Re: [core] Core Link Format

[Klaus Hartke <[e-mail]>]

I tried to remember where I had seen those broken Link Format
implementations. I've found some of them now again. It's Leshan and
Wakaama:

> * The rules for the context of a link ("...origin of the target
> URI...") [2] are a constant source of unawareness/confusion and
> mis-implementation.

Wakaama seems to completely ignore the existence of the 'anchor' attribute. [1]

> * The "hosts" link relation type [3] has really, really weird
> semantics (a featureless relation between a host and a resource, where
> determining the host relies on the weird rules for the link context).

Wakaama seems to completely ignore the existence of link relation types. [1]

> * Despite the appearance of simplicity, parsing Link Format is often
> more difficult than anticipated by implementers. For example, I've
> seen implementations that just split the input by ';' and assume that
> each resulting substring must be one link, regardless of where the ';'
> occurs.

(Of course, links are separated by ',' and not ';'.)

Leshan splits by ',', regardless of where the ',' occurs. [2]

> * Processing URI references correctly is also really hard to get
> right. On constrained devices, implementations often don't want to pay
> the price for a full URI parser and just do some simple string
> manipulation instead. For example, I've seen implementations that
> completely disregard the existence of percent encoding and dot
> segments.

Wakaama doesn't seem to do any kind of percent encoding or dot segment
processing. [1]

Overall, it seems that LwM2M implementations are interoperable on the
basis of an informally-specified subset of Link Format. That might
work out for LwM2M, but I think it highlights exactly the problem with
Link Format. :-)

Klaus

[1] https://github.com/eclipse/wakaama/blob/31d64c0c41fae9653c1fa53ef58d1a44e49017fa/core/registration.c#L1279-L1447

[2] https://github.com/eclipse/leshan/blob/f512b574797e852f8e883641ab0df852bebb7fcd/leshan-core/src/main/java/org/eclipse/leshan/Link.java#L119-L160