PC MMTA: External Configured for CommType=X25Eicon May Hang

• Microsoft Mail for PC Networks, version 3.2

SYMPTOMS

An MS-DOS message transfer agent (MTA) or OS/2 multitasking MTA (MMTA) session started with the Commtype=X25Eicon switch stops responding (hangs). No X.25 Call-Requests go out and all incoming calls in that session or other instances, if any, are rejected. Following the hang, the MMTA session typically will not respond to the ESC key to terminate.

CAUSE

An Eicon adapter tracks network control blocks (NCBs) by putting them in queues labeled Free, Active, Completed, and Unknown. On one Eicon adapter there can be a maximum of 245 NCBs allocated simultaneously. The NCBs are released to the Free queue when an X.25 application calls the Eicon application programming interface (API) routine X25DONE.

When initiating a transfer, the MMTA session issues a Call-Request to the Eicon adapter. The Eicon adapter allocates two NCBs per Call-Request, regardless of whether or not an X.25 virtual circuit (VC) has actually been established with a remote MMTA. Although the MMTA does issue an X25HANGUP call when a message transfer is complete, it does not call X25DONE.

This causes NCBs to accumulate in the Completed queue, never to return to the Free queue. When an MMTA session sends the 126th NCB (roughly), given two NCBs per Call-Request, the adapter hangs and does not process any more incoming or outgoing calls. Following this, the MMTA session that sent the last Call-Request hangs.

WORKAROUND

Currently, two workarounds are available:

• Use the -BR (break relative) switch on the MMTA command line to periodically terminate X.25 MMTA sessions.
• Restart the computer containing the Eicon adapter. This will re-initialize the adapter. If you are running OS/2, any additional X.25 MMTA instances will be disabled until this is done.

STATUS

Microsoft has confirmed this to be a problem in the MTA for version 3.2 of Microsoft Mail for PC Networks and in the MMTA for OS/2. This was fixed with External version 3.2.6 for both MS-DOS and OS/2.