Certainly, let's break down the improvements and changes made in the IPv6 header:

IPv6 Header Improvements:

• IPv6 introduces several improvements beyond addressing size expansion.
• The IPv6 header is simpler compared to the IPv4 header.

IPv6 Header Fields:

1. Version Field (4 bits): Similar to IPv4, it indicates the version of the IP protocol in use.
2. Traffic Class Field (8 bits): Defines the type of traffic and enables different traffic classes to receive different priorities.
3. Flow Label Field (20 bits): Used alongside the traffic class field, routers use this to make quality of service decisions for datagrams.
4. Payload Length Field (16 bits): Specifies the length of the data payload section of the datagram.
5. Next Header Field: A unique concept in IPv6. Specifies the type of header immediately following the current one. This is part of the design to keep the IPv6 header as compact as possible. Optional headers are abstracted away from the main header.
6. Hop Limit Field (8 bits): Similar in purpose to IPv4's TTL (Time to Live) field. It indicates the maximum number of hops (routers) the datagram can take before being discarded.
7. Source and Destination Address Fields (128 bits each): These are the IPv6 addresses of the source and destination devices.

Next Header Field:

• The next header is just actually the next header of IPv6 inside of it.
• In IPv6, optional headers (such as routing, fragmentation, extension headers) are abstracted from the main IPv6 header.
• The "Next Header" field specifies the type of header that immediately follows the current one.
• These additional headers form a chain, and each optional header has its own "Next Header" field, allowing for various configurations.

Data Payload:

• If the "Next Header" field points to another header, that header would follow immediately.