[vlc-devel] Re: vlc: svn commit r13006 (fkuehne)

Marian Durkovic md at bts.sk
Sat Oct 29 09:08:33 CEST 2005

On Sat, Oct 29, 2005 at 12:12:19AM +0200, Derk-Jan Hartman wrote:
> On 28-okt-2005, at 22:27, Marian Durkovic wrote:
> >   I really don't quite understand what's the problem here. In this  
> >context,
> >RTP is nothing else that UDP plus 12 bytes. But the huge difference  
> >is, that
> >it is standards based (RFC3550) and works properly over large networks
> >(read Internet). It uses exactly the same payload format - MPEG2-TS.
> I'm perfectly aware what it is. It is "different" from UDP, not just  
> UDP with something extra. that's like saying HTTP is just TCP. The  
> view is too simplistic.

No, actually not, you're confusing different protocol layers.
TCP, UDP and RTP are transport layer protocols, while HTTP, FTP, RTSP etc. 
are application layer protocols.

In other words, HTTP is a "language" which the client must speak in order to 
get any data from the server. Without sending HTTP request, parsing the 
responses etc. nothing happens.

On the other hand, 12-bytes long RTP header just carries some additional data,
which a simple client could easily skip and handle the rest as raw UDP.
> >   Could you please elaborate, what's the exact problem with  
> >replacing UDP by RTP >>in streaming wizard<< ?  
> 1: you didn't ask
> 2: UDP has much less overhead then RTP

This doesn't apply here. RTP overhead is a matter of concern for VoIP
applications, transferring 20 byte payload units (G.729 etc.), where
it increases packet size by 25 %. With MPEG-TS used for UDP streaming,
the addition of RTP header increases packet size by less than 0.9 % 
which is not worth any consideration.

> 3: RTP has undergone major changes. changes make something unstable  
> and therefore making this default is DEFINITELY not something that  
> should be done just before an 0.8.4 release

Those changes were heavily tested in our lab in various scenarios (reordered
packets, missing packets, duplicate packets - PIM ASSERT etc.), and proved 
to be working fine. Yes, Murphy is always around, but you'll never find this 
kind of bugs until many people really start using the code. As 0.8.4-test2
hasn't happened yet, there are at least 2 weeks to get feedback and fix
possible bugs before final 0.8.4 release. But since Felix removed the
code yesterday, we would not get the feedback anymore :-(

> 4: UDP is used a lot for various STB solutions

Yes, I'm aware of that and therefore I've never proposed to discontinue UDP 
support and have also not removed it from StreamOutput dialog. But things
called "Wizard" should IMHO guide the users to make best-possible choices, 
and RTP is definitely better than raw UDP. 

> 5: you didn't ask

So what's left is 1) and 5) - yes I didn't ask, but I have sent several
emails about this topic to the vlc lists in the last couple of months
and therefore expected the RTP vs. UDP topic is well understood.

	With kind regards,

----                                                                  ----
----   Marian Durkovic                       network  manager         ----
----                                                                  ----
----   Slovak Technical University           Tel: +421 2 524 51 301   ----
----   Computer Centre, Nam. Slobody 17      Fax: +421 2 524 94 351   ----
----   812 43 Bratislava, Slovak Republic    E-mail/sip: md at bts.sk    ----
----                                                                  ----

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