Re: [AVTCORE] [R-C] I-D Action: draft-perkins-avtcore-rtp-circuit-breakers-00.txt

On 6 Mar 2012, at 07:52, Harald Alvestrand wrote:
> On 03/06/2012 12:32 AM, Colin Perkins wrote:
>> 
>> Here's our initial attempt at a "circuit breakers" draft for RTCWeb. Comments welcome - this is very much a straw-man for discussion, rather than a final solution.
>> 
>> Colin
>> 
>> 
>> 
>> Begin forwarded message:
>>> From: internet-drafts@ietf.org
>>> Subject: I-D Action: draft-perkins-avtcore-rtp-circuit-breakers-00.txt
>>> Date: 5 March 2012 20:17:59 GMT
>>> To: i-d-announce@ietf.org
>>> Reply-To: internet-drafts@ietf.org
>>> 
>>> 
>>> A New Internet-Draft is available from the on-line Internet-Drafts directories.
>>> 
>>> 	Title           : RTP Congestion Control: Circuit Breakers for Unicast Sessions
>>> 	Author(s)       : Colin Perkins
>>>                          Varun Singh
>>> 	Filename        : draft-perkins-avtcore-rtp-circuit-breakers-00.txt
>>> 	Pages           : 14
>>> 	Date            : 2012-03-05
>>> 
>>>   The Real-time Transport Protocol (RTP) is widely used for telephony,
>>>   video conferencing, and telepresence applications.  These
>>>   applications are often used over best-effort UDP/IP networks.  If
>>>   congestion control is not implemented then network congestion will
>>>   deteriorate the user's multimedia experience.  This document does not
>>>   propose a congestion control algorithm.  Instead, it specifies a
>>>   minimal set of "circuit-breakers".  Circuit-breakers are conditions
>>>   under which an RTP flow should cease to transmit media to protect the
>>>   network from excessive congestion.  It is expected that all RTP
>>>   applications running on best-effort networks will be able to run
>>>   without triggering these circuit breakers in normal operation.
>>> 
>>> 
>>> A URL for this Internet-Draft is:
>>> http://www.ietf.org/internet-drafts/draft-perkins-avtcore-rtp-circuit-breakers-00.txt
> Thanks for this work!
> 
> A few questions (to make sure I get them noted down):
> 
> Section 4.1:
> 
>    Accordingly, if a
>    sender of RTP data packets receives two or more consecutive RTCP RR
>    packets from the same receiver that correspond to its transmission
>    and have a non-increasing extended highest sequence number received
>    field, then that sender SHOULD cease transmission.
> 
> If I see RTCP packets with
> 
> 1: highest sequence number = 2
> 2: highest sequence number = 2
> 3: highest sequence number = 2
> 
> do I cease transmission after packet 3 has arrived, or after packet 2 has arrived?
> I *think* the logical time is after packet 3 has arrived, but I'm a little unsure that the words are
> unambiguously saying that; it's not 100% clear to me whether packet 1 is considered included in the set of "non-increasing highest sequence number".

We should clarify. My feeling is that either way would work, depending on how aggressive we want the circuit breaker to be. 

> Section 4.2: Is it reasonable to replace, for the purposes of this calculation, "an order of magnitude" with "a factor of ten"? (for those who don't have a physics background, putting text somewhere that says that an order of magnitude is "somewhere around a factor of ten" might be appropriate.)

Sure, will clarify. 

> We might also want to add the words about doing a dramatically reduced rate if we can from section 4.1 here, factor it out as a general statement, or say that it is not appropriate here if it's not.

Yes.

> Security considerations (missing section): For an end node that implements this specification, an active attacker can cut the transmission by faking two RTCP packets that get accepted instead of the recipient's RTCP packets. This may be worthy of a note, and pointer to appropriate defenses.

Agreed.

> Good stuff!
> 
>                      Harald
> 
> 
> 
>  
> 
> 
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-- 
Colin Perkins
http://csperkins.org/