Re: [Tsv-art] Tsvart last call review of draft-ietf-mops-streaming-opcons-10

[Spencer Dawkins at IETF <[e-mail]>]

Hi, Michael,

Thanks for the helpful review!

Some of the comments in your review are at the level of "the authors need
to discuss this comment, and probably discuss this with the working group,
before adding Github issues", which is fine ("the truth is always
appropriate"). I'm meeting with Jake and Ali later this week, so we'll
definitely go through these, and come up with proposed responses, but PRs
for a couple of them may take longer.

But, seriously, thanks for the helpful review.

Best,

Spencer

On Fri, Apr 29, 2022 at 1:40 PM Michael Scharf via Datatracker <
[e-mail]> wrote:

> Reviewer: Michael Scharf
> Review result: Ready with Issues
>
> This document has been reviewed as part of the transport area review team's
> ongoing effort to review key IETF documents. These comments were written
> primarily for the transport area directors, but are copied to the
> document's
> authors and WG to allow them to address any issues raised and also to the
> IETF
> discussion list for information.
>
> When done at the time of IETF Last Call, the authors should consider this
> review as part of the last-call comments they receive. Please always CC
> [e-mail] if you reply to or forward this review.
>
> This informational document surveys network and transport protocol issues
> that
> affect quality of experience for streaming applications, in particular
> video.
>
> The overview covers many topics across different technologies. Readers may
> only
> be familiar with a subset of these topics and could therefore learn quite
> a bit
> from this kind of tutorial.
>
> Nonetheless, it remains somewhat unclear what the actual objective of some
> of
> the text is. For some topics, quite a bit of technical background is
> provided,
> but the discussion is not really comprehensive. In many sections, the
> document
> neither derives technical challenges, nor system/protocol requirements, nor
> practical usage guidance. It is just a kind of tutorial. Quite a bit of
> text
> also presents ongoing IETF work, and an RFC with this scope may thus get
> outdated soon.
>
> Section 6.2 deals with topics owned by TCPM. In similar past documents, I
> have
> asked for a TCPM presentation prior to an IETF last call in order to ensure
> that owners of running code are in the loop. I believe this strategy has
> worked
> well in the past.
>
> Having said this, as TSV-ART reviewer I don't strongly disagree with a
> publication. All these issues may just be OK for an informational RFC.
>
> Some more specific comments:
>
> * Section 3.2.1.  Recognizing Changes from an Expected Baseline
>
> This section apparently assumes relatively static path properties, e.g.,
> fixed
> network connectivity. It would be more useful to analyze the impact of
> Wifi and
> 4G/5G networks in one place. Some of this follows later in sections 3.2.1
> and
> 5.5.3. Yet, the split between these three sections is unclear. Having a
> discussion about today's Internet path characteristics and the dynamics at
> one
> place could be more useful.
>
> The text could also better distinguish between what matters to endpoints
> and
> considerations relevant only for middleboxes (such as passive monitors).
>
> * Section 3.3 Path Requirements
>
> The statement "to find the bandwidth requirements for a router on the
> delivery
> path" may assume that the bottleneck on the path will be routers. Yet, on a
> path the bottlenecks could also be a link layer device (e.g., an Ethernet
> Switch, a Wifi access point, etc.). RFC 6077 (specifically Section 3.1.3)
> also
> explains that issue. A better wording may be "to find the bandwidth
> requirements for a delivery path".
>
> * Section 3.6.  Unpredictable Usage Profiles / Section 3.7.  Extremely
> Unpredictable Usage Profiles
>
> I am not fully convinced by the distinction between "unpredictable" and
> "extremely unpredictable". To me, these two sections could be merged and
> maybe
> also be shortened. More specifically, Section 3.7 lists a lot of statistics
> from the past. What is IMHO a bit missing in Section 3.7 are actual
> operational
> considerations. For instance, are there any lessons learnt for the future?
> Note
> that Section 3.6 has a reasonable conclusion at the end.
>
> * Section 4.  Latency Considerations and Section 4.1.  Ultra Low-Latency
>
> I am surprised by the definition "ultra low-latency (less than 1 second)",
> as
> well as some of the other numbers. For other real-time communication use
> cases,
> "ultra low-latency" would probably imply a latency requirement of the
> order of
> *one millisecond*. For instance, isochronous traffic for motion control in
> industrial networks may require a latency of 1 ms, and such "ultra-low
> latency"
> requirements are discussed elsewhere, e.g., in the DetNet WG. The
> terminology
> in this section should be better explained to readers dealing with
> networked
> systems with much harder latency requirements. And a reference should be
> added
> for these definitions in the context of video streaming.
>
> * Section 5.5.2 Head-of-Line Blocking
>
> If Head-of-Line Blocking is indeed a relevant operational problem, it
> would be
> useful to add a corresponding reference (e.g., with measurements).
>
> * Section 5.5.3.  Wide and Rapid Variation in Path Capacity
>
> The statement "As many end devices have moved to wireless connectivity for
> the
> final hop (Wi-Fi, 5G, or LTE), new problems in bandwidth detction have
> emerged
> from radio interference and signal strength effects." could be moved to
> earlier
> parts of the document. Quite a bit of new computers apparently only have
> Wifi
> connectivity and no Ethernet port, i.e., Wifi may just be their default.
> Also,
> wireless may not only be used on the last hop, for instance in meshed
> setups.
> This may make the problem even harder.
>
> * Section 6.  Evolution of Transport Protocols and Transport Protocol
> Behaviors
>
> I really wonder whether UDP vs. TCP vs. QUIC is actually the relevant
> distinction. What may actually matter are the transport services provided
> by
> the protocol stack (e.g., RFC 8095). I fully agree to a related comment in
> the
> INTDIR review.
>
> * Section 6.1.  UDP and Its Behavior
>
> UDP is also used for encrypted tunnels (OpenVPN, Wireguard, etc.). Does
> encrypted tunneling really have no operational impact on streaming apps
> other
> than circuit breakers? (I am thinking about 4K streaming over OpenVPN and
> the
> like.)
>
> * Section 6.2.  TCP and Its Behavior
>
> This text should IMHO be presented and be discussed in TCPM. Personally, I
> am
> not convinced that this document is a good place for discussing the long
> history of TCP congestion control. If the objective of the document is to
> provide a tutorial, IMHO it would be more useful to briefly explain the
> state-of-the-art in year 2022. Some further specific comments on TCP
> congestion
> control:
>
> - It is surprising that RFC 5681 is not referenced.
>
> - 793bis has normative statements regarding congestion control, and most
> stacks
> are compatible to what is written in 793bis. Several operating systems use
> CUBIC by default and support standard Reno.
>
> - Another general good reference for TCP standards is RFC 7414.
>
> - Does ECN, DCTCP, etc. really not matter at all, for instance, if a data
> center hosts video playout servers?
>
> * Section 6.3.  QUIC and Its Behavior
>
> As already noted, the discussion about head-of-line-blocking would really
> benefit from backing by a reference.
>
> * Section 7. Streaming Encrypted Media
>
> As far as I know, streaming users sometimes use encrypted tunnels such as
> OpenVPN or WireGuard (or IPsec) to access video content. I may miss
> something,
> but it is unclear to me how that fits into the categories presented in
> Section
> 7.
>
>
>