Lecture 9 - Åbo Akademi

ACK/NACK flow control scheme
• when flits are sent on a link, a local copy is kept in a buffer by sender
• when ACK received by sender, it deletes copy of flit from its local buffer
• when NACK is received, sender rewinds its output queue and starts
resending flits, starting from the corrupted one
• implemented either end-to-end or switch-to-switch
• sender needs to have a buffer of size 2N + k
• N is number of buffers encountered between source and destination
• k depends on latency of logic at the sender and receiver
• overall a minimum of 3N + k buffers are required
• fault handling support comes at cost of greater power, area overhead