Topic 6: Routing (Network Layer)

1. Topic 6: Routing (Network Layer) Network Layer Routing in Packet Networks Shortest Path Routing Reference A. Leon-Garcia and I. Widjaja, Communication Networks, pp. 492-495, 515-520, 522-534. (Reserved in the DC library. Call No. TK5105. L46 2004.)

2. 6.1 Network Layer Network Layer Overview Network Layer Services Network Layer Functions

3. Network Layer Overview • Network Layer: the most complex layer • Requires the coordinated actions of multiple, geographically distributed network elements (switches & routers) • Must be able to deal with very large scales • Billions of users (people & communicating devices) • Biggest Challenges • Addressing: where should information be directed to? • Routing: what path should be used to get information there?

4. Messages Messages Segments Transport layer Transport layer Network service Network service Network layer Network layer Network layer Network layer Data link layer Data link layer Data link layer Data link layer End system β End system α Physical layer Physical layer Physical layer Physical layer Network Layer Services • Network layer can offer a variety of services to transport layer • Connection-oriented service or connectionless service • Best-effort or delay/loss guarantees

5. Network Layer Functions Essential • Routing: mechanisms for determining the set of best paths for routing packets requires the collaboration of network elements • Forwarding: transfer of packets from NE inputs to outputs • Priority & Scheduling: determining order of packet transmission in each NE Optional: congestion control, segmentation & reassembly, security

6. 6.2 Routing in Packet Networks Packet Networks Routing Tables Routing Algorithms IP Addressing

7. 1 3 6 4 2 Node (switch or router) 5 Packet Networks • Three possible (loopfree) routes from 1 to 6: • 1-3-6, 1-4-5-6, 1-2-5-6 • Which is “best”? • Min delay? Min hop? Max bandwidth? Min cost? Max reliability?

8. Node 3 Destination Next node Node 6 Node 1 1 1 Destination Next node Destination Next node 2 4 1 3 2 2 4 4 2 5 3 3 5 6 3 3 4 4 6 6 4 3 5 2 5 5 6 3 Node 4 Destination Next node 1 1 2 2 Node 2 Node 5 3 3 Destination Next node Destination Next node 5 5 1 1 1 4 6 3 3 1 2 2 4 4 3 4 5 5 4 4 6 5 6 6 Routing Tables

9. Routing Algorithm: Requirements • Rapid and accurate delivery of packets • Must operate correctly • Rapid convergence • Responsiveness to changes and avoid routing loops • Topology or bandwidth changes, congestion • Freedom from persistent loops • Optimality • Resource utilization, path length • Robustness • Continues working under high load, congestion, faults • Simplicity • Efficient implementation, reasonable processing load

10. A. Centralized vs Distributed Routing Routing Algorithm: Classification • Centralized Routing • All routes determined by a central node • All state information sent to central node • Problems adapting to frequent topology changes • Does not scale • Distributed Routing • Routes determined by routers using distributed algorithm • State information exchanged by routers • Adapts to topology and other changes • Better scalability

11. B. Static vs Dynamic Routing • Static Routing • Set up manually, do not change; requires administration • Works when traffic predictable & network is simple • Used to override some routes set by dynamic algorithm • Used to provide default router • Dynamic Routing • Adapt to changes in network conditions • Automated • Calculates routes based on received updated network state information

12. C. Flat vs Hierarchical Routing • Flat Routing • All routers are peers • Does not scale • Hierarchical Routing • Partitioning: Domains, autonomous systems, areas... • Some routers part of routing backbone • Some routers only communicate within an area • Scales

13. 0001 0100 1011 1110 0000 0111 1010 1101 1 4 3 R2 R1 5 2 0011 0101 1000 1111 0011 0110 1001 1100 0001 4 0100 4 1011 4 … … 0000 1 0111 1 1010 1 … … Example: Non-Hierarchical Addresses and Routing • No relationship between addresses & routing proximity • Routing tables require 16 entries each

14. 0100 0101 0110 0111 0000 0001 0010 0011 1 4 3 R2 R1 5 2 1100 1101 1110 1111 1000 1001 1010 1011 00 1 01 3 10 2 11 3 00 3 01 4 10 3 11 5 Example: Hierarchical Addresses and Routing • Prefix indicates network where host is attached • Routing tables require 4 entries each

15. IP Addressing • Each host on Internet has unique 32 bit IP address • Each address has two parts: netid and hostid • netid unique & administered by • American Registry for Internet Numbers (ARIN) • Reseaux IP Europeens (RIPE) • Asia Pacific Network Information Centre (APNIC) • Facilitates routing • Dotted-Decimal Notation: int1.int2.int3.int4 where intj = integer value of jth octet IP address of 10000000 10000111 01000100 00000101 is 128.135.68.5 in dotted-decimal notation

16. Classful Addresses Class A 7 bits 24 bits hostid netid 0 1.0.0.0 to 127.255.255.255 • 126 networks with up to 16 million hosts Class B 14 bits 16 bits hostid 0 netid 1 128.0.0.0 to 191.255.255.255 • 16,382 networks with up to 64,000 hosts

17. Class D 28 bits 0 1 1 1 multicast address 224.0.0.0 to 239.255.255.255 Class C 22 bits 8 bits 0 netid hostid 1 1 • 2 million networks with up to 254 hosts 192.0.0.0 to 223.255.255.255

18. Private IP Addresses • Specific ranges of IP addresses set aside for use in private networks (RFC 1918) • Use restricted to private internets; routers in public Internet discard packets with these addresses • Range 1: 10.0.0.0 to 10.255.255.255 • Range 2: 172.16.0.0 to 172.31.255.255 • Range 3: 192.168.0.0 to 192.168.255.255 • Network Address Translation (NAT) used to convert between private & global IP addresses

19. Example of IP Addressing 178.140.5.40 178.135.40.1 H Interface Address is 178.135.10.2 Interface Address is 178.140.5.35 H Network 178.135.0.0 Network 178.140.0.0 R H H H 178.135.10.20 178.135.10.21 178.140.5.36 Address with host ID=all 0s refers to the network Address with host ID=all 1s refers to a broadcast packet R = router H = host

20. Subnet Addressing • Subnet addressing introduces another hierarchical level • Transparent to remote networks • Simplifies management of multiplicity of LANs • Masking used to find subnet number

21. Subnetting Example • Organization has Class B address (16 host ID bits) with network ID: 150.100.0.0 • Create subnets with up to 100 hosts each • 7 bits sufficient for each subnet • 16-7=9 bits for subnet ID • Apply subnet mask to IP addresses to find corresponding subnet • Example: Find subnet for 150.100.12.176 • IP add = 10010110 01100100 00001100 10110000 • Mask = 11111111 11111111 11111111 10000000 • AND = 10010110 01100100 00001100 10000000 • Subnet = 150.100.12.128 • Subnet address used by routers within organization

22. H1 H2 150.100.12.154 150.100.12.176 150.100.12.128 150.100.12.129 150.100.0.0 R1 To the rest of H3 H4 150.100.12.4 the Internet 150.100.12.55 150.100.12.24 150.100.12.0 150.100.12.1 R2 H5 150.100.15.54 150.100.15.11 150.100.15.0 Subnet Example

23. Routing with Subnetworks • IP layer in hosts and routers maintain a routing table • Originating host: To send an IP packet, consult routing table • If destination host is in same network, send packet directly using appropriate network interface • Otherwise, send packet indirectly; typically, routing table indicates a default router • Router: Examine IP destination address in arriving packet • If dest IP address not own, router consults routing table to determine next-hop and associated network interface & forwards packet

24. 6.3 Shortest Path Routing Shortest Path Problem Routing Metrics Shortest Path Protocols

25. Shortest Paths Problem • Many possible paths connect any given source and to any given destination • Routing involves the selection of the path to be used to accomplish a given transfer • Typically it is possible to attach a cost or distance to a link connecting two nodes • Routing can then be posed as a shortest path problem

26. Routing Metrics Means for measuring desirability of a path • Path Length = sum of costs or distances • Possible metrics • Hop count: rough measure of resources used • Reliability: link availability; BER • Delay: sum of delays along path; complex & dynamic • Bandwidth: “available capacity” in a path • Load: Link & router utilization along path • Cost: $$$

27. Shortest Path Protocols Distance Vector Protocols • Neighbors exchange list of distances to destinations • Best next-hop determined for each destination • Bellman-Ford (distributed) shortest path algorithm Link State Protocols • Link state information flooded to all routers • Routers have complete topology information • Shortest path (& hence next hop) calculated • Dijkstra (centralized) shortest path algorithm

28. 6.3.1 Distance VectorDo you know the way to San Jose? San Jose 294 San Jose 392 San Jose 596 San Jose 250

29. Local Signpost Direction Distance Routing Table For each destination list: Next Node Distance Table Synthesis Neighbors exchange table entries Determine current best next hop Inform neighbors Periodically After changes dest next dist Distance Vector

30. j i Shortest Path to SJ Focus on how nodes find their shortest path to a given destination node, i.e. SJ San Jose Dj Cij Di If Diis the shortest distance to SJ from i and if j is a neighbor on the shortest path, then Di = Cij + Dj

31. San Jose j' j i j" But we don’t know the shortest paths i only has local info from neighbors Dj' Cij' Dj Cij Pick current shortest path Cij” Di Dj"

32. SJ sends accurate info San Jose Accurate info about SJ ripples across network, Shortest Path Converges Why Distance Vector Works 1 Hop From SJ 2 Hops From SJ 3 Hops From SJ Hop-1 nodes calculate current (next hop, dist), & send to neighbors

33. Bellman-Ford Algorithm • Consider computations for one destination d • Initialization • Each node table has 1 row for destination d • Distance of node d to itself is zero: Dd=0 • Distance of other node j to d is infinite: Dj=, for j d • Next hop node nj = -1 to indicate not yet defined for j d • Send Step • Send new distance vector to immediate neighbors across local link • Receive Step • At node j, find the next hop that gives the minimum distance to d, • Minj { Cij + Dj } • Replace old (nj, Dj(d)) by new (nj*, Dj*(d)) if new next node or distance • Go to send step

34. Bellman-Ford Algorithm • Now consider parallel computations for all destinations d • Initialization • Each node has 1 row for each destination d • Distance of node d to itself is zero: Dd(d)=0 • Distance of other node j to d is infinite: Dj(d)=  ,for j d • Next node nj = -1 since not yet defined • Send Step • Send new distance vector to immediate neighbors across local link • Receive Step • For each destination d, find the next hop that gives the minimum distance to d, • Minj { Cij+ Dj(d) } • Replace old (nj, Di(d)) by new (nj*, Dj*(d)) if new next node or distance found • Go to send step

35. 2 3 1 1 5 2 4 6 3 1 3 2 2 5 4 Table entry @ node 3 for dest SJ Table entry @ node 1 for dest SJ San Jose

36. D3=D6+1 n3=6 D6=0 2 3 1 1 5 2 4 6 3 1 3 2 2 5 4 D6=0 D5=D6+2 n5=6 1 0 San Jose 2

37. 2 3 1 1 5 2 4 6 3 1 3 2 2 5 4 3 1 3 0 San Jose 6 2

38. 2 3 1 1 5 2 4 6 3 1 3 2 2 5 4 1 3 3 0 San Jose 6 4 2

39. 2 3 1 1 5 2 4 6 3 1 3 2 2 5 4 1 5 3 3 0 San Jose 4 2 Network disconnected; Loop created between nodes 3 and 4

40. 2 3 1 1 5 2 4 6 3 1 3 2 2 5 4 5 7 3 5 3 0 San Jose 2 4 Node 4 could have chosen 2 as next node because of tie

41. 2 3 1 1 5 2 4 6 3 1 3 2 2 5 4 7 5 7 0 5 San Jose 2 4 6 Node 2 could have chosen 5 as next node because of tie

42. 2 3 1 1 5 2 4 6 3 1 3 2 2 5 4 7 7 9 5 0 San Jose 6 2 Node 1 could have chose 3 as next node because of tie

43. (a) 1 2 3 4 1 1 1 (b) 1 2 3 4 X 1 1 Counting to Infinity Problem Nodes believe best path is through each other (Destination is node 4)

44. Problem: Bad News Travels Slowly Remedies • Split Horizon • Do not report route to a destination to the neighbor from which route was learned • Poisoned Reverse • Report route to a destination to the neighbor from which route was learned, but with infinite distance • Breaks erroneous direct loops immediately • Does not work on some indirect loops

45. (a) 1 2 3 4 1 1 1 (b) 1 2 3 4 X 1 1 Split Horizon with Poison Reverse Nodes believe best path is through each other

46. 6.3.2. Link-State Algorithm • Basic idea: two step procedure • Each source node gets a map of all nodes and link metrics (link state) of the entire network • Find the shortest path on the map from the source node to all destination nodes • Broadcast of link-state information • Every node i in the network broadcasts to every other node in the network: • ID’s of its neighbors: Ni=set of neighbors of i • Distances to its neighbors: {Cij | j Ni} • Flooding is a popular method of broadcasting packets

47. w' x' z' w' w x x z s w" w" Dijkstra Algorithm: Finding shortest paths in order Find shortest paths from source s to all other destinations Closest node to s is 1 hop away 2nd closest node to s is 1 hop away from s or w” 3rd closest node to s is 1 hop away from s, w”, or x

48. Dijkstra’s algorithm • N: set of nodes for which shortest path already found • Initialization: (Start with source node s) • N = {s}, Ds = 0, “s is distance zero from itself” • Dj=Csj for all j  s, distances of directly-connected neighbors • Step A: (Find next closest node i) • Find i N such that • Di = min Dj for j  N • Add i to N • If N contains all the nodes, stop • Step B: (update minimum costs) • For each node j  N • Dj = min (Dj, Di+Cij) • Go to Step A Minimum distance from s to j through node i in N

49. 2 1 3 1 2 1 1 5 2 5 3 2 3 2 1 2 1 3 3 3 3 3 3 3 1 3 1 2 6 6 6 6 6 6 6 3 5 2 4 3 4 4 4 4 4 4 4 4 1 2 3 2 2 1 2 2 2 2 2 2 2 1 3 3 1 1 5 5 5 5 5 5 5 4 5 5 2 2 3 3 1 2 1 2 3 3 4 4 2 2 1 3 1 1 3 1 5 5 2 2 3 3 1 2 1 2 3 3 4 4 Execution of Dijkstra’s algorithm         

50. 2 1 3 1 2 1 1 5 2 5 3 2 3 1 2 3 3 3 3 3 3 3 1 2 6 6 6 6 6 6 3 4 4 4 4 4 4 4 4 2 2 1 2 2 2 2 2 2 1 3 3 1 1 5 5 5 5 5 5 5 5 2 2 3 3 1 2 1 2 3 3 4 4 2 2 1 3 1 1 3 1 5 5 2 2 3 3 1 2 1 2 3 3 4 4 Shortest Paths in Dijkstra’s Algorithm