Medium Start in TCP-Friendly Rate Control Protocol CS 217 Class Project Spring 04 Peter Leong & Michael Welch

TFRC Overview Congestion control scheme (CCID3) for transport layer protocols Mostly used for unreliable data flows Competes fairly with TCP for bandwidth Lower throughput variation over time than TCP Streaming multimedia Telephony data RFC 3448

TFRC Overview (cont) Streaming data are sent using UDP without congestion control Results an unfair sharing of the network bandwidth Transports without congestion control mechanisms run a higher risk of being selectively dropped by a router Provides feedback to sender about network condition

TFRC Algorithm The receiver measures the loss event rate and feeds this information back to the sender. The sender also uses these feedback messages to measure the round-trip time (RTT). The loss event rate and RTT are then fed into TFRC's throughput equation, giving the acceptable transmit rate. The sender then adjusts its transmit rate to match the calculated rate.

TFRC Algorithm -- Handling of idle periods Sender has a nofeedback timer Receiving a feedback packet resets the nofeedback timer to expire after 2*RTT Nofeedback timer expiration triggers halving of the send rate and timer reset e.g. An idle period of 10*RTT = halving send rate 5 times Exponential decrease of window size

TFRC Algorithm -- Handling of idle periods (cont) If the sender subsequently starts to send again, a normal slowstart phase will occur until the transmit rate reaches calculated fair rate. An idle period of several RTTs is not uncommon for voice-over-IP data transmission This means we often find ourselves sending data in slowstart (below our last measured fair rate) In general, we slowstart because we do not know current network conditions

Medium Start Assuming that fair rate is a good indicator of the actual available bandwidth within a reasonable time period, can we do better? With TFRC, we have a recent sample of the network through our fair rate calculation We can use this information to slowstart faster  Medium Start

Medium Start Algorithm Set T_idle := now – T_last_send F := (30 min - min(max(T_idle, 10 min), 30 min)) / 20 min Set ssthresh := F*rcvrate Set rate := max(4*size/RTT, rate) Set ssmult = 4 Perform traditional slowstart algorithm, including backoff mechanisms

Medium Start Evaluation We implemented Medium Start in ns-2 Compared achieved sending rate of TFRC with Medium Start vs. Stock TFRC Varied the loss rate, competing traffic type and density, and the source sending model. Results:

Evaluation Topology Node 0 : TFRC Sender Node 1 : Competing traffic source Node 2 : Router Node 3 : TFRC Receiver Links are 10Mb Losses occur on the 2-3 link only Router is DropTail Packetsize = 500 bytes

Evaluation 1 CBR sender No competing traffic 0.1 Mbps sending rate Link loss 15% X axis is time Y axis is sending rate in bytes

Evaluation 1 - Stock TFRC

Evaluation 1 – Medium Start

Evaluation 2 Exponentially distributed sender Mean burst time = 10s Mean idle time = 10s No competing traffic 0.1 Mbps burst sending rate Link loss 15% X axis is time Y axis is sending rate in bytes

Evaluation 2 – Stock TFRC

Evaluation 2 – Medium Start

Evaluation 3 Exponentially distributed sender Mean burst time = 10s Mean idle time = 10s 1 Mbps burst sending rate Exponentially distributed competing traffic Mean burst time = 5s Mean idle time = 5s 5 Mbps burst sending rate Link loss 10% X axis is time Y axis is sending rate in bytes

Evaluation 3 – Stock TFRC

Evaluation 3 – Medium Start

Benefits of Medium Start Utilizes recent measurements to reach our fair sending rate faster While still performing some conservative probing Allows higher fidelity for streaming media applications Allows higher utilization of the channel

Drawbacks of Medium Start Most test results show an increase in loss rate compared to Stock TFRC Relies on stable network dynamics May cause competing flows to experience additional loss  less TCP Friendly?

Problems and Open Issues ns-2 implementation of TFRC seems to deviate from the RFC Simulation does not allow us to perceive the improvement Medium Start may have on streaming multimedia and internet telephony An actual implementation would allow us to tweak the algorithm’s parameters through experimentation