[Bloat] ADSL, ATM drivers, bloat, education & confusion

[Kevin Darbyshire-Bryant <kevin@darbyshire-bryant.me.uk>]

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Hi Chaps,

Guess who  :-)  Following on from my probably rash assertion that BT 
here in the UK probably aren't as bad at bufferbloat from them to end 
user due to rate limiting as I think, it has reminded me of my parents' 
situation and efforts to help them.

Scenario is: ADSL very long line (hopefully they've got the copper 
repaired now) so on a good day the ADSL sync is something like 2Mbit 
down, 512kbit up.  I bought them a Netgear DGN3500 which is now running 
OpenWrt CC (+cake) This is not my preferred solution in hardware terms 
but it has a supported built-in ADSL modem so is a one box solution.

The problem I have is setting outbound rate limiting.  I was hoping that 
'cake' without the 'bandwidth' parameter would work on the 
'backpressure' from the ATM(?) driver, sadly this wasn't the case and so 
setting a bandwidth limit (I'm not in a position to test the new 
keywords for ATM encapsulation etc yet) was the only way forward.  
Unfortunately the ADSL link isn't rock solid stable so the negotiated 
rates for in & out flap around a bit.  So a few questions:

1) ATM interface 'backpressure' - is this Byte Queue Limits? (ifxmips_atm)
2) Do byte queue limits apply to outbound only?
3) Horrid thought - this probably isn't just the ATM driver as IP is 
provisioned over PPP.  PPP driver needs BQL too?


So another approach is to query the ATM interface on every IF up (which 
should get reflected into the the PPP interface up) for its current 
negotiated sync rates and use those as input into the SQM scripts.

There's a question here that's niggling me at the back of my mind: How 
is this supposed to work with a two box modem and router solution?  I 
guess the modem should ideally be running some sort of outbound AQM and 
dropping packets on its ethernet interface that just won't fit/queue too 
long.

Sorry, too many questions and I've not enough programming experience let 
alone linux kernel experience to be able to help.  I'll try if someone 
can point me in a direction.

Kevin




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