IPv6 Header Details

An IPv6 Packet consists of a fixed-sized Header and variable-sized Payload. IPv6 header is 40 bytes in size.

Version (4 bits):  This field is similar to the "Version" field in IPv4 Header. The version field indicates the version of the header and its value is set to 6 (Value 0110) for IPv6.

Traffic Class (8 bits): This field defines the prioritization of the Traffic. This field is Similar to the "TOS" field in IPv4 Header. 8 bits are divided into two parts. The most significant 6 bits are used for Type of Service (or DSCP). The least significant 2 bits are used for Explicit Congestion Notification (ECN).

Flow Label (20 bits): This a new field included in IPv6 header basically used to distinguish the flow of packets. A source device can label the packets by setting a non zero value in the Flow Label field. The packets marked with the same Flow Label value would be given the same treatment by various devices (Routers/L3 Switches) that exist in the path. Multiple active flows may exist from a source to a destination along with packets that are not associated with any flow i.e with Flow Label value 0.

Payload length (16 bits): This field indicates the size of the payload in bytes, including any extension headers. A Jumbo payload is indicated by the value zero in the Payload Length field, details explained in this field below. This field is similar to the "Total Length" field in IPv4 header but not exactly identical to it.
In IPv6,  The Payload Length = Variable Length Data - (minus) IPv6 header, 
In IPv4, Total Length = Variable Length Data size + IPv4 Header.

The total size of an IPv6 packet can be derived by adding 40bytes to the Payload Length field.

IPv6 Payload is also composed of Extension Headers along with Upper layer data. With 16 bits, up to 65535 bytes is the maximum length of Payload, but if the Extension Headers contain Hop-by-Hop Extension Header, then the payload may exceed 65535 bytes and this field is set to 0.  You can see more details about Extension Header in my other article "IPv6 Extension Headers".

Next Header (8 bits): This field is similar to the "Protocol Field" in the IPv4 Header. This field identifies the protocol running at the transport layer for e.g. TCP(6)/UDP(17)/OSPF(89)/EIGRP (88) etc. Also, note that This field is used to indicate either the type of Extension Header or if the Extension Header is not present then it indicates the Upper Layer Protocol at Transport layer.

Hop Limit (8 bits): This field is similar to "Time to Live (TTL)" field in IPv4 header. The source device set the value in this field and its value is decremented by one by each node in the path that forwards the packet.  The packet is discarded when the hop count gets zero. This field is used to prevent the packet to remain in an indefinite loop due to any routing related errors.

Source address (128 bits): Specifies the IPv6 address of the sending node.

Destination address (128 bits): Specifies the IPv6 address of the destination node.