An End-to-End Transport Protocol for Extreme Wireless Network Environments

1. An End-to-End Transport Protocol for Extreme Wireless Network Environments TCP Receiver TCP Sender Packets, FEC Status Reports Repairs Vijay Subramanian, Shiv Kalyanaraman (Rensselaer Polytechnic Institute) K. K. Ramakrishnan (AT&T) Networks Lab, RPI

2. Overall Motivation • TCP response to errors and congestion is the same: • drop the window, and thus reduce load on the network • In the worst case, timeout when particular sequence of packets get lost (retransmits, entire window) • TCP was designed for congestion, loss rate in the 1-2% max. range. • TCP suffers significant timeout penalties with erasure rates > 5%. • Wireless channels becoming more pervasive • With mesh networks (infrastructure or community) it is likely that more than the last hop will be wireless. • Wireless links: • individual links can experience loss that can be high (even 10-15%) in transient situations, until power and link rate adjustments kick in • interference can also result in high loss rates. • E.g., ad-hoc networks, Mesh network, WiLAN.

3. Performance of TCP-SACK • TCP-SACK Performance degrades beyond an error rate of 5% PER. • Performance is also sensitive to RTT .

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

5. TCP uses Loss Feedback to Estimate Available Capacity LT-TCP: Adaptive Mechanisms to Reinstate Performance Adaptive MSS/ Proactive and Reactive FEC Erasure Recovery/ Loss Estimation Capacity Used Capacity Used Available Capacity RECEIVER SENDER X X Loss Feedback Through Acknowledgements X– Packet Erasure

6. Approach • Tools available to us: • Method of getting congestion indication that is separate from packet loss due to errors: Explicit Congestion Notification (ECN) • Use error recovery methods beyond retransmission and timeouts to overcome packet loss, so that TCP’s performance is retained. • Use FEC on an end-end basis: • Dynamic knowledge of the loss information can be exploited by the end-system. • Track short term loss rates. • Protect data by using FEC proactively and reactively. • 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).

7. Building Blocks … • ECN-Only: We infer congestion solely from ECN markings. Window is cut in response to • ECN signals: which means that hosts/routers have to be ECN-capable. • Timeouts: The response to a timeout is the same as before. • Window Granulation and Adaptive MSS: We ensure that the window always has at least G segments at all times. • Window size in bytes initially is the same as normal SACK TCP. • Initial segment size is small to accommodate G segments. • Packet size is continually changed so that we have at least G segments. Once we have G segments, packet size increases with window size. • 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 EWMAapproach to estimating loss is used.

8. Building Blocks … • 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. • Proactive FEC based upon estimate of per-window loss rate (Adaptive) • Reactive FEC: Reactive FEC to complement retransmissions. • Upon receipt of 1 or 2 dupacks, Reactive FEC packets are sent based on the following criteria. • Number of Proactive FEC packets already sent. • Number of holes still left in the decoding block. • Loss rate currently estimated.

9. Proactive and Reactive FEC in Action.. • Data + PFEC are sent in the initial transmission. • Feed back from the receiver is used to determine strength of RFEC protection. • SACK retransmissions along with RFEC packets are used to recover the original data.

10. Recover K data packets! >= K of N received RS(N,K) FEC (N-K) Block Size (N) Lossy Network Data = K Reed-Solomon FEC: RS(N,K) Recovery possible if we receive at least K packets out of N

11. LT-TCP Big Picture

12. Simulation Setup

13. LT-TCP Performance • Performance of LT-TCP is much better compared to that of TCP-SACK • LT-TCP degrades gracefully (linear fall) • Relative insensitivity to RTT variation.

14. LT-TCP and TCP-SACK Performance • LT-TCP performance is good both with Uniform with Gilbert Loss Process. • Gilbert Loss Process • Error Rate toggles between 0.5p and 1.5p for an average PER of p. • Sojourn time is randomized around a mean period.

15. LT-TCP Component Contributions (Goodput) • Individual component contributions are shown. • Proactive FEC has the most significant impact.

16. LT-TCP Component Contributions (Timeout) • Contributions of components in reducing the incidences of timeouts is shown. • RFEC and PFEC are needed to reduce timeouts. • With just TCP-SACK and ECN schemes, timeouts are repeated and large though few in number.

17. LT-TCP Component Contributions (Cumulative Goodput) • Cumulative goodput for a representative pair of flows (1 TCP-SACK and 1 LT-TCP) are shown out of 10 flows total.

18. Fairness Comparisons • Instantaneous goodput for a representative pair of flows (1 TCP-SACK and 1 LT-TCP) are shown out of 10 flows total. • The goodput was measured in intervals of 100ms.

19. Comparison of Congestion Windows • Error free simulation with 5 TCP-SACK and 5 LT-TCP flows except for a 100ms period at t=50s with PER set to 50%. • Congestion Window for TCP-SACK is as shown • Following a timeout, TCP-SACK recovers quickly and regains its fair share of the bandwidth. • It does not get beaten down by LT-TCP’s behavior.

20. LT-TCP Congestion Window • Congestion Window for LT-TCP is as shown • LT-TCP suffers a loss event during the loss period but does not suffer a timeout.

21. Summary • LT-TCP provides robustness even under conditions of large and bursty loss rates. • Avoids timeouts • High Goodput • Increased Dynamic Range • Current and future work includes link-level enhancements that provide bounded delay, low residual loss rate and high goodput even under disruption scenarios.

22. Thanks! • Researchers: • Vijay Subramanian: • subrav@rpi.edu(Rensselaer Polytechnic Institute) • Shivkumar Kalyanaraman: • shivkuma@rpi.edu(Rensselaer Polytechnic Institute) • K.K. Ramakrishnan, • kkrama@research.att.com(AT&T Labs Research)