Packet fragmentation

[bolik]

Hi all. I'm having issue with one of my application servers running on SuSe Linux.

Network engineer was using some network sniffer and claimed that although MTU is set to 1500 (on SuSe ethernet interface) he found lots of packets of bigger size.

I'm wondering is it possible to enable packet fragmentation on SuSe and if yes, then how?

[hcvv]

When you talk about SuSE Linux, do you mean openSUSE or SLES/SLED?

I ask because SLED/SLES have their own forums:
SUSE Linux Enterprise Desktop (SLED) - NOVELL FORUMS
SUSE Linux Enterprise Server (SLES) - NOVELL FORUMS

OTOH, when somebody here on openSUSE thinks she/he might be able to help you, she/he will come forward.

[bolik]

Thanks for the forwrding to appropriate forums. But I think this issue is common to these systems.

[ken_yap]

The packet size depends on what kind of interface you have. Traditionally it cannot exceed 1500 (actually 1536 IIRC, but 36 goes to Ethernet headers) on Ethernet, but recent Gb Ethernet interfaces are capable of transmitting "jumbo frames". But I think this has to be explicitly enabled. Also if you are connected to a switch, the switch has to be able to handle jumbo frames too.

The problem is you have a vague claim by a network engineer without any additional detail. What you really need is somebody onsite with the knowledge to challenge your network engineer for clarification. It's not clear that these "large packets" (how large?) are due to Linux and even if so, what effects it has.

And it would be the same for any Linux distro, not just openSUSE, since the kernel is the same.

[deano_ferrari]

Network engineer was using some network sniffer and claimed that although MTU is set to 1500 (on SuSe ethernet interface) he found lots of packets of bigger size.

I find this hard to believe. In any case, you can always lower the MTU value for a given network interface. For example

ip link set dev eth0 mtu 1300

man ip for info on this command.

To make this permanent, you could edit /etc/sysconfig/network/ifcfg-eth0 (or similar) and enter a suitable value for MTU.

However, this is something I've never needed to play with - on any network.

[bolik]

The packet size depends on what kind of interface you have. Traditionally it cannot exceed 1500 (actually 1536 IIRC, but 36 goes to Ethernet headers) on Ethernet, but recent Gb Ethernet interfaces are capable of transmitting "jumbo frames". But I think this has to be explicitly enabled. Also if you are connected to a switch, the switch has to be able to handle jumbo frames too.

The problem is you have a vague claim by a network engineer without any additional detail. What you really need is somebody onsite with the knowledge to challenge your network engineer for clarification. It's not clear that these "large packets" (how large?) are due to Linux and even if so, what effects it has.

And it would be the same for any Linux distro, not just openSUSE, since the kernel is the same.

OK then. I'll refraze my question (with a help of another example):

I'm having one Linux system and trying to do the following:
ping -M do -s 1600 server1.
And as a result I get:
Frag needed and DF set (mtu = 1500)
I need to enable packet fragmentation. How should I?

[ken_yap]

Don't use the -M do option. Maybe try -M dont?

You told ping to forbid fragmentation (see the man page) and then you ask how to enable it.

[Christian Riechers]

On 10/10/2009 10:36 AM, bolik wrote:
>
> Hi all. I'm having issue with one of my application servers running on
> SuSe Linux.
>
> Network engineer was using some network sniffer and claimed that
> although MTU is set to 1500 (on SuSe ethernet interface) he found lots
> of packets of bigger size.
>
> I'm wondering is it possible to enable packet fragmentation on SuSe and
> if yes, then how?
>
>

Using the packet sniffer, it should be easy enough to check the source
IP of the 'big' packets, if they are really transmitted from the machine
in question.
What exactly is the issue you have with the application server?

--
Christian