IPv6 Packets

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Windows Embedded CE 6.0 R3

4/8/2010

An IPv6 packet, also known as an IPv6 datagram, consists of an IPv6 header and an IPv6 payload, as shown in the following illustration.

The IPv6 header consists of two parts, the IPv6 base header, and optional extension headers. Functionally, the optional extension headers and upper-layer protocols, for example TCP, are considered part of the IPv6 payload. The following table shows the fields in the IPv6 base header.

IPv6 header field	Length	Function
Version	4 bits	Identifies the version of the protocol. For IPv6, the version is 6.
Class	8 bits	Intended for originating nodes and forwarding routers to identify and distinguish between different classes or priorities of IPv6 packets.
Flow label	20 bits	Defines how traffic is handled and identified. A flow is a sequence of packets sent either to a unicast or a multicast destination. This field identifies packets that require special handling by the IPv6 node. The following list shows the ways the field is handled if a host or router does not support flow label field functions: If the packet is being sent, the field is set to zero. If the packet is being received, the field is ignored.
Payload length	16 bits	Identifies the length, in octets, of the payload This field is a 16-bit unsigned integer. The payload includes the optional extension headers, as well as the upper-layer protocols, for example, TCP.
Next Header	8 bits	Identifies the header immediately following the IPv6 header. The following list shows examples of the next header: 0 = Hop-by-Hop options 1 = ICMPv4 4 = IP in IP (encapsulation) 6 = TCP 17 = UDP 43 = Routing 44 = Fragment 50 = Encapsulating security payload 51 = Authentication 58 = ICMPv6 59 = None 60 = Destination options
Hop Limit	8 bits	Identifies the number of network segments, also known as links or subnets, on which the packet is allowed to travel before being discarded by a router. The Hop Limit is set by the sending host and is used to prevent packets from endlessly circulating on an IPv6 internetwork. When forwarding an IPv6 packet, IPv6 routers must decrease the Hop Limit by 1, and must discard the IPv6 packet when the Hop Limit is 0.
Source Address	128 bits	Identifies the IPv6 address of the original source of the IPv6 packet.
Destination Address	128 bits	Identifies the IPv6 address of the intermediate or final destination of the IPv6 packet.

IPv4 headers and IPv6 headers are not interoperable. A host or router must use an implementation of both IPv4 and IPv6 in order to recognize and process both header formats.

See Also

Concepts

Core Protocols of IPv6

Other Resources