[Sis-ams] notation for delivery point names

[Scott Burleigh]

Hi, gang.  At the October meetings we left one spec issue unresolved, 
because it seemed subtle and complex enough to deserve some off-line 
thought and discussion by email.  That issue was the notation we use for 
delivery point names.

The spec currently says that a delivery point name is the concatenation 
of a transport service name, an equals ('=') sign, and an endpoint name 
in transport-service-specific format.  Currently the syntax of the 
transport service name for TCP is the intuitively obvious "tcp", and 
endpoint names are in the similarly obvious format 
/hostname/:/portnumber/; likewise for UDP.  But for VxWorks message 
queues the spec says the transport service name is "vxmq/hostnumber/", 
with /msgqID /as the endpoint name.  The idea under discussion, as I 
recall, was to simplify the protocol by using the general structure 
/protocolname/:/qualifier/:/qualifier/:...:/qualifier /for all delivery 
point names, e.g. "tcp:amroc.jpl.nasa.gov:5453" and 
"vxmq:2113451256:11311876".

I've given this some thought and have concluded that, while I agree that 
the proposed notation's generality is appealing, this is one of those 
rare cases where the more intuitively elegant approach is 
disadvantageous in implementation.

The intent of the current design is to simplify the processing of 
delivery vectors in MAMS messages: a receiving MAMS entity can always 
know whether or not a given delivery point is one that it can use for 
message transmission to the sending entity, simply by looking for the 
'=' character and then comparing the preceding character string to the 
names of the transport services to which it has access.

The proposed design is, paradoxically, more complex.  In the case of 
"tcp:amroc.jpl.nasa.gov:5453" the receiving entity would know that the 
delivery point is usable if "tcp" -- the token preceding the first colon 
-- is one of its transport services.  But in the case of 
"vxmq:2113451256:11311876" the delivery point would be usable only if 
"vxmq:2113451256" -- the string preceding the *second* colon -- was one 
of its transport services, because message-queue transport is possible 
only between tasks residing on the same machine.  Tasks residing on 
different computers running VxWorks cannot exchange messages among 
themselves via message queue; "vxmq" alone would not be enough to assure 
connectivity.

In order to make the proposed syntax work, we would have to build 
transport-service-specific rules into the code that processes delivery 
vectors in inbound MAMS messages, indicating how much of the delivery 
point name constitutes the transport service name.  To avoid adding any 
more complexity to an already complex specification, I think we should 
stick with the current notation.  Does anyone have strong feelings 
otherwise?

Scott
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