Re: [ledbat] Suggestion re "How to deal with the modem buffer"

On Nov 20, 2008, at 2:32 PM, Robb Topolski wrote:

> Comment in response to Nick's VG presentation:  Even if there is no  
> silver bullet, as a "to do item" for the WG, in the recommendations  
> and current practices paper, we ought to address the issue that Nick  
> brings up on how to deal with large modem buffers.

The big thing is it needs to be parameterizeable by the ISP, with  
exposure to whatever ISP control mechanisms are available.

There needs to be a floor of a couple of packets for the DSL case, and  
nothing can really fix that.  Thus a floor of probably 4 packets *  
1500B MTU => ~8 kB as a floor.  OTOH, uploads from a 128 Kbps DSL link  
are going to be painful, period.

Once you shift to higher bandwidth networks, the buffering needs to be  
bandwidth * generic RTT.

A good generic RTT for most uses is probably 150ms, which puts a bound  
on additional latency & jitter for realtime uses (VoIP, games, SYNs  
for web surfing) of 150ms regardless of the TCP flows going  
simultaneously,  while ensuring that you can do a full rate TCP flow  
for any host <150ms away, and only lose throughput for hosts >150ms  
away.

Thus for a 1 Mbps DOCSIS link, it should be ~20 kB, and at a 4 Mbps  
link, it should be ~80 kB.

The key thing for the vendors is the following:  It is ENCOURAGED to  
oversize the buffer (eg, 256 kB), but the ISP MUST be able to remotely  
configure the access device to use less than the full buffer capacity,  
as this way the device can be parameterized for the actual network, as  
the same modem is used for different service-tiers.

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