Re: QUIC Invariants and QUIC-LB

Martin Thomson <mt@lowentropy.net> Fri, 22 May 2020 03:12 UTC

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Date: Fri, 22 May 2020 13:11:47 +1000
From: "Martin Thomson" <mt@lowentropy.net>
To: quic@ietf.org
Subject: Re: QUIC Invariants and QUIC-LB
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On Fri, May 22, 2020, at 10:00, Martin Duke wrote:
> I don't think we're going to get a 100% version-invariant QUIC-LB, 
> though we're pretty close.

Yep.

Based on this discussion, what I think is happening here is that we're defining a new set of invariants: the set of things that we can't really change between versions that support the same load balancers as QUIC version 1.  That this is a superset of the invariants work is a bit unfortunate, but the difference is small enough.

> It would be nice to give LBs somewhat more specific guidance for what 
> fields they can use for packets with non-compliant DCIDs. For example, 
> if we could at least safely assume the DCID won't change until the 
> server provides one, that would be way better than forcing them to make 
> assumptions about IP and UDP fields remaining stable.

Based on invariants + LB-invariants, I think that you have some good advice already:

If you didn't create the DCID, then apply some sort of "default" routing.  That routing will rely on the stability of a subset of fields.  I would select from (source IP, source port, destination IP, destination port, DCID) and no others.  Nothing else in the QUIC packet is good, except *perhaps* the version number.  Without understanding the version, SCID is totally out as it isn't guaranteed to be stable.  Add anything that isn't stable and you might get problems, so while I can see a case for including something like the IPv6 flow label, that's risky.

Generally speaking, it should be enough to route based on DCID alone, or to route just on the source address, but the more (stable) fields you include the more you allow concurrent connection attempts from the same address.

Finally, to speak to the first assumption, if it is a short header, and you decide to use the DCID for routing, then you are OK throwing the packet away, because you don't know how long the DCID is.  Similarly, if the DCID is empty and you need it, then throwing it out is justified.  But you can do QUICv1 without either if you only use addressing information, so that wouldn't need to be the same for every load balancer.