第三章计算机网体系结构55.pptVIP

协议与体系结构 硬件和软件相结合的分层结构，用于支持系统之间的数据交换以及分布式应用。 在协议结构的每一层，通信系统都要实现一个或多个通用协议。 协议所执行功能：封装，分段，重组，连接控制，顺序控制，流控制，差错控制，寻址以及复用。 开放系统互连（Open System Interconnection） 最广泛的协议TCP/IP协议组。 Protocol Characteristics Direct or indirect(直接—间接) Monolithic or structured(整体化—结构化) Symmetric or asymmetric(对称？) Standard or nonstandard(标准？) Direct or Indirect Direct Systems share a point to point link or Systems share a multi-point link Data can pass without intervening active agent Indirect Switched networks or Internetworks or internets Data transfer depend on other entities Monolithic or Structured Communications is a complex task Too complex for single unit Structured design breaks down problem into smaller units Layered structure Why a layered network model Symmetric or Asymmetric Symmetric Communication between peer entities Asymmetric Client/server Standard or Nonstandard Nonstandard protocols built for specific computers and tasks K sources and L receivers leads to K*L protocols and 2*K*L implementations If common protocol used, K + L implementations needed Use of Standard Protocols Protocol Functions Encapsulation—封装 Segmentation and reassmebly – 分段和重组 Connection control – 连接控制 Ordered delivery – 顺序发送 Flow control – 流控制 Error control – 差错控制 Addressing – 寻址 Multiplexing – 复用 Transmission services – 传输服务 Encapsulation Addition of control information to data Address information Error-detecting code Protocol control Segmentation (Fragmentation) Data blocks are of bounded size Application layer messages may be large Network packets may be smaller Splitting larger blocks into smaller ones is segmentation (or fragmentation in TCP/IP) ATM blocks (cells) are 53 octets long Ethernet blocks (frames) are up to 1526 octets long Checkpoints and restart/recovery(断点检查,重启动/恢复) Why Fragment? Advantages More efficient error control More equitable access to network facilities Shorter delays Smaller buffers needed Disadvantages Overheads Increased interrupts at receiver More p