Re: Babel RTT mechanism [Was: [RESEND PATCH v3] Babel: allow choosing link quality estimation] algorithm]
On Wed, Nov 08, 2023 at 11:56:32AM -0500, Nick Cao wrote: > I think it is the intended design. Link quality is about packet losses, > while RTT is about, well, RTT, they cover different characteristics of the > link, which should all be taken into account for metric calculation. RTT has two sources, propagation delay (which is just a question of distance) and queuing/retransmission/MAC delay (which depends on link quality). On wifi with worsening link quality / congestion it starts with raising RTT (due to MAC delay and L2 retransmissions) and fluently continues with raising packet loss, so i would say that both packet loss and RTT are parameters of link quality and should be handled together by one mechanism, instead of two independent mechanisms. > And babeld the reference implementation also allows enabling link quality > estimation and the RTT extension at the same time, matching the current > behavior of bird. I agree that it make sense to handle both packet loss and RTT information for purposes of metric estimation. I just do not think it makes sense to estimate two separate metrics and then sum it. Considering that ETX estimates number of retransmissions, then for algorithm that takes into account both RTT and packet loss it makes more sense to have RTT metric multiplied by ETX coeficient? > On 11/8/23 11:44, Ondrej Zajicek wrote: > > On Sat, Oct 28, 2023 at 09:24:56PM -0400, Nick Cao via Bird-users wrote: > > > Resend of the patch in > > > http://trubka.network.cz/pipermail/bird-users/2023-June/017058.html > > > > > > ETX link quality estimation algorithm is useful for link types other than > > > wireless, > > > especially when using babel with tunnels where packet losses do occur. > > > > Hi > > > > I looked at your patch with intent of merging it, i like a separate > > option for selection of dynamic metric algorithm, but i noticed that i > > misunderstood how recently added RTT mechanism interacts with the rest. > > > > I thought we have three alternative link quality / metric estimation > > algorithms: > > > > - fixed (k-out-of-j) > > - ETX > > - RTT > > > > But in fact RTT is independent metric, which is added on top of fixed/ETX > > metric. > > > > Is this intentional? Does it make sense? > > > > (This is mainly a question for Toke or Juliusz) > > -- Elen sila lumenn' omentielvo Ondrej 'Santiago' Zajicek (email: santi...@crfreenet.org) OpenPGP encrypted e-mails preferred (KeyID 0x11DEADC3, wwwkeys.pgp.net) "To err is human -- to blame it on a computer is even more so."
Re: Babel RTT mechanism [Was: [RESEND PATCH v3] Babel: allow choosing link quality estimation] algorithm]
I think it is the intended design. Link quality is about packet losses, while RTT is about, well, RTT, they cover different characteristics of the link, which should all be taken into account for metric calculation. And babeld the reference implementation also allows enabling link quality estimation and the RTT extension at the same time, matching the current behavior of bird. On 11/8/23 11:44, Ondrej Zajicek wrote: On Sat, Oct 28, 2023 at 09:24:56PM -0400, Nick Cao via Bird-users wrote: Resend of the patch in http://trubka.network.cz/pipermail/bird-users/2023-June/017058.html ETX link quality estimation algorithm is useful for link types other than wireless, especially when using babel with tunnels where packet losses do occur. Hi I looked at your patch with intent of merging it, i like a separate option for selection of dynamic metric algorithm, but i noticed that i misunderstood how recently added RTT mechanism interacts with the rest. I thought we have three alternative link quality / metric estimation algorithms: - fixed (k-out-of-j) - ETX - RTT But in fact RTT is independent metric, which is added on top of fixed/ETX metric. Is this intentional? Does it make sense? (This is mainly a question for Toke or Juliusz)
Re: Babel RTT mechanism [Was: [RESEND PATCH v3] Babel: allow choosing link quality estimation] algorithm]
>> And babeld the reference implementation also allows enabling link quality >> estimation and the RTT extension at the same time, matching the current >> behavior of bird. Exactly right. > Considering that ETX estimates number of retransmissions, then for > algorithm that takes into account both RTT and packet loss it makes more > sense to have RTT metric multiplied by ETX coeficient? The current algorithm is described here: https://datatracker.ietf.org/doc/html/draft-ietf-babel-rtt-extension My intuition is that it doesn't matter much. The goal is not to have an accurate model of reality, the goal is to choose optimal paths. And I would guess that in any realistic topology, the two algorithms will yield the same or almost the same set of paths. It looks like your intuition is different from mine: that's exciting, and it means that we need to design an experiment in order to find out who's right. Can you design a topology in which the algorithm described in the draft and the algorithm that you propose yield different sets of paths, and where your proposed algorithm leads to significantly better performance? If it turns out that you're right, I'll be thrilled to write a successor RFC, perhaps with you as co-author. -- Juliusz
Babel RTT mechanism [Was: [RESEND PATCH v3] Babel: allow choosing link quality estimation] algorithm]
On Sat, Oct 28, 2023 at 09:24:56PM -0400, Nick Cao via Bird-users wrote: > Resend of the patch in > http://trubka.network.cz/pipermail/bird-users/2023-June/017058.html > > ETX link quality estimation algorithm is useful for link types other than > wireless, > especially when using babel with tunnels where packet losses do occur. Hi I looked at your patch with intent of merging it, i like a separate option for selection of dynamic metric algorithm, but i noticed that i misunderstood how recently added RTT mechanism interacts with the rest. I thought we have three alternative link quality / metric estimation algorithms: - fixed (k-out-of-j) - ETX - RTT But in fact RTT is independent metric, which is added on top of fixed/ETX metric. Is this intentional? Does it make sense? (This is mainly a question for Toke or Juliusz) -- Elen sila lumenn' omentielvo Ondrej 'Santiago' Zajicek (email: santi...@crfreenet.org) OpenPGP encrypted e-mails preferred (KeyID 0x11DEADC3, wwwkeys.pgp.net) "To err is human -- to blame it on a computer is even more so."
