Code change:
If cross traffic that also traverses part or all of the same path topology increases or decreases, especially if this new cross <del> traffic is "inelastic," and does not, itself, respond to indications of path congestion. To recognize that a path carrying streaming media is not behaving the way it normally does, having an expected baseline that describes the way it normally does is fundamental.  Analytics that aid in that recognition can be more or less sophisticated and can be as simple as noticing that the apparent round trip times for media traffic carried over TCP transport on some paths are suddenly and significantly longer than usual.  Passive monitors can detect changes in the elapsed time between the acknowledgements for specific TCP segments from a TCP receiver since TCP octet sequence numbers and acknowledgements for those sequence numbers are carried in the clear, even if the TCP payload itself is encrypted.  See reliable-behavior for more information. As transport protocols evolve to encrypt their transport header fields, one side effect of increasing encryption is the kind of passive monitoring, or even "performance enhancement" (RFC3135) that was possible with the older transport protocols (UDP, described in unreliable-behavior and TCP, described in reliable-behavior) is no longer possible with newer transport protocols such as QUIC (described in quic-behavior).  The IETF has specified a "latency spin bit" mechanism in Section 17.4 of RFC9000 to allow passive latency monitoring from observation points on the network path throughout the duration of a connection, but currently chartered work in the IETF is focusing on endpoint monitoring and reporting, rather than on passive monitoring. One example is the "qlog" mechanism I-D.ietf-quic-qlog-main-schema, a protocol-agnostic mechanism used to provide better visibility for encrypted protocols such as QUIC (I-D.ietf-quic-qlog-quic-events) and for HTTP/3 (I-D.ietf-quic-qlog-h3-events). </del> <ins> traffic is "inelastic," and does not respond to indications of path congestion. Wireless links (Wi-Fi, 5G, LTE, etc.) often see rapid changes to capacity from changes in radio interference and signal strength as endpoints move. To recognize that a path carrying streaming media has experienced a change, maintaining a baseline that captures its prior properties is fundamental.  Analytics that aid in that recognition can be more or less sophisticated and can usefully operate on several different time scales, from milliseconds to hours or days. Useful properties to monitor for changes can include: round-trip times loss rate (and explicit congestion notification (ECN) (RFC3168 when in use) out of order packet rate packet and byte receive rate application level goodput properties of other connections carrying competing traffic, in addition to the connections carrying the streaming media externally provided measurements, for example from network cards or metrics collected by the operating system </ins> 3.3.

Related discussion:
text: This tries to address both of:
Discussion from editor's conference call: Can we provide a reference for useful properties? Network analytics, application analytics, or both? Currently application analytics, which is OK. Is that clear enough? Who is supposed to notice changes from an expected baseline? Is mentioning QLOG helpful? Have we said clearly enough that baselines themselves can fluctuate, before the kinds of path characteristic changes we're talking about kick in?
I think just "baseline" is clear enough to me, in context with this section listing some relevant stats. I guess "baseline pattern of some relevant statistics" is maybe more precise, but I'm not sure "baseline pattern" really gets there.
from NAME 's review (URL) 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.
This comment helpfully points out the changing network architecture (a LOT of wireless), and we should be clearer about what we're talking about in Section 3.2 (our mental was mostly wired) and in 5.5.3 (where we focused on wireless). This material should go in Section 3.2, and should be referenced in 5.5.3..
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From NAME (URL) 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 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.
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This works for me. I'll merge it.