Exit Zen
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../../protocols/ethernet.rst:176
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Two multi-mode optical fiber, 2 km maximum
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../../protocols/ethernet.rst:177
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1000Base-CX
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../../protocols/ethernet.rst:177
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Two pairs shielded twisted pair, 25m maximum
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../../protocols/ethernet.rst:178
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1000Base-SX
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../../protocols/ethernet.rst:178
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Two multi-mode or single mode optical fibers with lasers
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../../protocols/ethernet.rst:179 • ../../protocols/ethernet.rst:358
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10 Gbps
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../../protocols/ethernet.rst:179
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Optical fiber but also Category 6 UTP
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../../protocols/ethernet.rst:180
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40-100 Gbps
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../../protocols/ethernet.rst:180
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Optical fiber (experiences are performed with copper)
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../../protocols/ethernet.rst:192 • ../../protocols/ethernet.rst:308
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Footnotes
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../../protocols/ethernet.rst:193
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Additional information about the history of the Ethernet technology may be found at http://ethernethistory.typepad.com/
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../../protocols/ethernet.rst:195
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Initially, the OUIs were allocated by Xerox [DP1981]_. However, once Ethernet became an IEEE and later an ISO standard, the allocation of the OUIs moved to IEEE. The list of all OUI allocations may be found at http://standards.ieee.org/regauth/oui/index.shtml
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../../protocols/ethernet.rst:197
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The official list of all assigned Ethernet type values is available from http://standards.ieee.org/regauth/ethertype/eth.txt
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../../protocols/ethernet.rst:199
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The attentive reader may question the need for different `EtherTypes` for IPv4 and IPv6 while the IP header already contains a version field that can be used to distinguish between IPv4 and IPv6 packets. Theoretically, IPv4 and IPv6 could have used the same `EtherType`. Unfortunately, developers of the early IPv6 implementations found that some devices did not check the version field of the IPv4 packets that they received and parsed frames whose `EtherType` was set to `0x0800` as IPv4 packets. Sending IPv6 packets to such devices would have caused disruptions. To avoid this problem, the IETF decided to apply for a distinct `EtherType` value for IPv6. Such a choice is now mandated by :rfc:`6274` (section 3.1), although we can find a funny counter-example in :rfc:`6214`.
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../../protocols/ethernet.rst:201
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These network interfaces compute the TCP checksum while a segment is transferred from the host memory to the network interface [SH2004]_.
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../../protocols/ethernet.rst:203
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Fortunately, IEEE was able to define the [IEEE802.3]_ frame format while maintaining backward compatibility with the Ethernet [DIX]_ frame format. The trick was to only assign values above 1500 as `EtherType` values. When a host receives a frame, it can determine whether the frame's format by checking its `EtherType/Length` field. A value lower smaller than `1501` is clearly a length indicator and thus an [IEEE802.3]_ frame. A value larger than `1501` can only be type and thus a [DIX]_ frame.
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../../protocols/ethernet.rst:207
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Ethernet Switches
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../../protocols/ethernet.rst:211
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Increasing the physical layer bandwidth as in `Fast Ethernet` was only one of the solutions to improve the performance of Ethernet LANs. A second solution was to replace the hubs with more intelligent devices. As `Ethernet hubs` operate in the physical layer, they can only regenerate the electrical signal to extend the geographical reach of the network. From a performance perspective, it would be more interesting to have devices that operate in the datalink layer and can analyze the destination address of each frame and forward the frames selectively on the link that leads to the destination. Such devices are usually called `Ethernet switches` [#fbridges]_. An `Ethernet switch` is a relay that operates in the datalink layer as is illustrated in the figure below.
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../../protocols/ethernet.rst:248
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An `Ethernet switch` understands the format of the Ethernet frames and can selectively forward frames over each interface. For this, each `Ethernet switch` maintains a `MAC address table`. This table contains, for each MAC address known by the switch, the identifier of the switch's port over which a frame sent towards this address must be forwarded to reach its destination. This is illustrated below with the `MAC address table` of the bottom switch. When the switch receives a frame destined to address `B`, it forwards the frame on its South port. If it receives a frame destined to address `D`, it forwards it only on its North port.
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../../protocols/ethernet.rst:255
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Operation of Ethernet switches
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