[Sis-ams] flow label in AAMS messages

[Burleigh, Scott C]

Hi.  Tim Ray and Pat Donahue and I have been giving Remote AMS a lot of attention over the past couple of weeks, in the course of which I have slowly come to want to make a small change to the structure of the AAMS message.  The change would be to reclaim the reserved octet between Priority and the Checksum Flag to once again carry the Flow Label byte.

We dropped that byte from the message header a while back to save space, figuring it wasn't really important once its value had been passed down to the Transport Service that was going to carry the message.  Nothing else paid any attention to it, and we needed the extra bits because we were adding the source continuum number.

But the source continuum number has been trimmed back to 15 bits (it has to be, since its additive inverse gets used as a subject number and subject numbers are limited to 16 bits), so we've gotten 9 bits back, one of which is now the checksum flag.  If we carry the flow label in the other 8, a Remote AMS Gateway that receives a published AAMS message can use the flow label as a QOS hint when it invokes the RAMS network protocol to forward the message to its peer gateway(s).

Since there seems to be some potential benefit and no very great cost -- we've got another 8 reserved bits between the source node number and the context number -- I would like to go ahead with this change.  Anybody see a significant downside here that I'm missing?

Scott