[Iccrg] RE: TFRC-SP and maximum packet rate
Ingemar Johansson S
ingemar.s.johansson at ericsson.com
Tue Oct 12 08:42:20 BST 2010
Hi
Sorry about the delay. I have read the report below and it answers a lot of questions. My take is that even though TFRC may sometimes fail to be 100% fair against TCP it is anyway a lot better than nothing. I am very interested in your work so if you can send me a draft copy of your work I'd be more than happy.
>From my (wireless) perspective I still have some concerns as regards to how fast TFRC can react to the higher dynamics of for instance 3GPP access like LTE or HSDPA/EUL. I can imagine that one may need an additional emergency break to cope with this but I have no real work done to prove my point.
Some additional info:
My interest in TFRC is given by the initiative from Google et al to bring real time communication to the web platform (see http://www.alvestrand.no/mailman/listinfo/rtc-web )
My main interest in this area is congetsion control, a feature that I believe is very important as the real time media will in this case share the same bottlenecks as TCP.
/Ingemar
> -----Original Message-----
> From: Soo-Hyun Choi [mailto:S.Choi at cs.ucl.ac.uk]
> Sent: den 21 september 2010 13:59
> To: Ingemar Johansson S
> Cc: iccrg; tsvwg at ietf.org; kohler at cs.ucla.edu; floyd at icir.org
> Subject: Re: TFRC-SP and maximum packet rate
>
> Hi,
>
> >
> > An issue with modern video codecs is that they produce frames with
> > larger differences in frame size. For instance I-frames may
> be so big
> > that they need to be split up in several packets (for PMTU reasons)
> > while B- and P-frames are considerably smaller. The effect
> of this is
> > that even though the framerate of the video is 30Hz, the
> packet rate
> > may be considerably higher when I-frames are transmitted.
> But I guess
> > the only way to determine how TFRC-SP works in these cases
> is to try
> > it out.
>
> You are right - the packet size will significantly vary
> depending on the types of the frame (I, B, or, P) and video
> content as well. As I said earlier, using TFRC-SP for video
> streams without a modification may break the (proportional)
> flow-level fairness when competing with the standard TCP flows.
>
> There is another similar work that studied TFRC performance
> with the variable packet sizes -
> <http://www.icsi.berkeley.edu/ftp/pub/techreports/2000/tr-00-008.pdf>
> This work also mentioned the fairness issues and solve the
> problem by estimating the mean packet size (assuming PMTU is
> known a priori).
>
> However, it is unclear to me as to how much the approximated
> packet size is meaningful to the calculated send rate
> (because, the frame sizes are so varying as we know).
>
> I have an on-going work to solve these problems by
> re-introducing the TCP-like Ack-clocking mechanism while
> retaining the smoothness property in the send rate (of
> course, for the interactive video streaming applications). I
> should be able to publish the results somewhere in the next
> month or so. If you are interested, I can send you the draft
> copy sometimes early next month.
>
>
> Cheers,
> Soo-Hyun
>
>
>
>
>
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