The following list summarizes the features of the IPv6 protocol:
The IPv6 header has a new format that is designed to minimize header overhead. This is achieved by moving both nonessential and optional fields to extension headers that are placed after the IPv6 header. The streamlined IPv6 header is more efficiently processed at intermediate routers.
IPv4 headers and IPv6 headers are not interoperable. IPv6 is not a superset of functionality that is backward compatible with IPv4. A host or router must use an implementation of both IPv4 and IPv6 to recognize and process both header formats. The new IPv6 header is only twice the size of the IPv4 header, even though the number of bits in IPv6 addresses is four times larger than IPv4 addresses.
IPv6 has 128-bit (16-byte) source and destination addresses. Although 128 bits can express over 3.4 x 1038 possible combinations, the large address space of IPv6 has been designed to allow for multiple levels of subnetting and address allocation, from the Internet backbone to the individual subnets within an organization.
Even with all of the addresses currently allocated for use by hosts, there are plenty of addresses available for future use. With a much larger number of available addresses, address-conservation techniques, such as the deployment of NATs, are no longer necessary.
IPv6 global addresses used on the IPv6 portion of the Internet are designed to create an efficient, hierarchical, and summarizable routing infrastructure that is based on the common occurrence of multiple levels of ISPs. On the IPv6 Internet, backbone routers have much smaller routing tables, corresponding to the routing infrastructure of global ISPs. For more information, see "Aggregatable Global Unicast Addresses" in Chapter 3, "IPv6 Addressing."
To simplify host configuration, IPv6 supports both stateful address configuration (such as address configuration in the presence of a DHCPv6 server) and stateless address configuration (such as address configuration in the absence of a DHCPv6 server). With stateless address configuration, hosts on a link automatically configure themselves with IPv6 addresses for the link (called link-local addresses), addresses for IPv4 and IPv6 coexistence, and with addresses derived from prefixes advertised by local routers. Even in the absence of a router, hosts on the same link can automatically configure themselves with link- local addresses and communicate without manual configuration. Link-local addresses are autoconfigured within one second and communication with neighboring nodes on the link is possible immediately. In comparison, an IPv4 host using DHCP must wait a full minute before abandoning DHCP configuration and self-configuring an IPv4 address.
Support for IPSec is an IPv6 protocol suite requirement. This requirement provides a standards-based solution for network security needs and promotes interoperability between different IPv6 implementations. IPSec consists of two types of extension headers and a protocol to negotiate security settings. The Authentication header (AH) provides data integrity, data authentication, and replay protection for the entire IPv6 packet (excluding fields in the IPv6 header that must change in transit). The Encapsulating Security Payload (ESP) header and trailer provide data integrity, data authentication, data confidentiality, and replay protection for the ESP-encapsulated payload. The protocol typically used to negotiate IPSec security settings for unicast communication is the Internet Key Exchange (IKE) protocol.
New fields in the IPv6 header define how traffic is handled and identified. Traffic is prioritized using a Traffic Class field. A Flow Label field in the IPv6 header allows routers to identify and provide special handling for packets that belong to a flow (a series of packets between a source and destination). Because the traffic is identified in the IPv6 header, support for QoS can be achieved even when the packet payload is encrypted with IPSec and ESP.
The Neighbor Discovery protocol for IPv6 is a series of Internet Control Message Protocol for IPv6 (ICMPv6) messages that manages the interaction of neighboring nodes (nodes on the same link). Neighbor Discovery replaces the Address Resolution Protocol (ARP) (broadcast-based), ICMPv4 Router Discovery, and ICMPv4 Redirect messages with efficient multicast and unicast Neighbor Discovery messages.
IPv6 can easily be extended for new features by adding extension headers after the IPv6 header. Unlike options in the IPv4 header, which can support only 40 bytes of options, the size of IPv6 extension headers is constrained only by the size of the IPv6 packet.