**2.4.1 Static forwarding**

338 Telecommunications Networks – Current Status and Future Trends

link costs the triggered signalling can be used in a combination with exponential smoothing

Compared to unsolicited signalling, on-demand signalling works the other way around. When a node (called the requesting node) requires state information, it queries the other nodes (called the serving nodes) for this information. Thus, on-demand signalling yields the state information as recent as possible, with expected benefit for the routing decisions. Furthermore, the type of state information which is queried (e.g. capacity or buffer occupancy) may vary according to the type of route that must be computed. On the other hand, since the signalling procedure is triggered for each route computation, the amount of traffic generated by on-demand signalling is likely to be higher than with unsolicited signalling. Additionally, the requesting node has to gather complete information before initiating the route computation. On-demand signalling is more convenient for connection oriented networks, where the source node requests the network state information from other nodes before setting up a connection and then the route to destination node is computed. As the number of packets during a signalling session is high, additional mechanisms (caching, snooping) have to be devised, in order to limit the number of

In the case of per-hop packet-switched routing routes cannot be computed on demand. Instead, routing tables are pre-computed for all nodes periodically or in response to a significant change in link costs, thus defining routing update intervals. Link-cost metrics for the delay sensitive traffic are typical additive metrics, and thus the shortest routes are typically calculated using the Dijkstra algorithm. The main feature of an additive metric is

On the other hand, the link cost for the throughput sensitive traffic is a concave metric. Thus, the total cost for any path equals the one on the link with minimum cost. A typical optimization criterion for the throughput sensitive traffic is to find the paths within minimum hop count with the maximum available bandwidth. Minimum hop count is an additional constraint, which is used to minimize the use of resources. The Bellman-Ford shortest path algorithm is well suited to compute paths of the maximum available bandwidths within a minimum hop count. It is a property of the Bellman-Ford algorithm that, at its *hth* iteration, it identifies the optimal path (in our context the path with the maximum available bandwidth) between the source and each destination not more than *h* hops away. In other words, because the Bellman-Ford algorithm progresses by increasing the hop count, it provides the hop count of a path as a side result, which can be used as a

Regardless of the type of traffic the second shortest path with disjoint first link can be calculated by eliminating the first link on the shortest route (i.e. *LC*l is set to infinity for delay sensitive traffic and to 0 in the case of throughput sensitive traffic) and using Dijkstra and Bellman Ford algorithm on such modified network. The alternative paths are used in

that the total cost for any path is a sum of costs of individual links.

link-cost function or adaptive forwarding.

signalling packets (Franck & Maral, 2002a).

**2.3 Computing routes** 

second optimization criterion.

the case of adaptive forwarding.

**2.2.2 On-demand signalling** 

Two representatives of static forwarding policies originally developed for regular network topologies, such as exhibited by ISL networks, are alternate link routing with deflection in the source node (ALR-S) and alternate link routing with deflection in all nodes (ALR-A) (Mohorcic et al., 2000, 2001). Both policies are based on an iterative calculation of routing algorithm for determining alternative routes between satellite pairs. An additional restriction considered in static forwarding policies is that the alternative routes must consist of the same (i.e., minimum) number of hops, with a different link for the first hop. Such alternative routes with the same number of hops guarantee that the propagation delay increase for the second-choice route is kept within a well-defined limit.

After determination of alternative routes with the same number of hops between each pair of nodes (satellites) the selected forwarding policy decides which packets are forwarded along each of these routes. Different forwarding policies are depicted in Fig. 1

According to the routing table given in Table 1, the SPR policy is only forwarding user traffic along the shortest routes. This leads to very non-uniform traffic load particularly on links (A-D, B-E, and C-F).


Table 1. Alternative paths to Satellite F with the same minimum number of hops.

Fig. 1. Path selection with different forwarding policies.

Routing and Traffic Engineering in Dynamic Packet-Oriented Networks 341

For the throughput sensitive traffic we monitor the number of packets in outgoing queues (*n*). The alternative second shortest path is used only if it has the same or a smaller number of hops (*h*) to the destination and if the number of packets (*n*) in the outgoing queue on the shortest path (*n1*) is more than a given threshold *Δtr<sup>T</sup>* (where *T* is denoting throughput sensitive traffic) higher than the number of packets in the outgoing queue on the alternative

21 1 2 ( ) ( () () *<sup>T</sup>*

The significance of the threshold is that it regulates distribution of traffic between alternative paths based on local information about the link status, and thus differentiates between lightly and heavily loaded nodes. The higher the threshold value the more congested the shortest path needs to be to allow forwarding along the alternative second shortest path. In the extreme, setting the threshold value to infinity prevents forwarding along the second shortest path (i.e. adaptive forwarding deteriorates to SPR), while no threshold (i.e. *ΔtrT* = 0) means that packets are forwarded along the second shortest path as soon as the expected queuing delay

Routing with the proposed adaptive forwarding promises more uniform distribution of traffic load between links and the possibility to react quickly to link failure. However, packets belonging to the same session can be forwarded along different routes, even within the same routing update interval, so additional buffering is required in destination nodes to

As we have shown in previous section, the general routing and traffic engineering functions consist of many different algorithms, methods and policies that need to be carefully selected and adapted to the particular network characteristics as well as types of traffic to be used in the network. Clearly, the more dynamic and non-regular the network and the more different types of traffic, the more demanding is the task of optimising network performance, requiring good understanding of the fundamental network operating conditions and the traffic characteristics. The later largely affect the performance of routing and traffic engineering, typically requiring appropriate traffic models to be used in simulating, testing and benchmarking different routing and traffic engineering solutions. In the following a methodology is described for developing a global traffic model suitable for supporting the dimensioning and computer simulations of various procedures in the global networks but focusing in particular on the non-geostationary ISL networks, which are well suited for supporting asymmetric applications such as data, audio and video streaming, bulk data transfer, and multimedia applications with limited interactivity, as well as the broadband access to Internet services beyond densely populated areas. Such traffic models are an important input to network dimensioning tasks (Werner et al., 2001) as well as to simulators devoted to the performance evaluation of particular network functions such as routing and

A typical multimedia application contains a mix of packets from various sources. Purely mathematical traffic generators cannot capture the traffic characteristics of such applications in real networks to the extent that would allow detailed performance evaluation of the

for the corresponding link is smaller than the one on the shortest path.

traffic engineering (Mohorcic et al., 2001, 20021, Svigelj et al., 2004a).

reorder terminated packets and obtain the correct sequence.

**3. Traffic modelling for global networks** 

*tr h h nt nt* ≤ ∧ − <Δ (17)

path (*n2*), as given in Equation (17).

The ALR-S policy ensures a more uniform distribution of traffic load over the network, as it distinguishes between the packets passing through a particular node and the packets that are originating in that node. Packets originating in a particular node are forwarded on the link of the second shortest route (e.g. from A to F via B, from B to F via C), while packets passing through the node are forwarded on the link of the shortest route (e.g. through A to F via D, through B to F via E). By using the second-choice route only for originating packets, the delay is increased with respect to the shortest route only on the first hop, hence the increase in delay does not accumulate for the packets with a large number of hops. Between the consecutive updates of routing tables, all packets between a given pair of nodes follow the same route. Thus, ALR-S policy maintains the correct sequence of the packets within the routing interval, the same as the SPR forwarding policy.

The ALR-A policy promises an even more uniform distribution of traffic load and thus further improvement of link utilisation by alternating between the shortest and the second shortest route regardless of the packet origination node (this is denoted in Fig. 1 by dashed lines). However, packets belonging to the same session can be forwarded along different routes even within one routing table update interval, thus additional buffering is required in the destination nodes to re-order terminated packets and obtain the correct sequence.

The static forwarding policies, such as ALR-S and ALR-A, distribute packets according to a pre-selected rule. They allow significant reduction of traffic load fluctuation between links, however they do not adapt to the actual traffic load on alternative routes.

#### **2.4.2 Adaptive forwarding**

In contrast to static forwarding an adaptive forwarding policy has to take into account the link status information to support the selection of the most appropriate between the alternative outgoing links on the route to the destination. An example of such approach is adaptive forwarding policy based on local information about the link load (Svigelj et al, 2003, 2004b; Mohorcic et al. 2004). This policy selects the most suitable outgoing link taking into account routing tables with alternative routes, calculated using link costs obtained during the previous routing update interval, and current local information on the link status.

In particular, for delay sensitive traffic local information can be based on the expected queuing delay as defined in Equation (7). The expected queuing delay for a particular link can be calculated locally and does not require any information distribution between neighbouring nodes, thus enabling a very fast response to congestion on the link. Depending on this local information, packets are forwarded on the shortest or on the alternative second shortest path. The alternative second shortest path is used only if it has the same or a smaller number of hops (*h*) to the destination and if the expected queuing delay in the outgoing queue on the shortest path (*Texp1*) is more than a given threshold *Δtr<sup>D</sup>* (where *D* is denoting delay sensitive traffic) higher than the expected queuing delay in the outgoing queue on the second shortest path (*Texp2*). This condition for selecting the alternative second shortest path is given in Equation (16). Different threshold values can be used for different traffic types.

$$\text{tr}\left(h\_2 \le h\_1\right) \land \left(T\_{\text{exp1}}(t) - T\_{\text{exp2}}(t) < \Delta\_{tr}^D\right) \tag{16}$$

The ALR-S policy ensures a more uniform distribution of traffic load over the network, as it distinguishes between the packets passing through a particular node and the packets that are originating in that node. Packets originating in a particular node are forwarded on the link of the second shortest route (e.g. from A to F via B, from B to F via C), while packets passing through the node are forwarded on the link of the shortest route (e.g. through A to F via D, through B to F via E). By using the second-choice route only for originating packets, the delay is increased with respect to the shortest route only on the first hop, hence the increase in delay does not accumulate for the packets with a large number of hops. Between the consecutive updates of routing tables, all packets between a given pair of nodes follow the same route. Thus, ALR-S policy maintains the correct sequence of the packets within the

The ALR-A policy promises an even more uniform distribution of traffic load and thus further improvement of link utilisation by alternating between the shortest and the second shortest route regardless of the packet origination node (this is denoted in Fig. 1 by dashed lines). However, packets belonging to the same session can be forwarded along different routes even within one routing table update interval, thus additional buffering is required in the destination nodes to re-order terminated packets and obtain the correct sequence.

The static forwarding policies, such as ALR-S and ALR-A, distribute packets according to a pre-selected rule. They allow significant reduction of traffic load fluctuation between links,

In contrast to static forwarding an adaptive forwarding policy has to take into account the link status information to support the selection of the most appropriate between the alternative outgoing links on the route to the destination. An example of such approach is adaptive forwarding policy based on local information about the link load (Svigelj et al, 2003, 2004b; Mohorcic et al. 2004). This policy selects the most suitable outgoing link taking into account routing tables with alternative routes, calculated using link costs obtained during the previous routing update interval, and current local information on the link

In particular, for delay sensitive traffic local information can be based on the expected queuing delay as defined in Equation (7). The expected queuing delay for a particular link can be calculated locally and does not require any information distribution between neighbouring nodes, thus enabling a very fast response to congestion on the link. Depending on this local information, packets are forwarded on the shortest or on the alternative second shortest path. The alternative second shortest path is used only if it has the same or a smaller number of hops (*h*) to the destination and if the expected queuing delay in the outgoing queue on the shortest path (*Texp1*) is more than a given threshold *Δtr<sup>D</sup>* (where *D* is denoting delay sensitive traffic) higher than the expected queuing delay in the outgoing queue on the second shortest path (*Texp2*). This condition for selecting the alternative second shortest path is given in Equation (16). Different threshold values can be

2 1 exp1 exp2 ( ) ( () () *<sup>D</sup>*

*tr h h T tT t* ≤ ∧ − <Δ (16)

however they do not adapt to the actual traffic load on alternative routes.

routing interval, the same as the SPR forwarding policy.

**2.4.2 Adaptive forwarding** 

used for different traffic types.

status.

For the throughput sensitive traffic we monitor the number of packets in outgoing queues (*n*). The alternative second shortest path is used only if it has the same or a smaller number of hops (*h*) to the destination and if the number of packets (*n*) in the outgoing queue on the shortest path (*n1*) is more than a given threshold *Δtr<sup>T</sup>* (where *T* is denoting throughput sensitive traffic) higher than the number of packets in the outgoing queue on the alternative path (*n2*), as given in Equation (17).

$$(h\_2 \le h\_1) \land (n\_1(t) - n\_2(t) < \Delta\_{tr}^T) \tag{17}$$

The significance of the threshold is that it regulates distribution of traffic between alternative paths based on local information about the link status, and thus differentiates between lightly and heavily loaded nodes. The higher the threshold value the more congested the shortest path needs to be to allow forwarding along the alternative second shortest path. In the extreme, setting the threshold value to infinity prevents forwarding along the second shortest path (i.e. adaptive forwarding deteriorates to SPR), while no threshold (i.e. *ΔtrT* = 0) means that packets are forwarded along the second shortest path as soon as the expected queuing delay for the corresponding link is smaller than the one on the shortest path.

Routing with the proposed adaptive forwarding promises more uniform distribution of traffic load between links and the possibility to react quickly to link failure. However, packets belonging to the same session can be forwarded along different routes, even within the same routing update interval, so additional buffering is required in destination nodes to reorder terminated packets and obtain the correct sequence.
