  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 the first broad analysis of MPTCPv0 in the
Internet. 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 MPTCP-capable IPs, reaching 9k+ on IPv4 and a few dozen on IPv6. We
also discover a significant 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.
