IPv6 Packet Format

The IPv6 packet is composed of two main parts: the header and the payload.

Header

The header is in the first 40 octets (320 bits) of the packet and contains:

· Version - version 6 (4-bit IP version).

· Traffic class - packet priority (8-bits). Priority values subdivide into ranges: traffic where the source provides congestion control and non-congestion control traffic.

· Flow label - QoS management (20 bits). Originally created for giving real-time applications special service, but currently unused.

· Payload length - payload length in bytes (16 bits). When cleared to zero, the option is a "Jumbo payload" (hop-by-hop).

· Next header - Specifies the next encapsulated protocol. The values are compatible with those specified for the IPv4 protocol field (8 bits).

· Hop limit - replaces the time to live field of IPv4 (8 bits).

· Source and destination addresses - 128 bits each.

The payload can have a size of up to 64KiB in standard mode, or larger with a "jumbo payload" option.

Fragmentation is handled only in the sending host in IPv6: routers never fragment a packet, and hosts are expected to use PMTU discovery.

The protocol field of IPv4 is replaced with a Next Header field. This field usually specifies the transport layer protocol used by a packet's payload.

In the presence of options, however, the Next Header field specifies the presence of an extra options header, which then follows the IPv6 header; the payload's protocol itself is specified in a field of the options header. This insertion of an extra header to carry options is analogous to the handling of AH and ESP in IPsec for both IPv4 and IPv6.

Source:
http://en.wikipedia.org/wiki/IPv6