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An IPv6 packet is the smallest message entity exchanged via the Internet Protocol across an Internet Protocol version 6 (IPv6) network. Packets consist of control information for addressing and routing, and a payload consisting of user data. The control information in IPv6 packets is subdivided into a mandatory fixed header and optional extension headers. The payload of an IPv6 packet is typically a datagram or segment of the higher-level Transport Layer protocol, but may be data for an Internet Layer (e.g., ICMPv6) or Link Layer (e.g., OSPF) instead. IPv6 packets are typically transmitted over a Link Layer protocol, such as Ethernet which encapsulates each packet in a frame, but this may also be a higher layer tunneling protocol, such as IPv4 when using 6to4 or Teredo transition technologies. Routers do not fragment IPv6 packets, as they do for IPv4. Hosts are "strongly recommended"〔Deering, S.; Hinden, R. (December 1998). (Internet Protocol, version 6 (IPv6) Specification ). IETF. RFC2460.〕 to implement Path MTU Discovery to take advantage of MTUs greater than the smallest MTU of 1280 octets. A node may use the IPv6 Fragment header to fragment the packet at the source and have it reassembled at the destination(s).〔Deering, S.; Hinden, R. (December 1998). (Internet Protocol, version 6 (IPv6) Specification ). IETF. RFC2460.〕 ==Fixed header== The fixed header of an IPv6 packet consists of its first 40 octets (320 bits).〔 It has the following format: : ; ''Version'' (4 bits) : The constant 6 (bit sequence 0110). ; ''Traffic Class'' (8 bits) : The bits of this field hold two values. The 6 most-significant bits are used for differentiated services, which is used to classify packets.〔Nickols, K.; Blake, S.; Baker, F.; Black, D. (December 1998) ''(Definition of the Differentiated Service Field (DS Field) in the IPv4 and IPv6 Headers )'', IETF. RFC2474.〕〔Grossman, D. (April 2002) ''(New Terminology and Clarifications for DiffServ )'', IETF. RFC3260.〕 The remaining two bits are used for ECN;〔Ramakrishnan, K.; Floyd, S.; Black, D. (September 2001) ''(The Addition of Explicit Congestion Notification (ECN) to IP )'', IETF. RFC3168.〕 priority values subdivide into ranges: traffic where the source provides congestion control and non-congestion control traffic. ; ''Flow Label'' (20 bits) : Originally created for giving real-time applications special service.〔 The flow label when set to a non-zero value now serves as a hint to routers and switches with multiple outbound paths that these packets should stay on the same path so that they will not be reordered.〔Wijnen, B. (September 2003) ''(Textual Conventions for IPv6 Flow Label )'', IETF. RFC3595.〕〔Amante, S.; Carpenter, B.; Jiang, S.; Rajahalme, J. (November 2011) ''(IPv6 Flow Label Specification )'', IETF. RFC6437.〕 It has further been suggested that the flow label be used to help detect spoofed packets.〔(draft-blake-ipv6-flow-label-nonce-02 )〕 ; ''Payload Length'' (16 bits) : The size of the payload in octets, including any extension headers. The length is set to zero when a ''Hop-by-Hop'' extension header carries a Jumbo Payload option.〔Borman, D.; Deering, S.; Hinden, R. (August 1999). ''(IPv6 Jumbograms )''. IETF. RFC2675.〕 ; ''Next Header'' (8 bits) : Specifies the type of the next header. This field usually specifies the transport layer protocol used by a packet's payload. When extension headers are present in the packet this field indicates which extension header follows. The values are shared with those used for the IPv4 protocol field, as both fields have the same function (see List of IP protocol numbers). ; ''Hop Limit'' (8 bits) : Replaces the time to live field of IPv4. This value is decremented by one at each intermediate node visited by the packet. When the counter reaches 0 the packet is discarded. ; ''Source Address'' (128 bits) : The IPv6 address of the sending node. ; ''Destination Address'' (128 bits) : The IPv6 address of the destination node(s). In order to increase performance, and since current link layer technology and transport or application layer protocols are assumed to provide sufficient error detection,〔RFC 1726 section 6.2〕 the header has no checksum to protect it.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「IPv6 packet」の詳細全文を読む スポンサード リンク
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