Routing Functions in Mesh Networks

Routing Functions in Mesh Networks
2007년 5월 18일 이상환 국민대학교

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

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

Multi-hop Wireless Networks
Static Mobile Motivating scenario Community wireless networks Battlefield networks Cause of Tx Failure Bad Link Quality Node Movement Key challenge Improving network capacity Handling mobility, node failures, limited power.

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

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

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.

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

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

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

Challenge 1 : more hops, less throughput
6 1 2 3 4 5 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)

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

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

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

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

Maximize bottleneck throughput B Delivery ratio = 100% 50% A C 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:

Maximize end-to-end delivery ratio B 100% 51% A C 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:

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

Calculating Link ETX Assuming 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)

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)

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

ETX Properties Advantages Caveats
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

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 retransmits lost packets up to 7 times Lossy links will have higher RTT Disadvantages Expensive Self-interference due to queuing

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

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

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

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 s

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

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

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 Path Throughput SETT Red-Blue 6 Mbps 2.66 ms Red-Red 3 Mbps 2.66 ms

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

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)

BG-ETT Example BG-ETT favors high-throughput, channel-diverse paths.
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 Path Throughput Blue Sum Red Sum BG-ETT All Red 1.5 Mbps 5.33 ms 1 Blue 2 Mbps 1.33 ms 4 ms Red-Blue 3 Mbps 2.66 ms BG-ETT favors high-throughput, channel-diverse paths.

BG-ETT does not favor short paths
6 Mbps 6 Mbps D 6 Mbps 1.33 ms 1.33 ms 1.33 ms 6 Mbps 1.33 ms 4 ms 2 Mbps S D Path Throughput Blue Sum Red Sum BG-ETT 3-Hop 2 Mbps 4 ms 4-Hop 2 Mbps 4 ms

Path Metric: Putting it all together
SETT favors short paths BG-ETT favors channel diverse paths 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

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

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

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

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

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)

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

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

Illustration: BAF A packet from B to E
3 B E 3, {B-E} 2 2 3 ∞, {} 1 A C 3 F 1 H 3, {B-E, A-C} 4 3 2 2 ∞, {} 2 D G 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

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.

4 transmissions in total
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

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

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])

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

Replay

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

Is All-Wireless Office Feasible?
Up to 100+ users 19 user traces 13 orthogonal channels => 6 parallel transmission 19 * 6 = 114 users

Findings of Experiments
Routing metric Significant impact when the offered load grows close to network capacity. Metrics that make use of unicast probes : high overhead contention for the medium increases Server placement Direct effect on average path length Crucial for achieving good performance Hardware and IEEE band Can significantly impact delay. Additional delay : mostly under 20ms.

Research Issues (1) Scalability Better Performance Metrics
Hierarchical routing protocols can only partially solve this problem Geographic routing relies positioning technologies. New scalable routing protocols need to be developed. Better Performance Metrics New performance metrics need to be developed. Need to integrate multiple performance metrics into a routing protocol

Research Issues (2) Routing/MAC Cross-Layer Design Hybrid Routing
Needs to interact with the MAC layer, e.g. adopting multiple performance metrics from MAC layer. Merely exchanging parameters between them is not enough, merging certain functions of MAC and routing protocols is a promising approach. For multi-radio or multi-channel routing, the channel/radio selection in the MAC layer can help the path selection in the routing layer. Hybrid Routing Mesh routers and mesh clients have different constraints in power efficiency and mobility. Need to adaptively support mesh routers and mesh clients

References [1] MACAW: A Medium Access Protocol for Wireless LANs, by V. Bharghavan et al., ACM SIGCOMM '94 [2] Jinyang Li, charles Blake, Douglas S. J. De Couto, Hu Imm Lee, Robert Morris, Capacity of Ad Hoc Wireless Networks. In Mobicom 2001, Rome, Italy [3] D. Aguayo, J. Bicket, S. Biswas, G. Judd, and R. Morris. Link-level measurements from an b mesh network. In Proc. ACM Sigcomm, August 2004. [4] Kamal Jain Jitendra Padhye Venkat Padmanabhan Lili Qiu. The impact of interference on multi-hop wireless network performance. In Proc. ACM Mobicom, September 2003. [5] Douglas De Couto, Daniel Aguayo, John Bicket, and Robert Morris. A high throughput path metric for multi-hop wireless routing. In Proc. ACM Mobicom, September 2003.

References [6] Richard Draves, Jitendra Padhye, and Brian Zill. Comparison of routing metrics for static multi-hop wireless networks. In Proc. ACM Sigcomm, August 2004. [7] Richard Draves Jitendra Padhye Brian Zill. Routing in Multi-Radio, Multi-Hop Wireless Mesh Networks. In Proc. ACM Mobicom, September 2004. [8] Srihari Nelakuditi, Sanghwan Lee, Yinzhe Yu, Junling Wang, Zifei Zhong, Guor-Huar Lu, and Zhi-Li Zhang, "Blacklist-Aided Forwarding in Static Multihop Wireless Networks," In the Proceedings of IEEE SECON'05, Santa Clara, CA, Sep 2005 [9] I. Akyildiz, X. Wang, and W. Wang. Wireless mesh networks: A survey. In Elsevier Computer Networks, 2005. [10] J. Eriksson, S. Agarwal, P. Bahl, and J. Padhye, “Feasibility study of mesh networks for all-wireless offices,” in MobiSys’06, Uppsala, Sweden, June 2006 [11] Sanjit Biswas and Robert Morris. ExOR: Opportunistic Multi-Hop Routing for Wireless Networks. In Proc. ACM Sigcomm, August 2005.

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