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Re: Why is my gigabit ethernet so slow?

On Jan 28, 2010, at 7:01 PM, Aaron J. Grier wrote:

> On Wed, Jan 27, 2010 at 09:03:14PM -0500, Steven Bellovin wrote:
>> Sack is mostly of benefit on LFNs (Long, Fat, Networks).  On a LAN or
>> pair of LANs, it won't matter much, I suspect.  SACK is "selective
>> acknowledgment" -- it can resend just the missing segments, rather
>> than everything from the missing segment on forwards.  But that's only
>> of interest if there are packet drops, and with few routers, little
>> congestion, and steady-state sending, those more or less should not
>> happen very often.
> I wonder if it would make sense for the kernel to keep a list of
> networks to disable SACK on, or look at TTL values to determine that.

In theory, one should never have to disable it explicitly; its use is 
negotiated.  But there's no particular harm to using if if both sides support 
it; it's just that it's not likely to help much.

Looking at TTLs doesn't help, because you have no way of knowing what the 
original TTL was of the incoming packets, and it's not worth doing pings to 
find out.

                --Steve Bellovin,

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