Re: [Taps] Benjamin Kaduk's Discuss on draft-ietf-taps-minset-08: (with DISCUSS and COMMENT)

[Michael Welzl <[e-mail]>]

Dear Benjamin,

Thanks a lot for your comments! Answers below:

> Benjamin Kaduk has entered the following ballot position for
> draft-ietf-taps-minset-08: Discuss
> 
> Please refer to https://www.ietf.org/iesg/statement/discuss-criteria.html
> for more information about IESG DISCUSS and COMMENT positions.
> 
> 
> The document, along with other ballot positions, can be found here:
> https://datatracker.ietf.org/doc/draft-ietf-taps-minset/
> 
> 
> 
> ----------------------------------------------------------------------
> DISCUSS:
> ----------------------------------------------------------------------
> 
> [My general summary view of this document is not really relevant to these
> Discuss points and appears in the COMMENT section.]
> 
> This document includes a general discussion of the features/services that
> IETF transport protocols (TCP, MPTCP, SCTP, UDP, etc.) provide ... and also
> throws in LEDBAT congestion control, with no real coverage of all the other
> congestion control schemes the IETF provides.  It seems that there should
> be some text/justification of why LEDBAT (but none of the others) fall into
> the same categorization as the general transport features.  As far as I can
> tell, the idea is that there can be a "low-impact background data transfer"
> feature/service, which LEDBAT attempts to provide, but I'm basing that on
> inference and not something explictily stated in the document.

You're right about the technical rationale for LEDBAT. From the
point of view of this document however, the more important reason
for the specific choice of protocols + LEDBAT is that this document
only carries out an analysis based on what's in RFC 8303 + RFC 8304,
which is: TCP, MPTCP, UDP(-Lite), SCTP, LEDBAT.
Anyway, we now inserted a note about LEDBAT immediately after the
sentence (in intro, par. 2) that already says that this list of
protocols + LEDBAT is the basis, covered in RFCs 8303 and 8304.


> Section 3.3.2 and Appendix A.3.1 are limiting this "minimal set" of
> transport services to exclude discrete messages and allow only the
> provisioning for TCP-like byte streams.  While I can understand the desire
> to make TCP the "gold standard", the surrounding discussion, particularly
> in A.3.1, seems to be a layering violation.  That is, we are hearing that
> AFra-Bytestream requires receivers (i.e., applications) to be able to
> consume contiguous bytestreams.  But this seems to really be a requirement
> on the application protocol to be self-framing (and to provide its own
> sequence numbers if needed)!  Normally we think of an application protocol
> placing requirements on the transport, or a particular transport as being
> inappropriate for use with a given application protocol.  So I think this
> document needs to more explicitly acknowledge that this is not a "generic
> minimal set" of transport features, but rather a minimal set that is
> applicable for many, but not all, applications: some application
> protocols have requirements that are not met by this "minimal set".

Done, in the last paragraph of the introduction.


> In Section A.3.6, "Data encryption" and "source authenticity" are absent
> from the list of "security related transport features" (that are relegated
> to the other document); this seems like a fatal omission.

The list here are the security related transport features from A.1,
which are taken (literally, for consistency) from RFC 8303, which is
an analysis of what TCP, MPTCP, SCTP, UDP(-Lite) and LEDBAT offer.
Since none of them offer encryption and "Data encryption" is thus
not included in RFC 8303, we believe that this is a misunderstanding.
We have now clarified where this list is from by adding
"from Appendix A.1" in the sentence that introduces this list.


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

> As a former academic, I can see the appeal of abstracting away details to
> get to a generic set of features that could allow the backend to do
> protocol selection and increase the usage of more modern/better-featured
> protocols.  But in some sense this feels like it really wants to be
> defining an abstract API for transport services, making the feature set
> into an *interface*, and more directly enabling this automatic selection in
> the backend.  Unfortunately, this particular description is perhaps too
> abstract to be translateable into "interoperable implementations" of an
> abstract API (that is, so that an application written for one instantiation
> of the API would be able to be used with a different instantiation of the
> API using only a thin shim layer).  In particular, in Section 3.2 we are
> given the option for keeping grouping information within the transport
> abstraction (even when SCTP is not available) or reporting to the
> application that he attempt to group connections failed.  An abstract API
> would also need to be quite clear on the semantics of the exposed routines;
> e.g., "timeout event when data could not be delivered for too long" is
> predicated on reliable delivery being requested, so an application should
> not try to use that event as a failsafe abort timer, since UDP does not
> provide reliable delivery at all, and the application won't know that UDP
> is/is not in use.  So while my personal preference would be to see this
> document written in a very different way before publication, I have not
> technical grounds to insist on it, so my preference here is just a
> nonblocking objection.

We agree that there are several difficulties when trying to take this
to a real implementation; otherwise the next step currently taken by the
WG (to define an API and describe the implementation below) would be easy
(but it isn't). We note, though, that interoperability is essentially
solved by the requirement of "one-sided implementation over TCP".


> I agree with Ben that some of the content in the appendices probably
> ought to be pulled into the main body of the document.

Done.


> The Gen-ART review called out "ADDED" as being noteworthy; my only
> additional contribution is to ask whether "CHANGED FROM RFC8303" would be
> easier on the reader.

Done.


Per-section comments follow.

> I think we need a reference for LEDBAT in Section 1 when it first shows up.

Reference: not done now because, in this case, we should probably also have
references to TCP, MPTCP, UDP, UDP-Lite and SCTP - but they all are taken
from RFC 8303 here.


> (Also, RFC 6817 seems to say that the 'T' is "Transport", not "Transfer".)

The features are copied literally (for consistency) from RFC 8303, which
calls this the LEDBAT transport feature:
'Enable and configure "Low Extra Delay Background Transfer"'.


> Section 2 has an interesting definition of socket, but I don't have an
> alternate term handy to reflect this concept without overloading the
> connotations of a POSIX socket.

This definition is also taken from RFCs 8303.


> I may just be thinking too hard, but does "Hand over a message to reliably
> transfer (possibly multiple times) before connection establishment"
> mean that the data transfer of this message will complete prior to
> connection establishment, or merely that the application has handed data to
> the transport prior to connection establishment (but the data transfer will
> occur after connection establishment)?  (The Appendices do help clear this
> up with concrete examples, but perhaps the text earlier in the document
> could be clarified?)

It means the latter. After moving text from the appendix to the main
text, we think this is now clarified early enough.


> Section 3.1
> 
>   [...] This means that
>   unacknowledged data residing a transport system's send buffer may
>   have to be dropped from that buffer upon arrival of a "close" or
>   "abort" notification from the peer.
> 
> nit: Is this supposed to be "residing in"?

Yes, done.


> Section A.1.1
> 
> I'm a little confused how the multiple-streams establishment features are
> "automatable".  Coming at this from the perspective of an application that
> needs to know that multiple streams exist in order to concurrently send
> data on them, it seems non-automatable.  But I assume there's a different
> sense in which the application can't really tell whether those "multiple
> streams" are related at the transport protocol level or it's just the
> transport abstraction that's tracking the group.

Right, the notion of grouping needs to be there such that priorities
can be given - but whether this grouping really is available, and
really is implemented using multi-streaming, is a matter that can
be decided without application-specific knowledge. We changed the
text to point two references that describe how this can be done in
case of SCTP.


> I'm also a little unsure about "configure message fragmentation" being
> "automatable", given my understanding of how many fragmentation-intolrant
> networks there are.  (But that's far from my area of expertise and any data
> I have would be stale, so I don't really have anything useful to say here.)

From the text in "pass 1" in RFC 8303 it's clear that this is about
fragmentation in the host itself, by SCTP, not IP-level fragmentation.
In this draft, however, it's hard to understand that. We clarified
this in the text.


>   o  Disable checksum when sending
>      Protocols: UDP
>      Functional because application-specific knowledge is necessary to
>      decide whether it can be acceptable to lose data integrity.
>      Implementation: via SET_CHECKSUM_ENABLED.UDP.
>      Implementation over TCP: do nothing (TCP does not offer to disable
>      the checksum, but transmitting data with an intact checksum will
>      not yield a semantically wrong result).
> 
> (Also for receiving, "checksum coverage" topics, etc.).  Please clarify
> that this is "data integrity with respect to random corruption" (as opposed
> to "source authentication and integrity protection" which would require
> more crypto).

Done: added "with respect to random corruption" after "data integrity" everywhere.


> The notification of unsent/unacknowledged (part of a) message items are
> listed as automatable because the distinction is "network-specific".  I
> agree that the distinction is something that can be made automatically, but
> I don't really understand how what "network" it is supposed to be that
> matters for the distinction.

Agreed; we replaced this with "does not relate to application-specific knowledge".
(basically, applications don't have to care about this distinction)


> Section A.2
> 
>   the following list, we therefore precede a transport feature with
>   "T:" if an implementation over TCP is possible, "U:" if an
>   implementation over UDP is possible, and "TU:" if an implementation
>   over either TCP or UDP is possible.
> 
> nit: It looks like "T,U:" is what's actually used, with the comma.

Done.


> Section A.3.2
> 
>   With only these semantics necessary to represent, the interface to a
>   transport system becomes easier if we assume that connections may be
>   a transport protocol's connection or association, but could also be a
>   stream of an existing SCTP association, for example.
> 
> nit: is this missing a "not only" (or similar) in "may be not only a
> transport protocol's connection or association"?

Done.

Cheers,
Michael