[Sis-ams] Predictability concerns with AMS, and other questions too

[Scott Burleigh]

Scott Burleigh wrote:

> Marek Prochazka wrote:
>
>> Here are my notes:
>>
>> 1) Predictability issues. There is a number of places where I'm
>> concerned about predictability issues.
>
Marek, something that a note from Peter Shames reminded me of:

I'd say that real-time performance (a guaranteed upper bound on message 
delivery latency) is an element of Quality of Service, just as 
reliability (retransmission), preservation of data transmission order, 
etc. are elements of Quality of Service.  Underlying the AMS design is a 
commitment to the layering principle and a deliberate and resolute 
refusal to reinvent communications all over again, so a fundamental 
design principle of AMS is to rely on the underlying transport systems 
to provide QOS.  For example, AMS doesn't do retransmission itself: it 
relies on (say) TCP, where many thousands of hours of work have gone 
into a sound retransmission design.  So one reason you're not seeing a 
lot of discussion of real-time performance guarantees in the AMS spec is 
that AMS, by design, is going to rely on the real-time performance of 
underlying transport systems when applications deem real-time 
performance necessary.

The job of AMS is issuance of asynchronous messages in response to 
applications' directions; in a sense it's probably better to think of 
AMS as an enhancement of applications rather than an enhancement of 
transport.  AMS will send messages on their way at the direction of 
applications, but in itself it doesn't have a lot of control over when 
those messages will arrive.  The control it does have lies in selection 
of the underlying transport system to use (per application direction) 
and transmittal of application-specified QOS parameters - priority, flow 
label - to that transport system.  If the underlying transport system 
is, say, vxWorks messages queues - or TCONS - then the latency between 
the moment of transmission and the moment of arrival of each message 
will be quite predictable.  But it won't really be AMS that has provided 
that predictability: AMS has just conveyed the application's mandate to 
the transport layer.

Scott