Multiprotocol Label Switching
(MPLS)
References:
Juniper white papers on MPLS and
DiffServ at:
http://www.juniper.net/solutions/literature/
white_papers/
Outline
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General concepts
ATM LSR
Ships-in-the-night
DiffServ-MPLS architecture
Resource provisioning
High availability
MPLS: General Concepts
Outline:
• Historic perspective
• MPLS basics
• ATM LSR
A historical Perspective
What happened in mid-90’s?
• The problems with overlay models reveal
themselves:
– Overlay models create bottleneck from the lack of segmentation and
reassembly (SAR) functionality on interfaces faster than OC-48
– Overlay models increase complexity by requiring providers to
manage two separate control planes and two fundamentally different
types of networking equipment
– Overlay model results in an inefficient use of network bandwidth due
to the traditional ATM cell tax
– No QoS for LANE and the IP Diffserv approach to CoS does not
map well to the existing ATM QoS mechanims
– LANE requires a client-server model in place, e.g., LEC, LECS,
LES, and BUS
– Classical IP over ATM requires the deployment of n-squared routing
adjacencies.
A Historical Perspective (cont’d)
• Multi-layer switching solutions in the spotlight:
–
–
–
–
–
IP switching by Ipsilon/Nokia
Tag switching by Cisco
Aggregate routing-based IP switching (ARIS) by IBM
IP Navigator by Cascade/Ascend/Lucent
Cell Switching Router (CSR) by Toshiba
 All ATM based solutions
MPLS Basics
• MPLS switching concept:
IP ROUTER
CONTROL
IP Routing
Software
ATM-LSR
IP Routing
Software
ATM SWITCH
ATM Routing &
Signaling
Software
Signaling &
label binding
FORWARDING
Longest
Prefix Match
Label
Switching
Label
Switching
MPLS Basics (cont’d)
• Separation of control functions from
forwarding functions:
ROUTING
CONTROL
PLANE
MPLS CONTROL
DATA PLANE
SWITCHING CONTROL
MPLS Basics (cont’d)
• Label distribution and label swapping:
Ingress LER
LSR
Routing
CONTROL
PLAN
Routing
CONTROL
PLAN
Signaling
LAYER-2
TRANSPORT
Egress LER
CONTROL
PLAN
Signaling
LABELED
LABELED
SWITCHING
LABELED
SWITCHING
SWITCHING
Frames
13
20
LAYER-2
TRANSPORT
MPLS Basics (cont’d)
• Generic label and label stacking:
0
LAYER-3
0
LAYER-2
1
20
Label
3
1
8
exp
s
TTL
MPLS Basics (cont’d)
• ATM Based label and label stacking:
LAYER-3
0
0
+
0
LAYER-2
1
20
Label
3
1
8
exp
s
TTL
VPI/VCI
MPLS Basics (cont’d)
• MPLS signaling protocols: distribute labels and
maintain connectivity of an LSP
– Topology driven:
• LDP:
– distributes labels based on routing topology, i.e., label to FEC
binding where FEC = IP prefix.
– Using TCP and maintaining “hard state”.
– No QoS.
– Policy driven:
• RSVP-TE: Widely deployed
– distributes labels based on source routing
– Using raw IP and maintaining “soft state”
– Designed for QoS
• CR-LDP: not well accepted
– Distributes labels based on source routing
– Using TCP and maintaining “hard state”
– Designed for QoS
MPLS Basics (cont’d)
• Label distribution mechanisms:
– Downstream-on-demand (RSVP-TE, CR-LDP, LDP)
Label Request
Downstream LSR
Upstream LSR
Label Response
Next Hop to FEC
– Unsolicited-downstream (LDP)
Downstream LSR
Upstream LSR
Unsolicited
Label Response
Next Hop to FEC
MPLS Basics (cont’d)
• Label retention modes:
– Conservative label retention:
• an upstream LSR maintains the received label binding for an FEC
only if the label binding is received from the downstream LSR that
the upstream LSR has selected as the next-hop for that FEC
– Liberal label retention:
• An upstream LSR maintains the received label binding for an FEC
even if the label binding is received from a downstream LSR that
the upstream LSR has not selected as the next-hop for that FEC
• Pros and Cons of the two modes?
MPLS Basics (cont’d)
• Routing for MPLS
– Traditional shortest path based IP routing protocols
provide enough information for LDP signaling
– Policy driven signaling protocols set up an LSP based on
the policy:
• A policy may enforce a manually configured route
• A policy may use a route found by a constraint-based routing
protocol
• A policy may use a route found by a shortest-path based routing
protocol
• Constraint-based routing protocols:
– Find “best” routes meeting multiple criteria
– Pre-calculate or calculate on-demand
MPLS Basics (cont’d)
•
•
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•
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MPLS eliminates the potential for SAR bottlenecks by not
using ATM as a transport
MPLS eliminates the complexity of managing two separate
control planes and two fundamentally different types of
networking equipment.
MPLS eliminates the cell tax by not using ATM as a
transport
MPLS can support DiffServ CoS.
The peer-to-peer nature of IP routed MPLS eliminates the
need to manage a complex logical topology (n-squared
PVCs)
MPLS provides flexible traffic engineering features
MPLS Related Research Topics
•
•
•
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MPLS multicasting
Pseudo-wire over MPLS
MPLS VPN
MPLS high availability
MPLS Traffic engineering
Useful Resources:
Go to:
http://www3.uta.edu/library/ejournals/
and click on:
ACM Digital Library
IEEE Xplore