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Introduction to MPLS and Traffic Engineering
Zartash Afzal Uzmi
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CS 573: Network Protocols and Standards
Feb 14, 2008 Outline Traditional IP Routing Forwarding and routing Problems with IP routing Motivations behind MPLS MPLS Terminology and Operation MPLS Label, LSR and LSP, LFIB Vs FIB Transport of an IP packet over MPLS More MPLS terminology Traffic Engineering [with MPLS] Nomenclature Requirements Examples Feb 14, 2008 CS 573: Network Protocols and Standards
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CS 573: Network Protocols and Standards
Feb 14, 2008 Outline Traditional IP Routing Forwarding and routing Problems with IP routing Motivations behind MPLS MPLS Terminology and Operation MPLS Label, LSR and LSP, LFIB Vs FIB Transport of an IP packet over MPLS More MPLS terminology Traffic Engineering [with MPLS] Nomenclature Requirements Examples Feb 14, 2008 CS 573: Network Protocols and Standards
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Forwarding and routing
Passing a packet to the next hop router Routing: Computing the “best” path to the destination IP routing – includes routing and forwarding Each router makes the forwarding decision Each router makes the routing decision MPLS routing Only one router (source) makes the routing decision Intermediate routers make the forwarding decision Feb 14, 2008 CS 573: Network Protocols and Standards
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CS 573: Network Protocols and Standards
IP versus MPLS routing IP routing Each IP datagram is routed independently Routing and forwarding is destination-based Routers look at the destination addresses May lead to congestion in parts of the network MPLS routing A path is computed “in advance” and a “virtual circuit” is established from ingress to egress An MPLS path from ingress to egress node is called a label switched path (LSP) Feb 14, 2008 CS 573: Network Protocols and Standards
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CS 573: Network Protocols and Standards
How IP routing works Searching Longest Prefix Match in FIB (Too Slow) Feb 14, 2008 CS 573: Network Protocols and Standards
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Problems with IP routing
Too slow IP lookup (longest prefix matching) “was” a major bottleneck in high performance routers This was made worse by the fact that IP forwarding requires complex lookup operation at every hop along the path Too rigid – no flexibility Routing decisions are destination-based Not scalable in some desirable applications When mapping IP traffic onto ATM Feb 14, 2008 CS 573: Network Protocols and Standards
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IP routing rigidity example
1 1 A S A B B 1 2 C Packet 1: Destination A Packet 2: Destination B S computes shortest paths to A and B; finds D as next hop Both packets will follow the same path Leads to IP hotspots! Solution? Try to divert the traffic onto alternate paths Feb 14, 2008 CS 573: Network Protocols and Standards
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IP routing rigidity example
1 4 A S A B B 1 2 C Increase the cost of link DA from 1 to 4 Traffic is diverted away from node D A new IP hotspot is created! Solution(?): Network Engineering Put more bandwidth where the traffic is! Leads to underutilized links; not suitable for large networks Feb 14, 2008 CS 573: Network Protocols and Standards
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Motivations behind MPLS
Avoid [slow] IP lookup Led to the development of IP switching in 1996 Provide some scalability for IP over ATM Evolve routing functionality Control was too closely tied to forwarding Evolution of routing functionality led to some other benefits Explicit path routing Provision of service differentiation (QoS) Feb 14, 2008 CS 573: Network Protocols and Standards
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IP routing versus MPLS routing
Multiprotocol Label Switching (MPLS) Traditional IP Routing 1 2 S D 3 4 5 MPLS allows overriding shortest paths! Feb 14, 2008 CS 573: Network Protocols and Standards
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CS 573: Network Protocols and Standards
Feb 14, 2008 Outline Traditional IP Routing Forwarding and routing Problems with IP routing Motivations behind MPLS MPLS Terminology and Operation MPLS Label, LSR and LSP, LFIB Vs FIB Transport of an IP packet over MPLS More MPLS terminology Traffic Engineering [with MPLS] Nomenclature Requirements Examples Feb 14, 2008 CS 573: Network Protocols and Standards
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CS 573: Network Protocols and Standards
MPLS label To avoid IP lookup MPLS packets carry extra information called “Label” Packet forwarding decision is made using label-based lookups Labels have local significance only! How routing along explicit path works? Label IP Datagram Feb 14, 2008 CS 573: Network Protocols and Standards
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Routing along explicit paths
Idea: Let the source make the complete routing decision How is this accomplished? Let the ingress attach a label to the IP packet and let intermediate routers make forwarding decisions only On what basis should you choose different paths for different flows? Define some constraints and hope that the constraints will take “some” traffic away from the hotspot! Use CSPF instead of SPF (shortest path first) Feb 14, 2008 CS 573: Network Protocols and Standards
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CS 573: Network Protocols and Standards
Label, LSP and LSR Label Router that supports MPLS is known as label switching router (LSR) An “Edge” LSR is also known as LER (edge router) Path which is followed using labels is called LSP Label = 20 bits Exp = Experimental, 3 bits S = Bottom of stack, 1bit TTL = Time to live, 8 bits Label | Exp|S| TTL Feb 14, 2008 CS 573: Network Protocols and Standards
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CS 573: Network Protocols and Standards
LFIB versus FIB Labels are searched in LFIB whereas normal IP Routing uses FIB to search longest prefix match for a destination IP address Why switching based on labels is faster? LFIB has fewer entries Routing table FIB has larger number of entries??? In LFIB, label is an exact match In FIB, IP is longest prefix match Feb 14, 2008 CS 573: Network Protocols and Standards
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CS 573: Network Protocols and Standards
Mpls Flow Progress D R1 LSR4 R2 LSR1 D LSR6 destination LSR3 LSR2 R1 and R2 are regular routers LSR5 1 - R1 receives a packet for destination D connected to R2 Feb 14, 2008 CS 573: Network Protocols and Standards
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CS 573: Network Protocols and Standards
Mpls Flow Progress D R1 LSR4 R2 LSR1 D LSR6 destination LSR3 LSR2 LSR5 2 - R1 determines the next hop as LSR1 and forwards the packet (Makes a routing as well as a forwarding decision) Feb 14, 2008 CS 573: Network Protocols and Standards
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CS 573: Network Protocols and Standards
Mpls Flow Progress R1 LSR4 R2 LSR1 31 D D LSR6 destination LSR3 LSR2 LSR5 3 – LSR1 establishes a path to LSR6 and “PUSHES” a label (Makes a routing as well as a forwarding decision) Feb 14, 2008 CS 573: Network Protocols and Standards
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CS 573: Network Protocols and Standards
Mpls Flow Progress R1 LSR4 R2 LSR1 D LSR6 destination LSR3 17 D LSR2 Labels have local signifacance! LSR5 4 – LSR3 just looks at the incoming label LSR3 “SWAPS” with another label before forwarding Feb 14, 2008 CS 573: Network Protocols and Standards
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CS 573: Network Protocols and Standards
MPLS Flow Progress R1 LSR4 R2 LSR1 D LSR6 destination LSR3 17 D LSR2 Path within MPLS cloud is pre-established: LSP (label-switched path) LSR5 5 – LSR6 looks at the incoming label LSR6 “POPS” the label before forwarding to R2 Feb 14, 2008 CS 573: Network Protocols and Standards
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MPLS and explicit routing recap
Who establishes the LSPs in advance? Ingress routers How do ingress routers decide not to always take the shortest path? Ingress routers use CSPF (constrained shortest path first) instead of SPF Examples of constraints: Do not use links left with less than 7Mb/s bandwidth Do not use blue-colored links for this request Use a path with delay less than 130ms Feb 14, 2008 CS 573: Network Protocols and Standards
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CS 573: Network Protocols and Standards
CSPF What is the mechanism? (in typical cases!) First prune all links not fulfilling constrains Now find shortest path on the rest of the topology Requires some reservation mechanism Changing state of the network must also be recorded and propagated For example, ingress needs to know how much bandwidth is left on links The information is propagated by means of routing protocols and their extensions Feb 14, 2008 CS 573: Network Protocols and Standards
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CS 573: Network Protocols and Standards
More MPLS terminology Upstream Downstream /24 LSR1 LSR2 Data Feb 14, 2008 CS 573: Network Protocols and Standards
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Label advertisement Always downstream to upstream label advertisement and distribution Downstream Upstream Use label 5 for destination /24 /24 MPLS Data Packet with label 5 travels LSR2 LSR1 Feb 14, 2008 CS 573: Network Protocols and Standards
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CS 573: Network Protocols and Standards
Label advertisement Label advertisement can be downstream unsolicited or downstream on-demand Sends label Without any Request Downstream Upstream /24 LSR1 LSR2 Sends label ONLY after receiving request Downstream Upstream /24 LSR1 Request For label LSR2 Feb 14, 2008 CS 573: Network Protocols and Standards
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CS 573: Network Protocols and Standards
Label distribution Label distribution can be ordered or unordered First we see an example of ordered label distribution Ingress LSR Egress LSR Label Feb 14, 2008 CS 573: Network Protocols and Standards
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CS 573: Network Protocols and Standards
Label distribution Label distribution can be ordered or unordered Next we see an example of unordered label distribution Ingress LSR Egress LSR Label Label Feb 14, 2008 CS 573: Network Protocols and Standards
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CS 573: Network Protocols and Standards
Label retention modes Label retention can be conservative or liberal ? Destination Label LSR1 Label Feb 14, 2008 CS 573: Network Protocols and Standards
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CS 573: Network Protocols and Standards
Label operations Advertisement Downstream unsolicited Downstream on-demand Distribution Ordered Unordered Retention Liberal Conservative Feb 14, 2008 CS 573: Network Protocols and Standards
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CS 573: Network Protocols and Standards
Feb 14, 2008 Outline Traditional IP Routing Forwarding and routing Problems with IP routing Motivations behind MPLS MPLS Terminology and Operation MPLS Label, LSR and LSP, LFIB Vs FIB Transport of an IP packet over MPLS More MPLS terminology Traffic Engineering [with MPLS] Nomenclature Requirements Examples Feb 14, 2008 CS 573: Network Protocols and Standards
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Traffic Engineering with MPLS (Application of CSPF)
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What is traffic engineering?
Performance optimization of operational networks optimizing resource utilization optimizing traffic performance reliable network operation How is traffic engineered? measurement, modeling, characterization, and control of Internet traffic Why? high cost of network assets service differentiation Feb 14, 2008 CS 573: Network Protocols and Standards
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CS 573: Network Protocols and Standards
Traffic engineering Recall the IP hotspot problem The ability to move traffic away from the shortest path calculated by the IGP (such as OSPF) to a less congested path IP: changing a metric will cause ALL the traffic to divert to the less congested path MPLS: allows explicit routing (using CSPF) and setup of such explicitly computed LSPs Feb 14, 2008 CS 573: Network Protocols and Standards
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CS 573: Network Protocols and Standards
MPLS-TE: How to do it? LSPs are set up by LSRs based on information they learn from routing protocols (IGPs) This defeats the purpose! If we were to use “shortest path”, IGP was okay Feb 14, 2008 CS 573: Network Protocols and Standards
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MPLS TE: How we actually do it?
MPLS TE Requires: Enhancements to routing protocols OSPF-TE ISIS-TE Enhancement to signaling protocols to allow explicit constraint based routing RSVP-TE and CR-LDP Constraint based routing Explicit route selection Recovery mechanisms defined Feb 14, 2008 CS 573: Network Protocols and Standards
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CS 573: Network Protocols and Standards
Signaling mechanisms RSVP-TE Extensions to RSVP for traffic engineering BGP-4 Carrying label information in BGP-4 CR-LDP A label distribution protocol that distributes labels determined based on constraint based routing RSVP-TE and CR-LDP both do label distribution and path reservation – use any one of them! Feb 14, 2008 CS 573: Network Protocols and Standards
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CS 573: Network Protocols and Standards
RSVP-TE Basic flow of LSP set-up using RSVP Feb 14, 2008 CS 573: Network Protocols and Standards
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CS 573: Network Protocols and Standards
RSVP-TE PATH Message PATH message is used to establish state and request label assignment R1 transmits a PATH message addressed to R9 Feb 14, 2008 CS 573: Network Protocols and Standards
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CS 573: Network Protocols and Standards
RSVP-TE RESV Message RESV is used to distribute labels after reserving resources R9 transmits a RESV message, with label=3, to R8 R8 and R4 store “outbound” label and allocate an “inbound” label. They also transmits RESV with inbound label to upstream LSR R1 binds label to forwarding equivalence class (FEC) Feb 14, 2008 CS 573: Network Protocols and Standards
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CS 573: Network Protocols and Standards
Rerouting LSP tunnels When a more “optimal” route/path becomes available When a failure of a resource occurs along a TE LSP Make-before-break mechanism Adaptive, smooth rerouting and traffic transfer before tearing down the old LSP Not disruptive to traffic Feb 14, 2008 CS 573: Network Protocols and Standards
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Recovering LSP tunnels
LSP Set-up Feb 14, 2008 CS 573: Network Protocols and Standards
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CS 573: Network Protocols and Standards
Protection LSP set up Feb 14, 2008 CS 573: Network Protocols and Standards
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CS 573: Network Protocols and Standards
Protection LSP Feb 14, 2008 CS 573: Network Protocols and Standards
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CS 573: Network Protocols and Standards
References RFC 2702 “Requirements for Traffic Engineering Over MPLS” RFC 3031 “Multiprotocol Label Switching Architecture” RFC 3272 “Overview and Principles of Internet Traffic Engineering” RFC 3346 “Applicability Statement for Traffic Engineering with MPLS” MPLS Forum ( Feb 14, 2008 CS 573: Network Protocols and Standards
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