Tài liệu Mobile & wireless networking – lecture 10 ad hoc networks and mobile transport layer - geert heijenk

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262001 Mobile & Wireless Networking Lecture 10: Ad Hoc Networks & Mobile Transport Layer [Schiller, Section 8.3 & Section 9] [Reader, Part 8] Geert Heijenk Mobile and Wireless Networking 2009 / 2010 Outline of Lecture 10 Ad hoc networks     Concept Routing           Problem DSDV (Destination Sequenced Distance Vector) Ad-hoc On-demand Distance Vector (AODV) DSR (Dynamic Source Routing) Further alternatives Mobile transport layer     Motivation Approaches for improvement             Indirect TCP Snooping TCP Mobile TCP Selective retransmission Comparison Recommended TCP improvements for 2.5G/3G wireless 2 Mobile and Wireless Networking 2009 / 2010 Ad hoc network concept   Networks of wireless terminals that do not necessarily rely on existing infrastructure         Although interworking with infrastructure is possible Direct communication between terminals when needed Multi-hop communication Extended concept of mobility: network mobility (moving routers) 3 Mobile and Wireless Networking 2009 / 2010 Mobile ad hoc networks Standard Mobile IP needs an infrastructure     Home Agent/Foreign Agent in the fixed network DNS, routing etc. are not designed for mobility Sometimes there is no infrastructure!     remote areas, ad-hoc meetings, disaster areas cost can also be an argument against an infrastructure! Main topic: routing     no default router available every node should be able to forward A B C 4 Mobile and Wireless Networking 2009 / 2010 Routing examples Routing is a major topic   in principle, every node should be able to forward   dynamic topology   asymmetric links   redundant links: too many links when terminals are close N1 N1 N2 N3 N4 time = t1 N3 N2 N4 N5 good link weak link N5 time = t2 5 Mobile and Wireless Networking 2009 / 2010 Traditional routing algorithms   Distance Vector       periodic exchange of messages with all physical neighbors that contain information about who can be reached at what distance (monodirectional) selection of the shortest path if several paths available Link State     periodic notification of all routers about the current state of all physical links (bidirectional) router get a complete picture of the network 6 Mobile and Wireless Networking 2009 / 2010 Problems of traditional routing algorithms   Dynamic of the topology     Limited performance of mobile systems         frequent changes of connections, connection quality, participants periodic updates of routing tables need energy without contributing to the transmission of user data, sleep modes difficult to realize limited bandwidth of the system is reduced even more due to the exchange of routing information links can be asymmetric, i.e., they can have a direction dependent transmission quality Problem   protocols have been designed for fixed networks with infrequent changes and typically assume symmetric links 7 Mobile and Wireless Networking 2009 / 2010 Ad hoc routing algorithms   Pro-active Example:   Destination Sequenced Distance Vector (DSDV)   Re-active Example:   Ad-hoc On-demand Distance Vector (AODV)   Dynamic Source Routing (DSR) 8 Mobile and Wireless Networking 2009 / 2010 DSDV (Destination Sequenced Distance Vector)     Expansion of distance vector routing Sequence numbers for all routing updates       assures in-order execution of all updates avoids loops and inconsistencies Decrease of update frequency     store time between first and best announcement of a path inhibit update if it seems to be unstable (based on the stored time values) See [Schiller] for details 9 Mobile and Wireless Networking 2009 / 2010 Ad-hoc On-demand Distance Vector (AODV) •  Specified in IETF: RFC 3561 •  Forms the basis for DYMO (Dynamic On-demand MANET routing protocol) which is a planned IETF reactive routing protocol. •  Uses destination sequence numbers to avoid loops, and ensure proper updating of routes Storage of routes in Route Table Uses only symmetric links •  •  10 Mobile and Wireless Networking 2009 / 2010 Route Discovery •  Source broadcasts Route Request (RREQ): •  A node can reply to the RREQ if •  •  •  •  It is the destination It has a “fresh enough” route to the destination Otherwise it rebroadcasts the RREQ Nodes keep track of and discard redundant broadcasts Source: Perkins & Royer 11 Mobile and Wireless Networking 2009 / 2010 Reverse Path Setup •  •  •  Nodes update their Route Table with source node information before forwarding RREQ Reverse path entry used to forward Route Reply (RREP) back to source if one is received Expiration time is long enough for a RREP to be received and forwarded 12 Mobile and Wireless Networking 2009 / 2010 Forward Path Setup •  Destination, or intermediate node with current route to destination, unicasts Route Reply (RREP) back to source: •  •  •  Nodes along path record forward route in Route Table, use reverse route to forward RREP Source can begin sending data when it receives first RREP If it later receives a RREP with better metric, it updates its route entry 13 Mobile and Wireless Networking 2009 / 2010 Route Table •  Fields: •  •  •  •  •  •  •  •  Destination IP Address Destination Sequence Number HopCount Next Hop IP Address Active Neighbors Expiration time Each time a route entry is used to transmit data, the expiration time is updated to current time + active_route_timeout Route entries may be updated if a route with greater sequence number or smaller hopcount is discovered 14 Mobile and Wireless Networking 2009 / 2010 Path Maintenance •  •  •  Movement of nodes not along active path does not trigger protocol action If source node moves, it can reinitiate route discovery When destination or intermediate node moves, node upstream of break sends unsolicited RREP to all active upstream neighbors •  •  •  •  •  ∞ metric, incremented Seq# Used to flush stale routes RREP is propagated to their active neighbors, and so on back to source Source can reinitiate route discovery if route is still needed RREQs for reinitiated route discovery contain destination sequence number of one greater than last known number 15 Mobile and Wireless Networking 2009 / 2010 Path Maintenance (example) •  •  •  •  Node 3 moves to new location 3’ Node 2 notices loss of link, sends link failure to Node 1 Node1 forwards link failure to Source Source reinitiates route discovery, finds new route through Node 4 16 Mobile and Wireless Networking 2009 / 2010 Local Connectivity Management •  •  •  •  Node must periodically hear from active neighbors to know they are still within range Eavesdrop on neighbor transmissions If no other transmissions within hello_interval, broadcast Hello packet Failure to hear from a neighbor for (1+ allowed_hello_loss) * hello_interval indicates loss of link 17 Mobile and Wireless Networking 2009 / 2010 Optimizations •  Expanding Ring Search •  •  RREP generated by intermediate node •  •  •  by limiting TTL at first attempt, and increasing it at successive attempts. only if Seq# for route to destination ≥ Dest_Seq# of RREQ Maintaining Local Connectivity by means of layer 2 info. Local Repair •  node upstream of link failure tries to find a new route to destination by sending a RREQ (with reduced TTL, and incremented Dest_Seq#) 18 Mobile and Wireless Networking 2009 / 2010 DSR (Dynamic Source Routing) •  Similar to AODV •  Big difference: DSR uses Source Routing •  AODV relies on storing routing table entries in intermediate nodes  RREQ and RREP carry addresses of all intermediate nodes •  See [Schiller] for details 19 Mobile and Wireless Networking 2009 / 2010 Alternative metrics Not only the number of hops is important Also:           interference with other stations total energy consumption (against number of hops) balanced energy consumption energy availability per node price (accounting based on forwarding effort) 20 Mobile and Wireless Networking 2009 / 2010
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