Fibre Channel over Ethernet

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Mapping Fibre Channel to Ethernet

Fibre Channel over Ethernet (FCoE) is a proposed standard being developed by the International Committee for Information Technology Standards (INCITS). FCoE carries native Fibre Channel (FC) over full duplex IEEE 802.3 Ethernet connections. As with Fibre Channel over IP (FCIP), FCoE makes it possible to maintain existing FC infrastructure while providing a mechanism for interconnecting FC-based storage area networks (SAN) over a more commonly used networking technology. Where FCIP accomplishes this as a Layer 3 solution and is designed to support this connectivity over any IP network, FCoE implements a Layer 2 solution that can be deployed over any Ethernet-based network. That includes premises-based networks as well as Metro Ethernet or Carrier Ethernet services.

Fiber Channel Encapsulation in Ethernet

In FCoE, Fibre Channel is mapped into Ethernet. Fibre Channel and traditional networks have multiple layers where each layer in the stack represents a particular set of functions. The Fibre Channel stack comprises five layers, known as FC-0 through FC-4. Ethernet is typically considered to be a set of protocols and standards that operate at the Physical Layer (Layer 1) and Data Link Layer (Layer 2) of the OSI Reference Model (OSI-RM). FCoE essentially replaces FC-0 and FC-1 with Ethernet (as depicted in the figure to the right), which means that FC-2, FC-3 (if present) and FC-4 are carried within the Ethernet payload field. Ethernet becomes the highway for Fibre Channel payloads.

The FCoE Frame Structure

FCoE encapsulates a Fibre Channel frame within an Ethernet frame. Figure 3 represents the frame format as agreed to by the INCITS T11.3 standards committee. The fields of the frame are:

  • Destination MAC address: The first 48 bits in the frame are used to specify the Destination MAC address
  • Source MAC address: The next 48 bits specify the Source MAC Addresses. Both of these first two fields are defined by IEEE 802.3.
  • IEEE 802.1Q Tag: The next 32-bit field provides the same function as it does for any virtual LAN environment; it supports the creation of multiple virtual networks across a single physical infrastructure.
  • Ethernet Type (ET) field: The next 16-bit field, also defined by the IEEE 802.3 standard, can be either a length field or a type field. FCoE has its own assigned Ethernet type number, which is the final field in the formal Ethernet header. What follows is the payload of the Ethernet frame, in which we find the encapsulated Fibre Channel frame.
  • Version: As is commonly found in modern protocols, this 4-bit field identifies the version of FCoE being implemented. It provides a mechanism for evolving FCoE and having network equipment that can understand any format changes made in the protocol.
  • Reserved: The next 100-bits are reserved, another common practice in new protocols. A field is created "just in case." It may or may not be used by future versions.
  • Start of Frame (SOF) delimiter: This 8-bit field marks the start of the encapsulated FC frame and includes some control bits.
  • Encapsulated FC Frame: The actual FC frame, including the FC frame check sequence (FCS)
  • End-of Frame (EOF) delimiter: This 8-bit field marks the end of the encapsulated FC frame and includes some control bits.
  • Reserved: The next 24-bits are reserved for future use.
  • Ethernet FCS: The final field is part of the IEEE 802.3 frame proper. It contains a 32-bit cyclic redundancy check (CRC) that verifies the integrity of the Ethernet frame.

The encapsulated Fibre Channel frame consists of the original 24 byte FC header and the data being transported (including the Fibre Channel CRC). The FC frame structure is maintained so that, when a traditional FC Storage Area Network is connected to an FCoE capable switch, the frame is de-encapsulated and handed off seamlessly. This capability enables FCoE to integrate with existing FC SANs without the need for a gateway.

Frame size is also a factor in FCoE. A typical Fibre Channel frame has a 2112 byte payload, a header and an FCS. An Ethernet frame has a maximum payload capacity of 1500 bytes. To maintain good performance, FCoE must utilize jumbo frames (or the 2.5 KB “baby jumbo”) to prevent a Fibre Channel frame from being split into two Ethernet frames.

External Links

See Also


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