Lecture 16: Wireless Networks

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&6( *UDGXDWH1HWZRUNLQJ :LQWHU Lecture 16: Wireless Networks Geoffrey M. Voelker :LUHOHVV1HWZRUNLQJ Many topics in wireless networking Transport optimizations, ad hoc routing, MAC algorithms, QoS, mobility, etc. Survey papers on TCP optimizations, ad hoc routing Good mileage cover a lot of ground in just two papers Can often understand the problem and issues better when different approaches are compared and contrasted These papers reflect one goal of the course Pick up a paper like this and understand what all of the buzzwords mean» Reno, NewReno, SACK, distance vector, link state, etc. Look, ns! March 1, 2001 CSE 222 -- Lecture 16 Wireless Networks 2 1

&RPSDULQJ$SSURDFKHV These two papers are also good examples of methodologies for comparing approaches to problems If you re in a similar situation, think back to these papers What s involved? Likely will have to implement approaches yourself» Requires good understanding of all approaches Sometimes better than original developers!» A lot of work, both implementing and performing experiments» Have to anticipate claims that implementation is not correct What are your options? Pure implementation no need to model messy details Simulation can explore wide parameter ranges March 1, 2001 CSE 222 -- Lecture 16 Wireless Networks 3 7&32YHU:LUHOHVV Problem: Wireless links have new loss characteristics Key: Losses are not due to congestion» Sporadic bit-error rates (e.g., temporary blockage)» Losses due to handoffs Sender should not scale back its rate But this is exactly what TCP does in reaction to loss» TCP interprets loss solely as a congestion signal Approaches End-to-end: Only change TCP at end-hosts Link-layer: Rexmit at link-layer, hide loss from sender Split-connection: Use two TCP connections, split at base station, to separate congestion from wireless losses March 1, 2001 CSE 222 -- Lecture 16 Wireless Networks 4 2

(QG7R 7R(QG End-to-end approaches try to solve the problem by changing TCP at the end points E2E Reno: fast retransmission, fast recovery (base)» Timeouts when > 1 loss in window, redundant rexmits E2E-NewReno fast recovery with partial acks» Faster recovery, still redundant rexmits E2E-SACK Sender explicitly notified of missing data» Losses still interpreted as congestion, senders backs off E2E-ELN Explicit Loss Notification» Receiver tells sender that a loss is not from congestion, sender does not back off (does not shrink congestion window) E2E-ELN-RXMT ELN w/ faster retransmission» Rexmit on first dup ELN ack March 1, 2001 CSE 222 -- Lecture 16 Wireless Networks 5 /LQN/D\HU /D\HU Wireless networks are a shared resource It is essentially a bus, and all devices compete to use it Base station must mediate access to it Many algorithms to do the mediation» We skipped this topic» Ad hoc routing paper describes 802.11 (wireless ethernet) Link-layer often tries to recover from losses transparently from higher level layers Key is that link-layer can do retransmissions as well Seen by higher layers only as a longer delay for packet Trade-off between transport rexmits and link rexmits» Don t want link-layer to be perfectly reliable, just reliable enough» Back to the end-to-end principle March 1, 2001 CSE 222 -- Lecture 16 Wireless Networks 6 3

/LQN/D\HU /D\HU Four link-layer approaches LL Cumulative acks (based on TCP acks), link-layer does local rexmits from base station» LL rexmit timer much shorter than TCP timer, reacts faster» Dup acks still go back to sender, though (invoking cong. avoid.) LL-SACK Use SACK to only rexmit actual losses» Dup acks still go back to sender LL-TCPAWARE Suppress dup acks ( snoop, by authors)» Sender does not see wireless losses» Does this violate E2E? LL-OPT Suppress dup acks, only rexmit actual losses» TCPAWARE + SACK March 1, 2001 CSE 222 -- Lecture 16 Wireless Networks 7 6SOLW&RQQHFWLRQ Split connection approaches use two connections Sender ÄÅ base station ÄÅ receiver Note: Overhead of managing two connections SÄÅB connection experiences congestion losses Use normal TCP (Reno) BÄÅR connection experiences wireless losses Can use something better adapted to wireless link Two variants SPLIT Reno TCP over wireless SPLIT-SCAK SACK TCP over wireless March 1, 2001 CSE 222 -- Lecture 16 Wireless Networks 8 4

([SHULPHQWDO(YDOXDWLRQ Methodology Implemented all variants in BSD Experiments using implementation on real wireless network No need to model any of the underlying complexity» c.f. ad hoc routing paper Metrics Throughput total bytes through network Goodput useful bytes through network WAN tests, too Losses artificially induced Reasonable? March 1, 2001 CSE 222 -- Lecture 16 Wireless Networks 9 5HVXOWV Paper posed four questions at beginning 1. What combination of mechanisms results in best performance for each protocol class? 2. How important is it for link-layer schemes to be TCP-aware? 3. How useful is SACK? 4. Is splitting necessary for good performance? Also, socket buffer size matters March 1, 2001 CSE 222 -- Lecture 16 Wireless Networks 10 5

%HVW:LWKLQ$SSURDFKHV 1. What is the best variant in each approach? Link-layer Å LL-OPT (best overall) Wireless losses ideally handled, sender only sees congestion LL, LL-SACK not as effective no suppression Best wide-area performance across all approaches E2E Å E2E-SACK Much lower throughputs than LL (but excellent goodputs) Why would SACK be better than ELN? Split Å Split-SACK SACK necessary, otherwise performance is terrible March 1, 2001 CSE 222 -- Lecture 16 Wireless Networks 11 5HPDLQLQJ4XHVWLRQV«2. Should LL be TCP aware? Yes: 10-30% better throughput 3. How useful is SACK? Significantly benefits all approaches (message: SACK is good) 4. Is TCP splitting necessary? It gives excellent performance, but LL-TCP-AWARE and LL- OPT are just as good Splitting not necessary Is LL less complex than splitting? March 1, 2001 CSE 222 -- Lecture 16 Wireless Networks 12 6

'LVFXVVLRQ What does significant mean in results? LL wireless goodput is only 95.5%, significantly less than LL- TCP-AWARE s wireless goodput of 97.6% Wireless congestion What happens when you do have congestion in wireless? How bad is it to misinterpret a loss due to congestion? Constrained setup What about sending the other direction? Wireless losses I m also wondering what type of loss rates we get in these new wireless home lans. Is it substantial? March 1, 2001 CSE 222 -- Lecture 16 Wireless Networks 13 'LVFXVVLRQ Determining source of loss In practice, how can we identify which packets are lost due to errors on all lossy link? March 1, 2001 CSE 222 -- Lecture 16 Wireless Networks 14 7

$G+RF5RXWLQJ,Q:LUHOHVV1HWZRUNV Ad hoc networks are composed of mobile nodes with overlapping cells centered on each node Two nodes are connected by a link if their cells overlap No centralized supporting infrastructure (no base stations) Frequent changes in network topology Problem: How do proposed routing algorithms perform on these networks? Many routing algorithms proposed Not evaluated well individually Not evaluated on common ground March 1, 2001 CSE 222 -- Lecture 16 Wireless Networks 15 $SSURDFK 1. Build a detailed simulator (ns) of a wireless network Node mobility Physical layer with radio propagation model Interfaces with power and receiver sensitivity 802.11 MAC protocol for mediating access to media 2. Implement routing algorithms in ns Twice, independently, to validate (rare) Add improvements to algorithms based upon experimentation» Do a better job than original developers 3. Simulate over a wide range of conditions Topologies, movement, sources, etc. 210 scenarios (one advantage of simulation) 4. Write paper using microfont March 1, 2001 CSE 222 -- Lecture 16 Wireless Networks 16 8

5RXWLQJ$OJRULWKPV Four routing algorithms analyzed 1. Destination-Sequenced Distance Vector (DSDV) 2. Temporally-Ordered Routing Algorithm (TORA) 3. Dynamic Source Routing (DSR) 4. Ad Hoc On-Demand Distance Vector (AODV) Important phrase in paper: each based on different assumptions and intuitions Assumptions and intuitions can sink your ship (e.g., TORA) March 1, 2001 CSE 222 -- Lecture 16 Wireless Networks 17 '6'9 DSDV is distance vector with sequence numbers Forwarding tables with next-hop info broadcast to neighbors Entries contain sequence number in addition to metric Variants» Nodes announce even seq #s for themselves» Nodes announce odd seq #s, infinite metrics for next-hops to destinations when link to next-hop is broken» Destinations react to this with a higher seq #, metrics recomputed at all nodes» In this in-between time, destination is unreachable But loops are prevented DSDV Trigger updates only upon receipt of new metrics DSDV-SQ Trigger updates upon receipt of new seq # March 1, 2001 CSE 222 -- Lecture 16 Wireless Networks 18 9

725$ TORA s goals are to: Discover routes on demand Provide multiple routes to destinations Establish routes quickly Localizing reactions to topology changes In TORA Nodes broadcast query packets to discover routes Other nodes that have a route respond with an update» Either destination or intermediate node This update establishes a route from originator to responder When a link goes down, nodes connected send an update with an effectively infinite metric March 1, 2001 CSE 222 -- Lecture 16 Wireless Networks 19 725$ TORA layered on the Internet MANET Encapsulation protocol (IMEP) Provides reliable, in-order delivery of control messages» Does resends until it gets an ack Notifies upper layer when a link goes down» If it does not eventually receive an ack, it consider the link down Beacons are used to sense link status when no traffic is going across the link» Good or bad? IMEP turns out to be Achilles heel of TORA March 1, 2001 CSE 222 -- Lecture 16 Wireless Networks 20 10

'65 DSR uses source routing Routes discovered on demand Nodes forward packets according to routes in packets Route Discovery Broadcast Route Request, route returned in Route Reply Cache source routes to reduce need for discovery Route Maintenance Need to decide when a source route no longer works When a link breaks, send Route Error to senders in cached source routes Sender can use a different source route, or discover another March 1, 2001 CSE 222 -- Lecture 16 Wireless Networks 21 $2'9 AODV combines features of DSR and DSDV Route Requests are flooded to discover routes to destinations Nodes create reverse routes back to sender during flooding Route Replies contain hop count and a sequence number» Route Replies sent back to sender via reverse routes» Intermediate nodes create forward routes to destination» Both reverse and forward routes are next-hop (c.f., DSDV) Upon link failure, Unsolicited Route Reply sent upstream to recent senders with infinite metric to break forwarding path Variants AODV use beacon messages to detect failed links AODV-LL use link-layer feedback to detect failed links March 1, 2001 CSE 222 -- Lecture 16 Wireless Networks 22 11

([SHULPHQWDO(YDOXDWLRQ Goal: Determine how routing algorithms react to topology changes while delivering data Metrics Packet delivery ratio packets sent / packets recieved Routing overhead -- # routing packets sent (per-hop) Path optimality difference between actual path and optimal Sources Constant bit rate, not TCP With TCP, could not get comparable transmissions March 1, 2001 CSE 222 -- Lecture 16 Wireless Networks 23 5HVXOWV Packet delivery All do well except DSDV-SQ at small pause times» DSDV-SQ fails to converge at high rate of topology changes TORA breaks down with heavy workload (large # sources)» TORA creates short-lived routing loops that interfere with the ability of the network to converge Routing overhead DSDV-SQ has overhead independent of rate of change Others depend DSR smaller than DSDV, others >> larger TORA again breaks down with heavy workload» Sends 100x more packets than other algorithms» Unstable overhead dramatically increases as load increases March 1, 2001 CSE 222 -- Lecture 16 Wireless Networks 24 12

5HVXOWV Path optimality DSDV-SQ and DSR are close to optimal TORA, AODV-LL have small fraction of paths with 4+ hops How much does this matter? Lower speeds (1m/s max speed) All deliver 98.5% of packets at this speed DSR s cache even more effective IMEP a significant source of packets for TORA (beacons) March 1, 2001 CSE 222 -- Lecture 16 Wireless Networks 25 'LVFXVVLRQ Why use a speed of 20m/s instead of 1m/s? What do these speeds correspond to? Is workload too extreme? What do the pauses correspond to? Eventually want to ground this in reality Why would we not want to use these algorithms within and among ISPs? What makes ad hoc routing different? Conclusions What can we do with the drawbacks of those protocols? March 1, 2001 CSE 222 -- Lecture 16 Wireless Networks 26 13

)RU1H[W7LPH«Only ST-II/RTSP comparison is required reading March 1, 2001 CSE 222 -- Lecture 16 Wireless Networks 27 14