Network Working Group Juha Heinanen
INTERNET DRAFT Telia Finland
Expires May 1998 November 1997
Intra-area IP unicast among routers over legacy ATM
<draft-ietf-ion-intra-area-unicast-01.txt>
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Abstract
This document describes how IP unicast can be efficiently implemented
among routers belonging to the same area of a routing domain, where
the connectivity is provided by a legacy ATM network as defined by
the ATM Forum or ITU. This proposal is designed to be complementary
to IP multicast solutions such as the one described in [1].
1. Introduction
This document describes how a set of routers (such as the access/edge
routers of an ISP or enterprise) connected to a legacy ATM network
can in a dynamic and plug-and-play fashion optimize ATM connections
for efficient forwarding of unicast IP packets. The method can be
used in situations where the number of routers is so large that a
full mesh of point-to-point ATM VCs is not practical from technical
or economic reasons. In addition, it can be applied to smaller
router networks to automate the setup of a full mesh of ATM
connectivity between the routers.
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The set of routers must belong to the same area of a link state
routing protocol, such as OSPF or IS-IS, that floods topology and
reachability information to every router in the area.
This proposal only deals with IP unicast, but it complements and can
be used in conjunction with IP multicast solutions such as the one
described in [1].
2. Router configuration and behavior
Initialization
As introduced above, this document defines a method of dynamically
managing ATM connectivity among a set of routers that belong to the
same area of a routing domain, where a link state protocol, such as
OSPF or IS-IS, is used to exchange topology and reachability
information.
Before the dynamic management of ATM VCs can begin, the routers of
the area must be manually configured to exchange routing information
among themselves. There must thus be enough initial connectivity so
that at least one IP path exists from each router to each other
router in the area. This initial connectivity is also used to
forward IP packets when dynamic ATM VCs don't exist.
Note that the initial connectivity doesn't necessarily need to be
implemented over ATM and that not all routers of the area need to be
ATM attached. Furthermore, even if a router is ATM attached, it
doesn't need to participate in the dynamic management of ATM VCs.
The ATM routers of an area can thus be upgraded one at a time to
support the method described in this document.
Connection setup and teardown
After the initial connectivity has been established, ATM attached
routers that participate in this method start to dynamically create
and delete dynamic shortcut ATM VCs among themselves based on traffic
volumes. This can be accomplished, for example, as follows.
An ATM attached router R measures, how many bytes it has received
during the past M seconds, whose final hop router S within the area
is also ATM attached. If the number of bytes is less than N, R
forwards the packets according to its routing table. When the number
of bytes equals or exceeds N, and R doesn't yet have a dynamic ATM VC
to S, R creates such a VC and starts to forward S bound packets
directly.
Once the dynamic ATM VC from R to S has been created, R starts to
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measure the traffic along it. When R detects that during the past K
seconds the number of bytes along the dynamic ATM VC to S has fallen
below L, it deletes the dynamic ATM VC and returns to the initial
mode of operation that was described above.
The values of the constants K, L, M, and N control the rate of
dynamic ATM VC creation and teardown. They are assigned by the
network administrator and may differ from one ATM attached router to
another.
Setting the VC creation and deletion limits N and L to zero, turns
off the measurement process and causes the router to create a dynamic
VC to every other participating router. That can be the default in
small router networks that want to use this method to automate the
setup of a full mesh of ATM VCs.
If a router doesn't want to set up any dynamic ATM VCs, the VC
creation limit N is set to a positive value and the measurement
interval M is set to zero. Finally, if a router doesn't want to be a
destination of dynamic ATM VCs, it doesn't make its ATM address
available to the other routers for the purpose of this application.
Note that if a router is not capable in measuring traffic, it can
still participate as a destination of dynamic ATM VCs and can itself
set up dynamic VCs non-selectively to every other router.
Characteristics of dynamic ATM VCs
In order to keep the number of routing peers small and in order to
avoid frequent changes in topology information, the router that
establishes a dynamic ATM VC does not use it for the exchange of
routing information nor does it advertise the dynamic VC to its
routing peers.
Dynamic ATM VCs are unidirectional, because the source router that
established a dynamic ATM VC does not have information about the
traffic volumes to the reverse direction. Unidirectionality also
simplifies the method, since it allows the source router to manage
the dynamic ATM VC autonomously without coordination with the
destination router. Yet another advantage of unidirectionality is
that unidirectional VCs can be merged if more than one source router
sets up a connection to the same destination router.
The traffic category, traffic parameters, and protocol encapsulation
of dynamic ATM VCs are a local matter of the routers that establish
them. The default traffic category is UBR with peak cell rate set to
the link rate and minimal acceptable cell rate (if applicable) set to
zero. The default protocol encapsulation method is LLC Encapsulation
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as defined in [2]. Signalling is as specified in [3] for UNI 3.1 and
in [4] for UNI 4.0.
3. Address resolution
Since all the routers belong to the same area of a link state routing
domain, they learn each others' router IDs and the IP address
prefixes that are reachable via each router. In order to dynamically
create an ATM VC from one router to another, the source router also
needs to learn the ATM address of the destination router.
A router that wants to participate as a destination in the dynamic
management of ATM VCs, makes its ATM address known to the other
routers of the area by including in its link state advertisements a
field that contains an ATM address of the advertising router.
In OSPF, the router advertises its ATM address(es) using the Address
Resolution Advertisement (ARA) option [5]. The Opaque Type of the
ARA is Intra-area Router ARA (Opaque Type-1). One or more ATM
addresses or LIJ identifications (one per Vertex Association) can be
advertised in a single ARA. The Resolution Type of a Vertex
Association is ATM Address, if the Link Service Type is ATM Point-To-
Point, or ATM LIJ Call Identification, if the Link Service Type is
ATM MultiPoint-To-Point. See [5] for further details regarding the
ARA option.
A field similar to Opaque LSA could be easily defined for IS-IS.
Futher, it could be possible to use a well-known discretionary non-
transitive attribute of BGP to carry the address resolution
information, but the use of inter-domain routing protocols is outside
the scope of this document.
4. Discussion
The method proposed in this document allows efficient interconnection
of a set of routers over a legacy ATM network. After small amount of
manual configuration, the routers will automatically optimize direct
connectivity among themselves based on dynamic traffic load. Network
administrators can control the number of ATM VCs created by the
method taking into account scalability and cost.
As shown above, the method can readily exploit a multipoint-to-point
ATM signalling capability, which will reduce the number of ATM VCs
terminating at the destination routers. The method also benefits
from the capability to dynamically renegotiate the traffic parameters
of active ATM VCs. Both of these new capabilities are currently
under study in the ATM Forum.
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5. Security Considerations
Since the method described in this document allows data paths to be
established that bypass the normal hop-by-hop control path, the
location of any access filters should be decided carefully. To
ensure proper enforcement of filter policies, filters should be moved
to the edges of an area so that they may be applied on entry or exit
from the short-cut data path.
References
[1] Farinacci, D., Meyer, D., and Rekhter, Y., "Intra-LIS IP
multicast among routers over ATM using Sparse Mode PIM".
draft-ietf-ion-intralis-multicast-01.txt, August 1997.
[2] Heinanen, J., "Multiprotocol Encapsulation over ATM Adaptation
Layer 5". RFC 1483, July 1993.
[3] Perez, M, et al., "ATM Signalling Support for IP over ATM". RFC
1755, February 1995.
[4] Maher, M, "ATM Signalling Support for IP over ATM -
UNI Signalling 4.0 Update". draft-ietf-ion-sig-uni4.0-04.txt, May
1997.
[5] Coltun, R. and Heinanen, J., "The OSPF Address Resolution
Advertisement Option". Internet Draft, November 1997.
Acknowledgements
I would like to thank Rob Coltun and Lou Berger of Fore Systems for
their comments on earlier versions of this document.
Author Information
Juha Heinanen
Telia Finland, Inc.
Myyrmaentie 2
01600 VANTAA
Finland
Phone +358 303 994 808
Email jh@telia.fi
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