Internet Engineering Task Force T. Przygienda/P. Droz/R.
Haas
INTERNET DRAFT Bell
Labs/IBM
15 June
1999
OSPF over ATM and Proxy PAR
<draft-ietf-ospf-atm-02.txt>
Status of This Memo
This document is an Internet Draft and is in full conformance with
all provisions of Section 10 of RFC2026. Internet Drafts are working
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Abstract
This draft specifes for OSPF implementors and users mechanisms
describing how the protocol operates in ATM networks over PVC and
SVC meshes with the presence of Proxy PAR. These recommendations
do not require any protocol changes and allow for simpler, more
efficient and cost-effective network designs. It is recommended that
OSPF implementations should be able to support logical interfaces,
each consisting of one or more virtual circuits and used either
as numbered logical point-to-point links (one VC), logical NBMA
networks (more than one VC) or point-to-multipoint networks (more
than one VC), where a solution simulating broadcast interfaces is
not appropriate. PAR can help to distribute across the ATM cloud
configuration set-up and changes of such interfaces when OSPF capable
routers are (re-)configured. Proxy-PAR can in turn be used to
exchange this information between the ATM cloud and the routers
connected to it.
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1. Introduction
Proxy-PAR and PAR have been accepted as standards by the ATM Forum in
January 1999 [Dro99 ]. A more complete overview of Proxy PAR than in
the section below is given in [PD99 ].
1.1. Introduction to Proxy PAR
Proxy PAR [Dro99 ] is an extension allowing for different ATM
attached devices (like routers) to interact with PAR capable switches
and query information about non-ATM services without executing
PAR themselves. The Proxy PAR client side in the ATM attached
device is much simpler in terms of implementation complexity and
memory requirements than a complete PAR protocol stack (which
includes the full PNNI [AF96b ] protocol stack) and should allow
easy implementation in e.g. existing IP routers. Additionnaly,
clients can use Proxy PAR to register different non-ATM services
and protocols they support. Proxy PAR has consciously not been
included as part of ILMI [AF96a ] due to the complexity of PAR
information passed in the protocol and the fact that it is intended
for integration of non-ATM protocols and services only. A device
executing Proxy PAR does not necessarily need to execute ILMI or UNI
signaling, although this normally will be the case.
The protocol in itself does not specify how the distributed service
registration and data delivered to the client is supposed to be
driving other protocols so e.g. OSPF routers finding themselves
through Proxy PAR could use this information in a Classical IP over
ATM [ML98 ] fashion, forming a full mesh of point-to-point connections
to interact with each other to simulate broadcast interfaces. For
the same purpose LANE [AF95 ] or MARS [Arm96 ] could be used. As a
by-product, Proxy PAR could provide the ATM address resolution for
IP attached devices but such resolution can be achieved by other
protocols under specification at the IETF as well, e.g. [Col98 ].
And last but not least, it should be mentioned here that the protocol
coexists with and complements the ongoing work in IETF on server
detection via ILMI extensions [Dav99a , Dav99b , Dav99c ].
1.1.1. Proxy PAR Scopes
Any Proxy PAR registration is carried only within a defined scope
that is set during registration and is equivalent to the PNNI routing
level. Since no assumptions except scope values can be made about
the information distributed (e.g. IP addresses bound to NSAPs
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are not assumed to be aligned with them in any respect such as
encapsulation or functional mapping), registration information cannot
be summarized. This makes a careful handling of scopes necessary to
preserve the scalability. More details on the usage of scope can be
found in [PD99 ].
1.2. Introduction to OSPF
OSPF (Open Shortest Path First) is an Interior Gateway Protocol
(IGP) and described in [Moy98 ] from which most of the following
paragraphs has been taken almost literally. OSPF distributes routing
information between routers belonging to a single Autonomous System.
The OSPF protocol is based on link-state or SPF technology. It was
developed by the OSPF working group of the Internet Engineering
Task Force. It has been designed expressly for the TCP/IP internet
environment, including explicit support for IP subnetting, and
the tagging of externally-derived routing information. OSPF also
utilizes IP multicast when sending/receiving the updates. In
addition, much work has been done to produce a protocol that responds
quickly to topology changes, yet involves small amounts of routing
protocol traffic.
To cope with the needs of NBMA and demand circuits capable networks
such as Frame Relay or X.25, [Moy95 ] has been made available that
standardizes extensions to the protocol allowing for efficient
operation over on-demand circuits.
OSPF supports three types of networks today:
- Point-to-point networks: A network that joins a single pair
of routers. Point- to-point networks can either be numbered
or unnumbered in the latter case the interfaces do not have IP
addresses nor masks. Even when numbered, both sides of the link
do not have to agree on the IP subnet.
- Broadcast networks: Networks supporting many (more than two)
attached routers, together with the capability to address
a single physical message to all of the attached routers
(broadcast). Neighboring routers are discovered dynamically on
these networks using the OSPF Hello Protocol. The Hello Protocol
itself takes advantage of the broadcast capability. The protocol
makes further use of multicast capabilities, if they exist. An
Ethernet is an example of a broadcast network.
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- Non-broadcast networks: Networks supporting many (more than
two) attached routers, but having no broadcast capability.
Neighboring routers are maintained on these nets using
OSPF's Hello Protocol. However, due to the lack of broadcast
capability, some configuration information is necessary for the
correct operation of the Hello Protocol. On these networks, OSPF
protocol packets that are normally multicast need to be sent to
each neighboring router, in turn. An X.25 Public Data Network
(PDN) is an example of a non-broadcast network.
OSPF runs in one of two modes over non-broadcast networks. The
first mode, called non-broadcast multi-access (NBMA), simulates
the operation of OSPF on a broadcast network. The second mode,
called Point-to-MultiPoint, treats the non-broadcast network as a
collection of point-to-point links. Non-broadcast networks are
referred to as NBMA networks or Point-to-MultiPoint networks,
depending on OSPF's mode of operation over the network.
2. OSPF over ATM
2.1. Model
Contrary to broadcast-simulation based solutions such as LANE
[AF95 ] or Classical IP over ATM [ML98 ], this document elaborates
on how to handle virtual OSPF interfaces over ATM such as
NBMA, point-to-multipoint or point-to-point and allow for their
auto-configuration in presence of Proxy PAR. One advantage is the
circumvention of server solutions that often present single points of
failure or hold large amounts of configuration information.
The other main benefit is the possibility to execute OSPF on top
of NBMA and point-to-multpoint ATM networks, and still benefit from
the automatic discovery of OSPF neighbors. As opposed to broadcast
networks, broadcast-simulation based networks (like LANE or Classical IP
over ATM), and point-to-point networks, where an OSPF router dynamically
discovers its neighbors by sending Hello packets to the AllSPFRouters
multicast address, this is not the case on NBMA and point-to-multipoint
networks. On NBMA networks, the list of all other attached routers to
the same NBMA network has to be manually configured or discovered by
some other means: Proxy PAR allows to automate this configuration.
Also on point-to-multipoint networks, the set of routers that are
directly reachable must be configured: it can be dynamically discovered
by Proxy PAR or through mechanisms like Inverse ATMARP. In an ATM
network, (see 8.2 in [ML98 ]) Inverse ATMARP can be used to discover the
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IP address of the router at the remote end of a given PVC, whether or
not its ATM address is known. But Inverse ATMARP does not return for
instance whether the remote router is running OSPF, as opposed to Proxy
PAR.
Parallel to [dR94 ] that describes the recommended operation of
OSPF over Frame Relay networks, a similar model is assumed where
the underlying ATM network can be used to model single VCs as
point-to-point interfaces or collections of VCs as non-broadcast
interfaces, whether in NBMA or point-to-multipoint mode. Such
a VC or collection of VCs is called a logical interface and
specified through its type (either point-to-point, NBMA or
point-to-multipoint), VPN ID (the Virtual Private Network to which
interface belongs), address and mask. Layer 2 specific configuration
such as address resolution method, class and quality of service
of used circuits and other must be also included. As logical
consequence thereof, a single, physical interface could encompass
multiple IP subnets or even multiple VPNs. In contrary to layer 2
and IP addressing information, when running Proxy PAR, most of the
OSPF information needed to operate such a logical interface does not
have to be configured into routers statically but can be provided
through Proxy PAR queries. This allows for much more dynamic
configuration of VC meshes in OSPF environments than e.g. in Frame
Relay solutions.
Proxy PAR queries can also be issued with a subnet address set to
0.0.0.0, instead of a specific subnet address. This type of query
returns information on all OSPF routers available in all subnets, within
the scope specified in the query. This can be used for instance when
the IP addressing information has not been configured.
2.2. Configuration of OSPF interfaces with Proxy PAR
To achieve the goal of simplification of VC mesh reconfiguration,
Proxy PAR allows the router to learn automatically most of the
configuration that has to be provided to OSPF. Non-broadcast
and point-to-point interface information can be learned across
an ATM cloud as described in the ongoing sections. It is up to
the implementation to possibly allow for a mixture of Proxy PAR
autoconfiguration and manual configuration of neighbor information.
Moreover, manual configuration could e.g. override or complement
information derived from a Proxy PAR client. Additionally, OSPF
extensions to handle on-demand circuits [Moy95 ] can be used to allow
for graceful tearing down of VCs not carrying any OSPF traffic over
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prolonged periods of time. The different interactions are described
in sections 2.2.1, 2.2.2 and 2.2.3.
Even after autoconfiguration of interfaces has been provided, the
problem of VC setups in an ATM network is unsolved since none of the
normally used mechanisms such as Classical IP [ML98 ] or LANE [AF95 ]
are assumed to be present. Section 2.5 describes the behavior of
OSPF routers necessary to allow for router connectivity.
2.2.1. Autoconfiguration of Non-Broadcast Multiple-Access (NMBA)
Interfaces
Proxy PAR allows to autoconfigure the list of all routers residing on
the same IP network in the same VPN by simply querying the Proxy PAR
server. Each router can easily obtain the list of all OSPF routers
on the same subnet with their router priorities and corresponding
ATM addresses. This is the precondition for OSPF to work properly
across such logical NBMA interfaces. Note that this memberlist, when
learned through Proxy PAR queries, can dynamically change with PNNI
(in)stability and general ATM network behavior. It maybe preferable
for an implementation to withdraw list membership (de-register
itself as an OSPF router) e.g. much slower than detect new members
(done by querying). Relying on OSPF mechanism to discover lack of
reachability in the overlaying logical IP network could alleviate the
risk of thrashing DR elections and excessive information flooding.
Once the DR registration is completed and the router has not been
elected DR or BDR, an implementation of [Moy95 ] can ignore the fact
that all routers on the specific NBMA subnet are available in its
configuration since it only needs to maintain VCs to the DR and BDR.
Traditionally, router configuration for a NBMA network provides
the list of all neighboring routers to allow for proper protocol
operation. For stability purposes, the user may choose to provide a
list of neighbors through such static means but additionally enable
the operation of Proxy PAR protocol to complete the list. It is left
to specific router implementations whether the manual configuration
is used in addition to the information provided by Proxy PAR, used
as filter of the dynamic information or whether a concurrent mode
of operation is prohibited. In any case it should be obvious that
allowing for more flexibility may facilitate operation but provides
more possibilities for misconfiguration as well.
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2.2.2. Autoconfiguration of Point-to-Multipoint Interfaces
Point-to-Multipoint interfaces in ATM networks only make sense if
no VCs can be dynamically set up since an SVC-capable ATM network
normally presents a NBMA cloud to OSPF. This is e.g. the case if
OSPF executes over a network composed of a partial PVC or SPVC mesh
or pre-determined SVC meshes. Such a network could be modeled using
the point-to-multipoint OSPF interface and the neighbor detection
could be provided by Proxy PAR or other means. In the Proxy PAR case
the router queries for all OSPF routers on the same network in the
same VPN but it installs in the interface configuration only routers
that are already reachable through existing PVCs. The underlying
assumption is that a router knows the remote ATM address of a PVC
and can compare it with appropriate Proxy PAR registrations. If the
remote ATM address of the PVC is unknown, it can be discovered by
mechanisms like Inverse ARP [TB99 ].
Proxy PAR provides a true OSPF neighbor detection mechanism, whereas
a mechanism like Inverse ARP only returns addresses of directly
reachable routers (which are not necessarily running OSPF), in the
point-to-multipoint environment.
2.2.3. Autoconfiguration of Numbered Point-to-Point Interfaces
OSPF point-to-point links do not necessarily have an IP address
assigned and even when having one, the mask is undefined. As a
precondition to successfully register a service with Proxy PAR, IP
address and mask is required. Therefore, if a router desires to use
Proxy PAR to advertise the local end of a point-to-point link to the
router it intends to form an adjacency with, an IP address has to
be provided and a netmask set or a default of 255.255.255.254 (this
gives as the default case a subnet with 2 routers on it) assumed. To
allow the discovery of the remote end of the interface, IP address
of the remote side has to be provided and a netmask set or a default
of 255.255.255.254 assumed. Obviously the discovery can only be
successfull when both sides of the interface are configured with the
same network mask and are within the same IP network. The situation
where more than two possible neighbors are discovered through
queries and the interface type is set to point-to-point presents a
configuration error.
Sending multicast Hello packets on the point-to-point links allows
to automatically discover OSPF neighbors. On the other hand, using
Proxy PAR instead avoids sending Hello messages to routers which are not
necessarily running OSPF.
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2.2.4. Autoconfiguration of Unnumbered Point-to-Point Interfaces
For reasons given already in [dR94 ] using unnumbered point-to-point
interfaces with Proxy PAR is not a very attractive alternative
since the lack of an IP address prevents efficient registration and
retrieval of configuration information. Relying on the numbering
method based on MIB entries generates conflicts with the dynamic
nature of creation of such entries and is beyond the scope of this
work.
2.3. Registration of OSPF interfaces with Proxy PAR
To allow other routers to discover an OSPF interface automatically,
the IP address, mask, Area ID, interface type and router priority
information given must be registered with the Proxy PAR server at an
appropriate scope. A change in any of these parameters has to force
a reregistration with Proxy PAR.
It should be emphasized here that since the registration information
can be used by other routers to resolve IP addresses against NSAPs as
explained in section 2.4 already, whole IP address of the router must
be registered. It is not enough to just indicate the subnet up to
the mask length but all address bits must be provided.
2.3.1. Registration of Non-Broadcast Multiple-Access Interfaces
For an NBMA interface the appropriate parameters are available and
can be registered through Proxy PAR without further complications.
2.3.2. Registration of Point-to-Multipoint Interfaces
In case of a point-to-multipoint interface the router registers its
information in the same fashion as in the NBMA case except that the
interface type is modified accordingly.
2.3.3. Registration of Numbered Point-to-Point Interfaces
In case of point-to-point numbered interfaces the address mask is not
specified in the OSPF configuration. If the router has to use Proxy
PAR to advertise its capability, a mask must be defined or a default
value of 255.255.255.254 used.
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2.3.4. Registration of Unnumbered Point-to-Point Interfaces
Due to the lack of a configured IP address and difficulties generated
by this fact as described earlier, registration of unnumbered
point-to-point interfaces is not covered in this document.
2.4. IP address to NSAP Resolution Using Proxy PAR
As a byproduct of Proxy PAR presence, an OSPF implementation could
use the information in registrations for the resolution of IP
addresses to ATM NSAPs on a subnet without having to use static data
or mechanisms such as ATMARP [ML98 ]. This again should allow for
drastic simplification of number of mechanisms involved in operation
of OSPF over ATM to provide an IP overlay.
2.5. Connection Setup Mechanisms
This sections describes OSPF behavior in an ATM network under
different assumptions in terms of signaling capabilities and preset
connectivity.
2.5.1. OSPF in PVC Environments
In environments where only partial PVCs (or SPVCs) meshes are
available and modeled as point-to-multipoint interfaces, the routers
see reachable routers through autodiscovery provided by Proxy PAR.
This leads to expected OSPF behavior. In cases where a full mesh of
PVCs is present, such a network should preferably be modeled as NBMA.
2.5.2. OSPF in SVC Environments
In SVC-capable environments the routers can initiate VCs after having
discovered the appropriate neighbors, preferably driven by the need
to send data such as Hello-packets. Since this can lead to race
conditions where both sides can open a VC and it is desirable to
minimize this valuable resource, if the router with lower Router ID
detects that the VC initiated by the other side is bidirectional, it
is free to close its own VC and use the detected one.
Observe that this behavior operates correctly in case OSPF over
Demand Circuits extensions are used [Moy95 ] over SVC capable
interfaces.
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+ + +
| +---+ | |
+--+ |---|RTA|---| +-------+ | +--+
|H1|---| +---+ | | ATM | |---|H2|
+--+ | | +---+ | Cloud | +---+ | +--+
|LAN Y |---|RTB|-------------|RTC|---|
+ | +---+ | PPAR | +---+ |
+ +-------+ +
Figure 1: Simple Topology with Router B and Router C operating across
NBMA ATM interfaces with Proxy PAR
The existence of VCs used for OSPF exchanges is orthogonal to the
number and type of VCs the router chooses to use within the logical
interface to forward data to other routers. OSPF implementations are
free to use any of these VCs (1) to send packets if their endpoints
are adequate and must accept hello packets arriving on any of the VCs
belonging to the logical interface even if OSPF operating on such an
interface is not aware of their existence. An OSPF implementation
may not accept or close connections being initiated by another router
that has either not been discovered by Proxy PAR or whose Proxy PAR
registration is indicating that it is not adjacent.
As an example consider the topology in figure 2.5.2 where router
RTB and RTC are connected to a common ATM cloud offering Proxy PAR
services. Assuming that RTB's OSPF implementation is aware of SVCs
initiated on the interface and RTC only makes minimal use of Proxy
PAR information the following sequence could develop illustrating
some of the cases described above:
1. RTC and RTB register with ATM cloud as Proxy PAR capable and
discover each other as adjacent OSPF routers.
2. RTB sends a hello which forces it to establish a SVC connection
to RTC.
___________________________________________
1. in case they are aware of their existence
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3. RTC sends a hello to RTB but disregards the already existing VC
and establishes a new VC to RTB to deliver the packet.
4. RTB sees a new bi-directional VC and assuming here that RTC's
OSPF Id is higher, closes the VC originated in step 2.
5. Host H1 sends data to H2 and RTB establishes a new data SVC
between itself and RTC.
6. RTB sends a Hello to RTC and decides to do it using the newly
establish data SVC. RTC must accept the hello despite the minimal
implementation.
3. Acknowledgments
Comments and contributions from several sources, especially Rob
Coltun, Doug Dykeman and John Moy are included in this work.
4. Security Consideration
Several aspects are to be considered when talking about security of
operating OSPF over ATM and/or Proxy PAR. The security of registered
information handed to the ATM cloud must be guaranteed by the underlying
PNNI protocol. Extensions to PNNI are available and given their
implementation spoofing of registrations and/or denial-of-service issues
can be addressed [PB97 ]. The registration itself through proxy PAR is
not secured and appropriate mechanisms are for further study. However,
even if the security at the ATM layer is not guaranteed, OSPF security
mechanisms can be used to verify that detected neighbors are authorized
to interact with the entity discovering them.
References
[AF95] ATM-Forum. LAN Emulation over ATM 1.0. ATM Forum
af-lane-0021.000, January 1995.
[AF96a] ATM-Forum. Interim Local Management Interface (ILMI)
Specification 4.0. ATM Forum 95-0417R8, June 1996.
[AF96b] ATM-Forum. Private Network-Network Interface Specification
Version 1.0. ATM Forum af-pnni-0055.000, March 1996.
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[Arm96] G. Armitage. Support for Multicast over UNI 3.0/3.1 based
ATM networks, RFC 2022. Internet Engineering Task Force,
November 1996.
[Col98] R. Coltun. The OSPF Opaque LSA Option, RFC 2370. Internet
Engineering Task Force, July 1998.
[Dav99a] M. Davison. ILMI-Based Server Discovery for ATMARP, RFC
2601. Internet Engineering Task Force, June 1999.
[Dav99b] M. Davison. ILMI-Based Server Discovery for MARS, RFC 2602.
Internet Engineering Task Force, June 1999.
[Dav99c] M. Davison. ILMI-Based Server Discovery for NHRP, RFC 2603.
Internet Engineering Task Force, June 1999.
[dR94] O. deSouza and M. Rodrigues. Guidelines for Running OSPF
Over Frame Relay Networks, RFC 1586. Internet Engineering
Task Force, March 1994.
[Dro99] P. Droz. PNNI Augmented Routing (PAR) Version 1.0. ATM
Forum af-ra-0104.000, January 1999.
[ML98] J. Halpern M. Laubach. Classical IP and ARP over ATM, RFC
2225. Internet Engineering Task Force, April 1998.
[Moy95] J. Moy. Extending OSPF to Support Demand Circuits, RFC
1793. Internet Engineering Task Force, April 1995.
[Moy98] J. Moy. OSPF Version 2 - RFC 2328. Internet Engineering
Task Force, April 1998.
[PB97] T. Przygienda and C. Bullard. Baseline Text for PNNI Peer
Authentication and Cryptographic Data Integrity. ATM Forum
97-0472, July 1997.
[PD99] T. Przygienda P. Droz. Proxy PAR. Internet Draft
draft-ietf-ion-proxypar-arch-01, February 1999.
[TB99] A. Malis T. Bradley, C. Brown. Inverse Address Resolution
Protocol, RFC 2390. Internet Engineering Task Force,
September 1999.
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Authors' Addresses
Tony Przygienda
Bell Labs, Lucent Technologies
101 Crawfords Corner Road
Holmdel, NJ 07733-3030
prz@dnrc.bell-labs.com
Patrick Droz
IBM Research Division
Saumerstrasse 4
8803 Ruschlikon
Switzerland
dro@zurich.ibm.com
Robert Haas
IBM Research Division
Saumerstrasse 4
8803 Ruschlikon
Switzerland
rha@zurich.ibm.com
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