[conex] Benoit Claise's No Objection on draft-ietf-conex-tcp-modifications-09: (with COMMENT)

"Benoit Claise" <bclaise@cisco.com> Thu, 01 October 2015 10:22 UTC

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Subject: [conex] Benoit Claise's No Objection on draft-ietf-conex-tcp-modifications-09: (with COMMENT)
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Benoit Claise has entered the following ballot position for
draft-ietf-conex-tcp-modifications-09: No Objection

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----------------------------------------------------------------------
COMMENT:
----------------------------------------------------------------------

No objection to the document itself, but I really expect a new version
based on Warren's feedback, as agreed by Suresh (document shepherd)
Warren's feedback below.

Be ye not afraid -- I have reviewed this document as part of the
operations directorate's ongoing effort to review all IETF documents
being processed by the IESG.  These comments were written primarily
for the benefit of the operation area directors.  Document editors and
WG chairs should treat these comments just like any other last call
comments.


Version reviewed: draft-ietf-conex-tcp-modifications-09

Summary: I am not sure that this document is ready, and believe that
review is needed from the Ops ADs. I believe that interactions with
TCP feedback need to be performed very carefully, and I do not have
the knowledge to adequately evaluate these, hopefully others with this
experience (Transport ADs?) have also evaluated the document.
The document needs significant readability cleanup and nit fixing.



Detail: I found the document difficult to read -- there are a large
number of typos, areas with lack of precision, ambiguities and grammar
issues. This made a full evaluation difficult. I went to read the
'Document Quality' section of the Shepherd report to see if this was
noted, but it was simply boilerplate.

The document is marked as Experimental. I would be more concerned if
it were a different stream...

Open questions:

Section 7 Open Areas for Experimentation; "This decision was taken
because most network devices today expirience byte-congestion where
the memory is filled exactly with the number of bytes a packet
carries.  However, there are also devices that may allocate a certain
amount of memory per packet, no matter how larger a packet is."
Citation needed; many devices I am familiar with split packets into
multiple fixed size cells. e.g Juniper Q5 chips split packets into 96,
112, 128, 144, 160, or 176 byte cells and spray these across the
fabric.I believe the Cisco CRS (and similar architectures) perform
similar cell splitting. RFC7141 did not seem to contain this, but I
may have missed it.

In my opinion the document should discuss monitoring and reporting.
What parameters should implementations expose to the user / operator?

I think that the document should also discuss how this technique does
or does not suffer from the potential of global synchronization
(although this may be covered in other ConEx documents that I am not
familiar with).
The Security Considerations and Open Areas for Experimentation
sections sort of discuss coexistence / deployment, but not in much
depth





Nits are readability issues - in a number of cases I was not entirely
sure what was intended, so my suggestions may not be correct.


   Whereas loss has to be minimized, ECN can provide more fine-grained

[O] Whereas loss has to be minimized,
[P] Do you mean While, not whereas? I'm not sure how to parse this.


   feedback information.  ConEx-based traffic measurement or management
   mechanisms could benefit from this.
...

3.  Counting congestion
  ...

   The outstanding bytes counted based on ECN feedback information are
   maintained in the congestion exposure gauge (CEG), as explained in
   Section 3.2.

   When the sender sends a ConEx capable packet with the E or L flag set

[O] When the sender sends a ConEx capable packet with the E or L flag
set
[P] When the sender sends a ConEx capable packet with the E or L flag
set,
[R] punctuation

   it reduces the respective counter by the byte-size of the packet.
   This is explained for both counters in Section 4.1.

   Note that all bytes of an IP packet must be counted in the LEG or CEG
   to capture the right number of bytes that should be marked.
   Therefore the sender SHOULD take the payload and headers into
   account, up to and including the IP header.  However, in TCP the
   information how large the headers of an lost or marked pacekt were is

[O] information how large the headers of an lost or marked pacekt
[P] information regarding how large the headers of a lost or marked
packet
[R] readability, grammar, and typo (three changes)

   usually not available, as only payload data will be acknowledged.

   If equal-sized packets, or at least equally distributed packet sizes

[O] or at least equally distributed packet sizes
[P] or at least equally distributed packet sizes,
[R] grammar

   can be assumed, the sender MAY only add and subtract TCP payload
   bytes.
...

3.1.  Loss Detection

   This section applies whether or not SACK support is available.  The
   following subsection in addition handles the case when SACK is not

[O] The following subsection in addition handles
[P] The following subsection (3.1.1) handles
[R] Clarity

   available.

   A TCP sender detects losses and subsequently retransmits the lost
   data.  Therefore, ConEx sender can simply set the ConEx L flag on all
   retransmissions in order to at least cover the amount of bytes lost.
   If this aprroach is taken, no LEG is needed.

[O] aprroach
[P] approach
[R] spelling


   However, any retransmission may be spurious.  In this case more bytes
   have been marked than necessary.  To compensate this effect a ConEx

[O] To compensate this effect
[P] To compensate for this effect,
[R] grammar

   sender can maintain a local signed counter, the (LEG), that indicats

[O] indicats
[P] inidicates
[R] spelling

   the number of outstanding bytes to be sent with the ConEx L flag and
   also can become negative.

   Using the LEG, when a TCP sender decides that a data segment needs to
   be retransmitted, it will increase LEG by the size of the TCP payload
   bytes in the retransmission (assuming equal sized segments such that
   the retransmitted packet will have the same number of header bytes as
   the original ones):

   For each retransmision:

[O] retransmision
[P] retransmission
[R] spelling


   LEG += payload

   Note, how the LEG is reduced when the ConEx L marking are set is
   described in section Section 4.

   Further to accommodate spurious restransmissions, a ConEx sender

[O] restransmissions
[P] retransmissions
[R] spelling

   SHOULD make use of heuristics to detect such spurious retransmissions
   (e.g.  F-RTO [RFC5682], DSACK [RFC3708], and Eifel [RFC3522],
   [RFC4015]) if are already available in a given implementation.  If no

[O] if are already available in a given implementation.
[P] if already available in a given implementation.
[R] grammar


   mechanism for detecting spurious retransmissions is available, the
   ConEx sender MAY chose to implement one of the mechanism stated
   above.  However, given the inaccuracy that ConEx may have anyway and
   the timeliness of ConEx information, a ConEx MAY also chose to not
   componsate for spurious retransmission.  In this case if spurious

[O] componsate
[P] compensate
[R] spelling

   retransmissions occur, the ConEx sender simple has sent too much

[O] retransmissions occur, the ConEx sender simple has sent too much
[P] retransmissions occur, the ConEx sender simply has sent too many
[R] grammar x 2

   ConEx signals which e.g would decrease the congestion allowance in a
   ConEx policer unnecessary.

[O] in a ConEx policer unnecessary.
[R] ? Cannot parse.

   If a heuristic to detect spurious retransmission is used and has

[O] If a heuristic to detect spurious retransmission is used
[P] If a heuristic method is used to detect spurious retransmission
[R] readability, and was missing a noun after heuristic. Could use
analysis instead.

   determined that a certain number of packets were retransmitted
   erroneously, the ConEx sender subtracts the payload size of these TCP
   packets from LEG.

   If a spurious reransmission is detected:

[O] reransmission
[P] retransmission
[R] spelling

   LEG -= payload

   Note that the LEG can get negative, if too many L marking have

[O] Note that the LEG can get negative
[P] Note that LEG can become negative
[R] clarity

   already been sent.  This case is further discussed in section
   Section 6.

3.1.1.  Without SACK Support

   If multiple losses occur within one RTT and SACK is not used, it may
   take several RTTs until all lost data is retransmitted.  With the
   scheme described above, the ConEx information will be delayed
   considerably, but timeliness is important for ConEx.  However, for
   ConEx it is not important to know which data got lost but only how
   much.  During the first RTT after the initial loss detection, the

[O] However, for

   ConEx it is not important to know which data got lost but only how
   much.

[P] For ConEx, it is important to know how much data was lot; it is
not important to know what data is lost.
[R] readability

   amount of received data and thus also the amount of lost data can be
   estimated based on the number of received ACKs.

   ...

3.2.  ECN

   ...

   DeliveredData covers the number of bytes that has been newly
   delivered to the receiver.  Therefore on each arrival of an ACK,
   DeliveredData will be increased by the newly acknowledged bytes
   (acked_bytes) as indicated by the current ACK, relative to all past
   ACKs.  The formula depends on whether SACK is available: if SACK is
   not avaialble SACK_diff is always zero, whereas is ACK information is

[O] avaialble
[P] available
[R] spelling

   available is_dup and is_after_dup are always zero.

   With SACK, DeliveredData is increased by the number of bytes provided
   by (new) SACK information (SACK_diff).  Note, if less unacknowledged
   bytes are announced in the new SACK information than in the previous
   ACK, SACK_diff can be negative.  In this case, data is newly
   acknowledged (in acked_bytes), that has previously already been
   accumulated into DeliveredData based on SACK information.

   Otherwise without SACK, DeliveredData is increased by 1 SMSS on
   duplicate acknowledgements as duplicate acknowledgements do not

[O] as duplicate acknowledgements
[P] because duplicate acknowedgements
[R] grammar. Since would also be fine, instead of as.

   acknowlegde any new data (and acked_bytes will be zero).  For the

[O] acknowlegde
[P] acknowledge
[R] spelling

   subsequent partial or full ACK, acked_bytes cover all newly
   acknowledged bytes including the ones that where already accounted
   which the receiption of any duplicate acknowledgement.  Therefore

[O] the ones that where already accounted

   which the receiption of any duplicate acknowledgement

[P] those already accounted for with the receipt of any duplicate
acknowledgement.
[R] spelling, grammar, inability to parse the original. I *think* this
is what was meant.

   DeliveredData is reduced by one SMSS for each preceding duplicate
   ACK.  Consequently, is_dup is one if the current ACK is a duplicated
   ACK without SACK, and zero otherwise. is_after_dup is only one for
   the next full or partial ACK after a number of duplicated ACKs
   without SACK and num_dup counts the number of duplicated ACKs in a
   row (which usually is 3 or more).

   With classic ECN, one congestion marked packet causes continuous
   congestion feedback for a whole round trip, thus hiding the arrival
   of any further congestion marked packets during that round trip.  A
   more accurate ECN feedback scheme (AccECN) is needed to ensure that
   feedback properly reflects the extent of congestion marking.  The two
   cases, with and without a receiver capable of AccECN, are discussed
   in the following sections.

3.2.1.  Accurate ECN feedback

   With a more accurate ECN feedback scheme (AccECN) that is supported
   by the receiver either the number of marked packets or the number of

[O] by the receiver either the number
[P] by the receiver, either the number
[R] grammar/punctuation

  marked bytes will be feed back from the receiver to the sender and is

[O] feed back
[P] fed back
[R] spelling


   therefore know at sender-side.  In the latter case the CEG can

[O] therefore know at sender-side.  In the latter case the CEG can
[P] therefore know at sender-side. In the latter case, the CEG can
[R] grammar/punctuation

   directly be increased by the number of marked bytes.  Otherwise if D
   is assumed to be the number of marks, the gauge (CEG) will be
   conservatively increased by one SMSS for each marking or at max the
   number of newly acknowledged bytes:

   CEG += min(SMSS*D, DeliveredData)

3.2.2.  Classic ECN support
...

   To extract more than one ECE indication per RTT, a ConEx sender could
   set the CWR flag continuously to force the receiver to signal only
   one ECE per CE mark.  Unfortunately, the use of delayed ACKs
   [RFC5681] (which is common) will prevent feedback of every CE mark;
   if a CWR confirmation is received before the ECE can be sent out on
   the next ACK, ECN feedback information could get lost (depeding on

[O] depeding
[P] depending
[R] spelling

   the actual receiver implementation).  Thus a sender SHOULD set CWR
   only on those data segments that will presumably trigger a (delayed
   ACK.  The sender would need an additional control loop to estimated

[O] to estimated
[P] to estimate
[R] grammar

   which data segments will trigger an ACK in order to extract more
   timely congestion notifications.  Still the CEG SHOULD be increased

[O] Still the CEG SHOULD be increased
[P] Still, the CEG SHOULD be increased
[R] readability

   by DeliveredData, as one or more CE marked packets could be
   acknowledged by one delayed ACK.

4.  Setting the ConEx Flags

   By setting the X flag, a packet is marked as ConEx-capable.  All
   packets carrying payload MUST be marked with the X flag set,
   including retransmissions.  Only if no congestion feedback
   information is (currently) available, the X flag SHOULD be zero, such
   as for control packets on a connection that has not sent any (user)
   data for some time e.g., sending only pure ACKs which are not
   carrying any payload.
[O] Only if no congestion feedback

   information is (currently) available, the X flag SHOULD be zero, such
   as for control packets on a connection that has not sent any (user)
   data for some time e.g., sending only pure ACKs which are not
   carrying any payload.

[P] The X flag SHOULD be zero only if no congestion feedback
information is (currently) available (e.g. for control packets on a
connection that not sent any user data for some time and is sending
only pure ACKs that are not carrying any payload).

[R] grammar and readability

4.1.  Setting the E or the L Flag

   As described in section Section 3.1, the sender needs to maintain a
   CEG counter and might maintain a LEG counter.  If no LEG is used, all
   retransmission will be marked with the L flag.

   Further, as long as the LEG or CEG counter is positive, the sender
   marks each ConEx-capable packet with L or E respectively, and
   decreases the LEG or CEG counter by the TCP payload bytes carried in
   the marked packet (assuming headers are not being counted because
   packet sizes are regular).  No matter how small the value of LEG or
   CEG, if it is positive, the sender MUST NOT defer packet marking to
   ensure ConEx signals are timely.  Therefore the value of LEG and CEG

[O] No matter how small the value of LEG or

   CEG, if it is positive, the sender MUST NOT defer packet marking to
   ensure ConEx signals are timely.

[P] No matter how small the value of LEG or CEG, if the value is
positive the sender MUST NOT defer packet marking; this ensure ConEx
signals are timely.

[R] readability.

   will commonly be negative.

   If both LEG and CEG are positive, the sender MUST mark each ConEx-
   capable packet with both L and E.  If a credit signal is also pending
   (see next section), the C flag can be set as well.

4.2.  Setting the Credit Flag
...

   Recall that CSC will be decreased whenever congestion occurs,
   therefore CSC will need to be replenished as soon as CSC drops below

[O] Recall that CSC will be decreased whenever congestion occurs,

   therefore CSC will need

[P] CSC will be decreased whenever congestion occurs; therefore, CSC will
need
[R] grammar

   F.  Also recall that the sender can set the C flag on a ConEx-capable
   packet whether or not the E or L flags are also set.

   In TCP Slow Start, the congestion window might grow much larger than
   during the rest of the transmission.  Likely, a sender could consider
   sending fewer than F credits but risking being penalized by an audit
   function.  However, the credits should at least cover the increase in
   sending rate.  Given the exponential increase as implemented in the
   TCP Slow Start algorithm which means that the sending rate doubles
   every RTT, a ConEx sender should at least cover half the number of
   packets in flight by credits.

   Note that the number of losses or markings within one RTT does not
   solely depend on the sender's actions.  In general, the behavior of
   the cross traffic, whether active queue management (AQM) is used and
   how it is parameterized influence how many packets might be dropped
   or marked.  As long as any AQM encountered is not overly aggressive
   with ECN marking, sending half the flight size as credits should be
   sufficient whether congestion is signaled by loss or ECN.

   To maintain half of the packet in flight as credits, of course half

[O] To maintain half of the packet in flight as credits, of course half
[P] consider removing "of course" -- otherwise, put commas around it.
[R] readability/grammer

   of the packet of the initial window must be C marked.  In Slow Start
   marking every fourth packet introduces the correct amount of credit
   as can be seen in Figure 1.

...

5.  Loss of ConEx information

   Packets carrying ConEx signals could be discarded themselves.  This
   will be a second order problem (e.g. if the loss probability is 0.1%,
   the probability of losing a ConEx L signal will be 0.1% of 0.1% =
   0.01%).  Further, the penality an audit induces should be propotional

[O] Further, the penality an audit induces should be propotional
[P] Further, the penalty an audit induces should be proportionate
[R] spelling x 2

   to the mismatch of expected ConEx marks and observed congestion,
   therefore the audit might only slightly increase the loss level of
   this flow.  Therefore, an implementer MAY choose to ignore this
   problem, accepting instead the risk that an audit function might
   wrongly penalize a flow.

   Nonetheless, a ConEx sender is responsible to always signal

[O]  responsible to always signal
[P] responsible for always signalling
[R] grammar

   sufficient congestion feedback and therefore SHOULD remember which
   packet was marked with either the L, the E or the C flag.  If one of
   these packets is detected as lost, the sender SHOULD increase the
   respective gauge(s), LEG or CEG, by the number of lost payload bytes
   in addition to increasing LEG for the loss.

6.  Timeliness of the ConEx Signals

   ConEx signals will only be useful to a network node within a time
   delay of about one RTT after the congestion occurred.  To avoid
   further delays, a ConEx sender SHOULD send the ConEx signaling on the
   next available packet.

   Any or all of the ConEx flags can be used in the same packet, which
   allows delay to be minimised when multiple signals are pending.  The
   need to set multiple ConEx flags at the same time, can occur if e.g

[O] at the same time, can occur
[P] at the same time can occur
[R] unnecessary comma; grammar

   an ACK is received by the sender that simultaneously indicates that
   at least one ECN mark was received, and that one or more segements

[O] segements
[P] segments
[R] spelling

   were lost.  This may e.g. happen during excessive congestion, where

[O] may e.g. happen uring excessive congestion, where
[P] may happen during excessive congestion, if
[R] readability

   the queues overflow even though ECN was used and currently all
   forwarded packets are marked, while others have to be dropped
   nevertheless.  Another case when this might happen is when ACKs are

[O] nevertheless.
[P] [delete "nevertheless"]
[R] readability


   lost, so that a subsequent ACK carries summary information not
   previously available to the sender.

   If a flow becomes application-limited, there could be insufficient
   bytes to send to reduce the gauges to zero or below.  In such cases,
   the sender cannot help but delay ConEx signals.  Nonetheless, as long
   as the sender is marking all outgoing packets, an audit function is
   unlikely to penalize ConEx-marked packets.  Therefore, no matter how
   long a gauge has been positive, a sender MUST NOT reduce the gauge by
   more than the ConEx marked bytes it has sent.

   If the CEG or LEG counter is negative, the respective counter MAY be
   reset to zero within one RTT after it was decreased the last time or
   one RTT after recovery if no further congestion occurred.

7.  Open Areas for Experimentation

   All proposed mechanisms in this document are experimental, and
   therefore further large-scale experimentation in the Internet is
   required to evaluate if the signaling provided by these mechanisms is
   accurate and timely enough to produce value for ConEx-based (traffic
   management or other) mechanisms.

   The current ConEx specifications assume that congestion is counted in
   number of bytes (including the IP header that directly encapsulates
   the CDO and everything that IP header encapsulates)
   [draft-ietf-conex-destopt].  This decision was taken because most
   network devices today expirience byte-congestion where the memory is

[O] expirience
[P] experience
[R] spelling


   filled exactly with the number of bytes a packet carries.  However,
   there are also devices that may allocate a certain amount of memory
   per packet, no matter how larger a packet is.  These devices get
   congested based on the number of packets in their memory and
   therefore in this case congestion is determined by the number of
   packets that have been lost or marked.  Furthermore, a transport
   layer endpoint, such as a TCP sender or receiver, might not know the
   exact number of bytes that a lower layer was carrying.  Therefore a
   TCP endpoint may only be able to estimate the exact number of
   congested bytes (assuming that all lower layer header have the same
   length).  If this estimation is sufficient to work with the ConEx

[O] If this estimation is sufficient to work with the ConEx
[P] If this estimation is sufficient to work with, the ConEx
[R] readability

   signal needs to be further evaluated in tests in the Internet
   together with different auditor implementations.

   Further, the proposed marking schemes in this document are designed
   under the assumption that all TCP packets of a ConEx-capable flow are
   of equal size or that flows have a constant mean packet size over a
   rather small time frame, like one RTT or less.  In most
   implementations this assumption might be taken as well and probably
   is true for most of the traffic flows.  However, it should be
   evaluted how much the accuracy degrades if this precondition is not
   fulfilled, while the proposed scheme is used.  Especially evaluating
   this with real traffic from different application is important to
   make a decision if the proposed schemes are sufficient or a more
   complexe scheme is needed.

[O] However, it should be

   evaluted how much the accuracy degrades if this precondition is not
   fulfilled, while the proposed scheme is used.  Especially evaluating
   this with real traffic from different application is important to
   make a decision if the proposed schemes are sufficient or a more
   complexe scheme is needed.

[P] If this proposed scheme is used, it is necessary to evaluate how
much accuracy degrades if this precondition is not met. Evaluating
with real traffic from different applications is especially important
in making the decision regarding whether the proposed schemes are
sufficient or whether a more complex scheme is needed.

[R] grammar, spelling, readability

   In this context the proposed scheme to set credit markings in Slow
   Start runs a risk to provide an insufficient number of markings which
   can cause an audit function to penalize this flow.  Both the proposed
   credit scheme for Slow Start as well as the scheme in Congestion
   Avoidance must be evaluated together with one or more specific
   implementations of an ConEx auditor to ensure that both algorithms,
   in the sender and in the auditor, work propoerly together with a low

[O] propoerly
[P] properly
[R] spelling

   risk of false positives (which would lead to penalization of an
   honest sender).  However, if a sender is wrongly assumed to cheat,
   the penalization of the audit should be adequate and should allow an
   honest sender using a congestion control scheme that is commonly used
   today to recover quickly.

   Another open issue is the accuracy of the ECN feedback signal.  At
   time of publication of this document there is no AccECN mechanism s
   pecified yet, and further AccECN will also take some time to be

[O] s
  pecified yet,

[P] specified yet,
[R] spelling

   widely deployed.  This document proposes an advanced compatibility
   mode for Classic ECN.  The proposed mechanism can provide more
   accurate feedback by utilizing the way Classic ECN is speficed but

[O] speficed
[P] specified
[R] spelling

[O] loosing information
[P] losing information
[R] spelling

   this risk is in a real deployment scenario, further experimental
   evaluation is needed.  The following argument is intended to prove
   that suppressing repetitions of ECE, however, is still safe against
   possible congestion collapse due to lost congestion feedback and
   should be further proven in experimentation:

   Repetition of ECE in classic ECN is intended to ensure reliable
   delivery of congestion feedback.  However, with advanced
   compatibility mode, it is possible to miss congestion notifications.
   This can happen in some implementations if delayed acknowledgements
   are used.  Further an ACK containing ECE can simply get lost.  If

[O] Further an ACK
[P] Further, an ACK
[R] readability

   only a few CE marks are received within one congestion event (e.g.,
   only one), the loss of one acknowledgements due to (heavy) congestion
   on the reverse path can prevent that any congestion notification is
   received by the sender.

   However, if loss of feedback exacerbates congestion on the forward
   path, more forward packets will be CE marked, increasing the
   likelihood that feedback from at least one CE will get through per
   RTT.  As long as one ECE reaches the sender per RTT, the sender's
   congestion response will be the same as if CWR were not continuous.
   The only way that heavy congestion on the forward path could be
   completely hidden would be if all ACKs on the reverse path were lost.
   If total ACK loss persisted, the sender would time out and do a
   congestion response anyway.  Therefore, the problem seems confined to
   potential suppression of a congestion response during light
   congestion.

   Anyway, even if loss of all ECN feedback leads to no congestion

[O] Anyway
[P] Furthermore
[R] tone

   response, the worst that could happen would be loss instead of ECN-
   signaled congestion on the forward path.  Given compatibility mode
   does not affect loss feedback, there would be no risk of congestion
   collapse.


10.  Security Considerations

  ...

   However, if the receiver is solely interested in making the sender
   draw down its allowance, the net effect will depend on the sender's
   congestion control algorithm as permanetly adding more and more

[O] permanetly
[P] permanently