"Does TCP Need an Overhaul?" (internetevolution, via slashdot)

Paul Vixie paul at vix.com
Sat Apr 5 12:49:43 UTC 2008

> You've also got fast retransmit, New Reno, BIC/CUBIC, as well as host
> parameter caching to limit the affect of packet loss on recovery time.  I
> don't doubt that someone else could do a better job than I did in this
> field, but I'd be really curious to know how much of an effect a
> intermediary router can have on a TCP flow with SACK that doesn't cause more
> packet loss than anyone would put up with for interactive sessions.

my takeaway from the web site was that one of the ways p2p is bad is that
it tends to start several parallel tcp sessions from the same client (i guess
think of bittorrent where you're getting parts of the file from several folks
at once).  since each one has its own state machine, each will try to sense
the end to end bandwidth-delay product.  thus, on headroom-free links, each
will get 1/Nth of that link's bandwidth, which could be (M>1)/Nth aggregate,
and apparently this is unfair to the other users depending on that link.

i guess i can see the point, if i squint just right.  nobody wants to get
blown off the channel because someone else gamed the fairness mechanisms.
(on the other hand some tcp stacks are deliberately overaggressive in ways
that don't require M>1 connections to get (M>1)/Nth of a link's bandwidth.
on the internet, generally speaking, if someone else says fairness be damned,
then fairness will be damned.

however, i'm not sure that all TCP sessions having one endpoint in common or
even all those having both endpoints in common ought to share fate.  one of
those endpoints might be a NAT box with M>1 users behind it, for example.

in answer to your question about SACK, it looks like they simulate a slower
link speed for all TCP sessions that they guess are in the same flow-bundle.
thus, all sessions in that flow-bundle see a single shared contributed
bandwidth-delay product from any link served by one of their boxes.

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