According to a presentation delivered by Cisco engineer Ed Warnicke to the CNCF Technical Oversight Committee on April 9, Istio is suitable for layer 7 application data traffic. Network Service Mesh however is for layer 3 (IP) and layer 2 (Ethernet) payloads.
“Inspired by Istio, Network Service Mesh maps the concept of a Service Mesh to L2/L3 payloads as part of an attempt to re-imagine NFV in a Cloud-native way,” the Network Service Mesh github project page explains.