IP Routing: GGP and RIP

Nov 04, 2004 CS573: Network Protocols and Standards

1

IP Routing: GGP and RIP

Network Protocols and Standards

Autumn 2004-2005

Nov 04, 2004 CS573: Network Protocols and Standards 2

IP Routing Protocols Autonomous System Interior Gateway Protocols

GGP RIP OSPF

Exterior Gateway Protocols BGP EGP

IP Multicast Routing MPLS


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IP Routing Protocols

Autonomous Systems


Routing in the Internet Routing Algorithms

Bellman-Ford Dijkstra

Routing Protocols Distance Vector Link State

Routing Hierarchy Interior Gateway Protocols (RIP, OSPF, IGRP) Exterior Gateway Protocols (EGP, BGP, CIDR, Policy

Routing) Multicasting (IGMP)


Internet from the start First, there was ARPANET

Routers had complete information about all the possible destinations – core routers

GGP (gateway-to-gateway) protocol was used for routing – a distance vector protocol

R R

RR

H

H

H


Internet from the start Then, LANs were connected to ARPANET

R RR

ARPANET

LAN LAN LAN

Core Routers


Internet from the start Problems with above configuration:

Routing overhead increased with the number of connected routers

Number of routes increased with the number of connected segments

Frequency of routing exchanges increased Higher likelihood that something went wrong

somewhere requiring updates Number of different types of routers

increased Slow deployment of new versions of routing

algorithms


Internet from the start

Backbone Network

R1

Local Network

Core Router

R2 R3

Local Network Local Network R4 Local Network


Autonomous System

R RR

Backbone Network

AS AS AS

Core Routers

AS: Autonomous System


Autonomous System What is an autonomous system?

A set of routers and networks under the same administration. Examples:

A single router directly connecting one local network to the Internet

A corporate network linking several local networks through a corporate backbone

A set of client networks served by a single ISP

NOTE: From a routing point of view, all parts of an AS must remain connected


Autonomous System Internal connectivity within the AS means:

All routers must be connected Parts of network connected through core AS

(yes, core is an AS!) cannot form an AS All routers must exchange routing information

in order to maintain the connectivity (normally achieved by using a single routing protocol)

Routers inside an AS are called “interior gateway” and the protocol they use is called Interior Gateway Protocol (IGP)


Autonomous System In 1982, the IGP of choice was GGP IGPs in use today are:

RIP OSPF IGRP

Each AS is identified by a 16-bit number

Number is assigned by the numbering authorities


Autonomous System: Benefits Routing overhead is lower Network management becomes easy Easier computation of new routes Distribution of new software versions is

easier Failing elements can be isolated easily AS use an Exterior Gateway Protocol to

exchange information about reachability


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Gateway-to-Gateway ProtocolGGP


GGP The “old” ARPANET routing protocol Defined in RFC 823 A distance-vector routing protocol

Only core routers participate in GGP GGP messages travel in IP datagrams

with protocol type = 3 GGP measures distance in router hops.

i.e., the number of hops along a path refers to the number of routers


GGP Message Types 4 types of GGP messages

GGP Routing Update message (type 12)

GGP Acknowledgment message (type 2/10)

GGP Echo Request or Reply (type 0 or 8)


GGP Routing Update A router sends this message to advertise

the destination networks it knows how to reach

To keep the size of message small, networks are grouped by distance In the message “Distance” is followed by a list

of “Net” addresses that are at this distance Contains a field that tells how many distance

groups are being reported (3 in case below) D1 – Net1, Net5, Net11 D2 – Net4, Net2, Net7, Net16 D3 – Net6, Net9


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Routing Information ProtocolRIP


Routing Information Protocol A distance vector based IGP Similar to GGP Designed at UC Berkeley Based on Xerox XNS Distributed with 4BSD UNIX (routed) First RFC was 1058, current RFC is 2453 Started off in small networks and then

extended to larger networks See Huitema, Chapter 5


RIP Details Routers are active machines

Advertise their routes (IP NET, distance) to others

Hosts are passive machines They listen and update their routes but do

not advertise RIP uses hop count metric RIP messages are transmitted using

UDP at port 520


RIP Route Computation There is a cost associated with each link

Typically cost =1 i.e., number of hops Each router receives route advertisements from

its neighbors Advertisements show distances to all destinations in

the network For each destination in the network:

The router takes each received advertisement and adds to it the cost to reach that neighbor who sent this advertisement; this gives the distance to the destination

The router selects lowest of these as path/cost to that destination


Algorithm Properties Convergence is guaranteed in a finite time

given that topology remains static Starting value of distance estimates to each

destination can be any non-negative number No assumption is made as to when the updates

are sent or when the distances are computed Each router can work based on its own clock and send

its updates asynchronously If the network changes, routes converge to a

new equilibrium point


Example

RouterAdvertisement:Distance to A is 2Distance to B is 3Distance to C is 5

Advertisement:Distance to A is 1Distance to B is 4Distance to C is 1

Advertisement:Distance to A is 2Distance to B is 1Distance to C is 3

Cost = 2

Cost = 3Cost = 1

P1

P2

P3

Distance to

Through

Destination

Port P1

Port P2 Port P3

A 3 4 4

B 4 3 7

C 6 5 4


Counting to InfinityRoutes to Target:A: route via B, distance 3B: route via D, distance 2C: route via B, distance 3D: direct, distance 1

Assume that B to D link goes down, and B notices.

A

DB

C1

1 1

1

10

1

Target

di

D

C

C

Via

1

11

12

12

Dist

1

3

-

3

Dist

diD

BC

xB

BA

ViaFrom

1

4

4

4

Dist

di

A

C

C

Via

di

A

C

C

Via

1

6

6

6

Dist

di

A

C

C

Via

1

5

5

5

Dist

…

1

11

11

11

Dist

di

A

C

C

Via

x = destination unreachable; di = directly connected

What if the link from C to D also goes down? Counting to Infinity!!!

To r

each

targ

et

…


Some Solutions Split Horizon

If A reaches a destination through B, it makes no sense for B to reach the same destination through A

Instead of broadcasting the same distance vector on all links, send different versions on each outgoing link by removing the entries for the destinations that are reachable through that link

Split Horizon with Poisonous Reverse Include all the destinations in advertisements; even

those which were missing in split horizon, but… Set those vector distances to infinity that were

missing in the simple version of split horizon


Triggered Updates Split Horizon can work in loops with two

gateways, but not with three or more See example in book by Huitema

Another solution to deal with “count to Infinity” problem is triggered updates A gateway is required to send an immediate

update when any route changes. This reduces the occurrence of loops

Flood of triggered updates resolves loops faster when these happen


RIPv2 Message Format

MUST BE ZERO

AS NUMBER

AUTHENTICATION TYPE

NEXT HOP

DISTANCE TO NET 1

168 24VERSION (2)COMMAND (1-5)

FFFF

FAMILY OF NET 1

ADDRESS OF NET 1

MASK

AUTHENTICATION HEADER

31

… … … …


Message Format

Command Meaning

1 Request for partial or full routing information

2 Response containing network-distance pairs fromsender’s routing table

3 Turn on trace mode (obsolete)

4 Turn off trace mode (obsolete)

5 Reserved for Sun Microsystems Internal Use


RIPv2 Message Format Address format is not limited to TCP/IP RIP can be used with multiple network protocol

suites Family of net i:

Identifies the protocol family under which the network address should be interpreted

IP addresses are assigned value 2 Next hop

The sending router can specify another router’s IP address as next hop for the network

Set to 0.0.0.0 for sender itself Solves similar problem (extra hop) as ICMP redirect


RIP Metrics and Updates By default, RIP uses hop count as

the distance metric Integers 1 through 15 16 denotes infinity

Packets are normally sent every 30sec

If a route is not refreshed within 180 seconds, distance is set to infinity and later entry is removed


Input Processing How to process incoming RIP

packets? Examine entries one by one Validation check

Address is valid class A, B, or C Network number is not 127 Host port is not a “broadcast” address Metric is not larger than infinity (16)

Incorrect entries are ignored And should be reported as errors


Input Processing Metric for entry is increased by link cost Routing table is searched for an entry

corresponding to the destination If the entry is not present, it is added If the entry is present but with a larger

metric Entry is updated and timer restarted

Entry is present and next hop router is sender of response message

Metric is updated and timer restarted For all other cases, entry is ignored


RIP Responses A separate response is prepared for all connected

interfaces/ports Information sent on different ports may vary due to

Split Horizon processing Subnet summarization

For triggered updates: may include only those entries that have been updated since last transmission

Maximum message size: 512 bytes (up to 25 entries)

Multiple messages have to be sent if more than 512 bytes Source IP address is that of the interface on which the

message is sent Destination IP address is the broadcast address

Documents

IP Routing: GGP and RIP