第四章. Network Layer 應用層 network core
1. Forwarding vs. Routing
- forwarding: 查routing table，source to destination往哪走
- routing: 選擇routing路徑，決定和建立source to destination路徑
2. Network Layer Service
- Virtual circuits: 建connection，像telephone circuit，通了一定要給頻寬，但有時候因為多人佔線會導致不通
- Datagram networks: 不建connection，相同source的packet路徑可能不同；查表forwarding，利用prefix來比較
3. Inside a router
- Input Port: lookup, forwarding, queueing
- Switching
- via Memory: 兩次I/O時間
- via a Bus: Bus的頻寬要是所有port的總和
- via an Interconnection Network: Cisco 12000 60 Gbps
- Output Port:buffering, scheduling, discipline (如果datagram speed > line termination)
4. IP (Internet Protocol)
- IPv4 addressing:
- CIDR (Classless Inter Domain Routing): 不只用8/16/24來分subnet，a.b.c.d/x
- DHCP (Dynamic Host Configuration Protocol): plug-and-play
- NAT (Network Address Translation): 1個local network共享1個public IP，外部看不到內部server，靠port number分辨，最多65536個connections
- ICMP (Internet Control Message Protocol): 錯誤偵測及回報，Ping
- IPv6
5. Routing Algorithms:
- Link State
- Global，所有routers都有complete topology
- Dijsktra's Algorithm: 遞迴計算最短路徑
- 缺點: O(n^2)，可能會oscillations (重複計算的封包)
- Distance Vector
- Decentrailized, router只知道physically-connected neighbors
- Bellman-Ford Equation: x->y最短路徑為x->v + v->y最短路徑
- Good news travels fast/ Bad news travels slow (count to infinity)
- poisoned reverse: 如果z->x最短路徑經過y，就告訴y到x的路徑為“無限大”
- Hierarchical Routing: 網路太大，分層管理
- AS (autonomous system, 自治區)
- intra-AS: Interior Gateway Protocols (IGP)
- inter-AS
6. Routing in the Internet
- RIP (Routing Information Protocol): Distance Vector，每30秒 advertisement
- OSPF (Open Shortest Path First): Link State，Hierarchical in large domain，網路越快，cost越小；用擁擠，cost越大
- BGP (Border Gateway Protocol): 業界標準
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