27.9 Summary
The most commonly used of the 10 defined IPv4
options is the source route, but its use is dwindling these days
because of security concerns. Access to IPv4 header options is
through the IP_OPTIONS socket option.
IPv6 defines six extension headers. Access to
IPv6 extension headers is through a functional interface, obviating
the need to understand their actual format in the packet. These
extension headers are written as ancillary data with
sendmsg and returned as ancillary data with
recvmsg.
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