2. cnp3-ebook
3. principles/network
4. English
5. Zen

36 ../../principles/network.rst:225
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — 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. 560/10000 · 560
37 ../../principles/network.rst:227
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — 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. 457/10000 · 457
38 ../../principles/network.rst:230
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — Computing forwarding tables 27/10000 · 27
39 ../../principles/network.rst:232
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — Networks deployed several techniques to update the forwarding tables upon the arrival of a packet. In this section, we briefly present the principles that underlie three of these techniques. 190/10000 · 190
40 ../../principles/network.rst:234
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — 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. 369/10000 · 369
41 ../../principles/network.rst:238
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — 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. 748/10000 · 748
42 ../../principles/network.rst:240
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — 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. 1119/10000 · 1119
43 ../../principles/network.rst:242
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — To understand the operation of the port-address table, let us consider the example network shown in the figure below. This network contains three hosts: `A`, `B` and `C` and five routers, `R1` to `R5`. When the network boots, all the forwarding tables of the nodes are empty. 275/10000 · 275
44 ../../principles/network.rst:269
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — Host `A` sends a packet towards `B`. When receiving this packet, `R1` learns that `A` is reachable via its `West` interface. Since it does not have an entry for destination `B` in its port-address table, it forwards the packet to both `R2` and `R3`. When `R2` receives the packet, it updates its own forwarding table and forward the packet to `C`. Since `C` is not the intended recipient, it simply discards the received packet. Router `R3` also receives the packet. It learns that `A` is reachable via its `North-West` interface and broadcasts the packet to `R4` and `R5`. `R5` also updates its forwarding table and finally forwards it to destination `B`. Let us now consider what happens when `B` sends a reply to `A`. `R5` first learns that `B` is attached to its `North-East` port. It then consults its port-address table and finds that `A` is reachable via its `North-West` interface. The packet is then forwarded hop-by-hop to `A` without any broadcasting. Later on, if `C` sends a packet to `B`, this packet will reach `R1` that contains a valid forwarding entry in its forwarding table. 1095/10000 · 1095
45 ../../principles/network.rst:273
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — 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. 954/10000 · 954
46 ../../principles/network.rst:296
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — Assume that the network has started and all port-address and forwarding tables are empty. Host `A` sends a packet towards `B`. Upon reception of this packet, `R1` updates its port-address table. Since `B` is not present in the port-address table, the packet is broadcasted. Both `R2` and `R3` receive a copy of the packet sent by `A`. They both update their port-address table. Unfortunately, they also both broadcast the received packet. `B` receives a first copy of the packet, but `R3` and `R2` receive it again. `R3` will then broadcast this copy of the packet to `B` and `R1` while `R2` will broadcast its copy to `R1`. Although `B` has already received two copies of the packet, it is still inside the network and continues to loop. Due to the presence of the cycle, a single packet towards an unknown destination generates many copies of this packet that loop and will eventually saturate the network. Network operators who are using port-address tables to automatically compute the forwarding tables also use distributed algorithms to ensure that the network topology is always a tree. 1093/10000 · 1093
47 ../../principles/network.rst:300
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — Another technique called `source routing` can be used to automatically compute forwarding tables. It has been used in interconnecting Token Ring networks and in some wireless networks. Intuitively, `source routing` enables a destination to automatically discover the paths from a given source towards itself. This technique requires nodes to encode information inside some packets. For simplicity, let us assume that the `data plane` supports two types of packets : 465/10000 · 465
48 ../../principles/network.rst:302
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — the `data packets` 18/10000 · 18
49 ../../principles/network.rst:303
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — the `control packets` 21/10000 · 21
50 ../../principles/network.rst:305
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — `Data packets` are used to exchange data while `control packets` are used to discover the paths between hosts. With `source routing`, routers can be kept as simple as possible and all the complexity is placed on the hosts. This is in contrast with the previous technique where the nodes had to maintain a port-address and a forwarding table while the hosts simply sent and received packets. Each node is configured with one unique address and there is one identifier per outgoing link. For simplicity and to avoid cluttering the figures with those identifiers, we assume that each node uses as link identifiers north, west, south,... In practice, a node would associate one integer to each outgoing link. 704/10000 · 704
51 ../../principles/network.rst:329
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — In the network above, router `R2` is attached to two outgoing links. `R2` is connected to both `R1` and `R3`. `R2` can easily determine that it is connected to these two nodes by exchanging packets with them or observing the packets that it receives over each interface. Assume for example that when a node (either host or router) starts, it sends a special control packet over each of its interfaces to advertise its own address to its neighbors. When a node receives such a packet, it automatically replies with its own address. This exchange can also be used to verify whether a neighbor, either router or host, is still alive. With `source routing`, the data plane packets include a list of identifiers. This list is called a `source route`. It indicates the path to be followed by the packet as a sequence of link identifiers. When a node receives such a `data plane` packet, it first checks whether the packet's destination is a direct neighbor. In this case, the packet is forwarded to this neighbor. Otherwise, the node extracts the next address from the list and forwards it to the neighbor. This allows the source to specify the explicit path to be followed for each packet. For example, in the figure above there are two possible paths between `A` and `B`. To use the path via `R2`, `A` would send a packet that contains `R1,R2,R3` as source route. To avoid going via `R2`, `A` would place `R1,R3` as the source route in its transmitted packet. If `A` knows the complete network topology and all link identifiers, it can easily compute the source route towards each destination. It can even use different paths, e.g. for redundancy, to reach a given destination. However, in a real network hosts do not usually have a map of the entire network topology. 1764/10000 · 1764
52 ../../principles/network.rst:333
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — In networks that rely on source routing, hosts use control packets to automatically discover the best path(s). In addition to the source and destination addresses, `control packets` contain a list that records the intermediate nodes. This list is often called the `record route` because it allows recording the route followed by a given packet. When a node receives such a `control packet`, it first checks whether its address is included in the record route. If yes, the packet has already been forwarded by this node and it is silently discarded. Otherwise, it adds its own address to the `record route` and forwards the packet to all its interfaces, except the interface over which the packet has been received. Thanks to this, the `control packet` can explore all paths between a source and a given destination. 815/10000 · 815
53 ../../principles/network.rst:335
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — 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. 805/10000 · 805
54 ../../principles/network.rst:337
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — 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. 861/10000 · 861
55 ../../principles/network.rst:339
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — The third technique to compute forwarding tables is to rely on a control plane using a distributed algorithm. Routers exchange control messages to discover the network topology and build their forwarding table based on them. We dedicate a more detailed description of such distributed algorithms later in this section. 318/10000 · 318