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

[Harald Alvestrand <[e-mail]>]

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: [e-mail]
>> Subject: I-D Action: draft-perkins-avtcore-rtp-circuit-breakers-00.txt
>> Date: 5 March 2012 20:17:59 GMT
>> To: [e-mail]
>> Reply-To: [e-mail]
>>
>>
>> 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".

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.)

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.

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.

Good stuff!

                      Harald