1. MPLS
Technology Overview

2. Outline MPLS Overview MPLS Framework MPLS Applications
MPLS Architecture
Conclusion

3. MPLS Overview -- How it works
The IETF MPLS working group (created in 1997) is to standardize a base technology that integrates the label swapping forwarding paradigm with network layer routing.
Current status:
A framework document has been published as Internet draft. This draft discusses technical issue and requirements for the MPLS.
An architecture document has been published as Internet draft. This draft contains a draft protocol architecture for MPLS. The proposed architecture is based on the MPLS framework document.
An Internet draft that discuss MPLS with Frame Relay has been published.
Cisco System Inc. is the major contributor to the MPLS working group.
substitute “Label” for “Tag” in Tag MPLS

4. Core mechanisms of MPLS
Semantics assigned to a stream label
Labels are associated with specific streams of data.
Forwarding Methods
Forwarding is simplified by the use of the short fixed length labels to identify streams.
Forwarding may require simple functions such as looking up a label in a table, swapping labels, and possibly decrementing and checking a TTL.
In some case MPLS may direct uses of underlying layer 2 forwarding.
Label Distribution Methods
Allow nodes to determine which labels to use for specific streams.
This may use some sort of control exchange, and/or be piggybacked on a routing protocol.

5. Motivation for MPLS Benefits relative to use of a Router Core
Simplified forwarding
Efficient explicit routing
Traffic reengineering
QoS routing
Complex mappings from IP packet to forwarding equivalence class (FEC)
Partitioning of functionality
Single forwarding paradigm with several level differentiation
Benefits relative to use of an ATM or Frame Relay Core
Scaling of the routing protocol
Common operation over packet and cell media
Easier Management
Elimination of the ‘routing over Large Clouds’ issue

6. MPLS Related Protocols
Data forwarding
Label encapsulation
Label operations: PUSH, SWAP and POP
Label distribution protocols (RFC 3036)
Provide procedures by which one LSR informs another of the label/FEC binding
Extensions to routing protocols
Existing routing protocols can be extended to distribute traffic engineering information

7. MPLS Framework
The framework document discusses the core MPLS components, observations, issues, assumptions, and technical approach.
Core MPLS components:
the Basic routing approach, Labels, and Encapsulation
Observations, Issues, and Assumptions
Layer 2 versus Layer 3 forwarding, Scaling issues, Types of streams, and Data driven versus control driven label assignment.
Technical approach
Label distribution, Stream Merging, Loop handling, Interoperation with NHRP, Operation in a hierarchy, Interoperation with “conventional “ ATM, Multicast, Mutipath, Host interactions, Explicit Routing, Traceroute, LSP Control: Egress versus local, and security.

8. Key Terminology in MPLS
FEC (Forwarding Equivalence Class)
A group of IP packets which are forwarded in the same manner (e.g., over the same path, with the same priority and the same label)
Label
A short fixed length identifier which is used to identify a FEC
Label Swapping
Looking up the incoming label to determine the outgoing label, encapsulation and port
Label Switched Path (LSP)
Path through one or more LSRs for a particular FEC
Label Switching Router (LSR)
An MPLS capable router

9. What is a Label
The label can be carried in a layer 2 header (e.g., ATM and frame relay) or in a “shim” that sits between the layer 2 header and IP (e.g., LAN and PPP)
Layer 2
“shim” IP
Payload
Labels may be stacked. For example, one label can be used to steer the packet within an AS and another label can be used to steer the packet through Ases.
Label value (20 bits)
Exp S
TTL
4 Octets
Exp: Experimental (3 bits)
S: Bottom of label stack (1 bit)
TTL: Time-To-Live (8 bits)

10. Data Forwarding Edge LSR (Ingress) (Egress) LSR Label IP (Penultimate)
PUSH
POP
SWAP
L2 header

11. A simplified LSR forwarding engine
Next hop + port
Queuing and
Scheduling rules
Switching
Table
Output
Ports
Input
Ports
MPLS label MPLS payload

12. Ingress and Transit Operation
FEC Output
/16 port 4
PUSH label 40
Input Output
port 2 label 40 port 3
SWAP label 45
To:
Label: 40
Label: 40
Label: 45
Port 1
Port 4
Port 2
Port 3
Ingress LSR
LSR

13. Egress Operation The egress router has to do two table lookups
There is a concern that this might cause a performance penalty on the egress router
Solution: Penultimate Hop Popping (PHP)
Input Output
port 1 label POP
FEC Output Next Hop
/16 Port
Label: 45
To:
To:
Port 1
Port 4
Egress LSR

14. Routing Aggregation Access 1 4 1 Access 3 2 Access 2 5 3 Destination D

15. Per-Hop classification, queuing, and scheduling
Queue S
Classify
Port 1
Port N
Port M

16. PHP with Explicit NULL
Egress router returns a label value of 0 during signaling
Input Output
Port 2 label 40 Port 3
SWAP label 0
FEC Output Next Hop
/16 Port
Label: 40
Label: 0
Label: 0
To:
Port 2
Port 3
Port 1
Port 4
Penultimate LSR
Egress LSR

17. PHP with Implicit NULL
Egress router returns a label value of 3 during signaling
Penultimate LSR pops the label
Input Output
port 2 label 40 port 3
POP
FEC Output Next Hop
/16 Port
Label: 40
To:
To:
To:
Port 2
Port 3
Port 1
Port 4
Penultimate LSR
Egress LSR

18. Label Distribution Protocols
How do routers know what labels to use?
They need a label distribution protocol
There are a number of possible label distribution methods:
Manual
MPLS-BGP (MP-iBGP-4)
Resource Reservation Protocol-Traffic Engineering (RSVP-TE) (RFC 2205, RFC 2210)
Label Distribution Protocol (LDP)
Constraint-Based LDP (CR-LDP)

19. Label Distribution Modes
Downstream-on-Demand
LSR requests its next hop for a label for a particular FEC
Downstream Unsolicited
LSR distributes bindings to LSRs that have not explicitly requested them
For example, topology driven
Only LDP and MPLS-BGP support Downstream Unsolicited mode

20. Manual Configuration Labels are manually configured
Useful in testing or to get around signaling problems R1
(Ingress) R2 R3 R4
(Egress)
LSP
/16
Nexthop R2
Push 40
Label 40
Nexthop R3
Swap 45
Label 45
Nexthop R4
Swap 50
Label 50
Pop

21. MPLS-BGP
Use MP-iBGP-4 to distribute label information as well as VPN routes
BGP peers can send route updates and the associated labels at the same time
Route reflectors can also be used to distribute labels to increase scalability

22. Forwarding Component Label Stack and Forwarding Operations
The basic forwarding operation consists of looking up the incoming label to determine the outgoing label, encapsulation, port, and any additional information which may pertain to the stream such as a particular queue or other QoS related treatment. This operation is referred as label swap.
When a packet first enters an MPLS domain, the packet is associated with a label. It is referred as a label push. When a packet leaves an MPLS domain, the label is removed. It is referred as a label pop.
The label stack is useful within hierarchical routing domain.

23. Encapsulation
Label-based forwarding makes use of various pieces of information, including a label or stack of labels, and possibly additional information such as a TTL field.
These information can be carried in several forms.
The term “MPLS encapsulation” is used to refer to whatever form is used to encapsulate the label information and information used for label based forwarding.
An encapsulation scheme may make use of the following fields:
label, TTL, class of service, stack indicator, next header type indicator, and checksum

24. MPLS label stack encoding
Stack top
Stack bottom
Original
Packet
Label
(20 bits)
Label
(20 bits)
Label
(20 bits)
Exp
(3 bits)
Exp
(3 bits)
Exp
(3 bits)
...
COS S
(1 bit) S
(1 bit) S
(1 bit)
TTL
(8 bits)
TTL
(8 bits)
TTL
(8 bits)
MPLS frame delivered to link layer

25. Label Assignment Topology driven (Tag) Request driven (RSVP)
In response to normal processing of routing protocol control traffic
Labels are pre-assigned; no label setup latency at forwarding time.
Request driven (RSVP)
In response to normal processing of request based control traffic
May require a large number of labels to be assigned.
Traffic driven (Ipsilon)
The arrival of data at an LSR triggers label assignment and distribution.
Label setup latency; potential for packet reordering.

26. Label Distribution Explicit Label Distribution
Downstream label allocation
label allocation is done by the downstream LSR
most natural mechanism for unicast traffic
Upstream label allocation
label allocation is done by the upstream LSR
may be used for optimality for some multicast traffic
A unique label for an egress LSR within the MPLS domain
Any stream to a particular MPLS egress node could use the label of that node.

27. Label Distribution Explicit Label Distribution Protocol (LDP)
Reliability : by transport protocol (TCP) or as part of LDP.
Separate routing computation and label distribution.
Piggybacking on Other Control Messages
Use existing routing/control protocol for distributing routing/control and label information.
OSPF, BGP, RSVP, PIM
Combine routing and label distribution.
Label purge mechanisms
By time out
Exchange of MPLS control packets

28. Label Distribution Protocol
LDP Peer:
Two LSRs that exchange label/stream mapping information via LDP
LDP messages
Discovery messages (via UDP)
announce and maintain the presence of LSR
Session messages
maintain session between LDP peers
Advertisement message
label operation (Label distribution)
Notification message
advisory information and signal error information
Error notification: signal fatal errors
Advisory notification: status of the LDP session or some previous message received from the peer.

29. RSVP-TE Traffic engineering extensions added to RSVP
Sender and receiver are ingress and egress LSRs
New objects have been defined
Supports Downstream on Demand label distribution
PATH messages used by sender to solicit a label from downstream LSRs
RESV messages used by downstream LSRs to pass label upstream towards the sender

30. RSVP-TE Operation Edge LSR (Ingress) LSR LSR Edge LSR (Egress) PATH
(Label
Request)
PATH
(Label
Request)
PATH
(Label
Request)
RESV
Label = 40
RESV
Label = 45
RESV
Label = 50
RESVCONF
RESVCONF
RESVCONF

31. RSVP-TE Operation with PHP
Edge LSR
(Ingress)
LSR
LSR
(Penultimate)
Edge LSR
(Egress)
PATH
(Label
Request)
PATH
(Label
Request)
PATH
(Label
Request)
RESV
Label = 40
RESV
Label = 45
RESV
Label = 0 or 3
RESVCONF
RESVCONF
RESVCONF

32. LDP Supports Downstream on Demand and Downstream Unsolicited
No support for QoS or traffic engineering
UDP used for peer discovery
TCP used for session, advertisement and notification messages
Uses Type-Length-Value (TLV) encoding

33. CR – LDP
Extensions to LDP that convey resource reservation requests for user and network constraints
CR-LDP uses TCP sessions between LSR peers to send LDP messages
A mechanism for establishing explicitly routed LSPs
An Explicit Route is a Constrained Route
Ingress LSR calculates entire route based on Traffic Engineering Database (TED) and known constraints

34. CR-LDP Operation Edge LSR (Ingress) LSR LSR Edge LSR (Egress)
Label Request
Label Request
Label Request
Label Mapping
Label = 40
Label Mapping
Label = 45
Label Mapping
Label = 50

35. CR-LDP vs RSVP-TE Signaling Attributes LSP Attributes
Traffic Engineering Attributes
Reliability & Security Mechanisms

36. Signaling Attributes CR-LDP LDP TCP Hard Yes No RSVP-TE RSVP Raw IP
Soft
Yes No
Underlying Protocol
Transport Protocol
Protocol State
Multipoint-to-Point
Multicasting

37. LSP Attributes CR-LDP Strict & Loose Yes RSVP-TE Strict & Loose Yes
Explicit Routing
Route Pinning
LSP Re-Routing
LSP Preemption
LSP Protection
LSP Merging
LSP Stacking

38. Traffic Engineering Attributes
CR-LDP
Forward Path
RSVP-TE
Reverse Path
Traffic Control
CR-LDP
Negotiates resources during the Request process
Confirms resources during the Mapping process
LSPs are setup only if resources are available
Ability exists to allow for negotiation of resources

39. Traffic Engineering Attributes
CR-LDP
Forward Path
RSVP-TE
Reverse Path
Traffic Control
RSVP-TE
Passes resource requirements to the Egress LER
Egress LER converts the Tspec into a Rspec
Resource reservations occur on RESV process

40. Reliability & Security Attributes
CR-LDP
Yes
RSVP-TE
Yes
Link Failure Detection
Failure Recovery
Security Support

41. Signaling Protocols Each protocol has strengths & weaknesses
CR-LDP is based upon LDP giving it an advantage of using a common protocol
RSVP-TE is more deployed than CR-LDP giving it an early lead in the marketplace

42. Modifications to Routing Protocols
Modifications are also required to distribute traffic engineering topology including bandwidth and administrative constraints
Information used to build up the traffic engineering database
Available bandwidth
Metric
Resource class/color
OSPF-TE
Opaque LSAs used to carry TE information
IS-IS-TE
New TLVs used to carry TE information

43. Traffic Engineering Database
BW=100M
R,O
Metric=1
BW=100M
G,B,O
Metric=1
BW=10M
G,O
Metric=5
BW=1M
B,G,R
Metric=1
BW=50M
R,O
Metric=2
Ingress
Egress
BW=50M
G,R
Metric=1
BW=10M
G,R
Metric=2
G=Green
R=Red
O=Orange
B=Blue
BW=100M
G,R,O
Metric=1

44. Selecting a Path
How to select a 2M path which excludes any blue links?
First prune the links
BW=50M
G,R
Metric=1
R,O
Metric=2
BW=100M
G,R,O
BW=10M
G,O
Metric=5
Ingress
Egress

45. Selecting a Path Now select the shortest path BW=10M G,O Metric=5
G,R
Metric=1
R,O
Metric=2
BW=100M
G,R,O
BW=10M
G,O
Metric=5
Ingress
Egress

46. Explicit Route
Once the path has been determined, the ingress router will typically signal the path using the Explicit Route Option (ERO) or ER-TLV R1 R2 R3 R4 R5 R6
LSP to R6
strict R4
strict R5
PATH

47. FEC and LSP How does an LSR associate a FEC with an LSP? Independent
RFC3031 (Multiprotocol Label Switching Architecture)defines two possible methods
Independent
Each LSR, upon recognizing a particular FEC, makes an independent decision to bind a label to it
Ordered
An LSR binds a label to a FEC only if it is the egress LSR for that FEC, or if it has already received a label binding for that FEC from its next hop

48. Example: BGP and LSP
LSP1 R2 R1
LSP2
/16
/16 R3
AS 1
AS 2
R1 learns about the prefixes of AS2 from R2 via I-BGP and creates a LSP

49. Example: BGP and LSP
R1 learns about the prefixes of AS2 from R2 over IBGP session
Without MPLS, R1 forwards traffic destined for AS2 towards R2 based on the IGP topology database
With MPLS, one (or more) LSP tunnels are pre-established between R1 and R2
This is usually a manual process taking into account traffic engineering requirements
If multiple LSPs exist between the two BGP routers, route filtering can be used to direct the traffic
R1 binds the FEC ( /16) to LSP1 and uses it to forward traffic to R2

50. MPLS Applications Traffic engineering Virtual Private Networks (VPNs)
Layer 3 (BGP/MPLS VPN’s, RFC 2547)
Layer 2, point to point, Virtual Private Lan (Martini, Kompella, etc.)
Enhanced route protection
MPLS and DiffServ

51. Traffic Engineering Traditional routing selects the shortest path
All traffic between the ingress and egress nodes passes through the same links causing congestion
Traffic engineering allows a high degree of control over the path that packets take
Allows more efficient use of network resources
Traffic redirection through BGP or IGP shortcut
Improved resource utilization
Load balancing

52. Example: Traffic Redirection
LSPs can be established so that BGP and IGP traffic traverse different links
LSP Tunnel
BGP
router
traffic
IGP
In this example,
only BGP traffic
is allowed to use
the LSP

53. Example: Improved Utilization
Engineer LSP tunnels to avoid resources that are already congested
BGP or IGP will use LSP tunnel instead of the normal routed path
Congested node
LSP Tunnel

54. Load Balancing Two LSPs established along two paths
BGP traffic routed along both paths R5 R1

55. MPLS VPN Components
VPN Site
Provider Router
Provider Edge
Customer Edge CE CE P PE PE CE P PE
Customer Edge device: device located on customer premises
Provider Edge device: maintains VPN-related information, exchanges VPN information with other Provider Edge devices, encapsulates/decapsulates VPN traffic
Provider router: forwards traffic VPN-unaware

56. Layer 2 and Layer 3 MPLS VPNs
VPNs based on a layer 2 (Data Link Layer) technology and managed at that layer are defined as layer 2 VPNs
Martini, Kompella etc.
VPNs based on tunneling at layer 3 (Network or IP Layer) are Layer 3 VPNs
RFC 2547 bis

57. PE – CE Routing Connections
VPN Routing & Forwarding instance (VRF) for each VPN on each PE
Flexible addressing
Support overlapping IP addresses and private IP address space
Secure
Customer packets are only placed in customers VPN
Customers can use different IGP; Static, RIP, OSPF or BGP
Each VRF contains customer routes

58. PE – PE Routing ConnectionsCE CE P PE PE CE P PE
MP-iBGP used between PE’s to distribute VPN routing information.
PE routers are full mesh MP-iBGP
Multiprotocol Extensions of BGP propagate VPN-IPv4 routes
PE and P routers run IGP and label distribution protocol
P routers are VPN unaware

59. Traffic Forwarding Ingress PE router receives IP packet/Frame from CE
Ingress PE router does lookup and adds label stack
P router switches the packet/frame based on the top label (gray)
Egress PE router removes the top label
Egress PE router uses bottom label (red) to select VPN
Egress PE removes bottom label and forwards IP packet/frame to CE

60. Enhanced Route Protection
Head-end Reroute
If a link along the path fails, the ingress node is notified
The ingress node must recompute another path and then set up the new path
Protection Switching
Pre-establish two paths for an LSP for redundancy
If a link along the primary path fails, the ingress node switches over to the secondary path
Fast Reroute
Each node precomputes and preestablishes a path to bypass potential failures in the downstream link or node

61. Example: Protection Switching
Failure
Ingress
Router
Failure
Link failure
Primary Path
Secondary Path
When ingress router is notified of the link failure, it switches all traffic
to the secondary path.

62. DiffServ Scalability via Aggregation
Flows
Aggregation on Edge Many flows associated with a Class (marked with DSCP)
Aggregated Processing in Core Scheduling/Dropping (PHB) based on DSCP
DiffServ scalability comes from: - aggregation of traffic on Edge - processing of Aggregate only in Core

63. MPLS Scalability via Aggregation
Flows
Aggregation on Edge Many flows associated with a Forwarding Equivalent Class (marked with label)
Aggregated Processing in Core Forwarding based on label
MPLS scalability comes from: - aggregation of traffic on Edge - processing of Aggregate only in Core

64. MPLS and Diffserv
Flows
MPLS: flows associated with FEC, mapped into one label DS: flows associated with Class, mapped to DSCP
MPLS: Switching based on Label DS: scheduling/Dropping based on DSCP
Same scalability goals: - aggregation of traffic on Edge - processing of Aggregate only in Core

65. E-LSP and L-LSP
Information on DiffServ must be made visible to LSR in MPLS.
E-LSP (<= 8 PHB) (PHB = Per Hob Behavior)
EXP-Inferred-PSC LSP
A single LSP can support up to eight BA’s
EXP (3-bits) maps LSP using drop precedence (3-bits)
L-LSP (<= 64 PHB )
Label-Only-Inferred-PSC LSP
A separate LSP for a single FEC / BA (OA) pair
Label maps LSP using DSCP (6-bits)
Defined for both CR-LDP and RSVP-TE

66. MPLS Architecture
The draft MPLS architecture is based on the MPLS framework.
MPLS architecture document gives precise definitions and operations of the MPLS.
The details of this architecture are not introduced here.

67. Conclusion
MPLS has emerged as a promising technology that will improve the scalability of hop-by-hop routing and forwarding, and provide traffic engineering capabilities for better network provisioning.
It decouples forwarding from routing and allows multiprotocol support without requiring changes to the basic forwarding paradigm.