One of the most significant architectural improvements in IPv6 is the move from a complex, variable-length header (as seen in IPv4) to a streamlined, fixed-length base header of 40 bytes. Additional functionality that was previously handled by "Options" in IPv4 is now moved toExtension Headers, which are inserted between the IPv6 base header and the upper-layer protocol (TCP/UDP).
According to Juniper Networks technical documentation and RFC 8200, the following are valid IPv6 Extension Headers:
Hop-by-Hop Options (Option B):This header carries optional information that must be examined by every node along the delivery path. It is used for features like the Router Alert and Jumbo Payload options.
Fragment (Option E):Unlike IPv4, where any router can fragment a packet, in IPv6, fragmentation is performed only by the source node. The Fragment header contains the information necessary for the destination to reassemble the packet (Offset, Identification, and More Fragments flag).
Destination Options (Option A):This header carries information intended only for the destination node. It can appear twice: once before a routing header and once after.
Why other options are incorrect:
Protocol (Option C):In IPv4, this was a field in the header. In IPv6, this is replaced by theNext Headerfield, which identifies the type of the following header (whether it's an extension header or the upper-layer protocol).
Header Checksum (Option D):This field was entirely removed in IPv6. IPv6 relies on the data link layer (Ethernet) and the transport layer (TCP/UDP) to perform error detection, significantly reducing the processing overhead for routers in the core of a service provider network.
