Towards a Robust Protocol Stack for Diverse Wireless

Towards a Robust Protocol Stack for Diverse Wireless Networks Arun Venkataramani (in collaboration with Ming Li, Devesh Agrawal, Deepak Ganesan, Aruna Balasubramanian, Brian Levine, Xiaozheng Tie at UMass Amherst) UNIVERSITY OF MASSACHUSETTS , AMHERST • Department of Computer Science

Wireless network landscape today Diverse wireless networks coexist Adapt TCP/IP stack in different ways WLAN Mesh Little mobility Cellular MANET Some mobility DTN High mobility UNIVERSITY OF MASSACHUSETTS AMHERST • Department of Computer Science 2

Interconnecting diverse networks? Vision: A simple robust protocol stack to interconnect diverse wireless networks. WLAN Mesh Little mobility Cellular DTN MANET Some mobility High mobility UNIVERSITY OF MASSACHUSETTS AMHERST • Department of Computer Science 3

Why does TCP/IP not suffice? E 2 E route disruptions cause unavailability WLAN Mesh Little mobility Cellular DTN MANET Some mobility High mobility UNIVERSITY OF MASSACHUSETTS AMHERST • Department of Computer Science 4

Why does TCP/IP not suffice? Gap between what you buy vs. get WLAN Mesh Little mobility Cellular DTN MANET Some mobility High mobility Advertised capacity: 11 Mbps UNIVERSITY OF MASSACHUSETTS AMHERST • Department of Computer Science 5

Principle: Design for high uncertainty Observe what works at an extreme point in the design space--always-partitioned DTNs--for insights into a robust design. UNIVERSITY OF MASSACHUSETTS AMHERST • Department of Computer Science 6

Outline Motivation Block transport Replication routing Research challenges UNIVERSITY OF MASSACHUSETTS AMHERST • Department of Computer Science 7

1. E 2 E Transport E 2 E rate control is error-prone E 2 E retransmissions are wasteful E 2 E Feedback Loss Rate RTT Rate Control Source Congestion? Link loss? UNIVERSITY OF MASSACHUSETTS AMHERST • Department of Computer Science Dest 8

1. E 2 E Transport E 2 E rate control is error-prone E 2 E retransmissions are wasteful E 2 E Retransmissions P Source Redundant Transmissions UNIVERSITY OF MASSACHUSETTS AMHERST • Department of Computer Science Dest 9

2. Packet as Unit of Control Channel access Link layer ARQ Listen Backoff RTS/CTS UNIVERSITY OF MASSACHUSETTS AMHERST • Department of Computer Science 11

2. Packet as Unit of Control Channel access Link layer ARQ Timeout Backoff UNIVERSITY OF MASSACHUSETTS AMHERST • Department of Computer Science 12

3. Complex Cross-Layer Interaction Link-layer ARQ/backoff hurts TCP rate control Rate Control Transport Highly Variable RTT Link ARQ UNIVERSITY OF MASSACHUSETTS AMHERST • Department of Computer Science 13

Hop: Clean-Slate Transport Protocol End-To-End Hop-by-Hop Packets Blocks Complexity Minimize interaction Block-switched networks: A new paradigm for wireless networks, Li et al. [NSDI 2009] UNIVERSITY OF MASSACHUSETTS AMHERST • Department of Computer Science 14

Hop Design Virtual Retransmission Backpressure Multi-hop Per-hop ACK Withholding Micro-block Prioritization Reliable Block Transfer UNIVERSITY OF MASSACHUSETTS AMHERST • Department of Computer Science 15

Reliable Per-Hop Block Transfer Mechanisms Burst mode (TXOP) Block ACK based ARQ Benefits Amortizes control overhead B-SYN Request for B-ACK Packet bitmap CSMA TXOP UNIVERSITY OF MASSACHUSETTS AMHERST • Department of Computer Science 16

Hop Design Virtual Retransmission Backpressure Multi-hop Per-hop ACK Withholding Micro-block Prioritization Reliable Block Transfer UNIVERSITY OF MASSACHUSETTS AMHERST • Department of Computer Science 17

Virtual Retransmission (VTX) Mechanism Leverages in-network caching Re-transmits blocks only when unavailable in cache Benefits Fewer transmissions Low overhead Simple A B-SYN B-ACK DATA C B D E UNIVERSITY OF MASSACHUSETTS AMHERST • Department of Computer Science 18

Virtual Retransmission (VTX) Mechanism Leverages in-network caching Re-transmits blocks only when unavailable in cache Benefits Fewer transmissions B-SYNVTX Low overhead Simple A VTX Timer B-SYNVTX B-ACK B B-SYN with VTX flag set 2 E E E B- CK A D YN S UNIVERSITY OF MASSACHUSETTS AMHERST • Department of Computer Science 19

Hop Design Virtual Retransmission Backpressure Multi-hop Per-hop ACK Withholding Micro-block Prioritization Reliable Block Transfer UNIVERSITY OF MASSACHUSETTS AMHERST • Department of Computer Science 20

Backpressure Mechanism Limits outstanding blocks per-flow at forwarder Hold B-ACK A Source B C B-SYN D Slow E Dest Limit on outstanding blocks=2 UNIVERSITY OF MASSACHUSETTS AMHERST • Department of Computer Science 21

Backpressure Mechanism Limits outstanding blocks per-flow at forwarder Benefits Improves network utilization A 20 Mbps Source B 10 Mbps C D 20 Mbps E Dest 20 Mbps F 1 Mbps G Dest Aggregate goodput without backpressure: 6 Mbps UNIVERSITY OF MASSACHUSETTS AMHERST • Department of Computer Science 22

Backpressure Mechanism Limits #outstanding blocks per-flow at forwarder Benefits Improves network utilization A 20 Mbps Source B 10 Mbps C D 20 Mbps Dest 20 Mbps F E 1 Mbps Limit of Outstanding Blocks=1 G Dest Aggregate goodput with backpressure: 10 Mbps UNIVERSITY OF MASSACHUSETTS AMHERST • Department of Computer Science 23

Hop Design Virtual Retransmission Backpressure Multi-hop Per-hop ACK Withholding Micro-block Prioritization Reliable Block Transfer UNIVERSITY OF MASSACHUSETTS AMHERST • Department of Computer Science 24

Ack Withholding Mechanism: Receiver withholds all but one BACK A C B Benefit: Low overhead Less conservative Simple B-SYN B-ACK DATA B-SYN Withhold B-ACK DATA UNIVERSITY OF MASSACHUSETTS AMHERST • Department of Computer Science 25

Hop Design Virtual Retransmission Backpressure Multi-hop Per-hop ACK Withholding Micro-block Prioritization Reliable Block Transfer UNIVERSITY OF MASSACHUSETTS AMHERST • Department of Computer Science 26

Micro-block Prioritization Mechanisms Sender piggybacks small blocks to B-SYN Receiver prioritizes small block’s B-ACK Benefits Low delay for small blocks SSH Sender 64 B Receiver B-SYN DATA B-ACK Sender B-SYN FTP 1 MB UNIVERSITY OF MASSACHUSETTS AMHERST • Department of Computer Science 27

Testbed 20 nodes on the 2 nd floor of UMass CS building Apple Mac Mini 1. 8 GHz, 2 GB RAM, Atheros 802. 11 a/b/g card UNIVERSITY OF MASSACHUSETTS AMHERST • Department of Computer Science 28

Single-flow Single-hop Performance 1. 2 x TCP 1. 6 x Hop 28 x Hop achieves significant gains over TCP UNIVERSITY OF MASSACHUSETTS AMHERST • Department of Computer Science 29

Single-flow Multi-hop Performance 1. 9 x TCP 2. 3 x Hop 2. 7 x Hop achieves significant gains over TCP UNIVERSITY OF MASSACHUSETTS AMHERST • Department of Computer Science 30

Graceful Degradation with Loss Emulated link layer losses at the receiver TCP Hop TCP breaks down at moderate loss rates UNIVERSITY OF MASSACHUSETTS AMHERST • Department of Computer Science 31

Scalability to High Load 30 concurrent flows 2 x Mean goodput 20 x TCP 150 x Hop-by-hop TCP Hop is much fairer than TCP UNIVERSITY OF MASSACHUSETTS AMHERST • Department of Computer Science 32

Hop Performance Breakdown UNIVERSITY OF MASSACHUSETTS AMHERST • Department of Computer Science 33

Low Delay for Small Transfers 1 small transfer competing with 4 large AP Small transfer size (KB) Hop lowers delay across all file sizes UNIVERSITY OF MASSACHUSETTS AMHERST • Department of Computer Science 35

Related Work Fixing E 2 E rate-control Separating loss/congestion [Snoop, WTCP, Westwood+, ATCP, TCP-ELFN] Network-assisted rate control [ATP, NRED, IFRC, WCP] Hop circumvents rate control Backpressure ATM, theoretical work [Tassiulas et al. ] Tree/chain sensor data aggregation [Fusion, Flush] Reliable point-to-point transport [RAIN, CXCC, Horizon] Hop reduces backpressure overhead using blocks Batching Common optimization at link [802. 11 e/802. 11 n, Wi. LDNet, Kim 08, CMAP], transport [Delayed-ACK, DTN 2. 5], and network [Ex. OR] layers Hop leverages batching across layers UNIVERSITY OF MASSACHUSETTS AMHERST • Department of Computer Science 40

Outline Motivation Block transport Replication routing Research challenges UNIVERSITY OF MASSACHUSETTS AMHERST • Department of Computer Science 41

Replication routing Replication reduces delay under topology uncertainty Y P P P Z X P W UNIVERSITY OF MASSACHUSETTS AMHERST • Department of Computer Science 42

Routing as resource allocation problem Problem: Which packets to replicate given limited X bandwidth to optimize a specified metric? Y RAPID Protocol (X, Y): 1. Control channel: Exchange metadata 2. Direct Delivery: Deliver packets destined to each other Change in utility 3. Replication: Replicate in decreasing order of marginal utility Packet size 4. Termination: Until all packets replicated or nodes out of range DTN routing as a resource allocation problem, Balasubramanian et al. [SIGCOMM 2007] UNIVERSITY OF MASSACHUSETTS AMHERST • Department of Computer Science 43

Utility computation example Objective: Minimize average delay Define U(i) = -(T + D) T = time since created, D = expected remaining time to deliver Simplistic assumptions uniform exponential meeting with mean ¸ global view Z i X D=¸ i Y D = ¸/2 i W D =¸/3 UNIVERSITY OF MASSACHUSETTS AMHERST • Department of Computer Science 44

Utility computation example (cont’d) j j i W X Y Deadline of i < T Z Deadline of j = T 1 > T Metric: Min average delay Metric: Maximize #packets delivered within deadline Replicate i Replicate j UNIVERSITY OF MASSACHUSETTS AMHERST • Department of Computer Science 45

Deployment on Diesel. Net UNIVERSITY OF MASSACHUSETTS AMHERST • Department of Computer Science 46

Results based on Diesel. Net traces UNIVERSITY OF MASSACHUSETTS AMHERST • Department of Computer Science 47

Outline Motivation Block transport Replication routing Research challenges UNIVERSITY OF MASSACHUSETTS AMHERST • Department of Computer Science 48

C 1: When to replicate? relays Replication useful under X 1 Topology uncertainty Delay optimization Light load scenarios X 2 src Claim: Delay benefit of replication is unbounded. dst Xk Xi = r. v. for delay of path i Forwarding delay = mini(E[Xi]) Replication delay = E[mini(Xi)] WLAN Mesh Low uncertainty Forwarding suffices DTN MANET High uncertainty Replication useful UNIVERSITY OF MASSACHUSETTS AMHERST • Department of Computer Science 49