Transcript MPLS Architecture SMU CSE 8344

MPLS Architecture
SMU
CSE 8344
MPLS Network Model
Internet
LER
IP
LER
LSR
LSR
MPLS
LSR
MPLS
LSR
LER
LSR = Label Switched Router
LER
SMU= Label Edge Router
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IP
MPLS Benefits
Comparing 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: Interfacespecific 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
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link level
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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Hierarchical Routing in MPLS
•External Routers A,B,C,D,E,F - Talk BGP
•Internal Routers 1,2,3,4,5,6 - Talk OSPF
C
D
Domain #2
1
B
A
Domain #1
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2
3
4
5
E
F
Domain #3
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Note: Internal routers in CSE
domains
1 and 3 not shown
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
POLICY-BASED NETWORKING
IntServ
Multimedia
Video Conference,
Collaborative Computing
DiffServ
MPLS
VPNs
Hybrid
Signaling Techniques (RSVP, DSCP*, ATM (UNI/NNI))
Classification & Marking Techniques (DSCP, MPLS EXP, NBAR, etc.)
Congestion Avoidance Techniques (WRED)
Traffic Conditioners (Policing, Shaping)
Congestion Management Techniques (WFQ, CBWFQ, LLQ)
Link Efficiency Mechanisms (Compression, Fragmentation)
Frame
Relay
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PPP
HDLC
SDLC
ATM, POS
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FE,Gig.E
10GE
Wireless
Fixed,Mobile
BroadBand
Cable,xDSL
PROVISIONING & MONITORING
Mission Critical
Services
VoIP
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 microflows
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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
LDP/RSVP
LSR
LDP/RSVP
E-LSP
AF1
EF
•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
Referred to as Packet Classification or Coloring
Standard IPV4: Bits 0-2 Called IP Precedence (Three MSB)
(DiffServ Uses Six ToS bits…: Bits 0-5, with Two Reserved)
Version ToS
Len
Length 1 Byte
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ID
offset
TTL Proto FCS IP-SA IP-DA
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Data
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
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• 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
7
2
45
4
150
1
150
150
150
5
3
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6
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
R3
150
R4
150
R5
R2
150
R1
150
R6
R7
150
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Is Plain IP Enough?
R8
R3
150
R4
150
R5
R2
150
R1
150
R6
Under utilized
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R7
150
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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
R3
150
R4
150
R5
R2
150
R1
150
R6
R7
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
R2
X
R3
R5
R4
Even if the traffic is rerouted to R3, it will that
back to R1 since R3 is not aware of the failure
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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
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