The s.reply field

REPLYs and COHE_REPLYs also use a field called s.reply to indicate the response type. This field can have the following values:

REPLY

Successful completion of the desired action.

RAR

Request a retry: action could not be completed immediately due to transient condition, and demand should be retried. These are sent by a directory when its pending request buffer fills up or when certain negative-acknowledgment responses cannot be immediately resolved (as explained in Chapter 14).

NACK

Action could not be completed as specified (COHE_REPLYs only). These are sent on invalidation messages when the line is not present in the cache, or on write-back subset-enforcement messages when the L1 cache does not have the line in modified state.

NACK_PEND

Action could not be completed as specified due to a transient condition (COHE_REPLYs only). The cache sends these responses during certain races described in Chapter 13. The directory handles these replies by either retrying the corresponding COHE or reprocessing the original REQUEST.