15-441: Computer NetworkingWireless ChallengesOverviewCellular ReuseCSMA/CD Does Not WorkSlide 6Routing to Mobile NodesHow to Handle Mobile Nodes? (Addressing)How to Handle Mobile Nodes? (Naming)How to Handle Mobile Nodes? (Transport)How to Handle Mobile Nodes? (Link Layer)How to Handle Mobile Nodes? (Routing 1)How to Handle Mobile Nodes? (Routing 2)Basic Solution to Mobile RoutingInterceptionDeliveryMobile IP (RFC 2290)Mobile IP (MH at Home)Mobile IP (MH Moving)Mobile IP (MH Away – Foreign Agent)Mobile IP (MH Away - Collocated)Other Mobile IP IssuesSlide 23TCP Problems Over Noisy LinksConstraints & RequirementsChallenge #1: Wireless Bit-ErrorsPerformance DegradationProposed SolutionsApproach Styles (End-to-End)End-to-End: Selective AcksSlide 31Approach Styles (Split Connection)Split ConnectionSplit-Connection Congestion WindowApproach Styles (Link Layer)Hybrid Approach: Snoop ProtocolSnoop Protocol: CH to MHSlide 38Slide 39Slide 40Slide 41Slide 42Slide 43Slide 44Slide 45Slide 46Slide 47Snoop Data ProcessingSnoop ACK ProcessingSlide 50Adapting ApplicationsTranscoding ExampleSource Adaptation15-441: Computer NetworkingLecture 23: Mobile and Wireless NetworkingLecture 23: 11-27-01 2Wireless Challenges•Force us to rethink many assumptions•Need to share airwaves rather than wire•Don’t know what hosts are involved•Host may not be using same link technology•Mobility•Other characteristics of wireless•Noisy lots of losses•Slow•Interaction of multiple transmitters at receiver• Collisions, capture, interference•Multipath interferenceLecture 23: 11-27-01 3Overview•Link layer challenges•Internet mobility•TCP Over Noisy Links•Adapting Applications to Slow LinksLecture 23: 11-27-01 4Cellular Reuse•Transmissions decay over distance•Spectrum can be reused in different areas•Different “LANs”•Decay is 1/R2 in free space, 1/R4 in some situationsLecture 23: 11-27-01 5CSMA/CD Does Not Work•Carrier sense problems•Relevant contention at the receiver, not sender•Hidden terminal•Exposed terminal•Collision detection problems•Hard to build a radio that can transmit and receive at same timeABCABCDHidden ExposedLecture 23: 11-27-01 6Overview•Link layer challenges•Internet mobility•TCP Over Noisy Links•Adapting Applications to Slow LinksLecture 23: 11-27-01 7Routing to Mobile Nodes•Obvious solution: have mobile nodes advertise route to mobile address/32•Should work!!!•Why is this bad? consider forwarding tables on backbone routers•Would have an entry for each mobile host•Not very scalableLecture 23: 11-27-01 8How to Handle Mobile Nodes?(Addressing)•Dynamic Host Configuration (DHCP)•Host gets new IP address in new locations•Problems•Host does not have constant name/address how do others contact host•What happens to active transport connections?Lecture 23: 11-27-01 9How to Handle Mobile Nodes?(Naming)•Naming•Use DHCP and update name-address mapping whenever host changes address•Fixes contact problem but not broken transport connectionsLecture 23: 11-27-01 10How to Handle Mobile Nodes? (Transport) •TCP currently uses 4 tuple to describe connection•<Src Addr, Src port, Dst addr, Dst port> •Modify TCP to allow peer’s address to be changed during connection•Security issues•Can someone easily hijack connection?•Difficult deployment both ends must support mobilityLecture 23: 11-27-01 11How to Handle Mobile Nodes?(Link Layer)•Link layer mobility•Learning bridges can handle mobility this is how it is handled at CMU•Encapsulated PPP (PPTP) Have mobile host act like he is connected to original LAN•Works for IP AND other network protocolsLecture 23: 11-27-01 12How to Handle Mobile Nodes?(Routing 1)•Multicast•Solves similar problem how to route packets to different sets of hosts at different times•Can’t we just reuse same solutions?•Don’t really have solution for multicast either!Lecture 23: 11-27-01 13How to Handle Mobile Nodes?(Routing 2)•Allow mobile node to keep same address and name•How do we deliver IP packets when the endpoint moves?•Why can’t we just have nodes advertise route to their address?•What about packets from the mobile host?•Routing not a problem•What source address on packet?•Key design considerations•Scale•Incremental deploymentLecture 23: 11-27-01 14Basic Solution to Mobile Routing •Same as other problems in Computer Science•Add a level of indirection•Keep some part of the network informed about current location•Need technique to route packets through this location (interception)•Need to forward packets from this location to mobile host (delivery)Lecture 23: 11-27-01 15Interception•Somewhere along normal forwarding path•At source•Any router along path•Router to home network•Machine on home network (masquerading as mobile host)•Clever tricks to force packet to particular destination•“Mobile subnet” – assign mobiles a special address range and have special node advertise routeLecture 23: 11-27-01 16Delivery•Need to get packet to mobile’s current location•Tunnels•Tunnel endpoint = current location•Tunnel contents = original packets•Source routing•Loose source route through mobile current location•Network address translation (NAT)•What about packets from the mobile host?Lecture 23: 11-27-01 17Mobile IP (RFC 2290)•Interception•Typically home agent – hosts on home network•Delivery•Typically IP-in-IP tunneling•Endpoint – either temporary mobile address or foreign agent•Terminology•Mobile host (MH), correspondent host (CH), home agent (HA), foreign agent (FA)•Care-of-address, home addressLecture 23: 11-27-01 18Mobile IP (MH at Home)Mobile Host (MH)Visiting LocationHomeInternetCorrespondent Host (CH)PacketLecture 23: 11-27-01 19Mobile IP (MH Moving)Visiting LocationHomeInternetCorrespondent Host (CH)PacketHome Agent (HA)Mobile Host (MH)I am hereLecture 23: 11-27-01 20Mobile IP (MH Away – Foreign Agent)Visiting LocationHomeInternetCorrespondent Host (CH)PacketHome Agent (HA)Foreign Agent (FA)EncapsulatedMobile Host (MH)Lecture 23: 11-27-01 21Mobile IP (MH Away - Collocated)Visiting LocationHomeInternetCorrespondent Host (CH)PacketHome Agent (HA)Mobile Host (MH)EncapsulatedLecture 23: 11-27-01 22Other Mobile IP Issues•Route optimality•Resulting paths can be sub-optimal•Can be improved with route optimization•Unsolicited binding cache update to
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