Computer Networks · 免费预览
网际协议
网际协议（IP）几乎不作任何承诺，便将异构网络粘合为一个逻辑互联网络：其尽力而为的数据报服务可能丢失、重复、损坏或乱序分组，因而能运行在任何现存或未来可能发明的链路技术之上；其首部携带跳数限制的生存时间、协议解复用键、首部校验和以及完整的源地址和目的地址；分片与重组利用共享标识符和字节偏移，将过大的数据报跨越最大传输单元较小的网络传送；地址采用层次化结构，分为网络部分和主机部分，使路由器可按前缀转发；子网掩码和无类别域间路由收回了旧分类编址系统的地址浪费并聚合路由；而地址解析协议（ARP）和动态主机配置协议（DHCP）则补全了链路层转换与主机配置这些最后一公里的细节。
1. The Narrow Waist
The Narrow Waist
2. The Datagram Header
The Datagram Header
3. Fragmentation and Reassembly
Fragmentation and Reassembly
4. Addresses That Aggregate
Addresses That Aggregate
5. The best-effort bargain
Why did IP's designers choose a service model that may lose, duplicate, and reorder packets?
6. Time to live
What failure does the time-to-live field protect against, given that routers decrement it by one per hop and discard the packet at zero?
7. Reading the fragments
A 1400-byte payload must cross a link whose maximum transmission unit leaves 512 bytes per fragment. What offsets and more-fragments flags do the fragments carry?
8. Why hierarchy scales
Why do hierarchical network-plus-host addresses scale better than flat addresses for forwarding?
9. Subnets and supernets
A campus with thirty physical networks wants to appear as one network to the outside. Which mechanisms make that work, and in which direction does each operate?
10. The ARP exchange
A host must deliver a datagram to another host on the same Ethernet but knows only its IP address. How does the mapping get resolved?
11. The demultiplexing key
Which IP header field names the higher-level protocol that should receive the payload, with 6 for the stream protocol and 17 for the datagram protocol?
12. Offset units
In IP fragmentation, the offset field positions each fragment within the original datagram in units of how many bytes?
13. Reassembly site
Fragments of an IP datagram are reassembled at which node: the fragmenting router, the next router, or the destination host?
14. Cache lifetime
Roughly how many minutes does a host keep an address resolution mapping in its cache before timing it out?