AlternatePath and Device Prefix need charset restriction

[sbernard31]

LWM2M specification define Alternate Path (see LWM2M-v1.2.1@transport§6.4.1. Alternate Path)

And also Device Prefix when LWM2M Gateway is used (see Lightweight Machine to Machine Gateway Technical Specification)

Both can be find in a LWM2M path.
E.g. :

• alternate path : /lwm2m/3/0/1
• device prefix : /d01/3/0/1
• or both : /lwm2m/d01/3/0/1

AFIAK, there is only 1 restriction about how Alternate Path should looks like :

This link MUST NOT contain numerical URI segment.

And nothing about Device Prefix (but I maybe missed something)

I think it would be wiser to clearly define how path/prefix MUST look like else we risk to face some issue between different content format.

Currently this value can be see with different constraint in :

• Link Format (as URI reference https://datatracker.ietf.org/doc/html/rfc3986#appendix-A
• and SenML (see https://datatracker.ietf.org/doc/html/rfc8428#section-4.5.1)

In Link format, absolute path should follow :

path-absolute = "/" [ segment-nz *( "/" segment ) ]
segment       = *pchar
segment-nz    = 1*pchar

pchar         = unreserved / pct-encoded / sub-delims / ":" / "@"
pct-encoded   = "%" HEXDIG HEXDIG
unreserved    = ALPHA / DIGIT / "-" / "." / "_" / "~"
sub-delims    = "!" / "$" / "&" / "'" / "(" / ")"
            / "*" / "+" / "," / ";" / "="

And in SenML :

name    = firstchat *nextchar
firstchar  = ALPHA / DIGIT
nextchat = ALPHA / DIGIT / "-" /  ":" /  "." /  "/" / "_"

I propose some very restrictive constraint to avoid that kind of issues :

alternatePath  = "/" segment *(  "/" segment)
prefix = "/" segment
segment = ALPHA * (ALPHA / DIGIT) // force segment to start by ALPHA making LWM2M path parsing easier
                                  // no need to parse whole segment to know 
                                  // IF we are in the prefix/alternatePath OR in the Id of a LWM2M node
ALPHA  =  %x41-5A / %x61-7A     ; A-Z / a-z
DIGIT  =  %x30-39       ; 0-9

Any opinion ?

[dnav]

As an individual, not an OMA representative:

The Alternate path being a part of an URI path, I think it follows the format defined in RFC 3986:

   path          = path-abempty    ; begins with "/" or is empty
                 / path-absolute   ; begins with "/" but not "//"
                 / path-noscheme   ; begins with a non-colon segment
                 / path-rootless   ; begins with a segment
                 / path-empty      ; zero characters

   path-abempty  = *( "/" segment )
   path-absolute = "/" [ segment-nz *( "/" segment ) ]
   path-noscheme = segment-nz-nc *( "/" segment )
   path-rootless = segment-nz *( "/" segment )
   path-empty    = 0<pchar>

   segment       = *pchar
   segment-nz    = 1*pchar
   segment-nz-nc = 1*( unreserved / pct-encoded / sub-delims / "@" )
                 ; non-zero-length segment without any colon ":"

   pchar         = unreserved / pct-encoded / sub-delims / ":" / "@"

   query         = *( pchar / "/" / "?" )

   fragment      = *( pchar / "/" / "?" )

   pct-encoded   = "%" HEXDIG HEXDIG

   unreserved    = ALPHA / DIGIT / "-" / "." / "_" / "~"

[sbernard31]

Thx @dnav,

Don't you think that it lead to some issues ?

As it is very permissive , this could lead to situation where alternate path can not be express in given content format.
E.g. You can use characters which are not allowed by SenML 😬

And with grammar you propose you can have an alternate path which looks like : /120 and so don't follow : This link MUST NOT contain numerical URI segment.