commit bce57cda1e59944659d9cd36c1e23ce9043dfbe4
author Philipp Hancke <phancke@meta.com> Tue Jan 14 20:16:55 2025
committer WebRTC LUCI CQ <webrtc-scoped@luci-project-accounts.iam.gserviceaccount.com> Wed Jan 15 15:54:23 2025
tree e5afb650fbe124eb40d0ae48305ca52106831ddd
parent cfaba8fd2d1e4d3cd7f484e4c4509a60805497fc

Piggyback DTLS handshake in initial STUN packets

This change puts the DTLS handshake as payload of STUN packets with a custom STUN attribute (registered with the IANA) and starts the DTLS handshake before the ICE transport becomes writable. Effectively, STUN acts as a transport layer for DTLS during the handshake phase.

This will theoretically reduce the call setup time by one RTT for aggressive nomination or two RTTs for regular nomination.

The latest DTLS packet (flight) is cached and sent on every STUN request or response. DTLS packets are extracted from every authenticated STUN request or response and handled to the DTLS layer for processing.
The caching also increases the resilience to packet loss as STUN pacing is more aggressive (every 20ms) than the exponential backoff used by DTLS which should reduce call setup time in lossy networks.

If the other side of the connection does not support this feature the fallback to normal DTLS happens as soon as the ICE transport becomes writable. This also handles edge-cases like fragmentation of the DTLS handshake.

The feature is only supported when ECDSA certificates are used since RSA certificates are too large to transport as STUN attributes. The observed attributes for the server and client flights with the certificates were around 600 to 650 bytes. This may be further reduced by using raw public keys defined in RFC 7250.

This feature is disabled by default and guarded by the field trial
  WebRTC-IceHandshakeDtls
and requires experimentation and standardization before roll-out in the browser.

Parts of this landed in
  https://webrtc-review.googlesource.com/c/src/+/370679

BUG=webrtc:367395350

Change-Id: I4809438b2a267c4690a9b2bd6f1766d2f959500d
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/362480
Commit-Queue: Jonas Oreland <jonaso@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Jonas Oreland <jonaso@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#43742}