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MPLS: Traffic Engineering and Restoration Routing Basics

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October 8, 2004. MPLS: TE and Restoration. 1 ... Do not use links with blue color for this request. Use a path with delay less than 130ms ... – PowerPoint PPT presentation

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Title: MPLS: Traffic Engineering and Restoration Routing Basics


1
MPLS Traffic Engineering and Restoration Routing
Basics
  • Zartash Afzal Uzmi
  • Computer Science and Engineering Department
  • Lahore University of Management Sciences

2
Outline
  • Background
  • IP Routing and related problems
  • MPLS Routing Basics
  • Labels and label switched paths
  • Traffic Engineering
  • Restoration Routing
  • Our Research
  • Conclusions

3
Application Scenario
  • A service provider (ISP) with several points of
    presence (PoPs) geographically distributed
  • ISP provisions applications with strict network
    requirements (e.g., VoIP service)
  • Two major requirements
  • Guaranteed minimum bandwidth between a source and
    a destination
  • Less then 50ms recovery time in the event of any
    network element failure

4
Traditional (IP) Routing
  • Characterized by best effort service
  • Individual nodes (routers) take routing and
    forwarding decisions
  • Usually based on a pre-computed shortest path
  • Forwarding is destination based
  • When routers forward packets, they only look at
    the destination address
  • May lead to congestion in some parts of the
    network

5
IP Routing Example
D
1
1
A
A
S
B
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

6
IP Routing Example
D
1
4
A
A
S
B
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

7
IP Routing Vs MPLS
Traditional IP Routing
Multiprotocol Label Switching (MPLS)
2
1
S
D
3
5
4
MPLS allows overriding shortest paths!
8
Routing Along Parallel Paths
  • Idea
  • Let the source make the complete routing
    decision source decides the complete path for
    each flow
  • How this may be accomplished?
  • Attach a label to the IP packets let everyone
    make forwarding decision on that label
  • 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)

9
MPLS Basics
  • How did they route along parallel paths?
  • They did use a label
  • They also decided to use a new label at each hop
    to save on label space
  • Terminology
  • LSP Label switched path
  • LSR Label switch router

IP Datagram
Label
10
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
11
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)
12
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)
13
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
14
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
15
TE Capability 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 links with blue color for this request
  • Use a path with delay less than 130ms

16
MPLS Routing
2
1
S
D
3
5
4
MPLS allows routing on pre-established paths!
17
IP versus MPLS Summary
  • In IP Routing, each router makes its own routing
    and forwarding decisions
  • In MPLS, source makes the routing decision
  • Intermediate routers make forwarding decisions
  • In IP Routing, packets usually follow the SPF
  • In MPLS packets follow the CSPF
  • In IP Routing, restoration takes few seconds
  • In MPLS, restoration can be of the order of 10ms

18
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

19
Restoration Routing
  • Application of Traffic Engineering

20
Restoration in IP network
  • In traditional IP, what happens when a link or
    node fails?
  • Information needs to be disseminated in the
    network
  • During this time, packets may go in loops
  • Restoration latency is in the order of seconds

21
Restoration in MPLS
Path Protection
S
1
2
3
D
This type of path Protection still takes 100s
of ms.
Primary Path
Backup Path
22
Restoration in MPLS
Element Local Protection
S
1
2
3
D
Local Protection takes of order of 10ms
Primary Path
Backup Path
23
Opportunity Cost
  • Fast restoration requires that backup paths are
    established in advance
  • Backup provisioning requires bandwidth
    reservation along the backup paths
  • Backup bandwidth is taken from the primary
    bandwidth
  • Fewer primary LSPs can be established
  • Can we do something to avoid wasting so much
    bandwidth in backup paths?
  • Try to share the backup bandwidth!

24
BW Sharing in Backup Paths
  • Assumption
  • Two primary paths, whose backups are sharing
    bandwidth, must not fail together
  • Is this assumption realistic?
  • Failure is a low probability event
  • Once failure occurs, new primary paths with new
    backups are computed
  • Failure of another element in that time is
    unlikely

25
BW Sharing in Backup Paths
  • Example-

b1
S1
D1
LSR
max(b1, b2)
4
5
3
S2
D2
b2
26
Creation of Backup Paths
27
Types of Backup Paths
28
Backup Paths Definitions
  • A next-hop (nhop) backup path that spans
    link(i,j) is a backup path which
  • Originates at node i
  • Merges with the primary at node j
  • Provides restoration for one or more primary LSPs
    that traverse link(i,j) when
  • link(i,j) fails

29
Backup Paths Definitions
  • A next-next-hop (nnhop) backup path that spans
    link(i,j) and link(j,k) is a backup path which
  • Originates at node i
  • Merges with the primary at node k
  • Provides restoration for one or more primary LSPs
    that traverse link(i,j) and link(j,k) when
    either
  • Node j fails
  • Link(i,j) fails

30
Activation Sets
  • When an element fails, a number of backups are
    activated simultaneously
  • Such backups are in the activation set of that
    protected element
  • Backups is a single activation set can not share
    the bandwidth
  • Backups in different activation sets may share
    the bandwidth

31
Activation Set for node j
  • What paths are activated when node j fails?
  • NNhop paths that span link(x,j) and link(j,y) for
    all x,y

Note that a node is protected by nnhop paths only!
32
Activation Set for node j
33
Activation Set for link(i,j)
  • What paths are activated when link(i,j) fails
  • Nhop path that spans link(i,j)
  • Nhop path that spans link(j,i)
  • NNhop paths that span link(i,j) and link(j,x) for
    all x not equal to i,j
  • NNhop paths that span link(j,i) and link(i,x) for
    all x not equal to i,j

34
Activation Set for link(i,j)
35
Providing Protection
  • Suppose link(i,j) is traversed by a new primary
    LSP with bandwidth demand b
  • A backup path around the link(i,j) can either
    be
  • Nhop path (if node j is egress)
  • NNhop path (if node j is not egress)
  • In either case, point of local repair (PLR) is
    node i
  • We are protecting the LSP that traverses the
    triplet(PLR, facility, MP)
  • PLR is always node i
  • Facility is the entity being protected link(i,j)
    or node j
  • MP is either node j or some other node adjacent
    to node j

36
Providing Protection
  • Let the bandwidth corresponding to previously
    established LSPs traversing the triplet (PLR,
    facility, MP) is bold
  • The backup path is recomputed with bandwidth
    demand bnew boldb
  • Various computation algorithms can be deployed
    and have been studied

37
Computing the Backups
  • How much bandwidth can be shared?
  • Depends upon the routing information propagated
  • Aggregate information scenario
  • Fij BW reserved on link(i,j) for primary LSPs
  • Gij BW reserved on link(i,j) for backup LSPs
  • Rij Residual BW on link(i,j)
  • Link(i,j) will propagate above information
  • Note total primary BW on link(i,j) is FijFji

38
Computing the Backups
  • When the new backup path is nhop, how much is
    shareable on link(u,v)?
  • FijFji-bold is the maximum bandwidth that will
    simultaneously be active with new backup
  • The bandwith shareable on link(u,v) is
  • Suv max(0, Guv (FijFji-bold))
  • When the new backup path is nnhop, how much is
    shareable on link(u,v)?
  • Note that nnhop is protecting against a link as
    well as a node. Thus, the bandwidth required for
    both the activation sets must be computed
  • Max(FijFji-bold, ?Fxj-bold) is the maximum that
    will simultaneously be active with the new backup
  • The bandwidth shareable on link(u,v) is
  • Suv max(0, Guv max(FijFji-bold,
    ?Fxj-bold))

39
Computing the Backups
  • PLR knows Ruv and Suv for all links
  • PLR computes total bandwidth RuvSuv available to
    route the new backup path on each link(u,v)
  • All links for which RuvSuv lt bnew are pruned
  • For each remaining link(u,v), the additional
    bandwidth required is given by max(0, bnew-Suv)
  • PLR computes the route that requires minimum
    additional bandwidth
  • Note The computed path is sub-optimal

40
Simulation Parameters
  • 20 node ISP network
  • Each link with capacity 120 units
  • 380 possible pairs
  • LSP requests arrive one by one
  • Ingress/Egress chosen randomly
  • Bandwidth demand for each request is uniformly
    distributed between 1 and 6
  • Call holding time is infinite
  • 10 experiments with randomly selected
    ingress/egress pairs and traffic demands

41
Schemes Compared
  • Kinis scheme
  • Signalled path is suboptimal
  • Reservations made are corrective
  • Facility
  • Optimal path is signalled
  • Static pools for primary and backups
  • NPP
  • Primary and backups dynamically allocated
  • Optimal path is signalled

42
Results
43
Results
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