How to open HRMP with Rococo chain?

Question

hrmp.hrmpInitOpenChannel(
   recipient: 3000                    //the other parachain you want to open the channel with
   proposedMaxCapacity: 1000          // specifies how many messages can be in the channel at once
   proposed_max_message_size: 102400  //specifies the maximum size of the messages
)

There is example how to open HRMP between two siblings parachains. https://docs.substrate.io/reference/how-to-guides/parachains/add-hrmp-channels/ But what exactly instruction should I use to open between my parachain and Rococo/Kusama/Polkadot?

Answer 1

As Xavier mentioned, relaychain and parachain interact via VMP, so there is no need to open HRMP between parachain and relaychain(Rococo/Kusama/Polkadot), and there is no HRMP between parachain and relaychain. HRMP only can be opened between parachain and parachain.

Answer 2

H means horizontal.

V means vertical.

So, you should use VMP to interact with relaychain.

       releychain
     /            \
  (vmp)          (vmp)
   /                \
para a---(hrmp)---para b 

Also, check cross consensus protocols.

---

When you send an open HRMP request, you could see an event on relaychain.

ump (UpwardMessagesReceived), which means you send a message from the downside to the upside through the VMP.