Re: Consensus call for Late-Stage documents, pre-IETF 108 edition

[Gorry Fairhurst <[e-mail]>]

On 23/07/2020 22:02, Jana Iyengar wrote:
> I'm happy with the resolution that Ted proposed and Lars wrote up in 
> PR 3945 <https://github.com/quicwg/base-drafts/pull/3945>.
>
> We should not forget that endpoints will do what they choose to do on 
> this particular issue. It's perfectly reasonable to assume that port 
> changes are infrequent and do not reflect a path change commonly. In 
> the rare case that it happens and in the rarer case that it is in fact 
> a path change, both congestion controllers and RTT estimators are 
> adaptive. The text in the PR captures this possibility. That reflects 
> reality and is an adequately cautious recommendation, IMO.
>
As you expect I don't agree this is cautious: optimising for an 
infrequent/rare case is not cautious, it introduces more chance of 
undesriable side effects on paths that actually are different.

There seems to me to be little impact in re-initialising the RTT 
estimator, there's an initial handshake for the new path from which this 
can be initialised.

Christian: I also would love to see data about the frequency and 
pathology, but don't know of any.

Gorry

> On Thu, Jul 23, 2020 at 11:47 AM Christian Huitema 
> <[e-mail] <mailto:[e-mail]>> wrote:
>
>     On 7/23/2020 12:42 AM, Lars Eggert wrote:
>
>>     Hi,
>>
>>     On 2020-7-23, at 10:34, Gorry Fairhurst<[e-mail]>  <mailto:[e-mail]..uk>  wrote:
>>>     These methods often hash based on port, IP, and maybe other fields to select a queue - There are a finite set of queues, a rehash results in the traffic moving queue ... to chooses a different path or queue within the device. This mapping is statistical, this matters more if the multiplexing is low and a port change results in unbalance across quques/paths.
>>     I agree this is mostly a concern for low-mux'ing cases when a single rehash can create significant imbalance. How common are such deployments, and how much of a problem is such a temporary imbalance (the CC will adjust quickly). Esp. compared to forcing slow-start after every NAT rebind.
>
>
>     Gorry, Lars, where can we find data on the NAT behavior and the
>     relation between NAT rebinding and routing paths? In home routers,
>     NAT rebinding typically only change if the session is quiet for
>     some time. Do we observe a different behavior with CGNAT?
>
>     -- Christian Huitema
>