Networks Lab, Rensselaer Polytechnic Institute 1 LT-TCP: End-to-End Framework to Improve TCP Performance over Networks with Lossy Channels Omesh Tickoo, Vijay Subramanian,Shiv Kalyanaraman (Rensselaer Polytechnic Institute) K.K. Ramakrishnan (AT&T) Status Reports Packets, FEC Repairs Timeout

Networks Lab, Rensselaer Polytechnic Institute 2 Overview q TCP performance over wireless: q loss vs. congestion, heavy erasures q Building Blocks: q ECN congestion response q Adaptive maximum segment size (MSS) q Proactive and Reactive FEC q Performance Results: q Contribution of each building block q Comparisons to link-level support q Ongoing work

Networks Lab, Rensselaer Polytechnic Institute 3 TCP over wireless channels q TCP doesn’t distinguish between erasure and congestion loss q Bigger problem: TCP suffers significant timeout penalties with erasure rates > 5% q Qn: Can we “robustify” TCP to handle larger packet erasure rates: 30-50% ? q Wireless channels becoming more pervasive q With mesh networks (infrastructure or community) it is likely that more than the last hop will be wireless q Cannot just use the cross-layer techniques like TCP performance enhancing proxies (PEPs) to “fix” TCP’s performance

Networks Lab, Rensselaer Polytechnic Institute 4 SENDERSENDER RECEIVERRECEIVER Available Capacity Loss Feedback Through Acknowledgements Capacity Used TCP uses Loss Feedback to Estimate Available Capacity Capacity Used Interpreting Transmission Losses as Congestion Leads to Capacity Under-Estimation Capacity Used Erasure Recovery/ Loss Estimation Adaptive MSS/ Proactive and Reactive FEC LT-TCP: Adaptive Mechanisms to Reinstate Performance XX X – Packet Erasure

Networks Lab, Rensselaer Polytechnic Institute 5 Transport/Link Layer: Standard Reliability Model Packets Sequence Numbers CRC or Checksum Proactive FEC Status Reports ACKs NAKs, dupacks SACKs Bitmaps Rexmitted Packets Reactive FEC Repair pkts Timeout

Networks Lab, Rensselaer Polytechnic Institute 6 Reed-Solomon FEC: RS(N,K) Data = K FEC (N-K) Block Size (N) RS(N,K) >= K of N received Lossy Network Recover K data packets!

Networks Lab, Rensselaer Polytechnic Institute 7 Building Blocks: Goals q Congestion Response: q How should TCP respond to congestion notifications, q … but not respond to packet erasures that do not signal congestion? q Mix of Reliability Mechanisms: q What mix of TCP repair mechanisms should be used to achieve the TCP reliability objectives ? q What is the role of error correction (FEC) ? q How should be split between proactive and reactive repair? q Timeouts: q Timeouts: final fallback mechanism, but wasteful otherwise. q How to structure the mix to reduce timeouts?

Networks Lab, Rensselaer Polytechnic Institute 8 Building Blocks: Design q Congestion: respond only to ECN q Assumes ECN-enabled networks q Window granulation: at least G q Smaller PER for a given BER q More dupacks per burst loss event (SACK requires at least 3 dupacks) q MSS is adaptive. q FEC per-window: q Shortened RS-codes (see next slide) q Proactive FEC based upon estimate of per-window loss rate (Adaptive) q Reactive FEC to protect retransmissions q FEC packets can correct any data packets: totally K out of N needs to reach receiver q Timeout avoidance turns out to be harder than distinguishing erasure from congestion

Networks Lab, Rensselaer Polytechnic Institute 9 Shortened Reed Solomon FEC (per-Window) Proactive FEC (F) Data = d Window (W) Reactive FEC (R) Zeros (z) Block Size (N) K = d + z d z RS(N,K)

Networks Lab, Rensselaer Polytechnic Institute Complete Window Lost! Window XXXX Transmission Loss Timout Cause #1: Burst Errors + Large MSS

Networks Lab, Rensselaer Polytechnic Institute 11 1 Window XXXX Transmission Loss ACK Stream (assuming selective ACK) 654 Rexmins 3 Window Granulation Reduces the Risk of Losing the Complete Window

Networks Lab, Rensselaer Polytechnic Institute 12 1 XX Transmission Loss Window ACK Stream 2 DUPACK-1 Timeout because of insufficient dupacks 2 Timout Cause #2: Insufficient Dupacks => SACK not triggered X

Networks Lab, Rensselaer Polytechnic Institute 13 1 XX Transmission Loss P-FEC 4321 Window P-FEC Receiver FEC Decoder P-FEC Proactive FEC Reduces the Need for Dupacks Recover data packets…

Networks Lab, Rensselaer Polytechnic Institute 14 3 Window Transmission Loss XX ACK Stream 1 DUPACK-3 11 DUPACK-1DUPACK-2 2 Retransmission X Transmission Loss ReXMITS ESPECIALLY vulnerable! Timeout Cause #3: Loss of Retransmissions

Networks Lab, Rensselaer Polytechnic Institute 15 1 XX Transmissio n Loss Window ACK Stream 6 DUPACK-3 54 DUPACK-1DUPACK-2 R-FEC Receiver FEC Decoder R-FEC Reactive FEC: Protects Rexmits

Networks Lab, Rensselaer Polytechnic Institute 16 Loss Estimate FEC Computation (n,k) Parameter Estimation MSS Adaptation Granulated Window Size Window Size Application Data P-FEC Data Window Putting it Together….

Networks Lab, Rensselaer Polytechnic Institute 17 Building Block Behavior: Adaptive MSS (Window Granulation)  Congestion window (in segments) kept above G = 10  MSS increases when CWND grows,  MSS shrinks when CWND shrinks to maintain G

Networks Lab, Rensselaer Polytechnic Institute 18 Packet Erasure Rate EWMA Estimator: E =  *E latest + (1-  )*E Estimate is fairly accurate within small erasure rate variations Overestimate after spikes :  = E latest / (E latest + E) Trade off :Over- estimation leads to overhead. Overestimate Inefficiency Period BBlock Behavior: Per-Window Loss Estimator for P-FEC

Networks Lab, Rensselaer Polytechnic Institute 19 Simulation Configuration Lumped model: view multiple bottlenecks as 1 aggregate bottleneck

Networks Lab, Rensselaer Polytechnic Institute 20 Performance Results SACK LT-TCP:

Networks Lab, Rensselaer Polytechnic Institute 21 Performance Results (Contd)

Networks Lab, Rensselaer Polytechnic Institute 22 Contribution of Components (10% PER case) Source of gains: #1: Timeout reduction #2: Distinguishing erasures from congestion (w/ ECN)

Networks Lab, Rensselaer Polytechnic Institute 23 Comparison w/ Link Layer FEC, HARQ LL FEC: adaptive FEC based upon average PER HARQ: 10% FEC; ARQ persistence = 3 LT-TCP: end-to-end

Networks Lab, Rensselaer Polytechnic Institute 24 Summary TCP performance over wireless: residual erasure rates > 5% (short- or long-term) q E2E H-ARQ: q Granulation ensures better flow of ACKs especially in small window regime. q Adaptive FEC (proactive and reactive) can protect critical packets appropriately q Adaptive => No overhead when there is no loss q ECN to distinguish congestion from loss q Future Work: q Handle higher erasure rates (30%+) better q Optimal division of reliability functions between PHY,MAC, E2E

Networks Lab, Rensselaer Polytechnic Institute 25 Thanks! Researchers: Omesh Tickoo: Vijay Subramanian: Shiv Kalyanaraman: K.K. Ramakrishnan,