Subject: Re: How asymmetric are paths?

From: Vernon Schryver <[email-masked]>
Date: Wed, 1 Dec 1999 16:14:32 -0700 (MST)

> From: "Falk, Aaron" <[email-masked]>

> At the DC IETF it was suggested that TCP works just fine over the bandwidth
> asymmetries found today -- up to 200:1. I would like to hear whether there
> is any experience (operational or experimental, preferably not simulated)
> that supports or contradicts this. What are the symmetries out there? How
> does TCP perform? (and how do you know?)
> ...

As I've pointed out before, Hybrid Networks was running 10 Mbit/sec
downstream cable modem systems with 19.2 kbit/sec modems over plain old
telephone for the upstream half a dozen years ago in various places.
That's a ratio above 200:1.  They may still be running the system in
Cupertino, California.

See http://www.hybrid.com/PRODUCTS/onewaycabl.htm.

Vernon Schryver    [email-masked]

---

From: "Falk, Aaron" <[email-masked]>
Date: Wed, 1 Dec 1999 18:28:10 -0500

> 
> So I find 200:1 a bit on the high side. 

I believe that implied using delayed ACKs and header compression.

> As to link protocol assymetries, Surely this is an orthogonal 
> issue? From
> first principles, a couple of points stand out: If the 
> channels are implemented
> on different network technologies (likely with sats) then there is no
> direct coupling on link level overhead for the forward and 
> back channel: the
> relative cost of link protocol overhead is specific to each 
> path. 

I was a little too terse in my message. What I meant was that protocols that
share links (increasingly common in satellite and cable-modem systems) may
be tuned such that the upstream bandwidth allocated to a single flow is very
small. I'm wondering what kind of systems are in use that do this and how
small is the allocated share of bandwidth. 

Of course you can say the links should be tuned such that sufficient TCP
throughput is attained. And I agree with that for new systems. However, my
quesstions are meant to get a sense of what people are doing today on
existing systems that may be difficult or impossible to modify. It is my
idealistic hope is as link designers gain clue (perhaps by reading LINK)
they will make smarter choices in how they build their systems. If they
don't, perhaps a competitor will.

--aaron