# NAT gateway metrics and dimensions The following metrics are available for your NAT gateways. The description column includes a description of each metrics as well as the [units](https://docs.aws.amazon.com/AmazonCloudWatch/latest/monitoring/cloudwatch_concepts.html#Unit) and [statistics](https://docs.aws.amazon.com/AmazonCloudWatch/latest/monitoring/Statistics-definitions.html). | Metric | Description | | --- | --- | | ActiveConnectionCount | The total number of concurrent active TCP connections through the NAT gateway. A value of zero indicates that there are no active connections through the NAT gateway. Units: Count Statistics: The most useful statistic is `Max`. | | BytesInFromDestination | The number of bytes received by the NAT gateway from the destination. If the value for `BytesOutToSource` is less than the value for `BytesInFromDestination`, there might be data loss during NAT gateway processing, or traffic being actively blocked by the NAT gateway. Units: Bytes Statistics: The most useful statistic is `Sum`. | | BytesInFromSource | The number of bytes received by the NAT gateway from clients in your VPC. If the value for `BytesOutToDestination` is less than the value for `BytesInFromSource`, there might be data loss during NAT gateway processing. Units: Bytes Statistics: The most useful statistic is `Sum`. | | BytesOutToDestination | The number of bytes sent out through the NAT gateway to the destination. A value greater than zero indicates that there is traffic going to the internet from clients that are behind the NAT gateway. If the value for `BytesOutToDestination` is less than the value for `BytesInFromSource`, there might be data loss during NAT gateway processing. Unit: Bytes Statistics: The most useful statistic is `Sum`. | | BytesOutToSource | The number of bytes sent through the NAT gateway to the clients in your VPC. A value greater than zero indicates that there is traffic coming from the internet to clients that are behind the NAT gateway. If the value for `BytesOutToSource` is less than the value for `BytesInFromDestination`, there might be data loss during NAT gateway processing, or traffic being actively blocked by the NAT gateway. Units: Bytes Statistics: The most useful statistic is `Sum`. | | ConnectionAttemptCount | The number of connection attempts made through the NAT gateway. This includes only the initial SYN. In some cases, `ConnectionAttemptCount` may be lower than `ConnectionEstablishedCount` due to SYN retransmission. If the value for `ConnectionEstablishedCount` is less than the value for `ConnectionAttemptCount`, this indicates that clients behind the NAT gateway attempted to establish new connections for which there was no response. Unit: Count Statistics: The most useful statistic is `Sum`. | | ConnectionEstablishedCount | The number of connections established through the NAT gateway. This includes SYN and SYN retransmissions. If the value for `ConnectionEstablishedCount` is less than the value for `ConnectionAttemptCount`, this indicates that clients behind the NAT gateway attempted to establish new connections for which there was no response. Unit: Count Statistics: The most useful statistic is `Sum`. | | ErrorPortAllocation | The number of times the NAT gateway could not allocate a source port. A value greater than zero indicates that too many concurrent connections are open through the NAT gateway. Units: Count Statistics: The most useful statistic is `Sum`. | | IdleTimeoutCount | The number of connections that transitioned from the active state to the idle state. An active connection transitions to idle if it was not closed gracefully and there was no activity for the last 350 seconds. A value greater than zero indicates that there are connections that have been moved to an idle state. If the value for `IdleTimeoutCount` increases, it might indicate that clients behind the NAT gateway are re-using stale connections. Unit: Count Statistics: The most useful statistic is `Sum`. | | PacketsDropCount | The number of packets dropped by the NAT gateway. To calculate the number of dropped packets as a percentage of the overall packet traffic, use this formula: `PacketsDropCount/(PacketsInFromSource+PacketsInFromDestination)*100`. If this value exceeds 0.01 percent of the total traffic on the NAT gateway, there may be an issue with Amazon VPC service. Use the [AWS service health dashboard](http://status.aws.amazon.com/) to identify any issues with the service that may be causing NAT gateways to drop packets. Units: Count Statistics: The most useful statistic is `Sum`. | | PacketsInFromDestination | The number of packets received by the NAT gateway from the destination. If the value for `PacketsOutToSource` is less than the value for `PacketsInFromDestination`, there might be data loss during NAT gateway processing, or traffic being actively blocked by the NAT gateway. Unit: Count Statistics: The most useful statistic is `Sum`. | | PacketsInFromSource | The number of packets received by the NAT gateway from clients in your VPC. If the value for `PacketsOutToDestination` is less than the value for `PacketsInFromSource`, there might be data loss during NAT gateway processing. Unit: Count Statistics: The most useful statistic is `Sum`. | | PacketsOutToDestination | The number of packets sent out through the NAT gateway to the destination. A value greater than zero indicates that there is traffic going to the internet from clients that are behind the NAT gateway. If the value for `PacketsOutToDestination` is less than the value for `PacketsInFromSource`, there might be data loss during NAT gateway processing. Unit: Count Statistics: The most useful statistic is `Sum`. | | PacketsOutToSource | The number of packets sent through the NAT gateway to the clients in your VPC. A value greater than zero indicates that there is traffic coming from the internet to clients that are behind the NAT gateway. If the value for `PacketsOutToSource` is less than the value for `PacketsInFromDestination`, there might be data loss during NAT gateway processing, or traffic being actively blocked by the NAT gateway. Unit: Count Statistics: The most useful statistic is `Sum`. | | PeakBytesPerSecond | This metric reports the highest 10-second bytes per second average in a given minute. Units: Count Statistics: The most useful statistic is `Maximum`. | | PeakPacketsPerSecond | This metric calculates the average packet rate (packets processed per second) every 10 seconds for 60 seconds and then reports the maximum of the six rates (the highest average packet rate). Units: Count Statistics: The most useful statistic is `Maximum`. | To filter the metric data, use the following dimension. | Dimension | Description | | --- | --- | | NatGatewayId | Filter the metric data by the NAT gateway ID. |