RFC 2453

RIP 2(Routing Information Protocol)

Daher Kaiss

2

Motivation

Introduction

Review the Basic Protocol

RIP Characteristics

Protocol Extensions & Compatibility

3

IntroductionBasic Internet Model

R2

R3

H1

H2

N2

N1

N3

R1

4

Why RIP?

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

RIP 1 doesn’t consider :

– Autonomous Systems and IGP/EGP

interactions.

– Subnetting

– Authentication

6

The Basic Protocol

7

So, What is RIP ?

A routing protocol Based on Bellman-Ford Algorithm Uses a distance vector algorithm 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..)

RIP is working as an IGP (Interior Gatway Protocol) in moderate size AS’s

9

Limitations of the protocol - II

Only Networks with longest path (Network diameter) of 15 hops

Identifying Loops requires a lot of time and bandwidth

“Best route” doesn’t consider real-time parameters (e.g. delay, reliability, dollar cost, or load)

10

Distance Vectors Algorithms

Find a path from the sender to the destination

Forwarding inside Network or Subnet is the responsibility of Network technology

Network technology is Transparent to IP

11

Distance Vectors Algorithms (Cont..)

Each Router has it’s own data-base

about all the destinations

Each entry includes :

– Next router

– “Metric” (e.g. time delay, dollar cost)

Destinations are networks but could be

individual host

12

The router’s data base

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

Periodically, Send to others update

messages about your data base content

Take care to keep your data base

updated

14

So, what is “good” path ?

Goodness is determined according to

the value of the “metric” (1..15)

In simple networks : “metric” simply

counts the number of routers a

message must go through

In more complex : May consider delay,

cost and others

15

Calculating the minimal metric

Based on Bellman-Ford Algorithm

Formally :

– D(i,i) = 0 all i

– D(i,j) = mink [d(i,k) + D(k,j)] otherwise

* D(i,j) represents the metric of the best rout from I to j

Algorithm will converge in finite time

Assuming no topology change occurred

16

Calculating the minimal metric (Cont..)

Data is adaptively updated - No need to

keep the whole estimates

Only routers participate in the game -

No need for individual hosts information

Worse metric that comes from the next

router, should be considered

17

Calculating the minimal metric (Cont..)

R1

R2

R3N3 R2 5

Example :

N1

N2

N3

18

So Far So Good !! But …

The discussion assumes fixed topology

In practice routers and lines often fail

Algorithm needs modifications

19

Dealing with changes in topology

Main problem : If a router crashes, it

has no way to notify it’s neighbors

Solution : time-out paradigm

Details depend upon the protocol itself

In RIP : Send update messages to your

neighbors every 30 seconds

20

Dealing with changes in topology (Cont..)

If R1 doesn’t hear from R2 for 180

seconds,R2 is marked invalid

R1 notifies its neighbors the R2 is

unreachable

Unreachable metric = 16

21

Preventing Instability

Main problem : Mathematics proves that

the algorithm converges in finite time,

but it doesn’t tell how long does it take it

to converge !!!

“Count to infinity” method

Choose “infinity” value to be 16.

22

Preventing Instability (Cont..)

A B

C

D

Cost=1

Cost=1

Cost=1

Cost=1

Cost=10

Example

N Cost=1

23

Preventing Instability (Cont..)

A B

C

D

Cost=1

Cost=1

Cost=1

Cost=10

Example

N Cost=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

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Preventing Instability (Cont..)

If network becomes inaccessible, we

want to stop counting as soon as

possible

“infinity” was chosen to be as small as

possible

25

Preventing Instability (Cont..)

“Simple Split Horizon Scheme”

– Omit routes learned from one neighbor in

updates sent to that neighbor

“Split Horizon With Poisoned Reverse”

– Include such routs in update but set their

metric to infinity

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Protocol Characteristics

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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)

Must be Zero (4)

Metric (4)

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Handling messages 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

Authentication scheme uses the space of an entire RIP entry

Command (2) Unused (2)

0xFFFF

Authentication (16)

Version (2)

Authentication type (2)

32

Compatibility

Implemented by compatibility switch 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