Channels

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

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.