Transcript MPLS Traffic Engineering (TE) Tutorial

Introduction to MPLS
and Traffic Engineering
Zartash Afzal Uzmi
Outline
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Traditional IP Routing
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MPLS Terminology and Operation
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Forwarding and routing
Problems with IP routing
Motivations behind MPLS
MPLS Label, LSR and LSP, LFIB Vs FIB
Transport of an IP packet over MPLS
More MPLS terminology
Traffic Engineering [with MPLS]



Nomenclature
Requirements
Examples
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Outline

Traditional IP Routing




MPLS Terminology and Operation




Forwarding and routing
Problems with IP routing
Motivations behind MPLS
MPLS Label, LSR and LSP, LFIB Vs FIB
Transport of an IP packet over MPLS
More MPLS terminology
Traffic Engineering [with MPLS]



Nomenclature
Requirements
Examples
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Forwarding and routing
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Forwarding:
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Routing:
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Computing the “best” path to the destination
IP routing – includes routing and forwarding
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Passing a packet to the next hop router
Each router makes the forwarding decision
Each router makes the routing decision
MPLS routing
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Only one router (source) makes the routing decision
Intermediate router make the forwarding decision
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IP versus MPLS routing
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IP routing
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Each IP datagram is routed independently
Routing and forwarding is destination-based
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Routers look at the destination addresses
May lead to congestion in parts of the network
MPLS routing
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A path is computed “in advance” and a “virtual
circuit” is established from ingress to egress
An MPLS path from ingress to egress node is
called a label switched path (LSP)
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How IP routing works
Searching
Longest
Prefix Match
in FIB (Too
Slow)
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Problems with IP routing
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Too slow
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Too rigid – no flexibility
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IP lookup (longest prefix matching) “was” a
major bottleneck in high performance routers
This was made worse by the fact that IP
forwarding requires complex lookup operation
at every hop along the path
Routing decisions are destination-based
Not scalable in some desirable applications
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When mapping IP traffic onto ATM
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IP routing rigidity example
D
1
A
1
S
B
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1
C
B
2
Packet 1: Destination A
Packet 2: Destination B
S computes shortest paths to A and B; finds D as next hop
Both packets will follow the same path
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A
Leads to IP hotspots!
Solution?
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Try to divert the traffic onto alternate paths
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IP routing rigidity example
D
1
A
4
S
B
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A
1
C
B
2
Increase the cost of link DA from 1 to 4
Traffic is diverted away from node D
A new IP hotspot is created!
Solution(?): Network Engineering
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Put more bandwidth where the traffic is!
Leads to underutilized links; not suitable for large networks
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Motivations behind MPLS
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Avoid [slow] IP lookup
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Provide some scalability for IP over ATM
Evolve routing functionality
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Led to the development of IP switching in 1996
Control was too closely tied to forwarding
Evolution of routing functionality led to some
other benefits
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Explicit path routing
Provision of service differentiation (QoS)
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IP routing versus MPLS routing
Traditional IP Label
Routing
Multiprotocol
Switching (MPLS)
1
2
S
D
3
4
5
MPLS allows overriding shortest paths!
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Outline

Traditional IP Routing




MPLS Terminology and Operation




Forwarding and routing
Problems with IP routing
Motivations behind MPLS
MPLS Label, LSR and LSP, LFIB Vs FIB
Transport of an IP packet over MPLS
More MPLS terminology
Traffic Engineering [with MPLS]



Nomenclature
Requirements
Examples
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MPLS label
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To avoid IP lookup MPLS packets carry
extra information called “Label”
Packet forwarding decision is made using
label-based lookups
Label
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IP Datagram
Labels have local significance only!
How routing along explicit path works?
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Routing along explicit paths
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Idea: Let the source make the complete routing
decision
How is this accomplished?
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Let the ingress attach a label to the IP packet and let
intermediate routers make forwarding decisions only
On what basis should you choose different paths
for different flows?
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Define some constraints and hope that the constraints
will take “some” traffic away from the hotspot!
Use CSPF instead of SPF (shortest path first)
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Label, LSP and LSR
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Label
01234567890123456789012345678901
Label
| Exp|S|
TTL
Label = 20 bits
Exp = Experimental, 3 bits
S = Bottom of stack, 1bit
TTL = Time to live, 8 bits
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Router that supports MPLS is known as label
switching router (LSR)
An “Edge” LSR is also known as LER (edge router)
Path which is followed using labels is called LSP
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LFIB versus FIB
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Labels are searched in LFIB whereas normal IP
Routing uses FIB to search longest prefix match
for a destination IP address
Why switching based on labels is faster?
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LFIB has fewer entries
Routing table FIB has very large number of entries
In LFIB, label is an exact match
In FIB, IP is longest prefix match
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Mpls Flow Progress
D
R1
LSR4
R2
LSR1
D
LSR6
destination
LSR3
LSR2
R1 and R2 are
regular routers
LSR5
1 - R1 receives a packet for destination D connected to R2
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Mpls Flow Progress
R1
D
LSR4
R2
LSR1
D
LSR6
destination
LSR3
LSR2
LSR5
2 - R1 determines the next hop as LSR1 and forwards the packet
(Makes a routing as well as a forwarding decision)
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Mpls Flow Progress
R1
LSR4
LSR1
31
R2
D
D
LSR6
destination
LSR3
LSR2
LSR5
3 – LSR1 establishes a path to LSR6 and “PUSHES” a label
(Makes a routing as well as a forwarding decision)
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Mpls Flow Progress
R1
LSR4
R2
LSR1
D
LSR6
LSR3
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destination
D
LSR2
LSR5
Labels have local
signifacance!
4 – LSR3 just looks at the incoming label
LSR3 “SWAPS” with another label before forwarding
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MPLS Flow Progress
R1
LSR4
R2
LSR1
D
LSR6
LSR3
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destination
D
LSR2
LSR5
Path within MPLS cloud
is pre-established:
LSP (label-switched path)
5 – LSR6 looks at the incoming label
LSR6 “POPS” the label before forwarding to R2
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MPLS and explicit routing recap
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Who establishes the LSPs in advance?
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Ingress routers
How do ingress routers decide not to always take
the shortest path?
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Ingress routers use CSPF (constrained shortest path
first) instead of SPF
Examples of constraints:
 Do not use links left with less than 7Mb/s bandwidth
 Do not use blue-colored links for this request
 Use a path with delay less than 130ms
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CSPF
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What is the mechanism?
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First prune all links not fulfilling constrains
Now find shortest path on the rest of the topology
Requires some reservation mechanism
Changing state of the network must also be
recorded and propagated
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For example, ingress needs to know how much
bandwidth is left on links
The information is propagated by means of routing
protocols and their extensions
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More MPLS terminology
Upstream
Downstream
172.68.10/24
LSR1
LSR2
Data
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Label advertisement
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Always downstream to upstream label
advertisement and distribution
Upstream
Use label 5 for destination
171.68.32/24
Downstream
171.68.32/24
LSR1
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MPLS Data Packet
with label 5 travels
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LSR2
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Label advertisement
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Label advertisement can be downstream
unsolicited or downstream on-demand
Upstream
Sends label
Without any Request
Downstream
171.68.32/24
LSR2
LSR1
Upstream
Sends label ONLY after
receiving request
Downstream
171.68.32/24
LSR1
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Request For label
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LSR2
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Label distribution
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Label distribution can be ordered or unordered
First we see an example of ordered label distribution
Label
Egress LSR
Ingress LSR
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Label distribution
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Label distribution can be ordered or unordered
Next we see an example of unordered label distribution
Label
Label
Egress LSR
Ingress LSR
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Label retention modes
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Label retention can be conservative or liberal
?
Destination
Label
LSR1
Label
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Label operations
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Advertisement
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Distribution
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Downstream unsolicited
Downstream on-demand
Ordered
Unordered
Retention
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Liberal
Conservative
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Outline

Traditional IP Routing




MPLS Terminology and Operation




Forwarding and routing
Problems with IP routing
Motivations behind MPLS
MPLS Label, LSR and LSP, LFIB Vs FIB
Transport of an IP packet over MPLS
More MPLS terminology
Traffic Engineering [with MPLS]



Nomenclature
Requirements
Examples
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Traffic Engineering
Traffic Engineering with MPLS
(Application of CSPF)
What is traffic engineering?
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Performance optimization of operational networks
 optimizing resource utilization
 optimizing traffic performance
 reliable network operation
How is traffic engineered?
 measurement, modeling, characterization, and
control of Internet traffic
Why?
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high cost of network assets
service differentiation
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Traffic engineering
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Recall the IP hotspot problem
The ability to move traffic away from the
shortest path calculated by the IGP (such as
OSPF) to a less congested path
IP: changing a metric will cause ALL the traffic
to divert to the less congested path
MPLS: allows explicit routing (using CSPF) and
setup of such explicitly computed LSPs
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MPLS-TE: How to do it?
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LSPs are set up by LSRs based on information
they learn from routing protocols (IGPs)
This defeats the purpose!
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If we were to use “shortest path”, IGP was okay
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MPLS TE: How we actually do it?
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MPLS TE Requires:
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Enhancements to routing protocols
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Enhancement to signaling protocols to allow
explicit constraint based routing
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OSPF-TE
ISIS-TE
RSVP-TE and CR-LDP
Constraint based routing
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Explicit route selection
Recovery mechanisms defined
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Signaling mechanisms
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RSVP-TE
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BGP-4
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Carrying label information in BGP-4
CR-LDP
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Extensions to RSVP for traffic engineering
A label distribution protocol that distributes labels
determined based on constraint based routing
RSVP-TE and CR-LDP both do label distribution
and path reservation – use any one of them!
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RSVP-TE
Basic flow of LSP set-up using RSVP
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RSVP-TE PATH Message
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PATH message is used to establish state and
request label assignment
R1 transmits a PATH message addressed to R9
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RSVP-TE RESV Message
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RESV is used to distribute labels after reserving resources
R9 transmits a RESV message, with label=3, to R8
R8 and R4 store “outbound” label and allocate an “inbound” label.
They also transmits RESV with inbound label to upstream LSR
R1 binds label to forwarding equivalence class (FEC)
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Rerouting LSP tunnels
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When a more “optimal” route/path
becomes available
When a failure of a resource occurs along
a TE LSP
Make-before-break mechanism
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Adaptive, smooth rerouting and traffic
transfer before tearing down the old LSP
Not disruptive to traffic
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Recovering LSP tunnels
LSP Set-up
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Protection LSP set up
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Protection LSP
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References
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RFC 2702 “Requirements for Traffic
Engineering Over MPLS”
RFC 3031 “Multiprotocol Label Switching
Architecture”
RFC 3272 “Overview and Principles of
Internet Traffic Engineering”
RFC 3346 “Applicability Statement for
Traffic Engineering with MPLS”
MPLS Forum (http://www.mplsforum.org)
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