One Comment on “Which two ELS request and reply sequences advertise buffer-to-buffer credits between Fibre Channel devices?”

1. alexey says:

   May 18, 2016 at 12:49 pm

   BB_credits are used by Class 2 and Class 3 service and rely on the receiver sending back
   receiver-readies (R_RDY) to the transmitter. As was previously discussed, node pairs
   communicate their number of credits available during FLOGI/PLOGI. This value is used by the
   transmitter to track the consumption of receive buffers and pace transmissions if necessary.
   FICON directors track the available bb_credits in the following manner:
   • before any data frames are sent, the transmitter sets a counter equal to the BB_credit
   value communicated by its receiver during FLOGI,
   • for each data frame sent by the transmitter, the counter is decremented by one,
   • upon receipt of a data frame, the receiver sends a status frame (R_RDY) to the
   transmitter indicating that the data frame was received and the buffer is ready to receive
   another data frame, and
   • for each R_RDY received by the transmitter, the counter is incremented by one.

   
   
   
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