On 1/29/19 10:21 AM, Alex Balashov wrote:
Hi,
I am using a media relay which implements some internal loop detection
by adding a non-IANA-registered SDP attribute to any offer or answer
that passes through it:
a=CustomAttribute:GUID
From what I understood from RFC 4566 § 5.13, this isn't illegal, but
such an attribute must be ignored.
5.13 says: "Attributes MUST be registered with IANA", so why do you say
this isn't illegal?
The requirement to ignore unknown attributes is intended to cover the
case where there has been a legal extension (with registration) that is
used by the sender but not implemented by the receiver.
Now, there is a particular type of endpoint in use in this environment
which takes that attribute from an incoming initial INVITE--which is, by
definition, not locally intelligible--and copies it dumbly into its
2xx/SDP answer.
Commonsensically, this seems like broken behaviour; if it doesn't
understand the attribute, it must be ignored, per § 5.13. Beyond that,
is there any specific proscription against doing this in the standard?
Again consider this behavior in conjunction with a valid extension. The
inclusion of an attribute in an answer after receiving it in an offer is
often used as an indication of support for the extension. This depends
on answerers ignoring (not mirroring) attributes they don't understand.
So an endpoint with this behavior will break any peer using such a legal
extension.
Thanks,
Paul
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