Computer Networks (Lecture 5: Network Layer Protocols )

1. Computer Networks(Lecture 5: Network Layer Protocols ) Arzad Kherani (alam@cse.iitd.ac.in) Dept. of Computer Sc. And Engg. Indian Institute of Technology Delhi Computer Networks, Jan-May 2004

2. Outline • Connection-less vs. connection-oriented data transfer • Routing protocol • Congestion control • IP protocol • ICMP protocol Computer Networks, Jan-May 2004

3. The Network Layer • End-to-end data transfer • Addressing • Store-and-forward packet switching • Routing • Congestion control • Interconnection between networks Computer Networks, Jan-May 2004

4. Packet switching • Store-and-forwarding at intermediate nodes using connection-less or connection-oriented data transfer • Service-provider vs. customer-premise equipment Computer Networks, Jan-May 2004

5. Datagram Routing • No connection is established • Each packet is forwarded independent of others • Every packet carries a destination address • Intermediate routers maintain (and update) “routing tables” Computer Networks, Jan-May 2004

6. Routing connections • A connection is established before data transfer can take place • Route is fixed at the tie connection is established • And resources allocated • Connections are also known as virtual circuits Computer Networks, Jan-May 2004

7. Datagrams vs. virtual circuits Computer Networks, Jan-May 2004

8. H4 H5 H1 Router2 LAN H6 Router1 H2 LAN Router1 H3 Routing: the problem • Largely concerned with routing datagrams through a subnet • Between a pair of source-destination devices, packets may have to traverse several “subnets” • Routing tables are updated every T seconds Computer Networks, Jan-May 2004

9. Routing: the problem (2) • Correct • Simple • Robust • Address the problems of changing traffic conditions, changes to topology, failures (both transient and permanent) • Stable • In several cases route computation is an iterative process • In such cases the process must converge • Incremental changes in traffic/topology must result in increment changes in routes (I.e. there are no large swings in routes due to increnetal changes) • Fair • Optimal Computer Networks, Jan-May 2004

10. Routing: the problem (3) • Fairness vs. Optimality Computer Networks, Jan-May 2004

11. Routing: the problem (4) • Performance metrics: • Transit delay • Throughput • Number of hops • Security • Delay vs. throughput Computer Networks, Jan-May 2004

12. Routing algorithms static adaptive others Centralized (based on all info) Decentralized (on incomplete info) Routing protocols: classification • Static routes • Computed off-line • based on certain topology, traffic, performance metric • Not change, unless there is a major network overhaul • Adaptive routing • Routes adapt to changes in topology, traffic • On-line based on current measurements • Based on complete or partial knowledge • Distributed computation vs. centralized computation • Other algorithms • Flooding • Ho-potato Computer Networks, Jan-May 2004

13. Flooding • An incoming packet is sent on all incoming links • Limit the number of hops to avoid infinite loops • Or, forward packets only once using a packet ID • Or only on selected links (in the right direction) • Useful in case some data is to be “broadcasted” • Terribly expensive in terms of resource utilization • But, results in minimum delay Computer Networks, Jan-May 2004

14. Static routing • Shortest path routing using Dijkstra algorithm • Where “distance” is either delay, drop rate, or simply number of hops • Results in “rooted” tree with destination as the root Computer Networks, Jan-May 2004

15. Static routing: Dijkstra algorithm Computer Networks, Jan-May 2004

16. Adaptive routing • Distance-vector routing • Link-state routing • Others • Hierarchical routing • Standards • OSPF • BGP • MPLS and “traffic engineering” Computer Networks, Jan-May 2004

17. At node J Distance-vector routing • Also known as Bellman-Ford routing • Used in Arpanet, till 1979 • Each router maintains a routing table, with estimated “distance” to each destination (and updates it periodically) • Each router periodically exchanges this table with its neighbors Computer Networks, Jan-May 2004

18. At node J Distance-vector routing • Each router measures “distance” on each outgoing link • Using e.g. queue length, round-trip delay • It re-computes the routes as follows: Computer Networks, Jan-May 2004

19. Distance-vector routing • Several problems with Distance Vector routing: • Poor estimate of delays along each link • Count-to-infinity problem: • Good news spreads fast • Bad news travels slow, very slow Computer Networks, Jan-May 2004

20. Link State Routing • Every few seconds (or minutes), each router: • Re-discovers the neighborhood (and their addresses) • Estimate delays (or distances) to each of its neighbors • Construct a packet with above information • Send it to all routers in the network • Collate similar information from all routers in the network • Re-compute the “shortest” routes • Possibly using Dijkstra’s algorithm Computer Networks, Jan-May 2004

21. Two fundamental points • Routing schemes discussed thus far • Belong to “ routes for all source-destination pairs” • As opposed to “on-demand routing”, where a route is determined only if and when needed (as in wire-less networks, MPLS networks) • Belong to schemes where “routing tables” are used to route packets • As opposed to “source-routing”, where each packet carries the route that it must follow Computer Networks, Jan-May 2004

22. Link State Routing:Neighborhood discovery • Use “hello” packets on each outgoing links • Neighbors respond with an “ack” Computer Networks, Jan-May 2004

23. Link State Routing:Measuring Distances over Links • Use hello packets, and timers, to estimate delay • Start timer when the “hello” packet is put in the queue • Takes into account “load” • Or, when its transmission is started • Does not take into account “load” Computer Networks, Jan-May 2004

24. Link State Routing • Format of the link-state packet: • “seq no” helps with flooding the packet to all routers • Age, so that the information can be discarded after a while Computer Networks, Jan-May 2004

25. Link State Routing • Packet processing: • Re-sequencing of link-state info packets • Ignore packets with “lower” sequence numbers (as “stale”) • What if a packet is lost? • No big deal • Other problems • What if a sequence number is corrupted by noise? And this fact goes undetected • What if a router re-boots? • Each packet has an associated “age” in seconds (say 60 sec) • “age” is decremented every second by intermediate routers, and by the router that caches it • processing starts afresh if age  0 Computer Networks, Jan-May 2004

26. Link State Routing • Route computation: • Note every router has identical information • Use Dijkstra’s shortest path algorithm • Problems: • Stale information • Incorrect information • Incomplete information • Inconsistent routes  loops Computer Networks, Jan-May 2004

27. Link State Routing • Standards • IS-IS • Used with variety of protocols, including IP, IPX • OSPF • An Internet RFC Computer Networks, Jan-May 2004

28. Hierarchical Routing • Essentially solves “scalability” problem for large networks • Considers a network to consist of a connected network of regional networks • Routing is either within the local region, or across regions • Multiple levels of hierarchy ( 2 or more) Computer Networks, Jan-May 2004

29. Hierarchical Routing • Significant saving in size of routing tables • In example below, entries in table at 1A: • for local destination: 3 (size of local network) • For other regions: 4 (one for every other region) • For a network with say 720 routers organized as 8 regional networks, each consisting of 9 sub-nets, each of which contains 10 routers: • 10 entries, one for each router in its sub-net • 8 entries, one for every other sub-net • 7 entries, one for every other regional network Computer Networks, Jan-May 2004

30. source Broadcast routing: multi-destination routing • Send n-1 copies, one for every other router • Multi-destination routing (a smarter of sending one copy to every other router) • The source sends a packet, containing list of all n-1 destinations addresses • When a packet arrives at an intermediate router, the router identifies for each destination the “best” route, and then sends a packet on an outgoing line with the packet containing a list of sub-set of destination addresses • Both distance-vector and link-state routing algorithms will provide the necessary information Computer Networks, Jan-May 2004

31. source Broadcast routing: spanning-tree based • Intelligent form of broadcast, based on a spanning tree rooted-at-source • The problem with this router is: does each router know the spanning tree • Works with link-state routing, but not distance-vector routing Computer Networks, Jan-May 2004

32. Broadcast routing: reverse path forwarding • Essentially spanning tree based packet broadcast, except that the spanning tree is determined on-the-fly • Simple, efficient Computer Networks, Jan-May 2004

33. Multicast routing • Spanning tree vs. multicast tree • The latter includes nodes that are required to forward packets to all member nodes • Multi-destination routing to all K-1members • When a packet arrives at an intermediate router, the router identifies for each destination the “best” route, and then sends a packet on an outgoing line with the packet containing a list of sub-set of destination addresses • Both distance-vector and link-state routing algorithms will provide the necessary information Computer Networks, Jan-May 2004

34. Routing in peer-to-peer ad hoc networks • What is different about routing in ad hoc networks • Routing environment • Wireless, mobile hosts resulting in: • Greater probability of link, node failure • Changing topology • Frequent route changes • Every device is a potential router • Potentially different goals: • Stability of routes • Power consumption Computer Networks, Jan-May 2004

35. Classification of routing protocols • Multicast routing • Unicast routing • Proactive protocols • Where routes between every pair of nodes are computed a-priori • Examples: distance-vector, link-state rouitng in IP networks • Advantage: reduced latency • Dis-advantage: excessive overhead due to route computation • Reactive protocols • Routes are determined between a pair of devices only when required • As in MPLS networks • Advantage: overhead is minimized • Dis-advantage: Increased latency • Example routing protocols for ad hoc networks • Flooding • Dynamic source routing • AODV • … Computer Networks, Jan-May 2004

36. [S] S E F B C M L J A G H D K I N Dynamic Source Routing (DSR) • If source S does not have a route to destination D: • It initiates “route discovery” • Or, broadcasts (floods) Route Request (RREQ) • RREQ includes address S as “source address” Computer Networks, Jan-May 2004

37. [S,E] S E F B [S,B] C M L J A G [S,C] H D K I N Dynamic Source Routing (contd.) • Each node appends own identifier when forwarding RREQ • Issues concerning hidden terminal and of collisions again arise Computer Networks, Jan-May 2004

38. S E F [S,E,F] B C M L J A G H D K [S,C,G] I N Dynamic Source Routing (contd.) • DSR effectively uses flooding to discover a route to destination Computer Networks, Jan-May 2004

39. S E F B C M L J A G [S,E,F,J] H D K I N [S,C,G,K] Dynamic Source Routing (contd.) • Route discovery continues till node M has also attempted to find a route to destination D • Final route is say [S, E, F, J, D] Computer Networks, Jan-May 2004

40. RREP [S,E,F,J,D] S E F B C M L J A G H D K I N Dynamic Source Routing (contd.) • Destination sends a RREP (Route Reply) back to source S, together with route [S, E, F, J, D] • RREP is sent along route obtained by reversing discovered route, viz. [D, J,F, E, S] Computer Networks, Jan-May 2004

41. Dynamic Source Routing (contd.) • For DSR to succeed, links must be bi-directional: • RREP is sent along route obtained by reversing discovered route • Ensure: • Intermediate node forwards RREQ if it is received on a bi-directional link • Intermediate node forwards RREQ on links that are known to be bi-directional • If links (in general) are not bi-directional then • RREP is sent on a (new) discovered route from D to S • RREP is piggybacked onto RREQ packets for D to S • Links are bi-directional in IEEE 802.11 and in Bluetooth Computer Networks, Jan-May 2004

42. DATA [S,E,F,J,D] S E F B C M L J A G H D K I N Dynamic Source Routing (contd.) • Processing of RREP: • Source S caches the discovered route for subsequent packets • The route is included in each packet as “source route” Computer Networks, Jan-May 2004

43. DATA [E,F,J,D] S E F B C M L J A G H D K I N Dynamic Source Routing (contd.) • Intermediate nodes also cache relevant portions of the route for their use Computer Networks, Jan-May 2004

44. S E F J RREP [S,E,F,J,D] D Route Caching in DSR • All nodes along the discovered route deduce and cache a route by any means: • Given fact that RREP contains route [S, E, F, J, D]: • S has a route to E, F, J as well • E has a route [E, F, J, D] to D • So does E to F and to J, and F to J • Given fact that intermediate nodes need to forward RREP • Nodes D, J, F, E all have a route to S, and to intermediate nodes • Etc. Computer Networks, Jan-May 2004

45. S E F [S,E,F] B C M L J A G H D K [S,C,G] I N Route Caching in DSR (contd.) • Other nodes not on the discovered path also discover routes: • For example, when node K receives RREQ [S,C,G] for node D, node K learns route [K,G,C,S] to node S Computer Networks, Jan-May 2004

46. Route Caching in DSR (contd.) • Cached routes are used: • To route packets • To obtain alternate routes when a route in use is broken • speed up recovery • To respond to RREQ if a route is cached  • speed up route discovery • limit propagation of RREQ Computer Networks, Jan-May 2004

47. route [S,E,F,J,D] S E F B C M L J A G route [C, G, K, D] H D K I N Route Recovery in DSR • Speed up recovery: • If node E fails • S initiates route discovery • Node C responds immediately with RREP [S,C,G,K,D] • S routes data with source route [S,C,G,K,D] Computer Networks, Jan-May 2004

48. RERR [J-D] S E F B C M L J A G H D K I N Route Recovery in DSR (contd.) • Link failure is detected when node is unable to forward source-routed packet  • notification is sent up-stream • Source S and intermediate nodes remove all routes with broken link as one of the links Computer Networks, Jan-May 2004

49. Route Caching in DSR (contd.) • Cached routes may become invalid due to changes in topology (or mobility) • Stale, invalid cache pollute neighboring caches • Impact on performance • No route is available • Route is poor • Need to implement policy to “purge” stale/invalid cache entries Computer Networks, Jan-May 2004

50. DSR: pros and cons • Pros: • On demand routing • Caching speeds up route discovery • Route discovery uses flooding  discovers minimum delay routes • Routing tables are not maintained • Cons: • Requires entire route to be included in packet header • Requires symmetric links • Inherits all problems associated with flooding (too many RREQs, collisions, hidden terminals) • Stale, invalid cache Computer Networks, Jan-May 2004