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Opening HRMP Channels

In order to communicate over HRMP, parachains must establish channels by registering them on the Relay Chain. Like XCMP, HRMP is a message transport protocol, but passes all messages via the Relay Chain. When XCMP is implemented on Polkadot, HRMP is planned to be deprecated and phased out.

HRMP channels are uni-directional. Bi-directional communication between two parachains will require two channels, one in each direction.

Opening HRMP Channelsโ€‹

Opening a channel between two parachains is a two-phase process, with one chain first initiating a channel request and then the second chain accepting it. When neither chain is a system chain, they will use the hrmpInitOpenChannel and hrmpAcceptOpenChannel calls, respectively.

Each chain must dispatch the following calls on the Relay Chain from its parachain origin.

  1. hrmp > hrmpInitOpenChannel(recipient, proposedMaxCapacity, proposedMaxMessageSize): Initiates channel establishment by creating a channel request with a given configuration. Note that the max capacity and max message size must be within the configuration's limits.

  2. hrmp > hrmpAcceptOpenChannel(sender): Accept the channel open request from the given sender.

In order to dispatch a call from its sovereign origin, a parachain may use governance to send the encoded call in a Transact instruction to the Relay Chain, but it may also execute this logic autonomously (e.g. on the notification that a channel was requested).

Examples of HRMP Channel Managementโ€‹

There are several ways to trigger a specific message from a parachain's origin. The naive way is to write the program off-chain and submit it using the XCM pallet's send extrinsic. Sending arbitrary programs is gated by a privileged origin, so who can initiate that depends on each chain's configuration. The chain may need to go through governance to dispatch the extrinsic.

Another option is to write the programs that a chain will send ahead of time and incorporate them into the runtime. These programs could be behind extrinsics with their own privileged origins, or even unprivileged origins. As the extrinsic can perform any checks prior to sending the message, the runtime developer can program things like allowing any signed origin to dispatch a call accepting an open HRMP channel request with another parachain.

Note that this is actually how other extrinsics (e.g. to teleport assets) in the XCM pallet work; they construct XCM programs locally from a user's inputs and, assuming all checks pass, send the program to the destination.

In addition, the logic could be autonomous and react to other instructions that the chain receives. For example, see Polimec's implementation of the XcmExecutor, which handles notifications of channel requests and acceptance.

Opening HRMP Channels with System Parachainsโ€‹

HRMP channel management involving system parachains takes place entirely on the Relay Chain. No action is required from the parachain origin.

Opening an HRMP channel with a system parachain requires an OpenGov referendum using the General Admin Track.

Proposals should generally be a batchAll call containing two forceOpenHrmpChannel calls (one for each direction of the channel).

As an example, see Referendum 280, which opened two bi-directional channels with Asset Hub (one for Zeitgeist and one for Composable).


Please ensure that you use the new forceOpenHrmpChannel directly on the Relay Chain, rather than the old two-phase channel request/accept method.


As of Polkadot and Kusama runtimes 1,001,000, channel establishment with system chains will not require a deposit. However, for lower spec versions, each chain in the channel will need a free balance of at least the required channel deposits plus the existential deposit.

For example, on Polkadot the sender and recipient deposit are both 10 DOT and the existential deposit is 1 DOT. Each chain, as in both the system chain and its interlocutor, will need 21 DOT; 10 as a channel sender, 10 as a channel recipient, and 1 as existential.

Someone must transfer this DOT to the parachain sovereign accounts prior to the execution of the forceOpenHrmpChannel calls.

Opening HRMP Channels Between Two System Parachainsโ€‹

As of Polkadot and Kusama runtimes 1,001,000, anyone can call an establishSystemChannel extrinsic with two system parachains as arguments in order to establish a channel from the given sender to receiver.