Multi-Algorithm Bitrate Control Support

[andrescera]

Overview

Add support for multiple bitrate control ("balancing") algorithms at runtime.

Motivation

The current RTT + buffer heuristic is battle-tested but may not suit all network types. Different scenarios benefit from different algorithms:

• Stable networks → Fixed bitrate
• Variable mobile networks → BBR-style probing
• Ultra-low latency → Latency-target controller

Proposed CLI Interface

belacoder pipeline.txt host 4000 --balancer=rtt_buffer  # Current (default)
belacoder pipeline.txt host 4000 --balancer=aimd
belacoder pipeline.txt host 4000 --balancer=latency_target --rtt-target=150
belacoder pipeline.txt host 4000 --balancer=fixed --fixed-bitrate=4000000

Controller Interface

typedef struct {
    const char *name;
    void* (*init)(BalancerConfig *config);
    BalancerOutput (*step)(void *state, BalancerInput *input);
    void (*cleanup)(void *state);
} BalancerAlgorithm;

Candidate Algorithms

Algorithm	Complexity	Best For
RTT+Buffer (current)	Low	General use
AIMD	Low	Fair sharing
Fixed/Ladder	Minimal	Stable networks
Latency-Target	Medium	Ultra-low latency
BBR-style	High	Variable networks
PID	Medium	Fine control

Implementation Tasks

Phase 1: Interface

• Define BalancerAlgorithm interface
• Refactor current algorithm to use interface
• Add --balancer=<name> CLI flag

Phase 2: Basic Algorithms

• Add fixed mode (trivial, useful for testing)
• Add aimd mode (simple alternative)

Phase 3: Advanced

• Add latency_target mode
• Add config file support for algorithm parameters
• Consider BBR-style for advanced users

Metrics Available to Algorithms

Metric	Source	Frequency
RTT (ms)	srt_bstats()	Every 20 ms
Send buffer (packets)	SRTO_SNDDATA	Every 20 ms
Throughput (bps)	mbpsSendRate	Every 20 ms
Negotiated latency	SRTO_PEERLATENCY	Once at connect

[andrescera]

Completed in PR #6

Implemented Features

✅ Phase 1: Interface

• Defined BalancerAlgorithm interface in balancer.h
• Refactored current algorithm to use interface
• Added -a <algorithm> CLI flag

✅ Phase 2: Basic Algorithms

• Added fixed mode (constant bitrate)
• Added aimd mode (TCP-style AIMD)

✅ Phase 3: Partial

• Added config file support for algorithm parameters (-c flag)
• Algorithm tuning via [adaptive] and [aimd] sections

Available Algorithms

./belacoder -a adaptive pipeline.txt host 4000  # Default
./belacoder -a fixed pipeline.txt host 4000
./belacoder -a aimd pipeline.txt host 4000

Config File Example

[adaptive]
incr_step = 30        # Kbps
decr_step = 100       # Kbps
incr_interval = 500   # ms
decr_interval = 200   # ms

[aimd]
incr_step = 50        # Kbps
decr_mult = 0.75      # Reduce to 75%

Not Implemented (deferred)

• latency_target mode (complex edge cases with high-latency networks)
• BBR-style probing (high complexity)

[andrescera]

Implementation complete in PR #7

All Phase 1 and Phase 2 tasks are now complete:

✅ Phase 1: Interface

• BalancerAlgorithm interface defined in src/balancer.h
• Current algorithm refactored to use interface
• --balancer=<name> CLI flag implemented

✅ Phase 2: Basic Algorithms

• fixed mode implemented
• aimd mode implemented

✅ Bonus

• Config file support for algorithm parameters (belacoder.conf)
• Packet loss detection added to adaptive algorithm
• 16 integration tests covering all algorithms

The latency_target and BBR-style algorithms remain as potential future work.