The transaction layer is the upper layer of the PCI Express architecture, and its primary function is to accept, buffer, and forward transaction layer packets (TLPs). TLPs communicate information with the use of memory, I/O, configuration, and message transactions. To maximize the efficiency of communication between devices, the transaction layer enforces PCI compliant transaction ordering rules and supports relaxed ordering (RO) of received transactions. The transaction layer also manages TLP buffer space through credit-based flow control. The transaction layer implements built-in tag management for transmitted non-posted transactions. It also implements cut-through forwarding of transactions in the transmit (or outbound) direction.