Breadway/breadcast
1
0
Fork 0
Code Issues Pull requests Projects Releases Packages Wiki Activity Actions
breadcast/breadcast-caststream-sys/vendor/openscreen/PATCHES.md
Breadway bd511fea33 Measure real EnqueueFrame outcomes, then size the send window to fit RTT 
Every "fix" for the mirroring freezes so far has been reasoned from code
rather than measured, because the one counter that could have falsified any
of them was blind by construction: `enqueue_frame` returns as soon as a frame
is *posted* to openscreen's TaskRunner, long before `Sender::EnqueueFrame`
decides whether to accept it. The frame pump's `enqueued_fps` therefore read a
healthy 30fps through every freeze.

Add `BreadcastEnqueueStats` (new FFI accessor, no behaviour change): per-second
counts of OK / MAX_DURATION_IN_FLIGHT / REACHED_ID_SPAN_LIMIT /
PAYLOAD_TOO_LARGE, plus the in-flight window gauges and RTT sampled at the
enqueue attempt, all surfaced on the existing "frame pump rate" line as
`accepted_fps` / `rejected_*`.

Measured against the real Chromecast, that settles it: 12.2% of frames were
being rejected with MAX_DURATION_IN_FLIGHT, in 85% of all seconds -- steady,
not just during visible freezes. Since breadcast enqueues already-encoded
frames, each rejection silently breaks the H.264 reference chain rather than
merely dropping a frame.

The measurement also corrects the diagnosis. The send window is
clamp(2*RTT, kMinSenderInFlight, target_playout_delay/3); the assumption was
that a LAN pins it to the 66ms floor. It does not -- RTT to this receiver runs
42-189ms, so 2*RTT is 84-378ms and the window was pinned at the *ceiling*,
133ms at a 400ms playout delay. The ceiling was the binding constraint, so
raising the floor alone would have changed nothing.

So raise both, ceiling first: target playout delay 400ms -> 1200ms (ceiling
133ms -> 400ms) and kMinSenderInFlight 66ms -> 200ms for RTT dips. Measured
over a matched 65s steady-state window, rejections fall 12.2% -> 4.3% and
seconds containing a broken reference chain 85% -> 40%. Costs ~800ms of
added latency, which is unnoticeable for mirroring to a TV.

This is an improvement, not a cure. The residual rejections are bursts
(in-flight seen at 433ms against a 200ms window, RTT spiking to 221ms), and no
static window survives those. The real fix is the backpressure contract
sender.h documents and this facade still doesn't implement: consult
GetInFlightMediaDuration()/GetMaxInFlightMediaDuration() and throttle *before*
encoding, so a skipped frame never leaves a dangling reference behind.
2026-08-15 22:34:21 +08:00

4.9 KiB
Raw Blame History

Vendoring notes

This directory is a pruned subset of chromium/openscreen, pinned at the commit in PINNED_COMMIT, licensed BSD-3-Clause (see LICENSE). It contains only the files needed for the Cast Streaming sender data path (RTP/RTCP send, OFFER/ANSWER message types, AES frame encryption) plus the POSIX platform/util plumbing that path needs — not the receiver, not OSP discovery, not remoting/RPC, not TLS (breadcast's TLS CASTV2 control channel is handled by the existing rust_cast-based Rust code; this vendored code only drives the UDP RTP/RTCP data path once rust_cast has already negotiated OFFER/ANSWER and we know where to send packets).

It is compiled directly via the cc crate in ../build.rs, not GN/Ninja — there is no build system here to regenerate anything from.

Rolling the pin

There are no release/API-stability guarantees upstream. To update:

  1. Re-run the file selection against the new commit's cast/streaming/BUILD.gn (:common + :sender targets, minus public/sender_session.* and public/rpc_messenger.* — see "What's excluded" below) plus platform/BUILD.gn's :base/:api/:standalone_impl targets.
  2. Re-apply the patches below (they're small; check if upstream has since fixed the same problem and the patch can be dropped).
  3. Update PINNED_COMMIT and rebuild.

What's excluded and why

  • public/sender_session.{h,cc}, public/rpc_messenger.{h,cc}, public/receiver_session.*, receiver-side files, remoting.proto/ input.proto — SenderSession bundles mirroring negotiation together with RPC/remoting/input support, which pulls in protobuf (input.pb.h/remoting.pb.h) for no benefit here (breadcast only ever does one-way video mirroring). Instead, ../src/session.cc drives Offer/Answer/SessionConfig directly — logic adapted from sender_session.cc's CreateMirroringOffer/StartNegotiation/ SelectSenders, minus everything RPC/remoting/audio/input-related.
  • All TLS support (platform/impl/tls_*, platform/impl/stream_socket*) — unused; see above.
  • util/crypto/{certificate_utils,digest_sign,pem_helpers,rsa_private_key, secure_hash,sha2}.* — only needed for OSP discovery / X.509 certificate handling, not the RTP data path.
  • util/scoped_wake_lock_mac.cc — macOS-only.

Local patches (not upstream)

  1. build/build_config.h, build/buildflag.h — stand-ins for Chromium's GN-generated versions of these headers. Only define what the 3 callers here actually check (IS_POSIX, IS_LINUX, IS_APPLE, IS_ANDROID), hardcoded for Linux.

  2. patches/aes_ctr128_compat.cc — frame_crypto.cc calls AES_ctr128_encrypt(), a BoringSSL convenience wrapper not in system OpenSSL's public headers. Reimplemented from scratch (standard CTR mode over AES_encrypt(), which OpenSSL does still expose).

  3. platform/impl/platform_client_posix.{h,cc} — stripped the TlsDataRouterPosix member/accessor (see "All TLS support" above).

  4. util/crypto/openssl_util.{h,cc} — dropped SSLErrorCodeToError()/ GetSSLError(), which reference BoringSSL's SSL_error_description() (not in system OpenSSL). Unused for the same reason as (3).

  5. util/base64.cc — upstream implements this on third_party/modp_b64, which isn't fetched by a plain shallow clone (pulled in separately via gclient/DEPS in a full Chromium checkout). Reimplemented on EVP_EncodeBlock/EVP_DecodeBlock from system OpenSSL instead, same public interface.

  6. cast/streaming/impl/sender_impl.cc — raised kMinSenderInFlight from upstream's 66ms to 200ms. This is a behaviour patch, not a portability one, and is the only one here that changes what goes on the wire — so unlike the others it should not be silently re-applied when rolling the pin without re-measuring first.

    GetMaxInFlightMediaDuration() sizes the sender's send window as clamp(2*RTT, kMinSenderInFlight, target_playout_delay/3). Upstream's 66ms floor assumes the RTT is negligible, which holds for Chrome's own usage but not for breadcast's measured case: instrumenting real Sender::EnqueueFrame result codes (see BreadcastEnqueueStats in ../../src/facade.h) against a Chromecast over Wi-Fi showed RTT of 57-145ms and a steady 3-40% of frames per second rejected with MAX_DURATION_IN_FLIGHT. Because breadcast enqueues already-encoded frames, each such rejection silently breaks the H.264 reference chain rather than merely dropping a frame, which is what the user-visible multi-second picture freezes turned out to be.

    Pairs with kTargetPlayoutDelay in ../../src/session.cc, which raises the ceiling of that same clamp (the ceiling, not this floor, was the binding constraint at 400ms playout delay). Both are needed: this floor covers RTT dips, that ceiling covers the normal case.