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MPLS: Multi-protocol Label Switching

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Title: MPLS: Multi-protocol Label Switching


1
MPLS Multi-protocol Label Switching
2
Topics
  • Introduction
  • History and motivation
  • MPLS mechanisms
  • MPLS protocols
  • RSVP-TE/CR-LDP
  • MPLS applications
  • VPNSs, traffic engineering, restoration
  • Generalized MPLS

3
WHY MPLS ?
  • Ultra fast forwarding
  • Use switching instead of routing
  • IP Traffic Engineering
  • Constraint-based routing
  • Virtual Private Networks
  • Controllable tunneling mechanism
  • Protection and restoration

4
IP Forwarding Table
47.1..
1
2
1
3
2
1
47.2..
3
47.3..
2
5
Hop-by-Hop IP Forwarding
47.1
1
IP 47.1.1.1
2
IP 47.1.1.1
1
3
2
IP 47.1.1.1
1
47.2
3
47.3
2
6
Routing Lookup
Interface
Prefix
Next Hop
4
9.1..
67.1.2.2
8
9.1.1.
113.1.2.1
6
9.1.1.1
71.1.2.3
6
9.2.1.1
71.1.2.3
  • Longest prefix match is (was) expensive.
  • Label matching is much less expensive.

7
MPLS Labels
1
47.1
3
3
2
1
1
2
47.3
3
47.2
2
8
Label Switched Path
1
47.1
3
3
2
1
1
2
47.3
3
47.2
2
9
Forwarding Equivalence Classes
LSR
LSR
LER
LER
LSP
Packets are destined for different address
prefixes, but can be mapped to common path
  • FEC A subset of packets that are all treated
    the same way by a router
  • The concept of FECs provides for a great deal of
    flexibility and scalability
  • In conventional routing, a packet is assigned to
    a FEC at each hop (i.e. L3 look-up), in MPLS it
    is only done once at the network ingress

10
MPLS Terminology
  • LDP Label Distribution Protocol
  • LSP Label Switched Path
  • FEC Forwarding Equivalence Class
  • LSR Label Switching Router
  • LER Label Edge Router

11
Label Distribution Methods
Downstream Label Distribution
Downstream-on-Demand Label Distribution
LSR2
LSR1
LSR2
LSR1
Label-FEC Binding
Request for Binding
  • LSR2 discovers a next hop for a particular FEC
  • LSR2 generates a label for the FEC and
    communicates the binding to LSR1
  • LSR1 inserts the binding into its forwarding
    tables
  • If LSR2 is the next hop for the FEC, LSR1 can use
    that label knowing that its meaning is understood

Label-FEC Binding
  • LSR1 recognizes LSR2 as its next-hop for an FEC
  • A request is made to LSR2 for a binding between
    the FEC and a label
  • If LSR2 recognizes the FEC and has a next hop for
    it, it creates a binding and replies to LSR1
  • Both LSRs then have a common understanding

Both methods are supported, even in the same
network at the same time
12
Distribution Control
Next Hop (for FEC)
Incoming Label
Outgoing Label
Independent LSP Control
Ordered LSP Control
  • Label-FEC binding is communicated to peers if
  • - LSR is the egress LSR to particular FEC
  • - label binding has been received from
    upstream LSR
  • LSP formation flows from egress to ingress
  • Each LSR makes independent decision on when to
    generate labels and communicate them to upstream
    peers
  • Communicate label-FEC binding to peers once
    next-hop has been recognized
  • LSP is formed as incoming and outgoing labels are
    spliced together

Definition
  • Labels can be exchanged with less delay
  • Does not depend on availability of egress node
  • Granularity may not be consistent across the
    nodes at the start
  • May require separate loop detection/mitigation
    method
  • Requires more delay before packets can be
    forwarded along the LSP
  • Depends on availability of egress node
  • Mechanism for consistent granularity and freedom
    from loops
  • Used for explicit routing and multicast

Comparison
Both methods are supported in the standard and
can be fully interoperable
13
Label Retention Methods
Conservative Label Retention
Liberal Label Retention
LSR2
LSR2
Label Bindings for LSR5
Label Bindings for LSR5
LSR1
LSR1
LSR3
LSR3
LSR4s Label LSR3s Label LSR2s Label
LSR4s Label LSR3s Label LSR2s Label
LSR4
LSR4
Valid Next Hop
Valid Next Hop
  • LSR maintains bindings received from LSRs other
    than the valid next hop
  • If the next-hop changes, it may begin using these
    bindings immediately
  • May allow more rapid adaptation to routing
    changes
  • Requires an LSR to maintain many more labels
  • LSR only maintains bindings received from valid
    next hop
  • If the next-hop changes, binding must be
    requested from new next hop
  • Restricts adaptation to changes in routing
  • Fewer labels must be maintained by LSR

Label Retention method trades off between label
capacity and speed of adaptation to routing
changes
14
Label Encapsulation
ATM
FR
Ethernet
PPP
L2
VPI
VCI
DLCI
Shim Label
Label
Shim Label .
IP PAYLOAD
MPLS Encapsulation is specified over various
media types. Top labels may use existing format,
lower label(s) use a new shim label format.
15
Label Format
Label 20 bits
Exp 3 bits
Stack 1 bit
TTL 8 bits
  • Exp field used to identify the class of service
  • Stack bit is used identify the last label in the
    label stack
  • TTL field is used as a time-to-live counter.
    Special processing rules are used to mimic IP TTL
    semantics.

16
Label Distribution Protocols
  • Label Distribution Protocol (LDP)
  • Constraint-based Routing LDP (CR-LDP)
  • Extensions to RSVP
  • Extensions to BGP

17
LDPLabel Distribution Protocol
Label distribution ensures that adjacent routers
have a common view of FEC lt-gt label bindings
Routing Table Addr-prefix Next
Hop 47.0.0.0/8 LSR3
Routing Table Addr-prefix Next
Hop 47.0.0.0/8 LSR2
LSR1
LSR3
LSR2
IP Packet
47.80.55.3
Label Information Base Label-In FEC
Label-Out XX 47.0.0.0/8 17
For 47.0.0.0/8 use label 17
Label Information Base Label-In FEC
Label-Out 17 47.0.0.0/8 XX
Step 2 LSR communicates binding to adjacent LSR
Step 3 LSR inserts label value into forwarding
base
Step 1 LSR creates binding between FEC and
label value
Common understanding of which FEC the label is
referring to!
18
LDP Basic Characteristics
  • Provides LSR discovery mechanisms to enable LSR
    peers to find each other and establish
    communication
  • Defines four classes of messages
  • DISCOVERY deals with finding neighboring LSRs
  • ADJACENCY deals with initialization, keep alive,
    and shutdown of sessions
  • LABEL ADVERTISEMENT deals with label binding
    advertisements, request, withdrawal, and release
  • NOTIFICATION deals with advisory information and
    signal error information
  • Runs over TCP for for reliable delivery of
    messages, except for discovery, which uses UDP
    and IP multicast
  • Designed to be extensible, using messages
    specified as TLVs (type, value, length) encoded
    objects.

19
LDP Messages
  • INITIALIZATION
  • KEEPALIVE
  • LABEL MAPPING
  • LABEL WITHDRAWAL
  • LABEL RELEASE
  • LABEL REQUEST

20
Explicitly Routed LSP
1
47.1
3
3
2
1
1
2
47.3
3
47.2
2
21
ER LSP - Advantages
  • Operator has routing flexibility
  • policy-based, QoS-based
  • Can use routes other than shortest path
  • Can compute routes based on constraints in
    exactly the same manner as ATM based on
    distributed topology database.(traffic
    engineering)

22
ER LSP - discord!
  • Two signaling options proposed in the standards
    CR-LDP, RSVP extensions
  • CR-LDP LDP Explicit Route
  • RSVP ext Traditional RSVP Explicit Route
    Scalability Extensions
  • Market will probably have to resolve it
  • Survival of the fittest not such a bad thing.

23
MPLS and QoS in IP Network
  • Integrated Services
  • Differentiated Services

24
Integrated Services Internet
  • Applications specify traffic and service specs
  • Tspec traffic specs including peak rate, maximum
    packet size, burst size, and mean rate
  • Rspec service spec, specifically service rate
  • Two classes of service defined
  • Guaranteed service satisfies hard guarantees on
    bandwidth and delay
  • Controlled load service provides service similar
    to that in unloaded network
  • RSVP was extended to RSVP-TE support signaling
  • RSVP was further extend to add MPLS support

25
Differentiated Services Internet
  • IP packets carry 6-bit service code points (DSCP)
  • Potentially support 64-different classes of
    services
  • Routers map DSCP to per-hop-behavior (PHB)
  • PHBs can be standard or local
  • Standard PHBs include
  • Default No special treatment or best effort
  • Expedited forwarding (EF) Low delay and loss
  • Assured forwarding (AF) Multiple classes, each
    class with multiple drop priorities
  • LSRs dont sort based on IP headers, hence DSCPs
    need to be mapped to EXP field in MPLS shim
    header
  • Exp field is only 3-bit wide can support only 8
    DSCPs/PHBs
  • Labels can be used if more than 8 PHBs need to be
    supported
  • Same approach can be used for link layers which
    do not use Shim headers, e.g. ATM

26
Traffic Engineering with RSVP
PATH Tspec
PATH Tspec
PATH Tspec
PATH Tspec
Sender
RESV Rspec
RESV Rspec
RESV Rspec
RESV Rspec
Receiver
27
Label Distribution with RSVP-TE
PATH Tspec
PATH Tspec
PATH Tspec
PATH Tspec
Sender
RESV Rspec
RESV Rspec Label 5
RESV Rspec Label 10
PATH Tspec
RESV Rspec
28
MPLS Protection
  • End-to-end protection
  • Fast node and link reroute

29
MPLS Protection End-to-end Path Protection
F
Primary LSP
E
A
D
B
C
Backup LSP
Backup and primary LSPs should be route diverse
30
MPLS Protection Fast Reroute
Detour to avoid CD
Detour to avoid AB
Detour to avoid link DE
LSR F
LSR D
LSR B
LSR A
LSR E
LSR C
Detour to avoid DE
Detour to avoid BC
  • Detour around node or link failures
  • Example LSP shown traverses (A, B, C, D, E, F)
  • Each detour avoids
  • Immediate downstream node link towards it
  • Except for last detour only avoids link DE

31
Detour Merging
Detour to avoid AB
Merged detour to avoid AB and BC
Detour to avoid BC
LSR F
LSR A
LSR E
LSR C
LSR D
LSR B
  • Reduces state maintained
  • Improves resource utilization

32
MPLS Protection Types
  • 11 Backup LSP established in advance,
    resources dedicated, data simultaneously sent on
    both primary and backup
  • Switchover performed only by egress LSR
  • Fastest, but most resource intensive
  • 11 Same as 11 with the difference that data
    is not sent on the backup
  • Requires failure notification to the ingress LSR
    to start transmitting on backup
  • Notification may be send to egress also
  • Resources in the backup may be used by other
    traffic
  • Low priority traffic (e.g., plain IP traffic),
    shared by other backup paths

33
MPLS VPN The Problem
Customer 1 Site 2
Customer 1 Site 1
10.2/16
Provider Network
10.1/16
10.2/16
Customer 2 Site 2
10.1/16
Customer 2 Site 1
Customer 1 Site 3
10.3/16
Customer 2 Site 3
10.3/16
34
MPLS VPN The Model
Customer 1 Site 2
Customer 1 Site 1
10.2/16
Customer 1 Virtual Network
10.1/16
Customer 2 Site 2
10.2/16
Customer 2 Virtual Network
10.1/16
Customer 2 Site 1
Customer 1 Site 3
10.3/16
Customer 2 Site 3
10.3/16
35
MPLS VPN The Solution
Customer 1 Site 2
MPLS LSP
Customer 1 Site 1
10.2/16
VRF 1
10.1/16
VRF 1
10.2/16
VRF 2
Customer 2 Site 2
VRF 2
10.1/16
VRF 1
Customer 2 Site 1
VRF 2
Customer 1 Site 3
MPLS LSP
10.3/16
Customer 2 Site 3
10.3/16
36
Unified Control Plane
E-NNI
IP Network
UNI
Optical Network
Optical subnet
Optical subnet
IP Network
I-NNI
Optical subnet
E-NNI
E-NNI
ATM Network
UNI
IP Network
UNI - User-to-Network Interface I-NNI - Internal
Network-to-Network Interface E-NNI - External
Network-to-Network Interface
37
GMPLS Generalized MPLS
LSC Cloud
TDM Cloud
PSC Cloud
FSC Cloud
  • GMPLS Handles Nodes With Diverse Capabilities.
  • Packet Switch Capable (PSC)
  • Time Division Multiplexing Capable (TDM)
  • Lambda Switch Capable (LSC)
  • Fiber Switch Capable (FSC)
  • Each Node Is Treated As an MPLS Label-switching
    Router (LSR)
  • Lightpaths/TDM Circuits Are Considered Similar to
    Label-Switched Paths (LSPs)
  • Selection of ?s and OXC ports are considered
    similar to selection of labels
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