TCP challenges in multi-hop wireless networks. Why multi-hop?

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1 TCP challenges in multi-hop wireless networks Konstantinos Psounis Assistant Professor EE and CS departments, USC Joint work with Vlad Balan, Ramesh Govindan, Ki- Young Jang, Apoorva Jindal, Sumit Rangwala Why multi-hop? Easy to deploy Easy to upgrade Inexpensive The only option for some killer applications, e.g. disaster recovery networks vehicular ad hoc networks environmental monitoring (underwater, forests, ) 2 University of Southern California 1

2 Why not multi-hop? Bad performance e.g. consider a mesh network using TCP over de facto MAC standard (802.11) throughput reduces significantly after 3 hops severe capture effects which leads to extreme unfairness But, is this inherent to multi-hop, or we don t do things right? Specifically, is TCP regulating the end-to-end rates properly? 3 Congestion in the wired world f 1, r 1 l 1,C 1 f 2, r 2 In case of congestion TCP signals congestion to sources and fairly shares capacity 4 University of Southern California 2

3 Congestion in the multi-hop wireless world Assume, for simplicity, a single disk model for transmission and interference range C 1 =f(,r 3,r 4,r 5, ) f 4, r 4 f 1, r 1 f 3, r 3 S l 1,C 1 R f 2, r 2 f 5, r 5 5 An example 6 University of Southern California 3

4 Outline AIMD basics and TCP What s the cause of trouble and how to fix it Congestion is a neighborhood, not a single-link affair Simulation results Experimental results fixed AIMD scheme: evolution of TCP or a new protocol? 7 AIMD basics and TCP AIMD basics: r f : rate of flow f upon congestion r f = r f / β react again to congestion after t MD else r f = r f + α every t AI TCP: recall that r = W / RTT β = 2 r f = r f / 2 α = 1 / RTT r f = r f + 1 / RTT t MD = t AI = RTT 8 University of Southern California 4

5 What is wrong with TCP Congestion is a neighborhood, not a single link affair Define what is the neighborhood of a link Translate the basic AIMD features in the context of a neighborhood-centric world 9 Neighborhood of a link Neighbors (overhearing) All incoming and outgoing links from the sender, receiver, all the onehop neighbors of sender, and all the one-hop neighbors of receiver 10 University of Southern California 5

6 Neighborhood-centric world In a neighborhood-centric world we want to share information within the neighborhood: 1. Congestion notifications 2. Clocking information, i.e. t MD and t AI Congestion sharing is conceptually easy upon congestion inform neighboring nodes/links neighbors inform flows that traverse them sources react to congestion Sharing clocking information is a bit more involved 11 From flow RTT to neighborhood RTT TCP clocks window increases and decreases with each flow s RTT silent adaptation to network dynamics: as congestion increases, rate of increase reduces We want to do the same at a neighborhood level use the same value for all flows traversing a neighborhood One option: let RTT i j be the max RTT among all RTTs of flows traversing link i j let RTT i jn be the max RTT among all RTT i j s of links in the neighborhood of link i j use this neighborhood RTT 12 University of Southern California 6

7 Simulation setup Qualnet-based simulations b MAC with default parameters 11Mbps, 512bytes DATA packets Zero channel loses packet loses due to collision and interference do occur Buffer size: 64 packets 4 qualitatively different topologies Schemes: TCP with SACK and ECN Max-min optimal rate allocation Neighborhood-centric AIMD-scheme (which shares congestion and clocking information within a neighborhood) 13 Stack topology (flow in the middle) How to find if a link is congested number of links it interferes with number of asymmetric links S1 R1 S2 R2 14 University of Southern California 7

8 Stack topology (flow in the middle) TCP starves middle flow Sharing congestion within the neighborhood of link 4 5 improves performance dramatically nodes 2 and 8 react to congestion at link Diamond topology Congested links/neighborhoods are different from stack TCP again starves flows that traverse congested links Sharing congestion within the neighborhoods yields better performance flow in the middle traverses both congested neighborhoods result implies proportional fairness type of behavior 16 University of Southern California 8

9 Half-diamond topology TCP does well here Neighborhood AIMD is slightly worse because it is not particularly optimized 17 Chain-cross topology TCP again starves flows going through congested links Sharing congestion information within the neighborhoods yields close to optimal performance 18 University of Southern California 9

10 Experimental setup mini-pcs running Click and Linux ICOP ebox-3854 wireless cards running the madwifi driver Senao NMP-8602 one omni directional antenna per mini-pc, 18dBm transmission power some antennas covered with aluminum to reduce transmission range 19 Experimental setup (cont.) Parameters (radio rates, packet sizes, etc.) same as in simulations 3 qualitatively different topologies two man-made one arbitrary topology of 15 nodes All experiments performed at night-time for repeatability external interference limited but present 20 University of Southern California 10

11 Stack topology 3 nodes at each floor in 3 consecutive floors Experiments in line with simulations Note: Max-min optimal (computed using brute-force) does not yield equal rates because external interference is different for each flow depending on the building floor 21 Chain-cross topology Same behavior as before 22 University of Southern California 11

12 Arbitrary topology Similar behavior as before Note: brute-force computation of max-min rates harder to do here 23 Effect on application layer Consider the stack topology HTTP connection: download a number of pictures totaling 900K Three cases 1. no other flows 2. one FTP flow on two FTP flows, one on and one on Under case 3. the connection timeouts (no completion even after 10min) show videos 24 University of Southern California 12

13 Evolution or a new scheme? Summarize: TCP can be spectacularly bad over scheduled multihop networks Translating key AIMD features to a neighborhood-centric world can fix the problem Required changes could be retrofitted into TCP but are rather major Neighborhood-centric designs can be viewed as evolution of AIMD schemes to account for complex interference but not as evolution of TCP 25 University of Southern California 13