1. 인터넷 구조
2002년 2학기
장주욱

2. 참고자료 그림및 텍스트 발췌: Govindan Kurose Peterson & Davie Huitema Halabi
Retana, Slice & White

3. Hierarchical Design Principles
Right topology
Hierarchical design
Network core
Distribution layer
Access layer
Case study

4. Right topology Why do networks melt? Fast convergence after change
Factors for convergence
number of routers
amount of information

5. Hierarchical design Core layer: fast switching
Distribution layer: Summarize routes
Access layer: Policy control

6. Hierarchical design

7. Summarization is key!
1) Area affected by topology change should be minimized
2) Routers carry minimum information
Fig 1-2 Retana

8. Network core No policy-based routing
No complexity (one mistake is fatal)
Full reachability (Minimum latency)

9. What is policy-based routing?
Forwarding decision based on
source address
source/destination address pair
destination address
packet type(TCP, UDP, ICMP, …)
service type(Telnet, FTP, SMTP)
Precedence(Diff-Serv)

10. Example: Telnet and other traffic
Fig. 1-3 in Retana

11. Configuration example
1) Build a filter
access-list 150 permit any eq telnet any
access-list 150 permit any any eq telnet
2) Build a policy
route-map telnetthroughframe permit 10
match ip address 150
set ip next-hop x
3) Apply the policy to the traffic
interface ethernet 0
ip policy route-map telnetthroughframe

12. Full reachability Core redundancy Reduce suboptimal routing
Prevent routing loops

13. Example: Mesh core
Fig. 3-4 Retana

14. NAP(Network Access Point)
Interconnection points for ISPs
Number of routers connected for the purpose of traffic exchange
FDDI or ATM switch
ex) PacBell NAP

15. Typical NAP

16. Collapsed core Single router acts as a core Easy to manage
Bad scalability
Single point of failure (If only one router fails in the network, it will be the single core!)
sol) Core routers connected by high speed LAN or WAN links

17. Distribution layer Topology change isolation
Control routing table size
Traffic aggregation
Route summarization
Minimize core to distribution layer connections

18. Access layer Feed the traffic Control access
not to overflow the link to the distribution layer
place server so that traffic for inside should not go outside
never use access layer device for through-point for distribution layer
Control access
block packets not originating from inside (prevent spoofing)
No broadcast sources
No directed broadcast

19. Directed broadcast Destined to the broadcast of a subnet (10.1.4.255)
Other routers forwards it as a unicast
The router attached to the subnet replace the dest. IP as a broadcast address ( )
Used to find a server in a local subnet (HA?)
Sources of attack ( to prevent it, no ip directed broadcast)

20. Access layer security
Fig 1-4 Retana

21. Edge services
Tagging for QoS-based forwarding: voice or video for less delay
Terminate tunnels
Traffic metering
Policy-based routing

22. Connections to common services
Server farm or Internet access
Connected directly to network core : ex) server farm
Through buffer zone (DMZ): ex) Internet access

23. Example
Fig. 1-5 Retana

24. Is hierarchy important in switched network?
Retana 1-6

25. Hierarchical Form a spanning tree with C as a root
Block a port on B (arrowed) to prevent routing loop
It is better to block the other port considering E-A traffic
We know beforehand which path will be taken between any two links