RFC 2453 RIP 2 (Routing Information Protocol) Daher Kaiss

2 Motivation n Introduction n Review the Basic Protocol n RIP Characteristics n Protocol Extensions & Compatibility

3 Introduction Basic Internet Model R2 R3 H1 H2 N2 N1 N3 R1

4 Why RIP? n Although OSPF has a lot of advantages, we still need RIP : –Very little “overhead” in small networks Mainly in terms of bandwidth, Configuration and management time. –Very easy to implement

5 RIP 1 Limitations - I n RIP 1 doesn’t consider : –Autonomous Systems and IGP/EGP interactions. –Subnetting –Authentication

6 The Basic Protocol

7 So, What is RIP ? n A routing protocol n Based on Bellman-Ford Algorithm n Uses a distance vector algorithm n Historically : –Used since the early ARPANET –Based on the program “routed”, which is included in the Berkeley Unix. –An updated version was used by XNS (Xerox Network Systems)

8 RIP Basic Protocol (Cont..) n RIP is working as an IGP (Interior Gatway Protocol) in moderate size AS’s

9 Limitations of the protocol - II n Only Networks with longest path (Network diameter) of 15 hops n Identifying Loops requires a lot of time and bandwidth n “Best route” doesn’t consider real-time parameters (e.g. delay, reliability, dollar cost, or load)

10 Distance Vectors Algorithms n Find a path from the sender to the destination n Forwarding inside Network or Subnet is the responsibility of Network technology n Network technology is Transparent to IP

11 Distance Vectors Algorithms (Cont..) n Each Router has it’s own data-base about all the destinations n Each entry includes : –Next router –“Metric” (e.g. time delay, dollar cost) n Destinations are networks but could be individual host

12 The router’s data base n For each destination keep the following: –address (subnet): In IP implementation –router : First router along the path –interface : The physical network to be used to reach the first router –metric : indicating the distance –timer :amount of time since the entry was last updated –other flags...

13 The router’s rule n Periodically, Send to others update messages about your data base content n Take care to keep your data base updated

14 So, what is “good” path ? n Goodness is determined according to the value of the “metric” (1..15) n In simple networks : “metric” simply counts the number of routers a message must go through n In more complex : May consider delay, cost and others

15 Calculating the minimal metric n Based on Bellman-Ford Algorithm n Formally : –D(i,i) = 0 all i –D(i,j) = min k [d(i,k) + D(k,j)] otherwise * D(i,j) represents the metric of the best rout from I to j n Algorithm will converge in finite time n Assuming no topology change occurred

16 Calculating the minimal metric (Cont..) n Data is adaptively updated - No need to keep the whole estimates n Only routers participate in the game - No need for individual hosts information n Worse metric that comes from the next router, should be considered

17 Calculating the minimal metric (Cont..) R1 R2 R3 N3R2 5 Example : N1 N2 N3

18 So Far So Good !! n But … n The discussion assumes fixed topology n In practice routers and lines often fail Algorithm needs modifications 

19 Dealing with changes in topology n Main problem : If a router crashes, it has no way to notify it’s neighbors n Solution : time-out paradigm n Details depend upon the protocol itself n In RIP : Send update messages to your neighbors every 30 seconds

20 Dealing with changes in topology (Cont..) n If R1 doesn’t hear from R2 for 180 seconds,R2 is marked invalid n R1 notifies its neighbors the R2 is unreachable n Unreachable metric = 16

21 Preventing Instability n Main problem : Mathematics proves that the algorithm converges in finite time, but it doesn’t tell how long does it take it to converge !!! n “Count to infinity” method n Choose “infinity” value to be 16.

22 Preventing Instability (Cont..) AB C D Cost=1 Cost=10 Example NCost=1

23 Preventing Instability (Cont..) AB C D Cost=1 Cost=10 Example NCost=1 D: dir 1, dir 1 ….. dir 1 B: unreach, C 4, C5 … C12 C: B 3, A 4, A 5,...A 11, D 11 A: B 3, C 4, C 5, …,C 11, C 12

24 Preventing Instability (Cont..) n If network becomes inaccessible, we want to stop counting as soon as possible n “infinity” was chosen to be as small as possible

25 Preventing Instability (Cont..) n “Simple Split Horizon Scheme” –Omit routes learned from one neighbor in updates sent to that neighbor n “Split Horizon With Poisoned Reverse” –Include such routs in update but set their metric to infinity

26 Protocol Characteristics

27 Message Format Command (1) Version (1)Must be Zero (2) RIP Entry (20) RIP1 Packet Format Address Family identifier (2)Must be Zero (2) RIP1 Entry Format IPv4 address (4) Must be Zero (4) Metric (4)

28 Handling messages n Input processing –Request Messages –Response Messages

29 Protocol Extensions

30 Entry Format Address Family identifier (2)Rout Tag (2) IP address (4) Subnet Mask(4) Next Hop (4) Metric (4)

31 Authentication n Authentication scheme uses the space of an entire RIP entry Command (2)Unused (2) 0xFFFF Authentication (16) Version (2) Authentication type (2)

32 Compatibility n Implemented by compatibility switch n Necessary for two reasons : –Some implementations still follow RIP1 –Use of Multicasting (*) would prevent RIP1 from receiving RIP2 updates (*) Multicasting is used to reduce the unnecessary load on hosts not listening to RIP2 messages