1. W4140 Network Laboratory Lecture 5 Oct 9 - Fall 2006 Shlomo Hershkop Columbia University

2. Announcements  Please do not reset password, and let me know if you find someone has done it to your station  Please log out when done  how many groups are having hardware / software issues ?  this week’s lab if you have done part 1-4 of lab 4…..you have a free week!! else this week you have a chance to catch up also start to think about projects, the wireless equipment have arrived and so will setup more project proposals based on those  wireless bridges and configuration  security with wireless  wireless mesh networks  capacity, interference, and overlap with wireless systems

3.  ISSUES ??  is there any issues with the labs ?  please be aware that VOIP will be using the lab, so if you can, please coordinate with the TA’s about extra lab time…they will be in touch with VOIP  our lab times, represent reserved times, so feel free to ask people to move over (nicely please)

4. Dynamic routing protocols I 1.Overview of router architecture 2.Overview Dynamic Routing Protocols: Distance Vector Routing 3.Intra-Domain Routing Protocols: RIP

5. Routing and Forwarding  Forwarding is selecting the next-hop machine for each outgoing packet. Forwarding table, FIB (Forwarding Information Base).  Routing is the process of deciding the path from a source to a destination. Routing table, RIB (Routing Information Base).  Why two tables and not just one?

6. Routing and Forwarding Control plane: run routing protocols: (RIP, OSPF, BGP) Data plane: forwarding packets from incoming to outgoing link

7. Routing and Forwarding  Select the next-hop router. Find the outgoing interface. Find the MAC address of the next-hop router. In Unix, you specify the IP address of the next-hop router.  Longest-prefix first.  Default routing (implied by longest-prefix rule: default has prefix of length 0).

8. 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

9. 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

10. 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

11. Basic Architectural Components Per-packet processing I/O Ports Switching Fabric

12. 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

13. 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.

14. 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

15. 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

16. 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.

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

18. 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

19. Cisco Express Forwarding (distributed mode)

20. Cisco Express Forwarding Benefits  Scalability & Efficiency Adjacency Tables for local hosts (same network)  Layer 2 switching is faster. The line cards perform the express forwarding between port adapters, relieving the RSP (Route Switch Processing) of involvement in the switching operation.  Resilience No route cache: several data structures for CEF switching Line Cards maintain an identical copy of the FIB and adjacency tables.  More at Cisco on-line documentationCisco on-line documentation

21. 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

22. Dynamic Routing Protocols Distance Vector Routing

23. Routing Protocols  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 (We modify manually the Routes) Dynamic Routing: Routes are calculated by a routing protocol

24. Routing protocols vs 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

25.  IGP : interior gateway protocols used within an autonomous system 1. Distance-vector routing protocol 1. information on who is next to you and cost (hop) (route table) 2. share info 3. update info 1. relatively slow to propagate 2. can insert bad info 2. Link-state routing protocol 1. have a network map by everyone 2. calculate best path 1. can end up with loops if two points have different starting maps

26. Overview Routing Protocols 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 protocolRouting Algorithm

27. 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 destinations’s in routing tables! routing table exchange would swamp links

28. 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

29. 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.

30. Distance Vector Routing  Variations of Bellman-Ford algorithm.  Each router starts by knowing: Prefixes of its attached networks (“zero” distance). Its next hop routers (how to find them?)  Each router advertises only to its neighbors: All prefixes it knows about. Its distance from them.  Each router learns: All prefixes its neighbors know about. Their distance from them.  Each router figures out, for each destination prefix: The “distance” (how far away it is). The “vector” (the next hop router).

31. Distance Vector Routing Properties  DV Computes the Shortest Path  “Routing by rumor” Each router believes what its neighbors tell it.  In steady-state, each router has the “shortest” (smallest metric) path to the destination.  Convergence time is (on the average) proportional to the diameter of the network.  Any link change affects the entire network.

32. 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).

33. Bellman-Ford Algorithm 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

34. 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 ]

35. 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)

36. Distance vector algorithm: an example 11 1 11 1 1 1 Time = 0

37. Distance vector algorithm: an example Time = 1

38. Distance vector algorithm: an example Time = 2 (End)

39. 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

40. 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

41. 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)]

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

43. 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

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

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

46. The Count-to-Infinity Problem X  What happens on a link failure? A: 1,A B:0 C:1,C A: 0 B:1,B C:2,B A: 2,B B:1,B C:0 A: 1,A B:0 C:- A: 1,A B:0 C:3,A A: 0 B:1,B C:4,B A: 1,A B:0 C:5,A A: 0 B:1,B C:6,B

47. 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?

48. 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

49. 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.

50. Characteristics of D.V. 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.

51. Inter-domain Routing Protocols: RIP

52. 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

53. 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)

54. 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

55. 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

56. 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

57. 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

58. 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

59. 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.

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

61. 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