Dear Wesley, Dear All,
First of all our feedback regarding different "re-entering dropping
state" in the document and in the Linux implementation
(http://www.ietf.org/mail-archive/web/aqm/current/msg01686.html) was not
addressed.
As FQ-CoDel relies on CoDel, this issue is also (partly) relevant for
the FQ-CoDel document. In the introduction FQ-CoDel references ns-3 and
Linux implementations where the first one uses the re-entering logic
from the CoDel document while the second from CoDel Linux
implementation. The algorithm that has seen widespread testing according
to Section 7 is (I suppose) the Linux version. Is this situation
acceptable for an algorithm specification?
/[since this comment was supposed to be sent before the end of 2015,
feel free to (silently) ignore it]/
Second, unlike Rasool Al-Saadi (see
http://www.ietf.org/mail-archive/web/aqm/current/msg01693.html) I do not
like the document. Although I agree that the pseudocode is sufficient to
create a working implementation, however, in my opinion, the rest of the
document makes implementing CoDeL more confusing (at least without
reading [CODEL2012] first). Is it normal for a RFC, which, as I assume,
should primarily contain an algorithm specification to contain the
algorithm specification ONLY in form of pseudocode?
These are two items that I found the most confusing:
(1) section 3 introduces estimator, setpoint, and control loop which are
not clearly distinguishable in pseudocode. It would be nice if section 4
explained how the three entities transition into the routines in
pseudocode.
(2) IMHO, there is some missing explanations. For example, the document
never says how exactly "bad"/persistent queue is determined. The
document says in Section 3.1 that the minimum is tracked, but it never
says that persistent queue is when the minimum is above setpoint/target
for at least an interval. As another example, Section 3.3 could say how
the controller looks like exactly.
I also have some small comments and nits regarding the latest version (-03):
---
in the whole document there are several places of using "we" to
(supposedly) refer to the authors. According to Bob Briscoe's message
(http://www.ietf.org/mail-archive/web/aqm/current/msg01805.html), "we"
do not necessary refer to the authors in a rfc. Are these "we"s ok?
---
abstract:
This document describes a general*framework* called CoDel
IMHO the word "framework" is too overloaded and too broad. It might be
better to call CoDel an AQM or something that has "queue" in it. Other
ways (e.g., http://queue.acm.org/detail.cfm?id=2839461) probably also
use it to control a queue.
---
section 1:
(nit)
Despite this
awareness, the problem has only gotten worse as Moore's Law growth in
memory density fueled an exponential increase in buffer pool size.
Efforts to deploy AQM have been frustrated by difficult configuration
and negative impact on network utilization. *This problem,* recently
christened "bufferbloat", [TSV2011] [BB2011] has become increasingly
important throughout the Internet but particularly at the consumer
edge.
it reads like "this problem" refers to "efforts to deploy AQM" and not
to "increase in buffer pool size"
---
(nit)
To understand
queue management, it is critical to understand the difference between
the necessary, useful*"good" queue*, and the counterproductive*"bad" queue*.
strictly speaking "good" and "bad" queue were not defined either before
or in this sentence (it is explained in section 3 or in [CODEL2012])
---
(nit)
o treat "good queue" and "bad queue" differently, that is, keep
delay low while permitting necessary bursts of traffic
(1) see above
(2) I would say that the goal is "to keep delay low while ..." and to
treat good/bad queue differently is a mechanism used to achieve the goal
---
section 2:
(nit)
In this document, *the characters ">>" preceding an indented line(s)*
indicates a compliance requirement statement using the key words
listed above. This convention aids reviewers in quickly identifying
or finding the explicit compliance requirements of this RFC.
I maybe missing something, but I can't find any use of "characters ">>""
anywhere except this line in any (txt,html, pdf) of the versions.
---
section 3:
generally IMHO it would be nice to introduce /interval/ and /target/ so
it is more obvious that these are parameters (or some other kind of
variables). Interval is more or less obvious (although imho should be
more explicit). The target is first introduced at the end of section 3.2 as:
This results in a particularly simple form for the
setpoint: the ideal range for the permitted standing queue is between
5% and 10% of the TCP connection's RTT. Thus*target* is simply 5% of
the interval ofsection 3.1
<https://tools.ietf.org/html/draft-ietf-aqm-codel-03#section-3.1>.
It was not previously defined that /target/ is a parameter with a
meaning of /setpoint/. I don't think that this is clear for people who
do not know what target is in advance.
[It seems that it was intended to introduce /interval/ and /target/
parameters in section 4]
---
Instead of averages
*we recommend* tracking the minimum sojourn time, then,
"we recommend" or "CoDeL does"? According to the last paragraph in
Section 3.1, tracking minimum is a core part of CoDeL:
These two innovations, use of sojourn time as observed values*and the local
minimum as the statistic to monitor queue congestion* are key to
CoDel's estimator building block.
---
Since the peak queue delay*is simply f r*, power is solely a function
is it f/r ?
---
(nit)
The only remaining building block needed for a basic AQM is a
'control loop' algorithm to effectively drive the queueing system
from any*'persistent queue above target'* state to a state where the
*persistent queue is below target*.
It would be nice to explicitly state that the "persistent queue above
target" is indication of "bad" queue/congestion somewhere above or in
this line. I think that these terms are used in [codel2012] but were not
used before in the document. See also the next item.
---
In section 3.3 there are three "states":
*
'persistent queue above target' state
*
'has persistent queue' state
*
dropping state
It would be nice to a) use the same one b) explain how CoDeL gets in/out
of there
---
(probably nit)
When
*the minimum sojourn time first crosses the target* and*CoDel drops a
packet*, the earliest the controller could see the effect of the drop
from current sentence it could be inferred that the two events occur at
the same time which is not the case. Maybe " when persistent queue above
target is detected" would be better.
---
variation is less than the target, and so the*initial drop spacing SHOULD be set to the estimator's interval plus
twice the target (i.e., initial drop spacing = 1.1 * interval)* to ensure that the
this contradicts the pseudocode:
dodequeue_result codel_queue_t::dodequeue(time_t now)
...
first_above_time_ = now + interval_;
section 4:
To keep from
making drops when it would starve the output link, CoDel makes
another check before dropping to see if at least an MTU worth of
bytes remains in the buffer. If not, the packet SHOULD NOT be
dropped and,*currently*, CoDel exits the drop state.
what is the meaning of "currently" ? in the current experimental version?
---
(nit)
In practice, this value is not known or measured*(*though see
Section 6.2
<https://tools.ietf.org/html/draft-ietf-aqm-codel-03#section-6.2> for an application where interval is measured.
missing closing bracket
Section 5 / pseudocode: (all nits)
The variables are introduced as first_time_above, dropping, but are used
as first_time_above_, dropping_ (with underscores at end).
---
*Packet** CoDelQueue::dequeue()
a pointer to current packet was introduced as packet_t
*--- double* now = clock();;
time units were supposed to be of type time_t
*--- dodequeueResult* r = dodequeue(now);
compare with: typedef struct { ... } *dodequeue_result* below (present
in two routines)
---
dodequeueResult r = {*NULL, queue_t::dequeue() *};
shouldn't it be {queue_t::dequeue(), false} ?
Best Regards,
Polina
* note the reply by Dave Taht:
http://www.ietf.org/mail-archive/web/aqm/current/msg01687.html
On 03/20/2016 04:35 PM, Wesley Eddy wrote:
It looks like the WGLC feedback on the document body is incorporated,
so this is good.
Is there a reason to stay with Informational and not Experimental like
we've done with PIE an d FQ-CoDel?
Also, idnits has some problems with the references that should be
fixed (e.g. "NATAL2010" is probably supposed to be "NETAL2010"):
https://tools.ietf.org/idnits?url=https://tools.ietf.org/id/draft-ietf-aqm-codel-03.txt
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