How to build HrmpChannel between two parachains?

[jonathanxuu]

Want to use xToken to transfer assets between 2 parachains, but don't know how to build HrmpChannel.
I have no idea how to construct request message.

[bkchr]

CC @pepyakin

[pepyakin]

From a high level, here is the sequence of actions that should take place in order to establish a channel from parachain A to parachain B:

1. Parachain A calls hrmp_init_open_channel dispatchable with the parachain B as the recipient and the desired parameters of the channel.
2. Within the next N sessions parachain B calls hrmp_accept_open_channel. sender should be the ID of parachain A.
3. On the next session change, the channel A→B should be opened.

Note that these dispatchables has the ensure_parachain predicate for the origin. That means that the parachain itself should issue the request. In order to do that, the parachain should send a specially prepared upward message. The simplest way is to use sudo_send_upward_message.

When received, the relay-chain treats an upward message as an XCM. There is an Xcm::Transact, which will execute the given call (aka dispatchable) on the behalf of the parachain. So you need to instantiate the Xcm<C> where C is the Call of the polkadot (or whatever chain you are trying to pull this off on). You would need something like Call::Hrmp(parachains::Call::hrmp_init_open_channel(..)).

This is a bit involved. There were ideas of a dedicated XCM messages just for that. See paritytech/polkadot#3422

[jonathanxuu]

thanks , it works~

[samelamin]

@jonathanxuu were you able to get it working via the UI?

I cant seem to get the channels open on the UI so I am curious to how you got it working?