[ledbat] LEDBAT doesn't help sharing network beyond home gateway

[Bob Briscoe <[e-mail]>]

Jana,

You asked the basis of what I just said in TSVAREA. I said in a 
residential network, LEDBAT won't yield to traffic sharing any queue 
upstream of your own home gateway queue.

LEDBAT's target delay is 150ms.

A typical shared link in such a network will be 1Gb/s.

150ms of queuing into a 1Gb/s link is 12,500 packets of queue (if 
1500B per pkt).

The buffer will not be this big (unless there's more bloat than 
anyone would expect)! Therefore LEDBAT will drive the queue off the 
end of its tail, just like TCP does. Therefore LEDAT is not designed 
to yield to others in such a queue into a higher speed link.

The root cause is the fixed 150ms target delay.

Nonetheless, I believe this design choice makes sense for LEDBAT 
_today_: you only yield to others in your own home. You don't yield 
to others in other homes.

If LEDBAT yielded to others, users would reject it. Evidence is on 
the Bittorrent community postings when uTP was 'imposed' on 
BitTorrent users - it was only accepted because it was shown that 
performance didn't suffer unless it was yielding to self.

We could design a LEDBAT-like protocol that yields to others in any 
queue. But no-one would want it (yet). Incidentally, a goal of ConEx 
is to incentivise deployment of LEDBAT-like protocols. The suffix 
'-like' means that they would benefit from yielding to anyone, not 
just your family in your home.


Bob



________________________________________________________________
Bob Briscoe,                                BT Innovate & Design