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TDTS41 Computer Networks

Lecture 4: Network layer I Claudiu Duma, cladu@ida.liu.se IISLAB/IDA Linköpings universitet. TDTS41 Computer Networks . Network Layer. Goals: understand principles behind routing routing in the Internet. Introduction Routing algorithms Link state shortest path first Distance vector

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TDTS41 Computer Networks

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  1. Lecture 4: Network layer I Claudiu Duma, cladu@ida.liu.se IISLAB/IDA Linköpings universitet TDTS41 Computer Networks

  2. Network Layer Goals: • understand principles behind routing • routing in the Internet

  3. Introduction Routing algorithms Link state shortest path first Distance vector Hierarchical routing Routing in the Internet RIP OSPF BGP Outline

  4. Transport data from sending to receiving host IP datagram/packet Network layer protocols in every host, router network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical application transport network data link physical application transport network data link physical Network layer H1 H2

  5. Key Network-Layer Functions analogy: process of planning trip from source to dest process of getting through single interchange • 1st: routing: determine route taken by packets from source to dest. • 2nd: forwarding: move packets from router’s input to appropriate router output

  6. routing algorithm local forwarding table header value output link 0100 0101 0111 1001 3 2 2 1 value in arriving packet’s header 1 0111 2 3 Interplay between routing and forwarding

  7. Routing

  8. Routing Principles • Minimize the cost of the routing path • Scalability • Local changes should not affect globally • Administrative issues • Different networks belong to different organizations

  9. Routing Algorithms • Simple • Flooding • Those that minimize costs • Link-state shortest-path-first • Distance vector • Those that scale and meet administrative needs • Intra-/Inter-autonomous systems routing

  10. Routing algorithms that minimize cost

  11. 5 3 5 2 2 1 3 1 2 1 x z w u y v Graph abstraction • Set of nodes {u,v, w, ..} • Set of edges {(u,v), …} • Cost of links, e.g. c(u,v) = 2 • inverse to bandwidth • proportional to congestion • … Cost of path (x1, x2, x3,…, xp) = c(x1,x2) + c(x2,x3) + … + c(xp-1,xp) Question: What’s the shortest-path between u and w? Note: “shortest-path” vs. “least-cost-path”

  12. Link-state shortest-path-first • Routers find, by broadcasts, about all links in the net (costs). • Each router computes locally the shortest-paths from itself to all other routers. • Dijkstra algorithm

  13. Dijkstra’s Algorithm 1 Initialization: 2 N' = {u} 3 for all nodes v 4 if v adjacent to u 5 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 10 add w to N' 11 update D(v) for all v adjacent to w and not in N' : 12 D(v) = min( D(v), D(w) + c(w,v) ) 13 /* new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v */ 15 until all nodes in N' N': set of nodes whose least cost path is known D(v): current value of cost of path from source to dest. v

  14. Distance Vector Routing Bellman-Ford Equation (dynamic programming) Define dx(y) := cost of least-cost path from x to y Then dx(y) = min {c(x,v) + dv(y) } where min is taken over all neighbors v of x

  15. 5 3 5 2 2 1 3 1 2 1 x z w u y v Bellman-Ford Example dv(z) = 5, dx(z) = 3, dw(z) = 3 B-F equation says: du(z) = min { c(u,v) + dv(z), c(u,x) + dx(z), c(u,w) + dw(z) } = min {2 + 5, 1 + 3, 5 + 3} = 4 Node that achieves minimum is next hop in shortest path ➜ forwarding table

  16. Distance Vector Algorithm (1) • Dx(y) = estimate of least cost from x to y • Each node x knows: • Its distance vector (DV): Dx = [Dx(y): y є N ] • Cost to each neighbor v: c(x,v) • Its neighbors’ distance vectors: Dv

  17. Distance Vector Algorithm (2) Basic idea: • Each node periodically sends its own distance vector estimate to neighbors • When node x receives new DV estimate from neighbor, it updates its own DV using B-F equation: Dx(y) ← minv{c(x,v) + Dv(y)} for each node y ∊ N • Under minor, natural conditions, the estimate Dx(y) converges to the actual least costdx(y)

  18. Dx(z) = min{c(x,y) + Dy(z), c(x,z) + Dz(z)} = min{2+1 , 7+0} = 3 cost to x y z x 0 2 7 y x y z from ∞ ∞ ∞ x y z z ∞ ∞ ∞ x 0 2 7 x 0 2 3 y 2 0 1 y 2 0 1 2 1 z 7 1 0 z 3 1 0 x y z x y z x y y 7 x y z x y z x 0 2 7 x 0 2 3 y z 2 0 1 y x y 2 0 1 z 7 1 0 z 3 1 0 x y z x y z x y z x y z x y z x 0 2 7 x 0 2 3 y y 2 0 1 2 0 1 z z 7 1 0 3 1 0 y y z x y z x y z Dx(y) = min{c(x,y) + Dy(y), c(x,z) + Dz(y)} = min{2+0 , 7+1} = 2 node x 3 Next hop: y x y z node y x ∞ ∞ ∞ 2 0 1 y z ∞ ∞ ∞ x y z node z x ∞ ∞ ∞ y ∞ ∞ ∞ z 3 7 1 0 y

  19. 60 4 1 50 x z y Count to infinity problem After the cost change occurs … y z Dz(x) = 5 Dy(x) = 6 Dy(x) = 6 Dz(x) = 7 Dz(x) = 7 Dy(x) = 8 Solution: Poisoned reverse • If Z routes through Y to get to X, Z tells Y that Dz(x) = ∞. • A variant of split horizon • Do not advertise a route back to the interface from which you have learned about it! . . . Dy(x) = 8 Dz(x) = 7 . . . . . . Dz(x) = 49 Dy(x) = 50 Dy(x) = 50 Dz(x) = 50 But not through y!

  20. Message complexity LS-SPF: with n nodes, E links, O(nE) msgs sent DV: exchange between neighbors only convergence time varies Speed of Convergence LS-SPF: O(n2) algorithm requires O(nE) msgs may have oscillations DV: convergence time varies may have routing loops count-to-infinity problem Robustness: what happens if router malfunctions? LS-SPF: node can advertise incorrect link cost each node computes only its own table DV: DV node can advertise incorrect path cost each node’s table used by others error propagate thru network Comparison of LS-SPF and DV algorithms

  21. Hierarchical routing

  22. scale: with 200 million destinations: can’t store all dest’s in routing tables! routing table exchange would swamp links! administrative autonomy internet = network of networks each network admin may want to control routing in its own network Hierarchical Routing Our routing study thus far - idealization • all routers identical • network “flat” … not true in practice

  23. Internal and gateway routers Two routing protocols: intra-AS routing inter-AS routing Routers in same AS run same intra-AS routing protocol 2c 2b 3c 1b 1d 1c Forwarding table Inter-AS Routing algorithm Intra-AS Routing algorithm Autonomous systems (AS) 3a 3b 2a AS3 AS2 1a AS1

  24. Suppose a router in AS1 receives datagram for which dest is outside of AS1 Router should forward packet towards one of the gateway routers, but which one? AS1 needs: to learn which dests are reachable through AS2 and which through AS3 to propagate this reachability info to all routers in AS1 Job of inter-AS routing! 3a 3b 2a AS3 AS2 1a AS1 2c 2b 3c 1b 1d 1c Inter-AS tasks

  25. 2c 2b 3c 1b 1d 1c Example: Setting forwarding table in router 1d • Suppose AS1 learns from the inter-AS protocol that subnet x is reachable from AS3 (gateway 1c) but not from AS2. • Inter-AS protocol propagates reachability info to all internal routers. • Router 1d determines from intra-AS routing info that its interface I is on the least cost path to 1c. • Router 1d puts in forwarding table entry (x,I). 3a 3b 2a AS3 AS2 1a AS1

  26. Determine from forwarding table the interface I that leads to least-cost gateway. Enter (x,I) in forwarding table Use routing info from intra-AS protocol to determine costs of least-cost paths to each of the gateways Learn from inter-AS protocol that subnet x is reachable via multiple gateways Hot potato routing: Choose the gateway that has the smallest least cost Example: Choosing among multiple ASes • Now suppose AS1 learns from the inter-AS protocol that subnet x is reachable from AS3 and from AS2. • To configure forwarding table, router 1d must determine towards which gateway it should forward packets for dest x. • This is also the job of inter-AS routing protocol! • Hot potato routing: send packet towards closest of two routers.

  27. Intra-AS Routing

  28. Intra-AS Routing • Also known as Interior Gateway Protocols (IGP) • Most common Intra-AS routing protocols: • RIP: Routing Information Protocol • OSPF: Open Shortest Path First • IGRP: Interior Gateway Routing Protocol (Cisco proprietary)

  29. RIP ( Routing Information Protocol) • Distance vector algorithm • Included in BSD-UNIX Distribution in 1982 • Distance metric: # of hops (max = 15 hops) • RIP advertisements, containing DVs • Exchanged every 30 sec among neighbors • Each advertisement list of up to 25 destination nets within AS • If no advertisement heard after 180 sec --> neighbor/link declared dead

  30. routed routed RIP Table Processing • RIP routing tables managed by application-level process called route-d (daemon) • advertisements sent in UDP packets, periodically repeated (port 520) Transprt (UDP) Transprt (UDP) network forwarding (IP) table network (IP) forwarding table link link physical physical

  31. OSPF

  32. OSPF (Open Shortest Path First) • “open”: publicly available • Uses LS-SPF algorithm • LS packet dissemination • Topology map at each node • Route computation using Dijkstra’s algorithm • Link weights can be configured by net admin • OSPF advertisement carries one entry per neighbor router • Advertisements disseminated to entire AS (via flooding) • Carried in OSPF messages directly over IP (rather than TCP or UDP)

  33. OSPF “advanced” features (not in RIP) • Security: all OSPF messages authenticated (to prevent malicious intrusion) • Multiple same-cost paths allowed (only 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) • Hierarchical OSPF in large domains.

  34. Hierarchical OSPF Hierarchically structured OSPF autonomous system

  35. Inter-AS Routing

  36. Internet inter-AS routing: BGP • BGP (Border Gateway Protocol):the de facto standard • BGP provides each AS a means to: • Obtain subnet reachability information from neighboring ASs. • Propagate the reachability information to all routers internal to the AS. • Determine “good” routes to subnets based on reachability information and policy. • Allows a subnet to advertise its existence to rest of the Internet: “I am here”

  37. 3a 3b 2a AS3 AS2 1a 2c AS1 2b eBGP external session 3c 1b 1d 1c iBGP internal session BGP basics • Pairs of routers (BGP peers) exchange routing info over semi-permanent (“long-lived”) TCP connections: BGP sessions • When AS2 advertises a prefix to AS1, AS2 is promising it will forward any datagrams destined to that prefix towards the prefix. • AS2 can aggregate prefixes in its advertisement

  38. 3a 3b 2a AS3 AS2 1a 2c AS1 2b eBGP session 3c 1b 1d 1c iBGP session Distributing reachability info • Example: • 3a sends reach. info to 1c • 1c distributes this reach. info to all routers in AS1 • 1b can then re-advertise the new reach info to AS2 over the 1b-to-2a eBGP session

  39. Path attributes & BGP routes • When advertising a prefix, advert includes BGP attributes. • prefix + attributes = “route” • Two important attributes: • AS-PATH: contains the ASes through which the advert for the prefix passed: • E.g. AS 67 AS 17 • NEXT-HOP: Indicates the specific internal-AS router to next-hop AS.

  40. ASN examples • ASN Network 3356 Level3 3549 Global Crossing 2529 Demon UK 4589 Easynet 5459 LINX

  41. Some AS-Paths known by PeakWebHosting's BGP routers AS path: 6453[Teleglobe], 3356[Level3], 2529[Demon UK], 5459[LINX] AS path: 20248[NetVMG], 3356[Level3], 2529[Demon UK], 5459[LINX] AS path: 3356[Level3], 2529[Demon UK], 5459[LINX] AS path: 174[PSI/Cogent], 2914[Verio], 5413[GX Networks], 5459[LINX] AS path: 2914[Verio], 5413[GX Networks], 5459[LINX] AS path: 19151[WebUseNet], 3257[Tiscali Backbone], 5459[LINX] AS path: 6327[Shaw Cable], 4589[Easynet], 5459[LINX] AS path: 3549[Global Crossing], 5459[LINX]

  42. BGP route selection • Router may learn about more than 1 route to some prefix. Router must select route. • Elimination rules: • Local preference value attribute: policy decision • Shortest AS-PATH • Closest NEXT-HOP router: hot potato routing • Additional criteria

  43. BGP routing policy • A,B,C are provider networks • X,W,Y are customer (of provider networks) • X is dual-homed: attached to two networks • X does not want to route from B via X to C • .. so X will not advertise to B a route to C

  44. BGP routing policy (2) • A advertises to B the path AW • B advertises to X the path BAW • Should B advertise to C the path BAW? • No way! B gets no “revenue” for routing CBAW since neither W nor C are B’s customers • B wants to force C to route to w via A • B wants to route only to/from its customers!

  45. Why different Intra- and Inter-AS routing ? Policy: • Inter-AS: admin wants control over how its traffic routed, who routes through its net. • Intra-AS: single admin, so no policy decisions needed Scale: • hierarchical routing saves table size, reduced update traffic Performance: • Intra-AS: can focus on performance • Inter-AS: policy may dominate over performance

  46. What we’ve covered: Network layer’s functions Routing principles Hierarchical routing Internet routing protocols: RIP, OSPF, BGP Summary

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