PCI Express Avalon-ST Valid Data Bytes

I was told by altera support that it is usually safer to use the TLP fields rather than the signals that come out of the cores. First BE and Last BE indeed do tell you what bytes are valid. You also need to know what the address is, because the First BE applies to the start address, and last BE applies to the end address - remember that PCIe works in 32 bit long words.

The PCIe spec is not written by altera, so they dont explain how the headers work. I suggest getting the mindshare PCIe book, as it explains all aspects of PCIe (it is also recommended by altera support). It explains all fields in the header and how to use them.