
Extensions below). So, in addition to the standard information 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 (via a reference to the ARA Association
Table). 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.
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 an ARA
with an intra-area router association and RT5 has originated an ARA
with an intra-area network association for N5.
As a result of running the routing table calculation, RT1 would have
entries for N1-N8 in its routing table. The entry for N2 would refer-
ence the link-layer associations distributed in RT2's R-ARA, the
entries for N3, N4, N6, N7, N8 would reference the link-layer associa-
tions distributed in RT5's R-ARA and the entry for N5 would reference
the link-layer associations distributed in RT5's intra-area N-ARA.
Coltun, Heinanen [Page 8]
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+ ARA
| +---+ N3 N5
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 and RT6 support the ARA option and RT5 supports
the Opaque option. N3-N7 are AS external routes and RT5 and RT6 are
ASBRs. RT4 is an area-border router and originates LS Type-4 LSAs on
behalf of RT5 and RT6 and a LS Type-3 LSA for N8 into area 1.1.1.1.
RT1, RT2, RT3 and RT4 have originated intra-area R-ARAs, and RT6 has
originated an inter-area R-ARA.
As a result of running the routing table calculation, RT1 would have
entries for N1-N8 in its routing table. The entry for N2 would refer-
ence the link-layer associations distributed in RT3's R-ARA, the
entries for N3, N4, N5 and N8 would reference the link-layer associa-
tions distributed in RT4's intra-area R-ARA (i.e., R1 can't see link-
layer information beyond the area border router) and the entries for
N6 and N7 would reference the link-layer associations distributed in
RT6's inter-area R-ARA.
Coltun, Heinanen [Page 9]
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|
+ ARA ARA |
| +---+ +---+ | N3 N5
N1|--|RT1|----|RT2|\ | \ N4 /
| +---+ +---+ \ | \ | /
+ \ ARA \|/
\+---+ +---+
|RT4|------|RT5|
+---+ +---+
+ ARA / | | N6
| +---+ / | | /
N2|--|RT3|/ | ARA /
| +---+ | +---+/
+ | |RT6|----N7
| +---+
| |
| |
| +---+
| |RT7|
| +---+
| |
| +-----+
| N8
|
|
|
Area 1.1.1.1 | OSPF Backbone
------------------------|-------------------------
Figure 2: Sample Area Toplogy
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 to be used when more than one vir-
tual overlay network is present within the OSPF domain.
The ARA format allows more than one equivalent association to been
advertised by a router for a specific vertex. The association can
include a preference which identifies the advertising router's
Coltun, Heinanen [Page 10]
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relative preference for the equivalent associations. Equivalent asso-
ciations are ones that have identical link service type, integrated
service type and logical network identifier fields.
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 of the information may be
limited appropriately by the originator of the LSA. Opaque LSAs have
a sub-type which identifies the specific information that is carried
within the LSA. The ARA Opaque types are Opaque-type 1 and 2. The
flooding scope for specific vertex types is realized by embedding the
association in the appropriate Opaque LSA type.
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 router address resolution associations.
These associations are used to advertise the router's link-layer
attachments. A Vertex Type of 1 is identified by an Opaque type of 1.
The vertex identifier for a Router ARA (R-ARA) is the advertising
router field of the ARA header. Intra-area vertex type-1 advertise-
ments are included in Opaque Type-10 LSAs (their flooding scope is
area-local). Inter-area vertex type-1 advertisements are included in
Opaque Type-11 LSAs (their flooding scope is domain-wide). Inter-area
vertex type-1 advertisements are originated by as AS boundary routers.
Vertex type 2 ARAs advertise IP network address resolution associa-
tions (the network may be a IP multicast (class D) network). These
associations are used to advertise the link-layer associations for a
router's directly connected network. Vertex Type 2 advertisements are
be included in area-local (LS Type 10) advertisements. A Vertex Type
of 2 is identified by an Opaque type of 2. The vertex identifier (the
network and mask) for a Network ARA (N-ARA) is contained in the body
of the advertisement. A N-ARA may only identify a single network
(i.e., lists of networks are not permitted).
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.
7.0 Distribution Of ARA Information
Coltun, Heinanen [Page 11]
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In general, OSPF is composed of two components. It's transport com-
ponent deals with adjacency formation and reliable distribution of
topology information. The second component tracks topology informa-
tion changes and organizes the topology information that has gathered
from other routers into to a topology map which 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 information. This information may be used
directly by OSPF or by other protocols and mechanisms such as the ARA
option. Opaque LSAs use standard OSPF link-state database flooding
mechanisms for distribution. Each of the three Opaque types (LS Types
9, 10 and 11) have a scope associated with them so that distribution
of the information throughout the OSPF topology may be limited if
appropriate. ARA information has topology-specific scopes (i.e., dif-
ferent Opaque types) depending on the ARA vertex type (see the section
entitled Vertex Types and Vertex Identifiers).
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 routing table. Validation is needed
because topology information contained in the OSPF link-state database
may be stale (i.e., the originator of the information is no longer
reachable).
It is envisioned that an implementor designs an ARA user application
interface to which facilitates 1) flooding of ARA information to other
routers in the OSPF network, 2) receiving ARA information from other
routers in the OSPF network and 3) determining the validity (and
change of validity) of ARA information.
For the realization of 1 above, an implementation must provide an API
which allows the ARA user application to have its local OSPF entity
distribute resolution information in the ARA defined format (if the
scope is area-local, the area must also be supplied). 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 association 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
information 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
Coltun, Heinanen [Page 12]
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added to the routing table during the OSPF routing table calculation
(see the section on Adding ARA Routing Table Extensions below). 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.
The following section defines the mechanism to calculate the ARA rout-
ing table extensions.
8.0 ARA Routing Table Extensions
OSPF determines reachability and topology changes by performing the
algorithms described in the section entitled "Calculation of the rout-
ing table" (see section 16 of [OSPF]). ARAs are included in this cal-
culation 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-
to-MultiPoint call identifiers and QoS service specific information.
(See section Appendix A for a more complete description of the
specific link-layer information distributed in ARAs). The associa-
tions that are bound to a routing table entry are the associations
that are closest to the originator of the routing table's network
entry. The closest associations are determined during to the construc-
tion of the OSPF topology map. The associations that are bound to the
routing table entries are subsequently used by the ARA user applica-
tion to setup shortcut paths. Because a link-layer association may be
bound to more than one entry in the routing table, an ARA implementa-
tion keeps a table of ARA derived link-layer associations which is
referenced by the routing table entry. An entry in the ARA Associa-
tion Table consists of a list of all association for a specific vertex
and vertex type. The lookup keys to an entry in the table include the
vertex type, vertex ID and, if the scope is area-local, the area ID.
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.
Coltun, Heinanen [Page 13]
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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
vertex type will be type-2, the vertex identifier will consist of
a network and mask which are equivalent to those found in vertex
M and its area ID will be the one that is associated with the
shortest-path calculation.
o If an 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 type-1, the
vertex identifier will be vertex M's advertising router and its
area ID will be the one that is associated with the shortest-path
calculation.
o If an entry is found reference this entry in vertex M's link-
layer association field.
o If no link-layer association entries are found, and vertex 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 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 type-2, the vertex identifier will consist of a network
and mask which are equivalent to those found in the stub network
and its area ID will be the one that is associated with the
shortest-path calculation.
Coltun, Heinanen [Page 14]
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o If an entry is found copy the entry's 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 use the advertising router's information (else
the information field is left blank).
8.1.2 Modifications To The Inter-Area Route Calculations
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, locate the area border
router (ABR) that originated the advertisement. If link-layer
association information is available for the ABR entry, copy the
contents of the ABR's link-layer association information field
into the routing table entry's link-layer association field.
o If the LSA is a Type-4 summary LSA, lookup the entry in the ARA
Association Table. This entry's vertex type will be type-1, the
vertex identifier will the Type-4's Link-State ID (the link-state
ID identifies the ASBR) the area ID will not be used (the ARAs
have domain-wide scope).
o If an entry is found copy the entry's association information
into the routing table entry's link-layer association field.
o If an entry is not found locate the area border router (ABR)
that originated the Type-4 advertisement. Copy the contents of
the ABR's link-layer association information field into the rout-
ing table entry's link-layer association field.
8.1.3 Modifications To The AS External Route Calculations
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
Coltun, Heinanen [Page 15]
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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.
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. Note that the ASBR's association information may have been
added from an intra-area or domain-wide 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 newer invocation of with the same
contents, 2) a newer invocation with different contents, 3) a new ARA
or 4) a newer invocation that is being flushed by the ARA's origina-
tor. Actions need to be taken if the ARA is new, the ARA is being
purged or the contents of the ARA has changed.
o Lookup the ARA's vertex in the ARA Association Table. If the
ARA is a LS Type-10, the table entry will reference the ARA's
associated area. If the ARA is Type-11 the table entry will not
be associated with an area.
o If the ARA is new and there is an existing entry, add the asso-
ciations found in the newly received ARA to the entry. The ARA
application must be informed that new associations have been
added.
o If there is no existing entry, create one which contains the
associations found in the ARA. Reschedule the SPF calculation.
o If the ARA's contents have changed, modify the existing entry
and inform the ARA application that there has been a change to
the contents of the link-layer association entry for the speci-
fied vertex.
o If the ARA is being flushed and there is an existing entry,
remove the associations from the link-layer entry that were
included in the ARA. If the contents of the entry is now empty,
remove the entry from the table and reschedule the SPF calcula-
tion. If the entry still contains associations inform the ARA
user application that there has been a change to the contents of
the entry.
Coltun, Heinanen [Page 16]
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10.0 Additional Data Structures And APIs
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 Configuration flag to enable the OSPF ARA option.
o A ARA Association Table is added. An entry in this table con-
tains the list of link-layer associations for a specific vertex
and vertex type. The lookup keys to an entry in the table include
the vertex type, vertex ID and, if the scope is area-local, the
area ID. The table is referenced by the routing table and the
data structures used in the SPF calculation.
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 be able to purge the associations that were previously
distributed when they are no longer valid and send out new ver-
sions when the associations information has changed.
A.1 ARA Formats
This document defines three 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.1.1. Subse-
quent sections describe the specific advertisements and their content
(i.e., the formats of the resolution information).
Each ARA describes a link-layer association for a piece of the OSPF
routing domain. Any router may originate intra-area router ARAs and
network ARAs. These ARAs are distributed throughout the local area.
ASBRs may originate domain-wide router ARAs. An ARA capable router
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may use the ARAs to build shortcut paths to other ARA capable routers.
A.1.1 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 vertex identifier for a Router ARA (R-ARA) is the advertising
router field of the ARA header. The vertex identifier for a Network
ARA (N-ARA) is contained in the body of the advertisement. A N-ARA may
only identify a single network (i.e., lists of networks are not per-
mitted). A router may originate multiple (N or R) ARAs each contain-
ing different associations or may originate a single (N or R) ARA con-
taining 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. 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
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advertisement format. The AR LSA as defined in this docu-
ment may be Type-10 (for N-ARAs and R-ARAs with intra-area
scope) or Type-11 (for ASBR R-ARA with inter-domain 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 and 2. Opaque-type 1 advertisements are
Router Address Resolution Advertisements and contain associ-
ations for the advertising router. R-ARAs may be encapsu-
lated in type-10 (Opaque) LSAs when the associations are to
be distributed within the area or encapsulated in type-11
(Opaque) LSAs when the associations are to be coordinated
with inter-area ASBR (LS Type-4) advertisements (i.e., the
router is an ASBR).
Opaque-type 2 advertisements area Network Address Resolution
Advertisements and contain associations for one of the
advertising router's directly connected IP networks (or
hosts). N-ARAs are encapsulated in LS Type-10 (Opaque) LSAs
(i.e., their scope is area local).
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
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 is 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 LSA. This includes the 20 byte
ARA header.
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A.1.2 Router ARAs
Opaque-type 1 advertisements are Router Address Resolution Advertise-
ments and contain associations for the advertising router. R-ARAs may
be encapsulated in LS Type-10 (Opaque) LSAs when the associations are
to be distributed within the area or encapsulated in LS Type-11
(Opaque) LSAs when the associations are to be coordinated with inter-
area ASBR (LS Type-4) advertisements (i.e., the router is an ASBR).
All of the router's associations may be described in a single R-LSA or
they may distributed over several R-LSAs.
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 or 11 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 1 | Opaque ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 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.1.4) followed by association-
specific resolution information (described in Section A.1.5).
A.1.3 Network ARAs
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Opaque-type 2 advertisements area Network Address Resolution Adver-
tisements and contain associations for one of the advertising router's
directly connected IP networks (or hosts). N-ARAs are encapsulated in
LS Type-10 (Opaque) LSAs (i.e., their scope is area local).
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 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.1.4) followed by association-specific
resolution information (described in Section A.1.5).
A.1.4 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
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
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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 virtual overlay network is used to estab-
lish shortcut paths within the OSPF domain, this number
identifies a specific virtual overlay network. The router
must have an active attachment to the specific logical net-
work to use the associated resolution information. An ARA
capable router is configured with a list of Logical Network
IDs. The default value (i.e., only one virtual overlay net-
work 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.1.5 of this document. The
Vertex Association may include several resolution informa-
tion items.
A.1.5 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 |
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| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Res Type
Identifies the resolution being advertised. Resolution
Types 1 and 2 are defined by this specification. Resolution
Type 1 is defined in A.1.6, Type 2 is defined in A.1.7. 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 Identifier
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
The resolution type-specific data.
A.1.6 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 |
+---+---+---+---+---+---+---+---+
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| 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).
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 [AF] signalling specification.
References
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[AF] ATM Forum, "ATM User-Network Interface (UNI) Specification
Version 3.1.", ISBN 0-13-393828-X, Prentice-Hall, Inc., Upper
Saddle River, NJ, 07458, September, 1994.
[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>
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