Re: Packets not captured, tcp acking lost segments. Large packets

[Martin Visser <martinvisser99 () gmail com>]

Michael,

"Large packets" are perfectly legal. It is possible that you are
sending "jumbo" packets which are supported on gigabit ethernet.
Normally you would need to configure that on your servers explicitly
though and you need to make sure your clients are also similarly
configured to support these. (The reason I didn't mention it earlier
is that it I expect you would have mentioned jumbo packets if you were
using thhese) It would be good for you to confirm that your server is
using Large Segment Offload. This will in the NIC driver
configuration.
BTW Laura Chappell blogged about LSO at
http://laurachappell.blogspot.com/2010/09/analyzing-huge-packets-tsolro.html
and in fact I got a bit confused by it about a year or so ago.

Yep, that wiki entry is a bit brief, and could be enhanced. The issue
I guess is for to understand whether you found a bug or whether you
aren't using Wireshark correctly of not. (Having a driver using LSO
shouldn't be causing you to drop packets, I am not sure what is going
on there). I have just found though a post and response from April
2010 that might shed some light. Have a look at
http://www.wireshark.org/lists/wireshark-users/201004/msg00067.html
and possibly try out Graham's suggestion at the bottom of
http://www.wireshark.org/lists/wireshark-users/201004/msg00068.html

One other reason you are getting that could be you are using a teaming
adapter. If you are only using Wireshark to monitor a single physical
NIC you might not be seeing all the traffic.

Regards, Martin

MartinVisser99 () gmail com



On 7 January 2011 20:25, Michael Lynch <michaellynch511 () gmail com> wrote:
> Thanks Martin
>
> I realise that sniffing traffic on the wire is more practical, but I am in a
> test environment in this case.
>
> Wouldn't the machine on the mirrored port simply experience the same
> problem?
> (Oh wait, I see what you mean.. I am monitoring on the server... and the
> large packets are out-going, and so therefore if I was sniffing on the wire
> they would be of normal size.)
>
> However, I understand this is an open source project, and I really want to
> help (I beleive many other users have experienced this problem with perhaps
> no resolution as to what is really going on).
> But I'm not about to delve in the ins and outs or even the code of WinPCap.
>
> For a novice user it would be helpful if either a) Large packets at least
> had a mention, or b) The issue was at least mentioned in
> http://wiki.wireshark.org/TCP_ACKed_lost_segment
>
> Thanks for explaining Martin.
> I will post some net caps perhaps on monday back in the office.
> I opened the NetMon capture file in Wireshark and the large packets are
> listed and the "acked lost segments" are no longer there.
> However I still saw some "acked lost segment", god knows where they are
> comming from. (I think we have a few problems with this applications use of
> SOAP, which is what led me here in the first place!!)
>
> Cheers!
> Michael.
>
> ----- Original Message ----- From: "Martin Visser"
> <martinvisser99 () gmail com>
> To: "Community support list for Wireshark" <wireshark-users () wireshark org>
> Sent: Friday, January 07, 2011 3:14 PM
> Subject: Re: [Wireshark-users] Packets not captured, tcp acking lost
> segments. Large packets
>
>
> > Michael,
> >
> > Normally your server will be connected to a switch. If this is a
> > manageable switch, you should be able to configure it to port-mirror,
> > which means a copy of the traffic on one port is sent to another port.
> > This will enable easy monitoring of your traffic, and you will see
> > what is actually going on the wire. When I meant "avoid", it is more
> > about making sure you see what is on the wire rather than the tricks
> > that the driver might be doing. (I try to avoid installing Wireshark
> > or Net Mon on production servers - not that it doesn't work, but I
> > don't want my measuring application potentially affecting the normal
> > performance of the server).
> >
> > I'm not sure if there is possibly an issue with WinPcap library not
> > working properly on your box of not. You might want to post a small
> > capture file showing what you saw with Wireshark and what you captured
> > with Net Mon. (Also note that Wireshark can read Net Mon files - does
> > this show the difference as well?)
> >
> > Regards, Martin
> >
> > MartinVisser99 () gmail com
> >
> >
> >
> > On 7 January 2011 14:27, Michael Lynch <michaellynch511 () gmail com> wrote:
> > > Thanks Martin
> > >
> > > I read up on LSO. It explains how these >4K packets are appearing
> > >
> > > Yes I am running Wireshark on the application server. I had a hard time
> > > installing it on my switch!! No CD-rom drive!! :)
> > > (I am not sure what you mean by 'Server Switch')
> > >
> > > But why is MS Net Mon seeing these large packets?
> > >
> > > Wireshark is providing misleading information and I don't think i'm the
> > > only
> > > one that is suffering major confusion.
> > > I think my self lucky as I have witnessed the packets in NetMon.
> > > Most users on the net seem to have presumed that packets are being lost!
> > >
> > > > Wireshark will see the large segments go out.
> > >
> > > But its not...?
> > >
> > > > You might want to capture on your server switch rather than the server
> > > > to avoid seeing this.
> > >
> > > I don't want to avoid packets, I want to see the packets!
> > >
> > >
> > >
> > > Cheers
> > > Michael.
> > >
> > >
> > >
> > > ----- Original Message ----- From: "Martin Visser"
> > > <martinvisser99 () gmail com>
> > > To: "Community support list for Wireshark"
> > > <wireshark-users () wireshark org>
> > > Sent: Friday, January 07, 2011 1:46 PM
> > > Subject: Re: [Wireshark-users] Packets not captured, tcp acking lost
> > > segments. Large packets
> > >
> > >
> > > > It sounds like you are capturing traffic on the server rather than the
> > > > wire. If your server NIC and driver does Large Segment Offload, the
> > > > segmentation is done by the NIC, which allows the transfer from your
> > > > kernel to the NIC do be done in larger chunks, meaning a more
> > > > efficient transfer. Wireshark will see the large segments go out.
> > > >
> > > > You might want to capture on your server switch rather than the server
> > > > to avoid seeing this.
> > > >
> > > > Regards, Martin
> > > >
> > > > MartinVisser99 () gmail com
> > > >
> > > >
> > > >
> > > > On 7 January 2011 11:25, Michael Lynch <michaellynch511 () gmail com>
> > > > wrote:
> > > > > Hi All
> > > > >
> > > > > I think I've found something everyone may be interested in...
> > > > >
> > > > > In wireshark I am monitoring traffic of a SOAP application.
> > > > >
> > > > > Upon transfer of a BLOB, wire shark is showing many "Tcp ACKed lost
> > > > > segment"
> > > > > packets.
> > > > > On top of this there is no evidence of any of the SOAP data, other than
> > > > > the
> > > > > initial header.
> > > > >
> > > > > Now I've search for this lost segment business, and no forums really
> > > > > seem
> > > > > to
> > > > > have much of a solution other than perhaps disabling sequence analysis.
> > > > >
> > > > > However I think I have found the problem, but I have no understanding
> > > > > of
> > > > > the
> > > > > whats and whys.
> > > > >
> > > > > In Microsoft Net Mon, the data packets ARE THERE!!!
> > > > >
> > > > > i.e
> > > > > Sent packet: Captured Frame Length = 4434, Media Type = Ethernet...
> > > > > Continuaion to packet #76.
> > > > > Received packet: Ack
> > > > >
> > > > > The received packet is the only packet that shows up in Wireshark! (I
> > > > > have
> > > > > cross referenced the Packet ID)
> > > > > Wireshark is NOT COLLECTING LARGE PACKETS!!
> > > > >
> > > > > I have no idea how packets THAT LARGE got onto the wire IN THE FIRST
> > > > > PLACE!!
> > > > >
> > > > > What is going on??!! :)
> > > > >
> > > > > Cheers
> > > > > Michael
> > > > >
> > > > >
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