Adaptive session-level pacing

Adaptive session pacing allows the pacing window size to dynamically:

Increase to accommodate increased traffic
Decrease when congestion is in the receiving node

The pacing window sizes are adapted to buffer availability and demand on a session-by-session basis. The session stage endpoints exchange explicit pacing windows that can vary in size during the course of a session. The mechanism thus prevents any single session from monopolizing or exhausting the buffer resources of a session stage endpoint.

Adaptive pacing occurs between each pacing stage endpoint and can vary for each direction. In Figure 1, APPL1 is in session with a type 2.1 node. Two-stage pacing occurs for data flowing in both directions between the session partners.

Figure 1. Adaptive session pacing

If adaptive pacing is performed, the value determined during session setup for the pacing window size represents the minimum pacing window size to be used during normal operation.

Note: Before VTAM® Version 4 Release 4, VTAM did not dynamically vary the session partner send window size. VTAM always returns the same window size that was set during session establishment. However, VTAM honors the pacing windows sent by its pacing stage partners for adaptively paced sessions. The next window size used for data transmission is whatever value was received from the session partner in the pacing response.

An isolated pacing message (IPM) indicates whether the pacing window size should be changed. Pacing window sizes are changed by IPMs. The IPM, like the IPR used in fixed session pacing, is used to authorize the sending of the next window of messages units, but it also specifies whether the window size is to be changed or remain the same.

The following scenario describes how adaptive pacing works.

The transmitter requests a pacing response from the receiver when sending the first PIU in the current send window.
The receiver returns a pacing response that contains the next window size it considers appropriate for the next transmission. If the transmitter sends the entire window of PIUs before the pacing window response is received, the transmitter must wait until the pacing response is received.
The transmitter can request a larger window size by setting an indicator in the next transmission.
The receiver can ignore the transmitter request for a larger window or honor it when generating the pacing response.
The transmitter is obligated not to exceed the window size specified by the receiver.

Note: You must have NCP Version 5 Release 2, or higher, to support two-stage and adaptive-session pacing involving a 3745 communication controller, or Version 4 Release 3, or higher, for a 3725 communication controller.