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1、第十四章Reading MaterialMulti-Protocol Label Switching (MPLS)In the IPv4 networks as they are today,when a source sends out an IP packet to a remote destination,path taken by the packet will depend on the multiple routers it travels across.IP expects the network layer of every router route to carry out
2、exhaustive analysis of the layer 3-IP headers of each packet and take a decision on how to route the packet onwards to its final destination. This means a longish latency at every hop and there is also no assurance how the packet will travel.IP Routers also treat all packets just about equally and t
3、herefore their capabiliity to prioritize packets between different hosts and of different applications is non existent or primitive. MPLS is an IETF specified framework sought to be intergrated into the routers to provide a solution for these woes.1.1ntroductionThe objective of MPLS is to speed up t
4、he IP packet forwarding process by cutting down on the amout of processing(read latency) at every intermediate router and also to provide a mechanism to prioritize packet forwarding,thereby provide ability to engineer traffic flow and assure differential Quality of Service(QOS).All this is done with
5、out losing on the flexibility of IP-based network .MPLS performs the following functions:Affixes simple fixed-length labels on packets to indicate a certain path, which is used by different packet-forwarding & packet-switching technologies Specifies mechanisms to manange traffic flow of various gran
6、ularities , such as flows between different machines or applications.Remain independent of the Layer-2 and Layer-3 protocols.Interfaces to existing routing protocols such as Resource Reservation Protocol(RSVP) and Open Shortest Path First(OSPF).MPLS provides significant new capabilities in four area
7、s that have ensure its popularity: QoS support, Traffic Engineering, Virtual Private Network (VPNs) and Multiprotocol Support.Connection-Oriented QoS SupportThe requirement of QoS on the network , is critical certain category of users and applications.A real time business application demands a highe
8、r QoS Voice traffic that travels without certain minimum QoS is rarely of any use A connectionless network , such as in IPbased internetwork, cannot provide tmly firm QoS commitments. A connectionoriented network has powerful traffic management and QoS capabilities. MPLS imposes a connenction-orient
9、ed framework on an IP-based internet and thus provides the foundation for sophisticated and reliable QoS traffic contracts.Traffic EngineeringThe ability to dynamically define routes , plan resource commitments on the basis of known demand, and optimize network utilization is referred to as Traffic
10、Engineering(TE). With the basic IP mechanism, there is a primitive form of automated traffic engineering. MPLS is aware of not just individual packets,but flows of packets in which each flow has certain QoS requirements and a predictable traffic demand .With MPLS , it is possible to set up routes on
11、 the basis of these individual flows, with two different flows between the same endpoints perhaps following different routers . Further ,when congestion threatens, MPLS paths can be rerouted intelligently. That is ,instead of simply changing the route on a packet-by-packet basis, with MPLS , the rou
12、tes are changede on flow-by-flow basis , taking advantage of the known traffic demands of each flow. Effective use of traffic engineering can substantially in crease usable network capacit y.1.3 Multiprotocol SupportMPLS , which can be used on many networking technologies, is an enhancement to the w
13、ay a connectionless IP-based internet is operated , requiring an upgrade to IP routers to support the MPLS features MPLS enabled routers can coexist with ordinary IP routers , facilitating the introduction of evolution to MPLS schemes MPLS is also designed to work in ATM and Frame Relay networks Aga
14、in , MPLS-enabled ATM switches and MPLS-enabled Frame Relay switches can be configured to co-exist with ordinaiy switches . Furthermore , MPLS can be used in a pure IP-based internet, a pure ATM network , a pure Frame Relay network , or an internet that includes two or even all three technologies Th
15、is universal nature of MPLS should appeal to users who currently have mixed network technologies and seek ways to optimize resources and expand QoS support2 MPLS Operate2.1 PrincipleUnlike legacy routing , MPLS uses labels to forward traffic across the MPLS domains When packets enter the MPLS domain
16、 , labels are imposed on the packets , and the label (not the IP header ) determines the next hop Labels are removed at the egress of the MPLS domain .When a labeled packet arrives at a Label Switching Router (LSR), the incoming label will determine the path of this packet within the MPLS network . MPLS label forwarding will then swap this label to the appropriate outgoing label and send packets to the next hop These label
