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Section: New Results
New generation of OLSR, new services and protocols
Participants : Cédric Adjih, Aline Carneiro Viana, Emmanuel Baccelli, Thomas Clausen, Philippe Jacquet, Pascale Minet, Ichrak Amdouni, Ridha Soua, Erwan Livolant, Paul Mühlethaler, Yasser Toor.
Executive summary
The user of a mobile network very quickly experience problems with quality of service: links fade, connectivity disrupts, delays accumulate.
In a wireless network, the set of neighbors which with one node can communicate depends on transmission range, and numerous factors, and in addition the transmission range is often lower than the interference range (the range within which a node prevents correct transmissions of other nodes). Thus bandwidth reservation, a crucial step of quality of service, is an important and difficult problem.
The services and protocols that need careful adaptation are
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Connectivity continuity
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Bandwidth reservation
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Delay routing
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Connectivity control
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Autoconfiguration
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Security
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Energy efficiency
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Localization
The connectivity continuity is the most important problem. Trivial in the wired world where a link failure is a rare event, it becomes problematic in the mobile world where link failure caused by mobility are frequent and normal. The first experiments of mobile ad hoc networks with regular internet protocols miserably failed simply because either the protocol was to slow to recover link failure, or when tuned appropriately was generating such a huge overhead that the network collapsed under its own weight. A new generation of routing protocols has arised that allow a suitable control of connectivity in mobile networks. Among them the Optimized Link State Routing combines the optimization of overhead for mobile networks and the full internet legacy. It naturally provides path redundancy which accelerate link failure recovery.
The most important lesson that must be retained is that most of these optimization become NP complete, which is a significant complication compared to their counterpart in the classical wired world. The reason for the NP-completeness is two-sided: on one side the co-interferences make impossible an optimization link by link, on the other side, the large dispersion of performance measurement makes simple heuristic ineffective. As an example, routing with respect to shortest delay average does not guarantee smallest probability of high delay.
Since the bandwidth is scarce, any multimedia application such as video streaming is resource demanding. For example a TV broadcast that uses a mesh network will rapidly exhaust the bandwidth if all connections are point to point. In this case multicast protocols that allows to gather all these point to point connections in a single flow is a need.
There are two classes of multicast protocols: the tree based protocols and the network coding protocols. In the first class the protocols take advantage of the relatively small size of the recipient node set. One can show equivalent results of Gupta and Kumar scaling properties but in the multicast plan when the ratio of recipient versus network size is a fundamental parameter. When this ratio tends to one the performance naturally worsen.
When the recipient set is the whole network, one can apply the network coding scheme with random packet combination. In network coding the packets are no longer isolated: relay nodes makes linear combination of packets and transmitted mixed packets. In theory the performance of network coding is better than isolated packet multicast. In practice network coding is simpler to operate does not need topology management such as spanning trees or Connectedc Dominating Set. The reason for this is highly non intuitive, as if packet superposition was acting like state superposition in quantum mechanic, leading to non expected results.
Quality of service has become the central requirement that users expect from a network. High throughput, service continuity are critical issue for multimedia application over the wireless internet where the bandwidth is more scarce than in the wired world. A significant issue in the ad-hoc domain is that of the integrity of the network itself. Routing protocols allow, according to their specifications, any node to participate in the network - the assumption being that all nodes are behaving well and welcome. If that assumption fails - then the network may be subject to malicious nodes, and the integrity of the network fails. An important security service over mobile networks is to ensure that the integrity of the network is preserved even when attacks are launched against the integrity of the network.
Scientific achievements
Optimized Link State Routing (OLSR)
The routing protocol OLSR is universally known in the mobile wireless community (more than 475,000 hits on Google). It has numerous implementations and is used in many wireless networks. It is a proactive protocol with full internet legacy which is based on partial topology information exchange, that non the less provide optimal path with additive metrics (such as BGP/OSPF). It is an experimental RFC within IETF and soon will become a full standard under the name OLSRv2.
OSPF extension for wireless mesh networking
Long a near-future myth, ad hoc networks are now becoming a reality as a variety of wireless mesh networks are being deployed. Wireless mesh networks are a specific kind of ad hoc network, where terminals are essentially fixed. Even in such cases, which somewhat resembles usual networks, specific routing protocols have nevertheless to be employed, to cope with the characteristics of wireless, multi-hop communications. Such characteristics include scarce bandwidth over inherently unreliable, versatile, semi-broadcast links, and absence of a central authority in general. One of the main difficulties in this context is to cope with contradictory requirements such as, on one hand, dealing with bandwidth scarcity, which typically requires decreasing control traffic, while on the other hand, dealing with unreliable, versatile links which typically requires increasing control traffic. The two prominent routing protocols that have been developed for ad hoc networks and studied over the past decade, are the IETF standards AODV and OLSR. AODV is based on a reactive scheme (i.e. on-demand flooding to discover a path to a new destination), while OLSR is based on a proactive scheme, which is essentially an optimization of link state routing (i.e. pre- provisioning of paths to all possible destinations). OLSR is to date the most deployed such protocol, as it powers numerous wireless mesh community networks that currently flourish in various cities throughout Europe and North America. Based on this experience, the integration of ad hoc networking in the "standard" networking body is going further in several directions. One direction is the IEEE 802.11s standardization effort, which uses AODV and OLSR-derived algorithms to provide wireless mesh routing capabilities below IP. Another direction, spearheaded by the IETF, is the extension of IP routing standards such as OSPF to support ad hoc routing: in this realm we recently spun RFC 5449, as well as a series of academic publications on the subject. The idea behind extending OSPF to support ad hoc networks comes from a simple observation: OSPF is algorithmically quite similar to OLSR, as both are based on a proactive, link state approach. As on the other hand OSPF is a well-understood, widely deployed, industry-standard protocol, employing it to integrate ad hoc networks with existing infrastructure is considered by users as an easy migration path.
Multi-metric routing
Quality of service involves finding routes between two nodes in the network that satisfies a number of constraints. These constraints could be the requested bandwidth, the maximum delay, the minimum loss probability, the reliability of links, etc. This problem is NP-Complete because it combines additive metrics in the optimization problem. Hipercom proposed heuristics for finding routes that respect up to four metrics when calculating routes between source and destination. Another QoS issue is the creation of models that estimate the actual value of a metric. For example, computing the available bandwidth or the transfer delay on a link, etc. is very complex in a non-deterministic medium access such as WiFi. To resolve this problem, we developed a model for estimating the available bandwidth in a wireless network. This model is based on considering interfering links in cliques, after which we provide the maximum capacity that could be deployed in a clique. We may still enhance the model by adding a scaling factor to the clique equations in order to become more accurate when compared to real measurements.
In particular we have investigated the metric based on packet delay distribution. Since propagation delays between routers are negligible, most delays occur in queueing and medium access control processing. Contrary to previous common belief there is no need of network synchronization. The objective is to proactively determine the delay in absence of packet data traffic. The estimate of delay distribution is done via analytical method. In order to keep control on quality of service flows we use source routing forwarding options.