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Introduction to MPLS and Traffic Engineering

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Title: Introduction to MPLS and Traffic Engineering


1
Introduction to MPLS and Traffic Engineering
  • Zartash Afzal Uzmi

2
Outline
  • Traditional IP Routing
  • Forwarding and routing
  • Problems with IP routing
  • Motivations behind MPLS
  • MPLS Terminology and Operation
  • 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

3
Outline
  • Traditional IP Routing
  • Forwarding and routing
  • Problems with IP routing
  • Motivations behind MPLS
  • MPLS Terminology and Operation
  • 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

4
Forwarding and routing
  • Forwarding
  • Passing a packet to the next hop router
  • Routing
  • Computing the best path to the destination
  • IP routing includes routing and forwarding
  • Each router makes the forwarding decision
  • Each router makes the routing decision
  • MPLS routing
  • Only one router (source) makes the routing
    decision
  • Intermediate router make the forwarding decision

5
IP versus MPLS routing
  • IP routing
  • Each IP datagram is routed independently
  • Routing and forwarding is destination-based
  • Routers look at the destination addresses
  • May lead to congestion in parts of the network
  • MPLS routing
  • 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)

6
How IP routing works
Searching Longest Prefix Match in FIB (Too Slow)
7
Problems with IP routing
  • Too slow
  • 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
  • Too rigid no flexibility
  • Routing decisions are destination-based
  • Not scalable in some desirable applications
  • When mapping IP traffic onto ATM

8
IP routing rigidity example
D
1
1
A
A
B
S
B
1
2
C
  • 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
  • Leads to IP hotspots!
  • Solution?
  • Try to divert the traffic onto alternate paths

9
IP routing rigidity example
D
1
4
A
A
B
S
B
1
2
C
  • 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
  • Put more bandwidth where the traffic is!
  • Leads to underutilized links not suitable for
    large networks

10
Motivations behind MPLS
  • Avoid slow IP lookup
  • Led to the development of IP switching in 1996
  • Provide some scalability for IP over ATM
  • Evolve routing functionality
  • Control was too closely tied to forwarding
  • Evolution of routing functionality led to some
    other benefits
  • Explicit path routing
  • Provision of service differentiation (QoS)

11
IP routing versus MPLS routing
Traditional IP Routing
Multiprotocol Label Switching (MPLS)
2
1
S
D
3
5
4
MPLS allows overriding shortest paths!
12
Outline
  • Traditional IP Routing
  • Forwarding and routing
  • Problems with IP routing
  • Motivations behind MPLS
  • MPLS Terminology and Operation
  • 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

13
MPLS label
  • To avoid IP lookup MPLS packets carry extra
    information called Label
  • Packet forwarding decision is made using
    label-based lookups
  • Labels have local significance only!
  • How routing along explicit path works?

IP Datagram
Label
14
Routing along explicit paths
  • Idea Let the source make the complete routing
    decision
  • How is this accomplished?
  • 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?
  • Define some constraints and hope that the
    constraints will take some traffic away from
    the hotspot!
  • Use CSPF instead of SPF (shortest path first)

15
Label, LSP and LSR
  • Label
  • 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

16
LFIB versus FIB
  • 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?
  • 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

17
Mpls Flow Progress
D
R1
R2
LSR4
LSR1
D
destination
LSR6
LSR3
LSR2
R1 and R2 are regular routers
LSR5
1 - R1 receives a packet for destination D
connected to R2
18
Mpls Flow Progress
D
R1
R2
LSR4
LSR1
D
destination
LSR6
LSR3
LSR2
LSR5
2 - R1 determines the next hop as LSR1 and
forwards the packet (Makes a routing as well as
a forwarding decision)
19
Mpls Flow Progress
R1
R2
LSR4
LSR1
D
31
D
destination
LSR6
LSR3
LSR2
LSR5
3 LSR1 establishes a path to LSR6 and PUSHES
a label (Makes a routing as well as a forwarding
decision)
20
Mpls Flow Progress
R1
R2
LSR4
LSR1
D
destination
LSR6
LSR3
D
17
LSR2
Labels have local signifacance!
LSR5
4 LSR3 just looks at the incoming label LSR3
SWAPS with another label before forwarding
21
MPLS Flow Progress
R1
R2
LSR4
LSR1
D
destination
LSR6
LSR3
D
17
LSR2
Path within MPLS cloud is pre-established LSP
(label-switched path)
LSR5
5 LSR6 looks at the incoming label LSR6 POPS
the label before forwarding to R2
22
MPLS and explicit routing recap
  • Who establishes the LSPs in advance?
  • Ingress routers
  • How do ingress routers decide not to always take
    the shortest path?
  • 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

23
CSPF
  • What is the mechanism?
  • 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
  • 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

24
More MPLS terminology
Upstream
Downstream
172.68.10/24
LSR1
LSR2
25
Label advertisement
  • Always downstream to upstream label advertisement
    and distribution

Downstream
Upstream
171.68.32/24
LSR2
LSR1
26
Label advertisement
  • Label advertisement can be downstream unsolicited
    or downstream on-demand

Downstream
Upstream
171.68.32/24
LSR2
LSR1
Downstream
Upstream
171.68.32/24
LSR1
LSR2
27
Label distribution
  • Label distribution can be ordered or unordered
  • First we see an example of ordered label
    distribution

Label
28
Label distribution
  • Label distribution can be ordered or unordered
  • Next we see an example of unordered label
    distribution

Label
Label
29
Label retention modes
  • Label retention can be conservative or liberal

?
Destination
Label
LSR1
Label
30
Label operations
  • Advertisement
  • Downstream unsolicited
  • Downstream on-demand
  • Distribution
  • Ordered
  • Unordered
  • Retention
  • Liberal
  • Conservative

31
Outline
  • Traditional IP Routing
  • Forwarding and routing
  • Problems with IP routing
  • Motivations behind MPLS
  • MPLS Terminology and Operation
  • 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

32
Traffic Engineering
  • Traffic Engineering with MPLS
  • (Application of CSPF)

33
What is traffic engineering?
  • 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?
  • high cost of network assets
  • service differentiation

34
Traffic engineering
  • 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

35
MPLS-TE How to do it?
  • LSPs are set up by LSRs based on information they
    learn from routing protocols (IGPs)
  • This defeats the purpose!
  • If we were to use shortest path, IGP was okay

36
MPLS TE How we actually do it?
  • MPLS TE Requires
  • Enhancements to routing protocols
  • OSPF-TE
  • ISIS-TE
  • Enhancement to signaling protocols to allow
    explicit constraint based routing
  • RSVP-TE and CR-LDP
  • Constraint based routing
  • Explicit route selection
  • Recovery mechanisms defined

37
Signaling mechanisms
  • RSVP-TE
  • Extensions to RSVP for traffic engineering
  • BGP-4
  • Carrying label information in BGP-4
  • CR-LDP
  • 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!

38
RSVP-TE
Basic flow of LSP set-up using RSVP
39
RSVP-TE PATH Message
  • PATH message is used to establish state and
    request label assignment
  • R1 transmits a PATH message addressed to R9

40
RSVP-TE RESV Message
  • RESV is used to distribute labels after reserving
    resources
  • R9 transmits a RESV message, with label3, 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)

41
Rerouting LSP tunnels
  • When a more optimal route/path becomes
    available
  • When a failure of a resource occurs along a TE
    LSP
  • Make-before-break mechanism
  • Adaptive, smooth rerouting and traffic transfer
    before tearing down the old LSP
  • Not disruptive to traffic

42
Recovering LSP tunnels
LSP Set-up
43
Protection LSP set up
44
Protection LSP
45
References
  • 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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