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Network Routing: Link Metrics and Non-Traditional Routing
Y. Richard Yang 2/26/2009
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Admin. Homework 3 Project proposal:
March 6 by to and
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Recap: Routing Protocols
Proactive protocols distance vector e.g., DSDV link state link reversal e.g., partial link reversal, TORA Reactive (on-demand) protocols DSR AODV A E D C B F 2 1 3 5
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Recap: ETX ETX: The predicted number of data transmissions required to successfully transmit a packet over a link Link loss rate = p Expected number of transmissions
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ETX Performance DSDV DSR
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Problems of ETX ETX does not handle multirate 802.11 networks
ETX does not work out well when nodes have multiple radios that can operate at different channels
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Extending ETX: Multirate
In a multirate environment, need to consider link bandwidth: packet size = S, Link bandwidth = B each transmission lasts for S/B Multirate Multradio “Routing in Multi-radio, Multi-hop Wireless Mesh Network,” Richard Draves, Jitendra Padhye, and Brian Zill. Mobicom 2004.
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Extending ETX: Multirate
Add ETTs of all links on the path Use the sum as path metric Interpretation: pick a path with the lowest total network occupation time Q: under what condition is SETT the network occupation time?
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Problem of SETT In networks with multiple channels/radios, SETT does not consider channel reuse Path SETT Throughput Red-Red Red-Blue 2.66 ms 3 Mbps 2.66 ms 6 Mbps
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Observation Interference reduces throughput
throughput of a path is lower if many links are on the same channel path metric should be worse for non-diverse paths
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Extending SETT for Multiple Channels
Group links on a path according to channel assumes links on the same channel interfere with one another pessimistic for long paths Add ETTs of links in each group Find the group with largest sum (BG-ETT) this is the “bottleneck” group too many links, or links with high ETT (“poor quality” links) Use this largest sum as the path metric Lower value implies better path
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BG-ETT Example Path Blue Sum Red Sum BG-ETT Throughput All-red 1 Blue
5.33 ms 5.33 ms 1.5 Mbps 1.33 ms 4 ms 4 ms 2 Mbps 2.66 ms 2.66 ms 2.66 ms 3 Mbps
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BG-ETT May Select Long Paths
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Path Metric: Putting it all together
SETT favors short paths BG-ETT favors channel diverse paths β is a tunable parameter Higher value: more preference to channel diversity Lower value: more preference to shorter paths
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Implementation and such
Implemented in a source-routed, link-state protocol, Multi-Radio Link Quality Source Routing (MR-LQSR) Nodes discover links to its neighbors, measure quality of those links link information floods through the network each node has “full knowledge” of the topology sender selects “best path” packets are source routed using this path Measure loss rate and bandwidth loss rate measured using broadcast probes similar to ETX updated every second bandwidth estimated using periodic packet-pairs updated every 5 minutes
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Evaluations
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Median Throughput (Baseline, single radio)
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Median Throughput (Baseline, two radios)
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Impact of β value
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Summary Link metrics are still an active research area, in particular, due to interactions with (channel, spatial) diversity
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Summary: Traditional Routing
So far, all routing protocols in the framework of traditional wireline routing a graph representation of underlying network point-to-point graph, edges with costs select a lowest-cost route for a src-dest pair commit to a specific route before forwarding each node forwards a received packet as it is to next hop Problems: don’t fully exploit path (spatial) diversity and wireless broadcast opportunities
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Motivating Scenario: I
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Motivating Scenario: II
Motivating question: can we take advantage of transmissions that reach unexpectedly far or unexpectedly short? Traditional routing picks a single route, e.g., src -> B -> D -> dst Packets received off path are useless
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Motivating Scenario: III
Src A sends 1 packet to dst B; src B sends packet 3 to dst A The network needs to transmit 4 packets Motivating question: can we do better? A R B
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Motivating Scenario: III
If R has both packets 1 and 3, it can combine them and explore coding and broadcast nature of wireless A B R
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Outline Admin. Link metrics Non-traditional routing motivation
network coding: exploiting network broadcast
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Network Coding We have covered source coding (FEC, compression)
The new approach uses opportunistic network coding goal: increase the amount of information that is transported
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Opportunistic Coding: Basic Idea
Each node looks at the packets available in its buffer, and those its neighbors’ buffers It selects a set of packets, computes the XOR of the selected packets, and broadcasts the XOR
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Opportunistic Coding
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Outline Admin. Link metrics Non-traditional routing motivation
network coding: exploiting network broadcast opportunistic routing
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Key Issue in Opportunistic Routing
Key Issue: opportunistic forwarding may lead to duplicates.
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Extreme Opportunistic Routing (ExOR)
Basic idea: avoid duplicates by scheduling Instead of choosing a fix sequential path (e.g., src->B->D->dst), the source chooses a list of forwarders (a forwarder list in the packets) using ETX-like metric a background process collects ETX information via periodic link-state flooding Forwarders are prioritized by ETX-like metric to the destination
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ExOR: Forwarding Group packets into batches
The highest priority forwarder transmits when the batch ends The remaining forwarders transmit in prioritized order each forwarder forwards packets it receives yet not received by higher priority forwarders status collected by batch map
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Batch Map Batch map indicates, for each packet in a batch, the highest-priority node known to have received a copy of that packet
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ExOR: Example N2 N0 N3 N1
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ExOR: Stopping Rule A nodes stops sending the remaining packets in the batch if its batch map indicates over 90% of this batch has been received by higher priority nodes the remaining packets transferred with traditional routing
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Evaluations 65 Node pairs 1.0MByte file transfer
1 Mbit/s bit rate 1 KByte packets EXOR bacth size 100 1 kilometer
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Evaluation: 2x Overall Improvement
1.0 0.8 0.6 Cumulative Fraction of Node Pairs 0.4 0.2 ExOR Traditional spend a little more time on the 240 x say this is just for the median, and it’s a factor of 2! 200 400 600 800 Throughput (Kbits/sec) Median throughputs: Kbits/sec for ExOR, 121 Kbits/sec for Traditional
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OR uses links in parallel
possible question – why are there only 7 forwarders.(just say we thin out...) ExOR 7 forwarders 18 links Traditional Routing 3 forwarders 4 links
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OR moves packets farther
58% of Traditional Routing transmissions 0.6 ExOR Traditional Routing Fraction of Transmissions 0.2 25% of ExOR transmissions 0.1 lower is better. right circle – using lots of longer links, sum them up and it’s 25%. so, like ex 1, using lots of long links. zeros: before many packets made no progress, with exor at least some. 100 200 300 400 500 600 700 800 900 1000 Distance (meters) ExOR average: 422 meters/transmission Traditional Routing average: 205 meters/tx
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Comments: ExOR Pros Cons
takes advantage of link diversity (the probabilistic reception) to increase the throughput does not require changes in the MAC layer can cope well with unreliable wireless medium Cons scheduling is hard to scale in large networks overhead in packet header (batch info) batches increase delay
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Outline Admin. Link metrics Non-traditional routing motivation
network coding: exploiting network broadcast opportunistic routing ExOR MORE
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MORE: MAC-independent Opportunistic Routing & Encoding
Basic idea: Replace node coordination with network coding Trading structured scheduler for random packets combination Previous network coding technique is for inter-flow MORE is for intra-flow network coding
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Basic Idea: Source Chooses a list of forwarders (e.g., using ETX)
Breaks up file into K packets (p1, p2, …, pK) Generate random packets MORE header includes the code vector [cj1, cj2, …cjK] for coded packet pj’
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Basic Idea: Source
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Basic Idea: Forwarder Check if in the list of forwarders
Check if linearly independent of new packet with existing packet Re-coding and forward
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Basic Idea: Destination
Decode Send ACK back to src if success
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Key Practical Question: How many packets does a forwarder send?
Compute zi: the expected number of times that forwarder i should forward each packet
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Computes zs Єij: loss probability of the link between i and j Compute zs so that at least one forwarder that is closer to destination is expected to have received the packet :
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Compute zj for forwarder j
Only need to forward packets that are received by j sent by forwarders who are further from destination not received by any forwarder who is closer to destination
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Compute zj for forwarder j
To guarantee at least one forwarder closer to d receives the packet
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Evaluations 20 nodes distributed in a indoor building
Path between nodes are 1 ~ 5 hops in length Loss rate is 0% ~ 60%; average 27%
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Throughput
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Problem of MORE?
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Mesh Networks API So Far
Forward correct packets to destination PHY/LL Deliver correct packets
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570 bytes; 1 bit in 1000 incorrect
Motivation R1 10-3 BER 0% S D 0% 10-3 BER R2 570 bytes; 1 bit in 1000 incorrect Packet loss of 99%
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Opportunistic Routing 50 transmissions
Implication R1 99% (10-3 BER) Loss 0% S D Loss 0% 99% (10-3 BER) R2 Opportunistic Routing 50 transmissions
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Outline Admin. Link metrics Non-traditional routing motivation
network coding: exploiting network broadcast opportunistic routing ExOR MORE MIXIT
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New API PHY + LL Deliver correct symbols to higher layer Network
Forward correct symbols to destination
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What Should Each Router Forward?
P1 P1 P2 R2 P2 P1 P2
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What Should Each Router Forward?
P1 P2 P1 P1 P2 R2 P2 P1 P2 P1 P2 Forward everything Inefficient Coordinate Unscalable
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Symbol Level Network Coding
P1 P2 P1 R2 P2 P1 P2 Forward random combinations of correct symbols
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Symbol Level Network Coding
… … D R2 … … Routers create random combinations of correct symbols
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Symbol Level Network Coding
… D R2 … Solve 2 equations Destination decodes by solving linear equations
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Symbol Level Network Coding
… … D R2 … … Routers create random combinations of correct symbols
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Symbol Level Network Coding
… D R2 … Solve 2 equations Destination decodes by solving linear equations
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Destination needs to know which combinations it received
Use run length encoding Original Packets Coded Packet
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Destination needs to know which combinations it received
Use run length encoding Original Packets Coded Packet
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Destination needs to know which combinations it received
Use run length encoding Original Packets Coded Packet
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Destination needs to know which combinations it received
Use run length encoding Original Packets Coded Packet
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Destination needs to know which combinations it received
Use run length encoding
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Evaluation Implementation on GNURadio SDR and USRP
Zigbee (IEEE ) link layer 25 node indoor testbed, random flows Compared to: Shortest path routing based on ETX MORE: Packet-level opportunistic routing
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Throughput Comparison
CDF 2.1x MIXIT 3x MORE Shortest Path Throughput (Kbps)
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Backup Slides
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Motivation for a Better Metric
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Implementation and such
Modify DSDV or DSR Example evaluation: in DSDV w/ ETX, route table is a snapshot taken at end of 90 second warm-up period in DSR w/ ETX, source waits additional 15 sec before initiating the route request
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Where do the gains come from?
CDF MIXIT without concurrency 1.5x MORE Shortest Path Throughput (Kbps) Take concurrency away from MIXIT
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Where do the gains come from?
CDF MIXIT MORE Shortest Path Throughput (Kbps) Take concurrency away from MIXIT
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