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<a href=" https://wiki.asterisk.org/wiki/display/~mjordan " style="color:#326ca6;text-decoration:none;; color: #326ca6; text-decoration: none">Matt Jordan</a> edited the page:
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<td valign="top" style="font-size: 13px"> <h2 id="email-title-heading" style="font-size: 16px; line-height: 20px; min-height: 20px; margin: 0; padding: 0"> <a href="https://wiki.asterisk.org/wiki/display/AST/Media+Format+Rewrite" style="color: #326ca6; text-decoration: none"> <img class="icon" src="cid:page-icon" alt="" style="border: 0; padding: 0 5px 0 0; text-align: left; vertical-align: middle" /> <strong style="font-size:16px;line-height:20px;vertical-align:top;">Media Format Rewrite</strong> </a> </h2> </td>
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<p class="diff-context-placeholder" style="font-size: 13px">...</p>
<p class="diff-block-context" style="font-size: 13px">The following, for the most part, assumes that the channels use RTP for media and SIP for signalling. Most use cases, however, will translate to any VoIP channel driver. DAHDI, as always, is its own thing.</p>
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<th class="diff-macro-title" style="background-color: transparent; text-align: left; font-weight: normal;padding: 5px;; font-size: 13px"><span class="diff-html-added" id="added-diff-0" style="font-size: 100%; background-color: #ddfade;"><span class="icon macro-placeholder-icon" style="background-color: ;line-height: 20px;"><img src="https://wiki.asterisk.org/wiki/s/en_GB-1988229788/4252/6ac85e9b14675c5514a674e1aecae99c9505ed36.48/_/images/icons/macrobrowser/dropdown/note.png" style="padding-right: 5px; vertical-align: text-bottom;" /> </span>Note</span></th>
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<td class="diff-macro-body" style="background-color: #fff;border: 1px solid #dddddd;padding: 10px;; font-size: 13px"> <p style="font-size: 13px"> <span class="diff-html-added" style="font-size: 100%; background-color: #ddfade;">The Offer/Answer use cases below only apply to </span><code style="font-size: 13px"><span class="diff-html-added" style="font-size: 100%; background-color: #ddfade;">chan_pjsip</span></code><span class="diff-html-added" style="font-size: 100%; background-color: #ddfade;">. </span><code style="font-size: 13px"><span class="diff-html-added" style="font-size: 100%; background-color: #ddfade;">chan_sip</span></code><span class="diff-html-added" style="font-size: 100%; background-color: #ddfade;">, for better or worse, has its own fun rules about what codecs are offered and when.</span> </p>
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<th class="diff-macro-title" style="background-color: transparent; text-align: left; font-weight: normal;padding: 5px;; font-size: 13px"><span class="diff-html-added" style="font-size: 100%; background-color: #ddfade;"><span class="icon macro-placeholder-icon" style="background-color: ;line-height: 20px;"><img src="https://wiki.asterisk.org/wiki/s/en_GB-1988229788/4252/6ac85e9b14675c5514a674e1aecae99c9505ed36.48/_/plugins/servlet/confluence/placeholder/macro-icon?name=code" style="padding-right: 5px; vertical-align: text-bottom;" /> </span>Code Block</span></th>
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<td class="diff-macro-body" style="background-color: #fff;border: 1px solid #dddddd;padding: 10px;; font-size: 13px"> <pre style="font-size: 13px">
<span class="diff-html-added" style="font-size: 100%; background-color: #ddfade;">/* add_sdp: */
/* Now, start adding audio codecs. These are added in this order:
- First what was requested by the calling channel
- Then preferences in order from sip.conf device config for this peer/user
- Then other codecs in capabilities, including video
*/</span>
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</table> <p style="font-size: 13px"> <span class="diff-html-added" style="font-size: 100%; background-color: #ddfade;"> </span> </p> <p style="font-size: 13px"> <span class="diff-html-added" style="font-size: 100%; background-color: #ddfade;">Changing </span><code style="font-size: 13px"><span class="diff-html-added" style="font-size: 100%; background-color: #ddfade;">chan_sip</span></code><span class="diff-html-added" style="font-size: 100%; background-color: #ddfade;"> is fraught with peril. As such, we're going to try and give the power/flexibility of how things are offered/answered to where we can better maintain/control the behaviour, which means </span><code style="font-size: 13px"><span class="diff-html-added" style="font-size: 100%; background-color: #ddfade;">chan_pjsip</span></code><span class="diff-html-added" style="font-size: 100%; background-color: #ddfade;">.</span> </p> </td>
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<h2 id="MediaFormatRewrite-GeneralRules" class="diff-block-target diff-block-context"> <span class="diff-html-added" style="font-size: 100%; background-color: #ddfade;">General Rules</span> </h2>
<ol class="diff-block-target diff-block-context">
<li style="font-size: 13px"> <span class="diff-html-added" style="font-size: 100%; background-color: #ddfade;">For an inbound channel with a set of format capabilities, Asterisk should respond to that set of formats with the intersection of the offered capabilities and what is configured for the endpoint for that channel. The format preference order should be based on the configuration of the endpoint.</span>
<ol>
<li style="font-size: 13px"> <span class="diff-html-added" style="font-size: 100%; background-color: #ddfade;">If the system should accept a different set of codecs, a dialplan function and/or channel variable can be used to set which formats (and their preference order) are accepted on the channel at run-time. This would have to occur before the inbound channel is answered (via the MASTER_CHANNEL function and the U/M options in the dialplan).</span> </li>
<li style="font-size: 13px"> <span class="diff-html-added" style="font-size: 100%; background-color: #ddfade;">If the system would like to restrict in the device to a single format, a dialplan function and/or channel variable and/or configuration option can be set so that Asterisk will only ever respond with the preferred codec.</span> </li>
</ol> </li>
<li style="font-size: 13px"> <span class="diff-html-added" style="font-size: 100%; background-color: #ddfade;">For an outbound channel, Asterisk should offer the formats that have been configured for that endpoint, with the format preference order based on the configuration of that channel's endpoint.</span>
<ol>
<li style="font-size: 13px"> <span class="diff-html-added" style="font-size: 100%; background-color: #ddfade;">If the system would like to restrict the outbound channel such that it only has a single format, a dialplan function/channel variable/configuration option can be used such that Asterisk only offers a single format.</span> </li>
</ol> </li>
<li style="font-size: 13px"> <span class="diff-html-added" style="font-size: 100%; background-color: #ddfade;">Prior to entering a bridge, a dialplan function can be used to set whether or not that channel will attempt to make itself compatible with whatever is in the bridge with it. If a channel enters a bridge that has another channel in it with a format it supports, it will attempt to switch the channel to the bridged channel's format to facilitate native bridging. Note that this has no bearing in multi-party bridges, where everyone is transcoded.</span> </li>
<li style="font-size: 13px"> <span class="diff-html-added" style="font-size: 100%; background-color: #ddfade;">At any point in time, a dialplan function can be used to set the allowed set of formats on the channel, with whatever ordering. These formats should be a subset of the allowed formats configured on that channel's endpoint. This will cause the channel to re-negotiate to the set of formats specified by the function.</span> </li>
</ol>
<p class="diff-block-target diff-block-context" style="font-size: 13px"> <span class="diff-html-added" style="font-size: 100%; background-color: #ddfade;">The difference with this approach is that Asterisk will no longer always attempt to avoid transcoding. Instead, it will default to the rules configured in the </span><code style="font-size: 13px"><span class="diff-html-added" style="font-size: 100%; background-color: #ddfade;">.conf</span></code><span class="diff-html-added" style="font-size: 100%; background-color: #ddfade;"> files, overriding as it can via the dialplan. Transcoding may be more likely in poorly configured systems, but it will also allow for much greater flexibility in the behaviour of Asterisk.</span> </p>
<h2 id="MediaFormatRewrite-SingleChannel" class="diff-block-context">Single Channel</h2>
<h3 id="MediaFormatRewrite-NominalOffer%2FAnswer%28SingleMediaStream%29" class="diff-block-context">Nominal Offer/Answer (Single Media Stream)</h3>
<p class="diff-context-placeholder" style="font-size: 13px">...</p>
<ul class="diff-block-target">
<li style="font-size: 13px">Alice's phone offers a set of codecs in an INVITE request, where none of the codecs are supported by Alice's <span class="diff-html-removed" id="removed-diff-0" style="font-size: 100%; background-color: #ffe7e7; text-decoration: line-through;">endponit</span><span class="diff-html-added" id="added-diff-1" style="font-size: 100%; background-color: #ddfade;">endpoint</span> </li>
<li style="font-size: 13px">Asterisk responds with a 488.</li>
</ul>
<p class="diff-context-placeholder" style="font-size: 13px">...</p>
<p class="diff-block-context" style="font-size: 13px">All of the scenarios in Invalid Format Offer/Answer apply, only with a single audio and a single video stream. Streams can be declined, or the entire offer can be declined with a 488 as appropriate.</p>
<p class="diff-block-target diff-block-context" style="font-size: 13px"> <span class="diff-html-removed" id="removed-diff-1" style="font-size: 100%; background-color: #ffe7e7; text-decoration: line-through;"> </span> </p>
<h2 id="MediaFormatRewrite-MultipleChannels" class="diff-block-target diff-block-context"> <span class="diff-html-added" id="added-diff-2" style="font-size: 100%; background-color: #ddfade;">Multiple Channels</span> </h2>
<h3 id="MediaFormatRewrite-NominalOffer%2FAnswer" class="diff-block-target diff-block-context"> <span class="diff-html-added" style="font-size: 100%; background-color: #ddfade;">Nominal Offer/Answer</span> </h3>
<ul class="diff-block-target diff-block-context">
<li style="font-size: 13px"> <span class="diff-html-added" style="font-size: 100%; background-color: #ddfade;">Alice sends an INVITE request with an offer containing a set of codecs. The offer is a complete match with the set of codecs configured for Alice's endpoint.</span> </li>
<li style="font-size: 13px"> <span class="diff-html-added" style="font-size: 100%; background-color: #ddfade;">Asterisk dials Bob. Bob's endpoint is configured with the same set of codecs, in the same order.</span> </li>
<li style="font-size: 13px"> <span class="diff-html-added" style="font-size: 100%; background-color: #ddfade;">Bob's response to the INVITE request contains the same set of codecs as the offer. Asterisk responds to Alice with her set of configured codecs.</span> </li>
<li style="font-size: 13px"> <span class="diff-html-added" style="font-size: 100%; background-color: #ddfade;">Alice and Bob are bridged using the same formats, with the same priority order.</span> </li>
</ul>
<p class="diff-block-target diff-block-context" style="font-size: 13px"> <span class="diff-html-added" style="font-size: 100%; background-color: #ddfade;">Preferred codec only</span> </p>
<p class="diff-block-target diff-block-context" style="font-size: 13px"> <span class="diff-html-added" style="font-size: 100%; background-color: #ddfade;">Alice sends over an offer. Asterisk dials Bob with his codecs. Bob returns a set of codecs. Alice is sent as a reply on her preferred codec.</span> </p>
<p class="diff-block-target diff-block-context" style="font-size: 13px"> <span class="diff-html-added" style="font-size: 100%; background-color: #ddfade;">Transcoding</span> </p>
<p class="diff-block-target diff-block-context" style="font-size: 13px"> <span class="diff-html-added" style="font-size: 100%; background-color: #ddfade;">Alice sends over an offer. Asterisk dials Bob with his codecs, the preferred codec not being Alice. Alice get a reply with her codecs, and Asterisk transcodes.</span> </p>
<p class="diff-block-target diff-block-context" style="font-size: 13px"> <span class="diff-html-added" style="font-size: 100%; background-color: #ddfade;">Re-Invite to Native Bridge</span> </p>
<p class="diff-block-target diff-block-context" style="font-size: 13px"> <span class="diff-html-added" style="font-size: 100%; background-color: #ddfade;">Alice sends over an offer with a different codec priority than Bob. Asterisk Dials Bob. Bob responds back with a set of codecs in a different order than Alice. Asterisk Answers Alice with her preferred codecs. Prior to being Answered, we set something on Alice and/or Bob that says to 'negotiate to a common codec'. Bridging core does the rest and the two are natively bridged.</span> </p>
<p class="diff-block-target diff-block-context" style="font-size: 13px"> <span class="diff-html-added" style="font-size: 100%; background-color: #ddfade;">"Off nominal" Transcoding</span> </p>
<p class="diff-block-target diff-block-context" style="font-size: 13px"> <span class="diff-html-added" style="font-size: 100%; background-color: #ddfade;">Alice sends over an offer. Asterisk dials Bob with his codecs, which don't match Alice at all. Asterisk answers Alice with her codecs. Asterisk transcodes like crazy.</span> </p>
<p class="diff-block-target diff-block-context" style="font-size: 13px"> <span class="diff-html-added" style="font-size: 100%; background-color: #ddfade;">Modified outbound invite</span> </p>
<p class="diff-block-target diff-block-context" style="font-size: 13px"> <span class="diff-html-added" style="font-size: 100%; background-color: #ddfade;">Alice sends over an offer. Dialplan changes which codecs are offered on the outbound channel. Asterisk sends the codecs that it was told to send.</span> </p>
<p class="diff-block-target diff-block-context" style="font-size: 13px"> <span class="diff-html-added" style="font-size: 100%; background-color: #ddfade;">Modified inbound response</span> </p>
<p class="diff-block-target diff-block-context" style="font-size: 13px"> <span class="diff-html-added" style="font-size: 100%; background-color: #ddfade;">Alice sends over an offer. Dialplan changes the response. Asterisk responds with what you told it to respond with.</span> </p>
<p class="diff-block-context" style="font-size: 13px"> </p>
<p class="diff-block-context" style="font-size: 13px"> </p>
<h1 id="MediaFormatRewrite-Design" class="diff-block-context">Design</h1>
<h2 id="MediaFormatRewrite-ThePresent" class="diff-block-context">The Present</h2>
<h3 id="MediaFormatRewrite-structast_format" class="diff-block-context">struct ast_format</h3>
<p class="diff-context-placeholder" style="font-size: 13px">...</p>
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