Re: [core] A couple of late comments on Hop-Limit

[Adam Roach <[e-mail]>]

On 10/16/19 5:13 AM, Christer Holmberg wrote:
>
> Hi,
>
> I haven’t followed the hop-limit discussions, so I apologize if the 
> following has already been discussed. I also know that my comments 
> come very late in the process, so sorry for that.
>
> Adam may have an opinion regarding Q2.
>

I do!


> Q2:
>
> The draft says that a CoAP message with the Hop-Limit option value set 
> to 0 bigger than 255 must be rejected with a 4.00 Bad Request response.
>
>   * Is 4.00 really an appropriate response code? When reading the HTTP
>     definition for 400 Bad Request (CoAP inherits the semantics), it
>     is about a client error (miss formed message, etc), which is not
>     the case here.
>   * Based on my experience in SIP, it is VERY useful to have a
>     dedicated response code for this (SIP defines ‘483 Too Many
>     Hops’). Because, when this happens, it is often related to
>     **network configuration and/or routing issues** – not to client
>     implementation issues – so it is VERY useful information when
>     trying to figure out why the message was rejected.
>

So, keep in mind that properly-implemented instances of this 
specification will never try to put a zero on the wire: an 
implementation that receives a message with a Hop-Count of 1 that would 
otherwise forward it are required to forego such forwarding by the final 
paragraph of section 3. (The notion of "greater than 255" doesn't exist, 
since you can't communicate a value greater than 255 in an 8-bit 
unsigned field). This is generally going to trigger a TBA1 (probably 
5.08) response.

That means that a "Hop-Count" of zero is semantically nonsense, and 
arguably a malformed message. This is analogous to an HTTP proxy 
corrupting a message on the way to its destination such that it cannot 
be interpreted by the recipient, a situation that would also trigger a 
400 response.

All of which is to say that I think the 4.00 response to a Hop-Count of 
0 is the correct behavior.

/a