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In a network, a path can be defined as the list of all intermediate routers for a given source destination pair. For a given source/destination pair, the path can be derived by first consulting the forwarding table of the router attached to the source to determine the next router on the path towards the chosen destination. Then, the forwarding table of this router is queried for the same destination... The queries continue until the destination is reached. In a network that has valid forwarding tables, all the paths between all source/destination pairs contain a finite number of intermediate routers. However, if forwarding tables have not been correctly computed, two types of invalid paths can occur.
A path may lead to a `black hole`. In a network, a black hole is a router that receives packets for at least one given source/destination pair but does not have an entry inside its forwarding table for this destination. Since it does not know how to reach the destination, the router cannot forward the received packets and must discard them. Any centralized or distributed algorithm that computes forwarding tables must ensure that there are not black holes inside the network.
A second type of problem may exist in networks using the datagram organization. Consider a path that contains a cycle. For example, router `R1` sends all packets towards destination `D` via router `R2`. Router `R2` forwards these packets to router `R3` and finally router `R3`'s forwarding table uses router `R1` as its nexthop to reach destination `D`. In this case, if a packet destined to `D` is received by router `R1`, it will loop on the `R1 -> R2 -> R3 -> R1` cycle and will never reach its final destination. As in the black hole case, the destination is not reachable from all sources in the network. In practice the loop problem is more annoying than the black hole problem because when a packet is caught in a forwarding loop, it unnecessarily consumes bandwidth. In the black hole case, the problematic packet is quickly discarded. We will see later that network layer protocols include techniques to minimize the impact of such forwarding loops.
Any solution which is used to compute the forwarding tables of a network must ensure that all destinations are reachable from any source. This implies that it must guarantee the absence of black holes and forwarding loops.
The `forwarding tables` and the precise format of the packets that are exchanged inside the network are part of the `data plane` of the network. This `data plane` contains all the protocols and algorithms that are used by hosts and routers to create and process the packets that contain user data. On high-end routers, the data plane is often implemented in hardware for performance reasons.
Besides the `data plane`, a network is also characterized by its `control plane`. The control plane includes all the protocols and algorithms (often distributed) that compute the forwarding tables that are installed on all routers inside the network. While there is only one possible `data plane` for a given networking technology, different networks using the same technology may use different control planes.
The simplest `control plane` for a network is to manually compute the forwarding tables of all routers inside the network. This simple control plane is sufficient when the network is (very) small, usually up to a few routers.
An alternative to manually computed forwarding tables is to use a network management platform that tracks the network status and can push new forwarding tables on the routers when it detects any modification to the network topology. This solution gives some flexibility to the network managers in computing the paths inside their network. However, this solution only works if the network management platform is always capable of reaching all routers even when the network topology changes. This may require a dedicated network that allows the management platform to push information on the forwarding tables. Openflow is a modern example of such solutions [MAB2008]_. In a nutshell, Openflow is a protocol that enables a network controller to install specific entries in the forwarding tables of remote routers and much more.
Another interesting point that is worth being discussed is when the forwarding tables are computed. A widely used solution is to compute the entries of the forwarding tables for all destinations on all routers. This ensures that each router has a valid route towards each destination. These entries can be updated when an event occurs and the network topology changes. A drawback of this approach is that the forwarding tables can become large in large networks since each router must always maintain one entry for each destination inside its forwarding table.
Some networks use the arrival of packets as the trigger to compute the corresponding entries in the forwarding tables. Several technologies have been built upon this principle. When a packet arrives, the router consults its forwarding table to find a path towards the destination. If the destination is present in the forwarding table, the packet is forwarded. Otherwise, the router needs to find a way to forward the packet and update its forwarding table.
The first technique assumes that the underlying network topology is a tree. A tree is the simplest network to be considered when forwarding packets. The main advantage of using a tree is that there is only one path between any pair of nodes inside the network. Since a tree does not contain any cycle, it is impossible to have forwarding loops in a tree-shaped network.
In a tree-shaped network, it is relatively simple for each node to automatically compute its forwarding table by inspecting the packets that it receives. For this, each node uses the source and destination addresses present inside each packet. Thanks to the source address, a node can learn the location of the different sources inside the network. Each source has a unique address. When a node receives a packet over a given interface, it learns that the source (address) of this packet is reachable via this interface. The node maintains a data structure that maps each known source address to an incoming interface. This data structure is often called the `port-address table` since it indicates the interface (or port) to reach a given address.
Learning the location of the sources is not sufficient, nodes also need to forward packets towards their destination. When a node receives a packet whose destination address is already present inside its port-address table, it simply forwards the packet on the interface listed in the port-address table. In this case, the packet will follow the port-address table entries in the downstream nodes and will reach the destination. If the destination address is not included in the port-address table, the node simply forwards the packet on all its interfaces, except the interface from which the packet was received. Forwarding a packet over all interfaces is usually called `broadcasting` in the terminology of computer networks. Sending the packet over all interfaces except one is a costly operation since the packet is sent over links that do not reach the destination. Given the tree-shape of the network, the packet will explore all downstream branches of the tree and will finally reach its destination. In practice, the `broadcasting` operation does not occur too often and its performance impact remains limited.
By inspecting the source and destination addresses of packets, network nodes can automatically derive their forwarding tables. As we will discuss later, this technique is used in :term:`Ethernet` networks. Despite being widely used, it has two important drawbacks. First, packets sent to unknown destinations are broadcasted in the network even if the destination is not attached to the network. Consider the transmission of ten packets destined to `Z` in the network above. When a node receives a packet towards this destination, it can only broadcast that packet. Since `Z` is not attached to the network, no node will ever receive a packet whose source is `Z` to update its forwarding table. The second and more important problem is that few networks have a tree-shaped topology. It is interesting to analyze what happens when a port-address table is used in a network that contains a cycle. Consider the simple network shown below with a single host.
the `data packets`
the `control packets`
For example, consider again the network topology above. `A` sends a control packet towards `B`. The initial `record route` is empty. When `R1` receives the packet, it adds its own address to the `record route` and forwards a copy to `R2` and another to `R3`. `R2` receives the packet, adds itself to the `record route` and forwards it to `R3`. `R3` receives two copies of the packet. The first contains the `[R1,R2]` `record route` and the second `[R1]`. In the end, `B` will receive two control packets containing `[R1,R2,R3,R4]` and `[R1,R3,R4]` as `record routes`. `B` can keep these two paths or select the best one and discard the second. A popular heuristic is to select the `record route` of the first received packet as being the best one since this likely corresponds to the shortest delay path.
With the received `record route`, `B` can send a `data packet` to `A`. For this, it simply reverses the chosen `record route`. However, we still need to communicate the chosen path to `A`. This can be done by putting the `record route` inside a control packet which is sent back to `A` over the reverse path. An alternative is to simply send a `data packet` back to `A`. This packet will travel back to `A`. To allow `A` to inspect the entire path followed by the `data packet`, its `source route` must contain all intermediate routers when it is received by `A`. This can be achieved by encoding the `source route` using a data structure that contains an index and the ordered list of node addresses. The index always points to the next address in the `source route`. It is initialized at `0` when a packet is created and incremented by each intermediate node.
Flat or hierarchical addresses
The last, but important, point to discuss about the `data plane` of the networks that rely on the datagram mode is their addressing scheme. In the examples above, we have used letters to represent the addresses of the hosts and network nodes. In practice, all addresses are encoded as a bit string. Most network technologies use a fixed size bit string to represent source and destination address. These addresses can be organized in two different ways.