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MPLSMulti Protocol Label Switching (an introduction)
Karst Koymans
Informatics InstituteUniversity of Amsterdam
(version 4.3, 2015/03/09 13:07:57)
Thursday, March 12, 2015
Karst Koymans (UvA) MPLS Thursday, March 12, 2015 1 / 26
...1 Faster IP forwarding
ATM and MPLS
...2 MPLS Operation
Labels, paths and switching
MPLS Control and Label Management
...3 MPLS Applications
MPLS VPN techniques
Karst Koymans (UvA) MPLS Thursday, March 12, 2015 2 / 26
Faster IP forwarding
Outline
...1 Faster IP forwardingATM and MPLS
...2 MPLS OperationLabels, paths and switchingMPLS Control and Label Management
...3 MPLS ApplicationsMPLS VPN techniques
Karst Koymans (UvA) MPLS Thursday, March 12, 2015 3 / 26
Faster IP forwarding
IP Forwarding
Forwarding IP packetsNormally done by destination IP addressRouting protocols are used to distribute layer 3 informationDecisions are made hop-by-hop independently
Connectionless operationRouting protocol should offer consistency
ShortcomingsRouting tables may become very large
Search time is ≈ O(log N) for table of size N
Limited traffic engineering optionsLayer 2 and layer 3 information is unrelated
Karst Koymans (UvA) MPLS Thursday, March 12, 2015 4 / 26
Faster IP forwarding ATM and MPLS
Outline
...1 Faster IP forwardingATM and MPLS
...2 MPLS OperationLabels, paths and switchingMPLS Control and Label Management
...3 MPLS ApplicationsMPLS VPN techniques
Karst Koymans (UvA) MPLS Thursday, March 12, 2015 5 / 26
Faster IP forwarding ATM and MPLS
ATM (Asynchronous Transfer Mode)
Uses small fixed size cells to transport dataEach cell contains 53 bytes
A fixed size header of 5 bytesA fixed size payload of 48 bytes
Often data needs to be fragmented
Uses VCs (Virtual Circuits) between endpointsSwitching based on labels
An 8-bit (UNI) or 12-bit (NNI) Virtual Path Identifier (VPI)An 16-bit Virtual Channel Identifier (VCI)
Switching lookup is now O(1)Promises QoS, for instance low jitter
Higher level protocols use the ATM Adaptation Layer (AAL)Karst Koymans (UvA) MPLS Thursday, March 12, 2015 6 / 26
Faster IP forwarding ATM and MPLS
MPLS (Multi Protocol Label Switching)
Defined in RFC 3031Combine the best of both worlds
Packet forwardingCircuit switchingRoute at the edges, switch at the core
Multi ProtocolAgnostic to underlying protocols used at layer 2 or 3
Label SwitchingUses labels instead of IP addressesRouting at the speed of switching
Hybrid, “layer 2.5”, shim protocolKarst Koymans (UvA) MPLS Thursday, March 12, 2015 7 / 26
MPLS Operation
Outline
...1 Faster IP forwardingATM and MPLS
...2 MPLS OperationLabels, paths and switchingMPLS Control and Label Management
...3 MPLS ApplicationsMPLS VPN techniques
Karst Koymans (UvA) MPLS Thursday, March 12, 2015 8 / 26
MPLS Operation
MPLS Network
MPLS Network
Karst Koymans (UvA) MPLS Thursday, March 12, 2015 9 / 26
MPLS Operation Labels, paths and switching
Outline
...1 Faster IP forwardingATM and MPLS
...2 MPLS OperationLabels, paths and switchingMPLS Control and Label Management
...3 MPLS ApplicationsMPLS VPN techniques
Karst Koymans (UvA) MPLS Thursday, March 12, 2015 10 / 26
MPLS Operation Labels, paths and switching
MPLS Label (RFC 3032)0 19 20 22 23 24 31
Label Exp S TTL
.MPLS packet label fields..
......
Label Per router local semantics
Exp(erimental)a Class of Service
S(tack) Bottom of stack indication
T(ime) T(o) L(ive) Incoming IP TTL
aRenamed to TC (Traffic Class) by RFC 5462
Karst Koymans (UvA) MPLS Thursday, March 12, 2015 11 / 26
MPLS Operation Labels, paths and switching
LER (Label Edge Router)
LSR (Label Switching Router)
Label Edge RouterPositioned at the edge of the MPLS networkAdds or removes labels for incoming and outgoing trafficAlso called Edge Label Switch Router (eLSR)
Label Switch RouterPositioned in the core of the MPLS networkSwaps labels and forwards packetsDoes not need to know about IP forwarding
Karst Koymans (UvA) MPLS Thursday, March 12, 2015 12 / 26
MPLS Operation Labels, paths and switching
FEC (Forwarding Equivalence Class) and
LSP (Label Switched Path)
Forwarding Equivalence ClassPackets which are forwarded in the same mannerEach label on a link determines a unique FECEvery FEC is associated with a certain LSP
Label Switched PathSequence of intermediate routers on same stack depthon the path from ingress to egress router
Hop-by-hop routingExplicit routing
Karst Koymans (UvA) MPLS Thursday, March 12, 2015 13 / 26
MPLS Operation Labels, paths and switching
LIB (Label Information Base)
Mapping between previous hop (incoming port, label) and FEC
Mapping between FEC and next hop (outgoing port, label)
Each router has its own LIBDetails are implementation dependent
Generates an LFIB (Label Forwarding Information Base)
Input Port Incoming Port Label Output Port Outgoing Port Label
1 green 3 blue
2 red 1 orange
Karst Koymans (UvA) MPLS Thursday, March 12, 2015 14 / 26
MPLS Operation Labels, paths and switching
MPLS Operation (continued)
MPLS Network
1
3
2
4 5
6
...0 Receive IP packet at edge router
...1 Classify into FEC green
...2 Push (label green)
...3 Swap (label green, label red)
...4 Swap (label red, label blue)
...5 Pop (label blue)
...6 Forward IP packet to destination
Karst Koymans (UvA) MPLS Thursday, March 12, 2015 15 / 26
MPLS Operation MPLS Control and Label Management
Outline
...1 Faster IP forwardingATM and MPLS
...2 MPLS OperationLabels, paths and switchingMPLS Control and Label Management
...3 MPLS ApplicationsMPLS VPN techniques
Karst Koymans (UvA) MPLS Thursday, March 12, 2015 16 / 26
MPLS Operation MPLS Control and Label Management
MPLS Separation of Control and Data
Control PlaneExchange layer 3 and label information
Several label distribution protocols (LDP, CR-LDP, RSVP-TE)Also piggybacked on routing protocols (OSPF-TE, ISIS-TE, MP-BGP)
Data PlaneSwitch (forward) packets based on labels
Push (ingress)Swap (intermediate)Swap and push (start LSP tunnel)Pop and swap (end LSP tunnel)Pop (egress)
Karst Koymans (UvA) MPLS Thursday, March 12, 2015 17 / 26
MPLS Operation MPLS Control and Label Management
LDP (Label Distribution Protocol)
UDP discovery and TCP session with peers
Adjacent LSRs inform each other of the label bindingsDownstream LSR determines the labelOn demand or unsolicited
An IGP protocol is configured on all LSRs
New IGP routes lead to new label bindings
Labels can be withdrawn when IGP routes are no longer valid
Hard-stateExpected to work until explicitly torn down
Karst Koymans (UvA) MPLS Thursday, March 12, 2015 18 / 26
MPLS Operation MPLS Control and Label Management
RSVP-TE
(Resource Reservation Protocol – Traffic Engineering)
Extension of RSVPUnreliable delivery (protocol 46; sometimes UDP)Soft-state
Information needs to be periodically refreshed
An LSP is created by propagating an RSVP message to the endpoint
Paths are terminated if not refreshed on time
Karst Koymans (UvA) MPLS Thursday, March 12, 2015 19 / 26
MPLS Applications
Outline
...1 Faster IP forwardingATM and MPLS
...2 MPLS OperationLabels, paths and switchingMPLS Control and Label Management
...3 MPLS ApplicationsMPLS VPN techniques
Karst Koymans (UvA) MPLS Thursday, March 12, 2015 20 / 26
MPLS Applications
Applications (1)
TE (Traffic Engineering)Traffic steering across network in order to fully utilize resourcesWith the help of RSVP-TE and OSPF-TE or IS-IS-TEFull network view (OSPF, IS-IS) and extra information about link stateRSPV-TE creates LSPs on demand
QoS (Quality of Service)Different types of service across the networkEnforcing drop probability, bandwidth and delay (queueing)CoS bits or FEC (destination + class of service)
Karst Koymans (UvA) MPLS Thursday, March 12, 2015 21 / 26
MPLS Applications
Applications (2)
VPN (Virtual Private Network)Private connections over shared networksLayer 2 or layer 3 basedBusiness can choose the needed type of serviceStacked labels: egress router and VPN identityLDP (layer 2) or BGP (layer 3) is used to transport label information
Karst Koymans (UvA) MPLS Thursday, March 12, 2015 22 / 26
MPLS Applications MPLS VPN techniques
Outline
...1 Faster IP forwardingATM and MPLS
...2 MPLS OperationLabels, paths and switchingMPLS Control and Label Management
...3 MPLS ApplicationsMPLS VPN techniques
Karst Koymans (UvA) MPLS Thursday, March 12, 2015 23 / 26
MPLS Applications MPLS VPN techniques
Layer 2 VPN (1)
AToM (Any Transport over MPLS)Transports layer 2 frames over an MPLS Network
ATM, Frame-Relay, PPP, Ethernet
Point-to-point service
From ingress router/interface to egress router/interfaceIdentified by using an inner label
Karst Koymans (UvA) MPLS Thursday, March 12, 2015 24 / 26
MPLS Applications MPLS VPN techniques
Layer 2 VPN (2)
VPLS (Virtual Private LAN Service)Transports layer 2 frames over an MPLS NetworkWorks multipoint-to-multipointThe IP/MPLS core operates as a big virtual switch
Outer label used for normal MPLS forwardingInner label used for VPLS instances
Karst Koymans (UvA) MPLS Thursday, March 12, 2015 25 / 26
MPLS Applications MPLS VPN techniques
Layer 3 VPN
Uses the MP-BGP (Multi Protocol BGP) extension of BGP
Introduces the VPN-IPv4 address family
Extends the 32-bit IPv4 address with a 64-bit RD (route distinguisher)
A BGP route for VPN-IPv4 includes an MPLS labelEach VPN has its own VRF (VPN Routing and Forwarding) instance
Keeping separate routing tables for the VPNsAlso separate from the global routing table
MPLS forwarding uses stacked labelsOuter label for LSP forwardingInner label to differentiate between different VPNs
Karst Koymans (UvA) MPLS Thursday, March 12, 2015 26 / 26
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