Re: [Masque] ECN & Flow IDs

[Mirja Kuehlewind <[e-mail]>]

Hi David, hi all,

to also add to your question about multiple flow IDs. I guess we kind of agree that we don’t want flow ID for ECN information, as adding two bits is simple enough, however, then it is actually not fully clear to me why multiple flow IDs per connect request are needed. You briefly mentioned other extension below, however, not all, or only very few information, require per-packet information (as it was used for the ECN example). If you want to actually send multiple flows/connections to the same server you, can always send multiple connect requests. That’s slightly more overhead at connection establishment but not much, and I would say it’s architecturally more clean and therefore also simpler.

Mirja


From: Masque <[e-mail]> on behalf of David Schinazi <[e-mail]>
Date: Wednesday, 31. March 2021 at 02:36
To: Martin Duke <[e-mail]>
Cc: MASQUE <[e-mail]>
Subject: Re: [Masque] ECN & Flow IDs

The trivial other approach to solving ECN would be to prefix the UDP payload with a type byte that contains the two ECN bits and 6 unused bits.
That would definitely work, and therefore I don't think ECN is a good example to use as a discussion starter for how flow IDs are managed.
I wrote that draft to test out the extensibility of the CONNECT-UDP design, I'm not planning on moving it forward at this time.

More conceptually, the main question is whether we allow one client-initiated bidirectional stream to map to multiple datagram flow IDs.
The approach I've taken is to say yes: that allows us to reuse the flow ID multiplexing logic to encode additional information in the flow ID.
You could build something isomorphic to this where you say no and have one flow ID per stream, and then add a second varint right after the flow ID.
Since datagrams have to fit in a QUIC packet, adding more varints eats into available datagram MTU which makes me prefer the multiple flow IDs per stream approach.
Both are pretty much equivalent in terms of implementation complexity: either way you need a hash table mapping from a varint to what that varint means.
There is more complexity in how you convey that mapping (right now this is done using parameters on the Datagram-Flow-Id header), but I don't think
requiring one flow ID per stream solves any of that complexity, you'll still need a way to convey extension information. The ECN example is so simple that
it doesn't require sending much extension information, but if you look further at enabling optional IP header compression in CONNECT-IP, then you'll want
a way to associate a varint with which IP addresses you're compressing.

I think the question we have to answer becomes: do we want MASQUE protocols to be extensible? Our options are:
- disallow extensibility and slightly simplify the protocol
- allow extensibility via multiple flow IDs per stream, and deal with the slight complexity
- allow extensibility with a single flow ID per stream, and deal with the slight complexity
Personally, I feel strongly that we should allow extensibility.

David


On Tue, Mar 30, 2021 at 3:40 PM Martin Duke <[e-mail]<mailto:[e-mail]>> wrote:
Hello MASQUE,

At IETF 110 there was a lot of good discussion challenging the foundations of the CONNECT-UDP framework, including the relationship between streams and Flow-IDs. While CONNECT-UDP happens to use flow IDs somewhat incidentally, the real action with the controversial multiple-flow-id-per-CONNECT happens in David's (unadopted) ECN draft: https://datatracker.ietf.org/doc/draft-schinazi-masque-connect-udp-ecn/

Leading up to the interim, it would be great if one of the detractors of the flow-id mapping submitted their own approach to solve ECN. This would help to illuminate the tradeoffs. Speaking as an individual, I am also hoping to move the ECN work forward and having another design would help to do that.

Thanks,
Martin
--
Masque mailing list
[e-mail]<mailto:[e-mail]>
https://www.ietf.org/mailman/listinfo/masque