CS 520 - Fall 2003 - Lecture 15 MPLS and Its Applications (With Modifications For CS 520)
CS 520 - Fall 2003 - Lecture 15 MPLS and Its Applications (With Modifications For CS 520)
CS 520 - Fall 2003 - Lecture 15 MPLS and Its Applications (With Modifications For CS 520)
• What MPLS is
• What MPLS is good for
• MPLS protocol/mode comparisons & opinions
• Some applications of MPLS
• MPLS future
2
What This Lecture is NOT About
• General networking
• Product comparisons
• Marketing
3
Tutorial Outline
• Overview
• Label Encapsulations
• Label Distribution Protocols
• Constraint-Based Routing
• MPLS and VPNs
• Network Survivability
• Summary
4
What is MPLS?
• Encapsulations
— Labels used at the data plane to make forwarding decisions
• Signaling Protocols
— Distribution of Labels to establish Label Switched Paths
5
“Label Substitution” what is it?
Have a friend go to B ahead of you. At every road they
reserve a lane just for you. At every intersection they post
a big sign that says for a given lane which way to turn and
what new lane to take.
LANE#2
6
Label Switched Path (LSP)
#1 RIGHT #2
#2 LEFT #1 #1 RIGHT #2
#2 LEFT #1
7
What is a “LABEL”?
8
Label Switched Path
#3 Right #7
IP #3
#7
#7 LEFT #99
#99 RIGHT #9
#99
#9
#9 LEFT #4072
#4072 IP
9
Another context: An Optical Label Switched Path – “labels” are
colors of light
RED RIGHT BLUE
IP RED
BLUE
ORANGE
RED
IP
10
MPLS Labels
11
ROUTE AT EDGE, SWITCH IN CORE
12
Routers Do Both Routing and Switching
• Routing
— Deciding the next hop based on
the destination address.
— A Layer 3 (L3) function.
• Switching
— Moving a packet from an input
port to an output port and out.
— A layer 2 function.
INPUT PORTS OUTPUT PORTS
13
MPLS Turns Routing into Switching
• So we can avoid performing the layer 3 function.
— Use labels to decide next hops.
14
MPLS: Flexible Forwarding
IP: Packets are forwarded based on Destination Address (DA). We can
call this “destination based routing”.
IP DA IP DA IP DA IP DA IP DA
MPLS:
• Map packets to LSP based on (Source Address, Destination Address,
protocol, port, DSCP, interface, etc.)
• Forward packets based on the Label
15
MPLS-based Solutions
• IP Traffic Engineering
— Use constraint-based routing to adapt to latest network
loading and QoS performance
• L2/L3 Integration
— Integrate with L1 and L2 technologies like Optical Cross
Connects (OXC’s) and ATM
• Resilient Network Design
— Automatic Failover and Backup
16
BEST OF BOTH WORLDS
PACKET CIRCUIT
HYBRID SWITCHING
Forwarding
IP MPLS ATM
+IP
• MPLS + IP forms a middle ground that combines
the best of IP and the best of circuit switching
technologies.
17
MPLS Terminology
18
LER’s and LSR’s
19
Forwarding Equivalence Classes
• FEC = “A subset of packets that are all treated the same way by a router”
• The concept of FECs provides for a great deal of flexibility and
scalability. Traffic can be aggregated or differentiated in many ways
• In conventional routing, a packet is assigned to an FEC at each hop (i.e.
L3 look-up), in MPLS it is only done once at the network ingress.
LSR LSR
LER LER
LSP
IP1 IP1
IP1 #L1 IP1 #L2 IP1 #L3
IP2 #L1 IP2 #L2 IP2 #L3
IP2 IP2
#427
#216
#819
#18 #77
#963
#14
#612 #462
#99 #311
#5
#963
#14
#612 #462
#99 #311
#5
22
STANDARD IP
Dest Out
47.1 1
Dest Out 47.2 2
47.1 1 47.3 3
47.2 2
47.3 3
1 47.1
1 2
3
Dest Out
2
47.1 1
47.2 2
47.3 3
1
3 47.2
Network 47.3 2
2
IP 47.1.1.1
24
Label Switched Path (LSP)
IntfIn
IP 47.1.1.1
1 47.1
3 3
Intf D
2
1
1
2
47.3 3 47.2
2
IP 47.1.1.1
25
MPLS Label Distribution
IntfIn
1 47.1
Request: 47.1
3
47.1
Intf D
s t : 2
ue 3
Req 1
Mapping: 40
1 2
0
47.3 3 p i ng: 5 47.2
p
Ma
2
26
Benefits and Limitations
• Why might this approach be better than normal IP
forwarding that does not use MPLS?
— Remember, all packets still travel the same paths.
27
Route= EXPLICITLY ROUTED OR ER-LSP
{A,B,C}
#14 #972
#216
B
#14
C
A #972
#462
28
EXPLICITLY ROUTED LSP ER-LSP
This entry gives the
longest prefix match.
IntfIn
IP 47.1.1.1
1 47.1
Intf D
3 3
2
1
1
2
47.3 3 47.2
2
IP 47.1.1.1
29
ER LSP - Advantages
30
ER LSP - Discord!
• Two signaling options proposed in the standards: CR-LDP, RSVP
extensions:
– CR-LDP = Label Distribution Protocol (LDP) + Explicit Routing
– RSVP-TE = Traditional Resource Reservation Protocol (RSVP) + Explicit Route +
Scalability Extensions
31
Tutorial Outline
• Overview
• Label Encapsulations
• Label Distribution Protocols
• Constraint Based Routing
• MPLS and VPNs
• Network Survivability
• Summary
32
Upper Layer Consistency Across
Lower Layers
WDM
Ethernet GigEthernet Optical Cross Frame
Connect (OXC) ATM
Relay
33
MPLS Encapsulation - PPP & LAN Data Links
MPLS ‘Shim’ Headers (1-n)
n ••• 1
Layer 2 Header Network Layer Header
(eg. PPP, 802.3) and Packet (eg. IP)
4 Octets
Label Stack
Label Exp. S TTL
Entry Format
Label: Label Value, 20 bits (Values 0 through 16 are reserved)
Exp.: Experimental, 3 bits (was Class of Service)
S: Bottom of Stack, 1 bit (1 = last entry in label stack)
TTL: Time to Live, 8 bits
• Network layer must be inferable from value of bottom label of the stack
MPLS
MPLSononPPP
PPPlinks
linksand
andLANs
LANsuses
uses‘Shim’
‘Shim’Header
HeaderInserted
Inserted
Between Layer 2 and Layer 3 Headers
Between Layer 2 and Layer 3 Headers
(other
(othertechnologies
technologiesuse
usedifferent
differentapproaches)
approaches)
34
Tutorial Outline
• Overview
• Label Encapsulations
• Label Distribution Protocols
• Constraint-Based Routing
• MPLS and VPNs
• Network Survivability
• Summary
35
Label Distribution Protocols
36
Label Distribution Protocol (LDP) - Purpose
Label distribution ensures that adjacent routers have
a common view of bindings of FEC’s to labels
Routing
Routing Table:
Table:
Routing
Routing Table:
Table:
Addr-prefix
Addr-prefix Next Hop
Addr-prefix
Addr-prefix Next
Next Hop
Hop 47.0.0.0/8
47.0.0.0/8 LSR3
LSR3
47.0.0.0/8
47.0.0.0/8 LSR2
LSR2
IP Packet 47.80.55.3
Label
Label Information
Information Base:
Base:
For 47.0.0.0/8 Label
Label Information
Information Base:
Label-In
Label-In FEC
FEC Label-Out
Label-Out use label ‘17’
XX
XX 47.0.0.0/8
47.0.0.0/8 17
17 Label-In
Label-In FEC Label-Out
Label-Out
17
17 47.0.0.0/8
47.0.0.0/8 XX
Step 3: LSR inserts label Step 2: LSR communicates Step 1: LSR creates binding
value into forwarding base binding to adjacent LSR between FEC and label value
37
Labels are Downstream Assigned
• Note that label assignments are decided at the
downstream node and communicated to the upstream
node.
• Why does it need to be done this way?
38
Label Distribution Protocols
39
Traffic Engineering Requirements
Constraint-Based Routing is one method of Traffic Engineering. Traffic
Engineering seeks to engineer the best use of capacity.
RFC 2702:
• Strict & Loose ER
• Specification of QoS
• Specification of Traffic Parameters
• Route Pinning
• Preemption
• Failure Recovery
40
Constraint Based Routing using LDP (CR-
LDP)
• Built on existing LDP messages over TCP.
• Defines an Explicit Route:
— Detailed path that can traverse any links supporting CR-LDP.
42
CR-LDP Preemption
43
Preemption A.K.A. Bumping
This LSP must be
Route= preempted.
{A,B,C} Now this one can
proceed.
#216
B
#14
C
A #972
#462
44
Label Distribution Protocols
45
ER-LSP setup using RSVP-TE
• TE (Traffic Engineering) extensions to RSVP
• Built on RSVP messages over IP.
— In RSVP, a source requests resources along a path.
— Then the source regularly sends refresh messages to keep the reservations
active.
• Extensions to RSVP:
— Explicit Route Object
— Label Request
— Label Object
— Session Attribute
— Record Route Object
• Defines a set of constraints for LSP computation and admission:
— Expectation and Allocation of resources: Uses Inserv-style reservations
— Preemption Level: Setup and Holding Priority with respect to other LSPs.
46
Tutorial Outline
• Overview
• Label Encapsulations
• Label Distribution Protocols
• Constraint Based Routing
• MPLS and VPNs
• Network Survivability
• Summary
47
IP Follows a Tree to the Destination
Dest=a.b.c.d
a.b.c.d
Dest=a.b.c.d
Dest=a.b.c.d
#216
#963
#14
#612 #462
#99 #311
#5
#427
#216
#819
#18 #77
#963
#14
#612 #462
#99 #311
#5
& &
=
51
Traffic Engineering
B C
Demand
A D
Network
Topology
53
MPLS Traffic Engineering Methods
• The ingress LSR may be specify an LSP to have one or more
other LSR’s along the desired path
— hop-by-hop routing may be used to set up the rest of the path in between
those LSR’s
— a.k.a. loose source routing, less configuration required
54
WHEN SHOULD TE BE USED?
55
Reactive traffic engineering
S5
S6
61
Tutorial Outline
• Overview
• Label Encapsulations
• Label Distribution Protocols
• Constraint Based Routing
• MPLS and VPNs
• Network Survivability
• Summary
62
MPLS Provides Benefits for Establishing
Virtual Private Networks
• Virtual Private Network (VPN)
— Connects two or more separate sites over the Internet
— Allows them to function as if they were a single, private network.
— Key Features: Security, control over performance, management
ability.
• Overview
• Label Encapsulations
• Label Distribution Protocols
• Constraint Based Routing
• MPLS and VPNs
• Network Survivability
• Summary
64
The need for MPLS protection
• MPLS-based services have been growing
— VPN’s
— Migration of ATM and Frame Relay onto MPLS
— Traffic Engineering
65
The need for MPLS protection
• Layer 3 recovery is also too slow.
— OSPF, RIP, etc. require a redistribution updated link status
information in response to a fault.
— Then routers must recompute their routes.
— Takes on the order of seconds.
— Can have looping and lost packets in the meantime.
66
MPLS protection approaches
• Headend reroute
— Recompute LSP’s to find a new LSP after a failure.
— Initiated by the source or LER.
— Just has slow as using OSPF or RIP – not good.
67
Pre-signaled Standby LSP’s
70
Tutorial Outline
• Overview
• Label Encapsulations
• Label Distribution Protocols
• Constraint Based Routing
• MPLS and VPNs
• Network Survivability
• Summary
71
Summary of Motivations for MPLS
72
Summary of Motivations for MPLS
73
Summary of Motivations for MPLS
• 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
Forwarding
Based on:
MPLS Exact Match on Fixed Length Label
76