Spelling, grammar, and formatting fixes.

middleware/packet-forward-middleware/README.md

@@ -8,14 +8,14 @@ Asynchronous acknowledgements are utilized for atomic multi-hop packet flows. Th
 The packet-forward-middleware is an IBC middleware module built for Cosmos blockchains utilizing the IBC protocol. A chain which incorporates the 
 packet-forward-middleware is able to route incoming IBC packets from a source chain to a destination chain. As the Cosmos SDK/IBC become commonplace in the 
 blockchain space more and more zones will come online, these new zones joining are noticing a problem: they need to maintain a large amount of infrastructure 
-(archive nodes and relayers for each counterparty chain) to connect with all the chains in the ecosystem, a number that is continuing to increase quickly. Luckly 
-this problem has been anticipated and IBC has been architected to accomodate multi-hop transactions. However, a packet forwarding/routing feature was not in the 
+(archive nodes and relayers for each counterparty chain) to connect with all the chains in the ecosystem, a number that is continuing to increase quickly. Luckily 
+this problem has been anticipated and IBC has been architected to accommodate multi-hop transactions. However, a packet forwarding/routing feature was not in the 
 initial IBC release. 
 
 ## Sequence diagrams
 
 ### Multi-hop A->B->C->D success
-```
+```ascii
         channel-0 channel-1         channel-2 channel-3        channel-4 channel-5
 ┌───────┐       ibc        ┌───────┐        ibc       ┌───────┐        ibc       ┌───────┐
 │Chain A│◄────────────────►│Chain B│◄────────────────►│Chain C│◄────────────────►│Chain D│
@@ -27,7 +27,7 @@ initial IBC release.
 ```
 
 ### Multi-hop A->B->C->D, C->D `recv_packet` error, refund back to A
-```
+```ascii
         channel-0 channel-1         channel-2 channel-3        channel-4 channel-5
 ┌───────┐       ibc        ┌───────┐        ibc       ┌───────┐        ibc       ┌───────┐
 │Chain A│◄────────────────►│Chain B│◄────────────────►│Chain C│◄────────────────►│Chain D│
@@ -39,7 +39,7 @@ initial IBC release.
 ```
 
 ### Forward A->B->C with 1 retry, max timeouts occurs, refund back to A
-```
+```ascii
         channel-0 channel-1         channel-2 channel-3
 ┌───────┐       ibc        ┌───────┐        ibc       ┌───────┐
 │Chain A│◄────────────────►│Chain B│◄────────────────►│Chain C│
@@ -65,7 +65,7 @@ Utilizing the packet `memo` field, instructions can be encoded as JSON for multi
 - The packet `memo` is included in `MsgTransfer` by user on Chain A.
 
 memo:
-```
+```json
 {
   "forward": {
     "receiver": "chain-c-bech32-address",
@@ -88,7 +88,7 @@ In the case of a timeout after 10 minutes for either forward, the packet would b
 `next` is the `memo` to pass for the next transfer hop. Per `memo` intended usage of a JSON string, it should be either JSON which will be Marshaled retaining key order, or an escaped JSON string which will be passed directly.
 
 `next` as JSON
-```
+```json
 {
   "forward": {
     "receiver": "pfm", // purposely using invalid bech32 here*
@@ -110,7 +110,7 @@ In the case of a timeout after 10 minutes for either forward, the packet would b
 ```
 
 `next` as escaped JSON string
-```
+```json
 {
   "forward": {
     "receiver": "pfm", // purposely using invalid bech32 here*
@@ -131,8 +131,49 @@ To prevent accidentally sending funds to a chain which does not have PFM, it is
 
 The examples above show the intended usage of the `receiver` field for one or multiple intermediate PFM chains.
 
+## 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
+- <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>