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Call stanza acknowledgement

Signalling - call-ack

SIG-07 - status: review - audio, video, group

Acknowledging an inbound <call> stanza: a generic <ack> for every <call>, plus an <offer>-only <receipt><offer/></receipt>.

A <call> carries exactly one action child: <offer>, <offer_notice>, <preaccept>, <accept>, <reject>, <terminate>, <transport>, or <relaylatency>.

Level 1 — generic ack. Every received <call> MUST be acknowledged with a generic <ack> correlated by the stanza id attribute (distinct from the action's call-id), for all action types including <offer_notice>.

Level 2 — offer receipt. When and only when the action child is <offer>, the device MUST additionally send:

<receipt to="{caller}" id="{stanza_id}" from="{own_addressable_id}">
  <offer call-id="{call-id}" call-creator="{call-creator}"/>
</receipt>
  • to MUST equal the from JID of the inbound <call>.
  • id MUST equal the stanza id of the <call>, NOT the action's call-id.
  • from SHOULD be the device's own addressable id, chosen to match the address space of the inbound from JID: if from is a LID use the own LID, else the own phone-number JID. If no own addressable id exists, omit from.
  • The <offer> child MUST carry exactly two attributes copied verbatim from the inbound offer: call-id and call-creator. It MUST NOT echo the offer's media or capability children.

An offer receipt MUST NOT be sent for any non-<offer> action; <offer_notice> gets the Level-1 ack only.

An unrecognised action child MUST NOT fail the stanza and MUST NOT emit an offer receipt; it gets the Level-1 ack only.

Notes. The action's call-id is separate from the stanza id: the offer receipt correlates on stanza id, while later signalling (accept, transport, terminate) correlates on call-id + call-creator. Inbound envelope e == "1" marks offline delivery.

Requires: call-offer
Breakdown: flow-call-rejected, flow-incoming-1to1, flow-outgoing-1to1

Implemented by

Flavor Status Source Notes
whatsapp-rust working history - blame - commits 674e851 —
zapo-caller working — signalling layer consumes/produces the offer receipt

Annotation wacrg:SIG-07 — 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 - Whether the server expects (or rejects) an offer receipt that omits the from attribute when an own addressable id exists. - Whether any action other than <offer> (e.g. video offers carrying a <video> child) triggers an additional receipt beyond the generic ack.

References - RFC 6120 — XMPP Core (stanza acknowledgement model)

Changelog

  • 2026-06-21 — Initial spec entry.

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