1. Lecture 4: Dynamic routing protocols Today: 1.Overview of router architecture 2.RIP, OSPF, BGP 3.Notes on Lab 4 4.Midterm review

2. Router Architectures An overview of router architectures.

3. 3 Two key router functions Control plane: run routing protocols (RIP, OSPF, BGP) Data plane: forwarding packets from incoming to outgoing link

4. 4 Routing and Forwarding Routing functions include: –route calculation –maintenance of the routing table –execution of routing protocols On commercial routers handled by a single general purpose processor, called route processor IP forwarding is per-packet processing On high-end commercial routers, IP forwarding is distributed Most work is done on the interface cards

5. 5 Router Hardware Components Hardware components of a router: –Network interfaces –Switching fabrics –Processor with a memory and CPU Interface Card Switching fabric Interface Card Processor CPUMemory

6. 6 PC Router versus commercial router On a PC router: –Switching fabric is the (PCI) bus –Interface cards are NICs (e.g., Ethernet cards) –All forwarding and routing is done on central processor On Commercial routers: –Switching fabrics and interface cards can be sophisticated –Central processor is the route processor (only responsible for control functions) Interface Card Switching fabric Interface Card Processor CPUMemory

7. 7 Basic Architectural Components Per-packet processing

8. 8 Evolution of Router Architectures Early routers were essentially general purpose computers Today, high-performance routers resemble supercomputers Exploit parallelism Special hardware components Until 1980s (1 st generation): standard computer Early 1990s (2 nd generation):delegate to interfaces Late 1990s (3 rd generation): Distributed architecture Today: Distributed over multiple racks

9. 9 1 st Generation Routers (switching via memory) This architecture is still used in low end routers Arriving packets are copied to main memory via direct memory access (DMA) Switching fabric is a backplane (shared bus) All IP forwarding functions are performed in the central processor. Routing cache at processor can accelerate the routing table lookup.

10. 10 Drawbacks of 1 st Generation Routers Forwarding Performance is limited by memory and CPU Capacity of shared bus limits the number of interface cards that can be connected Input Port Output Port Memory System Bus

11. 11 2 nd Generation Routers (switching via a shared bus) Keeps shared bus architecture, but offloads most IP forwarding to interface cards Interface cards have local route cache and processing elements Fast path: If routing entry is found in local cache, forward packet directly to outgoing interface Slow path: If routing table entry is not in cache, packet must be handled by central CPU

12. 12 Another 2 nd Generation Architecture IP forwarding is done by separate components (Forwarding Engines) Forwarding operations: 1.Packet received on interface: Store the packet in local memory. Extracts IP header and sent to one forwarding engine 2.Forwarding engine does lookup, updates IP header, and sends it back to incoming interface 3.Packet is reconstructed and sent to outgoing interface.

13. 13 Drawbacks of 2 nd Generation Routers Bus contention limits throughput

14. 14 3 rd Generation Architecture Switching fabric is an interconnection network (e.g., a crossbar switch) Distributed architecture: –Interface cards operate independent of each other –No centralized processing for IP forwarding These routers can be scaled to many hundred interface cards and to aggregate capacity of > 1 Terabit per second

15. 15 Slotted Chassis Large routers are built as a slotted chassis –Interface cards are inserted in the slots –Route processor is also inserted as a slot This simplifies repairs and upgrades of components

16. Dynamic Routing Protocols Part 1: RIP Relates to Lab 4. The first module on dynamic routing protocols. This module introduces RIP.

17. 17 Routing Recall: There are two parts to routing IP packets: 1. How to pass a packet from an input interface to the output interface of a router (packet forwarding) ? 2.How to find and setup a route ? We already discussed the packet forwarding part –Longest prefix match There are two approaches for calculating the routing tables: –Static Routing (Lab 3) –Dynamic Routing: Routes are calculated by a routing protocol

18. 18 Routing protocols versus routing algorithms Routing protocols establish routing tables at routers. A routing protocol specifies –What messages are sent between routers –Under what conditions the messages are sent –How messages are processed to compute routing tables At the heart of any routing protocol is a routing algorithm that determines the path from a source to a destination

19. 19 What routing algorithms common routing protocols use Routing information protocol (RIP)Distance vector Interior Gateway routing protocol (IGRP, cisco proprietary) Distance vector Open shortest path first (OSPF)Link state Intermediate System-to-Intermediate System (IS-IS Link state Border gateway protocol (BGP)Path vector Routing protocol Routing algorithm

20. 20 Intra-domain routing versus inter-domain routing Recall Internet is a network of networks. Administrative autonomy –internet = network of networks –each network admin may want to control routing in its own network Scale: with 200 million destinations: –can’t store all dest’s in routing tables! –routing table exchange would swamp links

21. 21 Autonomous systems aggregate routers into regions, “autonomous systems” (AS) or domain routers in the same AS run the same routing protocol –“intra-AS” or intra-domain routing protocol –routers in different AS can run different intra-AS routing protocol

22. 22 Autonomous Systems An autonomous system is a region of the Internet that is administered by a single entity. Examples of autonomous regions are: UCI’s campus network MCI’s backbone network Regional Internet Service Provider Routing is done differently within an autonomous system (intradomain routing) and between autonomous system (interdomain routing). RIP, OSPF, IGRP, and IS-IS are intra-domain routing protocols. BGP is the only inter-domain routing protocol.

23. 23 RIP and OSPF computes shortest paths Shortest path routing algorithms Goal: Given a network where each link is assigned a cost. Find the path with the least cost between two nodes. a b cd 3 1 6 2

24. 24 Distance vector algorithm A decentralized algorithm –A router knows physically-connected neighbors and link costs to neighbors –A router does not have a global view of the network Path computation is iterative and mutually dependent. –A router sends its known distances to each destination (distance vector) to its neighbors. –A router updates the distance to a destination from all its neighbors’ distance vectors –A router sends its updated distance vector to its neighbors. –The process repeats until all routers’ distance vectors do not change (this condition is called convergence).

25. 25 A router updates its distance vectors using bellman-ford equation Bellman-Ford Equation Define d x (y) := cost of the least-cost path from x to y Then d x (y) = min v {c(x,v) + d v (y) }, where min is taken over all neighbors of node x

26. 26 Distance vector algorithm: initialization Let D x (y) be the estimate of least cost from x to y Initialization: –Each node x knows the cost to each neighbor: c(x,v). For each neighbor v of x, D x (v) = c(x,v) –D x (y) to other nodes are initialized as infinity. Each node x maintains a distance vector (DV): –D x = [D x (y): y 2 N ]

27. 27 Distance vector algorithm: updates Each node x sends its distance vector to its neighbors, either periodically, or triggered by a change in its DV. When a node x receives a new DV estimate from a neighbor v, it updates its own DV using B-F equation: –If c(x,v) + D v (y) < D x (y) then D x (y) = c(x,v) + D v (y) Sets the next hop to reach the destination y to the neighbor v Notify neighbors of the change The estimate D x (y) will converge to the actual least cost d x (y)

28. 28 Distance vector algorithm: an example t = 0 a = ((a, 0), (b, 3), (c, 6)) b = ((a, 3), (b, 0), (c,1)) c = ((a, 6), (b, 1), (c, 0) (d, 2)) d = ((c, 2), (d, 0)) a b cd 3 1 6 2 t = 1 a = ((a, 0), (b, 3), (c, 4), (d, 8)) b = ((a, 3), (b, 0), (c,1), (d, 3)) c = ((a, 4), (b, 1), (c, 0), (d, 2)) d = ((a, 8), (b, 3), (c, 2), (d,0)) t = 2 a = ((a, 0), (b, 3), (c, 4), (d, 6)) b = ((a, 3), (b, 0), (c,1), (d, 3)) c = ((a, 4), (b, 1), (c, 0), (d, 2)) d = ((a, 6), (b, 3), (c, 2), (d,0))

29. 29 How to map the abstract graph to the physical network Nodes (e.g., v, w, n) are routers, identified by IP addresses, e.g. 10.0.0.1 Nodes are connected by either a directed link or a broadcast link (Ethernet) Destinations are IP networks, represented by the network prefixes, e.g., 10.0.0.0/16 –Net(v,n) is the network directly connected to router v and n. Costs (e.g. c(v,n)) are associated with network interfaces. –Router1(config)# router rip –Router1(config-router)# offset-list 0 out 10 Ethernet0/0 –Router1(config-router)# offset-list 0 out 10 Ethernet0/1

30. 30 Distance vector routing protocol: Routing Table Net(v,w): Network address of the network between v and w c(v,w): cost to transmit on the interface to network Net(v,w) D(v,net) is v’s cost to Net

31. 31 Distance vector routing protocol: Messages Nodes send messages to their neighbors which contain distance vectors A message has the format: [Net, D(v,Net)] means“My cost to go to Net is D (v,Net)” v v n n [Net, D(v,Net)]

32. 32 Distance vector routing algorithm: Sending Updates Periodically, each node v sends the content of its routing table to its neighbors:

33. 33 Initiating Routing Table I Suppose a new node v becomes active. The cost to access directly connected networks is zero: –D (v, Net(v,m)) = 0 –D (v, Net(v,w)) = 0 –D (v, Net(v,n)) = 0

34. 34 Initiating Routing Table II Node v sends the routing table entry to all its neighbors:

35. 35 Initiating Routing Table III Node v receives the routing tables from other nodes and builds up its routing table

36. 36 Updating Routing Tables I Suppose node v receives a message from node m: [ Net,D(m,Net)] if ( D(m,Net) + c (v,m) < D (v,Net) ) { D new (v,Net) := D (m,Net) + c (v,m); Update routing table; send message [Net, D new (v,Net)] to all neighbors } Node v updates its routing table and sends out further messages if the message reduces the cost of a route:

37. 37 Updating Routing Tables II Before receiving the message: Suppose D (m,Net) + c (v,m) < D (v,Net):

38. 38 Example Router ARouter B Router CRouter D 10.0.2.0/2410.0.3.0/2410.0.4.0/2410.0.5.0/2410.0.1.0/24.1.2.1 Assume: - link cost is 1, i.e., c(v,w) = 1 - all updates, updates occur simultaneously - Initially, each router only knows the cost of connected interfaces t=0: 10.0.1.0 - 0 10.0.2.0 - 0 Net via cost t=0: 10.0.2.0 - 0 10.0.3.0 - 0 Net via cost t=0: 10.0.3.0 - 0 10.0.4.0 - 0 Net via cost t=0: 10.0.4.0 - 0 10.0.5.0 - 0 Net via cost t=1: 10.0.1.0 - 0 10.0.2.0 - 0 10.0.3.0 10.0.2.2 1 t=2: 10.0.1.0 - 0 10.0.2.0 - 0 10.0.3.0 10.0.2.2 1 10.0.4.0 10.0.2.2 2 t=2: 10.0.1.0 10.0.2.1 1 10.0.2.0 - 0 10.0.3.0 - 0 10.0.4.0 10.0.3.2 1 10.0.5.0 10.0.3.2 2 t=1: 10.0.1.0 10.0.2.1 1 10.0.2.0 - 0 10.0.3.0 - 0 10.0.4.0 10.0.3.2 1 t=2: 10.0.1.0 10.0.3.1 2 10.0.2.0 10.0.3.1 1 10.0.3.0 - 0 10.0.4.0 - 0 10.0.5.0 10.0.4.2 1 t=1: 10.0.2.0 10.0.3.1 1 10.0.3.0 - 0 10.0.4.0 - 0 10.0.5.0 10.0.4.2 1 t=2: 10.0.2.0 10.0.4.1 2 10.0.3.0 10.0.4.1 1 10.0.4.0 - 0 10.0.5.0 - 0 t=1: 10.0.3.0 10.0.4.1 1 10.0.4.0 - 0 10.0.5.0 - 0

39. 39 Example Router ARouter B Router CRouter D 10.0.2.0/2410.0.3.0/2410.0.4.0/2410.0.5.0/2410.0.1.0/24.1.2.1 t=3: 10.0.1.0 - 0 10.0.2.0 - 0 10.0.3.0 10.0.2.2 1 10.0.4.0 10.0.2.2 2 10.0.5.0 10.0.2.2 3 Net via cost t=3: 10.0.1.0 10.0.2.1 1 10.0.2.0 - 0 10.0.3.0 - 0 10.0.4.0 10.0.3.2 1 10.0.5.0 10.0.3.2 2 Net via cost t=3: 10.0.1.0 10.0.3.1 2 10.0.2.0 10.0.3.1 1 10.0.3.0 - 0 10.0.4.0 - 0 10.0.5.0 10.0.4.2 1 Net via cost t=3: 10.0.1.0 10.0.4.1 3 10.0.2.0 10.0.4.1 2 10.0.3.0 10.0.4.1 1 10.0.4.0 - 0 10.0.5.0 - 0 Net via cost Now, routing tables have converged ! t=2: 10.0.1.0 - 0 10.0.2.0 - 0 10.0.3.0 10.0.2.2 1 10.0.4.0 10.0.2.2 2 t=2: 10.0.1.0 10.0.2.1 1 10.0.2.0 - 0 10.0.3.0 - 0 10.0.4.0 10.0.3.2 1 10.0.5.0 10.0.3.2 2 t=2: 10.0.1.0 10.0.3.1 2 10.0.2.0 10.0.3.1 1 10.0.3.0 - 0 10.0.4.0 - 0 10.0.5.0 10.0.4.2 1 t=2: 10.0.2.0 10.0.4.1 2 10.0.3.0 10.0.4.1 1 10.0.4.0 - 0 10.0.5.0 - 0

40. 40 Characteristics of Distance Vector Routing Protocols Periodic Updates: Updates to the routing tables are sent at the end of a certain time period. A typical value is 30 seconds. Triggered Updates: If a metric changes on a link, a router immediately sends out an update without waiting for the end of the update period. Full Routing Table Update: Most distance vector routing protocol send their neighbors the entire routing table (not only entries which change). Route invalidation timers: Routing table entries are invalid if they are not refreshed. A typical value is to invalidate an entry if no update is received after 3-6 update periods.

41. 41 The Count-to-Infinity Problem A A B B C C 11 A's Routing TableB's Routing Table C tocost via (next hop) 2BC tocost via (next hop) 1C now link B-C goes down C2C C 1 -C2B CC3 C3AC- C4C C-C4B 1 1 1 1 1

42. 42 Count-to-Infinity The reason for the count-to-infinity problem is that each node only has a “next-hop-view” For example, in the first step, A did not realize that its route (with cost 2) to C went through node B How can the Count-to-Infinity problem be solved?

43. 43 Count-to-Infinity The reason for the count-to-infinity problem is that each node only has a “next-hop-view” For example, in the first step, A did not realize that its route (with cost 2) to C went through node B How can the Count-to-Infinity problem be solved? Solution 1: Always advertise the entire path in an update message to avoid loops (Path vectors) –BGP uses this solution

44. 44 Count-to-Infinity The reason for the count-to-infinity problem is that each node only has a “next-hop-view” For example, in the first step, A did not realize that its route (with cost 2) to C went through node B How can the Count-to-Infinity problem be solved? Solution 2: Never advertise the cost to a neighbor if this neighbor is the next hop on the current path (Split Horizon) –Example: A would not send the first routing update to B, since B is the next hop on A’s current route to C –Split Horizon does not solve count-to-infinity in all cases! »You can produce the count-to-infinity problem in Lab 4.

45. 45 RIP - Routing Information Protocol A simple intradomain protocol Straightforward implementation of Distance Vector Routing Each router advertises its distance vector every 30 seconds (or whenever its routing table changes) to all of its neighbors RIP always uses 1 as link metric Maximum hop count is 15, with “16” equal to “  ” Routes are timeout (set to 16) after 3 minutes if they are not updated

46. 46 RIP - History Late 1960s : Distance Vector protocols were used in the ARPANET Mid-1970s: XNS (Xerox Network system) routing protocol is the ancestor of RIP in IP (and Novell’s IPX RIP and Apple’s routing protocol) 1982Release of routed for BSD Unix 1988RIPv1 (RFC 1058) - classful routing 1993RIPv2 (RFC 1388) - adds subnet masks with each route entry - allows classless routing 1998Current version of RIPv2 (RFC 2453)

47. 47 RIPv1 Packet Format One RIP message can have up to 25 route entries 1: request 2: response 2: for IP Address of destination Cost (measured in hops) 1: RIPv1

48. 48 RIPv2 RIPv2 is an extends RIPv1: –Subnet masks are carried in the route information –Authentication of routing messages –Route information carries next-hop address –Uses IP multicasting Extensions of RIPv2 are carried in unused fields of RIPv1 messages

49. 49 RIPv2 Packet Format One RIP message can have up to 25 route entries 1: request 2: response 2: for IP Address of destination Cost (measured in hops) 2: RIPv2

50. 50 RIPv2 Packet Format Used to provide a method of separating "internal" RIP routes (routes for networks within the RIP routing domain) from "external" RIP routes Identifies a better next-hop address on the same subnet than the advertising router, if one exists (otherwise 0….0) 2: RIPv2 Subnet mask for IP address

51. 51 RIP Messages This is the operation of RIP in routed. Dedicated port for RIP is UDP port 520. Two types of messages: –Request messages used to ask neighboring nodes for an update –Response messages contains an update

52. 52 Routing with RIP Initialization: Send a request packet (command = 1, address family=0..0) on all interfaces: RIPv1 uses broadcast if possible, RIPv2 uses multicast address 224.0.0.9, if possible requesting routing tables from neighboring routers Request received: Routers that receive above request send their entire routing table Response received: Update the routing table Regular routing updates: Every 30 seconds, send all or part of the routing tables to every neighbor in an response message Triggered Updates: Whenever the metric for a route change, send entire routing table.

53. 53 RIP Security Issue: Sending bogus routing updates to a router RIPv1: No protection RIPv2: Simple authentication scheme 2: plaintext password

54. 54 RIP Problems RIP takes a long time to stabilize –Even for a small network, it takes several minutes until the routing tables have settled after a change RIP has all the problems of distance vector algorithms, e.g., count-to-Infinity »RIP uses split horizon to avoid count-to-infinity The maximum path in RIP is 15 hops

55. Relates to Lab 4. This module covers link state routing and the Open Shortest Path First (OSPF) routing protocol. Dynamic Routing Protocols II OSPF

56. 56 Distance Vector vs. Link State Routing With distance vector routing, each node has information only about the next hop: Node A: to reach F go to B Node B: to reach F go to D Node D: to reach F go to E Node E: go directly to F Distance vector routing makes poor routing decisions if directions are not completely correct (e.g., because a node is down). If parts of the directions incorrect, the routing may be incorrect until the routing algorithms has re-converged. A A B B C C D D E E F F

57. 57 Distance Vector vs. Link State Routing In link state routing, each node has a complete map of the topology If a node fails, each node can calculate the new route Difficulty: All nodes need to have a consistent view of the network A A B B C C D D E E F F ABC DE F ABC DE F ABC DE F ABC DE F ABC DE F ABC DE F

58. 58 Link State Routing: Properties Each node requires complete topology information Link state information must be flooded to all nodes Guaranteed to converge

59. 59 Link State Routing: Basic principles 1. Each router establishes a relationship (“adjacency”) with its neighbors 2. Each router generates link state advertisements (LSAs) which are distributed to all routers LSA = (link id, state of the link, cost, neighbors of the link) Each router sends its LSA to all routers in the network (using a method called reliable flooding) 3. Each router maintains a database of all received LSAs (topological database or link state database), which describes the network has a graph with weighted edges 4. Each router uses its link state database to run a shortest path algorithm (Dijikstra’s algorithm) to produce the shortest path to each network

60. 60 Link state routing: graphical illustration a b cd 3 1 6 2 a 3 6 b c a b c 3 1 a b cd 1 6 cd 2 a’s view b’s view c’s view d’s view Collecting all pieces yield a complete view of the network!

61. 61 Operation of a Link State Routing protocol Received LSAs IP Routing Table Dijkstra’s Algorithm Link State Database LSAs are flooded to other interfaces

62. 62 Dijkstra’s Shortest Path Algorithm for a Graph Input: Graph (N,E) with N the set of nodes and E the set of edges c vw link cost (c vw = 1 if (v,w)  E, c vv = 0) s source node. Output : D n cost of the least-cost path from node s to node n M = {s}; for each n  M D n = c sn ; while (M  all nodes) do Find w  M for which D w = min{D j ; j  M}; Add w to M; for each neighbor n of w and n  M D n = min[ D n, D w + c wn ]; Update route; enddo

63. 63 OSPF OSPF = Open Shortest Path First The OSPF routing protocol is the most important link state routing protocol on the Internet (another link state routing protocol is IS-IS (intermediate system to intermediate system) The complexity of OSPF is significant –RIP (RFC 2453 ~ 40 pages) –OSPF (RFC 2328 ~ 250 pages) History: –1989: RFC 1131 OSPF Version 1 –1991: RFC1247 OSPF Version 2 –1994: RFC 1583 OSPF Version 2 (revised) –1997: RFC 2178 OSPF Version 2 (revised) –1998: RFC 2328 OSPF Version 2 (current version)

64. 64 Features of OSPF Provides authentication of routing messages Enables load balancing by allowing traffic to be split evenly across routes with equal cost Type-of-Service routing allows to setup different routes dependent on the TOS field Supports subnetting Supports multicasting Allows hierarchical routing

65. 65 Hierarchical OSPF

66. 66 Hierarchical OSPF Two-level hierarchy: local area, backbone. –Link-state advertisements only in area –each nodes has detailed area topology; only know direction (shortest path) to nets in other areas. Area border routers: “summarize” distances to nets in own area, advertise to other Area Border routers. Backbone routers: run OSPF routing limited to backbone.

67. 67 Example Network Router IDs can be selected independent of interface addresses, but usually chosen to be the smallest interface address 3 42 5 1 1 32 Link costs are called Metric Metric is in the range [0, 2 16 ] Metric can be asymmetric 10.1.1.0 / 24.1.2 10.1.1.1 10.1.4.0 / 24 10.1.2.0 / 24.1.4 10.1.7.0 / 24 10.1.6.0 / 24 10.1.3.0 / 24 10.1.5.0/24 10.1.8.0 / 24.3.5.2.3.5.4.6 10.1.1.210.1.4.4 10.1.7.6 10.1.2.310.1.5.5

68. 68 Link State Advertisement (LSA) The LSA of router 10.1.1.1 is as follows: Link State ID: 10.1.1.1 = Router ID Advertising Router: 10.1.1.1 = Router ID Number of links: 3 = 2 links plus router itself Description of Link 1: Link ID = 10.1.1.2, Metric = 4 Description of Link 2: Link ID = 10.1.2.2, Metric = 3 Description of Link 3: Link ID = 10.1.1.1, Metric = 0 10.1.1.0 / 24.1.2 10.1.1.1 10.1.4.0 / 24 10.1.2.0 / 24.1.4 10.1.7.0 / 24 10.1.6.0 / 24 10.1.3.0 / 24 10.1.5.0/24 10.1.8.0 / 24.3.5.2.3.5.4.6 10.1.1.210.1.4.4 10.1.7.6 10.1.2.3 10.1.5.5 4 3 2

69. 69 Network and Link State Database Each router has a database which contains the LSAs from all other routers LS TypeLinkStateIDAdv. RouterChecksumLSSeqNoLS Age Router-LSA10.1.1.1 0x9b470x800000060 Router-LSA10.1.1.2 0x219e0x800000071618 Router-LSA10.1.2.3 0x6b530x800000031712 Router-LSA10.1.4.4 0xe39a0x8000003a20 Router-LSA10.1.5.5 0xd2a60x8000003818 Router-LSA10.1.7.6 0x05c30x800000051680 10.1.1.0 / 24.1.2 10.1.1.1 10.1.4.0 / 24 10.1.2.0 / 24.1.4 10.1.7.0 / 24 10.1.6.0 / 24 10.1.3.0 / 24 10.1.5.0/24 10.1.8.0 / 24.3.5.2.3.5.4.6 10.1.1.210.1.4.4 10.1.7.6 10.1.2.3 10.1.5.5

70. 70 Link State Database The collection of all LSAs is called the link-state database Each router has an identical link-state database –Useful for debugging: Each router has a complete description of the network If neighboring routers discover each other for the first time, they will exchange their link-state databases The link-state databases are synchronized using reliable flooding

71. 71 OSPF Packet Format Destination IP: neighbor’s IP address or 224.0.0.5 (ALLSPFRouters) or 224.0.0.6 (AllDRouters) TTL: set to 1 (in most cases) OSPF packets are not carried as UDP payload! OSPF has its own IP protocol number: 89

72. 72 OSPF Packet Format 2: current version is OSPF V2 Message types: 1: Hello (tests reachability) 2: Database description 3: Link Status request 4: Link state update 5: Link state acknowledgement ID of the Area from which the packet originated Standard IP checksum taken over entire packet 0: no authentication 1: Cleartext password 2: MD5 checksum (added to end packet) Authentication passwd = 1: 64 cleartext password Authentication passwd = 2: 0x0000 (16 bits) KeyID (8 bits) Length of MD5 checksum (8 bits) Nondecreasing sequence number (32 bits) Prevents replay attacks

73. 73 OSPF LSA Format LSA Header Link 1 Link 2

74. 74 Discovery of Neighbors Routers multicasts OSPF Hello packets on all OSPF-enabled interfaces. If two routers share a link, they can become neighbors, and establish an adjacency After becoming a neighbor, routers exchange their link state databases Scenario: Router 10.1.10.2 restarts

75. 75 Neighbor discovery and database synchronization Sends empty database description Scenario: Router 10.1.10.2 restarts Discovery of adjacency Sends database description. (description only contains LSA headers) Database description of 10.1.10.2 Acknowledges receipt of description After neighbors are discovered the nodes exchange their databases

76. 76 Regular LSA exchanges 10.1.10.2 explicitly requests each LSA from 10.1.10.1 10.1.10.1 sends requested LSAs 10.1.10.110.1.10.2 Link State Request packets, LSAs = Router-LSA,10.1.10.1, Router-LSA,10.1.10.2, Router-LSA,10.1.10.3, Router-LSA,10.1.10.4, Router-LSA,10.1.10.5, Router-LSA,10.1.10.6, Link State Update Packet, LSAs = Router-LSA, 10.1.10.1,0x80000006 Router-LSA, 10.1.10.2, 0x80000007 Router-LSA, 10.1.10.3, 0x80000003 Router-LSA, 10.1.10.4, 0x8000003a Router-LSA, 10.1.10.5, 0x80000038 Router-LSA, 10.1.10.6, 0x80000005

77. 77 Routing Data Distribution LSA-Updates are distributed to all other routers via Reliable Flooding Example: Flooding of LSA from 10.10.10.1 LSA Update database ACK LSA ACK LSA Update database ACK Update database 10.1.1.110.1.2.210.1.3.410.1.7.6 10.1.1.210.1.4.5

78. 78 Dissemination of LSA-Update A router sends and refloods LSA-Updates, whenever the topology or link cost changes. (If a received LSA does not contain new information, the router will not flood the packet) Exception: Infrequently (every 30 minutes), a router will flood LSAs even if there are not new changes. Acknowledgements of LSA-updates: explicit ACK, or implicit via reception of an LSA-Update Question: If a new node comes up, it could build the database from regular LSA-Updates (rather than exchange of database description). What role do the database description packets play?

79. Border Gateway protocol (BGP)

80. 80 BGP BGP = Border Gateway Protocol. Currently in version 4, specified in RFC 1771. (~ 60 pages) Note: In the context of BGP, a gateway is nothing else but an IP router that connects autonomous systems. Interdomain routing protocol for routing between autonomous systems Uses TCP to establish a BGP session and to send routing messages over the BGP session BGP is a path vector protocol. Routing messages in BGP contain complete routes. Network administrators can specify routing policies

81. 81 BGP policy routing BGP’s goal is to find any path (not an optimal one). Since the internals of the AS are never revealed, finding an optimal path is not feasible. Network administrator sets BGP’s policies to determine the best path to reach a destination network.

82. 82 BGP basics A route is defined as a unit of information that pairs a destination with the attributes of a path to that destination. EBGP session is a BGP session between two routers in different ASes. IBGP session is a BGP session between internal routers of an AS.

83. 83 EBGP and IBGP IBGP is organized into a full mesh topology, or IBGP sessions are relayed using a route reflector. 128.195.0.0/16 0 128.195.0.0/16 1 0 AS 0 AS 1 AS 2 AS 3 128.195.0.0/16 2 1 0 R1 R2 R3 R4 R5 R6 R7 R8

84. 84 Commonly BGP attributes Origin: whether it is an internal prefix or an prefix learned from BGP peers AS path Next hop Multi_Exit_Disc (MED, multiple exit discriminator): used to distinguish routes learned from different peers of the same neighboring AS Local_pref Community: group routes to communities

85. 85 BGP route selection process Input/output engine may filter routes or manipulate their attributes Input Policy Engine Decision process Best routes Out Policy Enigne Routes recved from peers Routes sent to peers

86. 86 Best path selection algorithm 1.If next hop is inaccessible, ignore routes 2.Prefer the route with the largest local preference value. 3.If local prefs are the same, prefer route with the shortest AS path 4.If AS_path is the same, prefer route with lowest origin (IGP < EGP < incomplete) 5.If origin is the same, prefer the route with lowest MED 6.IF MEDs are the same, prefer EBGP paths to IBGP paths 7.If all the above are the same, prefer the route that can be reached via the closest IGP neighbor. 8.If the IGP costs are the same, prefer the router with lowest router id.

87. 87 Example of BGP route selection Input Policy Engine Decision process Best routes Out Policy Enigne AS1 AS2 AS 5 AS3 AS4 128.195.0.0/16 0/0 128.195.0.0/16 0/0 Deny 0/0 from AS1 Give 128.195.0.0/16 From AS1 higher Local_pref Accept other routes Accept 0/0 from AS2 Use AS1 to reach 128.195.0.0/16 0/0 AS2 128.195.0.0/16 AS1 Do not propagate 0/0. 128.195.0.0/16

88. 88 Summary Router architectures Dynamic routing protocols: RIP, OSPF, BGP RIP uses distance vector algorithm, and converges slow (the count-to-infinity problem) OSPF uses link state algorithm, and converges fast. But it is more complicated than RIP. Both RIP and OSPF finds lowest-cost path. BGP uses path vector algorithm, and its path selection algorithm is complicated, and is influenced by policies.

89. Lab 4: dynamic routing protocols

90. 90 Exercise (4B): count-to-infinity is optional Time consuming to reproduce, but interesting. Why does count-to-infinity still exist with split horizon? Lab report due after midterm Router2 Router4 Router3 Router1 10 1 1 1 1 1

91. 91 Why does count-to-infinity still exist with split horizon? Router2 Router4 Router3 Router1 10 1 1 1 1 X 1 10.0.1.0/24 Router3’s routing table: 10.0.1.0/24 ?? 1 Router2’s routing table: 10.0.1.0/24 ?? 1 Router4’s routing table: 10.0.1.0/24 Router3 3 Router2 is not Router4’s next hop. Router4 sends to router2 the routing update Router2’s routing table: 10.0.1.0/24 Router 4 4 This lie will be told to Router3 and Circulates in the system  count-to-infinity Suppose updates happen in the following sequence: 1.The update from PC3 arrives at Router 2.The update from Router 3 arrives at Router 2 3.The update from Router 4 arrives at Router 2 PC3

92. Midterm review

93. 93 What you’ll be tested on Basic lab commands –E.g., ping, traceroute, tcpdump, ethereal, ifconfig, how to copy a file, how to list a directory Basic trouble shooting –E.g., I cannot ping 128.195.1.150, why? Basic networking concepts –E.g., layering principle, multiplexing, and encapsulation Protocols we’ve covered so far –ARP –ICMP –IP

94. 94 Address translation protocol What is it used for? What is an ARP cache used for?

95. 95 ICMP What is it used for? –E.g. error reporting, route redirect When will an ICMP message be triggered?

96. 96 IP Network order versus host order CIDR addressing Route aggregation Longest prefix match Fragmentation