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Document: draft-ietf-aqm-fq-codel-05.txt
Reviewer: Elwyn Davies
Review Date: 2016/03/06
IETF LC End Date: 2016/03/17
IESG Telechat date: (if known) 2016/03/17

Summary:
Almost ready.  There are a couple of minor issues that are barely above 
the level of nits plus a fair bit of editorial work.

I notice that the draft is on the IESG agenda on the day that last call 
ends.  If the authors wish to sort out the nits before the end of last 
call, I will be happy to work with them on this.

Major issues:
None.

Minor issues:
Treatment of packets that don't fit into the hashing classification 
scheme:  The default FQ-CoDel hashing mechanism uses the 
protocol/addresses/ports 5-tuple, but there will be packets that don't 
fit this scheme (especially ICMP).  There is no mention of what the 
classification would do with these packets.  I guess that one extra 
queue would probably suffice to hold any such outliers, but it would be 
wise to say something about how the packets from this/these queue(s) 
would be treated by the scheduler.  It might also be useful to say 
something about treatment of outliers in other classification schemes, 
if only to say that the scheme used needs to think about any such outliers.

s6.2, last para:  The proposal to add flow related marking to 
encapsulated packets needs to come with a very well exposed security and 
privacy health warning.. [It occurs to me that it might be possible to 
confine these markings to out of band notifications within the 
originating host which would allow fq_codel or similar to Flow Queue the 
packets getting them into a sensible order on the outgoing link.  Once 
the packets had been ordered in this way, a subsequent box (maybe an 
ADSL modem or suchlike) linking a home environment to the outside world 
could work purely on source address, thereby interspersing the packets 
from different hosts onto the external link but not needing to reorder 
the packets from each individual host.  Not sure if this is a sensible 
idea but it looks like a way to avoid the privacy issues.]

s11:  Internet Drafts are not scientific papers as such and do not 
usually have a Conclusions section.  I think it would be useful to move 
the paragraph in s11 as is to s1.  Since this draft is targeted for 
Experimental RFC status, it would be useful to suggest (maybe in s7) 
that experiments along the lines noted in s7 might be carried out and 
there results reported (where?  AQM WG?).  Given the developments with 
Cake, I am not sure whether the authors expect this version of FQ-CoDel 
to make it onto standards track or BCP, but to set out some sort of 
expected trajectory is desirable.

Nits/editorial comments:
General: s/i.e./i.e.,/, s/e.g./e.g.,/

Draft Title: The acronym CoDel with an uppercase D is used everywhere 
else in the document - Suggest the title should be FlowQueue-CoDel

Abstract:   Need to expand AQM. [DNS and and CPU are 'well-known' - 
http://www.rfc-editor.org/materials/abbrev.expansion.txt]

General: It would be helpful to capitalize Quantum throughout (or at 
least from s3 onwards)  to emphasise that it is a configured value. 
Likewise Interval and Target parameters.  Maybe also Flow and Queue as 
they a defined terms.

s1, para 1: It would be helpful to give the full title 
(FlowQueue-CoDel), expand AQM (again), provide a refernece explaining 
the term bufferbloat and give references for AQM, basic CoDel, DRR and 
the ns2/ns3 network simulators.

I would think one or both of the following would be useful (long term 
stable) refs for bufferbloat (the first is useful because the article is 
publically available rather than needing a sub to IEEE or ACM):

Jim Gettys. 2011. Bufferbloat: Dark buffers in the Internet. IEEE 
Internet Comput. 15, 3 (2011), 95–96. 
DOI:http://dx.doi.org/10.1109/MIC.2011.56 (freely available at 
http://www.bufferbloat.net/attachments/27/IC-15-03-Backspace.pdf)

and/or
Jim Gettys and Kathleen Nichols. 2012. Bufferbloat:Dark buffers in the 
Internet. Commun. ACM 55, 1 (2012), 1–15. 
DOI:http://dx.doi.org/10.1145/2063176.2063196

Suggest:
OLD:
    The FQ-CoDel algorithm is a combined packet scheduler and AQM
    developed as part of the bufferbloat-fighting community effort. It
    is based on a modified Deficit Round Robin (DRR) queue scheduler,
    with the CoDel AQM algorithm operating on each queue.  This document
    describes the combined algorithm; reference implementations are
    available for ns2 and ns3 and it is included in the mainline Linux
    kernel as the fq_codel queueing discipline.

NEW:
    The FlowQueue-CoDel (FQ-CoDel) algorithm is a combined packet 
scheduler and
    Active Queue Management (AQM) [RFC 3168] algorithm
    developed as part of the bufferbloat-fighting community effort (see 
http://www.bufferbloat.net).  It
    is based on a modified Deficit Round Robin (DRR) queue scheduler 
[DRR][DRRPP],
    with the CoDel AQM algorithm operating on each queue.  This document
    describes the combined algorithm; reference implementations are
    available for the ns2 (http://nsnam.sourceforge.net/wiki) and
    ns3 (https://www.nsnam.org/wiki) network simulators, and it is
    included in the mainline Linux kernel as the fq_codel queueing 
discipline.
END

s1.2, definition of Flow: s/protocol/protocol number/; also mac is 
ambiguous - s/mac address/media access control (MAC) address/

s1.2, definitions of Flow and Queue: To make the mentions of hash in the 
definition of Queue comprehensible, it would be sensible to mention 
'hash' in the definition of flow.  Suggest adding:
     The identifier(s) of a flow are mapped to a hash code used 
internally in FQ-CoDel to organize the packets into Queues.

s1.2: Probably worthwhile adding the references for CoDel and DRR to the 
definitions.

s1.2: It would help with subsequent understanding to provide definitions 
of the Interval and Target parameters of the CoD
el scheme.  Suggest:
ADD:
Interval: Characteristic time period used by the control loop of CoDel 
to age the value of the minimum sojourn time of packets in a Queue 
(i.e., the time spent by the packet in the local Queue) that is used to 
indicate when a persistent Queue is developing (see Section 4.3 of 
[CODELDRAFT]).

Target: Setpoint value of the minimum sojourn time of packets in a Queue 
used as the target of the control loop in CoDel (see Section 4.4 of 
[CODELDRAFT]).
END

s1.3, para 1: It would be helpful to expand SQF: s/SQF/Shortest Queue 
First (SQF)/

s1.3, para 1: s/differently than/differently from/ (the latter is good 
for both US and British English)

s1.3, para 2: Suggest starting the para with 'By default,' so that the 
'although' bit is not a surprise.

s1.3, para 2:  Adding a reference for the CoDel algorithm would help.  
In particular, linking to http://queue.acm.org/appendices/codel.html as 
well as the basic article would be helpful - I am not sure if the CoDel 
article is available for free to all but the appendix seems to be.

s1.3, para 4: s/current implementation/Linux implementation at the time 
of writing/

s1.3, para 5: s/rather than a packet-based./rather than packet-based./

s1.3, para 6 (next to last para):
> However, in practice many things that are
>     beneficial to have prioritised for typical internet use (ACKs, DNS
>     lookups, interactive SSH, HTTP requests, ARP, RA, ICMP, VoIP) _tend_
>     to fall in this category, which is why FQ-CoDel performs so well for
>     many practical applications.
I am doubtful as whether mentioning ARP, RA and ICMP in this statement 
is appropriate since they don't fit into the default flow classification 
scheme, and are not really flows as such.  As mentioned in Minor Issues 
above some discussion of non-flow packets is needed IMO.  They could be 
left out here without problem I think.

s3.1, para 1: s/buckets are configurable/buckets is configurable/

s3.1, para 3: s/This number is is/This number is/

s3.1, para 3:
OLD:
until the number of credits runs into the negative
NEW:
until the value of _byte credit_ becomes negative
END
What about _byte_credit_ == 0?  (Just asking!  Did possibly think that 
if the queue has the same number of bytes as the credit, it might be 
possible for the queue to jam in the active list while empty.)

s3.1,para 4: s/fairness queueing/fairness queueing scheme/

s3.1, last para: s/flow-building queues/queues that build a backlog/

s4.1, para 2: A reference for Jenkins hash functions would be 
desirable.  It seems the best we can probably do is Jenkins' web site: 
http://www.burtleburtle.net/bob/hash/doobs.html

s4.1:  The basic Jenkins has used in the Linux scheduler code appears to 
be targeted at IPv4 32 bit addresses.  Technically, the abstract 
definition doesn't need to worry about the lengths of addresses, but 
does anything special need to be said about IPv6?  I couldn't see in the 
Linux code where IPv6 is dealt with. Furthermore, looking at the code a 
bit more closely, I am not sure how the 112 bits (32 + 32 + 16 + 16 + 
16) of the 5-tuple are pushed into the hashing algorithm which has an 
input 'bandwidth' of 64 bits.  Again this doesn't desperately matter 
here but, if I understand correctly, the calculations in s5.3 of hash 
collision probabilities reported in para 2 of s5.3 relate to the Jenkins 
update3 has algorithm used in Linux.  This isn't explicitly stated. I am 
not sure if the probabilities would be different if the number of bits 
in the 5-tuple was greater - presumably this depends to some extent on 
how the Jenkins algorithms are used.  The core Linux algorithm just uses 
the 'final' part of the algorithm.  According to the Jenkins comments, 
one probably ought to use the 'mix' part to combine additional bits.  
Thoughts?

s4.1, para 2:  I am not sure that 'permuted by a random value' is very 
clear, and it doesn't explain why this is done.
How about 'salted by modular addition of a random value'?  It would also 
be helpful to note here that this is done to mitigate a possible DoS 
attack that could occur if the hash is externally predictable and point 
to the note about this in the Security Considerations (s8).

s4.1.1, para 1: s/mac address/MAC address/; s/diffserv/diffserve 
codepoint values/
[I am not sure what firewall specific markings might be - any references?]

s4.1.1, para 2:  Suggest:
OLD:
An alternative to changing the classification scheme is to perform
    decapsulation prior to hashing.
NEW:
An addition to the default classification scheme is to perform
    decapsulation of tunnelled packets prior to hashing on the 5-tuple 
in the encapsulated.
END

Figure in s4.2: Needs a caption and a Figure number.  It is also 
somewhat incomplete as a state diagram.  Both New and Old states have 
actions that result from both Arrival/Enqueue and Dequeue events, and 
there are 'loopback' actions when there are Arrival/Enqueue and Dequeue 
events that occur in both New and Old states when the conditions are not 
satisfied.

s5.2.1, para 1:
> to ensure that the measured minimum delay does not become too stale.
Without detailed knowledge of CoDel this didn't read very easily - it is 
taken out of context from the CoDel draft.
Suggest:
    to  ensure that the minimum sojourn time of packets in a queue used 
as an estimator by the CoDel control algorithm is a relatively 
up-to-date value.

s5.2.1, para 1: s/It SHOULD be set on the order/It SHOULD be set to be 
on the order/

s5.2.3, para 2: s/10GigE/10 Gigabit Ethernet/

s5.2.4, para 2: Expand TSO (TCP Segmentation Offload). TSO probably 
could do with a reference (Freimuth below should work).

s5.2.4, para 2: I think GRO-enabled should probably be GSO-enabled - 
whichever it is, it needs to be expanded (Generic Segmentation 
Offload?).   There is also a web page for GSO 
(https://lwn.net/Articles/188489/)

  s5.2.5, para 2 and s4.1, para 2: It would be sensible to use a common 
term for 'initialisation time' and 'load time'.

s5.2.6, title and body: Need to expand ECN on first use.  Also s/thus 
unresponsive/thus the number of unresponsive/

s5.2.7: Expand CE (Congestion Encountered) on first occurrence.

s5.2.7, para 1: Expand DCTCP acronym and provide reference [Alizadeh] 
below and/or [draft-ietf-tcpm-dctcp]....thus..
OLD:

    This parameter enables DCTCP-like processing to enable CE marking ECT
    packets at a lower setpoint than the default codel target.

NEW:

    This parameter enables Date Centre TCP (DCTCP)-like processing resulting in
    CE (Congestion Encountered) marking on ECN-Capable Transport (ECT)
    packets [RFC3168] starting at a lower sojourn delay setpoint than the default CoDel
    Target. Details of DCTCP can be found in [Alizadeh2011] and [I-D.draft-ietf-tcpm-dctcp].

END

s5.3, para 2: see the comments on s4.1 above regarding the link between 
probabilities and address lengths.

s5.3, para 3: Need to expand Cake acronym and providepointer to Section 
7 which expalisn about Cake.

s5.4: Is it possible to add some comments to the codel_vars structure?

s5.5, para 2: s/resolution below the target/resolution significantly 
finer than the CoDel Target setting/

s5.6: Need to expand SFQ, WFQ and QFQ.  There is a reference for SFQ 
below [McKenney]

s5.7, last para: s/doesn't miss a beat/reacts almost immediately/ 
(Original is rather too colloquial!)

s6.1, last para: s/classification/service classification/

s7: I might add 'Checking behaviour when two or more instantiations of 
FQ-CoDel are used in series.'

s12.2:  Various of the informative references could do with completion.  
I think the referecnes below are correct:

Kathleen Nichols and Van Jacobson. 2012. "Controlling Queue Delay". 
/Queue/ 10, 5 (May 2012), 20. DOI:http://dx.doi.org/10.1145/2208917.2209336

M. Shreedhar and George Varghese. 1996. "Efficient fair queuing using 
deficit round-robin". IEEE/ACM Trans. Netw. 4, 3 (June 1996), 375–385. 
DOI:http://dx.doi.org/10.1109/90.502236

M.H. MacGregor and W. Shi. 2000. "Deficits for bursty latency-critical 
flows: DRR++". In Proceedings IEEE International Conference on Networks 
2000 (ICON 2000). Networking Trends and Challenges in the New 
Millennium. IEEE Comput. Soc, 287–293. 
DOI:http://dx.doi.org/10.1109/ICON.2000.875803

Y. Gong, D. Rossi, C. Testa, S. Valenti, and M.D. Taht. 2013. "Fighting 
the bufferbloat: On the coexistence of AQM and low priority congestion 
control". In /2013 IEEE Conference on Computer Communications Workshops 
(INFOCOM WKSHPS)/. IEEE, 411–416. 
DOI:http://dx.doi.org/10.1109/INFCOMW.2013.6562885

Giovanna Carofiglio and Luca Muscariello. 2012. "On the Impact of TCP 
and Per-Flow Scheduling on Internet Performance". IEEE/ACM Trans. Netw. 
20, 2 (April 2012), 620–633. DOI:http://dx.doi.org/10.1109/TNET.2011.2164553

The following are extra references that I suggested above:

P.E. McKenney. 1990. Stochastic fairness queueing. In /Proceedings. IEEE 
INFOCOM ’90: Ninth Annual Joint Conference of the IEEE Computer and 
Communications Societies@m_The Multiple Facets of Integration/. IEEE 
Comput. Soc. Press, 733–740. DOI:http://dx.doi.org/10.1109/INFCOM.1990.91316

Doug Freimuth et al. 2005. Server network scalability and TCP offload. 
In /USENIX Annual Technical Conference/. USENIX Association, 209–222.

Mohammad Alizadeh et al. 2011. Data center TCP (DCTCP). /ACM SIGCOMM 
Comput. Commun. Rev./ 41, 4 (October 2011), 63–74. 
DOI:http://dx.doi.org/10.1145/2043164.1851192