CS 268: End-Host Mobility and Ad-Hoc RoutingOverviewWirelessProblemSolutionsModify Transport ProtocolModify Link LayerRetransmissionFECFEC & Packet ShrinkingFlex [Eckhardt &Steenkiste ’98]Hybrid: Indirect-TCP [Bakre & Badrinath ’94]Hybrid: Snoop-TCP [Balakrishnan et al. ’95]Slide 14Motivation and ProblemSlide 16Mobile IPMobile IP PropertiesMobile IP Route OptimizationTCP Migrate [SB00]i3 Based Mobility (Z+03)Other solutionsSummarySlide 24MotivationRoutingDSDVTORADSRAODVResultsRelated WorkConclusionsCS 268: End-Host Mobility and Ad-Hoc RoutingIon StoicaFeb 18, 20042OverviewWirelessEnd-host mobilityAd-hoc routing3WirelessWireless connectivity proliferating-Satellite, line-of-sight microwave, line-of-sight laser, cellular data (CDMA, GPRS, 3G), wireless LAN (802.11a/b), Bluetooth-More cell phones than currently allocated IP addressesWireless non-congestion related loss-Signal fading: distance, buildings, rain, lightning, microwave ovens, etc.Non-congestion related loss -Reduced efficiency for transport protocols that depend on loss as implicit congestion signal (e.g. TCP)4Problem0.0E+005.0E+051.0E+061.5E+062.0E+060 10 20 30 40 50 60Time (s)Sequence number (bytes)TCP Reno(280 Kbps)Best possible TCP with no errors(1.30 Mbps)2 MB wide-area TCP transfer over 2 Mbps Lucent WaveLAN (from Hari Balakrishnan)5SolutionsModify transport layerModify link layer protocolHybrid6Modify Transport Protocol Explicit Loss Signal-Distinguish non-congestion losses-Explicit Loss Notification (ELN) [BK98]-If packet lost due to interference, set header bit-Only needs to be deployed at wireless router-Need to modify end hosts-How to determine loss cause?-What if ELN gets lost?7Modify Link LayerAdvantages:-Limited changes: only link-layer affected-Preserve end-to-end (TCP) semanticsThree types of losses-Total packet loss-Partial packet loss-Packet corrupted by bit errorsThree methods to reduce packet loss-Packet retransmission-Forward error correction-Packet shrinking8RetransmissionAdvantages:-Optimal overhead: only lost packets are retransmittedDisadvantages: “nasty” interactions between TCP control look and link-level retransmission-Both TCP and link-layer can retransmit same packets -Can introduce packet reordering -Can introduce highly variable delays9FECForward Error Correction (FEC) codes-k data blocks, use code to generate n>k coded blocks-Can recover original k blocks from any k of the n blocks-n-k blocks of overhead-Trade bandwidth for loss-Can recover from loss in time independent of link RTT•Useful for links that have long RTT (e.g. satellite)-Pay n-k overhead whether loss or not•Need to adapt n, k depending on current channel conditions10FEC & Packet Shrinking Advantages:-No changes at end hosts or base-stations above link-layer-Decrease packet loss-Do not introduce variability Disadvantages:-Overhead can be quite high, e.g., packet segmentation/reassembly, encoding/decoding11Flex [Eckhardt &Steenkiste ’98]Combine the three types of error control seven policies (three fixed and four adaptive)Most sophisticated : Flex-When two or more packets in a window of ten are truncated reduces “safe” packet size by 85%-When three consecutive packets do not experience truncation linearly increase packet size-When two or more packets in a window of ten cannot be decoded decrease user data by 15% (more conservative coding)-When three consecutive packets can be decoded increase user data linearlyNote: adaptation exhibits a linear-increase multiplicative-decrease behavior12Hybrid: Indirect-TCP [Bakre & Badrinath ’94]Split TCP connection into 2 TCPsAdvantages-Optimize performance for wireless TCP-No changes to protocol for fixed hosts (transparent to fixed hosts)Disadvantages-Violate end-to-end TCP semantics (why?)-High overhead, because dual stack at BS-Might introduce high delays because packet buffering Fixed Host (FH)Base Station (BS)Mobile Host (MH)wireless TCPInternetregular TCP13Hybrid: Snoop-TCP [Balakrishnan et al. ’95]Insert a “snoop agent” between fixed host (FH) and mobile host (MH) -Monitor traffic, retransmit packets and discard acknowledgements Notes:-Avoid violating end-to-end semantics-What about layering?Fixed Host (FH)Base Station (BS)Mobile Host (MH)InternetTCPpacket retransmissions14OverviewWirelessEnd-host mobilityAd-hoc routing15Motivation and ProblemNetwork Layer mobility-Movement = IP address changeProblem:-Location•I take my cell phone to London•How do people reach me?-Migration•I walk between base stations while talking on my cell phone•I download or web surf while riding in car or public transit•How to maintain flow?16SolutionsMobile IP (v4 and v6)TCP MigrateMulticast17Mobile IPUse indirection to deal with location and migrationPoint of indirection: Home Agent (HA)-Resides in Mobile Host’s (MH) home network-Uses MH’s home IP address-As MH moves, it sends its current IP address to HACorrespondent Host (CH) contacts MH through HAHA tunnels packets to MH using encapsulationMH sends packets back to CH-Tunnels packets back to HA (bi-directional tunneling)-Sends directly to CH (triangle routing)18Mobile IP PropertiesAdvantages-Preserves location privacy-CH does not have to be modifiedDisadvantages-Triangle routing and especially bidirectional tunneling increase latency and consume bandwidth-HA is single point of failure19Mobile IP Route OptimizationCH uses HA to contact MH initiallyMH sends its location directly back to CHCH and MH communicate directlyLose location privacyCH must be modified20TCP Migrate [SB00]Location: uses dynamic DNS updates-When MH moves to new IP address, it updates its home DNS server with new hostname to IP address mappingMigration:-When MH moves, it sends update to CHAdvantage-No new infrastructure-Incremental deployable-Efficient routingDisadvantages-Only works for TCP-Both CH and MH need new TCP implementation-No location privacy21i3 Based Mobility (Z+03)Receiver R maintains a trigger (id, R) in the i3 infrastructure; sender sends packets to idAdvantages-Support simultaneous mobility-Efficient routing: receiver can chose id to map on a close i3 server-Ensure privacyDisadvantage-Require a new infrastructure SenderReceiver(R1)Receiver(R2)id R1id R222Other solutionsNetwork specific mobility schemes-Cellular phones, 802.11b -Cannot
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