  Driven by their quest to improve web performance, Content Delivery Networks
(CDNs) are known adaptors of performance optimizations. In this regard, TCP
congestion control and particularly its initial congestion window (IW) size is
one long-debated topic that can influence CDN performance. Its size is,
however, assumed to be static by IETF recommendations---despite being network-
and application-dependent---and only infrequently changed in its history. To
understand if the standardization and research perspective still meets Internet
reality, we study the IW configurations of major CDNs. Our study uses a
globally distributed infrastructure of VPNs giving access to residential access
links that enable to shed light on network-dependent configurations. We observe
that most CDNs are well aware of the IW's impact and find a high amount of
customization that is beyond current Internet standards. Further, we find CDNs
that utilize different IWs for different customers and content while others
resort to fixed values. We find various initial window configurations, most
below 50 segments yet with exceptions of up to 100 segments---the tenfold of
current standards. Our study highlights that Internet reality drifted away from
recommended and standardized practices.
