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)

Networks Lab, Rensselaer Polytechnic Institute 2 Overall Motivation q TCP response to errors and congestion is the same: q drop the window, and thus reduce load on the network q In the worst case, timeout when particular sequence of packets get lost (retransmits, entire window) q TCP was designed for congestion, loss rate in the 1-2% max. range. q TCP suffers significant timeout penalties with erasure rates > 5%. 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 Wireless links: q individual links can experience loss that can be high (even 10-15%) in transient situations, until power and link rate adjustments kick in q interference can also result in high loss rates. q E.g., ad-hoc networks, Mesh networks.

Networks Lab, Rensselaer Polytechnic Institute 3 Approach q Tools available to us: q Method of getting congestion indication that is separate from packet loss due to errors: Explicit Congestion Notification (ECN) q Use error recovery methods beyond retransmission and timeouts to overcome packet loss, so that TCP’s performance is retained. q Use FEC on an end-end basis: q Dynamic knowledge of the loss information can be exploited by the end-system. q Track short term loss rates. q Protect data by using FEC proactively and reactively. q FEC can work in a coordinated fashion with TCP’s window mechanisms to optimize the usage of FEC within a window (which is not available at the link level).

Networks Lab, Rensselaer Polytechnic Institute 4 Goals We pose the following questions.. q Dynamic Range: q Can we extend the dynamic range of TCP into high loss regimes? q Can TCP perform close to the theoretical capacity achievable under high loss rates? q Congestion Response: q How should TCP respond to notifications due to congestion.. q … but not respond to packet erasures that do not signal congestion? q Mix of Reliability Mechanisms: q What mechanisms should be used to extend the operating point of TCP into loss rates from 0% - 50 % packet loss rate? q How can Forward Error Correction (FEC) help? q How should the FEC be split between sending it proactively (insuring the data in anticipation of loss) and reactively (sending FEC in response to a loss)? q Timeout Avoidance: q Timeouts: Useful as a fall-back mechanism but wasteful otherwise especially under high loss rates. q How can we add mechanisms to minimize timeouts?

Networks Lab, Rensselaer Polytechnic Institute 5 SENDERSENDER RECEIVERRECEIVER Available Capacity Loss Feedback Through Acknowledgements Capacity Used TCP uses Loss Feedback to Estimate Available Capacity 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 6 Building Blocks… q ECN-Only: We infer congestion solely from ECN markings. Window is cut in response to q ECN signals: which means that hosts/routers have to be ECN-capable. q Timeouts: The response to a timeout is the same as before. q Window Granulation and Adaptive MSS: We ensure that the window always has at least G segments at all times. q Window size in bytes initially is the same as normal SACK TCP. q Initial segment size is small to accommodate G segments. q Packet size is continually so that we have at least G segments. Once we have G segments, packet size increases with window size. q Loss Estimation: The receiver continually tracks the loss rate and provides a running estimate of perceived loss back to the TCP sender through ACKs. An adaptive EWMA approach to estimating loss is used.

Networks Lab, Rensselaer Polytechnic Institute 7 Building Blocks … q Proactive FEC: TCP sender sends data in blocks where the block contains K data segments and R FEC packets. The amount of FEC protection (K) is determined by the current loss estimate. q Proactive FEC based upon estimate of per-window loss rate (Adaptive) q Reactive FEC: Upon receipt of 1 or 2 dupacks, Reactive FEC packets are sent based on the following criteria. q Number of Proactive FEC packets already sent. q Number of holes still left in the decoding block. q Loss rate currently estimated. q Reactive FEC to complement retransmissions

Networks Lab, Rensselaer Polytechnic Institute 8 Proactive and Reactive FEC in Action..

Networks Lab, Rensselaer Polytechnic Institute 9 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 Block Behavior: Per-Block Loss Estimator for P-FEC Estimation is done at receiver and fed-back to the sender

Networks Lab, Rensselaer Polytechnic Institute 10 Loss Tracking at Sender Sender can quickly and accurately track the loss rate based on feedback from the receiver % 0% 20% 20% 30% 0% 20% 10% ( Time ) Packet Error Rate

Networks Lab, Rensselaer Polytechnic Institute 11 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! Recovery possible if we receive at least K packets out of N

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

Networks Lab, Rensselaer Polytechnic Institute 13 1 Window XXXX Transmission Loss ACK Stream 654 Rexmins 3 Window Granulation Reduces the Risk of Losing the Complete Window

Networks Lab, Rensselaer Polytechnic Institute 14 1 XX Transmission Loss Window ACK Stream 3 DUPACK-1 Timeout because of insufficient dupacks 3 Timout Cause #2: Insufficient Dupacks => SACK not triggered X

Networks Lab, Rensselaer Polytechnic Institute 15 1 XX Transmission Loss P-FEC 4321 Window P-FEC Receiver FEC Decoder P-FEC Proactive FEC Recover data packets…

Networks Lab, Rensselaer Polytechnic Institute 16 3 Window Transmission Loss XX ACK Stream 2 DUPACK 3 22 DUPACK 1 DUPACK 2 2 Retransmission X Transmission Loss ReXMITS ESPECIALLY vulnerable! Timeout Cause #3: Loss of Retransmissions

Networks Lab, Rensselaer Polytechnic Institute 17 1 XX Transmission Loss Window ACK Stream 6 DUPACK 3 54 DUPACK 1 DUPACK 2 R-FEC Receiver FEC Decoder R-FEC Reactive FEC: Complements Rexmits Selective Acknowledgements

Networks Lab, Rensselaer Polytechnic Institute 18 Loss Estimate FEC Computation (n,k) Loss Estimation MSS Adaptation Granulated Window Size Window Size Application Data P-FEC Data Window Putting it Together….

Networks Lab, Rensselaer Polytechnic Institute 19 Simulation Configuration

Networks Lab, Rensselaer Polytechnic Institute 20 Performance Results SACK (Multiple Sources) LT-TCP (Multiple Sources)

Networks Lab, Rensselaer Polytechnic Institute 21 Contribution of Components (20% PER case (Single Source)  LT-TCP is able to  reduce timeouts drastically  keep the queue non-empty maximizing throughput and capacity utilization.  minimize use of FEC to level needed

Networks Lab, Rensselaer Polytechnic Institute 22 Comparison w/ Link Layer FEC, HARQ LL FEC: FEC based upon average PER HARQ: 10% FEC; ARQ persistence = 3 LT-TCP: end-to-end

Networks Lab, Rensselaer Polytechnic Institute 23 Summary q TCP performance over wireless with residual erasure rates 0- 50% (short- or long-term). q E2E FEC: 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 used to distinguish congestion from loss. q Near-optimal performance for wide range: from low to high loss rates. q Future Work: q Optimal division of reliability functions between PHY,MAC, E2E q Study of interaction between LT-TCP and link-layer schemes.

Networks Lab, Rensselaer Polytechnic Institute 24 Thanks! Researchers: Omesh Tickoo: Vijay Subramanian: Shiv Kalyanaraman: K.K. Ramakrishnan,

Networks Lab, Rensselaer Polytechnic Institute 25 Building Block Behavior: Adaptive MSS (Window Granulation) q Adaptive MSS behavior. q Congestion window (in segments) kept above G = 10 q MSS increases when CWND grows, q MSS shrinks when CWND shrinks to maintain G

Networks Lab, Rensselaer Polytechnic Institute 26 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 27 Performance Results.. LT-TCP (Single Source)  At 50 % error rate, timeouts increase drastically because..  Few Proactive FEC packets received.  Proactive FEC cannot counter variation in error patterns.  Reactive FEC is insufficient in this case to avoid timeouts.  This effect can be mitigated by increasing FEC protection. Drop in Performance from 40 to 50 %

Networks Lab, Rensselaer Polytechnic Institute 28 Changes w.r.t. submitted paper q FEC is now done on a block-by-block basis. q Proactive protection is determined solely by the loss estimate. (no arbitrary constants) q Reactive FEC packets may be wasted if they belong to the wrong block. q Conditions under which Reactive FEC packets are sent are restricted (discussed earlier). q Window granulation is done using the following rule q “Send as big a packet as possible while maintaining granularity” q Throughput and goodput are measured at the receiver for better accuracy. q On partial dupacks, we make sure that retransmission are not duplicated. We send new TCP data instead. q Loss tracking is now done whenever we receive an ACK. q Loss estimation at receiver has changed to accommodate block-by-block decoding.