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CMU 15441 Computer Networking - lecture

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Lecture 14 MPLS Intro to Transport, Reliability, and TCPMulti Protocol Label Switching - MPLSMPLS + IPMPLS core, IP interfaceMPLS use case #1: VPNsMPLS use case #2: Reduced State CoreMPLS use case #3: Traffic EngineeringMPLS MechanismsMPLS DiscussionFurther reading - MPLSPackets over SONETTake Home PointsTransport LayerTransport ProtocolsFunctionality SplitSlide 16UDP: User Datagram Protocol [RFC 768]UDP, cont.UDP ChecksumHigh-Level TCP CharacteristicsTCP HeaderEvolution of TCPTCP Through the 1990sOutlineStop and WaitRecovering from ErrorProblems with Stop and WaitHow to Recognize Resends?How to Keep the Pipe Full?Sliding WindowSender/Receiver StateSequence NumbersWindow Sliding – Common CaseLoss RecoveryGo-Back-N in ActionSelective RepeatSelective Repeat: Sender, Receiver WindowsImportant LessonsNext Lecture1Lecture 14MPLSIntro to Transport, Reliability, and TCP15-441 Networking, Spring 2007As usual, adapted from Srini Seshan and David Anderson, 15-441, Fall ‘062Multi Protocol Label Switching - MPLSSelective combination of VCs + IP»Today: MPLS useful for traffic engineering, reducing core complexity, and VPNsCore idea: Layer 2 carries VC label»Could be ATM (which has its own tag)»Could be a “shim” on top of Ethernet/etc.:»Existing routers could act as MPLS switches just by examining that shim -- no radical re-design. Gets flexibility benefits, though not cell switching advantagesLayer 2 headerLayer 3 (IP) headerLayer 2 headerLayer 3 (IP) headerMPLS label3MPLS + IPMap packet onto Forward Equivalence Class (FEC)»Simple case: longest prefix match of destination address»More complex if QoS of policy routing is usedIn MPLS, a label is associated with the packet when it enters the network and forwarding is based on the label in the network core.»Label is swapped (as ATM VCIs)Potential advantages.»Packet forwarding can be faster»Routing can be based on ingress router and port»Can use more complex routing decisions»Can force packets to followed a pinned route4MPLS core, IP interfaceABR2R1R3R4C1234333111224442DIPIPMPLS tag assignedIPIPMPLS forwarding in coreMPLS tag stripped5MPLS use case #1: VPNsABR2R1R3R4C1234333111224442D10.1.0.0/2410.1.0.0/2410.1.0.0/2410.1.0.0/24MPLS tags can differentiate green VPN from orange VPN.6MPLS use case #2: Reduced State CoreAR2R1R3R4C.EBGPEBGPAR2R1R3R4C1234333111224442EBGPIP CoreMPLS CoreA-> C pktInternal routers must know all C destinationsR1 uses MPLS tunnel to R4. R1 and R4 know routes, but R2 and R3 don’t.7MPLS use case #3: Traffic EngineeringAs discussed earlier -- can pick routes based upon more than just destinationUsed in practice by many ISPs, though certainly not all.8MPLS MechanismsMPLS packet forwarding: implementation of the label is technology specific.»Could be ATM VCI or a short extra “MPLS” headerSupports stacked labels.»Operations can be “swap” (normal label swapping), “push” and “pop” labels.–VERY flexible! Like creating tunnels, but much simpler -- only adds a small label.Label CoS S TTL20 3 189MPLS DiscussionOriginal motivation.»Fast packet forwarding:–Use of ATM hardware–Avoid complex “longest prefix” route lookup–Limitations of routing table sizes»Quality of serviceCurrently mostly used for traffic engineering and network management.»LSPs can be thought of as “programmable links” that can be set up under software control» on top of a simple, static hardware infrastructure10Further reading - MPLSMPLS isn’t in the book - sorry. Juniper has a few good presentations at NANOG (the North American Network Operators Group; a big collection of ISPs):»http://www.nanog.org/mtg-0310/minei.html»http://www.nanog.org/mtg-0402/minei.html»Practical and realistic view of what people are doing _today_ with MPLS.11Packets over SONETSame as statically configured ATM pipes, but pipes are SONET channels.Properties.–Bandwidth management is much less flexible+Much lower transmission overhead (no ATM headers)muxmuxmuxmuxmuxmuxOC-4812Take Home PointsCosts/benefits/goals of virtual circuitsCell switching (ATM)»Fixed-size pkts: Fast hardware»Packet size picked for low voice jitter. Understand trade-offs.»Beware packet shredder effect (drop entire pkt)Tag/label swapping»Basis for most VCs. »Makes label assignment link-local. Understand mechanism.MPLS - IP meets virtual circuits»MPLS tunnels used for VPNs, traffic engineering, reduced core routing table sizes13Transport LayerTransport introductionError recovery & flow control14Transport ProtocolsLowest level end-to-end protocol.»Header generated by sender is interpreted only by the destination»Routers view transport header as part of the payload776655776655TransportTransportIPIPDatalinkDatalinkPhysicalPhysicalTransportTransportIPIPDatalinkDatalinkPhysicalPhysicalIPIProuter2222111115Functionality SplitNetwork provides best-effort deliveryEnd-systems implement many functions»Reliability»In-order delivery»Demultiplexing»Message boundaries»Connection abstraction»Congestion control»…16Transport ProtocolsUDP provides just integrity and demuxTCP adds…»Connection-oriented»Reliable»Ordered»Point-to-point»Byte-stream»Full duplex»Flow and congestion controlled17UDP: User Datagram Protocol [RFC 768]“No frills,” “bare bones” Internet transport protocol“Best effort” service, UDP segments may be:»Lost»Delivered out of order to appConnectionless:»No handshaking between UDP sender, receiver»Each UDP segment handled independently of othersWhy is there a UDP?No connection establishment (which can add delay)Simple: no connection state at sender, receiverSmall headerNo congestion control: UDP can blast away as fast as desired18UDP, cont.Often used for streaming multimedia apps»Loss tolerant»Rate sensitiveOther UDP uses (why?):»DNS, SNMPReliable transfer over UDP»Must be at application layer»Application-specific error recoverySource port # Dest port #32 bitsApplicationdata (message)UDP segment formatLengthChecksumLength, inbytes of UDPsegment,includingheader19UDP ChecksumSender:Treat segment contents as sequence of 16-bit integersChecksum: addition (1’s complement sum) of segment contentsSender puts checksum value into UDP checksum fieldReceiver:Compute checksum of received segmentCheck if computed checksum equals checksum field value:»NO - error detected»YES - no error detectedBut


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CMU 15441 Computer Networking - lecture

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