[jira] [Created] (PROTON-1523) [proton c] API unclear how incoming_capacity and max_frame work

["Chuck Rolke (JIRA)" <ji...@apache.org>]

Chuck Rolke created PROTON-1523:
-----------------------------------

             Summary: [proton c] API unclear how incoming_capacity and max_frame work
                 Key: PROTON-1523
                 URL: https://issues.apache.org/jira/browse/PROTON-1523
             Project: Qpid Proton
          Issue Type: Bug
          Components: proton-c
    Affects Versions: 0.17.0
            Reporter: Chuck Rolke


Two settings are available to throttle incoming traffic. These operate in units of bytes:
* pn_transport_set_max_frame
* pn_session_set_incoming_capacity

Suppose a user sets a max_frame to 1 Mbyte and incoming_capacity to 2 Mbytes. How many 200-byte hello-world messages might a user expect to be able to receive before the local incoming-window closes? 10,000? The answer turns out to be 2.

The logical disconnect is that the user interface incoming_capacity operates in bytes but the AMQP protocol over the wire operates with an Incoming-Window in frames. In the user's case the API assumes full frames and sets  Incoming-Window = (incoming_capacity / max_frame), or 2. When the sending peer sends 200-byte messages the actual incoming capacity is 400 bytes and not 2Mbytes.

The rules (AFAICT) are:
{noformat}
  IF (max_size is specified)
    incoming_window = incoming_capacity / max_size
  ELSE
    incoming_window = 2147473647
  ENDIF
{noformat}

Noting:
* Incoming capacity is achieved only if the peer sends max_size frames.
* If max_frame is not set then the incoming_capacity setting is ignored.




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