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develop/parachain-devs/interoperability/open-message-passing-channels.md

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+---
+title: Open Message Passing Channels (HRMP)
+description: Learn how to open message passing channels between parachains for cross consensus operations.  This is known as horizontal relay routed message passing (HRMP).
+---
+
+In order to communicate over HRMP, parachains must establish channels by registering them on the Relay Chain. HRMP is a message transport protocol that passes all messages via the relay chain.
+
+HRMP channels are uni-directional. Bi-directional communication between two parachains will require
+two channels, one in each direction.
+
+## Opening HRMP Channels
+
+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` extrinsics.
+
+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 Management
+
+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](https://github.com/Polimec/polimec-node/blob/da9d1ee0062ead7a62f815647813ada48e4c2250/pallets/xcm-executor/src/lib.rs#L890){target=_blank}
+of the `XcmExecutor`, which handles notifications of channel requests and acceptance.
+
+## Opening HRMP Channels with System Parachains
+
+The
+[`establish_channel_with_system`](https://paritytech.github.io/polkadot-sdk/master/polkadot_runtime_parachains/hrmp/pallet/dispatchables/fn.establish_channel_with_system.html){target=_blank}
+call can be used to establish a bi-directional channel between any parachain and a system chain.
+This call needs to be dispatched from the parachain via an XCM program to execute on the Relay
+Chain.
+
+For instance, the call to be executed on the relay chain to establish a bi-directional channel
+between a parachain and the Asset Hub with `ParaID` 1000 is `0x3c0ae8030000`
+
+![asset-hub-hrmp-channel](../assets/asset-hub/establish-channel-asset-hub.png)
+
+Here is an example call with an XCM program sent from the parachain to Rococo relay chain to
+establish channel a bi-directional channel with Rococo Asset Hub -
+`0x0f001f000301000314000400000000070010a5d4e81300000000070010a5d4e80006000300c16678419c183c0ae8030000140d01000001003145`
+
+![asset-hub-hrmp-channel-xcm](../assets/asset-hub/asset-hub-establish-channel-xcm.png)
+
+## Opening HRMP Channels Between Two System Parachains
+
+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.