  Vehicles shipping sensors for onboard systems are gaining connectivity. This
enables information sharing to realize a more comprehensive understanding of
the environment. However, peer communication through public cellular networks
brings multiple networking hurdles to address, needing in-network systems to
relay communications and connect parties that cannot connect directly. Web
Real-Time Communication (WebRTC) is a good candidate for media streaming across
vehicles as it enables low latency communications, while bringing standard
protocols to security handshake, discovering public IPs and transverse Network
Address Translation (NAT) systems. However, the end-to-end Quality of Service
(QoS) adaptation in an infrastructure where transmission and reception are
decoupled by a relay, needs a mechanism to adapt the video stream to the
network capacity efficiently. To this end, this paper investigates a mechanism
to apply changes on resolution, framerate and bitrate by exploiting the Real
Time Transport Control Protocol (RTCP) metrics, such as bandwidth and
round-trip time. The solution aims to ensure that the receiving onboard system
gets relevant information in time. The impact on end-to-end throughput
efficiency and reaction time when applying different approaches to QoS
adaptation are analyzed in a real 5G testbed.
