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