[Standards] XEP-0167: Something, I can't understand

Константин Козлов yagiza at yandex.ru
Sat Jul 2 16:19:47 UTC 2016


Hello!

There's something, I still cannot understand in XEP-0167. I tried to ask 
in both jdev at conference.jabber.org and xsf at muc.xmpp.org but got no 
answer so far.

XEP-0167 chapter 5 says about *session-initiate* request:
/When the initiator sends a session-initiate message to the responder, 
the <description/> element includes all of the payload types that the 
initiator //*can send and/or receive*//for Jingle RTP, each one 
encapsulated in a separate <payload-type/> element//
/and later about *session-accept* request:
/The session-accept message SHOULD include a subset of the payload types 
sent by the initiator, i.e., a list of the offered payload types that 
the responder //*can send and/or receive*//./

I don't know how to understan that "and/or" construction, but I 
understand it as /"can either send or receive or both"/. If I understand 
correctly, there is no way to specify if the payload type could be 
received or sent or both. Also, "senders" attribute mentioned below, but 
this is an attribute of <content/> element, not <payload-type/>.

So, there could be a situation, when initiator, for example can send and 
receive iLBC and opus, also can send speex and receive g.722 and g.726. 
And it preferes opus. Responder can send and receive iLBC and speex, can 
also send opus and receive g.726. And it preferes speex. So, 
*session-initiate* request will contain some payload types with names:
opus, iLBC, speex, g.722, g.726-32, g726-40
and *session-accept* request will contain payload types with names:
speex, opus, iLBC, opus, g.726-32, g.726-40

Now, when session is accepted, initiator will start sending RTP data 
encoded with opus, which responder cannot receive and responder will 
start sending RTP data encoded with speex, which initiator cannot receive.
Or there's something I don't understand?!

The second question is preferences. XEP-0167 says that order of 
<payload-type/> elements within <description/> element indicates sending 
entity preferences. But there's no mention about what to do with that 
information. What other party should do with knowledge of this party's 
payload type prefenrences?

The third question is about *"clockrate"* attribute. According to 
XEP-0167, "name" attribute is "RECOMMENDED for static payload types, 
REQUIRED for dynamic payload types", but *"clockrate" *is just 
"RECOMMENDED". But according to RFC 4566 
<http://tools.ietf.org/html/rfc4566#section-6>, both name and clockrate 
are required for a=rtpmap: line of SDP message, which is is required for 
dynamic payload types. And I found nothing about default value to use if 
no *"clockrate"* attribute specified.

With my best regards,
                                    Konstantin
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