  In modern SD-WAN networks, a global controller is able to steer traffic on
different paths based on application requirements and global intents. However,
existing solutions cannot dynamically tune the way bandwidth is shared between
flows inside each overlay link, in particular when the available capacity is
uncertain due to cross traffic. In this context, we propose a global QoS
(Quality of Service) policy optimization model that dynamically adjusts rate
limits of applications based on their requirements to follow the evolution of
network conditions. It relies on a novel cross-traffic estimator for the
available bandwidth of overlay links that only exploits already available
measurements. We propose two local search algorithms, one centralized and one
distributed, that leverage cross-traffic estimation. We show in packet-level
simulations a significant performance improvement in terms of SLA (Service
Level Agreement) satisfaction. For instance, the adaptive tuning of load
balancing and QoS policies based on cross-traffic estimation can improve SLA
satisfaction by $40\%$ compared to static policies.
