Fibre Channel Frame
| S O F | HEADER | PAYLOAD | C R C | E O F |
|---|---|---|---|---|
| 4 | 24 | 2112 | 4 | 4 |
Fibre Channel places a restriction on the length of the data field of a frame at 528 transmission words, which is 2112 bytes. Larger amounts of data must be transmitted in several frames. This larger unit that consists of multiple frames is called a sequence. An entire transaction between two ports is made up of sequences administered by an even larger unit called an exchange.
The TYPE field (Word 2, bits 31- 24) tells which format to use. The Routing Control INFO bits (bits 27-24) determine how to interpret the payload.
Field Definitions
Routing Control
(R_CTL): first 8 bits of the header. They define the type of frame and its content or function. The first 4 bits (Bits 31-28) identify the frame type. The second four bits (Bits 27-24) define the contents of the frame or identify the function of the frame.
Destination_ID
(D_ID): 24-bit Port Identifier (PID) of the recipient. It could also be a well-known address such as the Name Server FFFFFC.
Class Specific Control Field
(CS_CTL): control necessary for the different classes of service. This field is always zero for classes 2 and 3 per the standards. Classes 1 and 4 use it.
Source_ID
(S_ID): 24-bit Port identifier (PID) of the source. It could be a well-known address.
Type:
identifies the protocol of the frame content for Data Frames (i.e FC_CT, FCP, IPFC).
Frame Control
(F_CTL): contains miscellaneous control information regarding the frame such as who owns initiative, first frame of the Exchange, last frame of the Exchange, etc.
Sequence ID
(SEQ_ID): used to identify and track all of the frames within a sequence between a source and destination port pair.
Data Field Control
(DF_CTL): indicates if any optional headers are present at the beginning of the data field of the frame. Optional headers are used for information that may be required by some applications or protocol mappings.
Sequence Count
(SEQ_CNT): used to indicate the sequential order of frame transmission within a sequence or multiple consecutive sequences within the same exchange. This is a counter that increments as a sequence of frames are transmitted.
Originator_ID
(OX_ID): Exchange ID assigned by the originator port.
Responder_ID
(RX_ID): Exchange ID assigned by the responder to the Exchange.
Data Field/Payload:
The maximum size is 2112 bytes.
