Open Shortest Path First(OSPF)

Presenter: Lilian Atieno

Course: ECE 697C (Fall 2002)

Instructor: Professor Linix Gao

Outline Introduction OSPF routing Protocol Overview

– Features– Processing of LSAs and data– OSPF packet Format

Black-box OSPF delay measurements– Testbed setup & Procedure– Actual experimental results

Conclusions

Introduction

OSPF is an internal Gateway Protocol– Determines how routing is done in an AS

Primary Characteristics– I)Protocol is open

• Specifications published in RFC

– ii)It’s a Link-state routing protocol• router broadcasts routing information to other

routers and so each router has complete view of topology

OSPF overview

Protocol overview

Internal details of an AS is invisible to other ASs

Each OSPF router maintains database of its AS topology.

Router builds shortest path tree. Routers obtain forwarding table (FIB)

from the tree. OSPF is a dynamic routing protocol

AS Splitting

OSPF allows AS to be configured into areas– Each area runs its own OSPF link-state routing

algorithm.• Router broadcasts its link state only to area routers

Topology of an area invisible to other areas Adv. of splitting:

– reduction of routing traffic

An OSPF AS Consists of Multiple Areas

Classification of routers

I)Internal routers• only perform intra AS routing

II)Area Border routers• Belong to both area and backbone

III)Backbone routers• Perform routing within the backbone

IV)Boundary routers• Exchange routing info. With routers in other Ass.

Backbone area

One area in the AS is configured to be backbone area.

• Consists of Area boarder routers (ABRs) and other non-ABRs.

• It must be contiguous otherwise virtual links have to be configured to restore connectivity.

• It routes traffic between areas in the AS.

Backbone topology invisible to intra- area routers.

Stub area

It’s one of the enhancements in OSPF protocol.

These are areas configured not be flooded by external advertisements.

All data traffic to external destinations will follow default route

Types of OSPF routing

I)Inter-area routing• Source & destination in different areas

ii)Intra-area routing• Source & destination in same area

OSPF also receives and transmits routes to other Ass through boundary routes.

Inter-area routing

Its like a star configuration• backbone-hub• areas-spokes

How does routers choose ABR for packet exit?

• Each ABR in an area advertise location of AS boundary router.

• Also advertise cost to ABRs in other areas

Router Model

Route Processor (CPU)

FIB

Interface card Interface card

Forwarding

SwitchingFabric

Data packet

Data packet

TopologyView

SPF Calculation

OSPF Process

LSA

LS Ack

LSA

Forwarding

LSA Processing

LSA Flooding

SPF Calculation

FIB Update

LSA ProcessingReceive an LS Update packet

New/duplicate?new

Update link state database

Schedule SPF calc. if reqd.

duplicate

Acknowledge LSA immed.Send LS Ack packet back

Get (next) LSA

Determine which interfaces LAS needs to be flooded out

FIB is updated

Next LSA exist?

LS Update processing over

Bundle LSAs into packet & flood it out

SPF calculation

Types of Link-state Advertisements Router link adv.

• Originated by all routers (flooded per area).• Describes states of the router’s interfaces

Network link adv• Originated by Designated router for multi-

access networks (flooded per area)• Contain list of all routers connected to the

network

Cont’

Summary link adv• Originated by ABRs (flooded per area)• Describe route to dest. Outside the area but in AS• 2 types:

– Type 3 – describe routes to networks

– Type 4 - describe routes to AS boundary routers

AS external adv• Originated by AS boundary routers (flooded in AS)• Describe routes to a dest in another AS

Packet format

Version Number - OSPF used Type - Hello, Database Description, Link-state request etc Packet length - Specifies packet length including the header Router ID - Identifies source of packet Area ID - Identifies area to which the packet belongs Checksum - Checks packet for damage suffered during transit Authentication type - Contains Auth. Type configured in the area Authentication - Contains authentication information

Routing protocol packets

Are given preference over regular data packets

Include:-• Hello• Database Description• Link-state Request• Link-state Update• Link-state Acknowledgement

Hello packet

Used to acquire neighbors– After a router is assured by the lower-layer

protocols that it’s interfaces are functional, it uses hello packet to acquire neighbors

It acts as keepalives– Sent periodically on all interfaces– Let routers know that other routers are

functional

Database Description packet

Type 2 OSPF packet Describe contents of topological

database These packets are exchanged when

adjacency is being initialized

Link-state Request

Are type 3 Packets Requests pieces of topological dbase

from neighbors Are sent when router discovers that part

of its topological database is outdated

Link-state Update

Type 4 Responds to link-state request

– Implement flooding of LSAs Several LSAs may be included in a

packet

Link-state Acknowledgement

Are type 5 It ensures reliability

– It acknowledges the link-state update packets

Packet Authentication

Authentication differs for each area– Autype 0 -No authentication– Autype 1 -Simple password– All others - Reserved for assignment by IANA

Additional authentication on per-interface basis.

• Example: Simple password for area and additional password configured for each network in the area

OSPF Delay Measurements

Why measure OSPF ?– Internal OSPF processing delays impact:-

• speed updates propagate through network• Amount of load on routers• time needed for routing to converge incase of

topology/configuration change

How to measure OSPF

Methods– Black-box - based on external observation– white-box - based on internal

instrumentation Key internal tasks considered in

measuring delays– Processing LSAs, performing SPF

calculations,updating FIB & Flooding LSAs

Black-box protocol measurements

3 techniques enforced in designing expt.– Use of OSPF emulator

• generates OSPF patterns

– Exploit features mandated by OSPF specs• duplicate LSAs must be acknowledged

immediately

– Set config. Parameters to ensure tasks in an order that allow for measurement

Methodology

PC1 Expect script TopTracker

Target router

Emulated topology

•Load emulated topology on target router•Initiate task of interest•Measure the time for task

(Testbed setup)

LSA

Top Tracker- OSPF topology emulatorTarget router-router whose OSPF impl. Is under test

PC2

VLAN1

VLAN2

Experimental Designs

Task Logical Connectivity

LSA Processing

LSA Flooding

SPF Calculation

FIB Update

PCI - target router on VLAN1

PCI - target router on VLAN1 & 2

PCI - target router on VLAN1

PCI - target router on VLAN1 & PC2 acts as pin generator on VLAN1

Measuring Task Time

top bracket event

bottom bracket event

task start time

task finish time

time

1. Use a black-box method to bracket task start and finish times

2. Subtract out intervals that precede and exceed these times

X

B

C

X = A - (B+C)

A

LSA Processing Time

Most complex– OSPF bundles LSAs into LS Update packets

For expt:– Top tracker sends 2 packets back to back

• 1st contain legitimate LSAs (probeLSAs)• 2nd contain duplicate LSA

– Top tracker logs time it sends packet (ts) and the time ack for duplicate LSA it received (tr)

LSA processing calculation

Ack for duplicate LSA arrives(tr)

Duplicate LSA arrives t2’

time

Target RouterTopTracker

Duplicate LSA sent t1’

Probe LSA Sent (ts)

Duplicate LSA processing over t3

• X = t2 - t1 =(tr-ts)-[(tr-t3)+(t3-t2)+(t1-ts)]• Estimate the overhead = [(tr-t3)+(t3-t2)+(t1-ts)]

Probe LSA arrive (t1)

Probe LSAs processing over t2

X

- To estimate overhead Top tracker sends one packet with duplicate and logs transmission and reception time

Cont’ Processing time for the packet depends

on:-– Number of links the LSA describes– The number of interfaces on which The

LSA must later be flooded to– Size of link state database

LSA Flood Time

This is the time target router takes to flood LSA after receiving it

Probe LSA is received (tr)

time

Target RouterTopTracker

Probe LSA Sent (ts)

Probe LSA is flooded out t3

Probe LSA arrive (t1)

Probe LSAs processing over t2

X

Cont’

X = t3 - t1 =(tr-ts)-[(tr-t3)+(t1-ts)] Estimate the overhead = [(tr-t3)+(t1-ts)]

The overhead is the round trip to forward a packet between the 2 Tracker interfaces via target router

Pacing-timer– Controls rate at which packets are transmitted out

an interface

SPF Calculation 2 parameters influence the scheduling of the

calculation• spf-delay-specifies how long OSPF waits between receiving

topology change and starting SPF comp.• spf-hold time-Enforces a lag time between consecutive comps.

Expt:-• Set both parameters to 0• Send a probe LSA whose receipt immediately initiates calc on

the target router• Send duplicate LSA whose role is to bracket the finish time if

SPF calculation.

Methodology for SPF Calculation

Ack for duplicate LSA arrives

Initiator LSA arrives

SPF calculation ends

SPF calculation starts

time

Target RouterTopTracker

Send initiator LSA

Send duplicate LSA

Load desired topology

Send ack for duplicate LSA

• X = A – (B + C + D + E)• Estimate the overhead = B + C + D + E•The overhead is therefore a round trip between tracker and router

A X

C

D

B

E

Estimating the Overhead

Remove SPF calculation from bracketset spf_delay = 60 seconds (very large value)

Ack for duplicate LSA arrives

Initiator LSA arrives

Initiator LSA processing done

Duplicate LSA arrivestime

Target RouterTopTracker

Send initiator LSASend duplicate LSA

Duplicate LSA processing done; send ack

SPF calculation starts

overhead = B + C + D + E.tracker logs sending time of probe and the reception time of ack for duplicate LSA

B

E

C

D

Overhead

Results for Cisco Routers

Similar results for two modelsSPF calculation time is O(n2)

Mean SPF time (Cisco GSR)

0

0.005

0.01

0.015

0.02

0.025

0.03

0 20 40 60 80 100

Number of nodes (n)

Tim

e (s

eco

nd

s)

Mean SPF Time (Cisco 7513)

0

0.005

0.01

0.015

0.02

0.025

0.03

0 20 40 60 80 100

Number of nodes (n)T

ime

(sec

on

ds)

Results for GateD

Black-box over-estimates white-box measurementBlack-box captures the characteristics very well

Mean SPF Time (GateD)

0

0.002

0.004

0.006

0.008

0.01

0.012

0.014

0.016

0.018

0 20 40 60 80 100

Number of nodes (n)

Tim

e (s

eco

nd

s)

Black-box

White-box

FIB Update Time

It’s time it takes to update FIB after it starts SPF calculation

Expt:– PC2 pings a destination addr d

every .01sec– Until FIB updated, all pings are dropped– When change is reflected in FIB, ping

packet is forwarded to PC1

FIB timer

First ping packet reaches(tr)

SPF computation starts t2

time

Target RouterTopTracker (PC1)

Probe LSA Sent (ts)

Ping packet reaches target router t5

• X = t4 - t1 =(tr-ts)-[(tr-t5)+(t5-t4)+(t2-t1)+(t1-ts)]

Probe LSA arrive (t1)

Route to d installed in FIB t4

X

SPF computation ends t3

PC2

ping d

OSPF Task Delays (Cisco)

LSA Processing 100-800 microseconds

LSA flooding30-40 millisecondspacing timer is the determining factor

SPF calculation 100-300 milliseconds

FIB update timeno dependence on the size of the topology1-40 milliseconds

Conclusions

OSPF routing protocol proved to be effective in serving large, heterogeneous internetworksBlack-box methods for estimating OSPF processing delays worked:

LSA processing and floodingSPF calculation and FIB Update

Applied techniques to Cisco GSR and 7513 routers as well as GateDFuture work

develop techniques for other protocols, in particular BGP

Any questions?