Computer Networks. Instructor: Niklas Carlsson

Transcription

1 Computer Networks Instructor: Niklas Carlsson Notes derived Computer Networking: A Top Down Approach, b Jim Kurose and Keith Ross, Addison-Wesle. The slides are adapted and modified based on slides the book s companion Web site, as well as modified slides b Anirban Mahanti and Care Williamson.

2 Routing Algorithms Link State Distance Vector Hierarchical Routing

3 Interpla between routing and forwarding routing algorithm local forwarding table header value output link Effectivel building the forwarding tables (and determining the routing path) value in arriving packet s header 0

4 Let s start with the case where we operate the full (sub) network c a b AS a AS c d b a c AS b 4

5 Graph abstraction Graph: G = (N,E) u v w N = set of routers = { u, v, w,,, } E = set of links ={ (u,v), (u,), (v,), (v,w), (,w), (,), (w,), (w,), (,) } Remark: Graph abstraction is useful in other network contets too Eample: PP, where N is set of peers and E is set of TCP connections

6 Graph abstraction: costs u v w c(, ) = cost of link (, ) - e.g., c(w,) = cost could alwas be, or inversel related to bandwidth, or inversel related to congestion Cost of path (,,,, p ) = c(, ) + c(, ) + + c( p-, p ) Question: What s the least-cost path between u and? Routing algorithm: find good paths source to destination router. 6

7 Routing Algorithm Classification. Global vs decentralied? Global: all routers have complete topolog, link cost info link state algorithms Decentralied: router knows about phsicallconnected neighbors Iterative, distributed computations distance vector algorithms. Static vs dnamic? Static: routes change slowl over time Dnamic: routes change more quickl periodic update in response to link cost changes. Load sensitivit? Man Internet routing algorithms are load insensitive 7

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9 A Link-State Routing Algorithm Dijkstra s algorithm net topolog, link costs known to all nodes accomplished via link state broadcast all nodes have same info computes least cost paths one node ( source ) to all other nodes gives forwarding table for that node iterative: after k iterations, know least cost path to k dest. s Notation: c(,): link cost node to ; = if not direct neighbors D(v): current value of cost of path source to dest. v p(v): predecessor node along path source to v N': set of nodes whose least cost path definitivel known 9

10 Dijsktra s Algorithm Initialiation: N' = {u} for all nodes v 4 if v adjacent to u then D(v) = c(u,v) 6 else D(v) = 7 8 Loop 9 find w not in N' such that D(w) is a minimum 0 add w to N' update D(v) for all v adjacent to w and not in N' : D(v) = min( D(v), D(w) + c(w,v) ) /* new v is either old v or known 4 shortest path w plus cost w to v */ until all nodes in N' 0

11 Dijkstra s algorithm: eample Step 0 4 N' u D(v),p(v),u D(w),p(w),u D(),p(),u D(),p() D(),p() u v w

12 Dijkstra s algorithm: eample Step 0 4 N' u u D(v),p(v),u D(w),p(w),u D(),p(),u D(),p() D(),p() u v w

13 Dijkstra s algorithm: eample Step 0 4 N' u u D(v),p(v),u,u D(w),p(w),u 4, D(),p(),u D(),p(), D(),p() u v w

14 Dijkstra s algorithm: eample Step 0 4 N' u u u D(v),p(v),u,u D(w),p(w),u 4, D(),p(),u D(),p(), D(),p() u v w 4

15 Dijkstra s algorithm: eample Step 0 4 N' u u u D(v),p(v),u,u,u D(w),p(w),u 4,, D(),p(),u D(),p(), D(),p() 4, u v w

16 Dijkstra s algorithm: eample Step 0 4 N' u u u uv D(v),p(v),u,u,u D(w),p(w),u 4,, D(),p(),u D(),p(), D(),p() 4, u v w 6

17 7 Dijkstra s algorithm: eample Step 0 4 N' u u u uv D(v),p(v),u,u,u D(w),p(w),u 4,,, D(),p(),u D(),p(), D(),p() 4, 4, u w v

18 8 Dijkstra s algorithm: eample Step 0 4 N' u u u uv uvw D(v),p(v),u,u,u D(w),p(w),u 4,,, D(),p(),u D(),p(), D(),p() 4, 4, u w v

19 9 Dijkstra s algorithm: eample Step 0 4 N' u u u uv uvw D(v),p(v),u,u,u D(w),p(w),u 4,,, D(),p(),u D(),p(), D(),p() 4, 4, 4, u w v

20 0 Dijkstra s algorithm: eample Step 0 4 N' u u u uv uvw uvw D(v),p(v),u,u,u D(w),p(w),u 4,,, D(),p(),u D(),p(), D(),p() 4, 4, 4, u w v

21 Dijkstra s algorithm, discussion Algorithm compleit: n nodes each iteration: need to check all nodes, w, not in N n(n+)/ comparisons: O(n ) more efficient implementations possible: O(nlogn) Oscillations possible: e.g., link cost = amount of carried traffic D A +e e C e initiall B D A +e 0 0 +e C 0 B recompute routing D A 0 +e 0 0 +e C B recompute D A +e 0 0 +e C 0 recompute B

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23 Distance Vector Algorithm () Bellman-Ford Equation (dnamic programming) Define d () := cost of least-cost path to

24 Distance Vector Algorithm () Bellman-Ford Equation (dnamic programming) Define d () := cost of least-cost path to Then d () = min {c(,v) + d v () } where min is taken over all neighbors of 4

25 Bellman-Ford eample () u v w Clearl, d v () =, d () =, d w () = B-F equation sas: d u () = min { c(u,v) + d v (), c(u,) + d (), c(u,w) + d w () } = min { +, +, + } = 4 Node that achieves minimum is net hop in shortest path forwarding table

26 Distance Vector Algorithm () D () = estimate of least cost to Distance vector: D = [D (): є N ] Node knows each neighbor v: c(,v) Node maintains D () Node also maintains its neighbors distance vectors (sent to b neighbors) For each neighbor v, maintains D v = [D v (): є N ] 6

27 Distance vector algorithm (4) Basic idea: Each node periodicall sends its own distance vector estimate to neighbors When node receives new DV estimate neighbor, it updates its own DV using B-F equation: D () min v {c(,v) + D v ()} for each node N Under some conditions, the estimate D () converge the actual least cost d () 7

28 Distance Vector Algorithm () Iterative, asnchronous: each local iteration caused b: local link cost change DV update message neighbor Distributed: each node notifies neighbors when its DV changes neighbors then notif their neighbors if necessar 8

29 Distance Vector Algorithm () Iterative, asnchronous: each local iteration caused b: local link cost change DV update message neighbor Distributed: each node notifies neighbors when its DV changes neighbors then notif their neighbors if necessar Each node: wait for (change in local link cost or msg neighbor) recompute estimates if DV to an dest has changed, notif neighbors 9

30 node table 0 7 node table 0 node table 7 0 time 7 0

31 node table 0 7 node table 0 node table time 7

32 node table 0 7 node table 0 node table time 7

33 node table 0 7 node table 0 node table time 7

34 D () = min{c(,) + D (), c(,) + D ()} = min{+0, 7+} = node table node table 0 node table time D () = min{c(,) + D (), c(,) + D ()} = min{+, 7+0} = 7 4

35 D () = min{c(,) + D (), c(,) + D ()} = min{+0, 7+} = node table node table 0 node table time D () = min{c(,) + D (), c(,) + D ()} = min{+, 7+0} = 7

36 D () = min{c(,) + D (), c(,) + D ()} = min{+0, 7+} = node table node table 0 node table time D () = min{c(,) + D (), c(,) + D ()} = min{+, 7+0} = 7 6

37 node table 0 7 node table 0 node table time 7 7

38 node table 0 7 node table 0 node table time 7 8

39 node table 0 7 node table 0 node table time 7 9

40 node table 0 7 node table 0 node table time 7 40

41 node table 0 7 node table 0 node table time 7 4

42 Mabe another eample 4

43 Distance Vector: link cost changes Link cost changes: node detects local link cost change updates routing info, recalculates distance vector 4 0 if DV changes, notif neighbors good news travels fast At time t 0, detects the link-cost change, updates its DV, and informs its neighbors. At time t, receives the update and updates its table. It computes a new least and sends its neighbors its DV. At time t, receives s update and updates its distance table. s least costs do not change and hence does not send an message to. 4

44 Distance Vector: link cost changes Link cost changes: node detects local link cost change updates routing info, recalculates distance vector 4 0 if DV changes, notif neighbors good news travels fast 44

45 Distance Vector: link cost changes Link cost changes: good news travels fast bad news travels slow - count to infinit problem! 44 iterations before algorithm stabilies: see tet

46 Distance Vector: link cost changes Link cost changes: good news travels fast bad news travels slow - count to infinit problem! 44 iterations before algorithm stabilies: see tet Poissoned reverse: If Z routes through Y to get to X : Z tells Y its (Z s) distance to X is infinite (so Y won t route to X via Z) will this completel solve count to infinit problem? 46

47 Comparison of LS and DV algorithms Message compleit LS: with n nodes, E links, O(nE) msgs sent DV: echange between neighbors onl Speed of Convergence LS: O(n ) algorithm requires O(nE) msgs ma have oscillations DV: convergence time varies ma have routing loops count-to-infinit problem Robustness: what happens if router malfunctions? LS: DV: node can advertise incorrect link cost each node computes onl its own table DV node can advertise incorrect path cost each node s table used b others error propagate thru network 47

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49 Hierarchical Routing and Routing on the Internet 49

50 Hierarchical Routing: Motivation Our routing stud thus far - idealiation all routers identical, network flat scale: with billions of destinations: can t store all dest s in routing tables! routing table echange would swamp links! administrative autonom internet = network of networks each network admin ma want to control routing in its own network 0

51 Hierarchical Routing aggregate routers into regions, autonomous sstems (AS) routers in same AS run same routing protocol intra-as routing protocol routers in different AS can run different intra- AS routing protocol Gatewa router Direct link to router in another AS Establishes a peering relationship Peers run an inter-as routing protocol

52 Interconnected ASes c a b AS a c d b Intra-AS Routing algorithm AS Forwarding table Inter-AS Routing algorithm a c AS b Forwarding table is configured b both intra- and inter-as routing algorithm Intra-AS sets entries for internal dests Inter-AS & Intra-As sets entries for eternal dests

53 Inter-AS tasks Obtain reachabilit information neighboring AS(s) Propagate this info to all routers within the AS All Internet gatewa routers run a protocol called BGPv4 (we will talk about this soon) c a b AS a c d b AS a c AS b

54 Intra-AS Routing Also known as Interior Gatewa Protocols (IGP) Most common Intra-AS routing protocols: RIP: Routing Information Protocol (DV protocol) OSPF: Open Shortest Path First (Link-State) Including hierarchical OSPF for large domains EIGRP: Enhanced Interior Gatewa Routing Protocol Cisco protocol: originall proprietar, made open in 0 4

55 Hierarchical OSPF

56 Internet inter-as routing: BGP BGP (Border Gatewa Protocol): the de facto standard BGP provides each AS a means to:. Obtain subnet reachabilit information neighboring ASs.. Propagate the reachabilit information to all routers internal to the AS.. Determine good routes to subnets based on reachabilit information and polic. Allows a subnet to advertise its eistence to rest of the Internet: I am here 6

57 BGP basics Pairs of routers (BGP peers) echange routing info over semipermanent TCP connections: BGP sessions Note that BGP sessions do not correspond to phsical links. When AS advertises a prefi to AS, AS is promising it will forward an datagrams destined to that prefi towards the prefi. AS can aggregate prefies in its advertisement c a b AS a AS c d b a c AS b ebgp session ibgp session 7

58 Path attributes & BGP routes When advertising a prefi, advert includes BGP attributes. prefi + attributes = route Two important attributes: AS-PATH: contains the ASs through which the advert for the prefi passed: AS 67 AS 7 NEXT-HOP: Indicates the specific internal-as router to net-hop AS. (There ma be multiple links current AS to net-hop-as.) When gatewa router receives route advert, uses import polic to accept/decline. 8

59 BGP route selection Router ma learn about more than route to some prefi. Router must select route. Elimination rules:. Local preference value attribute: polic decision. Shortest AS-PATH. Closest NEXT-HOP router: hot potato routing 4. Additional criteria 9

60 Qui based on China Telecom Incident Slides (0-46) used for qui found here: Attack described and characteried in R. Hiran, N. Carlsson, and P. Gill, Characteriing Large-scale Routing Anomalies: A Case Stud of the China Telecom Incident, Proc. PAM, Hong Kong, China, Mar

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62 Multicast/Broadcast duplicate R duplicate creation/transmission R R R duplicate R R4 R R4 (a) (b) Source-duplication versus in-network duplication. (a) source duplication, (b) in-network duplication 6

63 Network Laer Routing: summar What we ve covered: network laer services routing principles: link state and distance vector hierarchical routing Internet routing protocols RIP, OSPF, BGP what s inside a router? Net stop: the Data link laer! 6

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65 More slides 6

66 Eample: Choosing among multiple ASes Now suppose AS learns the inter-as protocol that subnet is reachable AS and AS. To configure forwarding table, router d must determine towards which gatewa it should forward packets for dest. This is also the job on inter-as routing protocol! Hot potato routing: send packet towards closest of two routers. Learn inter-as protocol that subnet is reachable via multiple gatewas Use routing info intra-as protocol to determine costs of least-cost paths to each of the gatewas Hot potato routing: Choose the gatewa that has the smallest least cost Determine forwarding table the interface I that leads to least-cost gatewa. Enter (,I) in forwarding table 66

67 Hierarchical Routing Routing in the Internet 67

68 RIP ( Routing Information Protocol) Distance vector algorithm Included in BSD-UNIX Distribution in 98 Distance metric: # of hops (ma = hops) u A C B D v w destination hops u v w 68

69 OSPF (Open Shortest Path First) open : publicl available Uses Link State algorithm LS packet dissemination Topolog map at each node Route computation using Dijkstra s algorithm OSPF advertisement carries one entr per neighbor router Advertisements disseminated to entire AS (via flooding) Carried in OSPF messages directl over IP (rather than TCP or UDP 69

70 OSPF advanced features (not in RIP) Securit: all OSPF messages authenticated (to prevent malicious intrusion) Multiple same-cost paths allowed (onl one path in RIP) For each link, multiple cost metrics for different TOS (e.g., satellite link cost set low for best effort; high for real time) Integrated uni- and multicast support: Multicast OSPF (MOSPF) uses same topolog data base as OSPF Hierarchical OSPF in large domains. 70