Packet Relaying

This section describes the configuration and commands that can be used to start the relayer and relay packets over one or multiple paths.

Table of Contents

The start Command

To relay packets only configure the mode section of the configuration file like so:

[global]
log_level = 'info'

[mode]

[mode.clients]
enabled = true
# ...

[mode.connections]
enabled = false

[mode.channels]
enabled = false

[mode.packets]
enabled = true
# ...

Then start hermes using the start command:

hermes start

The relayer sends packet transactions triggered by IBC packet events for all open channels between the configured chains. This is also referred to packet streaming.

Packet Streaming

After the relayer is started using the start command, it listens to IBC packet events emitted by any of the configured chains. Assuming the events are coming from a source chain, the relayer builds packets based on these events, packets that are then sent either to the source chain or the counterparty (destination) chain.

Current events and actions are:

  • send_packet: the relayer builds a packet message with the packet obtained from the event and any required proofs obtained from the counterparty of the chain where the message is sent. The concrete packet is:
    • MsgRecvPacket, sent to destination chain if the channel is in open state on the destination chain, and a timeout has not occurred,
    • MsgTimeout, sent to the source chain if the channel is in open state on the destination chain, but a timeout has occurred.
    • MsgTimeoutOnClose, sent to the source chain if the channel is in closed state on the destination chain.
  • write_acknowledgement: the relayer builds a MsgAcknowledgement packet that is sent to the destination chain.

In addition to these events, the relayer will also handle channel closing events:

  • chan_close_init: the relayer builds a MsgChannelCloseConfirm and sends it to the destination chain

Packet Delay

If the relay path is using a non-zero delay connection, then hermes will delay all packet transactions. The delay is relative to the submission time for the client update at the height required by the packet proof. The delay is used to prevent light client attacks and ensures that misbehavior detection finalizes before the transaction is submitted. For more information on the misbehavior detector see the misbehaviour section.