Re: [quicwg/base-drafts] Specify IPv6 flow label for QUIC (#2348)

[Brian Trammell <[e-mail]>]

Probably. There are (probably minor) caveats here.

If/when [RFC 6437](https://tools.ietf.org/html/6437) is widely deployed in the forwarding plane (I have neither data nor intuition about this), this will cause ECMP to operate on the DCID, so switching DCID will cause packets to be grouped along a different path. 

The upside of this is that in cases where ECMP is balanced across different _interdomain_ paths (this does happen, though I'm not sure how rarely -- though I know of recent work on traceroute enhancements to compensate for increasingly complex multipath topologies in the Internet), the cost of timing-correlation linkability across DCID change goes way up in the Internet core.

The downside is that a DCID switch will be more likely to cause latency discontinuity; when the switch hashes on to a faster path, there will be a spike in reordering at the receiver. This won't break anything per se (QUIC handles reordering just fine as specified), but it may lead to adverse performance in the few windows around a DCID switch that isn't currently there.

tl;dr it's certainly worth doing, but let's not be surprised by the (minor) effects of the decision.

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