Subject: How asymmetric are paths?

From: "Falk, Aaron" <[email-masked]>
Date: Wed, 1 Dec 1999 15:48:21 -0500

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?)

Venkat has recently suggested (on this list) that link protocols can create
severe asymmetries. Can anyone provide specifics of the kinds of bandwidth
asymmetries TCP will see on a shared link? What bandwidth? What link? 

--aaron

---

From: George Michaelson <[email-masked]>
Date: Thu, 02 Dec 1999 09:21:11 +1000

I did some grossly simplistic tests in PPP/Modem with unmodified FreeBSD
implementation of TCP and measured the return/ACK to payload ratio at 4%
for large object transfers in FTP and WWW. 

So I find 200:1 a bit on the high side. Maybe there is something in window
scale which gets you up there but for payloads which do small packet flow
I can't see that figure being sustainable. Streaming protocols surely might
achieve this but thats not TCP payload.

If the backchannel congests, the backoff alg. is going to make the on-effects
on the forward channel completely out of sync with its own congestion levels
if nothing else. (dropped ACK in place of real lost payload)

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. Secondly
if they are on a shared media, then for there to be an assymetry implies
that the link protocol itself takes a distinctly non-symmetric view of the
physical layer. If it does, perhaps the reflected assymetries of link overheads
are a function of that complexity and not directly a function of the link
protocol itself? 

Sorry if this is tangential or gibberish.

cheers
	-George
--
George Michaelson         |  DSTC Pty Ltd
Email: [email-masked]    |  University of Qld 4072
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