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MPLS Tutorial
Bilel N. Jamoussi, Ph.D.
Senior Network Architect
Carrier Data Networks
[email protected]
Tutorial Outline
• Overview
• Label Encapsulations
• Label Distribution Protocols
• MPLS and ATM
• IETF Status
• Nortel Networks Activity
• Summary
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MPLS Motivations
• Flexibility (L2/L3 Integration)
— Media Support: ATM, FR, Ethernet, PPP
— Operate IP over Multiservice ATM
— More than destination-based Forwarding
• IP Traffic Engineering
— Constraint-based Routing
• IP-VPN
— Tunneling mechanism
• VOIP
— Connection-oriented Paths and QoS
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All Nodes Run Standard IP Routing
Dest
47.1
47.2
47.3
Dest
47.1
47.2
47.3
Out
1
2
3
Out
1
2
3
1 47.1
3
1
Dest
47.1
47.2
47.3
Out
1
2
3
2
3
2
1
47.2
47.3 3
2
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IP Destination Lookup at Each Hop
Dest
47.1
47.2
47.3
Dest
47.1
47.2
47.3
Out
1
2
3
1 47.1
1
Dest
47.1
47.2
47.3
Out
1
2
3
IP 47.1.1.1
2
IP 47.1.1.1
3
Out
1
2
3
2
IP 47.1.1.1
1
47.2
47.3 3
2
IP 47.1.1.1
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Multiprotocol Label Switching (MPLS)
Edge Label Switch
Router (LSR)
Label Switch Router
Label Switch Router
Edge Label Switch
Router (LSR)
IP Packet
IP Packet
IP Packet
Label
IP Packet
IP Packet
Layer 3 Routing
Label
Label
Layer 2 Forwarding
Layer 3 Routing
MPLS involves routing at the edges, switching in the core
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MPLS Terminology
LDP:
FEC:
LSP:
LSR:
LER:
Label Distribution Protocol
Forwarding Equivalence Class
Label Switched Path
Label Switching Router
Label Edge Router (Note that LER is a Nortel Networks
term describing the edge LSR function)
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Forwarding Equivalence Classes
LSR
FEC
LSP
FEC
Packets are destined for different address prefixes, but can be
mapped to common egress router, treated as equivalent FEC
• 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
• In conventional routing, a packet is assigned to an FEC at each hop (i.e.,
L3 lookup); in MPLS, it is only done once at the network ingress
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Label Switched Path — Concept
Label Switched Path (LSP) Set Up Across Network
Interior Nodes
Forwarded Along LSP
Based on Labels
Incoming Packets
Classified, Labeled
Egress Node
Removes Label
Before Forwarding
Two types of Label Switched Paths:
• Hop-by-hop
• Explicit Routing
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MPLS Label Distribution
Intf Label Dest Intf Label
In In
Out Out
3
0.50 47.1 1
0.40
Intf
In
3
Label Dest Intf
In
Out
0.40 47.1 1
1
Request: 47.1
3
Intf Dest Intf Label
In
Out Out
3
47.1 1
0.50
3
2
1
1
47.1
Mapping: 0.40
2
47.3 3
47.2
2
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Label Switched Path (LSP)
Intf Label Dest Intf Label
In In
Out Out
3
0.50 47.1 1
0.40
Intf Dest Intf Label
In
Out Out
3
47.1 1
0.50
Intf
In
3
IP 47.1.1.1
1 47.1
3
3
1
1
Label Dest Intf
In
Out
0.40 47.1 1
2
2
47.3 3
47.2
2
IP 47.1.1.1
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LSPs: Explicit Routing
Explicit Routing
LSR A
Forward to
LSR B
LSR C
LSR D
LSR E
LSR D
LSR B
LSR E
LSR C
• Ingress node (or egress node) determines
path from ingress to egress
• Operator has routing flexibility (policy-based, QoS-based)
• Required for MPLS traffic engineering
• Two signaling options proposed in the standards: RSVP, CR-LDP
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Traffic Engineered Path
Intf Label Dest Intf Label
In In
Out Out
3
0.50 47.1 1
0.40
Intf
In
3
3
Dest
47.1.1
47.1
Intf
Out
2
1
Label
Out
1.33
0.50
Intf
In
3
IP 47.1.1.1
1 47.1
3
3
1
1
Label Dest Intf
In
Out
0.40 47.1 1
2
2
47.3 3
47.2
2
IP 47.1.1.1
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Tutorial Outline
• Overview
• Label Encapsulations
• Label Distribution Protocols
• MPLS & ATM
• IETF Status
• Nortel Networks Activity
• Summary
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Label Encapsulation
MPLS
L2
Label
ATM
FR
VPI VCI DLCI
Ethernet
PPP
“Shim”
MPLS Encapsulation is specified
over various media types
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MPLS Link Layers
• MPLS is intended to run over multiple link layers
• Specifications for the following link layers currently exist:
• ATM: label contained in VCI/VPI field of ATM header
• Frame Relay: label contained in DLCI field in FR header
• PPP/LAN: uses ‘shim’ header inserted between L2 and L3 headers
• Fields and functionality may vary between different link layers
— ATM/FR have to adapt to existing structure
— PPP/LAN header has more freedom to incorporate useful features (CoS, TTL)
• Translation between link-layers types must be supported
MPLS intended to be “multiprotocol” below as well as above
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MPLS Encapsulation — ATM
ATM LSR constrained by the cell format imposed by existing ATM standards
5 Octets
ATM Header
Format
Option 1
VPI
Label
PT
CLP
HEC
Label
Combined Label
Option 2
Option 3
VCI
ATM VPI (Tunnel)
Label
AAL 5 PDU Frame (nx48 bytes)
n
ATM
SAR
•••
1
Network Layer Header
and Packet (e.g., IP)
Generic Label Encap.
(PPP/LAN format)
AAL5 Trailer
48 Bytes
ATM Header
ATM Payload
48 Bytes
•••
• Top one or two labels are contained in the VPI/VCI fields of ATM header
— one in each or single label in combined field, negotiated by LDP
• Further fields in stack are encoded with ‘shim’ header in PPP/LAN format
— must be at least one, with bottom label distinguished with ‘explicit NULL’
• TTL is carried in top label in stack, as a proxy for ATM header (that lacks TTL)
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MPLS Encapsulation — Frame Relay
Generic Encap.
(PPP/LAN Format)
Q.922
Header
n
DLCI
C/ E
R A
DLCI
•••
FE BE D E
CN CN E A
Layer 3 Header and Packet
1
DLCI Size = 10, 17, 23 Bytes
• Current label value carried in DLCI field of Frame Relay header
• Can use either 2 or 4 octet Q.922 address (10, 17, 23 bytes)
• Generic encapsulation contains n labels for stack of depth n
— top label contains TTL (which FR header lacks), ‘explicit NULL’ label value
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MPLS Encapsulation — PPP & LAN Data Links
MPLS ‘Shim’ Headers (1-n)
n
•••
1
Network Layer Header
and Packet (e.g., IP)
Layer 2 Header
(e.g., PPP, 802.3)
4 Octets
Label Stack
Entry Format
Label
Exp.
S
TTL
Label: Label Value, 20 bits (0-16 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
• TTL must be set to the value of the IP TTL field when packet is first labeled
• When last label is popped off stack, MPLS TTL to be copied to IP TTL field
• Pushing multiple labels may cause length of frame to exceed layer-2 MTU
— LSR must support “Max. IP Datagram Size for Labeling” parameter
— any unlabeled datagram greater in size than this parameter is to be fragmented
MPLS on PPP links and LANs uses ‘Shim’ Header Inserted
Between Layer 2 and Layer 3 Headers
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Tutorial Outline
• Overview
• Label Encapsulations
• Label Distribution Protocols
• MPLS & ATM
• IETF Status
• Nortel Networks Activity
• Summary
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Label Distribution Protocols
• Overview of Hop-by-hop and Explicit
• Label Distribution Protocol (LDP)
• Constraint-based Routing LDP (CR-LDP)
• Extensions to RSVP
• Extensions to BGP
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LSPs: Hop-by-Hop vs. Explicit Routing
Hop-by-Hop Routing
LSR A
Forward to
LSR B
MPLS will form label switched paths by one of two methods
— hop-by-hop routing or explicit routing
LSR B
LSR D
LSR C
Forward to
LSR C
Forward to
LSR D
LSR E
Forward to
LSR E
Forward to
LSR ...
• Each node runs layer 3 routing protocol
• Routing decisions made independently at each node
Explicit Routing
LSR A
LSR D
LSR B
LSR E
LSR C
Forward to
LSR B
LSR C
LSR D
LSR E
• Also known as ‘source routing’ or ‘traffic steering’
• Ingress node (or egress node) determines path from ingress to egress
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Comparison — Hop-by-Hop vs. Explicit Routing
Hop-by-Hop Routing
Explicit Routing
• Distributes topology awareness
• Centralized topology awareness (in ingress
node)
• No path setup/tear-down/refresh required
• Path setup/tear-down/refresh required
• Automates routing using industry
standard protocols (e.g., OSPF, ISIS)
• Requires manual provisioning or creation of
new routing protocol
• Loop detection/prevention required
• Reroute on failure impacted by
convergence time of routing protocol
• Existing routing protocols are destination
prefix-based
• Backup paths may be preprovisioned for
rapid restoration
• Operator has routing flexibility (policy-based,
QoS-based)
• Easily used for traffic engineering
• Difficult to perform traffic engineering,
QoS-based routing
Explicit routing shows great promise for traffic engineering,
at the cost of operator involvement (or new routing protocols)
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Explicit Routing — MPLS vs. Traditional Routing
LSR A
LSR D
LSR B
LSR E
LSR C
Forward to
LSR B
LSR C
LSR D
LSR E
• Connectionless nature of IP implies that routing is based on information in each packet header
• Source routing is possible, but path must be contained in each IP header
— lengthy paths increase size of IP header, make it variable size, increase overhead
— some gigabit routers require ‘slow path’ option-based routing of IP packets
• Source routing has not been widely adopted in IP and is seen as impractical
— some network operators may filter source-routed packets for security reasons
• MPLS enables the use of source routing by its connection-oriented capabilities
— paths can be explicitly set up through the network
— the ‘label’ now can represent the explicitly routed path
• Loose and strict source routing can be supported
MPLS makes the use of source routing in the Internet practical
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Label Distribution Protocol (LDP) — Purpose
Label distribution ensures that adjacent routers have
a common view of FEC <-> label bindings
Routing Table:
Routing Table:
Addr-prefix
47.0.0.0/8
Addr-prefix
47.0.0.0/8
Next Hop
LSR2
Next Hop
LSR3
LSR1
IP Packet
LSR3
LSR2
47.80.55.3
Label Information Base:
Label-In FEC Label-Out
XX
47.0.0.0/8
17
Step 3: LSR inserts label
value into forwarding base
For 47.0.0.0/8
use label ‘17’
Label Information Base:
Label-In FEC Label-Out
17
47.0.0.0/8
XX
Step 2: LSR communicates
binding to adjacent LSR
Step 1: LSR creates binding
between FEC and label value
Common understanding of which FEC the label is referring to!
Label distribution can either piggyback on top of an existing routing protocol,
or a dedicated label distribution protocol (LDP) can be created
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Label Distribution — Methods
Label Distribution can take place using one of two possible methods
Downstream Label Distribution
Downstream-on-Demand Label Distribution
LSR2
LSR1
Label-FEC Binding
• LSR2 and LSR1 are said to have an “LDP
adjacency” (LSR2 being the downstream LSR)
LSR1
LSR2
Request for Binding
Label-FEC Binding
• LSR2 discovers a ‘next hop’ for a particular FEC
• LSR1 recognizes LSR2 as its next-hop for an FEC
• LSR2 generates a label for the FEC and
communicates the binding to LSR1
• A request is made to LSR2 for a binding between
the FEC and a label
• LSR1 inserts the binding into its forwarding tables
• If LSR2 recognizes the FEC and has a next hop for
it, it creates a binding and replies to LSR1
• If LSR2 is the next hop for the FEC, LSR1 can use
that label knowing that its meaning is understood
• Both LSRs then have a common understanding
Both methods are supported, even in the same network at the same time.
For any single adjacency, LDP negotiation must agree on a common method.
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Distribution Control: Ordered vs. Independent
MPLS path forms as associations
are made between FEC next-hops
and incoming and outgoing labels
Next Hop
(for FEC)
Incoming
Label
Independent LSP Control
Definition
Example
Comparison
Outgoing
Label
Ordered LSP Control
• Each LSR makes independent decision on when to
generate labels and communicate them to upstream
peers
• Communicate label-FEC binding to peers once
next-hop has been recognized
• LSP is formed as incoming and outgoing labels are
spliced together
• Label-FEC binding is communicated to peers if:
- LSR is the ‘egress’ LSR to particular FEC
- Label binding has been received from
upstream LSR
• Cisco’s Tag Switching
• IBM’s ARIS
• Labels can be exchanged with less delay
• Does not depend on availability of egress node
• Granularity may not be consistent across the nodes
at the start
• May require separate loop detection/mitigation
method
• Requires more delay before packets can be
forwarded along the LSP
• Depends on availability of egress node
• Mechanism for consistent granularity and freedom
from loops
• Used for explicit routing and multicast
• LSP formation ‘flows’ from egress to ingress
Both methods are supported in the standard and can be fully interoperable
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Label Retention Methods
Binding
for LSR5
An LSR may receive label
bindings from multiple LSRs
LSR2
LSR1
LSR5
Binding for LSR5
Some bindings may come
from LSRs that are not the
valid next-hop for that FEC
Binding
for LSR5
LSR4
Conservative Label Retention
Liberal Label Retention
Label Bindings
for LSR5
LSR3
LSR2
Label Bindings
for LSR5
LSR1
LSR3
LSR4’s Label
LSR3’s Label
LSR2’s Label
Valid
Next Hop
LSR4
• LSR maintains bindings received from LSRs
other than the valid next-hop
• If the next-hop changes, it may begin using
these bindings immediately
• May allow more rapid adaptation to routing
changes
• Requires an LSR to maintain many more
labels
LSR2
LSR1
LSR3
LSR4’s Label
LSR3’s Label
LSR2’s Label
Valid
Next Hop
LSR4
• LSR only maintains bindings received from
valid next-hop
• If the next-hop changes, binding must be
requested from new next-hop
• Restricts adaptation to changes in routing
• Fewer labels must be maintained by LSR
Label-Retention method trades-off between label capacity and speed of adaptation to routing changes
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LSPs: Hop-by-Hop
Hop-by-Hop Routing
LSR D
LSR B
LSR A
Forward to
LSR B
Forward to
LSR C
LSR E
LSR C
Forward to
LSR D
Forward to
LSR E
Forward to
LSR ...
• Each node runs layer 3 routing protocol
• Routing decisions made independently at each node
• Distributes topology awareness
• Automates routing using industry standard protocols
(e.g., OSPF, ISIS)
• Difficult to perform traffic engineering
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Outline
• CR-LDP Solution overview
• CR-LDP update
• CR-LDP QoS
• Summary
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ER-LSP Setup using CR-LDP
1. Label Request message. It
contains ER path < B,C,D>.
2. Request message processed
and next node determined.
Path list modified to <C,D>.
6. When LER A receives
label mapping, the ER
established.
LER A
Ingress
5. LSR C receives label to
use for sending data to LER
D. Label table updated.
LSR B
3. Request message
terminates.
4. Label mapping
message originates.
LSR C
ER Label
Switched Path
LER D
Egress
• Simple — part of the MPLS LDP protocol
• Robust — signaling built upon reliable TCP layer
• Scalable — no need to refresh LSP state
• Interoperable — proven multivendor interoperability
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MPLS Traffic Engineering
• Traffic Engineering requires a solution to route LSPs
according to various constraints
• Solution has to be:
— Scalable
— Reliable
• CRLDP use LDP messages to signal these various
constraints
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Constraint-based LSP Setup using LDP
• Uses LDP Messages & TLVs
— LDP runs on a reliable transport (TCP)
• Does NOT require hop-by-hop
— DOD-O can be used for loose segments
• Introduces additional TLVs to the base LDP specification
to signal ER, and other “constraints”
• TLVs for error handling & diagnostics
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Why CR-LDP?
• Runs on TCP
Reliable
• Hard State
Scalable
• QoS Support ATM-like, FR-like, & Diffserv
— More apt to integrate/migrate in existing FR and ATM networks and to
support emerging diffserev-based POS gigabit routers
• Demonstrated interoperability
• Simple protocol based on LDP, output of MPLS WG
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Latest CRLDP Revision
• Constraint-based routing overview section
• CR-TLV is broken in separate TLVs
— Explicit route, route pinning, pre-emption
• ER-Hop TLV encoding consistent with LDP
— 2-byte type, 2-byte length, variable length content
• Traffic TLVs and QoS
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CR-LDP TLVs
• CR-LSP FEC Element
— An opaque FEC element type 0x04 value (0 octet)
• LSPID TLV
— A CRLSP unique identifier within an MPLS network.
• ER-Hop Type (4) LSPID TLV
— The LSPID is used to identify the tunnel ingress point as the next hop
in the ER.
• Resource Class (Color) TLV
— 32 bit mask indicating which of the 32 "administrative groups" or
"colors" of links the CRLSP can traverse.
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CR-LDP Label Request Message
U F
Label Request
Message Length
Message ID TLV
Return Message ID TLV
FEC TLV
LSPID TLV
ER-TLV
Traffic Parameters TLV
Optional
Pinning TLV
"Resource Class" TLV
Pre-emption TLV
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CRLDP Traffic and QoS
• In the crldp-00 draft three service classes (delay sensitive,
throughput sensitive and best effort) were defined.
• This is inflexible and it's hard to map existing and new applications
onto these service definitions.
• In crldp-01 only CRLSP traffic and QoS parameters of a CRLSP are
defined. These describe the characteristics of the CRLSP.
Loosely routed
segment
Unlabeled IP
CRLDP MPLS domain
HBH only MPLS domain
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Traffic Parameters TLV
Flags control “negotiability” of
parameters
U F
Traf. Param. TLV
Flags
Frequency
Length
Reserved
Weight
Peak Data Rate (PDR)
Peak Burst Size (PBS)
Committed Data Rate (CDR)
Committed Burst Size (CBS)
Excess Burst Size (EBS)
32 bit fields are short IEEE floating point
numbers
Any parameter may be used or not used by
selecting appropriate values
Frequency constrains the variable
delay that may be introduced
Weight of the CRLSP in the
“relative share”
Peak rate (PDR+PBS) maximum
rate at which traffic should be sent
to the CRLSP
Committed rate (CDR+CBS) the
rate that the MPLS domain
commits to be available to the
CRLSP
Excess Burst Size (EBS) to
measure the extent by which the
traffic sent on a CRLSP exceeds
the committed rate
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CRLSP characteristics not edge functions
• The approach is like diffserv’s separation of PHB from
edge
• The parameters describe the “path behavior” of the
CRLSP, i.e., the CRLSP’s characteristics
• Dropping behavior is not signaled
— Dropping may be controlled by DS packet markings
• CRLSP characteristics may be combined with edge
functions (which are undefined in CRLDP) to create
services
— Edge functions can perform packet marking
— Example services are in an appendix
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Peak Rate
• The maximum rate at which traffic should be sent to the
CRLSP
• Defined by a token bucket with parameters
— Peak data rate (PDR)
— Peak burst size (PBS)
• Useful for resource allocation
• If a network uses the peak rate for resource allocation
then its edge function should regulate the peak rate
• May be unused by setting PDR or PBS or both to
positive infinity
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Committed Rate
• The rate that the MPLS domain commits to be available
to the CRLSP
• Defined by a token bucket with parameters
— Committed data rate (CDR)
— Committed burst size (CBS)
• Committed rate is the bandwidth that should be reserved
for the CRLSP
• CDR = 0 makes sense; CDR = + less so
• CBS describes the burstiness with which traffic may be
sent to the CRLSP
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Excess Burst Size
• Measure the extent by which the traffic sent on a CRLSP
exceeds the committed rate
• Defined as an additional limit on the committed rate’s
token bucket
• Can be useful for resource reservation
• If a network uses the excess burst size for resource
allocation then its edge function should regulate the
parameter and perhaps mark or drop packets
• EBS = 0 and EBS = + both make sense
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Frequency
• Specifies how frequently the committed rate should be
given to CRLSP
• Defined in terms of “granularity” of allocation of rate
• Constrains the variable delay that the network may
introduce
• Constrains the amount of buffering that an LSR may use
• Values:
— Very frequently: no more than one packet may be buffered
— Frequently: only a few packets may be buffered
— Unspecified: any amount of buffering is acceptable
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Weight
• Specifies the CRLSP’s weight in the “relative share
algorithm”
• Implied but not stated:
— CRLSPs with a larger weight get a bigger relative share of the
“excess bandwidth”
• Values:
— 0 — the weight is not specified
— 1-255 — weights; larger numbers are larger weights
• The definition of “relative share” is network specific
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Negotiation Flags
PDR Negotiation Flag
PBS Negotiation Flag
CDR Negotiation Flag
CBS Negotiation Flag
EBS Negotiation Flag
Weight Negotiation Flag
Res F6 F5 F4 F3 F2 F1
If a parameter is flagged as negotiable
then LSRs may replace the parameter
value with a smaller value in the label
request message. LSRs descover the
negotiated values in the label mapping
message.
Label request - possible
downward negotiation
Label mapping no negotiation
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ER-LSP Setup Using RSVP
2. New path state. Path
message sent to next node.
1. Path message. It contains
ER path < B,C,D>.
5. When LER A receives
Resv, the ER
established.
LER A
3. Resv message originates.
Contain the label to use and the
required traffic/QoS para.
4. New reservation state.
Resv message propagated
upstream.
6. ResvConf
message (o).
LSR B
LSR C
Per-hop Path and
Resv refresh unless
suppressed.
LER D
• More complex — signaling in addition to MPLS LDP protocol
• Unreliable — signaling built upon UDP
• Scalability concerns — Significant number of refresh messages to process
• Interoperability concerns — IETF draft underspecified, no proven
interoperability
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BGP Extensions
• A mechanism to exchange label binding information
among BGP peers by adding (piggybacking) the label
mapping information on the BGP route update
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Tutorial Outline
• Overview
• Label Encapsulations
• Label Distribution Protocols
• MPLS & ATM
• IETF Status
• Nortel Networks Activity
• Summary
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MPLS & ATM
• Various Modes of Operation
— Label-controlled ATM
— Tunneling through ATM
— Ships in the night with ATM
• ATM Merge
— VC merge
— VP merge
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MPLS & ATM
Several models for running MPLS on ATM:
1. Label-Controlled ATM:
• Use ATM hardware for label switching
• Replace ATM Forum SW by IP/MPLS
IP Routing
MPLS
ATM HW
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Label-Controlled ATM
• Label switching is used to forward network-layer packets
• It combines the fast, simple forwarding technique of ATM with network layer
routing and control of the TCP/IP protocol suite
Label Switching Router
Network Layer
Routing
(e.g., OSPF, BGP4)
Switched path topology
formed using network
layer routing
(i.e., TCP/IP technique)
Forwarding
Table
Forwarding
Table
B 17
C 05
•
•
•
Label
Port
A
C
IP Packet
05
Label
IP Packet
17
B
D
Packets forwarded
by swapping short,
fixed-length labels
(i.e., ATM technique)
ATM Label Switching is the combination of L3 routing and L2 ATM switching
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2. MPLS Over ATM
MPLS
MPLS
L
S
R
ATM Network
L
S
R
Two Models
VP
VC
Internet Draft:
VCID notification over ATM Link
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3. Ships in the Night
L
S
R
L
S
R
MPLS
ATM
ATM
SW
ATM
SW
• ATM Forum and MPLS control planes both run on the
same hardware but are isolated from each other, i.e.,
they do not interact.
• This allows a single device to simultaneously operate as
both an MPLS LSR and an ATM switch.
• Important for migrating MPLS into an ATM network.
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Ships in the Night Requirements
• Resource Management
— VPI.VCI Space Partitioning
— Traffic management
– Bandwidth Reservation
– Admission Control
– Queuing & Scheduling
– Shaping/Policing
— Processing Capacity
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Bandwidth Management
Port Capacity
A. Full Sharing
MPLS
Pool 1
• MPLS
• ATM
ATM
Available
B. Protocol Partition
Pool 1
• 50%
• ATM
MPLS
Available
Pool 2 ATM
• 50%
• rt-VBR
Available
C. Service Partition
MPLS
Pool 1
• 50%
• rt-VBR ATM
• COS2
Available
Pool 2 MPLS
• 50%
• nrt-VBR ATM
• COS1
Available
• Bandwidth Guarantees
• Flexibility
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ATM Merge
• Multipoint-to-point capability
• Motivation
— Stream Merge to achieve scalability in MPLS:
– O(n) VCs with Merge as opposed to O(n2) for full mesh
– Less labels required
— Reduce number of receive VCs on terminals
• Alternatives
— Frame-based VC Merge
— Cell-based VP Merge
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Stream Merge
Input cell streams
1 1 1
in out
1 7
2
3
2 2 2
3 3
6
9
6 7 9 6 7 9 6 7
Non-VC merging (Nin–Nout)
Input cell streams
1 1 1
2 2 2
3 3
in
1
2
3
out
7
7
7
7 7 7 7 7 7 7 7
AAL5 Cell Interleaving Problem
7 7 7 7 7 7 7 7
No Cell Interleaving
VC merging (Nin-1out)
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VC-Merge: Output Module
Reassembly buffers
Output buffer
Merge
Passport is VC-Merge Capable
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VP-Merge
VCI=1
Option 1: Dynamic VCI Mapping
VCI=2
VPI=1
No Cell Interleaving Problem
Since VCI is Unique
VCI=1
VCI=2
VPI=2
VCI=3
VPI=3
Option 2: Root
Assigned VCI
VCI=3
–merge multiple VPs into one VP
–use separate VCIs within VPs to distinguish frames
–less efficient use of VPI/VCI space, needs support of SVP
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Tutorial Outline
• Overview
• Label Encapsulations
• Label Distribution Protocols
• MPLS & ATM
• IETF Status
• Nortel Networks Activity
• Summary
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Proposed Standard RFCs
• MPLS Label Stack Encoding <draft-ietf-mpls-labelencaps-03.txt>
• Use of Label Switching on Frame Relay Networks
Specification <draft-ietf-mpls-fr-03.txt>
• MPLS using ATM VC Switching <draft-ietf-mpls-atm01.txt>
• Multiprotocol Label Switching Architecture <draft-ietfmpls-arch-04.txt>
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Last Call
• Gone through Last Call:
— Label Distribution Protocol
• Going to last call:
— Constraint-based Label Distribution Protocol
— Extensions to RSVP for LSP Tunnels
— RSVP Refresh Reduction Extensions
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Tutorial Outline
• Overview
• Label Encapsulations
• Label Distribution Protocols
• MPLS & ATM
• IETF Status
• Nortel Networks Activity
• Summary
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Nortel’s Activity
• IETF
• Interoperability Demonstration
— CR-LDP
• Implementation
— Traffic Engineering
— VPN
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Progress: Consensus Plus Running Code
• 14 vendors & ISPs collaborated on CRLDP
• MPLS WG document in Orlando
• CRLDP is included by reference in the LDP Specification
• LDP Spec has gone through last call
• Demonstrated interoperability among three Vendors’
implementations in November ’98
• CRLDP is simple, stable, robust, and easily extendible
• CR-LDP WG document is going to last call
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Leading Key MPLS Standards
• Label Distribution Protocol (LDP)
— Loa Andersson & Andre Fredette
• Constraint-based Routing LDP (CR-LDP)
— Bilel Jamoussi, Andre Fredette, Loa Andersson, Osama AbouldMagd, & Peter Ashwood-Smith
• QoS Resource Management in MPLS-Based Networks
— Osama Aboul-Magd & Bilel Jamoussi with Jerry Ash, AT&T
• MPLS using ATM VP Switching
— Bilel Jamoussi & Nancy Feldman, IBM
• Explicit Tree Routing
— Swee Loke
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Hosting MPLS Multivendor Interoperability
Demo
• MPLS over ATM
• Protocol implemented according to:
— CRLSP over LDP Spec.
— Explicit Routing (ER)
— Bw Reservation
— QoS signaling
• VC-Merge
• Ships in the Night
• Has been Tested for Interoperability with Bay BN router,
Ericsson & GDC
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Demo Description
• Demo of five node network
— Three MPLS LSRs based on ATM switches:
– Ericsson AXI537, GDC Apex, Nortel Networks Passport
— Two Nortel Networks MPLS LERs based on BN/ARE routers
• MPLS/IP links are OC3 ATM
• IP/Ethernet links are 10baseT
• All LERs/LSRs capable of LDP and CR-LDP functions
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Demo Interoperability Network
A4
A2
LSR 2
LSR 3
Nortel
Networks
Passport
Ericsson
AXD311
A3
A0
A1
PC1
PC2
E22
A5
LER 2
Nortel
Networks A51
BN/ARE
A4
A51
LER 1
LSR 1
A6
GDC
APEX
E22
A8
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Nortel
Networks
BN/ARE
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Experience Gained
• Clear intent and structure of LDP
— Fast implementation
— Simple implementation
• LDP flexibility
— Made implementing CR-LDP easy
— Frame format flexibility helped
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Promoting Open Standard
www.nortelnetworks.com/mpls
C Source code of LDP/CRLDP
message and TLV processing
According to the latest Specs:
LDP:
<draft-ietf-mpls-ldp-03>
CR-LDP: <draft-ietf-mpls-cr-ldp01>
Freely available to anyone
Objective: promote interoperability
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Passport 6400/7400/15000 MPLS
• Q399
— Passport 6400/7400/15000 LSR over ATM
– Strict ER
– Hop-by-hop
– QoS mapping
– Failure handling and recovery
– Interoperability with BN router
— Passport 6400/7400/15000 LER
– Support for terminating and initiating LSPs
– FEC configuration
– QoS-based mapping of traffic onto LSPs
– MVR over MPLS
• Q499
— MPLS over Frame Relay
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Passport 6400/7400/15000 as an LSR
• BN router can do the LER capability
• Passport current edge switch position in the network makes it an
LSR candidate
• Passport can intemperate with Cisco at edge based on MPLS
Standard LDP
LER
LSR
LER
FEC
LDP
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Passport 6400/7400/15000 as an LER
• Provides ability to interface to legacy non-MPLS literate
routers and take advantage of MPLS in the network
• Provides support for MPLS as a transport for MVR
LER
LSR
LER
FEC
LDP
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MPLS interconnecting MVRs
• LSPs established between CVRs
• Label Stacking between VRn and CVRx
• BGP or LDP sessions established to distribute
reachability and Label
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Tutorial Outline
• Overview
• Label Encapsulations
• Label Distribution Protocols
• MPLS & ATM
• IETF Status
• Nortel Networks Activity
• Summary
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Summary of Motivations for MPLS
•
Simplified forwarding based on exact match of fixed-length label
– Initial drive for MPLS was based on existance of cheap, fast ATM switches
•
Separation of routing and forwarding in IP networks
– Facilitates evolution of routing techniques by fixing the forwarding method
– New routing functionality can be deployed without changing the forwarding techniques of every
router in the Internet
•
Facilitates the integration of ATM and IP
– Allows carriers to leverage their large investment of ATM equipment
– Eliminates the adjacency problem of VC-mesh over ATM
•
Enables the use of explicit routing/source routing in IP networks
– Can be easily used for such things as traffic management, QoS routing
•
Promotes the partitioning of functionality within the network
– Move granular processing of packets to edge; restrict core to packet forwarding
– Assists in maintaining scalability of IP protocols in large networks
•
Improved routing scalability through stacking of labels
– Removes the need for full routing tables from interior routers in transit domain; only routes to
border routers are required
•
Applicability to both cell and packet link-layers
– Can be deployed on both cell (e.g., ATM) and packet (e.g., FR, Ethernet) media
– Common management and techniques simplifies engineering
Many drivers exist for MPLS above and beyond high-speed forwarding
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IP and ATM Integration
IP over ATM VCs
IP over MPLS
• ATM cloud invisible to Layer 3 Routing
• ATM network visible to Layer 3 Routing
• Full mesh of VCs within ATM cloud
• Singe adjacency possible with edge router
• Many adjacencies between edge routers
• Hierachical network design possible
• Topology change generates many route updates
• Reduces route update traffic and power
needed to process them
• Routing algorithm made more complex
MPLS eliminates the “n-squared” problem of IP over ATM VCs
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Traffic Engineering
B
Demand
C
A
D
Traffic engineering is the process of mapping traffic demand onto a network
Network
Topology
Purpose of traffic engineering:
• Maximize utilization of links and nodes throughout the network
• Engineer links to achieve required delay, grade-of-service
• Spread the network traffic across network links, minimize impact of single failure
• Ensure available spare-link capacity for rerouting traffic on failure
• Meet policy requirements imposed by the network operator
Traffic engineering key to optimizing cost/performance
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Traffic Engineering Alternatives
Current methods of traffic engineering:
Manipulating routing metrics
Difficult to manage
Use PVCs over an ATM backbone
Not scalable
Overprovision bandwidth
Not economical
MPLS provides a new method to do traffic engineering (traffic steering)
Example Network:
Ingress node
explicitly routes
traffic over
uncongested path
Chosen by Traffic Eng.
(least congestion)
Congested Node
Chosen by routing protocol
(least cost)
Potential benefits of MPLS for traffic engineering:
- Allows explicitly routed paths
- No “n-squared” problem
- Per FEC traffic monitoring
- Backup paths may be configured
operator control
scalable
granularity of feedback
redundancy/restoration
MPLS combines benefits of ATM and IP-layer traffic engineering
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MPLS Traffic Engineering Methods
• MPLS can use the source routing capability to steer traffic on desired path
• Operator may manually configure these in each LSR along the desired path
— Analogous to setting up PVCs in ATM switches
• Ingress LSR may be configured with the path, RSVP used to set up LSP
— Some vendors have extended RSVP for MPLS path setup
• Ingress LSR may be configured with the path, LDP used to set up LSP
— Many vendors believe RSVP not suited
• Ingress LSR may be configured with one or more LSRs along the desired path,
hop-by-hop routing may be used to set up the rest of the path
— A.k.a loose source routing, less configuration required
• If desired for control, route discovered by hop-by-hop routing can be frozen
— A.k.a “route pinning”
• In the future, constraint-based routing will offload traffic engineering tasks from
the operator to the network itself
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MPLS: Scalability Through Routing Hierarchy
AS1
BR2
AS2
TR1
BR1
AS3
TR2
BR3
TR4
Ingress router
receives packet
Packet labeled
based on
egress router
TR3
BR4
Forwarding in the interior
based on IGP route
Egress border
router pops
label and fwds.
• Border routers BR1-4 run an EGP, providing inter-domain routing
• Interior transit routers TR1-4 run an IGP, providing intra-domain routing
• Normal layer 3 forwarding requires interior routers to carry full routing tables
— Transit router must be able to identify the correct destination ASBR (BR1-4)
• Carrying full routing tables in all routers limits scalability of interior routing
— Slower convergence, larger routing tables, poorer fault isolation
• MPLS enables ingress node to identify egress router, label packet based on interior route
• Interior LSRs would only require enough information to forward packet to egress
MPLS increases scalability by partitioning exterior routing from interior routing
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MPLS: Partitioning Routing and Forwarding
Based on:
Routing
OSPF, IS-IS, BGP, RIP
Forwarding Table
Forwarding
Classful Addr. Prefix?
Classless Addr. Prefix?
Multicast Addr.?
Port No.?
ToS Field?
Based on:
MPLS
Exact Match on Fixed-Length Label
• Current network has multiple forwarding paradigms
— Class-ful longest prefix match (Class A,B,C boundaries)
— Classless longest prefix match (variable boundaries)
— Multicast (exact match on source and destination)
— Type-of-service (longest prefix. match on addr. + exact match on ToS)
• As new routing methods change, new route lookup algorithms are required
— Introduction of CIDR
• Next generation routers will be based on hardware for route lookup
— Changes will require new hardware with new algorithm
• MPLS has a consistent algorithm for all types of forwarding; partitions routing/forwarding
— Minimizes impact of the introduction of new forwarding methods
MPLS introduces flexibility through consistent forwarding paradigm
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Upper Layer Consistency Across Link Layers
Ethernet
PPP
(SONET, DS-3 etc.)
ATM
Frame
Relay
• MPLS is “multiprotocol” below (link layer) as well as above (network layer)
• Provides for consistent operations, engineering across multiple technologies
• Allows operators to leverage existing infrastructure
• Co-existence with other protocols is provided for
— e.g., “Ships in the Night” operation with ATM, muxing over PPP
MPLS positioned as end-to-end forwarding paradigm
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Summary
• MPLS is a promising emerging technology
• Basic functionality (Encapsulation and basic Label
Distribution) has been defined by the IETF
• Nortel Networks is taking an active role in defining key
aspects of MPLS standard and providing support of
MPLS on the Bay and Nortel Networks platforms
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