인터넷 구조 2002년 2학기 장주욱

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

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

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

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

Hierarchical design

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

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

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)

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

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 192.168.10.x 3) Apply the policy to the traffic interface ethernet 0 ip policy route-map telnetthroughframe

Full reachability Core redundancy Reduce suboptimal routing Prevent routing loops

Example: Mesh core Fig. 3-4 Retana

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

Typical NAP

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

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

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

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 (255.255.255.255) Used to find a server in a local subnet (HA?) Sources of attack ( to prevent it, no ip directed broadcast)

Access layer security Fig 1-4 Retana

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

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

Example Fig. 1-5 Retana

Is hierarchy important in switched network? Retana 1-6

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