Advanced Telecommunications

Transcription

1 ternet Routing - MPLS By Richard Harris MPLS Presentation line troduction Problems of ternet size Methods for overcoming potential problems What is MPLS? Overview MPLS terminology MPLS Architecture The label stack Switching Protocols MPLS Signalling Protocols Traffic Management in MPLS Semester Advanced Telecommunications Slide 2 troduction Huge growth in the ternet Figures vary from 2x to 10x per annum for traffic growth! Major ISPs (ternet Service Providers) have coped with this growth in a variety of different directions, viz: Network architecture Capacity expansion Traffic management methods ( Traffic Engineering ) Semester Advanced Telecommunications Slide 3 Advanced Telecommunications

2 Network Architecture This approach entails Development of scalable network architectures Defining suitable network topologies and structures Identification of abstract objects and classes within the network. Semester Advanced Telecommunications Slide 4 Capacity Expansion Most ISPs started with backbone capacities in the range of 44Mbps (in the USA this is DS3 ) during By 1999, the capacities used were about 622Mbps A recent announcement shows that ISPs are now introducing capacities in the region of 9.9Gbps (referred to as OC-192c) and using DWDM (Dense WDM). Semester Advanced Telecommunications Slide 5 Traffic Management/Engineering This direction is required because the previous two approaches are insufficient to solve the growth problem mentioned in the introduction! Architectures and capacity expansion cannot provide the high quality ternet services that are now being demanded by the many different customers of this network. Traffic engineering deals with performance optimisation issues and it has been realised how important this aspect is for operating such a huge multipurpose network. Semester Advanced Telecommunications Slide 6 Advanced Telecommunications

3 Traffic Engineering Limitations of the existing ternet - 1 Historically difficult to perform effective traffic engineering and management of ternet traffic. IP was conceived as a best effort service. Shortcomings of IP technologies: Lack of measurement capabilities Limitations of intra-domain routing control functions. Routing protocols like OSPF, EIGRP, IGP etc work on topology-driven per-packet progressive connection control. Each router makes independent decisions using a local version of a routing area link state database. Semester Advanced Telecommunications Slide 7 Traffic Engineering Limitations of the existing ternet 2 Route selection is based on shortest path calculations using simple additive link metrics. This approach is highly distributed and scalable, but flawed, because these protocols do not consider the characteristics of the offered traffic and the network capacity constraints when making routing decisions. Subsets of network resources become congested. Alternate paths become under-utilised. This suggests that there is poor resource allocation taking place. Traffic engineering is required to fix these problems. MPLS to the rescue! Semester Advanced Telecommunications Slide 8 What will MPLS do? Multi Protocol Switching (MPLS) is expected to improve the price/performance of network layer routing, improve the scalability of the network layer, provide greater flexibility in the delivery of (new) routing services (by allowing new routing services to be added without a change to the forwarding mechanism). Semester Advanced Telecommunications Slide 9 Advanced Telecommunications

4 History MPLS was originally proposed by a group of engineers from Ipsilon Networks, but their "IP Switching technology, which was defined only to work over ATM, did not achieve market dominance. Cisco introduced a related proposal, not restricted to ATM transmission, called "Tag Switching". It was a Cisco proprietary proposal, and was renamed " Switching". It was handed over to the IETF for open standardization. One original motivation was to allow the creation of simple high-speed switches, (itially it was impossible to forward IP packets entirely in hardware.) The advantages of MPLS primarily revolve around the ability to support multiple service models and perform traffic management. MPLS also offers a robust recovery framework. Semester Advanced Telecommunications Slide 10 What is MPLS? - 1 MPLS is a technique for combining the best features of IP and ATM, viz: IP routing ATM throughput It borrows heavily from methods used in ATM to rapidly transfer cells across the network. Supports destination oriented, multicast and hierarchical routing Supports explicit routing -transport, -stack Quality of Service (QoS) MPLS is not a specific layer 3 protocol it appears to lie between layers 2 and 3. MPLS supports many other layer 2 technologies (Frame Relay, ATM, Ethernet, Token Ring) Semester Advanced Telecommunications Slide 11 What is MPLS? - 2 A key concept of MPLS is the separation of an IP router s functions into two parts: Forwarding Control Forwarding: Responsible for how data packets are relayed between the network routers. It uses label swapping similar to ATM switches that use Virtual Path/Virtual Circuit identifiers although MPLS may be regarded as more general than VPI/VCI. A label is a short fixed length number, independent of the network layer. swapping essentially involves a table lookup of a packet s label to determine its route and new label value. It is much simpler than previous methods. Semester Advanced Telecommunications Slide 12 Advanced Telecommunications

5 What is MPLS? - 3 A router capable of MPLS is known as a Switching Router (LSR). A set of Switching Routers traversed by a packet is called a Switched Path (LSP). A contiguous set of LSPs under a single administration is known as an MPLS domain. A packet is transmitted across an MPLS domain based only on its label. Packet Semester Advanced Telecommunications Slide 13 Some MPLS Terminology LDP: Distribution Protocol LSP: Switched Path FEC: Forwarding Equivalence Class LSR: Switching Router LER: Edge Router Semester Advanced Telecommunications Slide 14 MPLS The label can: use an existing layer 2 header label (eg the VPI/VCI field in the ATM cell header) or inserted between the layer 2 and IP headers as a small shim label like the following: Value S Time-to-Live (TTL) 20-bits: value used bylsr to lookup either next-hop, operation to perform, or outgoing data-link encapsulation Type of service 1-bit: "Bottom of Stack" Flag 8-bits: TTL decremented by each LSR Semester Advanced Telecommunications Slide 15 Advanced Telecommunications

6 Route at edge, switch in core IP IP #L1 IP #L2 IP #L3 IP IP Forwarding LABEL SWITCHING IP Forwarding Semester Advanced Telecommunications Slide 16 Switched Path (LSP) tf Dest tf tf Dest tf tf Dest tf IP IP Semester Advanced Telecommunications Slide 17 Explicitly routed or ER-LSP Route= {A,B,C} #216 A #14 B #972 #14 #972 C #462 - ER-LSP follows a route that the source chooses. other words, the control message to establish the LSP (label request) is source routed. Semester Advanced Telecommunications Slide 18 Advanced Telecommunications

7 Explicitly Routed LSP (ER-LSP) tf D e s t tf L a b e l O u t O u t IP tf Dest tf tf Dest tf IP Semester Advanced Telecommunications Slide 19 ER LSP - advantages Operator has routing flexibility (policy-based, QoSbased) 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) Semester Advanced Telecommunications Slide 20 MPLS Stack Stack is LIFO Depth n stack illustrated below: Push stack 1 2 n-1 n 1 Last label Pop stack Semester Advanced Telecommunications Slide 21 Advanced Telecommunications

8 MPLS: Stack; Hierarchical Routing Company Network Company Network S D Leased Virtual Path Semester Advanced Telecommunications Slide 22 Forwarding Equivalence Classes More general than Virtual Circuits in ATM. Key features: All packets divided into subsets called FECs based on: IP source address IP destination address IP protocol TCP/UDP source/destination ports Time to Live (TTL) Type of service fields Granularity can be varied if desired for packet forwarding criteria. Mapping of FEC done only once at entry to MPLS domain. Semester Advanced Telecommunications Slide 23 Packet Processing Forwarding strictly according to labels! is removed at the egress LSR. Next Hop Forwarding Entries NHLFEs: going interface (next hop) going label (push/pop label stack) Data link encapsulation (optional) formation about resources (optional) Packet handling policies (optional) Semester Advanced Telecommunications Slide 24 Advanced Telecommunications

9 Quality of Service Routing The control part of MPLS consists of Network layer routing protocols to distribute routing information between LSRs; binding procedures for converting this routing information into the forwarding tables needed for label switching. MPLS is designed to work with existing routing protocols like OSPF, BGP etc. Major interest in QoS routing instead of just shortest path routing. Signalling is required to establish the routes. Semester Advanced Telecommunications Slide 25 Signalling Methods in MPLS There are two standardized protocols for managing MPLS paths: CR-LDP (Constraint-based Routing Distribution Protocol) and RSVP-TE, an extension of the RSVP protocol for traffic engineering. An MPLS header does not identify the type of data carried inside the MPLS path. If we need to carry two different types of traffic between the same two routers, with different treatment from the core routers for each type, we must establish a separate MPLS path for each type of traffic. Semester Advanced Telecommunications Slide 26 MPLS Signalling Protocols The signalling protocol informs the switches along the route which labels and links to use for each LSP. This information is used to program the switching fabric. RSVP-TE is used where traffic engineering is required. LDP is used when traffic engineering is not required, as it needs less management. Semester Advanced Telecommunications Slide 27 Advanced Telecommunications

10 MPLS Protocols OSPF/ ISIS Routing LDP TCP CR LDP/ RSVP-TE UDP IP Forwarding Link State formation MPLS Forwarding Physical Semester Advanced Telecommunications Slide 28 LDP Protocol - 1 MPLS LDP provides the means for LSRs to request, distribute, and release label prefix binding information to peer routers in a network. LDP enables LSRs to discover potential peers and to establish LDP sessions with those peers for the purpose of exchanging label binding information. MPLS LDP enables one LSR to inform another LSR of the label bindings it has made. Once a pair of routers communicates the LDP parameters, then they establish a label-switched path (LSP). MPLS LDP enables LSRs to distribute labels along normally routed paths to support MPLS forwarding. This method of label distribution is also called hop-by-hop forwarding. Semester Advanced Telecommunications Slide 29 LDP Protocol - 2 With IP forwarding, when a packet arrives at a router the router looks at the destination address in the IP header, performs a route lookup, and forwards the packet to the next hop. With MPLS forwarding, when a packet arrives at a router the router looks at the incoming label, looks up the label in a table, and then forwards the packet to the next hop. Semester Advanced Telecommunications Slide 30 Advanced Telecommunications

11 Merging The following edited extract from explains the process of label merging (this is essentially the aggregation process). Merging is the capability for forwarding two different packets belonging to the same FEC, but arriving with different labels, with the same outgoing label. An LSR is label merging capable if it can receive two packets from different incoming interfaces, and/or with different labels, and send both packets out the same outgoing interface with the same label. Once transmitted, the information that they arrive from different interfaces and/or with different labels is lost. Semester Advanced Telecommunications Slide 31 RSVP-TE Protocol RSVP-TE is used to establish MPLS LSPs when there are traffic engineering requirements. It is mainly used to provide QoS and load balancing across the network core, and includes the ability to control all-optical networks. RSVP allows the use of source routing where the ingress router determines the complete path through the network. The ingress router can use a Constrained Shortest Path First (CSPF) calculator to determine a path to the destination, ensuring that any QoS and Shared Risk Link Group (SRLG) requirements are met. The resulting path is then used to establish the LSP. Semester Advanced Telecommunications Slide 32 Merging FEC #1 Router #1 FEC #1 Router #2 L2 FEC #1 L2 FEC #1 Router #3 LDP Tunnel #1 LDP Tunnel #2 L3 FEC #1 Next GenerationNetwork Service ResiliencyProject L3 FEC #1 ASBR Gateway Gateway supporting FEC#1 Cannot distinguish between incoming packets from R1 and R2 as they have the same label Network: LDP subnetwork Author: Richard Harris Date: 02/12/2009 Company: Massey University Packets eminating from R1 and R2 towards the ASBR are part offec #1 as theyare destined for the same prefix. AtR3, their incoming labels (L2) are swapped for L3 giving the appearance ofbeing a single LSPto the ASBR Gateway. Semester Advanced Telecommunications Slide 33 Advanced Telecommunications

12 ER-LSP setup using RSVP 1. Path message. It contains ER path < B,C,D> 5. When LER A receives Resv, the ER established. 2. New path state. Path message sent to next node 4. New reservation state. Resv message propagated upstream 3. Resv message originates. Contain the label to use and the required traffic/qos para. Per-hop Path and Resv refresh unless suppressed LSR B LSR C LER A LER D Per-hop Path and Resv refresh unless suppressed Per-hop Path and Resv refresh unless suppressed Semester Advanced Telecommunications Slide 34 Constraint Routing LDP (CR-LDP) CR = Constraint based Routing eg: USE: (links with sufficient resources AND (links of type somecolor ) AND (links that have delay less than 200 ms) & & = Semester Advanced Telecommunications Slide 35 Requirements for Constraint Based Routing 1) A topology database that knows about link attributes. {a,b,c} z ANSWER: OSPF/ISIS + attribs{a,b,c} {a,b,c} z 2) A label distribution protocol that goes where it s told. ANSWER: LDP + Explicit Route{x,y,m,z} x y m z Semester Advanced Telecommunications Slide 36 Advanced Telecommunications

13 Traffic Engineering Demand A B C D Traffic engineering is the process of mapping traffic demand onto a network Network Topology Traffic engineering key to optimizing cost/performance Semester Advanced Telecommunications Slide 37 Constrained Shortest Path First (CSPF) CSPF is an extension of shortest path algorithms. The path computed using CSPF is a shortest path that satisfies a set of constraints. It runs a shortest path algorithm after pruning those links that violate a given set of constraint requirements. A constraint could be the minimum bandwidth required per link (also known as bandwidth guaranteed constraint), end-to-end delay, maximum number of links traversed, or a set of include/exclude nodes. CSPF is widely used in MPLS Traffic Engineering. The routing using CSPF is known as Constraint Based Routing (CBR). Semester Advanced Telecommunications Slide 38 Comment on Routes obtained with CSPF Note: The path computed using CSPF could be exactly same as that of computed from OSPF and IS-IS, or it could be completely different depending on the set of constraints to be met. Semester Advanced Telecommunications Slide 39 Advanced Telecommunications

14 Example of CSPF Router #1 50Mbps 100Mbps Router #3 Router #2 100Mbps 60Mbps 100Mbps Router #4 100Mbps 100Mbps Router #6 Router #5 Cost is 1 hop count Constrained to 50Mbps: Constrained to 55Mbps: Constrained to 90Mbps: Using hop count, OSPF/ISIS always gives Semester Advanced Telecommunications Slide 40 CSPF Path Selection - 1 Juniper documentation summarises the path selection process as follows: 1. Computes LSPs one at a time, beginning with the highest priority LSP (the one with the lowest setup priority value). Among LSPs of equal priority, CSPF starts with those that have the highest bandwidth requirement. 2. Prunes all the links that are not full duplex and do not have sufficient reservable bandwidth. 3. If the LSP configuration includes the include statement, prunes all links that do not share any included colours. 4. If the LSP configuration includes the exclude statement, prunes all links that contain excluded colours. If the link does not have a colour, it is accepted. Semester Advanced Telecommunications Slide 41 CSPF Path Selection Finds the shortest path toward the LSP s egress router, taking into account explicit-path constraints. For example, if the path must pass through Router A, two separate SPFs are computed, one from the ingress router to Router A, the other from Router A to the egress router. 6. If several paths have equal cost, chooses the one whose last-hop address is the same as the LSP s destination. 7. If several equal-cost paths remain, selects the one with the fewest number of hops. 8. If several equal-cost paths remain, applies the CSPF loadbalancing rule configured on the LSP (least fill, most fill, or random). Semester Advanced Telecommunications Slide 42 Advanced Telecommunications

15 Purpose of traffic engineering Maximize utilization of links and nodes throughout the network Engineer links to achieve required delay, grade-ofservice Spread the network traffic across network links, minimize impact of single failure Ensure available spare link capacity for re-routing : traffic on failure Meet policy requirements imposed by the network operator Semester Advanced Telecommunications Slide 43 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 gress LSR may be configured with the path, RSVP used to set up LSP - some vendors have extended RSVP for MPLS path set-up gress LSR may be configured with the path, LDP used to set up LSP - many vendors believe RSVP is not suitable. gress 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 - loose source routing, less configuration required. If desired for control, routes discovered by hop-by-hop routing can be frozen ( route pinning ) the future, constraint-based routing will off-load traffic engineering tasks from the operator to the network itself. Semester Advanced Telecommunications Slide 44 Advanced Telecommunications