Chapter 5 Introducing SDN Control in MPLS Networks - High ...

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181routers make decisions on LSP paths; label-distribution is downstream on-demand andordered; and resource management can be performed on the basis of LSPs [92]. MPLSbased flows are more restrictive than the SDN flow-abstraction; but it is fair to say thatflow-abstraction exists in some form in MPLS networks.We do not compare the SDN flow-abstraction to the MPLS datagram-based usagemodel(Sec. 5.1.1) for the following reasons: In the datagram-model, the only logicalassociation (or FEC or flow-definition) that matters network-wide is the IP-destinationaddress; forwarding decisions are still made independently router to router; and onecannot perform resource management at the flow level. However, even though the SDNflow-abstraction cannot (and should not) be compared to the MPLS datagram usagemodel;the former can still provide the service the latter enables – VPNs. We discuss thispoint in more detail in Sec. 5.4.5.2.2 MPLS and the Map-AbstractionWhen comparing MPLS network control with our control-architecture, it is important todistinguish between the map and the map-abstraction. We discuss them individually.Fig. 5.5 Network Maps in a) IP networks b) MPLS-TE networks c) SDNMaps: A map by our definition is an annotated graph of the network topology. Firstlet’s see how the maps themselves differ in different networks. In plain-vanilla IPnetworks, distributed routing protocols like OSPF and IS-IS distribute link state to each
182 CHAPTER 5. INTRODUCING SDN CONTROL IN MPLS NETWORKSrouter in the network. This allows each router in the network to build a map (Fig. 5.5a) oftheir own that is consistent with maps built by other routers. The map includes topologyinformation like nodes and links, as well as the ‘cost’ of each link for use in an SPFcalculation. The only other link-‘state’ that is distributed by the routing protocol iswhether the link is up-or-down. This is a very limited map, which allows the basicnetwork-functions of SPF routing and re-routing around failures, and not much more.In MPLS-TE networks, the same distributed routing protocols are used, but this timewith TE extensions that enable it to carry more link-state. Such state includes maximumreservable bandwidth on a link, un-reserved bandwidth per priority level, link attributesand administrative weights [92]. And so, the map that gets constructed in each router(Fig. 5.5b) has node and link information as before, but now with additional link-state.This allows a router to perform more sophisticated path calculations that take intoaccount bandwidth and other constraints on links when calculating TE-LSP paths.An SDN map has all the information about the topology and link-state discussed sofar. But in addition, it has a lot more information about the network-node internals likeswitch flow-tables, queues, ports and their state and statistics. It also has globalinformation on not just link-state but also switch-state, and if needed TE-tunnel andpacket-flow state (Fig. 5.5c, Fig. 1.10 & Sec. 2.2.1). The net result is that the map can beused to support not just traffic-engineering based features, but a host of other networkapplications.Such applications may include access-control, mobility management, VPNs,bandwidth-on-demand, QoS, datacenter backup/migration, and more. In short, the map inSDNs, together with the flexible flow abstraction, makes networks more programmable.Map-Abstraction: So far we have compared and discussed the differences in themaps themselves. Now let’s consider the map abstraction. The map-abstraction helpscentralize decision-making. It allows control-programs to be written in a centralized waywith full, global visibility into the network, without worrying about how that globalvisibility (the map) is created, supported (perhaps with multiple physical controllers) andkept up to date. MPLS networks lack such a map-abstraction. We have discussed the
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Page 12 and 13: 183benefits of the map-abstraction
Page 14 and 15: 185all upgraded at the same time. M
Page 16 and 17: 187However the tunnel’s reserved
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Page 24 and 25: 195the start of the tunnel, to the
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Page 34 and 35: 205In Fig. 5.14 we show two tunnels
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