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Re: [MMUSIC] [rtcweb] Updating JSEP and BUNDLE: Semantics of same port in multiple m- lines

Christer Holmberg <christer.holmberg@ericsson.com> Fri, 29 January 2021 18:16 UTC

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From: Christer Holmberg <christer.holmberg@ericsson.com>
To: Roman Shpount <roman@telurix.com>
CC: Christer Holmberg <christer.holmberg=40ericsson.com@dmarc.ietf.org>, Magnus Westerlund <magnus.westerlund=40ericsson.com@dmarc.ietf.org>, "mmusic@ietf.org" <mmusic@ietf.org>
Thread-Topic: [MMUSIC] [rtcweb] Updating JSEP and BUNDLE: Semantics of same port in multiple m- lines
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Date: Fri, 29 Jan 2021 18:16:02 +0000
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Subject: Re: [MMUSIC] [rtcweb] Updating JSEP and BUNDLE: Semantics of same port in multiple m- lines
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Hi,

> This is exactly the language I have remembered. So, it is not explicitly prohibited but the behavior is undefined.

...which is why you cannot make any assumptions regarding how the receiver will process such SDP - unless you have prior knowledge about the capabilities of the receiver.

Regards,

Christer




On Thu, Jan 28, 2021 at 4:26 AM Christer Holmberg <christer.holmberg@ericsson.com<mailto:christer.holmberg@ericsson.com>> wrote:
This is the test from Section 5.14 of RFC 4566:

      “The semantics of multiple "m=" lines using the same transport
      address are undefined.  This implies that, unlike limited past
      practice, there is no implicit grouping defined by such means and
      an explicit grouping framework (for example, [18]) should instead
      be used to express the intended semantics.”

([18] refers to the SDP grouping framework defined in RFC 5888)


This is the corresponding (slightly modified) text from Section 5.12 of RFC 8866:

     “This document gives no meaning to assigning the same media address
      to multiple media descriptions.  Doing so does not implicitly
      group those media descriptions in any way.  An explicit grouping
      framework (for example, [RFC5888]) should instead be used to
      express the intended semantics.  For instance, see [RFC8843].”

(RFC 8843 is BUNDLE)


So, again, it is ok to use the same port in multiple m- lines, if the semantics is defined, and if the participants support that semantics.

When you send an initial INVITE, from a standards viewpoint, you don’t know whether the participants in the path support that semantics.

Regards,

Christer



From: mmusic <mmusic-bounces@ietf.org<mailto:mmusic-bounces@ietf.org>> On Behalf Of Christer Holmberg
Sent: torstai 28. tammikuuta 2021 1.26
To: Roman Shpount <roman@telurix.com<mailto:roman@telurix.com>>
Cc: Magnus Westerlund <magnus.westerlund=40ericsson.com@dmarc.ietf.org<mailto:40ericsson.com@dmarc.ietf.org>>; mmusic@ietf.org<mailto:mmusic@ietf.org>
Subject: Re: [MMUSIC] [rtcweb] Updating JSEP and BUNDLE

Hi,

>Can you or anyone else provide the reference to the document which prohibits using the same port in multiple m= lines?

It is not prohibited, but the SDP spec says that the semantics is undefined.

Now, in BUNDLE we can define semantics for usage of multiple m= lines, and BUNDLE endpoints would support that semantics.

The problem is if you send an initial INVITE that reaches a non-BUNDLE endpoint. There is no way to know how that endpoint will process the offer, and we have seen that the offer gets rejected (in different ways).

Regards,

Christer

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