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全球互联网
将路由扩展到全球需要双重层次化：在域内，链路状态区域把泛洪限制在各区域内，由边界路由器将路由汇总至骨干区域；在域间，互联网是自治系统构成的图，每个自治系统归属单一管理机构，可自主运行内部路由；域间路由使用边界网关协议，该协议仅通告可达性并携带完整的自治系统号路径，通过拒绝任何包含自身的路径来防止环路，其存在是为了表达策略——偏好提供商、拒绝转接、对经济信息保密——而这些是任何度量都无法编码的。
1. Hierarchy Inside the Domain
Hierarchy Inside the Domain
2. A Network of Networks
A Network of Networks
3. The Border Gateway Protocol
The Border Gateway Protocol
4. The Shape That Emerged
The Shape That Emerged
5. The area border
In a domain split into link-state areas, what does an area border router do?
6. Why autonomous systems
What problem does dividing the Internet into autonomous systems solve?
7. Path as loop detector
How does BGP prevent routing loops without a time-to-live on its advertisements?
8. Stub and transit
A multihomed company connects to two providers for reliability but refuses to carry traffic between them. How is that refusal expressed?
9. Reachability not quality
Why do BGP advertisements announce only that a prefix is reachable, without any cost or quality metric?
10. When aggregation holds
A provider wants to advertise one prefix covering two thousand customer networks. What must be true for that to work?
11. Area zero
In hierarchical link-state routing, what is the special backbone area, to which all other areas must connect, commonly called?
12. Path vector
BGP is classed as what kind of routing protocol, named for its habit of carrying the complete list of traversed autonomous systems?
13. Withdrawal
What does a BGP speaker send to retract a previously advertised route that is no longer valid?
14. Policy secrecy
Interdomain routing policies are often kept private because they encode the operators' what, which competitors must not see?