MANET S. Ruffino
Internet-Draft P. Stupar
Expires: April 18, 2005 TILAB
T. Clausen
LIX
October 18, 2004
Autoconfiguration in a MANET: connectivity scenarios and technical
issues
draft-ruffino-manet-autoconf-scenarios-00
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Copyright (C) The Internet Society (2004).
Abstract
MANET interconnection with external networks enables a number of
usage scenarios, but generates also a number of technical issues,
mainly related with node autoconfiguration and global connectivity.
This Internet Draft aims at characterizing global connectivity
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scenarios and listing technical issues related to IP address
autoconfiguration which are implied by such scenarios and should be
addressed by a generic solution.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Scenarios . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1 Isolated MANET . . . . . . . . . . . . . . . . . . . . . . 6
3.2 MANET connected to an external network . . . . . . . . . . 6
3.2.1 Fixed Gateways . . . . . . . . . . . . . . . . . . . . 8
3.2.2 Mobile Gateways scenario . . . . . . . . . . . . . . . 9
4. Technical issues . . . . . . . . . . . . . . . . . . . . . . . 11
4.1 Isolated MANET . . . . . . . . . . . . . . . . . . . . . . 11
4.1.1 General Aspects . . . . . . . . . . . . . . . . . . . 11
4.1.2 Stateful Autoconfiguration . . . . . . . . . . . . . . 11
4.1.3 Stateless Autoconfiguration . . . . . . . . . . . . . 13
4.2 Connected MANET . . . . . . . . . . . . . . . . . . . . . 14
4.2.1 Prefix Advertisement methods . . . . . . . . . . . . . 14
4.2.2 Prefixes assignment to the gateways . . . . . . . . . 15
4.2.3 Multiple Gateways routing and addressing issues . . . 15
4.2.4 Stateful Autoconfiguration . . . . . . . . . . . . . . 17
4.2.5 Stateless Autoconfiguration . . . . . . . . . . . . . 17
4.2.6 NAT Considerations for IPv4 . . . . . . . . . . . . . 17
4.2.7 Ingress Filtering . . . . . . . . . . . . . . . . . . 17
4.2.8 Mobile IP . . . . . . . . . . . . . . . . . . . . . . 17
4.2.9 Data Forwarding . . . . . . . . . . . . . . . . . . . 19
5. Architectural considerations . . . . . . . . . . . . . . . . . 20
6. Security Considerations . . . . . . . . . . . . . . . . . . . 21
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 24
A. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 25
Intellectual Property and Copyright Statements . . . . . . . . 26
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1. Introduction
MANET were initially designed to be employed in highly dynamic and
unpredicatable environments, characterized by high mobility of users
and terminals. MANET are essentially autonomous, self-configuring,
self-healing networks, whose mobile nodes discover other nodes and
supported services in an automatic fashion. MANET routing protocols,
as studied in IETF, enable two generic MANET nodes to exchange data
traffic through multi-hop connections, if a 1-hop radio link between
them is not available. In this way, nodes can freely move within the
MANET: routing protocols dynamically react to movement and constantly
discover and choose the optimal path according to a predifined
metric, e.g. number of hops. If an intermediary node, which belongs
to a path between a source and a destination, fails traffic is
automatically re-routed through an alternative path.
RFC2501 [1] gives a definition of MANET and also introduces the
possibility to connect a MANET to an external network, by means of
gateways: in this case the MANET acts as a "stub" network, whose
nodes route traffic originating and/or terminating within the MANET
only.
Operators, Network and Service providers show increasing interest in
this type of network, as a consequence of the wide spreading of
low-cost radio technologies such as IEEE802.11a/b/g/h and the
increasing customer base. Commercial MANET deployments indicate that
MANETs are mainly employed as extensions of traditional
infrastructure networks, to realize the so-called hybrid networks:
MANET nodes exchange traffic between them using multi-hop paths and
can reach outside hosts and the Internet by means of the gateways,
which are equipped with two or more interfaces: a MANET interface and
typically an interface to one or more non-MANET networks.
An example of this are the so-called Mesh Networks, used to extend
the coverage area of a public hot-spot or to realize large-scale
low-cost wireless coverage in urban areas. A further interesting
application and research field is represented by multi-hop cellular
networks: MANETs connected to cellular WAN networks. In this case
MANETs can be used to realize an extended wireless coverage in areas
where "traditional" cellular network is not available.
Many proposals and projects that introduce an integration between
MANET and 3G+ networks exist in this area: among other [2], [3] and
[4].
In both isolated and hybrid MANETs, an important technical issue is
automatic configuration of network parameters on each node: IP
address, default route, DNS, etc. At least one unique IP address
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with local scope is needed to enable traffic exchange within a MANET.
In particular, if a MANET is connected to an external network, IP
address autoconfiguration rises an additional problem, known as
"Global Connectivity": the IP address should be globally routable, in
order for the node to be reachable both from MANET internal nodes and
external Internet hosts. The intrinsic high dynamicity of MANETs
rises additional issues: a MANET could split in two or more
sub-MANETs, or two or more MANETs could merge in one larger MANET.
In all these cases, a node could experience problems due to address
conflicts, reconfiguration and reestablishment of the default route.
So attention must be paid in such scenarios, expecially when multiple
gateways are used within the MANET.
Many solutions addressing autoconfiguration and global connectivity
issues have been proposed ([5], [8], [11], [6], [7], [9]). In the
authors' opinion, there is the need to define a number of MANET
connectivity scenarios and, consequently, a clear problem statement,
against which to validate proposed and future solutions.
This document is structured in the following way: in Section 2 a
glossary for commonly used terms is given; in Section 3 connectivity
scenarios for a MANET are listed. In this section particular
attention is given to the connection of a MANET with other external
wireless networks, by means of one or more fixed (Section 3.2.1) or
mobile wireless gateways (Section 3.2.2). In Section 4 IPv4 and IPv6
autoconfiguration and routing issues are derived from the previous
scenarios, both for an isolated (Section 4.1) and connected (Section
4.2) MANET. Some architectural considerations are reported in
Section 5.
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2. Terminology
Node
An IPv4/IPv6 device which is a MANET element: it runs a MANET
routing protocol and exchanges data with other nodes within a
MANET and with hosts located within external networks. A node has
at least one physical interface connecting it to the MANET.
Gateway
A node equipped with at least two interfaces, one of which
connects it to an external network, i.e. non-MANET, and can be
wired or wireless.
Host
An IPv4/IPv6 terminal/computer, external to the MANET. Host is
defined here as only "External" to differentiate it from the nodes
of the MANET.
Wireless Interface (or MANET interface)
The physical network interface that connects a node to the MANET.
Radio Interface (or Cellular Interface)
The physical network interface that may connect a gateway to an
external Wireless Wide Area Network, owned and administered by an
operator.
Stateful autoconfiguration
A mechanism of address autoconfiguration is defined stateful, when
a MANET node receives its IP address by another device, acting as
configuration server, which can be either within or outside the
MANET.
Stateless autoconfiguration
A mechanism of address autoconfiguration is defined stateless when
a node builds its own address without the partecipation of any
external device.
Default route forwarding
A node transmits all data directed to external networks to one of
its neighbors, elected as next-hop to the chosen gateway.
Source route forwarding
A node transmits all data directed to external networks explicitly
indicating in each packet's header which gateway must be used to
forward it. "Loose Source Routing" for IPv4 has been specified in
[16]; the "Routing Header" has been specified for IPv6 in [17].
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3. Scenarios
In this section, we describe the scenarios in which a MANET may
typically operate and where different address autoconfiguration
issues arise.
3.1 Isolated MANET
An isolated MANET is a network that is autonomously set-up between
wireless mobile nodes localized in the same geographical area. Nodes
activate Layer 2 radio links, by which they can exchange traffic with
their neighbors, and employ an ad-hoc routing protocol, which enables
multi-hop data forwarding through intermediate nodes. Routing
protocol constantly discovers routes between nodes, in a proactive
([12], [14]) or reactive fashion ([13], [15]): this enables each node
to route traffic to all other nodes within the MANET also during
movements.
In this type of MANET there is no connection to an external network:
all traffic is generated by MANET nodes and addressed to MANET nodes.
Typical applications of this scenario are temporary networks, that
must be set-up in areas where neither wireless coverage nor
infrastructure exist. Examples can be emergency networks used for
disaster recovery, battlefield applications, electronic surveillance.
Other examples can be found in occasional work meetings, where
networks are formed to enable file sharing between co-workers.
3.2 MANET connected to an external network
In this scenario a MANET is connected to an external network by means
of one or more gateways (Figure 1). A generic MANET node, running a
MANET routing protocol, can exchange data traffic with every other
node through multi-hop paths and communicate with hosts located in
the external network, routing its uplink traffic towards a gateway.
This, in turn, will receive return traffic from the host and will
route it to the source node.
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H1
|
+---------------+
| Internet |**
+---------------+ *
* * *
* * *
GW1** * GW3
| +--GW2-------+
| | |
---N1--------+ |
/ \ |
N4 \ N2
N3-----/
Figure 1: MANET interconnected to an external network
Gateways play a critical role here. If the number of nodes in the
MANET increases, gateways can become bottlenecks, as they route an
increasing and possibily huge amount of traffic. This also depends
on the available bandwidth on the uplink interface. Moreover,
gateways can be equipped with a number of additional features. For
example, they could participate to the external routing protocol, in
order to announce internal routes to external routers and hosts,
possibly performing some kind of aggregation. They can act as
enforcement points for security purposes: they can control access to
external networks and, following a common practice, they can enforce
Ingress Filtering on MANET generated traffic. Finally they can also
provide services like DNS to MANET nodes.
This scenario can be expanded, depending on the characteristics of
the network interface connecting gateways to the external network: it
can be either wired or wireless, which can, in turn, be of a
different type with respect to the MANET interface. In the first
case Gateways are fixed, while in the second case they can also be
mobile, as the other MANET nodes.
Moreover, a MANET can have only one gateway (fixed or mobile) or can
have multiple gateways (fixed or mobile). Other than guaranteeing a
higher degree of reliability and fault tolerance to the entire MANET,
the presence of multiple gateways permits load balancing among the
gateways themselves. This can be very useful especially when the
external network is a low-throughput cellular WAN, such as GPRS/EDGE,
in order to not overload a single gateway with traffic potentially
generated by many nodes at the same time. Single traffic flows of
multiple nodes or many flows of a single node can be routed through
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different gateways, consequently suggesting an improvement of the
overall performances of the MANET.
Gateways can also be equipped with additional resources in order to
grant better fault tolerance to the entire MANET: additional energy
resources, more processing power, more volatile and non-volatile
memory. This is especially true in case of fixed gateways, that can
be directly powered and directly operated. It is clearly more
difficult to enhance mobile gateways, due to their limited energy
resources.
The following sections detail usage scenarios for fixed and mobile
gateways.
3.2.1 Fixed Gateways
In this scenario, gateways are deployed in predefined positions
planned by the network operator. Each gateway is connected to the
external network by means of a wired or wireless interface.
Mesh networks and networks used for environmental surveillance can be
categorized under this scenario.
o Mesh Networks: these are probably the most widespread ad-hoc
networks. In a Mesh Network, user terminals (nodes) exchange
traffic between them directly through a layer-2 radio link and
using other nodes or fixed wireless Access Points as intermediate
relays. A Mesh Network is typically connected to an external
infrastructure network by means of fixed wired Access Points,
which act as gateways and typically connect the Mesh to an
external infrastructure network.
Mesh Networks can be further categorized, depending on if the mesh
is realized only between the wireless Access Points or also
between all the nodes, which, in this case, run a routing
protocol. In fact, in some deployments, intermediate access
points are equipped with two wireless interfaces: the first
interface forms the mesh with other peer access points,
participating in an ad-hoc routing protocol, the second interface
provides local connectivity to nodes, which cannot set-up a
network themselves, as they don't run any routing protocol.
Applications of this networks are Internet public access
(browsing, email etc.) by mobile users from outdoor areas,
wireless coverage of corporate building to give employees access
to shared data and commonly used services (email, Intranet
browswing). These solutions can bring to savings on cabling
costs.
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o Surveillance networks: several wireless nodes endowed with sensors
of varios kinds are spread over high enviromental risk areas (e.g.
fires). They communicate through multi-hop connections and run a
routing protocol. When an emergency situation arises, data
collected by sensors are transmitted from the collecting nodes
upwards one or more gateways (which can have both a wired or
wireless interface) and conveyed to a manned monitoring station.
Topologies of this kind of network are typically static, as the
nodes are installed in fixed positions within the monitored areas.
Moreover, these networks are characterized by multiple constant
low-throughput data flows going from the sensors to the gateways.
3.2.2 Mobile Gateways scenario
In this scenario, the gateway's radio interface, connecting the MANET
to the external network, can be a cellular WAN interface (GSM, GPRS,
EDGE, UMTS), a broadband wireless MAN (WMAN) interface (e.g.
802.16x, 802.20) or a WLAN interface (802.11a/b/g/h/j). In each of
these cases, gateways can forward uplink traffic outside the MANET
only if located within the transmission/reception range of one or
more Base Stations or Access points. Gateways can therefore not only
freely move within the coverage area, but they can also move outside
this area: in such case, the gateway can't forward uplink traffic
destined for external hosts anymore, nor downlink traffic destined
for internal nodes.
The primary benefit of coverage extension is that local communication
between two nodes of the MANET are preformed without using any
cellular radio resource, e.g. radio channels. Another benefit is
the possibility to grant network access also to those terminals that
are not equipped with a cellular radio interface (e.g. access
sharing). The implication of this business model on security and
accounting aspects are out of the scope of this draft.
A more advanced scenario can be realized when most of the nodes are
also equipped with two heterogeneous interfaces. In this case
gateways can be "occasional": they can be nodes that, after setting
up the connection towards the external network, whenever located
within its coverage area, can start forwarding other nodes' outbound
packets. In this kind of scenario, gateways can be "special" nodes
endowed with additional features, but they can also be ordinary MANET
nodes, such as mobile phones and PDAs. In this last case, gateways
are characterized by low computational power and limited energy
resources. Although the MANET can again exploit benefits given by
multiple gateways, additional issues arise: in fact, gateways are not
under operators control anymore. It's possible that the owner of the
gateway decides abruplty to turn his terminal off or to tear down the
connection towards the cellular network, in order to save battery
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life. Thus, the number and the position of gateways are higly
dynamic and this can cause frequent re-routing of uplink data flows.
In this situation, autoconfiguration operations performed by nodes
and gateways assume critical importance, as they have not only to be
performed by a node on joining the MANET, but also constantly
repeated, in order to repair broken routes to the gateway.
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4. Technical issues
The scenarios introduced in the previous sections, raise some
technical issues that must be addressed both to guarantee the normal
functionalities of a MANET and to assure an accettable level of
performance to the applications running on each single node. Among
those issues there is particullary the one of the nodes' IPv4/IPv6
address configuration.
The purpose of this section is not to propose of a solution to the
address configuration issue, but to characterize and describe the
technical aspects, which are related to such issue, affect the
scenarios descripted above and lack of a standard solution.
4.1 Isolated MANET
In an isolated MANET scenario, the nodes' addressing issue has been
already considered by many proposals which furnish a set of
solutions. See [21] for a complete and exaustive comparison of such
proposals. However, some of these proposals do not cope with all the
critical aspects that affect the autoconfiguration issue in an
isolated MANET.
The purpose of this section is to describe the main critical aspects
related to the addressing in this scenario.
4.1.1 General Aspects
As an isolated MANET is characterized by flat addressing: a node must
only be endowed with an address, unique within the MANET itself.
There are no limitations on the prefix of such address, both when
using IPv4 and when using IPv6.
Therefore, the problem can be reduced to the configuration of an
unique IP (IPv4 or IPv6) address, without any constraint on the
chosen prefix. Such configuration should be made in an efficient and
automatic way.
In the following, stateful and stateless approaches are considered
separately.
4.1.2 Stateful Autoconfiguration
In an isolated MANET, the role of the configuration server can be
only assigned within the MANET itself and can be performed by any
node of the network. It must be always guaranteed that within a
MANET there is a node acting as configuration server. This may not
be always true, as a MANET can experience a partition during its
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existence. If indeed there is only one node providing stateful
configuration within a MANET and such a MANET gets partitioned, one
of the newly-born networks will be without any connection to the
configuration server.
Several solutions have been proposed to counter this issue:
o Dynamic Election: the role of the configuration server is
dynamically assigned to one only MANET node.
o Server Redundancy: the role of configuration server is distributed
among more than one terminal of the MANET; it can also be
distributed among all the MANET nodes.
When an isolated MANET is endowed with a stateful address mechanism,
there is the problem of detecting, by means of a Service Discovery
mechanism, which of the MANET nodes acts as a configuration server.
This is true both when the configuration service is assigned to a
single node and when this role is distributed among several nodes.
Moreover, when the stateful configuration service is not distributed
among all the nodes, a MANET which has been created by a previous
partition can be without any configuration server: the problem of the
configuration server election within such MANET therefore arises.
Partitioning is not the only event creating some problems with
respect to addressing. It can't be excluded that two MANETS merge,
creating a new MANET, as such kind of networks are characterized by
the mobility of their elements. Moreover it can't be excluded that
two nodes, located in two different MANETs, own the same address (the
unicity of an address is indeed guaranteed only within the MANET the
node is connected to). If these two MANETs merge with each other,
there would be a newly-born MANET in which two nodes have configured
the same address. Such an event surely compromises the normal
functions of these two nodes, but also of the whole network. It is
therefore necessary that a node verifies the unicity of its address
after a merging. Such requirement implies the existence of a
merging-detection mechanism, or that every node keeps on verifying
the univocity of its address.
It is worth mentioning that when the stateful configuration is
executed by means of more than one server, it is possible that within
a MANET, two of such servers assign the same address to two nodes:
the problem of the DAD execution therefore arises, otherwise it is
necessary that a coordination mechanism exists among servers, which
assures that assigned addresses belong to disjoint addressing space.
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4.1.3 Stateless Autoconfiguration
If the configuration mechanism is stateless, there are several
issues, depending on the used Internet protocol. A stateless
configuration mechanism has been standardized both for IPv4 and for
IPv6. Such mechanisms are based upon the assumption that all the
nodes of the network are located on the same link: this assumption is
not valid for MANETs. Moreover, such solutions don't guarantee that
the configured address is unique, and therefore a DAD mechanism is
still needed. We assume here that a global prefix is not available
in the MANET.
o In case of IPv4, the standard stateless configuration mechanism is
ZEROCONF [19]: through its use, a node configures itself a
link-local address derived from the prefix 169.254.0.0/16.
o In the case of IPv6 the standard stateless configuration is
defined IPv6 stateless autoconfiguration [18] and assures the
automatic configuration of a link-local address on network
interfaces by using the well known prefix fe80::/16. The suffix
of such address is the EUI-64 identifier, which is derived from
IEEE 802 identifier of network interfaces through which the nodes
are connected to the network. The uniqueness of such addresses is
not guaranteed if the network is made of nodes having
heterogeneous interfaces, in particular when one or more of them
are connected to the network by means of interfaces that have no
IEEE 802 identifier (e.g. Bluetooth). Such nodes must be endowed
with a mechanism that lets them generate a suffix which will be
used to set up the link-local address through stateless
configuration. It is, however, not guaranteed that the generated
address is unique and that a DAD mechanism is therefore still
required. The issues related to the use of link-local addresses
within a MANET, listed in [5] , can be summarized by asserting
that IPv6 prevents the nodes from communicating with such type of
addresses along multi-hop paths.
o In ZEROCONF, the DAD mechanism is based upon ARP messages
exchange, which is used by the nodes to defend their address and
to detect the presence of a duplicate address. In IPv6 the same
mechanism is based upon exchange of Neighbor Solicitation and
Neighbor Advertisement messages. In both cases (IPv4 and IPv6),
the DAD mechanism has been defined to be executed within LANs,
which are networks whose elements are on the same link. As cited
before, in a MANET, such an assumption is not valid: it is
therefore necessary to introduce a standard DAD mechanism which is
able to detect that two nodes use the same address even when such
nodes are not directly connected on the same link.
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o It can't be excluded that two nodes located into two different
MANETS own the same address also when the configuration mechanism
is stateless and not only in the case of a stateful approach: if
these two MANET merge, there will be two nodes having the same
address in the new MANET. It is therefore necessary that MANET
nodes verify the uniqueness of their address not only when they
enter the MANET, but also after a merging. Such requirement
implies the necessity of a merging-detection mechanism or the
necessity of a continuos DAD execution, as stated in section
Section 4.1.2. For example, [19] and [18] define the continuos
defense of nodes' addresses.
Incidentally, partitioning do not imply any issue, as addresses
are trivially unique before and after the MANET gets partitioned.
4.2 Connected MANET
If the MANET is connected to an external IP network, e.g. the
Internet, the address configuration must let the MANET nodes to
communicate with hosts located in the external network. In turn,
this implies the configuration of an IPv4 or IPv6 global address: the
network interface partecipating to the MANET routing protocol must be
configured with at least one globally routable address, which can be
public or private (only for IPv4). In case of public addressing,
such an address must be derived from a prefix which must be globally
valid, while in the case of private IPv4 addressing there must be a
NAT agent somewhere. We assume here that prefixes are managed and
announced by gateways in the MANET.
Several issues related to global IP address configuration can be
characterized. Some of these are similar to those described in
section Section 4.1, however, there are some scenarios that create
further aspects that must be considered. The most complex scenarios
are particularly the ones characterized by the presence of multiple
gateways and multiple prefixes. In such situations a MANET indeed
experiences high dynamicity, as described in section Section 3.2.
4.2.1 Prefix Advertisement methods
It is firstly necessary considering the ways through which prefixes
are announced within a MANET. Two general approaches exist: prefix
information is inserted into routing protocol messages or inserted
into other protocol messages. In the first case it is necessary an
opportune extension of routing protocol: the solutions could not be
general enough, as the global connectivity is tied to every single
routing protocol. Using a routing protocol can optimize the
diffusion, the choice and the management of the prefixes that must be
used: a deeper description of such aspect is given in Section 4.2.3.
Viceversa, if prefix advertisement is independent from the routing
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protocol, it is necessary the definition of new mechanisms or the
modifications to existing ones. IPv6 Neighbor Discovery, for
example, must be adapted to the multi-hop nature of the MANET paths.
4.2.2 Prefixes assignment to the gateways
For this purpose, gateways must own the prefixes which they announce
within the MANET. It is possible to assume that they are manually
configured on gateways, but this approach is problematic if it is
applied to dynamic scenarios such as those described in Section
3.2.2. In this scenario, gateways can appear or disappear within a
MANET in an unpredictable way. Gateways should therefore
automatically receive the global prefixes associated to the MANET
they are connected to, e.g. through a prefix delegation mechanism
[23], that, moreover, lets external routers install automatically the
proper routes towards the MANET.
4.2.3 Multiple Gateways routing and addressing issues
The presence of multiple active gateways can improve the overall
robustness of the MANET, as described in Section 3.2. In fact,
gateway redudancy guarantees a higher fault tolerance: for example,
if a gateway fails, nodes, which are using such gateway, can
dynamically change their own default gateway and continue to
communicate with external hosts.
Multiple gateways can improve global MANET performance if nodes are
enabled to concurrently route traffic through more than one gateway.
If all the MANET nodes used the same gateway as default gateway,
leaving the others as pure back-up, load balancing among different
gateways would not be exploited.
Moreover, if uplink traffic is supposed to be sent towards different
gateways and if also downlink traffic is supposed to be received from
different gateways, the choice of the prefix used by a node to
configure its address assumes critical importance. In fact, IP
source address used for uplink traffic determines the gateway to
which the return traffic will be sent. It is desiderable that
traffic follows the same path uplink and downlink within the MANET:
this is because the uplink path towards the default gateway is
guaranteed to be optimal by routing the protocol.
However, in case of multiple gateways / multiple prefixes in the
MANET, the following can arise (as depicted in Figure 2): a node N1
elects a default gateway GW1, receives prefixes form it, configures
its global address, begins a data session with an external host H1.
Both uplink and downlink traffic are delivered through GW1. Then the
node moves away from its position and find itself near another
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gateway, GW2, which has a better routing metric than the previous
gateway, which is now many hops away (namely, N4 - N3 - N2). The
node elects this as default gateway, but keeps its old global
address, in order not to disrupt the session. Downlink traffic
follows in this case a non-optimal path within the MANET, which can
be potentially very long: the bandwidth available to the user (and to
other MANET nodes) may descrease dramatically. It is therefore
desiderable that the return traffic flows towards the default gateway
of the source node.
---------->H1
/ |
/ +---------------+
/ +-| Internet |---+
/ | +---------------+ |
| | |
| GW1++ +GW2
| | \ /
v | \N2----N3----N4/
N1
-----------H1<------------
/ | \
/ +---------------+ \
/ +-| Internet |---+ \
/ | +---------------+ | |
| | | |
| GW1++ +GW2+ |
\-----\ \ / | |
\ \N2----N3----N4+ | |
\--------------\ \----N1
------^
Figure 2: Multiple gateways non-optimal routing
In this scenario, a critical situation arises when one or more
gateways are no longer reachable from MANET nodes (e.g. because
MANET gets partitioned or gateways fail or simply shut-off). In this
case, nodes, whose global addresses are associated to such gateways ,
are no more globally reachable and should configure a new address.
Mobile IP can handle this address change. See Section 4.2.8 for
considerations on this approach.
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4.2.4 Stateful Autoconfiguration
Differently from what happens in an isolated MANET, in hybrid MANETs
the configuration server can also be located in the external network.
This option requires more attention and a deeper analysis,
particulary in the scenarios of Section 3.2.2, where gateways can
activate and deactivate abruptly. The major issue here comes from
the fact that a node may not reach the external server, thus
experimenting high delays and session disruption.
4.2.5 Stateless Autoconfiguration
If the connected MANET uses a stateless autoconfiguration mechanism,
same considerations as in Section 4.1.3 arise. In this case, the
node must be able to configure a global address: this must be derived
from a globally valid prefix, which is also used to install routes
towards the MANET within the external network. Issues related to
discovery of such prefixes and DAD have been described in Section
4.2.1 and Section 4.1.3, respectively.
4.2.6 NAT Considerations for IPv4
If any NAT mechanism is deployed, the problem of the prefix
diffusion, which is described in Section 4.2.1 and Section 4.2.2, is
avoided as the node in the MANET can choose a random address and use
it to partecipate to the routing protocol. However, the nodes of a
MANET whose gateways use NAT mechanism to achieve global
connectivity, as described in [24], experience a session break if
they change the gateway used to send uplink traffic.
4.2.7 Ingress Filtering
Ingress filtering is a security mechanism often implemented on sites
border routers, which could also be deployed on gateways of a
connected MANET. It consists in blocking all the outbound packets
whose source address does not belong to a prefix advertised within
the MANET. This means that in a MANET with multiple gateways, which
advertise multiple prefixes, nodes that have received a prefix from a
gateway must also use the same gateway to route uplink traffic. In
such a MANET, a node which changes its default gateway, must also
change its address, in order to be able to continue to transmit
traffic towards external networks.
4.2.8 Mobile IP
This section highligths some issues raised by the use of Mobile IP in
MANETs.
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In many of the scenarios described above, particularly in those of
Section 3.2, a MANET node may have to change its IP address. Reasons
for this can be the following:
o a node moves to a separate MANET, where different global prefixes
are advertised;
o the gateway, that is advertising the prefix used to configure a
node's address, fails: in this case downlink traffic directed to
such prefix cannot be delivered to the MANET anymore;
o ingress filtering (see Section 4.2.7 ) is deployed in the MANET to
improve security;
o as seen in Section 4.2.3 a node could change its address in order
to use always an optimal path both for uplink and downlink
traffic, thus acheiving better performances.
In order to handle this address changes, Mobile IP ([20]) can be
deployed on MANET nodes. Mobile IP is commonly used to avoid data
session disruption after a mobile node changes its IP address as a
consequence of a change of point of attachment to the Internet. A
MANET with multiple gateways can be considered as an overlapping set
of different access networks, if gateways advertise multiple global
prefixes.
The use of standard Mobile IP (either for IPv4 or IPv6) in this
scenario can imply several issues, described in the following.
o In the scenario described in Section 4.2.3, where multiple egress
gateways can concurrently be active, a node could change its
address frequently. This brings to the "Binding Storm" problem:
if many nodes in the MANET change their addresses at the same
time, a very high number of Binding Update messages (or
Registration Request, if Mobile IPv4 is used) will be generated,
therby overloading the MANET and gateways in particular.
o If Mobile IPv4 is used, some nodes with special functions, such as
Foreign Agent or DHCP server, must be located within the MANET in
order to assign a valid address to the nodes. If the Foreign
Agent approach is used, two more mechanisms are needed: a
discovery mechanism for Foreign Agents and a mechanism which
enables multi-hop transmission of Advertisement and Registration
Request/Response messages within the MANET. If the DHCP approach
is used, same consideration hold as those described in Section
4.2.4. Moreover, if robustness is needed and one employs
redundant agents, synchronization issues may arise.
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o If Mobile IPv6 is used, the major issue is related to Movement
Detection. In fact, Movement Detection is realized by means of
Router Advertisements: if these messages are not present in the
MANET, i.e. prefixes are advertised through MANET routing
messages, standard Mobile IPv6 procedure will not work. Moreover,
Router Advertisements were not designed to be multi-hop, so same
considerations as in Section 4.1.3 and Section 4.2.1 apply.
4.2.9 Data Forwarding
There can be two methods which a node can use to forward data traffic
towards external networks : default route and source routing. Both
methods, applied in the reference scenarios described above, have
pros and cons.
If the default route method is employed, intermediate nodes of a path
from a source node to a gateway may not have elected the same default
gateway as the source node. This means that data can flow towards a
different egress gateway: if ingress filtering is deployed,
communication is not possible. If source routing is used, data can
flow through a gateway explicitly chosen by source node.
Moreover, if a default route is used, load balancing in the MANET
could be problematic as, again, nodes can't control the path data
follow through the gateway.
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5. Architectural considerations
There are many different proposal that solve many of the issues
described above. As already mentioned, the objective of this draft
is not to give a survey of state of the art solutions, nor to address
problems that such solutions pose. It is, however, needed to clarify
what architectural choices are more effective, in order to design a
comprehensive solution for both autoconfiguration and global
connectivity problems. Two important aspects are the following.
o Layering : this means integration between different protocols
(autoconfiguration, mobility, routing). It can bring the
definition of optimized mechanims, exploiting information passed
from one protocol to another (e.g. to perform movement
detection). From a different point of view, it is however
desiderable to keep different functionalities separated from each
other, in order to get a modular approach.
o Nodes with specific functionalities: in some scenarios one can
deploy some special nodes, enabled with mechanisms such as DHCP,
NAT, Foreign Agent, gateway, that have, for example, a fixed
connection to external network and are equipped with more
energetic resources than "normal" nodes. In this case these
special nodes perform all "operational" tasks and normal nodes are
only source or destination for data traffic and are only requested
to run a MANET routing protocol. In this case, if a special node
fails, unpredictable results are possible, that can bring to the
failure of the entire MANET. Moreover, in a very large MANET
(i.e. big cardinality), it can be difficult to mantain an
acceptable level of performance with only one active gateway,
which can become a bottleneck for outbound traffic.
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6. Security Considerations
This document raises no security issue.
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7. IANA Considerations
This document has no actions for IANA.
8 References
[1] Corson, S. and J. Macker, "Mobile ad hoc networking (MANET):
Routing protocol performance issues and evaluation
considerations", RFC 2501, January 1999.
[2] Siebert, M., "On Ad Hoc Networks in the 4G Integration
Process", Med-Hoc 2004 , June 2004.
[3] "Ambient Networks", http://www.ambient-networks.org .
[4] "World Wireless Research Forum",
http://www.wireless-world-research.org .
[5] Wakikawa, R., Malinen, J., Perkins, C., Nilsson, A. and A.
Tuominen, "Global connectivity for IPv6 Mobile Ad Hoc
Networks", I-D draft-wakikawa-manet-globalv6-03.txt, October
2003.
[6] Cha, H., Park, J. and H. Kim, "Extended Support for Global
Connectivity for IPv6 Mobile Ad Hoc Networks", October 2003.
[7] Jeong, J., Park, J., Kim, H. and D. Kim, "Ad Hoc IP Address
Autoconfiguration", I-D
draft-jeong-adhoc-ip-addr-autoconf-02.txt, February 2004.
[8] Perkins, C., Malinen, J., Wakikawa, R. and E. Belding-Royer,
"IP Address Autoconfiguration for Ad Hoc Networks", I-D
draft-perkins-manet-autoconf-01.txt, November 2001.
[9] Singh, S., Kim, JH., Choi, YG., Kang, KL. and YS. Roh, "Mobile
multi-gateway support for IPv6 mobile ad hoc networks", I-D
draft-singh-manet-mmg-00.txt, June 2004.
[10] Paakkonen, P., Rantonen, M. and J. Latvakoski, "IPv6 addressing
in a heterogeneous MANET-network", I-D
draft-paakkonen-addressing-htr-manet-00.txt, December 2003.
[11] Jelger, C., Noel, T. and A. Frey, "Gateway and address
autoconfiguration for IPv6 adhoc networks", I-D
draft-jelger-manet-gateway-autoconf-v6-02.txt, April 2004.
[12] Clausen, T. and P. Jacquet, "Optimized link state routing
protocol", RFC 3626, October 2003.
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[13] Perkins, C., Belding-Royer, E. and S. Das, "Ad hoc On-Demand
Distance Vector (AODV) Routing", RFC 3561, July 2003.
[14] Ogier, R., Templin, F. and M. Lewis, "Topology Dissemination
Based on Reverse-Path Forwarding (TBRPF)", RFC 3684, February
2004.
[15] Johnson, D., Maltz, D. and Y. Hu, "The Dynamic Source Routing
Protocol for Mobile Ad Hoc Networks (DSR)", I-D
draft-ietf-manet-dsr-10.txt, July 2004.
[16] Postel, J., "Internet Protocol", STD 5, RFC 791, September
1981.
[17] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6)
Specification", RFC 2460, December 1998.
[18] Thomson, S. and T. Narten, "IPv6 Stateless Address
Autoconfiguration", RFC 2462, December 1998.
[19] Aboba, B., "Dynamic Configuration of Link-Local IPv4
Addresses", draft-ietf-zeroconf-ipv4-linklocal-17 (work in
progress), July 2004.
[20] Johnson, D., Perkins, C. and J. Arkko, "Mobility Support in
IPv6", RFC 3775, June 2004.
[21] Sun, Y. and E. Belding-Royer, "A study of dynamic addressing
techniques in mobile ad hod networks", I-D Wireless
communication and mobile computing, May 2004.
[22] Narten, T., Nordmark, E. and W. Simpson, "Neighbor Discovery
for IP Version 6 (IPv6)", RFC 2461, December 1998.
[23] Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic Host
Configuration Protocol (DHCP) version 6", RFC 3633, December
2003.
[24] Engelstad, P., T—nnesen, A., Hafslund, A. and G. Egeland,
"Internet Connectivity for Multi-Homed Proactive Ad Hoc
Networks", First IEEE International Conference on Sensor and Ad
hoc Communications and Networks , October 2004.
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Authors' Addresses
Simone Ruffino
Telecom Italia LAB
Via G.Reiss Romoli 274
Torino 10148
Italy
Phone: +39 011 228 7566
EMail: simone.ruffino@telecomitalia.it
Patrick Stupar
Telecom Italia LAB
Via G.Reiss Romoli 274
Torino 10148
Italy
Phone: +39 011 228 5727
EMail: patrick.stupar@telecomitalia.it
Thomas Heide Clausen
Laboratoire d'informatique
Ecole Polytechnique
Palaiseau Cedex 91128
France
Phone: +33 1 6933 2867
EMail: thomas.clausen@polytechnique.fr
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Appendix A. Acknowledgments
The authors would like to thank Ivano Guardini for his valuable
comments.
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