[e2e] Are we doing sliding window in the Internet?

lars.eggert@nokia.com lars.eggert at nokia.com
Thu Jan 11 07:43:16 PST 2007


sorry for jumping in late - big pile of unread mail over the holidays.

> This issue is minor compared to the widespread changes to 
> their TCP stack Microsoft made with adopting Compound TCP in Vista.
> http://www.microsoft.com/technet/community/columns/cableguy/cg
> 1105.mspx

The IETF has approached MS over this issue, and apparently C-TCP will
not be enabled by default on end-user Vista versions. The decision for
the server versions has not been made AFAIK.

Recent Linux versions, however, apparently enable BIC or CUBIC TCP by
default, which raises concerns.

In any case, these examples all illustrate that there seems to be
considerable interest in deploying "faster" TCP variants on the
Internet, or TCP variants that are "more optimal" across certain paths.
Many of these schemes can be significantly more aggressive than any
current congestion control standard.

The concern is that while many of these proposals look interesting, few
if any have been validated to the point where they can be recommended
for wide-spread deployment. (And let's be clear, stuff that gets shipped
in the most common stacks out there _is_ seeing wide-spread deployment,
especially if enabled by default.) Many modifications haven't even been
_documented_ to the point of allowing to analyze their impact, even ones
that are shipping.

We're on a slippery slope here. Yes, TCP is less than efficient in many
network scenarios that are becoming increasingly more common, and
modifications can have a positive impact. But they can also have a large
negative impact on the careful equilibrium that the VJ mechanism have
maintained for the last 15 years. Congestion control is arguably one of
the pillars of the Internet, and changes need to be thought through and
validated carefully, both by the proposers and the community at large.

The good news is that we do have research results and some limited
operational experience that looks promising. We need more of it. Before
wide-spread deployment, we need wide-spread experimentation. The recent
draft-floyd-tsvwg-cc-alt lists a number of important points that such
experiments need to discuss.

Some IETF transport folks have been discussing how to make progress in
this space. A first step seems to be that proposed modifications need to
be sufficiently documented by the proposers in a public forum, such that
the community can review them. Informational RFCs are a convenient form.
The community could then elect to further discuss and analyze promising
proposals, developing them towards specification for Experimental use. A
Standards Track effort would eventually follow.

We're planning to further discuss these issues and a proposed way
forward at the ICCRG and PFLDnet meetings in February and would welcome
participation from researchers, developers and other interested parties.

NEW EMAIL: lars.eggert at nokia.com
NEW MOBILE: +358 50 48 24461
NEW JABBER: lars.eggert at googlemail.com  
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