Routing Functions in Mesh Networks

1. Routing Functions in Mesh Networks 2007년 5월 18일 이상환 sanghwan@kookmin.ac.kr 국민대학교

2. Contents • Introduction • Link Quality Metric • ETX, ETT, WCETT • WMN Routing Protocols • LQSR, BAF, ExOR • Feasibility for All-Wireless Offices • 100+ users

3. What is Wireless Mesh Network (WMN)? • Nodes are static • Also called Static Wireless Network • Wireless channel for node to node transmission • External interference, channel fading, inclement weather • Quality of a link varies frequently over time • Many links may be in degraded state at any given time • Multi-hop transmission • The path from source to destination can be multi-hop

4. Multi-hop Wireless Networks

5. Why New Routing Protocols? • Routing protocols for wireless ad-hoc networks can be applied to WMN • TBRPF, DSR, AODV, DSDV, etc • Need research for several reasons • New performance metric • Limited scalability • Cross-layer interaction • Different requirements on power and mobility

6. Features of Routing Protocol (1) • Multiple Performance Metrics • Hop-count is not an effective routing metric. • Other performance metrics, e.g., link quality and round trip time (RTT), must be considered. • Scalability • Routing setup in large network is time consuming. • Node and link states on the path may change. • Scalability of routing protocol is critical in WMNs

7. Features of Routing Protocol (2) • Robustness • WMNs must be robust to link failures or congestion. • Routing protocols need to be fault tolerant with link failures and can achieve load balancing • Adaptive support of both mesh routers and mesh clients • Mesh routers : minimal mobility, no constraint of power consumption, routing is simpler • Mesh clients : mobility, power efficiency, routing is complicated • Need to design a routing protocol that can adaptively support both mesh routers and mesh clients.

8. better Better Performance Metric • Throughput ([6]) • How many packets are transmitted successfully 29 PC Testbed UDP throughput

9. Routing protocol ‘Best’ Can We Do Better? DSDV : Shortest Path in Hop count ‘Best’ for each pair is highest measured throughput of 10 promising static routes. There must be some protocol to achieve this.

10. 2 Phase Path Selection Strategy • Phase 1 : Link Quality Metric • Assign the quality of individual link • Phase 2 : Path Quality Metric • Combine link quality metrics on the path • Challenges • Multi-hop performance degradation • Lossy links • Asymmetric links

11. 1 2 3 5 6 4 Challenge 1 : more hops, less throughput Throughput over # of hops 1 hop = 1 2 hop = 1/2 3 hop = 1/3 • Links in route share radio spectrum MAC Interference among a chain of nodes. The Solid-line circle denotes transmission range (200m approx) and the dotted line circle denotes the interference range (550m approx)

12. Challenge 2: many links are lossy One-hop broadcast delivery ratios ‘Good’ ‘Bad’ Smooth link distribution complicates link classification.

13. Broadcast delivery ratios in both link directions. Very asymmetric link. Challenge 3 : many links are asymmetric Many links are good in one direction, but lossy in the other.

14. Contents • Introduction • Link Quality Metric • ETX, ETT, WCETT • WMN Routing Protocols • LQSR, BAF, ExOR • Feasibility for All-Wireless Offices

15. A straw-man route metric (1) B Product of link delivery ratio along path 100% 100% C A 51% A-B-C = 100% A-C = 51% Product: A-B-C : ABABAB = 2 tx A-C : AAAAAAAA = 1.96 tx Actual throughput:

16. A straw-man route metric (2) B Maximize bottleneck throughput Delivery ratio = 100% 50% C A 51% 51% D A-B-C = 50% A-D-C = 51% Bottleneck throughput: A-B-C : ABBABBABB = 3 tx A-D-C : AADDAADD = 4 tx Actual throughput:

17. A straw-man route metric (3) B Maximize end-to-end delivery ratio 100% 51% C A 50% A-B-C = 51% A-C = 50% End-to-end delivery ratio: A-B-C : ABBABBABB = 3 tx A-C : AAAAAAAA = 2 tx Actual throughput:

18. Expected Transmission Count ([5]) Minimize total transmissions per packet Link throughput  1/ Link ETX Delivery Ratio Link ETX Throughput 100% 100% 1 50% 50% 2 33% 33% 3

19. Calculating Link ETX • Assuming 802.11 link-layer acknowledgments (ACKs) and retransmissions: P(TX success) = P(Data success) ⅹP(ACK success) Link ETX = 1 / P(TX success) = 1 / [ P(Data success) ⅹ P(ACK success) ] • Estimating link ETX: P(Data success) ≈ measured fwd delivery ratio rfwd P(ACK success) ≈ measured rev delivery ratio rrev Link ETX ≈ 1 / (rfwd ⅹ rrev)

20. Measuring Delivery Ratios • Each node broadcasts small link probes (134 bytes), once per second • Nodes remember probes received over past 10 seconds • Reverse delivery ratios estimated as rrev pkts received / pkts sent • Forward delivery ratios obtained from neighbors (piggybacked on probes)

21. Route ETX Route ETX = Sum of link ETXs Route ETX Throughput 1 100% 2 50% 2 50% 3 33% 5 20%

22. ETX Properties • Advantages • ETX predicts throughput for short routes (1, 2, and 3 hops) • ETX quantifies loss, asymmetry, throughput reduction of longer routes • Caveats • ETX link probes are susceptible to MAC unfairness and hidden terminals • Route ETX measurements change under load • ETX estimates are based on measurements of a single link probe size (134 bytes) • Loss rate of broadcast probe packets is not the same as loss rate of data packets • Under-estimates data loss ratios, over-estimates ACK loss ratios • ETX assumes all links run at one bit-rate • Does not take data rate or link load into account

23. Per-hop RTT ([6]) • Node periodically pings each of its neighbors • Unicast probe/probe-reply pair • RTT samples are averaged using TCP-like low-pass filter • Path with least sum of RTTs is selected • Advantages • Easy to implement • Accounts for link load and bandwidth • Also accounts for link loss rate • 802.11 retransmits lost packets up to 7 times • Lossy links will have higher RTT • Disadvantages • Expensive • Self-interference due to queuing

24. Per-hop Packet-Pair ([6]) • Node periodically sends two back-to-back probes to each neighbor • First probe is small, second is large • Neighbor measures delay between the arrival of the two probes; reports back to the sender • Sender averages delay samples using low-pass filter • Path with least sum of delays is selected • Advantages • Self-interference due to queuing is not a problem • Implicitly takes load, bandwidth and loss rate into account • Disadvantages • More expensive than RTT

25. Considering Multiple Channel • ETX assumes single channel • One bit-rate • Self Interference among links No simultaneous transmission Simultaneous transmission

26. Existing Routing Metrics are Inadequate 2 Mbps 18 Mbps 18 Mbps 11 Mbps 11 Mbps Shortest path: 2 Mbps Path with fastest links: 9 Mbps Best path: 11 Mbps

27. Link Metric: Expected Transmission Time (ETT, [7]) • Link loss rate = p • Expected number of transmissions • Packet size = S, Link bandwidth = B • Each transmission lasts for S/B • Lower ETT implies better link Similar to airtime metric in 802.11s

28. ETT: Illustration 11 Mbps 5% loss 18 Mbps 10% loss 18 Mbps 50% loss 1000 Byte Packet ETT : 0.77 ms ETT : 0.40ms 1000 Byte Packet ETT : 0.77 ms ETT : 0.89 ms

29. Add ETTs of all links on the path Use the sum as path metric Combining Link Metric into Path Metric Proposal 1 SETT = Sum of ETTs of links on path (Lower SETT implies better path) Pro: Favors short paths Con: Does not favor channel diversity

30. SETT does not favor channel diversity 6 Mbps No Loss 6 Mbps No Loss 1.33ms 1.33ms 1.33ms 1.33ms 6 Mbps No Loss 6 Mbps No Loss

31. Impact of Interference • 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 • Assumption: All links that are on the same channel interfere with one another • Pessimistic for long paths

32. Combining Link Metric into Path Metric : Proposal 2 • Group links on a path according to channel • Links on same channel interfere • Add ETTs of links in each group • Find the group with largest sum. • 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 “Bottleneck Group ETT” (BG-ETT)

33. BG-ETT Example 6 Mbps 6 Mbps 6 Mbps 6 Mbps 6 Mbps 6 Mbps 1.33 ms 1.33 ms 1.33 ms 1.33 ms 1.33 ms 1.33 ms BG-ETT favors high-throughput, channel-diverse paths.

34. 6 Mbps 6 Mbps 6 Mbps 2 Mbps 4 ms 1.33 ms 1.33 ms 1.33 ms BG-ETT does not favor short paths S D 6 Mbps 6 Mbps 6 Mbps 1.33 ms 1.33 ms 1.33 ms S D

35. SETT favors short paths BG-ETT favors channel diverse paths Path Metric: Putting it all together • Weighted Cumulative ETT (WCETT) • WCETT = (1-β) * SETT + β * BG-ETT β is a tunable parameter Higher value: More preference to channel diversity Lower value: More preference to shorter paths

36. How to 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

37. Contents • Introduction • Link Quality Metric • ETX, ETT, WCETT • WMN Routing Protocols • LQSR, BAF, ExOR • Feasibility for All-Wireless Offices

38. Multi-Radio Link Quality Source Routing (MR-LQSR, [7]) • Implemented in a source-routed, link-state protocol • Derived from DSR : RREQ, RREP • Nodes discovers 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 • http://research.microsoft.com/mesh/

39. Blacklist Aided Forwarding ([8]) • Disseminate base topology infrequently globally • Base topology reflects the long-term state of each link • Convey short-term state of degraded links as far as necessary • Links with higher short-term cost w.r.t. base topology • Ensure loop-free forwarding to reachable destinations • Updating of links with better short-term cost is not essential • Usage of such links doesn’t cause loops even without link state updates • A scheme based on LOLS approach • Blacklist-Aided Forwarding

40. Blacklist Aided Forwarding (2) • Each packet carries a blacklist • A set of degraded links and their short-term costs • Each node maintains a blacklist cache • Adjacent degraded links • From forwarding failures or periodic probes • Non-adjacent degraded links • From blacklists of arriving packets • Purged after a refresh interval • Forwarding based on both destination and blacklist • (p.dest, p.blist)  (next hop, p.blist)

41. Forwarding under BAF • Imagine forwarding packets in two modes • Packets normally forwarded in greedy mode • Next hop along the path with decreasing long-term cost to destination • Switched to recovery mode upon hitting a deadend • In recovery mode, each packet carries a blacklist • Nexthop chosen after excluding the packet’s blacklist • Switched back to greedy mode on forward progress • When next hop is closer to the destination than any node visited so far

42. Updating of a Packet’s Blacklist • Blacklist initialized to empty at the packet’s source • Stays empty if forward progress w.r.t. base topology • A link XY is added to the packet’s blacklist if • Packet arrives at X and the nexthop is Y and • Link XY is currently degraded • A packet’s blacklist is reset to empty if • Cost from next hop to destination is the smallest so far • Blacklist grows if necessary and reset when possible • Minimal set of degraded links to ensure loop-freedom

43. Illustration: BAF 3 B E 3, {B-E} • A packet from B to E • gets caught in a loop under Shortest Path Forwarding • traverses B-A-D-C-E under BAF • BAF can forward packets between all pairs of nodes • without informing G,F,H about A-C or B-E and A,B,C,D,E about G-H 2 2 3 ∞, {} 1 A C 3 F 1 H 3, {B-E, A-C} 4 3 2 2 ∞, {} 2 D G

44. ExOR (1) • Ex Opportunistic Routing ([11]) • A Link/Network Layer diversity routing technique that uses standard radio hardware • Achieves substantial increase in throughput for large unicast transfers in mesh network.

45. ExOR (2) dst src • A complete schedule, undelivered packet are retried in subsequent one • A subset within a transmission batch is called Fragment (F) • After each batch destination sends packet just containing batch map 4 transmissions in total

46. Contents • Introduction • Link Quality Metric • ETX, ETT, WCETT • WMN Routing Protocols • LQSR, BAF, ExOR • Feasibility for All-Wireless Offices

47. Feasibility Evaluation • Questions • Can we use a wireless mesh network to support an entire office? At what scale and performance penalty? • How do various network design choices, such as node placement, hardware, wireless band and routing metrics impact application performance? • Current Approach • Deployed testbeds • Synthetic traces • Random traffic patterns • Need more realistic evaluation • CARE : Capture, Analysis, Replay, and Evaluation ([10])

48. Layered Service Provider in Windows XP • Socket level traffic capture • 27% missing traffic • SMB, RPC, NetBUI/NBT, LDAP and ICMP

49. Replay

50. Result Difference between wired transaction and wireless transaction 10 ms delay Performance Variation Across Repeated Runs of Medium Traffic Period, Distant Placement, WCETT Metric