| Re: PCIe-AS - Tutorial -- Is there a CRC for the address header ? [message #943 is a reply to message #928] |
Sun, 19 September 2004 20:57  |
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| David Slogsnat wrote on Wed, 15 September 2004 14:59 |
This is true. However, things get more complicated when looking at the ASI specification:
-The Turn Pointer is not included in the header CRC.
-The final receiver of an AS Packet has to check the CRC. The intermediate switches may check it, but they don't have to.
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All true. However, so far we ignored that ASI is a transaction layer build on top of the PCIe physical and data link layer. This adds
- a transaction layer sequence number
- a link layer CRC (LCRC)
- and a link layer ACK/NAK
The LCRC is checked and regenerated at the link level.
So a switch will eventually realize that a packet is corrupted. The question is now what happens if such a corrupted packet is being already cut-through forwarded. I guess, that the only possible action in this case is to
- make sure that the outgoing packet gets a bad LCRC too, to that it will eventually be dropped
- send a data link level NAK, so that the sender will eventually retransmit.
If this is true, the exclusion of some fields in the header CRC isn't a real issue. However, the above is a guess (or hope), I don't know what the standart says for this case.
W.F.J.Müller, GSI, CBM, Tel: 2766
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