[RPRWG] Single and dual transit buffer MACs

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I have some concern with the way in which co-existence of Single Transit Buffer (STB) MAC and Dual Transit Buffer (DTB) MACs on the same ring has been approached in draft 0. I thought I voice my concerns to the working group on the reflector to enable some dialog in resolving this concern while I also file my comments.

I think the best place to start would be at Clause 9.3, page 70, lines 3 and 4 which is: "If a ring consists of mixed RPR MACs (single and dual transit buffer nodes), the fairness scheme will need to interact without disadvantaging any other node on the ring."

First disadvantage stems from the special accommodations to be made in the DTB MACs to support performance requirements while using STB MAC at other nodes. This requirement is stipulated further down in the same Clause 9.3, page 70, lines 11 through 13. This stipulates that DTB MAC requires a rate shaper to throttle ring outgoing rate to no more than 95% of the link rate. The only way a loss-less MAC (as stipulated in 5.3.2, page 33, line 41) can do this is to buffer frames when instantaneous rates exceed 95%. Further, if such traffic shaper is classless than HP priority traffic could be subjected to further buffering in this traffic shaper leading to contradiction of clause 3.1, which requires HP traffic to have no more transit delay than one frame-time through transit nodes. In other words defining a RPR standard to accommodate STB MAC is not free of cost in terms of implementation and complexity in the standard.

Raj