Introduction to MPLS and Traffic Engineering Zartash Afzal Uzmi

Jan 11, 2006Lahore University of Management Sciences2 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

Jan 11, 2006Lahore University of Management Sciences3 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

Jan 11, 2006Lahore University of Management Sciences4 Forwarding and routing Forwarding: 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 router make the forwarding decision

Jan 11, 2006Lahore University of Management Sciences5 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)

Jan 11, 2006Lahore University of Management Sciences6 How IP routing works Searching Longest Prefix Match in FIB (Too Slow)

Jan 11, 2006Lahore University of Management Sciences7 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

Jan 11, 2006Lahore University of Management Sciences8 IP routing rigidity example 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 A B C A B S D

Jan 11, 2006Lahore University of Management Sciences9 IP routing rigidity example 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 A B C S A B D

Jan 11, 2006Lahore University of Management Sciences10 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)

Jan 11, 2006Lahore University of Management Sciences11 IP routing versus MPLS routing Traditional IP Routing Multiprotocol Label Switching (MPLS) SD MPLS allows overriding shortest paths!

Jan 11, 2006Lahore University of Management Sciences12 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

Jan 11, 2006Lahore University of Management Sciences13 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? IP DatagramLabel

Jan 11, 2006Lahore University of Management Sciences14 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)

Jan 11, 2006Lahore University of Management Sciences15 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

Jan 11, 2006Lahore University of Management Sciences16 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 very large number of entries In LFIB, label is an exact match In FIB, IP is longest prefix match

Jan 11, 2006Lahore University of Management Sciences17 Mpls Flow Progress LSR1 LSR2 LSR3 LSR5 LSR6 R1R2LSR4 D 1 - R1 receives a packet for destination D connected to R2 R1 and R2 are regular routers D destination

Jan 11, 2006Lahore University of Management Sciences18 Mpls Flow Progress LSR1 LSR2 LSR3 LSR5 LSR6 R1R2LSR4 D 2 - R1 determines the next hop as LSR1 and forwards the packet (Makes a routing as well as a forwarding decision) D destination

Jan 11, 2006Lahore University of Management Sciences19 Mpls Flow Progress LSR1 LSR2 LSR3 LSR5 LSR6 R1R2LSR4 D 3 – LSR1 establishes a path to LSR6 and “PUSHES” a label (Makes a routing as well as a forwarding decision) D destination 31

Jan 11, 2006Lahore University of Management Sciences20 Mpls Flow Progress LSR1 LSR2 LSR3 LSR5 LSR6 R1R2LSR4 D 4 – LSR3 just looks at the incoming label LSR3 “SWAPS” with another label before forwarding D destination 17 Labels have local signifacance!

Jan 11, 2006Lahore University of Management Sciences21 MPLS Flow Progress LSR1 LSR2 LSR3 LSR5 LSR6 R1R2LSR4 D 5 – LSR6 looks at the incoming label LSR6 “POPS” the label before forwarding to R2 D destination 17 Path within MPLS cloud is pre-established: LSP (label-switched path)

Jan 11, 2006Lahore University of Management Sciences22 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 Use a path with delay less than 130ms

Jan 11, 2006Lahore University of Management Sciences23 CSPF What is the typical mechanism? First prune all links not fulfilling constraints 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

Jan 11, 2006Lahore University of Management Sciences24 More MPLS terminology /24 LSR1LSR2 UpstreamDownstream Data

Jan 11, 2006Lahore University of Management Sciences25 Label advertisement Always downstream to upstream label advertisement and distribution /24 LSR1 LSR2 Use label 5 for destination /24 MPLS Data Packet with label 5 travels Upstream Downstream

Jan 11, 2006Lahore University of Management Sciences26 Label advertisement Label advertisement can be downstream unsolicited or downstream on-demand /24 LSR1 LSR2 Sends label Without any Request Upstream Downstream /24 LSR1LSR2 Sends label ONLY after receiving request Request For label Upstream Downstream

Jan 11, 2006Lahore University of Management Sciences27 Label distribution Label distribution can be ordered or unordered First we see an example of ordered label distribution Ingress LSR Egress LSR Label

Jan 11, 2006Lahore University of Management Sciences28 Label distribution Label distribution can be ordered or unordered Next we see an example of unordered label distribution Ingress LSR Egress LSR Label

Jan 11, 2006Lahore University of Management Sciences29 Label retention modes Label retention can be conservative or liberal Conservative: only keep labels you are using currently Liberal: also keep labels that you don’t need right now! LSR1 Destination Label ? Conservative retention makes more sense with downstream on-demand Liberal retention makes more sense with downstream unsolicited Only one can be next-hop to destination; What do I do with the other label?

Jan 11, 2006Lahore University of Management Sciences30 Label operations Advertisement Downstream unsolicited Downstream on-demand Distribution Ordered Unordered Retention Liberal Conservative

Jan 11, 2006Lahore University of Management Sciences31 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

Traffic Engineering Traffic Engineering with MPLS (Application of CSPF)

Jan 11, 2006Lahore University of Management Sciences33 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

Jan 11, 2006Lahore University of Management Sciences34 MPLS-TE: How to do it? LSPs may be 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

Jan 11, 2006Lahore University of Management Sciences35 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 Defined recovery mechanisms

Jan 11, 2006Lahore University of Management Sciences36 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!

Jan 11, 2006Lahore University of Management Sciences37 RSVP-TE Basic flow of LSP set-up using RSVP

Jan 11, 2006Lahore University of Management Sciences38 RSVP-TE PATH Message PATH message is used to establish state and request label assignment R1 transmits a PATH message addressed to R9

Jan 11, 2006Lahore University of Management Sciences39 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 destination/forwarding equivalence class (FEC)

Jan 11, 2006Lahore University of Management Sciences40 Summary: MPLS is used to establish virtual circuits Between routers of service providers MPLS allows to overcome shortest path constraint of IP routing This allows traffic engineering This also allows protection/backup paths In present day networks, MPLS still carries the IP traffic

Jan 11, 2006Lahore University of Management Sciences41 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 (