Dynamic Routing Protocols II OSPF

1

Relates to Lab 4. This module covers link state routing and the Open Shortest Path First (OSPF) routing protocol.

Dynamic Routing Protocols IIOSPF

2

Distance Vector vs. Link State Routing

• With distance vector routing, each node has information only about the next hop:

• Node A: to reach F go to B• Node B: to reach F go to D• Node D: to reach F go to E• Node E: go directly to F

• Distance vector routing makespoor routing decisions if directions are not completelycorrect (e.g., because a node is down).

• If parts of the directions incorrect, the routing may be incorrect until the routing algorithms has re-converged.

AA BB CC

DD EE FF

3

Distance Vector vs. Link State Routing

• In link state routing, each node has a complete map of the topology

• If a node fails, each node can calculate the new route

• Difficulty: All nodes need to have a consistent view of the network

AA BB CC

DD EE FF

A B C

D E F

A B C

D E F

A B C

D E F

A B C

D E F

A B C

D E F

A B C

D E F

4

Link State Routing: Properties

• Each node requires complete topology information• Link state information must be flooded to all nodes• Guaranteed to converge

5

Link State Routing: Basic princples

1. Each router establishes a relationship (“adjacency”) with its neighbors

2.Each router generates link state advertisements (LSAs) which are distributed to all routers

LSA = (link id, state of the link, cost, neighbors of the link)

3. Each router maintains a database of all received LSAs (topological database or link state database), which describes the network as a graph with weighted edges

4. Each router uses its link state database to run a shortest path algorithm (Dijikstra’s algorithm) to produce the shortest path to each network

6

Operation of a Link State Routing protocol

ReceivedLSAs

IP Routing Table

Dijkstra’s

Algorithm

Link StateDatabase

LSAs are flooded to other interfaces

7

Dijkstra’s Shortest Path Algorithm for a Graph

Input: Graph (N,E) with N the set of nodes and E the set of edges

dvw link cost (dvw = infinity if (v,w) E, dvv = 0)

s source node.

Output: Dn cost of the least-cost path from node s to node n

M = {s};

for each n M Dn = dsn;

while (M all nodes) do Find w M for which Dw = min{Dj ; j M};Add w to M;for each n M

Dn = minw [ Dn, Dw + dwn ];Update route;

enddo

8

OSPF

• OSPF = Open Shortest Path First• The OSPF routing protocol is the most important link state

routing protocol on the Internet• The complexity of OSPF is significant

• History:– 1989: RFC 1131 OSPF Version 1 – 1991: RFC 1247 OSPF Version 2– 1994: RFC 1583 OSPF Version 2 (revised)– 1997: RFC 2178 OSPF Version 2 (revised)– 1998: RFC 2328 OSPF Version 2 (current version)

9

Features of OSPF

• Provides authentication of routing messages• Enables load balancing by allowing traffic to be split evenly

across routes with equal cost• Type-of-Service routing allows to setup different routes

dependent on the TOS field• Supports subnetting• Supports multicasting• Allows hierarchical routing

10

Example Network

10.1.1.0 / 24

.1 .2 .2

10.10.10.1

10.1.4.0 / 24

.1

.4

10.1.7.0 / 24

10.1

.6.0

/ 24

10.1

.3.0

/ 24

10.1.5.0/24.3

.3 .5

.2

.3

.5

.5

.4

.4

.6

.6

10.10.10.2 10.10.10.4 10.10.10.6

10.10.10.2 10.10.10.5

Router IDs are selected independent of interface addresses

3

4 2

5

1

1

32

•Link costs are called Metric

• Metric is in the range [0 , 216]

• Metric can be asymmetric

11

10.1.1.0 / 24

.1 .2 .2

10.10.10.1

10.1.4.0 / 24

.1

.4

10.1.7.0 / 24

10.1

.6.0

/ 24

10.1

.3.0

/ 24

10.1.5.0/24.3

.3 .5

.2

.3

.5

.5

.4

.4

.6

.6

10.10.10.2 10.10.10.4 10.10.10.6

10.10.10.3 10.10.10.5

Link State Advertisement (LSA)

• The LSA of router 10.10.10.1 is as follows:

• Link State ID: 10.10.10.1 = Router ID

• Advertising Router: 10.10.10.1 = Router ID

• Number of links: 3 = 2 links plus router itself

• Description of Link 1: Link ID = 10.1.1.1, Metric = 4

• Description of Link 2: Link ID = 10.1.2.1, Metric = 3

• Description of Link 3: Link ID = 10.10.10.1, Metric = 0

3

4

2

Each router sends its LSA to all routers in the network(using a method called reliable flooding)

12

Network and Link State Database

10.1.1.0 / 24

.1 .2 .2

10.10.10.1

10.1.4.0 / 24

.1

.4

10.1.7.0 / 24

10.1

.6.0

/ 24

10.1

.3.0

/ 24

10.1.5.0/24.3

.3 .5

.2

.3

.5

.5

.4

.4

.6

.6

10.10.10.2 10.10.10.4 10.10.10.6

10.10.10.2 10.10.10.5LS Type Link StateID Adv. Router Checksum LS SeqNo LS Age

Router-LSA 10.1.10.1 10.1.10.1 0x9b47 0x80000006 0

Router-LSA 10.1.10.2 10.1.10.2 0x219e 0x80000007 1618

Router-LSA 10.1.10.3 10.1.10.3 0x6b53 0x80000003 1712

Router-LSA 10.1.10.4 10.1.10.4 0xe39a 0x8000003a 20

Router-LSA 10.1.10.5 10.1.10.5 0xd2a6 0x80000038 18

Router-LSA 10.1.10.6 10.1.10.6 0x05c3 0x80000005 1680

Each router has a database which contains the LSAs from all other routers

13

Link State Database

• The collection of all LSAs is called the link-state database• Each router has an identical link-state database

– Useful for debugging: Each router has a complete description of the network

• If neighboring routers discover each other for the first time, they will exchange their link-state databases

• The link-state databases are synchronized using reliable flooding

14

OSPF Packet Format

OSPF MessageIP header

Body of OSPF MessageOSPF MessageHeader

Message TypeSpecific Data

LSA LSALSA ...

LSAHeader

LSAData

...

Destination IP: neighbor’s IP address or 224.0.0.5 (ALLSPFRouters) or 224.0.0.6 (AllDRouters)

TTL: set to 1 (in most cases)

OSPF packets are not carried as UDP payload!OSPF has its own IP protocol number: 89

15

OSPF Packet Format

source router IP address

authentication

authentication

32 bits

version type message length

Area ID

checksum authentication type

Body of OSPF MessageOSPF MessageHeader

2: current version is OSPF V2

Message types:1: Hello (tests reachability)2: Database description3: Link Status request4: Link state update5: Link state acknowledgement

ID of the Area from which the packet originated

Standard IP checksum taken over entire packet

0: no authentication1: Cleartext password2: MD5 checksum(added to end packet)

Authentication passwd = 1: 64 cleartext password Authentication passwd = 2: 0x0000 (16 bits)

KeyID (8 bits) Length of MD5 checksum (8 bits) Nondecreasing sequence number (32 bits)

Prevents replay attacks

16

OSPF LSA Format

Link State ID

link sequence number

advertising router

Link Age Link Type

checksum length

Link ID

Link Data

Link Type Metric#TOS metrics

LSA

LSAHeader

LSAData

Link ID

Link Data

Link Type Metric#TOS metrics

LSA Header

Link 1

Link 2

17

Discovery of Neighbors

• Routers multicasts OSPF Hello packets on all OSPF-enabled interfaces.

• If two routers share a link, they can become neighbors, and establish an adjacency

• After becoming a neighbor, routers exchange their link state databases

OSPF Hello

OSPF Hello: I heard 10.1.10.2

10.1.10.1 10.1.10.2

Scenario:Router 10.1.10.2 restarts

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Neighbor discovery and database synchronization

OSPF Hello

OSPF Hello: I heard 10.1.10.2

Database Description: Sequence = X

10.1.10.1 10.1.10.2

Database Description: Sequence = X, 5 LSA headers = Router-LSA, 10.1.10.1, 0x80000006 Router-LSA, 10.1.10.2, 0x80000007 Router-LSA, 10.1.10.3, 0x80000003 Router-LSA, 10.1.10.4, 0x8000003a Router-LSA, 10.1.10.5, 0x80000038 Router-LSA, 10.1.10.6, 0x80000005

Database Description: Sequence = X+1, 1 LSA header= Router-LSA, 10.1.10.2, 0x80000005

Database Description: Sequence = X+1

Sends empty database description

Scenario:Router 10.1.10.2 restarts

Discovery of adjacency

Sends database description. (description only contains LSA headers)

Database description of 10.1.10.2Acknowledges

receipt of description

After neighbors are discovered the nodes exchange their databases

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Regular LSA exchanges

10.1.10.1 10.1.10.2

Link State Request packets, LSAs = Router-LSA, 10.1.10.1, Router-LSA, 10.1.10.2, Router-LSA, 10.1.10.3, Router-LSA, 10.1.10.4, Router-LSA, 10.1.10.5, Router-LSA, 10.1.10.6,

Link State Update Packet, LSA = Router-LSA, 10.1.1.6, 0x80000006

Link State Update Packet, LSAs = Router-LSA, 10.1.10.1, 0x80000006 Router-LSA, 10.1.10.2, 0x80000007 Router-LSA, 10.1.10.3, 0x80000003 Router-LSA, 10.1.10.4, 0x8000003a Router-LSA, 10.1.10.5, 0x80000038 Router-LSA, 10.1.10.6, 0x80000005

10.1.10.2 explicitly requests each LSA from 10.1.10.1

10.1.10.1 sends requested LSAs 10.1.10.2 has more

recent value for 10.0.1.6 and sends it to 10.1.10.1(with higher sequence number)

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Routing Data Distribution

• LSA-Updates are distributed to all other routers via Reliable Flooding

• Example: Flooding of LSA from 10.10.10.110.10.10.1 10.10.10.2 10.10.10.4 10.10.10.6

10.10.10.2 10.10.10.5

LSA

LSA

Updatedatabase

Updatedatabase

ACK

AC

K

LSA

LS

A

LSA

LS

A AC

K

AC

K

ACK

ACK

LSA

LS

A

LSA

LS

A

Updatedatabase

Updatedatabase

ACK

AC

K

ACK

AC

K

Updatedatabase

21

Dissemination of LSA-Update

• A router sends and refloods LSA-Updates, whenever the topology or link cost changes. (If a received LSA does not contain new information, the router will not flood the packet)

• Exception: Infrequently (every 30 minutes), a router will flood LSAs even if there are not new changes.

• Acknowledgements of LSA-updates:• explicit ACK, or• implicit via reception of an LSA-Update

• Question: If a new node comes up, it could build the database from regular LSA-Updates (rather than exchange of database description). What role do the database description packets play?

Relates to Lab 4. This module covers additional details on the Open Shortest Path First (OSPF) routing protocol.

Dynamic Routing Protocols IIIMore OSPF

Functional Requirements of OSPF

• Fast convergence and low consumption of network resources• A descriptive routing metric

– Configurable – Value ranges between 1 and 65,535– No restriction on network diameters (RIP has a limit of 15)

• Equal-cost multipath– A way to do load balancing

Functional Requirements of OSPF

• Routing Hierarchy– Support large routing domains

• Separate internal and external routes• Support of flexible subnetting schemes

– Route to arbitrary [address,mask] combinations using variable length subnet masks (VLSMs)

• Security• Type of Service Routing

OSPF Basics(The Essence)

• Distributed, replicated database model– Describes complete routing topology

• Link State Advertisements (LSAs, sometimes called Link State Announcements)– Carry local piece of routing topology

• Distribution of LSAs using reliable flooding• Link state database

– Identical for all the routers

OSPF Packet Format

OSPF MessageIP header

Body of OSPF MessageOSPF MessageHeader

Message TypeSpecific Data

LSA LSALSA ...

LSAHeader

LSAData

...

Destination IP: neighbor’s IP address or 224.0.0.5 (ALLSPFRouters) or 224.0.0.6 (AllDRouters)

TTL: set to 1 (in most cases)

OSPF packets are not carried as UDP payload!OSPF has its own IP protocol number: 89

OSPF Packet Format

source router IP address

authentication

authentication

32 bits

version type message length

Area ID

checksum authentication type

Body of OSPF MessageOSPF MessageHeader

2: current version is OSPF V2

Message types:1: Hello (tests reachability)2: Database description3: Link Status request4: Link state update5: Link state acknowledgement

ID of the Area from which the packet originated

Standard IP checksum taken over entire packet

0: no authentication1: Cleartext password2: MD5 checksum(added to end packet)

Authentication passwd = 1: 64 cleartext password Authentication passwd = 2: 0x0000 (16 bits)

KeyID (8 bits) Length of MD5 checksum (8 bits) Nondecreasing sequence number (32 bits)

Prevents replay attacks

OSPF LSA Format

Link State ID

link sequence number

advertising router

Link Age Link Type

checksum length

Link ID

Link Data

Link Type Metric#TOS metrics

LSA

LSAHeader

LSAData

Link ID

Link Data

Link Type Metric#TOS metrics

LSA Header

Link 1

Link 2

LSAs (1)

• Identifying LSAs– LS type field– Link State ID field

• Mostly carries addressing information• E.g. IP address of externally reachable network

– Advertising Router field• Originating router’s OSPF router ID

LSAs (2)

• Identifying LSA instances– Needed to update self-originated LSAs– LS Sequence Number field

• 32 bit values• Monotonically increasing until some max value• 600 years to roll over!• LSA checksum and LS Age guard against potential

problems

LSAs (3)

• Verifying LSA contents– LS Checksum field

• Computed by the originating router and left unchanged thereafter

• LS age field not included in checksum• Removing LSAs from databases

– LS Age field• Ranges from 0 to 30 min.• Max Age LSAs used to delete outdated LSAs

LSAs (4)

• Other LSA Header fields– Options field

• Sometimes used to give special treatment during flooding or routing calculations

– Length field• Includes LSA header and contents• Ranges from 20-65535 bytes

Link State Database

• Collection of all OSPF LSAs• Databases exchanged between neighbors• Synchronization thru reliable flooding• Gives the complete routing topology• Each OSPF router has identical link-state database

Reliable Flooding

• Robustness– Updates flooded over all the links , so failure of any link

does not affect database synchronization– LSAs refreshed every 30 minutes– LSA checksum field detects corruption– Flooding loops avoided by LS Age field– MinLSInterval limits rate of LSA origination– Receivers can refuse to accept LSA updates if they

received an update less than a second ago

Routing Calculations

• Link costs configurable by administrator• Smaller values for more preferred links• Must make sense to add link costs• Different costs for each link direction possible• Dijkstra’s shortest path algorithm

– Incrementally calculates tree of shortest paths– Each link in the network examined once– Computes multiple shortest paths (equal-cost multipath)

IP Multicast to Send/Receive Changes

• Multi-Access networks– All routers must accept packets sent to the AllSPFRouters

(224.0.0.5) address– All Designated Router (DR) and Backup Designated Router

(BDR) must accept packets sent to the AllDRouters (224.0.0.6) address

• Hello packets are sent to the AllSPFRouters address (Unicast for point-to-point and virtual links)

Hierarchical Routing

• Technique used to build large networks• Minimizes consumption of network resources:

– Router memory– Router computing resources– Link bandwidth

• Flat Routing: linear increase in routing table size• Hierarchical Routing: size increases logarithmically

An Example of Hierarchical Routing (1)

10.0.3

10.3.110.3.2

10.1.3

10.1.1 10.1.2

10.2.3

10.2.110.2.2

10.0.0.0/8

10.1.0.0/16

10.3.0.0/16

10.2.0.0/16

An Example of Hierarchical Routing (2)

• Consider a router in 10.1.1• Assume 16 entries in each of the first level partitions• With flat routing, 9*16 = 144 entries/router• With 3 level hierarchy, the router has 16 entries

within 10.1.1.0/24 + entries for 10.1.2.0/24, 10.1.3.0/24,10.2.0.0/16 and 10.3.0.0/16 for a total of 20 entries.

• Significant reduction in routing table size• But might lead to suboptimal routing

• Two-level hierarchical routing scheme through the use of areas

• Areas identified by 32-bit id• Each area has its own link state database

which is a collection of network-LSAs and router-LSAs

• Area’s topology hidden from all other areas• Interconnection of areas through area border

routers (ABRs) • ABR leaks IP addressing information to other

areas through summary LSAs

OSPF Areas

OSPF Areas

• Reduction in link state databases of an area• Reduction in amount of flooding traffic needed for

synchronization• Reduction in the cost of the shortest path

calculations• Increased robustness

Area Organization

• All the areas are connected to area 0.0.0.0 also called the Backbone Area

• Need not have a direct physical connection though– Virtual links provide logical link to backbone– Summary LSAs tunneled across non backbone areas

• Exchange of routing information between areas using Distance Vector Protocol– Absence of redundant paths between areas– Not subject to convergence problems

OSPF Areas

• Group of nodes/networks

• Per area topology DB– Invisible outside the area– Reduces routing traffic

• Backbone Area is contiguous– All others areas must connect to

the backbone

• Virtual Links Area 1Area 4

Area 0

Backbone Area

Area 2 Area 3

Router Classification

• Internal Router (IR)• Area Border Router

(ABR)• Backbone Router (BR)• Autonomous System

Border Router (ASBR)Area 1

IR/BR

Area 0

Area 2 Area 3

IR

ABR/BR

To another AS

ASBR

OSPF Route Types

Intra-Area Route– All routes within an area

Inter-Area Route– Routes announced from area to

another by an ABR

External Route– Routes imported into OSPF from

another protocol or Static routes

Area 0Area 2 Area 3

ABR

To Another AS

ASBR

Inter-Area Route Summarization

• Prefix or all subnets• Prefix or all networks

1.A 1.B 1.C

FDDI

Dual Ring

R1 (ABR)

R2

Network

1

Next Hop

R1

Network

1.A

1.B

1.C

Next Hop

R1

R1

R1

With

Summarization

Without

Summarization

Backbone

Area 0

Area 1

External Routes

• Redistributed into OSPF• Flooded without changes throughout the AS• OSPF supports two type of external metrics

– Type 1– Type 2 (Default)

RIP

IGRP

EIGRP

BGP

etc.

OSPF

Redistribute

Topology/Links-State DB

• A router has a separate DB for each area it belongs to

• All routers within an area have an identical DB

• SPF calculation is done separately for each area

• LSA flooding is limited to the particular area

Protocol Functionality

• Bringing up adjacencies• LSA Types• Area Classification

The Hello Protocol

• Responsible to establish and maintain neighbor relationships

• Elects designated router in multi-access networks

FDDI

Dual Ring

Hello

HelloHello

Designated Router (DR)

One per multi-access network

Generates network links advertisements

Assists in DB synchronization

Designated

Router

Backup

Designated

Router

Designated Router by Priority

• Configured priority (per interface)

• Otherwise determined by the highest router ID

144.254.3.5

R2 Router ID = 131.108.3.3

131.108.3.2 131.108.3.3

R1 Router ID = 144.254.3.5

DR

Neighbor States

• 2-way– The router sees itself in other Hello packets

– DR is selected from neighbors in state 2-way or greater

DR BDR

2-way

Neighbor States

• Full– Routers are fully adjacent– DB is synchronized– Relationship to the DR and

BDR

DR BDR

Full

When to Become Adjacent

• Underlying network is point-to-point• Underlying network type is virtual link• The router itself is the DR• The router itself is the BDR• The neighboring router is the DR• The neighboring router is the BDR

LSAs Propagate Along Adjacencies

• LSAs acknowledged along adjacencies

DR BDR

Different Types of LSAs

• Five LSA types– Type 1 : Router LSA– Type 2 : Network LSA– Type 3 y 4: Summary LSA– Type 5 y 7: External LSA

Router LSA (Type 1)

• Describes the state and cost of the router’s link to the area

• All the router’s links in an area must be described in a single LSA

• Flooded throughout the particular area and not beyond

• Router indicates whether it is an ASBR, ABR, or the end point of a virtual link

Network LSA (Type 2)

• Generated for every transit broadcast or NBMA network

• Describes all the routers attached to the network

• Only the DR originates this type of LSA• Flooded throughout the area and not beyond

Summary LSA (Type 3 y 4)

• Describes a destination outside the area but still within the AS

• Flooded throughout a single area• Originated by an ABR• Only intra-area routes are advertised into

the backbone (Area 0)• Type 4 is the information about the ASBR

External LSA (Type 5)

• Defines routes to destinations outside the AS• Default route is also sent as external• Two Types of external LSA:

• E1: Considers the total cost of to the external destination• E2: Considers only the cost of the outgoing interface to the

external destination

Issues not covered

• OSPF Network Types– Broadcast subnets– NBMA Subnets

• OSPF Extensions• Multicast Routing using OSPF (MOSPF)• OSPF Management• and a whole lot of others!

Further Reading

• John T. Moy, OSPF - An Anatomy of an Internet Routing Protocol

• Christian Huitema, Routing in the Internet• RFC 2178