Routing Within An Autonomous System (RIP, OSPF)

1. Routing Within An Autonomous System(RIP, OSPF) Chapter 15

2. Static vs Dynamic Interior Routes • Interior Routers • Two routers within an autonomous system • Are considered to be interior to one another • How do they learn about their own networks? • Small, slowly changing systems • Establish and modify routes by hand • Update table when new network added or deleted • Figure…routing is trivial

3. Figure 15.1

4. Disadvantages of manual system • Cannot accommodate rapid growth • Cannot accommodate rapid change • Need automated methods • Respond to change more easily • Improve reliability • Better response to failures

5. Figure 15.2

6. Multiple physical paths • Usually pick one to be primary • Router(s) fail along primary, must change • Manually: time-consuming & error prone • Even in small internets need automated system • For automation • Interior routers must communicate • Exchange routing information • Once data established, advertise • One interior router advertises to other autonomous systems via Exterior Gateway Protocol

7. No single interior protocol has emerged • Varied topologies and technologies • Tradeoffs between simplicity & functionality • Easy to install & configure; less functionality • Multiple protocols have become popular • Small AS • Choose a single one; use exclusively internally • Larger AS • Often choose a small set

8. Interior Gateway Protocol • Used as generic description • Refers to any algorithm used by interior routers • Routers may run BGP to advertise reachability • Need an IGP to obtain information within the AS

9. Figure 15.3

10. Routing Information Protocol (RIP) • One of most widely used IGPs • Also know as route-d • Came from Univ of CA – Berkeley • Developed for machines on their LANs • Relies on physical network broadcast to make routing exchanges quickly • Not designed to use on large WANs • Versions of RIP adapted for WANs are sold

11. Popularity not only due to technical merits • Was distributed with popular 4BSD UNIX systems • Lots of TCP/IP sites used without even considering technical aspects • Once installed it became the basis for local routing • RIP was built and adopted without a standard • Most implementations derived from Berkeley code • Interoperability was limited • Many undocumented details and subtleties • New versions led to more problems • Standard appeared in June 1988

12. RIP Operation • Straightforward distance-vector routing • Partitions participants into two categories • Active: • Advertise their routes • Only routers • Passive: • Do not advertise • Host must use passive mode

13. Active RIP routers broadcast every 30 seconds • Sends routing update message • Takes information from current routing database • Update is a set of pairs • (IP address, integer distance to that network) • Hop count is used as the distance metric • One hop: directly connected • Two hops: reachable through one other router • Hops = number of networks datagram will encounter • Hop count for shortest path not always optimal • 3 Ethernets faster than 2 satellites • Some RIP implementations allow assignment of artificially high hop counts when advertising slow networks

14. Active & passive routers listen to broadcasts • All update tables according to DV algorithm • May take some time for advertisements to propagate • Routers use hysteresis to improve performance • Does not replace a route with an equal cost route • Prevents oscillation among equal paths • Timers are used on all routes • Solves problem of routes through crashed routers • Start timer when install route in table • Restart whenever receive msg advertising that route • Route is invalid if 180 seconds pass without another advertisement

15. RIP must handle three types of errors • Routing loops • Algorithm does not explicitly detect routing loops • Must either assume participants can be trusted or take precautions to prevent • Instabilities • Must limit hop count to prevent • Maximum possible distance value is 16 • If legitimate hop count is higher, must divide the internet into sections or use an alternate protocol • Slow convergence • Routing messages propagate slowly across the network • Can lead to inconsistencies • Not unique to RIP; fundamental problem in any DV protocol • Hop count limit helps but does not eliminate

16. Figure 15.4

17. Solving slow convergence • “Good news travel quickly; bad news travel slowly” • Quick to install good route • Unreachable only after timeout; then learn and propagate new route • Split horizon update • Router does not propagate information over the interface from where the route arrived • R2 would not advertise route to network 1 to R1 • If R1 loses connectivity, it must quit advertising • After a few rounds of routing updates, all routers agree that the network is unreachable • Does not prevent all routing loops

18. Hold down • Router ignores information for a period of time • Typical time is 60 seconds • Done after receiving msg that network is unreachable • Wait so all machines can get bad news; keeps from mistakenly accepting an out-of-date message • All machines must have same idea of hold down • Otherwise, get routing loops • Disadvantages: • If routing loop occurs, will be preserved for the hold down • Also preserves incorrect routes during the hold down time • Even when alternatives exist

19. Poison reverse • When connection disappears • Advertising router keeps entry for few update periods • Puts infinite cost in the broadcasts • Combine with triggered updates • Router sends immediate broadcast when get bad news • Not wait for next broadcast time • Minimizes time it is vulnerable to believing good news

20. These techniques solve some problems; introduce others • Triggered updates • Suppose many routers share common network • Single broadcast changes all tables; triggering more • Broadcast avalanche • Broadcasts • Take substantial bandwidth themselves • Loops prevent stopping loops • Looping messages may prevent routing msgs to break loops • Hold down in WANs • Period so long, higher level protocol timers may expire • Breaks the connections

21. RIP1 message format • Messages are of two types • Routing information messages • Periodic broadcast of unsolicited response messages • Messages to request information • Routers or hosts can ask for info with request command • Routers reply using a response command • Both use same format

22. Figure 15.5

23. RIP2 Address Conventions • Skip…..

24. RIP route interpretation and aggregation • Version 1 contains no provision for subnet mask • Originally designed for classful addressing • Extended to allow subnetting • Important restriction: • Subnet routes can only go in updates sent across networks that are part of the subnetted prefix • Cannot use with variable-length subnet addresses or classless • Due to not having explicit subnet mask information • May have updates for networks in & out of prefix • Router must prepare different update messages

25. RIP2 extensions • Contains provisions for explicit subnet mask • Also include explicit next-hop information • Prevents routing loops • Prevents slow conversion • RIP2 message format • Puts new info in unused octets of address field • Router can use both versions simultaneously • Version number in same octet; inspect before process • Adds 16-bit ROUTE TAG • Identify the origin of the route

26. Figure 15.6

27. Transmitting RIP messages • Messages do not have explicit length field • Nor any explicit count of entries • Rely on delivery mechanism to tell length • With TCP/IP • Rely on UDP to tell receiver the message length • RIP operates on UDP port 520

28. Disadvantage of RIP hop counts • RIP restricts routing to a hop-count metric • RIP restricts size of any internet using it • Has small hop count value for infinity (16) • Limits span to at most 15 routers between hosts • Is not a limit on total number or density of routers • In any case, hop count is a crude measure • Not always get least delay or highest capacity routes • Makes routes static; cannot change due to load

29. The Hello Protocol • IGP that uses routing metric other than hops • Now obsolete • Historically, used in original NSFNET backbone “fuzzball” routers • Uses metric of delay • Provides two functions • Synchronizes clocks among a set of machines • Allows each machine to compute shortest delay paths

30. Messages carry timestamp as well as routing info • Each participating machine maintains table • Contains best estimate of neighboring machine clocks • Transmit timestamp with each packet • Receiver computes estimate of delay on the link by using the timestamp and its estimate of the sender’s clock • Periodically poll neighbors to update clock estimates • Standard D-V approach for update • Send table of destinations & estimated delays • Receiver’s update tables if cheaper route advertised

31. Delay Metrics & Oscillation • Is delay a good routing metric? • Would seem so • Worked well in the early Internet backbone • Instability is the reason most protocols do not use delay • Any protocol that changes routes quickly can become unstable

32. Hop counts fixed; delay is not • Minor variations in delay measurements occur • Hardware clock drift • CPU load during measurement • Delays by link-level synchronization • If react quickly to slight variations, get two-stage oscillation • Switch back and forth between alternate paths

33. Heuristics to help avoid oscillation • Hold down • Slows down changing • Round off measurements or use threshold • Ignore differences less than the threshold • Use average measurement • Keep average of recent measurements • Use K-out-of-N rule • K of the most recent N measurements must be less than the current delay before route can be changed

34. Can still have instability • Due to comparing delays on paths with different characteristics • Traffic has dramatic effect on delay • As load increases, delay grows rapidly • Fall into positive feedback cycle • Burst of traffic at one place increases delay • Protocol changes route • New traffic may cause another change in delay • Another route change occurs • Must have mechanism to dampen oscillation

35. Previous heuristics may not help • They help in simple case for paths with same throughput characteristics • Not good when paths have different delay and throughput characteristics • Compare serial line and satellite link • First, both paths idle; serial line have much less delay • Then, traffic quickly overloads low capacity line • Satellite delay will be less; change to it • High capacity; load not significantly change delay • But, unloaded serial line will now become attractive • Routing will change again and the cycle will continue • Oscillations do occur in practice • Difficult to manage

36. Combining RIP, Hello, and BGP • Single router may use multiple protocols • Interior Gateway Protocol • Gather routing information within AS • Exterior Gateway Protocol • Advertise routes to other ASs • Should be easy to combine the two • Technical and political obstacles exist

37. IGP protocols are routing protocols • RIP and HELLO used to update routing tables • Get info from other routers inside AS • routed implements RIP • Advertises information from local routing table • Updates local table when it receives updates • RIP trusts routers within the AS to send correct data • Exterior protocols (BGP) do not trust routers • Do not advertise all possible routes in local table • Keep database of reachability • Apply policy constraints when sending/receiving info

38. Ignoring policy constraints can make some parts of the internet unreachable • Example: • Suppose router running RIP • Propagates route to Purdue; actually has no route • Other RIP routers will accept and update • Will pass Purdue traffic to the erroneous router • Problem if EGP protocol not have policy constraints • Border router pass illegal route to other ASs • Purdue may become unreachable for parts of the internet

39. Gated: Inter-AS Communication • gated • Interface between autonomous systems • Understands multiple protocols • Both IGP’s and BGP • Ensures policy constraints are honored • Can accept RIP msgs and modify local table (routed) • Can advertise routes from within AS using BGP • Has rules on which networks it may & may not advertise • Also has rules on how to report distances to those networks • Links IGP with BGP

40. Open SPF Protocol (OSPF) • Chapter 13: link state algorithm • Uses SPF to compute shortest paths • Scales better than distance-vector algorithms • OSPF is an IGP using link state algorithm • Designed by Internet Engineering Task Force • To encourage adoption of link state technology • Tackles several ambitious goals

41. Open standard • Anyone can implement without license fees • Includes type of service routing • Have multiple routes for a given destination • Choose by TOS field in IP header • OSPF first among TCP/IP protocols to have this • Provides load balancing • Distributes traffic over multiple, same cost routes • Can partition routers and networks into areas • Permits growth; makes management easier

42. Allows exchanges to be authenticated • Variety of authentication schemes • Supports host-specific, subnet-specific, and classless routes • Accommodates multi-access nets (Ethernet) • Can describe network via virtual network • Abstracts away from details of physical connections • Provides flexibility for managers • Allows routers to exchange routing info learned from external sites • Distinguishes where information came from

43. Figure 15.7

44. Figure 15.8

45. Routers exchange database description msgs • Used to initialize network topology database • During exchange: • One router is master; other is slave • Slave acknowledges each description with a response • Topology database may be large • Can divide into several messages using I and M bits • I = 1: is initial message • M = 1: additional messages follow • Bit S indicates if sent by master (1) or slave (0) • Sequence numbers used to make sure all received

46. Figure 15.9

47. Link status request message • After exchanging DB descriptions, router may discover parts of its DB are out of date • Requests neighbor to send update • Lists specific links it wants info about • Neighbor responds with most current information about those links

48. Figure 15.10

49. Links from router to: - given area - specific network - single, subnetted IP network - networks at other sites Figure 15.11 Figure 15.12

50. Routing with Partial Information • Hosts can have partial information • Rely on routers • Not all routers have complete information • Usually single router in AS connects to others • Suppose site connects to global Internet • At least one router must have connection to an ISP • Routers inside AS know all destinations within • Have default route to send all traffic to the ISP