Starting today, you can enable Common Media Client Data (CMCD) fields in your CloudFront real-time logs. You can select key client-side performance parameters and CloudFront delivery performance parameters in the same log record. This can help you correlate variations in Quality of Experience (QoE) for your viewers to CloudFront performance at the granularity of single viewer sessions, simplifying the troubleshooting of QoE issues that impact your viewers engagement.
Source:: Amazon AWS