Re: [AVTCORE] WG last call for draft-ietf-avtcore-rtp-circuit-breakers-11

gorry@erg.abdn.ac.uk Sun, 29 November 2015 15:27 UTC

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Cc: magnus.westerlund@ericsson.com
Subject: Re: [AVTCORE] WG last call for draft-ietf-avtcore-rtp-circuit-breakers-11
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I have reviewed draft-ietf-avtcore-rtp-circuit-breakers-11.

This document appears to be useful to publish as a Standards Track
document, and appears to be complete. My review is from a  transport
perspective (rather than based on RTP expertise). I have the following
comments (below).

Gorry

----
/If congestion control is not implemented in the applications, then
network congestion will deteriorate the user's multimedia experience./
- This is likely true, but then it could be more important that a circuit
breaker is used to protect other traffic sharing the bottleneck, than just
the media of the flow. Is it possible to include something like or similar
to:
   This acts as a safety measure to prevent
   starvation of network resources denying other flows from access to
   the Internet, such measures are essential for an Internet that is
   heterogeneous and for traffic that is hard to predict in advance.
----
/resort to avoid persistent congestion/
- A requested change to the related draft in TSVWG is to use the text:
/resort to avoid persistent excessive congestion/
(This has the advantage of highlighting both “persistent” over time, and
“excessive” not a low level of loss/mark.)
----
NiT:
/so that it matches the available network bandwidth./
- I’d suggest the words for this is /not more than the capacity available
along the network path /
Is /available bandwidth estimator/ also /available capacity estimator/?
----
/ but there is potential to disrupt the network's operation/
- The use of "disrupt" seems like a little vague.
----
/p is the loss event rate, between 0.0 and 1.0,/
- strictly speaking this seem like it should be a ratio rather than a rate
per second?
----
/Implementations that sometimes need to interwork with endpoints that do
not support RTCP need to disable the RTP circuit breakers if they don't
receive some confirmation via signalling that the remote endpoint
implements RTCP (the presence of an SDP "a=rtcp:" attribute in an answer
might be such an indication).
- I think I understand the intention, but this somehow does not seem the
correct thing to write,
Later, the document says “SHOULD monitor packet loss to ensure that the
packet loss rate is within acceptable parameters”, which I think is
correct, hence this seems like a violation of a SHOULD, which to me seems
to imply careful wording to say do-this-if-it-is-possible, and then say
why this is clearly impossible in some cases.
---
/The remaining congestion signals are the packet loss fraction and the
cumulative number of packets lost.  /
- This para - or the set of paras here - could benefit from a few
sentences that explain that while such signals can be used immediately as
input to a CC, they need to be taken over a reasonable time scale when
used for a circuit breaker, because the measured loss/marks/delay can be
transient and could otherwise result in premature triggering.
draft-ietf-tsvwg-circuit-breaker could be a reference here.
- It also may be worth considering adding a note saying that continuous,
but low levels of loss/delay also may result from normal operation when
flows sharing a path, and that these too shouldn’t be used on their own as
a basis for terminating a flow. (i.e. a threshold is needed), also
draft-ietf-tsvwg-circuit-breaker could be a reference here.
---
NiT
/When a sender receives an RTCP packet indicating that the media it's
sending is being received/
- Is there an English problem?
Perhaps it may be possible to say something like:
/When a sender receives an RTCP packet that indicates reception of the
media it has been sending/
---
NiT
/the throughput a TCP connection would achieve over the path/
- I think this is the *maximum* throughput a TCP connection *could*
achieve or better written as
 /the throughput a *bulk* TCP connection would achieve over the path/
---
[RFC3448] has been obsoleted, do you specifically mean to refer to this
rather than RFC 5348?
---
/However, in pathological scenarios where the congestion
   circuit breaker does not stop the flow, it is desirable that the RTP
   application cease sending useless traffic. The role of the media
   usability circuit breaker is to protect the network in such cases./
- True, but “protect the network” is maybe a little veiled, this seems to
be more about
preventing the use of network resources denying other flows from access to
the Internet... anyway just saying protect seems wrong to me.
---
NiT
/What it means to cease transmission depends on the application, but
   the intention is that/
- A mangled sentence?
---
/restart the call/
Section 4.5 could point to draft-ietf-tsvwg-circuit-breaker for rules on
reseting
circuit breakers, which seem to be similar, and could benefit from a
cross-reference?
     “The reaction to a triggered Circuit Breaker MUST continue for a
      period that is at least the triggering interval.  Operator
      intervention will usually be required to restore a flow.  If an
      automated response is needed to reset the trigger, then this needs
      to not be immediate.  The design of an automated reset mechanism
      needs to be sufficiently conservative that it does not adversely
      interact with other mechanisms (including other Circuit Breaker
      algorithms that control traffic over a common path).  It SHOULD
      NOT perform an automated reset when there is evidence of continued
      congestion.“
---
Should the reference to I-D.ietf-tsvwg-circuit-breaker be normative, since
this is a BCP on the same topic, and I would expect any update to this
document would need to be consistent with this, or update this?
---