[Iccrg] Re: TFRC-SP and maximum packet rate

[Soo-Hyun Choi]

Hi,

> 
> I have a question regarding TFRC-SP (RFC4828). It states a maximum
> packet rate or 100 packets/s. What was the rationale behind this
> restriction ?. Suppose that a 2Mbps video codec use TFRC-SP (mainly
> because TFRC Inter packet Interval may be difficult to implement
> given various OS concerns). This would give a packet rate of
> ~170packets/s
> 
> My question... How does this affect the stability of TFRC-SP ?
> 


That's right - the OS's clock granularity can limit the calculated TFRC
rate. I would imagine that the issues with the OS's clock granularity is
something that you cannot get away from when using a rate-based protocol.


As you know, TFRC-SP, however, is mainly intended to be used for a VoIP
type of applications (e.g., with smaller packets). In general, video
packets are much bigger than the usual VoIP packets, so it might be
inappropriate to use it without careful consideration - TFRC-SP used
with 1460 bytes packet size would break fairness when competing with
standard TCP flows (see
<http://icapeople.epfl.ch/widmer/tfmcc/vp-tfmcc.html> for the detailed
reasoning behind this). It basically says the smaller the packet size,
the more the flow will receive bandwidth sharing (if not modifying the
loss event calculation method).

RFC4828 explained some of the reasoning there as well (see RFC4828
Section 4.3).


Cheers,
Soo-Hyun