Introduction

There are two fundamental levels of error detection in Fibre Channel. Link integrity focuses on the inherent quality of the received transmission signal. Sequence integrity focuses on the integrity of the received data, within a Frame, within a Sequence, and between Sequences in an Exchange. Error recovery procedures must be defined to operate at both of these levels.

When the integrity of the link is in question, a hierarchy of Primitive Sequences are used to reestablish link integrity, as described in the Link Recovery Protocols section, on page 104. When Primitive Sequence protocols are finished, or when data transmission shows errors with overall link integrity, data recovery on a Sequence-by-Sequence or Exchange basis may be required.

A separate type of error recovery must be implemented to recovery from end-to-end or buffer-to-buffer Credit errors that can occur as a result of transmission errors in Primitive Signals, Data Frames, and Link Control Frames. Credit recovery is handled separately from Sequence recovery, although when a sequence is aborted, any end-to-end Credit for outstanding ACK Frames of the Sequence should be recovered. End-to-end Credit loss is usually only detectable when either a receive buffer overflows (for too high a credit value) or (more commonly) when data transmission suffers or stops altogether due to lost credit. Recovery of credit using the Link Credit Reset Primitive Sequence is described in the Link Credit Reset (LCR) Frame section, on page 134, and in the End-to-End Flow Control section, on page 265 and the Buffer-to-Buffer Flow Control ...