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PCI Express System Architecture by MindShare, Inc, Ravi Budruk, Don Anderson, Tom Shanley

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Some More Examples

To demonstrate the reliable TLP delivery capability provided by the ACK/NAK Protocol, the following examples are provided.

Lost TLP

Consider Figure 5-14 on page 245 which shows the ACK/NAK protocol for handling lost TLPs.

  1. Device A transmits TLPs 4094, 4095, 0, 1, and 2.

  2. Device B receives TLPs 4094, 4095, and 0, for which it returns ACK 0. These TLPs are forwarded to the Transaction Layer. NEXT_RCV_SEQ is incremented and the next value of NEXT_RCV_SEQ count is 1. Device B is ready to receive TLP 1.

  3. Seeing ACK 0, Device A purges TLPs 4094, 4095, and 0 from its replay buffer.

  4. TLP 1 is lost en route.

  5. TLP 2 arrives instead. Upon performing a Sequence Number check, Device B realizes that TLP 2's Sequence Number is greater than NEXT_RCV_SEQ ...

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