B4.1.1 of the AMBA CHI Chip-to-Chip spec lists the field mapping requirements, but TxnID is not mentioned. Does it mean C2C will pass the original TxnID from requester chip to the home chip without remap? If it's the case, will a C2C interface generate multiple outstanding requests with identical TxnID to home chip (in requester chip they are from different RNs so it's ok to have same TxnID), which might be illegal to the on-chip CHI rules?
The C2C Specification leaves it up to implementations how they wish to handle incoming SrcIDs and TxnIDs from a remote chip.
The destination chip would need to ensure that it's on-chip transactions have a unique SrcID + TxnID combination. This could involve a flat mapping where the original SrcID and TxnID are used in some way, or a a full remapping by the gateway to ensure the on-chip requests follow the required rules.