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Re: NetBSD 5.1 TCP performance issue (lots of ACK)

Thor Lancelot Simon <> writes:

> On Thu, Oct 27, 2011 at 07:55:16PM +0200, Manuel Bouyer wrote:
>> The mbuf passed to the adapter is also passed to bpf_mtap(),
>> so bpf_mtap() gets the tso packet and not the on-wire packets when
>> tso is in use.
>> AFAIK bpf_mtap() doesn't do any attempts at doing something special
>> with TSO mbufs it could get. So I guess tcpdump is getting the TSO
>> buffer ...
> ...which should frequently exceed the MTU, shouldn't it?  So I would
> actually expect tcpdump to misbehave.

Perhaps, but bpf doesn't know about the interface MTU, and has its own
snaplen of how much, so if it is seeing a 32 KB TCP packet then that
should be ok.  But I saw no such packets in the trace.

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