Subject: RE: [TIME-LIST] RE: draft-ietf-pilc-asym-00.txt

From: Vernon Schryver <[email-masked]>
Date: Wed, 20 Oct 1999 14:12:04 -0600 (MDT)

> From: Jan Kullander <[email-masked]>

> ...
> > I don't understand how that might be.  Unless the sum of the advertised
> > windows of the individual TCP connections amounts to at least
> > bandwidth*RTT or more than 10 MByte, you cannot fill a 300 ms. by 30
> > Mbit/sec pipe.  ...

> We are aware that both TCP peers need bandwidth*RTT buffer space to
> fully utilise the channel. However even 16 kb buffer space (which we believe
> is common among Internet servers, please advise us otherwise) requires 
> more bandwidth on the reverse channel than GSM offers to sustain the
> uncompressed ack flow. We wanted to point out that even if the asymmetry
> is limited to a non extreme level in this case, the possible sharing of the
> reverse bottleneck channel exacerbates it.

Ok.
At the start,  I just wanted to point out that such problems are ancient
in Internet Time, and have easier, simpler work-arounds (and possibly even
solutions) than proposed.

Have you looked at those old work-arounds?  Are they known to not work
well enough to make more complicated systems such as proposed here
unnecessary and so undesirable?

> ...
> The receiving host could also be a laptop.

Even if the receiving host is a laptop with 4 GByte of RAM, I do not
understand how the claimed assymetry can exist.  Besides the TCP window
issue is something else that just occurred to me.  For you to have a 3000:1
assymetry, the bit rate between the two pipes must be 3000:1.  If one
direction is merely low, say only 10,000 bits/sec, then the other direction
must be awfully fast for most mobile links.

Yes, I've heard of things such direct from satellite in one direction
and various low speed links in the other.  Some have 3000:1 or higher
peak instantaneous ratios, but none that I've heard of have 3-digit
ratios measured over reasonable durations, such as the time required
for a single ACK to be sent on the slow link.

>                                            We agree that MByte sized
> buffering in a mobile phone seem a bit unlikely. Furthermore, we don't
> see much use of it.

Exactly.  A phone that could display the MBytes of HTTP data implied
by a 3000:1 ratio sounds a long way in the future.

> ...
> We see no need for a proxy in the asymmetrically connected host, just
> the need of large buffers as you pointed out.

I do not see any need of large buffers, unless you somehow have a real
need for that 3000:1 ratio.  A proxy was suggested as a way to justify
that ratio and those buffers.

> The proxy we were thinking of would be located on the opposite side of
> the asymmetric link (from the receiving host point of view). It could e.g.
> be an ordinary workstation with an Apache web proxy and a FreeBSD 
> operating system with timestamp  and SACK options enabled and large 
> buffer spaces allowed.
> ...

Proxies can be good things, but I don't see what proxies have to do with
3000:1 assymetries, neither in showing that such assymetries can exist
in practice nor in showing how they might be used.

Vernon Schryver    [email-masked]