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路由
路由是一种填充转发表的分布式计算；网络可以抽象为一张图，目标是寻找最低成本路径。距离向量路由让每个节点把自己到各处的距离向量告诉邻居，却会出现计数到无穷大的问题，水平分割只能部分缓解这一问题；链路状态路由则让每个节点可靠地洪泛其邻域拓扑，使所有节点都能构建出相同的完整网络图，并在本地运行迪杰斯特拉最短路径算法。无论是哪种算法，所使用的链路度量都必须反映延迟、容量和负载，同时避免振荡；而通过区域和自治系统实现的层次结构，则使整个体系保持在可控范围之内。
1. The Routing Problem
The Routing Problem
2. Counting to Infinity
Counting to Infinity
3. Link State
Link State
4. Metrics and Hierarchy
Metrics and Hierarchy
5. What neighbors hear
What fundamentally distinguishes what a distance-vector node tells its neighbors from what a link-state node tells them?
6. Counting to infinity
A link fails, and two nearby routers begin advertising the lost destination to each other with ever-increasing costs. What is happening?
7. Split horizon
What does the split-horizon rule forbid, and what loop does it eliminate?
8. Reliable flooding
In link-state routing, how does every node end up with the same map, and how do stale copies of link-state packets get discarded?
9. The metric trap
Why did measuring link cost by instantaneous queue length fail in the early ARPANET?
10. Why hierarchy
Why does OSPF divide a large network into areas?
11. Infinity in RIP
In the Routing Information Protocol, what hop count is defined to represent infinity, bounding the count-to-infinity pathology?
12. The shortest-path algorithm
Each link-state node computes least-cost paths on the shared map using which classic shortest-path algorithm?
13. Poison reverse
Which technique strengthens split horizon by advertising a route back to its teacher at infinite cost rather than staying silent?
14. The sequence number's job
In reliable flooding, which field of a link-state packet lets a node discard an older copy in favor of the freshest one?