IPv6 Packet - Base Header, Payload Length, and Next Header
An IPv6 packet uses a fixed 40-byte base header followed by payload. The Next Header field identifies either an extension header or the upper-layer protocol.
Key Takeaways
IPv6 base header is fixed at 40 bytes.
Payload Length excludes the base header.
Next Header may point to an extension header before TCP or UDP.
IPv6 removes the IPv4 header checksum.
IPv6 packet layout
The IPv6 base header contains version, traffic class, flow label, payload length, next header, hop limit, source address, and destination address. After that, payload bytes may begin with an extension header or an upper-layer protocol.
Next Header chains
IPv6 uses Next Header values to build a chain. A packet might go from base header to routing extension header, then to fragmentation header, and finally to TCP or UDP.
How IPv6 differs from IPv4
IPv6 simplifies the base header by removing header checksum and router fragmentation. Optional information is represented by extension headers rather than IPv4-style options.
Practical Reference
| Item | Value | Analysis Note |
|---|---|---|
| Base header | 40 bytes | Fixed size. |
| Payload length | Excludes base header | Different from IPv4 Total Length. |
| Next Header | Protocol or extension | Follow the chain carefully. |
| Hop Limit | Loop prevention | IPv6 equivalent of TTL. |
FAQ
Can IPv6 carry UDP?
Yes. IPv6 commonly carries UDP, TCP, ICMPv6, and other protocols through the Next Header field.
Why is IPv6 header parsing simpler?
The base header is fixed size, but extension headers can still require careful parsing.