How SNA Server Manages Send and Receive Window Sizes on SDLC

• Microsoft SNA Server for Windows NT, versions 2.0, 2.1, 2.11, and 3.0

SUMMARY

SNA Server Admin does not support configuration of the SDLC send and receive window sizes, as the 802.2 connection does with the Unacknowledged Send Limit and Receive Ack Threshold parameters. This article describes how SNA Server determines the send and receive window sizes on SDLC connections.

NOTE: Do not confuse these send and receive window sizes with the send and receive pacing counters configured on an APPC mode entry, or with the send and receive window sizes negotiated on a BIND exchange when establishing an LU-LU session.

MORE INFORMATION

The SDLC send window is defined as the number of frames that SNA Server sends before setting the P/F bit requesting an acknowledgment. By default, the SDLC send window is set to 7. You can override the SDLC send window value in SNA Server by modifying the value in byte 27 of Format 3 XID frames sent from the remote system. This is the field in which the XID sender specifies the receive window size. SNA Server then takes that value in the Format 3 XID exchange and uses it for the SNA Server send window size.

NOTE: The XID format is documented in the IBM SNA Formats Guide (IBM part number GA27-3136).

The SNA Server SDLC receive window is the number of frames which the SNA Server link service can receive before requiring an acknowledgement. For SNA Server SDLC connections there is no configured receive windows. The receive window is actually a function of the Credit messages that flow between the SNA Server SDLC link service and the SNA Server service. These Credit messages manage the internal flow control between the link service and the server and determine the receive window size at any given instant.