The fields for the IPv6 packet header shown in Figure 22-10 are as follows:
• Version —This field contains a 4-bit binary value set to 0110 that identifies this as an IP version 6 packet.
• Traffic Class — This 8-bit field is equivalent to the IPv4 Differentiated Services (DS) field.
• Flow Label — This 20-bit field suggests that all packets with the same flow label receive the same type of handling by routers.
• Payload Length — This 16-bit field indicates the length of the data portion or payload of the IPv6 packet. This does not include the length of the IPv6 header, which is a fixed 40-byte header.
• Next Header — This 8-bit field is equivalent to the IPv4 Protocol field. It indicates the data payload type that the packet is carrying, enabling the network layer to pass the data to the appropriate upper-layer protocol.
• Hop Limit — This 8-bit field replaces the IPv4 TTL field. This value is decremented by a value of 1 by each router that forwards the packet. When the counter reaches 0, the packet is discarded, and an ICMPv6 Time Exceeded message is forwarded to the sending host,. This indicates that the packet did not reach its destination because the hop limit was exceeded. Unlike IPv4, IPv6 does not include an IPv6 Header Checksum, because this function is performed at both the lower and upper layers. This means the checksum does not need to be recalculated by each router when it decrements the Hop Limit field, which also improves network performance.
• Source IPv6 Address — This 128-bit field identifies the IPv6 address of the sending host.
• Destination IPv6 Address — This 128-bit field identifies the IPv6 address of the receiving host.
An IPv6 packet may also contain extension headers (EH), which provide optional network layer information. Extension headers are optional and are placed between the IPv6 header and the payload. EHs are used for fragmentation, security, to support mobility and more.
Unlike IPv4, routers do not fragment routed IPv6 packets.
Video – Sample IPv6 Headers in Wireshark (22.3.5)
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Check Your Understanding – IPv6 Packet (22.3.6)
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