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Transport stanza

Signalling - call-transport

SIG-05 - status: review - audio, video

The action of the stanza negotiates ICE/relay candidates after the offer, carrying the relay token and network medium.

Wire format. Send a top-level <call to="<peer>"> wrapping a <transport> action child:

<call to="<peer>">
  <transport call-id="<call-id>" call-creator="<creator-jid>"
             [p2p-cand-round="<n>"]
             [transport-message-type="<t>"]>
    [ <te priority="1">…relay-token-bytes…</te> ]
    <net medium="2" [protocol="0"]/>
  </transport>
</call>

Identity (both required, echoed unchanged from the offer): - call-id MUST equal the offer's call-id. - call-creator MUST identify the call creator.

Children and order: - The action MUST contain a <net> child. - When a relay token is conveyed, a <te priority="1"> child (binary body = relay token blob) MUST precede <net>. - When no relay token is conveyed, <te> is omitted and <net> is the sole child.

<net> attributes: - MUST carry medium="2". - MUST carry protocol="0" unless transport-message-type="9"; when transport-message-type="9", protocol MUST be omitted.

transport-message-type (optional) selects the exchange role:

"1"  relay candidate
"3"  peer (ICE) candidate    ; callee replies with type "9"
"9"  keepalive / reply       ; <net> omits protocol

p2p-cand-round (optional): round number as a decimal string for round-based peer-to-peer candidate exchange.

Receiver: MUST read call-id and call-creator; MAY read p2p-cand-round and transport-message-type. A receiver that does not recognise the action MUST ignore the stanza rather than fail the call (forward-compatibility for future transport-message-type values).

Notes. The same <te> relay-token carrier appears in the accept (priority="2") and relaylatency exchanges, indicating a shared relay-token element across call actions.

Requires: call-offer, stun-relay
Breakdown: media-loop, call-mute, flow-outgoing-1to1

Implemented by

Flavor Status Source Notes
whatsapp-rust working history - blame - commits 674e851 —
zapo-caller working — —

Annotation wacrg:SIG-05 — a flavor marks its implementation site in source with this comment; a script clones the source, finds it, and attaches the commit blame/permalink.

Contributors

Contributor Role
Rajeh Taher Rajeh Taher wrote initial spec

protocol history / diff - blame

Open questions - Exact internal structure of the relay-token blob (token bytes vs. framed structure). - Full enumeration of transport-message-type values beyond 1 (relay candidate), 3 (peer ICE), and 9 (keepalive/reply). - Whether takes values other than 2 in the transport exchange, and the meaning of the medium scale. - Semantics of p2p-cand-round rounds and the termination condition for candidate-exchange rounds.

References - RFC 8445 — Interactive Connectivity Establishment (ICE) - RFC 7675 — STUN Usage for Consent Freshness

Changelog

  • 2026-06-21 — Initial spec entry.

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