# Heartbeat <0> # The [Heartbeat `<0>`](https://www.onixs.biz/fix-dictionary/4.4/msgType_0_0.html) monitors the status of the communication link and identifies when the last of a string of messages was not received. _The only supported heartbeat interval, as specified by the [Logon ``](https://www.onixs.biz/fix-dictionary/4.4/msgType_A_65.html) message, is 30 seconds_. When either end of a FIX connection has not sent any data for [HeartBtInt `<108>`](https://www.onixs.biz/fix-dictionary/4.4/tagNum_108.html) seconds, it will transmit a [Heartbeat `<0>`](https://www.onixs.biz/fix-dictionary/4.4/msgType_0_0.html) message. When either end of the connection has not received any data for ([HeartBtInt `<108>`](https://www.onixs.biz/fix-dictionary/4.4/tagNum_108.html) + "some reasonable transmission time") seconds, it will transmit a [Test Request `<1>`](https://www.onixs.biz/fix-dictionary/4.4/msgType_1_1.html) message. If there is still no [Heartbeat `<0>`](https://www.onixs.biz/fix-dictionary/4.4/msgType_0_0.html) message received after ([HeartBtInt `<108>`](https://www.onixs.biz/fix-dictionary/4.4/tagNum_108.html) + "some reasonable transmission time") seconds then the connection should be considered lost and corrective action be initiated. Note that a [Test Request `<1>`](https://www.onixs.biz/fix-dictionary/4.4/msgType_1_1.html) message can still be sent independent of the value of the [HeartBtInt `<108>`](https://www.onixs.biz/fix-dictionary/4.4/tagNum_108.html), which will force a [Heartbeat `<0>`](https://www.onixs.biz/fix-dictionary/4.4/msgType_0_0.html) message. Heartbeats issued as the result of [Test Request `<1>`](https://www.onixs.biz/fix-dictionary/4.4/msgType_1_1.html) must contain the [TestReqID `<112>`](https://www.onixs.biz/fix-dictionary/4.4/tagNum_112.html) transmitted in the [Test Request `<1>`](https://www.onixs.biz/fix-dictionary/4.4/msgType_1_1.html) message. This is useful to verify that the [Heartbeat `<0>`](https://www.onixs.biz/fix-dictionary/4.4/msgType_0_0.html) is the result of the [Test Request `<1>`](https://www.onixs.biz/fix-dictionary/4.4/msgType_1_1.html) and not as the result of a regular timeout. --- ### FIELDS | Tag | Name | Req | Description | | --- | --------------------------------------------------------------------- | --- | ------------------------------------------------------------------------------------------------------------------------------ | | | [Standard Header](/trading/fix/overview/standard-header) | Y | [MsgType](https://www.onixs.biz/fix-dictionary/4.4/msgs_by_msg_type.html) = `0` | | 112 | [TestReqID](https://www.onixs.biz/fix-dictionary/4.4/tagNum_112.html) | N\* | Required when the heartbeat is the result of a [Test Request `<1>`](https://www.onixs.biz/fix-dictionary/4.4/msgType_1_1.html) | | | [Standard Trailer](/trading/fix/overview/standard-trailer) | Y | |