Skip to content

Commit

Permalink
Describe how RTCRtpParameters dictionary is constructed in more detail.
Browse files Browse the repository at this point in the history
  • Loading branch information
@taylor-b
taylor-b committed May 18, 2017
1 parent 22b2ae1 commit be6c9c9
Showing 1 changed file with 95 additions and 33 deletions.
128 changes: 95 additions & 33 deletions webrtc.html
View file
Expand Up @@ -5663,19 +5663,53 @@ <h2>Methods</h2>
parameters for how <code>track</code> is encoded and transmitted
to a remote <code><a>RTCRtpReceiver</a></code>.</p>

<p>If a remote description for this sender has not yet been set,
<code>getParameters</code> will return an
<code><a>RTCRtpParameters</a></code> dictionary with <code><a
data-link-for="RTCRtpParameters">encodings</a></code> set to
the value of the [[<a>send encodings</a>]] internal slot, and
all other members <code>undefined</code>. If a remote
description for the sender has been set,
<code>getParameters</code> will also return
<code><a>RTCRtpCodecParameters</a></code> and
<code><a>RTCRtpHeaderExtensionParameters</a></code> from the
parsed remote description, for each codec and header extension
that the implementation supports.</p>

<p>When <code>getParameters</code> is called, the
<code><a>RTCRtpParameters</a></code> dictionary is
constructed as follows:</p>
<ul>
<li>
<code><a data-link-for="RTCRtpParameters">transactionId</a></code>
is set to a new unique identifier, used to match this
<code>getParameters</code> call to a
<code>setParameters</code> call that may occur later.
</li>
<li>
<code><a data-link-for="RTCRtpParameters">encodings</a></code>
is set to the value of the [[<a>send encodings</a>]] internal
slot.
</li>
<li>
The <code><a data-link-for="RTCRtpParameters">headerExtensions</a></code>
sequence is populated based on the header extensions that
have been negotiated for sending.
</li>
<li>
The <code><a data-link-for="RTCRtpParameters">codecs</a></code>
sequence is populated based on the codecs that have been
negotiated for sending, and which the user agent is currently
capable of sending. If <code>setParameters</code> has
removed or reordered codecs, <code>getParameters</code> MUST
return the shortened/reordered list. However, every time
codecs are renegotiated by a new offer/answer exchange, the
list of codecs MUST be restored to the full negotiated set,
in the priority order indicated by the remote description, in
effect discarding the effects of <code>setParameters</code>.
</li>
<li>
<code><a data-link-for="RTCRtpParameters">rtcp</a>.cname</code>
is set to the CNAME of the associated
<code><a>RTCPeerConnection</a></code>.
<code><a data-link-for="RTCRtpParameters">rtcp</a>.reducedSize</code>
is set to <code>true</code> if reduced-size RTCP has been negotiated
for sending, and <code>false</code> otherwise.
</li>
<li>
<code><a data-link-for="RTCRtpParameters">degradationPreference</a></code>
is set to the last value passed into <code>setParameters</code>, or
the default value of "balanced" if <code>setParameters</code> hasn't
been called.
</li>
</ul>
<p><code>getParameters</code> may be used with
<code>setParameters</code> to change the parameters in the
following way:</p>
Expand Down Expand Up @@ -6603,26 +6637,54 @@ <h2>Methods</h2>
<code>RTCRtpReceiver</code> object's current parameters for how
<code>track</code> is decoded.</p>

<p>The receiver's parameters are determined by the applied local
and remote descriptions; the
<code><a>RTCRtpCodecParameters</a></code> and
<code><a>RTCRtpHeaderExtensionParameters</a></code> are
determined by the local description, and the SSRCs in the
<code><a>RTCRtpEncodingParameters</a></code> may be populated
from the remote description, if SSRCs are signaled. A remote
description may also cause offered codecs/header extensions to be
removed. For example, if three codecs are offered, and the remote
endpoint only answers with two, the absent codec will be
removed from the receiver's parameters, as the receiver no longer
needs to be prepared to receive it.</p>

<p>Note that the receiver's parameters are determined solely by
the applied local and remote descriptions. This means that, for
instance, if SSRCs are unsignaled, the <code><a
data-link-for="RTCRtpEncodingParameters">ssrc</a></code>
member of the <code><a>RTCRtpEncodingParameters</a></code> will
be undefined, even after packets are received and an internal
SSRC/receiver mapping is created.</p>
<p>When <code>getParameters</code> is called, the
<code><a>RTCRtpParameters</a></code> dictionary is
constructed as follows:</p>
<ul>
<li>
<p><code><a data-link-for="RTCRtpParameters">encodings</a></code>
is populated based on SSRCs and RIDs present in the current
remote description, including SSRCs used for RTX and FEC, if
signaled. Every member of the
<code><a>RTCRtpEncodingParameters</a></code> dictionaries
other than the SSRC and RID fields is left
<code>undefined</code>.</p>
<div class="note">This means that if SSRCs are unsignaled,
the SSRC members will be <code>undefined</code>, even after
a packet is received and an internal SSRC/receiver mapping is
created.</div>
</li>
<li>
The <code><a data-link-for="RTCRtpParameters">headerExtensions</a></code>
sequence is populated based on the header extensions that the
receiver is currently prepared to receive.
</li>
<li>
<p>The <code><a data-link-for="RTCRtpParameters">codecs</a></code>
sequence is populated based on the codecs that the receiver
is currently prepared to receive.</p>
<div class="note">Both the local and remote description may
affect this list of codecs. For example, if three codecs are
offered, the receiver will be prepared to receive each of
them and will return them all from
<code>getParameters</code>. But if the remote endpoint only
answers with two, the absent codec will no longer be returned
by <code>getParameters</code> as the receiver no longer needs
to be prepared to receive it.</div>
</li>
<li>
<code><a data-link-for="RTCRtpParameters">rtcp</a>.reducedSize</code>
is set to <code>true</code> if the receiver is currently prepared to
receive reduced-size RTCP packets, and <code>false</code> otherwise.
<code><a data-link-for="RTCRtpParameters">rtcp</a>.cname</code> is
left <code>undefined</code>.
</li>
<li>
<code><a data-link-for="RTCRtpParameters">transactionId</a></code> and
<code><a data-link-for="RTCRtpParameters">degradationPreference</a></code>
are left <code>undefined</code>.
</li>
</ul>
<div>
<em>No parameters.</em>
</div>
Expand Down

0 comments on commit be6c9c9

Please sign in to comment.