Information Network 1 Routing (1) - NAIST · OSPF: Open Shortest Path First The link state routing...

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Information Network 1

Routing (1)

Youki Kadobayashi

NAIST

©2013 Suguru Yamaguchi and Youki Kadobayashi, All rights reserved.

Image: Part of the entire Internet topology based on

CAIDA dataset, using NAIST Internet viewer

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The Routing Problem

How do I get from source to destination?

Which path is best? In terms of:

Number of hops

Delay, bandwidth

Policy constraints, cost…

Who will make decision?

Router?

Source?

How can we detect failures?

How much the overhead will be?

S D


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The Routing Problem

How do I get from source to destination?

Which path is best? In terms of:

Number of hops

Delay, bandwidth

Policy constraints, cost…

Who will make decision?

Router?

Source?

How can we detect failures?

How much the overhead will be?

S D

Rogue ISP

500ms 500ms

unreliable


Solution design space for the routing problem

Represent network in:

a graph, or

a matrix

Collect information:

across the network, or

toward some routers, or

only locally among neighbors

Compute route at:

every router, or

some routers

… solutions are instantiated in routing systems.

4 ©2013 Suguru Yamaguchi and Youki Kadobayashi, All rights reserved.

Characterization of routing systems

Static routing

Compute route a priori

Dynamic routing

Reflect dynamic state of network

Source-based routing

Source node computes path to destination

Hop-by-hop routing

Every node computes next hop

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S D


Focus of this lecture: dynamic, hop-by-hop routing

Static routing

Compute route a priori

Dynamic routing

Reflect dynamic state of network

Source-based routing

Source node computes path to destination

Hop-by-hop routing

Every node computes next hop


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Routing system: its function

Provision end-to-end reachability

Automatically compute best path

Distribute traffic among multiple links

Avoid failing links

Isolate faults

Reflect administrative policies

… in one system. Isn’t it awesome.


Routing system characterization

Representation of network

Network topology

Attributes associated with each link

Exchange of information

Communication overhead

Propagation speed

Computation algorithm

Computation overhead

Convergence speed

Routing system: protocol + information + algorithm


On network topology

Topology: geometric configurations that are unaltered

by elastic deformations

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1

2

3

1

3

2

1 2 3

1

2

3

∞ 1 3

1 ∞ 2

3 2 ∞

Graph representation Matrix representation


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Routing system: its structure

Routing protocol

discovers neighbor router;

exchanges topology information;

exchanges link information

Routing algorithm

computes route (result: RIB - Routing Information Base)

Integrate information from multiple routing protocols

Multiple routing protocols → Multiple RIBs

Consolidate Multiple RIBs into single FIB (FIB: Forwarding Information Base)


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Gateway Model: a conceptual model of routing system

FIB

Input interfaces Output interfaces

Multiple RIBs

Routing software Topology info, Link status info

Topology info, Link status info


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Types of Routing Algorithm

1. Distance vector

2. Link state

3. Path vector

Key difference of these algorithms:

Topology representation

Propagation range / frequency / timing of link state

Algorithm for computing shortest path

Design trade-offs:

Scalability

Convergence time

Algorithm simplicity


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Questions?


Distance Vector Routing

©2013 Suguru Yamaguchi and Youki Kadobayashi, All rights reserved. 14

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Distance vector routing w/ Bellman-Ford algorithm

1. assign distance vector to myself 0; for others, assign ∞

2. send my distance vectors to all neighbor routers

3. router calculates minimum distance vectors by 1) distance vectors advertised by neighbor routers and 2) distance from myself to individual neighbor router

Initial condition bi

(0) ← ∞ (i ≠ D)

bD(0) ← 0

Repetition

bi(m) ← minj ∈ Vi{bj

(m-1) + dij}

Terminal condition bi

(m) = bi(m-1)

D: destination router, bi(m): distance between router “i” and “D” by m-times iterative

calculation, Vi: neighbor routers of router “i”, dij: distance of link between router “i” and j

DN p.397


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RIP: Routing Information Protocol

Distance vector routing

RIP-2 (IPv4), RIPng (IPv6)

Used in relatively small network

• Due to ease of implementation and operation

Textbook classics

RFC 2453, RFC 2080


Informal description of RIP

Each router sends a list of distance-vectors (route, cost) to each neighbor periodically

Every router selects the route with smallest metric

Metric: integer of 1..16, where 16 implies infinity


A B C cost: 2 cost: 1

Router metric

A 3

B 2

Router metric

A 1

C 2

Router metric

B 1

C 3

Hands on: see how RIP works

(description of GNS3)

(basic instruction to keep RIP up and running)


GNS3 Description

An open source software that simulate complex

networks

Devices list

View zone

Topology summary

Console

IOS Image Setting Edit -> ISO images and hypervisors

Select a image

Cisco Router: Basic Configuration

• enable

• configure terminal

• interface name_of_the_inteface

• ip address ip_address mask

• no shutdown

• show ip interface brief

RIPv2 Basic Configuration

In configuration mode:

(config)#router rip

(config)#version 2

(config)#network network_address

Show Running Config

Troubleshooting Ripv2 show ip route

show ip protocols

Hands on task: try to expand your RIP network

Add 3rd router to your RIP network

Example topology:


1

2

3

1 3 2

Adding A New Router

Router Configuration

Right click R3

R3 Configuration

IP address configuration

RIP configuration at R

Save Configuration

After

Configuration

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Problems of distance vector routing

Poor scalability. For the given number of routers N,

Time complexity: O(N3)

Traffic: O(N2)

Slow convergence speed, as it sends distance vector

periodically

Slow convergence induces inconsistent and transitional state

=> Counting to infinity problem


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Counting to infinity problem

A C B

Suppose link B-C went down.

B thinks: (C, inf)

A says: (C, 2)

B thinks: (C, 3) via A

B says: (C, 3)

A thinks: (C, 4) via B

…

1 1 → inf


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Split Horizon

A C B


B thinks: (C, inf)

A says: (C, 2) to everyone except B

…

1 1 → inf

Workaround for counting to infinity

Router doesn’t send learned information to source

router


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Limitation of split horizon

A C B


B thinks: (C, inf)

A says: (C, 2) to everyone except B

D thinks: (C, 3) via A

D says: (C, 3)

B thinks: (C, 4) via D

…

1 1 → inf

D

1 1


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Questions?

(Dumb questions welcomed while you’re a student)

Extra hands-on if you are further motivated:

Introduce link failure and observe counting-to-

infinity problem

Add 4th router as in the last slide and observe that

split-horizon does not save us


Lessons from RIP

Bellman-Ford: very simple

But with many traps and pitfalls… as in A. Koenig, “C Traps and Pitfalls”

Guiding principle for using RIP: avoid loops!

Loops can be easily formed, however.

Backup links

Guiding principle can be easily forgotten • OMG!

A better alternative: link state routing.


Do you know?

Link-State Routing


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Link-state routing

Collect router and link information

→ Directed graph: router as a node, link as an arc

… then create link state database (LSDB)

• A network map that collects link state information

Based on LSDB, calculate the shortest path with the

Dijkstra’s shortest path algorithm

Time complexity: O(N2)

• Can be further optimized by improved data structure


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A high level view

Router

Link

Router

Link

LSDB RIB Dijkstra

directed graph shortest path tree

fragments of directed graph


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Graph representation of routers and links

Source: OSPF Version 2, RFC 1583 ©2013 Suguru Yamaguchi and Youki Kadobayashi, All rights reserved.

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A more complex

network


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and its directed graph

representation


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Shortest path tree;

rooted at RT6


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Questions?


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From directed graph to shortest-path trees:

Dijkstra Algorithm

Initially, ds ← 0 dj ← csj (for every j ∈ V – {s}) P ← {s}

Find the next closest node:

di ← minj ∈ V – P {dj} P ← P ∪ {i}

Update labels: dk ← mink ∈ V – P {dk, di + cik}

Terminal condition: P = V

V: all routers, s: starting router, cik: link cost between “i” and “k”, dj: least cost between “s” to “j”, P: router with least cost determined


Interconnections p.223

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Pros and Cons of Link-State Routing

Traffic increases in proportion to the number of links and

routers

Storage complexity increases in proportion to the number of

links and routers

Time complexity: lower than distance vector routing

Convergence time: must be short, for transient loop issue

Counting to infinity doesn’t happen

(read: fewer traps and pitfalls)

Flexible configuration of link cost is possible

(no nonsense like 16 = infinity)


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OSPF: Open Shortest Path First

The link state routing protocol for the Internet

OSPFv2 (IPv4), OSPFv3 (IPv6)

Functions

Recognize neighbor router

Exchange link state information and create LSDB

Calculate shortest path tree (spanning tree)

+ Designated Router, Backup Designated Router

+ Hierarchical structure by area

+ Collaboration with EGP

RFC 2328, 5340


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OSPF Hello

Discover neighbor router on the same link

• send Hello packet to 224.0.0.5, ff02::5 (AllSPFRouters)

List of neighbor routers in Hello packet

• check bidirectional communication

（select designated router and backup designated router）

Send Hello packet periodically to detect link down

keeps pinging, as in ICMP


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Adjacency formation

Neighbor → Adjacent

Routers don’t exchange routing

information unless they are adjacent

Formation process:

Hello → Neighbor

Synchronize each LSDB

Synchronized → Adjacent


Hands on: say hello in OSPF!

(basic instruction to bring OSPF up and running)

(establish OSPF adjacency)

(add 1 or 2 routers)


Basic OSPF Configuration

• router ospf process_ID

• router-id router_ID

• network network_address wildcard-mask area

area_ID

• show ip protocols

• show ip ospf

• show ip ospf interface

• show ip ospf neighbor

• show ip route

• show ip ospf database

OSPF Configuration

R3 configuration

1. Set IP address

2. Set OSPF

R1 configuration

1. Set IP address

2. Set OSPF

area 0

area 28

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Topology Representation in OSPF

LSA (Link State Advertisement)

• Type 1: Router LSA

• Type 2: Network LSA

• Type 3: Summary LSA (network)

• Type 4: Summary LSA (AS boundary)

• Type 5: AS External LSA

LSA common header

• Validity period and sequence number in LSA header

• Helps routers to tell if given LSA is fresh


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Questions?


Summary


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Gateway Model Revisited

FIB

Input interfaces Output interfaces

RIP

Multiple RIBs

OSPF

Routing software Topology info, Link status info

Topology info, Link status info


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Summary

Routing system: design space, characterization

Distance vector routing

Bellman-Ford algorithm

RIP protocol, information model

Traps and pitfalls

Link state routing

Graph and its spanning trees

Dijkstra algorithm

OSPF protocol, algorithm, information model


Assignment


Assignment:


Assignment:

Question: If the router R1 has an OSPF configuration as

follows:

Which interfaces, shown in the exhibit, will participate

in OSFP according to this configuration statement?

Please give explanations on why.


R1(config)# network 192.168.12.64 0.0.0.63 area 0

Assignment:

Instructions

Please submit the report in PDF to our e-mail

address:

[email protected]

File nomenclature: Name_StudentID

Language: Japanese or English

Submission deadline: 2013/05/23(Thursday) at

23:59


Information Network 1 Routing (1) - NAIST · OSPF: Open Shortest Path First The link state routing...

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