mpls architecture
DESCRIPTION
MPLS Architecture. Internet. IP. LER. LER. LER. LSR. LSR. LSR. LSR. MPLS. IP. MPLS Network Model. MPLS. LSR = Label Switched Router LER = Label Edge Router. MPLS Benefits. Comparing MPLS with existing IP core and IP/ATM technologies, MPLS has many advantages and benefits: - PowerPoint PPT PresentationTRANSCRIPT
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MPLS Architecture
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MPLS Network Model
MPLS
LSR = Label Switched RouterLER = Label Edge Router
LER
LER
LSR
LER
LSRLSR
IP
MPLS
IP
Internet
LSR
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MPLS BenefitsComparing MPLS with existing IP core and
IP/ATM technologies, MPLS has many advantages and benefits:
• The performance characteristics of layer 2 networks
• The connectivity and network services of layer 3 networks
• Improves the price/performance of network layer routing
• Improved scalability
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MPLS Benefits (cont’d)
• Improves the possibilities for traffic engineering
• Supports the delivery of services with QoS guarantees
• Avoids need for coordination of IP and ATM address allocation and routing information
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Necessity of L3 Forwarding
• For security– To allow packet filtering at firewalls– Requires examination of packet
contents, including the IP header
• For forwarding at the initial router - used when hosts don’t do MPLS
• For Scaling– Forward on a finer granularity than
the labels can provide
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MPLS Architecture
• Down stream label assignment for unicast traffic– On demand– Unsolicited
• Path selection– Hop by hop– Explicit
• Ordered vs. independent control• Loop detection and prevention
mechanisms
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Label Distribution Protocol (LDP)
• Set of procedures used by LSRs to establish LSPs
• Mapping between network-layer routing information directly to data-link layer switched paths
• LDP peers: – Two LSRs which use LDP to exchange
label/stream mapping – Information exchange known as “LDP Session”
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LDP Messages
• Discovery messages – Used to announce and maintain the presence of
an LSR
• Session/Adjacency messages – Used to establish, maintain and terminate
sessions between LDP peers
• Advertisement messages– Used to create, change, and delete label
mappings
• Notification messages– Used to provide advisory information and to
signal error information
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Forwarding Equivalence Class (FEC)
• Introduced to denote packet forwarding classes
• Comprises traffic – To a particular destination– To destination with distinct service
requirements
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LSP - FEC Mapping
• FEC specified as a set of two elements – IP Address Prefix - any length from 0 – 32– Host Address - 32 bit IP address
• A given packet matches a particular LSP if and only if IP Address Prefix FEC element matches packet’s IP destination address
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Label Spaces
• Useful for assignment and distribution of labels
• Two types of label spaces– Per interface label space: Interface-
specific labels used for interfaces that use interface resources for labels
– Per platform label space: Platform-wide incoming labels used for interfaces that can share the same label space
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LDP Discovery• A mechanism that enables an LSR to
discover potential LDP peers• Avoids unnecessary explicit configuration
of LSR label switching peers • Two variants of the discovery mechanism
– Basic discovery mechanism: used to discover LSR neighbors that are directly connected at the link level
– Extended discovery mechanism: used to locate LSRs that are not directly connected at the link level
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LDP Discovery (Cont’d)• Basic discovery mechanism
– To engage - send LDP Hellos periodically– LDP Hellos sent as UDP packets for all routers
on that subnet
• Extended discovery mechanism– To engage - send LDP targeted Hellos
periodically– Targeted Hellos are sent to a specific address– Targeted LSR decides whether to respond or to
ignore the targeted Hello
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Session Establishment
• Exchange of LDP discovery Hellos triggers session establishment
• Two step process– Transport connection establishment
• If LSR1 does not already have a LDP session for the exchange of label spaces LSR1:a and LSR2:b, it attempts to open a TCP connection with LSR2
• LSR1 determines the transport addresses at its end (A1) and LSR2’s end (A2) of the TCP connection
• If A1>A2, LSR1 plays the active role; otherwise it is passive
– Session initialization• Negotiate session parameters by exchanging LDP
initialization messages
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Label Distribution and Management
• Two label distribution techniques– Downstream on demand label distribution:
An LSR can distribute a FEC label binding in response to an explicit request
– Downstream Unsolicited label distribution: Allows an LSR to distribute label bindings to LSRs that have not explicitly requested them
• Both can be used in the same network at the same time; however, each LSR must be aware of the distribution method used by its peer
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Label Distribution Control Mode
• Independent Label Distribution Control– Each LSR may advertise label mappings to its
neighbors at any time– Independent Downstream on Demand mode -
LSR answers without waiting for a label mapping from next hop
– Independent Downstream Unsolicited mode - LSR advertises label mapping for a FEC whenever it is prepared
– Consequence: upstream label can be advertised before a downstream label is received
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Distribution Control Mode (cont’d)
• Ordered Label Distribution Control– Initiates transmission of label mapping for a
FEC only if it has next FEC next hop or is the egress
– If not, the LSR waits till it gets a label from downstream LSR
– LSR acts as an egress for a particular FEC, if• Next hop router for FEC is outside of label switching
network• FEC elements are reachable by crossing a domain
boundary
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Label Retention Mode• Conservative Label Retention Mode
– Advertised label mappings are retained only if they are used for forwarding packets
– Downstream on Demand Mode typically used with Conservative Label Retention Mode
– Advantage: only labels required are maintained– Disadvantage: a change in routing causes
delay
• Liberal Retention Mode– All label mappings are retained regardless of
whether LSR is next hop or not– Faster reaction to routing changes
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Label Information Base
• LSR maintains learned labels in Label Information Base (LIB)
• Each entry of LIB associates an FEC with an (LDP Identifier, label) pair
• When next hop changes for a FEC, LSR will retrieve the label for the new next hop from the LIB
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Domain #3
Domain #2
Domain #1
Hierarchical Routing in MPLS
C
12 3 4 5
6
D
EBA F
•External Routers A,B,C,D,E,F - Talk BGP
•Internal Routers 1,2,3,4,5,6 - Talk OSPF
Note: Internal routers in domains 1 and 3 not shown
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Hierarchical Routing (cont’d)
• When IP packet traverses domain #2, it will contain two labels, encoded as a “label stack”
• Higher level label used between routers C and D, which is encapsulated inside a lower level label used within Domain #2
• Operation at C– C needs to swap BGP label to put label that D expects– C also needs to add an OSPF label that 1 expects– C therefore pushes down the BGP label and adds a
lower level label
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Explicit Routing in MPLS• Two options for route selection:
– Hop by hop routing– Explicit routing
• Explicit Routing (Source Routing) is a very powerful technique– With pure datagram routing, overhead of
carrying complete explicit route is prohibitive
– MPLS allows explicit route to be carried only at the time the LSP is setup, and not with each packet
– MPLS makes explicit routing practical
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Explicit Routing (Cont’d)
• In an explicitly routed LSP – LSP next hop is not chosen by the
local node– Selected by a single node, usually the
ingress• The sequence of LSRs may be
chosen by– Configuration (e.g., by an operator or
by a centralized server)
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Loops and Loop Handling
• Routing protocols used in conjunction with MPLS are based on distributed computation which may contain loops
• Loops handling - 3 categories– Loop Mitigation/Survival– Loop Detection– Loop Prevention
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Loop Mitigation
• Minimizes the impact of loops by limiting the amount of resources consumed by the loop
• Method– Based on use of TTL field which is
decremented at each hop– Use of dynamic routing protocol
converging rapidly to non-looping paths
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Loop Detection
• Loops may be setup but they are subsequently detected
• The detected loop is then broken by dropping label relationship
• Broken loops now necessitates packets to be forwarded using L3 forwarding
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Loop Detection (Cont’d)
• Method is based on transmitting a Loop Detection Control Packet (LDCP) whenever a route changes
• LDCP is forwarded towards the destination until– Last MPLS node along the path is reached– TTL of the LDCP expires– It returns to the node which originated it
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Loop Prevention
• Ensures that loops are never set up• Labels are not used until it is sure to be
loop free• Methods
– Labels are propagated starting at the egress switch
– Use source routing to set up label bindings from the egress switch to each ingress switch
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QoS in MPLS
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Strategy
• To support end-to-end QoS as in IP• MPLS not an end-to-end protocol• Efficient ways of mapping QoS to
LSPs• Traffic Engineering key to QoS
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QoS Models
• Best effort – Original IP service
• Int-serv.– Fist IP effort to support QoS
• Diff-serv.– Simple, scalable
• Future– Int+ Diff+ TE with e2e SLAs
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CISCO QoS Framework
PR
OV
ISIO
NIN
G &
MO
NIT
OR
ING
PR
OV
ISIO
NIN
G &
MO
NIT
OR
ING
VPNsVPNsMultimediaVideo Conference,
Collaborative Computing
MultimediaVideo Conference,
Collaborative Computing
Mission Critical Services
Mission Critical ServicesVoIPVoIP
HybridHybridMPLSMPLSDiffServDiffServIntServIntServ
Signaling Techniques (RSVP, DSCP*, ATM (UNI/NNI))Signaling Techniques (RSVP, DSCP*, ATM (UNI/NNI))
Link Efficiency Mechanisms (Compression, Fragmentation)Link Efficiency Mechanisms (Compression, Fragmentation)
Congestion Avoidance Techniques (WRED)Congestion Avoidance Techniques (WRED)
Congestion Management Techniques (WFQ, CBWFQ, LLQ)Congestion Management Techniques (WFQ, CBWFQ, LLQ)
Classification & Marking Techniques (DSCP, MPLS EXP, NBAR, etc.)Classification & Marking Techniques (DSCP, MPLS EXP, NBAR, etc.)
FrameRelay
FrameRelay
PPPHDLC
PPPHDLC SDLC
SDLCATM, POSATM, POS FE,Gig.E
10GE
FE,Gig.E 10GE
WirelessFixed,Mobile
WirelessFixed,Mobile
BroadBandCable,xDSL
BroadBandCable,xDSL
PO
LIC
Y-B
AS
ED
NETW
OR
KIN
GP
OLIC
Y-B
AS
ED
NETW
OR
KIN
G
Traffic Conditioners (Policing, Shaping)Traffic Conditioners (Policing, Shaping)
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Support of RSVP
• Very similar to tag switching• Bind labels to reserved flows
– Label object inside the RESV message– Labels propagate upstream
• Only the edge router need to know the packet to flow mapping– Can aggregate flows instead of micro-
flows
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RSVP Scalability
• Aggregation• Refresh reduction
– Use acknowledgements for refresh– Once received, increase the refresh
time– Summary refresh
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Diff-Serv Support
– E-LSP
– “Queue” inferred from Label and EXP field
– “Drop priority” inferred from label and EXP field
– L-LSP
– Queue” inferred exclusively from Label
– “Drop priority” inferred from EXP field
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E-LSP
E-LSP
LSRLDP/RSVP LDP/RSVP
EF
AF1
•E-LSPs established by various label binding protocols (LDP or RSVP)
•no new Signalling needed.
•EF and AF1 on a single E-LSP
•EF and AF1 packets travel on single LSP (single label) but are enqueued in different queues (different EXP values)
•Queue & Drop Precedence is selected based on EXP
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E-LSP
VersionLengthVersionLength
ToSToS1 Byte1 ByteToSToS
1 Byte1 Byte LenLen
Standard IPV4: Bits 0-2 Called IP Precedence (Three MSB)(DiffServ Uses Six ToS bits…: Bits 0-5, with Two Reserved)
IDID offsetoffset TTLTTL ProtoProto FCSFCS IP-SAIP-SA IP-DAIP-DA DataData
Referred to as Packet Classification or Coloring
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IP Precedence to Label EXP
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E-LSP vs. L-LSP
• PHB from EXP• No additional
signaling• EXP->PHB
configured• Shim header
required• Up to 8 PHBs per LSP
• PHB from label + Exp/CLP
• Signaled at LSP setup
• Label->PHB mapped • Shim or link layer
header used• Arbitrarily large
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Explicit Congestion Notification(ECN)
• TCP approach – based on packet drop– May not reflect the status– Resources could have been wasted
• Early notification– Mark packets– Receiver conveys information to sender
• Two bits used to deal with deployment disparity (CE & ECT)
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MPLS Support of ECN
• Could use two bits as before– May not be available– Usually 1 bit available– LSRs should have the understanding
on mapping
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Traffic Engineering in MPLS
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Traffic Engineering Objectives
• Traffic Engineering (TE) concerned with performance optimization
• The key performance objectives – traffic oriented e.g. minimization of
packet loss – resource oriented - optimization of
resource utilization e.g. efficient management of bandwidth
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Objectives (cont’d)
• Minimizing congestion is a major traffic and resource oriented performance objective
• Congestion manifest under two scenarios– Network resources insufficient or
inadequate• Solved by capacity expansion or classical
congestion control techniques
– Inefficient mapping of traffic streams onto available resources
• Reduced by adopting load balancing policies
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MPLS and Traffic Engineering
• Main components used– Traffic Trunk - aggregation of traffic
flows of the same class which are placed inside a Label Switched Path
– Induced MPLS Graph • Analogous to a virtual topology in an
overlay model• Logically mapped onto the physical
network • Set of LSRs as nodes of the graph • Set of LSPs providing logical point to
point connectivity between LSRs as edges
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Constraint Based Routing (CBR)
• Associate each path with set of constraints– Performance, administrative– Local information
• Routing algorithms – Optimizes various metrics– Ensures that the constraints are not
violated
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Can IP Routing Do CBR?
• Plain IP routing cannot– CBR has to be source based – each
source may have different constraint to same destination
– Link attributes need to be distributed– Need explicit routing instead of
“destination-based”• Can be augmented to support CBR
– Usually a combination is used
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CBR Components
• Mechanism for source based path computing
• Mechanism to collect necessary information– Constraints (local), attributes, topology
• Support forwarding along the computed paths
• Notification of residual resources after allocation
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Constrain-Based SPF
24
7
53
1
6
150
45
150
150
150150
150
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CSPF• Uses the following inputs
– Link attributes– Topology state information– Path constraints
• Basic approach– Prune resources that do not meet the
constraints– Run a shortest path algorithm on the
residual graph
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MPLS for Forwarding
• Ideal to use MPLS explicit routing capability
• Once the path is computed – Need to establish forwarding state along the
path– Reserve resources along the path
• Two approaches– RSVP extensions– CR-LDP
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CBR (cont’d)
• Strict & Loose Explicit Routes– Constraint Based LSP (CRLSP) is
calculated at one point at the edge of the network based on certain criteria
– special char. such as assigning certain bandwidth can be supported
– The route is encoded as a series of Explicit routed hops contained in a CR based route TLV
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CBR (cont’d)
• Comparison of RSVP and CR-LDP– Scalability– Signaling mechanism– Qos Models
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Application of CR in TE
• IP?• ATM• Overlay• MPLS
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TE in MPLS - II
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Fish Network
R8
R1
R5
R2
R3R4
R7R6
150
150
150
150
150
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Is Plain IP Enough?
R8
R1
R5
R2
R3R4
R7R6
150
150
150
150
150
Under utilized
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Why IP Routing Fails
• Based only on metric optimization– Shortest path– Administrative optimization– Split paths
• Per link constraints not taken into consideration
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TE in MPLS Using CBR • Define traffic trunks
– Collection of micro-flows that share same path and class of service
– These are not end-to-end paths, rather paths within a single service provider
• No. of trunks dependent only on the topology
• Forwarding table does not grow with the traffic
• Rerouting– RSVP, CR-LDP, or IGP
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Forwarding Packets
R1
R5
R2
R3R4
R7R6
150
150
150
150
150
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Fast Rerouting
• Total restoration time after failure– Failure detection time– Propagation– Computation of new path
• Usually the 2nd and 3rd steps are significantly slow
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Is FR possible with IP?
R1
R4R3
R2
R5
Even if the traffic is rerouted to R3, it will that back to R1 since R3 is not aware of the failure
X
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FR using CBR
• Compute protection LSP for every link
• When a failure happens– Traffic rerouted to the protection LSP– Use label stacking for the transit
within the protection LSP– Beyond the end-nodes labels original
labels remain in tact