Re: Is To-tag in non 2xx response mandatory

Paul Kyzivat wrote:
> The simple answer to your question is: you must include the to-tag to 
> conform to 3261.
>
> I think the question you are really asking is: "will the UAC and 
> intervening proxies do the right thing if the to-tag is not present in 
> the response?"
>
> The answer to that is "probably". RFC 2543 didn't have to/from tags. 
> So if the UAC calls a UAS that only conforms to 2543 then it will get 
> this situation. So any element that attempts to be 2543 compatible 
> will deal with this.

I'd like to point out that we've already taken the first steps of 
actually breaking 2543 compatibility with RFC 4916. I would not be 
surprised if we take further steps away from 2543 compatibility in the 
future.

In other words: I would be very cautious in relying on *any* 2543 
backwards-compatibility mechanisms.

/a
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