[PROTON-1523] [proton c] API unclear how incoming_capacity and max_frame work

• Type: Bug
• Status: Closed
• Priority: Major
• Resolution: Done
• Affects Version/s: proton-c-0.17.0
• Component/s: proton-c
• Assignee: Unassigned
• Reporter: Charles E. Rolke
• Created: 21/Jul/17 15:32
• Updated: 11/Apr/18 10:39
• Resolved: 11/Apr/18 10:39

Description

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:

  IF (max_size is specified)
    incoming_window = incoming_capacity / max_size
  ELSE
    incoming_window = 2147473647
  ENDIF

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.

Issue Links

• is part of: PROTON-636 remove confusing default for session capacity and allow disabling it
• is related to: PROTON-1809 [python, ruby] Unable to receive messages when max-frame-size is set to more than 2^20