[asterisk-dev] [Code Review] 4453: Asterisk 14: RTP improvements

[Matt Jordan]

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Review of https://wiki.asterisk.org/wiki/display/AST/RTP+engine+replacement

-- Section: A glossary of terms

{quote}
* RTP stream: RTP instances created by Steel Zebra will be referred to as RTP streams.
* RTP session: A structure created by Steel Zebra that contains related RTP streams and coordinates activities between streams where necessary.
{quote}

Since RTP instances are created separately by a channel driver, how will the RTP engine be notified that multiple RTP instances (that is, streams) are related? This may be answered in another section, but it might be good to know that this will require changes in the RTP engine here.

-- Section: Media Flow: Incoming Media

{quote}
Buffering/reordering

RTP may be received in bursts, out of order, or in other less-than-ideal ways. Asterisk will implement reception buffers to place incoming RTP traffic into, potentially reordering packets as necessary if they arrive out of order.
{quote}

While I'm not against buffering in the RTP stack, have you given any thought how that would be set up? As it adds delay, I would expect that not every RTP stream should be buffered; consequently, this would need to be driven by configuration or by some dialplan construct. Configuration may work in some cases (for example, when you know that some endpoint is always jittery); in other cases, dialplan is probably a better approach. In both cases however, these would require manipulation at a layer higher than the RTP stack itself, which would mean drilling down through the RTP engine into the RTP instance - which ends up sound something like our current jitter buffer approaches. What advantages are there to buffering in the stack itself, versus simply expanding the jitter buffers to handle more than just VOICE frames? Would we want to provide buffering in native RTP bridges, or let the far endpoints handle the re-ordering?

-- Section: Other Stuff

{quote}
Native local RTP bridges

Native local RTP bridges have a few considerations when implementing a new RTP engine.

First, bridge_native_rtp requires that the RTP engine's local_bridge method has to be the same for each of the bridged RTP instances. If we create a new RTP engine, it will not have the same local_bridge method as res_rtp_asterisk. This means that calls that use res_rtp_asterisk will not be able to be locally bridged with calls that use the new RTP engine. I think it is possible to rework the inner workings of native local bridges such that they can be between different RTP engines. However, if the goal is the total removal of res_rtp_asterisk from the codebase, then such considerations are not as necessary.

Second, native local RTP bridging is performed at the main RTP API layer by having the bridged RTP instances point at each other. It is up to the individual RTP instances to detect that this has occurred and act accordingly. It might work better if the job of setting bridges on RTP instances were passed down to the engines themselves in case they want to perform other side effects besides changing a pointer.
{quote}

I think it is arguable whether or not the local_bridge code should be in res_rtp_asterisk still. Ideally, an RTP implementation would simply have a "direct write/direct read" callbacks that the bridge itself would call into, rather than let the RTP implementation do the actual bridging. This has a few advantages:
(1) We could implement some other more interesting RTP bridges (such as a multi-party RTP forwarding bridge or a RTP bridge w/ RTP recording)
(2) It simplifies the thread boundaries. Right now, it's a little tricky managing the safety of calling into an RTP implementation from bridge_native_rtp. Having a more concrete boundary between the bridge and the RTP implementations would be advantageous.

Of course, if res_rtp_asterisk is refactored as opposed to replaced, that makes altering and/or supporting the native bridging in any fashion easier.

I think you might be referring to this in your second point, but I'm not entirely sure if this is what you meant in it.

- Matt Jordan


On Feb. 27, 2015, 12:47 p.m., Mark Michelson wrote:
> 
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> This is an automatically generated e-mail. To reply, visit:
> https://reviewboard.asterisk.org/r/4453/
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> 
> (Updated Feb. 27, 2015, 12:47 p.m.)
> 
> 
> Review request for Asterisk Developers.
> 
> 
> Description
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> 
> I've created a series of wiki pages that discuss the idea of writing an improved RTP architecture in Asterisk 14.
> 
> To regurgitate some details from the linked page, the current RTP engine in Asterisk (res_rtp_asterisk) gets the job done but has some issues. It is not architected in a way that allows for easy insertion of new features. It has dead code (or code that might as well be dead). And it has some general flaws in it with regards to following rules defined by fundamental RFCs.
> 
> I have approached these wiki pages with the idea of writing a replacement for res_rtp_asterisk.c. The reason for this is that there are interesting media-related IETF drafts (trickle ICE and BUNDLE, to name two) that would be difficult to implement in the current res_rtp_asterisk.c code correctly. Taking the opportunity to re-engineer the underlying architecture into something more layered and extendable would help in this regard. The goal also is to not disturb the high-level RTP engine API wherever possible, meaning that channel drivers will not be touched at all by this set of changes.
> 
> The main page where this is discussed is here: https://wiki.asterisk.org/wiki/display/AST/RTP+engine+replacement . This page has a subpage that has my informal rambling notes regarding a sampling of RTP and media-related RFCs and drafts I read. It also has a subpage with more informal and rambling notes about the current state of RTP in Asterisk. While these pages are not really part of the review, you may want to read them anyway just so you might have some idea of where I'm coming from when drawing up the ideas behind a new architecture.
> 
> I also have a task list page that details a list of high-level tasks that would need to be performed if a new RTP engine were to be written: https://wiki.asterisk.org/wiki/display/AST/RTP+task+list . This should give some idea of the amount of work required to make a new RTP engine a reality. The tasks with (?) around them are tasks that add new features to Asterisk's RTP support, and it is therefore questionable whether they fit in the scope of this work at this time.
> 
> Some things to consider when reading through this:
> * Refactor or rewrite? When considering current issues with RTP/RTCP in Asterisk, and considering the types of features that are coming down the pipe, which of these options seems more prudent?
> * Does the proposed architecture make sense from a high level? Is there confusion about how certain areas are intended to work?
> * Are there any glaring details you can think of that have been left out?
> * Are there any questions about how specific features would fit into the described architecture?
> 
> 
> Diffs
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> Diff: https://reviewboard.asterisk.org/r/4453/diff/
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> 
> Testing
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> 
> Thanks,
> 
> Mark Michelson
> 
>

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