IP封装、分段与重组.ppt

* 路由器将数据报成小块，称为片段。每个片段使用了IP数据报的格式。独立发送的每个片段。 * 重组创建副本的原始数据报从碎片的过程被称为重组。IP指定的最终目标主机应重组片段。 * 识别一个数据报发送方放置一个唯一的标识号，每个传出数据报中的标识字段。当一个路由器片段数据报，路由器的识别号码复制到每个片段。 * fragment Loss Some fragment are delayed or lost. Receiver must save the fragments. IP specifies a maximum time to hold fragments. Either all fragments arrive and IP reassembles the datagram, or IP discards the complete datagram. There is no mechanism for a receiver to tell the sender which fragments have arrived. If the sender did retransmit the datagram,routes may be different. * 再分片 仔细规划分片方案已使人们有可能进一步再分片。 * * 第四章 网络层 4.2.5 IP Encapsulation, Fragmentation, and Reassembly 4.2.5 I P封装、分段与重组 注:可借助google翻译 / 重组 内容提要 数据报封装 分片 1 2 3 一、数据报封装1 Datagram Transmission and Frames IP software first selects the next hop, Then transmits the datagram across a physical network. Network hardware does not understand datagram format or Internet addressing. Each hardware technology defines a frame format and a physical addressing scheme. 2 Encapsulation encapsulation. The hardware does not examine or change the contents of the frame data area. The sender and receiver must agree on the value used in the frame type field. The sender must supply the physical address of the next computer to which the datagram should be sent. 3 Transmission Across an Internet When a frame reaches the next hop,the receiving software extracts the IP datagram and discards the frame header. If the datagram must be forwarded across another network , a new frame is created. Frame headers do not accumulate during a trip through the internet. Hosts and routers store a datagram in memory with no additional header. IP1 HA1 HA5 HA4 HA3 HA6 HA2 IP6 主机 H1 主机 H2 路由器 R1 IP 层上的互联网 IP2 IP4 IP3 IP5 路由器 R2 IP1 → IP2 IP1 → IP2 IP1 → IP2 MAC 帧 从 HA1 到 HA3 从 HA4 到 HA5 从 HA6 到 HA2 MAC 帧 MAC 帧 IP 数据报 在具体的物理网络的链路层 只能看见 MAC 帧而看不见 IP 数据报 二、分段（分片）1 分片原因 ?数据链路层协议规定了数据链路层数据帧的最大长度（MTU），限制帧的长度不是由于网络媒体的物理特性，而是为交互使用提供足够快的响应时间。 MTU:Maximum Transfer Unit 一帧数据=帧首部+MTU+帧