# Channels

> A channel is one exchange within a scope: one request, a stream of responses, and a finish that only the responder sends.

Canonical: https://provider.diverge.network/2.3.0/frames/channels/
Specification revision: 2.3.0

A **channel** is one exchange within an existing scope: one request,
and the stream of responses that answers it.

- **Opening.** Either party may open a channel, by sending a channel
  request (type `4`) naming the scope. The opener chooses the
  channel's number.
- **Numbering is per opener, per scope.** A channel number identifies
  a channel only together with the scope it was opened in and the
  party that opened it. The same number therefore names distinct
  channels across scopes, across openers, or both: the client's
  channel *n* in one scope, the client's channel *n* in another, and
  the server's channel *n* in each are four distinct channels, all
  four may be open at once, and none collides with any other.
- **Uniqueness.** A channel number names one channel, for its opener
  in its scope, from that channel's request until that channel's
  response finish; after the finish the number is free to bear a new
  channel. The server does not send a channel request whose channel
  field names a channel it still has open in the named scope.
- **A channel request naming an open channel is disregarded.** The
  server disregards a channel request from the client whose scope and
  channel fields name a channel the client already has open. The
  disregarded request is never answered, and the channel it named
  continues, unaffected.
- **One request per channel.** A channel carries exactly one request;
  a second request frame never appears on a live channel.
- **Answering.** The responder — the party that received the channel
  request — answers with zero or more channel responses (type `5`) and
  ends the channel with exactly one channel response finish (type
  `6`), each quoting the opener's channel number. The responses form
  one ordered stream, and the finish is its end.
- **Only the responder ends a channel.** The opener has no frame with
  which to stop asking, cancel, or abandon the exchange. A channel is
  open from its request until its finish.
- **A finish with no response before it is meaningful.** It states
  that the request was received and could not be served, exactly as a
  scope's is.
- **A bidirectional exchange is two channels.** An exchange in which
  both parties send data is composed of two channels, one opened by
  each party and each finished by its own responder. **Remark.** A
  channel carries one request, and only its responder ends it, so one
  channel cannot carry data in both directions.