Free Cdn

What it means (definitions and thresholds)

Free CDN (in this document) = a third-party caching/networking layer that offers no-cost tiers, or low-cost tiers, for HTTP delivery with limited guarantees on egress, throughput and support. It usually provides edge caches, TLS termination and HTTP acceleration but not enterprise SLAs, comprehensive logs, or guaranteed capacity.

• Scale thresholds (practical):
  • Small: up to 500 concurrent viewers (typical for product demos, small VOD catalogs).
  • Medium: 500–10,000 concurrent viewers. Expect variability: free tiers often throttle or limit egress here.
  • Large: >10,000 concurrent viewers. Use paid CDN or multi-CDN.
• Latency classes (end-to-end, encoder → viewer):
  • Sub-second (0.2–0.8 s): WebRTC or specialized RTP-based stacks; not achievable with standard free-CDN HLS/TLS caches.
  • Low-latency (1–3 s): CMAF chunked / LL-HLS or aggressively tuned DASH; requires packager and player support plus cache-control tuning.
  • Near-real-time (3–10 s): LL-HLS with larger parts or short-segment HLS; feasible with careful tuning on free CDN but fragile at scale.
  • Standard HLS (15–30 s): default HLS with 3–6 segments of 2–6s each; robust across generic CDNs.
• Bandwidth math (quick):
  • Average bitrate per viewer = choose ladder (e.g., 720p ≈ 3 Mbps). For N viewers: egress ≈ N * bitrate. 1,000 viewers at 3 Mbps ≈ 3 Gbps egress.
  • Rule of thumb: free-CDN egress caps or soft limits commonly start to bind between 100 Mbps and a few Gbps — verify provider limits.

Decision guide

Direct, actionable decision steps — follow these sequentially.

1. Classify your workload:
   • VOD vs Live vs Interactive. VOD is most forgiving.
2. Quantify peak concurrency and geography:
   • Calculate peak egress = peak_concurrency * average_bitrate.
   • If peak_e gress > 2 Gbps or viewers are global, treat free CDN as temporary.
3. Check latency requirements:
   • If you need <3 s end-to-end, you must control packager + player + edge TTL. Free CDNs can work for low-latency in single-region, low-concurrency scenarios; for sub-second interactivity, choose WebRTC or managed low-latency pathways.
4. Security / DRM / Geo-control:
   • If you need DRM, signed URLs, or strict geo-blocking, confirm the free tier supports those features — most do not.
5. Make a recommendation:
   • VOD, small audience, single-region → free CDN acceptable with correct headers and versioning.
   • Small live events (concurrency <500) and latency >3 s → free CDN OK with short segments and tight cache control.
   • Monetized live, global audience, high concurrency or required SLA → migrate to a commercial CDN or an integrated worldwide CDN. See product options: Edge CDN, Video API, Live production.

Latency budget / architecture budget

Build a simple latency budget table and keep it visible during design. Below are measured ranges and a template you can use.

• Latency components (typical ranges):
  • Capture & ingest buffer: 10–200 ms (camera, grabber, audio capture).
  • Encoder & pre-processing: 50–400 ms depending on encoder speed and settings (x264 veryfast vs ultrafast).
  • Network (encoder → origin): 20–200 ms (WAN dependent).
  • Packager / segmenter: 50 ms–2 s (depends on part/segment size).
  • CDN edge propagation / cache hit: 20–200 ms for local edge; cache-miss to origin adds origin RTT and origin processing.
  • Player buffer (start-up + safety): 0.5 s–6 s depending on target latency.
• Budget examples:
  • WebRTC target (0.3–0.8 s): capture 50 ms + encode 100–200 ms + network 50–150 ms + player jitter buffer 100–200 ms.
  • Low-latency CMAF/LL-HLS (1.5–3 s): capture 50 ms + encode 150–300 ms + packaging 300–700 ms (parts) + CDN 100 ms + buffer 500–1,000 ms.
  • Standard HLS (15–30 s): packaging dominated by segment length (e.g., 3 seg × 6 s = 18 s) + normal buffer.

Action: create a one-line budget for your application and measure every component. See deeper guidance in our latency doc: Latency budget.

Practical recipes

Three tested recipes for realistic use-cases. Each recipe lists exact settings to try, verification steps and what to watch for on a free CDN.

Recipe A: Small VOD catalog served via a free CDN

1. Encode a standard ABR ladder using H.264 (AVC):
   • 240p: 400 kbps VBR, audio 64 kbps AAC
   • 360p: 750 kbps VBR, audio 64 kbps
   • 480p: 1.2 Mbps VBR, audio 96 kbps
   • 720p: 3 Mbps VBR, audio 128 kbps
   • 1080p: 5 Mbps VBR, audio 192 kbps
2. Keyframe/GOP: 2 s GOP (e.g., for 30 fps, gop=60). Use 'keyint' = 2 × FPS and avoid scene-cut driven keyframes when possible.
3. Packaging: HLS fMP4 or TS, segment length 6 s, target playlist entries 3–5.
4. HTTP headers for origin objects:
   • Media segments: Cache-Control: public, max-age=86400, immutable
   • Master and variant playlists: Cache-Control: public, max-age=60, s-maxage=60
   • Use ETag or versioned object names on re-publish.
5. Verify: curl -I 'https://origin/path/playlist.m3u8' and confirm cache-control and content-type. Test startup time on 3 consumer networks and validate median TTFF < 4 s.

Recipe B: Small live event with low-latency ambition (1–3 s)

1. Use a packager that supports CMAF chunked or LL-HLS with PART tags. Recommended packager targets: part size 200–500 ms, segment duration 1–2 s.
2. Encoder settings:
   • GOP: 1–2 s (keyframes every 1–2 s)
   • Rate control: target VBR with max-rate = 1.5 × target, bufsize = 2 × max-rate
3. HTTP headers and CDN behavior:
   • Master/playlist: Cache-Control: no-cache, must-revalidate, s-maxage=1
   • Parts/segments: s-maxage=3, public — short TTL but allow edge caching of older parts.
4. Player: request 3 parts before playback; set stable ABR to prioritize lower variance rendition on startup.
5. Verify:
   • Measure end-to-end delay with a timestamp overlay or packet-sequence id. Target <3 s on a local region test.
   • Check CDN cache hit ratio for parts; if hit ratio <80% your edge geography may be too sparse for free tier.

Recipe C: Small live event with resilience (failover to paid path)

1. Primary: free CDN edge for cost efficiency; Secondary: paid CDN or multi-CDN route.
   • Configure DNS TTLs for fast failover (60 s) and health checks on origin endpoints.
2. Use origin-level signing and upload versioned playlists to avoid stale caches.
   • Keep signed URLs short-lived (TTL 60–300 s) and rotate keys on predictable windows.
3. Verification: Execute a failover drill (disable free CDN for 60 s) and measure viewer impact. Validate automated DNS or routing changes within the expected window.

Practical configuration targets

Below are explicit, actionable configuration targets to copy into CI or a QA runbook.

• Encoder:
  • Codec: H.264 (baseline compatibility) or H.265/AV1 if clients support it.
  • GOP / keyframe: 2 s; for very low-latency use 1 s.
  • Rate control: VBR with max-rate = 1.5 × target bitrate; bufsize = 2 × max-rate.
  • Preset: choose 'veryfast' or 'faster' for software x264 to keep encode latency <300 ms.
• ABR ladder (recommended targets):
  • 1080p@30fps: 5 Mbps target, max 8 Mbps
  • 720p@30fps: 3 Mbps target, max 4.5 Mbps
  • 480p@30fps: 1.2 Mbps target, max 1.5 Mbps
  • 360p: 750 kbps
  • 240p: 400–500 kbps
• Packaging:
  • Standard HLS: segment length 6 s (6 s target), playlist entries 3–5.
  • Low-latency: segment 1 s, part size 200–500 ms (CMAF/LL-HLS).
  • Use fMP4 (CMAF) for better chunking and byte-range support.
• Player buffers:
  • Low-latency: warm 3 parts (~600–1500 ms); maintain 3–5 parts during playback.
  • Standard: 2–3 segments startup; playback buffer 15–30 s for reliability.
• HTTP headers to set at origin:
  • Master playlist: Cache-Control: public, max-age=1, s-maxage=1, must-revalidate
  • Variant playlists: Cache-Control: public, max-age=3, s-maxage=3
  • Media segments (live parts): Cache-Control: public, max-age=30, s-maxage=3, stale-while-revalidate=30
  • VOD segments: Cache-Control: public, max-age=86400, immutable

Limitations and trade-offs

Free CDNs can be useful but come with known limitations. Each limitation below is actionable — decide mitigation now.

• Unpredictable egress caps:
  • Mitigation: run a traffic test (see rollout checklist) and enforce ingress rate limiting at the origin or throttle encoders if egress limits are hit.
• Geographic gaps:
  • Mitigation: restrict your launch to regions with edge presence, or use a paid CDN for missing regions.
• Limited logging & observability:
  • Mitigation: instrument origin with detailed logs; emit playback telemetry from player to a central collector.
• Cache churn on live:
  • Mitigation: use longer segments or batch small parts into larger chunks at the expense of latency, or move to a CDN with better cache control for small segment workloads.
• Security features: signed URLs, DRM or token-based access may not be available or limited.
  • Mitigation: perform origin-level signing or use temporary object URLs from storage providers and plan migration to paid CDN for DRM.

Common mistakes and fixes

Actionable list: identify and fix these quickly.

• Manifest cached too long — symptom: viewers see stale playlists after a stream restart.
  • Fix: set playlist Cache-Control to s-maxage=1 or use versioned playlist filenames.
• GOP too long — symptom: slow seeking, poor bitrate switch behavior at low-latency targets.
  • Fix: enforce GOP = 1–2 s, align keyframes to segment boundaries.
• Too small part/segment count on free CDN — symptom: huge number of HTTP requests and edge throttling.
  • Fix: increase part size marginally (e.g., 200 ms → 400 ms) or move to a CDN that supports HTTP/2 multiplexing at scale.
• Ignoring ABR and client bandwidth conditions — symptom: rebuffering spikes.
  • Fix: tune ABR logic to prefer stable lower-bitrate on startup; cap initial bitrate to 1.5 Mbps for most mobile audiences.

Rollout checklist

Pre-launch checklist you can paste into your runbook.

1. Define target latency and concurrency numbers and compute expected egress.
2. Run a controlled load test: simulate 2× expected concurrency for 30 minutes and measure edge TTFB, cache hit ratio, and origin CPU/memory.
3. Verify HTTP headers for playlists and segments with curl: curl -I 'https://edge/path/playlist.m3u8'.
4. Check geographic coverage: run synthetic tests from at least 5 target regions (use public cloud instances or remote testers).
5. Confirm monitoring and alerts: alert on 95th percentile TTFB > 500 ms, rebuffer ratio > 1%, or error-rate > 0.5%.
6. Run failover drill if using hybrid model (disable primary CDN for a period and measure time to recover on secondary).
7. Schedule post-launch performance review at 1 hour, 24 hours, and 7 days to verify assumptions.

Example architectures

Three textual architecture patterns to copy and adapt.

1) Small VOD (cost-sensitive)

Storage (S3 or object store) → Origin HTTP endpoints with versioned URLs → Free CDN (edge caching of segments) → Viewers. Use long TTLs for segments and short TTLs for playlists. Suitable for catalogs, tutorials, recorded webinars.

2) Single-region live event

Encoder/OBS → Ingest (SRT or RTMP) → Packager producing CMAF parts → Origin (short TTL headers) → Free CDN edges (regional) → Viewers. Monitor cache hit ratio and be prepared to increase part size if edges show request throttling.

3) Growth / enterprise migration (recommended path)

Encoder → managed ingest (consider using a Video API to centralize ingest and packaging) → origin with origin-shield → multi-CDN or paid worldwide CDN for edge delivery. For an upgrade path, evaluate product pages early: Video API for centralized ingest and packaging orchestration, Edge CDN for predictable worldwide delivery, and Live production solutions for managed events.

Troubleshooting quick wins

Short checklist to try when a live stream is suffering or a VOD item is slow to start.

• Check playlist cache headers: curl -I 'https://edge/path/playlist.m3u8'. Ensure playlist TTL is < 5 s for live.
• Measure TTFB and median response time from multiple regions. Use 'curl -w "%{time_starttransfer}\ " -o /dev/null -s 'https://edge/path/segment.m4s'' to get start transfer time.
• If rebuffering spikes, reduce initial bitrate to 1–1.5 Mbps for mobile clients and improve ABR startup behavior.
• If CDN edges return 503/429, inspect request rate and combine segments or increase segment duration to reduce request rate.
• If you see stale playlists after redeploy, use versioned filenames or increase playlist immutability with explicit object purges where supported.
• Check TLS handshake & keep-alive: ensure edge supports TLS1.2+/ALPN and that origin uses keepalive to reduce TCP/TLS setup overhead.
• Consult CDN docs for specific cache-key behavior to avoid cookies and query-strings causing cache-bypass. See CDN config guidelines: CDN configuration.

Next step

If your requirements are limited and the recipes above cover you, proceed with a staged rollout following the checklist. If you anticipate growth, global viewers, DRM, or strict SLAs, schedule a migration plan. Contact us to run a quick capacity assessment or trial our managed delivery options. Relevant pages:

• Product overview (Video API): https://callaba.io/product/video-api
• Edge CDN: https://callaba.io/product/edge-cdn
• Live production and event services: https://callaba.io/product/live-production
• Docs: encoding recommendations: https://callaba.io/docs/encoding-settings
• Docs: latency budgeting: https://callaba.io/docs/latency-budget
• Docs: CDN configuration and headers: https://callaba.io/docs/cdn-configuration

Ready to move from a free CDN to a predictable worldwide video CDN with SLAs, analytics and global points of presence? Start a trial or request a capacity review at https://callaba.io/contact-sales or sign up to evaluate our delivery at https://callaba.io/signup. If you want hands-on help converting your current free-CDN setup into a resilient production deployment, open a ticket with engineering and include the following: sample master playlist URL, one representative media segment URL, and your expected peak concurrency.