[core] SenML FETCH/PATCH without globally unique names

[Ari Keränen <[e-mail]>]

CoRE WG,

One issue on SenML FETCH/PATCH that came up during the joint IETF / OMA 
meeting was that it would be useful to be able to use PATCH payload 
names without the "globally unique prefix".

SenML RFC has a requirement that all SenML names are globally unique 
[1]. This enables the SenML Packs to be easily moved between systems 
without knowing the context they were originally retrieved at.

However, when SenML (i)PATCH is applied, the context is always known by 
the receiver since the receiver is hosting the resources that are being 
patched.

For example with LwM2M composite write using iPATCH, the content of the 
Patch Pack in JSON format could be as follows:

[{"bn":"/3303/0/", "n":"5700","v":22.4},
{"n":"5601","v":12.2},
{"n":"5602","v":34.2},
{"bn":"/3303/1/", "n":"5700","v":18.2},
{"n":"5601","v":9.2},
{"n":"5602","v":27.2}]

(Here you have a LwM2M client with two instances of a IPSO temperature 
sensor, Object ID 3303, supporting following resources 5700: Sensor 
Value, 5601: Min Measure Value, 5602: Max Measured Value).

If we keep the same requirement as SenML Pack names, both "bn" field 
values would have to be prefixed with the LwM2M endpoint ID, e.g., 
"urn:dev:mac:0024befffe804ff1". This would consume a fair amount of 
bytes on wire.

Our suggestion is that we relax the requirement for the names in Patch 
Packs so that the names are, for example, "evaluated in the context of 
the target resource". That is, all base names could be automatically 
prefixed by the unique endpoint name and target resource path.

Comments welcome! This will be discussed in the CoRE interim in one hour.


Cheers,
Ari

[1] https://tools.ietf.org/html/rfc8428#section-4.2