Re: Exercising Version Negotiation

[Christian Huitema <[e-mail]>]

On 3/23/2018 10:26 PM, Ryan Hamilton wrote:

> Maybe. :) I'm just trying to get my arms around a concrete proposal.
> For example, if the server always advertises both versions, but only
> supports 1 version based on hash(client IP), and those client
> typically prefer the other version, then they'll incur an extra RTT on
> every connection, which seems undesirable. But I'm not sure if that's
> the proposal.

There is of course no way to exercise version negotiation without
incurring a 1-RTT penalty. Following the "use it or lose it" paradigm,
we know that if we do not use version negotiation for a while, various
middle-boxes will "learn" that it is not needed and will start blocking
VN packets, in the same way that various middle boxes block various ICMP
packets today. Greasing VN has a cost. If we grease VN for X% of
connections, X% of connections will suffer an additional 1-RTT delay.
There is just no way around it.

Let's assume that we will grease VN on X% of connections. How do we
decide how much that X% is? If it is too low, middle-boxes will learn
that blocking VN only affects a small fraction of connections. They will
think that they can block it, and clients will probably react by
retrying with a different version. Grease fails. If it is too high, too
many clients suffer. So we need some kind of Goldilocks value, not too
big, not too high, just right. Anything below 1% will probably be too
low, but I don't know what would be too high. 5%? 10%? The trade-off is
not very appealing.

Perhaps we should look at it differently. What do we really want to
teach to the middle-boxes? I think the message should really be, "do not
try to lock on a specific version number". And doing that with just two
versions will not work. The middle boxes will just learn those two
versions, and we will not be able to introduce a third one. We want to
see plenty of mostly unpredictable versions proposed by clients, and we
want these unpredictable version numbers to result in successful
connections most of the time.

-- Christian Huitema