Requested revision
Standard: | 802.1AS | Clause: | 11.2.13.3 |
Clause title: | State diagram |
Rationale for revision
Email was received from an 802.1AS user that described a scenario where loss of a single Folllow_Up message can lead to Sync receipt timeout. A presentation that describes this in detail will be prepared for the July, 2013 802.1 TSN TG meeting. Essentially, the current MDSyncReceiveSM state machine, of Figure 11-6/802.1AS requires waiting for a Follow_Up message for a time equal to one mean Sync interval. If the next Sync arrives slightly early, before the expiration of this Sync interval, the next Sync will be ignored because the state machine will still be waiting for Follow_Up. If the Sync after that arrives slightly late, it also will be considered lost, and sync receipt timeout will occur.
Note that this item was discussed in the June 19, 2013 TSN call. It was the opinion of those on the call that this behavior was not intended and is not desirable. One possible fix, mentioned in the call, might be to assume that a Follow_Up is effectively lost if the next Sync arrives and Follow_Up has not been received, and to then wait for Follow_Up for the next Sync. In any case, it was decided that the editor should enter this item in the maintenance database, with the intent of addressing it in 802.1ASbt.
Proposed text
The exact fix is not decided at present, though the fix described above is one possibility. In any case, the MDSyncReceiveSM state machine will be modified so that this behavior does not occur.
Impact on existing networks
This will improve the performance of existing networks by making it less likely that loss of only a single Follow_Up could result in Sync receipt timeout. There will be no impact on interoperability of time-aware systems.
Originator
Name: | Geoffrey M. Garner | Email: | gmgarner@alum.mit.edu |
Affiliation: | Broadcom, Hirschmann, Marvell, Siemens | ||
Submitted: | 2013-06-24 |