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?

2 Answers 2


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.


H means horizontal.

V means vertical.

So, you should use VMP to interact with relaychain.

     /            \
  (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.

  • Thanks. But to be sure. Should we open HRMP between our parachain and Rococo?
    – rust.dev
    Commented Sep 15, 2022 at 12:23
  • There is no way to open HRMP between parachain and relaychain. HRMP is designed for parachain <> parachain.
    – AurevoirXavier
    Commented Sep 15, 2022 at 16:27

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