1、 IPv6As the Internet began to grow at a dramatic rate during the late 1980s and early 1990s,engineers realized that the current version of the IP protocol would not be adequate to meet the demands of the Internets growth.Of particular concern was the availability of adequate IP addresses for all kin
2、ds of devices accessing the Internet.dramatic a.喜剧的，引人注目的version n.版本protocol n.协议，草案availability n.可用性，有效性With 32 bits,IPv4 can theoretically provide up to 4.2 billion addresses.However,even with uniform allocation,this is insufficient.Allocation policies in practice make this worse;a few organizat
3、ions have an overabundance of addresses,whereas many others have ended up with too few.uniform a.统一的，一致 的allocation n.分配，配置insufficient a.不足的，缺乏能力的overabundance n.过多，过于丰富According to the report released by Gartner,there will be 25 billion IoT devices by 2020,and a large number of perceptive terminal
4、s or machines will be connected to the mobile network or the Internet,which will inevitably cause a huge demand for IP addresses.IPv6 is the fundamental way to solve the address problem.The designers of IPv6 have taken full account of the existing advantages of IPv4 and made a lot of improvements an
5、d extensions to make it more powerful and efficient than IPv4s processing performance.perceptive a.知觉的，领悟的inevitably a.不可避免的，必然的fundamental a.基本的，根本的IPv6 also provides additional features.Address auto-configuration allows hosts to autoconfigure their IP addresses without the need for a centralized s
6、erver.The header,which contains the essential fields for the IP protocol to work,has been simplified for more efficient processing;even with 40 bytes,the IPv6 header is more amenable to header compression than the IPv4 header.Extensions to the base IPv6 header provide better support for security and
7、 mobility:the Mobility Header in particular has been designed to support the Mobile IP protocol and its enhancements.configuration n.配置，结构autoconfiguration 自动配置simplify v.简化，使简易amenable a.有责任的，顺从的compression n.压缩，浓缩With the basic addressing support and additional support,especially for security and
8、mobility,the new IPv6 protocol can form the basis of the next-generation Internet.It is,hence,important to understand some of its basic operations.The Internet protocols often define standards for hosts and routers to communicate.IPv6 is one such important protocol.In order for a standard to be impl
9、ementable as well as interoperable,it needs to define unambiguous messages that the protocol uses.implementable a.可执行的interoperable a.互操作的unambiguous a.不含糊的，清楚的The messages carry information for the IP software to interpret,as well as payload a user generated such as web pages,streamed content and V
10、oIP.The information carried for the IP software can be generally referred to as the header,which specifies how the message must be interpreted and processed.The header definition and semantics must be unambiguous so that the sender and the receiver can communicate.A header format does just that.It a
11、llows a sender to construct and fill the fields in the header in a way that the receiver can understand and interpret.specify v.指定，详细说明interpret v.解释，翻译definition n.定义semantics n.语义学，语义论IPv6 packet consists of three parts:header,extended header and upper protocol data unit.The fixed header contains
12、eight fields with a fixed total length of 40 bytes.The following section provides a description of the IPv6 header.The header in an IPv6 packet is shown in Figure 1.The description of each of the header fields follows.Each“+-”corresponds to 1 bit.Hence,the Version field consists of 4 bits.The Versio
13、n field is set to 6 to indicate the protocol version.description n.描述，描写The Traffic Class field actually consists of two subfields.The first 6bits of this field constitute the Differentiated Services Code Point(DSCP),and are used for providing different forwarding treatment(Quality of Service,QoS)to
14、 traffic.A source code may mark these bits to request certain“differentiated services”,and a router may provide the corresponding forwarding behavior.These bits may be remarked by intermediate routers to ensure that the traffic is compliant to what is agreed upon.The remaining two bits of the field
15、are reserved for“Explicit Congestion Notification(ECN)”,which is used to inform in advance the transport protocol of congestion along the path a packet takes.subfield n.子域，分区forward v.发送，转寄differentiated a.分化的，可区分的remark v.备注，评论 intermediate a.中间的，过度的compliant a.顺从的，服从的The length of the rest of the
16、packet following the IPv6 header is denoted by Payload Length.The type of the header immediately following the IPv6 header is identified by Next Header.For data,this is typically TCP or Unreliable Datagram Protocol（UDP）.However,IPv6 defines multiple extension header which may be present.Hop Limit de
17、termines how far a packet should traverse on the Internet.Its value is decremented by one by each node that forwards the packet.When the value reaches zero,the packet is decremented.denote v.表示，指示 hop n.单足短距离跳跃 determine traverse v.穿过，来回移动 decrement n.渐减，减缩The Source and Destination Addresses specif
18、y the originator and the receiver of the packet,respectively.The Destination Address does not specify the ultimate receiver if the Routing Header is present.There are three types of addresses in IPv6:unicast,anycast,and multicast.Each of these addresses can have different scopes,which limits their a
19、pplicability and usage.There are no broadcast addresses in IPv6.ultimate a.最终的，终极的 scope n.范围，余地With the rapid development of Internet,IPv4 protocol can no longer meet the needs of users.This is mainly due to the limitations of IPv4 in terms of addresses,routing and security.Correspondingly,IPv6 has
20、 the advantage of large address space,security,mobility,quality of service and so on.So IPv6 protocol has become the inevitable trend of network development.inevitable a.必然的，不可避免的trend n.趋势，倾向However IPv4 and IPv6 are incompatible protocols,so a solution to transition is required.Because it is impos
21、sible to change their network to IPv6 at once,IPv4 and IPv6 will coexist for a few years.In order to achieve smooth and stepwise transition,IETF recommends three kinds of transition mechanisms:dual stack,tunneling and translation technology.incompatible a.不相容的，矛盾的coexist v.共存，和平共处stepwise a.逐步的，逐渐的dual a.双的，双重的stack n.一堆，堆栈tunneling n.隧道效应，隧道技术
