Re: [conex] WGLC for draft-ietf-conex-abstract-mech-06.txt

[Ingemar Johansson S <[e-mail]>]

Hi

Thanks for the explanation now I get it!

/Ingemar

> -----Original Message-----
> From: Bob Briscoe [mailto:[e-mail]]
> Sent: den 13 december 2012 14:25
> To: Ingemar Johansson S
> Cc: [e-mail]
> Subject: RE: [conex] WGLC for draft-ietf-conex-abstract-mech-06.txt
> 
> Ingemar,
> 
> At 09:46 13/12/2012, Ingemar Johansson S wrote:
> > > We should make it clear that the only state that re-creates
> > itself is the flow-
> > > ID state, not the credit associated with it. I wrote that
> > sentence (not Matt),
> > > and my motivation was to explain that a switch to a different
> > audit wouldn't
> > > be catastrophic for the flow. I wasn't trying to claim that it
> > would hand-over
> > > without a glitch. The audit would drop some packets, so the flow
> > > would send some Re-Echo-Loss, which would establish some credit and
> > > the flow could continue.
> >[IJ] Hmm, perhaps I did not get that part, what you say is that if the
> >audit drops a packet, that will count as a credit ?.  This credit thing
> >is is still a mystery so I probably need to get this explained
> >better...
> 
> OK, I think I promised Marcelo I would write a draft about audit, with a
> couple of example algorithms. I suspect that would help clear up many of
> these questions.
> 
> For now, example stateful and stateless audit pseudocode and evaluations
> are here:
> <http://www.bobbriscoe.net/pubs.html#refb-dis> (Section 7.6)
> 
> Here's an overview:
> 
> Very brief (for those who mostly get it already):
> ----------
> The audit function doesn't count its own losses when balancing Re-Echo-Loss
> against Loss, because they are /audit/ losses, not congestion losses.
> 
> So, if a flow is re-routed to a new audit function that doesn't hold any state
> about this flow, it will start discarding  a low level of packets (audit discard).
> This will trigger the sender to Re-Echo-Loss in the next round trip, which will
> make the flow's audit balance positive and stop further audit discards.
> 
> So, the flow has naturally created new state at the new audit function
> without the flow explicitly knowing that the burst of loss was due to a re-
> route into a new audit function which caused the flow's credit state at the
> original audit function to be lost.
> 
> 
> Brief but fuller explanation:
> ----------------------------
> Re-Echo is always at least a round trip behind (longer for e.g.
> RTCP). The audit function makes the sender responsible for making the
> allowance for its feedback delay, by having to mark sufficient credit packets.
> Then the audit function can just test instantaneous negativity, without
> having to allow for RTT or feedback delays that it doesn't know.
> 
> I'll assume a stateful audit function (stateless is possible, but it has to be less
> precise). For the duration of a flow, a stateful audit function counts bytes of
> different ConEx markings, and it will start dropping packets if
> * the balance of (Credit + Re-Echo-ECN  - ECN)  goes negative, or
> * the balance of (Credit + Re-Echo-Loss - Loss) goes negative. {See note}
> 
> Consider a re-route where a flow is switched to a different stateful audit
> function, and assume the audit functions don't hand-over their state.
> 
> * As soon as the flow suffers any loss or ECN in the network, the flow will go
> negative at the new audit function for at least a RTT, because it has no credit
> and no Re-Echo for at least a round trip.
> * So audit starts dropping packets (a very small but growing proportion).
> * Importantly, the audit doesn't count these discards as loss because they
> are its own /audit/ losses not congestion losses.
> * The sender gets the loss feedback from the receiver and sends some Re-
> Echo-Loss markings to balance.
> * These make the flow positive at the new audit function, because it didn't
> count its own discards.
> 
> So, the flow has naturally created new state at the new audit function
> without explicitly knowing that the burst of loss was due to a re-route.
> 
> 
> Cheers
> 
> 
> Bob
> 
> {Note: Let's set aside the question of how an audit function knows how
> much loss an upstream node has done - abstract-mech talks about that
> separately.}
> 
> 
> __________________________________________________________
> ______
> Bob Briscoe,                                BT Innovate & Design