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Multi-Protocol Label Switching:
Basics & Applications
Dr. Vishal SharmaEmail: [email protected]: http://www.metanoia-inc.com
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The Start: Routing Process at a Router
Destination address (DA)based forwarding
Longest prefix matching
Routing Table
DA=my_add orDA= IP brdcst add.
?
RT entry =complete DA?
RT entry =Destn. n/w id?
Default entryexists?
No
No
No
No
Yes
Yes
Yes
Yes
Deliver datagram toprotocol module(TCP/UDP) specifiedin IP hdr.
Send pkt. to next-hop
router or to directlyconnected interface.
Send pkt. to next-hop router or todirectly connected
interface.
Send pkt. tonext-hop router.
Datagram undeliverable. (Use ICMP to inform source.)
Receive incoming pkt.
DA Next hoprouter
NetworkInterface
Host entry 198.168.7.3 X 2
Host entry 198.168.7.4 X 3
Host entry 198.168.7.1 198.168.7.5 1
Host entry 198.168.7.2 198.168.7.5 1
N/w entry 198.100.x.x 198.100.9.1 4
N/w entry 128.72.x.x 128.72.55.4 5
Default x.x.x.x 128.84.73.1 6
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How Routing Works Today
How do routers build their routing tables?
By exchanging information with each other using routing protocols
DA Next hoprouter
N/wInt.
Host entry 198.168.7.3 X 2
Host entry 198.168.7.4 X 3
Host entry 198.168.7.1 198.168.7.5 1
Host entry 198.168.7.2 198.168.7.5 1
N/w entry 198.100.x.x 198.100.9.1 4
N/w entry 128.72.x.x 128.72.55.4 5
Default x.x.x.x 128.84.73.1 6
Routing table (RT) at 198.168.7.6
Longest prefix matchgives next hop routeras 198.100.9.1 and
outgoing interface as
4.
198.100.9.75
198.100.9.75
198.100.9.75
198.100.9.75
198.100.9.75198.168.7.4
198.168.7.3
198.168.7.1
198.168.7.2
198.168.7.5
198.168.7.6
198.100.x.x
198.100.9.1
128.72.x.x
128.72.55.4
128.84.x.x
128.84.73.1
23
4
56
1
198.100.9.7
5
DA = 198.100.9.75
Packet generated
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How it Would be with Labels
How do routers learn the labels?
By interpreting routing information and through signaling
(as we will learn later)
DA = 198.100.9.75Packet generated
Exact matchinglabelswapping gives outgoing
label as and outgoing
interface as 4.
Incominglabel
Outgoinglabel
Address pref ix N/wInt.
X 2
198.100.x.x 4
128.72.x.x 5
Label Forwarding Table at 198.168.7.6
198.168.7.4
198.168.7.3
198.168.7.1
198.168.7.2
198.168.7.5
198.168.7.6
198.100.x.x
198.100.9.1
128.72.x.x
128.72.55.4
128.84.x.x
128.84.73.1
23
4
56
1
198.100.9.7
5
Attach label
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Shortest-Path Routing: Little Flexibility
Shortest path converges traffic on a few network links
Significant increase in congestion
Unbalanced resource utilization
DA Next hop
router
N/w
Int.
Host entry 198.168.7.4 X 3
Host entry 198.168.7.1 198.168.7.5 1
Host entry 198.168.7.2 198.168.7.5 1
N/w entry 198.101.x.x 198.168.7.4 3
N/w entry 198.100.x.x 198.100.9.1 4
N/w entry 128.72.x.x 128.72.55.4 5
Default x.x.x.x 128.84.73.1 6
Routing table (RT) at 198.168.7.6
198.168.7.4198.168.7.1
198.168.7.2
198.168.7.5
198.168.7.6
198.100.x.x
198.100.9.1
128.72.x.x
128.72.55.4
128.84.x.x
128.84.73.1
3
45
6
1
198.100.9.7
5
198.101.84.21
R1
R2
R3
R4
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Labels De-couple Routing andForwarding: Much more Flexibility
Labels enable:
Differentiation based on criteria other than shortest path
Permit policy routing
198.168.7.4198.168.7.1
198.168.7.2
198.168.7.5
198.168.7.6
198.100.x.x
198.100.9.1
128.72.x.x
128.72.55.4
128.84.x.x
128.84.73.1
3
4
56
1
198.100.9.7
5
198.101.84.21
Incominglabel
Outgoinglabel Address Prefix N/wInt.
X 2
198.101.x.x 4
198.101.x.x 3
Label Forwarding Table at 198.168.7.6
R3
R2
R1
R4
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Basic Concept of MPLS
Routing fills routing table
Signaling fills label forwarding table
DA Next hoprouter
N/wInt.
128.89.10.x 198.168.7.6 1
179.69.x.x 198.168.7.6 1
128.89.10.x
1
179.69.x.x
2
1
128.89.10.12
179.69.42.3
198.168.7.6
In
label
Outlabel
Address Prefix N/w
Int.
Advertises binding
Advertises binding
128.89.10.x5 1
179.69.x.x7 2
Advertises bindings
128.89.10.x3 1
179.69.x.x4 1
3
4
X
X
DA Next hoprouter
N/wInt.
128.89.10.x 128.89.10.1 1
179.69.x.x 179.69.42.3 2
Routing Table
In
label
Outlabel
Address Prefix N/w
Int.
Label Table
R1 R2
R3
R4
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Basic Concept of MPLS
128.89.10.x
1
179.69.x.x
2
1
128.89.10.12
179.69.42.3
198.168.7.6
In
label
Outlabel
Address Prefix N/w
Int.
In
label
Outlabel
Address Prefix N/w
Int.
128.89.10.x5 1
179.69.x.x7 2128.89.10.x3 1
179.69.x.x4 1
3
4
X
X
3
5
Packet arrives
DA=128.89.10.25
3Push
Label
5Pop
labelForward
packet
553
Swap
Label
R2R1
R3
R3 R4
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A Word on Network Layer Routing
Control
Plane
Forwarding
/Data Plane
Control Component
Responsible forconstruction and maintenanceof forwarding table. Consists of:
Routing protocols for exchange of routing info.
Algorithms to convert this into forwarding table
Forwarding/data ComponentAlgos. used to make forwarding decision on packet
The algorithms define:
Information from packet used to find an entry inthe forwarding table
Exact procedures used to find that entryFor unicast routing
Information = Network layer (IP) address
Procedure = Longest prefix matching
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So What about MPLS Control andForwarding?
Superset of conventional router control
Distribute routing info. via network layer routing (OSPF, BGP, etc.)
Algos. to convert routing info. into forwarding table for fwding component
Create binding from FEC (derived from routing info.) --> label
Assign and distribute labels to peer LSRs via signaling
Uses a label switching forwarding table (or LIB), looking as:
Forwarding algorithm = label swapping, independentof control
component (implementable in optimized hardware or software)
Control
Component
Forwarding
Component
First Subentry Second Subentry
(formulticast orload balancing)
Incoming Label Map
Next hop label forwarding entry (NHFLE)
Outgoing label
Outgoing inf.
Next hop address
Outgoing label
Outgoing inf.
Next hop address
Incoming Label
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What does a label represent?The issue of label granularity
Packets treated identically by participating routers form
Forwarding Equivalence Class (FEC
Assigned the same label
Membership of a FEC must be determinable from
IP header
Info. that ingress router has about the packet
Entities grouped into a FEC are flexible, and could involve
A connection between two IP ports on two hosts
All traffic between two IP hosts
All traffic headed for a particular network with same TOS bits
All destination networks with a certain prefix
All traffic headed to a particular router (e.g. an egress)
A manually configured connection and many others
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Lets Recap: Elements of MPLS
Label Forwarding
Use data link addressing, e.g. ATM VPI/VCI, FR DLCI
Put shim header between data link and IP header
Label Creation and Binding
Label Assignment and Distribution
Ride piggyback on routing protocols, where possible (BGP)
Use separate label distribution protocol RSVP, LDP/CR-LDP
Reliability: TCP or separate ACK/NACK
Variable
L2 header L3 IP headerMPLS shim
headerHigher Layers
4 bytes 20 bytes
Label CoS TTLS
20 bits 3 bits 8 bits
Data
Plane
Control
Plane
EXP/
1 bit
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Benefits over Conventional Routing
MPLS forwarding possible by:
Switches incapable of analyzing network layer headers
Unable to do so at adequate speeds
Ingress can use any info.about packet to assign to FEC/LSP
Conventional forwarding only considers info. in the packet
Forwarding decisions can depend on ingress router
Conventional routing, identity of ingress router does not travel with packet
Packet FEC assignment can use complex decision process No impact on forwarding of labeled packets!
Explicit routing packet need not carry encoding of entire route Unlike source routing in conventional IP forwarding
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MPLS Header over POS or IEEE 802.3
Label
(20 bits)
TTL
(8 bits)
EXP
(3 bits)
S
(1 bit)
4 octets
MPLS
Shim
Header
IP
HeaderIP Payload
Layer 2 Hdr
(e.g., PPP
or 802.3)
For labeled packets, Layer 2 header indicates whether it is MPLS
unicast packet or MPLS multicast packet
The label stack: sequence of 4-octet label stack entries (no limit
on stack depth)
Network layer packet immediately follows the label stack entry that
has the S bit set. Label values 0 -->16 are reserved
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MPLS Header over ATM
Top stack of shim carries placeholder label value of 0. VPI/VCI value in header
represent actual label value (no SNAP/LLC encapsulation used)
Upstream LSR connected to first ATM-LSR adjusts TTL value based on how
many ATM-LSRs are consecutively connected downstream (learnt via LDP)
For ATM LSRs, UNI gives 24-bit VPI/VCI label, NNI gives 28-bit VPI/VCI label
If two ATM-LSRs connected via VPC through ATM cloud, 16-bit VCI label used
Label=0
(20 bits)
TTL
(8 bits)
EXP
(3 bits)
S
(1 bit)
4 octets
MPLS
Shim
Header
IP
Header
IP PayloadAAL5 Trailer
(length, CRC-
32, ...)
ATM
Header
ATM
PayloadATM
Header
ATM
Payload
48 octets 48 octets
M i I
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Label Assignment and Distribution(Control Component)
Downstream Upstream
Ordered Solicited (On Demand)Unsolicited
SolicitedUnsolicited
Independent Solicited (On Demand)Unsolicited
SolicitedUnsolicited
Directionfrom which labels flow
Refers to whether LSR distributes
labels on demand or voluntarily
Whether LSR waits to hear from
its upstream/downstream nbrs.
before responding to a request
for label(s)
Label Retention: Liberal or Conservative
Whether LSR keeps labels from a neighbor
who is not currently the next hop for a FEC
Labels
Data
Labels
Data
M t i I
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Example Label Assignment andDistribution Modes
4
33
Edge LSR
Edge LSR
Downstream-on-demand
with Independent Control
1 Requests
2
2Assignments
Edge LSR
2
35
6
Edge LSR
Downstream-on-demand
with Ordered Control
1 Requests
4
Assignments
M t i I
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Comparison of ATM Switch, IPRouter, LSR, and Optical X-connect
ATMswitch
IP router LSR OXC
ControlPlane
Dynamic routingprotocol for
route exchange
PNNI BGP,OSPF, IS-IS, RIP
BGP,OSPF,IS-IS,
RIP
OSPF, IS-IS
Signalingprotocols
UNI, PNNI RSVP LDP/CR-LDP,extendedRSVP
LDP/CR-LDP,extendedRSVP
Data
Plane
Forwarding
Engine
ASICs Software,ASICs
Software,ASICs
ASICs
Switched entity ATM SVC,PVC.
IP packetsor flows
LSPs SONETchannels,Wavelengths,fibers
M t i I
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More on the MPLS Control Plane:Hop-by-hop Routed LSPs
LSPs whose routes are determined by IP routing protocolsShortest path, based on destination IP address of packet
Effectively creates labels for each route in forwarding table
Label distribution for hop-by-hop routed LSPs
LDP (Label Distribution Protocol)
Defined by IETF MPLS Working Group
LDP messages:
Notification, Hello, Initialization, KeepAlive, Address, Address Withdraw, Label
Mapping, Label Request, Label Withdraw, Label Release
Peer discovery msgs. over UDP, rest over TCP for reliability
Piggyback on existing IP routing protocols
Example: Add label information to BGP
Not all IP interfaces may be enabled for dynamic routing protocols
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Hop-by-hop Routed LSP Setupusing LDP
Edge LSR
Edge LSR
Label Req.
Label Req.
Label Req.
Label Mapping.
Label Mapping.
Label Mapping.
LSR1 learns new IP
network prefix
1.1.1.0/24 via
dynamic IP routing
Each LSR forwards Label Req. along hop-by-hop routedpath to 1.1.1.0/24
Path established via a dynamic IP routing protocol
When next hop to 1.1.1.0/24 changes in LSR2 (e.g. due to
topology or link metric change)
LSR2 releases original LSP
Starts setting up new LSP from that point on
Several other options available
1.1.1.0/24.
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ER-LSPs: Explicitly Routed LSPs
Routes determined by operators or n/w management appsBased on specific TE policy, QoS, or VPN membership
Significantly more efficient than conventional IP source routing
Label distribution for ER-LSPs
Extended RSVP (significantly different from original RSVP)
Associates labels with RSVP flows, supports aggregate flows
Control messages run on raw IP transport, requiring refreshes
CR-LDP (Constraint-based Routed LDP)
Now mostly of historical value
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Strict ER-LSP Setup using CR-LDP
Edge LSR
Edge LSR
Label Req.
Label Req.
Label Req.
Label Mapping
Label Mapping
Label Mapping
Network operator or
network management
creates ER-LSP
request with path andtraffic parameters
Traffic parameter TLV contains: Frequency, weight
Peak data rate, Peak burst rate Committed data rate, committed burst rate, excess burst size
Frequency specifies granularity at which CDR is made available
Weight determines excess bandwidth possible above CDR
1.1.1.2 2.2.2.2
3.3.3.2
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Loose ER-LSP Setup using CR-LDP
Edge LSR
Edge LSR
Label Req.
Label Req.
Label Req.
Label Mapping.
Label Mapping.
Label Mapping.
Network operator or
network management
creates ER-LSP
request with path and
traffic parameters
4.4.5.6 4.4.5.7
3.3.3.2
AS100
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A th i li ti f
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Are there any implications forhardware/ASICS?
Label stacking depth (if any) supported depends on hardware
processing capabilities and speeds
Hardware engine needs ability to examine both EXP bits and LABEL,
and map it to any control hardware used for scheduling MPLS
packets
Ability to push and/or pop labels determines whether switch can be
an edge LSR, or only a core LSR (doing only swapping)
Number of queues in the switch/router determines per-label queueing
or per-class queueing ability
Label merging capabilitydetermined by ability to re-assemble
packets from interleaved cells
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Advantages of MPLS
Original justification was fast, amortized, ATM hardware
Eliminated by hardware forwarding engines at multi-gigabit rates
Current justifications include:
Separates forwarding from control, enabling
Evolution of routing functionality independently of forwarding algorithm
(which can continue to be label swapping)
Use of MPLS to control non-packet technologies like SONET/SDH
channels or optical light-paths
Facilitates scalable hierarchical routing(via label stacking)
Scalability by reducing number of IP peers/neighbors
Provides explicit, manageable IP routes: enablespolicy routingand
traffic engineering(can setup routes different than default shortest-path)
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Reducing number of IP Peers
VCs between routers connected overATM network
O(n^2) VCs for full adjacency
O(n^4) routing info. exchange
overwhelms routers and network
LSR (runs
IP routing)
IP routing
peers
Interior switches participate in IProuting protocolsminimizes IP nbrs.
Eliminates full VC mesh for adjacency,
as LSRs run IP routing protocols
Router
ATM Switch
IP routing
peers
ATM Network
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Hierarchical Label Stacking/Switching
Inside transit AS each interior router must keep trackof all networks reachable through it
With hierarchical labels, an arrangement is possible
where only Border Routers need to know whatnetworks might eventually be reached through them
All transit traffic can tunneled through interior routers
of the AS using LSPs with stacked labels
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Utility of Hierarchical Label Switching
Interior LSRs
Border LSRs
Swap andPush
Pop
Swap
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E li it M bl R t
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Explicit Manageable Routes --Policy Routing, Traffic Engineering
Carriers want certain traffic to go overcertain routesThis type of network engineering:
Keeps network loads balanced
Enhances network stability and reliability
Enables better QoS and performance assurances
Allows carriers to meet SLAs
Constraint-based routing + MPLS
Allows carriers to bind specific traffic to an LSP
Place (or route) LSP over a desired sequence of LSRs
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Constraint Based Routing
A class of routing systems that computes routes through a
network subject to a set of constraints and requirements
QoS-based Routing
Path of flows determined by
Knowledge of resource
availability in network
QoS requirements of flows
Policy-based Routing
Path/routing decision based
on administrative policy
Can be on-line or off-line
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CB Routing System
Inputs
Flow/path attributes:
required b/w, hop count, ...
Resource attributes:
properties of nodes/links
Network topology & state
Outputs
Computed feasible path
Explicit route of the path
Constraint-Based
Routing Process
Attributes
Resources
Topology
Feasible Path
ERO {1,3,4,5}
1
3
4
5
2
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TE T l R l
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TE Topology versus RegularRouted Topology
A
B
C D
E
Network Diagram
Regular Routed
Topology
A
B
C
D
E
4
11
1
3
2
3
Link weights
A
B
CD
E
Traffic Engineering
Topology
OC-3
OC-12
OC-192OC-12
DS3
Best effort shortest
path from D to E
TE Path from D Eavoiding green linkswith at least STS-3 b/w
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LSP ID = L2
Automatic Reroute Using MPLSRSVP-TE
Rerouting is done when
A better path is available
Upon failure along LSP
Use SESSION Obj. & SE style
Tunnel uniquely identified by
Destination IP address
Tunnel ID
Ingress IP address
Tunnel ingress made to appear
as 2 different senders to the
RSVP session (via LSP ID)
Src
Rcvr
LSP ID = L1
On these links the
LSPs share resources
Tunnel ID inSession Obj
Originates LSPs
with IDs 1 and 2
Here they are treated as different
LSPs within the same Session
LSPs 1 and 2 have a common SESSION Obj, but
a new LSP ID in the SENDER_TEMPLATE and a
different ERO (with possibly common hops)
Metanoia, Inc.
So what did we look at?
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Critical Systems Thinking
Copyright 2002 2005 34
So what did we look at?Lets summarize
Looked at conventional IP routing and its limitations
Saw how labels decouple data plane from control plane
Examined basics of MPLS
Control and forwarding components
Label granularity (forwarding equivalence class, FEC)
Benefits over conventional routing
Label assignment and distribution methods
Downstream-on-demand, with ordered or independent control
Hop-by-hop routed LSPs, strict- and loosely explicitly-routed LSPs
Advantages of MPLS efficient hierarchical routing, reduces
number of IP peers, facilitates explicit routing
Use of MPLS for traffic engineering, protection, automatic
rerouting