Subject: [AVT] Does RTCP_MIN_TIME still apply when b=RR is specified?

From: "Anders Klemets" <[email-masked]>
Date: Sat, 4 Dec 2004 12:14:46 -0800

The algorithm for computing the RTCP reporting interval in RFC-3550 has
a constant called RTCP_MIN_TIME.  The purpose of this constant (its
value is 5 seconds) is to limit the RTCP reporting interval.  It has the
effect of limiting the actual RTCP bandwidth used by any single RTP
session participant.  

RFC-3556 defines the SDP attributes "b=RR" and "b=RS".  These attributes
can be used to increase, or decrease, the bandwidth available for RTCP
reports.  But when I read RFC-3556, it is not clear if the RTCP_MIN_TIME
parameter still applies, or not.  This seems to be left entirely to the
reader's imagination.  

When I started reading RFC-3556, I imagined that RTCP_MIN_TIME would not
apply.  RFC-3556 says that some applications may want to increase the
RTCP bandwidth beyond 5% of the session bandwidth, and that the solution
to that problem is provided by "b=RR" and "b=RS".  But RTCP_MIN_TIME is
effectively imposing a hard upper limit on the actual RTCP bandwidth
that anyone is allowed to use.  

So, because RTCP_MIN_TIME seems to work against the goal of being able
to increase the RTCP bandwidth, I thought that it would not apply in
this case.  I asked some other knowledgeable people, and they said the
same thing.

But after reading the "Security Considerations" section at the end of
RFC-3556, I now have a different impression.  It says that although
"b=RR" and "b=RS" can be used to increase the RTCP bandwidth, this is
not a new security problem, because the same thing can happen using
RFC-3550, if someone just specifies an artificially large session
bandwidth using "b=AS".  

But in a RFC 3550-only implementation, if someone specifies a large
session bandwidth using "b=AS", RTCP_MIN_TIME will still limit the
actual RTCP bandwidth used, preventing it from becoming "excessive".
And the "Security Considerations" section in RFC-3556 says that "b=RR"
and "b=RS" do not make this problem any worse.  So, I must conclude that
RTCP_MIN_TIME will still limit the actual RTCP bandwidth that can be
used, even when "b=RR" and "b=RS" are used to specify a larger
bandwidth.

Did I understand this correctly?

(Luckily, I read the Security Considerations section, or I might have
missed this important implicit information.)

Anders

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