The RACK-TLP Loss Detection Algorithm for TCP
draft-ietf-tcpm-rack-15

[Ballot comment]
Thanks for presenting this complicated topic in a very easy-to-read
manner!

Section 3.3.2

  1.  The reordering window SHOULD be set to zero if no reordering has
      been observed on the connection so far, and either (a) three
      segments have been delivered out of order since the last recovery

I assume there's some subtle technical difference between "reordering"
and "segments delivered out of order" that makes this a reasonable thing
to say ... but it would be nice if the distincion was made more clear
(whether here or in earlier text).

Section 6.2

  Among all the segments newly ACKed or SACKed by this ACK that pass
  the checks above, update the RACK.rtt to be the RTT sample calculated
  using this ACK.  Furthermore, record the most recent Segment.xmit_ts
  in RACK.xmit_ts if it is ahead of RACK.xmit_ts.  If Segment.xmit_ts
  equals RACK.xmit_ts (e.g. due to clock granularity limits) then
  compare Segment.end_seq and RACK.end_seq to break the tie.

Perhaps we should state what the result of breaking the tie is used for
(i.e., updating RACK.segment & co.)?

  To avoid the issue above, RACK dynamically adapts to higher degrees
  of reordering using DSACK options from the receiver.  Receiving an
  ACK with a DSACK option indicates a possible spurious retransmission,
  suggesting that RACK.reo_wnd may be too small.  The RACK.reo_wnd
  increases linearly for every round trip in which the sender receives
  some DSACK option, so that after N distinct round trips in which a
  DSACK is received, the RACK.reo_wnd becomes (N+1) * min_RTT / 4, with
  an upper-bound of SRTT.

What constitutes a "distinct round trip"?

      Return min(RACK.min_RTT / 4 * RACK.reo_wnd_mult, SRTT)

I suggest reordering the expression to be min(RACK.reo_wnd_mult *
RACK.min_RTT / 4, SRTT), to avoid needing to consider the order of
operations and operator precedence in the pseudocode.  (soapbox: in
formal mathematics, there is no "division" operation, just
multiplication by the multiplicative inverse, in part because it makes
dealing with associativity and commutativity of operations harder to
reason about.)

Section 8

  sender SHOULD cancel any other pending timer(s).  An implementation
  is to have one timer with an additional state variable indicating the
  type of the timer.

nit: maybe "is expected to have one timer", to avoid any risk of being
interpreted as over-specifying implementation behavior?

Is there anything to say about increasing the usage of fast recovery
(vs RTO) potentially having an aggregate effect globally on how much
data is in flight and increasing the overall risk of congestion?  (I
mostly assume not, but it's kind of my job to ask.)

Section 13.1

In terms of how we reference it, RFC 3042 seems like it could be
informative.

[Ballot comment]
Section 1:

In this document, these words will appear
  with that interpretation only when in UPPER CASE.  Lower case uses of
  these words are not to be interpreted as carrying [RFC2119]
  significance.

This addition to the RFC 8174 boiler text appears redundant and can be removed. I assume it predates RFC 8174.

[Ballot comment]
Hi,

I didn't manage to review the algorithm in detail (way outside my area), but surrounding text was clear, understandable and interesting for someone without TCP expertise, so thank you for that.

One minor NIT would be the change the footer from "RACK" to "RACK-TLP".

Section 9.5 talks about using RACK for other protocols and I wanted to confirm that it does mean only RACK and not RACK-TLP for this case.

Regards,
Rob

[Ballot comment]
Thanks for this work.  It's interesting stuff!

Please expand DUPACK on first use.

Section 2 is curious.  As I read it, it establishes a normative recommendation to use it, but doesn't update either of its antecedents (RFC 5681 or RFC 6675) such that someone reading those might be referred to this.

[Ballot comment]
[[ nits ]]

[ section 3.1 ]

* "Conceptually, RACK puts a virtual timer for"

  Instead of "puts" perhaps "uses", "keeps", "imagines", or something?

[ section 4 ]

* "a timestamp whose granularity that is finer" ->
  "a timestamp with a granularity that is finer",  or
  "a timestamp whose granularity is finer", perhaps

[ section 9.4 ]

* "sender takes longer time" -> "sender takes a longer time"?

[Ballot comment]
Just a very small comment:

— Section 1 —

  In this document, these words will appear
  with that interpretation only when in UPPER CASE.  Lower case uses of
  these words are not to be interpreted as carrying [RFC2119]
  significance.

I don’t particularly object to that quoted text, but it’s redundant: the (correct) BCP 14 boilerplate before it already says that.  What’s the purpose of adding this text?  (I’m guessing it’s a remnant: it was there with the old BCP 14 boilerplate frim 2119, and when you switched to 8174 you didn’t remove this.)

(Via drafts-lastcall@iana.org): IESG/Authors/WG Chairs:

The IANA Functions Operator has reviewed draft-ietf-tcpm-rack-13, which is currently in Last Call, and has the following comments:

We understand that this document doesn't require any registry actions.

While it's often helpful for a document's IANA Considerations section to remain in place upon publication even if there are no actions, if the authors strongly prefer to remove it, we do not object.

If this assessment is not accurate, please respond as soon as possible.

Thank you,

Sabrina Tanamal
Senior IANA Services Specialist

The following Last Call announcement was sent out (ends 2020-11-30):

From: The IESG
To: IETF-Announce
CC: draft-ietf-tcpm-rack.all@ietf.org, tcpm@ietf.org, tcpm-chairs@ietf.org, draft-ietf-tcpm-rack@ietf.org, martin.h.duke@gmail.com, tuexen@fh-muenster.de
Reply-To: last-call@ietf.org
Sender:
Subject: Last Call:  (The RACK-TLP loss detection algorithm for TCP) to Proposed Standard


The IESG has received a request from the TCP Maintenance and Minor Extensions
WG (tcpm) to consider the following document: - 'The RACK-TLP loss detection
algorithm for TCP'
  as Proposed Standard

The IESG plans to make a decision in the next few weeks, and solicits final
comments on this action. Please send substantive comments to the
last-call@ietf.org mailing lists by 2020-11-30. Exceptionally, comments may
be sent to iesg@ietf.org instead. In either case, please retain the beginning
of the Subject line to allow automated sorting.

Abstract


  This document presents the RACK-TLP loss detection algorithm for TCP.
  RACK-TLP uses per-segment transmit timestamps and selective
  acknowledgements (SACK) and has two parts: RACK ("Recent
  ACKnowledgment") starts fast recovery quickly using time-based
  inferences derived from ACK feedback.  TLP ("Tail Loss Probe")
  leverages RACK and sends a probe packet to trigger ACK feedback to
  avoid retransmission timeout (RTO) events.  Compared to the widely
  used DUPACK threshold approach, RACK-TLP detects losses more
  efficiently when there are application-limited flights of data, lost
  retransmissions, or data packet reordering events.  It is intended to
  be an alternative to the DUPACK threshold approach.




The file can be obtained via
https://datatracker.ietf.org/doc/draft-ietf-tcpm-rack/



No IPR declarations have been submitted directly on this I-D.

As required by RFC 4858, this is the current template for the Document
Shepherd Write-Up. Changes are expected over time.

This version is dated 1 November 2019.

(1) What type of RFC is being requested (BCP, Proposed Standard, Internet Standard, Informational, Experimental, or Historic)? Why is this the proper type of RFC? Is this type of RFC indicated in the title page header?

The intended status is Proposed Standard. This is the proper state since
the document specifies a loss recover mechanism, which is implemented and
and has a substantial deployment. Therefore Experimental is not appropriate.
This explicitly decided by the working group.
The type indicated in the title page header is Standards Track.

(2) The IESG approval announcement includes a Document Announcement Write-Up. Please provide such a Document Announcement Write-Up. Recent examples can be found in the "Action" announcements for approved documents. The approval announcement contains the following sections:

Technical Summary:

Relevant content can frequently be found in the abstract and/or introduction of the document. If not, this may be an indication that there are deficiencies in the abstract or introduction.

This document specifies a loss recovery algorithm for TCP, which detects losses more efficiently than the widely used duplicate acknowledgement based algorithm, when there are application-limited flights of data, lost retransmissions, or data packet reordering events.

Working Group Summary:

There is very strong support for this document in the working group.

Document Quality:

Are there existing implementations of the protocol? Have a significant number of vendors indicated their plan to implement the specification? Are there any reviewers that merit special mention as having done a thorough review, e.g., one that resulted in important changes or a conclusion that the document had no substantive issues? If there was a MIB Doctor, YANG Doctor, Media Type or other expert review, what was its course (briefly)? In the case of a Media Type review, on what date was the request posted?

The core mechanisms described in the document are implemented in FreeBSD, Linux, and Windows. They are in active use on these platforms. RACK can also be used by other transport protocols. QUIC loss recovery uses the same ideas and there exists also an implementation for SCTP within a simulation environment.
The document got substantial comments during the first working group last Call,
which were addressed and a second working group last call made sure that the
working group is fine with the changes.

Personnel:

The Document Shepherd is Michael Tüxen (tuexen@fh-muenster.de).
The Responsible Area Director is Martin Duke (martin.h.duke@gmail.com).

(3) Briefly describe the review of this document that was performed by the Document Shepherd. If this version of the document is not ready for publication, please explain why the document is being forwarded to the IESG.

I read the document to especially ensure that the description of the algorithm allows not only implementations in TCP stacks but provides enough background to implement it for other transport protocols. This document is ready for publication.

(4) Does the document Shepherd have any concerns about the depth or breadth of the reviews that have been performed?

No.

(5) Do portions of the document need review from a particular or from broader perspective, e.g., security, operational complexity, AAA, DNS, DHCP, XML, or internationalization? If so, describe the review that took place.

No.

(6) Describe any specific concerns or issues that the Document Shepherd has with this document that the Responsible Area Director and/or the IESG should be aware of? For example, perhaps he or she is uncomfortable with certain parts of the document, or has concerns whether there really is a need for it. In any event, if the WG has discussed those issues and has indicated that it still wishes to advance the document, detail those concerns here.

There are no issues.

(7) Has each author confirmed that any and all appropriate IPR disclosures required for full conformance with the provisions of BCP 78 and BCP 79 have already been filed. If not, explain why?

All authors have confirmed that all IPR disclosures required have already been filed.

(8) Has an IPR disclosure been filed that references this document? If so, summarize any WG discussion and conclusion regarding the IPR disclosures.

No IPR disclosures are found by https://datatracker.ietf.org/ipr/search/?draft=draft-ietf-tcpm-rack&submit=draft&rfc=&doctitle=&group=&holder=&iprtitle=&patent=

(9) How solid is the WG consensus behind this document? Does it represent the strong concurrence of a few individuals, with others being silent, or does the WG as a whole understand and agree with it?

The WG consensus is very strong and the document is supported by a large number of key contributors of the WG.

(10) Has anyone threatened an appeal or otherwise indicated extreme discontent? If so, please summarise the areas of conflict in separate email messages to the Responsible Area Director. (It should be in a separate email because this questionnaire is publicly available.)

No.

(11) Identify any ID nits the Document Shepherd has found in this document. (See http://www.ietf.org/tools/idnits/ and the Internet-Drafts Checklist). Boilerplate checks are not enough; this check needs to be thorough.

Two false positives are reported:
* Pseudocode is detected as code due to comments. Therefore the too suggests to use and , but this is not appropriate, since it is pseudo code.
* A problem with references is reported due to the fact that the title of RFC 8174 contains the string RFC 2119.

(12) Describe how the document meets any required formal review criteria, such as the MIB Doctor, YANG Doctor, media type, and URI type reviews.

Not required, since none of the above is used in the document.

(13) Have all references within this document been identified as either normative or informative?

Yes.

(14) Are there normative references to documents that are not ready for advancement or are otherwise in an unclear state? If such normative references exist, what is the plan for their completion?

No.

(15) Are there downward normative references references (see RFC 3967)? If so, list these downward references to support the Area Director in the Last Call procedure.

No.

(16) Will publication of this document change the status of any existing RFCs? Are those RFCs listed on the title page header, listed in the abstract, and discussed in the introduction? If the RFCs are not listed in the Abstract and Introduction, explain why, and point to the part of the document where the relationship of this document to the other RFCs is discussed. If this information is not in the document, explain why the WG considers it unnecessary.

No, this document does not change the status of any existing RFC.

(17) Describe the Document Shepherd's review of the IANA considerations section, especially with regard to its consistency with the body of the document. Confirm that all protocol extensions that the document makes are associated with the appropriate reservations in IANA registries. Confirm that any referenced IANA registries have been clearly identified. Confirm that newly created IANA registries include a detailed specification of the initial contents for the registry, that allocations procedures for future registrations are defined, and a reasonable name for the new registry has been suggested (see RFC 8126).

There are no IANA actions required by this document. This is stated clearly in the IANA section.

(18) List any new IANA registries that require Expert Review for future allocations. Provide any public guidance that the IESG would find useful in selecting the IANA Experts for these new registries.

None.

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No such reviews where made, since the document does not contain sections written in a formal language.

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The document does not contain a YANG module.