IPv6 Overview

IP version 6 (IPv6) is designed to eventually supersede IP version 4 (IPv4). The intent of this design change is not to take a radical step away from IPv4, but to enhance IP addressing and maintain other IPv4 functions that work well.

The differences between IPv4 and IPv6 include the following:

• Expanded addressing capabilities

  IPv6 increases the size of the IP address from 32 bits to 128 bits. This increased size provides a larger address space and a much larger number of addressable nodes.

• Simplified header format

  Reducing some common processing costs associated with packet handling and streamlining the bandwidth cost of the larger IPv6 header, some IPv4-specific header fields either no longer exist or are now optional in the IPv6 header.

• Traffic flow labeling capabilities

  The ability to label packets for specific traffic flows exists in the IPv6 packet. These labels allow for nondefault quality of service (QoS) or the possibility of “real-time” services.

• Authentication capabilities

  Authentication provides the ability to use extensions for some authentication and data integrity applications.

IPv6 continues to provide the basic packet delivery service for all TCP/IP networks. As a connectionless protocol, IPv6 does not exchange control information to establish an end-to-end connection before transmitting data. Instead, just like its IPv4 predecessor, IPv6 continues to rely on protocols in other layers to establish the connection if connection-oriented services are required and to provide error detection and error recovery.

In addition to supporting a revised header structure and an expanded addressing format, the E Series router supports the following IPv6 features:

• Static routes
• ICMPv6
• Ping
• Traceroute
• Routing policy (See JunosE IP Services Configuration Guide for details.)
• IPv6 B-RAS (See the JunosE Broadband Access Configuration Guide for details.)
• IPv6 tunnel routing tables

IPv6 Packet Headers

An IPv6 packet is a block of data that contains a header and a payload. The header is the information necessary to deliver the packet to a destination address; the payload is the data that you want to deliver. IPv6 packets can use a standard or an extended format.

IPv4 and IPv6 Header Differences

The main difference between IPv4 and IPv6 resides in their headers. Figure 1 provides a comparison between the two protocol versions.

Figure 1: IPv4 and IPv6 Header Comparison

Standard IPv6 Headers

IPv6 packet headers contain many of the fields found in IPv4 packet headers; some of these fields differ from IPv4. (See Figure 1.)

The 40-byte IPv6 header consists of the following eight fields:

• Version—Indicates the version of the Internet Protocol.
• Traffic class—Previously the type-of-service (ToS) field in IPv4, the traffic class field defines the class-of-service priority of the packet. However, the semantics for this field (for example, Differentiated Services (DiffServ) code points) are identical to IPv4.
• Flow label—The flow label identifies all packets belonging to a specific flow (that is, packet flows requiring a specific class of service (CoS)); routers can identify these packets and handle them in a similar fashion.
• Payload length—Previously the total length field in IPv4, the payload length field specifies the length of the IPv6 payload.
• Next header—Previously the protocol field in IPv4, the Next Header field indicates the next extension header to examine.
• Hop limit—Previously the time-to-live (TTL) field in IPv4, the hop limit indicates the maximum number of hops allowed.
• Source address—Identifies the address of the source node sending the packet.
• Destination address—Identifies the final destination node address for the packet.

Extension Headers

In IPv6, extension headers are used to encode optional Internet-layer information. Extension headers are placed between the IPv6 header and the upper-layer header in a packet.

IPv6 enables you to chain extension headers together by using the next header field. The next header field, located in the IPv6 header, indicates to the router which extension header to expect next. If there are no more extension headers, the next header field indicates the upper-layer header (TCP header, UDP header, ICMPv6 header, an encapsulated IP packet, or other items).