Re: [dispatch] RFC 3261 section 14.2 - "brand new call" does not specify whether the SDP should modify media attributes of an existing session containing a=sendonly or a=recvonly

[Paul Kyzivat <[e-mail]>]

Shaun,

Take a look at RFC6337 (especially section 5.1) and see if it helps.
That RFC was written to respond to many questions about O/A that came up 
over time. It is not normative, but rather simply clarifies things that 
are implicit upon analyzing an assortment of normative RFCs.

BTW, dispatch isn't really the right place for a question like this. A 
better place is <[e-mail]>.

	Thanks,
	Paul

On 9/5/20 5:43 AM, Shaun Stokes wrote:
> Hi,
> 
> We are struggling with some of the interpretation used in RFC 3261 
> section 14.2 which is creating a conflict between the software we use 
> (FreeSWITCH) and another environment (based on Cisco previously 
> Broadsoft equipment) both of which claim to be RFC 3261 compliant and 
> claim that the other is not.
> 
> The problem occurs when the 3rd party sends an SDP with the media 
> attribute 'a=sendonly' on an existing session then follow with a 
> RE-INVITE with-out an SDP, they claim that our 2xx offer in response 
> should contain an SDP with-out 'a=sendonly' (or replace with 
> 'a=sendrecv') based on the interpretation of a "brand new call" used 
> below. Anthony Minessale II (FreeSWITCH lead) claims that "brand new 
> call" is only intended to refer to codecs (not all media attributes) and 
> that the 3rd party (Broadsoft) invented this concept on their own.
> 
>     RFC 3261
> 
>     14.2 UAS Behavior
>     A UAS providing an offer in a 2xx (because the INVITE did not contain
>     an offer) SHOULD construct the offer as if the UAS were making a
>     brand new call, subject to the constraints of sending an offer that
>     updates an existing session, as described in [13] in the case of SDP.
>     Specifically, this means that it SHOULD include as many media formats
>     and media types that the UA is willing to support.  The UAS MUST
>     ensure that the session description overlaps with its previous
>     session description in media formats, transports, or other parameters
>     that require support from the peer.  This is to avoid the need for
>     the peer to reject the session description.  If, however, it is
>     unacceptable to the UAC, the UAC SHOULD generate an answer with a
>     valid session description, and then send a BYE to terminate the
>     session.
> 
>     Source: https://tools.ietf.org/html/rfc3261#section-14.2
> 
> 
> The 3rd party have also stated that this isn't a call going on hold as 
> it's routing to an ACD group, according to RFC 6337 section 5.3 "the use 
> of sendonly/recvonly is not limited to hold".
> 
> In the following discussion on this subject involving the authors of RFC 
> 3261 there is a clear indication that a RE-INVITE with-out an SDP should 
> not modify 'a=sendonly', unfortunately this isn't enough to support our 
> argument and our service providers protocol lead have determined that 
> the 3rd party is acting correctly and have asked for more evidence.
> http://marc.info/?t=98738614300001&r=1&w=2
> 
> We have also pointed out that RFC 3264 section 8 states that the offer 
> MAY be identical to the last SDP provided but are promptly referred back 
> to "brand new call" in RFC 3261 section 14.2.
> 
>     RFC 3264
> 
>     8 Modifying the Session
>     At any point during the session, either participant MAY issue a new
>     offer to modify characteristics of the session.  It is fundamental to
>     the operation of the offer/answer model that the exact same
>     offer/answer procedure defined above is used for modifying parameters
>     of an existing session.
> 
>     The offer MAY be identical to the last SDP provided to the other
>     party (which may have been provided in an offer or an answer), or it
>     MAY be different.  We refer to the last SDP provided as the "previous
>     SDP".  If the offer is the same, the answer MAY be the same as the
>     previous SDP from the answerer, or it MAY be different.  If the
>     offered SDP is different from the previous SDP, some constraints are
>     placed on its construction, discussed below.
> 
>     Source: https://tools.ietf.org/html/rfc3264#section-8
> 
> 
> I'm struggling to find anything more in RFC which can support our 
> argument, is it possible to update RFC 3261 14.2 to be more specific in 
> the terminology for "brand new call" or is this answered elsewhere?
> 
> Hope someone here can help.
> 
> Regards,
> Shaun
> 
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