| IPv6 Network - Extension Headers |
IPv6 Network - Extension Headers
To allow special processing along the way, IPv4 allows the IP header to be extended with one or more options. These options are rarely used today, both because they don’t really solve common problems but also because packets with options can’t be processed in the “fast path,” and many routers and firewalls block some or all options. Not unlike the checkout counters at a grocery store, many routers have several “paths” that packets may follow: a fast one, implemented in hardware or highly optimized software, that supports only the most common operations (“no checks”), and one or more slower paths that use more advanced but slower software code that supports less common operations such as looking at IP options. However, many modern routers only have a “fast path,” so using additional features doesn’t lead to a performance penalty.
Because the header is fixed length in IPv6, options can’t be tagged onto the IP header as in IPv4. Instead, they’re put in a header of their own that sits between the IPv6 header and the TCP or UDP (or other higher level protocol) header. The most common extension headers are Hop-by-Hop Options: See the section that follows.
Routing: Similar to the Source Route option in IPv4. Fragment: Used for fragmentation; see later in this chapter. Authentication: Authenticates the user data and most header fields. Encapsulating Security Payload (ESP): Encrypts and/or authenticates user data. Destination Options: See the section that follows. The Hop-by-Hop Options and Destination Options headers are container headers: they have room for multiple suboptions. The Hop-by-Hop Options are processed by all routers along the way. All other options are normally ignored by routers and processed only by the destination. Obviously firewalls, or routers configured to perform filtering, may also look at these options. The Hop-by-Hop Options, Routing, Fragment, and Destination Options extensionheaders are defined in RFC 2460.
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