6.13 IPv6

As described above, IPv4 limits address space to 32 bits. Unfortunately, 32 bits proved a severe limitation on the rapid expansion of Internet addresses, so the IETF began work on the next generation, known as IPv6. IPv6 increases the address space to 128 bits, or 16 bytes.

6.13.1 Features of IPv6

IPv6 does not provide fragmentation support for transit packets in routers. The terminal hosts are required to perform PMTU to avoid fragmentation. In addition, IPv6 has enhanced options support. The options are defined in separate headers, instead of being a field in the IP header. Known as header chaining , this format inserts the IP option headers between the IP header and the transport header.

The IPv6 header fields (shown in Figure 6-5) can be described as follows :

Version

A four-bit field describing the IP version (in this case, IPv6).

Traffic class

Similar to the Type-of-Service field in IPv4.

Flow label

This experimental 20-bit field is under development to signal special processing in routers.

Payload length

This 16-bit field indicates the length of the data payload.

Next header

This is similar to the Protocol field in the IPv4 header, but it also includes the Options header.

Hop limit

This eight-bit field serves a purpose similar to the TTL field in the IPv4 header.

Source and destination address

128-bit fields that represent the source and destination addresses in IPv6 format.

Data

Includes the information payload.

Figure 6-5. Representation of IPv6 header fields

6.13.2 IPv6 Addressing

IPv6 has an updated addressing scheme that accommodates the geometric expansion of the Internet. IPv4 used decimal notation to represent a 32-bit address, such as 255.255.255.0. In contrast, IPv6 uses hexadecimal numbers , separated by colons. An example of this would be as follows:

 1844:3FFE:B00:1:4389:EEDF:45AB:1029 

6.13.3 Security Aspects of IPv6

One growth area of IPv6 is expected to be in wireless devices such as cellular phones and PDAs, which benefit from the enlarged address space. However, some experts have raised privacy concerns. For example, the IPv6 address space in some cases uses a unique identifier (ID) derived from your hardware (e.g., handheld phone) that allows packets to be traced back to your device. This can be a problem: the IPv6 ID can also be used to determine the manufacturer, make, model number, and value of the hardware equipment being used.

As a workaround, the IETF published RFC 3041, "Privacy Extensions for Stateless Address Autoconfiguration in IPv6." The RFC describes an algorithm to generate randomized interface identifiers and temporary addressees during a user session.