multiprotocol label switching (mpls)alfuqaha/fall11/cs6570/... · mpls terminology ldp: label...
TRANSCRIPT
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Multiprotocol Label Switching (MPLS)
Based on slides from Prof. Shivkumar Kalyanaraman Prof. Raj Jain (OSU) , Kireeti Kompella, Juniper networks, Peter Ashwood-Smith and Bilel Jamoussi (Nortel Networks),
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Overview
IP-over-ATM to MPLS: History of IP Switching
MPLS: generalization of labels, de-coupling of control plane
Label distribution/setup protocols: RSVP, LDP
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IP: “Best-Effort Philosophy”
Well architected, not necessarily worked out in detailRealization: can’t predict the futureArchitectural decisions:
Make it reasonableMake it flexibleMake it extensible
stuff above
transportnetwork
stuff below
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IP Control Plane EvolutionAgain, just good enough (best-effort) …
But again, flexible, extensible
Distance Vector routing was fine for quite a whileJust in time, along came link state (OSPF and IS-IS)
Now a burning question in OSPF/IS-IS is: Convergence “in a few seconds” is not good enough?See NANOG June 2002 for interesting videos and papers on how to fix LS-routing for fast convergence
Goal: “Business” IP for service providers…Make me money – new services, GoSDon’t lose me money – uptime, SLAsOSPF/BGP not originally designed to support QoS or multiple services (eg: VoIP, VPNs)
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ATM – Perfectionist’s Dream
stuff abovetransportnetwork
ATM
AAL 1
AAL 2
AAL 3
/4AA
L 5Connection-orientedDoes everything and does it wellAnticipated all future uses and factored them inPhilosophical mismatch with IP
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Overlay Model for IP-over-ATM Internetworking
Goal: Run IP over ATM core networksWhy? ATM switches offered performance, predictable behavior and services (CBR, VBR, GFR…)ISPs created “overlay” networks that presented a virtual topology to the edge routers in their networkUsing ATM virtual circuits, the virtual network could be reengineered without changing the physical networkBenefits
Full traffic controlPer-circuit statisticsMore balanced flow of traffic across links
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Overlay Model (Contd)ATM core ringed by routersPVCs overlaid onto physical network
PhysicalView
A
BC
A
B
CLogicalView
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Summary: IP-to-ATM Overlay Model Drawbacks
IP-to-ATM: control-plane mapping issuesNeed a full mesh of ATM PVCs for mapping IP routingBoth NBMA and Pt-Mpt mapping models have drawbacks
IP-to-ATM: data-plane mapping issuesAddress resolution (eg: LANE, RFC 1577, MPOA, NHRP) requires a complex distributed server and multicast VC infrastructureSegmentation-and-Reassembly (SAR) of IP packets into ATM cells can have a multiplier-effect on performance even if one cell in a packet is lost
ATM SAR has trouble scaling to OC-48 and OC-192 speedsPacket-over-SONET (POS) emerged as an alternative at the link layer
ATM + AAL5 overhead (20%) deemed excessive
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Re-examining Basics: Routing vs Switching
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IP Routing vs IP Switching
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MPLS: Best of Both Worlds
PACKETROUTING
CIRCUITSWITCHING
MPLS+IP
IP ATM
HYBRID
Caveat: one cares about combining the best of both worlds only for large ISP networks that need both features!Note: the “hybrid” also happens to be a solution that bypasses IP- over- ATM mapping woes!
TDM
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MPLS Broad Concept: Route at Edge, Switch in Core
IP ForwardingLABEL SWITCHINGIP Forwarding
IP IP #L1 IP #L2 IP #L3 IP
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MPLS TerminologyLDP: Label Distribution Protocol
LSP: Label Switched Path
FEC: Forwarding Equivalence ClassLSR: Label Switching Router
LER: Label Edge Router (Useful term not in standards)
MPLS is “multi- protocol” both in terms of the protocols it supports ABOVE it and BELOW it in the protocol stack!
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MPLS HeaderIP packet is encapsulated in MPLS header and sent down LSP
IP packet is restored at end of LSP by egress router
TTL is adjusted by default
…IP Packet
32-bitMPLS Header
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MPLS Label Stack Concept
Allows nested tunnels, that are opaque, I.e. do not know or care what protocol data they carry (a.k.a multi- protocol)
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MPLS Header
TTLLabel EXP S
LabelUsed to match packet to LSP
Experimental bitsCarries packet queuing priority (CoS)
Stacking bit: can build “stacks” of labelsGoal: nested tunnels!
Time to liveCopied from IP TTL
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Multi-protocol operationThe abstract notion of a “label” can be mapped to multiple circuit- or VC-oriented technologies!
• ATM - label is called VPI/VCI and travels with cell.
• Frame Relay - label is called a DLCI and travels with frame.
• TDM - label is called a timeslot its implied, like a lane.
• X25 - a label is an LCN
• Proprietary labels: TAG (in tag switching) etc..
• Frequency or Wavelength substitution where “label” is a light frequency/wavelength? (idea in G-MPLS)
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Label EncapsulationATM FR Ethernet PPPL2
VPI VCI DLCI “Shim Label”Label
“Shim Label” …….
IP | PAYLOAD
MPLS Encapsulation is specified over various media types. Top labels may use existing format, lower label(s) use a new “shim” label format.
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MPLS Encapsulation - ATMATM LSR constrained by the cell format imposed by existing ATM standardsATM LSR constrained by the cell format imposed by existing ATM standards
VPI PT CLP HEC
5 OctetsATM Header
Format VCI
AAL5 Trailer
•••Network Layer Header
and Packet (eg. IP)
1n
AAL 5 PDU Frame (nx48 bytes)
Generic Label Encap.(PPP/LAN format)
ATMSAR
ATM HeaderATM Payload • • •
• Top 1 or 2 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)
48 Bytes
48 Bytes
Label LabelOption 1Option 2 Combined Label
Option 3 LabelATM VPI (Tunnel)
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MPLS Encapsulation - Frame Relay
•••n 1
DLCI C/R
EA DLCI FE
CNBECN
DE
EA
Q.922Header
Generic Encap.(PPP/LAN Format) Layer 3 Header and Packet
DLCI Size = 10, 17, 23 Bits
• 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
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
Layer 2 Header(eg. PPP, 802.3)
•••Network Layer Header
and Packet (eg. IP)
4 Octets
MPLS ‘Shim’ Headers (1-n)
1n
• 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 labelled• 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 Labelling” parameter- any unlabelled 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
MPLS on PPP links and LANs uses ‘Shim’ Header Inserted Between Layer 2 and Layer 3 Headers
Label StackEntry Format
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MPLS Forwarding: ExampleAn IP packet destined to 134.112.1.5/32 arrives in SFSan Francisco has route for 134.112/16
Next hop is the LSP to New York
SanFrancisco
New YorkIP
Santa Fe
134.112/16
134.112.1.5
19651026
0
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MPLS Forwarding ExampleSan Francisco pre-pends MPLS header onto IP packet and sends packet to first transit router in the path
SanFrancisco
New York
Santa Fe
134.112/16
IP1965
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MPLS Forwarding ExampleBecause the packet arrived at Santa Fe with an MPLS header, Santa Fe forwards it using the MPLS forwarding tableMPLS forwarding table derived from mpls.0 switching table
SanFrancisco
New York
Santa Fe
134.112/16
IP1026
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MPLS Forwarding ExamplePacket arrives from penultimate router with label 0Egress router sees label 0 and strips MPLS headerEgress router performs standard IP forwarding decision
SanFrancisco
New York
Santa Fe
IP134.112/16
IP0
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Label Setup/Signaling: MPLS Using IP Routing Protocols
47.1
47.247.3
D e s t O u t4 7 . 1 14 7 . 2 24 7 . 3 3
1
23
D e s t O u t4 7 . 1 14 7 . 2 24 7 . 3 3
D e s t O u t4 7 . 1 14 7 . 2 24 7 . 3 3
1
23
1
2
3
• Destination based forwarding tables as built by OSPF, IS-IS, RIP, etc.
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Regular IP Forwarding
47.1
47.247.3
IP 47.1.1.1
D e s t O u t4 7 .1 14 7 .2 24 7 .3 3
1
23
D e s t O u t4 7 . 1 14 7 . 2 24 7 . 3 3
1
2
1
2
3
IP 47.1.1.1
IP 47.1.1.1IP 47.1.1.1
D e s t O u t4 7 . 1 14 7 . 2 24 7 . 3 3
IP destination address unchanged in packet header!
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MPLS Label Distribution
IntfIn
LabelIn
Dest IntfOut
3 0.40 47.1 1
IntfIn
LabelIn
Dest IntfOut
LabelOut
3 0.50 47.1 1 0.40
47.1
47.247.3
1
2
31
2
1
23
3IntfIn
Dest IntfOut
LabelOut
3 47.1 1 0.50 Mapping: 0.40
Request: 47.1
Mapping: 0.50
Request: 47.1
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Label Switched Path (LSP)
IntfIn
LabelIn
Dest IntfOut
3 0.40 47.1 1
IntfIn
LabelIn
Dest IntfOut
LabelOut
3 0.50 47.1 1 0.40
47.1
47.247.3
1
2
31
2
1
23
3IntfIn
Dest IntfOut
LabelOut
3 47.1 1 0.50
IP 47.1.1.1
IP 47.1.1.1
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A General Vanilla LSP
#216
#612
#5#311
#14
#99
#963
#462
- A Vanilla LSP is actually part of a tree from every source to that destination (unidirectional).
- Vanilla LDP builds that tree using existing IP forwarding tables to route the control messages.
#963
#14
#99
#311
#311
#311
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#216
#14
#462
ER-LSP follows route that source chooses. In other words, the control message to establish the LSP (label request) is source routed.
#972
#14 #972
A
B
C
Route={A,B,C}
Explicitly Routed (ER-) LSP
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Explicitly Routed (ER-) LSP Contd
IntfIn
LabelIn
Dest IntfOut
3 0.40 47.1 1
IntfIn
LabelIn
Dest IntfOut
LabelOut
3 0.50 47.1 1 0.40
47.1
47.247.3
1
2
31
2
1
23
3
In tfIn
D e s t In tfO u t
L a b e lO u t
3 4 7 .1 .1 2 1 .3 33 4 7 .1 1 0 .5 0
IP 47.1.1.1
IP 47.1.1.1
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ER LSP - advantages
•Operator has routing flexibility (policy-based, QoS-based)
•Can use routes other than shortest path•Can compute routes based on constraints in exactly the same manner as ATM based on distributed topology database.(traffic engineering)
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ER LSP - discord!• Two signaling options proposed in the standards: CR-LDP, RSVP extensions:
— CR-LDP = LDP + Explicit Route— RSVP ext = Traditional RSVP + Explicit Route +
Scalability Extensions• Not going to be resolved any time soon, market will probably have to resolve it.
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Summary: Key Benifits of MPLS
Goal: Low-overhead virtual circuits for IPOriginally designed to make routers faster by leveraging ATM switch cores (bypasses IP-over-ATM overlay problems)Fixed label lookup faster than longest match used by IP routing
Caveat: Not true anymore! IP forwarding has broken terabit/s speeds through innovative data-structures (next class) !PPP-over-SONET (POS) provides a link layer!