Chapter 14IPv6 (Overview)
The Internet Protocol, version 6 (IPv6), is a new version
of Internet Protocol (IP) that is designed to be an evolutionary step from
the current version, IPv4. IPv6 is a natural increment to IPv4. Deploying
IPv6 with defined transition mechanisms does not disrupt current operations.
IPv6 adds increased address space. IPv6 also improves Internet capability
by using a simplified header format, support for authentication and privacy,
autoconfiguration of address assignments, and new quality-of-service capabilities.
This chapter contains the following information:
IPv6 Features
Most of the changes from IPv4 to IPv6 are described in the following
categories:
Expanded routing and addressing capabilities - IPv6 increases the IP address size from 32 bits to 128
bits to support more levels of addressing hierarchy. IPv6 provides a much
greater number of addressable nodes and employs simpler autoconfiguration
of addresses.
The addition of a scope field improves the scalability
of multicast routing to multicast addresses.
IPv6 defines a new type of address that is called an anycast address. An anycast address identifies sets of nodes. A packet
that is sent to an anycast address is delivered to one of the nodes. The use
of anycast addresses in the IPv6 source route allows nodes to control the
path over which their traffic flows.
Header format simplification -
Some IPv4 header fields have been dropped or have been made optional. This
change reduces the common-case processing cost of packet handling. This change
also keeps the bandwidth cost of the IPv6 header as low as possible, despite
the increased size of the addresses. Even though the IPv6 addresses are four
times longer than the IPv4 addresses, the IPv6 header is only twice the size
of the IPv4 header.
Improved support for options -
Changes in the way IP header options are encoded allow for more efficient
forwarding. Also, the length of options has less stringent limits. The changes
also provide greater flexibility for introducing new options in the future.
Quality-of-service capabilities -
A new capability is added to enable the labeling of packets that belong to
particular traffic flows for which the sender requests
special handling. For example, the sender can request nondefault quality of
service or real-time service.
Authentication and privacy capabilities - IPv6 includes the definition of extensions that provide
support for authentication, data integrity, and confidentiality.
IPv6 Header and Extensions
The IPv6 protocol defines a set of headers, including the basic IPv6
header and the IPv6 extension headers.
Header Format
The following figure shows the elements that appear in the IPv6 header
and the order in which the elements appear.
Figure 14-1 IPv6 Header Format
 The following list describes the function of each header field.
Version - 4-bit Version
number of Internet Protocol = 6.
Traffic Class - 8-bit
traffic class field (see Traffic Class).
Flow Label - 20-bit
field (see IPv6 Quality-of-Service Capabilities).
Payload Length -
16-bit unsigned integer, which is the rest of the packet that follows the
IPv6 header, in octets.
Next Header - 8-bit
selector. Identifies the type of header immediately following the IPv6 header.
Uses the same values as the IPv4 protocol field (see Extension Headers).
Hop Limit - 8-bit
unsigned integer. Decremented by one by each node that forwards the packet.
The packet is discarded if Hop Limit is decremented to zero.
Source Address -
128 bits. The address of the initial sender of the packet (see IPv6 Addressing).
Destination Address -
128 bits. The address of the intended recipient of the packet. The intended
recipient is not necessarily the recipient if an optional Routing Header is
present.
Extension Headers
IPv6 includes an improved option mechanism over IPv4. IPv6 options are
placed in separate extension headers that are located between the IPv6 header
and the transport-layer header in a packet. Most IPv6 extension headers are
not examined or processed by any router along a packet's delivery path until
the packet arrives at its final destination. This feature is a major improvement
in router performance for packets that contain options. In IPv4, the presence
of any options requires the router to examine all options.
The other improvement is that, unlike IPv4 options,
IPv6 extension headers can be of arbitrary length. Also, the number of options
that a packet carries are not limited to 40 bytes. This feature, plus the
manner in which IPv6 options are processed, permits IPv6 options to be used
for functions that are not practical in IPv4. A good example of IPv6 options
is the IPv6 authentication and security encapsulation options.
To improve performance when handling subsequent option headers,
and the transport protocol that follows, IPv6 options are always an integer
multiple of eight octets long. The integer multiple of eight octets retains
the alignment of subsequent headers.
The following IPv6 extension headers are currently defined.
Routing - Extended
routing (like IPv4 loose source route)
Fragmentation - Fragmentation
and reassembly
Authentication -
Integrity and authentication, security
Encapsulation - Confidentiality
Hop-by-Hop Option -
Special options that require hop-by-hop processing
Destination Options -
Optional information to be examined by the destination node
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