27.1 Introduction
IPv4 allows up to 40 bytes of options to follow
the fixed 20-byte header. Although 10 different options are
defined, the most commonly used is the source route option. Access
to these options is through the IP_OPTIONS socket option
and we will demonstrate this with an example that uses source
routing.
IPv6 allows extension headers to occur between
the fixed 40-byte IPv6 header and the transport-layer header (e.g.,
ICMPv6, TCP, or UDP). Six different extension headers are currently
defined. Unlike the IPv4 approach, access to the IPv6 extension
headers is through a functional interface instead of forcing the
user to understand the actual details of how the headers appear in
the IPv6 packet.
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