cnp3-ebook/protocols/ethernet — English @ Weblate

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61 ../../protocols/ethernet.rst:287

	English Security issues with Ethernet hubs and switches 47/10000 · 47
62 ../../protocols/ethernet.rst:289

	English From a security perspective, Ethernet hubs have the same drawbacks as the older coaxial cable. A host attached to a hub will be able to capture all the frames exchanged between any pair of hosts attached to the same hub. Ethernet switches are much better from this perspective thanks to the selective forwarding, a host will usually only receive the frames destined to itself as well as the multicast, broadcast and unknown frames. However, this does not imply that switches are completely secure. There are, unfortunately, attacks against Ethernet switches. From a security perspective, the `MAC address table` is one of the fragile elements of an Ethernet switch. This table has a fixed size. Some low-end switches can store a few tens or a few hundreds of addresses while higher-end switches can store tens of thousands of addresses or more. From a security point of view, a limited resource can be the target of Denial of Service attacks. Unfortunately, such attacks are also possible on Ethernet switches. A malicious host could overflow the `MAC address table` of the switch by generating thousands of frames with random source addresses. Once the `MAC address table` is full, the switch needs to broadcast all the frames that it receives. At this point, an attacker will receive unicast frames that are not destined to its address. The ARP attack discussed in the previous chapter could also occur with Ethernet switches [Vyncke2007]_. Recent switches implement several types of defenses against these attacks, but they need to be carefully configured by the network administrator. See [Vyncke2007]_ for a detailed discussion on security issues with Ethernet switches. 1675/10000 · 1675
63 ../../protocols/ethernet.rst:293

	English The `MAC address learning` algorithm combined with the forwarding algorithm work well in a tree-shaped network such as the one shown above. However, to deal with link and switch failures, network administrators often add redundant links to ensure that their network remains connected even after a failure. Let us consider what happens in the Ethernet network shown in the figure below. 385/10000 · 385
64 ../../protocols/ethernet.rst:300

	English Ethernet switches in a loop 27/10000 · 27
65 ../../protocols/ethernet.rst:303

	English When all switches boot, their `MAC address table` is empty. Assume that host `A` sends a frame towards host `C`. Upon reception of this frame, switch1 updates its `MAC address table` to remember that address `A` is reachable via its West port. As there is no entry for address `C` in switch1's `MAC address table`, the frame is forwarded to both switch2 and switch3. When switch2 receives the frame, its updates its `MAC address table` for address `A` and forwards the frame to host `C` as well as to switch3. switch3 has thus received two copies of the same frame. As switch3 does not know how to reach the destination address, it forwards the frame received from switch1 to switch2 and the frame received from switch2 to switch1... The single frame sent by host `A` will be continuously duplicated by the switches until their `MAC address table` contains an entry for address `C`. Quickly, all the available link bandwidth will be used to forward all the copies of this frame. As Ethernet does not contain any `TTL` or `HopLimit`, this loop will never stop. 1059/10000 · 1059
66 ../../protocols/ethernet.rst:305

	English The `MAC address learning` algorithm allows switches to be plug-and-play. Unfortunately, the loops that arise when the network topology is not a tree are a severe problem. Forcing the switches to only be used in tree-shaped networks as hubs would be a severe limitation. To solve this problem, the inventors of Ethernet switches have developed the `Spanning Tree Protocol`. This protocol allows switches to automatically disable ports on Ethernet switches to ensure that the network does not contain any cycle that could cause frames to loop forever. 550/10000 · 550
67 ../../protocols/ethernet.rst:309

	English The first Ethernet relays that operated in the datalink layers were called `bridges`. In practice, the main difference between switches and bridges is that bridges were usually implemented in software while switches are hardware-based devices. Throughout this text, we always use `switch` when referring to a relay in the datalink layer, but you might still see the word `bridge`. 380/10000 · 380
68 ../../protocols/ethernet.rst:314

	English The Spanning Tree Protocol (802.1d) 35/10000 · 35
69 ../../protocols/ethernet.rst:316

	English The `Spanning Tree Protocol` (STP), proposed in [Perlman1985]_, is a distributed protocol that is used by switches to reduce the network topology to a spanning tree, so that there are no cycles in the topology. For example, consider the network shown in the figure below. In this figure, each bold line corresponds to an Ethernet to which two Ethernet switches are attached. This network contains several cycles that must be broken to allow Ethernet switches, using the MAC address learning algorithm, to exchange frames. 521/10000 · 521
70 ../../protocols/ethernet.rst:324

	English Spanning tree computed in a switched Ethernet network 53/10000 · 53
71 ../../protocols/ethernet.rst:327

	English In this network, the STP will compute the following spanning tree. `Switch1` will be the root of the tree. All the interfaces of `Switch1`, `Switch2` and `Switch7` are part of the spanning tree. Only the interface connected to `LAN B` will be active on `Switch9`. `LAN H` will only be served by `Switch7` and the port of `Switch44` on `LAN G` will be disabled. A frame originating on `LAN B` and destined for `LAN A` will be forwarded by `Switch7` on `LAN C`, then by `Switch1` on `LAN E`, then by `Switch44` on `LAN F` and eventually by `Switch2` on `LAN A`. 559/10000 · 559
72 ../../protocols/ethernet.rst:329

	English Switches running the `Spanning Tree Protocol` exchange `BPDUs`. These `BPDUs` are always sent as frames with destination MAC address as the `ALL_BRIDGES` reserved multicast MAC address. Each switch has a unique 64 bit `identifier`. To ensure uniqueness, the lower 48 bits of the identifier are set to the unique MAC address allocated to the switch by its manufacturer. The high order 16 bits of the switch identifier can be configured by the network administrator to influence the topology of the spanning tree. The default value for these high order bits is 32768. 565/10000 · 565
73 ../../protocols/ethernet.rst:331

	English The switches exchange `BPDUs` to build the spanning tree. Intuitively, the spanning tree is built by first selecting the switch with the smallest `identifier` as the root of the tree. The branches of the spanning tree are then composed of the shortest paths that allow all of the switches that compose the network to be reached. The `BPDUs` exchanged by the switches contain the following information : 402/10000 · 402
74 ../../protocols/ethernet.rst:334

	English the `identifier` of the root switch (`R`) 41/10000 · 41
75 ../../protocols/ethernet.rst:335

	English the `cost` of the shortest path between the switch that sent the `BPDU` and the root switch (`c`) 97/10000 · 97
76 ../../protocols/ethernet.rst:336

	English the `identifier` of the switch that sent the `BPDU` (`T`) 57/10000 · 57
77 ../../protocols/ethernet.rst:337

	English the number of the switch port over which the `BPDU` was sent (`p`) 66/10000 · 66
78 ../../protocols/ethernet.rst:339

	English We will use the notation `<R,c,T,p>` to represent a `BPDU` whose `root identifier` is `R`, `cost` is `c` and that was sent from the port `p` of switch `T`. The construction of the spanning tree depends on an ordering relationship among the `BPDUs`. This ordering relationship could be implemented by the Python function below. 327/10000 · 327
79 ../../protocols/ethernet.rst:350

	English In addition to the `identifier` discussed above, the network administrator can also configure a `cost` to be associated to each switch port. Usually, the `cost` of a port depends on its bandwidth and the [IEEE802.1d]_ standard recommends the values below. Of course, the network administrator may choose other values. We will use the notation `cost[p]` to indicate the cost associated to port `p` in this section. 413/10000 · 413
80 ../../protocols/ethernet.rst:353

	English Bandwidth 9/10000 · 9

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