Routing Concept

Sirak Kaewjamnong Computer Network Tech and Security

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Agenda

• Basic concepts

• Routing components

• Classes of routing protocol

• Internet routing protocol

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What’s Routing

Routing - path finding from one end to the other

• Routing occurs at layer 3• Bridging occurs at layer 2

Physical link

Data link layer

Network layer

Transport layer

Session layer

Presentation layer

Application layer

Physical link

Data link layer

Network layer

Transport layer

Session layer

Presentation layer

Application layer

Physical link

Data link layer

Network layer

Network A Network B

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IP Routing

• IP performs:• search for a matching host address

• search for a matching network address

• search for a default entry

• Routing done by IP router, when it searches the routing table and decide which interface to end a packet out.

incoming

which interface ?

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Routing Tables

• Routing is carried out in a router by consulting routing table.

• No unique format for routing tables, typically table contains:– address of a destination

– IP address of next hop router

– network interface to be used

– subnet mask for the this interface

– distance to the destination

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Routing Component

• Three important routing elements :– algorithm– database– protocol

• Algorithm : can be differentiate based on several key characteristics

• Database : table in routers or routing table• Protocol: the way information for routing to be

gathered and distributed

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Routing Algorithm

Design goals• Optimality - compute the best route• Simplicity/low overhead - efficient with a

minimum software and utilization overhead • Robustness/stability- perform correctly in the face

of unusual circumstances• Rapid convergence- responds quickly when the

network changes• Flexibility- accurate adapt to a variety of network

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Routing Protocols

• Routing protocol : protocol to exchange of information between routers about the current state of the network

• Routing protocol jobs– create routing table entries– keep routing table up-to-date – compute the best choice for the next hop router

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Routing Metrics

• How do we decide that one route is better than another?

• Solution : using a metric as a measurement to compare routes

• Metrics may be distance, throughput, delay, error rate, and cost.

• Today, IP supports Delay, Throughput, Reliability and Cost (DTRC)

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Hop Count

• A hop is defined as a passage through one router

R1 R2

R3

1 hop 1 hop

1 hop 1 hop

2 hops

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Routing Algorithm Types

• Static V.S. Dynamic

• Source routing V.S. Hop-by-hop

• Centralize V.S. Distributed

• Distance vector V.S. Link state

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Routing Algorithm: Static Route

• Manually configuration routing table• Can’t react dynamically to network change such

as router’s crash• Work well with small network or simple topology• Unix hosts use command route to add an entry

point to point connection

route to this way only, no needfor update

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Routing Algorithm: Static Technique

Flooding• Every incoming packet is sent out every outgoing• Retransmit on all outgoing at each node• Simple technique, require no network information• Generate vast numbers of duplicate packet

incoming

flooding

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Routing Algorithm: Dynamic Route

Dynamic route• Network protocol adjusts automatically for

topology or traffic changes• Unix hosts run routing daemon routed or gated

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Routing Algorithm: Dynamic Route operation

• Routing protocol maintains and distributes routing information

Update Routing Information

Routing Table

Routing Table

Routing Protocol

Routing Protocol

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Routing Algorithm: Source Routing

• Source routing– Source will determine the entire route– Routers only act as store-forward devices

• Hop-by-hop– Routers determine the path based on theirs own

calculation

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Routing Algorithm: Distance Vector

• Distance means routing metric

• Vector means destination

• Flood routing table only to its neighbors

• RIP is an example

• Also known as Bellmann-Ford algorithm or Ford-Fulkerson algorithm

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Distance Vector Algorithm

• Using hop count as a metric• Each router periodically sends a copy of its

routing table to neighbors• send <network X, hopcount Y>

routing table W 0 X 0 Y 1 Z 2

routing table W 1 X 0 Y 0 Z 1

routing table W 2 X 1 Y 0 Z 0

W X Y ZR1 R2 R3

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Distance Vector Routing Update

• Step by step from router to router• Slow convergence

R1 R2 R3

topology change

Œ

recompute R3’s routing table

R3 sends out the updated table

Ž

recompute R2’s routing table

R2 sends out the updated table

recompute R1’s routing table

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Distance Vector: Broadcast (I)

• The first roundR1

R2

R3

R4

IJ

K

LM

I, 1 hopJ, 1 hop

I, 1 hopK, 1 hopL, 1 hop

N

J, 1 hopK, 1 hopM, 1 hopN, 1 hop

L, 1 hopM, 1 hopO 1 hop

O

R5

N, 1 hopO, 1 hop

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Distance Vector: Broadcast (II)

• The second roundR1

R2

R3

R4

IJ

K

LM

I, 1 hopJ, 1 hopK, 2 hopsL, 2 hopsM, 2 hopsN, 2 hops

I, 1 hopK, 1 hopL, 1 hopJ, 2 hopsM, 2 hopsN, 2 hopsO, 2 hops

N

J, 1 hopK, 1 hopM, 1 hopN, 1 hopI, 2 hopsL, 2 hopsO, 2 hops

L, 1 hopM, 1 hopO, 1 hopI, 2 hopsK, 2 hopsJ, 2 hopsN, 2 hops

O

R5

N, 1 hopO, 1 hopJ, 2 hopsK, 2 hopsM, 1 hopL, 2 hops

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Distance Vector: Broadcast (III)

• The third roundR1

R2

R3

R4

IJ

K

LM

I, 1 hopJ, 1 hopK, 2 hopsL, 2 hopsM, 2 hopsN, 2 hopsO, 3 hops

I, 1 hopK, 1 hopL, 1 hopJ, 2 hopsM, 2 hopsN, 2 hopsO, 2 hops

N

J, 1 hopK, 1 hopM, 1 hopN, 1 hopI, 2 hopsL, 2 hopsO, 2 hops

L, 1 hopM, 1 hopO, 1 hopI, 2 hopsK, 2 hopsJ, 2 hopsN, 2 hops

O

R5

N, 1 hopO, 1 hopJ, 2 hopsK, 2 hopsM, 1 hopL, 2 hopsI, 3 hops

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Distance Vector: Crashed Recovery

• R3 crashed• New complete route of R1

R1

R2 R3

R4

IJ

K

LM

N

O

R5

R1 routing table

hop via1 N/A 1 N/A2 R22 R22 R32 R33 R5

netIJKLMNO

hop via1 N/A 1 N/A2 R22 R23 R24 R23 R2

netIJKLMNO

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Count to Infinity

• R2 does not hear any thing from R3• R1 says : don’t worry, I can reach R3 in 2 hops,

R2 update hop count to 3• R1 sees R2’s update, then update itself to 4 and so

on……

R1 R2 R3

hop count to R3

2 1 initial2 3 1st round 4 3 2nd round4 5 3rd round

I JR3 crashed

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Split Horizon

• Solve by set distance “16” as infinity• No destination can be more than 15 hops away

from any other• Distance to X is not reported on the line that

packet for X are sent• Actually, it reports with infinity

R1 R2 R3I J R3 crashed

to R3to R3

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Dijkstra’s Shortest Path First Algorithm

• Routers send out update messages whenever the

state of a link changes. Hence the name: “Link

State” algorithm.

• Each router calculates lowest cost path to all

others, starting from itself.

• At each step of the algorithm, router adds the next

shortest (i.e. lowest-cost) path to the tree.

• Finds spanning tree routed on source router.

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Open Shortest Path First (OSPF)

• RIP limited in large internets

• OSPF preferred interior routing protocol for TCP/IP based internets

• Link state routing used

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Routing Algorithm: Link State

• Flood routing information to all nodes

• Each router finds who is up and flood this

information to the entire routers

• Use the link state to build a shortest path map to

everybody

• OSPF is an example

• Also known as Shortest Path First (SPF) algorithm

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Flooding• Packet sent by source router to every neighbor• Incoming packet resent to all outgoing links

except source link• Duplicate packets already transmitted are

discarded– Prevent incessant retransmission

• All possible routes tried so packet will get through if route exists– Highly robust

• At least one packet follows minimum delay route– Reach all routers quickly

• All nodes connected to source are visited– All routers get information to build routing table

• High traffic load

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Link State Overview

• Using cost as a metric• Exchange its connection and cost to its neighbors• Each router compute the set of optimum path to all

destination (Shortest Path First)

link state W 0 X 0

link state X 0 Y 0

link state Y 0 Z 0

W X Y Z

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Link State Concept

• Each router initially begins with directly connected network

• Determine full knowledge of distant routers and theirs connection

R2

R1

R3

R4

exchange link state packets

RoutingTable

Œ

build topologicaldatabase

Žcompute SPF update routing

table

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Link State Routing Update

• Send information to other routers

• Fast convergence

R2

R1

R3

R4

topology change

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OSPF Overview

• Router maintains descriptions of state of local links

• Transmits updated state information to all routers it knows about

• Router receiving update must acknowledge– Lots of traffic generated

• Each router maintains database– Directed graph

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Link Costs

• Cost of each hop in each direction is called routing metric

• OSPF provides flexible metric scheme based on type of service (TOS)– Normal (TOS) 0– Minimize monetary cost (TOS 2)– Maximize reliability (TOS 4)– Maximize throughput (TOS 8)– Minimize delay (TOS 16)

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Areas

• Make large internets more manageable• Configure as backbone and multiple areas• Area – Collection of contiguous networks

and hosts plus routers connected to any included network

• Backbone – contiguous collection of networks not contained in any area, their attached routers and routers belonging to multiple areas

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Operation of Areas

• Each are runs a separate copy of the link state algorithm– Topological database and graph of just that area– Link state information broadcast to other

routers in area– Reduces traffic– Intra-area routing relies solely on local link

state information

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Comparison

Distance Vector Link State

pass a copy of pass links state update whole routing table

add metric from calculate the shortest pathrouter to router to other routers

frequent periodic update: event updated: fast slow convergence convergence

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Internet Routing Architecture

AutonomousSystem

AutonomousSystem

Autonomous System

IGP

IGP

IGP

BGP4

BGP4

BGP4

IGP

IGP

IGP IGPIGP IGP

EGP/BGP EGP/BGP EGP/BGP EGP/BGP

EGP/BGPEGP/BGP

AutonomousSystem Autonomous

System

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Routing in the Internet

The Internet uses hierarchical routing• The Internet is split into Autonomous Systems (AS’s)

• Within an AS, the administrator chooses an Interior Gateway Protocol (IGP)

• Examples of IGPs: RIP (rfc 1058), OSPF (rfc 1247).

• Between AS’s, the Internet uses an Exterior Gateway Protocol

• AS’s today use the Border Gateway Protocol, BGP-4 (rfc 1771)

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Autonomous System

• AS is a collectionof LANs and WANs and the interconnectting routers which under the control of one management authority

• The same AS runs the same Interior Gateway Protocol

• Why setting up AS? - establish a direct link to each other rather than route through the core Internet

• How to select AS? - register and get the AS number from IAB

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Gateway Protocol

• Interior gateway protocol– exchange routing information between routers

within a single AS– RIP, RIP II, OSPF

• Exterior gateway protocol– collect network-reachablity information for the

AS– EGP, BGP

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Interior Routing Protocols• RIP

• Uses distributed Bellman-Ford algorithm.• Updates sent every 30 seconds.• No authentication.• Originally in BSD UNIX.

• OSPF• Link-state updates sent (using flooding) as and when required.• Every router runs Dijkstra’s algorithm.• Authenticated updates.• Autonomous system may be partitioned into “areas”.

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Exterior Routing ProtocolsProblems:

• Topology: The Internet is a complex mesh of different AS’s with very little structure.

• Autonomy of AS’s: Each AS defines link costs in different ways, so not possible to find lowest cost paths.

• Trust: Some AS’s can’t trust others to advertise good routes (e.g. two competing backbone providers), or to protect the privacy of their traffic (e.g. two warring nations).

• Policies: Different AS’s have different objectives (e.g. route over fewest hops; use one provider rather than another).

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Border Gateway Protocol (BGP-4)

• BGP is not a link-state or distance-vector routing protocol.

• BGP advertises complete paths (a list of AS’s).• Example of path advertisement:

• “The network 171.64/16 can be reached via the path {AS1, AS5, AS13}”.

• Paths with loops are detected locally and ignored.• Local policies pick the preferred path among

options.• When a link/router fails, the path is “withdrawn”.

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References

• http://www.cisco.com/en/US/products/hw/routers/ps274/index.html

• http://www.cisco.com/en/US/products/hw/routers/ps274/products_data_sheet09186a008010fba1.html

• Computer Networks with Internet TechnologyBy William Stallings Chapter 11 Interior Routing Protocols