[e2e] Link Aggregation

[Spencer Dawkins]

Dear Valentin,

The problem would be that once the "last router" starts putting
packets from a single flow on two different ports, we KNOW that
the packets will not be routed along the same path, and there's
no "resequencer" (short of the destination host) that will
ensure that we're not generating spurious Fast Retransmits at
the destination host.

Packets that are out-of-sequence by three packets aren't that
hard to generate, if you really have two gigabit streams being
independently routed through the network.

There are limited environments where what you're talking about
might work (3GPP uses a sequenced tunneling protocol, for
instance), but the general case is harder.

In the scenario you describe, if you spend much time in Fast
Recovery, you quickly lose the advantage of the second link
(because each Fast Recovery reduces cwnd by 50%).

TSVWG has active proposals for a "response to spurious Fast
Retransmit detection" (see
http://www.ietf.org/internet-drafts/draft-ietf-tsvwg-tcp-eifel-response-03.txt),
but I believe the proposals for adjusting the threshold for Fast
Retransmit are still floating around the research community.

I hope this is useful to you,

Spencer Dawkins

--- Valentin Ossman <valentin at tehutinetworks.com> wrote:
> Or, even better, let's suppose that the last router (connected
> to the
> station network) knows about this link aggregation and is
> distributing
> the packets in-order and evenly between the 2 ports to achieve
> higher
> bandwidth.
> 
> Valentin