MTU size

2016-01-29 Thread Christer Solskogen 
Hi!

On all of my x86_64 Linux boxes the default MTU is 1500. But with SuSE
(both 11 and 12) on s390 they seems to be 1492. How come?

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Re: MTU size

2016-01-29 Thread Donald J. 
More important is too large an MSS which can cause fragmentation.
You can test your network route from z/OS with this ping:
ping 192.168.20.54(PMTU yes LENGTH 1472 Verbose

'PMTU yes' says 'do not fragment'.  On my host 1472 is max value that
does not fragment.  1473 will need fragmentation.  Going over a
VPN will have larger TCP/IP headers giving a smaller MSS.
For dynamic detection to work, your routers must pass the necessary
ICMP packets.  By default, some do not.

Good article here:
http://www.networkworld.com/article/2224654/cisco-subnet/mtu-size-issues.html

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  Donald J.
  dona...@4email.net

On Fri, Jan 29, 2016, at 07:35 AM, Cohen, Sam wrote:
> Chris,
>
> If you look in the hardware documentation for the Open Systems Adapter (OSA), 
> it has a "normal" MTU of 1492, so your Linux is matching the hardware 
> settings.  The OSA does support jumbo frames (8992) if your network is 
> configured to use them.
>
> Thanks,
>
>
> Sam Cohen
> Levi, Ray & Shoup, Inc.
>
>
>
> -Original Message-
> From: Linux on 390 Port [mailto:LINUX-390@VM.MARIST.EDU] On Behalf Of 
> Christer Solskogen
> Sent: Friday, January 29, 2016 1:56 AM
> To: LINUX-390@VM.MARIST.EDU
> Subject: MTU size
>
> Hi!
>
> On all of my x86_64 Linux boxes the default MTU is 1500. But with SuSE (both 
> 11 and 12) on s390 they seems to be 1492. How come?
>
> --
> For LINUX-390 subscribe / signoff / archive access instructions, send email 
> to lists...@vm.marist.edu with the message: INFO LINUX-390 or visit
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Re: MTU size

2016-01-29 Thread Alan Altmark 
On Friday, 01/29/2016 at 04:28 GMT, "Donald J."  
wrote:
> More important is too large an MSS which can cause fragmentation.
> You can test your network route from z/OS with this ping:
> ping 192.168.20.54(PMTU yes LENGTH 1472 Verbose
>
> 'PMTU yes' says 'do not fragment'.  On my host 1472 is max value that
> does not fragment.  1473 will need fragmentation.  Going over a
> VPN will have larger TCP/IP headers giving a smaller MSS.
> For dynamic detection to work, your routers must pass the necessary
> ICMP packets.  By default, some do not.
>
> Good article here:
> 
http://www.networkworld.com/article/2224654/cisco-subnet/mtu-size-issues.html

Even with all of that in place, you still need to get your local MTU value 
correct.  It represents a ceiling on the MTU size PMTU discovery will try, 
and it sets the limit on the TCP MSS.  Everyone connected to the same LAN 
segment needs to have the same MTU on that LAN segment.

This is still very much a stone-knives-and-bearskins issue with TCP/IP. 
IMO, it's something IEEE should address since the mothership for the MTU 
is the local ethernet switch.  That's where the MTU is configured ("Are we 
using jumbo frames or not?").  There's a smart device at the other end of 
the wire.  There are a variety of things negotiated at a low level 
(duplex, speed, VEPA) and there's no practical reason "maximum frame size" 
can't be discovered.  MTU for a particular protocol is a function of the 
MFS.  (We're familiar with MFS from HiperSockets.)

Alan Altmark

Senior Managing z/VM and Linux Consultant
Lab Services System z Delivery Practice
IBM Systems & Technology Group
ibm.com/systems/services/labservices
office: 607.429.3323
mobile; 607.321.7556
alan_altm...@us.ibm.com
IBM Endicott

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Re: MTU size

2016-01-29 Thread Cohen, Sam 
Chris,

If you look in the hardware documentation for the Open Systems Adapter (OSA), 
it has a "normal" MTU of 1492, so your Linux is matching the hardware settings. 
 The OSA does support jumbo frames (8992) if your network is configured to use 
them.

Thanks,


Sam Cohen
Levi, Ray & Shoup, Inc.



-Original Message-
From: Linux on 390 Port [mailto:LINUX-390@VM.MARIST.EDU] On Behalf Of Christer 
Solskogen
Sent: Friday, January 29, 2016 1:56 AM
To: LINUX-390@VM.MARIST.EDU
Subject: MTU size

Hi!

On all of my x86_64 Linux boxes the default MTU is 1500. But with SuSE (both 11 
and 12) on s390 they seems to be 1492. How come?

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Re: brtfs -u issue

2016-01-29 Thread Rick Troth 
On 01/28/2016 10:26 AM, PHILIP TULLY wrote:
> According to the documentation to change the uuid of a btrfs
> filesystem in SLES 12 s390x, we should be using the  btrfstune with
> the  –u option, but this doesn't seem to work.   The result is we are
> unable to mount a duplicate copy of a disk with BTRFS in two different
> mounts on the same system. 

Need more detail.
Can you share how you're trying to mount it? actual command sequence?

Forgive me ... couldn't help but think you *might* be trying to
mount-by-UUID,
which would not work if the two vols have the same UUID.
(i.e. stamping a new UUID on the dup vol failed)
I presume you would have thought of this, but ... gotta ask.

-- R; <><

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Re: MTU size

2016-01-29 Thread Alan Altmark 
On Friday, 01/29/2016 at 09:58 GMT, Christer Solskogen 
 wrote:
> On all of my x86_64 Linux boxes the default MTU is 1500. But with SuSE
> (both 11 and 12) on s390 they seems to be 1492. How come?

Historically, it's because there are two different kinds of non-jumbo 
ethernet frames.  The traditional "DIX" frame has room for a 1500-byte 
packet.  The other, defined by IEEE 802.3 has a slightly longer frame 
header that enables multiple multi-protocol hosts to share a single 
network adapter, leaving room for only 1492 bytes.  (See RFC 1042.) Bottom 
line, if you're on an IEEE 802.3 network with SNAP headers, you have to 
use MTU 1492.

And in the Before Times, the MTU also dictated the maximum packet size a 
host could *receive*.  (Hmmm...so other guy's MTU has to match your MRU.) 
But that's usually not an issue any more.  Not, at least, in the 
mainframes -- dunno about x86 adapters.  And we have dynamic path MTU 
discovery to help us.

All of that said, as of the z13, OSAs no longer support IEEE 802.3 
networks, so 1500 is the correct default from here on out.

OSAs are capable of telling the host the MTU size, so Linux *can* get that 
value from the OSA adapter.  It's just that this isn't something other 
platforms can do and so the configuration process for the IP layer isn't 
really amenable to self-discovery.

All of this fancy local MTU discovery was based on ARP.  When you ARP for 
a IP address, you include "hardware type" in the ARP packet.  There is a 
value for 802.2 (which covers 802.3) and a value for Ethernet (DIX). First 
you ARP with hw=802.2.  If you get a response, you use MTU 1492.  If you 
don't you try again with hw=Ethernet.  If you get a response, 1500 it is 
(unless you have configured for Jumbo frames).

This same issue drives the jumbo frame configuration of 9000 vs. 8992. 
There's that 8-byte difference, even though 802.2 networks can't carry 
jumbo frames, and IEEE has been otherwise derelict in their duty to 
standardize them.

Alan Altmark

Senior Managing z/VM and Linux Consultant
Lab Services System z Delivery Practice
IBM Systems & Technology Group
ibm.com/systems/services/labservices
office: 607.429.3323
mobile; 607.321.7556
alan_altm...@us.ibm.com
IBM Endicott

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Re: MTU size

2016-01-29 Thread Ron Wells 
OSA Gig supports jumbo..BUT routers/switches in the path must be set to do so 
as well.

-Original Message-
From: Linux on 390 Port [mailto:LINUX-390@VM.MARIST.EDU] On Behalf Of Cohen, Sam
Sent: Friday, January 29, 2016 9:36 AM
To: LINUX-390@VM.MARIST.EDU
Subject: Re: MTU size

Chris,



If you look in the hardware documentation for the Open Systems Adapter (OSA), 
it has a "normal" MTU of 1492, so your Linux is matching the hardware settings. 
 The OSA does support jumbo frames (8992) if your network is configured to use 
them.



Thanks,





Sam Cohen

Levi, Ray & Shoup, Inc.







-Original Message-

From: Linux on 390 Port [mailto:LINUX-390@VM.MARIST.EDU] On Behalf Of Christer 
Solskogen

Sent: Friday, January 29, 2016 1:56 AM

To: LINUX-390@VM.MARIST.EDU

Subject: MTU size



Hi!



On all of my x86_64 Linux boxes the default MTU is 1500. But with SuSE (both 11 
and 12) on s390 they seems to be 1492. How come?



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Re: brtfs -u issue

2016-01-29 Thread Mark Post 
>>> On 1/28/2016 at 10:26 AM, PHILIP TULLY  wrote: 
> According to the documentation to change the uuid of a btrfs filesystem 
> in SLES 12 s390x, we should be using the  btrfstune with the  *u 
> option, but this doesn't seem to work.   The result is we are unable to 
> mount a duplicate copy of a disk with BTRFS in two different mounts on 
> the same system.

My understanding is that it is -U, not -u.  Also, that it is not available in 
SLES12, but is in SLES12 SP1.  Whether it actually works there or not, I cannot 
say.

> Does anyone have any suggestions?

Try SLES12 SP1?  Not ideal, of course, but it might give you/us a data point.


Mark Post

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Re: MTU size

2016-01-29 Thread Christer Solskogen 
Thanks everyone! I've learned something today as well!

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