[IPv6] IPv6 Header

IPv6 Header

  * Struct of an IPv6 Packet 

   

IPv6 Header     The IPv6 header is always present and is a fixed size of 40 bytes. The fields in the IPv6 header are described in the “IPv6 Header” section in this chapter.

Extension Headers     Zero or more extension headers can be present and are of varying lengths. If extension headers are present, a Next Header field in the IPv6 header indicates the first extension header. Within each extension header is another Next Header field, indicating the next extension header. The last extension header indicates the header for the upper-layer protocol—such as Transmission Control Protocol (TCP), User Datagram Protocol (UDP), or Internet Control Message Protocol for version 6 (ICMPv6)—contained within the upper-layer protocol data unit.
The IPv6 header and extension headers replace the existing IPv4 header and its options. The new extension header format allows IPv6 to be enhanced to support future needs and capabilities. Unlike options in the IPv4 header, IPv6 extension headers have no maximum size and can expand to accommodate all the extension data needed for IPv6 communication.

Upper-Layer Protocol Data Unit     The upper-layer protocol data unit (PDU) typically consists of an upper-layer protocol header and its payload (for example, an ICMPv6 message, a TCP segment, or a UDP message).
The IPv6 packet payload is the combination of the IPv6 extension headers and the
upper-layer PDU. Normally, it can be up to 65,535 bytes long. IPv6 packets with
payloads larger than 65,535 bytes in length, known as jumbograms, can also be sent.

  * IPv4 Header

   

  * IPv6 header

   

  * IPv4와 IPv6 헤더의 비교

  * IPv6 Extension Header

Hop-by-Hop Options Header

The Hop-by-Hop Options header is used to specify delivery parameters at each hop on the path to the destination. It is identified by the value of 0 in the IPv6 header’s Next Header field.
The Hop-by-Hop Options header consists of a Next Header field, a Header Extension Length field, and an Options field that contains one or more options. The value of the Header Extension Length field is the number of 8-byte blocks in the Hop-by-Hop Options extension header, not including the first 8 bytes. Therefore, for an 8-byte Hop-by-Hop Options header, the value of the Header Extension Length field is 0. Padding options are used to ensure 8-byte boundaries.

Destination Options Header

The Destination Options header is used to specify packet delivery parameters for either intermediate destinations or the final destination. This header is identified by the value of 60 in the previous header’s Next Header field. The Destination Options header has the same structure as the Hop-by-Hop Options header.

The Destination Options header is used in two ways:
1. If a Routing header is present, it specifies delivery or processing options at each intermediate destination. In this case, the Destination Options header occurs before the Routing header.
2. If no Routing header is present, or if this header occurs after the Routing header, this header specifies delivery or processing options at the final destination. An example of a destination option is the Home Address destination option for Mobile IPv6.

Routing Header

IPv4 defines strict source routing, in which each intermediate destination must be only one hop away, and loose source routing, in which each intermediate destination can be one or more hops away. IPv6 source nodes can use the Routing header to specify a source route, which is a list of intermediate destinations for the packet to travel to on its path to the final destination. The Routing header is identified by the value of 43 in the previous header’s Next Header field.

The Routing header consists of a Next Header field, a Header Extension Length field (defined in the same way as the Hop-by-Hop Options extension header), a Routing Type field, a Segments Left field that indicates the number of intermediate destinations that are still to be visited, and routing type-specific data.

Fragment Header

The Fragment header is used for IPv6 fragmentation and reassembly services. This header is identified by the value of 44 in the previous header’s Next Header field. The Fragment header includes a Next Header field, a 13-bit Fragment Offset field, a More Fragments flag, and a 32-bit Identification field. The Fragment Offset, More Fragments flag, and Identification fields are used in the same way as the corresponding fields in the IPv4 header. Because the use of the Fragment Offset field is defined for 8-byte fragment blocks, the Fragment header cannot be used for jumbograms. The maximum number that can be expressed with the 13-bit Fragment Offset field is 8191. Therefore, Fragment Offset can be used to indicate only a fragment data starting position of up to 8191 × 8, or 65,528.

Authentication Header

The Authentication header provides data authentication (verification of the node that sent the packet), data integrity (verification that the data was not modified in transit), and antireplay protection (assurance that captured packets cannot be retransmitted and accepted as valid data) for the IPv6 packet including the fields in the IPv6 header that do not change in transit across an IPv6 internetwork. The Authentication header, described in RFC 2402, is part of the security architecture for IP, as defined in RFC 2401. The Authentication header is identified by the value of 51 in the previous header’s Next Header field.
The Authentication header contains a Next Header field, a Payload Length field (the number of 4-byte blocks in the Authentication header, not counting the first two), a Reserved field, a Security Parameters Index (SPI) field that helps identify a specific IP Security (IPsec) security association (SA), a Sequence Number field that provides antireplay protection, and an Authentication Data field that contains an integrity value check (ICV). The ICV provides data authentication and data integrity.