1. Rate Adaptation Protocol for Real-time Streams Goal: develop an end-to-end TCP-friendly RAP for semi-reliable rate-based applications (e.g. playback of real-time streams) RAP employs an additive-increase, multiplicative- decrease (AIMD) algorithm with implicit loss feedback to control congestion RAP separates congestion control from error control RAP is fair as long as TCP operates in a predictable AIMD mode Fine-grain rate adaptation extends range of fairness RED enhances fairness between TCP and RAP traffic RAP does not exhibit inherent instability

2. The RAP Protocol RAP is implemented at source host Each ACK packet contains sequence number of corresponding delivered data packet From ACKs, RAP source can detect losses and sample RTT Decision Function: if no congestion detected, periodically increase rate if congestion detected, immediately decrease rate Congestion detected through timeouts, and gaps in sequence space Timeout calculated based on Jacobson/Karel algorithm using RTT estimate (SRTT)

3. Decision Function (cont’d) RAP couples timer-based loss detection to packet transmission - before sending a new packet, source checks for a potential timeout among outstanding packets using most recent SRTT A packet is considered lost if an ACK implies delivery of 3 packets after the missing one (cf. fast recovery) RAP provides robustness against ACK losses by adding redundancy to ACK packets

4. Increase/Decrease Algorithm In absence of packet loss, increase rate additively in a step-like fashion Upon detecting congestion, decrease rate multiplicatively Rate controlled by adjusting inter-packet gap (IPG)

5. Decision Frequency RAP adjusts IPG once every SRTT If rate is increased by one packet, then slope of rate is inversely related to the square of SRTT (cf. linear increase of TCP) RAP emulates the coarse-grain rate adjustment of TCP RAP is unfair to flows with longer RTT as TCP

6. Clustered Losses Right after loss of first packet, loss of following outstanding packets are silently ignored (cf. TCP- SACK) Cluster-loss-mode terminated as soon as ACK for a packet after that cluster is received

7. Fine-Grain Rate Adaption Goal: make RAP more stable and responsive to transient congestion Emulate a degree of congestion avoidance that TCP obtains due to ack-clocking (self-limiting) During a given step, multiply IPG by ratio of short- term average RTT to long-term average RTT

8. RED A TCP flow suffers if it experiences multiple losses within a window RAP always follows AIMD and reacts only to first loss in an RTT RED limits divergence of TCP from AIMD

9. Simulations RAP is in general TCP-friendly, even without fine- grain rate adaptation The more TCP diverges from AIMD, the less bandwidth it obtains RAP compared to TCP-SACK to avoid TCP’s inherent performance problems Measure inter-protocol fairness: ratio of average RAP bandwidth to average TCP bandwidth Resources scaled linearly with number of flows to maintain share per flow fixed and operate TCP in its well-behaved mode Varying number of flows and RTT

10. Simulations (cont’d) For a wide range of RTT, increasing number of flows improves fairness (ratio converges to 1) Not true for small RTT (or small pipe size) due to the small size of TCP’s congestion window Fine-grain rate adaptation prevents RAP flows from overshooting the available bandwidth share Thus, reducing loss for TCP flows and improving fairness RED, if configured correctly (i.e. maxP not too large or too small), improves fairness between RAP and TCP