# JMS Message Header Fields

The header contains predefined fields that contain values used to route and deliver messages.

Header Field | Set by | Comments
--- | --- | ---
`JMSDestination` | send or publish method | Destination to which message is sent
`JMSDeliveryMode` | send or publish method | Persistent or non-persistent message. The default is persistent. EMS extends the delivery mode to include a `RELIABLE_DELIVERY` mode.
`JMSExpiration` | send or publish method | Length of time that message will live before expiration. If set to 0, message does not expire. The time-to-live is specified in milliseconds. If the server `expiration` property is set for a destination, it will override the `JMSExpiration` value set by the message producer.
`JMSDeliveryTime` | send or publish method | Read-only field. If the message producer has a delivery delay set, then the time returned here after calling the send method represents the earliest time when the EMS server will deliver the message to consumers. Once the message has been received, it carries that same value. This value is calculated by adding the delivery delay value held by the message producer to the time the message was sent. For transactions, the delivery time is calculated using the time the client sends the message, not the time the transaction is committed. For more information, see Delivery Delay.
`JMSPriority` | send or publish method | Uses a numerical ranking, between 0 and 9, to define message priority as normal or expedited. Larger numbers represent higher priority. See Message Priority for more information.
`JMSMessageID` | send or publish method | Value uniquely identifies each message sent by a provider.
`JMSTimestamp` | send or publish method | Timestamp of time when message was handed off to a provider to be sent. Message may actually be sent later than this timestamp.
`JMSCorrelationID` | message client | This ID can be used to link messages, such as linking a response message to a request message. Entering a value in this field is optional. The JMS Correlation ID has a recommended maximum of 4 KB. Higher values may result in the message being rejected.
`JMSReplyTo` | message client | A destination to which a message reply should be sent. Entering a value for this field is optional.
`JMSType` | message client | Message type identifier.
`JMSRedelivered` | JMS provider | If this field is set, it is possible that the message was delivered to the client earlier, but not acknowledged at that time.

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