TCP in Wireless Ad Hoc Networks 2009

Summary: TCP Congestion Control When CongWin is below Threshold, sender in slow-start phase, window grows exponentially. When CongWin is above Threshold, sender is in congestion-avoidance phase, window grows linearly. AIMD When a triple duplicate ACK occurs, Threshold set to CongWin/2 and CongWin set to Threshold. AIMD When timeout occurs, Threshold set to CongWin/2 and CongWin is set to 1 MSS.

TCP Problem over Wireless Networks In TCP Packet Loss <=> Congestion But in Mobile Networks Packet Loss <=> Congestion (Net) ??? high bit error rate (Phy) - No access contention (MAC) - ? disconnection (Net) - No handoff (Net) - No The TCP end-to-end performance is degraded seriously in wireless networks

Problems with TCP in ad hoc networks Multihop - throughput reduction mobility - path breaks and forces TCP to timeout Random interference/jamming causes packet loss => timeout Source cannot discriminate between congestion loss and random loss => drive TCP window to zero! Interaction between TCP backoff and MAC backoff may cause unfairness and “capture”

Impact of Multi-Hop Wireless Paths TCP Throughput using 2 Mbps 802.11 MAC, transmission range = one hop For large number of hops throughput stabilizes (pipelining effect)

Throughput Degradations with Increasing Number of Hops Packet transmission can occur on at most one hop among three consecutive hops Increasing the number of hops from 1 to 2, 3 results in increased delay, and decreased throughput Increasing number of hops beyond 3 allows simultaneous transmissions on more than one link, however, degradation continues due to contention between TCP Data and Acks traveling in opposite directions When number of hops is large enough, the throughput stabilizes due to effective pipelining

Impact of Mobility on TCP Mobility causes route changes Throughput generally degrades with increasing speed … Ideal Average throughput over 50 runs Actual Speed (m/s)

How to Improve Throughput Network feedback Inform TCP of route failure by explicit message Let TCP know when route is repaired Probing (eg, persistent pkt retransmissions) Explicit link repair notification Alleviates repeated TCP timeouts and backoff

Performance with Explicit Notification

TCP-over-wireless End-to-end approaches Purely Sender-centric: TCP-Westwood, TCP-SPC Involving both sender and receiver: WTCP Purely Receiver-centric: e.g. RCP, WebTP Split-Connection/Base-Station Oriented e.g. Indirect-TCP, Snoop etc. Link-layer approaches Reliable link layer Misc. e.g. Loss Discrimination using AQM, Explicit Notification etc.

TCP Westwood: Efficient Transport for High-speed wired/wireless Networks (Mobicom 2001)

TCP Westwood (Mobicom 2001) Key Idea: Enhance congestion control via the Rate Estimate (RE) Estimate is computed at the sender by sampling and exponential filtering Samples are determined from ACK inter-arrival times and info in ACKs regarding amounts of bytes delivered RE is used by sender to properly set cwnd and ssthresh after packet loss (indicated by 3 DUPACKs, or Timeout)

Rate Estimation (BE-> RE) Receiver Sender Internet Bottleneck packets ACKs measure Ideally, would like to determine the connection fair share of the bottleneck bandwidth Since fair share is difficult (to define or determine), we instead estimate the achieved rate: Rate Estimate (RE)

“Original” Rate estimation (BE-> RE) tk-1 tk dk First TCPW version used a “bandwidth like” estimator (BE) given by: sample exponential filter filter gain RE/BE Estimation is similar to Keshav Packet Pair estimation

TCP Westwood: the control algorithm TCPW Algorithm Outline: When three duplicate ACKs are detected: set ssthresh=BE*RTTmin (instead of ssthresh=cwin/2 as in Reno) if (cwin > ssthresh) set cwin=ssthresh When a TIMEOUT expires: set ssthresh=BE*RTTmin (instead of ssthresh=cwnd/2 as in Reno) and cwin=1 Note: RTTmin = min round trip delay experienced by the connection

TCP Westwood Benefits Reno overreacts to random loss (cwin cut by half) TCPW less sensitive to random loss (1) a small fraction of “randomly” lost packets minimally impacts the rate estimate RE (2) Thus, cwin = RE x RTT remains unchanged As a result, TCPW throughput is higher than Reno What do we gain by using RE “feedback” in addition to packet loss? (a) better performance with random loss (ie, loss caused by random errors as opposed to overflow) (b) ability to distinguish random loss from buffer loss (c) using RE to estimate bottleneck bdw during slow start

TCPW in a wireless lossy environment Efficiency: Improvement significant on high (Bdw x Length) paths Fairness: better fairness than RENO under varying RTT Friendliness: TCPW is friendly to TCP Reno

TCPW in presence of random loss: Analysis and Simulation

TCPW Rate Estimation (TCP RE) dk-1 dk tk T is the sample interval T Rate estimate (RE) is obtained by aggregating the data ACKed during the interval T (typically = RTT): sample exponential filter filter gain

TCPW RE or BE interaction with RENO One TCPW RE or BE and one Reno share a 5Mbps bottleneck No errors (bottleneck gets saturated) Errors (0.5%), no congestion fair share* fair share BE overestimates fair rate ( = 2.5 Mbps) TCPW BE Not friendly to NewReno! RE underestimates fair rate (=3.6 Mbps) TCPW RE does not improve thruput! (*) TCPW fair share > 50% because NewReno is incapable of getting 50%

TCPW with adaptive filter (AF) Neither RE or BE estimator are optimal for all situations BE is more effective in random loss RE is more appropriate in congestion loss (ie, buffer overflow) KEY IDEA: dynamically select the aggressive estimate (BE) or the conservative estimate (RE) depending on current channel status (congestion or random loss?) NEEDED: a “congestion measure” that gives us an idea of the most probable cause of packet loss (congestion or random) The Adaptive Filter actually provides a smooth transition from aggressive to conservative measure

Summary Introduced the concept of Rate Estimation and related work Reviewed end-to-end estimation based congestion control methods Presented TCP Westwood, and the evolution of “fair rate” estimate to improve the performance; showed simulation results to evaluate the method Compared TCPW with other methods