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Date:      Mon, 24 Mar 2014 11:54:00 -0700
From:      John-Mark Gurney <jmg@funkthat.com>
To:        soumya panigrahi <soumyapanigrahi@gmail.com>
Cc:        Giuseppe Lettieri <giuseppe.lettieri73@gmail.com>, Luigi Rizzo <rizzo@iet.unipi.it>, Vincenzo Maffione <v.maffione@gmail.com>, "freebsd-net@freebsd.org" <net@freebsd.org>
Subject:   Re: Handling Jumbo Packets
Message-ID:  <20140324185400.GQ32089@funkthat.com>
In-Reply-To: <CADDTU%2BDYxnEwu%2BtWxfofrykd21JLpc3ps8Y28WwF4tgwXqid5A@mail.gmail.com>
References:  <CADDTU%2BCxQdkzktGnvj4SqpG2N-ccN1vNThtQUoue1H00Dkaf5A@mail.gmail.com> <CA%2BhQ2%2Bjjihzy1P=s%2BAtSzZfTmL-yLRgCh36331azw2OUTafiaQ@mail.gmail.com> <CADDTU%2BDYxnEwu%2BtWxfofrykd21JLpc3ps8Y28WwF4tgwXqid5A@mail.gmail.com>

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soumya panigrahi wrote this message on Mon, Mar 24, 2014 at 15:17 +0530:
> On Mon, Mar 24, 2014 at 2:45 PM, Luigi Rizzo <rizzo@iet.unipi.it> wrote:
> 
> > On Mon, Mar 24, 2014 at 7:40 AM, soumya panigrahi <
> > soumyapanigrahi@gmail.com> wrote:
> >
> >> First of all I would like to thank you all for developing an awesome
> >> packet capturing framework.
> >> I have been using it recently and find it very very fast and efficient.
> >>
> >> However off late I am trying to capture Jumbo packets on the network for
> >> the following scenario's but that doesn't seem to be working.
> >
> > the sysctl tree dev.netmap (on FreeBSD)
> > or the filesystem
> > /sys/module/netmap_lin/parameters
> > (on Linux)
> > have a number of parameters that control buffer numbers and sizes.
> > In particular "buf_size" is used to allocate packet buffers and
> > you can bump it up from 2048 to something larger to accommodate
> > jumbo buffers.
> >
> > Depending on the NIC you may need to do something in the initialization
> > of the NIC itself to allow sending/receiving frames larger than 1518 bytes,
> > but I'd just try and see whether it works for you now.
> 
> Thanks a lot for the quick response.
> I will also try to see if we can handle Jumbo frames by tweaking the
> buf_size parameter on Linux.
> In addition can you advice if NetMap can de-fragment Jumbo packets that
> were fragmented as IP packets by intermediate IP device because of MTU size
> limitation?
> For example a 9000 Bytes single ethernet frame was fragmented into 1516
> Bytes multiple ethernet frames at IP layer.
> Can netmap re-assemble these fragmented ethernet frames into a single Jumbo
> frame before passing it the application?
> If so how?

There really isn't a way to tell the difference between a packet that
was sent fragemented originally, and one that was fragmented along the
route...

Now you might be able to tell with some weird situations when the
original fragemented packet was refragmented by a router, but this
isn't the case you're asking about...  look at the Fragmentation
section of the IPv4 wikipedia entry for an example:
https://en.wikipedia.org/wiki/Ipv4#Fragmentation

And as Luigi said, all fragment reassembly has to be done by the netmap
client.. that's the point of netmap, is that the kernel doesn't touch
the packets...  if it did do the reassembly, then the kernel would have
to look at every packet, defeating the point of netmap...

-- 
  John-Mark Gurney				Voice: +1 415 225 5579

     "All that I will do, has been done, All that I have, has not."



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