115-441 Computer NetworkingLecture 29 – Final Review12-12-06Lecture 29: Review 2Fun• TCP/IP drinking game• (http://infohost.nmt.edu/~val/tcpip.html)• Internet map • (http://xkcd.com/c195.html)12-12-06Lecture 29: Review 3Outline – Lec 14• The recurring IP address space problem• IPv6.• NAT.• Tunneling / Overlays• Network Management• Autoconfiguration• SNMP (notes only)12-12-06Lecture 29: Review 4• “Next generation” IP.• Most urgent issue: increasing address space.• 128 bit addresses • Simplified header for faster processing:• No checksum (why not?)• No fragmentation (?)• Support for guaranteed services: priority and flow id• Options handled as “next header”• reduces overhead of handling optionsV/PrV/PrFlow labelFlow labelLengthLengthNextNextHop LHop LSource IP addressSource IP addressDestination IP addressDestination IP addressIP v6212-12-06Lecture 29: Review 5B IPB IPNetwork Address TranslationPubl A IPPubl A IPB IPB IPA Port’A Port’B PortB PortPriv A IPPriv A IPB IPB IPA PortA PortB PortB PortPubl A IPPubl A IPB PortB PortB IPB IPPriv A IPPriv A IPB PortB PortA PortA PortA Port’A Port’AB• NAT maps (private source IP, source port) onto (public source IP, unique source port)• reverse mapping on the way back• destination host does not know that this process is happening• Very simple working solution.• NAT functionality fits well with firewalls12-12-06Lecture 29: Review 6Tunneling• Force a packet to go to a specific point in the network.• Path taken is different from the regular routing• Achieved by adding an extra IP header to the packet with a new destination address.• Similar to putting a letter in another envelope• preferable to using IP source routing option• Used increasingly to deal with special routing requirements or new features.• Mobile IP,..• Multicast, IPv6, research, ..Data IP1 IP2IP2IP112-12-06Lecture 29: Review 7DHCP• DHCPOFFER• IP addressing information• Boot file/server information (for network booting)• DNS name servers• Lots of other stuff - protocol is extensible; half of the options reserved for local site definition and use.DHCPDISCOVER - broadcastDHCPOFFERDHCPREQUESTDHCPACK12-12-06Lecture 29: Review 8Outline – Lec 15• Exam discussion• Layering review (bridges, routers, etc.)• Exam section C.• Circuit switching refresher• Virtual Circuits - general• Why virtual circuits?• How virtual circuits? -- tag switching!• Two modern implementations• ATM - teleco-style virtual circuits• MPLS - IP-style virtual circuits312-12-06Lecture 29: Review 9• Global VC ID allocation -- ICK! Solution: Per-link uniqueness. Change VCI each hop.Input Port Input VCI Output Port Output VCIR1: 1 5 3 9R2: 2 9 4 2R4: 1 2 3 5ABR2R1R3R4 Dst1234333111224442Virtual Circuit Switching:Label (“tag”) Swapping12-12-06Lecture 29: Review 10Multi 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 label12-12-06Lecture 29: Review 11MPLS + 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 route12-12-06Lecture 29: Review 12Take 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 sizes412-12-06Lecture 29: Review 13Outline – Lec 16• Transport introduction• Error recovery & flow control12-12-06Lecture 29: Review 14Important Lessons• Transport service• UDP Æ mostly just IP service• TCP Æ congestion controlled, reliable, byte stream• Types of ARQ protocols• Stop-and-wait Æ slow, simple• Go-back-n Æ can keep link utilized (except w/ losses)• Selective repeat Æ efficient loss recovery• Sliding window flow control• Addresses buffering issues and keeps link utilized12-12-06Lecture 29: Review 15ReceiverReceiverSenderSenderSender/Receiver State……Sent & Acked Sent Not AckedOK to Send Not Usable……Max acceptableReceiver window Max ACK received Next seqnumReceived & Acked Acceptable PacketNot UsableSender windowNext expected12-12-06Lecture 29: Review 16Outline – Lec 17• TCP flow control• Congestion sources and collapse• Congestion control basics512-12-06Lecture 29: Review 17Important Lessons• Why is congestion control needed?• How to evaluate congestion control algorithms?• Why is AIMD the right choice for congestion control?• TCP flow control• Sliding window Æ mapping to packet headers• 32bit sequence numbers (bytes)12-12-06Lecture 29: Review 18acknowledged sent to be sent outside windowSource PortSource PortDest. PortDest. PortSequence NumberSequence NumberAcknowledgmentAcknowledgmentHL/FlagsHL/FlagsWindowWindowD. ChecksumD. ChecksumUrgent PointerUrgent PointerOptions…Options…Source PortSource PortDest. PortDest. PortSequence NumberSequence NumberAcknowledgmentAcknowledgmentHL/FlagsHL/FlagsWindowWindowD. ChecksumD. ChecksumUrgent PointerUrgent PointerOptions...Options...Packet SentPacket ReceivedApp writeWindow Flow Control: Send Side12-12-06Lecture 29: Review 19Causes & Costs of Congestion• When packet dropped, any “upstream transmission capacity used for that packet was
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