IPv6 Header Format

In the network, IP addresses are the foundation, so the packet header format is also very important. Although the format of IPv6 headers has been standardized A long time ago, IPv6 is gradually put into use. Let's talk about the structure of IPv6 headers.

IPv6 Header Format

IPv6 headers are divided into the following parts:

1. Version Number (4-bit). The IPv6 protocol Version value is 6. The size of this Sub-segment is the same as that of the version number field in IPv4. However, the use of this field is limited. The information packages of IPv6 and IPv4 are not distinguished by the version value of the version field, but by the protocol type in layer-2 encapsulation, such as Ethernet or PPP.

2. Traffic level TrafficClasses, 8-bit). The traffic level domain in the IPv6 Header enables the source node or packet forwarding router to identify and distinguish different levels or priorities of IPv6 information packets. No agreement has been reached on the definitions of common IPv6 traffic types and levels. In RFC1883, this domain has only four bits, which are called Priority) and defines eight information packet priorities. In RFC2640, the traffic level domain is expanded to eight bits, which is also a manifestation of the increase in the traffic type.

The following conditions must be met when a traffic level domain is used:

1. In an IPv6 node, the IPv6 service interface must support the traffic level in the information packet generated by the upper layer protocol. The default value of IPv6 traffic level is that all 8 bits are 0.

2. IPv6 nodes that support some or all of the traffic levels allow you to modify the value of the traffic level in the information packages they generate, forward, or receive. When these nodes do not support special usage, the traffic level bit in the information package will be ignored or not modified.

3. The upper-layer protocol does not need to assume that the traffic level value in the received information package is the same as the value when the source node sends the package. 3. Stream flag: FlowLabel, 20 bits). The flow mark in the IPv6 Header is used to mark the order of information packets that require special processing by the IPv6 router, these special processes include non-default quality services or real-time services. The stream tag field of IPv6 is experimental when RFC2640 is written, and changes with the demand for Internet streaming support. A host or router that does not support the stream marking Domain Function sets this domain to 0 when generating an information package, and does not change this domain when forwarding an information package, this domain is ignored when an information packet is forwarded.

In RFC1883, the stream tag domain length is 24 bits, and the value in RFC2640 is changed to 20 bits.

4. PayloadLength, 16 bits). The payload length is expressed by a 16-bit unsigned integer, representing the length of the rest of the information packet except the IPv6 Header, in bytes, that is, 8 bits. Because it is 16-bit, the IPv6 information package's payload length is 64 KB. It is worth noting that any extension header is considered a part of the payload and will be included.

5. Next header NextHeader, 8-bit). This 8-bit selector is used to identify the type of the header followed by the IPv6 Header. Its usage is the same as that of the next header domain in IPv6.

6. route segment restriction: HopLimit, 8 bits). This field is represented by an 8-bit unsigned integer. When the forwarded information package passes through a node, this value is reduced by 1, when it is reduced to 0, the information package is discarded.

7. Source Address: SourceAddress, 128 bits), the address of the sender of the information package.

8. Destination Address: DestinationAddress, 128 bits), the address of the expected receiver of the information package. If there is a Routing header, this address may not be the address of the final receiver of the information package.