Youki Kadobayashi NAIST

Transcription

1 Information Network 1 Routing (1) Image: Part of the entire Internet topology based on CAIDA dataset, using NAIST Internet viewer Youki Kadobayashi NAIST 1 The Routing Problem! How do I get from source to destination? S D 2 1

2 The Routing Problem: add some realistic constraints! How do I get from source to destination?! Which path is best? In terms of:! Number of hops! Delay, bandwidth! Policy constraints, cost! Who will make decision?! Router?! Source? S 500ms 500ms D! How can we detect failures?! How much the overhead will be? Rogue ISP unreliable 3 Routing problem can be solved in many ways! Represent network in:! a graph, or! a matrix! Collect information:! across the network, or! toward some routers, or! only locally among neighbors! Compute route at:! every router, or! some routers solutions are instantiated in routing systems. 4 2

3 Characterization of routing systems! Static routing! Compute route a priori! Dynamic routing! Reflect dynamic state of network S D! Source-based routing! Source node computes path to destination! Hop-by-hop routing! Every node computes next hop 5 Focus of this lecture: dynamic, hop-by-hop routing! Static routing! Compute route a priori! Dynamic routing! Reflect dynamic state of network! Source-based routing! Source node computes path to destination! Hop-by-hop routing! Every node computes next hop 6 3

4 Routing system has many functions! Provision end-to-end reachability! Automatically compute best path! Distribute traffic among multiple links! Avoid failing links! Isolate faults! Reflect administrative policies in one system. Isn t it awesome. 7 Routing system can be characterized by! Representation of network! Network topology! Attributes associated with each link! Exchange of information! Communication overhead! Propagation speed! Computation algorithm! Computation overhead! Convergence speed! Routing system: protocol + information + algorithm 8 4

5 Routing system: its structure! Routing protocol! discovers neighbor router;! exchanges topology information;! exchanges link information! Routing algorithm! computes route (result: RIB - Routing Information Base)! Integrate information from multiple routing protocols! Multiple routing protocols Multiple RIBs! Consolidate Multiple RIBs into single FIB (FIB: Forwarding Information Base) 9 Typically, routing protocol discovers network topology! Topology: geometric configurations that are unaltered by elastic deformations Graph representation Matrix representation 10 5

6 Gateway Model: a conceptual model of routing system Topology info, Link status info Routing software Multiple RIBs (routing information base) Topology info, Link status info FIB (forwarding information base) Input interfaces Output interfaces 11 Types of Routing Algorithm 1. Distance vector 2. Link state 3. Path vector! Key difference of these algorithms:! Topology representation! Propagation range / frequency / timing of link state! Algorithm for computing shortest path! Design trade-offs:! Scalability! Convergence time! Algorithm simplicity 12 6

7 Questions? 13 Distance Vector Routing 14 7

8 Distance vector routing w/ Bellman-Ford algorithm! 1. assign distance vector to myself 0; for others, assign DN p.397! 2. send my distance vectors to all neighbor routers! 3. router calculates minimum distance vectors by 1) distance vectors advertised by neighbor routers and 2) distance from myself to individual neighbor router! Initial condition! b i (0) (i D)! b D (0) 0! Repetition! b i (m) min j Vi {b j (m-1) + d ij }! Terminal condition! b i (m) = b i (m-1)! D: destination router, b i (m) : distance between router i and D by m-times iterative calculation, V i : neighbor routers of router i, d ij : distance of link between router i and j 15 RIP: Routing Information Protocol RFC 2453, RFC 2080! Distance vector routing! RIP-2 (IPv4), RIPng (IPv6)! Used in relatively small network Due to ease of implementation and operation! Textbook classics 16 8

9 Informal description of RIP! Each router sends a list of distance-vectors (route, cost) to each neighbor periodically! Every router selects the route with smallest metric! Metric: integer of 1..16, where 16 implies infinity A cost: 1 B cost: 2 C Router metric B 1 C 3 Router metric A 1 C 2 Router metric A 3 B 2 17 Hands on: see how RIP works! (description of GNS3)! (basic instruction to keep RIP up and running) 18 9

10 Hands on task: try to expand your RIP network! Add 3 rd router to your RIP network! Example topology: Problems of distance vector routing! Poor scalability. For the given number of routers N,! Time complexity: O(N 3 )! Traffic: O(N 2 )! Slow convergence speed, as it sends distance vector periodically! Slow convergence induces inconsistent and transitional state! => Counting to infinity problem 20 10

11 Counting to infinity problem A B 1 1 inf C Suppose link B-C went down. B thinks: (C, inf) A says: (C, 2) B thinks: (C, 3) via A B says: (C, 3) A thinks: (C, 4) via B 21 Split Horizon! Workaround for counting to infinity! Router doesn t send learned information to source router A B 1 1 inf C Suppose link B-C went down. B thinks: (C, inf) A says: (C, 2) to everyone except B 22 11

12 Limitation of split horizon A B 1 1 inf C 1 D 1 Suppose link B-C went down. B thinks: (C, inf) A says: (C, 2) to everyone except B D thinks: (C, 3) via A D says: (C, 3) B thinks: (C, 4) via D 23 Questions?! (Dumb questions welcomed while you re a student)! Extra hands-on if you are further motivated:! Introduce link failure and observe counting-toinfinity problem! Add 4 th router as in the last slide and observe that split-horizon does not save us 24 12

13 Lessons from RIP! Bellman-Ford: very simple! But with many traps and pitfalls! Guiding principle for using RIP: avoid loops!! Loops can be easily formed, however.! Backup links! Guiding principle can be easily forgotten OMG!! A better alternative: link state routing. 25 Link-State Routing 26 13

14 Link-state routing! Collect router and link information Directed graph: router as a node, link as an arc! then create link state database (LSDB) A network map that collects link state information! Based on LSDB, calculate the shortest path with the Dijkstra s shortest path algorithm! Time complexity: O(N 2 ) Can be further optimized by improved data structure 27 A high level view of link-state routing Router Router LSDB Dijkstra RIB Link Link directed graph shortest path tree fragments of directed graph 28 14

15 Graph representation of routers and links Source: OSPF Version 2, RFC A more complex network 30 15

16 and its directed graph representation 31 Shortest path tree; rooted at RT

17 Questions? 33 From directed graph to shortest-path trees: Dijkstra Algorithm! Initially,! d s 0! d j c sj (for every j V {s})! P {s} Interconnections p.223! Find the next closest node:! d i min j V P {d j }! P P {i}! Update labels:! d k min k V P {d k, d i + c ik }! Terminal condition:! P = V! V: all routers, s: starting router, c ik : link cost between i and k, d j : least cost between s to j, P: router with least cost determined 34 17

18 Pros and Cons of Link-State Routing! Traffic increases in proportion to the number of links and routers! Storage complexity increases in proportion to the number of links and routers! Time complexity: lower than distance vector routing! Convergence time: must be short, for transient loop issue! Counting to infinity doesn t happen (read: fewer traps and pitfalls)! Flexible configuration of link cost is possible (no nonsense like 16 = infinity) 35 OSPF: Open Shortest Path First RFC 2328, 5340! The link state routing protocol for the Internet! OSPFv2 (IPv4), OSPFv3 (IPv6)! Functions! Recognize neighbor router! Exchange link state information and create LSDB! Calculate shortest path tree (spanning tree)! + Designated Router, Backup Designated Router! + Hierarchical structure by area! + Collaboration with EGP 36 18

19 Discovering neighbor routers with OSPF Hello! Discover neighbor router on the same link send Hello packet to , ff02::5 (AllSPFRouters)! List of neighbor routers in Hello packet check bidirectional communication! (select designated router and backup designated router)! Send Hello packet periodically to detect link down! keeps pinging, as in ICMP 37 Information exchange among routers! Neighbor Adjacent! Routers don t exchange routing information unless they are adjacent! Formation process:! Hello Neighbor! Synchronize each LSDB! Synchronized Adjacent 38 19

20 Hands on: say hello in OSPF!! (basic instruction to bring OSPF up and running)! (establish OSPF adjacency)! (add 1 or 2 routers) 39 Topology Representation in OSPF! LSA (Link State Advertisement) Type 1: Router LSA Type 2: Network LSA Type 3: Summary LSA (network) Type 4: Summary LSA (AS boundary) Type 5: AS External LSA! LSA common header Validity period and sequence number in LSA header Helps routers to tell if given LSA is fresh 40 20

21 Hands on:! (some more extra work that lays foundation for Assignment) 41 Questions? 42 21

22 Summary 43 Gateway Model Revisited Topology info, Link status info Routing software Multiple RIBs Topology info, Link status info RIP OSPF FIB Input interfaces Output interfaces 44 22

23 Summary! Routing system: design space, characterization! Distance vector routing! Bellman-Ford algorithm! RIP protocol, information model! Traps and pitfalls! Link state routing! Graph and its spanning trees! Dijkstra algorithm! OSPF protocol, algorithm, information model 45 Assignment 46 23

24 Assignment: 47 24