Frame Sizes
There is both a minimum and a maximum size of Ethernet frames. The minimum is 64 bytes, requiring a minimum data area (payload) length of 48 bytes (no tags). In cases where the payload is smaller, pad bytes (value 0) are appended to the end of the payload portion to ensure that the minimum length is enforced.
Note
The minimum was important for the original 10Mb/s Ethernet using CSMA/CD. In order for a transmitting station to know which frame encountered a collision, a limit of 2500m (five 500m cable segments with four repeaters) was placed upon the length of an Ethernet network. Given that the propagation rate for electrons in copper is about .77c or 231M m/s, and given the transmission time of 64 bytes to be (64 * 8/10,000,000) = 51.2μs at 10Mb/s, a minimum-size frame could consume about 11,000m of cable. With a maximum of 2500m of cable, the maximum round-trip distance from one station to another is 5000m. The designers of Ethernet included a factor of 2 overdesign in fixing the minimum frame size, so in all compliant cases (and many noncompliant cases), the last bit of an outgoing frame would still be in the process of being transmitted after the time required for its signal to arrive at a maximally distant receiver and return. If a collision is detected, the transmitting station thus knows with certainty which frame collided—the one it is currently transmitting. In this case, the station sends a jamming signal (high voltage) to alert other stations, which then initiate a random binary exponential backoff procedure.
The maximum frame size of conventional Ethernet is 1518 bytes (including the 4-byte CRC and 14-byte header). This value represents a sort of trade-off: if a frame contains an error (detected on receipt by an incorrect CRC), only 1.5KB need to be retransmitted to repair the problem. On the other hand, the size limits the MTU to not more than 1500 bytes. In order to send a larger message, multiple frames are required (e.g., 64KB, a common larger size used with TCP/IP networks, would require at least 44 frames).
802.1p/q: Virtual LANs and QoS Tagging
With the growing use of switched Ethernet, it has become possible to interconnect every computer at a site on the same Ethernet LAN. The advantage of doing this is that any host can directly communicate with any other host, using IP and other network-layer protocols, and requiring little or no administrator configuration. In addition, broadcast and multicast traffic (see Chapter 9) is distributed to all hosts that may wish to receive it without having to set up special multicast routing protocols. While these represent some of the advantages of placing many stations on the same Ethernet, having broadcast traffic go to every computer can create an undesirable amount of network traffic when many hosts use broadcast, and there may be some security reasons to disallow complete any-to-any station communication.
The Linux command for manipulating 802.1p/q information is called vconfig. It can be used to add and remove virtual interfaces associating VLAN IDs to physical interfaces. It can also be used to set 802.1p priorities, change the way virtual interfaces are identified, and influence the mapping between packets tagged with certain VLAN IDs and how they are prioritized during protocol processing in the operating system.
802.1AX: Link Aggregation (Formerly 802.3ad)
Some systems equipped with multiple network interfaces are capable of bonding or link aggregation. With link aggregation, two or more interfaces are treated as one in order to achieve greater reliability through redundancy or greater performance by splitting (striping) data across multiple interfaces. The IEEE Amendment 802.1AX [802.1AX-2008] defines the most common method for performing link aggregation and the Link Aggregation Control Protocol (LACP) to manage such links. LACP uses IEEE 802 frames of a particular format (called LACPDUs).
Using link aggregation on Ethernet switches that support it can be a costeffective alternative to investing in switches with high-speed network ports. If more than one port can be aggregated to provide adequate bandwidth, higher-speed ports may not be required. Link aggregation may be supported not only on network switches but across multiple network interface cards (NICs) on a host computer. Often, aggregated ports must be of the same type, operating in the same mode (i.e., half- or full-duplex).
Full Duplex, Power Save, Autonegotiation, and 802.1X Flow Control
When Ethernet was first developed, it operated only in half-duplex mode using a shared cable. That is, data could be sent only one way at one time, so only one station was sending a frame at any given point in time. With the development of switched Ethernet, the network was no longer a single piece of shared wire, but instead many sets of links. As a result, multiple pairs of stations could exchange data simultaneously. In addition, Ethernet was modified to operate in full duplex, effectively disabling the collision detection circuitry. This also allowed the physical length of the Ethernet to be extended, because the timing constraints associated with half-duplex operation and collision detection were removed.