Adding to what Brett says...
An issue with 3261 is that there is no precise definition of the
duration over which a "Supported:xyz" is applicable. It might be for the
lifetime of the device sending it, it might be for the duration of the
dialog in which it is sent, or it might only be for the duration of the
transaction in which it is sent.
So worst case, it might change from transaction to transaction. When
assuming it applies for longer, you are wise to be prepared for your
assumption to be wrong.
If the device sending the 420 is just an "ordinary device", and nothing
is really changed since the prior invite, then IMO it is acting
perversely, but is not technically violating anything.
Thanks,
Paul
On 1/17/14 7:11 AM, Brett Tate wrote:
>> Is it OK to include Require header with 100rel
>> in the re-INVITE, UA1 has already showed that
>> it supports this extension.
>
> Yes; however you might receive a 420.
>
>
>> Why should UA1 reject this message with 420 response.
>
> Among other potential reasons, a B2BUA is involved and re-INVITE reached a
> different UAS which doesn't support 100rel.
>
> And for completeness since you are attempting to use 100rel for re-INVITE, be
> aware that RFC 6141 updates RFC 3261.
>
>
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