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.