It has taken longer than anybody might have liked, but the IPv6 protocol is
slowly displacing IPv4 across the Internet.  A quick, highly scientific
"grep the access logs" test shows that about 16% of the traffic to
LWN.net is currently using IPv6, and many large corporate networks are
using IPv6 exclusively internally.  This version of the IP protocol was
designed to be more flexible than IPv4 in a number of ways; the "extension
header" mechanism is one way in which that flexibility is achieved.  A
proposal to formalize extension-header processing in the kernel's
networking stack has led to some concerns, though, about how this feature
will be used and what role Linux should play in its development.