1: semantics 语义

一：Introduction

    1. Changes form IPv6 to IPv6.

        1) Expanded Addressing Capabilities

            32 bits to 128 bits to support

            a) more levels of addressing hiearchy

            b) a much greater number of address nodes

            c) simpler auto-configration of address nodes.

            d) add a "scope" field to multicast addresses.

            e) define a new type of addresses called "anycast address" to send a packet to anyone of a group of nodes.

        2) Header Format Simplication

           Drop or make optional  some IPv4 header field.

            a) Reduce the common-case processing cost of packet handling.

            b) limit the bandwidth cost of the IPv6 header.

        3) Improved Support for Extensions and Options

            Changes in the way IP header options

            a) more efficient forwarding.

            b) less stringent limits on the length of options. (选项长度放松)

            c) greater flexibility for introducing new options in the future.

        4) Flow Labeling Capability

            a) add a capability to enable the labeling of packets, for which the sender requests special handling,

                such as non-default quality or "real-time" service.

        5) Authentication and Privacy Capabilities

            a) Extensions to support authentication data integraty (数据完整)，

               optional data confidentiality(数据机密性)。

二、 Terminology

        1: node - a device that implements IPv6.

        2: router - a node that forwards IPv6 packets not explicitly addressed to itself.

        3: host - any node that is not a router.

        4: upper layer - a protocol layer immediatly above IPv6, ICMP, TCP, UDP, OSPF, IPX, AppleTalk or IPv6 intself.

        5. link - a communication facility at the same link.

                    below IPv6.

                    Ethernets(bridged), PPP links, X.25, Frame, Relay.

        6. neighbors - nodes attached to the same link. 

        7. interface - a node‘s attachment to a link.

        8. packet - an IPv6 header plus payload.

        9. link MTU - the maximum transmission unit.

                          maximum packet size in octets.

        10. path MTU - the minimum link MTU of all the links in a path between a source node and a destination node.

三、 IPv6 Header Format  (40 octets)

       8 parts

      a) Version, Traffic class, Flow Label(4, 8, 20)

      b) Payload, Next Header, Hop Limit(16, 8, 8)

      c) Source Address(128).

      d) Destination Address(128). 

    1: Version - 4 bits

    2: Traffic Class -8 bits

    3: Flow Label -20 bits

    4: Payload -16 bits

    5: Next Header -8 bits

    6: Hop limit -8 bits   The packet is discarded if Hop limit is decremented to zero.

    7. Source Address - 128 bits.

    8. Destination Address -128 bits.

四、 IPv6 Extension Header

     Optional IPv6 internet-layer is encoded in separate headers that may be placed between the IPv6 header and the upper layer header in a packet.

    ?

    With one excepition(Hop by Hop), extension headers are not examined or processed by any node along a packets delivery path,

    until the packet reaches the node(or each set of nodes in the case of municast).

    Extension Headers must be processed strictly in the order they appear in the packet.

    Hop by hop option carries information that must be examined and processed by every node along a packets delivery path,

    including the source and destination nodes.