Offer no WebTransport subprotocol when requesting draft-14 - #6
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Previously webtransport_protocol_offer returned a two-token list
("moq-00", "moqt-16") for --draft 14 and ("moqt-16", "moq-00") for
--draft 16. The secondary token was a fallback intended to help
negotiation succeed when the server did not advertise the primary
token, but WebTransport subprotocol selection is at the server's
discretion: when both tokens overlapped with the server's advertised
set, the server could legitimately pick the "wrong" one for the draft
the client was about to speak.
In practice relays that advertise only moqt-16/moqt-15 picked
moqt-16 from moqxr's --draft 14 offer, completed the WebTransport
handshake, and then rejected the draft-14 CLIENT_SETUP bytes
(version 0xff00000e) as a control-stream parse error because the
negotiated subprotocol implied draft-16.
Draft-16 now offers only "moqt-16", which matches the versioned-token
convention introduced in draft-15+.
Draft-14 predates the versioned-token convention and is the legacy
"moq-00" subprotocol. Some deployments accept "moq-00" as a WebTransport
subprotocol; others do not and only accept an empty subprotocol offer,
falling back to determining the version from CLIENT_SETUP/SERVER_SETUP.
Offering no subprotocol is the broadest interoperable path for
draft-14 over WebTransport and is the draft-14-over-WT handshake
documented by Akamai's probe tool. Verified end-to-end against a
draft-14-capable relay.
mondain
approved these changes
Apr 14, 2026
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Previously webtransport_protocol_offer returned a two-token list ("moq-00", "moqt-16") for --draft 14 and ("moqt-16", "moq-00") for --draft 16. The secondary token was a fallback intended to help negotiation succeed when the server did not advertise the primary token, but WebTransport subprotocol selection is at the server's discretion: when both tokens overlapped with the server's advertised set, the server could legitimately pick the "wrong" one for the draft the client was about to speak.
In practice relays that advertise only moqt-16/moqt-15 picked moqt-16 from moqxr's --draft 14 offer, completed the WebTransport handshake, and then rejected the draft-14 CLIENT_SETUP bytes (version 0xff00000e) as a control-stream parse error because the negotiated subprotocol implied draft-16.
Draft-16 now offers only "moqt-16", which matches the versioned-token convention introduced in draft-15+.
Draft-14 predates the versioned-token convention and is the legacy "moq-00" subprotocol. Some deployments accept "moq-00" as a WebTransport subprotocol; others do not and only accept an empty subprotocol offer, falling back to determining the version from CLIENT_SETUP/SERVER_SETUP. Offering no subprotocol is the broadest interoperable path for draft-14 over WebTransport and is the draft-14-over-WT handshake documented by Akamai's probe tool. Verified end-to-end against a draft-14-capable relay.