some very detailed flows sequences

README.md

@@ -119,8 +119,47 @@ In the case of a timeout after 10 minutes for either forward, the packet would b
 }
 ```
 
+## Implementation details
+
+Flow sequence mainly encoded in [middleware](router/ibc_middleware.go) and in [keeper](router/keeper/keeper.go). 
+
+Describes `A` sending to `C` via `B` in several scenarios with operational opened channels, enabled denom composition, fees and available to refund, but no retries.
+
+Generally without `memo` to handle, all handling by this module is delegated to ICS-020. ICS-020 ACK are written and parsed in any case (ACK are backwarded).
+
+# A -> B -> C full success
+
+1. `A` This sends packet over underlying ICS-004 wrapper with memo as is.
+2. `B` This receives packet and parses it into ICS-020 packet.
+3. `B` Validates `forward` packet on this step, return `ACK` error if fails.
+4. `B` If other middleware not yet called ICS-020, call it and ACK error on fail. Tokens minted or unescrowed here.
+5. `B` Handle denom. If denom prefix is from `B`, remove it. If denom prefix is other chain - add `B` prefix.
+6. `B` Take fee, create new ICS-004 packet with timeout from forward for next step, and remaining inner `memo`.
+7. `B` Send transfer to `C` with parameters obtained from `memo`. Tokens burnt or escrowed here.
+8.  `B` Store tracking `in flight packet` under next `(channel, port, ICS-20 transfer sequence)`, do not `ACK` packet yet.
+9.  `C` Handle ICS-020 packet as usual.
+10. `B` On ICS-020 ACK from `C` find `in flight packet`, delete it and write `ACK` for original packet from `A`.
+11. `A` Handle ICS-020 `ACK` as usual 
+
+# A -> B -> C with C error ACK
+
+10. `B` On ICS-020 ACK from `C` find `in flight packet`, delete it 
+11. `B` Burns or escrows tokens.
+12. `B` And write error `ACK` for original packet from `A`.
+13. `A` Handle ICS-020 timeout as usual
+14. `C` writes success `ACK` for packet from `B`
+
+Same behavior in case of timeout on `C`
+
+### A packet timeouts on B before C timeouts packet from B
+
+10. `A` Cannot timeout because `in flight packet` has proof on `B` of packet inclusion.
+11. `B` waits for ACK or timeout from `C`.
+12. `B` timeout from `C` becomes fail `ACK` on `B` for `A`
+13. `A` receives success or fail `ACK`, but not timeout
+
+In this case `A` assets `hang` until final hop timeouts or ACK.
+
 ## References
 
-- https://www.mintscan.io/cosmos/proposals/56
-- https://github.com/cosmos/ibc-go/pull/373
 - https://github.com/strangelove-ventures/governance/blob/master/proposals/2021-09-hub-ibc-router/README.md