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Concept
ATA over Ethernet
Summary
ATA over Ethernet (AoE) is a network protocol developed by the Brantley Coile Company, designed for simple, high-performance access of block storage devices over Ethernet networks. It is used to build storage area networks (SANs) with low-cost, standard technologies.
AoE runs on layer 2 Ethernet. AoE does not use Internet Protocol (IP); it cannot be accessed over the Internet or other IP networks. In this regard it is more comparable to Fibre Channel over Ethernet than iSCSI.
With fewer protocol layers, this approach makes AoE fast and lightweight. It also makes the protocol relatively easy to implement and offers linear scalability with high performance. The AoE specification is 12 pages compared with iSCSI's 257 pages.
AoE Header Format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
0 | Ethernet Destination MAC Address |
4 | Ethernet Destination (cont) | Ethernet Source MAC Address |
8 | Ethernet Source MAC Address (cont) |
12 | Ethernet Type (0x88A2) | Ver | Flags | Error |
16 | Major | Minor | Command |
20 | Tag |
24 | Arg |
AoE has the IEEE assigned EtherType 0x88A2.
SATA (and older PATA) hard drives use the Advanced Technology Attachment (ATA) protocol to issue commands, such as read, write, and status. AoE encapsulates those commands inside Ethernet frames and lets them travel over an Ethernet network instead of a SATA or 40-pin ribbon cable. Although internally AoE uses the ATA protocol, it presents the disks as SCSI to the operating system. Also the actual disks can be SCSI or any other kind, AoE is not limited to disks that use the ATA command set. By using an AoE driver, the host operating system is able to access a remote disk as if it were directly attached.
The encapsulation of ATA provided by AoE is simple and low-level, allowing the translation to happen either at high performance or inside a small, embedded device, or both.
AoE is a layer 2 protocol running at the data-link layer, unlike some other SAN protocols which run on top of layer 3 utilizing IP.
Official source
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