[aqm] Review of draft-ietf-aqm-ecn-benefits-03

[Mirja Kühlewind <[e-mail]>]

Hi Gorry, hi Michael,

as promised here is my review of draft-ietf-aqm-ecn-benefits-03.

My overall comment is that even after reading the document (or even slightly 
more than before) I'm not completely sure what the purpose of this document is 
and also what the audience is this documented is directed to. Currently this 
document seems to do two things: 1. it list benefits (which is interesting for 
someone who thinks about enabling ECN) and 2. it kind of outlines needed steps 
for deployment (which would be directed to someone who gets the task from his 
manager to turn on ECN). However, the second point is not clearly spelled out 
and therefore it might be rather confusing for some people to read the second 
part of the document. Also the second part is to some extend still 
work-in-progress, therefor I would recommend to only focus this document on the 
first part.

For the first part (listing benefits) it might also be good to make 
clear/distinguish who has these benefits. I think all benefits that are 
currently listed are only advantageous for the end host/application. Are there 
any benefits for a network operator? Would it be possible to write this document 
such that I could also use it to point network operators to and give them an 
incentive to enable ECN?

Another high level comment is that you say in the introduction that this 
document "also identifies some potential problems that might occur when ECN is 
used" but then you don't really discuss them. I think to show both sides of the 
coin in this document would make the document more useful (and more honest). One 
point that you mention slightly here is that cheating is easier than with loss 
by not providing the feedback. Another point might be fairness between ECN and 
non-ECN traffic as marking will not reduce the queue length and therefore might 
lead to a higher loss rate for the non-ECN traffic instaed. I guess there are 
papers about this; don't have any by hand right now. Are there any other 
problems that should be mentioned?

Find more detailed comment by section below:

Abstract
--------
...says "...potential benefits when applications enable Explicit Congestion 
Notification (ECN)" -> usually an application cannot able ECN because usually 
it's a system setting...?

Section 1
---------
... says "..separate
    configuration of the drop and mark thresholds is known to be
    supported in some network devices and this is recommended
    [RFC2309.bis]."
RFC2309bis does not recommend different settings, it only say that it should be 
possible have different configuration of both. Further, I think this should not 
only concern THE threshold (whatever this is) but usually there are several 
parameters you might want to set independent of each other, e.g. the max 
mark/drop probability in RED.


Section 2
----------
1) I'm not sure I understand the purpose of this section or maybe just the title 
is wrong. I'm currently seeing this section rather as a section that provides 
the needed background knowledge than is talking about deployment. For this 
purpose I'd put all references and potentially a brief summary to other 
RFC/drafts on ECN in this section including RFC2884, RFC4774, RFC5562, RFC6040, 
RFC6679, draft-briscoe-tsvwg-ecn-encap-guidelines and 
draft-ietf-tcpm-accecn-reqs (and rename it).

2) Second paragraph says:
"Network devices must not drop packets solely because these codepoints are used 
[RFC2309.bis]."
Not sure this is the right document to says this (because currently it not seems 
to be directed to network operator/equipment vendors but admins/application 
developers). However, if it says this, it should also say that network devices 
should not bleach these bits.

3) First bullet in list says
"A recent survey reported growing support for ECN on common network paths [TR15]."
This sounds like TR15 shows that ECN is actually used in the Internet. However, 
TR15 only shows that there are only very few cases left where ECN packets are 
dropped or incorrectly altered. Please clarify or remove this sentence here.

4) You could cite draft-bensley-tcpm-dctcp-00 instead of the DCTCP Sigcomm paper 
(or both).

5) I would remove the subsection headings (both 2.1 and 2.2) and just add the 
text there to the main part of the section.

6) "An AQM algorithm that supports ECN needs to define
    the threshold and algorithm for ECN-marking."
This is kind of self-redundant and therefore does not really makes sense to me 
to say; of course an algo that supports ECN needs to say something about ECN...

7) You can use TR15 to provide a reference for the first paragraph in section 2.2:
"Cases have been noted where a sending endpoint marks a packet with a
    non-zero ECN mark, but the packet is received with a zero ECN value
    by the remote endpoint."

8) I'd move the second paragraph of section 2.2. ("The current..") to a 
potentially new problems section, talking about known/previous deployment problems.

9) I would simply remove paragraph 3-4 of section 2.2 because this was basically 
as already mentioned by referring to 2309bis and rfc6040 in section 2.1.

Section 3.2
------------
1) Don't understand why there is a listing here...? Just remove the listing and 
make text out of it...?

2) The sentence "This also
       avoids the inefficiency of dropping data that has already made it
       across at least part of the network path."
does not belong in this section. This sentence should just be moved to section 
3.1 (or in an own section) and must be further explained, saying that dropping 
packet at the of the path has already blocked resources that other traffic could 
have used otherwise.

Section 3.3
-----------
1) I'd say this section misses on part of the discussion. It is true that if by 
chance your last  packet(s) get lost ECN can help. However, this section reads a 
little like, with ECN it is save to send packet bursts. Which is not true 
because even if ECN is used by a network device, the queue might be too small to 
hold the whole burst. I believe this case happen very often which might be a 
reason for the higher tail loss probability that sometimes is experienced with 
IW10. Please add this point to the discussion.

2) I don't really get the point of the second paragraph. First of all it is 
confusion that this paragraph starts which "In addition to avoiding HOL 
blocking,.."; I guess that is left over from a previous version of this text...? 
And then you talk about a connection that is currently idle, so why is the 
performance of this connection that is currently not sending anything reduced?

3) I don't understand what "applications that send intermittent bursts of data, 
and rely upon timer-based recovery of packet loss" are...? Isn't the transport 
responsible to not send bursts and care about recovery...?

4) For the last paragraph in section 3.3 note that stacks often remember RTT 
measurements for a certain IP address and set the initial RTO based on this 
information.

Section 3.4
-----------
You still need FEC or some kind of error concealment even if ECN is used because 
you can never be sure that your packet are not get dropped (by non-ECN-enable 
devices or other reasons). Therefore using ECN will clearly not reduce 
complexity. The only thing you can do is to potentially reduce the amount of 
redundancy you send if you know that a certain path is ECN enables or don't see 
losses at the beginning of a connection. This can save network resources but 
actually might not improve user experience; in fact the user experience might be 
worse in case there are sudden losses.

Further the text says "negative impact of using loss-hiding mechanisms"; I don't 
really think that FEC has a negative impact as long as you've send enough 
redundancy...? Error concealment might but is used less and less. I'd recommend 
to talk about error concealment only in this last paragraph and explain a little 
further.

Section 3.5
----------
"Recording the presence of CE-marked packets can therefore provide
    information about the performance of the network path."
Would change to:
"Recording the presence of CE-marked packets in absence of loss can therefore 
provide
    information about the performance of the network path."
And also say more concretely what is meant with 'performance of the network 
path' -> congestion level or no drops by other middleboxes on this path...

Section 3.6
-----------
1) I like the section but I would phrase it differently; also it's not clear who 
needs to support what in this case. I'd like to propose the following text [not 
sure about the heading...]:

"3.6 Opportunity to provide an improved congestion feedback signal

Loss and ECN marking are both used as an indication for congestion. However, 
while the amount of feedback that is provided by loss should naturally be 
minimized, this is not the case for ECN. With ECN a network node could provide 
richer and more frequent feedback on the congestion state of a link which then 
could be used by the control mechanisms implemented in end host to make a more 
appropriate decision on how to react to congestion and to react faster to 
changes in congestion state.

Further while drop-based AQM mechanisms usually operate on a smoothed queue 
length estimation (instead of the instantaneous queue length) and therefore 
slightly delay the feedback signal to avoid unnecessary losses in case of 
transient congestion, this would be not necessary for ECN. If congestion is only 
transient due to short traffic bursts that are active for less than one RTT, the 
congestion signal would reach the sender at a time where the congestion is 
already cleared up. However, instead delaying the feedback in the network, the 
end host could reduce its sending rate incrementally based on the extend of 
congestion (that was experienced over e.g. the last RTT) similar as DCTCP. In 
case if the congestion is only transient, the end host would only reduce its 
rate slightly and be able to catch up quickly again. However, in case the 
congestion is persistent, this would help to remove additional delays from the 
network and resolve congestion faster which after all reduces the average 
queuing delay.

However, current ECN is defined as a 'drop equivalent' in RFC3168. To change the 
semantics of ECN both the AQM in the network nodes and the control mechanism in 
the end hosts would still need to cope with nodes or end hosts that rely on the 
old semantics. Therefore changing the semantics can be done more easily in 
confined environment such as a data center. DCTCP is an example that changes 
both the configuration of the used AQM as well as the congestion response in the 
end host and relies on that fact that all nodes in data center are configured 
the same way. [Deployment strategies to change the semantics of ECN in the 
Internet are currently under discussion in the IETF.]"

2) I'd move the 1. and 2. paragraph of section 3.6.1 to the 
background/deployment section or to the intro depending what you going to do 
with section 2.

Sections 4 & 5
---------
First sentence talks about "operational
    difficulties when the network only partially supports the use of ECN,
    or to respond to the challenges due to misbehaving network devices
    and/or endpoints".
I think these are to very different things. Misbehaving network devices is a 
point for a  problems section (where the lesson learned is that we didn't think 
carefully enough about incremental deployment in the first place but do now). 
However, partial deployment is not a problem but is a thing we simply have to 
cope with. The text sound as if the goal would be that every router in the whole 
Internet would at some point of time be ECN-enabled. I don't think this will 
ever happen and is also not the goal for me. Routers that are very unlikely to 
ever get congested should no be required to look at the ECN bits or monitor the 
queue length to calculate a mark/drop probability.

However as I said at the beginning I don't really thing that sections 4 and 5 
belong in this document. If you decided to keep them (you have to change the 
abstract) and I'd recommend to rename them e.g 4. 'Incremental Deployment 
Strategy' or 'Requirements to enable Incremental Deployment' and 5. 
'Recommendations for enabling ECN in network nodes and end hosts'.


I hope that's helpful! Let me know if you have any questions!

Mirja