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Multipath TCP (MPTCP) extends traditional TCP to enable simultaneous use ofmultiple connection endpoints at the source and destination. MPTCP has beenunder active development since its standardization in 2013, and more recentlyin February 2020, MPTCP was upstreamed to the Linux kernel. In this paper, weprovide an in-depth analysis of MPTCPv0 in the Internet and the first analysisof MPTCPv1 to date. We probe the entire IPv4 address space and an IPv6 hitlistto detect MPTCP-enabled systems operational on port 80 and 443. Our scansreveal a steady increase in MPTCPv0-capable IPs, reaching 13k+ on IPv4(2$\times$ increase in one year) and 1k on IPv6 (40$\times$ increase). MPTCPv1deployment is comparatively low with $\approx$100 supporting hosts in IPv4 andIPv6, most of which belong to Apple. We also discover a substantial share ofseemingly MPTCP-capable hosts, an artifact of middleboxes mirroring TCPoptions. We conduct targeted HTTP(S) measurements towards select hosts and findthat middleboxes can aggressively impact the perceived quality of applicationsutilizing MPTCP. Finally, we analyze two complementary traffic traces fromCAIDA and MAWI to shed light on the real-world usage of MPTCP. We find thatwhile MPTCP usage has increased by a factor of 20 over the past few years, itstraffic share is still quite low.