MPLS In Perspective
Kireeti Kompella
Distinguished Engineer
Juniper Networks
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Menu
IP salad with horseradish dressing
ATM flambee
MPLS stewed in its own juice
For afters
Services double espresso
Revenue a la mode
Copyright Juniper Networks,
2001
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IP – Good Enough ™
Well-architected,
worked out in detail
– NOT!
Realization: can’t
predict the future
Make it reasonable
Make it flexible
Make it extensible
stuff above
transport
network
stuff below
Copyright Juniper Networks,
2001
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So Easy to Forget
IP started out with e-mail …
… and data services
ftp
news
Now: the “Web”, voice, video, …
Also, SLAs, grades of service, …
Copyright Juniper Networks,
2001
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IP Control Plane
Again, just good enough
But again, flexible, extensible
DV routing was fine for quite a while
Just in time, along came link state
Now: is convergence “in a few
seconds” good enough?
Copyright Juniper Networks,
2001
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Good Enough™ Can Get Better
Fast to ultrafast convergence
“Bullet-proof” IP
Hitless restart?
“Business” IP
Make me money – new services, GoS
Don’t lose me money – uptime, SLAs
Copyright Juniper Networks,
2001
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ATM – Perfectionist’s Dream
Connection-oriented
Does everything and
does it well
Anticipated all
future uses and
factored them in
Philosophical
mismatch with IP
stuff above
transport
network
Copyright Juniper Networks,
2001
ATM
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MPLS
If (ATM = Frame Relay on steroids)
then (MPLS = ATM on happy juice)
Make it just Good Enough ™
Despite all efforts to make it perfect
IP control plane
IP philosophy
Copyright Juniper Networks,
2001
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What Does MPLS Offer?
Tunnels
Drop a packet in, and out it comes at the
other end without being IP routed
Explicit (source) routing (circuits)
Label stack
2-label stack: “outer” label defines the
tunnel; “inner” label demultiplexes
Layer 2 independence
Copyright Juniper Networks,
2001
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Why Tunnels?
Can’t IP route
Non-IP packets
IP packets with private addresses
Don’t want to IP route
“BGP-free” core
Multicast 
Copyright Juniper Networks,
2001
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How Tunnels?
MPLS: LDP – “automagic” tunnels that
follow IP routing
IP: IP-in-IP, GRE, IPSec, UTI
Can one tunnel do multiple things?
Tunnel demux
Copyright Juniper Networks,
2001
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Tunnel Comparison
MPLS (LDP) tunnels
Small header
Label stacking
Signaling for demux
Automagic tunnels
Tracks IP routing
Harder to spoof
No data security
IP tunnels
Big header
No stacking (*)
No signaling (yet)
Configured tunnels
Duh!
Spoofable
IPSec
Copyright Juniper Networks,
2001
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Bottom Line on Tunnels
Don’t need MPLS for tunnels
But MPLS tunnels have some nice
properties
Decision (should be) based on cost of
deploying new protocol vs. benefits
Copyright Juniper Networks,
2001
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Why Explicit Routing?
Traffic Engineering
Fast reroute
Guaranteed bandwidth
Probably others
Connection-oriented paradigm nicely
complements IP’s connectionlessness
Copyright Juniper Networks,
2001
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Traffic Engineering
Is ATM the best way to engineer
traffic?
Or is it MPLS?
Or can we do just fine with IP?
First question: do you need traffic
engineering? What part of network?
Copyright Juniper Networks,
2001
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Traffic Engineering Steps
First, determine how to lay out
traffic on the physical topology
Measure traffic (e.g., city-pair-wise)
Crunch numbers
Second, do something to convince the
packets to follow your plan
Copyright Juniper Networks,
2001
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Traffic Engineering Options
BGP – play with communities, filtering
IGP – play with metrics
Linear programming can help
Source routing
ATM
MPLS
Copyright Juniper Networks,
2001
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Traffic Engineering
Warning: read at your own risk!
Fine-grained Traffic Engineering
needs some form of source routing
Specific incremental changes much
easier with source routing
Change a single city-pair flow
Reacting to a link failure
Copyright Juniper Networks,
2001
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Linear Programming
TE among N cities: N² city pairs
Set up N² by N² matrix for LP
Matrix multiplication/inversion is
O(M³) for M x M matrix;
simplex is O(M³) matrix operations
So, LP problem is O(N12)
Also can’t deal with “looped routes”
Copyright Juniper Networks,
2001
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Fast Reroute
Can MPLS re-route as fast as SONET
(50ms)?
Can IP re-route as fast as MPLS?
Do packets get dizzy if they are rerouted too fast?
Copyright Juniper Networks,
2001
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Fast Reroute (2)
First question: how fast is fast?
Do you really need 50 ms failover?
Second question: can you reroute
really quickly while maintaining
network stability?
Third question: what are the
scalability issues with fast reroute?
Copyright Juniper Networks,
2001
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Fast Reroute Comparison
IP
All nodes must be
told of failure
Fast propagation,
fast SPF trigger:
how stable?
One step to full
reconvergence
MPLS (RSVP-TE)
Only the two ends
of the link need be
told (no signaling)
Local operation:
explicit routing;
more stable
Two step process:
detour + converge
Copyright Juniper Networks,
2001
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Fast Reroute: MPLS vs. IP
C
10
pkt to B
1000
A
10
B
IP routing to B
MPLS detour to B
Copyright Juniper Networks,
2001
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Guaranteed Bandwidth
Again, first question: do you need it?
If so, you need source routing, CAC
and some way of signaling b/w
RSVP-TE can do this
ATM could probably do it better
Copyright Juniper Networks,
2001
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“MPLS” Services
IP VPNs (RFC 2547 et al)
Layer 2 transport
Layer 2 VPNs
Transparent LAN Service
TDM over MPLS over TDM over …
Electricity over photons?
Have we gotten a little carried away?
Copyright Juniper Networks,
2001
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“MPLS” Services (2)
Most of these services need tunnels
Not really MPLS services
MPLS-geeks definitely responsible
Some of these services enhanced by
source routing
More services may mean more revenue,
could also keep you awake at night
Copyright Juniper Networks,
2001
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Revenue
RFC 2547
New service – recent deployment
Give it a shot, or run like hell? Or wait?
Layer 2 VPNs
Old service – lots of deployment
New transport – is it Good Enough?
Guaranteed bandwidth, Diff Serv, …?
Copyright Juniper Networks,
2001
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Things to Ponder
Can Good Enough™ IP stay ahead of
the curve?
Even if so, can MPLS help?
Is MPLS a support, a crutch or a banana
peel?
Is connection-orientedness a useful
addition to connectionless IP?
What services, when, how far to go?
Copyright Juniper Networks,
2001
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My Biases
Vendor
MPLS geek
Protocols freak
Neutral about ATM
IP rules!
Reasonably agnostic
Copyright Juniper Networks,
2001
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Thank you!
http://www.juniper.net
kireeti@juniper.net
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