Chapter 5
MPLS
Labels
There are many examples of label substitution
protocols already in existence.
• ATM - label is called VPI/VCI and travels with cell.
• Frame Relay - label is called a DLCI and travels with frame.
• TDM - label is called a timeslot its implied, like a lane.
• X25 - a label is an LCN
• Proprietary PORS, TAG etc..
• One day perhaps Frequency substitution where label is a
light frequency?
What is MPLS?
• Hop-by-hop or source (explicit)
routing to establish labels
• Uses label native to the media
• Multilevel label substitution transport
What is MPLS?
 Virtual circuit layer underneath IP
 Virtual circuit = virtual wire = label switched path
IP Network
(Voice)
IP Network
(ATM)
IP Network
(Data)
MPLS
(Virtual Point-to-Point Circuits)
Physical Infrastructure
(Point-to-Point Circuits)
What is MPLS?
 Offer service above IP
 Converged network
 Realtime voice
 Best-effort data
 High priority transactions (ATM, control …)
 On the same physical infrastructure
What is MPLS?
 MPLS Characteristics




Mechanisms to manage traffic flows
Is independent of Layer-2 and Layer-3 protocols
Maps IP-addresses to fixed length labels
Interfaces to existing routing protocols (RSVP,
OSPF)
 Supports ATM, Frame-Relay and Ethernet
Why MPLS?
 Leverage existing ATM hardware
 Ultra fast forwarding
 IP Traffic Engineering
 Constraint-based Routing
 Virtual Private Networks
 Controllable tunneling mechanism
 Voice/Video on IP
 Delay variation + QoS constraints
7
ROUTE AT EDGE, SWITCH
IN CORE
IP
IP
IP Forwarding
#L1
IP
#L2
LABEL SWITCHING
IP
#L3
IP
IP Forwarding
MPLS Terminology
 LDP: Label Distribution Protocol
 LSP: Label Switched Path
 FEC: Forwarding Equivalence Class
 LSR: Label Switching Router
 LER: Label Edge Router
WHAT IS A LABEL?
 “labels” called a label stack.
 A label is a short, four-byte, fixed-length, locally-significant identifier
which is used to identify a Forwarding Equivalence Class (FEC).
 The label which is put on a particular packet represents the FEC to which
that packet is assigned.
 Each label stack entry contains four fields:
Label - a 20-bit label value.
Exp - a 3-bit Traffic Class field for QoS (quality of service) priority (experimental) and ECN
(Explicit Congestion Notification).
S
- a 1-bit bottom of stack flag. If this is set, it signifies that the current label is the last in the
stack.
TTL - an 8-bit TTL (time to live) field.
FORWARDING EQUIVALENCE
CLASS (FEC)

Any subset of packets are treated the same way by a router
 Forwarded out the same interface with the same next hop and label
 Given the same class of service, output on same queue, given same drop
preference, and any other option available to the network operator.



When a packet enters the MPLS network at the ingress node, the packet
is mapped into an FEC. The mapping can also be done on a wide variety
of parameters (as specified by network manager); address prefix (or
host), source/destination IP address pair, port numbers, IP protocol ID or
ingress interface. This greater flexibility adds functionality to MPLS that is
not available in traditional IP routing.
FECs also allow for greater scalability in MPLS. The aggregation of flows
into FECs of variable granularity provides scalability that meets the
demands of the public Internet as well as enterprise applications.
In the current LDP specification, only 3 types of FECs are specified:
 IP Address Prefix
 Router ID
 Flow (port, dest-addr, src-addr etc.)

The spec. states that new elements can be added as required.
How Does MPLS Work?
 Packets are tagged and routed based on tags.
 All traffic with the same label treated the same
13
5
IP Routing
Layer
Payload
Payload
IP Routing
Layer
LSR
Payload
Payload
13
Payload
5
Payload
LER
LER
13
Payload
Payload
5
Payload
Payload
Other Features of MPLS
 Label forwarding distinct from IP
forwarding
 May make non-shortest paths
 Label routing linked to IP routing
IP Forwarding
LER
(Perform Labeling)
LSR Cloud
(Forward by label)
IP Forwarding
LER
(Remove Label)
LSR Cloud
(Forward by label)
MPLS BUILT ON STANDARD IP
Dest
47.1
47.2
47.3
Dest
47.1
47.2
47.3
Out
1
2
3
Out
1
2
3
1 47.1
3
1
Dest
47.1
47.2
47.3
Out
1
2
3
2
3
2
1
47.2
47.3 3
2
• Destination based forwarding tables as built by OSPF, IS-IS, RIP, etc.
IP FORWARDING USED BY HOPBY-HOP CONTROL
Dest
47.1
47.2
47.3
Dest
47.1
47.2
47.3
Out
1
2
3
1 47.1
1
Dest
47.1
47.2
47.3
Out
1
2
3
IP 47.1.1.1
2
IP 47.1.1.1
3
Out
1
2
3
2
IP 47.1.1.1
1
47.2
47.3 3
2
IP 47.1.1.1
MPLS Label Distribution
Intf Label Dest Intf Label
In In
Out Out
3
0.50 47.1 1
0.40
Intf
In
3
Label Dest Intf
In
Out
0.40 47.1 1
1
Request: 47.1
3
Intf Dest Intf Label
In
Out Out
3
47.1 1
0.50
3
2
1
1
47.3 3
47.1
Mapping: 0.40
2
47.2
2
Label Switched Path (LSP)
Intf Label Dest Intf Label
In In
Out Out
3
0.50 47.1 1
0.40
Intf Dest Intf Label
In
Out Out
3
47.1 1
0.50
2
2
47.2
2
IP 47.1.1.1
3
1
47.3 3
Label Dest Intf
In
Out
0.40 47.1 1
IP 47.1.1.1
1 47.1
3
1
Intf
In
3
LABEL EDGE ROUTER
(LER)
 Can be an ATM switch or a router
 Ingress LER performs the following:
 Receives the packet
 Adds label
 Forwards the packet into the MPLS domain
 Egress LER removes the label and delivers
the packet
LABEL EDGE ROUTER
(LER)
LABEL SWITCHING
ROUTER (LSR)




A router/switch that supports MPLS
Can be a router
Can be an ATM switch + label switch controller
Label swapping
 Each LSR examines the label on top of the stack
 Uses the Label Information Base (LIB) to decide the
outgoing path and the outgoing label
 Removes the old label and attaches the new label
 Forwards the packet on the predetermined path
LABEL SWITCHING
ROUTER (LSR)
 Upstream Router (Ru) – router that sends packets
 Downstream Router (Rd) – router that receives packets
 Need not be an end router
 Rd for one link can be the Ru for the other
Ru
Rd
Ru
Rd
LABEL SWITCHING ROUTER
(LSR)
POSITIONS OF LERs &
LSRs
LABEL SWITCHED PATH
(LSP)
 LSP defines the path through LSRs from ingress to
egress router
 FEC is determined at the LER-ingress
 LSPs are unidirectional
LABEL SWITCHED PATH
(LSP)
LSP
LSP
ROUTE SELECTION
 Refers to the method of selecting an LSP for a particular FEC
 Done by LDP
 Set of procedures and messages
 Messages exchanged between LSRs to establish an LSP
 LSRs associate an FEC with each LSP created
 Two types of LDP
 Hop by hop routing
 Explicit routing
ROUTE SELECTION
Hop-by-Hop Routing
Explicit Routing
• Distributes routing of control traffic
• Source routing of control traffic
• Builds a set of trees either fragment
by fragment like a random fill, or
backwards, or forwards in organized
manner.
• Builds a path from source to destination
• Reroute on failure impacted by
convergence time of routing protocol
• LSPs can be ranked so some reroute
very quickly and/or backup paths may
be pre-provisioned for rapid restoration
• Existing routing protocols are
destination prefix based
• Difficult to perform traffic
engineering, QoS-based routing
• Requires manual provisioning, or
automated creation mechanisms.
• Operator has routing flexibility (policybased, QoS-based,
• Adapts well to traffic engineering
Explicit routing shows great promise for traffic engineering
Explicit Routing - MPLS vs. Traditional
Routing
•Connectionless nature of IP implies that routing is based on information in
each packet header
•Source routing is possible, but path must be contained in each IP header
•Lengthy paths increase size of IP header, make it variable size, increase
overhead
•Some gigabit routers require ‘slow path’ option-based routing of IP packets
•Source routing has not been widely adopted in IP and is seen as impractical
•Some network operators may filter source routed packets for security
reasons
•MPLS’s enables the use of source routing by its connection-oriented
capabilities
- paths can be explicitly set up through the network
- the ‘label’ can now represent the explicitly routed path
•Loose and strict source routing can be supported
MPLS makes the use of source routing in the Internet practical
Label Distribution Protocol
(LDP)
 Label Distribution Protocol (LDP)
 set of procedures by which LSRs 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”
Label Distribution Protocol (LDP) Purpose
Label distribution ensures that adjacent routers have
a common view of FEC <-> label bindings
Routing Table:
Routing Table:
Addr-prefix
47.0.0.0/8
Addr-prefix
47.0.0.0/8
Next Hop
LSR2
Next Hop
LSR3
LSR1
IP Packet
LSR3
LSR2
47.80.55.3
Label Information Base:
Label-In FEC Label-Out
XX
47.0.0.0/8
17
Step 3: LSR inserts label
value into forwarding base
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 1: LSR creates binding
between FEC and label value
Common understanding of which FEC the label is referring to!
Label distribution can either piggyback on top of an existing routing protocol,
or a dedicated label distribution protocol (LDP) can be created
Label Distribution - Methods
Label Distribution can take place using one of two possible methods
Downstream Label Distribution
LSR1
Downstream-on-Demand Label Distribution
LSR2
Label-FEC Binding
• LSR2 and LSR1 are said to have an “LDP
adjacency” (LSR2 being the downstream LSR)
LSR1
LSR2
Request for Binding
Label-FEC Binding
• LSR2 discovers a ‘next hop’ for a particular FEC
• LSR1 recognizes LSR2 as its next-hop for an FEC
• LSR2 generates a label for the FEC and
communicates the binding to LSR1
• A request is made to LSR2 for a binding between
the FEC and a label
• LSR1 inserts the binding into its forwarding tables
• If LSR2 recognizes the FEC and has a next hop for
it, it creates a binding and replies to LSR1
• If LSR2 is the next hop for the FEC, LSR1 can use
that label knowing that its meaning is understood
• Both LSRs then have a common understanding
Both methods are supported, even in the same network at the same time
For any single adjacency, LDP negotiation must agree on a common method
Distribution Control: Ordered v.
Independent
MPLS path forms as associations
are made between FEC next-hops
and incoming and outgoing labels
Next Hop
(for FEC)
Incoming
Label
Independent LSP Control
Definition
Comparison
• 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
• 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
Outgoing
Label
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
• 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
Both methods are supported in the standard and can be fully interoperable
Label Retention Methods
An LSR may receive label
bindings from multiple LSRs
Binding
for LSR5
LSR1
LSR5
Some bindings may come
from LSRs that are not the
valid next-hop for that FEC
Binding for LSR5
Binding
for LSR5
Liberal Label Retention
Label Bindings
for LSR5
LSR4
Label Bindings
for LSR5
LSR1
LSR3
Valid
Next Hop
LSR3
Conservative Label Retention
LSR2
LSR4’s Label
LSR3’s Label
LSR2’s Label
LSR2
LSR4
• 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
LSR2
LSR1
LSR3
LSR4’s Label
LSR3’s Label
LSR2’s Label
Valid
Next Hop
LSR4
• 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
MPLS Header
 Lightweight




8 bit TTL
20 bit label tag
3 bit QoS tag
1 bit stack
 Indicates last LSR tag
 Allows heirarchical tagging
13
Payload
13
5 13
Payload
8 13
Payload
Payload
Provisioning vs. Signaling
 Signaling
 Seconds
 Provisioning
 Minutes to days
 Separate control message protocol
 Distribute labels and forwarding info
 RSVP
 Label Distribution Protocol
Comparing MPLS to IP
 IP over MPLS vs IP only





Qos
Performance
Tunneling
VPN
Traffic Engineering
MPLS vs IP: QoS
 MPLS
 Per hop QoS
 Using labels to prioritize
 20 bit identifier space
 IP
 Per hop QoS
 Use IP&TCP header
 104 bit identifier space
MPLS vs IP: Performance
 MPLS
 Forward on short tags
 Not prefix match on address
 IP
 Routers can forward at gigabit/s
MPLS vs IP: Tunneling (VPN)
 MPLS
 Lightweight tunnels
 32 bit header
 No security
 IP
 Heavyweight tunnels
 ~160 (?) bit header
 No security
 (without IPSEC)
MPLS vs IP: Traffic
Engineering
 MPLS




Arbitrary (non-shortest) paths
Virtual circuits
MPLS routing linked to IP routing
Flexible aggregation
 IP
 Route announcement manipulation
 Path cost manipulation
MPLS vs IP: Future QoS
 MPLS
 Propagate QoS between networks
 RSVP
 IP
 Propagate QoS between networks
 RSVP
MPLS Advantages &
Disadvantages
 Advantages:






Improves packet-forwarding performance in the network
Supports QoS and CoS for service differentiation
Supports network scalability
Improves the possibilities for traffic engineering
Integrates IP and ATM in the network
Builds interoperable networks
 Disadvantages:
 An additional layer is added
 The router has to understand MPLS
Summary of Motivations for MPLS
• Simplified forwarding based on exact match of fixed length label
- initial drive for MPLS was based on existance of cheap, fast ATM switches
• Separation of routing and forwarding in IP networks
- facilitates evolution of routing techniques by fixing the forwarding method
- new routing functionality can be deployed without changing the forwarding
techniques of every router in the Internet
• Facilitates the integration of ATM and IP
- allows carriers to leverage their large investment of ATM equipment
- eliminates the adjacency problem of VC-mesh over ATM
•Enables the use of explicit routing/source routing in IP networks
- can be easily used for such things as traffic management, QoS routing
•Promotes the partitioning of functionality within the network
- move granular processing of packets to edge; restrict core to packet forwarding
- assists in maintaining scalability of IP protocols in large networks
•Improved routing scalability through stacking of labels
- removes the need for full routing tables from interior routers in transit domain;
only routes to border routers are required
•Applicability to both cell and packet link-layers
- can be deployed on both cell (eg. ATM) and packet (eg. FR, Ethernet) media
- common management and techniques simplifies engineering
Many drivers exist for MPLS above and beyond high speed forwarding