multi-protocol label switching: basics and applications
DESCRIPTION
Multi-Protocol Label Switching has become by far one of the most important Internet technologies of the last 15 years. From humble beginnings back in 1996-97, it is literally the defacto standard in a large majority of service provider networks today. This presentation, delivered to executives at MTNL, Mumbai (a large regional carrier in India), explains the key operational principles behind MPLS, and its significant applications.TRANSCRIPT
Multi-Protocol Label Switching:Multi-Protocol Label Switching:Basics & ApplicationsBasics & Applications
Dr. Vishal Sharma Email: [email protected] Web: http://www.metanoia-inc.com
Metanoia, Inc.Critical Systems Thinking™
© Copyright 2002-2005All Rights Reserved
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The Start: Routing Process at a Router
Destination address (DA) based forwarding
Longest prefix matching
Routing Table
DA=my_add or DA= IP brdcst add. ?
RT entry = complete DA?
RT entry = Destn. n/w id?
Default entry exists?
No
No
No
No
Yes
Yes
Yes
Yes
Deliver datagram to protocol module (TCP/UDP) specified in IP hdr.
Send pkt. to next-hop router or to directly connected interface.
Send pkt. to next-hop router or to directly connected interface.
Send pkt. to next-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 match gives next hop router as 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
198.100.9.75
DA = 198.100.9.75Packet 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 matching label
swapping gives outgoing label as and outgoing interface as 4.
Incominglabel
Outgoinglabel
Address prefix 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
198.100.9.75
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 hoprouter
N/wInt.
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
4
56
1
198.100.9.75
198.101.84.21
R1
R2
R3
R4
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Labels De-couple Routing and Forwarding: 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
198.100.9.75
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 hop router
N/w Int.
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
21
128.89.10.12
179.69.42.3
198.168.7.6
Inlabel
Outlabel
Address Prefix N/wInt.
Advertises binding<5, 128.89.10.x>
Advertises binding<7, 179.69.x.x>
128.89.10.x 5 1
179.69.x.x 7 2
Advertises bindings<3, 128.89.10.x> <4, 179.69.x.x>
128.89.10.x 3 1
179.69.x.x 4 1
3
4
X
X
DA Next hop router
N/w Int.
128.89.10.x 128.89.10.1 1
179.69.x.x 179.69.42.3 2
Routing Table
Inlabel
Outlabel
Address Prefix N/wInt.
Label Table
R1 R2
R3
R4
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Basic Concept of MPLS
128.89.10.x
1
179.69.x.x
21
128.89.10.12
179.69.42.3
198.168.7.6
Inlabel
Outlabel
Address Prefix N/wInt.
Inlabel
Outlabel
Address Prefix N/wInt.
128.89.10.x 5 1
179.69.x.x 7 2128.89.10.x 3 1
179.69.x.x 4 1
3
4
X
X
3
5
Packet arrives DA=128.89.10.25
3Push Label
5Pop label
Forward packet
553
Swap Label
R2R1
R3
R3 R4
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A Word on Network Layer Routing
Control Plane
Forwarding/Data Plane
Control ComponentResponsible for construction 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 in the forwarding table
• Exact procedures used to find that entry For unicast routing … Information = Network layer (IP) address Procedure = Longest prefix matching
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So What about MPLS Control and Forwarding?
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, independent of control component (implementable in optimized hardware or software)
ControlComponent
ForwardingComponent
First Subentry Second Subentry(for multicast or load balancing)
Incoming Label Map
Next hop label forwarding entry (NHFLE)
Outgoing labelOutgoing inf.Next hop address
Outgoing labelOutgoing 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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Let’s 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 header MPLS “shim” header
Higher Layers
4 bytes 20 bytes
Label CoS TTL S
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
MPLSShim
Header
IPHeader
IP PayloadLayer 2 Hdr(e.g., PPPor 802.3)
For labeled packets, Layer 2 header indicates whether it is MPLSunicast packet or MPLS multicast packet
The label stack: sequence of 4-octet label stack entries (no limiton stack depth)
Network layer packet immediately follows the label stack entry thathas 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 headerrepresent actual label value (no SNAP/LLC encapsulation used)
Upstream LSR connected to first ATM-LSR adjusts TTL value based on howmany 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
MPLSShim
Header
IPHeader
IP PayloadAAL5 Trailer(length, CRC-
32, ...)
ATMHeader
ATMPayload
ATMHeader
ATMPayload
48 octets 48 octets
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Label Assignment and Distribution (Control Component)
Downstream Upstream
Ordered Solicited (On Demand)Unsolicited
SolicitedUnsolicited
Independent Solicited (On Demand)Unsolicited
SolicitedUnsolicited
Direction from 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 requestfor 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
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Example Label Assignment and Distribution Modes
4
33’
Edge LSR
Edge LSR
Downstream-on-demand with Independent Control
1 Requests
2
2’Assignments
Edge LSR
2
35
6
Edge LSR
Downstream-on-demand with Ordered Control
1 Requests
4
Assignments
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Comparison of ATM Switch, IP Router, LSR, and Optical X-connect
ATM switch
IP router LSR OXC
Control Plane
Dynamic routing protocol for route exchange
PNNI BGP, OSPF, IS-IS, RIP
BGP, OSPF, IS-IS, RIP
OSPF, IS-IS
Signaling protocols
UNI, PNNI RSVP LDP/CR-LDP, extended RSVP
LDP/CR-LDP, extended RSVP
Data Plane
Forwarding “Engine”
ASICs Software, ASICs
Software, ASICs
ASICs
Switched entity ATM SVC, PVC.
IP packets or flows
LSPs SONET channels, Wavelengths, fibers
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More on the MPLS Control Plane: Hop-by-hop Routed LSPs
LSPs whose routes are determined by IP routing protocols
Shortest 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 Setup using 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 routed path 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 apps
Based 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.<1.1.1.2, 2.2.2.2, 3.3.3.2>
Label Req.<2.2.2.2, 3.3.3.2>
Label Req.<3.3.3.2>
Label Mapping
Label Mapping
Label Mapping
Network operator or network management creates ER-LSP request with path and traffic 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.<as100, 3.3.3.2> Label Req.
<as100, 3.3.3.2>
Label Req.<3.3.3.2>
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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Are there any implications for hardware/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 capability determined 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: enables policy routing and traffic engineering (can setup routes different than default shortest-path)
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Reducing number of IP Peers
• VCs between routers connected over ATM network
• O(n^2) VCs for full adjacency
• O(n^4) routing info. exchange
overwhelms routers and network
LSR (runsIP routing)
IP routing peers
• Interior switches participate in IP routing protocols minimizes 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 track of all networks reachable through it
With hierarchical labels, an arrangement is possible where only Border Routers need to know what networks 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 and Push Pop
Swap
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Explicit Manageable Routes -- Policy Routing, Traffic Engineering
Carriers want certain traffic to go over certain routes This 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-BasedRouting Process
Attributes
Resources
Topology
Feasible PathERO {1,3,4,5}
1
3
4
5
2
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TE Topology versus Regular Routed Topology
A
B
C D
E
Network Diagram
Regular Routed Topology
A
B
CD
E
4
11
1
3
2
3
Link weights
A
B
CD
E
Traffic EngineeringTopology
OC-3
OC-12
OC-192OC-12
DS3
Best effort shortestpath from D to E
TE Path from D Eavoiding green links
with at least STS-3 b/w
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LSP ID = L2
Automatic Reroute Using MPLS RSVP-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 theLSPs share resources
Tunnel ID inSession Obj
Originates LSPswith IDs 1 and 2
Here they are treated as differentLSPs within the same Session
LSPs 1 and 2 have a common SESSION Obj, buta new LSP ID in the SENDER_TEMPLATE and adifferent ERO (with possibly common hops)
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So what did we look at? Let’s 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