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CHI G - Txn ID re-use before comp ack being sent in case of ExpCompAck transfers

Karthik Ganesan

Scenario: 

1. Requester sends a Read transaction with unique Src ID and TXN ID with Expcompack set to 1. 

2. The data flow continues, and the requester receives the comp data packet with same TXN ID and DBID. 

3. The requester sends back the compack response with received DBID as TXN ID. 

The question is can the TXN ID be re-used after #2 or #3 has to be completed before the TXN ID can be re-used. 

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  • 0 Arm Employee Badge

    From the CHI Specification Section B2.4.2:

    "A Requester is permitted to reuse a TxnID value after receiving either:

    • All responses associated with a previous transaction that have used the same value.

    • A RetryAck response for a previous transaction that used the same value."

    Therefore, this can be reused after #2.

    As the CompAck has no response, it would be possible for a new request to overtake the CompAck, and so the HN must be able to handle the TxnID being reused before the CompAck was received.

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  • 0 Arm Employee Badge

    From the CHI Specification Section B2.4.2:

    "A Requester is permitted to reuse a TxnID value after receiving either:

    • All responses associated with a previous transaction that have used the same value.

    • A RetryAck response for a previous transaction that used the same value."

    Therefore, this can be reused after #2.

    As the CompAck has no response, it would be possible for a new request to overtake the CompAck, and so the HN must be able to handle the TxnID being reused before the CompAck was received.

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