[Tmrg] Now, where were we...?

lachlan.andrew at gmail.com (Lachlan Andrew) Tue, 24 November 2009 22:16 UTC

From: "lachlan.andrew at gmail.com"
Date: Wed, 25 Nov 2009 09:16:55 +1100
Subject: [Tmrg] Now, where were we...?
In-Reply-To: <1e41a3230911241017p26af6480sd8b4ff358e2363ef@mail.gmail.com>
References: <BE0E1358-7C27-46A8-AF1E-D8D7CC834A52@ifi.uio.no> <4B05A502.4050402@gmx.at> <1e41a3230911241017p26af6480sd8b4ff358e2363ef@mail.gmail.com>
Message-ID: <aa7d2c6d0911241416q5ca1315br4f66da0b77e304c3@mail.gmail.com>

2009/11/25 John Heffner <johnwheffner at gmail.com>:
> Another issue with delay as a signal is that, absent a means to
> measure one-way delay, it measures the forward and reverse paths
> together, whereas loss will be from only the forward path.

True. The fact that loss on the reverse path is ignored is a mixed
blessing, though.  With highly asymmetric paths, like ADSL, the
reverse path can become highly congested without triggering any
reduction in the cause of that congestion.

Our charter in this group is to do modelling for evaluation transport
protocols, rather than designing the congestion mechanisms.  Should we
conclude that tests should include:
- Variable reverse-path traffic
- Asymmetric links

> Delay measurements are also very problematic on any
> multi-path flow.

That sounds very interesting.  The current test-suite draft doesn't
consider multi-path at all, and we should fix that.

If we don't know the path that a packet takes (or tries to take before
getting lost), we can't deduce per-path congestion from either
measurement, can we?

How are delay measurements more problematic than loss measurements?
Did you mean that we need to match the RTT of a packet to the per-path
propagation delay, or that ACKs for one path will be delayed while
waiting for packets from another path?

Cheers,
Lachlan

-- 
Lachlan Andrew  Centre for Advanced Internet Architectures (CAIA)
Swinburne University of Technology, Melbourne, Australia
<http://caia.swin.edu.au/cv/landrew> <http://netlab.caltech.edu/lachlan>
Ph +61 3 9214 4837