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IS-IS and OSPFA Comparative Anatomy

Dave Katz, Juniper Networks

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Overview

Protocol History

Nuts and Bolts

Scalability Issues

Pragmatic ConsiderationsConclusions

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Protocol History

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Protocol History

1989

OSPF v.1 RFC published

Proteon ships OSPF

IS-IS becomes ISO proposed standard

Public bickering ensues--OSPF and IS-IS areblessed as equals by IETF, with OSPFsomewhat more equal

Private cooperation improves both protocols

1990

Dual-mode IS-IS RFC published

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Protocol History

1991

OSPF v.2 RFC published

Cisco ships OSPF

Cisco ships OSI-only IS-IS1992

Cisco ships dual IS-IS (part of DEC Brouter)

Lots of OSPF deployed, but very little IS-IS

1993Novell publishes NLSP (IPX IS-IS knockoff)

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Protocol History

1994

Cisco ships NLSP (rewriting IS-IS as sideeffect)

Large ISPs need an IGP; IS-IS is recommendeddue to recent rewrite and OSPF field experience(and to lesser extent, NSF CLNP mandate)

1995

ISPs begin deployment of IS-IS, Ciscoimplementation firms up, protocol starts tobecome popular in niche

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Protocol History

1996-1998

IS-IS niche popularity continues to grow (someISPs switch to it from OSPF)

IS-IS becomes barrier to entry for routervendors targeting large ISPs

Juniper and other vendors ship IS-IS capablerouters

1999-2000

Extensions continue for both protocols (e.g,Traffic Engineering)

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Nuts and Bolts

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Nuts and Bolts

10,000 foot view

Protocols are recognizably similar in functionand mechanism (unsurprising, given commonheritage)

Link state algorithms (network map isdistributed, each router calculates routesindependently based on the map)

Two level hierarchies

Designated Router on LANs

Widely deployed (for some value of wide)

Multiple interoperable implementations

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Nuts and Bolts

10,000 foot view

OSPF is for the most part more optimized(and therefore significantly more complex)

IS-IS was not designed from the start as an IProuting protocol (and is therefore a bit clunkyin places)

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Nuts and Bolts

Encapsulation

OSPF runs on top of IP

Traditional IP routing protocol approach

Allows virtual links (if you like them)Relies on IP fragmentation for large LSAs

Subject to spoofing and DoS attacks (use ofauthentication is strongly advised)

Allows use of ATM VCmux encapsulation (soTCP acks fit in one ATM cell)

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Nuts and Bolts

Encapsulation

IS-IS runs directly over L2 (next to IP)

Sort of makes sense, architecturally

Partition repair requires tunneling (rarelyimplemented)

More difficult to spoof or attack

More difficult to implement in someenvironments

Requires ATM SNAP encapsulation, forcingtwo-cell TCP acks (but Henk Smits NLPIDhack fixes this)

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Nuts and Bolts

Media support

Both protocols support LANs and point-to-pointlinks in similar ways

IS-IS has no direct NBMA support--expects O/Sto present NBMA network as either pseudo-LAN(bad idea) or set of point-to-point links

OSPF NBMA mode is configuration-heavy andrisky (all routers must be able to reach DR; bad

news if VC fails)OSPF P2MP mode models NBMA as point-to-

point links (if O/S wont help)

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Nuts and Bolts

Packet Encoding

OSPF is efficiently encoded

Positional fields

Holy 32-bit alignment provides tidy packetpictures, but not much else

Only LSAs are extensible (not Hellos, etc.)

Unrecognized LSA types not flooded (thoughopaque LSAs can suffice, if implementeduniversally, and IS-IS-like encoding canprovide good granularity)

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Nuts and Bolts

Packet Encoding

IS-IS is mostly Type-Length-Value encoded

No particular alignment

Extensible from the start (unknown typesignored but still flooded)

All packet types are extensible

Nested TLVs provide structure for moregranular extension (though base spec doesnot use them; OSPF is starting to do so)

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Nuts and Bolts

Area Architecture

Both protocols support two-level hierarchy ofareas (to reduce SPF graph complexity, andpotentially to allow route aggregation)

OSPF area boundaries fall within a router

Interfaces bound to areas

Router may be in many areas

Router must calculate SPF per area

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Nuts and Bolts

Area Architecture

IS-IS area boundaries fall on links

Router is in only one area, plus perhaps theL2 backbone (area)

Biased toward large areas, area migration

Requires router per area (unless multiplevirtual routers are implemented)

Historically proven somewhat difficult forusers to grasp

Little or no multilevel deployment (large flatareas work so far)

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Database Granularity

OSPF database node is an LSAdvertisement

LSAs are mostly numerous and small (oneexternal per LSA, one summary per LSA)

Network and Router LSAs can become large

LSAs grouped into LSUpdates duringflooding

LSUpdates are built individually at each hop

Small changes can yield small packets (butRouter, Network LSAs can be large)

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Database Granularity

IS-IS database node is an LSPacket

LSPs are clumps of topology informationorganized by the originating router

Always flooded intact, unchanged across allflooding hops (so LSP MTU is anarchitectural constant--it must fit across alllinks)

Small topology changes always yield entireLSPs (though packet size turns out to bemuch less of an issue than packet count)

Implementations can attempt clever packing

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Neighbor Establishment

Both protocols use periodic multicast Hellopackets, I heard you mechanism to establish2-way communication

Both protocols have settable hello/holdingtimers to allow tradeoff between stability,overhead, and responsiveness

OSPF requires hello and holding timers to

match on all routers on the same subnet (sideeffect of DR election algorithm) making itdifficult to change timers without disruption

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Neighbor Establishment

IS-IS requires padding of Hello packets to fullMTU size under some conditions (to detectmedia with MTUs smaller than 1497 bytes)--

this is effectively useless and causes needlesssupport calls (deprecated in practice)

OSPF requires routers to have matching MTUsin order to become adjacent (or LSA flooding

may fail, since LSUpdates are built at each hopand may be MTU-sized)

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Neighbor Adjacency Establishment

The goal--synchronize databases

The method--tell your neighbor everythingyouve got

You (or your neighbor) will figure out whatyoure missing and make sure that you get it

Each protocols approach is driven by databasegranularity

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Neighbor Adjacency Establishment

OSPF uses complex, multistate process tosynchronize databases between neighbors

Intended to minimize transient routingproblems by ensuring that a newborn routerhas nearly complete routing informationbefore it begins carrying traffic

Accounts for a significant portion of OSPFs

implementation complexityPartially a side effect of granular database(requires many DBD packets)

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Neighbor Adjacency Establishment

IS-IS essentially uses its regular floodingtechniques to synchronize neighbors (thatswhat flooding naturally does)

Coarse database granularity makes this easy(just a few CSNPs)

Transient routing issues can be reduced(albeit nondeterministically) by judicious use

of the overload bit (one of a number ofopportunistic hacks)

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Designated Routers and Adjacency

Both protocols elect a designated router onmultiaccess networks to remove O(N^2) linkproblem (by creating a pseudonode) and to

reduce flooding traffic (DR ensures floodingreliability)

OSPF elects both a DR and a Backup DR, eachof which becomes adjacent with all other

routersBDR takes over if DR fails

DRship is sticky, not deterministic

Complex algorithm

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Designated Routers and Adjacency

In IS-IS all routers are adjacent (but adjacencyis far less stateful)

If DR dies, new DR must be elected, withshort connectivity loss (synchronization isfast)

DRship is deterministic (highest priority,highest MAC address always wins)

DRship can be made sticky by cool NLSPpriority hack (DR increases its DR priority)

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LAN Flooding

OSPF uses multicast send, unicast ack from DR

Reduces flood traffic by 50% (uninteresting)

Requires per-neighbor state (forretransmissions)

Interesting (but complex) acknowledgementsuppression

Flood traffic grows as O(N)

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LAN Flooding

IS-IS uses multicast LSP from all routers, CSNPfrom DR

Periodic CSNPs ensure databases are synced(tractable because of coarse databasegranularity)

Flood traffic constant regardless of numberof neighbors on LAN

But big LANs are uninteresting

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Nuts and Bolts

Routes and Metrics

IS-IS base spec used 6-bit metrics on links

Allowed an uninteresting SPF optimization(CPUs are fast these days)

Proved difficult to assign meaningful metricsin large networks

Wide metric extension addresses this

Dual IS-IS spec advertises only default into L1

areas

Interarea traffic routed suboptimally

Route leaking extension addresses this

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Nuts and Bolts

Authentication and Security

Both support cryptographic authentication

OSPF really needs this (packet bombs)

Successful IGP attacks will be catastrophic (orworse, subtle)

Use packet filtering, particularly with OSPF

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MPLS Traffic Engineering extensions

Protocols carry around TE link information(available bandwidth, link color, etc.) on behalfof MPLS but dont use the data themselves

TE functionality is identical for the twoprotocols (by design)

TE functions are IGP-independent, somechanisms ought to be identical

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Scalability Issues

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Scalability Issues

Database Size

OSPF topologies limited by Network and RouterLSA size (max 64KB) to O(5000) links

External and Interarea routes are essentially

unbounded

IS-IS topologies limited by LSP count (256fragments * 1470 bytes) for all route types

Various hacks (fake pseudonodes, etc.)

could make this biggerUltimately a non-issue for even slightly sane

topologies

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Scalability Issues

Database Churn

Both protocols have time-limited databaseentries and therefore require refreshing

IS-IS lifetime field is 16 bits, giving 18.7-hour

lifetimes (with refresh times close to this)

OSPF age (counts up) has an architecturallifetime limit of 1 hour (80,000 LSAs yield arefresh every 23 milliseconds)

Do-not-age LSAs are not backwardcompatible

Dont inject zillions of routes into your IGP

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Scalability Issues

Flooding load--the only serious issue

Full-mesh topologies are worst-case for both

N^2 copies of each update (each of which isO(N) in size)

Link failure: O(N^3) information

Router failure: O(N^4) information

IS-IS mesh group hack provides backward-compatible way of pruning flooding topology

OSPF has no solution without protocol change

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Pragmatic Considerations

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Pragmatic Considerations

OSPF spec is an excellent implementationguide

If followed to the letter, a working, if nave,implementation will likely result

Spec is complex but has almost no whyinformation, so other (potentially morescalable) implementation approaches are at theimplementors own risk

Barrier to entry in high-end router market (youneed to know the protocol intuitively)

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Pragmatic Considerations

IS-IS spec uses arcane ISOspeak and hasvery few implementation hints

Spec is inherently simple (once you get thelingo), with fewer implementation issues

Boilerplate at front and back of spec meansthat you can lose pages without affectingcontent

Barrier to entry in high-end router market (you

need to know the protocol intuitively)

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Pragmatic Considerations

Extensibility

Despite anti-OSI FUD, IS-IS has proven mucheasier politically to extend (primarily due tosmall constituency and IETF disinterest)

Self-interest of router vendors and large ISPsbrings extensions more quickly in IS-IS andpromotes implementation stability, scalability,and interoperability

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Pragmatic Considerations

ExtensibilityOSPFs encoding scheme difficult to extend

Difficult compatibility issues

Explicitly and proudly optimized for IPv4

IPv6 requires a completely new protocol

IS-IS encoding inefficient and simple-minded

But proven to be easy to extend, at least insome ways

IPv6-Ready (also IPX-Ready)

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Pragmatic Considerations

OptimalityOSPF was optimized for things that dont

matter any more (link bandwidth, CPUalignment)

IS-IS was optimized for things that dontmatter any more (large LANs, SPF cost)

Optimizations turn out to add complexity butnot much value

A lot has changed in 10 years...

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Pragmatic Considerations

OSPF is much more widely understood Broadly deployed in enterprise market

Many books of varying quality available

Preserves our investment in terminology

IS-IS is well understood within a niche

Broadly deployed within the large ISP market

Folks who build very large, very visible

networks are comfortable with it

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Conclusions

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Conclusions

For all but extreme cases (large full-meshnetworks), protocols are pretty muchequivalent in scalability and functionality

Stability and scalability are largelyartifacts of implementation, not protocoldesign

Familiarity and comfort in both

engineering and operations is probably thebiggest factor in choosing

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Conclusions

Does the world really need two protocols?Nearly complete overlap in functionality means

(ironically) that few people are motivated toswitch

Entrenched constituencies (large ISPs;everyone else) ensure that installed bases willcontinue to exist

As long as there are two, people will never

agree on only oneNot even the oft-predicted demise of IPv4 will

suffice

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Conclusions

Both protocols are over 10 years old, usinggraph theory thats at least 40 years old

Both protocols are (even still) works in

progressCherish Diversity (and job security)

Theyre both good protocols

Use the one that makes the most sense to

you

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