[AVTCORE] Spencer Dawkins' Discuss on draft-ietf-avtcore-rtp-circuit-breakers-14: (with DISCUSS)

"Spencer Dawkins" <spencer.dawkins@huawei.com> Wed, 20 April 2016 01:20 UTC

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Subject: [AVTCORE] Spencer Dawkins' Discuss on draft-ietf-avtcore-rtp-circuit-breakers-14: (with DISCUSS)
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Spencer Dawkins has entered the following ballot position for
draft-ietf-avtcore-rtp-circuit-breakers-14: Discuss

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I really like this specification, and have two questions I'd like to
understand before balloting YES ...

I'm looking at this text:

4.5.  Ceasing Transmission

   What it means to cease transmission depends on the application.  The
   intention is that the application will stop sending RTP data packets
   to a particular destination 3-tuple (transport protocol, destination
   port, IP address), until the user makes an explicit attempt to
   restart the call.  It is important that a human user is involved in
   the decision to try to restart the call, since that user will
   eventually give up if the calls repeatedly trigger the circuit
   breaker.  This will help avoid problems with automatic redial systems
   from congesting the network.  Accordingly, RTP flows halted by the
   circuit breaker SHOULD NOT be restarted automatically unless the
   sender has received information that the congestion has dissipated,
   or can reasonably be expected to have dissipated. 
and trying to understand why this is not MUST NOT. I'm trying to
reconcile this with the RFC 2119 definition of SHOULD NOT, which is

4. SHOULD NOT   This phrase, or the phrase "NOT RECOMMENDED" mean that
   there may exist valid reasons in particular circumstances when the
   particular behavior is acceptable or even useful, but the full
   implications should be understood and the case carefully weighed
   before implementing any behavior described with this label.
Could you help me understand when automatic restarts might be "acceptable
or even useful"?

Reading on, I'm wondering if this text is anticipating 

   It is recognised that the RTP implementation in some systems might
   not be able to determine if a call set-up request was initiated by a
   human user, or automatically by some scripted higher-level component
   of the system.
but definitely want to understand what you're thinking here.

I have a similar question about this text 

   ECN-CE marked packets SHOULD be treated as if it were lost for the
   purposes of congestion control, when determining the optimal media
   sending rate for an RTP flow.  If an RTP sender has negotiated ECN
   support for an RTP session, and has successfully initiated ECN use on
   the path to the receiver [RFC6679], then ECN-CE marked packets SHOULD
   be treated as if they were lost when calculating if the congestion-
   based RTP circuit breaker (Section 4.3) has been met.  

Could you help me understand why an implementation wouldn't do this?