Amazon CloudFront now offers three additional data fields in CloudFront real-time logs: Origin first-byte latency, Origin last-byte latency, and autonomous system number (ASN). CloudFront real-time logs contain detailed information about requests delivered by CloudFront such as the HTTP status codes of the response, or whether the response was cached. With the three new data fields, customers can get more granular insights on CloudFront performance while analyzing real-time logs, or in the dashboards created using the logs. The Origin first-byte latency provides the time taken in seconds by the origin server to respond back with the first byte of the response. The Origin last-byte latency indicates the time taken in seconds by the origin server to respond back with the last byte of the response. ASN is a unique number that identifies the network, such as an internet service provider (ISP) network, that provides the viewer IP address.
Source:: Amazon AWS